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Reduction of Alkenes: Asymmetric Catalytic Hydrogenation02:17

Reduction of Alkenes: Asymmetric Catalytic Hydrogenation

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Catalytic hydrogenation of alkenes is a transition-metal catalyzed reduction of the double bond using molecular hydrogen to give alkanes. The mode of hydrogen addition follows syn stereochemistry.
The metal catalyst used can be either heterogeneous or homogeneous. When hydrogenation of an alkene generates a chiral center, a pair of enantiomeric products is expected to form. However, an enantiomeric excess of one of the products can be facilitated using an enantioselective reaction or an...
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Catalysis02:50

Catalysis

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The presence of a catalyst affects the rate of a chemical reaction. A catalyst is a substance that can increase the reaction rate without being consumed during the process. A basic comprehension of a catalysts’ role during chemical reactions can be understood from the concept of reaction mechanisms and energy diagrams.
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Reduction of Alkenes: Catalytic Hydrogenation02:13

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Alkenes undergo reduction by the addition of molecular hydrogen to give alkanes. Because the process generally occurs in the presence of a transition-metal catalyst, the reaction is called catalytic hydrogenation.
Metals like palladium, platinum, and nickel are commonly used in their solid forms — fine powder on an inert surface. As these catalysts remain insoluble in the reaction mixture, they are referred to as heterogeneous catalysts.
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Reduction of Alkynes to cis-Alkenes: Catalytic Hydrogenation02:24

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Introduction
Like alkenes, alkynes can be reduced to alkanes in the presence of transition metal catalysts such as Pt, Pd, or Ni. The reaction involves two sequential syn additions of hydrogen via a cis-alkene intermediate.
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Alkenes are converted to 1,2-diols or glycols through a process called dihydroxylation. It involves the addition of two hydroxyl groups across the double bond with two different stereochemical approaches, namely anti and syn. Dihydroxylation using osmium tetroxide progresses with syn stereochemistry.
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Reduction of Benzene to Cyclohexane: Catalytic Hydrogenation01:28

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Unlike the easy catalytic hydrogenation of an alkene double bond, hydrogenation of a benzene double bond under similar reaction conditions does not take place easily. For example, in the reduction of stilbene, the benzene ring remains unaffected while the alkene bond gets reduced. Hydrogenation of an alkene double bond is exothermic and a favorable process. In contrast, to hydrogenate the first unsaturated bond of benzene, an energy input is needed; that is, the process is endothermic. This is...
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Catalytic Reactions at Amine-Stabilized and Ligand-Free Platinum Nanoparticles Supported on Titania During Hydrogenation of Alkenes and Aldehydes
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Homogeneous Metastable Hexagonal Phase Iridium Enhances Hydrogen Evolution Catalysis.

Shize Geng1,2, Yujin Ji3, Jiaqi Su1

  • 1College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu, 215123, China.

Advanced Science (Weinheim, Baden-Wurttemberg, Germany)
|February 12, 2023
PubMed
Summary
This summary is machine-generated.

Researchers developed a new method to create metastable hexagonal close-packed (hcp) Iridium, enhancing its performance in the hydrogen evolution reaction (HER) for catalysis.

Keywords:
electrocatalysishydrogen evolution reactioniridiummetastable phase

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Area of Science:

  • Materials Science
  • Catalysis
  • Electrochemistry

Background:

  • Catalytic reactions are highly sensitive to surface properties.
  • Achieving specific metastable phases in metals for catalysis is difficult.
  • Hexagonal close-packed (hcp) structures are crucial for catalytic activity.

Purpose of the Study:

  • To synthesize metastable hexagonal close-packed (hcp) Iridium (Ir) for improved catalytic applications.
  • To investigate the catalytic performance of the synthesized hcp Ir in the hydrogen evolution reaction (HER).
  • To understand the role of crystal phase in noble metal nanomaterials for catalysis.

Main Methods:

  • Epitaxial growth of metastable hcp Ir onto metastable hcp Nickel (Ni).
  • Characterization using spherical aberration electron microscopy.
  • Electrochemical measurements for HER activity and stability.
  • Theoretical calculations (DFT) to elucidate reaction mechanisms.

Main Results:

  • Successfully fabricated metastable hcp Ir epitaxially grown on hcp Ni.
  • Demonstrated high intrinsic activity for the alkaline HER with a low overpotential (17 mV at 10 mA cm⁻²).
  • Achieved high specific activity (8.55 mA cm⁻²) and turnover frequency (38.26 s⁻¹).
  • Observed excellent stability due to the epitaxial structure.
  • Theoretical calculations confirmed enhanced H₂O adsorption and dissociation.

Conclusions:

  • Metastable hcp Ir exhibits superior HER performance and stability.
  • Crystal phase control is a viable strategy for designing advanced noble metal catalysts.
  • The findings provide insights into phase-dependent catalysis for future nanomaterial development.