Scholarly article on topic 'The potential use of HNO3-treated clinoptilolite in the preparation of Pt/CeO2-Clinoptilolite nanostructured catalyst used in toluene abatement from waste gas stream at low temperature'

The potential use of HNO3-treated clinoptilolite in the preparation of Pt/CeO2-Clinoptilolite nanostructured catalyst used in toluene abatement from waste gas stream at low temperature Academic research paper on "Chemical sciences"

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{Clinoptilolite / Pt/CeO2-Clinoptilolite / Toluene / "Total oxidation" / "Air treatment"}

Abstract of research paper on Chemical sciences, author of scientific article — Majid Amereh, Mohammad Haghighi, Pooya Estifaee

Abstract In this paper, CeO2(30%)/Clinoptilolite was synthesized via HNO3 treatment and co-precipitation methods and then 1%Pt was dispersed over support by ultrasound assisted wet impregnation. The synthesized samples were characterized using XRF, XRD, FESEM, N2 adsorption and FTIR techniques. The obtained results from XRD revealed the main phase in utilized clay is clinoptilolite and also formation of amorphous structure was proved after acid treatment. FESEM micrographs confirmed significant structural changes in clinoptilolite after acid treatment and also formation of ceria nanoparticles. N2 adsorption presented large enough surface area for Pt/CeO2-Clinoptilolite nanostructured catalyst to be used for toluene oxidation. Finally, activity results indicated that synthesized nanostructured catalyst was highly active and stable and could eliminate more than 90% of toluene at 130°C. Utilizing inexpensive materials in synthesis and assisting ultrasound in dispersion of active phase have made Pt(1%)/CeO2(30%)-Clinoptilolite a nominee to be used for catalytic oxidation of toluene.

Academic research paper on topic "The potential use of HNO3-treated clinoptilolite in the preparation of Pt/CeO2-Clinoptilolite nanostructured catalyst used in toluene abatement from waste gas stream at low temperature"

Arabian Journal of Chemistry (2015) xxx, xxx-xxx

King Saud University Arabian Journal of Chemistry

www.ksu.edu.sa www.sciencedirect.com

ORIGINAL ARTICLE

The potential use of HNO3-treated clinoptilolite in the preparation of Pt/CeO2-Clinoptilolite nanostructured catalyst used in toluene abatement from waste gas stream at low temperature

Majid Amereh, Mohammad Haghighi *, Pooya Estifaee

Chemical Engineering Faculty, Sahand University of Technology, P.O. Box 51335-1996, Sahand New Town, Tabriz, Iran Reactor and Catalysis Research Center (RCRC), Sahand University of Technology, P.O. Box 51335-1996, Sahand New Town, Tabriz, Iran

Received 12 July 2013; accepted 6 February 2015

KEYWORDS

Clinoptilolite;

Pt/CeO2-Clinoptilolite;

Toluene;

Total oxidation;

Air treatment

Abstract In this paper, Ce02(30%)/Clinoptilolite was synthesized via HNO3 treatment and co-precipitation methods and then 1% Pt was dispersed over support by ultrasound assisted wet impregnation. The synthesized samples were characterized using XRF, XRD, FESEM, N2 adsorption and FTIR techniques. The obtained results from XRD revealed the main phase in utilized clay is clinoptilolite and also formation of amorphous structure was proved after acid treatment. FESEM micrographs confirmed significant structural changes in clinoptilolite after acid treatment and also formation of ceria nanoparticles. N2 adsorption presented large enough surface area for Pt/Ce02-Clinoptilolite nanostructured catalyst to be used for toluene oxidation. Finally, activity results indicated that synthesized nanostructured catalyst was highly active and stable and could eliminate more than 90% of toluene at 130 °C. Utilizing inexpensive materials in synthesis and assisting ultrasound in dispersion of active phase have made Pt(1%)/Ce02(30%)-Clinoptilolite a nominee to be used for catalytic oxidation of toluene.

© 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of King Saud University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Corresponding author at: Reactor and Catalysis Research Center (RCRC), Sahand University of Technology, P.O. Box 51335-1996, Sahand New Town, Tabriz, Iran. Tel.: +98 41 33458096, +98 41 33459152; fax: +98 41 33444355. E-mail address: haghighi@sut.ac.ir (M. Haghighi). URL: http://rcrc.sut.ac.ir (M. Haghighi). Peer review under responsibility of King Saud University.

