TiO2 Catalysts for CO Oxidation

Jul 30, 2014 - Propylene epoxidation over biogenic Au/TS-1 catalysts by Cinnamomum camphora extract in the presence of H 2 and O 2. Mingming Du , Jial...
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Influence of Au Particle Size on Au/TiO2 Catalysts for CO Oxidation Mingming Du,† Daohua Sun,*,† Hongwei Yang,§ Jiale Huang,† Xiaolian Jing,† Tareque Odoom-Wubah,† Haitao Wang,† Lishan Jia,† and Qingbiao Li*,†,‡,∥ †

Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, and ‡National Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, Xiamen University, Xiamen 361005, P. R. China § The State Key Laboratory of Advanced Technologies for Comprehensive Utilization of Platinum Metals, Kunming Institute of Precious Metals, Kunming 361005, P. R. China ∥ College of Chemistry & Life Science, Quanzhou Normal University, Quanzhou 362000, P. R. China S Supporting Information *

ABSTRACT: A series of Au/TiO2 catalysts for CO oxidation with same Au loading but different Au nanoparticles (NPs) sizes were prepared by varying the calcination temperatures and biomass concentration via a biosynthetic approach. The resulting catalysts were characterized by DRUV−vis, TEM, and TG techniques. The experimental results showed that the activity of the gold catalysts for CO oxidation was very sensitive to the particle size. Among the tested catalysts, the one with mean size of 3.8 nm was the most active. As determined by TEM, the contact boundary between the Au NPs and the TiO2 support was related to the size of the Au NPs. For the most active catalyst, hemispherical Au NPs (3.8 ± 0.6 nm) had the best contact boundary with the TiO2 support, yielding the longest perimeter interface, suggesting that the contact boundary was the most critical factor for the CO oxidation. The in-situ FTIR study of CO adsorption on the catalysts showed that CO was not adsorbed on the Au surface. This might be due to the modification of the Au/TiO2 catalysts by the residual biomass. The intensity of the peak at 2185 cm−1 for the Au/TiO2 catalysts with the longest perimeter interface was highest, demonstrating that the Au−TiO2 contact boundary played an important role in the adsorption of CO. al.3 found that the DP method can produce hemispherical Au particles with flat planes and in good contact with TiO2, yielding longer perimeter interface, whereas the impregnation method produced spherical particles which were just loaded on the metal oxide support. In addition, the catalysts prepared by the DP method showed better catalytic performance than that by the impregnation method. Haruta found that the preparation method affected the Au−support contact boundary; regrettably, the relation between the Au NPs sizes and the Au−support interaction was not further clarified. In the studies mentioned above, calcination at different temperatures and atmosphere was employed to obtain the Au nanoparticles with different sizes. It is worth noting that calcination also results in different molar ratios of Au0/Auδ+.4 It has been well established that the molar ratio of Au0/Auδ+ is also an important factor affecting the catalytic performance of Au catalysts. Therefore, the difference in catalytic performance of the catalysts prepared by the DP method was a result of multiple factors. Biosynthesis of metal catalysts has received great attention in the past decade, as an emerging highlight of

1. INTRODUCTION Among the noble metals, gold was long thought to be catalytically inert and attracted less attention compared to the other noble metals. However, Au catalysis has been in the limelight since Haruta et al. first demonstrated that very small Au nanoparticles (NPs) can be catalytically active if supported over transition metal oxides.1−3 CO oxidation is one of the representative reactions that supported nanogold catalysts are applicable to.1−5 The size of Au NPs has great influence on the catalytic performance of Au catalysts for CO oxidation.4,6,7 However, there still remains controversy on the optimal particle size of Au NPs for CO oxidation.4−12 Louis et al.11 found that maximum activity of Au/TiO2 catalysts occurred at Au NPs sizes of 2 nm. Delgass et al.12 reported that the most active Au NPs among a series of Au/TS1 catalysts prepared by the deposition−precipitation (DP) method for CO oxidation ranged from 2 to 5 nm. Shen et al.4 found that the activity of Au catalysts for CO oxidation could be greatly improved if the size of gold particle is kept in the range 3.9−7.5 nm. Although these researchers strongly believe that the size of Au NPs greatly influences the catalytic performance by changing the interaction between Au and their respective supports,4,12 the phenomenon behind these interactions has not been thoroughly investigated. Haruta et © XXXX American Chemical Society

