## 3 Results and Discussion
### Catalytic Performance
A high and stable H\({}_{2}\) yield from 51 to 81% was obtained on the A catalyst, with a one-stage synthesis and activation in a vacuum being preferable to the polyol method and calcination in air. It was established [32] that an important factor providing the catalytic stability of the reduced Ni/MgO was the solid solution without \"free\" NiO. It can be assumed that preliminary vacuum calcination of the S-samples (one-stage synthesis) ensured the formation of a NiO-MgO solution that was supported by higher values of the crystal lattice parameter (Table 2); therefore, a large amount of Ni\({}^{2+}\) was in the bulk of the solid solution, which offered strength in the sintering and production of coke. The C-series catalyst exhibited a low activity, which, however, increased over time. According to XRD and XPS data, that sample was not reduced in a 30% H\({}_{2}\)/Ar flow at 800 \({}^{\circ}\)C (Tables 3 and 4). Apparently, Ni\({}^{0}\) active centers were blocked by salt impurities, which could be removed from the surface by water vapor during the reaction. Thus, the long induction period of the reaction was probably associated with the clearing of active centers and the reduction of Ni\({}^{2+}\). At the same time, the quantity of active Ni\({}^{0}\) particles increased with an increase in reaction time from 1 to 6 h, and the H\({}_{2}\) yield increased from 0 to 33%. Pd-promoted catalysts were self-activated under the reaction conditions, as evidenced by an increase in the CH\({}_{4}\) to CO\({}_{2}\) conversion (Figure S8) and in the H\({}_{2}\) yield from 55 to 76% within 2 h of reaction. Such differences between monometallic (Ni/CeO\({}_{2}\)) and bimetallic (Pt-Ni/CeO\({}_{2}\)) catalysts were also observed in DMR [15]. This was consistent with the coating of the reduced catalyst with chemisorbed hydrogen. The activity of the promoted catalysts was retained during the day of operation, which indicated that they were promising for the CSMR reaction. The activity of catalysts during long-term testing was controlled by two processes: the sintering of nanoparticles and carbon formation. The extent of nickel leaching from the catalyst was negligible due to the strong MSI. The mass of carbon deposited on spent catalysts was calculated from the TGA (Figure 7). A weight loss was observed in the low-temperature range associated with the desorption of volatile reaction compounds, mainly water. In the range from 200 to 325 \({}^{\circ}\)C, the weight loss was associated with the removal of the template applied in the synthesis, which was confirmed by the thermal analysis data of fresh catalysts. In addition, in the higher temperature range of 325-470 \({}^{\circ}\)C, an increase in the sample weight occurred, accompanied by an exothermic effect (T\({}_{\rm DTA}\) = 390 \({}^{\circ}\)C), which was associated with the oxidation of Ni\({}^{0}\) and the formation of oxide particles. Figure 6: Hydrogen yield during CSMR at 750 \({}^{\circ}\)C. 1—Ni/CeMg-C-S-O; 2—Ni/CeMg-A-S-V; 3—Ni/CeMg-A-S-O; 4—Ni-Pd/CeMg-A-S-V; 5—Ni/CeMg-A-P-O. For the C catalyst (chloride support precursor), no weight increase was observed, which indicated a predominantly oxidized state of nickel and corresponded to XPS and XRD data (Tables 3 and 4). For the A catalyst (cerium acetate support precursor) after activation in vacuum, there was also no weight gain step, which was associated with the removal of a larger amount of template. It is known that a weight loss above 470 \({}^{\circ}\)C on the TG-DTA curves indicates the formation of coke [64] on the catalysts. The rate of carbon formation in the A catalysts rose in the following order: Ni/CeMg-A-P-O \(<\) Ni/CeMg-A-S-O \(<\) Ni-Pd/CeMg-A-S-V \(<\) Ni/CeMg-A-S-V (Figure 7). Despite the larger amount of deposited carbon, the catalyst activated in vacuum (Table 4) showed a relatively stable CH\({}_{4}\) conversion compared with the catalysts after activation in air. The average crystallite sizes of the spent catalysts were calculated from XRD (Table 3).