Adsorption kinetic measurements

Figure D3.6.8 Drop growth at the tip of a capillary and subsequent drop detachment during adsorption kinetics measurements using the drop volume technique (DVT).

Grenier, Ph., Malka-Edery, A., and Bourdin, V., A temperature frequency response method for adsorption kinetic measurements. Adsorption, 5, 135-143, 1999. 

Recently, thermal-desorption [300,301] and adsorption kinetics measurements on heterogeneous surfaces [302-304] have been proposed for evaluating the energy distribution functions. These methods have been tested extensively during the last few years. 

This demonstrates the possibility of EWIF to perform adsorption kinetics measurements. The present time resolution is about 2 minutes if a complete R(A" ) versus A" curve is needed and only a few seconds for data collection at fixed A. This is several orders of magnitude faster than SANS in which the low flux of neutrons requires a counting time of a few hours. 

If Langmuir adsorption occurs, then a plot of 9 versus p for a particular isothenn will display the fonn of equation (Al.7.3). Measurements of isothenns are routinely employed in this manner in order to detennine adsorption kinetics. 

Adsorption Kinetics. In zeoHte adsorption processes the adsorbates migrate into the zeoHte crystals. First, transport must occur between crystals contained in a compact or peUet, and second, diffusion must occur within the crystals. Diffusion coefficients are measured by various methods, including the measurement of adsorption rates and the deterniination of jump times as derived from nmr results. Factors affecting kinetics and diffusion include channel geometry and dimensions molecular size, shape, and polarity zeoHte cation distribution and charge temperature adsorbate concentration impurity molecules and crystal-surface defects. 

Surface SHG [4.307] produces frequency-doubled radiation from a single pulsed laser beam. Intensity, polarization dependence, and rotational anisotropy of the SHG provide information about the surface concentration and orientation of adsorbed molecules and on the symmetry of surface structures. SHG has been successfully used for analysis of adsorption kinetics and ordering effects at surfaces and interfaces, reconstruction of solid surfaces and other surface phase transitions, and potential-induced phenomena at electrode surfaces. For example, orientation measurements were used to probe the intermolecular structure at air-methanol, air-water, and alkane-water interfaces and within mono- and multilayer molecular films. Time-resolved investigations have revealed the orientational dynamics at liquid-liquid, liquid-solid, liquid-air, and air-solid interfaces [4.307]. 

Much of the pioneering work which led to the discovery of efficient catalysts for modern Industrial catalytic processes was performed at a time when advanced analytical Instrumentation was not available. Insights Into catalytic phenomena were achieved through gas adsorption, molecular reaction probes, and macroscopic kinetic measurements. Although Sabatier postulated the existence of unstable reaction Intermediates at the turn of this century. It was not until the 1950 s that such species were actually observed on solid surfaces by Elschens and co-workers (2.) using Infrared spectroscopy. Today, scientists have the luxury of using a multitude of sophisticated surface analytical techniques to study catalytic phenomena on a molecular level. Nevertheless, kinetic measurements using chemically specific probe molecules are still the... 

Combined with their kinetic measurements, the authors proposed CO from the gas phase could directly react with oxygen atoms in the surface oxides, accounting for relatively high reactivity of this phase for CO oxidation. This mechanism, termed as Mars-Van Krevelen mechanism, challenges the general concept that CO oxidation on Pt group metals is dominated by the Langmuir-Hinshelwood mechanism, which proceeds via (1) the adsorption of CO and the dissociative adsorption of 02 and (2) surface diffusion of COa(j and Oa(j atoms to ultimately form C02. 

Hence, it appeared that the story was complete and the adsorption of hydrogen on platinum understood. However, considerable controversy was arising even as the definitive experiments were being performed. In 1959, Schuldiner carried out kinetic measurements on the H2-evolution reaction... 

