Comparative study of ageing condition effects on Pd/Ce0.5Zr0.5O2 and Pd/Al2O3 catalysts: Catalytic activity, palladium nanoparticle structure and Pd-support interaction

• Effects of ageing conditions on Pd supported catalysts are studied systematically.
• Sintering of Pd species in Pd/Ce0.5Zr0.5O2 is inhibited in air and cyclic ageing.
• Sintering of Pd species in Pd/Al2O3 is severe regardless of the ageing condition.
• PdOCe bond works with the involvement of oxygen to inhibit the sintering of Pd. PdOCe bond works during cooling down from high temperature.

Pd/Ce0.5Zr0.5O2 and Pd/Al2O3 catalysts were prepared by impregnation method and aged in different atmospheres (air, N2, H2 and cyclic (air–H2)) at 1050 °C. The CO oxidation activities of the catalysts were evaluated. The states of palladium nanoparticles were characterized by step scanning X-ray diffraction (XRD), CO chemisorption, H2 temperature-programmed reduction (TPR), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy (Raman). The Pd/Ce0.5Zr0.5O2 catalysts aged in air and cyclic conditions exhibit much better catalytic activities than those aged in N2 and H2, but the Pd/Al2O3 catalysts aged in different atmospheres show similar activities. Small PdO crystallites are observed in Pd/Ce0.5Zr0.5O2 catalysts after air and cyclic ageing, while larger Pd crystallites are found after N2 and H2 ageing. The sintering of Pd species is inhibited with the presence of oxygen in Pd/Ce0.5Zr0.5O2 catalysts. This is due to the strong interaction between Pd and CZ support in the form of PdOCe bond, which works during the cooling stage (from 1050 °C to room temperature). However, the thermal ageing deactivates the Pd/Al2O3 catalysts regardless of the ageing condition. The Pd/PdO particles grow to around 100 nm in all the aged Pd/Al2O3 catalysts, leading to the worse activity compared with the Pd/Ce0.5Zr0.5O2 catalysts.

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