Graham Wallidge

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⩽x⩽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°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°C. These clearly show that the Zr environment depends on ambient moisture in addition to heat treatment.

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

A combination of 29Si and 17O 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(IV) 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. 17O MAS NMR and EXAFS have proven valuable techniques for gauging the level of atomic mixing in these materials.

Advanced physical characterisation of the structural evolution of amorphous (TiO2) x (SiO2)1−x sol-gel materials

Amorphous (TiO2)x(SiO2)1−x (x = 0.08, 0.18, and 0.41) sol-gel formed materials have been characterised by a combination of X-ray and neutron diffraction, infra-red and 29Si and 17O magic angle spinning NMR spectroscopy. This combination allows new insight into the fundamental structural role titanium additions play in silica, entering the network at x = 0.08 but largely phase separating at x = 0.41. At an intermediate value of x = 0.18 there is complex coordination behaviour with initially some Ti—O—Ti linkages forming and both TiO4 and TiO6 coordinations present. It appears that at 500°C for the x = 0.18 sample all titanium is present in Ti—O—Si linkages but that this situation is unstable on further calcination. The new information presented here is amalgamated with that from our previous EXAFS, XANES and SAXS on the same samples to produce the most complete view to date of the local and mesoscopic structural behaviour of the (TiO2)x(SiO2)1−x system produced by the sol-gel method.

Comparing the atomic structures of binary MO2-SiO2 (M = Ti, Zr or Hf) xerogels

The incorporation of transition-metal oxides into silica can give materials with useful optical, electronic or catalytic properties. For example, ZrO2-SiO2 and HfO2-SiO2 materials are of interest due to their high dielectric constants. Here we present a comparison of extended X-ray absorption fine structure and small-angle X-ray scattering results for acid-catalysed binary (MO2)x(SiO2)1 − x (M = Ti, Zr or Hf) xerogels, with x up to 0.4 and heat treatments up to 750°C. Detailed observations for TiO2-SiO2 and ZrO2-SiO2 xerogels provide a basis for interpretation of new results for HfO2-SiO2 xerogels. At low concentrations metal atoms are homogeneously incorporated into the silica network. Ti adopts coordinations of 4 or 6, and Zr and Hf both adopt higher coordination of 6 or 7 (the larger coordinations being due to ambient moisture). At higher concentrations, phase separation of metal oxide occurs. Such regions become clearly separated from the silica network for TiO2, but remain very finely mixed with silica network for ZrO2 and HfO2.

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

A combination of 29Si and 17O 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

Abstract 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 TiO 2 content. Unheated xerogels contain Ti with isolated, distorted 6-fold coordination ( [6] Ti ). Initial heating causes [6] Ti to be rapidly converted into 4-fold coordinated Ti( [4] Ti ), which upon cooling reverts to [6] Ti . Increased heat treatment creates more stable [4] Ti , 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 [4] Ti , and hence for understanding catalytic properties.

In situ high temperature x-ray diffraction measurements on a (TiO2)0.18(SiO2)0.82 xerogel using a curved image-plate

In situ high temperature x-ray diffraction measurements have been performed on a (TiO2 )0.18 (SiO2 )0.82 xerogel using a 185 mm radius curved image-plate. The results clearly show that the coordination of Ti in this material changes from predominantly sixfold to predominantly fourfold as the temperature is increased from 25 °C to 310 °C. An increase in the average O-O distance associated with this change is also identified. The use of the curved image-plate is shown to be a valuable technique for in situ studies of structural changes associated with thermal treatment of 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.

Transition metal atom sites in ternary ZrO2-TiO2-SiO2 xerogels

There has been much work on the binary TiO2-SiO2 and ZrO2-SiO2 materials prepared by sol-gel because of the beneficial properties resulting from incorporation of Ti and Zr. In contrast the ternary TiO2-ZrO2-SiO2 xerogels have been relatively little studied. We report the results of a study of those xerogels having Zr:Ti:Si ratios of 5:15:80, 10:10:80 and 15:5:80 heated to 750°C and to 1000°C. The study includes X-ray diffraction, small angle X-ray scattering, X-ray absorption spectroscopy at Ti and Zr K-edges, and 17O MAS-NMR. The study has benefited from close comparison with similar previous studies of the binary systems. The metal atoms in the ternary systems are shown to be predominantly homogeneously mixed in the silica network, as observed for the respective binary systems. The clear exception is for the sample with a minority of Zr, which after heat treatment at 750°C shows the presence of phase separation attributed to the formation of an amorphous precursor of ZrTiO4; at 1000°C this phase crystallises. In samples with higher Zr content the crystallisation of a ZrO2 tetragonal phase was observed. The data obtained illustrate well the strength of a research methodology in which a common batch of samples is studied using a coherent suite of modern structural probes.

In 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.