Coprecipitation of hydroxides

Homogeneous distribution can be attained by controlled coprecipitation of hydroxides which are then decomposed by calciaation yielding powders of fine particle si2es. Active siaterable powders are produced commercially, usually by hydrolysis of a mixture of ZrOCl2 and YCl to precipitate the mixed... 

The mixed oxides CeNixOv were prepared by coprecipitation of hydroxides from mixtures of cerium and nickel nitrate using triethylamine (TEA) as precipitating agent, drying at 323 K and calcination in air at 773 K [22]. The metal loading has been verified by microanalysis. The solids will be called CeNix. 

Figure 1. Ferrites prepared by coprecipitation of hydroxides. Atomic ratio Fe M in...

Many stoichiometric double salts (citrates, oxalates) are suitable as precursors for mixed oxides. If stoichiometries are needed in an oxide for which no double salts are available, coprecipitation of hydroxides can yield a precursor that has the metal ions atomically mixed in the proper amounts so that no excessive reaction times are necessary in the calcining step. 

The mobility of arsenic compounds in soils is affected by sorp-tion/desorption on/from soil components or co-precipitation with metal ions. The importance of oxides (mainly Fe-oxides) in controlling the mobility and concentration of arsenic in natural environments has been studied for a long time (Livesey and Huang 1981 Frankenberger 2002 and references there in Smedley and Kinniburgh 2002). Because the elements which correlate best with arsenic in soils and sediments are iron, aluminum and manganese, the use of Fe salts (as well as Al and Mn salts) is a common practice in water treatment for the removal of arsenic. The coprecipitation of arsenic with ferric or aluminum hydroxide has been a practical and effective technique to remove this toxic element from polluted waters... 

Willis 93) extracted lead directly from 200 ml of urine with APDC into 1.5 ml of methyl-n-amyl ketone. He was able to determine as little as 0.02 ppm of lead. Kopito and Shwachman 141>, on the other hand, co-precipitate the lead from urine with bismuth nitrate by adding ammonia. The precipitated bismuth hydroxide is dissolved in acid and this solution is aspirated. Coprecipitation of the lead is not quantitative, and so standards should be prepared in the same manner. It should be possible to employ this procedure with protein free filtrates of blood without the necessity of close pH control. 

Coprecipitation of the metals is usually achieved from an aqueous solution of nitrates upon addition of anions such as carbonates, citrates, or oxalates (10)(24-27). First reports in this field have underlined the necessity to neutralize the pH of the solution in order to obtain complete precipitation of barium or strontium. Also, oxalate or citrate ligands may bind to two different cations. This should allow a better mixing at a microscopic level. However, care should be taken since some cations such as Y or La may precipitate as double salt complexes with alkaline ions that have been added to the solution as hydroxides in order to control the pH (24). 

An even more intimate mixture of starting materials can be made by the coprecipitation of solids. A stoichiometric mixture of soluble salts of the metal ions is dissolved and then precipitated as hydroxides, citrates, oxalates, or formates. This mixture is filtered, dried, and then heated to give the final product. 

Natural clay catalysts were replaced by amorphous synthetic silica-alumina catalysts5,11 prepared by coprecipitation of orthosilicic acid and aluminum hydroxide. After calcining, the final active catalyst contained 10-15% alumina and 85-90% silica. Alumina content was later increased to 25%. Active catalysts are obtained only from the partially dehydrated mixtures of the hydroxides. Silica-magnesia was applied in industry, too. 

The copper chromium oxide (Cu/Cr = 1) has been prepared by coprecipitation of copper and chromium nitrates with ammonium hydroxide, followed by thermal decomposition in flowing nitrogen up to the final temperature (370"C), according to a previously described method (8). The apparatus and the catalytic procedure have also been described elsewhere in case of gas phase reactions (5) and liquid phase reactions (7). 

Coprecipitation of titanium and silicon hydroxides from their precursors, possibly by sol-gel techniques. 

In support of that explanation, X-ray analysis of the catalyst after use indicated the presence of MgO. Hence, the catalytically active phase was finely divided copper in intimate contact with magnesia, quasi as carrier. The same phenomenon was observed with the Zintl-phase alloys of silver and magnesium. Such catalysts were then deliberately prepared by coprecipitation of copper and silver oxides with magnesium hydroxide, followed by dehydration and reduction. Table I shows that these supported catalysts had the same activation energies as those formed by in situ decomposition of copper and silver alloys with magnesium. 

The situation appears to be very different for the catalyst where the support was prepared by coprecipitation of a mixed tin and aluminum hydroxide. In this case it appears that a high fraction of the tin is present in the interior of the solid, with the fraction located on the surface being small. Thus, when Pt is added to the surface by impregnation, even when the bulk Sn/Pt ratio is on the order of 1 to 4, the actual surface ratio of Sn/Pt will be very much smaller. 

The catalyst. The catalyst was prepared by coprecipitation of iron(III)hydroxide and zinc(II)hydroxide from a nitrate solution with ammonia. After the precipitate was dried and calcined, about 0.006 weight percent of iron sulphate was impregnated on the material. In earlier studies this impregnation proved to stabilize the catalyst. The catalyst was in situ reduced in pure hydrogen at 625 K and 100 kPa. Prior to the experiments, the reduced catalyst was activated in a continuous stream of synthesis gas (20% CO 20% H2 60% He) at 550 K. 

It should be noted that the sol-gel process is particularly attractive for the synthesis of multicomponent particles with binary or ternary compositions using double alkoxides (two metals in one molecule) or mixed alkoxides (with mixed metaloxane bonds between two metals). Atomic homogeneity is not easily achieved by coprecipitating colloidal hydroxides from a mixture of salt solutions, since it is difficult to constmct double metaloxane bonds from metal salt. Hybrid... 

If water samples containing a high concentration of Fe are to be speciated for As, the samples should be preliminarly acidified to prevent coprecipitation of Fe oxo-hydroxides and As. The pH should be set at 1-2 however, further acidification should be avoided because of the sensitivity of the As(III)-As(V) redox balance to lower pH values [105, 106]. 

In the coprecipitation method, trace metals are concentrated by adsorbing onto the surface of a precipitate. As the precipitate, hydroxides of Fe(III), Mn(IV), Al(IH), Bi(II), and Zr(IV) and sulfides of Co(II), Pb(II) and Fe(II) are frequently used. In AAS the use of hydroxide precipitates is more popular than that of sulfides. Coprecipitation using the hydroxide precipitates is generally unspecific, and many trace metals may be concentrated simultaneously. However, some limitations are often found in the coprecipitation technique [10],... 

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