NO DISSOCIATION AND NO- INFLUENCE OF THE SURFACE-STRUCTURE AND COMPOSITION ON THE REACTION-RATES(CO REACTION ON RH(110) )

The initial variations in the rates of NO dissociation and CO + NO rea ction on Rh(110) and their relation to the changes in the surface stru cture and composition have been studied by means of mass spectrometry, thermal-programmed desorption (TPD) and low-energy electron diffracti on (LEED). The changes in the partial pressures of the reaction produc ts, N2 and CO2, have been used as a measure of the reaction rates for three catalyst temperatures: 460, 510 and 670 K. The composition of th e surface layer at various stages of the reaction was determined from the TPD spectra of N2 and O2. Various LEED patterns were observed in t he course of the reactions which pass through several stages depending on the reaction temperature and CO/NO partial pressure ratios. In the case of NO dissociation the variations in the rate of N2 production a t 460 and 510 K correlate well with structural changes. The LEED shows N-related 3 x 1 and 2 x 1 structures in the early stages of the react ion and an oxygen-induced 1 x n reconstruction of the surface in the l ater stages. The latter leads to a successive formation of structures, from p(2 x 2) and c(2 x 4), related to mixed O + N layers, to (2 x 2) pg and c(2 x 6) related to oxygen alone. At 460 and 5 1 0 K, when the steady state is reached, the catalyst surface contains oxygen in a c(2 x 6) structure with some nitrogen possibly buried subsurface. Similar stages in the rate of N2 production and related surface structures we re observed during the CO + NO reaction. Because of the ''cleaning'' e ffect of CO at steady state the catalyst surface contains oxygen and n itrogen in a c(2 x 4) structure. At 670 K the variations in the reacti on rates and surface structures are governed only by oxygen. The obser ved anti-phase variations in the rates of N2 and CO2 production, when the CO + NO reaction is carried out at 460 and 510 K, are correlated t o the changes in the structure and composition of the catalyst surface . Models for some of the structures observed in the course of the reac tions are suggested.