Small Molecule Activation by Actinide Species and Transition Metal Species
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Authors
Panthi, Dipak
Issue Date
2022
Type
Dissertation
Language
Keywords
Alternative Title
Abstract
Nitrogen reduction, water, and hydrogen activation by actinide species: Small molecules like O2, N2, H2, CO2, H2O, and CH4 are generally abundant and accessible; however, their transformation into valuable chemical intermediates is often complex. We have used scalar relativistic and spin-orbit coupled density functional theory (DFT) calculations for the four-electron reduction of dinitrogen (N2 → N24-) and hydrogen evolution reaction by trans-uranium organometallic actinide complexes. We show that modifying the bonds between the actinide and ligand can improve reaction energies for the Np and Pu analogs. We concluded that small-molecule activation with organometallic trans-uranium species is achievable with a guided choice of ligands. Methane to methanol conversion in metal-exchanged zeolites: Copper-exchanged zeolites are interesting for directly converting methane to methanol. As such, a detailed understanding of electronic structures, formation and reactivities, spectroscopic properties, C-H activation, over-oxidation mechanisms of Cu-exchanged zeolites, and the discovery of heterometallic species is essential for the guided design of new and improved active sites. Using density functional theory (DFT) computations, the formation and reactivities of [Cu�"O�"M]2+ species (M = Ti�"Cu, Zr�"Mo, and Ru�"Ag) in metal-exchanged zeolites, as well as the stabilities of these species towards autoreduction by O2 elimination are investigated. Moreover, we examine methane over-oxidation by dicopper [Cu2O]2+ and [Cu2O2]2+ sites in zeolite mordenite (MOR) and found that [Cu2O2]2+ significantly higher methane C-H activation barrier than [Cu2O]2+ in the 8MR. Furthermore, we report that tricopper [Cu3O3]2+ active sites can over-oxidize methane in an intra-site fashion compared to the inter-site over-oxidation mechanisms of the dicopper sites. We used time-dependent density functional theory (TD-DFT) computation to examine the optical spectra of [CuO]+, [Cu2O]2+, [Cu2O2]2+, and [Cu3O3]2+ species implicated in MMC in zeolites. We performed electronic structure analysis and localized orbital bonding analysis to probe the redox chemistry of its Cu and μ-oxo sites. Also, the X-ray absorption near-edge structure, XANES, of methane activation in [Cu3O3]2+ is compared to that of the more ubiquitous [Cu2O]2+. Additionally, we compare the electronic structures and reactivities of [Cu3O3]2+ with the newly described [Cu2AlO3]2+, which is more active for methane oxidation than all other active sites.