Abstract
Alumina-supported manganese catalysts with cryptomelane and/or birnessite structure have been prepared using a simple method based on the thermal decomposition of potassium permanganate. The samples have been characterized by XRD, FTIR, TGA, DSC, N2 adsorption at −196 °C, SEM, H2-TPR and XPS, and their catalytic activity for soot combustion has been tested and compared to that of a reference Pt/alumina catalyst. The thermal decomposition of alumina-supported KMnO4 yields a mixture of supported birnessite and potassium manganate which is the most effective, among those prepared, to lower the soot combustion temperature. However, this material is not useful for soot combustion because the accelerating effect is not based on a catalytic process but on the oxidation of soot by potassium manganate. A suitable soot combustion catalyst is obtained after potassium manganate is removed by water washing, yielding only the birnessite phase on the γ-Al2O3 support. This birnessite phase can be transformed into cryptomelane by calcination at 600 °C. These two samples, γ-Al2O3-supported birnessite and cryptomelane are suitable catalysts for soot combustion in NOx/O2 mixtures, as their catalytic activity is based on the NO2-assited mechanism, that is, both catalysts accelerate the oxidation of NO to NO2 and NO2 promotes soot oxidation. The soot combustion temperatures obtained with these birnessite/cryptomelane alumina-supported catalysts are similar to that obtained with the reference Pt/alumina catalyst.
Highlights
The goal of the current study is to evaluate if alumina-supported manganese catalysts with cryptomelane and/or birnessite structure can be prepared by a simple method based on the thermal decomposition of potassium permanganate [6]
From the results presented and discussed in this paper, it can be concluded that different alumina-supported manganese oxide catalysts have been obtained by impregnation of potassium permanganate on γ-Al2O3 and subsequent thermal decomposition
Some of the prepared materials can be considered for the regeneration of Diesel Particulate Filters (DPFs) filters since they are able to catalytically accelerate the soot combustion process
Summary
One of the main problems related with the development of the XXI century society is environmental pollution; the emission of nano/micro particles of carbon (soot) by diesel-engines being one of the unsolved issues to overcome.Diesel Particulate Filters (DPFs) are able to remove soot from exhaust gas, and these filters must be continuously or periodically regenerated by thermal or catalytic combustion of the soot collected [1].Among the different catalysts proposed for this application alkali, alkali-earth, transition and rare-earth metals are the most useful [2,3,4,5,6,7,8,9,10,11,12,13]. In a previous study [5], manganese catalysts with birnessite and cryptomelane structure were prepared, characterized, and tested as catalysts for soot combustion with NOx/O2. In a further study [6], manganese oxides with cryptomelane and/or birnessite structure were successfully prepared by a simple method based on the thermal decomposition of potassium permanganate. Their catalytic activity to accelerate soot combustion was confirmed, and it was concluded that the catalytic combustion of soot occurs by the NO2-assisted mechanism, which is the most convenient to operate under real conditions
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