Field promoted and surface catalyzed formation of H 3 and NH 3 on transition metal surfaces: A pulsed-laser imaging atom-probe study

Abstract

The surface field must play an important role in the reactivity of a surface. This effect can be probed in field ion emission experiments where the applied field is an adjustable parameter and it is of the same order of magnitude as the surface field. We report here a study of field promoted and surface catalyzed formation of H 3 and NH 3 on 20 transition metals, using the pulsed-laser imaging atom-probe. We find that in an applied field of about 2.0 V/Å, H 3 can be formed most readily on surfaces of hep metals, and metals on the left hand side of the periodic table. Atomic chemisorption of hydrogen favors, but does not guarantee, the formation of H 3. In N 2H 2 mixed gases, a small amount of NH 3 can be formed in an applied field of 1.2 to 2.5 V/Å on surfaces of metals which are known to promote such a reaction in field free conditions. However, only on Fe and Co surfaces, a substantial amount of NH 3 can be formed. From the dependence of the relative abundances of the various observed ionic species on the composition of the mixed gas and on the gas pressure, we conclude that NH 3 is formed from the chemisorbed nitrogen and hydrogen atoms, whereas the pulsed-laser field desorbed NH 3 + originates from the field adsorbed NH 3, or the physisorbed NH 3.