Molybdenum VI oxide on alumina—chemisorbed water

Molybdenum trioxide supported on alumina is the precursor of molybdenum disulfide based hydrodesulflirisation catalysts. Sulfiding of supported M0O3 is facilitated by bound water. INS spectroscopy was used to determine the nature of the water in hydrated M0O3/AI2O3 [92, 93]. The librational modes of co-ordinated water (Fig. 7.22) are observed in INS spectra when the water molecule is bound through the oxygen atom ( 9.2). 

In a hydrated molybdenum oxide/alumina catalyst the water modes appeared as a broad peak with its maximum near 450 cm and the first overtone near 900 cm. The peak intensity was proportional to the molybdenum concentration in the catalyst therefore the 450 cm peak was assigned to water bound to molybdenum. The effective mass, 4 amu,, of water was calculated from the ratio of the intensities of the fundamental (5o-i)and first overtone (5o-2), Eq. (5.10) f 1 A f 1  

This value indicates water coordinated to molybdenum. 

When the calcined catalyst was exposed to air, so becoming hydrated, the Mo=0 stretching vibration in the infrared shifted to lower wavenumber (1000 to 950 cm ) [94]. The shift to lower wavenumber was attributed to an increase of molybdenum coordination number from four to six as a result of binding of water molecules. Fig. 7. 23. 

The downward shift of the Mo=0 vibration, observed in the Raman and infrared spectra of the catalyst, was concomitant with the development of the water twisting mode observed in the INS spectrum. The alternative possibility, that adsorption of water on calcined molybdate/ alumina was dissociative, was not consistent with the spectra. Dissociative chemisorption implies the formation of a Mo-OH group. Such groups have been identified by INS of hydrogen molybdenum 

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