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Reduction of Alkenes: Asymmetric Catalytic Hydrogenation02:17

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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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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.
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Redox Equilibria: Overview01:23

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A reduction-oxidation reaction is commonly called a redox reaction. In a redox reaction, electrons are transferred from one species to another rather than being shared between or among atoms. The reducing agent or reductant is the species that loses electrons and gets oxidized in the process. The species that gains electrons and gets reduced in the process is the oxidizing agent or oxidant. Redox reactions are represented as two separate equations called half-reactions, where one equation...
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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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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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Oxidation-reduction or redox reactions involve the transfer of electrons from one molecule or atom to another. When an atom gains an electron, another atom must lose an electron, meaning oxidation and reduction must occur together. Since the redox occurs in pairs, the atom that gets oxidized is also called the reducing agent or reductant, and the atom that is reduced is also called the oxidizing agent or oxidant. A straightforward way to remember the definitions of oxidation and reduction is...
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Defect-Driven Redox Interplay on Anatase TiO2: Surface-Structure Dependent Activation for CO2 Hydrogenation

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Titanium dioxide (TiO2) catalysts can be activated for reactions like RWGS by engineering oxygen vacancies. This study reveals how H2 reduces TiO2 and CO2 replenishes it, enabling tailored catalyst design.

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Area of Science:

  • Materials Science
  • Surface Chemistry
  • Catalysis

Background:

  • Titanium dioxide (TiO2) is a key material in energy and environmental catalysis.
  • Atomistic mechanisms of TiO2's dynamic response to reactive environments are poorly understood.
  • Understanding these mechanisms is crucial for optimizing TiO2-based catalysts.

Purpose of the Study:

  • To elucidate the atomistic mechanisms of anatase TiO2's dynamic response to H2 and CO2.
  • To investigate the interplay between oxygen loss and replenishment on TiO2 surfaces.
  • To correlate surface structure, defect dynamics, and reactivity for catalyst design.

Main Methods:

  • In situ environmental transmission electron microscopy (ETEM)
  • Synchrotron X-ray diffraction (XRD)
  • Ambient-pressure X-ray photoelectron spectroscopy (AP-XPS)
  • Temperature-programmed reduction (TPR)
  • Reactivity measurements
  • Theoretical modeling

Main Results:

  • H2 exposure causes TiO2 reduction via lattice oxygen loss, forming Ti3O5.
  • CO2 exposure leads to oxygen replenishment, reversing TiO2 stoichiometry.
  • The reverse water-gas shift (RWGS) reaction is selective to stepped/high-indexed TiO2 surfaces.
  • H2 pretreatment creates oxygen vacancies, activating inert TiO2(101) facets for RWGS catalysis.

Conclusions:

  • Defect engineering, specifically creating oxygen vacancies, can activate inert TiO2 facets.
  • Understanding the atomic-scale competition between reduction and oxidation pathways is key.
  • This work provides insights for designing adaptive catalysts for sustainable fuel synthesis.