Quantum chemical concepts in heterogeneous catalysis

Abstract

Four concepts of electronic change of relevance to heterogeneous catalysis are discussed. The Schwab effect of the second kind applies to changes in the reactivity of metal particles dispersed in a zeolite. The secondary ensemble effect applies to changes in the geometry of chemisorption if the composition of a metal surface changes because of alloying. The geometry of adsorbed molecules, e.g. atop vs. bridge coordination of CO, is affected by the interaction of doubly occupied CO 5σ orbitals and highly filled surface valence d-orbitals. Depletion of antibonding orbitals formed upon bonding is a function of adsorption geometry, and leads to nonclassical adsorption geometries. Orbital symmetry effects are also illustrated by discussing the orientation of an oxygen molecule on the (111) surface of Ag. The surface ligand effect is illustrated by calculations of changes in the electronic structure of the transition state of chemisorbed oxygen and ethylene in the presence and absence of subsurface oxygen. The surface ligand effect is responsible for the low activation energy for epoxidation formed on silver surfaces with sub-surface oxygen present. Hartree-Fock-Slater LCAO and Extended Hückel calculations have been used.