Surfaces Chemisorption specific types

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 catalytic fibers, presented in this work, are prepared by dip-coating of FeCrAlloy-type fibers into a slurry made of Pd/y-AlaOs. The catalytic activities of the samples are tested in total combustion of methane and, in order to correlate the catalytic performances with the solid state properties, they are characterized by XRD, XPS, CO chemisorption, specific surface area and porosity measurements. The importance of O2/H2 pretreatment and the implications on both physico-chemical properties and catalytic activity of the catalysts is discussed. 

In contrast to physisorption, chemisorption creates localized and directional chemical bonds between the adsorbate and the surface. Chemisorption occurs at specific geometric features of the surface, known as sites. Adsorbates can often bind in more than one type of site, and it then becomes important to identify not only which sites can bind the adsorbate, but also the relative energetics of these binding sites, which influences the likelihood of each site to be populated by the adsorbate. 

This description is traditional, and some further comment is in order. The flat region of the type I isotherm has never been observed up to pressures approaching this type typically is observed in chemisorption, at pressures far below P. Types II and III approach the line asymptotically experimentally, such behavior is observed for adsorption on powdered samples, and the approach toward infinite film thickness is actually due to interparticle condensation [36] (see Section X-6B), although such behavior is expected even for adsorption on a flat surface if bulk liquid adsorbate wets the adsorbent. Types FV and V specifically refer to porous solids. There is a need to recognize at least the two additional isotherm types shown in Fig. XVII-8. These are two simple types possible for adsorption on a flat surface for the case where bulk liquid adsorbate rests on the adsorbent with a finite contact angle [37, 38]. 

Adsorption and Surface Chemical Grafting. As with siHca and many other siHcate minerals, the surface of asbestos fibers exhibit a significant chemical reactivity. In particular, the highly polar surface of chrysotile fibers promotes adsorption (physi- or chemisorption) of various types of organic or inorganic substances (22). Moreover, specific chemical reactions can be performed with the surface functional groups (OH groups from bmcite or exposed siHca). 

In some specific cases one would like to convert the chemisorption data into an averaged particle size. In that case, the number of surface atoms per unit surface area (density of surface atoms) is an essential parameter. Since this number depends on the type of the crystallographic plane, (see Table 3.7), one also needs information on the types of crystallographic planes exposed to the gas phase. This is also important for another reason the adsorption stoichiometry may depend on the crystallographic plane. 

The adsorption of gas can be of different types. The gas molecule may adsorb as a kind of condensation process it may under other circumstances react with the solid surface (chemical adsorption or chemisorption). In the case of chemiadsorption, a chemical bond formation can almost be expected. On carbon, while oxygen adsorbs (or chemisorbs), one can desorb CO or C02. Experimental data can provide information on the type of adsorption. On porous solid surfaces, the adsorption may give rise to capillary condensation. This indicates that porous solid surfaces will exhibit some specific properties. Catalytic reactions (e.g., formation of NH3 from N2 and Hj) give the most adsorption process in industry. 

The zeolites are also known as molecular sieves because of their capacity to discriminate between molecules they find numerous uses in separation and catalytic processes. Although they appear to be solid particles to the naked eye, they are highly porous, with a typical specific surface area of about 1000 m2/g. Catalysis is discussed in Chapter 9, but the scope of that chapter does not permit detailed discussions of the various types of catalysts and the role of physisorption and chemisorption in catalysis this vignette provides a glimpse of the rationale used in the molecular design of new materials of interest in surface chemistry and how the concepts introduced in Chapter 1 and Chapter 9 fit into the larger scheme. 

Recently, in order to understand processes on the catalyst surface, in particular structural formations, it has become a frequent practice to apply theories accounting for the interaction of adsorbed atoms. An important microscopic model of such a type is the lattice gas model. Its specific peculiarity is that this model accounts for the interaction of the nearer surface molecules (lateral interactions). It is this model that was applied in refs. 86 and 87. They should be specially emphasized as having exerted a great influence on the interpretation of thermodesorption experiments. The lattice gas model is used, e.g. in a series of investigations by Tovbin and Fedyanin [88, 89] devoted to the kinetics of chemisorption and reactions on catalyst surfaces. In terms of this model, one can interpret the complicated reaction rate dependences of surface coverage observed experimentally... 

In general, two types of adsorption are distinguished, physical adsorption and chemisorption, which depend on the type of interaction established between the adsorbent and the adsorptive. In a chemisorption process, specific chemical interactions between the adsorbent and the adsorptive occur, and the process is not reversible. On the other hand, physical adsorption includes attractive dispersion forces and, at very short distances, repulsive forces, as well as contribution from polarization and electrostatic forces between permanent electrical moments and the electrical field of the solid, if the adsorptive or the adsorbent has a polar nature. In this case, the process is fully reversible (or almost reversible). Thus, the overall interaction energy ( >(z) of a molecule of adsorptive at a distance z from the surface of the adsorbent is given by the general expression... 

Nevertheless, C02 is an extremely valuable probe molecule because the infrared spectra of the chemisorbed species respond very sensitively to their environments. Thus, the frequency separation of the typical band pairs of the carbonate structures may be taken as a measure of the local asymmetry at the chemisorption site. The application of 13C-FT-NMR should be extremely valuable for a still more extensive study of the nature of sites by C02 adsorption. Due to the very detailed information on the structure of sites on oxide surfaces that can be obtained by C02 chemisorption studies, this compound should in some cases also be applicable as a specific poison. A very careful study of the type of interaction with the surface, however, has to be undertaken for each particular system before any conclusive interpretation of poisoning experiments becomes meaningful. 

Adsorption methods may be used to provide information about the total surface area of a catalyst, the surface area of the phase carrying the active sites, or possibly even the type and number of active sites. The interaction between the adsorbate and the adsorbent may be chemical (chemisorption) or physical (phys-isorption) in nature and ideally should be a surface-specific interaction. It is necessary to be aware, however, that in some cases the interaction between the adsorbate and the adsorbent can lead to a chemical reaction in which more than just the surface layer of the adsorbent is involved. For example, when using oxidizing compounds as adsorbates (O2 or N2O) with metals such as copper or nickel or sulfides, subsurface oxidation may occur. 

Aqueous radionuclide species and other solutes can sorb to mineral surfaces by forming chemical bonds directly with the amphoteric sites or may be separated from the surface by a layer of water molecules and be bound through longer-range electrostatic interactions. In the TLM, complexes of the former type are often called inner-sphere complexes those of the latter type are called outer-sphere complexes (Davis and Kent, 1990). The TLM includes an inner plane (o-plane), an outer plane (/8-plane), and a diffuse layer that extends from the /8-plane to the bulk solution. Sorption via formation of inner-sphere complexes is often referred to chemisorption or specific... 

---