Adsorption and surface reactions in heterogeneous catalysis

Heterogeneous catalysts are those which are present in a phase different than the reactants of the reaction. These reactions are hence called catalysis.

Note that phase here doesn’t only refer to a different state (solid, liquid, gas) but also certain immiscible liquids such as oil and water mixtures.

Majority of catalysts used in large scale reactions are solids, while the reactants are mostly liquids or gases. Let’s learn a bit more about how these reactions take place.

heterogeneous catalysis

Surface adsorption

This is usually the first step where a gas or liquid reactant binds to the surface of the catalyst. The opposite of this binding is called desorption which takes place after a product is formed.

As such, adsorption is of two types; physisorption and chemisorption. The first one usually takes place at the beginning, quickly followed by the other.

In physisorption, or physical adsorption, only small changes occur, binding the reactant weakly to the surface of the catalyst.  Van der Waals forces are at play here.

In contrast, chemisorption or chemical adsorption is the one where strong attachment occurs, by usually breaking a bond. This is described through Lennard-Jones potential, including various cases such as molecular adsorption and dissociation adsorption.

Surface reactions

The reaction often occurs on the surface of the catalyst, especially in case of solid metal catalysts. Here are three main mechanisms which may occur:

  1. Langmuir-Hinshelwood mechanism

Both reactants, let’s say reactants A and B, are adsorbed on the surface of the catalyst. This allows the reactants to interact with each other and bond together. The product C is then desorbed from the surface.

  1. Rideal-Eley mechanism

In this mechanism, only one of the two reactants is adsorbed on the surface. The second reactant, B, then meets the adsorbed reactant, A, react to form a bond. Finally, the product C is desorbed from the surface.

  1. Precursor mechanism

In this mechanism too, only one of the reactants (A) gets adsorbed. The other reactant, B, collides on the surface, resulting in a precursor state. It then collides with the adsorbed reactant to form a bond. The product gets desorbed following its synthesis.

One or a combination of these mechanisms is involved in most of these catalytic reactions.

How can you improve the reaction efficiency further?

phosphine ligandsOrganic synthesis can be made more efficient by the use of phosphine ligands.

Following three ligands are the ones most often used in reactions:

At Watson International, you can get consistent supply of high quality chemicals including metal catalysts and phosphine ligands. You can also order homogeneous and other catalysts, enzymes and a growing list of chemicals through our website.