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ELSEVIER
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
ICATALYSIS
Methane dry reforming over ceria-zirconia supported Ni catalysts
Astrid Wolfbeissera, Onsulang Sophiphunb, Johannes Bernardic, Jatuporn Wittayakunb, Karin Föttinger3 *, Günther Rupprechtera
a Institute of Materials Chemistry, Technische Universität Wien, Getreidemarkt 9, 1060 Vienna, Austria
b School of Chemistry, Institute of Science, Suranaree University of Technology, 111 University Ave. Muang District, Nakhon Ratchasima, 300000, Thailand c University Service Center for Transmission Electron Microscopy, Technische Universität Wien, Wiedner Hauptstraße 8-10, Vienna, 1040, Austria
ABSTRACT
Nickel nanoparticles supported on Ce1-xZrxO2 mixed oxides prepared by different synthesis methods, as well as Ni-ZrO2 and Ni-CeO2, were evaluated for their catalytic performance in methane dry reforming (MDR). MDR is an interesting model reaction to evaluate the reactivity and surface chemistry of mixed oxides. Textural and structural properties were studied by N2 adsorption and XRD. Mixed oxide preparation by co-precipitation resulted in catalysts with higher surface area than that of pure ZrO2 or CeO2. XRD analysis showed the formation of different Cei-xZrxO2 solid solutions depending on using a surfactant or not. The catalyst prepared by surfactant assisted co-precipitation was not active for methane dry reforming most likely because of the encapsulation of Ni particles by ceria-zirconia particles, as revealed by TEM and H2 chemisorption. The catalytic activity of the catalyst prepared by co-precipitation without surfactant was comparable to Ni-ZrO2. Clearly, catalyst activity strongly depends on preparation and on the resulting phase composition rather than on nominal composition. Compared to Ni-ZrO2 the ceria-zirconia supported Ni catalyst did not achieve higher activity or stability for methane dry reforming but, nevertheless, the formation of filamentous carbon was strongly reduced (100 times less carbonaceous species). Consequently, using ceria-zirconia as a support material decreases the risk of reactor tube blocking.
© 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/).
ARTICLE INFO
Article history:
Received 11 January 2016
Received in revised form 30 March 2016
Accepted 19 April 2016
Available online xxx
Keywords:
Methane dry reforming Nickel nanoparticles Ceria-zirconia Mixed oxides Coke
Coprecipitation
1. Introduction
Methane dry reforming (MDR) has attracted attention in recent years, not only because of synthesis gas production, but also due to environmental aspects. The MDR reaction (1.1) directly converts the greenhouse gases CH4 and CO2 to synthesis gas. Since a lower H2:CO ratio is achieved by MDR than by methane steam reforming (MSR), synthesis gas produced by MDR is effective for olefins hydroformylation and carbonylation reactions. Additionally, MDR has great potential to be used for energy transformation and storage due to its reversibility via methanation [1].
CH4 + CO2^ 2H2 + 2COAH0 (25°C) = +247 kJmol-1 (1.1)
* Corresponding author. E-mail addresses: astrid.wolfbeisser@tuwien.ac.at (A. Wolfbeisser), onsulang.111@hotmail.com (O. Sophiphun), bernardi@ustem.tuwien.ac.at (J. Bernardi), jatuporn@sut.ac.th (J. Wittayakun), karin.foettinger@tuwien.ac.at (K. Föttinger), guenther.rupprechter@tuwien.ac.at (G. Rupprechter).
However, other possible reactions such as reverse water-gas-shift RWGS (1.2), steam reforming of CH4 (1.3), methane decomposition (1.4) and CO disproportionation (1.5) must be considered, resulting in H2:CO ratios different from 1 and leading to carbon deposition on the catalyst.
H2 + CO2^H2O + CO AH0(25°C) =+41 kJmol-1 (1.2)
H2O + CH4^3H2 + CO AH0(25°C) = +206 kJmol-1 (1.3) CH4 ^2H2 + C A H0(25°C) = +75 kJmol-1 (1.4)
2CO^C + CO2 AH0(25°C) = - 171 kJmol - 1 (1.5)
The effect of carbon deposition is a major problem and may lead to catalyst deactivation [2,3], by decoration (poisoning) ofthe active metallic sites and/or physical blockage of the tubes in fixed bed reformers [4]. Hence, intense research efforts have been focused on the development of catalysts that show high activity but are also resistant to carbon formation and sintering. Fischer and Trop-sch showed already in 1928 that most group VIII metals display a significant activity for methane dry reforming [5]. Many years
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later various transition metals (Ni, Ru, Rh, Pd, Ir, and Pt) have been tested [6-10]. Even though Rh was the most stable and active of group VIII metals rather Pt catalysts have been developed as excellent methane dry reforming catalysts [11-15]. Besides the highly active noble metal catalysts, nickel based catalysts are the choice for industrial applications, due to better availability, lower cost and comparable activity [16]. Nevertheless, the active nickel particles tend to form coke, leading to catalyst deactivation [17].
Nickel-based catalysts have been proven to be sensitive to doping and structure modification by other metals, such as Cu, which affects the structure and morphology of the produced carbon [18]. We have studied CH4 reforming activities and selectivity on Ni and CuNi-based catalyst systems by combining in situ spectroscopic techniques such as near atmospheric pressure X-ray photoelectron spectroscopy (NAP-XPS), Fourier transform infrared spectroscopy (FTIR) and X-ray absorption spectroscopy (XAS) [19-21]. The formation of CuNi alloy nanoparticles enhanced the stability against carbon deposition, but the CuNi alloy showed limited stability (i.e. Ni segregation) under reaction conditions [20,21]. For in situ studies on other bimetallics (PdZn, Pd-Ga, Pd(Pt)-Cu) we refer to [22-26]. Boron promotion of Ni-based catalysts was also reported to inhibit the formation of bulk carbide and weaken the on-surface carbon binding energies [27].
