*2.3. Carbon Deposition*

Table 2 presented the amount of carbon deposition over the series catalysts determined by the temperature programmed oxidation (TPO)-infrared spectrum (IR) analysis. It can be seen that the amount of carbon deposition is varied with the introduction of Ni and Co. Clearly, the amount of carbon deposition over CA catalyst is larger than others, and the values drop off with the Co content increased. For the bi-metallic C12(C15)-NCA catalyst, the amount of carbon deposition (<4.0 mg/g-fuel) is much less than conventional Ni-alumina catalysts. The possible reasons are that the addition of Co reduces the acidity of the CA catalyst and carbon dioxide adsorption, and decreases carbon deposition generation during *n*-decane steam reforming process [48]. According to the literature [57], Co addition can increase the dispersion state of Ni, prevent metal sintering, and inhibit carbon deposition, which is consistent with our results in this work.


**Table 2.** The amount of carbon deposition over used catalysts.

#### *2.4. Used Catalyst Characterization*

The formation of carbon deposition on the used catalyst was also evidenced by SEM observations. In Figure 8a–g, the micrographs of used catalysts are presented. Carbon deposition grows on the surface of catalysts in different range. It is observed from Figure 8h that the Ni species is occupied at the tip of the filamentous carbon and the Ni metal particle size is found to be ca. 30 nm. This can demonstrate that active metal has removed from the surface of catalyst with the generation of filamentous carbon. The results are consistent with the mechanisms of filamentous carbon growth in literature [58]. In addition, the C content over the used CA, NCA, C3-NCA, C6-NCA, C9-NCA, C12-NCA, C15-NCA catalysts are 91.14%, 63.90%, 56.46%, 49.89%, 34.79%, 14.94% and 8.31%, respectively. It clearly indicates that the C content over the used catalysts has a close relationship with the surface area. Obviously, the Brunauer–Emmett–Teller (BET) specific surface area decrease caused by the formation of the carbon deposition is another main reason of the activity difference among these catalysts.

**Figure 8.** Field emission scanning electron microscopy (FESEM) micrographs of the used catalysts: CA (**a**), NCA (**b**), C3-NCA (**c**), C6-NCA (**d**), C9-NCA (**e**), C12-NCA (**f**), C15-NCA (**g**) and C15-NCA (**h**).
