LangmuirHinshelwoodtyp

The Langmuir–Hinshelwood type is a classical model of heterogeneous catalytic reactions in which all reacting species first adsorb onto a solid surface, react while bound to adsorbed states, and then desorb as products. It contrasts with mechanisms where a gas-phase molecule directly reacts with an adsorbed species (the Eley–Rideal pathway).

Key features of the Langmuir–Hinshelwood mechanism include the use of Langmuir adsorption isotherms to describe surface coverages, a fixed number of equivalent active sites, and a surface reaction step between adsorbed species. The mechanism typically assumes that adsorption steps reach quasi-equilibrium and that the surface reaction occurs between adsorbed species without substantial contribution from surface diffusion constraints or lateral interactions among adsorbates. The overall rate depends on the concentrations of adsorbed reactants rather than gas-phase concentrations alone.

For a bimolecular surface reaction A* + B* → products, the rate is often expressed as r = k_r θ_A θ_B, where θ_A and θ_B are the fractional coverages of A and B on the surface and k_r is the rate constant for the surface reaction. If multiple adsorbates compete for sites, θ_i follow θ_i = (K_i P_i) / (1 + Σ_j K_j P_j), with K_i the adsorption equilibrium constant and P_i the partial pressure of species i. The model can be reduced to first- or pseudo-first-order forms under specific conditions, such as one reactant in large excess.

Limitations include neglect of lateral interactions, surface heterogeneity, and diffusion limitations; applicability is strongest under moderate coverages and near steady-state conditions. The Langmuir–Hinshelwood framework remains a foundational tool for interpreting catalytic kinetics on metal surfaces.