Plasma-Catalytic CO₂ Hydrogenation over a Pd/ZnO Catalyst: <i>In Situ</i> Probing of Gas-Phase and Surface Reactions

Plasma-catalytic CO2 hydrogenation is a complex chemical process combining plasma-assisted gas-phase and surface reactions. Herein, we investigated CO2 hydrogenation over Pd/ZnO and ZnO in a tubular dielectric barrier discharge (DBD) reactor at ambient pressure. Compared to the CO2 hydrogenation using Plasma Only or Plasma + ZnO, placing Pd/ZnO in the DBD almost doubled the conversion of CO2 (36.7%) and CO yield (35.5%). The reaction pathways in the plasma-enhanced catalytic hydrogenation of CO2 were investigated by in situ Fourier transform infrared (FTIR) spectroscopy using a novel integrated in situ DBD/FTIR gas cell reactor, combined with online mass spectrometry (MS) analysis, kinetic analysis, and emission spectroscopic measurements. In plasma CO2 hydrogenation over Pd/ZnO, the hydrogenation of adsorbed surface CO2 on Pd/ZnO is the dominant reaction route for the enhanced CO2 conversion, which can be ascribed to the generation of a ZnO x overlay as a result of the strong metal-support interactions (SMSI) at the Pd-ZnO interface and the presence of abundant H species at the surface of Pd/ZnO; however, this important surface reaction can be limited in the Plasma + ZnO system due to a lack of active H species present on the ZnO surface and the absence of the SMSI. Instead, CO2 splitting to CO, both in the plasma gas phase and on the surface of ZnO, is believed to make an important contribution to the conversion of CO2 in the Plasma + ZnO system.