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 f rom 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 s intering of SnO2 crystals during calcination for preparation A, resulting i n homogeneous systems with high specific areas. The solids were characteriz ed by FTIR, temperature programmed reduction (TPR), DRS-UV, XPS and X-ray d iffraction (XRD). The molybdenum coverage was determined by oxygen pulses a fter reduction at 400 degrees C under hydrogen. The results indicated two s tructurally different superficial sites. Four-coordinated molybdates were p referentially formed on the surface of co-precipitated catalysts at low mol ybdenum loading, while six-coordinated polymolybdates were obtained in othe r cases. Bulk MoO3 oxide was also observed at very high loading. The turnov er numbers (TONs) measured for ethanol oxidative dehydrogenation suggested that dispersed, four-coordinated molybdates were the active phase. These sp ecies also gave higher selectivity to acetic acid. (C) 2000 Elsevier Scienc e B.V. All rights reserved.