Aerobic Oxidation of d-Glucose on Support-Free Nanoporous Gold

Jun 10, 2008 - Nanoporous gold (NPG) catalysts, made by dealloying Ag/Au alloys, were found to be novel unsupported Au nanocatalysts that exhibited ...
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J. Phys. Chem. C 2008, 112, 9673–9678

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Aerobic Oxidation of D-Glucose on Support-Free Nanoporous Gold Huiming Yin, Cunqi Zhou, Caixia Xu, Pengpeng Liu, Xiaohong Xu,* and Yi Ding* Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong UniVersity, Jinan 250100, China ReceiVed: March 6, 2008; ReVised Manuscript ReceiVed: April 28, 2008

Nanoporous gold (NPG) catalysts, made by dealloying Ag/Au alloys, were found to be novel unsupported Au nanocatalysts that exhibited effective catalytic activity and high selectivity (∼99%) for the aerobic oxidation of D-glucose to D-gluconic acid under mild conditions. Systematic studies have been carried out to discuss this new catalytic system, including the activity dependence as functions of pH value, temperature and NPG ligament size, reaction active sites, and reaction kinetics. The possible contribution from the residual Ag atoms trapped in the NPG ligaments was also discussed, which turned out to be unfavorable for the glucose oxidation. The unexpected observation of the catalytic activity from NPG with a ligament size as large as 60 nm indicated that the low-coordinated surface Au atoms should be the reaction active sites for glucose oxidation. 1. Introduction Selective catalytic oxidation of D-glucose with molecular oxygen is an environmentally benign process to produce D-gluconic acid which is widely used in the food, detergent, and pharmaceutical industries. The procedures based on supported palladium or platinum catalysts have been investigated intensively in the past years.1–5 However, these catalysts often suffer from drawbacks of low catalyst durability and relatively low selectivity, which stimulates the exploration of new catalytic systems that possess excellent activity and durability under mild reaction conditions. The discovery of nanogold having catalytic activity for many important reactions offers an alternative strategy for the oxidation of D-glucose to D-gluconic acid with molecular oxygen.6 Many studies have recently been devoted to employ Au as catalysts to achieve selective aerobic oxidation of D-glucose under mild conditions, and superior selectivity, high catalytic activity, and long-term stability were observed.7–11 Being highly active, the Au catalysts need to be effectively dispersed as nanoparticles supported on carbon black or metal oxides, or as colloid forms protected by surfactants.12 Comotti et al. recently reported that “naked” Au nanoparticles (∼3.6 nm) exhibited interesting catalytic activity with an initial turnover frequency of ∼50 120 h-1, which is comparable to that of enzymatic systems.11 They also observed that, for particles ranging from 3 to 6 nm, there was a good correlation between activity and particle size, which indicated that only surface Au atoms were associated with the glucose oxidation reaction. However, these unprotected Au colloids were so active that they could maintain catalytic activity for only a few minutes, after which their performance quickly declined as a result of particle aggregation. Recently, we reported that nanoporous gold (NPG), made by dealloying of Ag from Ag/Au alloys, may work as a new type of high surface area Au nanocatalyst for CO oxidation.13,14 Possessing a three-dimensional spongy morphology with tunable pore and ligament sizes at nanometer scale,15 NPG has a series of unique characteristics as compared with supported Au catalysts: (i) it is bulk in nature yet nanoscale in microstructure, * Corresponding author. E-mail: [email protected] (Y.D.); xhxu@ sdu.edu.cn (X.X.).

