| 1. |
- Baran, Tomasz, et al.
(author)
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Copper oxide-based photocatalysts and photocathodes: Fundamentals and recent advances
- 2021
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In: Molecules. - : MDPI AG. - 1420-3049 .- 1420-3049 .- 1431-5157. ; 26:23
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Research review (peer-reviewed)abstract
- This work aims at reviewing the most impactful results obtained on the development of Cu-based photocathodes. The need of a sustainable exploitation of renewable energy sources and the parallel request of reducing pollutant emissions in airborne streams and in waters call for new technologies based on the use of efficient, abundant, low-toxicity and low-cost materials. Photoelectrochemical devices that adopts abundant element-based photoelectrodes might respond to these requests being an enabling technology for the direct use of sunlight to the production of energy fuels form water electrolysis (H2) and CO2 reduction (to alcohols, light hydrocarbons), as well as for the degradation of pollutants. This review analyses the physical chemical properties of Cu2O (and CuO) and the possible strategies to tune them (doping, lattice strain). Combining Cu with other elements in multinary oxides or in composite photoelectrodes is also discussed in detail. Finally, a short overview on the possible applications of these materials is presented.
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| 2. |
- Busch, Michael, 1983, et al.
(author)
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A Generalized Picture of C-C Cross-Coupling
- 2017
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In: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 7:9, s. 5643-5653
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Journal article (peer-reviewed)abstract
- Transition metal catalyzed cross-coupling reactions occupy a privileged position in chemistry because of their ability to link myriad functional groups. The numerous variants of this class of reactions (e.g., Suzuki, Kumada, Negishi, etc.) differ in the transmetalation agent used to transfer "R" groups onto the catalyst. While understanding any single variant (e.g., Suzuki coupling) can be accomplished through direct analysis of the catalytic cycle, a comprehensive picture that illustrates the interrelationships between the different types of cross coupling reactions remains absent. Here, using a tool built upon a three-dimensional volcano plot, we create a generalized thermodynamic picture of C-C cross-coupling reactions. This "cross-coupling" genome not only facilitates better understanding of catalytic behavior but also outlines strategies for developing new reaction protocols through the manipulation of easily computed descriptor variables.
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| 3. |
- Busch, Michael, 1983, et al.
(author)
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Activation Energies in Computational Chemistry - A case study
- 2011
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In: Rate Constant Calculation of Thermal Reactions: Methods and Applications, Herbert DaCosta (Ed.), John Wiley & Sons. - Hoboken, NJ, USA : John Wiley & Sons, Inc.. - 9780470582305
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Book chapter (other academic/artistic)abstract
- A straight-forward way to learn how different complementary properties of a catalyst control a resulting activation energy is proposed within the frame work of Density Functional Theory. It is argued that in special cases the activation energy can be approximated from the crossing of the two vibrational modes’ harmonic oscillator potentials corresponding to the reactant and the product, respectively. The procedure is argued to be applicable in cases were traditional transition state search algorithms such as the synchronous transit or the nudged elastic band methods are of limited use. The constraints of the present approach include accessibility of reactant and product structures as well as availability of normal modes pointing towards the transition state. The usefulness of the proposed procedure is demonstrated for the O-O bond formation step in the water oxidation reaction (OER). A comparative study of the activation energy for said reaction is understaken, employing (i) a molecular manganese dimer, (ii) an embedded manganese dimer, and (iii) an embedded cobalt dimer. In case of the two latter, an MgOx(OH)y is used as support. It is shown how the activation barrier for said reaction step is influenced by mainly two factors, (a) the flexibility of a catalyst and (b) the equilibrium O—O distance of the dioxo- species. It is demonstrated how in case of a flexible molecular catalyst, the influence of the O—O distance is negliable, while it is decisive to the activation energy in case of a more stiff embedded catalyst.
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| 4. |
- Busch, Michael, 1983, et al.
(author)
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Adsorption of NO on Fe3O4(111)
- 2018
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In: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 693, s. 84-87
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Journal article (peer-reviewed)abstract
- Adsorption of NO on Fe 3 O 4 (111) is studied by density functional theory (DFT) calculations. NO is preferably adsorbed atop the octahedral site which has a clearly higher adsorption energy than the tetrahedral site. The difference in adsorption energy correlates with differences in adsorption geometries and N–O stretch vibrations. The results are in good agreement with temperature programmed desorption (TPD) and reflection-adsorption IR spectroscopy (RAIRS) measurements and provide an explanation of the observation of only one vibrational mode despite two distinct TPD features.
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| 5. |
- Busch, Michael, 1983, et al.
