Coprecipitation method

The method of preparation of a support material has a tremendous effect on its properties (11). For example, zeoHtes, which are highly stmctured aluminosihcates, are known to be extremely sensitive to the conditions employed both during and after crystallization (12). Also, when siUca—titania is precipitated by a coprecipitation method using ammonia, in which localized hydroxide ion gradients are estabUshed by the precipitation process itself, the product is much more acidic than when it is precipitated using urea, which suppHes hydroxide ion slowly and uniformly during precipitation (13). 

Results are different when solutions are used instead of suspensions. Chitosan-hydroxyapatite composites with a homogeneous nanostructure have been prepared by a coprecipitation method by Yamaguchi et al. [170]. 

On the basis of the experimental results, Sn02-Zr02 catalysts were prepared by the coprecipitation method. That is, Sn02 and Z1O2 were mixed chemically rather than physically. The effects of the Sn/Zr molar ratio and reaction temperature on the SO2 conversion and sulfur yield for Sn02-Zr02 catalysts were... 

The anticancer agent Methotrexate (MTX) was conjugated with LDH by a coprecipitation method. To synthesize the MTX-LDH conjugate, powdered MTX was dissolved in decarbonated water, and titrated with NaOH (0.5 M) solution to give a 0.043 M solution of MTX at pH 7. The mixed metal solution of Mg(N03)2-6H20 and A1 (N03)3-9H20 with molar ratio Mg/Al = 2/1 was added to the MTX solution and the solution was titrated with NaOH solution until pH 9.5 to produce yellowish precipitates [7,26]. 

Fig. 15.4 (A) Urease adsorption isotherms for (a) Zn2AI-CI, (b) Zn3AI-CI, (c) Zn4AI-CI (B) amount of immobilized urease in Zn3-AI by the coprecipitation method.

Selenium (IV) adsorbed as selenotrisulfate was then eluted from the column with either 0.1 M penicillamine or 0.1M cysteine. The eluate was then subjected to an acid digestion procedure to reduce selenium to the tetravalent state with diaminonaphthalene for fluorometric determination. Approximate agreement with the tellurium coprecipitation method was obtained. The application of both methods to the analysis of estuarine waters permitted the separate determination of both selenium (IV) and selenium (VI), since the tellurium coprecipitation methods did not differentiate between the two species. 

In the oxalate coprecipitation method, oxalate acid reacts with metal cations to form the precipitate, which is subsequently calcined to obtain the products. The advantage of oxalate acid as a precipitant is that, unlike hydroxides, oxalates are less sensitive to the treatment conditions, such as washing and drying. In addition, the... 

Nevertheless, the oxalate coprecipitation method has some problems. For example, this method usually results in rodlike doped ceria particles, which are agglomerations of smaller particles with irregular shapes. Hence, the green density of the compact body is relatively low, so it is difficult to fabricate a dense electrolyte film or membrane. In addition, the poor flow of the rodlike powder makes forming difficult. 

The copper-ceria catalysts in WGS were found to be nonpyrophoric and stable, showing little or no deactivation during the experiments. The Civ,2Ce(J802 y catalyst prepared by coprecipitation method showed good catalytic activity for the WGS reaction. The Cu0, Ce0 902 v catalysts prepared by the sol-gel method were found to be less active, which could be due to the lower number of active... 

By carrying out selective CO oxidation with some addition of C02 and H20 to the feed gas over a similar nanostructured CTy.Cej x02 catalyst prepared by a coprecipitation method [56], 15 vol% of C02 in the feed gas decreases the activity of the catalyst. Under these conditions the same values of activity and selectivity were obtained at temperatures 15 to 35°C higher. The addition of 10 vol% of H20 shifted the activity and selectivity curves to temperatures 20 to 40°C higher with respect to the curves where only CO, 02, H2, and He were used in the feed. 

Fig. 7 XRD patterns of a an Mg/Mn - CO3 LDH formed by aerial oxidation of Mg(N03)2/MnC03 mixtures at constant pH and b an analogous material prepared by traditional coprecipitation methods. Reprinted with permission from [117]. Copyright The Mineralogical Society...

This method has also been successfully applied in the synthesis of Cu(II)-containing LDHs, although well crystallized materials are difficult to prepare as a consequence of the Jahn-Teller distortion found in the coordination shell of Cu(II) [70,71]. Incorporation of Ni(II) in the layers was found to improve the crystallinity and structural stability of such LDHs. For the synthesis of Cu/Ni/Al - CO3 and Cu/Ni/Mg/Al - CO3 LDHs [70] by the method with separate nucleation and aging steps, LDHs with both smaller particle size and narrower distribution of particle size were obtained compared with those prepared using a conventional coprecipitation method, similar to the case for Mg/Al-C03 LDHs [20]. Well crystallized Cu/Ni/Cr-COs LDHs [71] were obtained when the Cu/Ni/Cr atom ratio ranged from 1 2 1 to 1 3 1 in the reaction mixture with hydrothermal aging conditions at 180 °C for 4 h. 

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)... 

Table 2 Chemical analysis of M2 Al/Cl (M = Ca, Zn) and the organic derivatives obtained by direct incorporation using the coprecipitation method. The basal spacing is displayed (Reprinted from [70] by permission of Elsevier)...

Choy et al. [193] reported that vitamin A (retinoic acid), vitamin C (ascorbic acid) and vitamin E (tocopherol) could be intercalated into Zn/Al LDHs by the coprecipitation method. In solutions, these vitamins are normally all sensitive to light, heat and oxygen, and it was proposed that incorporating the molecules into a layered inorganic lattice may lead to their stabilization, resulting in a wider range of potential apphcations. 

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