| 1. |
- Hedenstedt, Kristoffer, 1979, et al.
(författare)
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Study of Hypochlorite Reduction Related to the Sodium Chlorate Process
- 2016
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Ingår i: Electrocatalysis. - : Springer Science and Business Media LLC. - 1868-2529 .- 1868-5994. ; 7:4, s. 326-335
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Tidskriftsartikel (refereegranskat)abstract
- Reduction of hypochlorite is the most important side reaction in the sodium chlorate reactor leading to high energy losses. Today chromate is added to the reactor solution to minimize the hypochlorite reduction but a replacement is necessary due to health and environmental risks with chromate. In order to understand the effect of different substrates on the hypochlorite reduction, α-FeOOH, γ-FeOOH, Cr2O3 and CrOH3 were electrodeposited on titanium and subjected to electrochemical investigations. These substances are commonly found on cathodes in the chlorate process and can serve as model substances for the experimental investigation. The mechanism of hypochlorite reduction was also studied using DFT calculations in which the reaction at Fe(III) and Cr(III) surface sites were considered in order to single out the electrocatalytic properties. The experimental results clearly demonstrated that the chromium films completely block the reduction of hypochlorite, while for the iron oxyhydroxides the process can readily occur. Since the electrocatalytic properties per se were shown by the DFT calculations to be very similar for Fe(III) and Cr(III) sites in the oxide matrix, other explanations for the blocking ability of chromium films are addressed and discussed in the context of surface charging, reduction of anions and conduction in the deposited films. The main conclusion is that the combined effect of electronic properties and reduction of negatively charged ions can explain the reduction kinetics of hypochlorite and the effect of chromate in the chlorate process.
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| 2. |
- Wiberg, Cedrik, 1989, et al.
(författare)
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The electrochemical response of core-functionalized naphthalene Diimides (NDI) – a combined computational and experimental investigation
- 2021
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Ingår i: Electrochimica Acta. - : Elsevier BV. - 0013-4686 .- 1873-3859. ; 367
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Tidskriftsartikel (refereegranskat)abstract
- © 2020 Aqueous organic redox flow batteries (AORFBs) have attracted increased interest as sustainable energy storage devices due to the desire of increasing electricity production from renewable energy sources. Several organic systems have been tested as redox active systems in AORFBs but few fundamental electrochemical studies exist. This article provides reduction potentials and acid constants, pKa, of nine different core-substituted naphthalene diimides (NDI), calculated using density functional theory (DFT). Reduction potentials were acquired at each oxidation state for the nine species and were used to achieve a correlation between the electron donating ability of the substituents and the potential. Cyclic voltammograms were simulated using the scheme-of-squares framework to include both electron and proton transfer processes. The results show that the anion radical is unprotonated in the entire pH range, while the dianion can be protonated in one or two steps depending on the substituent. The core substituents may also have acid-base properties. and optimization of the redox properties for battery applications can therefore be obtained both by changing the core substituent and by changing pH of the electrolyte. Three core-substituted NDI molecules were studied experimentally and good qualitative agreement with the theoretically predicted behaviour was demonstrated. For 2,6-di(dimethylamino)-naphthalene diimide (2DMA-NDI), the calculations showed that one of the DMA substituents could be protonated in the accessible pH range and pKa was determined to 3.95 using 1H NMR spectroscopy. The redox mechanism of each molecule was explored and the qualitative agreement between theory and experiment clearly shows that this combination provides a better understanding of the systems and offers opportunities for further developments. The applicability of NDI for redox flow batteries is finally discussed.
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| 3. |
- Wodrich, Matthew D., et al.
(författare)
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On the Generality of Molecular Volcano Plots
- 2018
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Ingår i: ChemCatChem. - : Wiley. - 1867-3899 .- 1867-3880. ; 10:7, s. 1586-1591
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Tidskriftsartikel (refereegranskat)abstract
- In homogeneous catalysis, the structure and electronic configuration of the active catalysts can vary significantly. Changes in ligation, oxidation, and spin states have the potential to influence the catalytic cycle energetics strongly that, to a large degree, dictate the catalytic performance. With the increased use of computational screening strategies aimed towards the identification of new catalysts, ambiguity surrounding structure/electronic configurations can be problematic, as it is unclear which species should be computed to determine a catalyst's properties. Here, we show that a single volcano plot constructed from linear free energy scaling relationships is able to account for variations in ligation, oxidation, and spin state. These linear scaling relationships can also be used to predict the free energies associated with a specific structure and electronic configuration of a catalyst based on a single descriptor. As a result, a single volcano plot can be used to screen prospective new catalysts rapidly.
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| 4. |
- Busch, Michael, 1983, et al.
(författare)
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Water Oxidation on MnOx and IrOx: Why Similar Performance?
- 2013
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 117:1, s. 288-292
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Tidskriftsartikel (refereegranskat)abstract
- The critical steps in water oxidation at a binuclear Mn(II–IV) oxide site are revisited. Ideal stabilities of intermediates are confirmed by comparing to results for a binuclear Ir(III–V) system. The latter in turn is known to be an excellent water oxidation catalyst. The inefficiency of the binuclear Mn(II–IV) site is owing to the high activation energy for the chemical step whereby MnIV═O double bonds on adjacent sites are broken prior to forming the MnIII—O—O—MnIII peroxy moiety. A rationale for Mn(II–IV)—Mn(III–V) mixed oxidation state for water oxidation catalysis, analogous to mixed transition metal oxide systems, is offered. Possible virtues of the kinetic stability of the binuclear MnIV═O moiety are discussed, utilizing its oxidizing power by sidestepping oxygen evolution.
