Activation and properties of Mo=O bonds in Mo/SiO2 catalysts |
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Authors: | C Louis M Che M Anoo |
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Institution: | 1. Laboratoire de Réactivité de Surface et Structure, URA 1106 CNRS, Université P. et M. Curie, 4 place Jussieu, 75252, Paris, Cedex 05, France 2. Department of Applied Chemistry, University of Osaka Prefecture, Sakai, Osaka 591, Japan
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Abstract: | The existence of the charge transfer excited triplet state Mo5+-O-] produced by UV-irradiation of Mo/SiO2 catalysts, and its reactivity are evidenced by experiments of photoluminescence, photoinduced metathesis, and photoreduction
of CO. Mo5+ ions can be produced separately by thermal activation and O- ions by further adsorption of N2O on those Mo5+ ions. The latter of which are adsorbed on Mo6+ ions are found to be more reactive than O2- of Mo6+ =O2-] bond. They are able either to add a molecule such as CO or C2H4, or to abstract hydrogen from H2, CH4 or trans-dicyanoethylene, or a CN group form tetracyanoethylene (TCNE). The Mo5+ ions are able to coordinate gas phase ligands when their coordination sphere possesses vacant sites. This is the case for
tetracoordinated Mo5+
4c ions arising from reduction of tetrahedral Mo6+ ions (Eq. (7)). These Mo5+
4c ions are similar to those produced by UV-irradiaiion (Eq. (2)). In addition, if the adsorbed molecule has a sufficiently
large electron affinity, such as TCNE or O2, an electron transfer can occur (Eq. (9) and (17)). The Mo5+-O-] bond obtained by thermal activation is more difficult to evidence than that obtained with UV-activation because it is not
detectable by EPR. However, the EPR results obtained at low temperature show that the O- ions adsorbed on Mo/SiO2 catalysts as well as the Mo5+-O-] excited triplet state obtained by UV-irradiation of 1Mo6+=O2] interact with methanol (Eq. (16)). They are consistent with the mechanism of methanol oxidation occurring at high temperature
(Eq. (4)). |
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