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Theoretical Studies on Water Oxidation Catalysts - from Solvent to Interfaces
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH).ORCID iD: 0000-0002-6383-1771
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: urn:nbn:se:kth:diva-250296ISBN: 978-91-7873-201-2 (print)OAI: oai:DiVA.org:kth-250296DiVA, id: diva2:1307935
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
List of papers
1. Why Is There a Barrier in the Coupling of Two Radicals in the Water Oxidation Reaction?
Open this publication in new window or tab >>Why Is There a Barrier in the Coupling of Two Radicals in the Water Oxidation Reaction?
2016 (English)In: ACS Catalysis, ISSN 2155-5435, E-ISSN 2155-5435, Vol. 6, no 12, p. 8308-8312Article in journal (Refereed) Published
Abstract [en]

Two radicals can form a bond without an energetic barrier. However, the radical coupling mechanism in ruthenium catalyzed water oxidation has been found to be associated with substantial activation energies. Here we have investigated the coupling reaction of [Ru=O(bda)L-2](+) catalysts with different axial L ligands. The interaction between the two oxo radical moieties at the Ru(V) state was found to have a favorable interaction in the transition state in comparison to the prereactive complex. To further understand the existence of the activation energy, the activation energy has been decomposed into distortion energy and interaction energy. No correlation between the experimental rates and the calculated coupling barriers of different axial L was found, showing that more aspects such as solvation, supramolecular properties, and solvent dynamics likely play important roles in the equilibrium between the free Ru-v=0 monomer and the [Ru-v=O center dot center dot center dot O=Ru-v] dimer. On the basis of our findings, we give general guidelines for the design of catalysts that operate by the radical coupling mechanism.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2016
Keywords
water oxidation, radical coupling, I2M, DFT, bda, catalysis, ruthenium
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-199489 (URN)10.1021/acscatal.6b02697 (DOI)000389399400034 ()2-s2.0-85046818884 (Scopus ID)
Note

QC 20170119

Available from: 2017-01-19 Created: 2017-01-09 Last updated: 2019-04-29Bibliographically approved
2. Capturing the Role of Explicit Solvent in the Dimerization of Ru-V(bda) Water Oxidation Catalysts
Open this publication in new window or tab >>Capturing the Role of Explicit Solvent in the Dimerization of Ru-V(bda) Water Oxidation Catalysts
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2017 (English)In: Angewandte Chemie International Edition, ISSN 1433-7851, E-ISSN 1521-3773, Vol. 56, no 24, p. 6962-6965Article in journal (Refereed) Published
Abstract [en]

A ground-breaking empirical valence bond study for a soluble transition-metal complex is presented. The full reaction of catalyst monomers approaching and reacting in the Ru-V oxidation state were studied. Analysis of the solvation shell in the reactant and along the reaction coordinate revealed that the oxo itself is hydrophobic, which adds a significant driving force to form the dimer. The effect of the solvent on the reaction between the prereactive dimer and the product was small. The solvent seems to lower the barrier for the isoquinoline (isoq) complex while it is increased for pyridines. By comparing the reaction in the gas phase and solution, the proposed p-stacking interaction of the isoq ligands is found to be entirely driven by the water medium.

Place, publisher, year, edition, pages
Wiley-VCH Verlagsgesellschaft, 2017
Keywords
diradical coupling reaction, empirical valence bond, hydrophobic oxo, solvation effect, water oxidation
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-209064 (URN)10.1002/anie.201701488 (DOI)000402523900049 ()28493633 (PubMedID)2-s2.0-85019990678 (Scopus ID)
Funder
Knut and Alice Wallenberg Foundation
Note

QC 20170620

Available from: 2017-06-20 Created: 2017-06-20 Last updated: 2019-04-29Bibliographically approved
3. Dynamics with Explicit Solvation Reveals Formation of the Prereactive Dimer as Sole Determining Factor for the Efficiency of Ru(bda)L-2 Catalysts
Open this publication in new window or tab >>Dynamics with Explicit Solvation Reveals Formation of the Prereactive Dimer as Sole Determining Factor for the Efficiency of Ru(bda)L-2 Catalysts
2018 (English)In: ACS Catalysis, ISSN 2155-5435, E-ISSN 2155-5435, Vol. 8, no 9, p. 8642-8648Article in journal (Refereed) Published
Abstract [en]

This report describes all key steps in the O-O bond formation from two separated [Ru-V=O(bda)L-2](+) cations to form the dinuclear [(bda)L2RuIV-O-Ru-IV(bda)L-2](2+) in explicit solvent. The three steps involve the diffusion of the catalysts in the water phase, formation of the prereactive dimer, and the bond formation between the two catalysts. On the basis of the calculated parameters, we compute the rate constant of two catalysts with different L-ligands, isoquinoline and picoline, and the computed values are in excellent agreement with the experimental ones. The interaction of the axial ligands is key to the improved rates of the larger ligand, mainly by facilitating the formation of the prereactive dimer from the solvated monomer. By comparing the binding free energy of hydrophilic Ru-IV-OH and hydrophobic Ru-V=O, the hydrophobic driving force of Ru-V=O in this system has been estimated to 1 kcal mol(-1).

