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    Öğe
    H2 Evolution Reaction Pathway of a Cobalt Bis-bipyridyl Planar Catalyst Revealed by Ab Initio Simulations
    (Amer Chemical Soc, 2024) Güçlü, Ipek; Kiser, Ayas; Gurdal, Yeliz
    Ligand type and ligand coordination around cobalt photocatalysts influence the reaction mechanism of H2 production via water splitting. We investigated the H2 production mechanisms of a cobalt bis-bipyridyl planar water reduction catalyst in an aqueous solution using ab initio molecular dynamics, metadynamics, density functional theory, and free energy perturbation theory. Modeling each intermediate step of the reaction mechanism reveals that the cobalt-based planar catalyst exhibits a preference for a pathway involving two successive reductions, followed by two protonation steps. The reason for this preference is the presence of an energy barrier for the conformational change of water molecules in the first solvation shell following the first reduction. This barrier prevents direct coordination of cobalt with protons, which is necessary for the initial protonation step in the sequential electron- and proton-transfer mechanisms. In the mechanism involving two successive reductions followed by two protonations, however, the second reduction step facilitates direct interaction between cobalt and protons, enabling the initial protonation. Following the second protonation of this mechanism, H2 is produced and released into the solvent. Reduction free energy calculations reveal that, following the first reduction, the singlet spin state of the system is more favorable than the triplet spin state. The reduction free energy of the second electron transfer is 0.19 eV less energetic than that of the first reduction reaction.
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    Öğe
    Ideal octahedral geometry leads to poor H2 evolution performance: Theoretical insights on reaction intermediates of cobalt-pentapyridyl molecular catalyst
    (Pergamon-Elsevier Science Ltd, 2025) Kiser, Ayas; Gurdal, Yeliz
    The type and coordination of ligands surrounding water reduction photo-catalysts affect the H2 production reaction pathway. In this respect, we examined two successive reductions followed by two protonations mechanism of H2 evolution reaction catalyzed by Cobalt-based pentapyridyl molecular catalyst in an aqueous solution employing Ab-initio Molecular Dynamics (AIMD), Density Functional Theory (DFT), and Free Energy Perturbation Theory. Each intermediate step of the H2 evolution reaction mechanism was simulated to determine the allowable spin states and solvent response surrounding the reaction center of the catalyst. A single electron transfer to the catalyst, leading to a singlet spin state, induces a conformational change in the water molecule in the first solvation shell, facilitating the formation of a Co-proton bond. Subsequent to the second electron transfer, the Co center acquires the proton from this water molecule, while the remaining OH- ion diffuses swiftly through the solvent. The first protonation step was determined to continue adopting a quartet spin state. Due to the catalyst's cage-like structure around the Co center, following the second protonation, H2 forms; however, it fails to diffuse from the reaction center into the solvent, suggesting the importance of maintaining an open structure around the Co center when designing ligand structures for water- splitting catalysts. The first and the second reduction free energies were calculated as -2 eV and -1.3 eV, respectively.
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    Öğe
    Theoretical modeling of heterolytic mechanism for H2 production: A detailed investigation of Cobalt(II)-polypyridyl catalyst
    (Adana Alparslan Türkeş Bilim ve Teknoloji Üniversitesi, 2022) Kiser, Ayas; Durğun, Yeliz Gürdal
    Hidrojen (H2) molekülü biyokütle, su ve biyokarbonun içerisinde doğal olarak bulunan bir enerji taşıyıcısıdır. Güneş enerjisi ise bitkilerin fotosentez mekanizmalarında doğal olarak kullandıkları en yaygın bulunan enerji kaynağıdır. Araştırmacılar doğanın önemli bir olayı olan fotosentezin ışıklı reaksiyonlarındaki kusursuz H2 üretme yolağından ilham almışlardır. Fotosentezin su indirgeme mekanizmasını taklit etmek amacıyla çok çeşitli geçiş metalleri kullanılmıştır. Buna rağmen araştırmacılar, kobalt bazlı fotokatalizörleri keşfedene kadar, fiyat/performans açısından tatmin edici bir cevap bulamamışlardır. Yakın zamanda, Kobalt bazlı fotokatalizörler kullanılarak fotokatalitik su indirgemesi üzerine bir çok araştırma yapılmıştır. Bu bağlamda, tezimizde düzgün dörtyüzlü koordinasyona ve çoklu piridil grubuna sahip Co-bazlı katalizörün su indirgeme, bir diğer adıyla, H2 üretme mekanızması Ab-initio moleküler dinamik, Yoğunluk Fonksiyoneli Teorisi ve Serbest Enerji Pertürbasyon Teorisi kullanılarak incelenmiştir. Bu tezdeki amacımız, oktahedral koordinasyondaki düzenlemenin etkisini açıklığa kavuşturmak ve ayrıca, heterolitik H2 üretme mekanizmasının yolaklarından ikisi olan ECEC ve EECC mekanizmalarını karşılaştırmaktır. Dolayısıyla, çalışma boyunca, hidrojen üretim döngüsünün ECEC ve EECC mekanizmaları değerlendirilerek, mekanizmanın her ara adımı simüle edilmiş ve ayrıca Co-bazlı su indirgeme katalizörünün çevresindeki çözelti tepkisi incelenmiştir. Buna ek olarak, her iki mekanizmanın her bir adımının elektronik yapıları ek simülasyonlar uygulanarak analiz edilmiştir. Elektron transferi için Marcus Teorisi kullanılmış ve indirgenme serbest enerjileri hesaplanmıştır.
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    Öğe
    Why the Perfectly Symmetric Cobalt-Pentapyridyl Loses the H2 Production Challenge: Theoretical Insight into Reaction Mechanism and Reduction Free Energies
    (Wiley-V C H Verlag Gmbh, 2024) Kiser, Ayas; Gurdal, Yeliz
    Researchers have extensively investigated photo-catalytic water reduction utilizing Cobalt-based catalysts with polypyridyl ligands. While catalysts exhibiting distorted polypyridyl ligand demonstrate higher H-2 production yields, those with ideal octahedral coordination display poor performance. This outcome suggests the crucial role of ligand framework in catalytic activity, yet reasons behind the disparity in H-2 production rates for catalysts with octahedral geometries remain unclear. We theoretically examined the water reduction mechanism of Co-based poly-pyridyl catalyst, CoPy5, having perfect octahedral coordination. We clarified the effect of octahedral coordination by utilizing each intermediate step of ECEC mechanism. We determined spin states, solvent response, electronic structures, and reduction free energies. CoPy5 with perfect octahedral coordination, alongside its distorted counterparts, exhibit similar spin states as the reaction progresses through each intermediate step. However, the first reduction free energy obtained for the CoPy5 is slightly higher than that of its distorted counterparts. Following the second protonation, resulting H-2 molecule experiences limited diffusion from the Co center due to the compact structure of the CoPy5, which blocks the Co center for the next H-2 production cycle. Catalysts having distorted octahedral geometries facilitate fast removal of H-2 into the solvent. Thus, the reaction center becomes immediately available for subsequent H-2 production.