Gold Supported on Iron Oxide Nanodisk as Efficient Catalyst for The

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Gold Supported on Iron Oxide Nanodisk as Efficient Catalyst for The Removal of Toluene Wen Han,† Jiguang Deng,*,† Shaohua Xie,† Huanggen Yang,† Hongxing Dai,*,† and Chak Tong Au‡ †

Key Laboratory of Beijing on Regional Air Pollution Control and Laboratory of Catalysis Chemistry and Nanoscience, Department of Chemistry and Chemical Engineering, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China ‡ Department of Chemistry and Center for Surface Analysis and Research, Hong Kong Baptist University, Kowloon Tong, Kowloon, Hong Kong, China ABSTRACT: In the present study, nanodisk-like Fe2O3 was first prepared using the hydrothermal method, and then its supported gold catalysts (xAu/Fe2O3 nanodisk, x = 0.71−6.55 wt %) were fabricated using the polyvinyl alcohol-protected reduction method. Under the reaction conditions of toluene concentration = 1000 ppm, toluene/O2 molar ratio = 1/400, and space velocity = 20 000 mL/(g h), 6.55Au/Fe2O3 nanodisk performed the best (T50% = 200 °C and T90% = 260 °C). The apparent activation energies (46−50 kJ/mol) obtained over the xAu/Fe2O3 nanodisk were smaller than that (65 kJ/mol) obtained over the Fe2O3 nanodisk for toluene oxidation. We conclude that the high oxygen adspecies concentration, good lowtemperature reducibility, and strong interaction between Au nanoparticles and the Fe2O3 nanodisk are responsible for the high catalytic performance of the 6.55Au/Fe2O3 nanodisk.

1. INTRODUCTION Volatile organic compounds (VOCs, e.g., toluene) cause harmful effects on the atmosphere and human health. A number of strategies including physical and chemical methods have been applied for the removal of VOCs, among which catalytic oxidation is believed to be an effective pathway. The catalyst plays an important role in determining the efficiency of such a process. In the past years, oxidation of VOCs over transition metal oxide-supported precious metal catalysts has been studied. These materials are catalytically active for the oxidation of VOCs. For example, Wang et al. pointed out that the good catalytic performance for o-xylene oxidation of Pd/porous Co3O4 was associated with the Pd particle size, oxidized Pd species, and oxygen vacancies in Co3O4.1 Hao and co-workers found that morphology of the support exerted a significant effect on the catalytic activity of cobalt oxide-supported Au for low-temperature oxidation of trace ethylene, and concluded that the catalytic activity was related to the exposed planes of morphologically different Co3O4.2 Solsona et al. prepared the Au-deposited partially ordered mesoporous Co3O4 catalysts and observed high catalytic activities for toluene oxidation.3 Ji and co-workers prepared Au nanoparticles (NPs) deposited on Mn3O4 nanocrystallites with distinct morphologies, and found a strong morphological effect of Mn3O4 nanocrystallites on the catalytic performance of Au/Mn3O4 for benzene combustion.4 Fe2O3 is an important catalytic material due to its low processing cost, nontoxicity, and high resistance to corrosion.5 Since the physicochemical property of a material is often dependent upon its particle size and morphology, investigations on the size- and shape-controlled synthesis of nanomaterials are of great interest. Recently, many efforts have been devoted to the synthesis of nanostructured iron oxides. To date, welldefined nanostructures of iron oxides with different morphol© 2014 American Chemical Society

