*3.2. Catalytic Performances*

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]. *Processes* **2021**, *9*, x FOR PEER REVIEW 9 of 14

**Figure 6.** Catalytic activity in the ethanol steam reforming on NiLaCe P (**left**) and NiLaCe M (**right**) catalysts in comparison with the non-doped samples: ethanol conversion (full line) and hydrogen yield (dotted line). **Figure 6.** Catalytic activity in the ethanol steam reforming on NiLaCe P (**left**) and NiLaCe M (**right**) catalysts in comparison with the non-doped samples: ethanol conversion (full line) and hydrogen yield (dotted line).

Nevertheless, both the catalysts suffered a progressive deactivation over time on stream. However, the deactivation presented a different degree: after 2.5 h, NiCe M completely lost its activity for hydrogen production, maintaining at the same time a very low ethanol conversion. The catalyst prepared via precipitation kept 50% conversion and 30% hydrogen yield after 5 h on stream. The behavior of lanthanum-doped samples was very Nevertheless, both the catalysts suffered a progressive deactivation over time on stream. However, the deactivation presented a different degree: after 2.5 h, NiCe M completely lost its activity for hydrogen production, maintaining at the same time a very low ethanol conversion. The catalyst prepared via precipitation kept 50% conversion and 30% hydrogen yield after 5 h on stream. The behavior of lanthanum-doped samples was very similar to the corresponding non-doped catalysts under the tested reaction conditions.

similar to the corresponding non-doped catalysts under the tested reaction conditions. Characterizations of catalyst samples recovered after reaction were also performed to understand more deeply the evolution of catalytic behavior. Examples of SEM images of used samples are shown in Figure 7. As can be seen, the catalyst prepared via microemulsion, NiCe M, was almost completely covered by carbon, while the sample prepared by precipitation, NiCe P, still showed a very clear surface despite the fact that it had some dark agglomerates associated with carbon deposits. Although Carbon was not formed as nanotubes or nanowires, it was in a more compact form, either polymeric or graphitic. These results perfectly matched with the catalytic results: the complete activity loss for the NiCe M sample is due to the complete coverage of active sites by carbon. On the contrary, only a small portion of metals was covered by coke deposits, and therefore the sam-Characterizations of catalyst samples recovered after reaction were also performed to understand more deeply the evolution of catalytic behavior. Examples of SEM images of used samples are shown in Figure 7. As can be seen, the catalyst prepared via microemulsion, NiCe M, was almost completely covered by carbon, while the sample prepared by precipitation, NiCe P, still showed a very clear surface despite the fact that it had some dark agglomerates associated with carbon deposits. Although Carbon was not formed as nanotubes or nanowires, it was in a more compact form, either polymeric or graphitic. These results perfectly matched with the catalytic results: the complete activity loss for the NiCe M sample is due to the complete coverage of active sites by carbon. On the contrary, only a small portion of metals was covered by coke deposits, and therefore the sample was still active after 5 h of reaction.

ple was still active after 5 h of reaction.

**Figure 6.** Catalytic activity in the ethanol steam reforming on NiLaCe P (**left**) and NiLaCe M (**right**) catalysts in comparison

Nevertheless, both the catalysts suffered a progressive deactivation over time on stream. However, the deactivation presented a different degree: after 2.5 h, NiCe M completely lost its activity for hydrogen production, maintaining at the same time a very low ethanol conversion. The catalyst prepared via precipitation kept 50% conversion and 30% hydrogen yield after 5 h on stream. The behavior of lanthanum-doped samples was very similar to the corresponding non-doped catalysts under the tested reaction conditions.

Characterizations of catalyst samples recovered after reaction were also performed to understand more deeply the evolution of catalytic behavior. Examples of SEM images of used samples are shown in Figure 7. As can be seen, the catalyst prepared via microemulsion, NiCe M, was almost completely covered by carbon, while the sample prepared by precipitation, NiCe P, still showed a very clear surface despite the fact that it had some dark agglomerates associated with carbon deposits. Although Carbon was not formed as nanotubes or nanowires, it was in a more compact form, either polymeric or graphitic. These results perfectly matched with the catalytic results: the complete activity loss for the NiCe M sample is due to the complete coverage of active sites by carbon. On the contrary, only a small portion of metals was covered by coke deposits, and therefore the sam-

with the non-doped samples: ethanol conversion (full line) and hydrogen yield (dotted line).

ple was still active after 5 h of reaction.

**Figure 7.** SEM images of used catalyst samples recovered after ethanol steam reforming tests: (**a**) Used NiCe P and (**b**) Used NiCe M. **Figure 7.** SEM images of used catalyst samples recovered after ethanol steam reforming tests: (**a**) Used NiCe P and (**b**) Used NiCe M.

