Catalytic hydrolysis of sodium borohydride for hydrogen production using phosphorylated silica particles

Hydrolysis of sodium borohydride (NaBH4) offers substantial applications in the production of hydrogen but requires an inexpensive catalyst. Herein, silica (SP) and phosphorylated silica (SP-PA) are used as a catalyst for the generation of hydrogen from NaBH4 hydrolysis. The catalyst is prepared by sol–gel route synthesis by taking tetraethyl orthosilicate as the precursor of silica whereas phosphoric acid served as the gelation and phosphorylating agent. The prepared catalyst is characterized by FT-IR spectroscopy, XRD, SEM, and EDAX. The hydrogen generation rate at SP-PA particles (762.4 mL min−1 g−1) is higher than that of silica particles (133 mL min−1 g−1 of catalyst). The higher catalytic activity of SP-PA particles might be due to the acidic functionalities that enhance the hydrogen production rate. The kinetic parameters (activation energy and pre-exponential factor) are calculated from the Arrhenius plot and the thermodynamic parameters (enthalpy, entropy, and free energy change) are evaluated using the Erying plot. The calculated activation energy for NaBH4 hydrolysis at SP-PA catalyst is 29.92 kJ.mol−1 suggesting the high catalytic activity of SP-PA particles. The obtained entropy of activation (ΔS‡ =  − 97.75 JK−1) suggested the Langmuir–Hinshelwood type associative mechanism for the hydrolysis of NaBH4 at SP-PA particles.


