*3.3. Temperature-Programmed Reduction (H2-TPR)*

interaction with the support (forming spinel) [26].

reduction of Co2+ species with strong support interaction.

*3.3. Temperature-Programmed Reduction (H2-TPR)*  In order to evaluate the reducibility of the species present in the catalyst, temperatureprogrammed reduction with hydrogen was employed. As shown in Figure 3a,b, Ni and/or Co In order to evaluate the reducibility of the species present in the catalyst, temperature-programmed reduction with hydrogen was employed. As shown in Figure 3a,b, Ni and/or Co catalysts calcined at

Ni–Co-800 123 0.47 142

550 °C (Figure 3a), a broad and pronounced reduction peak is observed for Ni-550 at 450–700 °C with a peak centered at 590 °C. This indicates that the Ni2+ species are difficult to reduce due to their

It is well known that the TPR profiles of cobalt catalysts demonstrate two distinct metal oxide species being reduced at specific temperature. First, region (<400 °C) is assigned to the reduction of Co3O4 to CoO. The second region (400–500 °C) corresponds to the reduction of CoO to metallic Co0 [27]. Therefore, the H2 reduction peak with a maximum around 870 °C can be attributed to the

catalysts calcined at 550 °C and 800 °C undergo a single-step reduction. NiO and CoO/Co3O4 are

550 ◦C and 800 ◦C undergo a single-step reduction. NiO and CoO/Co3O<sup>4</sup> are reducible species and can be categorized on the basis of their reduction temperature. As found in literature, bulk NiO is reduced between 300 and 400 ◦C [24,25]. In the case of the catalysts calcined at 550 ◦C (Figure 3a), a broad and pronounced reduction peak is observed for Ni-550 at 450–700 ◦C with a peak centered at 590 ◦C. This indicates that the Ni2+ species are difficult to reduce due to their interaction with the support (forming spinel) [26].

It is well known that the TPR profiles of cobalt catalysts demonstrate two distinct metal oxide species being reduced at specific temperature. First, region (<400 ◦C) is assigned to the reduction of Co3O<sup>4</sup> to CoO. The second region (400–500 ◦C) corresponds to the reduction of CoO to metallic Co<sup>0</sup> [27]. Therefore, the H<sup>2</sup> reduction peak with a maximum around 870 ◦C can be attributed to the reduction of Co2+ species with strong support interaction. *Processes* **2019**, *7*, 141 7 of 15

With regard to the bimetallic Ni–Co-550 catalyst, the combination of both Ni and Co enhances the reducibility of Co. The broadening of the reduction peak in the high-temperature zone may be attributed to the reduction of Ni–Co2O<sup>4</sup> species forming Co–Ni alloy, having strong interaction with Al2O<sup>3</sup> + ZrO<sup>2</sup> as proposed by our previous study [28]. It is noteworthy that the TPR peak area of Ni–Co-550 is higher than the monometallic one, suggesting that it possesses more reducible species. On the other hand, H2-TPR conducted for the catalysts calcined at 800 ◦C (Figure 3b) followed the same trend. However, the position and the intensity of the peaks were different. In the case of Co-800, the single reduction peak may be designated to the overlapping of two-stage reduction of Co3O<sup>4</sup> → CoO → Co metal [29,30]. The shift of reduction peaks suggests the existence of strong interaction between Co2+ and support due to calcination. The same shift was also observed for both Ni and bimetallic Ni–Co. Moreover, Co-800 was found to have the highest intensity and peak shift as it was found for the samples calcined at 550 ◦C. With regard to the bimetallic Ni–Co-550 catalyst, the combination of both Ni and Co enhances the reducibility of Co. The broadening of the reduction peak in the high-temperature zone may be attributed to the reduction of Ni–Co2O4 species forming Co–Ni alloy, having strong interaction with Al2O3 + ZrO2 as proposed by our previous study [28]. It is noteworthy that the TPR peak area of Ni– Co-550 is higher than the monometallic one, suggesting that it possesses more reducible species. On the other hand, H2-TPR conducted for the catalysts calcined at 800 °C (Figure 3b) followed the same trend. However, the position and the intensity of the peaks were different. In the case of Co-800, the single reduction peak may be designated to the overlapping of two-stage reduction of Co3O4 → CoO → Co metal [29,30]. The shift of reduction peaks suggests the existence of strong interaction between Co2+ and support due to calcination. The same shift was also observed for both Ni and bimetallic Ni– Co. Moreover, Co-800 was found to have the highest intensity and peak shift as it was found for the samples calcined at 550 °C.

