Dry Reforming of Methane on Ni/Nanorod-CeO 2 Catalysts Prepared by One-Pot Hydrothermal Synthesis: The Effect of Ni Content on Structure, Activity, and Stability

: The nanorod morphology of the CeO 2 support has been recognized as more beneﬁcial than other morphologies for catalytic activity in the dry reforming of methane. Ni/nanorod-CeO 2 catalysts with different Ni contents were prepared by one-pot hydrothermal synthesis. Samples were characterized by X-ray diffraction (XRD), H 2 -temperature-programmed reduction (H 2 -TPR), H 2 - temperature-programmed desorption (H 2 -TPD), ﬁeld emission scanning electron microscopy/energy dispersive spectroscopy (FE-SEM/EDS), Brunauer–Emmet–Teller (BET) and Barrett–Joyner–Halenda (BHJ) analysis. The effect of Ni content on the size and the intrinsic strain of ceria was analyzed by the Size–Strain plot and Williamson–Hall plot of XRD data. The average Ni particle size and Ni dispersion were determined by H 2 -TPD. XRD and H 2 -TPR analysis revealed a strong Ni–support interaction that limited nickel sintering. The activity for the dry reforming of methane was tested with the stoichiometric mixture CO 2 :CH 4 :N 2 :He = 20:20:20:140, gas hourly space velocity (GHSV) = 300 L g − 1 h − 1 , and temperatures in the range of 545–800 ◦ C. The turnover frequency (TOF) value increased linearly with the average Ni particle size in the range of 5.5–33 nm, suggesting the structure sensitivity of the reaction. Samples with Ni loading of 4–12 wt.% showed high H 2 /CO selectivity and stability over time on stream, whereas the sample with a Ni loading of 2 wt.% was less selective and underwent rapid deactivation. Only a small amount of nanotubular carbon was observed by FE-SEM after the time-on-stream experiment. Deactivation of the low-Ni-content sample is ascribed to the easier oxidation of the small Ni particles.


