Synergistic effects of plasma-catalyst interactions for CH₄ activation

The elucidation of catalyst surface-plasma interactions is a challenging endeavor and therefore requires thorough and rigorous assessment of the reaction dynamics on the catalyst in the plasma environment. The first step in quantifying and defining catalyst-plasma interactions is a detailed kinetic study that can be used to verify appropriate reaction conditions for comparison and to discover any unexpected behavior of plasma-assisted reactions that might prevent direct comparison. In this paper, we provide a kinetic evaluation of CH₄ activation in a dielectric barrier discharge plasma in order to quantify plasma-catalyst interactions via kinetic parameters. The dry reforming of CH₄ with CO₂ was studied as a model reaction using Ni supported on γ-Al₂O₃ at temperatures of 790-890 K under atmospheric pressure, where the partial pressures of CH₄ (or CO₂) were varied over a range of ≤25.3 kPa. Reaction performance was monitored by varying gas hourly space velocity, plasma power, bulk gas temperature, and reactant concentration. After correcting for gas-phase plasma reactions, a linear relationship was observed in the log of the measured rate constant with respect to reciprocal power (1/power). Although thermal catalysis displays typical Arrhenius behavior for this reaction, plasma-assisted catalysis occurs from a complex mixture of sources and shows non-Arrhenius behavior. However, an energy barrier was obtained from the relationship between the reaction rate constant and input power to exhibit ≤∼20 kJ mol^-1 (compared to ∼70 kJ mol^-1 for thermal catalysis). Of additional importance, the energy barriers measured during plasma-assisted catalysis were relatively consistent with respect to variations in total flow rates, types of diluent, or bulk reaction temperature. These experimental results suggest that plasma-generated vibrationally-excited CH₄ favorably interacts with Ni sites at elevated temperatures, which helps reduce the energy barrier required to activate CH₄ and enhance CH₄ reforming rates.