Preparation of Bulk Transition-Metal Sulfides

To further the understanding of the TMS, it is necessary to synthesize catalysts supported and unsupported, using methods that offer a good degree of control over their properties and stoichiometry. In the following section, we 

Group VIII sulfides are obtained by the low-temperature precipitation described above. However, a preferred synthesis involves the direct sulfidation with hydrogen sulfide of ammonium hexachlorometallates. A good example is found in the synthesis of RuS2 and Rh2S3 (28). However, synthetic techniques and the solid-state chemistry of the Group VIII sulfides are not as well developed as those for the Group VII sulfides, and much research needs 

The second metal, for example, the promoter, may also be added by subsequent impregnation of binary sulfide. When a nonreactive promoter precursor, for example, metal nitrate, is used it is necessary to resulfide the impregnated sulfide in order to decompose the precursor. Another variation of this method consists in using reactive promoter precursors that will react with the surface of the binary sulfide. In this case, further treatment of the catalyst may not be required. Good precursors include metal carbonyls and metal alkyls (32, 33). The precursor decomposition approach been most widely applied to the MoS2-based systems. However, it has also been extended to the mixed noble-metal sulfides by Breysse and co-workers (34) at Lyon following the work of Passaretti et al (35). 

Finally, it should be mentioned that highly crystalline material and/or doped single crystals can be obtained from the elements by vapor transport techniques (36) or by impregnation of the highly crystalline binary sulfide (37). These materials serve as important models for higher-surface-area materials and often yield characterization information that is clear compared to more complicated catalysts. 

The use of precursor synthesis techniques as described above is driven by the fact that decomposition of the precursor material into the catalyst often results in catalysts that have activity or selectivity superior to that of preferred products. The amine thiomolybdate decomposition described above results in MoS2 catalysts with surface areas exceeding several hundred square meters per gram (27). The HDS activity increases correspondingly and unpromoted catalysts have activities approaching those of promoted systems. 

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