Introduction
Limiting the ever-increasing concentration of greenhouse gases in the atmosphere is urgent and highly required to avoid global warming effects. The major greenhouse gases released into the atmosphere are water vapor, CO 2 , CH 4 , N 2 O, and ozone (EPA 2016). The dry reforming of methane (DRM) with CO 2 to form syngas (CO 2 + CH 4 → 2H 2 + 2CO; ∆H • = 247 kJ mol −1 ) has emerged as a very attractive process for the mitigation of two of the major contributors to global warming, producing a mixture of CO and H 2 with a H 2 /CO ratio equal to one, highly required for the industrial synthesis of hydrocarbons and oxygenated hydrocarbons using the Fischer-Tropsch process. Critical issues for industrial applications of DRM are the occurrence of several simultaneous reactions: the reverse water-gas shift (RWGS) which consumes H 2 and produces CO, resulting in a H 2 /CO molar ratio less than unity; and CH 4 decomposition and CO disproportionation (Boudouard reaction), both of which produce coke, which may lead to catalyst deactivation or reactor plugging [1][2][3][4][5].
with water until a neutral pH was obtained, and washed with ethanol as the last step. The precipitate was dried in air at 110 • C and then calcined in air at 500 • C (heating ramp 2 • C min −1 ) for 5 h in a muffle furnace (CWF, Carbolite, Hope Sheffield, England).
The one-pot hydrothermal synthesis of Ni/r-CeO 2 catalysts consists of the addition of nickel during the synthesis of nanorod-CeO 2 through the co-precipitation of the nickel and cerium precursors. The synthesis in detail: stoichiometric quantities of cerium (III) nitrate hexahydrate and nickel (II) nitrate hexahydrate, corresponding to 2 wt.%, 4 wt.%, 8 wt.%, and 12 wt.% (Ni mass × 100/catalyst mass), were dissolved in water under stirring. The complete precipitation of the salts was obtained by adding a 12 M NaOH solution drop by drop and stirring the suspension for 30 min. The suspension was then heated up to 110 • C (heating ramp 10 • C min −1 ) in hydrothermal conditions for 24 h, then cooled, filtered, washed, dried, and finally calcined at 500 • C with the same procedure and conditions used for pure r-ceria synthesis, obtaining NiO/r-CeO 2 . For the sake of brevity, the catalysts are indicated with the abbreviation xNiCe, where x is the nominal nickel content in wt.%. H 2 -temperature programmed reduction (H 2 -TPR) and H 2 -temperature programmed desorption (H 2 -TPD) characterizations were performed using a TPDRO1100 (Thermo Fisher Scientific, Waltham, MA, USA) instrument. The sample (0.100 g) was pre-treated in a 5% O 2 /He flow mixture (20 cm 3 min −1 ) at 400 • C for 30 min. The reduction was achieved by flowing a 5% H 2 /Ar (30 cm 3 min −1 ) mixture and raising the temperature from 50 • C up to 800 • C (heating rate of 10 • C min −1 ), maintaining the final temperature for 1 h. After H 2 -TPR at 800 • C, the sample was cooled to 50 • C in a hydrogen flow (30 cm 3 min −1 ) to promote the chemisorption of hydrogen. The sample was then heated in Ar flow (30 cm 3 min −1 , heating rate 20 • C min −1 ) from 50 • C up to 800 • C, obtaining the desorption of the hydrogen chemisorbed on the surface (H 2 -TPD). The H 2 consumption or evolution was measured by a thermal conductivity detector (TCD), which was calibrated by the reduction of a known mass of CuO (purity 99.99%). From the H 2 adsorbed on nickel, we can estimate the particle size, d Ni TPD (nm), and dispersion, D Ni (%), of active nickel by Equations (1) where f = 6, the geometrical factor for a spherical particle; W Ni = wt.% of Ni; ρ Ni = 8.9 g/cm 3 , density of Ni; η = H 2 adsorbed on Ni (mol/g); N A = 6.023 × 10 23 ; S f = 2, stoichiometric factor for the chemisorption of H 2 (Ni mol/H 2 mol); A Ni = 6.51 × 10 −16 cm 2 , surface area occupied by a nickel atom; and M Ni = 58.69 g/mol, atomic mass of Ni. X-ray diffractograms (XRD) of the powders were recorded using a Scintag X1diffractometer (Scintag Inc., San Francisco, CA, USA) equipped with a Cu Kα source (λ = 1.5406 Å) in the Bragg-Brentano configuration θ-θ, with 2θ = 20-80 intervals, 0.05 • increments, and 3 s acquisition time. The NiO and Ni 0 crystallite sizes were estimated from the NiO (200) and Ni (111) reflections by the Scherrer Equation (3), where 0.9 is the dimensionless crystalliteshape factor, λ is the wavelength of the X-ray, β is the full-width half-maximum (FWHM) after subtracting the instrumental broadening, and θ is the Bragg angle.
The size and intrinsic strain of CeO 2 crystallites were calculated by the Williamson-Hall plot (W-H) and the Uniform Deformation Model (UDM), as reported in [24], where the full width at half maximum, β T , of the diffraction peaks depends on the strain-induced broadening ε, according to the following Equation (4): The slope of the linear fitting of the W-H plot provides the intrinsic strain, whereas the intercept (Kλ/d) gives the average size of the nanocrystals.
The size and strain of CeO 2 crystallites were also calculated by the Size-Strain plot (SSP) using Equation (5), where β hkl is the full width at half maximum of the XRD peak fitted as a Lorentzian curve, and d hkl is defined in Equation (6) [24]: The slope of the linear fitting of the SSP provides the average size, whereas the intercept gives the intrinsic strain of the crystallites.
The lattice parameter, a(Å), was calculated by Equation (7): The morphology of calcined samples was examined using a field emission scanning electron microscope (FE-SEM) Zeiss Sigma 300 VP (Carl Zeiss Microscopy, Jena, Germany). The morphology of catalysts after the time on stream (TOS) experiment was investigated using an ultra-high-resolution FE-SEM LEO 1530 (Carl Zeiss Microscopy, Jena, Germany). To carry out the analysis, the sample was dispersed in isopropyl alcohol using an ultrasonic bath and deposited on a graphite film.
The adsorption-desorption isotherms of N 2 were obtained at a temperature of 77 K using a Gemini V (Micromeritics Instrument Corporation, Norcross, GA, USA) apparatus. The surface area (S.A.) was calculated with the Brunauer-Emmett-Teller (BET) method in the equilibrium pressure range of 0.05 < p/p • < 0.5. The pore size distribution (PSD) was obtained from the desorption branch of the isotherm using the Barrett-Joyner-Halenda (BJH) method, and the total pore volume was calculated from the maximum adsorption point at p/p • = 0.99. Before N 2 adsorption, the sample (40-45 mg) was degassed in He flow at 350 • C for 1 h.
The activity for the dry reforming of methane with CO 2 was tested in a fixed-bed quartz reactor (diameter = 6 mm) inserted in a tubular furnace and connected to a flow apparatus at atmospheric pressure with independent mass flow regulators (170A MKS Instruments Inc., Munich, Germany). The temperature of the catalytic bed was measured by a thermocouple in contact with the catalyst. The gas mixture was analyzed by an Agilent 7820 gas chromatograph (Agilent Technologies, Santa Clara, CA, USA) equipped with Molecular Sieve X13 and Hayesep Q (Sigma-Aldrich, Darmstadt, Germany) columns and a TCD detector. The sample (40 mg) was pressed at 4 tons, ground in an agate mortar, and then mixed with 100 mg of silicon carbide. The sample was pre-reduced in situ with H 2 /Ar flow 50 vol.% (100 cm 3 min −1 ), increasing the furnace temperature from 50 • C up to 800 • C at a ramp of 10 • C min −1 and maintaining it at 800 • C for 1 h. The reagent mixture CO 2 :CH 4 :N 2 :He = 20:20:20:140 vol.% (stoichiometric ratio, total flow rate = 200 cm 3 min −1 , gas hourly space velocity (GHSV) = 300 L g −1 h −1 ) was introduced into the reactor at 800 • C. The catalytic test was performed in the temperature range of 800-545 • C, decreasing the temperature by steps of 50 • C. The percentage conversions of CH 4 and CO 2 (X i %) were calculated by Equation (8), and the percentage yields (Y%) were determined by Equations (9) and (10), using N 2 as the internal standard, where %i 0 and %N 2 0 are the inlet concentrations, and %i and %N 2 are the outlet concentrations of the reactants (i = CH 4 or CO 2 ) and of N 2 , respectively.
The H 2 /CO ratio was calculated as the molar ratio. The carbon balance was calculated by Equation (11), where F 0 and F are the total flow inlet and outlet of the reactor, respectively: The reaction rate for CH 4 consumption (-r CH 4 ) was calculated with Equation (12), where F • CH 4 is the inlet flow of CH 4 , and m (g) is the catalyst mass. The apparent activation energy, E a , was estimated by the Arrhenius plot, based on Equation (13), where R is 8.31 JK −1 mol −1 , T (K) is the reaction temperature, and C is the natural logarithm of the pre-exponential factor.
The turnover frequency, TOF (s −1 ), as the number of CH 4 molecules reacted per second per surface nickel site, was calculated using the dispersion value of nickel on the surface obtained from the H 2 -TPD analysis, D Ni , with Equation (14): where M Ni = 58.69 g/mol, the atomic mass of Ni; m is the catalyst mass (g); and W Ni = wt.% of Ni.

