Fractional surface coverage defined

Thus, the surface area A in Eqs. (10.13) and (10.14) must be replaced by the available (free, unoccupied) surface area. This surface area, which is free for the electrode reaction, can be expressed in terms of the surface coverage 9. The fractional surface coverage 6 of the electrode surface by an adsorbed substance is defined as... 

Initially, expressions for the individual rates of the elementary steps in Scheme I are sought and can be written according to mass action kinetics [17]. Unfortunately, because of the nonuniformity of the adsorption of all species, the values of fractional surface coverage (0,) and rate constants (, ) are not single-valued and will depend on the adsorptive properties of each discrete site for the three species. In general, the adsorptive properties of any given site toward A, B or C are distinct and independent of the adsorptive properties of that site for another molecule. These complexities have been previously addressed [16]. Briefly, we define the adsorptive property of interest to be the heat of adsorption for each species on any given site, denoted as qg and species A, B and C, respectively. Minimum and maximum... 

It is seen that n deviates most from Wg at the intraface. Thus the gas molecules that make up the surfaee exeess eoneentration F reside very close to the interface. One may now assume a model where the adsorbed gas molecules are supposed to be stuek to the solid surfaee. A quantity 0, the fractional surface coverage, is thus defined as... 

The concentration of the oil at the surface is expressed in terms of fractional surface coverage, 0, defined as follows ... 

In the case of adsorption of an intermediate in electrode kinetics, we have discussed the extent of adsorption in terms of the fractional surface coverage, 0, which was defined as ... 

It represents the equilibrium between a species in solution and the same species adsorbed on the surface. The fractional surface coverage is defined as... 

The equation of state can be furnished if the partial molar fraction xf can be described in terms of the real surface excess The molar fraction is defined as the molar concentration of the component in the surface layer divided by the total molar concentration of all components in the layer. Since the molar concentrations can also be expressed in terms of the surface area, the molar fraction is equivalent to the partial coverage 9i of the component. Now we have x f = 6i = Ficoi. In terms of the surface coverage 9i and the bulk concentration Ci of the zth component, Eqs. 6 and 7 can be transformed to give... 

To model the process, a mass balance is made for the adsorbed species, and the partial pressures of the reactants are considered as constant. The surface coverage of each adsorbed species is defined as the fraction of the total number of available adsorption sites occupied by that species and denoted by 0A or 0B. The differential equations for this mechanism are ... 

The surface coverage 8 of the i-mer is defined as the fraction of propylene oxide adsorption sites covered by any of the i units of the i-mer. 

In Section 4-1, it was shown that isotherm linearity is determined by 6, the fractional coverage of the surface by sample. The maximum value of 0 for isotherm linearity (the linear capacity 0o.i) in turn depends upon adsorbent heterogeneity specifically, the types of adsorbent sites (as defined by their Ki values) and their relative concentrations (iV,). Nowhere have we considered the variation of linear capacity with the extent of sample adsorption, i.e., the dependence of 60.1 on A . For a Langmuir isotherm Eq. (4-1) shows that linear capacity corresponds to 10% surface coverage, regardless of the value of X . Similarly, for a surface which contains several different site types (differing in values), it may easily be shown that if the Kf values of all sites are increased by a constant factor Q, K° is... 

If there are surface sites on a clean surface and we adsorb N adsorbates on that surface, we define the fractional coverage as 6=N/N. hi terms of this coverage, the average adsorption energy is defined as... 

Densities of molecules adsorbed on surfaces are in moles per unit area, which we win give the symbol rij for species j. Typical units of nj are moles per cm. There is a maximum density rijo of molecules packed in a two-dimensional layer on a flat surface when all molecules touch or reach Hquidlike or soHdlike densities. We call this density a monolayer, whose density is approximately the inverse of the square of the molecular diameter, which is less than 1 x lO molecules/cm2 for all molecules (Figure 7-21). We wiU find it convenient also to define a coverage of adsorbed molecules as the fraction of the monolayer density dj,... 

This makes a convenient point of contact with theory since models for adsorption inevitably subdivide the surface into an array of adsorption sites that gradually fill as the pressure increases. If 6 is defined as the fraction of sites filled, then 6 = 1 corresponds to monolayer coverage, with 8 < 1 or 6 > 1 to submonolayer and multilayer coverages, respectively. Theoretical isotherms predict how 6 varies with p in terms of some particular model for adsorption. It turns out that a set of experimental points can often be fitted by more than one theoretical isotherm, at least over part of the range of the data that is, theoretical isotherms are not highly sensitive to the model on which they are based. A comparison between theory and experiment with respect to the temperature dependence of adsorption is somewhat more discriminating than the isotherms themselves. 

K is defined in terms of standard states consisting of an ideal solution at unit mole fraction and for the surface an ideal two-dimensional gas with unit coverage. Therefore,... 

The fractional coverage 6 is defined as the fraction of total surface adsorption sites that are occupied by A. If [ ]q represents the number density of all adsorption sites (vacant and occupied) on a catalyst surface, then ... 

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