eley rideal mechanisms - Catalysis

What is the Eley-Rideal Mechanism?

The Eley-Rideal (ER) mechanism is a type of catalytic reaction mechanism named after D.D. Eley and E.K. Rideal, who proposed it in the early 20th century. In this mechanism, one reactant is adsorbed on the surface of a catalyst, while the other reactant remains in the gas phase and directly reacts with the adsorbed species. This contrasts with the Langmuir-Hinshelwood mechanism, where both reactants are adsorbed on the catalyst surface before reacting.

How Does the Eley-Rideal Mechanism Work?

The ER mechanism can be summarized in a few key steps:
1. Adsorption: One of the reactants, say A, is adsorbed onto the catalyst surface.
2. Direct Reaction: The other reactant, B, in the gas phase approaches and reacts directly with the adsorbed species A.
3. Desorption: The product of the reaction, AB, desorbs from the surface, leaving the catalyst ready for another cycle.

Why is the Eley-Rideal Mechanism Important?

The ER mechanism is crucial for understanding reactions where one of the reactants is difficult to adsorb or where the surface coverage is too low for both reactants to adsorb simultaneously. It provides insights into reaction kinetics, catalyst design, and the overall efficiency of catalytic processes.

What are the Conditions Favoring the Eley-Rideal Mechanism?

Several conditions favor the ER mechanism:
- Low Surface Coverage: When the surface coverage of one reactant is low, making it difficult for both reactants to adsorb simultaneously.
- High Reactivity of Gas Phase Reactant: When the gas-phase reactant is highly reactive and can readily react with the adsorbed species.
- Temperature: The reaction temperature can influence the likelihood of the ER mechanism. Higher temperatures can enhance the mobility of adsorbed species, facilitating the direct reaction with the gas-phase reactant.

Examples of Eley-Rideal Mechanism

Several catalytic processes are thought to follow the ER mechanism:
- Hydrogenation Reactions: The hydrogenation of ethylene on metal surfaces where hydrogen remains in the gas phase and reacts with adsorbed ethylene.
- Oxidation Reactions: Certain oxidation reactions where oxygen remains in the gas phase and reacts with adsorbed hydrocarbons.

How is the Eley-Rideal Mechanism Studied?

The ER mechanism is studied using various experimental and theoretical techniques:
- Surface Science Techniques: Methods like Temperature Programmed Desorption (TPD) and Infrared Spectroscopy help in understanding the adsorption characteristics.
- Computational Modeling: Density Functional Theory (DFT) and molecular dynamics simulations provide insights into the reaction pathways and energetics.
- Kinetic Studies: Experimental kinetic studies help in distinguishing between different catalytic mechanisms by analyzing reaction rates and dependencies on reactant concentrations.

Challenges and Limitations

While the ER mechanism provides a valuable framework, it also has limitations:
- Surface Heterogeneity: Real catalyst surfaces are often heterogeneous, complicating the simple picture of the ER mechanism.
- Reaction Intermediates: Identifying and characterizing reaction intermediates can be challenging, affecting the mechanistic understanding.
- Experimental Verification: Direct experimental verification of the ER mechanism can be difficult, requiring sophisticated techniques and careful interpretation of results.

Conclusion

The Eley-Rideal mechanism is a fundamental concept in the field of catalysis, offering insights into the behavior of reactions where one reactant remains in the gas phase. Understanding this mechanism aids in the design of more efficient catalysts and the optimization of catalytic processes. Despite challenges in its study, the ER mechanism remains a pivotal model for explaining a variety of catalytic reactions.