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    A comparative study of CO catalytic oxidation on the single vacancy and di-vacancy graphene supported single-atom iridium catalysts: A DFT analysis
    (Elsevier B.V., 2021) Akça, Aykan; Karaman, Onur; Karaman, Ceren; Atar, Necip; Yola, Mehmet Lütfi
    Engineering of high-performance catalysts is of great importance for reducing the greenhouse gas emission by the electrocatalytic oxidation of CO. Single-atom-catalysts (SACs) have gained substantial attention thanks to their superior catalytic activity for CO oxidation, and graphene has been considered as one most promising supporting material owing to its peculiar physicochemical properties. In this work, the mechanism of CO oxidation over iridium (Ir) embedded on both single vacancy graphene (Ir-GN(SV)) and di-vacancy graphene (Ir-GN(DV)) has been investigated with the aid of density functional theory (DFT). The structural properties of Ir-GN(SV) and Ir-GN(DV) were analyzed by Bader charge analysis and electron density difference map. The calculated adsorption energy values of CO and O-2 molecules on both the Ir-GNSV and Ir-GN(DV) have validated that both molecules can be molecularly adsorbed on the surface of each catalyst at room temperature. The results put forth that the reaction mechanism of CO + O-2 -> OOCO -> CO2 + O* prefers to Langmuir Hinshelwood (LH) mechanism. The activation energy for the transition-state for Ir-GNSV has been calculated to be 0.31 eV, whereas the first transition state (TS1) and the second transition state (TS2) of Ir-GN(DV) have been determined as 0.30 eV and 0.26 eV, respectively. Moreover, the results have confirmed that Ir-GN(SV) and Ir-GN(DV) surfaces have high catalytic activity and selectivity towards CO oxidation. On the basis of these findings, the proposed Ir-GN(SV) and Ir-GN(DV) catalysts are considered to be promising SAC for CO oxidation at low-temperature. It can be speculated that this work paves the way for the engineering of boosted-performance Ir-based heterogeneous catalysts by providing deeper mechanistic insights.
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    Ammonia free catalytic reduction of nitric oxide on Ni-embedded graphene nanostructure: A density functional theory investigation
    (Elsevier B.V., 2023) Genç, Ali Emre; Akça, Aykan; Karaman, Ceren; Camarada, Maria B.; Dragoi, Elena-Niculina
    In this study, the catalytic reduction reaction of NO (directly) without the presence of ammonia (NH3) was studied on the Ni-embedded graphene (Ni@GN) layer using periodic Density Functional Theory (DFT) calculations. Ni-embedded graphene surface can be synthesized experimentally and it is predicted that it will cost much less than single crystal surfaces due to the economic usage of the transition metal atoms. First of all, by optimizing the geometric structure of the Ni@GN layer, crucial geometric features and electron density differences (EDD) were obtained. Based on the different adsorption configurations of NO molecule, the reduction reaction was investigated by Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) based mechanisms. Finally, N2O degradation was analyzed in detail. It is shown that the Eley-Rideal model is a more dominant mechanism on the Ni@GN surface than the other model. In addition, all proposed reaction pathways for NO reduction are exothermic. This information can be used for the research and development of graphene-based materials for NO reduction; paves the way for finding new Ni-based catalysts based on active single transition metal atom embedded on different kind of defects.
