Effect of surface and bulk palladium doping on the catalytic activity of La2Sn2O7 pyrochlore oxides for diesel soot oxidation

The soot combustion on Pd-doped La2Sn2O7 pyrochlore oxides was studied. Palladium promoter was doped into the pyrochlore system via two ways: bulk substitution (La2Sn1.9Pd0.1O7) and surface impregnation (Pd/La2Sn2O7). The catalysts were characterized by XRD, BET, SEM, HRTEM, PL, XPS and H-2-TPR measurements. Pd modifications increased the concentration of surface oxygen vacancy, especially Pd/La2Sn2O7. The different morphologies of palladium promoters were revealed in the two palladium-doped catalysts. Palladium was present primarily in the form of a lattice Pd2+ in the bulk-doping sample, with trace of metal palladium deposited on the surface. As for the surface-doping sample, PdO was the predominant form, accompanied with some of highly dispersed metallic palladium. Due to the adhesion of massive surface palladium species on Pd/La2Sn2O7, a strong metal oxide interaction occurred in the interface between Pd species and La2Sn2O7. The activity evaluation results show that Pd/La2Sn2O7 possesses the highest performance, exhibiting the lowest activation energy (98.83kJ/mol) for soot combustion. Surface Pd/PdO redox couples as well as Pd-O-Sn interface species were identified to be the active phases for the superior catalytic behavior of Pd/La2Sn2O7. A possible oxidation mechanism for the reactions has been speculated.