Catalysis and Its Classification | Chemistry for EmSAT Achieve PDF Download

What is Catalyst in Chemistry?

In Chemistry, catalysts are substances that modify the rate of a reaction by altering its pathway. Typically, catalysts accelerate the reaction, though at a deeper level, they facilitate the breaking or formation of chemical bonds between atoms within molecules of different substances. Essentially, catalysts prompt molecules to react, simplifying and enhancing the overall reaction process.

Key features of catalysts include:

• Catalysts do not initiate reactions.
• Catalysts are not consumed during reactions.
• They interact with reactants to form intermediates, aiding in the production of final reaction products, and can regenerate after the process.
• Catalysts can exist in solid, liquid, or gaseous states. Solid catalysts may include metals, metal oxides, sulphides, halides, and semi-metallic elements like boron, aluminium, and silicon. Liquid and gaseous catalysts may be pure elements or combined with suitable solvents or carriers.

A reaction involving a catalyst is termed a catalytic reaction, wherein the catalyst interacts with a reactant to form chemical intermediates that can readily combine with each other or with another reactant to yield a product. The catalyst is regenerated during this process.
Catalytic reactions can take various forms, including acid-base reactions, oxidation-reduction reactions, coordination complexes formation, and the generation of free radicals. Solid catalysts, in particular, exhibit complex reaction mechanisms influenced by surface properties and electronic or crystal structures. Certain solid catalysts, like polyfunctional catalysts, may engage in multiple reaction modes with reactants.

Types of Catalysts with Examples

There are several types of catalysts that can be used depending on the need or requirement of the chemical reaction. They are explained below.

Positive Catalysts

Positive catalysts are substances that accelerate the rate of a chemical reaction. They achieve this by reducing the activation energy required for the reaction, thereby facilitating the conversion of a higher proportion of reactant molecules into products and increasing the overall yield of products.
Example of a positive catalyst: In the Haber process for the synthesis of ammonia, iron oxide serves as a positive catalyst, enhancing the production of ammonia despite nitrogen being less reactive.

Negative Catalysts

Negative catalysts, on the other hand, hinder the rate of a chemical reaction by increasing the activation energy barrier required for the reaction to occur. This impedes the transformation of reactant molecules into products, resulting in a decrease in the reaction rate.
Example of a negative catalyst: Acetanilide retards the decomposition of hydrogen peroxide into water and oxygen, functioning as a negative catalyst by impeding the decomposition process.

Promoters or Accelerators

Promoters or accelerators are substances that augment the activity of a catalyst, thereby enhancing its effectiveness in facilitating a chemical reaction.
Example: In the Haber process, molybdenum or a combination of potassium and aluminum oxides serve as promoters, amplifying the catalytic activity.

Catalyst Poisons or Inhibitors

Catalyst poisons or inhibitors are substances that diminish the activity of a catalyst, hindering its effectiveness in promoting a chemical reaction.
Example: In the hydrogenation of alkynes to alkenes, barium sulfate in quinoline solution poisons the catalyst palladium, stopping the reaction at the alkene stage. This type of catalyst is referred to as Lindler's catalyst.

Units

The SI unit for quantifying the catalytic activity of a catalyst is the "katal," measured in moles per second. The productivity of a catalyst can be described using the turnover number (TON), while the catalytic activity is expressed through the turnover frequency (TOF), representing TON per unit time. Additionally, the enzyme unit serves as its biochemical equivalent.

Catalysis

When a catalyst is used to increase the rate of a chemical reaction, this phenomenon is known as catalysis.

What Are the Types of Catalysis?

Catalysis can be classified into three types based on the nature and physical state of the substances involved in the chemical reaction:

1. Heterogeneous Catalysis

In this type of catalysis, the reacting substances in a reaction and the catalyst employed in that reaction are not in the same state of matter.
Examples: Preparation of ammonia by Haber’s process.

