Chemisorption

Chemisorption is a form of adsorption in which the adsorbate undergoes a chemical reaction with the surface, resulting in the formation of new chemical bonds. Unlike physisorption, which involves weak van der Waals interactions and leaves the chemical identities of the species largely unchanged, chemisorption involves the creation of ionic or covalent bonds between the adsorbate and the surface. This process can be observed in phenomena ranging from large-scale corrosion to highly specialised surface interactions that underpin heterogeneous catalysis. Because chemisorption is specific to both the adsorbate and the structural properties of the surface, its characteristics vary greatly across chemical systems. The generally accepted energetic boundary between physisorption and chemisorption is approximately 0.5 eV per adsorbed species.

Characteristics and Mechanisms

Chemisorption typically proceeds through several kinetic stages. First, the gas-phase or liquid-phase particle approaches and collides with the surface. If the collision is elastic, the particle returns to the bulk phase. If sufficient momentum is lost through inelastic interactions, the particle becomes temporarily trapped in a weakly bound precursor state, resembling physisorption. From this state, the particle diffuses across the surface until it locates a deeper chemisorption potential well where a chemical reaction with the surface atoms occurs.
The thermodynamics of chemisorption follow the Gibbs free-energy condition for spontaneity, requiring a negative change in free energy. Because adsorption reduces the entropy of the particle by confining it to a surface, the enthalpy change must be sufficiently negative, meaning the process is exothermic. The potential energy landscape is often represented by a transition from a Lennard–Jones potential (physisorption) to a Morse potential (chemisorption), with a crossover point that may involve an activation barrier depending on the specific system.
In metals such as copper, oxygen chemisorption can be particularly strong, forming bonds of approximately 4 eV and causing significant restructuring of the surface, including missing-row reconstructions. Chemisorption can induce relaxation or reconstruction of the solid surface, altering interplanar distances or reorganising atomic arrangements.

Uses and Functional Importance

One of the most significant applications of chemisorption is in heterogeneous catalysis. Many catalytic reactions proceed through chemisorbed intermediates: reactant molecules bind to the catalyst surface, undergo transformation, and then desorb as products. For example, during the hydrogenation of alkenes, both the alkene and hydrogen molecules chemisorb onto a metal catalyst, enabling bond-making and bond-breaking processes that generate the final product.
Chemisorption is also fundamental to the formation of self-assembled monolayers (SAMs). In these systems, reactive molecules form ordered layers on solid surfaces. Thiols, for instance, chemisorb onto gold surfaces, forming robust Au–S bonds and releasing hydrogen. The resulting monolayers protect the surface and provide functional interfaces for further chemical modification.

Gas–Surface Chemisorption and Kinetics

Gas-phase chemisorption follows adsorption kinetics influenced by collision dynamics and surface mobility. The sticking probability quantifies how many incident particles successfully adsorb. Factors such as temperature, surface irregularities and defect sites can alter the probability and the ease with which chemisorption occurs.
Theoretical modelling is challenging because real surfaces are not perfectly uniform and often contain imperfections that affect adsorption energies. Multidimensional potential energy surfaces (PES) are therefore employed to describe the interaction between adsorbates and surfaces. These models consider electronic energy contributions and ion–ion interactions but may omit rotational or vibrational degrees of freedom in simplified forms. Surface-reaction models include the Langmuir model, which assumes both reactants are adsorbed, and alternative schemes in which one reactant remains in the gas phase while the other is surface-bound.

Surface Bonding and Structural Effects

The strong interaction between adsorbates and surfaces can significantly alter the substrate. Adsorbates may cause:

• surface relaxation, in which interlayer distances change;
• surface reconstruction, in which the atomic arrangement is modified;
• changes in electronic structure, especially on metal surfaces where chemisorption can generate new electronic states.

Direct experimental evidence of transitions between physisorption and chemisorption has been obtained through atomic force microscopy, such as measuring the interaction of a single CO molecule with an isolated iron atom.

Dissociative Chemisorption

Dissociative chemisorption occurs when diatomic molecules, such as hydrogen, oxygen or nitrogen, break their internal bonds upon adsorption and form new bonds with the surface. Many metal surfaces permit dissociative chemisorption through the precursor-mediated mechanism: the molecule first enters a precursor state, diffuses to reactive sites and dissociates once sufficient translational or vibrational energy is available to overcome the activation energy barrier.
The hydrogen–copper system is an extensively studied example. Dissociation on copper surfaces requires activation energies typically between 0.35 and 0.85 eV. Vibrational excitation greatly enhances dissociation efficiency, particularly on low-index surfaces.