Effect of Ozone Pretreatment on CO Oxidation Catalysts

Ka Yee HO
Environmental Engineering Program, the Hong Kong University of Science & Technology

*King Lun YEUNG
Department of Chemical Engineering, the Hong Kong University of Science & Technology

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     Last modified: February 1, 2006

Abstract
Titanium dioxide supported gold catalysts exhibit excellent activity for CO oxidation even at temperatures as low as 90 K [1]. The key is the high dispersion of the nanostructured gold particles over the semiconducting TiO2 support. The potential applicationsof ambient temperature CO oxidation catalysts include air purifier, gas sensor and fuel cell [2]. This work investigates the performance of Au/TiO2 pretreated by calcination and by ozone.

The nanostructured TiO2 was prepared by a modified sol-gel method [3]. The titanium isopropoxide was hydrolysed to form gel spheres of well-defined size and shape and the nanometer-sized, Anatase TiO2 particles of controlled size and crystallinity were crystallized from the amorphous gel spheres by thermal or hydrothermal treatment methods [4]. The TiO2 with crystal size of 13 nm and aggregate size of 100 nm was selected as gold support for this study. Highly dispersed, nanostructured gold catalysts were deposited onto TiO2. Besides the nanostructured TiO2 prepared by sol-gel process, commercial P25 TiO2 was also tested as support for Au/TiO2. Gold catalysts were pretreated at 473 K for 5 h either in air or in 100 ppm O3/O2 mixture. The catalysts were tested for CO oxidation in a flow reactor using a 2.5 % CO in dry air mixture at a fixed flow rate of 200 sccm. The reaction was conducted at 298, 323, 373 and 473 K with 75 minutes duration at each temperature. After completing the temperature program, the gas inlet and outlet to the reactor were shut off to isolate the catalyst. The catalyst was allowed to cool down to room temperature and left overnight for 16 h. The reaction was resumed next day by flowing in the reaction mixture at room temperature.

The calcined or ozone treated Au/Nano-TiO2 were twice as active compared to the ozone treated Au/P25. After heating program, the O2-treated Au/Nano-TiO2 lost half of its activity after 45 minutes of reaction, while O3/O2-treated Au/Nano-TiO2 displays a constant activity. It is clear from these results that Au/Nano-TiO2 catalyst pretreated with ozone exhibits better and more stable catalyst performance. The transmission electron microscope pictures of the O2-treated Au/Nano-TiO2 display an increase in the gold particle size to as large as 17 nm after the reaction, while the O3/O2-treated catalyst did not show any changes in the gold particle size.

Reference:
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