*2.1. The Catalytic Performance of Ni-xSi/ZrO2 Catalysts*

The catalytic performance of Ni-xSi/ZrO2 catalysts was investigated over the temperature range from 200 to 400 ◦C under atmospheric pressure. The results of the catalytic test were presented in Figure 1, including the conversion of H2 and CO2, and the selectivity and yield of CH4. The catalytic activity increased as the temperature increased until it was

thermodynamically limited by the equilibrium. Among all catalysts, the Ni-0.1Si/ZrO2 catalyst was the most active over the whole temperature range, followed by the Ni/ZrO2 catalyst, which exhibited an excellent catalytic activity at 250 ◦C. On the unpromoted Ni/ZrO2 catalyst, the CO2 conversion and CH4 yield were 66.6% and 66.2%, respectively with the 99.4% CH4 selectivity. Si-promoted Ni/ZrO2 catalysts exhibited a different catalytic activity, which changed with the amount of Si. The Ni-0.1Si/ZrO2 catalyst exhibited the highest CO2 conversion of 72.5% and H2 conversion of 73.4% among the studied catalysts. Simultaneously, it showed the highest CH4 selectivity of 99.6% and the highest CH4 yield of 72.2%, whose catalytic activity was about 6% higher than the Ni/ZrO2 catalyst. However, the Ni-0.5Si/ZrO2 catalyst showed a rather low activity, with 10% H2 conversion and 9.8% CO2 conversion. The lowest catalytic activity was obtained on the Ni-1Si/ZrO2 catalyst, with only 1% CH4 yield. In general, adding the appropriate Si was beneficial to CO2 methanation.

**Figure 1.** The conversion of H2 (**A**) and CO2 (**B**), the selectivity of CH4 (**C**), and the yield of CH4 (**D**) on Ni-xSi/ZrO2 catalysts over the temperature range from 200 to 400 ◦C.

Then, the catalytic stability of Ni-xSi/ZrO2 catalysts was investigated at 250 ◦C where the catalyst exhibited a high activity. The results of the catalytic test were presented in Figure 2. All the catalysts exhibited the high stability with 10 h on stream. TG-DSC analysis results showed that the weight of all spent catalysts did not decrease during the heating process (Figure S1), which illustrated that there was no carbon deposition on the catalysts after reaction.

**Figure 2.** The conversion of H2 (**A**) and CO2 (**B**), the selectivity of CH4 (**C**), and the yield of CH4 (**D**) on Ni-xSi/ZrO2 catalysts at 250 ◦C with a time on stream (H2/CO2 = 4/1, F = 150 mL/min).
