The Origin of Chemo- and Enantioselectivity in the Hydrogenation of Diketones on Platinum

Revealing Catalytically Relevant Surface Species by Kinetic Isotope Effect Spectroscopy: H-Bonding to Ester Carbonyl of trans-Ethyl Pyruvate Controls Enantioselectivity on a Cinchona-Modified Pt Catalyst

Monitoring active surface species on an operating technical catalyst is a challenging task due to the presence of multiple different adsorption sites and the abundance of bulk species. In this work, kinetic isotope effect (KIE) spectroscopy is introduced to capture the signals of catalytically relevant hydrogenation species from the IR spectroscopic detection in attenuated total reflection mode. The catalytic interface formed between a cinchona-modified Pt/Al₂O₃ catalyst and the solvent toluene is sensitively probed directly at the rate limiting step(s) during the asymmetric hydrogenation of ethyl pyruvate by measuring the effects of substituting H₂ by D₂ kinetically and spectroscopically in the same operando experiment. The application of KIE spectroscopy provides unprecedented molecular level insight into the structure of the diastereomeric intermediate surface complex and the phenomenon rate enhancement, which revolutionizes our understanding of chirally modified metal catalysts.

Self-assembled hybrid metal oxide base catalysts prepared by simply mixing with organic modifiers

Multidentate materials formed by simply mixing heterogeneous and homogeneous components are promising for construction of versatile active sites on the surface of heterogeneous compounds, however, to the best of our knowledge, there are no reports on such materials. Self-assembly of hetero-hybrid catalytic materials occurs when heterogeneous catalysts having adjacent Lewis acid-Lewis base sites are mixed with an organic modifier that contains at least two Lewis base functional groups. Here we demonstrate the strategy by combining cerium oxide and 2-cyanopyridine that self-assembles to form a charge-transfer complex in methanol that exhibits a 2,000-fold increase in reaction rate for hydromethoxylation of acrylonitrile with high selectivity compared with cerium oxide or 2-cyanopyridine alone. The catalytic system is applied to the transesterification and Knoevenagel condensation affording 14-fold and 11-fold higher activity, respectively, than cerium oxide alone. These results demonstrate the potential versatility of the catalytic system and the generality of the catalyst preparation strategy.

Self-assembled heterogeneous/homogeneous hybrid materials may offer ease of preparation and tunable catalytic performance. Here, the authors report the accelerated selective hydromethoxylation of acrylonitrile in the presence of CeO₂ and 2-cyanopyridine, resulting from the proposed supramolecular complex.

Nano-assemblies consisting of Pd/Pt nanodendrites and poly (diallyldimethylammonium chloride)-coated reduced graphene oxide on glassy carbon electrode for hydrogen peroxide sensors

Non-enzymatic hydrogen peroxide (H2O2) sensors were fabricated on the basis of glassy carbon (GC) electrode modified with palladium (Pd) core-platinum (Pt) nanodendrites (Pt-NDs) and poly (diallyldimethylammonium chloride) (PDDA)-coated reduced graphene oxide (rGO). A facile wet-chemical method was developed for preparing Pd core-Pt nanodendrites. In this approach, the growth of Pt NDs was directed by Pd nanocrystal which could be regarded as seed. The PDDA-coated rGO could form uniform film on the surface of GC electrode, which provided a support for Pd core- Pt NDs adsorption by self-assembly. The morphologies of the nanocomposites were characterized by transmission electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction (spectrum). Electrocatalytic ability of the nanocomposites was evaluated by cyclic voltammetry and chronoamperometric methods. The sensor fabricated by Pd core-Pt NDs/PDDA-rGO/GCE exhibited high sensitivity (672.753 μA mM(-1) cm(-2)), low detection limit (0.027 μM), wider linear range (0.005-0.5mM) and rapid response time (within 5s). Besides, it also exhibited superior reproducibility, excellent anti-interference performance and long-term stability. The present work could afford a viable method and efficient platform for fabricating all kinds of amperometric sensors and biosensors.

A simple route to CoFe2O4 nanoparticles with shape and size control and their tunable peroxidase-like activity

A new precursor-mediated growth of monodisperse magnetic CoFe₂O₄ NPs with controlled size and shape which exhibit size and shape dependent peroxidase like activity towards TMB in the presence of H₂O₂.

Adsorption and stability of chiral modifiers based on 1-(1-naphthyl)-ethylamine for Pt catalysed heterogeneous asymmetric hydrogenations

Catalytic and in situ ATR-IR spectroscopic investigations provide mechanistic insight relevant to heterogeneous asymmetric hydrogenation on Pt catalyst using naphthylethylamine-based modifiers.

Chiral Recognition of l-Gramicidine on Chiraly Methionine-Modified Au(111)

A chiral pentadecapeptide, l-gramicidine, has been shown to undergo specific adsorption onto gold surfaces modified by chiral amino acids, d- and l-methionine, upon adsorption in the liquid phase. There is a chiral recognition only between the slightly anionic l-peptide and the l-modified surface exhibiting cationic molecules. However, whatever the probe and target molecule charges, very weak recognition was observed between molecules of opposite chirality.

