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Article

Development of a Niobium-Based Coordination Compound with Catalytic Applications for Green Hydrogen Evolution

by
Emily Pacheco Squizzatto
1,2,
Tatianny de Araujo Andrade
1,2,
Renata Pereira Lopes Moreira
2,
Luciano de Moura Guimarães
3,
Márcio José da Silva
2,
Fábio Junior Moreira Novaes
2 and
Jemmyson Romário de Jesus
1,*
1
Research Laboratory in Bionanomaterials, LPbio, Department of Chemistry, Federal University of Viçosa, Viçosa 36570-900, Minas Gerais, Brazil
2
Department of Chemistry, Federal University of Viçosa, Viçosa 36570-900, Minas Gerais, Brazil
3
Department of Physics, Federal University of Viçosa, Viçosa 36570-900, Minas Gerais, Brazil
*
Author to whom correspondence should be addressed.
Processes 2024, 12(12), 2677; https://doi.org/10.3390/pr12122677
Submission received: 31 August 2024 / Revised: 6 November 2024 / Accepted: 19 November 2024 / Published: 27 November 2024
(This article belongs to the Section Catalysis Enhanced Processes)
Browse Figures
Versions Notes

Abstract

Green hydrogen (H2) offers a sustainable alternative to non-renewable energy sources. This study focuses on enhancing H2 generation from sodium borohydride (NaBH4) using a platinum nanoparticle (Pt-NP) catalyst supported on a niobium-based coordination compound, [Nb(BDC)0.9(PDC)0.1]n, synthesized via a solvothermal method with 1,4-benzenedicarboxylic acid (BDC) and 2,5-pyridinedicarboxylic acid (PDC). Characterization techniques including Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), Brunauer–Emmett–Teller (BET) analysis, Raman spectroscopy, and X-ray diffraction (XRD) confirm the morphology, composition, surface area (398.583 m2g−1), and crystallinity of the material. The in situ synthesized Pt-NPs showed a hydrogen generation rate (HGR) of 86.588 mL min−1 g−1 when alone, while the supported catalyst achieved an enhanced HGR of 119.020 mL min−1 g−1 under optimal conditions (10 mmol% Pt, 0.5 mmol NaBH4, 303.15 K). The low activation energy (Ea) of 16.38 kJ mol−1 indicates efficient catalysis. The catalyst maintained stable performance in recycling tests, demonstrating its potential for practical applications in H2 evolution from NaBH4.

1. Introduction

Decarbonization and reduction of greenhouse gas emissions, both major contributors to the greenhouse effect, have been the focus of extensive research aimed at achieving a green and circular economy [1]. Since the Industrial Revolution, environmental impacts have increased due to the improper disposal of toxic industrial waste into rivers and the uncontrolled release of pollutant gases into the atmosphere [2]. In the 18th century, coal emerged as the dominant fossil fuel, becoming the main global source of energy [3]. In 1859, oil exploration introduced an additional energy resource [4]. Today, oil remains the most commercially viable fossil fuel, with its derivatives, produced through refining, serving as essential raw materials for gasoline, diesel, kerosene, and liquefied petroleum gas (LPG), as well as in the production of plastics and rubber. However, the emission of pollutant gases from fossil fuel combustion has been a major contributor to severe climate change [5].
Since the establishment of the Paris Agreement in 2015 [6], research has increasingly focused on facilitating the use of green hydrogen (H2) as a sustainable fuel. The most common method for storing and transporting H2 involves pressurization, but the volatility and flammability of the gas pose significant challenges. An alternative approach is to store H2 in metal hydrides such as sodium borohydride (NaBH4) [7,8]. This storage method is promising due to its high stability and non-toxicity [7,9,10]. The hydrolysis reaction of NaBH4 can be represented by Equation (1) [11]:
NaBH4 (aq) + 2H2O (l) ⇌ NaBO2 (aq) + 4H2 (g) + 216.18 kJ
Although the hydrolysis reaction of NaBH4 is relatively slow, recent studies have investigated the use of heterogeneous catalysts to improve its reaction kinetics [12]. Metals such as platinum (Pt), palladium (Pd), cobalt (Co), and nickel (Ni) are commonly employed as catalysts, yielding promising results [10,13]. However, there is still significant potential to further explore the applicability of these catalysts when incorporated into supported nanomaterials such as coordination compounds or metal–organic frameworks (MOFs) [14].
MOFs are characterized by the binding of metal clusters or metal ions with one or more polydentate organic ligands [15]. These crystalline materials can have one-, two-, or three-dimensional structures and have high surface area, high porosity, thermal and chemical stability, post-synthesis functionalization capability, and recyclability, meeting the demands of sustainable chemical analysis [16,17]. Metal–organic frameworks (MOFs) find applications in various sectors, including catalytic processes [18,19,20,21,22,23]. For instance, Kanchanakanho et al. [24] employed cobalt–ZIF frameworks synthesized with different morphologies to enhance H2 evolution from NaBH4 hydrolysis. The study demonstrated that 2D cobalt–ZIF frameworks exhibited higher hydrogen generation rates (HGR) compared to their 3D counterparts [24]. Similarly, Mirshafiee et al. [25] developed Co/Fe3O4@GO nanocatalysts to improve the kinetics of H2 production via NaBH4 hydrolysis. Their results confirmed that Co/Fe3O4@GO acted as an efficient catalyst, achieving a high HGR rate and low activation energy, meeting the desired performance criteria [25].
Given that Brazil holds one of the largest niobium reserves in the world, there is significant potential to exploit this resource for innovative technological applications. Despite its promising properties, niobium chemistry remains underutilized, particularly in the catalytic field. This work presents a novel niobium-based metal–organic framework (Nb-MOF) developed to serve as a catalytic support for hydrogen evolution from NaBH4. By leveraging the unique characteristics of Nb, such as its high thermal stability, we aim to advance clean and sustainable energy generation technologies. This study not only proposes a practical application for Nb but also contributes to diversifying H2 production solutions, aligning with global sustainability goals.

