Active sites for ethanol oxidation over SnO2-supported molybdenum oxides

SnO2-supported molybdenum oxides with varying coverage were synthesized and used for the catalytic oxidation of ethanol. The catalysts were obtained from precipitation of SnCl4 by ammonia in the presence of(NH4)(2)MO7O24 (A) Some catalysts were also prepared by impregnation of (NH4)(2)MO7O24 On SnO2 (B) for comparison. It was verified that molybdenum oxides inhibited the sintering of SnO2 crystals during calcination for preparation A, resulting in homogeneous systems with high specific areas. The solids were characterized by FTIR, temperature programmed reduction (TPR), DRS-UV, XPS and X-ray diffraction (XRD). The molybdenum coverage was determined by oxygen pulses after reduction at 400 degrees C under hydrogen. The results indicated two structurally different superficial sites. Four-coordinated molybdates were preferentially formed on the surface of co-precipitated catalysts at low molybdenum loading, while six-coordinated polymolybdates were obtained in other cases. Bulk MoO3 oxide was also observed at very high loading. The turnover numbers (TONs) measured for ethanol oxidative dehydrogenation suggested that dispersed, four-coordinated molybdates were the active phase. These species also gave higher selectivity to acetic acid. (C) 2000 Elsevier Science B.V. All rights reserved.