Adsorption reversible reactions

Fig. 3.1 (Kapteijn et al., 1999) shows the model commonly u.sed to pre.sent a reversible reaction (A B) taking place on the surface of a solid catalyst. Three elementary steps are distinguished, i.e. adsorption of A on an active site, reaction of this adsorbed complex to adsorbed complex B, and desorption of B from the active site.

Consider the reversible reaction A B which proceeds via three elemental steps, viz. adsorption of A on an active site, reaction of adsorbed A, and desorption of product B ... 

Adsorption/desorption Reactions that involve solute becoming chemically bonded to the surface of a solid. The reverse process releases solutes from the surface of a solid... 

Figure 2.69 compares the theoretical responses of an adsorption coupled reaction with the simple reaction of a dissolved redox couple, for a reversible case. Obviously, the adsorption enhances considerably the response, making the oxidation process more difficult. The forward component of reaction (2.144) is a sharp peak, with a lower peak width compared to reaction (2.157). The relative position of the peak potentials of the forward and backward components of the adsorption comph-cated reaction is inverse compared to simple reaction of a dissolved redox couple. Finally, the peak current of the stripping (forward) component of adsorption coupled reaction is lower than the backward one, the ratio being 0.816. The corresponding value for reaction of a dissolved couple is 1.84. This anomaly is a consequence of the current sampling procedure and immobilization of the reactant, as explained in the Sect. 2.5.1. 

Fig. 2.69 Theoretical voltammograms for a reversible reaction coupled with adsorption of the reactant (2.144) and simple reversible reaction of a dissolved redox couple (2.157). Conditions of the simulations are /3r = 0.1 cm, /= 100 Hz, AF = 5 mV, tiE y, = 50 mV, 7) = 5 x 10 cm s . The numbers 1, 2, and 3 designate the forward, backward and net component of the response, respectively...

Fig. 2.70 Reversible reaction coupled with adsorption of the reactant. ElFect of / on the produet...

The response of a reversible reaction (2.146) depends on two dimensionless adsorption parameters, Pr and po. When pR = po the adsorbed species accomplish instantaneously a redox equilibrium after application of each potential pulse, thus no current remains to be sampled at the end of the potential pulses. The only current measured is due to the flux of the dissolved forms of both reactant and product of the reaction. For these reasons, the response of a reversible reaction of an adsorbed redox couple is identical to the response of the simple reaction of a dissolved redox couple (2.157), provided Pr = po- As a consequence, the real net peak current depends linearly on /J, and the peak potential is independent of the frequency. If the adsorption strength of the product decreases, i.e., the ratio increases, the net peak current starts to increase (Fig. 2.73). Under these conditions, the establishment of equilibrium between the adsorbed redox forms is prevented by the mass transfer of the product from the electrode surface. Thus, the redox reaction of adsorbed species contributes to the overall response, causing an increase of the current. In the hmiting case, when ]8o —0, the reaction (2.146) simplifies to reaction (2.144). 

The chemical composihons of the zeolites such as Si/Al ratio and the type of cation can significantly affect the performance of the zeolite/polymer mixed-matrix membranes. MiUer and coworkers discovered that low silica-to-alumina molar ratio non-zeolitic smaU-pore molecular sieves could be properly dispersed within a continuous polymer phase to form a mixed-matrix membrane without defects. The resulting mixed-matrix membranes exhibited more than 10% increase in selectivity relative to the corresponding pure polymer membranes for CO2/CH4, O2/N2 and CO2/N2 separations [48]. Recently, Li and coworkers proposed a new ion exchange treatment approach to change the physical and chemical adsorption properties of the penetrants in the zeolites that are used as the dispersed phase in the mixed-matrix membranes [56]. It was demonstrated that mixed-matrix membranes prepared from the AgA or CuA zeolite and polyethersulfone showed increased CO2/CH4 selectivity compared to the neat polyethersulfone membrane. They proposed that the selectivity enhancement is due to the reversible reaction between CO2 and the noble metal ions in zeolite A and the formation of a 7i-bonded complex. 

On the other hand, adsorption on a bonded stationary phase may be regarded formally as a reversible reaction involving the eluite, E, and the binding surface, B, to form a complex, EB ... 

Adsorption and reaction of C2H4. Ethylene reversibly adsorbed on clean Au(lll) at 95 K. Desorption of ethylene in TPD from the clean Au(lll) surface was observed from approximately 100 - 250 K, with a large peak at 104 K. The reason for the broad desorption peak is uncertain. Outka and Madix (8) have shown that a broad range of desorption is typical of C2 hydrocarbons on Au(llO). They propose that these molecules are weakly bound and therefore occupy a variety of binding sites with different activation energies, consequently they desorb at different temperatures. We observed no H2 evolution from the surface up to temperatures of 700 K and AES showed no residual carbon on the surface after heating. Thus, C2H4 is reversibly, and most likely molecularly, adsorbed on clean Au(lll). 

