Browsing by Author "Warrier, KG"

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    Aluminosilicates with varying alumina-silica ratios: synthesis via a hybrid sol-gel route and structural characterisation
    (DALTON TRANSACTIONS, 2010) Nampi, PP; Moothetty, P; Berry, FJ; Mortimer, M; Warrier, KG
    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.
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    Calcination and associated structural modifications in boehmite and their influence on high temperature densification of alumina
    (CERAMICS INTERNATIONAL, 2011) Nampi, PP; Ghosh, S; Warrier, KG
    A systematic study is reported on the calcination of boehmite and its associated structural changes, and their effect on densification features. Boehmite precursor gels have been calcined in the temperature range 250-1200 degrees C. The associated structural changes are identified by FTIR and XRD. The specific surface area measurements indicated a relatively high value of 169 m(2)/g for bochmite calcined at 400 degrees C; this value reduced to 4 m(2)/g on calcination at 1200 degrees C. In the temperature range 400-1000 degrees C, the coordination of aluminium changes from a quasioctahedral to a tetrahedral nature, which reverts to octahedral at 1200 degrees C. The precursor containing gamma-alumina gives a 92.1% theoretical density, on sintering at 1500 degrees C due to the highly unstable quasioctahedral coordination. Boehmite precursors calcined at 400 degrees C and 1000 degrees C produced a density of 88.2% and 96.9%, respectively, in the sintered compact at 1500 degrees C. Boehmite calcined to a-alumina (1200 degrees C) possesses an octahedral structure having a density of 97.6% at 1500 degrees C. (C) 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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    High-Surface-Area Alumina-Silica Nanocatalysts Prepared by a Hybrid Sol-Gel Route Using a Boehmite Precursor
    (JOURNAL OF THE AMERICAN CERAMIC SOCIETY, 2010) Nampi, PP; Moothetty, P; Wunderlich, W; Berry, FJ; Mortimer, M; Creamer, NJ; Warrier, KG
    High-surface-area alumina-silica mixed oxide (Al(2)O(3):SiO(2)) 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 degrees C, result in mixed oxides with specific surface areas of 287 m2/g (Al(2)O(3):SiO(2)=3:1) and 262 m2/g (Al(2)O(3):SiO(2)=3:4). Further heating to 600 degrees C produces materials with specific surface areas of 237 and 205 m2/g, respectively. The larger specific surface areas characteristic of the 3Al(2)O(3):SiO(2) samples are attributed, via transmission electron micrograph investigations, to the presence of similar to 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 degrees C induced an approximately 5% decrease in the total pore volume for the 3Al(2)O(3):SiO(2) sample. In contrast, the material with Al(2)O(3):SiO(2)=3:4 showed an approximately 12% increase in pore volume when heated at 600 degrees C. The pore-size distribution was in the range 1-3.5 nm with r(max) at similar to 2 and similar to 2.5 nm at 600 degrees and 800 degrees 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 Al(2)O(3): SiO(2)=3:4) are observed for silica-rich compositions consistent with their application as efficient acid catalysts.