Thermal analysis data recorded from silicotungstic acid show the existence of stable phases of composition H4SiW12O40 center dot nH(2)O where n = 24, 14,6 and 0. FT-IR data confirms the presence of the Keggin anion, SiW12O404-, in each of these phases. In addition, dehydration of the higher hydrates (n > 6) is shown to occur via the loss of zeolitic type water followed by (n < 6) the removal of water associated with the acidic protons. Each of the hydrated phases, as well as the anhydrous phase, has been characterised by X-ray powder diffraction and the unit cell parameters are reported. Si-29 MAS NMR spectroscopy is consistent with a single silicon site in each of the four single phases and deshielding of the silicon nucleus in the range n = 6 to n = 0 is associated with the transfer of acidic protons from the H5O2+ ions to random locations on oxygen atoms in the Keggin unit. The observed increase in magnitude of T-1 (Si-29) for the hexahydrate is interpreted in terms of a reduction in motional freedom of the water molecules in this hydrate compared to those in the higher hydrates. Nominal hydrated states between n = 6 and 14, as well as those between n = 14 and 24, are shown by X-ray powder diffraction to be composed of the stable hydrated phases in the given ranges. (C) 2013 Published by Elsevier Ltd.

High‐surface‐area alumina–silica mixed oxide (Al2O3:SiO2) nanocatalysts have been prepared by a hybrid sol–gel method using boehmite (synthesized from aluminum nitrate) as the source of alumina and tetraethyl orthosilicate as the source of silica. The gels, after calcination at 400°C, result in mixed oxides with specific surface areas of 287 m2/g (Al2O3:SiO2=3:1) and 262 m2/g (Al2O3:SiO2=3:4). Further heating to 600°C produces materials with specific surface areas of 237 and 205 m2/g, respectively. The larger specific surface areas characteristic of the 3Al2O3:SiO2 samples are attributed, via transmission electron micrograph investigations, to the presence of ∼10 nm size, needle‐like particles having an aspect ratio of 1:50. Further addition of silica leads to the formation of larger needles of 20–75 nm size. Calcination at 600°C induced an approximately 5% decrease in the total pore volume for the 3Al2O3:SiO2 sample. In contrast, the material with Al2O3:SiO2=3:4 showed an approximately 12% increase in pore volume when heated at 600°C. The pore‐size distribution was in the range 1–3.5 nm with rmax at ∼2 and ∼2.5 nm at 600° and 800°C, respectively. Adsorption isotherms and pore‐size distribution analyses are discussed in some detail for the aluminosilicates at different calcination temperatures. This discussion is supported by structural information determined from FTIR and 27Al MAS NMR studies. Relatively high acidity values (0.234 mmol/g for Al2O3: SiO2=3:4) are observed for silica‐rich compositions consistent with their application as efficient acid catalysts.

Aluminosilicates with varying Al(2)O(3):SiO(2) molar ratios (3 : 1, 3 : 2, 3 : 3 and 3 : 4) have been synthesized using a hybrid sol-gel route using boehmite sol as the precursor for alumina and tetraethyl orthosilicate (TEOS) as the precursor for silica. The synthesis of boehmite sol from aluminium nitrate, and its use as the alumina precursor, is cost effective compared to alkoxide precursors. Structural aspects, including bonding and coordination, are studied in detail for samples calcined in the temperature range 400-1400 degrees C using both NMR and FTIR spectroscopy: the results are correlated with phase formation data (spinel and high temperature phases) obtained from XRD and thermal analysis. FTIR results show a broadening of peaks at 800 degrees C indicating a disordered distribution of octahedral sites caused by crosslinking between AlO(6) octahedral and SiO(4) tetrahedral units prior to the formation of mullite. (27)Al MAS NMR spectra are consistent with a progressive decrease in the number of AlO(6) polyhedra with increasing temperature corresponding to Al in these units being forced to adopt a tetrahedral coordination due to the increasing presence of similarly coordinated Si species. XRD results confirm the formation of pure mullite at 1250 degrees C for a 3Al(2)O(3):2SiO(2) system. At 1400 degrees C, phase pure mullite is observed for all compositions except 3Al(2)O(3):SiO(2) where alpha-Al(2)O(3) is the major phase with traces of mullite. The synthesis of aluminosilicates through a hybrid sol-gel route and the detailed insight into structural features gained from spectroscopic and diffraction techniques contributes further to the development of these materials in applications ranging from nanocatalysts to high-temperature ceramics.

