Kinetics of adsorption, replacement and catalysis in the interaction of O2 and CO with clean nickel surfaces

Abstract

The kinetics of the interaction of O₂ with CO on Ni films under ultra-high vacuum conditions has been investigated utilizing an apparatus designed for sticking probability profile measurement. With it, absolute rates of adsorption, replacement and catalysis, and surface coverages in the reacting species were simultaneously measured. The results reveal marked differences in the chemical reactivity of the different binding states in the adlayer. The catalytic reaction of preadsorbed O₂ with gaseous CO to form CO₂ was found to proceed only when the coverage θ in the preadsorbed O₂ layer was < ¼ monolayer. The catalytically active species β′-O₂ is converted to a more tightly bound, inert species β-O₂ at θ_O2 > ¼ monolayer, this transformation having a marked effect on the sticking probability profile for O₂ on Ni.⁴ Reaction kinetics indicate that the β′-O₂ state is non-dissociatively adsorbed. When gaseous O₂ is introduced to Ni films with preadsorbed CO, the CO is efficiently and quantitatively displaced from the surface as Co and CO₂. The strongly bound β-CO state yields CO₂ and a more weakly bound α-CO state yields CO, both processes being preceded by a short induction period. Reaction mechanisms are proposed which are supported by infra-red data obtained from supported nickel-on-silica samples. The continuous catalytic reaction of CO with O₂ to form CO₂ to form CO₂ over O₂-presaturated Ni films was also examined, and a possible reaction mechanism is discussed.