Influence of Supported Gold on the Dynamics of Reduction and

Oct 22, 2011 - CIEP, Facultad de Ciencias Químicas, Universidad Autónoma de San Luis Potosí, San Luis Potosí, Mexico, 78210. §. Departamento de ...
0 downloads 0 Views 2MB Size
Download PDF
ARTICLE pubs.acs.org/JPCC

Influence of Supported Gold on the Dynamics of Reduction and Crystallization of Iron Oxides: A Dispersive X-ray Absorption Near Edge Structure Spectroscopy and X-ray Diffraction Study Sergio A. Jimenez-Lam,† María G. Cardenas-Galindo,‡ Brent E. Handy,‡ Sergio A. Gomez,§ Gustavo A. Fuentes,§ and Juan C. Fierro-Gonzalez*,† †

Departamento de Ingeniería Química, Instituto Tecnologico de Celaya, Celaya, GTO, Mexico, 38010 CIEP, Facultad de Ciencias Químicas, Universidad Autonoma de San Luis Potosí, San Luis Potosí, Mexico, 78210 § Departamento de Ingeniería de Procesos e Hidraulica, Universidad A. Metropolitana—Iztapalapa, A.P. 55-534, Mexico, DF, Mexico, 09340 ‡

bS Supporting Information ABSTRACT: Iron oxide-supported gold samples were characterized by X-ray absorption near edge structure (XANES) spectroscopy during treatments in flowing H2 at increasing temperature. Spectra were recorded at the Au LIII and Fe K edges to monitor the reduction of both metals and to determine the influence of gold on the reducibility of the support. The results show that reduction of Fe3+ to Fe2+ on the support occurs at lower temperatures in samples containing gold than on samples of the bare support, with the reduction temperature being dependent on the gold content. X-ray diffraction patterns characterizing samples after H2 treatments at various temperatures complement the XANES data and indicate that the presence of gold favors the crystallization of the support to give Fe3O4. Our data emphasize the power of XANES spectroscopy in following changes in the oxidation states of both gold and iron and suggest that the role that gold might have in promoting the reduction and crystallization of iron oxide support is to provide sites for hydrogen dissociation. Hydrogen moieties might spillover from the gold nanoparticles to the support, promoting its reduction and ensuing structural changes.

’ INTRODUCTION Supported gold catalysts have recently attracted attention because they are active for many industrially relevant chemical reactions and because their catalytic properties were unexpected, as gold is the most inert metal in its bulk form. Among the many reactions catalyzed by supported gold, the water gas shift1 3 and the oxidations of carbon monoxide4 6 and alcohols7 9 have been widely investigated. For these reactions, it has been observed that the catalysts are typically more active when the gold particles are dispersed on reducible metal oxides (e.g., Fe2O3,4,10,11 TiO2,5,12,13 CeO2,1,2,14 La2O3,5,15 etc.16) than when they are on nonreducible metal oxides (e.g., γ-Al2O3,6 MgO,17 SiO2,18 etc.16). This observation has led some authors19 21 to conclude that the interaction between the gold and the support is important in determining the activity of the catalysts, with the existence of a synergistic effect between the gold and reducible metal oxides at the gold-support interface. It has been hypothesized that such effect might involve redox processes, in which the gold particles favor the reduction of the metal oxide, thus allowing lattice oxygen atoms from the support to become activated species available for the oxidation reactions.19 21 r 2011 American Chemical Society

Various authors22 24 have used H2-temperature-programmed reduction (H2-TPR) to investigate how the presence of gold particles on iron oxide affects the reduction of iron. From H2-TPR data it has been concluded that the presence of gold lowers the temperature at which the reduction of Fe3+ occurs, but H2-TPR techniques have various limitations. Typically, the consumption of H2 is interpreted as evidence of reduction of metals (either from the supported particles or from the support), but this interpretation has the limitation that the consumption of H2 might not necessarily be caused only by reduction processes of those metals, but it could also be related to the reaction of H2 with any surface species. Indeed, it has been reported that residual species such as carbonate, nitrate and chlorine ions react with hydrogen during H2-TPR experiments.25 27 Moreover, when the catalysts to be reduced under H2 at increasing temperature are prepared at low temperatures (as it is the case for supported gold catalysts prepared by coprecipitation), adsorbed Received: September 15, 2011 Revised: October 20, 2011 Published: October 22, 2011 23519

