Niobium Metal–Organic Framework Is an Efficient Catalytic Support for the Green Hydrogen Evolution Process from Metal Hydride

Abstract

Herein, the development of a niobium-based metal–organic framework (Nb-MOF) designed to serve as a catalytic support for the production of hydrogen (H₂) from sodium borohydride (NaBH₄) is reported. The Nb-MOF was synthesized via a solvothermal method using niobium ammoniacal oxalate (AmOxaNb) as the metal source and 1,4-benzenedicarboxylic acid (BDC) as the ligand. The resulting MOF was characterized by Fourier transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). The characterization study confirmed the successful synthesis of Nb-MOF. The catalytic activity was optimized by examining five key factors: (i) platinum (Pt) and cobalt (Co) bimetallic compositions (ranging from 1:0 to 0:1 mmol), (ii) NaBH₄ concentration (0.2, 0.3, 0.4, and 0.5 mol L⁻¹), (iii) the Nb-MOF/Pt–Co catalyst dose (0.05, 0.10, 0.20, and 0.40 mmol), (iv) sodium hydroxide (NaOH) concentration (0.01, 0.05, 0.1, and 0.2 mol L⁻¹), and (v) system temperature (293.15, 298.15, 303.15, 313.15, and 323.15 K). The optimal catalyst was identified as Nb-MOF supporting a Pt-Co bimetallic composition in a 0.4:0.6 mmol ratio, achieving a hydrogen generation rate (HGR) of 1473 mL min⁻¹ g_cat⁻¹ and an activation energy of 19.2 kJ mol⁻¹. Furthermore, this catalyst maintained its efficiency over 20 cycles, demonstrating significant potential as a sustainable solution for H₂ evolution from NaBH₄.

1. Introduction

Excessive consumption of fossil fuels and its severe climate consequences highlight the urgent need for more sustainable energy sources [1]. Hydrogen (H₂) stands out as one of the most environmentally friendly alternatives, due to its high energy density and combustion that releases only water, unlike hydrocarbon-based fuels that release carbon dioxide [2]. However, the transport and storage of H₂ still present significant challenges for large-scale implementation. One of the safest ways to store H₂ is in solid form, in hydrides, such as sodium borohydride (NaBH₄) [3,4]. This compound has been widely studied due to its high chemical stability, purity of the gas released, and high storage and release capacity for H₂ [5,6]. NaBH₄ is particularly promising due to its theoretical H₂ storage capacity, which can reach 10.8% by mass, with 1 mol of NaBH₄ releasing up to 4 mols of H₂ [7,8]. However, the high activation energy of NaBH₄ requires the use of suitable catalysts to enable its autohydrolysis [9]. The most common catalysts include noble metals such as platinum (Pt), palladium (Pd), and ruthenium (Ru), as well as transition metals such as cobalt (Co), nickel (Ni), copper (Cu), and iron (Fe) [4,9,10]. In addition, multimetallic compositions have been widely studied due to the synergistic effect that can be obtained between noble metals and transition metals [11]. This synergistic effect takes advantage of the superior properties of noble metals, while reducing costs by incorporating transition metals while maintaining catalytic efficiency [4,11].

In recent years, the use of metal–organic frameworks (MOFs) as catalyst supports for H₂ evolution has shown to be a promising approach [5,12]. This is due to their unique properties, which include a high specific surface area, adjustable porosity, and a highly ordered structure, which allow a uniform dispersion of the metal catalysts. These characteristics facilitate the interaction between the reagents and the active sites, improving the efficiency of the catalytic process and reducing the number of metals used. In addition, MOFs can be functionalized with various chemical groups, further increasing their ability to optimize catalytic activity. For instance, Gao et al. [13] were the first to report the introduction of a novel flower-shaped Ni-MOF with good thermostability in MgH₂ to improve its catalytic performance in the dehydrogenation process. As a result, it was observed that the maximum dehydrogenation temperature of MgH₂ using the Ni-MOF was 78 °C lower than that of pure MgH₂. In addition, the Ni-MOF exhibited faster dehydrogenation kinetics, releasing 6.4 wt% H₂ at 300 °C in 600 s. According to the authors, the high catalytic activity of the flower-shaped Ni-MOF can be attributed to the combined effect of in situ generated Mg₂Ni/Mg₂NiH₄, MgO nanoparticles, amorphous Ni-MOF, and Ni-MOF in the remaining layer [13]. In another study, Cu-MOF was used as a support for a copper (Cu) catalyst applied in the methanol steam reforming (MSR) process at low temperatures (130 and 250 °C) [14]. In this study, the effects of various metals were evaluated on the catalytic activities of the Cu/XeCu(BDC) catalysts (X = Ce, Zn, Gd, Sm, La, Y, Pr). As a result, it was observed that Ce/SmeCu(BDC) supports exhibited the highest activities, the lowest reduction temperatures, and largest specific surface areas, which resulted in the highest distribution of active copper nanoparticles on the supports [14].