Production and hosting by Elsevier

1. Introduction

During past decade, volatile organic compounds (VOCs) have received increasing attention due to their high toxic potential and their easiness of spreading over large areas through the atmosphere (Alifanti et al., 2007). BTX (Benzene, Toluene, Xylene), as major VOCs, are main group of air pollutants especially in middle east, where is contaminated by refined petrochemical products (BankoviC et al., 2009; Mester and Kosson, 1996; Zou et al., 2006). An efficient and economical

http://dx.doi.org/10.1016/j.arabjc.2015.02.003

1878-5352 © 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of King Saud University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

way for VOC abatement is their complete catalytic oxidation to harmless products as well as CO2 and H2O (Li et al., 2008; Luo et al., 2007; Spivey, 1987). In addition, in comparison with thermal methods, catalytic oxidation needs lower temperature leading to lower NOX emission (Gluhoi and Nieuwenhuys, 2007). During recent years, catalytic oxidation of VOCs has been widely studied (Huang et al., 2008; Kovanda and Jiratova, 2011; Li et al., 2013a; Nogueira et al., 2011; Tabakova et al., 2013). According to the literature, supported noble metal catalysts have been known as highly active catalysts for VOC removal (Oliveira et al., 2008; Wang et al., 2011), CO oxidation (Estifaee et al., 2014; Tomita et al., 2012) and methanol oxidation (Arnby et al., 2004; Malakhova et al., 2000). They have been preferentially used in commercial practices in spite of their cost (Masui et al., 2010; Venezia et al., 2001). Among different noble metals, Pd and Pt have been widely studied for this purpose (Ozcelik et al., 2009; Rahmani et al., 2014a). Pd is more active for oxidation of short-chain hydrocarbons, while Pt exhibits higher activity toward long-chain hydrocarbons and aromatics (Abbasi et al., 2011). The catalytic performance of these supported catalysts is mostly dependent on the catalyst preparation method and used promoters, determining the dispersion, activity, and stability of the catalyst. Regarding the fact that catalytic process is a surface phenomenon, a sufficiently high dispersion of metal active species along with high stability is normally required. In this regard, some innovative treatments such as plasma (Liu et al., 1999; Rahemi et al., 2013a, 2013b) and ultrasound methods (George et al., 2008; Kumar et al., 2010) and various promoter elements such as ceria have been employed. Unique properties, for instance providing lattice oxygen and excellent thermal and mechanical resistance and finally, preventing the sintering of noble metals have made CeO2-based catalysts essential in numerous environmental catalytic processes (Drenchev et al., 2013; Nousir et al., in press (in press, available online 30 July 2011); Tang et al., 2013; Yu et al., 2010). The reduction of Ce4 + to Ce3 + feature helps the dispersion and oxidation/ reduction of noble metals along with the decrease in coke formation on the catalyst surface. As particle size decreases, the concentration of Ce3 + relative to Ce4 + increases, hence more oxygen vacancies and more active sites are produced (Jamalzadeh et al., 2013).

Over the few past decades, the use of clay minerals has expanded. Natural zeolites (i.e. clinoptilolite) have indicated good potential for waste water treatment (Herney-Ramirez et al., 2010), gas separation (Rahmani et al., 2014b), gas drying (Jenkins et al., 2002; Li et al., 2013b) and other catalytic reactions (Baroi and Dalai, 2014; Li et al., 2014; Otomo et al., 2014). Because of high density of active acid sites, high thermal and mechanical stability and porous structure, zeolite based catalysts are desired for VOC abatement (Dammak et al., 2013; Jansson et al., in press (in press, available online 24 October 2014); Lopez-Fonseca et al., 2003; Rahmani et al., 2014a). The other advantage of using clay as catalyst support is clay's composition. Clay consists of octahedral aluminum and tetrahedral silicon dioxide and the ratio of Si/Al is relatively steady, which makes them to perform as molecular sieves (Gao and Xu, 2006).