which means they can be easily employed and recovered; (ii) its structural unit is tunable in a wide range from a few nanometers to many microns, which allows the study of the size dependence very easily; (iii) processed in concentrated nitric acid, NPG has extremely clean surfaces, which rules out the possible poisoning or passivating effects from unwanted molecules or ions such as polymer surfactants and Cl-, as often seen in other Au nanocatalysts; (iv) being truly unsupported and nanostructured, NPG completely avoids the particle aggregation and the possible contributions from the catalyst supports. Indeed, more recent studies have indicated that NPG can work as a very effective catalyst for many important reactions, such as low-temperature CO oxidation, methanol electro-oxidation, and reduction of oxygen and hydrogen peroxide.16–18 In this paper, we report on the aerobic oxidation of glucose over NPG. The catalytic performance of NPG for glucose oxidation was investigated in detail as a function of reaction temperature, pH value, NPG ligament size, and residual Ag atoms in the structure. The active sites of NPG catalysts and reaction kinetics were also discussed. 2. Experimental Section 2.1. Preparation and Characterization of NPG Catalysts. NPG catalysts were fabricated by selective dissolution (dealloying) of Ag from Ag/Au alloys, which was similar to that reported in previous work.10,11 Before dealloying, Ag/Au foils (58:42, wt %, 25 µm thick) were annealed at 1123 K for 20 h and subsequently degreased by sonicating in acetone and rinsed with ultrapure water several times. NPG catalysts with different sizes were obtained using various methods. NPG with a ligament size of ∼6 nm was obtained by electrochemical dealloying in concentrated nitric acid (∼67%) under constant potential (350 mV vs reversible hydrogen electrode (RHE)) for 900 s on a CHI 1130 potentiostat. Thirty-nanometer NPG was prepared by immersing Ag/Au foils into concentrated nitric acid at 303 K for 24 h (free corrosion). NPG with even larger ligament sizes (60 and 250 nm) were made by annealing the free corroded NPG samples at 473 and 573 K for 1 h, respectively. NPG catalysts with different Ag residual contents were achieved by free corrosion at 303 K for a desired period of time. All NPG

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9674 J. Phys. Chem. C, Vol. 112, No. 26, 2008 samples were washed to neutral with ultrapure water and dried in a vacuum oven at room temperature for 12 h before utilization. The morphologies of NPG before and after catalytic reaction were observed by a field emission scanning electron microscope (FESEM, JEOL JSM-6700F), equipped with an Oxford INCA x-sight energy-dispersive X-ray spectrometer (EDS) for compositional analysis. 2.2. Oxidation of D-Glucose. D-Glucose oxidation experiments were carried out in a Pyrex glass reactor (200 mL), within which a ring shaped polytetrafluoroethylene pipe was laid at the bottom, from which a constant flux oxygen gas was bubbled through tiny holes on the pipe-wall into the reaction mixture. The pH value was controlled by a ZD-2A automatic potentiometric titrator using 0.3 M NaOH as the titration reagent. In a typical run, NPG catalysts were first crushed into fragments and then ultrasonically dispersed into micron-scale grains. Then 80 mL 0.1 M D-Glucose solution and 20 mg of NPG catalyst were charged into the reactor. The stirred mixture was kept at the set temperature and fixed pH value, with the oxygen gas bubbling into the suspension from the bottom of the reactor at a constant flow rate of 100 cm3 min-1 at atmospheric pressure. The reaction mixture (1 mL) was sampled every hour and subsequently filtered through a 0.45 µm Millipore filter to remove the catalyst grains. The samples were analyzed on a Shimadzu LT-20AT HPLC, equipped with a 20A UV (210 nm) detector and a 10A refractive index detector. A Hypersil APS-25 µm NH2 column (Thermo, 250 × 4.6 mm) was used with aqueous 0.02 M H3PO4 (pH 2) as the eluent (flow rate 1 mL min-1).5 The absence of isomerization of D-glucose to fructose and other byproduct after reaction with or without NPG catalysts was confirmed by employing high-performance liquid chromatography (HPLC) analysis, which revealed a selectivity value better than 99% for the oxidation of D-glucose to D-gluconic acid under the present reaction conditions. The parallel control experiments without catalyst were also performed, and, only at pH 9 (323 K), D-glucose conversion of ∼1% was detected after reaction for 7 h. By subtracting these values, all data presented here can be considered as the net contribution from the NPG catalysts. 3. Results and Discussion 3.1. Effect of pH Value. Previous studies on the supported Au catalysts have proved their superior catalytic performance for D-glucose oxidation by having better selectivity and higher activity to form D-gluconic acid as compared to traditional platinum- or palladium-based catalysts. They are also less sensitive to chemical poisoning and can exhibit activity over a wide pH range (7-9.5).7 In our test, NPG catalysts also exhibited very high (>99%) selectivity toward D-gluconic acid at pH 7-9, and no byproduct such as fructose was detected by HPLC. Figure 1a illustrates the pH value dependence of D-glucose oxidation catalyzed by 30 nm NPG at 323 K. Analogous to the supported Au catalysts,19 the D-glucose oxidation is favored under alkaline conditions over NPG. At pH 7, one observes a sharp increase of the apparent conversion to over 5% in less than one hour, after which the reaction slows down and eventually saturates at a conversion around 9%. At pH 8, the catalysts show similar activity toward D-glucose oxidation during the first hour, and can reach ∼18% conversion after 7 h. This effect is even more pronounced at pH 9, where the NPG catalysts can still remain active at a conversion of ∼25%. Similar behavior was observed from NPG catalysts with smaller sizes. As expected, the 6 nm NPG catalyst is significantly more active than the 30 nm one. In particular, in the neutral medium (pH 7), this smaller size NPG catalyst can reach a conversion of ∼33%.

