| Abstract: | The object of our studies was a NiMo/Al2O3 catalyst thermally activated in oxygen, hydrogen, and in a mixture of hydrogen and hydrogen sulfide. On the surface of the oxide form of the catalyst there are different species, among them dimeric and polymeric octahedrally coordinated Mo6+ ions containing oxygen bridges are found. The presence of structures resembling α- and β-NiMoO4 and 6-heteropolymolybdenates was determined. Reduction with hydrogen leads to formation of two nuclear structures with single and double bridges of Mo25+ O3] and Mo5+ O4] type, respectively, and partially reduced 6-heteropolymolybdenates. On the other hand, when the reduction and sulfiding take place at the same time, then terminal oxide ions are partially replaced by sulfide ones. The presence of MoO2S2]2− species has been observed, however, there is no evidence of the exchange —OH groups for —SH ones and exchange of oxygen bridged ions for sulfur ions. It has been also noted that the subsurface structures remained intact after reduction and sulfiding. |
