CHEMSUSCHEM FULL PAPERS DOI: 10.1002/cssc.201400050

Porous Cube-Aggregated Co3O4 Microsphere-Supported Gold Nanoparticles for Oxidation of Carbon Monoxide and Toluene Huanggen Yang,[a] Hongxing Dai,*[a] Jiguang Deng,*[a] Shaohua Xie,[a] Wen Han,[a] Wei Tan,[a] Yang Jiang,[a] and Chak Tong Au[b] Porous cube-aggregated monodisperse Co3O4 microspheres and their supported gold (xAu/Co3O4 microsphere, x = 1.6– 7.4 wt %) nanoparticles (NPs) were fabricated using the glycerol-assisted solvothermal and polyvinyl alcohol-protected reduction methods. Physicochemical properties of the materials were characterized by means of numerous analytical techniques, and their catalytic activities were evaluated for the oxidation of toluene and CO. It is shown that the cubic Co3O4 microspheres were composed of aggregated cubes with a porous structure. The gold NPs with a size of 3.2–3.9 nm were uniformly deposited on the surface of Co3O4 microspheres. Among the Co3O4 microsphere and xAu/Co3O4 microsphere samples, the 7.4Au/Co3O4 microspheres performed the best, giving T90 % values (the temperature required for achiev-

ing a CO or toluene conversion of 90 % at a weight hourly space velocity of 20 000 mL g1 h1) of 8 and 250 8C for CO and toluene oxidation, respectively. In the case of 3.0 vol % water vapor introduction, a positive effect on CO oxidation and a small negative effect on toluene oxidation were observed over the 7.4Au/Co3O4 microsphere sample. The apparent activation energies obtained over the xAu/Co3O4 microsphere samples were in the ranges of 40.7–53.6 kJ mol1 for toluene oxidation and 21.6–34.6 kJ mol1 for CO oxidation. It is concluded that the higher oxygen adspecies concentration, better low-temperature reducibility, and stronger interaction between gold NPs and Co3O4 as well as the porous microspherical structure were responsible for the excellent catalytic performance of 7.4Au/Co3O4 microsphere.

Introduction Carbon monoxide (CO) and volatile organic compounds (VOCs) emitted from industrial and transportation activities are recognized as major pollutants harmful to the atmosphere and human health. Catalytic oxidation is one of the most effective pathways for the reduction of CO and VOC concentrations. Development of high-performance catalysts becomes the focus in environmental catalysis. Supported noble metals and transition-metal oxides are promising materials in catalyzing the oxidation of CO and VOCs.[1, 2] Although the supported platinum, palladium, or rhodium catalysts exhibit excellent activities at low temperatures for CO and VOCs removal, their high cost and easy-poisoning tendency limit their wide applications. Due [a] H. Yang, Prof. H. Dai, Dr. J. Deng, S. Xie, W. Han, W. Tan, Y. Jiang Laboratory of Catalysis Chemistry and Nanoscience Department of Chemistry and Chemical Engineering College of Environmental and Energy Engineering Beijing University of Technology Beijing 100124 (PR China) Fax: (+ 86) 10-6739-1983 E-mail: [email protected] [email protected] [b] C. T. Au Department of Chemistry Hong Kong Baptist University Kowloon Tong, Kowloon, Hong Kong (PR China) Part of a Special Issue for the 6th Asia-Pacific Catalysis Congress (APCAT6). A link to the full Table of Contents will appear here. Supporting Information for this article is available on the WWW under http://dx.doi.org/ 10.1002/cssc.201400050.

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to the advantages of lower price and good catalytic performance at relatively high temperatures, transition-metal oxides have attracted a great attention in recent years. As diffusion is often a key factor affecting the performance of a bulk catalyst, making a catalyst with a porous structure is expected to significantly increase the number of accessible active sites and thus enhance the catalytic efficiency.[3] For example, Tysz et al. observed that the ordered mesoporous Co3O4 was highly active for the oxidation of CO at low temperatures.[3] Bruce et al. claimed that mesoporous Co3O4, Cr2O3, Fe2O3, Mn2O3, and Mn3O4 showed good catalytic performance for CO oxidation.[4] Our group also found that mesoporous cobalt oxide and manganese oxide performed well in the combustion of toluene.[5, 6] These porous materials are, however, usually prepared through nanocasting routes, and the fabrication approaches are quite complex due to the use of hard templates and various subsequent treatment steps.[7] A number of investigations have demonstrated that the catalytic properties of a nanocrystal are sensitive to its size, shape, and porous structure. The fabrication of a material with a controlled size, shape, and porous structure is still a big challenge.[8, 9] Gold is usually considered to be catalytically inactive. However, Hutchings and co-workers found that supported gold was an efficient catalyst for acetylene hydrochlorination.[10, 11] Haruta’s group reported that transition metal (e.g., iron, cobalt, or nickel) oxide-supported ultrafine gold particles exhibited excellent catalytic performance in the oxidation of CO.[12] ThereChemSusChem 0000, 00, 1 – 11

