Structural Investigation of Au Catalysts on TiO2 ... - ACS Publications

bulk mesoporous SiO2 with TiO2.13 Au can be deposited upon this mixed oxide support using standard DP with HAuCl4 at. pH 7 even when the Ti:Si molar r...
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J. Phys. Chem. C 2007, 111, 17322-17332

Structural Investigation of Au Catalysts on TiO2-SiO2 Supports: Nature of the Local Structure of Ti and Au Atoms by EXAFS and XANES Viviane Schwartz,* David R. Mullins, Wenfu Yan,† Haoguo Zhu, Sheng Dai, and Steven H. Overbury Chemical Sciences DiVision and CNMS DiVision, Oak Ridge National Laboratory, P. O. Box 2008, MS-6201, Oak Ridge, Tennessee 37831-6201 ReceiVed: June 7, 2007; In Final Form: August 15, 2007

X-ray absorption spectroscopy (XAS) was utilized to investigate both the Au particle size on several supports composed by silica and titanium oxide, and the coordination of TiO2 in the support phase. Particularly, we wanted to utilize the technique to probe mixing in the support phase, by using different synthetic methods such as by functionalizing silica or by ALD (atomic layer deposition) techniques as prepared in our laboratories, and the growth and stability of Au nanoparticles deposited on these supports. The study using cosynthesis techniques to dope bulk mesoporous SiO2 with TiO2 resulted in TiO2 being dispersed in the SiO2 matrix; however, a second phase starts forming as the TiO2 content increases as indicated by the EXAFS Ti-O shell shift in position and increase of coordination number. On the supports prepared by cosynthesis, Au particles were smaller and more stable. The study using the surface sol-gel technique for deposition of single monolayers of an oxide such as TiO2 produced Ti environments in which the Ti-O shell and the next two Ti-Ti shells lie on the same position as expected for an anatase structure. Although undercoordinated, the presence of the Ti-Ti shells indicate that the titania species are not molecularly dispersed on the SiO2 surface as hypothesized, but there is indeed a cross-linking of the titania moieties.

1. Introduction Catalysis by small gold particles has been the object of considerable attention in the past few years, and their activity is well-known for size-dependent behavior.1-3 It is also widely recognized that oxide supports play key roles in the activity and stability of Au nanocatalysts.1,4-6 The support structure and its nature can affect the nucleation of Au particles during deposition and the stabilization of the Au particles during subsequent treatments and reactions.7 Additionally, the support can participate in the adsorption and exchange of oxygen via defect oxygen sites in the support lattice. SiO2 is potentially a very useful support because it has the advantage that it can be readily synthesized into a variety of morphologies with high surface area and the porosity and pore size can be controlled over a wide range, being more mechanically and thermally stable than other oxides such as TiO2. However, pure SiO2 has the limitation that the standard method of deposition-precipitation (DP) using HAuCl4 does not work well because SiO2 has a low isoelectronic point (IEP).8 We and others have explored methods to engineer the surface of SiO2 to alter its behavior with respect to Au DP and to alter the stability and activity of the resulting Au catalysts. One approach is to functionalize mesoporous or nonporous SiO2 with functional ligands which interact with Au precursor during DP or during cosynthesis.9-11 Another approach is to functionalize SiO2 by using a surface sol-gel technique for deposition of single monolayers of an oxide such as TiO2.7 With such supports it is found that TiO2 facilitates deposition of Au by the DP * To whom correspondence should be addressed. Telephone: (865) 5766749. E-mail: [email protected]. † Current address: State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, China.

method and leads to stable and active catalysts12 even though the surface is only partly covered by the TiO2. Such an oxide overlayer may be more stable than a functional organic ligand, and it may interact more strongly with reduced Au particles. In another approach, we have used cosynthesis techniques to dope bulk mesoporous SiO2 with TiO2.13 Au can be deposited upon this mixed oxide support using standard DP with HAuCl4 at pH 7 even when the Ti:Si molar ratio is only 1:5.5. The result is an active catalyst which maintains high activity even after calcination to 700 °C. It is therefore of interest to understand better how the TiO2 doping alters the structure of the Au catalysts, the interaction between the TiO2, SiO2, and Au phases, and whether the Au anchors primarily at the titania sites. It is difficult to determine, for example by transmission electron microscopy (TEM), the morphology of a layer of supported titanium oxide on silica, how the titania is distributed between the surface and bulk of the silica, or how this distribution depends upon the method of preparation. X-ray diffraction is also very hard to apply in the case of very small particles and/or dispersed amorphous phases. Other techniques may be required.14 In this paper we apply X-ray absorption techniques to explore the question of the local environment of the TiO2 species present in each of these SiO2based samples. This technique has been demonstrated to be sensitive to the environment of Ti cations in supported catalysts and in titanosilicates.15-19 Therefore, we have used near-edge and extended X-ray absorption spectroscopy (XAS) at the Ti K-edge to examine the environment of the Ti cations following calcination and dehydration both with and without deposited Au. Absorption at the Au LIII-edge was used to investigate the effect of the TiO2 upon the Au-support interaction and the size and oxidation state of the Au particles. The goal is to correlate changes of Au particle size with variations of titanium oxide

10.1021/jp074426c CCC: $37.00 © 2007 American Chemical Society Published on Web 10/25/2007

