Adsorbent molecular solids

We call the adsorbing species the adsorbate. An adsorbate is atomic, ionic or molecular solid, liquid or gas. 

The terms in equation (7.3) have previously been defined asW = weight of adsorbate, M = adsorbate molecular weight, S, = solid surface area and P = equilibrium pressure. 

The theory of Brunauer, Emmett and Teller167 is an extension of the Langmuir treatment to allow for multilayer adsorption on non-porous solid surfaces. The BET equation is derived by balancing the rates of evaporation and condensation for the various adsorbed molecular layers, and is based on the simplifying assumption that a characteristic heat of adsorption A Hi applies to the first monolayer, while the heat of liquefaction, AHL, of the vapour in question applies to adsorption in the second and subsequent molecular layers. The equation is usually written in the form... 

In the first monolayer of conjugated model material, a model molecular solid or a polymer adsorbate, assume that no chemistry (covalent bonding) occurs, since, in the absence of, for example, mechanical rupturing, the bonds at the surface of the molecular film are completely satisfied. This assumption is supported by the fact that, at least for condensed molecular solids, vapor-deposited films may be re-evaporated (removed) from the surface by gentle heating in UHV. 

Adsorption fiom liquids can involve both physisorption and chemisorption however, one needs to consider the molecular interactions between the adsorbing species and the liquid, as well as those with the solid. The relative magnitudes of the liquid/adsorbate and solid/adsorbate interactions will determine the extent... 

Carborane and decaborane show similar behavior in that both molecnles adsorb molecularly on Pt(lll) at 85 K, nndergo some strnctnral changes at low temperatures, and lose hydrogen in stages when the temperature is raised, as determined by both RAIRS and TPD. The RAIR spectra show remarkably sharp B-H stretch peaks. The fact that the individual B-H stretch peaks can be resolved here is in marked contrast to experimental IR spectra of solid carborane , which show only a broad feature about 100 cm wide centered at 2620 cm Both C2B10H12 and BioHu are assumed to be adsorbed molecularly on Pt(lll) at 85 K, because the RAIR spectra for submonolayer and multilayer coverages do not differ that much from each other as far as the B-H stretch positions are concerned. 

For our purposes, adsorption from solution is of more direct relevance than gas adsorption. Most, if not all, topics in the five volumes of FICS Involve one or more elements of it. In the present chapter, the basic elements will be introduced, restricting ourselves to low molecular weight, uncharged adsorbates and solid surfaces. Adsorption of charged species leads to the formation of electrical double layers, which will be treated in chapter 3. Adsorption at fluld/fluid Interfaces follows in Volume III. Adsorption of macromolecules will be Introduced in chapter 5. Between monomers, short oligomers, longer oligomers and polymers there is no sharp transition in the present chapter we shall go as far as non-ionic surfactants, but omit most of the association and micelle formation features, which will be addressed in a later Volume. There will be some emphasis on aqueous systems. 

Quantum-mechanical calculations of gas-solid interactions, and analysis of experimental kinetic data suggest existence of a certain precursor state which is a weakly adsorbed molecular species. In a next step the precursor species dissociates into adsorbed atoms. Let us assume that the rate determining step of this kinetic process is the transition from the precursor state to the adsorbed atoms. 

In this section the discussion will be confined to the examination of the behavior of isotopic molecular hydrogens adsorbed on solid surfaces at low temperatures. A considerable amount of experimental data is available for these isotopes and furthermore, the approximations em-... 

Because of the long radiative lifetime of the lowest triplet state, most phosphorescence in fluid solutions is obviated by collisional quenching, especially by dissolved molecular oxygen. Phosphorescence, when it occurs, is usually observed at low temperatures (e.g., that of liquid nitrogen) in rigid matrices where it may demonstrate high quantum yields. In the past three decades, much interest has been focused on phosphorescence at room temperature (RTP), which sometimes can be observed in samples adsorbed on solid substrates such as filter paper. Unfortunately, the quantum yields observed in room temperature phosphorescence are low, leading to poor analytical sensitivity, and the method has not enjoyed wide popularity. Phosphorescent measurements at low temperatures... 

In evaluating the mechanistic models for sorption by coals, one is well advised to consider the contrast of the two general classes of sorbents physical adsorption (sometimes called physisorption) will likely alter the surface structure of a molecular solid adsorbent (such as ice, paraffin, and polymers), but not that of high surface energy, refractory solids (such as the usual metals and metal oxides, and carbon black) (9). Adamson (27) has proposed... 

Chemical kinetics also plays a basic role in the study of the nature of catalytic activity. Studies of the catalyst and reactants in the absence of appreciable over-all reaction, such as studies of the electronic properties of catalytic solids or optical studies of adsorbed molecular species can provide valuable information about these materials. In most cases, however, kinetic data are ultimately needed to establish the relation and relevance of any information derived from such studies to the catalytic reaction itself. For example, a particular adsorbed species may be observed and studied by a spectral technique yet it need not play any essential role in the catalytic reaction since adsorption is a more general phenomenon than catalytic activity. On the other hand, kinetics studies can provide information about the variation, as a function of experimental conditions, of the relative number of adsorbed species that play a basic role in the reaction. Consequently, such information may make it possible to identify which, if any, of the adsorbed species studied by the use of a direct analytical technique are relevant to the reaction. As another example, when studies are made of the solid state properties of a given catalytic solid, the question as to which, if any, of these properties are related to catalytic activity must ultimately be answered in terms of consistency with the observed behavior of the reaction system. 

The specific surface area is defined as the accessible area of solid surface per unit mass of material. It is also dependant on the method employed and the size of the probe used (e.g. adsorbate, molecular probe, wavelength of radiation, etc.). The recorded value will also depend on the assumptions inherent in the simplified models applied to interpret experimental data. This consideration is of particular significance for materials containing micropores. 

Adsorption defines the accumulation of a substance, or material, at tlie interface between a solid surface and a bathing solution (Sparks, 2002). Within the adsorption framework, the individual components are referred to as the adsorbate, the accumulating material at the interface, and the adsorbent, or solid surface (Sparks, 2002). If adsorption occurs and results in the formation of a stable molecular phase at the interface, this entity can be described as a surface complex. Two general surface complexes exist and are described by the configuration geometry of the adsorbate at the adsorbent surface. These are the iiuier-and outer-sphere surface complexes, defined by the presence, or absence, of the hydration sphere of the adsorbate molecule upon interaction. When at least one water molecule of the hydration sphere is retained upon adsorption, the surface complex is referred to as an outer-sphere complex (Sposito, 1984) when an ion or molecule is bound directly to the adsorbent without the presence of the hydration sphere, an inner-sphere complex is formed. 

In an attempt to lower their surface energy, solids almost always will adsorb small molecules, such as H2O or O2, on their surfaces. Molecules which form molecular solids have much smaller surface tensions than other solids. The study of surfaces is a difficult one, it can be seen. Fortunately a number of very special experimental methods have been developed, and surfaces may be studied in great detail. Such studies are useful in many important practical areas, such as adhesion, lubrication, corrosion and adsorption. However, the most important area is probably heterogeneous catalysis. 

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