In our previous works, the effect of different loadings of CeO2 on CeO2/Clinoptilolite for p-xylene elimination was investigated and the results showed the highest conversion

for 30% CeO2 content (Asgari et al., 2013a). In another paper different Pd/CeO2-Clinoptilolite catalysts with various Pd loadings were synthesized and the results depicted that Pd(1%)/CeO2(30%)-Clinoptilolite is a potential nominee for VOC oxidation (Asgari et al., 2013b). Furthermore, Abbasi et al. (2011) investigated the effect of different loadings of ceria over alumina in Pt(1%)/Al2O3-CeO2 catalyst with nanocharac-teristics for catalytic oxidation of BTX. The obtained results showed Pt(1%)/Al2O3-CeO2(30%) is the most appropriate catalyst, with more than 90% conversion at 230 0C. According to our previous researches, in a continuing effort to find a highly active and economic catalyst for VOC abatement, the objective of this paper is to investigate an effective nanostructured catalyst with 1% Pt as active phase, 30% cerium oxide as a co-support and promoter and HNO3-treated clinoptilolite as an inexpensive support to be utilized for total catalytic oxidation of toluene. The structure, morphology, surface area and other physicochemical properties of synthesized samples were studied by XRF, XRD, FESEM, BET and FTIR techniques. The activity test was carried out and parameters such as effect of temperature, toluene concentration and GHSV were investigated on toluene removal. Finally, time on stream behavior test was carried out for the synthesized nanostructured catalyst.

2. Materials and methods

2.1. Materials

The natural zeolite used in this work was clinoptilolite from Mianeh mine (East Azerbaijan, Iran) and modified by HNO3 (65%). Cerium oxide was synthesized over clinoptilolite via co-precipitation method using Ce(NO3)2-6H2O and NH3 aqueous solution (25%). The noble metal (Pt) utilized in this work as active phase was obtained from H2Cl6Pt-6H2O. In addition, toluene was used as VOC pollutant. The materials were from Merck (Darmstadt, Germany) and were not purified further. It should be noted that the following abbreviations are used for samples throughout the article: non-treated clinoptilolite: Cln; HNO3-treated clinoptilolite: Cln-T; CeO2/treated clinoptilolite: Ce-Cln-T and Pt/CeO2-treated Clinoptilolite: Pt-Ce-Cln-T.

2.2. Nanocatalyst preparation and procedures

Fig. 1 shows schematic flowchart of nanostructured catalyst preparation steps and procedures. As shown on the figure, there are four main steps for nanocatalyst preparation. In part (a) Cln and HNO3 solution (mL solution g_1 zeolite = 20) with 8 mol L_1 concentration were introduced to a flask and boiled at 80 0C for 8 h with stirring. The suspensions were separated in centrifugal system and filtered. Next, the samples were dried for 24 h at 110 0C and then calcined at 500 0C for 3 h under air stream.

In second step (b), by dissolving Ce(NO3)2-6H2O into water, 0.5 M solution was made. Next, CeO2 was loaded over clinoptilolite using precipitation method (c). In this step, Ce(NO3)2-6H2O and NH3 aqueous solutions were added to clinoptilolite aqueous suspension in 30 min until pH = 9 to get white/yellowish precipitate of ceria over clinoptilolite. Next, the solution was mixed for 1 h at 50 0C and dried at

(a) Clinoptilolite treatment using HNO3

(b) Preparation of ceria solution

(d) Addition of Pt to support using ultrasound energy

Figure 1 Schematic flow chart for the preparation steps of Pt/CeO2-Clinoptilolite nanocatalyst via HNO3-treatment, CeO2 precipitation and ultrasound assisted impregnation of Pt over support.

110 0C for 12 h under air flow. After drying, the samples were calcined at 500 0C for 5 h under air flow.

In final step, platinum was dispersed over CeO2/ Clinoptilolite by ultrasound. Fig. 2 reveals schematic diagram of utilized experimental setup in ultrasound assisted impregnation of Pt over CeO2-Clinoptilolite. Because of high cost of noble metals, low loadings of them are used in nanocatalyst structure. In this regard, utilizing novel technologies such as ultrasound is an efficient way to modify active phase dispersion. In final step (d), H2Cl6Pt-6H2O and Ce-Cln-T were mixed in distilled water and ultrasonic source irradiated for 45. The ultrasonic treatment has been performed using a sonicator with 20 kHz frequency, 90 W effective input power and a tip diameter of 13 mm. Finally, calcined samples were shaped cylindrically to be used as nanostructured catalysts at toluene abatement process.

