Reaction of hydrogen with adsorbed oxygen on Ni(110)

Abstract

We have titrated preadsorbed oxygen on Ni(110) with hydrogen, observing oxygen coverage as a function of time via Auger Electron Spectroscopy (AES). The experiments were performed for hydrogen pressures from 1 × 10 −6 to 4 × 10 −4 Torr, temperatures from 410 to 610 K, and initial oxygen coverages from 0.25 to 0.85 of a monolayer (ML). The titration curves display an induction period followed by a period of rapid reaction, with the reaction rate dependent on initial oxygen coverage as well as instantaneous coverage. A simple Monte Carlo simulation based on the formation of oxygen islands with reaction along the perimeters gives a qualitative explanation of the data. The reaction has an overall activation energy of 0.5 eV. Half order dependence of the rate upon H 2 pressure eliminates as possible mechanisms H 2(g) + O(a) → H 2O, H 2(a) + O(a) → H 2O, and 2H(a) + O(a) → H 2O. Other mechanisms, involving dissociative adsorption of hydrogen and OH(a) intermediates, remain possibilities.