*2.5. H2-TPR*

The reduction behavior of the different catalyst samples was investigated using H2-TPR and the profiles are presented in Figure 3. The nickel reduction peaks for Ti-CAT-x (x = I, II, III) samples containing Ni combined with other metals, are characterized by three reduction regions at low, medium and high temperature ranges. Their ranges are dependent on the degree of dispersion and interaction of the active metal with the support. The nickel phase reducibility was influenced by the combination of the metal oxides. The reduction peak in the temperature range of 280–380 ◦C is assigned to the reduction of NiO having weak interaction with the support. Higher temperature peaks (600–700 ◦C) are likely due to the reduction of NiO species having strong interactions with the support. The reduction peak of Ni2+ derived from spinel is found at around 810 ◦C [35].

For Ti-CAT-V (the catalyst with only Ni), the NiO reduction peaks appeared narrower and more intense in temperature ranges lower than those of combined metal counterparts.

Only two reduction peaks are observed for Ti-CAT-VI at temperature ranges centered at 260 and 325 ◦C. Similar reduction peaks are expected for CeO2 promoted samples, but appear to have merged with the peaks for NiO that appeared around that temperature range.

**Figure 3.** TPR profiles of the promoted and un-promoted catalysts.

#### *2.6. Effect of MgO and CeO2 Combination on the Catalytic Performance*

The effects of combining CeO2 and MgO on Ti-CAT-V and their catalytic performance were studied by comparing the activities of Ti-CAT-V catalyst with that of Ti-CAT-I, Ti-CAT-II, and Ti-CAT-III. CH4, CO2 conversions, H2/CO mole ratio, and their selectivity at 700 ◦C, for 7.0 h time-on-stream for DRM were calculated and plotted as shown in Figure 4. All the promoted catalysts have CH4 and CO2 conversions higher than that of the Ti-CAT-V catalyst except for Ti-CAT-VI and Ti-CAT-IV, which showed no sign of reaction during the DRM. Ti-CAT-II had the highest CH4 conversion at the start of the reaction (~55%) and maintained stability at around 52%. The high specific surface area of the catalyst (283 m2/g) enhanced the adsorption, diffusion, and contact of the reactant gases. The high average pore diameter and pore volume of Ti-CAT-II is a likely factor for the best-in-class performance. The Ce and Mg promoted catalysts enhanced the activity. The improvement of the activity is accompanied by the formation of graphitic carbon in comparison with the unpromoted catalysts, as depicted in the TG analysis.

**Figure 4.** Catalytic performance of Ti-CAT-I, Ti-CAT-II, Ti-CAT-III, and Ti-CAT-V (**a**) CH4 conversion (**b**) CO2 conversion and (**c**) H2/CO ratio and (**d**) H2, CO selectivity.

The same trend was observed for CO2 conversion, with the Ti-Cat-V catalyst showing the least conversion. For all the catalysts under investigation, CO2 conversion was observed to be higher than CH4 conversion, which is suggestive of the occurrence of reverse water gas shift (RWGS) reaction. Wang et al. gave the same observation in their study on catalytic hydrogenation of carbon dioxide [36]. *Catalysts* **2019**, *9*, 188

$$\text{H}\_2 + \text{CO}\_2 \rightarrow \text{CO} + \text{H}\_2\text{O} \qquad \qquad \qquad \Delta \text{H}\_{298} = +41.2 \text{ kj/mol}$$

In addition, the H2/CO mole ratio showed values less than 1 for all the catalysts. The deviation from the stoichiometric ratio is also suggestive of the occurrence of RWGS reaction. Ti-CAT-II appeared to be the most selective towards H2 (~48%) and least selective towards CO (~52%), while Ti-CAT-V has the least H2 selectivity (~45%) but the highest CO selectivity (~55%). In all cases, the as-prepared catalysts showed higher CO selectivity than H2.

Ti-CAT-II catalyst resulted in a H2/CO mole ratio value closest to 1, compared to the tested catalysts. The desirable value of the syngas ratio suitable for downstream Fischer-Tropsch synthesis is unity [37], thus making it the best option for the dry reforming.
