Oxidation of carbon on vicinal surfaces of nickel

Abstract

The role of bulk carbon in determining the high temperature stability of oxygen on carbon‐free Ni surfaces has been investigated. Oxygen adsorbed on a room temperature carbon‐free Ni surface reacts, upon subsequent heating, with bulk carbon to form a CO desorption product. We have studied this reaction over a wide range of oxygen exposures on clean Ni(111) and on clean and sulphur‐covered Ni(111) 10°[11̄0] using Auger electron spectroscopy (AES) and temperature programmed desorption (TPD). TPD data for oxygen‐covered Ni surfaces reveal three principal CO desorption states. The first state (α) desorbs between 350 and 500 K and appears to be the result of molecular adsorption of CO from the background gas. The second state (β1) desorbs between 550 and 700 K. There is evidence to suggest that the β1 state arises from the dissociative adsorption of CO from the background gas at or near surface step and defect sites. The third state (β2) desorbs between 600 and 1100 K and the desorption peak temperature increases with increasing O2 exposure. A fit of the low temperature side of the β2 peak to a bulk carbon diffusion rate limiting desorption model yields a value of ∠36.5 kcal/mol for the activation energy for bulk diffusion of carbon in nickel.