Charge Redistribution at the Embedded Rh−Alumina Interface - The

J. Phys. Chem. C 2009, 113, 18069–18074

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Charge Redistribution at the Embedded Rh-Alumina Interface Loredana De Rogatis,† Erik Vesselli,‡ Alessandro Baraldi,‡ Maria F. Casula,§ Tiziano Montini,% Giovanni Comelli,‡ Mauro Graziani,% and Paolo Fornasiero*,% Chemistry Department, Center of Excellence for Nanostructured Materials (CENMAT), Italian Consortium on Materials Science and Technology (INSTM), Trieste Research Unit and Institute of Chemistry of Organometallic Compounds (ICCOM) CNR, Trieste Research Unit, UniVersity of Trieste, Via L. Giorgieri 1, I-34127, Trieste, Italy, Laboratorio Nazionale CNR TASC-INFM, Area Science Park, SS 14 km 163.5, I-34012, BasoVizza (TS), Italy, Physics Department, UniVersity of Trieste, Via A. Valerio 2, I-34127, Trieste, Italy, and Chemistry Department and INSTM, Cagliari Research Unit, UniVersity of Cagliari, Monserrato, I-09042, Cagliari, Italy ReceiVed: June 18, 2009; ReVised Manuscript ReceiVed: August 25, 2009

An X-ray photoelectron spectroscopy (XPS) study was performed on model systems of Rh nanoparticles embedded into Al2O3. One of the main tasks was the investigation of the possibility to distinguish the Rh particles embedded into alumina matrix with respect to those on the surface of the support by means of their different electronic properties. A new component in the XPS spectra was found in the embedded sample after H2 treatment at 750 °C for 2 h. On the basis of these results and of other data, obtained by means of X-ray diffraction, temperature programmed reduction, N2 physisorption, and H2 chemisorption experiments, we suggest that this particular feature is associated with the encapsulation process of the metal nanoparticles rather than to the particle size effect. The anomalous charge redistribution of the embedded metallic clusters detected in the XPS measurements may be therefore used as a spectroscopic fingerprint of the embedding of Rh nanoparticles in Al2O3. Introduction In the last years, many efforts have been devoted to the synthesis of a novel class of heterogeneous catalysts, resistant to the sinterization under severe reaction conditions. A successful strategy relies on embedding nanosized metal particles into porous oxides.1 The encapsulation of preformed metal nanoparticles can reduce their mobility under medium and high working temperatures while the porous nature of the support prevents their total occlusion, thus favoring the access of the reactants to the catalytic sites. Following this approach, Budroni et al.2 and Yeung et al.3 developed respectively Au- and Ptbased catalysts for the water gas shift reaction. The advantages and the efficacy of this strategy were also demonstrated for Al2O3 supported rhodium catalysts.4 Indeed, under reaction conditions, that is, during catalytic methane partial oxidation (MPO), the embedded Rh(1 wt %)@Al2O3, obtained by a simple and low cost coprecipitation method, was proven to be thermally more stable than a reference catalyst prepared by conventional incipient wetness impregnation. This significant improvement is correlated to the protection offered to the active metal phase by surrounding layer of aluminum oxide, which prevents extensive sintering. Moreover, the partial deactivation observed in the embedded system during MPO, mainly due to coke deposition, is essentially reversible. This means that it is possible to restore in situ to a large extent the properties of the catalysts, * Corresponding author. E-mail: [email protected]. † Chemistry Department, CENMAT University of Trieste and CNR TASC-INFM. ‡ Physics Department, CENMAT University of Trieste and CNR TASCINFM. § Chemistry Department, University of Cagliari and INSTM. % Chemistry Department, CENMAT University of Trieste, INSTM and CNR ICCOM.

