Theoretical Studies on Water Oxidation Catalysts - from Solvent to Interfaces
Abstract: Water splitting contains two half-reactions, the water oxidation reaction and the hydrogen reduction reaction. In water oxidation, protons and electrons will be generated to offer two elemental components for production of fuels, such as H2 and CH3OH. To overcome the overpotential of the reaction, a large amount of water oxidation catalysts (WOCs) have been synthesized. In the second chapter, a variety of homogeneous and heterogenous catalysts have been introduced. The homogeneous catalysts include Ru-based catalysts, Ir-based catalysts, and first-row transition metal-based catalysts. Among these catalysts, a family of Ru(bda)L2 complexes was found in experiment to have a comparable turnover frequency (TOF) at acidic pH with photosystem II. A similar catalyst, Ru(tda)(py)2, was found to have an impressive TOF of 50 000 s-1 at pH 10.0. The heterogenous catalysts include heterogenous oxide and heterogenized molecular catalysts that catalyze the reaction using either electrochemical driving force or photoelectrochemical driving force.Understanding the details of the mechanism can help to design a better catalyst with high catalytic performance. For this purpose, several theoretical methods have been applied. Density functional theory (DFT) was employed to study the rate limiting reaction in implicit solvent. The empirical valence bond (EVB) method is a powerful tool for describing environment effects. This approach was used to get insight into the solvent and surface effects on the reaction pathway. Molecular dynamics (MD) and potential of mean force (PMF) methods are applied to perform simulations for large systems at long time-scales. The Ru(bda)L2 catalysts have been found to have high TOF, up to 1000 s-1 and react via an I2M (Interaction of two metal centers) pathway. By using B3LYP-D3 functional to study the diradical coupling of the O-O bond formation, we found that there is no intrinsic barrier between the two RuV=O fragments of RuV=O complexes. On the basis of the study of the solvent role on the reaction using an EVB-MD model, the oxo of the RuV=O species was shown to be hydrophobic. The hydrophobic oxo explained why the Ru(bda)L2 complexes proceed the reaction via the I2M pathway. To study the full dimerization of two separated RuV=O species in fully explicit solvent, we calculated the diffusion of individual catalysts from MD simulations, association of pre-reactive dimer from PMF simulations, and the coupling reaction in explicit solvation using the EVB approach. The formation of the prereactive dimer was found to be the sole determining factor for the efficiency of the Ru(bda)L2 catalysts. In the study of four Ru complexes with different equatorial ligands, the secondary coordination environments, such as flexibility, hydrophilicity were proposed to be the affecting the different catalytic pathways.To make an efficient electrocatalyst, the Ru(bda)L2 catalyst has been modified by Sun and co-workers with pyrene groups at the axial L-ligands to be adhered on the CNT functionalized electrodes. A computational model of the RuV=O catalyst tethered on the CNT surface was built to study the O-O bond formation in heterogenous system. By using the same combination of MD, PMF, and EVB, we studied the full dimerization reaction of the catalyst at CNT-water interfaces with full dynamics. The reasons for the lower the TOF of the surface catalyst and methods to improve the lower TOF were addressed in this study.With the pH dependent Ru(tda)(py)2 complex, we used the same combination methods and proposed a conceptually new function of the dangling carboxylate – the oxide relay. The oxide relay provides a highly nucleophilic oxygen atom close to the oxo to facilitate the O-O bond formation at the first step, and a highly electrophilic center to react with the OH- even at neutral pH at the second step. The rate-limiting step is the O-O bond formation at high pH, OH- nucleophilic attack at neutral pH.In summary, several key properties of the water oxidation catalyzed by Ru-based complexes, such as solvent and surface effects, hydrophobicity, and oxide relay have been investigated in detail by using several computational techniques. Our studies can shed light on the design of molecular WOCs with high catalytic activity and will help the development of artificial photosysnthesis devices.
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