Received: May 12, 2014 Revised: July 30, 2014

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the intersection of biotechnology and nanotechnology.13−20 The Au NPs were obtained by the reduction of chloroauric acid with plant extract (bioreduction). This method is advantageous because the Au NPs are reduced before they are loaded onto the support, so there are usually no oxidized Au species on the surface of the catalysts before calcination. Another advantage is that the size of the Au NPs can be controlled easily by changing the plant extract concentration owing to the presence of plant extract which plays dual roles as both reductant and stabilizer.16,17,19 Hence, in the present work, a series of Au/TiO2 catalysts for CO oxidation, with the same Au loading but different mean Au NPs sizes, were prepared via changing the calcination temperature and biomass concentration of the plant extract used and then characterized by DRUV−vis, TEM, and TG techniques. The effect of Au NPs size on the Au−support contact boundary and the catalytic reaction process was studied in detail.

windows in a Nicolet 6700 FTIR spectrometer using an MCT/ A detector. Prior to the adsorption experiment, the samples were pretreated in situ by heating them to 300 °C in a vacuum, and then the samples were cooled to 50 °C. The spectra of the samples at 50 °C prior to the adsorption were used as background in a vacuum. 2.4. Activity Measurements. The activity measurements were carried out in a continuous flow fixed-bed microreactor using a reactant concentration of 1% CO, 1% O2, and 98% N2 with a space velocity of 18 000 mL h−1 gcat−1. A 0.2 g amount of catalyst was used. The gas leaving the reactor was analyzed online by a chromatograph, equipped with a thermal conductivity detector (TCD), using a MS 5A packed column (2 mm × 3 m).

3. RESULTS AND DISCUSSION 3.1. Effect of Calcination Temperature on the Catalytic Performance of the Au/TiO2 Catalysts. 3.1.1. Diffuse Reflectance UV−vis Spectroscopy. First, the Au/TiO2 catalysts with different sizes of Au NPs were prepared by changing the calcination temperature. 3.3 ± 0.3 nm Au NPs were biologically synthesized by the reduction of HAuCl4 with CC extract at room temperature and were immobilized onto the TiO2 supports. The obtained Au/TiO2 catalyst was calcined at different temperatures. Figure 1 shows the representative

2. EXPERIMENTAL SECTION 2.1. Materials. The freshly Cinnamomum camphora (CC) leaves, planted by Xiamen Peony Perfume & Chemicals Industry Co. Ltd., China., were completely dried, then milled, and screened by a sieve of 20 mesh for experiments. Other chemicals were bought from Sinopharm Chemical Reagent Co. Ltd., China. 2.2. Preparation of the Au/TiO2 Catalysts. The Au NPs were prepared by the biological method using the plant extract (sol-immobilization (SI) method).16,19 In order to obtain CC extract, the biomass of 4.0 g was added to deionized (DI) water (400 mL) in a conical flask. The mixture was then shaken at 30 °C under the condition of rotation rate 150 rpm for 2 h and then filtrated to obtain the plant extract (10 g/L). In a typical preparation of Au NPs, as follows, 0.52 mL of HAuCl4 aqueous solution (48.56 mM) was added to 50 mL of CC extract, and then the mixed solution was stirred for 1 h to obtain the Au colloid. Afterward, the Au colloid was acidified at pH 2 with sulfuric acid, and then 0.5 g of TiO2 (P25) was added to the Au colloid with vigorous stirring. After 1 h, the suspension was filtered using a membrane of pores 0.8 mm, and the retained solid was washed totally using 400 mL of DI water and then dried at 50 °C in a vacuum for 24 h. Finally, the obtained catalysts were calcined at different temperatures (300, 375, 450, 550, and 650 °C). 2.3. Characterization of the Catalysts. The DRUV−vis spectra of the Au/TiO2 catalysts were obtained on a Varian Cary-5000 spectrometer, which had a diffuse-reflectance fitting, using dried BaSO4 as a reference from 200 to 800 nm. Transmission electron microscopy (TEM) images of the samples were obtained on a Tecnai F30 microscope. The sizes of the as-produced nanoparticles were obtained based on the statistical average from TEM images using the software SigmaScan Pro software. UV−vis spectroscopy studies of the Au/TiO2 catalysts were carried out on a UNICAM UV-300 spectrophotometer (Thermo Spectronic) with a range of 330− 800 nm. Atomic absorption spectroscopy (AAS) was used to acquire the definite Au loadings of the samples. X-ray photoelectron spectroscopy (XPS) characterization was obtained on a PHI-Quantum 2000 spectrometer. TG studies were carried out on a Netzsch TG209F1 thermobalance under flowing air atmosphere at a heating rate of 10 °C min−1. The insitu FT-IR experiments were performed using a home-built high temperature in-situ IR cell with quartz lining and CaF2

