Unravelling the Nature of Gold Surface Sites by Combining IR

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Unravelling the Nature of Gold Surface Sites by Combining IR Spectroscopy and DFT Calculations. Implications in Catalysis M. Boronat, P. Concepcioń, and A. Corma* Instituto de Tecnologıá Quı´mica (UPV-CSIC), AV. de los Naranjos s/n, 46022 Valencia, Spain ReceiVed: June 2, 2009; ReVised Manuscript ReceiVed: July 21, 2009

A combination of XPS, IR spectroscopy of adsorbed CO, and DFT calculations has allowed us to identify different gold sites on Au/TiO2 catalysts, and to analyze the electronic and structural changes induced on the supported gold nanoparticles when adsorbing CO. The characteristics of low coordinated neutral Au0, slightly positively charged Auδ+, and cationic AuIII species have been studied, and the IR bands associated with CO adsorbed on these species have been determined. The shift in the νCO frequency not only depends on the charge on the particle or on the particular gold atom to which CO binds but also on the degree of coordination of gold atoms, that is, particle shape, and the capability of CO to modify this particle shape. It has been unequivocally shown by combining IR/DFT results and H/D isotopic exchange experiments that, among the different gold species identified, only the low coordinated neutral gold atoms not involved in Au-O-Ti linkages are responsible for H2 dissociation on Au/TiO2 catalysts. Introduction Gold nanoparticles finely dispersed on inorganic oxide supports are active catalysts, in contrast to bulk gold that is chemically inactive.1 CO oxidation is the most studied reaction on gold, either experimentally2-8 or theoretically.9-13 However, catalysis by gold has been opened to other reactions and increased interest has been attracted by selective alcohol14-16 and olefin17-20 oxidations, C-C bond forming reactions,21-23 direct synthesis of hydrogen peroxide from H2 and O2,24 and selective hydrogenations of olefins,25,26 aldehydes,27-29 and nitroaromatics.30,31 The catalytic activity of supported gold nanoparticles is attributed to a number of morphological and electronic effects such as particle size and shape, oxidation state of gold, charge transfer between the support and the gold nanoparticle, and quantum size effects. A clear understanding of the contribution of each of these factors toward catalytic activity still remains unclear. It has been proposed that the increasing activity of supported gold catalysts with decreasing gold particle size can be directly related to the number of low coordinated gold atoms present at the particles, on which reactants are more strongly adsorbed and activated.27,32-36 However, a decrease in the CO oxidation reaction rate when the catalyst particle size decreases below 3 nm has been reported, indicating that the presence of low coordinated corner or edge sites alone is not the only factor influencing the catalyst activity, at least for this reaction.3a,37,38 Particle morphology rather than particle size has been proposed as a key parameter the catalytic properties of gold, both for hydrogenation34,35,39 and for CO oxidation, for which a very high reaction rate has been observed on supported Au nanoparticles with a highly ordered Au bilayer structure.38,40 There is also general agreement concerning the higher catalytic activity of gold supported on reducible oxides such as TiO2, Fe2O3, or CeO2.2-4,6,41,42 However, it is under debate if the role of the support is limited to the stabilization of small * To whom correspondence should be addressed. E-mail: acorma@ itq.upv.es. Phone: 34 96 387 7800. Fax: 34 96 3879444. E-mail: [email protected].

particles with large amounts of low coordinated gold sites3,13,43 or some of the reactants are stabilized or activated at the metal-support interface or on the support itself.41,44-46 On the other hand, while it has been demonstrated that metallic gold nanoparticles are active for hydrogenation39 and oxidation reactions,47 the role of ionic gold surface sites (Au+1,+3) has also been proposed and discussed,22,46,48-51 though both types of gold sites can be two faces of the same coin. Oxidic gold species are formed during sample preparation through partial lattice filling of vacant sites by gold ions, and therefore, the method of preparation of the catalyst and the calcination or reduction pretreatments strongly influence its activity.2,4,52-54 IR spectroscopy of CO adsorption has been widely used in the literature to determine the nature and electronic state of the active sites in gold catalysts, since the vibrational stretching mode of CO adsorbed on gold shifts to lower frequency on electron-rich particles and to higher frequency on electrondeficient particles in relation to metallic gold. However, care is required in the interpretation of vibrational spectra because a combination of several effects is influencing the CO stretching vibrations.32 It is generally accepted that CO adsorbs weakly on top of gold atoms in single crystal gold surfaces, with stretching frequencies around 2110 cm-1.55,56 IR studies of CO adsorption on gold nanoparticles supported on TiO2 show, in most cases, a single band at ∼2100-2110 cm-1 assigned to CO adsorbed on top of gold metallic sites. When the experiments are carried out in the presence of O2, the band appears at 2120-2130 cm-1, and is assigned to CO coadsorbed with oxygen on the gold particles or to CO adsorbed on gold atoms at the metal-support interface.53,57,58 In some cases, a signal appearing around 2140-2160 cm-1 has been attributed to CO adsorbed on Au+ sites,53 although the exact nature and origin of these sites is not known. Another question that has been considered is whether interaction with CO leaves the catalyst surface in its original state or on the contrary CO modifies the surface electronically or morphologically. It has been reported that at high CO pressures gold particles are irreversibly spread on the TiO2 support and, as a consequence, additional CO adsorption sites

10.1021/jp905157r CCC: $40.75  2009 American Chemical Society Published on Web 08/28/2009

