A fluorescence-yield near-edge spectroscopy (FYNES) investigation of the reaction kinetics of NiO/Ni(100) with hydrogen

Abstract

Abstract The method of fluorescence-yield near-edge X-ray absorption spectroscopy (FYNES) has been applied to investigate the thermal stability of NiO films on a Ni(100) substrate and the reaction kinetics of this surface with hydrogen gas. The FYNES results indicate that the total oxygen content near the surface region remains constant after heating a NiO/Ni(100) layer above a phase separation temperature of 525 K, indicating that there is no significant thermally induced diffusion of oxygen into the bulk Ni(100) lattice. These results support conclusions from previous EELS and XPS experiments that the NiO/Ni(100) layer prepared at 300 K undergoes a thermally induced phase separation to produce a surface with mixtures of roughly 50 A NiO(100) crystalline clusters and c(2 × 2)−O chemisorbed phases. The FYNES investigation of the reaction kinetics of NiO/Ni(100) with H2 in the temperature range of 600–800 K indicates that the oxygen removal rate from NiO clusters is roughly zero order with respect to the initial oxygen content, and that the subsurface oxygen is removed completely. In addition, the removal rate of oxygen is found to be approximately first order with respect to H2 pressure. The reaction of NiO/Ni(100) with H2 has a rather weak temperature dependence, as characterized by an activation energy of 1.8 kcal/mol with a pre-exponential factor of 9.4 × 1013 molecules cm−2 s−1. These observations indicate that the rate-determining step for the removal of oxygen is most likely the initial HH bond breaking, and that the removal of oxygen does not occur preferentially at the edges of NiO islands.