Transient kinetics during the isothermal reduction of NO by CO on Rh(111) as studied with effusive collimated molecular beams

Abstract

The transient kinetics of the reaction between NO and CO on clean Rh(111) surfaces have been studied using molecular beams in conjunction with mass spectrometry detection. The changes in the partial pressures of the reactants (CO and NO) and products (N₂ and CO₂) as a function of time have been used as a measure of the evolution of the uptake and desorption rates, respectively, for temperatures between 350 and 1000 K and for NO:CO mixture ratios between 4:1 and 1:99. Post-mortem temperature programmed desorption (TPD) and CO titration experiments were also performed in order to estimate the surface coverages of atomic nitrogen and oxygen left on the Rh(111) surface by the gas mixture. Systematic variations were observed during the transition from the clean surface to the steady-state catalytic regime that correlate well with the overall reaction rates in the latter. Specifically, there is a time delay in the production of molecular nitrogen because of the need to build up a threshold atomic nitrogen coverage on the surface before the start of the desorption of N₂. This atomic nitrogen coverage, as calculated by the time delay in the transient, corresponds to that estimated by TPD after the reaction, and displays a dependence on the NO:CO ratio in the reaction mixture, increasing at a given temperature as the beam becomes richer in CO. Initial sticking coefficients were also determined for both NO and CO in NO + CO mixtures as a function of surface temperature and beam composition.