Unravelling the Nature of Gold Surface Sites characterized by a signal at 2060 cm-1 appear.58 It has also been observed that CO adsorption on Au(111) occurs only above 1 Torr CO pressure at RT, with an adsorbate induced modification of the surface morphology that is evidenced by STM, and with the νCO stretching frequency being shifted to 2060 cm-1. In this case, the shift to lower frequencies cannot be related to electron-rich or negatively charged species that have taken electron density from the inorganic oxide support. Instead, theoretical calculations indicate that the νCO stretching frequency shifts to lower frequencies as the coordination number of the CO-bonding gold atom decreases.59 Finally, it should be taken into account that very small particles or gold atoms are not restructured but are modified electronically by interaction with CO.60 The aim of the present work is to analyze in depth the interaction of CO with nanocrystalline Au/TiO2 catalysts, abording the question from different perspectives. On one hand, the changes in the νCO stretching frequency associated with undercoordination, localization, or charging of the gold atoms interacting with CO have been investigated by means of density functional theory (DFT) calculations. The calculated frequencies have been successfully compared with those obtained experimentally by infrared spectroscopy on different types of Au/ TiO2 catalysts, and in this way, the nature of different sites interacting with CO and responsible for different bands in the IR spectra has been established. On the other hand, the electronic and morphological changes induced on the gold nanoparticles by interaction with CO have also been investigated, and it has been demonstrated that the particle shape is irreversibly modified by CO adsorption. Thus, characterization of gold catalysts by IR spectroscopy of CO should be done with care, since it provides information about the catalyst not in its initial state but after interaction with CO. Finally, the rate of H/D exchange of a series of Au/TiO2 catalysts has been measured and correlated with the amount of gold sites characterized by IR bands in the 2077-2110 cm-1 range present in these samples, confirming that among the different gold sites, only low coordinated gold atoms not directly bonded to O and which are characterized by νCO stretching frequencies between 2070 and 2110 cm-1 are active for H2 dissociation. Theoretical Basis Adsorption of CO on TiO2 surfaces, Au atoms, and Au nanoparticles supported on TiO2 was studied by means of periodic slab models using large supercells to avoid interaction between the periodically repeated Au atoms or particles or between the adsorbates. The TiO2 support was represented by means of (2 × 2) and (4 × 4) supercell slab models of the most reactive (001) facet of the anatase polymorph,61,62 with unit cells containing three TiO2 layers, that is, nine atomic layers and a vacuum region larger than 20 Å placed between vertically repeated slabs. Besides the stoichiometric surface (TiO2 model), defective surfaces containing a OH group (TiO2-TiOH), an O vacancy defect (TiO2-Ovac), and one, two, or three O adatoms (TiO2-nOad) were also considered. A gold atom and a gold nanoparticle containing 13 gold atoms, Au13, were then placed on the stoichiometric (TiO2), the reduced (TiO2-Ovac), and the oxidized (TiO2-nOad) surface models. The atomic positions of all Au atoms and of the Ti and O atoms of the two uppermost atomic layers of the support were fully relaxed, and spinpolarization was included when necessary. Finally, CO was placed near different gold atoms in each of the models, and the positions of the C and O atoms of CO, of all Au atoms, and of the Ti and O atoms of the two uppermost layers of the support were fully optimized.

J. Phys. Chem. C, Vol. 113, No. 38, 2009 16773 TABLE 1: Physicochemical Properties of the Au/TiO2 Samples sample

pH

T (K)

% wta

GT4 GT33 GT5 GT2 GT27 GT37 WGC GT25 GT36 GT21

9.0 7.0 9.0 6.3 6.3 7.0 7.0 6.3 6.3 6.3

473 373 473 473 473 473 673 673 473 673

0.47 1.06 1.26 1.26 2.90 1.06 1.52 2.9 1.21 2.8

D (nm) TEMb

2.3 3.2 3.2 3.3 3.5 5.1 6.1 19

Au4f7/2 BE (XPS)c

R parameter (XPS)d

84.4 eV 84.1 eV

n.d. n.d.

84.2 eV

2098.2

84.3 eV 84.3 eV

2099.3

a Weight percent of gold as determined by atomic adsorption spectroscopy. b Particle size determined by TEM. c BE referred to Ti2p3/2 ) 457.5 eV. d XPS auger parameter determined as BE(Au4f7/2) + KE(Au:M5N67N67).

All calculations are based on density functional theory (DFT) and were carried out with the VASP code,63 using the Perdew-Wang (PW91)64 exchange-correlation functional within the generalized gradient approximation (GGA). The KohnSham orbitals used to obtain the electron density were expanded in a plane wave basis set with a kinetic energy cutoff of 415 eV, and the effect of the core electrons was taken into account by means of the projected augmented wave (PAW) method.65 Calculations were carried out using a (3 × 3x1) Monkhorst-Pack mesh66 for the (2 × 2) supercell models and at the Γ k-point of the Brillouin zone for the (4 × 4) supercell models. The atomic positions were optimized by means of a conjugate-gradient algorithm until atomic forces were smaller than 0.01 eV/Å. Vibrational frequencies were calculated by diagonalizing the block Hessian matrix corresponding to displacements of the C and O atoms of CO and the Au or Ti atoms to which CO is bonded. Charge distributions were estimated by making use of the theory of atoms in molecules (AIM) of Bader.67 Experimental Section Sample Preparation. Gold catalysts were synthesized by a deposition-precipitation technique of the gold nanoparticles ontotheTiO2 surface(Degussa,P-25).Thedeposition-precipitation procedure was carried out by adding to the support an aqueous solution of HAuCl4 (0.01 M) containing a proper amount of gold. The resulting mixture was kept 2 h at 343 K under vigorous stirring, controlling the pH of the solution with NaOH at a specific set point. Then, the resulting powder is filtered, washed with distilled water to remove chlorides, dried in an oven at 373 K, and, finally, calcined in air at the desired temperature. Depending on the pH of deposition, loading of gold, and calcination temperature, different gold particle sizes can be obtained (see Table 1). WGC catalyst refers to the World Gold Council gold reference catalyst. The gold loading in the samples was determined by atomic adsorption spectroscopy using a Varian SpectraA-10Plus equipment. FTIR Experiments. Fourier transform infrared (FTIR) spectra have been obtained on a Biorad FTS-40A spectrometer equipped with a DTGS detector. The experiments have been carried out in a homemade IR cell able to work in the high and low (77 K) temperature range. Prior to CO adsorption experiments, the sample has been evacuated at 298 K in a vacuum (10-6 mbar) for 1 h. In one case, the catalyst has been treated in a 4% CO/He flow at 298 and 373 K for 1 h prior to the CO adsorption experiment. CO adsorption experiments have been performed at 77 K in the 0.2-20 mbar range. Spectra were

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Figure 1. XPS Au/Ti surface composition for Au/TiO2 catalysts as a function of bulk Au/Ti atomic ratio.