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Figure 1. The catalytic activity of (a) 30 nm and (b) 6 nm NPG at different pH values. The measurements were carried out using a 20 mg catalyst at 323 K.

Figure 2. The catalytic activity of 30 nm NPG under different reaction temperatures at pH 9. The measurement was performed using 20 mg catalyst in the temperature range of 283-323 K.

Reaction at a higher pH value of 9 results in a 78% conversion to D-gluconic acid (Figure 1 b). 3.2. Effect of Reaction Temperature. The effect of reaction temperature on the oxidation of D-glucose was evaluated at pH 9 using 30 nm NPG catalysts. As expected, increasing the temperature dramatically accelerates the reaction (Figure 2). At 283 K, the reaction proceeds very slowly, whereas, at 303 K, the reaction is much faster, and a 2-fold increase in reaction rate is observed during the first hour. Further increasing the temperature to 323 K, the reaction rate can increase for another factor of 2, and after just 1 h of reaction, the conversion can reach nearly 10%. It needs to be emphasized that under all these conditions, HPLC analysis confirmed a high selectivity (>99%) toward D-gluconic acid. Considering that previous investigations on supported Au catalysts have revealed that higher temperature (T > 323 K) and strong alkaline conditions (pH > 9.5) would

Aerobic Oxidation of D-Glucose on Support-Free NPG

Figure 3. SEM images of NPG samples with different ligament sizes. (a) electrochemically dealloyed at 350 mV for 900s; (b) free corroded at 303 K for 24 h; (c) free corroded samples annealed at 473 K and (d) 573 K for 1 h.

Figure 4. Catalytic activity of NPG catalysts with different ligament sizes (6, 30, 60, and 250 nm). Experiments were carried out at 323 K and pH 9 using 20 mg catalyst.

result in side reactions,7,19 the typical reaction conditions taken in this work were chosen to be 323 K and pH 9, unless otherwise mentioned. 3.3. Catalytic Activity of NPG with Different Size. The FESEM images in Figure 3 illustrate four different NPG samples with ligament sizes of about 6, 30, 60, and 250 nm, which characterize a uniform bicontinuous porous structure. Figure 4 shows typical catalytic performances of these NPG samples for D-glucose oxidation at 323 K in alkaline solution (pH 9). It is generally recognized that the catalytic properties of Au strongly depend on the particle size, and only particles with size smaller than 10 nm are catalytically active.20,21 For instance, Rossi et al. reported that both carbon-supported and naked colloid Au nanoparticles with a mean diameter of 3.6 nm exhibited very high activity in converting D-glucose to D-gluconic acid.11 Unfortunately, the unsupported Au colloids rapidly deactivated within several hundred seconds, which was attributed to the increasing of particle size over 10 nm due to the agglomeration of Au nanocrystallites. To improve the catalytic performance, catalyst supports such as carbon black are needed to stabilize the structure and activity of colloidal Au nanoparticles. In our test, we have also observed the prominent size effect on the catalytic activity for D-glucose oxidation, i.e., 6 nm NPG sample shows activity superior to that of the other three samples. Although this activity is still lower than that for supported or unsupported Au nanoparticles, considering the slower kinetic