Figure 1. DRUV−vis spectra of the Au/TiO2 catalysts synthesized using CC at different calcination temperatures: (a) TiO2, (b) 1.0Au/ TiO2-uncalcined, (c) 1.0Au/TiO2-300 °C, (d) 1.0Au/TiO2-375 °C, (e) 1.0Au/TiO2-450 °C, (f) 1.0Au/TiO2-550 °C, (g) 1.0Au/TiO2-650 °C.

DRUV−vis spectra of the Au/TiO2 catalysts, calcined at 300, 375, 450, 550, and 650 °C. The spectrum of the TiO2 (Figure 1a) only showed the absorbance below 400 nm. The spectra of the uncalcined Au/TiO2 catalyst (Figure 1b) exhibited another band around 560 nm, which is characteristic for the plasmon resonance of metallic Au NPs.11 Such resonances are seen when the wavelength of the incident light far exceeds the particle diameter.11 The absorbance peak at 560 nm did not change while increasing the calcination temperature to 375 °C, indicating no obvious change in Au NPs size. However, the absorbance peak shifted from 560 to 580 nm when the calcination temperature further increased from 375 to 650 °C, suggesting that the size of the Au NPs increased. 3.1.2. Transmission Electron Microscopy. The TEM images of the catalysts calcined at different temperatures are shown in Figure 2. It can be seen that the Au NPs with mean diameters around 3.3 nm were well dispersed on the surfaces of support. Finally, the Au NPs almost retained their sizes after B

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Figure 2. TEM images and corresponding histograms of size distributions of the Au/TiO2 catalysts synthesized using CC at different calcination temperatures: (A) 1.0Au/TiO2-uncalcined, (B) 1.0Au/TiO2-300 °C, (C) 1.0Au/TiO2-375 °C, (D) 1.0Au/TiO2-450 °C, (E) 1.0Au/TiO2-550 °C, (F) 1.0Au/TiO2-650 °C.

immobilization on TiO2, even if the calcination temperature was increased to 375 °C. Continuing to increase the calcination temperature from 450 to 650 °C, the Au NPs mean size increased from 3.3 to 4.5 nm, consistent with the DRUV−vis results. The size distribution of the Au NPs also became broader while increasing the calcination temperature. That also indicates that the Au NPs sizes retained their sizes when the calcination temperature is less than 375 °C and then increases at temperatures more than 375 °C. Therefore, through this, we were able to obtain Au/TiO2 catalysts with different Au NPs sizes. 3.1.3. Thermogravimetric Analysis. The residual plant biomass on the bioreduced catalysts calcined at different temperatures was analyzed by TG analysis, as shown in Figure 3. It can be seen that a gradual decrease in the amount of plant biomass occurred with an increase in calcination temperature of up to 650 °C. The plant biomass weighed 4.84 wt % on the uncalcined catalyst with Au loading of 1.0 wt %. When increasing the calcination temperature to 375 °C, the residual plant biomass decreased to 0.77 wt %, without changing Au NPs size, as evidenced by the DRUV−vis spectra (Figure 1)

and TEM (Figure 2). A further increase in the temperature from 375 to 600 °C resulted in the residual plant biomass decreasing from 0.77 to 0.32 wt %, increasing the Au NPs mean size from 3.3 to 4.5 nm. From the analysis above, we can see that the Au NPs retained their size when the residual plant biomass weighed more than 0.77 wt % and then increased when the residual plant biomass weighed less than 0.77 wt %. Our previous studies have shown that residual plant biomass stabilizes bioreduced Au catalysts by preventing them from agglomeration.16,17 The catalytic activity of the Au/TiO2 catalysts, pretreated at different temperatures (uncalcined, 300, 375, 450, 550, and 650 °C), is shown in Figure 4. There was almost no catalytic performance for the catalyst without calcination, possibly because Au NPs were covered by excess amount of plant biomass (4.84 wt %), as demonstrated by the TG measurement. The treatment of calcination benefits the removal of significant amount of biomolecules residual on catalysts and thus helps the exposure of Au NPs.16 In comparison, the Au/ TiO2-375 °C catalyst showed better performance than that of the Au/TiO2-uncalcined and Au/TiO2-300 °C catalysts, although they had the same size of Au NPs (3.3 nm). Increasing the temperature from 375 to 450 °C resulted in an increase in the catalytic activity of the Au/TiO2-450 °C catalyst with 3.7 nm Au NPs mean size; it also showed the best catalytic performance. However, further increasing the temperature from 450 to 650 °C, the catalytic activity decreased. The drop in activity for the catalysts might be related to the growth of the particles, as demonstrated by the TEM result. The mean size of the Au NPs gradually increased from 3.7 to 4.5 nm, while increasing the temperature from 450 to 650 °C. The results above suggests that the dramatic difference in catalytic performance for the catalysts pretreated at different temperature (375, 450, 550, and 650 °C) might be attributed to the variation in the size of GNPs. 3.2. Effect of the CC Concentration on the Catalytic Performance of the Au/TiO2 Catalysts. In order to further

