2023 Theses Doctoral
Hybrid Catalytic Systems for the Sustainable Reduction of Carbon Dioxide to Value-Added Oxygenates
Atmospheric carbon dioxide (CO₂) concentrations have increased rapidly in recent decades due to the burning of fossil fuels, deforestation, and other industrial practices. The excessive accumulation of CO₂ in the atmosphere leads to global warming, ocean acidification, and other environmental imbalances, which may ultimately have wider societal implications. One potential solution to closing the carbon cycle is utilizing CO₂, rather than fossil fuels, as the carbon source for fuels and chemicals production. This lowers atmospheric CO₂ levels while simultaneously providing an economic incentive for capturing and converting CO₂ into more valuable products. This dissertation includes studies on three hybrid catalytic reactor systems coupling electrochemistry, thermochemistry, and plasma chemistry for the conversion of CO₂ into value-added oxygenates, such as methanol and C3 oxygenates (propanal and 1-propanol).
First, a tandem two-stage system is described where CO₂ is electrochemically reduced into syngas followed by the thermochemical methanol synthesis reaction. The work here specifically focuses on the electrochemical CO₂ reduction reaction to produce syngas with tunable H₂/CO ratios. Using a combination of electrochemical experiments, in-situ characterization, and density functional theory calculations, palladium-, gold-, and silver-modified transition metal carbides and nitrides were found to be promising catalysts for enhancing electrochemical activity while reducing the overall precious metal loading.
Second, another tandem two-stage system is demonstrated where CO₂ is electrochemically reduced into ethylene and syngas followed by the thermochemical hydroformylation reaction to produce propanal and 1-propanol. The CO₂ electrolyzer was evaluated with Cu catalysts containing different oxidation states and with modifications to the gas diffusion layer hydrophobicity, while the hydroformylation reactor was tested over a Rh₁Co₃/MCM-41 catalyst. The tandem configuration achieved a C₃ oxygenate selectivity of ~18%, representing over a 4-fold improvement compared to direct electrochemical CO₂ conversion to 1-propanol in flow cells.
Third, a hybrid plasma-catalytic system is investigated where CO₂ and ethane are directly converted into multi-carbon oxygenates in a one-step process under ambient conditions. Oxygenate selectivity was enhanced at lower plasma powers and higher CO₂ to C₂H₆ ratios, and the addition of a Rh₁Co₃/MCM-41 catalyst increased the oxygenate selectivity at early timescales. Plasma chemical kinetic modeling, isotopically-labeled CO₂ experiments, and in-situ spectroscopy were also used to probe the reaction pathways, revealing that alcohol formation occurred via the oxidation of ethane-derived activated species rather than a CO₂ hydrogenation pathway.
It is critical to assess whether the proposed CO₂ conversion strategies consume more CO₂ than they emit. A comparative analysis of the energy costs and net CO₂ emissions is conducted for various reaction schemes, including four hybrid pathways (thermocatalytic-thermocatalytic, plasma-thermocatalytic, electrocatalytic-thermocatalytic, and electrocatalytic-electrocatalytic) for converting CO₂ into C₃ oxygenates. The hybrid processes can achieve a net reduction in CO₂ provided that low-carbon energy sources are used, however further catalyst improvements and engineering optimizations are necessary. Hybrid catalytic systems can provide an alternative approach to traditional processes, and these concepts can be extended to other chemical reactions and products, thereby opening new opportunities for innovative CO₂ utilization technologies.
This item is currently under embargo. It will be available starting 2023-12-23.
More About This Work
- Academic Units
- Chemical Engineering
- Thesis Advisors
- Chen, Jingguang
- Ph.D., Columbia University
- Published Here
- January 4, 2023