*2.2. Reduction Behavior of the Studied Catalysts (H2-TPR)*

In order to investigate the effect of the barium promoter content on the reducibility of the cobalt catalysts, temperature-programmed reduction measurements were performed. Figure 1 shows the reduction profiles for the studied catalysts. The area of the graph presented in Figure 1 was divided into areas marked as I, II, IIIa, and IIIb to simplify the description of the obtained signals. In the reduction profile of the CoCe sample, which in the oxidized form is a mixture of Co3O<sup>4</sup> and CeO<sup>2</sup> oxides, two peaks (marked in Figure 1 as II and III) are observed, with maxima at 289 ◦C and 479 ◦C, respectively. These signals correspond to a two-step reduction of cobalt oxide to metallic cobalt [30,31], in accordance with Equations (1) and (2):

$$\text{Ca}\_3\text{O}\_4 + \text{H}\_2 \to \text{3CoO} + \text{H}\_2\text{O} \tag{1}$$

$$\text{3\%}\text{CoO} + \text{3H}\_2 \rightarrow \text{3\textdegree Co} + \text{3H}\_2\text{O} \tag{2}$$

Under the measurement conditions (temperature increase from 30 to 700 ◦C at a constant rate of 10 ◦C min−<sup>1</sup> , 10 vol.% H2/Ar), cerium (IV) oxide did not undergo reduction, which has been reported in previous studies [15,28]. It is noted that the introduction of the barium promoter to the systems containing cobalt (II,III) oxide and cerium (IV) oxide causes a change in the course of their reduction (Figure 1). The TPR profiles of the samples containing barium show a small peak (I) of constant area and maximum in the range of 210 ◦C ± 15 ◦C, which may be related to the decomposition of the barium salt. The position of peak II is not influenced by the barium content in the system. Its maximum occurs at the temperature of 299 ◦C ± 8 ◦C. However, in the case of the samples containing barium in the

amount of 1.4 mmol gCo <sup>−</sup><sup>1</sup> and more, the intensity of peak II slightly increases. Presumably, it results from the overlapping of the peaks related to the reduction of Co3O<sup>4</sup> to CoO and further decomposition of the barium salt. In the profiles of the samples containing 0.2–1.1 mmol Ba gCo −1 , the maximum of peak III shifts towards higher temperatures with increasing barium promoter content. For the samples containing 1.4 mmol Ba gCo <sup>−</sup><sup>1</sup> and more, two peaks (IIIa and IIIb) are observed instead of one in the region where peak III is present. The maximum of peak IIIa occurs in a constant temperature range, i.e., 435 ◦C ± 5 ◦C, while the maximum of peak IIIb shifts from the position at 554 ◦C for CoCeBa (1.4) towards lower temperatures with increasing barium content. Moreover, with the addition of more barium promoter, the decrease in the intensity of peak IIIb is observed, accompanied by the increase in the intensity of peak IIIa. Finally, in the reduction profile of CoCeBa(2.6), a very high intensity of peak IIIa and a negligibly small IIIb peak are observed. sumably, it results from the overlapping of the peaks related to the reduction of Co3O4 to CoO and further decomposition of the barium salt. In the profiles of the samples containing 0.2–1.1 mmol Ba gCo −1, the maximum of peak III shifts towards higher temperatures with increasing barium promoter content. For the samples containing 1.4 mmol Ba gCo −<sup>1</sup> and more, two peaks (IIIa and IIIb) are observed instead of one in the region where peak III is present. The maximum of peak IIIa occurs in a constant temperature range, i.e., 435 °C ± 5 °C, while the maximum of peak IIIb shifts from the position at 554 °C for CoCeBa (1.4) towards lower temperatures with increasing barium content. Moreover, with the addition of more barium promoter, the decrease in the intensity of peak IIIb is observed, accompanied by the increase in the intensity of peak IIIa. Finally, in the reduction profile of CoCeBa(2.6), a very high intensity of peak IIIa and a negligibly small IIIb peak are observed.

the barium promoter to the systems containing cobalt (II,III) oxide and cerium (IV) oxide causes a change in the course of their reduction (Figure 1). The TPR profiles of the samples containing barium show a small peak (I) of constant area and maximum in the range of 210 °C ± 15 °C, which may be related to the decomposition of the barium salt. The position of peak II is not influenced by the barium content in the system. Its maximum occurs at the temperature of 299 °C ± 8 °C. However, in the case of the samples containing barium in the amount of 1.4 mmol gCo −1 and more, the intensity of peak II slightly increases. Pre-

*Catalysts* **2022**, *12*, x FOR PEER REVIEW 4 of 15

**Figure 1.** Reduction profiles of the cobalt catalyst promoted with cerium and with a barium loading in the range: 0–2.6 mmol gCo <sup>−</sup><sup>1</sup> (measurement conditions: 30–700 °C, 10° C min<sup>−</sup>1, 10 vol.% H2/Ar). **Figure 1.** Reduction profiles of the cobalt catalyst promoted with cerium and with a barium loading in the range: 0–2.6 mmol gCo −1 (measurement conditions: 30–700 ◦C, 10◦ C min−<sup>1</sup> , 10 vol.% H2/Ar).

It can be clearly stated that the addition of barium in the range of 0.2–2.2 mmol Ba gCo−1 hinders the process of the CoCeBa catalysts reduction. It requires a longer time and ends at a higher temperature than in the case of a sample without barium (CoCe). Additional peaks and complexity of the CoCeBa catalyst precursor reduction profiles are most likely related to the decomposition of barium salts. It can be clearly stated that the addition of barium in the range of 0.2–2.2 mmol Ba gCo −1 hinders the process of the CoCeBa catalysts reduction. It requires a longer time and ends at a higher temperature than in the case of a sample without barium (CoCe). Additional peaks and complexity of the CoCeBa catalyst precursor reduction profiles are most likely related to the decomposition of barium salts.