dx.doi.org/10.1021/jp208936r | J. Phys. Chem. C 2011, 115, 23519–23526

The Journal of Physical Chemistry C species may leave the catalyst surface during the temperature ramp, thus affecting the TCD signal.25,26 Furthermore, for some iron oxide-supported gold samples it has been observed that H2-TPR peaks attributed to the reduction of Fe3+ and Au3+ overlap, complicating the interpretation of the data.22 24,28 Because of the limitations of TPR, there is a motivation to use spectroscopic methods to obtain specific information about the oxidation states of both gold and iron. X-ray absorption near edge structure (XANES) spectroscopy is a powerful technique that allows the investigation of the oxidation state(s) of metals under reactive environments at conditions that mimic those of typical H2-TPR experiments. This technique has been used successfully to follow the reduction of supported gold during thermal treatments in various atmospheres.30 34 Here we report the use of dispersive XANES spectroscopy to characterize the reduction of both gold and iron in iron oxidesupported gold samples as they were exposed to flowing H2 at increasing temperature. We combined the information about the local electronic structure of gold and iron in the materials with the structural information provided by X-ray diffraction (XRD). Recent reports35 show the strengths of complementing XANES results with XRD data to characterize electronic and structural changes of solid catalysts. Our results show that the presence of gold in the samples lowers the temperature of reduction and crystallization of the iron oxide support. Our data emphasize the power of XANES spectroscopy to follow redox processes occurring on supported metals and on the supports themselves by providing unambiguous information about the local electronic structure of metals.

’ EXPERIMENTAL SECTION Synthesis of Samples. Samples were prepared by a coprecipitation method.36 An aqueous solution of Na2CO3 (granular, g99.5%, Sigma-Aldrich) was added dropwise to an aqueous solution of HAuCl4 (99.999%, Sigma-Aldrich) and Fe(NO3)3 (g98%, Sigma-Aldrich) that was kept under vigorous stirring at 60 °C. A precipitate formed when the pH of the mixture reached a value of 8.0. The precipitate was aged during 5 h, and then it was recovered by filtration and washed with deionized water. Finally, the solid was dried overnight at 110 °C. The amount of HAuCl4 was calculated to give gold loadings of 2 or 6 wt %. Samples of the bare support were prepared by the same method but without adding the gold precursor. Dispersive XANES. Dispersive XANES spectra at the Au LIII and Fe K edges were recorded separately at the X-ray beamline D06A-DXAS at the Brazilian Synchrotron Light National Laboratory. The beamline was equipped with a curved Si (111) monochromator. For the experiments, a furnace with halogen lamps and a tubular quartz flow reactor sealed with Kapton windows was used. Self-supporting wafers of the samples were placed in a stainless steel holder inside the quartz reactor. XANES spectra were recorded in transmission mode with a cryogenically cooled charge-coupled device detector of 1340  1300 pixels. Each pixel has 20  20 μm2 for a total image area of 26.8  26.0 mm2. Conversion of data from pixel to energy was performed by comparing measurements of XANES spectra characterizing reference materials (i.e., iron and gold foils) that were done in conventional mode with those recorded in dispersive mode. The energy step-size in the resulting spectra was approximately 0.3 eV. XANES spectra were recorded as the samples were treated in a flowing mixture of 5% H2/He (100 mL min 1) and

ARTICLE

Figure 1. Au LIII-edge XANES spectra characterizing the initially prepared iron oxide-supported gold samples. (a) 2 wt % Au and (b) 6 wt % Au.

the temperature increased at a rate of 5 °C min 1 from room temperature to 400 °C at atmospheric pressure. Data reduction and analysis was done with the software Athena.37 Au LIII- and Fe K-edge XANES spectra were calibrated with gold and iron foils measured in transmission mode, respectively. The edges are represented as the inflection point of the first absorption peaks, at nearly 11919 eV for gold and 7112 eV for iron. The exact value of the absorption edge for each spectrum was found by determining the maximum of the first derivative of the data as a function of energy in the region where the absorbance increased drastically. The data were normalized by dividing the absorption intensity by the height of the absorption edge. XRD. The bare support and the iron oxide-supported gold samples were characterized by XRD after treatments in H2 at various temperatures. Powder XRD data were recorded in the 2θ range between 15 and 70° using a Rikagu DMAX-2200 with Ni-filtered Cu Kα radiation operated at 36 kV and 30 mA.

