OSCILLATORY BEHAVIOR OF THE REDUCTION OF NITRIC-OXIDE BY AMMONIA OVER THE PT(100) SINGLE-CRYSTAL SURFACE - THE ROLE OF OXYGEN, COMPARISON WITH THE NO-H-2 REACTION AND A GENERAL REACTION-MECHANISM FOR NO REDUCTION BY NH3 OVER PT

The reaction between NO and NH3 over a Pt(100) single-crystal surface was studied at low pressures (1.5 x 10(-6)-6 x 10(-5) Torr), at temperatures between 420 and 485 K, and with NO/NH3 ratios ranging from 0.1 to 4.3. It is shown that this reaction shows oscillatory behaviour. Besides single-peaked rate oscillations also various multiple-peaked and aperiodic oscillations were observed. The influence of temperature, NO/NH3 ratio, pressure and the addition of oxygen on the oscillations were assessed. In order to elucidate more details concerning the mechanism of the reduction reaction, labelled compounds ((NH3)-N-15 and (O2)-O-18 were used. Based on the results obtained, a model for the reaction mechanism is proposed which can explain all experimental observations. The model involves the formation of NO islands on the surface. The reaction between NO and ammonia proceeds rapidly at the boundaries between the NO islands, with parts of the crystal where vacancies and NH3 fragments are present. It was found that oxygen plays a minor role at the temperatures and pressures used, but that it is not just a spectator molecule. Addition of oxygen at up to 83% of the reaction mixture does not stop the oscillations observed. A short comparison between the NO-H2 and the NO-NH3 reaction is given. Furthermore, it cannot be ruled out that the reversible (5 x 20) <-> (1 x 1) surface reconstruction, which is known to take place upon NO adsorption on Pt(100), plays a role in the appearance of oscillations, but, as will be shown, this is not necessarily the case. A model for the oscillations is proposed in which surface vacant sites are the key factors determining the oscillating behaviour of the NO reduction by NH3 or H-2.