W 4f and O 1s X-ray photoelectron spectra for silicotungstic acid, H4SiW12O40, in pure and silica-supported form are reported. W 4f XP spectra for the supported acid are analysed in terms of contributions from two W(VI) spin–orbit doublets arising from tungsten atoms in terminal WO bonds some of which directly interact with the silica surface. At low loading (3.2wt.%) significant changes in the relative contributions and binding energies of the two spin–orbit doublets are taken as evidence of a strong interaction of individual [SiW12O40]4− anions with highly active sites on the silica surface. It is suggested that selective ordering of silanol groups can occur on the silica surface in order to accommodate the adsorption of individual [SiW12O40]4− anions.

The preparation of a new oxide fluoride of composition Ba2SnO2.5F3·xH2O (x≈0.5) from the low-temperature (240°C) reaction between Ba2SnO4 and ZnF2 is reported. X-ray and neutron powder diffraction showed fluorination to result in a significant enlargement along the c-axis (by ca. 3Å) of the unit cell of the precursor oxide. A structural model based on the perovskite-related K2NiF4-type structure of this oxide is proposed in which there is direct replacement of oxygen in octahedral SnO6 units by fluorine, as well as the presence of F– at interstitial sites between BaO rock salt layers. Atomistic computer modelling indicates that apical fluorine substitution is favoured. The structural model is supported by the results of 19F and 119Sn MAS NMR spectroscopy as well as tin K- and barium K-edge EXAFS. Thermal analysis revealed the presence of water in the synthesized material and this is assigned to interstitial sites. 119Tin Mössbauer spectroscopy and tin K-edge XANES are consistent with enhanced withdrawal by substituted fluorine of electron density from Sn4+.

A case is presented for kinetic compensation effects (KCEs) reported for sets of heterogeneous rate processes to be classified. Because many of these KCEs do not conform to the ideal of isokinetic behavior, a quantitative method for describing the degree of compensation (kappa) is introduced. in the case of solid-promoted catalytic reactions, it is suggested that (unlike simple homogeneous reactions) the effective concentrations and dispositions of surface intermediates can vary with temperature. Thus, the rate constants vary differently with temperature for the individual reactions of a set and, thereby, influence apparent magnitudes of the measured Arrhenius parameters. As a consequence, compensation is observed for sets of chemically related reactions occurring within a common temperature interval. KCEs are thus linked to mechanistic information and, depending on the magnitude of K, for identifying shared features of catalytic rate controls. In this respect, the classification of KCEs is useful and can be based on similarities or differences between the reactants, catalysts, or experimental conditions in a given set of measurements. These ideas are demonstrated for selected sets of catalytic reactions. KCEs reported from a range of thermal decompositions of solids are also discussed. It is argued that, for mathematical as well as term definition reasons, compensation in this type of heterogeneous process is often artifactual and, therefore, cannot provide information of mechanistic value. Overall, it is concluded that a more thorough interpretation of compensation trends should lead to greater understanding of heterogeneous processes, particularly at the mechanistic level. (c) 2006 Wiley Periodicals, Inc.

The kinetic compensation effect (KCE) is a well-known behavior pattern wherein a set of related reactions show a linear relationship between the calculated Arrhenius parameters, log(10)A and E-a. Although various theoretical explanations have been advanced, none has yet found general acceptance. The present paper reports multiple rate measurements for the heterogeneously catalyzed oxidation of CO on several identically prepared samples of supported noble metal (Pd, Pt, or Rh) catalysts. Distinctive KCEs were observed for all three metals; these are discussed with reference to a new parameter, the degree of compensation (kappa), which quantitatively measures deviation from the ideal isokinetic relationship (kappa = 1.00 KCE). For carbon monoxide oxidation on each of the three metals, kappa was greater than 0.85 KCE, regarded as significant compensation. These KCEs are discussed in the context of published kinetic and mechanistic studies of CO oxidation, from which a theoretical explanation of the observed pattern of rate characteristics is proposed. overall kinetic control is ascribed to a common dominant, rate-determining process on each metal, resulting in approximately isokinetic behavior. Temperature-dependent secondary controls are identified as modifying the kinetics of surface reactions involving adsorbed intermediates, and thus the apparent magnitudes of Arrhenius parameters. Different contributions from secondary controls, between the individual reactions of each set, are attributed to temperature-determined variations of concentrations, equilibria, and mobilities of adsorbed participants in the (dominant) rate-controlling process. A modified Arrhenius equation, applicable to KCE reaction sets, is suggested in which the preexponential term varies with temperature. (c) 2006 Wiley Periodicals, Inc.