recorded once complete coverage of CO at the specified CO partial pressure has been achieved. Deconvolution of the IR spectra has been performed in the Origin software using Gaussian curves where the full width at half-maximum (fwhm) of the individual bands has been taken as constant. The peak areas are normalized to the sample weight. XPS Spectra. X-ray photoelectron spectroscopy (XPS) spectra have been collected using a SPECS spectrometer equipped with a Phoibos 9MCD detector and pass energy of 50 eV. The spectra have been collected using Mg KR (1253.6 eV) as the excitation source working at 200 W. Prior to spectra acquisition, the samples have been outgased at 10-10 mbar for 8 h. TEM. Transmission electron microscopy (TEM) was performed using a Philips-CM10 electron microscope at an accelerating voltage of 100 kV. Samples for TEM observations were directly supported on a copper mesh with a carbon microgrid. Particle size distributions were determined from an average of 150 particles obtained from different micrographs. Isotopic H/D Exchange Experiments. Hydrogen/deuterium (H/D) exchange experiments were carried out in a flow reactor at 298, 348, and 393 K, according to the procedure depicted in ref 34. The feed gas consisted of 2 mL/min H2, 2 mL/min D2, and 6 mL/min argon, and the total weight of catalyst was 125 mg. Reaction products (H2, HD, and D2) were analyzed with a mass spectrometer (Omnistar, Balzers). Prior to catalytic testing, the samples were activated in flowing argon (10 mL/min) at room temperature (298 K) for 30 min. Then, the gas feed was changed to the reactant gas composition, while the temperature was maintained at 298 K for 30 additional minutes. At this point, the temperature was increased at 10 K/min to 348 K, kept 30 min at that temperature, and further increased to 393 K at 10 K/min. Results and Discussion The physicochemical properties of the catalysts are given in Table 1, while the surface composition determined by XPS is shown in Figure 1. A homogeneous distribution of gold is observed on catalysts with a low weight percent of gold, while at higher amounts of gold three-dimensional particles are favored, giving an apparent enrichment of gold on the catalyst surface. The Au4f7/2 BE of different samples is given in Table 1. In all cases, a value of 84.1 ( 0.3 eV is observed which corresponds to metallic gold, in agreement with their auger parameter of 2099.8 eV. No contribution of cationic gold surface sites, related to gold in a formal oxidation state of I or III (AuI,

Boronat et al. AuIII), has been observed in any case. Thus, XPS results show the presence of metallic gold species on all samples, while no differences in their electronic properties can be inferred. In the samples with low Au content (GT4 and GT33), determination of the auger parameter was not possible, and information about the charge of the gold atoms could not be obtained in this case. Since XPS is not sensitive to identify gold atoms with slightly different electronic properties related to different coordination, FTIR spectroscopy of adsorbed CO as a probe molecule has been used, since CO has been shown to be very sensitive to the electronic properties of the adsorption site. Experimental results obtained by FTIR spectroscopy of CO in combination with theoretical DFT calculations have been used in order to identify the different gold sites on the catalyst surface. 1. Experimental Results Obtained by FTIR Spectroscopy. Although particle size is one of the most important parameters determining the catalytic activity of nanosized gold catalysts, this is not the only one and, as already indicated in the Introduction, some authors attribute the catalytic behavior of gold to particle morphology rather than to particle size. In this work, catalysts with different gold particle sizes have been synthesized, and our attention has been focused on the characterization of Au/TiO2 samples presenting the same particle size as determined from TEM results (WGC (3.4 nm), GT37 (3.3 nm), GT27 (3.2 nm), GT2 (3.2 nm), see Table 1). As an example, the TEM photography as well as the particle size distribution of two selected samples (GT37 and GT27) are shown in Figure 2. Our results indicate that, although a similar particle size distribution is observed in the samples named above, the nature of gold surface sites is different as determined from FTIR spectroscopy of CO adsorption. The IR spectra of CO adsorbed at 77 K on samples WGC, GT37, GT27, and GT2 are presented in Figure 3. In the IR frequency range above the CO gas phase stretching frequency, IR bands at 2177, 2163, and 2148-2150 cm-1 are observed. The IR bands at 2177 and 2162 cm-1 have been assigned to CO adsorbed on five coordinated TiIV sites located on different faces of the anatase surface.68 On the other hand, the IR band at 2150 cm-1 can be ascribed to CO adsorbed on OH groups. In fact, a shift of the νOH group on the titania surface from 3636 to 3583 cm-1 is observed in parallel to the increase of the 2150 cm-1 IR band (spectra not shown). However, at very low CO coverage, no shift of the hydroxyl group is observed while a weak band at 2148 cm-1 is still present (see difference spectra (a and b) in the νOH IR region (inset of Figure 3a) compared to their respective spectra in the CO stretching vibration region. Thus, the contribution of surface species other than the OH groups to the 2150 cm-1 IR band should be considered. Indeed, literature data assigned cationic Au+-CO surface sites (with gold in a formal AuI or AuIII state) as being responsible for a 2148 cm-1 IR band.53 A careful study of the IR spectra at low CO coverage leads us to conclude the presence of this type of cationic gold surface sites in all samples in addition to TiIV surface sites and Ti-OH groups. The relative concentration of these cationic gold sites in our samples cannot be determined from the IR spectra due to the contribution of the OH groups to the same IR band. Nevertheless, the amount of cationic gold sites should be relatively low, since XPS spectra do not reveal the presence of any contribution due to AuI or AuIII species in the Au/TiO2 samples. On the other hand, the presence of TiIII in Au/TiO2 catalysts cannot be determined from IR spectroscopy. Several authors argue that TiIII will be immediately oxidized under the experimental conditions of CO adsorption at 77 K and thus will not be detectable by IR spectroscopy of CO,69 while other

Unravelling the Nature of Gold Surface Sites

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Figure 2. TEM micrographs of GT27 and GT37 samples and their respective particle size distributions.

Figure 3. FTIR spectra of CO adsorbed at 77 K on WGC (a), GT37 (b), GT27 (c), and GT2 (d) samples at increasing coverages (1-10 mbar). Inset of Figure 3a: difference spectra in the νOH IR region, with respect to the sample without CO adsorption.

authors assigned IR bands at 2120-2110 cm-1 to CO-TiIII carbonyl complexes.70 In our case, even if the CO-TiIII carbonyl complex was present, the IR bands associated with it would be overlapped by those associated with CO interacting with gold surface sites (IR bands at 2135-2070 cm-1). Thus, we cannot conclude by IR spectroscopy of adsorbed CO the presence or

absence of TiIII surface sites in our samples, but XPS spectra in the Ti2p3/2 band do not show any contribution due to TiIII. We have then to conclude that if TiIII species are present in our catalysts their concentration should be very low. At frequencies below 2145 cm-1, IR bands at 2132, 2125, 2110, 2097, and 2077 cm-1, associated with gold surface sites,

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TABLE 2: Relative Concentration of Surface Sites Determined by FTIR Spectroscopy on Au/TiO2 Samples with Similar Particle Size (3.2 ( 0.2 nm)a species OH, AuI, AuIII

TiIV

a

Au0

sample

2176 (cm-1)

2162 (cm-1)

2150 (cm-1)

2135 (cm-1)

2125 (cm-1)

2110 (cm-1)

2096 (cm-1)

2077 (cm-1)

WGC GT37 GT27 GT2

0 0.953 0.089 0.706

0.151 0 0.10 0

0.124 0.752 0.361 1.53

0.07 0 0.117 0.417

0 0.203 0.044 0

0.148 0.170 0 0.127

0.246 0.120 0 0

0.109 0 0 0

IR peak areas, normalized to sample weight, at CO saturation coverage on the Au/TiO2 samples.

