Reactions on surfaces

Reactions on surfaces are reactions in which at least one of the steps of the reaction mechanism is the adsorption of one or more reactants. The mechanisms for these reactions, and the rate equations are of extreme importance for heterogeneous catalysis. Via scanning tunneling microscopy, it is possible to observe reactions at the solid|gas interface in real space, if the time scale of the reaction is in the correct range. Reactions at the solid|gas interface are in some cases related to catalysis.

If a reaction occurs through these steps:

where A is the reactant and S is an adsorption site on the surface and the respective rate constants for the adsorption, desorption and reaction are k₁, k₋₁ and k₂, then the global reaction rate is:

where:

${\displaystyle C_{\mathrm {S} }}$ is highly related to the total surface area of the adsorbent: the greater the surface area, the more sites and the faster the reaction. This is the reason why heterogeneous catalysts are usually chosen to have great surface areas (in the order of a hundred m²/gram)

If we apply the steady state approximation to AS, then:

and

Note that, with ${\displaystyle K_{1}={\frac {k_{1}}{k_{-1}}}}$, the formula was divided by ${\displaystyle k_{-1}}$.

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