Figure 3. TG curves of the Au/TiO2 catalysts at different calcination temperatures. C

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Figure 4. CO oxidation over Au/TiO2 catalysts at different calcination temperatures: (■) 1.0Au/TiO2-uncalcined; (●) 1.0Au/TiO2-300 °C; (▲) 1.0Au/TiO2-375 °C; (▼) 1.0Au/TiO2-450 °C; (◆) 1.0Au/TiO2-550 °C; (★) 1.0Au/TiO2-650 °C. Feed gas: CO/O2/N2 = 1/1/98 (vol %); space velocity: 18 000 mL h−1 gcat−1.

Figure 5. TEM images and their corresponding size distribution histograms of the Au NPs synthesized using CC at different biomass concentrations of (A) 20, (B) 15, (C) 10, (D) 9, (E) 7, and (F) 5 g/L.

elucidate the effect of GNPs size in CO oxidation, different sizes of Au NPs synthesized were first immobilized onto support and then calcined at the same temperature to exclude the influence of the different interactions between Au and supports, caused by calcination at different temperatures.21 First, the Au NPs with different sizes were synthesized by changing the CC concentration (20, 15, 10, 9, 7, and 5 g/L) as shown in Figure 5. It can be seen that Au NPs with mean sizes ranging from 2.4 to 4.9 nm were obtained, and then these Au NPs were immobilized onto the supports and pretreated at the same calcination temperature (450 °C) to obtain the Au/TiO2 catalysts with different Au NPs sizes. Figure 6 shows the representative DRUV−vis spectra of the Au/TiO2 catalysts, prepared by different CC concentrations. The absorbance peak, characteristic for the plasmon resonance of metallic Au NPs,6 occurred gradually red-shifting with decreasing CC concentration from 20 to 5 g/L, indicating the increase in the Au NPs size. The representative TEM images of the Au/TiO2 catalysts prepared using different CC concentration in Figure 7 clearly illustrate that the mean size of the Au

Figure 6. DRUV−vis spectra of the Au/TiO2 catalysts synthesized using CC at different biomass concentrations: (a) TiO2, (b) 1.0Au/ TiO2-5 g/L, (c) 1.0Au/TiO2-7 g/L, (d) 1.0Au/TiO2-9 g/L, (e) 1.0Au/ TiO2-10 g/L, (f) 1.0Au/TiO2-15 g/L, (g) 1.0Au/TiO2-20 g/L.

NPs on the TS-1 surfaces increased from 2.9 to 5.1 nm when the CC concentration was decreased from 20 to 5 g/L. Therefore, Au/TiO2 catalysts with different Au NPs sizes were D

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Figure 7. TEM images and corresponding size distribution histograms of the Au/TiO2 catalysts synthesized using CC at different biomass concentrations: (A) 1.0Au/TiO2-20 g/L, (B) 1.0Au/TiO2-15 g/L, (C) 1.0Au/TiO2-10 g/L, (D) 1.0Au/TiO2-9 g/L, (E) 1.0Au/TiO2-7 g/L, (F) 1.0Au/TiO2-5 g/L.

