How the incorporation synthesis method of the active phase affects the catalytic behavior: Ni/ceo₂ nanowires towards ethanol steam reforming

Abstract

The controlled synthesis of well-defined nanoparticles is crucial for producing optimized catalysts with enhanced properties and maximizing the desired product in catalysis. Therefore, the incorporation of Ni onto CeO₂ nanowires via four different routes was utilized to investigate the effect of metal-support interaction, particle size, and distribution of Ni over the CeO₂ nanowires. This aimed to correlate the physicochemical properties of the obtained Ni/CeO₂ nanowires with their high performance in the steam reforming of ethanol. Specifically, Ni/CeO₂ nanowires were obtained through: (i) incipient wet impregnation, (ii) nanoparticle impregnation, (iii) urea-assisted deposition, and (iv) NaBH₄-assisted deposition. These methods resulted in four nanostructures with distinct Ni particle sizes and distributions on the surface of CeO₂ nanowires, leading to differences in textural properties, the concentration of oxygen vacancies, and the degree of Ni-CeO₂ interaction. The best catalyst, obtained via incipient wet impregnation, exhibited high selectivity to H₂ (65%, considering that the maximum theoretical selectivity is 66%) in the catalytic tests, along with 100% ethanol conversion and minimal deactivation (6% within 24 h and 20% within 48 h) during the catalytic test. Thus, it was observed that controlled synthesis enabled the production of four different nanomaterials with enhanced properties, such as textural properties and concentration of oxygen vacancies, resulting in upgraded catalytic performance.

Graphical abstract

Illustration of the synthesis of Ni/CeO₂ nanowires by four different methods of incorporation for application as heterogeneous catalyst towards ethanol steam reforming to hydrogen

1 Introduction

Nanomaterials have revolutionized the material science field due to their enhanced properties compared to bulk materials, exhibiting improved quantum effects, bioactivity, and surface properties, such as catalytic activity [1]. Particularly, the surface properties of the nanomaterials emerge from the reduction of particle size, leading to direct changes in electronic and structural properties, including lattice parameters, the surface-to-volume ratio, and surface energy of the nanomaterial [2, 3]. Moreover, the surface energy important to determine the intensity of the chemical bond between the nanomaterial and a molecule or an intermediate during a chemical reaction, such as in heterogeneously catalyzed systems [4, 5]. Supported heterogeneous catalysts have been developed to minimize cost and particle size, and maximize dispersion of the active phase and catalytic activity [6,7,8,9,10]. However, the synthesis methods of these catalysts still face limitations in controlling the textural and physicochemical properties, primarily due to irregular surface structures that display varying surface energy on each exposed crystalline plane [4, 11].

The controlled synthesis of nanomaterials can optimize their properties, allowing for precise control over each component of supported catalysts, including support and active phase. Various methodologies have been employed to synthesize solid supports, such as precipitation [12], sonochemical [13, 14], and sol-gel [15]. Among the methods, the hydrothermal method stands out as a promisor methodology due to its low cost and ability to generate different morphologies with well-defined size, shape, and structure [16,17,18,19,20]. For instance, a study conducted by Soykal et al. investigated the impact of support particle size on the ethanol steam reforming reaction [21, 22]. They observed that CeO₂ microparticles ranging from 20 nm to 0.2 μm, and CeO₂ nanoparticles of 5–8 nm, demonstrated variances in ethanol conversion and H₂ yield. Specifically, CeO₂ nanoparticles exhibited superior ethanol conversion and H₂ yield compared to microparticles [21,22,23].

The reduction of particle size from macro and microscale, referred to as bulk materials, to nanoscale significantly impacts the properties of the nanoparticles, leading to substantial alterations in their optical, magnetic, electronic, and surface properties [10, 24,25,26]. In macro form, bulk materials exhibit a limited incorporation of surface atoms within the total atom count, resulting in a lower specific surface area and fewer exposed atoms. In contrast, at the nanoscale, the smaller particle size results in an enhanced specific surface area and a higher number of exposed atoms, promoting distortions in geometric and electronic factors and leading to enhanced properties [27,28,29].

Additionally, Vecchietti et al. emphasized that the shape of nanostructures significantly influences the catalytic behavior of nanomaterials [30]. They observed that CeO₂ nanooctahedra and nanocubes predominantly expose (111) and (100) surfaces, respectively. Nanocubes were found to produce 2.4 times more hydrogen compared to nanooctahedra, attributed to differences in adsorbed ethoxy species. On CeO₂(111) surfaces, Vecchietti et al. identified monodentate type I and type II ethoxy species, with alkyl chains oriented perpendicular or parallel to the surface. Conversely, CeO₂(100) surfaces exhibited bidentate and monodentate type III ethoxy species, distributed on the O-terminated checkerboard surface and on the pyramid of the reconstructed (100) surface, respectively. The more reactive surface ethoxy species, such as monodentate type I or III on CeO₂ nanooctahedra and CeO₂ nanocubes, respectively, reacted to form acetate species that decomposed into CO₂ and CH₄, with H₂ generated via the recombination of hydroxyl species. Furthermore, the more stable monodentate type II and bidentate ethoxy species on CeO₂ nanooctahedra and nanocubes, respectively, formed an ethylenedioxy intermediate. The binding of this intermediate was facet-dependent: on the (111) facet, the less strongly bound ethylenedioxy desorbed as ethylene, while on the (100) facet, the more strongly bound intermediate also produced CO₂ and H₂ via formate species. In our research, we utilized the hydrothermal method to synthesize CeO₂ nanowires with a narrow size distribution (11 nm ± 2 nm in width) and enhanced surface properties, including oxygen vacancies [18, 19, 31].

Nevertheless, the active phase can be synthesized through two routes: homogeneous or heterogeneous nucleation. In homogeneous nucleation, crystals form uniformly throughout the parent phase in the bulk of the reaction medium, while heterogeneous nucleation occurs in the presence of a second phase (support surface) [32,33,34,35]. Homogeneous nucleation typically requires higher energy than heterogeneous nucleation for crystal nucleation [3, 32,33,34,35]. In controlled synthesis, the active phase can be synthesized through homogeneous nucleation and subsequently incorporated by adsorption onto the solid support, a process known as impregnation [36]. This method allows for the synthesis of diverse morphologies, such as urchin [37, 38], flowers [39, 40], and hollow nanoparticles [10, 41,42,43], as well as different sizes [44,45,46], ranging from larger nanoparticles to thin layers [18]. In contrast, heterogeneous nucleation involves the active phase nucleating over the solid support, offering the ability to achieve different metal-support interactions and particle sizes. However, spherical morphologies are typically obtained [46,47,48]. Additionally, various methodologies can be employed to synthesize the active phase, including chemical reduction [49], precipitation [50], electrochemical reduction [51], and photo-reduction [52]. Chemical reduction emerges as a prominent method for synthesizing metallic active phases, facilitating the direct formation of metal nanoparticles from metal ions without additional steps. Nickel (Ni) is especially favored as a metallic active phase due to its affordability and distinctive properties, including high surface energy and catalytic activity [53,54,55,56,57,58].

Furthermore, the size of metallic active phase particles and the interaction between the metal and support are significant factors in conducting a well-controlled catalytic reaction, achieving high hydrogen yield while preventing carbon deposition on the catalyst. Da Silva et al. reported on the effect of metallic particle size using cobalt supported on carbon nanofibers in ethanol steam reforming [59]. To isolate the metallic active phase, they utilized carbon nanofibers that do not exhibit relevant ethanol conversion (~ 5%). By manipulating the cobalt loading, they were able to control the particle size. Increasing the cobalt loading led to an increase in particle size, ranging from 1.0 to 22 wt% and 2.4 to 16 nm, respectively. As the particle size decreases, the turnover frequency increases due to the higher number of surface atoms, which are more active than those in larger particles [31]. The catalyst containing larger cobalt particles, larger than 4 nm, exhibited a significant decrease in ethanol conversion. Moreover, cobalt particles with the highest size (16 nm) displayed significant deactivation and amorphous carbon deposition on the catalyst surface after 3 h of reaction [31, 59].

The nanoscale introduces various modifications in geometric and electronic characteristics due to the increased percentage of exposed surface atoms and higher free surface energy, leading to enhanced properties. These modifications promote surface defects, such as kinks, steps, and vacancies, which are more active catalytic sites. For instance, smaller spherical nanoparticles with diameters of 2, 6, and 10 nm can have between 200 and 400, 7,000–10,000, and 34,000–47,000 atoms, respectively [60], corresponding to 50% t 10% o exposed surface atoms [61]. These characteristics can result in higher interaction with the support and more active catalytic sites [62].

The metal-support interaction (MSI) plays a crucial role in heterogeneous catalysts that facilitate reforming reactions for producing renewable hydrogen. This interaction involves modifying the geometric and electronic structure of metal species when they come into contact with supports [31, 63, 64]. Tailoring supports with specific features has been successful in stabilizing metal species, adjusting their geometric and electronic configurations, and improving mass transfer efficiency. Importantly, placing active metal species close to support defects significantly enhances the activation of reactant adsorption, optimizes the transition state of adsorbates, and facilitates the conversion of reaction intermediates [31, 64, 65]. Wang et al. demonstrated these effects by depositing nickel nanoparticles of varying sizes to manipulate the quantity of oxygen vacancies and the metal-support interaction [31]. As the size of nickel particles decreases, there is an increase in nickel dispersion and the number of oxygen vacancies, resulting in enhanced metal-support interaction and higher hydrogen yield [31, 64, 66].

