Synthesis of Titania-Silica Aerogel Monolith for PCO Reactions

Shengli Cao
Environmental Engineering Program, the Hong Kong University of Science and Technology

King Lun Yeung
Department of Chemical Engineering, the Hong Kong University of Science and Technology

Po Lock Yue
Department of Chemical Engineering, the Hong Kong University of Science and Technology

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     Last modified: February 13, 2006

Abstract
Abstract

Titanium dioxide has been widely used as a catalyst for the photocatalytic oxidation (PCO) of organic pollutants in air streams [1]. TiO2 powder is coated onto plates, ceramic honeycombs and reticulated glass supports to prevent the loss of catalyst and to increase the contact and irradiated areas. However, the presence of a support can significantly reduce light penetration and adds to the total cost and weight of the reactor. This work investigates the preparation, characterization and performance of a lightweight freestanding titania-silica aerogel monolith for the remediation of airborne pollutants.

The titania-silica aerogel was prepared by a modified sol-gel method. Tetramethyl orthosilicate (TMOS) in ethanol was prehydrolyzed by HNO3 solution. Organic ligand was added into titanium isopropoxide (TIP)/ethanol solution in order to inhibit its hydrolysis rate. The prehydrolyzed TMOS and modified TIP were mixed together in 1 Ti:1 Si atomic ratio and further hydrolyzed with ammonia solution. The clear solution turned into a solid alcogel and further aged, followed by ethanol/CO2 supercritical drying and calcination in air.

Crack-free monolithic titania-silica aerogels can be obtained by both high-temperature ethanol supercritical drying and low-temperature CO2 supercritical drying. The samples are lightweight (ca. 0.5 g cm-3) and freestanding. Both samples display mesoporous pore structure, larger surface area (> 300 m2/g) and pore volume (> 1 cm3/g) compared to traditional TiO2 powders (i.e., Degussa P25). X-ray diffraction shows that 10.5 nm anatase TiO2 crystals are detected after ethanol supercritical drying and calcination but only amorphous TiO2 is observed after CO2 supercritical drying and calcination.

Photocatalytic oxidations of trichloroethylene (TCE) and isopropanol (IPA) were carried out for the monolithic titania-silica aerogels. The samples weighed 0.137 g, had a diameter of 12 mm, a thickness of 3 mm and a Ti/Si ratio of 1. The aerogel prepared by low-temperature CO2 supercritical drying did not contain anatase TiO2 phase and was not active for PCO reactions. However, the aerogel prepared by high-temperature ethanol supercritical drying was active. Photocatalytic oxidation tests were performed in a flat, rectangular stainless steel photoreactor [2], in which air flowed over the aerogel monolith surface. The removal efficiencies were 9.7 % and 10.1 %, respectively for TCE (113 ppm, 400 sccm of air) and IPA (268 ppm, 400 sccm of air), which were much higher than Degussa P25 with similar morphology under same reaction conditions (1.5 % and 1.3 %, respectively). Tests were also conducted in a stainless steel cell reactor, in which air was forced to pass through the network of the aerogel monolith body. The removal efficiency could also reach over 10 % for TCE (113 ppm, 40 sccm of air). These results showed that the freestanding, titania-silica aerogel monolith is applicable as the self-support catalyst for photocatalytic oxidation of airborne organic pollutants.


References

[1] J. Peral, X. Domenech, D. F. Ollis, J. Chem. Technol. Biotechnol., 70 (1997) 117.
[2] A. J. Maira, K. L. Yeung, C. Y. Lee, P. L. Yue, C. K. Chan, J. Catal., 192 (2000) 185.