Adsorption, Chemisorption

For suspended adsorbents, stirred vessels are generally used, and the process is easily designed. For scale-up the erosion behavior of the adsorbent and its separability from the liquid are important. The amount of energy to be introduced is determined experimentally and should be high enough that suspension just takes place. Slow-running stirrers with propeller or inclined blades of large diameter are favorable. 

Mechanical energy introduced Energy for suspension is proportional 0.2-1.0 kW m to the content of crystallizate 

In most cases adsorbents are used in the form of fixed beds. A number of theoretical treatments are available for the adsorption equilibrium and the adsorption kinetics. However, it is recommended to determine these parameters, the possible loading, and the breakthrough behavior experimentally. This also applies for the desorption 

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The solid-gas interface and the important topics of physical adsorption, chemisorption, and catalysis are addressed in Chapters XVI-XVIII. These subjects marry fundamental molecular studies with problems of great practical importance. Again the emphasis is on the basic aspects of the problems and those areas where modeling complements experiment. 

Adsorption of ions from the solution. There are two types of ionic adsorption from solutions onto electrode surfaces an electrostatic (physical) adsorption under the effect of the charge on the metal surface, and a specific adsorption (chemisorption) under the effect of chemical (nonelectrostatic) forces. Specifically adsorbing ions are called surface active. Specific adsorption is more pronounced with anions. 

The first step of oxide-layer formation is oxygen adsorption (chemisorption). In the case of platinum, the process stops at this stage, and depending on the conditions, an incomplete or complete monolayer of adsorbed oxygen is present on the platinum surface. In the case of other metals, layer formation continues. When its thickness 5 has attained two to three atomic diameters, the layer is converted to an individual surface phase that is crystalline (more seldom, amorphous) and has properties analogous to those of the corresponding bulk oxides. 

Juza and Blanke (125) investigated the reaction of carbon and sulfur between 100 and 1000° at various pressures. They thought it unlikely that there was genuine chemical bonding. The phenomenon of sulfur fixation was ascribed to capillary condensation, adsorption, chemisorption, and solution in the carbon structure. 

The adsorption (chemisorption) of hydrogen on clean metal surfaces is almost always accompanied by absorption of hydrogen into the interior of the structure. This absorption is a slow activated process and has in the past been mistaken for activated adsorption of hydrogen on the surface. 

Adsorption/chemisorption at the surface (adsorption of gas molecules occurs on the solid surface because of attractive forces between them). Gas molecules approaching the surface may lose some of their momentum (in the component normal to the surface) and become trapped in the potential well. The energy required to overcome the attractive potential barrier of the surface and the attraction of neighbouring molecules is the heat of adsorption (Van der Wall forces) and several monolayers may be adsorbed. However, if there is some interaction or electron transfer between the gas molecule and the surface (forming, e.g., a surface compound), it is defined as chemisorption. The heat of chemisorption is usually greater than the heat of adsorption. The extent of chemisorption depends upon the specific nature of the solids and gases. 

Normally, catalytic activity is expressed as the reaction rate per unit area of active surface (expressed as metre per gram) under given conditions. In a chemical reaction, catalytic conversion is defined as the fraction of reactants converted to products and selectivity is a function of the rate of formation of a desired product with respect to the overall conversion of the initial reactants. The reactant molecules transfer to the catalyst surface where adsorption may occur on an active site , with possible rearrangement of their bonds leading to a chemical adsorption (chemisorption), gas-catalyst reaction and the subsequent desorption of new species. The active site or phase is of high activity and selectivity for the desired products. Thus, the nature of the active sites is important. In many cases, it is not enough to have just activity. Selectivity to desired products is important and often modifiers or promoters are needed both to improve the... 

Au NPs functionalized with biomolecules can be synthesized using different methods, depending on factors inducing the interactions promoted between the nanoparticle and the biomolecule. These interactions can be classified as electrostatic adsorption, chemisorption and covalent binding and, finally, specific affinity interactions. Some examples are given in the following paragraphs (Scheme 3.23). 

