*3.2. Characterization Methods*

XRD: X-ray powder diffraction (XRD) was carried out using a Rigaku (Miniflex) diffractometer (Rigaku, Bahrain, Saudi Arabia). The unit was set, using a Cu K α radiation run at 40 kV and 40 mA, to investigate the diffraction peaks of the catalysts before and after the reaction. The 2θ step and the scanning range were set at 0.02◦ and 10–85◦, respectively. The instrumental raw data was analyzed via X'pert HighScore Plus software (3.0.5, Malvern Panalytical Ltd, Malvern, UK, 2017). An ASCII file of the peak intensity was produced at granularity 8, bending factor 5, minimum, peak significance 1, minimum peak width 0.40, maximum tip width 1, and peak base width 2 by minimum second derivatives. Additional dissimilar phases with their marks were accorded using the JCPDS data bank (N.B.S\*AIDS83, International Center for Diffraction Data, Newtown Square, PA, USA, 1980) and X'pert HighScore Plus software.

TEM: Samples were arranged for transmission electron microscopy (TEM) investigation by crushing the powders between clean glass slides and then scattering them onto a lacey carbon film held on a Cu mesh grid. Bright-field transmission electron microscopy (BF-TEM) and selected area electron diffraction (SAED) experiments were performed using a JEOL 2000FX microscope (JEOL, Peabody, MA, USA) fitted with a thermionic LaB6 source working at 200 kV.

LRS: Laser Raman spectroscopy apparatus was represented by a highly sensitive CCD Q15 spectrograph (Thorlabs, Munich, Germany) having two 215 excitation lasers of 532 and 785 nm with CCD cooling temperature reaching up to 60 ◦C and high-throughput laboratory fiber optic probes. Moreover, the scanning was fixed between 250 and 2350 cm<sup>−</sup>1.

N2-physisorption: The distribution of the pore size and the specific surface area of the catalysts were computed from N2 adsorption–desorption data. Measurements of the BET surface area were conducted by nitrogen adsorption at −196 ◦C using a Micromeritics Tristar II 3020 (Micromeritics, Riyadh, Saudi Arabia) surface area and porosity analyzer. In each test, 300 mg of catalyst was degassed at 300 ◦C for 3 h to remove the wetness and adsorbed gases from the catalyst surface. Pore size distribution was computed via the Barrett-Joyner-Halenda (BJH) technique.

H2-TPR: Micromeritics Auto Chem II apparatus (Micromeritics, Riyadh, Saudi Arabia) was employed in the temperature programmed reduction (TPR) to examine the reducibility by taking 0.07 g for each test. High purity argon flow was first passed thought the samples at 150 ◦C for 30 min. Then, the samples were brought to 25 ◦C. Lastly, the furnace temperature was set to 1000 ◦C at a 10 ◦C/min rate while running 40 mL/min flow rate of H2/Ar mixture that had 10 vol % of H2. A thermal conductivity detector (TCD) checked the H2 consumption signals.

TGA: The quantitative investigation of coke formation on the used catalysts after the duration of 90, 180, 360, and 720 min reaction at 700 ◦C was conducted using thermogravimetric analysis (TGA) in the presence of air via a Shimadzu TGA (Thermo-gravimetric/Differential) analyzer (Shimadzu, Jebel Ali Free Zone, Dubai). The temperature of the spent catalysts weighing 10–15 mg was increased from 25 ◦C to 1000 ◦C at a heating rate of 20 ◦C/min, and the mass reduction was recorded.

AAS: Atomic absorption spectrometry was employed to examine samples of Fe and Co components in the catalysts. The AAS analyses were conducted using a Model 951 AA/AE (Berkeley Nucleonics corporation, San Rafael, CA, USA) spectrophotometer with graphite furnace and Model

254 Auto sampler. The composition of Co and Fe in catalysts were theoretically determined and compared to those experimentally obtained by AAS.

#### *3.3. Activity Test (Regeneration Procedure)*

CMD activity tests were done in a fixed-bed quartz reactor (id = 9 mm). Before the CMD reaction, 0.3 g of the catalyst was reduced in situ under hydrogen flow (40 mL/min) at 500 ◦C for 60 min. Then 20 min of N2 flushing was carried out. Subsequently, the reactor temperature was raised to 700 ◦C under N2. For all runs, a fixed feed gas composed of 15 mL of CH4and 10 mL of N2was used.

The regeneration cycles were performed in situ at 700 ◦C under diluted O2 gasifying agen<sup>t</sup> (10%O2/N2), followed by inert treatment under N2. The obtained regenerated catalyst was tested again in CMD reaction. First, the CMD reaction was carried out for 90 min in CMD reaction. Then, the spent catalyst (labeled SP-90 min) was removed for characterization as a reference. Then, the reactor was charged with fresh catalyst and the above steps were repeated for each of the following experiments:

In the first regeneration experiment: the CMD reaction was performed for 90 min. At this point, 10 mL/min of O2 was added to the system for 10 min. Then, the reactor was fed with N2 at a rate of 20 mL/min for 20 min and the CMD reaction was continued for another 90 min to finally obtain 180 min (= 2 × 90 min). Then, the spent/regenerated catalyst (noted as SP-180 min) was removed for characterization.

In the last regeneration experiment: the reactor was again recharged with a fresh catalyst. The CMD reaction was allowed to run for 90 min. Then, 10 mL/min of O2 was used to regenerate the catalyst for 10 min. Then, 20 mL/min of N2 was used to flush the reactor for 20 min and the reaction was continued for another 90 min. After that, the above addition of O2 and N2 was periodically repeated for eight cycles to give a total time of reaction/regeneration of 720 min (= 8 × 90 min). Then, the spent/regenerated catalyst (labeled as SP-720 min) was removed for characterization.

Finally, in this work, three regenerations of periodic cycling (with 2×, 4×, and 8× of 90 min) were chosen and the spent samples at 180, 360, and 720 min were removed for characterizations.

The CH4 reactant and H2 product were evaluated using an online GC (Alpha MOS PR 2100, Alpha MOS, Toulouse, France,) fitted with a sampling valve and two thermal conductivity detectors for examining heavier and lighter gases. CH4 conversion and H2 yield were computed using Equations (1) and (2):

$$\text{CH}\_4 \text{ conversion}(\%) = \frac{\text{CH}\_{4\text{in}} - \text{CH}\_{4\text{out}}}{\text{CH}\_{4\text{in}}} \times 100,\tag{1}$$

$$\text{H}\_2\text{Yield}(\%) = \frac{\text{H}\_{2\text{out}}}{2 \times \text{CH}\_4 \text{ converted}} \times 100. \tag{2}$$
