Nanostructured Oxide-Based Powders: Investigation of the Growth

Russian Journal of Physical Chemistry A 2018 92 (6), 1087-1098 ... G.N. Gerasimov , V.F. Gromov , O.J. Ilegbusi , L.I. Trakhtenberg. Sensors and Actua...
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J. Phys. Chem. B 2006, 110, 2515-2521

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Nanostructured Oxide-Based Powders: Investigation of the Growth Mode of the CeO2 Clusters on the YSZ Surface Marta M. Natile* and Antonella Glisenti Dipartimento di Scienze Chimiche, UniVersita` di PadoVa, Via F. Marzolo, 1-35131 PadoVa, Italy ReceiVed: July 19, 2005; In Final Form: NoVember 9, 2005

CeO2/YSZ nanocomposite powders, characterized by increasing Ce/Zr atomic ratio, were obtained by depositing, by wet impregnation, different amounts of CeO2 on the yttria-stabilized zirconia (YSZ) surface. These powders were characterized by means of X-ray photoelectron spectroscopy, transmission electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. Experimental results allow us to obtain interesting information concerning the growth mode, the morphology, and the dimensions of the CeO2 clusters on the YSZ supporting surface. A 3-D growing mechanism was observed for the CeO2 nanoparticles. With increasing Ce/Zr atomic ratio the CeO2 clusters become more and more spherical. Moreover, XPS data also show the presence of Ce(III) and Ce(IV) ions at the interface supported/supporting oxides.

Introduction During the past decade, nanostructured materials have received considerable attention because of their attractive properties entirely different from those of the bulk state. These materials exhibiting unique physical and chemical properties due to their higher surface area (higher fraction of atoms lying on or near the surface), limited size, more polyhedral shapes, and high density of corner and edge surface sites can advantageously be used in applications when the surface properties are paramount, as in the case of the heterogeneous catalysis.1,2 A particular interest is directed toward heterogeneous nanostructured catalysts consisting of nanoparticles of an active metal oxide on a supporting one that is also an active catalyst.3 Their properties, in fact, are strongly different when compared to those of the corresponding bulk oxides. Beyond the nanodimension of the supported oxide and consequently its higher reactivity, if the overlayer does not completely wet the substrate, it is also possible to take advantage of the reactivity of the supporting oxide and/or the formation of new active sites3,4 at the interface of the two oxides. This paper deals with ceria/yttria-stabilized zirconia nanocomposite powders; the aim of this work is to investigate the possibility of depositing clusters of ceria on the surface of YSZ powders. As is known, ceria- and ceria-containing materials are used as catalysts and promoters in several heterogeneous catalytic reactions.5 The oxygen storage capacity (OSC), due to the ability of cerium to move between Ce(IV) and Ce(III), is one of the key properties of these materials. The OSC makes ceria a main component in the three-way catalysts (TWCs), which are used for the treatment of automotive exhaust gases. CeO2-Al2O3 is one of the first investigated systems for application in TWCs. The effect of ceria on the properties and reactivity of CeO2Al2O3 was extensively studied6 as well as its influence on the dispersion, stability, and reactivity of Pt, Rh, and other supported metal catalysts.7 Moreover, cerium oxide (exhibiting also ionic * Address correspondence to this author. Phone: ++39-049-8275196. Fax: ++39-049-8275161. E-mail: [email protected].

and electronic conductivity (n-type)) is used as a base material for electrolytes and electrodes in solid oxide fuel cells (SOFCs).8,9 Zirconium oxide is a very important ceramic material and finds application in a number of technologies, including fuel cells, catalysis, buffer layer of superconductors growth, and oxygen sensor.10,11 The cubic and tetragonal zirconia phases are unstable at room temperature, but are more valuable for the technological applications mentioned above than the monoclinic one (stable at room temperature). It is possible to stabilize the cubic phase by doping with rare earth oxides such as Y2O3: the minimum Y2O3 amount required is about 8 mol %.8 The substitution of Y(III) for Zr(IV) in the lattice also results in the introduction of oxygen anion vacancies. The presence of these vacancies enhances the catalytic activity of zirconia. Moreover, anion vacancies lead to the unusually high oxygen ion conductivity of yttria-stabilized zirconia (YSZ).12 Several studies have shown that both the thermal stability and the oxygen storage capacity of ceria can be enhanced by mixing with zirconia:13,14 consistently, the use of ceria-zirconia based materials in TWCs is markedly increased.5,15,16 In particular, the study of Putna et al.14 concerning the deposition of the ceria films on the YSZ substrate shows that YSZ helps to enhance the OSC of ceria by promoting the formation and stabilization of more active sites than those present on bulk ceria. For the above-mentioned reasons the CeO2/YSZ nanocomposite catalysts may be an interesting choice. It is worth underlining that despite the CeO2-ZrO2 system being one of the most studied mixed metal oxides in the literature,5 this is the first paper concerning a CeO2/YSZ nanocomposite powder obtained by depositing CeO2 clusters on the YSZ surface. It is known that the supported metal oxide can grow on a supporting surface with a variety of structures strongly depending on the experimental conditions such as the preparation method, the amount of supported oxide, the deposition temperature, the heat treatment, and the surface morphology of the support. Moreover, the growth mechanism of the supported metal oxide is thermodynamically driven in order to lower the surface free energy of the nanocomposite oxide system.17,18 Concerning the thin metal films the following growing modes are observed: (1) three-dimensional (3-D) clustering (Volmer-

