Probing the metal sites of a vanadium oxide–Pd(111) ‘inverse catalyst’: adsorption of CO

The adsorption of CO on V-oxide covered Pd(111) multicomponent surfaces, so-called ‘inverse catalyst’ surfaces, has been studied by high-resolution X-ray photoelectron spectroscopy using synchrotron radiation. Since CO does not chemisorb on V oxide surfaces at room temperature, the adsorption of CO reflects the reactivity of the free Pd sites of this inverse catalyst surface. The C 1s and Pd 3d core level spectra have been monitored to follow the CO surface coverage as a function of CO exposure and V oxide coverage and to investigate the CO adsorption sites. The results indicate that the adsorption of CO on V-oxide/Pd(111) surfaces is kinetically influenced by the V oxide phase via spill-over of CO molecules from a mobile physisorbed precursor state on the oxide surface on to Pd sites. As a result of this spill-over effect the initial sticking probability of CO per free Pd site increases beyond unity with increasing oxide coverage. The analysis of C 1s binding energies as a function of CO and oxide coverage suggests that the distribution of CO adsorption sites is altered by the V-oxide/Pd phase boundary.