Other Adsorbents

A limited number of synthetic zeolites are currently used as commercial adsorbents i.e.. Type A and Type X. 

In the crystal structures of Type A, shown in Fig. 2.8, exchangeable cations are located near the window between neighboring cells. The 4A type zeolite contains Na ion at this site, which permits the entry of molecules smaller than 4 A. This effect is called the molecular sieving effect, and is schematically illustrated in Fig. 2.9. If the K ion, which is larger than the Na ion, is introduced to this position, effective window (aperture) size becomes 3 A and only H 0 and NH can penetrate through the window. Then this type is called 3A zeolite. On the other hand, if a Ca ion, which has two valences, is introduced, the effective aperture becomes larger. The zeolite of this type is called SA zeolite. 

Type X zeolite has much larger windows made up of 12-membered rings (Fig. 2.8), and is usually called 13X zeolite. 

Dry bones free of flesh together with fat and oil of animals crushed, screened and freed from miscellaneous foreign elements are put in an airtight iron retort and heated at 600-900° C for about 8 h. The volatile gases evolved by this process contain ammonium, tar and noncondensible gas. The remaining char is cooled in inert atmosphere then taken out for further crushing and screening. The yield of bone char is about 60%. 

Calcium hydroxyapatite is an interesting adsorbent which collects cations of heavy metals such as lead and cadmium. Suzuki (1985) suggested that hydroxyapatite is capable of ion exchange not only with 

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SERS. A phenomenon that certainly involves the adsorbent-adsorbate interaction is that of surface-enhanced resonance Raman spectroscopy, or SERS. The basic observation is that for pyridine adsorbed on surface-roughened silver, there is an amazing enhancement of the resonance Raman intensity (see Refs. 124—128). More recent work has involved other adsorbates and colloidal... 

Some general points are the following. One precondition for a vertical step in an isotherm is presumably that the surface be sufficiently uniform that the transition does not occur at different pressures on different portions, with a resulting smearing out of the step feature. It is partly on this basis that graphitized carbon, BN, MgO, and certain other adsorbents have been considered to have rather uniform surfaces. Sharp LEED patterns are another indication. 

In this maimer, it can also be seen that molecules will desorb as the surface temperature is raised. This is the phenomenon employed for TPD spectroscopy (see section Al.7.5.4 and section BT25). Note tliat some adsorbates may adsorb and desorb reversibly, i.e. the heats of adsorption and desorption are equal. Other adsorbates, however, will adsorb and desorb via different pathways. 

There are interactions between the adsorbates themselves, which greatly affect the structure of the adsorbates [32]. If surface difhision is sufficiently facile during or following the adsorption step, attractive interactions can induce the adsorbates to fomi islands in which the local adsorbate concentration is quite high. Other adsorbates may repel each other at low coverages fomiing structures in which the distance between adsorbates... 

A puzzling feature is that obedience to the Gurvitch rule is by no means universal, the (liquid) volume of water at saturation usually being less than that of the other adsorbate. In Fig. 5.14 for example, the volume of water at... 

The most widely used pitch control method is the addition of pitch dispersants, which can be either organic, ie, typically anionic polymers such as naphthalene sulfonates, ligninsulfonates, and polyacrylates (33,34), or inorganic, ie, typically clay or talc. The polymers maintain the pitch as a fine dispersion in the pulp, preventing agglomeration and potential deposition on the paper machine or the sheet. When talc, clay, or other adsorbent fillers are added to the furnish, moderate amounts of pitch can adsorb on these materials, producing a nontacky soHd that can be retained in the sheet. 

Various types of detector tubes have been devised. The NIOSH standard number S-311 employs a tube filled with 420—840 p.m (20/40 mesh) activated charcoal. A known volume of air is passed through the tube by either a handheld or vacuum pump. Carbon disulfide is used as the desorbing solvent and the solution is then analyzed by gc using a flame-ionization detector (88). Other adsorbents such as siUca gel and desorbents such as acetone have been employed. Passive (diffuse samplers) have also been developed. Passive samplers are useful for determining the time-weighted average (TWA) concentration of benzene vapor (89). Passive dosimeters allow permeation or diffusion-controlled mass transport across a membrane or adsorbent bed, ie, activated charcoal. The activated charcoal is removed, extracted with solvent, and analyzed by gc. Passive dosimeters with instant readout capabiUty have also been devised (85). 

