Molybdenum oxides, Raman studies

A. Christodoulakis, E. Heracleous, A.A. Lemonidou and S. Boghosian, An operando Raman study of structure and reactivity of alumina-supported molybdenum oxide catalysts for the oxidative dehydrogenation of ethane, J. 

In order to circumvent this problem the rate-determining step was bypassed by using more reactive reagents, allyl alcohol and allyl iodide. These allylic probes were expected to adsorb on the molybdenum oxide surface to provide, respectively, M-O-C and M-C bonded intermediates. These studies were carried out with a sample of 9.0 wt% Mo03/Si02, which Raman spectroscopy and x-ray diffraction showed to consist of fine (- 5nm) crystallites of M0O3 (23). 

In contrast, recent work (4-12) has shown that Raman spectroscopy can be used to study Ti) adsorption on oxides, oxide supported metals and on bulk metals [including an unusual effect sometimes termed "enhanced Raman scattering" wherein signals of the order of 10 - 106 more intense than anticipated have been reported for certain molecules adsorbed on silver], (ii) catalytic processes on zeolites, and (iii) the surface properties of supported molybdenum oxide desulfurization catalysts. Further, the technique is unique in its ability to obtain vibrational data for adsorbed species at the water-solid interface. It is to these topics that we will turn our attention. We will mainly confine our discussion to work since 1977 (including unpublished work from our laboratory) because two early reviews (13,14) have covered work before 1974 and two short recent reviews have discussed work up to 1977 (15,16). 

Catalytic and single-turnover experiments with the R. sphaeroides DMSO reductase, 0-labeled DMSO and l,3,5-triaza-7-phosphatricyclo[3.3.1.1]decane as an oxygen atom acceptor have been used to demonstrate that the enzyme is an oxotransferase. Complementary resonance Raman studies have been interpreted on the basis of a direct mechanism for OAT, with the active site cycling between mono-oxo-Mo(VI) and des-oxo-Mo(IV) forms via a DMSO-bound Mo(IV) intermediate. Both MPT dithiolene groups stay firmly attached to the molybdenum throughout the catalytic cycle. However, EPR and UV/visible spectroscopic evidence has been interpreted on the basis of the species formed upon the addition of DMS to oxidized... 

K. Chen, S. Xie, A.T. Bell and E. Iglesia, Structure and properties of oxidative dehydrogenation catalysts based on M0O3/AI2O3, J. Catal, 198(2), 232-242, 2001. H.C. Hu and l.E. Wachs, Catalytic properties of supported molybdenum oxide catalysts In situ Raman and methanol oxidation studies, J. Phys. Chem., 99(27), 10911-10922, 1995. 

Chiistodoulakis, A., Heracleous, E., Lemonidou, A., etal (2006). An Operando Raman Study of Structure and Reactivity of Alumina-supported Molybdenum Oxide Catalysts for the Oxidative Dehydrogenation of Ethane, J. Catal., 242, pp. 16-25. 

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