Au Catalysts on TiO2-SiO2 Supports structure. We find that there are clear differences in the Ti environment depending on the synthesis method applied and how the Ti mixes and disperses over the SiO2, and that smaller and more stable Au particles were formed when supported on a mixed mesoporous SiO2-TiO2 in which most of the TiO2 species are four-coordinated and dispersed on the silica matrix. 2. Experimental Section 2.1. Preparation of Titania-Containing Supports. Three different forms of pure TiO2 were used in this study. Rutile TiO2 was synthesized using an ultrasonic synthesis method from TiCl4 precursors as described previously.20 Mesoporous TiO2 was synthesized as described previously.21 Anatase TiO2 was purchased from Aldrich. Two other reference compounds used were Ba2TiSi2O8 (fresnoite) and Ba2TiO4. The latter was synthesized from a 2:1 molar ratio mixture of BaCO3 and TiO2 by calcining to 1200 °C for 2 h. The Ba2TiSi2O8 was synthesized from a 2:1:2 molar ratio mixture of BaCO3,TiO2, and SiO2 and by calcining to 1550 °C for 1 h. The synthesized compounds were analyzed by X-ray diffraction (XRD), which verified that the desired structures were obtained. Two different pure SiO2 supports were used. One was amorphous fumed silica (Cab-o-Sil, Cabot Corporation), which was used without further treatment. Mesoporous SiO2 (SBA15) was the same as that used in our previous work8 and was synthesized according to the method described by Zhao et al.22 Briefly, it was synthesized under acidic conditions using Pluronic (P123) as a surfactant and tetraethyl orthosilicate (TEOS) as the Si precursor. The as-synthesized SBA-15 was filtered, dried at 90 °C, and finally calcined at temperatures between 500 and 600 °C. These two silicas were used for subsequent preparation of layered structures. Layered supports consisting of TiO2 deposited onto SiO2 were synthesized as described in our earlier work using the surface sol-gel synthesis method.23 Briefly, high surface area SiO2 was added to an anhydrous mixture of methanol and toluene and refluxed along with titanium tetrabutoxide, Ti(OBu)4, for 3 h to deposit a single layer of the Ti precursor onto the surface. The resulting powder was filtered, washed with pure ethanol to remove the Ti(OBu)4 and followed by water to hydrolyze all surface Ti species, and then dried at 130 °C for 10 h. For samples prepared in this way, it has been demonstrated that the titania precursors attach at silanol groups, are highly dispersed without cross-linking,7 and incompletely cover the silica surface. This sample is designated TiO2/SiO2. The anhydrous deposition process was repeated in a subsequent deposition to produce the “double layer” of TiO2, designated TiO2/TiO2/SiO2. This second deposition covers the support with a higher loading than the single layer.7 Double-layer samples with aluminum oxide in the outer or inner layer were also prepared using the same procedure but with Al(OBu)4 as the Al precursor in one of the deposition steps. This route leads to higher loadings of AlO- compared to the TiO2.7 Mesoporous, mixed TiO2-SiO2 supports were synthesized using methods that we have reported previously,13and three different Si:Ti atomic ratios (10, 5.5, and 2.7) were examined. The synthesis is based upon a one-pot coassembly method designed to control the hydrolysis of Si precursor (Si(OEt)4) and Ti precursor (TiOEt)4 using a block copolymer (F108) as a structure-directing template. Following drying and gelation at room temperature, each sample was ramped to 350 °C (at 1 °C/min) and held for 3 h before ramping to 600 °C (or 500 °C) and held for 5 h. At low Ti concentrations (Si:Ti ) 10), the Ti is believed to be mixed into the silica matrix, but at

J. Phys. Chem. C, Vol. 111, No. 46, 2007 17323 higher Ti concentrations it is believed to be incorporated as small crystallites of anatase located within the mesopores.13 2.2. Au Deposition onto Supports. Au was deposited onto the samples using the deposition-precipitation (DP) method with HAuCl4 precursor. Briefly, chloroauric acid was diluted in water, the pH was adjusted with KOH, and then the support was added (pH 7 for mixed TiO2-SiO2; pH 10 for layered samples). The mixture was stirred at 70 °C for 2 h and then centrifuged and washed several times with deionized water. It was then dried at 40 °C overnight and stored without further calcination in this as-synthesized state until used in these experiments. 2.3. X-ray Absorption Spectroscopy (XAS) Measurements. The XAS data at the Ti K-edge (4965 eV) and Au LIII-edge (11 918 eV) were collected at beamlines X18B and X19A at the NSLS, Brookhaven National Laboratory. At X19A, a Si(111) double crystal monochromator was used and detuned by 40-50% at the Ti K-edge and by 25% at the Au LIII-edge to reject higher harmonics. The X-ray absorption was measured in transmission mode, by using an ion chamber placed in the beam path, and in fluorescence mode by using a large area passivated implanted planar silicon (PIPS) detector perpendicular to the beam path. In most cases, the concentration of the absorption element was high enough that transmission data were utilized for the analysis. In the case of Au absorption measurements, the ion chambers for measuring Io and It were filled with air (or nitrogen) and Ar, respectively. In the case of Ti K-edge, the ion chambers for measuring Io and It were filled with He (containing some amounts of impurities) and air, respectively. Two different cells were used during measurements at X19A. In one case, our homemade quartz tube apparatus (50 cm long by 2.4 cm diameter) for low-temperature/high-temperature in situ studies was used. The cell allowed control of the atmosphere (reducing or oxidizing) and temperature while operating in transmission mode. The fresh catalyst was held inside the reactor in a quartz holder in which it has been loosely pressed into a 12 mm diameter pellet and mixed with BN to provide structural integrity to the wafer. Heating of the quartz reactor was accomplished by placing a clamshell furnace around it, and the temperature could be controlled from room temperature to 800 °C (maximum temperature used in our experiments). The clamshell furnace was substituted by a cooling jacket made from aluminum alloy in which liquid nitrogen flowed through internal channels in order to collect XAFS data close to liquid nitrogen temperatures. Temperatures as low as -182 °C were reached inside the reactor tube using this arrangement. In another setup, a Nashner-Adler24,25 reaction cell for transmission and fluorescence modes was utilized. The catalyst was inserted as a 13 mm diameter pellet into a copper holder. A gas flow pipe terminates near the sample’s surface to allow in situ studies of temperature and gas-flow controlled experiments. A liquid nitrogen Dewar was placed on the top of the cell to ensure absorption measurements at low temperature (the lowest temperature reached was about -110 °C) and, consequently, minimize thermal disorder effects. Higher temperatures up to 300 °C for in situ treatments were achieved by placing a heating element in place of the liquid nitrogen Dewar. At X18B, a Si(111) channel cut monochromator was used and the crystal was detuned by 20% at the Au LIII-edge and by 40% at the Ti K-edge. In the case of Au measurements, the X-ray absorption was measured in transmission mode, by using two ion chambers placed in the beam path. The ion chambers for measuring Io and It were filled with N2 and Ar, respectively. In the case of Ti K-edge measurements, the X-ray absorption