Given the importance of MOFs in catalytic H₂ evolution processes, this study aims to present the synthesis and application of a new niobium-based MOF (Nb-MOF) as a catalytic support for the dehydrogenation of NaBH₄. The novelty of this work lies in the development of an Nb-MOF for this specific catalytic application, expanding the potential of MOFs in hydrogen-related technologies. Nb is a transition metal with unique properties that make it interesting in the synthesis of MOFs. Its high thermal and chemical stability, in addition to the ability to form complexes with a variety of organic ligands, allows the creation of stable and versatile MOFs. In the synthesis of MOFs, Nb acts as a metallic node, coordinating with organic ligands to form porous three-dimensional networks. These structures are highly desirable for applications in catalytic activities due to their high specific surface area [15,16]. In addition, Brazil holds more than 92% of the world production of Nb mines in the world [17,18]. Therefore, using catalysts based on Nb can contribute to the technological and sustainable growth of the country.

2. Experimental

2.1. Reagents

Niobium ammoniacal oxalate (AmOxaNb) was sourced from CBMM (MG, Brazil); 1,4-benzenedicarboxylic acid (BDC) and hexachloroplatinic acid (H₂PtCl₆·6H₂O) were obtained from Sigma-Aldrich (MO, USA); ethylene glycol (C₂H₆O₂) was acquired from Êxodo Científica (SP, Brazil); and sodium borohydride (NaBH₄), sodium hydroxide (NaOH), and cobalt (II) nitrate hexahydrate (Co(NO₃)₂·6H₂O) were purchased from Vetec (RJ, Brazil).

2.2. Synthesis of Nb-MOF

The Nb-MOF was produced via solvothermal synthesis as described by de Jesus et al. [19] with minor modifications. Briefly, 1.25 g of BDC was dissolved in 9.0 mL of deionized water (18.2 MΩ·cm). Then, 5.0 mL of NaOH solution (8.00 mol L⁻¹) was added to adjust the basic medium (pH 10) and deprotonate the organic ligand (BDC). After that, 1.0 g of AmNbOx oxalate was added and stirred. The mixture was then kept under stirring and heating (80 °C) until reaching a final volume of 9.0 mL. Then, 6.0 mL of ethylene glycol was added to the reaction medium. The solution was transferred to a Teflon reactor and introduced into an autoclave. The system was inserted into an oven and heated at 200 °C for 24 h. After the synthesis time, the material was centrifuged for 30 min at 4000 rpm to separate the solid. Subsequently, the solid obtained was washed three times with deionized water and once with ethanol, and then dried in an oven at 60 °C for 6 h. Figure 1 provides a schematic representation of the Nb-MOF synthesis steps.

2.3. In Situ Synthesis of Metal Nanoparticles on Nb-MOF

The metal catalyst was supported on the Nb-MOF through a chemical reduction process as described by Sperandio et al. [5]. Initially, 50.0 mg of the synthesized Nb-MOF was suspended in 10.0 mL deionized water (18.2 MΩ·cm) and stirred for 10 min. Pt and Co were then added with various metal combinations. The tested Pt:Co ratios were 1:0, 0.8:0.2, 0.6:0.4, 0.5:0.5, 0.4:0.6, 0.2:0.8, and 0:1 mmol. For the chemical reduction of these metals on the surface of the Nb-MOF, an excess of NaBH₄ (40.0 mg) was used due to its high reduction capacity. After 10 min of stirring, the catalyst was centrifuged for 4000 rpm for 10 min, and the supernatant was removed to avoid excessive H₂ release from NaBH₄. Then, 10.0 mL of deionized water was added, and the catalyst was subjected to an ultrasonic bath for approximately 5 min. After this process, the catalyst was ready for use in H₂ evolution.

