Promotion effect of niobium on ceria catalyst for selective catalytic reduction of NO with NH3

Abstract

The CeO2, Ce–Nb–Ox and Nb2O5 catalysts were synthesized by citric acid method and the promotion effect of Nb on ceria for selective catalytic reduction (SCR) of NO with NH3 was investigated. The catalytic activity measurements indicate that the mixed oxide Ce–Nb–Ox presents a higher SCR activity than the single oxide CeO2 or Nb2O5 catalyst. In addition, the Ce–Nb–Ox catalyst shows high resistance towards H2O and SO2 at 280 °C. The Raman, X-ray photoelectron spectra and temperature programmed reduction with H2 results indicate that the incorporation of Nb provides abundant oxygen vacancies for capturing more surface adsorbed oxygen, which provides a superior redox capability and accelerates the renewal of active sites. Furthermore, the Fourier transform infrared spectra and temperature programmed desorption of NH3 results suggest that niobium pentoxide shows high surface acidity, which is partly retained in the Ce–Nb–Ox catalyst possessing a high content of Lewis and Brønsted acid sites. Therefore, the incorporation of Nb improves both the redox and acidic capacities of Ce–Nb–Ox catalyst for the SCR reaction. Here, the redox behavior is primarily taken on Ce and the acidity is well improved by Nb, so the synergistic effect should exist between Ce and Nb. In terms of the reaction mechanism, in situ DRIFT experiments suggest that both NH3 on Lewis acid sites and NH4+ on Brønsted acid sites can react with NO species, and adsorbed NO and NO2 species can both be reduced by NH3. In the SCR process, O2 primarily acts as the accelerant to improve the redox and acid cycles and plays an important role. This work proves that the combination of redox and acidic properties of different constituents can be feasible for catalyst design to obtain a superior SCR performance.