Kinetics of ammonia decomposition on nickel

Two kinetic regimes were observed in the decomposition of NH3 over nickel wires at temperatures between 750 and 1350K and NH3 pressures between 5.3 and 133 Pa. At temperatures below 1000K, the decomposition is highly activated (211 ± 20 kJ mol−1), and the rate is independent of NH3 pressure. Above 1000K a transition to first-order kinetics occurs as the apparent activation energy decreases with increasing temperature. The data were closely predicted by a rate expression which was derived from an analysis of the elementary reaction steps involved in NH3 decomposition. The analysis of the data indicates that the desorption of strongly adsorbed nitrogen is the rate-controlling step in the zero-order regime, and the adsorption of NH3 is the rate-controlling step in the first-order regime.