2.4. Evolution of Hydrogen from NaBH₄

The catalytic hydrolysis of NaBH₄ was quantified using the water displacement method as described by Junior et al. [6]. Briefly, the prepared catalyst was dispersed in 10.0 mL of deionized water (18.2 MΩ·cm) inside a kitassato, sealed with a rubber septum. The system was kept under constant stirring (300 rpm) and controlled temperature, initially 298 K. Subsequently, the H₂ gas collector was connected through a rubber hose to the side outlet of the kitassato. After this setup, 1.0 mL of NaBH₄ solution (initially 0.5 mol L⁻¹) was introduced into the system using a syringe. The displacement of the water volume in the burette was recorded, and the data were plotted. Figure S1 shows as general scheme of the system used.

The H₂ evolution from NaBH₄ was optimized evaluating four factors, including: (i) the molar ratio of bimetallic nanoparticles (Pt and Co), ranging from 1:0 to 0:1 mmol; (ii) the concentration of NaBH₄ (0.2, 0.3, 0.4, and 0.5 mol L⁻¹); (iii) the dose of Nb-MOF/Pt–Co catalyst (0.05, 0.10, 0.20, and 0.4 mmol), (iv) the concentration of NaOH (0.01, 0.05, 0.1, and 0.2 mol L⁻¹), and (v) the temperature of the system (293.15, 298.15, 303.15, 313.15, and 323.15 K).

The hydrogen generation rate (HGR) was measured using the Equation (1):

$$HGR={\displaystyle \frac{V_{H_2}}{t_{min}\times m_{catalyst}}}$$

(1)

where V_H2 is the volume of H₂ (mL) by the reaction, t_min is the time in minutes of H₂ production, and m_catalyst is the total mass of the catalyst (g).

2.5. Activation Energy

HGR is directly influenced by temperature. Therefore, the activation energy was determined using the Arrhenius equation (Equation (2)):

$$\ln k=\ln A-{\displaystyle \frac{E_a}{R}}$$

(2)

where k is the kinetic constant of the reaction, A is the pre-exponential factor constant, E_a is the apparent activation energy in kJ mol⁻¹, R is the universal constant of the gases, and T is the absolute temperature (K). The slope of the Arrhenius graph (log k × 1/T) provides factors to calculate the catalyst activation energy. Physically, the equation illustrates that higher temperatures impart more energy to molecules, increasing the number of collisions with enough energy to overcome the activation energy barrier, thereby speeding up the reaction. Thus, the Arrhenius equation fundamentally explains the relationship between molecular dynamics, temperature, and reaction rates [8].

2.6. Kinetic Isotope Effect (KIE) Evaluation

To investigate the catalytic reaction mechanism, the method described by Sperandio et al. [5] was used. Briefly, Nb-MOF/Pt–Co catalyst was freshly prepared, rinsed with acetone, and then dried under vacuum. Then, 50.0 mg of the Nb-MOF/Pt–Co catalyst was placed in a 10.0 mL Schlenk flask. The flask was sealed and connected to a burette filled with water. Subsequently, 1.0 mL of a freshly prepared solution of 0.5 mol L⁻¹ NaBH₄ in deuterated water (D₂O) was added using a syringe. All reactions were carried out at a constant temperature of 298.15 K. KIE was calculated according to Equation (3):

$$KIE={\displaystyle \frac{k_{D_{2}O}}{k_{H_{2O}}}}$$

(3)

where $k_{D_{2}O}$ is the angular coefficient of linear regression of evolution made with deuterium oxide and $k_{H_{2O}}$ is the angular coefficient of linear regression of evolution made with deionized water.

2.7. Durability of the Catalyst

The durability of the catalyst was evaluated in optimal conditions. The optimal condition included 50.0 mg of Nb-MOF mass, a dose of NPs Pt–Co (4:6) of 0.05 mmol (Pt + Co), 1.0 mL of NaBH₄ (0.50 mol L⁻¹) prepared in solution of NaOH (0.050 mol L⁻¹), continuous stirring (300 rpm), in a H₂ cycle. At the end of each cycle, the system was opened by removing the rubber septum for total release of the produced H₂. Then, a new solution injection with NaBH₄ was made for reassessment. The study involved 20 cycles.

2.8. Characterization

The MOF and catalyst were characterized using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FT-IR), powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), and surface area analysis.

