Spectroscopic Studies Of Metal Oxide And Supported Palladium Based Catalysts For CO Catalytic Conversion

Research about structure-acivity relationship is always the center of surface chemistry.Model studies combinded with theoretical calculation have provided profound understanding of reaction mechanisms at the atomic scale.Owing to the technology limitation,traditional model reactions usually perform under ultra-low pressure,which may be inconsistant with the results under atmosphere or reaction conditions.In-situ and ex-situ techniques can make high pressure in-situ characterization possible and aviod the suface exposure to atmosphere after consequential treatments.Surface sensitive analysis that can work under atmosphere or higher pressure is useful for understanding the structure-acivity relationship of a realistic catalyst.The research focuses on the fundamental understanding of surface compositions and oxidation states of CeO2 and TiO2 supported Pd,Cu,Pd-Au and Pd-Cu catalysts,using XPS and HS-LEIS equipments equipped with an ex-situ treatment cell.Combined with in-situ IR,surface adsorbate and transformation routes on ZnCrOx-Al2O3 under CO hydrogenation process have been investigated.Pd/TiO2 and Pd/CeO2 are compared after similar reaction process.The results indicate that CeO2 supported Pd nanoparticles are easily to be oxidized to PdO,but PdO is harder to be reduced.CeO2 and palladium have strong metal-support interaction,which decreases the reduction temperature of Ce4+.Such two supports have different effects on the oxidation states of palladium under CO oxidation conditions.The pre-reduced 5wt%Pd/TiO2 shows Pd0 only during the reaction.The pre-oxidized surface also intends to form metallic palladium too.But there is always present a mixture of Pd0 and Pd2+ for CeO2 supported one during CO oxidation.Surface composition is essential for understanding the structure-activity relationship.In order to have a deep insight to the dynamic changment of a realistic catalyst,phase diagrams of the surface composition as a function of the bulk one,and the relationship between the surface composition and activity are obtained for Pd-Au/TiO2 using HS-LEIS.The results indicate that Pd becomes hard to be oxidized after alloying with Au.For Pd-Au bimetal nanoparticles,Pd was preferred to be rich on the surface under O2/N2,H2 and CO/O2/He.To reduce the surface composition of Pd,higher amount of Au is needed for the the tunning of activity for CO oxidation.CO oxidation activity decreases rapidly when the amount of the surface Pd exceeds 65%.Pd-Cu/CeO2 is used for comparing reduction and oxidation process.Here,we try to figure out the role of copper during CO oxidation.The activity for CO oxidation as a function of the Cu amount appears as a volcano plot.Small amount of copper is efficient for improving the overall activity.XPS and TPR results indicate that PdO is harder to be reduced with increasing amount of copper.According to the HS-LEIS results,we found the surface segregation of Cu during both oxidation and reduction treatments.That is to say,a large amount of Cu might block the Pd site since such segregation.For ZnCrOx-Al2O3 mix-oxides,we aim to study the surface active sites and the reaction intermediate during CO hydrogenation.The results indicate that,doping with Cr can enhance the dissociation of CO and adsorption of oxygen-containing carbon species.Zn2+plays an important role in CO and H2 dissociatin.After XPS/lR/TPD characterization,we confirm the formation of dissociation carbon,hydrocarbon and formate species.The formate species comes from the hydrogenation of carbonate or bicarbonate,and can be removed after further hydrogenation.