Clearly, in addition to the metallic component of the catalyst, the support material also plays an important role in the catalytic activity and stability towards carbon deposition. Indeed, nickel has been supported on various supports such as MgO [3,16,28-30], Al2O3 [3,30-34], SiO2 [3,16,35], TiO2 [3,16], La2O3 [31,32], CeO2 [30,32,36], ZrO2 [32,33,37,38] and Ce1-xZrxO2 [39-49]. Ce1-xZrxO2 solid solution is considered as a promising support [39-46] for Ni-based MDR catalysts. Mixing ZrO2 and CeO2 has been found to improve the thermal stability, catalytic activity and oxygen storage capacity. The latter refers to the ability to deliver oxygen from the lattice to the gas phase or to solid (adsorbed) carbon [39,46,50]. Several methods have been used to prepare Ce1-xZrxO2 solid solutions for catalytic applications. These include the high-temperature milling of a mixture of the oxides [51], co-precipitation [39-41,44,50,52-56], surfactant-assisted co-precipitation [42,43,45,57,58], thermal hydrolysis [59-61], sol-gel [46,62,63] and pseudo-sol-gel techniques [47,48]. Among these methods, the surfactant-assisted co-precipitation route has attracted considerable interest because of its effective soft-templating, reproducibility and simplicity [58]. However, detailed correlations of MDR catalytic activity with the oxides microstructures (e.g. determined by Raman spectroscopy or powder XRD via the Rietvield analysis) are still lacking. In order to understand the beneficial effect of the Ce1-xZrxO2 support is ofgreat importance to establish relationships between the surface structural and physico-chemical properties with the catalytic activity.
Recent studies by Makri et al. [64] provided very important insights into the carbon formation pathways and carbon reactivity over 5 wt% Ni/Ce0.8Zr0.2O2-8 and 5wt% Ni/Ce0.5Zr0.5O2-8 MDR catalysts. Using temperature-programmed methods and transient isotopic experiments it was demonstrated that the reaction temperature and support chemical composition had a strong effect of on the relative contribution of the CH4 or the CO2 activation routes towards carbon formation, as well as on the reactivity of the various carbons towards H2 and O2.
In this contribution we have revisited the mixed oxide support and have compared the catalytic activity, stability and coke formation tendency of Ni based catalysts supported on Ce0.6Zr0.4O2 (prepared by co-precipitation and surfactant assisted co-precipitation) with those supported by the individual supports ZrO2 and CeO2. MDR is an interesting model reaction to compare the reactivity and surface chemistry of different mixed oxides. The main aim of this work was to understand some of the influ-
encing factors by correlating structure (governed mainly by the synthesis procedure) with catalytic performance. Ce0.6Zr0.4O2 was chosen since Kumar et al. [42] reported Ni-Ce06Zr0.4O2 to be the most effective methane dry reforming catalyst, as its activity was stable for up to 100 h at 650 and 700 °C. We have tested these catalysts during MDR at 600 °C for up to 24 h and characterized them carefully before and after exposure to the reaction mixture. The mixed oxide catalysts prepared by different co-precipitation methods showed very different catalytic behaviour. Even though the catalytic activity of the active ceria-zirconia supported Ni catalyst was not higher than that of Ni-ZrO2, the Ce0.6Zr0.4O2 support still strongly decreased deposition of carbonaceous species.
2. Experimental
2.1. Sample preparation
Ni-ZrO2, Ni-CeO2 and two types of Ni-Ce06Zr0.4O2 catalysts were prepared by different methods. ZrO2 was prepared from Zr(OH)4 (MEL Chemicals, XZO 880/01) by heating from room temperature (RT) to 650 °C at a ramp of 2°C/min and kept at 650 °C for 2 h. Commercial CeO2 (Schuchardt, CE026) was calcined at 650°C at a ramp of 2°C/min and kept at 650°C for 2h. Two further binary ceria-zirconia materials were prepared by a co-precipitation (cp) method, both Ce0.6Zr0.4O2 were composed of 60mol% CeO2 and 40mol% ZrO2. For preparation of the first oxide support (labelled as Ce1-xZrxO2_cp) stoichiometric quantities of zirconyl nitrate hexahydrate (ZrO(NO3)2-6H2O, Fluka) and cerium(III) nitrate hexahydrate (Ce(NO3)3-6H2O, Aldrich) were dissolved in distilled water. The resulting solution was transferred to a round bottom flask and stirred for 1 h at 80 °C. Then, an aqueous solution of 20% w/w KOH was added drop-wise at 80 °C with constant stirring until a pH of 10.5 was reached. After digesting the precipitate at 80 °C for 72 h, it was filtrated and washed several times with distilled water to remove any potassium impurity. Then, it was air-dried for 72 h followed by drying at 120 °C for 6 h. Finally, the oxide was heated at a ramp of 2 °C/min and kept at 650 °C for 2 h. The second mixed oxide, labelled as Ce1-xZrxO2_ctab, was prepared by surfactant-assisted co-precipitation with cetyltrimethylammo-nium bromide (CTAB, Roth). The synthesis route was adapted from Sunkonket et al. [45] with a molar ratio c(CTAB)/c(Ce + Zr) = 1 and dropwise addition of aqueous ammonia for precipitation.
A nominal 5% w/w Ni was loaded over all supports by wet impregnation, using nickel nitrate hexahydrate (Ni(NO3)26H2O, Merck) aqueous solution. After drying overnight at 100 ° C the catalysts were calcined at 450 °C for 2 h.
2.2. Catalyst characterization
2.2.1. X-ray diffraction measurements
Powder XRD patterns were recorded on a XPERT-PRO diffrac-tometer with Cu Ka radiation operating at 40 kV and 40 mA with a 20 scanning from 5 to 90° and a step size of 0.02°. The crystallite size t was estimated from X-ray line broadening using the Scherrer's formula [65] t = 0.9-\/(B • cos(0)), where \ is the X-ray wavelength (Cu Ka radiation: 0.154 nm) and B the full-width half-maximum of the Bragg diffraction angle 0.