We reported the impact of nanostructures on catalytic activity by employing Ni/CeO₂-Sm₂O₃ nanowires and their bulk analog. The Ni/CeO₂-Sm₂O₃ nanowires exhibited remarkable stability, with 192 h of continuous activity, 60% hydrogen selectivity, and 100% ethanol conversion in ethanol steam reforming [18]. Subsequently, we explored the influence of the nature of the solid support on the catalytic performance of Rh/Mn_xO_y (M = Ce, Ti, Si, Zn, and Al) in ethanol steam reforming [67]. In this paper, we systematically investigate the impact of different incorporation methods of the active phase on the catalytic behavior of Ni/CeO₂ nanowires in ethanol steam reforming, serving as model reaction. The CeO₂ nanowires were synthesized via the hydrothermal method, and we evaluated four different synthetic approaches for incorporating Ni onto the surfaces of the CeO₂ nanowires: (i) wet impregnation of Ni precursor, (ii) wet impregnation of preformed Ni nanoparticles, (iii) deposition assisted by urea, and (iv) deposition assisted by sodium borohydride. Subsequently, we analyzed the physical and chemical properties of the catalysts using various characterization techniques. Finally, we applied the Ni/CeO₂ nanowires catalysts in ethanol steam reforming as a model reaction to produce hydrogen (H₂). Specifically, we aimed to investigate how different incorporation methods of the active phase would impact the catalytic performance of the Ni/CeO₂ nanowires catalysts in terms of activity, selectivity, and stability. Our findings reveal that the incorporation methods of the active phase significantly influence the physicochemical properties of the catalysts, including nanoparticle size, metal dispersion, metal-support interaction. Moreover, the catalytic behavior in ethanol steam reforming, encompassing activity, selectivity, stability, and carbon deposition, is strongly influenced by each employed incorporation methodology.

2 Experimental

2.1 Materials

Analytical grade cerium (III) nitrate hexahydrate (Ce(NO₃)₃·6H₂O, 99.5%, Sigma-Aldrich), ethanol (C₂H₆O, 99.5%, Sigma-Aldrich), ethylene glycol (HOCH₂CH₂OH, 99.8%, Sigma-Aldrich), nickel (II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O, 99.5%, Sigma-Aldrich), Polyvinylpyrrolidone ((C₆H₉NO)_n, 55,000 g.mol⁻¹, Sigma-Aldrich), Sodium borohydride (NaBH₄, 98.0%, Sigma-Aldrich), sodium hydroxide (NaOH, 99%, Sigma-Aldrich), urea (CH₄N₂O, 99.75%, Vetec), hydrochloric acid (HCl, 37%, Vetec), and nitric acid (HNO₃, 70%, Vetec), were used as received. All glassware were cleaned with aqua regia (HCl: HNO₃ = 3:1) before the synthesis to prevent contamination.

2.2 Synthesis of CeO₂ nanowires

The CeO₂ nanowires were synthesized using the hydrothermal method [19]. In this process, 19.6 g of NaOH were dissolved in 35.0 mL of Milli-Q water in a Teflon bottle under magnetic stirring. Subsequently, 15 mL of an aqueous Ce(NO₃)₃·6H₂O solution at 1.4 M was added dropwise (1 drop per second) to the alkaline solution under stirring and maintained for 15 min at room temperature. Then, the magnetic stir bar was removed, and the Teflon bottle was transferred to a stainless-steel autoclave, sealed, and submitted to thermal treatment at 110 °C for 24 h in a preheated furnace. After the period, the autoclave was removed from the furnace and naturally cooling down at room temperature. The supernatant was removed and the obtained solid was washed five times with Milli-Q water and twice with ethanol at 10,000 rpm for 10 min in a centrifuge. The ethanol wash step is used to improve the drying process at 120 °C for 2 h (Fig. S1A).

2.3 Synthesis of Ni/CeO₂ via incipient wet impregnation

The impregnation of Ni onto CeO₂ nanowires was carried out using a wet impregnation method [18]. Initially, 2.0 g of CeO₂ nanowires were dispersed in 36.6 mL of Milli-Q water and ultrasonicated for 15 min to well disperse the CeO₂ nanowires. Then, 13.4 mL of an aqueous solution of Ni(NO₃)₂·6H₂O at 25.0 mM was added to the suspension under magnetic stirring. In sequence, the mixer was heated at 80 °C under stirring until all water was evaporated. The solid was dried at 120 °C for 2 h and calcined in air at 450 °C for 2 h (Fig. S1B).

2.4 Synthesis of Ni/CeO₂ via nanoparticles impregnation

The nanoparticles impregnation method was also employed for the impregnation of preformed Ni nanoparticles onto CeO₂ nanowires. Initially, 500 mL of an aqueous PVP solution at 0.5 M was stirred magnetically at 40 °C for 10 min in a round-bottomed flask immersed in an oil bath to achieve a homogeneous temperature solution. Subsequently, 13.4 mL of an aqueous Ni(NO₃)₂·6H₂O solution at 25 mM were added to the solution, followed by the carefully one-shot addition of 46.5 mL of a fresh aqueous NaBH₄ solution at 0.1 M, and the mixture was stirred at 40 °C for 4 h. After synthesizing the Ni nanoparticle, the suspension was added dropwise (1 drop per second) into a suspension of 2.0 g CeO₂ nanowires in 10.0 mL water and 10.0 mL ethanol under stirring at room temperature. The ethanol helps to avoid bobble formations due to the presence of PVP. After completing the addition, the reaction medium was stirred at room temperature for 4 h, the solid was washed five times with Milli-Q water and twice with ethanol at 10,000 rpm for 10 min in a centrifuge. Then, the solid was dried at 110 °C for 2 h and calcined in air at 450 °C for 2 h (Fig. S1C) [68].

2.5 Synthesis of Ni/CeO₂ via urea deposition method

The deposition of Ni nanoparticles on CeO₂ nanowires was carried out using a deposition-precipitation method assisted by urea [67]. In a round-bottomed flask, 2.0 g CeO₂ nanowires was dispersed in 100.0 mL Milli-Q water and ultrasonicated for 15 min. Then, the suspension was stirred magnetically for 15 min at 90 °C in an oil bath. Subsequently, 0.6 g urea was added to the suspension and 13.4 mL of an aqueous Ni(NO₃)₂·6H₂O solution at 25 mM was added dropwise (1 drop per second), and maintained under stirring at 90 °C for 4 h. After the period the glassware was removed from oil bath to naturally cooling down at room temperature. The solid was washed five times with Milli-Q water and twice with ethanol at 10,000 rpm for 10 min in a centrifuge. The solid was dried at 120 °C for 2 h and calcined in air at 450 °C for 2 h (Fig. S1D).

2.6 Synthesis of Ni/CeO₂ via NaBH₄ deposition method

The deposition of the Ni nanoparticles onto CeO₂ nanowires was carried out using a chemical reduction method assisted by NaBH₄ [69]. In a round-bottomed flask, 2.0 g CeO₂ nanowires were dispersed in 100 mL of a PVP/ethylene glycol solution at 1.3 g L⁻¹ and ultrasonicated for 15 min. Then, the glassware was immersed in an oil bath at 90 °C for 15 min under magnetic stirring with a condenser trap to maintain a constant the concentration and volume of the reaction. Subsequently, 13.4 mL of an aqueous Ni(NO₃)₂·6H₂O solution at 25 mM was added to the mixture and 30.0 mL of a fresh NaBH₄ aqueous solution at 120.0 mM were one shot carefully added to the mixture and stirred at 90 °C for 4 h with a condenser trap to maintain a constant the concentration and volume of the reaction. After the period, the glassware was removed from the oil bath to cool down at room temperature. The solid was washed five times with Milli-Q water and twice with ethanol at 10,000 rpm for 10 min in a centrifuge. The solid was dried at 120 °C for 2 h and calcined in air at 450 °C for 2 h (Fig. S1E).

2.7 Characterization methods

Scanning electron microscopy (SEM) images were obtained using a JEOL Neoscope JCM-5000 field emission gun microscope operated at 5 kV. The samples were not pre-treated before SEM analysis. The samples were dispersed in a mixture of 50% ethanol/water and ultrasonicated for 15 min. Then, the suspension was drop-casting the nanostructures over a silicon wafer and drying under ambient conditions. Before SEM analysis, the silicon wafer was washed with aquaria to prevent contamination.

Transmission Electron Microscopy (TEM) images were obtained with a JEOL JEM 2100 microscope operated at 200 kV. Before TEM analysis, the catalysts were treated under a H₂ atmosphere at 450 °C for 1 h under a flow rate of 50 mL·min⁻¹. After that, the catalysts were passivated under oxygen (O₂) (5%) diluted in a helium (He) atmosphere at 25 °C for 12 h under a flow rate of 50 mL·min⁻¹ to emulate the catalytic reaction condition. After that, the samples were dispersed in a 50% ethanol/water mixture and ultrasonicated for 15 min. The suspension was drop-cast over a carbon-coated copper grid, followed by drying under ambient conditions.

Energy Dispersive X-ray Spectroscopy (EDS) is used for elementary maps using a Tescan VEGA 3 LMU scanning electron microscope operated at 20 kV equipped with an Oxford detector with a 20 mm window. Samples were prepared by drop-casting an aqueous suspension containing the nanostructures over a silicon wafer, then drying under ambient conditions. Before EDS analysis, the silicon wafer was washed with aqua regia to prevent contamination.

Atomic Absorption Spectrometry (AAS) measured the Ni mass fractions using a ContrAA 300 Analitk Jena. Typically, 0.1 g of a catalyst was dissolved in an aqua regia solution (chloridric acid : nitric acid = 3 : 1) at 150 °C. After that, the sample was swelled to 50.0 mL with an HCl at 1.0 M and NH₄Cl at 0.1 M solution.

Thermogravimetric (TGA) measurements were carried out using a HITACHI STA 7300 equipment in the range of 25 –1000 °C using a heating rate of 10 °C·min⁻¹ under air flow.

The X-ray diffraction (XRD) data were obtained using a Rigaku Miniflex X-Ray diffractometer and Cu K-alpha radiation (λ = 1.5418 Å). The diffraction patterns were obtained between Bragg’s angle of 20° to 90° with a 0.05° min⁻¹ angular speed scan. The diffractograms were analyzed with the aid of crystallographic files from the JCPDS database (Joint Committee on Powder Diffraction Standards) of the JADE 5 software.

