Intrinsic reactivity of alkaline and alkaline-earth metal oxide catalysts for oxidation of soot

The reactivity of selected alkaline (Na, K, Cs) and alkaline-earth (Ca, Ba, Mg) oxide catalysts in the oxidation of a model soot sample (Printex U) is investigated in this study. With the aim of obtaining information about the intrinsic reactivity of these elements, samples have been prepared in which the active elements have been directly deposited on the soot (‘‘full contact’’ conditions). In this way the reactivity of the catalytic elements is not controlled by the type of contact with the soot. It is found that the combustion of soot is greatly enhanced by the presence of either alkaline or alkaline-earth oxides, with Cs and Mg exhibiting the highest and the lowest activity, respectively. Notably, the reactivity in the soot combustion is found to nicely correlate with the electropositivity of the investigated metal ions, in line with literature indications pointing out the relevance of the electron-donor characteristics of the active elements. The correlation between electropositivity and activity in the soot combustion is not apparent in the absence of a close contact, i.e. under loose contact conditions. Under these conditions it is suggested that the catalytic activity is governed by the mobility of the surface species, which favors the soot–catalyst contact. Accordingly the poor performances observed in the case of the alkaline-earth metal oxides suggest for these elements a poor mobility of the active surface species, as opposite to alkaline oxide based catalysts showing a high reactivity even under loose contact conditions.