Morphology-Dependent Catalytic Surface Chemistry Of Cu₂O And CeO₂ Nanocrystals

Catalysis plays a key role in modern chemical industry and environment.In order to overcome the energy source shortage and environmental pollution,catalysts must be not only high-active at mild reaction conditions but also high-selective in specific product.Fundamental understanding of structure-performance relationship at the atomic and molecular level is of great importance for the design of high-efficient catalysts.Single-crystal model catalysts with uniform surface structures have been tradiontally used as model catalysts for studies of structure-performance relationship.However,there are "material gap" and "pressure gap" between model catalysts and powder catalysts,and the acquired fundamental understanding from single crystal model catalysts can not be simply extended to powder catalysts.In recent years,the rapid development in nanotechnology has realized successful synthesis of nanocrystals with uniform morphology and exposed facets that can be used as model catalysts.Moreover,nanocrystal model catalysts can bridge the "material gap" and "pressure gap"between single crystal model catalysts and powder catalysts.Based on the above ideas,in this thesis uniform Cu2O and CeO2 nanocrystals have been employed as model catalysts for studies of catalytic surface chemistry of Cu2O-and CeO2-based catalysts.The acquired major conclusions are as the following:(1)CO and CO2 chemisorption studied with in-situ DRIFTS spectra are successfully developed as an effective probe to characterize surface structures of uniform Cu2O nanocrystals with different morphologies.CO and CO2 species adsorbed at copper sites and bidentate and bridge carbonates adsorbed at oxygen sites were found to be sensitive to surface composition and surface copper and oxygen coordination environment dependent on the exposed Cu2O facets on Cu2O nanocrystals.Then PVP-capped and oleic aeid(OA)-capped Cu-2O octahedra and capping ligands-free Cu2O octahedra were used to study the influence of capping-ligands on Cu2O in CO oxidation.PVP was found as a promoter while OA was found as a poison.Via combined in-situ DRIFTS characterization and DFT calculations,PVP was identified to promote O2 adsorption on Cu2O surface,and the resulting Cu3c sites act as the active site.(2)Pd-Cu2O composites various Pd structures,including single atoms,clusters,Pd nanoparticles with small sizes,PdCu bimetal with large sizes,were synthesized via the Cu2O-Pd2+ interfacial redox reaction with controlled concentrations of Pd precursor.Size-dependent activity and selectivity of Pd-Cu2O composites were observed in C2H2 hydrogenation reactions.Pd single atoms in the form of Pd2+ are inactive,Pd clusters have high catalytic activity and 100%ethylene selectivity,smally-sized Pd nanoparticles have high catalytic activity and mediLum ethylene selectivity,and largely-sized PdCu bimetal nanoparticles have high catalytic activity but no ethylene selectivity.In-situ aggregation and formation of smally-sized Pd-Cu nanoparticles of Pd clusters,and re-dispersion of smally-sized Pd nanoparticles were observed to occur during the C2H2 hydrogenation reaction due to the surface reduction of Cu2O substrate.Both resulting Pd structures exhibit similar ethylene selectivity of about 85%in catalyzing C2H2 hydrogenation,suggesting the presence of "twin" Pd active sites with different structures but similar catalytic selectivities.(3)Reaction mechanisms of sacetylene semhydrogenation catalyzed by commercial CeO2 nanoparticles have been successfully elucidated employing in-situ DRIFTS spectroscopy.Various types of surface species were observed to form upon C2H2 and C2H4 adsorption on CeO2 at various temperatures.including molecularly-adsorbed π-bonded and di-a-bonded species,dissociatively-adsorbed species respectively of C2H and C2H3,carbonates,formate species,and oligomers species.During the C2H2 semihydrogenation reaction,the CeO2 surface is partially reduced and strongly hydroxylated.Both O and Ce sites on CeO2 are active in catalyzing C2H2 semihydrogenation reaction to C2H4,and the O site is more active than the Ce site.The reaction mechanism was elucidated with observed molecularly-adsorbed C2H2 species,C2H3 intermediate and adsorbed C2H4 species on the CeO2 surface.The π-bonded C2H2 species at the O site was identified as the dominant surface species for the CeO2-catalvzed C2H2,semihydrogenation reaction.F’urthermore.facet effects of CeO2 nanocrystals in catalyzing acetylene semihydrogenation were studied.The catalytic performance follows an order of CeO2 rods mainly exposing(110)and(100)facets>CeO2 cube exposing(100)facets>CeO2 nanoparticles mainly exposing(111)facets.TPRS and in-situ DRIFTS results demonstrate that acetylene semhydrogenation catalyzed by CeO2 nanocrystals with different morphologies follows the same reaction mechanism and the catalytic performance of CeO2 nanocrystals can be related to their activity to activate H2.