Low temperature coprecipitation

Palladium black was prepared from palladium nitrate and formaldehyde solution by dropwise addition of potassium hydroxide solution (50 wt. %) at about 10°. The solution and precipitate were warmed at about 60° and the precipitate washed several times by decantation. It was then placed in a Soxhlet extractor and washed for 48 hr. (about 100 times). The precipitate was then dryed at 110°. The palladium-silver system is known to be one of complete miscibility (3). Alloys of silver-palladium were prepared following a procedure discussed elsewhere 4). Their preparation involved a low-temperature coprecipitation of both metals from a solution containing proper amounts of their nitrates. Alloy formation was checked by means of x-ray diffraction patterns which were obtained with Cu-Ka radiation. The computed lattice constants are shown in Fig. 1 to be a linear function of the alloy composition. Hydrogen, used for pretreatment of all samples, was obtained from a commercial tank and purified by passage through a Deoxo unit, magnesium perchlorate, and a charcoal trap immersed in liquid nitrogen. 

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). 

However, in some cases oxidic gold species may be the active sites for CO oxidation. Gates reported that oxidic gold dispersed on La203 by using GG of Au acac complex is active at room temperature [43]. On the other hand, we have recently found that over Au/La coprecipitates calcined at temperatures below 500 K are active even at 193 K [46]. The EXAFS and XANES analyses of the active samples showed that oxidic gold stabilized by La(OH)3 is responsible for low-temperature activity. 

Morse JW, Bender ML (1990) Partition coefficients in calcite Examination of factors irrflnencing the validity of experimental resnlts and their application to natural systems. Chem Geol 82 265-277 Mucci A, Morse JW (1990) The chemistry of low temperature abiotic calcites Experimental studies on coprecipitation, stability and fractionation. Rev Aquatic Sci 3 217-254 Musgrove ML, Barmer JL, Mack LE, Combs DM, James EW, Cheng H, Edwards RL (2001) Geochronology of late Pleistocene to Holocene speleothems from central Texas Implications for regional paleoclimate. Geol Soc Am Bull 113 1532-1543... 

Because the Ba2+/Ca2+ ratio In the solution changed during the course of an experiment, the Doerner-Hosklns equation (32) was used to calculate partition coefficients. The Doerner-Hosklns model of coprecipitation without equilibration of the interior of a crystal to the evolving composition of the solution has proved appropriate to low-temperature aqueous coprecipitation. 

M. Haruta, N. Yamada, T. Kobayashi, and S. Lijima, Gold catalysts prepared by coprecipitation for low-temperature oxidation of hydrogen and of carbon monoxide, J. Catal. 115(2), 301-309 (1989). 

The porosity of a collagen-GAG copolymer is an indispensable component of its biological activity [61]. Pores are incorporated by first freezing a very dilute suspension of the collagen-GAG coprecipitate and then inducing sublimation of the ice crystals by exposing to vacuum at low temperatures [62], The resulting... 

Mazur P (1980) Limits to life at low temperatures and at reduced water contents and water activities. Orig Life 10 137-159 McCaffrey MA, Lazar B, Holland HD (1987) The evaporation path of seawater and the coprecipitation of Br and K+ with halite. J Sed Petrol 57 928-937... 

Figure 10.4 Compensation plot for various supported gold catalysts the cation of the support is shown. The filled point is for a Cu/ZnO-A Os low-temperature shift catalyst the points for Ti02-supported catalysts on the upper line were made by deposition-participation all those on the lower line were made by coprecipitation. See Table 10.2 for further details.

Skeletal Cu-Zn catalysts show great potential as alternatives to coprecipitated Cu0-Zn0-Al203 catalysts used commercially for low temperature methanol synthesis and water gas shift (WGS) reactions. They can also be used for other reactions such as steam reforming of methanol, methyl formate production by dehydrogenation of methanol, and hydrogenolysis of alkyl formates to produce alcohols. In all these reactions zinc oxide-promoted skeletal copper catalysts have been found to have high activity and selectivity. 

The sol-gel method has been conveniently employed for the synthesis of 123 compounds such as YBa2Cu307 and the bismuth cuprates. Materials prepared by such low-temperature methods have to be annealed or heated under suitable conditions to obtain the desired oxygen stoichiometry as well as the characteristic high Tc value. 124 cuprates, lead cuprates and even thallium cuprates have been made by the sol-gel method the first two are particularly difficult to make by the ceramic method. Coprecipitation of all the cations in the form of a sparingly soluble salt such as carbonate in a proper medium (e.g. using tetraethyl-ammonium oxalate), followed by thermal decomposition of the dried precipitate has been employed by many workers to prepare cuprates. 

Bismuth vanadate can be produced by either a wet process or high-temperature calcination. By the first method, an acidic solution of bismuth nitrate, Bi(N03)3. is mixed with an alkaline solution of sodium vanadate, Na3V04. The resultant gel is filtered, washed, and converted to crystalline form by low-temperature calcination at 200-500°C. Multiple-phase pigments can be created by coprecipitation of bismuth vanadate with molybdenum, tungsten, or niobium compounds. 

Low temperature synthesis of lead zirconate titanate (PZT) can also be obtained via a microemulsion process [161]. The microemulsion, containing cations of lead zirconium and titanium in the aqueous phase, was coprecipitated as hydroxide precursors by the addition of ammonium solution. Crystalhne tetragonal PZT powders were then obtained by calcining the precursors at a temperature as low as 450 °C in air without forming any intermediate phases. 

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