as opposed to traditional impregnated samples where, because of sintering, a partially irreversible deactivation takes place. All of these results highlight the potential of this new class of noble metal embedded catalysts as feasible choice at industrial scale, despite their high cost which often, up to now, has limited wide scale-up. In order to investigate the Rh-alumina interaction in the embedded catalysts (i.e., the effect of the thickness of the protective layer on the metal-support interaction and the nature of the Rh species), an X-ray photoelectron spectroscopy (XPS) study was performed on properly designed embedded model systems. Specifically, the samples presented here simulate the first stages of the growth of protective oxide layers around metal particles. The materials used in this work are therefore model systems and not real industrial catalysts. For the latter ones, indeed, the low metal content (usually e1 wt %) together with the well-known insulating properties of the alumina support would make a characterization by means of XPS more difficult, requiring the use of an additional charge neutralizing system. However, an accurate and efficient XPS investigation of the interaction between Rh nanoparticles and the first layers of the alumina can be more easily performed for the present proposed model systems, since the charging effects under XPS measurements are reduced to a few electronvolts. The investigation was carried out both on calcined/oxidized and on reduced samples. Notably, the prereduction treatment represents a standard activation procedure used to transform the catalyst precursor in the active metal phase. Our results show how the catalyst design deeply affects the Rh-alumina charge distribution, allowing embedded and supported metal nanoparticles to be identified by means of XPS measurements.

10.1021/jp905736q CCC: $40.75  2009 American Chemical Society Published on Web 09/25/2009

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TABLE 1: Embedded Rh@Al2O3 Model Systems and Calculated Number of Alumina Layers around the Metal Nanoparticles

a

metal loading wt %a

number of layers

33 53 67

5 2.5 1

with respect to the amount of alumina in the final system.

Experimental Section Materials. Rh@Al2O3 samples were prepared according to the procedure described in details in ref 4. Specifically, the Rh(x wt %)@Al2O3 model systems were prepared by embedding protected Rh nanoparticles into a nanostructured oxidic matrix. The growth of the porous oxide around the Rh nanoparticles was obtained by a single step precipitation of the corresponding metal hydroxide in the presence of the colloidal suspension of Rh nanoparticles. Stabilized Rh nanoparticles were prepared under Ar at 25 °C, according to the procedure reported by Schultz et al.5 Briefly, an aqueous solution containing NaBH4 (98+%, ACROS) and the cationic surfactant HEAC16Br was quickly added under vigorous stirring to an aqueous solution of Rh(NO3)3 · 2H2O (Johnson Matthey). The suspension was then stirred for 2 h to decompose the excess of reductant, and finally, Al(NO3)3 · 9H2O (g98.0%, Fluka) was added. The Al hydroxide was precipitated by dropping the obtained suspension into a 10% (w/w) NH4OH solution under vigorous stirring. After aging for 2 h, the precipitate was washed 4 times with NH4OH/NH4NO3 buffer solution (pH ) 10) to remove the Br- ions. The final solid was aged overnight at room temperature, centrifuged, suspended in 2-propanol (electronic grade, Carlo Erba) and refluxed for 5 h in order to stabilize the textural framework of the support.6-8 After filtration, the solid was dried at 120 °C overnight, crushed and sieved to collect the fraction smaller than 180 µm and calcined in a static oven, first at 500 °C for 5 h and, finally, at 900 °C for 5 h (heating rate 3 °C min-1). Model embedded samples (Table 1) were prepared with three different metal loadings, in order to investigate the influence of the capping oxide thickness. Rh nanoparticles surrounded by an incomplete or thin porous alumina layer or by a thicker protecting oxide capping were synthesized. The nominal number of the alumina layers around each particle was roughly estimated as follows. The metal particles were assumed to be spheres with an average diameter of 2 nm, that is, slightly larger than that of the protected nanoparticles in solution, and similar to that we observed on Rh(1 wt %)@Al2O3 after reduction.4 On the basis of these assumptions, we calculated the exposed metal surface area of the Rh(1 wt %)@Al2O3 sample. The area of one alumina unit cell, assuming for simplicity the (0001) surface termination of R-Al2O3, and the resulting number of unit cells needed to cover the surface area of the Rh nanoparticles were calculated. Given the amount of alumina needed to obtain a nominal monolayer, the sample chemical composition was chosen in order to yield about 1, 2.5, and 5 monolayer of alumina capping (Table 1). As reference, a standard impregnated Rh(33 wt %)/Al2O3 catalyst was prepared. The Al2O3 support was synthesized using the procedure described above, with the exception that the Rh salt was not present. The obtained hydroxide was dried at 120 °C for 12 h. After calcination at 900 °C for 5 h, the metal was deposited by multiple incipient wetness impregnation using a

