1212 coprecipitation at high supersaturation during

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Unformatted text preview: uration During the precipitation at high supersaturation the metal ions containing solution is simply quickly added to the basic solution or the solutions are mixed together rapidly. The concentrations of the solutions are rather high (> 2 mol/l). The precipitants are generally not very crystalline and sometimes even amorphous, owing to the high number of crystallisation nuclei created by the rapid mixing and the high concentrations of the solutions. Coprecipitation at low supersaturation Coprecipitation at low supersaturation at constant pH is the method most frequently used in the preparation of HTs. It usually yields the most crystalline precipitate. Normally, both the solution containing the metal salts containing solution and the base solution are low in concentration (0.5 – 2 mol/l). They are mixed together slowly in a 1. Introduction 11 ytrtrghrhrhhrhrhhfffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffpffffffffffffffffffffffffffffffffffffffffffffffff separate vessel at constant pH and at temperatures between 60-80°C. This method ensures the best condition to obtain crystalline material of pure HT structure. This is mainly due to the fact that at constant pH the rate of crystal growth is generally lower than the rate of nucleation. This method is most often used, as it is the most effective one. For example in the preparation of Cu/Co/Al hydrotalcites catalysts, Marchi et al. obtained an amorphous precipitate by coprecipitation at high supersaturation while under the conditions of low supersaturation a crystalline HT was precipitated [1]. In all three methods of coprecipitation the precipitants are washed with distilled water in order to eliminate adsorbed ions and are dried at temperatures up to 120°C [1, 25]. To improve the degree of crystallinity of the freshly precipitated HTs and to turn amorphous phases into crystalline ones, the precipitates can be hydrothermally treated. Two different forms of hydrothermal treatment are frequently applied [1, 25]. Hydrothermal treatment at low...
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