The Adsorption Equilibria

Many organic electrode processes require the adsorption of the electroactive species at the electrode surface before the electron transfer can occur. This adsorption may take the form of physical or reversible chemical adsorption, as has been commonly observed at a mercury/water interface, or it may take the form of irreversible, dissociative chemical adsorption where bond fracture occurs during the adsorption process and often leads to the complete destruction of the molecule. This latter type of adsorption is particularly prevalent at metals in the platinum group and accounts for their activity as heterogeneous catalysts and as 

Many molecules undergo partial oxidation on adsorption and many alkanes and alkenes are believed to yield an adsorbed CHO group on adsorption (Petrii, 1968). These processes usually lead to the complete oxidation of the organic molecule to carbon dioxide and few workers have attempted to halt the reaction at an intermediate stage. Hence, although there are undoubtedly possibilities for using dissociative chemisorption for synthetic reactions, this chapter will not consider these processes further. 

The capacitance is a readily measured interfacial property and it gives qualitative information on the adsorption of species at the electrode surface. Since the surface charge density, q, is a function of the potential and of coverage, the measured capacitance may be expressed as the sum of a true (high frequency) capacitance and an adsorption pseudocapacitance, i.e. q i(E,6) and hence 

The techniques described above are not entirely suitable for the study of adsorption when a faradaic current is flowing. Hence, little fully 

More usually, the participation of adsorbed reactants and intermediates is inferred indirectly from kinetic data (Bockris, 1954). Thus the observation of reactions having a low or zero order with respect to the reactant concentration imphes adsorption of the reactant. A reaction scheme 

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The method appears to be particularly suitable for the study of expensive compounds and/or species available in very low amounts. Figure 10.19 shows the application of IM to the study of the adsorption equilibria of a polypeptide, nociceptin/orphanin FQ, on a C,g column [40]. Isotherms (Figure 10.20) were obtained by numerical procedure through the fitting of the overloaded band profiles obtained under different conditions. Adsorption data allowed for the prediction of the band profile under overloaded gradient conditions (Figure 10.21) with a minimal amount of compound consumption. 

Then, for the fraction of surface left free from adsorbed molecules we have (1 - a -a ), and, equating for each gas the rate of condensation on the uncovered surface and the rate of evaporation from that part of the surface which it occupies, we have for the adsorption equilibria... 

From the point of interest in the properties of the interfacial region between an electrode and a solution, the adsorption of species from the solution at the interface has been well investigated in several aspects, e.g. the thermodynamics and the formulation of isotherms [20]. This is also true for the kinetics and the modes of transport via which the adsorption equilibria are established [134—136]. It can be stated that this branch of electrochemical science has been developed almost independently from the field of electrode kinetics. 

A somewhat different picture emerges from the adsorption studies of Matsuura and Schuit 117). They have attempted to elucidate the active adsorption sites on -y-bismuth molybdate by measuring the adsorption equilibria of butene, propylene, oxygen, water, butadiene, acrolein, and... 

From this, it can be concluded that the adsorption equilibria occur quickly. However, the pseudo-limiting-current of the prewave of hydrogen peroxide is not further influenced by the addition of a second additive to a solution that already contains an additive, unless the most recently added additive mixture contains components with a stronger current-lowering effect than the first. 

More recently, Kander and Paulaitis (16) have studied the adsorption of phenol onto activated carbon and measured its sorption equilibria from dense C02. These researchers found that temperature controlled the adsorption equilibria and that phenol uptake was negligibly effected by changes in the gas phase density. Such a result indicates that factors other then a solute s solubility in a dense gas play a key role in defining the adsorption equilibrium which accompany such processes. 

Because of the surface charge reversal as the electrolyte concentration increases, it is more realistic to also include the adsorption of anions among the adsorption equilibria, since they can have a relatively high concentration near a positively charged surface. 

In supercritical adsorption processes the crucial problem encountered is that, summing up to fluid phase solute concentration, the adsorption equilibria is influenced by the system temperature and by the supercritical fluid density. So, the variation of the parameters in isotherm models as a function of both temperature and density limits the applicability of the equations when they are used for fitting experimental data. To date, due partly to insufficient data, the density and temperature dependence of the isotherm parameters has not been established. 

Zhou, L., Zhang, J.-Sh. and Zhou, Y.-P. A Simple Isotherm Equation for Modeling the Adsorption Equilibria on Porous Solids over Wide Temperature Ranges, Langmuir 17 (2001) pp.5503-5507. 

Since 1916 when Langmuir published his fundamental paper on adsorption, the Theory of Activated Adsorption/Desorption Kinetics (TAAD) has, almost exclusively, been used for the interpretation of adsorption/desorption kinetics. However, contraiy to the success of Langmuir equation to represent the adsorption equilibria, a dramatic failure of TAAD was observed to represent by the Langmuir kinetic equation and its further modifications, [1-3] the monitored adsorption/desorption kinetics. 

