Regulating Metal-support Interaction Over CeO₂-based Catalvst And Its Performance In CO Oxidation

The CeO2-based metal oxide catalyst is widely used in mang catalytic reactions,such as CO low-temperature oxidation and CO-PROX.Pd or Cu is often employed as active site in these reactions.Owing to the special properties of CeO2,it is generally accepted that there is a strong metal-support interaction(SMSI)between CeO2 and metal oxides.The interaction would induce the change in morphology and electronic properties of the catalyst.The resulting special active site and synergistic effect is the key to catalytic activity.In order to design highly active catalyst,considerable attention had been attracted on regulating the interface and then optimizing the SMSI.On the other hand,there are the challenges in the identification of active site and mechanism due to the complex changes caused by morphology and electronic properties.In this dissertation,CeO2 was choosed to support Pd and Cu.By modifying the preparation method,the SMSI of the catalyst was well adjusted.Then,a series of characterization was used to explore the key structure causing SMSI.Based on the above,the variations of active sites were distinguished and the "structure-activity"relationship of the PdO/CeO2 and CuO/CeO2 catalyst was established.The main content is as follows:Using an improved reactive deposition method,a series of PdO/CeO2 catalysts with different SMSI were synthesized.It was found that an ultra-thin mixed Pd-Ce oxides nanocluster was obtained if the CeO2 with large oxygen defect was used.This Pd-Ce oxides nanocluster had special Pd adsorption sites and good redox ability.The Pd-Ce oxides are the key to the SMSI.During the CO oxidation,the active oxygen formed due to the improvement of redox ability.CO adsorbed on the Pd site and then reacted with the surrounding active oxygen to form CO2.O2 would quickly fill into the catalyst.The whole process was accompanied by the mutual conversion of Pd2+/Pd4+.Because the adsorption of CO and the dissociation of O2 occurred at different sites,the Pd-Ce oxide followed the non-competitive mechanism,showing much high activity.However,when CeO2 has no oxygen defect,typical PdO cluster was formed.The reaction occurs on the reduced PdO.CO and O2 simultaneously adsorbed on the Pd surface and reacted to form CO2.The competitive adsorption of CO and O2 made the CO oxidation process follow the competitive mechanism.PdO showed low activity.Due to the different proportion of Pd-Ce oxide and PdO,the catalysts showed distinct catalytic performance.By treating CeO2 with different amount of hydrogen peroxide,a series of catalysts with different SMSI was obtained.The characterization proved that the surface two-electron defect was the key structure to the SMSI.Firstly,the two-electron defect adsorbed oxygen to form the peroxide.Cu2+ could connect with the peroxide to form the Cu-[O]x-Ce.Due to the dispersion of the extra two electrons on Cu-[O]x-Ce,pairs of Cu+-Ce3+were produced.The changes in interface structure and electronic properties represented the SMSI between CuO and CeO2.The unsaturated CuO cluster was obtained and the catalyst revealed good redox ability.With the amount decrease of the two-electron defect,the unsaturated CuO cluster gradually transformed into typical stoichiometric CuO.Thus,the redox ability of the catalyst gradually weakened and disappeared,leading to the decrease of CO-PROX activity.In addition to the two-electron defect,CeO2 revealed different crystal planes.Since the {110} and {100} planes strongly adsorbed CuO,the electronic transfer from Ce to Cu occurred.Even if the {110} and {100} planes had no surface two-electron defect,the strong interaction was obtained between CuO and CeO2,resulting in good activity.The {111} crystal plane weakly adsorbed CuO,leading to the CuO aggregation.The formation of stoichiometric CuO had no SMSI with CeO2 and revealed poor activity.Ultrasonic-assisted co-precipitation method was used to prepare the CuO/CeO2 catalyst.The amount of surface two-electron defect was promoted by controlling the ultrasonic time.More Cu-[O]x-Ce were formed,and enhanced SMSI resulted in the catalyst with good CO-PROX activity.Then,there was an in-depth study about the catalyst mechanism.It was found that CO absorbed on the Cu+ site and directly oxidized by adjacent active oxygen around the Ce3+site.The gaseous oxygen was then quickly captured and dissociated to continuously produce new active oxygen species.The whole process was accompanied by the conversion of Cu2+/Cu+and Ce4+/Ce3+.Thus,the catalyst followed the non-competitive mechanism and had good activity.It was interesting to see that the structural changes caused by different SMSI mainly changed the the mobility of active oxygen,but did not change the amount of adsorption Cu+site.There was a linear correlation between the the mobility of active oxygen and the reaction rate.The mobility of active oxygen is one of the key factors affecting the catalytic activity.Finally,the SMSI was also affected by CuO coverage.In addition to finding the optimal CuO loading,the structure and properties of CuO were further explored.It was found that different Cu species existed in the catalyst,having distinct redox abilities and leading to different effects in the CO-PROX reaction.The Cu-[O]x-Ce structure had the strongest interaction with CeO2,and was most favorable for the reaction.The amorphous CuOx cluster had weak interaction with CeO2,and could lower the selectivity.The aggregated CuOx cluster had the weakest interaction with CeO2,showing the lowest redox ability and activity.The catalytic performance was the comprehensive result of the performance of these Cu species.