ogies (e.g., nanotubes, nanocubes, nanorods, nanobelts, and hollow spheres) have been fabricated using hydrothermal approaches.6−14 Gold NPs supported on reducible transitionmetal oxides (e.g., Fe2O3) show good catalytic activities, which is ascribed to the activation and supplement of oxygen for the reaction. The Fe2O3-supported noble metal catalysts were more active than those supported on other materials.15 Recently, Ji and co-workers prepared the FeOx hollow nanorod-supported Au catalysts and observed a substantial pretreatment effect on CO oxidation.16 However, there have been no reports on the use of Fe2O3 nanodisk-supported gold catalysts for the oxidation of toluene. The deposition−precipitation method is a facile strategy for preparing supported gold catalysts. Previously, we reported that three-dimensionally ordered mesoporous β-MnO2-supported Au nanocatalysts derived from the deposition−precipitation route exhibited high catalytic performance for the oxidation of carbon monoxide, benzene, and toluene.17 With the protection of polyvinyl alcohol (PVA), however, we found that the gas bubble-assisted PVA-protected reduction is a good strategy that could generate narrowly sized Au NPs and higher actual Au loadings, which would be beneficial for improvement in catalytic performance of the as-prepared supported catalysts for the removal of VOCs.18,19 As an extension of this work, we herein adopted the PVA-protected reduction method to fabricate the Au/Fe2O3 nanodisk catalysts and investigated their catalytic performance for toluene oxidation. Received: Revised: Accepted: Published: 3486

January 5, 2014 February 13, 2014 February 14, 2014 February 14, 2014 dx.doi.org/10.1021/ie5000505 | Ind. Eng. Chem. Res. 2014, 53, 3486−3494

Industrial & Engineering Chemistry Research

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2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Nanodisk-like Fe2O3 was fabricated according to the procedures described by Peng and co-workers.20 A 1.37 g sample of FeCl3·6H2O was dissolved in 40 mL of ethanol, and 3.60 g of sodium acetate was added to the above solution under stirring. The mixture was transferred into a Teflon-lined stainless steel autoclave for hydrothermal treatment at 200 °C for 12 h. For the elimination of impurity ions, the product was filtered and washed with deionized water and ethanol three times, dried at 80 °C overnight, and calcined in air at 500 °C for 12 h. Using a polyvinyl alcohol (PVA)protected reduction method,18,19 the nanodisk-like Fe2O3supported gold NPs with different Au loadings (xAu/Fe2O3 nanodisk, theoretical gold loading x = 1, 3, 5, and 8 wt %) were obtained. As shown in Table 1, the real gold loading, calculated

analyzer) technique with Mg Kα (hv = 1253.6 eV) as the excitation source. Hydrogen temperature-programmed reduction (H2-TPR) experiments were carried out on a chemical adsorption analyzer (Autochem II 2920, Micromeritics) from room temperature (RT) to 700 °C, with the catalyst exposed to a 5% H2−95% Ar (v/v) flow (50 mL/min). The reduction peaks were calibrated against that of the complete reduction of a standard CuO powder (Aldrich, 99.995%) sample. 2.3. Catalytic Evaluation. Catalytic activities of the samples for the complete oxidation of toluene were tested in a continuous flow fixed-bed quartz microreactor (i.d. = 4 mm). To minimize the effect of hot spots, 0.25 g of quartz sands (40−60 mesh) was mixed with 50 mg of the catalyst (40−60 mesh). The catalyst was pretreated in 30 mL/min O2 flow at 250 °C for 1 h before the test. After being cooled to RT, the reactant mixture (1000 ppm toluene + O2 + N2 (balance)) with a flow rate of 16.7 mL/min was passed through the catalyst bed. The toluene/O2 molar ratio was 1/400, and the space velocity (SV) was ca. 20 000 mL/(g h). We changed the catalyst mass for the alteration of the SV. In the case of water vapor addition, we passed the feed stream through a water saturator at a certain temperature, so that 1.0, 3.0, or 5.0 vol % concentration of H2O could be introduced to the reaction system. The reactants and products were analyzed online on a gas chromatograph (GC2010, Shimadzu), equipped with a stabilwax@-DA column (30 m in length) for organic separation and a Carboxen 1000 column (3 m in length) for permanent gas separation. The carbon balance was estimated to be 99.5 % throughout the investigations.

