Bridge adsorption model

Scheme 6. Reaction pathways for the ORR in acid medium for three different adsorption modes, the Griffith adsorption model (pathway 1), the Pauling adsorption model (pathway 2) and a bridged adsorption model (pathway 3).

Figure 9.6. Interactions platinum-oxygen in the case of Griffith and Bridge adsorption models.

Fig. 16. Left A one-dimensional intensity profile along the line AB of the image shown in Fig. 15 (b). Lower part of the figure is the result of the curve-fitting using Gaussians. Right The adsorption model illustrating the bidentate and bridging forms of the acetate. [159].

As noted before, thin film lubrication (TFL) is a transition lubrication state between the elastohydrodynamic lubrication (EHL) and the boundary lubrication (BL). It is widely accepted that in addition to piezo-viscous effect and solid elastic deformation, EHL is featured with viscous fluid films and it is based upon a continuum mechanism. Boundary lubrication, however, featured with adsorption films, is either due to physisorption or chemisorption, and it is based on surface physical/chemical properties [14]. It will be of great importance to bridge the gap between EHL and BL regarding the work mechanism and study methods, by considering TFL as a specihc lubrication state. In TFL modeling, the microstructure of the fluids and the surface effects are two major factors to be taken into consideration. 

Differences in the structure of monocrystalline, threshold or bridge type polycrystalline adsorbents are to be manifested in the shape of adsorption - caused response of electrophysical characteristics [25]. The basic models of adsorption - induced response of monocrystalline and barrier poly crystal line adsorbents have been considered in Chapter 1. Here we describe various theoretical models of adsorption-induced response of polycrystalline adsorbents having intercrystalline contacts of the bridge type and their comparison with experimental results. 

Thus, we have considered in detail various theoretical models of effect of adsorption of molecular, atom and radical particles on electric conductivity of semiconductor adsorbents of various crystalline types. Special attention has been paid to sintered and partially reduced oxide adsorbents characterized by the bridge type of intercrystalline contacts with the dominant content of bridges of open type because of wide domain of application of this very type of adsorbents as sensitive elements used in our physical and chemical studies. 

Moreover, calculation shows that molecular oxygen can be adsorbed on the PANI surface only when both oxygen atoms form bonds to surface atoms, i.e. a bridge model of adsorption is most probably (Figure 4). 

Figure 4. Bridge model of oxygen adsorption on PANI.

A specialized MOPAC computer software package and, in particular, its PM3 quantum-chemical program has been successfully applied in calculations. The results of calculations have shown that both oxygen atoms form bonds with two more active carbon atoms of CP molecular cluster (so-called bridge model of adsorption). The total energy of system after a chemical adsorption at such active atoms is minimal. 

The problems associated with the application of this (or any other) model have been discussed. Because of the form of the typical isotherm, which exhibits a broad plateau region, fitting of experimental results to the model requires that data be obtained over a very broad range of concentrations. This is often very difficult to accomplish in practice, especially when difference methods are used to determine the amount of polymer adsorbed. Evaluation of adsorption in real systems is further complicated by a lack of knowledge of the available solid surface area. The latter may be affected by particle size, shape and surface topography and by polymer bridging between particles. 

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