Solid-State NMR Studies Of Surface Structure Of Ceria

Metal oxide nanomaterials are widely used in catalysis.The surface structure in particular,the exposed facets and defects such as oxygen vacancy,are the key to control the catalytic properties.A variety of methods have been developed to characterize their surface structure and shed light on the structure-property relationships.Solid-state NMR(ssNMR)is a powerful tool to probe the local structure of solid materials with short-range and medium-range order,with which the original phase of the sample can be retained without any damage and quantitative research can be realized.In this dissertation,we have applied ssNMR as the major technique,in combination with different characterization methods,to study the surface structure of ceria and ceria supported Pt catalysts.The results obtained from this study provide some new insights on applying ssNMR to probe the surface structure of metal oxide nanomaterials.The ceria nanocrystals with different morphologies and exposed facets have been prepared by a hydrothermal method.31P MAS NMR combined with trimethylphosphine(TMP)probe molecule was used to characterize the surface structure and properties of ceria nanocrystals.The data show that TMP is adsorbed differently on ceria nanocrystals exposing different facets.Since the 31P chemical shifts of TMP adsorbed on the(111)and(110)facets of ceria are distinct,this approach can be used to differentiate ceria nanocrystals exposing different facets.Pt supported on CeO2 nanocrystals with different morphologies and specific exposed facets prepared and the catalytic properties were tested by using CO catalytic oxidation.17O NMR spectroscopy,combined with isotopic labeling,was used to study the structure of surface oxygen and attempts were made to distinguish these oxygen species.The influence of Pt on the surface oxygen were investigated and the data show that a shift to higher frequency may occur when Pt is supported on CeO2.The largest shift is observed for CeO2 nanocubes exposing(100)facets,which is in agreement with the largest change on the CO oxidation catalytic properties after Pt loading.