Specific surface area, average pore diameter, and pore volume were determined by physisorption of N₂ at − 196 °C (77 K) in a Micromeritics ASAP 2020. Typically, 0.5 g of catalyst was pretreated under a 1.10⁻⁶ mmHg vacuum at 300 °C for 24 h. The adsorption and desorption isotherms were obtained by plotting the N₂ amount assimilated by the material as a function of the partial pressure. The specific areas were estimated by the Brunauer-Emmett-Teller model (BET method), and average pore diameter and pore volume were calculated by the Barret-Joyner-Halenda model (BJH method).

Raman spectra were collected using a in a Renishaw InVia Reflex coupled to a Leica DM 2500 M microscope and a CCD detector. The laser excitations used were 632.8 nm (He/Ne source) and the objective was a 50 × objective (NA = 0.9).

Temperature-programmed reduction with hydrogen (H₂-TPR) was carried out in a multipurpose unit with a Pfeiffer Vaccum Prisma detector mass spectrometer. Typically, 0.1 g of a catalyst was dried with He flow at 300 °C for 1 h and then cooled down to room temperature. The TPR profiles were obtained between 30 °C and 1000 °C in a flow of 2% H₂/He, at a rate of 10 °C·min⁻¹. H₂ consumption is calculated using H₂-TPR by comparing the reduction to a standard copper oxide.

H₂ chemisorption was carried out in a multipurpose unit with a mass spectrometer Pfeiffer Vaccum Prisma detector. Typically, 0.1 g of a catalyst was reduced with H₂ flow at 450 °C for 1 h, then kept under He flow for 30 min and cooled down to room temperature. H₂ was titrated in the catalysts until it did not present chemisorption.

Temperature-programmed desorption with H₂ (H₂-TPD) was carried out in a multipurpose unit with a Pfeiffer Vaccum Prisma detector mass spectrometer. After H₂ chemisorption, the temperature-programmed desorption was performed by heating in a stream of He at 50 mL·min⁻¹ between 50 °C and 1100 °C with a heating rate of 10 °C·min⁻¹.

X-Ray photoelectron spectroscopy (XPS) data of the samples was obtained with a SPECSLAB II (Phoibos-Hsa 3500 150, 9 channeltrons) SPECS spectrometer, with Al Kα source (E = 1486.6 eV) operating at 12 kV, pass energy (Epass) = 40 eV, 0.1 eV energy step and acquisition time of 1 s per point. The samples were placed on stainless steel sample-holders and transferred under an inert atmosphere to the XPS pre-chamber and held for 2 h in a vacuum atmosphere. The residual pressure inside the analysis chamber was ~ 1 × 10⁻⁹ Torr. The binding energies (BE) of the Ce 3d and O 1s spectral peaks were referenced to the C 1s peak, at 284.5 eV, providing accuracy within ± 0.2 eV. The binding energies were calibrated by using the containment carbon (C1s = 284.6 eV) and the data was carried out using Fityk software with Gaussian function. Atomic Ce³⁺ concentration is calculated by XPS via the integration of the areas of the individual peaks to obtain the total area of the Ce³⁺ (Eq. 1) and Ce⁴⁺ (Eq. 2) [70]. The determination of surface oxygen was done via Eq. 3, where O_S is the surface oxygen, O_W is the adsorbed water, and O_L is the lattice oxygen [18, 19, 71,72,73,74]. The determination of surface Ni was done via Eq. 4, where Ni_S is the surface nickel, Ni_Non is the nonlocal nickel, and Ni_B is the bulk nickel [75,76,77,78,79].

$$\% \ {Ce}^{3+}=\frac{{A}_{{Ce}^{3+}}}{({A}_{{Ce}^{3+}}+{A}_{{Ce}^{4+}})}$$

(1)

$${A}_{{Ce}^{4+}}=U+{U}^{\prime\prime}+{U^{\prime\prime\prime}}+V+{V}^{\prime\prime}+{V^{\prime\prime\prime}}$$

(2)

$$\%\:{O}_{S}=\frac{{A}_{{O}_{S}}}{({A}_{{O}_{S}}+{A}_{{O}_{L}})}$$

(3)

$$\% \ {Ni}_{S}=\frac{{Ni}_{S}}{({Ni}_{Non}+{Ni}_{S}+{Ni}_{B})}$$

(4)

2.8 Catalytic experiments: ethanol steam reforming

Catalytic experiments were conducted at atmospheric pressure in a fixed-bed quartz tubular reactor with a 5 mm inner diameter. The reactor was packed with quartz wool and 30, 50, or 100 mg of catalyst powder, resulting in bed heights of 1, 3, and 5 mm, respectively. The reactor was placed in a vertical oven equipped with a thermocouple to control the temperature at 600 °C. Before reaction, the catalysts were reduced under pure H₂ atmosphere at 400 °C for 1 h, at a flow rate of 50 mL·min⁻¹. The reduction temperature was determined through the H₂–TPR (Fig. S3). Water and ethanol (CH₃CH₂OH) steams were generated using two bubblers: one containing water and the other ethanol, to obtain a H₂O/CH₃CH₂OH molar ratio of 3 and fed into the reactor. The feed composition (assessed via ethanol vapor pressure, P_feed) was determined by Antoine equation (Eq. 5) with parameters: water (A = 4.65, B = 1435.26, C = − 64.85, and T = 65 °C) and ethanol (A = 5.37, B = 1670.41, C = − 40.19, and T = 15 °C), and Raoult’s law (P_atm = 1.01325 bar) (Eq. 6). The He inlet flow rates of the individual gas streams were controlled with mass flow meters (MKS Instruments) ranging from 10 to 80 mL·min⁻¹ to ethanol and from 23 to 181 mL·min⁻¹ to water, obtain a total flow rate from 33 to 265 mL·min⁻¹. At 33 mL·min⁻¹ and 100 mg of catalyst were used to conventional literature experiment (contact time (W/F) of 3 mg·min·mL⁻¹, Fig. S4) [18, 67, 80, 81]. At 261 mL·min⁻¹ with 30, 50, and 100 mg of catalyst were used to contact time experiments (contact time (W/F) of 0.11, 0.19, and 0.39 mg·min·mL⁻¹, Fig. S5). At 261 mL·min⁻¹ with 50 mg of catalyst were used to deactivation experiments (contact time (W/F) of 0.19 mg·min·mL⁻¹, Fig. 6). The reactants and the reaction products were analyzed by gas chromatography (GC-2014 Shimadzu) equipped with two columns. The first column was a RQTPlot-Q (30 m, 0.32 mm i.d.) that separated the heavier products before being analyzed by an FID. Some lighter compounds, mainly H₂, CO and CO₂, are not detectable by the FID, such that the FID outlet stream was treated in a cold trap and then injected into another column (Carboxen 1010, 30 m, 0.53 mm i.d.) and then into a TCD. The tubes were kept at 120 °C, monitored by thermocouples to several sections to prevent condensation, fouling, and cold spots. Data processing was carried out by the software related to the chromatograph (GCSolution, v. 2.32), which provided online measurements of the catalytic performance.

$$log{P}_{feed}=A-\frac{B}{T+C}$$

(5)

$${P}_{feed}={X}_{feed}\ast{P}_{atm}$$

(6)

The analytic calibration curve was determined for ethanol (CH₃CH₂OH), hydrogen (H₂), carbon dioxide (CO₂), carbon monoxide (CO), methane (CH₄), acetone (CH₃(CO)CH₃), acetaldehyde (CH₃CHO), ethene (C₂H₄), ethane (C₂H₆), propene (C₃H₆), and propane (C₃H₈). The calibration curve was established using 5 points. The liquids were calibrated using a bubbler and the mole fraction was adjusted by varying the bubbler temperature determine via Antoine equation (Eq. 5), Raoult’s law (Eq. 6), and Ideal gas law. Additionally, the molar fraction of the gases was adjusted by diluting them with the He carrier gas. The catalytic performance was evaluated by the conversion profiles of ethanol (X_EtOH) as in Eq. 7, where X_EtOH is the observed ethanol conversion given respectively the inlet (mol_EtOH,_in) and outlet (mol_EtOH,_out) ethanol flow rates. Selectivity for the obtained products (S_i) was evaluated as a function of the reaction time as in Eq. 8, where S_i refers to the product distribution of species i and C_i is the concentration of species i. The denominator represents the sum of the concentrations of every detected species n in the reactor outlet, like hydrogen (H₂), carbon dioxide (CO₂), carbon monoxide (CO), methane (CH₄), acetone (CH₃(CO)CH₃), acetaldehyde (CH₃CHO), and ethene (C₂H₄). The yield of i species (Y_i) is the product of ethanol conversion and species selectivity as described by Eq. 9.

$${X}_{EtOH}\:\left[\%\right]=\frac{\left({mol}_{EtOH,\:\:\:in}-{mol}_{EtOH,\:\:\:out}\right)}{{mol}_{EtOH,\:\:\:in}}\times 100$$

(7)

$$S_i\;\left[\%\right]=\frac{C_i}{\sum_{m=1}^nC_p}\times100$$

(8)

$${Y}_{i}\:\left[\%\right]={X}_{EtOH}\ast{S}_{i}$$

(9)

3 Results and discussion

The study started with the synthesis of CeO₂ nanowires and Ni/CeO₂ nanowires catalysts employing various methods to produce Ni nanoparticles on CeO₂ nanowires. In the initial step, well-defined CeO₂ nanowires with specific shape and size were synthesized through a straightforward and robust procedure based on the hydrothermal method [19], as illustrated in Fig. S1A. The CeO₂ nanowires were formed in a single step: initially, the precursor salt solution underwent precipitation upon contact with the alkali solution under vigorous stirring, forming a precipitate complex (Eq. 10). Subsequently, inside a stainless-steel autoclave, the increased temperature and pressure enhanced the reactivity of the reaction medium, creating a favorable environment for hydrothermal treatment. This treatment decomposed the Ce complex, facilitating the growth of CeO₂ nanowires (Eq. 11). The obtained CeO₂ nanowires, as shown in Fig. S2, exhibited a narrow particle size distribution according to SEM and HRTEM images, and particle size distribution. The triplicate SEM images showed measurements of 15.4 nm ± 0.6 nm, 15.5 nm ± 0.8 nm, 15.2 nm ± 1.2 nm, with an average width of 15.3 nm ± 1.1 nm and a length of a few micrometers. Each batch was synthesized with a high yield of 97%, roducing around 2.8 g of CeO₂ nanowires. The first triplicate of the CeO₂ nanowires, which exhibited the narrowest particle size distribution, was used in the following analysis. In the high-resolution transmission electron microscopy (HRTEM) image of an individual CeO₂ nanowire, the lattice fringes revealed a monocrystalline structure with interplanar spaces of 0.30 nm, corresponding to a uniform growth direction along the [82] axis [19, 83,84,85].