Pure and dry nitrogen and hydrogen gases in a 1:3 ratio are directed through a compressor, where a high pressure of 200-30 atmospheres is sustained. Iron oxide is utilized as a catalyst in this procedure. It is a solid oxide utilized in a reaction where the reactants exist in a gaseous state. Nitrogen gas reacts with hydrogen gas to produce ammonia gas under the influence of solid iron oxide, making it an example of heterogeneous catalysis.
Example 2: Manufacture of sulphuric acid by contact process.
In this process, the oxidation of sulphur dioxide is a major step. In this oxidation, sulphur dioxide and oxygen are gases, while vanadium pentoxide is a solid catalyst. Besides, reactants and catalysts are in different states of matter.

Mechanism of Heterogeneous Catalyst

Heterogeneous catalysis encompasses both adsorption and the formation of intermediate compounds. Reactant molecules are adsorbed onto the activation site of the catalyst's surface, leading to the creation of an activated complex, which acts as an intermediate compound. This complex subsequently decomposes to yield products. Once formed, the products promptly desorb from the surface without any delay. In essence, heterogeneous catalysis begins with the adsorption of reactants onto the catalyst's surface, followed by the formation of intermediate compounds and their subsequent dissociation into products.

Example: Hydrogenation of ethene into ethane on the surface of the nickel.

• Ether and hydrogen molecules are adsorbed on the surface of the catalyst.
• Hydrogen occupies most of the activation centre and is known as occlusion.
• Ethane molecules attack at their double bond region to form an activated complex.
• Ether reacts with active hydrogen to form ethane.
• This ethane gets desorbed on the surface of the catalyst.

Electrocatalysts

In the realm of electrochemistry, particularly within fuel cell engineering, various types of metal-based catalysts find application. These catalysts primarily serve to accelerate the rates of the half-reactions taking place within a fuel cell. Among the commonly utilized electrocatalysts in fuel cells are those predominantly composed of platinum nanoparticles, often supported on slightly larger carbon particles. When these catalysts interact with one of the electrodes within a fuel cell, platinum enhances the rate of oxygen reduction, leading either to the formation of water or hydroxide ions (as well as hydrogen peroxide).

Homogeneous Catalysis

• Homogeneous catalysis involves a scenario where the catalyst and the reactants share the same state of matter. These catalysts are typically in the same phase as the reactants, often dissolved in a solvent. An example of homogeneous catalysis is the influence of H+ ions on the esterification of carboxylic acids, such as the conversion of acetic acid and methanol into methyl acetate. 
• Processes like hydroformylation, hydrosilylation, and hydrocyanation, which involve high-volume production, often employ homogeneous catalysts. While homogeneous catalysis is often associated with organometallic catalysts in inorganic chemistry, it's worth noting that many homogeneous catalysts do not contain organometallic compounds. For instance, cobalt salts are used to catalyze the oxidation of p-xylene to terephthalic acid.
• In catalysis research, transition metals receive significant attention, yet it's noteworthy that small organic molecules devoid of metals can also display catalytic properties. This is evident from the absence of transition metals in numerous enzymes. Organic catalysts typically require a higher loading (catalyst quantity per unit of reactant, expressed in mol percentage) compared to transition metal-based catalysts. 
• However, organic catalysts are often available in bulk, leading to reduced costs. These organocatalysts, often regarded as a "new breed" in the early 2000s, compete favorably with conventional metal-containing catalysts.

Homogeneous Catalysis Mechanism

Homogeneous catalysis operates through an intermediate compound formation process. Consider the lead chamber process for oxidizing SO2 into SO3, where nitric oxide gas serves as the catalyst. Initially, NO reacts with SO₂ to produce an intermediate compound, NO₂.
First step: Nitric oxide combines with oxygen to yield nitrogen dioxide (NO₂), which acts as the intermediate compound.\
2NO(g) + O₂(g) → 2NO₂(g) (intermediate compound)
Subsequently, this NO₂ reacts with SO₂ to generate sulfur trioxide and NO.
2SO₂ + 2NO₂ → 2SO₃(g) + 2NO(g)

Photocatalysts: Photocatalysis involves a catalyst being activated to an excited state upon exposure to light, typically visible light.

Autocatalysis: Autocatalytic reactions don't require a specific external catalyst; rather, one of the products catalyzes the reaction, accelerating the production of further products.