2. Material and Methods

2.1. Reagents

Ammonium niobium oxalate was sourced from CBMM (Araxá, MG, Brazil). 1,4-benzenedicarboxylic acid (BDC) and 2,5-pyridinedicarboxylic acid (PDC) were purchased from Sigma-Aldrich (MO, USA). Sodium borohydride (NaBH4, 98%), nickel sulfate heptahydrate (NiSO4·7H2O), cobalt (II) nitrate hexahydrate (Co(NO3)2 · 6H2O, 99,9%), and sodium hydroxide (NaOH) were obtained from VETEC (Rio de Janeiro, RJ, Brazil). Deuterium oxide (D2O), potassium tetrachloropalladate (II) (K2PdCl4), and potassium hexachloroplatinate (IV) (K2PtCl6) were acquired from Sigma-Aldrich (Waltham, MO, USA). All aqueous solutions were prepared with Milli-Q water (Millipore Corporation, Darmstadt, HE, Germany).

2.2. Synthesis of the Coordination Compound ([Nb(BDC)0.9(PDC)0.1]n)

The material was synthesized according to the solvothermal method described by de Jesus et al. [19], with small adjustments in the proportions between the organic ligands (BDC and PDC) and the metallic source (Nb) [19]. The synthesis was carried out in two steps (Figure 1). First, 0.75 g of BDC and 0.75 g of PDC were dissolved in 10.00 mL of deionized water at room temperature. After the ligands were fully dissolved, the solution’s pH was adjusted to 12 using an 8 mol L−1 NaOH solution to promote ligand deprotonation. Subsequently, 0.5 g of niobium ammonium oxalate was slowly added to the mixture, with continuous stirring until complete dissolution. The resulting solution was transferred to a Teflon reactor, and 4.00 mL of ethylene glycol was added. The Teflon solution containing the resulting material was then placed inside an autoclave and heated at 200 °C for 24 h. After that, the system was cooled to room temperature. The solid formed was washed three times with Milli-Q deionized water and once with ethanol and centrifuged at 4000 rpm for 10 min during each wash. Finally, the material was dried at 50 °C for 8 h in an oven.

2.3. MOF Chemical Characterization

The synthesized [Nb(BDC)0.9(PDC)0.1]n was characterized using various techniques. Nitrogen adsorption and desorption isotherms were measured using a Nova 600 Series instrument (Anton Paar, Graz, Austria). The specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method. Raman spectra were obtained using a MicroRaman instrument (Renishaw, Wotton-under-Edge, UK). The analysis was performed in five repetitions with a 633 nm wavelength laser, operated at 3 mW power, with an integration time of 30 s. Thermogravimetric analysis was conducted using Pyris TGA 9 Instruments (PerkinElmer, Hopkinton, MA, USA), in the temperature range from 25 to 900 °C, with a heating rate of 10 °C min−1, under a nitrogen gas atmosphere. Scanning Electron Microscopy (SEM) was performed using a JEOL instrument (JSM-6010LA, Akishima, Tokyo, Japan), equipped with an Everhart–Thornley detector for secondary electron imaging and a solid-state detector for backscattered electrons, allowing contrast variations related to topography, composition, and shading. The analysis was conducted with a resolution of 4 nm, operating at 20 kV, with magnifications ranging from 8 to 300 times, an acceleration voltage from 500 V to 20 kV, and a pre-centered electron gun with a tungsten filament. The sample was dispersed in Milli-Q® water, and 10 μL was then deposited onto grids coated with a Formvar carbon support film. Analyses were conducted using a Tecnai G2-20 SuperTwin 200 kV Transmission Electron Microscope (TEM) (Hillsoboro, OR, USA). Particle size distribution was measured by dynamic light scattering (DLS) with a Litesizer DLS 500 instrument from Anton Paar (Austria). Prior to analysis, the [Nb(BDC)0.9(PDC)0.1]ₙ/Pt-NP sample was dispersed in ultrapure water to ensure accurate measurements. The crystal structures were examined using room temperature powder X-ray diffraction (XRD) performed on Rigaku diffractometer (The Woodlands, TX, USA) with Bragg–Brentano geometry in continuous mode, scanning in a 2θ range from 20° to 100° with CuKα radiation.

2.4. Hydrogen Evolution from NaBH4

The experimental conditions were adapted from Junior et al. [26], with minor modifications. To prepare the catalyst, the previously synthesized [Nb(BDC)0.9(PDC)0.1]ₙ was dispersed in 5.00 mL of Type 1 water, under stirring for 10 min. The Pt salt was then added to the reaction medium and kept under constant stirring for another 10 min. Next, 1.00 mL of a 1 mol L⁻¹ NaBH4 solution was introduced to reduce the platinum on the Nb-MOF. After this period, the resulting catalyst was washed with Type 1 water, and the precipitate was separated by centrifugation at 4000 rpm in preparation for the H2 evolution process. The H2 evolution was conducted in the reaction system illustrated in Figure S1. Briefly, in a Schlenk tube, the freshly prepared catalyst was dispersed in 2.00 mL of Type 1 water. The tube was sealed with a rubber septum, and its side outlet was connected via tubing to collect the H2 gas. The system was subjected to constant stirring with temperature control. Subsequently, 1.00 mL of a 0.500 mol L−1 NaBH4 solution was introduced into the system using a syringe, and the reaction time was recorded.

2.5. Parameters Optimization

The optimization of H2 evolution from NaBH4 was conducted by evaluating the following parameters: (i) composition of monometallic catalysts; (ii) variation in metal content; (iii) NaBH4 concentration; (iv) NaOH concentration; (v) temperature variation; and (vi) catalyst reuse.

