Catalytic reduction of NO by CO over rhodium catalysts 1. Adsorption and displacement characteristics investigated by in situ FTIR and transient-MS techniques

Adsorption of NO(CO) and displacement by CO(NO) has been investigated at 25 0 degrees C on Rh catalysts supported on undoped and W6+-doped TiO2, employ ing transient mass spectroscopy and FTIR techniques. It is found that, unde r the experimental conditions employed, four kinds of nitrogen oxide specie s may coexist in the adsorbed mode, namely, Rh-NO-(high), Rh-NO-(low), Rh(N O)(2), and Rh-NO+, giving rise to LR bands located at 1770, 1660, 1830/1725 , and 1908 cm(-1), respectively. Both negatively charged species readily di ssociate on reduced surface sites, yielding nitride, and are mainly respons ible for dinitrogen formation in the gas phase. The dinitrosyl species, the formation of which is favored over partially oxidized surfaces, is related to the production of nitrous oxide. The formation of both N-2 and N2O requ ires the presence of reduced surface sites. In the absence of Rh-0, dissoci ative adsorption of NO stops and Rh-NO+ species dominate the catalyst surfa ce. Doping TiO2 with W6+ cations alters the electronic properties of suppor ted Rh crystallites and, concomitantly, the chemisorptive behavior of the c atalyst toward NO and CO. In particular, doping results in blue shifts in t he stretching frequencies of N-O and C-O bonds contained in Rh-NO+, Rh(NO)( 2), Rh-CO, and Rh(CO)(2) species, indicating a weaker bonding of the adsorb ed molecules with the surface. This is also evidenced by the significantly lower amounts of accumulated species, desorbed in TPD experiments. In contr ast, the N-O bond of the Rh-NO- species is weakened by doping, resulting in higher rates of dissociation and, therefore, in higher transient yields of N-2 production in the gas phase, compared to the undoped catalyst. (C) 200 0 Academic Press.