Decomposition of ammonia on a tungsten surface. Effects of carbon and oxygen

Abstract

In this work the effects of carbon and oxygen on the decomposition of ammonia on a tungsten surface have been studied. The composition of the surface was examined by Auger electron spectroscopy (A.e.s.) and the surface structure by low-energy electron diffraction (l.e.e.d.). The rate of the reaction, the extent of nitrogen adsorption during the reaction, and the extent to which molecular nitrogen is adsorbed, all decreased linearly with increasing amount of carbon on the surface, finally approaching zero at full coverage. The carbon on the surface thus decreased the number of effective active sites for ammonia decomposition and nitrogen adsorption on the surface. On the other hand, as oxygen coverage increased, the decomposition rate increased initially and then decreased. This is interpreted as an activation of the N₂ desorption rate by the presence of oxygen on the surface. From the l.e.e.d. studies, a C(2 × 2) structure under reaction conditions was observed on the clean tungsten (100) surface. A superposition structure of C(2 × 2)–N and (4 × 1)–O or C(2 × 2)–N and (2 × 1)–O was obtained in the steady state of the reaction on the oxygen preadsorbed surface.