Infrared spectroscopic studies of the reactions of alcohols over group IVB metal oxide catalysts. Part 3.—Ethanol over TiO2, ZrO2and HfO2, and general conclusions from parts 1 to 3

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Abstract
The dehydrogenation and dehydration reactions of ethanol over TiO2, ZrO2 or HfO2 catalysts has been monitored in the gas phase and on the surfaces by infrared spectroscopy. The reaction pathways closely parallel those of methanol reported in the prevous paper (Part 2) with the addition of the direct dehydration reaction C2H5OH → C2H4+ H2O and the production of benzene as a minor product. The infrared spectroscopic analysis of the decomposition of diethyl ether as initial reagent over the TiO2(500) catalyst confirms that the ethane product is derived from the ether precursor. As with methane from methanol, it is probably produced by reduction of the ether to the alkane plus water by hydrogen derived from the parallel dehydrogenation reaction. A summary is given of probable mechanisms for the catalysed reactions of the three alcohols, methanol, ethanol and propan-2-ol based on the gas-phase products and surface species identified by infrared spectroscopy. The general importance of alkoxide surface intermediates is emphasised. Alkoxides of a given formula occur on different spectroscopically distinguishable sites with different reactivities.

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