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2516 J. Phys. Chem. B, Vol. 110, No. 6, 2006 Weber growth mode) where the overlayer does not wet the substrate; (2) complete wetting of the supporting surface by a monolayer followed by growth in 3-D clusters (Stranski Krastanov growth mode); and (3) growth in a layer-by-layer fashion (Franck-van der Merwe growth mode).17 Several studies concern the deposition of CeO2 over YSZ single crystal; the epitaxial growth, the reactivity with respect to several interesting molecules, as well as other interesting properties and features also have been extensively investigated.14,19-21 Shi et al.,22 as an example, investigated the growth modes of the CeO2 films deposited by pulsed laser deposition on (001) yttria-stabilized zirconia single-crystal substrate. They observed that the CeO2 film growth mode changes with the deposition temperature: at 1063 K (the optimum epitaxial temperature) the CeO2 films have a layer-by-layer growth mode up to a thickness of 100 nm; at 1048 and 1078 K, instead, the growth mode is initially layer-by-layer (up a thickness of 10 nm), but then changes to island growth mode (by a thickness of 100 nm). Up to now, in contrast, the growing mechanism on the powder system has not been investigated. With this study we want to show that also in the case of powder systems it is possible to know and to monitor the growing mechanism of a supported oxide on a supporting one. Several powders with increasing Ce/Zr atomic ratio were synthesized and characterized. In this paper, particular attention was devoted to the investigation of the growth mode of CeO2 on the supporting YSZ surface. As mentioned above, heterogeneous catalysts made up of active nonoparticles supported on a substrate are becoming more and more important because of the possibility of using different shapes and sizes to catalyze different reactions.23,24 Consistently, detailed knowledge of the nanocomposite surface at the molecular level is of particular interest since the catalytic reaction takes place at the surface and the surface itself is most prone to both structural and topological changes. With this purpose further work is underway in our laboratories to evaluate the surface reactivity of the CeO2/ YSZ nanopowders toward oxidation reactions. In particular, the study of the reactivity with respect to methanol and carbon monoxide is in progress. The possibility of controlling the growing mechanism can be highly significant in heterogeneous catalysis. This advanced technology, in fact, is fundamental to designing and producing material with specific functional properties. Experimental Section Synthesis. Nine CeO2/YSZ nanocomposite powders were obtained by depositing, by wet impregnation,25,26 different amounts of CeO2 on yttria-stabilized zirconia (YSZ). Commercial yttria (8% mol)-stabilized zirconia powder (TOSOH, TZ-Y8) with a specific BET surface area of 13 m2/g, pore volume of 0.1258 cm3/g, and crystallite average diameter of 25 nm was impregnated with aqueous solutions containing increasing quantities of Ce(NO3)3‚6H2O (Strem Chemicals, 99.9%): [Ce/Zr]nominal ) 0.003, 0.005, 0.010, 0.015, 0.020, 0.035, 0.050, 0.080, 0.135 (nominal atomic ratios are obtained from the precursors weighed quantities). The amount of water used is such that every impregnating solution is 0.014 mol/L of Ce(NO3)3‚6H2O. The obtained suspensions were maintained under stirring for 2 days and then kept at rest for 1 day. Water was then evaporated in air at 338 K and the solids were dried at 373 K for 5 h and calcined at 523 K for 5 h (in air). This preparation procedure, repeated several times, resulted highly reproducible results.27 Characterization. XPS spectra were recorded on a PerkinElmer PHI 5600 ci spectrometer with a standard Al KR source

Natile and Glisenti

Figure 1. XRD patterns of the CeO2/YSZ supported oxides with CeO2 loading increasing from bottom to top (from [Ce/Zr]nominal ) 0.020 to 0.135) compared with the ones recorded for YSZ and CeO2 (heated at 523 K). The peaks of cubic CeO2 are evidenced by an asterisk.

