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.

Using a combination of state-of-the-art computational modeling and Fourier transform infrared (FTIR) spectroscopy study of the surface species formed during interaction of CO2 or CO with activated (stoichiometric), reduced, and hydroxylated ceria, CeO2, we assigned various experimentally observed vibrational modes to individual types of surface species. We considered carbonates CO3 2–, formates HCO2 –, and hydrogen carbonates CO2(OH)− bound in various ways to the surface of a ceria nanoparticle. Since the structure of the surface carbonate species is particularly versatile, we introduced a notation of different types of such species and computationally determined the regions where the characteristic vibrational frequencies of each type of species can be found. The complementary FTIR measurements of the surface species produced under different conditions revealed the actual experimental vibrational peaks and allowed estimation of the accuracy of the computational method to reproduce the frequencies of different vibrational modes. Thus, combining computed and experimental data we suggest a sound, partly new assignment of the vibrational bands in the complex IR spectra of surface (hydrogen)carbonate and formate species on ceria. The proposed reassignment of the vibrational peaks enables reliable detection of the surface species on ceria surface using vibrational spectroscopy. This is critical for the meaningful analysis of the reactivity of these species and the clarification of the mechanisms of the rich variety of surface processes on ceria.

We address the formation of the energetically most favourable single oxygen vacancies in ceria nanoparticles (CeO 2 ) n focusing on their size dependence. We study a series of structures with increasing number of CeO 2 units (n ¼ 21, 30, 40 and 80) that, according to well tested interatomic-potential calculations, approach the global minima for these particle sizes. The structures thus obtained are refined by means of density functional (DF) methods, modified by the on-site Coulomb correction. Subsequent DF calculations are performed to quantify and analyse the depletion of atomic O from the nanoparticles that results in the formation of a vacancy O vac . We show that (i) removal of a low-(two-)coordinate O atom from ceria species requires the lowest energy, in line with evidence from other metal oxides; (ii) the depletion of such O atoms from the nanoparticles is strongly facilitated compared to extended (even irregular) surfaces; (iii) increase of the particle size is accompanied by a dramatic decrease of the O vac formation energy, implying that at certain sizes this energy should reach a minimum; (iv) the size dependence of the O vac formation energy is driven by the electrostatics, thus enabling the prediction of the most easily removable O atoms by analysing the distribution of the electrostatic potential in the pristine stoichiometric (vacancy-free) ceria systems. Our findings provide a key to rationalize the observed spectacularly enhanced reactivity of ceria nanostructures.

The formation of oxygen vacancies in nanoparticles Ce(n)O(2n) (n < or = 80), studied using density-functional calculations, is found to be greatly facilitated compared to extended surfaces, which explains the observed spectacular reactivity of nanostructured ceria.