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 wa s probed both by temperature-programmed desorption (TPD) experiments a nd by isothermal measurements using an extension of the so-called King and Wells collimated beam method. The TPD studies corroborated previo usly reported results, including the existence of two distinct N-2 des orption peaks, the first of which displays apparent first-order kineti cs. The isothermal work proved that the adsorption of NO is precursor mediated at low temperatures, and that it is not affected significantl y by the presence of coadsorbed nitrogen and/or oxygen atoms at any te mperature below 900 K. The rate of molecular nitrogen production was f ound to be significant above 450 K and to be controlled by the recombi nation 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. S uch an observation is interpreted here as being the result of the slow diffusion of nitrogen atoms across the surface prior to their recombi nation. A strong additional effect due to lateral repulsion between ni trogen and/or oxygen atoms was also inferred from the data. (C) 1998 A cademic Press.