Langmuir—Hinshelwood recombination

The desire to understand catalytic chemistry was one of the motivating forces underlying the development of surface science. In a catalytic reaction, the reactants first adsorb onto the surface and then react with each other to fonn volatile product(s). The substrate itself is not affected by the reaction, but the reaction would not occur without its presence. Types of catalytic reactions include exchange, recombination, unimolecular decomposition, and bimolecular reactions. A reaction would be considered to be of the Langmuir-Hinshelwood type if both reactants first adsorbed onto the surface, and then reacted to fonn the products. If one reactant first adsorbs, and the other then reacts with it directly from the gas phase, the reaction is of the Eley-Ridel type. Catalytic reactions are discussed in more detail in section A3.10 and section C2.8. 

We can conclude now that one electron returns to the conductivity band during each act of formation of the vacant site to adsorb sensitizer. Because adsorption centers Zr(. ) are not accounted for by (2.81) the energetics of the process does not depend on the manner in which R is closing in A, i.e. on the fact which recombination mechanism (either Langmuire-Hinshelwood or Ili-Ridil) takes place. 

In the case of very strong chemisorption [see Fig. 2(d)], the desorption of molecules is even more activated than the desorption of adatoms. At temperatures not high enough for appreciable desorption of adatoms, the very large value of — AU2 t ensures values of 6 close to unity. The virtual absence of molecular desorption rules out the Langmuir— Hinshelwood process, so that recombination is restricted to the Rideal— Eley mechanism, which is now activated, causing recombination to be... 

Besides volume processes wall collisions of hydrogen particles can contribute to the vibrational population. A direct process is the interaction of already vi-brationally excited molecules with the surface (v) +wall —> ff2(w) mostly depopulating the vibrational levels. Further fundamental mechanisms are the Langmuir-Hinshelwood and the Eley-Rideal mechanism. They are based on recombining hydrogen atoms or ions Hads/gas + Hads —> H2(v). In the first case an adsorbed particle at the surface recombines with another adsorbed particle (Langmuir-Hinshelwood mechanism). In the second case one particle from the gas phase recombines with an adsorbed particle (Eley-Rideal mechanism). For these processes the data base is scarce and often not determined from plasma material interaction experiments. A dependence on particle densities, surface material and surface treatment as well as surface temperature can be expected. 

Keywords Photocatalysis and catalysis Photocatalytic activity Band structure and excitation Energy conversion Langmuir-Hinshelwood mechanism Electron-hole recombination Quantum efficiency Physical property-activity correlation Synergetic effect. 

Before A and B can react, they must both adsorb on the catalyst surface. The next event is an elementary step that proceeds through a reaction of adsorbed intermediates and is often referred to as a Langmuir-Hinshelwood step. The rate expression for the bimolecular reaction depends on the number density of adsorbed A molecules that are adjacent to adsorbed B molecules on the catalyst surface. This case is similar to the one developed previously for the recombinative desorption of diatomic gases [reverse reaction step in Equation (5.2.20)] except that two different atomic species are present on the surface. A simplified rate expression for the bimolecular reaction is ... 

These oxygenated species promote the oxidation of the adsorbate by either a direct recombination (reaction (V)) of the species through a Langmuir-Hinshelwood mechanism [47] (Scheme 2.1)... 

Instead of etching, radicals can recombine on the surface (reaction R24 in Table 4). This may be an important loss mechanism of potential etchant species, especially at low pressures. Recombination can occur between two radicals adsorbed on the surface at adjacent sites, (Langmuir-Hinshelwood mechanism [65]). The product of recombination desorbs into the gas. 

Upon the chemisorption of the reactants, Oad + COad may recombine according to the Langmuir-Hinshelwood mechanism by overcoming an activation barrier of lOOkJ/mol (which is only about half as high at higher coverages). The above reaction mechanism and its energetics were confirmed by density functional theory. 

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