’ RESULTS AND DISCUSSION Evidence of Cationic Gold and Fe3+ in the Initially Prepared Samples. XANES spectra at the Au LIII edge characteriz-

ing the as-prepared iron oxide-supported gold samples clearly indicate the presence of cationic gold, identified by the intense peak at 4 eV (white line), a shoulder at 15 eV, and a broad shoulder at 50 eV above the X ray absorption edge38 (Figure 1). XANES spectra at the Fe K edge characterizing the bare support and the supported gold samples are shown in Figure 2. In all cases, the absorption edge is located at 7126.5 eV, and the spectra include a pre-edge feature at 7113.8 eV and a peak at 7133.3 eV. These features have been observed in XANES spectra of ferrihydrite39 and are consistent with the presence of Fe3+ in the fresh samples. XRD patterns for the bare support and the supported gold samples (Figure 3) are in agreement with the interpretation of the Fe K-edge XANES results and show that, although the samples were essentially amorphous, there are weak 23520

dx.doi.org/10.1021/jp208936r |J. Phys. Chem. C 2011, 115, 23519–23526

The Journal of Physical Chemistry C

ARTICLE

Figure 2. Fe K-edge XANES spectra characterizing (a) the bare support and the initially prepared iron oxide-supported gold samples containing (b) 2 and (c) 6 wt % Au.

Figure 4. Au LIII edge XANES characterizing iron oxide-supported gold samples containing (A) 2 and (B) 6 wt % Au as they were treated in flowing H2 at increasing temperature. Figure 3. XRD patterns characterizing (a) the bare support and the initially prepared iron oxide-supported gold samples containing (b) 2 and (c) 6 wt % Au.

peaks at 2θ = 35.7, 40.2, 61.2, and 62.7, characteristic of ferrihydrite.40 These results indicate that the presence of gold did not affect the oxidation state of the iron cations or the crystal structure of the support in the fresh samples. Reduction of Supported Cationic Gold to Zerovalent Gold. When the supported gold samples were treated in flowing H2, a decrease in the intensity of the white line with increasing temperature was observed in the XANES spectra at the Au LIII edge, starting at approximately 52 °C for the sample containing 2% Au and at approximately 60 °C for the sample containing 6% Au (Figure 4). This observation indicates increasing reduction of the gold in both samples by conversion of cationic gold

into zerovalent gold during the treatment. The complete reduction of cationic gold to Au0 was observed at approximately 70 °C for the sample containing 2% Au and at approximately 85 °C for the sample containing 6% Au, as evidenced by the disappearance of the features characteristic of cationic gold and the appearance of features characteristic of Au0 (Figure 4).38 The new features are a shoulder at 15 eV and intense peaks at 25 and 50 eV higher than that of the Au LIII edge (11919 eV).38 Influence of Gold on the Reduction and Crystallization of the Support. A comparison of Fe K-edge XANES spectra of the bare support and the supported gold samples at selected temperatures during the treatments in flowing H2 is shown in Figure 5 (the complete Fe K-edge XANES spectra for all samples during the H2 treatments are included in Supporting Information). When H2 flow started at room temperature, XANES spectra of all samples indicated the presence of Fe3+, as evidenced 23521

dx.doi.org/10.1021/jp208936r |J. Phys. Chem. C 2011, 115, 23519–23526

The Journal of Physical Chemistry C

ARTICLE

Figure 5. Fe K-edge XANES spectra characterizing the bare support and the iron oxide-supported gold samples as they were treated in flowing H2 at (A) room temperature, (B) 100, (C) 200, (D) 300, and (E) 400 °C.

by the position of the absorption edge at 7126.5 eV and the presence of a peak at 7133.3 eV (Figure 5A). These results indicate that H2 did not reduce the Fe3+ present in the samples at room temperature.

