Catalysis | What do Catalysts do?

Catalysis is the process of increasing the rate of a chemical reaction by adding a catalyst. The rate of a chemical reaction can also be increased by raising the pressure or/and temperature of the reaction (Le Chatelier’s principle), but the reactors in which such conditions can be safely maintained become progressively more expensive and difficult to make.

Catalysts, on the other hand, accelerate reactions by order of magnitude, enabling them to be carried out under the most favorable thermodynamic conditions and at lower temperatures and pressures. Thus, efficient catalysts are the key factor in reducing the operational cost of a chemical process.

What do Catalysts actually do?

The catalysts provide an alternative mechanism by providing a different transition state and lowering the activation energy which is the minimum energy required to overcome the ‘reaction potential’ barrier.

For a chemical reaction; X + Y → Z

The catalyst changes the rate of thermodynamically feasible reaction (ΔG < 0) but cannot change the position of the thermodynamic equilibrium. (ΔG = -RT ln(K), where k is equilibrium constant).

The catalytic reaction is a cyclic process.

Catalyst Cycle

Every catalytic reaction is a sequence of elementary steps i.e., when the reactant molecules bind to the catalyst, where they react, after which the product detaches from the catalyst, liberating the latter (product) and getting ready for the next cycle.

A catalyst in a chemical reaction performs many cycles before it becomes deactivated, so it can be said that a catalyst does not get used up in a chemical reaction.

The number of cycles that a catalyst goes through the reaction before it is deactivated is called the turnover number. A good catalyst must have a large turnover number. Similarly, the turnover frequency of a catalyst is the turnover number in a certain period of time.

A catalyst is considered active in a catalytic reaction if it has certain properties.

Properties of an active catalyst

• Results in faster reactions even at low concentrations.
• Has a large turnover frequency.

Related Topics:

• Types of Catalysts
• Factors affecting the Reaction Rate

Classification of Catalysts

Based on the physical states, a catalyst can be classified as solid, liquid, or gas.

Based on the way catalyst work, they are classified into:

• Acid-Base catalysts
• Enzyme catalysts
• Metallic catalysts
• Autocatalysts
• Negative catalysts

In almost all of these classes, there are two main categories of catalysis; homogeneous catalysis and heterogeneous catalysis. The main differences between the two catalysis types are summarized in the below table:

Heterogenous catalysts are extensively used in industry and have a much greater economic impact than homogenous catalysts because many of these solid catalysts are robust at high temperatures and pressure (tolerate a wider range of operating conditions), and require no separation of the product from the catalyst, at the end.

Solid state catalysis (A Heterogenous catalysis)

The majority of industrial catalytic processes involve heterogeneous catalysis. It is a surface process (adsorption) in which catalytic reactions happen at the active sites on the surface of the solid catalyst.

In general, it is divided into two main categories:

1. Unsupported catalysts (Bulk).
2. Supported catalysts (to increase the surface area).

Nano Catalysis

It is a nanomaterial catalyst-based reaction, where instead of large-volume cubes or flakes of catalysts, nanoparticles are used to increase the surface area and enhance the catalytic activity.

Concepts Berg

What are the advantages of using catalysts?

Using a catalyst leads to a faster reaction by providing a new reaction mechanism with lower activation energy. Also, catalysts (homogeneous catalysts) have an important property called selectivity, which increases the production of the desired compound and reduces the concentration of side products.

What are the properties of catalysts?

1. A catalyst increases the rate of a reaction.
2. A catalyst takes part in the reaction even though it will not be consumed or used up in the course of the reaction.
3. A catalyst makes the reaction faster by giving an alternative path with lower activation energy.
4. Catalysts may undergo physical changes but they never go for chemical changes.
5. Small quantities of catalysts are sufficient for catalysis.
6. The catalyst activates the rate of reaction but cannot initiate it.

What are some common catalysts in daily life?

1. Enzymes are biochemical catalysts (biocatalysts) that play a major part in life, e.g. the Pepsin enzyme breaks down protein into peptides in the stomach during the digestion process.
2. Catalytic converters in vehicles {Platinium catalyst is used to transform toxic gas- (CO) to less toxic gas(CO₂)}
3. The haber process converts nitrogen and hydrogen into ammonia using an iron catalyst.

How do you tell if a substance is a catalyst?

A catalyst is neither a reactant nor a product. It is shown in a chemical equation by being written above the yield arrow. Moreover, it is used at the beginning of the reaction and takes part in the reaction by providing an alternate mechanism but is always regenerated at the end.

Can nanoparticles be used in catalysts?

Nanoparticles are extensively used in catalysts to improve catalytic activity, due to their small size. They will have a higher fraction of surface metal atoms where the reaction occurs and this generally leads to higher catalytic activity.

Why are Carbon nanotubes used as catalysts?

Carbon nanotubes (CNTs) have exceptional physical properties: large specific surface areas, high electronic and thermal conductivity, and good chemical inertness, and such properties make them a good support material for heterogeneous catalysis.

References

• A chemical engineering article (knowino.org)
• Surface Temperature Excess in Heterogeneous Catalysis by Lianjie Zhu (Master of Science in Physical Chemistry, Jilin University, Changchun, P.R. China Geboren te Beizhen, P.R. China) [Tianjin University of TechnologyChemistry &amp; Chemical Engineering, Surface Temperature Excess in Heterogeneous Catalysis, page 6 — ISBN: 90-9020-087-8]
• Imperial College London, Module 1410: Green Chemistry, Lecture 3: Catalysis.
• S.Kathirvelu, Louis D’Souza, and Bharathi Dhurai, Nanotechnology applications in textiles, Indian Journal of Science and Technology, Vol.1 No 5 (Oct. 2008)