Kinetics of NH3 decomposition over well dispersed Ru

Turnover frequencies for NH3 decomposition were an order of magnitude lower on small carbon-supported Ru crystallites, compared to similarly well-dispersed Ru on alumina, while activation energies did not vary much (21-24 kcal/mole). The lower initial activity for the Ru/C catalyst and a continual increase in activity as exposure time to H2 increased is very likely due to the removal of Cl ions, as indicated by XPS measurements. A near first-order dependence on NH3 and a dependence on H2 that fell between negative 3/2 and negative second-order was observed with small (1-2 nm) Ru crystallites on carbon. To overcome the inability of previous models for NH3 decomposition on metal surfaces to account for a strong H2 inhibition, as well as a near first-order NH3 dependence, a kinetic model is presented that proposes that both NH2-H bond cleavage and recombinative nitrogen desorption are slow kinetic steps, the NH2-H bond cleavage step is reversible, and nitrogen atoms are the most abundant reaction intermediate. This model is qualitatively consistent with other results in the literature and provides an excellent statistical fit to experimental kinetic data obtained at two temperatures under our reaction conditions employing much higher H2 and NH3 pressures. Copyright © 1997 by Academic Press. All rights of reproduction in any form reserved.