4.4.3. Methane Decomposition (CH4/He) Reaction

The reduced CeO2-supported Ni catalysts were exposed to 20% CH4/He for 30 min in order to measure the initial rate of CH<sup>4</sup> decomposition and its subsequent rate evolution, which is one of the main routes of inactive carbon formation under DRM conditions. The transient responses of H<sup>2</sup> (m/z = 2), CH<sup>4</sup> (m/z = 15), and CO (m/z = 28) were followed during the step-gas switch He → 20% CH4/He (750 ◦C, 30 min, 50 NmL min−<sup>1</sup> ; GHSV ~30,000 h−<sup>1</sup> ) by online MS. The latter switch was followed by a 10 min He purge, while the temperature was increased to 800 ◦C (until background values were reached for the CO and CO<sup>2</sup> MS signals). The catalyst was then cooled down to 200 ◦C and the feed was switched from He to 10 vol% O2/He (50 NmL min−<sup>1</sup> ) to perform a TPO run (β = 30 ◦C min−<sup>1</sup> ). The transient evolution of CO (m/z = 28) and CO<sup>2</sup> (m/z = 44) was continuously monitored with MS, and their quantification was made using certified calibration gas mixtures (1.06 vol% CO/1.02 vol% CH4/0.95 vol% H2/He and 2.55 vol% CO2/He).

### 4.4.4. Carbon Monoxide Dissociation (CO/He) Reaction

The second main route of inactive carbon formation during DRM, that of reverse Boudouard reaction, was investigated by performing over the 5 wt% Ni/CeO<sup>2</sup> catalysts the step-gas switch He → 20% CO/He (750 ◦C, 30 min, 50 NmL min−<sup>1</sup> ; GHSV ~30,000 h−<sup>1</sup> ), where the evolution of CO (m/z = 28) and CO<sup>2</sup> (m/z = 44) were continuously monitored with MS. The latter gas switch was followed by a He purge (10 min) and temperature increase to 800 ◦C, where the catalyst was kept at this temperature until the CO and CO<sup>2</sup> MS signals reached their respective background value. The reactor's temperature was then decreased to 200 ◦C, where a switch to 10 vol% O2/He (50 NmL min−<sup>1</sup> ) gas mixture was made for a TPO run to 800 ◦C (β = 30 ◦C min−<sup>1</sup> ). During TPO, the mass numbers (m/z) of 28 and 44 were followed by MS, and quantification was made by considering certified calibration gas mixtures (1.06 vol% CO/He and 2.55 vol% CO2/He).

#### 4.4.5. Isotopically Labelled Competitive (13CO/ <sup>12</sup>CH4) Reaction towards Carbon Formation

The relative contribution of the two main routes towards inactive carbon accumulation under DRM reaction conditions (CH<sup>4</sup> decomposition and reverse Boudouard reaction) was investigated by exposing the catalysts over an isotopically labelled mixture consisting of 2.5 vol% <sup>13</sup>CO/2.5 vol% <sup>12</sup>CH4/2 vol% Kr/Ar/He (50 NmL min−<sup>1</sup> ; GHSV ~30,000 h−<sup>1</sup> ) at 750 ◦C for 20 min. The gas-flow was then switched to He for a 10 min purge and the temperature was increased to 800 ◦C until the <sup>12</sup>CO, <sup>13</sup>CO, <sup>12</sup>CO2, and <sup>13</sup>CO<sup>2</sup> MS signals reached their respective background value. The catalyst was then cooled in He flow to 200 ◦C and a switch to 10 vol% O2/He (50 NmL min−<sup>1</sup> ) gas mixture was made for a TPO run (increase T to 800 ◦C, β = 30 ◦C min−<sup>1</sup> ). The effluent gas stream was continuously monitored by MS 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), and quantification of the MS signals was made by using the previously mentioned (Section 4.4.2) calibration gas mixtures. It's worth mentioning that the <sup>12</sup>C-containing TPO traces refer to the <sup>12</sup>CH<sup>4</sup> contribution on the amount of carbon accumulation, whereas the <sup>13</sup>C-containing TPO traces refer to the <sup>13</sup>CO route.

