Selective catalytic hydrogenation of C₂H₂ from plasma-driven CH₄ coupling without extra heat: mechanistic insights...

We study the selective catalytic hydrogenation of C₂H₂, the main product from non-oxidative CH₄ coupling in gas-phase plasmas, to C₂H₄, a cornerstone of the global chemical industry, by experiments and temperature-dependent micro-kinetic modelling. The model is validated against new experimental data from a nanosecond pulsed plasma reactor integrated with a downstream catalytic bed consisting of Pd/Al₂O₃. We explore the effects of varying Pd loadings (0.1, 0.5, and 1 wt%) on the catalyst activity and the C₂H₄/C₂H₆ product distribution. Consistent with the experimental data, our surface micro-kinetic model shows that while higher Pd loadings lower the catalyst activation temperature for C₂H₂ conversion, they also induce over-hydrogenation to C₂H₆ at lower temperatures and increase oligomerisation in the experiments, which are detrimental to the C₂H₄ yield. The model also elucidates reaction mechanisms and pathways across different temperature regimes, expanding our understanding of the hydrogenation process beyond the experimental range. Besides highlighting the importance of optimising the metal loading to balance C₂H₄ and C₂H₆ selectivity, our findings demonstrate the effective implementation of post-plasma catalysis using a simple catalyst bed heated by hot gas from the plasma region. This study opens possibilities for testing different plasma sources, catalysts, gas flow magnitude and patterns, and catalyst bed-to-plasma distances.