Institute of Chemistry

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Author: search.filters.author.Koleva, VioletaLanguage: search.filters.language.English

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    Facile Synthesis of Cu<sub>x</sub>S Electrocatalysts for CO<sub>2</sub> Conversion into Formate and Study of Relations Between Cu and S with the Selectivity
    (Wiley, 2024-10-25)
    Stojkovikj, Sasho
    ;
    El‐Nagar, Gumaa A.
    ;
    Gupta, Siddharth
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    ;
    Koleva, Violeta
    <jats:title>Abstract</jats:title><jats:p>The conversion of CO<jats:sub>2</jats:sub> into formate (HCOO<jats:sup>−</jats:sup>), a techno‐economically feasible product, can be achieved using earth‐abundant Cu<jats:sub>x</jats:sub>S electrocatalysts, but questions remain regarding how catalyst structure, composition, and reaction environment influence product selectivity. A novel synthesis method based on electrodeposition of Cu foam and its subsequent sulfidation via immersion in sulfur saturated toluene solution resulted in Cu<jats:sub>x</jats:sub>S foams. Catalytic activity studies found that HCOO<jats:sup>−</jats:sup> selectivity is dependent on electrochemical activation at higher overpotentials. To understand the effects of activation, determine the active forms of the catalysts, and identify the role of sulfur, the electrodes are carefully characterized as well as gaseous and sulfur dissolved in electrolyte. This included study of the effects of intentional addition of solution sulfur species, identification of the sulfur loss, determination of the electrode composition and relating sulfur speciation to observed product selectivity. It is found that residual sulfur stabilizes Cu<jats:sup>+</jats:sup> during electrolysis at potentials favoring HCOO<jats:sup>−</jats:sup> production, in contrast to pristine Cu that undergoes complete reduction and shows poor HCOO<jats:sup>−</jats:sup> selectivity. Sulfur in both the catalyst and dissolved in electrolyte are of dynamic nature, and surface residues of SO<jats:sub>4</jats:sub><jats:sup>2−</jats:sup> species are identified in all activated catalysts which correspond with enhanced HCOO<jats:sup>−</jats:sup> production.</jats:p>
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    Thermal behavior of acid phosphate salts Ca2MH7(PO4)4⋅2H2O (M = K+, NH4 +) and CaK3H(PO4)2
    (Elsevier, 2023-06)
    Koleva, Violeta
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    Najkov, Kosta
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    ;
    New knowledge about various aspects of little-known acid salts is of great scientific and practical importance in view of their potential application in different areas such as proton conductors. For the first time, the thermal behavior of three acid phosphate salts, dicalcium potassium heptahydrogen tetrakis(phosphate) dihydrate (Ca2KH7(PO4)4⋅2H2O), dicalcium ammonium heptahydrogen tetrakis(phosphate) dihydrate (Ca2(NH4)H7(PO4)4⋅2H2O) and calcium tripotassium hydrogenbis(phosphate) (CaK3H(PO4)2), has been investigated. By means of simultaneous thermogravimetry, differential thermal and mass spectrometry analyses (TG/DTA/MS) he schemes of their thermal decomposition have been proposed. The two isostructural compounds Ca2KH7(PO4)4⋅2H2O and Ca2(NH4)H7(PO4)4⋅2H2O are stable up to 90 – 95 ◦C and then undergo multiple-steps thermal decomposition process owing to dehydration of the crystallization water and dehydration-condensation. The anhydrous salt CaK3H(PO4)2 exhibits very high thermal stability up to 530 ◦C. The products of the thermal decomposition have been identified.