Types of Adsorption
Physisorption, Chemisorption, and the Isobar Traps.
Not all molecules stick to surfaces for the same reason. Depending on the type of attractive forces operating between the adsorbate (the gas/liquid) and the adsorbent (the surface), adsorption is broadly classified into two distinct types: Physisorption and Chemisorption. Distinguishing between these two is the highest-yield theoretical concept in this chapter.
1. Physical Adsorption (Physisorption)
Physisorption occurs when the adsorbate molecules accumulate on the surface of the adsorbent strictly on account of weak van der Waals forces.
Key Characteristics:
- Nature of Forces: Weak van der Waals forces (dispersion forces, dipole-dipole interactions).
- Lack of Specificity: Since van der Waals forces are universal, any gas will physisorb on any solid surface under the right conditions. It is not specific.
- Reversibility: Because the forces are weak, the process is highly reversible. Decreasing pressure or increasing temperature easily causes desorption.
- Enthalpy of Adsorption ($\Delta H$): Very low. Only about $20 - 40 \text{ kJ/mol}$ is released, similar to the heat of condensation.
- Activation Energy: None required. It is an instantaneous process.
Because van der Waals forces can act over long distances and between identical gas molecules, once the first layer of adsorbate covers the surface, a second layer can form on top of the first, and so on. Therefore, Physisorption results in Multi-molecular layers (especially under high pressure).
2. Chemical Adsorption (Chemisorption)
Chemisorption occurs when the gas molecules or atoms are held to the solid surface by chemical bonds (which can be ionic or covalent).
Key Characteristics:
- Nature of Forces: Strong chemical bonds.
- High Specificity: It will only occur if the gas is capable of forming a chemical bond with the specific surface material. For example, oxygen chemisorbs on metals to form oxides, while hydrogen chemisorbs on transition metals (like Ni or Pt) forming hydrides. It is highly specific.
- Irreversibility: Because true chemical bonds are formed, it is generally irreversible. Heating might break the bond, but it often changes the chemical nature of the species (e.g., desorbing $O_2$ from tungsten might yield $WO_3$ gas instead of pure $O_2$).
- Enthalpy of Adsorption ($\Delta H$): Very high. Around $80 - 240 \text{ kJ/mol}$, typical of chemical bond formation.
- Activation Energy ($E_a$): Just like any chemical reaction, forming the bond requires an initial energy input. Therefore, it requires high activation energy (often called activated adsorption).
Chemical bonds can only form between the surface atoms and the first layer of gas molecules. Once the surface is completely covered, there are no more free valencies available for bonding. Therefore, Chemisorption strictly results in a Uni-molecular (single) layer.
Figure 1: Molecular view of Multi-layer Physisorption vs. Uni-layer Chemisorption.
3. Adsorption Isobars (Effect of Temperature)
An Adsorption Isobar is a graph plotting the extent of adsorption ($\frac{x}{m}$) against Temperature ($T$) at a constant pressure. ($x$ = mass of adsorbate, $m$ = mass of adsorbent). The shapes of these graphs are radically different and highly tested.
Physisorption: Because it is an exothermic process ($\Delta H \lt 0$) with no activation energy, Le Chatelier's principle dictates that increasing temperature favors desorption. It decreases continuously with an increase in temperature.
Chemisorption: It initially increases with temperature because heat provides the necessary Activation Energy ($E_a$) to form the chemical bonds. After a peak, further heating increases kinetic energy too much, breaking the bonds and causing desorption.
4. Master Comparison Summary
| Parameter | Physisorption | Chemisorption |
|---|---|---|
| Forces Involved | Weak van der Waals forces | Strong Chemical bonds |
| Specificity | Not specific (any gas on any solid) | Highly specific (requires bond formation) |
| Reversibility | Reversible | Irreversible |
| Enthalpy ($\Delta H$) | Low ($20 - 40 \text{ kJ/mol}$) | High ($80 - 240 \text{ kJ/mol}$) |
| Activation Energy | Almost zero | High (Activated adsorption) |
| Temperature Effect | Decreases continuously | Increases initially, then decreases |
| Molecular Layers | Multi-molecular layers | Uni-molecular layer strictly |
| Effect of Surface Area | Increases with an increase in SA | Increases with an increase in SA |
Can Physisorption change into Chemisorption? Yes!
At low temperatures, $H_2$ gas is physisorbed onto finely divided Nickel via weak van der Waals forces. As the temperature is increased, the $H_2$ molecules gain enough activation energy to break the $H-H$ bond and form strong $Ni-H$ chemical bonds. The process transitions smoothly from physical to chemical adsorption.
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