Adsorption chemical energy

The effect is to write the adsorption free energy or, approximately, the energy of adsoiption Q as a sum of electrostatic and chemical contributions. A review is provided by Ref. 156. [Pg.412]

The applications of this simple measure of surface adsorbate coverage have been quite widespread and diverse. It has been possible, for example, to measure adsorption isothemis in many systems. From these measurements, one may obtain important infomiation such as the adsorption free energy, A G° = -RTln(K ) [21]. One can also monitor tire kinetics of adsorption and desorption to obtain rates. In conjunction with temperature-dependent data, one may frirther infer activation energies and pre-exponential factors [73, 74]. Knowledge of such kinetic parameters is useful for teclmological applications, such as semiconductor growth and synthesis of chemical compounds [75]. Second-order nonlinear optics may also play a role in the investigation of physical kinetics, such as the rates and mechanisms of transport processes across interfaces [76]. [Pg.1289]

This is our principal result for the rate of desorption from an adsorbate that remains in quasi-equihbrium throughout desorption. Noteworthy is the clear separation into a dynamic factor, the sticking coefficient S 6, T), and a thermodynamic factor involving single-particle partition functions and the chemical potential of the adsorbate. The sticking coefficient is a measure of the efficiency of energy transfer in adsorption. Since energy supply from the... [Pg.442]

The interaction between the adsorbed molecules and a chemical species present in the opposite side of the interface is clearly seen in the effect of the counterion species on the HTMA adsorption. Electrocapillary curves in Fig. 6 show that the interfacial tension at a given potential in the presence of the HTMA ion adsorption depends on the anionic species in the aqueous side of the interface and decreases in the order, F, CP, and Br [40]. By changing the counterions from F to CP or Br, the adsorption free energy of HTMA increase by 1.2 or 4.6 kJmoP. This greater effect of Br ions is in harmony with the results obtained at the air-water interface [43]. We note that this effect of the counterion species from the opposite side of the interface does not necessarily mean the interfacial ion-pair formation, which seems to suppose the presence of salt formation at the boundary layer [44-46]. A thermodynamic criterion of the interfacial ion-pair formation has been discussed in detail [40]. [Pg.130]

Kds are the constants of rates of chemical reactions of oxygen adsorption and desorbtion from ZnO film and Aq are electron work function from ZnO before oxygen gets adsorbed and its variation caused by dipole moment of adsorbed complexes being formed U is the adsorption activation energy of non-electrostatic nature [ M] is the concentration of solvent molecules. Apparently we can write down the following expression for the stationary system ... [Pg.211]

Adsorption is typically exothermic (i.e., releases energy in the process of bonding), but can be endothermic, and can be classified into two groups, based on the energies involved chemical adsorption and physical adsorption. Chemical adsorption is more significant for heavy metals, either in the form of ion exchange or interactions involving metal complexes. [Pg.795]

The free energy of interaction of protons (and other ions) with a charged surface is the sum of the chemical energy of adsorption and an electrostatic component, i. e. [Pg.229]

The surface complexation models quantify adsorption with experimentally determined equilibrium constants. Another, less widely used approach considers the relationship between the equilibrium constant for the adsorption reaction and the associated free energy change (James and Healy, 1972). Attempts have been made to determine the chemical contribution to the overall adsorption free energy by fitting adsorption isotherms to the experimental data values of -50, -33 and —45 kj mol were found for the change in chemical free energy associated with adsorption of Cr, Ni and Zn, respectively, on ferrihydrite (Crawford et al., 1993). Values ranging from -21 to 241 kJ mol were found for Ni on hematite the actual value depended upon the hydrolysis species that were assumed to exist (Fuerstenau and Osseo-Assare, 1987). [Pg.258]

As seen from the above equations, specific adsorption involves direct coordination of the adsorbing species to the surface metal atom of the solid, i. e. there is no solvent molecule between the adsorbed species and the surface. Thus, the linkage has a large proportion of covalent character. The chemical component of the adsorption free energy predominates as a result, adsorption may take place on a neutral surface... [Pg.261]

The constant in Equation (94) is easily shown to be proportional to a Boltzmann factor in which the exponential energy consists of two contributions zei/ s, the electrical energy associated with the ion in the Stern layer, and , the specific chemical energy associated with the adsorption ... [Pg.528]

