*3.1. Catalyst Preparation*

A series of Al2O3 supported Ni-Mo sulfide catalysts with different Ni/Mo molar ratios was synthesized. Al2O3 with surface area about 310 m<sup>2</sup>/g was grinded and then sieved to 40−60 mesh. In order to prepare the precursors of Ni-Mo sulfide supported on Al2O3, Al2O3 was impregnated by a mixed solution of nickel nitrate and ammonium molybdate at room temperature. The total loading of the oxide precursor was 5 wt%. After the impregnation, the samples were dried at 120 ◦C for 8 h, and then calcined at 500 ◦C for 3 h. After calcination, the samples were sulfided in 60 vol% H2S/H2 flow (80 mL/min) for 3 h. The catalysts were denoted as xNi(8 − x)Mo/Al2O3, where x/(8 − x) was the Ni/Mo molar ratio.

#### *3.2. Catalyst Evaluation for Plasma-Induced CO2-H2S Conversion*

A dielectric barrier discharge (DBD) reactor was used to generate non-thermal plasma at atmospheric pressure in this work. The configuration of reactor has been illustrated in detail in our previous study [12,13]. A high voltage generator was applied to supply a voltage from 0 to 20 kV with a sinusoidal waveform at a frequency of about 10 kHz. This reactor consisted of one quartz tube and two electrodes. The discharge volume of DBD reactor is 15 mL. The stainless-steel rod was used as a high voltage electrode on the axis of the tubular reactor and connected to the plasma generator. The aluminum foil was used as a grounding electrode, and wrapped around the quartz tube and grounded by wires. An amount of 15 mL of the Ni-Mo sulfide/Al2O3 catalyst with 40–60 mesh was placed in the gap between the quartz tube and the high voltage electrode. The discharge power was determined by the Q-V Lissajous pattern, which was measured by the digital oscilloscope. In the conversion of CO2 and H2S to syngas, elemental sulfur was produced. In order to prevent sulfur deposition on the catalyst surface, the reactor was immersed in the oil bath at 120 ◦C to turn the generated sulfur into liquid phase and out of the catalyst bed. The feed gas was flowed through the catalyst bed while the non-thermal plasma was generated by high-voltage discharge.

A cold trap was placed at the exit of the DBD reactor for the condensation of any liquid products. The gas products were analyzed by a two-channel gas chromatography equipped with two thermal conductivity detections (TCD). The first channel contained a Porapak Q column for the measurement of CO2, H2S, and C2−C4 hydrocarbons, while the second channel was equipped with a Molecular Sieve 5A column for the separation of H2, CO, and CH4. The gas chromatography was calibrated for a wide range of concentrations for each gaseous component using reference gas mixtures from the calibrated gas mixes.

For the H2S and CO2 conversion, the conversions of H2S (xH2S) and CO2 (xCO2) were defined as:

$$\text{x}\_{\text{H}\_2\text{S}} = \frac{\text{H}\_2\text{S converted}}{\text{H}\_2\text{S input}} \times 100\% \text{/} \tag{2}$$

$$\chi\_{\text{CO}\_2} = \frac{\text{CO}\_2 \text{ converted}}{\text{CO}\_2 \text{ input}} \times 100\%,\tag{3}$$

The H2/CO molar ratios were defined as:

$$\text{H}\_2/\text{CO} = \frac{\text{H}\_2 \text{ produced}}{\text{CO produced}'} \tag{4}$$

The gas product distributions (C, %) were calculated by the selectivity (S, %) of the products:

$$\text{S}\_{\text{CO}} = \frac{\text{CO produced}}{\text{CO}\_2 \text{ converted}} \times 100\% \tag{5}$$

$$\text{S}\_{\text{H}\_2} = \frac{\text{H}\_2 \text{ produced}}{\text{H}\_2\text{S} \text{ converted}} \times 100\%,\tag{6}$$

$$\text{S}\_{\text{CrMy}} = \frac{\text{x} \times \text{C}\_{\text{x}}\text{H}\_{\text{Y}} \text{ produced}}{\text{CO}\_{2} \text{ converted}} \times 100\text{\%}\_{\text{t}}\tag{7}$$

$$=\frac{\text{H}\_2\text{ produced}}{\text{H}\_2\text{ produced} + \text{CO produced} \times (1 + \sum\_{\text{x$$

$$\text{C}\_{\text{CO}} = \frac{\text{CO produced}}{\text{III} \cdot \text{I} \cdot \text{I} \cdot \text{I} \cdot \text{I} \cdot \text{I} \cdot \text{I} \cdot \text{I} \cdot \text{I} \cdot \text{I}^2 \cdot \text{I}^2} \times 100\% \tag{9}$$

$$^{\text{C-CO}}\text{ H2 produced + CO produced} \times (1 + \sum\_{\text{x=Score}}^{\text{S}\_{\text{CuMg}}}) \text{ }^{\text{C-CO}}\text{H2}$$

$$\text{C}\_{\text{others}} = \frac{\text{CO produced} \times \sum \frac{\text{S}\_{\text{C}\text{H}\_{\text{Y}}}}{\text{x} \times \text{S}\_{\text{CO}}}}{\text{H}\_{2} \text{ produced } + \text{ CO produced} \times (1 + \sum \frac{\text{S}\_{\text{C}\text{H}\_{\text{Y}}}}{\text{x} \times \text{S}\_{\text{CO}}})} \times 100\% \tag{10}$$

The sulfur and carbon balances were defined as:

CH2

$$\mathrm{B\_{sulfur}} = \frac{[\mathrm{H\_2S}]\_{\mathrm{out}} + [\mathrm{Sulphur}]\_{\mathrm{out}}}{[\mathrm{H\_2S}]\_{\mathrm{into}}} \times 100\% \,\mathrm{} \tag{11}$$

$$\text{B}\_{\text{carbon}} = \frac{[\text{CO}\_2]\_{\text{out}} + [\text{CO}]\_{\text{out}} + [\text{C}\_x\text{H}\_\text{Y}]\_{\text{out}}}{[\text{CO}\_2]\_{\text{into}}} \times 100\text{\%} \tag{12}$$

The sulfur and carbon balances were based on sulfur-atom and carbon-atom, respectively. The error of the balances were within 5% and typically better than this.

The area of the Lissajous diagram measures the energy dissipated in the discharge during one period of the voltage. The charge was determined by measuring the voltage across the capacitor of 0.47 μF connected in series to the ground line of the plasma reactor. The discharge power was

calculated from the area of charge–voltage parallelogram and the frequency of discharge. Specific energy input (SEI, J/L), which measures the energy input in the plasma process, was calculated by:

$$\text{SEI} = \frac{\text{P}}{\text{V}'} \tag{13}$$

where P is the discharge power (W), and V is the gas flow rate (L/s).