Place, publisher, year, edition, pages
AMER CHEMICAL SOC, 2018
Keywords
water oxidation, binding free energy, diffusion rate, O-O bond formation, rate constant, molecular dynamics, empirical valence bond
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-235594 (URN)10.1021/acscatal.8b02519 (DOI)000444364800095 ()2-s2.0-85052393349 (Scopus ID)
Note

QC 20181001

Available from: 2018-10-01 Created: 2018-10-01 Last updated: 2019-04-29Bibliographically approved
4. Modifying Ru-bda Backbone with Steric Hindrance and Hydrophilicity: Influence of Secondary Coordination Environments on Water-Oxidation Mechanism
Open this publication in new window or tab >>Modifying Ru-bda Backbone with Steric Hindrance and Hydrophilicity: Influence of Secondary Coordination Environments on Water-Oxidation Mechanism
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(English)In: Article in journal (Other academic) Submitted
Abstract [en]

Understanding the seven coordination and O-O coupling pathway of the distinguished Ru-bda catalysts is essential for the development of next generation efficient water-oxidation catalysts based on earth-abundant metals. This work reports the synthesis, characterization and catalytic properties of a monomeric ruthenium catalyst Ru-bnda (H2bnda = 2,2'-bi(nicotinic acid)-6,6'-dicarboxylic acid) featuring steric hindrance and enhanced hydrophilicity on the backbone. Combining experimental evidence with systematic density functional theory calculations on the Ru-bnda and related catalysts Ru-bda, Ru-pda and Ru-biqa, we emphasized that seven coordination clearly determines presence of RuV=O with high spin density on the ORuV=O atom, i.e. oxo with radical properties, which is one of the necessary conditions for reacting through the O-O coupling pathway. However, an additional factor to make the condition sufficient is the favorable intermolecular face-to-face interaction for the generation of the pre-reactive [RuV=O...O=RuV], which is significantly influenced by the secondary coordination environments. This work provides a new understanding of the structure-activity relationship of water-oxidation catalysts and their potential to adopt I2M pathway for O-O bond formation.

National Category
Natural Sciences
Identifiers
urn:nbn:se:kth:diva-250301 (URN)
Note

QCR 20190619

Available from: 2019-04-29 Created: 2019-04-29 Last updated: 2019-06-18Bibliographically approved
5. Dynamics and Reactions of Molecular Ru Catalysts at Carbon Nanotube-Water Interfaces
Open this publication in new window or tab >>Dynamics and Reactions of Molecular Ru Catalysts at Carbon Nanotube-Water Interfaces
2018 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 140, no 24, p. 7498-7503Article in journal (Refereed) Published
Abstract [en]

Immobilization of molecular catalysts to electrode surfaces can improve the recyclability and electron transfer rates. The drawback is that most experimental techniques and theoretical methods are not applicable. Here we present results from a study of a ruthenium water oxidation catalyst [(RuO)-O-V(bda)L-2] in explicit water at a carbon nanotube water interface, forming the key O-O bond between two 128 atom catalysts, all fully dynamically. Our methodology is based on a recently developed empirical valence bond (EVB) model. We follow the key steps of the reaction including diffusion of the catalysts at the interface, formation of the prereactive dimer, and the bond formation between the two catalysts. On the basis of the calculated parameters, we compute the turnover frequency (TOF) at the experimental loading, in excellent agreement with the experiments. The key O-O bond formation was significantly retarded at the surface, and limiting components were identified that could be improved by catalyst modification.

Place, publisher, year, edition, pages
AMER CHEMICAL SOC, 2018
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-232254 (URN)10.1021/jacs.8b00433 (DOI)000436211600026 ()29798669 (PubMedID)2-s2.0-85047638616 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20180720

Available from: 2018-07-20 Created: 2018-07-20 Last updated: 2019-04-29Bibliographically approved
6. Nucleophilic Attack by OH2 or OH-: A Detailed Investigation on pH Dependent Performance of a Ru Catalyst
Open this publication in new window or tab >>Nucleophilic Attack by OH2 or OH-: A Detailed Investigation on pH Dependent Performance of a Ru Catalyst
2019 (English)In: Organometallics, ISSN 0276-7333, E-ISSN 1520-6041, Vol. 38, no 6, p. 1264-1268Article in journal (Refereed) Published
Abstract [en]

The considerable rate enhancements along with the increase in pH values may be due to the direct involvement of hydroxide anion in attacking electrophilic [Ru-V(tda)(py)(2)O](+) (1; tda = [2,2':6',2 ''-terpyridine]-6,6 ''-dicarboxylate, py = pyridine). The enhanced reaction rate is well in agreement with the descending activation barriers in our calculation. The addition of four extra water molecules in the geometry optimization plays a key role in stabilizing hydroxide anion as well as building a reasonable hydrogen-bonding network, and three of these molecules are required to stabilize the OH as an anion instead of a radical.

Place, publisher, year, edition, pages
AMER CHEMICAL SOC, 2019
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-249813 (URN)10.1021/acs.organomet.8b00544 (DOI)000462944200013 ()2-s2.0-85054136598 (Scopus ID)
Note

QC 20190423

Available from: 2019-04-23 Created: 2019-04-23 Last updated: 2019-04-29Bibliographically approved
7. The Carboxylate Ligand as an Oxide Relay in Catalytic Water Oxidation.
Open this publication in new window or tab >>The Carboxylate Ligand as an Oxide Relay in Catalytic Water Oxidation.
(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:nbn:se:kth:diva-250303 (URN)
Note

QC 20190521

Available from: 2019-04-29 Created: 2019-04-29 Last updated: 2019-05-21Bibliographically approved

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