Morphology and Chemical Composition of Reduced NiCe Sample
The FE-SEM images of the NiCe catalysts show the formation of a well-developed nanorod morphology ( Figure 1). The range of values of length and width are reported in Table 1. The nanorod dimensions appear to increase with the increase in Ni content; nanorods appear partially attached and fused in the most concentrated sample, 12NiCe.  The chemical composition of the samples was analyzed by energy dispersive X-ray spectrometry (EDS) (Supplementary Materials: Figure S1). The analysis, carried out on a series of points with a diameter of 10 × 10 μm 2 of the powder sample, gives the atomic percentage composition as an average value, which confirms the nominal composition with good approximation ( Table 1). The elemental mapping of samples (Supplementary Materials: Figure S2) shows the nickel element uniformly dispersed on ceria for all samples.

Material Reducibility
Since the active species for the dry reforming reaction is the metallic nickel phase of the reduced samples, we investigated the nickel oxide reduction process of calcined NiO/r-CeO2 samples by temperature-programmed reduction (H2-TPR) experiments.
The reduction profiles of xNiCe catalysts at temperatures up to 800 °C, carried out in the same conditions as the in situ catalytic pre-treatment, are reported in Figure 2, together with the profile of the pure r-CeO2 support reduced at temperatures up to 1010 °C. Pure r-CeO2 shows peaks at 200 °C and 400 °C due to the surface reduction Ce(IV) → Ce(III) and a peak above 550 °C due to bulk ceria reduction. The total consumption of hydrogen for r-CeO2 up to 800 °C corresponds to a reduction of about 23% of Ce(IV) based on the stoichiometry in Equation (15) ( Table 2). The chemical composition of the samples was analyzed by energy dispersive X-ray spectrometry (EDS) (Supplementary Materials: Figure S1). The analysis, carried out on a series of points with a diameter of 10 × 10 µm 2 of the powder sample, gives the atomic percentage composition as an average value, which confirms the nominal composition with good approximation ( Table 1). The elemental mapping of samples (Supplementary Materials: Figure S2) shows the nickel element uniformly dispersed on ceria for all samples.

