Chemisorption

Chemisorption is a classification of adsorption characterized by a strong interaction between an adsorbate and a substrate surface, as opposed to physisorption which is characterized by a weak Van der Waals force. A distinction between the two can be difficult and it is conventionally accepted that it is around 0.5eV of binding energy per atom or molecule. The types of strong interactions include chemical bonds of the ionic or covalent variety, depending on the species involved.

It is characterised by:

• High temperatures.
• Type of interaction: strong; covalent bond between adsorbate and surface.
• High enthalpy: 50 kJ/mol <ΔH< 800 kJ/mol
• Adsorption takes place only in a monolayer.
• High activation energy
• Increase in electron density in the adsorbent-adsorbate interface.
• Reversible only at high temperature.

Due to specificity, the nature of chemisorption can greatly differ from system to system, depending on the chemical identity and the surface structure.

Uses

The main way in which most chemists utilise the effect of chemisorption is in catalysed reactions. The process of chemisorption is actually pivotal to the role of heterogeneous catalysis where the catalyst is in a solid phase—particularly transition metal catalysts. In many cases the chemical reagents will both bind to the catalytic surface. The chemical bonds then form and draw electrons away from the chemisorption bonds. The molecule then desorbs and is free to leave the surface.

Examples

Self Assembled Monolayers (SAMs) are often formed by chemisorbing thiols (RS-H) onto gold surfaces forming Au-SR bonds.

O₂ on carbon at high temperatures

Research on the adsorption of hydrogen onto carbon nanotubes with the aim of producing a fuel cell that can eventually replace our dependence on fossil fuels

Gas-surface Chemisorption

Adsorption Kinetics

As a subset of adsorption, chemisorption follows the adsorption process. The first stage is for the adsorbate particle to come into contact with the surface. The particle needs to be trapped onto the surface by not possessing enough energy to leave the gas-surface potential well. If it elastically collides with the surface, then it would return to the bulk gas. If it loses enough momentum through an inelastic collision, then it “sticks” onto the surface, forming a precursor state bonded to the surface by weak forces, similar to physisorption. The particle diffuses on the surface until it finds a deep chemisorption potential well. Then it reacts with the surface or simply desorbs after enough energy and time.

The reaction with the surface is dependent on the chemical species involved. Applying Gibb’s Free Energy equation for reactions:

$\Delta G=\Delta H-T\Delta S$

General thermodynamics states that for reactions, the change in free energy should be negative. Since a free particle is restrained to a surface, and unless the surface atom is highly mobile, entropy is lowered. This means that the enthalpy term must be negative, implying an exothermic reaction.

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Figure 1: Graph of energy curves of physisorption (W-0₂) and chemisorption (W-O). Chemisorption has a deeper energy well. The graphs are shown to either cross above the zero-energy line or below it.

Figure 1 is a graph of physisorption and chemisorption energy curves of tungsten and oxygen. Physisorption is given as a Lennard-Jones potential and chemisorption is given as a Morse potential. There exists a point of crossover between the physisorption and chemisorption, meaning a point of transfer. It can occur above or below the zero-energy line (with a difference in the Morse potential, a), representing an activation energy requirement or lack of. Most simple gases on clean metal surfaces lack the activation energy requirement.

Modeling

For experimental setups of chemisorption, the amount of adsorption of a particular system is quantified by a sticking probability value.

However, chemisorption is very difficult to theorize. A multidimensional potential energy surface (PES) derived from effective medium theory is used to describe the effect of the surface on absorbtion, but only certain parts of it are used depending on what is to be studied. A simple example of a PES, which takes the total of the energy as a function of location:

$E(\{R_{i}\})=E_{el}(\{R_{i}\})+V_{\text{ion-ion}}(\{R_{i}\})$

where E_el is the energy eigenvalue of the Schroedinger Equation for the electronic degrees of freedom. and V_ion-ion is the ion interactions. This expression is without translation energy, rotational energy, vibrational excitations, and other such considerations.

There exist several models to describe surface reactions: the Langmuir-Hinschelwood mechanism where both reacting species are adsorbed, and the Eley-Rideal mechanism where one is adsorbed and the other reacts with it.

Real systems have many irregularities, making theoretical calculations more difficult:

• Solid surfaces are not necessarily at equilibrium.
• They may be perturbed and irregular, defects and such.
• Distribution of adsorption energies and odd adsorption sites.
• Bonds formed between the adsorbates.

Compared to physisorption where adsorbates are simply sitting on the surface, the adsorbates can change the surface, along with its structure. The structure can go through relaxation, where the first few layers change interplanar distances without changing the surface structure, or reconstruction where the surface structure is changed.

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Figure 2: STM image of oxygen adsorbed onto Cu(110). Several defects can be seen, gaps and missing rows. The oxygen is arrayed in the 001 direction.

For example oxygen can form very strong bonds (~4eV) with metals, such as Cu(110). This comes with the breaking apart of surface bonds in forming surface-adsorbate bonds. A large restructuring occurs by missing row as seen in Figure 2.

Dissociation Chemisorption

A particular brand of gas-surface chemisorption is the dissociation of diatomic gas molecules, such as hydrogen, oxygen, and nitrogen. One model used to describe the process is precursor-mediation. The absorbed molecule is adsorbed onto a surface into a precursor state. The molecule then diffuses across the surface to the chemisorption sites. They break the molecular bond in favor of new bonds to the surface. The energy to overcome the activation potential of dissociation usually comes from the translational energy and vibrational energy.

And example is the Hydrogen and Copper system, one that has been studied many times over. It has a large activation energy of .35 - .85eV. The vibrational excitation of the hydrogen molecule promotes dissociation on low index surfaces of copper.

See also

• Adsorption
• Physisorption