2.5.1. Composition of Monometallic Catalysts

Initially, H₂ evolution experiments were conducted using different metallic nanoparticles (Pt, Co, Ni, and Pd) supported on [Nb(BDC)0.9(PDC)0.1]ₙ. In these experiments, 20 mg of [Nb(BDC)0.9(PDC)0.1]n was used as support, and 1.00 mL of NaBH4 (0.500 mol L−1) and a temperature of 303.15 K were kept constant.

2.5.2. Effect of Catalyst Dose

Among the tested metals, Pt showed the best performance when combined with the [Nb(BDC)0.9(PDC)0.1]ₙ support. Therefore, different concentrations of platinum nanoparticles (Pt-NPs) (0.00005, 0.005, 0.025, and 0.05 mmol) were evaluated. During these tests, the following parameters were kept constant: 20 mg of [Nb(BDC)0.9(PDC)0.1]ₙ as the catalyst support, 1.00 mL of a 0.500 mol L−1 NaBH4 solution, and a temperature of 303.15 K.

2.5.3. Effect of NaBH4 Concentration

The effect of NaBH4 concentration on H2 evolution was investigated using concentrations of 0.50, 0.75, 1.00, and 1.50 mol L⁻¹. The amount of Pt-NP catalyst was kept constant at 0.05 mmol. All other conditions, including 20 mg of the support material ([Nb(BDC)0.9(PDC)0.1]ₙ), 1.00 mL of NaBH4 solution, and a temperature of 303.15 K were maintained unchanged.

2.5.4. Effect of Temperature

For this H₂ evolution optimization phase, the following temperatures were tested: 293.15, 303.15, 313.15, 323.15, and 333.15 K. During these experiments, all other parameters were kept constant, including 0.05 mmol of Pt-NPs, 20 mg of [Nb(BDC)0.9(PDC)0.1]ₙ as support, and 1.00 mL of a 0.500 mol L−1 NaBH4 solution.
The activation energy (Ea, in kJ mol−1) was calculated using Equation (2), based on the ln(k) values.
ln(k) = ln(A) − Ea/RT
where, k is the reaction rate constant, A is the pre-exponential factor, R is the universal gas constant, and T is the temperature in Kelvin.

2.5.5. Effect of NaOH Concentration

To evaluate the effect of NaOH concentration on H2 evolution, four solutions were prepared in Type 1 water with concentrations of 0.010, 0.050, 0.100, and 0.200 mol L−1. In each experiment, the catalyst ([Nb(BDC)0.9(PDC)0.1]ₙ/Pt-NP) was dispersed in 2.00 mL of the respective alkaline solution. All other parameters were kept consistent across the tests, including 20 mg of the support material, 0.05 mmol of Pt-NP, 1.00 mL of NaBH4 solution (0.500 mol L−1), and a constant temperature of 303.15 K.

2.6. Catalyst Reuse

Catalyst reusability was evaluated under the following conditions: 20 mg of support material, 0.050 mmol of Pt-NPs, 1.00 mL of NaBH4 solution (0.500 mol L−1), and temperature of 303.15 K. H2 evolution was conducted in multiple cycles. After each cycle, the reaction system suspension was washed with 15.00 mL of Type 1 water, followed by centrifugation at 4000 rpm for 5 min to recover the solid catalyst. The recovered solid was then redispersed in 2.00 mL of Type 1 water and reintroduced into the reaction system for subsequent cycles. This procedure was repeated for a total of eight cycles.

2.7. Effect of Deuterated Water (D2O)

This study aimed to investigate the kinetic isotope effect (KIE) of the catalyst under pre-established conditions. The experimental setup included 20 mg of support material, 0.050 mmol of Pt-NPs, 1.00 mL of NaBH4 solution (0.500 mol L−1), and a temperature of 303.15 K. For the test, the Pt-NPs were dispersed in 2.00 mL of D2O and then introduced into a Schlenk tube. All other parameters were kept consistent, including the support mass of [Nb(BDC)0.9(PDC)0.1]ₙ (20 mg), the addition of 0.050 mmol of Pt-NPs, 1.00 mL of NaBH4 solution (0.500 mol L−1), and a temperature of 303.15 K. The KIE was determined using Equation (3) to elucidate the mechanism of the H2 evolution reaction.
KIE = kH2O/kD2O

