Scholarly article on topic 'Review of material design and reactor engineering on TiO2 photocatalysis for CO2 reduction'

Review of material design and reactor engineering on TiO2 photocatalysis for CO2 reduction Academic research paper on "Chemical engineering"

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{"Co2 photoreduction" / Photocatalysis / "Photocatalytic reactor" / "Solar fuel production" / "Titanium dioxide" / "Artificial photosynthesis"}

Abstract of research paper on Chemical engineering, author of scientific article — Oluwafunmilola Ola, M.Mercedes Maroto-Valer

Abstract The continuous combustion of non-renewable fossil fuels and depletion of existing resources is intensifying the research and development of alternative future energy options that can directly abate and process ever-increasing carbon dioxide (CO2) emissions. Since CO2 is a thermodynamically stable compound, its reduction must not consume additional energy or increase net CO2 emissions. Renewable sources like solar energy provide readily available and continuous light supply required for driving this conversion process. Therefore, the use of solar energy to drive CO2 photocatalytic reactions simultaneously addresses the aforementioned challenges, while producing sustainable fuels or chemicals suitable for use in existing energy infrastructure. Recent progress in this area has focused on the development and testing of promising TiO2 based photocatalysts in different reactor configurations due to their unique physicochemical properties for CO2 photoreduction. TiO2 nanostructured materials with different morphological and textural properties modified by using organic and inorganic compounds as photosensitizers (dye sensitization), coupling semiconductors of different energy levels or doping with metals or non-metals have been tested. This review presents contemporary views on state of the art in photocatalytic CO2 reduction over titanium oxide (TiO2) nanostructured materials, with emphasis on material design and reactor configurations. In this review, we discuss existing and recent TiO2 based supports, encompassing comparative analysis of existing systems, novel designs being employed to improve selectivity and photoconversion rates as well as emerging opportunities for future development, crucial to the field of CO2 photocatalytic reduction. The influence of different operating and morphological variables on the selectivity and efficiency of CO2 photoreduction is reviewed. Finally, perspectives on the progress of TiO2 induced photocatalysis for CO2 photoreduction will be presented.

Academic research paper on topic "Review of material design and reactor engineering on TiO2 photocatalysis for CO2 reduction"

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Journal of Photochemistry and Photobiology C: Photochemistry Reviews

journal homepage www.elsevier.com/locate/jphotochemrev

Review

Review of material design and reactor engineering on TiO2 photocatalysis for CO2 reduction

Oluwafunmilola Ola*, M.Mercedes Maroto-Valer

Centre for Innovation in Carbon Capture and Storage (CICCS), School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, United Kingdom

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ARTICLE INFO

ABSTRACT

Article history:

Received 25 February 2015

Received in revised form 19 May 2015

Accepted 10 June 2015

Available online 23 June 2015

Keywords: Co2 photoreduction Photocatalysis Photocatalytic reactor Solar fuel production Titanium dioxide Artificial photosynthesis

The continuous combustion of non-renewable fossil fuels and depletion of existing resources is intensifying the research and development of alternative future energy options that can directly abate and process ever-increasing carbon dioxide (CO2) emissions. Since CO2 is a thermodynamically stable compound, its reduction must not consume additional energy or increase net CO2 emissions. Renewable sources like solar energy provide readily available and continuous light supply required for driving this conversion process. Therefore, the use of solar energy to drive CO2 photocatalytic reactions simultaneously addresses the aforementioned challenges, while producing sustainable fuels or chemicals suitable for use in existing energy infrastructure. Recent progress in this area has focused on the development and testing of promising TiO2 based photocatalysts in different reactor configurations due to their unique physicochemical properties for CO2 photoreduction. TiO2 nanostructured materials with different morphological and textural properties modified by using organic and inorganic compounds as photosensitizers (dye sensitization), coupling semiconductors of different energy levels or doping with metals or non-metals have been tested. This review presents contemporary views on state of the art in photocatalytic CO2 reduction over titanium oxide (TiO2) nanostructured materials, with emphasis on material design and reactor configurations. In this review, we discuss existing and recent TiO2 based supports, encompassing comparative analysis of existing systems, novel designs being employed to improve selectivity and photoconversion rates as well as emerging opportunities for future development, crucial to the field of CO2 photocatalytic reduction. The influence of different operating and morphological variables on the selectivity and efficiency of CO2 photoreduction is reviewed. Finally, perspectives on the progress ofTiO2 induced photocatalysis forCO2 photoreduction will be presented.

© 2015 Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/

licenses/by/4.0/).

Contents

1. Introduction ....................................................................................................... 17

1.1. Scope of the review ........................................................................................... 17

1.2. CO2 photocatalysis ............................................................................................ 18

1.3. TiO2 ........................................................................................................ 20

2. Modified TiO2 catalysts ..............................................................................................20

2.1. Dye sensitization..............................................................................................20

2.2. Coupling of semiconductors ..................................................................................... 23

2.3. Metal and non-metal modifications .............................................................................. 24

2.3.1. Metal doping..........................................................................................24

2.3.2. Metal semiconductor modification ........................................................................ 26

2.3.3. Non-metal modification ................................................................................. 26

2.3.4. Co-doping ............................................................................................ 27

* Corresponding author at: School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, United Kingdom. E-mail address: O.O.Ola@hw.ac.uk (O. Ola).

http://dx.doi.org/10.1016/jjphotochemrev.2015.06.001

1389-5567/© 2015 Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

3. Influence of operating parameters on CO2 reduction.......................................................................28

3.1. Effect of reductant ............................................................................................28

3.2. Effect of temperature ..........................................................................................28

3.3. Effect of pressure .............................................................................................29

3.4. Effect of particle size ..........................................................................................29

4. Catalyst configuration: supports .......................................................................................30

4.1. Glass .......................................................................................................30

4.2. Optical fibers.................................................................................................30

4.3. Monoliths ...................................................................................................31

4.4. Other supports ...............................................................................................32

5. Support immobilization techniques ....................................................................................33

5.1. Sol-gel method ...............................................................................................33

5.1.1. Thermal treatment .....................................................................................34

5.1.2. Influence of organic contaminants ........................................................................35

5.2. Vapor deposition..............................................................................................35

6. Photoreactor design and configuration ..................................................................................35

6.1. Fluidized and slurry reactor designs ..............................................................................36

6.2. Fixed bed reactor designs .......................................................................................37

7. Conclusions and future perspectives....................................................................................37

Acknowledgements .................................................................................................38

References ........................................................................................................38

Dr Oluwafunmilola Ola joined the Centre for Innovation in Carbon Capture and Storage at Heriot-Watt University as Research Associate in CO2 conversion and solar fuels. She is currently working on an EPSRC funded project to engineer novel photoreactors that can achieve efficient hydrocarbon conversion and separation from CO2 for solar fuel production. Prior to this, she obtained MSc. (with distinction in Environmental Engineering at the University of Nottingham in 2010 and Ph. D. in Chemical Engineering at Heriot-Watt University. Her work on solar fuel production from CO2 has resulted in over 25 (7 journals and 19 conference papers) publications that have been cited over 46 times. She is also a reviewer for 3 journals (Catalysis Science and Technology, Renewable Energy and ChemCatChem). Her contribution to this research field within the interface of materials chemistry and chemical engineering has led to the award of 10 travel grants and prizes such as RSC Energy Sector Ph.D. Thesis Award, RSC Solar Fuels Symposium Best Poster Prize, UKERC 3rd Place Poster Award and Engineering Research Showcase Highest Merit for Poster Presentation Award.

Prof M. Mercedes Maroto-Valer is the first Robert Buchan Chair in Sustainable Energy Engineering at Heriot-Watt University. This is a joint appointment between the School of Engineering and Physical Sciences and the Institute of Petroleum Engineering. At Heriot-Watt, she is the Head of the Institute for Mechanical, Processing and Energy Engineering (School of Engineering and Physical Sciences) and leads the pan-University Energy Academy. She is also Director of the EPSRC funded Centre for Innovation in Carbon Capture and Storage (CICCS). She is a member of the Directorate of the Scottish Carbon Capture and Storage (SCCS).She obtained a BSc with Honours (First Class) in Applied Chemistry in 1993 and then a Ph.D. in 1997 at the University of Strathclyde (Scotland). Following a one-year postdoctoral fellowship at the Centre for Applied Energy Research (CAER) at the University of Kentucky in US, she moved to the Pennsylvania State University in US, where she worked as Research Fellow and from 2001 as Assistant Professor and became Program Coordinator for Sustainable Energy. She joined the University of Nottingham as Reader in 2005 and within 3 years she was promoted to Professor in Energy Technologies. During her time at Nottingham she was the Head of the Energy and Sustainability Research Division at the Faculty of Engineering.She has over 300 publications, including editor of 3 books, 90 refereed publications in journals and book chapters and over 200 contributions to other journals and conference proceedings. She holds leading positions in professional societies and editorial boards, including FRSC, Chair of the RSC Energy Sector, Member of the Scientific Advisory Committee of the RCUK Energy Programme and of the UKERC Research Committee and Editor-in-Chief of Greenhouse Gases: Science and Technology.Her outstanding contributions, publication record and service to the chemical sciences and energy engineering have been recognized with numerous international prizes

and awards, including 2013 Hong Kong University William Mong Distinguished Lecture, 2011 RSC Environment, Sustainability and Energy Division Early Career Award, 2009 Philip Leverhulme Prize, 2005 U.S. Department of Energy Award for Innovative Development, 1997 Ritchie Prize, 1996 Glenn Award—Fuel Chemistry Division of the American Chemical Society and the 1993 ICI Chemical & Polymers Group Andersonian Centenary Prize.

1. Introduction

1.1. Scope of the review

The pressures arising from the need for improved living standards triggered mainly by economic and population growth have detrimental effects on the environment through the continuous consumption of finite fossil fuel reserves linked to increasing CO2 emissions [1,2]. The need to meet global energy demand predicted to increase due to a rising global population has led to the development of different strategies by which CO2 emissions can be reduced. These can be achieved through the use of non-fossil fuels such as hydrogen, renewable and nuclear energy, increased energy efficiency, reduced deforestation and capture and storage of CO2 emissions or by using a combination of these options. Although nuclear energy can supply low carbon energy, there are concerns with regards to waste generation [3,4]. Public acceptance and limited water availability are also key issues associated with this technology. There are several challenges, including capital cost, source and seasonal availability, economic barriers, geographical distribution and environmental issues as major constraints in the use of renewable energy like biomass, hydropower, solar and wind energy [1,5].

Conversely, the use of hydrogen energy eliminates the constraints associated with environmental impacts, but requires full optimization and energy input as its production is mainly from steam reforming and water electrolysis. Additional drawbacks are related to storage and hydrogen fueling infrastructures such as fuel

cell vehicles and fueling stations which are still being developed [6]. Carbon dioxide capture and storage (CCS) serve as a means of reducing CO2 emissions, where CO2 is removed and captured from large point sources of industrial processes, such as petrochemical plants, power generation, cement, iron and steel production and others. This is followed by its subsequent transport, injection and storage in various sinks, such as geological storage (underground saline aquifers, depleted oil and gas reservoirs and deep coal seams), mineral carbonation and ocean storage. The CO2 separated and captured is then considered to be stored for prolonged periods ranging from centuries to millions of years [7]. Although the technologies associated with CO2 capture and separation show great potential with regards to reducing cost and energy penalty [8], they still require further development. Bachu et al. compared the aforementioned storage options and identified geological storage as the preferable option owing to the significant quantity of CO2 that can be sequestered, long retention time and great depth of experience from the oil and gas industry that would accelerate the immediate deployment after full-scale implementation [9]. CCS still needs optimization in order to fill the gap in knowledge with regards to the location and capacity of possible geological locations and possible leakage that could occur during or after injection. Public acceptance has also been recognized as a key factor that can pose barriers to the implementation of geological storage as the public can accelerate CCS development [9,10].

As CCS is still in the demonstration phase and may be uneconomical for emissions from small to medium sized sources, other sustainable alternatives with little or no environmental impacts and zero CO2 emissions need to be developed. These technologies which can offset the cost associated with CO2 capture and utilize CO2 for chemicals and fuels production rely solely on technological breakthroughs and market competitiveness due to their versatile applications. At present, utilization of CO2 accounts for approximately 2% of emissions and forecasts predict 700 megatons of CO2/year could be mitigated [11]. Alternatives processes such as photocatalysis, direct photolysis, and electrochemical reduction can utilize CO2 as opposed to geological storage [12,13]. The separated CO2 stream from the capture plant will serve as a feedstock for these conversion methods. The synthesized products such as methane, methanol, ethanol etc., can be used as chemicals, feedstock in fuel cells or hydrogen sources for electricity. Mikkelsen et al. have highlighted the difference between photo-catalysis and electrochemical reduction as the source of electrons which is obtained from irradiating semiconductors under light in the former and the application of an applied current in the latter [14]. Although electrochemical cells can convert CO2, Yano et al. reported low efficiencies resulting from the deactivation of electrodes as a major drawback. This is due to the deposition of poisoning species, i.e., adsorbed organic compounds, on the electrode [15,16]. The need for an inexpensive hydrogen source and high energy photons have been reported as drawbacks in direct hydrogenation and photolysis [17,18].

In this review, CO2 utilization by direct catalytic conversion of CO2 driven by light energy is described. Although CO2 conversion to energy rich and chemically useful products is endothermic, renewable carbon free sources like solar energy provide readily available and continuous light supply required for driving this conversion process under ambient conditions. Thus, carbon based fuels and chemicals suitable for end-use infrastructure can be produced from the conversion of CO2 and water by semiconductor photocatalysts capable of simultaneously driving chemical reactions and utilizing solar energy. These value added products can be used directly or supplement feed stocks in hydrocarbon production or chemical processes. Amongst semiconductor photocatalysts, titanium dioxide (TiO2) has been frequently used for UV induced photocatalysis due to its abundance, low cost and chemical

stability. However, its use is limited due to its large band gap; as it can only be activated by ultraviolet (UV) light which represents 2-5% of sunlight [19]. Attempts to improve the efficiency of this catalyst for CO2 photocatalysis are limited to the overall process efficiency being largely dependent on two factors—the physico-chemical properties of the catalyst and reactor configuration. The optical and electronic properties of TiO2 can be modified through the addition of metals or their oxides such as Cu [20], Ag [21], Pd [22] and Rh [23] and non-metals, such as nitrogen [24] and iodine [25]. When these metal and non-metal atoms occupy the interstitial sites, replace Ti in the substitutional sites or form aggregates on the surface of TiO2, they can cause changes in the properties of TiO2 [26], where the band structures and properties of TiO2 have been reported to be tailored by this process. These metals also serve as a source of charge-carrier traps which can increase the life span of separated electron-hole pairs, and thus enhancing the efficiency and product selectivity for CO2 photore-duction [13]. Furthermore, the textural properties such as the surface and bulk crystal structure, particle size and morphology can also be modified. However, it still remains largely unknown how the interaction of metal dopants or their oxides modify the surface chemistry and reaction mechanism of TiO2 for CO2 photoreduction. The configuration of catalyst particles in a photoreactor system is also another factor that can influence the overall photocatalytic efficiency of TiO2 [27,28]. This review discusses the current conditions, limitations, correlations and possibilities of existing systems i.e., photocatalysts and reactors. The concept and mechanism of CO2 photocatalysis using titanium dioxide (TiO2) is presented in Section 1. Section 2 is focused on the route by which the physicochemical properties of TiO2 can be modified. The influence of different operational variables i.e., reductant, temperature, pressure and particle size, on the photo-activity of TiO2 is addressed in Section 3 while Section 4 reviews various techniques for the fabrication of immobilized semiconductor photocatalysts. Section 5 covers different catalyst configurations by which the textural properties of TiO2 can be enhanced. Section 6 describes the current conditions, limitations and possibilities of existing photoreactor configurations available for CO2 photocatalysis. Comparative analyses of existing systems crucial to the field of CO2 photocatalytic reduction are discussed under Sections 4-6. Finally, Section 7 summarizes the review with future projections required for driving the field of CO2 photo-catalysis.

