Atomic Structure Analysis of Nanocrystalline Boehmite AlO(OH)

Abstract

Nanocrystalline n-AlO(OH) (Disperal P2 ® of Sasol) was investigated by means of the atomic pair distribution function (PDF). The PDF is derived from powder diffraction data, an ideally resolved PDF is obtained from a synchrotron source which provides a large maximal scattering vector length Q max > 300 nm -1 . Here, however, we were able to reveal atomic structure details of the ~ 4 nm particles from in-house diffraction data (Q max = 130 nm -1 ): PDF least squares model refinements show that n-AlO(OH) is of the layered Boehmite structure type (oC16, Cmcm). But the structure is uniformly distorted in domains of ~ 2 nm size within the nano particles. The hydrogen bonds between the layers are widened up significantly by +13 pm, accounting for the higher reactivity when compared to microcrystalline Boehmite. Our results from only one "nanocrystallographic" experiment are consistent with a trend which was found via the Rietveld technique on a series of AlO(OH) of different crystallite size (Bokhimi et al., 2001 [2]). In addition, the PDF contains information on structural distortion as a function of (coherence) domain size within the nano particles.

Synthesis and characterisation of new MO(OH)2(M = Zr, Hf) oxyhydroxides and related Li2MO3salts

“…19 Yawata reported on recovering lithium ions from hot spring water using H 2 TiO 3 , obtained from Li 2 TiO 3 by an ion exchange reaction. 20 Recently, we showed that all three MO(OH) 2 (M = Ti, Zr, Hf ) oxyhydroxides can be synthesised in solid form by an ion exchange reaction in aqueous acetic acid solution from Li 2 MO 3 (M = Ti, Zr, Hf ) precursors. 19,[21][22][23] It is worth mentioning that the precursors in ref.…”

“…[19][20][21][22][23] have been obtained by standard solid-state synthesis at temperatures 600-700°C, 19,21 which are significantly lower than the temperatures used during the solid-state synthesis of the above-mentioned lithium salts. [24][25][26][27][28][29][30] In the case of TiO(OH) 2 we showed that decreasing the synthesis temperature is crucial for obtaining oxyhydroxides. Indeed, only precursors annealed at low temperatures allow the complete exchange of Li + by H + (the extent of exchange of lithium by protons in Li 2 TiO 3 obtained at 800-1000°C was found to be less than 50%).…”

“…21,22 We proposed that this difference is probably caused by a high concentration of planar defects in the low-temperature modification of the Li 2 TiO 3 precursor. 23 However, till now there are neither data concerning details of the crystal structure of precursors which present the possibility of an ion exchange reaction nor data concerning the crystal structure of MO(OH) 2 (M = Zr, Hf ). So far, only a crystallographic study of TiO(OH) 2 has been performed.…”

“…23 However, till now there are neither data concerning details of the crystal structure of precursors which present the possibility of an ion exchange reaction nor data concerning the crystal structure of MO(OH) 2 (M = Zr, Hf ). So far, only a crystallographic study of TiO(OH) 2 has been performed. Tarakina et al 23 showed that titanium oxyhydroxide belongs to the family of layered double hydroxides and can be described as a stacking of charge-neutral metal oxyhydroxide slabs [(OH) 2 OTi 2 O(OH) 2 ].…”

“…So far, only a crystallographic study of TiO(OH) 2 has been performed. Tarakina et al 23 showed that titanium oxyhydroxide belongs to the family of layered double hydroxides and can be described as a stacking of charge-neutral metal oxyhydroxide slabs [(OH) 2 OTi 2 O(OH) 2 ]. It possesses a lot of disorder in the structure, partly preserved from the structure of the Li 2 TiO 3 precursor.…”