**1. Introduction**

Hydrogen, one of the most useful intermediate products, can be the ideal candidate to solve the environmental problems [1]. Although, it is not a fuel by itself, it is considered as the future energy vector owing to its great potential for generating electricity by fuel cells [2]. A critical issue in the use of hydrogen for energy applications is its production method. In fact, despite hydrogen being the most abundant element in the universe, it does not exist in significant amounts in its elemental form [3,4]. Therefore, it must be produced from other sources. Nowadays, 96% of the hydrogen produced worldwide derives from the conversion of fossil resources [5], mainly from natural gas by steam reforming. Nevertheless, one potential for the future is the possibility of hydrogen generation from renewable sources. Among the most attractive processes, the steam reforming of light alcohols such as methanol and ethanol plays a key role [6]. Indeed, methanol can be produced by syngas derived from biomass, while ethanol can be generated by fermentation of carbohydrate sources [7]. In addition to the said use of renewable raw materials, the use of alcohol as the main resource for hydrogen production has many advantages such as low cost, easy transportation in liquid form and the possibility of its conversion to hydrogen in

**Citation:** Pizzolitto, C.; Menegazzo, F.; Ghedini, E.; Martínez Arias, A.; Cortés Corberán, V.; Signoretto, M. Microemulsion vs. Precipitation: Which Is the Best Synthesis of Nickel–Ceria Catalysts for Ethanol Steam Reforming? *Processes* **2021**, *9*, 77. https://doi.org/10.3390/ pr9010077

Received: 13 November 2020 Accepted: 26 December 2020 Published: 31 December 2020

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relatively mild reaction conditions [8] with highly efficient and cost-effective processes. For example, conversion of ethanol into hydrogen via steam reforming provides six moles of hydrogen per mole of ethanol because it can extract hydrogen not only from ethanol but also from water (CH3CH2OH + 3H2O = 2CO<sup>2</sup> + 6H2) [9–11]. However, ethanol steam reforming (ESR) follows a complex reaction pathway, summarized in Figure 1. As may be seen, several by-products such as carbon monoxide, methane, ethylene, acetaldehyde and more complex carbon species can be generated under reaction conditions [12]. For this reason, the catalyst formulation is not a trivial task: it should be properly formulated to be functional to direct the reaction to maximize hydrogen yield and, at the same time, to suppress the unwanted side reactions. Common catalysts for ESR are metals, such as Pt, Pd, Rh, Ni, Co and Cu, [13,14] supported on different oxides, mainly Al2O3, SiO2, CeO2, ZrO2, TiO2, MgO and La2O<sup>3</sup> [15–17]. Among them, nickel is an attractive active phase for its low cost and high activity, comparable to that of noble metals. In addition, ceria is an interesting support since it belongs to the partially reducible oxides (PROs) [8]. Indeed, it is commonly used in different oxidation reactions such as CO oxidation [18], preferential oxidation (PROX) of CO for hydrogen purification [19], water gas shift (WGS) reaction, as well as oxygen-conducting membranes for solid oxide fuel cells and many other processes [20,21]. Thanks to the redox ability and strong interaction with nickel, ceria has been extensively studied in the reforming field [22]. Ceria redox ability can be modulated by a careful control of structural defects [23–25]: the higher the number of defective sites, the more effective the redox pump. Therefore, lanthanum oxide has been added as promoter due to its possible substitution as La3+ in the Ce4+ lattice [18] which may lead to the formation of defective sites. drogen in relatively mild reaction conditions [8] with highly efficient and cost-effective processes. For example, conversion of ethanol into hydrogen via steam reforming provides six moles of hydrogen per mole of ethanol because it can extract hydrogen not only from ethanol but also from water (CH3CH2OH + 3H2O = 2CO2 + 6H2) [9–11]. However, ethanol steam reforming (ESR) follows a complex reaction pathway, summarized in Figure 1. As may be seen, several by-products such as carbon monoxide, methane, ethylene, acetaldehyde and more complex carbon species can be generated under reaction conditions [12]. For this reason, the catalyst formulation is not a trivial task: it should be properly formulated to be functional to direct the reaction to maximize hydrogen yield and, at the same time, to suppress the unwanted side reactions. Common catalysts for ESR are metals, such as Pt, Pd, Rh, Ni, Co and Cu, [13,14] supported on different oxides, mainly Al2O3, SiO2, CeO2, ZrO2, TiO2, MgO and La2O3 [15–17]. Among them, nickel is an attractive active phase for its low cost and high activity, comparable to that of noble metals. In addition, ceria is an interesting support since it belongs to the partially reducible oxides (PROs) [8]. Indeed, it is commonly used in different oxidation reactions such as CO oxidation [18], preferential oxidation (PROX) of CO for hydrogen purification [19], water gas shift (WGS) reaction, as well as oxygen-conducting membranes for solid oxide fuel cells and many other processes [20,21]. Thanks to the redox ability and strong interaction with nickel, ceria has been extensively studied in the reforming field [22]. Ceria redox ability can be modulated by a careful control of structural defects [23–25]: the higher the number of defective sites, the more effective the redox pump. Therefore, lanthanum oxide has been added as promoter due to its possible substitution as La3+ in the Ce4+ lattice [18]which may lead to the formation of defective sites.

use of alcohol as the main resource for hydrogen production has many advantages such as low cost, easy transportation in liquid form and the possibility of its conversion to hy-

*Processes* **2021**, *9*, x FOR PEER REVIEW 2 of 14

**Figure 1.** Reaction pathways involved in the ethanol steam reforming (ESR) process. **Figure 1.** Reaction pathways involved in the ethanol steam reforming (ESR) process.

This work has been focused on the preparation method of the support for nickel– ceria-based catalysts. Two different synthetic methods have been investigated for ceria supports preparation: precipitation and reverse microemulsion. Precipitation is the standard approach used for metal oxides preparation. With this method, however, it is difficult to control particle size distribution. On the contrary, reverse microemulsion can be an innovative way to modulate and control the textural properties of new materials. This approach is based on the formation of nanospherical micelles inside which the precipitation This work has been focused on the preparation method of the support for nickel–ceriabased catalysts. Two different synthetic methods have been investigated for ceria supports preparation: precipitation and reverse microemulsion. Precipitation is the standard approach used for metal oxides preparation. With this method, however, it is difficult to control particle size distribution. On the contrary, reverse microemulsion can be an innovative way to modulate and control the textural properties of new materials. This approach is based on the formation of nanospherical micelles inside which the precipitation of the oxide takes place. In this way, as reported by Eriksson et al. [26], a suitable environment for producing small nanoparticles with narrow size distribution can be generated. Accordingly, the motivation of this work is to focus on the investigation of the influence of the prepa-

ration method on the activity, stability and regenerability of nickel–ceria-based catalysts for hydrogen production via ESR, that was investigated under severe Gas Hourly Space Velocity (GHSV) conditions. To the best of our knowledge this is the first use in ESR of Ni supported on La doped CeO<sup>2</sup> prepared by reverse microemulsion. To achieve this goal, different characterization approaches were used. In particular, the correlation between the synthetic method and structural and morphological properties was investigated by N2-physisorption, SEM, XRD, and H2-temperature programmed reduction (TPR). The effects of lanthanum doping have been investigated too.
