| 1. |
- Agarwala, Hemlata, et al.
(författare)
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Alternating Metal-Ligand Coordination Improves Electrocatalytic CO2 Reduction by a Mononuclear Ru Catalyst**
- 2023
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Ingår i: Angewandte Chemie International Edition. - : Wiley. - 1433-7851 .- 1521-3773. ; 62:17
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Tidskriftsartikel (refereegranskat)abstract
- Molecular electrocatalysts for CO2-to-CO conversion often operate at large overpotentials, due to the large barrier for C−O bond cleavage. Illustrated with ruthenium polypyridyl catalysts, we herein propose a mechanistic route that involves one metal center that acts as both Lewis base and Lewis acid at different stages of the catalytic cycle, by density functional theory in corroboration with experimental FTIR. The nucleophilic character of the Ru center manifests itself in the initial attack on CO2 to form [Ru-CO2]0, while its electrophilic character allows for the formation of a 5-membered metallacyclic intermediate, [Ru-CO2CO2]0,c, by addition of a second CO2 molecule and intramolecular cyclization. The calculated activation barrier for C−O bond cleavage via the metallacycle is decreased by 34.9 kcal mol−1 as compared to the non-cyclic adduct in the two electron reduced state of complex 1. Such metallacyclic intermediates in electrocatalytic CO2 reduction offer a new design feature that can be implemented consciously in future catalyst designs.
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| 2. |
- Agarwala, Hemlata, et al.
(författare)
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An Elusive Intermediate Uncovered in the Pathway for Electrochemical Carbon Dioxide Reduction by Ruthenium Polypyridyl Catalyst - Combined Spectroscopic and Computational Investigation
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- A scrutinous study of the catalytic cycle for electrochemical CO2 reduction by the ruthenium 2,2:6,2-terpyridine (tpy) 2,2-bipyridine (bpy) class of catalysts is presented. An unprecedented 2-(C,O)-carboxycarboxylatoruthenium(II) metalacyclic intermediate, critical for C-O bond dissociation at low overpotentials, so far precluded from mechanistic considerations of polypyridyl transition metal complex catalysts, is unearthed by infra-red spectroscopy coupled to controlled potential electrolysis in corroboration with density functional theory (DFT) investigations. Thermodynamic and kinetic analyses of the intermediate reveal the important role of the structural flexibility of polypyridyl ligands and fine electronic tunability of the metal center, along with kinetic trans effect, in propelling catalysis at lower overpotentials. The choice of metal center, Ru in the present case, points to the fact that the requirement of an additional Lewis acid to enhance C-O bond dissociation, hence increase the catalytic rate or turnover, can be circumvented.
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| 3. |
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| 4. |
- Ahlstrand, David A., et al.
(författare)
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Csp(3)-H Activation without Chelation Assistance in an Iridium Pincer Complex Forming Cyclometallated Products
- 2017
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Ingår i: Chemistry - A European Journal. - : WILEY-V C H VERLAG GMBH. - 0947-6539 .- 1521-3765. ; 23:8, s. 1748-1751
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Tidskriftsartikel (refereegranskat)abstract
- Cyclometallation of 8-methylquinoline and 2-(dimethylamino)-pyridine in an iridium-based pincer complex is described. The C-H activation of 2-(dimethylamino) pyridine is not chelation assisted, which has not been described before for Csp(3)-H bonds in cyclometallation reactions. The mechanism of the cyclometallation of 2-(dimethylamino) pyridine was studied by DFT calculations and kinetic measurements.
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| 5. |
- Castner, Ashleigh T., et al.
