Adsorption on Catalysts

In heterogeneously catalysed reactions, Langmuir explained the role of adsorption on catalysts. Contact catalysis refers to the adsorption of the reactant molecules on the catalyst surfaces. The adsorption concept of catalysis claimed that reactants in their gaseous state are adsorbed onto the surface of a solid catalyst. The pace of the reaction is accelerated by increasing the surface concentration of the reactants. Adsorption of the reactants on the catalyst’s surface is thought to generate an active complex that decomposes and releases the product.

What is Catalysis?

Catalysis is defined as the process in which the reaction rate is altered (either increased or decreased) due to the presence of a substance that remains chemically unchanged during the reaction. Catalyst refers to the substance that affects the rate of reaction. A catalyst can be positive (increasing the reaction rate) or negative (decreasing the reaction rate).

Features of Catalysts

1. The catalyst’s volume and chemical composition remain unchanged.
2. In reversible processes, the catalyst does not affect the equilibrium composition of the reaction mixture. It just serves to speed the process up of establishing and maintaining equilibrium.

Different Types of Catalysis

There are three types of catalytic reactions:

a) Autocatalysis.

b) Homogeneous Catalysis.

c) Heterogeneous Catalysis.

a) Autocatalysis

Autocatalysis occurs while one of the products of a reaction process serves as a catalyst.

b) Homogeneous Catalysis

Homogeneous catalysis is a catalytic process in which the catalysts and reactants are all in the same physical condition.

c) Heterogeneous Catalysis

Heterogeneous catalysis can be defined as a type of catalysis when the catalyst’s phase differs from the reactants. It’s sometimes called surface or contact catalysis. The mechanism of a heterogeneous catalyst is explained using this hypothesis. This theory combines the traditional adsorption on catalysts’ theory with the concept of intermediate compound formation.

Most heterogeneous catalysts are solids, while most reactants are gases or liquids. Heterogeneous catalysis is vital in a wide variety of energy and chemical industries. In 1918, Fritz Haber and Carl Bosch won the Nobel Prize in chemistry for their work on heterogeneous catalysis, as did Irving Langmuir in 1932 and Gerhard Ertl in 2007.

Adsorption

Adsorption is frequently a prerequisite for heterogeneous catalysis. Because adsorption on catalysts is an exothermic reaction, the heat created during the process is also used to dissolve or break the bindings between the reactants, thereby favouring new bonds. Intermediate compound production explains homogeneous catalysis. Both the catalyst and the reactant are in the same stage in this reaction.

Adsorption on catalysts refers to the concentration of molecules at the surface instead of being in a solid or liquid mass due to unbalanced attraction forces. The molecular species that adsorbs to the surface is referred to as an adsorbate, and the substance on which the adsorption occurs is referred to as an adsorbent.

(i) Charcoal adsorbs O2, H2, C12, and NH3 gases.

(ii) Silica gels effectively absorb water molecules from the air.

Desorption is the reversal of this process (in which the adsorbate separates from the adsorbent). The adsorption on catalysts is the adsorbate, while the supporting is the adsorbent.

Heterogeneous Catalysis Mechanism:

A five-step process is required for heterogeneous catalysis according to the Surface Adsorption Hypothesis:

Step 1: Reactant Diffusion to Surface:

Reactant molecules diffuse the solid catalytic surface. The surface reaction rate is influenced by bulk absorption and the size of the border layer.

Step 2: Reactant adsorption

Physical adsorption of reactant molecules on the surface occurs initially. Afterwards, they become chemisorbed. Bonds form as the reactant(s) adsorbed onto the catalytic surface. These molecules are then adsorbed to the accessible adsorption sites. The potential of an atom or molecule to apply to a surface is radiantly referred to as the Sticking Coefficient.

Step 3: Chemical Reaction:

The adjacent chemisorbed molecules react to form the products. When reactants are attached to the surface, they have a greater chance of interacting and forming an intermediate molecule. On the exterior, bonds develop between atoms and molecules.

Step 4: Product Desorption:

This is the inverse of the adsorption process. The resulting molecules are initially chemisorbed onto the surface following the reaction. Following this process, the transitional chemical is desorbed from the surface, which becomes available for further adsorption of additional molecules. Bonds are broken as the product(s) desorbs. Physical adsorption and ultimate separation from the surface occur at this point.

Step 5: Product release:

There is a diffusion of product molecules from the surface. The transitional chemical then disintegrates to generate the final products, eventually diffusing from the catalyst’s interior pores and the exterior surface. The molecules are then adsorbed from the catalyst’s surface.

Applications of Adsorption

• To create a high-vacuum environment.
• Coal miners breathe through gas masks that contain activated charcoal. They are capable of adsorbing harmful gases.
• Adsorbents like silica and aluminium gels help reduce humidity.
• Colourant removal from solutions.
• It is a catalytic intermediate in heterogeneous catalysis.
• For inert gas separation.
• As for indicators of adsorption.
• In the context of chromatographic analysis.
• Qualitative analysis.

Conclusion

Surface Chemistry is a subfield of chemistry that studies surface phenomena, such as corrosion: catalysis, crystallisation, and similar processes. Heterogeneous catalysis is defined as a surface phenomenon in which the catalyst’s catalytic activity is concentrated on the catalyst’s surface. The adsorption on catalysts theory explains why the catalyst gives the surface and why the reaction occurs. The reactant diffuses across the catalyst’s surface and adsorbs on the catalyst’s active sites.