Boosting Hydrogen Generation with Platinum Nanoparticles Decorated on HTiNbO₅ via NaBH₄ Hydrolysis

Abstract

In this study, we report the preparation of platinum nanoparticles (Pt NPs) deposited on HTiNbO₅ and the application of the resultant material in the catalytic decomposition of sodium borohydride (NaBH₄) to generate hydrogen. The starting material, KTiNbO₅, was prepared through a solid-state process involving Nb₂O₅, K₂CO₃, and TiO₂. The subsequent treatment with HNO₃ resulted in the exchange of potassium by protons, rendering HTiNbO₅. This material served as support for Pt nanoparticles (3.6 ± 0.7 nm), producing Pt NPs/HTiNbO₅. All compounds were characterized using TGA, FTIR, XRD, Raman, SEM-EDS, and HRTEM. The influence of different factors on the reaction kinetics was evaluated, resulting in a hydrogen generation rate (HGR) of 22,790.18 $\mathrm{m}\mathrm{L} {\mathrm{m}\mathrm{i}\mathrm{n}}^{-1}{\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$ at 50 °C. The activation energy (41.83 kJ mol⁻¹) was also determined. A mechanistic study with deuterated water revealed a kinetic isotopic effect (KIE) value of 1.27, indicating the dissociation of B-H from BH₄⁻ as the rate-determining step of the process. Furthermore, the reuse and durability of the material were evaluated, revealing a catalyst performance close to 100% over the 10 tested cycles. Therefore, it can be concluded that the synthesized material, Pt-nanoparticles dispersed on HTiNbO₅, exhibits excellent performance and is suitable for hydrogen evolution from NaBH₄.

1. Introduction

In recent years, significant changes have been observed in the transition from fossil fuel-based energy matrices to sustainable options. In this scenario, the use of hydrogen as an energy source stands out due to its heat capacity being approximately three times that of gasoline [1,2]. Furthermore, its conversion into electricity only releases water vapor. Initially, the terms “green hydrogen” and “gray hydrogen” were used to describe different types of hydrogen. Green hydrogen is considered environmentally friendly because it is produced without CO₂ emissions, while gray hydrogen is derived from natural gas. However, hydrogen can be produced through various processes, including electrolysis, steam methane reforming, and gasification, among other processes. A broader range of terms is needed to distinguish the different sources and production methods, known as the “hydrogen rainbow” [3].

Despite the excellent energetic performance of hydrogen in comparison to fossil fuels, ethanol, etc., it is highly explosive, and has a low boiling point and density, drawbacks that prevent its use. Therefore, complex hydrides, such as sodium borohydride, as a solid-state hydrogen storage material, have been receiving increasing attention [4,5]. One of the biggest obstacles to the use of hydrogen is the infrastructure for its implementation [6]. NaBH₄ is a solid and stable compound at room temperature and can undergo hydrolysis to produce gaseous hydrogen. The hydrolysis reaction is described by Equation (1), which shows that each mole of NaBH₄ yields 4 moles of gaseous hydrogen. Although hydrolysis is a spontaneous reaction, it is a slow process, requiring catalysts.

NaBH₄(aq) + 4H₂O(l) → NaB(OH)₄(aq) + 4H₂(g)

(1)

Some of the most used catalysts include platinum, nickel, and cobalt nanoparticles [1,2]. However, these nanoparticles often tend to agglomerate, requiring the deposition on a support material to maintain effectiveness and stability. In this context, since the discovery and study of graphene’s properties, other two-dimensional chemical species have become the focus of extensive research. Some of them are the titanium niobate-based materials mentioned in the literature since the first preparation of KTiNbO₅ [7]. This double oxide-possessing species displays a remarkable ion-exchange ability, which can be explored to prepare other chemical entities [8]. These 2D inorganic polymers can be exfoliated using Bu₄NOH, resulting in the formation of infinite layers of TiNbO₅⁻, intercalated by Bu₄N⁺ cations [9]. Furthermore, they have the versatility of hosting various chemical species, organic or metallic cations, by displacing the Bu₄N⁺ ions [9,10]. This property makes 2D titanoniobates promising candidates as starting reagents for many new materials, leading to a range of innovative applications.

