Understanding the deposition and reaction mechanism of ammonium bisulfate on a vanadia SCR catalyst: A combined DFT and experimental study

Abstract

The deactivation of NH3-selective catalytic reduction (SCR) catalysts due to NH4HSO4 deposition at low temperatures (< 300 °C) is still a significant challenge. In this work, we present a comprehensive mechanism describing the formation, deposition, and reaction of NH4HSO4 on a V2O5/TiO2 catalyst using a combination of theoretical and experimental methods. The results show that NH4HSO4 is mainly formed in the gas phase through the nucleation of SO3, H2O, and NH3 and then deposits onto the catalyst surface. The decomposition of NH4HSO4 on the surface of the V2O5/TiO2 catalyst consists of two steps: NO is reduced by the NH4+ of NH4HSO4 forming N2 and H2O by transferring an electron to the adjacent vanadium site, followed by a reoxidation of the reduced vanadium site by either O2 or NO2. At low temperatures, due to the weak reoxidizing ability of O2, the reaction of NH4HSO4 with NO in the NO/O2 mixture is rather slow. Adding NO2 can remarkably enhance the decomposition of NH4HSO4 on the catalyst surface. Our results reveal that the rate-determining step of the reaction between NH4HSO4 and NO/O2 is the reoxidation of the reduced vanadium site and that NO2 is a better reoxidizing agent than O2, which has been confirmed by X-ray photoelectron spectroscopy analysis and the designed transient response method experiments. Finally, the catalyst sulfur tolerance test has proven that the commercial V2O5-WO3/TiO2 catalyst can successfully maintain its long-term activity for NOx reduction in SO2-contained flue gas at 250 °C due to the rapid decomposition of deposited NH4HSO4 on the catalyst surface by the NO/NO2 mixture.