Controlled electrochemically-assisted deposition of sol-gel biocomposite on electrospun platinum nanofibers

The modification of platinum nanofibers by silica using the electrochemically-assisted deposition is reported here. Pt nanofibers are obtained by electrospinning and deposited on a glass substrate. The electrochemically-assisted deposition of the sol-gel material then gives the unique possibility to finely tune the silica film thickness around these nanofibers. It also allows the successful encapsulation of a biomolecule (glucose oxidase was chosen here as a model) while retaining its biological activity, as pointed out via the electrochemical monitoring of H(2)O(2) produced upon addition of glucose in the medium. This silica-glucose oxidase composite offers the possibility of comparing systematically the influence of the deposition time on the bioelectrode response and to compare it with the particular features of the deposits. It was found that the film first grew uniformly around the nanofibers and then started to deposit between them, covering the whole sample (fibers and glass substrate), and tended to fully embed the nanofibers for prolonged deposition. The thickness of the silica film is critical for the electroactivity of the biocomposite, the best response being obtained for a silica layer thickness in the range of the fiber diameter (∼50 nm).

Hierarchical paramecium-like hollow and solid Au/Pt bimetallic nanostructures constructed using goethite as template

Novel hollow and solid paramecium-like hierarchical Au/Pt bimetallic nanostructures were constructed using goethite as template via a seed-mediated growth method. Transmission electron microscopy (TEM), xi-potential measurement, UV-vis spectroscopy, energy dispersive x-ray spectroscopy (EDS), ICP-AES measurement, x-ray powder diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) were utilized to systematically characterize the bimetallic nanostructures. It is found that the core structure of the paramecium-like bimetallic nanomaterial is closely related to reducing agent. When ascorbic acid is used as reducing agent, goethite serves as in situ sacrificed template and hollow paramecium-like bimetallic structure is obtained. When NH(2)OH.HCl is used, solid nanostructure with preserved goethite core is produced. Heating the reaction solution is necessary to obtain the paramecium-like morphology with rough interconnected Pt cilia shell. The thickness of Pt cilia layer can be controlled by adjusting the molar ratio of H(2)PtCl(6) to Au nanoseeds. The overgrowth of the rough Pt cilia is proposed to be via an autocatalytic and three-dimensional heterogeneous nucleation process first through flower-like morphology. Both the hollow and solid hierarchical paramecium-like Au/Pt bimetallic nanostructures show good catalytic activities.

Chemoenzymatic synthesis of alpha-hydroxy-beta-methyl-gamma-hydroxy esters: role of the keto-enol equilibrium to control the stereoselective hydrogenation in a key step

Alpha-hydroxy-beta-methyl-gamma-hydroxy esters not only are found in many natural products and potent drugs but also are useful intermediates in organic synthesis due to their highly functionalized skeleton that can be further manipulated and applied in the synthesis of many compounds with remarkable biological activities. This work was based on a chemoenzymatic approach to obtain these molecules with three contiguous stereogenic centers in a highly enantio- and diastereoselective way. Two distinct linear routes were proposed in which the key steps in both routes consisted of initial stereocontrolled ketoester bioreduction followed by unsaturated carbonyl bioreduction or reduction with Pd-C. Other key reactions in the synthesis include a Wasserman protocol for chain homologation and a Mannich-type olefination with maintenance of enantiomeric excess for all intermediates during the sequence. Whereas route A gave exclusively the skeleton with 3R,4R,5S configuration (99% ee and 11.5% global yield after 7 steps), route B gave the skeleton with 3R,4R,5S and 3R,4S,5R configurations (dr 1:12, 98% ee and 20% global yield after 5 steps).

Three-dimensional Pt-on-Pd bimetallic nanodendrites supported on graphene nanosheet: facile synthesis and used as an advanced nanoelectrocatalyst for methanol oxidation

Graphene nanosheet, the hottest material in physics and materials science, has been studied extensively because of its unique electronic, thermal, mechanical, and chemical properties arising from its strictly 2D structure and because of its potential technical applications. Particularly, these remarkable characteristics enable it to be a promising candidate as a new 2D support to load metal nanoparticles (NPs) for application in fuel cells. However, constructing high-quality graphene/bimetallic NP hybrids with high electrochemical surface area (ECSA) remains a great challenge to date. In this paper, we demonstrate for the first time a wet-chemical approach for the synthesis of high-quality three-dimensional (3D) Pt-on-Pd bimetallic nanodendrites supported on graphene nanosheets (TP-BNGN), which represents a new type of graphene/metal heterostructure. The resulting hybrids were characterized by atomic force microscopy (AFM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), energy-dispersive X-ray (EDX) spectroscopy, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), Raman spectroscopy, and electrochemical technique. It is found that small single-crystal Pt nanobranches supported on Pd NCs with porous structure and good dispersion were directly grown onto the surface of graphene nanosheets, which exhibits high electrochemical active area. Furthermore, the number of nanobranches for Pt-on-Pd bimetallic nanodendrites on the surface of graphene nanosheets could be easily controlled via simply changing the synthetic parameters, thus resulting in the tunable catalytic properties. Most importantly, the electrochemical data indicate that the as-prepared graphene/bimetallic nanodendrite hybrids exhibited much higher electrocatalytic activity toward methanol oxidation reaction than the platinum black (PB) and commercial E-TEK Pt/C catalysts.