No abstract

Interactions of metal particles with oxide supports can radically enhance the performance of supported catalysts. At the microscopic level, the details of such metal-oxide interactions usually remain obscure. This study identifies two types of oxidative metal-oxide interaction on well-defined models of technologically important Pt-ceria catalysts: (1) electron transfer from the Pt nanoparticle to the support, and (2) oxygen transfer from ceria to Pt. The electron transfer is favourable on ceria supports, irrespective of their morphology. Remarkably, the oxygen transfer is shown to require the presence of nanostructured ceria in close contact with Pt and, thus, is inherently a nanoscale effect. Our findings enable us to detail the formation mechanism of the catalytically indispensable Pt-O species on ceria and to elucidate the extraordinary structure-activity dependence of ceria-based catalysts in general.

To check the accuracy of our computational method, we optimized the geometry of the cluster compound Ir 4 (CO) 12 in the gas phase. The structure of this compound has been explored experimentally with EXAFS 1 and X-ray diffraction (XRD). 2 We started the geometry optimization with the XRD structure and applied C 3 symmetry constraints because the space group of the crystal studied by X-ray diffraction was assigned as P3. 2 In Table S1, we compare calculated structural characteristics of the optimized geometry with the corresponding experimental results. Inspection of Table S1 shows that the applied computational theoretical method describes very well the structure of the metal moiety and the complex. Ir-Ir bonds, calculated at 272 pm, are 3 pm longer than the XRD value, 269 pm; also, longer, non-bonding Ir-O distances, calculated at 307 pm, slightly overestimate the corresponding experimental value, by 6 pm. Thus; not untypical for a GGA density functional method, bonding distances are calculated 1% longer than XRD values; for nonbonding distances that deviation is 2%. The Ir-C distance is not directly available from the XRD analysis. As expected, the corresponding distances from a previous computational study, 3 where a local density approximation (LDA) was used for the exchange-correlation functional, are somewhat shorter and slightly closer to the XRD values (Table S1). The present GGA results also fit very well the EXAFS data 1 for unsupported Ir 4 (CO) 12 .

With density functional calculations, we explored the successive adsorption of hydrogen on tetrahedrally shaped zeolite-supported M 4 clusters (M ) Rh, Ir, and Pt). Similarly to our earlier results for models of Ir 4 , hydrogen adsorption on Rh 4 and Pt 4 causes an increase of the metal-metal distances. The type of metal strongly affects the adsorption energy of hydrogen, and the optimum H/M ratio varies with the metal, being ∼2 for Rh and ∼3 for Ir and Pt. As judged by the core-level shifts and atomic charges, Rh and Pt clusters are oxidized through interactions with the support and the hydrogen ligands, similarly to our earlier findings for Ir 4 .