Cadmium, adsorbed layers

The cadmium electrodeposition on the cadmium electrode from water-ethanol [222, 223], water-DMSO [224], and water-acetonitrile mixtures [225-229] was studied intensively. It was found that promotion of Cd(II) electrodeposition [222] was caused by the formation of unstable solvates of Cd(II) ions with adsorbed alcohol molecules or by interaction with adsorbed perchlorate anions. In the presence of 1 anions, the formation of activated Cd(II)-I complex in adsorbed layer accelerated the electrode reaction [223]. 

K ozarac, Z., and B. Cosovic 1984), Interaction of Cadmium with the Adsorbed Layer of Biogenic Surface Active Substances at the Mercury Electrode, Bioelectrochem. Bioenerg 12, 353 363. 

Adiponitrile is readily hydrogenated catalytically to hexamethylenediamine, which is an important starting material for the prodnction of nylons and other plastics. The electrochemical production of adiponitrile was started in the United States in 1965 at present its volume is about 200 kilotons per year. The reaction occurs at lead or cadmium cathodes with current densities of np to 200 mA/cm in phosphate buffer solutions of pH 8.5 to 9. Salts of tetrabntylammonium [N(C4H9)4] are added to the solution this cation is specihcally adsorbed on the cathode and displaces water molecules from the first solution layer at the snrface. Therefore, the concentration of proton donors is drastically rednced in the reaction zone, and the reaction follows the scheme of (15.36) rather than that of (15.35), which wonld yield propi-onitrile. 

The ion that determines the p>otential of the compact layer is called the potential-determining ion. In cases in which the potentied of compact layer is determined by the dissociation reaction of adsorbed hydroxyl groups, the potential -determining ions are hydrated protons or hydroxide ions. For cadmium sulfide electrodes, the potential-determining ions are not hydrated protons but hydrated sulfide ions the iso-electric point is at the sulfide ion concentration of 4 x 10 M [Ginley-Butler, 1978]. 

It should be pointed out that this deposition was carried out for films ca. 50 nm thick the study was carried out with CdS window layers for solar cells in mind, which are usually thin. It is possible that much longer depositions result in different impurities. Thus the sparingly soluble cadmium carbonate and cyanamide will be converted to CdS if enough sulphide ion is formed with time (or, for the complex-decomposition mechanism, if enough adsorbed thiourea decomposes on the surface of the solid phases). Of course, longer time also means more thiourea decomposition products. 

Adsorption experiments were conducted on chromium, platinum, cadmium, and zinc the sources and preparation of these metal specimens have been reported previously (16). In preparing adsorbed, mono-molecular layers by adsorption directly from the molten pure acid (5), the clean adsorbing substrate was first heated to a temperature just above the melting point of the acid (see Table I), a few crystals of the acid were sprinkled on the surface, and the resulting pool of molten acid was teased over the whole surface with a previously freshly flamed platinum wire. If spontaneous retraction of the liquid acid did not occur, the specimen was allowed to cool and all of the solidified material adhering on top of the adsorbed monolayer was removed by appropriate solvent treatments as discussed below. 

Experiments with lead and cadmium on sediment samples from the oxidized surface layer of mudflats in the South San Francisco Bay estuary, where the reaction systems were equilibrated for 24 hours at the appropriate pH for approximately 90% metal adsorption as determined by prior experiments, indicate slow release of adsorbed cadmium within a time frame of 96 hours, whereas lead was substantially non-labile... 

Cadmium does not adsorb hydrogen. After full coverage of nickel nanoparticles by zerovalent cadmium atoms, the bimetallic Cd-Ni particle surface no longer activates hydrogen and reaction (18.1) is no longer possible. However, if the deposited atom (M ) is also able to activate the hydrogen molecule, reaction (18.1) must be continued by the formation of several M layers over nickel. This is indeed what has been observed when M =Ni or Co (Scheme 18.21). 

When complexation holds the gegen ions preferentially in solution, the ratio of the cations in bulk solution to die cations in the diffuse and Stern layer increases. Even chloride ions are strong enough to hold an increased amount of cadmium ions in bulk solution (Fig. 7). The calculated amount of cadmium ions in the Stern and diffuse layer agrees with the measured amounts of cadmium ions adsorbed [41]. The fraction of cadmium ions in solution increases from 3.6% in 0.01 M NaCl to 56% in 0.05 M NaCl the adsorption in the Stern layer decreases from 90.4% to 43% (calculated values, totd concentration of Cd ions 0.13—1.07 pM). In the presence of CIO which does not form complexes with Cd, the fraction of Cd in solution is 1.9% in 0.01 M NaC104 and 30.7% in 0.05 M NaC104. 

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