*2.8. Chemical State of the Elements on Ni-xSi/ZrO2 Catalysts*

The surface element composition and chemical state of reduced Ni-xSi/ZrO2 catalysts were obtained by the XPS experiment, which were shown in Figure 9. There were four main peaks in the Ni 2p spectra. The peak at around 852 eV was assigned to the characteristic peak of Ni0 [7]. There were two characteristic peaks of Ni2+, P1 (about 854 eV) was the low energy peak and belonged to the peak of NiO, while P2 (about 856 eV) corresponded to Ni (OH)2 [25]. The peak at around 860 eV was the companioning peak of Ni2+, produced by the orbital spin splitting [50]. The percentages of different elements on the surface of catalysts were summarized in Table 6. The surface content of Ni0 was the highest on the

Ni-0.1Si/ZrO2 catalyst, which was 2.15%. More Ni0 could provide more active sites and facilitate CO2 methanation [58]. It could be found that the surface content of Si was higher than its actual loading, suggesting that Si enriched on the catalysts surface and part of Ni might be covered by Si, especially on Ni-0.5Si/ZrO2 and Ni-1Si/ZrO2 catalysts. The O 1s spectrum exhibited two types of oxygen species, as shown in Figure 9B. The peak at 530–530.46 eV was attributed to the lattice oxygen (Oα) and the peak at 528.9–529.1 eV belonged to the surface oxygen (Oβ) [28,59]. Based on the areas of O<sup>α</sup> and Oβ, the ratios of the oxygen vacancies could be obtained by calculating the ratio of O<sup>β</sup> to OT (OT = O<sup>α</sup> + Oβ) [5,22,38]. In Table 6, the reduced Ni-0.1Si/ZrO2 catalyst exhibited the highest ratio (0.589) of O<sup>β</sup> to OT compared with other catalysts, suggesting the highest amount of oxygen vacancies. The presence of oxygen vacancies was beneficial to the adsorption of CO2, which could promote CO2 activation [23,58]. Jiang et al. [17] found that the content of surface oxygen on the Mn promoted Ni/bentonite catalyst was 83.55%, which was higher than that of the unpromoted Ni/bentonite catalyst (74.85%), representing the higher amount of oxygen vacancies. The increased oxygen vacancies were helpful to the adsorption and dissociation of CO2 on the catalyst. To sum up, the Ni-0.1Si/ZrO2 catalyst exhibited the highest catalytic activity and stability due to the highest amount of Ni<sup>0</sup> and oxygen vacancies on the surface.

**Figure 8.** TEM images of 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.

**Figure 9.** XPS spectra of reduced Ni-xSi/ZrO2 catalysts (**A**) Ni 2p and (**B**) O 1s.


**Table 6.** Surface contents on reduced Ni-xSi/ZrO2 catalysts.

## **3. Materials and Methods**

## *3.1. Catalysts Preparation*

The ZrO2 support was prepared by the precipitation method. A certain amount of Zr (NO3)4·5H2O (ChengduKelong, China) was dissolved in deionized water with continuous stirring until dissolved completely. Then, NH3·H2O was added to the above solution to achieve a pH value of 9. After stirring for 2 h, the mixture was aged for 24 h at room temperature. After that, the mixture was filtered and washed three times with deionized water. The obtained sample was dried at 110 ◦C for 4 h and then calcined at 500 ◦C for 5 h (with the rate of 2 ◦C /min) to obtain the ZrO2 support.

The impregnation method was used to synthesize the Ni-xSi/ZrO2 catalysts (ZrO2 was used as the support and Si was used as the promoter with the load of 0, 0.1, 0.5, and 1 wt%). The designed amount of Ni (NO3)2·6H2O (ChengduKelong, Chengdu, China) and different amounts of (C2H5O)4Si (Alfa Aesar Chemicals, Shanghai, China) were dissolved in a certain amount of absolute ethanol (Chengdu Chron Chemical, Chengdu, China). The ZrO2 support was impregnated with the aforesaid ethanol solution for 24 h at room temperature. Then, these samples were dried at 80 ◦C for 2 h and 110 ◦C for 4 h. Finally, the above mixtures were heated to 500 ◦C (with the rate of 2 ◦C/min) and calcined at 500 ◦C for 5 h under air flow, then, the Ni-xSi/ZrO2 catalysts were obtained (x = 0, 0.1, 0.5, 1 wt%).