1.2. CO2 photocatalysis

Since CO2 is a chemically stable compound due to its carbon-oxygen bonds (bond enthalpy of C=O in CO2 is +805kJ/mol), its conversion to carbon based fuels requires substantial energy input for bond cleavage [29]. Renewable carbon free sources like solar energy provide readily available and continuous energy supply required for driving this conversion process. CO2 photocatalysis offers the possibility of utilizing captured CO2 to synthesize chemicals and fuels with the aid of semiconductor catalyst(s) under light irradiation. Apart from solar energy, other readily accessible light sources can be used. Fig. 1 highlights the typical photocatalytic process showing band gap formation in a typical semiconductor photocatalyst when exposed to light radiation. As shown in Fig. 1, the band gap is the energy region extending from the bottom of the empty conduction band (CB) to the top of the occupied valence band (VB). When an electron excited by light energy migrates from the fully occupied valence band of the semiconductor located at an energy level (Ev) to a higher energy (Ec) empty conduction band, electron-hole pairs are created if the absorbed light energy (hv) is greater than or equal to the band gap (Eg) of the semiconductor [12,30]. Eq. (1) presents the formation of

Fig. 1. Schematic of TiO2 photocatalyzed reaction where CB and VB represents the conduction band, and valence band, respectively.

electron-hole pairs [31] where e_, hv and h+ represents the conduction band electron, photon energy and hole in the valence band, respectively.

Photocatalyst

hue +h+

e- + h+ -> heat

Eg = Ec - Ev

Eq. (2) shows that the charge carriers may also recombine in the surface or bulk before reacting with adsorbed species, dissipating energy as heat or light while Eq. (3) shows the band gap energy (Eg) which is equal to the difference between the energy of the conduction (Ec) and the valence band (Ev). The reduction potential of photo-generated electrons is the energy level at the bottom of conduction band while the energy level at the top of valence band determines the oxidizing ability of photo-generated holes which determine the ability of the semiconductors to undergo oxidations and reductions [19]. The redox potential levels of the adsorbate species and the band gap energy determine the likelihood and rate of the charge transfer processes for electrons and holes [32]. For an electron to be donated to the vacant hole, the redox potential level of the donor is thermodynamically required to be above the VB position of the semiconductor, while that of the acceptor should be below the CB position. The reduction potentials for CO2 photore-duction with H2O to various products with reference to NHE at pH

7 are given in Eqs. (4)-(13) below [33,34]. 2H+ + 2e- ! H2 E0 = -0.41

H2O ! 1o2 + 2H+ + 2e- E0 = 0.82

CO2 + e- ! CO2' E0 = -1.90

CO2 + H+ + 2e- ! HCO2' E0 = -0.49

CO2 + 2H+ + 2e- ! CO + H2O E0 = -0.53

CO2 + 4H+ + 4e- ! HCHO + H2O E0 = -0.48

CO2 + 6H+ + 6e- = CH3OH + H2O

2CO2 + 8H+ + 12e- = C2H4 + 2H2O

2 CO2 +9H+ + 12e-

(8) (9)

CO2 + 8H+ + 8e- ! CH42H2O + H2O E0 = -0.24 (11)

C2H5OH + 3H2O E0 = -0.33 (13)

The band gaps of some of the most commonly used photo-catalysts are shown in Fig. 2 [35-38]. Although some of these semiconductor photocatalysts such as hematite (Fe2O3) are low cost and possess suitable band gap energies for visible light absorption, they suffer from different limitations. Metal chalco-genides semiconductors e.g., CdS, PbS, CdSe etc., have been reported as being susceptible to photocorrosion and low stability especially in aqueous media [31,39]. The addition of sulphide or sulfite to the contacting solution has been described to suppress photocorrosion. These semiconductors have also been reported to show some toxicity [40]. Since semiconductors like WO3, Fe2O3 and SnO2 possess conduction band edge values below the

Fig. 2. Band gap of some photocatalysts with respect to the redox potential of different chemical species measured at pH of 7. Adapted from Refs. [35-38].

hydrogen potential and Gupta and Tripathi pointed out the use of an external electrical bias as a requirement needed to achieve hydrogen evolution during water splitting [41]. Fe2O3 has also been reported by Fox and Dulay to show lower photoactivity compared to TiO2 and ZnO due to corrosion or the formation of short lived ligand-to-metal or metal-to-ligand charge transfer states [42]. The formation of Zn(OH)2 on the surface of the semiconductor ZnO observed from its dissolution in water has been reported by Bahnemann et al. to cause instability and deactivation over time [43]. On the other hand, TiO2 appears to be corrosion resistant and chemically stable [19].

Jeyalakshmi et al. also stated that large band gap semiconductors like TiO2 are more suitable for CO2 photoreduction due to sufficient positive and negative redox potentials in the VB and CB, respectively, compared to smaller band gap semiconductors like CdS where the energy levels of either the VB or CB tend to be unsuitable for water oxidation and/or CO2 photoreduction [44]. The photogenerated electrons and holes may recombine to generate heat energy or become trapped in surface sites, where reactions with electron accepting or donating species adsorbed on the surface of the semiconductor photocatalyst can occur [30,45]. For redox reactions to take place, electron-hole recombination must be minimized.

Several researchers have studied CO2 photoreduction using different semiconductors, including single catalysts like TiO2 [46] and ZrO2 [47], double catalysts like Cu-Fe/TiO2-SiO2 [48] and Cu-ZnO/Pt-K2Ti6O13 [49], metal and compound oxides such as CuO [50], LaCoO3 [51], Ga2O3 [52] and ATaO3 where A represents Na, Li and K [53]. The basic characteristics of an ideal semiconductor photocatalyst were reviewed by Refs. [13,54-59]. These properties include the presence of a large surface area, cost-effectiveness, accessibility, resistance to photocorrosion or production of toxic by-products and the ability of the redox potentials of the photogenerated valence band and conduction band to be positive and negative in order for the electrons to act as an acceptor and donor, respectively. The most frequently used semiconductors are ZnO, CdS, TiO2, WO3 and NiO. However, compared to these semiconductors, TiO2 still remains the most researched semiconductor photocatalyst due to its availability, chemical stability, low cost, high photocatalytic activity and resistance to corrosion.

1.3. TiO2

The most common crystalline phases of TiO2 are rutile, anatase and brookite. The bulk properties of the crystalline forms of TiO2 are presented in Table 1. Anatase and rutile have lattice parameters (a and c) of 0.3733/0.4584 nm and 0.9370/0.2953 nm, respectively, in the unit cell based on the body centered tetragonal structure.

The anatase form is the most suitable for photocatalytic reactions due to its larger surface area, stability and higher activity compared to the rutile form [60-63]. The brookite form is not commonly accessible, difficult to synthesize and has not been proved for photocatalytic reactions [64], [65]. Bouras et al. reported that optimal photocatalytic efficiency can be obtained from a mixture of anatase with a small percentage of rutile through a synergistic effect between the two crystalline phases as electron hole recombination is prevented by the creation of energy wells which serve as an electron trap formed from the lower band gap of rutile [62]. Although TiO2 has several unique features, its use is limited due to its large band gap (see Table 1); as it can only be activated by ultraviolet light which represents 2-5% of sunlight [54,66]. Since visible light accounts for 45% of the solar spectrum [51,67], there is a need to develop titania based photocatalysts which are active under the visible light spectrum.

2. Modified TiO2 catalysts

Since the time scale of electron-hole recombination of TiO2 has been reported to be higher than the desirable redox reactions [33], it is crucial to modify the physicochemical properties of TiO2 to improve process efficiency. Suitable modification of the optical and electronic properties of TiO2 results in not only the reduction of the band width via the incorporation of addition energy levels but increased lifetime of the photogenerated electrons and holes via effective charge carrier separation and suppression of electron-hole recombination. Furthermore, the textural properties such as the surface and bulk crystal structure, particle size and morphology can also be modified. The photocatalytic activity of TiO2 for visible light can be increased by using organic and inorganic compounds as photosensitizers (dye sensitization), coupling semiconductors of different energy levels or doping with metals or non-metals to suppress recombination rate and thus increasing quantum yield [54,56,68,69]. Table 2 highlights a summary of literature on CO2 photoreduction using TiO2 modifications [20,22,25,70-121]. All these strategies are described below.

2.1. Dye sensitization

Dye sensitization is a means of increasing absorption toward the visible light region through the inducement of the photo-excited dye molecule [19,56,122,123]. Various dyes which harvest visible light that have been used as sensitizers include rhodamine B, porphyrins, thionine, rose bengal, erythrosine B etc. [124-126]. Electrons are transferred from the dye molecule to the conduction band of the semiconductor when the energy level of the dye molecule was more negative than the semiconductor. Fig. 3 and

Table 1

Structural properties of crystalline structures of TiO2.

Properties Crystalline forms

Anatase Rutile Brookite

Crystalline structure Tetragonal Tetragonal Rhombohedral

Lattice constants (nm) a=b=0.3733 a = b = 0.4584 a = 0.5436

c = 0.9370 c = 0.2953 b = 0.9166

c = 0.5135

Bravais lattice Simple, Body centred Simple, body centred Simple

Density (g/cm~3) 3.83 4.24 4.17

Melting point (°C) Turning into rutile 1870 Turning into rutile

Boiling point (°C) 2927a - -

Band gap (eV) 3.2 3.0 -

Refractive index (ng) 2.5688 2.9467 2.8090

Standard heat capacity, Cop 55.52 55.60 -

Dielectric constant 55 110-117 78

a Pressure at pO2 is 101.325 KPa.

Table 2

Modifications of TiO2 for CO2 photoreduction.

Modifications Photocatalyst Light source Reductant Products References

Dye sensitization Dye sensitized (perylene diimide derivatives) Pt 75 W daylight lamp H2O 0.74 mmol/gcatal CH4 [70]

impregnated on TiO2

N3 dye (Ru"(2,2'-bipyridyl-4,4'-dicarboxylate)2-Cu Solar concentrator H2O vapor 0.617 mmol/gcatal h CH4 [71]

(0.5 wt%)-Fe (0.5 wt%)/TiO2

Ru/RuOx sensitized TiO2 Solar simulator H2O 900 mLh-1 CH4 [72]

N719/TiO2 300 WXe lamp H2O/2 M NaOH 0.1781 mmol/cm2 CH3OH [73]

0.1292 mmol/cm2 CH2O

Semiconductor CdSe quantum dot (QD)-Pt/TiO2 films 300 WXe arc lamp, H2O 48ppmg-1h-1 CH4, [74]

coupling <100mW/cm2 3.3ppmg-1h-1 CH3OH

23.2 wt% AgBr-TiO2 150WXe lamp 0.2 M KHCO3 128.56 mmolg CH4 [75]

77.87 mmolgCH3OH

13.28 mmolgC2H5OH

32.14 mmol g CO

PdS quantum dot (QD)-Cu/TiO2 300WXe lamp H2O 0.82 mmolg-1 h-1 CO [76]

0.58 mmolg-1 h-1 CH4

0.31 mmol g-1 h-1 C2H6

CdS/TiO2 nanotubes 500WXe lamp 0.8 g NaOH 159.5 mmolg/catal CH3OH [77]

Bi2S3/TiO2 nanotubes 2.52 g Na2SO3 224.6 mmol g/catal CH3OH

CeO2/TiO2 SBA-15 300WXe lamp H2O <12mmol/gcatal CH4 [78]

45 wt% CdS/TiO2 125 WHg lamp H2O -16 mmolg/catal CH3OH [79]

-3 mmolg/catal CH3OH

TiO2/ZrO2 8 W Hg lamp 0.2 mpl/L NaOH —16 mmolg /catal CH4 [80]

—175 mmolg /catal H2

GaP/TiO2 1500WXe lamp H2O 118 mM/gcatal CH4 [81]

Metal doping 0.5 wt% Ru-TiO2 100WHg lamp 1 M 2-propanol 200 mmolg-Ti-1 CH4 [82]

-250 mmolg-Ti-1 H2

TiO2 pellets UVC lamp H2O vapor 0.16 mmol/h H2 [83]

0.25 mmol/h CH4

1 wt% Ag-TiO2 Solar concentrator H2O vapor 4.12 mmol/gcatalh CH3OH [84,85]

3 wt% CuO-TiO2 6 (10W) UV lamps, H2O 2655 mmol/gcatal CH3OH [20,86]

2450 mW/cm2

2 wt% Cu-TiO2 8WHg lamp 0.2 M NaOH 1000 mmol/gcatal CH3OH [87,88]

—20 mmol O2

TiO2 pellets 3 UVC lamps H2O 0.25 mmolh-1 CH4 [89]

5.2 wt% Ag-TiO2 300WHg lamp 0.2 M NaOH >10 mmol/gcatal CH4 + CH3OH [90]

0.15% Pt-TiO2 nanotubes 8WHg lamp H2O 4.8 mmolh-1/gTi-1 CH4 [91]

Pd/RuO2/TiO2 450 W Xe short arc lamp 0.05 M NaOH 72ppm HCOO- [92]

0.05 M Na2SO3

2 wt% Pd-TiO2 500WHg lamp H2O 24.7 x 10-8 mol CH4 [22]

2 wt% Cu-TiO2-SBA 15 400 W halide lamp 0.1 M NaOH and l_J n 627 mmolg-1h-1 CH4 [93]

0.5 wt% Cu/TiO2-SiO2 Xe arc lamp, 2.4 mW/cm2 H2O H2O 60 mmolg-1h-1 CH4, [94]

10 mmolg-1 h-1 CO

Kaolinite/TiO2 8WHg lamp 0.2 M NaOH 4.5 mmol gcatal CH4, [95]

2.5 mmol gcatal CO,

-5 mmol gcatal H2

0.1 wt% Y-TiO2 300WHg lamp 0.2 M NaOH 384.62 mmol gcatai HCHO [96]

1 wt% and 3 wt% Ce-TiO2 SBA 15 450 WXe lamp H2O 1 mmolg-1 CO [97]

3 wt% Ag-TiO2 8WHg lamp H2O -100 mmol/gcatal H2 [98]

-6 mmol/gcatal CH4

-14 mmol/gcatal C3H6

Ni-TiO2 (0.1 mol%) 6 (3W/cm2) UV lamps H2O 14 mmolgcatal CH4 [99]

La2O3/TiO2 300WXe Lamp H2O 4.57 mmol CH4 [100]

CeF3-TiO2 500WXe lamp H2O 162 mmolgcatal CH3OH [101]