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fore, catalytic applications of supported gold materials have increased in recent years.[13, 14] Compared to other precious metals (e.g., platinum, rhodium, and palladium), gold is considerably less expensive and more abundant. Supported gold nanoparticle (NP) catalysts have attracted considerable attention for the removal of CO and VOCs. For example, Ma et al. reported that mesoporous Co3O4 and its supported gold NP catalysts showed high activities for the oxidation of ethylene at 0 8C.[15] Xue et al. found that Au/nanorod-like Co3O4 exhibited excellent catalytic activity (93.7 % ethylene conversion was achieved at 0 8C).[16] Solsona et al. observed a high catalytic activity for the oxidation of toluene, propane, and CO over partially ordered mesoporous Co3O4-supported gold NP catalysts.[17] It is known that supported gold NPs are catalytically active for redox reactions and that their performance is significantly influenced by the nature of the support.[18] Up to now, however, there have been no reports on the preparation and catalytic application of porous cube-aggregated monodisperse Co3O4 Figure 1. XRD patterns of A) bulk Co3O4, B) Co3O4 microspheres, C) 1.6Au/ microsphere-supported gold NPs for the oxidation of CO and Co3O4 microspheres, D) 3.7Au/Co3O4 microspheres, E) 6.2Au/Co3O4 microVOCs. spheres, F) 7.4Au/Co3O4 microspheres, and G) 7.9Au/bulk Co3O4. Previously, our group has synthesized a number of porous or micro/nanostructured materials with well-defined morpholoTable 1. BET surface areas, pore volumes, average crystallite sizes (DCo3O4), average gold particle sizes, and gies (e.g., Ce0.6Zr0.3Y0.1O2,[19] manactual gold content of the Co3O4 and xAu/Co3O4 samples. ganese oxide,[20] NiO,[21] and [22] Co3O4 ) using the hydrothermal Pore volume DCo O [a] Au particle Au content[c] Sample Surface area or microemulsion method and [cm3 g1] [nm] size[b] [nm] [wt %] [m2 g1] investigated their physicochemibulk Co3O4 8.4 – 156.6 – – cal properties. It was found that 17.4 0.13 44.3 – – Co3O4 microspheres 18.3 0.11 42.3 3.2 1.6 1.6Au/Co3O4 microspheres most of the porous materials O microspheres 15.4 0.09 40.4 3.4 3.7 3.7Au/Co 3 4 performed well in catalyzing the 21.5 0.11 39.0 3.7 6.2 6.2Au/Co3O4 microspheres oxidation of CO and/or typical 7.4Au/Co3O4 microspheres 22.4 0.12 35.6 3.9 7.4 VOCs. In this work, we report 8.6 – 156.7 4.9 7.9 7.9Au/bulk Co3O4 the preparation, characterization, [a] Data determined based on the XRD results according to the Scherrer equation using the Full width at half and catalytic properties of maximum (FWHM) of the (311) line of Co3O4. [b] Estimated according to TEM images. [c] Determined by ICPporous cube-aggregated monoAES. disperse Co3O4 microsphere-supported gold NPs for CO and toluene oxidation. and xAu/Co3O4 microsphere samples were 35.6–44.3 nm, whereas they were approximately 157 nm in the bulk Co3O4 and 7.9Au/bulk Co3O4 samples. 3 4

Results and Discussion Crystal phase composition

Morphology, pore structure, and surface area

Figure 1 shows the XRD patterns of the Co3O4 and xAu/Co3O4 samples. By comparing the XRD pattern of the standard cobalt oxide sample (JCPDS PDF# 42-1467), one can realize that the Co3O4 and xAu/Co3O4 samples exhibited a cubic Co3O4 crystal structure. The diffraction signals at 2q = 19.48, 31.58, 36.98, 38.98, 44.98, 56.28, 59.48, 65.48, and 77.88 were due to the reflection of the (111), (220), (311), (222), (400), (422), (511), (440), and (128) crystal planes, respectively. Similar results were also reported by other researchers.[23] The loading of gold did not result in clear changes in the crystal structure of Co3O4. The weak diffraction peak of Au(111) at 2q = 38.58 was covered by that of Co3O4(222) at 2q = 38.98. As shown in Table 1, the calculated grain sizes of Co3O4 crystallites in the Co3O4 microsphere

Figure 2 shows the SEM images of the Co3O4 microsphere and bulk Co3O4 samples. It can be clearly seen that the Co3O4 microsphere sample displayed a uniform and monodisperse microspherical morphology with a size of approximately 10 mm; each microsphere was composed of porous cube-aggregated entities and was formed through the disordered aggregation of NPs with a size of 30–50 nm (Figure 2 A–C). The bulk Co3O4 sample contained nano/macroparticles (Figure 2 D) with irregular morphologies. As shown in the high-resolution TEM images of the xAu/Co3O4 microsphere samples, there were a number of uniform gold NPs highly dispersed on the surface of Co3O4 microspheres (Figure 3 B–E); the gold NPs on the surface of bulk Co3O4 (Figure 3 F), however, were bigger

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Figure 2. SEM images of A–C) Co3O4 microsphere and D) bulk Co3O4.

www.chemsuschem.org ples suggests that the Co3O4 and xAu/Co3O4 samples were polycrystalline. The porous structures of the Co3O4 microsphere and xAu/ Co3O4 microsphere samples were further confirmed by the results of N2 adsorption–desorption isotherms and pore-size distributions of the samples, as shown in Figure 4. It is observed that each of the samples displayed a type IV isotherm with a hysteresis loop in the relative pressure (p/p0) range of 0.8– 1.0, indicating the presence of mesopores.[24] In addition, there was also a hysteresis loop in the p/p0 range of 0.2–0.8. The results indicate the presence of two types of mesopores,[25] in agreement with two pore-size distributions shown in Figure 4 B. The generation of porous structures in the Co3O4 microsphere and xAu/Co3O4 microsphere samples would be beneficial for the enhancement of catalytic performance as easy diffusion of the reactants and facile accessibility of the active sites are expected to be achieved.[3] Table 1 summarizes the textural parameters of the Co3O4 and xAu/Co3O4 samples. The Brunauer–Emmett–Teller (BET) surface areas (15.4–22.4 m2 g1) of the porous Co3O4 microsphere samples were much higher than that (8.4 m2 g1) of the nonporous bulk counterpart. The pore volumes of the Co3O4 microsphere and xAu/Co3O4 microsphere samples were in the range of 0.09–0.13 cm3 g1. Surface composition, metal oxidation state, and oxygen species

