Influence of Ordered Mesoporous Oxides in Plasma-Assisted Ammonia Synthesis

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 (gamma-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 gamma-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 gamma-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.