1.5 wt% NiO-TiO2 200WHg lamp H2O 19.51 mmol/gcatalh CH3OH [102]

8.7 at% Pt/9.6at% Cu-TiO2 AM 1.5G solar simulator H2O >180ppm/cm2h H2 [103]

49 ppm/cm2 h CH4

< 25ppm/cm2 h CO

Ce-TiO2 8WHg lamp 0.2 N NaOH 16 mmol/gcatal CH4 [104]

750 mmol/gcatal H2

Pt/TiO2 500WXe Lamp H2O 389.2 ppm H2 [105]

277.2 ppm CH4

12.4ppm C2H6

785.3 ppm O2

Ti-KIT-6/Si-Ti = 100 300 WUV lamp H2O 4.14 mmol/gcatalh CH4 [106]

2.55 mmol/gcatalh H2

1.45 mmol/gcatalh CO

Pt/SrTiO3-Rh/Pt/CuAlGaO4 AM 1.5G 2 mM FeCl2/FeCl3 0.52 mmol CH3OH [107]

WO3 0.12 mmol H2

—5 mmol O2

Pt/SrTiO3-Rh/Pt/CuAlGaO4 AM 1.5G 2 mM FeCl2/FeCl3 8 mmol/g CH3OH [108]

WO3 -1 mmol/g H2

12 mmol/g O2

Degussa P25 TiO2 1000WXe lamp H2O [109]

Table 2 (Continued)

Modifications Photocatalyst

Light source

Reductant

Products

References

Non-metal doping

Ag/BaLa4ri4O15 CeO2/TiO2

Pt/MgO/TiO2 nanotubes ln/TiO2

TiO2 (20%)/KlT6

N doped TiO2/Ni N doped TiO2/Pt-Cu N doped TiO2 nanotubes

C doped TiO2 l doped TiO2 TiO2/N-100

g-CaN4-N-TiO2 (CT-70) N-TiO2/spirulina

400 WHg lamp

500 WXe lamp 300 WHg lamp 500 WHg lamp 300 W lamp

15 W UV lamp/ incandescent lamp AM 1.5 outdoor sunlight, 75-102 mW/cm2 500 W tungsten/ halogen lamp

175 WHg lamp 450WXe lamp

300WXe Lamp 13 W lamp

0.1 mol/L KHCO3

0.2 mol/L NaOH and Na2SO3 H2O

0.1 N NaOH

Na2SO3

H2O H2O H2O H2O

0.1 mmol/h CH4 1.4 mmol/h H2 2.7 mmol/h CO

0.7 mmolHCOOH [110]

10 mmol H2

22 mmol CO 16 mmol O2

2.75 mmol/gh H2/CH4 [111]

1.28 mmol/gh O2

100.22 ppm/hcm2 CH4 [112]

10.4ppm/hcm2 CO

244 mmolg"1 h"1 CH4 [113]

81 mmolg"1 h"1 CO

44.56 mmol/g H2 [114]

44.56 mmol/g CH4

1.09 mmol/g CH3OH

120.54 mmol/g CO

482 mmolgcatal CH3OH [115]

111 ppm/cm2h (CO,H2, etc.) [116]

1132.6 mmolgcatal CH3OH [117] 921.6 mmolgcatai HCHO 12475.8 mmolgcatal HCOOH

2610.98 mmolgcatal HCOOH [118]

2.4 mmolg"1 h"1 CO [25]

23 mmolg"1 h"1 CH3OH [119] 14.73 mmol CO [120] 144.99 mmol/g H2 [121] 0.48 mmol/g CH4

0.12 mmol/g C2H4 0.17 mmol/g C2H6

Fig. 3. Excitation steps with a photosensitizer, where A and D represent the electron acceptor and electron donor, respectively.

Eqs. (14)-(16) illustrate the reactions involved, including photoexcitation, injection of electrons and regeneration of dyes, respectively [122].

dye !hndye* (14)

dye !Ti02dye+ +e- (15)

dye++e~ ! dye (16)

As shown in Fig. 3, the transferred electron reduces the organic electron acceptor (EA) adsorbed on the surface. An ideal photosensitizer must undergo slow backward reactions and fast electron injection to attain high efficiency [122]. The

photosensitizer must also have high absorption spectrum in the visible light and infrared regions, with the excitation state having a long lifetime as well [69,127,128]. The rate of electron injection and back electron transfer reactions from the dye molecule to the photocatalyst depend on the characteristics of the dye molecule and the properties of TiO2 and its interactions with the dye. Gupta and Tripathi [41] reported TiO2 as an ideal semiconductor for dye sensitized solar cells due to its stability, high refractive index which facilitates increased light absorption, high dielectric constant for electrostatic shielding of the injected electron from the dye molecule to the electrolyte and suitable conduction band edge below the energy level of several dyes. Gratzel et al. conducted CO2 photoreduction using Ru/RuOx sensitized TiO2 and obtained approximately 900 mLh-1 of methane using a solar simulator with light intensity of 0.08 Wcm-2 [72]. Ozcan et al. also demonstrated the effect of dye sensitized (perylene diimide derivatives) Pt impregnated on TiO2 films on CO2 photoreduction [70]. It was observed that methane production rate was enhanced to a maximum value of 0.74 mmol/gcatai by adsorbing dye molecules to Pt-TiO2. When Pt was not loaded on TiO2 films, inactivity was observed in the presence of dye sensitizers. The N3 dye (Ru"(2,2'-bipyridyl-4,4'-dicarboxylate)2-(NCS)2) coated with Cu-Fe/TiO2 utilized by Nguyen et al. was found to be capable of visible light absorption, producing 0.617 mmol/gcatajh of CH4 after 5.5 h [71]. Data comparison between this sample and one without dye showed the stability of N3 dye sample over a wide light spectrum. Yuan et al. investigated the photoreduction of CO2 with H2O using a Cu(I) dye sensitized TiO2 based system [129]. The introduction of the Cu(I) bipyridine complex was reported to be beneficial for charge separation in TiO2 under full sun illumination (AM 1.5G). Maximum CH4 production rate of ca. 7 mmol/g-1 was observed following 24 h of visible light irradiation with no CH4 detected when the pure Cu(I) dye complex was used. However, instability, light and thermal degradation of dye molecules and

disposal of undesired intermediates formed during reactions have been reported as a major drawbacks associated with dye sensitization [41,74,130].

2.2. Coupling of semiconductors

During heterojunction formation in semiconductor based photocatalysts, the direction of transfer of photogenerated charge carriers from the coupled semiconductors will depend on the position of the CB and VB. TiO2 can be coupled with semiconductors via direct or indirect Z scheme. In direct Z-scheme, spatial charge separation occurs when electrons and holes are injected to CB and VB of different semiconductors in opposite directions (Fig. 4A), while charge separation does not occur in indirect Z-scheme due to electron and hole transfer occurring in the same direction for different semiconductors (Fig. 4B) [122,131-133]. The coupling of these semiconductors result in the balance of their Fermi levels (i.e., energy midway between the conduction and the valence band edges) such that electron flow is from the semiconductor with the higher Fermi level to the one with the lower Fermi level [134]. Excess negative charges are created in the semiconductor with the lower Fermi level while excess positive charges are created in the semiconductor with the highest Fermi level due to charge transfer. Thus, coupled semiconductors benefit from extended band widths in the visible light

and increased charge separation. Sigmund et al. [134] reported that the injection of electrons or holes and their direction is dependent on the Fermi level and the band gap combination of the semiconductors. The requirements for successful coupling of semiconductors are efficient and fast electron injection; ability of the small band gap semiconductor to be excited by visible light with its conduction band being more negative than that of the other semiconductor; proper positioning of the Fermi energy level and insusceptibility of the semiconductors to photocorrosion [56,134].

Wang et al. [74] conducted studies of CO2 photocatalytic reduction over CdSe quantum dot (QD) loaded with Pt impregnated TiO2 films. Typical product yields of 48 ppmg-1 h_1 (methane) and 3.3 ppmg-1 h_1 (methanol) were observed in gas phase using visible light irradiation of 420 nm. They observed that charge injection into TiO2 was facilitated by the shift of the conduction band of CdSe into higher energy which initiated CO2 reduction. Process efficiency was also increased via charge separation due to electron transfer from CdSe toTiO2. No activity was recorded when both semiconductors were used independently and using the same wavelength of light. Recently, ordered mesoporous silica SBA-15/ TiO2 composites with varying ratios of CeO2 were synthesized by Wang et al. [78] for the reduction of CO2 with H2O under simulated solar irradiation. The addition of CeO2 was found to not only influence the light harvesting properties of TiO2 toward the visible

Fig. 4. Coupling of TiO2 with semiconductors (SC) illustrating direct (A) and indirect (B) Z-scheme.

light region but enhance the photocatalytic performance as well. The improved performance was ascribed to the separation of photogenerated charge carriers induced from the drift of TiO2 electrons to CeO2.

Li et al. [77] utilized either CdS or Bi2S3 in the modification of the properties of TiO2 nanotubes for CO2 reduction under visible light irradiation. The addition of either semiconductor was found to enhance visible light absorbance and photocatalytic activity of TiO2 nanotubes, with Bi2S3 exhibiting superior activity due to better surface area and CO2 adsorption. Optimum methanol yields of 224.6 mmol/gcatal and 159.5 mmol/gcatal was observed using TiO2 nanotubes coated with Bi2S3 and CdS, respectively. Heterojunction formation between the semiconductors was reported to play a crucial role in prolonging the lifetime of charge carriers and preventing electron/hole recombination. Despite its promising results, this technique is not widely applied due to its drawbacks. These include photo-corrosion in aqueous phase which affects the durability and stability of the catalyst through leaching out of dopant [135,136] and difficulty in finding appropriate semiconductor pairs such that recombination of charge carriers can be reduced [137].

2.3. Metal and non-metal modifications

The optical and electrochemical properties of TiO2 can be enhanced by the addition of metal and non-metal ion(s). The band gap and properties of TiO2 have been reported to be modified when metals or non-metals occupy non-lattice sites (i.e. interstitial), replace Ti in the substitutional sites or form aggregates on the surface of TiO2 [26]. The redox potential of photogenerated charge carriers and visible light absorption will be determined by the spectral distribution of the modified photocatalysts, which is invariably determined by their chemical states [137]. Apart from these metals possessing their own catalytic activity, they also serve as a source of charge-carrier traps which can increase the life span of separated electron-hole pairs, and thus enhance the efficiency and product selectivity for CO2 photoreduction [13,32].

Whether the metal ions are present in the lattice or TiO2 surface is dependent on two key factors: the preparation procedure where the amount and homogeneity of the metal ions in its host oxide are key parameters and the firing temperatures to which the samples have been subjected. Diffusion of the metal ions in TiO2 lattice is influenced by temperature; with higher temperatures favouring diffusion due to high thermal energy of the atoms. The mechanisms for metal and non-metal modification are described below.

2.3.1. Metal doping

Neamen [138] described doping as the process of adding foreign or impurity atoms into the crystal lattice of a semiconductor material. Alterations to the properties of the semiconductor can occur when controlled amounts of dopants are added to the semiconductor. Fig. 5 shows the schematic representation of these lattice defects. When these impurity atoms are located at normal lattice sites i.e., substitution of the host atom occurs, they are referred to as substitutional doping. Substitutional doping can occur when one or more of the following criteria are met: the differences in atomic radii of the atom types are less than 15%, dopant and host metals have similar crystal structures and electronegativity or comparable valences to ensure solubility [139,140]. Conversely, when these impurity atoms are present between normal lattice sites, they are referred to as interstitial doping i.e., the host atom dislodged from its normal lattice sites is forced into voids between atoms [138,139]. The likelihood of an atom occupying an interstitial site can be predicted by comparison of the radii of the interstitial dopant to the host metal [140]. The greater difference between these atomic radii results in the

M" cation N"- anion

• 7i4* ion at lattice site

• O2' ion at lattice site

Fig. 5. Two dimensional representation of a single TiO2 crystal lattice showing substitutional and interstitial doping.

dopants positioning itself in an interstitial site. The ionic radius ratio of the cation/anion (r+/r~) determines the preference of cations to occupy certain interstitial sites [140]. Interstitial sites may consist of cations with coordination numbers such as 4 (tetrahedral), 6 (octahedral) etc., based on the radius ratio of these ions. As the values of the ionic radius ratio increase, the number of anions packed around the cation increase. In tetrahedral holes, the cations are packed between planes of anions in close-packed structures if the ionic radius ratio falls within 0.225 and 0.414. Whilst the radius ratio falls within 0.414 and 0.732, if they are packed in octahedral holes [138].

According to Pagot and Clerjaud [141 ] and Seebauer and Kratzer [142], the local distortion of the crystal lattice can occur in substitutional and interstitial doping due to the difference in the atomic radii of the dopants compared with the host atoms and their chemical affinity with their surrounding atoms. In these lattice defects, the change in electric properties is caused by the disruption of the chemical bonding between the atoms and distortion of the geometric arrangement of atoms [138]. Vacancies may be created during the catalyst preparation process due to impurity atoms hopping from one vacancy to the other, thus remaining permanently in the substitutional lattice sites after calcination. Dopants can be introduced into sol-gel derived samples at molecular level through the mixing of titanium precursors with soluble dopant compounds. The introduction of dopants has been found to alter the degree of crystallinity and phase transformation, thereby, subsequently altering the peak heights, areas and relative intensities [143]. Phase transformation can be facilitated or inhibited by substitutional dopants when cations enter the anatase lattice and cause an increase or decrease in the level of oxygen vacancies through valence or reduction/ oxidation effects. This leads to the subsequent rearrangement of atoms in the lattice of TiO2 through the substitution of Ti4+ with cations. Conversely, the formation of Ti interstitials may distort the anatase lattice thus restricting the lattice contraction involved in the phase transformation to rutile [143]. The reactions of metal doping are described by the following equations, where Mn represents the metal ion dopant [56].

Mn+ + hv !M(n+1) + e-CB (17)

Mn+ + hv !M(n-1) + h+VB (18)

Mn+ + eCB~ M(n-1)+ as electron trap (19)

Mn+ + h+VB M(n+1)+ as hole trap (20)

Eqs. (17) and (18) depict the formation of energy levels in the band gap of TiO2, while (19) and (20) represent the transfer of electrons between TiO2 and metal ions. However, the energy level (M" +/M("~1)+) must be less negative than conduction band (CB) of TiO2, while the energy level (M"+/M("+1)+) must be less positive than valence band of TiO2. The influence of iodine doping on the phase transformation and photocatalytic activity of TiO2 for CO2 reduction was evaluated by Ref. [25]. The anatase fraction of their iodine doped samples was found to increase with increased iodine concentration and calcination temperature while the brookite fraction was found to decrease under the aforementioned conditions. They attributed the optimal visible light activity of their 5 wt% l-TiO2, calcined at 648 K sample to combinational effect of increased surface area, improved visible light absorption and enhanced charge separation from the substitution of Ti4+ with 15+ which led to the generation of titania surface states trapping electrons and suppressing recombination. The incorporation of substitutional or interstitial metal dopants in the titania structure generates trap levels in the band gap and thus modifying the band gap after doping. As shown in Fig. 6, the trap levels usually in the form of narrow bands are located below the lower conduction band edge. After modification, required energy level becomes hv > (Eg -Et) where Et represents lower edge of the trap band level as opposed to hv > Eg which is required for photon excitation before modification [134]. Consequently, electrons excited at these levels become trapped, with the holes having enough time for OH" generation such that electron/hole recombination is suppressed and overall process efficiency improved. The choice of metal dopant is determined by the ability of the metal to exhibit multiple oxidation states, possess ionic radii and M"+/M("+1) energy levels closer to Ti4+ and the capacity to trap either electrons or holes.

