**3. Results and Discussion**

### *3.1. Catalysts Characterization*

The specific surface area is one of the most important parameters in the design of a heterogeneous catalyst: a high surface area greatly improves the dispersion of the active phase [29,30]. Figure 2 shows N2-physisorption isotherms of the samples, while the calculated values of specific surface area, mean pore radius and pore volume are reported in Table 1. *Processes* **2021**, *9*, x FOR PEER REVIEW 5 of 14

> NiCe P 64 7.0 0.11 27 NiLaCe P 71 6.8 0.12 25 NiCe M 48 10.5 0.12 15 NiLaCe M 42 12.0 0.16 14 (a) Specific surface area calculated via BET; (b) average pore diameter, and (c) pore volume calcu-

All samples exhibited the IV-type isotherm that is typical of mesoporous materials according to IUPAC classification [31]. However, the shapes and hysteresis loops of the isotherms were quite different. The hysteresis loop of precipitated samples is H2 type, characteristic of solids with pores of irregular shape and dimension. Conversely, the catalysts obtained by the microemulsion approach show a hysteresis profile more difficult to classify being a combination of H2 and H3 hysteresis loops associated with a complex pores structure. H3 loops are, generally, associated with non-rigid aggregates of plate-like particles (e.g., certain clays) or with a pore network consisting of macropores that are not completely filled with pore condensate [32]. The pore size distribution obtained by BJH is consistent with the N2 adsorption-desorption profiles, in fact the pore size distribution for the "M" type catalysts was broader and at higher values, at the limit of macroporosity, than for LaCe P and NiLaCe P samples. Moreover, both precipitated samples exhibit a higher BET surface area than the catalysts obtained by the microemulsion approach (Table

Analytical Ni amount was the same for all the samples (7.5 wt %), only slightly lower than the nominal value of 8 wt %. As for La amount, it was around 5 wt %. The particles morphology and catalyst size were determined using microscopy techniques. Figure 3 shows SEM images of the fresh samples. As can be seen, the appearance of the materials prepared by different techniques was notably different. Samples synthesized by precipitation were made of agglomerated spherical particles of 1.8–2 μm, while catalysts prepared by microemulsion presented a wrinkled surface covered by small superficial cubic

**Figure 2.** N2-physisorption isotherms of catalysts (**a**) and their pore size distributions (**b**): NiCe P (red squares); NiCe M **Figure 2.** N<sup>2</sup> -physisorption isotherms of catalysts (**a**) and their pore size distributions (**b**): NiCe P (red squares); NiCe M (green cross); NiLaCe P (blue rhombs); NiLaCe M (violet circle).

lated via BJH method; (d) calculated by Scherrer Equation.

**Table 1.** Physico-chemical properties of the catalysts.


**Table 1.** Physico-chemical properties of the catalysts.

<sup>a</sup> Specific surface area calculated via BET; <sup>b</sup> average pore diameter, and <sup>c</sup> pore volume calculated via BJH method; d calculated by Scherrer Equation.

All samples exhibited the IV-type isotherm that is typical of mesoporous materials according to IUPAC classification [31]. However, the shapes and hysteresis loops of the isotherms were quite different. The hysteresis loop of precipitated samples is H2 type, characteristic of solids with pores of irregular shape and dimension. Conversely, the catalysts obtained by the microemulsion approach show a hysteresis profile more difficult to classify being a combination of H2 and H3 hysteresis loops associated with a complex pores structure. H3 loops are, generally, associated with non-rigid aggregates of plate-like particles (e.g., certain clays) or with a pore network consisting of macropores that are not completely filled with pore condensate [32]. The pore size distribution obtained by BJH is consistent with the N<sup>2</sup> adsorption-desorption profiles, in fact the pore size distribution for the "M" type catalysts was broader and at higher values, at the limit of macroporosity, than for LaCe P and NiLaCe P samples. Moreover, both precipitated samples exhibit a higher BET surface area than the catalysts obtained by the microemulsion approach (Table 1).

Analytical Ni amount was the same for all the samples (7.5 wt %), only slightly lower than the nominal value of 8 wt %. As for La amount, it was around 5 wt %. The particles morphology and catalyst size were determined using microscopy techniques. Figure 3 shows SEM images of the fresh samples. As can be seen, the appearance of the materials prepared by different techniques was notably different. Samples synthesized by precipitation were made of agglomerated spherical particles of 1.8–2 µm, while catalysts prepared by microemulsion presented a wrinkled surface covered by small superficial cubic particles.