Introduction
Hydrogen is the most significant renewable energy source because of its cleanness with zero emission, non-toxic nature, high efficiency compared to diesel or natural gas and reduce the emission of greenhouse gases (Lee et al. 2005;Şahin et al. 2021;Zhou et al. 2022). Hydrogen can be generated through various methods like electrolysis of water (Patil et al. 2020), photocatalytic production (Nouruzi et al. 2022a, b;Gholipour et al. 2022), photo electrolysis (Zhang et al. 2021), thermolysis (Hasani et al. 2019), and biological processes (Zhou et al. 2021). However, practical applications of hydrogen are limited due to difficulty in hydrogen storage (Abdalla et al. 2018;Dawood et al. 2020). In comparison with many hydrogen storage methods, chemical hydrides offer much greater advantages in storing hydrogen because of their safety and their ambient reaction condition to produce molecular hydrogen (Kojima 2019;Kim 2018). Hydrolysis of sodium borohydride (NaBH 4 ) is of great interest among researchers due to its high storage capacity (~ 10.6 wt%), pure hydrogen production, and environmentally friendly by-products (Demirci 2015;Liu and Li 2009). Besides, the hydrolysis of NaBH 4 is a nonflammable reaction and the amount of hydrogen production can be easily controlled (Deng et al. 2021). The extremely slow kinetics of NaBH 4 hydrolysis triggers the researchers to find a suitable catalyst to enhance the production of hydrogen. Noble metal nanoparticles find much greater catalytic Responsible Editor George Z. Kyzas activity towards NaBH 4 hydrolysis, yet the availability and price render them to be used in large-scale applications (Patel et al. 2008;Huff et al. 2017). To reduce the cost, various catalysts prepared using low-cost transition metals like Co and Ni are extensively studied for NaBH 4 hydrolysis (Bozkurt et al. 2018;Durano et al. 2017;Seven and Sahiner 2014;Wei et al. 2018). However, extensive research is still underway to improve the catalytic efficiency of hydrogen production rate via NaBH 4 hydrolysis.
Many catalysts have been prepared and utilized for the hydrolysis of NaBH 4 to improve the hydrogen production rate and the long-term stability of the catalyst (Fangaj and Ceyhan 2020;Yang et al. 2020;Akti 2021;Abdelhamid 2020;Abdelhamid 2021;Kıpçak et al. 2020;2020a, b;Hayagreevan et al. 2021]. Fangaj and Ceyhan (2020) used phosphoric acid-treated apricot kernel shell waste as the catalyst for the generation of hydrogen from NaBH 4 hydrolysis. Yang et al. (2020) prepared poly(ethylene imine)-coated silica nanoparticles for the generation of hydrogen. Akti (2021) reported the generation of silica xerogel supported cobalt catalyst for the hydrolysis of NaBH 4 to produce molecular hydrogen and the catalyst shows very low activation energy of 15.2 kJ. mol −1 . Abdelhamid (2021) synthesized a hierarchical porous zeolitic imidazolate framework using terephthalic acid as a source and utilized it as catalyst for hydrogen generation for NaBH 4 hydrolysis. The prepared catalyst enhanced the hydrogen production rate (2333 mL min −1 g −1 of catalyst) (Abdelhamid 2020). Abdelhamid (2021) also used cobalt-embedded zeolitic imidazolate frameworks as the catalyst for dehydrogenation of NaBH 4 and the catalyst showed a high hydrogen generation rate of 7230 mL min −1 g −1 of catalyst. Kıpçak and Kalpazan (2020) prepared CoB catalysts supported on Naexchanged bentonite clays and the catalyst showed a hydrogen generation rate of 921.94 mL min −1 g −1 of catalyst for 5% NaBH 4 solution. Saka et al. (2020a, b) reported CoB-doped acid-modified zeolite catalyst for the generation of hydrogen from NaBH 4 hydrolysis, and the catalyst showed an activation energy of 42.45 kJ mol −1 . Previously, our research group reported sulphonic acid-functionalized silica (SS) and SS/ carbon (SSC) catalysts synthesized via a simple sol-gel technique using conc. H 2 SO 4 as sulphonating and gelation agent (Hayagreevan et al. 2021). Both SS and SSC acted as the good catalysts for the hydrolysis of NaBH 4 . In addition to NaBH 4 hydrolysis for H 2 generation, molecules like ammonia borane and formic acid were also used for H 2 production using suitable catalysts (Nouruzi et al. 2022a, b;Farajzadeh et al. 2020;Alamgholiloo et al. 2020;Doustkhah et al. 2020Doustkhah et al. , 2018. Silica-supported phosphoric acid has been extensively used as a heterogeneous catalyst in many organic reactions (Santander et al. 2019;Gulbinski et al. 2020;Qin et al. 2012) and proton exchange membranes for fuel cells (Jin et al. 2009;Zeng et al. 2013). Elghniji et al. (2018) reported industrial phosphoric acid-supported silica particles for the hydrolysis of NaBH 4 . They prepared the catalyst by addition industrial phosphoric acid into silica gel in a chloroform medium. In the present work, phosphoric acid is used as the phosphorylating agent and gelation agent for the sol-gel preparation of silicaphosphoric acid (SP-PA) particles from tetraethyl orthosilicate (TEOS). It is attempted to use phosphorylated silica prepared via a simple sol-gel procedure and used as a heterogeneous molecular catalyst for the hydrolysis of NaBH 4 . It is expected that the acidic functionality of the catalyst enhances the hydrogen production rate in NaBH 4 hydrolysis. The mechanism of hydrogen generation from NaBH 4 hydrolysis is investigated via calculating kinetic and thermodynamic parameters using Arrhenius and Erying plots, respectively.

Materials
Tetraethyl orthosilicate (TEOS, 98%), orthophosphoric acid (85%), and sodium borohydride (NaBH 4 ) were procured from Merck. Other common chemicals were procured from CDH and the chemicals were used as received without further purification. All the experiments were carried out using double distilled water.

Instrumentation
FT-IR spectra of the samples were measured using JASCO FT-IR 460 plus model under ambient conditions. Spectroscopic grade KBr was used for the preparation of pellets for FT-IR measurements. The particle size and shape were analyzed using VEGA3 TESCAN Scanning electron microscope. For elemental analysis, energy dispersive X-ray analysis (EDX) was carried out using Brucker Nano, GmbH, Berlin, Germany. XRD spectra were measured using Siemens D5000 using Cu-Kα radiation (λ = 1.5406 Å) and Ni filter.