The type of metal dopant added will determine whether the dominant charge carrier in the semiconductor will be either holes in the valence band or electron in the conduction band [138]. Koci et al. [12] reported doping as a means of increasing the level of holes in the band gap to permit the excitation of electrons where mobile holes are created in the valence band (p-type) or addition of an energy level fully occupied with electrons in the band gap which accelerates excitation into the conduction band (n-type). Carp et al. [19] described n-type and p-type dopants where the former acts as a donor centre of electrons and the latter conversely acts as acceptor centers of holes. Recombination centers are formed in p-type dopant as they have an affinity for hole formation once negatively charged, while electron-hole recombination in n-type occurs due to increase in concentration of conduction electrons.

Fig. 6. Band structure of titania (a) before doping and (b) after doping where Eg,Ef, F, x, and s represent the band gap energy, Fermi level, work function, electron affinity and semiconductor. Reprinted from Ref. [134] with permission.

They also reported that charge separation could be improved by co-doping to produce an overall beneficial effect.

Extensive studies on the improvement of the electronic structure of TiO2 have been performed by doping with high energy transition metals by several researchers [130,144-147]. Their results established that the amount and type of metal dopant as well as the method of synthesis were key factors in determining photocatalytic activity and the extent of red shift that can be achieved in the visible light region. Comparisons between the absorption spectra of metal doped TiO2 synthesized by chemical doping (impregnation) and metal ion implantation were made. Samples synthesized by the latter method exhibited shifts in their absorption band toward visible light region (~600nm) caused by intense distance interaction between the metal ion and TiO2. In contrast, the samples prepared by the impregnation method experienced no shift, but absorption shoulders in the UV/Vis spectra. This was caused by the creation of impurity energy levels with the amount of metal ions dopant used determining their intensity.

The electronic structure of TiO2 doped with transition metals (Cr, Fe, Co, V, Ni and Mn) was also examined by Umebayashi et al. [148] using ab initio band gap calculations based on density functional theory. According to their work, a shift of the localized level to a lower energy was observed based on the increase of the atomic number of the dopant. Incorporation of metal or its oxide into TiO2 structure has been reported to cause an increase in the recombination rate between photogenerated electrons and holes via the impurity energy level. Therefore, doping can be effective if the metal ions are placed near the photocatalyst surface where efficient charge transfer of the trapped electrons and holes can occur [41].

In the field of CO2 photoreduction, Nie et al. [149] presented the formation of smaller particles as a way by which doping can alter recombination rate. As smaller particles have large surface to volume ratio, the migratory path is shorter such that the probability of the generated electrons and holes from the bulk undergoing recombination is reduced before reaching the surface. The dopant loading level plays a key role in CO2 photocatalytic activity as increased product yield can be obtained due to red shift towards visible light [144]. However, doping at high concentrations results in the metal ions becoming recombination centres. Gupta and Tripathi [41] further explained that increasing doping concentration results in a narrowed space charge layer where electron-hole pairs within this region can be efficiently separated by the electric field before recombination. However, exceeding the optimum doping concentration results in an extremely narrow space charge layer such that light penetration depth exceeds the width of this space charge layer. Consequently, recombination rate increases due to the lack of a driving force to separate them.

Koci et al. [150] used different loading ratios of Ag/TiO2 and observed an increase in product yield of methane and methanol, with 7% Ag/TiO2 showing the highest product yield compared to lower loading ratios of 1%, 3% and 5%. Conversely, Sasirekha et al. [82] obtained an optimal Ru loading value of 0.5 wt%, after which photocatalytic activity decreased for 1.0 wt% due to increased electron-hole recombination. Slamet et al. [20,86] also reported that Cu dopant in excess of 3 wt% could reduce photocatalytic activity by reducing the depth of light penetration, and thus inhibiting interfacial charge transfer. When the doping content of Fe3+ exceeded 0.03 wt%, Xin et al. recorded a decrease in

photocatalytic activity due to electron-hole recombination, while the opposite was observed for lower loading content (<0.03 wt%).

Regarding product formation, extensive studies into the use of doped TiO2 in CO2 photocatalysis have been conducted using various metals such as chromium [152], copper [86,153], silver [154], platinum [91], palladium [155] and ruthenium [92]. Several

primary products with their yields includes methane (4.8 mmolh^1/gTi"1 [91], methanol (2655 mmol/gcatal [86] and 4.12 mmol/gcatal [84] and formate (72.3 ppm [92]), respectively have been obtained. However, thermal instability and increase in recombination centers are drawbacks linked with this process [24,156].

2.3.2. Metal semiconductor modification

The overall efficiency and surface properties of a semiconductor can be altered by the addition of a metal which is not chemically bonded to TiO2 [134]. These metals act as an electron scavenger and thus facilitate the generation of holes. Ren and Valsaraj [157] and Usubharatana et al. [13] described the addition of metals as a source of charge-carrier traps which increased the life span of separated electron-hole pairs and enhanced reaction rate. Fig. 7 illustrates the band structure of TiO2 before and after contact with a metal. As shown in Fig. 7, the comparison of a semiconductor with work function Fs to a metal with work function Fm> Fs results in the Fermi level of the semiconductor, EFs being higher than the Fermi level of the metal EFm. When this metal is brought in contact with TiO2 (Fig. 7b), electrons will flow from the semiconductor to the metal until the two Fermi energy levels reach an equilibrium. This results in an upward band bending formed due to an excess of positive charges inTiO2 generated from the migrating electrons [158]. Consequently, this bending at the metal-semiconductor interface creates a small barrier known as the Schottky barrier [32]. The Schottky barrier serves as an electron trap which prevents migrating electrons from crossing back to the semiconductor and thus preventing recombination. The schematic in Fig. 8 also illustrates the mechanism of a metal modified semiconductor for photocatalysis where recombination is

Fig. 7. Band structure of titania (a) before contact and (b) after contact with a metal, where the Schottky barrier is formed. (Fm and Efint represent the metal work function and Fermi level if titania is an intrinsic semiconductor respectively. Reprinted from Ref. [134] with permission.

Fig. 8. Metal modified semiconductor photocatalyst.

suppressed by the Schottky barrier of the metal in contact with the surface of the semiconductor.

As a result of this trapping mechanism, the photogenerated electrons then diffuse to the surface of the adsorbed species where reduction takes place. Photoreactivity can be negatively influenced by either a high concentration of metallic islands on the semiconductor surface or an enhancement of their size [158]. When this occurs, reduced surface illumination of catalysts and increased recombination rate is observed. Krejcikova et al. [90] conducted CO2 reduction studies using different loading ratios of Ag/TiO2and observed increased product yield of methane in the gas phase and methanol in the liquid phase with increasing Ag concentration under 254 nm UV irradiation over a 24 h period. The increase in product yield compared to both commercial and synthesized pure TiO2 was attributed to higher Fermi level of TiO2 and Schottky barrier formation which facilitated electron transfer from the TiO2 conduction band to Ag particles and improved charge separation, respectively. Tseng et al. [87] synthesized Cu loaded TiO2 nanoparticles for the photocatalytic reduction of CO2 under UV irradiation using NaOH as a reductant. CH3OH production was found to increase with increasing Cu concentration, after which a markedly decrease was observed when the loading ratio exceeded 2 wt%. Optimum CH3OH production of 118mmol/g was obtained using the 2wt% Cu-TiO2 sample following 6 h of UV illumination. The formation of the Schottky barrier between Cu and TiO2 and the electric charge redistribution via semiconductor-metal contact was reported to facilitate electron trapping and thus promoting improved photo-efficiency.

Other than Schottky barrier, loading of noble metals such as Ag [159], Cu and Au [160] on the TiO2 can enhance visible light absorption via localized surface plasmonic resonance (SPR) effect. This phenomenon can occur either by collective oscillation of valence electrons in plasmonic nanostructures in resonance with electric field part of inbound radiation or metallic elements creating trap sites that propagates light within the semiconducting material [161]. Morphology and size of the plasmonic nano-structures influence the SPR frequency and intensity as well as the resonant wavelength. Gas-phase photochemical reduction of CO2 using mesoporous TiO2 modified with bimetallic Au/Cu nano-structures was studied for CH4 production under UV irradiation [162]. The bimetallic nanocomposites were reported to exhibit higher activity (CH4 yield of ~11 mmol/gcatal) compared to Au/TiO2 and Cu/TiO2 (CH4 yield <4 mmol/gcatal).

2.3.3. Non-metal modification

Doping with non-metals creates heteroatomic surface structures and can modify the properties and activity of TiO2 toward

visible light [137]. Some of the non-metals that have been used include nitrogen (N) [24], carbon (C) [118], sulphur (S) [163,164], fluorine (F) [165]. Asahi et al. [24] and Asahi and Morikawa [156] described doping with anions as being more efficient for photo-catalytic activity compared to cations because they do not form recombination centers caused by the presence of d states deep in the band gap of TiO2. Liu et al. [137] reported that effective band gap narrowing can only occur by anion dopants if the non-metal has a comparable radius with O atoms and lower electronegativities than O, with the aim of facilitating uniform distribution and elevating the valence band.

The band gap energy of TiO2 has been reported to be narrowed by a mixture of p states of the non-metal dopant with the O 2p states of TiO2 via substitutional or interstitial doping [24,156]. Conversely, Valentin et al. [166,167] proposed that substitutional doping with N results in the formation of localized levels within the band gap, with the catalyst synthesis conditions determining whether either interstitial or substitutional nitrogen exists in the lattice of TiO2. On the other hand, Serpone et al. [168] attributed the origin of visible light absorption in their titania samples to the existence of color centers instead of band gap narrowing via mixing of states, as proposed by Liu et al. [137].

First principle calculations by Asahi etal. [24] using anions (F, N, P, S and C) indicated the superior activity of N owing to the p states influencing band gap narrowing through combination with O 2p states. Although S doping showed similar photoresponse as N, they found the ionic radius of S too bulky to be integrated into the lattice of TiO2. Zhang et al. [25] tested iodine doped TiO2 synthesized by the hydrothermal method and found that the calcination temperature influenced the rate of CO2 photoreduction under visible light irradiation. They observed that increased calcination led to reduced surface area. An optimal yield of CO (2.4 mmol g_1 h_1) was observed for the 10wt% sample calcined at 375 °C. Xue et al. [118] examined carbon doping for CO2 photoreduction using citric acid as the carbon source and Na2SO3 as the reductant. After 6 h irradiation using high pressure 175W mercury lamp, 2610.98 mmol/gcatal of CH2O2 was produced. This was significantly higher than the undoped TiO2.

Compared to other non-metals, N doped TiO2 (TiO2_xNx) has been extensively studied because of its photoactivity toward visible light [137,169]. Zhao et al. [117] prepared N doped TiO2 nanotubes via hydrothermal method at different calcination temperatures. N doping into TiO2 nanotube framework was found to be effective for increasing the photoactivity of TiO2 in the visible light region compared to pure TiO2 and N doped TiO2. Optimum total organic carbon content (sum of the product yields of formaldehyde, methanol and formic acid) of 14,530 mmol/gcatal was observed using a N-TiO2 nanotube sample calcined at 500°C for CO2 reduction with 0.1 N NaOH as reductant following 12 h of light irradiation.

Since its quantum efficiency of anion doping is still low, investigations have been conducted by codoping with metals to enhance the reaction rate [170,171]. Several transition metals such as Pd, Fe and Pt have been used in the photodegradation of pollutants and dyes [172-174]. The results of these researchers showed the increased photocatalytic activity and absorbance of visible light by the metal ion modified TiO2_xNx compared to bare TiO2_xNx. CO2 photoreduction studies have been carried out by Varghese et al. [116] using N-TiO2 nanotube arrays with metals (Pt and Cu) under outdoor AM 1.5 sunlight. They found the optimal nitrogen concentration to be 0.75 atom% with Cu doping generating greater hydrocarbon product yield of 104 ppm/ (cm2 h) compared to Pt doping. As both Pt and Cu have varying effects toward product selectivity; the combination of both metals resulted in an optimal yield of 111 ppm/cm2 h.

2.3.4. Co-doping

The properties of TiO2 can also be modified via co-doping, which can be achieved via the combination of metal/metal, non-metal/metal or non-metal/non-metal pairs. A synergistic effect can be obtained with an appropriate combination of co-dopants compared to their single ion doped or undoped TiO2 [128]. During co-doping, the non-metal can cause a red shift in the visible light region, while the metal can facilitate the transfer of photogenerated charge carriers thus suppressing recombination. Apart from co-dopants facilitating band gap narrowing, their combination can result in the formation of different heterostructures (i.e., different electronic structures) with respect to TiO2 [44]. A heterostructure consisting of different combinations of non-metal and metal has the capacity for improved charge separation (metal) and visible light absorption (non-metal). Factors crucial for successful co-doping are the selection of the compatible co-dopants and the method of introducing the dopants which affects the doping level [44]. Several metallic and non-metallic combinations such as N-I [175], C-vanadium (V) [176] and Ag-V [177] have been used in the photodegradation of pollutants and dyes. The results of these researchers showed the increased photo-catalytic activity and absorbance of visible light by the metal combinations compared to un-doped and single doped TiO2 systems.

For metallic combinations for CO2 reduction, the catalytic activity of sol-gel derived Mn-Cu/TiO2 nanocomposites of varying metal concentrations was evaluated by Richardson et al. [178]. After 24 h of UV irradiation, the photocatalytic activity of CO2 using 0.1 M NaOH and 0.25 M KHCO3 was found to be promoted based on the coupling of Mn and Cu doped titania photocatalysts compared to either commercial TiO2-P25 or single metal loaded samples. Improved results were due to electron transport to the dopant which suppressed electron/hole recombination. Maximum CH3OH yield of 238.6 mmol/gcatal was achieved using the 0.22 wt% Mn/0.78 wt% Cu-TiO2 sample. The same trend was observed in a further study conducted by Richardson et al. [179] using different sol-gel derived Cu-Ga/TiO2 nanocomposites of varying metal concentrations. The photocatalytic activity was improved when Cu and Ga doped photocatalysts were used such that the 0.78 wt% Cu/0.22 wt% Ga-TiO2 sample gave the maximum HCHO yield of 394 mmol/gcatal when compared to single metal loaded samples or TiO2-P25. They reported that their optimized results were due to the rapid transfer of high energy electrons in their catalytic structures.

For metallic and non-metallic combinations in CO2 reduction, co-doped N and Ni were introduced onto TiO2 framework for CO2 reduction using 0.2 mol/Lof NaOH and Na2SO3 [115]. An increased red shift toward the visible light was observed using the co-doped samples compared to pure titania and individually doped samples of Ni-TiO2 and N-TiO2. An optimal methanol yield of 482 mmol/gcatal was observed after 8 h of UV light irradiation using the 4wt% N-6wt% Ni/TiO2 sample compared to the methanol yield of the individually doped samples of 245.4 mmol/gcatal of 4wt% N-TiO2 and 214.4 mmol/gcatal of Ni-TiO2. They suggested that improved activity was due to the improved properties (surface area and crystallinity) of the co-doped samples and the synergy created by the metal (Ni) acting as an electron trap and the non-metal (N) facilitating increased visible light absorption.