Periodic ab initio HF calculations using the CRYSTAL code have been used to calculate 23Na NMR quadrupole parameters for a wide range of crystalline sodium compounds including Na3OCl. An approach is developed that can be used routinely as an alternative to point-charge modelling schemes for the assignment of distinct lines in 23Na NMR spectra to specific crystallographic sodium sites. The calculations are based on standard 3-21G and 6-21G molecular basis sets and in each case the same modified basis set for sodium is used for all compounds. The general approach is extendable to other quadrupolar nuclei. For the 3-21G calculations a 1:1 linear correlation between experimental and calculated values of CQ(23Na) is obtained. The 6-21G calculations, including the addition of d-polarisation functions, give better accuracy in the calculation of η(23Na). The sensitivity of η(23Na) to hydrogen atom location is shown to be useful in testing the reported hydrogen-bonded structure of Na2HPO4.

Alumina–silica mixed oxide nano-catalyst materials with compositions 83.6wt.% Al2O3–16.4wt.% SiO2 (3Al2O3·1SiO2), 71.82wt.% Al2O3–28.18wt.% SiO2 (3Al2O3·2SiO2), 62.84wt.% Al2O3–37.16wt.% SiO2 (3Al2O3·3SiO2) and 56.03wt.% Al2O3–43.97wt.% SiO2 (3Al2O3·4SiO2) have been prepared by a hybrid sol–gel technique using boehmite as the precursor for alumina and tetraethoxysilane as that for silica. The bonding characteristics and coordination features around Al and Si in the mixed oxide catalysts have been studied using FTIR and 27Al MAS NMR after calcination at 400°C which is the temperature region where cross-condensation is seen to take place. A high BET specific surface area of 287m2g−1 is obtained for 3Al2O3·1SiO2 mixed oxide composition. The porosity features are further established by BET adsorption isotherms and pore size distribution analysis. The temperature-programmed desorption studies showed more surface active sites for the silica-rich composition, suggesting enhanced catalytic potential. The TEM features of the mixed oxides showed a homogeneous distribution of alumina and silica phases with particle sizes in the nano-range. The low silica-containing mixed oxide showed a needle-like morphology with a high aspect ratio of 1:50 and ∼10nm particle size while the silica-rich composition had particle size in a wide range (∼20–75nm).

As part of a study of ion migration mechanisms in feldspars, the dynamical behaviour of the alkali metal cations ions in albite and K-feldspar has been investigated using a combination of dielectric spectroscopy and atomistic computer simulation techniques. The low-frequency dielectric properties of these minerals have been studied from room temperature to 1100 K. At each temperature, the dielectric constant, conductivity and dielectric loss were determined over a range of frequencies from 100 Hz to 10 MHz. At high temperatures a distinct Debye-type relaxation in the dielectric loss spectra was observed for both albite and K-feldspar; the activation energy for these processes was determined to be 1.33 eV in both albite and K–feldspar. Atomistic simulation techniques were used to elucidate the mechanism and energetics of the cation migration processes. Mechanisms involving the conventional hopping of Na+ and K+ ions between cation sites in the (010) plane were found to give calculated energy barriers in good agreement with the experimentally determined activation energies. These results assist in understanding the nature of the processes responsible for the observed dielectric behaviour.

