Catalytic activity for soot combustion of birnessite and cryptomelane

Birnessite and cryptomelane samples have been prepared and characterised by N2 adsorption at −196°C, XRD, XRF, XPS and H2-TPR. The activity of birnessite and cryptomelane for catalysed combustion of soot under NOx/O2 gas mixtures has been studied and compared with that of some other commercially available pure manganese oxides, namely MnO, MnO2, Mn2O3, Mn3O4, and natural MnO2. Birnessite and cryptomelane are more active for soot combustion than all the commercial oxides tested. Among the manganese oxides studied, birnessite shows the highest catalytic activity, lowering the soot combustion temperature by 150°C regarding the uncatalysed reaction, and cryptomelane lowers this temperature by 120°C. The NOx removal profiles suggest that birnessite chemisorbs NOx and forms potassium nitrate, which melts and reacts with soot. On the contrary, the main soot combustion pathway for cryptomelane and for the commercial manganese oxide tested is based on the catalytic oxidation of NO to NO2. Birnessite and cryptomelane only yield CO2 as carbon-containing soot combustion product.