Transition metal chalcogenides Chevrel phase

Perspectives for fabrication of improved oxygen electrodes at a low cost have been offered by non-noble, transition metal catalysts, although their intrinsic catalytic activity and stability are lower in comparison with those of Pt and Pt-alloys. The vast majority of these materials comprise (1) macrocyclic metal transition complexes of the N4-type having Fe or Co as the central metal ion, i.e., porphyrins, phthalocyanines, and tetraazaannulenes [6-8] (2) transition metal carbides, nitrides, and oxides (e.g., FeCjc, TaOjcNy, MnOx) and (3) transition metal chalcogenide cluster compounds based on Chevrel phases, and Ru-based cluster/amorphous systems that contain chalcogen elements, mostly selenium. 

Figure 2.25. Structure of Mo4Ru2Se8 and interaction of O2 at the Chevrel phase clusters-electrolyte interface [62]. (Reprinted from J Phys IV, Alonso-Vante N, Fieber-Erdmann M, Rossner H, Holub-Krappe E, Giorgetti Ch, Tadjeddine A, et al. The catalytic center of transition metal chalcogenides vis-a-vis the oxygen reduction reaction an in situ electrochemical EXAFS study, 7 887-9. 1997 with permission of EDP Sciences.)...

Figure 15.11. Interaction of molecular oxygen in acid medium at the Chevrel phase cluster-electrolyte interface [6]. (Reprinted from Journal de Physique, vol. 7, C2, The Catalytic Centre of Transition Metal Chalcogenides vis- -vis the Oxygen Reduction Reaction An In situ Electrochemical EXAFS Study, Alonso-Vante, N. et al. 31997, with permission from EDP Sciences.)...

Most of the chalcogenide studies have involved binary, ternary, and pseudo ternary metal clusters. The ternary Chevrel-phases can be represented by MxMo6X8 (M = transition metal, X = chalcogen atom), where the transition metal fills the cavities in the Mo6X8 crystal structure. The pseudo-ternary (or pseudo-binary) Chevrel-phase clusters can be represented by MxMo6 xX8, where the Mo atoms are substituted by transition metal atoms.185 The crystal structure of ternary and pseudo ternary Chevrel phases are shown in Fig. 4. The carbonyl-chalcogenide clusters189 192 will be discussed in the next section. 

Trends in the electronic structure of the chalcogenide catalysts have proved to be helpful in the design and understanding of the catalyst clusters. During ORR, the molecular oxygen has been found to react with the cluster as a whole, rather than individual metal atoms.177 The overall number of electrons per cluster unit (NEC) in the valence bond has been shown to have a factor in the activity and stability of the cluster catalysts.177,181 The unsubstituted Chevrel phases have a NEC of 20.177,181 Substituting or intercalating other transition metals into the crystal lattice to make ternary or pseudo-binary Chevrel phases allows for the increase of NEC. It has been found that as the NEC approaches 24, the catalytic activity improves.181 Alonso-Vante compiled the results from his previous studies to show the effect of NEC in... 

---