2.6.3. Transient Methane Decomposition (CH4/He) Reaction

Figure 5 shows transient evolution rates of CH<sup>4</sup> consumption and H<sup>2</sup> and CO gas formation (the only gaseous reaction products observed), obtained during the step gas switch He → 20 vol% CH4/1% Ar/He (30 min) made at 750 ◦C over the four 5 wt% Ni supported on CeO<sup>2</sup> carriers prepared by different synthesis methods. The differences in the initial transient rate values, but also their shapes, are apparent. It should be mentioned at this point that the latter rates appeared very small when the reaction was performed over the supports alone. The different kinetics of CH<sup>4</sup> decomposition, over each of the four catalytic surfaces presented in Figure 5A, led also to different H<sup>2</sup> transient formation rates (Figure 5B), similar in shape with those of CH<sup>4</sup> consumption (Figure 5A). On the other hand, the rate of CO formation was the result of carbon removal by the support's lattice oxygen, which followed largely different kinetics (compare Figure 5B,C). In particular, the H<sup>2</sup> transient rates in the case of Ni/CeO2-TD and Ni/CeO2-PT passed through a maximum a short time after the switch (<10 s), as opposed to Ni/CeO2-HT and Ni/CeO2-SG, which passed through a maximum after 25 s in CH4/Ar/He feed gas stream. Also, the latter catalyst presented only a slight decrease in the reaction rates after maximum rate was achieved (practically a plateau in the rate is obtained (Figure 5Ad,Bd,Cd). It has been discussed that these transient features reflect the Ni metal surface's ability to decompose methane over the remaining empty sites with time on stream, leading to carbon structure dependent deposition with different kinetics [18,20]. *Catalysts* **2019**, *9*, x FOR PEER REVIEW 7 of 24 On the other hand, the rate of CO formation was the result of carbon removal by the support's lattice oxygen, which followed largely different kinetics (compare Figure 5B and 5C). In particular, the H2 transient rates in the case of Ni/CeO2-TD and Ni/CeO2-PT passed through a maximum a short time after the switch (<10 s), as opposed to Ni/CeO2-HT and Ni/CeO2-SG, which passed through a maximum after 25 s in CH4/Ar/He feed gas stream. Also, the latter catalyst presented only a slight decrease in the reaction rates after maximum rate was achieved (practically a plateau in the rate is obtained (Figure 5Ad,Bd,Cd). It has been discussed that these transient features reflect the Ni metal surface's ability to decompose methane over the remaining empty sites with time on stream, leading to carbon structure dependent deposition with different kinetics [18,20].

**Figure 5.** Transient rates (μmol g<sup>−</sup>1s−1) of CH4 consumption (**A**), H2 (**B**), and CO (**C**) formation, as a function of time after the gas switch He → 20 vol% CH4/1% Ar/He (50 NmL min<sup>−</sup>1; GHSV ∼ 30,000 h<sup>−</sup>1) at 750 °C. (**D**) Transient response curves of CO2 concentration obtained during TPO of carbon formed after 30 min of methane decomposition (20% CH4/1% Ar/He) at 750 °C over 5 wt% Ni/CeO2 prepared by (a) Thermal decomposition (TD), (b) Precipitation (PT), (c) Hydrothermal (HT), and (d) Sol-Gel (SG) method. **Figure 5.** Transient rates (µmol g−<sup>1</sup> s −1 ) of CH<sup>4</sup> consumption (**A**), H<sup>2</sup> (**B**), and CO (**C**) formation, as a function of time after the gas switch He → 20 vol% CH<sup>4</sup> /1% Ar/He (50 NmL min−<sup>1</sup> ; GHSV ~30,000 h−<sup>1</sup> ) at 750 ◦C. (**D**) Transient response curves of CO<sup>2</sup> concentration obtained during TPO of carbon formed after 30 min of methane decomposition (20% CH<sup>4</sup> /1% Ar/He) at 750 ◦C over 5 wt% Ni/CeO<sup>2</sup> prepared by (a) Thermal decomposition (TD), (b) Precipitation (PT), (c) Hydrothermal (HT), and (d) Sol-Gel (SG) method.

