Irreversible adsorption, experimental

It is interesting that by taking into account eqn. (11), the inequality (13) can be written as (x — y)2 > 0, where x and y are the concentrations of AZ and BZ in the internal steady-state points. This condition seems to be trivial, but it gives possibilities for the following statement. If the catalytic reaction mechanism is of type (8) with irreversible adsorption steps and the experimental data have revealed one steady state with a non-zero reaction rate, i.e. W ds = ksxy and, hence, x 0, y 0, andx y (or vice versa) then already from this one fact it follows that (a) there exist two boundary steady states (x = 1, y = 0) and (y = 1, x = 0) with a zero reaction rate and one... 

The problem of decrease in catalyst activity due an irreversible adsorption of poison was solved numerically using a single point collocation approximation. The numerical results are compared with experimental data obtained by measuring concentration changes due to thiophene poisoning of Ni/AljO in benzene hydrogenation. 

The Study of ammonia adsorption was carried out in a flow adsorption microcalorimeter under dynamic conditions. Some of the parameters from these experiments are collected in Table 2.. In all cases the amount desorbed was much smaller than the adsorbed one, and so was the absolute value of the heat evolved. This clearly points out that ammonia adsorption consists of two different components. One is related to chemisorption (irreversible adsorption) and the other one (more labile or reversible) to physisorption in the pores. Another important feature about these experiments is that heat was still released long afterwards the NH3 uptake was negligible. This heat evolution, already reported in other experimental systems [4,7,8], is due to diffusion of adsorbed ammonia from low energy sites to higher energy sites having low accessibility. The lack of further uptake of NH3 may be due to irreversibly adsorbed molecules on the borders of micropores blocking NH3 towards the end of the adsorption process. 

The irreversible adsorption of organic solutes, which is of great importance in the regeneration of the adsorbents, is due to stronger interactions than dispersion or hydrophobic interactions. In the case of aromatic compounds such as phenol, it could involve a charge-transfer mechanism between the carbon surface and the adsorbate and/or its polymerization under certain experimental conditions. Therefore, further research is warranted in this area. 

Protein purifiers often liven up their coffee breaks with complaints about the disappearing activity of their protein. The activity disappears because of irreversible adsorption of protein to the column materials, proteolytic digestion, and/or unstable conformation. Cofactors are treacherous—of whose existence the experimenter has no idea but that are nevertheless important for the stability of the protein and that get lost during the purification. The existence of a low molecular cofactor may be suspected if the protein is stable in the raw cell or membrane extract but loses—activity during dialysis against extraction buffer. 

In parallel with complications induced by irreversible adsorption, new approaches to the experimental studies of surface excesses are developed. Namely, the adsorbate s state and coverage usually remain unchanged when the potential is switched off and the electrode withdrawn from solution. Moreover, some adsorbed layers remain stable even under UHV conditions, and a number of highly informative ex situ spectroscopic techniques were successfully applied to the studies of adsorption on d-metals [89-91]. 

Unlike a transmission cell that samples both the solution and the electrode surface, the LOPTLC configuration allows the separation of surface and solution processes (indeed a cell that could be used in both transmission and LOPTLC modes simultaneously might be very interesting). For example, irreversible adsorption processes may be studied [78] using a double logarithmic method. Three experimental cases were... 

The reversibility of the adsorption steps affects the number of steady states. Analysis shows that boundary steady states are absent if both adsorption steps are reversible. If one of the adsorption steps is irreversible, there is one boundary steady state with the irreversibly adsorbed intermediate occupying all active catalyst sites. If both adsorption steps are irreversible, two boundary steady states exist, one with complete coverage of the catalyst surface by AZ and one with complete coverage by BZ. If the reaction orders of these irreversible adsorption steps are equal, there is a line of internal steady states connecting the two boundary steady states. In this case, the steady state is very sensitive to the initial conditions and experimental data may be irreproducible. 

The mechanism described above, with irreversible adsorption of reactants and irreversible desorption of the Mari explained very satisfactorily all the experimental data of Tamaru et and Boudart et for the decomposition rate of ammonia at high temperatures and low pressures on tungsten and molybdenum respectively with simultaneous measurement of the surface concentration of N by Auger electron spectroscopy. Disturbing the stationary state by flashing desorption of the metal catalysts, the rate of returning to steady state of the two postulated irreversible steps of adsorption and desorption could be evaluated separately. 

It also was found experimentally, in accordance with theoretical predictions, that the jamming coverage of hard spheres adsorbed irreversibly (in the limit of negligible electrostatic interactions) approached 55%. This value strongly decreases for lower ionic strength, hilly in accordance with the effective hard-particle concept. This hnding conhrmed the decisive role of the lateral electrostatic interactions in irreversible adsorption of colloid particles. 

The conditions utilized to generate the isotherms were chosen to induce minimal change to the surface and to somewhat enhance chemisorption relative to physisorption. Degas conditions utilized were similar to what would be used for samples prior to BET surface area analysis. It should be noted that chemisorption in the present context is used to denote irreversible adsorption under the conditions of the experimental protocol. It s not meant to imply chemical bond formation as one might determine for example on a metal catalyst with carbon monoxide at high temperature. 

Hurst (19) discusses the similarity in action of the pyrethrins and of DDT as indicated by a dispersant action on the lipids of insect cuticle and internal tissue. He has developed an elaborate theory of contact insecticidal action but provides no experimental data. Hurst believes that the susceptibility to insecticides depends partially on the cuticular permeability, but more fundamentally on the effects on internal tissue receptors which control oxidative metabolism or oxidative enzyme systems. The access of pyrethrins to insects, for example, is facilitated by adsorption and storage in the lipophilic layers of the epicuticle. The epicuticle is to be regarded as a lipoprotein mosaic consisting of alternating patches of lipid and protein receptors which are sites of oxidase activity. Such a condition exists in both the hydrophilic type of cuticle found in larvae of Calliphora and Phormia and in the waxy cuticle of Tenebrio larvae. Hurst explains pyrethrinization as a preliminary narcosis or knockdown phase in which oxidase action is blocked by adsorption of the insecticide on the lipoprotein tissue components, followed by death when further dispersant action of the insecticide results in an irreversible increase in the phenoloxidase activity as a result of the displacement of protective lipids. This increase in phenoloxidase activity is accompanied by the accumulation of toxic quinoid metabolites in the blood and tissues—for example, O-quinones which would block substrate access to normal enzyme systems. The varying degrees of susceptibility shown by different insect species to an insecticide may be explainable not only in terms of differences in cuticle make-up but also as internal factors associated with the stability of oxidase systems. 

It is generally accepted that the time required for desorption of adsorbed polymer is very long, and this process seems to appear to be irreversible(ljO. Accordingly, it is expected that the high adsorption values which appeared near the LCST may be held for a long time under different temperature conditions. In Table 3, experimental results for irreversibility of adsorption in the HPC-latex systems are shown. After the HPC samples and the latex particles were mixed for 2 hrs at 1+8 °C under the same condition as in the case of the adsorption process, one portion of one of the samples was separated immediately by centrifugation at 1+8 °C. The other half portion of the HPC-coated latex suspension was kept at room temperature for 1+8 hrs and then centrifuged at 6 °C. As... 

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