In our system ZrO<sup>2</sup> obviously doesn't interact with Al2O<sup>3</sup> strongly and the interaction between ZrO<sup>2</sup> and Ni is weak as was found by J. Asencios et al. [31] so Ni and/or Co is able to interact with Al2O<sup>3</sup> to form Ni and/or CoAl2O<sup>4</sup> and by this the Ni and/or Co are highly dispersed and as they are small crystals it is not possible to observe by XRD. The extent of this transformation increased with calcination temperature and is evidenced by the shift in reduction temperature for samples calcined at different temperatures. Also, G. P. Berrocal et al. [10] found that Ni strongly interacts with aluminum forming small NiAl2O<sup>4</sup> particles that have the highest reduction temperature. At the same time, this sample showed the highest catalytic activity for the partial oxidation of methane. We observed similar dependency in our results. In our system ZrO2 obviously doesn't interact with Al2O3 strongly and the interaction between ZrO2 and Ni is weak as was found by J. Asencios et al. [31] so Ni and/or Co is able to interact with Al2O3 to form Ni and/or CoAl2O4 and by this the Ni and/or Co are highly dispersed and as they are small crystals it is not possible to observe by XRD. The extent of this transformation increased with calcination temperature and is evidenced by the shift in reduction temperature for samples calcined at different temperatures. Also, G. P. Berrocal et al., [10] found that Ni strongly interacts with aluminum forming small NiAl2O4 particles that have the highest reduction temperature. At the same time, this sample showed the highest catalytic activity for the partial oxidation of methane. We observed similar dependency in our results.

**Figure 3.** TPR profiles for fresh monometallic Ni or Co and bimetallic Ni–Co-based catalysts (**a**) calcined at 550 °C and (**b**) calcined at 800 °C. **Figure 3.** TPR profiles for fresh monometallic Ni or Co and bimetallic Ni–Co-based catalysts (**a**) calcined at 550 ◦C and (**b**) calcined at 800 ◦C.

#### *3.4. Thermal Analysis for Carbon Deposition 3.4. Thermal Analysis for Carbon Deposition*

TGA analysis was conducted to quantify the deposited carbon over the spent catalysts. In Figure 4a, 4b, the TGA profiles illustrate the weight loss (%) as a function of temperature for all recovered TGA analysis was conducted to quantify the deposited carbon over the spent catalysts. In Figure 4a,b, the TGA profiles illustrate the weight loss (%) as a function of temperature for all

catalysts from tests at 700 and 800 °C, respectively. In general, the amount of deposited carbon was relatively low for all the tested catalysts due to the presence of zirconia which is well known for high

800 < Ni–Co-550 (Figure 4b). For all catalysts, the burning of carbon starts at the same temperatures around 500 °C except for Ni–Co-550. From Figure 4 it is clear that Co-550 was the least prone to carbon deposition at both reaction temperatures 700 °C and 800 °C because cobalt is recognized as a strong oxidizing catalyst which can tackle the soot formation [32]. Interestingly, all catalysts calcined at 800 °C were found to have lower and similar amount of carbon deposits after reaction at 800 °C (encircled in Figure 4b), which can be associated to the strong interaction of metal species with composite support as it has been discussed in Section 3.3 [14]. Consequently, it can be deduced that higher calcination and reaction temperatures pose no adverse effect to our catalysts because they were less susceptible to carbon deposition. We assume that increasing the calcination temperature

