→ CH3CHO on a palladium catalyst. CO + OH → CO2 + H+ + e− on a platinum catalyst. Eley–Rideal mechanism In this mechanism, proposed in 1938 by D. D. Eley and E. K. Rideal, only one of the molecules adsorbs and the other one reacts with it directly from the gas phase, without adsorbing ("nonthermal surface reaction"): A(g) + S(s) ⇌ AS(s) AS(s) + B(g) → Products Constants are and and rate equation is . Applying steady state approximation to AS and proceeding as before (considering the reaction the limiting step once more) we get . The order is one with respect to B. There are two possibilities, depending on the concentration of reactant A: At low concentrations of A, , and the order is one with respect to A. At high concentrations of A, , and the order is zero with respect to A. The following reactions follow an Eley–Rideal mechanism: C2H4 + ½ O2 (adsorbed) → (CH2CH2)O The dissociative adsorption of oxygen is also possible, which leads to secondary products carbon dioxide and water. CO2 + H2 (ads.) → H2O + CO 2 NH3 + 1½ O2 (ads.) → N2 + 3H2O on a platinum catalyst C2H2 + H2 (ads.) → C2H4 on nickel or iron catalysts See also Diffusion-controlled reaction References Graphic models of Eley Rideal and Langmuir Hinshelwood mechanisms Surface science Chemical kinetics Chemical reaction engineering