Material Reducibility
Since the active species for the dry reforming reaction is the metallic nickel phase of the reduced samples, we investigated the nickel oxide reduction process of calcined NiO/r-CeO 2 samples by temperature-programmed reduction (H 2 -TPR) experiments.
The reduction profiles of xNiCe catalysts at temperatures up to 800 • C, carried out in the same conditions as the in situ catalytic pre-treatment, are reported in Figure 2, together with the profile of the pure r-CeO 2 support reduced at temperatures up to 1010 • C. Pure r-CeO 2 shows peaks at 200 • C and 400 • C due to the surface reduction Ce(IV) → Ce(III) and a peak above 550 • C due to bulk ceria reduction. The total consumption of hydrogen for r-CeO 2 up to 800 • C corresponds to a reduction of about 23% of Ce(IV) based on the stoichiometry in Equation (15) ( Table 2).  The H2-TPR profiles of xNiCe samples at temperatures up to 800 °C show several nickel reduction peaks in two temperature ranges: 200-300 °C and 300-450 °C. Peaks at low temperatures are the main fraction of H2 consumption in more diluted samples, and their intensity decreases with the Ni content ( Table 2). These peaks are not observed on the pure r-CeO2 support, suggesting that they are related to the presence of nickel, which may even facilitate the surface reduction of CeO2. For these reasons, they are attributed to the reduction of highly dispersed Ni-O-Ce species and to the surface reduction Ce(IV) → Ce(III). The intense peak in the range of 300-450 °C, the intensity of which increases with Ni content, is assigned to the reduction of NiO dispersed on ceria and its strong interactions with the support. The weak hydrogen consumption in the range of 500-800 °C is due to the reduction of bulk ceria. The profiles and attributions are in agreement with literature data [25].
The theoretical consumption of hydrogen for the total reduction of Ni corresponds to the molar ratio H2/Ni = 1 in Equation (16). The total hydrogen consumption increases with the nickel content (Table 2), reaching a H2/Ni molar ratio slightly greater than 1, except for the most concentrated sample, which showed H2/Ni = 1.
The shape and the temperature of the peaks suggest that NiO on calcined samples was completely reduced below 450 °C; therefore, the excess in H2 consumption is attributed to the reduction of Ce(IV) and permits calculating the percentage of Ce(IV) reduced to Ce(III). Ce(III) corresponds to 21% of cerium for the most diluted 2NiCe sample, very close to the value of pure r-CeO2, and decreases as the nickel content increases, up to a minimum of 3.4% of ceria reduction for the most concentrated 12NiCe sample. These findings are in agreement with the significant surface area decrease with the increase in nickel content (see the following paragraphs) and with a homogeneous nickel dispersion on the surface in all samples. The H 2 -TPR profiles of xNiCe samples at temperatures up to 800 • C show several nickel reduction peaks in two temperature ranges: 200-300 • C and 300-450 • C. Peaks at low temperatures are the main fraction of H 2 consumption in more diluted samples, and their intensity decreases with the Ni content ( Table 2). These peaks are not observed on the pure r-CeO 2 support, suggesting that they are related to the presence of nickel, which may even facilitate the surface reduction of CeO 2 . For these reasons, they are attributed to the reduction of highly dispersed Ni-O-Ce species and to the surface reduction Ce(IV) → Ce(III). The intense peak in the range of 300-450 • C, the intensity of which increases with Ni content, is assigned to the reduction of NiO dispersed on ceria and its strong interactions with the support. The weak hydrogen consumption in the range of 500-800 • C is due to the reduction of bulk ceria. The profiles and attributions are in agreement with literature data [25].
The theoretical consumption of hydrogen for the total reduction of Ni corresponds to the molar ratio H 2 /Ni = 1 in Equation (16). The total hydrogen consumption increases with the nickel content (Table 2), reaching a H 2 /Ni molar ratio slightly greater than 1, except for the most concentrated sample, which showed H 2 /Ni = 1.
The shape and the temperature of the peaks suggest that NiO on calcined samples was completely reduced below 450 • C; therefore, the excess in H 2 consumption is attributed to the reduction of Ce(IV) and permits calculating the percentage of Ce(IV) reduced to Ce(III). Ce(III) corresponds to 21% of cerium for the most diluted 2NiCe sample, very close to the value of pure r-CeO 2 , and decreases as the nickel content increases, up to a minimum of 3.4% of ceria reduction for the most concentrated 12NiCe sample. These findings are in agreement with the significant surface area decrease with the increase in nickel content (see the following paragraphs) and with a homogeneous nickel dispersion on the surface in all samples.

