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

Abstract

Gadolinium (Gd) has been used as a dopant to modify MnOx for enhanced catalytic performance and sulfur resistance in the application of the selective catalytic reduction (SCR) of NOx with NH3 for the first time. The results show that the introduction of Gd with proper amount can effectively restrain the crystallization of MnOx, 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 MnOx 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 NOx 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 MnOx 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-NOx mechanism and the SO2 resistance mechanism of MnOx catalysts.