XRD analyses using Scherrer refinement were carried out to determine the crystal size of the support and the metal phases in the samples. Figure 4 compares the XRD patterns of the four NiCe samples prepared via different support preparation methods.

As for the fresh samples, XRD profiles showed a fluorite-type phase of ceria with characteristic reflections at 2θ = 28◦ , 33◦ , 47◦ , 56◦ , 59◦ , and 69◦ associated with (111), (200), (220), (311), (222) and (400) planes of the cubic phase, respectively [33]. No diffraction lines related to lanthanum nor lanthanum oxide can be evidenced in XRD spectra, despite its almost 5 wt % loading. This could be reasonably due to the incorporation of La3+ ions in the ceria lattice [18]. A mean size of 11 and 9 nm was calculated by Sherrer for the crystal particle size of CeO2, respectively, in NiCeP and NiCeM. La addition does not significantly affect ceria size. Regarding the active phase, the occurrence of NiO was clearly detected, with the characteristic diffraction lines at 2θ 37◦ and 43.4◦ [34]. The presence of nickel in its oxidic form was not unexpected, because the analyses have been performed on calcined catalysts, as the samples charged in the reactor for catalytic ESR testing. Table 1 shows the crystal size of NiO, calculated from the analysis of the most intense diffractions, corresponding to 2θ = 43.4◦ . The patterns of the precipitated samples showed sharper and more intense diffraction lines, meaning that the particles of ceria and NiO were bigger and more crystalline than for the catalysts prepared via microemulsion (Table 1).

**Figure 3.** Representative SEM images of fresh catalysts (**a**) NiCe P, (**b**) NiCe M, (**c**) NiLaCe P and (**d**) NiLaCe M. **Figure 3.** Representative SEM images of fresh catalysts (**a**) NiCe P, (**b**) NiCe M, (**c**) NiLaCe P and (**d**) NiLaCe M. *Processes* **2021**, *9*, x FOR PEER REVIEW 7 of 14

**Figure 4.** XRD patterns of fresh catalysts after calcination. (\* denoted peaks of ceria crystal phases.) **Figure 4.** XRD patterns of fresh catalysts after calcination. (\* denoted peaks of ceria crystal phases.)

As for the fresh samples, XRD profiles showed a fluorite-type phase of ceria with characteristic reflections at 2θ = 28°, 33°, 47°, 56°, 59°, and 69° associated with (111), (200),

These preliminary analyses indicated that the support preparation method strongly affected morphological and structural features of the final catalysts. Therefore, further characterizations were performed. TPR technique was used to identify the different NiO species on the ceria surface and their reduction features and to determine the support reduction temperature. The TPR profiles are reported in Figure 5. The most evident difference between the TPR profiles of the two different techniques is the number of NiO species interacting with the support. The profiles of samples prepared by precipitation presented three broad reduction peaks at 204 °C, 249 °C and 329 °C that can be associated with NiO reduction. On the contrary, the TPR curves of samples synthesized by microemulsion showed only one sharp peak centered at 470 °C. Therefore, TPR analyses clearly showed that the synthetic approach has a deep effect on Ni reducibility, which could affect their catalytic behavior. NiO was reduced at temperatures below 400 °C for samples prepared by precipitation, while for catalysts synthesized by microemulsion the temperature needed for NiO reduction is at least 400 °C. This difference can be reasonably ascribable to the different metal–support interactions that can be formed during the oxide precipitation: the stronger the interaction, the higher the reduction temperature. The broad maxima at higher temperatures are related to the support, since it is known that the ceria can be reduced from Ce4+ to Ce3+ at temperatures above 700 °C [35]. As reported in the literature, one can envisage, for the precipitated supports, one small and broad peak at 800 °C, while

crystalline than for the catalysts prepared via microemulsion (Table 1).

