Chemisorption and incorporation of oxygen at a nickel surface

Abstract

A continuous recording (static capacitor) device, with a response time of 0.1 s, has been used to measure the surface potential changes occurring during oxygen chemisorption on clean nickel films. At 77 and 90 °K, the surface potential is decreased to – 1.6 V, followed by an increase of 0.1 V at an oxygen pressure of ca . 10 -2 torr. This increase is the result of physical adsorption of oxygen molecules in a second layer. Oxygen doses added at higher temperatures cause a surface potential decrease, followed immediately by a slow decrease due to incorporation of oxygen in the metal lattice. The kinetics of the incorporation process are affected by the surface electric field formed by the oxygen-nickel double layer. Analysis of the kinetic curves, corrected for this field effect, shows that the average charge per oxygen atom is 0.3 ± 0.04 of an electronic charge. The experimental rate of incorporation is smaller, by a factor of approximately 10 -12 , than that calculated from the absolute rate equation and the use of the experimental value of the activation energy of 1 kcal/mole. This discrepancy is attributed to the very small probability of creating a low-energy barrier by the simultaneous movement apart of surface nickel atoms, allowing passage inwards of the oxygen adatoms. The calculated probability for this concerted motion is in agreement with the experimental value; the magnitude of the effective surface energy barrier is then evaluated as 7.3 ±1 kcal/mole. At temperatures between 350 and 403 °K, the kinetics of incorporation change, and their autocatalytic character corresponds to that of a nucleation and growth process. At these temperatures the surface potential increases to +0.7 V, and the adsorptive capacity is markedly reduced. These changes are attributed to the formation of oxide-like patches containing positively charged nickel atoms in their external surface.