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.). 

The action of molecular sieves is shghtly different from that of other adsorbents in that selectivity is determined more by the pore-size hm-... 

Adsorption of lA of POMs with CV and Malachite Green (MG) on the polyurethane foams (PF) and some other adsorbents is investigated. While lA is fully adsorbed on the PF in wide pH range (0,4 M H SO - pH 4) extent of dye adsoi ption does not exceed 0,4%. lA are adsorbed faster then POMs. Extent of sorption of lA is 60-70% at 5 minutes and is complete after 15 minutes. lA can be eluted from PF most effectively by methylbutylketone, acetone or alcohols can be used too. 

Graded Adsorbents and Solvents. Materials used in columns for adsorption chromatography are grouped in Table 12 in an approximate order of effectiveness. Other adsorbents sometimes used include barium carbonate, calcium sulfate, calcium phosphate, charcoal (usually mixed with Kieselguhr or other form of diatomaceous earth, for example, the filter aid Celite) and cellulose. The alumina can be prepared in several grades of activity (see below). 

In order to discuss the nature of the interaction between an adsorbed molecule and a surface it is important that the surface coverage be less than one monolayer since in multimolecular adsorption and capillary condensation the spectrum of the adsorbate molecule perturbed by interaction with other adsorbate molecules may mask the spectrum of the adsorbate molecule perturbed by interaction with the adsorbent. Surface coverages may be determined by obtaining an adsorption isotherm with the adsorbate... 

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. 

We present expressions for reaction rates and steady-state concentrations using the simplified assumption that Cads hydrogenation to CH4 occurs in one reaction step. We also assume that Oads removal is fast and that hydrogen adsorption is not influenced by the other adsorbates. 

The amount of species of the adsorbed substance j (adsorbate) per unit area of the trae surface area of the electrode or of any other adsorbent will be labeled Aj and will be called real adsorption (in contrast to the notion of Gibbs adsorption T see Section 10.4.1). In the limiting case, all adsorbed particles are packed right again the adsorbent s surface. This limiting case is called monolayer adsorption. In other cases, several layers of the adsorbate can form on the adsorbent s surface multilayer adsorption). 

The influence of water can be included by adding water molecules to the DFT calculation. Whereas the interaction with water will be discussed in more detail later, in short, the water interaction will be most important for adsorbates that easily form hydrogen bonds, react with water, or form strong ionic bonds to the surface. For other adsorbates, such as H, the effect of water is negligible [Jerkiewicz, 1998 Roudgar and Gross, 2005]. 

Thus, it is quite natural to consider the properties of other acceptor particles, for example, atoms of nitrogen, aminoradicals, hydroxyl radicals, and many others, adsorbed on oxide semiconductors. However, the properties of these particles are not studied yet. As to adsorbed donor particles, it was found in our experiments that liquid media with different values of the dielectric constant do not have any influence on the properties of adsorbed atoms of hydrogen. 

The retention mechanism of organic solutes by porous polymer beads remains ambiguous [478]. At low temperatures adso tion dominates but at higher temperatures the polymer beads could behave as a highly extended liquid with solvation interactions. The evidence for a partition mechanism is not very strong and its importance, at present, remains speculative. Like other adsorbents it has proven possible to control retention and enhance efficiency by diluting porous polymers with an inert support material (479). 

Figure 9.10 STM images of a triangular single-layer M0S2 nanocluster showing the adsorption of thiophene at low temperatures, (a) Below 200 K there are two states, both molecular, one adsorbed on top of the bright rim associated with an edge (Type B) and the other adsorbed at the perimeter of the nanocrystal (Type A) in (b), only Type A exists between 200 and 240 K (c) above 240 K no thiophene is present. (Reproduced from Ref. 33).

In NPLC, which refers to the use of adsorption, i.e. liquid-solid chromatography (LSC), the surface of microparticulate silica (or other adsorbent) constitutes the most commonly used polar stationary phase normal bonded-phase chromatography (N-BPC) is typified by nitrile- or amino-bonded stationary phases. Silica columns with a broad range of properties are commercially available (with standard particle sizes of 3, 5 and 10 im, and pore sizes of about 6-15nm). A typical HPLC column is packed with a stationary phase of a pore size of 10 nm and contains a surface area of between 100 and 150m2 mL-1 of mobile phase volume. 

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