Catalytic combustion of soot. Effects of added alkali metals on CaO–MgO physical mixtures

The effect of adding alkali metals (Li, Na, K) to a CaO–MgO mixture, on the catalytic combustion of carbon black (CB), a model compound for soot, was studied. Catalysts were prepared by the Sol–Gel method and characterized by surface (BET surface area, XPS, DRIFTS) and bulk (AAS, XRD and TPR) techniques. Samples with a 4:1 catalyst-CB ratio were subjected to catalytic oxidation in a thermo-gravimetric apparatus and the temperature Tm, at which combustion occurs at its maximum rate, was recorded for comparison of catalytic activity. The addition of alkali metals (Li, Na, K) over the CaO–MgO mixture significantly increased the catalytic activity, due to the formation of surface oxygenated species that enhanced the oxidizing properties of the catalyst surface. That activity for CB combustion increases with the atomic number of the alkali metal contained in the catalyst. The presence of alkali metals also diminished the amount and stability of carbonates formed on the catalyst. The K-containing catalyst showed the largest activity for the catalytic CB combustion, because it shows the largest capacity to enrich its surface with α-oxygen type and promotes best the surface dissociation of that oxygen. Furthermore, surface-adsorbed OH and carbonate groups that disable the active sites and prevent the oxygen adsorption and dissociation, were less abundant and desorbed at lower temperatures, showing to be less stable on this K-containing catalyst.