17324 J. Phys. Chem. C, Vol. 111, No. 46, 2007 was measured in fluorescence mode by using a Ge multielement detector perpendicular to the beam path. Au absorption measurements were made in our homemade quartz tube (described previously), which could be heated for sample treatments under reducing or oxidizing atmosphere or cooled for EXAFS measurements. For the Ti measurements, the catalysts were pretreated in the same quartz reactor tube in which the temperature and the flowing gases were controlled. However, following the pretreatment, the catalysts were transferred through ambient air and covered with Kapton tape for XAS experiments in air under fluorescence mode. The program XDAP, version 3.2, was used to analyze and fit the data.26 The data reduction procedure consisted of the following steps: pre-edge subtraction, background determination, normalization, and spectra averaging. First, the pre-edge was approximated by a modified Victoreen curve and subtracted from the whole data range. The edge position is defined to be the first inflection point on the leading absorption peak. This was calibrated to be 11 918 eV for the LIII-edge of a Au reference foil. The background in the EXAFS region was approximated using a cubic spline routine and optimized according to the criteria described by Cook and Sayers.27 Then, the spectra were normalized by the edge step of the background fitted curve at 50 eV after the absorption edge. The k3-weighted and k1-weighted EXAFS functions were Fourier transformed, filtered, and fitted in R space. Fourier filtering was used to isolate the contributions of specific shells and to eliminate lowfrequency background and high-frequency noise. Fourier filtering is done by choosing a window in the Fourier transform spectrum and calculating the inverse Fourier transform of the selected R range. The interatomic distance r, the first nearestneighbor coordination number (1NN), the difference of the Debye-Waller factor from the reference (∆σ2), and the correction of the threshold energy (∆E°) were treated as free parameters during the fitting. The quality of the fit was estimated from the values of k3 variance (Vk3). The variance represents the residual between the observed and calculated spectra in the fitted range. Low values of variance indicate a good agreement between data and model. Fitting analysis in both k1- and k3weighted Fourier transforms was applied in order to obtain a unique set of coordination number (CN) and ∆σ2 parameters. Those parameters are highly correlated and there are a number of different combinations of CN and ∆σ2 that can lead to similar fits; however, the set of combinations depends on the k weight factor. Therefore, a unique set of parameters can be found by fitting on both k1- and k3-weighted Fourier transforms. To analyze the spectra, simulations of reference compounds using FEFF8.128 were used to calculate phase shifts and backscattering amplitude. FEFF references were obtained for Au-Au by utilizing crystallographic data of Au metal, and for Ti-O and Ti-Ti by utilizing crystallographic data of TiO2 (anatase). 3. Results 3.1. Titania Support. X-ray absorption near-edge structures (XANES) around the Ti K-edge of two reference compounds, anatase and rutile, and of the synthesized supports, monolayer TiO2 on SiO2 (Cab-o-Sil), monolayer TiO2 on SiO2 (SBA), and mixed TiO2-SiO2 oxides, are shown in Figure 1. The pre-edge features of the Ti XANES spectra (Figure 1B) can provide information about the symmetry environment of the absorbing atom.15 In the case of octahedral structures, such as anatase and rutile, the pre-edge features give rise to three prepeaks usually denoted A1, A2, and A3. The intensity of the A2 peak increases with increasing site distortion. The first peak, A1, is less well

Schwartz et al.

Figure 1. (A) Ti K-edge absorption spectra after dehydration of (a) anatase, (b) rutile, (c) monolayer TiO2/SiO2 (Cab), (d) mixed oxides Si:Ti ) 5.5, and (e) monolayer TiO2/SiO2 (SBA). The curves were offset vertically. (B) Details of the pre-edge region.

resolved for layered TiOx on Cab-o-Sil (curve C) or SBA-15 (curve E) than for crystalline anatase (curve A) or rutile (curve B). In the case of the mixed oxide (curve D), the A2 feature is even more intense and shifted to lower energies, which is indicative of lower coordination numbers since a single, intense pre-edge feature is observed in tetra- and pentacoordinated Ti compounds. Determination of both the pre-edge A2 energy and its normalized intensity can be used to distinguish between fourfold, fivefold, and sixfold coordination symmetry around the Ti atom. The three defined domains for each coordination environment are shown in Figure 2, which includes our own measurements of standard compounds, such as anatase (sixfold coordination), Ba2TiSi2O8 (fivefold coordination), and Ba2TiO4 (fourfold coordination), combined with reference compounds gathered from the work of Farges et al.29 The reason for using standards was to compare our near-edge measurements against those in the literature so that our assessment of the unknown compounds could be verified. The reference compounds representing fourand fivefold coordination showed a variation not higher than 0.05 eV for the energy position, whereas the anatase showed a higher deviation of 0.3 eV. The precision of our measurements was systematically verified by repeating a standard (such as anatase), as can be observed by two representative measurements plotted in Figure 2. From the comparison with the reference compounds measured by our group and by Farges et al.,29 the pre-edge features observed for the layered and mixed oxides suggest that the Ti atoms are present in mixtures of at least two Ti coordinations. In their previous studies, Farges et al. collected an extensive

Au Catalysts on TiO2-SiO2 Supports

J. Phys. Chem. C, Vol. 111, No. 46, 2007 17325

Figure 2. Normalized pre-edge height versus energy position for the A2 peak for mixed and layered oxides after treatment at 300 or 150 °C. These results are compared with those for model compounds from the literature (solid black symbols) and from current experiments, showing the fourfold, fivefold, and sixfold coordination domains.

set of pre-edge spectra of mixtures of model compounds with different coordinations and demonstrated that both the pre-edge position and its intensity give a more reliable estimation of samples with a mixture of coordinations. The near-edge features shown in Figure 1 and plotted as a function of normalized intensity and energy in Figure 2 correspond to spectra taken upon heating the samples to temperatures of about 300 °C. This procedure was adopted due to a dramatic change observed for some of the support samples during heat treatment (Figure 3) indicating a clear modification of the site symmetry of Ti atoms. This change is more evident for the mixed oxide sample (Figure 3a) than for the layered titania sample (Figure 3b) and clearly indicates a significant modification of the Ti coordination sphere as a consequence of sample treatment. We ascribe the changes to H2O removal as has been observed by others.16,30-32 In addition to the pre-edge features, the post-edge features (as depicted in Figure 1) are also an indication of the Ti local structure. The reference compounds anatase and rutile appear quite differently in this region. Although both the anatase and rutile exhibit a white line peak at about 4987 eV, the rutile exhibits an additional maximum about 5 eV higher energy while the anatase exhibits three resolvable maxima in the range from 4995 to 5003 eV, in agreement with previous reports.17 None of the supported titania or the mixed SiO2-TiO2 oxides present these additional peaks, and the white line peak is less pronounced. The damping effect on features in the post-edge region is generally indicative of loss of long- and medium-range order that usually broadens the edge crest significantly.15 The lack of features in this range for materials except the reference compounds may be due to smaller clusters and/or disordered materials, indicating that titania is not likely present as crystalline anatase particles. This interpretation is consistent with the titania moiety being highly dispersed as expected for the supported samples or mixed throughout the silica as expected for the mixed mesoporous TiO2-SiO2 samples. In order to further investigate the titania structure, the EXAFS region above the Ti K-edge was also collected and analyzed. Figure 4A shows the EXAFS oscillations for the synthesized mesoporous titania, monolayer TiO2 on Cab-o-Sil, monolayer TiO2 on SBA, mixed TiO2-SiO2, and several reference compounds. Ti EXAFS were taken after heating to temperatures not greater than 300 °C. The heat treatment was interrupted

Figure 3. Ti K-edge absorption spectra of (a) mixed oxide (Si:Ti ) 10) and (b) monolayer TiO2/SiO2, after different treatment conditions.