3. Results and Discussion

3.1. Characterization of [Nb(BDC)0.9(PDC)0.1]ₙ

The surface area and porosity of the synthesized material, [Nb(BDC)0.9(PDC)0.1]ₙ,were evaluated using the BET technique. The adsorption–desorption curve, presented in Figure 2, shows that the adsorption–desorption of N2 follows a type IV isotherm, which indicates multilayer adsorption typical of mesoporous structures. Furthermore, the presence of H1-type hysteresis confirms a well-defined pore structure, highlighting the suitability of the material for applications that benefit from high surface area and porosity, such as catalysis [27,28]. Additionally, the BET analysis revealed a specific surface area of 398.583 m2g−1, which is significant for enhancing surface contact. In a related study, Hassan et al. [29] reported the synthesis of Ag-MOF, V2CTx, and Ag-MOF@V2CTx materials as supercapacitors for hydrogen evolution [29]. Their findings indicated that materials with high pore volume and large specific surface area exhibited superior performance in H₂ evolution. Similarly, the significant surface area and mesoporosity of [Nb(BDC)0.9(PDC)0.1]ₙ likely enhance its effectiveness in catalytic processes. A larger surface area allows for greater exposure of the catalytic sites to NaBH4 as well as better adsorption of H2 molecules, thereby increasing the efficiency of H2 evolution. These attributes make [Nb(BDC)0.9(PDC)0.1]ₙ a promising candidate for applications requiring efficient surface interactions.
The morphology of [Nb(BDC)0.9(PDC)0.1]ₙ was analyzed using SEM, revealing a combination of irregular rod-shaped and spherical structures of varying sizes (Figure 3a). These observations align with the findings of Zang et al. [30], who reported similar morphologies in Co-MOFs, characterized by irregular spherical shapes. Similarly, Sun et al. [31] identified irregular rod-shaped structures, similar to those observed in the micrograph shown in Figure 3a. Furthermore, EDS analysis confirmed the presence of Nb within [Nb(BDC)₀.₉(PDC)₀.₁]ₙ (Figure 3b), supporting the successful synthesis of the material.
The TGA analysis revealed two main thermal events (Figure 4). The first event occurs at 173.48 °C, resulting in a substantial mass loss of 35.67%. This loss corresponds to the initial degradation of the BDC and PDC binders [32,33]. The second thermal event is observed at 375.28 °C, with a mass loss of 4.99%, indicating the complete decomposition of the PDC binder, which is fully degraded at 375.23 °C. These findings provide valuable insights into the thermal stability and decomposition behavior of the [Nb(BDC)0.9(PDC)0.1]ₙ material, which are essential for its potential applications under varying thermal conditions. Understanding the decomposition temperature of the material allows determining the maximum operating temperature to ensure its integrity [34,35,36].
The Raman spectrum of [Nb(BDC)0.9(PDC)0.1]ₙ exhibited two prominent peaks at 462 and 700 cm−1 (Figure 5). These bands are attributed to the presence of Nb and are consistent with findings from other studies. For instance, Wang et al. [37] identified a vibration at 670 cm−1 corresponding to the symmetric stretching mode of Nb polyhedra, while the Raman bands between 200 and 300 cm⁻¹ are typically associated with the vibrational modes of Nb–O–Nb bonds in T-Nb2O5. Additionally, Su et al. [38] reported NbO-related bands at 689, 1350, and 1450 cm−1, while Choudhury et al. [39] observed vibrational modes of NbO at 576 and 168 cm−1 in Nb(BTC) MOF. The bands observed in the ranges of 800–1000 cm−1 and 1380–1400 cm−1 indicate the presence of the PDC ligand. Choudhury et al. [39] also identified a band at 824 cm−1, attributed to the benzene ring. Peaks between 1000 and 1550 cm−1 correspond to the BDC ligand within the sample. Supporting this, Abuzalat et al. [40] detected a peak at 1420 cm−1 in the spectrum of the (Fe/Co)-BDC compound, indicative of the carboxylate group. Furthermore, Li et al. [41] identified peaks at 1140 and 900 cm−1 associated with the C–H bond of the benzene ring in the BDC ligand [40,41]. These spectral features collectively confirm the successful incorporation of BDC and PDC ligands, along with Nb, into the material structure.
The experimental XRD pattern of [Nb(BDC)0.9(PDC)0.1]ₙ exhibited good agreement with the standard structure of NbO3 (Figure 6), confirming that the material has achieved phase stability and structural integrity. The primary XRD peaks for [Nb(BDC)0.9(PDC)0.1]ₙ were observed at 20.4° and 30.3°, corresponding to the indexed positions of 20° and 30.3° [42].

3.2. Characterization of [Nb(BDC)0.9(PDC)0.1]ₙ/Pt-NP

TEM images of the catalyst, [Nb(BDC)0.9(PDC)0.1]ₙ/Pt-NP, (Figure 7a,b) reveal that the Pt-NP, highlighted by a yellow circle (Figure 7a), exhibits a spherical morphology. Additionally, the micrographs indicate that the Pt-NPs are dispersed and incorporated on the surface of [Nb(BDC)0.9(PDC)0.1]n. This uniform distribution and size control are crucial for catalytic applications as they influence the active surface area and, consequently, the catalytic efficiency [43,44,45]. Furthermore, the EDS analysis presented in Figure 7c confirmed the presence of Pt-NPs in the synthesized material. The copper detected in the analysis is attributed to the grid used for the TEM analysis [46]. DLS analysis (Figure 7d) revealed an average particle size of 181.73 nm, which indicates aggregation. Nanoparticle size is critical for evaluating material efficiency in catalytic applications as nanoparticle size directly impacts active surface area and therefore catalytic performance [47,48].
The adsorption/desorption analysis of [Nb(BDC)0.9(PDC)0.1]ₙ/Pt-NP revealed a specific surface area of 344.277 m²/g, evidencing the decrease in the catalyst surface area compared to [Nb(BDC)0.9(PDC)0.1]ₙ (Figure 8). This reduction can be attributed to the synergy between the Nb-MOF and Pt-NP. The presence of Pt-NP, along with potential reaction byproducts, may partially block or obstruct the pores of the structure, limiting surface accessibility and leading to a decrease in the total available surface area. The observation of high relative pressures (p/p0 > 0.8) suggests that the isotherm is characteristic of mesoporous materials [43].

3.3. Application of [Nb(BDC)0.9(PDC)0.1]ₙ in H2 Evolution from NaBH4

[Nb(BDC)0.9(PDC)0.1]ₙ was employed as a support for catalysts used in H2 evolution from NaBH4. Initially, a study was conducted to evaluate the catalytic performance of four monometallic catalysts, including Pt, Ni, Co, and Pd. For comparison, a control test was also performed with each metal alone, without the support [Nb(BDC)0.9(PDC)0.1]n, to evaluate the influence of the support on the catalytic activity. As shown in Figure 9a, the MOF-supported Pt-NPs exhibited the highest H2 evolution efficiency, demonstrating enhanced kinetics with a HGR of 119,020 mL min−¹ g−¹. In comparison, the Pt-NPs without the support achieved a lower HGR of 86,588 mL min−¹ g−¹ (Figure 9b). The superior performance can be attributed to the larger contact area between the catalyst and NaBH4 when supported on [Nb(BDC)0.9(PDC)0.1]n, which improves the dispersion of Pt-NPs and prevents their agglomeration. This improved dispersion facilitates more efficient hydrolysis of NaBH4, leading to a higher HGR [49].
Wu et al. [50] reported Pt supported on CeO2 and Co7Ni2O, forming the hybrid catalyst Pt/CeO2–Co7Ni2Ox, which showed an HGR of 7834.8 mL min−1 gcat−1. Bozkurt et al. [12] also reported Pt-NPs coupled with the Co3O4 support, utilizing the polyol method with microwave irradiation for green hydrogen evolution from NaBH4 hydrolysis. The authors evaluated that the higher the dose of Pt–Co3O4 (100 mg), the higher the HGR obtained (3763 mL min−1 gcat−1) [12].