The functional groups on the MOF were identified by FT-IR in the mid-infrared region (4000–400 cm⁻¹) using a VARIAN 660-IR spectrometer (Agilent, Santa Clara, CA, USA) paired with a PIKE GladiATR (Wisconsin, USA) attenuated total reflectance accessory equipped with a diamond crystal. The crystal structures were examined by powder XRD using a Rigaku diffractometer (MA, USA) at room temperature, covering a 2θ range of 25° to 70°, with CuKα radiation. SEM images were captured using a JEOL JSM-360-LV microscope (MA, USA) operating at 20 kV with secondary electron imaging. The samples were mounted on a Cu–Zn metal-coated support and coated with a 10 nm gold layer using a Metalizer Quorum Q150R S (Hertfordshire, UK). TGA was conducted with a Pyris TGA 9 Instruments (PerkinElmer, Massachusetts, USA). Approximately 3.0 mg of the synthesized MOF was placed on alumina sample supports for analysis. The samples were subjected to a nitrogen flow of 50.0 mL min⁻¹ and heated from 25 °C to 900 °C at a rate of 10 °C min⁻¹. To determine the Brunauer–Emmett–Teller (BET) surface area and pore size distribution, an analyzer for surface area and pore size distribution (Anton Paar, Graz, Austria) was employed. The measurements were carried out using nitrogen adsorption at 77 °K, covering a relative pressure range of 0.0001 ≤ p/p₀ ≤ 0.9918.

3. Results and Discussion

3.1. Characterization of the Catalyst

During the MOF synthesis, the appearance of white precipitates indicated the formation of [Nb(BDC)]_n. The BDC ligands coordinated to the Nb centers as shown in Figure 1, resulting in the formation of metalloligands. These metalloligands act as building blocks, facilitating the assembly of Nb-MOFs, specifically [Nb(BDC)]_n. The coordination of BDC to Nb is a crucial step in MOF synthesis, as it determines the structural integrity and stability of the resulting structure. The carboxylate groups of the BDC ligand interact with the Nb centers, leading to the formation of metalloligands. These metalloligands further polymerize via coordination bonds, creating an extended network structure characteristic of MOFs. The specific arrangement and coordination geometry conferred by the Nb–BDC interaction plays a significant role in defining the physical and chemical properties of the MOF, such as its pore size, surface area, and potential catalytic properties.

The success of the synthesis of Nb-MOF was confirmed by FT-IR spectroscopy, which also detailed the presence of functional groups in the [Nb(BDC)]_n material. Figure 2 shows the FT-IR spectra of both the synthesized MOF and the ligand (BDC). Significant shifts in the absorption bands around 500 cm⁻¹ likely indicate the stretching and bending modes associated with the Nb–O bonds [5]. Furthermore, the shifts in the peaks at 1104 cm⁻¹ and 1250 cm⁻¹ can be attributed to the stretching vibrations of the C–O and C–C bonds, respectively [19]. The deformation of the bands between 1750 cm⁻¹ and 1500 cm⁻¹ in the as-synthesized compound suggests deprotonation of the –COOH group, supporting the formation of the MOF as the metal center binds to these negatively charged sites [5,19]. The broad band observed at 3400 cm⁻¹ can be attributed to the surface-absorbed water and hydroxyl groups within the [Nb(BDC)]_n structure [20]. When comparing the spectra of the synthesized Nb-MOF with that of the BDC ligand, a remarkable broadening of the band associated with the Nb-MOF is observed. This broadening suggests subtle interactions between the ligand and the metal, reflected not only in a shift in frequency but also in significant changes in the shape and width of the band. These modifications suggest coordination between the metal center and the ligand.

SEM was used to examine the morphology of [Nb(BDC)]_n. Figure 3 display the morphology of the Nb-MOF (Figure 3A) and Nb-MOF/Pt–Co (Figure 3B), highlighting its distinct cube-shaped crystal structure. In conjunction with micrograph analysis, EDS analysis was conducted to identify the metals (Nb, Pt, Co) involved in the synthesis of Nb-MOF (Figure 3C) and the Nb-MOF/Pt–Co catalyst (Figure 3D). EDS analysis confirmed the presence of Nb in the synthesized MOF, verifying the successful formation of Nb-MOF with Nb as the metal center. Furthermore, the analysis demonstrated the deposition of Pt and Co nanoparticles on the surface of Nb-MOF, validating the successful preparation of Nb-MOF/Pt–Co catalyst. The structure of the synthesized MOF was also characterized by XRD. Figure 3E illustrates the resulting XRD patterns for the synthesized material. The material exhibited intense and sharp peaks at various 2θ values, indicating a polycrystalline structure characterized by multiple diffraction planes. These distinct peaks reflect the well-ordered arrangement and crystallinity of the material. The crystalline phase of Nb-MOF, as revealed by XRD, has significant implications for its properties and effectiveness as a catalyst. The structural order and arrangement of MOF components directly influence key factors such as surface area, porosity, active site availability, and electron transport, all of which are crucial for catalytic performance. In other words, a well-ordered crystalline phase enhances the ability of the MOF to act as an efficient and robust catalyst, particularly in complex reactions such as those involving hydrogen evolution [5]. Figure 3E(I) represents the experimental data, while Figure 3E(II) shows the patterns obtained from the literature. By comparison, it is observed that the obtained material presents great similarity to sodium niobate, confirming the successful synthesis of [Nb(BDC)]_n.