recovered catalysts from tests at 700 and 800 ◦C, respectively. In general, the amount of deposited carbon was relatively low for all the tested catalysts due to the presence of zirconia which is well known for high oxygen storage capacity and the presence of basic centers. The relative carbon deposition after reaction at 700 ◦C can be assigned in the following order: Ni-800 ≈ Co-550 < Co-800 < Ni-550 < Ni–Co-800 < Ni–Co-550 (Figure 4b). For all catalysts, the burning of carbon starts at the same temperatures around 500 ◦C except for Ni–Co-550. From Figure 4 it is clear that Co-550 was the least prone to carbon deposition at both reaction temperatures 700 ◦C and 800 ◦C because cobalt is recognized as a strong oxidizing catalyst which can tackle the soot formation [32]. Interestingly, all catalysts calcined at 800 ◦C were found to have lower and similar amount of carbon deposits after reaction at 800 ◦C (encircled in Figure 4b), which can be associated to the strong interaction of metal species with composite support as it has been discussed in Section 3.3 [14]. Consequently, it can be deduced that higher calcination and reaction temperatures pose no adverse effect to our catalysts because they were less susceptible to carbon deposition. We assume that increasing the calcination temperature from 550 to 800 ◦C may form new surface sites due to the strong metal-support interaction. This might stabilize the high Ni and/or Co dispersion against metal agglomeration and deactivation. Apart from this, ZrO<sup>2</sup> might activate the oxidation of coke at high temperature and prevent the catalysts from coking. Also, Co-800 catalyst operated at 800 ◦C had excellent stability for 24 h on stream without deactivation (as it will be discussed latter). Henceforth, monometallic catalysts presented better performance with higher calcination temperature than bimetallic ones. In addition, the rate of coking over Ni–Co-800 was higher in comparison with mono-metallic catalysts which is consistent with the findings reported in the literature [33,34]. Moreover, this effect was more pronounced for the catalysts calcined at 550 ◦C (Figure 4a). *Processes* **2019**, *7*, 141 8 of 15 from 550 to 800 °C may form new surface sites due to the strong metal-support interaction. This might stabilize the high Ni and/or Co dispersion against metal agglomeration and deactivation. Apart from this, ZrO2 might activate the oxidation of coke at high temperature and prevent the catalysts from coking. Also, Co-800 catalyst operated at 800 °C had excellent stability for 24 h on stream without deactivation (as it will be discussed latter). Henceforth, monometallic catalysts presented better performance with higher calcination temperature than bimetallic ones. In addition, the rate of coking over Ni–Co-800 was higher in comparison with mono-metallic catalysts which is consistent with the findings reported in the literature [33,34]. Moreover, this effect was more pronounced for the catalysts calcined at 550 °C (Figure 4a).

**Figure 4.** TGA profiles for spent Ni and/or Co-based catalysts calcined at 550 °C and at 800 °C after tests at (**a**) 700 °C and (**b**) 800 °C. **Figure 4.** TGA profiles for spent Ni and/or Co-based catalysts calcined at 550 ◦C and at 800 ◦C after tests at (**a**) 700 ◦C and (**b**) 800 ◦C.

#### *3.5. Temperature-Programmed Desorption of CO2 (CO2-TPD) 3.5. Temperature-Programmed Desorption of CO<sup>2</sup> (CO2-TPD)*

and coke removal.

(**b**) 800 °C.

The basicity of the Ni and/or Co-containing catalysts was evaluated by adsorption and desorption of CO2 on the basic sites at different temperatures. Figure 5 represents the CO2-TPD profiles of the catalysts. The strength of basic sites can be classified by the temperature of the corresponding desorption peak of CO2: weakly basic in the range of 50–200 °C, intermediate basic (200–400 °C), strongly basic (400–650 °C) and very strong basic sites (>650 °C) [35]. In fact, all these basic sites are evident from CO2-TPD profiles, which reveal the strong basic character of the catalysts (Figure 5). Al2O3 as an acidic support favors coke formation. Therefore, ZrO2 addition has rendered the catalysts basic character, which in turn escalated CO2 adsorption contributing to higher activity The basicity of the Ni and/or Co-containing catalysts was evaluated by adsorption and desorption of CO<sup>2</sup> on the basic sites at different temperatures. Figure 5 represents the CO2-TPD profiles of the catalysts. The strength of basic sites can be classified by the temperature of the corresponding desorption peak of CO2: weakly basic in the range of 50–200 ◦C, intermediate basic (200–400 ◦C), strongly basic (400–650 ◦C) and very strong basic sites (>650 ◦C) [35]. In fact, all these basic sites are evident from CO2-TPD profiles, which reveal the strong basic character of the catalysts (Figure 5). Al2O<sup>3</sup> as an acidic support favors coke formation. Therefore, ZrO<sup>2</sup> addition has rendered the catalysts basic character, which in turn escalated CO<sup>2</sup> adsorption contributing to higher activity and coke removal.

**Figure 5.** CO2-TPD profiles for fresh Ni and/or Co-based catalysts after calcination at (**a**) 550 °C and

calcined at 550 °C (Figure 4a).

tests at (**a**) 700 °C and (**b**) 800 °C.

