In a catalysed process, the catalyst usually reacts chemically with the reactants but is eventually regenerated, thus the catalyst level remains constant. Because the catalyst isn’t consumed, each molecule of catalyst can cause the transformation of a large number of reactant molecules. The number of molecules changed every minute by a single molecule of active catalyst can be as high as several million.
The distribution of products may be changed by the employment of a catalyst that selectively accelerates one reaction relative to the other when a given material or a mixture of substances experiences two or more simultaneous reactions that generate distinct products (s).
A certain reaction can be induced to occur to the point that it practically excludes another by using the right catalyst. This type of selectivity is used in a lot of significant catalytic applications.
What is Catalyst?
Catalyst is a term that we have come across several times while studying chemistry, particularly when learning about chemical reactions. While some chemical reactions occur quickly, others take a long time and necessitate the use of additional materials or effort. This is where a catalyst can help. When heated to a high temperature, potassium chlorate slowly decomposes, releasing dioxygen.
The decomposition takes place between 653 and 873K as,
2KClO3 → 2KCl + 3O2
When a small amount of manganese dioxide is added, however, the decomposition occurs at a much lower temperature range, i.e., 473-633K, and at a much faster rate. In terms of mass and composition, the manganese dioxide that has been added remains unchanged.
Similarly, the mere presence of a foreign substance can alter the rates of a variety of chemical reactions. Berzelius conducted the first systematic study of the effect of various foreign substances on the rates of chemical reactions in 1835.
Promoters and poisons: Promoters are substances that increase the activity of a catalyst, whereas poisons decrease its activity. Molybdenum, for example, acts as a promoter for iron, which is used as a catalyst in Haber’s ammonia manufacturing process.
Characteristics of Catalyst
- A chemical reaction is not started by a catalyst.
- The reaction does not consume a catalyst.
- Catalysts tend to react with reactants to form intermediates while also facilitating the production of the final reaction product. A catalyst can regenerate after the entire process.
Types of Catalysts
- Positive Catalysts: Positive catalysts are those that increase the rate of a chemical reaction. It accelerates the reaction by lowering the activation energy barriers, allowing a large number of reaction molecules to be converted into products, increasing the percentage of product yield. e.g. Iron oxide acts as a positive catalyst in Haber’s process, increasing the yield of ammonia despite less nitrogen reaction.
- Negative Catalysts: Catalysts that slow down the rate of the reaction, as well as negative catalysts It reduces the rate of reaction by increasing the activation energy barrier, which reduces the number of reactant molecules that can be converted into products and thus the rate of reaction. For example, the decomposition of hydrogen peroxide into water and oxygen is slowed by the use of acetanilide, which acts as a negative catalyst to slow the rate of hydrogen peroxide decomposition.
Types of Catalysis
Catalysis can be broadly divided into two groups:
Homogeneous catalysis occurs when the reactants and the catalyst are in the same phase. Some examples of homogeneous catalysis are as follows:
- In the lead chamber process, sulphur dioxide is oxidised to sulphur trioxide with dioxygen in the presence of nitrogen oxides as the catalyst. All of the reactants, sulphur dioxide and oxygen, as well as the catalyst, nitric oxide, are in the same phase.
2SO2 (g) + O2(g) → 2SO3 (g) (in the presence of gaseous NO)
- H+ ions supplied by hydrochloric acid catalyze the hydrolysis of methyl acetate. The reactants and catalyst are both in the same phase.
CH3COOCH3 (l) + H2O(l) → CH3COOH(aq) + CH3OH(aq) (in the presence of aqueous HCl)
Mechanism of Heterogeneous Catalyst
Adsorption and intermediate compound formation are both involved in heterogeneous catalysis. The reactant molecule becomes adsorbent on the surface of the catalyst’s activation centre. This results in the formation of an activated complex, which is an intermediate compound. This compound degrades to produce products. As soon as the products are formed, they are desorbed from the surface with no time-lapse. Adsorption of reactants on the surface of the catalyst is the first step in heterogeneous catalysis, followed by intermediate compound formation and dissociation into a product.
Heterogeneous catalysis refers to the catalytic process in which the reactants and catalysts are in different phases. The following are some examples of heterogeneous catalysis:
- In the presence of Pt, sulphur dioxide is oxidised to sulphur trioxide. The reactant is in a gaseous state, while the catalyst is solid.
2SO2 (g) → 2SO3 (g) (in presence of Pt(s))
- In Haber’s process, dinitrogen and dihydrogen combine to form ammonia in the presence of finely divided iron. The reactants are in a gaseous state, while the catalyst is solid.
4NH3 (g) + 5O2 (g → 4NO(g) + 6H2O(g) (in presence of Pt(s))
- Hydrogenation of vegetable oils with finely divided nickel as a catalyst. One of the reactants is in a liquid state, the other is in a gaseous state, and the catalyst is solid.