When the treatment temperature reached 100 °C, XANES spectra characterizing the bare support and the sample containing 2% Au did not change and still included features characteristic of Fe3+ (Figure 5B). In contrast, XANES spectra characterizing 23522

dx.doi.org/10.1021/jp208936r |J. Phys. Chem. C 2011, 115, 23519–23526

The Journal of Physical Chemistry C

ARTICLE

Figure 6. XRD patterns characterizing the bare support and the iron oxide-supported gold samples after they had been treated in flowing H2 at (A) 100, (B) 200, (C) 300, and (D) 400 °C.

the sample containing 6% Au at 100 °C showed that the absorption edge had been shifted to lower energy by approximately 1.6 eV (from 7126.5 to 7124.9 eV) and that the first peak above the edge had also shifted from 7133.3 to 7132 eV (Figure 5B). It has been reported41 46 that the shift in the energy of the Fe K edge to lower values is consistent with a decrease in the oxidation state of the iron. Also, Waychunas et al.45 found that the first peak above the edge is typically centered at higher energies in minerals containing Fe3+ than in minerals containing Fe2+. Therefore, our results indicate that some Fe3+ in the sample containing 6% Au had started to reduce in the presence of flowing H2 at 100 °C. XRD patterns characterizing the samples after treatment in H2 at 100 °C (Figure 6A) show that all samples remained essentially amorphous, with weak peaks that indicate the presence of ferrihydrite. These XRD patterns were undistinguishable from those characterizing the initially prepared samples (Figure 3).47 When the treatment temperature reached 200 °C, XANES spectra characterizing the support remained practically unchanged and again showed features characteristic of Fe3+ (Figure 5C). The XRD results characterizing the bare support after treatment in H2 at 200 °C (Figure 6B) are consistent with the XANES data,

showing that the support remained essentially amorphous with weak peaks characteristic of ferrihydrite. In contrast, the energy of the Fe K edge in the XANES spectra characterizing the supported gold samples at 200 °C in flowing H2 shifted to lower values (to 7123.7 eV for the sample containing 2 wt % Au and to 7123.3 eV for the sample containing 6 wt % Au). These results are consistent with the increasing reduction of some Fe3+ to Fe2+ on the supported gold samples. At this temperature, XRD patterns characterizing the supported gold samples show clear evidence of crystallization (Figure 6B), as indicated by the presence of intense peaks at 2θ = 30, 35.5, 43.2, 53.6, 57.1, and 62.8°, characteristic of magnetite (Fe3O4).40 Because the iron in Fe3O4 is present as a mixture of Fe3+ and Fe2+, the XRD results are consistent with the XANES data. In addition, because the support still contained ferrihydrite when it was treated in H2 at 200 °C (Figure 6B), these results also provide clear evidence that the presence of gold promoted the transformation of ferrihydrite to magnetite. XANES spectra characterizing the samples when the H2 treatment had reached 300 °C (Figure 5D) show that the absorption edge in the spectra characterizing the bare support 23523

