Adsorption – Physisorption and Chemisorption Explained

What is adsorption?

The term adsorption describes the accumulation of molecules or atoms of a substance on the surface of a solid or liquid. These molecules typically originate from a gaseous or liquid phase and adhere to the interface between the phases due to intermolecular forces. While the adsorbing material is called an adsorbent, the attached molecules in the bound state are called adsorbate. However, as long as they are still in the fluid phase, they are referred to as adsorptive.

Adsorption is an interfacial phenomenon and differs from absorption, in which the molecules penetrate into the interior of a material. Both processes fall under the umbrella term sorption, which describes the interaction of substances on and in materials.

Basic concepts of adsorption

For a better understanding of the adsorption process, the components involved are clearly defined:

Adsorbent

The adsorbent is a material, usually a solid, that has a large surface area to bind molecules from a gaseous or liquid phase. Examples of adsorbents are activated carbon, silica gel and zeolites. The porosity and surface structure of the adsorbent are decisive for the adsorption capacity.

Adsorbate

The adsorbate refers to the molecules or atoms that are attached to the surface of the adsorbent. This term is also used to describe the combination of adsorbent and adsorbed molecules.

Adsorptive

The adsorptive describes the molecules or atoms in their original phase before they are bound to the adsorbent. These are still in the liquid or gaseous phase.

Adsorption as part of sorption

Adsorption is an important sub-process of sorption and is triggered by physical interactions at interfaces. This makes adsorption particularly suitable for separation and purification processes, in which specific substances are to be removed from a mixture, and opens up space for innovation.

The efficiency of the adsorption depends heavily on the size of the surface area of the adsorbent and its pore structure, which is already taken into account in pre-engineering. Materials with a large specific surface area — such as activated carbon or molecular sieves — enable particularly high accumulation of molecules.

Adsorption is important in various areas of technology and science, whether in analytical chemistry, separation processes or environmental technology. It plays an essential role in the purification of gases and liquids as well as in the chemical industry.

By clearly separating terms and processes, adsorption provides a structured basis for the development and optimization of technical processes.

Physisorption — Physical Adsorption

Physisorption is an adsorption process based on weak interactions between a surface (adsorbent) and the adsorbed molecules (adsorbate). The chemical properties of the adsorbate molecules remain unchanged, as no chemical bonds are formed. Instead, weak intermolecular forces such as van der Waals forces, dipole-dipole interactions or hydrogen bonds act. Due to the weak bonding forces, the process can easily be reversed (desorption).

Properties of physisorption:

1. Binding energy: The binding energy is below 100 kJ/mol, which reflects the weak interaction between adsorbate and adsorbent.
2. Exothermic reaction: Adsorption is exothermic, as kinetic energy of the adsorbed particles is released as heat.
3. Reversibility: Physisorption is completely reversible as no chemical bonds are formed. Desorption often takes place by increasing the temperature or lowering the pressure.
4. Multi-layer adsorption: Due to the weak and long-range binding forces, several adsorption layers can form on top of each other.
5. No high activation energy: The process does not require high activation energy and is therefore relatively fast.

Factors influencing the balance:

• Surface size: The amount of adsorbed substance depends heavily on the available surface area of the adsorbent. Materials with a high specific surface area, such as activated carbon or zeolites, are particularly effective.
• Temperature: As the temperature rises, the adsorption rate is reduced by increased thermal movement of the molecules.
• Partial pressure or concentration: In the case of gases, the partial pressure of the adsorbent influences the equilibrium. Higher pressure or higher concentration promote adsorption.

Dynamics on the surface:

Adsorbed molecules do not remain rigidly at their initial binding site, but move along the surface. This mobility makes it possible to fill particularly attractive areas, such as grid defects or pores. Such defects significantly increase the adsorption capacity, as they represent preferred anchor points for molecules.

Transition to condensation:

After the formation of a monomolecular adsorption layer, a multi-layered structure can form with the further addition of adsorptive. As soon as the entire surface is covered, adsorption turns into condensation. This is dominated by interactions between the adsorbate molecules, whose energy is lower than the adsorption energy to the surface.

Typical adsorbents:

• Activated carbon: High performance due to amorphous structure with numerous defects.
• Zeolites: Crystalline materials with close-meshed channel systems that enable selective adsorption.
• Silica gel and molecular sieves: Granules with a high inner surface area, ideal for gas purification and separation processes.

Chemisorption — chemical adsorption

Chemisorption, also known as chemical adsorption, is a form of adsorption that creates chemical bonds between the molecules or atoms of the adsorbate and the surface of the adsorbent. These bonds are significantly stronger than the forces involved in physical adsorption and are in the range of around 200—800 kJ/mol. As a result of the bond strength, the electronic structure of the adsorbate changes during this process.

Chemical adsorption is generally chemically selective, i.e. it only occurs with specific combinations of adsorbate and adsorbent. At most one monomolecular layer (monolayer) is formed on the surface of the adsorbent.