Table 1. BET Surface Areas, Average Crystallite Sizes (DFe2O3), Average Au Particle Sizes, and Real Au Contents of the Fe2O3 Nanodisk and xAu/Fe2O3 samples sample Fe2O3 nanodisk 0.71Au/Fe2O3 nanodisk 2.52Au/Fe2O3 nanodisk 4.51Au/Fe2O3 nanodisk 6.55Au/Fe2O3 nanodisk 6.82Au/bulk Fe2O3

surface area (m2/g)

DFe2O3a (nm)

Au particle sizeb (nm)

Au contentc (wt%)

19.2 14.8

30 36

3.4

0.71

13.2

28

2.4

2.52

16.1

34

2.9

4.51

18.9

35

2.2

6.55

6.2

40

5.2

6.82

3. RESULTS AND DISCUSSION 3.1. Crystal Structure, Morphology, and Surface Area. From the XRD patterns of the samples shown in Figure 1, one can see that the crystal structure of the as-obtained samples could be indexed to the rhombohedral Fe2O3 phase (JCPDS PDF no. 86-2368), and the loading of Au did not change the crystal structure of Fe2O3 nanodisk. The detection of a weak diffraction peak at 2θ = 38.5° due to the Au (111) plane

a

Data determined according to the Scherrer equation using the fwhm of the (104) line of Fe2O3. bData estimated according to the TEM images; and the standard deviation in particle size measurement was ±0.5 nm. cData determined by the ICP−AES technique.

from the results of inductively coupled plasma atomic emission spectroscopic (ICP-AES, Thermo Electron IRIS Intrepid ER/ S) investigations, was 0.71, 2.52, 4.51, and 6.55 wt % for the xAu/Fe2O3 nanodisk samples. Compared to the expected gold loadings, the corresponding real gold loadings were less due to the loss of gold during the processes of filtration and washing with deionized water. Owing to the low surface area of the Fe2O3 nanodisk, the loss of gold became significant for the preparation of xAu/Fe2O3 nanodisk samples with a higher Au loading. For comparison purposes, bulk Fe2O3 was first prepared by the thermal decomposition of ferric nitrate at 650 °C for 3 h, and then the 6.82 wt % Au/bulk Fe2O3 catalyst was fabricated via the similar PVA-protected reduction route. 2.2. Catalyst Characterization. X-ray diffraction (XRD) patterns of the catalysts were recorded on a Bruker D8 Advance diffractometer with Cu Kα radiation and nickel filter (λ = 0.15406 nm). The Brunauer−Emmett−Teller surface areas of the samples were calculated on the basis of the N2 adsorption− desorption isotherms obtained on a Micromeritics ASAP 2020 analyzer. The scanning electron microscopic (SEM) and transmission electron microscopic (TEM) images of the samples were recorded on a Gemini Zeiss Supra 55 apparatus (operating at 10 kV) and a JEOL-2010 equipment (operating at 200 kV), respectively. The Fe 2p, O 1s, Au 4f, and C 1s binding energies (BEs) of surface species were measured using the Xray photoelectron spectroscopic (XPS, VG CLAM 4 MCD

Figure 1. XRD patterns of (a) Fe2O3 nanodisk, (b) 0.71Au/Fe2O3 nanodisk, (c) 2.52Au/Fe2O3 nanodisk, (d) 4.51Au/Fe2O3 nanodisk, (e) 6.55Au/Fe2O3 nanodisk, and (f) 6.82Au/bulk Fe2O3. 3487

dx.doi.org/10.1021/ie5000505 | Ind. Eng. Chem. Res. 2014, 53, 3486−3494


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Figure 2. SEM images of (a) Fe2O3 nanodisk, (b) 0.71Au/Fe2O3 nanodisk, (c) 2.52Au/Fe2O3 nanodisk, (d) 4.51Au/Fe2O3 nanodisk, (e) 6.55Au/ Fe2O3 nanodisk, and (f) 6.82Au/bulk Fe2O3.

Figure 3. TEM images of (a) 0.71Au/Fe2O3 nanodisk, (b) 2.52Au/Fe2O3 nanodisk, (c) 4.51Au/Fe2O3 nanodisk, and (d) 6.55Au/Fe2O3 nanodisk.