The thermal stability of the synthesized compound was also assessed using thermogravimetric analysis. The resulting thermogram, presented in Figure 4, shows three notable mass loss events. The first event, with a mass loss of 14.4%, occurs at 567 °C, which is attributed to the initial decomposition of the BDC ligands. The second event, involving a mass loss of 7.2%, occurs at 661 °C, likely indicating the further loss of BDC residues. Above 800 °C, the remaining mass loss is associated with the decomposition of other MOF byproducts.

To examine the pore size distribution, surface area, porosity type, and total pore volume of [Nb(BDC)]_n, the BET method was employed. Nitrogen adsorption–desorption isotherms were obtained at 77 K to assess the material specific surface area, with the pore characteristics being calculated based on the BET model. The N₂ adsorption–desorption curves shown in Figure 5 indicate that [Nb(BDC)]_n follows a type III isotherm. From the BET data, the MOF surface area and total pore volume were determined to be 1200 m² g⁻¹ and 0.0454 cm³ g⁻¹, respectively. The average pore size, calculated at 6.42 nm, confirmed the microporous nature of the synthesized material.

3.2. Evolution of Hydrogen from NaBH₄ Using Nb-MOF

Initially, the production of H₂ from NaBH₄ was tested using Nb-MOF alone. The results of this preliminary experiment are shown in Figure 6A, where it is evident that Nb-MOF alone did not exhibit satisfactory catalytic activity. These results can be attributed to the saturation of catalytic sites by metaborate, a byproduct of the hydrolysis reaction. This accumulation makes it difficult to renew the substrate on the catalyst surface, thus reducing its efficiency. To mitigate these limitations, a potential approach is to modify the surface of the Nb-MOF or incorporate noble metals such as Pt or Co. These metals can serve as active centers for the dehydrogenation reaction, ultimately increasing the overall catalytic activity [5]. In light of these findings, Nb-MOF was subsequently evaluated as a support for bimetallic compositions of Pt and Co in different ratios. For these tests, 50.0 mg of Nb-MOF, 1.0 mL of NaBH₄ (0.5 mol L⁻¹), continuous stirring at 300 rpm, and a constant temperature of 298.15 K were maintained. The results, shown in Figure 6B, indicate that the combination of Nb-MOF with Pt and Co significantly improved catalytic performance when compared to MOF alone (Figure 6A).

The Pt–Co composition (1.0–0.0 mmol) showed the highest H₂ production in approximately 9 min of reaction, but its HGR was relatively low (77.7 mL min⁻¹ g⁻¹) (Figure S2). On the other hand, the Pt–Co composition (0.0–1.0 mmol) displayed similar kinetics with 25% lower H₂ production at the same time (Figure 6B). Some bimetallic Pt–Co compositions (0.2–0.8 mmol and 0.4–0.6 mmol) exhibited more interesting catalytic activity than the monometallic compositions, reaching higher HGR values of 507.9 mL min⁻¹ g⁻¹ and 510.9 mL min⁻¹ g⁻¹, with the reaction peak in less time and satisfactory yield values (Figure 6B and Figure S2). Among the compositions, the Pt_0.4–Co_0.6 mmol ratio had its maximum hydrolysis close to 1.5 min and presented an HGR of 513.9 mL min⁻¹ g⁻¹ (Figure S2). To evaluate the synergy between the Nb-MOF and the bimetallic nanoparticle composition (Pt:Co), an H₂ evolution assay was conducted using unsupported bimetallic Pt:Co nanoparticles. The assays were performed with Pt:Co ratios of 1:0, 0.4:0.6, and 0:1 mmol. The results are shown in Figure S3. The assay revealed that, while the bimetallic Pt–Co composition demonstrated satisfactory catalytic activity (HGR > 127 mL min⁻¹ g⁻¹), the H₂ production rate was lower compared to the experiment using Nb-MOF-supported Pt_0.4–Co_0.6 nanoparticles (Figure 6B and Figure S2). This difference can be attributed to the increased surface area and catalytic sites provided by the Nb-MOF. As a result, this specific composition was chosen for the subsequent steps of the study. The Pt_0.4–Co_0.6 mmol ratio is particularly advantageous as it reduces the amount of Pt, a more expensive metal, while still maintaining its catalytic benefits. Therefore, this composition was used in the subsequent stages of the work. This bimetallic ratio (Pt_0.4–Co_0.6 mmol) is interesting because it uses less Pt, a more expensive metal, while still taking advantage of the catalytic properties of this metal.