(author)
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Electrocatalytic Oxygen Evolution from Water on a Mn(III-V) Dimer Model Catalyst - A DFT Perspective
- 2011
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In: Physical Chemistry Chemical Physics. - 1463-9076 .- 1463-9084. ; 13, s. 15069-15076
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Journal article (peer-reviewed)abstract
- A complete water oxidation and oxygen evolution reaction (OER) cycle is monitored by means of Density Functional Theory (DFT). A biomimetic model catalyst is employed, comprising a m-OH bridged Mn(III-V) dimer truncated by acetylacetonate ligand analogs and hydroxides. The reaction cycle is divided into four electrochemical hydrogen abstraction steps followed by a series of chemical steps. The former employ the Tyrosine/Tyrosyl radical acting as electron and proton sink thus determining the reference potential. Stripping hydrogen from water leads to the formation of two highly unstable Mn(V)=O/Mn(IV)-O· moieties, which subsequently combine to form a peroxy O-O bond. O2 evolution results from subsequent consecutive replacement of remaining Mn-O bonds by water. Conditions for the validities of GGA DFT and self-interaction error corrected hybrid DFT predictions despite the presence of a manifold of near-degenerate spin states, are discussed in some detail. The applicability of the former is extended to include the rate limiting steps in the OER.
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| 6. |
- Busch, Michael, 1983, et al.
(author)
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Hydroxide Oxidation and Peroxide Formation at Embedded Binuclear Transition Metal Sites; TM=Cr, Mn, Fe, Co
- 2011
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In: Physical Chemistry Chemical Physics. - 1463-9076 .- 1463-9084. ; 13, s. 15062-15068
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Journal article (peer-reviewed)abstract
- Key steps in electro-catalytic water oxidation on binuclear Transition Metal (TM) sites are addressed. These comprise (a) two one-electron oxidation steps of TM-OH moieties to form the corresponding two TM=O oxy-groups, and (b) a chemical step whereby the two oxy-species form a TM-O-O-TM peroxy-bridge. A test rig representing a generic low crystal field oxide support is described and employed. The energetics for said reactions are compared for homo-nuclear Cr(IIIV), Mn(III-V), Fe(II-IV) and Co(II-IV) sites. The uniqueness of the TyrO/TyrOH reference potential for driving said oxidation steps is demonstrated. Hydroxide oxidation on binuclear Mn and Co candidates require an overpotential of approximately 0.5 V relative to the chosen reference potential. Correspondingly, the subsequent O-O bond formation becomes strongly exothermic of the order of 1 eV. Hydroxide oxidation on binuclear CrCr and FeFe systems are exothermic by 0.2-0.4 eV relative to the TyrO/TyrOH reference potential. Consequently, the chemical step for transforming the TM=O moieties to the peroxo species is found to be endothermic on the order of 0.7 eV. These findings suggest that hetero-binuclear sites containing one TM from each class would be optimal. The validity of this concept is demonstrated for the FeCo binuclear site. The results are discussed in the context of experimental observations, which display preference for mixed oxide systems.
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| 7. |
- Busch, Michael, 1983, et al.
(author)
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Improving the Thermodynamic Profiles of Prospective Suzuki–Miyaura Cross-Coupling Catalysts by Altering the Electrophilic Coupling Component
- 2018
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In: ChemCatChem. - : Wiley. - 1867-3899 .- 1867-3880. ; 10:7, s. 1592-1597
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Journal article (peer-reviewed)abstract
- As a heavily used technique for forming new C−C bonds, developing new catalysts and reaction conditions for Suzuki–Miyaura cross-couplings is highly desirable. By using molecular volcano plots, the influence of the electrophilic coupling component in catalytic cycle thermodynamics is revealed. Less reactive electrophiles, such as iodine, broaden the volcano plateau, which leads to a larger number of catalysts having appealing thermodynamic profiles. Whereas, fluorine and other more reactive electrophiles compress the volcano plateau, which shrinks the pool of catalysts having good thermodynamic profiles. As a result, judicious selection of the electrophile employed in a reaction may represent an appealing strategy for further tuning the thermodynamics of cross-coupling reactions.
