Pre-adsorption

TPD Temperature programmed desorption After pre-adsorption of gases on a surface, the desorption and/or reaction products are measured while the temperature Increases linearly with time. Coverages, kinetic parameters, reaction mechanism... 

Adsorption of nitrogen at 77 K on a microptorous carbon after pre-adsorption of n-nonane (cf. Fig. 4.13)... 

Fig. 4.13 The pre-adsorption method (a) adsorption isotherms of nitrogen at 77 K on a sample of Mogul I carbon black charged with different amounts x of pre-adsorbed nonane. Values ofx (mg g (A) 63 (B)48 (C) 29 (D) 16 (E) 0. (See Table 4.5.) (Some points at low pressures omitted for the sake of clarity.)...

Fig. 4.15 a,-plots for the adsorption of nitrogen on a sample of microporous titania, before and after nonane treatment. Curve (A), before nonane pre-adsorption curves (B), (C), (D) after nonane pre-adsorption, followed by outgassing at (B) 250° (C) 150°C (D) 25°C. The a,-plots were based on standard nitrogen isotherms having the same c-values as the isotherms of... 

Fig. 4.16 Comparison plots for a microporous sample of y-Mn02 after outgassing at various temperatures, also after pre-adsorption of nonane. The adsorption on the sample is plotted against the adsorption on a reference sample of synthetic MnOOH. (Courtesy Lee and Newnham.) Outgassing temperature (K) Curve A, 9, room B, O, 393 C, , 443 D, A, 493 K. Curve E, pre-treated with nonane.

A major difficulty in testing the validity of predictions from the DR equation is that independent estimates of the relevant parameters—the total micropore volume and the pore size distribution—are so often lacking. However, Marsh and Rand compared the extrapolated value for from DR plots of CO2 on a series of activated carbons, with the micropore volume estimated by the pre-adsorption of nonane. They found that except in one case, the value from the DR plot was below, often much below, the nonane figure (Table 4.9). 

These procedures proposed by Dubinin and by Stoeckli arc, as yet, in the pioneer stage. Before they can be regarded as established as a means of evaluating pore size distribution, a wide-ranging study is needed, involving model micropore systems contained in a variety of chemical substances. The relationship between the structural constant B and the actual dimensions of the micropores, together with their distribution, would have to be demonstrated. The micropore volume would need to be evaluated independently from the known structure of the solid, or by the nonane pre-adsorption method, or with the aid of a range of molecular probes. 

As remarked on p. 214, the validity of the nonane pre-adsorption method when adsorptives other than nitrogen are employed for determination of the isotherms, has been examined by Tayyab. Two organic adsorptives, /i-hexane and carbon tetrachloride, which could be used at or near room temperature, were selected and the adsorbents were the ammonium salts of... 

Fig. 4J0 Adsorption isotherms on ammonium silicomolybdate powder. (I), (4). nitrogen at 77 K (2), (3), /t-hexane at 298 K. Isotherms I and 2 were measured before, and 3 and 4 after, pre-adsorption of n-nonane. Open symbols, adsorption solid symbols, desorption. (Adsorption is expressed in mm (liquid.)...

It follows that the applicability of the nonane pre-adsorption method for the evaluation of microporosity is restricted to adsorptives such as nitrogen which are used at temperatures far below ambient and which have negligible solubility in soUd or liquid nonane. 

The strength of dispersion interaction of a solid with a gas molecule is determined not only by the chemical composition of the surface of the solid, but also by the surface density of the force centres. If therefore this surface density can be sufficiently reduced by the pre-adsorption of a suitable substance, the isotherm may be converted from Type II to Type III. An example is rutile, modified by the pre-adsorption of a monolayer of ethanol the isotherm of pentane, which is of Type II on the unmodified rutile (Fig. 5.3, curve A), changes to Type III on the treated sample (cf. Fig. 5.3 curve B). Similar results were found with hexane-l-ol as pre-adsorbate. Another example is the pre-adsorption of amyl alcohol on a quartz powder... 

