*2.2. Temperature-Programmed Reduction (TPR)*

Figure 2 and Table 1 illustrates the temperature-programmed reduction (TPR) and the hydrogen consumptions values of LCG and LKCG-x (x = 0.1 and 0.2) catalysts. Seen from the Figure 2, all the three catalysts contain two major hydrogen-consuming peaks, one at 400–500 ◦C and the other at 600–800 ◦C. According to the literature, the H2-consuming peak of LaCoO3 at 500 ◦C can be attributed to Co3+ → Co2+, while that above 500 ◦C to Co2+ → Co0 [22–25]. Herein, seen from Table 1, the area ratio of the low temperature peak to the high temperature peak (TL/TH) is 1/2. Therefore, we believe that the peak of 400–500 ◦C in all the three catalysts is classified to Co3+ → Co2+, and the peak of 600–800 ◦C can be assigned to Co2+ → Co0. At the same time, the similar theoretical and experimental H2-consumption also confirmed the attribution of the above reduction peak.

**Figure 2.** The temperature-programmed reduction (TPR) curves of catalysts after calcination.

Compared to the TPR results of LaCoO3 in the literature, the H2-consuming peak of Co2+ → Co0 migrates to higher temperatures in the LCG catalyst, suggesting that the existence of Ga in LaCoO3 will restrain the conversion of Co2+ to Co0 and a strong effect between Co and Ga exists for LCG [16].

Seen from Figure 2, with the addition of K ions in LCG, the H2-consuming peaks at around 750 ◦C moved to lower temperature, illustrating the addition of K promotes the reduction of Co2+ → Co0. While for LKCG-0.2, a shoulder peak around 620 ◦C appeared. According to XRPD, the catalyst contains Co3O4. Therefore, this small shoulder can be attributed to the reduction of Co2+ → Co<sup>0</sup> in Co3O4. The presence of the small shoulder also confirms the XRPD results. For the other samples, no shoulders can be observed, which indicates that all the Co ions have entered into the crystal lattice of the perovskite.

**Table 1.** Theoretical and experimental H2 consumption value of the LKCG-x (x = 0.1 and 0.2) and LCG catalysts.


<sup>a</sup> Calculated from the TPR results. <sup>b</sup> The unit of H2 consumption value is mmol H2/50 mg. TL and TH represent low and high temperature, respectively.
