Catalysis: Definition, Types, Function and Examples

Catalysis

Edited By Shivani Poonia | Updated on Jul 02, 2025 06:05 PM IST

Interestingly enough, catalysis is considered to be one extremely captivating and very topically important concept of chemistry since it has a great influence on our everyday lives and on many industrial processes. For instance, just think of the frustration that would be in case one needs to wait for some hours for an automobile catalytic converter to cause full neutralization of toxic gases or in case industrial chemical reactions need days instead of minutes. All these difficulties are solved by catalysts, which are substances able to speed up chemical reactions without their consumption.

This Story also Contains

Homogeneous and Heterogeneous Catalysis
Promoters and Poisons
Shape Selective Catalysis
Surface Roughening of Zeolites
Some Solved Examples
Conclusion

Catalysis

Homogeneous and Heterogeneous Catalysis

Homogeneous Catalysis

The reaction is homogeneous if the phase of the catalyst is at the same phase as that of the reactants. Usually, the phase is a liquid solution, in this case, the esterification is produced by reacting carboxylic acids and alcohols under acid catalysis.

Heterogeneous Catalysis

In heterogeneous catalysis, the catalysts act in a phase that is different from that of the reactants. It normally involves solids with gaseous or liquid reactants, for example, platinum in catalytic converters for reducing auto emissions.

Promoters and Poisons

Promoters: These are substances that, when added to a catalyst, enhance its activity. The addition of minute quantities of, for example, molybdenum to catalysts for the Haber process, iron, increases greatly ammonia production.

Poisons

Catalyst Poison: A catalyst poison is a substance that depresses or hinders the catalytic activity. For example, sulfur compounds act as poisons for some metal catalysts, as these cake the main active sites of the catalyst and thus lower its efficiency.

Some of the unique properties of solid catalysts are that they may be porous with a high area and contain some active sites that may catalyze reactions very effectively. Because of this reason, these catalysts are very vital in many applications—for example—in the cracking of petroleum hydrocarbons using zeolites as catalysts.

Shape Selective Catalysis

This way, a catalyst can afford reaction selectivity depending on size and shape of reactant molecules. One of its examples comes from a catalyst used in shaping selective catalysis that takes place during hydrocarbon processing. Zeolites, used for converting low-octane feedstock into gasoline, are a nice example of this.

Surface Roughening of Zeolites

Enzymes are biological catalysts and drive nearly all of the chemical reactions that power life. They are highly selective and work at a rapid pace in even the mildest conditions. Enzymes are able to digest food, respire within cells, or catalyze any other reaction just as important, by putting these minute powers of nature into play.

Some Solved Examples

Q.1Which of the following is not an example of a heterogeneous catalyst?

1)Haber's process of NH₃ synthesis

2)Catalytic conversion of SO₂ to SO₃ in contact process

3)Oxidation of ammonia into nitric oxide in Ostwald’s process.

4) (correct)Acidic hydrolysis of methyl acetate

Solution:

In the hydrolysis of esters, all reactants, products, as well as Catalysts (H⁺), are in the aqueous phase and hence it is an example of Homogenous catalysis in Haber's process, the catalyst is Fe(s)In the Contact process, the catalyst is V₂O₅(s)In Ostwald's process, the catalyst is Pt(s)In all the above reactions, the reactant(s) and product(s) are gaseous.

Hence, the answer is the option (4).

Example 2: In homogeneous catalyst reactions, the rate of reactions:

1. Depends upon the concentration of the catalyst(correct)
2. Independent of the concentration of the catalyst
3. Depends upon the free energy change
4. Depends upon the physical state of the catalyst

Solution:

In homogeneous catalysis, the catalyst is in the same phase as that of the reactants, so its concentration will affect the rate of reaction.

Hence, the answer is option (1).

Example 3: Which of the following is not an example of a heterogeneous catalytic reaction?

1. Ostwald's process
2. Combustion of coal(correct)
3. Hydrogenation of vegetable oils
4. Haber's process

Solution:

Heterogeneous Catalysis: The catalytic process in which the reactants and catalyst are in different phases is known as heterogeneous catalysis.

As we have learned in combustion, we do not require any catalysts for the combustion of coal. So, there will be no sense of a heterogeneous catalytic reaction.

Hence, the answer is option (2).

Conclusion

Homogeneous and heterogeneous catalysis, promoters, and poisons, solid catalysts characteristics, shape-selective catalysis, and enzyme catalysis, all discussed within the article, broadly encompass catalysis. From all these concepts taken for review in this paper, we could observe how the catalysts are meant to accelerate chemical reactions, producing great effects on both industrial processes and, for the benefit of the latter, on the environment and biological systems. Of course, the extracted relevance toward the real-life application of catalysis underpins the relevance of the advancement of science and technology.

