*2.5. The Results of CO2-TPD on Ni-xSi/ZrO2 Catalysts*

The CO2 desorption capability was explored by the CO2-TPD measurement and the profiles were shown in Figure 6, which was used to describe the basicity of catalysts usually. Two main CO2 desorption peaks could be observed over each catalyst, which were classified as the adsorbed CO2 at weak basic sites (361–410 ◦C) and strong basic sites (577–604 ◦C), respectively. Based on the pioneer studies, the CO2 adsorbed on the weak basic sites could be desorbed at a low temperature and that absorbed on the strong basic sites could be desorbed at a high temperature [44]. The contents of weak and strong basic sites were calculated by the area of the peaks (Table 4). It could be seen that the content of strong basic sites on Ni-0.1Si/ZrO2 was the highest (71%) among all the catalysts studied, which could inhabit carbon formation effectively [54,55]. Le et al. [56] found that the stronger CO2 desorption peak around 500 ◦C on the Ni-CeO2 catalyst suggested the strong CO2 adsorption ability, which had a positive effect on the catalytic performance. In addition, the CO2 adsorption peak of strong basic sites slightly shifted to a higher temperature with the increasing of Si, suggesting that the addition of Si promoted the adsorption of CO2. The intensified surface basicity of the Ni-0.1Si/ZrO2 catalyst could promote the adsorption of CO2 thus promoting the CO2 methanation reaction [42,44,46].

**Figure 6.** CO2-TPD profiles of Ni-xSi/ZrO2 catalysts.

**Table 4.** Peak position and basic sites of Ni-xSi/ZrO2 catalysts obtained by CO2-TPD.

