chemisorption

Chemisorption is a form of adsorption in which the adsorbate forms a chemical bond with the surface, typically involving electron sharing or transfer. The interaction is localized at specific surface sites and is generally much stronger and more specific than physisorption, which is governed by van der Waals forces. Chemisorption often involves an activation barrier, and the process can be dissociative (a molecule splits upon adsorption) or non-dissociative (the intact molecule binds through a bond to the surface).

Because chemical bonding is involved, chemisorption is highly sensitive to temperature, surface structure, and defect sites. The adsorption energies are commonly tens to hundreds of kilojoules per mole, and desorption occurs at higher temperatures than for physisorption. Chemisorption is typically irreversible under mild conditions, though desorption is possible with heat or chemical treatment.

Sites on metals, semiconductors, and oxides control chemisorption. Defect sites, steps, and kinks often bind adsorbates more strongly than flat terraces. The process can activate adsorbates for further reactions, making it central to heterogeneous catalysis and surface chemistry. Common examples include dissociative chemisorption of H2 on transition-metal surfaces to produce adsorbed hydrogen atoms, and chemisorption of CO on Pt, Ni, or other metals, sometimes followed by dissociation or reaction with oxygen or other species.

Characterization methods include temperature-programmed desorption, X-ray photoelectron spectroscopy, infrared spectroscopy, and scanning probe or electron microscopy techniques. Models of chemisorption include site-occupancy frameworks adapted for strong, specific bonding, with corrections for heterogeneity and dissociation.