Decomposition of ammonia on rhodium crystals

Abstract

The rates of ammonia decomposition of (110), (100), and (111) single-crystal faces of rhodium were measured over the temperature range 580–725 K for ammonia pressures ranging from 1 × 10 −3 to 500 × 10 −3 Torr and hydrogen and nitrogen pressures varying from 1 × 10 −3 to 150 × 10 −3 Torr and 1 × 10 −3 to 250 × 10 −3 Torr, respectively. The decomposition rates were proportional to P NH 3 1 2 and P NH 3 at low and high hydrogen pressure, respectively. The H 2 kinetic order varied from 0 (low P H 2 ) to −1.0 (high p H 2 ). The rate was independent of nitrogen pressure. Isotope studies indicated that NH 3 decomposes about 1.5 times faster than ND 3 on the (110) and (111) faces. A large face specificity is evident with the rates on the (110) surface over 10 times as great as those on the (111) surface. LEED, AES, and flash desorption experiments indicated that boron (B) was a significant surface “poison” and that the Rh (110) surface under reaction conditions might contain at most moderate coverages of nitrogen adatoms, but that coverages of other species were very small. The LEED/AES characterizations were done in a different system from that used in the kinetic and thermal desorption experiments. Rate expressions consistent with the observed kinetics are derived from a model involving surface species Rh 2NH, RhH, and RhN, with only the RhN coverage being appreciable under the experimental conditions used. The kinetic data for the Rh (110) face are semiquantitatively fit to these rate expressions via a superposition technique and also to an empirical rate expression which functionally is similar to the mechanism rate equation. A decreasing NH 3 order ( < 1 2 ) at high P NH 3 and low T is attributed to the buildup of surface nitrogen. Kinetic orders and activation energies obtained are generally consfistent with most other literature data.