Adsorbate-substrate bond strong

Some adsorbates do not form strong chemical bonds with substrate atoms. This situation is called physical adsorption or physisorption. For these adsorbates the adsorbate-adsorbate interactions can dominate the adsorbate-substrate interactions, and the optimum adsorbate-substrate bonding geometry can be overridden by the lateral adsorbate-adsorbate interactions, yielding, for example, incommensurate structures in which the overlayer and the substrate have independent lattices. When... 

Strong adsorbate-substrate forces lead to chemisorption, in which a chemical bond is fomied. By contrast, weak forces result inphysisorption, as one calls non-chemical physical adsorption. 

The balance between these different types of bonds has a strong bearing on the resulting ordering or disordering of the surface. For adsorbates, the relative strength of adsorbate-substrate and adsorbate-adsorbate interactions is particularly important. Wlien adsorbate-substrate interactions dominate, well ordered overlayer structures are induced that are arranged in a superlattice, i.e. a periodicity which is closely related to that of the substrate lattice one then speaks of commensurate overlayers. This results from the tendency for each adsorbate to seek out the same type of adsorption site on the surface, which means that all adsorbates attempt to bond in the same maimer to substrate atoms. 

In chemisorbed systems, the molecular orbitals of the adsorbate are mixed with the electronic states of the substrate, producing strong adsorption bonds, i.e. the frequency of the adsorbate mode is well above the highest phonon frequency of the substrate. The relaxation of these vibrational excited states via emission of substrate phonons has only a low probability, because many phonons have to be enoitted during the decay. Non-radiative damping by electron-hole pair excitation appears to be the dominant relaxation path in these systems. 

We define the formation of a surface chemical bond to be adsorption accompanied by charge transfer and charge redistribution between the adsorbate and the substrate, producing strong bonds of covalent or ionic character. Heats of adsorption on the order of 63 kJ/mole (15 kcal /mole) or larger would certainly indicate the formation of a chemical bond, leading to long surface residence times r [r = tq exp (A//ajs/ 7 )], even at elevated temperatures, compared to to (to == 10 sec) related to vibrational times for surface atoms. 

Whereas the bond strength of adsorbed atoms varies strongly with the metal substrate, the adsorption bond strength of molecules varies much less. Within the same row of the periodic system, transition metals with less d-valence electrons are more reactive. Open surfaces, with atoms of decreased coordination, are more reactive than the more dense surfaces. 

Chemisorption occurs when the attractive potential well is large so that upon adsorption a strong chemical bond to a surface is fonued. Chemisorption involves changes to both the molecule and surface electronic states. For example, when oxygen adsorbs onto a metal surface, a partially ionic bond is created as charge transfers from the substrate to the oxygen atom. Other chemisorbed species interact in a more covalent maimer by sharing electrons, but this still involves perturbations to the electronic system. 

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