dx.doi.org/10.1021/jp208936r |J. Phys. Chem. C 2011, 115, 23519–23526

The Journal of Physical Chemistry C had shifted to 7125 eV, while that of the sample containing 2% Au shifted slightly to 7123.3 eV and that of the sample containing 6% Au remained at 7123.3 eV. These results indicate that the iron in the sample containing 6% Au was not further reduced in the temperature range between 200 and 300 °C. XRD patterns characterizing the samples that had been treated in flowing H2 at 300 °C are shown in Figure 6C. For all cases, the patterns correspond to crystalline samples. The pattern for the bare support shows peaks at 2θ = 24.1, 33.2, 35.6, 40.9, 49.5, 54.2, 57.5, and 64.2°, characteristic of hematite (α-Fe2O3),40 as well as peaks characteristic of magnetite (Fe3O4). The XRD patterns for the supported gold samples show evidence of crystalline magnetite, as in the case of 200 °C H2-treated samples. The XRD pattern characterizing the sample containing 6% Au includes a peak at 38.2° (Figure 6C), which has been attributed to metallic gold.48 Because this peak is not distinguishable in the XRD pattern of the sample containing 2% Au (Figure 6C), we conclude that a higher degree of sinterization of the gold occurred in the sample with higher gold loading. Finally, XANES spectra of all the samples recorded when the H2 treatment reached 400 °C are essentially undistinguishable from each other (Figure 5E), with the absorption edge at approximately 7123.3 eV. These results indicate that after treatment at 400 °C in flowing H2, all samples contained a mixture of Fe3+ and Fe2+. Again, the XANES and XRD data (Figure 6D) are consistent with each other, showing that all samples contained magnetite (Fe3O4), in which the oxidation states of iron are 2+ and 3+. The diffraction peak at 38.2° (characteristic of gold particles) observed in the XRD pattern of the sample containing 6% Au (Figure 6D) increased in intensity with respect to that observed when the same sample had been treated in H2 at 300 °C (Figure 6C). This observation is consistent with an increasing sinterization of the gold particles on that sample with increasing temperature. The dependence between the position of the Fe K edge and the treatment temperature for all the samples is shown in Figure 7. It is clear that the onset of reduction of Fe3+ (as evidenced by the shift of the absorption edge to lower energies) occurred at lower temperatures in the gold-containing samples, with higher gold loading leading to reduction of iron at lower temperature. Taken together, the results shown in Figures 5 7 indicate that the presence of gold lowered the temperature at which (a) Fe3+ was reduced and (b) crystallization of the support occurred in the presence of flowing H2. These results are consistent with previous reports22,49 51 suggesting that the redox properties of transition metal oxides are strongly affected by the inclusion of metals (e.g., Pt, Ru, and Pd)49 52 on their surface. A comparison between Figures 4 and 7 reveals that the reduction of cationic gold started at lower temperature than that of Fe3+ for both supported gold samples. This observation indicates that H2 reacted with the gold prior to reducing the support. Because supported gold is catalytically active for various hydrogenation reactions,53 56 one would expect that gold is capable of activating hydrogen. Indeed, there are reports57 61 showing that hydrogen molecules are chemisorbed dissociatively on gold films57,58 and on supported gold nanoparticles.59 61 Given the ability of zerovalent gold to dissociate H2 and the fact that our results indicate that the reduction of Fe3+ to Fe2+ only occurred until Au0 was formed in our samples, we propose that reduced gold particles on the support serve as sites for the adsorption and dissociation of H2. Some hydrogen atoms could

ARTICLE

Figure 7. Dependence of the position of the Fe K edge with respect to the treatment temperature in flowing H2 for the bare support (circles) and the supported gold samples. 2% Au (squares) and 6% Au (triangles).

then spillover from zerovalent gold to the support, favoring its reduction at lower temperatures than in the absence of gold. Analogous processes for the reduction of ZrO2 and CeO2 in ZrO2and CeO2-supported platinum samples have been proposed.52,62 It has been reported that the reduction of surface Fe3+ ions in ferrihydrite proceeds by the removal of surface oxygen atoms,63,64 leading to the restructuring of the oxide into Fe3O4. Our results are consistent with that proposal, by showing that the presence of gold favors both the reduction of Fe3+ and the formation of Fe3O4 from ferrihydrite. Because the reduction of Fe3+ on the surface of ferrihydrite involves removal of oxygen atoms, the influence of gold on the reducibility of the support has implications for catalysis, as surface oxygen atoms become labile and might participate as activated species during oxidation reactions at lower temperatures when gold is present. Indeed, various authors have attributed the higher activity of gold supported on reducible metal oxides with respect to gold on nonreducible metal oxides to a possible role of the former supports as providers of lattice oxygen atoms for oxidation reactions.19 21 Recently, there have been various attempts to make quantitative descriptions from XANES data to determine the fractions of metals in specific oxidation states in samples of metal oxides and oxide-supported metals.65 67 These attempts use principal component analysis (PCA), in which XANES spectra of problem samples are described as the linear combination of independent components (i.e., XANES spectra of reference compounds). However, a recent report suggests that PCA might fail to describe accurately the surfaces of supported metals, as the reference materials that are used in the analysis differ significantly from the investigated samples, which are structurally complex and highly amorphous.68 Therefore, quantitative descriptions from XANES spectra of supported metals are not reliable and the use of complementary methods is recommended. In spite of its limitations, TPR methods could provide quantitative information regarding the fraction of metals in specific oxidation states. Therefore, the use of XANES spectroscopy and TPR methods in tandem is a good alternative to avoid misleading interpretations that might arise from the use of both methods separately. 23524

