The reduction of oxygen-pretreated Ni(1 1 1) by NH3: a study with MIES and UPS data through Monte Carlo simulations

Metastable induced electron spectroscopy (MIES) in combination with ultraviolet photoelectron spectroscopy (UPS) was applied to study the oxygen-pretreated Ni(1 1 1) reduction by ammonia at 600 K. Experimental data showed the perfect reduction of the surface, achieved by water desorption. However, no intermediate species like OH or NHx were observed in our spectra. Thus, ammonia dissociation reaction should be considered as rate limiting. The reduction process kinetics was investigated by measurement of the 3d band intensity. Due to the relatively large escape depth of the emitted electrons UPS provides spectroscopic information about several layers contrarily to MIES, which is only sensitive to the outermost layer. This allowed us to discriminate the surface and the subsurface processes. MIES and UPS data presented two different behaviours. Our experimental data indicated a less numerous oxygen atoms in the innerlayers whereas the surface remained unchanged. The substitution of the surface-desorbed oxygen atoms by subsurface-embedded ones is very likely. These experiment results were interpreted by using Monte Carlo simulation, which fitted well our data under the following local condition: during the upward diffusion process to the surface, the subsurface oxygen atoms slightly move laterally.