Time-resolved in-situ IR and DFT study: NH3 adsorption and redox cycle of acid site on vanadium-based catalysts for NO abatement via selective catalytic reduction

Abstract

Transient in-situ IR spectroscopy was performed to analyze the molecular behavior of selective catalytic reduction of NO with NH3 on V2O5-WO3/TiO2 catalyst. During NH3 adsorption, an unexpected growth in the intensity of Lewis acid sites was observed due to the hydrogen migration, which was verified by the DFT calculation and the existence of intermediate species. The comparison between the isolated atmosphere experiments and the simultaneous exposure of NH3-NO-O2 on catalytic monolayer demonstrated that: (i) there exists a pathway for Lewis to Brønsted acid sites transformation at T > 300 °C, (ii) the rate determining step of the SCR cycle involves in the re-oxidation of acid sites by O2 at 200–300 °C. Generally, the Lewis acids are crucial at high temperatures, while Brønsted acids dominate the overall reaction at lower temperatures. Based on the experimental data and the DFT calculation, a mechanism for ammonia adsorption, acidity conversion and re-oxidation for the redox loop of the de-NOx reaction was proposed. The findings shed light on the understanding of relative strength of NO reduction and acid re-oxidation at different temperatures.