(1486.6 eV) working at 350 W. The working pressure was less than 1 × 10-8 Pa. The spectrometer was calibrated by assuming the binding energy (BE) of the Au 4f7/2 line to lie at 84.0 eV with respect to the Fermi level. Extended spectra (survey) were collected in the range 0-1350 eV (187.85 eV pass energy; 0.4 eV step; 0.05 s step-1). Detailed spectra were recorded for the following regions: Zr 3d, Y 3d, Ce 3d, Ce 4d, O 1s, and C 1s (11.75 eV pass energy, 0.1 eV step, 0.1 s step-1). The standard deviation in the BE values of the XPS line is 0.10 eV. The atomic percentage, after a Shirley-type background subtraction,28 was evaluated by using the PHI sensitivity factors.29 To take into consideration charging problems the C 1s peak at 285.0 eV was considered and the peaks BE differences were evaluated. The sample for the XPS analysis was processed as a pellet by pressing the catalyst powder at ca. 7 × 106 Pa for 10 min; the pellet was then evacuated for 12 h at ca. 1 × 103 Pa. Particular attention was paid to avoid the ceria reduction problem under XPS conditions, which are well documented in the literature.30,31 In particular, spectra were always acquired within the minimum time required to obtain an optimum signal-to-noise ratio. This time, always much smaller than that observed to be effective in modifying the Ce 3d spectrum, was determined by means of repeated measures at increasing irradiation time. Transmission electron micrographs were obtained with a Philips JRM 2010 electron microscope, using 200 kV primary voltage. The samples used for TEM observations were prepared by dispersing some products in ethanol followed by ultrasonic vibration for 30 min, then placing a drop of the dispersion onto a copper grid (200 Cu) coated with a layer of amorphous carbon. Energy dispersive spectroscopy (EDS) measurements were carried out by means of a LINK INCA 100 microanalysis system. The diameter of the analyzed spot was 5-15 nm. Powder X-ray diffraction patterns were recorded with a Bruker D8 Advance diffractometer with Bragg-Brentano geometry, using a Cu KR radiation (40 kV, 40 mA, λ ) 0.154 nm). Results and Discussion In Figure 1 the X-ray diffraction patterns obtained for a series of the nanocomposite oxide samples are reported. For comparison, the corresponding spectra of the pure YSZ and CeO2 treated a 523 K (i.e., the same temperature used for the heat treatment of the CeO2/YSZ powders) are also shown. No diffraction peaks due to crystalline CeO2 with fluorite structure are evident in

529.8 531.8 529.8 531.6 529.8 532.0 529.8 532.0 529.8 531.8 529.9 531.6 M ) Ce(VI), Ce(III), Zr(VI), Y(III).

O 1s

)

Ce4d(Ce3+

Ce4d(Ce4+)

a

108.1 111.7 530.0 531.9

108.5 111.7 530.0 531.8

121.7 125.1

903.9 Ce 3d3/2

Zr 3d5/2 Zr 3d3/2 Y 3d5/2 Y 3d3/2 Ce 3d5/2

V V′ V′′ V′′′ U U′ U′′ U′′′ A B C X′′′ W′′′ D E F M-Oa M-OHa

885.8 889.1

529.8 531.8

122.0 125.1 121.8 125.0 121.8 125.0 121.7 125.1

916.9 108.7 111.8 916.9

121.8 125.1

907.4 916.9 108.6 111.8 907.5 917.0 108.8 111.8

121.7 125.0

530.1 532.1

529.6 531.6

121.7 125.0

907.4 916.8 108.7 112.1

105.4 108.0 111.3 530.7

903.9-904.1

885.4 889.0 898.7 901.0 889.0 898.7 901.1 889.1 898.7 901.2

182.0 184.3 157.0 159.0 882.7 885.8 889.0 898.7 901.1 904.2 907.5 916.8 108.7 111.8 182.0 184.4 157.1 159.0 882.7 885.7 889.0 898.8 901.1 904.2 182.2 184.5 157.2 159.1

