2015 Theses Doctoral

Carbon Dioxide Reduction using Supported Catalysts and Metal-Modified Carbides

Porosoff, Marc

To sustain future population and economic growth, the global energy supply is expected to increase by 60% by 2040, but the associated CO₂ emissions are a major concern. Converting CO2 into a commodity through a CO₂-neutral process has the potential to create a sustainable carbon energy economy; however, the high stability of CO₂ requires the discovery of active, selective and stable catalysts.
To initially probe the performance of catalysts for CO₂ reduction, CO₂ is activated with H₂, which produces CO and CH₄ as the primary products. For this study, CO is desired for its ability to be used in the Fischer-Tropsch process, while CH₄ is undesired because of its low volumetric energy density and abundance. Precious bimetallic catalysts synthesized on a reducible support (CeO₂) show higher activity than on an irreducible support (γ-Al₂O₃) and the selectivity, represented as CO:CH₄ ratio, is correlated to electronic properties of the supported catalysts with the surface d-band center value of the metal component.
Because the high cost of precious metals is unsuitable for a large-scale CO₂ conversion process, further catalyst development for CO₂ reduction focuses on active, selective and low-cost materials. Molybdenum carbide (Mo₂C) outperforms precious bimetallic catalysts and is highly active and selective for CO₂ conversion to CO. These results are further extended to other transition metal carbides (TMCs), which are found to be a class of promising catalysts and their activity is correlated with oxygen binding energy (OBE) and reducibility as shown by density functional theory (DFT) calculations and in-situ measurements. Because TMCs are made from much more abundant elements than precious metals, the catalysts can be manufactured at a much lower cost, which is critical for achieving a substantial reduction of CO₂ levels.
In the aforementioned examples, sustainable CO₂ reduction requires renewable H₂, 95% of which is currently produced from hydrocarbon based-feedstocks, resulting in CO₂ emissions as a byproduct. Alternatively, CO₂ can be reduced with ethane from shale gas, which produces either synthesis gas (CO + H₂) or ethylene with high selectivity. Pt/CeO₂ is a promising catalyst to produce synthesis gas, while Mo₂C based materials preserve the C-C bond of ethane to produce ethylene. Ethylene and higher olefins are desirable for their high demand as commodity chemicals; therefore, future studies into CO₂ reduction must identify new low-cost materials that are active and stable with higher selectivity toward the production of light olefins.