Li et al. [180] demonstrated that co-doped mesoporous Pt-N/ TiO2 photocatalysts had some inherent advantages over undoped TiO2. Using the optimum loading ratio of 0.2 wt% Pt for the synthesis of the co-doped samples under NH3 atmosphere, they observed increased CH4 evolution rate with increasing nitridation temperature up to 525 °C. After this temperature was exceeded; a subsequent decrease in CH4 evolution was observed. Optimal CH4

production rate of ca. 2.6 mmol/gcatal_1 was observed using the 0.2 wt% Pt-TiO2 sample when the amount of doped N was 0.84% on the basis of lattice oxygen atoms under visible light irradiation. Improved activity under increasing nitridation temperature was due to the doping of more N atoms in the lattice position of oxygen in TiO2, which gave rise to improved visible light absorption. Above the optimum N doping concentration, decreased photocatalytic activity was observed due to increased defect sites and non-stoichiometry of the samples.

Co-doped samples were synthesized by Wang et al. [74] using commercial P25 TiO2 nanoparticles and CdSe quantum dots (QDs). Pt was further incorporated by the wet impregnation methods onto the CdSe-TiO2 samples for the experimental investigation for CO2 reduction using H2O. They found that the use of co-catalyst, Pt with CdSe quantum dot (QD)-sensitized TiO2 heterostructures led to increased visible light absorption greater than their individual photoresponse. No photocatalytic activity was also observed when either CdSe or Pt doped on TiO2 was employed for CO2 reduction. However, the synergy between CdSe-Pt/TiO2 heterostructures was found to influence methane and methanol production under visible light irradiation with wavelength of 420 nm. In order to further demonstrate the need for heterostructure formation in co-doped samples, Wang and co-workers synthesized PbS-Cu/TiO2 samples with different sizes of quantum dots. Although they achieved optimal activity with the 4 nm co-doped heterostructure, the drawback of photocorrosion observed from oxidation in their previous and current studies could not be surmounted. Zhang et al. [181] prepared co-doped Cu/I-TiO2 samples with different concentrations using wet impregnation and hydrothermal methods. Under UV-vis and visible light irradiation, the photoactivity of the co-modified sample was found to be higher than either of the single ion modified catalysts (Cu-TiO2/I-TiO2). For CO production under visible light, the optimum yield of 6.74 mmol/g-1 was observed on the 1 wt% Cu-10 wt% I-TiO2 while the optimum yield of 12^mol/g_1 was observed on the 0.1 wt% Cu-10 wt% I-TiO2 sample under UV-vis light irradiation. The presence of the dopant was reported to reduce the crystal size and influence visible light absorption while Cu facilitated charge transfer and enhanced CO2 reduction.

3. Influence of operating parameters on CO2 reduction

The following operating parameters listed below have been shown to influence CO2 photoreduction: type of reductant, temperature, pressure and particle size. These factors are discussed in the following sections.

3.1. Effect of reductant

Several types of reducing agents such as H2O, NaOH, and C3H7OH amongst others have been tested for CO2 photocatalytic reduction [182-184]. For CO2 photoreduction using TiO2 to become economically feasible, readily available sources of hydrogen are needed. H2O still remains the most naturally abundant source of hydrogen that is available and inexpensive [13]. Other reductants such as NH3, pure H2 gas etc., which serve as hydrogen sources are not readily available as primary feedstock and require prior preparation [185]. However, the drawback of utilizing water is the low solubility of CO2 in H2O (2 g/L) and the competition of the CO2 photoreduction process with hydrogen formation, as shown in Eqs. (4) and (6), which indicate that it is thermodynamically more favorable to reduce H2O than CO2 [33,186].

Liu et al. [182] investigated the role of solvents on the product selectivity for CO2 reduction in an attempt to increase reaction yield. Their results indicated that the use of solvents with low dielectric constants, such as CCCl4 and CH2Cl2, led to CO2- anion

radicals being strongly absorbed on Ti sites due to the anions showing little solubility in these solvents of low polarity and therefore, CO was the major product observed during this reaction. When a high dielectric solvent such as H2O was used, CO2" - anion radicals were greatly stabilized by the solvents which led to weak interactions with the surface of the photocatalyst, and thus formate was observed, as the main product via the reaction of a proton with the carbon atom of the CO2'- anion radical.

Zhao et al. [187] employed titania supported cobalt phthalocy-anine (CoPc) nanoparticles for CO2 reduction in either NaOH or Na2SO3 solutions. Maximum production of formic acid and formaldehyde was observed at concentration of 0.15 M due to increased solubility of CO2 in NaOH and the OH- ions produced from NaOH acting as strong hole scavengers during the OH radical formation. They further explained that electron/hole recombination could be suppressed through the longer decay time of electrons, since the holes are preoccupied in HCO3- formation in the CO2 saturated system. Further addition of an optimal concentration of Na2SO3 (0.1 M) led to an increase in formal acid production through increased hole scavenging and proton concentration within the semiconductor particle for CO2 reduction. The same phenomenon was also observed by Tseng et al. [87] in their CO2 reduction studies using NaOH solution.

The study conducted by Koci et al. [188] demonstrated the influence of the volume of reductant on CO2 photocatalytic studies using TiO2. The use of NaOH was reported to not only enhance CO2 solubility, but also facilitate improved CO2 reduction via OH-radical formation, which promoted the longer decay time of electrons. The production rate of CH4 and CH3OH was found to increase when the volume of NaOH increased from 50 to 100 mL, and then markedly decreased above these values. For example, the CH4 yield increased from 7.5 mmol/gcatal to >8 mmol/gcatal when the volume of NaOH increased from 50 to 100 mL, then decreased to <2 mmol/gcatal when 250 mL of NaOH was used. Ti-MCM-41 mesoporous photocatalysts with Si/Ti molar ratios of 50, 100 and 200 were tested for CO2 photoreduction using NaOH, deionized H2O and monoethanolamine (MEA) as reducing agents. For the best photocatalyst within the series tested (Ti-MCM-41 with Si/Ti ratio of 50), maximum CH4 yield of 62.42 mmol/gcatal was observed when MEA was used as a reductant compared to 5.62 mmol/gcata of CH4 over H2O after 8 h of UV illumination. The lowest CH4 yield of 1.96 mmol/gcatal over NaOH was due to the formation and precipitation of sodium bicarbonate (NaHCO3) in solution after contact with CO2 gas stream. Although the use of solvents other than water as hole scavengers can increase product selectivity and yield, they still remain economically unsustainable due to their potential to increase cost.

3.2. Effect of temperature

CO2 photocatalysis is generally conducted at ambient conditions, i.e., room temperature because solubility decreases with increasing temperature and the formation of electron/hole pairs occurs by photon (light energy) activation. An increase in reaction rate has been reported to occur at high temperatures due to increased collision frequency and diffusion rate [188]. The optimum temperature required for photocatalysis is within 293-353 K, with decreasing activity occurring outside this range [189]. This is due to exothermic adsorption of reactants being the rate limiting step as the temperature approaches the boiling point of H2O. Yamashita et al. [190] demonstrated that photocatalytic reactions proceed more efficiently at temperatures higher than 275 K by using anchored titanium oxide catalysts for CO2 reduction. They observed increased production rates of CH4, CO and CH3OH under UV irradiation at 323 K compared to 275 K. Saladin and Alxineit [191 ] studied the effect of temperature on CO2

reduction using titania samples irradiated under UV light for 4 h. They found that the production rate of CH4 increased when the temperature rises from 298 to 473 K. Based on model calculations, the reaction rate was not expected to be substantially improved after the maximum temperature of 473 K due to hindered absorption of reactants. They also concluded that thermal activation processes such as product desorption played a crucial role in improved reaction rate at 473 K. Product desorption readily occurred at higher temperatures (473 K) compared to lower temperatures (298 K).

Guan et al. [49] investigated the use of a hybrid catalyst Pt loaded potassium hexatitanate (K2Ti6O13) combined with Fe based catalyst supported on Y zeolite (Fe-Cu-K/day) for CO2 reduction under concentrated sunlight. They found that the Pt/K2Ti6O13 catalyst produced H2 from water decomposition, while the Fe-Cu-K/day catalyst reduced CO2 with the resulting H2 converted into CH4, HCOOH and HCHO. The reaction temperature was found to promote the generation of the products listed above in addition to C2H5OH and CH3OH over the hybrid catalyst on temperature increase from 534 to 590 K. They claimed that the simultaneous supply of photons and thermal energy from the solar concentrator was responsible for the optimal production of reaction products observed at 590 K. The same phenomenon with higher production rate was also observed by Guan et al. [192] when they used a hybrid catalyst Pt loaded potassium hexatitanate (K2Ti6O13) combined with Cu/ZnO catalyst under concentrated sunlight at 583 K. Increased methanol yield on temperature increase within the range of 333-373 K was also observed by other researchers during CO2 reduction studies [20,193].

Photoreduction studies conducted by Kaneco et al. [194] demonstrated that temperature had no effect on the catalytic activity of their samples. The photocatalytic activity of TiO2 suspended in supercritical CO2 was investigated. Formic acid production observed in the liquid phase was attributed to the reaction of water with reaction intermediates on the surface of TiO2. An increase in temperature at the rate of 278 K from 308 to 323 K led to the steady state formation of formic acid. Kocietal. [188] reported that a temperature increase of 10 K from 299 K to 309 K did not influence the hydrocarbon production rate for the photocatalytic reduction of CO2 using TiO2 following 4 h of UV irradiation. Although increasing reaction temperatures have been reported to facilitate increased production rates, the cost of fabricating sophisticated high temperature photoreactor systems capable of maintaining the selected optimum temperatures and the source of thermal energy required to heat up solvents i.e., water has a high specific heat capacity, still poses a problem.

3.3. Effect of pressure

Improved product selectivity has been reported to occur due to increased CO2 concentration resulting from an increase in CO2 pressure in aqueous media [188]. Experimental studies conducted by Mizuno et al. [195] demonstrated the effect of CO2 pressure on CO2 reduction using TiO2 suspensions in H2O and NaOH. CH4, C2H4 and C2H6 were observed in the gas phase under pressurized conditions (2500 kPa), with no hydrocarbon production detected under ambient pressure. CH4 production was found to increase sharply with increased CO2 pressure from 500 to 2500 kPa. Slight increase in formic acid production was observed in the liquid phase under similar pressurized conditions. Overall, increased CO2 pressure accelerated CO2 reduction when both H2O and NaOH were used.

On the other hand, CHOOH and CH3OH production was observed in the liquid phase at ambient pressure. While a linear increase of formic acid was observed with slight pressure increase, CH3OH yield was found to reach an optimal rate at 1000 kPa.

Overall, the gross amount of hydrocarbons produced in the liquid phase exceeded that of the gaseous phase. Similar trend of increased hydrocarbon production was observed when NaOH was used as the hole scavenger under CO2 pressurized conditions. Additional reaction products such as C2H5OH and CH3CHO were also detected in the liquid phase. They suggested that improved reaction yield was due to the availability of CO2 on the surface of TiO2, which accelerated CO2 reduction.

Koci et al. [188] demonstrated that the pressure influenced the rate of product yield obtained. They observed that the amount of CH3OH yield in the liquid phase increased when CO2 pressure was increased from 110 kPa (0.75 mmol/gcatal) to 130 kPa (1.5 mmol/ gcatal). Further pressure increase to 140 kPa led to reduced CH3OH yield. Conversely, CH4 yield in the gas phase increased with increasing CO2 pressure from 120 kPa (2.75 mmol/gcatal) to 140 kPa (4.5 mmol/gcatal). Kaneco et al. [186] studied the photocatalytic reduction of CO2 under various pressures using TiO2 suspended in iso-propyl alcohol medium. The results show that the pressure increase from 200 to 2800 kPa can increase CH4 production linearly. Conversely, the increased pressure conditions were found to inhibit CHOOH production, with reaction product observed at only 750 kPa. The lack of C2H4 formation was attributed to the accelerated formation of CH4. An increase in CH3OH formation from 175 to 230 mmol/g-cat was observed by Tseng et al. [87] when the CO2 pressure increased from 110 kPa to 125 kPa. Further pressure increase above 125 kPa led to decreased CH3OH production rate of 85 mmol/g-cat. Cost of fabricating sophisticated high pressure systems must be considered when selecting parameters for reactor designs in CO2 reduction.

3.4. Effect of particle size

Particle size is a key parameter in photocatalytic processes since the interaction between the amount of absorbed and reflected photons and the reactants depends on it. Apart from nanostructured photocatalyst possessing high surface area, they also benefit from low refractive index which minimizes light reflection, high surface to volume ratio and rapid charge transfer [196,197]. Several researchers have established that photo-catalysts in the form of nanoparticles are more effective than bulk powders [198-200]. The rate of electron-hole recombination has been reported to be controlled by particle size since extremely small ultrafine particle (within the diameter range of few nanometres) experience surface recombination as opposed to large particles where volume recombination predominates [200]. The problem of volume recombination can be overcome by reducing the particle size. During surface recombination, most of electron-hole pairs photogenerated close to the surface undergo rapid recombination due to their shorter migratory paths, abundant surface trapping sites and limited driving force for charge separation [138]. This phenomenon has been reported to occur within certain size reduction. Zhang et al. [200] further demonstrated that the number of available active surface sites and transfer rate of surface charge carrier increased with smaller particle sizes due to their larger surface area. They observed increased photoactivity in the decomposition of CHCl3 when TiO2 particle size decreased from 21 to 11 nm. Decreased photoactivity was also observed when the particle was further reduced to 6 nm. Optimal photoactivity was demonstrated with the 10 nm particle. On the other hand, Koci et al [198] proposed that the 14 nm TiO2 particle was the optimal value for CO2 reduction, since they obtained maximum CH4 and CH3OH production using this particle size. A decrease was observed on further increase to 29 nm. They attributed the decreased photoactivity observed in samples with particle size <14 nm to rapid flocculation which decreased availability of active sites.

The model developed by Almquist and Biswas [199] was used to elucidate the effect of particle size on TiO2 activity for the photo-oxidation of phenol. Particle sizes ranging from 5 to 165 nm were prepared from flame synthesized TiO2 and commercially available P25 and anatase TiO2. The results also highlighted the strong dependence of particle size on photo-activity. Increased photo-oxidation occurred when the particle size increased from 5 to 30nm and decreased when the particle size increased beyond this value. The optimal particle size range reported was within the range of 25-40 nm. Band gap of photocatalysts has been reported to be influenced by particle size [41,138,197]. They proposed that semiconductor particles within the nanometre range experienced energy shift according to the size quantum effect which could accelerate reduction and oxidation reactions via the conduction and valence band, respectively. This size quantization effect expected to cause an increase in the band gap energy results in a shift to larger redox potentials, which increases rate constants for surface charge transfer. Banerjee [201] also reported that the large fraction of surface atoms and high surface to volume ratio found in nanoparticles are responsible for enhanced light absorption through indirect electron transition at the boundary of the crystal i.e., surface or interface between two crystals. Particle size of photocatalysts must be carefully considered during catalyst synthesis since it is invariably linked to surface area and photocatalytic efficiency.

