ABSTRACT
The dissociative interaction of oxygen with structurally well-defined monolayer Pd(x)Ag(1-x)/Pd(111) surface alloys of different compositions, with well-known distributions of the respective surface atoms (A. K. Engstfeld et al., Phys. Chem. Chem. Phys. 2012, 14, 10754-10761), and the coadsorption of/reaction with CO on oxygen pre-covered surfaces were studied by high-resolution electron energy loss spectroscopy (HREELS) and temperature-programmed desorption/reaction spectroscopy (TPD/TPR). The impact of geometric ensemble effects as well as electronic ligand and strain effects on the adsorption and reaction behaviour of the respective species on the bimetallic surfaces is elucidated and compared with related systems such as CO adsorption on similar surfaces and oxygen adsorption on a Pd(67)Ag(33)(111) bulk alloy surface. The data show a clear dominance of ensemble effects on the oxygen adsorption and CO coadsorption behaviour, with oxygen adsorption limited to threefold-hollow sites on Pd(3) sites, while the combined electronic effects, as evident from modifications in the adsorption and reaction characteristics on the Pd sites, are small.
ABSTRACT
The interaction of oxygen with a carburized Mo(100) surface was investigated at different temperatures (300-1000 K). The different information depths of low-energy ion-scattering (LEIS) spectroscopy, with topmost layer sensitivity, Auger electron spectroscopy (AES), and angle-resolved X-ray photoelectron spectroscopy (ARXPS) allowed us to discriminate between reactions on the topmost layer and subsurface transformations. According to ARXPS measurements, a carbide overlayer was prepared by the high-temperature decomposition of C(2)H(4) on Mo(100), and the carbon distribution proved to be homogeneous with a Mo(2)C stoichiometry down to the information depth of XPS. O(2) adsorbs dissociatively on the carbide layer at room temperature. One part of the chemisorbed oxygen is bound to both C and Mo sites, indicated by LEIS. Another fraction of oxygen atoms probably resides in the hollow sites not occupied by C. The removal of C from the outermost layer by O(2), in the form of CO, detected by mass spectroscopy (MS), was observed at 500-600 K. The carbon-depleted first layer is able to adsorb more oxygen compared to the Mo(2)C/Mo(100) surface. Applying higher doses of O(2) at 800 K results in the inward diffusion of O and the partial oxidation of Mo atoms. This process, however, is not accompanied by the removal of C from subsurface sites. The depletion of C from the bulk starts only at 900 K (as shown by MS, AES, and XPS), very probably by the diffusion of C to the surface followed by its reaction with oxygen. At T(ads) = 1000 K, the carbon content of the sample, down to the information depth of XPS, decreased further, accompanied by the attenuation of the C concentration gradient and a substantially decreased amount of oxygen.
