Adsorption

• Adsorption is the process of accumulation of molecules, ions, or atoms on the surface of a solid or a liquid.
• It can be of two types: physical adsorption (physisorption) and chemical adsorption (chemisorption).
• Physisorption involves weak Vander Waals forces between adsorbate and adsorbent.
• Chemisorption involves chemical bonding between adsorbate and adsorbent.

Factors Affecting Adsorption

Adsorption is influenced by the following factors:

1. Nature of adsorbate and adsorbent

2. Surface area

3. Temperature

4. Pressure

5. Concentration of adsorbate

Langmuir Isotherm

• Langmuir isotherm is used to describe the adsorption phenomenon.
• According to this isotherm, the rate of adsorption is directly proportional to the pressure of the gas and the available surface area.
• The equation for the Langmuir isotherm is:

Langmuir-Hinshelwood Mechanism

The Langmuir-Hinshelwood mechanism involves the following steps:

1. Adsorption of reactant molecules on the surface.

2. Diffusion of reactants on the surface.

3. Chemical reaction between the adsorbed reactants.

4. Desorption of reaction products from the surface.

Langmuir-Hinshelwood Mechanism: Step 1 - Adsorption

• Reactant molecules adsorb onto the surface of the catalyst.
• This occurs due to weak Vander Waals forces between the reactants and the surface.

Langmuir-Hinshelwood Mechanism: Step 2 - Diffusion

• The adsorbed reactants diffuse on the surface of the catalyst.
• This step is crucial for bringing the reactants in close proximity for the reaction to occur.

Langmuir-Hinshelwood Mechanism: Step 3 - Chemical Reaction

• The adsorbed reactants undergo a chemical reaction.
• This reaction can involve bond formation, bond breaking, or other chemical transformations.

Langmuir-Hinshelwood Mechanism: Step 4 - Desorption

• Products of the chemical reaction desorb from the catalyst’s surface.
• This allows new reactant molecules to adsorb and continue the reaction cycle.

Examples of Langmuir-Hinshelwood Mechanism

• Hydrogenation of ethylene using a metal catalyst.
• Oxidation of carbon monoxide on metal oxide catalysts.
• Nitrogen fixation in the Haber process.
• Fischer-Tropsch synthesis for the production of synthetic hydrocarbons.

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Catalytic Poisoning

• Catalytic poisoning refers to the deactivation of catalysts due to the presence of certain substances.
• Substances that can poison catalysts include sulfur, lead, and arsenic.
• These substances can adsorb onto the catalyst surface and block the active sites, preventing the catalytic reaction from occurring.

Promoters

• Promoters are substances that enhance the activity of catalysts.
• They are added to catalysts to increase their efficiency and selectivity.
• Promoters can improve the dispersion of the active catalyst surface or increase the number of active sites available for the reaction.

Heterogeneous Catalysts

• Heterogeneous catalysts are catalysts that exist in a different phase than the reactants.
• They are usually solid catalysts reacting with gaseous or liquid reactants.
• These catalysts provide a surface for the reactant molecules to adsorb and react.

Homogeneous Catalysts

• Homogeneous catalysts are catalysts that exist in the same phase as the reactants.
• They are usually present in solution and can interact directly with the reactant molecules.
• These catalysts can undergo changes during the reaction but are regenerated at the end.

Enzymes as Catalysts

• Enzymes are biological catalysts that accelerate chemical reactions in living organisms.
• They are usually large protein molecules with specific active sites.
• Enzymes are highly selective and can catalyze specific reactions with high efficiency.

Catalytic Selectivity

• Catalytic selectivity refers to the ability of a catalyst to selectively promote a desired reaction.
• In some cases, catalysts can also promote undesired side reactions.
• The selectivity of a catalyst can be influenced by its structure, composition, and reaction conditions.

Industrial Applications of Catalysts

• Catalysts play a crucial role in various industrial processes:
  • Petroleum refining for the production of gasoline and other fuels.
  • Production of fertilizers through the Haber process.
  • Manufacture of plastics, such as the polymerization of ethylene.
  • Catalytic converters in automobiles to reduce harmful emissions.

Kinetics of Heterogeneous Catalysis

• The rate of a heterogeneous catalytic reaction is influenced by both the rate of adsorption and the rate of surface reaction.
• The overall rate can be determined using rate equations based on the Langmuir-Hinshelwood mechanism.
• A catalyst’s effectiveness is measured by its turnover frequency (TOF), which represents the number of reactant molecules converted per catalyst active site per unit time.

Catalyst Regeneration

• Catalysts can become deactivated or poisoned over time due to various factors.
• Regeneration refers to the restoration of catalyst activity by removing the poison or cleaning the catalyst surface.
• Techniques such as washing, leaching, and calcination are used to regenerate catalysts.

Summary

• Surface chemistry involves the study of adsorption and reactions occurring at surfaces and interfaces.
• The Langmuir-Hinshelwood mechanism explains the stepwise process of adsorption, diffusion, reaction, and desorption on a solid surface.
• Catalysts are substances that accelerate chemical reactions without being consumed.
• Heterogeneous catalysts are solid catalysts acting on gaseous or liquid reactants, while homogeneous catalysts are in the same phase as reactants.
• Catalysts can be poisoned or promoted by certain substances. They play a crucial role in industry and can be regenerated.