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

The catalysts Pd/TiO2, Pd/CeO2, and Pd/CeO2–TiO2 for CO oxidation at low temperature together with their corresponding supports TiO2, CeO2, and CeO2–TiO2 prepared by sol–gel precipitation followed by supercritical fluid drying were characterized by means of N2 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 Pd2+ and Pd0 species coexist in Pd/CeO2, while only Pd0 is detected in Pd/TiO2 and Pd/CeO2–TiO2. H2-TPR showed that no bulk CeO2 exists in CeO2–TiO2 and the reduction of CeO2–TiO2 is more difficult than that of the surface oxygen in the individual CeO2 probably due to the formation of CeO2–TiO2 solid solution. PdO in Pd/TiO2 can be fully reduced by H2 at ambient temperature, whereas PdO in Pd/CeO2 and Pd/CeO2–TiO2 is reduced respectively at 162 and 80 °C, accompanied by the reduction of surface CeO2. CO-TPR showed that the reduction of PdO in PdO/TiO2 is limited within the outermost layer at ambient temperature and the core PdO can only be reduced with a further increase of temperature. PdO/CeO2 is also reduced both at ambient temperature and with the increase of temperature during the CO-TPR process, and the reduction temperature of CeO2 decreased significantly in the presence of PdO, which may be due to the different degrees of interaction between PdO and CeO2. In contrast, a complete reduction of PdO in Pd/CeO2–TiO2 by CO is observed at ambient temperature, accompanied by the partial reduction of CeO2, which may indicate that the special Pd–Ce–Ti interaction in Pd/CeO2–TiO2 is favorable for the reduction of PdO and interfacial CeO2 species. The water–gas shift (WGS) between CO and the hydroxyl groups in the catalysts is detected at a temperature higher than 210 °C on all of the Pd-supported catalysts as well as the individual CeO2 and TiO2 supports.