Adsorption Theory of Heterogeneous Catalysis

1.0Introduction

The adsorption theory of heterogeneous catalysis explains how solid catalysts speed up chemical reactions by providing an active surface where reactants can adsorb, react, and then desorb as products.

According to this theory:

1. The catalyst surface contains active sites.
2. Reactant molecules are adsorbed onto these sites (by physisorption or chemisorption).
3. Adsorption weakens the existing bonds in reactant molecules, thereby reducing activation energy.
4. Reaction occurs between adsorbed molecules, forming new products.
5. Desorption takes place, releasing products from the surface and regenerating the active sites for further reaction.

This cyclic mechanism explains why even a small amount of catalyst can convert a large amount of reactants. It also clarifies the role of specific catalysts in industrial processes like the Haber’s process, Contact process, and hydrogenation reactions.

2.0Steps Involved in Adsorption Theory

The mechanism of heterogeneous catalysis can be broken down into five distinct steps:

1. Diffusion of Reactants: The reactant molecules (adsorbates) diffuse from the bulk phase (e.g., gas or liquid) to the surface of the solid catalyst.
2. Adsorption of Reactants: The reactant molecules get adsorbed onto the active sites on the catalyst's surface. This is a crucial step that often involves the formation of a weak bond between the reactant and the catalyst, known as chemisorption.
3. Reaction on the Surface: The adsorbed reactants react to form an intermediate or product. This reaction occurs on the catalyst's surface.
4. Desorption of Products: The newly formed product molecules detach (desorb) from the catalyst surface. This frees up the active sites for new reactant molecules.
5. Diffusion of Products: The product molecules diffuse away from the catalyst's surface into the bulk phase.

3.0Factors Affecting Heterogeneous Catalysis

Several factors influence the efficiency of a heterogeneous catalyst:

• Surface Area: A larger surface area of the catalyst provides more active sites for adsorption, leading to a faster reaction rate. Porous materials are often used for this reason.
• Pore Size and Structure: The size and shape of the pores in the catalyst can affect the diffusion of reactants and products, influencing overall efficiency.
• Temperature: Increasing the temperature generally increases the reaction rate, but excessively high temperatures can lead to the catalyst's deactivation.
• Promoters and Inhibitors: Promoters are substances that enhance a catalyst's activity, while inhibitors(or catalytic poisons) reduce or destroy its activity.

4.0Applications and Examples

Heterogeneous catalysis is vital in many industrial processes. Some key examples include:

• Haber-Bosch Process: The synthesis of ammonia  NH₃​ , from nitrogen N₂​ and hydrogen H₂​ , using an iron catalyst. Equation: N₂​(g)+3H₂​(g)⇌2NH₃​(g)
• Contact Process: The production of sulfuric acid H₂SO₄ , using V₂O₅ as a catalyst for the oxidation of SO₂ and SO₃. Equation: 2SO_2​(g)+O_2​(g)⇌2SO_3​(g)
• Catalytic Converters: Used in vehicles to convert toxic gases like carbon monoxide (CO), nitrogen oxides NO₂, and unburnt hydrocarbons into less harmful substances using platinum, palladium, and rhodium catalysts.