(707e) Effect of Framework and Al Incorporation in Ordered Mesoporous Silicas on Non-Thermal Plasma-Assisted Ammonia Reactions

Alternatives to industrial high temperature and pressure Haber-Bosch ammonia (NH₃) synthesis include use of direct electricity from renewable sources in dielectric barrier discharge (DBD)-assisted catalysis. However, DBD-assisted NH₃ energy yields are currently low; moreover, porous oxide supports surprisingly account for most of the NH₃ yield, indicating that presence and identity of packed beds alter plasma dynamics and properties. Prior work has shown γ-alumina exhibits higher NH₃ yields than silica, while also providing Lewis acid sites that can adsorb and shield NH₃ from decomposition. Ordered structures, like silica-based SBA-15, have demonstrated promising performance, but ordered γ-alumina has not been similarly studied due to its complex syntheses. Thus, we explored the systematic effects of different ordered structures and acid site incorporation on plasma-assisted NH₃ production. We synthesized SBA-15 with conformal γ-alumina coatings (Al₂O₃-SBA-15) at various weight loadings (0-15 wt. % Al) and compared them against MCM-41 and Al-MCM-41, which has Al incorporated tetrahedrally into the silica backbone. Al₂O₃-SBA-15 exhibited higher steady-state and overall energy yields than SBA-15, where overall accounts for NH₃ recovered after the reaction via temperature-swing desorption. MCM-41 materials demonstrated similar steady-state energy yields to each other, but Al-MCM-41 had a higher overall energy yield, analogous to trends for SBA-15 materials. While mass-normalized steady-state energy yields were lower for both MCM-41 materials than SBA-15 or Al₂O₃-SBA-15, Al-MCM-41 exhibited similar mass-normalized overall energy yield to that of Al₂O₃-SBA-15, highlighting the importance of product shielding by adsorption at acid sites. That being said, Al-MCM-41 demonstrated less NH₃ recovery than Al₂O₃-SBA-15, possibly due to the different type (Bronsted and Lewis, respectively) and density of acid sites found in the Al-incorporated silica ordered structures. These results inform the rational design of porosity and functionality of porous oxide supports to optimize plasma properties, catalytic activity, NH₃ uptake, and therefore the energy yield of DBD-assisted catalysis.