2.2.2. Surface area analysis
The Brunauer-Emmet-Teller (BET) specific surface areas for all catalysts were obtained by N2 physisorption acquired at liquid N2 temperature using a Micromeritics ASAP 2020 apparatus. Prior to analysis, all powders were outgassed at 300 °C and <13 |ibar for 1 h prior to acquisition of the adsorption isotherm. The average pore size and average pore volume were analysed by the BJH method
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using the adsorption branch of the N2 isotherm, as well as by NLDFT (model: cylindrical pores in an oxide surface).
2.2.3. Metal dispersion measurements
The metal dispersion and metallic surface area of the catalyst samples were estimated by H2 chemisorption at 35 °C using a Micromeritics ASAP 2020C instrument with 0.5 g catalyst. Prior to analysis, the samples were heated in an oxygen flow at 500 °C for 1 h. After cooling down in vacuum to 300 °C the samples were reduced in hydrogen flow at 400°C. After 20min reduction the samples were evacuated for 30min at 400°C. Chemisorp-tion was performed at 35 °C at hydrogen pressures between 75 and 775 mbar and repeated in order to isolate the chemisorp-tion isotherm. The same analysis procedure was applied after reducing at 600 °C. The difference of the first isotherm data (reversible + irreversible adsorption) and the repeated isotherm data (only reversible adsorption) was utilized to calculate the quantity of irreversibly adsorbed hydrogen [66,67] (for further details see Supporting information).
2.2.4. Infrared spectroscopy
The surface sites and oxidation states of the metals were investigated by FTIR spectroscopy of chemisorbed CO. IR spectra were recorded in transmission using a Bruker Vertex 70 spectrometer with an MCT detector. Samples were pressed to small discs and placed in the IR cell. All infrared spectra were collected at a resolution of 4 cm-1 in the 4000-900 cm-1 range by averaging 128 scans to achieve good signal to noise ratios. The results shown herein are difference spectra with the spectrum of the clean sample was taken as a background before adsorption. The pretreatment was carried out in the IR cell by heating at 10 C/min from room temperature to 500 °C under 100mbar O2 pressure and holding that temperature for 1 h, followed by a treatment in 5 mbar H2 and 900 mbar N2 at 400 °C for 30 min. Then, the samples were outgassed at 400 °C below 10-5 mbar for 30min. Spectra before and after exposure to 5 mbar CO were recorded at 30 °C.
2.2.5. Transmission electron microscopy (TEM)
TEM was utilized to obtain information about the size, morphology and distribution of the nickel particles on the oxide supports. Prior to microscopy the catalysts were oxidized at 500 °C and reduced at 600 °C. To characterize the used catalysts, samples were exposed to methane and carbon dioxide at 600 °C for three hours. For TEM measurements the samples were suspended in ethanol and deposited on carbon coated Cu grids. TEM was performed on an analytical TECNAI F20 field emission TEM operated at 200 kV equipped with an energy dispersive X-ray detector (EDX). EDX was used to identify the Ni particles on the support materials. The structural analysis of the recorded images has been performed using ImageJ 1.48. The digital diffractograms reported herein correspond to the log- scaled power spectrum of the corresponding fast Fourier transforms.
2.2.6. X-ray absorption spectroscopy (XAS): data acquisition and analysis
XAS spectra were recorded at the Ni K-edge (E0=8333eV) at Beamline 8 of the Synchrotron Light Research Institute, Thailand. Spectra of catalysts reduced in 20 vol% H2 in Ar at 600 °C (labelled as: "red.") and samples after exposure to the MDR-reaction mixture (10 vol% CH4, 10 vol% CO2 and 80vol% Ar) at 600 °C for 3h (labelled as: "used") were recorded at room temperature in fluorescence mode. Each sample was pressed into a frame covered by polyimide tape and mounted onto a sample holder. The experimental setup and the measurement procedure were described in [66].
X-ray absorption data were processed and analysed using the IFFEFIT [67] software package (version 0.9.21). With each recorded XAS spectrum, the absorption edge position (E0) was set equal with the position of the highest maximum of the 1st derivative of | (E) in the X-ray energy E. Data processing was done by fitting a linear and a polynomial function to the pre- and post-edge region, respectively, resulting in a normalized | (E). Five | (E) data scans were merged into one data file to improve the signal-to-noise ratio. The fraction of Ni present in reduced and used catalysts were determined by fitting to linear combinations of reference spectra, i.e. normalized Ni K-edge XANES spectra of Ni foil and NiO references. Further details are given in the supporting information.
2.3. Catalytic testing
Activity studies of methane dry reforming (MDR) were carried out in a quartz flow reactor. 25 mg catalyst were mixed with 75 mg quartz sand and loaded into the glass reactor in-between two plugs of quartz wool. The catalysts were oxidized in 20vol% O2 in Ar at 500°C for 30 min and reduced in 10vol% H2 in Ar at 600°C for 30 min. Before exposure to the reaction mixture the sample was kept in Ar at 600 °C for 30 min. Then, the sample was exposed to a mixture of 10% CH4, 10% CO2 and 80% Ar with a total flow of 25 ml/min at 600 °C for 24 h. For quantitative determination of the reaction rates the reaction products were analysed under steady state conditions by a gas chromatograph equipped with a HP-PLOT Q column, a thermal conductivity detector (TCD) and a flame ion-ization detector (FID).
After the MDR experiment the sample was cooled to room temperature in Ar. Subsequently, temperature programmed oxidation (TPO) was performed in 20% O2 in Ar applying a heating rate of 5 °C up to 900 °C. The amount of CO2 formed by oxidation of C was monitored by mass spectrometry (Pfeiffer QMS 200) detection.