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    Mechanism of methanol decomposition on the Cu-Embedded graphene: A DFT study
    (Elsevier Ltd, 2023) Akça, Aykan; Karaman, Onur; Karimi-Maleh, Hassan; Karimi, Fatemeh; Karaman, Ceren; Atar, Necip; Yola, Mehmet Lütfi; Erk, Nevin
    The methanol decomposition reaction has gained substantial attention due to the wide range of applications that its intermediates offer. In this work, methanol (CH3OH) decomposition on Copper-embedded graphene (CuG) surface has been investigated via density functional theory with Grimme-D2 dispersion correction. The charge density of the CuG surface has been analyzed and the redistribution of the electron density of the surface has been represented via the electron density difference (EDD) map. Moreover, the decomposition reaction mechanism of CH3OH on the CuG surface through the cleavage of C–H, O–H and C–O bonds has been investigated in detail. In the initial state, the C–O and O–H bonds of CH3OH have similar activation barriers, thereby the adsorption and degradation mechanism of the intermediate states arising through O–H bond cleavage on the CuG surface has been investigated. In addition, the charge density calculations of the transition state geometries have been conducted and examined with EDD maps. The results have revealed that the previously adsorbed oxygen molecule exhibited high catalytic activity towards O–H decomposition compared to the bare surface. The CuG surface has offered higher activity on the C–H bonds compared to the C–O bonds of the intermediate states generated by CH3OH decomposition. The results revealed that the proposed CuG structure can be utilized as an alternative electrode catalyst that can prevent the CO poisoning issue in direct methanol fuel cells.
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    Öğe
    Mechanistic insights into catalytic reduction of N2O by CO over cu-embedded graphene: A density functional theory perspective
    (IOP Publishing Ltd, 2021) Akça, Aykan; Karaman, Onur; Karaman, Ceren
    In this study, the mechanism of N2O reduction by CO over Cu-embedded graphene(CuG) surface was examined through Density Functional Theory(DFT) with Grimme-D2 dispersion correction. Cu-embedded graphene networks can be synthesized experimentally, and are less costly than plain graphene by virtue of the limited use of Cu atoms. Cu atoms strongly bond to defective structures and make the structure more stable. The binding energy between the defective graphene structure and the Cu atom was calculated as -3.92 eV. The Bader analysis was performed for CuG surface characteristics, and adsorption geometries of N2O and electron density difference maps were created. The results showed that the charge density of Cu atoms provided a high catalytic activity for reduction reactions. O*atom adsorbed to the surface renders O transfer easier. The results indicated that there were 0.16 ?e? and 0.02 ?e? electron were transferred from the surface to the N-terminated and O-terminated N2O molecule, respectively. The calculations proved that the surface possessed a high catalytic activity on O*+N2O ? N2 + O2 and CO + N2O ? CO2 + N2 reduction reactions. This study paves the way for tailoring a high-performance electrocatalyst for NO2 reduction reaction by considering the high electrocatalytic activity and superior physicochemical properties of Cu-embedded graphene.
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    Theoretical insights into the NH3 decomposition mechanism on the Cu- and Pt- embedded graphene surfaces: A DFT approach
    (IOP Publishing Ltd, 2021) Akça, Aykan; Küçük, Hilal; Karaman, Ceren; Atar, Necip; Yola, Mehmet Lütfi
    Herein, the catalytic activities of Cu-and Pt-embedded graphene surfaces on the sequential decomposition reaction of NH3 molecule were investigated by density functional theory (DFT). Partial charge changes on the surfaces by embedding Cu and Pt atoms on the bare graphene surface were analyzed by the Bader charge analysis and depicted by the electron density difference maps. Grimme-D2 dispersion correction was employed for weak interactions between adsorbates and both graphene surfaces. The most stable geometries of the adsorption of NHx (x = 0 ? 3) and H species and their fragmented co-adsorption structures on both graphene surfaces were obtained. The internal energy barrier calculations required for the sequential decomposition of NH3 on both graphene surfaces were calculated by the CINEB method and the results obtained for complete decomposition of NH3 were illustrated by relative energy diagram. The findings revealed that the decomposition of NH3 to NH2, NH, and N on the Cu-embedded graphene surface had relatively lower activation barriers of 1.52 eV, 0.72 eV, and 0.64 eV, respectively, compared to the Pt-embedded graphene surface. The Cu-embedded graphene surface was of high selectivity over the NH3 sequential decomposition reaction. This information may paw the way for different strategies for the development of Cu-based catalysts for NH3 decomposition.