Date of Award
Doctor of Philosophy
Methane is an abundant resource existing in the form of natural and shale gas, and molybdenum-based catalysts, including molybdenum oxides and carbides, are the commonly used components in catalysts for converting methane to value-added chemicals. Therefore, understanding the catalytic mechanism underlying the methane conversion over molybdenum-based catalysts is key to designing highly efficient catalysts and optimizing the operating conditions. In this dissertation, I focus on the structural evolution from oxide to carbides and catalytic reactivity of the molybdenum-based catalysts for methane conversion based on the result from density functional theory (DFT) computational studies.First, the surface chemistry and reactivity of α-MoO3 toward C-H bond activation of methane by breaking the first C-H bond on the MoO3 (010) surface were used to evaluate various functionals of the DFT method. Our results indicate that surface reduction of α-MoO3 (010) occurs preferably through releasing the terminal oxygen atoms, generating oxygen vacancies while exposing the reduced Mo centers. These oxygen vacancies tend to be separated from each other at a higher density due to the repulsive interactions. Furthermore, the reduced α-MoO3 (010) surface promotes methane activation kinetically and thermodynamically by reducing the activation barrier for the first C-H bond breaking and stabilizing the product state as compared with those on the stoichiometric surface. There is a synergy between the reduced Mo active site and surface lattice oxygen for C-H bond cleavage. In addition, the performance of different functionals, including the pure-GGA PBE functional with the semi-empirical vdW correction and the meta-GGA SCAN functional, has been investigated. With the meta-GGA functional, we can predict the bulk structure of α-MoO3 more accurately while reproducing the thermal chemistry of MoO3. On the other hand, the reactivity based on the PBE functional is qualitatively consistent with that from the SCAN functional.We then conducted a systematic study of methane activation and conversion over the Mo-terminated surfaces derived from different phases of Mo2C carbides, i.e. the (001) surface of α-Mo2C and the (100) surface of β-Mo2C. The results show that Mo-terminated Mo2C with lower carburization in its subsurface (β-Mo2C) possesses a superior reactivity toward methane activation, resulting in a complete dissociation of methane to carbon adatom on the surface. This carbon adatom causes further carburization of the surface, lowering the reactivity toward methane activation. Moreover, the carburization occurs more easily in the near surface layers of Mo2C than in the bulk. Although carburization lowers the activities for methane activation, it promotes C-C coupling for dimerization of the (CH)ad species, resulting in (C2H2)ad on the Mo-terminated surfaces. On the deep carburized molybdenum carbide (MoC) surfaces, we mapped out the elementary steps of CH4 dissociation and possible mechanisms for forming the C2 species. The results indicate that the Mo-terminated MoC surfaces derived from different bulk phases (α- and δ-) of MoC possess a similar mechanism to that on the noble-metal surfaces for methane dissociation, i.e., CH4 dissociates sequentially to (CH)ad with both kinetic and thermodynamic feasibilities while breaking the last C-H bond in (CH)ad is highly activated. As such, C-C coupling through dimerization of the (CH)ad species occurs more readily, resulting in (C2H2)ad on the Mo-terminated surfaces. Such (C2H2)ad species can dehydrogenate easily to other C2 adsorbates such as (C2H)ad and (C2)ad. Consequently, these C2 species from CH4 dissociation will likely be the precursors for producing long chain hydrocarbons and/or aromatics on molybdenum carbide based catalysts.
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