*2.7. H2-Pulse Chemisorption*

To understand the effect of the active Ni component on the catalytic performance of the best two catalysts, Ti-CAT-I and Ti-CAT-II, we carried out H2 pulse chemisorption to determine the degree of Ni dispersion on the surface of the support and Ni metallic surface area. The results of H2-pulse chemisorption are displayed in Table 3. We found that both catalysts had high Ni metallic surface areas of ~90% and good dispersion of ~13%, which is responsible for the good catalytic performance of the two catalysts. The small relative higher catalytic performance of Ti-CAT-II than that of Ti-CAT-I could be attributed to the slightly higher Ni metallic surface area and dispersion of Ti-CAT-II.

**Table 3.** Ni metallic surface area and dispersion obtained by H2 chemisorption.


#### *2.8. Temperature Programmed Oxidation (TPO) of the Spent Catalysts*

TPO is a useful technique that can be employed to determine the nature of the carbon deposited onto the surface of the catalysts. Several forms of carbon deposition have been reported in dry reforming reactions—ranging from atomic carbon, to graphitic, and amorphous carbon. The carbon can undergo gasification to form CO2 under oxidative atmosphere and at different temperature ranges. The atomic carbon, amorphous, and graphitic carbon can be gasified at temperatures less than 250, 250–600, and >600 ◦C, respectively [38]. The TPO profiles of Ti-CAT-I and Ti-CAT-II spent catalysts are shown in Figure 5. Each of the catalysts exhibited a broad peak near 600 ◦C and a low-intensity shoulder at 100–250 ◦C. According to the TPO results, the carbon deposited on both Ti-CAT-I and Ti-CAT-II spent catalysts revealed the formation of carbon atoms, mostly amorphous carbon, and a small amount of extent graphitic carbon.

**Figure 5.** TPO profiles for both Ti-CAT-I and Ti-CAT-II.

#### *2.9. SEM and TG Analysis*

We used scanning electron microscopy (SEM) to determine the change in morphology of the spent catalysts. Figure 6 shows the SEM micrographs for the best two catalysts: Ti-CAT-I and Ti-CAT-II. Similar morphology, based on agglomerated, spherical nanoparticles, was observed for both fresh catalysts (Figure 6A,B). Such an observation was expected, because both catalysts were synthesized using an identical preparation procedure and had similar components.

**Figure 6.** SEM micrographs for fresh catalysts (**A**) Ti-CAT-I, (**B**) Ti-CAT-II, and spent catalysts (**C**) Ti-CAT-I, (**D**) Ti-CAT-II. White circles are for some areas where CNTs are present.

The morphology of the spent catalysts was similar to that one of the fresh samples, except for the presence of carbon nanotubes (CNTs) on the surface of the spent catalysts (Figure 6C,D). Detection of CNTs on the surface of the spent catalyst is in agreemen<sup>t</sup> with the TPO results (c.f. Figure 5) and confirms the results of TGA of spent catalysts (Figure 7). The presence of CNTs on the surface of the spent catalysts could be attributed to Boudouard reaction, which in turn would be responsible for reducing the catalytic performance.

**Figure 7.** TGA profiles for the spent catalysts.

After the 7 h reaction, we analyzed the used catalysts by thermal gravemetric analysis (TGA), a quantitative analysis that determines the amount of carbon deposition. Figure 7 shows the result of the analysis. Catalysts that showed no sign of reaction were not reported. The weight loss (%) for virtually all the catalysts began at around 620 ◦C. The TGA profiles revealed that both Ti-CAT-V and Ti-CAT-III catalysts had the lowest weight loss, ~15.0%, while the two most reactive catalysts (Ti-CAT-II and Ti-CAT-I) had the highest amount of carbon deposition, corresponding to a weight loss of 25.0%.

From these TGA results of spent catalysts, it can be inferred that the combined metal catalysts, namely Ti-CAT-II and Ti-CAT-I, enhance the feed conversion capacity of the catalysts and gasify the carbon deposited over the surface to a considerable extent.

#### *2.10. Effect of Space Velocity*

The effect of gas hourly space velocity (GHSV) was studied on the catalyst that showed the best performance in the previous section (i.e., Ti-CAT-II catalyst). GHSV of 19,500 and 78,000feed flow rate mass of cat. - mL g· h were considered at 700 ◦C and time-on-stream over 7.0 h for DRM, while keeping the mass of the catalyst constant. These GHSV values are half and twice as much as the initial GHSV of 39,000 mL g<sup>−</sup><sup>1</sup> <sup>h</sup>−1, respectively. The results, in terms of CO2 and CH4 conversions, as well as H2/CO mole ratio, were calculated and plotted in Figure 8A,B. As the GHSV increased, the CH4 and CO2 conversions decreased, with the highest conversions for both CH4 and CO2 being obtained at a GHSV of 19,500 feed flow rate mass of cat. - mL g· h . The decrease in conversions can be attributed to the feed having less residence time at higher GHSV [39]. A similar trend was observed with H2/CO mole ratio, where it decreased from a ratio of 1 to ~0.8. However, the results at GHSV of 39,000 were the most stable in comparison to those obtained at other GHSV values.

