Surface adsorbed phase

Firstly, the two phases considered by Ettelaie do not have the same chemical composition. One phase (the surface-adsorbed phase) contains the macroion, and the other phase (the solution) does not contain the macroion. The condition for thermal equilibrium in these circumstances has been clearly stated by Guggenheim... 

The relative magnitudes of He and Kj (Table I) indicate that the removal of both PCP and TCP occurs mainly by adsorption on the air-water interface of the air bubbles while that of NAPH occurs mostly as the vapor phase inside the air bubbles. Since the volatile fraction carried inside the air bubbles is lost to the air at the top of the aqueous phase and the surface adsorbed phase is deposited at the top of the aqueous phase, bubble fractionation removal of NAPH involves simultaneous gas desorption (air stripping) and enrichment at the top. Bubble fractionation is an integral part of most operations conducted in bubble columns. 

In theoretical studies, two different concepts of the adsorption system are considered. The first assumes that the adsorbed film forms an individual thermodynamic phase, being in thermal equilibrium with the bulk uniform part of the system. This model has been very effectively used to describe various adsorption systems [5,6]. It allows one to derive the relatively simple equations for adsorption equilibrium by utilizing the quality of the chemical potentials of a given component in both phases. Numerous models of the surface (adsorbed) phase are considered it may be assumed to be a monolayer or multilayer and either localized, mobile, or partially mobile, molecular interaction can be taken into account or neglected, and so on. However, the thermodynamical correctness of the concept of surface phase is coniroversial. 

Employing the Gibbs adsorption isotherm for gas adsorption on the surface of a solid adsorbent and the equivalent of the ideal gas equation of state for the surface adsorbed phase, obtain Henry s law for pure gas adsorption in the form of (<7( /S ) = H, P, where H is the Henry s law constant for species i. Henry s law is valid for dilute/low-pressure adsorption systems. 

Equation (409) is in a form which clearly defines the adsorption separation factor. However, Eq. (409) is not a very convenient adsorption equation since it contains four unknown variables Xl> i>X2 62 ( 1 2 unknown constant). Elimination of two unknowns is possible using solution and surface adsorbed mass balance equations. Two mass balance equations over both species, 1 and 2, in the solution and surface adsorbed phases can be written, respectively, as... 

In general, one should allow for nonideality in the adsorbed phase (as well as in solution), and various authors have developed this topic [5,137,145-149]. Also, the adsorbent surface may be heterogeneous, and Sircar [150] has pointed out that a given set of data may equally well be represented by nonideality of the adsorbed layer on a uniform surface or by an ideal adsorbed layer on a heterogeneous surface. 

To characterize the state of the adsorbed phase, it is useful to evaluate its molar entropy, s , defined as the mean molar value for all the molecules adsorbed over the complete range of surface coverage up to the given amount adsorbed. The molar integral entropy of adsorption. As, is then defined as... 

Henry s law corresponds physically to the situation in which the adsorbed phase is so dilute that there is neither competition for surface sites nor any significant interaction between adsorbed molecules. At higher concentrations both of these effects become important and the form of the isotherm becomes more complex. The isotherms have been classified into five different types (9) (Eig. 4). Isotherms for a microporous adsorbent are generally of type I the more complex forms are associated with multilayer adsorption and capillary condensation. 

The desorptive process may be analyzed before boiling. The key assumption is that the vapor and adsorbed phases are ia equiUbrium ia the bulk of the bed. This assumption eliminates iatraparticle resistances from further consideration and is reasonable for rotary kiln appHcations. The two remaining resistances are associated with hydrocarbon diffusion out of the bed and with convection from the bed surface to the bulk gases. The flux of species Fi from the desorbiag bed becomes... 

D5 adsorbed-phase (sohd, surface, particle, or micropore) diffusion... 

Numerical values for solid diffusivities D,j in adsorbents are sparse and disperse. Moreover, they may be strongly dependent on the adsorbed phase concentration of solute. Hence, locally conducted experiments and interpretation must be used to a great extent. Summaries of available data for surface diffusivities in activated carbon and other adsorbent materials and for micropore diffusivities in zeolites are given in Ruthven, Yang, Suzuki, and Karger and Ruthven (gen. refs.). 

