Metal Oxide Impact on Thermodynamic and Steric Behavior of Adsorbates for Sustainable Energy and Carbon Transformation

Carrington Moore

doi:10.7273/000007430

Metal oxides have the potential to catalyze carbon-based resource transformation chemistries that are prerequisites to a sustainable energy economy. While metal-oxides have demonstrated carbon transformation benefits and been widely adopted, the nanoscale understanding of surface chemistry still has knowledge gaps that have yet to be elucidated. The computational studies contained in this thesis are sub-grouped into three applications of metal-oxide catalysts: those that target (i) biomass transformations, (ii) plastic upcycling, and (iii) hydrogen production. The associated computational studies of these systems provide important thermodynamic, steric, and energic information to real-world experimental efforts to innovate such catalyst systems. Biomass transformation encompasses the primary focus of this work. One application is sustainable aviation fuel production through the dehydration of short-chain alcohols. 2,3-butanediol (BDO) as a fuel precursor to or as a value-added by-product of upstream processes in an ethanol-to-jet process can be dehydrated on a rutile RuO2(110) catalyst. Using density functional theory (DFT) calculations, a P-T phase diagram was constructed for BDO adsorbed on rutile RuO2(110), revealing the thermodynamically stable ground state configurations of BDO that can be expected under experimental conditions. From this, it was determined that a high-pressure reactor would be necessary as the phase diagram predicts BDO will desorb before reaching the elevated temperatures (~500 K) needed for dehydration. The relevant ground state configurations of BDO expose the most favorable site as one with the BDO hydroxyl functional groups bonded to five-fold coordinated surface ruthenium (“Ru5c”) atoms with BDO’s methyl groups in gauche conformation. Lateral interactions between adsorbed BDO molecules were extracted from all BDO configurations. They showed that the only attractive lateral interaction with the surface is from BDOs interaction separated by a bridge-bonded O2c atom row. All other interactions are repulsive and dictate which configurations of BDO are most favorable up to saturation coverage. We also investigated the stability and acid-base properties of anatase TiO2 nanocrystals with low-index facets (101) and (001) for dehydration reactions. Surface models of both facets were analyzed with respect to H2O and OH adsorption. Computational modeling allowed the assignments of the experimental in situ DRIFTS spectra of both facets. In the (101) facet, the OH* region reveals a previously misrepresented, red-shifted terminal v(OH) (OH stretch) at 3605 cm-1. The (001) facet undergoes reconstruction under UHV conditions due to strain. Although this was not observed experimentally due to surface contamination, the stabilized hydroxylated stoichiometric surface yields peak assignments for undercoordinated edge titania. A broad range of water peaks from 2400-3400 cm-1 exhibit delocalization with bridging v(OH) and vs(OH) (symmetric OH stretch in a molecular water molecule), with the experimental peak identified at 3133 cm-1. A partially hydroxylated anatase (101) surface was utilized to absorb methanol as a probe molecule for the acid-base chemistry of TiO2(101). By projecting vibrational normal modes into internal molecular coordinates, we could differentiate bridging (MeOb) versus terminal (MeOt) methoxy and methanol in the IR spectra. The frequencies associated to the vibration methyl groups of adsorbed methanol and methoxy follow the order MeOH > MeOb > MeOt trend of MeOH>MeOb>MeOt. Partially deuterated methanol species were also analyzed computationally and experimentally to determine the symmetry of the methanol adsorbates, as well as offering an unobscured view of the methyl stretching modes for assignment. Symmetry point groups give spatial and molecular orbital information on adsorbates and demonstrated differences between methanol and methoxy. Our findings indicated that methanol possessed C1, the lowest symmetry point group, whereas methoxy displayed Cs symmetry, indicative of a mirror plane within the molecule. This information enhances our understanding of spatial positioning and provides tools to distinguish between dissociated versus molecular water and methanol adsorbates for further chemical transformation. Metal oxides serve also as a support for metal nanoparticles in a variety of catalytic applications, such as hydrogenolysis. Plastic upcycling hydrogenolysis can convert waste plastics into usable carbon chains for lubricants and fuels. Nanoparticle metals, particularly ruthenium, are desirable candidates for this reaction due to their high surface-to-volume ratio. CeO2 has been demonstrated as a support for Ru nanoparticle clusters in hydrogenolysis. TiO2 is tested as an additional support for Ru nanoparticle clusters and used as a control to evaluate the support advantages of CeO2. Computational modeling found that the wetting capability (flatness of the Ru clusters) of the Ru nanoparticles on CeO2 is significantly stronger, influenced by the characteristic oxygen vacancies that are present under reaction conditions. For both CeO2 and TiO2 supports, only small nanoparticle sizes remained stable when wetting the surface (Ru7 for CeO2, Ru4 for TiO2). The larger size of the Ru clusters on CeO2 was evident from the low-loading wetting observed experimentally. In all the carbon transformations mentioned, additional hydrogen is required to make the chemistry accessible. However, the main form of hydrogen used industrially comes from environmentally costly steam reformation in the petrochemical industry. Therefore, there is a push for hydrogen, which comes from the electrochemical water-splitting reaction. The water-splitting hinges on the hydrogen evolution reaction, whose bottleneck is the oxygen evolution reaction (OER). A new class of materials called topological Weyl semi-metals are candidate materials for this reaction as they are poison-resistant and stable. Co3In2S2 has been proven to be a strong material and, in this work, was the modeled catalyst for the OER reaction. The catalyst formed an indium oxide complex layer during the reaction that was poison-resistant and effective at carrying out the OER reaction. The modeled reaction energy pathway determined that the OER reaction is inaccessible on the surface without this amorphous layer. First-principles analysis of metal oxide systems is used to analyze and compare to experiments and provide foundational understanding in the applications of i) biomass transformation ii) plastics upcycling and iii) hydrogen production. Through the analysis provided, thermodynamic and steric information highlights the importance of metal and oxygen in carbon transformation for sustainable energy. This was achieved by illustrating the stability at various temperatures and pressures and affixing the adsorbates in a favorable confirmation. Adaptations toward sustainable energy are necessary for increased energy demand without destroying organic life through fossil fuel production. The work contained in this dissertation provides a pathway for metal oxide catalyst technology to be implemented for sustainable energy and carbon transformation for environmental and societal benefits.

Metal Oxide Impact on Thermodynamic and Steric Behavior of Adsorbates for Sustainable Energy and Carbon Transformation

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