when no changes in the XANES region were observed, indicating complete dehydration. The k3-weighted EXAFS spectra, presented in Figure 4A as a function of the wave vector k, are the result of data reduction and signal averaging of at least three scans. The Fourier transforms of the EXAFS functions are shown in Figure 4B. Visual inspection of the Fourier transform spectra can give an indication of the presence or absence of neighbors in a given coordination shell and their positions relative to the absorber atom. The intensities of the peaks are roughly related to the number of atoms contained in a coordination shell; however, other factors such as thermal and static disorders (embedded in the Debye-Waller factor) and the many-body parameter (So2) are strongly correlated. Applying higher k-weightings is used as a method to compensate for the reduction of the EXAFS oscillation in the high k-region. In this case, the purpose of high k-weighting is to amplify the Ti-Ti absorber-scatterer pair contribution. The first large peak in the Fourier transform spectrum of anatase (Figure 4B) corresponding to the first coordination shell around Ti is assigned to the six O atoms at 1.95 Å. The next two peaks appearing between 2 and 4 Å (not phase corrected) are attributable to four Ti atoms located at 3.04 Å and another four Ti atoms located at 3.785 Å in the known anatase structure.33 Rutile has a crystal structure slightly different from

17326 J. Phys. Chem. C, Vol. 111, No. 46, 2007

Figure 4. Comparison of Ti EXAFS data after treatment with 4% H2/He at 150 °C (or 300 °C) with the standards anatase and rutile: (A) k3-weighted χ(k); (B) Fourier transform k3-weighted EXAFS function.

that of anatase, and the differences are reflected in the spectrum in Figure 4B. Similar to anatase, the first peak corresponds to six nearest-neighbor O atoms at about 1.96 Å. The most pronounced peak in the rutile spectrum appearing at about 3.0 Å (not phase corrected) is consistent with the eight Ti neighbors at 3.569 Å, while the small peak near 2.5 Å (not phase corrected) corresponds to two nearest Ti neighbors at 2.95 Å.34 The Fourier transform spectra presented in Figure 4B indicate that most of the qualitative differences for the various samples lie in the region of 2-4 Å. In a qualitative inspection, the positions for the peaks (indicated by dotted lines in Figure 4B) are in general agreement with the position of the Ti-Ti shells corresponding to an anatase structure rather than a rutile structure. In particular, the mesoporous TiO2 fairly closely mimics the anatase spectrum. The mixed TiO2-SiO2, however, shows weak features in this region and less closely resembles the anatase. Tables 1 and 2 present the results of the fitting analysis for two different sets of experimental data collected at X-19A (run 1) and X-18B (run 2) at NSLS. Analysis was carried out by Fourier transformation and subsequent Fourier filtering so that

Schwartz et al. single-scattering contributions can be isolated from multiplescattering ones. A typical curve-fitting analysis is depicted in Figure 5. An anatase experimental spectrum, in which the coordination numbers are known, was used to refine and fix the So2 parameter so that the only free parameters during the fitting were the interatomic distance r, the first nearest-neighbor coordination number (1NN), the difference of the DebyeWaller factor from the reference (∆σ2), and the correction of the threshold energy (∆E°). In general, the layered compounds and the meso-TiO2 present the O and Ti shells lying on almost the same positions that are found for the anatase structure. The first oxygen shell presents coordination numbers close to or less than a sixfold coordination, which was expected based on the XANES results. The Ti-Ti shells are significantly less populated than an anatase-type structure, which again confirms the visual inspection of these higher shells (Figure 4B). The undercoordination of the Ti-Ti shells has been observed also by Asakura et al.17 and is a strong indication of a layer model in which higher shells are significantly reduced instead of the presence of bulk crystallites of anatase. However, the clear presence of the Ti-Ti interactions at about 3.04 and 3.78 Å reveals that there is indeed a crosslink between the TiO2 entities, and the model where the octahedral TiO2 entities would be spread on the surface of the SiO2 was not confirmed. The coordination numbers for the mesoporous TiO2 sample were also smaller than a bulk anataselike compound. The reason could lie in a high degree of disorder due to the inhomogeneity of the titania structure. This material exhibits only very broad features in the XRD consistent with poorly organized anatase, and clear anatase features do not appear until it is calcined to higher temperatures. This lack of organized anatase would lead to broad weak features in the EXAFS. In addition, this is a high surface area material, so a large fraction of the Ti is at the surface, leading to lower average coordination. Surface areas for the layered materials ranged from 157 to 245 m2/g. No indication of Ti-Au interaction arose with higher Au loadings (Table 1), and the reasons for the observed lower Ti-O coordination numbers are not clear. The addition of Al2O3 prior to deposition of TiO2 induced the dispersion of the TiO2 moieties as evidenced by the disappearance of the second Ti-Ti shell. In the case of the mixed oxides, the two main distinctive observations compared to the other compounds are that the higher shells were nearly absent and the Ti-O shell distances were shorter (in the range of 1.81-1.88 Å). The two observations are strong indications that most of the Ti atoms are dispersed in the silica structure. Regarding the Ti-O distance, the average Ti-O distance in sixfold coordination structures such as anatase is 1.95 Å. It is found in the literature35 that compounds containing fivefold coordinated Ti can be present in a square pyramidal structure with one short TidO double bond distance (about 1.70 Å) and four longer Ti-O bonds (about 1.97 Å). For fourfold coordinated Ti compounds, the Ti-O bond distances can spread out from 1.76 to 1.84 Å.36 The reported average Ti-O distances for fourfold coordinated Ti-containing silicalite structures are in the range of 1.80-1.88 Å,31,37 whereas the reported average Ti-O distances in sixfold coordinated silicates are between 1.94 and 2.06 Å.30 Therefore, the shorter Ti-O distances are strong indications of the presence of a fourfold phase, although our data cannot be used to distinguish between highly dispersed surface titania moieties and a framework environment. Another important observation in the case of mixed oxides is that the Ti-O distance increases with higher amounts of Ti

