The Role of Alkali Metal in α-MnO2 Catalyzed Ammonia-Selective Catalysis.

The unexpected phenomenon and mechanism of the alkali metal involved NH3 selective catalysis are reported. Incorporation of K+ (4.22 wt %) in the tunnels of α-MnO2 greatly improved its activity at low temperature (50-200 °C, 100 % conversion of NOx vs. 50.6 % conversion over pristine α-MnO2 at 150 °C). Experiment and theory demonstrated the atomic role of incorporated K+ in α-MnO2 . Results showed that K+ in the tunnels could form a stable coordination with eight nearby O sp 3 atoms. The columbic interaction between the trapped K+ and O atoms can rearrange the charge population of nearby Mn and O atoms, thus making the topmost five-coordinated unsaturated Mn cations (Mn5c , the Lewis acid sites) more positive. Therefore, the more positively charged Mn5c can better chemically adsorb and activate the NH3 molecules compared with its pristine counterpart, which is crucial for subsequent reactions.