Chemisorption of Nitric Oxide at a Zn(0001) Surface and the Role of Water Vapour in Its Hydrogenation

The techniques of X-ray and u.v.-photoelectron spectroscopy have been used to investigate the chemisorption of the individual molecules nitric oxide, oxygen, and water, and also of coadsorbed nitric oxide and water, at Zn(0001) surfaces. The core and valence level spectra established that at 77 K, nitric oxide existed in both dissociative and molecular states, oxygen generated both surface O^-(a) and subsurface O^2-(b) species, while water was only physically adsorbed. At 160 K and above, Zn(0001) surfaces were unreactive to water vapour, while at 295 K only the dissociative state of nitric oxide was present. The coadsorbed water-nitric oxide adlayer was reactive below 200 K. Surface hydroxyls were formed through the activation of molecularly adsorbed water by chemisorbed oxygen generated in the dissociative chemisorption of nitric oxide. These surface hydroxyls participated in a hydrogen transfer reaction leading to hydrogenation of the chemisorbed nitrogen at 273 K followed by desorption. In the absence of water, chemisorbed nitrogen adatoms are by comparison thermally stable. A detailed mechanism, with each surface species spectroscopically identified, is proposed for the hydrogenation of nitric oxide. Analysis of the X-ray induced core level spectra enabled the concentrations of the surface species to be calculated.