Mechanochemical Synthesis of RuCo/MgTiO₃Catalysts for Nonthermal Plasma-Assisted Ammonia Synthesis

The development of robust yet effective catalysts is vital for promoting nonthermal plasma (NTP)-assisted ammonia (NH3) synthesis toward practical applications. Herein, we report a simple mechanochemical synthesis method (via ball milling) for preparing bimetallic RuCo supported on MgTiO3 catalyst (i.e., RuCo/MgTiO3-6-500) to be used in NTP-assisted catalytic NH3 synthesis. The physicochemical properties of the developed catalysts were investigated systematically with respect to the relevant process parameters of ball milling. Catalytic tests showed that, at a low specific energy input of 15 kJ L-1, the developed RuCo/MgTiO3-6-500 catalyst exhibited a high NH3 synthesis rate and an energy efficiency of ∼50 μmol min-1 gcat-1 and 1.54 gNH3kWh-1, respectively, which outperformed the monometallic Ru or Co control catalysts. Additionally, the energy barriers were calculated as a function of N2/H2 ratio for the NTP-catalytic NH3 synthesis over RuCo/MgTiO3-6-500, and the results were interpreted together with the activity data measured during the catalysis. It was found that the energy barrier approach may not be appropriate for discussing the energy required for the dynamics of interest in the NTP-catalytic system. Compared to MgTiO3-packed and plasma-alone systems, results by in situ optical emission spectroscopic (OES) diagnosis of the plasma-catalytic systems showed that the presence of active metal sites (i.e., Ru and/or Co) could intensify the activation and dissociation rates of N2 and H2 molecules, showing the relatively higher peak intensity of NH, N2*, N2+, and Hα species. However, based on the obtained information, the possible roles of Ru and/or Co in NTP-assisted ammonia synthesis system are difficult to be identified.