To further understand the reasons of this discrepancy in catalytic behavior, X-ray diffractograms of the used catalysts were obtained*.* Figure 8 compares the XRD patterns of fresh and used catalysts from both preparation methods. The patterns of used samples presented reflections attributed to carbon species at 2θ 26.4° [36] probably with a graphitelike structure. For the used catalysts, the crystal particle size of CeO2 slightly increased (11 and 10 nm, respectively, for used NiCe P and NiCe M) and the reflections attributed to metallic Ni appear at 2θ of 44.5° and 51.8°, while those of NiO at 43.4° disappear (see inset in Figure 8). This evidenced that the reaction mixture, that is the reactant ethanol, allowed the reduction in the metal phase, thus activating the catalysts for the reaction. To further understand the reasons of this discrepancy in catalytic behavior, X-ray diffractograms of the used catalysts were obtained. Figure 8 compares the XRD patterns of fresh and used catalysts from both preparation methods. The patterns of used samples presented reflections attributed to carbon species at 2θ 26.4◦ [36] probably with a graphitelike structure. For the used catalysts, the crystal particle size of CeO<sup>2</sup> slightly increased (11 and 10 nm, respectively, for used NiCe P and NiCe M) and the reflections attributed to metallic Ni appear at 2θ of 44.5◦ and 51.8◦ , while those of NiO at 43.4◦ disappear (see inset in Figure 8). This evidenced that the reaction mixture, that is the reactant ethanol, allowed the reduction in the metal phase, thus activating the catalysts for the reaction.

*Processes* **2021**, *9*, x FOR PEER REVIEW 11 of 14

**Figure 8.** (**a**) XRD patterns of fresh and used catalysts after calcination. (Reflections at 2θ 35° are due to remains of SiC used as diluent in the catalytic bed.) and (**b**) magnification on reflections of NiO at 2θ 43.3 and 44.5°. **Figure 8.** (**a**) XRD patterns of fresh and used catalysts after calcination. (Reflections at 2θ 35◦ are due to remains of SiC used as diluent in the catalytic bed.) and (**b**) magnification on reflections of NiO at 2θ 43.3 and 44.5◦ .

Moreover, to determine the possibility of reusability of the catalysts, reactivation of used catalysts was carried out, followed by a second run of the catalytic test. Figure 9 compares ethanol conversion and hydrogen yield of fresh and used catalysts for both undoped materials. After the regeneration in air, the initial H2 yield on NiCe P, the most active one, was lower than that of the fresh sample, decreasing to 17% after 5 h of reaction (instead of 30% of the first run). Therefore, catalyst regeneration did not allow its complete reactivation, indicating either possible sintering of the catalyst or incomplete removal of the carbonaceous deposits. In the case of NiCe M, it was completely reactivated after the regeneration step, as the curve of ethanol conversion of the regenerated sample practically overlapped with that of the fresh one. However, considering hydrogen yield, an even faster deactivation is visible, with no hydrogen production after just 2 h. This apparent discrepancy, of the same conversion but different yield, is due to a different selectivity, indicating that some changes in the nature of the material occurred. Moreover, to determine the possibility of reusability of the catalysts, reactivation of used catalysts was carried out, followed by a second run of the catalytic test. Figure 9 compares ethanol conversion and hydrogen yield of fresh and used catalysts for both undoped materials. After the regeneration in air, the initial H<sup>2</sup> yield on NiCe P, the most active one, was lower than that of the fresh sample, decreasing to 17% after 5 h of reaction (instead of 30% of the first run). Therefore, catalyst regeneration did not allow its complete reactivation, indicating either possible sintering of the catalyst or incomplete removal of the carbonaceous deposits. In the case of NiCe M, it was completely reactivated after the regeneration step, as the curve of ethanol conversion of the regenerated sample practically overlapped with that of the fresh one. However, considering hydrogen yield, an even faster deactivation is visible, with no hydrogen production after just 2 h. This apparent discrepancy, of the same conversion but different yield, is due to a different selectivity, indicating that some changes in the nature of the material occurred.

**Figure 9.** Ethanol steam reforming on fresh (solid lines) and reactivated (dotted lines) catalysts. Ethanol conversion (**a**) NiCe P and (**b**) NiCe M and hydrogen yield (**c**) NiCe P and (**d**) NiCe M vs. reaction time. **Figure 9.** Ethanol steam reforming on fresh (solid lines) and reactivated (dotted lines) catalysts. Ethanol conversion (**a**) NiCe P and (**b**) NiCe M and hydrogen yield (**c**) NiCe P and (**d**) NiCe M vs. reaction time.