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Figure 5. SEM images of NPG sample before (a,c) and after (b,d) catalytic reactions. Images a and b were photographed from 6 nm NPG, while c and d were from 30 nm NPG. The experiments were carried out at 323 K and pH 9 using 20 mg NPG.

for the mass transfer into the pores and relatively lower surface area-to-volume ratio (∼30 m2 g-1) as compared to Au nanoparticles with size around 3 nm, it is very impressive that 6 nm NPG was able to convert 80% of D-glucose into D-gluconic acid without an apparent loss of activity after 7 h. On the other hand, being less active, 30 and 60 nm NPG samples still show catalytic capability for this reaction, although their structure sizes are much larger than the traditional threshold of 10 nm. Finally, the NPG with the ligament size of 250 nm is almost inactive. The inverse relation between catalytic activity and ligament size of NPG indicates that the low-coordinated surface Au atoms are the possible reaction active sites for glucose oxidation. 3.4. Catalytic Stability of NPG. NPG catalysts, composed of micron-scale grains and nanometer-scale pores/ligaments, show greatly enhanced catalytic stability, which motivated us to observe the structure evolution of NPG during the catalytic reaction. Figure 5 illustrates the FESEM images of two NPG samples (6 and 30 nm) before and after 7-h D-glucose oxidation. It is very apparent that, over the same period of reaction time, the ligament size for the first sample quickly increases from 6 to about 15 nm, while for the 30 nm sample, there is almost no obvious change in structure. For this reason the 30 nm NPG catalyst was recycled by removing the reaction solution and washing in ultrapure water, and then used in a new cycle of glucose oxidation. Figure 6 shows the catalytic activity of recycled NPG compared to the freshly prepared one. The little difference in converion rate indicates that the NPG catalyst well retains its activity during the reaction. While reconstruction of catalyst surface is a common phenomenon, this effect is significantly less severe in NPG as compared to that in nanoparticles. This can be understood by the fact that, in spongy structures such as NPG, the coarsening process is mainly governed by the diffusion of surface atoms,15 while, in traditional catalysts, the active nanoparticles can undergo a much more complicated sintering process such as ripening, migration and coalescence.22 Furthermore, the coarsening of a surface-clean porous structure such as NPG will generally generate a selfsimilar bicontinuous framework as seen in Figure 5, which may explain that in situ coarsened or ex situ coarsened NPG samples can all exhibit very effective catalytic activities in glucose oxidation. We have also explored the stability of NPG catalysts upon room temperature aging. Figure 7 compares the activity of a freshly prepared 30 nm NPG catalyst and that of another

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Figure 6. Catalytic activity of recycled NPG catalyst compared to freshly prepared one. Experiments were operated at 323 K and pH 9 using 20 mg NPG catalyst with ligament size of 30 nm.

Figure 7. Effect of storage time on the catalytic activity of NPG. Experiments were operated at 323 K and pH 9 using 20 mg NPG catalyst with ligament size of 30 nm.

sample that has been stored at room temperature for 15 days. While FESEM observation did not reveal obvious morphological change under this condition, we were surprised to see that the long-term stored NPG exhibited almost the same activity for D-glucose oxidation as the freshly prepared one, especially when one considers that no special preactivation process is required in using this kind of catalyst. The deactivation of Au catalyst in D-glucose oxidation process is usually ascribed to the strong interaction of the active sites with free D-gluconic acid molecules on surface. Recently, based on the density functional calculations, Hvolbæk et al. found that the activity of a Au catalyst mainly relies on the low-coordinated Au atoms, i.e., the atoms on the corners and edges of Au nanoparticles.23 More recently, Zeis et al. have investigated the catalytic reduction of oxygen and hydrogen peroxide over NPG, and they also suggested that the active sites of NPG are Au atoms located in the crystal surface defect sites.18 In NPG, the abundant highly curved nanoscale ligaments provide a rich source of active surface sites, such as step edges. Additionally, the smaller the ligament size, the more the step edge sites and the larger the total surface area. The strong dependency of the catalytic activity on the size of NPG with respect to D-glucose oxidation as shown in Figure 3 is also consistent with our previous observations that 6 nm NPG was much more active for methanol electro-oxidation and CO oxidation than samples with larger ligament sizes.13,14,16 Taking full consideration of all these unique structure features that NPG has, it is reasonable to find that NPG has much enhanced catalytic stability as compared to Au nanoparticles, even at pH 7 where there exists a great amount of free D-gluconic acid molecules.

