Catalytic reduction of NO by CO over rhodium catalysts 2. Effect of oxygenon the nature, population, and reactivity of surface species formed under reaction conditions

The effect of oxygen on the nature, population, and reactivity of surface s pecies formed during reduction of NO by CO over Rh/TiO2 catalysts has been examined employing FTIR and transient RIS techniques. It has been found tha t the activity of Rh is hindered by accumulation of surface oxygen originat ing from NO decomposition and gas-phase oxygen in the feed. Adsorbed CO and reduced TiO2-x species in the vicinity of Rh particles act as oxygen atom scavengers and, under fuel-rich conditions, remove atomic oxygen from the s urface and restore the catalytic properties. Results of the present study p rovide additional evidence that production of N-2 is related to dissociatio n of adsorbed Rh-NO- while production of N2O is related to the presence of Rh(NO)(2). The presence of reduced Rh-0 sites is necessary for the formatio n of both reduction products. In the absence of oxygen in the feed, surface isocyanate species are also observed under reaction conditions. Their form ation requires the presence of adjacent Rh-0-CO and reduced Rh-0 sites. Alt hough these species are favored under conditions in which NO conversion to reduction products is observed, there is no evidence that they are catalyti cally active species. (C) Academic Press.