2.3. Nanocatalyst characterization Techniques

Chemical analysis was carried out on Philips XRF Magixpro to investigate the components of Cln and Cln-T. X-ray diffraction (XRD) was employed to identify the compounds and verify the crystalline structure of nanostructured catalysts. It was carried out on a D-5000, Siemens diffractometer

employing Cu Ka radiation coupled to an X-ray tube operated at 30 kV, 40 mA and the scanning range of 2h = 5-500. The phase identification was made by comparison with Joint Committee on Powder Diffraction Standards (JCPDSs). Microstructure, particle size and morphology were investigated by field emission scanning electron microscopy (VEGA\\tESCAN, BSE DETECTOR). The specific surface area (BET) of nanocomposites was characterized by N2 adsorption and desorption isotherms obtained at —196 0C using a Quantachrome ChemBET-3000. To investigate diagnosing surface functional groups, Fourier Transform Infrared Spectroscopy (FTIR, UNICAM 4600) was conducted in the range of 400-4000 cm—1 wave numbers.

2.4. Experimental setup for catalytic performance test

Experimental setup for testing activity of nanostructured Pt-Ce-Cln-T catalyst total toward catalytic oxidation of toluene is the same as the setup used previously by our research group (Abbasi et al., 2011; Rahmani et al., 2014a). In our experiments, feedstock consisted of fresh air stream saturated bubbling through a saturator which was filled with liquid toluene and was kept at —4 0C. To this stage, feedstock was ready and the concentrations were always confirmed by GC

........

........

Notations:

I. Probe system display Ultrasonic probe pH probe Thermocouple Septum sampling Teflon cover Water jacket Stirrer

Cooling water inlet 10. Cooling water outlet

II. Stand

Figure 2 Schematic flow chart of ultrasound experimental setup used in ultrasound assisted impregnation of Pt over CeO2-Clinoptilolite.

before each experiment. The reactor was tubular plug flow with 6 mm inside diameter and made of Pyrex glass and operated in continuous mode at atmospheric pressure. The reactor was placed in an electrical furnace which provides the required temperature (70-300 0C) for catalytic reaction. Next, 0.5 g of sample was placed in reactor for each run. In addition, all flows were controlled by MFCs (Sevenstar-D07). Prior to catalytic tests, the nanostructured catalyst was pre-treated under reactant mixture flows at 70 0C to prevent overestimation of toluene conversion caused by adsorption of toluene in the initial stages of the test. The feed and the reaction products were monitored by on-line gas chromatograph (GC Chrom, Teif Gostar Faraz, Iran) equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). The products were analyzed with HP-PLOT U and HP-PLOT Molsieve capillary columns (Agilent Co.). The primary products detected by GC were CO2 and H2O only. It should be considered that the Gas Chromatography system used in current work was equipped with methanator followed by a flame ionization detector. Finally, toluene conversion calculated as:

N — Nf Converswn(%) = N^rNf x ^0

where Nin and Nf are the initial and final moles of toluene, respectively.

3. Results and discussions

3.1. Nanocatalyst characterizations 3.1.1. Clinoptilolite characterizations

Fig. 3 shows XRD pattern of Iranian clinoptilolite in comparison with reference pattern. XRD peaks revealed the main phase in applied zeolite is clinoptilolite and is in good agreement with the ones given by the 025-1349 JCPDS card.

In addition to zeolitic phase, according to peak positions and JCPDS cards, other phases such as feldspar and biotite were detected (Ozcelik et al., 2009). Fig. 4 depicts chemical analysis of raw and HNO3 treated clinoptilolite. The analysis, indicated the presence of silica (SiO2), alumina (Al2O3), hematite (Fe2O3) and lime (CaO) as main minerals. A comparative analysis between raw and treated clinoptilolite shows the SiO2/Al2O3 ratio has increased from 4.62 to 12.39. Such an observation has been reported in other papers which contained acid treatment (Cobzaru et al., 2008; Davila-Jimenez et al., 2008). In addition, decreasing SiO2/Al2O3 mass ratio leads to smaller loss of ignition (L.O.I.) (Mohsen and El-maghraby, 2010). As can be seen in Fig. 4, L.O.I. decreases significantly from 8.7 to 1.3 after HNO3 acid treatment. Iron oxide is the main colorant in the clays, which is responsible for reddish color after burning (Mohsen and El-maghraby, 2010).