$${4Ce}^{3+}+4\left(y-1\right){OH}^{-}+\left(4x+2\right){H}_{2}O+{O}_{2}\rightarrow4{\left[{Ce\left({OH}^{-}\right)}_{y}{\left({H}_{2}O\right)}_{x}\right]}^{(4-y)+}$$

(10)

$$4{\left[{Ce\left({OH}^{-}\right)}_{y}{\left({H}_{2}O\right)}_{x}\right]}^{(4-y)+}+4\left(y-1\right){OH}^{-}\rightarrow{CeO}_{2}+(x+2){H}_{2}O$$

(11)

In the second step, with a focus on achieving well-dispersed Ni on the support, CeO₂ nanowires were chosen as a high surface area support to incorporate Ni using four different synthesis methods. The objective was to visualize and analyze the influence of each method on the properties of Ni nanoparticles and their consequence in surface reactivity. Figure 1 presents Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), High-Resolution Transmission Electron Microscope (HRTEM), and Energy Dispersive X-ray Spectroscopy (EDS) map images of Ni/CeO₂ nanowires reduced under the same pretreatment conditions before catalytic experiments and passivated. In the first column, SEM images of Ni/CeO₂ nanowires indicate that the different synthesis methods do not influence the morphology and size of the CeO₂ nanowires, demonstrating the robustness of the synthesis. The second and third columns display TEM and HRTEM images of Ni/CeO₂, revealing the Ni nanoparticles. The fourth column presents the elemental distribution of Ni, Ce, and O through EDS. The scale bar in the EDS maps is equivalent to 2 μm.

Fig. 1

SEM (First column), TEM (Second column), HRTEM (Third column), and EDS (Fourth column) images of Ni/CeO₂ nanowires synthesized via wet impregnation (A-G), nanoparticles impregnation (H-N), urea deposition (O-U), and BH₄ deposition (V-AB) methods. The scale bar in the fourth column is 2 μm. The white arrows in second column indicate the Ni nanoparticles

The first two synthesis methods are impregnation techniques, involving the incorporation of the Ni²⁺ salt (Ni(NO₃)₂) precursor and preformed nanoparticles onto the CeO₂ nanowires through the adsorption of Ni²⁺ on CeO₂ nanowires. In the first method, illustrated in Fig. S1B, Ni was added through a wet impregnation route, known for its effectiveness in achieving high metallic dispersion on the support surface by producing a thin Ni layer on CeO₂ nanowires. Figure 1A-G display the results for Ni/CeO₂ wet impregnation, revealing a Ni monolayer on the CeO₂ nanowires, with lattice fringes at a distance of 0.30 nm corresponding to CeO₂ nanowires. Additionally, EDS mapping images demonstrate the incorporation of a thin Ni monolayer onto the CeO₂ nanowires.

The second impregnation method, depicted in Fig. S1C, involves initially producing Ni⁰ nanoparticles through chemical reduction by NaBH₄ (Eq. 12) via homogeneous nucleation [86]. Subsequently, the Ni nanoparticles are incorporated onto CeO₂ nanowires through nanoparticle impregnation via adsorption between the metal and the support. Figure 1H-N illustrate Ni/CeO₂ nanoparticles impregnation, where the Ni nanoparticles exhibit a quasi-spherical shape with a diameter of 4.6 nm ± 0.3 nm and are well-dispersed on the CeO₂ nanowires.

The other two synthesis methods involve deposition techniques where Ni nanoparticles heterogeneously nucleate and grow on the CeO₂ nanowires through a chemical reaction. The third method, illustrated in Fig. S1D, is based on deposition-precipitation using urea, exploiting the reduction potential of Ni (ΔE° = -0.26 V). In this method, urea is hydrolyzed to form OH^‒ (Eq. 13) [86], which precipitates the Ni²⁺ ions as Ni(OH)₂ (Eq. 14). At a 90 °C in aqueous medium, urea is quickly hydrolyzed, changing the reaction color from light yellow, due to the CeO₂ nanowires dispersed, to black when in contact with Ni²⁺ ions, due to the Ni nanoparticles deposited on the surface of CeO₂ nanowires, as depicted in Fig. 1O-U. The Ni nanoparticles have a size of 12.9 nm ± 4.5 nm and a polydisperse shape. In Fig. 1P, some smaller Ni nanoparticles can be seen aggregating and merging into larger nanoparticles via the Ostwald ripening mechanism to minimize surface energy, likely due to heat treatment during calcination and/or pre-catalytic treatment. The small Ni nanoparticles formed as a result of the high temperature of the aqueous medium, which increased the hydrolysis rate and OH⁻ ions availability. The slow addition of Ni²⁺ ions promoted controlled precursor supply, leading to rapid deposition to the Ni nanoparticles nuclei without leaving remaining Ni²⁺ ions to grow.

The fourth and final method, illustrated in Fig. S1E, is based on the polyol method modified into deposition via chemical reduction by NaBH₄. In this approach, Ni²⁺ ions in the solution in ethylene glycol (EG) are reduced by BH₄⁻ to form Ni⁰ metallic nanoparticles (Eq. 15) [87]. Figure 1V-AB illustrate the Ni/CeO₂ BH₄ deposition. The Ni nanoparticles exhibit a larger size of 36.8 nm ± 6.5 nm, attributed to the high reduction kinetics promoted by NaBH₄ and the lower nucleation energy required to deposit and nucleate the nanoparticles on the surface of CeO₂ nanowires. The lattice fringes of face-centered cubic (fcc) NiO (111) planes, approximately 0.21 nm, are evident in the Ni nanoparticles [88,89,90,91].

$${{4\text{Ni}}^{2+}}_{\left(\text{aq}\right)}+{\text{BH}_4^-}_{\left(\text{aq}\right)}+{{3\text{H}}_2\text{O}}_{\left(\text{l}\right)}\rightarrow\:{4\text{Ni}}_{\left(\text{s}\right)}+{{\text{B}\left(\text{OH}\right)}_3}_{\left(\text{aq}\right)}+{{7\text{H}}^+}_{\left(\text{aq}\right)}$$

(12)

$${\text{CO}{\left({\text{NH}}_2\right)}_2}_{\left(\text{aq}\right)}+{3{\text{H}}_2\text{O}}_{\left(\text{l}\right)}\rightarrow{2\text{NH}_4^+}_{\left(\text{aq}\right)}+{{\text{CO}}_2}_{\left(\text{g}\right)}+{{2\text{OH}}^-}_{\left(\text{aq}\right)}$$

(13)

$${\text{Ni}^{2+}}_{\left(\text{aq}\right)}+{{2\text{OH}}^-}_{\left(\text{aq}\right)}\rightarrow{{\text{Ni}\left(\text{OH}\right)}_2}_{\left(\text{s}\right)}$$

(14)

$$2{\text{Ni}^{2+}}_{\left(\text{eg}\right)}+{\text{BH}_4^-}_{\left(\text{s}\right)}+{2{\text{H}}_2\text{O}}_{\left(\text{eg}\right)}+{2\text{nPVP}}_{\left(\text{eg}\right)}$$

(15)

Table 1 summarizes the textural and physical-chemical properties of Ni/CeO₂ nanowires synthesized using different methods. Crystallite size was determined through X-Ray Diffraction (XRD), while HR-TEM and SEM provided particle size information. Specific surface area, average pore size, and average pore volume were obtained via N₂ physisorption using the BET and BJH models. Reducibility was assessed through temperature-programmed reduction with H₂ (H₂-TPR). H₂ chemisorption and H₂-TPD were utilized to determine metallic surface area, dispersion, H₂ adsorption, and desorption. Lastly, Ni content was measured using Atomic Absorption Spectroscopy (AAS). The XRD pattern, N₂ Isotherm, Raman spectra, H₂-TPR, and H₂-TPD of the samples are depicted in Figs. 2 and 3.

Table 1 The obtained catalysts’ structural and surface properties were measured by XRD, HRTEM, N₂ physisorption, H₂-TPR, H₂ Chemisorption, H₂-TPD, and atomic absorption spectroscopy

Fig. 2

XRD pattern (A), N₂ isotherms (B), and pore size distribution (C) for Ni/CeO₂ nanowires synthesized via impregnation and deposition methods, along with CeO₂ nanowires. In XRD pattern (A) is depicted the CeO₂ fcc fluorite structure JCPDS 34–0394

Fig. 3

Raman spectra (A), H₂-TPR (B), and H₂-TPD (C) for Ni/CeO₂ synthesized using different methods and CeO₂ nanowires

After synthesizing and confirming the morphology and size of the nanoparticles, the Ni amount incorporated in Ni/CeO₂ nanowires was quantified by AAS, as shown in Table 1. The Ni mass fractions in Ni/CeO₂ catalysts correspond to 0.91%, 1.0%, 0.89%, and 0.95% for Ni/CeO₂ wet impregnation, nanoparticles impregnation, urea deposition, and BH₄ deposition, respectively. The AAS data demonstrate the capability of the synthesis methods to decorate the metal on the support.

XRD profiles presented exhibit well-defined peaks assigned to their corresponding crystalline phases without contaminants. All the diffraction peaks (Fig. 2A) index to 2θ values of 28° (111), 33° (200), 47° (220), 56° (311), 58° (222), 69° (400), 76° (331), 79° (420), and 88° (422) planes, corresponding to a face-centered cubic (fcc) fluorite structure for CeO₂ [92]. After Ni incorporation, the XRD profiles for the Ni/CeO₂ catalysts changed due to thermal treatment and metal-support interaction, as evident in the crystallite size calculated using Scherrer’s equation, which resulted in values of 21.7 nm, 22.4 nm, 23.8 nm, 20.2 nm, and 15.3 nm for Ni/CeO₂ wet impregnation, nanoparticles impregnation, urea deposition, BH₄ deposition, and pure CeO₂ nanowires, respectively.

