Mixed oxide aerogels

Single-source precursor approaches are becoming more and more attractive in the sol-gel synthesis of mixed oxide materials. Well-mixed Si/Ti binary oxides have been synthesized by Miller et al. They chose to use a diethoxysiloxane-ethyltitanate copolymer as a network-forming precursor since the Ti—O—Si linkage is already present in the precursor. This synthetic route avoids the problem associated with unequal hydrolysis rates of metal alkoxide precursors commonly used to prepare mixed oxide catalysts [60]. A more detailed discussion can be found in Section 25.4. 

In 2009, Yeung and coworkers reported the possibility to prepare freestanding Ti02—Si02 monoliths with ultralow densities and well-defined hierarchical pore structures [65]. For this purpose, they mixed a solution of titanium isopropoxide 

Another approach has been presented by Sun et al. Their methods rely on hydrolysis and condensation reactions in the highly viscous polymer matrix of polypropylene. Phase separation into silica and titania domains is prevented as 

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With the prehydrolysis of TEOS, we expect a homogeneous distribution of the two oxides in our titania-silica support Coupled with the fact that silica is the major component, this sample should behave as the "silica-like" sample prepared by Handy et al. (8) with a two-stage hydrolysis procedure. In other words, there are no crystalline titania domains in the sample, consistent with the X-ray diffraction and Raman results. This titania-silica mixed oxide aerogel is therefore less effective in stabilizing surface vanadia species (see Figure 3) and less active in SCR (see Table II) than pure titania aerogel as observed. 

Mixed oxide aerogels composed of titania, vanadia, and niobia have been shown to be highly active catalysts for the selective catalytic reduction (SCR) of nitric oxides with ammonia. These gels were characterized by FTIR, Raman, photoelectron spectroscopy, and secondary ion mass spectrometry [57-59]. 

Transition-metal mixed oxides active in combustion catalysis have been prepared by two main procedures i) classical coprecipitation / calcination procedures starting from metal nitrates and/ or alkoxides ii) preparation based on the supercritical drying of gels prepared from organic complexes (alkoxides, acetylacetonates or acetates), producing aerogels . Details on the second preparation can be found in Ref. 13. 

Phosphate and sulphate modifiers were incorporated by the addition of appropriate amounts of 0.01 M sulphuric or phosphoric acid to a pre-calcined aerogel followed by further calcination at 873 K. Samples are labeled as X-SiZr (y) where X refers to either sulphated (S) or phosphated (P) samples, and y refers to the mole ratio of sulphate/phosphate relative to zirconium in the preparation method. For comparison purposes, samples of zirconia and sulphated zirconia were also prepared. This was achieved via precipitation from zirconium isopropoxide (Aldrich 70 wt.%). The same H2O Zr propanol ratios were employed as used during the preparation of the mixed oxides. A sulphated zirconia, prepared by the use of sulphuric acid as hydrolysis catalyst was prepared for comparative purposes and had a nominal S Zr ratio of 0.30 1. A further sample was prepared where segregation of components was induced by thermal treatment by calcination at 1373 K for 6 h.of the non-treated SiZr (0)... 

Investigation of the sol-gel derived bismuih-molybdcnum-titanium xerogel and aerogel mixed oxide as catalyst for the oxidation of butadiene to furan l 14J 15] indicated activities and selectivities comparable to other suitable catalysts. The unique microstructure and good catalytic performance of the bismuth molybdenum oxide particles are attributed to lilania matrix. How ever, these favorable properties are limited to low temperature reaction conditions since both xerogels and aerogels are prone to rapid restructuring at elevated temperatures, which result in the loss of their unique redox properties. 

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