Raman spectroscopic study of Co-Mo/. gamma. -Al/sub 2/O/sub 3/ catalysts

Laser Raman spectra of oxide precursors of hydrodesulfurization catalysts containing 0-30% molybdenum trioxide and/or 0-10% cobalt oxide on two alumina and two silica supports showed that with increasing molybdenum coverage of the support the molybdenum formed monomeric molybdate tetrahedra (MoO/sub 4/), polymolybdate with distorted octahedra (polymeric MoO/sub 4/), subsurface aluminum molybdate (Al/sub 2/MoO/sub 4/)/sub 3/, and free molybdenum trioxide (MoO/sub 3/) as distorted actahedra; that cobalt forms spinel (CoAl/sub 2/O/sub 4/) and tetrahedrally coordinated cobalt(II) (Co/sub 3/O/sub 4/), of which the latter increases with increasing cobalt coverage of the support; and that with increasing cobalt content of alumina-supported molybdenum, cobalt first forms subsurface cobalt-aluminum spinel, then a small amount of microcrystalline Co/sub 3/O/sub 4/, then subsurface cobalt molybdate (CoMoO/sub 4/) at the expense of subsurface aluminum molybdate.