Soot combustion manganese catalysts prepared by thermal decomposition of KMnO4

Abstract

Manganese oxides with cryptomelane and/or birnessite structure have been successfully prepared by thermal decomposition of potassium permanganate. The samples have been characterized by XRD, FTIR, TGA, N2 adsortion at −196°C, H2-TPR and XPS, and have been evaluated as catalysts for soot combustion in a NOx/O2 gas mixture. KMnO4 decomposition at 400°C yields birnessite and potassium manganate, and the latter can be removed by water washing. Further calcination at 600°C of the washed birnessite leads to the increase of the birnessite crystallinity and to the partial transformation into cryptomelane. For the unwashed sample, the catalytic effect of potassium, which forms low melting temperature compounds upon NOx chemisorption, controls the catalytic combustion of soot in a NOx/O2 gas mixture. On the contrary, the combustion of soot assisted by these birnessite and cryptomelane catalysts (once manganese salts have been removed) occurs by the NO2 mechanism, and these catalysts seem promising candidates for practical application in the regeneration of DPF filters.