Three decades of research have documented the evolution of novel synthetic procedures and described their functionalities and promising applications. One can mention, for instance, the catalytic and photocatalytic activities [11,12], and application in electrochemistry [13,14], among others. Materials produced by the deposition of Au nanoparticles on KTiNbO₅ or HTiNbO₅ induced water splitting, yielding hydrogen [15,16]. In addition, it was observed that nanosheets (ns) of TiNbO₅⁻ improve the water splitting in comparison with the bulk potassium titanoniobate [17]. Finally, a TiO₂/HTiNbO₅-pillared composite was very efficient in the photocatalytic production of hydrogen.

Therefore, in view of the growing interest in graphene-like materials, this study aimed to investigate the decomposition of NaBH₄ activated by KTiNbO₅ (1), HTiNbO₅ (2), or ns-HTiNbO₅ (3) impregnated with platinum nanoparticles.

2. Materials and Methods

2.1. Materials and Reagents

All the reagents used in this work were of analytical grade. H₂PtCl₆·6H₂O (≥37.5% Pt, CAS No: 18497-13-7), Bu₄NOH·30H₂O (98%, CAS No: 47741-30-8), and NaBH₄ (≥98.0%, CAS No: 16940-99-2) were obtained from Sigma Aldrich (St. Louis, MO, USA). K₂CO₃ (P.A., CAS No: 584-08-7), TiO₂ (P.A., CAS No: 3463-67-7), and absolute ethanol (P.A., CAS No: 64-17-5) were purchased from Synth (Diadema, SP, Brazil). Nb₂O₅ (P.A., CAS No: 1313-96-8) was obtained from Merck (Darmstadt, Germany). HNO₃ was acquired from Qhemis (Lençóis Paulista, SP, Brazil; 65%, P.A.; CAS No. 7697-37-2).

KTiNbO₅, HTiNbO₅, or ns-HTiNbO₅ (ns = nanosheets) were prepared, with minor modifications, following the literature procedure [7,18].

2.2. Synthesis of KTiNbO₅, HTiNbO₅, or Ns-HTiNbO₅ Decorated with Platinum Nanoparticles

The synthesis of Pt nanoparticles (Pt NPs) was carried out using NaBH₄ as reducing reagent, according to previous reports [4,5]. KTiNbO₅, HTiNbO₅, or ns-HTiNbO₅ (10 mg) were added to three beakers, followed by the addition of water (10 mL) and an appropriate volume of a H₂PtCl₂.6H₂O solution (22 mmol L⁻¹). After stirring the mixture for 15 min, NaBH₄ (1.00 mol L⁻¹, 1.00 mL) was vigorously mixed. The resulting material was centrifuged (4000 rpm, 5 min), the solid was filtered, and washed with type II water.

2.3. Characterization of the Materials

The materials were characterized using various techniques. For X-ray diffraction (XRD), the data were collected at room temperature, in a Rigaku Ultima IV diffractometer (Rigaku Corporation, Tokyo, Japan), using Cu Kα radiation (0.154 nm), with instrument power of 40 kV and 30 mA, and 2θ scan range from 2° to 90°, with a speed of 0.5° min⁻¹.

Fourier transform infrared spectroscopy (FTIR) studies were conducted at room temperature over a range of 550 to 4000 cm⁻¹ using a PerkinElmer Frontier Spectrometer (PerkinElmer, Shelton, CT, USA).

Thermal characterization was performed using simultaneous thermogravimetry (TG) and differential thermal analysis (DTA) on a Shimadzu DTG60H thermobalance (Shimadzu Corporation, Kyoto, Japan) in an inert atmosphere of synthetic nitrogen, with a flow rate of 50 mL min⁻¹, over a temperature range of 30 to 800 °C. An open alumina crucible was employed for the analysis. During the heating cycle, the temperature was increased at a rate of 10 °C min⁻¹. The TG curves were differentiated to the first order (DTG) to confirm the thermal phenomena.