Fig. 53 Adsorption of pentane vap>our at 273 K on a sample of nonporous rutile before and after modification of the surface by pre-adsorption of ethanol. Curve (A), unmodified surface curve (B), surface containing 52 pmol of ethanol. (After Parfitt.)...

If a Type I isotherm exhibits a nearly constant adsorption at high relative pressure, the micropore volume is given by the amount adsorbed (converted to a liquid volume) in the plateau region, since the mesopore volume and the external surface are both relatively small. In the more usual case where the Type I isotherm has a finite slope at high relative pressures, both the external area and the micropore volume can be evaluated by the a,-method provided that a standard isotherm on a suitable non-porous reference solid is available. Alternatively, the nonane pre-adsorption method may be used in appropriate cases to separate the processes of micropore filling and surface coverage. At present, however, there is no reliable procedure for the computation of micropore size distribution from a single isotherm but if the size extends down to micropores of molecular dimensions, adsorptive molecules of selected size can be employed as molecular probes. 

Comparison of the volume/volume composition data with the relative pressure data shows that although C2-C4 hydrocarbons are present to the greatest volume percent, their actual pressures are an order of magnitude lower than the C5 plus hydrocarbons. Hence, the C5 plus hydrocarbons would be adsorbed in preference to the C2-C4 hydrocarbons and would displace them over a number of cycles. It is apparent therefore that the C5 plus hydrocarbons must be considered the primary target gases for pre-adsorption in guard bed systems... 

Figure 10.5 CO-stripping voltammograms (sweep rate 0.10 V s ) at pure Pt and stabilized Pt5gCo42 in 0.1 M HCIO4, and PtgoRu4o in 0.05 M H2SO4 at room temperature. The CO pre-adsorption was performed in CO-saturated solution at 0.075 V for 5 minutes. (From Wakisaka et al. [2006], reproduced by permission of the American Chemical Society.)...

In eadi specific case the choice of an adsorbent, electrophysical parameters and the method of registration of its change as well as the choice of various pre-adsorption treatment techniques of the surface of adsorbent is dictated by the type and nature of analytical problem to be solved. For instance, if particles active from the standpoint of the change in electrophysical parameters of semiconductor adsorbent occur on the surface of the latter due to development of a chemical reaction involving active particles, it is natural to use either semiconductor material catalyzing the reaction in question or if this is not possible specific surface dopes accelerating the reaction. Above substances are used as operational element of the sensor. If such particles occur as a result of adsorption from adjacent volume, one can use semiconductor materials with maximum adsorption sensitivity to the chosen electrophysical parameter with respect to a specific gas as operational element. 

If acceptor adsorption particles with concentrations Nta possessing a value of affinity to electron Eta appear on the surface of adsorbent characterized by pre-adsorption band bending (1.49), then the condition of their transition into the charged form is provided by inequality... 

Calculation yields that in these experiments the stationary concentration of radicals [ 4] for the maximum intensity of light of a mercury lamp SVDSh-500 was no more than 10 radicals/cm, the stationary conductivity of a ZnO film at 300 C changing by 300 to 400% compared to its initial pre-adsorption value. Thus, in accordance with the above formula (3.21), concentrations of radicals equal to 10 -10 radicals/cm may be detected by means of semiconductor sensors which is 7 magnitudes lower than existing methods would allow. 

For simplicity, up to now, first-order kinetics have been assumed, but obviously other rate laws may apply. Further complications can be generated by the presence of multiple paths for M on a variety of sites exhibiting different kinetics [5,11] or sequential enzymatic processes [100], Some complexes, labelled as lipophilics , have been shown to cross the membrane without the need for specific pre-adsorption sites [5,11,18,19,50] see also Chapters 5, 6 and 10 in this volume. Fortin and Campbell [76] have recently reported the accidental uptake of Ag+ induced by thiosulfate ligand. 

We are interested in the total electronic energy of the substrate chain in the pre-adsorption situation, when f3a = 0, so that the adatom is isolated from the chain and no surface states exist, i.e., s < 1, and we are only concerned with in-band states, for which 6k is real. In Fig. 1.2, a small increment es in — zs causes a correspondingly small decrement —5k in 6k. Thus, (1.60) reads ... 

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