EleyRideal

Eley–Rideal is a reaction mechanism in heterogeneous catalysis and surface science in which a gas-phase reactant reacts directly with a species already adsorbed on a solid surface, without first adsorbing the gas-phase partner or relying on surface diffusion of the gas-phase molecule. The mechanism was proposed by D. D. Eley and R. Rideal in 1938 to explain catalytic processes that could not be adequately described by the Langmuir–Hinshelwood framework.

In the Eley–Rideal process, the surface holds an adsorbed species (A*) and a second reactant from the gas phase (B(g)) collides with the surface and reacts with A*. The reaction products typically desorb from the surface, and the surface site is regenerated. The rate of an Eley–Rideal step is generally proportional to the gas-phase concentration of B and to the fraction of surface sites occupied by A*, and it does not require the gas-phase molecule to adsorb or diffuse on the surface prior to reaction.

Eley–Rideal is distinguished from Langmuir–Hinshelwood mechanisms, where both reactants must adsorb and diffuse on the surface to meet. In practice, real systems may exhibit contributions from both pathways, and the dominant mechanism can depend on factors such as surface coverage, adsorption strengths, temperature, and reactant flux. The ER mechanism is particularly relevant when adsorbed species are highly reactive with gas-phase partners, or under conditions of high gas flux and low surface coverage of the reacting partner.

Today, Eley–Rideal remains a standard concept in kinetic modeling of surface reactions and is used alongside Langmuir–Hinshelwood analyses to interpret experimental rate data and mechanism.