Catalytic oxidation of soot over alkaline niobates

Abstract

The lack of studies in the current literature about the assessment of alkaline niobates as catalysts for soot oxidation has motivated this research. In this study, the synthesis, characterization and assessment of alkaline metal niobates as catalysts for soot combustion are reported. The solids MNbO3 (M=Li, Na, K, Rb) are synthesized by a citrate method, calcined at 450°C, 550°C, 650°C, 750°C, and characterized by AAS, N2 adsorption, XRD, O2-TPD, FTIR and SEM. All the alkaline niobates show catalytic activity for soot combustion, and the activity depends basically on the nature of the alkaline metal and the calcination temperature. The highest catalytic activity, expressed as the temperature at which combustion of carbon black occurs at the maximum rate, is shown by KNbO3 calcined at 650°C. At this calcination temperature, the catalytic activity follows an order dependent on the atomic number, namely: KNbO3>NaNbO3>LiNbO3. The RbNbO3 solid do not follow this trend presumably due to the perovskite structure was not reached. The highest catalytic activity shown by of KNbO3, despite the lower apparent activation energy of NaNbO3, stress the importance of the metal nature and suggests the hypothesis that K+ ions are the active sites for soot combustion. It must be pointed out that alkaline niobate subjected to consecutive soot combustion cycles does not show deactivation by metal loss, due to the stabilization of the alkaline metal inside the perovskite structure.