Rogatis et al. Rh(NO3)3 solution. After drying at 120 °C overnight, the material was calcined at 500 °C for 5 h (heating rate 3 °C min-1). Thereinafter, samples are indicated with the Rh/Al2O3 and Rh@Al2O3 notations for the impregnated and the encapsulated systems, respectively. Methods. XPS measurements were performed in a multipurpose apparatus equipped with a conventional Mg KR X-ray source (hν ) 1253.6 eV, ∆E ) 0.9 eV) and a VG MKII hemispherical electron energy analyzer. The background pressure in the analysis chamber was kept around 5 × 10-8 mbar during data acquisition. Powder X-ray diffraction (XRD) patterns were recorded using an X3000 Seifert diffractometer equipped with a graphite monochromator on the diffracted beam. The scans were collected with Cu KR radiation (λ ) 0.154 nm). BET surface area and H2 chemisorption measurements were conducted using a Micromeritics ASAP 2020C analyzer. N2 physisorption isotherms were collected at liquid nitrogen temperature from 0.1 g of sample, after overnight evacuation at 350 °C. H2 chemisorption experiments were carried out at 35 °C, after cleaning pretreatment at 500 °C for 1 h under O2 (5%) in Ar flow, followed by reduction at 750 °C in H2(5%)/Ar for 2 h and evacuation at 400 °C for 4 h. Typically, 0.1 g samples were used, and an equilibration time of 10 min was employed. Adsorbed volumes were determined by extrapolation to zero pressure of the linear part of the adsorption isotherm (150-400 Torr), after elimination of the so-called reversible hydrogen adsorption. A chemisorption stoichiometry H:Rh ) 1:1 was assumed. Temperature programmed reduction (TPR) experiments were performed according to ref 4. Transmission electron microscopy (TEM) bright field (BF) and dark field (DF) images and selected-area electron diffraction (SAED) patterns were obtained on a JEOL 200CX transmission electron microscope equipped with a tungsten cathode operating at 200 kV. Finely ground samples were dispersed by sonication in n-octane and dropped and dried on a carbon-coated copper grid. Results and Discussion Among the three model embedded systems, the Rh(33 wt %)@Al2O3 shows the most interesting features. Indeed, its behavior is noteworthy with respect to the corresponding impregnated sample. Figure 1 displays the Rh 3d spin-orbit split XPS spectra corresponding to the Rh(33 wt %)@Al2O3 (A) and Rh(33 wt %)/Al2O3 (A′). For clarity, among the various components obtained by the fitting procedure, only the ones contributing to the 5/2 feature are depicted, while those corresponding to the 3/2 contribution are not shown.9 The binding energies were calibrated relatively to the C 1s peak from carbon contamination of the samples at 284.8 eV. After calcination in air at 900 °C for 5 h (top panel in Figure 1), in both systems, all Rh is oxidized to Rh3+ (i.e., Rh2O3) as indicated by the binding energy value of the Rh 3d5/2 peak at 308.2 eV.10-13 The presence of the bulk oxide phase is consistent with the fact that the catalysts were calcined in air at relatively high temperature (900 °C). In agreement, X-ray diffraction (XRD) patterns show the characteristic diffraction peaks of the Rh2O3 phase. A minor component at 310.0 eV is also found in the XPS spectra, indicative of the presence of Rh4+ species (i.e., RhO2)11 in a concentration which is below the XRD detection limit.

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Figure 1. Comparison between the Rh 3d XPS spectra of (A) Rh(33%)@Al2O3 and (A′) Rh(33%)/Al2O3, after calcination in air at 900 °C for 5 h (top panel) and reduction in H2 at 750 °C for 2 h (bottom panel).

After reduction/activation in H2 at 750 °C for 2 h, the XPS spectra are significantly different (Figure 1, bottom panels). In the case of the impregnated sample, a new peak grows at 307.1 eV (blue curve), attributed to Rh0 on the basis of the values reported in literature for similar systems.13-15 Notably, the feature at 308.2 eV associated to the presence of Rh3+ species does not vanish completely. By taking into account the fact that the samples were exposed to air before XPS measurements, a partial Rh0 reoxidation is expected, especially for small particles. Furthermore, we can not exclude that some rather large or deeply buried Rh oxide particles have not been completely reduced under the adopted procedure (see below). On the other hand, in the case of the embedded system, a second new component is observed at 306.3 eV (red curve) in addition to the 307.1 eV peak. The feature at 308.2 eV is still present in the spectra. In the case of Rh loading of 53% and 67%, the peak at 306.3 eV is absent. In order to gain insight into the origin of the XPS component at low binding energy and to determine a possible relation with the embedded system and/or with the size distribution of the metal particles, N2 physisorption and H2 chemisorption experiments, TEM analysis, and TPR measurements were performed. Table 2 summarizes the main textural properties of the embedded systems after calcination at 900 °C in air. All samples show type IV isotherms with hysteresis loops, typical of mesoporous materials.16 The t-plot analysis indicates that the microporous volume is always negligible, while the BJH analysis reveals that the materials have a pore distribution centered approximately around 10 nm. Rh(53%)@Al2O3 have a specific surface area of ∼70 m2 g-1, while for the sample

