3.1.5. Temperature-Programmed Reduction (TPR)

Temperature-programmed reduction or TPR is a handy tool to analyze the reducibility– metal-support interaction and to find the reduction or activation temperature required to generate metallic particles prior to catalytic reaction. The reduction profiles shown in Figure 4 indicate the variation in reducibility and metal–support interaction when Ce is replaced with Sr. The small reduction peak, below 150 ◦C for the CeNiO3 catalyst is related to the reduction in adsorbed oxygen species. Reduction peaks appearing between 200 and 500 ◦C correspond to the reduction of Ni3+ to Ni2+, while the small shoulders at higher temperatures (>500 ◦C) are attributed to the reduction of Ni2+ to Ni<sup>0</sup> [38,39]. On the contrary, significant changes in the reduction peak temperatures (~245 and 345 ◦C) were observed for SrNiO3. Additionally, the three-fold decrease in peak height of SrNiO3, when compared to CeNiO3, indicates that number of reducible species was suppressed by the replacement of Ce with Sr. This was also evident from the total amounts of hydrogen consumed during TPR (Table 2). The degree of reduction was significantly lower in SrNiO3 (50.6%) than CeNiO3 (Table 2), which can be attributed to poor dispersion of Ni within the SrNiO3. The reduction in both CeNiO3 and SrNiO3 can be expressed as (CeNiO3 + H2 → Ni<sup>0</sup> + CeO2 + H2O) and (SrNiO3 + 2H2 → Ni<sup>0</sup> + SrO + 2H2O). The role of these findings in influencing the catalytic activity is discussed in Section 3.

**Figure 3.** TEM images of (**a**) fresh CeNiO3, (**b**) reduced CeNiO3, (**c**) used CeNiO3, (**d**) fresh SrNiO3, (**e**) reduced SrNiO3, and (**f**) used SrNiO3.

**Figure 4.** H2-TPR profiles of SrNiO3 and CeNiO3 perovskites.


**Table 2.** Textural properties and deactivation factors of SrNiO3 and CeNiO3 perovskites.

a Deactivation Factor (D.F., %) = 100 × (CH4 conversioninitial − CH4 conversionfinal)/(CH4 conversioninitial); b from TPR; c the ratio of amount of hydrogen consumed in TPR to the theoretical amount of hydrogen required to completely reduce the catalyst.