Properties of chemisorption:

• Binding energy and temperature dependence: The high binding energy means that chemisorption is often irreversible. Desorption can generally only take place at high temperatures. In some cases, adsorption requires high activation energy, which is why it often takes place at elevated temperatures.
• Irreversibility: Due to chemical changes in the adsorbate and the bond strength, it is often not possible to return the adsorbed material to its original state. This fundamentally distinguishes chemisorption from physical adsorption.
• Catalytic activity: Chemisorption is a central mechanism in heterogeneous catalysis. It facilitates chemical reactions by weakening bonds in the adsorbate or converting the adsorbate into an activated state.

Mechanisms and processes:

• Dissociative chemisorption: Some molecules, such as hydrogen (H₂), break down into individual atoms when in contact with certain metal surfaces (e.g. iron, platinum, palladium). These atoms then chemically bond to the surface. This decay is known as dissociative chemisorption.
• Directional dependency of the bond: In some cases, such as carbon monoxide (CO), the bond is made specifically via a specific atom (e.g. the carbon atom in CO on metal surfaces). This directed chemisorption is another characteristic feature of this process.
• Activated adsorption: Chemisorption can be energetically demanding, as high transition states must be overcome. This distinguishes it from physisorption, which generally requires no additional activation energy.

Influence on the adsorbate:

The chemical changes due to chemisorption can weaken or break bonds within the adsorbate or change its spatial structure. This makes it possible to facilitate chemical reactions or even make them possible in the first place. For example, chemical reactions can take place at lower temperatures as a result of chemisorption on a catalyst.

Chemisorption in catalysis:

Chemisorption is a central part of many catalytic processes. It enables reactants to bind to the surface of a catalyst, causing their chemical activation. This can increase the rate and efficiency of chemical reactions. One example is the activation of hydrogen by metals, which is crucial for many industrial reactions.

Chemisorption restrictions:

A common problem with chemisorption is the possibility of irreversible binding, in which the adsorbate is difficult to remove from the surface. Such processes can result in the deactivation of catalysts, a phenomenon known as catalyst poison.

Adsorption isotherm

An adsorption isotherm describes the relationship between the quantity of an adsorbate bound to a surface and its concentration or partial pressure at a constant temperature. This representation makes it possible to characterize the interactions between the adsorbate and the adsorbent and to record them in the documentation.

Commonly used models

1. Henry isotherm:
   • Description for low concentrations.
   • Linear relationship: q = K_H * c_Eq, where q is the load, c_Eq is the equilibrium concentration and K_H is Henry's constant.
   • Interactions between adsorbate molecules are not considered.
2. Freundlich isotherm:
   • Considers limited absorption and interactions.
   • Mathematical form: q = K_F * C_eq^ (1/n), where K_F is the Freundlich coefficient and n is the exponent.
   • Complete saturation cannot be described.
3. Langmuir isotherm:
   • Describes adsorption on a homogeneous surface with a limited number of adsorption sites.
   • Formula: q = q_mono_max * (K * C_eq)/(1 + K * C_eq), where q_mono_max is the maximum load when fully covered.
   • Interactions between adsorbate molecules are not considered.
4. BET isotherm:
   • Extension of the Langmuir isotherms to describe several adsorption layers.
   • More complex mathematical form that also takes capillary condensation into account.

What is the degree of coverage?

The degree of coverage (Θ) is a dimensionless variable that describes the ratio of occupied adsorption sites to the maximum possible number.

Definition:

Θ = N/n_M

• N: Number of occupied adsorption sites
• n_M: Maximum number of adsorption sites
• Θ = 0: No adsorption
• Θ = 1: All adsorption sites are occupied

Relation to loading:

The load q can be expressed as a function of the degree of coverage:
q = Θ * Γ_mono_max * S

• Γ_mono_max: concentration of a complete monolayer
• S: Specific surface area of the adsorbent

The degree of coverage is a central concept for the analysis of adsorption processes and is used in many adsorption isotherms to quantitatively describe the binding of the adsorbate to the surface.

Applications of adsorption

Adsorption is a central technology in process plant engineering, as it offers an efficient way of separating and purifying substances or extracting specific components from complex mixtures — often accompanied by professional consulting. But adsorption is also of great importance in many other sectors and industries. The application of physical and chemical adsorption is described in detail below.

Applications of physical adsorption

Heat pumps and adsorption chillers

In the area of adsorption chillers and heat pumps, physisorption is used to store or release heat through adsorption and desorption. These machines change the aggregate state of fluids and are suitable for energy-efficient solutions in the cooling and heating sector. Plant designers take into account the thermodynamic properties of the adsorbents and the operating conditions in order to achieve maximum efficiency — both in basic and detailed engineering.

