Coprecipitation sequential precipitation

The conditions and procedures of precipitation play a significant role in catalyst morphology, texture, pore structure, physical strength, and consequently the performance (activity, selectivity, and stability) of the catalyst. By merely changing the sequence of solution addition, catalyst components can be precipitated simultaneously or sequentially. The methods of precipitation most often used are constant pH coprecipitation, sequential precipitation, acid-to-base precipitation, and base-to-acid precipitation. 

If catalysts are prepared by coprecipitation, the relative solubilities of the precipitates and the possibility for the formation of defined mixed phases are essential. If one of the components is much more soluble than the other, there is a possibility that sequential precipitation occurs. This leads to concentration gradients in the product and less intimate mixing of the components. If this effect is not compensated by adsorption or occlusion of the more soluble component, the precipitation should be carried out at high supersaturation in order to exceed the solubility product for both components simultaneously. Precipitation of the less soluble product will proceed slightly faster, and the initially formed particles can act as nucleation sites for the more soluble precipitate which forms by heterogeneous precipitation. The problem is less crucial if both components form a defined, insoluble species. This is for instance the case for the coprecipitation of nickel and aluminum which can form defined compounds of the hydrotalcite type (see the extensive review by Cavani et al. [9] and the summary by Andrew [10]). 

Figure 3. Possible implementations of precipitation processes (after [14]). In the batchwise process (a) the pH and all other parameters except for the temperature change continuously during the precipitation due to consumption of the metal species. Coprecipitation should be carried out in the reversed arrangement by addition of the metal species to the precipitating agent to avoid sequential precipitation. In process (b) the pH is kept constant, but the batch composition and the residence time of the precipitate change continuously. In process (c) all parameters are kept constant.

Another problem with the use of coprecipitation to prepare supported catalysts is that this procedure places a relatively large amount of the active metal inside the particles of the support material and, therefore, unavailable for reaction. A more efficient arrangement is to have the active material on or near the surface of the support particles. One way of accomplishing this is through a sequential precipitation procedure in which, for instance, a precipitate of Ni(OH)2 is formed on a freshly prepared suspension of Al(OH)3.20 in coprecipitation the metal content of the precipitate is reasonably continuous throughout the precipitate. As illustrated in Fig. 13.2, though, in a sequential precipitation the first drops of... 

Ternary systems have been prepared in this way as well. The sequential precipitation of aluminum hydroxide, lanthanum hydroxide and, finally, nickel hydroxide gave, after calcination and reduction, a lanthamun activated Ni/Al203 catalyst which had smaller metal crystallites and was somewhat more active than a catalyst prepared by the simultaneous coprecipitation of the three... 

After isolation the supported precipitate is washed, dried and usually calcined to produce a supported oxide which is then reduced, commonly in a hydrogen stream. Reduction of these supported oxides generally proceeds more readily than the mixed oxides produced by coprecipitation since there is only a monolayer in which there is a direct interaction of the active component with the support. This monolayer can be considered to be a silicate or aluminate which is more difficult to reduce than the oxide or hydroxide found in the outer metal-containing layers.33 Precipitation-deposition gives catalysts having compositions similar to those produced by sequential precipitation as shown in Fig. 13.2. 

In another method, designated the sequential precipitation method, an aqueous solution of ruthenium trichloride is contacted with the ammoniacal hydrazine solution (3). The resulting precipitate is filtered out of solution and reslurried in water, after which a solution of copper nitrate is added to the slurry. On subsequent addition of ammoniacal hydrazine solution, further precipitation occurs in the presence of the original precipitate. The total precipitate is then dried and reduced in the same manner as the coprecipitated preparations. 

There is a common misconception that coprecipitation generates a mix that is homogeneous on an atomic level. However, most coprecipitation reactions precipitate the metal ions sequentially, not simultaneously. Also, while atomically mixed batches would form crystals at much lower temperatures than mixtures of coarser particles, the coprecipitated batches seldom lower the reaction temperature by more than 100 Celsius degrees. 

A series of copper-zirconia catalysts have been prepared by methods of sequential precipitation, coprecipitation and deposition precipitation. The influence of various pretreatments and of the copper zirconia ratio on the structural and chemical properties of these samples are examined. High activity and selectivity of the catalysts is shown to be correlated to the presence of amorphous zirconia which is stabilized by copper ions. The results indicate that the structural and chemical properties of the support and particularly the interface copper/zirconia are most decisive in governing the catalytic properties of these methanol synthesis catalysts. 

Sample A was prepared in the same way except that zirconia was substituted by alumina. Pure zirconia was prepared analogously by precipitation of zirconyl nitrate. Sample C was made by coprecipitation instead of sequential precipitation. Sample H was prepared by the method of deposition precipitation using urea. A suitable amount of amorphous zirconia was suspended in deionised water. After the addition of copper(II)nitrat and urea the temperatme was brought to 363 K under constant stirring. The reaction was accompanied by a rise of the pH to a final value of 8. The final product was treated in the same way as the sequentially precipitated catalysts. 

Several additional coprecipitates were prepared with Nl/Al, Cu/Al and Cu/Cr in which sequential precipitation was conducted for atomic ratios of 0.1-0.2 with oxalic present prior to addition of the Ni or Cu to the A1 or Cr hydroxide. The results for these catalysts are summarized in Tables 3... 

Inclusions, occlusions, and surface adsorbates are called coprecipitates because they represent soluble species that are brought into solid form along with the desired precipitate. Another source of impurities occurs when other species in solution precipitate under the conditions of the analysis. Solution conditions necessary to minimize the solubility of a desired precipitate may lead to the formation of an additional precipitate that interferes in the analysis. For example, the precipitation of nickel dimethylgloxime requires a plT that is slightly basic. Under these conditions, however, any Fe + that might be present precipitates as Fe(01T)3. Finally, since most precipitants are not selective toward a single analyte, there is always a risk that the precipitant will react, sequentially, with more than one species. 

A series of sequential coprecipitations was conducted also with Cu/Al in an initial atomic ratio of 1 1 and with a molecular ratio of 1 1 with citric acid or oxalic acid (Table 1). In these systems Cu loadings in the range of 3.9-5.8 wt.% were obtained at pH values of 4.0 and 7.4 respectively. In this sequence the presence of citric acid resulted in a decreased state of dispersion as indicated by the BET area of the coprecipitate calcined at 350° C when compared with the reference system with no anionic present. On the other hand the system prepared with the oxalic acid present preadsorbed on the initially precipitated A1 hydroxide provided an order of magnitude increase in the dispersion as indicated by the BET area of the calcined oxide. 

---