Systems with Three Adsorbable Components

FIGURE 9.5. Equilibrium diagram for ternary Langmuir system showing possible coherent composition paths. 

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The site occupation variables, c, take on a value of 1 or —1, following the Ising convention, and in this application of the cluster expansion to a two-dimensional (2D) surface-adsorbate system, 1 represents an occupied site and —1 represents a vacant site. Eqn 2.11 is valid for a system with any number of non-equivalent lattice sites, as long as each site is occupied by the same pair of species (in this case the adsorbate and vacancy). Generalizations can be made to multi-component systems with three or more species per site or multisublattice systems where different sites may contain different pairs of species, but we do not discuss these elaborations in this chapter. 

In an actual exhaust system controlled by the signal of the oxygen sensor, stoichiometry is never maintained, rather, it cycles periodically rich and lean one to three times per second, ie, one-half of the time there is too much oxygen and one-half of the time there is too Httle. Incorporation of cerium oxide or other oxygen storage components solves this problem. The ceria adsorbs O2 that would otherwise escape during the lean half cycle, and during the rich half cycle the CO reacts with the adsorbed O2 (32,44,59—63). The TWC catalyst effectiveness is dependent on the use of Rh to reduce NO and... 

Transfer matrix calculations of the adsorbate chemical potential have been done for up to four sites (ontop, bridge, hollow, etc.) or four states per unit cell, and for 2-, 3-, and 4-body interactions up to fifth neighbor on primitive lattices. Here the various states can correspond to quite different physical systems. Thus a 3-state, 1-site system may be a two-component adsorbate, e.g., atoms and their diatomic molecules on the surface, for which the occupations on a site are no particles, an atom, or a molecule. On the other hand, the three states could correspond to a molecular species with two bond orientations, perpendicular and tilted, with respect to the surface. An -state system could also be an ( - 1) layer system with ontop stacking. The construction of the transfer matrices and associated numerical procedures are essentially the same for these systems, and such calculations are done routinely [33]. If there are two or more non-reacting (but interacting) species on the surface then the partial coverages depend on the chemical potentials specified for each species. 

According to Gibbs theory, adsorption is a three-phase system consisting of the stationary phase (adsorbent), the mobile phase and the adsorbed phase (adsorbt) (Fig. 2.1). The adsorbt is viewed as a 2-dimensional boundary layer between the other phases. It is also assumed that the activated surface of the adsorbent is identical for all components and does not change its properties. This reduces the adsorption to a two-phase system with an interaction between the adsorbt and the mobile phase. 

Tablets have been the most common matrix for the SFE of active components. Pure supercritical CO2 has been used in the extraction of megestrol acetate (145), caffeine and vanillin (146), and ibuprofen (147). In the first case, a comparison between two SFE systems was done and various support materials were assessed for their suitability as adsorbents. Methanol was used as the modifier for the extraction of benzodiazepines from tablets or capsules (148) in a static and dynamic mode for 5 and 10 min, respectively, at 65°C and 101 bar. The eluate was analyzed using GC-MS. Psoralen and isopsoralen have been extracted from Baishi pills and Baidianfeng capsules-belonging, to traditional Chinese medicines—using supercritical CO2 modified with tricloromethane (149). The determination of the analytes was performed by GC. By the standard additions method, recoveries were 97.8 101.2% with RSDs (three replicates) less than 2.01%. In the same way, SFE has been applied to the isolation of pentosalen (also known as imperatorin) from Cnidium monnieri cusson powder (a traditional Chinese medicine) (150). This experiment was carried out with a homemade SFE system with the use of replicates) were 3.9% and 1.7%. The SFE method was more efficient than the...

