Effect of surface and bulk palladium doping on the catalytic activity of La2Sn2O7 pyrochlore oxides for diesel soot oxidation
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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 H2-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 (98.83 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/La2Sn2O7. A possible oxidation mechanism for the reactions has been speculated.
This work was financially supported by the National Natural Science Foundation of China (No. 21777055), Shandong Provincial Natural Science Foundation (ZR2017BB004), Shandong Province Key Research and Development Plan (2017GGX202004) and Shandong Province Major Science and Technology Innovation Project (2017CXGC1004).
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