To use all functions of this page, please activate cookies in your browser.
my.chemeurope.com
With an accout for my.chemeurope.com you can always see everything at a glance – and you can configure your own website and individual newsletter.
 My watch list
 My saved searches
 My saved topics
 My newsletter
Reactions on surfacesBy 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
Simple decompositionIf 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 k_{1}, k_{1} and k_{2} then, the global reaction rate is: where C_{AS} is the concentration of occupied sites, θ is the surface coverage and C_{S} is the total number of sites (occupied or not). C_{S} 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 m^{2}/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 . The order respect to A is 1. Examples of this mechanism are N_{2}O on gold and HI on platinum
so which is just Langmuir isotherm and . Depending on the concentration of the reactant the rate changes:
Bimolecular reactionLangmuirHinshelwood mechanismThis 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 k_{1},k_{ − 1},k_{2},k_{ − 2} and k for adsorption of A, adsorption of B, and reaction. The rate law is: Proceeding as before we get , where θ_{E} is the fraction of empty sites, so θ_{A} + θ_{B} + θ_{E} = 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 θ_{A} = K_{1}C_{A}θ_{E}, with K_{i} = k_{i} / k_{ − 1}, which is nothing but Langmuir isotherm for two adsorbed gases, with adsorption constants K_{1} and K_{2}. Calculating θ_{E} from θ_{A} and θ_{B} 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 > > K_{1}C_{A},K_{2}C_{B}, so . The order is one respect to both the reactants
In this case K_{1}C_{A},1 > > K_{2}C_{B}, 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. K_{1}C_{A} > > 1,K_{2}C_{B}, so . The reaction order is 1 respect to B and 1 respect to A. Reactant A inhibits the reaction at all concentrations.
EleyRideal mechanismThis 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 k_{1},k_{ − 1} and k and rate equation is r = kC_{S}θ_{A}C_{A}C_{B}. 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:
ReferencesGraphic models of Eley Rideal and Langmuir Hinshelwood mechanisms German page with mechanisms, rate equation graphics and references Categories: Surface chemistry  Chemical kinetics 

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. 