(författare)
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Microscopic Insights into Cation-Coupled Electron HoppingTransport in a Metal-Organic Framework br
- 2022
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 144:13, s. 5910-5920
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Tidskriftsartikel (refereegranskat)abstract
- Electron transport through metal-organic frameworks by ahopping mechanism between discrete redox active sites is coupled to diffusion-migration of charge-balancing counter cations. Experimentally determinedapparent diffusion coefficients,Deapp, that characterize this form of chargetransport thus contain contributions from both processes. While this is wellestablished for MOFs, microscopic descriptions of this process are largelylacking. Herein, we systematically lay out different scenarios for cation-coupledelectron transfer processes that are at the heart of charge diffusion throughMOFs. Through systematic variations of solvents and electrolyte cations, it isshown that theDeappfor charge migration through a PIZOF-type MOF,Zr(dcphOH-NDI) that is composed of redox-active naphthalenediimide(NDI) linkers, spans over 2 orders of magnitude. More importantly, however,the microscopic mechanisms for cation-coupled electron propagation arecontingent on differing factors depending on the size of the cation and its propensity to engage in ion pairs with reduced linkers,either non-specifically or in defined structural arrangements. Based on computations and in agreement with experimental results, weshow that ion pairing generally has an adverse effect on cation transport, thereby slowing down charge transport. In Zr(dcphOH-NDI), however, specific cation-linker interactions can open pathways for concerted cation-coupled electron transfer processes thatcan outcompete limitations from reduced cationflux.
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| 6. |
- Chen, Xiaoyu, et al.
(författare)
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Aggregation and Significant Difference in Reactivity Therein : Blocking the CO2-to-CH3OH Reaction
- 2021
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Ingår i: Organometallics. - : American Chemical Society (ACS). - 0276-7333 .- 1520-6041. ; 40:17, s. 3087-3093
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Tidskriftsartikel (refereegranskat)abstract
- A CoPc/CNT system has been only recently reported to transform CO2 to methanol via electrochemical reductions, despite the fact that catalyst has been studied extensively since the 1980s. The explanation of high methanol selectivity lies behind the fact that in the new report CoPc exists mainly as a monomer, while in earlier works aggregates dominate. Here, we have studied the reactivity of monomeric and dimeric CoPc by DFT. The mechanism involves rate-limiting CO2 association, with the C-O cleavage step having very similar activation free energy. Once the Co-CO-intermediate is formed, the reaction bifurcates with two possible paths: (1) CO dissociation or (2) one additional reduction follows a protonation to give the Co-CHO-intermediate, which then leads to methanol by further reactions. For the monomeric species at low reduction potentials, CO dissociation is favored, but the formation of Co-CHO-becomes competitive at more negative applied potentials. For the dimer, the CO dissociation is always favored, and the reduction needed to form the C-H bond is negative enough for it not to be observed. The more difficult reduction stems from repulsive interactions between the CoPc units and lower solvent stabilization of the charge in the aggregate.
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| 7. |
- Chen, Xiaoyu, et al.
(författare)
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Aggregation and the Siginificant Difference in Reactivity therein: Blocking the CO2-to-CH3OH Reaction Pathway
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- A CoPc/CNT system was recently reported to transform CO2 to methanol via electrochemical reductions, despite the catalyst has been studied since the 1980s, such observations were not reported earlier. A clue to the high methanol selectivity is that CoPc exist as mainly as monomers in the new report while in earlier works CoPc aggregates dominate. Here we have studied the reactivity of monomeric and dimeric CoPc by DFT. The mechanism involves rate limiting CO2 association, with the C-O cleavage step being having very similar activation free energy. Once the Co-CO- intermediate is formed the reaction bifurcates with two possible paths, CO dissociation or further reduction and protonation to give the Co-CHO- intermediate, which then leads to methanol by further reactions. For the monomeric species at low reduction potentials CO dissociation is favored, but the formation of Co-CHO- becomes competitive at more negative applied potentials. For the dimer the CO dissociation is always favored, and the reduction needed to form the C-H bond is negative enough for it not to be observed. The more difficult reduction stems from repulsive interac- tions between the Co-Pc units and lower solvent stabilization of the charge in the aggregate.
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| 8. |
- Chen, Xiaoyu, et al.
(författare)
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Deconstructing the Enhancing Effect on CO2 Activation in the Electric Double Layer with EVB Dynamic Reaction Modeling
- 2020
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 124:41, s. 22479-22487
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Tidskriftsartikel (refereegranskat)abstract
- The reactivities of the same molecular electrocatalyst under homogeneous and heterogeneous conditions can be dramatically different, highlighting that the reaction environment plays an important role in catalysis. For catalysis on solid electrodes, reactions take place in the electric double layer (EDL), where a strong electric field is experienced. In this work, empirical valence bond molecular dynamics (EVB-MD) was used to explore CO2binding in the EDL. It allows explicit descriptions of the solvent, electrolyte, catalyst–reactant, and the electrode surface material, as well as an unbiased description of the applied electric field. The strong local electric field concentrates cations, which in turn stabilizes the bound CO2. Furthermore, controlled computational experiments suggest that neither the electric field nor the cations alone can produce significant stabilization, but that the combination leads to a dramatic stabilization of the CO2 bound state.