Frequently Asked Questions (FAQs)

1. What is Catalysis?

A catalyst is a substance that increases the rate of chemical reaction since it is not consumed in the reaction.

2. What is Catalysis?

Catalysis is the process of increasing the rate of a chemical reaction by using a substance called a catalyst. The catalyst lowers the activation energy of the reaction without being consumed in the process, allowing the reaction to proceed more quickly or under milder conditions.

3. How does homogeneous catalysis differ from heterogeneous catalysis?

The catalysts used in homogeneous catalysis exist in the same phase as the reactants whereas heterogeneous catalysis uses catalysts that exist in a phase different from the reactant.

4. Explain how promoters and poisons affect catalysis.

Promoters increase work activity in catalysts, but they reduce the catalytic activity or even nullify it in poisons.

5. What are some of the properties of solid catalysts?

Solid catalysts usually have high surface areas, porosities, and particular special active sites encouraging chemical reactions.

6. How do enzymes serve as catalysts?

Enzymes are used as biological catalysts to speed up biochemical reactions with very high sensitivity and efficiency at levels that can be termed mild.

7. What is the difference between homogeneous and heterogeneous catalysis?

Homogeneous catalysis occurs when the catalyst is in the same phase as the reactants (e.g., all in solution), while heterogeneous catalysis involves the catalyst and reactants in different phases (e.g., a solid catalyst with gaseous or liquid reactants). Heterogeneous catalysis often occurs on the surface of solid catalysts.

8. How does a catalyst work?

A catalyst works by providing an alternative reaction pathway with lower activation energy. It does this by interacting with reactants, forming temporary intermediate compounds, and then releasing the products while regenerating itself. This process allows the reaction to occur more rapidly without the catalyst being used up.

9. Can you explain the concept of turnover frequency in catalysis?

Turnover frequency (TOF) is a measure of catalyst efficiency, defined as the number of reactant molecules converted to products per active site per unit time. It represents how quickly a catalyst can complete one catalytic cycle. A higher TOF indicates a more efficient catalyst, capable of processing more reactants in a given time period.

10. What is autocatalysis?

Autocatalysis is a type of catalysis where one of the products of a reaction acts as a catalyst for the same reaction. This leads to a characteristic acceleration of the reaction rate as more product (and thus more catalyst) is formed. Autocatalytic reactions often exhibit sigmoidal kinetics, with a slow start followed by rapid acceleration and then a slowdown as reactants are depleted.

11. What is catalyst poisoning?

Catalyst poisoning occurs when certain substances, called poisons, bind strongly to the active sites of a catalyst, reducing or eliminating its catalytic activity. This can happen through chemical bonding or physical blockage of the active sites, preventing reactants from accessing them and thus inhibiting the catalytic process.

12. How do promoters enhance catalyst performance?

Promoters are substances added to catalysts in small amounts to enhance their performance. They work by modifying the electronic or structural properties of the catalyst, which can increase activity, selectivity, or stability. Promoters may improve adsorption of reactants, facilitate electron transfer, or prevent catalyst deactivation.

13. What is the difference between a catalyst and an inhibitor?

A catalyst increases the rate of a chemical reaction by lowering the activation energy, while an inhibitor decreases the reaction rate by interfering with the reaction mechanism or binding to active sites. Catalysts promote reactions without being consumed, whereas inhibitors may or may not be consumed in the process.

14. What is catalyst deactivation?

Catalyst deactivation is the loss of catalytic activity or selectivity over time. It can occur through various mechanisms, including poisoning (strong adsorption of contaminants), fouling (physical blockage of active sites), sintering (agglomeration of catalyst particles), and chemical transformation of the catalyst. Understanding and mitigating deactivation is crucial for maintaining long-term catalyst performance in industrial processes.

15. How does temperature affect catalyst performance?

Temperature can significantly impact catalyst performance. Generally, increasing temperature increases the rate of a catalyzed reaction by providing more energy for molecules to overcome the activation energy barrier. However, extremely high temperatures can lead to catalyst deactivation through sintering (fusing of particles) or decomposition. The optimal temperature depends on the specific catalyst and reaction.

16. How do shape-selective catalysts work?

Shape-selective catalysts, such as zeolites, have pores or channels of specific sizes and shapes that allow only certain molecules to enter and react. This selectivity is based on the size and shape of reactants, products, or transition states. Shape-selective catalysis can control which products are formed or prevent unwanted side reactions by excluding molecules that don't fit the catalyst's pore structure.

17. Can you explain the concept of catalyst selectivity?

Catalyst selectivity refers to the ability of a catalyst to direct a reaction towards a specific product when multiple reaction pathways are possible. A selective catalyst promotes the formation of desired products while minimizing unwanted side reactions, improving the efficiency and yield of the target product.

