*3.4. Discussion*

The as-synthesized perovskites were analyzed prior to reaction to predict their performance during reaction. TG-DTG data show that the precursors were converted into perovskite after being calcined at the temperatures demonstrated by the DTGs (Figure 1 and Table 1). Both SrNiO3 and CeNiO3 perovskites were formed when they were calcined at 700 ◦C. XRD diffraction patterns (Figure 2) have shown the existence of perovskite structure and oxides of nickel and cerium along with oxide and carbides of strontium. The textural properties (Table 2), analyzed using nitrogen adsorption-desorption isotherms, demonstrated specific surface areas of 12.2 and 20.7 m2/g (SrNiO3 and CeNiO3, respectively). Morphological analysis using TEM (Figure 3) displayed spherical particles with different sizes as strontium is replaced with cerium. The TPR profiles (Figure 4) aimed to find out the reduction behavior of the perovskites and it was evident that the reduction in oxides of nickel was easier for CeNiO3, while it became difficult in the case of SrNiO3 which is in agreemen<sup>t</sup> with the TG-DTG results (Figure 1). Based on the analyses of perovskites prior to the reforming reaction, it was inferred that CeNiO3 exhibited a higher specific surface area, number of reducible species, and a wider range of particle sizes in comparison with SrNiO3. Hence, CeNiO3 perovskite was expected to show higher activity which was evidenced by CH4 and CO2 conversions (Figure 5). It is well known that the dry reforming reaction mechanism needs adsorption of reactants on the active sites, which then dissociate and react to give products, followed by product desorption [4]. Metallic nickel is the main active site for CH4 adsorption. From catalyst activity results, it was found that SrNiO3 showed a decrease in both CH4 and CO2 conversions which can be attributed to the loss of nickel active sites due to agglomeration during calcination, and/or the covering of nickel with strontium oxide or carbonate. From the TPR and TEM images, it can be seen that SrNiO3 has higher reduction temperatures and there is evidence of sintering during calcination. Similarly, the XRD patterns show that clear peaks of oxides and carbonates of strontium are found for SrNiO3 perovskite, which supports the hypothesis that nickel active sites are covered.

Figure 5 also shows the activity results as a function of reaction time which is associated with the durability of the perovskites. It is evident that both perovskites showed deactivation over time, which can be attributed to both sintering and carbon deposition, as evidenced by the TPO and TEM images of the spent catalysts. The extent of the sintering is almost the same for both perovskites during DRM. This suggests that the carbon deposition resulted from methane decomposition, which is a prevalent side reaction at high reaction temperatures and is considered to be the main cause of deactivation. This is in agreemen<sup>t</sup> with the TPO results, where it is evident that carbon gasification over the surface of CeNiO3 requires a higher temperature when compared to SrNiO3. Moreover, CO2 conversions for CeNiO3 are higher than for SrNiO3 which implies that the oxidative environment suitable for carbon gasification is predominant in CeNiO3, suggesting easier carbon removal and no deactivation in this catalyst. Rynkowski et al. [42] investigated DRM in reduced La2−<sup>x</sup>SrxNiO4 perovskite oxides and concluded that the smaller amounts of strontium exhibited less activity and more stability when compared to strontium-free catalysts. Choudhary et al. [43] also studied the influence of the partial substitution of La and Ni in LaNiO3 perovskites and found that catalytic activity is lost after La is partially substituted by Sr in LaNiO3 perovskite.
