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Ceria nanocrystallites with different morphologies and crystal planes were hydrothermally prepared, and the effects of ceria supports on the physicochemical and catalytic properties of Pd/CeO2 for the CO and propane oxidation were examined. The results showed that the structure and chemical state of Pd on ceria were affected by ceria crystal planes. The Pd species on CeO2-R (rods) and CeO2-C (cubes) mainly formed PdxCe1–xO2−σ solid solution with −Pd2+–O2––Ce4+– linkage. In addition, the PdOx nanoparticles were dominated on the surface of Pd/CeO2-O (octahedrons). For the CO oxidation, the Pd/CeO2-R catalyst showed the highest catalytic activity among three catalysts, its reaction rate reached 2.07 × 10–4 mol gPd–1 s–1 at 50 °C, in which CeO2-R mainly exposed the (110) and (100) facets with low oxygen vacancy formation energy, strong reducibility, and high surface oxygen mobility. TOF of Pd/CeO2-R (3.78 × 10–2 s–1) was much higher than that of Pd/CeO2-C (6.40 × 10–3 s–1) and Pd/CeO2-O (1.24 × 10–3 s–1) at 50 °C, and its activation energy (Ea) was 40.4 kJ/mol. For propane oxidation, the highest reaction rate (8.08 × 10–5 mol gPd–1 s–1 at 300 °C) was obtained over the Pd/CeO2-O catalyst, in which CeO2-O mainly exposed the (111) facet. There are strong surface Ce–O bonds on the ceria (111) facet, which favors the existence of PdO particles and propane activation. The turnover frequency (TOF) of the Pd/CeO2-O catalyst was highest (3.52 × 10–2 s–1) at 300 °C and its Ea value was 49.1 kJ/mol. These results demonstrate the inverse facet sensitivity of ceria for the CO and propane oxidation over Pd/ceria.
Zong et al. (Tue,) studied this question.