$$\rightleftharpoonup{xx}$$
$$\longleftharp{xx}$$,
$$\longrightharp{xx}$$,
Adsorption is a process where molecules, known as the adsorbates, accumulate on a surface, which is referred to as the adsorbent or substrate. Occurring at the solid-gas interface, this phenomenon is crucial in various scientific and industrial contexts. The reverse of adsorption is desorption.
Two types of adsorptions exist: physical (physisorption) and chemical (chemisorption). Physisorption involves gas molecules held to the solid's surface by relatively weak intermolecular van der Waals forces, while chemisorption entails a chemical reaction at the solid's surface, with the gas held by strong chemical bonds.
Physisorption is nonspecific; for instance, nitrogen will be physically adsorbed on any solid at low temperatures. Conversely, chemisorption is highly specific, akin to ordinary chemical reactions. For example, N2 is chemisorbed at room temperature on Iron (Fe), Tungsten (W), Calcium (Ca), and Titanium (Ti) but not on Nickel (Ni), Silver (Ag), Copper (Cu), or Lead (Pb).
Chemisorption generally involves larger enthalpy changes than physisorption due to the breaking and forming of chemical bonds. Chemisorption forms a monolayer on the solid's surface, while physical adsorption can form multilayers due to intermolecular interactions. In practice, once a chemisorbed monolayer forms, further adsorption may proceed by physisorption on top of that layer.
During chemisorption, some molecules dissociate while others remain intact. Molecules such as H₂ tend to dissociate because breaking the internal bond allows the atoms to form stronger, more stable bonds with surface atoms. In contrast, molecules like NH₃ or CO often adsorb without dissociation because they possess lone pairs or multiple bonds that can interact directly with the surface, making bond cleavage unnecessary. Whether dissociation occurs depends on the molecule’s bonding, the energy required to break its internal bonds, and the electronic structure of the surface.
The extent of surface coverage, either by physisorption or chemisorption, is commonly expressed as fractional coverage, θ. It is calculated from the relation θ = V/Vmon, where V is the volume of adsorbate adsorbed, and Vmon is the volume of adsorbate corresponding to complete monolayer coverage. Both volumes are measured for the free gas under the same conditions of temperature and pressure.
The adsorption rate is directly proportional to the gas reactant's concentration and the number of available surface positions or adsorption sites. After reacting on the surface, the gas molecules desorb, a zeroth-order reaction related only to coverage.
Adsorption isotherms are used in experimental analysis to plot the amount of gas adsorbed against gas pressure at a fixed temperature. Type I isotherms, common for chemisorption, indicate monolayer formation, while Type II isotherms, typical for physical adsorption, depict multilayer adsorption. Chemisorption has a high binding energy, so when a space on the substrate is available, it will bind quickly; this causes a sharp spike in volume adsorbed before it levels off when no more sites are available (restricted to a monolayer). For physisorption, the binding energy is low, so the volume increases more slowly. Additionally, it can form multilayers, so there isn’t a maximum threshold that is quickly reached–the volume of adsorbed gas will continue to increase with pressure.
Understanding the adsorption of gases on solids is crucial for various applications, including catalysis, gas storage, and environmental remediation. For instance, exposing molecules such as alkyl thiols (RSH), where R represents an alkyl chain, to a Gold (Au) surface leads to the formation of a highly ordered monolayer on the surface by the reaction of the thiol group with the surface.
