Frequency dependence Adsorption kinetics

Pt surface, as studied by Pajkossy [1994], almost ideally capacitative behavior can, in fact, be observed this is obviously a critical result indicating that it is not inseparable coupling between solution resistance and capacitance at a roughened (Pt) electrode surface (Pajkossy [1994]) that is the origin of dispersion effects. This led (Pajkossy [1994]) to the conclusion that it is ion (anion) adsorption that plays a crucial role in capacitance dispersion, because of frequency-dependent adsorption pseudocapacitance associated with anion chemisorption and associated kinetics of that process (Pajkossy [1994], Pajkossy et al. [1996]). 

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]. 

Adsorption kinetics. We can also study the adsorption kinetics of the nitrile component. This is illustrated by the IRRAS spectra shown in Figure 3, which demonstrate the influence of electrode potential on the competitive adsorption of CO and CjH CN. Curves a and b show the control experiments, in which spectra were recorded-at different potentials in saturated CO electrolyte with no nitrile added. A saturated CO layer is produced in both cases, but the frequency is different at the two potentials i.e., v(CO) 2085 cm at 0.55V, vs. v(C0) 2070 cm at 0.05 V. The magnitude of this shift is in agreement with the potential dependence of v(C0) discussed above. 

The region of the CMC (n (c +o c ) c l) requires special consideration. Substitution of of t2 and ti from Eq. (5.264) into (5.272), and the transition to the limit c -> 0 leads to the dynamic surface elasticity of sub-micellar solutions [165] and thus to a rather obvious conclusion if a solution contains mainly monomers, micelles do not influence the dynamic surface properties. Therefore, even for low frequencies (diffusion controlled adsorption kinetics) there is a concentration range close to the CMC where the surface elasticity is almost constant and begins to increase gradually only at further increasing concentration. Finally the surface elasticity takes values given by relations (5.275) - (5.278). This concentration dependence was observed in experiments with nonionic surfactants [95]. The oscillating barrier... 

The adsorption kinetics of individual proteins, reflected in changes in the frequency diminution A/(0 as a function of time (Figure 6.33), depends on the chemical natme of the surfaces studied and their roughness (Figures 6.34 through 6.36). 

Useful information about the adsorption kinetics, mobility of the adsorbed polynucleotide segments, and mechanism of electrode processes can be obtained by measurement of the frequency dependence of the impedance of the electrode double layer (EIS) [31, 88, 207-209]. If the adsorption/desorption process is slow with respect to the period of the a.c. potential used for the impedance measurement, the measured capacitance values decrease with increasing frequency (dispersion of the capacity). The frequency effect is most remarkable around the potentials of adsorption/desorption peaks. With more flexible ss polynucleotides, the frequency effect is larger than with the more rigid ds ones [210]. 

Adsorption kinetic. (A) Small deviations from equilibrium-frequency dependence. At small deviations from equilibrium, the value of the adsorption pseudocapacitance. Cad (see Eq. (16)), when measured by the a.c. method is the function of the frequency decreasing from the equilibrium value at very low frequency to zero at high frequencies. The... 

Abstract The physical principles and basic experimental techniques of impedance spectroscopy, i. e. static or frequency dependent dielectric permittivity measurements of sorbent/sorbate systems are given. These measurements can be used to characterize the state of a sorbent material in industrial adsorption processes. Combined with either manometric or gravimetric measurements of adsorption equilibria leading to calibration curves, permittivity measurements also allow fairly simple and quick measurements of gas adsorption equilibria. Kinetic processes and catalytic reactions inside a sorbent/sorbate system also can be observed. Pros and cons of dielectric measurements are discussed. List of Symbols. References. 

Unlike porous amorphous carbons, the high ratio of the external surface area to the total surface area of CNTs provides fast adsorption/desorption of electrolyte ions associated with the process of the formation of the electric double layer due to no ion-sieving effect occurring (Arulepp et al. 2006). Sorption of ions onto external surface area of CNTs makes the double-layer capacitance of CNT-based actuators less dependent on the ionic hquid species (ion dimensions) than the capacitance of amorphous carbon-based actuators, where the ion transport into the pores depends on the pore size and the size of electrolyte ions. The frequency dependence of generated strain has been attributed to the elecfrochemical kinetics different deflection amplitudes are the result of different ionic conductivities of EL species (Imaizumi et al. 2012). 

