N2O decomposition on CoOx, CuOx, FeOx or MnOx supported on ZrO2: The effect of zirconia doping with sulfates or K+ on catalytic activity

Zirconia doped with sulfates or K+ were prepared by impregnation with (NH4)2SO4, or KNO3 aqueous solutions. MeOx/ZrO2 and MeOx/doped-ZrO2 catalysts (Me = Co, Cu, Fe or Mn) were prepared by wet impregnation of zirconia and doped-zirconia supports. The effect of doping on MeOx properties was studied by XRD, UV–vis DRS, H2-TPR and FTIR and the influence of doping on the catalytic activity for N2O decomposition was investigated under ideal conditions (N2O in He) and under real reaction conditions (addition of NO, O2, and water vapour to the reactant mixture). Characterization results indicated that all samples contained mainly dispersed Men+ species interacting with the support. In MeOx/sulfated-ZrO2 the doping with electron-withdrawing sulfates stabilized the Men+ oxidation state. In CoOx/K-ZrO2 samples the doping with electron-releasing K+ increased the poly-nuclear CoOx reducibility. FTIR characterization suggested that the electron-donor capacity of Co2+ site had the order CoOx/sulfated-ZrO2 < CoOx/ZrO2 < CoOx/K-ZrO2. Catalytic results showed that dispersed Men+ are the active site for N2O decomposition on MeOx/ZrO2. The “twin peak pattern” of catalytic activity versus tmi d-electron number suggests that formation and stability of the intermediate Me(n+1)+O- surface complex, requiring the mobility of Men+ oxidation state, are key factors for activity. Because the electron-withdrawing sulfates lowered the electron-donor capacity of tmi, hindering the formation of Me(n+1)+O-, the effect of sulfate-doping was to decrease the deN2O activity in MeOx/sulfated-ZrO2. Conversely, because the electron-releasing potassium cation increased electron-donor capacity of Co2+, yielding an easier formation of the intermediate surface complex, the effect of K-doping was to increase the deN2O activity in CoOx/K-ZrO2. From an applied viewpoint, cobalt supported on ZrO2 and K-doped ZrO2 systems, that were affected by reversible inhibitory effect in real reaction conditions, are interesting catalysts for N2O abatement and for simultaneous abatement of N2O and NO with hydrocarbons.