The
Langmuir-Hinshelwood mechanism is a model that describes the kinetics of heterogeneous catalysis. It is particularly useful for understanding reactions that occur on the surface of a solid catalyst. According to this mechanism, both reactants are adsorbed onto the catalyst surface, where they undergo a reaction to form products. This mechanism was proposed by
Irving Langmuir and further developed by
Cyril Hinshelwood.
1. Adsorption Equilibrium: The reactants adsorb onto the catalyst surface and achieve a dynamic equilibrium.
2. Surface Reaction: The actual chemical reaction takes place on the catalyst surface.
3. Desorption: The products desorb from the catalyst surface after the reaction.
1. Adsorption of Reactants: The reactants (A and B) adsorb onto active sites of the catalyst, forming adsorbed species (A* and B*).
2. Surface Reaction: The adsorbed reactants react to form an intermediate or directly form the products (P*).
3. Desorption of Products: The product molecules desorb from the catalyst surface, freeing up active sites for new reactants.
In the Langmuir-Hinshelwood mechanism, the rate-determining step (RDS) can be either the adsorption, surface reaction, or desorption process. However, it is often the surface reaction that acts as the RDS, influencing the overall reaction rate.
The rate of reaction can be expressed in terms of the surface coverages of the adsorbed species. Assuming that adsorption follows Langmuir isotherm, the rate equation can be written as:
\[ r = \frac{k \theta_A \theta_B}{1 + K_A P_A + K_B P_B} \]
where:
- \( r \) is the rate of reaction
- \( k \) is the rate constant for the surface reaction
- \( \theta_A \) and \( \theta_B \) are the surface coverages of reactants A and B
- \( K_A \) and \( K_B \) are the adsorption equilibrium constants for A and B
- \( P_A \) and \( P_B \) are the partial pressures of A and B
While the Langmuir-Hinshelwood mechanism is widely useful, it has some limitations:
- It assumes a uniform surface with identical active sites, which is often not the case in real-world catalysts.
- It does not account for potential
inhibition effects where adsorbed species block active sites.
- It assumes that the surface reaction step is the RDS, which may not always hold true.
The Langmuir-Hinshelwood mechanism is often compared to the
Eley-Rideal mechanism, where one reactant is adsorbed on the catalyst surface and the other reacts directly from the gas phase without being adsorbed. While the Langmuir-Hinshelwood mechanism involves adsorption of both reactants, the Eley-Rideal mechanism simplifies the process but is less commonly observed.
Conclusion
The Langmuir-Hinshelwood mechanism provides a fundamental framework for understanding the kinetics of surface-catalyzed reactions. By considering adsorption, surface reaction, and desorption steps, it helps in deriving rate equations and optimizing catalytic processes. Despite its limitations, it remains a cornerstone in the field of
heterogeneous catalysis.
