High-Throughput Screening of Earth-Abundant Water Reduction Catalysts toward Photocatalytic Hydrogen Evolution

Noble-metal
photosensitizers and water reduction co-catalysts (WRCs)
still present the highest activity in homogeneous photocatalytic hydrogen
production. The search for earth-abundant alternatives is usually
limited by the time required to screen new catalyst combinations;
however, here, we utilize newly designed and developed high-throughput
photoreactors for the parallel synthesis of novel WRCs and colorimetric
screening of hydrogen evolution. This unique approach allowed rapid
optimization of photocatalytic water reduction using the organic photosensitizer
Eosin Y and the archetypal cobaloxime WRC [Co­(GL1)2pyCl], where GL1 is dimethylglyoxime and py is
pyridine. Subsequent combinatorial synthesis generated 646 unique
cobalt complexes of the type [Co­(LL)2pyCl],
where LL is a bidentate ligand, that identified promising
new WRC candidates for hydrogen production. Density functional theory
(DFT) calculations performed on such cobaloxime derivative complexes
demonstrated that reactivity depends on hydride affinity. Alkyl-substituted
glyoximes were necessary for hydrogen production and showed increased
activity when paired with ligands containing strong hydrogen-bond
donors.