In their study, Biehler et al. [21] reported the synthesis of a Pt-decorated mesoporous carbon nanocomposite from a sustainable source and applied it as a catalyst in H₂ evolution from NaBH₄. As a result, an HGR of 37.4 mL min⁻¹ g_cat⁻¹ was achieved. In another study, Pope et al. [22] produced chitosan spheres encapsulating active Co catalyst species under ambient conditions. This material acts as a catalyst in H₂ evolution from the hydrolysis of KBH₄. As a result, the authors report an HGR of 128 mL min⁻¹ g_cat⁻¹.

When we compare the results obtained in our study with those reported in the literature, the catalyst composition of Nb-MOF/Pt_0.4–Co_0.6 emerges as a promising alternative for producing H₂ from NaBH₄.

3.2.1. Effect of NaBH₄ Concentration

After establishing the optimal condition of catalyst composition (Nb-MOF/Pt_0.4–Co_0.6), the effect of different concentrations of NaBH₄ was studied, and the results are shown in Figure S4A. Concentrations of 0.2, 0.3, 0.4, and 0.5 mol L⁻¹ of NaBH₄ were evaluated. The findings indicate that H₂ produced increases with higher concentration of NaBH₄, and the the HGR of the reaction increases linearly with the concentration of NaBH₄ (Figure S4B). According to the results from the ln k vs. ln [NaBH₄] graph shown in Figure S4C, the slope of the linear model for the data was determined, showing a value of −0.81, indicating that the reaction is zero-order with respect to NaBH₄ concentration [5,23]. This suggests that the activation of NaBH₄ can be excluded as the rate-determining step in the reaction. Therefore, subsequent tests were performed at a NaBH₄ of 0.5 mol L⁻¹ and Nb-MOF/Pt_0.4:Co_0.6.

3.2.2. Effect of Catalyst Dosage

To reduce the amount of metals (Pt and Co) used in the catalyst, a study was conducted by varying the metal loading in the Nb-MOF at levels of 5, 10, 20, and 40 mmol %. The tests were performed under the same conditions described previously, with 50.0 mg of support, 1.0 mL of NaBH₄ (0.50 mol L⁻¹), continuous stirring at 300 rpm, and a temperature of 298.15 K. Figure S5 presents the results obtained. As illustrated in the graph in Figure S5A, the reaction showed a higher yield when 10 mmol% of (Pt–Co) NPs was used. However, when using a 5 mmol% load of nanoparticles, a higher HGR (1408 mL min⁻¹ g⁻¹) was obtained, when compared to 10 mmol%, where the HGR was 700 mL min⁻¹ g⁻¹ (Figure S5B). This indicates that, in a smaller amount, there is greater H₂ evolution, even with slower reaction kinetics. These results suggest that a metal loading of 5 mmol% provides a sufficient amount of active sites to promote catalysis of the occurrence, in addition to facilitating the substitution of borohydride on its surface, resulting in a satisfactory yield. Furthermore, the plot of ln k vs. ln (catalyst dosage) (Figure S5C) showed linearization of the data with a slope of −0.11. This fact indicates that the H₂ evolution process behaves as a zero-order reaction with respect to the catalyst dosage. Therefore, subsequent tests were performed at a metal catalyst ratio of 5 mmol% in relation to the amount of NaBH₄ and Nb-MOF/Pt_0.4:Co_0.6.

