What are Heterogeneous Catalysts?
Heterogeneous catalysts are chemical catalysts whose physical phase is different from the physical phase of the reactants and/or products that take part in the catalyzed chemical reaction. Typically, solid phase heterogeneous catalysts are employed in order to facilitate the chemical reaction between two gaseous reactants. In such reactions, the catalysis takes place over the following three steps:
- The adsorption of the gaseous reactants on the surface of the solid heterogeneous catalyst.
- The chemical reaction between the adsorbed reactants, resulting in the formation of the product.
- Desorption of the product compound from the surface of the catalyst, resulting in the regeneration of the active catalytic surface.
Heterogeneous catalysts are extremely useful because they enable production of several commercially important products on a relatively large scale. For example, oxides of iron placed on alumina (a chemical compound with the formula Al2O3) are widely used as heterogeneous catalysts in the Haber process for the industrial production of ammonia.
How does the Adsorption of the Reactants on the Surface of the Heterogeneous Catalyst take place?
Adsorption is a critical step during the process of heterogeneous catalysis. Adsorption can be visualised as the mechanism through which the adsorbate (the reactant molecule, usually present in the gaseous phase) binds to the atoms at the surface of the adsorbent (the heterogeneous catalyst, usually present in the solid phase). The opposite of adsorption is commonly known as desorption. It refers to the breaking apart of the adsorbents and the adsorbate. In the chemical reactions that are induced by heterogeneous catalysis, the reactants are the adsorbate and the heterogeneous catalyst is usually the adsorbent in the process.
The adsorption of the reactant molecules onto the surface of the heterogeneous catalyst can occur through two different adsorption processes, namely physisorption and chemisorption.
Heterogeneous Catalysis involving Physisorption
When the molecules of the adsorbate are attracted to the atoms/molecules of the adsorbent as a result of the Van der Waals forces that arise between them, the type of adsorption is commonly referred to as physisorption. The types of forces that can contribute towards physisorption include:
- Interactions between two dipoles
- Interactions between a dipole and an induced dipole
- London dispersion forces
It is important to note that no chemical bonds are formed between the adsorbent and adsorbate when physisorption takes place. During heterogeneous catalysis, the Van der Waals forces that arise between the reactant molecules and the atoms/molecules present on the surface of the heterogeneous catalyst result in the physisorption of the reactant molecules onto the surface of the heterogeneous catalyst. Now, the reactant molecules can go on to be more powerfully adsorbed onto the surface of the catalysts via a process known as chemisorption, or they can undergo desorption and move away from the surface of the heterogeneous catalyst. It can also be noted that the reactant molecules, having undergone physisorption onto the surface of the catalyst, can migrate across the surface of the catalyst as well.
Heterogeneous Catalysis involving Chemisorption
When the molecules of the adsorbate are brought close enough to the atoms or molecules that are present on the surface of the adsorbent, an overlap of the electron clouds of the adsorbate and the adsorbent can take place. This process is commonly referred to as chemisorption. During chemisorption, it is not uncommon for the adsorbate and the adsorbent to share electrons. This implies that the process of chemisorption involves the formation of new chemical bonds.
When it comes to heterogeneous catalysis via chemisorption, the reactant molecules are usually adsorbed onto the surface of the heterogeneous catalyst to the point that an overlap of their electron clouds takes place. Thus, it is not uncommon for chemical bonds to be formed between the reactants and the heterogeneous catalysts in such reactions. It can be noted that the chemisorption of the reactant molecules onto the surface of the catalyst can take place in one of two different ways:
The molecular adsorption pathway – here, the molecular structure of the adsorbate (the reactant) remains uncompromised. The reactant molecule forms bonds with the heterogeneous catalyst without breaking any of the chemical bonds between the atoms that constitute it. An example of molecular adsorption can be observed in the binding of alkenes to the surface of activated platinum catalysts.
The dissociative adsorption pathway – in this type of chemisorption, the molecular structure of the adsorbate (or the reactant) is affected. The chemical bonds between the atoms that constitute the molecule are broken or altered in order to accommodate the formation of new bonds with the heterogeneous catalyst. Therefore, the rate of this type of adsorption is dependent on the rate of dissociation of the bonds in the adsorbent. A common example of dissociative adsorption can be observed in the binding of dihydrogen molecules to a platinum catalyst. Upon being adsorbed onto the platinum surface, the hydrogen-hydrogen bond in the H2 molecule is broken.
Examples of Heterogeneous Catalysis
Some common examples of reactions that involve heterogeneous catalysis (reactions in which the physical states of the reactants and the catalysts are different) are provided below.
- The contact process for the synthesis of sulfuric acid, which involves the reaction between oxygen and sulfur dioxide, catalyzed by oxides of vanadium.
- The Haber-Bosch process for the industrial production of ammonia, which involves the reaction between hydrogen and nitrogen, catalysed by oxides of iron on alumina.
- The Ostwald process for the synthesis of nitric acid, which involves the reaction between ammonia and oxygen, catalyzed by an unsupported platinum-rhodium gauze.
- Steam reforming of methane for the production of hydrogen, which involves the reaction between methane and water, catalyzed by nickel or potassium oxide.
- The synthesis of ethylene oxide, which involves the reaction between ethylene and oxygen, catalyzed by silver on alumina along with several other promoters.
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