Rate determining step interface

In a later publication, Kolbel et al. (K16) have proposed a less empirical model based on the assumption that the rate-determining steps for a slurry process are the catalytic reaction and the mass transfer across the gas-liquid interface. When used for the hydrogenation of carbon monoxide to methane, the process rate is expressed as moles carbon monoxide consumed per hour and per cubic meter of slurry ... 

Calderbank et al. (C6) studied the Fischer-Tropsch reaction in slurry reactors of 2- and 10-in. diameters, at pressures of 11 and 22 atm, and at a temperature of 265°C. It was assumed that the liquid-film diffusion of hydrogen from the gas-liquid interface is a rate-determining step, whereas the mass transfer of hydrogen from the bulk liquid to the catalyst was believed to be rapid because of the high ratio between catalyst exterior surface area and bubble surface area. The experimental data were not in complete agreement with a theoretical model based on these assumptions. 

The Na ions remain in the aqueous phase they cannot cross. The Q ions do cross the interface and carry an anion with them. At the beginning of the reaction the chief anion present is CN . This gets carried into the organic phase (equilibrium 1) where it reacts with RCl to produce RCN and Cl . The Cl then gets carried into the aqueous phase (equilibrium 2). Equilibrium 3, taking place entirely in the aqueous phase, allows Q" CN to be regenerated. All the equilibria are normally reached much faster than the actual Reaction (4), so the latter is the rate-determining step. 

The effectiveness of a crude oil demulsifier is correlated with the lowering of the shear viscosity and the dynamic tension gradient of the oil-water interface. The interfacial tension relaxation occurs faster with an effective demulsifier [1714]. Short relaxation times imply that interfacial tension gradients at slow film thinning are suppressed. Electron spin resonance experiments with labeled demulsifiers indicate that the demulsifiers form reverse micellelike clusters in the bulk oil [1275]. The slow unclustering of the demulsifier at the interface appears to be the rate-determining step in the tension relaxation process. 

Kinetics of chemical reactions at liquid interfaces has often proven difficult to study because they include processes that occur on a variety of time scales [1]. The reactions depend on diffusion of reactants to the interface prior to reaction and diffusion of products away from the interface after the reaction. As a result, relatively little information about the interface dependent kinetic step can be gleaned because this step is usually faster than diffusion. This often leads to diffusion controlled interfacial rates. While often not the rate-determining step in interfacial chemical reactions, the dynamics at the interface still play an important and interesting role in interfacial chemical processes. Chemists interested in interfacial kinetics have devised a variety of complex reaction vessels to eliminate diffusion effects systematically and access the interfacial kinetics. However, deconvolution of two slow bulk diffusion processes to access the desired the fast interfacial kinetics, especially ultrafast processes, is generally not an effective way to measure the fast interfacial dynamics. Thus, methodology to probe the interface specifically has been developed. 

Note that Eqs. (4) and (5) implicitly consider the transfer across the interface as the rate-determining step in the ion transfer processes [51], and neglect other steps involved in the process such as the ion transport across the diffusion boundary layers [55] and across the diffuse electrical double layer [50]. 

Aguilella, V., Belaya, M. and Levadny, V. (1996). Ion permeability of a membrane with soft polar interfaces. 1. The hydrophobic layer as the rate-determining step, Langmuir, 12, 4817 -827. 

The interface where the reaction takes place must be accessible to these reactants. In such cases, the diffusion of reactants to the reaction front becomes an important factor and diffusion of reactions to each other is rate determining step. 

When the molar volume of product, i.e. solid (AB) is less than that of solid (A), the product layer will be porous and the rate determining step is the chemical process occurring at the interface of solid (A). Such reactions are also known as topochemical reactions. The rate of reaction may be determined by the available surface area of A. For example, if reaction involves spherical particle, the rate of reaction (i.e. - dV/dt, where V is the volume of particle at time t) is given as... 

