Titania thermal stability

Madia, G., Elsener, M., Koebel, M. et al. (2002) Thermal stability of vanadia-tungsta-titania catalysts in the SCR process, Appl. Catal. B, 39, 181. 

The thermal stability of mesoporous frameworks substantially increases with an increase in the wall thickness and pore size, which can be varied even for the same template by changing the processing conditions. Ozin et al.55 developed a way to prepare crystalline titania films with a 2D-hexagonal architecture by replacement of ethanol in the Pluronic-containing precursor solution with more hydrophobic butanol-1. The latter promotes phase separation at low surfactant-to-titania ratios, resulting in thicker pore walls, which are more compatible with the crystal growth during subsequent calcination. 

Crepaldi, E. L. Soler-Illia, G. Grosso, D. Sanchez, M. 2003. Nanocrystallised titania and zirconia mesoporous thin films exhibiting enhanced thermal stability. New J. Chem. 27 9-13. 

Commercial SCR catalysts are made of homogeneous mixtures of titania, tungsta and vanadia (or molybdena). Titania in the anatase form is used as a high surface and sulfur-resistant carrier to disperse the active components. Tungsta or molybdena is employed in large amounts (10 and 6% w/w, respectively) to increase the surface acidity and the thermal stability of the catalyst and to limit the oxidation of SO2. Vanadia is responsible for the activity in the reduction of NO, but it is also active in the oxidation of SO2. Accordingly, its content is kept low, usually below 1-2% w/w. 

Fig. IS. A drop in surface area marks the onset of sintering in a series of cogelled Cr/ siiica-titania catalysts. Titania decreases the thermal stability of the catalyst.

TP-Raman spectroscopy was used to evaluate the thermal stability of titanium oxide nanotubes (Blume, 2001). Titania nanotubes are more stable in oxidizing than in inert atmospheres. Titania nanotubes were transformed into anatase at 250 °C in an inert atmosphere, whereas the nanotube structure remained stable up to a temperature of 400 °C in the presence of air. It was proposed that oxygen prevents segregation of hydroxyl and Ti4+ ions and the ensuing reduction to Ti3+, which would otherwise lead to the generation of a nonstoichio metric anatase phase (Blume, 2001). 

The potential of rare earth compounds as catalytically active phases and promoters in pollution control, catalytic combustion, polymer production and in the fuel and chemical manufacture and thermal stabilizers for catalyst supports (alumina, silica-alumina, titania) need to be mentioned. Application of rare earths in alternate fuels technology (Fisher-Tropsch Processes, natural gas to transport fuel pathways) is also promising. 

Zr oxide. Recently, Chen et al also reported that doping with 10 mol % titania could significantly increase the thermal stability of Ti02-Zr02 mixed sample after post treatment with phosphoric acid solution [5]. Hereafter the 1 1 molar ratio was chosen to synthesize the Ti-Zr samples. 

It is well known that some auxiliary organic components such as mesitylene can expand the mesopores of titania [6] and silica [7]. Here, DDA, TritonX-100, triethanolamine and ethanol were chosen to control the mesophase structure of the Ti-Zr samples. Considering the fast condensation of Ti- or Zr- components, these organic components may help to slow down the condensation and improve the thermal stability. 

Since natural sunlight can only penetrate a few microns depth, the use of thin films of titania applied to ceramic or metallic supports as maintenance free decontamination catalysts for the photocatalytic oxidation of volatile organic compounds is of interest for the abatement or control of these emissions. The sol-gel technology can be readily incorporated as a washcoating step of the catalyst supports that may be subsequently heat-treated to fix the titania to the support. The surface area, porosity and crystalline phases present in these gels is important in controlling their catalytic activity. Furthermore, the thermal stability and development of porosity with heat-treatment was important if the sol-gel route is to be used as a washcoating step to produce thin films. 

Thermal stability. Thermal stability of several common ceramic and metallic membrane materials has been briefly reviewed in Chapter 4. The materials include alumina, glass, silica, zirconia, titania and palladium. As the reactor temperature increases, phase transition of the membrane material may occur. Even if the temperature has not reached but is approaching the phase transition temperature, the membrane may still undergo some structural change which could result in corresponding permeability and permselectivity changes. These issues for the more common ceramic membranes will be further discussed here. 

A sharp change in porosity of the titania membranes occurs at 350 C. This may be attributed to the crystallization of the constituent titania particles [Xu and Anderson, 1994]. Thus crystallization is likely to be a key factor affecting thermal stability of sol-gel derived titania membranes. 

On the other hand, the oder of thermal stability for model Ag catalysts in vacuum is somewhat different than for Pt, namely Ag/silica > Ag/alumina > Ag/carbon but in agreement with the findings of Seyedmonir et al. who found that thermal stability of conventional supported Ag catalysts in Oj decreases in the order Ag/silica > Ag/titania > Ag/alumina. ... 

PVA addition to zirconia precursor solutions resulted in an increase of the membrane pore volume and the pore size compared to samples without PVA. Similar effects were found for titania membranes but at lower temperatures indicating a relatively small thermal stability of these titania membranes. This can be improved by doping with alumina [39,40]. 

Kumar [39] reports a considerably larger thermal stability for titania membranes in the rutile phase instead of the usual anatase form. The effect of the support on thermal stability has been reported by Kumar et al. [40,41]. Pure, non-supported titania (anatase) membranes lose their porosity completely when calcined at 600°C for 8 h, where as the supported titania membrane retained ca 30% porosity at 900°C (8 h). Unsupported titania-(50 wt%)alumina composite membranes retained a porosity of ca 40% at 700°C (8 h), supported ones retained porosity even at 900°C. 

K. N.P. Kumar, K. Keizer and A.J. Burggraaf, Stabilization of the porous texture of nano structured titania by avoiding a phase transformation. /. Mater. Sci. Lett., 59 (1994) 13. Y. S. Lin, C.H. Chang and R. Gopalan, Improvement of thermal stability of porous... 

The thermal stability of the titania layer was found to vary with the metal substrate. On Pt, this oxide layer was found to diffuse into the bulk at high temperatures and segregate back to the surface at low temperatures.(19) On Rh, the titania layer did not disappear from the surface at high temperatures however, we did observe evidence for the migration of oxide... 

A disadvantage of HPA as catalysts lies in their relatively low thermal stability. It has been tried to stabilize them by supporting the HPA on several carriers as silica, alumina, titania [2, 3] and functionalized silica [4]. Nevertheless, the leaching of supported heteropolycompounds cannot beforehand be excluded when catalysts are used in heterogeneous liquid reactions. 

A titania (TiOa) sample with a large surface area (300 m g ) was chemically modified with 3-aminopropyltrimethoxysilane (APS) under a reflux of toluene. The thermal stability of the modified TiOa (APS-Ti02) and the adsorptivity of metavanadate anion (VO3 ) on APS-TiOz from an aqueous solution were investigated. Modification with APS suppressed crystal growth and transformation of anatase crystallite to rutile upon calcination, and the anatase phase was preserved even after calcination at 1000°C, while transformation to rutile in the unmodified TiOz samples was observed at around 800°C. Since there was little crystal growth in APS-TiOz, it possessed a large surface area of 205 m g after calcination at 700°C. The amount of adsorbed on APS-TiOz was ca. 

The use of other supports such as aluminas or aluminophosphates produces a similar Cr(VI) saturation behavior, although the exact saturation levels may differ somewhat for each temperature. Addition of promoters, to improve activity or modify polymerization behavior, such as fluoride, sulfate, phosphate, titania, etc., can also affect the thermal stability of the Cr(VI) species. These subjects are addressed separately in later sections. 

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