dx.doi.org/10.1021/jp208936r |J. Phys. Chem. C 2011, 115, 23519–23526

The Journal of Physical Chemistry C

’ CONCLUSIONS In summary, we used dispersive XANES spectroscopy to monitor the reduction processes of both gold and iron in iron oxide-supported gold samples during treatments in flowing H2 at increasing temperatures. Our results, comparing the reduction of Fe3+ on samples with and without gold, clearly show that the presence of gold favored the reduction of the support. XRD data characterizing the samples indicate that the presence of gold also promoted the crystallization of the support to give preferentially Fe3O4. Because the reduction of gold occurred at lower temperatures than those at which the onset of reduction of Fe3+ was observed, we propose that reduced gold provides sites for H2 adsorption and dissociation. Some hydrogen atoms might then spillover from the gold to the support, favoring its reduction and restructuring. ’ ASSOCIATED CONTENT

bS

Supporting Information. Fe K-edge XANES spectra recorded during H2 treatments for the bare support and the iron oxide-supported gold samples. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: jcfi[email protected]. Fax: +52 (461) 611 7744.

’ ACKNOWLEDGMENT We acknowledge the Brazilian Synchrotron Light National Laboratory for financial support and the staff of beamline D06ADXAS for assisting with the XANES measurements. We also acknowledge the Consejo Nacional de Ciencia y Tecnología (CB-2006, Contract No. 61856), Consejo de Ciencia y Tecnología del Estado de Guanajuato (FOMIX-GTO2007 and 08-09K662-117 A03), and PIFI-2009-CA32 (UASLP) for financial support. ’ REFERENCES (1) Fu, S.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Science 2003, 301, 935–938. (2) Leppelt, R.; Schumacher, B.; Plzak, V.; Kinne, N.; Behm, R. J. J. Catal. 2006, 244, 137–152. (3) Tabakova, T.; Idakiev, V.; Andreeva, D.; Mitov, I. Appl. Catal., A 2000, 202, 91–97. (4) Hutchings, G. J.; Hall, M. S.; Carley, A. F.; Landon, P.; Solsona, B. E.; Kiely, C. J.; Herzing, A.; Makkee, M.; Moulijn, J. A.; Overweg, A.; Fierro-Gonzalez, J. C.; Guzman, J.; Gates, B. C. J. Catal. 2006, 242, 71–81. (5) Guzman, J.; Kuba, S.; Fierro-Gonzalez, J. C.; Gates, B. C. Catal. Lett. 2004, 95, 77–86. (6) Costello, C. K.; Yang, J. H.; Law, H. Y.; Wang, Y.; Lin, J.-N.; Marks, L. D.; Kung, M. C.; Kung, H. H. Appl. Catal., A 2003, 243, 15–24. (7) Prati, L.; Rossi, M. J. Catal. 1998, 176, 552–560. (8) Prati, L.; Martra, G. Gold Bull. 1999, 32, 96–101. (9) Martinez-Ramirez, Z.; Jimenez-Lam, S. A.; Fierro-Gonzalez, J. C. J. Mol. Catal. A 2011, 344, 47–52. (10) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 16, 405–408. (11) Finch, R. M.; Hodge, N. A.; Hutchings, G. J.; Meagher, A.; Pankhusrt, Q. A.; Siddiqui, M. R. H.; Wagner, F. E.; Whyman, R. Phys. Chem. Chem. Phys. 1999, 1, 485–489.