Nickel Dispersion by H 2 -TPD
The amount of H 2 adsorbed on reduced nickel and the temperature of its desorption provide information about particle size and metal dispersion on the support after the reduction treatment, parameters that strongly influence the catalytic activity. Figure 3a shows the H 2 -TPD profiles of the r-CeO 2 support and of NiCe samples reduced at 800 • C. H 2 -TPD of Ni-CeO 2 systems is not common in the literature due to the difficulty of analyzing the profile in the presence of the H 2 spill-over phenomenon on ceria-based catalysts. In Figure 3a, H 2 desorption from the r-CeO 2 bare support starts at 300 • C and produces a uniform peak centered at about 550 • C, which could be attributed to the hydrogen adsorbed on CeO 2 as hydroxyl. H 2 desorption from Ni/r-CeO 2 catalysts appears more complex and moves at the lower temperature range of 75-500 • C. As the nickel content increases, the total hydrogen uptake decreases from 198 to 135 mmol g −1 ( Table 3) and especially decreases the intensity of the peak centered at 400 • C, which, for this reason, may be attributed to H 2 desorption from the support. H 2 desorption at low temperatures (75-250 • C) could be tentatively attributed to hydrogen adsorbed on nickel atoms. To support this hypothesis, we report the H 2 -TPD profile of the 2NiCe sample reduced at temperatures up to 500 • C as a reference, which underwent less pronounced nickel sintering and supports smaller Ni particles (14 nm instead of 21 nm, evaluated by XRD analysis) (Figure 3a, profile 6). The peak position of profile 6, combined with its greater intensity, strongly suggests the attribution of peaks at 75-250 • C to hydrogen adsorbed on Ni sites. With the deconvolution of the H 2 -TPD profiles (Figure 3b), it is possible to measure the area of the α and β components, attributed to H 2 desorbed from nickel species, from which the average particle size of active nickel, d Ni TPD (nm), and the nickel dispersion, D Ni (%), can be estimated based on Equations (1) and (2), assuming Ni:H = 1:1 as chemisorption stoichiometry. Table 3 reports the data and calculated values.
As expected, the nickel particle size increases with the nickel content from 5.5 to 36 nm. In agreement, the corresponding nickel dispersion decreases from 18% to 2.8%. As expected, the nickel particle size increases with the nickel content from 5.5 to 36 nm. In agreement, the corresponding nickel dispersion decreases from 18% to 2.8%.    both methods shows the same trend with the nickel content: dCeO2 increases with the Ni content from 14 nm up to about 25-23 nm (SS plot) and from 16 nm up to 32-40 nm (W-H plot). The lattice strain gives information on the level of stress of materials. The results from the SS and W-H plots agree, both showing that the lattice strain ε increases with the nickel content. In agreement, the lattice parameter slightly decreases from 5.41 to 5.40 with the nickel content. All of the results suggest an intimate connection and a strong interaction between nickel oxide and cerium oxide in calcined samples.  The average crystallite size of CeO 2 , d CeO 2 , and the lattice strain, ε, were calculated from the linear fitting of both the Size-Strain plot (SS plot) (Supplementary Materials: Figure S3) and the Williamson-Hall plot (W-H plot) (Supplementary Materials: Figure S4) and are reported in Table 4, with the R 2 of the straight line used as an evaluation of the quality of the spectrum analysis. The average size calculated from the SSP plot is considered closer to the true value, as it gives more importance to low-angle reflections, where measures of position and broadening are more accurate and precise than those at high diffraction angles. On the contrary, the strain calculated from the W-H plot is considered more accurate than that calculated from the SSP plot, which appears to be more intense [24]. The average crystallite size of CeO 2 on calcined NiO/CeO 2 samples obtained from both methods shows the same trend with the nickel content: d CeO 2 increases with the Ni content from 14 nm up to about 25-23 nm (SS plot) and from 16 nm up to 32-40 nm (W-H plot). The lattice strain gives information on the level of stress of materials. The results from the SS and W-H plots agree, both showing that the lattice strain ε increases with the nickel content. In agreement, the lattice parameter slightly decreases from 5.41 to 5.40 with the nickel content. All of the results suggest an intimate connection and a strong interaction between nickel oxide and cerium oxide in calcined samples.
The CeO 2 crystallites of reduced Ni/CeO 2 samples show marked sintering, about doubling their size up to about 28-45 nm (SS plot) or up to 32-87 nm (W-H plot), dependent on the Ni content. The intrinsic lattice strain ε is lower than the corresponding value of the calcined sample, and the lattice parameter "a" of the more concentrated samples (8-12%) returns to a value of 5.41, characteristic of pure ceria, in accordance with the occurrence of the nickel sintering phenomenon, which, by increasing the separation of the NiO and CeO 2 phases, decreases the Ni-support interaction.
The reduced samples show N 2 adsorption-desorption isotherms classified as type II with a type H1 hysteresis loop, with almost parallel vertical branches, characteristic of uniform agglomerates and narrow distributions of pore sizes, typical of non-porous or macroporous material (Figure 5a). The morphological properties of the reduced samples are shown in Table 5. The specific surface area of nanorod-ceria is 94 m 2 g −1 . The specific area of xNiCe catalysts decreases considerably as a function of the nickel content, from 73 m 2 g −1 of the 2NiCe sample to 28 m 2 g −1 of the 12NiCe sample. To explain the trends of surface area, pore volume, and average diameter, it must be taken into account that these experimental values are a result of the combination of changes in cerium oxide morphology and metallic nickel content. As the nickel content increases, the surface area decreases, because the ceria is increasingly covered by nickel, causing a partial pore occlusion. However, the pore volume decreases with a bell-shaped trend as a function of Ni content, with a maximum of 0.68 cm 3 g −1 for the 4NiCe sample. We know that the nickel particles grow as the nickel content increases, modifying the morphology of ceria nanorods, which increase in dimensions. Small and well-dispersed Ni particles, modifying the particle network, contribute to increasing the overall pore volume; the increase in pore volume is also evident from the higher pore content in Figure 5b. Above a certain nickel content, the coverage of the surface area by nickel, with the occlusion of pores, prevails. The average pore diameter is given by the combination of pore volume and surface area (pore diameter = 4 pore volume/S.A.); consequently, it shows a similar bell-shaped trend.
In agreement, the pore size distributions (PSDs) show a very small fraction of small pores with a diameter of 2-4 nm, assigned to the r-CeO 2 structure, and a large fraction of pores in the range 20-90 nm, assigned to the agglomeration of the material (Figure 5b). In agreement with the attribution, the number of small pores is approximately the same in all samples, while the number of pores in the 20-90 nm range varies greatly, depending on the shape of nanorods.