182.1 184.5 157.2 159.0 882.7 885.9 889.1 899.2 901.2 904.2

182.0 184.3 157.0 158.9 882.7 885.8 889.0 898.8 901.1 904.2 907.4 916.9 108.7 111.9

182.0 184.3 157.0 158.9 882.6 885.7 889.0 898.6 901.0 904.2 907.4 916.8 108.7 111.8

182.0 184.4 157.0 159.0 882.7 885.8 889.0 898.6 901.0 904.3 907.4 916.8 108.7 111.9

182.0 184.4 156.9 158.9 882.7

181.9 184.3 156.9 158.9 882.7

182.2 184.6 157.3 159.1

882.8

CeO2(523K) YSZ [Ce/Zr] ) 0.135 [Ce/Zr] ) 0.080 [Ce/Zr] ) 0.050 [Ce/Zr] ) 0.035 [Ce/Zr] ) 0.020 [Ce/Zr] ) 0.015 [Ce/Zr] ) 0.010 [Ce/Zr] ) 0.005 [Ce/Zr] ) 0.003 XPS peaks

TABLE 1: XP Peak Positions (Binding Energy, eV) Obtained for the Pure and the Supported Oxides, with Literature Data Reported for Comparison

the samples with lower CeO2 loading. The characteristic peaks of cubic CeO2 become visible in the samples with [Ce/Zr]nominal g 0.035. With increasing Ce/Zr nominal atomic ratio the pattern of cubic CeO2 is more and more evident. The absence in the XRD patterns of the samples characterized by lower CeO2 loading of contributions due to cerium oxide suggests that cerium oxide exists as highly dispersed or amorphous surface species or the cerium oxide amount is low.32 The average diameter of CeO2/YSZ crystallites, evaluated by means of the Scherrer formula,33 is about 25 nm, like that of the YSZ used as a support. This suggests that the deposition of ceria clusters does not significantly modify the particle dimension (this is also consistent with the TEM images). The XP peak positions obtained for all the supported powders are summarized in Table 1, whereas the spectra are shown in Figures 2-5. For comparison, the corresponding spectra of pure YSZ and CeO2 treated at 523 K are also reported. The two contributions of Zr 3d (Zr 3d5/2 and 3d3/2) and Y 3d (Y 3d5/2 and 3d3/2) XP peaks for the sample with [Ce/Zr]nominal ) 0.003 are located at 182.2, 184.5 eV and 157.2, 159.1 eV, respectively, and agree with the values observed for the pure YSZ (182.2, 184.6 eV and 157.3, 159.1 eV, respectively) (see Table 1 and Figures 2 and 3). With an increase in the Ce/Zr nominal atomic ratio the Zr 3d peak positions change. The Zr 3d peak of the sample characterized by [Ce/Zr]nominal ) 0.135 is shifted 0.3 eV toward lower binding energy when compared with the pure YSZ. The same behavior has been observed for the Y 3d peaks (chemical shift is 0.4 eV with respect to the pure YSZ) (Figure 3). The above-mentioned chemical shifts in Zr 3d and Y 3d core levels could be related to the progressive wetting of the surface from ceria: the charge transfer between the yttrium, zirconium, and cerium ions at the interface can cause these changes in binding energy values.36 Figure 4 depicts the evolution of the Ce 3d XP peak with the increase of the Ce/Zr nominal atomic ratio. The Ce 3d level has a very complicated structure: six peaks corresponding to three pairs of spin-orbit doublets (noted as (V, U), (V′′, U′′), and (V′′′, U′′′)) can be identified in the Ce 3d spectrum of Ce(IV) oxide,34,37 while four peaks due to two pairs of doublets (noted as (V°, U°) and (V′, U′)) characterize the Ce 3d spectrum of Ce(III) oxide.34 The labels follow the convention established by Burroughs et al.:38 V(n) and U(n) refer to the 3d5/2 and 3d3/2 spin-orbit components, respectively. The comparison of the Ce 3d peaks obtained for the supported oxides with that of the pure CeO2 (Figure 4) and with literature data30,31,34,35 (Table 1) reveals the presence of both Ce(IV) and Ce(III) cations on the surface. The peak shape of the nanocomposite sample with [Ce/ Zr]nominal ) 0.003 recalls that of Ce(III) compounds.30,31,34,35 As the Ce/Zr nominal atomic ratio increases, features characteristic of Ce(IV) (contributions V, U, V′′, U′′, V′′′, U′′′) become more and more evident in the Ce 3d spectra. The Ce 3d spectra of the samples with the highest Ce/Zr ratio ([Ce/Zr]nominal ) 0.080, 0.135) are more similar to that of the pure CeO2.30,31 This behavior means that as the amount of cerium increases the cerium oxide deposited is more and more oxidized. An attempt was also made to determine the relative amount of Ce(III) in CeO2/YSZ supported oxides by using the procedure suggested by El Fallah et al.;39 the corresponding results are reported in Table 2. The Ce 4d shape (Figure 4) confirms this trend: with increased amounts of cerium oxide the X′′′ and W′′′ contributions characteristic of Ce(IV) also become more and more intense and sharp.34 The labels X′′′ and W′′′ are the same

