A clear understanding of the growth mechanism involved in the shape-controlled synthesis of noble-metal nanocrystals with concave surfaces can provide useful information for the rational design of novel anisotropic nanostructures with controllable properties. In this paper, we conducted a systematic study of the detailed growth mechanism of the Pd arrow-headed tripods and revealed how the formation of the concave Pd nanocrystals was collectively controlled by the reduction kinetics, concentration gradient of Pd precursors, and surface diffusion of atoms. The formation of the arrow-headed tripods can be attributed to an auto-catalytic tip overgrowth process, where the Pd triangular nanoplate seeds formed under a suitably slow reduction rate can auto-catalyze the dehydrogenation of benzyl alcohol to generate hydrogen atoms [H]. The presence of [H] further dramatically accelerates the reduction of Pd(acac)2, which introduces a concentration gradient of Pd precursors in our non-stirring synthesis system and facilitates the kinetically-controlled tip overgrowth under a concentration gradient to form tripods with troughs on the arms. The final shapes of the concave nanocrystals depend on the relative rate of atom deposition and surface diffusion of atoms, which can be tuned by manipulating the reaction conditions such as the reaction temperature and the stirring conditions. This study presents a new possibility for the rational synthesis of various Pd nanostructures by manipulating the auto-catalytic process and tuning the relative rate of atom deposition and surface diffusion of atoms, which provides useful information for understanding the growth mechanism and the design of other anisotropic noble-metal nanostructures.

Single crystalline palladium arrow-headed tripods prepared via a simple one-pot strategy exhibit high electro-activity in formic acid oxidation, which could be a promising anodic catalyst for direct formic acid fuel cells.

This review mainly summarizes our latest progress in the preparation of free-standing mesoporous metal catalysts and their applications in heterogeneous enantioselective hydrogenations. Owing to exclusion of the support effect, free-standing mesoporous metal could be a suitable experimental model for a mechanistic understanding, which would have significant benefits for the design of real catalysts with high performance. We start with a brief survey on the main synthetic approaches available for the preparation of free-standing mesoporous metal catalysts. Next, we select a few mesoporous metal catalysts to demonstrate their improved catalytic performance in the enantioselective hydrogenation of ketones or aromatic ketones. The factors influencing the activity and enantioselectivity, such as the modification conditions and confinement effect, are discussed. Finally, we introduce DFT calculations to gain insight into the molecular interaction involved in the adsorption process. We conclude the review with a personal perspective on the challenges and opportunities for the future development of efficient heterogeneous enantioselective metal catalysts.

Mesoporous Ni–P amorphous alloy nanospheres with controllable sizes and compositions were synthesized by chemical reduction of Ni(OH)2 colloidal particles co-assembling with surfactant hexadecyl-trimethyl-ammonium bromide in liquid crystal mesophase using hypophosphite as reductant. The effects of the synthesis conditions on the particle size, composition and mesostructure of the mesoporous Ni–P nanospheres were systematically studied. It was found that the size of the mesoporous Ni–P nanospheres could be tuned from 35 to 90 nm by changing the reduction temperature, and the phosphorus content of the Ni–P products could be adjusted in the range of 20.1 to 27.6 % by changing the molar ratio of H2PO2−/Ni2+. The active surface area and the thermal stability of the mesoporous Ni–P nanosphere catalyst are much higher than those for the conventional nonporous Ni–P amorphous alloy. In the liquid phase hydrogenation of nitrobenzene, the typical mesoporous Ni–P nanosphere catalyst exhibits much higher activity and better selectivity than the conventional nonporous Ni–P. The correlation between the catalytic performance and the structural properties is discussed based on the results of detailed characterization.

Three-dimensionally ordered mesoporous Pd networks fabricated by a simple reduction method in solution using a face centered cubic silica super crystal as template exhibit high electroactivity in formic acid oxidation.

•Free-standing mesoporous Pd was used in asymmetric hydrogenation of aromatic ketone.•Enantioselectivity (ee) was effectively tuned by altering the confinement effect.•Confinement effect was optimized by good control of nanoporosity and lattice property.•Optimized Pd catalyst exhibited 40–73% ee at 273 K under atmospheric pressure of H2.•DFT study found ee was linearly correlated with ΔE between prochiral-R & -S complexes.The confinement effect on enantioselective hydrogenation of acetophenone and its derivatives over free-standing mesoporous Pd network catalysts was systematically studied. It was found for the first time that the enantiomeric excess (ee) could be effectively tuned by altering the confinement effect optimized by precise control of the topology, pore size, and lattice structure of mesoporous Pd catalysts. The double gyroid structure with proper pore size and desired lattice structure formed by KBH4 reduction provided suitable microenvironment to generate optimized confinement effect. The optimized catalyst exhibited ee of 40–73% at 273 K under atmospheric pressure of H2. DFT study revealed that the major enantiomeric product could be predicted by comparing relative energies of prochiral-R and -S complexes formed by acetophenone derivatives with S-proline. The energetically favored complex led to the formation of the corresponding enantiomer in excess upon hydrogenation, and ee was found to be linearly correlated with the energy difference between prochiral-R and -S complexes.Download high-res image (293KB)Download full-size image

This review mainly summarizes our latest progress in the preparation of free-standing mesoporous metal catalysts and their applications in heterogeneous enantioselective hydrogenations. Owing to exclusion of the support effect, free-standing mesoporous metal could be a suitable experimental model for a mechanistic understanding, which would have significant benefits for the design of real catalysts with high performance. We start with a brief survey on the main synthetic approaches available for the preparation of free-standing mesoporous metal catalysts. Next, we select a few mesoporous metal catalysts to demonstrate their improved catalytic performance in the enantioselective hydrogenation of ketones or aromatic ketones. The factors influencing the activity and enantioselectivity, such as the modification conditions and confinement effect, are discussed. Finally, we introduce DFT calculations to gain insight into the molecular interaction involved in the adsorption process. We conclude the review with a personal perspective on the challenges and opportunities for the future development of efficient heterogeneous enantioselective metal catalysts.

Three-dimensionally ordered mesoporous Pd networks fabricated by a simple reduction method in solution using a face centered cubic silica super crystal as template exhibit high electroactivity in formic acid oxidation.

Single crystalline palladium arrow-headed tripods prepared via a simple one-pot strategy exhibit high electro-activity in formic acid oxidation, which could be a promising anodic catalyst for direct formic acid fuel cells.