The carbon accumulated over the ceria-supported Ni catalytic surface can diffuse towards the Ni-support interface, where it was gasified to form CO(g) by the support's labile lattice oxygen, and this chemical process was likely responsible for the delay in the peak maximum, as shown in Figure The carbon accumulated over the ceria-supported Ni catalytic surface can diffuse towards the Ni-support interface, where it was gasified to form CO(g) by the support's labile lattice oxygen, and this chemical process was likely responsible for the delay in the peak maximum, as shown in Figure 5C.

5C. However, lattice oxygen diffusion towards carbon formed on Ni and/or Ni-ceria support interface can also be considered, as discussed in Section 3.2. The H- and C-material balances close within less

Ni/CeO2-SG and Ni/CeO2-TD, ca. 10 and 12.2 mmol g−1, respectively. On the other hand, the amount of CO formed was found to be 0.9 mmol gcat−1 in the cases of Ni/CeO2-PT and Ni/CeO2-HT, but slightly lower in the case of Ni/CeO2-TD and Ni/CeO2-SG, ca. 0.8 and 0.6 mmol gcat−1, respectively. The amount of H2 produced was lower in the case of Ni/CeO2 -HT (14.9 mmol gcat−1) compared to Ni/CeO2-PT (16.4 mmol gcat−1), Ni/CeO2-SG (22 mmol gcat−1), and Ni/CeO2-TD (26.4 mmol gcat−1). Of interest is the amount of labile oxygen of the ceria support contributing to the gasification of carbon towards CO(g), which could be quantified by estimating the ratio between the CO production and CH4 consumption, as However, lattice oxygen diffusion towards carbon formed on Ni and/or Ni-ceria support interface can also be considered, as discussed in Section 3.2. The H- and C-material balances close within less than 5% (Table 1). In particular, the amount of CH<sup>4</sup> decomposed was found to be the same (7.8 mmol g−<sup>1</sup> ) for Ni/CeO2-PT and Ni/CeO2-HT, an amount which increases by about 1.3 and 1.6 times for Ni/CeO2-SG and Ni/CeO2-TD, ca. 10 and 12.2 mmol g−<sup>1</sup> , respectively. On the other hand, the amount of CO formed was found to be 0.9 mmol gcat −1 in the cases of Ni/CeO2-PT and Ni/CeO2-HT, but slightly lower in the case of Ni/CeO2-TD and Ni/CeO2-SG, ca. 0.8 and 0.6 mmol gcat −1 , respectively. The amount of H<sup>2</sup> produced was lower in the case of Ni/CeO<sup>2</sup> -HT (14.9 mmol gcat −1 ) compared to Ni/CeO2-PT (16.4 mmol gcat −1 ), Ni/CeO2-SG (22 mmol gcat −1 ), and Ni/CeO2-TD (26.4 mmol gcat −1 ). Of interest is the amount of labile oxygen of the ceria support contributing to the gasification of carbon towards CO(g), which could be quantified by estimating the ratio between the CO production and CH<sup>4</sup> consumption, as shown in Table 1. This ratio was found to be the same (0.12) for the Ni/CeO2-PT and Ni/CeO2-HT catalysts, but significantly lower in the case of Ni/CeO2-TD (0.07) and Ni/CeO2-SG (0.06), showing clearly the lower contribution of O<sup>L</sup> (active labile oxygen) towards CO(g).

**Table 1.** Quantity of CH<sup>4</sup> consumed, H<sup>2</sup> and CO formed (mmol g−<sup>1</sup> ), and molar ratio of CO/CH<sup>4</sup> obtained after 30 min of methane decomposition (20% CH<sup>4</sup> /He) conducted at 750 ◦C. Also shown is the amount of carbon deposited (mmol g−<sup>1</sup> ), which was obtained after TPO following 30 min of methane decomposition.


Following the 30 min CH<sup>4</sup> decomposition performed at 750 ◦C over the Ni/CeO<sup>2</sup> catalysts, temperature-programmed oxidation was performed to estimate the amount of carbon and its reactivity towards oxygen. The TPO traces in terms of CO<sup>2</sup> concentration (mol%) are depicted in Figure 5D, and the amount of carbon deposited (mmol g−<sup>1</sup> ) is reported in Table 1. The latter results were in harmony with the amount of CH<sup>4</sup> consumed and H<sup>2</sup> produced, as reported above. However, it should be mentioned at this point that these values did not agree with the results regarding the amount of carbon deposited during DRM, where Ni/CeO2-HT was found to accumulate more carbon. As it will be discussed in the following sections, we argue that the carbon formation rate, and that of carbon removal towards CO formation, are largely influenced when both CH<sup>4</sup> and CO<sup>2</sup> (or CO) are present over the ceria-supported Ni catalyst surface to be compared to the case when CH4, CO2, or CO is only present.

