4.2.3. H<sup>2</sup> Temperature-Programmed Desorption (TPD)

The effect of Ni supported on different CeO<sup>2</sup> supports on the hydrogen chemisorption and desorption behavior was investigated by using the H2-TPD technique. A 0.3 g sample was first reduced in situ in hydrogen gas at 700 ◦C for 2 h prior to He-purge and increase of its temperature to 750 ◦C, until H<sup>2</sup> signal reached its background value (desorption of any spilled-over hydrogen on the support). The reactor was then cooled down to room temperature and a switch from He to 0.5 vol% H2/He (30 min) gas mixture was performed. The catalyst was then purged for 10 min in He flow, and the temperature was subsequently increased to 750 ◦C (TPD, β = 30 ◦C min−<sup>1</sup> , 50 NmL min−<sup>1</sup> ). The H<sup>2</sup> (m/z = 2) signal was continuously monitored with online mass spectrometer (MS, Balzers, Omnistar 1–200 amu, Pfeiffer Vacuum, Asslar, Germany), and the MS signal was converted into concentration (ppm) by using a certified standard gas mixture (0.95 vol% H2/He). The Ni dispersion (DNi, %) was estimated after assuming an H<sup>2</sup> chemisorption stoichiometry of H/Ni<sup>s</sup> = 1, where the Ni mean primary particle size (dNi, nm) was estimated by using the following Equation (4) [76]:

dNi (nm) = 0.97/DNi. (4)

#### 4.2.4. Transmission Electron Microscopy (TEM)

The fresh 5 wt% Ni/CeO2-HT supported Ni catalyst was characterized with a JEOL (JEM-2100) high-resolution transmission electron microscopy system (HR-TEM) (Tokyo, Japan), operated at 200 kV (resolution point 0.23 nm, lattice 0.14 nm). Selected specimens were prepared by dispersion of the powdered catalyst in water, and spread onto a carbon-coated copper grid (200 mesh), while images were recorded by means of films (Kodak SO-163).

#### 4.2.5. Scanning Electron Microscopy (SEM)

The morphology of the fresh CeO2-supported Ni solids was characterized by using scanning electron microscope (SEM, JEOL JSM-6610 LV, Tokyo, Japan), equipped with a BRUKER type QUANTAX 200 energy dispersive spectrometer (EDS). The effect of different method of synthesis of the CeO<sup>2</sup> was studied using secondary electron images (SEI). EDS analysis was performed for determining the chemical composition of the solids.

### *4.3. Catalytic Performance of CeO2-Supported Ni in DRM*

Catalytic measurements were performed using a Micro-activity reactor system (MA-REF from PID Eng & Tech, Madrid, Spain) equipped with a tubular quartz reactor (i.d. = 6 mm), and the experimental apparatus used was described elsewhere [20]. The catalytic bed was prepared by grinding (grain powder size less than 106 µm) and mixing an appropriate amount of Ni/CeO<sup>2</sup> catalyst with SiC (1 cat:1 SiC (*w*/*w*)) in order to achieve a gas hourly space velocity (GHSV) of ~30,000 h−<sup>1</sup> . Due to the differences in the ceria solid powder prepared by the four different methods, the amount of catalyst for each Ni/CeO<sup>2</sup> was varied (Wcat = 0.072–0.167), while the total gas flow rate was kept the same (50 NmL min−<sup>1</sup> ). The catalytic performance of the solids was examined at 750 ◦C for 30 min with a DRM feed gas composition of 20 vol% CH4/20 vol% CO2/60 vol% He. The conversions of CH<sup>4</sup> and CO<sup>2</sup> (XCH4 and XCO2, %) were calculated by using Equation (5). The effluent gas stream from the micro-reactor was analyzed through online MS and infrared gas analyzers (Horiba, VA-3000, Kyoto, Japan) for H<sup>2</sup> (m/z = 2), CH<sup>4</sup> (m/z = 15) and CO, CO2, respectively. Calibration of the signals from the MS and IR gas analyzers was made by using certified calibration gas mixtures (1.06 vol% CO/1.02 vol% CH4/0.95 vol% H2/He and 2.55 vol% CO2/He). The product yields (YH2 and YCO, %) were estimated via Equations (6) and (7):

