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Abstract

New catalyst preparation methods which avoid the formation of waste water are desirable for environmental reasons, especially in the case of catalysts produced in amounts as large as those in the DeNO_x sector. Thermal spreading has recently been applied in the preparation of titania-supported tungsten and molybdenum monolayer catalysts. In this paper we compare catalysts which are prepared via ball milling of mixtures of the individual precursor oxides with subsequent thermal spreading to catalysts which are prepared by a conventional coprecipitation process. Titania (anatase) was milled in the presence of either tungsten trioxide or molybdenum trioxide and these mixtures were subsequently calcined in a stream of oxygen which contained water vapor. After calcination, no crystalline WO₃ or MoO₃ was detected by XRD or DRIFTS, indicating a good dispersion of the active oxides. The overtones of tungsten and molybdenum oxygen double bond vibrations, which are representative for the formation of surface polyoxo compounds, were detected with DRIFT spectroscopy for samples prepared either by coprecipitation or via ball milling. The intensity distribution of the hydroxyl groups for ball mill prepared samples was different from the distribution for coprecipitated samples. When the physical mixtures were milled for several hours, the anatase was transformed into rutile as shown by X-ray diffraction. Furthermore, a decrease in surface area and changes in the pore size distribution were observed in correlation with the milling.