Chemisorption surface forces

Forces of Adsorption. Adsorption may be classified as chemisorption or physical adsorption, depending on the nature of the surface forces. In physical adsorption the forces are relatively weak, involving mainly van der Waals (induced dipole—induced dipole) interactions, supplemented in many cases by electrostatic contributions from field gradient—dipole or —quadmpole interactions. By contrast, in chemisorption there is significant electron transfer, equivalent to the formation of a chemical bond between the sorbate and the soHd surface. Such interactions are both stronger and more specific than the forces of physical adsorption and are obviously limited to monolayer coverage. The differences in the general features of physical and chemisorption systems (Table 1) can be understood on the basis of this difference in the nature of the surface forces. 

Adsorption is conveniently considered as either physical adsorption or chemisorption, depending on the nature and strength of the surface forces. Chemisorption can be considered as the formation of a chemical bond between the sorbate and the solid surface. Such interactions are strong, highly specific, and often not easily reversible. 

Forces of Adsorption. Adsorption may be classified as chemisorption or physical adsorption, depending on the nature of the surface forces. 

The acid monolayers adsorb via physical forces [30] however, the interactions between the head group and the surface are very strong [29]. While chemisorption controls the SAMs created from alkylthiols or silanes, it is often preceded by a physical adsorption step [42]. This has been shown quantitatively by FTIR for siloxane polymers chemisorbing to alumina illustrated in Fig. XI-2. The fact that irreversible chemisorption is preceded by physical adsorption explains the utility of equilibrium adsorption models for these processes. 

Note that the van der Waals forces tliat hold a physisorbed molecule to a surface exist for all atoms and molecules interacting with a surface. The physisorption energy is usually insignificant if the particle is attached to the surface by a much stronger chemisorption bond, as discussed below. Often, however, just before a molecule fonus a strong chemical bond to a surface, it exists in a physisorbed precursor state for a short period of time, as discussed below in section AL7.3.3. 

NakatsujI H, Nakal H and Fukunishi Y 1991 Dipped adcluster model for chemisorptions and catalytic reactions on a metal surface Image force correction and applications to Pd-02 adclusters J. Chem. Phys. 95 640-7 NakatsujI H and Nakal H 1992 Dipped adcluster model study for the end-on chemisorption of O2 on an Ag surface Can. J. Chem. 70 404-8... 

The reaction kinetics approximation is mechanistically correct for systems where the reaction step at pore surfaces or other fluid-solid interfaces is controlling. This may occur in the case of chemisorption on porous catalysts and in affinity adsorbents that involve veiy slow binding steps. In these cases, the mass-transfer parameter k is replaced by a second-order reaction rate constant k. The driving force is written for a constant separation fac tor isotherm (column 4 in Table 16-12). When diffusion steps control the process, it is still possible to describe the system hy its apparent second-order kinetic behavior, since it usually provides a good approximation to a more complex exact form for single transition systems (see Fixed Bed Transitions ). 

Adsorption of ions from the solution. There are two types of ionic adsorption from solutions onto electrode surfaces an electrostatic (physical) adsorption under the effect of the charge on the metal surface, and a specific adsorption (chemisorption) under the effect of chemical (nonelectrostatic) forces. Specifically adsorbing ions are called surface active. Specific adsorption is more pronounced with anions. 

It is most convenient to explain catalysis using an example. We have chosen a hydrogenation catalysed by nickel in the metallic state. According to the schematic of Fig. 3.1 the first step in the actual catalysis is adsorption . It is useful to distinguish physisorption and chemisorption . In the former case weak, physical forces and in the latter case relatively strong, chemical forces play a role. When the molecules adsorb at an active site physisorption or chemisorption can occur. In catalysis often physisorption followed by chemisorption is the start of the catalytic cycle. This can be understood from Fig. 3.2, which illustrates the adsorption of hydrogen on a nickel surface. 

First H2 is physically adsorbed on the surface. The forces are weak and the adsorption enthalpy is only slightly negative. Subsequently, chemical bonding between Ni atoms and the hydrogen atoms of H2 molecule takes place while simultaneous dissociation of the H2 molecule takes place. The chemisorption enthalpy is strongly negative (exothermic reaction). 

Fig. 3.3 gives a model for the adsorption states. In physical adsorption the molecule is adsorbed as such (without dissociation) the forces are of the van der Waals type. In chemisorption chemical bonds have been formed for H2 this is only possible at the cost of dissociation. In the transition state the H-atoms are bonded both to each other and to Ni-atoms in the surface of the metal crystal. 

Usually adsorption, i.e. binding of foreign particles to the surface of a solid body, is distinguished as physical and chemical the difference lying in the type of adsorbate - adsorbent interaction. Physical adsorption is assumed to be a surface binding caused by polarization dipole-dipole Van-der-Vaals interaction whereas chemical adsorption, as any chemical interaction, stems from covalent forces with plausible involvement of electrostatic interaction. In contrast to chemisorption in which, as it has been already mentioned, an absorbed particle and adsorbent itself become a unified quantum mechanical system, the physical absorption only leads to a weak perturbation of the lattice of a solid body. 

The adsorption action of activated carbon may be explained in terms of the surface tension (or energy per unit surface area) exhibited by the activated particles whose specific surface area is very large. The molecules on the surface of the particles are subjected to unbalanced forces due to unsatisfied bonds and this is responsible for the attachment of other molecules to the surface. The attractive forces are, however, relatively weak and short range, and are called Van der Waals forces, and the adsorption process under these conditions is termed as a physical adsorption (physisorption) process. In this case, the adsorbed molecules are readily desorbed from the surface. Adsorption resulting from chemical interaction with surface molecules is termed as chemisorption. In contrast to the physical process described for the adsorption on carbon, the chemisorption process is characterized by stronger forces and irreversibility. It may, however, be mentioned that many adsorption phenomena involve both physical and chemical processes. They are, therefore, not easily classified, and the general term, sorption, is used to designate the mechanism of the process. 

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