Coverage, fractional

The molecules in an adsorbed layer interact not only with the solid, hut also with their neighbours within the layer. The effect is negligible when the fractional coverage 0 of the surface is small and the adsorbed molecules are therefore far apart, but it becomes increasingly significant as the monolayer becomes more and more crowded. A densely occupied monolayer will act in some degree as an extension of the solid, and will be able to attract further molecules from the gas phase in the manner already described, though more... 

Fig. 2.22 Adsorption isotherms of argon on graphitized carbon black at a number of temperatures," plotted as fractional coverage 0 against relative pressure p/p°. (Courtesy Prenzlow and Halsey.)...

Fig. 5.2 Type III isotherms, (a) n-hexane on PTFE at 25°C (b) n-octane on PTFE at 20 C (c) water on polymethylmethacrylate at 20°C (d) water on bis(A-polycarbonate) (Lexan) at 20°C. The insets in (c) and (d) give the curves of heat of adsorption against fractional coverage the horizontal line marks the molar heat of liquefaction. (Redrawn from diagrams in the original papers, with omission of experimental points.)...

The flux of flie adsorbed species to die catalyst from flie gaseous phase affects die catalytic activity because die fractional coverage by die reactants on die surface of die catalyst, which is determined by die heat of adsorption, also determines die amount of uncovered surface and hence die reactive area of die catalyst. Strong adsorption of a reactant usually leads to high coverage, accompanied by a low mobility of die adsorbed species on die surface, which... 

Fractional coverage (% Phase state y,v by JKR method (mJ/m-) Contact angle hysteresis (mJ/m ) ... 

Figure 5.17. To derive the BET isotherm the surface is divided into regions covered by / adsorbate layers each region is characterized by a fractional coverage Oj...

Perrin et al. have denoted the fractional coverages of the surface sites as 6, and 4 for =SiH, =Si, and =SiHSiH3, respectively [317], with 0 + 6q 6 = 1. These three fractional coverages can be found by expressing them as rates in terms of creation and destruction. This gives, with the incident SiH3 flux density per surface site [439],... 

If the composition of the outermost layer is different from that of the bulk (for example, as a result of surface or grain boundary segregation), a different approach has to be made. For example, if a partial overlayer of element A of fractional coverage 9a covers a substrate of element B, the spectrum contains three contributions that from the overlayer, that from the exposed part of the substrate, and that from the covered part of the substrate. 

Category II. The rate of chemical reaction on the surface is so rapid that adsorption equilibrium is not achieved, but a steady-state condition is reached in which the amount of adsorbed material remains constant at some value less than the equilibrium value. This value is presumed to be that corresponding to equilibrium for the surface reaction at the appropriate fractional coverages of the other species involved in the surface reaction. The rate of adsorption or desorption of one species is presumed to be much slower than that of any other species. This step is then the rate limiting step in the overall reaction. 

Formate Fractional Coverage 0formate at Steady-State Low-Temperature Shift Conditions at 225°C or 185°C... 

Note that we consider unit area in the general case S and Sex denote fractional coverage. At short times Sex 1, the clusters do not touch, and S Sex- At long times Sex -> oo and S - 1, and the whole surface is covered by a monolayer. For a proof of Avrami s theorem we refer to his original paper [4] (see also Problem 2). 

The mass adsorbed is proportional to the fractional coverage of The proportionality factor is embodied in the constant of Eq 6.7. 

So the fractional coverage of the total adsorption sites 0 becomes... 

Before reviewing reactions of the individual hydrocarbons, some overall trends will be discussed. In general, post-reaction surface analysis by AES showed the presence of submonolayer coverages of carbon, with a fractional coverage fairly independent of reaction conditions for a given alkane. The amount of carbon left on the surface increased approximately linearly with the size of the parent hydrocarbon molecule. This carbon residue is not believed to be an important intermediate in the hydrogenolysis reactions , however,... 

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