2.3.1. Equations used for calculation of catalytic results
The CH4 and CO2 conversion, H2 formation rates, H2:CO ratio, carbon-balance, H2 selectivity and turn-over-frequency (TOF), based on the metallic Ni surface area after reduction at 600 °C, are defined as follows:
% CH4 conversion : XCH4 = f (CH4,i") ~f (CH4,out) x 100% (2.1)
% CO2 conversion : XCO2 =
f (CH4>in) f (CÛ2,in) - f (CÛ2,oUt )
f (CO2,in) H2 production rate : rH2 = FT x yH2 CO production rate : rCO = FT x yCO
H2 : CO ratio : R = Гн2
x 100% (2.2)
_ f(CH4,out) + f(CO2,out) + f (COout)
Carbon ratio : Cbaiance = —1--т--т--¡—^-т- (2.6)
% H2 selectivity : SH2 =
f (CH4,in) + f (CO2,in) f (H2,out)
Turn-over-frequency : TOF =
2 (f (CH4,in) -f (CH4,o„t)) Nh2
Number of produced H2 molecules per s : NH2 f (H2,out) xp (H2)
M (H2)
x 100% (2.7) (2.8)
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Fig. 1. XRD patterns of Ni-ZrO2, Ni-CeO2, Ni-Ce1-xZrxO2_cp and Ni-Ce1-xZrxO2_ctab after ex-situ reduction. ZrO2 exhibits characteristics of monoclinic zirconia, CeO2 and Ce1-xZrxO2_ctab show characteristics of cubic CeO2 or cubic Ce-Zr-oxide and Ce1-xZrxO2_cp shows reflections of both cubic and tetragonal Ce-Zr-oxides.
Number of surface Ni atoms : NNi = mcat x ANi x NNi/m2 (2.10)
for f (iin) and f (iout) being the volume flow rates of each component in the feed or effluent in m3/s, FT being the total molar flow rate (inmol/s) measured at room T and 1 atm, yH2 is the mole fraction of hydrogen and yCO that of CO in the product gas mixture, p (H2) the density of hydrogen, M (H2) the molar mass of hydrogen, mcat is the mass of the catalyst in the reactor of 0,025 g, ANi is the Ni surface area (m2/g of catalyst), and NNi/m2 is the number of surface nickel atoms per square meter of catalyst.
3. Results
3.1. Textural and structural properties
The crystalline structure of the samples was determined by XRD. The X-ray diffractograms of Ni-ZrO2, Ni-CeO2, Ni-Ce1-xZrxO2_cp and Ni-Ce1-xZrxO2_ctab after ex-situ reduction at 600 °C are shown in Fig. 1. The ZrO2 supported catalyst shows reflections assigned to monoclinic ZrO2 while the diffractogram of Ni-CeO2 shows reflections assigned to cubic CeO2. The ceria-zirconia mixed oxides vary in their phase composition: The support material prepared by co-precipitation without surfactant is composed of a mixture of a cubic ceria-rich mixed oxide phase and a tetragonal zirconia-rich mixed oxide phase. The lattice parameter of the cubic phase is 539 pm, indicating a Ce:Zr ratio of 90:10 and the lattice parameters of the tetragonal phase are 363 pm and 522 pm, indicating a Ce:Zr ratio of 12:88. The phase separation indicates insufficient mixing of the
cerium and zirconium precursor during co-precipitation. The support prepared by co-precipitation with CTAB as surfactant consists of a single cubic ceria-zirconia-mixed oxide phase with a lattice parameter of 530 pm and a Ce:Zr ratio of 60:40. Broader reflections indicate smaller ceria-zirconia crystallites compared to the other samples. The average crystallite sizes of the support materials estimated by using the Scherrer-equation are given in Table 1. We also performed the Rietveld refinement for CeO2 as well and obtained lower crystallite size (12 nm as compared to 30 nm according to the Scherrer equation). Thus, the absolute values have to be treated with care and we focus our discussion on and potential reaction-induced changes. The reflection positions at about 38°, 43° and 63° 20 of cubic NiO are marked in Fig. 1 and are observed as small reflections on all samples. Since the samples were exposed to air between ex-situ reduction and XRD measurements, the Ni particles were (partially) re-oxidized to NiO. The average crystallite sizes of NiO estimated by the Scherrer-equation are 27 nm onNi-ZrO2,21 nm on Ni-CeO2 and 16 nm on both ceria-zirconia supported catalysts. For all samples the presence of additional amorphous phases cannot be excluded, however.
Surface area analysis by nitrogen physisorption was carried out for the samples ZrO2, CeO2, Ce1-xZrxO2_cp and Ce1-xZrxO2_ctab. The results of the calculations of the specific surface areas using the method of Brunauer, Emmet and Teller (BET) and of the pore size and pore volume using the method of Barret, Joyner and Hal-enda (BJH) [68] as well as a comparison to the pore size and pore volume obtained by NLDFT are included in Table 1. The specific surface area of both oxides prepared by co-precipitation was significantly higher than that of pure CeO2 and ZrO2. The mesopores volume reached the highest value for the ceria-zirconia supports while the lowest value was obtained for ZrO2. The average pore size of ZrO2 of 21 nm is clearly larger than that of the other samples. In general, all the samples are mesoporous, and hardly any micropores were detected.
3.2. TEM study
Fig. 2 shows representative TEM images of a) Ni-ZrO2, b) Ni-Ce1-xZrxO2_cp andc)Ni-Ce1-xZrxO2_ctab.The samples were ex-situ reduced at 600 °C prior to the microscopy but exposure to air during sample transport led to (partial) re-oxidation of the Ni particles.