3.3.1. Evaluation of the Effect of NaBH4 Concentration

The effect of NaBH4 concentration on H2 evolution in the presence of the [Nb(BDC)0.9(PDC)0.1]ₙ/Pt-NPs catalyst was also investigated. As illustrated in Figure 10a, the H2/NaBH4 ratio decreased with increasing NaBH4 concentration. Based on these findings, subsequent experiments were carried out using 0.5 mmol of NaBH4, as this dosage demonstrated the highest HGR, as shown in Figure S2.
To determine the reaction order with respect to NaBH₄ concentration, a plot of ln[k] versus ln[NaBH4] concentration was generated, as shown in Figure 10b. The slope of the linear fit (−0.79) suggests a zero-order reaction, indicating that NaBH₄ concentration does not limit the rate of the H₂ evolution reaction. These findings are consistent with the results of Ababaii et al. [51], who also reported that variations in NaBH₄ concentration had minimal impact on the hydrogen evolution process.

3.3.2. Evaluation of Catalyst Dosage

The catalyst dosage (Pt-NPs) effect on NaBH4 hydrolysis was evaluated from 0.10 to 10.0 mmol% (Figure 11a). The highest HGR observed was 119,020 mL min−¹ g−¹ at a dosage of 10 mmol% (Figure 11b). The data revealed that HGR increased with higher catalyst dosages, suggesting that H2 generation is dosage-dependent the catalyst. Ababaii et al. [51] obtained similar results in the study of Ni–B–Cr catalyst dosage for H2 evolution. They determined that higher catalyst dosages led to higher HGR values and improved reaction kinetics [51].

3.3.3. Evaluation of Effect of NaOH Concentration

The results of H₂ evolution at varying NaOH concentrations are presented in Figure 12a, with concentrations ranging from 0.01 to 0.2 mol L−1. As illustrated in Figure 12b, the highest HGR of 119,020 mL min⁻¹ g⁻¹ was achieved in the absence of NaOH. This evaluation is typically performed to determine whether the addition of NaOH is necessary to stabilize NaBH4. In this case, the observed decline in reaction yield with increasing NaOH concentration suggests that borate anions have accumulated within the catalyst pores, leading to blockages that impede mass transfer and hinder NaBH4 hydrolysis. In the study by Al-shaikh et al. [52], they reported a decrease in H2 generation efficiency with increasing NaOH concentration when using a Pd NPs@[KIT-6]-PEG-imid catalyst for de H2 evolution from NaBH4, with NaOH concentrations ranging from 0.05 to 0.2 mM at 298 K [52]. Additionally, as presented by Churikov et al. [53] in their H2 evolution study, the addition of NaOH can increase the viscosity of the reaction medium, thereby reducing H2 gas production yield [53].

3.3.4. Evaluation of the Temperature Effect

Temperature is a critical factor in many catalytic reactions, as optimal temperatures can significantly increase reaction rates by increasing the frequency of molecular collisions. Figure 13a demonstrates that temperature has a relatively minor effect on H2 evolution during the hydrolysis of NaBH4 catalyzed by [Nb(BDC)0.9(PDC)0.1]n/Pt-NPs. This observation is further supported by the HGR values presented in Figure S3. The kinetic constants (Table 1) were determined for each temperature evaluated (Figure 13a), with which the Arrhenius plot was constructed (Figure 13b) accompanied by a fitted line equation model. A linear regression on the experimental data resulted in an activation energy of 16.38 kJ mol−¹. The low energy activation suggests that the catalyst [Nb(BDC)0.9(PDC)0.1]n/Pt-NPs has a reduced energy barrier, leading to faster reaction kinetics during the reaction process. Churikov et al. [53] examined the dependence of the NaBH4 hydrolysis rate on temperature and concentration, focusing on the constant (k). Their findings indicated that increasing temperature accelerates the hydrolysis of NaBH4, and this relationship aligns with the Arrhenius equation [53].

3.4. Evaluation of Catalyst Reuse

The catalyst reusability was tested for eight consecutive cycles. As shown in Figure 14, there was a 7.18% reduction in HGR from the first to the eighth cycle, indicating that the catalyst maintained good performance over multiple uses. Although small fluctuations were observed during the reactions, these variations can be attributed to the gradual deactivation of active catalytic sites, probably due to NaBO2 poisoning [54]. Demirci et al. [55] reported a similar trend in their reuse tests of the PEI-Ni catalyst, where only a slight decrease in catalytic activity was observed over consecutive cycles. These findings highlight the durability and effectiveness of the [Nb(BDC)0.9 (PDC)0.1]n/Pt-NPs catalyst, though further investigation into the causes of efficiency fluctuations could help optimize its long-term performance.

3.5. Effect of Deuterated Water (D2O)

Kinetic Isotope Effect (KIE) is necessary to analyze the mechanism of a chemical reaction [56]. It is classified as first order if 2 < KIE < 7 or second order if 0.7 < KIE < 1.5 [57]. In the H2 evolution process described by Equation (3), the first-order kinetic isotope effect (KIE) involves the water-facilitated production of H2 gas. On the other hand, the second-order KIE, as analyzed in Equation (1), indicates that NaBH4 plays a key role in driving the production of H2 gas. The KIE was evaluated for [Nb(BDC)0.9(PDC)0.1]n/Pt-NPs, and according to the results presented in Figure 15, a KIE of 1.17 was obtained. This indicates that the step involving water is not the rate-determining step, rather, it is the dissociation of BH from BH4-, making it a second-order reaction.

