Kinetics of chemisorption

Chemisorption, a phenomenon activated by temperature, does not take place instantaneously and has a certain velocity just like a chemical reaction. 

We stick to the assumptions that led to Langmuir-type isotherms and regard adsorption as an elementary step (or at least like an equivalent reaction within the meaning of section 7.9.2). We consider dissociative adsorption and distinguish two cases depending on whether the two fiagments of the molecule are adsoihed on identical or different sites. 

Adsorption of the two fragments on identical sites The reaction of adsorption can be written as  

Remark.- The reaction of adsorption being regarded as equivalent to an elementary step, so we are not authorized to express this reaction in the following form  

It was noted in Section XVII-1 that chemisorption may become slow at low temperatures so that even though it is favored thermodynamically, the only process actually observed may be that of physical adsorption. Such slowness implies an activation energy for chemisorption, and the nature of this effect has been much discussed. 

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In considering isotherm models for chemisorption, it is important to remember the types of systems that are involved. As pointed out, conditions are generally such that physical adsorption is not important, nor is multilayer adsorption, in determining the equilibrium state, although the former especially can play a role in the kinetics of chemisorption. 

NMR spectroscopy has been used to detect hydrides on various oxide-supported metals in the presence of H2 and on La203-supported Ir4, in the absence of H2 [37]. The kinetics of chemisorption of H2 supports the inference of hydride formation by dissociative adsorption of H2 [38]. 

Hence, BSS recharging substantially hinders the adsorption-induced change in degree of barrier disarrangement of adsorbent and in several cases can bring about small sensitivity of electrophysical characteristics of adsorbent to the adsorption process. We should note that above BSS recharging can be one of reasons of notable effect of adsorbent manufacturing prehistory of and the methods to treat its surface on the value and kinetics of chemisorption response. 

Fig. 1.16. The qualitative type of dependencies ait) (curve 1) and flit) (curve 2) in case of linear kinetics of chemisorption of donors.

At a sufficient distance from the equilibrium, where the reverse reaction can be neglected, we obtain for the kinetics of chemisorption of oxygen on NiO... 

Recently, in order to understand processes on the catalyst surface, in particular structural formations, it has become a frequent practice to apply theories accounting for the interaction of adsorbed atoms. An important microscopic model of such a type is the lattice gas model. Its specific peculiarity is that this model accounts for the interaction of the nearer surface molecules (lateral interactions). It is this model that was applied in refs. 86 and 87. They should be specially emphasized as having exerted a great influence on the interpretation of thermodesorption experiments. The lattice gas model is used, e.g. in a series of investigations by Tovbin and Fedyanin [88, 89] devoted to the kinetics of chemisorption and reactions on catalyst surfaces. In terms of this model, one can interpret the complicated reaction rate dependences of surface coverage observed experimentally... 

Ungarish, M., and Aharoni, C. (1981). Kinetics of chemisorption Deducing kinetic laws from experimental data. /. Chem. Soc., Faraday Trans., 1 pp. 975-979. 

Undoubtedly the overall Eqs. (47) and (48) as weli as the relations derived from them represent an approximate account of the electrochemical situation. However no information is obtained about the actual elementary steps of chemisorption which lead to creation of the field and space charge, or about the slow partial step. In the following we shall consider the kinetics of chemisorption on a p-type oxide on which electron-acceptor A particles are chemisorbed ... 

As noted in Section 2.1.2, measurement of the kinetics of chemisorption on clean metal surfaces generally requires ultra high vacuum techniques, in order to accomplish the experiment in a reasonable period of time. The variant of the classical adsorption method known as the flash-filament technique has been developed by several groups of workers and recently summarised by Ehrlich . 

Already at 1 = 2, this model describes well enough the kinetics of chemisorption of CH3OH, CH3NH2, H2O and NH3 on the dehydroxylated silica surface at a constant temperature [26]. The use of Eq. 32 to describe the organosilicon compounds chemisorption kinetics on the silica surface is presented in Section 6.2. 

Section 1 presents the studies both on the structure of the formed surface compounds and the main regularities of the chemisorption processes and on kinetics of chemisorption from the gas phase on oxide surfaces. Prospective ways for the immobillization of different active compounds have been also proposed. 

The adsorption of the several gases, which participate in the catalysis, onto the catalyst surface has been measured separately, and the thermodynamics and kinetics of chemisorption have been studied in various systems by many investigators. Chemisorption from gas mixtures, however, has been studied only in a limited number of cases and no measurements of adsorption on the catalyst in its working state has been carried out until recent years. 

An attempt has been made, on the basis of gravimetric and spectroscopic investigations of the kinetics of chemisorption of Me3SiCl on silicas of differing degrees of hydration, to explain the chemistry involved in the modification of the surface structure according to the equation ... 

Bansal RC, Vastola FJ, Walker Jr PL. Kinetics of chemisorption of oxygen on Graphon. J Coll InterfSci 1970 32(2) 187-194. 

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