Introduction to the Eley-Rideal Mechanism
The
Eley-Rideal mechanism is one of the fundamental models describing reactions on catalytic surfaces. Unlike the
Langmuir-Hinshelwood mechanism, where both reactants adsorb on the catalyst surface, the Eley-Rideal mechanism involves a direct interaction between an adsorbed species and a reactant from the gas phase.
How Does the Eley-Rideal Mechanism Work?
In the Eley-Rideal mechanism, one reactant is adsorbed on the surface of the
catalyst, while the other reactant remains in the gas phase. The gas-phase molecule directly collides with the adsorbed species, leading to a reaction. This model can be particularly relevant in high-vacuum conditions or when one of the reactants has a very low adsorption probability.
Key Characteristics
1. Single Adsorption: Only one of the reactants is adsorbed on the catalytic surface, unlike dual adsorption in the Langmuir-Hinshelwood model.
2. Direct Reaction: The reaction occurs through a direct collision between the gas-phase molecule and the adsorbed species.
3. Rate-Determining Step: The rate of the reaction is primarily determined by the concentration of the gas-phase reactant and the surface coverage of the adsorbed species.Mathematical Representation
The rate of the reaction in the Eley-Rideal mechanism can often be described by the following rate law:
\[ r = k' \theta_A P_B \]
where:
- \( r \) is the reaction rate.
- \( k' \) is the rate constant.
- \( \theta_A \) is the surface coverage of the adsorbed species A.
- \( P_B \) is the partial pressure of the gas-phase reactant B.Advantages and Limitations
Advantages:
- Simplified Model: The Eley-Rideal mechanism provides a simplified model for reactions involving a gas-phase and an adsorbed reactant.
- Applicability: It is particularly useful for describing reactions in conditions where one reactant does not adsorb significantly on the surface.Limitations:
- Specific Conditions: It is less applicable to systems where both reactants have high adsorption tendencies.
- Experimental Verification: Experimental verification can be challenging due to the difficulty in isolating the contributions of gas-phase collisions.
Experimental Evidence and Applications
Experimental evidence for the Eley-Rideal mechanism is often found in studies involving
surface science and
ultrahigh vacuum conditions. One well-known example is the reaction between adsorbed hydrogen atoms and gas-phase oxygen molecules on metal surfaces.
Comparison with Other Mechanisms
The Eley-Rideal mechanism is often compared with the Langmuir-Hinshelwood mechanism. While the Langmuir-Hinshelwood mechanism involves dual adsorption and surface diffusion of both reactants, the Eley-Rideal mechanism simplifies the reaction to a single adsorption event followed by a direct collision. Additionally, the
Mars-van Krevelen mechanism involves lattice oxygen in the catalyst participating in the reaction, which is distinct from both the Eley-Rideal and Langmuir-Hinshelwood models.
Conclusion
The Eley-Rideal mechanism offers a valuable perspective for understanding catalytic reactions involving gas-phase and adsorbed reactants. While it may not be universally applicable, it provides critical insights into specific catalytic processes, especially under conditions where traditional dual-adsorption models fall short. Further research and experimental validation continue to expand its relevance in the field of catalysis.
