The probability of an incident molecule impacting a site with a valid configuration has the form
The shape of the coverage function over time is different for each kinetic order, so assuming desorption is negligible, dissociative adsorption for a system following the Langmuir model can be determined by monitoring the adsorption rate as a function of time under a constant impinging flux.
Often the adsorbing molecule does not dissociate directly upon contact with the surface, but is instead first bound to an intermediate precursor state.
If extrinsic and intrinsic sites are assumed energetically equivalent and the adsorption rate to the precursor state is assumed to follow the Langmuir model, the following expression for the coverage dependence of the overall sticking coefficient is obtained:
[2] The behaviour of the sticking coefficient as a function of temperature is governed by the shape of the potential energy surface of adsorption.
[2] To detect dissociation on the surface, additional techniques that can distinguish surface ordering due to the interaction of dissociated fragments, identify desorbed particles,[3][4] determine the order of kinetics[5] or measure the chemical bond energies of the adsorbed species are required[6][7].
In many experiments, a combination of multiple methods that probe different surface properties is used to form a complete picture of the adsorbed species.
Often, the coverage can also be related to a change in the surface work function,[9] which can enable faster measurements in otherwise challenging conditions.
[4] The shape of the isotherms is sensitive to the order of kinetics of the adsorption and desorption processes,[2] and though the exact forms can be difficult to find, simulations have been used to find general functional forms for isotherms of dissociative adsorption for specific systems.
[10] XPS is a surface sensitive method that allows the direct probing of the chemical bonds of the surface atoms, thus being capable of differentiating bond energies corresponding to intact molecules or dissociated fragments.
A challenge with this method is that the incident photons can induce surface modifications that are difficult to separate from the effects to be measured.
[7][11] LEED patterns are often combined with other measurements to verify surface structure and recognize ordering of the adsorbates.
[5] Presence of masses different from the original molecules, or the detection of additional desorption peaks with higher order kinetics can indicate that the adsorption is dissociative.
Density functional theory (DFT) can be used to calculate the change in energy caused by the adsorption and dissociation of molecules.
[9] Another approach for considering the stretching and dissociation of adsorbates is through the charge-transfer between the electron bands near the Fermi surface using molecular orbital (MO) theory.
A strong charge transfer caused by overlap of unoccupied and occupied orbitals weakens the molecular bonds, which lowers or eliminates the barrier for dissociation.
The charge transfer can be local or delocalized in terms of the substrate electrons, depending on which orbitals participate in the interaction.
[12] In atmospheric conditions, the adsorption of water and oxygen on transition metal surfaces is a well studied phenomenon.
This is explained by the oxygen atoms binding with one hydrogen of the adsorbing water molecule to form an energetically favourable hydroxyl group.
[13] Likewise, molecular pre-adsorbed water can be used to lower the barrier for dissociation of oxygen that is needed in metal-catalyzed oxidation reactions.
[15] On clean close-packed surfaces of Ag, Au, Pt, Rh and Ni, dissociated oxygen prefers adsorption to hollow sites.
[13] The formation and dissociation of water on transition metals like palladium has important applications in reactions for obtaining hydrogen and for the operation of proton-exchange membrane fuel cells, and much research has been conducted to understand the phenomenon.
However, details of the specific adsorption sites and preferred reaction pathways for water formation have been difficult to determine.
[16] The oxidation of carbon monoxide in catalytic converters utilizes a transition metal surface as a catalyst in the reaction This system has been extensively studied to minimize the emissions of toxic CO from internal combustion engines, and there is a trade-off in the preparation of the Pt catalyst surface between the dissociative adsorption of oxygen and the sticking of CO to the metal surface.
A larger step density increases the dissociation of oxygen, but at the same time decreases the probability of CO oxidation.
The optimal configuration for the reaction is with a CO on a flat terrace and a dissociated O at a step edge.
[12] The most prevalent method for hydrogen production, steam reforming, relies on transition metal catalysts which dissociatively adsorb the initial molecules of the reaction to form intermediates, which then can recombine to form gaseous hydrogen.
Kinetic models of the possible dissociative adsorption paths have been used to simulate the properties of the reaction.
The hydrogen gas dissociates on the surface of the film, after which the individual atoms are able to diffuse through the metal, and recombine to form a higher hydrogen content atmosphere inside the low-pressure receiving vessel.
If enough partial pressure builds up inside the material, this can cause cracks, blistering or embrittlement of the walls.