Surface Chemistry - Physisorption and Chemisorption

Introduction

  • Surface chemistry focuses on the study of chemical reactions that occur at the interface of two phases.
  • Physisorption and chemisorption are two types of adsorption that take place on solid surfaces.

Physisorption (Physical Adsorption)

  • Physisorption is a weak and reversible type of adsorption.
  • It involves the formation of weak intermolecular forces between the adsorbate and the adsorbent.
  • Examples of physisorption include the adsorption of gases on activated carbon and the adsorption of water on silica gel.

Features of Physisorption

  • Low enthalpy of adsorption
  • Adsorption is usually reversible
  • Occurs at low temperatures and high pressures
  • Multilayer adsorption is common

Factors Affecting Physisorption

  • Surface area of the adsorbent
  • Nature of the adsorbate and the adsorbent
  • Temperature and pressure

Isotherm for Physisorption

A plot of the amount of adsorbate adsorbed per gram of adsorbent against the pressure at constant temperature. Example: Adsorption of nitrogen gas on activated charcoal.

##IMPORTANT SLIDE Liquid-gas surface This demostrates ‘A liquid has definite volume and definite shape’ and ‘GAS state , so atoms and molecules can move to all the places and fill up the space here.’

Chemisorption (Chemical Adsorption)

  • Chemisorption is a strong and irreversible type of adsorption.
  • It involves the formation of chemical bonds between the adsorbate and the adsorbent.
  • Examples of chemisorption include the adsorption of hydrogen gas on a platinum catalyst and the adsorption of oxygen gas on an iron surface.

Features of Chemisorption

  • High enthalpy of adsorption
  • Adsorption is usually irreversible
  • Occurs at high temperatures and low pressures
  • Usually monolayer adsorption

Factors Affecting Chemisorption

  • Nature of the adsorbate and the adsorbent
  • Surface area and structure of the adsorbent
  • Activation energy required for chemisorption

Isotherm for Chemisorption

A plot of the amount of adsorbate adsorbed per gram of adsorbent against the pressure at constant temperature. Example: Adsorption of hydrogen gas on a platinum catalyst.

Differences between Physisorption and Chemisorption

Physisorption:

  • Weak intermolecular forces
  • Reversible adsorption
  • Occurs at low temperatures and high pressures
  • Multilayer adsorption Chemisorption:
  • Chemical bonds formation
  • Irreversible adsorption
  • Occurs at high temperatures and low pressures
  • Monolayer adsorption

Applications of Physisorption

Physisorption is widely used in various applications, including:

  • Gas separation and purification
  • Catalyst support
  • Solid fuel adsorption
  • Drug delivery systems

Applications of Chemisorption

Chemisorption is used in several important processes, such as:

  • Catalysis
  • Corrosion prevention
  • Electroplating
  • Gas sensing

Importance of Surface Chemistry

  • Surface chemistry plays a crucial role in many natural and industrial processes.
  • It is essential in catalysis, where a catalyst increases the rate of a reaction by providing an alternative reaction pathway.
  • It also affects the characteristics of materials such as metals, ceramics, and polymers.

Industrial Examples of Surface Chemistry

  • Haber’s process: Catalytic reaction between nitrogen and hydrogen to produce ammonia.
  • Contact process: Conversion of sulfur dioxide to sulfur trioxide using a vanadium pentoxide catalyst.
  • Hydrogenation of oils: Adsorption of hydrogen gas on a metal catalyst to convert unsaturated fats to saturated fats.

Summary

  • Surface chemistry involves physisorption and chemisorption.
  • Physisorption involves weak intermolecular forces, while chemisorption involves chemical bonds formation.
  • Factors affecting adsorption include surface area, nature of adsorbate and adsorbent, and temperature and pressure.
  • Both physisorption and chemisorption have various applications in industries and natural processes.

Factors Influencing Physisorption

  • Surface area: A greater surface area of the adsorbent increases the amount of physisorption.
  • Nature of the adsorbate: Adsorption is favored when intermolecular forces between the adsorbate and the adsorbent are present.
  • Nature of the adsorbent: Adsorption is favored on surfaces with weaker intermolecular forces.
  • Temperature: Physisorption decreases with increasing temperature due to increased kinetic energy of the adsorbate molecules.
  • Pressure: Physisorption increases with increasing pressure due to the greater concentration of adsorbate molecules.

Example of Physisorption

  • Adsorption of nitrogen gas on activated carbon: Nitrogen molecules are adsorbed on the surface of the activated carbon due to weak intermolecular forces.
  • Adsorption of water molecules on silica gel: Water molecules are adsorbed on the surface of the silica gel due to hydrogen bonding.

Isotherm for Physisorption

  • Langmuir isotherm: Describes the ideal behavior of monolayer physisorption.
  • BET isotherm: Describes multilayer physisorption.
  • Freundlich isotherm: Describes the non-ideal behavior of physisorption.

Factors Influencing Chemisorption

  • Nature of the adsorbate: Certain molecules have a higher tendency to chemisorb due to their ability to form stable chemical bonds.
  • Nature of the adsorbent: The surface of the adsorbent must possess active sites that can facilitate chemical reactions.
  • Surface area and structure: A higher surface area and unique structure of the adsorbent help maximize chemisorption.
  • Activation energy: Certain chemisorption reactions require a specific activation energy to proceed.

Example of Chemisorption

  • Adsorption of hydrogen gas on a platinum catalyst: Hydrogen molecules chemisorb on the surface of the platinum catalyst, forming strong chemical bonds.
  • Adsorption of oxygen gas on an iron surface: Oxygen molecules chemisorb on the surface of the iron, forming oxides.

Isotherm for Chemisorption

  • Langmuir-Hinshelwood isotherm: Describes the chemisorption of one species on a surface.
  • Eley-Rideal isotherm: Describes the chemisorption involving two species, one in gas phase and the other on the surface.

Application of Physisorption in Gas Separation

  • Adsorption technology is used to separate and purify different gases.
  • Activated carbon is commonly used as an adsorbent due to its large surface area and strong adsorption capacity.
  • Examples include the removal of carbon dioxide and water vapor from natural gas.

Application of Physisorption in Catalyst Support

  • Physisorption is utilized to anchor a catalyst to a solid support.
  • Silica gel, alumina, or zeolites are commonly used as catalyst supports due to their high surface area and ability to adsorb reactant molecules.
  • These supports increase the overall surface area available for catalytic reactions.

Application of Chemisorption in Catalysis

  • Chemisorption plays a crucial role in heterogeneous catalysis.
  • Catalysts provide a surface for reactant molecules to adsorb and undergo chemical reactions.
  • Examples include the Haber process for ammonia synthesis and the oxidation of carbon monoxide by platinum catalysts.

Application of Chemisorption in Corrosion Prevention

  • Chemisorption can form protective surface layers that prevent corrosion of metals.
  • For example, the formation of a passivating layer of aluminum oxide on aluminum surfaces protects the metal from further oxidation.
  • This layer acts as a barrier to corrosive substances, preventing further degradation of the metal.