Figure 4. FTIR spectra of 0.6 mbar (A) and 6 mbar (B) CO adsorbed at 77 K on the GT33 sample without CO pretreatment (a) and with CO pretreatment at 298 K (b) and 353 K (c).

are observed. Assignation of the specific gold surface sites to each of the above-reported bands is not at all clear in the literature, and therefore, we have performed DFT calculations on model gold catalysts in order to identify the nature of the gold surface sites interacting with CO. These results will be reported below. Independent of the exact nature of surface sites, Table 2 shows the intensity of the IR band, determined by deconvolution of the IR spectra, in the different samples. It can be seen there that there are differences in the relative amount of different gold surface sites despite the fact that the same particle size is observed for all samples. Thus, we can conclude that the nature of surface sites is not only related to particle size, but other factors like particle morphology or metal-support interactions should also have an influence. It has been reported that CO interaction with gold surface sites modifies electronically and/or morphologically the particle. In fact, Figure 4 shows the IR spectra at 77 K of CO adsorbed at low (Figure 4a) and high CO coverage (Figure 4b) on sample GT33 without CO pretreatment (spectrum a) and after CO pretreatment at 298 K (spectrum b) and 353 K (spectrum c), respectively. At low CO coverage, no interaction of CO with hydroxyl groups is observed in the νOH IR region; thus, the IR band at 2148 cm-1 in Figure 4a can unambiguously be associated with cationic gold surface sites due to gold in a formal AuI or AuIII state. According to Figure 4a, the amount of cationic gold surface sites decreases slightly after CO treatment at 298 K and more drastically after CO treatment at 353 K. This could be related to a reduction of AuI and AuIII surface sites to Au0 due to CO interaction at a temperature >298 K. In fact, new IR bands associated with Au0 surface sites have been observed in spectra b (IR band at 2125 cm-1) and c (IR band at 2110 cm-1) of Figure 4a. On the other hand, at CO saturation coverage (see Figure 4b), a total increase in the amount of coordinated

unsaturated TiIV surface sites (IR band at 2174 cm-1) due to CO treatment at 298 and 373 K is clearly observed, as well as an apparent increase in the total amount of hydroxyl groups in the sample (IR band at 2150 cm-1) and the presence of new gold surface sites (IR bands at 2125 and 2110 cm-1). Thus, the surface of gold and the surface of TiO2 are modified by CO adsorption. 2. Different Gold Sites and Their Interaction with CO as per DFT Calculations. According to the IR results presented above, different gold surface sites have been considered in this part of the work, and their interaction with CO has been studied by DFT calculations. The validity of the methodology has been first proven for the TiO2 support, and then applied to gold atoms and gold particles. 2.1. TiO2 Support. In a first step, CO was adsorbed on TiIV sites located at the stoichiometric (001) and (101) faces of anatase, on a TiIII site associated with a oxygen vacancy defect, and on a surface hydroxyl group (see Figure 5). The adsorption energies listed in Table 3 indicate a weak interaction with the oxide surface in all cases. The calculated νCO stretching modes for CO adsorbed on the stoichiometric surfaces, 2177 and 2166 cm-1, are in very good agreement with the experimental values of 2176 and 2162 cm-1 assigned to CO adsorbed on five coordinated TiIV sites located on different faces of anatase.68 A good agreement between the calculated (2155 cm-1) and the measured (2150 cm-1) vibration frequencies is also obtained for CO adsorbed on surface OH groups, demonstrating the validity of the methodology employed for the theoretical study. The calculated vibration frequency for CO adsorbed on a TiIII site associated with an oxygen vacancy defect is 1999 cm-1, somewhat lower than that reported by IR spectroscopy in the literature.69 Such a band is not observed in our case in any of the samples, probably because the concentration of TiIII sites is too low, as indicated by the XPS spectra, and because gold



Figure 5. Optimized structures of CO adsorbed on stoichiometric (001) and (101) and defective (001) surfaces of anatase.

TABLE 3: CO Adsorption on Stoichiometric and Defective Surfaces of Anatase, on Some Species of Gold on Anatase, and on an Isolated Au13 Nanoparticlea TiO2 TiO2 (101)c TiO2–Ovac TiO2–TiOH Au/TiO2 Au/TiO2–Ovac Au/TiO2–Oad Au–TiO2 Au–TiO2–Ovac Au–TiO2–2Ovac Au13

Eads (eV)

νCO (cm-1)

d(CO) (Å)

d(C-M)b (Å)

qAu0 (e)

qAu-CO (e)

–0.318 –0.257 –0.585 –0.135 –1.727 –0.304 –2.511 –0.273 –2.434 –1.370 –0.931

2177 2166 1999 2155 2084 2026 2107 2145 1704,1126 2091 2072

1.139 1.139 1.159 1.140 1.155 1.154 1.152 1.144 1.213,1.306 1.153 1.150

2.295 2.519 2.178 2.129 1.883 2.154 1.872 1.959 2.050 1.898 1.946

0.368 –0.490 0.467 1.271 0.846 0.476 0

0.473 –0.386 0.611 1.270 0.536 0.616 0

a The calculated νCO stretching frequency for gas phase CO is 2143 cm-1. qAu0 is the net atomic charge on gold in the initial catalyst, and qAu-CO is the net atomic charge on the gold atom in the CO adsorption complex. b M ) Ti or Au. c The anatase (001) surface is considered in all other cases.