Catalytic Activity for Dry Reforming of Methane
The conversions of CH 4 and CO 2 (X i ) and the production of H 2 and CO (Y i ), together with the carbon balance, as a function of temperature in the range from 450-525 • C to 800 • C, of the four xNiCe catalysts are shown in Figure 6. The DRM reaction (CO 2 + CH 4 → 2CO + 2H 2 ) is endothermic and therefore favored at high temperatures; consequently, the conversions increase with temperature. The reaction products are only CO and H 2 . As expected, due to the occurrence of the reverse water gas shift (RWGS) reaction, which produces CO and consumes CO 2 and H 2 , the CO 2 conversion is always slightly greater than the CH 4 conversion at all temperatures. The carbon balance is 100 ± 4% at all temperatures, suggesting limited carbon deposition on the surface. In Figure 7, the catalytic properties of samples with different nickel contents are reported for comparison. The CO2 and CH4 conversions increase with nickel content up to Ni 8 wt.%: at 800 °C, XCH4 reaches 60% over the 2NiCe catalyst and 80-82% over the 8NiCe and 12NiCe catalysts (Figure 7a,b).
The simultaneous occurrence of the RWGS reaction results in a selectivity, defined as the H2/CO ratio, less than unity. As expected from the theoretical calculation of the equi- In Figure 7, the catalytic properties of samples with different nickel contents are reported for comparison. The CO 2 and CH 4 conversions increase with nickel content up to Ni 8 wt.%: at 800 • C, X CH 4 reaches 60% over the 2NiCe catalyst and 80-82% over the 8NiCe and 12NiCe catalysts (Figure 7a,b).
The simultaneous occurrence of the RWGS reaction results in a selectivity, defined as the H 2 /CO ratio, less than unity. As expected from the theoretical calculation of the equilibrium conversions of simultaneous DRM and RWGS reactions [1], the selectivity increases with temperature: at 800 • C, over NiCe samples, the selectivity reaches 0.8 and 1.1 as a function of the nickel content. The slight exceeding (10%) of the theoretical value of 1.0 observed at 800 • C over 8NiCe and 12NiCe, where H 2 /CO is 1.09-1.10, is attributed to some uncertainty in the gas chromatographic analysis. The selectivity is greater but approximately equal in the concentrated samples (Ni 8 and 12 wt.%) containing larger and similar Ni crystallite sizes (Figure 7c).  Figure 7d shows the natural logarithm of the reaction rate of CH4 conversion as a function of 1/T. The apparent activation energy, Ea, is calculated from the slope of the straight lines in conditions far from equilibrium, that is, values lower than 15% of conversion (Table 6). Ea appears to be independent of the metal content and, in turn, of the particle size and in agreement with values reported for Ni supported on ceria or other supports [7,26,27].  Figure 7d shows the natural logarithm of the reaction rate of CH 4 conversion as a function of 1/T. The apparent activation energy, Ea, is calculated from the slope of the straight lines in conditions far from equilibrium, that is, values lower than 15% of conversion (Table 6). Ea appears to be independent of the metal content and, in turn, of the particle size and in agreement with values reported for Ni supported on ceria or other supports [7,26,27]. The turnover frequency (TOF) values were calculated from the conversion of CH 4 , since methane is not involved in other side reactions, such as in the case of CO 2 . TOF, namely, the number of CH 4 molecules consumed per second per surface Ni site, was calculated from Equation (14) using the dispersion of nickel on the surface obtained from the H 2 -TPD experiments, D Ni (Table 6).
To evaluate whether the particle size affects the specific activity of the nickel atoms, i.e., whether the reaction is structure sensitive towards particle dimension, the TOF was related to the average particle size of Ni calculated by H 2 -TPD experiments. In Figure 8a,b, CH 4 conversion as a function of nickel content and the corresponding TOF as a function of Ni particle size at 600, 650, and 700 • C are compared. The TOF value increases linearly with the average Ni particle size in the range of 5.5-33 nm, namely, up to nickel content of 8 wt.%, appearing dependent on the nickel particle size (Figure 8b). Over the 12NiCe catalyst, with an average particle size of 36 nm, the TOF values appear to be lower, suggesting that a larger amount of nickel does not increase the number of active sites exposed on the surface.  To evaluate whether the particle size affects the specific activity of the nickel atoms, i.e., whether the reaction is structure sensitive towards particle dimension, the TOF was related to the average particle size of Ni calculated by H2-TPD experiments. In Figure 8a,b, CH4 conversion as a function of nickel content and the corresponding TOF as a function of Ni particle size at 600, 650, and 700 °C are compared. The TOF value increases linearly with the average Ni particle size in the range of 5.5-33 nm, namely, up to nickel content of 8 wt.%, appearing dependent on the nickel particle size (Figure 8b). Over the 12NiCe catalyst, with an average particle size of 36 nm, the TOF values appear to be lower, suggesting that a larger amount of nickel does not increase the number of active sites exposed on the surface.