Au Catalysts on TiO2-SiO2 Supports

J. Phys. Chem. C, Vol. 111, No. 46, 2007 17327

TABLE 1: Results of the Curve Fitting of the Ti K-Edge of the Layered TiO2 and Pure Mesoporous TiO2 Samplesa-c samples monolayer TiO2/SiO2 (Cab) monolayer TiO2/SiO2 (Cab) with Au < 4% monolayer TiO2/SiO2 (Cab) with Au < 8% double-layer Al2O3/TiO2/SiO2 double-layer TiO2/Al2O3/SiO2 double-layer TiO2/TiO2/SiO2 monolayer TiO2/SiO2 (SBA) meso-TiO2 anatased rutiled

shell

CN run 1/run 2

R, Å run 1/run 2

∆σ2/10-4 Å2 run 1/run 2

∆E°/eV run 1/run 2

Vk3-fit run 1/run 2

Ti-O Ti-Ti Ti-Ti Ti-O Ti-Ti Ti-Ti Ti-O Ti-Ti Ti-Ti Ti-O Ti-Ti Ti-Ti Ti-O Ti-Ti Ti-O Ti-Ti Ti-Ti Ti-O Ti-Ti Ti-Ti Ti-O Ti-Ti Ti-Ti Ti-O Ti-Ti Ti-Ti Ti-O Ti-Ti Ti-Ti

5.9/5.5 2.7/3.5 1.1/0.9 5.5/5.5 3.3/2.9 2.5/1.7 3.6/4.2 4.8/2.6 1.4/0.6 3.4/5.5 2.8/2.4 2.4/0.9 na/2.3 na/1.6 6.3/5.3 3.1/2.8 1.3/0.9 na/4.5 na/2.6 na/0.7 na/4.5 na/2.7 na/0.9 6 4 4 6 2 8

1.93/1.95 3.04/3.04 3.81/3.75 1.93/1.93 3.03/3.05 3.81/3.79 1.93/1.94 3.02/3.04 3.78/3.78 1.96/1.95 3.04/3.04 3.77/3.77 na/1.93 na/3.02 1.94/1.95 3.04/3.04 3.78/3.80 na/1.92 na/3.03 na/3.83 na/1.94 na/3.03 na/3.81 1.95 3.04 3.78 1.96 2.95 3.57

93/76 80/85 54/55 98/71 79/60 114/170 97/46 124/57 44/10 36/48 26/26 100/11 na/70 na/89 106/69 70/52 25/10 na/76 na/93 na/32 na/61 na/40 na/20

8.7/7.0 10/11 4.7/8.7 9.0/9.6 12/10 5.2/6.7 13/7.5 14/11 7.6/5.8 7.2/6.9 11/11 11/9.1 na/5.9 na/12 9.0/7.2 10/10 7.5/5.7 na/8.7 na/13 na/0.1 na/8.3 na/10 na/5.6

3.4/2.7 2.6/2.2 2.8/2.2 2.1/1.3 na/2.5 3.5/2.5 na/1.9 na/3.4

a Samples were heated to temperatures up to 300 °C. Comparison with the coordination numbers and distances for anatase and rutile based on crystallographic data from literature. b The errors in the final parameters are expected to be for CN, (10%; for R, (0.02 Å, for ∆σ2, (20%; and for ∆E°, (20%. Run 1 was performed at X19A in an in situ cell at transmission mode, and run 2 was perfomed at X18B at fluorescence mode. So2 was refined for anatase experimental spectra in run 1 and run 2 to give coordination numbers close to the nominal ones. c na, EXAFS was not run. d Average distances and CN obtained from literature crystallographic data.33

TABLE 2: Results of the Curve Fitting of the Ti K-Edge of the Mixed SiO2-TiO2 after Heating to 300 °Ca

a

mixed oxide samples

shell

CN

R, Å

∆σ2/10-4 Å2

∆E°/eV

Vk3-fit

Si:Ti ) 10 Si:Ti ) 5.5 Si:Ti ) 2.7

Ti-O Ti-O Ti-O

4.6 4.7 5.1

1.81 1.87 1.88

55 114 126

11 12 12

4.5 4.3 3.3

The errors in the final parameters are expected to be for CN, (10%; for R, (0.02 Å; for ∆σ2, (20%; and for ∆E°, (20%.

atoms (lower Si:Ti ratio), which can suggest the presence of mixed phases as the Ti content increases. In one phase, most

Figure 5. Ti EXAFS data of double-layer TiO2/TiO2/SiO2: comparison between the Fourier filtered data and the best-fit EXAFS curves.

of the Ti is in a fourfold structure dispersed in the silica matrix, and in the other phase Ti atoms agglomerate, forming an anatase-like structure isolated from the silica. This hypothesis is supported by the XANES region (Figure 2) in which the mixed Si:Ti ) 10 is located in the fourfold region whereas the mixed Si:Ti ) 5.5 lies in the intermediate region pertaining to a mixed phase. Changes of the XANES region during dehydration (Figure 3a) would indicate that many of the dispersed Ti atoms are present on the surface. Additionally, the high surface area of the mesoporous material would contribute to the presence of surface Ti atoms. In the case of the mixed oxides, the surface area decreased with higher amounts of TiO2, ranging from 239 m2/g (Si:Ti ) 2.7) to 392 m2/g (Si:Ti ) 10). The EXAFS region can do little to detect the presence of fivefold coordinated Ti at the surface mixed with a fourfold coordinated Ti at the bulk. One of the inherent characteristics of EXAFS it is that it is a bulk technique and averages over all the atoms. Therefore, the coordination number obtained during fitting is an average value and fitting can be subject to interpretations when different phases are present and mixed. For instance, the extended XAFS region was also taken for the mixed Si:Ti (ratio equals 10) as inserted and after heating to 300 °C (Figure 6). The Ti-O first shell shows very little change in intensity or position. This is very likely the case when a

17328 J. Phys. Chem. C, Vol. 111, No. 46, 2007

Figure 6. Comparison of the Ti-O shell of the mixed Si:Ti ()10) as inserted and after heating in He at 300 °C.

shorter Ti-O bond is added with a bond distance slightly different from that of the other four oxygen atoms surrounding the central Ti atom. Additionally, the presence of variable Ti-O bond lengths significantly broadens the peak. In this case, the XANES is a better indication of the presence of mixed phases. Although Ti seems to be dispersed in the silica matrix and at the surface, it was not possible to assign any higher shell due to a contribution from a Si backscatterer. High Debye-Waller factors were obtained during our simulation between 2 and 4 Å; therefore, neither a Ti-Ti nor a Ti-Si contribution could be assigned in that range. This may be due to very low intensity oscillations at higher k due to the large variety of distances in disordered systems leading to a very high Debye-Waller factor. In the case of using a Ti-Si pair, the problem becomes worse because Si is a weaker backscatterer. The absence of a clear Ti-Si shell in dispersed and mixed oxides was also observed by others.17,32,38 3.2. Au Nanoparticles. The redox behavior of Au on all samples was investigated by observing changes in the intensity of the white line in a XANES spectrum. As noted in a previous publication,39 Au is usually reduced after heat treatment at 150 °C under a reducing atmosphere (4% H2/He). Additionally, it remains reduced under reaction conditions and no reoxidation is observed even after exposure to highly oxidizing atmospheres at high temperatures. This behavior is observed in the case of Au supported on the mixed TiO2-SiO2, layered structures, and mesoporous titania. Some variations related to the reduction behavior were observed among different samples. In some cases, Au particles were easily reduced even at very mild temperatures (close to room temperature) or displayed an autoreduction behavior that can be due to sensitivity to light or X-rays, for instance. Alternatively, for the case of Au on the mixed oxide supports, complete reduction occurred at higher temperatures (only around 150 °C). Besides the support nature, the reducibility behavior can be attributed to other factors such as Au loading and particle size. Figure 7 presents the Au LIII-edge EXAFS results for Au supported on the layered and mixed oxide supports after two consecutive treatments: (a) reduction in 4% H2/He at 150 °C and (b) air treatment at high temperatures at 500 °C. The first treatment was to ensure that all the catalysts contained Au in a fully reduced active state. The aim of the second treatment was to induce growth and ensure stability of the Au particles under high-temperature oxidative conditions. Visual inspection reveals that smaller particles, as indicated by the smaller peak intensities, were obtained for the Au supported on the mixed oxide (Si:Ti ) 5.5) and on the double-