Figure 6.16 Reconformation of adsorbed cationic polyacrylamide (MW 4 x 106) on cellulose fibres as shown by the kinetics of adsorption and adsorption stoichiometry (measured by counter ion release).

D. Axelrod, R. M. Fulbright, and E. H. Hellen, Adsorption kinetics on biological membranes Measurement by total internal reflection fluorescence, in Applications of Fluorescence in the Biomedical Sciences (D. L. Taylor, A. S. Waggoner, F. Lanni, R. F. Murphy, and R. Birge, eds.), pp. 461-467, Alan R. Liss, New York (1986). 

The competitive data, obtained with pairs of simultaneously reacting members of the series, are less subject to experimental error and to changes in catalyst activity, especially when cross checking is achieved by altering the pairs and extent of conversion. However, relative data measured in this way constitute in most cases a product of the rate constant and adsorption coefficient divided by the same product for the reference compound. Some authors (cf., e.g., 36, 37) were able to separate these products into relative rate constants and relative adsorption coefficients by conducting separate individual kinetic measurements with at least one member of the series. 

From this expression, the relative adsorption coefficients of the starting compounds 1 and = KJK2 can be calculated when the values of the rate constants k and 2 are known from individual kinetic measurements. However, the success of this procedure depends very much on the reliability of the estimation of the rate constants. Sometimes simple measurements are conducted under the assiunption that the reaction is zero order with respect to the concentration of the organic compound. When this assumption has not been adequately tested, this third source of data must be judged with care. 

A rapid reaction kinetic technique (time scale = 10-1000 ps) that typically uses a Van de Graff accelerator or a microwave linear electron accelerator to promptly generate a pulse of electrons at sufficient power levels for excitation and ionization of target substances by electron impact. The technique is the direct radiation chemical analog of flash photolysis and the ensuing kinetic measurements are accomplished optically by IR/visible/UV adsorption spectroscopy or by fluorescence spectroscopy. 

Adsorption has a significant impact on the movement of allelo-chemical substances in soil. Such movement in soil by water is important from the standpoint of mechanism of phytotoxin activity in the receiving species at a site remote from the donor plant. Adsorption reduces the solute concentration in the soil solution and consequently minimizes redistribution in the environment. Solute transport has been described by Pick s second law of diffusion and the kinetic models for adsorption and degradation of reactive solutes (, 44). The contribution of adsorption is measured and expressed as the retardation factor, R. 

We see in these formulae that the kinetically measured velocity constant has only in one special case, namely, in the case of zero order, the meaning of a real velocity constant. In all the other cases, it contains implicit adsorption coefficients, e.g., in the form kb = const., or (in retarded reactions) kb/b = const. Only in the case of broken order is it possible to determine k and b separately from kinetic measurements, as e.g., Schwab and Zorn (5) did for the ethylene hydrogenation. 

Che and Naccache (199) have studied the kinetics of 02 formed on slightly reduced anatase using EPR. They found that the adsorption could be explained on the basis of different formation rates for 02 adsorbed at different sites, with zero- and first-order kinetics for the oxygen and Ti3+ concentrations, respectively. Using the same approach, Hauser (200) has extended this work and proposed different models to explain the kinetics based on the formation of 02, O2-, and 02 ions for which activation energies around 1 kcal/mol were obtained. Nikisha et al. (201) have studied the oxygen adsorption kinetics using EPR, conductivity, and volumetric measurements. 

One general method may always be used to reduce the effect of impurity adsorption on electrodes, and that is to work only for shorl times. Impurities take substantial times to adsorb. If the time in which the measurement is made is short enough, the adsorption aspect of impurity interference with electrode kinetic measurements can be reduced. Many of the techniques for doing this are described in Chapter 8 (transients). However, this approach does not eliminate the difficulty that at low current densities impurities in the solution may compete with electrons from the electrode. Further, although transient measurements may greatly reduce the adsorption of impurities during the measurement, it is difficult to arrange techniques so that the electrode is in contact with the solution for seconds only. 

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