Mechanistic Insights for Plasma-Catalytic CO₂ Reduction over TiO₂ in a Dielectric Barrier Discharge Reactor

Reaction kinetics experiments coupled with phenomenological kinetic modeling and parameter estimation are used to elicit insights into the mechanism and active sites for the plasma-catalytic dissociation of CO₂ on TiO₂. Experimental and model insights showed that gas-phase reactions contribute at least two-thirds of the overall product formation at explored conditions; weak temperature dependence, strong sensitivity to specific energy input (SEI), apparent first order in CO₂, and positive influence of cofed argon (Ar) and oxygen (O₂) for the gas-phase contributions all suggest that expected plasma reaction steps such as electron-impact and high-energy collisions are the dominant modes for CO₂ dissociation. The Arrhenius-like expression for gas contributions resulted in a preexponential of 4.40 × 10^-3 s^-1, an E _SEI,g of 7.90 × 10^-4 mol/kJ, and an E _a,g of 1.00 × 10^-3 J/mol. For surface contributions, the small apparent barrier of 16.3 kJ/mol, relatively weaker dependence on SEI, first-order dependence on CO₂, and insensitivity to cofed Ar and O₂ all point to CO₂ dissociation on TiO₂ surface facets without vacancies and aided by plasma (leading to vibrationally excited CO₂ and/or a reactive surface with significant surface charge accumulation). The Arrhenius-like expression resulted in a preexponential of 7.81 × 10^-2 s^-1, an E _SEI,s of 1.90 × 10^-3 mol/kJ, and an E _a,s of 1.63 × 10⁴ J/mol. The derived kinetic model further enabled a systematic evaluation of the effect of inputs (plasma power, flow rate, CO₂ inlet concentration, and temperature) to identify process trends and optimal operating conditions.