Atom superposition and electron delocalization (ASED) theory for catalysis. Dissociative properties of acetylene on Fe and Ni(100) with coadsorbed O, S, Se and implications for Te

Abstract

Using an atom superposition and electron delocalization (ASED) theory, acetylene is found to bond to the fourfold site on Ni(100) more strongly and with greater CC bond lengthening distortion than on Ni(111). Half-monolayer c(2 × 2) O, S, Se, and Te coverages poison the surface toward acetylene chemisorption. Quarter-monolayer (2 × 2) O and S allow weakened adsorption. A specific destabilization of an acetylene π orbital when bridging two O atoms activates CC bond scission barrier reduction. This is of catalytic significance. Increasing or decreasing the surface Ni charge, as occurs when bonding to more electronegative or electropositive atoms or on an electrode in a dielectric medium, also serves to weaken the barrier. The bonding of S and Se to Fe(100) is treated. The fourfold site is preferred because a high-lying orbital loses its unpaired electron in this site. On quarter-monolayer covered Fe(100), acetylene CC bond scission is accelerated in chalcogen bridging sites by the same wedging effect even though the chemisorption energy is weakened. Chemisorption occurs only on the half-monolayer c(2 × 2) O-covered surface if a high-lying orbital loses its unpaired electron. This possibility is discussed. The relationship of the theory and photoemission spectra is discussed.