**Figure 8.** (**A**) CO2 conversion for Ti-CAT-II at different gas hourly space velocity (**B**) CH4 and H2/CO ratio for Ti-CAT-II catalyst at different space velocities.

#### *2.11. Effect of GHSV on Carbon Deposition*

Quantitative analysis of carbon deposition was performed on the catalyst Ti-CAT-II used in methane dry reforming at 3 different space velocities 19,500, 39,000 and 78,000 mL g<sup>−</sup><sup>1</sup> h−1.

The results obtained after the completion of the reactions are shown in Figure 9. The analysis for the reaction performed at 19,500 mL g<sup>−</sup><sup>1</sup> h−<sup>1</sup> showed the least amount of carbon deposition of about 18%, which shows that, relatively, more contact time between the catalyst and the feed stream was allowed at this space velocity, giving room for gasification of the coke that was deposited during the reaction. The reactions carried out at 39,000 and 78,000 mL g<sup>−</sup><sup>1</sup> h−<sup>1</sup> showed higher carbon deposition of about 26 and 25%, respectively. This is an indication that at the higher space velocities, the residence time was not enough for the gasification of the carbon deposit, which variably continued to pile up. Lalit et al. reported similar findings in their study of the effect of GHSV on the conversion of CH4 and CO2 [39].

**Figure 9.** TGA curves for Ti-Cat-II at 19,500, 39,000 and 78,000 mL/(g·h) GHSV values.

From the results of the investigation, it can be inferred that deactivation is expected at all the space velocity investigated, since carbon deposit was evident; however, at 19,500 mL/(g·h) space velocity, the catalyst will stay active for a longer time than at 39,000 and 78,000 mL/(g·h).

#### *2.12. Effect of Different CO2/CH4 Ratios*

The mole ratio of CO2 to CH4 was changed at a fixed total flow rate to study the performance of Ti-CAT-II catalyst when CH4 was supposed to act as the limiting reagen<sup>t</sup> in excess of CO2 at 700 ◦C and 39,000 mL g<sup>−</sup><sup>1</sup> h−<sup>1</sup> GHSV. The results are shown in Figure 10a–c. The highest CH4 conversion of about 78% was obtained when CO2 was 20% in excess of CH4, while the least conversion of CH4 (~43%) resulted when the amount of CO2 was 50% of the required stoichiometric amount in the feed. This observation was expected, as CH4 would have enough CO2 to undergo dry reforming. On the other hand, the highest CO2 conversion of (~90%) was observed when CO2 was the limiting reagent. This observation could be due to excess CH4 present in the feed. The CO2 conversion was reduced with the reaction time-on-stream. Such observation could be ascribed to the disproportionation of carbon monoxide into CO2 and graphite, a transformation known as the Boudouard reaction:

$$\text{2CO}\_{\text{(g)}} = \text{CO}\_{2(g)} + \text{C}\_{\text{(s)}}$$

Comparing the different CO2/CH4 ratios, it was observed that CH4 conversion increased with the ratio up to 1.2 (i.e., 0.5 < 1.0 < 1.2) and then declined slightly at 1.5. However, the conversion for CO2 was observed to decrease as the ratio increased (i.e., 1.5 < 1.2 < 1.0 < 0.5).

Figure 10c displays the H2/CO mole ratio results. It was observed that at the lowest CO2/CH4 mole ratio, the H2/CO mole ratio was greater than one. This observation could be owing to the insufficient amount of CO2 for complete dry reforming of the available CH4 and to the thermal decomposition of unreformed CH4, giving more H2 than the stoichiometric amount. Moreover, the Boudouard reaction might contribute to the increase of hydrogen production, because the formed CO2 from Boudouard reaction would shift the DRM equilibrium to the product side.

On the other hand, H2/CO mole ratio was close to one of the cases where CO2 was in excess of CH4, where it was noticed that the H2/CO mole ratio increased with the reaction time-on-stream. Once again, the Boudouard reaction might be responsible for such observation.

**Figure 10.** (**a**) CH4 (**b**) CO2 conversions, and (**c**) H2/CO ratio for different CO2/CH4 ratio over Ti-CAT-II.