These observations suggest that the heterogeneous effect in the S02-modenite system represents an extreme case, so much so that chemisorp-tive bonds may be stipulated (probably between the S02 and the cations). These effects would, of course, involve energy emission and show up in the calorimetric measurements. However, the specificity of the adsorption would tend to show a relatively temperature-insensitive isotherm in the low-pressure region, thus rendering the isosteric techniques of obtaining heats of adsorption/chemisorption ineffective. 

In electrode kinetics, interface reactions have been extensively modeled by electrochemists [K.J. Vetter (1967)]. Adsorption, chemisorption, dissociation, electron transfer, and tunneling may all be rate determining steps. At crystal/crystal interfaces, one expects the kinetic parameters of these steps to depend on the energy levels of the electrons (Fig. 7-4) and the particular conformation of the interface, and thus on the crystal s relative orientation. It follows then that a polycrystalline, that is, a (structurally) inhomogeneous thin film, cannot be characterized by a single rate law. 

In chemical adsorption(chemisorption) definite chemical bonds are produced betw the atoms molecules on the surface of solid absorbents and the molecules or atoms of "adsorbates . Chemisorption is usually accompained by an enormous evolution of heat(of the order of tens of thousands kcal per mole) and is very difficult to reverse. As an example of chemisorption, may be cited adsorption of oxygen on incandescent tungsten or of hydrogen nitrogen by tungsten even in the cold... 

Chemisorption and Physisorption. One classification of adsorption phenomena is based on the adsorption energy the energy of the adsorbate-surface interaction. In this classification there are two basic types of adsorption chemisorption (an abbreviation of chemical adsorption) and physisorption (an abbreviation of physical adsorption). In chemisorption the chemical attractive forces of adsorption are acting between surface and adsorbate (usually covalent bonds). Thus, there is a chemical combination between the substrate and the adsorbate where electrons are shared and/or transferred. New electronic configurations are formed by this sharing of electrons. In physisorption the physical forces of adsorption, van der Waals or pure electrostatic forces, operate between the surface and the adsorbate there is no electron transfer and no electron sharing. 

The lubrication system is extremely complex. The mechanism of lubrication is partly dictated by the nature of interactions between the lubricant and the solid surface. Additives blended into lubricating oil formulations either adsorb onto the sliding surfaces, eg., fatty alcohols, fatty amines, amides, phosphoric acid esters (friction modifiers), or react with the surface, eg., ZDDP, MoDTC, MoDDP organic phosphates (extreme pressure). Some interactions affecting the surfaces of metals include adsorption, chemisorption, and tribochemical reactions-these form new compounds on the surface and lubrication by reaction products (Bhushan and Gupta, 1991 Briscoe et al., 1973 Briscoe and Evens, 1982 Heinicke, 1984 Hsu and Klaus, 1978 and 1979 Klaus and Tewksbury, 1987 Lansdown, 1990 Liston, 1993 McFadden et al., 1998 Studt, 1989). 

In adsorption, we call the gas or solution solute the adsorbate and the solid the adsorbent. Monolayer adsorption involves up to one layer of adsorbate on the adsorbent, whereas multilayer adsorption involves more than one layer of adsorbate. Adsorption may be either physical adsorption (physisorption), where the adsorbate is bound to the surface by relatively weak physical forces (AT/desorp < 40 kJ/mol) or chemical adsorption (chemisorption), where the binding forces are stronger (A//desorb > 40 kJ/mol).6 Because chemical adsorption involves chemical-type bonds between adsorbate and adsorbent, it is limited to the first monolayer on the surface. Physical adsorption can involve multiple layers and physical adsorption can occur on top of chemisorbed layer. 

Atoms or molecules are physisorbed into a porous structure -such as a zeolite or a sample of coal- or onto a surface, and the amount of gas adsorbed is a measure of the surface area available for adsorption. Chemisorption of atoms or molecules on surfaces yields surface concentration of selected elements or adsorption sites. 

