Dielectric Barrier Discharge (DBD)-Assisted Ammonia Decomposition in the Presence of Zeolites with Varying Composition and Pore Structure

Abstract

H2 is a clean fuel that can circumvent the dispersed and intermittent nature of renewable energy sources, yet challenges in storing and transporting H2 restrict its current utilization. H2 can be converted to the more easily liquefied NH3 for distribution to its point of use where it is decomposed back to H2 sustainably via electrified processes, e.g., dielectric barrier discharge (DBD)-assisted reactors. Catalyst packed beds within the DBD can improve the low energy yields associated with these reactors by selectively facilitating surface reactions and interacting synergistically with the DBD. This thesis studies DBD-assisted NH3 decomposition with earth-abundant, inexpensive zeolites, as their ordered aluminosilicate nature and high dielectric constants are favored in DBD systems. Specifically, we probe zeolite elemental composition and pore structure effects on the H2 energy yield by systematically quantifying dilute NH3 decomposition rates and efficiencies within a single-stage, coaxial AC-powered reactor under similar experimental conditions in the presence of different zeolites. We systematically evaluate a suite of MFI-framework samples (X-MFI-Y, where X represents the cation (H+ or NH4+), and Y represents the Si/Al ratio (40 or 25)) as well as previously reported LTA (5A) and FAU (13X) frameworks. H-MFI-25 has higher steady-state decomposition rates and H2 energy yield than H-MFI-40, as well as the zeolite 13X benchmark. H-MFI-25 further has a higher H2 energy yield than the zeolite 5A benchmark, with a slightly lower but comparable steady-state decomposition rate. Steady-state decomposition rates on H-MFI-40 and H-MFI-25 trend logarithmically with NH3 feed concentration, similar to DBD-only reactions, yet the rates are higher in the presence of MFI zeolites at similar residence times. On average, steady-state DBD power is highest for the empty tube and lowest for H-MFI-25, while the inverse is true for H2 energy yield. H-MFI-25 further exhibits higher NH3 decomposition rates (0.41 μmol/s) and H2 energy yield (22 g/kWh) as well as a lower steady-state DBD power across feed concentrations compared to H-MFI-40. These results suggest that a lower Si/Al ratio (higher Al content) influences the bulk DBD properties, which could affect both the DBD-phase reaction rate as well as the DBD-zeolite interactions mediating the surface-facilitated reactions. Future deconvolution of zeolite and bulk DBD contributions to the decomposition rate is key for quantifying the overall surface-facilitated reaction kinetics and the underlying DBD-assisted mechanism of NH3 decomposition; this better understanding of the complex interactions in plasma-assisted chemistries is needed to overcome limiting efficiencies in sustainable green H2 production.