Pd/CeO2-TiO2 catalyst for CO oxidation at low temperature:a TPR study with H2 and CO as reducing agents

摘要

The catalysts Pd/TiO2,Pd/CeO_2,and Pd/CeO_2-TiO_2 for CO oxidation at low temperature together with their corresponding supports TiO_2,CeO_2,and CeO_2-TiO2 prepared by sol-gel precipitation followed by supercritical fluid drying were characterized by means of N_2 adsorption,XRD,diffuse reflectance infrared Fourier transform spectroscopy(DRIFTS)of CO adsorption and temperature-programmed reduction(TPR)with H2 and CO as reducing agents.The results showed that PdO is finely dispersed on the supports with high surface area.DRIFTS of CO adsorption further indicated that both Pd~2+ and Pddeg species coexist in Pd/CeO2,while only Pddeg is detected in Pd/TiO_2 and Pd/CeO_2-TiO_2.H_2-TPR showed that no bulk CeO_2 exists in CeO2-TiO2 and the reduction of CeO_2-TiO_2 is more difficult than that of the surface oxygen in the individual CeO_2 probably due to the formation of CeO_2-TiO_2 solid solution.PdO in Pd/TiO_2 can be fully reduced by H_2 at ambient temperature,whereas PdO in Pd/CeO_2 and Pd/CeO_2-TiO_2 is reduced respectively at 162 and 80 degC,accompanied by the reduction of surface CeO_2-CO-TPR showed that the reduction of PdO in PdO/TiO_2 is limited within the outermost layer at ambient temperature and the core PdO can only be reduced with a further increase of temperature.PdO/CeO_2 is also reduced both at ambient temperature and with the increase of temperature during the CO-TPR process,and the reduction temperature of CeO_2 decreased significantly in the presence of PdO,which may be due to the different degrees of interaction between PdO and CeO_2.In contrast,a complete reduction of PdO in Pd/CeO_2-TiO_2 by CO is observed at ambient temperature,accompanied by the partial reduction of CeO_2,which may indicate that the special Pd-Ce-Ti interaction in Pd/CeO_2-TiO_2 is favorable for the reduction of PdO and interfacial CeO_2 species.The water-gas shift(WGS)between CO and the hydroxyl groups in the catalysts is detected at a temperature higher than 210 degC on all of the Pd-supported catalysts as well as the individual CeO_2 and TiO_2 supports.