X-Ray absorption near-edge structure, extended X-ray absorption fine structure and X-ray photoelectron spectroscopy have been used to investigate the interaction of vanadium with both NH4+- and Eu3+-exchanged, dealuminated zeolite-Y. In both cases, samples were subjected to either calcination or steam treatment at temperatures in the range 100 to 800 °C. Following deposition of vanadium, in the form of vanadyl(IV) acetylacetonate, there is evidence of a weak interaction between vanadium and the zeolite-Y framework. Under the conditions used in the work, treatment at increasing temperature, either in air or steam, results in the complete oxidation of V(IV) to V(V) at temperatures in the range 300 to 500 °C. At temperatures exceeding 500 °C, V(V) is present in tetrahedrally-distorted vanadium oxide species containing terminal VO bonds, which are highly dispersed but bonded to the zeolite framework. These results show that the migration of vanadium into the zeolite-Y framework is not dependent on the presence of steam, nor is there any detectable influence of the exchanged Eu3+ cations.

Aluminium titanate precursor gels have been prepared by the sol–gel route using monohydroxy aluminium oxide (boehmite) sol and titanium isopropoxide. The influence of calcination temperature of the precursors on phase formation, densification and microstructure development in sol–gel aluminium titanate have been studied. Studies of pre-calcined precursor gels at 600, 800, 1000 and 1200°C show a significant increase in density on raising the calcination temperature from 600 to 1000°C. The treatment at 1000°C favours a phase composition most conducive to low temperature densification of aluminium titanate at 1350°C.

Using a combination of dielectric spectroscopy and atomistic computer simulation techniques, the dynamical behaviour of the loosely bound (Na+ and K+) channel ions in nepheline has been investigated. The low-frequency dielectric properties of a natural Bancroft nepheline have been studied from room temperature to 1100 K. At each temperature, the dielectric constant, conductivity and dielectric loss were determined over a range of frequencies from 100 Hz to 10 MHz. At high temperatures a distinct Debye-type relaxation in the dielectric loss spectrum was observed; the activation energy for this process was determined to be 1.38 ± 0.02 eV. Atomistic simulation techniques were used to elucidate the mechanism and energetics of cation migration. A mechanism involving the hopping of Na+ ions between oval sites and partially occupied hexagonal (K+) sites, via a bottleneck consisting of a distorted sixfold ring of (Al,Si)O4 tetrahedra, was found to give a calculated energy barrier in very good agreement with the experimentally determined activation energy. These results confirm the nature of the process responsible for the observed dielectric behaviour. Overall, this study demonstrates the intrinsic, microscopic control of cation diffusion processes in rock-forming minerals. Identifying specific energy barriers and preferred diffusion pathways is fundamental to the prediction of diffusion energetics.

Ab initio HF and DFT calculations, based upon both cluster and periodic modelling approaches, are reported for the efg tensor at sodium in NaNO2. Calculations based on different-sized clusters are compared and it is shown that resonable agreement with experiment can be obtained for a symmetrical cluster that extends beyond the immediate coordination environment of the sodium cation. The accuracy of calculations based on this cluster are, however, very dependent upon basis set. Hybrid DFT methods were not found to give significantly better results than HF methods. The periodic calculations are shown to give very good agreement with experiment for both a mid-sized, as well as a smaller standard 6-21G, basis set. In both cases the presence of d-polarisation functions on Na and O is essential to the accuracy of the calculations. HF methods were found to be superior to DFT methods in the approach used. It is suggested that periodic ab initio HF calculations using the standard 6-21G basis set, with suitable basis set optimisation to take into account the cationic nature of sodium, can provide a routine and consistent method for predicting sodium efg tensor information for ionic sodium compounds. Such a method would be important for assignment purposes in materials in which there are different crystallographic sites for the sodium cation.

The extended x-ray absorption fine structure (EXAFS) recorded from tin-doped alpha-Fe2O3, prepared by the mechanical milling of tin dioxide and alpha-Fe2O3 and by the hydrothermal processing of iron- acid tin-containing precipitates, can be interpreted in terms of a model in which tin occupies both substitutional octahedral sites and the interstitial octahedral sites in the corundum-related alpha-Fe2O3 structure. The EXAFS and Sn-119 Mossbauer spectra suggest that structural models derived from x-ray powder diffraction data do not adequately describe the complexity of the local environment of tin in alpha-Fe2O3. In particular, the EXAFS and Sn-119 Mossbauer spectra recorded from materials made by mechanical milling show evidence of more disorder. The Sn-119 Mossbauer spectra also indicate that the degree of order in the materials made by both methods is far from perfect and that the microstructural defects are highly sensitive to tin content and the preparative method.