Schwartz et al. layer Al2O3/TiO2/SiO2. Although not shown in Figure 7 for the other mixed Si:Ti ratios, the Au on mixed oxide supports consistently presented very low Au-Au peak intensities with correspondingly small coordination numbers indicative of small Au particles. EXAFS oscillations and corresponding Fourier transform peak intensities (Figure 7b) became more intense upon heating at 500 °C, indicating particle growth; however, differences in stability and growth were also observed for different supports. Fits of the first peak resulted in bulk-like coordination numbers for the Au on monolayer TiO2/SiO2 and double-layer TiO2/TiO2/SiO2 after heating to 500 °C. Au metal presents 12 Au atoms on the first coordination sphere at a distance of 2.87 Å. Au on mixed Si:Ti still produced a small coordination number (1NN ) 7.6) after heating to 500 °C. This coordination number can be related to a mean Au particle size by assigning a geometrical model for the Au particle structure. For instance, if the particles are assumed to grow as hemispherical cuboctahedra truncated by a basal plane,40 then a coordination number of 7.6 would correspond to particles of about 11 Å. The presence of very small gold particles especially after reduction at 150 °C is clearly indicated by the observed low Au-Au signal and shorter bond distances, in agreement with our previous work39 and other reports.41-43 In many samples, the EXAFS data were best fit with two populations of Au particles of different sizes as illustrated in Figure 8 and reported by others.42 This bimodal distribution is based on the assumption that each of the Au-Au scattering distances belongs to a different particle, instead of a model that incorporates an inhomogeneity of the Au-Au bond distances within the same particle. The latter model would be better fitted with a unique Au-Au distance and larger Debye-Waller factors. Another model attempted in our analysis was one that includes a shell of oxygen atoms as contributing backscatterers at a distance corresponding to the shorter Au-Au shell. We were especially interested in trying to find Au-O distances characteristic of oxygen atoms in direct contact with low- or zero-valence metal clusters. Long metal-oxygen distances of typically 2.7 Å have been previously reported in supported metal catalysts,44,45 and it could be a good indication of a strong metal-support interaction. However, the presence of a Au-O shell in addition to a Au-Au shell was not supported by either fitting with variable k-weightings or by investigation of the shapes of the magnitude and imaginary parts of the uncorrected and phase-corrected Fourier transforms. As opposed to the case of Au supported on mixed oxide, a clear and well-defined single shell fitting was obtained for the case of Au on layered TiO2/SiO2 (Figure 9b). In this case, both the coordination number and the bond distance corresponded to the presence of bigger Au particles. Table 3 summarizes the results of the fitting analysis presented in Figures 7-9 for the two consecutive treatments, the reduction in 4% H2/He at 150 °C, and the air treatment at high temperatures at 500 °C. 4. Discussion Our overarching goal is to tailor support materials for activation, control of particle size, and stabilization of Au nanoparticles. To achieve this purpose, we investigated two different synthesis methods involving TiO2 and SiO2. The first method resulted in the samples referred to as layered TiO2 on SiO2, and the second resulted in the ones referred to as mixed mesoporous SiO2-TiO2. XAFS enables the investigation of the local environment of the TiO2 species present in each of the SiO2-based samples and the stability of Au as formed in each of these environments. Moreover, the XAFS results obtained

Au Catalysts on TiO2-SiO2 Supports

J. Phys. Chem. C, Vol. 111, No. 46, 2007 17329

Figure 7. Magnitude of the Au LIII-edge k3-weighted Fourier transform of Au on layered supports and mixed Si/Ti oxide after (a) reductive treatment at 150 °C and (b) air treatment at 500 °C.

for the Ti edge and the Au edge should correspond to the structure of the Ti and Au species related to the catalyst in the most active state since the temperature used for the dehydration of TiO2 would also be sufficient to ensure complete reduction of Au particles. Besides, higher temperatures as used to induce Au agglomeration did not result in any observable change in the Ti XAFS spectra. The combination of the EXAFS and XANES results indicates that the mixed oxides have a large portion of the Ti cations at the silica surface or occupying silica-like sites at the surfaces of the mesoporous material. There are five indicators of this conclusion: (1) The intensity and energy of the pre-edge features of the mixed oxides support the presence of four-coordinated Ti species especially for Si:Ti ) 10 (Figure 2). (2) The changes during heat treatment clearly show changes of coordination environment that can be due to dehydration of the titania located mainly at the surface. (3) The lack of features in the post-edge region corroborates the idea of smaller clusters and/or disordered materials, indicating that most of the Ti atoms are highly dispersed on the silica structure and not present as crystalline anatase particles. (4) Ti-O bond distances in the mixed oxide are shorter than in the anatase or rutile TiO2 crystalline structures

(1.81-1.88 Å for mixed oxide compared to 1.95 Å for anatase), and the coordination numbers vary between 4 and 5. (5) A neighboring Ti shell near 3 Å was not observable (Figure 4). The high dispersion of Ti within the silica matrix was concluded previously from XRD analysis of the mixed oxides.8 Annealing the oxide with Si:Ti ) 5.5 to 700 °C yields little or no indication of anatase features in the XRD, although a physical mixture of mesoporous TiO2 and SiO2 in the same molar ratio and annealed to 700 °C yields clear anatase XRD features.8 The state of the Ti in the samples with Si:Ti ) 10 and 5.5 was previously not clear since no anatase form was detectable by XRD. The present results demonstrate that the majority of Ti cations for these samples are not cross-linked to other Ti cations but instead are highly dispersed on the silica surface and/or incorporated within the silica framework. In titanium silicalite structures such as TS-1, only a low fraction of Ti atoms (about 3 wt % TiO2) can be inserted into the framework.46 In TiO2SiO2 glasses, Greegor et al. suggested that the Ti atoms possess predominantly tetrahedral coordination in the range of 0.05-9 wt % TiO2 with a small amount of Ti in octahedral coordination (less than 5% of the total Ti atoms).47 As observed by our measurements, as the TiO2 amount increases in the mixture,

17330 J. Phys. Chem. C, Vol. 111, No. 46, 2007

Schwartz et al.

