Identifying Regimes During Plasma Catalytic Ammonia Synthesis

Herein, we demonstrate that the performance of mesoporous silica SBA-15 and SBA-15-Ag during plasma ammonia synthesis depends on the plasma conditions. At high power, the mesoporous silica SBA-15 without Ag produces the largest amount of ammonia, but the addition of Ag provides a minor benefit at lower powers. Plasma conditions were analyzed through optical emission spectroscopy using N₂, N₂⁺, and NH molecular bands and H_α line. Stark broadening of H_α line was used to find electron density, and N₂ molecular bands were used to assess N₂ vibrational excitation, important for plasma nitrogen decomposition. At similar input conditions, reactors with SBA-15 have higher electron density and higher N₂ vibrational temperature. Consistent with higher electron density, SBA-15 reactors have stronger N₂⁺ emission intensity relative to the neutral N₂. The addition of Ag results in higher N₂ rotational temperature, possibly due to localized heating. From the materials point of view, SBA-15 is a more robust catalyst with good surface area retention after plasma exposure due to the lack of local heating generated when a metal is in the structure. We identify two possible regimes during ammonia synthesis, a metal and a surface-plasma driven. At lower plasma densities, the addition of metal is beneficial, while at higher power and plasma density, the best performance is achieved without the aid of a metal catalyst. Graphical Abstract: Mesoporous materials for Plasma Catalytic Ammonia Synthesis, at certain plasma conditions lead to different regimes, a plasma/surface and a metal dominated regimes of ammonia production.[Figure not available: see fulltext.].