18 21 24 28 (degree)

Figure 3 XRD patterns of Cln and reference pattern.

g 40-<

□ Cln (raw clinoptilolite)

□ Cln-T (HNO3-treated clinoptilolite)

SiO2 Al2O3 Fe2O3 TiO2 CaO MgO Na2O K2O L.O.I Components

Figure 4 Chemical analysis of Cln and Cln-T.

3.1.2. XRD analysis

XRD patterns of Cln (a), Cln-T (b), Ce-Cln-T (c) and (d) Pt-Ce-Cln-T are shown in Fig. 5. As mentioned above, spectra (a) confirmed the presence of clinoptilolite in monoclinic phase. Comparison between (a) and the other samples shows no significant changes in peak positions. A comparative analysis between (a) and (b) reveals less intense peak for treated sample. This observation can be explained by the fact that some amorphous structure is formed during acid extraction (Jamalzadeh et al., 2013). A slight view over pattern (c) reveals that the height of most of the peaks has decreased considerably. In addition, there are no sharp peaks for CeO2 but it seems that there are three small peaks at 2h = 28.8, 33.2 and 42.0 that confirm formation of CeO2 (01-075-0076) in cubic phase (Estifaee et al., 2014). As shown in pattern (d) due to the low loading or high metal dispersion no evidence of Pt peak is obtained. Furthermore, according to XRD no impurities were detected (See Table 1).

Figure 5 XRD patterns of (a) Cln, (b) Cln-T, (c) Ce-Cln-T and (d) Pt-Ce-Cln-T.

as well as nanosized particles will be a desirable choice for VOC abatement.

3.1.3. FESEM analysis

FESEM micrographs of (a) Cln, (b) Cln-T, (c) Ce-Cln-T and (d) Pt-Ce-Cln-T are shown in Fig. 6. A general view over image (a) shows un-uniform particles in irregular shapes. Comparing micrographs (a) and (b) reveals that acid treatment resulted in some morphological changes in clinoptilolite structure. This observation is in consistent with the literature (Asgari et al., 2013a,b; Jamalzadeh et al., 2013; Khajeh Talkhoncheh and Haghighi, 2015). In image (c), a dense structure composed of nanosized particles was observed. The particle size of cerium oxide and/or platinum can be found among clinoptilolite support with particle size of <100 nm. Nanoparticles provide more reactive and reducible sites and results to a high catalytic performance of the nanocatalyst (Abbasi et al., 2011).

Generally, deep oxidation of most hydrocarbons over Pt and palladium catalysts highly depends on metal particle size (Jossens and Petersen, 1982; Labalme et al., 1996; Marécot et al., 1994; Papaefthimiou et al., 1998; Pliangos et al., 1997). Song et al. investigated the significant effect of Pt particle size on turnover rate and activation energy on some reactions (Somorjai and Park, 2008). In a distinct paper, Radic et al. revealed that both kinetic form and turnover frequency are affected by Pt crystallite size (Radic et al., 2004). It should be mentioned that selecting a catalyst with good morphology

3.1.4. BET analysis

Table 1 represents BET surface areas of Cln, Cln-T, Ce-Cln-T and Pt-Ce-Cln-T. Surface area is a key parameter in nanos-tructured catalysts and has a considerable effect on catalytic performance. The larger the surface area, the more the reaction sites available and hence the higher conversion is obtained (Asgari et al., 2013a). Results show BET area for Cln has increased significantly from 13 to 61 m2/gr after acid treatment. According to the literature, this phenomenon can be addressed to acid leaching which caused pore openings during modifications of the zeolite. Acid washing of natural zeolites may remove impurities that block the pores, and increase both porosity and adsorption capacity leading to increased catalytic activity of the support (Jamalzadeh et al., 2013) such a trend as a result of acid treatment was observed in other papers (Asgari et al., 2013a,b). Adding 30% ceria content showed 68% increase in surface area. Furthermore, as 1% Pt content is added to Ce-Cln-T, surface area decreases about 9%. This observation seems to be due to high dispersion of Pt and covering the support surface. A similar trend was observed in our previous investigations on zeolite based catalysts (Asgari et al., 2013a,b; Jamalzadeh et al., 2013; Rahmani et al., 2014b). In contrast, utilizing ultrasonic irradiation on depositing Pt on support is an important factor that effects

Table 1 BET surface area analysis of Cln, Cln-T, Ce-Cln-T and Pt-Ce-Cln-T.

