DISTRIBUTION OF MOLYBDENUM OXIDATION-STATES IN REDUCED MO/TIO2 CATALYSTS - CORRELATION WITH BENZENE HYDROGENATION ACTIVITY

A 5 wt % MoO3/TiO2 catalyst was reduced in hydrogen at various temperatures to produce a surface with average Mo oxidation states between +6 and 0. The changes in molybdenum oxidation states as a function of the extent of reduction were monitored by gravimetric analyses and X-ray photoelectron spectroscopy (XPS, ESCA), and the results were correlated with benzene hydrogenation activity. ESCA Mo 3d5/2 binding energy values for the various Mo oxidation states on a 5 wt % MoO3/TiO2 catalyst show a shift of 5.1 eV between Mo+6 (232.7 eV) and Mo0 (227.6 eV). The benzene hydrogenation activity was found to depend strongly on the extent of reduction of the Mo phase. When the catalyst was reduced at temperatures of ≤350°C (average Mo oxidation state ≥4), the benzene hydrogenation rate was very low (<0.2 mmol of cyclohexane/(h g of MoO3)). The rate increased sharply for reduction temperatures of 400-500°C and reached a maximum of ∼6 mmol of cyclohexane/(h g of MoO3) after 500-550°C reduction. A sharp decline in benzene hydrogenation activity was observed when the catalyst was reduced at temperatures of >550°C. Comparison of benzene hydrogenation activity with the distribution of Mo oxidation states determined by ESCA suggests that molybdenum ions with an oxidation state of +2 are the most active species. © 1990 American Chemical Society.