Potential of mixed halides and vanadates as catalysts for soot combustion

Several soot combustion catalysts, based on a single halide and potassium vanadate supported on alpha-Al2O3, have been prepared, characterized, and tested. Three catalysts were selected on the basis of their prevalent activity and thermal stability: KCl + KVO3, RbCl + KVO3, and CsCl + KVO3. These compositions are more environmentally acceptable than previously studied Cu-based catalysts. The CsCl + KVO3 catalyst showed the highest catalytic activity and appreciable resistance to thermal degradation. The catalyst based on RbCl + KVO3 showed the best thermal stability (comparatively low loss of chlorides at high temperatures). The KCl + KVO3 catalyst, despite its slightly lower activity and thermal stability, is to be particularly appreciated for its low cost. The reaction mechanism of the catalysts is primarily linked to the formation of eutectic liquids between halides and metavanadates. Such liquids wet the carbon and promote its combustion according to redox processes in which metavanadates should play a major role.