Figure 6. Optimized structures of CO adsorbed on some species of gold on anatase.

nanoparticles initially nucleate on oxygen vacancy defects, impeding or making difficult the adsorption of CO on TiIII sites. 2.2. Highly Dispersed Supported Gold. CO adsorption on six different models that simulate highly dispersed supported gold was studied in a second step (see Figure 6). Three of these models consist of a single gold atom placed either on a Ti atom of the stoichiometric TiO2 surface (Au/TiO2), in the middle of an oxygen vacancy defect (Au/TiO2-Ovac), or beside one oxygen atom adsorbed on the oxide surface (Au/TiO2-Oad) and simulate

gold atoms deposited on a stoichiometric, a reduced, and an oxidized TiO2 surface, respectively. The other three models are obtained by substituting one Ti atom in the metal oxide by a gold atom (Au-TiO2), and creating one (Au-TiO2-Ovac) or two (Au-TiO2-2Ovac) oxygen vacancy defects beside the Au atom. While the three former gold species initially correspond to metallic gold, the Au atoms substituting Ti atoms in the oxide are formally AuIII, although the presence of oxygen vacancy defects considerably modifies the net atomic charge on these

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Figure 7. Correlation between calculated νCO stretching frequency and net atomic charge on gold.

gold atoms (see Table 3). Thus, gold atoms deposited on the stoichiometric and oxidized surfaces bear a net positive charge of ∼0.4 e, while gold atoms placed in an oxygen vacancy defect become negative Au- species. The net positive charge on the AuIII atoms substituting TiIV or TiIII in the oxide structure decreases from 1.271 in the Au-TiO2 model to 0.846 and 0.476 in the Au-TiO2-Ovac and Au-TiO2-2Ovac models, respectively, suggesting a formal reduction of AuIII to AuI as the number of oxygen vacancy defects increases. The calculated adsorption energies indicate that CO interacts weakly with negatively charged metallic gold (in Au/TiO2-Ovac) and with cationic AuIII gold species (in Au-TiO2), and more strongly with slightly positive metallic Au0 species (with qAu ∼ 0.4 e). The largest adsorption energy corresponds to CO adsorbed on gold deposited on an oxidized TiO2 surface (Au/TiO2-Oad), in agreement with results obtained for gold nanoparticles supported on TiO2(110).71 The optimized geometry of the complex is similar to that obtained for CO adsorbed on Au/TiO2, which also forms a quite stable structure. In both systems, the gold atom is bonded to CO and one oxygen atom of the surface, with a linear arrangement of the OsAusCtO atoms involved. The gold atom in the oxidized surface is more positive than in the stoichiometric one, and accordingly, the calculated stretching frequency is shifted to larger values (2107 versus 2084 cm-1). On the other hand, interaction of CO with the positively charged AuIII species in Au-TiO2 results in a calculated νCO stretching frequency of 2145 cm-1, suggesting that the experimentally observed band at 2148 cm-1 could be related to these AuIII cations present in the oxide structure. A very stable complex is also obtained by CO adsorption on the Au-TiO2-Ovac model, but as depicted in Figure 6, the optimized geometry corresponds to a carbonate-like species, as well as the calculated vibration frequency around 1700 cm-1. Except for this structure, a clear linear correlation has been found between the net atomic charge on gold and the νCO stretching frequency, both for single gold atoms, either deposited on the support surface or substituting a Ti atom, and for gold atoms in an isolated Au13 nanoparticle (Figure 7). Interestingly, a similar linear correlation between net atomic charge on different metal cations and the νCO stretching frequency has already been determined experimentally by IR spectroscopy72 but not observed until now on single gold atoms. 2.3. Supported Gold Nanoparticles. CO adsorption on gold nanoparticles is different from that on isolated gold atoms, and

Boronat et al. a relatively large number of nonequivalent gold sites has been characterized and described in this section. We know from previous work36 that gold nanoparticles become positively charged when supported on stoichiometric and oxidized TiO2 surfaces, and negatively charged when adsorbed on reduced surfaces. Moreover, the net charge on the nanoparticle is not equally distributed among all atoms but clearly localized on some of them. Thus, while most gold atoms in supported nanoparticles are neutral, those Au atoms directly bonded to an O atom of the support forming a Au-O-Ti bridge exhibit a net positive charge, and only medium coordinated Au atoms bonded to a Ti atom of the reduced surface are negatively charged. In this work, an irregular Au13 particle containing gold atoms with different coordination numbers, geometrical arrangements, net atomic charges, and neighboring atoms has been chosen to represent gold nanoparticles supported on stoichiometric and reduced titanium oxide surfaces (Au13/TiO2 and Au13/TiO2-Ovac models in Figure 8, respectively), and a Au13 particle formed around an O adatom on the stoichiometric TiO2 surface (Au13/ TiO2-Oad model) has also been considered. Two or four gold atoms, labeled as a-d in Figure 8, were chosen in each of the considered models. CO was initially placed at 2.0 Å from the Au site, and the atomic positions of the CO molecule, all Au atoms in the particle, and the two uppermost layers of the oxide support were fully optimized. The resulting structures are depicted in Figures 9-11, and the most relevant geometric, energetic, and charge distribution information, together with the calculated νCO vibration frequencies, are summarized in Tables 4-6. 2.3.1. Stoichiometric Support. CO adsorption on stoichiometric Au13/TiO2 is a highly exothermic process, with calculated adsorption energies ranging between -1.4 and -1.8 eV (Eads CO, Table 4). In all cases, no matter the initial coordination number (CN) or net atomic charge on gold, CO adsorbs on top of a gold atom, forming a complex with optimized Au-C distances around 1.95 Å and with the CO bond slightly elongated with respect to the gas phase value calculated at the same theoretical level, 1.142 Å. As a result of this interaction, the νCO stretching mode shifts and appears in the region between 2080 and 2100 cm-1, corresponding to CO adsorbed on low coordinated metallic Au0 gold atoms. Although the initial atomic charges on the four selected gold atoms, q(Auatom)0 in Table 4, were quite different, interaction with CO causes a reorganization of the electron density, and the calculated atomic charges on these gold atoms in the complexes (q(Auatom)-CO) become similar and positive. The most remarkable effect of CO adsorption, however, is observed in the optimized geometry of the gold nanoparticles depicted in Figure 9. Gold atom a in the Au13/TiO2 model is a highly coordinated terrace atom with a negligible net atomic charge. CO adsorption on this site causes a weakening of some Au-Au bonds and a slight deformation of the particle shape, but reoptimization of the particle after removal of CO leads to the initial Au13 structure (Figure 9a, right). Gold atom b is a corner atom with a negligible atomic charge, and its coordination number is not modified upon CO adsorption. However, atom d, that was directly bonded to an O atom of the support in the initial Au13/TiO2 model, moves upward and situates far from the oxide surface. When CO is removed from the model and the particle geometry is reoptimized, the Au-O bond is not recovered, and a different structure more stable than the initial model is obtained. The same effect has adsorption of CO on sites c and d. Both gold atoms are directly bonded to an O atom of the support and bear a net



Figure 8. Sites considered for CO adsorption on different Au/TiO2 catalyst models.

