Effect on adsorption kinetics

Jin X, Talbot J and Wang N FI L 1994 Analysis of steric hindrance effects on adsorption kinetics and equilibria AlChE J. 40 1685-96... 

The second term on the left of Eq. (7A.17) is associated with the decrease of surfactant concentration at the outer boundary of the DL. This effect decreases with increasing r(0). Thus the applicability of the adsorption kinetics given by Eq. (7A.15) is restricted both at the initial stage and aroimd equilibrium. Nevertheless, the approximation Eq. (7A.15) is a useful solution to the problem of the DL effect on adsorption kinetics because Eq. (7A.8) holds at strong electrostatic retardation. 

To get the main idea of the charge effect on adsorption kinetics, it is sufficient to consider an aqueous solution of a symmetric (z z) ionic surfactant in the presence of an additional indifferent symmetric (z z) electrolyte. When a new interface is created or the equilibrium state of an interfacial layer disturbed a diffusion transport of surface active ions, counterions and coions sets in. This transport is affected by the electric field in the DEL. According to Borwankar and Wasan [102], the Gouy plane as the dividing surface marks the boundary between the diffuse and Stem layers (see Fig. 4.10). When we denote the surfactant ion, the counterion and the coion, respectively, with the indices / = 1, 2 and 3, the transport of the ionic species with valency Z/ and diffusion coefficient A, under the influence of electrical potential i, is described by the equation [2, 33] ... 

Platinum is the only acceptable electrocatalyst for most of the primary intermediate steps in the electrooxidation of methanol. It allows the dissociation of the methanol molecule hy breaking the C-H bonds during the adsorption steps. However, as seen earlier, this dissociation leads spontaneously to the formation of CO, which is due to its strong adsorption on Pt this species is a catalyst poison for the subsequent steps in the overall reaction of electrooxidation of CHjOH. The adsorption properties of the platinum surface must be modified to improve the kinetics of the overall reaction and hence to remove the poisoning species. Two different consequences can be envisaged from this modification prevention of the formation of the strongly adsorbed species, or increasing the kinetics of its oxidation. Such a modification will have an effect on the kinetics of steps (23) and (24) instead of step (21) in the first case and of step (26) in the second case. 

Temperature has a strong influence on fcasignificant effect on the kinetics of adsorption, then energy conservation equations may have to be included in the analysis to establish the temperature to every point in the bed. In these circumstances the additional space coefficients, kx, ht, and ke must be added to the previous list. A point coefficient analogous to k d8 is not included, because it has been assumed that the temperature behavior of the particle can be faithfully represented by assuming a homogenous material. With this simplification it is not... 

To verify the effect of the ions adsorption on the regularities of photoexcitation relaxation, we studied the temperature effect on the kinetics of the ultradispersed CdS photobleaching relaxation at the addition of electron acceptors of various nature. Fig. 2.13 presents the kinetic curves of the colloidal CdS photobleaching relaxation prepared with an excess of cadmium ions at different temperatures and at the addition of different... 

Additives to starch exert varying effects on the kinetics of water sorption. For example, lipids do not significantly affect the content of adsorbed water. The mode of starch defatting can also influence moisture sorption by molecules of the defatting solvent occupying active centers of sorption.389 The addition of either sucrose or lipids to starch has the same effect on both branches of the hysteresis curve.386,398 Some additives, such as dimethyl sulfoxide or ammonium rhodanide, induce selectivity of the adsorption and solvation of starch 411 Sulfur dioxide accelerates water sorption regardless of the temperature.412 Pretreatment of starch with sulfur dioxide usually increases the water sorption.413 Studies on the sorption of components of water-alcohol mixtures are discussed in Section IV. 