(JCPDS PDF no. 04-0784) indicates the generation of cubic Au NPs on the support surface. According to the Scherrer equation using the fwhm of the (104) line of Fe2O3, we found that the crystallite sizes of Fe2O3 in the xAu/Fe2O3 nanodisk and 6.82Au/bulk Fe2O3 samples were 28−36 and 40 nm (Table 1), respectively. Surface areas of the xAu/Fe2O3 samples were in the range of 13.2−19.2 m2/g, whereas that of the 6.82Au/bulk Fe2O3 sample was 6.2 m2/g. From the representative SEM images (Figure 2), one can see that the xAu/Fe2O3 nanodisk samples displayed a uniform

nanodisk-like architecture, with the typical width and thickness being 176 and 27 nm, respectively. The loading of Au NPs did not induce a significant alteration in particle morphology. The bulk Fe2O3 sample was composed of irregularly morphological nano/macroparticles (Figure 2f). The formation of a goodquality nanodisk-like structure in xAu/Fe2O3 nanodisk was confirmed by their TEM images (Figure 3). The Au NPs were well dispersed on the surface of Fe2O3 nanodisk (Figure 3a−d). The average Au NP diameter of each sample was calculated by making a statistic analysis on the sizes of more than 200 Au 3488



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Figure 4. Au particle size distributions of (a) 0.71Au/Fe2O3 nanodisk, (b) 2.52Au/Fe2O3 nanodisk, (c) 4.51Au/Fe2O3 nanodisk, (d) 6.55Au/Fe2O3 nanodisk, and (e) 6.82Au/bulk Fe2O3.

Figure 5. (A) Fe 2p3/2, (B) O 1s, and (C) Au 4f XPS spectra of (a) Fe2O3 nanodisk, (b) 0.71Au/Fe2O3 nanodisk, (c) 2.52Au/Fe2O3 nanodisk, (d) 4.51Au/Fe2O3 nanodisk, (e) 6.55Au/Fe2O3 nanodisk, and (f) 6.82Au/bulk Fe2O3.

can see linearly aligned bright spots, indicating the formation of single crystal structure. 3.2. Surface Element Composition and Oxygen Species. Using the XPS technique, we obtained the surface element compositions, metal oxidation states, and adsorbed oxygen species of the as-prepared catalysts, as shown in Figure 5. The asymmetrical Fe 2p3/2 XPS signal could be decomposed to two components at BE = 710.3 and 711.6 eV (Figure 5A), assignable to the surface Fe 2+ and Fe3+ species,21−23 respectively. The detection of one weak shakeup satellite signal at BE = 717.4 eV was indicative of the presence of surface Fe2+ species.24 With a rise in Au loading, the surface Fe3+/Fe2+ molar ratio decreased (Table 2). The asymmetrical O 1s XPS peak could be decomposed into three components at BE = 529.4,

NPs in the TEM images. The average Au particle sizes (Figure 4) of the xAu/Fe2O3 nanodisk (x = 0.71, 2.52, 4.52, and 6.55 wt %) and 6.82Au/bulk Fe2O3 samples were 3.4, 2.4, 2.9, 2.2, and 5.2 nm, respectively. The unusual observation in average Au NP diameters of 0.71Au/Fe2O3 nanodisk and 6.55Au/ Fe2O3 nanodisk might be associated with the statistical limitation of the used TEM images. The intraplanar spacing (d value, 0.270−0.271 nm) of Fe2O3 nanodisk was rather close to that (0.270 nm) of the (104) crystal plane of the standard Fe2O3 sample (JCPDS PDF no. 86-2368), and the d value (0.235−0.236 nm) of Au NPs was not far away from that (0.235 nm) of the (111) crystal plane of the standard Au sample (JCPDS PDF no. 04-0784). In the SAED patterns (insets of Figure 3) of the xAu/Fe2O3 nanodisk samples, one 3489



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Table 2. Surface Element Compositions, H2 Consumption, and Catalytic Activities at SV = 20 000 mL/(g h) of the Fe2O3 Nanodisk and xAu/Fe2O3 Samples H2 consumption (mmol/g)a sample Fe2O3 nanodisk 0.71Au/Fe2O3 nanodisk 2.52Au/Fe2O3 nanodisk 4.51Au/Fe2O3 nanodisk 6.55Au/Fe2O3 nanodisk 6.82Au/bulk Fe2O3 a

Fe3+/Fe2+ molar ratio

Auδ+/Au0 molar ratio

Oads/Olatt molar ratio

toluene oxidation activity and apparent activation energy

≤360 °C

> 360 °C

total

T10% (°C)