*3.5. Temperature-Programmed Desorption of CO2 (CO2-TPD)* 

**Figure 4.** TGA profiles for spent Ni and/or Co-based catalysts calcined at 550 °C and at 800 °C after

The basicity of the Ni and/or Co-containing catalysts was evaluated by adsorption and desorption of CO2 on the basic sites at different temperatures. Figure 5 represents the CO2-TPD profiles of the catalysts. The strength of basic sites can be classified by the temperature of the corresponding desorption peak of CO2: weakly basic in the range of 50–200 °C, intermediate basic (200–400 °C), strongly basic (400–650 °C) and very strong basic sites (>650 °C) [35]. In fact, all these basic sites are evident from CO2-TPD profiles, which reveal the strong basic character of the catalysts (Figure 5). Al2O3 as an acidic support favors coke formation. Therefore, ZrO2 addition has rendered

from 550 to 800 °C may form new surface sites due to the strong metal-support interaction. This might stabilize the high Ni and/or Co dispersion against metal agglomeration and deactivation. Apart from this, ZrO2 might activate the oxidation of coke at high temperature and prevent the catalysts from coking. Also, Co-800 catalyst operated at 800 °C had excellent stability for 24 h on stream without deactivation (as it will be discussed latter). Henceforth, monometallic catalysts presented better performance with higher calcination temperature than bimetallic ones. In addition, the rate of coking over Ni–Co-800 was higher in comparison with mono-metallic catalysts which is consistent with the findings reported in the literature [33,34]. Moreover, this effect was more pronounced for the catalysts

**Figure 5.** CO2-TPD profiles for fresh Ni and/or Co-based catalysts after calcination at (**a**) 550 °C and (**b**) 800 °C. **Figure 5.** CO<sup>2</sup> -TPD profiles for fresh Ni and/or Co-based catalysts after calcination at (**a**) 550 ◦C and (**b**) 800 ◦C. *Processes* **2019**, *7*, 141 9 of 15

#### *3.6. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) 3.6. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM)*

Figure 6 displays SEM images of fresh and spent catalysts obtained after five hours on stream at 700 ◦C and corresponding samples calcined at 550 ◦C. The fresh catalyst surface shows a fairly good distribution of the particles while the spent catalyst shows agglomeration of the particles and therefore the surface area and Ni dispersion decrease. The catalytic activity is strongly affected by carbon deposition over the catalysts' surfaces, finally deactivating the catalyst. Figure 6 displays SEM images of fresh and spent catalysts obtained after five hours on stream at 700 °C and corresponding samples calcined at 550 °C. The fresh catalyst surface shows a fairly good distribution of the particles while the spent catalyst shows agglomeration of the particles and therefore the surface area and Ni dispersion decrease. The catalytic activity is strongly affected by carbon deposition over the catalysts' surfaces, finally deactivating the catalyst.

TEM of Co/(Al2O3–ZrO2) catalyst calcined at 800 ◦C used in the long term POM test at 800 ◦C reveals presence of filamentous coke and the size of carbon nanotubes (CNTs) is determined by the size of starting metallic species. These CNTs gradually grow and metallic Co species settled on the tip of the CNTs. As the metallic species are still exposed to the reacting gases, these CNTs do not show an adverse effect on activity because metallic species are still accessible [28]. TEM of Co/(Al2O3–ZrO2) catalyst calcined at 800 °C used in the long term POM test at 800 °C reveals presence of filamentous coke and the size of carbon nanotubes (CNTs) is determined by the size of starting metallic species. These CNTs gradually grow and metallic Co species settled on the tip of the CNTs. As the metallic species are still exposed to the reacting gases, these CNTs do not show an adverse effect on activity because metallic species are still accessible [28].

**Figure 6.** SEM images of Co–Ni/(Al2O3–ZrO2) catalyst. (**A**) fresh catalyst, (**B**) spent catalyst calcined at 800 °C and operated at 700 °C reaction for 5 h and (**C**) TEM of Co/(Al2O3–ZrO2) catalyst calcined at 800 °C after recovery from long term POM test (24 h) at 800 °C. **Figure 6.** SEM images of Co–Ni/(Al2O3–ZrO<sup>2</sup> ) catalyst. (**A**) fresh catalyst, (**B**) spent catalyst calcined at 800 ◦C and operated at 700 ◦C reaction for 5 h and (**C**) TEM of Co/(Al2O3–ZrO<sup>2</sup> ) catalyst calcined at 800 ◦C after recovery from long term POM test (24 h) at 800 ◦C.

#### *3.7. Catalytic Activity 3.7. Catalytic Activity*

contributes to the rise in selectivity to H2.

