Vapor phase synthesis of Al-doped titania powders

Abstract

The role of aluminum as dopant in gas phase synthesis of titania powders was experimentally investigated in an aerosol flow reactor between 1300 and 1700 K. Titania was produced by vapor phase oxidation of titanium tetrachloride in the presence of dopant aluminum trichloride vapor. The presence of aluminum altered the particle morphology from polyhedral to irregular crystals. Energy dispersive analysis and transmission electron microscopy indicated that the powders were mixtures of crystalline titania and amorphous alumina. Analysis by XPS indicated significant enrichment of aluminum on the particle surface. Some aluminum titanate (up to 17% by volume) was formed at 1700 K when a high concentration of AlCl3 was used (AlCl3/TiCl4 ≥ 0.07). Measurements of lattice parameters by x-ray diffraction indicated that aluminum formed a solid solution in titania. While titania synthesized in the absence of aluminum was about 90% anatase, the introduction of aluminum resulted in pure rutile at AlCl3/TiCl4 = 0.07. The effects of aluminum on titania phase composition and morphology are explained by the creation of oxygen vacancies in the titania crystallites and by the enhancement of the sintring rate of titania grains.

Characterization of Fumed Alumina/Silica/Titania in the Gas Phase and in Aqueous Suspension

“…Individual fumed alumina consists of the amorphous (Ϸ80%) and crystalline (Ϸ20%, ␥-Al 2 O 3 ) phases, but in the case of fumed alumina/silica, alumina and silica are totally amorphous (6), as well as in fumed Al 2 O 3 /SiO 2 /TiO 2 . One can assume that silica in Al 2 O 3 /SiO 2 /TiO 2 stabilizes the anatase phase in contrast to fumed alumina/titania (5), with the stabilized rutile phase due to the influence of alumina (with no silica) on the titania crystalline structure. Notice that the stabilization of the anatase phase and inhibition of the phase transition from anatase to rutile were observed (by means of the XRD method) for titania/silica previously (6).…”

“…can be synthesized by employing different methods, such as solgel, high-temperature hydrolysis, and chemical vapor deposition (CVD), and features of the synthetic techniques can strongly affect the structural and other physicochemical properties of these materials (1)(2)(3). Binary oxides Al 2 O 3 /SiO 2 (AS) and TiO 2 /SiO 2 (TS) have been studied considerably more (1,2) than Al 2 O 3 /TiO 2 (AT) or ternary Al 2 O 3 /SiO 2 /TiO 2 (AST) prepared by using the pyrogenic, plasma-sprayed, sol-gel, metal oxidation methods (3)(4)(5). Typically binary oxides Al 2 O 3 /TiO 2 are employed as catalysts or as catalyst supports (3).…”

“…For example, Akhtar et al (5) used AlCl 3 as a dopant during the pyrogenic synthesis (high-temperature hydrolysis of an admixture of TiCl 4 and AlCl 3 in an oxygen/hydrogen flame at ϳ1300 K) of titania and showed that it can alter the particle morphology to irregular crystals with a blend of crystalline titania and amorphous alumina and, according to the XRD measurements, aluminum forms a solid solution in titania. Usually, individual fumed titania consists of a major fraction with anatase (70 -85%) and a minor portion with rutile (2, 6), but the introduction of aluminum during the pyrogenic synthesis results in pure rutile that has been explained by the creation of oxygen vacancies in the rutile crystallites and by the enhancement of the sintering rate of titania grains (5). Also, Khalil et al (3m) synthesized bioinert ceramics with Al 2 O 3 / SiO 2 /TiO 2 using a hydrolysis sol-gel method and compared mullite, tialite, and other compositions in the ternary oxide region between aluminum titanate, silica, and titania.…”

“…Also, Khalil et al (3m) synthesized bioinert ceramics with Al 2 O 3 / SiO 2 /TiO 2 using a hydrolysis sol-gel method and compared mullite, tialite, and other compositions in the ternary oxide region between aluminum titanate, silica, and titania. Thus, on the basis of the known results (1)(2)(3)(4)(5)(6), one can assume that fumed Al 2 O 3 /SiO 2 /TiO 2 has considerable promise for different applications as an adsorbent, catalyst, catalyst support, etc.…”

“…Additionally, these oxides demonstrate not only Brønsted acid sites but also Lewis (L) acid sites (incomplete O-coordinated metal atoms), which (i.e., B-plus L-sites) are of importance not only for catalytic purposes but also for control of the particle size distribution in the aqueous suspensions, interaction with macromolecules in filled polymer systems, etc. (1)(2)(3)(4)(5)(6). Despite the numerous investigations of mixed oxides, many of the properties of such fumed binary and ternary oxides as alumina/titania and Al 2 O 3 /SiO 2 /TiO 2 have not been adequately explored in terms of the characteristics of the particle surfaces in air (oxide phase and active site distributions) or treated dispersions (particle size distribution, potential, dispersion stability, and durability).…”