It is worth noting that the difference between particle size measured by SEM microscopy and crystallite size in Table 1 is because microscopy measures the size of the whole nanoparticles and distinguishes the CeO₂ nanowires and Ni nanoparticles, while XRD measures the crystallite size. Furthermore, no peaks corresponding to Ni-based species were detected, which aligns with their small sizes and low Ni loadings in the samples. Additionally, as known, the scale decreases as the surface energy of a material increases due to the surface-to-volume ratio and the presence of dangling bonds, causing the contraction of the crystalline lattice in nanomaterials, which can be seen in CeO₂ nanowires crystallite size in Table 1 [67, 93, 94]. The incorporation of the metal on the support promotes metal-support interaction based on electron transfer and attractive forces. The equilibrium in the electron transfer satisfies the dangling bonds and relax the tensioned bonds and increase the crystallite size [95, 96]. This phenomenon is depicted in Table 1, while incorporating Ni on the surface of the CeO₂ nanowires promotes an increment in the crystallite size of the Ni/CeO₂ nanowires, indicating a presence of metal-support interaction [95, 96].

Figure 2B; Table 1 present the N₂ adsorption-desorption isotherm curve of Ni/CeO₂ synthesized using different methods, CeO₂ nanowires, and detailed textural parameters. The isotherms of all catalysts exhibited II-type isotherms, characteristic of mesoporous materials, and the hysteresis loop shape corresponded to type H1 according to the IUPAC classification [19]. This corresponds to macropores that may be attributed to the aggregation of CeO₂ nanowires and a narrow range of uniform mesopores. A previous report by our group depicted TEM images of circular pores in CeO₂ nanowires with a diameter of ~ 2 nm and N₂ adsorption isotherm [19].

The catalysts exhibited specific surface areas, average pore sizes, and average pore volumes in the range of 32 to 56 m²·g⁻¹, 219 to 323 Å, and 0.25 to 0.38 cm³·g⁻¹, respectively. Moreover, the internal and external surface areas clarify the nanostructures disposal over both surfaces. These broad ranges are a consequence of the different sizes of Ni nanostructures. For instance, Ni/CeO₂ wet impregnation forms a thin layer, resulting in a specific surface area similar to that of CeO₂ nanowires. Here, internal surface is increased probably due to the formation of nanostructures with a rough surface inconsequence of a remained condensed Ni²⁺ solution into the pores. Ni/CeO₂ nanoparticles impregnation, being smaller in structure than other methods, shows lower values for textural properties, indicating potential nanoparticle blockage of pores. The internal surface area corroborates depicting that the Ni nanoparticles are filling the pores, and minor modifications in the external area in comparation of CeO₂ nanowires.

Ni/CeO₂ urea and BH₄ deposition methods produce intermediate and larger-sized Ni nanoparticles, contributing to higher specific surface areas. Figure 2C illustrates the pore diameter distribution for Ni/CeO₂ nanowires. The pore distribution in CeO₂ nanowires exhibits different pores with higher pore diameter, potentially corresponding to twisted CeO₂ nanowires. After Ni incorporation, a peak at a lower pore diameter increases to a narrow peak, suggesting that small nanoparticles block the pores and decrease the pore diameter. Moreover, both deposition methodologies show an increase in the external surface area, indicating that the Ni nanoparticles were beneficially deposited on the external surface.

Raman Spectroscopy is employed to determine the molecular structure and oxygen vacancies by analyzing molecular vibrational and rotational modes, providing a structural fingerprint for each system [97,98,99]. Figure 3A displays the Raman spectra normalized for Ni/CeO₂ and CeO₂ nanowires. CeO₂ exhibits an intense and narrow peak at 470 cm⁻¹ associated with the fluorite structure in the F_2g mode. This peak is assigned to the symmetric stretching vibration of the eight oxygen atoms around Ce⁴⁺ ions due to the well-stoichiometric ratio [67]. The substitution of Ce⁴⁺ cations by Ce³⁺ causes an expansion of the lattice, leading to a broad and shifted F_2g peak at 556 cm⁻¹. Additionally, a low-intensity and broad peak at 638 cm⁻¹ is related to oxygen vacancies produced by the reduction of cerium ions [19, 69]. The NiO spectra show two peaks at 556 cm⁻¹ and 638 cm⁻¹, associated with the Raman symmetric stretching mode of Ni – O [100, 101]. However, as the CeO₂ and NiO exhibit a peak at 638 cm⁻¹, it is no able to verify the oxygen vacancies via Raman spectroscopy for Ni/CeO₂ nanostructures. Due to the low amount of Ni, small particle size, and high metal dispersion of Ni on the CeO₂ nanowire surfaces, the peaks attributed to the Ni nanoparticles exhibit low intensity for Ni/CeO₂ wet impregnation, nanoparticle impregnation, and urea deposition. However, Ni/CeO₂ BH₄ deposition, with a larger particle size, contributes to higher intensity peaks at 556 and 638 cm⁻¹, possibly indicating a higher concentration of oxygen vacancies.

The Temperature-Programmed Reduction with hydrogen (H₂-TPR) measures the reduction temperature and identifies the type and strength of the metal-support interaction in Ni/CeO₂ synthesized using different methods and CeO₂ nanowires. The reducibility of the CeO₂ nanowire support is a crucial parameter in catalytic applications, as metals can interact with metal oxides, altering their redox properties and influencing catalytic activity [71, 73, 102]. Pure CeO₂ typically exhibits two or three peaks related to the reduction of surface and bulk oxide. The first peak, occurring at temperatures below 500 °C, is linked to the reduction of surface oxide and active oxygen species. The second peak, typically observed in the temperature range between 500 °C and 650 °C, is attributed to the reduction of non-stoichiometric oxides with a composition of Ce_yO_x. The third and final peak occurs at temperatures higher than 800 °C and is associated with the bulk reduction of CeO₂ to Ce₂O₃ [71, 73, 102].

According to the literature, Du et al. describe that NiO exhibits two peaks during H₂-TPR analysis [103]. The first peak, ranging from 265 °C to 285 °C, is attributed to the reduction of weakly interactive NiO species with CeO₂ support. The second peak, observed at 360 °C to 380 °C, is related to the reduction of intensely interactive NiO species with CeO₂ support [101, 104]. Atzori et al. describe that NiO has two peaks [105]. The first peak is located at temperatures lower than 350 °C, ascribed to oxygen species reduction in oxygen vacancies generated by weak interaction with nickel. The locality of the second peak is at a temperature range between 400 °C and 600 °C, ascribed to the reduction of NiO species strongly interacting with CeO₂ [104,105,106,107].

Figure 3B; Table 1 present the H₂-TPR profiles of Ni/CeO₂ synthesized using different methods and CeO₂ nanowires, along with quantitative reducibility. CeO₂ nanowires exhibit two region peak temperatures between 400 °C and 600 °C and 600 °C and 1000 °C. The first region shows two peaks centered at 487 °C and 526 °C, corresponding to surface reduction of non-stoichiometric CeO₂ with active oxygen, surface Ce³⁺ (Ce³⁺−O − Ce⁴⁺), surface oxygen defects, and surface oxygen of stoichiometric CeO₂ (Ce⁴⁺−O − Ce⁴⁺). The second region displays two peaks at 770 °C and 980 °C, corresponding to the reduction of bulk CeO₂ [71, 73, 102]. For all Ni/CeO₂ synthesis methods, the second region is evident, with peak centers between 940 °C and 960 °C. However, the Ni/CeO₂ synthesis methods exhibit different temperatures for the first region, suggesting varying intensities of metal-support interaction due to the differences in synthetic methodologies [93].

The Ni/CeO₂ wet impregnation exhibited two peaks at 287 °C and 365 °C, corresponding to the reduction of NiO with weak interaction with the CeO₂ surface. Both peaks contribute to 52.2% of the reducibility. In contrast, Ni/CeO₂ nanoparticles impregnation displayed a single intense peak centered at 653 °C, indicating NiO reduction with strong interaction and 49.1% reducibility. Ni/CeO₂ urea deposition revealed a peak at 270 °C with 52.5% reducibility and weak interaction. Ni/CeO₂ BH₄ deposition exhibited two peaks at 400 °C and 530 °C, representing reductions of NiO with mild-strong interaction and reduction of the CeO₂ surface, contributing to 27.3% of the reducibility.

The difference in H₂-TPR profiles can be attributed to varying intensities of the metal-support interaction, influenced by differences in synthetic methodologies.

During wet impregnation, slow evaporation of the solvent facilitates the formation of a well-dispersed Ni thin film. This method promotes a minimized metal-support interaction, thereby exhibiting a weak interaction between Ni and CeO₂. In contrast, nanoparticles impregnation involves the adsorption of pre-formed small Ni nanoparticles onto the surface of CeO₂. This method does not chemically modify the CeO₂ surface and allows for a strong interaction between Ni nanoparticles and unmodified, strong negative oxygen vacancies, as observed in the CeO₂ nanowires profile. However, deposition methods involving chemical reactions to deposit Ni nanoparticles can modify the CeO₂ surface. Urea deposition results in uncontrolled nanoparticle formation, suggesting that Ni nanoparticles undergo an energy minimization mechanism, promoting a weak metal-support interaction. On the other hand, BH₄ deposition involves the reduction of Ni²⁺ ions, which can act as nucleation sites on the CeO₂ surface, leading to a moderate to strong metal-support interaction, as indicated by the H₂-TPR profile. These adjustments aim to clarify the influence of synthetic methods on the metal-support interaction in our study.

