Wenjie Shen

Stabilized Gold Nanoparticles on Ceria Nanorods by Strong Interfacial Anchoring

Au/CeO(2) catalysts are highly active for low-temperature CO oxidation and water-gas shift reaction, but they deactivate rapidly because of sintering of gold nanoparticles, linked to the collapse or restructuring of the gold-ceria interfacial perimeters. To date, a detailed atomic-level insight into the restructuring of the active gold-ceria interfaces is still lacking. Here, we report that gold particles of 2-4 nm size, strongly anchored onto rod-shaped CeO(2), are not only highly active but also distinctively stable under realistic reaction conditions. Environmental transmission electron microscopy analyses identified that the gold nanoparticles, in response to alternating oxidizing and reducing atmospheres, changed their shapes but did not sinter at temperatures up to 573 K. This finding offers a new strategy to stabilize gold nanoparticles on ceria by engineering the gold-ceria interfacial structure, which could be extended to other oxide-supported metal nanocatalysts.

Rod-Shaped Fe2O3 as an Efficient Catalyst for the Selective Reduction of Nitrogen Oxide by Ammonia

[zhang, bingsen; su, dang sheng] chinese acad sci, inst met res, shenyang natl lab mat sci, shenyang 110016, peoples r china. [mou, xiaoling; li, yong; wei, xuejiao; shen, wenjie] chinese acad sci, dalian inst chem phys, state key lab catalysis, dalian 116023, peoples r china. [yao, lide; su, dang sheng] max planck soc, dept inorgan chem, fritz haber inst, d-14195 berlin, germany.;su, ds (reprint author), chinese acad sci, inst met res, shenyang natl lab mat sci, shenyang 110016, peoples r china.;dangsheng@fhi-berlin.mpg.de; shen98@dicp.ac.cn

Tuning crystal-phase and shape of Fe2O3 nanoparticles for catalytic applications

The design and fabrication of solid nanomaterials is the core issue in heterogeneous catalysis to achieve desired performance. Traditionally, the main theme is to reduce the size of the catalyst particles as small as possible for maximizing the number of active sites. In recent years, the rapid advancement in materials science has enabled us to fabricate catalyst particles with tunable morphologies. Consequently, both size modulation and morphology control of catalyst particles at the nanometer level can be achieved independently or synergistically to optimize their catalytic performance. In particular, morphological control of catalyst nanoparticles can selectively expose reactive crystal planes, and hence drastically promote their reaction efficiency. We highlight, in this review article, the recent progress on crystal-phase and shape control of Fe2O3 nanomaterials that act as essential components in heterogeneous catalysts. We initially summarize the major synthetic strategies of shape-controlled α- and γ- Fe2O3 nanomaterials. We then survey morphology- and crystal-phase-dependent nanocatalysis of these ferric oxides for a couple of chemical reactions. In this context, we stress that the catalytic property of Fe2O3 nanomaterials is closely linked to the surface atomic configurations that are determined both by the shape and the crystal-phase. Finally, we provide our perspectives on the future development of Fe2O3 nanomaterials through tailoring their shape and crystal-phase. The fundamental understanding of crystal-phase- and morphology-tunable nanostructures that are enclosed by reactive facets is expected to direct the development of highly efficient nanocatalysts.

Stability Enhancement of H-Mordenite in Dimethyl Ether Carbonylation to Methyl Acetate by Pre-adsorption of Pyridine

The carbonylation of dimethyl ether to methyl acetate over H-mordenite (HMOR) and pyridine-modified HMOR was compared. The catalytic stability of HMOR was improved significantly by pyridine pre-adsorption, and a yield of methyl acetate ∼30% was still obtained after 48 h on stream at 473 K. In situ infrared spectroscopy and ammonia temperature-programmed desorption revealed that pyridine preferentially occupied the acidic sites in 12-membered ring pores but not the acidic sites in 8-membered ring pores. 129Xe NMR studies suggested that the channels of HMOR were blocked by coke in the reaction but those in the pyridine-modified HMOR were not. The acidic sites in the 12-membered ring pores were responsible for the deactivation of HMOR, and the reaction can be directed to occur mainly on the acidic sties in the 8-membered ring pores by the selective adsorption of pyridine in the 12-membered ring pores.

