Silica-titania powders

IR-ATR, UV/vis, XPS, and XAFS measurements led to the conclusion that up to a Si Ti ratio of 35 1, well-dispersed titanium centers in a macro/mesoporous Si02 network with a specific surface area up to 582 m and mainly fourfold coordinated Ti centers have been obtained. An increase of the Ti content resulted in a decrease of the specific surface area, as well as a loss of the cellular character of the macroporous network and an increasing content of Ti in octahedral coordination. With the Si Ti ratio of 1 1, silica-titania powders with 100 m g and anatase domains within the Si02 matrix were obtained. 

Figure 12. Isoelectric points of DS-coated titanias O, titania dispersed in aqueous silica , dried, coated titania powders. The hose titania was 20 m2/g.

The modified electrode was a CP electrode with the organofunctionahzed silica-titania as modifier and was prepared as follows 1.0 g of graphite (powder), three different modifier masses (0.002, 0.004, and 0.005 g), as well as 0.32 g of mineral oil were ground in a mortar. Better results were observed for the 0.005 g electrode. 

The surface hydration-hydroxylation structure of titania, proved previously mainly by IR studies using dry titania powders, also seems to hold when these powders are dispersed in water. An interesting approach, therefore, is to probe directly the uptake of water from the gas phase by DS-coated rutile surfaces [42]. Water adsorption isotherms are presented in Figure 52.14. The dual nature of titania surface sites, a property not seen with other common oxides such as silica and alumina, leads to an unusual type of water adsorption isotherm for titania. The isotherm shows two distinct knees (Figure 52.14) connected by a region where adsorption increases linearly with the partial vapor pressure of water. The explanation for this adsorption behavior is rather complex [42] and beyond the scope of this chapter. This behavior is believed to be due to the presence of hydrated surface cation sites. 

Titania-supported vanadia catalysts have been widely used in the selective catalytic reduction (SCR) of nitric oxide by ammonia (1, 2). In an attempt to improve the catalytic performance, many researchers in recent years have used different preparation methods to examine the structure-activity relationship in this system. For example, Ozkan et al (3) used different temperature-programmed methods to obtain vanadia particles exposing different crystal planes to study the effect of crystal morphology. Nickl et al (4) deposited vanadia on titania by the vapor deposition of vanadyl alkoxide instead of the conventional impregnation technique. Other workers have focused on the synthesis of titania by alternative methods in attempts to increase the surface area or improve its porosity. Ciambelli et al (5) used laser-activated pyrolysis to produce non-porous titania powders in the anatase phase with high specific surface area and uniform particle size. Solar et al have stabilized titania by depositing it onto silica (6). In fact, the new SCR catalyst developed by W. R. Grace Co.-Conn., SYNOX , is based on a titania/silica support (7). 

Various aerosol processes have been developed for the generation of ultrafine powders at laboratory s e, such as flame (2), tube furnace (5), gas-condensation (4), thermal plasma (5), laser, sputtering and a variety of other aerosol processes named after the energy sources which are applied to provide the high temperatures during gas-to-particle conversion. However, until now, only flame processes have been scaled up to produce commercial quantities of ceramic particulates, such as silica, titania, etc., at low cost (about 1/lb). 

Titania powders are extensively used in pigments, as catalyst supports, and more recently in synthesis of inorganic membranes and as photocatalyst in gas and water purification. Silica particles have found applications as optical fibers, fillers. 

Xiong, Y., Akhtar, M.K., Pratsinis, S.E., 1993. Formation of agglomerate particles by coagulation and sintering—part II. The evolution of the morphology of aerosol-made titania, silica and sihca-doped titania powders. J. Aerosol Sci. 24 (3), 301-313. 

Titania powders not only with particulate morphology in different nano-sizes but also with fibrous morphology were synthesized. Even synthesis of nanotubes was reported under hydrothermal conditions from NaOH solution [26-30] and also nanofibers from KOH solution [31,32]. Both nanotubes and nanofibers thus prepared were later clarified to be protonated titania (titanate) [33-36]. A comprehensive review was published on protonated titanate nanotubes [37]. Effects of remnant sodium content and annealing temperature were studied on the structure and photoactivity of the nanotubes [38]. Titanate nanowires and nanoribbons were also reportedly formed [39,40]. Nano-sized Ti02 powders were obtained by annealing of titanate nanotubes and nanofibers [41]. Mesoporous anatase-type Ti02 powder was prepared by selective dissolution of silica component in Ti-Si binary oxides [42]. 

These processes are very rapid and allow the preparation of inorganic supports in one step. This technique allows large-scale manufacturing of supports such as titania, fumed silica, and aluminas. Sometimes the properties of the material differ from the conventional preparation routes and make this approach unique. Multicomponent systems can be also prepared, either by multimetallic solutions or by using a two-nozzle system fed with monometallic solutions [22]. The as-prepared powder can be directly deposited onto substrates, and the process is termed combustion chemical vapor deposition [23]. 

Titania, goethite and silica were used as adsorbents. Titanium dioxide (anatase) and silica powders were obtained commercially. Goethite was prepared from an FefNC solution by the precipitation of ferrihydrite.8 The suspension was held in a closed flask at 70°C for 60 hours. During this period the red brown suspension of ferrihydrite was converted to a yellow brown goethite. These oxides were washed with double distilled water to remove impurities, until the supernatant conductivity was below 2pS x cnT1. 

V. M. Gun ko, V. M. Bogatyrev, V. V. Turov, R. Leboda, J. Skubiszewska-Ziqba, L. V. Petrus, G. R. Yurchenko, O. I. Oranska, and V. A. Pokrovsky, Composite powders with titania grafted onto modified fumed silica, Powder Technology, submitted for publication. 

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