An Exceptionally Fast Homogeneous Carbon-Free Cobalt-Based Water Oxidation Catalyst

An all-inorganic, oxidatively and
thermally stable, homogeneous
water oxidation catalyst based on redox-active (vanadate­(V)-centered)
polyoxometalate ligands, Na10[Co4(H2O)2(VW9O34)2]·35H2O (Na101-V2, sodium salt of the polyanion 1-V2), was synthesized, thoroughly characterized and shown
to catalyze water oxidation in dark and visible-light-driven conditions.
This synthetic catalyst is exceptionally fast under mild conditions
(TOF > 1 × 103 s–1). Under light-driven
conditions using [Ru­(bpy)3]2+ as a photosensitizer
and persulfate as a sacrificial electron acceptor, 1-V2 exhibits higher selectivity for water oxidation versus bpy ligand
oxidation, the final O2 yield by 1-V2 is twice
as high as that of using [Co4(H2O)2(PW9O34)2]10– (1-P2), and the quantum efficiency of O2 formation
at 6.0 μM 1-V2 reaches ∼68%. Multiple experimental
results (e.g., UV–vis absorption, FT-IR, 51V NMR,
dynamic light scattering, tetra-n-heptylammonium
nitrate-toluene extraction, effect of pH, buffer, and buffer concentration,
etc.) confirm that the polyanion unit (1-V2) itself is
the dominant active catalyst and not Co2+(aq) or cobalt
oxide.