ACTIVATION OF CARBON-MONOXIDE ON NICKEL ALUMINUM-ALLOY SURFACES AND BY INTERSTITIAL TRAPPING IN A NICKEL MATRIX - STRUCTURE AND ELECTRONIC FACTORS FROM MOLECULAR-ORBITAL THEORY

An atom superposition and electron delocalization molecular orbital study of an isolated CO molecule bound to a cluster model of the Ni(111) surface shows the site preference order is 3-fold > 2-fold > 1-fold and the vibrational frequencies match well those observed by Campuzano and Greenler. CO is attracted to a coadsorbed Al atom, forming a nonlinear Al-OC-Nix complex with low vibrational frequency, a result of 5σ and π donation to empty Al 3p orbitale which allows increased π* back donation. This gives an electronic explanation for the activation of CO on Ni-Al alloy surfaces observed by Rao et al. and Yates and co-workers. When trapped in Ni matrices CO again exhibits a large decrease in vibrational frequency and this is shown to stem not from a Nix-CO-Niy structure, which favors σ interactions, but rather from an interstitial defect structure where Ni atoms contact both C and O ends and back donate to the empty π* orbitals. © 1990 American Chemical Society.