Synergy of the successive modification of cryptomelane MnO2 by potassium insertion and nitrogen doping for catalytic formaldehyde oxidation

Defect engineering is a powerful way to achieve the structural manipulation of MnO2 with rich surface active oxygen species for formaldehyde elimination under ambient conditions. Herein, a two-step protocol for consecutive modification of alpha-MnO2 structure by potassium insertion and ammonium treatment was successfully implemented, leading to a synergetic enhancement of formaldehyde oxidation efficiency. For K modification alone, only slight activity increase was realized at moderate K level and an even higher K addition jeopardized the catalytic performance. Neither did NH4+ treatment alone work well for promoting formaldehyde oxidation. However, for the sample containing a higher K content (> 3.07 wt%), the followingNH(4)(+)+ treatment could induce structural deformation via substitution of lattice oxygen by N doping, resulting in the formation of abundant reactive oxygen vacancies that could continuously transform inert gaseous dioxygen into oxidative surface adsorbed oxygen. Catalyst characterizations proposed that activity of the oxygen vacancy was the main factor contributing to the excellent formaldehyde oxidation performance. The best-performed K-MnO2-NH sample with 76% removal efficiency for similar to 400 mg/m(3) HCHO at 30?degrees C not only surpassed a benchmark commercial purifier in terms of formaldehyde mineralization, but also was much more resistant to the interference from the concomitant benzene pollutant than the commercial catalyst. Reaction pathways were proposed through comprehensive experimental investigations on the surface intermediates of the spent catalysts.