Figure 8. EXAFS above the Au LIII-edge of the Au on mixed Si/Ti (Si:Ti ) 5.5) oxide sample after reduction at 150 °C. (a) The magnitude and imaginary component of the Fourier transform k3-weighted (uncorrected for phase shift) and comparison with the Fourier filtered data (gray line) after applying a window around the first shell from 1.4 to 3.4 Å. (b) Fourier filtered (black curve) and curve-fitting results (gray curve) with one Au-Au shell (CN ) 6.2, d ) 2.84 Å, Vk3 ) 20). (c) Fourier filtered (black curve) and curve-fitting results (gray curve) with two Au-Au shells (CN ) 1.4, d ) 2.71 Å; and CN ) 3.8, d ) 2.84 Å; Vk3 ) 0.42).

TABLE 3: Results of the Curve Fitting of the First Shell of the Au LIII-Edge after Heating in 4% H2/He at 150 °C and after Consecutive Treatment in Air at 500 °Ca shell

CN

R, Å

∆σ2/10-4 Å2

∆E°/eV

Vk3-fit

7 wt % Au/TiO2/TiO2/SiO2 (Cab)

4% H2/150 air/500 4% H2/150 air/500 4% H2/150 air/500 4% H2/150

Au-Au Au-Au Au-Au Au-Au Au-Au Au-Au Au-Au

air/500 4% H2/150

Au-Au Au-Au

5 wt % Au/mixed Si:Ti ) 5.5

air/500 4% H2/150

Au-Au Au-Au

air/500

Au-Au

2.85 2.87 2.85 2.86 2.85 2.86 2.79 2.87 2.87 2.73 2.85 2.85 2.71 2.84 2.84

46 33 59 39 52 38 73 35 33 47 46 53 39 52 51

-5.3 -4.0 -7.3 -4.5 -3.8 -5.1 0.2 -7.5 -4.1 2.3 -5.4 -5.9 1.6 -4.4 -5.1

0.44 0.45 1.76 0.62 0.61 0.33 0.66

9 wt % Au/Al2O3/TiO2/SiO2 (Cab)

8.1 11.8 7.1 10.6 6.5 10.7 4.9 1.8 11.8 2.3 3.2 8.5 1.4 3.8 6.2

Au EXAFS samples 6 wt % Au/TiO2/SiO2 (Cab) 10 wt % Au/TiO2/Al2O3/SiO2 (Cab) 4 wt % Au/TiO2//SiO2 (SBA)

a

treatment temp/°C

The errors in the final parameters are expected to be for CN, (10%; for R, (0.02 Å; for

the Ti-O distance becomes longer and the shell becomes more populated as nanocrystallites of anatase start forming by agglomeration and separation from the silica matrix. Davis et al.38 also reported an increase of Ti-O distances with higher amounts of SiO2 in a previous study of a series of high surface area Ti-Si mixed oxides. Additionally, in the case of the mesoporous material, and especially before any heat treatment, the large surface areas would generate large amounts of Ti atoms at the surface that may present higher coordination numbers due to hydration. In the case of lower Ti content, divergences of the Ti-O coordination number from a pure fourfold coordination environment (Table 2) may be due in part to the

∆σ2,

0.33 1.02 1.12 0.42 0.55

(20%; and for ∆E°, (20%.

typical statistical error in the first shell analysis (typically 10%) combined with the fact that EXAFS averages out all the species present. In the case of the layered supports, the Ti XANES features suggest that the Ti atoms are in a mixture of five- and sixfold coordinate environments (Figure 2). The Ti EXAFS region of the monolayer support (Figure 5) reveals that the Ti-O neighbors are at about 1.93 Å and two Ti shells at about 3.03 and 3.8 Å. The positions of the atomic shells correlate well with an anatase crystal structure. By using a surface sol-gel technique for deposition of single monolayers of an oxide such as TiO2, it is expected that the titania precursor would attach at

Au Catalysts on TiO2-SiO2 Supports

J. Phys. Chem. C, Vol. 111, No. 46, 2007 17331 particle growth. In the case of the double-layer supports containing Al2O3, the order of deposition affects the titania structure and the size and stability of Au. Smaller and stable Au particles were formed upon deposition on the Al2O3/TiO2/ SiO2. Factors other than the support nature might influence the particle size, such as the Au loading. For the samples analyzed, the loading varied in the range of 5-10 wt % and, in this range, we found no correlation between particle size and loading. For instance, the mixed TiO2-SiO2 support (Si:Ti ) 5.5) with 5 wt % Au had the smallest Au particle sizes measured (Figure 7) but the comparably loaded monolayer TiO2/SiO2 (about 6 wt % Au) showed the largest particle size. The presence of Au atoms did not seem to alter the observed environment of the titania, although as the Au loading increased on the TiO2/SiO2 (Cab), lower Ti-O CNs were observed (Table 1). The reason for this variation is not clear, since no Ti-Au shell was detectable in the Ti EXAFS, nor was a Ti or Si shell in the Au EXAFS spectrum. Because of the inability to detect Ti-Au or Si-Au neighbors, it is not possible to ascertain that the Au has a preference to nucleate at silica sites vs TiO sites on the titanosilica supports. However, because of the consistently smaller Au particle size on the mixed oxides, it appears that the highly dispersed, low coordinate Ti cations favor nucleation and stabilization of many, small precursor particles under the DP condition. 5. Conclusions

Figure 9. EXAFS above the Au LIII-edge of the Au on layered TiO2/ SiO2 sample after reduction at 150 °C. (a) The magnitude and imaginary component of the Fourier transform k3-weighted (uncorrected for phase shift) and comparison with the Fourier filtered data (gray line) after applying a window around the first shell from 1.4 to 3.4 Å. (b) Fourier filtered (black curve) and curve-fitting results (gray curve) with one Au-Au shell (CN ) 8.1, d ) 2.85 Å, Vk3 ) 0.44).