ARTICLE

(12) Boccuzzi, F.; Chiorino, A.; Manzoli, M.; Lu, P.; Akita, T.; Ichikawa, S.; Haruta, M. J. Catal. 2001, 202, 256–267. (13) Fu, L.; Wu, N. Q.; Yang, J. H.; Qu, F.; Johnson, D. L.; Kung, M. C.; Kung, H. H.; Dravid, V. P. J. Phys. Chem. B 2005, 109, 3704– 3706. (14) Lee, Y.; He, G.; Akey, A. J.; Si, R.; Flytzani-Stephanopoulos, M.; Herman, I. P. J. Am. Chem. Soc. 2011, 133, 12952–12955. (15) Fierro-Gonzalez, J. C.; Bhirud, V. A.; Gates, B. C. Chem. Commun. 2005, 5275–5277. (16) Bond, G. C.; Thompson, D. T. Cat. Rev. Sci. Eng. 1999, 41, 319–388. (17) Schubert, M. M.; Hackenberg, S.; van Veen, A. C.; Muhler, M.; Plzak, V.; Behm, R. J. J. Catal. 2001, 197, 113–122. (18) Lin, S. D.; Bollinger, M.; Vannice, M. A. Catal. Lett. 1993, 17, 245–262. (19) Haruta, M. Catal. Today 1997, 36, 153–166. (20) Bond, G. C. Catal. Today 2002, 72, 5–9. (21) Hutchings, G. J. Gold Bull. 1996, 29, 123–130. (22) Khoudiakov, M.; Gupta, M. X.; Deevi, S. Appl. Catal., A 2005, 291, 151–161. (23) Minic o, S.; Scire, S.; Crisafulli, C.; Maggiore, R.; Galvano, S. Appl. Catal., B 2000, 28, 245–251. (24) Neri, G.; Visco, A. M.; Galvano, S.; Donato, A.; Panzalorto, M. Thermochim. Acta 1999, 329, 39–46. (25) Zotin, F. M.; Tournayan, L.; Varloud, J.; Perrichon, V.; Frety, R. Appl. Catal., A 1993, 98, 99–114. (26) Tichit, D.; Medina, F.; Coq, B.; Dutartre, R. Appl. Catal., A 1997, 159, 241–258. (27) Niemantsverdriet, J. W. Spectroscopy in Catalysis. An Introduction; VCH: Weinheim, NY, 1993; pp 223 225. (28) This observation is not exclusive for supported gold catalysts, as the reduction of other supported metals also occur at temperatures that are similar to those at which the support is reduced in the presence of flowing H2. For example, the reduction of platinum was recorded to occur at almost the same temperature as that of iron oxide, resulting in overlapping H2-TPR peaks.29 (29) Liu, L.; Zhou, F.; Wang, L.; Qi, X.; Shi, F.; Deng, Y. J. Catal. 2010, 274, 1–10. (30) Guzman, J.; Gates, B. C. J. Am. Chem. Soc. 2004, 126, 2672– 2673. (31) Guzman, J.; Gates, B. C. J. Phys. Chem. B 2002, 106, 7659–7665. (32) Calla, J. T.; Davis, R. J. Catal. Lett. 2005, 99, 21–26. (33) Costello, C. K.; Guzman, J.; Yang, J. H.; Wang, Y. M.; Kung, M. C.; Gates, B. C.; Kung, H. H. J. Phys. Chem. B 2004, 108, 12529–12536. (34) Jacobs, G.; Ricote, S.; Davis, B. H. Appl. Catal., A 2006, 302, 14–21. (35) Frenkel, A. I.; Wang, Q.; Marinkovic, N.; Chen, J. G.; Barrio, L.; Si, R.; Lopez Camara, A.; Estrella, M.; Rodriguez, J. A.; Hanson, J. C. J. Phys. Chem. C. 2011, 115, 17884–17890. (36) Al-Sayari, S.; Carley, A. F.; Taylor, S. H.; Hutchings, G. J. Top. Catal. 2007, 44, 123–128. (37) Ravel, B.; Newville, M. J. Synchrotron Radiat. 2005, 12, 537–541. (38) Salama, T. M.; Shido, T.; Ohnishi, R.; Ichikawa, M. J. Phys. Chem. 1996, 100, 3688–3694. (39) Wilke, M.; Farges, F.; Petit, P. E.; Brown, G. E., Jr; Martin, F. J. Am. Miner. 2001, 86, 714–730. .; Sajo, I. Appl. Catal. A: (40) Sarkany, A.; Schay, Z.; Frey, K.; Szeles, E Gen. 2010, 380, 133–141. (41) Battiston, A. A.; Bitter, J. H.; Koningsberger, D. C. Catal. Lett. 2000, 66, 75–79. (42) Wong, S. T.; Lee, J. F.; Cheng, S.; Mou, C. Y. Appl. Catal., A 2000, 198, 115–126. (43) Mudarra Navarro, A. M.; Bilovol, V.; Cabrera, A. F.; Rodríguez Torres, C. E.; Sanchez, F. H. Physica B 2009, 404, 2838–2840. (44) Zhao, J.; Huggins, F. E.; Feng, Z.; Lu, F.; Shah, N.; Huffman, G. P. J. Catal. 1993, 143, 499–509. 23525