18. What is the role of support materials in heterogeneous catalysis?

Support materials in heterogeneous catalysis serve several important functions: 1) They provide a high surface area for dispersing the active catalyst, increasing the number of accessible active sites. 2) They can enhance catalyst stability, preventing sintering or agglomeration. 3) Some supports interact with the active phase, modifying its electronic properties and potentially enhancing activity or selectivity. 4) Supports can improve the mechanical strength and heat transfer properties of the catalyst.

19. How do bifunctional catalysts work?

Bifunctional catalysts contain two different types of active sites that work together to catalyze a reaction. Each site performs a different step in the overall reaction mechanism. For example, in catalytic reforming, platinum sites catalyze dehydrogenation while acidic sites on the support catalyze isomerization. This combination of functions allows for more efficient and selective catalysis of complex reactions.

20. How do enzymes function as biological catalysts?

Enzymes are protein molecules that act as highly efficient biological catalysts. They work by binding specific reactants (substrates) in their active sites, which have a unique shape and chemical environment. This binding lowers the activation energy for the reaction, allowing it to proceed much faster than it would without the enzyme.

21. What is the role of surface area in heterogeneous catalysis?

Surface area plays a crucial role in heterogeneous catalysis because the catalytic reactions occur on the surface of the solid catalyst. A larger surface area provides more active sites for reactants to adsorb and react, increasing the overall rate of the reaction. This is why many industrial catalysts are designed with high surface area materials or nanoparticles.

22. What is the significance of the Sabatier principle in catalysis?

The Sabatier principle states that the most effective catalyst will bind reactants neither too strongly nor too weakly. If binding is too weak, reactants won't adsorb sufficiently for reaction. If binding is too strong, products won't desorb easily, blocking active sites. The principle helps in understanding and designing catalysts with optimal binding strengths for maximum activity.

23. How do nanocatalysts differ from traditional catalysts?

Nanocatalysts are catalytic materials with dimensions in the nanometer range (1-100 nm). Compared to traditional catalysts, nanocatalysts often exhibit higher activity and selectivity due to their large surface area-to-volume ratio and unique electronic properties. They can also have different catalytic properties than their bulk counterparts due to quantum size effects and increased number of edge and corner sites.

24. What is catalyst regeneration?

Catalyst regeneration is the process of restoring the activity of a deactivated or partially deactivated catalyst. This can involve removing accumulated contaminants, reversing chemical changes, or restructuring the catalyst surface. Regeneration is important for maintaining catalyst efficiency and extending its useful lifetime in industrial processes.

25. How does pressure affect heterogeneous catalysis?

Pressure can significantly impact heterogeneous catalysis, especially for gas-phase reactions. Increasing pressure generally increases the rate of catalytic reactions by increasing the concentration of reactants at the catalyst surface. This enhances the probability of reactant adsorption and reaction. However, very high pressures can sometimes hinder product desorption or cause undesired side reactions.

26. What is the difference between a catalyst and a reaction intermediate?

A catalyst and a reaction intermediate are both involved in the reaction mechanism, but they differ in key ways. A catalyst is not consumed in the reaction and is regenerated at the end of each catalytic cycle. In contrast, a reaction intermediate is formed and consumed during the reaction. While intermediates are part of the reaction stoichiometry, catalysts are not included in the overall reaction equation.

27. How do acid-base catalysts function?

Acid-base catalysts work by donating or accepting protons (Brønsted acids/bases) or electrons (Lewis acids/bases) to/from reactants. This can activate molecules by making them more electrophilic or nucleophilic, lowering activation energies for various reactions. Examples include the catalysis of ester hydrolysis by acids or bases, and the use of Lewis acids like AlCl3 in Friedel-Crafts reactions.

28. What is meant by catalyst loading, and how does it affect reaction rates?

Catalyst loading refers to the amount of catalyst used relative to the amount of reactant or reaction mixture. Generally, increasing catalyst loading increases the reaction rate due to the greater number of active sites available. However, there's often a point of diminishing returns where further increases in loading provide minimal rate enhancement. Optimal loading balances reaction rate with catalyst cost and potential mass transfer limitations.

29. How do phase transfer catalysts work?

Phase transfer catalysts facilitate reactions between reactants in immiscible phases, typically an aqueous phase and an organic phase. These catalysts, often quaternary ammonium salts, can transport reactive species (usually anions) from one phase to another. This allows reactions to occur that would otherwise be limited by the inability of reactants to come into contact in a single phase.

30. What is the concept of catalyst turnover number?

The turnover number (TON) is the total number of reactant molecules that a single catalytic site can convert to product over its lifetime. It's a measure of catalyst efficiency and longevity, indicating how many catalytic cycles a catalyst can perform before becoming inactive. A high TON is desirable for industrial catalysts, as it implies that a small amount of catalyst can process a large amount of reactant.

