Crystalline precipitates coprecipitation with

Coprecipitation with the hydrous oxides, such as of iron(III) and aluminum, occurs by adsorption and possibly also by compound formation. The precipitates, coming down in either amorphous or finely crystalline form with extensive surface, adsorb large amounts of water and adsorb hydroxide ions as potential-determining ions. Figure 9-1 illustrates the effect of varying the concentration of ammonium chloride and ammonium hydroxide on the amount of coprecipitation of divalent metal ions with hydrous iron(III) oxide. When the concentration of ammonium chloride is increased at a constant ammonia concentration, the adsorption is decreased... 

The extent of mixed-ci-ystal contamination is governed by the law of mass action and increases as the ratio of contaminant to analyte concentration increases. Mixed-crystal fomiation is a particularly troublesome type of coprecipitation because little can be done about it when certain combinations of ions are present in a sample matrix. This problem is encountered with both colloidal suspensions and crystalline precipitates. When mixed-crystal formation occurs, the interfering ion may have to be separated before the final precipitation step. Alternatively, a different precipitating reagent that does not give mixed crystals with the ions in question may be used. 

When creating supersaturation levels sufficient to induce particle formation, precipitation of sparingly soluble salts and sol-gel processes are viewed differently. Precipitation normally involves mixing a cation solution with a precipitant solution. For example, consider preparation of an oxalate precursor to a CoO- and MnO-doped ZnO powder. In this process, the Zn, Mn, and Co are coprecipitated with controlled stoichiometry and the precipitate is calcined to the oxide. To form the oxalate, a state of supersaturation is created by mixing an aqueous solution of the metal nitrates or chlorides with an oxalate precipitant solution. The system is supersaturated with respect to the different metal oxalate phases and a crystalline coprecipitate forms. Depending on precipitation conditions (pH, concentrations, temperature, etc.), different metal complexes are present in solution. The form and concentration of these complexes determine the phase, morphology, and particle size distribution of the resulting precipitate. 

Using coprecipitation methods with a suitable mixture of solutions described above, the resulting LDH materials are often poorly crystallized and exhibit compositional fluctuations due mainly to the difference in the values of the pH at which the precipitation of M(II)(OH)2 and M(III)(OH)3 hydroxides occurs. Consequently, the chemical formula of the final material may not reflect the composition of the solution prior to the precipitation as noted in Chapter 1. Controlling the amount of anion incorporated under such conditions is very difficult. A "chimie douce method has been proposed by Delmas et al. in an effort to overcome this problem [181,182]. The process is illustrated schematically in Fig. 8. Since the synthesis starts from a highly crystalline layered y-oxyhydroxide precursor, it was suggested that this favored the formation of very crystalline LDHs with controllable M(1I)/M(III)... 

The low-pressure methanol synthesis process utilizes ternary catalysts based on copper, zinc oxide, and another oxide, such as alumina or chromia, prepared by coprecipitation. Cu-Zn0-Al203 and Cu-Zn0-Cr203 are usually the most important industrial catalysts. A significant advance was made when a two-stage precipitation was suggested in which ZnAl2C>4, a crystalline zinc aluminate spinel, was prepared prior to the main precipitation of copper-zinc species.372 This alteration resulted in an increase in catalyst stability for long-term performance with respect to deactivation. Catalyst lifetimes industrially are typically about 2 years. 

Adsorption is a common source of coprecipitation and is likely to cause significant contamination of precipitates with large specific surface areas—that is, coagulated colloids (see Feature 12-1 for definition of specific area). Although adsorption does occur in crystalline solids, its effects on purity are usually undetectable because of the relatively small specific surface area of these solids. 

Unless otherwise noted, catalysts were prepared by coprecipitating the hydrous oxides of uranium, antimony, and a tetravalent metal from a hydrocholoric acid solution of their salts by the addition of ammonium hydroxide. The precipitates were washed, oven dried, then calcined at 910 C overnight or at 930 C for two hours to form crystalline phases. Attrition resistant catalysts, containing 50% by weight silica binder, were prepared by slurrying the washed precipitate with silica-sol prior to drying. In some cases, small amounts of molybdenum or vanadium were added by impregnating the oven dried material with ammonium paramolybdate or ammonium metavanadate solution. The details of these preparations may be found elsewhere (5-8). 

We have already discussed 1. To coprecipitate a raw material, we would use Na2Si03 as a solution in water along with the proper combination of BaCl2 and PbCl2 in solution. We then precipitate at 100 °C. so as to obtain a crystalline product, adding the silieate solution to the cation solution. The precipitate is filtered, and the NaCl removed by washing. We then must add 1.00 mol of Si02 to the raw material and then fire the mixture. 

Precipitation (or coprecipitation) method is also a simple and efficient wet-chemical route for preparing ceramic nanoparticles. A carefuUy developed precipitation method that optimizes processing parameters such as reactants and their concentrations, pH, temperature, and calcination conditions can produce a massive and reproducible quantity of ceramic nanoparticles with high purity and crystallinity [10]. For example, one of the most important ceramics in orthopedic applications, nanocrystaUine hydroxyapatite (Ca,(,(PO )/OH)2, HA), can be produced in large quantity through the aqueous reaction ... 

Additionally, LDH particles formed in a one-step process have been shown to exhibit increased crystallinity. This is presumably due to their direct formation from solution rather than by transformation of a pre-existing solid, such as A1(0H)3, which will offer numerous sites for heteronucleation (101). Thus, LDH formed by the coprecipitation or aluminate method are of higher crystallinity than that formed by precipitation at varying pH (380). MAS NMR studies confirm that the appropriate number of divalent metals surround the trivalent metal, forming a homogeneous metal hydroxide layer, for materials formed via aluminate, compared to a less than ideal ratio in materials prepared by precipitation at varying pH (122). Yet another factor that influences the crystallinity of the material formed is the level of ion saturation in solution. In more concentrated solutions, the LDH is formed with poorer crystallinity (381). 

A classical method of the preparative fractionation of polymera consists of fractional precipitation based on the separation of molecules with respect to their molecular weight between two liquid phases, namely, the more concentrated gel phase and the dilute sol phase. Wth nylons (similar to other crystalline polymers) the separation efficiency with respect to molecular weight is unfavorably affected in numerous systems by allegation of polymer molecules near the precipitation point The partly crystalline gel phase may then contain a certain amount of the coprecipitated lower-molecular weight fractions. 

The diffraction pattern of the continuously precipitated precursor that was continuously aged for 60 min is almost identical to that observed for the precursor coprecipitated and aged for 2 h in the usual batch process. Thus, it can be assumed that this precursor composition represents a thermodynamic minimum that all ternary precursors of the same molar ratios approach with increasing time, independent of the precipitation method. This is also reflected in the increasing crystallinity with increasing aging time compared to the initially X-ray amorphons precnrsor mixture. 

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