Liquid-phase precipitation coprecipitation

In the geological and soil science literature, ion exchange and precipitation are frequently considered as adsorption and thermodynamically described by adsorption equations, or isotherms. This is not correct because, as shown previously, the processes are principally different adsorption is directed by the decrease of surface energy, and it takes place on the free surface sites ion exchange is just a competitive process on an already covered surface, determined by the ionic composition of the liquid phase. Precipitation, including colloid formation, is governed by the composition of the liquid phase, the crystal structure (coprecipitation), or primary chemical forces. 

Precipitation or coprecipitation methods are also often used. Suh et al. [40] analyzed the effect of the oxygen surface functionalities of carbon supports on the properties of Pd/C catalysts prepared by the alkali-assisted precipitation of palladium chloride on carbon supports, followed by liquid-phase reduction of the hydrolyzed salt with a saturated solution of formaldehyde. They observed that the metal dispersion increased with increasing amount of oxygen surface groups. Nitta et al. [41] also used a deposition-precipitation method, with sodium carbonate and cobalt chloride or nitrate, to prepare carbon-supported Co catalysts for the selective hydrogenation of acrolein. 

Scheme 13.3 was then adopted by us and other different research groups also for gas-phase applications. Coprecipitation, deposition-precipitation and colloidal particles immobilisation, discussed in Section 13.2, were the preferred methods for preparing catalysts. In our case, supporting finely dispersed gold on carbon by means of metal sol immobilisation led to the discovery of an active and selective catalyst for liquid phase oxidation. 

Mixed copper/zinc catalysts with high copper-to-zinc ratios are widely used as catalysts for low-pressure methanol production and for low-temperature shift reaction [2, 31], see also Chapter 15. These catalysts are commonly made by coprecipitating mixed-metal nitrate solutions by addition of alkali. Li and Inui [32] showed that apart from chemical composition, pH and temperature are key process parameters. Catalyst precursors were prepared by mixing aqueous solutions of copper, zinc, and aluminum nitrates (total concentration 1 mol/1) and a solution of sodium carbonate (1 mol/1). pH was kept at the desired level by adjusting the relative flow rate of the two liquids. After precipitation was complete, the slurry was aged for at least 0.5 h. When the precipitation was conducted at pH 7.0, the precipitate consisted of a malachite-like phase (Cu,Zn)C03(0H)2 and the resulting catalysts were very active, while at pH < 6 the formation of hydroxynitrates was favored, which led to catalysts less active than those prepared at pH 7.0 (Figure 7.8). 

Zhanlai D, Cunran A, Qiang L, Zhezhe H, Jianqiang W, Haibo Q, et al. Precipitation of ITO nanoparticle by liquid phase coprecipitation method. J Nanomater, Doi 10.1155/2010/543601. 

A classical method of the preparative fractionation of polymera consists of fractional precipitation based on the separation of molecules with respect to their molecular weight between two liquid phases, namely, the more concentrated gel phase and the dilute sol phase. Wth nylons (similar to other crystalline polymers) the separation efficiency with respect to molecular weight is unfavorably affected in numerous systems by allegation of polymer molecules near the precipitation point The partly crystalline gel phase may then contain a certain amount of the coprecipitated lower-molecular weight fractions. 

In the analysis of high-purity substances, general matrix removal is often very important so as to pre-concentrate the elements to be determined. To this aim all separation techniques such as ion exchange, liquid-liquid extraction of a metal complex with organic solvents, fractionated crystallization, precipitation and coprecipitation as well as electrochemical methods may be used (for a systematic treatment, see Ref. [300]). These principles can also be applied in on-line systems, as is now possible with solid phase extraction. Here matrix elements or the analytes can be adsorbed as complexes onto the column and eluted for direct determination by AAS. 

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