positive charge. Adsorption of CO causes the breaking of the Au-O bond, that is not recovered when CO desorbs. The final particles are more stable and less positively charged than the initial model, and have one more low coordinated gold atom accessible to interact with other molecules. This means, on one hand, that CO interaction with highly coordinated gold atoms is strong enough to modify the particle shape and to generate new gold atoms in the low coordinated state and, on the other hand, that the presence of initially highly coordinated gold atoms cannot always be detected by IR spectroscopy because the calculated or measured frequencies reflect the final state of the gold atoms. 2.3.2. Reduced Support. Adsorption of CO on the reduced Au13/TiO2-Ovac model is not as exothermic as on stoichiometric Au13/TiO2, and does not cause such important deformations in the shape and bonding to the support of the gold nanoparticle. CO interacts with low coordinated corner atoms (cases a and d, Table 5 and Figure 10), forming linear complexes with optimized Au-C distances around 1.96 Å, and with calculated adsorption energies between -1.0 and -1.2 eV. Again, the initial atomic charge on the selected gold atom q(Auatom)0 is modified upon CO adsorption, and both the initially neutral atom a and the initially negatively charged atom d bear a positive charge of ∼0.2 in the adsorption complex. The calculated νCO vibration frequencies, ∼2090 cm-1, correspond to CO adsorbed on low coordinated metallic gold atoms. The situation is different when CO is initially placed near atom b, which is a medium coordinated edge atom with a negligible net atomic charge. In this case, the interaction with CO is not strong enough to weaken any Au-Au bond and modify the particle shape, so that CO is finally placed in a bridge position between two gold atoms. The CO bond is noticeably longer than in the isolated molecule, and therefore, the calculated νCO vibration frequency

appears at 1927 cm-1. The fact that no band at around 1930 cm-1 is observed in the measured spectra is not surprising, since this complex is considerably less stable than the other three considered here and therefore its concentration, if it was formed, should be low. Finally, atom c is a low coordinated atom directly bonded to an O atom of the support and bearing a net positive charge. It has been previously described that adsorption of CO on atom c in the Au13/TiO2 model causes the breaking of the Au-O bond and the displacement of the gold atom upward and far from the oxide surface. In the Au13/TiO2-Ovac model, however, the interaction of CO with gold atom c is not strong enough to break the Au-O bond, and the particle shape is not modified. The resulting complex is as stable as those formed on atoms a and d, but the net positive atomic charge on the gold atom is larger, the Au-C distance is shorter, and the calculated νCO vibration frequency is higher, 2119 cm-1. This value has been experimentally related to positively charged Au0 atoms, directly bonded or close to O atoms.57,58 The IR band at 2125 cm-1 observed in some of our spectra could be originated by this gold species directly bonded to the support through a Au-O-Ti bridge. 2.3.3. Oxidized Support. Data in Table 6 indicate that CO adsorbs on neutral gold atoms at the top layer of the Au13/ TiO2-Oad particle and, as occurred on the Au13/TiO2 model, the electron density in the particle reorganizes, and the particle shape is modified by this interaction (see Figures 8 and 11). The coordination number of the gold atom in site a decreases from 7 to 3 upon CO adsorption, and the calculated νCO stretching mode appears at 2091 cm-1, in the region corresponding to CO adsorbed on low coordinated gold atoms. Adsorption on site b is energetically less favorable, and as previously described for site c in the Au13/TiO2-Ovac model, the interaction between CO and gold is not strong enough to

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Boronat et al. TABLE 5: CO Adsorption on Different Sites of the Au13/ TiO2-Ovac Model CN q(Auatom)0 Eads CO (eV) νCO (cm-1) rCO (Å) rAu-C (Å) q(Auatom)-CO

a

b

c

d

3 -0.088 -1.02 2088 1.150 1.961 +0.178

4 +0.054 -0.79 1927 1.168 2.10, 2.13 +0.179

4 +0.280 -1.18 2119 1.149 1.886 +0.564

4 -0.175 -1.18 2089 1.148 1.956 +0.206

frequency is only 13 cm-1 shifted in relation to that calculated for site a. This indicates that CO is less sensitive to identify slightly different gold sites on oxidized surfaces than on reduced ones. Since the IR band at 2135 cm-1 observed in some of the IR spectra is still not assigned, we also considered coadsorption of CO and molecular O2 on the Au13/TiO2 and Au13/TiO2-Ovac models. In a first step, O2 interacts strongly with a Ti atom of the support wih O2 adsorption energies larger than -2 eV, and in the case of the Au13/TiO2-Ovac model, a strong O-Au bond is also formed, as depicted in Figure 11. O2 adsorption involves a net charge transfer from the catalyst to the adsorbate of -0.547 in the stoichiometric surface and of -0.901 in the reduced one, and as a result, the gold nanoparticles become positively charged by +0.822 and +0.251 in the Au13/TiO2-peroxo and Au13/ TiO2-Ovac-peroxo models, respectively. O2 dissociation on the gold nanoparticle supported on the reduced surface, generating the Au13/TiO2-Ovac-2O model depicted in Figure 11, was also considered. After dissociation, one of the O atoms remains, forming a Ti-O-Au bridge, while the other one is located in a hollow position on the gold nanoparticle, forming three Au-O bonds. The net positive charge on the gold particle in this system is +1.044, and the structure is 0.49 eV more stable than the Au13/TiO2-Ovac-peroxo model. CO adsorption on the Au/TiO2-peroxo complex is clearly exothermic and leads to a structure with CO directly bonded to

Figure 9. CO adsorption on different sites of the Au13/TiO2 model (left) and particle restructuration (right).

TABLE 4: CO Adsorption on Different Sites of the Au13/ TiO2 Model CN q(Auatom)0 Eads CO (eV) νCO (cm-1) rCO (Å) rAu-C (Å) q(Auatom)-CO ∆Ereconstruction (eV) q(Auatom)f q(Au13 particle)f

a

b

c

d

7 +0.043 -1.44 2087 1.149 1.954 +0.182 0.00 +0.043 +0.533

3 -0.033 -1.79 2077 1.151 1.950 +0.179 -0.54 -0.024 +0.421

4 +0.310 -1.53 2102 1.147 1.945 +0.268 -0.18 +0.039 +0.480

3 +0.108 -1.65 2099 1.148 1.960 +0.177 -0.69 +0.025 +0.318

break the Au-O bond, and the particle shape is not modified. The net atomic charge on the gold atom in the adsorption complex is high, 0.547, but the calculated νCO vibration

Figure 10. CO adsorption on different sites of the Au13/TiO2-Ovac model.