4. Catalyst configuration: supports

TiO2 can be synthesized as powders (nanospheres, microspheres), crystals, films or immobilized by dip or spin coating onto substrates such as fibers, membranes, glass [202], monolithic ceramics [155], silica [203] and clays such as zeolite [204], kaolinite [95], montmorillonite [205] etc. Several materials have been used as TiO2 support for CO2 reduction. The use of supports eliminates the need for post treatment separation, provides high surface area and mass transfer rate [202]. The product selectivity, structure and electronic properties of TiO2 can be modified by the use of supports. However, the photocatalyst must be strongly adhered to the support and have light absorption properties to be effective. An ideal support must be resistant to degradation induced by the immobilization technique and should provide firm adhesion between the support and the catalyst [206,207]. Mass transfer limitations and low light utilization efficiency due to little or no light absorption in the pores or channels of the catalyst coated supports are key limitations that have been identified with the use of supports [208]. Many researchers have focused on ways of anchoring photocatalysts onto supports since high photoconversion efficiencies and improved light harvesting can only be achieved through the combined use of optimized photoreactor and photocatalyst configurations. An overview of some commonly used supports is presented below, including glass, optical fibers and monoliths.

4.1. Glass

Several types of glass substrates such as beads [209], plates [210], microfiber filter [94,98,211 ] and plates [102] have been used for CO2 reduction due to the transparency of the substrates to light irradiation. The use of conductive materials like glass as supports have been extensively studied due to their ability to prevent total internal reflection through surface roughening which also provides better catalyst adhesion to the glass substrate and increases the amount of immobilized catalyst per unit area [28]. Furthermore, Ray and Beenackers [28] reported that utilizing conductive materials serve as a means by which light can be transmitted to

the catalyst film which is connected to an external potential that can move excited electrons, and thereby, reducing electron-hole recombination to improve efficiency.

Highly dispersed titanium oxide anchored onto Vycor glass was tested for the photocatalytic reduction of CO2 with H2O [190]. The supports were prepared through facile reaction between surface OH groups of a transparent porous Vycor glass and TiCl4. UV irradiation of the support led to the formation of C1 compounds such as CH4, CH3OH and CO as major products and trace amounts of C2 compounds (C2H4 and C2H6) at 323 K. Cu nanoparticles were deposited on transparent conductive fluorinated tin oxide (FTO) glass substrates for CO2 reduction to CH4 under UV irradiation [212]. Cu-TiO2 films were reported to exhibit higher yields compared to pure TiO2 and TiO2 P25. Enhanced light absorption and increased diffusion length of photoinduced electrons were amongst some of the reasons for enhanced CO2 photoconversion rates. TiO2 pellets (Aerolyst 7708) were affixed to a flat glass tray by Tan et al. [83,89,213] to increase absorption capacity and contact area for CO2 photoreduction. The product yield of CH4 (200 ppm) using ultraviolet light C (UVC) wavelength of 253.7 nm was reduced to values lower than 100 ppm on switching to ultraviolet light A (UVA) wavelength of 365 nm after 48 h of irradiation.

Platinum (Pt)-TiO2 nanostructured thin films with different deposition times were prepared by Wang et al. [213] for immobilized onto indium tin oxide (ITO)-coated aluminumosi-licate glass using RF magnetron sputtering and gas-phase deposition method. The films which had a one-dimensional structure of TiO2 single crystals with ultrafine Pt nanoparticles (NPs, 0.5-2 nm) were found to exhibit enhanced CO2 photoreduction efficiency with selective CH4 yield of 1361 mmol/gcatal h. The fast electron-transfer rate in TiO2 single crystals and the efficient electron-hole separation by the Pt NPs were the main reasons reported to be attributable for this enhancement. Mesoporous Cu-TiO2 nanocomposites synthesized by a one-pot sol-gel method were loaded onto glass fiber filters as thick films for CO2 photoreduction to CO and CH4 [94]. CH4 and CO peak production rates of 10 and 60 mmol/gcatalh were achieved over the 0.5% Cu/TiO2-SiO2 composite. Improved results were reported to be influenced by the synergistic effect resulting from the combination of the SiO2 substrate and Cu deposition loaded onto the glass fiber filter.

The effect of Ag/TiO2 nanoparticles deposited on glass microfiber filter for UV light induced CO2 photoreduction using water vapor as electron donor was performed by Collado et al. [98]. Deposition of Ag on TiO2 surface led to an enhancement in the production of C1-C3 compounds which increased as Ag loading increased from 1.5-3.0 wt%. Better catalytic performance and selectivity were observed over glass filters containing Ag samples prepared by wet impregnation than incipient wetness impregnation procedure. Enhanced hydrocarbon production was reported to be due to lower recombination rates and synergistic effect between TiO2 and Ag nanoparticles. The transparency of the glass material used can also limit the overall efficiency due to the catalyst receiving insufficient light e.g., Pyrex glass can cut off UV light below 300 nm [214]. On the other hand, quartz glass is a better alternative as a light conducting material because of its excellent light transmission properties and its ability for increased contact efficiency, thus creating more active sites.

4.2. Optical fibers

The use of a single or bundle of optical fibers for the remote delivery of light to reactive sites of coated photocatalysts has been studied by several researchers for wastewater treatment and CO2 photocatalysis [48,154,215-217]. All researchers observed

increased degradation and conversion rates when the optical fibers were simultaneously used as a support and light distributing guide. In optical fiber, light is transmitted along the fiber core by the cladding with lower refractive index that traps light in the core through total internal reflection. Light can be primarily emitted at the end of the fiber (end emitting) or through the leakage of light as it travels from the fiber core to the cladding via the side surfaces (side emitting) [218].

Catalytic performance of 120 catalyst coated optical fibers was evaluated for CO2 reduction under 365 nm UV irradiation. Maximum CH3OH yield of 0.45 mmol/gcatalh was achieved using 1.2 wt% Cu-TiO2 catalyst coated fibers. Properties of the optical fibers such as external surface area and light transmittance were reported to influence the processing capacity of the catalysts. The influence of the optical fibers in delivering photons required to activate different catalyst combinations such as Cu-Fe/TiO2, Cu-Fe/TiO2-SiO2 and N3 dye-Cu-Fe/TiO2 was further demonstrated [48,71,219]. Overall, maximum CH4 production rate of

I.86 mmol/gcatalh was obtained using Cu (0.5wt%)-Fe (0.5 wt %)/TiO2-SiO2 catalyst coated fibers while ethylene production rate of 0.575 mmol/gcatalh was achieved with Cu (0.5wt%)-Fe (0.5 wt%)/TiO2 coated fibers under UVA irradiation. During gas phase CO2 reduction studies, Wang et al. [193] obtained yields of

II.3 mmol/gcatalh and 11.3 mmol/gcatalh for methanol production under visible light irradiation and sunlight, respectively, when the optical fibers were coated with NiO/InTaO4.

32 optical fibers coated with inverse opal Cu-TiO2 inserted in a stainless tube were tested for the photoreduction of CO2 to CH3OH [157]. CH3OH product rate of 0.036mmol/gcatalh was achieved after UV irradiation with light intensity of 113.65 mW/cm2. The inverse opal configuration was reported to enhance catalyst activation via increased contact time of light within the photo-catalyst. Although high catalyst loading and direct light excitation of coated catalyst films can be achieved when a bundle of optical fibers are coated, fragility of the optical fibers and the durability of their coatings has been described as drawbacks associated with their use [197]. The durability and performance of these fibers are directly related to the adhesion of the catalyst coatings on the fibers and thickness of the coated layer which may not withstand severe gas/liquid continuous flow conditions [220,221]. Even though roughening of the fiber surface has been reported to increase durability of these coatings, the problem of uneven catalyst and light distribution has also emerged [220,221]. Heat build-up from the bundled array of fibers can result in catalyst deactivation [222].

4.3. Monoliths

The use of interconnected three-dimensional structures like honeycomb monoliths containing parallel straight channels has been exploited for industrial processes due to its potentially high surface to volume ratio, easy of scale-up through an increase of its dimensions and channels, control of structural parameters (i.e., pore volume, pore size and surface area) etc. [223,224]. Different types of metal oxides and mesoporous materials have been immobilized on TiO2 coated monolithic materials to improve catalytic performance for CO2 photoreduction. Tahir and Amin [225] deposited montmorillonite (MMT) based TiO2 onto monolithic structure to improve surface area and adsorption of gaseous species for CO2 photoreduction with H2O. The addition of MMT into TiO2 matrix was reported to increase surface area from 42.98 m2/g for pure TiO2 to 51.79 m2/g for MMT/TiO2. Higher CO (52 mmol/gcatal_1 h"1) and CH4(139 mmol/gcatal_1 h"1) production was achieved over the MMT/TiO2 monolith compared to pure TiO2 (CO, 47 mmol/gcatal^1 h"1 and CH4, 82 mmol/gcatal^1 h"1) after 10 h of UV light irradiation. Photocatalytic activity was reported to be influenced by increased CO2 adsorption originating from surface hydroxyl (OH) groups in MMT/TiO2 framework. The effect of monolithic geometry such as cell density and channel length on UV induced CO2 photocatalysis was further evaluated. The product rates were reported to be influenced by the geometry of the monolith since maximum product rates of CO and CH4 were observed over the monoliths with lower cell density of 100 cells per square inch (cpsi) and channel length of 2.5 cm compared to the monolith with higher cell density (400 cpsi) and lengths of 1.2,1.7 and 5 cm. The monolith channel length was reported to be linked to light distribution. The effect of In loading on TiO2 coated monoliths was further tested for photocatalytic reduction of CO2 under UV irradiation by the same research group [226]. The introduction of indium to TiO2 framework not only increased surface area and reduced particle size, but also facilitated charge transfer. Maximum CH4 production rate of 55.4 mmolg"1 h"1 was observed over 10wt% In/TiO2 monolith with 100cpsi after 10 h of UV irradiation.

Photocatalytic studies conducted using monoliths as catalytic support for wastewater treatment, NO and CO2 reduction have identified low light utilization efficiency due to little or no light absorption in the pores or channels of the honeycomb monolith as a major drawback associated with its use [155,208,223]. Not all immobilized photocatalyst may be activated due to limited light distribution arising from the catalyst coated on the outer surface absorbing most of the light and its intensity decaying rapidly along the opaque channels of the monolith [207,227]. In a

Fig. 9. Schematic of light propagation in a single channel of a coated honeycomb monolith threaded with a non-coated side light emitting optical fiber.

mathematical model developed by Hossain et al [228] for influx of UV light within a square channel monolith, half of the incoming light flux was reported to be lost due to light shadowing effect at the entrance of the channel of the monolith wall. The UV light flux was also reported to decrease sharply with increasing distance in the monolith channel. A strategy for improving light distribution in monolithic structures was originally proposed by Du et al. [220], where non coated side-light emitting fibers were evenly distributed in each TiO2 coated channel to ensure light refracted out of the surface of the fiber could reach the catalyst-reactant interface without attenuation. Fig. 9 shows the schematic where the gaseous reactants diffuse into each coated interconnected monolithic channel, adsorb and react with catalyst activated by a non-coated side-light fiber to form desorbed products and intermediates which diffuse back into the bulk gas stream (CO2 saturated with H2O).

In recent years, studies on CO2 reduction using non coated side-light emitting fibers with geometric notches in the core-cladding system were reported to improve photocatalytic activity [229,230]. Vapor phase CO2 with H2O was reduced to CH3OH by NiO/InTaO4 coated monoliths containing no fibers, bare fibers and fibers with tip-reflection and mid-carves under visible light irradiation. Highest CH3OH rate of ~0.16 mmolg^h-1 was observed over 1 wt% NiO/InTaO4 monolith containing fibers with tip-reflection and mid-carves compared to the monolith containing no fibers and bare fibers. The results are linked with fibers with tip-reflection and mid-carves having the highest side light emission percentage of 98% compared to no fiber (84%) and bare fiber (93.8%) amongst configurations. Overall, maximum acetal-dehyde conversion rate of 0.3 mmol g_1 h_1 was achieved with the 2.6 wt% NiO/InTaO4 monolith containing fibers with tip-reflection and mid-carves by simulated sunlight AM1.5G at 70 °C. The loading of different metal-based TiO2 nanomaterials onto monolithic structures threaded with optical fibers were tested for UV and visible light induced CO2 photocatalysis [102,230]. The loading of metal or metal oxide on the surface of TiO2 via the introduction of defects into the lattice was reported to tailor its band width towards the visible light and alter its particle properties. For example, sol-gel derived 1 wt% Ni2+-based TiO2 monolith containing optical fibers showed improved CH3OH production rate of 13 mmol/gcatalh compared to pure TiO2 coated monolith. Addition of Ni2+ influenced activity and selectivity of TiO2 toward UV and visible light region due to the substitutional metal ions not only causing changes in the electronic structure and light absorption properties of TiO2, but also altering the surface area, grain size and degree of phase transformation.

4.4. Other supports

The effect of co-catalyst (Cu-Pt)-sensitized TiO2 nanoparticle wafer on CO2 photocatalytic conversion was studied under full sun illumination (AM 1.5G) [103]. Coated wafers which had randomly connected pores were used as flow through membrane such that reactants (CO2 and H2O) pass through one end of the membrane and products collected at the other end. Improved product rates were achieved due to back reactions limited by the diffusion of reaction products to the outlet. Optimum amount of catalyst loading on the TiO2 wafer were 9.6 at% Cu and 8.7 at% Pt. Hence, maximum conversion of CO2 to CH4 (49 ppm/cm2 h) was achieved over TiO2 wafer sputtered with both Cu and Pt layers than Pt (~28 ppm/cm2 h) and Cu (38 ppm/cm2 h). Thin layer of CdS-TiO2 nanocomposite was coated on a stainless steel support to improve CO2 reduction performance. Performance of catalyst coated supports was dependent on metal concentration and size of the nanoparticles. Maximum CO and CH4 production rates of 10.5 and 1.5 mmol/gcatal under visible light irradiation were

observed for the stainless steel support coated with 45% CdS-TiO2 nanocomposite.