#### *4.5. Participation of Support's Lattice Oxygen in DRM Reaction Conditions*

The partial <sup>16</sup>O/ <sup>18</sup>O isotopic exchange of ceria support's lattice <sup>16</sup>O was performed over pre-reduced Ni/CeO<sup>2</sup> catalysts (Wcat = 0.02 g) at 750 ◦C for 10 min prior to the dry reforming of methane reaction. This designed experiment probes for the extent of contribution of support's lattice oxygen in the carbon-path under DRM conditions [18,19]. More precisely, after 2 h reduction of catalyst with pure H<sup>2</sup> (1 bar) at 700 ◦C, the feed was switched to Ar for 10 min with subsequent increase of the temperature to 750 ◦C, until no H<sup>2</sup> (m/z = 2), <sup>16</sup>O<sup>2</sup> (m/z = 32), and <sup>16</sup>O18O (m/z = 34) MS signals were recorded. The exchange of support lattice oxygen and the oxidation of Ni/NiO<sup>x</sup> to Ni18O with <sup>18</sup>O2(g) was then made by exposing the catalyst to 2 vol% <sup>18</sup>O2/2 vol% Kr/Ar/He (10 min, 50 NmL min−<sup>1</sup> ). During the exchange process, the signals of <sup>16</sup>O2, <sup>16</sup>O18O, <sup>18</sup>O2, and Kr (m/z = 32, 34, 36, and 84, respectively) were recorded continuously with online MS, which then converted into concentration (mol%) by using appropriate material balances [18] from which the amount of oxygen exchanged (mol <sup>16</sup>O g−<sup>1</sup> ) can be estimated. A 10 min He purge then followed, and the feed gas was then switched to 20 vol% CH4/20 vol% CO2/He (50 NmL min−<sup>1</sup> ). During the latter DRM reaction step, the MS signals of 30, 44, 46, 48, and 84 (C18O, C16O2, C16O18O, C18O2, and Kr, respectively) were continuously monitored, and then converted into concentration (mol%) by using appropriate calibration gases. It was assumed same sensitivities for the C16O and C18O (m/z = 30) gases. The contribution of C18O<sup>2</sup> (m/z = 48) and C16O18O (m/z = 46) to the m/z=30 were carefully subtracted from the m/z = 30 (C18O) signal recorded by using a standard C16O2/He gas mixture and considering the same contribution of m/z = 44 to m/z = 28 for the m/z = 48 and m/z = 46 to m/z = 30. The formation of C18O(g) during DRM, following the oxygen <sup>16</sup>O/ <sup>18</sup>O isotopic exchange step, is clearly described in our previous publications [20,46], where *<sup>18</sup>O<sup>L</sup> of support* can react with carbon formed on the catalyst surface.

#### **5. Conclusions**

The main conclusions derived from the present work are as follows:


**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4344/9/7/621/s1, Figure S1: Powder X-ray diffractograms of 5 wt% Ni supported on (a) CeO<sup>2</sup> -TD, (b) CeO<sup>2</sup> -PT, (c) CeO<sup>2</sup> -HT and (d) CeO<sup>2</sup> -SG carriers in the (A) 20–70◦ 2θ and (B) 35–45◦ 2θ region (diffraction peaks of NiO), Figure S2: Representative HR-TEM images of the calcined (air, 750 ◦C/4 h) 5 wt% Ni/CeO<sup>2</sup> -HT catalyst. Left graph: magnification at 50 nm unit scale; Right graph: magnification at 10 nm unit scale, Figure S3: SEM images of the fresh Ni/CeO<sup>2</sup> -SG (top left), Ni/CeO<sup>2</sup> -PT (top right), Ni/CeO<sup>2</sup> -HT (down left) and Ni/CeO<sup>2</sup> -TD (down right), Table S1: Textural and structural characterization of 5 wt% Ni/CeO<sup>2</sup> (-TD, -PT, -HT and -SG) DRM fresh catalysts, Table S2: Catalytic activity in terms of CH<sup>4</sup> , CO<sup>2</sup> conversion (XCH4, XCO2, %), H<sup>2</sup> Yield (%) and H<sup>2</sup> /CO gas product ratio obtained after 30 min in DRM at 750 ◦C for the four ceria supports prepared by different methods, Table S3: Catalytic stability performance in terms of CH<sup>4</sup> , CO<sup>2</sup> conversion (XCH4, XCO2, %), H<sup>2</sup> Yield (%), H<sup>2</sup> /CO gas product ratio and carbon deposition (mg C g−<sup>1</sup> cat) obtained during DRM (20% CH<sup>4</sup> /20% CO<sup>2</sup> /He) at 750 ◦C over the 5 wt% Ni/CeO<sup>2</sup> -PT solid, Table S4: Catalytic activity in terms of CH<sup>4</sup> , CO<sup>2</sup> conversion (XCH4, XCO2, %), H<sup>2</sup> Yield (%) and H<sup>2</sup> /CO gas product ratio obtained after 30 min in DRM (5 vol% <sup>13</sup>CO<sup>2</sup> /5 vol% <sup>12</sup>CH<sup>4</sup> /He) at 750 ◦C, Table S5: Carbon accumulation (mg C gcat −1 ) estimated via TPO followed individual reactions over all catalysts at 750 ◦C; 20 vol% CO<sup>2</sup> /20 vol% CH<sup>4</sup> /He (12 h), 5 vol% <sup>13</sup>CO<sup>2</sup> /5 vol% <sup>12</sup>CH<sup>4</sup> /He (30 min), 20 vol% CH<sup>4</sup> /He (30 min), 20 vol% CO/He (30 min), 2.5 vol% <sup>13</sup>CO/2.5 vol% <sup>12</sup>CH<sup>4</sup> /He (20 min), Table S6: 18O consumption (mmol g−<sup>1</sup> ) during <sup>16</sup>O/ <sup>18</sup>O exchange, C18O formation (mmol g−<sup>1</sup> ) during DRM following <sup>16</sup>O/ <sup>18</sup>O oxygen exchange, and C18O/ <sup>18</sup>O ratio.

**Author Contributions:** Conceptualization, A.M.E., V.N.S., and M.A.V.; methodology, A.M.E., V.N.S., and M.A.V.; validation, M.A.V. and C.M.D.; formal analysis, C.M.D.; investigation, C.M.D.; resources, A.M.E.; data curation, C.M.D.; writing—original draft preparation, M.A.V.; writing—review and editing, A.M.E.; visualization, A.M.E. and M.A.V.; supervision, A.M.E. and M.A.V.; project administration, M.A.V. and C.M.D.; funding acquisition, A.M.E.

**Funding:** This research was funded by the Research Committee of the University of Cyprus.

**Acknowledgments:** The authors are grateful to Maria Kollia (Research Associate) of the University of Patras for performing the HR-TEM studies.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


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