Figure 8. CO oxidation over Au/TiO2 catalysts with different concentrations of CC: (■) 1.0Au/TiO2-20 g/L, (●) 1.0Au/TiO2-15 g/L, (▲) 1.0Au/ TiO2-10 g/L, (▼) 1.0Au/TiO2-9 g/L, (◆) 1.0Au/TiO2-7 g/L, and (★) 1.0Au/TiO2-5 g/L. Feed gas: CO/O2/N2 = 1/1/98 (vol %); space velocity: 18 000 mL h−1 gcat−1.

obtained at same calcination temperature, and there is almost no difference in the specific areas (SBET, SLangmuir, and SBJH) for these Au/TiO2 catalysts (Figure S1 and Table S1 of the Supporting Information). The catalytic performance for the catalysts with different particle mean sizes from 2.9 to 5.1 nm is shown in Figures 8 and 9. The catalyst with 3.8 ± 0.6 nm Au NPs showed the best catalytic performance. The catalytic activity decreased with increasing Au NPs particle size from 3.8 ± 0.6 to 5.1 ± 0.9 nm or decreasing Au NPs size from 3.8 ± 0.6 to 2.9 ± 0.6 nm. The dramatic difference in the catalyst activity due to the variation in the size of gold particle further validates the size-dependent effect of Au NPs where the catalytic performance is higher while the size of GNPs is not too big or small. 3.3. Essence of Size Effect. In order to study how the Au particle size affects the catalytic reaction, we focused on FTIR study of CO adsorption on supported Au catalysts at 20 °C. Figure 10 shows the FTIR spectra in the 2250−2050 cm−1 region of the samples interactions with CO at 20 °C. The catalyst 1.0Au/TiO2-DP catalyst was prepared by the DP

Figure 9. CO oxidation over Au/TiO2 catalysts with different Au NPs sizes. Reaction conditions: catalyst, 0.2 g; reaction temperature, 60 °C; feed gas: CO/O2/N2 = 1/1/98 (vol %); space velocity: 18 000 mL h−1 gcat−1.

method22 with 1 wt % of gold loading for comparison. In Figure 10a, a band at 2185 cm−1 was observed. Boccuzzi et al.23 E

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were also observed in the region of 2800−2950 cm−1, which can be assigned to the C−H stretching vibration. In addition, a broad peak at around 3416 cm−1 can be assigned to the stretching vibration of O−H.28 To a large extent, the functional groups, such as −C−O−C−, −CC−, −C−O−, and −CO come from the heterocyclic compounds, e.g., alkaloids, flavones, and anthracenes,26 which are water-soluble components in the CC extract, which might be the capping ligands of the Au NPs. After the calcination of Au NPs at 450 °C, the peak at 1727 cm−1, ascribed to the stretch vibration of −CO, disappeared; however, a number of vibration bands for −C−O−C−, −C− O−, −CC−, and C−H still remain on the surface of Au NPs (Figure 11b). From the TG analysis (Figure 3), it can be seen that there was still about 0.49 wt % of residual plant biomass on the surface of the Au/TiO2-10 g/L catalyst after calcination at 450 °C. The residual biomass on the Au/TiO2 catalysts was speculated to play dual roles as both Au NPs stabilizer and Au/ TiO2 catalysts surface modifier. Modification of the catalysts by the residual biomolecules changed the surface property of the catalysts,16,17,19 which might have prevented the adsorption of CO. Although there was a peak at 2109 cm−1 for the 1.0Au/ TiO2-DP catalyst, after the catalyst was pretreated in solution of CC extract no peak was observed at 2109 cm−1 (Figure 10), further demonstrating that the residual biomolecules can modify the catalysts. Admittedly, because of the complicate composition of plant biomass, the exact role of residual biomolecules on the surface of the catalysts remains unclear, and we will make some efforts on achieving this goal. We also found that the intensity of the band at 2185 cm−1 for all the three catalysts 1.0Au/TiO2-20g/L, 1.0Au/TiO2-9g/L, and 1.0Au/TiO2-5g/L was higher than that of the pure TiO2, suggesting that the Au−TiO2 contact boundary can promote the adsorption of CO onto the surface of the catalyst. The 1.0Au/TiO2-9g/L catalyst with the highest catalytic performance also had the highest peak intensity at 2185 cm−1 among the three catalysts, indicating that the size of gold particle significantly affects the Au−TiO2 interface.4 The different Au−TiO2 interface for the three catalysts is shown in Figure 12. It can been seen that the 1.0Au/TiO2-9g/L catalyst with 3.8 ± 0.6 nm Au NPs produced hemispherical Au NPs in good contact with the TiO2 support, yielding the longest perimeter interface. An increase or decrease in the Au NPs size decreases the Au−TiO2 perimeter interface. The 1.0Au/TiO2-5g/L catalyst with 5.1 ± 0.9 nm Au NPs resulted in spherical particles which were just simply loaded on the metal oxide support. The significant difference in catalytic performance between the above three catalysts in CO oxidation suggests that the reactions may take place at the perimeter interfaces around the Au NPs. CO is usually adsorbed on the surface of Au NPs29,30 in the reaction process; here we found that the Au−TiO2 contact boundary played an important role in the adsorption of CO.

