Yunsheng Ma

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.

Photocatalytic Cross-Coupling of Methanol and Formaldehyde on a Rutile TiO2(110) Surface

The photocatalytic oxidation of methanol on a rutile TiO2(110) surface was studied by means of thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS). The combined TDS and XPS results unambiguously identify methyl formate as the product in addition to formaldehyde. By monitoring the evolution of various surface species during the photocatalytic oxidation of methanol on TiO2(110), XPS results give direct spectroscopic evidence for the formation of methyl formate as the product of photocatalytic cross-coupling of chemisorbed formaldehyde with chemisorbed methoxy species and clearly demonstrate that the photocatalytic dissociation of chemisorbed methanol to methoxy species occurs and contributes to the photocatalytic oxidation of methanol. These results not only greatly broaden and deepen the fundamental understanding of photochemistry of methanol on the TiO2 surface but also demonstrate a novel green and benign photocatalytic route for the synthesis of esters directly from alcohols or from alcohols and aldehydes.

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.

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.

Reaction mechanism of WGS and PROX reactions catalyzed by Pt/oxide catalysts revealed by an FeO(111)/Pt(111) inverse model catalyst

We have employed XPS and TDS to study the adsorption and surface reactions of H2O, CO and HCOOH on an FeO(111)/Pt(111) inverse model catalyst. The FeO(111)-Pt(111) interface of the FeO(111)/Pt(111) inverse model catalyst exposes coordination-unsaturated Fe(II) cations (Fe(II)CUS) and the Fe(II)CUS cations are capable of modifying the reactivity of neighbouring Pt sites. Water facilely dissociates on the Fe(II)CUS cations at the FeO(111)-Pt(111) interface to form hydroxyls that react to form both water and H2 upon heating. Hydroxyls on the Fe(II)CUS cations can react with CO(a) on the neighbouring Pt(111) sites to produce CO2 at low temperatures. Hydroxyls act as the co-catalyst in the CO oxidation by hydroxyls to CO2 (PROX reaction), while they act as one of the reactants in the CO oxidation by hydroxyls to CO2 and H2 (WGS reaction), and the recombinative reaction of hydroxyls to produce H2 is the rate-limiting step in the WGS reaction. A comparison of reaction behaviors between the interfacial CO(a) + OH reaction and the formate decomposition reaction suggest that formate is the likely surface intermediate of the CO(a) + OH reaction. These results provide some solid experimental evidence for the associative reaction mechanism of WGS and PROX reactions catalyzed by Pt/oxide catalysts.

Preparation and properties of the SmOx/Rh(100) model surface

The preparation of SmOx/Rh(100) and CO adsorption on this model surface have been investigated with Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption spectroscopy (TDS). The oxygen adsorption on the SmRh alloy surface leads to the aggregation of Sm on the surface. The thermal treatment of this oxidized surface induces the further agglomeration of SmOx on the Rh(100) surface. Compared with CO TDS on the clean Rh(100) surface, three additional CO desorption peaks can be observed at 176, 331 and 600 K on the SmOx/Rh(100) surface. The CO desorption peak at 176 K may originate from CO adsorbed on SmOx islands, while the appearance of the CO adsorption peaks at 331 and 600 K, depending on the oxidation state of Sm, is attributed to CO species located at the interface of SmOx/Rh(100).

Catalytic Performance of MnOx Nanorods in Aerobic Oxidation of Benzyl Alcohol

Various manganese oxide nanorods with similar one‐dimensional morphology were prepared by calcination of MnOOH nanorods under different gas atmosphere and at different temperatures, which were synthesized by a hydrothermal route. The morphology and structure of MnOx catalysts were characterized by a series of techniques including X‐ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, and temperature programmed reduction (TPR). The catalytic activities of the prepared MnOx nanorods were tested in the liquid phase aerobic oxidation of benzyl alcohol, which follow a sequence as MnO2>Mn2O3≈Mn3O4>MnOOH with benzaldehyde being the main product. On the basis of H2‐TPR results, the superior activity of MnO2 is ascribed to its lower reduction temperature and therefore high oxygen mobility and excellent redox ability. Moreover, a good recycling ability was observed over MnO2 catalysts by simply thermal treatment in air.