X-ray photoelectron spectroscopy (XPS) is an effective technique to investigate the surface element compositions, metal oxidation states, and adsorbed species of a solid sample. Figure 5 shows the Co 2p3/2, O 1s, and Au 4f XPS spectra of the Co3O4 and xAu/Co3O4 samples. The asymmetrical Co 2p3/2 signal of each sample (Figure 5 A) could be decomposed into two comFigure 3. TEM images of A) Co3O4 microspheres, B) 1.6Au/Co3O4 microspheres, C) 3.7Au/Co3O4 microspheres, ponents at binding energy (BE) D) 6.2Au/Co3O4 microspheres, E) 7.4Au/Co3O4 microspheres, and F) 7.9Au/bulk Co3O4. corresponding to 780.2 and 781.8 eV, which are assignable to the surface Co3 + and Co2 + spe[26] cies, respectively. A weak satellite signal at BE = 786.5 eV incompared to those on the surface of Co3O4 microspheres. The average gold particle sizes of xAu/Co3O4 microsphere (x = 1.6, dicates the presence of Co2 + .[27] Table 2 lists the surface Co3 + 3.7, 6.2, and 7.4 wt %) and 7.9Au/bulk Co3O4 were 3.2, 3.4, 3.7, /Co2 + molar ratios of the samples. The surface Co3 + /Co2 + 3.9, and 4.9 nm (Table 1 and Figure S1 of the Supporting Informolar ratio decreased (i.e., the surface Co2 + concentration inmation), respectively. The intraplanar spacings (d values) were creased) after deposition of gold NPs on the surface of Co3O4 measured to be approximately 0.46 and 0.23 nm (Figure 3 B, C, microspheres. The surface Co3 + /Co2 + molar ratio (1.47) of bulk E, and F), consistence with those of the (111) crystal planes of Co3O4 was much higher than that (1.36) of Co3O4 microspheres, the standard Co3O4 (JCPDS PDF# 42-1467) and gold (JCPDS and the surface Co3 + /Co2 + molar ratio (1.32) of 7.9Au/bulk PDF# 04-0784) samples, respectively. The measuring of multiCo3O4 was also higher than those (0.92–1.27) of xAu/Co3O4 miple bright electron diffraction rings in the selected-area eleccrospheres. The results demonstrate that there was a larger tron diffraction (SAED) patterns (insets of Figure 3) of the samamount of surface oxygen vacancies on Co3O4 and xAu/Co3O4  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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www.chemsuschem.org existence of several types of surface oxygen species. The oxygen species at BE = 529.9, 531.6, and 533.5 eV were attributed to the surface lattice oxygen (Olatt), adsorbed oxygen (Oads, e.g., O2 , O22, or O), and adsorbed water species, respectively.[30, 31] With loading of gold on the surface of Co3O4, the surface Oads/Olatt molar ratio increased remarkably (Table 2). Compared to the Oads/ Olatt molar ratio (0.56) of the bulk Co3O4 sample, those (1.10–1.86) of the Co3O4 microsphere and xAu/Co3O4 microsphere samples

Figure 4. A) Nitrogen adsorption–desorption isotherms and B) pore-size distributions of a) Co3O4 microspheres, b) 1.6Au/Co3O4 microspheres, c) 3.7Au/Co3O4 microspheres, d) 6.2Au/Co3O4 microspheres, and e) 7.4Au/Co3O4 microspheres.

Figure 5. A) Co 2p3/2, B) O 1s, and C) Au 4f XPS spectra of a) bulk Co3O4, b) Co3O4 microspheres, c) 1.6Au/Co3O4 microspheres, d) 3.7Au/Co3O4 microspheres, e) 6.2Au/Co3O4 microspheres, f) 7.4Au/Co3O4 microspheres, and g) 7.9Au/bulk Co3O4.

Table 2. Surface element compositions, H2 consumption, and catalytic activities of the Co3O4 and xAu/Co3O4 samples. Sample Co3 + /Co2 + bulk Co3O4 Co3O4 microspheres 1.6Au/Co3O4 microspheres 3.7Au/Co3O4 microspheres 6.2Au/Co3O4 microspheres 7.4Au/Co3O4 microspheres 7.9Au/bulk Co3O4

1.47 1.36 1.27 1.13 1.04 0.92 1.32

Molar ratios Aud + /Au0 – – 0.38 0.45 0.60 0.66 0.31

Oads/Olatt

H2 consumption[a] [mmol g1]

0.56 1.11 1.37 1.48 1.61 1.86 1.30

18.0 16.9 16.4 16.1 16.0 15.6 16.7

T10 % [8C]

CO oxidation T50 % T90 % Ea [8C] [8C] [kJ mol1]

T10 % [8C]

Toluene oxidation T50 % T90 % Ea [8C] [8C] [kJ mol1]

