Rate enhancing role of molybdenum chloride in molten KCl for methane activation

Molten salts have been recently used as catalysts for methane pyrolysis to generate hydrogen and carbon. It was found that molten alkali chlorides, which are poor catalysts, become rather active upon the addition of a small amount of FeCl3. Calculations have shown that this takes place through an unusual mechanism. Due to fluctuations in the position of the ions, the charge on the iron fluctuates between Fe3+ and Fe2+. The electron required for forming Fe2+ is donated by several chlorine ions, and this causes them to be active in breaking the carbon hydrogen bond in methane. It was suggested that this mechanism is general: it takes place whenever the dopant is such that a lower charge state is possible. We test this hypothesis here by examining KCl doped with MoCl5, MoCl4, or MoCl3. We find that MoCl5 and MoCl4 activate KCl by the mechanism described above. MoCl3 does not because Mo3+ is not converted by fluctuations to Mo2+. We also show that the gas-phase dissociation energy of molybdenum chloride can be used as a descriptor to assess its ability to activate methane. Because it is easy to remove one Cl from MoCl5 or MoCl4, they are effective dopants for methane activation. The energy required for the dissociation of MoCl3 is very high, and doping with MoCl3 does not improve catalytic activity.