Coadsorption isotherms

Figure 17.5 Coadsorption isotherms of different organic solutes onto cellulose fibers after treatment with ODTMA, at pH 6.5-7. ...

The adsorption isotherms (7.24, 7.26, 7.30) easily can be generalized to multi-component adsorption equilibria. In view of space limitations and lack of elucidating technical examples we here restrict the discussion to a mention of the coadsorption isotherm corresponding to the dual site isotherm (7.24) only, cp. also Sect. 2.1.1. The mass of component (i) adsorbed on a two site adsorbent is given by... 

Coadsorption isotherms of type (7.34) have proved to be useful for the correlations of binary and ternary coadsorption data of natural gas mixtures (CH4, N2, CO2, CO, H2) on activated carbons. Details are given in [7.26]. 

After activation by heating under dynamic vacuum, the sample is brought to the adsorption temperature. The gas mixture is prepared in the flask of volume by introducing each component of the mixture, one after the other one, while controlling the partial pressures. The flask is then slightly heated in order to homogenize the mixture by convection. The composition of the mixture is controlled by GPC or MS. Coadsorption isotherms are obtained by successive introduction of small doses of... 

For associative (not dissociative) coadsorption of A and B on a catalyst surface the Langmuir isotherm takes the form ... 

This type of isotherm is more realistic for describing chemisorption at intermediate 0a values but quickly leads to mathematically cumbersome or intractable expressions with many unknown parameters when one considers coadsorption of two gases. One needs to know how -AHa is affected both by 0A and by the coverages of all other adsorbates. Thus for all practical purposes the LHHW kinetics represent even today the only viable approach for formulating mathematically tractable, albeit usually highly inaccurate, rate expressions for catalytic kinetics. In Chapter 6 we will see a new, medium field type, approach which generalizes the LHHW kinetics by accounting also for lateral interactions. 

The study of thiourea adsorption on an Hg electrode from ethanolic solutions shows that different supporting electrolytes can make a comparison of the adsorption parameters more difficult. The data obtained for various electrolytes (KF, KPFe, LiCl, NH4NO3, and KCNS) suggest that coadsorption, size, and polarizability of ions strongly influence the interfacial behavior of TU, at high concentration of the ions in particular. The effect of the electrolyte on AG° and parameter A in the Frumkin isotherm is illustrated in Table 6. 

Hydrogen evolution reaction, mechanism, 1135, 1151, 1163, 1164, 1189 catalytic pathway, 1163,1194, 1255 electrocatalysis, 1280 Frumkin-Temkin isotherm, 1194 Langmuir isotherm, 1194 Hydrogen coadsorption... 

Nikitas, isotherms, 936, 952, 1195 Nitrobenzene reduction, 1376 Nonaqueous solutions, coadsorption of hydrogen and organic molecules, 13-10 see also hydrogen coadsorption Non -faradaic electrochemical modification of catalytic activity, 1371 Nonlocalized adsorption, 928, 958 Nonpolarizable interfaces, 812, 857, 1055, 1060, 1111... 

O Brien. 1235 Ohmic drop, 811, 1089, 1108 Ohmic resistance, 1175 Ohm s law, 1127. 1172 Open circuit cell, 1350 Open circuit decay method, 1412 Order of electrodic reaction, definition 1187. 1188 cathodic reaction, 1188 anodic reaction, 1188 Organic adsorption. 968. 978. 1339 additives, electrodeposition, 1339 aliphatic molecules, 978, 979 and the almost-null current test. 971 aromatic compounds, 979 charge transfer reaction, 969, 970 chemical potential, 975 as corrosion inhibitors, 968, 1192 electrode properties and, 979 electrolyte properties and, 979 forces involved in, 971, 972 977, 978 free energy, 971 functional groups in, 979 heterogeneity of the electrode, 983, 1195 hydrocarbon chains, 978, 979 hydrogen coadsorption and, 1340 hydrophilicity and, 982 importance, 968 and industrial processes, 968 irreversible. 969. 970 isotherms and, 982, 983... 

The second, and most important, success of the effective double-layer isotherm is that when considering the coadsorption and rate-controlling surface reaction of two reactants, A and D, one obtains the following analytical expression for the catalytic rate ... 

