Effect of coverage and temperature on the kinetics of nitrogen desorption from Rh(111) surfaces

Abstract

The kinetics of molecular nitrogen desorption from Rh(111) single-crystal surfaces covered with atomic nitrogen were characterized by both isothermal and temperature-programmed experiments. A complex kinetic behavior was observed for this system not describable by typical Arrhenius expressions even if coverage-dependent activation energies are considered. In fact, the possibility of developing an analytical expression for those desorption rates is voided by the fact that they depend not only on surface temperature and nitrogen coverage but also on the mode of preparation of the system. Isotope labeling experiments support the idea of the formation of surface nitrogen islands and of preferential recombination and desorption at their edges. Coadsorbed oxygen, a typical by-product in NO reduction catalysis, behaves kinetically in a way similar to surface nitrogen as far as N₂ desorption is concerned. The recombination of two surface nitrogen atoms is typically slower than the reaction between one nitrogen and one adsorbed NO molecule, and therefore the catalytic reduction of NO most likely occurs via the latter pathway.