*3.3. Catalysts Characterization*

The specific surface areas measured by Brunauer-Emmett-Teller (BET) isotherms of all of the catalysts before and after DRM reaction are given in Table 1. The NY catalyst is found to have highest specific surface area (573.3 m2/g) before reaction but NY catalyst also showed significant loss (13%) in specific surface area during reaction. The NA catalyst showed a loss of 18% in specific surface area while NH catalyst exhibited a minimum loss of 5%. The loss in specific surface area is accounted for carbon formation over the catalyst surface during DRM reaction. The minimal loss in specific surface area for NH catalyst indicates its superior stability under reaction conditions.

In order to estimate the crystallinity and different phases of the as-synthesized catalysts, X-ray diffraction (XRD) study was conducted over NA, NY and NH fresh catalysts and NA used catalyst. From Figure 3, the characteristic peaks of NiO were found over all the catalysts at two theta values of 44 and 63◦ (JCPDS: 01-073-1519), except for the fact that NY and NH catalysts showed additional peak of NiO at 37◦ (JCPDS: 01-073-1519). The peaks at 42 and 66◦ in the case of NA catalysts are characteristics of alumina support (JCPDS: 00-004-0875). The diffraction peaks had higher intensities in case of NY and NH catalysts as compared with NA catalyst showing higher crystallinity of the former catalysts. The NA catalyst after reaction revealed the formation of carbon during reaction as indicated by strong diffraction peak at 26◦ associated with graphite. Moreover, in addition to Ni0, the NA catalyst after reaction also showed a characteristic peak of spinel NiAl2O4 (JCPDS: 01-073-0239) at 45 and 78◦.

**Figure 3.** XRD patterns for Ni supported on alumina (NA), Ni supported on Y-zeolite (NY), and Ni supported on H-ZSM-5 (NH) fresh and Ni supported on alumina (NA) used catalysts.

The temperature-programmed oxidation (TPO) is a useful technique to estimate the amount of carbon formed during reaction by oxidizing the carbon as function of temperature. TPO also provides significant details about the structure, morphology and composition of accumulated carbon. Figure 4 displays the TPO profiles as a function of temperature for all the three catalysts (NA, NY and NH respectively). The spent or used catalysts exhibited carbon gasification peaks in the temperature range from 400 to 640 ◦C associated with different types of carbon including carbon nanofibers and carbon nanotubes.

**Figure 4.** TPO profiles for Ni supported on alumina (NA), Ni supported on Y-zeolite (NY), and Ni supported on H-ZSM-5 (NH) spent catalysts.

The following four side reactions are most probably responsible for carbon deposition during DRM reaction.

$$2\text{CO} \leftrightarrow \text{CO}\_2 + \text{C} \tag{3}$$

$$\text{CH}\_4 \leftrightarrow 2\text{H}\_2 + \text{C} \tag{4}$$

$$\text{C} \text{O} + \text{H}\_2 \leftrightarrow \text{H}\_2\text{O} + \text{C} \tag{5}$$

$$\text{CO}\_2 + 2\text{H}\_2 \leftrightarrow 2\text{H}\_2\text{O} + \text{C} \tag{6}$$

As mentioned earlier, the first two reactions are favorable at high temperatures while the last two reactions (reactions (5) and (6)) require lower temperatures [36]. The peak temperature indicates the type of carbon formed, and hence it is obvious that the carbon formed over the surface of NY (corresponding peak temperature of ~460 ◦C) and NY (corresponding peak temperature of ~490 ◦C) is less crystalline and easily gasified as compared with crystalline carbon deposited over the surface of NH catalyst (corresponding peak temperature of ~545 ◦C). The peak intensity shows the amount of carbon formed, and it is obvious that NH catalyst had less amount of carbon deposition among all of the catalysts [37].

The extent of interaction between the metal and support plays a vital role in catalytic activity. Temperature-programmed reduction (TPR) using hydrogen is utilized to measure the metal-support interaction for NA, NY, and NH catalysts and extent of reducibility of each catalyst. Figure 5 shows the reduction profiles for NA, NY, and NH fresh catalysts as a function of temperature. NY catalyst exhibited two peaks at 350 and 440 ◦C along with a shoulder centered at 550 ◦C. The two low temperature peaks are assigned to the reduction of NiO species weakly interacting with the support while shoulder is ascribed to the reduction of NiO species having medium interaction with the support (Y-zeolite). Also, the NY catalyst shows NiO species which are easier to reduce as compared with the NA and NH catalysts.

**Figure 5.** TPR profiles for Ni supported on alumina (NA), Ni supported on Y-zeolite (NY), and Ni supported on H-ZSM-5 (NH) fresh catalysts.

In case of NH catalyst, the first smaller peak is centered at 330 ◦C and a second large peak has maxima at 450 ◦C while the shoulder is centered at 560 ◦C. This suggests that the strength of NiO species over NH catalyst is not significantly different than the NY catalyst. On the contrary, NA catalyst gave only one distinct reduction peak at 455 ◦C with two smaller shoulders centered at 560 and 740 ◦C, respectively. It is interesting to note that lower temperature peak appearing in the 330–350 ◦C range is disappeared in NA catalyst while a new shoulder at 740 ◦C is observed. These results sugges<sup>t</sup> that NA catalyst showed medium to strong metal-support interactions. The shoulder at 740 ◦C is assigned to the reduction of spinel NiAl2O4 species.

The extent of basic sites and their strength for fresh NA, NY, and NH catalysts was measured by employing temperature-programmed desorption (TPD) using CO2. The basicity of the catalyst is found to influence the carbon deposition and a higher number of basic sites led to lesser amount of carbon deposition [38]. The enhanced basicity promotes CO2 activation over the surface of the catalyst which reacts with carbon formed due to side reactions. Consequently, reverse Boudouard reaction (2CO - CO2 + C) converts the carbonaceous species into CO. Hence, the catalyst with higher basicity is expected to show the minimum carbon deposition. CO2-TPD profiles are shown in Figure 6 for fresh NA, NY, and NH catalysts.

NA catalyst presents two peaks, centered at 80 and 260 ◦C, along with a shoulder centered at 560 ◦C. The two peaks which appeared at lower temperatures are associated with weak basic sites and the shoulder represents strong basic sites. In the case of the NY catalyst, a small peak appears at 150 ◦C while a broad peak is centered at 275 ◦C and both are assigned to weak basic sites. NH catalyst shows two peaks with peak maxima at 130 and 300 ◦C respectively along with a shoulder centered at 550 ◦C. The first peak represents weak basic sites and the second peak is associated with medium basic sites while the shoulder is assigned to strong basic sites. It is noteworthy that the peaks for NH catalysts are broader than NY and NA catalysts and hence exhibit more amount of CO2 adsorbed which leads to lesser carbon formation. This is in agreemen<sup>t</sup> with the TGA and TPO results.

**Figure 6.** CO2-TPD for Ni supported on alumina (NA), Ni supported on Y-zeolite (NY), and Ni supported on H-ZSM-5 (NH) fresh catalysts.
