Introduction to the Eley-Rideal Model
The
Eley-Rideal model is a kinetic model used to describe the mechanism of heterogeneous catalysis. It is particularly significant for reactions occurring on the surface of solid catalysts. In this model, a reactant in the gas phase directly interacts with another reactant that is already adsorbed on the catalyst surface to form the product.
How Does the Eley-Rideal Model Work?
In the Eley-Rideal mechanism, a molecule from the gas phase (A) approaches the catalyst surface where another molecule (B) is already adsorbed. The gas-phase molecule then reacts directly with the adsorbed molecule to form the product (P), which subsequently desorbs from the surface. This model contrasts with the
Langmuir-Hinshelwood model, where both reactants need to be adsorbed on the surface before reacting.
Key Assumptions of the Eley-Rideal Model
1. Gas-Surface Interaction: One reactant is in the gas phase.
2. Surface Adsorption: The other reactant is adsorbed on the catalyst surface.
3. Direct Reaction: The gas-phase reactant reacts directly with the adsorbed species without needing to be adsorbed first.
4. Rate Determination: The rate of reaction is determined by the concentration of the gas-phase reactant and the surface coverage of the adsorbed reactant.Why is the Eley-Rideal Model Important?
The Eley-Rideal model is crucial for understanding reactions where the adsorption of the second reactant is either energetically unfavorable or kinetically hindered. It explains the kinetics of certain reactions more accurately than other models, providing insights into
reaction mechanisms that occur on catalyst surfaces.
Applications of the Eley-Rideal Model
The model is applied in various fields, including:
- Catalytic Converters: In automotive exhaust systems where pollutants react with catalysts.
- Fuel Cells: For understanding the reactions occurring on the electrode surfaces.
- Chemical Synthesis: In processes like ammonia synthesis on iron catalysts.What are the Limitations of the Eley-Rideal Model?
Although the Eley-Rideal model is useful, it has limitations:
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Rare Occurrence: It is less common compared to the Langmuir-Hinshelwood mechanism.
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Surface Heterogeneity: It assumes a uniform catalyst surface, which is not always the case.
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Complex Reactions: It may not accurately describe multi-step or complex reactions.
Comparing Eley-Rideal and Langmuir-Hinshelwood Models
In the
Langmuir-Hinshelwood model, both reactants must be adsorbed on the catalyst surface before reacting, making it more suitable for reactions where both species have strong adsorption affinities. In contrast, the Eley-Rideal model is applicable when one reactant remains in the gas phase and directly interacts with the adsorbed species. This distinction allows for different interpretations of reaction kinetics and mechanisms.
Conclusion
The Eley-Rideal model is a valuable tool in the realm of
heterogeneous catalysis. It provides a simplified yet insightful approach to understanding surface reactions involving gas-phase reactants. Despite its limitations, it complements other models like the Langmuir-Hinshelwood mechanism, enriching our understanding of catalytic processes.
