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The Carboxylate Ligand as an Oxide Relay in Catalytic Water Oxidation.
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology.
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology.ORCID iD: 0000-0002-1553-4027
(English)In: Article in journal (Other academic) Submitted
Abstract [en]

Carboxylate groups have diverse functionality in ligands of transiton metal catalysts. Here we present a conceptuallydifferent function of the carboxylates – the oxide relay. It functions by providing an intramolecular nucleophilic oxygen close tothe oxo group to facilitate O-O bond formation, and at a later stage a remote electrophilic center to facilitate OH- nucleophilic attack.EVB-MD models were generated for key bond forming steps, diffusion coefficients and binding free energies from potentialof mean force estimations were calculated from MD simulations, activation free energies of chemical steps were calculated usingdensity functional theory. The catalyst studied is the extremely active Ru(tda)(py)2 water oxidation catalyst. The combination ofsimulation methods allowed for estimation of the turnover frequencies, which were within one order of magnitude from the experimentalresults at different pH values. From the calculated reaction rates we find that at low pH the OH- anion nucleophilic attack isthe rate-limiting step, which changes at high pH to the O-O bond formation step. Both steps are extremely rapid and key to theefficiency is the oxide relay functionality of a pendant carboxylate group. The functionality was discovered for a ruthenium catalyst,but since there is nothing in the mechanism restricting it to this metal, the oxide relay functionality could open new ways todesign the next generation water oxidation catalysts with improved activity.

National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:kth:diva-250303OAI: oai:DiVA.org:kth-250303DiVA, id: diva2:1307743
Note

QC 20190521

Available from: 2019-04-29 Created: 2019-04-29 Last updated: 2019-05-21Bibliographically approved
In thesis
1. Theoretical Studies on Water Oxidation Catalysts - from Solvent to Interfaces
Open this publication in new window or tab >>Theoretical Studies on Water Oxidation Catalysts - from Solvent to Interfaces
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Kungliga Tekniska högskolan, 2019
Series
TRITA-CBH-FOU ; 33
National Category
Natural Sciences
Research subject
Theoretical Chemistry and Biology
Identifiers
urn:nbn:se:kth:diva-250296 (URN)978-91-7873-201-2 (ISBN)
Public defence
2019-05-28, FD5, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20190506

Available from: 2019-05-06 Created: 2019-04-29 Last updated: 2019-05-06Bibliographically approved

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