$$X\_Y(\%) = \frac{F\_Y^{in} - F\_Y^{out}}{F\_Y^{in}} \times 100\tag{5}$$

$$\text{Y}\_{H\_2}(\text{\textquotedblleft}\_0) = \frac{\text{F}\_{H\_2}^{\text{out}}}{\text{2F}\_{\text{CH}\_4}^{\text{in}}} \times 100\tag{6}$$

$$Y\_{\text{CO}}(\%) = \frac{F\_{\text{CO}}^{\text{out}}}{F\_{\text{CH}\_4}^{\text{in}} - F\_{\text{CO}}^{\text{in}}} \times 100\tag{7}$$

where, *F in* and *F out* are the molar flow rates (mol s−<sup>1</sup> ) of reactant Y (CH<sup>4</sup> or CO2) and product (H<sup>2</sup> or CO) at the inlet and outlet of the reactor, respectively. The *F out* was estimated based on the total volume flow rate at the outlet of the reactor (measured at 1 bar and room T), and the mole fraction of the component measured by the above-mentioned gas analysis system.

#### *4.4. Characterization of Carbon Formed under Di*ff*erent Reaction Conditions*

#### 4.4.1. Dry Reforming of Methane (12CO2/ <sup>12</sup>CH4) Reaction

The reactivity of carbon towards oxygen and its amount (mg C gcat <sup>−</sup><sup>1</sup> or wt%) accumulated after 12 h of DRM at 750 ◦C over the catalysts investigated in this work were estimated by performing temperature-programmed oxidation (TPO) experiments following DRM. A purge with He (20 min) was applied after the 12 h DRM reaction with the reactor's temperature to increase to 800 ◦C until no MS signals was identified for CH4, CO2, H2, and CO. The catalyst's temperature was then decreased to 100 ◦C followed by a feed gas switched from He to 10 vol% O2/He (50 NmL min−<sup>1</sup> ). The catalyst's temperature was subsequently increased to 800 ◦C with a heating ramp of 30 ◦C min−<sup>1</sup> (TPO). During the latter switch, the signals of CO (m/z = 28) and CO<sup>2</sup> (m/z = 44) were continuously monitored with the MS and CO/CO<sup>2</sup> infrared gas analyzer, and then converted into mol% based on certified calibration gas mixtures (1.06 vol% CO/He and 2.55 vol% CO2/He).

#### 4.4.2. Isotopically Labelled Dry Reforming of Methane (13CO2/ <sup>12</sup>CH4) Reaction

Isotopically labelled DRM mixture (5 vol% <sup>13</sup>CO2/5 vol% <sup>12</sup>CH4/45 vol% Ar/45 vol% He; 50 NmL min−<sup>1</sup> ; GHSV ~30,000 h−<sup>1</sup> ) was used for 30 min at 750 ◦C, followed by TPO, in order to investigate the relative contribution of CH<sup>4</sup> and CO<sup>2</sup> activation routes towards carbon accumulation (µmol g−<sup>1</sup> and mg gcat −1 ) over the examined ceria-supported Ni catalytic systems. The <sup>12</sup>C-containing TPO traces referred to the CH<sup>4</sup> activation route contribution on the amount of carbon, whereas the <sup>13</sup>C-containing TPO traces referred to the CO<sup>2</sup> activation route. More precisely, after 30 min in DRM, a He purge was performed for 10 min prior to the temperature increase to 800 ◦C (until no MS signals for CO and CO<sup>2</sup> were observed). The reactor was then cooled down to 200 ◦C, and the feed gas was switched from He to 10 vol% O2/He (50 NmL min−<sup>1</sup> ), followed by TPO to 800 ◦C (β = 30 ◦C min−<sup>1</sup> ). The signals for <sup>12</sup>CO, <sup>13</sup>CO, <sup>12</sup>CO2, and <sup>13</sup>CO<sup>2</sup> (m/z = 28, 29, 44, and 45, respectively) were continuously monitored by MS and their quantification (mol%) was made by using certified gas mixtures (1.06 vol% <sup>12</sup>CO/He, 10 vol% <sup>13</sup>CO/Ar, 2.55 vol% <sup>12</sup>CO2/He, and 10 vol% <sup>13</sup>CO2/Ar).