Yin et al.

Figure 8. Initial reaction rates of gluconic acid formation at different NPG concentrations with respective ligament sizes of 6 and 30 nm. Catalytic experiments were performed at 323 K and pH 9.

Figure 9. Plot of glucose conversion rates versus initial glucose concentrations after reacting for 1 h. Catalytic experiments were performed at 323 K and pH 9 using 10 mg NPG catalyst with respective ligament size of 6 and 30 nm.

3.5. Reaction Kinetics. Previous to the kinetic measurements, the effect of external mass transfer was removed by optimizing the stirring rate and the flow rate of oxygen gas. In order to characterize the kinetics of D-glucose oxidation catalyzed by NPG, a series of experiments with different concentrations of catalyst were carried out at 323 K and pH 9. Figure 8 represents the conversion rates of D-glucose in the first hour of reaction. A nearly linear correlation between Au concentration and catalytic activity was observed for both NPG catalysts, and, as expected, NPG with a smaller ligament size of ∼6 nm is more active because it contains a greater amount of active sites on surface. Moreover, the result shows the D-glucose oxidation proceeded with a similar initial rate at low NPG concentration (6 × 10-4 M). Testing the catalytic dependence on substrate concentration (Figure 9) generates very similar response for both catalysts, and an optimal glucose concentration range was found to be between 0.2 and 0.3 mol L-1 at pH 9 and 323 K. Liquid phase oxidation of alcohols and aldehydes is usually considered to undergo an oxidehydrogenation mechanism.19,24 During this process, the classical kinetic model, the LangmuirHinshelwood-type model, can be used to account for the D-glucose oxidation kinetics catalyzed by Au. For highly stirred and mixed systems, where the external diffusion can be ruled out, the whole D-glucose oxidation reaction over Au/C catalysts is suggested to involve three elementary reactions, namely, adsorption of D-glucose, surface reaction of D-glucose, and desorption of D-gluconic acid.19 In the case of NPG, the internal

