Ammonia Synthesis via Membrane Dielectric-Barrier Discharge Reactor Integrated with Metal Catalyst

Abstract

The synthesis of ammonia using non-thermal plasma can present distinct advantages for distributed stand-alone operations powered by electricity from renewable energy sources. We present the synthesis of ammonia from nitrogen and hydrogen using a membrane Dielectric-Barrier Discharge (mDBD) reactor integrated with metal catalyst. The reactor used a porous alumina membrane as a dielectric-barrier and as a distributor of H₂, a configuration that leads to greater NH₃ production than using pre-mixed N₂ and H₂. The membrane is surrounded by catalyst powder held by glass wool as porous dielectric support filling the plasma region. We evaluated nickel, cobalt, and bimetallic nickel-cobalt as catalysts due to their predicted lower activation energy under non-thermal plasma conditions as determined through Density Functional Theory (DFT) calculations. The catalysts were loaded at 5% by weight on alumina powder. The performance of the catalytic mDBD reactor was assessed using electrical, optical, and spectroscopic diagnostics, as well as Fourier-Transform Infrared spectroscopy. Experimental results showed that the glass wool support suppresses microdischarges, generally leading to greater ammonia production. The Ni-Co/Al₂O₃ catalyst produced the greatest energy yield of 0.87 g-NH₃/kWh, compared to a maximum of 0.82 and 0.78 g-NH₃/kWh for the Co/Al₂O₃ and Ni/Al₂O₃ catalysts, respectively. Although the differences in performance among the three metal catalysts are small, they corroborate the predictions by DFT. Moreover, the maximum energy yield for bare Al₂O₃ (no metal catalyst) with dielectric support was 0.38 g-NH₃/kWh, for mDBD operation with no metal catalyst or dielectric support was 0.28 g-NH₃/kWh, and for standard DBD operation (no membrane, dielectric support, or catalyst) was 0.08 g-NH₃/kWh, i.e., 2.1, 3.1, and 11 times lower, respectively, than the maximum energy yield for the Ni-Co/Al₂O₃ catalyst with dielectric support. The study shows that the integration of dielectric membrane and metal catalyst is an effective approach at enhancing ammonia production in a DBD reactor.