On Ni-ZrO2, Ni orNiO particles of 10-30 nm diameter were identified as displayed in Fig. 2a. On Ni-Ce1-xZrxO2_cp, besides rather large 50 nm Ni or NiO particles, mainly particles of about 10 nm size were detected, as shown in Fig. 2b. According to elemental maps (shown in the Supporting information) Ni particles were located both on ceria- enriched and zirconia-enriched grains of the mixed oxide support. On Ni-Ce1-xZrxO2xtab, Ni was again detected by EDX analysis but, in contrast to the other catalysts, no Ni or NiO particles could be found on the border of the agglomerates (even after intensive search). Thus, it seems that these particles are well dispersed and in strong interaction with the mixed oxide support (or
Table 1
BET surface area, BJH average pore size and volume, mean pore size and total pore volume obtained by NLDFT, and average support and metal crystallite size (estimated by using Scherrefs equation) of Ni-ZrO2, Ni-CeO2, Ni-Ce1-xZrxO2_cp and Ni-Ce1-xZrxO2_ctab.
sample
BET surface area (m2/g)
BJH average pore size (nm)
BJH average pore volume (cm3/g)
NLDFT mean pore size (nm)
NLDFT total pore volume (cm3/g)
Oxide structure and average crystallite size (nm)
Average NiO crystallite size (nm)
ZrO2 37
CeO2 56
Ce,-xZrxO2-cp 91
Ce,_xZrxO2_ 93 ctab
21 0.023
7 0.074
7 0.13
9 0.25
16 0.016
6.7 0.059
7.8 0.15
10.1 0.11
Monoclinic: 36 27
cubic: 30 21
tetragonal: 9 16 cubic: 19
cubic: 10 16
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Fig. 2. TEM images of a) Ni-ZrO2 b) Ni-Ce1-xZrxO2_cp and c) Ni-Ce1-xZrxO2_ctab.
Table 2
Ni dispersion and Ni surface area of Ni-ZrO2, Ni-CeO2, Ni-Ce1-xZrxO2_cp and Ni-Ce1-xZrxO2_ctab after reduction at 400 °C and 600 °C, based on H2 chemisorption.
sample Ni dispersion (%) Ni surface area (AM) (m2 /g of catalyst)
400 °C red. 600 °C red. 400 °C red. 600 °C red.
Ni-ZrO2 4.7 1.75 1.57 0.58
Ni-CeO2 2.6 1.4 0.88 0.46
Ni-Ce1-xZrxO2_cp 3.3 1.0 1.08 0.34
Ni-Ce1-xZrxO2_ctab 0.1 - 0.03 -
even partly encapsulated which would decrease the accessibility of nickel; see below).
3.3. Adsorption properties
3.3.1. H2-chemisorption
The accessible nickel surface area was determined by selective chemisorption of H2. The nickel dispersion and metallic nickel surface area per gram of sample after reduction at 400 °C and 600 °C are shown in Table 2. The highest Ni surface area upon reduction at 400 °C is observed on the zirconia supported Ni catalyst while on Ni-Ce1-xZrxO2_ctab only very little H2 adsorbs. The very small uptake of hydrogen by the Ni-Ce1-xZrxO2_ctab catalyst could be due to activated H2 chemisorption or to a strong SMSI effect after oxidation at 500°C and reduction at 400°C, induced by the specific interactions of Ni with this support (we will discuss the origin of this effect later in Section 4). After reduction at 600 °C the nickel surface area decreases suggesting sintering of the supported nickel particles. TPR confirmed that Ni reduction was complete at 600 °C for all catalysts. The metallic Ni surface area decreases more on Ni-ZrO2 than on Ni-CeO2 indicating that the ceria support increases the stability of Ni against sintering.
Table 3
Turn-over frequencies forH2 production normalized by the number of surface nickel atoms after reduction at 600 °C during MDR at 600 °C.
sample turn-over frequency (TOF) (s-1)
initial after 3 h after 24 h
Ni-ZrO2 Ni-CeO2 1.2 1.2 1.0
0.8 0.8 0.5
Ni-Ce1-xZrxO2.cp 1.8 1.6 0.9
2131-2145 and vas(CO) at 2100-2083 cm-1) [75,77-79] and tri-carbonyls (2156, 2124 and 2109 cm-1) [75] with Ni+. This unusual oxidation state is stabilized by CO. The stretching vibrations of linear adsorbed CO on metallic nickel appear at 2050-2094 cm-1 [72,78-84] and bridged carbonyls are observed below 1960 cm-1 [72,80,84].
On Ni-ZrO2 and Ni-Ce1-xZrxO2_cp, Ni2+-CO is observed as a shoulder of the larger Zr4+-CO peak at 2177 and 2171 cm-1, respectively. The peaks of CO adsorbed as mono-, di- or tricarbonyls on surface-Ni+ can be observed on Ni-ZrO2 and the mixed oxide supported samples but not on Ni-CeO2. On all catalysts CO peaks of linear and bridged adsorbed CO on metallic nickel are present. On Ni-CeO2 CO only adsorbs on metallic nickel which indicates that all the Ni on the surface is fully reduced at 400 °C. The ability of Ni-Ce1-xZrxO2_ctab to adsorb CO but not H2 (cf. Table 3) is currently not understood.
After evacuation Ni+-CO as well as linear and bridged CO on Ni0 remain on the surface of Ni-ZrO2. On Ni-CeO2 a rather high amount of linear and bridged CO on Ni0 remains on the surface in contrast to the Ce1-xZrxO2 supported samples, where only little CO remains on the reduced Ni surface.
3.4. Catalyst performance evaluation
3.3.2. Infrared spectroscopy of CO adsorption
IR spectra of room temperature CO adsorption on reduced Ni-ZrO2,Ni-CeO2,Ni-Ce1-xZrxO2_cp andNi-Ce1-xZrxO2_ctab are shown in Fig. 3. Note that reduction was carried out directly in the IR cell prior to adsorption without air exposure, in contrast to XRD and TEM measurements. According to literature data [15,33,69-71] adsorption of CO on ZrO2 results in a band at about 2190 cm-1 which can be observed on Ni-ZrO2 as well as on the mixed oxide supported samples. Different oxidation states of Ni can be distinguished by the vibrational frequency of adsorbed CO. Bands attributed to Nin+-CO surface complexes cover a broad spectral range, such as linear monocarbonyls but also bridged di- and tricarbonyl species. The weak adsorption band of CO on Ni2+ is observed around 2150-2180 cm-1 [33,70,72-74]. CO forms stable monocarbonyls (2160-2100 cm-1) [75-78], dicarbonyls (vs(CO) at
The results obtained for methane dry reforming at 600 °C for 24 h are presented in Fig. 4. Concerning stability, it can be stated that the CH4 and CO2 conversion over Ni-ZrO2 hardly changed with time while the conversion over Ni-CeO2 rapidly decreased and that over Ni-Ce1-xZrxO2_cp continuously decreased. Ni-Ce1-xZrxO2_ctab was hardly active during the first 30 min and then completely deactivated. The oxide support materials without nickel were inactive under these conditions, too.
