As our world continues to develop and contend with the impacts of climate change, the scale up renewable energy technologies has never been more urgent. Alternative fuels derived from biomass-derived oxygenates and water splitting offer promising solutions for the transition towards sustainable chemical feedstocks and integration of clean renewable energy sources. However, this technology continues to be hampered by the need for scarce and costly precious metal catalysts. The work done in this thesis explored the facet-dependence of glycerol electrooxidation and studied the application of earth-abundant transition metal carbides (TMCs) and nitrides (TMNs) for reducing precious metal catalyst loadings in water electrolysis and electrooxidation of methanol and glycerol. Glycerol valorization has drawn significant interest in recent years due to the growth in biodiesel production leading to the market saturation of glycerol.
While this molecule can be converted into a variety of value-added products, the possibilities have been limited by poor selectivity for C-C bond scission. The breaking of the C-C bonds in glycerol allows for complete extraction of energy from the molecule via complete glycerol oxidation, thereby opening the door for utilizing glycerol as an electrochemical fuel. While platinum (Pt) has been among the most popular catalysts, its tendency for poisoning due to adsorbed CO has hindered its activity. Previously demonstrated to enhance the catalytic activity of platinum (Pt) by reducing CO binding energy and increasing C-C bond scission selectivity in ethanol electrooxidation, TMCs were employed as catalyst supports for the glycerol electrooxidation reaction. This work used electrochemical techniques and in-situ IRRAS to study various loadings of Pt/TaC and Pt/WC to find enhanced C-C bond scission activity at reduced Pt loading because of the synergistic effects between Pt and TMCs.
While Pt has remained the benchmark catalyst for glycerol electrooxidation due to its high C-C scission activity, gold (Au) has also found popularity with its high catalytic activity attributed to greater resistance to CO poisoning, despite its favorability for partial glycerol oxidation. Previous studies have hinted at the significance of Au surface facets on glycerol oxidation activity and product selectivity, but none had used nanoparticles with controlled surface facets. This thesis sought to bridge the knowledge gap using precisely-synthesized Au nanocrystals with well-characterized {100}, {110}, and {111} surface facets to provide insight into glycerol electrooxidation on Au. Electrochemical techniques were used in parallel with in-situ IRRAS analysis to uncover the differences in product selectivity and oxidation activity between the three Au surfaces, with Au {111} exhibiting the greatest activity for C-C bond scission, while Au {110} showed the lowest onset potential due to facile AuOH- formation.
Hydrogen (H₂) fulfills a critical role in modern society, not only as a renewable fuel, but also as a key chemical feedstock. Production of H₂ from water electrolysis creates opportunities for storing excess energy from renewable sources as an energy-dense fuel and reducing the environmental footprint of chemical processes requiring H₂. However, efforts have been hampered by the dependence on scarce Pt-group catalyst materials. This thesis explores the application of TMNs as an earth-abundant material for enhancing the activity of Pt in the hydrogen evolution reaction (HER). Combined with DFT calculations, the HER activity of monolayer Pt- and Au-modified TMN thin films was correlated with the ΔGH* values in a volcano-type relationship. Electrocatalytic experiments in acidic electrolyte showed that TMN-supported monolayer Pt exhibited similar HER activity to the Pt foil, correlating with intermediate hydrogen adsorption strength. TiN-supported Pt and Au powders were studied to extend the correlations from thin films. Furthermore, the electrochemical stability of TMNs was studied across a wide range of potentials and pH values to generate pseudo-Pourbaix diagrams and identify TMN candidates for HER, alcohol oxidation, ORR and OER applications.
Using the pseudo-Pourbaix findings, Pt/TMN catalysts were selected for studying methanol electrooxidation activity. Methanol electrooxidation has drawn significant attention particularly due to interest in direct alcohol fuel cells. Much like the case for glycerol oxidation, while Pt has been the benchmark catalyst, it has been hindered by strong adsorption of CO. As the modification of Pt with other materials, such as ruthenium, has shown promising enhancements to methanol electrooxidation activity, the synergistic effects of Pt modification with TMNs were studied in this work. In the resulting electrochemical experiments, Pt/Mo₂N was found to exhibit negligible activity likely because of its oxidative instability. In contrast, Pt/TiN showed enhanced activity, and in-situ IRRAS experiments suggest that Pt/TiN enhanced the COads-free pathway leading to increased formic acid selectivity.
This thesis demonstrated avenues for developing more optimized catalysts with reduced loadings of Pt and other precious metals for applications in alternative fuel production and utilization. The influence of Au surface facets on glycerol oxidation was examined and the synergistic effects between Pt and earth-abundant TMC and TMN materials were used to enhance the electrooxidation of biomass-derived oxygenates and H₂ production from water electrolysis. These electrochemical stability and activity trends can guide future catalyst design for other critical reactions such as oxygen evolution and challenging applications like glycerol electroreduction.
| Identifer | oai:union.ndltd.org:columbia.edu/oai:academiccommons.columbia.edu:10.7916/6t57-ee35 |
| Date | January 2024 |
| Creators | Mou, Hansen |
| Source Sets | Columbia University |
| Language | English |
| Detected Language | English |
| Type | Theses |
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