Preparation and coating of molybdenum oxide on alumina submicrospheres by sonochemical method
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Preparation and coating of molybdenum oxide on alumina submicrospheres by sonochemical method Z.Y. Zhong Department of Chemistry, Bar-Ilan University, Ramat-Gan, 52900 Israel
Y. Mastai Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot, 76100 Israel
R.A. Salkar, Y. Koltypin, and A. Gedankena) Department of Chemistry, Bar-Ilan University, Ramat-Gan, 52900 Israel (Received 21 July 1999; accepted 1 December 1999)
In this work, we coated molybdenum oxide on submicrospheres of amorphous alumina and crystalline alumina, using a sonochemical method. The sonication products were characterized by transmission electron microscopy, x-ray diffraction, differential scanning calorimetry, thermogravimetric analysis, ultraviolet–visible, Raman, Fourier transform infrared spectroscopy, and surface area (Brunauer–Emmett–Teller) measurements. We found that on crystalline alumina, the blue oxide of molybdenum was formed, while on amorphous alumina, the presence of an isolated tetrahedrally coordinated Mo oxide species was confirmed. It seems that the amount and types of surface hydroxyl groups on the surface of alumina play an important role in both the oxidation of Mo and the relative content of Mo species in the sonication product. The surface area of the alumina-coated Mo oxide is about 11 times larger than that of the bare alumina. An explanation for this change is offered.
I. INTRODUCTION
Supported molybdenum oxide catalysts show important catalytic properties in reactions such as selective dehydrogenation of alcohol,1 conversion of ethane to ethene and acetaldehyde,2 methane to methanol and formaldehyde,3 and propane to propene and high organic species.4 Another important related catalytic reaction is the hydrodesulfurization (HDS) of petroleum feedstocks to produce clean fuels for environmental protection and, particularly, for the reduction of sulfur emissions.5 It is believed that the active phase of HDS catalysts is best described by well-dispersed MoS2, which is usually obtained through the sulfidation of an Mo/Al2O3 oxide precursor. It is well known that the catalytic properties are strongly dependent on the structural features of the molybdenum species in catalysts as well as on their precursor, and these structural features are closely correlated to the preparation methods. There are various methods for the preparation of supported molybdenum oxide-based catalysts. The most usual methods are coprecipitation, or wet impregnation, and the mechanical mixing of MoO3 with the oxide support. The latter shows spontaneous spreading of MoO3 a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 15, No. 2, Feb 2000
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on ␥–Al2O3 above 670 K.6 An alternate approach for silica-supported catalysts, based on the reaction between molybdenum compounds containing labile ligands, such as MoCl5, Mo(3–C3H5)4, and Mo2(3–C3H5)4 and silica hydroxyl groups, has also be
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