The Key Ru^V=O Intermediate of Site-Isolated Mononuclear Water Oxidation Catalyst Detected by in Situ X-ray Absorption Spectroscopy

Abstract

Improvement of the oxygen evolution reaction (OER) is a challenging step toward the development of sustainable energy technologies. Enhancing the OER rate and efficiency relies on understanding the water oxidation mechanism, which entails the characterization of the reaction intermediates. Very active Ru-bda type (bda is 2,2'-bipyridine-6,6'-dicarboxylate) molecular OER catalysts are proposed to operate via a transient 7-coordinate Ru═O intermediate, which so far has never been detected due to its high reactivity. Here we prepare and characterize a well-defined supported Ru(bda) catalyst on porous indium tin oxide (ITO) electrode. Site isolation of the catalyst molecules on the electrode surface allows trapping of the key 7-coordinate Ru═O intermediate at potentials above 1.34 V vs NHE at pH 1, which is characterized by electron paramagnetic resonance and in situ X-ray absorption spectroscopies. The in situ extended X-ray absorption fine structure analysis shows a Ru═O bond distance of 1.75 ± 0.02 Å, consistent with computational results. Electrochemical studies and density functional theory calculations suggest that the water nucleophilic attack on the surface-bound Ru═O intermediate (O-O bond formation) is the rate limiting step for OER catalysis at low pH.

Amphiphilic Oxo-Bridged Ruthenium “Green Dimer” for Water Oxidation

“…X-ray absorption near edge structure (XANES), powerful in monitoring the valence state of metal complexes, was performed to affirm the valence state of monomer 1 and dimer 2 (Moonshiram et al, 2012;Pushkar et al, 2014;Yang et al, 2017;Lebedev et al, 2017). As shown in Figure 3A, the absorption edge of the Ru metal center of 2 shifted to a higher energy at 22,129.0 eV, indicating an increase in the average valence of Ru center in 2.…”

Quantum Chemical Study of the Mechanism of Water Oxidation Catalyzed by a Heterotrinuclear Ru2Mn Complex

“…The lower oxidation state transitions from A to B and B to C are not observed experimentally probably owing to the low quality of the electrochemical response for this complex. [66][67][68] OÀObondformation OÀOb ond formation in water oxidationb yR u [17][18][19][20][76][77][78][79][80] and Mn [62,[69][70][71][72][81][82][83][84][85][86] catalysts have been widelys tudied by quantum chemicalc alculations.T he two popularp athways, namely direct coupling (DC)o ft wo adjacent oxygen units andw ater nucleophilic attack (WNA), were studied herein. [66][67][68] OÀObondformation OÀOb ond formation in water oxidationb yR u [17][18][19][20][76][77][78][79][80] and Mn [62,[69][70][71][72][81][82][83][84][85][86] catalysts have been widelys tudied by quantum chemicalc alculations...…”

“…[41] It is worth mentioning here that the active species, derived from A,t hat are responsible for the OÀOb ond formation, reach formal the oxidation state Ru1 IV Mn V Ru2 IV .T his is in good agreement with the OEC-PSII,w here af ormal Mn V is also neededi nt he S 4 state. [66][67][68] OÀObondformation OÀOb ond formation in water oxidationb yR u [17][18][19][20][76][77][78][79][80] and Mn [62,[69][70][71][72][81][82][83][84][85][86] catalysts have been widelys tudied by quantum chemicalc alculations.T he two popularp athways, namely direct coupling (DC)o ft wo adjacent oxygen units andw ater nucleophilic attack (WNA), were studied herein.…”

Evidence for tetranuclear bis-μ-oxo cubane species in molecular iridium-based water oxidation catalysts from XAS analysis

“…XAS has been successfully employed in several cases to gain structural insight into functional WOCs, including the CaMn 4 oxo-cubane OEC in PSII, 27 CoO x films 14,28 and molecular Ru WOCs immobilized on surfaces. 29 Here we investigated the activated form of [Cp*Ir(dimethyl-pyridinealkoxide)Cl] obtained by stirring with 50 equivalents of NaIO 4 in water at room temperature for 16 hours (termed Ir-pyalk WOC), conditions which have previously been shown to yield a homogeneous iridium species without any bound Cp* but retaining the pyalk ligand. 19,24,30 XAS measurements were performed directly on freshly prepared 5 mM aqueous solutions, and on cellulose pellets of the dark blue solid obtained after slow evaporation of these solutions to dryness (yielding identical spectra).…”