Catalysis Homogeneous and Heterogeneous

Catalysis:

Catalysis is the increase in the rate of a chemical reaction due to the participation of an additional substance called a catalyst.

Example:

· Potassium chlorate, when heated strongly decomposes slowly giving dioxygen.

2KClO₃ → 2KCl + 3O₂

· when a little of manganese dioxide is added, the decomposition takes place at a considerably lower temperature range

Catalyst:

Substances, which accelerate the rate of a chemical reaction and themselves remain chemically and quantitatively unchanged after the reaction, are known as catalysts

· The phenomenon is known as catalysis.

Promoters and Poisons:

Promoters:

Promoters are substances that enhance the activity of a catalyst

· By itself the promoter has little or no catalytic effect

· Some promoters interact with active components of catalyst and thereby alter their chemical effect on the catalyzed substance

· Commonly used promoters are metallic ions incorporated into metals and metallic oxide catalysts

Example:

In Haber’s process for manufacture of ammonia, molybdenum acts as a promoter for iron which is used as a catalyst.

N₂ (g) + 3H₂ (g) 2NH₃ (g)

Poisons:

The substances which decrease the activity of catalyst are called catalytic poisons or inhibitors

· Catalyst poisoning refers to the partial or total deactivation of a catalyst caused by exposure to a range of chemical compounds

· Poisoning refers specifically to chemical deactivation, rather than other mechanism of catalyst degradation such as thermal decomposition or physical damage.

· Poisoning involves compounds which bonds chemically to the active surface sites of a catalyst

Example:

Arsenic acts as catalytic poison in the manufacture of sulphuric acid by ‘contact process.

Types of Catalysis:

1. Homogeneous catalysis

2. Heterogeneous catalysis

Homogenous Catalysis:

When the reactants and the catalyst are in the same phase the process is said to be homogeneous catalysis.

· Homogeneous catalysis applies to reactions in the gas phase and even in solids

· Heterogeneous catalysis is the alternative to homogeneous catalysis

· The term is used almost exclusively to describe solutions and often implies catalysis by organomettalic.

Example:

1. Hydrolysis of methyl acetate is catalysed by H+ ions furnished by hydrochloric acid.

CH₃COOCH₃ (l) + H₂ O (l) HCI (l) CH₃COOH (aq) + CH₃OH (aq)

Heterogeneous Catalysis:

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

· This may refer to phase-solid, liquid and gas

· Heterogeneous catalysts can be more easily recycled than homogeneous catalyst

· Characterization of the catalyst and optimization of properties can be more difficult

Adsorption Theory of Heterogeneous Catalysis:

Adsorption theory explains the mechanism of heterogeneous catalysis.

The mechanism involves five steps:

· Diffusion of reactants characteristics of catalyst

· Adsorption of reactant molecules on the surface of the catalyst.

· Occurrence of chemical reaction on the catalyst’s surface through formation of an intermediate

· Desorption of reaction products from the catalyst surface, and thereby, making the surface available again for more reaction to occur.

· Diffusion of reaction products away from the catalyst’s surface.

Advantage and Disadvantage of Catalysis:

Property	Homogeneous	Heterogeneous
Catalyst recovery	Difficult and expensive	Easy and cheap
Thermal stability	Poor	good
Selectivity	Good or excellent-single active site	Good or poor-multiple active