Adsorption of Gases on Solids

Physisorption involves weak van der Waals intermolecular forces and is nonspecific, allowing gases like nitrogen to adsorb on any solid at low temperatures. Chemisorption forms strong chemical bonds and is highly specific, depending on the solid type. Chemisorption generally produces larger enthalpy changes due to breaking and forming chemical bonds.

Dissociation depends on the molecule's bonding, energy required to break internal bonds, and the surface's electronic structure. Hydrogen dissociates because breaking its bond allows atoms to form stronger bonds with surface atoms. Ammonia and carbon monoxide adsorb without dissociation because their lone pairs or multiple bonds interact directly with the surface, making bond cleavage unnecessary.

Chemisorption forms only a monolayer on the surface due to strong chemical bonding. Physisorption creates multilayers because weak intermolecular forces allow additional layers to build on top of the surface. After chemisorption establishes a monolayer, physisorption can continue forming additional layers above it.

Fractional coverage, represented as θ, is calculated as the ratio of the volume of adsorbate adsorbed to the volume required for complete monolayer coverage. Both volumes are measured for the free gas under identical temperature and pressure conditions. This metric quantifies how much of the available surface has been occupied by adsorbed molecules.

The adsorption rate is directly proportional to both the gas reactant's concentration and the number of available adsorption sites on the surface. After molecules react on the surface, they desorb in a zeroth-order reaction dependent only on coverage. This relationship shows that more gas molecules and more empty sites increase the rate of adsorption.

Type I isotherms, common for chemisorption, show a sharp spike in volume adsorbed before leveling off, indicating monolayer formation with high binding energy. Type II isotherms, typical for physisorption, show slower volume increase with pressure and no maximum threshold because multilayer formation continues. These patterns reflect the different binding strengths and layer-forming capabilities of each adsorption type.

Adsorption is crucial for catalysis, gas storage, and environmental remediation. For example, alkyl thiols exposed to a gold surface form highly ordered monolayers through thiol group reactions with the surface. Understanding adsorption mechanisms enables design of materials for heterogeneous catalysis and efficient gas capture systems.