Cerium Based Shells With Palladium Cores Encapsulated: An Efficient Catalyst For Carbon Monoxide Oxidation

Catalytic oxidation of CO has been studied extensively because of the manyimportant practical applications, such as gas masks, removal of CO from exhauststreams, and preferential oxidation of CO during the water-gas-shift reaction. Ceriumbased supported catalysts, especially Pd/CeO2and Pt/CeO2, are usually applied to COoxidation reaction. However, these catalysts usually exhibited undesirable activity atlow temperature due to insufficient interfaces between metal particles and CeO2,which played key role in the CO oxidation reaction. Therefore, core-shellconfiguration maximizing the interfaces between precious metal cores and CeO2shells was introduced to improve the low-temperature activity. Pd@Zr/CeO2core-shell catalyst prepared by hydrothermal method was applied in CO oxidationreaction, exhibiting high CO oxidation activity at low temperature. XRD (X-raydiffraction) analysis demonstrated that the remarkable enhancement of the catalyticperformance was found to depend on the presence of more oxygen vacancies in thecore-shell structure, which contributing higher content and readily release of activeoxygen species at low temperature, confirmed by H2-TPR (Temperature programedreduction) results. Therefore, Pd@CeO2sample has higher catalytic activity than thePd@CeO2one. Interestingly, introducing a small amount of zirconium (0.5wt%)exhibited a significant improvement of catalytic activity while exhibiting almost noinfluence on the catalytic activity of Pd/Zr/CeO2one. The reason is that theintroduction of Zr further improving the amount of crystal defects and promoting themigration of oxygen species.After high-temperature hydrothermal aging, the core-shell structure of thePd@CeO2catalyst was destroyed seriously. The sphere morphology of the Pd@CeO2was not maintained and many core-shell structures aggregated together (demonstratedby TEM characterization). The Pd@CeO2catalyst’s surface area declineddramatically after aging and was almost the same with the Pd/CeO2and Pd/Zr/CeO2one. More importantly, the high-temperature hydrothermal aging had an effect on itsmicrocrystal structure, therefore, the oxygen vacancy in the Pd@CeO2was declinedand the reduction of its active oxygen species was more difficult. Thus, the COoxidation catalytic ability of Pd@CeO2was as low as Pd/CeO2. However, theintroduction of small amount of Zr could largely enhance the Pd@CeO2’s stability against the high-temperature hydrothermal aging. This is because the introduction ofZr could inhibit the declining of surface area of CeO2, but also prevent the damage ofmicrocrystal of core-shell nanostructure and facilitate the migration of bulk oxygenspecies to surface oxygen species, therefore, proving more oxygen for CO oxidationaction.Above all, comparing to the traditional Pd/CeO2catalyst, the fresh Pd@CeO2one had more active oxygen species due to its special structure, which leading to itshigh CO oxidation catalytic activity. The modification with Zr helped to facilitate themigration of oxygen species, giving the Pd@Zr/CeO2catalyst higher catalytic activity.High temperature hydrothermal aging seriously destroyed the Pd@CeO2structure,therefore, its advantage given by the core-shell structure was lost, and its catalyticactivity was similar to the Pd/CeO2one. But the modification with Zr not onlyimproved its structure’s stability, also promoted the migration of active oxygenspecies. Therefore, the aged Pd@Zr/CeO2catalyst had much higher catalytic activitythan the Pd/Zr/CeO2one.