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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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    Electrocatalytic decomposition of formic acid catalyzed by M-Embedded graphene (M = Ni and Cu): A DFT Study
    (Springer, 2022) Akça, Aykan; Karaman, Onur
    In this study, the HCOOH decomposition reaction on nickel (Ni)- and copper (Cu)-embedded graphene surfaces was computationally modeled using density functional theory. The charge density of both graphene surfaces was investigated by bader charge analysis and demonstrated by an electron density difference map. The results proved that HCOOH, HCOO, COOH, HCO, H2O, CO, OH and H structures chemically bonded to both graphene sheets. Moreover, the minimum energy reaction path from HCOOH to CO2 and CO on both graphene surfaces was investigated by breaking the C–O, C–H and O–H bonds. The main intermediate of HCOOH dissociation on Ni and Cu embedded graphene substrates was determined as HCOO. The main product of HCOO decomposition on both graphene surfaces was CO2. In comparison to cis-COOH and trans-COOH, CO2 recovery from HCOO on graphene substrates was less favored.The breakdown of trans-COOH on graphene surfaces was of a more minimal-energy reaction pathway than cis-COOH. In addition, the main product of HCO decomposition on both graphene surfaces was determined to be CO. Finally, it was determined that the minimum energy reaction pathway for HCOOH dissociation on both graphene surfaces was HCOOH ? HCOO ? CO2.
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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.