The direct conversion of CO2 over TiO2 coated stainless steel webnets of varying sizes was investigated under UV irradiation [231]. High surface area and good utilization of UV light were observed on TiO2 films deposited on the webnets. An increase in mesh size resulted in increased TiO2 surface area and reduced penetration of UV light. Evaluation of the photocatalytic activities of TiO2 coated on three different mesh sizes of stainless steel webnets for CO2 photoreduction resulted in higher product rate for TiO2 coated on 120 mesh size than TiO2 coatings on 60 and 200 mesh sizes. Nishimura et al [232] dip coated TiO2 on a silica-alumina gas separation membrane to obtain 3.5ppmV/h of CO after 336h, while Pathak et al. [233] used the hydrophilic structural cavities in Nafion-117 membrane films to host TiO2 coated with nanoscale silver and obtained 0.071 mg-1 and 0.031 mg-1 of methanol and formic acid after 5 h. Reproducible results were obtained when these films were reused. Cybula et al. [234] employed a flat perforated steel or plastic tray as a support for the dispersion of TiO2 in a tubular reactor designed for CO2 photoreduction studies. They observed that the type of support used not only played a critical role in determining the amount of immobilized catalyst but also influenced the photoconversion rate when the same coating procedure was used. A decrease in catalyst loading and methane production (from 90ppm to 34ppm) was observed when the support was switched from steel to plastic due to weaker adhesion of TiO2 on plastic compared to steel. CO2 photoreduction studies by Shioya et al. [203] and Li et al. [94] employed silica as supports due to its even composition and orderly mesoporous structure with small channels. Sasirekha et al. [82], Yang et al. [93] and Li et al. [94] attempted to improve this arrangement by doping with metals such as Ru and Cu. They found that the combination of metals with mesoporous silica enhanced the reaction rate due to effective TiO2 dispersion and improved absorption of CO2 and H2O on the surface of SiO2. Product yields of 60 mmolg-TiO2-1 h-1 and 10mmolg-TiO2-1 h-1 were obtained for CH4 and CO, respectively, in a continuous flow photoreactor at an optimal doping ratio of 0.5 wt% Cu using Xe arc light source [94]. Maximum CH4 production of 627 mmol g-1 h-1 was observed by Yang et al. [93] with 2wt% Cu after 8 h reaction time. Improved surface area and better dispersion of cerium (Ce)-TiO2 on mesoporous silica (SBA-15) was also demonstrated by Zhao et al. [97] in their CO2 photoreduction studies following 4 h of UV-vis irradiation. They found that an optimal amount of 3% Ce-TiO2 dispersed on the silica matrix (Ti:Si—1:4) not only facilitated improved textural properties compared to pure TiO2, but also resulted in an order of magnitude increase in CO (7.5 mmolg-1) and CH4 (7.9 mmolg-1) production. They reported that the adsorption properties of silica resulting from its unique mesoporous structure was one of the contributing factors due to the increased localized CO2 concentration near TiO2 surface where photocatalysis could occur. Clays have been extensively used as supports in photocatalytic studies because of their low-cost and strong absorption capacity [235]. Koci et al. [95] used kaolinite/TiO2 in CO2 photoreduction and obtained CH4 and CH3OH yields of 4.5 mmol/gcatal and ~2.5 mmol/gcatal after 24 h of irradiation. Kaolinite prevented particle aggregation and modified the acid-basic properties of the surface of TiO2. The use of montmorillonite as support in CO2 photoreduction has also been examined by Kozak et al. [205]. CH4, CH3OH and CO production were observed over ZnS after 24 h of irradiation. Carbon based materials such as graphene/graphene oxide [236-239], carbon nanotubes (CNT) [240] and fullerenes amongst others have attracted wide attention as support materials for TiO2 induced CO2 photocatalysis, due to their high specific surface area,

electronic properties and enhanced transport of photogenerated electrons and visible light absorption [241-244]. Recently, hollow spheres consisting of alternating titania (Ti0.91O2) and graphene nanosheets were tested for CO2 reduction to CO and CH4 in the presence of water vapor [243]. CH4 and CO production rates over Ti0.91O2-graphene were 1.14 and 8.91 mmol/gh, respectively, which was reported to be five times higher than Ti0.91O2. Multi walled CNT (MWCNT)/TiO2 nanocomposites were reported to exhibit superior photocatalytic activity for CO2 photoreduction compared to anatase TiO2 and pure MWCNT [244]. Maximum CH4 yield of 0.178.91 mmol/gcatalh was achieved after 6h of visible light irradiation. Thus, the choice of an adequate support is of utmost importance since the overall process efficiency of the photoreactor is predominantly determined by the amount of activated photocatalysts. It is therefore imperative to utilize versatile materials with excellent light transmission properties that can simultaneously serve as catalyst carrier and provide high light transfer area via light distribution from the source to the photocatalyst present within the photoreactor.

5. Support immobilization techniques

TiO2 based catalysts can be deposited on structured substrates through aqueous or gaseous routes. Some examples of aqueous methods include sol-gel and electrophoretic deposition, while gas phase methods include spray pyrolysis deposition, chemical vapor deposition and physical vapor deposition. Table 3 highlights the advantages and disadvantages of different methods used for immobilising TiO2 catalysts [245-247].

5.1. Sol-gel method

Sol-gel technique is amongst the most widely used procedure for preparingTiO2 photocatalysts. This technique is not only noteworthy for achieving excellent chemical homogeneity but, also deriving unique stable structures at low temperatures as well [30,248-250]. The compositional and microstructural properties of the nano-sized samples can be tailored through the control of the precursor chemistry and processing conditions. Inorganic metal salts like titanyl sulphate, titanium tetrachloride etc., (non-alkoxide) and metal alkoxides e.g., titanium(IV) butoxide are usually employed as chemical precursors. Conversion from the liquid sol phase into the solid gel phase occurs due to solvent loss and complete polymerization. The pH of the reaction medium, water:alkoxide ratio and reaction temperatures are factors that influence the sol-gel

procedure [251]. Watson et al. [64] demonstrated the preparation of more uniform and pure photocatalysts via the alkoxide route, while Sivakumar et al. [252] used ammonium nitrate and titanyl sulphate as precursors. The rapid hydrolysis rate of titanium alkoxide has been reported as a major drawback that makes this process difficult to control [253]. The sol-gel process is initiated via hydrolysis and polycondensation of metal precursors (Eqs. (21)-(25)) where R stands for C4H9 [254,255].

Apart from esterification, the hydrolysing water can also be introduced and controlled through oxolation, as shown in Eq. (22). During condensation, the crystal of the metal oxide can be formed when the constituent particles of the gel are pulled into a compact mass. Additionally, acetic acid modifies and stabilizes the hydrolysis process by altering the alkoxide precursor at molecular level and acting as a chelating ligand, such that Ti—OH bonds were formed when the bidental ligand was broken off (Eq. (25)). The decrease in the hydrolysis rate results in the formation of fine particles of titania which are uniformly dispersed in solution. Appropriate amount of metal precursor(s) can also be introduced within the hydrolysis and polycondensation phase depending on the weight percent loading calculated from the amount of precursors used in the procedure. Sols are usually deposited on the substrates via dip-coating, spin coating, doctor blade techniques amongst others. The withdrawal speed of the substrate, number of coating cycles and the sol viscosity determines the catalyst film thickness. TiO2 was immobilized onto gas separation membrane by sol-gel dip coating method to study the CO2 reduction performance [256]. The gas separation membranes were dipped into TiO2 sol with different withdrawal speeds. Maximum CO production yield of >250ppmV was observed over the TiO2 film coated with the withdrawal speed of 0.66 mm/s after 72 h of UV irradiation. The improved activity using this optimum coating condition was attributed to TiO2 films being thin and even. The synthesized nano-sized TiO2 films have been to have special catalytic properties due to the integration of the active metal during the gelation stage.

■ Esterification:

ROH + CH3COOH H2O + RCOOC2H5

■ Oxolation:

Ti(OR)3 (OH) +Ti(OR)3 (OH)! (RO)3 Ti-O-Ti(OR)3 + H2O (22)

Table 3

Advantages and disadvantages of different methods used for immobilizing TiO2 photocatalysts.

Catalyst preparation method

Advantages

Disadvantages

References

Sol-gel

High purity of materials Hydrolysis rate is difficult to control

Homogeneity Longer processing time compared to CVD and PVD

Versatile means of processing and control of parameters Calcination at high temperatures results in the decomposition of

substrates

Large surface area Multiple coating cycles is required depending on the sol viscosity Chemical bonding results in strong adherence of coating to the substrate

[248-251]

Physical vapor deposition

Does not require complex chemical reactions Low to medium deposition temperature

Expensive as vacuum systems are used [19,245]

Difficulty in deposition of multiple source precursors due to various evaporation times

Chemical vapor Suitable for uniform and pure films with high deposition Could be expensive if vacuum systems are used

deposition rate

Short processing time Difficulty in deposition of multiple source precursors due to various

evaporation times

High deposition temperature (>600 °C) is required

[245-247]

■ Hydrolysis:

Ti(OR)4 + H2O ! Ti(OR)3 (OH) + ROH

■ Condensation dehydration:

Ti(OR)3 (OH) + Ti(OR)4 !(RO)3 Ti-O-Ti(OR)3 + ROH

■ Chelation:

RO i OR

/ \ HR

-4 ROH | + 2CH3COOH R

yO \ yO

J V./ \

-C Ti C — CH3

The series of Cr doped TiO2 samples synthesized by the sol-gel method was found to promote the CO2 reforming performance of TiO2. Under their experimental conditions, the Cr-doped samples showed improved photoresponse in the visible light in their study compared to the pure TiO2 film. Optimum product yields of 92.5 mmol/gcatal (CO), 15.1 mmol/gcatal (CH4) and 19.1 mmol/gcatal (C2H6) were obtained using the 7wt% sample. Copper supported on unstructured and inverse opal titania (templates of polystyrene spheres) films coated onto optical fibers by sol-gel dip-coating technique were employed for the photoreduction of gaseous CO2 to CH3OH in the presence of water vapor and UV light [157]. Although the methanol production rates of the Cu films supported on inverse opal titania (0.0364 mmol/gcatalh) were comparable to the films supported on unstructured titania, much higher quantum efficiency was achieved using the inverse opal film due to improved photon utilization rate observed at a lower light intensity.

5.1.1. Thermal treatment

Calcination is one of the means by which crystal growth can be controlled. The crystal growth influences the phase, shape, size and surface area of photocatalysts. Sol-gel derived precipitates tend to be amorphous in nature and require heat treatment to remove organic molecules from the final products and induce crystallization [251,255]. Amorphous TiO2 can be converted to anatase phase due to pore collapse and crystal growth of the TiO2 powder during calcination. With increasing temperatures, calcinations may result in phase transformation, reduced surface area, grain growth and

particle aggregation, which affects the microstructure and textural properties (crystallinity, surface area, morphology) of TiO2 [41,255]. The removal of organics has been reported to occur at temperatures >673K [254]. Conversely, well crystallized anatase TiO2 films can be synthesized by the peptization of tetraisopropyl orthotitanate in acidic conditions at low temperatures [257].

Since photoconversion can only occur when the photogenerated holes and electrons are present on TiO2 surface, the surface phase ofTiO2 exposed to reactant and light has been found to play a critical role in determining the rate of photoconversion by several researchers [62,75,258]. Consequently, a reasonable calcination temperature must be selected such that increased crystallinity is achieved with the surface area remaining intact and unchanged. Su et al. [255], Vijayan et al. [259] and Schulte et al. [260] demonstrated that their optimum activities were strongly dependent on the crystallinity of their nanostructures. Su et al. [255] showed that optimal photocatalytic activity for decomposition of salicylic acid can be achieved with the sample calcined at 773 K. The photoactivity decrease was observed beyond this temperature due to reduced surface area and decreasing anatase fraction. The anatase to rutile fraction was found to decrease with increasing temperatures.

Conversely, Vijayan et al. [259] observed increased methane production using titania nanotubes calcined between 473-673 K for CO2 reduction owing to the combined effects of crystallinity and surface area. Declined activity was observed on further temperature increase. The anatase content was also found to play a critical role in the UV activation of nanotubes prepared by Schulte et al. [260] for CO2 reduction. They also observed declined reactivity with increasing calcination temperature. Increased rutile content at near 953 K was found to tune the photoresponse toward the visible light region which further optimized reactivity of the samples. Amongst the crystalline phases of titania that can be formed during calcination, anatase has been reported to possess better photocatalytic activity for CO2 reduction compared to rutile due to higher surface areas and improved hole trapping arising from steeper band bending [251]. Phase transformation from anatase to stable rutile can occur upon thermal treatment between temperatures of 623-1373 K owing to different processing methods, precursors and techniques of determining this transition temperature [143,261]. The presence of mixed crystalline phases of titania (i.e., anatase and rutile) has also been reported to show improved photocatalytic activity due to synergistic effect derived from better charge separation and high surface area [262]. Improved charge separation and high reactivity at the anatase to rutile interface occurs during electron transfer from rutile to anatase at this interface where defect sites with unique charge trapping and adsorption properties can be created [62,262,263].

Zhang et al. [258] further investigated the relationship between the effect of calcination temperature and time on the surface phases and photocatalytic activities of TiO2. The transformation from anatase bulk phase to rutile occurred at 823 K, with the anatase phase still being maintained on the surface till 680°C. Further temperature increase to >973 K led to the complete conversion of the bulk anatase phase into rutile, with only 44% of the anatase phase being present on the TiO2 surface. Maximum hydrogen production was observed for samples calcined between 973-1023 K due to catalyst's surface consisting of a mixed phase of anatase and rutile particle, with the bulk consisting of almost pure rutile. Similar results were also observed for the samples calcined at 873 K for different time periods between 20 and 80 h. These results were due to the formation of the surface-phase junction which was found to promote electron transfer from the conduction band of rutile to the trapping sites of anatase. Cybula et al. [234] synthesized titania nanoparticles using TiO2 P25 and found the rate of CO2 photoreduction to methane was much higher on

temperature increase from 353 to 393 K and drying times from 5 to 20 h. A marked decrease in photoreduction efficiency was observed on further temperature increase to 433 K and drying time of 35 h. Asi et al. [75] synthesized AgBr doped TiO2 that exhibited good crystallinity and optimal hydrocarbon production rate for the photoreduction of CO2 under visible light at calcination temperature of 773 K and doping concentration of 23.2 wt%. They found that increasing the calcination temperature to 973 K resulted in the aggregation of the doped nanoparticles which explained the decreased in hydrocarbon production rate.

5.1.2. Influence of organic contaminants

Organic impurities originating from chemical precursors used during catalyst synthesis have been reported to influence the activity and selectivity of CO2 photoreduction. These organic compounds in the form of carbon and chlorine species which adsorb on the surface of TiO2 can serve as a carbon or chlorine source for the formation of hydrocarbons or undesired products such as CH3Cl. By using a combination of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) with isotope labelled 13CO2, Yang et al. [264] demonstrated that carbon residues present on the catalyst surface were involved in the photocatalytic reduction of CO2 to CO. It was observed that prolonged exposure of the catalyst surface to UV irradiation and H2O vapor was more effective for removal of these carbon residues which originated from Ti alkoxides and polyethylene glycol (PEG) than thermal treatment in air. Since adsorbed 12CO species were observed as the main product compared to 13CO over Cu(I)/TiO2 in the absence of 12CO2, it was concluded that 12C originating from carbon residues was the predominant carbon source. Isotopic labelling results of Ag, Au and Pd-TiO2 samples tested for CO2 reduction also confirmed that CH4 was formed from organic impurities rather than 13CO2 [265]. The formation of chloromethane (CH3Cl) as a result of CO2 photoreduction with Cu/I-TiO2 synthesized from a chlorinated precursor (CuCl2) was observed [181]. CH3Cl was formed from the reaction of methyl radical (CH3") with chlorine radical (Cl). Adsorbed carbon species were reported to be intermediates for CH4 and CH3Cl formation.

