*2.3. The Reducibility of Ni-xSi/ZrO2 Catalysts*

The TPR profiles of calcined Ni-xSi/ZrO2 catalysts and support ZrO2 were presented in Figure 4. There was a broad peak at around 500 ◦C on the ZrO2. Three main different reduction peaks (α, β, and γ) could be observed on the Ni-xSi/ZrO2 catalysts, which suggested that there were three kinds of nickel oxide species. The α peak (317–327 ◦C) was attributed to the reduction of the bulk NiO species on the surface of the catalysts. The β peak (340–366 ◦C) and γ peak (419–428 ◦C) were assigned to the reduction of NiO species that interacted weakly and strongly with support, respectively. The amount of each NiO species was summarized in Table 2. The minimum amount of bulk NiO species could be observed, indicating that a little bulk NiO species existed on all catalysts. The amount of β peak was over 60% on both Ni-0.5Si/ZrO2 and Ni-1Si/ZrO2 catalysts. Associated with the catalytic activity, the NiO species that interacted with ZrO2 weakly was not good for the CO2 methanation reaction after reduction, resulting in a lower catalytic activity. While Ni-0.1Si/ZrO2 and Ni/ZrO2 catalysts showed the higher amount of the γ peak contributing to a higher catalytic activity, suggesting that the strong interaction between the nickel and the support could promote the dispersion of Ni on the support [51]. Furthermore, the β and γ peaks of promoted Ni/ZrO2 catalysts shifted to a lower temperature compared with that of the Ni/ZrO2 catalyst, suggesting that the appropriate Si (0.1 wt%) promoted the dispersion of Ni [50,51]. The peaks of the Ni-0.1Si/ZrO2 catalyst shifted to the lowest temperature (the β at 340 ◦C and the γ at 419 ◦C), indicating the highest Ni dispersion, which could increase the catalytic activity of catalysts. Therefore, the strong nickel-support interaction promoted the dispersion of Ni on the support, which was beneficial to the catalytic performance [1,50].

**Figure 4.** H2-TPR profiles of Ni-xSi/ZrO2 catalysts and ZrO2.


**Table 2.** The amount of each nickel oxide species on the Ni-xSi/ZrO2 catalysts.