Aerobic Oxidation of D-Glucose on Support-Free NPG diffusion of substrate and product in pores should be taken into consideration. Obviously, at low pH, desorption of D-gluconic acid should control the overall reaction rate, i.e., the selfpoisoning should be responsible for the deactivation of NPG catalyst. With the pH increased to 9, desorption of D-gluconic acid is greatly accelerated as a result of the fast deprotonation. Consequently, the rate-determining step appears to not be limited by desorption of D-gluconic acid. According to Figures 8 and 9, by using NPG with a ligament size of either 6 or 30 nm, the initial rate increases with both catalyst amount and substrate concentration. Considering the high viscosity of solution and the size of glucose molecules (radius ∼0.36 nm), we are inclined to suggest that the overall reaction rate should be determined by the internal diffusion in pores and the adsorption of D-glucose on the catalyst surface, especially for smaller pore size samples in alkaline medium. When the initial glucose concentration increases, the internal diffusion and adsorption rate of glucose molecules will be enhanced because of the increasing of the concentration gradient, which results in an increase of the overall reaction rate. On the other hand, increasing the glucose concentration would also lead to higher viscosity25 which should subsequently hinder the diffusion of glucose molecules in pores. Therefore, the glucose oxidation in this system has an optimum concentration in the median range, as depicted in Figure 9. 3.6. Effects of Residual Ag in the NPG Catalysts. The NPG catalysts used in our test were prepared by selective etching of Ag from Ag/Au alloys. Theoretically, there always exists a certain amount of Ag atoms trapped within Au-rich ligaments, and the amount of the residual Ag depends on the etching conditions. For NPG samples discussed above, energy-dispersive X-ray (EDX) analysis also revealed a tiny amount of residual Ag, typically ∼1 wt %. Regarding the possible contribution of silver to gold catalysis, Wang et al. recently reported in a series of research that Ag could remarkably enhance the activity of supported-Au with respect to CO oxidation,26,27 especially when the nominal Au/Ag ratio in nanoparticles approaches 3:1. Ag was suggested to play a synergistic role in improving the adsorption and activation of oxygen.27 In a more recent review on Au catalysis, Haruta suspected that the high catalytic activity of NPG for CO oxidation might be attributed to the interaction between Au and residual Ag surface atoms, which can be easily oxidized into Ag2O.28 In order to evaluate the role of residual Ag, the D-glucose oxidation experiments were carried out over NPG with different Ag contents at 323 K and pH 9. It should be emphasized that the displayed plots have been normalized to 20 mg Au in order to have a clearer sense of whether and how Ag contributes to this reaction. The residual Ag contents in the NPG catalysts were controlled by varying the dealloying time. Figure 10 shows the FESEM images of NPG formed by dealloying for 8, 16, and 32 min, from which the corresponding ligament sizes of 14, 16, and 20 nm could be observed, respectively. The residual Ag contents were 37.9, 12.8, and 6.8 wt %, decreasing as the dealloying time increased. Figure 11 illustrates the performance of Dglucose oxidation over these nanoporous Au/Ag alloy catalysts. It is very obvious that, although the 8 min NPG sample (37.9 wt % Ag) has the smallest ligament size (14 nm) and therefore the largest surface area among those three samples, it is the least active for D-glucose oxidation. And the catalytic activity of NPG was remarkably improved with the decrease in Ag content in spite of the increasing ligament size. For the 32 min NPG (6.8 wt % Ag, 20 nm), the conversion can reach ∼35%, which is higher than that of 24 h NPG. But considering that the 32 min sample has a surface area almost twice of that of

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Figure 10. FESEM images of NPG samples upon free dealloying for (a) 8, (b) 16, and (c) 32 min. The corresponding ligament sizes and residual Ag contents are 14, 16, and 20 nm, and 37.9, 12.8, and 6.8 wt %, respectively.

Figure 11. Catalytic activity of NPG with different Ag contents. The experiments were carried out using 20 mg catalyst at 323 K and pH 9 with Ag contents ranging from 6.8 to 37.9 wt %, and the shown data have been normalized to 20 mg Au for fair comparison.

the 24 h NPG (30 nm), its effective catalytic activity is lower than those of NPG samples with no or little Ag residuals. Therefore, it can be concluded that the residual Ag atoms in NPG ligaments do not contribute to the observed activity for the catalytic oxidation of D-glucose. This further suggests that the Au atoms on the corners and edges of NPG may play a central role as the active sites in D-glucose oxidation reaction. 4. Conclusions NPG catalysts were fabricated by a simple dealloying method with different ligament sizes and Ag residual contents. This unsupported Au was found to be active and highly selective for the aerobic oxidation of D-glucose to D-gluconic acid, even when the ligament sizes are larger than 10 nm. NPG catalyst with a ligament size of 6 nm exhibits the highest catalytic activity and is more resistant to deactivation. Additionally, the 30 nm sample was found to have better structure stability during the whole catalytic process, while keeping decent catalytic

9678 J. Phys. Chem. C, Vol. 112, No. 26, 2008 activity. The presence of residual Ag atoms does not seem to contribute to the activity of NPG for glucose oxidation. On the basis of the above results, we suggest that the active sites of NPG are Au atoms on the corners and step edges. The investigation on reaction kinetics suggests that internal diffusion in pores as well as the adsorption of glucose molecules determine the overall reaction rate on NPG. Finally, although the catalytic activity of NPG can not match that of Au nanoparticles due to their completely different structures and reaction configurations, NPG holds additional advantages as an catalyst, in a way that it can be easily prepared, recovered, and recycled. Their structural continuity and excellent electric conductivity also allow surface functionalization with other materials to design and develop novel devices such as membrane reactors for industrial applications. Acknowledgment. This work was supported by the National Science Foundation of China (NSFC 20776079), the National 863 (2006AA03Z222) and 973 (2007CB936602) Program Projects of China, and the Natural Science Foundation of Shandong Province (2007ZRB01117, Y2004F04, 2006BS04018). Y.D. is a Tai-Shan Scholar supported by the SEM-NCET and SRF-ROCS Programs. References and Notes (1) Mallat, T.; Baiker, A. Chem. ReV. 2004, 104, 3037. (2) Muzart, J. Tetrahedron 2003, 59, 5789. (3) Karski, S.; Witon˜ska, I. J. Mol. Catal. A: Chem. 2003, 191, 87. (4) Wenkin, M.; Renard, C.; Ruiz, P.; Delmon, B.; Devillers, M. Stud. Surf. Sci. Catal. 1997, 110, 517. (5) Nikov, I.; Paev, K. Catal. Today 1995, 24, 41.

