The role of the Cu dopant on a Mn3O4 spinel SCR catalyst: Improvement of low-temperature activity and sulfur resistance

Mn-based oxides are regarded as one of the most promising catalysts for selective catalytic reduction (SCR) of NOx by NH3 at low temperatures, but their applications are extremely restricted by the irreversible poisoning of SO2. Improving the SO2 tolerance of Mn-based catalyst has longtime received the most attentions from both academia and industry. In this work, a series of Cu-modified Mn3O4 spinels were synthesized, and the roles of the Cu dopant were investigated. The (Cu1.0Mn2.0)1–δO4 spinel showed both excellent SCR performance and SO2 resistance at low temperature. Cu doping improved the BET surface area, the quantities of active Mn4+ and the acid sites of Mn3O4 spinels, all of which contributed to the increase in low-temperature SCR activity. The formation of MnSO4 was mainly responsible for the irreversible deactivation of the Mn3O4 spinel upon exposure to SO2. DFT calculations suggested that SO2 was more likely to be adsorbed as “–Mn–O–S–O–Mn–” on Mn3O4 and (Cu1.0Mn2.0)1–δO4 spinels. Therefore, the formation of MnSO4 on the (Cu1.0Mn2.0)1–δO4 spinel was significantly mitigated by Cu doping, mainly due to reduced amounts of adjacent Mn. Moreover, resulting from the electronic transfer between copper and manganese cations within the spinel lattice (Cu2+ + Mn3+ ⇄ Mn4++ Cu+), the (Cu1.0Mn2.0)1–δO4 spinel retained a high surface ratio of Mn4+/Mntotal, which maintained an excellent low-temperature SCR activity under the SO2-containing condition. This work shows that doping with the low–valence dopant of Cu can significantly improve the low-temperature SCR activity and SO2 tolerance of the Mn3O4 spinel, which could be a strategy for the further design of Mn-based SCR catalysts.