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 La 2 Sn 2 O 7 pyrochlore oxides was studied. Palladium promoter was doped into the pyrochlore system via two ways: bulk substitution (La 2 Sn 1.9 Pd 0.1 O 7 ) and surface impregnation (Pd/La 2 Sn 2 O 7 ). 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/La 2 Sn 2 O 7 . The different morphologies of palladium promoters were revealed in the two palladium-doped catalysts. Palladium was present primarily in the form of a lattice Pd 2+ 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/La 2 Sn 2 O 7 , a strong metal oxide interaction occurred in the interface between Pd species and La 2 Sn 2 O 7 . The activity evaluation results show that Pd/La 2 Sn 2 O 7 possesses the highest performance, exhibiting the lowest activation energy (98.83 kJ/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/La 2 Sn 2 O 7 . A possible oxidation mechanism for the reactions has been speculated.