Preparation sequential 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]). 

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... 

Colloidal Pt/RuO c- (C5 0.4nm) stabilized by a surfactant was prepared by co-hydrolysis of PtCU and RuCls under basic conditions. The Pt Ru ratio in the colloids can be between 1 4 and 4 1 by variation of the stoichiometry of the transition metal salts. The corresponding zerovalent metal colloids are obtained by the subsequent application of H2 to the colloidal Pt/Ru oxides (optionally in the immobilized form). Additional metals have been included in the metal oxide concept [Eq. (10)] in order to prepare binary and ternary mixed metal oxides in the colloidal form. Pt/Ru/WO c is regarded as a good precatalyst especially for the application in DMECs. Main-group elements such as A1 have been included in multimetallic alloy systems in order to improve the durability of fuel-cell catalysts. PtsAlCo.s alloyed with Cr, Mo, or W particles of 4—7-nm size has been prepared by sequential precipitation on conductant carbon supports such as highly disperse Vulcan XC72 [70]. Alternatively, colloidal precursors composed of Pt/Ru/Al allow... 

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. 

The preparation method significantly influence the crystallinity of the materials synthesised. Compounds synthesised under low supersaturation (LS) conditions are more ciystalline than by sequential precipitation (SP). Furthermore, the ciystallinity also increases with increase in atomic ratio. Hydrothermal treatments increased the ciystallinity of the material. This result is corroborated with the reduction in the surface area of the hydrothermally treated samples. However, hydrothermal treatments performed on Co-Fe-COg-HTlcs resulted in the... 

The crystallinity of the material is also dependent on the natiure of the trivalent metal ion present in the network. C0-AI-CO3-HT are more oystalline in comparison with Fe and Cr containing samples. In the case of C0- -CO3-HT, preparation by sequential precipitation yielded amorphous material whereas preparation under low supersaturation resulted in a better oystalline material. In our Co-Fe containing samples, it is not completely possible to exclude the presence of hexagonal Co(OH)2 prepared under sequential precipitation. These results indicated that presence of Arfavours the formation of crystalline HTlc phase which is in accordance with the results observed by Clause et al ftn nickel containing hydrotalcites [7]. TEM results showed spherical to hexagonal platelets of thin and wide nature characteristic of these materials [3]. 

The effect of the preparation method (co-precipitation or deposition-precipitation) on the performance of PROX reactions was rationalized on the basis of sequential precipitation, which occurred due to the different solubilities of the hydroxide species formed during the CP stage. With Au/ceria, the gold hydroxide, with a much lower solubility product ( p = 5 x 10 ), should be formed before Ce(OH)3 with = 1.6 x lO , thus favoring the aggregation of gold... 

Titration of a solution containing salts of the metal cations to be precipitated with a basic solution, usually an alkaline metal hydroxide or carbonate [98, 99] is one of the simplest methods to prepare LDHs. A sequential precipitation of the metal hydroxides (forming particles with the less soluble hydroxide in the nuclei and an external shell formed by the hydroxide of the divalent cation), could be expected, but it is generally found that a true mixed hydroxide is formed it seems that precipitation of M(OH)j induces the simultaneous precipitation of MCOH). ... 

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... 

After the solution is diluted with 1200 mL of water, the pH is adjusted to 1-2 with coned hydrochloric acid and the mixture stirred at 5 C for 1 hr. The precipitated dlphenylphosphinic acid is isolated by filtration and washed with 50 mL of water (Note 6). The filtrate is placed in a 2000-mL, round-bottomed flask, equipped with a reflux condenser and an efficient mechanical stirrer. A solution of 20 mL of benzaldehyde in 50 mL of ether is added and the mixture is stirred vigorously for 20 min. The precipitate that forms is isolated by filtration and washed sequentially with 50 mL of water and 50 mL of ether to yield 19.6 g (69%) of 7-henzylideneaminotheophylline, mp 207-209°C.3 An analytical sample may be prepared by recrystallization from ethanol (mp 209°C). 

The following procedure describes the preparation and analysis of the (R)-a-methylbenzylamide of (R)-a-methylbenzenepropanoic add. A flame-dried, 10-mL, round-bottomed flask equipped with a Teflon-coated magnetic stirring bar and a rubber septum is charged with 25 mg (0.15 mmol) of (R)-a-methylbenzenepropanoic acid, 31 mg (0.23 mmol) of 1-hydroxybenzotriazole hydrate, 44 mg (0.23 mmol) of 1-(3-dimethylamino)propyl-3-ethylcarbodiimide hydrochloride, and 0.50 mL of anhydrous N,N-dimethylformamide. This mixture is stirred at 23°C for 10 min, then cooled to 0°C in an ice-water bath. To the cooled solution, 24 pL (0.19 mmol) of R-(+)-a-methylbenzylamine and 86 pL (0.62 mmol) of triethylamine are added. Within 1 min, a fine white precipitate appears. The mixture is stirred for 1 hr at 0°C, then warmed to 23°C. After stirring for 20 hr at 23°C, the mixture is transferred to a 30-mL separatory funnel with 10 mL of dichloromethane. The product solution is extracted, sequentially, with four 10-mL portions of 1 N aqueous hydrochloric acid solution, 10 mL of saturated... 

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