3.2.3. Effect of NaOH Concentration

The hydrolysis of NaBH₄ occurs spontaneously in aqueous solutions, which can reduce the amount of H₂ produced during the catalyzed reaction, thus decreasing the overall reaction yield. This problem can be mitigated by using an alkaline medium to stabilize NaBH₄ and control its hydrolysis [24]. To investigate this parameter, NaOH solutions with concentrations of 0.01, 0.05, 0.1, and 0.2 mol L⁻¹ were used to alkalize the medium during H₂ evolution. The results, shown in Figure S6, demonstrate that increasing the concentration of NaOH from 0.01 to 0.1 mol L⁻¹ increases the reaction rate, with HGR values increasing from 515.9 mL min⁻¹ g⁻¹, using 0.01 mol L⁻¹ of NaOH, to 851.2 mL min⁻¹ g⁻¹, employing 0.05 mol L⁻¹ of NaOH. When employing 0.1 mol L⁻¹ of NaOH, an HGR of 1252.4 mL min⁻¹ g⁻¹ was obtained. However, at a concentration of 0.2 mol L⁻¹ NaOH, a reduction in HGR (642.9 mL min⁻¹ g⁻¹) was obtained. This decrease in efficiency is probably due to the increase in viscosity of the medium at higher NaOH concentrations. The increase in viscosity impairs mass transfer and leads to pore blockage by the accumulation of borate anions at the active sites, thus hindering H₂ generation [6]. Based on these results, the optimal NaOH concentration for maximizing reaction efficiency was determined to be 0.1 mol L⁻¹.

3.2.4. Effect of Temperature

The effect of temperature on the hydrolysis of NaBH₄ was also evaluated. For this purpose, a temperature range from 293.15 to 323.15 K was tested. The results, shown in Figure S7A,B, demonstrate a direct proportional relationship between temperature and H₂ yield, with the reaction yield increasing as the temperature increases. The reaction rate constants for each temperature are shown in

Table 1. Kinetic constants (k_obs) for hydrogen evolution using the catalyst (Nb-MOF/Pt0.4:Co0.6) at different reaction temperatures.

Temperature (K)	kobs/s−1
293.75	0.0279
305.85	0.0437
314.15	0.0504
324.15	0.0581

.

Based on the rate constants, an Arrhenius plot was constructed (Figure S7C). By using linear regression on the experimental data, the activation energy of the reaction was determined (19.2 kJ mol⁻¹). The slope of the Arrhenius graph (log k × 1/T) (Figure S7C) provides us with factors to calculate the catalyst activation energy [8]. This trend indicates that higher temperatures significantly increase the H₂ production. Thus, at 313 K, the HGR reached 3321.1 mL min⁻¹ g⁻¹, indicating higher yield. Similar data were obtained by Abraham et al. [25], who synthesized cobalt phospho-boride (CoPB) within MOF using hydrothermal and chemical reduction methodologies. This compound was applied as a catalyst in a NaBH₄ hydrolysis reaction, obtaining 20.7 KJ mol⁻¹ of activation energy.

3.3. Durability of Nb-MOF/Pt_0.4–Co_0.6

To assess the efficiency of the catalyst, it is fundamental to evaluate its stability and durability. Therefore, the durability of Nb-MOF/Pt_0.4–Co_0.6 was tested through catalyst cyclization experiments. The catalyst was reused for 20 cycles under optimal conditions. After each cycle, a fresh NaBH₄ solution was reinjected into the system after a 5 min interval with the system open, ensuring complete dissipation of any residual H₂ gas. The results are presented in Figure 7; the catalyst maintained its efficiency throughout the cycles, consistently achieving the same yield with an average reaction time of approximately 60 s. These findings confirm the durability and sustained efficiency of the catalyst over multiple uses.

3.4. Kinetic Isotope Effect (KIE) Evaluation

The kinetic isotope effect (KIE) is an important tool for investigating the reaction mechanism, indicating the rate-determining step of the reaction [26]. KIEs can be classified as primary (with values between 2 and 7) or secondary (with values between 0.7 and 1.5) [27]. A primary KIE suggests that the reaction involving the isotopically labeled atom is the rate-determining step, whereas a secondary KIE indicates additional complexity in identifying the rate-determining step [28,29]. The KIE for Nb-MOF/Pt_0.4–Co_0.6 was evaluated. The results are shown in Figure 8. H₂ evolution using H₂O and D₂O is illustrated in Figure 8A, while linear regression indicating the KIE of the reaction is shown in Figure 8B. A KIE of 4.36 was obtained, indicating that the dissociation of O–H/O–D from H₂O/D₂O is affected by the reaction. This also suggests that the step involving water is the rate-determining one in the process.

