Synthesis, characterisation and performance of (TiO2)(0.18)(SiO2)(0.82) xerogel catalysts

The synthesis of high surface area xerogels has been achieved using the sol-gel route. Heptane washing was used during the stages of drying to minimise capillary pressures and hence preserve pore structure and maximise the surface area. SAXS data have identified that heptane washing during drying, in general, results in a preservation of the pore structure and surface areas of up to 450 m(2) g(-1). O-17 NMR showed that Ti is fully mixed into the silica network in all of the samples. XANES data confirm that reversible 4-fold Ti sites are more prevalent in samples with high surface areas, as expected. The calcined xerogels were tested for their catalytic activity using the epoxidation of cyclohexene with tert-butyl hydroperoxide (TBHP) as a test reaction, with excellent selectivities and reasonable percentage conversions. FT-IR spectroscopy has revealed that the catalytic activity is correlated with the intensity of the Si-O-Ti signal, after accounting for variations in Si-OH and Si-O-Si. The most effective catalyst was produced with heptane washing, a calcination temperature of 500 degreesC, and a heating rate of 5 degreesC min(-1)

Structure of (ZrO2)(x)(SiO2)(1-x) xerogels (x=0.1, 0.2, 0.3 and 0.4) from FTIR, Si-29 and O-17 MAS NMR and EXAFS

A combination of Si-29 and O-17 MAS NMR, EXAFS and FTIR spectroscopy has been used to study the atomic structure of(ZrO2)(x)(SiO2)(1-x) (x = 0.1, 0.2, 0.3 and 0.4) xerogels prepared by reacting partially hydrolysed tetraethyl orthosilicate with zirconium(rv) propoxide. Results from (ZrO2)(0.1)(SiO2)(0.9) samples reveal the oxides to be atomically mixed with no evidence of phase separation. In these samples, the nearest neighbour environment of zirconium is similar to that found in cubic zirconia. In the (ZrO2)(0.4)(SiO2)(0.6) samples, phase separation occurs with a significant proportion of the zirconium present as amorphous ZrO2 with a local structure similar to that of monoclinic zirconia. O-17 MAS NMR and EXAFS have proven valuable techniques for gauging the level of atomic mixing in these materials

Inhomogeneities in acid-catalyzed titania-silica and zirconia-silica xerogels as revealed by small-angle x-ray scattering

The small-angle x-ray scattering (SAXS) technique was used to investigate inhomogeneities on the scale of 10 to 600 Angstrom in acid-catalyzed titania-silica and zirconia-silica xerogels. SAXS of (TiO2)(SiO2)(1-x) and (ZrO2)(x)(SiO2)(1-x) xerogels with x 0.3 showed the presence of phase-separated regions of metal oxide, which were initially amorphous and crystallized at higher temperatures. A (TiO2)(0.18)(SiO2)(0.82) xerogel that was not initially phase separated became phase separated after heat treatment at 750 degrees C due to reduced solubility of Ti in the silica network

A structural study of(TiO2)(x)(SiO2)(1-x) (x=0.18, 0.30 and 0.41)xerogels prepared using acetylacetone

A combination of Si-29 and O-17 MAS NMR, EXAFS and FTIR spectroscopy have been used to study the atomic structure of (TiO2)(x)(SiO2)(1-x) (x = 0.18, 0.30 and 0.41) xerogels prepared using an acetylacetone stabilised Ti(OPri)(4) precursor. In the as-prepared materials, Ti is present in a;distorted octahedral coordination with a significant proportion still complexed by acetylacetone. For the first time in such an amorphous solid, a O-17 NMR resonance has been observed at 110 ppm which is attributed to (O-5)Ti-O Si groups. Heat treatment of these xerogels tends to convert TiO6 into TiO4 where the Ti is substituted into the silica network. Our results are in accord with previous work which suggests that the upper limit for solubility of TiO2 in SiO2 is cn. 15 mol%. Clear evidence of some phase separation in the (TiO2)(0.41)(SiO2)(0.59) sample after heat treatment to 750 degrees C is presented, although the O-17 MAS NMR results show that the use of acetylacetone significantly increases the amount of Ti-O-Si bonding at this composition compared to samples prepared without it

Structure of (Ta2O5)(x)(SiO2)(1-x) xerogels (x=0.05, 0.11, 0.18, 0.25 and 1.0) from FTIR, Si-29 and O-17 MAS NMR and EXAFS

A combination of Si-29 and O-17 MAS NMR, EXAFS and FT-IR spectroscopy has been used to study the atomic structure of (Ta2O5)(x)(SiO2)(1 - x) (x = 0.05, 0.11, 0.18 and 0.25) xerogels prepared by reacting partially-hydrolysed tetraethyl orthosilicate with tantalum(V) ethoxide. Amorphous tantala, a-Ta2O5, xerogels have also been prepared and their structures studied in detail for the first time. Results have shown that in all these materials, Ta adopts predominantly 5-fold coordination with respect to oxygen. For the mixed oxide xerogels, partial phase separation of the two component oxides occurs for x > 0.11

In-situ high-temperature XANES observations of rapid and reversible changes in Ti coordination in titania-silica xerogels