Atomic force microscopy has been up to now only scarcely used by the plasma processing community. Results mainly concern low-resolution measurements, that is modification of the surface roughness induced by the plasma [43,44], Micro masking effects have been observed when processing Si with a SF6 plasma beam at low temperature (Fig. 11) and correlated to the multi-layer adsorption of plasma species as observed by XPS [45], Further development of vacuum techniques should allow high resolution surface probe microscopy measurements on plasma-treated samples, and possibly lead to complementary information on adsorption kinetics, surface density of states. 

The increase in the bulk concentration of methanol from 0.01 to 0.1 M (Fig. 30) has a clearly measurable effect on adsorption rates at short times up to 30 min at 0.2 V and 90 min at 0.1 V. Above these time values, adsorption tails at a similar rate, showing a surface state limitation rather than kinetic limitation. 

Rudzinski W. and Panczyk T. Sur ce Heterogeneity Effects on Adsorption Equilibria and Kinetics Rationalisation of Elovich Equation. In SCHWARZ J. and CONTESCU C. (eds.) Surfaces of Nanoparticles and Porous Materials (Marcel Dekker, 1999), pp.355-390. 

Water affects the reaction rate through its effect on reaction kinetics and protein hydration, which is required for optimal enzyme conformation and activity. Enzymes need a small amount of water to maintain their activity however, increasing the water content can decrease the reaction rate as a result of hydrophilic hin-drance/barrier to the hydrophobic substrate, or because of denaturation of the enzyme (189). These opposite effects result in an optimum water content for each enzyme. In SCFs, both the water content of the enzyme support and water solubilized in the supercritical phase determine the enzyme activity. Water content of the enzyme support is, in turn, determined by the distribution/partition of water between the enzyme and solvent, which can be estimated from water adsorption isotherms (141, 152). The solubility of water in the supercritical phase, operating conditions, and composition of the system (i.e., ethanol content) can affect the water distribution and, hence, determine the total amount of water that needs to be introduced into the system to attain the optimum water content of the support. The optimum water content of the enzyme is not affected by the reaction media, as demonstrated by Marty et al. (152), for esterification reaction using immobilized lipase in n-hexane and SCC02- Enzyme activity in different solvents should, thus, be compared at similar water content of the enzyme support. 

Kinoshita has also shown that ORR data for supported catalysts in hot, concentrated H3PO4 (180 °C, 97-98% acid) reported in three different studies were also fit by this model. Since the physical basis for the crystallite size effect in sulfuric acid is anion adsorption, it would be a considerable reach to suggest that the same physical basis applies to this size effect, i.e., structure-sensitive anion adsorption. There are, nonetheless, indications that this is the case. Anion adsorption in dilute phosphoric [43] has a very similar structure sensitivity as sulfate adsorption, i.e., strongest adsorption on the (111) face, and on poly-Pt anion adsorption and/or neutral molecule adsorption in dilute phosphoric has a strongly inhibiting effect on the kinetics of the ORR [43]. Sattler and Ross [16] report a similar crystallite size dependence of the ORR on supported Pt in dilute phosphoric acid at ambient temperature as that found in hot, concentrated acid with the same catalysts. But it is unclear whether similar adsorption chemistry would exist in the extreme conditions of hot, concentrated phosphoric acid. 

To evaluate the predominant pH effect on the kinetics of reductive dissolution of oxide minerals, it is indispensable to examine the pH dependence of both the rate, R, and the overall rate constant, ka, of reductive dissolution. The rate constant differs from the rate by the surface concentration of the reductant, R = ka RedadJ. If the rate constant ka is independent of pH, then the pH dependence of the rate is solely due to the pH dependence of adsorption of the reductant at the (hydr)oxide surface. If, on the contrary, the rate constant is dependent on pH, then other pH effects have to be considered. 

Campbell and Paffett reported that adsorbed Cl atoms had no effect on the kinetics of adsorption and desorption of oxygen on Ag(llO) [40]. To an extent, this result was not surprising since Campbell and Paffett dosed the Cl atoms on to the Ag(llO) from CI2 gas at 300 K, producing ordered overlayers of Cl, discemable by LEED, and areas that were Cl free [40]. 

