Reaction mechanism of ammonia decomposition on tungsten

Abstract

The mechanism of ammonia decomposition on a clean tungsten surface was studied by direct and simultaneous measurements of the reaction rate and the amounts and behaviour of adsorbed species during the course of the reaction in the temperature range 773–1473 K under an ammonia pressure of 10^–6–10^–3 Pa. The order of the reaction rate with respect to ammonia pressure changed with temperature from first order to fractional at 1473 and 773 K, respectively. No hydrogen was adsorbed on the surface in any form above 973 K and an increase in the hydrogen partial pressure (P_H2) during the reaction had no effect on either the reaction rate or on the amount of surface nitrogen (θ_N). Under higher ammonia pressures, up to 100 Pa, thick surface nitride layers were formed during the decomposition, which decomposed at the same rate in vacuo as in the steady-state NH₃ decomposition, provided the uptake of nitrogen was the same. It was also found that the rate of nitrogen desorption from the surface depended only on θ_N, being faster with increasing θ_N, whereas the rate of nitrogen uptake from ammonia decreased with increasing θ_N and increased with ammonia pressure (P_NH3). On the basis of these data, it was concluded that the overall reaction proceeds through a reaction mechanism of “dynamic balance” between two consecutive ratedetermining steps; the supply and consumption of surface nitrogen.