Tuning the shape of ceria nanomaterials for catalytic applications

Abstract The design and fabrication of catalytic materials is a key issue in heterogeneous catalysis to achieve desired performance. Traditionally, the main theme is to reduce the size of the catalyst particles as small as possible for increasing the number of active sites. In recent years, the rapid advancement in materials science has enabled us to fabricate catalyst particles with tunable shape at nanometer level. Through morphology control of nanoparticles by exposing highly reactive crystal planes, their catalytic properties can be drastically enhanced. Therefore, both size modulation and shape control of catalyst nanoparticles can be achieved independently or synergistically to optimize their catalytic behavior. We highlight, in this review, the recent progress in shape control of CeO 2 materials that are widely used as crucial components or structural and electronic promoters in heterogeneous catalysts. We first summarize the major synthetic strategies and characteristics of shape-controlled CeO 2 nanomaterials. We then survey morphology-dependent nanocatalysis of CeO 2 and Au-CeO 2 catalysts. We understand now that the enhanced catalytic property of the Au-CeO 2 system is closely related to the unique interaction between the gold nanoparticles and the ceria support; such an interaction originates from the particular shape of ceria, especially the exposed facets. Finally, we present our understanding of the morphology-dependent nanocatalysis and provide our perspectives on their future potential and development. The fundamental understanding of the nature of the intrinsic active sites of the shape-tunable ceria nanostructures, enclosed by reactive crystal planes/facets with unique properties, is expected to provide highly efficient nanocatalysts for practical applications.

Synthesis of Hollow Co Structures with Netlike Framework

Hollow Co structures with the size of 4-10 microm were fabricated by a simple solvothermal process using stearic acid as surfactant. Cobalt stearate formed at the initial stage and further self-assembled to micelles as a soft template. This precursor controlled the growth rate of Co crystal to form the primary nanorods attaching on the surface of the micelles. These nanorods then assembled into hollow Co spheres with a dense shell. Because of the acidic etching effect of stearic acid, however, the hollow Co spheres were further developed to Co nests constructed by netlike frameworks. Stearic acid acted as structure-directing and acidic etching agents in the formation of these novel hollow structures constructed by nanorods. The Co nests showed quite promising catalytic performance in hydrogenolysis of glycerol, demonstrating the potential application in heterogeneous catalysis.

Redox properties and catalytic performance of ceria–zirconia nanorods

The redox properties and catalytic performance of Ce1−xZrxO2 (0 ≤ x ≤ 0.2) nanorods, mainly exposing {110} and {100} planes, were comparatively examined with spherical Ce1−xZrxO2 nanoparticles that predominantly exposed {111} planes. The CeO2 nanorods had a superior redox property and much higher activity towards CO oxidation than the CeO2 nanoparticles, primarily because of the preferential exposure of the reactive {110} planes. However, this shape effect was weakened considerably in Ce1−xZrxO2 (x = 0.05–0.20) nanomaterials. ZrO2-doping promoted the reducibility of the nanoparticles more signifciantly than that of the nanorods, involving different rate-determining steps in the reduction process. The activity for CO oxidation enhanced with increasing ZrO2 content on the nanoparticles but decreased over the nanorods. These results demonstrate that the shape effect of Ce1−xZrxO2 nanomaterials is associated with the amount of zirconia that is incorporated into the ceria lattice.

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.

Structural Properties and Catalytic Activity of Sr-Substituted LaFeO3 Perovskite

Partial substitution of La3+ by Sr2+ in LaFeO3 resulted in significant changes in its structure and catalytic activity. The perovskite structure was changed from orthorhombic in LaFeO3 to nearly cubic in La0.8Sr0.2FeO3. Replacement of La3+ by Sr2+ induced a positive charge deficiency that was compensated for by the oxidation of some Fe3+ to Fe4+ and the generation of oxygen vacancies, which greatly promoted the reducibility of the perovskite. La0.8Sr0.2FeO3 gave considerably enhanced activity in CO oxidation and methane combustion because the oxygen vacancies accelerated the dissociation of gaseous oxygen on the surface in CO oxidation and facilitated the diffusion of lattice oxygen from the bulk to the surface during CH4 combustion.

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.