Scanning electron microscopy (SEM) images were obtained using a Thermo Fisher FEI Quanta 200 FEG microscope (Thermo Fisher Scientific, Hillsboro, OR, USA), operating at an acceleration voltage ranging from 2 kV to 30 kV, to investigate the morphology of the materials. The KTiNbO₅ and HTiNbO₅ samples were deposited on carbon film grids, while the ns-HTiNbO₅ sample was dispersed in isopropyl alcohol using ultrasound before being placed on silicon support grids.

Energy-dispersive X-ray spectroscopy (EDS) was performed using a JEOL JSM-6010LA electron microscope (JEOL Ltd., Tokyo, Japan), equipped with an EDS detector and operating at an accelerating voltage of 20 kV. The samples were also deposited on carbon film grids and coated with gold.

The nanoparticle size was evaluated by high-resolution transmission electron microscopy (HRTEM) using the FEI Tecnai G2-20 Supertwin equipment (FEI Company, Hillsboro, OR, USA), operating at a voltage of 200 kV. The equipment is fitted with an energy-dispersive spectroscopy (EDS) detector. The particle size was determined using ImageJ software (Version 1.8.0).

2.4. Application of Materials in Hydrogen Evolution from NaBH₄

The NaBH₄ hydrolysis reaction was carried out in the apparatus shown in Figure S1. The hydrogen evolution experiments were carried out following the methodology of Sperandio et al. (2024) [19], with modifications. The reaction occurred in a Schlenk tube, into which freshly synthesized material was added, containing a volume of 3 mL of water. After sealing the entire system and leveling the burette volume at zero, 1.0 mL of a NaBH₄ solution at the desired concentration was injected. The hydrogen gas produced displaces a water column in the burette. The amount of hydrogen was determined according to Equation (2).

$${\mathrm{n}}_{{\mathrm{H}}_2}={\displaystyle \frac{{\mathrm{P}}_{{\mathrm{H}}_2}\mathrm{V}}{\mathrm{R}\mathrm{T}}}$$

(2)

where $P_{H_2}$ (Pa) is the pressure that the gas exerts in the container, V (m³) is the volume of gas produced, n is the amount of the gas (mol), T is the temperature (K), and R is the universal gas constant (8.145 J mol⁻¹ K⁻¹).

The partial pressure of H₂ $\left(P_{H_2}\right)$ was determined according to Equation (3).

$${\mathrm{P}}_{{\mathrm{H}}_2}={\mathrm{P}}_{\mathrm{a}\mathrm{t}\mathrm{m}.}+\rho \mathrm{g}∆\mathrm{V}$$

(3)

where $P_{atm}$ refers to the local atmospheric pressure (0.93 atm), ρ refers to the density of the water, and ∆V is the volume of water displaced in the burette.

The hydrogen generation rate (HGR) was determined according to Equation (4).

$$\mathrm{H}\mathrm{G}\mathrm{R}\left(\mathrm{m}\mathrm{L} {\mathrm{m}\mathrm{i}\mathrm{n}}^{-1}{\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}.}^{-1}\right)={\displaystyle \frac{\mathrm{V}}{\mathrm{m}\times \mathrm{t}}}$$

(4)

where V (mL) is the volume of H₂ produced, t (min) is the reaction time, and m (g) is the mass of the catalyst (Pt NPs).

2.5. Evaluation of the Effect of Pt NPs Dose

The effect of the Pt nanoparticle (NP) dose was assessed only for the material that displayed the best performance in the catalytic decomposition of NaBH₄. Doses of 0.1, 1.0, 5.0, and 10.0 mmol% relative to NaBH₄ were evaluated, which were deposited on 10 mg of HTiNbO₅. The freshly prepared materials were dispersed in 5.0 mL of type II water. In each experiment, 1.00 mL of NaBH₄ solution (1.00 mol L⁻¹) prepared with type II water was injected. All experiments were conducted at a controlled temperature (28.4 °C).