The height and position of the pre-edge peak in Ti K-edge X-ray absorption near-edge structure (XANES) is a sensitive indicator of the coordination of Ti. This method is used in situ to investigate Ti coordination in titania-silica xerogels with low TiO2 content. Unheated xerogels contain Ti with isolated, distorted 6-fold coordination (Ti-[6]). Initial heating causes [Ti-[6] to be rapidly converted into 4-fold coordinated Ti(Ti-[4]), which upon cooling reverts to Ti-[6]. Increased heat treatment creates more stable Ti-[4], which remains after cooling. Thus, the coordination of Ti depends on ambient conditions in addition to heat treatment. In-situ XANES is important for distinguishing different kinds of Ti-[4], and hence for understanding catalytic properties. (C) 1999 Elsevier Science B.V. All rights reserved

n Situ Studies of the Processing of Sol-Gel Produced Amorphous Materials Using Xanes, Saxs and Curved Image Plate XRD

Sol-gel produced mixed oxide materials have been extensively studied using conventional, ex situ structural techniques. Because the structure of these materials is complex and dependent on preparation conditions, there is much to be gained from in situ techniques: the high brightness of synchrotron X-ray sources makes it possible to probe atomic structure on a short timescale, and hence in situ. Here we report recent results for mixed titania- (and some zirconia-) silica gels and xerogels. Titania contents were in the range 8-18 mol%, and heat treatments up to 500 degrees C were applied. The results have been obtained from intrinsically rapid synchrotron X-ray experiments: i) time-resolved small angle scattering, using a quadrant detector, to follow the initial stages of aggregation between the sol and the gel; ii) the use of a curved image plate detector in diffraction, which allowed the simultaneous collection of data across a wide range of scattering at high count rate, to study heat treatments; and iii) X-ray absorption spectroscopy to explore the effects of ambient moisture on transition metal sites

A rare earth L-3-edge EXAFS and L-1-edge XANES study of Ce, Nd and Eu phosphate glasses and crystals in the composition range from metaphosphate to ultraphosphate

Rare earth (R) phosphate glasses, (R2O3)(x)(P2O5)(1-x), can be prepared with compositions in the range from ultra-phosphate,x = 0.17 (RP5O14), to metaphosphate, x = 0.25 (RP3O9), and it is important to know whether the R-O coordination changes significantly with composition. In rare earth phosphate crystals, the number of nearest neighbour oxygens changes from eight for ultraphosphate to six for metaphosphate. These R-O correlations are clearly distinguished in L-3-edge extended X-ray absorption spectroscopy (EXAFS) of NdP5O14 and EuP3O9 crystals. Samples of Ce (x = 0.197 and 0.235), Nd (x = 0.187) and Eu (x = 0.218) phosphate glasses all show the same EXAFS and L-1-edge X-ray absorption near edge structures (XANES) results. The results indicate an R-O coordination with six nearest neighbour oxygens, a similar level of static disorder to that in rare earth phosphate crystals and reduced centrosymmetry. There is no evidence for a change in R-O coordination with changing x. (C) 2001 Elsevier Science B.V. All rights reserved

Changes in the Zr environment in zirconia-silica xerogels with composition and heat treatment as revealed by Zr K-edge XANES and EXAFS

X-ray absorption spectroscopy at the Zr K-edge is an important technique for probing the environment of Zr. Here it is applied to zirconia-silica xerogels with composition 0.07 less than or equal to x less than or equal to 0.40, where x is the molar ratio Zr : (Zr + Si). Reference samples of crystalline ZrO2, ZrSiO4, BaZrO3 and liquid Zr n-propoxide were also examined. New XANES (X-ray adsorption near edge structure) results are presented for zirconia-silica xerogels, and compared with previous EXAFS (extended X-ray absorption fine structure) results. For high Zr contents (x = 0.4) there is a separate, amorphous ZrO2 phase, which before heat treatment is similar to Zr hydroxide, and after heat treatment at 750 degrees C is similar to an amorphous precursor of tetragonal ZrO2. For low Zr contents (x = 0.1) there is atomic mixing of Zr in the SiO2 network, and the environment of Zr is more similar to that in Zr n-propoxide compared to other reference samples. New in situ XANES and EXAFS results are presented for x = 0.1 xerogels heated at 250 degrees C. These clearly show that the Zr environment depends on ambient moisture in addition to heat treatment

Synchrotron-based studies of transition metal incorporation into silica-based sol-gel materials

Previous structural studies on titania- and zirconia-silica xerogels have shown the occurrence of homogeneous mixing at low metal content, and phase separation at high metal content. The use of additional, complementary, synchrotron-based methods can contribute to a fuller structural description of these materials. We present new X-ray absorption near edge structure (XANES) and SAXS results for (ZrO2)(x)(SiO2)(1-x) xerogels and compare them with previous results for (TiO2)(x)(SiO2)(1-x) xerogels. Significant differences between (TiO2)(x)(SiO2)(1-x) and (ZrO2)(x)(SiO2)(1-x) xerogels are observed in the affects of heat treatment on the coordination of homogeneously mixed metal atoms, and in the development of phase separated metal oxide regions