1214 hydrothermal treatment at low temperature

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Unformatted text preview: temperature Hydrothermal treatment at low temperature is also often referred as ‘ageing’. In its most simple form, it does not need special equipment. The freshly precipitated HT is kept in a heated aqueous suspension at temperatures below 100°C. Stirring can be applied [1, 25]. It generally increases the crystallinity of the precipitates. However, it has been reported for Cu containing HTs that this treatment led to the partial destruction of the HT structure with the formation of a malachite-like side phase [2, 25]. Hydrothermal treatment at high temperature Autoclaves must be used, because this treatment requires temperatures above 100°C and high pressure. The efficiency of the hydrothermal treatment at high temperature is 12 1. Introduction ytrtrghrhrhhrhrhhfffffffffffffffffffffffffffffffffpfffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff generally far larger than that at low temperatures. Under the conditions of hydrothermal treatment at high temperatures even the preparation of HTs from mechanical mixtures of the metal oxides in aqueous solution in the presence of CO 2 has been reported in the literature [1, 25]. Synthesis of HTs containing anions other than carbonate The synthesis of HTs with a wide range of different anions has been reported in the literature [1, 2, 4, 25, 27, 31, 33, 34, 35, 36, 38, 39, 40, 41, 42, 43, 45, 46, 47, 48, 49, 50, 51, 52]. These can be divided into five classes: - Simple halides and oxyanions [Cl-, ClO4-, CO32-, NO3-, SO42-, OH-, CrO42-, WO42-, S2O32-, etc.] - Iso- and heteropolyanions [V10O286-, Mo7O246-, PMo12O403-, PW 12O403-, NaP5W 30O11016- etc.] - Complex anions [Fe(CN)63-, Fe(CN)64- etc.] - Organic anions [carboxylates and dicarboxylates, benzenecarboxylates, alkylsulphates, chlorocinnamate etc.] - Layer compounds like the mineral chlorite [(Mg, Fe, Al)6[(Si, Al)4O10](OH)8] Of special interest is the synthesis of HTs with large and highly charged anions, because they increase the spacing between the brucite-like layers (gallery height) and decrease the anion population in the interlayer (Tab. 3). This can have several purposes like the enlargement of t...
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