Nonthermal Plasma Catalytic Ammonia Synthesis over a Ni Catalyst Supported on MgO/SBA-15

Nonthermal plasma (NTP)-enabled ammonia synthesis has been recently considered a sustainable technique as compared to the Haber-Bosch (HB) process. Herein, we demonstrated the NTP catalytic ammonia synthesis using mesoporous silica (SBA-15)-supported Ni catalysts under ambient conditions. Specifically, two types of MgO-modified SBA-15 were developed (as the catalyst support) by the in situ doping and incipient wetness impregnation (IWI) methods, respectively. Experimental results demonstrated that the addition of Mg in SBA-15 via the IWI method favored the ammonia synthesis rate under NTP conditions. The developed Ni-Mg_0.02/SBA-15-IWI catalyst exhibited the highest ammonia synthesis rate and energy efficiency value of 4.4 mmol h^-1g_cat^-1and 1.05 g_NH3kWh^-1, respectively, outperforming the Ni/SBA-15 and Ni-Mg_0.02/SBA-15-In situ catalysts (i.e., the doping of Mg via the in situ method). High-resolution transmission electron microscopy (HRTEM) and energy-dispersive system (EDS) mapping analysis showed that the addition of Mg (on SBA-15) via the IWI method favored the dispersion of Ni active sites on the surface of the catalyst and the interaction between Ni and the SBA-15 support, i.e., uniform distribution of Ni nanoparticles of 5.1 ± 1.1 nm in the Ni-Mg_0.02/SBA-15-IWI catalyst, which enhanced the ammonia synthesis performance. Finally, the developed Ni-Mg_0.02/SBA-15-IWI catalyst displayed a slight decrease in the ammonia synthesis rate from ∼4.42 to ∼3.89 mmol h^-1g_cat^-1over 40 h on stream, which could be attributed to the aggregation of Ni particles based on the postreaction HRTEM analysis.