Sulfide homogeneous precipitant

The rate-limiting step in CD for the first two mechanisms is almost always formation of the chalcogenide ion. This reaction should be slow otherwise fast, homogeneous precipitation of the metal chalcogenide will occur with little fihn formation. (Even rapid precipitation can lead to a film however, this film will be extremely thin and in most cases not visible.) Almost all the literature on CD is limited to sulfides (mostly), selenides, and oxides (including hydrated oxides and hydroxides). Anion-forming reactions are described in this section. 

Anion or cation generation Anions can be generated slowly in solution to bring about homogeneous precipitation. Swift and Butler reviewed precipitation of the metal sulfides by use of thioacetamide or thiourea. PFHS of sulfides of cadmium, mercury, zinc, and nickel have been studied more recently by Swift and others. Phosphate can be generated by hydrolysis of triethyl phosphate, oxalate by hydrolysis of methyl oxalate, and sulfate by hydrolysis of diethyl sulfate or sulfamic acid. 

Gallagher, D., Heady, W.E., Racz, J.M., and Bhargava, R.N., Homogeneous precipitation of doped zinc sulfide nanocrystals for photonic applications, J. Mater. Res., 10, 870, 1995. 

Precipitation of lead sulfide homogeneously by addition of thloacetamlde has been reviewed by Flnston and Mlskel (P2). 

The previous EXAFS studies were restricted to supported catalysts. Furthermore, the structural properties determined by MES and EXAFS were mainly related to the HDS activity and not to the other catalytic functions. Presently, we will report EXAFS (both Mo and Co), MES, HDS and hydrogenation activity studies of unsupported Co-Mo catalysts. These catalysts have been prepared by the homogeneous sulfide precipitation method (l8) which permits large amounts of Co to be present as Co-Mo-S. The choice of unsupported catalysts allows one to avoid some of the effects which inherently will be present in alumina supported catalysts, where support interactions may result in both structural and catalytic complexities. 

Sample Preparation. The preparation of the unsupported Co-Mo catalysts has been carried out using the homogeneous sulfide precipitation (HSP) method as described earlier (l8) and only few details will be given here. A hot (335 3 5 K) solution of a mixture of cobalt nitrate and ammonium heptamolybdate with a predetermined Co/Mo ratio is poured into a hot (335 3it5 K) solution of 20 ammonium sulfide under vigorous stirring. The hot slurry formed is continuously stirred until all the water has evaporated and a dry product remains. This product is finally heated in a flow of 2% H2S in H2 at 675 K and kept at this temperature for at least b hr. Catalysts with the following Co/Mo atomic ratios were prepared 0.0, 0.0625, 0.125, 0.25, 0.50, 0.75, and 1.0. 

Zinc and Cadmium Sulfides and Sulfoselenides. The raw materials for the production of these sulfide phosphors are high-purity zinc and cadmium sulfides, which are precipitated from purified salt solutions by hydrogen sulfide or ammonium sulfide [5.291], [5.296], [5.307], [5.310], The concentration of contaminants such as Fe, Co, or Ni must be below 1 % of the activator concentration. The Zn, Cd S can be produced by mixing precipitated zinc sulfide and cadmium sulfide. However, coprecipitation from mixed zinc-cadmium salt solutions is preferred because of the better homogeneity. 

H., Synthesis of controlled spherical zinc sulfide particles by precipitation from homogeneous solutions, J. Am. Ceram. Soc., 81, 2699, 1998. 

Ceramic powders of exceptional purity with very fine particle sizes (ca. 10 nm) have been prepared by precipitation of hydrolyzed oxides from high purity alkoxides . Yttria-stabilized zirconia is prepared by mixing high purity zirconium and yttrium isopropoxides in n-hexane, with water added dropwise to precipitate the oxides quantitatively. The homogeneity of the oxides results in stabilized cubic Zr02 at very low temperatures. Cyclic organoaluminum amides are used to prepare nonoxides such as sulfides and AIN- . 

With the exception of the alkali metals and the alkaline earth metals, most cations form sparingly soluble sulfides whose solubilities differ greatly from one another. Because it is relatively easy to control the sulfide ion concentration of an aqueous solution of H2S by adjustment of pH (see Section 11C-2), separations based on the formation of sulfides have found extensive use. Sulfides can be conveniently precipitated from homogeneous solution, with the anion being generated by the hydrolysis of thioacetamide (see Table 12-1). 

For many of the more abundant elements, such as Al, Fe, and Mn, precipitation of mineral forms is common and may greatly influence or even control their solubility. For most trace elements, direct precipitation from solution through homogeneous nucleation appears to be less likely than adsorption-desorption, by virtue of the low concentration of these metals and metalloids in soil solutions in well-aerated dryland soils. When soils become heavily polluted, metal solubility may reach a level to satisfy the solubility product to cause precipitation. Precipitation may also occur in the immediate vicinity of the phosphate fertilizer zone, where the concentration of heavy metals and metalloids present as impurities may be sufficiently high. Precipitation of trace metals as sulfides may have a significant role in metal transformation in reduced environments where the solution sulfide concentration is sufficiently high to satisfy the solubility product constants of metal sulfides (Robert and Berthelin, 1986). 

Over the decades that have passed since La Mer s work numerous examples of monodispersed particles of various composition, morphologies and properties, as well as methods for their preparation (not limited to condensational formation), were described in the literature. Extensive studies in this area were carried out by E. Matijevic and T. Sugimoto. Examples of monodisperse systems formed by precipitation from homogeneous solutions include dispersions of uniform particles of simple composition having different morphologies, such as metal halides, sulfides, phosphates, (hydrous) oxides, etc, various composite particles, including particles of internally mixed composition and coated particles. Both crystalline and amorphous materials can be obtained. Electron micrographs of some characteristic examples of monodispersed colloids are shown in Fig. IV-14. 

Mixed-Metal Sulfides. Mixed-metal sulfides are generally prepared by one of the four following methods 1) comaceration, (2) homogeneous sulfide precipitation, (3) mixed-metal salts or organometallic clusters decomposition, and 4) impregnation of a binary sulfide. Most of these techniques were developed for the synthesis of Co/Mo or Ni/Mo catalysts but are applicable to other systems. 

The homogeneous sulfide precipitation (HSP) corresponds to a low temperature coprecipitation. In that particular case, two or more salts are dissolved prior to the addition of the sulfiding agent. The synthesis of unsupported Co-Mo sulfide provides a good example of the HSP method a solution of cobalt nitrate and ammonium heptamolybdate is poured into a hot solution of ammonium sulfide, then, the slurry is then evaporated to dryness (37). 

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