Molybdenum-cobalt clusters

TOF-SIMS images (Figs. 13.5 and 13.6) illustrate the ability to detect changes in the dispersion (uniform or presence of metal clusters) of the active phase in supported-oxide catalysts. Figure 13.5 shows nearly uniform distribution of molybdenum. The surface contamination with NH4+ ions coming from a precursor, which were not removed during the catalyst preparation process, is also observed. Cobalt clusters in the range of several micrometers are clearly visible in Fig. 13.6. 

B7.7 Electron-deficient molybdenum/cobalt/sulfide clusters ... 

Many carbonyl and carbonyl metallate complexes of the second and third row, in low oxidation states, are basic in nature and, for this reason, adequate intermediates for the formation of metal— metal bonds of a donor-acceptor nature. Furthermore, the structural similarity and isolobal relationship between the proton and group 11 cations has lead to the synthesis of a high number of cluster complexes with silver—metal bonds.1534"1535 Thus, silver(I) binds to ruthenium,15 1556 osmium,1557-1560 rhodium,1561,1562 iron,1563-1572 cobalt,1573 chromium, molybdenum, or tungsten,1574-1576 rhe-nium, niobium or tantalum, or nickel. Some examples are shown in Figure 17. 

Solid-state sulfide compounds MMo2S4 (M = V, Cr, Fe, Co) contain infinite chains of Mo4S6 clusters with intracluster Mo-Mo distances (2.756-2.989 A) and intercluster distance (2.960 A) for the cobalt derivative (112). As the valence of M is +2, judging from magnetic measurements, and the average oxidation state of molybdenum +3, the CVE for M2Mo4S8 is 12. This agrees with five intracluster and one intercluster... 

By applying an approach similar to the one taken with the unpromoted M0S2 nanoclusters, we recently managed to synthesize a model system for the promoted CoMoS catalyst 119). As a result of co-deposition of molybdenum and cobalt onto the Au(l 1 1) crystal during exposure to an H2S atmosphere and subsequent annealing, crystalline CoMoS clusters formed on the Au(l 1 1) terraces. As shown in Fig. 24, the main new finding is that the CoMoS nanoclusters now adopt a... 

Fig. 25. (a) Ball model of the proposed CoMoS structure. The CoMoS cluster is shown in top view, exposing the unpromoted molybdenum edge and a cobalt-promoted sulfur edge (molybdenum dark sulfur bright cobalt dark with white spot). Also shown on the basal plane is a single cobalt inclusion. 

This electronic explanation of the synergy between Co or Ni and Mo does not give any direct information on the structure of the active phase except that cobalt or nickel atoms should be bonded to molybdenum atoms through bridging sulfur atoms according to the molecular model used by the authors. The precise structure of this pseudobinary cluster is as yet unknown, but... 

---