140 82 15 12 41 52 39

195 132 95 43 15 25 110

255 240 190 183 170 155 212

288 266 262 258 244 242 264

210 153 120 79 2 8 143

51.2 34.6 32.6 30.3 29.1 21.6 32.7

320 285 275 270 258 250 280

86.5 57.8 53.6 51.0 41.9 40.7 53.9

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

microspheres than on bulk Co3O4 and 7.9Au/bulk Co3O4.[28] Such structural defects could facilitate the adsorption and activation of gas-phase oxygen molecules, hence enhancing the catalytic performance of the materials for CO and toluene oxidation.[29] In Figure 5 B, a broad and asymmetrical O 1s XPS peak for each of the samples can be identified, indicating the  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

were much higher. Figure 5 C shows the Au 4f XPS spectra of the xAu/Co3O4 samples. The components at BE = 84.0 and 87.5 eV could be ascribed to surface metallic gold (Au0), whereas the ones at BE = 84.9 and 88.8 eV were attributed to surface-oxidized gold (Aud + ).[32] The surface Aud + /Au0 molar ratio (0.31) of 7.9Au/bulk Co3O4 was lower than those (0.38–0.66) of ChemSusChem 0000, 00, 1 – 11

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CHEMSUSCHEM FULL PAPERS xAu/Co3O4 microspheres (Table 2). The differences in surface Co3 + /Co2 + , Oads/Olatt, and Aud + /Au0 molar ratios suggest the presence of a strong interaction between gold NPs and Co3O4 support[33] as electron transfer from gold NPs to Co3O4 could occur through the reaction of Au0 + Co3 + !Aud + + Co2 + . The increased proportion of Co2 + species could give rise to the formation of abundant oxygen vacancies (and hence Oads species) on the surface, which would be responsible for the high catalytic activity for CO and toluene oxidation.[34] Reducibility As reducibility is an important factor influencing the catalytic performance of a material for redox-involving reactions,[6] temperature-programmed reduction (TPR) experiments were performed to investigate the reducibility of the Co3O4 and xAu/ Co3O4 samples. Figure 6 A shows the H2-TPR profiles of the samples. There were two reduction peaks for the bulk Co3O4 sample: the one centered at 460 8C was due to the reduction of Co3O4 to CoO, whereas the one centered at 545 8C was attributable to the reduction of CoO to Co0.[35] Compared to the bulk Co3O4 sample, the reduction peaks of the Co3O4 microsphere sample was shifted to lower temperatures, indicating

www.chemsuschem.org and 13.3 mmol g1, respectively. As revealed in our H2-TPR results, the H2 consumption of the samples was in the range of 15.6–18.0 mmol g1, confirming the co-presence of Co3 + and Co2 + species in Co3O4 and xAu/Co3O4 and the Co3O4 microsphere and xAu/Co3O4 samples possessed more amounts of Co2 + than the bulk Co3O4 sample, which is consistent with the results of the XPS investigations. It is better to use the initial (for which less than 25 % oxygen for the first reduction peak in the sample was removed) H2 consumption rate to evaluate the low-temperature reducibility of the samples (Figure 6 B).[36, 37] The initial H2 consumption rate followed a sequence of bulk Co3O4 < 7.9Au/bulk Co3O4 < Co3O4 microsphere < 1.6Au/Co3O4 microsphere < 3.7Au/Co3O4 microsphere < 6.2Au/Co3O4 microsphere < 7.4Au/Co3O4 microsphere, coinciding with their orders in Oads concentration and catalytic performance (shown below). Catalytic performance

We performed the blank experiment (only quartz sand was loaded in the microreactor) below 400 8C under the following conditions: CO concentration 1.0 vol %, CO/O2 molar ratio 1:20, weight hourly space velocity (SV) of 20 000 mL g1 h1; or toluene concentration 1000 ppm, toluene/O2 molar ratio 1:400, and SV = 20 000 mL g1 h1. No significant conversion of CO or toluene was detected, indicating that there were no significant homogeneous reactions occurring under the adopted reaction conditions. Figure 7 A and B shows the catalytic performance of the Co3O4 and xAu/Co3O4 samples for the oxidation of CO and toluene, respectively. Clearly, CO or toluene conversion increased monotonously with the increase in reaction temperature; the Co3O4 microsphere sample Figure 6. A) H2-TPR profiles and B) initial H2 consumption rate as function of inverse temperature of a) bulk Co3O4, displayed better catalytic activity b) Co3O4 microspheres, c) 1.6Au/Co3O4 microspheres, d) 3.7Au/Co3O4 microspheres, e) 6.2Au/Co3O4 microspheres, than the bulk Co3O4 sample, and f) 7.4Au/Co3O4 microspheres, and g) 7.9Au/bulk Co3O4. the xAu/Co3O4 microsphere samples outperformed the 7.9Au/ that the porous microspherical Co3O4 sample was more reducibulk Co3O4 sample, especially for the oxidation of CO. It is ble. After the gold NPs were supported on bulk Co3O4 or better to use the reaction temperatures T10 %, T50 %, and T90 % Co3O4 microspheres, the reduction peaks of the xAu/Co3O4 (corresponding to CO or toluene conversion of 10, 50, and 90 %) to evaluate the catalytic performance of the samples, as samples were further shifted to lower temperature, especially summarized in Table 2. Apparently, the 7.4Au/Co3O4 microfor the xAu/Co3O4 microsphere samples. This result implies that a strong interaction between Au NPs and Co3O4 was pressphere sample showed the highest catalytic activity, giving T10 %, T50 %, and T90 % values of 52, 25, and 8 8C for CO oxient, leading to the improvement in low-temperature reducibility of the xAu/Co3O4 microsphere samples. By quantifying the dation, and 155, 242, and 250 8C for toluene oxidation, respectively. Over the 7.9Au/bulk Co3O4 sample, however, the T10 %, reduction peaks, the total H2 consumption of the bulk Co3O4, Co3O4 microsphere, and xAu/Co3O4 samples was 18.0, 16.9, and T50 %, and T90 % values were 39, 110, and 143 8C for CO oxidation, 15.6–16.7 mmol g1 (Table 2), respectively. Assuming that the and 212, 264, and 280 8C for toluene oxidation, respectively. The CO or toluene consumption rates versus reaction temperacobalt ions in cobalt oxide were only Co3 + or only Co2 + , and ture over the Co3O4 and xAu/Co3O4 samples are shown in Figreduced to Co0, the theoretical H2 consumption would be 18.1  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 7. A) CO conversion and B) toluene conversion as a function of reaction temperature over (*) bulk Co3O4, (*) Co3O4 microspheres, (~) 1.6Au/Co3O4 microspheres, (~) 3.7Au/Co3O4 microspheres, (&) 6.2Au/Co3O4 microspheres, (&) 7.4Au/Co3O4 microspheres, and (^) 7.9Au/bulk Co3O4 under the conditions of CO concentration 1.0 vol %, CO/O2 molar ratio 1:20, and SV = 20 000 mL g1 h1; or toluene concentration 1000 ppm, toluene/O2 molar ratio 1:400, and SV = 20 000 mL g1 h1.

