Role of Structural Deficiency of Nanoscaled Perovskites Prepared by Reactive Grinding on the Catalytic Purification of Exhaust Pollutants

It is well known that the catalytic characteristics of perovskites for various redox reactions depend primarily on the preparation procedure. The conventional method for perovskite preparation, the so-called "ceramic method," involves a calcination step with a temperature of at least 800 degrees C, resulting in large grain size and low specific surface area (usually several m(2)/g). Recently, a new method for perovskite preparation designated as reactive grinding has been proposed by our group, generating a large variety of perovskites at room temperature with extraordinarily high specific surface areas on the order of 100 m(2)/g when grinding additives are used. Additionally, this novel technology is favorable to yield perovskites with an abundant deficiency structure simultaneously with a nanosized crystallite domain. Series of La(Co, Mn)(1-x)(Cu)(x)O-3 perovskites were prepared by reactive grinding and characterized by XRD, O-2-TPD, and H-2-TPR showing anion deficiency (O-2 vacancy) in lanthanum cobaltites and cation deficiency (O-2 excess) in lanthanum manganites. These samples were thereafter used for catalytic purification of NO, CO and soot pollutants coming from an automobile. For NO reduction by CO, a better catalytic performance was found over LaCoO3 compared to LaMnO3. The deNO(x) activity of LaCoO3 can be considerably improved via 20% Cu substitution, leading to a 97% N-2 yield and nearly complete CO conversion at 450 degrees C. This improvement was ascribed to the ease of generation of anion deficiencies after Cu incorporation, which plays a crucial role in NO adsorption and dissociation. A mechanism was proposed with dissociation of chemisorbed NO upon oxygen vacancies forming N-2 and/or N2O, and oxidized perovskite surface, with continuous reduction by CO with the production of CO2. For soot combustion, the better activity was observed again in the case of LaCoO3 with respect to LaMnO3. A mechanism was proposed with an attack of soot by O-species which immigrates from the perovskite surface. Cation deficiency of lanthanum manganites associates with overstoichiometric oxygen from the perovskite lattice, which can be only utilized for an oxidation process but less active compared to molecular oxygen formed upon anion vacancies. In summary, the anion deficiency of perovskite-typed oxides seems to make more contribution for both NO reduction and soot oxidation in comparison with cation deficiency.