The principles of methanol synthesis catalysts revolve around their ability to facilitate the hydrogenation of carbon monoxide (CO) and carbon dioxide (CO₂) into methanol (CH₃OH) under specific conditions, primarily through optimized active sites, structural properties, and reaction pathways. Below is a detailed explanation:

1. Core Reaction Mechanism: Hydrogenation of CO/CO₂

Methanol synthesis involves two primary reactions:

• CO Hydrogenation:

• CO₂ Hydrogenation:

Key Steps:

1. Adsorption: CO/CO₂ and H₂ adsorb onto the catalyst surface. For example:
   • CO adsorbs linearly or in a bridged configuration on copper (Cu) surfaces.
   • CO₂ may dissociate into CO and O or form formate (${\text{HCOO}}^{-}$) intermediates.
2. Activation: Adsorbed species undergo activation (e.g., CO dissociation into C and O).
3. Hydrogenation: Activated species react with surface-bound hydrogen to form methanol.

2. Catalyst Composition: Active Components and Promoters

Methanol synthesis catalysts are typically copper-based (e.g., Cu/ZnO/Al₂O₃) or zinc-chromium-based (ZnO/Cr₂O₃), with copper-based catalysts dominating modern industrial applications due to higher activity and selectivity.

A. Copper-Based Catalysts (Cu/ZnO/Al₂O₃)

• Active Phase: Metallic copper (Cu⁰) serves as the primary active site for CO/CO₂ adsorption and hydrogenation.
• Support and Promoters:
  • ZnO: Enhances Cu dispersion, stabilizes the catalyst structure, and acts as a physical support. It also participates in CO₂ activation by forming Zn-O-Cu interfaces.
  • Al₂O₃: Improves thermal stability, prevents sintering, and provides additional acid-base sites for intermediate stabilization.
• Synergistic Effects: The Cu-ZnO interface promotes CO₂ dissociation and formate intermediate formation, while Al₂O₃ modulates surface acidity to suppress by-product (e.g., dimethyl ether) formation.

B. Zinc-Chromium Catalysts (ZnO/Cr₂O₃)

• Active Phase: ZnO and Cr₂O₃ form a solid solution, with Cr³⁺ providing redox sites for CO activation.
• Limitations: Lower activity and selectivity compared to Cu-based catalysts, now largely replaced in industrial applications.

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