Au/a-Fe2O3 (T90 % = 180 8C),[41] and 4.9 wt % Au/ZnO (T90 % = 30 8C)[42] but inferior to that over 2.5 wt % Au/Fe(OH)x (T90 % = 20 8C)[43] and 4.9 wt % Au/Mn2O3 (T90 % = 45 8C);[44] in addition, the catalytic performance (T90 % = 274 8C) for the oxidation of toluene over 7.4Au/Co3O4 microsphere was much better than that over 1.5 wt % Au/7.5 wt % CeO2/Al2O3 (T90 % = 300 8C),[45] 1.5 wt % Au/ TiO2 (T90 % = 350 8C),[45] 1.5 wt % Au/Al2O3 (T90 % = 401 8C),[45] and 5 wt % Au/CeO2 (T90 % = 300 8C)[46] but inferior to that over mesoporous Co3O4 (T90 % = 198 8C),[5] NiO/ SiO2 (T90 % = 247 8C),[47] and over 30 wt % Co3O4–CeO2 (T90 % =

258 8C).[48] To examine the catalytic stability, 72 h on-stream reaction experiments were conducted over the best-performing 7.4Au/ Co3O4 microsphere sample at 0 8C for CO oxidation and 250 8C for toluene oxidation (Figure S3 in the Supporting Information), respectively. No significant loss in catalytic activity was observed either for CO oxidation or for toluene oxidation. Therefore, the 7.4Au/Co3O4 microsphere sample was catalytically durable under the adopted conditions. Based on the consumption rates and the amounts of Co3O4 or loaded gold, we calculated the turnover frequencies (TOFs) of the samples at 80 8C for CO oxidation or at 240 and 260 8C for toluene oxidation, as listed in Table 3. Clearly, the xAu/Co3O4 microsphere samples showed a much higher TOFCo3O4 than the bulk Co3O4 sample either at 240 or 260 8C in toluene oxidation. With increase in Au loading from 1.6 to 7.4 wt %, the TOFCo3O4 increased from 0.011  103 to 0.024  103 s1 at 240 8C and from 0.022  103 to 0.056  103 s1 at 260 8C. However, TOFAu over the xAu/Co3O4 microFigure 8. Effect of SV on A) CO and B) toluene conversions over the 7.4Au/Co3O4 microsphere sample under the sphere samples decreased from conditions of CO concentration 1.0 vol %, and CO/O2 molar ratio 1:20; or toluene concentration = 1000 ppm and 0.561  103 to 0.248  103 s1 at toluene/O2 molar ratio1:400. 240 8C and from 1.1  103 to 0.57  103 s1 at 260 8C, a result extension of contact time between the sample and reactant possibly due to the increase in gold loading. In addition, the molecules. Table S1 of the Supporting Information summarizes toluene reaction rate normalized per gram of Au in the xAu/ the catalytic activities of various materials reported in the literCo3O4 microsphere samples decreased from 2.85  106 to ature for the oxidation of CO and toluene. Under similar reac1.26  106 mol gAu1 s1 at 240 8C and from 5.7  106 to 2.9  tion conditions, the 7.4Au/Co3O4 microsphere sample exhibited 106 mol gAu1 s1 at 260 8C. Conversely, TOFCo3O4, TOFAu, and CO a catalytic activity (T90 % = 8 8C) for the oxidation of CO that reaction rate normalized per gram of Au at 80 8C in CO oxidation over 3.7Au/Co3O4 microsphere were much higher than was considerably better than that over Au/LaCoO3 (T90 % = 82 8C),[38] Au/LaMnO3 (T90 % = 78 8C),[39] Au/CeO2 (T90 % = 60 8C),[40] those over 1.6Au/Co3O4 microspheres. These results imply that ure S2 of the Supporting Information. The change in trend in CO or toluene consumption rate versus temperature was similar to that in CO or toluene conversion versus temperature. These results suggest that the porous and microspherical structures, the gold particle size, and gold loading played important promotional roles in enhancing the catalytic performance. The effect of SV on catalytic activity of the best-performing 7.4Au/Co3O4 microsphere sample for the oxidation of CO and toluene was investigated (Figure 8). As expected, the catalytic activity decreased with the increase in SV, a result due to the

