Adsorption Theory of Heterogeneous Catalyst: Definition, Mechanism, and Examples

Adsorption theory of heterogeneous catalysis states that the catalyst binds to the reactants on the catalyst's surface, and the reactants subsequently react with each other to generate the product. Heterogeneous catalysis, as used in chemistry, is catalysis in which the phases of the catalysts and the reactants or products are different. In contrast, in homogeneous catalysis, the catalyst, reactants, and products are all present in the same phase. 

• A catalyst's mechanism is described by the heterogeneous catalysis adsorption theory. 
• According to the notion, the catalyst binds to the reactants on its surface, which then undergo a chemical reaction to produce the product. 
• Langmuir outlined the function of adsorption on catalysts in reactions that are heterogeneously catalyzed. 
• Adsorption of the reactant molecules on the catalyst surfaces is referred to as contact catalysis.

Key Terms: Heterogeneous catalysis, Adsorption, Catalysts, Equilibrium, Exothermic, Chemisorbed, Haber-Bosch process

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What is Catalysis?

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Catalysis is the process in which the reaction rate is changed (either raised or lowered) because the material is present throughout the reaction while remaining chemically unaltered. The component that affects a reaction's rate is referred to as a catalyst.

• Both the catalyst's volume and chemical makeup are unaltered. 
• The catalyst has no impact on the equilibrium composition of the reaction mixture in reversible processes. 
• It only contributes to hastening the establishment and maintenance of equilibrium.
• Catalysis may be classified into two categories based on the phases of catalysts and reactants-- Homogeneous Catalysis and Heterogeneous Catalysis.
• A homogeneous catalyst is one that is present in the same phase as the reactants in the reaction, and this form of catalysis process is known as homogeneous catalysis.

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Definition of Heterogeneous Catalysis and Catalysts

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When the phases of the catalyst and reactants are different, this is known as heterogeneous catalysis. It is also known as surface catalysis or contact catalysis. Using this supposition, the workings of a heterogeneous catalyst are described. The traditional notion of adsorption on catalysts is combined in this theory with the idea of intermediate compound production.

• While most reactants are gases or liquids, the majority of heterogeneous catalysts are solids. 
• Numerous chemical and energy businesses depend on heterogeneous catalysis. 
• Catalysts are chemicals that hasten chemical reactions without consuming any of the ingredients involved. 
• They accomplish this by offering a surface on which the reactants can interact, hence lowering the energy required for the reaction to take place.

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Adsorption Theory of Heterogeneous Catalysis

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Heterogeneous catalysis typically requires adsorption as a precondition. Adsorption on catalysts is an exothermic reaction, and the heat produced by this reaction is utilized to dissolve or break the bonds that hold the reactants together, encouraging the formation of new bonds.

A model that explains the operation of a catalyst is the adsorption theory of heterogeneous catalysis. According to the theory, the catalyst binds to the reactants on its surface, and the reactants then interact to create the product.

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Heterogeneous Catalysis Mechanism

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1. Reactant Diffusion to Surface– The solid catalytic surface is diffused by reactant molecules. Bulk absorption and the thickness of the border layer have an impact on the rate of the surface reaction.
2. Reactant adsorption– Reactant molecules initially adhere physically to the surface. They then become chemisorbed. As the reactant or reactants are adsorbed onto the catalytic surface, bonds are formed. Then, these molecules are taken up by the available adsorption sites. The Sticking Coefficient is a glowing term for an atom or molecule's capacity to adhere to a surface.
3. Chemical Reaction– The products are formed by the reactions of the nearby chemisorbed molecules. Reactants have a better probability of interacting and generating an intermediary molecule when they are connected to the surface. Bonds form between atoms and molecules on the surface.
4. Product Desorption– This is how the adsorption process is reversed. Following the reaction, the resultant molecules are initially chemisorbed onto the surface. Following this procedure, the transitional chemical is desorbed from the surface, making it open to subsequent molecule adsorption. While the product(s) desorbs, bonds are broken. At this stage, physical adsorption and final separation from the surface take place.
5. Product release– The molecules of the product are diffusing from the surface. The ultimate products are then produced as the transitional chemical breaks down, gradually diffusing from the surface and interior pores of the catalyst. Following that, the molecules are absorbed from the catalyst's surface.

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Examples of Heterogeneous Catalysis

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A few examples of heterogeneous catalysts are–

1. The ammonia and oxygen reaction in the Ostwald method to make nitric acid is catalyzed by a platinum-rhodium gauze that is not supported.
2. Methane and water react in the steam reforming of methane to produce hydrogen, which is catalyzed by nickel or potassium oxide.
3. The reaction between ethylene and oxygen, which results in the synthesis of ethylene oxide, is catalyzed by silver on alumina along with a number of other promoters.
4. Vanadium oxides operate as catalysts in the contact method for the manufacture of sulfuric acid, which involves the reaction of sulfur dioxide and oxygen.
5. Iron oxides in alumina serve as the catalyst in the Haber-Bosch process, which produces ammonia in industrial quantities by combining hydrogen and nitrogen.

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Things to Remember

• A branch of chemistry called surface chemistry examines processes including crystallization, catalysis, and other surface phenomena like corrosion. 
• A surface phenomenon known as heterogeneous catalysis is when the catalyst's catalytic activity is concentrated on its surface. 
• The catalyst provides the surface, and the principle of adsorption on catalysts explains why the reaction takes place. 
• The catalyst's reactant diffuses across its surface and binds to its active sites.
• Homogeneous catalysis is a term used to describe a catalytic process in which the catalyst is present in the same phase as the reactants in the reaction.
• The two types of catalysis that can be distinguished based on the phases of the catalysts and the reactants are: Homogeneous Catalysis, Heterogeneous Catalysis 

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