# Nanosized Ni/SBA-15 Catalysts for CO\({}_{2}\) Reforming of CH\({}_{4}\)
## 3 Results and Discussion
The first peak was assigned to NiO species that are weakly attached to the support, while the one at 530 \({}^{\circ}\)C was for the NiO species that are strongly interacting with the support [15; 32]. For Co/S15, two low temperature peaks were observed for the two-step reduction of Co\({}^{3+}\) to Co\({}^{2+}\), and then to Co\({}^{0}\). Additionally, another peak was observed at 850 \({}^{\circ}\)C, which indicates a firm interaction between smaller Co oxide crystals and the support, or the formation of cobalt silicates. The profile of Co-Ni/S15 showed two low temperature peaks at 217 and 370 \({}^{\circ}\)C which could be due to the formation of an alloy between Co and Ni. Hence, reduction occurs more easily in the bimetallic system than in the monometallic system. It seems that the addition of Co as an active component weakens the interaction between Ni and the support. Therefore, catalyst reduction takes place easily, and more active sites are provided for the catalytic reaction [15]. The incorporation of Zn and Ce oxides led to a better efficiency and better reduction of the active sites at lower temperatures. The TPR profiles of Co-Ni/Ce-S15 and Co-Ni/Zn-S15 indicate the presence of four and three peaks, respectively, in contrast to two peaks only for Co-Ni/S15. NiO and/or Co in Co-Ni/Ce-S15 and Co-Ni/Zn-S15 catalysts were reduced at 220 and 240 \({}^{\circ}\)C, respectively, as shown in Figure 3. The cobalt oxides were reduced at lower temperatures in Co-Ni/Ce-S15 compared to Co-Ni/S15. Figure 3 shows that the catalyst doped with Ce was more completely reduced at lower temperatures than the Zn doped catalyst, indicating that the surface oxygen species on Co-Ni/Ce-S15 samples have higher mobility and activity due to the redox property of Ce [12]. Based on the TPR results, 500 \({}^{\circ}\)C was chosen as the reduction temperature at which all the free NiO was reduced. However, Co could not be completely reduced at this temperature, and higher temperature levels were avoided to reduce the sintering effect. Figures 4 and 5 illustrate the CH\({}_{4}\) and CO\({}_{2}\) conversions versus time-on-stream respectively, calculated from the onset of the reaction (CH\({}_{4}\)/CO\({}_{2}\)/N\({}_{2}\) mixture flowing). The reported conversion of both reactants over different catalysts varied significantly depending on the catalyst. This indicates that the promoter/dopants exerted a remarkable influence on the catalyst activity. As indicated by the TPD and TPR results for NH\({}_{3}\), the variation of the degree of reduction and acidity of the catalyst surface  
Figure 3: Temperature-programmed reduction using a mixture of hydrogen and argon. may affect the catalyst selectivity. The catalytic activity based on the conversion of both reactants was observed according to the following order (from high to low): Ni/S15 > Co-Ni/S15 > Co-Ni/Ce-S15 > Co-Ni/Zn-S15 > Co/S15. Figure 4: CH\({}_{4}\) conversion against time-on-stream (min). Figure 5: CO\({}_{2}\) conversion versus time-on-stream (min). Figures 4 and 5 show that Ni/S15 exhibited the maximum activity among the tested catalysts, which can be assigned to the high amount of nickel supported on S15, leading to the accessibility of more nickel active sites for the reaction compared to that of the other catalysts. In addition, Ni/S15 exhibited complete reduction and fewer acidic sites. CH\({}_{4}\) and CO\({}_{2}\) conversions were 90%, 78%, and 72% over Ni/S15, Co-Ni/S15, and Co-Ni/Ce-S15, respectively. For Co-Ni/Zn-S15, the CH\({}_{4}\) and CO\({}_{2}\) conversion was 20% and 37%, respectively. For Co/S15, the CH\({}_{4}\) and CO\({}_{2}\) conversion was 10% and 13%, respectively. The conversion of CO\({}_{2}\) was slightly greater than that of CH\({}_{4}\) due to the involvement of a reversed water-gas shift (RWGS) reaction. The conversion achieved with Co-Ni/S15 and Co-Ni/Ce-S15 was slightly less than that obtained with Ni/S15. The low conversion observed with Co-Ni/S15 and Co-Ni/Ce-S15 was attributed to the lower amount of nickel.