Redox-Mediator-Assisted Electrocatalytic Hydrogen Evolution from Water by a Molybdenum Sulfide-Functionalized Metal–Organic Framework

The
Zr6-based metal–organic framework NU-1000
was successfully functionalized with candidate catalystsMoSx unitsvia SIM (solvothermal deposition
in MOFs) of molybdenum­(VI), followed by reaction with H2S gas. The structure of the material, named MoSx-SIM, was characterized
spectroscopically and through a single-crystal X-ray diffraction measurement.
These measurements and others established that the catalyst is monometallic,
with mixed oxygen and sulfur coordination, and that it forms from
a MOF-node-supported molybdenum-based cluster featuring only oxy ligands.
Notably, the formal potential for the MOF-grafted complex, like that
for the metal–sulfur active site of hydrogenase, is nearly
coincident with the formal potential for the hydrogen couple. Its
effective concentration within the mesoporous MOF is several hundred
millimolar, and its porous-framework-based immobilization/heterogenization
obviates the need for aqueous solubility as a condition for use as
a well-defined catalyst. MoSx-SIM was evaluated as an electrocatalyst
for evolution of molecular hydrogen from aqueous acid. Although the
MoSx-functionalized framework exhibits
catalytic behavior, the highly insulating nature of the support inhibits
high electrocatalytic performance. Introduction of an archetypal redox
mediator (RM), methyl viologen (MV2+), resulted in more
than 20-fold enhancement in its catalytic performance on a turnover
frequency basis, implying efficient RM-assisted electron transfer
to otherwise electrochemically non-addressable MoSx moieties. Electrochemical kinetic studies with additional
viologens as mediators reveal an unexpected square-root dependence
of overall reaction rate on mediator concentration, as well as sensitivity
to the strength of RM•+ as a reductant. These observations,
together with observations of potential-dependent H/D isotope effects
and potential-dependent pH effects, provide useful insights into the
catalysis mechanism and help to explain how the MOF-affixed monometallic
catalyst can effectively catalyze a two-electron reduction reaction,
i.e., hydrogen evolution from acidified water.