Synthesis of silica particles.
The preparation of silica and silica-phosphoric acid follows the reported sol-gel procedure with slight modifications (Nandan et al. 2011). Briefly, 4 mL of TEOS (precursor of silica particles) is mixed with 10 mL of ethanol under stirring conditions. Then, 4 mL of deionized water was introduced to the reaction mixture followed by 2 mL conc. HCl to initiate the gelation process. Furthermore, stirring of the reaction mixture was continued for another 3 h and the mixture was aged for 12 h. As a result of the gelation process, the white gel was formed, centrifuged, and collected. To remove acid impurity, the obtained particles were washed with ethanol and distilled water and dried at 383 K. The formed silica particles were denoted as SP.

Synthesis of silica-phosphoric acid.
For the preparation of silica-phosphoric acid particles, 4 mL of TEOS (precursor of silica particles) is mixed with 10 mL of ethanol under stirring conditions. Then, 4 mL of deionized water was introduced to the reaction mixture followed by the addition of 2 mL phosphoric acid. Then, the stirring was continued for 3 h and the mixture was aged for 12 h. Then, the formed gel was washed with distilled water until the filtrate is free from phosphate ions (tested via ammonium molybdate test) and the purified gel was dried to obtain phosphorylated silica particles. The formed silica-phosphoric acid is abbreviated as SP-PA particles. Scheme 1 illustrated the schematic representation for the preparation of SP and SP-PA particles.

Calculation of acid density
To determine the number of acidic functionalities or acid density of SP-PA particles, back titration was used. 0.05 g SP-PA particles were mixed with standardized NaOH solution and magnetically stirred for 20 min. The solution is filtered off and titrated against standardized H 2 SO 4 solution using phenolphthalein as an indicator. From the titration value, the acid density was calculated using the following formula.
where, V a volume of H 2 SO 4 consumed for 20 mL of NaOH in the absence of SP-PA particles. (1) volume of H 2 SO 4 consumed for 20 mL of NaOH treated with 0.05 g of SP-PA particles. S NaOH normality of NaOH. m weight of SP-PA particles taken.

Probing the catalytic activity of SP-PA particles towards NaBH 4 hydrolysis
The effectiveness of the catalyst towards the dehydrogenation of NaBH 4 was analyzed in a neutral medium by the down-displacement of water. The experimental setup used for the hydrolysis of NaBH 4 is schematically represented in Scheme 2. In a typical experiment, 5 mg of the catalyst was taken along with 2 wt% NaBH 4 in a dry three-neck flask. To monitor the reaction temperature and to introduce water, the set-up was equipped with a thermometer and syringe, respectively. The amount of hydrogen produced after introducing water into the reaction chamber was monitored via the down-displacement of water from the inverted burette filled with water as shown in Scheme 2. The hydrogen generation rate was calculated from the linear portion of the plot using the following formula (Rambabu et al. 2019),

Calculation of activation energy
Arrhenius plot is used to obtain the activation energy (E a ) of the SP-PA particles towards the hydrolysis of NaBH 4 . The Arrhenius equation is given by the following: (2) Hydrogen generation rate ( where k, hydrogen generation rate (mL min −1 g −1 ) (calculated from the linear portion of the plot); A 0 , collision factor; E a , activation energy; R, gas constant (8.314 J K −1 mol −1 ); and T, absolute temperature (K). The integrated form of Eq. (3) is given by the following: while plotting log k versus 1000∕T gives a straight line. From the slope { −Ea∕(1000 × 2.303 × R) } and the intercept of the straight line, activation energy, and collision factor or pre-exponential factor can be calculated, respectively.