In agreement with the reactant conversions, the H2 and CO production rates were stable over Ni-ZrO2 and decreased with time on stream over Ni-CeO2 and Ni-Ce1-xZrxO2_cp. Over Ni-Ce1-xZrxO2_ctab only little initial H2 and CO production was observed before the catalyst was completely inactive for methane dry reforming. The H2 selectivity was around 75% both for Ni-ZrO2 and Ni-Ce1-xZrxO2_cp during the entire reaction time indicating
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Fig. 3. IR spectra of a) 5 mbar CO adsorbed on reduced Ni-ZrO2, Ni-CeO2, Ni-Ce1-xZrxO2_cp and Ni-Ce1-xZrxO2_ctab at room temperature and b) after evacuation below 10-5 mbar.
Fig. 4. Screening of Ni-ZrO2, Ni-CeO2, Ni-Ce1-xZrxO2_cp and Ni-Ce1-xZrxO2_ctab catalysts for methane dry reforming reaction at 600°C; feed composition: CH4:CO2:Ar=10/10/80; feed flow rate = 25ml/min.
RWGS during reaction, whereas over Ni-CeO2 the H2 selectivity strongly changed with time.
The H2:CO ratio achieved was below 1 due to H2O production by reverse water gas shift as a side reaction. The highest value was obtained for Ni-CeO2 but it strongly decreased with time on stream. The C-balance, being an indicator for coke formation when below 1, was increasing to (nearly) 1 within the first 2 h.
The turn-over-frequencies (TOF) for H2 production based on the metallic Ni surface area after reduction at 600 °C and normalized to the number of surface nickel atoms assuming 1.59-1019 nickel atoms/m2 [85] are presented in Table 3. Reporting TOF values is in general a difficult matter because the nature and amount of active sites and the reaction mechanism would have to be exactly known.
CO2 activation likely occurs on the support and/or metal-oxide interface [64,86]. Recently, Makri et al. [64] performed a detailed mechanistic study and reported that MDR on Ni/Ce1-xMxO2_8 (M = Zr4+, Pr3+) very likely follows a bi-functional mechanism. CH4 activation is suggested to occur practically only on the Ni surface, whereas CO2 activation is proposed to proceed both on Ni but also on oxygen vacancies at the Ni-support interface. There is, however, broad agreement that CH4 activation on Ni is the rate determining step in MDR. Therefore, we normalized the rate of H2 production by the accessible Ni sites to be able to compare the catalysts, taking into account the different Ni particle sizes. Nevertheless, the TOF values have to be considered with care.
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1E-11 .
y 620 °C
300 °C J 490 °c
Ni- ' / \ /
Ce,Zr02_cpj/ L/ \ Ni-Ce02
____ Ni-Ce1; ZrO, ctab x y 2—
400 500 600 temperature (°C)
800 900
Fig. 5. Temperature Programmed Oxidation with 20% O2 in Ar, performed after 24 h methane dry reforming at 600 °C.
3.5. Post-reaction characterization
Temperature programmed oxidation (TPO) was performed to characterize coke formation after 24 h reaction in CH4 and CO2.The amount of carbon dioxide formed during TPO quantifies the amount of coke formed during the reaction and the temperature needed to burn off the carbon is characteristic for the carbon bond strength to the catalyst's surface. Fig. 5 shows the production of carbon dioxide during TPO in 20% O2 in Ar after MDR. Over Ni-ZrO2 clearly the highest amount of coke was formed and temperatures of 600 °C are needed to oxidize most of it. We comparedTPO measurements after 3 h and after 24 h of reaction time. Interestingly, we observed that most of the coking occurs during the first three hours over Ni-ZrO2 and Ni-Ce1-xZrxO2_cp. Longer reaction times did not significantly increase the amount of carbonaceous species deposited on these samples (Fig. S4 in the Supporting information). The small maximum of CO2 evolution at 300 °C over the Ni-CeO2 catalyst can be assigned to the oxidation of amorphous carbon [87,88] and around 500°C graphitic carbon is oxidized [16]. The CO2 evolution around 600 °C can be assigned to the oxidation of whisker type carbon which does not deactivate the nickel surface but rather causes a breakdown of the catalyst by pore plugging [10].
In addition to carbon oxidation, carbonates can also contribute to the CO2 TPO signal at temperatures > 600 °C (i.e. the temperature at which we purged the samples in Ar after reaction). This would lead to an overestimation of the amount of carbon. Therefore, we rather focus on the relative difference in the amount of carbon-containing species over the various samples. Nevertheless, most of the CO2 produced during TPO is attributed to carbon oxidation, because O2 consumption (also detected by mass spec) occurred at the same temperature as CO2 production. Furthermore, reference experiments using Ar instead of O2/Ar indicated the absence of CO2 evolution in the absence of O2, which excludes a significant TPO contribution by carbonate decomposition (Fig. S5 in Supporting information).
The amount of CO2 formed over Ni-ZrO2 is about 20 times more than on Ni-CeO2 and about 200 times more than on Ni-Ce1-xZrxO2_cp (note the logarithmic scale). When the overall amount of carbon converted within 24 h reaction time is considered (i.e. 67.1 mmolC converted over Ni-ZrO2, 33.4mmolC over Ni-CeO2, and 34.2 mmol C over Ni-Ce1-xZrxO2_cp) somewhat lower but nevertheless considerable differences in the amount of carbonaceous species deposited on the catalysts were observed: the amount of CO2 formed in TPO over Ni-ZrO2 is about 10 times
higher than over Ni-CeO2 and about 100 times more than on Ni-Ce1-xZrxO2_cp. This is clearly a huge advantage of the Ni-Ce1-xZrxO2_cp catalyst over Ni-ZrO2 (even though the large amount of carbonaceous species formed during 24 h reaction hardly affects the Ni-ZrO2 activity), since the major problem of carbon deposition in a continuous flow reactor is the possibility of physical blocking of the tube reactor. Such "clogging" was indeed observed when the amount of catalyst used for catalytic tests was increased. Thus, using a mixed oxide support for the Ni particles is beneficial for the MDR performance. As expected, on Ni-Ce1-xZrxO2_ctab hardly any CO2 is formed during TPO, not surprising in light of its almost inactivity.
