Isothermal kinetic study of the decomposition of nitric oxide over Rh(111) surfaces

The kinetics of the thermal decomposition of NO on Rh(lll) surfaces was probed both by temperature-programmed desorption (TPD) experiments and by isothermal measurements using an extension of the so-called King and Wells collimated beam method. The TPD studies corroborated previously reported results, including the existence of two distinct N-2 desorption peaks, the first of which displays apparent first-order kinetics. The isothermal work proved that the adsorption of NO is precursor mediated at low temperatures, and that it is not affected significantly by the presence of coadsorbed nitrogen and/or oxygen atoms at any temperature below 900 K. The rate of molecular nitrogen production was found to be significant above 450 K and to be controlled by the recombination of atomic nitrogen below 600 K, but the experimental data could not be reproduced in a satisfactory manner by any empirical rate law unless the order in nitrogen coverage was set to be less than unity. Such an observation is interpreted here as being the result of the slow diffusion of nitrogen atoms across the surface prior to their recombination. A strong additional effect due to lateral repulsion between nitrogen and/or oxygen atoms was also inferred from the data. (C) 1998 Academic Press.