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

Low-temperature (LT) selective catalytic reduction (SCR) of NOx 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 alpha-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 NOx 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.