Erying plot
Erying equation is used to calculate the enthalpy of activation ( ΔH ‡ ) and entropy of activation ( ΔS ‡ ) and the equation is given by the following: where k denotes the hydrogen generation rate (obtained from the slope of the linear part of hydrogen evolution plot), T is the absolute temperature (K), ΔH ‡ is the enthalpy of activation of activated complex, k B is the Boltzmann constant, h is the Planck's constant, R is universal gas constant, and ΔS ‡ is the activation entropy. From the straight line obtained by plotting ln k∕T versus 1∕T , ΔH ‡ , and ΔS ‡ can be calculated from the slope and intercept, respectively.

Mechanism of formation of silica and silica-phosphoric acid
To prepare SP and SP-PA particles, acid catalyzed hydrolysis of TEOS method was used. Partial hydrolysis of TEOS results in the formation of silanol and the complete hydrolysis leads to the formation of silicic acid. While using phosphoric acid as the acid source, phosphorylation of silica takes place by the condensation between − OH groups of silicic acid and phosphoric acid resulting in the formation of SP-PA particles (Eq. 6).

Scheme 2
The reaction set-up used for the hydrolysis of NaBH 4

Acid density of SP and SP-PA
The total acid density of SP-PA was obtained by a simple back titration method. The obtained value is 2.2 mmol g −1 of SP-PA particles. The value of acid density indicates the number of the acidic phosphoric acid groups present on the surface of SP-PA particles.

FT-IR characterization of SP and SP-PA particles
Initially, FT-IR spectroscopy was used for the characterization of the prepared SP and SP-PA particles. Figure 1 shows -OH stretching bond of Si-OH besides intercalated water particles. In addition, the characteristic Si-H 2 O flexion band of SiO 2 was observed at 1645 cm −1 . These observations indicate the successful preparation of silica particles in SP and SP-PA. For SP-PA particles, a peak at 1398 cm −1 was observed corresponding to the stretching band of P = O bond present in the phosphoric acid functionality of SP-PA signifying the successful phosphorylation of silica. Table 1 summarizes the FT-IR spectral assignments obtained for SP and SP-PA particles.

Characterization of SP and SP-PA by XRD
XRD spectral study was used to evaluate the amorphous and crystalline properties of SP and SP-PA particles. XRD spectra of SP and SP-PA particles was shown in Fig. 2. XRD patterns of SP and SP-PA showed a very broad peak at around 23.5°. This indicated the amorphous nature of silica particles with a small degree of crystalline properties. The absence of any other peaks in the XRD spectra of SP and SP-PA indicates the high purity of SP and SP-PA particles.

Morphological characterization of SP and SP-PA using SEM and TEM
The size and morphology of SP and SP-PA particles were investigated using SEM analysis. Figure 3 shows the SEM images of SP (Fig. 3A) and SP-PA (Fig. 3B) particles. The particles of SP and SP-PA were spherical in shape with slight aggregation of particles. Compared to SP, SEM images of SP-PA particles showed much-agglomerated structures might be due to the phosphorylation of silica that influences the aggregation of silica particles during the gelation process. Figure 4 shows the TEM images of SP and SP-PA particles. The SP particles (Fig. 4A) show less aggregation compared to SP-PA particles (Fig. 4B). Furthermore, both the particles showed a slightly porous nature which was further confirmed using BET analysis.  Nitrogen physisorption studies-BET analysis N 2 adsorption and desorption (BET analysis) were carried out to know the surface area and the porous nature of SP and SP-PA. Figure 5 shows the N 2 adsorption/desorption isotherms of SP and SP-PA. N 2 sorption isotherms of both the particles are of type IV curves with a H 2 -type hysteresis loop indicating the porous nature of both SP and SP-PA particles. The obtained BET surface area of SP-PA (453.4 m 2 g −1 ) is higher than that of SP (414.2 m 2 g −1 ). On the other hand, the pore diameter of both SP and SP-PA are ~ 5 and 5.5 nm, respectively (inset: Fig. 5). The data obtained in N 2 adsorption/desorption studies are summarized in Table S1.

