Field-Emission Study of the Adsorption and Decomposition of Ammonia on Tungsten

The catalytic decomposition of ammonia on tungsten has been investigated by field electron-emission microscopy. Physisorbed ammonia causes a large lowering of work function (∼ − 2.8 eV). Ammonia chemisorbs at low temperatures by the formation of a coordinate bond and the heat of adsorption and activation energy for decomposition appear to be highest on planes of highest clean work function. Decomposition to nitrogen and hydrogen occurs in the interval 200°–400°K. Interaction of ammonia with a clean tungsten surface at higher temperatures causes the successive appearance of three species, presently interpreted as N, NNH2, and NNH3+. The interaction of ammonia with nitrogen adatoms to produce the additional species will proceed at lower temperatures and therefore requires little activation energy. The rate-limiting step in the decomposition of ammonia on tungsten is the decomposition on the surface of NNH2. The proposed scheme for decomposition satisfies the existing kinetic data for tungsten. The implication of these results on the structure of nitrogen adatoms is discussed.