Hoftyzer and van Krevelen [100] investigated the combination of mass transfer together with chemical reactions in polycondensation, and deduced the ratedetermining factors from the description of gas absorption processes. They proposed three possible cases for poly condensation reactions, i.e. (1) the polycondensation takes place in the bulk of the polymer melt and the volatile compound produced has to be removed by a physical desorption process, (2) the polycondensation takes place exclusively in the vicinity of the interface at a rate determined by both reaction and diffusion, and (3) the reaction zone is located close to the interface and mass transport of the reactants to this zone is the rate-determining step. 

In many reactions, transfer of the anion across the interface and subsequent diffusion into the bulk of the organic phase will not be the rate-determining step when lipophilic catalysts are used, but the effect of less lipophilic catalysts may be influenced more by the anion and the mechanism of the transfer process. Thus, for example, the reactive anion is frequently produced in base-initiated reactions by proton extraction from the substrate at the two-phase interface and diffusion of the ion-pair contributes to the overall kinetics of the reaction. Additionally, the reactivity of the anion depends on its degree of hydration and on its association with the quaternary ammonium cation. In most situations, the activity of the transferred anion is enhanced, compared with its reactivity in aqueous media, as its degree of hydration is reduced, whereas a relatively weak electrostatic interaction between the two ions resulting from the bulkiness of the cation enhances the reactivity of the anion by making it more available for reaction and will be a major factor in the ratedetermining step. 

Another effect involves charge transport resistivity at the semiconductor-semi-conductor interface. The charge transport of the Ti02 photoelectrode, limited by its poor conductivity (about 0.1 cm2 V-1 s-1) [94], is the rate-determining step for the power-conversion efficiency in DSSCs [95]. As mention above, an usual strategy to improve charge transport is to add CNTs to the DSSC photoelectrode. It could be expected that the effect is proportional to the conductivity of the CNT, but Guai et al. 

Case 3 There are two interfacial rate-determining steps, consisting of 1) formation of an interfacial complex between the interfacially adsorbed molecules of the extractant and the metal ion and (2) transfer of the interfacial complex from the interface to the bulk organic phase and simultaneous replacement of the interfacial vacancy with bulk organic molecules of the extractant. For this mechanism, we distinguish two possibilities. The first (case 3.1) describes the reaction with the dissociated anion of the extracting reagent, B"(ad). The second (case 3.2) describes the reation with the undissociated extractant, BH(ad). 

Case 3.2 When the interfacially reactive species are the undissociated molecules of the extractant adsorbed at the interface [i.e., the first rate-determining step of the two-step mechanism is the reaction between the metal ion and HB(ad)], the following equations will hold ... 

The slowest step, or rate-determining step, can be either (a) electron transfer at the electrode-solution interface or (b) formation of atoms at the electrode surface. The activation polarization component of the overpotential, r)a, is related to the actual rate of oxidation or reduction, i, and the exchange current density ... 

Charge density of electrode, determination, 858 Charge of double layer, 1217 Charge transfer, 1213 mechanism, 1294 overpotential and, 1172 rate determining step and, 1179 steady state and. 1213 transport in electrolyte, 1211 Charge transfer, equilibrium at interface kinetic treatment. 1058 Nemst s equilibrium treatment, 1058 polarography, 1240 thermodynamics, 1057... 

Now, this quantity impedance (Z) turns out upon detailed analysis to contain within the characteristics of its variation with frequency,48 properties of the reaction occurring at the electrode/solution interface. For example, if a reaction occurring there has as its rale-determining step the electron transfer, then the variation of the impedance with frequency will have certain characteristics different from those shown in the Z — log to plot if the rate-determining step involves instead diffusion in the solution. So, by working out how Z varies with log CD according to a chosen mechanism... 

There are several places in electrochemical reactions where rate-determining steps can occur. First, if a cathode potential is sufficiently negative, transport of reactants to the electrode will not be able to keep pace with the events that transfer charge as the electrode demands. Then, transport in solution and the electrode events have to be satisfied with what the transport rate can bring to the interface. Transport is the rds. 

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