T50% (°C)

T90% (°C)

Ea (kJ/mol)

3.09 2.90

0.25

0.35 0.72

1.05 1.85

16.10 13.70

17.20 15.55

300 291

350 315

410 380

65.1 50.1

2.75

0.25

0.75

2.09

13.30

15.39

265

310

348

46.5

2.45

0.26

0.79

2.01

12.80

14.81

249

290

330

46.1

2.26

0.34

1.19

2.19

13.30

15.49

135

200

260

45.7

3.16

0.29

1.10

2.04

15.10

17.14

295

335

400

59.9

The data were estimated by quantitatively analyzing the H2-TPR profiles.

Figure 6. (A) H2-TPR profiles and (B) initial H2 consumption rate as a function of inverse temperature of (a) Fe2O3 nanodisk, (b) 0.71Au/Fe2O3 nanodisk, (c) 2.52Au/Fe2O3 nanodisk, (d) 4.51Au/Fe2O3 nanodisk, (e) 6.55Au/Fe2O3 nanodisk, and (f) 6.82Au/bulk Fe2O3.

3.3. Reducibility. Shown in Figure 6 are the H2-TPR results of the as-prepared catalysts. Usually, the reduction of iron oxide proceeds according to the sequence of Fe2O3 → Fe3O4 → Fe0.22,31 For the present catalysts, there were two reduction steps in the ranges of 200−360 and 360−700 °C (Figure 6A), which were ascribed to the reduction of Fe2O3 to Fe3O4 and of Fe3O4 to Fe0.22,32 After loading Au onto the Fe2O3 surface, all of the initial reduction peaks shifted to lower temperatures, a result due to a strong interaction between Au NPs and Fe2O3. Furthermore, the detection of an initial reduction peak at 213 °C over the 6.55Au/Fe2O3 nanodisk sample implies that this sample exhibited the best lowtemperature reducibility. The H2 consumption of the samples was calculated according to the quantitative analysis of the H2TPR profiles. As summarized in Table 2, the H2 consumption of the xAu/Fe2O3 samples was in the range of 14.81−17.20 mmol/g. By assuming that Fe2O3 contained only Fe3+ and FeO contained only Fe2+, and were reduced to Fe0, the H2 consumption would be 18.75 and 6.25 mmol/g, respectively. It is apparent that the H2 consumption of the xAu/Fe2O3 samples was between 6.25 and 18.75 mmol/g. That is to say,

531.5, and 533.3 eV (Figure 5B), ascribable to the surface lattice oxygen (Olatt), adsorbed oxygen (Oads), and adsorbed water species,23,25 respectively. Because of the possible interaction between Au NPs and the Fe2O3 nanodisk, the surface Oads/Olatt molar ratio (0.72−1.19) increased (Table 2) when 0.71−6.55 wt % Au NPs were loaded on the Fe2O3 nanodisk surface, and the highest surface Oads/Olatt molar ratio (1.19) was achieved on the 6.55Au/Fe2O3 nanodisk sample. A rise in oxygen adspecies concentration would enhance the catalytic performance of a sample.26,27 As shown in the Au 4f XPS spectrum (Figure 5C), the signals at BE = 83.4 and 87.2 eV and at BE = 84.4 and 88.2 eV were assigned to the surface Au0 and Auδ+ (i.e., Au+ or Au3+) species,28,29 respectively. There was small difference in the surface Auδ+/Au0 molar ratio of the 0.71−4.51Au/Fe2O3 nanodisk catalysts. However, the surface Auδ+/Au0 molar ratio of the 6.55Au/Fe2O3 nanodisk increased the highest (0.34) due to strong interaction between Au NPs and the Fe2O3 nanodisk. It has been reported that the Auδ+ species might be more active than the Au0 species.30 Therefore, the xAu/Fe2O3 nanodisk catalyst with a higher Au loading would exhibit better catalytic activity. 3490



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Figure 7. (A) Toluene conversion as a function of reaction temperature over the Fe2O3 nanodisk, xAu/Fe2O3 nanodisk, and 6.82Au/bulk Fe2O3 samples under the conditions of toluene concentration = 1000 ppm, toluene/O2 molar ratio = 1/400, and SV = 20 000 mL/(g h); and (B) effect of SV on the catalytic activity over the 6.55Au/Fe2O3 nanodisk sample under the conditions of toluene concentration = 1000 ppm and toluene/O2 molar ratio = 1/400.