Characterization of SP and SP-PA by EDAX
EDAX characterization of SP and SP-PA particles was carried out to identify the composition of elements and also to know the purity of the as-prepared particles.

A B
peaks at 0.5 keV and 1.7 keV were observed due to the presence of oxygen and silicon in silica particles, respectively. In addition to O and Si peaks, SP-PA particles showed a peak at 2.1 keV indicating the presence of the P element. This confirms the successful phosphorylation of silica particles. The absence of any other peaks in both Fig. 6A and B indicates the high purity of SP and SP-PA particles. Elemental mapping of SP and SP-PA is shown in Fig. S1. From the mapping of SP-PA particles, it is clear that the phosphoric acid groups were uniformly distributed over silica particles. The elemental composition of SP and SP-PA particles obtained from EDAX studies are summarized in Table S2. The obtained compositions of the element are in good agreement with the concentration of elements taken for the synthesis of SP and SP-PA particles.

Probing the hydrolysis of NaBH 4 at SP and SP-PA particles
The investigation of NaBH 4 hydrolysis using phosphoric acid-modified silica particles is the primary focus of the present investigation. Hence, the catalytic activity of SP and SP-PA particles was studied towards the hydrolysis of NaBH 4 (Fig. 7). The hydrolysis of NaBH 4 was a predominantly slow reaction in the absence of any particles as it is evidenced from Fig. 7a. The slow kinetics of NaBH 4 hydrolysis was significantly improved while using SP particles as a catalyst due to the presence of silanol groups (Fig. 7b). The hydrogen evolution rate was calculated from the slope of the linear portion of the plot using Eq. 2 as 133.3 mL min −1 g −1 of SP particles (Fig. S2). While using SP-PA particles, the hydrogen production rate of 762.4 mL min −1 g −1 was obtained which is significantly higher than that obtained for SP particles towards NaBH 4 hydrolysis (Fig. 7c). The presence of phosphoric acid functionality and silica greatly enhanced the hydrogen production due to the presence of acidic proton in the former that uptake hydride ion from borohydride ion and the latter provides surface area for the borohydride ion and water molecules to bind which synergistically enhance the production of hydrogen. The obtained hydrogen production rate at SP and SP-PA particles are summarized in Table 2.

Effect of NaBH 4 concentration on NaBH 4 hydrolysis
To determine the effect of concentration of NaBH 4 concentration on the hydrogen production rate at SP-PA catalyst, the NaBH 4 hydrolysis was studied at different concentrations of NaBH 4 concentration by keeping the amount of catalyst constant and the results are shown in Fig. 8. As the concentration of NaBH 4 increases from 2 to 15% (0.54 to 4.054 M), the rate of hydrogen generation was increased (Table S3). The plot of ln k versus the NaBH 4 concentration is linear with a slope of 0.44 indicating the deviation of NaBH 4 hydrolysis from zero-order kinetics. The increase in viscosity that limits the mass transfer, the accumulation of sodium metaborate by-product on the catalyst surface, and the increase in pH that deactivates the acidic functionality of the catalyst are the probable cause for the deviation of NaBH 4 hydrolysis from zero-order kinetics.

Effect of catalyst concentration on NaBH 4 hydrolysis
Furthermore, the effect of catalyst dosage on NaBH 4 hydrolysis was evaluated by the hydrolysis of a 2% solution of NaBH 4 using SP-PA catalyst (5, 10, 15, and 20 mg) at room temperature. Figure 9 shows the variation of volume of hydrogen generation with the different dosages of SP-PA particles. As the dosage of catalyst increases, the rate of hydrogen production rate slightly increases while increasing the catalyst amount from 5 to 10 mg which might be due to the increase in the active surface area while increasing the dosage of SP-PA catalyst (Table S4). Further increase in catalytic amount decreases the rate of hydrogen generation ate which might be due to the saturation of the catalyst. Beyond 10 mg, the catalytic activity is decreased which might be due to the clogging of the catalytic surface.