dx.doi.org/10.1021/jp208936r |J. Phys. Chem. C 2011, 115, 23519–23526

The Journal of Physical Chemistry C

ARTICLE

(45) Waychunas, G. A.; Apted, M. J.; Brown, G. E., Jr. Phys. Chem. Minerals 1983, 10, 1–9. (46) Figueroa, S. J. A.; Stewart, S. J.; Rueda, T.; Hernando, A.; de la Presa, P. J. Phys. Chem. C 2011, 115, 5500–5508. (47) XANES spectra characterizing the sample containing 6% Au at 100 °C show evidence of mixtures of Fe3+ and Fe2+, indicating that part of the Fe3+ present in the initially prepared sample had been reduced at that temperature. The lack of XRD evidence of crystalline iron oxides containing Fe2+ in that sample indicates that those ions were present either as amorphous species or as microcrystals in amounts that were lower than the detection limit of XRD. (48) Solsona, B. E.; Garcia, T.; Jones, C.; Taylor, S. H.; Carley, A. F.; Hutchings, G. J. Appl. Catal., A 2006, 312, 67–76. (49) Passos, F. B.; Oliveira, E. R.; Mattos, L. V.; Noronhe, F. B. Catal. Lett. 2006, 110, 261–267. (50) Jozwiak, W. K.; Maniecki, T. P.; Basinska, A.; Goralski, J.; Fiedorow, R. Kinet. Catal. 2004, 45, 879–889. (51) Sekine, Y.; Chihara, T.; Watanabe, R.; Sakamoto, Y.; Matsukata, M.; Kikuchi, E. Catal. Lett. 2010, 140, 184–188. (52) Hoang, D.-L.; Berndt, H.; Lieske, H. Catal. Lett. 1995, 31, 165–172. (53) Guzman, J; Gates, B. C. Angew. Chem., Int. Ed. 2003, 42, 690–693. (54) Jia, J.; Haraki, K.; Kondo, J. N.; Domen, K.; Tamaru, K. J. Phys. Chem. B 2000, 104, 11153–11156. (55) Mohr, C.; Hofmeister, H.; Radnik, J.; Claus, P. J. Am. Chem. Soc. 2003, 125, 1905–1911. (56) Milone, C.; Tropeano, M. L.; Guline, G.; Neri, G.; Ingoglia, R.; Galvagno, S. Chem. Commun. 2002, 868–869. (57) Stobinski, L.; Zommer, L.; Dus, R. Appl. Surf. Sci. 1999, 141, 319–325. (58) Okada, M.; Nakamura, M.; Moritani, K.; Kasai, T. Surf. Sci. 2003, 523, 218–230. (59) Lin, S.; Vannice, M. A. Catal. Lett. 1991, 10, 47–61. (60) Bus, E.; Miller, J. T.; Bokhoven, J. A. J. Phys. Chem. B 2005, 109, 14581–14587. (61) Conte, M.; Miyamura, H.; Kobayashi, S.; Chechik, V. J. Am. Chem. Soc. 2009, 131, 7189–7196. (62) Jacobs, G.; Graham, U. M.; Chenu, E.; Patterson, P. M.; Dozier, A.; Davis, B. H. J. Catal. 2005, 229, 499–512. (63) Udovic, T. J.; Dumesic, J. A. J. Catal. 1984, 89, 303–313. (64) Channing, D. A; Graham, M. J. Corros. Sci. 1972, 12, 271–289. (65) Gamarra, D.; Fernandez-Garcıa, M.; Belver, C.; Martinez-Arias, A. J. Phys. Chem. C 2010, 114, 18576–18582. (66) Piovano, A.; Agostini, G.; Frenkel, A. I.; Bertier, T.; Prestipino, C.; Ceretti, M.; Paulus, W.; Lamberti, C. J. Phys. Chem. C 2011, 115, 1311–1322. (67) McPeak, K. M.; Becker, M. A.; Britton, N. G.; Majidi, N.; Bunker, B. A.; Baxter, J. B. Chem. Mater. 2010, 22, 6162–6170. (68) Bazin, D.; Rehr, J. J. J. Phys. Chem. C 2011.

23526

dx.doi.org/10.1021/jp208936r |J. Phys. Chem. C 2011, 115, 23519–23526