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Thermal Sigmatropic Reactions: Overview

Thermal Sigmatropic Reactions: Overview

Sigmatropic rearrangements are a class of pericyclic reactions in which a σ bond migrates from one part of a π system to another. These are intramolecular rearrangements where the total number of σ and π bonds remain unchanged.
Sigmatropic shifts are classified based on an order term [i, j ], where i and j indicate the number of atoms across which each end of the σ bond migrates. Below are examples of a [3,3] sigmatropic shift in...

Woodward–Hoffmann Selection Rules and Microscopic Reversibility

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Electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are concerted pericyclic reactions that proceed via a cyclic transition state. These reactions are stereospecific and regioselective. The stereochemistry of the products depends on the symmetry characteristics of the interacting orbitals and the reaction conditions. Accordingly, pericyclic reactions are classified as either symmetry-allowed or symmetry-forbidden. Woodward and Hoffmann presented the selection criteria for...

Thermal Electrocyclic Reactions: Stereochemistry

Thermal Electrocyclic Reactions: Stereochemistry

The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.

[3,3] Sigmatropic Rearrangement of 1,5-Dienes: Cope Rearrangement

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The Cope rearrangement is classified as a [3,3] sigmatropic shift in 1,5-dienes, leading to a more stable, isomeric 1,5-diene. The reaction involves a concerted movement of six electrons, four from two π bonds and two from a σ bond, via an energetically favorable chair-like transition state.

Photochemical Electrocyclic Reactions: Stereochemistry

Photochemical Electrocyclic Reactions: Stereochemistry

The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
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Catalysis

Catalysis

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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Active-Site Ensemble for the Reverse Water-Gas Shift Reaction over Pd/TiO₂: Two Is Better than One or More.

Gui-Chang Wang¹

¹College of Chemistry, Nankai University, Tianjin 300071, China.

ACS Applied Materials & Interfaces

|February 11, 2025

Summary

The optimal active ensemble for the reverse water-gas shift (rWGS) reaction over palladium clusters on TiO2 is a dual-atom site (Pd2). This Pd2 cluster exhibits superior catalytic activity compared to single atoms or larger clusters, offering insights for catalyst design.

Keywords:

DFT Pd/TiO2 active-site ensemble number dual-atom microkinetic modeling rWGS

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Area of Science:

Catalysis science
Materials science
Surface chemistry

Background:

Determining the active site size for catalytic reactions is crucial but challenging.
The reverse water-gas shift (rWGS) reaction is important for CO2 utilization.

Purpose of the Study:

To systematically investigate the optimal active ensemble number for the rWGS reaction.
To explore the catalytic performance of palladium clusters (Pd_n) supported on anatase TiO2 (A-TiO2), with n = 1, 2, 3, 4.

Main Methods:

Density functional theory (DFT) calculations.
Mean-field microkinetic modeling.
Analysis of reaction energy diagrams and intermediate binding energies.

Main Results:

Palladium dimer (Pd2) demonstrated the highest catalytic activity for the rWGS reaction.
Single Pd atoms (Pd1) showed limited activity due to insufficient active sites for carboxyl formation.
Larger clusters (Pd3, Pd4) exhibited reduced activity due to excessively strong H* binding, hindering carboxyl formation.
Pd2 provided the most stable configuration for key intermediates ([COOH* + H*]).

Conclusions:

The active ensemble size significantly impacts rWGS catalytic activity.
Hydrogen binding energy serves as a descriptor for rWGS activity, aligning with the Sabatier principle.
Pd2 clusters are promising for rWGS catalysis and potentially other reactions like the water-gas shift (WGS).