*3.2. Catalytic Activity Test*

The catalytic activity test was carried out in a fixed-bed continuous flow micro-quartztube reactor with 10 mm in diameter at atmospheric pressure. There was a thermocouple near the reactor close to the fixed-bed, which was used to follow the temperature of the catalysts during the test. Before the activity test, 0.50 g of the catalyst was heated up to 450 ◦C (10 ◦C/min) and then reduced at a constant temperature of 450 ◦C for 1 h in H2/Ar (F(H2) = F(Ar) = 30 mL/min) mixture gas. After that, the catalyst was cooled down to the reaction temperature before the introduction of the mixture of reactants (H2/CO2 = 4, F = 150 mL/min) for the CO2 methanation reaction. The effluent from the reactor passed through a condensing device and was analyzed online by a gas chromatograph (plot-C2000 capillary column) per hour.

$$X\_{CO\_2} = \frac{n\_{CO\_2,in} - n\_{CO\_2,out}}{n\_{CO\_2,in}} \times 100\%\_{\prime} \tag{1}$$

$$X\_{H\_2} = \frac{n\_{H\_2,in} - n\_{H\_2,out}}{n\_{H\_2,in}} \times 100\% \tag{2}$$

$$S\_{CH\_4} = \frac{n\_{CH\_4,our}}{n\_{CH\_4,our} + n\_{CO,our}} \times 100\%,\tag{3}$$

$$Y\_{CH\_4} = X\_{H\_2} \times S\_{CH\_4} \tag{4}$$

where *XCO*<sup>2</sup> and *XH*<sup>2</sup> were the conversion of *CO*<sup>2</sup> and *H*2, *SCH*<sup>4</sup> was the selectivity of *CH*4, and *YCH*<sup>4</sup> was the yield of *CH*4.

#### *3.3. Catalysts Characterization*

The physical property test was conducted in the Micromeritics Tristar II 3020 instrument by using the N2 adsorption-desorption method. Before the measurements, about 0.1 g samples were outgassed at 150 ◦C for 2 h and then at 300 ◦C for 2 h under a vacuum.

The actual Ni and Si loadings of the fresh catalysts were determined by the X-ray fluorescence (XRF) test. Ni and Si were detected by the Ni Kα line and Si Kα line, respectively.

The hydrogen temperature-programmed reduction (H2-TPR) measurement was carried out with a Micromeritics AutoChem II Chemisorption Analyzer. At first, about 100 mg of the catalyst was pretreated with Ar flow at 150 ◦C for 30 min. Then, the TPR experiment was performed from 50 to 800 ◦C in the H2/Ar (10/90 vol%) flow with the heating rate of 8 ◦C/min. The TCD detector was used to monitor the H2 consumption.

The temperature-programmed desorption of H2 (H2-TPD) was performed on the same equipment for H2-TPR. About 100 mg of the reduced catalyst was pretreated at 500 ◦C for 1 h in Ar flow. Next, H2 was absorbed at 50 ◦C for 1 h in 10% H2/Ar. After cleaning the excess unabsorbed H2, the catalyst was heated to 800 ◦C with a heating rate of 10 ◦C /min under Ar flow. The results were detected by a TCD detector. The H2 pulse chemisorption was also processed on the equipment. About 100 mg of the reduced catalyst was pretreated at 450 ◦C for 1 h in Ar flow and cooled down to 50 ◦C at the same atmosphere for beginning the H2 adsorption. A gas mixture of 10% H2 balance Ar was pulsed over the catalyst for chemisorption measurements.

Before the temperature-programmed desorption of CO2 (CO2-TPD), about 100 mg of the reduced catalyst (reduced at 450 ◦C) was pretreated at 500 ◦C in He flow for 1 h to remove surface impurities. Then, CO2 was absorbed at 50 ◦C for 1 h in 10% CO2/He. After cleaning the excess unabsorbed CO2, the catalyst was heated to 900 ◦C with a heating rate of 10 ◦C/min in He flow. The observed curves were fitted into two Gaussian peaks.

The X-ray diffraction (XRD) was performed on a DX-1000 CSC diffractometer instrument, operating at 40 kV and 25 mA with a Cu Kα radiation source for the calcined and reduced catalysts. The data was recorded over the scattering angle range of 2θ from 10 to 80◦, with a scan step with of 0.03◦.

Transmission electron microscopy (TEM) was used to characterize the reduced catalysts on the Tecnai G2 F20 machine. The twin instrument with the 0.20 nm resolution was used, and the acceleration voltage was 200 Kv.

The analysis of X-ray photoelectron spectroscopy (XPS) was carried out on a KRATOS spectrometer with an AXIS Ultra DLD. The Al Kα monochromatized line was operated at the accelerating power of 25 W. In addition, the binding energy was calibrated with C 1 s 284.6 eV.

Thermogravimetric (TG) and differential scanning calorimetry analysis (DSC) was used to characterize the deposited carbon of the spent Ni-xSi/ZrO2 catalysts, using the NETZSCH TG209F1 instrument. Before the test, the sample was placed until a better gas equilibrium. Then, the temperature was increased from 30 to 800 ◦C with a 5 ◦C·min−<sup>1</sup> heating rate in air flow with a rate of 60 mL·min<sup>−</sup>1.

#### **4. Conclusions**

Adding the appropriate amount of Si could increase the catalytic activity of Ni/ZrO2 catalyst, and the Ni-0.1Si/ZrO2 catalyst showed the highest catalytic activity and stability. The strong interaction between Ni and ZrO2 could promote the dispersion of Ni on the support, and the strong basic sites on the catalyst were beneficial to the absorption of CO2, thus to the CO2 methanation reaction on the Ni-0.1Si/ZrO2 catalyst. In addition, the higher amount of surface Ni0 could provide more active sites, and the more oxygen vacancies were beneficial to the absorption and activation of CO2 on the 0.1Si/ZrO2 catalyst.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/2073-4 344/11/1/67/s1. Figure S1. The TG-DSC profile of the spent Ni-xSi/ZrO2 catalysts (A) Ni /ZrO2 catalyst, (B) Ni-0.1Si/ZrO2 catalyst, (C)Ni-0.5Si/ZrO2 catalyst, and (D) Ni-1Si/ZrO2 catalyst.

**Author Contributions:** Conceptualization, methodology, L.L., Y.W. and C.H.; software, validation, formal analysis, investigation, resources, data curation, writing—original draft preparation, L.L. and Q.Z.; writing—review and editing, visualization, supervision, Y.W. and C.H.; project administration, funding acquisition, C.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the National Key R and D Program of China (2018YFB1501404), the 111 program (B17030), and Fundamental Research Funds for the Central Universities.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** We would like to thank the Analytical and Testing center of Sichuan University for the characterization and we are grateful to Yunfei Tian for his help in the XPS experiments.

**Conflicts of Interest:** The authors declare no conflict of interest.