It was shown in [39, 40, 42], by use of the Delahay and Senda scheme, that the frequency dependence d the impedance and phase angle components make it possible to establish the nature of the electrode processes and to calculate their kinetic and adsorption parameters. In the absence of adsorption of the reaction components cot0 increases linearly with wV2 [29] ... 

In general, it seems more reasonable to suppose that in chemisorption specific sites are involved and that therefore definite potential barriers to lateral motion should be present. The adsorption should therefore obey the statistical thermodynamics of a localized state. On the other hand, the kinetics of adsorption and of catalytic processes will depend greatly on the frequency and nature of such surface jumps as do occur. A film can be fairly mobile in this kinetic sense and yet not be expected to show any significant deviation from the configurational entropy of a localized state. 

The theory for the reaction of an adsorbed redox couple (2.146) has been exemplified by experiments with methylene blue [92], and azobenzene [79], Both redox couples, methylene blue/leucomethylene, and azobenzene/hydrazobenzene adsorb strongly on the mercury electrode surface. The reduction of methlylene blue involves a very fast two-step redox reaction with a standard rate constants of 3000 s and 6000 s for the first and second step, respectively. Thus, for / < 50 Hz, the kinetic parameter for the first electron transfer is log(m) > 1.8, implying that the reaction appears reversible. Therefore, regardless of the adsorptive accumulation, the net response of methylene blue is a small peak, the peak current of which depends linearly on /J. Increasing the frequency above 50 Hz, the electrochemical... 

As far as the adsorption and skeletal isomerization of cyclopropane and the product propene are concerned, results mainly obtained by infrared spectroscopy, volumetric adsorption experiments and kinetic studies [1-4], revealed that (i) both cyclopropane and propene are adsorbed in front of the exchangeable cations of the zeolite (ii) adsorption of propene proved to be reversible accompanied by cation-dependent red shift of the C=C stretching frequency (iii) a "face-on" sorption complex between the cyclopropane and the cation is formed (iv) the rate of cyclopropane isomerization is affected by the cation type (v) a reactant shape selectivity is observed for the cyclopropane/NaA system (vi) a peculiar catalytic behaviour is found for LiA (vii) only Co ions located in the large cavity act as active sites in cyclopropane isomerization. On the other hand, only few theoretical investigations dealing with the quantitative description of adsorption process have been carried out. 

Table V collects values for the activation energies of isomerization and protonation as deduced by De Gauw and van Santen [138] from kinetic measurements. A comparison of the turnover frequency per proton (TOF) and / iso is made in Table V. One notes that the large differences in measured overall TOFs of different zeolites disappear for the elementary rate constant fciso. This implies that the difference in apparent acidity of the zeolite is due mainly to the difference in adsorption isotherms of the different zeolites. One notes the small variation in activation and protonation energy values, which implies a slight dependence of protonation on the micropore channel size and dimension.

Reductive removal of these oxygen layers is a slow kinetic process, commencing at potentials well below the characteristic potential for the layer formation on each metal. Thus, adsorption results based on the commonly used triangular potential sweep method can depend on the anodic potential excursions, the frequency of potential cycling and the number of cycles, that is, the catalyst surface history. Similarly, kinetic studies of oxygen reduction can be influenced by the dependence of the oxygen layer formation... 

Por the computation we have used the integral method using cubic spline and the combined gradient method of Levenberg-Marquardt [57, 58]. The kinetic models chosen describe well the hydrogenation kinetics. In the formulas presented in Table 3.1 k is the kinetic parameter of the reaction and Q takes into account the coordination (adsorption) of the product (LN) and substrate (DHL) with the catalyst (the ratio of the adsorption-desoprtion equilibrium constants for LN and DHL). Parameters of the Arrhenius equation, apparent activation energy kj mol , and frequency factor k, have been determined from the data on activities at different temperatures. The frequency factor is derived from the ordinate intercept of the Arrhenius dependence and provides a measure of the number of collisions or active centers on the surface of catalytic nanoparticles. 

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