Figure S3 illustrates the H₂-TPR profiles with 2% H₂/He and pure H₂. The profiles remain consistent with a shift toward lower temperatures, with the peak corresponding to the reduction of bulk CeO₂ splitting into two peaks. The first may correspond to the reduction of the CeO₂ surface, and the second to bulk reduction. The reduction profile provides insights into the size of Ni nanoparticles. Wet impregnation, forming thin layers, results in weak metal-support interaction. Nanoparticle impregnation, with smaller sizes, leads to strong metal-support interaction, evident from the high-temperature peak. Urea deposition, potentially producing small-sized Ni nanoparticles with high surface energy, exhibits an early peak. In contrast, BH₄ deposition, with the largest nanoparticles, shows intermediate metal-support interaction.

The reduction of catalysts occurred under a pure H₂ flow rate, and H₂-TPR was carried out with diluted H₂, known to increase the reduction rate and potentially produce different phases. This approach aims to gain further insights into the catalyst under catalytic conditions. Figure S3 presents a comparison between the reduction profiles with diluted H₂ (Fig. S3A) and pure H₂ (Fig. S3B). The reduction profile remains consistent below 450 °C, with a slight shift of the two initial peaks at 287 °C and 365 °C to 218 °C and 272 °C. Notably, the reduction peak associated with CeO₂ at 958 °C splits into two peaks at 655 °C and 1,000 °C, possibly linked to the reduction of CeO₂ to Ce₂O₃, as reported in previous studies [71, 73, 102]. Moreover, it is important analyze Fig. S3B to determine the reduction temperature of the nickel oxides to metallic nickel nanoparticles, which the first reduction area ends at 400 °C.

Figure 3C; Table 1 present the Temperature-Programmed Desorption of hydrogen (H₂-TPD) for the Ni/CeO₂ synthesis methods, providing quantitative information on reducibility. H₂-TPD offers insights into variations in metal-support interactions. According to the literature, H₂ desorption peaks at low temperatures (≤ 250 °C) are attributed to hydrogen species weakly bonded to active nickel sites [107, 108]. The temperature range between 250 °C and 450 °C is associated with strong bonding to nickel, while temperatures exceeding 450 °C indicate very strong chemisorption on the nickel surface [107, 108]. The H₂-TPD profiles for Ni/CeO₂ synthesis methods exhibit desorption peaks below 130 °C, corresponding to weak interactions. Peaks between 300 °C and 450 °C indicate strong interactions, and peaks above 450 °C suggest very strong interactions or may be related to nickel and other transition metals serving as hydrogen reservoirs. These metals can adsorb and store substantial amounts of hydrogen on the metal-support interface or as spilled-over species. The higher desorbed amounts of hydrogen than adsorbed amounts are attributed to H₂ molecules retained on the Ni nanostructures during pretreatment, as depict in H₂ adsorbed and desorbed in Table 1 [82, 107,108,109,110,111].

The Ni/CeO₂ wet impregnation exhibited two desorption peaks at 78 °C and 127 °C, attributed to two different intensities of H-Ni interaction—very weak interaction and weak interaction, respectively. The Ni/CeO₂ nanoparticles impregnation exhibited four desorption peaks, including two intensity peaks at 70 °C and 125 °C, and two lower intensity peaks at 623 °C and 955 °C. The peaks at low temperatures are associated with very weak and weak interactions, while the peaks at high temperatures are associated with very strong interaction or H₂ retention.

The Ni/CeO₂ urea deposition exhibits three prominent peaks at 114, 337, and 984 °C. The peak at low temperature is associated with weak interaction, the intermediate peak is associated with strong interaction, and the high-temperature peak is associated with very strong interaction. When comparing impregnation methods, the first desorption peak can be associated with mild interaction, and the second desorption peak with strong interaction and H₂ retention.

Lastly, Ni/CeO₂ BH₄ deposition exhibits five desorption peaks at 118, 370, 550, 767, and 962 °C. Classical nickel-based catalysts typically show two or more peaks at different temperatures due to the formation of several active sites. The first peak can be associated with mild interaction, while the other peaks are associated with strong and very strong interaction, as well as H₂ retention. These results indicate that the impregnation methods result in weak H-Ni interaction at metal sites, and the impregnated catalysts desorb H₂ at low temperatures, demonstrating good H₂ lability. In contrast, the deposition methods show considerable H-Ni solid interaction at metal sites, indicating less favorable H₂ lability. It is possible that the catalysts prepared through deposition methods have higher interaction with H₂ on the active sites.

X-ray Photoelectron Spectroscopy (XPS) analyses (Fig. 4, Tables 2, and S1) were performed to identify surface composition and gain further insights into the Ni/CeO₂ different synthesis methods and CeO₂ nanowires. Figure 4 presents the Ce 3d (Fig. 4A), O 1s (Fig. 4B), and Ni 2p (Fig. 4C) core-level spectra for Ni/CeO₂ different synthesis methods and CeO₂ nanowires. The Ce 3d region exhibits two multiplets with ten peaks, where V and U indicate the spin-orbit coupling Ce 3d_5/2 and Ce 3d_3/2, respectively. The Ce 3d spectra consist of peaks assigned to Ce⁴⁺ and Ce³⁺ species on the surface. The peaks V, V’’, V’’’, U, U’’, and U’’’ correspond to the Ce⁴⁺ 3d¹⁰ 4f⁰ state, while V₀, V’, U₀, and U’ correspond to the Ce³⁺ 3d¹⁰ 4f¹ state (Table S1) [19, 72, 73, 102].

Fig. 4

XPS spectra of (A) Ce 3d, (B) O 1s, and (C) Ni 2p for Ni/CeO₂ synthesized using different methods and CeO₂ nanowires

Table 2 XPS parameters for Ce³⁺ content (at%), O_S content (at%), and Ni_S (%) were obtained from the ce 3d, O 1s, and Ni 2p regions for the Ni/CeO₂ different synthesis methods and CeO₂ nanowires

When the dimensions of CeO₂ are reduced from the bulk to the nanoscale, the geometric and electronic parameters are influenced by the increased exposed atoms on the surface and the dangling bond. This promotes a lattice tension that raises the surface free energy [67, 93, 94]. Consequently, the high tension in the electrotonic structure and crystalline lattice creates an unfavorable environment for stoichiometric oxide [112, 113]. Unstable atomic bonds lead to the release of oxygen atoms, leaving an excess of electrons in the material. The release of oxygen atoms results in the formation of oxygen vacancies [2]. The electronic structure of the crystal is stabilized as empty levels capture of cation and redistribute the electrons resulting from the released oxygen. This process changes the oxidation state of cerium ions from Ce⁴⁺ to Ce³⁺ [2, 114,115,116]. The Ce³⁺ ions are distributed in regions of sesquioxide Ce₂O₃ around oxygen vacancies in CeO₂. XPS deconvolution and integration of the peaks are used to determine the atomic Ce³⁺ concentration [70]. Table 2 summarizes the surface composition of the Ni/CeO₂ different synthesis methods and CeO₂ nanowires. The percentage of Ce³⁺ onto the CeO₂ nanowires is 24.53%, and amount of Ce³⁺ increases withthe size.

To validate this observation, we examined the O 1s XPS spectra (Fig. 4B). The O 1s XPS spectra consist of three peaks representing lattice oxygen (O_L) between 531.2 and 530.3 eV, surface oxygen ions, or oxygen vacancies (O_S) between 532.2 and 531.9 eV, and adsorbed water (O_W) between 534.6 and 534.2 eV (Table S1) [19, 117]. The percentage of O_S in the samples, as shown in Table 2, reveals the proportion of surface oxygen in the samples [18, 19, 71,72,73,74]. It is important to note that the percentage of the O_S is the sum of surface oxygen in the CeO₂ nanowires and oxidized Ni nanoparticles as NiO.

The data indicates that the wet impregnation method does not extremely modify the surface of CeO₂ nanowires with the addition of Ni, as analyzed by the Ce³⁺ and O_S contentes. However, in the nanoparticle impregnation method, small Ni nanoparticles (4.6 nm) were adsorbed on the surface of CeO₂ nanowires, potentially occupying more electronic sites and promoting a strong metal-supporting interaction, as observed in the H₂-TPR and Ce³⁺ content. The deposition methods depicted a higher O_S content, suggesting a chemical interaction between nickel and support involving higher electron transfer or potential formation of a Ni − O−Ce spinel at the interface [118]. Additionally, larger Ni nanoparticles may have a higher amount of NiO on the surface.

Figure 4C displays the Ni 2p_3/2 XPS spectra. The electronic structure of NiO can be described as a rearrangement of the Ni final state of the 3d⁸, 3d⁹L, and 3d¹⁰L² (where L denotes a hole in the ligand band and c denotes a Ni 2p core level hole) level of the electrons in the valence band configuration affected in photoemission [75,76,77,78,79]. The Ni 2p_3/2 XPS spectra consist of four peaks corresponding to a configuration screened by an oxygen electron at the bulk cluster (Ni_B; red line; c3d⁹L:3d⁸), oxygen electron at the surface (Ni_S; green line; 3d⁸:c3d⁹L), non-local electron coming from the neighboring Ni ion (Ni_i; blue; c3d⁹:3d⁷ and 3d⁷:c3d⁹), and a satellite peak (cyan). The shifts in the Ni 2p spectra can be attributed to significant hydroxylation associated with the surface-state effects and residual charging [75,76,77,78,79]. As indicated in Fig. 3, the percentage of surface Ni increases with smaller nanostructures, as thin film to wet impregnation and 4.6 nm to nanoparticles impregnation, and is bigger nanoparticles, as urea and BH₄ depositions with 12.9 and 36.8 nm, respectively, from 35.48%, 36.30%, 3.85%, ad 29.12 .

Here, we understand that the Ni/CeO₂ nanomaterial has influenced the different synthesis methods to create four structures and nanoparticles of various sizes, affecting different nickel properties such as reducibility, surface composition, oxygen vacancies, and metal-support interaction. It highlights the combination of exceptional properties makes possible the application of Ni/CeO₂ synthesized through different methods as an effective catalyst for ethanol steam reforming and potentially for many other reactions. Subsequently, following the synthesis and characterization of Ni/CeO₂ catalysts via various methods, our focus shifted to investigating their catalytic properties, performance, and the impact of synthesis methods on surface reactivity in ethanol steam reforming, as illustrated in Fig. 5A for the chemical reaction and depicted coproducts in Fig. 5C.