The remaining vacuum pumps to be discussed in this chapter fall into a group which remove gas particles from systems by sorption effects such as adsorption, chemisorption/gettering and implantation. They tend to be used on systems where any contamination of the vacuum by pump fluids, lubricants, etc. must be avoided. However, those pumps that remove gas particles exclusively by temperature-dependent gas adsorption on molecular sieves or A1203 (adsorption pumps) will not be discussed. 

Water adsorbs into the walls of a glass container, but that adsorption is the extent of its adhesion. Some adsorbed molecules react chemically with some types of containers in a process called chemical adsorption (chemisorption. For example, carbon monoxide chemisorbs with palladium, but not with gold). The bonds resulting from chemisorption can hold molecules to the surface with far greater force than would exist with only physical attraction. It is also possible for a molecule (that normally would not chemisorb with the container wall) to break up when hitting the wall s surface. At that point the molecule s constituent parts chemisorb with the container walls. When an adsorbed gas reacts with the materials of a container, it is called reconstruction (for example, the reconstruction of iron with oxygen is rust). 

Also called secondary , irreversible , and, if it takes place slowly at temperatures not too far above room tomperatures, activated adsorption. Chemisorption, dofined as co-valent combination of the adsorbate with the surface atoms, is very often irreversible, in the sense that the gas originally adsorbed comes off in a different state (e.g. as atoms instead of molecules, or as compounds with the atoms of the substratum). It need not necessarily be irreversible, however. 

On the basis of the rate equations, differential equations can be set up. Due to the adsorption/chemisorption and feedback steps the solution of this set of differential equations is periodic or chaotic. Usually the occurrence of adsorption/chemisorption leads to nonlinearity and negative impedance. 

The surface layer can be formed by adsorption, chemisorption, chemical reaction (covalent bonding),... 

Chemisorption or specific adsorption involves greater forces of attraction than physical adsorption. As hydrogen bonding or n rbital interactions are utilised, the adsorbed species lose their hydrated spheres and can approach the surface as close as the ionic radius. Whereas multilayer adsorption is possible in physical adsorption, chemisorption is necessarily limited to monolayer coverage. 

For example, one criterion is based on heats of adsorption chemisorption has high heats while physical adsorption has low heats. We know of chemisorption phenomena in which the heats of adsorption are probably zero and some that are endothermic. Another criterion is based on rates of adsorption, fast for physical, slow for chemical. There are chemisorption phenomena that are too fast to be measured, and also some physical adsorption phenomena are too slow to be measured. If 1 understand Dr. Hauffe, his criterion would be that physical adsorption does not perturb the electronic economy of the solid while chemisorption does.. Mignolet [Rec. tray, chlm., 74, 685 (1955)] found that noble gases adsorbed on evaporated metal films changed the work function by approximately one volt. 

The thermodynanucs of gas-phase species adsorbing on a surface can be separated into the classes of physical adsorption (physisorption) and chemical adsorption (chemisorption). The difference between the two classes of adsorption is related to the heat of adsorption, A Hads. In physisorption, weak attractive forces (van der Waals) such as dipole-dipole and induced dipole interactions drive the adsorption of species from the gas phase. For chemisorbed systems, chemical bonds associated with electron transfer between adsorbate and substrate are formed upon adsorption. Each of these interactions can be described by a two-dimensional potential-energy diagram as in Figures 10 and 11. 

Bibler and Marson 1992). Retention of uranium by the soil may be due to adsorption, chemisorption, ion exchange, or a combination of mechanisms (Allard et al. 1982). Any soil property that alters the sorption mechanism will also alter the mobility of uranium in the soil. The sorption of uranium in most soils is such that it may not leach readily from soil surface to groundwater, particularly in soils containing clay and iron oxide (Sheppard et al. 1987), although other geological materials such as silica, shale, and granite have poor sorption characteristics (Bibler and Marson 1992 Erdal et al. 1979 Silva et al. 1979 Tichnor 1994). 

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