TABLE 2: Textural Properties of Embedded Rh@Al2O3 Model Systems and Impregnated Rh/Al2O3 after Calcination at 900 °C in Air for 5 h sample Rh(33 Rh(33 Rh(53 Rh(67

wt wt wt wt

%)/Al2O3 %)@Al2O3 %)@Al2O3 %)@Al2O3

cumulative pore SBETa (m2 g-1) dMb (nm) volume (mL g-1) 98 105 72 38

9 9 10 10

0.31 0.30 0.28 0.22

a

BET surface area. b Pore diameter: maximum of the pore distribution obtained from the desorption branch of the adsorption.

with the lower alumina content, the total surface area further decreases to ∼40 m2 g-1. Similarly, the pore volume decreases with increasing Rh content. Therefore, the major contribution to the surface area of the present materials arises, as expected, from the alumina. Selective H2 chemisorption experiments were performed with the aim of further investigating the accessibility of the metal nanoparticles. The obtained results, detailed in Table 3, indicate that on Rh(33%)/Al2O3 the metal dispersion (ratio between adsorbed H atoms and total atoms of Rh) is 18.9% which corresponds to an exposed metallic surface of 27.4 m2 g-1. An average particle diameter of 5.8 nm was estimated by assuming a spherical geometry. A Rh dispersion and a particle size of 5.8% and 18.8 nm, respectively, are found for the corresponding embedded sample. Notably, the undoped support showed no hydrogen chemisorption under the same experimental conditions. The larger exposed Rh surface area is observed for the Rh(53 wt %)@Al2O3 (Table 3). This can be explained as the result of

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TABLE 3: Hydrogen Chemisorption on Embedded Rh@Al2O3 Model Systems and Impregnated Rh/Al2O3 sample

H/Rh (%)

Rh surface (m2 g-1)

Φa(nm)

Rh(33%)/Al2O3 Rh(33%)@Al2O3 Rh(53%)@Al2O3 Rh(67%)@Al2O3

18.9 5.8 4.4 2.0

27.4 8.5 9.7 6.0

5.8 18.8 26.4 53.9

a

Apparent Rh particle size determined from H2 chemisorption assuming spherical geometry.

a combination of two opposite effects: (i) the increase of metal loading and (ii) the increase of the average metal particle size. Figure 2 (top panel) shows a representative TEM image of a relatively large part of the sample together with a detail of a Rh nanoparticle for Rh(33 wt %)@Al2O3 (A) and Rh(33 wt %)/Al2O3 (A′). Even if not conclusive, the data are compatible with the idea that in the Rh(33 wt %)@Al2O3 sample most of the metal particles are embedded into the alumina, as a thin layer of matrix surrounding the nanoparticles can be imaged thanks to the different contrast of the two phases. Consistently, this shadow effect is almost absent in the corresponding impregnated system, suggesting that the majority of the Rh nanoparticles are exposed on the surface of the support, as expected on the basis of the preparation procedure. In order to further corroborate these observations, additional TEM images of the embedded and impregnated samples are included in Supporting Information. The average diameter of Rh particles is 4.7 and 10.6 nm for the impregnated and embedded systems, respectively, as determined from the size distribution histogram (medium panel in Figure 2). These values are quite different from those obtained by H2 chemisorption measurements (Table 3). This discrepancy can be interpreted on the basis of the intrinsic characteristics/limitations of the two techniques: indeed, while chemisorption is sensitive only to the contribution of Rh metal particles which are accessible to H2 molecule, TEM averages over all particles (i.e., also embedded particles, buried particles, or both). In the case of the standard impregnated sample, H2 chemisorption overestimates the average particles diameter by about 20% (5.8 vs 4.7 nm). This could be reasonable considering the fact that a portion of the metal particles, that in direct contact with the support, is certainly not accessible to the probe molecule. In the case of the embedded Rh(33%)@Al2O3, the difference between the two techniques is significantly higher (about 44%, 10.6 vs 18.8 nm). This is perfectly consistent with the hypothesis that the particles are mostly embedded. In fact, in this situation, the larger contact between the surrounding porous support and the nanoparticle reduces the accessible area of the system. Unfortunately, we do not have access to a STEM facility, which has been proved to be able to unequivocally characterize complex systems such as the present embedded ones.17 Broader size distributions (see Supporting Information) shifted toward larger particle diameter (>20 nm) are obtained by statistical analysis of TEM images of the other two model systems. In these latter cases, because of the very high metal loading, it is more difficult to establish whether the metal particles are actually embedded into the alumina matrix. Finally, the high metal content and the surface area of all samples used in this work intrinsically limit the possibility of evaluating the metal dispersion by TEM analysis. It should be pointed out that even though a narrower metal size distribution with a smaller average particle diameter is observed in the reduced impregnated sample with respect to

