Influence of support acidity on electronic state of platinum in oxide systems promoted by SO42- anions

The electronic state of platinum supported on SO4/ZrO2, SO4/TiO2, SO4/Al2O3, and SO4/SiO2 systems and on systems unpromoted by sulfur was investigated by diffuse-reflectance IR spectroscopy using CO as the probe molecule. The introduction of SO42- anions increases the electron deficit on platinum particles. This suppresses the formation of bridging CO complexes with the metal, leads to the high-frequency shift of absorption maxima of CO adsorbed in the linear form, and stabilizes positively charged metal species (Ptdelta+ and Pt+) during the reduction process. The formation of the positively charged species includes the interaction between the acidic protons and the metal particles yielding [Pt-H](delta+) adducts. The extent of the influence of the support on the electronic state of the metal increases in the series SO4/SiO2 < SO4/Al2O3 < SO4/TiO2 < SO4/ZrO2 in parallel with an increase in the strength of the acid sites in the system.