2.6. Evaluation of the Effect of NaBH₄ Concentration

The evaluated concentrations were 0.19, 0.24, 0.37, and 0.50 mol L⁻¹. Initially, 10 mg of Pt NPs/HTiNbO₅, freshly prepared, containing 5.0 mmol% of the catalyst relative to NaBH₄, were dispersed in 5.0 mL of type II water. Then, in each experiment, 1.00 mL of NaBH₄ solution at the desired concentration was injected. All experiments were conducted at a controlled temperature (28.4 °C).

2.7. Evaluation of the Effect of NaOH

The effect of NaOH concentration on the catalytic hydrolysis of NaBH₄ was evaluated. Then, solutions of NaBH₄ (1.00 mL, 0.500 mol L⁻¹) were prepared at various NaOH concentrations (0.010, 0.025, 0.050, and 0.075 mol L⁻¹). All other parameters were kept constant, as in the previous item.

2.8. Kinetic Evaluation

The hydrogen evolution rate can be related to temperature, concentrations of NaBH₄, Pt NPs/HTiNbO₅ dosage, and NaOH concentration, according to Equation (5) [19].

$$HGR=-4{\displaystyle \frac{d\left[\mathrm{N}\mathrm{a}\mathrm{B}{\mathrm{H}}_4\right]}{dt}}={\displaystyle \frac{d\left[{\mathrm{H}}_2\right]}{dt}}=k_{overall} {\left[\mathrm{N}\mathrm{a}\mathrm{B}{\mathrm{H}}_4\right]}^a{\left[\mathrm{P}\mathrm{t} \mathrm{N}\mathrm{P}\mathrm{s}/\mathrm{H}\mathrm{T}\mathrm{i}\mathrm{N}\mathrm{b}{\mathrm{O}}_5\right]}^b{\left[\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}\right]}^c$$

(5)

where a, b, and c are the orders of the reaction.

2.9. Evaluation of the Effect of Temperature

The effect of temperature, set at 20, 25, 28.4, 45, and 50 °C, on the kinetics of NaBH₄ hydrolysis catalyzed by Pt NPs/HTiNbO₅ was also determined. The remaining parameters were kept constant, such as 5.0 mmol% catalyst dose relative to NaBH₄ (0.500 mol L⁻¹). From the data obtained for each temperature, the kinetic constant was determined. The data were plotted according to the Arrhenius equation (Equation (6)), i.e., ln k versus 1/T.

$$\ln \mathrm{k}=\ln \mathrm{A}-{\displaystyle \frac{{\mathrm{E}}_{\mathrm{a}}}{\mathrm{R}\mathrm{T}}}$$

(6)

where k refers to the rate constant, A is the pre-exponential factor, Ea is the activation energy (J mol⁻¹), T is the absolute temperature (K), and R is the universal gas constant (8.3145 J mol⁻¹ K⁻¹).

2.10. Durability and Reuse Assays

The durability of the Pt NPs/HTiNbO₅ catalyst was determined by using 10 mg of HTiNbO₅ dispersed in 5.0 mL of type II water. A fresh dose of 5.0 mmol% of Pt NPs relative to NaBH₄ (reducing reagent) was deposited on this matrix. Then, 1.00 mL of NaBH₄ solution (0.500 mol L⁻¹) (source of H₂) was injected, keeping the temperature at 50 °C. A new NaBH₄ solution (1.00 mL, 0.500 mol L⁻¹) was added to the system after each catalytic cycle without removing the previous solution, and this procedure was repeated up to the 10th cycle. A similar procedure was employed for the reuse assay. However, at the end of each cycle, the Pt NPs/HTiNbO₅ sample was centrifuged (10,000 rpm for 10 min), and the material was filtered and washed with type II water between cycles.

