Gd-modified MnO_x for the selective catalytic reduction of NO by NH₃: The promoting effect of Gd on the catalytic performance and sulfur resistance

Gadolinium (Gd) has been used as a dopant to modify MnO_x for enhanced catalytic performance and sulfur resistance in the application of the selective catalytic reduction (SCR) of NO_x with NH₃ for the first time. The results show that the introduction of Gd with proper amount can effectively restrain the crystallization of MnO_x, enhance the specific surface area, increase the concentrations of surface Mn⁴⁺ and chemisorbed oxygen species, and enhance the amount and the strength of surface acid sites. The MnGdO-2 catalyst (Gd-modified MnO_x with the mole ratio of Gd/Mn = 0.1) exhibits optimal catalytic performance among all prepared catalysts with a 100% NO conversion performance in a wide temperature window from 120 to 330 °C and a 100% N₂ selectivity from 150 to 300 °C under a high space velocity of 36,000 h⁻¹. In-situ DRIFT spectra reveal that the Gd doping can promote the NH₃ adsorption on the catalyst pre-adsorbed with NO_x species, facilitating the reactive NH₄ ⁺ species taking part in the SCR reaction. More importantly, MnGdO-2 catalyst presents stronger resistance to water vapor or sulfur poisoning in comparison with pure MnO_x catalyst, which can be ascribed to these facts that Gd-modification restrains the transformation of MnO₂ to Mn₂O₃ and the generation of MnSO₄, impedes the decrease in Lewis acid sites and the increase in Brønsted acid sites, and alleviates the competitive adsorption between the NO and SO₂. This work may provide new insights into the effects of rare earth modification on the de-NO_x mechanism and the SO₂ resistance mechanism of MnO_x catalysts.