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Table 3. Reaction rates and TOF values for CO and toluene oxidation over the Co3O4 and xAu/Co3O4 samples at different temperatures. Sample

CO oxidation at 80 8C Toluene combustion at 240 8C Toluene combustion at 260 8C reaction rate TOFAu TOFCo3O4 reaction rate TOFAu TOFCo3O4 reaction rate TOFAu TOFCo3O4 [106 mol gAu1s1] [103 s1] [103 s1] [106 mol gAu1s1] [103 s1] [103 s1] [106 mol gAu1s1] [103 s1] [103 s1]

bulk Co3O4 Co3O4 microspheres 1.6Au/Co3O4 microspheres 3.7Au/Co3O4 microspheres 6.2Au/Co3O4 microspheres 7.4Au/Co3O4 microspheres 7.9Au/bulk Co3O4

– – 108.9 110.5 – – 13.61

– – 21.47 21.77 – – 2.681

0.0018 0.093 0.427 1.023 – – 0.281

– – 2.85 1.30 1.28 1.26 0.38

– – 0.561 0.255 0.252 0.248 0.075

0.0027 0.0052 0.011 0.012 0.020 0.024 0.0079

– – 5.7 3.4 3.3 2.9 0.96

– – 1.1 0.67 0.66 0.57 0.19

0.0056 0.016 0.022 0.032 0.053 0.056 0.020

the gold NPs might be the active sites for the oxidation of CO but not for the oxidation of toluene. Gold NPs highly dispersed on the surface of Co3O4 microsphere could enable the generation of more oxygen vacancies and exhibit better low-temperature reducibility of the xAu/Co3O4 microsphere samples, resulting in better catalytic performance for the oxidation of CO and toluene. Figure 9 shows the toluene consumption rate and TOFCo3O4 value at 220 or 260 8C as a function of Oads/Olatt molar ratio of the samples. Apparently, a higher Oads concentration was favor-

uene oxidation. When water vapor was cut off, CO conversion was almost restored to its initial values in the absence of H2O; however, toluene conversion was maintained at approximately 87 %, which was slightly lower than that (90 %) in the absence of water vapor. Previous studies reported that in the presence of moisture, water was adsorbed on the metal oxide support rather than on the precious metal.[50] The positive effect of moisture in CO oxidation might be associated with its two possible roles: one was the activation of oxygen whereas the other was the decomposition of formed carbonates.[51] The small negative effect of moisture on catalytic toluene oxidation was due to the competitive adsorption of water and toluene on the surface of the 7.4Au/ Co3O4 microsphere sample. It is well known that the performance of supported gold catalysts is associated with several factors, such as size and shape of gold NPs,[12] chemical state of active gold species,[32] nature of the support,[18] interaction between gold NPs and support,[33] preparation approach, and preFigure 9. A) Toluene consumption rate and B) TOFCo O at SV = 20 000 mL g1 h1 and different temperatures as treatment conditions. The presa function of Oads/Olatt molar ratio of (*) bulk Co3O4, (*) Co3O4 microspheres, (~) 1.6Au/Co3O4 microspheres, (~) ence of a porous structure in the 3.7Au/Co3O4 microspheres, (&) 6.2Au/Co3O4 microspheres, (&) 7.4Au/Co3O4 microspheres, and (^) 7.9Au/bulk Co3O4 microsphere samples can Co3O4. increase the contact surface area and enable a better diffusion of able for the enhancement in catalytic performance of the reactants and thereby access to the active sites, thus enhancsample. Furthermore, the reducibility of metal oxides was assoing catalytic efficiency.[3] It has been well established that the ciated with the formation of oxygen vacancies. Kleitz et al. oxidation of CO and hydrocarbons over transition-metal oxides claimed that if a metal oxide was more reducible, it could involves a Mars–van Krevelen mechanism.[52] The reactant molinduce surface defects more readily (which would lead to ecules are oxidized by lattice oxygen of the metal oxide, and a higher Oads concentration).[49] at the same time the gas-phase oxygen molecules are activated to turn into the Oads species (e.g., O2, O22, or O), and reThe effect of moisture on the catalytic activity of the 7.4Au/ Co3O4 microsphere sample for CO and toluene oxidation was oxidize the partially reduced metal oxide. In the case of Co3O4, examined, and the results are shown in Figure 10. After the the redox process corresponds to the recycling of Co3 + $Co2 + . 1 1 catalytic oxidation of CO at 30 8C and SV = 40 000 mL g h or The strong redox ability of Co3O4 guarantees the recyclability toluene at 250 8C and SV = 20 000 mL g1 h1 became stable, of Co ions with two oxidation states. Therefore, higher Oads 3.0 vol % water vapor in the feedstock was introduced to the concentrations and better low-temperature reducibility would reaction system. The addition of water vapor led to a 4 % inbe favorable for the oxidation of CO and toluene. Many recrease of T90 % in CO oxidation but a 7 % decrease of T90 % in tolsearchers have proposed the presence of strong metal–sup3 4

