Effects of W6+ doping of TiO2 on the reactivity of supported Rh toward NO: Transient FTIR and mass spectroscopy studies

Abstract

The transient adsorption and dissociation of NO over Rh supported on undoped and W6+-doped TiO2 in the temperature range of 25-450°C has been studied by FTIR and mass spectroscopy. It was found that at 25 °C three kinds of adsorbed NO species are formed on the Rh surface of Rh/TiO2 catalyst, namely Rh-NO+ (1920 cm-1), Rh-NO (1840 cm-1), and Rh-NO- (1750 cm-1), while several adsorbed nitrate species are formed on the TiO2 support (1610-1240 cm-1). On the other hand, in the case of Rh supported on W6+-doped TiO2, two additional infrared bands at 1680 and 1620 cm-1 are produced (not observed over W6+-doped TiO2 alone). The infrared band at 1680 cm-1 is suggested to be due to a Rh-NO- species adsorbed on a different site than that corresponding to 1750 cm-1, while that at 1620 cm-1 is due to a nitrate species associated with an oxygen adatom on the Rh surface and a surface oxygen atom of the support. In the temperature range of 100-450°C, the effect of carrier doping is to largely increase the extent of transient NO decomposition and selectivity towards N2 formation. The influence of W6+-doping of the TiO2 carrier on the reactivity of Rh crystallites toward NO is discussed considering the fact that doping modifies the electronic structure of the TiO2 carrier, which in turn modifies the electronic state of surface Rh atoms via electronic interactions at the metal-support interface. Furthermore, because of the significant enhancement of the concentration of oxygen vacancies in the W6+-doped TiO2, as compared to the undoped TiO2 surface, oxygen adatoms formed by dissociation of NO on the Rh surface can migrate onto these vacancies at high temperatures. As a result, higher NO dissociation rates with prolonged reaction periods are obtained.