3.5. Mechanistic Proposal for NaBH₄ Hydrolysis Reaction Catalyzed by Nb-MOF/Pt_0.4–Co_0.6

Although a mechanistic proposal to elucidate the catalytic reaction for H₂ generation from NaBH₄ using Nb-MOF/Pt_0.4–Co_0.6 has not been fully established, it is believed that the synergy between Pt and Co nanoparticles and the Nb-MOF support is crucial for the success of the process. Pt possesses electronic properties that enhance the adsorption of NaBH₄, leading to the formation of reactive intermediates (unidentified) that are essential for the reaction. In addition, Co can further enhance the catalytic performance by providing extra active sites and improving the overall electronic structure of the catalyst [30,31]. The bimetallic composition of Pt and Co can induce synergistic effects, where their combined properties produce higher catalytic activity than when they are used individually. This synergy likely increases the H₂ evolution rate, as the presence of Co can optimize the adsorption energy of NaBH₄ on the catalyst surface. Furthermore, the interaction between the Nb-MOF support and the bimetallic catalyst plays a significant role in the overall catalytic efficiency [30,31]. The distinct structural features of Nb-MOF, including its high surface area and porosity, promote the effective distribution and stabilization of Pt-Co nanoparticles, maximizing their exposure to reactants [5]. This interplay between the metal composition and the MOF support is vital to achieving enhanced catalytic activity in the dehydrogenation of NaBH₄.

3.6. Performance of the Catalyst

Comparing the material obtained in this work with others described in the literature, as detailed in

Table 2. Comparison of different MOF-based catalysts applied in H₂ evolution from metal hydride hydrolysis.

Catalyst	EA (kJ mol−1)	HGR (mL min−1 gcat−1)	Reuse	Ref.
H-Co/N/C-Ru@CT	26.9	9815.82	81.3% efficiency in the 25th cycle	[4]
Co-BDC	25.4	1886.8	Reused at least 10 times	[32]
CoPB-MOF	20.7	3600	Recycling with no signs of deactivation.	[25]
Ru-(ZIF-67/JUC-505)	23.9	34,790	91.0% of its initial performance after 10 cycles	[33]
Ru/Co6Ce1@ZIF-67	53.0	5726.1	Does not change considerably after 5 cycles	[34]
B–NiCoP/NC	33.92	5853.9	68.7% after 6 cycles	[35]
Nb-MOF/Pt0.4–Co0.6	19.2	1473	Up to 20 cycles without loss of efficiency	This work

, reveals several important insights. The performance of our material, particularly in terms of HGR and overall catalytic efficiency, closely aligns with those reported in other studies. This comparability suggests that the synthesis method and catalytic design employed in our research are on par with established approaches, validating the effectiveness of the prepared catalyst. However, it is important to note that while our material demonstrates similar or even superior catalytic properties in certain metrics, the specific conditions under which each study was conducted, such as temperature, pH, and reactant concentrations, can vary significantly. These differences may influence the direct comparability of the results. In addition, while the performance of our material is comparable, it offers unique advantages, such as its specific compositional tunability and the potential for further optimization. These factors may increase its applicability in various H₂ production scenarios. Thus, the material (Nb-MOF/Pt_0.4–Co_0.6) developed in this work not only matches the standards set by previous studies, but also provides a solid basis for future research and potential improvements in catalytic performance.

4. Conclusions

In this study, Nb-MOF was successfully synthesized using the solvothermal method, demonstrating a well-defined crystal structure as confirmed by SEM characterization. EDS analysis further identified the presence of metals within the MOF structure. Among the various metal catalysts tested, the bimetallic composition of Pt_0.4–Co_0.6 exhibited the highest yield in H₂ production and a metal content of 5 mmol% relative to the amount of NaBH₄. Kinetic studies revealed that H₂ from water plays a critical role in the bond-breaking and bond-formation processes within the reaction mechanism. The activation energy of the reaction was determined to be 19.2 kJ K⁻¹ mol⁻¹ at the optimum temperature of 313.15 K. Furthermore, the material demonstrated excellent durability, maintaining its catalytic efficiency over 20 cycles. These findings highlight the potential of Nb-MOF/Pt_0.4–Co_0.6 as a highly effective and durable catalyst for H₂ production from NaBH₄.