Figure 22.5(a) shows the variety of isotherms (Type I by Brunauer classification [19]) available for adsorption of pure CO2 on the activated carbons of Table 22.3 and on 5A zeolite [18]. Table 22.4 gives the corresponding Henry s law constants and selectivities for adsorption of CO2 and H2 at 303 K as well as the isosteric heats of adsorption of pure CO2 in the Henry s law regions [18]. These data show that (a) the strength of CO2 adsorption and the coadsorption of Hj from CO2 + H2 mixtures can vary significantly, and (b) the adsorption of COj on the 5A zeolite is too strong to be useful in a PSA process. 

Stephanie-Victoire F, Goulay A-M and Cohen de Lara E (1998), Adsorption and coadsorption of molecular hydrogen isotopes in zeolites. 1. Isotherms of H2, HD, and D2 in NaA by thermomicrogravrmetry , Langmuir, 14, 7255. 

Figure 17.4 Adsorption isotherm of ODTMA and coadsorption of 2-naphthol onto cellulose fibers. ...

Fig. 1-38. Influence of pressure on the course of the adsorption isotherm for the coadsorption of two gases [0.8].

Computational efforts to describe lateral interactions and compare the results with experiment in terms of isotherms started much earlier"" than the experimental information was well prepared. Qualitatively, these initial resirlts obtained in frames of lattice-gas model are interesting, as they demonstrate a possibihty of attractive H-H interactions in the case of coadsorption with repulsing anions. Modelling of mixed adlayers at various computational levels was systematically developed by Koper and coworkers, and now represents a rather general approach. It can be probably applied to mixed adlayers with short-range interactions of completely different type, earlier considered at phenomenological level (mixed ionic or organic" adlayers). 

Standard nanosilicas A-300 and A-380 (Sbet=337 and 378 mVg, pore volume Vp=0.714 and 0.943 cm /g, respectively, estimated from LT [77.4 K] adsorption isotherms of nitrogen) were heated at 673 K for several hours to remove adsorbed organics and residnal HCl and rehydrated in air. Silica 200DF (Crosfield, Sbet=540 mVg, Fp=0.34 cmVg) possessing nanopores and narrow mesopores was used to compare intrapartiele (200DF) and textural (nanosilica) porosity effects on the behavior of bound water and coadsorption of water and methane. Bidistilled water and conunercial methane (99% purity) were used (Gun ko et al. 2011c). 

Figure 4.17. Coadsorption equilibria data for a binary gasmixture CO2 / CH4 with yco2 = 20.4 %mol, Ych4 = 79.6 %mol on AC D47/3 at 293 K. The quantity In (p/n) is correlated to the absolute amount adsorbed (n) for n = nco2 " > d n = n,o, = nco2 + ncH4. .. . Lines refer to adsorption isotherms of the generalized Langmuir type for adsorbents with fractal dimensions cp. Chap. 7, [4.17].

The masses of these have to be approximately determined from pure adsorption isotherms of both components (1, 2) and calculations of coadsorption equilibria including only pure components parameters as for example the lAST-formalism [4.10,4.4,4.15,4.17]. 

For all isotherms mentioned in Table 7.3, extensions to multicomponent systems exist and have been mentioned in Sects. 2-4. However, it must be emphasized that in mixture adsorption it is often very difficult to predict or calculate the amounts of the lesser (or weakly) adsorbed components. Hence reliable measurements of coadsorption equilibria are recommended again. 

This book is the result of a fruitful collaboration of a theoretician (JUK) and an experimentalist (RS) over more than twelve years in the field of gas adsorption systems at the Institute of Fluid- and Thermodynamics (IFT) at the University of Siegen, Siegen, Germany. This collaboration resulted in the development of several new methods to measure not only pure gas adsorption, but gas mixture or coadsorption equilibria on inert porous solids. Also several new theoretical results could be achieved leading to new types of so-called adsorption isotherms based on the concepts of molecular association and -phenomenologically speaking - on that of thermodynamic phases of fractal dimension. Naturally, results of international collaboration of the authors over the years (1980-2000) also are included. 

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