the embedded one, this feature does not affect the better catalytic performance of the latter. The main difference between the embedded and the conventional real catalyst lies in the deactivation behavior. In fact, the primary goal of the embedding strategy is the inhibition of metal sinterization phenomena during the subsequent chemical and thermal treatments (i.e., reaction conditions). As reported for the Rh(1 wt %)@Al2O3 catalyst, the embedded metal particles significantly preserve their nanostructure (i.e., particle size) after aging at high temperature under MPO, thus avoiding the extensive sintering, typical of the corresponding impregnated sample.4 In Figure 2 (bottom panel), the TPR profiles of the two Rh(33 wt %) samples are also compared. The Rh(33 wt %)@Al2O3 system presents a reduction peak centered at 185 °C with shoulder around 135 °C. On the contrary, the reduction of the impregnated material starts immediately (below 100 °C), so that at 200 °C, all RhOx species were reduced, while for the corresponding embedded sample, the reduction process was not complete yet. Two features, at 140 and 240 °C, are observed in the case of Rh(53 wt %)@Al2O3, while a single component at 215 °C is found for Rh(67 wt %)@Al2O3, (see Supporting Information). A clear attribution of each reduction peak is not straightforward as several factors are involved (i.e., species nature, size effect). Indeed, the redox properties can vary appreciably with the metal loading, because of the different extent of the interaction between the RhOx species and the Al2O3 support. The metal content also influences the particle size, which can in turn affect the reduction properties. This can be attributed to the fact that, at low Rh content, the small rhodium particles exhibit a smaller tendency to agglomerate, while, at higher loading, partial sintering takes place because of a significant Rh particles density. A common feature of all samples discussed in this work is the absence of high temperature reduction peaks (T > 700 °C), suggesting that Rh-Al mixed oxide phases, Al-Rh alloys, or both are not formed during the calcination nor the reduction treatments (see Supporting Information). Consistently, there is no evidence for the formation of the above-mentioned species either in XRD or in TEM analyses. Unfortunately, the quantification of the TPR data is complex for several reasons. First of all, the high metal loading requires the use of very small amount of material, leading to appreciable errors in the sample weight. Furthermore, the reduction of very small Rh particles occurs even at room temperature, as evidenced in the case of the impregnated sample. Finally, H-desorption peaks and the buoyancy effect generate a complex baseline, which introduce arbitrariness in the integration procedure. Nevertheless, a rough estimate from the model embedded systems suggests that almost all Rh is reduced (87-94%). A partial Rh0 reoxidation, thus, takes place, upon exposure to air at room temperature (RT), before XPS measurements. The relative Rh3+/Rh0 ratio as determined from the XPS spectra is different in all samples (see Supporting Information). In particular, it increases as the Rh loading decreases, indicating that the metal particles are more easily oxidized in Rh(33 wt %)@Al2O3, in agreement with its smaller particle size with respect to Rh(53 wt %)@Al2O3 and Rh(67 wt %)@Al2O3, as indicated by TEM and chemisorption analysis. However, because of the baseline trend in TPR profile, a reduction of some large or deeply embedded/buried Rh2O3 particles between 750 and 1000 °C cannot be excluded. Consequently, after treatment in H2 at 750 °C, a small fraction of unreduced Rh could remain. This could explain the larger contribution of oxidized Rh species in the reduced Rh(33 wt %)@Al2O3 with respect to the reduced Rh(33 wt %)/Al2O3.

