Transition metal sulfides relationships

The chapter Fundamental Studies of Transition-Metal Sulfide Catalytic Materials by Chianelli, Daage, and Ledoux reviews current understanding of the relationship between structural and other properties of these catalysts and their catalytic activity and selectivity in hydrodesulfurization. In view of increasing environmental demands, this field has been heavily researched. The authors show how systematic studies and applications of novel methods can provide considerable understanding of these important catalysts. 

In [9], we compared the values of AH of the compounds of transition metals with Al, Sb, and Sn, and we found that the enthalpy of atomization of these compounds increased along the iron-cobalt-nickel series. TTiis was compared with the postulated rise of the electron affinity along the same series of the iron-group transition elements. In [10], we drew attention to the fact that the same relationship was obeyed by silicon alloys rich in transition metals (these alloys were characterized by relatively strong metallic interaction). This relationship was not obeyed by the compounds of transition metals with nonmetals (such as transition-metal sulfides). 

We need to develop methods to understand trends for complex reactions with many reaction steps. This should preferentially be done by developing models to understand trends, since it will be extremely difficult to perform experiments or DFT calculations for all systems of interest. Many catalysts are not metallic, and we need to develop the concepts that have allowed us to understand and develop models for trends in reactions on transition metal surfaces to other classes of surfaces oxides, carbides, nitrides, and sulfides. It would also be extremely interesting to develop the concepts that would allow us to understand the relationships between heterogeneous catalysis and homogeneous catalysis or enzyme catalysis. Finally, the theoretical methods need further development. The level of accuracy is now so that we can describe some trends in reactivity for transition metals, but a higher accuracy is needed to describe the finer details including possibly catalyst selectivity. The reliable description of some oxides and other insulators may also not be possible unless the theoretical methods to treat exchange and correlation effects are further improved. 

Fernandez E, Moses PG, Toftelund A, et al. Scaling Relationships for Adsorption Energies on Transition Metal Oxide, Sulfide and Nitride Surfaces. Angew Chem Int Ed. 2008 47 4683-6. 

The metal phosphorus trisulfide phases, MPS3, form layered structures with M = Mg, V, Mn, Fe, Co, Ni, Zn, Pd, and Cd. All of the first transition series sulfides adopt the FePSs structure that is based on a cubic close-packed anion array with alternate layers of cation sites vacant. Within a layer, the cation sites are occupied by M + cations and P2 pairs, as shown in Figure 20. The two different types of octahedra are ordered so that each P2S6 octahedron is surrounded by six MSe octahedra and each MSe octahedron has three each of MSe and P2S6 neighbors. To emphasize the structural relationship to the dichalcogenides, the composition can be written M2/3(P2)i/3S2. 

Fernandez EM, Moses PG, Toftelund A, Hansen HA, Martinez JI, Abild-Pedersen F, Kleis J, Hinnemann B, Rossmeisl J, Bligaard T, Nprskov JK. Scaling relationships for adsorption energies on transition metal oxide, sulfide, and nitride surfaces. Angew Chem Int Ed 2008 47 4683-f686. 

It can also be observed from the schematic of the interactions shown in Fig. 10 that interaction zones that provide the possibility for sharing sulfur atoms by the two different phases exist. The structure of these interaction zones are crucial to understanding the interaction of sulfides with supports and promoter phases. The basal plane interaction with a second phase can be expected to be weak, but charge transfer might be expected if metal atoms are exposed on the second phase. Such an interaction would be similar to an intercalation interaction. The edge interaction will be much stronger, and here a transition zone with a possible epitaxial relationship between MoS2 and the second phase is expected. It is in this zone that we can expect to find the surface phases as described above (for example, the CoMoS phase). But in the cases described here, the surface phase becomes a line phase at the boundary between the two bulk phases. It is our belief that the detailed study of these phases represents a key area for future research in TMS catalysis. 

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