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| 5. |
- Steegstra, Patrick, 1978, et al.
(författare)
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Revisiting the Redox Properties of Hydrous Iridium Oxide Films in the Context of Oxygen Evolution
- 2013
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 117:40, s. 20975-20981
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Tidskriftsartikel (refereegranskat)abstract
- The electrochemistry of hydrous iridium oxide films (HIROF) is revisited. Cyclic voltammograms of HIROFs display two reversible redox couples commonly assigned to the Ir(III)/Ir(IV) and Ir(IV)/Ir(V) transitions, respectively. However, compared to the first, the second redox couple has significantly less charge associated to it. This effect is interpreted as partial oxidation of Ir(IV) as limited by nearest neighbor repulsion of resulting Ir(V) sites. Thus, the redox process is divided into two steps: one preceding and one overlapping the oxygen evolution reaction (OER). Here, the ``super-nernstian'' pH dependence of the redox processes in the HIROF is used to expose how pH controls the overpotential for oxygen evolution, as evidenced by the complementary increased formation of Ir(V) oxide. A recently formulated binuclear mechanism for the OER is employed to illustrate how hydrogen bonding may suppress the OER, thus implicitly favoring Ir(V) oxide formation above the thermodynamic onset potential for the OER at low pH.
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| 6. |
- Visibile, Alberto, et al.
(författare)
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Influence of Strain on the Band Gap of Cu2O
- 2019
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 1520-5002 .- 0897-4756. ; 31:13, s. 4787-4792
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Tidskriftsartikel (refereegranskat)abstract
- Cu2O has been considered as a candidate material for transparent conducting oxides and photocatalytic water splitting. Both applications require suitably tuned band gaps. Here we explore the influence of compressive and tensile strain on the band gap by means of density functional theory (DFT) modeling. Our results indicate that the band gap decreases under tensile strain while it increases to a maximum under moderate compressive strain and decreases again under extreme compressive strain. This peculiar behavior is rationalized through a detailed analysis of the electronic structure by means of density of states (DOS), density overlap region indicators (DORI), and crystal overlap Hamilton populations (COHP). Contrary to previous studies we do not find any indications that the band gap is determined by d10-d10 interactions. Instead, our analysis clearly shows that both the conduction and the valence band edges are determined by Cu-O antibonding states. The band gap decrease under extreme compressive strain is associated with the appearance of Cu 4sp states in the conduction band region.
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| 7. |
- Busch, Michael, 1983, et al.
(författare)
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Adsorption of NO on Fe3O4(111)
- 2018
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 693, s. 84-87
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Tidskriftsartikel (refereegranskat)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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| 8. |
- Busch, Michael, 1983
(författare)
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Water oxidation: From mechanisms to limitations
- 2018
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Ingår i: Current Opinion in Electrochemistry. - : Elsevier BV. - 2451-9111 .- 2451-9103. ; 9, s. 278-284
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Forskningsöversikt (refereegranskat)abstract
- Water oxidation is a central reaction for energy conversion and storage. On the basis of the detailed analysis of different reaction mechanisms the limitations associated with the O2evolution reaction is analysed. We find that overpotentials lower than the 0.4 eV, which have been observed for a mono-nuclear mechanism, are possible if the problematic *-OOH intermediate is avoided. This is the case for the bi-nuclear and bi-functional reaction paths. 3-dimensional volcano plots are constructed and used to explore the performance of these reaction mechanisms. Our results suggest, that in contrast to previous analysis of the mono-nuclear mechanism, the true top of the water oxidation volcano is placed at slightly lower *=O binding energies of only 2.46 eV if combined with a suitable co-catalyst. On the basis of this new analysis, design criteria for improved catalysts are developed.
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| 9. |
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| 10. |
- Patra, Anuttam, et al.
(författare)
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Selective flotation of calcium minerals using double-headed collectors
- 2019
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Ingår i: Journal of Dispersion Science and Technology. - : Taylor & Francis. - 0193-2691 .- 1532-2351. ; 40:8, s. 1205-1216
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Tidskriftsartikel (refereegranskat)abstract
- A study was performed involving a series of double-headed carboxylate collectors with varying distance between the head groups (one, two or three carbon atoms). A collector with the same alkyl chain length but with only one carboxylate group was also included. All these were amino-acid based amphiphiles and the polar head group was connected to the hydrophobic tail via an amide linkage. Selective flotation recovery of different calcium minerals using these collectors was investigated. The double-headed collector with one carbon atom between the carboxylate groups was an apatite and fluorite specific reagent while the monocarboxylate surfactant showed high specificity for calcite. The flotation behavior of a simple conventional collector of the same alkyl chain length, a fatty acid salt, was also determined under identical flotation conditions in order to understand the effect of the amide group. Complementary experiments (ζ potential measurements, adsorption isotherm determinations) were also performed for these reagents. In order to shed light on the selectivity obtained with the dicarboxylate surfactants, the distances between the head groups were calculated and compared with the distances between neighboring calcium atoms on the surface of the minerals. It was found that the high degree of selectivity could be rationalized by perfect matching of these distances. To the best of our knowledge this is the first study where flotation selectivity in complex calcium mineral systems has been explained in terms of molecular recognition governing the interaction between the collector and the mineral surface.
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