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Figure 2. From top to bottom: TEM images, particle size distribution and TPR spectra of Rh(33%)@Al2O3 (A) and Rh(33%)/Al2O3 (A′) after reduction in H2 at 750 °C for 2 h.

A value of 306.3 eV for the binding energy of the Rh 3d5/2 core level component, which is by ∼0.8 eV lower than that usually found for metallic bulk Rh, was never observed in the case of conventional impregnated catalysts. A value lower than 307.0 eV was reported by Ojeda et al.18 for Rh(2 wt %)/γ-Al2O3 prepared by the microemulsion method, after reduction at 500 °C in H2. It was associated to Rh0, but its remarkably low binding energy value was not discussed. It is noteworthy that the microemulsion technique may yield partially embedded nanostructured materials, since it is conceptually similar to the catalyst design discussed above.

The absence of the 306.3 eV peak in the case of the Rh(53 wt %)@Al2O3 and Rh(67 wt %)@Al2O3 samples can be explained considering a reduction of the effectiveness of the embedding approach due to the increase of the metal loading. It is likely that a low relative amount of Al2O3 does not allow complete surrounding of the metal particles. The electronic density of states distribution in transition metal clusters can show remarkable deviations with respect to the corresponding bulk material, for a number of reasons. First, finite size effects lead to a redistribution of the d-band electron density of states close to the Fermi level, thus influencing the core level

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binding energies.19 Second, supported clusters, when deposited on an insulating template like alumina, may suffer charging effects during measurements with an ionizing technique like XPS (positive binding energy shift). Finally, the Coulomb energy of the charged cluster suppresses the conduction electron screening of the core hole, thus contributing with an additional core level shift due to final state effects.19 When dealing with embedded clusters, an additional point has to be considered. Indeed, the dielectric field at the interface between the metal and the surrounding matrix (alumina) is associated with an electron spillout and with a d-core electron polarization, which may influence the measured core electron binding energy both in the initial and in the final states.20-23 It has been shown that these effects may generate a negative shift (i.e., toward lower values) of the core level binding energy.24 A remarkable point is that the sign of the measured shift indicates that the dielectric effects due to the embedding overwhelm the expected trend associated with the finite cluster size. The effect of aging under MPO conditions (run-up experiment with subsequent isotherm at 900 °C for 2 h) is negligible on all samples. Indeed, no significant modifications in the XPS spectra were observed (see Supporting Information). Rh remains in the reduced state as H2 is produced during the reaction at high temperature. Conclusions The observed anomalous shift of the Rh 3d component associated with the charge redistribution of the embedded metallic clusters is the result of a complex interplay among the electronic effects discussed above. This may be used as a spectroscopic fingerprint of the embedding of Rh nanoparticles in an alumina matrix. Notably, the observed charging effects related to the particular environment of metal nanoparticles in the embedded systems cannot be used to improve the activity of a heterogeneous thermally activated catalyst but open new opportunities, for instance, in photocatalysis.25,26 Indeed, in the latter case, the enhanced metal-support charge transfer can play a key role in hydrogen photoproduction, yielding the possibility of benchmarking new reaction pathways which could not be explored before. Acknowledgment. University of Trieste and Cagliari, ICCOM-CNR, INSTM, The Fondazione CRTrieste, PRIN2007 “Sustainable processes of 2nd generation for the production of H2 from renewable resources” and FISR2002 “Nanosistemi inorganici ed ibridi per lo sviluppo e l’innovazione di celle a combustibile” are gratefully acknowledged for financial support. Supporting Information Available: Sample preparation and characterization, TEM images, particle size distribution, and

