In Situ DRIFTS Studies of NH ₃-SCR Mechanism over V ₂O ₅-CeO ₂/TiO ₂-ZrO ₂ Catalysts for Selective Catalytic Reduction of NO _x

Abstract

TiO ₂-ZrO ₂ (Ti-Zr) carrier was prepared by a co-precipitation method and 1 wt. % V ₂O ₅ and 0.2 CeO ₂ (the Mole ratio of Ce to Ti-Zr) was impregnated to obtain the V ₂O ₅-CeO ₂/TiO ₂-ZrO ₂ catalyst for the selective catalytic reduction of NO _x by NH ₃. The transient activity tests and the in situ DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) analyses were employed to explore the NH ₃-SCR (selective catalytic reduction) mechanism systematically, and by designing various conditions of single or mixing feeding gas and pre-treatment ways, a possible pathway of NO _x reduction was proposed. It was found that NH ₃ exhibited a competitive advantage over NO in its adsorption on the catalyst surface, and could form an active intermediate substance of -NH ₂. More acid sites and intermediate reaction species (-NH ₂), at lower temperatures, significantly promoted the SCR activity of the V ₂O ₅-0.2CeO ₂/TiO ₂-ZrO ₂ catalyst. The presence of O ₂ could promote the conversion of NO to NO ₂, while NO ₂ was easier to reduce. The co-existence of NH ₃ and O ₂ resulted in the NH ₃ adsorption strength being lower, as compared to tests without O ₂, since O ₂ could occupy a part of the active site. Due to CeO _2’s excellent oxygen storage-release capacity, NH ₃ adsorption was weakened, in comparison to the 1 wt. % V ₂O ₅-0.2CeO ₂/TiO ₂-ZrO ₂ catalyst. If NO _x were to be pre-adsorbed in the catalyst, the formation of nitrate and nitro species would be difficult to desorb, which would greatly hinder the SCR reaction. All the findings concluded that NH ₃-SCR worked mainly through the Eley-Rideal (E-R) mechanism.