Doping effect of Sm on the TiO2/CeSmOx catalyst in the NH3-SCR reaction: structure–activity relationship, reaction mechanism and SO2 tolerance

Abstract

A series of environmentally benign TiO₂/CeSmO_x catalysts were prepared by combining coprecipitation and impregnation methods. The effect of doping Sm species into CeO₂ for selective catalytic reduction of NO by NH₃ (NH₃-SCR) was investigated. The results of catalytic tests displayed that the catalyst with the optimal molar ratio (Ce : Sm = 20 : 1) exhibited the best deNO_x performance. Moreover, the NH₃-SCR efficiency of TiO₂/CeSmO_x (Ce : Sm = 20 : 1) was higher than that of TiO₂/CeO₂. The obtained samples were characterized by BET, XRD, Raman, HAADF-STEM, XPS, H₂-TPR, NH₃-TPD and in situ DRIFTS techniques to investigate the effect of Sm doping on the physiochemical properties of the TiO₂/CeSmO_x catalyst. The XRD, Raman and HAADF-STEM results revealed that the Sm dopant had been incorporated into the CeO₂ lattice successfully, and the Ti species were highly dispersed on the CeSmO_x support. The H₂-TPR, NH₃-TPD and in situ DRIFTS results suggested that the redox properties and surface acidity of the catalyst were adjusted to a good balance by the introduction of Sm species, which were beneficial to the enhancement of the catalytic activity at low temperatures (<300 °C). In addition, the NH₃-SCR reaction followed the Langmuir–Hinshelwood mechanism on the TiO₂/CeO₂ catalyst surface in the whole temperature range. The introduction of Sm species changed the reaction pathway, and both the Eley–Rideal and Langmuir–Hinshelwood mechanisms existed when the temperature was higher than 250 °C. Furthermore, the addition of Sm to the TiO₂/CeO₂ catalyst improved the H₂O and SO₂ resistance ability of the catalyst due to suppression of the bulk sulfate formation.