LangmuirHinshelwoodModell

The Langmuir–Hinshelwood mechanism is a model for heterogeneous catalytic reactions in which reactants first adsorb on a catalyst surface, react while adsorbed, and then the product desorbs. It was developed from early 20th-century studies of surface chemistry by Irving Langmuir and Cyril Hinshelwood and is a foundational concept in heterogeneous catalysis and surface science.

In the canonical Langmuir–Hinshelwood picture, two reactant molecules occupy adjacent adsorption sites and undergo a surface-mediated elementary step to form product(s). The reaction rate is therefore controlled by adsorption equilibria, surface coverages, and the intrinsic surface reaction rate. Using the Langmuir adsorption isotherm to relate gas- or solution-phase concentrations to site coverages yields characteristic rate laws that can display zero-, first-, or fractional-order dependence on reactant concentrations, depending on adsorption strengths and site occupancies.

The mechanism is typically contrasted with the Eley–Rideal mechanism, where a gas-phase species reacts directly with an adsorbed species without first adsorbing. Langmuir–Hinshelwood behavior is most applicable when both reacting species adsorb and diffuse on the surface. It is widely invoked in modeling processes such as oxidation, hydrogenation, reforming, and photocatalytic degradation.

Limitations include assumptions of uniform adsorption sites, rapid equilibration of adsorption steps, and neglect of surface heterogeneity, strong lateral interactions, or subsurface processes. Extensions and alternative models (e.g., Temkin or Mars–van Krevelen descriptions) are used when those assumptions break down.