Several authors have employed different spectroscopic techniques such as DRIFTS, GC-MS, NMR or LC-MS with isotope labelled 13CO2 as the reactant for verifying the carbon source of final products generated from photocatalytic CO2 reduction. For example, Yui et al. [266] observed that CO2 and CO32" were the main carbon sources of CH4 produced using Pd-TiO2 treated by calcination and washing procedures. Signal of m/z = 17 attributed to 13CH4 detected by the GC-MS when 13CO2 was used as a reactant clearly demonstrated that CH4 formation was from CO2 photore-duction and not from residual carbon species. To verify the source of evolved CO and O2 from CO2 photoreduction with H2O, Teramura et al. performed labelling experiments with 13CO2 and H218O as reactants using GC-MS [267]. After photo irradiation, peaks of 16O18O and 13CO were detected. The effect of several solvents on CO2 photoreduction with Q-TiO2/SiO2 films was also studied by using 13C NMR and 13CO2 to identify the carbon source for CO and C1-C2 oxygenates [268]. Labelling experiments confirmed formate and CO were produced from CO2 and not from the solvents (CCl4, 2-propanol and acetonitrile). Fu et al. used isotopic 13CO2 for the photocatalytic reduction of CO2 over titanium metal organic framework (MOF) catalysts where obtained products were identified by 13C NMR spectroscopy [269]. Isotopic 13CO2 reaction confirmed that the reduced product, HCOO" originated from CO2 rather than residual carbon species. Ohno et al. also demonstrated that that CO2 was the carbon source for CH3OH evolution with 1H NMR spectroscopy over Au-TiO2 (brookite) nanorods and hybrid photocatalyst composed of WO3

and graphitic carbon nitride [270,271 ]. In summary, these findings clearly indicate that the choice of catalyst precursors must be carefully considered during catalyst synthesis with systematically designed control experiment [226,266,272] or carbon source verification by GC-MS, NMR or LC-MS with isotope labelled 13CO2 in place.

5.2. Vapor deposition

Chemical vapor deposition (CVD) has been widely used in surface coating in CO2 photoreduction and is prepared via the vapor phase, while the formation of films in physical vapor deposition (PVD) occurs when no chemical reaction takes place. Silija et al. [273] described CVD as a better technique when compared to PVD because better durability, adhesion and uniformity can be achieved with this technique compared to the latter. The need for aging, drying and reduction is also eliminated with CVD. Extensive studies conducted by Choy [245] detailed several variants of CVD and noted the complexity of chemical process as a major difference between CVD and physical vapor deposition (PVD) due to the chemical precursors and reactions used. The kind of precursor and substrate used with the desired properties serves as a determining factor for the type of variant used. The thermal and chemical stability of the support and operating conditions of the volatile precursors i.e., temperature required for crystallization must be carefullyconsidered. Thin films are usually formed in CVD when the surface of the substrate is exposed to volatile precursor(s) in inert atmosphere under controlled temperature and pressure. Nishida et al. [274] demonstrated the use of plasma enhanced CVD for the preparation of thin films of TiO2 while Galindo et al. [246] reported the use of pulsed injected metal organic CVD technique toward the deposition of multilayer thin films.

Wang et al. [213] synthesized platinized TiO2 films via the aerosol chemical vapor deposition (ACVD) technique. The synthesized films which have unique one-dimensional structure of TiO2 single crystals coated with Pt nanoparticles were reported to exhibit high photoefficiency for CO2 reduction with water vapor following 4 h of UV irradiation. Maximum CH4 yield of 1361 mmol/ gcatal h"1 was exhibited by the Pt film with deposition time of 20 s. Overall, high surface area, single crystallinity of the one dimensional TiO2 films and efficient hole separation were the main reasons described by the authors for the enhanced photo-activity of the films compared toTiO2. Asi et al. [75] demonstrated the visible light reduction of CO2 to fuels using AgBr/CNT nanocomposites. Multi-walled carbon nanotubes (CNT) were synthesized by CVD while AgBr was introduced to the CNT framework via deposition-precipitation method in the presence of cation surfactant. Higher product yields were obtained over AgBr/ CNT nanocomposites compared to AgBr crystals. The product yield also increased with increasing nanotube length due to efficient charge transport demonstrated by longer nanotubes which was confirmed by electrochemical impedance spectroscopy measurements.

6. Photoreactor design and configuration

The configuration of catalyst particles in a photoreactor system is also another important factor that can influence the overall photocatalytic efficiency [27,28]. Photoreactors are vessels where reaction products are generated from the contact between photocatalysts, reactants and photons. The two key parameters which determine the types of photoreactors utilized in CO2 photoreduction are the phases involved (i.e., multiphase (gassolid, liquid-solid, gas-liquid-solid etc.) and the mode of operation (i.e., batch, semi-batch or continuous) [158].

Fig. 10. Schematic of (A) slurry reactor design with top illumination, (B) optical fiber reactor design with side illumination and (C) internally illuminated reactor with top illumination.

Photocatalysts can be generally tested in either suspended or immobilized forms in reactor configurations, as shown in Fig. 10. An ideal photoreactor must have uniform light distribution throughout the entire system in order to achieve optimum results. Currently, comparative analysis of product yield and reactor configurations in CO2 reduction is primarily reported in terms of quantum efficiency. The advantages and disadvantages of different types of photoreactor systems currently used in CO2 photoreduction are summarized in Table 4 [275-279].

6.1. Fluidized and slurry reactor designs

When powders are dispersed/suspended in a liquid medium; the quantum efficiency of the catalyst, absorption properties of both reactants and non-reactants in solution and surface light intensity determines the rate of reaction [28]. Some of the key advantages of slurry system are that there is entire external surface illumination during reaction time if the particle size of the catalyst is small with phase segregation not occurring if the solution is homogeneously mixed [27]. Although slurry system designs offer high catalyst loading and ease of construction; separation of catalyst particles from the reaction mixture is a major drawback [221,279]. The size of the catalyst crystals or aggregates (0.05 mm to a few mm) will determine the nature of separation process required which could be expensive and time consuming [158,279]. However, the penetration depth of UV light into the reaction medium can also be limited by the strong light absorption of organic species and catalyst particles [28,219]. A large proportion of catalyst surface area might be inactive due to low photon energy received, as most of the light irradiation is lost due to absorption by liquid when light approaches the catalyst through the bulk liquid phase [280,281]. Light distribution can be better controlled via external or internal illumination. Hydrocarbon formation (CO, CH3CHO and CH3CHO2) was observed over hybrid catalyst, TiO2: Rh-LHCII tested in a stirred batch reactor under visible light irradiation [272]. Hybrid catalyst, TiO2:Rh-LHCll was formulated by attaching light-harvesting complexes (LHCII) extracted from spinach to the surface of Rh-doped TiO2 (TiO2:Rh) in order to

Table 4

Advantages and disadvantages of photoreactor systems.

Reactor design Advantages Disadvantages References

Fluidized and slurry (I) Temperature gradients inside the beds can be reduced through (I) Filters (liquid phase) and scrubbers (gas) are needed [13]

reactor vigorous movements caused by the solid passing through the fluids

(multiphase)

(II) Heat and mass transfer rates increase considerable due to agitated (II) Flooding tends to reduce the effectiveness of the

movement of solid particles catalyst

(III) High catalyst loading (III) Difficulty of separating the catalyst from the reaction [219,220]

mixture

(IV) Low light utilization efficiency due to absorption and

scattering of the light by the reaction medium

V) Restricted processing capacities due to mass transport

limitations

Fixed bed reactor (I) High surface area (I) Temperature gradient between gas and solid surface is [275,280]

(II) Fast reaction time common

(III) The conversion rate per unit mass of the catalyst is high due to the

flow regime close to plug flow

(IV) Low operating costs due to low pressure drop

Variants of fixed bed designs

Monolith reactor (I) High surface to volume ratio and low pressure drop with high flow rate (I) Low light efficiency due to opacity of channels of the [276,277]

can be achieved monolith

(II) Configuration can be easily modified

Optical fiber reactor (I) High surface area and light utilization efficiency (I) Maximum use of the reactor volume is not achieved [277,278]

(II) Efficient processing capacities of the catalyst (II) Heat build-up of fibers can lead to rapid catalyst

deactivation

enhance the light absorption and increase yields. The quantum efficiency of 0.0411% was achieved using this configuration.

Slurry reactor design with two separate components for oxygen and hydrocarbon evolution separated by a Nafion membrane was tested by Lee at al. [107] under AM 1.5G irradiation. WO3 was chosen as the oxidation catalyst while Pt/CuGaAlO4 and Pt/SrTiO3: Rh as reduction catalyst. The dual photocatalyst system containing both reduction and oxidation catalysts demonstrated higher quantum efficiency of 0.0051% compared to the single photo-catalyst system with Pt/CuGaAlO4 as reduction catalyst only (0.0019%). Photocatalytic reduction of CO2 with monoethanol-amine (MEA) as reductants to form CH4 using Ti-MCM-41 meso-porous catalysts was studied and compared to other reductants such as NaOH and H2O in slurry reactor designs [184]. Photoreduction efficiency of Ti-MCM-41(50) sample 8 h of UV illumination using H2O, NaOH and MEA as reductants were 0.83, 0.29 and 9.18%, respectively. Copper or cobalt incorporated TiO2 supported ZSM-5 catalysts were tested in a slurry photoreactor where 0.1 M NaHCO3 was the reductant [282]. When Cu-TiO2/ZSM-5 was used as the catalyst, the quantum efficiency of CH3OH reached 10.11% after 12 h reaction time. High absorption ability of ZSM-5 was reported to influence the photoconversion rates.

6.2. Fixed bed reactor designs

The drawback of catalyst separation can be avoided by fixed bed reactor designs where catalysts are immobilized onto fixed supports e.g., plate, beads, fibers, monolith etc. In these systems, photocatalysts are coated on a support matrix placed inside the reactor around the light source or directly on the photoreactor wall. Light distribution becomes a limiting factor in this system which is influenced by the geometry of the light source and spatial distance between this light source and photocatalysts [197]. Overall, gas phase systems offer more flexibility compared to the slurry systems if the design considers the spatial relationship between the reactor and the light source when choosing the support of choice. Several researchers have designed photoreactors using optical fibers as supports [217,278,283]. The conventional optical fiber reactor (OFR) has been modified by using fibers with different cores and coatings [218,284], increasing their diameter to create ease of handling and the use of cooling systems [285] to eliminate the limitation of heat build-up. Wu et al. [154,286] have conducted CO2 photoreduction studies using the optical fiber reactor system. They coated optical fibers with Cu/TiO2 and Ag/TiO2 catalysts in the gaseous phase, respectively. A maximum yield of 0.91 mmol/gcatal h was observed using the loading ratio of 0.5 wt% Cu-Fe/TiO2 for methane production. Maximum methanol yield of 0.45 mmol/gcatalh was observed by Wu et al. [286] using 1.2 wt% Cu/TiO2 under light intensity of 16 W/cm2 while methanol yield of 4.12 mmol/gcatalh was observed by Wu et al. [154] using 1 wt% Ag/TiO2 under light intensity of 10 W/cm2. In particular, previous CO2 photoreduction studies conducted by Nguyen and Wu [219] using optical fibers coated with Cu-Fe/TiO2 catalysts in the gaseous phase have demonstrated that the number of optical fibers can determine the rate of ethylene production and selectively increase or decrease the quantum yield. A maximum yield of 0.91 mmol/gcatal h was observed using the loading ratio of 0.5 wt% Cu-Fe/TiO2 for CH4 production. CO2 photocatalytic activity of NiO/lnTaO4 catalysts dispersed in aqueous solution of NaOH (slurry designs) and immobilized in a fixed bed reactor design containing 216 optical fibers was evaluated by Wang et al. [193]. The quantum efficiency for methanol production was 14 times higher in optical-fiber reactor (0.053%) than that of the aqueous-phase reactor (0.0045%). The higher quantum efficiency was attributed to improved light efficiency by the films coated on optical fibers. The comparison between the photocatalytic

reduction of CO2 for 1 wt% Pd/0.01 wt% Rh-TiO2 in a slurry batch annular reactor system and internally illuminated photoreactor system was evaluated [155]. The quantum efficiency of the internally illuminated monolith reactor (0.049%) was near one order of magnitude higher than the slurry batch annular reactor (0.002%). The performance of TiO2 for CO production was evaluated in cell type and multichannel monolith reactor [226]. The performance comparison between the gas phase photoreactors revealed 8 fold higher yield of CO in the monolith compared to cell type reactor. Quantum efficiency in microchannel monolith reactor was 0.0301% compared to 0.0028% in the cell type reactor. lmproved quantum efficiency was reported to be due to higher illuminated surface area, higher photon energy consumption and better utilization of reactor volume.

7. Conclusions and future perspectives

The utilization of CO2 as a direct feedstock for photocatalytic conversion into fuels over different variants of pure and modified TiO2 synthesized by various routes and tested in various photo-reactor designs has been highlighted in this review. Application of TiO2 induced photocatalysis for the challenging conversion of CO2 remains a promising pathway as it can be activated by solar energy at relatively mild conditions to form valuable products. Although recent progress focused on the use of pure and modified photoactivated TiO2 materials has induced fuel generation from CO2 reduction with H2O; expected improvement required for scalable fuel production has not been achieved. To this end, design and synthesis of novel TiO2 photocatalysts with higher stability, selectivity and efficiency requires improvements in synthetic procedures offering improved control over physicochemical properties. Analytical techniques such as in-situ surface and bulk spectroscopies must be employed to provide valuable insight into fundamental steps occurring in CO2 photocatalytic reduction, rate limiting steps, formation and stability of surface reaction intermediates as well as adsorption and desorption of both reactant and product species. Besides the materials science aspect of CO2 photocatalytic reduction, the engineering challenge of optimal CO2 photoreactor design needs a step change transformation to reach its crucial role in the overall process performance. From this review, it can be concluded that CO2 photocatalysis is still not feasible due to the absence of scalable reactor designs able to simultaneously introduce reactants, light and efficient visible light responsive catalysts to effect production of specific fuels in significant quantities. Since the overall process efficiency is largely dependent on two factors—the reactor configuration and physico-chemical properties of the catalyst, it is desirable to scale up this system based on the design and development of these parameters incorporating maximal light efficiency and mass transfer. Performance of current photoreactor designs is primarily reported and evaluated in terms of quantum efficiency without consideration of light transport to active site which is critical for scale-up and quantification of energy losses due to light absorption by reaction media and reactor components. A deep understanding of engineering aspects of CO2 reduction is still required for the development of highly efficient photoreactor designs. ln order to achieve high conversion efficiency, photoreactor designs must take into account the material of construction, its thickness, mass of catalyst, reactor geometry (length, volume etc.), flow rate and the relationship between the reactor and irradiation source. The modelling of the effect of reactor designs and operation parameters on CO2 reduction is also required to extrapolate results for the design of pilot scale systems. Furthermore, this work can be extended to include the use of flue gas generated from power plants as a feedstock for CO2 reduction. Different compositions of flue gas streams can be used directly or indirectly in order to

ascertain the effect of impurities and the concentration of CO2 required to achieve maximum conversion rates. Results from using concentrated CO2 gas streams derived from the flue gas can also be tested and compared to pure flue gas streams to determine the most suitable option for scalable fuel production.

Acknowledgements

The authors thank the financial support provided by the School of Engineering and Physical Sciences and the Centre for Innovation in Carbon Capture and Storage (EPSRC grant number EP/K021796/ 1) at Heriot-Watt University.

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