What is a catalyst?

Catalyst is a common word that you come across when studying chemistry, especially when learning about chemical reactions. While some chemical reactions happen quickly, some take longer and require additional materials or effort. This is where a catalyst comes into play.

Catalysts are not consumed in chemical reactions but can act repeatedly. Often only very small amounts of catalyst are needed. The global demand for catalysts in 2010 was estimated at approximately 29.5 billion US dollars because they are widely used to accelerate chemical reactions in various industries.

What is the role of the catalyst?

In chemistry, catalysts are substances that change the reaction rate by changing the reaction path. Catalysts are often used to speed up or increase the rate of a reaction. However, if we go to a deeper level, catalysts are used to break or rebuild chemical bonds between atoms that exist in molecules of different elements or compounds. In fact, catalysts encourage molecules to react and make the entire reaction process easy and efficient.

Some of the important properties of catalysts are:
A catalyst does not start a chemical reaction.
The catalyst is not consumed in the reaction.

Catalysts tend to react with the reactants to form intermediates and at the same time facilitate the production of the final product of the reaction. After the whole catalysis process can be repeated again.

The catalyst can be solid, liquid or gas. Some solid catalysts include metals or their oxides, including sulfides and halides. Semi-metallic elements such as boron, aluminum and silicon are also used as catalysts. Likewise, liquid and gaseous elements that are pure are used as catalysts. Sometimes, these elements are also used together with suitable solvents or carriers. Reactions that include a catalyst in their system are known as catalytic reactions.

Classification of catalysts

There are different types of catalysts that can be used in a chemical reaction depending on the need. They are as follows:

Positive catalysts

Catalysts that increase the rate of a chemical reaction are positive catalysts. These compounds increase the speed of the reaction by reducing the activation energy barriers so that a large number of reaction molecules are converted into products, thereby increasing the yield and yield of the products.

Example: In the preparation of NH3 by the Haber process, iron oxide acts as a positive catalyst and increases the yield of ammonia despite the lower reactivity of nitrogen.

Negative catalysts

Catalysts that slow down the reaction rate are called negative catalysts. This action slows down the reaction rate by increasing the activation energy barrier, which reduces the number of reactant molecules to convert into products.

Example: The decomposition of hydrogen peroxide into water and oxygen is reduced by using stanilide. This substance acts as a negative catalyst to slow down the decomposition of hydrogen peroxide.

promoters or accelerators

A substance that increases the activity of a catalyst is known as a promoter or accelerator.

Example: In the Haber process, molybdenum or a mixture of potassium and aluminum oxides act as promoters.

poisons or catalyst inhibitors

Substances that reduce catalyst activity are known as catalyst poisons or inhibitors.

Example: In the hydrogenation of an alkyne to an alkene, the palladium catalyst is poisoned with barium sulfate in the quinolone solution and the reaction stops at the alkene level. This catalyst is known as Lindler catalyst.

Types of catalysts

Based on the nature and physical state of the substance used in the chemical reaction, catalysts are divided into four types:

Homogeneous catalyst
Heterogeneous catalyst
Automatic catalyst
Bio catalyst

What is a heterogeneous catalyst?

In this type of catalyst, the reactants in a reaction and the catalyst are not in the same substance or phase.

Example 1: Preparation of ammonia by the Haber process

Pure and dry nitrogen and hydrogen gases in a ratio of 1:3 are transferred through a compressor, where a high pressure of 30 to 200 atmospheres is maintained. In this process, iron oxide is used as a catalyst. This oxide is a solid used in a process where the reactants are in the gaseous state. Nitrogen (g) reacts with hydrogen (g) to form ammonia (g). Therefore, iron oxide is a heterogeneous catalyst.

Example 2: Sulfuric acid production through contact process.

In this process, the oxidation of sulfur dioxide is a major step. In the oxidation reaction, sulfur dioxide is one gas and oxygen is another gas, while vanadium pentoxide is a solid catalyst. In this process, reactants and catalysts are in different states of matter.

Heterogeneous catalyst mechanism

Heterogeneous catalysis involves adsorption as well as formation of an intermediate compound. The reactant molecule is adsorbed in the activation center of the catalyst surface. These compounds react together to form an activated complex which is an intermediate compound. This compound is broken down to produce products.

