Plasma-catalytic hydrogenation of CO₂: elucidating thermodynamically- or kinetically-limited step

     CO₂ conversion to useful chemicals attracts keen attention. Particularly, the use of electricity to drive catalytic conversion of CO₂ is recognized as key decarbonizing technology because renewable energy is used directly to activate stable molecules. Low-temperature catalysis, due to electron-driven chemistry, is critically important to mitigate overall CO₂ emission because the high-temperature heat is no longer necessary and thus eliminates the combustion process from the system. Nonthermal plasma catalysis becomes one of the feasible chemical conversion technology in addition to photocatalysis, electrochemistry, and a combination of those.

   DBD (dielectric barrier discharge) combined heterogeneous catalytic reaction was employed to convert CO₂ to CO at 5-10 kPa, known as Reverse Water Gas Shift (R-WGS) reaction (CO₂ + H₂ = CO + H₂O|_vapor: ΔH = 40.6 kJ/mol, ΔG = 24.4 kJ/mol at 400 K). The dependence of R-WGS on total pressure is negligible. The CO₂ conversion at 650 K is promoted beyond thermal equilibrium when Pd₂Ga/SiO₂ (10wt%) alloy catalyst was used, while other catalysts (Pt₃Mo/SiO₂, Pd/SiO₂) did not show synergism. Based on in situ transmission IR spectroscopy under the influence of DBD, together with DFT (density functional theory) analysis, the formation of monodentate formate (m-HCOO) was promoted by direct interaction between adsorbed hydrogen on Pd and bending mode vibrationally excited CO₂ (Eley-Rideal mechanism). More interestingly, the dissociation of m-HCOO, which is known to be a rate-determining step, seems to be promoted by DBD. Such unique reaction behavior was also confirmed by the fluidized-bed DBD reactor, showing the activation energy of 81 kJ/mol for thermal catalysis and 47 kJ/mol for plasma catalysis. DFT analysis indicates the bidentate formate (b-HCOO) is quite stable thermodynamically and to be a spectator: b-HCOO was not decomposed even under the DBD condition which was confirmed by in situ IR spectroscopy. The individual role of DBD and alloy catalyst (or alloying multiple elements) yielding synergistic effect was clarified, enabling a new catalyst design suitable for plasma catalytic conversion of CO₂.