The reaction mechanism for the SCR process on monomer V5+ sites and the effect of modified Brønsted acidity

Abstract

The energetics, structures and activity of a monomeric VO₃H/TiO₂(001) catalyst are investigated for the selective catalytic reduction (SCR) reaction by the use of density functional theory (DFT). Furthermore we study the influences of a dopant substitute in the TiO₂ support and its effects on the known properties of the SCR system such as Brønsted acidity and reducibility of vanadium. We find for the reduction part of the SCR mechanism that it involves two Ti–O–V oxygen sites. One is a hydroxyl possessing Brønsted acidity which contributes to the formation of NH₄⁺, while the other accepts a proton which charge stabilizes the reduced active site. In the reduction the proton is donated to the latter due to a reaction between NH₃ and NO that forms a H₂NNO molecule which decomposes into N₂(g) and H₂O(g). A dopant substitution of 10 different dopants: Si, Ge, Se, Zr, Sn, Te, Hf, V, Mo and W at each of the sites, which participate in the reaction, modifies the energetics and therefore the SCR activity. We find that Brønsted acidity is a descriptor for the SCR activity at low temperatures. Based on this descriptor we find that Zr, Hf and Sn have a positive effect as they decrease the activation energy for the SCR reaction.