3.6. Mechanistic Proposal for H2 Generation from NaBH4 Catalyzed by [Nb(BDC)0.9(PDC)0.1]n/Pt-NPs

Although a comprehensive mechanistic proposal to elucidate the catalytic reaction for H2 generation from NaBH4 using Nb-MOF has yet to be fully established, the synergy between Pt-NP and the [Nb(BDC)0.9(PDC)0.1]n/Pt-NPs support is believed to play a crucial role in the efficacy of the process [14]. Pt is known for its unique electronic properties, which enhance the adsorption of NaBH4 on its surface [10]. This increased adsorption is essential for the formation of reactive intermediates, although these intermediates remain unidentified. The interaction of NaBH4 with Pt facilitates the initial steps of the catalytic reaction, promoting the release of H2 [10,14]. Furthermore, the distinct structural characteristics of [Nb(BDC)0.9(PDC)0.1]n/Pt-NPs, such as its high surface area and porosity, provide a favorable environment for the effective distribution and stabilization of Pt nanoparticles. This structural architecture maximizes the exposure of Pt to the reactants, allowing for more efficient catalysis. This interplay between the metal composition and the MOF support is vital for achieving enhanced catalytic activity in the dehydrogenation of NaBH4 [10]. The collaborative effect of Pt-NP and the [Nb(BDC)0.9(PDC)0.1]n/Pt-NPs not only optimizes the catalytic process but also contributes to the sustainable production of H2 gas.

3.7. Performance of the Catalyst

The [Nb(BDC)0.9(PDC)0.1]ₙ/Pt-NPs catalyst demonstrated competitive performance compared to other MOF-based catalysts in terms of H2 evolution efficiency. Table 2 presents a comparison between our catalyst and several materials reported in the literature, highlighting its effectiveness. Using the [Nb(BDC)0.9(PDC)0.1]n/Pt-NPs, an HGR of 119,020 mL min−1 g−1 was achieved, maintaining high activity with minimal degradation over multiple cycles. The greater stability and sustained activity of the [Nb(BDC)0.9(PDC)0.1]n/Pt-NPs catalyst can be attributed to its unique structural characteristics, including its well-distributed porosity and high surface area, which likely enhance its resistance to poisoning and deactivation, making it a promising candidate for sustainable H2 production.
In summary, the performance of the [Nb(BDC)0.9(PDC)0.1]n catalyst highlights the potential of Nb-based MOFs as promising candidates for the development of advanced catalytic systems in clean energy applications.

4. Conclusions

This study demonstrated the effectiveness of an Nb-based MOF, [Nb(BDC)0.9(PDC)0.1]n, as a support for Pt-NPs to serve as a catalyst for H2 evolution from NaBH4 hydrolysis. The [Nb(BDC)0.9(PDC)0.1]n/Pt-NPs catalyst was fully characterized using BET, SEM, TGA, and Raman spectroscopy, confirming the successful synthesis and structural integrity of the material. During H2 evolution tests, the catalyst achieved a HGR of 119,020 mL min−¹ g−¹ and an activation energy of 16.38 kJ mol−¹, highlighting its catalytic efficiency. Furthermore, the catalyst maintained stable performance over eight reuse cycles, demonstrating its durability. These findings suggest that [Nb(BDC)0.9(PDC)0.1]ₙ/Pt-NPs is a promising candidate for H2 production from NaBH4, offering excellent catalytic activity, stability, energy efficiency, and favorable reaction kinetics.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/pr12122677/s1, Figure S1: The system used for the H2 evolution from [Nb(BDC)0.9(PDC)0.1]n/Pt-NPs catalyzed NaBH4 hydrolysis. Figure S2: Effect of NaBH4 concentration on hydrogen generation rate (HRG) catalyzed by [Nb(BDC)0.9(PDC)0.1]n/Pt-NPs. Figure S3: Effect of temperature on hydrogen generation rate (HRG) catalyzed by [Nb(BDC)0.9(PDC)0.1]n/Pt-NPs on NaBH4 hydrolysis.

Author Contributions

J.R.d.J., F.J.M.N. and R.P.L.M.: Conceptualization, Methodology, Funding acquisition. J.R.d.J., E.P.S., T.d.A.A., L.d.M.G., M.J.d.S. and F.J.M.N.: Data curation, Validation, Investigation. J.R.d.J., E.P.S. and T.d.A.A.: Writing—Original draft preparation. J.R.d.J., E.P.S., T.d.A.A., F.J.M.N. and R.P.L.M.: Writing—Reviewing and Editing. All authors have read and agreed to the published version of the manuscript.

