Metals, adsorption studies

Prior to the early 1990s, all structural studies of alkali-metal chemisorption found the adatom located at high coordination sites at which the alkali-metal atom is bound in three- or four-fold hollow sites. A comprehensive survey of alkali-metal adsorption studies prior to 1988 may be found in the book edited by Bonzel (Bonzel et al., 1989). Several more recent LEED, SEXAFS and X-ray studies have implicated low coordination (top) sites, as in the case of Cu(lll)p(2x2)-Cs, or substitutional behavior. These results may signal that the current understanding of the alkali-metal bonding at surfaces is incomplete. 

The application of the reflection technique to adsorption on single crystals will possibly offer a new lease on life to the use of infrared in fundamental gas-metal adsorption studies. Infrared studies that have contributed so much to CO adsorption studies, particularly those closely allied to catalytic systems, are likely, by application of the reflection technique, to be unique in their designation of adsorption complexes on single crystal surfaces. 

Metal adsorption studies were performed by treating the polymeric materials with solutions of nickel acetate at known concentrations followed by a determination of the concentration of the nickel solution after equilibrimn was reached. The exchange capacity of two commercially available resins was also determined. Results appear in Table 1. 

Diehl R D and McGrath R 1996 Structural studies of alkali metal adsorption and coadsorption on metal surfaces Surf. Sc/. Rep. 23 43... 

The hydrophobic character exhibited by dehydroxylated silica is not shared by the metal oxides on which detailed adsorption studies have been made, in particular the oxides of Al, Cr, Fe, Mg, Ti and Zn. With these oxides, the progressive removal of chemisorbed water leads to an increase, rather than a decrease, in the affinity for water. In recent years much attention has been devoted, notably by use of spectroscopic and adsorption techniques, to the elucidation of the mechanism of the physisorption and chemisorption of water by those oxides the following brief account brings out some of the salient features. 

The lack of a well-defined specular direction for polycrystalline metal samples decreases the signal levels by 10 —10, and restricts the symmetry information on adsorbates, but many studies using these substrates have proven useful for identifying adsorbates. Charging, beam broadening, and the high probability for excitation of phonon modes of the substrate relative to modes of the adsorbate make it more difficult to carry out adsorption studies on nonmetallic materials. But, this has been done previously for a number of metal oxides and compounds, and also semicon-... 

This is a process that takes place via specific chemical forces, and the process is unique to the adsorbent or adsorbate used. In general, it is studied at temperatures much higher than those of the boiling point of the adsorbate consequently, if supported metals are studied, little or no physical adsorption of the chemisorbing gas takes place on the high surface area support. 

Simultaneous adsorption studies on supported metals and on their support, where both have been treated with reactants In exactly the same manner. 

The chemistry of superheavy elements has made some considerable progress in the last decade [457]. As the recently synthesized elements with nuclear charge 112 (eka-Hg), 114 (eka-Pb) and 118 (eka-Rn) are predicted to be chemically quite inert [458], experiments on these elements focus on adsorption studies on metal surfaces like gold [459]. DFT calculations predict that the equilibrium adsorption temperature for element 112 is predicted 100 °C below that of Hg, and the reactivity of element 112 is expected to be somewhere between those of Hg and Rn [460, 461]. This is somewhat in contradiction to recent experiments [459], and DFT may not be able to simulate accurately the physisorption of element 112 on gold. More accurate wavefunction based methods are needed to clarify this situation. Similar experiments are planned for element 114. 

There is further emphasis on adsorption isotherms, the nature of the adsorption process, with measurements of heats of adsorption providing evidence for different adsorption processes - physical adsorption and activated adsorption -and surface mobility. We see the emergence of physics-based experimental methods for the study of adsorption, with Becker at Bell Telephone Laboratories applying thermionic emission methods and work function changes for alkali metal adsorption on tungsten. 

Less complex techniques have been reported to be useful to study the acidic and alkaline treatment processes of biosorbents and the role of carboxyl and carboxylate groups in metal adsorption. Rakhshaee and coworkers101 used potentiometric titration curves to assess the content of such groups in L. minor biomass treated with NaOH and HC1. The results showed an increase (up to 25%) in the adsorption of Hg(II), Cr(III), Cr(VI), and Cu(II) with NaOH-treated biomass as a consequence of an increase of -COO- groups (0.92-2.42 mmol/g). On the contrary, the -COOH groups increase observed (1.50-2.41 mmol/g) due to the acidic treatment led to a decrease in the metal ions uptake (up to 33%) despite activation by the chloride salts. 