Figure 3a shows the spectra of CO adsorbed at room temperature on a typical Cr(II)/Si02 sample. At low equilibrium pressure (bold black curve), the spectrum shows two bands at 2180 and 2191 cm Upon increasing the CO pressure, the 2191 cm component grows up to saturation without frequency change. Conversely, the 2180 cm component evolves into an intense band at 2184 cm and a shoulder at 2179 cm The bands at 2191, 2184, and 2179 cm which are the only present at room temperature for pressures lower than 40 Torr, are commonly termed the room temperature triplet and are considered the finger print of the Cr(ll)/Si02 system (grey curve in Fig. 3). A new weak band at around 2100 cm appears at room temperature only at higher CO pressure. As this peak gains intensity at lower temperature, it will be discussed later. The relative intensity of the three components change as a function of the OH content (i.e., with the activation temperature and/or the activation time) [17].

Since the components in the adsorbed polyelectrolyte layer are considered to be the same as the bulk phase with a three component system which consists of polyelectrolyte, simple salt, and water, we calculate the adsorbances of polyelectrolyte and salt by assuming the Donnan equilibrium between the bulk phase and the adsorbed polyelectrolyte layer, as described previously (5). 

A suggestion for the existence of at least three populations of adsorbed Ru(II) comes from the time evolution of the transient UV-vis absorption spectra. These spectra show that the recovery of the initial Ru(II) spectra occurs with two parallel (fast and slow) second-order components. The rate constants for these two components show remarkably little dependence on the nature of the coordinating ligands. Both of these components are attributed to recombination of the adsorbed Ru(III) with the injected electrons. Thus there is a small luminescent population of Ru(II) that does not engage in electron injection, a non-luminescent population that injects and recombines rapidly, and a third population that injects rapidly and recombines slowly. A detailed picture of the nature of the ligand/semiconductor interaction and how it affects the behavior of these systems awaits further study. 

The first ellipsometric measurement of the thickness of the adsorbed layer and the adsorbance of a polyelectrolyte and a negative adsorbance of salt onto a solid surface was reported by Takahashi et al.U4) They measured the adsorption of sodium poly(acrylate) (M = 950 x 103) onto a platinum plate as a function of the concentration of added sodium bromide. In an aqueous polyelectrolyte solution with an added simple salt, the bulk phase is a three-component system which consists of a polyelectrolyte, a simple salt, and water. The adsorbed layer on the solid surface is a three-component phase as well. The adsorbance of polyelectrolytes thus cannot easily be determined from measurements of the refractive index nf of the adsorbed phase. Hence, it was assumed that the adsorbed layer is a homogeneous layer of thickness t and further that nf is represented by the Lorenz-Lorentz equation as follows ... 

A lean NOx trap (LNT) (or NOx adsorber) is similar to a three-way catalyst. However, part of the catalyst contains some sorbent components which can store NOx. Unlike catalysts, which involve continuous conversion, a trap stores NO and (primarily) N02 under lean exhaust conditions and releases and catalytically reduces them to nitrogen under rich conditions. The shift from lean to rich combustion, and vice versa, is achieved by a dedicated fuel control strategy. Typical sorbents include barium and rare earth metals (e.g. yttrium). An LNT does not require a separate reagent (urea) for NOx reduction and hence has an advantage over SCR. However, the urea infrastructure has now developed in Europe and USA, and SCR has become the system of choice for diesel vehicles because of its easier control and better long-term performance compared with LNT. NOx adsorbers have, however, found application in GDI engines where lower NOx-reduction efficiencies are required, and the switch between the lean and rich modes for regeneration is easier to achieve. 

The aim of this book is to provide the reader with an overview of interfacial supramolecular chemistry. Supramolecular assemblies of the kind considered in this text are truly interfacial, not only because they separate solid- and solution-phase components but also because they represent the junctions where biology, chemistry, physics and engineering meet. In true interfacial supramolecular systems, individual moieties, e.g. the supporting surface and an adsorbed luminophore, interact co-operatively to produce a new function or property. In addition, these two- and three-dimensional structures remain an important step in the evolution of structure from discrete molecules, to interacting assemblies, and finally to solids. In this last chapter, the future of interfacial assemblies will be briefly considered. This discussion will focus on the possibility of integrating such assemblies into practical devices and the identification of the important scientific challenges. 

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