(730g) Microwave-Assisted Dry Reforming for Energy Efficient CO2 and CH4 Conversion

Microwave-assisted catalysis is an emerging concept for promoting thermally-challenging reactions. This talk describes NETL’s efforts to develop microwave-active catalysts for the dry reforming of methane (DRM: CO₂ + CH₄ à 2CO + 2H₂). DRM is an appealing reaction because it converts CO₂ and CH₄ into a carbon neutral industrial commodity (syngas), but high reaction temperatures currently make it impractical in most cases. Microwave-assisted catalysis can overcome this limitation by selectively heating the catalyst bed to high temperatures (900 ^oC) at extremely fast rates (200-300 ⁰C/min). This rapid on/off cycling will allow utilization of excess renewable energy and reduces heat management issues compared with traditional (always on) thermal reactors. We based our catalyst system on the perovskite oxide La_0.8Sr_0.2CoO₃ (LSC). This catalyst shows good microwave absorptivity and high temperature stability. We found that incorporating first row-transition metals (Mn, Fe, Ni and Cu) into the LSC structure dramatically impacted catalytic activity and stability. Ex situ O K-edge x-ray absorption spectroscopy, temperature programmed reduction, and density functional theory correlated the dopant electronegativity to catalyst activity and overall stability. Incorporating dopants that were less reducible than Co (Mn and Fe) improved LSC performance by strengthening metal-oxygen bonding and stabilizing the catalyst structure. On the other hand, incorporating more reducible dopants (Ni and Cu) led to severe phase segregation during DRM and rapid catalyst deactivation. Our best in class catalyst, Mn-doped LSC, demonstrated over 90% single-pass CO₂ and CH₄ conversion with good long-term stability. In situ synchrotron-based X-ray diffraction evaluated catalyst structure during DRM and identified the formation of small Co nanoparticles at the catalyst surface as key reaction centers. The increased performance of LSC-Mn was directly related to Mn-dopants stabilizing the perovskite structure and preventing sintering of catalytically active Co nanoparticles. Scaling studies show improved performance at higher flow rates and energy efficiencies of 10 kWh/kg_CO2 that approach the efficiency of electrochemical CO₂ conversion technologies. Our results demonstrate the potential for microwave-assisted DRM. The ability to use intermittent renewable energy to promote thermally-demanding catalytic processes may allow the development of new reactor configurations and reconsideration of currently impractical reactions. The results were published in Applied Catalysis B, 2021, 284, article: 119711