When a gas comes in contact with a solid surface, under suitable conditions of temperature and pressure, the concentration of the gas (the adsorbate) is always found to be greater near the surface (the adsorbent) than in the bulk of the gas phase. This process is known as adsorption. In all solids, the surface atoms are influenced by unbalanced attractive forces normal to the surface plane adsorption of gas molecules at the interface partially restores the balance of forces. Adsorption is spontaneous and is accompanied by a decrease in the free energy of the system. In the gas phase the adsorbate has three degrees of freedom in the adsorbed phase it has only two. This decrease in entropy means that the adsorption process is always exothermic. Adsorption may be either physical or chemical in nature. In the former, the process is dominated by molecular interaction forces, e.g., van der Waals and dispersion forces. The formation of the physically adsorbed layer is analogous to the condensation of a vapor into a liquid in fret, the heat of adsorption for this process is similar to that of liquefoction. 

Current use of statistical thermodynamics implies that the adsorption system can be effectively separated into the gas phase and the adsorbed phase, which means that the partition function of motions normal to the surface can be represented with sufficient accuracy by that of oscillators confined to the surface. This becomes less valid, the shorter is the mean adsorption time of adatoms, i.e. the higher is the desorption temperature. Thus, near the end of the desorption experiment, especially with high heating rates, another treatment of equilibria should be used, dealing with the whole system as a single phase, the adsorbent being a boundary. This is the approach of the gas-surface virial expansion of adsorption isotherms (51, 53) or of some more general treatment of this kind. 

In general, the concentration of the reactant will decrease from CAo in the bulk of the fluid to CAi at the surface of the particle, to give a concentration driving force of CAo - CAi)-Thus, within the pellet, the concentration will fall progressively from CAi with distance from the surface. This presupposes that no distinct adsorbed phase is formed in the pores. In this section the combined effects of mass transfer and chemical reaction within the particle are considered, and the effects of external mass transfer are discussed in Section J 0.8.4. 

The importance of one other type of reaction that metal ions undergo has been recognized and studied extensively in the past 40 years. This reaction is adsorption, in which metal ions bind to the surface of particulate matter and are thereby transported as part of a solid phase even though they do not form an identifiable precipitate. Conceptually, these reactions can be thought of as hybrids between complexation and precipitation reactions. Most studies of these reactions have used metal oxides or hydroxides as the solid (adsorbent) phase, and the... 

This would be consistent with a transformation of a COad-rich (2 x 2)(2CO + O) adsorbate phase [Schiffer et al., 1997] into an oxygen-rich, but stiU COad-containing phase, for example the (2 x 2)(CO + 20) phase known from UHV experiments [Narloch et al., 1994], CO adsorption on the Ru(OOOl) surface at 0.7 V is essentially inhibited [Wang et al., 2001], most likely as a result of surface blocking by OHad/ Oad species. 

NMR signals are highly sensitive to the unusual behavior of pore fluids because of the characteristic effect of pore confinement on surface adsorption and molecular motion. Increased surface adsorption leads to modifications of the spin-lattice (T,) and spin-spin (T2) relaxation times, enhances NMR signal intensities and produces distinct chemical shifts for gaseous versus adsorbed phases [17-22]. Changes in molecular motions due to molecular collision frequencies and altered adsorbate residence times again modify the relaxation times [26], and also result in a time-dependence of the NMR measured molecular diffusion coefficient [26-27]. 

The measured NMR signal amplitude is directly proportional to the mass of adsorbate present, and the NMR signal versus pressure (measured at a fixed temperature) is then equivalent to the adsorption isotherm (mass of adsorbate versus pressure) [24-25]. As in conventional BET measurements, this assumes that the proportion of fluid in the adsorbed phase is significantly higher than the gaseous phase. It is therefore possible to correlate each relaxation time measurement with the calculated number of molecular layers of adsorbate, N (where N = 1 is monolayer coverage), also known as fractional surface coverage. 

In applying RAIRS to CO adsorption, the contribution from CO molecules in the gas phase to the absorption spectrum at CO pressures above 10-3 mbar completely obscures the weak absorption signal of surface adsorbed CO. Beitel et al. found it possible to subtract out the gas phase absorption by coding the surface absorption signal by means of the polarization modulation (PM) technique applied to a conventional RAIRS spectrometer, p-polarised light produces a net surface electric field which can interact with adsorbed molecules, whereas both polarization states are equally sensitive to gas phase absorption because gas phase molecules are randomly oriented. By electronic filtering a differential spectrum is computed which does not show contributions from the gas phase and which has much higher surface sensitivity than a conventional RAIRS setup. 

Transfer hydrogenation in the alcohol-ketone system on metal catalysts was investigated by Patterson et al. In particular, by studying the reaction between 2-propanol and butanone on Cu they concluded that it must be a direct surface reaction (11), the mechanism being essentially a proton transfer in the adsorbed phase (Scheme 2). 

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