ADSORPTION AND REACTION OF CO, NO AND MIXTURES OF CO/NO ON RH(110)

The reactions of CO, NO and mixtures of CO/NO with a Rh{110} single-crystal surface have been studied by temperature programmed desorption (TPD), work-function measurements (DELTA-phi) and low-energy electron diffraction (LEED). Adsorption of CO occurs only molecularly. By contrast, exposures of up to 0.8 L of NO undergo complete dissociation. This is evidenced by the evolution of only N2 and O2 in TPD spectra. The reaction between NO and CO is found to proceed steadily only at temperatures above 670 K, i.e., after completion of N2 thermal desorption. Under isothermal conditions, but only above 670 K and under a constant flow of the reactants (flow ratio CO/NO approximately 10:1), a hysteresis-like behaviour in the CO2 production may be observed. Rate oscillations have been found at unusually high temperatures, i.e., above 900 K. Periods in the oscillating CO2 production are in the range of 5-20 min and coincide with electronic change at the surface as indicated by work-function measurements. The feedback process probably involves different states of oxygen which is dissociatively chemisorbed at the surface or absorbed beneath. These states have been characterized by TPD spectra after high exposures of pure O2 or NO to Rh{110}.