The adsorption of anions on solid surfaces is of considerable interest, mainly because of its effect on the kinetics of electrochemical reactions. Several in-situ techniques have been applied toward this purpose. Infrared measurements were used to identify adsorbed species, estimate anion adsorption isotherms, and to gain information on anion interaction with electrode surfaces. " Sulfuric acid anions are possibly the conunonest anion adsorbates because of their specific adsorption on metal surfaces. Depending on the metal, its surface orientation, and the concentration of anion, either sulfate or bisulfate can be specifically adsorbed on the surface. Identifying the predominant adsorbate on platinum-group metals has engendered some controversy. While STM studies show that... 

The rate controlling step for reaction involves methane adsorption. Catalyst structure has a marked effect on the kinetics of the reaction. Thus, under certain conditions the rate of reaction over a Ni/Kieselguhr catalyst at 911 K is first order with respect to the partial pressure of CH4 and independent of H2O and product partial P, while for other nickel catalysts the rate depends on the partial P of H2O, H2, and CO. ... 

All solid surfaces exhibit structural features that can have significant effects on the kinetics of charge transfer reactions and on the stability of the interfacial region. In the case of metals, the most significant structural features for "smooth" surfaces are emergent dislocations, kink sites, steps, and ledges. It has long been known, for example, that the kinetics of some electrodissolution and electrodeposition reactions depend on the density of such sites at the surface, but the exact mechanisms by which the effects occur have not been established. The role of "adion" in these processes is also unclear, as is the sequence of the dehydration-electronation-adsorption-diffusion-incorporation processes, even for the simplest of metals. 

A numerical solution, based on the model presented for a formation-dissolution mechanism, was derived by Miller (1981). The following two Figs 4.13 and 4.14 demonstrate the effect of micelles on adsorption kinetics. The effect of the rate of formation and dissolution of micelles, represented by the dimensionless coefficient nkfC Tj /D, becomes remarkable for a value larger than 0.1. Under the given conditions (D /D, =1, c /c , =10, n=20) the fast micelle kinetics accelerates the adsorption kinetics by one order of magnitude. 

At first glance, when taking into account Eq. (7.19), it seems to be ill-defined to set Co Cg. However, this condition does not effect the given estimate. Although the relation for k becomes more complex, its value does not change so much. A more exact analysis of the nonequilibrium DL and its influence on adsorption kinetics is given in Appendix 7A. 

The rate of the exchange process of surfactant molecules between the surface of a bubble (drop) and the bulk solution is determined not only by convective diffusion but in the general case also by the kinetics of the adsorption step itself Details of the physical model of the adsorption process are given in chapter 2 and 4. A method which takes into account the effect of adsorption kinetics on the formation of the dynamic adsorption layer was developed by Levich (1962). Using this method, attempts were made to generalize the theory of the dynamic adsorption layer of bouyant bubbles (Dukhin 1965). 

The above considerations on reaction kinetics seem to show that surface charging must exert an effect on the kinetics of catalytic and adsorption processes. 

It must be stressed that eqn. (72) represents an ideal desorption process, where both v and Ed are coverage-independent parameters. Unfortunately, very few systems behave in this ideal fashion desorption is the reverse process of adsorption and, as has been described above for adsorption, several properties of the adlayer severely affect the kinetics of the basic desorption process. Thus, in the following sections, the effects on desorption kinetics of surface inhomogeneity, changes in desorption mechanism, precursor states and lateral interactions between adspecies, will be considered. The effects which these parameters have are considerable... 

In Sect. 3.2.2, the effects of precursor states on adsorption kinetics have been discussed. Since even the earliest adsorption experiments show evidence for the influence of weakly bound intermediate states, following the principle of microscopic reversibility it might be expected that desorption kinetics would show the influence of such species. However, desorption experiments are usually carried out at considerably higher temperatures and average lifetimes in such states will be much lower. 

Effect Of Adsorption Kinetics On Fischer Tropsch Synthesis... 

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