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| 9. |
- Chen, Xiaoyu, 1993-
(författare)
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Theoretical Studies on CO2 Reduction Electrocatalysts
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The atmospheric CO2 concentration has increased by more than 20% since 1980s and has now reached the highest level than at any point in the past 800 000 years. Electrochemical CO2 reductions are receiving particular in- terest as the apparatus are relatively easy to maintain and cheap to operate. However, the direct reduction of CO2 into CO2 radical requires a very high over-potential, meaning a substantial waste in energy. In order to lower the over-potential required, a large number of catalysts has been synthesised and studied. Among these catalysts, three are studied in this work due to their interesting reactivities. We believe the further understanding gained in our studies will benefit the development of new and better catalysts.Ru(6-Mebpy)(tBu3-tpy) reduces CO2 at its first reduction potential and can therefore lower the over-potential required significantly. This observation is unique for Ru(tpy)(bpy) type of catalysts. Density functional theory (DFT) cal- culations revealed that the steric hindrance provided by the 6-methyl group weakens Ru-solvent interactions and hence allows solvent detachment to take place after only one reduction, which is otherwise not possible. Furthermore, we proposed a new mechanism for CO2 to CO reduction at the first reduc- tion potential and identified a cyclic intermediate by Infra-red spectroscopy in collaboration with experimentalists. Such intermediate was not reported pre- viously for Ru-based electrocatalysts.Co(TPP)/CNTs as a heterogenous catalyst exhibits superior reactivity as compared to in solution. DFT calculations with implicit solvent model ac- counts its enhanced reactivity to the increased proton concentration in water. The inverse-loading effect was studied by potential mean force (PME) sam- pling. Our results suggest that aggregation is triggered by the strong inter- molecular p - p interactions among the catalysts. Flatter nanotubes have better contact with Co(TPP) and hence reduces aggregation tendency. The same cat- alyst was also used as an example to study catalysis at interfaces in an electric field. Our full-explicit EVB -MD (Empirical Valence Bond-Molecular Dynam- ics) model illustrates that the electric double layer concentrates cations, which significantly stabilises polarised CO2 at a higher concentration and hence eases CO2 binding. Furthermore, we have also shown that either the electric field or the cations along provides only a minor, almost negligible stabilisation.In 2019, CoPc/CNTs was reported to be the first early-period transition metal complex that can catalyse CO2-to-CH3OH conversion at a decent yield. Literature search on previous work suggests that the presence of well-dispersed, monomeric CoPc is crucial to further reduce CO into CH3OH. We calculated the reaction profiles for both monomeric CoPc and dimeric CoPc, which is the simplest form of aggregates. Our DFT results demonstrate that after the formation of catalyst-CO- complex, monomers tend to go though further reac- tions to afford CH3OH while dimers tend to dissociate CO as reductions are slightly harder, which in turn, is raised from a less degree of solvation stabili- sation upon reductions.
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| 10. |
- Chen, Xiaoyu, et al.
(författare)
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Understanding the Enhanced Catalytic CO2 Reduction upon Adhering Cobalt Porphyrin to Carbon Nanotubes and the Inverse Loading Effect
- 2020
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Ingår i: Organometallics. - : American Chemical Society (ACS). - 0276-7333 .- 1520-6041. ; 39:9, s. 1634-1641
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Tidskriftsartikel (refereegranskat)abstract
- Adhering a cobalt porphyrin (Co(TPP)) catalyst on a carbon nanotube (CNT) supporting material greatly enhances its reactivity and enables catalysis in water, which is otherwise impossible. However, the effect of solvent as well as supporting materials on catalysis is still elusive. On the basis of computational results we found that water as a reaction medium lowers the reductive potential required due to the stabilization of intermediates and transition states, and provides higher availability of protons. To understand the effect of the support materials, we combine computations and experiments and illustrate that the curvature of the nanotubes plays an essential role in aggregation through the competition between the Ï-πinteractions between the porphyrin rings as well as between the Co(TPP) and the nanotube, providing an insight into lessening the degree of aggregation.
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