Question

What is “occlusion”?

Answer 1

Occlusion is a form of adsorption and occurs in a process of coprecipitation where an adsorbate impurity that is weakly adsorbed to the surface of precipitate gets physically trapped inside the crystals during its formation.

Complete answer:
Surface Chemistry is the branch of chemistry which deals with the phenomenon that occurs on the surfaces or interfaces of matters, such as corrosion, catalysis, crystallisation, etc.
The atoms or molecules present at the surface of a solid or liquid experience a net downward force of attraction due to the other atoms or molecules present around them. On the other hand, the molecules present inside the bulk of the matter experiences no net force due to the cancellation of forces exerted by atoms or molecules present around them. Due to this unbalanced attraction forces, accumulation of another molecular species can occur at the surface rather than in the bulk of a solid or liquid and this process is termed as adsorption.
The molecular species which accumulates at the surface is termed as adsorbate and the substance on the surface of which the adsorption takes place is called adsorbent, e.g., Silica gels adsorb water molecules from air on its surface and thus used to remove moisture from surrounding.
Adsorption is a spontaneous process and is always exothermic.
Adsorption of gases on the surface of metals is known as occlusion. An example is the process of hydrogenation of alkenes that involves the adsorption of hydrogen gas over Platinum catalyst.

Note:
Adsorption is very specific and selective in nature because chemical bond formation takes place between an adsorbent and an adsorbate during this process that is specific and selective for different types of elements.