It must be emphasized again that the mid-peak potential is equal to E° for a simple, reversible redox reaction when neither any experimental artifact nor kinetic effect (ohmic drop effect, capacitive current, adsorption side reactions, etc.) occurs, and macroscopic inlaid disc electrodes are used, that is, the thickness of the diffusion layer is much higher than that of the diameter of the electrode. 

Rate equations for simple reversible reactions are often developed from mechanistic models on the assumption that the kinetics of elementary steps can be described in terms of rate constants and surface concentrations of intermediates. An application of the Langmuir adsorption theory for such development was described in the classic text by Hougen and Watson (/ ), and was used for constructing rate equations for a number of heterogeneous catalytic reactions. In their treatment it was assumed that one step would be rate-controlling for a unique mechanism with the other steps at equilibrium. 

H. Eichorn Proved that the adsorption of ions by clays and zeolites constitutes a reversible reaction. 1858... 

First-order adsorption kinetics model A simple first-order reaction model is based on a reversible reaction with equilibrium state being established between two phases (A— fluid, B—solid) ... 

The best catalysts for olefin hydration are not necessarily those which have proved most satisfactory for the reverse reaction. Some of the successful hydration catalysts are not typical dehydration catalysts. The more obvious reasons are (i) different adsorption characteristics of the catalyst is desirable, e.g. stronger adsorption of olefin relative to alcohol, (ii) under the conditions used for the hydration, ether formation cannot be suppressed as readily as in the dehydration, (iii) at high pressures, the olefins tend to polymerise much more than at the low pressures used for the dehydration. 

The principal results have been those of Krauss,268 who worked in Bodenstein s laboratory, and of Trautz and Dalai,125 and are confined to a temperature range of 258-288°K. The upper limit is established by the rate of the reverse reaction, and the lower limit is due to adsorption on the walls. The data of Trautz and Dalai showed considerable scatter, so that it was impossible to deduce an activation energy from their results. Their average value at 0°C was log ks = 3.48, with a mean deviation of 25%. This spread in results was almost as large as the change in rate over the entire temperature range studied. Trautz... 

In this section, the interpretation of interfacial admittance data in the case of an a.c. reversible reaction with adsorption of O is briefly described. The relationships expressing the frequency dependence were derived some time ago [15, 17], but the essential meaning of the parameters involved was fully explained only recently [143], The brief description here is derived from the latter reference. 

The reaction direction, taken as forward, in particular cases may correspond to either the forward or reverse direction of stage 1 of scheme (188). Therefore, we distinguish the directions of stage 1 as follows. The direction that results in the occupation of a free site on the surface is called the adsorption direction, and the direction that results in a site becoming unoccupied is called the desorption direction. The rate of stage 1 in adsorption direction [i.e., in the forward direction in our record (188)] is denoted as rA and the rate in desorption direction [i.e., in the reverse direction in scheme (188)] is denoted as rB. When applied to concrete reactions one of these values will stand for the forward reaction rate, r+, and the other will stand for the reverse reaction rate, r. Transfer coefficients for adsorption and desorption directions will be denoted as a and / , respectively so a + / = 1. 

To examine the occurrence of gas-phase water pnotolysis in the dry state, CO was added to the system. H2 and COz are formed ir a ratio of 2 1, indicating that /the reaction, H20 -r CO — H2 + C02, takes place.n) Since this reaction does not occur on Ti02 alone, H2 should be formed at Pt.sites and C02 at TiOz sites t>y the reaction of CO with the oxidation products of water such as oxygen moiecules or OH radicals. Interestingly, a small amount of 02 is also formed when CO pressure is low, as shown m Fig. 13.3.n) After the complete consumption of CO, 02 as well as H2 decreases. This result implies that gas-phase water photolysis can occur on Pt/Ti02 in the dry state if the reverse reaction is inhibited by, for example, CO adsorption on Pt. It was also demonstrated that the reaction of QH4 with gas-phase... 

To derive the corresponding kinetic expressions for a bimolecular-unimolecular reversible reaction proceeding via an Eley-Rideal mechanism (adsorbed A reacts with gaseous or physically adsorbed B), the term K Pt should be omitted from the adsorption term. When the surface reaction controls the rate the adsorption term is not squared and the term KgKg is omitted. 

Sedlak and Andren (1991b) modeled hydroxyl radical reaction kinetics in the presence of particulate. They found that the reaction kinetics for PCB oxidation in the presence of particulate resulted from the complex interplay between solution-phase OH reactions and reversible adsorption-desorption reactions. A model predicting the reaction kinetics can be described by the following equation ... 

The removal of Ra by adsorption has been attributed to ion exchange reactions, electrostatic interactions with potential-determining ions at mineral surfaces, and surface- precipitation with BaSO 4. The adsorptive behavior of Ra2+ is similar to that of other divalent cationic metals in that it decreases with an increase in pH and is subject to competitive interactions with other ions in solution for adsorption sites. In the latter case, Ra is more mobile in groundwater that has a high total dissolved solids (TDS) content. It also appears that the adsorption of Ra + by soils and rocks may not be a completely reversible reaction (Benes et al. 1984, 1985 Landa and Reid 1982). 

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