(292d) Design of Ordered Mesoporous Oxides for Plasma-Assisted Catalysis of Ammonia Synthesis

Energetic electrons in dielectric barrier discharges (DBDs) have been proposed to collide with and activate the N₂ triple bond,¹ enabling ambient NH₃ synthesis powered by renewable electricity but plagued by low energy yields, even with heterogenous catalysts. Porous oxides exhibit similar N₂ conversions with or without supported metal nanoparticles,² indicating significant support contributions that need to be quantified. We synthesized SBA-15³ (a silica-based ordered mesoporous material (OMM) with tunable pore sizes) to probe porosity effects on N₂ conversion and plasma properties (i.e., electron densities). γ-alumina has not been similarly studied due to complex OMM syntheses and their lower surface areas, despite its higher dielectric constant (that improves DBD-catalyst synergy) and acid sites (that are catalytically active and adsorb product NH₃). We coated SBA-15 with different γ-alumina loadings³ (5-15 wt. % Al) for the desired surface functionality on an identical framework, which all displayed complete coverage of the SBA-15 surface and ordered porosity, as evidenced from FTIR and N₂ physisorption, respectively. A sharp decrease in pore size and surface area indicated 10 wt. % Al is the threshold loading for a relatively thin coating. This composite, alongside SBA-15, was pressed and sieved into 1-2 mm pellets to promote beneficial packing in the DBD reactor, in which they exhibited similar N₂ conversions. However, shielding NH₃ from decomposition was only demonstrated for the composite, as evidenced from temperature-swing desorption after reaction, similar to effects observed on zeolites.⁴ Both OMM pellets exhibited higher N₂ conversions relative to pelletized powders of non-ordered oxides, consistent with previous results for silica.⁵ The composite also showed higher conversion than commonly used beads (silica and γ-alumina) despite the fact that interparticle voids between the smaller OMM pellets likely inhibit plasma generation (i.e., higher breakdown voltage) hence a partial discharge. This is in agreement with the lower electron densities and N₂ conversions measured on pelletized commercial samples relative to beads. These results inform the rational design of OMM porosity and functionality to optimize plasma properties, catalytic activity, NH₃ uptake, and therefore the energy yield of DBD-assisted catalysis.

References

(1) Mehta, P.; Barboun, P. M.; Engelmann, Y.; et al. ACS Catal 2020, 10 (12), 6726–6734.

(2) Chen, Z.; Koel, B. E.; Sundaresan, S. J Phys D Appl Phys 2022, 55 (5), 055202.

(3) Babaei, Z.; Najafi Chermahini, A.; Dinari, M. Chem Eng J 2018, 352, 45–52.

(4) Rouwenhorst, K. H. R.; Mani, S.; Lefferts, L. ACS Sustain Chem Eng 2022, 10 (6), 1994–2000.

(5) Gorky, F.; Guthrie, S. R.; Smoljan, C. S.; et al. J Phys D Appl Phys 2021, 54 (26), 264003.