TABLE 6: CO Adsorption on Different Sites of the Au13/TiO2-Oad Model, on the Peroxo Complexes Formed on the Au13/TiO2 and Au13/TiO2-Ovac Models, and on Different Sites of the Au13/TiO2-Ovac-2O Models Oad

Oad

Ovac-2O

Ovac-2O

Au13/TiO2-

a

b

peroxo

Ovac-peroxo

a

b

CN q(Auatom)0 Eads CO (eV) νCO (cm-1) rCO (Å) rAu-C (Å) q(Auatom)-CO

7 +0.033 -1.86 2091 1.149 1.955 +0.190

5 +0.338 -1.03 2103 1.150 1.897 +0.547

3 +0.304 -1.01 2114 1.146 1.947 +0.254

4 +0.145 -0.49 2118 1.143 2.013 +0.367

2 +0.512 -0.27 2137 1.142 3.150 +0.488

5 +0.276 -1.97 2097 1.151 1.897 +0.495

a low coordinated Au atom not in direct contact with O2 (Table 6 and Figure 11). However, the presence of coadsorbed O2 influences the way in which electron density is reorganized in the gold nanoparticle, the net atomic charge on the gold atom to which CO is coordinated is high, and, as a result, the νCO stretching frequency is shifted to 2114 cm-1. The shift is more pronounced in the Au13/TiO2-Ovac-peroxo model, since in this case CO is adsorbed on a gold atom that is in direct contact with O2. The calculated νCO is 2118 cm-1, and therefore, this species could also be contributing to the IR band at 2125 cm-1.

Figure 11. CO adsorption on different sites of the Au13/TiO2-Oad (top), Au13/TiO2-peroxo (middle, left), Au13/TiO2-Ovac-peroxo (middle, right), and Au13/TiO2-Ovac-2O (bottom) models.

Two different sites were finally explored on the Au13/ TiO2-Ovac-2O model obtained by dissociation of molecular O2. Site a is the gold atom situated between the two adsorbed oxygen atoms. It is also directly bonded to a Ti atom of the support and to one gold atom of the nanoparticle, and exhibits a really high net positive charge. Site b is a gold atom on top of the particle also in direct contact with the adsorbed O atom, and exhibits a positive charge of 0.276. As depicted in Figure 11, CO interacts strongly with site b and modifies the coordination of this atom and the shape of the upper part of the particle but does not bind directly to site a and in this case remains weakly adsorbed on the particle. The calculated adsorption energies reflect the different behavior of both sites, as well as the calculated νCO stretching frequencies: 2097 cm-1, corresponding to CO adsorption on a neutral low coordinated gold atom for site b, and 2137 cm-1, only slightly shifted with respect to the gas phase value, for site a. The IR band at ∼2135 cm-1 that was attributed by several authors to physisorbed CO molecules could be associated with this weakly bonded complex. In conclusion, the theoretical study indicates that CO adsorption on supported gold nanoparticles is more complex than that on isolated gold atoms, because the electron density distribution in the particle reorganizes upon CO adsorption and, in most cases, the particle morphology is also modified. The shift in the calculated νCO stretching frequency cannot be clearly and only related to the net atomic charge on the gold atom to which CO binds (see Figure 12) but also depends on the particle morphology and on the capability of the Au-CO interaction to modify this particle morphology. Thus, the calculated or measured frequencies provide information about the catalyst not in its initial state but after interaction with CO, and IR results

Figure 12. Correlation between calculated νCO stretching frequency and net atomic charge on gold.

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Boronat et al.

Figure 13. Relationship between the H/D exchange determined on different Au/TiO2 samples and the area of the IR bands at 2110-2077 cm-1 (left) and 2136-2124 cm-1 (right).

have to be considered with care and discussed in combination with DFT results. By doing this, we have found that CO adsorption on top of low coordinated gold atoms in supported gold nanoparticles is always energetically favorable and leads to calculated νCO stretching frequencies between 2070 and 2100 cm-1 (bands at 2077, 2096, and 2110 cm-1 experimentally observed in the IR spectra). In some cases, CO interaction with highly coordinated gold atoms is strong enough to modify the particle shape and to generate new gold atoms in a low coordination state, demonstrating the capability of CO to modify Au/TiO2 catalysts. We have found that the IR band appearing at 2125 cm-1 can be assigned to gold atoms bonded to the support through a Au-O-Ti bridge or to gold atoms directly interacting with a peroxo species formed by adsorption of molecular O2 at the metal-support interface. A slightly higher IR frequency of 2135 cm-1 observed in some of the IR has been related to CO weakly adsorbed on highly positive gold atoms that are bonded to two oxygen atoms. Finally, the IR band at 2148 cm-1 observed at low CO coverage and not related to surface hydroxyl groups has been assigned to isolated AuIII cations substituting Ti atoms in the oxide structure. 3. Nature of Gold Active Sites for H2 Dissociation. We have recently shown that the rate determining step for hydrogenation of nitroaromatics on Au/TiO2 is the dissociation of H2 on the gold surface.73 If this is the case, and in order to increase the catalyst activity, one should find which are the gold sites responsible for hydrogen dissociation and then increase the number of these sites during catalyst preparation. In this part of the work, we have first studied which, among all the different sites identified by combining IR spectroscopy of CO on gold and the corresponding DFT calculations, are active for H2 dissociation. Then, the concentration of these sites on the different catalysts has been obtained from the area of the IR bands and has been correlated with their catalytic activity for H/D exchange. Thus, in relation with the catalytic activity of different gold sites, we found by theoretical studies36 that the active sites for adsorption and dissociation of H2 on gold nanoparticles supported on TiO2 are neutral gold atoms in low coordination state, not directly bonded to O, and not belonging to the first atomic layer in contact with the support. According to the findings in the present work, these atoms should be those characterized by calculated νCO stretching frequencies between 2070 and 2100 cm-1. Therefore, the intensity of these bands should directly correlate with the activity of the catalysts for H2 dissociation. To check this hypothesis, we have measured the rate of H/D exchange on the different catalysts studied by IR spectroscopy

