EFFECT OF COBALT OXIDE ON SURFACE-STRUCTURE OF ALUMINA-SUPPORTED MOLYBDENA CATALYSTS STUDIED BY INSITU RAMAN-SPECTROSCOPY

A set of Co promoted 10% Mo/Al2O3 samples have been characterized by m eans of Raman spectroscopy under ambient as well as in situ dehydrated conditions. Under ambient conditions, the degree of the polymerizatio n of surface molybdenum oxide species decreases with increasing Co loa ding. Under dehydrated conditions, the polymeric molybdenum oxide spec ies is absent with the addition of only 0.2% Co. At low Co loadings (l ess-than-or-equal-to 2%), before the formation of CoMoO4 compound, the spectral features are very similar under ambient conditions. Dehydrat ion causes the upward shift of the Mo=O symmetric stretching mode. A b road band around 920-930 cm-1 was thus observed. This band has been su ggested to be associated with the Co-Mo interaction species. In contra st to crystalline CoMoO4, this Species shows a reversibility on H-2 re duction-O2 reoxidation treatments. From the results obtained, it is pr oposed that cobalt oxide interacts with the most polymerized molybdenu m oxide species to form Co-Mo interaction species and/or crystalline C oMoO4; therefore, the amount of the surface molybdenum oxide species d ecreases with a change in the molecular structure as a function of the Co concentration.