Funding

The authors thanks to Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG, grant numbers APQ-01574-22, APQ-01786-22; APQ-05429-23, and RED-00144-22), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, grant number 05828/2022-5), and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for financial support.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. General scheme of coordination compound of niobium synthesis, employing 1,4-benzenedicarboxylic acid (BDC) and 2,5-pyridinedicarboxylic acid (PDC) as ligand and Ammonium niobium oxalate as Nb source.
Figure 1. General scheme of coordination compound of niobium synthesis, employing 1,4-benzenedicarboxylic acid (BDC) and 2,5-pyridinedicarboxylic acid (PDC) as ligand and Ammonium niobium oxalate as Nb source.
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Figure 2. Adsorption–desorption isotherm curves for [Nb(BDC)0.9(PDC)0.1].
Figure 2. Adsorption–desorption isotherm curves for [Nb(BDC)0.9(PDC)0.1].
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Figure 3. (a) SEM image for [Nb(BDC)0.9(PDC)0.1]. (b) EDS analysis of [Nb(BDC)0.9(PDC)0.1] confirming the elemental composition, including the presence of Nb.
Figure 3. (a) SEM image for [Nb(BDC)0.9(PDC)0.1]. (b) EDS analysis of [Nb(BDC)0.9(PDC)0.1] confirming the elemental composition, including the presence of Nb.
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Figure 4. Thermogram illustrating the thermal decomposition of [Nb(BDC)0.9(PDC)0.1]n.
Figure 4. Thermogram illustrating the thermal decomposition of [Nb(BDC)0.9(PDC)0.1]n.
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Figure 5. Raman spectrum of [Nb(BDC)0.9(PDC)0.1]ₙ confirming the successful synthesis of the material.
Figure 5. Raman spectrum of [Nb(BDC)0.9(PDC)0.1]ₙ confirming the successful synthesis of the material.
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Figure 6. Experimental X-ray diffraction pattern of [Nb(BDC)0.9(PDC)0.1]ₙ. The horizontal bars mean the standard pattern to NbO3 found in the database of the X-ray diffractometer (ICSD 028564).
Figure 6. Experimental X-ray diffraction pattern of [Nb(BDC)0.9(PDC)0.1]ₙ. The horizontal bars mean the standard pattern to NbO3 found in the database of the X-ray diffractometer (ICSD 028564).
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Figure 7. TEM images confirming the incorporation of Pt-NP into [Nb(BDC)0.9(PDC)0.1] at different magnifications: (a) 0.5 µm and (b) 100 nm. (c) EDS analysis indicating the presence of Nb and Pt in the material. (d) DLS results for [Nb(BDC)0.9(PDC)0.1]/Pt-NP showing the particle size distribution.
Figure 7. TEM images confirming the incorporation of Pt-NP into [Nb(BDC)0.9(PDC)0.1] at different magnifications: (a) 0.5 µm and (b) 100 nm. (c) EDS analysis indicating the presence of Nb and Pt in the material. (d) DLS results for [Nb(BDC)0.9(PDC)0.1]/Pt-NP showing the particle size distribution.
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Figure 8. Adsorption–desorption isotherm curves for [Nb(BDC)0.9(PDC)0.1]ₙ/Pt-NP.
Figure 8. Adsorption–desorption isotherm curves for [Nb(BDC)0.9(PDC)0.1]ₙ/Pt-NP.
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Figure 9. Hydrogen evolution from NaBH4 using 10 mmol% monometallic nanoparticles, both unsupported and supported on [Nb(BDC)0.9(PDC)0.1]n. (a) Kinetics of H₂ evolution and (b) Hydrogen Generation Rate (HGR) results. Experimental conditions: 20 mg of [Nb(BDC)0.9(PDC)0.1]ₙ and 1.00 mL of NaBH4 solution (0.500 mol L−1) at 303.15 K.
Figure 9. Hydrogen evolution from NaBH4 using 10 mmol% monometallic nanoparticles, both unsupported and supported on [Nb(BDC)0.9(PDC)0.1]n. (a) Kinetics of H₂ evolution and (b) Hydrogen Generation Rate (HGR) results. Experimental conditions: 20 mg of [Nb(BDC)0.9(PDC)0.1]ₙ and 1.00 mL of NaBH4 solution (0.500 mol L−1) at 303.15 K.
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Figure 10. Effect of NaBH4 concentration on the hydrogen evolution using [Nb(BDC)0.9(PDC)0.1]n/Pt-NPs catalyst: (a) Kinetic behavior and (b) linear model applied to the results. Experimental conditions: 20 mg of [Nb(BDC)0.9(PDC)0.1]n and 10 mmol% pure Pt-NPs at 303.15 K, with 1.00 mL of NaBH4 under different concentrations (0.50, 0.75, 1.00, and 1.50 mol L−1).
Figure 10. Effect of NaBH4 concentration on the hydrogen evolution using [Nb(BDC)0.9(PDC)0.1]n/Pt-NPs catalyst: (a) Kinetic behavior and (b) linear model applied to the results. Experimental conditions: 20 mg of [Nb(BDC)0.9(PDC)0.1]n and 10 mmol% pure Pt-NPs at 303.15 K, with 1.00 mL of NaBH4 under different concentrations (0.50, 0.75, 1.00, and 1.50 mol L−1).
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Figure 11. Effect of [Nb(BDC)0.9(PDC)0.1]n/Pt-NPs catalyst dosage on the hydrogen evolution: (a) Kinetic behavior and (b) Hydrogen Generation Rate (HGR) results. Experimental conditions: 20 mg of [Nb(BDC)0.9(PDC)0.1]n and 1.00 mL of NaBH4 (0.500 mol L−1) at 303.15 K, varying pure Pt monometallic NPs dosages (10.0, 5.0, 1.0, and 0.1 mmol%).
Figure 11. Effect of [Nb(BDC)0.9(PDC)0.1]n/Pt-NPs catalyst dosage on the hydrogen evolution: (a) Kinetic behavior and (b) Hydrogen Generation Rate (HGR) results. Experimental conditions: 20 mg of [Nb(BDC)0.9(PDC)0.1]n and 1.00 mL of NaBH4 (0.500 mol L−1) at 303.15 K, varying pure Pt monometallic NPs dosages (10.0, 5.0, 1.0, and 0.1 mmol%).