2.11. Kinetic Isotope Effect (KIE) Evaluation

The mechanism of the catalytic reaction was investigated. The Pt NPs/HTiNbO₅ catalyst was freshly synthesized, followed by rinsing with acetone and drying under vacuum. Subsequently, 10 mg of Pt NPs/HTiNbO₅ were added to a 10 mL Schlenk flask. After sealing the flask, it was connected to a burette filled with water. Then, a freshly prepared solution of NaBH₄ in deuterated water (D₂O) or type 1 water (1.0 mL, 0.500 mol L⁻¹) was introduced using a syringe. All reactions were conducted at a temperature of 298.15 K. The KIE was calculated according to Equation (7).

$$\mathrm{K}\mathrm{I}\mathrm{E}={\displaystyle \frac{{\mathrm{k}}_{{\mathrm{H}}_2\mathrm{O}}}{{\mathrm{k}}_{{\mathrm{D}}_2\mathrm{O}}}}$$

(7)

3. Results

3.1. Material Characterization

The thermal behavior of KTiNbO₅, HTiNbO₅, and ns-HTiNbO₅ was investigated through thermogravimetric experiments, and the findings agreed with the results discussed in the literature. A total mass loss of 6.87% is observed for KTiNbO₅ between 32 °C and 800 °C, corresponding to adsorbed water on the surface and possibly a small amount of structural water. The TG curve (Figure S2) for the HTiNbO₅ sample indicates a mass loss of 3.33% up to 366.71 °C, which corresponds to the loss of a molecule of H₂O and the formation of Ti₂Nb₂O₉, according to Equation (8), as previously observed in the literature [20,21,22].

2HTiNbO₅ ⟶ Ti₂Nb₂O₉ + H₂O

(8)

The diffraction patterns obtained for KTiNbO₅ (1), HTiNbO₅ (2), and ns-HTiNbO₅ (3) samples are displayed in Figure 1. These results enabled the study of the layered structure and crystallinity of (1)–(3) and confirmed the identity of the obtained compounds.

The XRD pattern of KTiNbO₅ (1) (Figure 1) matches the data previously published in the literature (PDF-2 ICDD No. 01-071-1747) [23]. The high crystallinity of (1) yields narrow and sharp peaks. The XRD pattern of HTiNbO₅ (2) agrees well with the data of the literature (PDF-2 ICDD 01-075-2062). The changes in the position of the peaks of (1) and (2) are small; however, the sharp signal at 2θ = 10.19° (002) moves to a higher angle in (2), which is indicative of the exchange of K⁺ to H⁺ [24]. This cation exchange caused the layer spacing for d002, calculated with Bragg’s equation (Equation (9)), which is reduced to 0.87 nm in (2), i.e., 0.07 nm smaller compared to (1).

$$d_{hkl}={\displaystyle \frac{n\lambda }{2sin\theta }}$$

(9)

The acidification of an aqueous suspension of Bu₄NTiNbO₅ produces nanosheets of HTiNbO₅ (3), (ns-HTiNbO₅). The diffraction peaks of (3) are less intense and wider in comparison with (1) and (2), possibly due to a less ordered crystalline arrangement in comparison to the other materials. In addition, the peak (002) moved to a smaller angle, 2θ = 8.08°, and the interlayer distance increased to 1.11 nm.

The infrared spectra of (1)–(3) were recorded between 4000 and 550 cm⁻¹ (Figure 2). A group of signals observed in the range of 1000–550 cm⁻¹ is attributed to the vibration modes of the TiO₆ and NbO₆ octahedral fragments, as discussed in the literature [25].

The bands varying from 1100 to 1000 cm⁻¹ in the spectra of (2) and (3) are assigned to the M-O-H vibrations {M = Nb(V) or Ti(V)}. These spectra also display the –O–H stretching frequencies as broad bands between 3500 and 2500 cm⁻¹ [25]. Scanning electron microscopy analysis was performed to compare the morphology of the products (Figure 3) with KTiNbO₅, with a structure made of irregularly sized plates (Figure 3b), which was also observed by G. H. Du (2003) [26]. HTiNbO₅ and ns-HTiNbO₅ (Figure 3d,f) showed similar results to those reported in the literature [15,27] (Figure 3). All images showed disorganized aggregates with varied particle sizes. The exfoliation of ns-HTiNbO₅ is clearly observed in Figure 3f, as described in the literature [21].

The materials were also analyzed by EDS, demonstrating that KTiNbO₅ contains the elements niobium, titanium, oxygen, and potassium in its composition, as shown in Figure S3a. As expected, HTiNbO₅ and the exfoliated material (ns-HTiNbO₅) display a similar composition.