In addition to analysis GC may be used to study structure of chemical compounds, determine the mechanisms and kinetics of chemical reactions, and measure isotherms, heats of solution, heats of adsorption, free energy of solution md/or adsorption, activity coefficients, and diffusion constants (see Chapter 11). [Pg.16]

The source of this discrepancy is unknown to us. Equation (349) is, undoubtedly, adequate for the description of the reaction kinetics on an iron-chromium oxide catalyst. The fact that in one of the works (124) magnetite without the addition of chromium oxide served as a catalyst can hardly be of consequence since a study of adsorption-chemical equilibrium (344) on an iron-chromium oxide catalyst (7% Cr203) (52) led to the value of the average energy of liberation of a surface oxygen atom that practically coincides with that found earlier (50) for an iron oxide catalyst with no chromium oxide. It may be suspected that in the first work (124) the catalyst was poisoned with sulfur of H2S that possibly was contained in unpurified C02... [Pg.266]

In fact, we have rewritten the adsorption free energy, Q, in the Langmuir equation as a siun of electrostatic and chemical contributions (i.e., + < ). The chemical interactions are contained in the term tf>. [Pg.389]

Fig. 12J Schematic representation of the variation of water orientation in the interphase with charge density. (/) only the energy of interaction between the dipole and the field in the d.l. is considered (2) an entropy term is added (3) lateral interactions considered 4) the difference in chemical energy of adsorption in the two orientations is also taken into account.

This is the BDM isotherm, not taking into account the small shift between the potential of zero charge and the potential of maximum adsorption, which arises from the different chemical energies of interaction of water with the surface in its two allowed orientations. [Pg.492]

Fig. 8. Dependence of standard adsorption free energy, AG , on applied potential for (O) tetra-, ( ) hexa-, and (3) octa-oxyethylene glycol monododecyl ethers at nitrobenzene-water interface at 25 °C. (Reprinted from [88] with permission. Copyright The Chemical Society of Japan).

Most authors apply some extrapolation procedure to determine the Intrinsic (or non-Coulomblc or "chemical") part of the standard adsorption Gibbs energy. This may entail extrapolation to zero charge or to Infinite salt concentration. James and Parks proposed a double extrapolation procedure. [Pg.327]

Silica membranes have received extensive attention in recent years because of their excellent chemical and thermal stability, especially in the application of gas separation and catalytic membrane reactor processes. And the separation of high purity H2 from the mixed gas, is very important to convert the chemical energy to the electric energy, such as fuel cells. The final objective of this study is to understand the adsorption and separation mechanism in the MTES templating composite silica membrane, which can get hi purity H2 from CO2 and CH4 mixture. [Pg.529]

The key step in the derivation by Reuter et al. of their lattice model is the use of detailed balance to determine the sticking coefficients for each species on each type of site.31 The total adsorption rate at a particular site can be expressed as Tad = SI(p, T), where S is the local sticking coefficient and I(p,T) is the impingement rate of the species of interest from a gas phase with partial pressure p and temperature T. At steady state, the total adsorption and desorption rates must satisfy the detailed balance condition TdesjTad = exp[(Fb—/j,(T, p))/kT, where Fb is the free energy of the adsorbed species and fi(T, p) is the chemical potential of the gas phase species. The adsorption free energy is well approximated by the adsorption enthalpy, which is simply the adsorption energy calculated by a DFT calculation. This approach provides a direct link between the adsorption and desorption rates and the pressure and temperature of the bulk gas phase. [Pg.112]

In the latter case the adsorption sites (energy sites) show spatial arrangement and form architectonic compositions similarly to the case of silica gels esterified by alcohols [28]. The problems connected with the mechanism of molecular retention on such adsorbents as well as with testing unmodified and chemically modified adsorbents have been dealt with in many works [28,30,68,71]. [Pg.136]

Additional examples of controlled adsorption are given in Reference 11, where the cohesion energy parameters for over 860 chemicals are given. There are also over 450 correlations for solubility, swelling, adsorption, chemical resistance, barrier properties, etc. [Pg.555]

Kulkova and Temkin have determined the bond energies of oxygen with various metals Qaus (377). Table XXV is based on their data, which were obtained by the method of adsorption-chemical equilibria. [Pg.176]

Bond Energies Qo—i - kcal) Obtained by the Method of Adsorption-Chemical Equilibria... [Pg.176]

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