Development of efficient and economic water oxidation catalysts (WOCs) remains a crucial bottleneck on the way to artificial photosynthesis applications. Over the past few decades, WOC research has turned into a fascinating interdisciplinary field that ranges from bio-inspired molecular design over nanomaterials and thin films to solid materials tuning. Under the umbrella of WOC optimization, advanced in situ/operando analytical techniques are being developed as increasingly powerful tools to elucidate the controversial discussions about the molecular or nanoscale nature of many WOCs. More and more of these approaches also enable the monitoring of possible key intermediates as an essential prerequisite for proposing catalytic mechanisms. This review is organized in three main parts: first, recent highlights outline frontiers in WOC development, such as the benefits of connecting molecular WOCs with solids along with the introduction of molecular concepts into heterogeneous WOC research. Next, a brief overview of emerging in situ/operando approaches demonstrates new options for monitoring WOC transformations. Finally, selected monitoring studies over the entire WOC dimensionality spectrum illustrate interesting cases of catalytic border crossings as new input for WOC construction.
The future of artificial photosynthesis depends on economic and robust water oxidation catalysts (WOCs). Cobalt-based WOCs are especially promising for knowledge transfer between homogeneous and heterogeneous catalyst design. We introduce the active and stable {CoO} cubane [Co(dpy{OH}O)(OAc)(HO)](ClO) (CoO-dpk) as the first molecular WOC with the characteristic {HO-Co(OR)-OH} edge-site motif representing the sine qua non moiety of the most efficient heterogeneous Co-oxide WOCs. DFT-MD modelings as well as in situ EXAFS measurements indicate the stability of the cubane cage in solution. The stability of CoO-dpk under photocatalytic conditions ([Ru(bpy)]/SO) was underscored with a wide range of further analytical methods and recycling tests. FT-IR monitoring and HR-ESI-MS spectra point to a stable coordination of the acetate ligands, and DFT-MD simulations along with H/H exchange experiments highlight a favorable intramolecular base functionality of the dpy{OH}O ligands. All three ligand types enhance proton mobility at the edge site through a unique bioinspired environment with multiple hydrogen-bonding interactions. In situ XANES experiments under photocatalytic conditions show that the {CoO} core undergoes oxidation to Co(III) or higher valent states, which recover rather slowly to Co(II). Complementary ex situ chemical oxidation experiments with [Ru(bpy)] furthermore indicate that the oxidation of all Co(II) centers of CoO-dpk to Co(III) is not a mandatory prerequisite for oxygen evolution. Moreover, we present the [CoNi(dpy{OH}O)(OAc)(HO)](ClO) (CoNiO-dpk) series as the first mixed Co/Ni-cubane WOCs. They newly bridge homogeneous and heterogeneous catalyst design through fine-tuned edge-site environments of the Co centers.
N,N'-Bis(salicylidene)ethylenediaminecobalt(II) (1) has been investigated as a highly efficient water oxidation precatalyst with a TON of 854 at pH = 9.0, using [Ru(bpy)3](ClO4)2 as a photosensitizer and Na2S2O8 as a sacrificial electron acceptor.
An eleven iron-containing nanoscale inorganic polyanionic oxide cluster was reported as the first example for exceptional photocatalytic water oxidation. Under optimal conditions, a remarkable turn-over number (TON) of 1815 ± 50 and a turn-over frequency (TOF(initial)) of 6.3 s(-1) over were achieved for water oxidation.
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