Catalyst Stability
The catalytic stability of all samples was tested at 800 °C by time on stream (TOS) experiments conducted for 15 h. The CH4 and CO2 conversions as a function of time on stream are reported in Figure 9a,b, respectively. Over samples with nickel content greater than or equal to 4 wt.%, the activity remains constant for 15 h, while over the catalyst with a Ni content of 2 wt.%, the activity gradually decays over time.

Catalyst Stability
The catalytic stability of all samples was tested at 800 • C by time on stream (TOS) experiments conducted for 15 h. The CH 4 and CO 2 conversions as a function of time on stream are reported in Figure 9a,b, respectively. Over samples with nickel content greater than or equal to 4 wt.%, the activity remains constant for 15 h, while over the catalyst with a Ni content of 2 wt.%, the activity gradually decays over time.

FE-SEM Characterization after TOS Experiment
Samples used in the TOS experiments were analyzed by FE-SEM microscopy to check the effect of the operating time (15 h) at an operating temperature of 800 °C on the catalyst morphology and on the formation of carbon residues ( Figure 10).
All samples underwent significant sintering with the destruction of the nanorod morphology and the formation of cuboidal fragments, even with flat surfaces. Only the sample with the highest nickel content (12 wt.%) seems to have partly preserved the nanorod structure of ceria. A limited amount of carbon with a nanotubular structure is present on 2NiCe, 4NiCe, and 8NiCe samples. The 12NiCe sample appears to have a greater amount of carbon nanotubes, in agreement with the higher content of nickel, which has a high affinity for coke formation and contains larger nickel nanoparticles. The mechanism of the DRM over Ni involves the dehydrogenation of CH 4 on Ni 0 sites to generate CH x * and C* reactive carbon species on the surface. Coke can be formed by converting C* atoms into less reactive carbon species, which can encapsulate Ni 0 or form carbon filaments growing underneath the Ni 0 particle and causing the nickel to detach from the support. To check for any carbon formation on the surface, the samples tested in the TOS experiment were analyzed by FE-SEM.

FE-SEM Characterization after TOS Experiment
Samples used in the TOS experiments were analyzed by FE-SEM microscopy to check the effect of the operating time (15 h) at an operating temperature of 800 • C on the catalyst morphology and on the formation of carbon residues ( Figure 10).
All samples underwent significant sintering with the destruction of the nanorod morphology and the formation of cuboidal fragments, even with flat surfaces. Only the sample with the highest nickel content (12 wt.%) seems to have partly preserved the nanorod structure of ceria. A limited amount of carbon with a nanotubular structure is present on 2NiCe, 4NiCe, and 8NiCe samples. The 12NiCe sample appears to have a greater amount of carbon nanotubes, in agreement with the higher content of nickel, which has a high affinity for coke formation and contains larger nickel nanoparticles.

Discussion
All NiO/CeO2 catalysts prepared by one-pot hydrothermal synthesis at 110 °C followed by calcination at 500 °C present a nanorod morphology of CeO2. In calcined samples, the NiO particle size increases with the nickel content. The presence of NiO causes slight stress in the ceria structure, which increases with nickel content and suggests an intimate junction between nickel oxide and cerium oxide, excluding, however, the formation of a solid solution by the value of the lattice parameter. The FE-SEM, EDS, H2-TPR, and XRD characterizations of calcined samples agree in confirming a strong nickel oxidecerium oxide interaction and a homogeneous nickel dispersion.
The reduction treatment at 800 °C causes the sintering of ceria crystallites, which increase by doubling their sizes, with a decrease in lattice stress. This finding is also explained by the occurrence of nickel sintering, which decreases the nickel-support interaction, separating the nickel phase from ceria. The average Ni 0 particle dimension was

