2.4.1. XRD Analysis

Figure 15 shows the XRD of deposited carbon on the used catalyst after the DR and CC reactions. The peak at 2θ = 26◦ confirms that the carbon formed is graphitic, no peaks of oxides have been detected, and only Ni and Fe were present in the patters, which proves that their oxides were reduced during the reactions. Carbide formation was expected because carbides are known to be the precursor of CNF especially with iron-based catalysts. Fe3C is metastable under the reaction conditions so it is decomposed to CNF and α-Fe [35]. Nickel plays a key role in the formation of CNF because, when UGSO was used alone, catalyst activity was low [30]. When nickel was added, the quantity and quality of CNF were found to have improved. Nickel catalyzes the C–C bond cleavage, thus producing carbon species radicals and atomic carbon that diffuses and dissolves in the iron to form a solid solution of iron carbides [36].
