The structure around cobalt in a series of CO-MO/Al2O3 catalysts was s
tudied by means of the X-ray Absorption Fine Structure technique. Low
Co loading yields the dominant phase of a Co-O-Mo (I) structure, in wh
ich six-coordinated Co atoms on the Al2O3 surface have appreciable int
eraction with Mo atoms. An increase of the Co loading leads to pronoun
ced formation of a CoAl2O4-like structure which is mainly present at t
he subsurface of Al2O3. CoMoO4 microcrystallites (Co-O-Mo (II)) are fo
und to be formed independent of the Co/Mo ratio in the catalysts with
high Mo loadings. Both Co-O-Mo (I) and Co-O-Mo (II) are postulated as
precursors which transfer to the active ''Co-Mo-S'' phase during sulfi
ding of die catalysts.