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 250°C on Rh catalysts supported on undoped and W6+-doped TiO2, employing transient mass spectroscopy and FTIR techniques. It is found that, under the experimental conditions employed, four kinds of nitrogen oxide species may coexist in the adsorbed mode, namely, Rh–NO−(high), Rh–NO−(low), Rh(NO)2, and Rh–NO+, giving rise to IR bands located at 1770, 1660, 1830/1725, and 1908 cm−1, respectively. Both negatively charged species readily dissociate on reduced surface sites, yielding nitride, and are mainly responsible 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 N2 and N2O requires the presence of reduced surface sites. In the absence of Rh0, dissociative adsorption of NO stops and Rh–NO+ species dominate the catalyst surface. Doping TiO2 with W6+ cations alters the electronic properties of supported Rh crystallites and, concomitantly, the chemisorptive behavior of the catalyst toward NO and CO. In particular, doping results in blue shifts in the 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 adsorbed molecules with the surface. This is also evidenced by the significantly lower amounts of accumulated species, desorbed in TPD experiments. In contrast, 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 N2 production in the gas phase, compared to the undoped catalyst.