J. Phys. Chem. B, Vol. 110, No. 6, 2006 2517 Ce2O330,31,34,35

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Natile and Glisenti

Figure 2. Zr 3d XP spectra obtained on the CeO2/YSZ supported oxides with CeO2 loading increasing from bottom to top (from [Ce/ Zr]nominal ) 0.003 to 0.135) compared with the corresponding spectrum of YSZ.

Figure 4. Ce 3d and Ce 4d XP spectra obtained on the CeO2/YSZ supported oxides with CeO2 loading increasing from bottom to top (from [Ce/Zr]nominal ) 0.003 to 0.135) compared with the corresponding spectra of CeO2 (heated at 523 K). Figure 3. Y 3d XP spectra obtained on the CeO2/YSZ supported oxides with CeO2 loading increasing from bottom to top (from [Ce/Zr]nominal ) 0.003 to 0.135) compared with the corresponding spectrum of YSZ.

used by Burroughs et al.38 and refer to the Ce 4d5/2 and Ce 4d3/2 components, respectively. This high presence of Ce(III) cations in the samples characterized by the lowest Ce/Zr atomic ratio can be due both to the low particle dimensions and to the low degree of crystallinity of cerium oxide clusters. Theoretical calculations, in fact, show that the energy required to reduce CeO2 increases with particles size.40 Moreover, the reduction of the ceria nanoparticles is structure sensitive, being easier in systems that have a low crystallinity.41 These results agree with the above-reported XRD outcomes. The fitting procedure of the O 1s XP peak shows the presence of two contributions (Figure 5). In the samples with lower Ce/ Zr nominal atomic ratio the most intense contribution (529.9530.0 eV, see Table 1) is in good agreement with the values reported in the literature for the lattice oxygen of cerium(III) oxide (Ce2O3) and those observed for yttria-stabilized zirconia. As the cerium content increases a broadness of this peak and its shift toward lower binding energy, characteristics of cerium(IV) oxide, are observed. The contribution at higher binding energy (531.6-532.0 eV) can be ascribed to the presence of hydroxyl groups.42 A slight increase of hydroxyl groups is

Figure 5. O 1s XP spectra obtained on the CeO2/YSZ supported oxides with CeO2 loading increasing from bottom to top (from [Ce/Zr]nominal ) 0.003 to 0.135) compared with the corresponding spectra of YSZ and CeO2 (heated at 523 K).

observed on the nanocomposite surfaces with respect to the supporting YSZ.

Nanostructured Oxide-Based Powders

J. Phys. Chem. B, Vol. 110, No. 6, 2006 2519

TABLE 2: Nominal and XP Atomic Composition of the Supported Oxides and the Amount of Ce(III) [Ce/Zr]nominal