Table 3. Toluene Oxidation Rates and TOF Values of the Fe2O3 Nanodisk and xAu/Fe2O3 Samples at Different Temperatures toluene combustion at 200 °C

toluene combustion at 260 °C

sample

reaction rate (×10−8 mol/(g s))

TOFAu (×10−4 s−1)

TOFFe2O3 (×10−6 s−1)

Fe2O3 nanodisk 0.71Au/Fe2O3 nanodisk 2.52Au/Fe2O3 nanodisk 4.51Au/Fe2O3 nanodisk 6.55Au/Fe2O3 nanodisk 6.82Au/bulk Fe2O3

0.25 0.45 0.48 0.95 11.5 0.34

1.20 0.37 0.43 3.40 1.00

0.39 0.71 0.79 1.64 19.7 0.59

the Fe2O3 in each of the catalysts contained Fe3+ and Fe2+, in agreement with the results of XPS investigations. The initial (where less than 25% oxygen in the sample was removed for the first reduction peak) H2 consumption rate is an important index for low-temperature reducibility evaluation.33,34 From Figure 6B, one can see that the initial H2 consumption rate increased in the order of Fe2O3 nanodisk < 6.82Au/bulk Fe2O3 < 0.71Au/Fe2O3 nanodisk < 2.52Au/Fe2O3 nanodisk < 4.51Au/Fe2O3 nanodisk < 6.55Au/Fe2O3 nanodisk. In other words, the Fe2O3 nanodisk-supported Au catalysts showed better low-temperature reducibility than the bulk Fe2O3supported Au catalyst, and the 6.55Au/Fe2O3 nanodisk sample exhibited the best low-temperature reducibility. 3.4. Catalytic Performance and Apparent Activation Energy. Under the conditions of toluene concentration = 1000 ppm, toluene/O2 molar ratio = 1/400, SV = 20 000 mL/(g h), and temperature < 400 °C, no significant conversion of toluene was detected over the quartz sands loaded in the microreactor. That is to say, the complete oxidation of toluene over the present catalysts was a catalytic process. As can be seen from Figure 7A, toluene conversion increased with a rise in temperature. For better comparison purposes, the reaction temperatures T10%, T50%, and T90% (corresponding to toluene conversion = 10, 50, and 90%) were summarized in Table 2.

reaction rate (×10−7 mol/(g s))

TOFAu (×10−4 s−1)

TOFFe2O3 (×10−6 s−1)

0.13 0.14 0.23 0.26 2.10 0.14

3.77 1.75 2.00 6.10 0.40

2.10 2.20 3.70 7.60 35.6 2.40

Over the Fe2O3 nanodisk sample, T10%, T50%, and T90% were 300, 350, and 410 °C, respectively. Previously, Fe2O3 was employed as catalyst for the removal of methanol, acetone, diethyl ether, 2-propanol, and toluene.35,36 For example, Scirè and co-workers observed a T90% of 380 °C over Fe2O3 for toluene oxidation at SV = 186 mL/(g h).36 Duran et al. reported that 80% toluene conversion could be achieved over Fe2O3 at 365 °C and 20 000 mL/(g h).37 Obviously, our Fe2O3 nanodisk sample outperformed the Fe2O3 samples reported above.36,37 In terms of T10%, T50%, and T90%, the xAu/Fe2O3 nanodisk samples showed better catalytic activities than the 6.82Au/bulk Fe2O3 sample, with the 6.55Au/Fe2O3 nanodisk sample performing the best. The T10%, T50%, and T90% over 6.55Au/Fe2O3 nanodisk were 135, 200, and 260 °C, whereas those over 6.82Au/bulk Fe2O3 were 295, 335, and 400 °C, respectively. It should be noted that a further increase in Au loading (higher than 6.55 wt %) could not obviously enhance the catalytic performance of xAu/Fe2O3. In terms of toluene reaction rate normalized per gram of catalyst at 200 or 260 °C (Table 3), the catalytic activity increased in the order of Fe2O3 nanodisk < 6.82Au/bulk Fe2O3 < 0.71Au/Fe2O3 nanodisk < 2.52Au/Fe2O3 nanodisk < 4.51Au/Fe2O3 nanodisk < 6.55Au/ Fe2O3 nanodisk, coinciding with the sequence of lowtemperature reducibility (Figure 6B). Owing to the presence 3491



