Methane, an abundant resource in the U.S., has very strong C-H bonds, which are hard to activate. Current catalytic processes used to upgrade methane to higher value products are facing two major issues: (i) high temperatures (ca. 900 K) are required, and (ii) the lifetime of the catalyst is limited due to sintering and coking. We aim to address these challenges and pursue a novel low temperature process, which leverages a molten carbonate electrochemical cell to convert methane to methanol. The two main advantages of this process are that the rate of CO32- delivery can be controlled by adjusting the cell potential or current density, and CO32- reduction leads again to CO2, which readily desorbs instead of overoxidizing CH4. We have studied various reaction pathways by performing density functional theory (DFT) calculations on Ni(111) and NiO(100) using the Vienna Ab-initio Simulation Package (VASP). The reaction enthalpies and activation barriers of all proposed elementary steps were calculated with and without applied electric fields. Results indicate that the interaction between the externally applied electric field and the surface dipole moment plays the dominant role in altering the binding behavior; the same external electric field may simultaneously stabilize some adsorbates, while destabilizing other surface species, depending on the orientation of their respective dipole moments. The potential energy diagrams of CO3-assisted and O-assisted reaction pathways on both Ni(111) and NiO(100) indicate that on Ni(111) the O-assisted reaction pathway is expected to be the dominant mechanism, while on the NiO(100) surface, the CO3-assisted reaction pathway is likely to dominate. We have quantified the extent to which electric fields impact the thermodynamics of CH4 oxidation reactions on Ni and NiO surfaces. Our results suggest that an electrochemical cell using CO32- as an oxidant offers unique advantages for upgrading methane to value-added products by preventing the overoxidation of methane. This is particularly interesting for small-scale methane conversion and distributed manufacturing when sustainable sources of electricity are available. Future work will focus on the optimization of electric fields. For instance, negative and positive electric fields promote different elementary steps of the CO3-assisted reaction pathway on NiO(100), making oscillating electric fields an interesting option to pursue.

Density functional theory investigation of electrochemical methane activation on Ni(111) and NiO(100)

ACS Spring Meeting 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.