Number of molecules adsorbed

Plotting P/[Va(Po - P)] versus P/Pq yields a straight line with slope a= x l)lxVo, crossing the y-axis at = I/ Vq. The volume adsorbed in the first monolayer is found as Vb = l/(ct + rj). The volume Vq can be converted into the number of molecules adsorbed by No = PVq/RT and if we know how big an area each molecule occupies (j4o) then the total area, A= NqAo, can be found. 

Henry constant F can be evaluated by the method of Tsvitering and Krevelyn (see [105]) based on the concept of mean lifetime of a molecule absorbed on a surface proposed by De Boer [106]. The number of molecules adsorbed on a surface is... 

If Type I adsorption behavior is obeyed, a plot of PA/v versus PA should be linear with slope l/vm. Once the volume corresponding to a mono-layer has been determined, it can be converted to the number of molecules adsorbed by dividing by the molal volume at the reference conditions and multiplying by Avogadro s number (N0). When this number of molecules is multiplied in turn by the area covered per adsorbed molecule (a), the total surface area of the catalyst (S) is obtained. Thus,... 

The strength of the segment-surface site interaction has more effect on the conformation of the adsorbed molecules than on the number of molecules adsorbed. 

At high temperatures the surface is only sparsely occupied by adsorbate, and the number of molecules adsorbed probably varies significantly with the... 

The natural line of inquiry is to study the progress of a given reaction on various catalytic surfaces, to determine the relative numbers of molecules adsorbed on each surface, and to seek a correlation between the heat of activation, using provisionally the apparent value as a sufficiently good approximation to the true value, and the velocity of change referred to equal numbers of adsorbed molecules. Unfortunately, no example has hitherto been found suitable for experimental investigation, in which both the adsorptions and the reaction velocities can be measured. Thus no really valid test can be made. The existence of centres of varying activity would still further complicate the interpretation even of direct measurements of adsorption. 

But it is not at all difficult to admit that the number of molecules adsorbed on parts of the glass surface possessing catalytic virtue may be ten thousand times smaller than the number which rhodium can accommodate. The matter is thus left open. 

The values 32,500 and 29,000 for the heats of activation are uncorrected for the change with temperature of the number of molecules adsorbed, and this obviously detracts from the conclusiveness of the results. In the other two instances which we proposed to discuss this uncertainty can, fortunately, be eliminated, or at least considerably reduced. 

It should be realized that selective chemisorption titrates only atoms on the surface, but that the surface composition can be changed upon adsorption (75). A difficulty in the interpretation of chemisorptive titration is the possible presence of electronic effects, which can alter the chemical nature of the titrated atoms. Adsorbates can also show strong interactions with each other on pure metals, and decreases in this interaction can cause an increase in the number of molecules adsorbed per active metal atom exposed. The surface composition of active metal atoms deduced from these measurements will then be too high. 

The total number of molecules adsorbed on the surface per unit mass of adsorbent is... 

Asp is the specific surface area of the adsorbent, ax is the cross-sectional area of an adsorbate molecule, and na is the number of adsorption sites, identical to the total number of molecules adsorbed at naax without vex effects, supposing a uniform monolayer thickness of solute at saturation. 

Once the density profile is defined for a given pressure, the amount adsorbed by a particular pore can be obtained from the area under the curve. The surface excess number of molecules adsorbed, which as we have seen corresponds to the experimentally determined adsorption, is then given by [p(r) - pB] dr, where pB is the density of the bulk phase at (p, T). 

A significant problem in surface complexation models is the definition of adsorption sites, The total number of proton-exchangeable sites can be determined by rapid tritium exchange with the oxide surface (25). Although surface equilibria are usually written in terms of one surface site, e.g. Equations 5, 6, 8, 9, adsorption isotherms for many ions show that the number of molecules adsorbed at maximum surface coverage (fmax) is less than the total number of surface sites. For example, uptake of Se(VI) and Cr(VI) ions on Fe(0H)3(am) at T ax 1/3 and 1/4 the total... 

Consider, by way of example, the simple case of localized monolayer adsorption without lateral interaction. Let there be /V j sites of type j. Tliese sites are distinguishable in the present situation we do not yet have to discriminate between patchwise and random distribution of these sites, but this becomes important as soon as lateral interaction has to be accounted for. We have Ns = Sj sj If Is the number of molecules adsorbed on sites j in... 

Fig. 1. Computer simulations of the number of molecules ( , adsorbed in the different layers and 7 as a function of time t by numerically solving Eqs. (6) and (7) in the case of two layers with different values of the binding parameters, n designates the total number of molecules adsorbed, is taken to be 100. The values of J approach the asymptotical value much faster then the values of n, indicating that Jantti s method is useful for the saving of time.

Fig. 2a gives the saturation capacity of the CS-C22 ra-alkanes, expressed in number of molecules adsorbed per unit cell. Fig. 2b gives the total number of carbon atoms per unit cell. For pentane and hexane, about 7.7 molecules are adsorbed per unit cell of ZSM-S. This corresponds to 39 C atoms for pentane and 4S C atoms for hexane. Although the number of adsorbed carbon atoms is the same for heptane as compared to hexane, only about 6.4 heptane molecules are adsorbed per unit cell. A sudden drop is observed between heptane and octane only 4.5 octane molecules are adsorbed per unit cell. From octane on, the number of C atoms adsorbed per unit cell increases steadily, to reach a plateau of about 55 C atoms adsorbed unit cell. 

For the second and subsequent layers, the adsorption energy is constant and equal to the heat of condensation. This amounts to saying that the interaction with the solid becomes negligible beyond a monolayer the phenomenon is reversible at all times indicating a permanent equilibrium between the number of molecules adsorbed and desorbed. 

The filament is then allowed to cool and the pressure drops due to adsorption until a new steady state is established as shown in Fig. 5. The net flow of gas into the system, (No—N)Sp, is now balanced by the net loss of gas to the sample (Fp — NSp). The number of molecules adsorbed at time t is given by... 

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