Therefore, the heterogeneous catalyst includes the initial adsorption of reactants on the surface of the catalyst, the formation of an intermediate compound, and separation into a product.

Example: hydrogenation of ethene to ethane on nickel surface.

Ether and hydrogen molecules are adsorbed on the surface of the catalyst. Hydrogen occupies most of the activation center and is known as blocking. The ethane molecule attacks its double bond region and forms an active complex. Ether reacts with active hydrogen to form ethane. Ethane is quickly removed from the catalyst surface.

Homogeneous catalyst

In a reaction where the catalyst used in the reaction and the reactants are in a similar state of matter, this process is referred to as homogeneous catalysis. In this process, homogeneous reactants and catalysts act in one phase.

Usually, homogeneous catalysts are dissolved with substrates in a solvent. The effect of H + in the esterification of carboxylic acids, such as the formation of methyl acetate from acetic acid and methanol, is one of the examples of homogeneous catalysis. Hydroformylation, hydrosilylation, hydrocyanation include high volume processes and require homogeneous catalysts.

Homogeneous catalysis is often synonymous with organometallic catalysts for inorganic chemists. However, many homogeneous catalysts are not metallic, as catalyzed the oxidation of p-xylene to terephthalic acid using cobalt salts.

What is a catalyst and what is its role in chemical reactions?

It is better to know:

While transition metals attract much attention in the study of catalysts, small organic molecules without metals may also exhibit catalytic properties, and the absence of transition metals is also evident in many enzymes.

Organic catalysts are usually required in larger amounts (amount of catalyst per unit amount of reactant) than transition metal (-ion) based catalysts, but these catalysts are purchased and processed in bulk and help reduce costs. . The price of the catalyst varies depending on its type in the market.

Such organic catalysts were considered as a “new breed” in the early 2000s and are competitive with conventional metal (-ion) catalysts.

Example: Hydrolysis of ethyl acetate in the presence of dilute acid.

Ethyl acetate is a liquid that contains an ester functional group. It reacts with water in the presence of dilute sulfuric acid to produce ethyl alcohol and acetic acid. In this process, the reactant and the catalyst are in the same phase.


Homogeneous catalyst mechanism

A homogeneous catalyst acts as an intermediate in the reaction. Let us illustrate this process using the oxidation process of SO2 to SO3 in a lead chamber. In this reaction, nitric oxide gas is the catalyst. NO first reacts with SO2 and forms SO2 and “NO2” as an intermediate compound.

2SO2(g)+O2(g)⟶ 2SO3 g

In the first stage, nitric oxide combines with oxygen and creates nitrogen dioxide (NO2). NO2 then acts as an intermediate compound that reacts with SO2 to form sulfur trioxide and NO.

(2NO(g) + O2(g) → 2NO2(g).

(2SO2 + 2NO2 → 2SO3(g) + 2NO(g

Automatic catalyst

In the autocatalytic reaction, no special catalyst is added to the reaction. Instead, one of the products acts as a catalyst and increases the rate of formation of the products.

Example 1: Decomposition of arsenic (AsH3) by the arsenic formed in the “autocatalyst” reactor.

2As H3 → 2As + 3H2

Example 2: Oxidation of oxalic acid by KMnO4

When permanganate is added to an acidic solution, oxidation of oxalate ions (or oxalic acid) occurs. This reaction leads to the formation of Mn+2 ions and catalyzes the reaction automatically. The reaction rate between potassium permanganate and oxalate acid solution is initially slow. Mn+2 ions, which are created during the reaction, help to increase the reaction rate.

What is a catalyst and what is its role in chemical reactions?

Bio catalyst

Biocatalysts are enzymes that catalyze biochemical reactions in the body. Most enzymes have a protein structure and have different names based on the reaction they catalyze.

What is a petrochemical catalyst?

Catalysts are one of the most important and key components for the operations of the oil, gas, petrochemical and chemical industries of countries, and considering that catalysts play an important role in the production of various fuels and many intermediate and final products, their importance is increasing day by day in It is increasing and scientists are looking for better catalysts at affordable costs.