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.
[1][2] 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:
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 The result is equivalent to the Michaelis–Menten kinetics of reactions catalyzed at a site on an enzyme.
The rate equation is complex, and the reaction order is not clear.
In experimental work, usually two extreme cases are looked for in order to prove the mechanism.
In them, the rate-determining step can be: The order respect to A is 1.
Examples of this mechanism are N2O on gold and HI on platinum The last expression is the Langmuir isotherm for the surface coverage.
The adsorption equilibrium constant
, and the numerator and denominator have each been divided by
Depending on the concentration of the reactant the rate changes: In this mechanism, suggested by Irving Langmuir in 1921 and further developed by Cyril Hinshelwood in 1926, two molecules adsorb on neighboring sites and the adsorbed molecules undergo a bimolecular reaction:[3] The rate constants are
for adsorption and desorption of A respectively,
for adsorption and desorption of B, and
for the reaction generating the final products.
The rate law is:
is the fraction of empty sites, so
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.
, which is nothing but Langmuir isotherm for two adsorbed gases, with adsorption constants
we finally get The rate law is complex and there is no clear order with respect to either reactant, but we can consider different values of the constants, for which it is easy to measure integer orders: That means that
The reaction order is 1 with respect to B.
There are two extreme possibilities for the order with respect to A: One of the reactants has very high adsorption and the other one doesn't adsorb strongly.
The reaction order is 1 with respect to B and −1 with respect to A. Reactant A inhibits the reaction at all concentrations.
The following reactions follow a Langmuir–Hinshelwood mechanism:[4] In this mechanism, proposed in 1922 by Irving Langmuir and later expanded upon by Eric Rideal, only one of the molecules adsorbs and the other one reacts with it directly from the gas phase, without adsorbing ("nonthermal surface reaction"): Constants are
Applying steady state approximation to AS and proceeding as before (considering the reaction the limiting step once more) we get
There are two possibilities, depending on the concentration of reactant A: The following reactions follow an Langmuir–Rideal mechanism:[4] The Langmuir-Rideal mechanism is often, incorrectly, attributed to Dan Eley as the Eley-Rideal mechanism.
[5] The actual Eley-Rideal mechanism, studied in the thesis of Dan Eley and proposed by Eric Rideal in 1939, was the reaction between a chemisorbed and a physisorbed molecule.
[6] As opposed to the Langmuir-Rideal mechanism, in this mechanism the physisorbed molecule is in thermal equilibrium with the surface.