*3.2. Catalytic Performances*

The as-synthesized SrNiO3 and CeNiO3 perovskites were investigated for their catalytic performance at 700 ◦C. Due to the fact that the dry reforming reaction requires metallic nickel crystallites as active sites, all the perovskites were reduced under a hydrogen atmosphere prior to the reaction study. The activity results, in terms of CH4 and CO2 conversions as a function of time, are shown in Figure 5a,b, respectively. From Figure 5a, it is evident that CeNiO3 deactivates over time, despite displaying relatively higher CH4 conversion. CeNiO3 demonstrates an initial CH4 conversion of 54.3% which reaches 50.1% after 440 min time-on-stream, resulting in deactivation factor of 7.7% (Table 2). Strontium incorporation clearly influences CH4 conversion, as initial conversion decreased from 54.3% (CeNiO3) to 22% (SrNiO3). Interestingly, significant deactivation is observed for the strontium incorporated perovskite (SrNiO3) and hence a deactivation factor of 64.4% is found for SrNiO3 (Table 2). The deactivation of the catalysts and the factors behind it are analyzed by characterizing the spent catalysts, as discussed in Section 3.3. A similar trend was found in Figure 5b for CO2 conversions versus time-on-stream. Initial CO2 conversions of 64.8 and 34.7% were demonstrated by CeNiO3 and SrNiO3, respectively, which reached final conversions of 58 and 11.5%, respectively. It is also worth observing from Figure 5 that CO2 conversions are higher than those of CH4. This result implies the simultaneous existence of the reverse water-gas shift reaction (CO2 + H2 → CO + H2O) that generates H2/CO molar ratios lower than the stoichiometric one (H2/CO = 1.0) due to the fact that hydrogen consumes CO2 and CO in a disproportionation or Boudouard reaction (2CO → CO2 + C). A separate section is dedicated to the discussion of catalytic activity results in relation to their analysis findings in Section 3.4.

**Figure 5.** (**a**) CH4 conversion, and (**b**) CO2 conversion versus time-on-stream (TOS) of SrNiO3 and CeNiO3 perovskites.

#### *3.3. Characterization of Spent Perovskites*

The perovskites, after being investigated for dry reforming reaction, were further analyzed to understand their catalytic performance results. CeNiO3 showed deactivation while all the perovskites showed CO2 conversions higher than CH4 conversions (Figure 5), which gave rise to side reactions such as reverse water-gas shift and CO disproportionation. The perovskites were then analyzed using temperature-programmed oxidation and transition electron microscope to assess the modifications to perovskites during reaction.

## 3.3.1. Temperature-Programmed Oxidation (TPO)

In order to verify the possibility of carbon deposition over the surface of the perovskites, TPO analysis was carried out after the reforming reaction. Figure 6 presents the TPO results of the SrNiO3 and CeNiO3 perovskites. Both of the perovskites showed one broad peak in the temperature range of from 170 to 550 ◦C. The peak maximum temperatures are ~320 and 335 ◦C for SrNiO3 and CeNiO3, respectively. These peaks were attributed to the polymeric species of carbon deposited and/or less reactive surface carbides formed during the reaction, as reported earlier [40,41]. The peak temperatures correspond to the degree of hydrogenation of surface carbon species and the surface carbon changing to be graphitic in nature as the peak temperature increased. It is evident from peak temperatures that both perovskites have shown the formation of mainly polymeric carbon species and that their interaction with the catalyst surface changes, becoming stronger with strontium replacement by cerium, as demonstrated by the increase in peak temperatures from 320 to 335 ◦C. It could also be observed that cerium oxide played a role in controlling the carbon deposition, carbon alleviation and the degree of interaction between carbon and the catalyst surface.

**Figure 6.** TPO profiles of SNiO3 and CeNiO3 perovskites.

3.3.2. Transition Electron Microscopy (TEM)

To further verify the causes of deactivation or modification of the perovskites during the reforming reaction, transition electron microscopic analysis was carried out. The TEM images in Figure 3c,f show particle sizes of 10–70 nm and 25–77 nm for CeNiO3 and SrNiO3 spent catalysts, respectively. The TEM results manifest the formation of carbon over spent catalysts as well as noticeable agglomeration of the perovskite particles. Hence, sintering also contributes to the deactivation of SrNiO3 and CeNiO3. These findings are discussed below in Section 3.4.