3.2. Material Applications

The obtained materials were evaluated for hydrogen evolution via NaBH₄ hydrolysis (Figure S4). In the absence of Pt nanoparticles, the catalytic effect of (1)–(3) on the evolution of hydrogen was not too different; however, the following tendency was observed: KTiNbO₅ < HTiNbO₅ < ns-HTiNbO₅. The decomposition of NaBH₄ was significantly enhanced in the presence of Pt nanoparticles deposited on (1)–(3). In this case, there was a better yield of H₂ production for Pt NPs/HTiNbO₅ (HGR of 3114 $\mathrm{m}\mathrm{L} {\mathrm{m}\mathrm{i}\mathrm{n}}^{-1}{\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$). Although unsupported Pt NPs exhibited a higher HGR, it is noteworthy that the reaction yield was greater for Pt NPs/HTiNbO₅ (~10%).

In addition, HRTEM images were obtained for Pt NPs/HTiNbO₅ (Figure 4). It can be observed that the nanoparticles (3.6 ± 0.7 nm) are uniformly distributed on the HTiNbO₅ support. The average of the interplanar distance for HTiNbO₅ was 0.269 nm, indexed to the (111) plane of the fcc structure, as previously discussed in the literature for a similar material [28]. The EDS analysis (Figure S5) confirmed in the material the presence of platinum (8%), oxygen (37.5%), niobium (33.3%), and titanium (20.6%), and a small amount of potassium (0.4%), due to an unreacted amount of KTiNbO₅.

Increasing loads of platinum were dispersed on the HTiNbO₅ (Figure 5a), improving both yield and the kinetics. It can be observed that the highest HGR (7230.5 $\mathrm{m}\mathrm{L} {\mathrm{m}\mathrm{i}\mathrm{n}}^{-1}{\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$) was obtained for the 5 mmol% composition (Figure 5b). These results were quite satisfactory if compared with the literature. Zhang et al. achieved 8943 $\mathrm{m}\mathrm{L} {\mathrm{m}\mathrm{i}\mathrm{n}}^{-1}{\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$ using carbon nanospheres (CNSs) supporting ultrafine bimetallic Pt-Co nanoparticles (CNSs@Pt0.1Co0.9) [29]. Farrag and Ali synthesized a (Pd0.5–Pt0.5)n(SG)m)/Co₃O₄ catalyst, which achieves a remarkable hydrogen generation rate (HGR) of 8333 $\mathrm{m}\mathrm{L} {\mathrm{m}\mathrm{i}\mathrm{n}}^{-1}{\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$.

Figure S6 shows the plot of the ln HGR versus the ln catalyst dosage, with a slope of 0.1431. The rate law for the catalytic hydrolysis of NaBH₄ using Pt NPs/HTiNbO₅, as a function of catalyst dosage, is provided in Equation (10). The effect of an increasing amount of catalyst on the reaction rate indicates more active sites and, therefore, an improvement in the kinetics. In general, the reaction order is expected to be positive. Junior et al. reported similar results for bimetallic nanoparticles (Ni–Co) supported on recycled Zn–C battery electrolyte paste [4]. Then, the next experiments were conducted using 5 mmol% of Pt NPs.

$$\mathrm{r}\propto {[\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}]}^{0.1431}$$

(10)

The effect of borohydride concentration on the system was analyzed, as shown in Figure 6a. Lower concentrations of NaBH₄ resulted in higher HGRs, stabilizing at 0.5 mol L⁻¹. Figure 6b shows that ln HGR is inversely proportional to concentration, as indicated by a slope of −0.783. Demirci and Miele suggest that the negative order might be due to the low desorption of borates from the catalyst surface, which affects the rate of site renewal [30]. Consequently, the rate-determining step should be the desorption of by-products and, subsequently, the adsorption of BH₄⁻. Other factors, such as viscosity, may also influence the process. Zhang et al. evaluated the kinetics of NaBH₄ hydrolysis using a Ni-supported catalyst [31]. The authors found an order of −0.456 for the BH₄⁻ ion. The rate law for the catalytic hydrolysis of NaBH₄ by Pt NPs/HTiNbO₅ is shown in Equation (11).