The product H2/CO ratio for all catalysts is slightly higher than the stoichiometric value of 2 (Figure 7), owing to the incomplete conversion of CO2 (from combustion, Equation (1)) to CO. Also, a part of CO was consumed by the side reactions such as water-gas shift (Equation (5)) and Boudouard reactions (Equation (6)). Consequently, both of these effects lower the CO selectivity (Figure 8b) with time on stream and thus increase the H2/CO ratio at 700 °C. Moreover, Co-800 gave the lowest CO2 selectivity (15.2%) and the highest CO selectivity (85%), eventually attained H2/CO ratio approaching the stoichiometric value of 2. It is worth to mention that the selectivity for hydrogen reached 98.6% for all catalysts at 700 °C. The product H2/CO ratio for all catalysts is slightly higher than the stoichiometric value of 2 (Figure 7), owing to the incomplete conversion of CO2 (from combustion, Equation (1)) to CO. Also, a part of CO was consumed by the side reactions such as water-gas shift (Equation (5)) and Boudouard reactions (Equation (6)). Consequently, both of these effects lower the CO selectivity (Figure 8b) with time on stream and thus increase the H2/CO ratio at 700 ◦C. Moreover, Co-800 gave the lowest CO2 selectivity (15.2%) and the highest CO selectivity (85%), eventually attained H2/CO ratio approaching the stoichiometric value of 2. It is worth to mention that the selectivity for hydrogen reached 98.6% for all catalysts at 700 ◦C.

formed on this catalyst as shown in Figure 4. On the other hand, the activity of the bimetallic catalyst suffered from the formation of carbon deposits (Figure 4a, 4b). The lower activity of Ni–Co-800 catalyst may also be due to the formation of spinel phase as it was discussed with the help of TPR (Figure 1). These species are irreducible and do not contribute to methane conversion [36]. Since Al3+ and Ni2+ are located in the same lattice, the generation of the solid solution of NiAl2O4 spinel is conducive under higher calcination temperatures what about ZrO2 [11]. Moreover, the highest selectivity for H2 of 99% is achieved with all the catalysts when operated at 700 °C. As steam reforming (Equation (3)) is thermodynamically feasible at this temperature and water is available, it

The performance of the Ni/ZrO2–Al2O3, Co/ZrO2–Al2O3 and Co–Ni/ZrO2–Al2O3 catalysts calcined at 550 °C and 800 °C was tested at 700 °C and 800 °C (Figure 8). Generally, the activity of catalysts progressively increases with rise in the reaction temperature. The oxygen conversion was unaltered

*Processes* **2019**, *7*, 141 10 of 15

**Figure 7.** Conversion, selectivity and H2/CO ratio obtained for Ni and/or Co-based catalysts operated at (**a**) 700 °C and (**b**) 800 °C. **Figure 7.** Conversion, selectivity and H2/CO ratio obtained for Ni and/or Co-based catalysts operated at (**a**) 700 ◦C and (**b**) 800 ◦C. **Figure 7.** Conversion, selectivity and H2/CO ratio obtained for Ni and/or Co-based catalysts operated at (**a**) 700 °C and (**b**) 800 °C.

**Figure 8.** (**a**) CH4 conversion and (**b**) CO selectivity with time on stream in POM over Ni and/or Cobased catalysts at 700 °C; (**c**) CH4 conversion and (**d**) CO selectivity with time on stream in POM over Ni and/or Co-based catalysts at 800 °C. **Figure 8.** (**a**) CH4 conversion and (**b**) CO selectivity with time on stream in POM over Ni and/or Cobased catalysts at 700 °C; (**c**) CH4 conversion and (**d**) CO selectivity with time on stream in POM over Ni and/or Co-based catalysts at 800 °C. **Figure 8.** (**a**) CH<sup>4</sup> conversion and (**b**) CO selectivity with time on stream in POM over Ni and/or Co-based catalysts at 700 ◦C; (**c**) CH<sup>4</sup> conversion and (**d**) CO selectivity with time on stream in POM over Ni and/or Co-based catalysts at 800 ◦C.

