Utilizing hot phonons to drive methane pyrolysis for H2 and carbon nanotube formation

Electrifying thermocatalytic reactions emerges as a promising avenue for addressing the limitations associated with thermal-driven reactions characterized as inefficient and chaotic. This study focuses on utilizing microwave radiation to produce non-equilibrium hot phonons in a catalytic susceptor material to drive surface reactions. To understand hot phonons in driving surface reactions, methane pyrolysis to produce hydrogen and carbon nanotubes was used as a probe reaction. We aim to develop a bi-functional catalyst that consists of a microwave susceptor support for hot phonon generation coupled with an intermetallic compound (IMC) that efficiently drives CNT formation.
Studies thus far have focused upon understanding the hot phonon behavior of SiC, whether inductive heating effects in supported pure metal nanoparticles are beneficial, and initial IMC catalyst designs using Fe and Al. Initial performance tests indicate that 100% methane conversion was possible over SiC (square channel monolith of 1/2 inch cylinder of 1.7 inch length) under 450W microwave conditions. These results indicate that CH4 can be activated via hot phonons generated either at the adsorbate-surface interface or in the surface lattice and transferred to CH4. Comparisons with thermally-driven reaction conditions illustrated that at similar vibrational temperatures, microwave-driven reactions are more energy efficient. The efficiency of converting microwaves to hot phonons correlated well with the crystallite size in various different SiC monoliths, with similar high CH4 conversion achieved at microwave powers as low as 200W. These results indicate that grain boundaries are energy sinks for hot phonon production. Observation of CH4 coupling product selectivity (ethane vs. ethylene) in thermal vs. microwave-driven reaction conditions illustrate the effects of entropy ( promoting H2 formation) and the “mass selective” activation of bonds that contain light elements (e.g., hydrogen) with ethylene production preferred under microwave reaction conditions. Results further showed that transition metal nanoparticles grown on SiC exhibited inductive heating effects that predominantly raised the equilibrium temperature and did not aid in the production or lifetime of hot phonons.