Nitrogen adsorption and ammonia decomposition on polycrystalline molybdenum

The equilibrium adsorption of dinitrogen and the decomposition rate of ammonia were studied on a polycrystalline molybdenum foil, with constant monitoring of surface cleanliness and determination of nitrogen surface coverage by Auger Electron Spectroscopy. The mechanism of the reaction on molybdenum is identical to that on tungsten. On both metals the decomposition proceeds through two irreversible steps, the adsorption of NH3 and the desorption of N2 on a surface with adsorbed N as most abundant reaction intermediate. Isosteric heats of adsorption of dinitrogen vary strongly with coverage indicating a thermodynamically broadly non-uniform surface. The rate data were also well described by a non-uniform surface kinetic treatment. The dependence of the adsorption heat of N2 and the activation energy for desorption of N2 on nitrogen coverage θ at values of θ less than 1/2 is too strong to be due to lateral interactions and indicates a strongly heterogeneous surface.