[Ce3d/Zr3d]XPS

[O1s/(Ce 3d + Z 3d + Y 3d)]XPS

Ce(III), %

0.003 0.005 0.010 0.015 0.020 0.035 0.050 0.080 0.135

0.12 0.15 0.22 0.26 0.31 0.44 0.52 0.63 0.73

2.62 2.36 2.48 2.34 2.20 2.30 2.10 2.12 2.25

40 39 39 30 29 29 27 20 20

XP atomic compositions are shown, as a function of the corresponding nominal ones (calculated from the weighed quantities), in Figure 6 and summarized in Table 2. Consistently with the surface specific character of the XPS and the preparation procedure, the XP Ce/Zr atomic ratios are always higher than the corresponding nominal values. This allows us to exclude the diffusion of the deposited oxide inside the supporting one. The curve trend, tending to a plateau, suggests a 3D or VolmerWeber growing mechanism.17,43 It is noteworthy that a similar behavior with respect to the nominal composition was also observed for the Zr 3d and Y 3d peak shift, thus supporting the hypothesis of a charge-transfer mechanism. The O 1s/(Ce 3d + Zr 3d + Y 3d) atomic ratio is always higher than the nominal value confirming the surface hydroxylation. The O 1s/(Ce 3d + Zr 3d + Y 3d) atomic ratio progressively decreases as the Ce/Zr atomic ratio increases. This behavior could be attributed to changes in cerium surroundings; the different hydroxylation with respect to the ceria content also has to be considered: surface hydroxylation was observed to be higher on the samples with a lower Ce/Zr atomic ratio (the possible formation of amorphous Ce(III) hydroxide was already hypothesized). Transmission electron microscopy (TEM) images of two CeO2/YSZ powders confirm the growing mechanism suggested by the XPS data. Figures 7 and 8 show the transmission electron microscopy images of the CeO2/YSZ supported oxides characterized by [Ce/Zr]nominal ) 0.020 and 0.080. The images show that the 3-dimensional cerium oxide particles directly nucleate and grow on the substrate grain boundaries. The energy dispersive (ED) spectra collected for the CeO2/YSZ supported oxides characterized by [Ce/Zr]nominal ) 0.020 on the boundary (Figure 9a) and on the center (Figure 9b) of the YSZ grain confirm the above-mentioned growth mode. With reference to the already cited paper of Shi et al.,22 the different preparation procedure can easily explain the different growing mechanism.

Figure 7. TEM image of the CeO2/YSZ supported oxide with [Ce/ Zr]nominal ) 0.020.

Figure 8. TEM (A) and HRTEM (B) images of the CeO2/YSZ supported oxide with [Ce/Zr]nominal ) 0.080.

Figure 6. XP atomic ratios displayed as a function of the nominal ones.

Concerning the sample characterized by a lower CeO2 loading (Figure 7), the supported clusters have an elliptical shape (the average diameter is 4.4 × 2.7 nm) with the longer axis parallel to the YSZ surface. With an increase in the amount of CeO2 the clusters become more spherical in shape (the average diameter is 4.4 nm) (Figure 8). This growing mechanism suggests that the supported atoms are more strongly bound to each other than to the substrate. Finally, the HRTEM image

2520 J. Phys. Chem. B, Vol. 110, No. 6, 2006

Figure 9. ED spectra collected on the boundary (a) and on the center (b) of YSZ grain.

(Figure 8B) of the CeO2/YSZ sample richer in CeO2 suggests a good crystallinity of the supported oxide. These results confirm the XRD results. Conclusions Several CeO2/YSZ nanocomposite powders differing in the Ce/Zr atomic ratio were synthesized by wet impregnation and characterized by means of XPS, TEM, EDS, and XRD. XPS data allow us to recognize the presence of Ce(III) and Ce(IV) ions at the interface supported/supporting oxides. The Ce(III) is more evident in the samples with low cerium content. A 3-D growing mechanism has been observed for the CeO2/ YSZ nanocomposite powders. At first small and elliptic 3-D islands form on the supporting YSZ; with an increase in the amount of cerium oxide these building clusters grow becoming more and more spherical. This 3-D growing mechanism, similar to the Volmer-Weber mode of metal thin film, suggests that the supported atoms are more strongly bound to each other than to the substrate. The diffusion of the deposited oxide inside the supporting one can be excluded. TEM and XRD data are consistent with the formation of cubic CeO2 on supporting YSZ at high CeO2 loading. Acknowledgment. The authors thank Professor E. Tondello for his helpful discussions and the I.N.S.T.M. for financial support. References and Notes (1) Klabunde, K. J. Nanoscale Materials in Chemistry; Wiley: New York, 2001, and references therein. (2) Rao, C. N. R.; Mu¨ller, A.; Cheetham, A. K. The Chemistry of Nanomaterials; Wiley-VCH: Weinheim, Germany, 2004. (3) Natile, M. M.; Glisenti, A. Chem. Mater. 2003, 15, 2502. (4) Natile, M. M.; Glisenti, A. J. Mol. Catal. A: Chem. 2004, 217, 175. (5) Trovarelli, A. Catal. ReV. Sci. Eng. 1996, 38, 439 and reference therein. (6) Sayle, D. C.; Sayle, T. X. T.; Parker, S. C.; Harding, J. H.; Catlow, C. R. A. Surf. Sci. 1995, 334, 170. Morterra, C.; Magnacca, G. J. Chem. Soc., Faraday Trans. 1996, 92, 5111. Martin, D.; Duprez, D. J. Phys. Chem.

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