#### 2.6.4. Transient Carbon Monoxide Dissociation (CO/He) Reaction

The transient rates of CO(g) consumption obtained during the step-gas switch He → 20 vol% CO/1 vol% Ar/He (750 ◦C, 30 min) over the four catalysts are presented in Figure 6A. It was clearly shown that during the reverse Boudouard reaction, two peak maxima were present, as opposed to the case of CH<sup>4</sup> decomposition reaction (Figure 5A). The first very sharp peak was formed immediately (tmax ~5 s) after the switch from inert He to CO/Ar/He, followed by a fast decay, while the second peak appeared at tmax ~20 s and was followed by a slower rate of CO consumption. Thus, the kinetics involved in both the initial very sharp and the slower transient rates of carbon monoxide dissociation are strongly affected by differences in the four catalytic surfaces.

**Figure 6.** Transient rates (μmol g<sup>−</sup>1s−1) of (**A**) CO consumption as a function of time after the gas switch He → 20 vol% CO/1 vol% Ar/He (50 NmL min<sup>−</sup>1; GHSV ∼ 30,000 h<sup>−</sup>1) at 750 °C. (**B**) Transient response curves of CO2 concentration obtained during TPO of carbon formed after 30 min of CO dissociation at 750 °C over 5 wt% Ni/CeO2 prepared by (a) Thermal decomposition (TD), (b) Precipitation (PT), (c) Hydrothermal (HT), and (d) Sol-Gel (SG) method. **Figure 6.** Transient rates (µmol g−<sup>1</sup> s −1 ) of (**A**) CO consumption as a function of time after the gas switch He <sup>→</sup> 20 vol% CO/1 vol% Ar/He (50 NmL min−<sup>1</sup> ; GHSV ~30,000 h−<sup>1</sup> ) at 750 ◦C. (**B**) Transient response curves of CO<sup>2</sup> concentration obtained during TPO of carbon formed after 30 min of CO dissociation at 750 ◦C over 5 wt% Ni/CeO<sup>2</sup> prepared by (a) Thermal decomposition (TD), (b) Precipitation (PT), (c) Hydrothermal (HT), and (d) Sol-Gel (SG) method.

Figure 6B presents the TPO traces obtained after the reaction with 20 vol% CO/1 vol% Ar/He (30 min) at 750 °C. The four catalytic surfaces showed one main peak centered at ~ 600 °C with shoulders, revealing likely the existence of different carbon structures, oxidized with different kinetics. Table 2 presents the amount of carbon deposited during the 30 min reaction with CO/Ar/He. The largest amount of carbon deposited was found to be on the Ni/CeO2-PT (2.7 mmol g−1) followed by the Ni/CeO2-TD (1.1 mmol g−1), Ni/CeO2-HT (0.8 mmol g−1), and Ni/CeO2-SG (0.6 mmol g−1) catalysts. The C-material balance closes within less than 5% in all cases (Table 2). Figure 6B presents the TPO traces obtained after the reaction with 20 vol% CO/1 vol% Ar/He (30 min) at 750 ◦C. The four catalytic surfaces showed one main peak centered at ~600 ◦C with shoulders, revealing likely the existence of different carbon structures, oxidized with different kinetics. Table 2 presents the amount of carbon deposited during the 30 min reaction with CO/Ar/He. The largest amount of carbon deposited was found to be on the Ni/CeO2-PT (2.7 mmol g−<sup>1</sup> ) followed by the Ni/CeO2-TD (1.1 mmol g−<sup>1</sup> ), Ni/CeO2-HT (0.8 mmol g−<sup>1</sup> ), and Ni/CeO2-SG (0.6 mmol g−<sup>1</sup> ) catalysts. The C-material balance closes within less than 5% in all cases (Table 2).

**Table 2.** Quantity of CO consumption and CO2 formation (mmol g<sup>−</sup>1) obtained after 30 min 20% CO/He at 750 °C. Also shown is the amount of carbon deposition (mmol g<sup>−</sup>1) obtained after TPO following 30 min of CO disproportionation. **Table 2.** Quantity of CO consumption and CO<sup>2</sup> formation (mmol g−<sup>1</sup> ) obtained after 30 min 20% CO/He at 750 ◦C. Also shown is the amount of carbon deposition (mmol g−<sup>1</sup> ) obtained after TPO following 30 min of CO disproportionation.


CeO2 - HT 1.7 0.8 0.8 2.6.5. Isotopically Labelled Competitive (13CO/ <sup>12</sup>CH4) Reaction towards Carbon Formation

CeO2 - SG 1.1 - 0.6 2.6.5. Isotopically Labelled Competitive (13CO/12CH4) Reaction towards Carbon Formation Figure 7 presents CO (C,D) and CO2 (A,B) concentration (mol%) profiles obtained after temperature-programmed oxidation (TPO) following a 20-min treatment of the Ni/CeO2-TD (a), Ni/CeO2-PT (b), Ni/CeO2-HT (c), and Ni/CeO2-SG (d) catalysts with 2.5 vol% 12CH4/2.5 vol% 13CO/2 vol% Kr/Ar/He gas mixture at 750 °C. The amount of deposited carbon, but also its reactivity towards oxygen (shape and position of TPO trace), were clearly different among the four catalysts. It was shown that all catalytic systems exhibited differences in the shape of 12CO2/12CO and 13CO2/13CO response curves as the result of the oxidation of carbon originated from the 12CH4 decomposition and Figure 7 presents CO (C,D) and CO<sup>2</sup> (A,B) concentration (mol%) profiles obtained after temperature-programmed oxidation (TPO) following a 20-min treatment of the Ni/CeO2-TD (a), Ni/CeO2-PT (b), Ni/CeO2-HT (c), and Ni/CeO2-SG (d) catalysts with 2.5 vol% <sup>12</sup>CH4/2.5 vol% <sup>13</sup>CO/2 vol% Kr/Ar/He gas mixture at 750 ◦C. The amount of deposited carbon, but also its reactivity towards oxygen (shape and position of TPO trace), were clearly different among the four catalysts. It was shown that all catalytic systems exhibited differences in the shape of <sup>12</sup>CO2/ <sup>12</sup>CO and <sup>13</sup>CO2/ <sup>13</sup>CO response curves as the result of the oxidation of carbon originated from the <sup>12</sup>CH<sup>4</sup> decomposition and <sup>13</sup>CO dissociation routes, respectively. Moreover, the amount of carbon derived from each route is different (area under the TPO trace), and the contribution of each route to the formation of carbon is estimated by considering the <sup>12</sup>C/ <sup>13</sup>C ratio. It is illustrated that in all cases, except Ni/CeO2-TD (0.97), CH<sup>4</sup> decomposition was dominant but to a different extent. In particular, Ni/CeO2-PT showed a ratio of <sup>12</sup>C/ <sup>13</sup>C = 1.06, Ni/CeO2-HT <sup>12</sup>C/ <sup>13</sup>C = 1.61, and the Ni/CeO2-SG a ratio of <sup>12</sup>C/ <sup>13</sup>C = 1.5 (Table 3).

13CO dissociation routes, respectively. Moreover, the amount of carbon derived from each route is different (area under the TPO trace), and the contribution of each route to the formation of carbon is estimated by considering the 12C/13C ratio. It is illustrated that in all cases, except Ni/CeO2-TD (0.97), CH4 decomposition was dominant but to a different extent. In particular, Ni/CeO2-PT showed a ratio of 12C/13C = 1.06, Ni/CeO2-HT 12C/13C = 1.61, and the Ni/CeO2-SG a ratio of 12C/13C = 1.5 (Table 3).

rates.

**Catalyst 5 wt% Ni/CeO2**

**12CO Production (mmol g−1)** 

**13CO Production (mmol g−1)** 

**Table 3.** Quantity of 12CO, 13CO, 12CO2, and 13CO2 (mmol g−1) formed during TPO following 20 min of reaction with 2.5 vol% 13CO/2.5 vol% 12CH4/2 vol% Kr/Ar/He at 750 °C over all the Ni/CeO2 catalysts. Also shown is the total amount of "carbon" (mmol g<sup>−</sup>1), and the ratio 12C to 13C in the products.

> **12CO2 Production (mmol g−1)**

In addition, the largest amount of carbon was deposited over the Ni/CeO2-HT catalyst, ca. 1.6, 1.5, and 1.3 times higher compared to Ni/CeO2-PT, Ni/CeO2-TD, and Ni/CeO2-SG (8.7 vs. 5.3, 5.6, and 6.5 mmol g−1, respectively). Furthermore, multiple peaks/shoulders appeared in the TPO traces, showing that at least two kinds of carbon were formed after CH4/CO gas treatment of the four catalysts, but to a different extent. More precisely, the Ni/CeO2-HT (c) and Ni/CeO2-SG (d), which revealed the highest amount of carbon deposition during DRM and CH4/CO gas treatments, reveal reaction of carbon with oxygen in the range 300–800 °C as opposed to Ni/CeO2-TD (a) and Ni/CeO2- PT (b), where carbon oxidation occurs in the 500–800 °C range (more strongly bound carbon species but of lower amount). The latter results are in harmony with the results obtained under DRM reaction

CeO2-TD 0.14 0.23 2.6 2.6 0.97 5.6 CeO2-PT 0.14 0.18 2.6 2.4 1.06 5.3 CeO2-HT 0.16 0.23 5.2 3.1 1.61 8.7 CeO2-SG 0.39 0.25 3.5 2.4 1.50 6.5

**13CO2 Production (mmol g−1)** 

**12C/13C** 

**Carbon Deposition (mmol g−1)**

**Figure 7.** Temperature-programmed oxidation (TPO) of carbon to 12C- and 13C-containing CO2 (**A**,**B**) and CO (**C,D**) formed after 20 min in 2.5 vol% 13CO/2.5 vol% 12CH4/2 vol% Kr/Ar/He (50 NmL min−1; GHSV ∼ 30,000 h<sup>−</sup>1) at 750 °C over 5 wt% Ni/CeO2 prepared by (a) Thermal decomposition (TD), (b) Precipitation (PT), (c) Hydrothermal (HT), and (d) Sol-gel (SG) method. **Figure 7.** Temperature-programmed oxidation (TPO) of carbon to <sup>12</sup>C- and <sup>13</sup>C-containing CO<sup>2</sup> (**A**,**B**) and CO (**C,D**) formed after 20 min in 2.5 vol% <sup>13</sup>CO/2.5 vol% <sup>12</sup>CH<sup>4</sup> /2 vol% Kr/Ar/He (50 NmL min−<sup>1</sup> ; GHSV ~30,000 h−<sup>1</sup> ) at 750 ◦C over 5 wt% Ni/CeO<sup>2</sup> prepared by (a) Thermal decomposition (TD), (b) Precipitation (PT), (c) Hydrothermal (HT), and (d) Sol-gel (SG) method.

*2.7. Participation of Support's Lattice Oxygen under DRM Conditions*  **Table 3.** Quantity of <sup>12</sup>CO, <sup>13</sup>CO, <sup>12</sup>CO<sup>2</sup> , and <sup>13</sup>CO<sup>2</sup> (mmol g−<sup>1</sup> ) formed during TPO following 20 min of reaction with 2.5 vol% <sup>13</sup>CO/2.5 vol% <sup>12</sup>CH<sup>4</sup> /2 vol% Kr/Ar/He at 750 ◦C over all the Ni/CeO<sup>2</sup> catalysts. Also shown is the total amount of "carbon" (mmol g−<sup>1</sup> ), and the ratio <sup>12</sup>C to <sup>13</sup>C in the products.


In addition, the largest amount of carbon was deposited over the Ni/CeO2-HT catalyst, ca. 1.6, 1.5, and 1.3 times higher compared to Ni/CeO2-PT, Ni/CeO2-TD, and Ni/CeO2-SG (8.7 vs. 5.3, 5.6, and 6.5 mmol g−<sup>1</sup> , respectively). Furthermore, multiple peaks/shoulders appeared in the TPO traces, showing that at least two kinds of carbon were formed after CH4/CO gas treatment of the four catalysts, but to a different extent. More precisely, the Ni/CeO2-HT (c) and Ni/CeO2-SG (d), which revealed the highest amount of carbon deposition during DRM and CH4/CO gas treatments, reveal reaction of carbon with oxygen in the range 300–800 ◦C as opposed to Ni/CeO2-TD (a) and Ni/CeO2-PT (b), where carbon oxidation occurs in the 500–800 ◦C range (more strongly bound carbon species but of lower amount). The latter results are in harmony with the results obtained under DRM reaction conditions (see Sections 2.6.1 and 2.6.2), and these will be discussed next in relation to the competitive contribution of CH<sup>4</sup> decomposition and CO dissociation towards carbon formation and removal rates.

#### *2.7. Participation of Support's Lattice Oxygen under DRM Conditions Catalysts* **2019**, *9*, x FOR PEER REVIEW 11 of 24

Figure 8A shows the transient evolution rate of <sup>18</sup>O<sup>2</sup> consumption estimated upon the 10 min isotopic exchange of the <sup>16</sup>O ceria lattice oxygen (surface and bulk) with gaseous <sup>18</sup>O2, and that due mainly to the oxidation of Ni to Ni18O (less to the exchange of <sup>16</sup>O with <sup>18</sup>O<sup>2</sup> in Ni16Ox, see Section 4.5) at the step-gas switch Ar <sup>→</sup> 2 vol% <sup>18</sup>O2/2 vol% Kr/Ar at 750 ◦C. It is seen that the ceria-supported Ni catalysts show similar <sup>18</sup>O<sup>2</sup> consumption rates during the 10 min exchange, except the Ni/CeO2-PT, but all four catalysts showed a similar exchangeable amount of <sup>16</sup>O which was found to be between 10.2–12.4 mmol O g−<sup>1</sup> (within less than 20%), as shown in Table S6. It should be noted that the maximum amount of <sup>18</sup>O consumed, and which is related to Ni oxidation, was 0.85 mmol g-1. The latter illustrates that both the initial rates of <sup>16</sup>O/ <sup>18</sup>O exchanged, but also the surface and bulk mobility of <sup>16</sup>O species that were exchanged with <sup>18</sup>O<sup>2</sup> were influenced by the CeO<sup>2</sup> synthesis method and Ni particles size only, to a small extent. Figure 8A shows the transient evolution rate of 18O2 consumption estimated upon the 10 min isotopic exchange of the 16O ceria lattice oxygen (surface and bulk) with gaseous 18O2, and that due mainly to the oxidation of Ni to Ni18O (less to the exchange of 16O with 18O2 in Ni16Ox, see Section 4.5) at the step-gas switch Ar → 2 vol% 18O2/2 vol% Kr/Ar at 750 °C. It is seen that the ceria-supported Ni catalysts show similar 18O2 consumption rates during the 10 min exchange, except the Ni/CeO2-PT, but all four catalysts showed a similar exchangeable amount of 16O which was found to be between 10.2–12.4 mmol O g−1 (within less than 20%), as shown in Table S6. It should be noted that the maximum amount of 18O consumed, and which is related to Ni oxidation, was 0.85 mmol g-1. The latter illustrates that both the initial rates of 16O/18O exchanged, but also the surface and bulk mobility of 16O species that were exchanged with 18O2 were influenced by the CeO2 synthesis method and Ni particles size only, to a small extent.

**Figure 8.** Transient rates (μmol g<sup>−</sup>1s−1) of (**A**) 18O2 consumption during 16O/18O exchange at 750 °C after the gas switch: Ar → 2% 18O2/2% Kr/Ar, (**B**) C18O formation obtained during the switch from Ar → 20% CH4/20% CO2/2% Kr/Ar/He (t) over 5 wt% Ni/CeO2 prepared by (a) Thermal decomposition (TD), (b) Precipitation (PT), (c) Hydrothermal (HT), and (d) Sol-Gel (SG) method. Wcat = 0.02 g. **Figure 8.** Transient rates (µmol g−<sup>1</sup> s −1 ) of (**A**) <sup>18</sup>O<sup>2</sup> consumption during <sup>16</sup>O/ <sup>18</sup>O exchange at 750 ◦C after the gas switch: Ar <sup>→</sup> 2% <sup>18</sup>O<sup>2</sup> /2% Kr/Ar, (**B**) C18O formation obtained during the switch from Ar → 20% CH<sup>4</sup> /20% CO<sup>2</sup> /2% Kr/Ar/He (t) over 5 wt% Ni/CeO<sup>2</sup> prepared by (a) Thermal decomposition (TD), (b) Precipitation (PT), (c) Hydrothermal (HT), and (d) Sol-Gel (SG) method. Wcat = 0.02 g.

Figure 8B presents the transient rates of C18O(g) formation over the four ceria-supported Ni catalysts obtained during the step-gas switch Ar → 20 vol% CH4/20 vol% CO2/2 vol% Kr/Ar at 750 °C, following the 10 min 16O/18O exchange (Figure 8A). It was observed that all four catalysts present similar shapes of the transient rate of C18O(g) formation, however, they differ on their time delays and quantity (area under the transient curve). In particular, the Ni/CeO2-TD and Ni/CeO2-HT exhibit similar time delays (∼ 5 s), followed by the Ni/CeO2-PT (∼10 s) and Ni/CeO2-SG (∼ 15 s). In addition, the amount of C18O(g) produced over the four catalysts, after subtracting the equivalent amount of 18O stored in the Ni during the 18O2 gas treatment (0.85 mmol g−1), was found to be 1.05 times larger for Ni/CeO2-HT (6.5 mmol g−1) compared to Ni/CeO2-TD (5.9 mmol g−1), 1.85 times larger compared to Ni/CeO2-PT (3.5 mmol g−1), and 1.96 times larger compared to Ni/CeO2-SG (3.3 mmol g−1). Considering the ratio of equivalent 18O in C18O (Figure 8B) to 18O exchanged (Figure 8A) as the contribution of the available amount of 18O to the carbon gasification, the highest value was found to result from the Ni/CeO2-HT (0.57), followed by Ni/CeO2-TD (0.48), Ni/CeO2-SG (0.32), and Ni/CeO2- PT (0.31). The latter results (shown also in Table S6) will be discussed in the next section regarding the importance of participation of support's lattice oxygen to the carbon gasification rate. Figure 8B presents the transient rates of C18O(g) formation over the four ceria-supported Ni catalysts obtained during the step-gas switch Ar → 20 vol% CH4/20 vol% CO2/2 vol% Kr/Ar at 750 ◦C, following the 10 min <sup>16</sup>O/ <sup>18</sup>O exchange (Figure 8A). It was observed that all four catalysts present similar shapes of the transient rate of C18O(g) formation, however, they differ on their time delays and quantity (area under the transient curve). In particular, the Ni/CeO2-TD and Ni/CeO2-HT exhibit similar time delays (~5 s), followed by the Ni/CeO2-PT (~10 s) and Ni/CeO2-SG (~15 s). In addition, the amount of C18O(g) produced over the four catalysts, after subtracting the equivalent amount of <sup>18</sup>O stored in the Ni during the <sup>18</sup>O<sup>2</sup> gas treatment (0.85 mmol g−<sup>1</sup> ), was found to be 1.05 times larger for Ni/CeO2-HT (6.5 mmol g−<sup>1</sup> ) compared to Ni/CeO2-TD (5.9 mmol g−<sup>1</sup> ), 1.85 times larger compared to Ni/CeO2-PT (3.5 mmol g−<sup>1</sup> ), and 1.96 times larger compared to Ni/CeO2-SG (3.3 mmol g−<sup>1</sup> ). Considering the ratio of equivalent <sup>18</sup>O in C18O (Figure 8B) to <sup>18</sup>O exchanged (Figure 8A) as the contribution of the available amount of <sup>18</sup>O to the carbon gasification, the highest value was found to result from the Ni/CeO2-HT (0.57), followed by Ni/CeO2-TD (0.48), Ni/CeO2-SG (0.32), and Ni/CeO2-PT (0.31). The latter results (shown also in Table S6) will be discussed in the next section regarding the importance of participation of support's lattice oxygen to the carbon gasification rate.

#### **3. Discussion 3. Discussion**
