Deep insights into the promotion role of Sm doping in the sulfur resistance of Fe2O3 catalyst for NH3-SCR: A combined experimental and DFT study

To investigate the inherent role of Sm doping in metal sulfation over the surface of Fe2O3 catalyst and reveal the reason for the SO2 tolerance enhancement of Sm/Fe2O3 catalyst at low temperatures (<320 °C), sulfated Fe2O3 and Sm/Fe2O3 catalysts were prepared by pretreating fresh catalysts in SO2+O2 atmosphere. Selective catalytic reduction (SCR) of NOx by NH3 activities and physicochemical characteristics of fresh and sulfated catalysts were studied, and the reaction mechanisms on sulfated catalysts analyzed. The minor shift of Fe signal to high value and larger shift of S signal to low value of sulfated Sm/Fe2O3 compared with sulfated Fe2O3 was observed with X-ray photoelectron spectroscopy (XPS), indicating that Sm modification could weaken the sulfation of active Fe sites. Consistent with the XPS results, density functional theory calculations showed that SO2 is more easily adsorbed on the Sm/Fe2O3 catalyst with the adsorption sites located at Sm atom and its neighboring Fe atoms. In-situ DRIFTS results showed that the SCR reaction over the sulfated Fe2O3 catalyst followed both L-H (the adsorbed NH3 species react with adsorbed nitrate species) and E-R (the adsorbed NH3 species react with gaseous NO) mechanisms at 250 °C, with the L-H route much faster, whereas only the L-H pathway occurred over sulfated Sm/Fe2O3 catalyst because of too strong affinity of NH3 on the sulfated sites to inhibit NH3 activation – this was also the reason for its enhanced Lewis and Brønsted acidity. More reactive nitrate species were formed on the sulfated Sm/Fe2O3 catalyst due to the existence of more un-sulfated Fe sites, making the Sm/Fe2O3 catalyst resistant to SO2 poisoning at relatively low temperatures.