Plasma-Catalytic CO2 Hydrogenation over a Pd/ZnO Catalyst: In Situ 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 ZnOx 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.


Experimental setup
. Schematic diagram of the experimental setup.

Kinetic analysis
To understand the effect of CO2 and H2 on the plasma-catalytic CO2 reduction, kinetic analysis was carried out to provide insights into the governing rate expression for the rate of CO production. According to previous studies [1] , the power-law expression for the CO production rate in the plasma-catalytic CO2 hydrogenation can be described by Eq. 9. Therefore, the reaction orders (a for CO2 and b for H2) can be determined by changing the partial pressure of one reactant (CO2 or H2) in excess of the other reactant (H2 or CO2). The partial pressure of H2 (or CO2) supplied was reduced and replaced with Ar to keep a constant total flow rate of 120 mL min -1 when keeping a constant CO2 (or H2) fraction and pressure (discharge power: 20 W, packing material: 0.5 g). The ratio of CO2/(Ar + H2) was kept at 2:1 when varying pH2, while the ratio of (Ar + CO2)/H2 was maintained at 1:5 when changing pCO2 (keeping the conversion of H2 or however, the contribution of Ar could be minimized due to the low partial pressures of the reactants in this study. This assumption was also considered in the previous study of Barboun et al. [1] . Figure S2a shows the binding energies of Zn 2p3/2 shifted from ∼1021.6 eV to ∼1021.0 eV after the reduction of the catalyst, demonstrating the strong metal-support interaction (SMSI) between Pd and ZnO, which can partially reduce surface Zn 2+ species to form a ZnOx overlayer with the generation of abundant oxygen vacancies [2,3] . As shown in Figure S2b, three peaks at ~531, ~532 and ~533 eV can be assigned to surface lattice oxygen (α, Olatt), chemisorbed oxygen species on oxygen vacancies (β, Oads), and hydroxyl-like groups (γ, OOH), respectively [4] . Compared to the calcined Pd/ZnO catalyst, the portion of peak β increased from 10.1% to 15.3% after the reduction of the catalyst, indicating more oxygen vacancies were formed on ZnOx of the reduced Pd/ZnO catalyst due to the SMSI between Pd and ZnO (Table S1).

Characterization of the Pd-ZnO interface
Moreover, the HRTEM images of the reduced Pd/ZnO confirm the formation of a ZnOx overlayer on the catalyst surface ( Figure S3).

Catalyst characterization results
The XRD patterns of the calcined, reduced and spent catalysts only exhibited ZnO peaks, suggesting that Pd nanoparticles (NPs) could be highly dispersed on the catalyst surface (Figures S14-S15). Figure S16 shows the XPS analysis of the surface Pd chemical state of Pd/ZnO. The calcined Pd/ZnO catalyst showed a peak of Pd 2+ at 336.2 eV, while the XPS of the reduced and spent Pd/ZnO exhibited a peak at 335.1 eV, which can be associated with the formation of Pd NPs on the ZnO surfaces [5] . The BET specific surface area of calcined, reduced and spent Pd/ZnO catalysts was 24.9, 18.4 and 16.2 m 2 /g, respectively ( Figure S17 and Table S4). characterization showed that the properties (pore size, crystal structure, Pd surface state and morphology) of the Pd/ZnO catalyst were almost unchanged after 6 h plasma reaction ( Figures S14-S19).   Figure S16. XPS spectra of Pd 3d for calcined, reduced and spent Pd/ZnO catalysts.   a "Total" means the H2 desorption amount calculated by α + β + γ.
b "I" means the amount of H2 desorption (determined by the peak area) normalized against the peak area (area intensity of 100) marked in Figure 2a. a "Total" means the CO2 desorption amount calculated by α + β + γ.
b "I" means the amount of H2 desorption (determined by the peak area) normalized against the peak area (area intensity of 100) marked in Figure 2b.