Modulating active oxygen species on α-MnO₂ with K and Pb for SCR of NO at low temperatures

Low-temperature (LT) selective catalytic reduction (SCR) of NO_x has been a research hotspot. However, the roles of oxygen vacancies and active oxygen species in the LT SCR process are still unclear. In this study, we categorized surface-active oxygen species into three groups: O_ng (capable of oxidizing NH₃ into nitrogen gas), O_lg (capable of oxidizing NH₃ into laughing gas), and O_no (capable of oxidizing NH₃ into nitric oxide). Accordingly, the influence of Pb and K doping on the active oxygen species of α-MnO₂ and its LT SCR performance were investigated. The results demonstrate that K doping reduces the generation of O_no while enhancing the production of O_ng and O_lg. In contrast, Pb doping inhibits the formation of O_ng species and decreases the temperature threshold for O_no species generation. Thus, K doping facilitated the SCR performance, while Pb doping improved the NH₃ oxidation and reduced the NO_x conversion. DFT calculations revealed that K incorporated in the tunnels of MnO₂ facilitated NH₃ adsorption and reduced the energy barrier for activation, while Pb doping inhibited NH₃ adsorption and increased the energy barrier for NH₃ activation. The findings provide valuable insight into the roles of active oxygen species and oxygen vacancies in the low-temperature SCR process.