Adsorption sites, metal-support interactions, and oxygen spillover identified by vibrational spectroscopy of adsorbed CO: A model study on Pt/ceria catalysts

Abstract

Vibrational spectroscopy of adsorbed CO is commonly used for the characterization of oxide-supported metal catalysts, in spite of persisting ambiguities concerning the band assignment. Here, we present reference data on well-defined Pt/CeO2(111)/Cu(111) model catalysts aiming at clear identification of the related features. Growth and morphology of the Pt particles are investigated by scanning tunneling microscopy (STM), the adsorption properties are probed by temperature programmed desorption (TPD), and the electronic structure has previously been studied by photoelectron spectroscopy (PES) and resonant photoelectron spectroscopy (RPES). Combining molecular beam (MB) methods and infrared reflection absorption spectroscopy (IRAS), we correlate this information to the interaction with CO.Two types of defect adsorption sites are identified on the ceria support (2097cm−1, 2120cm−1), which are assigned to structural defects and reduced Ce3+ centers. The relative abundance of these sites changes upon thermal treatment. On the Pt nanoparticles, CO first adsorbs on-top at step and edge sites (2066cm−1), before adsorption on the (111) facets occurs in bridging (1875cm−1) and on-top (2080–2097cm−1) geometry. For the first time, we identify vibrational CO features, which arise from electronic metal-support interactions and from oxygen reverse spillover. On reduced CeO2−x(111), a red-shifted band at 2053cm−1 appears, which is attributed to increased π-backbonding from Pt to CO, as a result of a change in electronic metal-oxide interaction. Thermal treatment also activates oxygen reverse spillover from the support to the Pt nanoparticle, which gives rise to a blue-shifted feature in the CO spectrum (2090–2105cm−1) due to coadsorbed oxygen next to CO sites.