The Study Of PdO-CeO₂ Composite Catalysts For CO And CH₄ Catalytic Oxidation

A series of PdO-CeO₂ composite catalysts were prepared using a co-precipitation method and tested for CO and CH₄ oxidation.The catalysts were further characterized by X-ray diffraction(XRD),the Brunauer-Emmett-Teller(BET) technique,chemisorption of CO,temperature programmed reduction(TPR),X-Ray Photoelectron Spectroscopy(XPS) and a pulse reaction technique.The information about active materials on CO and CH₄ oxidation over catalysts was obtained.XRD results indicated that a Ce_xPd_1-xO_2-δ solid solution was formed when the catalysts were calcined at 400-800℃,while PdO or metal Pd species re-migrated to the catalyst surface when it was calcined at a higher temperature(1000℃).The turnover frequency(TOF) for the CO oxidation reaction over these catalysts was found to be independent of the Pd particle size and it increased with increasing content of Ce_xPd_1-xO_2-δ solid solution in the catalyst.This suggested that the solid solution might be the active phase for the reaction.Furthermore,pulse reaction results indicated that oxidized Pd species were more active than metallic Pd.For the PdO-CeO₂ catalysts calcined at different temperatures,it was found that increasing calcination temperature led to sintering of catalyst and a decline of catalyst surface area.The redox property of CeO₂ in catalyst decreased with increasing calcination temperature,which was responsible for the decline of reactivity of CH₄ oxidation over the PdO-CeO₂ catalysts.This suggested that surface property of CeO₂ was essential for the reaction.Also,the activity for the CH₄ oxidation reaction over the PdO-CeO₂ catalysts decreased with increasing content of Ce_xPd_1-xO_2-δ solid solution in the catalyst,implying that the solid solution was unfavorable to the reaction.The XPS results showed that the relative content of Pd⁰ increased with increasing calcinations temperature,while that of Pd²⁺ decreased.It was correlated to the reactivity of CH₄ oxidation,indicating that metallic Pd is more active than oxidized Pd. CeO₂ with higher surface area had better redox property,which was beneficial to the cyclic transformation of PdO-Pd-PdO in the catalysts.Therefore,the catalyst had a strong ability to supply Pd⁰ dynamically,which led to the high activity for the CH₄ oxidation; While the redox property of CeO₂ was suppressed decreasing surface area(increasing calcinations temperature),and the cyclic transformation of PdO-Pd-PdO was hindered, which could explain the decline in the reactivity of CH₄ oxidation.However,the content of metallic Pd⁰ greatly increased when the catalyst was calcined at 1000℃by thermal decomposition,resulting in a high activity.Furthermore,pulse reaction results indicated that metallic Pd were more active than oxidized Pd.