Activity of KNbO3 as catalyst for soot combustion: Effect of the preparation method

Abstract

The effect of the preparation method and calcination temperature on the stability and activity of KNbO3 as catalysts for soot oxidation is reported. The solids were prepared by three different procedures: (i) by a citrate method using NbCl5 as niobium precursor; (ii) by a citrate method using NbF5 as niobium precursor; and (iii) by an autocombustion method using Nb2O5 as niobium precursor. The powders were calcined at 600°C and 700°C and characterized by N2 physisorption, oxygen desorption profiles, XRD and SEM. The catalytic activity is discussed in terms of both the Tm value, the temperature at which combustion of carbon black occurs at the maximum rate, and the apparent activation energy. It is found that KNbO3 calcined at 600°C shows the higher catalytic activity. Also, a strong dependence on the preparation method is observed. The solids prepared by the citrate method show the higher catalytic activities, being the most active the one that use NbCl5 as niobium precursor. This highest catalytic activity is discussed in terms of nanosized perovskite structure and rough interfaces associated to the oxygen's vacancies. Moreover, lower apparent activation energies were obtained for the solids prepared by the citrate method. In addition, it is observed that no deactivation by metal loss after consecutive soot combustion cycles because of the stabilization of the alkaline metal inside the perovskite structure.