Catalyst Nomenclature Sbet (m2/g)

Raw clinoptilolite Cln 13

HNO3 treated clinoptilolite Cln-T 61

CeO2/HNO3 treated clinoptilolite Ce-Cln-T 103

Pt/CeO2-HNO3 treated clinoptilolite Pt-Ce-Cln-T 94

Figure 6 FESEM images of (a) Cln, (b) Cln-T, (c) Ce-Cln-T and (d) Pt-Ce-Cln-T.

dispersion of active phase. Although, surface area of Ce-Cln-T has decreased after active phase addition but Pt-Ce-Cln-T has still high surface area to be used at catalytic oxidation of toluene.

3.1.5. FTIR analysis

Fig. 7 presents FTIR spectra of (a) Cln, (b) Cln-T, (c) Ce-Cln-T and (d) Pt-Ce-Cln-T. Generally, there are two groups of vibration frequencies in all samples. The first one is vibration of external linkages between tetrahedral in the zeolites due to topology and the mode of structure arrangement and the second one is internal vibrations. The broad absorption peaks at 1405, 1640 and 3450 cm-1 are assigned to the bending, combination, and symmetric/asymmetric stretching modes of adsorbed water (Sa and Anderson, 2008). A comparison between treated and non-treated sample shows a small displacement at 1060 cm. This observation can reveal elution of a portion of Al3+ by HNO3 acid treatment (Christidis et al., 2003; Mozgawa et al., 2002).

The recorded FTIR spectra for (c) and (d) show a significant change near 700 cm-1, which is attributed to Ce-O bonding and stretching around 500 cm-1. In addition, the other peak below 700 cm-1 is due to the envelope of the phonon bond of the metal oxide network (Mozgawa et al., 2002). These results support and complement the XRD data. In addition, in comparison with support patterns, the incorporation of the structures of zeolite and ceria declined the intensity of Fourier IR peaks in the catalyst pattern to decline (Jamalzadeh et al., 2013).

3.2. Catalytic performance study toward toluene abatement from waste gas stream

3.2.1. Effect of temperatures

The most widespread way of catalytic activity evaluation in deep oxidation studies is tracing ignition (light-off) curves. In these curves, percentage of eliminated toluene is traced versus temperature (Rahmani et al., 2014a). Fig. 8 depicts effect of temperature on total oxidation of toluene over Cln-T, Ce-Cln-T and Pt-Ce-Cln-T. As expected, the synthesized sample with Pt as active phase, cerium oxide as a co-support and promoter and HNO3-treated clinoptilolite has the best low-temperature activity. In other words, to obtain the same toluene abatement, Pt-Ce-Cln-T required about 120 0C lower reaction temperature in comparison with C-Cln-T. In addition, comparing activity results of Pt-Ce-Cln-T with similar nanocatalyst (Pt(1%)/Ce02(30%)-Al203) which was synthesized via impregnation method shows Pt-Ce-Cln-T needs ca. 70 0C lower reaction temperature to reach 90% of total oxidation (Abbasi et al., 2011). This observation reveals the positive effect of utilizing clinoptilolite as an effective and inexpensive support and also using ultrasound to dope Pt over support. Properties such as microporous structure, high thermal stability, high surface area and high stability in large range of pH have made clinoptilolite an effective support for catalysts. On the other hand, alumina's high potential to transfer phase from c to a has limited its application and in many cases promoters such as Zr02 and MgO are used to overcome the problem (Alipour et al., 2014; Khajeh Talkhoncheh and Haghighi, 2015; Luengnaruemitchai and Kaengsilalai, 2008).

Figure 7 FTIR spectra of (a) Cln, (b) Cln-T, (c) Ce-Cln-T and (d) Pt-Ce-Cln-T (Pt/CeO2-HNO3 treated clinoptilolite).

