*2.3. TEM Analysis*

The morphologies and microstructures of the Ni-Mo sulfide/Al2O3 catalyst with Ni/Mo of 5/3 are presented in Figure 3. The TEM images sugges<sup>t</sup> that the highly distributed nanoparticles were dispersed on the supports, which can prevent their grain growth. Furthermore, it also reveals that the average particle size was about 10 nm, which was in agreemen<sup>t</sup> with the particle size estimated from Scherrer equation (shown in Table 1). As shown in Figure 3d, the observed interlayer spacing of 6.1 Å, which was identical to the lattice fringe of Ni-Mo-Sx, was bigger than that of the (0 0 2) plane of MoS2 (6.0 Å) [18]. It can be attributed to the bigger radius of Ni2<sup>+</sup> ions. Meanwhile, the interplanar spacings were about 2.8 and 3.2 Å, which correspond to the (2 0 0) and (1 1 1) planes of NiS2, respectively [19,20]. These results of TEM analysis are in good agreemen<sup>t</sup> with the XRD analysis.

In addition, the porosity of the catalyst also plays an important role in the generation of an electric field in non-thermal plasma. As shown in Table 1, the obtained Ni-Mo sulfide/Al2O3 catalysts had high surface areas (>200 m<sup>2</sup>/g). As pointed by Fridman [21], the porous material in the gap refracts the electric field, enhancing the local field by a factor of over 10 depending on the porosity of the materials. The electric field can excite the Ni-Mo sulfide semiconductor to generate electron–hole pairs, which plays an extremely important role in converting CO2 and H2S. Simultaneously, the strong electric field is beneficial for delaying the recombination of electron–hole pairs, thereby extending their lifetime.

**Figure 3.** TEM and HR-TEM images of the 5Ni-3Mo/Al2O3 catalyst (**a**) scaleplate of 100 nm; (**b**) scaleplate of 50 nm; (**c**) scaleplate of 20 nm; (**d**) scaleplate of 10 nm.
