FUNDAMENTALS OF GAS-SURFACE INTERACTIONS ON METAL-OXIDES

In an attempt to understand the fundamental ways in which Molecules interact with metal-oxide surfaces, chemisorption has been studied on single-crystal oxide samples. Chemisorption behavior is determined both by the electronic configuration of surface cations and by the geometric structure of the surface. Stoichiometric, well ordered oxide surfaces are, for the most part, relatively unreactive (although striking exceptions exist). However, point defects, which consist predominantly of oxygen vacancies, significantly change the surface electronic structure on most oxides and are the active sites for many types of chemisorption. The interaction between 02 and transition-metal-oxide surfaces is determined primarily by cation electronic structure, while the dissociation of H2O seems to be promoted by particular structural features at defect sites and is less sensitive to electronic structure. For organic molecule chemisorption, a general feature of oxide surfaces is the rather facile breaking of C-H bonds, but not of C-C bonds; this is important in selective oxidation catalysis.