Processes 12 02677 g011
Processes 12 02677 g012
Figure 12. Effect of NaOH concentration on the hydrogen evolution using [Nb(BDC)0.9(PDC)0.1]n/Pt-NPs catalyst. (a) Kinetic behavior and (b) Hydrogen Generation Rate (HGR) results. Experimental conditions: 20 mg of [Nb(BDC)0.9(PDC)0.1]n with 10 mmol% pure Pt-NPs and 1.00 mL of NaBH4 (0.500 mol L̶ 1) at 303.15 K, under absence or presence of NaOH in different concentrations (0.2, 0.1, 0.05, 0.01mol L−1).
Figure 12. Effect of NaOH concentration on the hydrogen evolution using [Nb(BDC)0.9(PDC)0.1]n/Pt-NPs catalyst. (a) Kinetic behavior and (b) Hydrogen Generation Rate (HGR) results. Experimental conditions: 20 mg of [Nb(BDC)0.9(PDC)0.1]n with 10 mmol% pure Pt-NPs and 1.00 mL of NaBH4 (0.500 mol L̶ 1) at 303.15 K, under absence or presence of NaOH in different concentrations (0.2, 0.1, 0.05, 0.01mol L−1).
Processes 12 02677 g012
Processes 12 02677 g013
Figure 13. Effect of temperature on the hydrogen evolution using [Nb(BDC)0.9(PDC)0.1]n/Pt-NPs catalyst. (a) Kinetic behavior and the (b) linear model applied to the results of Table 1. Experimental conditions: 20 mg of [Nb(BDC)0.9(PDC)0.1]n with 10 mmol% pure Pt-NPs and 1.00 mL of NaBH4 (0.500 mol L−1).
Figure 13. Effect of temperature on the hydrogen evolution using [Nb(BDC)0.9(PDC)0.1]n/Pt-NPs catalyst. (a) Kinetic behavior and the (b) linear model applied to the results of Table 1. Experimental conditions: 20 mg of [Nb(BDC)0.9(PDC)0.1]n with 10 mmol% pure Pt-NPs and 1.00 mL of NaBH4 (0.500 mol L−1).
Processes 12 02677 g013
Processes 12 02677 g014
Figure 14. Efficiency of the [Nb(BDC)0.9(PDC)0.1]ₙ/Pt-NPs catalyst over eight cycles. Experimental conditions: 20 mg of [Nb(BDC)0.9(PDC)0.1]n with 10 mmol% of pure Pt-NPs and 1.00 mL of NaBH4 (0.500 mol L−1) at 303.15 K.
Figure 14. Efficiency of the [Nb(BDC)0.9(PDC)0.1]ₙ/Pt-NPs catalyst over eight cycles. Experimental conditions: 20 mg of [Nb(BDC)0.9(PDC)0.1]n with 10 mmol% of pure Pt-NPs and 1.00 mL of NaBH4 (0.500 mol L−1) at 303.15 K.
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Processes 12 02677 g015
Figure 15. Results of hydrogen evolution in the Kinetic Isotope Effect (KIE) of the Pt- [Nb(BDC)0.9(PDC)0.1]n/Pt-NPs catalyst.
Figure 15. Results of hydrogen evolution in the Kinetic Isotope Effect (KIE) of the Pt- [Nb(BDC)0.9(PDC)0.1]n/Pt-NPs catalyst.
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Table 1. Kinetic constants of H2 evolution reaction from NaBH4 using [Nb(BDC)0.9(PDC)0.1]n/Pt-NPs catalyst at different temperatures.
Table 1. Kinetic constants of H2 evolution reaction from NaBH4 using [Nb(BDC)0.9(PDC)0.1]n/Pt-NPs catalyst at different temperatures.
Temperature (Kelvin)Kinetic Constants (s−1)
303.152.9755
313.155.2268
323.154.7531
333.155.8433
Table 2. Comparison of different MOF-based catalysts applied in H2 evolution from NaBH4 hydrolysis.
Table 2. Comparison of different MOF-based catalysts applied in H2 evolution from NaBH4 hydrolysis.
CatalystEA
(kJ mol−1)		HGR
(mL min−1 gcat−1)		ReuseRef.
MOF-derived cobalt-phospho-boride for rapid hydrogen generation via NaBH4 hydrolysis20.71800~98% efficiency in the 5th cycle[58]
Cerium-Organic Framework (CeOF) for hydrogen generation via the hydrolysis of NaBH458.81800 ~100%efficiency in the 4th cycle[59]
Modulating effect of urea/melamine on Co2+/Co3+ ratio of Co3O4 microplates for rapid hydrogen generation via NaBH4 hydrolysis46.92042~100% efficiency in the 5th cycle[60]
Magnetic recyclable catalysts with dual protection of hollow Co/N/C framework and surface carbon film for hydrogen production from NaBH4 hydrolysis26.9 9815.8281.3% efficiency in the 25th cycle [61]
Hydrogen evolution from NaBH4 using novel Ni/Pt nanoparticles decorated on a niobium-based composite23.11782~100% efficiency in the 16th cycle[62]
Niobium coordination compound with catalytic application for green hydrogen evolution is developed16.38119,02092.82% efficiency in the 8th cycleThis work
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Squizzatto, E.P.; Andrade, T.d.A.; Lopes Moreira, R.P.; Guimarães, L.d.M.; da Silva, M.J.; Novaes, F.J.M.; de Jesus, J.R. Development of a Niobium-Based Coordination Compound with Catalytic Applications for Green Hydrogen Evolution. Processes 2024, 12, 2677. https://doi.org/10.3390/pr12122677

AMA Style

Squizzatto EP, Andrade TdA, Lopes Moreira RP, Guimarães LdM, da Silva MJ, Novaes FJM, de Jesus JR. Development of a Niobium-Based Coordination Compound with Catalytic Applications for Green Hydrogen Evolution. Processes. 2024; 12(12):2677. https://doi.org/10.3390/pr12122677

Chicago/Turabian Style

Squizzatto, Emily Pacheco, Tatianny de Araujo Andrade, Renata Pereira Lopes Moreira, Luciano de Moura Guimarães, Márcio José da Silva, Fábio Junior Moreira Novaes, and Jemmyson Romário de Jesus. 2024. "Development of a Niobium-Based Coordination Compound with Catalytic Applications for Green Hydrogen Evolution" Processes 12, no. 12: 2677. https://doi.org/10.3390/pr12122677

APA Style

Squizzatto, E. P., Andrade, T. d. A., Lopes Moreira, R. P., Guimarães, L. d. M., da Silva, M. J., Novaes, F. J. M., & de Jesus, J. R. (2024). Development of a Niobium-Based Coordination Compound with Catalytic Applications for Green Hydrogen Evolution. Processes, 12(12), 2677. https://doi.org/10.3390/pr12122677

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