Direct evidence for the instability and deactivation of mixed-oxide systems: influence of surface segregation and subsurface diffusion

Abstract

In the current contribution, we provide a direct demonstration of the thermally induced surface structural transformations of an alkaline-earth oxide/transition metal oxide interface that is detrimental to the essential catalytic functionality of such mixed-oxide systems toward particular reactants. The BaOx/TiO2/Pt(111) surface was chosen as a model interfacial system where the enrichment of the surface elemental composition with Ti atoms and the facile diffusion of Ba atoms into the underlying TiO2 matrix within 523 873 K leads to the formation of perovskite type surface species (BaTiO3/Ba2TiO4/BaxTiyOz). At elevated temperatures (T > 973 K), excessive surface segregation of Ti atoms results in an exclusively TiO2/TiOx-terminated surface which is almost free of Ba species. Although the freshly prepared BaOx/TiO2/Pt(111) surface can strongly adsorb ubiquitous catalytic adsorbates such as NO2 and CO2, a thermally deactivated surface at T > 973 K practically loses all of its NO2/CO2 adsorption capacity due to the deficiency of surface BaOx domains.