Effect of temperature on the hydrolysis NaBH 4 at SP-PA catalyst
Since SP-PA particles enhanced NaBH 4 hydrolysis for hydrogen generation, further studies were carried out to understand the kinetics of NaBH 4 hydrolysis at SP-PA particles. To obtain the activation energy for NaBH 4 hydrolysis, the reaction was carried out at various temperatures (298, 308, 318, 328, and 338 K) using 5 mg SP-PA particles.   the hydrogen evolution rate is also increased indicating the temperature dependence of NaBH 4 hydrolysis at the SP-PA catalyst.
The hydrogen production rate obtained at various temperatures towards the hydrolysis of NaBH 4 was summarized in Table S5. Arrhenius plot is used to calculate the activation energy of NaBH 4 hydrolysis at SP-PA catalyst (Fig. 11). While plotting log K with 1000/T, a straight line was obtained with a slope of − 1.5625. From the slope value, the activated energy of SP-PA catalyst towards NaBH 4 hydrolysis was calculated, and it is found to be 29.92 kJ mol −1 . The activation energy of NaBH 4 hydrolysis at SP-PA particles was compared with values reported in the literature (Table 3). The appreciable activation energy at SP-PA particles compared to many other catalysts in the literature indicates the high catalytic ability of SP-PA particles towards the hydrolysis of NaBH 4 . In addition, the collision factor or the pre-exponential factor was obtained from the intercept of the Arrhenius plot (Fig. 11). The collision factor at SP-PA catalyst is 1.41 × 10 8 towards NaBH 4 hydrolysis.

Calculation of thermodynamic parameters
Erying plot is used to calculate the Thermodynamic parameters like ΔH ‡ , ΔS ‡ , and ΔG ‡ for the formation of an activated complex to predict the mechanism of NaBH 4 hydrolysis at SP-PA catalyst. Figure 12 shows the Erying plot of NaBH 4 hydrolysis at SP-PA catalyst. ΔH ‡ is calculated from the slope of Erying plot and ΔS ‡ is calculated from the intercept of the Erying plot. The calculated ΔH ‡ and ΔS ‡ are 27.28 kJ mol −1 and − 97.75 J K −1 , respectively for NaBH 4 hydrolysis at SP-PA catalyst. The positive value of ΔH ‡ suggested that the formation of an activated complex from the initial reactants and the catalyst is an endothermic reaction. The ΔG ‡ (free energy change of activated complex) was also calculated from the values of ΔH ‡ and ΔS ‡ and obtained for NaBH 4 hydrolysis at SP-PA catalyst by taking the average temperature (318 K) using the equation ΔG ‡ = ΔH ‡ − TΔS ‡ . The calculated ΔG ‡ for SP-PA catalyst for NaBH 4 hydrolysis is 58.36 kJ mol −1 suggesting that the formation of the activated complex as a result of the reaction between NaBH 4 , SP-PA particles, and water is a non-spontaneous process. The obtained kinetics and thermodynamic parameters of NaBH 4 hydrolysis at SP-PA particles were summarized in Table 4. From Table 4, it is noted that the difference in activation energy and enthalpy of activation is 2.64 kJ mol −1 which is termed the enthalpy of adsorption ΔH ads . The positive ΔH ads depicts that the adsorption of borohydride ions and water molecules at SP-PA particles is an endothermic process.
Furthermore, the sign of ΔS ‡ gives an idea about the mechanism of a reaction, i.e., to say whether the reactants are associated or dissociated during the formation of the activated complex. The negative value of ΔS ‡ suggests a more ordered transition state than the reactants in the initial ground state (Al-Thabaiti et al. 2019). It signifies the Langmuir-Hinshelwood associative mechanism. The proposed mechanism for the hydrolysis of NaBH 4 at  SP-PA particles is schematically illustrated in Scheme 3. Initially, the reactants BH 4 − ion and H 2 O are adsorbed on the surface of SP-PA particles. Then, the hydride ion of BH 4 − takes up the acidic proton from the phosphoric acid group of SP-PA particles to form molecular hydrogen. Then, B in BH 4 − abstracts OH − ion from the adsorbed H 2 O molecule at SP-PA particles to form BH 3 (OH) − and the proton from water combines with phosphate ions resulting in the degeneration of the catalytic surface. The successive steps leads to the production of four molecules of hydrogen and by-product NaB(OH) 4 .

