Soot combustion: Reactivity of alkaline and alkaline earth metal oxides in full contact with soot

The activity in the soot combustion of K- and Ba-based catalysts in “loose” and “full” contact is investigated in this study. Under loose contact systems (obtained by mechanically mixing the soot and catalysts), the K-containing samples are very active in the oxidation of soot, as opposite to the Ba-containing samples. Also, a synergistic effect in this reaction is seen between K and Pt for the case of the K-containing samples. Under full contact conditions (obtained by impregnation of the soot with aqueous metal solutions), the reactivity of soot increases; in these conditions the performance of the poorly reactive Ba-containing samples becomes similar to the very active K-based system. This result points out the key-role played by both the selected alkaline or alkaline earth metal oxides in the soot combustion, and suggests that the poor activity of Ba under loose conditions is possibly due to the low mobility of the Ba surface species. Finally, it is found that the presence of NO favors the soot oxidation under loose contact, specifically over noble metal-containing samples, due to the oxidation of NO to NO2. However the presence of the alkaline and alkaline earth oxide also affects the soot oxidation activity in the presence of NO. This has been ascribed to the formation of Ba- and K-nitrite/nitrate species which participate in the soot combustion process by giving rise to NOx upon nitrite/nitrate decomposition and/or by directly reacting with soot.