Rogatis et al. TPR and XPS spectra. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) De Rogatis, L.; Montini, T.; Gombac, V.; Cargnello, M.; Fornasiero, P. In Nanorods, Nanotubes and Nanomaterials Research Progress, Chapter 2, (Eds. Prescott, W. V. Schwartz A. I.) Nova Science Publishers, 2008; pp 71-129. (2) Budroni, G.; Corma, A. Angew. Chem., Int. Ed. 2006, 45, 3328. (3) Yeung, C. M. Y.; Meunier, F.; Burch, R.; Thompsett, D.; Tsang, S. C. J. Phys. Chem. B 2006, 110, 8540. (4) Montini, T.; Condo`, A. M.; Hockey, N.; Lovey, F. C.; De Rogatis, L.; Fornasiero, P.; Graziani, M. Appl. Catal. B-EnViron. 2007, 73, 84. (5) Schulz, J.; Roucoux, A.; Patin, H. Chem.-A Eur. J. 2000, 6, 618. (6) Jones, S. L.; Norman, C. J. Am. Chem. Soc. 1988, 71, C190. (7) Segal, D. J. Mater. Chem. 1997, 7, 1297. (8) Tadokoro, S. K.; Muccillo, E. N. S. J. Eur. Ceram. Soc. 2002, 22, 1723. (9) Rh 3d spectra were fitted, after linear background subtraction, by Doniach-Sunjic envelopes convoluted with a Gaussian. In order to reduce the number of fitting parameters, for all the components the single peak lineshapes (lorentzian width and asymmetry parameter) of the Rh 3d 5/2 and 3/2 doublet, respectively, were fixed at the values obtained from the spectra of the bulk metallic rhodium. Only the Gaussian width was allowed to vary, in order to account for inhomogeneity and charging effects. (10) Suhonen, S.; Valden, M.; Hietikko, M.; Laitinen, R.; Savimaki, A.; Harkonen, M. Appl. Catal. A-Gen. 2001, 218, 151. (11) Weng-Sieh, Z.; Gronsky, R.; Bell, A. T. J. Catal. 1997, 170, 62. (12) Suhonen, S.; Polvinen, R.; Valden, M.; Kallinen, K.; Harkonen, M. Appl. Surf. Sci. 2002, 200, 48. (13) Zimowska, M.; Wagner, J. B.; Dziedzic, J.; Camra, J.; BorzeckaProkop, B.; Najbar, M. Chem. Phys. Lett. 2007, 417, 137. (14) Briggs, D.; Seah, M. P. Practical Surface Analysis; Wiley: Chichester, 1990; p 612. (15) Barr, T. L. J. Phys. Chem. 1978, 82, 1801. (16) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; J. Rouque´rol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (17) Wang, S.; Borisevich, A. Y.; Rashkeev, S. N.; Glazoff, M. V.; Sohlberg, K.; Pennycook, S. J.; Pantelides, S. T. Nat. Mater. 2004, 3, 143. (18) Ojeda, M.; Rojas, S.; Boutonnet, M.; Perez-Alonso, F. J.; GarciaGarcia, F. J.; Fierro, J. L. G. Appl. Catal. A-Gen. 2004, 274, 33. (19) Wertheim, G. K.; DiCenzo, S. B.; Buchanan, D. N. E. Phys. ReV. B 1986, 33, 5384. (20) Liao, H. B.; Xiao, R. F.; Fu, J. S.; Yu, P.; Wong, G. K. L.; Sheng, P. Appl. Phys. Lett. 1997, 70, 1. (21) Lerme´, J.; Palpant, B.; Pre´vel, B.; Pellarin, M.; Treilleux, M.; Vialle, J. L.; Perez, A.; Broyer, M. Phys. ReV. Lett. 1998, 80, 5105. (22) Celep, G.; Cottancin, E.; Lerme´, J.; Pellarin, M.; Arnaud, L.; Huntzinger, J. R.; Vialle, J. L.; Broyer, M.; Palpant, B.; Boisron, O.; Me´linon, P. Phys. ReV. B 2004, 70, 165409. (23) Dhara, S.; Sundaravel, B.; Ravindran, T. R.; Nair, K. G. M.; David, C.; Panigrahi, B. K.; Magudapathy, P.; Chen, K. H. Chem. Phys. Lett. 2004, 399, 354. (24) Berko´, A.; Ulrich, I.; Prince, K. C. J. Phys. Chem. B 1998, 102, 3379. (25) Gombac V.; Montini T.; Polizzi S.; Delgado J. J.; Hameed A.; Fornasiero P. Nanosci. Nanotechnol. Lett. 2009, in press. (26) Gombac, V.; Sordelli, L.; Montini, T.; Delgado, J. J.; Adamski, A.; Adami, G.; Cargnello, M.; Bernal, S.; Fornasiero, P. J. Phys. Chem. A 2009, DOI: 10.1021/jp907242q.

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