Huizhi Bao

Crystal-Plane-Controlled Surface Restructuring and Catalytic Performance of Oxide Nanocrystals†

Catalysts forheterogeneous catalytic reactions operate under pressures upto several hundred atmospheres and at temperatures up toseveral hundred degrees Celsius, so that the catalyst nano-particles caneasily undergosurfacerestructuringtoadopt thethermodynamically most stable structure. Thus, it is crucial toexplore the restructuring process of catalyst surfaces underthe reaction conditions to understand catalytic processes atthe microscopic level; moreover, it is desirable to tune thecatalytic performance of the catalyst nanoparticles by con-trolling the surface restructuring process in reactive atmos-pheres.The direct study of catalyst nanoparticles is challengingbecause of the complexity of their structures. Model catalystssuch as single crystals and vicinal surfaces have beenextensively investigated to understand the surface restructur-ing phenomenon, and these studies have provided deepinsights. Strongly chemisorbed adsorbates are well known toinduce structural changes to metal surfaces.

Compositions, Structures, and Catalytic Activities of CeO2@Cu2O Nanocomposites Prepared by the Template-Assisted Method

CeO2@Cu2O nanocomposites were prepared from Cu2O cubes and octahedra by the template-assisted method involving the liquid (Ce(IV))-solid (Cu2O) interfacial reaction. Their compositions, structures, and catalytic activities in CO oxidation were studied in detail. Under the same reaction conditions, CeO2@Cu2O nanocomposites prepared from cubic and octahedral Cu2O templates exhibit different compositions and structures. With an increasing amount of Ce(IV) reactant, a smooth CeO2-CuOx shell develops on the surface of Cu2O cubes and eventually void cubic core/multishell Cu2O/CeO2-CuOx nanocomposites form; however, a rough CeO2-CuOx shell develops on the surface of Cu2O octahedra, and eventually hollow octahedral CeO2-CuOx nanocages form. The formation of different compositions and structures of CeO2@Cu2O nanocomposites was correlated with the different exposed crystal planes and surface reactivities of Cu2O cubes and octahedra. The catalytic activity of CeO2@Cu2O nanocomposites in CO oxidation depends on their compositions and structures. The most active CeO2@Cu2O nanocomposites become active at 70 °C and achieve a 100% CO conversion at 170 °C. These results broaden the versatility of Cu2O nanocrystals as the sacrificial template for the fabrication of novel nanocomposites with core/shell and hollow nanostructures and exemplify the morphology effect of Cu2O nanocrystals in liquid-solid interfacial reactions with respect to the composition, structure, and properties of nanocomposites prepared by the template-assisted method.

Catalytically active structures of SiO2-supported Au nanoparticles in low-temperature CO oxidation

Various Au/SiO2 catalysts have been prepared by the deposition–precipitation method followed by calcination in air or reduction in H2. The structures of supported Au nanoparticles were characterized in detail by XRD, TEM, XPS, in situ XANES and operando DRIFTS of CO chemisorption, and their catalytic activity in CO oxidation was evaluated. Calcined in air, the gold precursor decomposes into Au(I) species at low temperatures and further to Au(0) at elevated temperatures, forming supported Au nanoparticles mostly larger than 4.5 nm. Reduced in H2, the gold precursor can be facilely reduced to Au(0) at low temperatures, forming supported Au nanoparticles with different size distributions depending on the reduction temperature. Supported Au nanoparticles around 3–4.5 nm with both abundant low-coordinated Au atoms and bulk Au-like electronic structure effectively chemisorb CO and catalyze CO oxidation at room temperature (RT). Larger supported Au nanoparticles with bulk Au-like electronic structure but few low-coordinated Au atoms do not chemisorb CO and catalyze CO oxidation at RT, and finer supported Au nanoparticles with abundant low-coordinated Au atoms but bulk Au-unlike electronic structure also do not chemisorb CO and catalyze CO oxidation at RT. These results provide solid and comprehensive experimental evidence that supported Au nanoparticles with both abundant low-coordinated Au atoms and bulk Au-like electronic structure are the catalytic active structures for catalyzing CO oxidation at RT without the involvement of oxide supports. The density of low-coordinated Au atoms increases with the decrease of their size, but their electronic structure eventually deviates from bulk Au-like electronic structure; therefore, the catalytic activity of SiO2-supported Au nanoparticles in low-temperature CO oxidation inevitably exhibits a volcano-shaped dependence on their size with the optimum size between 3 and 4.5 nm.