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of a strong interaction between Au NPs and Fe2O3, partial Au0 might react with Fe3+ to generate Auδ+ and Fe2+ species, causing the surface Fe2+ and Auδ+ concentrations to increase. According to the electroneutrality principle, an increase in Fe2+ content can give a rise in the amount of oxygen vacancies (Table 2). The generation of surface oxygen vacancies facilitates the activation of gas-phase oxygen molecules and the easy replenishment of Oads species. The oxidation of organic compounds usually proceeds through the interaction of organic compounds with Oads species at oxygen vacancies. During the oxidation process of toluene over the xAu/Fe2O3 nanodisk, the reduction of transition metal ions was associated with the reducibility of the xAu/Fe2O3 nanodisk, and the generation of oxygen vacancies took place concurrently. That is to say, an enhancement in reducibility would be beneficial for the generation of oxygen vacancies, and then for the activation of gas-phase oxygen molecules and the replenishment of Oads species. Therefore, it is understandable that the better lowtemperature reducibility led to better catalytic performance in the present study. The toluene reaction rate over the 6.55Au/ Fe2O3 nanodisk was much higher than that over 1.5 wt % Au/ ZnO,38 but inferior to those over 4.3 wt % Au/CeO239 and 0.96 wt % Au/TiO2.40 There are several kinds of active sites (noble metal, transition metal oxide, and noble metal−transition metal oxide) in the reducible metal oxide-supported noble metal catalysts. It is hard to identify a single active site. Therefore, the accurate calculation of the turnover frequencies (TOFs) is difficult to perform. For comparison convenience, we calculated the TOFs according to the activity data and molar amounts of Au and Fe in xAu/Fe2O3, and the results are listed in Table 3. Among the as-prepared samples, 6.55Au/Fe2O3 nanodisk exhibited the highest turnover frequencies (TOFAu and TOFFe2O3) at 200 or 260 °C, due to the increase in number of active Au sites. Furthermore, the Au sites seem to be more efficient than the Fe2O3 support in enhancing toluene oxidation efficiency. As we know, the Au particle size is an important factor influencing catalytic activity of a supported Au sample.41 In the present study, the average sizes of Au NPs in the xAu/ Fe2O3 nanodisk were in the range of 2.2−3.4 nm, whereas that in 6.82Au/bulk Fe2O3 was 5.2 nm. Therefore, it is understandable that the xAu/Fe2O3 nanodisk samples performed better than 6.82Au/bulk Fe2O3 for toluene oxidation. Toluene conversion increased with the drop in SV (Figure 7B), a result due to the elongation of contact time between reactant molecules and catalyst. To evaluate the catalytic stability, a 50 h on-stream toluene oxidation experiment at 260 °C over the 6.55Au/Fe2O3 nanodisk sample was carried out. Under the adopted reaction conditions, no significant loss in catalytic activity was detected within 50 h of on-stream reaction (Figure 8), demonstrating that the 6.55Au/Fe2O3 nanodisk sample was catalytically durable. The effect of water vapor (1.0, 3.0, or 5.0 vol %) on the catalytic performance of 6.55Au/Fe2O3 nanodisk was investigated, and the result is shown in Figure 9. At SV = 20 000 mL/(g h) and temperature = 260 °C, there were no significant changes in toluene conversion after 1.0 vol % water vapor was introduced to the reaction system; when 3.0 or 5.0 vol % water vapor was cofed, however, there was ca. 5 or 8% decrease in toluene conversion. The inhibition by water vapor might be due to the competitive adsorption of water and toluene as well as oxygen molecules.42 Fortunately, toluene conversions could be restored after cutting off water vapor.