Fig. 5

Ethanol Steam Reforming catalytic test showing the ethanol conversion, selectivity, and H₂ yield is employing the CeO₂ nanowires. The experiments were performed at 600 °C, H₂O/CH₃CH₂OH molar ratio = 3, 50 mg of catalyst, and gas flow = 261 mL·min^-1

We conducted the catalytic experiments under the standard conditions reported in the literature: reactions were set with an H₂O/CH₃CH₂OH molar ratio of 3, using 100 mg of catalyst, at 600 °C, and with a gas flow of 32 mL·min⁻¹, to achieve a contact time (W/F) of 3 mg·min·mL⁻¹ (Fig. S4) [18, 24, 67, 80, 81]. However, all catalysts exhibit 100% converion during the 24 h reaction period, which did not differentiate the influence of synthesis methods on the surface reactivity of these catalysts. To gain further insight into the surface reactivity of Ni/CeO₂ catalysts, the contact time (Weight / Flow rate) was reduced by 16 times, from 3.0 to 0.2 mg·min·mL⁻¹. Figure S5 illustrates the influence of contact time on the conversion and selectivity of Ni/CeO₂ catalysts. It is interesting to note that at the initial point of analysis, 5 min into the reaction, there is a minimal difference in ethanol conversion of 100% at 3 m·min·mL⁻¹ (Fig. S4) and 97% at 0.1 mg·min·mL⁻¹ (Fig. S5). These small differences in ethanol conversion indicate that the kinetic regime has not been reached and the catalysts are still in the diffusional regime.

To ensure data reliability, measurements were taken during low contact time reactions. Figure S6A displays the average and error distribution for conversion and selectivity of the Ni/CeO₂ wet impregnation catalyst triplicates (Fig. S6B-D). The consistent average conversion and selectivity, coupled with low error distribution, indicate reproducible catalytic experiments. Calculating the main error from the triplicate conversion during the 48 h catalytic test yielded a value of 1.6%.

The catalytic experiments were conducted at the maximum conditions supported by experimental catalytic system at 0.19 mg·min·mL⁻¹ under a gas feed flow of 261 mL·min⁻¹ and using 50 mg of catalyst. Figures 5 and 6 depict the ethanol conversion and selectivity for CeO₂ nanowires and Ni/CeO₂ nanowires, respectively. The catalytic test lasted 48 h, aiming to distinguish the activity performance and stability of the catalysts under various conditions. The CeO₂ nanowires revealed high ethanol conversion around 100% and stable selectivity throughout the entire reaction period. As expected, the CeO₂ nanowires exhibit higher selectivity to C₂ compounds, such as ethene, and lower selectivity to C₁ compounds, such as methane, and H₂ at higher contact times [119,120,121]. These data align with literature findings, as ethanol is adsorbed on the CeO₂ surfaces through dissociative adsorption in a dehydrogenation reaction to form adsorbed ethoxy (CH₃CH₂OH → CH₃CH₂O* + H*), followed by the dehydroxylation of ethoxy to ethene (CH₃CH₂O* → CH₂CH_2(g) + OH*) [119,120,121]. It is noteworthy that acetaldehyde can be adsorbed on the surface due to oxygen in the carbonyl group, while ethylene does not form a stable adsorption product and desorbs immediately [119, 120]. According to Mudiyanselage et al. [120], surface oxygen defects accelerate the ethanol reaction, explaining the high ethanol conversion for CeO₂ nanowires and corroborating the XPS data regarding the high oxygen surface content.

Fig. 6

Illustration of the ethanol steam reforming reaction (A), the reaction products (B), and catalytic reaction tests showing ethanol conversion, selectivity, and H₂ yield employing Ni/CeO₂ synthesized via wet impregnation (C), nanoparticle impregnation (D), urea precipitation deposition (E), and BH₄ reduction deposition (F). The experiments were performed at 600 °C, with a H₂O/CH₃CH₂OH molar ratio of 3, 50 mg of catalyst, and gas flow of 261 mL·min⁻¹

Here, after nickel incorporation onto the CeO₂ nanowires, the different synthesis methods of Ni/CeO₂ promoted the ethanol steam reforming reaction (Eq. 20 and depicted in Fig. 6A). It is notable that the different synthesis methods influenced the catalytic activity of Ni/CeO₂. The impregnation methods displayed a similar catalytic profile with high ethanol conversion and H₂ selectivity. Furthermore, at the end of the first 24 h, wet impregnation exhibits 6% deactivation, while nanoparticle impregnation exhibits 14% deactivation. After 48 h, Ni/CeO₂ wet impregnation (Fig. 6C) exhibited 20% deactivation and 61% H₂ selectivity, whereas Ni/CeO₂ nanoparticles impregnation (Fig. 6D) exhibited 27% deactivation and 55% H₂ selectivity. Both catalysts exhibit low selectivity for C₂ compounds, less than 3% for acetaldehyde and ethene, demonstrating high surface reactivity to completely follow the reaction pathway to final products. However, the deposition methods displayed strong deactivation and different reaction profiles. After 48 h of the experiment, Fig. 6E shows that the Ni/CeO₂ urea deposition catalytic profile exhibits 86% deactivation, low H₂ selectivity at 23%, and considerable selectivity towards acetaldehyde (39%) and ethene (23%). Figure 6F shows the Ni/CeO₂ BH₄ deposition catalytic profile, exhibiting strong deactivation (62%) but high H₂ selectivity (55%). However, the Ni/CeO₂ urea deposition depicts a change in the reaction mechanism. Initially, the mechanism focused on ethene, methane, and H₂ production. After strong deactivation, the mechanism shifts to acetaldehyde production.

Catalytic profiles are influenced by changes in active sites and the reducibility of catalysts. Wet impregnation resulted in two distinct peaks at lower temperatures (287 and 368 °C), indicative of strong metal-support interactions and enhanced activity sites. Additionally, H₂-TPD analysis revealed merged desorption peaks at lower temperatures (78 and 127 °C), suggesting facile H₂ release, possibly due to well-dispersed Ni forming a thin film with uniform active sites. Nanoparticles impregnation exhibited a prominent reduction peak at higher temperature (663 °C), indicating weaker metal-support interactions and fewer active sites. Correspondingly, H₂-TPD showed smaller peaks at higher temperatures (623 and 955 °C), suggesting poor H₂ utilization and site deactivation. Urea deposition exhibited a reduction peak at a lower temperature (270 °C), signifying strong metal-support interactions and increased active sites. However, H₂-TPD showed a significant peak at higher temperatures (600 to 1,000 °C), indicating strong hydrogen interaction with active sites. These strong sites deactivated initially during catalyst testing, with lower intensity active sites operating in the subsequent mechanism [27, 122, 123]. BH₄ deposition displayed two merged peaks between 100 and 600 °C, indicative of non-uniform active sites, which was confirmed by H₂-TPD showing a range-like profile with multiple desorption peaks. In conclusion, utilizing H₂-TPR and H₂-TPD to estimate metal-support interactions and active sites reveals a trend where strong metal-support interactions lead to more active sites. However, the homogeneity and intensity of these active sites significantly influence catalytic behavior, with more intense desorption peaks at higher temperatures indicating faster deactivation compared to weaker desorption sites.

According to the literature, the ethanol steam reforming mechanism can change depending on factors such as adsorption energy, acidity and basicity of the catalyst, active metal, catalyst support, H₂O/ethanol ratio, and the ability of the active metal to cleave C–C, C–H, and C–O bonds [56, 124,125,126]. Essentially, the mechanism begins with the dissociative adsorption of ethanol in a dehydrogenation process forming adsorbed ethoxy (CH₃CH₂OH → CH₃CH₂O*). Then, ethanol can undergo decomposition (Eq. 16), dehydrogenation (Eq. 17), or dehydration (Eq. 18) to produce CH₄, CO, H₂, acetaldehyde, and ethene, respectively [127,128,129,130]. These products can either desorb or continue reacting. Ethene can further decompose into solid carbon and H₂ (Eq. 19) [123, 126]. Acetaldehyde may decompose to methane and CO (Eq. 20) or undergo steam reforming to produce H₂ and CO (Eq. 21) [131, 132]. Finally, several reactions involving C₁ compounds may occur, such as methanation (Eqs. 22 and 23), water gas shift (Eq. 24), carbon hydrogenation (Eq. 25) (references [133, 134]), and methane steam reforming (Eq. 26), among others [128, 133].

$${{\text{C}}_2{\text{H}}_5\text{OH}}_{\left(\text{ads}\right)}\rightarrow\:{{\text{CH}}_4}_{\left(\text{g}\right)}+{\text{CO}}_{\left(\text{g}\right)}+{{\text{H}}_2}_{\left(\text{g}\right)}$$

(16)

$$\:{{\text{CH}}_3{\text{CH}}_2\text{OH}}_{\left(\text{g}\right)}\rightarrow{{\text{CH}}_3\text{CHO}}_{\left(\text{g}\right)}+{{\text{H}}_2}_{\left(\text{g}\right)}$$

(17)

$${{\text{CH}}_3{\text{CH}}_2\text{OH}}_{\left(\text{g}\right)}\rightarrow{{\text{C}}_2{\text{H}}_4}_{\left(\text{g}\right)}+{{\text{H}}_2\text{O}}_{\left(\text{g}\right)}$$

(18)

$${{\text{C}}_2{\text{H}}_4}_{\left(\text{ads}\right)}\rightarrow2 \ {\text{C}}_{\left(\text{s}\right)}+{2{\text{H}}_2}_{\left(\text{g}\right)}$$

(19)

$${{\text{CH}}_3\text{CHO}}_{\left(\text{g}\right)}\rightarrow{{\text{CH}}_4}_{\left(\text{g}\right)}+{\text{CO}}_{\left(\text{g}\right)}$$

(20)

$${{\text{CH}}_3\text{COH}}_{\left(\text{ads}\right)}+{{\text{H}}_2\text{O}}_{\left(\text{ads}\right)}\rightarrow{3{\text{H}}_2}_{\left(\text{g}\right)}+{2\text{CO}}_{\left(\text{g}\right)}$$

