HCl-doping of V/TiO2-based catalysts reveals the promotion of NH3-SCR and the rate limiting role of NO oxidative activation

NH3-SCR is the best available technology for NOx abatement from power plant, incinerators, gasifiers, and a well-established technology for on board NOx emission control; in the last 40 years, the reaction mechanism and kinetics have been deeply investigated, but the understanding of the nature of surface intermediates and rate determining steps is still object of a lively discussion. A major factor, though, has been largely neglected in the scientific literature, that is the presence of HCl in the flue gases generated by coal, biomass and plastics. In this work, the effect of HCl on NH3-SCR over V2O5 catalysts was investigated through combined kinetic analysis and operando FTIR. Unexpectedly, after HCl-doping, the catalyst intrinsic activity of a commercial catalyst increased by a factor of 5. Besides, it was found that while the NH3 surface coverage is little affected by HCl-doping, the chemisorption capacity of NO + O2 undergoes a remarkable promotion that scales with the promotion of the reaction rate. The HCl-effect provided thus an indirect but unambiguous piece of evidence that NO-activation by an O-species is a key rate controlling step of NH3-SCR