Post-plasma catalysis: charge effect on product selectivity in conversion of methane and nitrogen plasma to ethylene and ammonia

Abstract

The main challenge in simultaneous conversion of methane and nitrogen to ethylene and ammonia under plasma conditions is the low selectivity. This is largely due to the difficulties in controlling the reactions among the plasma-excited reactive intermediates. To address this challenge, we explored an innovative strategy – post-plasma catalysis outside a microwave-enhanced plasma zone (MWP). Experimentally, the post-plasma species, such as CH_x, NH_x, CN, and C₂H_x, from a feed-gas mixture of methane, nitrogen and argon were identified using an optical emission spectrometer. In the absence of a catalyst but under plasma conditions, methane and nitrogen could only convert to hydrogen cyanide, acetylene, and hydrogen. After placing the thermally heated Ag–Pd/CeO₂ catalyst in the post plasma region, the selectivity to hydrogen cyanide was reduced by 9%, while the active nitrogen-based species simultaneously generated a steady amount of ammonia (selectivity = 9%). The selectivity to ethylene increased from 3% to 37%, the highest among the C₂ products. Meanwhile under conventional thermal heating, in the absence of plasma, the same catalyst over-hydrogenated acetylene to ethane and no ammonia was produced from nitrogen. Theoretically, density functional theory calculations (DFT) determined that the post-plasma species-induced positive charge over the Pd–Ag catalytic surface could enhance the energetics of forming ethylene but suppress the energetics of its further hydrogenation to ethane. Overall, this combined experimental and theoretical study advanced the fundamental understanding of the effects of post-plasma species on optimizing the selectivity in heterogeneous catalysis.