The sol-gel synthesis of metal oxides can be performed via the hydrolysis and condensation of metal cations in aqueous solutions. This inorganic route provides a cheap and reliable way for industrial uses. However, many hydrolyzed precursors can be present simultaneously in the precursor solution and very few data are available on the formation of condensed species. The so-called Partial Charge Model provides a useful guide to describe and predict hydrolysis and condensation reactions in aqueous solutions. A charge-pH diagram can be established. It shows how condensation can be initiated via acid-base or redox reactions. The two basic condensation processes, olation and oxolation, are then discussed in terms of a charge-electronegativity diagram. They can lead either to small solute condensed species (polyanions or polycations) or to the formation of an infinite network (colloids, gels, or precipitates). Complexation by anionic species is finally discussed in the frame of an electronegativity-pH diagram. It also plays an important role during the formation of solid phases.

Aqueous chemistry of metal cations: Hydrolysis, condensation and complexation

\ensuremath{\gamma}-alumina $(\ensuremath{\gamma}\ensuremath{-}{\mathrm{Al}}_{2}{\mathrm{O}}_{3})$ derived from boehmite has historically been described as having a cubic spinel structure with $\mathrm{Fd}3\ifmmode\bar\else\textasciimacron\fi{}m$ symmetry, despite reports of tetragonal distortion in the structure. Based on neutron diffraction, transmission electron microscopy, and magic angle spinning NMR data, we propose a tetragonal model for the structure of boehmite-derived $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ with ${I4}_{1}/amd$ space group symmetry, a maximal subgroup of $\mathrm{Fd}3\ifmmode\bar\else\textasciimacron\fi{}m.$ It is also demonstrated that an accurate average structural model cannot be achieved if the cations are restricted to spinel positions.

Tetragonal structure model for boehmite-derived γ-alumina

R-Alumina results from the complete dehydration of several minerals of the form Al2O3‚nH2O. The "transition" aluminas, A-alumina, -alumina, and ‰-alumina are known to have a spinel structure but the possibility that they contain hydrogen (H) has been the subject of debate. We present a series of density- functional theory calculations which, together with available experimental data, show that the spinel aluminas exist over a range of hydrogen content captured by the empirical formula H3mAl2-mO3, with different greek- letter phases corresponding to different distributions of the Aluminum (Al) ions on the two cation sublattices. Calculations of densities and vibrational frequencies of bulk OH bonds are in excellent agreement with available data. The theory reconciles seemingly inconsistent data and reveals a remarkable property of the spinel aluminas: They are "reactive sponges" in that they can store and release water in a reactive way. This chemical activity offers a basis for understanding long-standing puzzles in the behavior of aluminas in catalytic systems.

Hydrogen and the Structure of the Transition Aluminas

Hydrated boehmite with nano-sized crystallites AlO(OH)·08H2O are obtained through a very reproducible sol–gel procedure and characterized using powder X-ray diffraction, in-situ IR spectroscopy, thermal analysis, BET surface area and photoelectron spectroscopy The unit cell parameters, obtained after deleting the first shifted (020) diffraction peak, are a = 3686 A, b = 12179 A and c = 2855 A They are consistent with well-crystallized boehmite and with the position of the harmonic (080) diffraction peak For the (020) peak, the correlation between position and peak width is confirmed for nano-crystallites The average shape of the crystallites, determined from three peak widths, corresponds to slabs with dimensions 79 × 27 × 87 nm Based on this crystallite shape, a model of hydrated boehmite is proposed and the formula corresponding to a full monolayer of chemically adsorbed water molecules is AlO(OH)·055H2OThe thermal evolution of hydrated boehmite leads first to hydrated γ-alumina Al2O3·033H2O = Al2O267(OH)066 and the number of remaining hydroxyl groups is critical for the porosity Between Al2O3·033H2O and Al2O3·02H2O the surface area remains approximately constant (300 m2 g−1) and the hydroxyl density decreases deeply For calcination temperature higher than 800 K, the loss of the remainder of the water leads to a strong decrease of this area The limiting value corresponds to about 9–10 OH-groups nm−2 and can be related to the hydrogen spinel HAl5O8 The same type of isolated OH groups are present on hydrated boehmite and hydrated transition alumina (IR bands at 3670 and 3730 cm−1) A simple model of partial dehydroxylation of boehmite is proposed, in agreement with the remaining water and unit cell parameters

Preparation of alumina through a sol–gel process. Synthesis, characterization, thermal evolution and model of intermediate boehmite

We have performed an extensive computational study of {gamma}-Al{sub 2}O{sub 3}, beginning with the geometric analysis of approximately 1.47 billion spinel-based structural candidates, followed by derivative method energy minimization calculations of approximately 122 000 structures. Optimization of the spinel-based structural models demonstrated that structures exhibiting nonspinel site occupancy after simulation were more energetically favorable, as suggested in other computational studies. More importantly, none of the spinel structures exhibited simulated diffraction patterns that were characteristic of {gamma}-Al{sub 2}O{sub 3}. This suggests that cations of {gamma}-Al{sub 2}O{sub 3} are not exclusively held in spinel positions, that the spinel model of {gamma}-Al{sub 2}O{sub 3} does not accurately reflect its structure, and that a representative structure cannot be achieved from molecular modeling when the spinel representation is used as the starting structure. The latter two of these three findings are extremely important when trying to accurately model the structure. A second set of starting models were generated with a large number of cations occupying c symmetry positions, based on the findings from recent experiments. Optimization of the new c symmetry-based structural models resulted in simulated diffraction patterns that were characteristic of {gamma}-Al{sub 2}O{sub 3}. The modeling, conducted using supercells, yields a more accurate andmore » complete determination of the defect structure of {gamma}-Al{sub 2}O{sub 3} than can be achieved with current experimental techniques. The results show that on average over 40% of the cations in the structure occupy nonspinel positions, and approximately two-thirds of these occupy c symmetry positions. The structures exhibit variable occupancy in the site positions that follow local symmetry exclusion rules. This variation was predominantly represented by a migration of cations away from a symmetry positions to other tetrahedral site positions during optimization which were found not to affect the diffraction pattern. This study has provided further insight of the defect structure of {gamma}-Al{sub 2}O{sub 3} which is necessary for the understanding and optimization of properties. This work also demonstrates the advantages of prior use of geometric analysis and interatomic potentials to assess a large number of structural possibilities, before striving to achieve high accuracy with DFT on promising cases.« less

Determination of the structure of y-alumina from interatomic potential and first principles calculations: The requirement of significant numbers of nonspinel positions to achieve an accurate structural model

R.M. Pearson

Papers

Water Content of Pseudoboehmite: A New Model for Its Structure

Wide line nuclear magnetic resonance studies on transition aluminas—Distribution of protons between surface and bulk phases

Measurement of Bronsted acid sites on aluminum oxide surfaces using deuterated pyridine and wide line nuclear magnetic resonance