Fig. 6 shows TEM images ofNi-ZrO2 (a) andNi-Ce1-xZrxO2_cp (b) after dry reforming at 600 °C for 3 h. On both catalysts filamentous carbon has formed and some Ni particles are on top of these carbon nanofibers, tubes or rods. On Ni-CeO2 and Ni-Ce1-xZrxO2_ctab no filamentous carbon was detected.
Fig. 7 shows XRD patterns of the samples after reaction at 600 °C, for the range of 20-55° 20. In contrast to the XRD patterns shown in Fig. 1, the reflections of NiO have disappeared or - in case ofNi-CeO2 decreased in intensity, and the reflection of Ni(111) at 44.3 20 is observed on Ni-CeO2 and the mixed oxide supported samples. On Ni-ZrO2 this reflection is overlapping with one of monoclinic ZrO2. The crystallite sizes calculated with Scherrer's equation are 27 nm, 23 nm and 21 nm on Ni-CeO2, Ni-Ce1-xZrxO2_cp and Ni-Ce1-xZrxO2_ctab, respectively, and thus not much larger than the NiO crystallites of the catalysts after ex-situ reduction. The reflection at 26.1 20 in the diffractogram of the used Ni-ZrO2 catalyst is assigned to graphite. On the other samples much less carbon was observed by TPO and it may be rather amorphous. The XRD patterns of the used Ni-Ce1-xZrxO2 catalysts indicate no change of the lattice parameters of the cubic phases and thus of the Ce/Zr ratios of the catalysts.
Fig. 8 shows graphical representations of the Ni K-edge normalized |(E) in the XANES region a) and the first derivative of the |(E) data in the XANES region b).The latter reveal several local maxima with two larger ones around 8333 eV and/or 8343 eV. The position of the largest maximum value with the first derivative of the |(E) data was chosen as the value of E0. With the Ni fcc reference foil the first maximum at 8333 eV is the highest one. With NiO reference and the reduced samples the highest maximum is apparently at 8343 eV and with the used samples it is also at 8343 eV.
The XANES spectra of the catalysts reduced ex situ in a H2/Ar mixture at 600°C for 30min resemble that of the NiO reference. Linear combination analysis of the spectra shown in Fig. 8 confirms that the Ni on all catalysts after ex situ H2 reduction is in the oxidized state. This proves that these catalysts are fully re-oxidized after exposure to air.
Linear combination analysis of the used catalysts, which were also exposed to air in between reaction and XAS measurement, revealed an amount of metallic nickel of 86%, 84%, 78% and 70% inNi-ZrO2, Ni-CeO2, Ni-Ce1-xZrxO2_cp and Ni-Ce1-xZrxO2_ctab, respectively. In contrast to the reduced samples most of the Ni has remained metallic and did not get re-oxidized upon air exposure. Since the Ni particles analysed by TEM and XRD were not significantly larger after reaction than before, sintering can be excluded. Possible explanations for the XANES spectra can be either that the carbon (which has formed during reaction and was deposited on the catalyst) stabilizes the metallic state of nickel (e.g. by decorating sites at which O2 dissociation and oxidation would initiate). Alternatively, a nickel carbide phase may have formed. The XANES spectrum of Ni3C would resemble the XANES of metallic Ni rather than NiO but shows significant differences in the Fourier transforms of the x(k). Ni3C was obtained and detected by XAS during Ni car-burization [89,90] and during low temperature steam reforming of ethanol over a ceria supported Ni catalyst [91]. Besides carbon,
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Fig. 6. TEM images of a) Ni-ZrO2 and b) Ni-Ce1-xZrxO2_cp after exposure to methane and carbon dioxide at 600 °C for3 h.
Fig. 7. XRD patterns ofthe Ni-ZrO2, Ni-CeO2, Ni-Ce1-xZrxO2.cp and Ni-Ce1-xZrxO2_ctab after MDR for3 h at 600°C.
Table 4
Structural parameters of the first Ni-Ni coordination shell resulting from multiple shell fits in R-space to EXAFS data of Ni foil and post-reaction Ni supported catalysts.
sample
ct2 (A2)
R factor
Ni foil 2.46 ±0.00 12a 0.006 0.087
Ni-ZrO2 2.52 ±0.05 9.6 0.005 0.073
Ni-CeO2 2.51 ±0.05 11.1 0.006 0.084
Ni-Cei-xZr„O2jcp 2.55 ± 0.05 11.1 0.007 0.072
Ni-Ce1-xZrxO2_ctab 2.53 ±0.05 10.9 0.006 0.065
a Constrained for Ni foil.
nickel carbide formation is favoured in a dry mixture of methane and carbon dioxide [1] and Ni3C formation was indeed observed by Swaan et. al [92]. However, Goula et. al [93] discarded the hypothesis of bulk nickel carbide formation during methane dry reforming.
The k2-weighted k and r space EXAFS with their respective fits are shown in Fig. 9 and the best fitting structural parameters are presented in Table 4. According to these fittings, all samples are well fitted by using fcc Ni. The formation of Ni3 C seems not to take
place, since the carbon signal from the first Ni-C coordination shell at the phase-uncorrected distance ofR ~1.2Ais missing.
