Dynamics of plasma and catalyst interfacial reactions: in situ IR spectroscopy of CO₂ hydrogenation

The use of renewable electricity to catalyze CO₂ to useful chemicals is recognized as a promising decarbonizing technology [1]. We propose nonthermal plasma combined heterogeneous catalysis as a new concept of an electricity-driven chemical reaction while utilizing renewable energy [2,3]. The catalytic surface reaction is modified by vibrationally-excited CO₂, leading to smaller activation energy than that of thermal catalysis [4]. In this study, DBD (dielectric barrier discharge) combined bimetallic alloy catalyst was applied to Reverse Water Gas Shift (RWGS) reaction (CO₂ + H₂ = CO + H₂O ΔH = 41 kJ/mol). The CO₂ conversion exceeded the thermal equilibrium limit when Pd₂Ga/SiO₂ (10wt%) alloy catalyst was coupled with 100 kHz DBD. The in situ transmission infrared (TIR) spectroscopy was employed during DBD exposure over the alloy catalyst. The formation of monodentate formate (m-HCOO) was promoted by direct interaction between adsorbed hydrogen and vibrationally excited CO₂ via the Eley-Rideal pathway. Moreover, the dissociation of m-HCOO, as a rate-determining step, was promoted clearly by DBD under hydrogen-rich conditions. Such unique reaction behavior was confirmed kinetically by the fluidized-bed DBD reactor [5], showing the activation energy of 75 kJ/mol for thermal catalysis and a drastic reduction to 44 kJ/mol for plasma catalysis. Density functional theory (DFT) calculation supports consistently in situ TIR spectroscopy and kinetic analysis of plasma catalysis of RWGS reaction. The individual role of DBD and alloy catalyst over the synergistic effect is discussed.

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