Yin et al. (6) Murzina, E. V.; Tokarev, A. V.; Krisztia´n, K.; Karhu, H.; Mikkola, J. P.; Murzin, D. Y. Catal. Today 2008, 131, 385. (7) Biella, S.; Prati, L.; Rossi, M. J. Catal. 2002, 206, 242. (8) Mirescu, A.; Prusse, U. Catal. Commun. 2006, 7, 11. (9) Beltrame, P.; Comotti, M.; Pina, C. D.; Rossi, M. Appl. Catal. A: Gen. 2006, 297, 1. (10) Comotti, M.; Pina, C. D.; Rossi, M. J. Mol. Catal. A: Chem. 2006, 251, 89. (11) Comotti, M.; Pina, C. D.; Matarrese, R.; Rossi, M. Angew. Chem., Int. Ed. 2004, 43, 5812. (12) Porta, F.; Rossi, M. J. Mol. Catal. A: Chem. 2003, 204, 553. (13) Xu, C. X.; Su, J. X.; Xu, X. H.; Liu, P. P.; Zhao, H. J.; Tian, F.; Ding, Y. J. Am. Chem. Soc. 2007, 129, 42. (14) Xu, C. X.; Xu, X. H.; Su, J. X.; Ding, Y. J. Catal. 2007, 252, 243. (15) Ding, Y.; Kim, Y. J.; Erlebacher, J. AdV. Mater. 2004, 16, 1897. (16) Zhang, J. T.; Liu, P. P.; Ma, H. Y.; Ding, Y. J. Phys. Chem. C 2007, 111, 10382. (17) Zielasek, V.; Ju¨rgens, B.; Schulz, C.; Biener, J.; Biener, M. M.; Hamza, A. V.; Ba¨umer, M. Angew. Chem., Int. Ed. 2006, 45, 1. (18) Zeis, R.; Lei, T.; Sieradzki, K.; Snyder, J.; Erlebacher, J. J. Catal. 2008, 253, 132. ¨ nal, Y.; Schimpf, S.; Claus, P. J. Catal. 2004, 223, 122. (19) O (20) Haruta, M. Catal. Today 1997, 36, 153. (21) Haruta, M.; Date, M. Appl. Catal. A: Gen. 2001, 222, 427. (22) Liu, R.; Crozier, P. A.; Smith, C. M.; Hucul, D. A.; Blackson, J.; Salaita, G. Microsc. Microanal. 2004, 10, 77. (23) Hvolbæk, B.; Janssens, T. V. W.; Clausen, B. S.; Falsig, H.; Christensen, C. H.; Nørskov, J. K. Nanotoday 2007, 2, 14. (24) Mallat, T.; Baiker, A. Catal. Today 1994, 19, 247. (25) Converti, A.; Zilli, M.; Arni, S.; Felice, R. D.; Borghi, M. D. Biochem. Eng. J. 1999, 4, 81. (26) Wang, A. Q.; Hsieh, Y. P.; Chen, Y. F.; Mou, C. Y. J. Catal. 2006, 237, 197. (27) Liu, J. H.; Wang, A. Q.; Chi, Y. S.; Lin, H. P.; Mou, C. Y. J. Phys. Chem. B 2005, 109, 40. (28) Haruta, M. Chem. Phys. Chem. 2007, 8, 1911.

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