The effects of H2O on a vanadium-based catalyst for NH3-SCR at low temperatures: a quantitative study of the reaction pathway and active sites

Abstract

The influences of H₂O on the adsorption amounts of NO, NO₂, and NH₃ on V₂O₅/WO₃–TiO₂ and the reaction pathway at low temperatures for selective catalytic reduction of nitrogen oxides by NH₃ (NH₃-SCR) were quantitatively obtained using the temperature programmed surface reaction as well as the transient response method (TRM). The active site distribution on V₂O₅/WO₃–TiO₂ was also clarified quantitatively. NO_x adsorption took place on [V⁴⁺]–OH + [V⁵⁺]–O, [V⁵⁺]–OH, and [Ti⁴⁺]₂–O, the surface concentrations of which were 24, 28 and 26 μmol g⁻¹, respectively, whereas NH₃ was activated on [V⁴⁺]–OH + [V⁵⁺]–O, [V⁵⁺]–OH, [Ti⁴⁺]₂–O and W species, with concentrations of 24, 28, 26 and 274 μmol g⁻¹, respectively. The V and W species did not distribute as a monolayer. The “nitrite path”, referring to the reaction between adsorbed NH₃ and nitrite species to produce N₂ and water (H₂O), contributed to standard and fast SCR mechanisms, but the “NH₄NO₃ path”, referring to the reaction between the NO gas and NH₄NO₃ formed from the surface nitrates and adsorbed NH₃ species to emit NO₂, H₂O and N₂, did not occur at low temperatures without H₂O. Both the “NH₄NO₃ path” and the “nitrite path” contributed to NH₃-SCR with H₂O in the gas feed. The presence of H₂O increased the amounts of the adsorbed nitrates and ammonia at 150 °C, but hindered the reaction between the adsorbed NH₃ and NO gas, lowering the SCR activity.