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.