Comparative study of nanometric Co-, Mn- and Fe-based perovskite-type complex oxide catalysts for the simultaneous elimination of soot and NOₓ from diesel engine exhaust

In this feature article, the effects of the intrinsic factors including structure, nanometric size effect, nature of B-site ions, A-site ion substitution, redox properties of three systems of nanometric La₁₋ₓMₓCoO₃, La₁₋ₓMₓMnO₃, La₁₋ₓKₓFeO₃ perovskite-type oxide catalysts, and the external factors containing contacting model of catalyst and soot, the content of O₂ or NO, the ratio of catalyst and the gas flow rate on their catalytic performances for the simultaneous elimination of soot particles and NO were systematically investigated. The catalysts were characterized by means of XRD, FT-IR, SEM, XPS and H₂-TPR, and the catalytic performances for the simultaneous removal of nitrogen oxides and diesel soot were evaluated by temperature-programmed oxidation reactions. All the prepared samples had the perovskite structure, possessed nano-sizes and gave high activities for the simultaneous elimination of soot particles and NO. On the one hand, the number of surface atoms is much greater than the normal bulk catalysts and the size of nanoparticle is small. These characters are favorable for the adsorption and activation of reactant molecules including the O₂ or NO, even soot particle. On the other hand, the surface atoms of small nano-particles formed on the surface of the nanometric perovskite oxide samples possess high free energy, which improves the mobility of catalyst, and thus it favors the contact between the catalysts and soot. Besides the intrinsic factors, the external factors of reaction conditions containing contact model of soot and catalyst, the content of O₂ or NO, the ratio of catalyst to soot and gas flow rate also play important roles in affecting the catalytic performances for the simultaneous removal of NO and soot. The reaction mechanisms for soot oxidation and the simultaneous removal of soot particles and NO over La₁₋ₓKₓMnO₃ were also briefly discussed.