Determination of active site densities and mechanisms for soot combustion with O2 on Fe-doped CeO2 mixed oxides

Fe-doped CeO2 mixed oxides were studied for soot combustion with O2 under tight contact conditions. They show increased activity compared to that of pure CeO2 and Fe2O3. The optimum Fe content according to soot ignition temperatures is Fe/(Ce + Fe) = 10 at.%. However, on the basis of turnover frequencies, the samples with a Fe/(Ce + Fe) ratio between 5 and 20 at.% show similar activity. Characterization of the catalysts and a kinetic study show that the reaction proceeds via a redox mechanism. The active sites were determined to be composed of Fe–O–Ce species, and the active oxygen was quantified using isothermal anaerobic titrations with soot as a probe molecule. The redox property for the Fe–O–Ce species is much stronger than for the Ce–O–Ce species. The methodology for quantifying active redox sites can be extended to soot combustion on all similar oxide systems.