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Reactions on surfaces
By reactions on surfaces it is understood 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
Additional recommended knowledge
If a reaction occurs through these steps:
A + S ⇌ AS → Products
Where A is the reactant and S is an adsorption site on the surface. If the rate constants for the adsorption, desorption and reaction are k1, k-1 and k2 then, the global reaction rate is:
where CAS is the concentration of occupied sites, θ is the surface coverage and CS is the total number of sites (occupied or not).
CS is highly related to the total surface area of the adsorbent: the bigger 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 hundred m2/gram)
If we apply the steady state approximation to AS, then
so and . Please notice that, with , the formula was divided by k − 1.
so which is just Langmuir isotherm and . Depending on the concentration of the reactant the rate changes:
This mechanism proposes that both molecules adsorb and the adsorbed molecules undergo a bimolecular reaction:
A + S ⇌ AS
B + S ⇌ BS
AS + BS → Products
The rate constants are now k1,k − 1,k2,k − 2 and k for adsorption of A, adsorption of B, and reaction. The rate law is:
Proceeding as before we get , where θE is the fraction of empty sites, so θA + θB + θE = 1. Let us assume now that the rate limiting step is the reaction of the adsorbed molecules, which is easily understood: the probability of two adsorbed molecules colliding is low. Then θA = K1CAθE, with Ki = ki / k − 1, which is nothing but Langmuir isotherm for two adsorbed gases, with adsorption constants K1 and K2. Calculating θE from θA and θB we finally get
The rate law is complex and there is no clear order respect to any of the reactants but we can consider different values of the constants, for which it is easy to measure integer orders:
That means that 1 > > K1CA,K2CB, so . The order is one respect to both the reactants
In this case K1CA,1 > > K2CB, so . The reaction order is 1 respect to B. There are two extreme possibilities now:
One of the reactants has very high adsorption and the other one doesn't adsorb strongly.
K1CA > > 1,K2CB, so . The reaction order is 1 respect to B and -1 respect to A. Reactant A inhibits the reaction at all concentrations.
This mechanism proposes that only one of the molecules adsorbs and the other one reacts with it directly, without adsorbing:
A + S ⇌ AS
AS + B → Products
Constants are k1,k − 1 and k and rate equation is r = kCSθACACB. Applying steady state approximation to AS and proceeding as before (considering the reaction the limiting step once more) we get . The order is one respect to B. There are two possibilities, depending on the concetration of reactant A:
Graphic models of Eley Rideal and Langmuir Hinshelwood mechanisms
German page with mechanisms, rate equation graphics and references
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Reactions_on_surfaces". A list of authors is available in Wikipedia.|