NOx-assisted soot combustion over dually substituted perovskite catalysts La1-xKxCo1-yPdyO3-δ

A series of dually substituted perovskite catalysts La1-xKxCo1-yPdyO3-delta (x=0, 0.1; y=0, 0.05) were successfully synthesized through a citrate-based sol-gel process, and employed for soot combustion in the presence of NOx. The physicochemical properties of them were systematically characterized by N-2-sorption, XRD, XPS, SEM, HRTEM, XANES, EXAFS, H-2-TPR, soot-TPR, FT-IR and TG/DTA. The activity evaluation results show that among all catalysts La0.9K0.1Co0.95Pd0.05O3-delta possesses the highest performance, exhibiting the lowest T-i and T-m (219 degrees C and 360 degrees C), the narrowest temperature range (T-f - T-i = 162 degrees C) and the lowest activation energy (93.6 kJ/mol) for soot combustion. The catalyst La0.9K0.1Co0.95Pd0.05O3-delta shows relatively larger BET surface area, smaller crystallite size and higher dispersion of Pd. Additionally, this catalyst also possesses the best reducibility and highest oxidibility as revealed by H-2-TPR and soot-TPR. The Pd ions with high valence (Pd3+, Pd4+) in distorted octahedral coordination environment as demonstrated by XPS, XANES and EXAFS are much more active for NO oxidation and soot combustion than the bivalent Pd ions with square-planar coordination symmetry. Based upon the characterization results and catalytic performance, a mechanism containing two reaction pathways namely direct soot oxidation by surface adsorbed oxygen species in oxygen vacancies and the NO2-assisted soot oxidation is proposed. (c) 2013 Elsevier B.V. All rights reserved.