The CO-NO and CO-O-2 reactions studied over Pt-Rh single crystal surfaces under steady state conditions

The CO-NO reaction was studied in the 10(-7) mbar range as a function of temperature and CO/NO ratio on the (111), (100), (410) and (210) surfaces of a Pt-0.25-Rh-0.75 single crystal. At low temperature (T < 500k) the reaction rate increases in the order (111)>(100)>(410)>(210). The results are compared with those obtained for the CO-O-2 reaction over the same surfaces and the effects of the surfaces composition and structure on both reactions are discussed. This comparison shows that the dissociation of NO is not the rate determining step in NO reduction by CO at low temperature. However, at T > 500K desorption of NO from Pt-Rh(111) becomes competitive with the dissociation of NO. As a result, the CO-NO reaction rate decreases much more strongly with increasing temperature over Pt-Rh(111) at T > 500K than rate of the CO-O-2 reaction.