almost 5 wt % loading. This could be reasonably due to the incorporation of La3+ ions in the ceria lattice [18]. A mean size of 11 and 9 nm was calculated by Sherrer for the crystal particle size of CeO2, respectively, in NiCeP and NiCeM. La addition does not significantly affect ceria size. Regarding the active phase, the occurrence of NiO was clearly detected, with the characteristic diffraction lines at 2θ 37° and 43.4° [34]. The presence of nickel in its oxidic form was not unexpected, because the analyses have been performed on calcined catalysts, as the samples charged in the reactor for catalytic ESR testing. Table 1 shows the crystal size of NiO, calculated from the analysis of the most intense diffractions, corresponding to 2θ = 43.4°. The patterns of the precipitated samples showed sharper and more intense diffraction lines, meaning that the particles of ceria and NiO were bigger and more

These preliminary analyses indicated that the support preparation method strongly affected morphological and structural features of the final catalysts. Therefore, further characterizations were performed. TPR technique was used to identify the different NiO species on the ceria surface and their reduction features and to determine the support reduction temperature. The TPR profiles are reported in Figure 5. The most evident difference between the TPR profiles of the two different techniques is the number of NiO species interacting with the support. The profiles of samples prepared by precipitation presented three broad reduction peaks at 204 ◦C, 249 ◦C and 329 ◦C that can be associated with NiO reduction. On the contrary, the TPR curves of samples synthesized by microemulsion showed only one sharp peak centered at 470 ◦C. Therefore, TPR analyses clearly showed that the synthetic approach has a deep effect on Ni reducibility, which could affect their catalytic behavior. NiO was reduced at temperatures below 400 ◦C for samples prepared by precipitation, while for catalysts synthesized by microemulsion the temperature needed for NiO reduction is at least 400 ◦C. This difference can be reasonably ascribable to the different metal–support interactions that can be formed during the oxide precipitation: the stronger the interaction, the higher the reduction temperature. The broad maxima at higher temperatures are related to the support, since it is known that the ceria can be reduced from Ce4+ to Ce3+ at temperatures above 700 ◦C [35]. As reported in the literature, one can envisage, for the precipitated supports, one small and broad peak at 800 ◦C, while for the support synthesized by microemulsion, there are two small and broad peaks at 750 and 1000 ◦C, respectively. *Processes* **2021**, *9*, x FOR PEER REVIEW 8 of 14 for the support synthesized by microemulsion, there are two small and broad peaks at 750 and 1000 °C, respectively.

**Figure 5.** Temperature programmed reduction (TPR) profiles of catalysts. **Figure 5.** Temperature programmed reduction (TPR) profiles of catalysts.

From all these characterization results, remarkable differences between two methodologies have arisen. Precipitation results in slightly higher surface areas, well-defined spherical-shaped particles and high crystallinity. At the same time, catalysts prepared by this technique presented a weaker metal support interaction, and a consequent easier metal reducibility. In contrast, NiCe M and NiLaCe M showed smaller NiO dimensions and only one stronger NiO interaction with the support, which could strongly affect NiO reducibility in reaction conditions. The effect of lanthanum addition is minimal with respect the difference in the preparation methodology. Consequently, it can be affirmed that From all these characterization results, remarkable differences between two methodologies have arisen. Precipitation results in slightly higher surface areas, well-defined spherical-shaped particles and high crystallinity. At the same time, catalysts prepared by this technique presented a weaker metal support interaction, and a consequent easier metal reducibility. In contrast, NiCe M and NiLaCe M showed smaller NiO dimensions and only one stronger NiO interaction with the support, which could strongly affect NiO reducibility in reaction conditions. The effect of lanthanum addition is minimal with respect the difference in the preparation methodology. Consequently, it can be affirmed

the support preparation method strongly influences morphological, structural, and chem-

ESR catalytic tests were carried out at 500 °C and atmospheric pressure using severe GHSV conditions. Figure 6 reports ethanol conversion (straight line) and hydrogen yield (dotted lines) along 5 h of time on stream. Both NiCe P and NiCe M catalysts presented a high initial activity with nearly complete ethanol conversion and 60% of hydrogen yield, considering that NiCe M has a slightly lower performance than NiCe P. This could be due to a more difficult reducibility of Ni in the sample prepared by microemulsion, as was demonstrated by TPR analyses. In fact, it should be noted that these materials were not reduced before the reaction test and it is relevant to verify the reducing power of ethanol,

confirming what was previously demonstrated by Pizzolitto et al. [18].

ical properties of the final catalysts.

*3.2. Catalytic Performances* 

that the support preparation method strongly influences morphological, structural, and chemical properties of the final catalysts.
