Cooperative effect of Ce and Mn in MnCe/Al2O3 environmental catalysts

Abstract

Three samples, Mn/Al2O3, Ce/Al2O3 and MnCe/Al2O3 (containing 10wt.% metal) were prepared by impregnation and comparatively studied by means of FTIR spectroscopy, other characterization techniques and catalytic measurements. It was established that the active phase deposition led to (i) partial disappearance of all kinds of the alumina OH groups and (ii) blocking of part of the Al3+ Lewis acid sites. The results indicated that manganese was better dispersed on the MnCe/Al2O3 sample as compared to Mn/Al2O3. As a result, the bimetallic sample was the most active one in a series of catalytic reactions: NO decomposition, CO oxidation and CO+NO interaction.Interaction of all samples with CO leads to formation of carbonates and hydrogencarbonates increasing in amount with the temperature of interaction and in the sequence Al2O3<Ce/Al2O3<Mn/Al2O3≪MnCe/Al2O3. When the samples interacted with NO, different nitrogen oxo-compounds were formed and manganese nitrosyls appeared with the Mn-containing samples. In agreement with the higher Mn dispersion, the amount of the latter was larger with the bimetallic sample. Interaction between CO and NO results in formation of isocyanates, (hydrogen)carbonates and nitrogen oxo-compounds (nitrosyls, nitro-nitrito and nitrate species). When the interaction occurs at ambient temperature, the surfaces of the Al2O3, Ce/Al2O3 and Mn/Al2O3 are covered mainly by nitrates and nitro-nitrito species. With increase of the interaction temperature these species are gradually replaced by (hydrogen)carbonates. The temperature of this replacement decreases in the order Al2O3–Ce/Al2O3–Mn/Al2O3. With the MnCe/Al2O3 sample carbonates dominate even at 373K interaction. Consequently, the NO+CO catalytic conversion occurs in two regimes. The low temperature regime (the temperature interval depends on the catalyst) is associated with NOx precovered surface, while at higher temperatures the catalysts surfaces are precovered with (hydrogen)carbonates.