Heterogeneous Catalyst: Structure, Adsorption, Steps & Examples

Heterogeneous catalyst is not in the same phase as the reactants. Most reactions involve the solid surface as the catalyst while the reactants are either liquids or gases. 

• Different materials are used as catalysts like platinum, nickel, iron, and copper, and certain metal oxides such as ferric oxide, zinc oxide, molybdenum oxide, etc. 
• Solid catalysts are carefully prepared to ensure a clean surface and large surface area.
• Heterogeneous catalysis, in the field of chemistry, is catalysis in which the phases of the catalysts and the reactants or products are different. 
• In contrast, the catalyst, reactants, and products are all present in the same phase during homogeneous catalysis. 
• Phase is capable of distinguishing between elements that are solid, liquid, or gas, as well as immiscible mixtures (such as oil and water), or any area where an interface is present.

Key Terms: Catalyst, Homogeneous, Reactants, Molecules, Electrons, Adsorption, Haloalkanes

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Definition of Heterogeneous Catalysts

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Chemical catalysts that differ physically from the reactants and/or products involved in the catalyzed chemical reaction are referred to as heterogeneous catalysts. The chemical interaction between two gaseous reactants is frequently facilitated by the use of solid-phase heterogeneous catalysts. The catalysis in these reactions occurs over the following steps:

• The gaseous reactants' adsorption on the solid heterogeneous catalyst's surface.
• The product is created as a result of the chemical reaction between the reactants that have been adsorbed.
• The catalyst's active catalytic surface is renewed as a result of the desorption of the product compound from its surface.
• Because they allow for the relatively large-scale manufacturing of a number of commercially significant goods, heterogeneous catalysts are incredibly helpful. 
• For instance, in the Haber process for the commercial generation of ammonia, iron oxides coated on alumina (a chemical compound with the formula Al₂O₃) are frequently utilized as heterogeneous catalysts.

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Structure and Function of Heterogeneous Catalysts

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Heterogeneous Catalysts have enormous surface areas between 1 to 500 m²/g for contact. It may be noted that many reactions that occur on a metal surface such as the decomposition of HI on a gold surface, and the decomposition of ammonia on tungsten, molybdenum, or platinum are found to be of zero order.

• This is because the rate-determining step occurs on the surface itself.
• In all these reactions, the reactant gases get absorbed on the surface of the catalyst and the surface of the catalyst is completely covered.
• Some of the reactant molecules get converted into products.
• However, immediately new molecules take up their positions, and thus the concentration of the reactant molecule remains constant on the surface.
• Consequently, the rate of reactions remains constant irrespective of the total amount of the gaseous reactants taken initially.

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Adsorption

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The process of adsorption is crucial to heterogeneous catalysis. A gas (or solution) phase molecule (the adsorbate) attaches to the surface atoms of a solid (or liquid) by the process of adsorption (the adsorbent). 

• Desorption, in which the adsorbate separates from the adsorbent, is the opposite of adsorption. 
• Heterogeneous catalysis facilitates reactions when the reactants are the adsorbate and the catalyst is the adsorbent.

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Types of Adsorptions

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There are two distinct adsorption processes, physisorption, and chemisorption, that might cause the reactant molecules to bind to the surface of the heterogeneous catalyst.

1. Physisorption – The form of adsorption is known as physisorption when the molecules of the adsorbate are drawn to the atoms or molecules of the adsorbent as a result of the Van der Waals forces that develop between them. 

• It is significant to remember that when physisorption occurs, no chemical bonds are created between the adsorbent and adsorbate. 
• The reactant molecules physisorbed onto the surface of a heterogeneous catalyst during heterogeneous catalysis.
• This is the result of the Van der Waals forces that form between both the reactant molecules as well as the atoms/molecules on the surface of the heterogeneous catalyst.

2. Chemisorption – The electron clouds of the adsorbent and the adsorbate can overlap if the molecules of adsorbate are placed closer sufficient to the atoms or molecules on the surface of the adsorbent. 

• Chemisorption is a term used frequently to describe this phenomenon. 
• It is usual for the adsorbate as well as the adsorbent to share electrons during chemisorption. 
• This suggests that new chemical bonds are created during the chemisorption process.

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Steps of Heterogeneous Catalysts

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The whole process can be described in a few steps–

• Reactants diffuse to the catalysts' surface. 
• The reactants first come into touch with the exterior surface in this process, from which some of them escape and enter the inner exposed surface, which includes the external surface's pathways and fractures.
• Then, these molecules attach themselves to the appropriate adsorption sites.
• The reactants have a better chance of interacting with one another when bonded to the surface, and this reaction results in the formation of an intermediate molecule.
• Following this procedure, the intermediate chemical is desorbed from the surface, which makes it once again available for subsequent molecule adsorption.
• The final products are created when the intermediate component is broken down, and they then diffuse from the catalyst's pores and surface on the inside.

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Examples of Heterogeneous Catalysis

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Below are a few typical instances of reactions involving heterogeneous catalysis (reactions where the physical states of reactants and catalysts differ).

• Vanadium oxides catalyze the contact process for the manufacture of sulfuric acid, which involves the reaction between oxygen and sulfur dioxide.
• Iron oxides in alumina serve as the catalyst in the Haber-Bosch process for the industrial synthesis of ammonia, which involves the reaction between hydrogen and nitrogen.
• A platinum-rhodium gauze that is not supported catalyzes the ammonia and oxygen reaction in the Ostwald technique to produce nitric acid.
• Methane and water combine to make hydrogen during the steam reformation of methane, which is catalyzed by nickel or potassium oxide.
• Silver on alumina and a variety of other promoters work together to catalyze the interaction between ethylene and oxygen, which produces the synthesis of ethylene oxide.

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Things to Remember

• The reactant and the catalyst are in separate phases during the heterogeneous catalytic process.
• Catalysts that are heterogeneous play a big part in the simplification of industrial chemical processing. 
• In homogeneous catalysis, the catalyst and reactants happen simultaneously. 
• In heterogeneous catalysis, the catalyst is involved in a different stage than the reactants. 
• The most typical type of heterogeneous catalyst is a solid catalyst.
• Typically, solid-phase catalysts and gas-phase reactants are used in heterogeneous catalysis.
• Heterogeneous gold catalysis is the term used to describe the use of gold to catalyze chemical reactions, usually supported on metal oxide substrates.

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