Gas treatment and waste gas purification

Physisorption is widely used in the gas phase. The following applications play an important role in the planning and construction of plants:

• Waste gas purification: Process for removing hydrocarbons (e.g. solvent vapors or gasoline vapors) and sulfur compounds such as hydrogen sulphide and sulfur dioxide. Adsorptive filters also ensure the removal of odorous substances.
• Gas separation: Processes such as temperature swing or pressure swing adsorption (TSA/PSA) separate gas mixtures. For example, CO2 is removed from biogas or high-purity hydrogen is obtained from synthesis gas. These processes are essential in plants with high purity requirements, such as in the chemical industry.
• Filter technology: Respiratory filters and gas filters use materials such as activated carbon, activated coke, silica gel or zeolites to efficiently adsorb pollutants. These technologies are integrated into safety-critical systems.

Chromatographic processes

Adsorption chromatography uses physisorption to efficiently separate mixtures of substances. In process engineering planning, chromatographic separation methods are taken into account in the analysis and processing of complex substances, particularly in the pharmaceutical and chemical industries.

Water treatment and environmental technology

Physisorption is used in the liquid phase to remove pollutants from water or to recover valuable substances. Typical applications include:

• Water treatment: Removal of organic and inorganic pollutants from drinking water.
• Wastewater treatment: Use of activated carbon and similar adsorbents to treat industrial waste water.
• Groundwater remediation: Adsorption helps clean contaminated groundwater from hazardous chemicals.
• Industrial water management: Recovery of substances from process water.

Material characterization

When planning systems that use adsorbents, the characterization of the materials is essential. Gas sorption measurements, which measure and evaluate gas sorption isotherms based on models, are used to optimally adapt micro and mesoporous materials to application requirements.

Applications of chemical adsorption

Heterogeneous catalysis

Chemisorption is an essential process in catalytic processes. It is integrated into numerous process engineering plants:

• Haber-Bosch process: The synthesis of ammonia uses the chemisorption of hydrogen on catalysts such as iron.
• Fischer-Tropsch synthesis: Carbon monoxide and hydrogen react by chemisorptive bonding to catalysts to form hydrocarbons.
• Automotive catalytic converters: These plants use chemisorptive processes to convert pollutants such as nitrogen oxides into less hazardous substances.

Drying agent

Chemisorption is used in drying processes, e.g. through molecular sieves or bentonite. These chemically bind water molecules and are suitable for drying under demanding conditions, such as in petrochemical plants or when processing hygroscopic substances.

Specific applications in chemistry and petrochemistry

Chemisorption is used to promote targeted reactions, for example when hydrogenating C=C double bonds in organic chemistry. Adsorbents such as Raney nickel or nickel-aluminum alloys are central components of such processes.

FAQ: Important questions about adsorption

What is the difference between absorption and adsorption?

absorption means the absorption of a substance into the interior of another material, for example the penetration of a gas or a liquid into a solid or a liquid. A typical example is the absorption of water by a sponge.

adsorption On the other hand, describes the accumulation of molecules or atoms on the surface of a solid or a liquid. It is a surface process in which the adsorbed particles (adsorbates) are bound to the surface by physical or chemical forces.

What does desorption mean?

Desorption is the process by which previously adsorbed molecules, atoms, or ions are released from a surface. This often happens as a result of changes in temperature, pressure, or the chemical composition of the environment. One example is the evaporation of water that has previously condensed on a surface.

What can you adsorb?

You can adsorb a wide range of substances, including:

• Gases: e.g. carbon dioxide (CO₂), nitrogen (N₂) or oxygen (O₂).
• Liquids: e.g. organic solvents such as ethanol or acetone.
• Ions and molecules in solutions: e.g. heavy metal ions or dyes.

Adsorption is particularly relevant in environmental technology, such as the purification of water or waste gas treatment.

What is competitive adsorption?

Competing adsorption occurs when several substances compete for the same adsorption sites on a surface. This can reduce the efficiency of adsorbing a particular substance. For example, in a mixture of gases, some gases may be more strongly adsorbed than others, depending on their binding affinity to the surface of the adsorbent.

Where does absorption occur in everyday life?

You encounter absorption in many everyday situations, including:

• Drinks: Sugar or carbonic acid dissolves in liquids.
• Skincare: Creams or oils are absorbed by the skin.
• Sponges: Water is absorbed by sponges or other porous materials.
• Gas masks: Harmful substances are absorbed in special materials.

What is the area between physical and chemical adsorption called?

The transition area between physical and chemical adsorption is called activated adsorption. This involves a mixture of weak physical interactions (van der Waals forces) and stronger chemical bonds. This area can show properties of both types of adsorption.

Are the coverage level and the occupancy rate the same?

No, they're different. The degree of coverage describes the proportion of adsorption sites occupied by adsorbates based on the entire surface area. The occupancy rate may also include chemical or structural aspects, such as the number of adsorbate molecules per adsorption site. The terms are used differently depending on the scientific context.

Which process engineering plants use adsorption?

Adsorption is used in many industrial plants, including:

• Activated carbon filter: To purify water and air.
• Silica gel dryer: To remove moisture from gases.
• Gas treatment plants: To remove unwanted gases or contaminants.
• Catalysts: For chemical reactions that take place at adsorption sites.
• Exhaust gas purification systems: To remove pollutants from industrial exhaust gases.

These systems make use of the versatile properties of adsorption to efficiently separate or remove substances.