Date of Award
2020
Document Type
Open Access Dissertation
Degree Name
Doctor of Philosophy in Materials Science and Engineering (PhD)
Administrative Home Department
Department of Materials Science and Engineering
Advisor 1
Yun Hang Hu
Committee Member 1
Stephen Hackney
Committee Member 2
Ranjit Pati
Committee Member 3
Loredana Valenzano-Slough
Abstract
The surge of methane as the major component of natural gas and a dominant greenhouse gas calls for the development of efficient strategy to convert it into valuable liquid chemicals such as methanol. However, the current industrial method for methanol synthesis, which consists of the stream reforming of methane (SRM) to syngas and the followed reaction of syngas to methanol, is rather energy-intensive. Direct conversion of methane into methanol (DMTM) is highly desirable in terms of energy efficiency and economy. DMTM with water as oxidant, i.e. SRM to methanol, is a promising solution but the development highly efficient and selective catalyst remains a critical challenge. Mo6S8-based catalysts have shown possible potential for this reaction process. In this thesis, for the first time, the feasibility of using Mo6S8 cluster and a series of single metal atom (K, Ti, Co, Fe, Ni, Cu, Rh) doped Mo6S8 as catalysts for SRM to methanol were evaluated via DFT calculation.
Chapter 4 provided the structure of Mo6S8 and M-Mo6S8 (M=K, Ti, Co, Fe, Ni, Cu, Rh) clusters and confirmed that they were stable under the reaction condition of SRM.
The catalytic behaviors of Mo6S8 and M-Mo6S8 clusters toward CH4 adsorption and dissociation were evaluated in chapter 5. All metal dopants, except K and Rh, showed enhanced CH4 adsorption compared to bare Mo6S8 via the ensemble effect (the direct participation of M in binding), while CH4 adsorption was weakened on K- and Rh-Mo6S8 due to the ligand effect (the modification of the electronic structure of Mδ+ and Moδ+). Meanwhile, the doping of Co, Fe, Ti, Ni, and K accelerated the first hydrogen abstraction of CH4 while all the metals suppressed the further dissociation of *CH3, suggesting their great potential for selective CH3OH synthesis and high coking resistance.
In chapter 6, the catalytic behaviors of Mo6S8 and M-Mo6S8 clusters toward H2O adsorption and dissociation were evaluated. H2O adsorption was enhanced by all metal dopants except Rh. The ensemble effect was dominating in enhancing the H2O adsorption. Compared to the bare Mo6S8 cluster, the first hydrogen abstraction was enhanced by all the single metal atom dopants, whereas the second abstraction was mostly suppressed except Ti and Fe. Furthermore, since H2O possessed stronger adsorption than CH4 on the same active site, the initial step for CH3OH synthesis via SRM was considered to be the dissociation of *H2O to *OH and/or *O species.
Based on the results obtained in the above chapters, in chapter 7, the reaction pathways of SRM to methanol on Mo6S8 and M-Mo6S8 were set up and the energy barrier for each elementary step was calculated.
Recommended Citation
Zhang, H. T. Zhang, "MO6S8-BASED SINGLE-METAL-ATOM CATALYSTS FOR METHANOL SYNTHESIS FROM STEAM REFORMING OF METHANE", Open Access Dissertation, Michigan Technological University, 2020.