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| 8. |
- Busch, Michael, 1983
(author)
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Mechanistic Insights into Transition Metal Oxide Catalyzed Water Oxidation
- 2012
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Doctoral thesis (other academic/artistic)abstract
- A binuclear mechanism was proposed and evaluated by means of Density Functional Theory calculations. The central reaction steps were found to be the oxidation of the transition metal TM-OH moieties to TM=O and the subsequent intramolecular O-O bond formation between two TM=O groups. These step were employed as descriptors for the performance of a set of 3d transition metal oxides studied in a MgOx(OH)y test rig embedding. Two classes of oxygen evolution reaction (OER) catalysts were found to emerge at the TyrOH/TyrO reference potential. The first class, referred to as [+/-], which contains Mn(III-V), Co(II-IV) and Ni(II-IV), shows an endothermic oxidation step combined with exothermic O-O bond formation. The members of this class were argued to be active towards the (OER), i.e. the oxy moieties desorb spontaneously as O2 at the expense of high overpotentials. The second class, called [-/+], comprising V(III-V), Cr(III-V) and Fe(II-IV), was found to show opposite behavior. Thus, poor performance is expected due to a highly unfavorable O-O bond formation step. Improved performance was predicted by mixing [+/-] with [-/+] transition metal oxides. Intermediate behavior, meaning oxidation of the TM-OH moiety to TM=O at the TyrOH/TyrO potential combined with a thermoneutral O-O bond formation, is found for Ir(III-V) and Mn(II-IV). While the former displays high activity towards the water oxidation reaction the latter is argued to be inactive due to unfavorable kinetics. The idea of mixing transition metal oxides was generalized for mixed oxidation state systems and evaluated for a Mn(II-IV) Mn(III-V) system. Improved performance was found suggesting, that this reaction path is relevant for the (OER). The generality of the mechanism was shown by direct comparison with experimental findings on iridium oxide and RuO2.
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| 9. |
- Busch, Michael, 1983, et al.
(author)
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The Influence of Inert Ions on the Reactivity of Manganese Oxides
- 2018
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In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 122:1, s. 216-226
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Journal article (peer-reviewed)abstract
- Inert ion doping is a possible method to modify electrical conductivity and catalytic activity of transition-metal oxide electrocatalysts. Despite the importance of dopants, little is known about the underlying mechanisms for the change of the system properties. We have performed density functional theory calculations to investigate the influence of different valent ions on the O 2 evolution reaction activity of β-MnO 2 and Mn 2 O 3 . While Mn 2 O 3 is unaffected by dopants, β-MnO 2 is strongly affected by ions placed in a subsurface position. Doping does not affect the ion charge at the active site, but instead it effects the bond character. This is concluded through an analysis of the density overlap regions indicator and the density of states showing that the partially covalent nature of the bonds in β-MnO 2 is responsible for the changes in the adsorbate binding energies. This mechanism is not active in the mostly ionic Mn 2 O 3 . These results highlight the need to explicitly consider the detailed bonding situation and to go beyond simple charge transfer considerations when describing doping of transition metal oxide catalysts.
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| 10. |
- Busch, Michael, 1983, et al.
(author)
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Validation of binuclear descriptor for mixed transition metal oxide supported electrocatalytic water oxidation
- 2013
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In: Catalysis Today. - : Elsevier BV. - 0920-5861 .- 1873-4308. ; 202:1, s. 114-119
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Journal article (peer-reviewed)abstract
- The energy profiles of the di-hydroxo – di-oxo – peroxo pathway are discussed for a set of 3d transition metal oxides comprising V(III–V), Cr(III–V), Mn(II–IV, Mn(III–V), Fe(II–IV), Co(II–IV) and Ni(II–IV) using density functional theory (DFT). Two classes of oxides were identified. The first class, comprising V(III–V), Cr(III–V) and Fe(II–IV), displays exothermicity for the oxidation of di-hydroxo to di-oxo versus the tyrosine/tyrosyl-radical (TyrOH/TyrO) couple and endothermicity for the subsequent O-O bond formation ([−/+] class), while the second class, comprising Mn(III–V), Co(II–IV) and Ni(II–IV), shows endothermicity with respect to the oxidation step and exothermicity for the O-O bond formation ([+/−] class). The energetics of the endothermicity (exothermicity) for the oxidation step is reflected in the exothermicity (endothermicity) of the subsequent O-O bond formation step. Mn(II–IV) is not part of any of the two classes. Instead it shows zero exothermicity with respect to TyrOH/TyrO for the oxidation step and a small endothermicity for the O-O bond formation step. Despite the promising energy profile Mn(II–IV) is argued to be inactive due to a large activation barrier. A set of improved hetero-nuclear candidate catalysts is predicted by mixing [−/+] with [+/−] transition metal oxides. A simple and efficient method to estimate the energy profile of mixed transition metal oxides from the homo-nuclear systems is demonstrated. The validity of this procedure is checked and agreement with the explicitly calculated values is found. All considered heteronuclear candidate catalysts display enhanced performance compared to the pure homonuclear systems.
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