Studies on cu containing hydrotalcites by extended x

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Unformatted text preview: ies on Cu containing hydrotalcites by extended X-ray adsorption fine structure 8 1. Introduction jjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjj (EXAFS), X-ray adsorption near edge structure (XANES), electron spin resonance (ESR) and X-ray photoemission spectroscopy (XPS) revealed that the hydroxide ions surrounding the Cu2+ ions are situated at the vertices of a distorted octahedra [29, 30, 31]. Tab. 2: Ionic radius of some divalent and trivalent cations (pm) [1, 25] M2+ Mg Cu Ni Co Zn Fe Mn Cd Ca 30 65 69 72 74 74 76 80 97 98 Al Ga Ni Co Fe Mn Cr V Ti In 50 M3+ Be 62 62 63 64 80 69 74 76 81 The second rule for the preparation of HTs concerns the ratio of the cations M(II)1-xM(III)x. The synthesis of pure HTs has been claimed in the range x = 0.1 to 0.5, but the formation of amorphous side phases not detectable by powder X-ray diffraction (XRD) may have occurred. These amorphous side-phases may be detected by thermogravimetry (TG), electron microscopy and solid state 27 Al MAS-NMR (magic angle spin-nuclear magnetic resonance) [1, 2, 25, 32]. The range 0.2 ≤ x ≤ 0.33 is generally acknowledged to be suitable for the synthesis of HT compounds. According to Brindley and Kikkawa [1, 2, 25], at x > 0.33 the increased number of neighbouring Al-containing octahedra leads to the formation of Al(OH)3 and, analogously, values of x < 0.2 lead to a high density of Mg-containing octahedra in the brucite-like sheets resulting in the precipitation of Mg(OH)2. However, these limits of the value of x must be regarded as the maximum interval, which can be narrower depending on the composition of the HT. For instance the successful synthesis of HTs of the metal composition Ni1-xAlx was reported only in the range of 0.25 ≤ x ≤ 0.33 [4, 6, 33]. 1. Introduction 9 The third rule is that the positive charge of the brucite-type layers due to the trivalent metal ions must be neutralised by the intercalation of anions according to: [ A n− ] 1 = ≤1 [M3+ ] n [An-] = amount of anions, n = charge of anion, [M3+] = amount of trivalent metal ions Though numerous anions have been intercalated there are some general limitations [1, 2, 25, 34, 35, 36]. Most important is that the anions a...
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