3.2.2. Effect ofGHSV

The influence of contact time on toluene abatement for Pt-Ce-Cln-T was investigated by varying the gas hourly space velocity (GHSV), as shown in Fig. 9. Increasing GHSV has an inverse effect on toluene conversion. In other words, contact time decreases by increasing GHSV, and toluene conversion is reduced. At high GHSVs, residence time is limited and there is not enough time for toluene to interact with Pt particles inside the nanocatalyst pores. Proper toluene conversion even

Temperature (°C)

Figure 8 Total oxidation of toluene at different temperatures over synthesized nanocatalysts at different temperatures: Cln-T, Ce-Cln-T and Pt-Ce-Cln-T.

10000 h 15000 h

50 100 150 200

Temperature (°C)

Figure 9 Toluene oxidation performance of nanocatalyst at different GHSVs over Pt-Ce-Cln-T.

Pt-Ce-Cln-T CeO2 loading = 30% Pt loading = 1% Toluene = 1500 ppm

synthesized

Pt-Ce-Cln-T CeO2 loading = 30% Pt loading = 1% Temperature = 300°C Toluene = 1500 ppm GHSV = 10000 h-1

900 1200 Time (min)

Figure 11 Time on stream performance of Pt-Ce-Cln-T nanocatalyst in terms of toluene abatement from waste gas stream.

at high GHSVs (15,000 h can be attributed to structural properties of synthesized catalyst as a result of good dispersion of active phase, small particle size and proper surface area. Therefore, for Pt-Ce-Cln-T nanocatalyst, even at high GHSVs, reactants can meet active phase and mass transport can done well enough.

3.2.3. Effect of waste gas stream composition The effect of feed composition on toluene abatement over Pt-Ce-Cln-T nanostructured catalyst is shown in Fig. 10 for (a) 500, (b) 1500 and (c) 3000 ppm of toluene in feedstock. As expected, conversion decreases by increasing toluene concentration which is a normal phenomenon in VOC oxidation. As can be seen ca. 90% removal of toluene could be achieved in the concentration of 500 ppm at 100 0C while in 1500 ppm and 3000 ppm the conversion was about 10% and 0% respectively. As concentration increases, toluene-oxygen interactions become lower because of higher toluene accumulation on catalyst sites. However, the obtained results show that even at high

Pt-Ce-Cln-T 500 ppm

CeO2 loading = 30% 1500 ppm

Pt loading = 1% ^

GHSV = 10000 h-1 3000 ppm

150 200

Temperature (°C)

Figure 10 Toluene oxidation performance of synthesized nanocatalyst at different waste gas stream compositions over Pt-Ce-Cln-T.

concentrations of toluene (3000 ppm) the nanostructured catalyst has still enough destruction ability to reduce the pollutant.

3.2.4. Time on stream performance

In order to investigate time on stream behavior of Pt-Ce-Cln-T, stability test was carried out at 300 0C and toluene concentration of 1500 ppm for 2100 min and results are presented in Fig. 11. The obtained data revealed there was no significant deactivation during all 2100 min. This means, besides enhancing catalytic activity, utilizing ultrasound in dispersion of Pt over CeO2-Clinoptilolite, has led to synthesis of a stable nanocatalyst.

4. Conclusions

Abatement of VOC is one of the greatest environmental concerns in our century. In order to eliminate toluene, Pt/CeO2-Clinoptilolite synthesized successfully and physiochemical characterization of synthesized nanostructured catalysts confirmed formation of amorphous structure both for ceria and clinoptilolite. Moreover, SEM micrographs showed most of the Ce and/or Pt particles are in <100 nm range. According to BET analysis, the surface area of catalysis is large enough for catalytic oxidation of toluene. Activity test revealed high potential of synthesized nanocatalyst for toluene abatement in low temperatures. In addition, effect of feed concentration, GHSV and stability of nanostructured catalyst revealed Pt/CeO2-Clinoptilolite has still high potential to enough destruction ability to reduce the pollution even at high concentrations and long periods of time. Consequently, the abundance of raw materials, ease of synthesizing method cost-effective nanocatalyst support and low-temperature activity in comparison with similar samples, are mentioned as the advantages of this work.

Acknowledgments

The authors gratefully acknowledge Sahand University of Technology for the financial support of the research as well as Iran Nanotechnology Initiative Council for complementary financial supports.

Appendix A. Supplementary material

Supplementary data associated with this article can be found,

in the online version, at http://dx.doi.org/10.1016/_j.arabjc. 2015.02.003.

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