$$\mathrm{r}\propto {[{\mathrm{B}\mathrm{H}}_4^{-}]}^{-0.783}$$

(11)

The effect of NaOH concentration was determined, and the results are shown in Figure S7. NaBH₄ can be stabilized in a basic medium, reducing its self-dehydrogenation [4,32]. Furthermore, the coordination of hydroxide anions on the surface of nanoparticles results in an electron-rich surface. Consequently, the oxidative addition of O–H, a typical step in metal-mediated sodium borohydride hydrolysis, may be facilitated [2]. It can be observed that there is a slight increase in HGR in the presence of NaOH, rising from 7230.5 $\mathrm{m}\mathrm{L} {\mathrm{m}\mathrm{i}\mathrm{n}}^{-1}{\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$ (without NaOH) to 8132.1 $\mathrm{m}\mathrm{L} {\mathrm{m}\mathrm{i}\mathrm{n}}^{-1}{\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$ (0.01 mol L⁻¹ NaOH). However, a further increase in NaOH concentration results in a decrease in HGR. Figure S7b presents the plot of ln (HGR) versus ln [NaOH], with a slope of −0.1795. The rate law for the catalytic hydrolysis of NaBH₄ using Pt NPs/HTiNbO₅, as a function of NaOH concentration, is given in Equation (12). The reduction in reaction yield with increasing NaOH concentration suggests the accumulation of borate anions within the catalyst pores, causing blockage, reducing mass transfer, and NaBH₄ hydrolysis [19]. Additionally, increasing NaOH concentration increases the viscosity of the medium [2].

$$\mathrm{r}\propto {[\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}]}^{-0.1795}$$

(12)

After evaluating the parameters of catalyst dosage, NaBH₄ concentration, and NaOH concentration, the rate law was proposed, as shown in Equation (13).

$$\mathrm{r}=\mathrm{k}\times {\left[\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}\right]}^{0.1431}{\times \left[{\mathrm{B}\mathrm{H}}_4^{-}\right]}^{-0.783}\times {[\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}]}^{-0.1795}$$

(13)

The effect of temperature on hydrogen evolution was evaluated, and it can be observed that as the temperature increases, the rate of evolution increases, reaching 22,790.22 $\mathrm{m}\mathrm{L} {\mathrm{m}\mathrm{i}\mathrm{n}}^{-1}{\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$ at 50 °C (Figure 7a). A Vant’Hoff plot was constructed to determine the activation energy of the system, and the outcomes are shown in Figure 7b. The activation energy (Ea) was calculated as 41.83 kJ mol⁻¹. Notably, the results obtained in this study surpass those reported in the literature, as shown in

Table 1. Evolution of hydrogen from NaBH₄ by different catalysts.

Catalyst	HGR/mLH2·min−1·g−1	Activation Energy (Ea)	Reference
Niobium-based nanocomposite decorated with Ni/Pt nanoparticles.	1782	23.1	[19]
Carbon nanospheres (CNSs) supporting ultrafine bimetallic Pt-Co nanoparticles (CNSs@Pt0.1Co0.9)	8943	38.0	[29]
Pd/Pt quantum dots over CO3O4 nanoparticles	8333	Not informed.	[33]
Pt/TiO2	800 (50 °C)	53.2	[34]
Multi-walled carbon nanotube supported platinum catalyst (Pt/MWCNT)	3561.6 *	27	[35]
Bimetallic Pt-Ni nanoparticles confined in porous titanium oxide cage	10,164.3 (29 °C)	28.7	[36]
Mesoporous silica nanosphere-supported platinum nanoparticles	19,000 (80 °C)	40.1	[37]
Carbon-supported platinum catalysts (Pt/C)	29,600	Not informed.	[38]
Platinum nanoparticles decorated on HTiNbO5	22,790.18 (50 °C)	41.83	This work

* The value was calculated from the value reported in the work: 159 mmol/(min gcat) at 67 °C.

