Multiple doping of cryptomelane catalysts by Co, Cu, Ag and Ca for efficient soot oxidation and its effect on NO2 formation and SO2 resistance

Two series of cryptomelane doped with Cu2+ and Ag+ ions were synthesized by coprecipitation method, and additionally impregnated with Ca2+, Co2+ and Ag+ ions to obtain bi- and tri-metallic catalysts. The samples were thoroughly characterized by XRD, RS, TEM/EDX, XRF and XPS. The catalysts were tested for soot oxidation in the loose contact mode in various reaction conditions (O2, O2 + NO, O2 + SO2, and O2 + NO + SO2). The role of doping was examined and discussed in terms of its influence on catalytic soot oxidation performance, NO2 formation in the course of the combustion, and resistance to SO2. It was found that NO2 production in the course of soot oxidation is strongly enhanced upon doping, and clearly depends on the dopants nature. However, it does not directly correlate with the catalysts activity, which in the O2/NO mixture is rather weakly sensitive to the type of doping. This observation was interpreted in terms of a critical amount of NO2, which actually can be utilized effectively for soot combustion. The excess of NO2 gains in importance in the presence of SO2, accounting for the beneficial role of doping in improving the resistance to sulphur poisoning of the cryptomelane catalyst.