The performance of the Ni/ZrO2–Al2O3, Co/ZrO2–Al2O<sup>3</sup> and Co–Ni/ZrO2–Al2O<sup>3</sup> catalysts calcined at 550 ◦C and 800 ◦C was tested at 700 ◦C and 800 ◦C (Figure 8). Generally, the activity of catalysts progressively increases with rise in the reaction temperature. The oxygen conversion was unaltered (nearly 98%) for all catalysts irrespective of calcination temperatures. At 700 ◦C (Figure 8a) maximum conversion of 71.5% was achieved with the Ni-550 catalyst. This might be attributed to its high surface area compared to the other catalysts (Figure 2). It may also be due to minimum carbon deposition formed on this catalyst as shown in Figure 4. On the other hand, the activity of the bimetallic catalyst suffered from the formation of carbon deposits (Figure 4a,b). The lower activity of Ni–Co-800 catalyst may also be due to the formation of spinel phase as it was discussed with the help of TPR (Figure 1). These species are irreducible and do not contribute to methane conversion [36]. Since Al3+ and Ni2+ are located in the same lattice, the generation of the solid solution of NiAl2O<sup>4</sup> spinel is conducive under higher calcination temperatures what about ZrO<sup>2</sup> [11]. Moreover, the highest selectivity for H<sup>2</sup> of 99% is achieved with all the catalysts when operated at 700 ◦C. As steam reforming (Equation (3)) is thermodynamically feasible at this temperature and water is available, it contributes to the rise in selectivity to H2.

Generally, when the partial oxidation was carried out at 800 ◦C, both CH<sup>4</sup> conversion and CO selectivity were remarkably increased (Figure 7b). Moreover, methane conversion for all catalysts was found to be in the order: Co-800 > Ni–Co-550 > Ni-800 > Ni–Co-800 ≈ Ni-550 = Co-550. The selectivities to CO and H<sup>2</sup> achieved with all the catalysts exceeded 99% at 800 ◦C. In addition, the amount of CO<sup>2</sup> was minimum (<1%) in the product stream which implies that CO<sup>2</sup> has been converted into CO. All the catalysts maintained their activity throughout the test duration, which can be associated to the higher calcination temperature and the presence of ZrO2. An intimate contact with metal species is developed by the presence of ZrO<sup>2</sup> due to strong electrostatic attraction between them. This fact is also evident from the TPR profiles. Furthermore, the strong metal-support interaction in these catalysts is responsible for the low carbon deposition.

Interestingly, when comparing to other catalysts, it was found that the monometallic Co-800 was the most active (84% CH<sup>4</sup> conversion) and stable catalyst (Figure 8c). In the presence of ZrO2, the interaction between Al2O<sup>3</sup> and Ni and/or Co increases. Ni and/or Co deposit on the support and develop an intimate contact which results in the modification of Al2O<sup>3</sup> support [6]. The TPR of the monometallic sample Co-800 showed the highest reduction temperature due to the formation of stable spinel structures with the support. These interactions probably assist in dispersing the metals and coke formation resistance. The slight decline in the methane conversion of Ni-800 may be ascribed to blocking of active sites by carbon deposits (Figure 4) and relatively lower basicity (Figure 5). Similarly, at 800 ◦C the selectivity to CO remained constant throughout the stability test for all tested catalysts (Figure 8d). Consequently, the rise in the CO selectivity at 800 ◦C shifted the H2/CO ratio to a value closer to 2. It is worth to mention that the reaction temperature of 800 ◦C is most favorable for reduction of the tested metal oxides as can be seen in TPR profiles (Figure 3). A similar study was conducted using the same catalyst Ni/(ZrO<sup>2</sup> + Al2O3) but employing a higher metal loading (8%) and a calcination temperature of 550 ◦C. The catalyst achieved almost comparable methane conversions, but higher amount of carbon deposits and significantly lower selectivity to CO and H<sup>2</sup> [10]. The comparison of this result with the present study suggests that calcination temperature has a significant influence on the catalytic performance.

The higher activity of bimetallic catalyst Ni–Co-550 can be attributed to the synergistic effect between Ni and Co which is in agreement with several findings [14]. This effect induces higher BET surface area, smaller crystallite size (XRD) and improved degree of reducibility (TPR). Co- and Ni–Co-based catalysts presented higher catalytic activity than Ni-based catalysts. This finding is consistent with recent studies conducted by Zagaynov and co-workers using (Ni, Co and Co–Ni)/–Gd0.1Ti0.1Zr0.1Ce0.7O<sup>2</sup> catalysts [14]. However, the decline in activity of Ni–Co-800 calcined at 800 ◦C may be ascribed to the formation of spinel phases as described above. On the basis of catalytic activity, Co-800 is the most promising catalyst giving higher conversion and excellent selectivity to CO (85%) as well as H<sup>2</sup> (98.6%) even at 700 ◦C, and this selectivity can reach 100% at 800 ◦C reaction temperature. Therefore, it is evident that the monometallic catalysts gave better performance with higher calcination temperature while bimetallic catalysts exhibit higher activity with lower calcination temperature.
