Influence of Ordered Mesoporous Oxides in Plasma-Assisted Ammonia Synthesis

Abstract

Widespread implementation of dielectric barrier discharge (DBD)-assisted NH₃ synthesis, a nascent technology operating under sustainable, ambient conditions, is hindered by low energy yields due to, in part, poor fundamental understanding. Porous oxides used to support metal nanoparticle catalysts have shown significant energy yield contributions for DBD-assisted NH₃ synthesis even without metal. Using an AC-powered, coaxial, single-stage reactor at 16 kV with equimolar (N₂/H₂) feed, we measured NH₃ synthesis rates in the presence of different nonordered oxides, ordered SiO₂ structures (SBA-15 and MCM-41), and ordered Al-incorporated analogues (γ-Al₂O₃-coated with varying Al-loadings and Al-substitution, respectively: Al₂O₃-SBA-15 and Al-MCM-41). We systematically quantified NH₃ energy yield dependence on pore structures and material identities (i.e., ordered pores and Al incorporation) known to facilitate higher DBD-assisted NH₃ synthesis rates. SBA-15 displayed a higher steady-state energy yield than MCM-41, indicating that framework type is a crucial factor, with both ordered porous systems outperforming fumed SiO₂. 10 wt % Al maximized in situ NH₃ uptake among the various Al loadings, exhibiting a higher steady-state energy yield and similar power to SBA-15. However, Al-MCM-41 had a similar steady-state energy yield and lower power than MCM-41, likely due to the extended γ-Al₂O₃ surface that has a dielectric constant higher than that of SiO₂. Both Al-incorporated analogues benefit from surface acid sites that can adsorb NH₃ in situ, resulting in higher overall NH₃ energy yields than that of their parent ordered SiO₂. Al₂O₃-SBA-15 shielded more NH₃ than Al-MCM-41, likely due to a higher acid site density than the acid site identity. Thus, Al incorporation via γ-Al₂O₃ coating more successfully improves the NH₃ energy yield; together with the high-performing ordered framework, these analogues are potential metal catalyst supports with promising energy yields for DBD-assisted synthesis of NH₃ and other chemicals.