4. Discussion
We have characterized nickel-based catalysts, including ceria-zirconia mixed oxide supports prepared by different synthesis methods. The catalysts were evaluated for their catalytic performance in methane dry reforming. Large differences in the catalytic performance of the two mixed oxide supported Ni catalysts of the same nominal elemental composition were observed. The catalyst prepared by surfactant assisted co-precipitation showed promising textural properties such as high surface area which is often connected with a good metal dispersion. Nevertheless, after reduction at 600 °C hardly any H2 adsorbed, as shown by hydrogen chemisorption measurements, indicating (partly) inaccessible (encapsulated) nickel on this catalyst. Since the surfactant CTAB was used for the synthesis of the mixed oxide support material, residual bromide was considered to potentially affect the catalytic activity. Residual chloride from Cl-containing Pt precursors was
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Fig. 8. a) Normalized XANES spectra of reference samples and reduced and used samples and b) first derivative of the spectra shown in a). The dashed lines in b) mark the positions of two local maxima at 8333 eV and 8343 eV. The asterisk (*) labels the absolute maximum.
Fig. 9. k2-weighted EXAFS functions x(k) and magnitude and imaginary part of Fourier transforms (uncorrected for phase shifts) of x(k) of Ni foil and the supported nickel catalysts after methane dry reforming reaction at 600 °C. Solid lines: experimental data, hollowed spheres: fits in R-space and back transformed into k-space.
reported to have an enormous effect on the selectivity of the catalyst [94], thus a similar effect of bromide is possible. Even though no bromide was detected by elemental analysis, an additional catalyst was prepared by impregnating the co-precipitation support with ammonium bromide (Merck) before adding Ni. The amount of Br was the same as used for the preparation of Ni-Ce1-xZrxO2_ctab assuming the maximum theoretical loading. The methane conversion over this brominated catalyst was about the
same as over Ni-Ce1-xZrxO2_cp, i.e. 56.7% initially (57.4% over Ni-Ce1-xZrxO2_cp) and 46.5% after 3 h time on stream (47.7% over Ni-Ce1-xZrxO2_cp). Therefore, a negative effect due to the utilization of the bromide containing surfactant can be excluded. Thus, a reasonable explanation for the lack of catalytic activity or the extremely fast deactivation of Ni-Ce1-xZrxO2_ctab is mostly inaccessible Ni particles, based on the extremely low Ni surface area observed by H2 chemisorption and the TEM observation that Ni par-
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ticles were always surrounded by the mixed oxide support which has a structure different from Ce1-xZrxO2_cp. Considering the active Ni-Ce1-xZrxO2_cp catalyst, no advantage compared to Ni-ZrO2 in terms of conversion or H2 selectivity was observed but the formation of filamentous carbon was strongly suppressed (100 times). As reported in literature, the ceria-zirconia support material leads to acceleration of gasification of coke deposits on metal particles which is related to the increase of the oxygen accessibility/mobility on the surface [95] and the higher surface density of active sites on the ternary catalyst [41].
Recently, Makri et al. [64] reported important insights into the carbon chemistry of 5wt% Ni/Ce0.8Zr0.2O2-8 and 5wt% Ni/Ce0.5Zr0 5O2-8 MDR catalysts. Using temperature-programmed methods and transient isotopic experiments they demonstrated that the relative amount of the carbon species (formed either via the CH4 or the CO2 activation routes) as well as the reactivity ofthe various carbons towards H2 and O2 were strongly influenced by the support chemical composition (Ce/Zr ratio). In addition, the support composition affected the nickel particle size, which in turn influenced the origin and the reactivity of carbon deposited under MDR conditions. Particularly, the metal-oxide interface was suggested to play an important role for reacting off carbon. While CH4 activation is found to occur only on Ni, CO2 is activated also at oxygen vacancies at and/or near the Ni-oxide interface. Thus, the amount of vacancies near the interface and an improved rate of oxygen transfer towards the interface strongly affect the catalytic properties and carbon accumulation during MDR [64]. In view of the work of Makri et al., the huge differences in MDR catalytic activity, which we observed for Ni supported on the two differently prepared ceria-zirconia materials, may also be explained by an effect of the different oxide compositions on the abundance of oxygen vacancies and on the participation of labile surface/subsurface lattice oxygen around the Ni particles, which may be key for the removal of carbon near or at the metal-support interface.
5. Conclusions
In this study we have prepared two different ceria-zirconia mixed oxide support materials which were impregnated with nickel and compared with zirconia and ceria supported nickel catalysts in terms of textural and adsorption properties, catalytic behaviour during methane dry reforming and post-reaction properties. Both ceria-zirconia supported catalysts showed promising high surface area. Ni was accessible by CO adsorption on all catalysts after reduction. Nevertheless, the catalyst prepared by surfactant assisted co-precipitation (Ni-Cei-xZrxO2_ctab) was not active for methane dry reforming while the activity of the one prepared by co-precipitation without surfactant (Ni-Cei-xZrxO2_cp) was comparable to Ni-ZrO2, which was the most active and most stable of the tested catalysts.
The inactivity of the Ni-Cei-xZrxO2_ctab catalyst is most likely explained by the inaccessibility of metallic Ni for hydrogen adsorption after reduction at 600 °C and by the TEM observation of Ni particles being covered by ceria-zirconia particles. Thus, as generally accepted, catalyst activity depends strongly on the preparation. Alternatively, the differing compositions of the ceria-zirconia phases obtained by the respective preparation routes may affect the presence of oxygen vacancies and of labile surface/subsurface lattice oxygen around the Ni particles which may be essential for the removal of carbon near/at the metal-support interface.
Considering the active Ni-Ce1-xZrxO2_cp catalyst no improvement of catalytic activity was observed when compared to Ni-ZrO2, but the stability against filamentous coke formation was significantly increased. Consequently, the risk of reactor tube blockage
in a flow system can be decreased which justifies using mixed ceria-zirconia as a support material.
Acknowledgements
This work was supported by the Austrian Science Fund (FWF) via SFB FOXSI (F4502-N16) and project DryRef (I 942-N17). We thank Dr. Klaudia Hradil and DI Werner Artner (X-ray center at TUW) for assistance with the XRD measurements and data evaluation. We are grateful to the Synchrotron Light Research Institute (SLRI), Thailand for providing beamtime at Beamline 8 forX-ray absorption analysis. A.W. acknowledges the ASEA-UNINET for a travelling grant.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/jxattod.2016.04. 025.
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