Figure 8. Toluene conversion as a function of on-stream reaction time over the 6.55Au/Fe2O3 nanodisk catalyst under the conditions of toluene concentration = 1000 ppm, toluene/O2 molar ratio = 1/400, and SV = 20 000 mL/(g h) and reaction temperature = 260 °C.

Figure 9. Effect of water vapor addition on the catalytic activity of the 6.55Au/Fe2O3 nanodisk sample under the conditions of toluene concentration = 1000 ppm, toluene/O2 molar ratio = 1/400, and SV = 20 000 mL/(g h) and reaction temperature = 260 °C.

It has been reported that VOCs oxidation over the transitionmetal oxide catalysts obeys a first-order reaction with respect to VOC concentration and zero-order reaction with respect to oxygen concentration.43,44 In the presence of excess oxygen, one can reasonably assume that toluene oxidation over the xAu/Fe2O3 nanodisk catalysts would obey a first-order reaction mechanism with respect to toluene concentration (c), and the reaction rate is: r = −kc = (−A exp(−Ea/RT))c, where r, k, A, and Ea are the reaction rate (mol/s), rate constant (s−1), preexponential factor, and apparent activation energy (kJ/mol), respectively. Shown in Figure 10 are the Arrhenius plots for toluene oxidation over the xAu/Fe2O3 nanodisk catalysts. The Ea values are summarized in Table 2. Although the Ea value (59.9 kJ/mol) obtained over 6.82Au/bulk Fe2O3 was lower than that (65.1 kJ/mol) obtained over the Fe2O3 nanodisk, the Ea value obtained over 6.82Au/bulk Fe2O3 was much higher than those (45.7−50.1 kJ/mol) obtained over the xAu/Fe2O3 nanodisk. Furthermore, the Ea values obtained over the xAu/ 3492



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on the Creative Research Team Construction Promotion Project of Beijing Municipal Institutions.

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Figure 10. Arrhenius plots for the oxidation of toluene over the Fe2O3 nanodisk, xAu/Fe2O3 nanodisk, and 6.82Au/bulk Fe2O3 samples.

Fe2O3 nanodisk decreased with a rise in Au loading, with the 6.55Au/Fe2O3 nanodisk sample exhibiting the lowest Ea value (45.7 kJ/mol). Therefore, the 6.55Au/Fe2O3 nanodisk sample performed the best for toluene oxidation.

4. CONCLUSIONS The Fe2O3 nanodisk was prepared using the hydrothermal method, and 0.71−6.55 wt % Au NPs (2.2−3.4 nm in particle size) were loaded onto the Fe2O3 nanodisk surface via the PVAprotected reduction route. The xAu/Fe2O3 nanodisk catalysts possessed a rhombohedral crystal structure and a surface area of 13−19 m2/g. Catalytic activity for toluene oxidation enhanced with the loading of Au NPs, with the 6.55Au/Fe2O3 nanodisk sample showing the highest catalytic activity (T50% = 200 °C and T90% = 260 °C at SV = 20 000 mL/(g h)). The Ea values obtained over the xAu/Fe2O3 samples were in the range of 45.7−65.1 kJ/mol. It is concluded that the good catalytic performance of the 6.55Au/Fe2O3 nanodisk was associated with its larger oxygen adspecies concentration and better lowtemperature reducibility as well as the strong interaction between Au NPs and the Fe2O3 nanodisk.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-10-6739-6118. Fax: +86-10-6739-1983. E-email: [email protected]. *Tel.: +86-10-6739-6118. Fax: +86-10-6739-1983. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work described was supported by the NSF of China (21103005 and 21377008), 2013 Education and Teaching− Postgraduate Students Education−2011 Beijing Municipality Excellent Ph.D. Thesis Supervisor (20111000501), 2013 Education and Teaching−Postgraduate Students Cultivation− National Excellent Ph.D. Thesis Supervisor and Cultivation Base Construction (005000542513551), and the Foundation 3493



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Gold Supported on Iron Oxide Nanodisk as Efficient Catalyst for The

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