Motivated by the woric of Kuznicki and Thrush [6], we have carried out a systematic study on the adsorption equilibria and kinetics of several heavy metal ions including Pb, ... 

The adsorption equilibria were measured using a gravimetric method and were expressed as isotherms. A chromatographic method was used to get the initial slope of the isotherms. In the simulation, GCMC method was used to calculate amounts adsorbed for various conditions. When the experiment result and simulation result of chloroform are compared, the simulation for the acid site model was most agreement with chromatographic data and baratron data. The simulation result of tetrachloroethylene with three models corresponded mostly for the non-polar molecule, and above all the acid site model was the closest to the experiment result. Therefore, to get better coincidence between experimental data and simulation, it was found to be necessary to account for aluminum rather than silanol nest. 

Adsorption equilibria and adsorption dynamics in supercritical fluids have been reported recently and it will be possible to apply the supercritical fluid to some new adsorptive separation processes. Fundamental informations on adsorption under supercritical conditions are necessary to design such processes. Supercritical chromatography has been used for study on the adsorption equilibria and adsorption dynamics. Adsorption of organics, i.e., benzene, toluene and m xylene, respectively, on MSC under supercritical conditions has already been reported in reference (Chihara, 1995). In the previous study, chromatographic measurements were made for the adsorption of benzene, toluene and m-xylene on MSC in supercritical CO2 mixed with benzene, toluene and m-xylene respectively. Moment analysis of the chromatogram was carried out. In the study, the organics used in the form of pulse were the same as organics mixed with supercritical CO2. The dependencies of adsorption equilibrium and micropore difliisivities on the amount adsorbed were obtained. 

The adsorption equilibria of methane, ethane and their mixture into single-walled carbon nanotuhes (SWNTs) were studied by using a Grand Canonical Monte Carlo (GCMC) method. The equilibrium isotherms of methane and ethane and the selectivity from their equimolar mixture were reported. 

Up to now, numerous studies have been conducted on their synthesis [9,10], treatment [5,13] and physical properties [4], However only limited number of studies has been carried out on die adsorption of gas in CNTs, including experimental works [8,11] and molecular simulations [3,7,14-lS]. Adsorption behavior depends strongly on the microporous structure of the particular adsorbent. In this work the effect of pore size on the adsorption behavior is of interest. The adsorption equilibria of methane, ethane and their mixture into SWNTs were studied by using a Grand Canonical Monte Carlo (GCMC) method. We reported equilibrium isotherms of methane and ethane, and the selectivity from their equimolar mixture. 

Increasing the temperature may influence the slowest chemical reaction step and the adsorption equilibria of substrates and products. Depending on the relation of the two latter steps and on the temperature range investigated, the measured activation energy, E, may be zero or larger or smaller than zero [74-79]. Since these factors depend on the detailed nature of the photocatalyst, minor modifications may induce variations of Fa- Accordingly, values of 1.7 and 4.2 kcal mol" were reported for two different CdS samples used in the photodimerization of A-vinylcarbazole [80]. 

Benton and White (35) established the adsorption equilibria of hydrogen on reduced nickel by approaching them from both higher and lower pressure at the upper part of the adsorption isotherm keeping the... 

These authors consider the increase of the methanol formation rate to result from the direct contribution of electrons and positive holes produced by gamma irradiation in the solid. The free carriers are able to modify the adsorption equilibria of Ha and CO, because these reactants, according to the authors, are adsorbed as ions on the surface. They consider that the observed unit G may be explained by admitting 20% of the electrons produced by radiation in the solid to be effective for catalytic reaction, 20 e.v. being necessary for the production of one electron. 

The final elements of the mass balances are the adsorption equilibria or the adsorption kinetics. According to Eqs. 6.5 and 6.6, adsorption and desorption steps are modeled as reactions with finite rate. The volumetric reaction rate based on the solid volume of all particles (Eq. 6.20) in the volume element is... 

Therefore, the basic correlations for situations present in the different sections of the SMB process, combined with a simplified short-cut approach, will be presented. Without knowing the adsorption equilibria, an operating point can be generated with a minimum number of experiments. All considerations made in the following are valid only for Langmuir-type isotherms. 

The dynamics of ad(de)sorption of water vapour from a carrier gas in a packed alumina column is governed by the adsorption equilibria and kinetics for the system. For the adsorption process, a mass transfer zone (MTZ) for water is formed within the column which propagates from the feed gas end to the column exit. The MTZ is generally constant... 

Chemisorption on the catalyst surface during the progress of reaction cannot be estimated from the adsorption equilibria of reactants and products measured separately with each species. It depends not only upon the interaction among the adsorbed species and the catalyst surface, but also upon the mechanism of the reaction, or the kinetic structure of the overall reaction. 

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