Effects of W6+ Doping of TiO2 on the Reactivity of Supported Rh toward NO: Transient FTIR and Mass Spectroscopy Studies

The transient adsorption and dissociation of NO over Rh supported on undoped and W 6+ -doped TiO 2 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/TiO 2 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 TiO 2 support (1610−1240 cm -1 ). On the other hand, in the case of Rh supported on W 6+ -doped TiO 2, two additional infrared bands at 1680 and 1620 cm -1 are produced (not observed over W 6+ -doped TiO 2 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 N 2 formation. The influence of W 6+ -doping of the TiO 2 carrier on the reactivity of Rh crystallites toward NO is discussed considering the fact that doping modifies the electronic structure of the TiO 2 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 W 6+ -doped TiO 2, as compared to the undoped TiO 2 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.