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 ESEI,g of 7.90 × 10–4 mol/kJ, and an Ea,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 ESEI,s of 1.90 × 10–3 mol/kJ, and an Ea,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.