31. How do photocatalysts differ from traditional catalysts?

Photocatalysts are materials that catalyze reactions when exposed to light. Unlike traditional catalysts that rely on thermal energy, photocatalysts use light energy to activate reactants or generate reactive species. They often involve semiconductor materials that create electron-hole pairs upon light absorption. These charge carriers can then participate in redox reactions at the catalyst surface. Photocatalysts are particularly important in environmental applications and solar energy conversion.

32. What is the role of transition metals in homogeneous catalysis?

Transition metals are widely used in homogeneous catalysis due to their ability to form complexes with variable oxidation states. Their partially filled d-orbitals allow them to bond with and activate a variety of substrates. Transition metal catalysts can facilitate electron transfer, bond breaking/forming, and can stabilize reaction intermediates. Their catalytic properties can be fine-tuned by modifying the ligands attached to the metal center.

33. How does catalyst particle size affect catalytic activity?

Catalyst particle size can significantly impact catalytic activity, especially in nanoscale catalysts. Smaller particles generally provide higher surface area-to-volume ratios, exposing more active sites for catalysis. Additionally, as particle size decreases, the proportion of edge and corner atoms (which often have higher catalytic activity) increases. However, very small particles can sometimes exhibit different electronic properties or decreased stability, potentially altering their catalytic behavior.

34. What is the significance of the active site in catalysis?

The active site is the specific location on a catalyst where the catalytic reaction occurs. It's where reactants bind and are transformed into products. The structure and properties of the active site determine the catalyst's activity and selectivity. Understanding the nature of active sites is crucial for catalyst design and optimization. In heterogeneous catalysis, active sites are often found at specific crystal faces, edges, or defects on the catalyst surface.

35. How do catalysts affect reaction equilibrium?

Catalysts do not affect the equilibrium position of a reversible reaction. They increase the rate of both the forward and reverse reactions equally, allowing the system to reach equilibrium faster. However, catalysts can affect the kinetics of approaching equilibrium and may influence which products are formed more quickly in complex reaction networks. This kinetic control can be used to selectively produce desired products in some processes.

36. What is catalyst characterization, and why is it important?

Catalyst characterization involves analyzing the physical and chemical properties of catalysts using various techniques. This includes determining surface area, pore structure, elemental composition, oxidation states, and active site distribution. Characterization is crucial for understanding how catalysts work, optimizing their performance, and developing new catalytic materials. It helps in identifying the relationship between catalyst structure and activity, guiding catalyst design and troubleshooting.

37. How do solid acid catalysts work?

Solid acid catalysts function similarly to liquid acids but in a heterogeneous system. They have surface sites that can donate protons or accept electron pairs (Brønsted or Lewis acid sites). These sites can activate reactants by protonation or by forming electron-deficient complexes. Solid acid catalysts are widely used in petrochemical processes for reactions like cracking, isomerization, and alkylation. Examples include zeolites, sulfated zirconia, and acidic clays.

38. What is the concept of catalyst poisoning, and how can it be prevented?

Catalyst poisoning occurs when certain substances (poisons) strongly adsorb to or react with the active sites of a catalyst, reducing its activity. Common poisons include sulfur compounds, heavy metals, and carbon deposits. Prevention strategies include: 1) Purifying feedstocks to remove potential poisons. 2) Using guard beds to trap poisons before they reach the main catalyst. 3) Designing poison-resistant catalysts. 4) Periodic regeneration to remove accumulated poisons. Understanding the specific poisons for each catalyst system is crucial for effective prevention.

39. How do catalysts influence reaction mechanisms?

Catalysts influence reaction mechanisms by providing alternative reaction pathways with lower activation energies. They can:

40. What is the difference between a catalyst and a cocatalyst?

A catalyst is the primary substance responsible for increasing the rate of a chemical reaction. A cocatalyst, on the other hand, is a substance that works in conjunction with the main catalyst to enhance its effectiveness. Cocatalysts often activate the main catalyst or assist in one step of the catalytic cycle. They may not have significant catalytic activity on their own but can greatly improve the performance of the primary catalyst when used together.

41. How do zeolites function as catalysts?

Zeolites are crystalline aluminosilicates that function as effective catalysts due to several properties:

42. What is catalyst sintering, and how does it affect catalyst performance?

Catalyst sintering is the process where small catalyst particles agglomerate into larger ones at high temperatures. This results in:

43. How do catalysts affect activation energy?

Catalysts lower the activation energy of a reaction by providing an alternative reaction pathway. This is achieved through:

44. What is the role of catalyst support interactions in heterogeneous catalysis?

Catalyst support interactions play a crucial role in heterogeneous catalysis:

45. How do catalysts affect reaction selectivity?

Catalysts can influence reaction selectivity in several ways: