*3.1. Structural Properties and Catalytic Performance of Ni Supported on CeO<sup>2</sup> Solids*

*3.1. Structural Properties and Catalytic Performance of Ni Supported on CeO2 Solids*  The 5 wt% Ni supported on CeO2 carriers prepared by different preparation methods exhibited largely different catalytic activity under DRM (20 vol% CO2/20 vol% CH4/He, 750 °C) (Figure 2) and structural and morphological differences were apparent among them. The high conversions achieved The 5 wt% Ni supported on CeO<sup>2</sup> carriers prepared by different preparation methods exhibited largely different catalytic activity under DRM (20 vol% CO2/20 vol% CH4/He, 750 ◦C) (Figure 2) and structural and morphological differences were apparent among them. The high conversions achieved

in our work, are close to the calculated equilibrium values for the used feed gas composition at 1 atm total pressure [52]. The 5 wt% Ni/CeO2-HT catalyst with the largest surface area (~ 50 m2 g−1) consisted

in our work, are close to the calculated equilibrium values for the used feed gas composition at 1 atm total pressure [52]. The 5 wt% Ni/CeO2-HT catalyst with the largest surface area (~50 m<sup>2</sup> g −1 ) consisted of smaller ceria mean primary crystallite size (~11.5 nm), smaller Ni mean particle size (~8.4 nm), and 11.5% Ni dispersion (Table S1, Figure 9). On the other hand, the 5 wt% Ni/CeO2-SG with smaller surface area (~14.5 m<sup>2</sup> g −1 ), consisted of larger ceria primary crystallite size (~43.1 nm) and Ni mean particle size (~20.8 nm; Ni dispersion 4.7%). The latter results are in good agreement with the literature [17,19,20,44,46,53–56]. The structural heterogeneity of the CeO<sup>2</sup> surface had strong effect on the deposition of Ni species. In fact, it was reported [45] that NiO (10 wt%) deposited on ceria nanoparticles of cubic shape was homogeneously dispersed. Yahi et al. [43] used three different preparation methods (microemulsion, sol-gel, and autocombustion) to synthesize CeO2, on which 15 wt% of Ni was deposited. They clearly showed, via XRD and TPR studies, that NiO could be present with good crystallization in different phases (i.e., monoclinic and cubic phase for the auto-combustion and sol-gel, and cubic only phase for the microemulsion), which depended on the different preparation method of the ceria support. The authors [43] also reported different pore volume, surface area, and particle size by changing the preparation method, results of which are in good agreement with the present work. Xu et al. [55] prepared three Ni/Al2O<sup>3</sup> catalytic systems of the same nominal composition by varying the preparation method, namely: Impregnation, water-in-oil-microemulsion, and sol-gel. By using XRD, TEM, and TPR, they showed crystalline structural differences, both for the support and Ni, which led to similar catalytic performance, but differences in the coking resistant, and thus in catalyst stability, as seen also in the present work. They argued that the latter differences might be due to strong metal–support interactions leading to differences on Ni particles size and dispersion. The latter results are in good agreement with those reported by other research groups [44,45,57], where a non-conventional synthesis method, namely the precipitation ionic exchange, led to cubic phases of CeO<sup>2</sup> and NiO (verified via FEG-SEM and XRD). In a recent study, Lykaki et al. [28] showed that the hydrothermal method (among other research works which are well reported there-in) led to well defined ceria nanorods of high specific surface area and with improved redox properties. *Catalysts* **2019**, *9*, x FOR PEER REVIEW 13 of 24 A possible explanation on the above results regarding carbon deposition on Ni/CeO2 as influenced differently by the DRM, CO/He, and CH4/He reaction conditions seems to be the competition of CH4 and CO for the same Ni catalytic active sites, even though under DRM higher energy barriers are needed during the first and second steps of CH4 dissociation (CH4 → CH3 + H and CH3 → CH2 + H), and carbon dimer formation (C2, carbon dimer is the first crucial step for inactive carbon formation). It was reported that both CH4 and CO preferentially dissociate on Ni(111) surface, with the former to also favorably dissociate on Ni(100) and Ni(110) surfaces to a similar extent [66– 74]. The main focus of this work was the effect of preparation method of CeO2 used as support of Ni (5 wt% Ni/CeO2) on the carbon deposition rates of the main routes (CH4 decomposition and reverse Boudouard reaction), and on carbon removal rate (participation of support's OL). It was shown that the preparation method influenced the Ni particle size and its morphology, and in turn its surface electronic structure, thus its catalytic performance for the DRM reaction at 750 °C. The various temperature-programmed and step-gas concentration transient experiments (including the use of isotopes) provided important information for the better understanding of the carbon pathways during DRM, to be discussed next.

**Figure 9.** Comparative graph of the crystallite size (dCeO2 and dNi, nm) and amount of carbon deposition (mg C g<sup>−</sup>1) after 12 h in DRM as a function of ceria support synthesis method. **Figure 9.** Comparative graph of the crystallite size (dCeO2 and dNi, nm) and amount of carbon deposition (mg C g−<sup>1</sup> ) after 12 h in DRM as a function of ceria support synthesis method.

*3.2. Rates of Carbon Deposition and Removal under DRM Reaction Conditions*  As mentioned in the Introduction section, supported Ni catalysts suffer from large amounts of carbon deposition under DRM reaction conditions. The amount of carbon deposited over the catalytic surface should be considered as the net rate between carbon formation (CH4 decomposition and Boudouard reaction) and the carbon removal (e.g., participation of support's lattice oxygen). The carbon removal rate via the participation of lattice oxygen was probed by the transient response curves depicted in Figure 8B after partially exchanging active ceria support lattice 16OL with 18OL. The carbon removal rate by this chemical step can be written to a first approximation as shown by the following Equation (1): The H2-TPD traces (Figure 1) obtained over the four catalytic systems suggest large differences in the electronic structure of the Ni supported metal surfaces (distribution of binding strength between H and surface Ni atoms (ENi-H, kcal mol−<sup>1</sup> )). The electronic structure of Ni surface (different faces and defects), as well shown in the literature, influenced the rates of carbon formation and diffusion on the Ni surface and towards the support during both CH<sup>4</sup> decomposition and Boudouard reaction [58–65]. The morphological differences presented in the CeO<sup>2</sup> carrier through SEM images (Figure S3) seem to play a role for the induced differences in the electronic structure of the Ni surface (Figure 1), as reported also in previous publications [27,46]. HR-TEM (Figure S2) also suggested different morphologies for the Ni nanoparticles for the given Ni/CeO2-HT catalyst, where ceria preparation method also influenced the mean Ni particle size as illustrated in Figure 9. The amount of carbon measured after

For reaction step (2), s is a catalytic site at the metal-support interface, the support or both, and

where k is an effective rate constant for the reaction step (2), θOL is the surface coverage of support lattice oxygen able to participate in reaction step (2), and θC is the surface coverage of carbon formed

of carbon, whilst θOL (t) is also determined by the rate of surface OL diffusion towards carbon. These two important kinetic parameters describing the rate of carbon gasification via Eq. (1) are likely to

depend on the Ni particle size/morphology as well as CeO2 primary particle size.

VO is a surface oxygen vacancy of ceria support.

RC18O (t) = k θOL(t) θC(t) (1)

C-s + 18O-L → C18O(g) + s + Vo (2)

12 h of DRM (Figure 9, Table S5), and the transient results of CH<sup>4</sup> and CO decomposition reactions (Figures 5–8), tend therefore to suggest that the different morphology of ceria, as the result of different preparation method applied, induced Ni morphological/surface structural differences, thus surface nickel electronic modifications. These in turn govern the DRM activity behavior and carbon deposition rate of the various ceria-supported Ni catalysts [58].

Based on the CH4-activity results reported in Figure 2 and the Ni dispersion values over the same catalysts (Table S1), it appears that Ni/CeO2-HT contained a smaller number of active sites than Ni/CeO2-SG catalyst. On the other hand, the former catalyst produced more carbon (larger carbon accumulation rates) under DRM reaction conditions (Figures 3 and 9), whereas the opposite was seen under transient methane decomposition reaction conditions (Figure 5); the initial rates of CH<sup>4</sup> dissociation to H<sup>2</sup> and the deposited amount of carbon (Figure 5D) were higher on Ni/CeO2-HT than Ni/CeO2-SG catalysts (Table 1). As illustrated in Figure 9, there seemed to be no clear relationship between dCeO2 and dNi and the amount of carbon deposition for at least 12 h after DRM. As is discussed below, the carbon formation and removal rates during DRM cannot be influenced only by the Ni metal and ceria support particle size in a clear monotonic way. The Ni-C strength, diffusion of carbon species on the Ni surface, and oxygen diffusion from the ceria support that the Ni phase and Ni-ceria interface (where carbon is formed) should all influence carbon deposition rate [46].

A possible explanation on the above results regarding carbon deposition on Ni/CeO<sup>2</sup> as influenced differently by the DRM, CO/He, and CH4/He reaction conditions seems to be the competition of CH<sup>4</sup> and CO for the same Ni catalytic active sites, even though under DRM higher energy barriers are needed during the first and second steps of CH<sup>4</sup> dissociation (CH<sup>4</sup> → CH<sup>3</sup> + H and CH<sup>3</sup> → CH<sup>2</sup> + H), and carbon dimer formation (C2, carbon dimer is the first crucial step for inactive carbon formation). It was reported that both CH<sup>4</sup> and CO preferentially dissociate on Ni(111) surface, with the former to also favorably dissociate on Ni(100) and Ni(110) surfaces to a similar extent [66–74].

The main focus of this work was the effect of preparation method of CeO<sup>2</sup> used as support of Ni (5 wt% Ni/CeO2) on the carbon deposition rates of the main routes (CH<sup>4</sup> decomposition and reverse Boudouard reaction), and on carbon removal rate (participation of support's OL). It was shown that the preparation method influenced the Ni particle size and its morphology, and in turn its surface electronic structure, thus its catalytic performance for the DRM reaction at 750 ◦C. The various temperature-programmed and step-gas concentration transient experiments (including the use of isotopes) provided important information for the better understanding of the carbon pathways during DRM, to be discussed next.

#### *3.2. Rates of Carbon Deposition and Removal under DRM Reaction Conditions*

As mentioned in the Introduction section, supported Ni catalysts suffer from large amounts of carbon deposition under DRM reaction conditions. The amount of carbon deposited over the catalytic surface should be considered as the net rate between carbon formation (CH<sup>4</sup> decomposition and Boudouard reaction) and the carbon removal (e.g., participation of support's lattice oxygen). The carbon removal rate via the participation of lattice oxygen was probed by the transient response curves depicted in Figure 8B after partially exchanging active ceria support lattice <sup>16</sup>O<sup>L</sup> with <sup>18</sup>OL. The carbon removal rate by this chemical step can be written to a first approximation as shown by the following Equation (1):

$$\mathbf{R\_{C18O}}\ (\mathbf{t}) = \mathbf{k}\ \boldsymbol{\theta}\_{\text{OL}}\ (\mathbf{t})\ \boldsymbol{\theta}\_{\text{C}}\ (\mathbf{t})\tag{1}$$

where k is an effective rate constant for the reaction step (2), θOL is the surface coverage of support lattice oxygen able to participate in reaction step (2), and θ<sup>C</sup> is the surface coverage of carbon formed during DRM. In Equation (1), k might be considered as an average reactivity of more than one kind of carbon, whilst θOL (t) is also determined by the rate of surface O<sup>L</sup> diffusion towards carbon. These two important kinetic parameters describing the rate of carbon gasification via Equation (1) are likely to depend on the Ni particle size/morphology as well as CeO<sup>2</sup> primary particle size.

$$\rm{C-s} + \rm{^{18}O-}\_{\rm{L}} \rightarrow \rm{C}^{18}O(g) + s + \rm{Vo} \tag{2}$$

For reaction step (2), s is a catalytic site at the metal-support interface, the support or both, and V<sup>O</sup> is a surface oxygen vacancy of ceria support.

Initial carbon formation rates (recorded over a clean catalyst surface) and total amount of carbon accumulated during 30 min treatment of the catalysts were measured by performing transient experiments at 750 ◦C with 20 vol% of CH<sup>4</sup> reactant in the feed (Figure 5), similar to DRM conditions, and by the reverse Boudouard reaction or the CO dissociation alone (Figure 6), using 20 vol% CO (similar composition obtained in the DRM depicted in Figure 2). In addition, the individual amount of carbon derived from each route (CH<sup>4</sup> vs. CO) when both gases were present in the feed stream was also estimated for probing any synergy effects on the accumulation of carbon (Figure 7).

It is clearly shown that both the initial rate of carbon formation (Figures 5A and 6A) and the total amount of carbon formed (Table S5) over the four catalytic surfaces was at least 10 times larger in the case of CH<sup>4</sup> decomposition compared to the reverse Boudouard reaction. This result is in very good agreement with the TPO results obtained following the isotopic DRM reaction (13CO2/ <sup>12</sup>CH4/He, Figure 4) and the isotopic <sup>13</sup>CO/ <sup>12</sup>CH4/He experiment (Figure 7), which both quantified the origin of carbon accumulation. Thus, the first conclusion is that CH<sup>4</sup> activation route was dominant and the one controlling the rate of carbon formation, however, the competition of CH<sup>4</sup> and CO activation for same catalytic sites, as clearly demonstrated, should be highly considered. In particular, the Ni/CeO2-HT catalyst (CeO<sup>2</sup> prepared by the hydrothermal method) led to a smaller (~1.8 times) initial rate of carbon formation via CH<sup>4</sup> decomposition (Figure 5A) and CO dissociation (~3.5 times, Figure 6A) compared to the Ni/CeO2-TD and Ni/CeO2-PT catalysts, respectively. At this point it would be of interest to mention the effect of DRM reaction temperature on the origin of carbon deposition (CH<sup>4</sup> vs. CO<sup>2</sup> activation route). Vasiliades et al. [17,20] reported similar <sup>12</sup>CH4/ <sup>13</sup>CO2/He isotopic DRM experiments as those reported in Section 2.6.2 (Figure 4) at 550 and 750 ◦C over 5 wt% Ni/Ce1−xMxO<sup>2</sup> (M <sup>=</sup> Zr4+, Pr3+) catalysts, the support of which (including pure CeO2) was prepared by the citrate sol-gel method. It was illustrated that at the low-T of 550 ◦C, a higher contribution to carbon deposition was obtained via the CO<sup>2</sup> activation route (reverse Boudouard reaction: 2 CO → CO<sup>2</sup> + C) as opposed to the reaction T of 750 ◦C.

A careful comparison could be also made on the transient rates of CO formation during the CH4/He treatment (Figure 5C), where the Ni/CeO2-HT catalyst revealed significantly larger initial rate (~1.5 times) of its labile oxygen towards carbon gasification to CO(g) compared to the Ni/CeO2-PT and Ni/CeO2-TD catalysts, and even larger (~3 times) in the case of Ni/CeO2-SG catalyst. The latter results are in a good agreement with the experimental findings shown in Figure 8B, where gasification of the formed carbon towards C18O(g) formation under DRM reaction conditions takes place by the participation of support's <sup>18</sup>OL. The amount of available labile oxygen for <sup>16</sup>O/ <sup>18</sup>O isotopic exchange was found to be similar for the four supported Ni catalysts, a fact that suggests that morphological differences in their metal and support do not influence this specific process at 750 ◦C.

Considering the transient rates of C18O(g) obtained over the four catalytic systems (Figure 8B), it was apparent that Ni/CeO2-HT catalyst had activated a higher amount of lattice <sup>18</sup>O (11.4 mmol g−<sup>1</sup> ) by a factor of ~1.1 compared to Ni/CeO2-SG (10.2 mmol g−<sup>1</sup> , Table S6). Given the fact that the amount of carbon accumulated during DRM after 12 h was ~1.5 times larger in the case of Ni/CeO2-HT compared to Ni/CeO2-SG (see Section 2.6.1), it might be suggested that the effective rate constant k (Equation (1)) must be considered larger in the former than the latter catalyst. This result is important since it can prove that during DRM, the rate of carbon deposition on Ni/CeO2-HT must be considered larger than on Ni/CeO2-SG, a result in harmony with the transient CH<sup>4</sup> decomposition studies described in Section 2.6.3 (Figure 5A). Moreover, considering the integral rates of CH<sup>4</sup> conversion reported in Figure 2A, the carbon deposited by CH<sup>4</sup> during the 2 min transient shown in Figure 8B (end of rate of carbon removal by <sup>18</sup>O lattice oxygen) could be estimated. Then, the ratio of the amount of carbon removed by <sup>18</sup>O lattice oxygen as C18O (see Figure 8B, Table S6) to the amount of carbon deposited via

CH<sup>4</sup> decomposition could be estimated. This ratio was found to follow the order: CeO2-HT > CeO2-TD > CeO2-SG > CeO2-PT. The implication of this is that the reason that Ni/CeO2-HT experienced the largest amount of carbon accumulation after 12 h in DRM (see Table S5), ~3.8 times larger than that of Ni/CeO2-PT, should not be considered to be due to its inferior ability compared to the other ceria supports to provide mobile lattice oxygen for carbon gasification, at least for the first 30 min of TOS. It was suggested that carbon deposition and removal rates could change with longer time-on-stream as Ni surface and ceria support start to accommodate carbon deposits. Thus, deep understanding of the carbon accumulation with TOS and the intrinsic reasons for this is required for the given DRM ceria-supported Ni catalytic system.

It is noteworthy to be mentioned at this point that the differences in the delays of C18O(g) that appeared during the switch from the inert gas to the DRM feed gas among the different catalysts (Figure 8B) were due to the different transient kinetics of reduction of the initially oxidized Ni surface (after <sup>16</sup>O/ <sup>18</sup>O exchange), as previously reported [46,75].

The temperature-programmed oxidation profiles of the carbon accumulation over a reduced metal surface after CH<sup>4</sup> decomposition or CO disproportionation alone or in the presence of both carbon sources illustrated that the co-presence of CH<sup>4</sup> and CO largely enhances the rate of carbon deposition.

#### **4. Materials and Methods**

#### *4.1. Catalysts Synthesis*

### 4.1.1. Cerium Dioxide (CeO2) Supports

#### Sol-Gel Method

The CeO2-SG metal oxide support was prepared using the modified citrate sol-gel method. The Ce metal precursor of Ce(NO3)3.6H2O (Sigma Aldrich, > 99% purity) was diluted in a beaker containing 100 mL solution of 1:1 (*v*/*v*) ratio of deionized H2O and propanol-1. Citric acid (CA) was added for the creation of 1:1.5 Mtot:CA, where Mtot refers to the total molar concentration of metal ions in the solution, and similarly, the CA (molar concentration of citric acid). The pH of the solution was continuously adjusted (pH ~2.0) by adding HNO<sup>3</sup> (5M), with the solution to be under stirring at 70 ◦C. The resulting gel-like yellowish material was dried at 120 ◦C for 12 h, prior to its thermal heating with 1 ◦C min−<sup>1</sup> under static air from room T to 500 ◦C. The sample was then kept at 500 ◦C for 6 h and its temperature was further increased to 750 ◦C (β = 5 ◦C min−<sup>1</sup> ) and kept for additional 4 h before cooled down to room T.

#### Thermal Decomposition Method

The CeO2-TD metal oxide support was prepared using the thermal decomposition method. An appropriate amount of Ce(NO3)3.6H2O was dried in static air at 120 ◦C for 12 h, and after being cooled down to room T, its temperature was increased with 1 ◦C min−<sup>1</sup> to 500 ◦C, where it was kept for 6 h. The temperature of the resulting material was then further increased to 750 ◦C (β = 5 ◦C min−<sup>1</sup> ), where it was kept for additional 4 h before cooled down to room T.

#### Hydrothermal Method

The CeO2-HT metal oxide support was prepared using the hydrothermal method. During this method, 40 M NaOH (pH ~12.5) and 0.13 M Ce(NO3)3·6H2O aqueous solutions were mixed (75 mL:175 mL), under vigorous stirring until a purplish milky slurry was formed. The milky slurry with total volume of 250 mL was kept under continuous stirring for 1 h and then transferred in a 1 L Teflon bottle and heated for 48 h at 90 ◦C. The reaction product was then cooled down to room temperature and the solid product was collected by filtration. The collected solid was rinsed with deionized water until pH neutralization to remove any co-precipitate salts. Drying and calcination procedures were performed as described in the thermal decomposition method.

#### Precipitation Method

The CeO2-PT metal oxide support was prepared using the precipitation method. In the latter method, ammonia solution (25% *v*/*v*) as precipitation agent was added dropwise at room temperature and under continuous stirring in a 0.5 M aqueous solution of Ce(NO3)3·6H2O until pH reached the value of 10, conditions that were controlled for 3 h. The resulting solution was then filtered, and the precipitate material was dried and calcined as described in the thermal decomposition method.

### 4.1.2. Wetness Impregnation of CeO<sup>2</sup> Supports with Ni (5 wt% Ni/CeO2)

The resulting CeO<sup>2</sup> supports from the various synthesis procedures were grinded prior to Ni metal deposition. A given amount of each of the oxidic ceria support was diluted in an aqueous solution of Ni(NO3)2·6H2O (Sigma-Aldrich, >99% purity) so as to be impregnated with 5 wt% Ni nominal loading. The resulting slurry was dried overnight at 120 ◦C, followed by cooling to room T. The temperature of the solid was then increased under static air to 750 ◦C, where it was kept for 4 h. The resulting material was named "fresh catalyst", and prior to any catalytic measurements it was in situ reduced in pure H<sup>2</sup> gas (1 bar, 50 NmL min−<sup>1</sup> ) at 700 ◦C for 2 h.

#### *4.2. Catalysts Characterization*

#### 4.2.1. Powder X-ray Diffraction (XRD)

Powder X-ray diffractograms of the calcined CeO2-supported Ni catalysts were recorded by using a Shimadzu 6000 Series diffractometer (CuKa radiation, λ = 0.15418 nm, Kyoto, Japan) in the 20–80◦ 2θ range (2◦ min−<sup>1</sup> , 0.02◦ increment). By using the Scherrer equation [17], the lattice parameter (α, Å), the mean primary crystallite size (dc, nm) of the ceria pseudo-cubic structure, and the mean crystal size of NiO were estimated. The latter was used to estimate the Ni mean particle size (dNi, nm) as of Equation (3), after the assumption that Ni and NiO preserve the same particle geometrical shape:

$$\text{d (Ni, nm)} = \text{d (NiO, nm)} \times 0.847.\tag{3}$$

#### 4.2.2. Surface Texture (BET/BJH)

The BET specific surface area (SSA, m<sup>2</sup> g −1 ), the total pore volume (Vp, cm<sup>3</sup> g −1 ), and the mean pore size (dp, nm) of the CeO2-supported nickel catalysts and their supports alone were determined based on N<sup>2</sup> adsorption/desorption isotherms measured at 77 K with a Micromeritics Gemini 2360 surface area and pore size analyzer (Norcross, Georgia, United States). Prior to any measurements, the sample was degassed in N<sup>2</sup> gas flow at 300 ◦C for 4 h.
