Studies on cu containing hydrotalcites by extended x

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

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...
View Full Document

Ask a homework question - tutors are online