Water oxidation surface mechanisms replicated by a totally inorganic tetraruthenium–oxo molecular complex

Abstract

Solar-to-fuel energy conversion relies on the invention of efficient catalysts enabling water oxidation through low-energy pathways. Our aerobic life is based on this strategy, mastered by the natural Photosystem II enzyme, using a tetranuclear Mn-oxo complex as oxygen evolving center. Within artificial devices, water can be oxidized efficiently on tailored metal-oxide surfaces such as RuO 2 . The quest for catalyst optimization in vitro is plagued by the elusive description of the active sites on bulk oxides. Although molecular mimics of the natural catalyst have been proposed, they generally suffer from oxidative degradation under multiturnover regime. Here we investigate a nano-sized Ru 4 -polyoxometalate standing as an efficient artificial catalyst featuring a totally inorganic molecular structure with enhanced stability. Experimental and computational evidence reported herein indicates that this is a unique molecular species mimicking oxygenic RuO 2 surfaces. Ru 4 -polyoxometalate bridges the gap between homogeneous and heterogeneous water oxidation catalysis, leading to a breakthrough system. Density functional theory calculations show that the catalytic efficiency stems from the optimal distribution of the free energy cost to form reaction intermediates, in analogy with metal-oxide catalysts, thus providing a unifying picture for the two realms of water oxidation catalysis. These correlations among the mechanism of reaction, thermodynamic efficiency, and local structure of the active sites provide the key guidelines for the rational design of superior molecular catalysts and composite materials designed with a bottom-up approach and atomic control.artificial photosynthesis | ab initio simulations | X-ray absorption spectroscopy | electrocatalysis P hotocatalytic water splitting offers a bioinspired strategy for replacing fossil fuels with clean energy vectors (1-3). The overall reaction entails a sequence of light-promoted electron and proton transfers coupled with cleavage and formation of molecular bonds, ultimately splitting H 2 O molecules into O 2 and H 2 . The application of such technology for a viable solar-fuel economy is at the forefront of a very intense research effort. The main issue is the design and optimization of innovative catalysts enabling the half reaction of water oxidation (2H 2 O → O 2 + 4H + + 4e − ) (1) at low overpotential, high turnover frequencies, and longterm operation stability. Considering its high thermodynamic cost [E 0 = −1.23 V at pH = 0 vs. normal hydrogen electrode (NHE)] and mechanistic complexity, water oxidation catalysis (WOC) poses severe challenges for artificial photosynthesis applications.In plants, water oxidation is catalyzed by the Mn 4 CaO 4 oxygen evolving complex of Photosystem II (PSII) enzymes. The natural catalyst exhibits a functional asset of four redox active metal centers with adjacent μ-oxo bridges to enable water oxidation with a maximal turnover frequency of 400 s −1 per O 2 molecule (4). The drawback lies in the intrinsic weakness of the biol...

Quantum Chemical Modeling of Homogeneous Water Oxidation Catalysis

“…Twopossible catalyticcycles for water oxidation by Ru 4 -POM. [163] ChemSusChem À was the major species under experimental conditions.…”

“…[160,161] Piccinin et al used ac ombined QM/MM approach to investigate the water oxidation mechanism. [162,163] To save some computational cost, the POMl igandsw ere replaced by four chloride ions. Startings tructure S 0 has four Ru IV ions, with each one bound to one water ligand.T wo catalytic cycles were investigated, with the first one starting from the S 2 state (Figure 42), which Figure 39.…”

What Influences the Water Oxidation Activity of a Bioinspired Molecular CoII4O4 Cubane? An In-Depth Exploration of Catalytic Pathways

“…We calculated the free energy difference of equation (1) to amount to 102.6 kcal/mol and used it as "computational thermodynamic limit" of water oxidation rather than the experimental value of 113.5 kcal/mol. 76 We are however aware of the (selfinteraction) error introduced by calculating molecular oxygen. For both pathways, the third catalytic state already provides enough free energy to oxidize water.…”

Oxygenation by Ruthenium Monosubstituted Polyoxotungstates in Aqueous Solution: Experimental and Computational Dissection of a Ru(III)–Ru(V) Catalytic Cycle

“…[96,100] In this perspective the POM alternative offers the unprecedented opportunity to replicate bioinspired pathways on inorganic molecular metal-oxides, thus bridging the gap with heterogeneous catalysis by active surfaces. [105] Experimental Section [ 43] In …”