As a matter of fact, the causes of catalysts deactivation were quite different for two samples. In fact, the catalyst prepared via microemulsions, despite the faster deactivation with time, seemed to be reactivated during the regeneration step. Therefore, its complete deactivation in the first run was probably due to a reversible coke deposition. SEM analyses performed on used NiCe M catalyst had shown that it was almost completely covered by a compact form of carbon, either polymeric or graphitic. Such coke can be oxidized during regeneration step. On the contrary, the catalyst prepared via precipitation is more stable over time, but it cannot be fully regenerated in air. For this used sample, SEM pictures have shown that only a small portion of metal was covered by coke deposits, and it could be easily removed during the oxidative treatment of regeneration. Hence, this is not the main cause of deactivation, and sintering of the active phase had probably occurred. As demonstrated by TPR technique, NiCe P presented a lower interaction between support and active phase, and this could have determined its easier sintering. As a matter of fact, the causes of catalysts deactivation were quite different for two samples. In fact, the catalyst prepared via microemulsions, despite the faster deactivation with time, seemed to be reactivated during the regeneration step. Therefore, its complete deactivation in the first run was probably due to a reversible coke deposition. SEM analyses performed on used NiCe M catalyst had shown that it was almost completely covered by a compact form of carbon, either polymeric or graphitic. Such coke can be oxidized during regeneration step. On the contrary, the catalyst prepared via precipitation is more stable over time, but it cannot be fully regenerated in air. For this used sample, SEM pictures have shown that only a small portion of metal was covered by coke deposits, and it could be easily removed during the oxidative treatment of regeneration. Hence, this is not the main cause of deactivation, and sintering of the active phase had probably occurred. As demonstrated by TPR technique, NiCe P presented a lower interaction between support and active phase, and this could have determined its easier sintering.

## **4. Conclusions**

**4. Conclusions**  The effect of the preparation method, namely, precipitation and microemulsion synthesis, has been evaluated for nickel–ceria-based catalysts. As expected, the microemulsion approach allowed us to prepare materials with smaller NiO dimensions and a defined interaction between NiO and support stronger than in the catalysts with precipitated supports, as evidenced by TPR analyses. At the same time, the samples prepared via precipitation had higher surface areas, well-defined spherical particles, and higher crystallinity. Therefore, the preparation method strongly affected the structural and chemical properties of catalysts. In ethanol steam reforming, the catalysts prepared via precipitation showed higher catalytic activity and stability, while those prepared via microemulsion deactivated very fast. Similar trends were found for La-promoted supports. Nevertheless, after the oxidative regeneration treatment, the NiCe P catalyst did not fully regain its properties, while NiCe M was completely reactivated. This indicated that the reasons for catalyst deactivation should be quite different. For catalysts prepared by precipitation, the The effect of the preparation method, namely, precipitation and microemulsion synthesis, has been evaluated for nickel–ceria-based catalysts. As expected, the microemulsion approach allowed us to prepare materials with smaller NiO dimensions and a defined interaction between NiO and support stronger than in the catalysts with precipitated supports, as evidenced by TPR analyses. At the same time, the samples prepared via precipitation had higher surface areas, well-defined spherical particles, and higher crystallinity. Therefore, the preparation method strongly affected the structural and chemical properties of catalysts. In ethanol steam reforming, the catalysts prepared via precipitation showed higher catalytic activity and stability, while those prepared via microemulsion deactivated very fast. Similar trends were found for La-promoted supports. Nevertheless, after the oxidative regeneration treatment, the NiCe P catalyst did not fully regain its properties, while NiCe M was completely reactivated. This indicated that the reasons for catalyst deactivation should be quite different. For catalysts prepared by precipitation, the deactivation was mostly due to sintering of the nickel particles that were not strongly interacting with the support. On the contrary, strong interaction between active phase and

support in NiCe M preserved the material from sintering. However, the lower surface area and the low degree of crystallinity led to a rapid deactivation of the material caused by coke deposition.

**Author Contributions:** Conceptualization, M.S. and V.C.C.; methodology, A.M.A.; formal analysis, C.P.; investigation, E.G.; data curation, F.M.; writing original draft preparation, review and editing, F.M.; supervision, M.S.; funding acquisition M.S. and V.C.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by MINECO project CTQ2015-71823-R and MICINN project RTI2018-101604-B-I00 (Spain).

**Acknowledgments:** The technical help of M. Sanchez with SEM measurements is gratefully acknowledged.

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