Ensheng Zhan

Au/ZrO2 catalysts for low-temperature water gas shift reaction: Influence of particle sizes

The size effects of Au and ZrO2 particles on the structural property and the catalytic performance of Au/ZrO2 catalysts for the water gas shift reaction were extensively investigated. It was found that the Au-ZrO2 contact boundaries played essential roles in determining the catalytic reactivity. By keeping the size of Au particle to be ∼3 nm, the increase in the particle size of ZrO2 from ∼7 nm to ∼55 nm caused significant decrease in the reaction rate. When the particle size of ZrO2 was fixed at ∼20 nm, the conversion of CO decreased greatly with increasing the size of gold particle from 2.9 to 6.2 nm. IR spectroscopy and kinetic study revealed that the water gas shift reaction occurred at the Au-ZrO2 contact boundaries, where CO is adsorbed on the Au species and H2O is activated on the surface of ZrO2 through the formation of formate species, acting as key reaction intermediates.

Heterogeneous asymmetric hydrogenation over chiral molecule-modified metal particles

Heterogeneous asymmetric hydrogenation of CO and CC bonds over chiral molecule-modified metal particles represents an important route for the production of chiral compounds. In this mini review, we first briefly introduced the background of heterogeneous asymmetric hydrogenation and the remaining challenges in this field. Then, we highlighted recent important progress in the understanding of the reaction mechanism in terms of the acid–base properties of supports and the effects of the size/shape of metal particles. Finally, we summarized the possible models proposed for the substrate–modifier adsorption and their interaction in asymmetric hydrogenation reactions.

Anatase TiO2 hollow nanosheets: dual roles of F−, formation mechanism, and thermal stability

Anatase TiO2 hollow nanosheets with a width of 550 nm, a thickness of 100 nm, and a hole diameter of 350 nm were hydrothermally fabricated in an aqueous solution containing NH4VO3, HF, and HCl at an appropriate composition. Structural analyses on the products obtained at different intervals during the synthesis revealed that the formation of the anatase TiO2 hollow nanosheets consisted of three steps: oriented assembly of square-like NH4TiOF3 nanoparticles, topochemical conversion of NH4TiOF3 to anatase TiO2, and selective etching by F− on the flat nanosheets. The fluorine anion was involved in the formation of NH4TiOF3 as the key intermediate, it directed the construction of the nanosheet, and participated in the etching process to generate the hollow structure. The resultant anatase TiO2 hollow nanosheets exhibited a rather high thermal stability, maintaining the anatase crystallite structure and the hollow shape up to 1073 K.

Morphology-controlled synthesis of α-MoO3 nanomaterials for ethanol oxidation

α-MoO3 nanobelts of about 8 nm in thickness, 60 nm in width and 0.7–7.5 μm in length were synthesized through a hydrothermal process. It was found that the pH value and the concentration of the surfactant (P123) played crucial roles in determining the crystal phase and the shape of the MoO3 nanomaterials. Small-sized nanobelts were produced at a lower pH value and a suitable P123 concentration in the synthetic solution. Further increase in the surfactant concentration directed self-assembly of the nanobelts into hierarchical microflowers. The α-MoO3 nanobelts showed a relatively higher activity but a lower selectivity toward acetaldehyde in ethanol oxidation than the microflowers, primarily due to the large exposure of the (010) planes.

The shape effect of TiO2 in VOx/TiO2 catalysts for selective reduction of NO by NH3

Anatase TiO2 nanosheets exposing 74% of {001} facets and nanospindles exposing 81% of {101} facets were hydrothermally synthesized with the aid of F− and CH3COO−, respectively. Upon vanadia loading at a monolayer amount level, the {001} facets on TiO2 nanosheets favored the deposition of octahedral vanadia species, but the {101} facets on TiO2 nanospindles resulted in the formation of tetrahedral vanadia species. The shape effect of TiO2, in terms of its predominantly exposed crystal facets, on the catalytic performance of VOx/TiO2 samples for selective reduction of NO with NH3 was examined. The octahedral vanadia species on TiO2 nanosheets showed a significantly higher activity than the tetrahedral vanadia species on TiO2 nanospindles.

Coking on micrometer- and nanometer-sized mordenite during dimethyl ether carbonylation to methyl acetate

As compared to conventional micrometer-sized mordenite particles, nanometer-sized mordenrte gave a much higher activity and better stability in dimethyl ether carbonylation to methyl acetate by suppressing the deposition of hard coke. A structural analysis revealed that intra-crystalline diffusion limitation of the reactants and products to and from the active sites inside the channels was greatly decreased by reducing the zeolite size to the nanometer size.

Vanadia directed synthesis of anatase TiO2 truncated bipyramids with preferential exposure of the reactive {001} facet

Anatase TiO2 truncated bipyramids that dominantly exposed the reactive {001} facet were hydrothermally synthesized using vanadia as the structure-directing agent. The exposed fraction of the {001} facet approached 53% upon adjusting the V/Ti molar ratio of the synthetic solution. Mechanistic investigation, together with control experiments, verified that vanadia stabilized the {001} facet and induced the construction of the truncated bipyramids. After calcination at 723 K in air, the resulting VOx/TiO2 truncated bipyramids effectively catalyzed the selective reduction of NO by ammonia.

Synthesis of mordenite nanosheets with shortened channel lengths and enhanced catalytic activity

Mordenite nanosheets of 20–40 nm thickness significantly shortened the lengths of parallel 12-member and 8-member ring channels and greatly facilitated the diffusion of molecules inside the micropores, and therefore remarkably enhanced the catalytic activity for the carbonylation of dimethyl ether.

Enantioselective hydrogenation of α,β-unsaturated carboxylic acids on Pd nanocubes

Pd nanocubes of 6–19 nm in size were synthesized using a seeded growth method and examined for enantioselective hydrogenation of α,β-unsaturated carboxylic acids. It was found that the Pd nanocubes had two types of active sites on the planes and at the edges, respectively. Small nanocubes having a higher edge/plane ratio were more active in enantioselective hydrogenation of α,β-unsaturated carboxylic acids, but afforded a lower enantioselectivity because their sharp edges could not offer stable adsorption of the chiral modifier and the reaction intermediates. In contrast, large nanocubes with a higher fraction of flat planes provided a higher enantioselectivity but a much lower activity.

Carbonylation of dimethyl ether over Co-HMOR

Incorporation of Co2+ into the framework of HMOR significantly enhanced the activity for the carbonylation of dimethyl ether to methyl acetate. About 68% of the Co2+ cations are located at site A in the 8-membered ring (8-MR) pores, while 32% of the metal cations are incorporated at site E of the 12-MR pores. Although the amount of the Bronsted acid sites in the 8-MR pores that are intrinsically active for DME carbonylation decreased considerably upon Co-doping, the conversion of DME increased remarkably, almost doubled. The promotional role of Co2+ in the 8-MR channels was to facilitate the adsorption/activation of both CO and DME molecules. Meanwhile, the Co2+ cations located in the 12-MR channels effectively suppressed coke deposition and thus improved the stability of the Co-HMOR catalyst.