Comparing the CO oxidation activity of free PdO and Pd2+ ions over PdO-CeO2/SiO2 catalysts

•Free PdO and Pd2+ ions were segregated efficiently by nitric acid treatment.•Turnover frequency of PdO and Pd2+ ions was quantitatively evaluated.•CO oxidation activity of free PdO was higher than that of Pd2+ ions.•Synergetic effect could improve CO catalytic activity of PdO-CeO2/SiO2 catalyst.

Aiming at comparing the CO oxidation activity of free PdO and Pd2+ ions, a series of PdO-CeO2/SiO2 catalysts were prepared via an impregnation method and the turnover frequencies of them were quantitatively evaluated. The catalysts were further characterized by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), temperature-programmed reduction (TPR), Raman technique and CO chemisorption. It was found that free PdO species and Pd2+ ions could be segregated efficiently by nitric acid treatment. The PdO-CeO2/SiO2 catalyst calcined at 600 °C showed the highest activity. The results of CO chemisorption suggested that free PdO species had a higher activity than Pd2+ ions, and the turnover frequencies of them were 3.27 × 10−2 s−1 and 0.49 × 10−2 s−1, respectively. The activity of PdO-CeO2/SiO2 catalyst was higher than that of PdO/SiO2 catalyst. Moreover, the CO catalytic activity of PdO-CeO2/SiO2 catalyst could be assigned to the synergetic effect of free PdO and Pd2+ ions, and this effect could improve the CO catalytic activity of catalysts.

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