Kinetics of decomposition of ammonia at low pressures on metal surfaces

Abstract

The catalytic decomposition of ammonia at 10^–5-10^–4 torr and in an ultra-high vacuum was studied on nickel, rhodium, tantalum and tungsten wires. The promoting and poisoning effect of carbon when present in wires in limited and excess amounts, respectively, generally confirmed previously reported observations for platinum. A decay of activity occurred after flashing a wire and was attributed to a depletion of carbon in the surface region of the wire as a result of reaction with ammonia (or with a surface nitride). With rhodium, ammonia was observed mass-spectrometrically to react with a wire poisoned by excess carbon to give hydrogen cyanide. Absolute rate theory calculations indicate that at low temperatures where the activity of a rhodium wire decreases rapidly with decreasing temperature, the rate-limiting step in ammonia decomposition is the desorption of nitrogen from the catalyst surface. Such kinetic behaviour is inconsistent with decomposition rates observed at temperatures in excess of about 650°K. The activation energies on tantalum (14 kcal mole^–1) and tungsten (25 kcal mole^–1) are thought to contain terms for migration of the adsorbed ammonia molecules, formation of active sites and evaporation from the surface.