Figure 10. FTIR spectra of CO absorbed on the Au/TiO2 catalysts at 20 °C in the 2250−2050 cm−1 range: (a) TiO2, (b) 1.0Au/TiO2-20 g/ L, (c) 1.0Au/TiO2-9 g/L, (d) 1.0Au/TiO2-5 g/L, (e) 1.0Au/TiO2-DP, and (f) 1.0Au/TiO2-DP-CC.

assigned the peak to CO species adsorbed on surface cations, possibly Ti4+ or Au3+ ions. Here the peak should be ascribed to linear CO species adsorbed on Ti4+ cations24 because the sample is pure TiO2. For that of 1.0Au/TiO2-DP catalyst, another band at 2109 cm−1 was seen; this could be assigned to CO chemisorbed on metallic gold sites, according to the data from previous literature.23−25 However, for the catalysts 1.0Au/ TiO2-20g/L, 1.0Au/TiO2-9g/L, and 1.0Au/TiO2-5g/L (Figure 9b−d), there was only one observable peak at 2185 cm−1, with no peak at about 2109 cm−1 indicating that CO was not adsorbed on the Au surface, totally different from previous studies.8,22 Boccuzzi et al. observed a peak at 2109 cm−1 for Au catalysts with mean particle diameters of 2.4 or 2.5 nm, but that was not in the case for catalyst with mean Au particle diameters of 10.6 nm. He thought that the gold terrace sites for the Au NPs of 10.6 nm, which constituted a large majority of the exposed gold sites, did not adsorb CO at all. This cannot explain why the Au NPs ranging from 2 to 5 nm prepared by biosynthesis method did not adsorb CO at all. From the FTIR spectrum of the obtained AuNPs synthesized using CC extract, as shown in Figure 11a, it shows several bands

Figure 11. FTIR absorption spectra of the Au NPs: (a) uncalcined and (b) calcined at 450 °C.

4. CONCLUSIONS In summary, we employed the biosynthetic method to prepare Au/TiO2 catalysts with different Au NPs size to investigate the influence of Au particle size on the as-synthesized catalysts for CO oxidation. The Au/TiO2 catalyst calcined at 450 °C showed the best catalytic performance among the ones annealed at different temperatures. On this basis, Au NPs with different sizes, tuned by CC concentration, were immobilized onto TiO2 and then pretreated at the same

located at about 1075, 1239, 1416, 1613, 1727, 2928, and 3416 cm−1 in the region 1000−4000 cm−1. The peaks at around 1075 cm−1 can be attributed to the absorption bands of −C−O− or −C−O−C−.26 The band centered at 1239 and 1416 cm−1 is due to C−H deformation vibrations. The absorption peak at about 1613 cm−1 can be assigned as the −CC− stretch vibration. The absorbance peak at 1727 cm−1 could be ascribed to the stretch vibration of −CO.27 Moreover, two bands F

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temperature (450 °C) to obtain a series of Au/TiO2 catalysts with mean Au NPs diameters ranging from 2.9 to 5.1 nm. The Au/TiO2 catalyst with 3.8 nm Au NPs mean size showed the best catalytic performance. The FTIR data revealed that CO was not adsorbed on the Au surface; this might be due to the modification of the Au/TiO2 catalyst by the residual biomass. And the significant difference in catalytic performance for the catalysts with different Au NPs sizes might be due to the different Au−TiO2 interface, caused by the size of the gold particles. We believe that this method can be applied to study the effect of Au NPs sizes on other catalytic reactions.

ASSOCIATED CONTENT

S Supporting Information *

Additional data regarding the BET analysis of Au/TiO2 catalysts with different Au NPs sizes. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