Discussion
All NiO/CeO 2 catalysts prepared by one-pot hydrothermal synthesis at 110 • C followed by calcination at 500 • C present a nanorod morphology of CeO 2 . In calcined samples, the NiO particle size increases with the nickel content. The presence of NiO causes slight stress in the ceria structure, which increases with nickel content and suggests an intimate junction between nickel oxide and cerium oxide, excluding, however, the formation of a solid solution by the value of the lattice parameter. The FE-SEM, EDS, H 2 -TPR, and XRD characterizations of calcined samples agree in confirming a strong nickel oxide-cerium oxide interaction and a homogeneous nickel dispersion.
The reduction treatment at 800 • C causes the sintering of ceria crystallites, which increase by doubling their sizes, with a decrease in lattice stress. This finding is also explained by the occurrence of nickel sintering, which decreases the nickel-support interaction, sepa-rating the nickel phase from ceria. The average Ni 0 particle dimension was estimated by XRD analysis and H 2 -TPD experiments. The particle sizes calculated by the two techniques completely agree for the high-Ni-content samples, giving values of 30-33 nm for 8NiCe and 34-36 nm for the 12NiCe sample, whereas they disagree for the low-content samples: 5.5 or 21 nm for Ni 2% and 14 or 30 nm for Ni 4%. Taking into account that the XRD peak of Ni in the most diluted sample is very weak and difficult to analyze, that the quantity of a gas selectively adsorbed on the metal indicates the exposed metal surface area, and that the resulting average dimension of particles is a theoretical calculation based on the assumption that metal particles are spherical with a very narrow size distribution, the differences in the average particle dimension calculated by XRD and TPD could be explained by a larger size distribution, with a fraction of bigger crystallites revealed by XRD; on the contrary, an excellent agreement between the results from the two techniques indicate that the metal particles are less polycrystalline. The average Ni size and the Ni dispersion calculated from the H 2 -TPD experiment appear to be dependent on the nickel content and are strictly proportional to the moles of CH 4 reacted by time and per Ni site (TOF).
The nickel loading causes a decrease in catalyst surface area, affecting the pore volume and diameter and partially occluding pores in the most concentrated samples.
The dry reforming of methane with CO 2 over Ni/r-CeO 2 showed the following relevant catalytic behaviors depending on the nickel content: (1) The activity increases with Ni content up to about Ni 8 wt.%; (2) The turnover frequency (TOF) for CH 4 conversion increases linearly with the average Ni particle size in the range of 5.5-33 nm; (3) The H 2 /CO selectivity increases with nickel content due to the lower amount of produced CO; (4) Only the low-Ni-content sample (Ni 2 wt.%) is deactivated over time on stream.
The commonly accepted mechanism of the DRM reaction over Ni-based catalysts involves the following steps: (i) the dehydrogenation of CH 4 on Ni 0 sites generates hydrogen and CH x * and C* reactive carbon species on the surface; (ii) the adsorbed H species are then recombined in H 2 molecules that desorb; (ii) the activation of CO 2 on Ni 0 or on CeO 2 support, or on interfacial perimeters, produces surface reactive oxygen; and (iii) the reaction of carbon C* with oxygen produces CO.
The linear increase in the TOF value with the average nickel particle size in the range of 5.5-33 nm suggests that the DRM reaction on Ni/r-CeO 2 is structure sensitive to the Ni particle dimension. The best specific activity for the surface is shown by the sample with 8 wt.% of Ni loading with an average particle dimension of 33 nm. The volcanotype behavior of the specific activity regarding Ni particle size has been reported for the CO 2 methanation reaction over Ni/nanorod-CeO 2 [12,28] and for the DRM reaction on Ni/CeO 2 [29] and is attributed to competitive structural features and, in particular, to the different distributions of more active under-coordinated Ni sites on the surface [28,29]. The lower specific activity of the most concentrated sample, 12NiCe, with an average particle size of about 36 nm, may depend on several structural aspects: (i) the formation of larger Ni particles may not correspond to a longer Ni-CeO 2 perimeter and thus may not correspond to a greater interfacial area; (ii) the increase in particle dimension may correspond to a decrease in the more active sites as step-edge and corner sites, with an increase in the less active terrace sites [29]; and (iii) the high nickel loading causes a sharp decrease in 12NiCe surface area, which may hinder the CO 2 activation occurring on the ceria support.
The lower H 2 /CO selectivity of the samples with low Ni content is explained by the high activity of smaller Ni particles for the RWGS reaction [30], CO 2 + H 2 → CO + H 2 O, producing CO, consuming H 2 , and consequently, lowering H 2 /CO selectivity.
After 15 h of the TOS experiment, only a limited amount of carbon with a nanotubular structure is present on all samples, in agreement with the carbon balance between reagents and products being very close to 100%. The most concentrated sample (Ni 12 wt.%) appears to have a greater amount of carbon nanotubes, in agreement with the high affinity of Ni for coke formation. The reduction treatment and 15 h of the TOS test cause the partial destruction of the nanorod morphology with the formation of cuboidal fragments, even with flat surfaces. Due to the absence of large amounts of carbon nanotubes or other carbon deposits, the deactivation of the 2NiCe sample can be explained by the progressive oxidation of small Ni 0 particles that strongly interact with the support.

Conclusions
One-pot hydrothermal synthesis allows the preparation of Ni/CeO 2 catalysts with nanorod morphology, strong nickel-support interaction, and homogeneous nickel dispersion.
The catalytic activity for the dry reforming of methane increases with the nickel content up to Ni 8 wt%. The TOF increases linearly with the average nickel particle size in the range of 5.5-33 nm, suggesting that the DRM reaction is structure sensitive over Ni/rCeO 2 catalysts. The sample with Ni loading of 8 wt.% and an average Ni particle size of 33 nm shows the best specific activity for the surface Ni site.
Samples with Ni content of 4-12 wt.% show high H 2 /CO selectivity and high stability over time on stream at 800 • C due to poor carbon deposition.
Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/reactions3030025/s1. Figure S1: EDS spectra of samples; Figure S2: EDS elementary mapping of cerium and nickel of xNiCe samples; Figure S3: Size-Stain plot of calcined and reduced xNiCe samples; Figure S4: H-W plot of calcined and reduced xNiCe samples.

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