.

Studies have consistently shown that sodium borohydride functions as a hydride donor in two steps [4]. This phenomenon can be classified as either primary (KIE between 2 and 7) or secondary (KIE between 0.7 and 1.5) [32]. A mechanistic study was performed using deuterated water, with results depicted in Figure S8. From these results, the kinetic constants in water (${\mathrm{k}}_{{\mathrm{H}}_2\mathrm{O}}$) and deuterated water (${\mathrm{k}}_{{\mathrm{D}}_2\mathrm{O}}$) were calculated, leading to a KIE value of 1.27. This indicates that the dissociation of O-H/O-D from H₂O/D₂O is affected by the reaction but suggests that the step involving water is not the rate-determining step. The rate-determining step of the reaction is likely to be the dissociation of B-H from BH₄⁻. Similar results were obtained by Sperandio and coworkers, who used Ni/Pt nanoparticles supported on a niobium-based composite to study the evolution of hydrogen from NaBH₄, achieving a KIE value of 1.148 [19].

The durability of a catalyst over time is a crucial factor for its practical application. To assess how the catalytic activity of the Pt NPs/HTiNbO₅ catalyst degrades over time, multiple cycles of NaBH₄ hydrolysis were performed under identical conditions. The results are shown in Figure 8.

The efficiency remained close to 100% over the ten cycles of the material. However, the kinetics were slightly reduced, as observed in Figure S9. This decrease may be attributed to the saturation of the material’s active sites by sodium metaborate, a byproduct of the reaction. The efficiency of the reuse and durability remained close to 100% over the 10 cycles. However, the kinetics were better compared to the durability test. This improvement may be attributed to the cleaning and regeneration of the active sites after washing. XRD analyses performed after the reuse cycles showed no noticeable structural changes in the catalyst.

The results obtained in this work were compared to other works in the literature [19,29,33,34,35,36,37,38], whose data are shown in Table 1.

To better understand the factors that may influence the catalytic performance of HTiNbO₅-based materials, some relevant studies can be highlighted. Takagaki reported the catalytic transformation of glucose into 5-hydroxymethylfurfural using HNbWO₆, HNb₃O₈, and HTiNbO₅, comparing their performance with that of zeolites [12]. While zeolites may lose part of their Brønsted acidity in aqueous media, such behavior is not observed for these layered metal oxides. According to the author, these materials can act as water-tolerant solid acids as well as insoluble heteropolyacids, which enhances their catalytic activity. Similarly, Fan et al. attributed the high photocatalytic performance of TiO₂/HTiNbO₅ to the material’s high porosity [18]. In this work, however, it is difficult to establish a clear correlation between the catalytic activity of Pt/HTiNbO₅ and specific factors such as acidity/basicity, surface area, or even electronic structure. Any interpretation at this stage would be speculative due to the lack of direct data. Future work will focus on a systematic investigation of titanium niobate-based materials to correlate their surface acidity or basicity and pore distribution with electronic density maps obtained from X-ray diffraction analysis.

4. Conclusions

This work reviewed the preparation of niobium-based materials using a relatively simple approach. The authors have also provided a robust characterization process of these materials using various techniques and the subsequent discussion of the results. HTiNbO₅ demonstrated a synergistic effect with Pt NPs in the hydrogen evolution from NaBH₄, exhibiting good kinetics and yield at room temperatures and pressures. The performance of the catalyst surpassed that of several works in literature, particularly regarding the hydrogen generation rate (HGR), which reached approximately 23,000 mL H₂· mL·min⁻¹·g⁻¹ at 50 °C. Furthermore, the materials are stable and maintain durability and the potential for reuse for at least 10 cycles, with excellent efficiency. Importantly, the synthesis route is straightforward, uses inexpensive and environmentally benign precursors, and can be easily scaled up, which makes the preparation of Pt/HTiNbO₅ catalysts potentially cost-effective compared with conventional supports. Therefore, it can be concluded that the synthesized material is an excellent candidate for hydrogen evolution from NaBH₄.