Vegetable oils(l) + H2 (g) → vegetable ghee (s) (in presence of solid Nickel)
Mechanism of Homogeneous Catalysis
The homogeneous catalysis takes place by intermediate compound formatter theory.
Adsorption Theory of Heterogeneous Catalysis
This theory explains how heterogeneous catalysis works. The adsorption theory of catalysis held that reactants in gaseous or solution form are adsorbed on the surface of the solid catalyst. The rate of reaction increases as the concentration of the reactants on the surface increases. Because adsorption is an exothermic process, the heat of adsorption is used to increase the rate of the reaction. The catalytic action can be explained in terms of the formation of intermediate compounds.
The modern adsorption theory is a synthesis of the intermediate compound formation theory and the old adsorption theory. The catalytic activity is concentrated on the catalyst’s surface. The mechanism consists of five steps:
- Diffusion of reactants to the catalyst’s surface.
- Adsorption of reactant molecules on the catalyst’s surface.
- Chemical reaction on the catalyst’s surface that results in the formation of an intermediate.
- Desorption of reaction products from the catalyst surface, allowing the surface to be used for further reactions.
- Diffusion of reaction products away from the surface of the catalyst. Unlike the interior of the bulk, the surface of the catalyst has free valencies that serve as a seat for chemical forces of attraction. When the gas comes into contact with such a surface, the molecules are held in place by a loose chemical combination. If different molecules are adsorbed next to each other, they may react, resulting in the formation of new molecules. As a result, formed molecules may evaporate, freeing up space on the surface for new reactant molecules.
This theory explains why the catalyst, even in small quantities, remains unchanged in mass and chemical composition at the end of the reaction. It does not, however, explain how catalytic promoters and catalytic poisons work.
Important features of Solid Catalysts
- Activity: To a large extent, the activity of a catalyst is determined by the strength of chemisorption. To become active, the reactants must be strongly adsorbed on the catalyst. They must not, however, be so strongly adsorbed that they become immobilized and leave no space on the catalyst’s surface for other reactants to adsorb. It has been discovered that for hydrogenation reactions, the catalytic activity increases from Group 5 to Group 11 metals, with Group 7-9 elements of the periodic table exhibiting the highest activity.
- Selectivity: A catalyst’s selectivity is its ability to direct a reaction to yield a specific product selectively when many products are possible under the same reaction conditions. The selectivity of different catalysts for the same reactants varies. It can be deduced that a catalyst’s action is highly selective in nature. As a result, a substance that acts as a catalyst in one reaction may be unable to catalyse another.
Shape Selective Catalysis by Zeolites
Shape-selective catalysis refers to a catalytic reaction that is influenced by the pore structure of the catalyst as well as the size of the reactant and product molecules. Because of their honeycomb-like structures, zeolites are excellent shape-selective catalysts. They are microporous aluminosilicates with a three-dimensional silicate network in which some silicon atoms are replaced by aluminium atoms, resulting in an Al–O–Si framework. The reactions that occur in zeolites are influenced by the size and shape of the reactant and product molecules, as well as the pores and cavities of the zeolites. They can be found in nature and synthesized for catalytic selectivity.
In the petrochemical industry, zeolites are commonly used as catalysts for hydrocarbon cracking and isomerization. ZSM-5 is a significant zeolite catalyst used in the petroleum industry. It dehydrates alcohol to produce a mixture of hydrocarbons, which it then converts directly into gasoline (petrol).
Question 1: How can a positive catalyst alter the reaction?
A positive catalyst increases the rate of the reaction by changing the path of the reaction and decreasing the activation energy basis. As a result, many reactant molecules are converted into products.
Question 2: What is the role of catalyst poison in Rosenmund reaction?
Aldehyde is produced in the Rosenmund reaction by reducing acid halides with hydrogen gas in the presence of palladium. If the catalyst is not poisoned, the reaction does not stop at the aldehyde level, which is a feather reducer of alcohol. To come to a halt at the aldehyde level. Barium sulphate is used to poison palladium.
Question 3: What is the role of promoters in Haber’s process?
Promoters and accelerators boost the activity of a catalyst in a process. Nitrogen reacts with hydrogen to form NH3 in Haber’s ammonia manufacturing process. Because nitrogen is very less reactive and the yield of ammonia is very low, NO is used as a promoter to increase the percentage yield of ammonia formed.
Question 4: What is autocatalysis?
There is no specific catalyst used in the autocatalytic reaction. Instead, one of the products acts as a catalyst, increasing the rate of product formation.
Question 5: Give an example of a shape-selective catalyst.
Shape-selective catalysis refers to the catalyst reaction in which small molecules are absorbed in the pores and cavities of selective adsorbents such as zeolites.
Question 6: Define electrophoresis.
When an electric current is passed through a colloidal solution, positively charged particles move to the cathode, while negatively charged particles move to the anode, where they lose their charge and coagulate. Electrophoresis is the name given to this phenomenon.