Pulsed microwave plasma coupled with MoO₃-based heterogeneous catalysts for nitrogen fixation

Nitrogen fixation (NF) is a crucial industrial process to convert nitrogen into valuable compounds, such as NH₃ or NO_x. The renowned Haber–Bosch’s (H–B) process to synthesize NH₃ relies on fossil fuel and releases CO₂. Plasma-based processes are a green and efficient technology that could be an alternative. Taking advantage of a synergy between plasma and catalyst is a promising approach to improve the performances of the plasma-based processes.

Nowadays, researches are in progress to optimize NF process based on this approach¹. High–frequency plasmas are well suited to gas conversion processes by generating large concentrations of vibrationally excited species². Also, Gicquel et al. found that NO_x formation is catalysed by MoO₃ in an RF plasma and gives higher yield and energy efficiency³.

In this study, a catalyst–assisted pulsed microwave plasma at low–pressure is applied for NO_x formation. MoO₃ catalysts supported on γ–alumina were prepared, characterized and applied in the post-discharge of the N₂/O₂ plasma. The products (NO and NO₂) concentrations were measured by in–situ gas phase FTIR spectroscopy. Effect of discharge parameters plus catalyst properties are discussed and correlated to the NO_x formation. The best conditions up–to–now are a total flow rate of 2 lit/min, 1 kHz pules frequency, 25%_on–75%_off duty cycle and a catalyst with 10 wt% MoO₃. In these conditions, vibrational, and rotational temperatures of plasma are respectively about 4000 K and 1000 K. The γ–alumina support is covered by a monolayer of MoO₃. A reaction yield of 8% and an energy efficiency of 1% are obtained.