Plasma-enabled mode-selective activation of CH4 for dry reforming: First touch on the kinetic analysis

Abstract

A mode-selective control of the surface reaction is expected to be a promising approach in the heterogeneous catalysis. Nonthermal plasma is a vital solution for the generation of vibrationally excited molecules, thereby enhancing mode-selective surface chemistry. Especially, plasma-enabled promotion of heterogeneous catalysis for CH4 conversion attracts keen attention because the strong C–H bond breaking is possible via vibrational excitation of CH4 at low temperature. Similarly, vibrational excitation of CO2 possesses unique reactivity in heterogeneous catalysts. Herein, we provide a rigorous determination of kinetic parameters of CH4 dry reforming to elucidate the drastic reaction promotion mechanism enabled by plasma-catalyst interaction. Lanthanum-modified Ni/Al2O3 catalyst was combined with dielectric barrier discharge (DBD) at 5 kPa and 400–700 °C without dilution gas. Reaction order for CH4 and CO2 were determined respectively as 0.68 and −0.17; these values were kept unchanged by DBD, indicating the surface coverage of CH4 and CO2 was not influenced by nonthermal plasma. The Arrhenius plot for forward CH4 rate constant revealed that 12 kHz DBD hybrid reaction is characterized as mixed catalysis where plasma and thermal catalysis are not decoupled. The apparent activation energy was influenced only slightly by the specific energy input (SEI, eV/molecules) and gaseous hourly space velocity (GHSV, h−1), because the electrical properties of streamer swarm are not influenced to a large extent by either SEI or GHSV at fixed frequency. In contrast, 100 kHz DBD yielded significant improvement of CH4 and CO2 conversion via vibrational excitation. Activation energy decreased from 91 kJ/mol to 44.7 kJ/mol which was well correlated with the state-specific gas-surface reactivity of vibrationally excited CH4 on Ni surfaces.