Stability of SP-PA catalyst
To analyze the stability and durability of SP-PA catalyst towards the hydrolysis of NaBH 4 , the SP-PA particles were collected via centrifugation and digested with dil. HCl in order to restore the acidic phosphoric acid groups, washed with water, dried in the oven, and examined the hydrolysis of NaBH 4 for four cycles (Fig. 13). The catalytic activity of SP-PA catalyst decreases slightly for four cycles which might be due to loss of acidic functionality of SP-PA catalyst besides the adsorption of sodium metaborate on SP-PA surface. Hence, the SP-PA catalyst could be used for NaBH 4 hydrolysis for four cycles with a slight loss of catalytic activity.

Conclusion
Molecular acidic catalysts SP and SP-PA for NaBH 4 hydrolysis were successfully prepared by the sol-gel method. For the preparation of SP-PA particles, conc. H 3 PO 4 was used for catalytic gel preparation as well as the phosphorylation agent for silica. The obtained acid density was 2.2 mmol g −1 for SP-PA particles indicating the acidic nature of SP-PA particles due to the presence of surface-active phosphoric acid groups. Furthermore, the as-prepared catalysts were characterized by FT-IR, XRD, SEM, and EDAX. XRD spectra of catalysts confirmed the amorphous nature with some crystalline nature of silica particles. SEM and TEM studies revealed the slightly agglomerated structures of SP and SP-PA particles. N2 adsorption/desorption studies showed that the surface area of SP-PA particles (453.4 m 2 g −1 ) is Scheme 3 Schematic illustration for the proposed Langmuir-Hinshelwood associative mechanism of NaBH 4 hydrolysis at SP-PA catalysts The reusability of SP-PA particles for the hydrolysis of 2% NaBH 4 at 298 K in neutral pH (~ 7) using 5 mg of catalyst for four cycles higher than that of SP particles (414.2 m 2 g −1 ). Furthermore, the catalytic efficiency of SP-PA particles towards the hydrolysis of NaBH 4 is higher than that of SP. The hydrogen production rate of NaBH 4 hydrolysis at SP-PA (762.4 mL min −1 g −1 of catalyst) is significantly higher than that of SP (133.3 mL min −1 g −1 of catalyst). The enhanced catalytic activity of SP-PA particles might be due to the acidic functionalities of SP-PA particles that abstract the hydride ion from adsorbed borohydride ion on the surface of SP-PA particles besides the surface area provided by silica particles. Using the Arrhenius plot, the activation energy of 29.92 kJ mol −1 was obtained for the SP-PA catalyst for NaBH 4 hydrolysis. Furthermore, the thermodynamic parameter values of ΔH ‡ = 27/28 kJ mol −1 , ΔS ‡ = − 97.75 J K −1 and ΔG ‡ = 58.36 kJ mol −1 were calculated using the Erying plot for the formation of the activated complex at SP-PA particles by the adsorption of borohydride ion and water molecules. The negative ΔS ‡ suggested the Langmuir-Hinshelwood associative mechanism towards NaBH 4 hydrolysis at SP-PA catalyst. This material may open up new avenues for the production of molecular hydrogen from the hydrolysis of NaBH 4 .