EXAFS STUDY OF THE LOCAL-STRUCTURE AROUND COBALT IN CO-MO/AL2O3 CATALYSTS

The structure around cobalt in a series of CO-MO/Al2O3 catalysts was studied 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 which six-coordinated Co atoms on the Al2O3 surface have appreciable interaction with Mo atoms. An increase of the Co loading leads to pronounced formation of a CoAl2O4-like structure which is mainly present at the subsurface of Al2O3. CoMoO4 microcrystallites (Co-O-Mo (II)) are found 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 sulfiding of die catalysts.