(1) Bondzie, V. A.; Parker, S. C.; Campbell, C. T. The Kinetics of CO Oxidation by Adsorbed Oxygen on Well-Defined Gold Particles on TiO2(110). Catal. Lett. 1999, 63, 143−151. (2) Okumura, M.; Nakamura, S.; Tsubota, S.; Nakamura, T.; Azuma, M.; Haruta, M. Chemical Vapor Deposition of Gold on Al2O3, SiO2, and TiO2 for the Oxidation of CO and of H2. Catal. Lett. 1998, 51, 53−58. (3) Bamwenda, G. R.; Tsubota, S.; Nakamura, T.; Haruta, M. The Influence of the Preparation Methods on the Catalytic Activity of Platinum and Gold Supported on TiO2 for CO Oxidation. Catal. Lett. 1997, 44, 83−87. (4) Tana; Wang, F. G.; Li, H. J.; Shen, W. J. Influence of Au Particle Size on Au/CeO2 Catalysts for CO Oxidation. Catal. Today 2011, 175, 541−545. (5) Wang, W. H.; Cao, G. Y. Size Effect of Gold Sol/GammaAlumina on the Catalytic Activities of CO Oxidation. Chin. J. Chem. 2006, 24, 817−821. (6) Zhang, X.; Xu, B. Q. Nano-Size Effect of Zirconia in Au/ZrO2 Catalyst for CO Oxidation. Acta Chim. Sin. 2005, 63, 86−90. (7) Lee, S. S.; Fan, C. Y.; Wu, T. P.; Anderson, S. L. CO Oxidation on Au-n/TiO2 Catalysts Produced by Size-Selected Cluster Deposition. J. Am. Chem. Soc. 2004, 126, 5682−5683. (8) Boccuzzi, F.; Chiorino, A.; Manzoli, M. Au/TiO2 Nanostructured Catalyst: Effects of Gold Particle Sizes on CO Oxidation at 90 K. Mater. Sci. Eng., C 2001, 15, 215−217. (9) Schuth, F.; Comotti, M.; Li, W. C.; Spliethoff, B. Support Effect in High Activity Gold Catalysts for CO Oxidation. J. Am. Chem. Soc. 2006, 128, 917−924. (10) Bokhimi, X.; Zanella, R.; Morales, A. Au/Rutile Catalysts: Effect of the Activation Air Flux on the Catalytic Activity for CO Oxidation and the Gold Crystallite Size. J. Phys. Chem. C 2008, 112, 12463− 12467. (11) Zanella, R.; Giorgio, S.; Shin, C. H.; Henry, C. R.; Louis, C. Characterization and Reactivity in CO Oxidation of Gold Nanoparticles Supported on TiO2 Prepared by Deposition-Precipitation with NaOH and Urea. J. Catal. 2004, 222, 357−367. (12) Lee, W. S.; Zhang, R.; Akatay, M. C.; Baertsch, C. D.; Stach, E. A.; Ribeiro, F. H.; Delgass, W. N. Differences in Catalytic Sites for CO Oxidation and Propylene Epoxidation on Au Nanoparticles. ACS Catal. 2011, 1, 1327−1330. (13) Ahmad, T.; Wani, I. A.; Ahmed, J.; Al-Hartomy, O. A. Effect of Gold Ion Concentration on Size and Properties of Gold Nanoparticles in Triton X-100 Based Inverse Microemulsions. Appl. Nanosci. 2014, 4, 491−498. (14) Wani, I. A.; Ahmad, T. Size and Shape Dependant Antifungal Activity of Gold Nanoparticles: A Case Study of Candida. Colloids Surf., B 2013, 101, 162−170. (15) Ahmad, T.; Wani, I. A.; Manzoor, N.; Ahmed, J.; Asiri, A. M. Biosynthesis, Structural Characterization and Antimicrobial Activity of gold and Silver Nanoparticles. Colloids Surf., B 2013, 107, 227−234. (16) Zhan, G. W.; Du, M. M.; Huang, J. L.; Li, Q. B. Green Synthesis of Au/TS-1 Catalysts via Two Novel Modes and Their Surprising Performance for Propylene Epoxidation. Catal. Commun. 2011, 12, 830−833. (17) Du, M. M.; Zhan, G. W.; Yang, X.; Wang, H. X.; Lin, W. S.; Zhou, Y.; Zhu, J.; Lin, L. Q.; Huang, J. L.; Sun, D. H.; et al. Ionic Liquid-Enhanced Immobilization of Biosynthesized Au Nanoparticles on TS-1 Toward Efficient Catalysts for Propylene Epoxidation. J. Catal. 2011, 283, 192−201. (18) Zhan, G. W.; Hong, Y. L.; Mbah, V. T.; Huang, J. L.; Ibrahim, A. R.; Du, M. M.; Li, Q. B. Bimetallic Au-Pd/MgO as Efficient Catalysts for Aerobic Oxidation of Benzyl Alcohol: A Green Bio-Reducing Preparation Method. Appl. Catal., A 2012, 439, 179−186. (19) Zhan, G. W.; Du, M. M.; Sun, D. H.; Huang, J. L.; Yang, X.; Ma, Y.; Ibrahim, A. R.; Li, Q. B. Vapor-Phase Propylene Epoxidation with H2/O2 over Bioreduction Au/TS-1 Catalysts: Synthesis, Characterization, and Optimization. Ind. Eng. Chem. Res. 2011, 50, 9019−9026.

