Surface chemistry Eley-Rideal mechanism

In surface chemistry, adsorption isotherms describe the equiUhrium situation. However, just as in the consideration of the gas-phase chemistry in the interstellar medium, it is the kinetics of surface processes which are more relevant. Two mechanisms for surface-catalysed reactions can be distinguished and are illustrated by the cartoons in Fig. 1.6. In the Eley-Rideal mechanism, it is assumed that reaction occurs when a species (say, A) from the gas-phase impacts on a species (say, B) that is adsorbed on the surface. At significant surface coverage, the rate of reaction will be proportional to the product of the fraction of the surface covered in B (6b) and the pressure (p ) of the species A, which will be proportional to the rate of collisions of A with unit area of the solid surface. An alternative picture is encapsulated in the Langmuir-Hinshelwood mechanism. Here it is assumed that reaction occurs in encounters between species both of which are adsorbed on the surface. Then the rate of reaction will be proportional to the product of the fractions of the surface covered by A and by B that is proportional to 0a b-... 

Le Bourlot J, Le Petit F, Pinto C, Roueff E, Roy F (2012) Surface chemistry in the interstellar medium - I - H2 formation by Langmuir-Hinshelwood and Eley-Rideal mechanisms. Astron Astrophys 54LA76... 

The Eley-Rideal (ER) mechanism of surface chemistry is the antithesis of the LH mechanism. In this case, an incoming gas phase species directly reacts with an adsorbate equilibrated on the surface. Of course, if a species impinges on the surface near an adsorbate, it can make a few bounces on the surface before reacting with the adsorbate. When it retains most of its initial energy of adsorption prior to reacting in this fashion, it is called a hot atom (HA) mechanism. There is no rigid division between ER and HA, so they will be considered together. In the discussion of H + H/Cu(lll), the difference between the two will be clarified. 

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