Electrochemical reactions are driven by the potential difference at the solid liquid interface, which is established by the electrochemical double layer composed, in a simple case, of water and two types of counter ions. Thus, provided the electrochemical interface is preserved upon emersion and transfer, one always has to deal with a complex coadsorption experiment. In contrast to the solid/vacuum interface, where for instance metal adsorption can be studied by evaporating a metal onto the surface, electrochemical metal deposition is always a coadsorption of metal ions, counter ions, and probably water dipols, which together cause the potential difference at the surface. This complex situation has to be taken into account when interpreting XPS data of emersed electrode surfaces in terms of chemical shifts or binding energies. 

Cavallaro and McBride (1984a) observed that the removal of Fe oxides from two clay soils reduced Zn adsorption. Shuman (1976) reported that the removal of Fe oxides resulted in an increase or decrease in Zn adsorption, but later in another similar study (1988) he found that the removal of either amorphous or crystalline Fe oxides increased Zn adsorption capacity and decreased Zn-bonding energy. The author explained that adsorption sites on the Fe oxide coatings were not as numerous as those released when the coatings were removed. Elliott et al. (1986) observed that DCB extraction of Fe oxides from two subsoils of the Atlantic Coastal Plain increased heavy metal adsorption. Wu et al. (1999) found that Cu adsorption on the fine clay fraction increased after dithionite treatment with possible exposure of much more high-affinity sites for Cu on the fine clay. 

Recent work by Stipp and Hochella (1991) provide evidence for the processes of reconfiguration and hydration at the calcite surface. These results may provide a basis for future spectroscopic studies of trace metal adsorption and subsequent solid-solution formation. 

Benard, 1983] J. Benard, Adsorption on Metal Surface, Studies in Surface Science and Catalysis 13, p. 150, Elsevier Sd. Pub. Co., Amsterdam, (1983). 

A clean, solid surface is actually an active center for adsorption from the surroundings (e.g., air or liquid). A perfectly cleaned metal surface, when exposed to air, will adsorb a single layer of oxygen or nitrogen (or water). Or, when a completely dry glass surface is exposed to air (with some moisture), the surface will adsorb a mono-layer of water. In other words, the solid surface is not as inert as it may seem to the naked eye. This has many consequences in industry, such as with corrosion control. Accordingly, solid surfaces should always be exposed to vacuum prior to any kind of adsorption studies. 

Gharacterization of inorganic sulfur speci-ation in marine and freshwater porewaters is critical to our understanding of metal and sulfur cycling in sediments. Since coprecipitation and/or adsorption on FeS(g) and formation of discrete authigenic sulfide minerals can effectively remove trace metals, many metal cycling studies are... 

Sawaya etal. [365] have described the local density approximation studies of semiconductor metal adsorption characteristics Ge/Ag(100). 

Considering first adsorption of metal ions or neutral species directly on the snbstrate, there are a nnmber of possible mechanisms for this process. Most simply, there will an equilibrinm between metal species in solution and a solid snr-face leading to dynamic adsorption of the metal. Adsorption of metal ions onto solid snrfaces has been extensively studied, to a large extent becanse of the nse of oxide snrfaces to adsorb heavy metal ions and remove them from solntion (see Ref. 55 for an example and list of other references on this snbject). This adsorption may go even farther with ion exchange between the solntion metal ions and ions in the substrate (again, glass is a good example of where this may occnr). 

The top layer of a (111) surface actually has sixfold symmetry, but the rotational symmetry of the top layers together is threefold. Since the near surface region can influence where gases adsorb on the surface and the LEED intensities exhibit threefold rotational symmetry at normal incidence, the (111) surface will be considered to have threefold rotational symmetry. Although most of the adsorption studies have been carried out on fee and bcc crystals, there have been several studies reported on hep crystals. For hep metals the basal or (0001) plane is the surface most frequently studied by LEED investigations and it is the most densely packed plane having threefold rotational symmetry. 

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