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

Abstract

A series of dually substituted perovskite catalysts La1−xKxCo1−yPdyO3−δ (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 N2-sorption, XRD, XPS, SEM, HRTEM, XANES, EXAFS, H2-TPR, soot-TPR, FT-IR and TG/DTA. The activity evaluation results show that among all catalysts La0.9K0.1Co0.95Pd0.05O3−δ possesses the highest performance, exhibiting the lowest Ti and Tm (219̊C and 360̊C), the narrowest temperature range (Tf−Ti=162̊C) and the lowest activation energy (93.6kJ/mol) for soot combustion. The catalyst La0.9K0.1Co0.95Pd0.05O3−δ 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 H2-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.