| Summary: | 博士 === 國立臺灣科技大學 === 化學工程系 === 107 === Abstract
To design an efficient catalyst for methane conversion, studying the chemical nature of methane on metal cluster surface is an alternative solution. Catalytic conversion of methane requires processes such as activation and possibly dissociation. Boron nitrogen co-doped graphene surface decorated by Irand Pt cluster exhibited higher catalytic activity for dehydrogenation of methane. In this thesis, we have investigated adsorption, dissociation of methane and possibly the coupling reactions on boron nitrogen co-doped graphene surface decorated by Ir4 cluster, Pt4 cluster and Ir13 cluster using density functional theory (DFT) methods. The most stable adsorption configuration and adsorption energies of CHX (0-4) on boron nitrogen co-doped graphene surface decorated by Ir4 cluster, Pt4 cluster and Ir13 cluster have been investigated. Moreover, the interactions between the surface and adsorbate discussed by the electron density difference contour plot (EDD) and density of sates (DOS). Methane molecularly adsorbs on the surface through agostic interactions anddehydrogenation of methaneinall surfaces have been also studied via precursor mediated mechanism. The result reveals that activation of methane on BNG-Ir4 cluster, which occurs at low temperature condition, is more facile and thermodynamically favorable than that of BNG-Pt4 cluster. The first dehydrogenation step of methane on BNG-Ir4 cluster has been found to have low kinetic barrier (0.17 eV) and high adsorption energy of methane (-0.58 eV), indicating easier methane dissociation than direct desorption. The third and the second dehydrogenation step is the rate determining step on BNG-Ir4 cluster and BNG-Pt4 cluster, respectively.
The results reveals that the coupling barriers of CH3/CH3 and CH2/CH2 on BNG-Ir4 cluster are 1.23 eV and 0.64 eV, respectively, indicating that the formation of ethane and ethylene is possible on BNG-Ir4 cluster. However, the desorption energies of ethane and ethylene are 0.53 eV and 2.00 eV, where the desorption of ethylene is very difficult on BNG-Ir4 cluster. Thus by controlling the reaction temperatures, producing of ethane is possible on BNG-Ir4 cluster.Furthermore, recombination of hydrogen to produce hydrogen molecule has been considered on both clusters and the result confirms that hydrogen molecule can be formed on BNG-Ir4 clusterand BNG-Pt4 cluster at mild temperature conditions and can easily be desorbed from the surface.
In the present study, we predict that BNG-Ir13 cluster can efficiently activate methane and promote the C-C coupling reactions. Top site of Ir on BNG-Ir13 cluster is considered as the most stable adsorption site of methane with the stable adsorption energy of -0.45 eV. Methane is activated with lower activation energy barrier of 0.16 eV and the reaction energy is -0.54 eV. It is the most facile step and is thermodynamically favorable, which is likely to occur at low temperature conditions. By controlling the reaction temperature, which inhibit further dehydrogenation of methane, the C-C coupling reactions have been studied on BNG-Ir13 cluster. Based on the DFT calculations, selective conversion of methane and self-coupling reactions of methyl formed ethane with a lower kinetic barrier, which is likely to occur on BNG-Ir13 cluster.
Furthermore, we found out low oxygen coverage of BNG-Ir13 cluster has higher adsorption energy of methane and lower activation energy barrier of methane dissociation throughout the calculations compared to high oxygen coverage of BNG-Ir13 cluster. The result reveals that the adsorption energy of methane in low oxygen coverage of BNG-Ir13 cluster is -0.44 eV and the second dehydrogenation of methane is the rate determining step (1.24 eV), which is lower than that of high oxygen coverage of BNG-Ir13 cluster. As a result, by controlling the reaction temperature, CH3 and CH2 species are the most abundant species in oxygen pre-covered BNG-Ir13 cluster and the C-O coupling reactions have been considered. Based on the DFT calculations, methanol and formaldehyde are formed with lower activation energy barrier in low oxygen coverage of BNG-Ir13 cluster (BNG-Ir13O cluster) and can be occurred at moderate temperature conditions compared to high oxygen coverage of BNG-Ir13 cluster. Low oxygen pre-covered BNG-Ir13 cluster is a promising catalyst for selective conversion of methane to methanol, formaldehyde and production of hydrogen.
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