Figure 12. TEM images (high resolution) and corresponding diagrams of the Au/TiO2 catalysts synthesized using CC at different biomass concentrations: (A, D) 1.0Au/TiO2-5 g/L, (B, E) 1.0Au/TiO2-9 g/L, and (C, F) 1.0Au/TiO2-20 g/L.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]; Tel (+86) 592-2183088; fax (+86)592-2184822 (D.S.). *E-mail [email protected]; Tel (+86) 592-2189595; fax (+86) 592-2184822 (Q.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Nature Science Foundation (21036004, 21206140), the fund of the State Key Laboratory of Advanced Technologies for Comprehensive Utilization of Platinum Metals (SKL-SPM-201210), and the outstanding PhD program of Xiamen University (2012). G

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(20) Vilchis-Nestor, A. R.; Avalos-Borja, M.; Gomez, S. A.; Hernandez, J. A.; Olivas, A.; Zepeda, T. A. Alternative Bio-reduction Synthesis Method for the Preparation of Au(AgAu)/SiO2-Al2O3 Catalysts: Oxidation and Hydrogenation of CO. Appl. Catal., B 2009, 90, 64−73. (21) Tsubota, S.; Nakamura, T.; Tanaka, K.; Haruta, M. Effect of Calcination Temperature on the Catalytic Activity of Au Colloids Mechanically Mixed with TiO2 Powder for CO Oxidation. Catal. Lett. 1998, 56, 131−135. (22) Boccuzzi, F.; Chiorino, A.; Manzoli, M.; Lu, P.; Akita, T.; Ichikawa, S.; Haruta, M. Au/TiO2 Nanosized Samples: A catalytic, TEM, and FTIR Study of the Effect of Calcination Temperature on the CO Oxidation. J. Catal. 2001, 202, 256−267. (23) Boccuzzi, F.; Chiorino, A. FTIR study of CO Oxidation on Au/ TiO2 at 90 K and Room Temperature. An Insight into the Nature of the Reaction Centers. J. Phys. Chem. B 2000, 104, 5414−5416. (24) Derrouiche, S.; Gravejat, P.; Bianchi, D. Heats of Adsorption of Linear CO Species Adsorbed on the Au-Circle and Ti+Delta Sites of a 1% Au/TiO2 Catalyst Using in Situ FTIR Spectroscopy under Adsorption Equilibrium. J. Am. Chem. Soc. 2004, 126, 13010−13015. (25) Chang, B. K.; Jang, B. W.; Dai, S.; Overbury, S. H. Transient Studies of the Mechanisms of CO Oxidation over Au/TiO2 Using Time-Resolved FTIR Spectroscopy and Product Analysis. J. Catal. 2005, 236, 392−400. (26) Huang, J. L.; Li, Q. B.; Sun, D. H.; Lu, Y. H.; Su, Y. B.; Yang, X.; Wang, H. X.; Wang, Y. P.; Shao, W. Y.; He, N.; et al. Biosynthesis of Silver and Gold Nanoparticles by Novel Sundried Cinnamomum Camphora Leaf. Nanotechnology 2007, 18. (27) Huang, J. L.; Liu, C.; Sun, D. H.; Hong, Y. L.; Du, M. M.; Odoom-Wubah, T.; Fang, W. P.; Li, Q. B. Biosynthesized Gold Nanoparticles Supported over TS-1 toward Efficient Catalyst for Epoxidation of Styrene. Chem. Eng. J. 2014, 235, 215−223. (28) Zhang, G. W.; Du, M. M.; Li, Q. B.; Li, X.; Huang, J. L.; Jiang, X. D.; Sun, D. H. Green Synthesis of Au-Ag Alloy Nanoparticles using Cacumen Platycladi Extract. RSC Adv. 2013, 3, 1878−1884. (29) Haruta, M. Size- and Support-Dependency in the Catalysis of Gold. Catal. Today 1997, 36, 153−166. (30) Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M. J.; Delmon, B. Low-Temperature Oxidation of CO over Gold Supported on TiO2-γ-Fe2O3, and Co3O4. J. Catal. 1993, 144, 175−192.

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