Unraveling the different regimes arisen during plasma ammonia synthesis on mesoporous silica SBA-15 through plasma diagnostics

Herein we demonstrate that the performance of mesoporous silica SBA-15 and SBA-15-Ag impregnated during plasma ammonia synthesis depend on the plasma conditions. At high power the mesoporous silica SBA-15 without Ag produces the largest amount of ammonia observed in our experiments, but the addition of Ag provides a minor benefit at lower powers. Plasma conditions were analyzed using optical emission spectroscopy using N2, N2+ and NH molecular bands and H{\alpha} lines. The analysis of optical emission, specifically Ha lines and N2 molecular bands data shows that SBA-15 has higher electron density, and higher N2 vibrational temperature. Stronger N2+ emission relative to the neutral N2 provides additional evidence for higher electron density in the SBA-15 packed bed reactors. The addition of Ag resulted in a lower concentration of atomic hydrogen. Where Ag acts as a hydrogen sink facilitating surface reactions with nitrogen. The presence NH in the spectrum may indicate a higher concentration of H in the SBA-15 reactor. From the materials point of view SBA-15 is a more robust catalyst with a commendable surface area retention after plasma exposure due to the lack of local heating generated when a metal is in the structure.