(21)

$${3{\text{H}}_2}_{\left(\text{g}\right)}+{\text{CO}}_{\left(\text{g}\right)}\rightarrow{{\text{CH}}_4}_{\left(\text{g}\right)}+{{\text{H}}_2\text{O}}_{\left(\text{g}\right)}$$

(22)

$${4{\text{H}}_2}_{\left(\text{ads}\right)}+{{\text{CO}}_2}_{\left(\text{ads}\right)}\rightarrow{{\text{CH}}_4}_{\left(\text{g}\right)}+{2{\text{H}}_2\text{O}}_{\left(\text{g}\right)}$$

(23)

$${\text{CO}}_{\left(\text{ads}\right)}+{{\text{H}}_2\text{O}}_{\left(\text{ads}\right)}\rightarrow{{\text{CO}}_2}_{\left(\text{g}\right)}+{{\text{H}}_2}_{\left(\text{g}\right)}$$

(24)

$${\text{C}}_{\left(\text{s}\right)}+{{\text{H}}_2}_{\left(\text{ads}\right)}\rightarrow{{\text{CH}}_4}_{\left(\text{g}\right)}$$

(25)

$${{\text{CH}}_4}_{\left(\text{ads}\right)}+{{\text{H}}_2\text{O}}_{\left(\text{ads}\right)}\rightarrow{\text{CO}}_{\left(\text{g}\right)}+{3{\text{H}}_2}_{\left(\text{g}\right)}$$

(26)

The catalyst deactivation follows the trend: CeO₂ nanowires < wet impregnation < nanoparticles impregnation < BH₄ deposition < urea deposition, displaying deactivation of 3%, 20%, 27%, 62%, and 86%, respectively. The CeO₂ nanowires exhibited the lowest catalytic deactivation, likely because of its significant oxygen storage capability and oxygen mobility at the surface due to the presence of Ce³⁺ and Ce⁴⁺ ions. These ions are associated with preventing the deposition of solid carbon compounds at the active sites. This characteristic is evident in the XPS analysis, which shows that CeO₂ nanowires have 70% of oxygen on the surface. Furthermore, the oxygen vacancies at the surface likely adsorb ethanol via hydroxyls as part of the reaction mechanism, leading to the scission of the O – C bond to form ethene in high proportions, as depicted in ethene selectivity and yield. It is important to note that CeO₂ nanowires can help prevent catalytic deactivation in Ni/CeO₂. However, the metal-support interaction can alter the activity of CeO₂ nanowires.

After the catalytic experiments, all Ni/CeO₂ synthesized using different methods and CeO₂ nanowires catalysts were characterized by SEM images, XRD spectra, TGA profiles, and Raman spectra (Fig. 7). The SEM images (Fig. 7A-E) reveal that the overall sizes and shapes of Ni/CeO₂ and CeO₂ nanowires remained unchanged after 48 h of catalytic experiments at 261 mL·min⁻¹, 50 mg, and 600 °C. A careful analysis shows different amounts of carbon deposits on the surface of Ni/CeO₂ and CeO₂ nanowires. XRD analyses (Fig. 7F) indicate well-defined peaks assigned to CeO₂, suggesting that the overall structures of Ni/CeO₂ and CeO₂ nanowires remained unchanged after the catalytic experiments. These data demonstrate the robustness of CeO₂ nanowires. TGA (Fig. 7G) depicts the investigation for the formation of solid carbon-based structures. Raman analyses (Fig. 7H) present the deposition of solid carbon-based materials with different structures over the catalysts. Ethanol steam reforming can also produce carbon structures through reactions such as the decomposition of methane (Eq. 27), ethene (Eq. 28), acetaldehyde (Eq. 29), and acetone (Eq. 30) [126, 134].

$${CH_4}_{\left(ads\right)}\rightarrow C_{\left(s\right)}+{2H_2}_{\left(g\right)}$$

(27)

$${{C}_{2}{H}_{4}}_{\left(ads\right)}\rightarrow2 \ {C}_{\left(s\right)}+{2{H}_{2}}_{\left(g\right)}$$

(28)

$${{C}_{3}CHO}_{\left(ads\right)}\rightarrow 4 \ {C}_{\left(s\right)}$$

(29)

$${C{H}_{3}\left(CO\right)C{H}_{3}}_{\left(ads\right)}\rightarrow {C}_{\left(s\right)}$$

(30)

Fig. 7

SEM images of the CeO₂ nanowires (A), Ni/CeO₂ wet impregnation (B), nanoparticles impregnation (C), urea deposition (D), and BH₄ deposition (E), along with XRD spectra (F), TGA profile (G), and Raman spectra (H) for Ni/CeO₂ synthesized using different methods and CeO₂ nanowires after the catalytic experiment

The CeO₂ nanowires show a small amount of carbon mass on the surface, with mass loss exhibiting a narrow main event around 350 °C and progressive loss until 1,000 °C. The Raman spectra indicate well-crystallized carbon deposition with an I_D/I_G ratio of 0.9. This corroborates the catalytic experiment, suggesting that the CeO₂ nanowires remained active for the entire 48 h, and XPS analysis indicates that oxygen vacancies are crucial in preventing excessive carbon deposition and deactivation. Furthermore, an efficient synthesis method to incorporate an active metal on the support surface can enhance catalytic activity and surface reactivity to be more stable. This is evident in the Ni/CeO₂ wet impregnation method, which shows a clear catalyst surface with minimal carbon deposition, lower mass loss compared to CeO₂ nanowires, and no detection of the D and G bands in the Raman spectra. TGA analysis reveals that Ni/CeO₂ wet impregnation exhibits the lowest mass loss (13.3%) with a well-defined event at 500 °C, and SEM images confirm the absence of a considerable amount of solid carbon on the surfaces of the nanowires in the Ni/CeO₂ wet impregnation sample.

In sequence, the effect of size and metal-support interaction may contribute to carbon deposition. Comparatively, the Ni/CeO₂ wet impregnation is characterized by a thin layer and weak metal-support interaction, contributing to low carbon deposition. This can be attributed to the thin layer promoting homogeneous active sites, as observed in H₂-TPD, and providing support to prevent carbon deposition [27, 122]. However, modifying the structure to spherical Ni nanoparticles with a size of 5 nm and weak metal-support interaction, the Ni/CeO₂ nanoparticles impregnation exhibited higher catalytic deactivation due to increased carbon deposition (Fig. 7B) and a mass loss of 25.6%. Incrasing particle size and enhancing metal-support interaction, the Ni/CeO₂ urea deposition formed agglomerated nanoparticles around 15 nm with very strong metal-support interaction, resulting in a significant amount of carbon deposition. Figure 7C displays a thick carbon mass layer covering the Ni/CeO₂ urea deposition and the most pronounced mass loss among the catalysts. This explains the highest deactivation observed within the first 24 h of the catalytic experiment. It may be associated with the previous supposition that high energetic sites are titrated by chemisorbed reactants and products, as evidenced by H₂-TPD where H₂ was desorbed at a higher temperature. Additionally, a lower metal-support interaction in the Ni/CeO₂ BH₄ deposition results in intermediate carbon deposition with carbon spots on the catalyst (Fig. 7D) and 43.0% mass oss. The Raman spectra show similar carbon structures for Ni/CeO₂ nanoparticles impregnation (I_D/I_G = 1.5), urea deposition (I_D/I_G = 1.4), and BH₄ deposition (I_D/I_G = 1.5). This demonstrates the influence of the incorporation synthesis methods of the active phase on changing the surface reactivity of the catalysts.

4 Conclusions

In summary, our results demonstrate the synthesis of Ni/CeO₂ using four different methods: (i) wet impregnation, (ii) nanoparticles impregnation, (iii) urea deposition, and (iv) BH₄ deposition, resulting in well-defined shapes, sizes, and compositions, all showing high performance in ethanol steam reforming. These synthesis methods produced four distinct nanostructures and particle sizes for nickel: (i) a thin layer on the CeO₂ nanowires, (ii) small nanoparticles of 5 nm, (iii) agglomerated nanoparticles of 15 nm, and (iv) large nanoparticles of 35 nm. In conclusion, three synthesis methods—wet impregnation, nanoparticles impregnation, and BH₄ deposition—yielded catalysts with controlled characteristics and properties after heating at 450 °C under H₂ atmosphere, while urea deposition showed agglomeration. In terms of catalytic performance, all Ni/CeO₂ catalysts exhibited 100% ethanol conversion throughout the catalytic experiment and high (~ 70%) H₂ selectivity under high contact time conditions: wet impregnation (68.8%), BH4 deposition (67.6%), nanoparticles impregnation (66.6%), urea deposition (65.2%), and CeO₂ nanowires (9.9%). The obtained catalysts were relatively similar in composition and catalytic activity but differed in synthesis methods. However, in low contact time experiments, the Ni/CeO₂ catalysts displayed differentiated surface reactivity. Specifically, catalytic properties depended on physicochemical properties such as nanoparticle size, metal dispersion, metal-support interaction, and synthesis methods. In terms of stability, impregnation methods exhibited high stability, with ethanol conversion rates above 78% and 60% for wet impregnation and nanoparticles impregnation, respectively, after 48 h of catalytic experiment. However, BH₄ deposition exhibited intermediate stability, with an ethanol conversion rate around 37% after 48 h, while urea deposition showed strong deactivation of 86% after the test. In terms of H₂ yield and stability, the Ni/CeO₂ wet impregnation method was the most effective catalyst, showing a high H₂ yield close to the theoretical maximum and lower deactivation. The average H₂ yield for each catalyst was: wet impregnation (66%), BH₄ deposition (65%), nanoparticles impregnation (54%), urea deposition (22%), and CeO₂ nanowire (7%). Considering experimental error, wet impregnation, nanoparticles impregnation, and BH₄ deposition yielded similar H₂ yields. These results suggest that the choice of synthesis method for heterogeneous nanocatalysts is crucial for preparing nanomaterials with exceptional catalytic performance. We believe that our findings make a significant contribution to the synthesis of supported catalysts for gas-phase transformations, such as ethanol steam reforming.