Activity and Selectivity of Solid Catalysts

Solid catalysts simplify the separation of catalysts from processes and provide for the potential of reusing those catalysts. Catalyst selectivity and activity are significant concepts. In nature, catalytic surfaces are usually inactive. The catalyst is activated whenever a reactant is adsorbed on its surface. This initiates the subsequent reaction process. 

Solid acid catalysts are highly specialised; what works as a catalyst for one reaction may act as an inhibitor for the other. Specific catalysts using the same combination of reactants produce different products than others.

What Is Catalysis?

A catalyst is a substance that increases the rate of chemical reactions without changing its mass or chemical structure after the reaction. Catalysis is the process of maximising the speed of a reaction with a catalyst, which is unaffected by the catalysed process and can act repeatedly.

Small amounts of catalyst are usually enough to achieve this result. In addition, catalysed reactions typically produce a transitory intermediate that regenerates the original catalyst.

For a long time, solid catalysts have been the primary energy source for most industrial chemical processes. In addition, thousands of enzymes work as catalysts in living cells in the human body. Enzymes (catalysts) are essential for the survival of every living organism.

Catalysis Mechanism

A catalyst reduces the amount of free energy required to achieve the transition state, while the overall amount of free energy transferred from reactants to products remains unchanged. A catalyst can be involved in a variety of chemical processes. For example, the effectiveness of a catalyst might vary depending on the presence of additional compounds known as inhibitors (which inhibit catalysis) or promoters (which induce the activity of the reaction).

An uncatalysed reaction can proceed more quickly at the same temperature and reactant concentration if it has lower activation energy than a catalysed reaction. It’s the same for any chemical reaction: The reaction rate is determined by how often the reactants come into contact with each other in the step that determines how quickly the reaction goes.

Typically, the catalyst is involved in this last and slowest stage. The rate of a reaction is proportional to the quantity of catalyst present. Secondary processes can inhibit, deactivate, or destroy catalysts, even when the reaction doesn’t consume them.

The mechanism followed during solid catalysis is known as the adsorption mechanism. The reactants are adsorbed on the surface of the catalyst and their bonds are weakened. This facilitates the formation of new bonds and makes the process faster.

Synthesis of Solid Catalysts

Solid catalysts are particularly complex in the world of catalysis. Solid catalysts, unlike liquid-phase catalysts, can be more complicated to characterise and more challenging to create because they often contain particles of one substance close to particles of other materials.

Research on solid catalysts is primarily focused on their role in facilitating chemical processes. However, some researchers are gaining a molecular understanding of the synthetic procedures used to create those composite materials. In their endeavour to understand and regulate the size, dispersion, shape, stability and other aspects of catalytic materials, they are encouraging others to join them.

Today’s large-scale chemical processes rely on solid catalysts, also called heterogeneous catalysts, due to their insoluble nature in the gaseous and liquid reagents they change.

Key Features of Solid Catalysts

Metals, alloys, metal oxides, and metal sulphides are all examples of solid catalysts. The effectiveness of a catalyst is determined by two critical characteristics: activity and selectivity.

1. Catalyst Activity

A catalyst’s activity is defined as its capacity to accelerate a specific reaction. Chemisorption is the primary determinant of a catalyst’s activity. It should be just powerful enough to activate the catalyst. Catalytic surfaces and their reactants must produce neither strong nor weak bonds.

As long as it isn’t so powerful that the molecules of the reactants get adsorbed on the catalytic surface, there won’t be any more space for new molecules.

The tungsten catalyst accelerates the breakdown of ammonia by ten times more, which is usually a slow process. During the formation of water, the action of a catalyst becomes increasingly essential. It can take years for hydrogen and oxygen to build up, but there won’t be even a drop of water in the world without a catalyst. However, the platinum catalyst causes hydrogen and oxygen to explode, resulting in the formation of water.

For instance, in platinum, H2 and O2 combine explosively to generate water.

2H2 + O2 + Pt → 2H2O

2. Catalyst Selectivity

Catalysts are extremely specialised molecules. They can control the reaction so that a specific product is produced. The reaction may produce other products when the same reactants are used, but a different catalyst is used. This is referred to as the catalyst’s selectivity. In nature, Solid catalysts are pretty selective. They can enhance one reaction while inhibiting another. So, we may state that a specific catalyst can catalyse only one type of reaction. Therefore, it may fail to catalyse a similar reaction.

For the reaction of H2 and CO, we can obtain different products by using several catalysts.

For solid catalyst examples, ethanol can be dehydrogenated and dehydrated. However, only one reaction is obtained when a proper catalyst is present.

When different catalysts are applied, various products are obtained.

CO+3H4 → CH4+H2O reaction with Ni as a catalyst

CO+H2 → HCHO  reaction with Cu as a catalyst

Different Types of Catalysts

A catalyst is a chemical molecule that accelerates the rate of a reaction by lowering the activation energy barrier. As a result, it is not absorbed during the reaction.

Homogeneous Catalyst:

This type of catalyst has the same physical state as the reactants and products and is usually in liquid phase.

Heterogeneous Catalyst:

This type of catalyst is in a different physical state than the reactants and products. It is usually a solid catalyst.

Acid-Base Catalysis:

Acid catalysis uses H+ ions as solid acid catalysts. On the other hand, OH- ions are usually the catalysts in base catalysis.

The hydrolysis of sucrose, more commonly known as table sugar, is a reaction that acid can aid. Sucrose is a compound sugar (or monosaccharide) composed of two simpler sugars, glucose and fructose. Sucrose can be converted to glucose and fructose with an acid or even an enzyme-like sucrase.

Conclusion

Solid catalysts play an essential role in everything from basic research to industrial usage. Solid catalysts are associated with the terms ‘activity’ and ‘selectivity’. The majority of heterogeneous solid catalysts are composed of base or basic oxides with a high specific surface area. Therefore, it is more effective to use solid-base catalysts than solid-acid catalysts.