Crystal‐Plane‐Controlled Surface Chemistry and Catalytic Performance of Surfactant‐Free Cu2O Nanocrystals

Surfactant-free Cu2 O nanocrystals, including cubes exposing {100} crystal planes, octahedra exposing {111} crystal planes, and rhombic dodecahedra exposing {110} crystal planes, were used as model catalysts to study the effect of the crystal plane on the surface chemistry and catalytic performance for CO oxidation of Cu2 O nanocrystals. The catalytic performance follows the order of octahedra rhombic dodecahedra>cubes; this suggests that Cu2 O(111) is most active in catalyzing CO oxidation among Cu2 O (111), (110), and (100) surfaces. CO temperature-programmed reduction results demonstrate that Cu2 O octahedra are the most easily reduced of the Cu2 O cubes, octahedra, and rhombic dodecahedra. Diffuse reflectance FTIR spectra show that CO chemisorption on Cu2 O nanocrystals depends on their shape and the chemisorption temperature. CO chemisorption is strongest on rhombic dodecahedra at 30°C, but at 150°C on octahedra. Both the reducibility and chemisorption ability of various Cu2 O nanocrystals toward CO are consistent with their catalytic performance in CO oxidation. The observed surface chemistry and catalytic performance in CO oxidation of various Cu2 O nanocrystals can be well correlated with their exposed crystal plane and surface composition/structure. Cu2 O octahedra expose the {111} crystal plane with coordinated, unsaturated Cu(I) sites, and thus, are most active in chemisorbing CO and catalyzing CO oxidation. These results nicely demonstrate the crystal-plane-controlled surface chemistry and catalytic performance of oxide catalysts.

Reduction of Cu2O nanocrystals: reactant-dependent influence of capping ligands and coupling between adjacent crystal planes

A systematic study of the reduction behavior of various types of uniform Cu2O nanocrystals reveals that the surface blocking effect exerted by the capping ligand on the reduction behavior of the Cu2O nanocrystals depends not only on the type of capping ligand but also on the reactant and that coupling between adjacent crystal planes occurs for Cu2O nanocrystals exposing two types of crystal planes during the reduction reactions and exerts great influence on the reduction kinetics of the involved crystal planes.

Evolution of surface and bulk structures of CexTi1-xO2 oxide composites

Abstract A series of Ce x Ti 1- x O 2 oxide composites were synthesized using a coprecipitation method, and their structures were investigated using X-ray diffraction, N 2 adsorption-desorption isotherms, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, H 2 temperature-programmed reduction, selective chemisorption of methyl orange, and isoelectric point measurements. The selective chemisorption of methyl orange and isoelectric point measurements successfully characterized the outmost surface structures of the Ce x Ti 1- x O 2 oxide composites, and the term “equivalent CeO 2 surface coverage” in a monolayer was introduced to describe the outmost surface compositions. Ce x Ti 1- x O 2 oxide composites with x ≥ 0.7 form a cubic fluorite phase solid solution, the Ce 0.3 Ti 0.7 O 2 oxide composite is a pure monoclinic compound, and the other oxide composites have mixed phase structures. The outmost surface structure evolves in a different way from the bulk structure. A cubic fluorite Ce 0.7 Ti 0.3 O 2 solid solution partially undergoes cubic fluorite solid solution-to-monoclinic Ce 0.3 Ti 0.7 O 2 phase transition on its outmost surface, and Ce 0.3 Ti 0.7 O 2 on the outmost surface of Ce 0.7 Ti 0.3 O 2 grows from the surface to the bulk. Cubic fluorite Ce x Ti 1- x O 2 solid solutions exhibit good reducibilities at relatively low temperatures, whereas Ce 0.3 Ti 0.7 O 2 exhibits good reducibility at relatively high temperatures. These results provide comprehensive and in-depth structural information for important Ce x Ti 1- x O 2 oxide composites.

Article (Dedicated to Professor Yi Chen on the occasion of his 80th birthday)Evolution of surface and bulk structures of CexTi1-xO2 oxide composites

Abstract A series of Ce x Ti 1- x O 2 oxide composites were synthesized using a coprecipitation method, and their structures were investigated using X-ray diffraction, N 2 adsorption-desorption isotherms, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, H 2 temperature-programmed reduction, selective chemisorption of methyl orange, and isoelectric point measurements. The selective chemisorption of methyl orange and isoelectric point measurements successfully characterized the outmost surface structures of the Ce x Ti 1- x O 2 oxide composites, and the term “equivalent CeO 2 surface coverage” in a monolayer was introduced to describe the outmost surface compositions. Ce x Ti 1- x O 2 oxide composites with x ≥ 0.7 form a cubic fluorite phase solid solution, the Ce 0.3 Ti 0.7 O 2 oxide composite is a pure monoclinic compound, and the other oxide composites have mixed phase structures. The outmost surface structure evolves in a different way from the bulk structure. A cubic fluorite Ce 0.7 Ti 0.3 O 2 solid solution partially undergoes cubic fluorite solid solution-to-monoclinic Ce 0.3 Ti 0.7 O 2 phase transition on its outmost surface, and Ce 0.3 Ti 0.7 O 2 on the outmost surface of Ce 0.7 Ti 0.3 O 2 grows from the surface to the bulk. Cubic fluorite Ce x Ti 1- x O 2 solid solutions exhibit good reducibilities at relatively low temperatures, whereas Ce 0.3 Ti 0.7 O 2 exhibits good reducibility at relatively high temperatures. These results provide comprehensive and in-depth structural information for important Ce x Ti 1- x O 2 oxide composites.