**1. Introduction**

Methane (CH4) is an important constituent of natural and biogas and plays an important role in C<sup>1</sup> chemistry. Its utilization is expected to increase in the future because of the weaker greenhouse gas effect (CO<sup>2</sup> release) compared to other fossil resources. However, it is well known that the direct conversion of methane yields less valuable petrochemical products and hence it is necessary to resort to an indirect process that initially involves the generation of synthesis gas (H<sup>2</sup> and CO) [1–4]. Synthesis gas is widely used in the production of hydrogen, synthetic fuels, alcohols and other chemicals. It can be produced by partial oxidation of hydrocarbons, particularly methane, via (i) steam reforming or (ii) dry reforming (DRM) or (iii) autothermal reforming. Specifically, the catalytic partial oxidation of methane has been recognized as a beneficial process from both technical and economic perspective; as it requires

less energy and capital cost due to low endothermic nature of the process [5]. In addition, the H2/CO ratio of 2 is suitable for methanol synthesis and higher hydrocarbons through the Fischer-Tropsch process [6].

Various reaction mechanisms have been suggested for the partial oxidation of methane. The first is a direct route (Equation (1)) while the second mechanism comprises combustion and two reforming reactions. In the latter pathway, combustion of methane is accomplished (Equation (2)). Subsequently, steam and dry reforming of methane take place in the presence of the newly produced CO<sup>2</sup> and H2O, respectively (Equations (3) and (4)) to render syngas.

$$\text{CH}\_4 + 0.5\text{O}\_2 \rightarrow \text{CO} + 2\text{H}\_2 \qquad \Delta H\_{298k}^\circ = -35.7 \text{ kJ/mol} \tag{1}$$

$$\text{CH}\_4 + 2\text{O}\_2 \rightarrow \text{CO}\_2 + 2\text{H}\_2\text{O} \qquad \Delta H\_{298\text{k}}^\circ = -802.3 \text{ kJ/mol} \tag{2}$$

$$\text{CH}\_4 + \text{H}\_2\text{O} \leftrightharpoons \text{CO} + 3\text{H}\_2 \qquad \Delta H\_{298\text{k}}^\circ = +226 \text{ kJ/mol} \tag{3}$$

$$\text{CH}\_4 + \text{CO}\_2 \leftrightharpoons 2\text{CO} + 2\text{H}\_2 \qquad \Delta H\_{298k}^\circ = +261 \text{ kJ/mol} \tag{4}$$

Moreover, some side reactions, such as water gas shift reaction (Equation (5)) and Boudouard reaction (Equation (6)) can also occur along with main reactions.

$$\text{CO} + \text{H}\_2\text{O} \rightleftharpoons \text{CO}\_2 + \text{H}\_2 \qquad \Delta\text{H}\_{298k}^\circ = -41.2 \text{ kJ/mol} \tag{5}$$

$$\mathbf{2CO} \leftrightharpoons \mathbf{C} + \mathbf{CO}\_2 \qquad \Delta H\_{298k}^{\circ} = -\mathbf{172.8 kJ/mol} \tag{6}$$

The water gas shift and Boudouard reactions are exothermic in nature and take place at lower temperature. However, the respective reverse reactions occur upon increasing the reaction temperature.

Among the efficient catalysts for partial oxidation of methane (POM) are transition metals such as Ni, Pt, and Co supported on alumina, zirconia etc. However, these catalysts deactivate as a result of carbon formation [7,8]. It has been established that the activity of the Ni and/or Co catalysts not only relies on the structure and the nature of the active metals but selection of the support also plays a significant role. Al2O<sup>3</sup> is extensively utilized as a support for reforming reactions. However, when Al2O<sup>3</sup> is employed alone as a support for such type of catalysts, problems arise such as carbon deposition on active sites and development of inactive spinel phase (NiAl2O4) [9]. The modification of support, therefore, can be a promising route to enhance the catalytic performance. Among the prevalent materials, ZrO<sup>2</sup> has drawn considerable attention due to its excellent characteristics like acid-base properties, oxygen storage capacity and thermal stability [10]. It also inhibits the formation of spinels like NiAl2O<sup>4</sup> by impeding the incorporation of active species into Al2O<sup>3</sup> lattice [11,12]. Tetragonal zirconia is unstable at ambient temperature, but it can be stabilized by addition of Al2O<sup>3</sup> to ZrO2. Moreover, this binary system has a higher modulus of elasticity compared to neat ZrO<sup>2</sup> [11,13].

Several studies have been carried out on the formation of synthesis gas by using Ni and Co-based catalysts. Zagaynov et al. [14] examined Ni (Co)–Gd0.1Ti0.1Zr0.1Ce0.7O2 mesoporous catalysts obtained by co-precipitation for partial oxidation and dry reforming of methane. Surprisingly, the results showed that Co and Ni–Co-containing catalysts were more active in partial oxidation of methane than the Ni sample, while Ni-catalysts were more active in dry reforming of methane. Calcination temperature, on the other hand, affects the active metal particle size and therefore alters the stability of the catalysts by changing the diffusion path. Moreover, the calcination temperature has a significant impact on the structural and catalytic properties of the catalysts, which interact strongly with the metal oxide support. Other researchers [15,16] also highlighted the effect of pretreatment of catalysts at calcination temperature. On the other hand, other studies have demonstrated comparable performance at high temperatures or by using precious metals. For instance, Dedov and co-workers utilized neodymium-calcium cobaltate-based catalysts for syngas production via partial oxidation of methane [17]. They reported to attain 85% methane conversion and selectivity of CO and H<sup>2</sup> close to 100% at very a high temperature (925 ◦C). Likewise, another study used Ni(Co)-Gd0.1Ti0.1Zr0.1Ce0.7O2 catalyst and

obtained comparable H<sup>2</sup> selectivity at a higher temperature (900 ◦C) for the production of syngas via partial oxidation of methane [14]. The present work is driven by our previous work [18] where it is has been shown that by using a single catalysis system of cobalt over CeO<sup>2</sup> and ZrO<sup>2</sup> supports; the hydrogen yield only up to 60% and 75 respectively was achieved for this system. Moreover, CeO<sup>2</sup> support yield low hydrogen and cobalt alone is considered less reforming catalysis. Therefore, in this work, the effect of binary metal system and support has been studied. It was observed that this system performs much better than single catalyst where hydrogen production was achieved up to 100%. Several studies have employed Co-based catalysts for reforming reactions [14,19,20]. For instance, Zagaynov et al. [14] examined (Ni, Co and Co-Ni)/-Gd0.1Ti0.1Zr0.1Ce0.7O2 mesoporous catalysts obtained by co-precipitation for partial oxidation and dry reforming of methane. Interestingly, the results showed that the Co- and Ni–Co- containing catalysts exhibited excellent catalytic performance in partial oxidation of methane than the Ni sample, while the Ni-catalysts demonstrated tremendous catalytic performance in dry reforming of methane.

Accordingly, the significance of this research contribution was to obtain a high catalytic performance at relatively low temperature using mono and bimetallic Co and Ni supported on (ZrO<sup>2</sup> + Al2O3) which are capable of producing syngas via partial oxidation of methane. In addition, they must be stable to overcome the deactivation processes like carbon accumulation, metal agglomeration and thermal sintering. The study of catalyst design started with a systematic investigation of the desired reaction together with potential side reactions. The sol-gel method of preparation was proposed to generate strong metal-support interaction (MSI) and to produce smaller metal particles, which is expected to be active in the catalytic reaction.