silanol groups, yielding high dispersion on the silica without cross-linking. However, the first and second Ti coordination shells identified by EXAFS at 3.05 and 3.8 Å on the monolayer and double-layer TiO2/SiO2 materials indicate that the titania species are cross-linking, although the number of nearest neighbor is decreased in the second shell. Decreasing the CN with distance probably indicates the presence of layered anataselike species instead of thick crystallites of anatase. The second Ti-Ti shell completely disappears (Table 1) upon adding a layer of Al2O3 prior to depositing the TiO2, indicating that the alumina surface enhances dispersion of the TiO2 species. It is clear that the order of deposition of Al2O3 and TiO2 affects the nucleation and coordination environment of the Ti atoms upon comparison of the two double-layer compounds containing Al2O3 and TiO2 (Table 1). We have observed differences in the size and stability of the Au particles for the different supports. On the mixed mesoporous SiO2-TiO2, after reducing at 150 °C, the Au-Au first shell CN was consistently low, indicative of small particles, regardless of the Si:Ti ratio. The shorter Au-Au bond distances are another indication of the presence of small Au particles.41 In contrast, the Au EXAFS region of the layered supports presented stronger features and fitting of the first Au-Au shell resulted in higher coordination numbers when compared to the mixed oxide supports. Moreover, the Au particles showed dramatic growth in the case of the layered supports after heating to 500 °C, and the first coordination numbers found during the fitting were close to a bulk value of 12. Au particles on the mixed oxide supports showed more stability upon heating to 500 °C despite some

In this study, we investigated the mixing and the local environment of the TiO2 species present in SiO2-based samples and how the environment could affect the particle size and growth of Au nanoparticles. Two distinct approaches were used to synthesize the oxide supports. One approach was using cosynthesis techniques to dope bulk mesoporous SiO2 with TiO2. Another approach was to functionalize SiO2 by using a surface sol-gel technique for deposition of single monolayers of an oxide such as TiO2. The first approach resulted in TiO2 being highly dispersed on the SiO2 structure as indicated by the preedge position, the EXAFS Ti-O shell position, and the lack of a Ti-Ti interaction. The large surface area of the mesoporous structure yielded most likely large amounts of titania species at the surface. Therefore, the higher coordination environment of the Ti atom evidenced by the pre-edge region would be likely due to hydroxyl groups present prior to any heat treatment. Higher amounts of TiO2 mixed (lower Si:Ti ratio) during synthesis increased the Ti-O distance and coordination number, suggesting the presence of mixed phases as the Ti content increases. In one phase, most of the Ti is in a fourfold structure dispersed in the silica matrix, and in the other phase Ti atoms agglomerate to form an anatase-like structure isolated from the silica. The second approach produced Ti environments in which the Ti-O shell and the next two Ti-Ti shells lie on the same position as expected for an anatase structure. The Ti-Ti shells were reduced (undercoordinated) compared to a bulk anatase structure; however, their presence is a clear indication of crosslinking of the titania moieties, contradicting the hypothesis of molecularly dispersed TiO2 species on the SiO2 surface. The Au particles were found to be smaller and more stable when supported on the mixed mesoporous SiO2-TiO2, where the titania was tetracoordinated and highly dispersed in the silica structure, compared to the layered TiO2/SiO2. Acknowledgment. Research sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy, under

17332 J. Phys. Chem. C, Vol. 111, No. 46, 2007 Contract No. DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC. W.Y. and H.Z. are sponsored by an appointment to the Oak Ridge National Laboratory Postdoctoral Research Associates Program administered jointly by the Oak Ridge Institute for Science and Education and Oak Ridge National Laboratory. A portion of this work was performed at the National Synchrotron Light Source at Brookhaven National Laboratory, which is supported by Basic Energy Sciences, U.S. Department of Energy, under Contract No. DE-AC02-98CH10886. References and Notes (1) Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M. J.; Delmon, B. J. Catal. 1993, 144, 175. (2) Chen, M. S.; Goodman, D. W. Catal. Today 2006, 111, 22. (3) Bond, G. C.; Louis, C.; Thompson, D. T. Catalysis by Gold; Imperial College Press: London, 2006. (4) Bamwenda, G. R.; Tsubota, S.; Nakamura, T.; Haruta, M. Catal. Lett. 1997, 44, 83. (5) Grunwaldt, J. D.; Maciejewski, M.; Becker, O. S.; Fabrizioli, P.; Baiker, A. J. Catal. 1999, 186, 458. (6) Overbury, S. H.; Ortiz-Soto, L.; Zhu, H. G.; Lee, B.; Amiridis, M. D.; Dai, S. Catal. Lett. 2004, 95, 99. (7) Yan, W. F.; Mahurin, S. M.; Chen, B.; Overbury, S. H.; Dai, S. J. Phys. Chem. B 2005, 109, 15489. (8) Zhu, H. G.; Liang, C. D.; Yan, W. F.; Overbury, S. H.; Dai, S. J. Phys. Chem. B 2006, 110, 10842. (9) Chi, Y.-S.; Lin, H.-P.; Mou, C.-Y. Appl. Catal., A: Gen. 2005, 284, 199. (10) Yang, C.-m.; Kalwei, M.; Schu¨th, F.; Chao, K.-j. Appl. Catal., A: Gen. 2003, 254, 289. (11) Lee, B.; Zhu, H. G.; Zhang, Z. T.; Overbury, S. H.; Dai, S. Microporous Mesoporous Mater. 2004, 70, 71. (12) Yan, W. F.; Mahurin, S. M.; Overbury, S. H.; Dai, S. Top. Catal. 2006, in press. (13) Zhu, H. G.; Pan, Z. W.; Chen, B.; Lee, B.; Mahurin, S. M.; Overbury, S. H.; Dai, S. J. Phys. Chem. B 2004, 108, 20038. (14) Dmowski, W.; Egami, T.; Swider-Lyons, K.; Yan, W.; Dai, S.; Overbury, S. H. Submitted for publication in Z. Kristallogr. 2007. (15) Farges, F.; Brown, G. E., Jr.; Rehr, J. J. Phys. ReV. B 1997, 56, 1809. (16) Gao, X. T.; Bare, S. R.; Fierro, J. L. G.; Banares, M. A.; Wachs, I. E. J. Phys. Chem. B 1998, 102, 5653. (17) Asakura, K.; Inukai, J.; Iwasawa, Y. J. Phys. Chem. 1992, 96, 829. (18) Thomas, J. M.; Sankar, G. Acc. Chem. Res. 2001, 34, 571. (19) Sankar, G.; Thomas, J. M.; Catlow, C. R. A.; Barker, C. M.; Gleeson, D.; Kaltsoyannis, M. J. Phys. Chem. B 2001, 105, 9028. (20) Yan, W. F.; Chen, B.; Mahurin, S. M.; Schwartz, V.; Mullins, D. R.; Lupini, A. R.; Pennycook, S. J.; Dai, S.; Overbury, S. H. J. Phys. Chem. B 2005, 109, 10676.

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