Study on the Simultaneous Catalytic Removal of Diesel Soot Particulates and Nitrogen Oxides

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The present study has aimed to develop a new after-treatment catalytic system for diesel exhausts, socalled the simultaneous removal of NOx and diesel particulates (PM). The catalytic activity of perovskite-type (ABO_3), K_2NiF_4-type (A_2BO_4) and spinel-type (AB_2O_4) oxides for the simultaneous NOx-PM removal depended significantly on A- and B-site metal cations. Generally speaking, mixed metal oxides containing Co and Mn showed high activity but low selectivity to NOx reduction into N_2, while those containing Cu and Fe were excellent catalysts with moderate activity and high selectivity. The addition of alkali metal ions, especially K,was quite effective in promoting both the activity and selectivity. The mixed metal oxides were superior to transition metal simple oxides, their mechanical mixtures and supported Pt catalysts in the selectivity for NOx reduction.The mechanism of the simultaneous NOx-PM removal reaction was investigated by measuring the effect of gaseous composition … More of NOx and O_2. Nitrogen dioxide (NO_2) was effective for the ignition temperature of PM to decrease, and coexistence of O_2 with NOx was indispensable to enhance the NOx reduction into N_2. Based on the results, a possible reaction mechanism for the simultaneous NOx-PM removal was proposed which involves the oxidation of NO into NO_2 by O_2, the dissociative adsorption of NO_2 to give active oxygen species (O_<ad>), the formation of surface carbon species (intermediate) activated by O_<ad> and the attack of gaseous oxygen to intermediate.Honeycomb PM filter loaded with perovskite-type La_<0.9>K_<0.1>CoO_3 catalyst (catalytic filter) was prepared as a prototype of practical catalysts. After PM being trapped on the catalytic filter directly from the real diesel exhaust, the simultaneous NOx-PM removal reaction was performed in the flow of simulated diesel exhaust. The result strongly suggests the potentiality in the practical application of the present simultaneous NOx-PM removal process. Less