94 J. Adv. Oxid. Technol. Vol. 10, No. 1, 2007 ISSN 1203-8407 © 2007 Science & Technology Network, Inc. TiO 2 -Based Materials for Toluene Photocalytic Oxidation: Water Vapor Influence Christophe Stavrakakis, Cécile Raillard, Valérie Héquet*, and Pierre Le Cloirec GEPEA, UMR CNRS 6144, Ecole des Mines de Nantes, DSEE, 4 rue Alfred Kastler, BP 20722, 44307 Nantes Cedex 3, France Abstract: Two photoactive TiO 2 -supported materials are considered for toluene degradation in the gas phase. A kinetic study is performed over these two catalysts. The mechanisms involved are well represented by the Langmuir-Hinshelwood (L-H) relation at initial time. The kinetic rates and adsorption constants are then calculated to compare the catalysts. Particular attention focuses on the role of the humidity on the photocatalytic activity. Conversion rates of the toluene into carbon dioxide are measured and reveal a singular difference between the catalysts. Benzaldehyde and benzoïc acid are identified as major by-products in the gas phase. The production of stable intermediates is also shown. It is concluded that not only the presence of water vapor but also the characteristics of the titanium materials have to be taken into account to explain the kinetic behavior of photocatalytic oxidation. Introduction Numerous studies deal with the use of TiO 2 - based photocatalysts in the form of powder or pellets (1, 3). This form of TiO 2 involves several technical constraints, particularly since a filtration step for recovery is necessary after the reaction. A major pro- blem associated when using unsupported photo- catalysts is that a large amount of catalyst is needed in reactors but only a thin layer absorbs the irradia- tion. For these reasons, and for a general improve- ment of photocatalytic processes, TiO 2 -supported catalysts are developed. Generally a thin layer of TiO 2 is coated on various supports such as for instance glass plates, silicon wafers, zeolites, ceramic membranes, and activated carbon fiber (3-5). The effect of humidity on the photochemical oxidation reactions in the gas phase has also been investigated because of its impact on the degradation rates (6). The presence of water vapor has both inhibiting and activating capabilities on the degradation of various volatile organic compounds (VOCs). Water vapor has been shown to react according to two main phenomena. On the one hand, water molecules are transformed into hydroxyl radicals $OH which are adsorbed at the titania surface, leading to a higher reaction rate. On the other hand, as the concentration of water vapor increases, a competitive adsorption between pollutants and water molecules occurs (7). It has also been reported that TiO 2 catalyst may be deactivated during the course of the VOC photo- oxidation. In many cases, catalyst deactivation has *Corresponding author; E-mail address: valerie.hequet@emn.fr been ascribed to the intermediates adsorbed on TiO 2 surfaces (1). Previous studies invoke the necessity of water for the regeneration of the catalyst (2, 7, 8). The objectives of the present study are to assess the behavior of toluene degradation on two different TiO 2 -based photocatalysts and the influence of water vapor in this particular case. Experimental and Methods TiO 2 -based materials Two different TiO 2 -based catalysts named GP25 (Degussa P25 TiO 2 on glass support) and SFA (ana- tase on fibrous support) are used for this assessment. The latter catalyst is an industrial commercialized material composed of mixed TiO 2 -SiO 2 , with an atomic ratio Ti/Si of 2/3 and deposited on a non- woven support. This support is constituted of cellu- lose and synthetic fibers with an organic binder. Titanium dioxide is of the anatase crystalline form and the amount of deposited TiO 2 is about 20 g.m S2 . More details such as optical photomicrographic observations are given in a previous paper (9). The catalyst GP25 is composed of powdered TiO 2 deposited on optically transparent glass slides as described elsewhere (10). Thus TiO 2 is a mixture of anatase (70%) and rutile (30%) crystalline phases. The average amount of TiO 2 deposited is 3.2 g.m S2 . Photocatalytic Batch Reactor The removal of toluene was performed in the gas phase over the two materials at 30 ± 1°C in a batch reactor. The 12-L reactor is equipped with a medium pressure mercury lamp (Heraeus TQ 718 Z4, 700 W, λ max = 365 nm). Relative humidity was set at 30% to Brought to you by | University of Sydney Authenticated Download Date | 8/6/18 3:53 PM