NO_x-assisted soot combustion over dually substituted perovskite catalysts La_(1-x)K_xCo_(1-y)PdyO_(3-δ)

摘要

A series of dually substituted perovskite catalysts La_(1-x)K_xCo_(1-y)PdyO_(3-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 NO_x. 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 La_(0.9)K_(0.1)Co_(0.95)Pd_(0.05)O_(3-δ) possesses the highest performance, exhibiting the lowest T_i and T_m (219 °C and 360 °C), the narrowest temperature range (Tf- T, = 162°C) and the lowest activation energy (93.6kJ/mol) for soot combustion. The catalyst La_(0.9)K_(0.1)Co_(0.95)Pd_(0.05)O_(3-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 (Pd~(3+) Pd~(4+)) 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.