Chemical desorption step

In the chemical desorption step the adsorbed H atoms diffuse about on the metal surface, either by threading their way through adsorbed water molecules or by pushing them aside, until two collide to form an Hj molecule which escapes into the solution. This chemical step will be independent of overpotential, since charge transfer is not involved, and the rate will be proportional to the concentration or coverage of adsorbed H,, (see equation 20.39) and may occur at coverages that range from very small to almost complete. 

Now although the chemical desorption step is independent of rj, the surface coverage will increase as rj becomes more negative and this will affect tic.D - Since two electrons will be required for the overall reaction, equation 20.98 can be expressed in terms of i... 

Step 2a Chemical desorption step (CD step) or atom-atom combination step ... 

This trend for a rate-determining proton discharge followed by a chemical desorption step at low q s and an electrochemical one at higher rj s seems to survive the change to alkaline solution shown here of course, the proton discharge occurs from water ... 

If chemical desorption is rate determining, the rate will be independent of overpotential since no charge transfer occurs in this step, and... 

In the case of coupled heterogeneous catalytic reactions the form of the concentration curves of analytically determined gaseous or liquid components in the course of the reaction strongly depends on the relation between the rates of adsorption-desorption steps and the rates of surface chemical reactions. This is associated with the fact that even in the case of the simplest consecutive or parallel catalytic reaction the elementary steps (adsorption, surface reaction, and desorption) always constitute a system of both consecutive and parallel processes. If the slowest, i.e. ratedetermining steps, are surface reactions of adsorbed compounds, the concentration curves of the compounds in bulk phase will be qualitatively of the same form as the curves typical for noncatalytic consecutive (cf. Fig. 3b) or parallel reactions. However, anomalies in the course of bulk concentration curves may occur if the rate of one or more steps of adsorption-desorption character becomes comparable or even significantly lower then the rates of surface reactions, i.e. when surface and bulk concentration are not in equilibrium. 

The simplest case to be analyzed is the process in which the rate of one of the adsorption or desorption steps is so slow that it becomes itself rate determining in overall transformation. The composition of the reaction mixture in the course of the reaction is then not determined by kinetic, but by thermodynamic factors, i.e. by equilibria of the fast steps, surface chemical reactions, and the other adsorption and desorption processes. Concentration dependencies of several types of consecutive and parallel (branched) catalytic reactions 52, 53) were calculated, corresponding to schemes (Ila) and (lib), assuming that they are controlled by the rate of adsorption of either of the reactants A and X, desorption of any of the products B, C, and Y, or by simultaneous desorption of compounds B and C. 

The concept of SPME was first introduced by Belardi and Pawliszyn in 1989. A fiber (usually fused silica) which has been coated on the outside with a suitable polymer sorbent (e.g., polydimethylsiloxane) is dipped into the headspace above the sample or directly into the liquid sample. The pesticides are partitioned from the sample into the sorbent and an equilibrium between the gas or liquid and the sorbent is established. The analytes are thermally desorbed in a GC injector or liquid desorbed in a liquid chromatography (LC) injector. The autosampler has to be specially modified for SPME but otherwise the technique is simple to use, rapid, inexpensive and solvent free. Optimization of the procedure will involve the correct choice of phase, extraction time, ionic strength of the extraction step, temperature and the time and temperature of the desorption step. According to the chemical characteristics of the pesticides determined, the extraction efficiency is often influenced by the sample matrix and pH. 

In an interesting analysis of the effects of reduction of dimensionality on rates of adsorption/desorption reactions (26), the bimolecular rate of 10 M- s- has been reported as the lower limit of diffusion control. Based on this value, the rates given in Table III indicate the desorption step is chemical-reaction-controlled, likely controlled by the chemical activation energy of breaking the surface complex bond. On the other hand, the coupled adsorption step is probably diffusion controlled. 

Thus, there are two kinetic paths for the hydrogen evolution. The first path consists of charge transfer (CT) followed by chemical desorption (CD) path CT-CD. The second path consists of charge transfer (CT) followed by electrochemical desorption (ED) path CT-ED. Within each path, either of the consecutive steps can be slow and thus can be the rate-determining step (RDS). Each of these paths has two pKJSsible mechanisms. 

Non-linearities arising from non-reactive interactions between adsorbed species will not be our main concern. In this section we return to variations of the Langmuir-Hinshelwood model, so the adsorption and desorption processes are not dependent on the surface coverage. We are now interested in establishing which properties of the chemical reaction step (12.2) may lead to multiplicity of stationary states. In particular we will investigate situations where the reaction step requires the involvement of additional vacant sites. Thus the reaction step can be represented in the general form... 

The narrow double-layer zone is the site of all the chemical and physicochemical processes that attend the electrode reaction. These may include, in addition to the electron transfer itself, adsorption and desorption steps, as well as chemical transformation between O and R (which are the species stable in the bulk of the solution) and modified species (with less solvation, perhaps, or with different configurations) o and r which are adsorbable. Thus, the complete train of events may be... 

Gas-adsorption processes Involve the selective concentration (adsorption) of one or more components (adsorbates) of a gas (or vapor) at the surface of a microporous solid (adsorbent) The attractive forces causing the adsorption are generally weaker than those of chemical bonds and are such that, by Increasing the temperature of the adsorbent or reducing an adsorbate s partial pressure, the adsorbate can be desorbed The desorption step Is quite Important in the overall process First, desorption allows recovery of adsorbates In those separations where they are valuable, and second, It permits reuse of the adsorbent for further cycles ... 

At first it was believed that the main factor responsible for the kinetic regularities is the displacement or the "competition of reaction mixture components for the catalyst surface sites. An additional assumption was made concerning the high rate of the adsorption and desorption steps compared with the chemical transformations proper. 

Hence, an alternative concept CCU is proposed to address the energy penalty problem in the CCS process. The essence is to directly use the captured C02 i.e., activated one as a feedstock to synthesize value-added chemicals, getting rid of the desorption step. Very recently, Huang et al. described a strategy of capturing C02 by utilizing imidazolium IL/MEA system and then electrochemical reduction of MEAH+ ion to H2 simultaneously leading to carbamate salts formation (Scheme 6.1) [18]. 

Arve and Liapis [34] suggest estimating the parameters characterizing the intraparticle diffusion and the adsorption-desorption step mechanisms of affinity chromatography from the experimental data obtained in a batch system. The numerical simulations of the chromatographic process will use the values of the parameters of the adsorption isotherm and those of the effective pore diffusion as determined from stirred tank experiments together with the film mass transfer coefficients calculated from chemical engineering expressions found in the literature. 

The desorption step serves to reduce the residual moisture content in the product such that it will no longer support biological growth or chemical reactions [1]. The final residual moisture content in the product will depend on its adsorption and desorption isotherms [8,9]. The equilibrium moisture content of the product depends on its temperature, the vapour pressure on its surface and its potential chemical interactions with water vapour molecules. For a given product, the final moisture content can be reduced by increasing the shelf temperature at a constant water vapour pressure or by decreasing the water vapour pressure on the product surface at a constant temperature. 

This step is intended to reduce residual moisture to levels allowing no microbial growth or chemical reactions of the end product. The amount of residual moisture present in a product depends on its desorption isotherms. Such isotherms in turn depend on various factors including the product temperature, pressure chamber, partial vapour pressure in the container and nature of the interaction of the water vapour with the interstitial material formed in the freezing step. The computer should be fed with information on the target sample component. For example, if the component of interest is a protein, then overdrying may alter its configuration and decrease the potency of the end product. Consequently, the computer should control not only the final product temperature but also the partial water vapour pressure and the duration of the desorption step. 

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