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tion toward CO or VOC concentration (c) and zero-order reaction toward oxygen concentration.[56, 57] Therefore, we can reasonably assume that the oxidation of CO or toluene in the presence of excessive oxygen (CO/O2 molar ratio = 1:20 or toluene/O2 molar ratio = 1:400) would follow a first-order reaction mechanism with respect to CO or VOC concentration: r = k c = [A exp(Ea/RT)]c, where r, k, A, and Ea are the reaction rate (mol s1), rate constant (s1), pre-exponential factor, and apparent activation energy (kJ mol1), respectively, obtained by using the Arrhenius equation. Figure 11 shows the Arrhenius plots for CO and toluene oxidation over the Co3O4 and xAu/Co3O4 samples, and the calculated Ea values are summarized in Table 3. The Ea values for either CO or toluene oxidation decreased in the sequence bulk Co3O4 > Co3O4 microsphere > 7.9Au/bulk Co3O4 > 1.6Au/Co3O4 microsphere > 3.7Au/Co3O4 microsphere > 6.2Au/Co3O4 microsphere > 7.4Au/Co3O4 microsphere. Clearly, the Ea value over Figure 10. Effect of 3.0 vol % water vapor on the catalytic activity of the 7.4Au/Co3O4 microsphere sample under the conditions of CO concentration the xAu/Co3O4 microsphere samples were much lower than 1.0 vol %, CO/O2 molar ratio 1:20, and SV = 40 000 mL g1 h1; or toluene conthose of the bulk counterparts. With the increase in Au loading centration 1000 ppm, toluene/O2 molar ratio 1:400, and from 1.6 to 7.4 wt %, the Ea value of the xAu/Co3O4 micro1 1 SV = 20 000 mL g h . sphere sample decreased from 32.6 to 21.6 kJ mol1 for CO oxidation, which is lower than those (41–64 kJ mol1) over Au/ SiO2,[58, 59] similar to those (21– 36 kJ mol1) over Au/TiO2,[60] Au/ Al2O3,[61] and Au/Mn2O3[44] but higher than that (18 kJ mol1) over Au/TiO2.[44] As for the oxidation of toluene, when the Au loading was increased from 1.6 to 7.4 wt %, the Ea value over the xAu/Co3O4 microsphere sample decreased from 53.6 to 40.7 kJ mol1, which is much lower than those (120–144 kJ mol1) over CuO/Al2O3 and MnOx/ Figure 11. Arrhenius plots for the oxidation of A) CO and B) toluene over a) bulk Co3O4, b) Co3O4 microspheres, Al2O3.[62] These results indicate c) 1.6Au/Co3O4 microspheres, d) 3.7Au/Co3O4 microspheres, e) 6.2Au/Co3O4 microspheres, f) 7.4Au/Co3O4 microthat CO or toluene oxidation spheres, and g) 7.9Au/bulk Co3O4 under the conditions of CO concentration 1.0 vol %, CO/O2 molar ratio 1:20, and proceeded more readily over the SV = 20 000 mL g1 h1; or toluene concentration 1000 ppm, toluene/O2 molar ratio 1:400, and porous Co3O4 microsphere-supSV = 20 000 mL g1 h1. ported gold samples. port interactions (SMSIs).[53, 54] The SMSIs between gold NPs and Co3O4 and the formation of oxygen vacancies in Co3O4 might favor the activation of O2 molecules to the Oads species, thus be beneficial for the improvement in catalytic activity of the supported gold NPs.[55] According to the results of characterization and activity evaluation, we conclude that the excellent catalytic performance of 7.4Au/Co3O4 microspheres was associated with its high Oads concentration, good low-temperature reducibility, and strong interactions between gold NPs and Co3O4 microspheres. Apparent activation energy It has been generally accepted that the oxidation of CO or VOCs over the transition-metal oxides obey a first-order reac 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Conclusions Porous cube-aggregated monodisperse Co3O4 microspheres and their supported gold (xAu/Co3O4 microsphere; x = 1.6– 7.4 wt %) NPs could be successfully prepared through a facile glycerol-assisted solvothermal and polyvinyl alcohol (PVA)-protected reduction routes, respectively. The Co3O4 microsphere displayed a porous cube-aggregated monodisperse microspherical morphology, and the sizes of gold NPs were in the range of 3.2–3.9 nm. Among the xAu/Co3O4 microsphere samples, the 7.4Au/Co3O4 microspheres had the highest Oads concentration and the best low-temperature reducibility, thus showing the highest catalytic activity for CO oxidation (T90 % = 8 8C at SV = 20 000 mL g1 h1) and toluene oxidation (T90 % = 250 8C at SV = 20 000 mL g1 h1). When 3.0 vol % water vapor ChemSusChem 0000, 00, 1 – 11

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CHEMSUSCHEM FULL PAPERS was introduced to the reaction system, a positive effect in CO oxidation and a small negative effect in toluene oxidation were observed over the 7.4Au/Co3O4 microsphere sample. The xAu/Co3O4 microsphere samples exhibited an Ea value of 40.7– 53.6 kJ mol1 for toluene oxidation and an Ea value of 21.6– 34.6 kJ mol1 for CO oxidation. We concluded that the excellent catalytic performance of 7.4Au/Co3O4 microspheres might be associated with its higher Oads concentration, better lowtemperature reducibility, and stronger interaction between gold NPs and Co3O4 as well as the porous microspherical structure.

