Zhiquan Jiang

Morphological evolution of Cu2O nanocrystals in an acid solution: stability of different crystal planes.

The morphological evolution of uniform Cu(2)O nanocrystals with different morphologies in a weak acetic acid solution (pH = 3.5) has been studied for cubic, octahedral, rhombic dodecahedral, {100} truncated octahedral, and {110} truncated octahedral nanocrystals. Cu(2)O nanocrystals undergo oxidative dissolution in weak acid solution, but their morphological changes depend on the exposed crystal planes. We found that the stability of Cu(2)O crystal planes in weak acid solution follows the order of {100} ≫ {111} > {110} and determines how the morphology of Cu(2)O nanocrystals evolves. The stable {100} crystal planes remain, and new {100} facets form at the expense of the less stable {111} and {110} crystal planes on the surface of Cu(2)O nanocrystals. Density functional theory calculations reveal that the Cu-O bond on Cu(2)O(100) surface has the shortest bond length. These results clearly exemplify that the morphology of inorganic crystals will evolve with the change of local chemical environment, shedding light on fundamentally understanding the morphological evolution of natural minerals and providing novel insights into the geomimetic synthesis of inorganic materials in the laboratory.

Size-dependent interaction of the poly(N-vinyl-2-pyrrolidone) capping ligand with Pd nanocrystals.

Pd nanocrystals were prepared by the reduction of a H(2)PdCl(4) aqueous solution with C(2)H(4) in the presence of different amounts of poly(N-vinyl-2-pyrrolidone) (PVP). Their average size decreases monotonically as the PVP monomer/Pd molar ratio increases up to 1.0 and then does not vary much at higher PVP monomer/Pd molar ratios. Infrared spectroscopy and X-ray photoelectron spectroscopy results reveal the interesting size-dependent interaction of PVP molecules with Pd nanocrystals. For fine Pd nanocrystals capped with a large number of PVP molecules, each PVP molecule chemisorbs with its oxygen atom in the ring; for large Pd nanocrystals capped by a small number of PVP molecules, each PVP molecule chemisorbs with both the oxygen atom and nitrogen atom in the ring, which obviously affects the structure of chemisorbed PVP molecules and even results in the breaking of involved C-N bonds of some chemisorbed PVP molecules. Charge transfer always occurs from a chemisorbed PVP ligand to Pd nanocrystals. These results provide novel insights into the PVP-metal nanocrystal interaction, which are of great importance in the fundamental understanding of surface-mediated properties of PVP-capped metal nanocrystals.

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.

Crystal‐Plane‐Controlled Selectivity of Cu2O Catalysts in Propylene Oxidation with Molecular Oxygen

The selective oxidation of propylene with O2 to propylene oxide and acrolein is of great interest and importance. We report the crystal-plane-controlled selectivity of uniform capping-ligand-free Cu2 O octahedra, cubes, and rhombic dodecahedra in catalyzing propylene oxidation with O2 : Cu2 O octahedra exposing {111} crystal planes are most selective for acrolein; Cu2 O cubes exposing {100} crystal planes are most selective for CO2 ; Cu2 O rhombic dodecahedra exposing {110} crystal planes are most selective for propylene oxide. One-coordinated Cu on Cu2 O(111), three-coordinated O on Cu2 O(110), and two-coordinated O on Cu2 O(100) were identified as the catalytically active sites for the production of acrolein, propylene oxide, and CO2 , respectively. These results reveal that crystal-plane engineering of oxide catalysts could be a useful strategy for developing selective catalysts and for gaining fundamental understanding of complex heterogeneous catalytic reactions at the molecular level.

Direct Evidence for the Interfacial Oxidation of CO with Hydroxyls Catalyzed by Pt/Oxide Nanocatalysts

By rational design of the FeO(111)/Pt(111) inverse model catalyst and the control experiments, we report for the first time direct experimental evidence for the interfacial CO(ads) + OH(ads) reaction to produce CO(2) at the Pt-oxide interface at low temperatures, providing deep insights into the reaction mechanism and active site of the important low-temperature water-gas shift and preferential CO oxidation reactions catalyzed by Pt/oxide nanocatalysts at the molecular level.

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.

Restructuring-Induced Activity of SiO2-Supported Large Au Nanoparticles in Low-Temperature CO Oxidation

Large Au nanoparticles with an average size of approximately 10 nm supported on inert SiO(2) become active in low-temperature CO oxidation after the addition of NaNO(3). The catalyst structures have been characterized in detail by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and X-ray absorption spectroscopy. The NaNO(3) additive in Au/SiO(2) catalysts does not lead to the formation of fine Au nanoparticles, which are generally considered to be inevitable in low-temperature CO oxidation catalyzed by gold, nor does it alter the electronic structure of Au. The NaNO(3)-induced restructuring of large Au nanoparticles was proposed to create low-coordinated Au sites on the surface capable of catalyzing low-temperature CO oxidation. These results experimentally prove that the activity of supported Au nanoparticles in low-temperature CO oxidation could solely arise from their geometric structure, which greatly deepens the fundamental understandings of Au nanocatalysis.

Effect of Calcination Temperature on Surface Oxygen Vacancies and Catalytic Performance Towards CO Oxidation of Co3O4 Nanoparticles Supported on SiO2

Co3O4/SiO2 catalysts for CO oxidation were prepared by conventional incipient wetness impregnation followed by calcination at various temperatures. Their structures were characterized with X-ray diffraction (XRD), laser Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR) and X-ray absorption fine structure (XAFS) spectroscopy. Both XRD and Raman spectroscopy only detect the existence of Co3O4 crystallites in all catalysts. However, XPS results indicate that excess Co2+ ions are present on the surface of Co3O4 in Co3O4(200)/SiO2 as compared with bulk Co3O4. Meanwhile, TPR results suggest the presence of surface oxygen vacancies on Co3O4 in Co3O4(200)/SiO2, and XAFS results demonstrate that Co3O4 in Co3O4(200)/SiO2 contains excess Co2+. Increasing calcination temperature results in oxidation of excess Co2+ and the decrease of the concentration of surface oxygen vacancies, consequently the formation of stoichiometric Co3O4 on supported catalysts. Among all Co3O4/SiO2 catalysts, Co3O4(200)/SiO2 exhibits the best catalytic performance towards CO oxidation, demonstrating that excess Co2+ and surface oxygen vacancies can enhance the catalytic activity of Co3O4 towards CO oxidation. These results nicely demonstrate the effect of calcination temperature on the structure and catalytic performance towards CO oxidation of silica-supported Co3O4 catalysts and highlight the important role of surface oxygen vacancies on Co3O4.

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.