Physical Adsorption vs. Chemisorption

The phenomenon of adsorption is essentially an attraction of adsorbate molecules to an adsorbent surface. The preferential concentration of molecules in the proximity of a surface arises because the surface forces of an adsorbent solid are unsaturated. Both repulsive and attractive forces become balanced when adsorption occurs.

Adsorption is nearly always an exothermic process.

We can distinguish between 2 types of adsorption process depending on which of these 2 force types plays the bigger role in the process. Adsorption processes can be classified as either physical adsorption (van der Waals adsorption) or chemisorption (activated adsorption) depending on the type of forces between the adsorbate and the adsorbent.

In physical adsorption, the individuality of the adsorbate and the adsorbent are preserved. In chemisorption, there is a transfer or sharing of electron, or breakage of the adsorbate into atoms or radicals which are bound separately.

Physical adsorption from a gas occurs when the inter-molecular attractive forces between molecules of the solid adsorbent and the gas are greater than those between molecules of the gas itself. In effect, the resulting adsorption is like condensation, which is exothermic and thus is accompanied by the release of heat.

Physical adsorption occurs quickly and may be mono-molecular (unimolecular) layer or monolayer, or 2, 3 or more layers thick (multi-molecular). As physical adsorption takes place, it begins as a monolayer. It can then become multi-layer, and then, if the pores are close to the size of the molecules, more adsorption occurs until the pores are filled with adsorbate. Accordingly, the maximum capacity of a porous adsorbent can be more related to the pore volume than to the surface area.

In contrast, chemisorption involves the formation of chemical bonds between the adsorbate and adsorbent is a monolayer, often with a release of heat much larger than the heat of condensation. Chemisorption from a gas generally takes place only at temperatures greater than 200 oC, and may be slow and irreversible. Most commercial adsorbents rely on physical adsorption; while catalysis relies on chemisorption.

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Although the theoretical difference between physical and chemical adsorption is clear, the distinction is not as simple as it may seem. The following parameters can be used to evaluate an adsorbate-adsorbent system to establish the type of adsorption:

The heat of physical adsorption is in the same order of magnitude as the heat of condensation, while the heat of chemisorption is of the same order as of the corresponding.

Physical adsorption will occur under suitable temperature-pressure conditions in any gas solid system, while chemisorption takes place only if the gas is capable of forming a chemical bond with the surface.

A physically adsorbed molecule can be removed unchanged at a reduced pressure at the same temperature where the adsorption took place. The removal of the chemisorbed layer is far more difficult.

Physical adsorption can involve the formation of multi-molecular layers, while chemisorption is always completed by the formation of a monolayer. In some cases, physical adsorption may take place on the top of a chemisorbed monolayer.

Physical adsorption is instantaneous (although the diffusion into porous adsorbent may be time consuming), while chemisorption, though may be instantaneous, generally requires activation energy.

Heat Effects

When a molecule adsorbs from the gas phase, the heat it liberates is perhaps 2 to 3 times larger than the heat of vapourization, while a molecule adsorbing from the liquid phase liberates an amount of heat nearly as great. There are only 2 ways in which the heat of adsorption can be dissipated: by heating up the adsorbent (and the vessel containing it) or by heating up the fluid exiting from the vessel.

When the concentration of adsorbate in the feed is low, say only a few wt% or less, then a large fraction of the heat of adsorption passes out with the effluent stream, and the adsorbent remains close to the temperature of the incoming fluid.

When the concentration of the adsorbate is high, say 10% wt or more, a large fraction of heat will be trapped in the bed and cause large temperature increase with time.

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