Experimental Section Sample preparation The porous cube-aggregated monodisperse Co3O4 microspheres were fabricated using the modified solvothermal method.[23] In a typical preparation process, CoCl2·6 H2O (1.0 g) was dissolved in a mixed solution of deionized H2O (20 mL), glycerol (60 mL), and urea (2.0 g) under stirring for 1 h. The obtained red solution was transferred into a Teflon-lined stainless autoclave (100 mL) for solvothermal treatment at 120 8C for 12 h. The resulting product was centrifuged and washed with pure ethanol and deionized water three times and dried in an oven at 80 8C overnight. Finally, the obtained pink powders were calcined in air by heating from room temperature (RT) to 500 8C at a ramp of 1 8C min1 and maintaining this temperature for 3 h to obtain the porous cube-aggregated monodisperse Co3O4 microspheres. The porous cube-aggregated monodisperse Co3O4 microspheresupported gold samples were prepared through a PVA-protected reduction method.[63] The typical preparation procedures were as follows: a desired amount of PVA (MW = 10 000 g mol1) was added to a HAuCl4 aqueous solution (100 mg L1; Au/PVA mass ratio 1.5:1) at RT under vigorous stirring for 10 min. After rapid injecting the 0.1 m NaBH4 aqueous solution (Au/NaBH4 molar ratio 1:5), a dark orange-brown solution (termed gold sol) was obtained. A desired amount of the porous cube-aggregated monodisperse Co3O4 microsphere support was added to a given amount of the gold sol (theoretical Au loading 2, 5, 8, and 11 wt %) under stirring until complete adsorption (decoloration of the solution) of colloidal gold occurred. The solid was filtered, washed with deionized water, and dried at 80 8C for 12 h to obtain the xAu/Co3O4 microsphere samples. The results of inductively coupled plasma atomic emission spectroscopic (ICP-AES) investigations reveal that the real Au loading (x) was 1.6, 3.7, 6.2, and 7.4 wt % in the xAu/Co3O4 microsphere samples, respectively. For comparison purposes, the bulk Co3O4 and 7.9Au/bulk Co3O4 samples were also prepared through the thermal decomposition of cobalt nitrate at 600 8C for 4 h and the PVA-protected reduction route. All chemicals (A.R. in purity) were purchased from Beijing Sinopharm Chemical Reagents Company and used without further purification.

www.chemsuschem.org Catalytic evaluation The catalytic activities of the samples were evaluated in a continuous flow fixed-bed quartz microreactor (i.d. 4 mm). To minimize the effect of hot spots, the sample (50 mg, 40–60 mesh) was diluted with quartz sands (0.25 g; 40–60 mesh). Prior to the test, the sample was treated in O2 (20 mL min1) at 250 8C for 1 h. After cooling to a given temperature, the reactant gas containing CO or toluene was passed through the sample bed. For CO oxidation, the reactant feed was 1.0 vol % CO + 20 vol % O2 + N2 (balance), and the total flow rate was 16.7 mL min1, giving a SV  20 000 mL g1 h1. In the case of water vapor introduction, 3.0 vol % H2O was introduced by passing the feed stream through a water saturator at a certain temperature. Catalytic activities of the samples for CO oxidation at low temperatures (below RT) were measured by immersing the microreactor in an ethanol–liquid N2 mixture at certain volumetric ratios. Reactants and products were analyzed online by using a gas chromatograph (GC-14C, Shimadzu) equipped with a thermal conductivity detector (TCD) using a 13X column. For toluene oxidation, the total flow rate of the reactant mixture [1000 ppm toluene + O2 + N2 (balance)] was 16.7 mL min1, giving a toluene/O2 molar ratio of 1:400 and a SV  20 000 mL g1 h1. The 1000 ppm toluene sample was generated by passing a N2 flow through a bottle containing pure toluene chilled in an ice–water isothermal bath. For the change in SV, we altered the total flow rate of the feed gas mixture. Reactants and products were analyzed online by using a gas chromatograph (GC-2010, Shimadzu) equipped with a flame ionization detector (FID) and a TCD, using a stabilwax@-DA column (30 m in length) for VOC separation and a 1/8 in Carboxen 1000 column (3 m in length) for permanent gas separation. The balance of carbon throughout the study was approximately 99.5 %.

Acknowledgements The work described was supported by the NSF of China (21377008 and 21103005), 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 on the Creative Research Team Construction Promotion Project of Beijing Municipal Institutions. Keywords: cobalt oxide · co oxidation · microspheres · nanoparticles · solvothermal synthesis [1] [2] [3] [4] [5] [6]

Sample characterization Physicochemical properties of the samples were characterized by means of ICP-AES, XRD, N2 adsorption-desorption (BET), SEM, TEM, SAED, XPS, and H2-TPR. Detailed procedures are described in the Supporting Information.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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FULL PAPERS In the world of microspheres and cubes: Porous Co3O4 monodisperse microspheres and xAu/Co3O4 microsphere (x = 1.6–7.4 wt %) are prepared using glycerol-assisted solvothermal and polyvinyl alcohol-protected reduction methods, respectively. The higher oxygen adspecies concentration, better low-temperature reducibility, and strong interaction between gold and Co3O4 are responsible for the excellent catalytic performance of 7.4Au/Co3O4 microspheres.

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H. Yang, H. Dai,* J. Deng,* S. Xie, W. Han, W. Tan, Y. Jiang, C. T. Au && – && Porous Cube-Aggregated Co3O4 Microsphere-Supported Gold Nanoparticles for Oxidation of Carbon Monoxide and Toluene

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