in the first part of the work and plotted it against the concentration of the different gold species identified. As shown in Figure 13, there is indeed a linear correlation between the rate of H/D exchange and the amount of metallic gold sites responsible for the experimental bands at 2077-2110 cm-1, but there is no correlation with the amount of positively charged Auδ+ gold sites associated with the bands at 2125 and 2135 cm-1. Notice that the highest H/D exchange rate is obtained on the catalyst having the largest concentration of Au0 sites (Figure 13, left) and a negligible amount of Auδ+ sites (Figure 13, right). This is the experimental confirmation that gold atoms directly bonded to O do not dissociate H2 and are therefore not active in hydrogenation reactions. It also shows that the morphology of the nanoparticles and the disposition of the gold atoms in the catalyst can strongly affect the activity of supported gold catalysts, independently of the particle size. Conclusions A number of Au/TiO2 catalysts with similar gold particle sizes have been characterized by IR spectroscopy of CO adsorption, and a different distribution of sites has been found that has to be related to differences in particle morphology. With the help of DFT calculations, the nature of these different sites has been elucidated and all bands observed in the IR spectra assigned to particular gold species. IR bands between 2070 and 2110 cm-1 correspond to CO adsorbed on low coordinated neutral Au0 species, bands at ∼2120 cm-1 are assigned to CO interacting with slightly positively charged Auδ+ gold atoms involved in Au-O-Ti bonds at the metal-support interface, and the IR band appearing at 2135 cm-1 has been related to CO weakly adsorbed on positively charged Auδ+ gold atoms in contact with O adatoms. Moreover, a band at 2148 cm-1 that is difficult to distinguish from the 2150 cm-1 signal corresponding to surface Ti-OH groups has been assigned to CO interacting with highly dispersed AuIII species. Such cationic sites are not found on nanoparticles but can only be stabilized as isolated Au atoms substituting Ti atoms in the oxide structure. These species are reduced to metallic gold atoms at the metal-support interface (band at 2125 cm-1) and metallic gold atoms on top of the particles and far from the support (bands between 2070 and 2110 cm-1) by CO treatment at increasing temperatures. At the same time, the amount of TiIV surface sites and hydroxyl groups accessible to CO increases, indicating that metallic gold nanoparticles are being formed from highly dispersed cationic gold. This could explain the strong influence of the method of preparation and of calcination or reduction pretreatment on the activity of supported gold catalysts.

Unravelling the Nature of Gold Surface Sites While for highly dispersed or isolated gold atoms the νCO stretching frequency can be directly related to the net atomic charge on gold, CO interaction with supported gold nanoparticles is more difficult to analyze. CO adsorption causes a reorganization of the electron density distribution in the particle and, in most cases, a modification in the particle shape that remains after CO desorption, thus causing an irreversible change of the particle morphology. The shift in the νCO stretching frequency depends not only on the charge on the particle or on the particular gold atom to which CO binds but also on the degree of coordination of gold atoms, that is, particle shape, and on the capability of CO to modify this particle shape. Thus, it should be taken into account that the calculated or measured νCO frequencies provide information about the catalyst not in its initial state but after interaction with CO. Finally, it has been demonstrated by isotopic H/D exchange experiments that only low coordinated neutral gold atoms not involved in Au-O-Ti linkages and responsible for the IR bands between 2070 and 2110 cm-1 are able to dissociate H2, as previously proposed by theory. This means that, besides particle size, particle morphology and the disposition of the gold atoms in it can strongly influence the activity of supported gold catalysts. Acknowledgment. The authors thank CICYT (MAT 200614274-C02-01) and Prometeo from the Generalitat Valenciana for financial support and Barcelona Supercomputing Center (BSC) for computational resources and technical assistance. References and Notes (1) Haruta, M. Chem. Rec. 2003, 3, 75. (2) (a) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 405. (b) Haruta, M. Catal. Today 1997, 36, 153. (3) (a) Valden, M.; Lai, X.; Goodman, D. M. Science 1998, 281, 1647. (b) Chen, M. S.; Goodman, D. W. Science 2004, 306, 252. (4) (a) Bond, G. C.; Thomson, D. T. Catal. ReV. Sci. Eng. 1999, 41, 319. (b) Bond, G. C.; Thomson, D. T. Gold Bull. 2000, 33, 41. (5) (a) Hutchings, G. J. Gold Bull. 2004, 37, 3. (b) Hutchings, G. J. Catal. Today 2005, 100, 55. (c) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem., Int. Ed. 2006, 45, 7896. (6) (a) Carretin, S.; Concepcioń, P.; Corma, A.; Nieto, J. M. L.; Puntes, V. F. Angew. Chem., Int. Ed. 2004, 43, 2538. (b) Guzman, J.; Carrettin, S.; Corma, A. J. Am. Chem. Soc. 2005, 127, 3286. (7) Schumacher, B.; Denkwitz, Y.; Plzak, V.; Kinne, M.; Behm, R. J. J. Catal. 2004, 224, 449. (8) Weiher, N.; Beesley, A. M.; Tsapatsaris, N.; Delannoy, L.; Louis, C.; van Bokhoven, J. A.; Schroeder, S. L. M. J. Am. Chem. Soc. 2007, 129, 2240. (9) Hakkinen, H.; Abbet, S.; Sanchez, A.; Heiz, U.; Landman, U. Angew. Chem., Int. Ed. 2003, 42, 1297. (10) Wang, J. G.; Hammer, B. Phys. ReV. Lett. 2006, 97, 136107. (11) (a) Lo´pez, N.; Janssens, T. V. J.; Clausen, B. S.; Xu, Y.; Mavrikakis, M.; Bligaard, T.; Nørskov, J. K. J. Catal. 2004, 223, 232. (b) Remediakis, I. N.; Lopez, N.; Nørskov, J. K. Angew. Chem., Int. Ed. 2005, 44, 1824. (12) Hernańdez, N. C.; Sanz, J. F.; Rodriguez, J. A. J. Am. Chem. Soc. 2006, 128, 15600. (13) Janssens, T. V. W.; Clausen, B. S.; Hvolbaek, B.; Falsig, H.; Christensen, C. H.; Bligaard, T.; Nørskov, J. K. Top. Catal. 2007, 44, 15, and references therein. (14) Comotti, M.; Della Pina, C.; Matarrese, R.; Rossi, M. Angew. Chem., Int. Ed. 2004, 43, 5812. (15) Enache, D. I.; Edwards, J. K.; Landon, P.; Solsona-Espriu, B.; Carley, A. F.; Herzing, A. A.; Watanabe, M.; Kiely, C. J.; Knight, D. W.; Hutching, G. J. Science 2006, 311, 362. (16) (a) Abad, A.; Almela, C.; Corma, A.; Garcıá, H. Chem. Commun. 2006, 3178. (b) Abad, A.; Corma, A.; Garcıá, H. Chem.sEur. J. 2008, 14, 212. (17) Hughes, M. D.; Xu, Yin-Jun; Jenkins, P.; McMorn, P.; Landon, P.; Enache, D. I.; Carley, A. F.; Attard, G. A.; Hutchings, G. J.; King, F.; Stitt, E. H.; Johnston, P.; Griffin, K.; Kiely, C. J. Nature 2005, 437, 1132. (18) Nijhuis, T. A.; Visser, T.; Weckhuysen, B. M. Angew. Chem., Int. Ed. 2005, 44, 1115. (19) Joshi, A. M.; Delgass, W. N.; Thomson, K. T. J. Phys. Chem. C 2007, 111, 7841.

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