Mechanism of formation of silica and silica-phosphoric acid
In order to prepare SP and SP-PA particles, acid catalysed 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).
Acid density of SP and SP-PA
The total acid density of SP-PA was obtained by 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 acidic phosphoric acid group present in 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 the FT-IR spectra of SP and SP-PA particles. Both SP and SP-PA particles showed peaks at 801 cm− 1 and 957 cm− 1 which is attributed to bending vibrations of Si-O and Si-OH vibrations of silica particles, respectively (Fig. 1A and B). Another Peak at 1095 cm− 1 was observed indicating the asymmetric stretching vibrations of Si-O-Si bond present in both SP and SP-PA particles. Further, a broad band at 3450 cm− 1 was observed due to the -OH stretching bond of Si-OH besides intercalated water particles. In addition, the characteristic Si-H2O flexion band of SiO2 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 stretching band of P = O bond present in 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.
Table 1
SP (cm− 1) | SP-PA (cm− 1) | Assignment |
3458 | 3452 | ν (O–H) |
1644 | 1649 | ν (Si-H2O) flexion for SP and SP-PA |
-- | 1398 | ν (P = O) |
1096 | 1099 | ν (Si-O-Si) |
957 | 966 | ν (Si-OH) |
810 | 809 | ν (Si-O) bending |
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 23.50. This indicated the amorphous nature of silica particles with a small degree of crystalline properties. The absence 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
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 gelation process.
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. Figure 4 shows the EDAX spectra obtained for SP and SP-PA particles. For both SP and SP-PA particles, well-defined 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 P element. This confirms the successful phosphorylation of silica particles. The absence of any other peaks in both Fig. 4A and Fig. 4B indicates the high purity of SP and SP-PA particles. Elemental mapping of SP and SP-PA are 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 S1. The obtained compositions of element are in good agreement with the concentration of elements taken for synthesis of SP and SP-PA particles.
Probing the hydrolysis of NaBH4 at SP and SP-PA particles
The investigation of NaBH4 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 NaBH4 (Fig. 5). The hydrolysis of NaBH4 was predominantly slow reaction in the absence of any particles as it is evidenced from Fig. 5a. The slow kinetics of NaBH4 hydrolysis was significantly improved while using SP particles as catalyst due to the presence of silanol groups (Fig. 5b). The hydrogen evolution rate was calculated from the slope of linear portion of plot using Eq. 2 as 133.3 mL min− 1 g− 1 of SP particles. 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 of obtained for SP particles towards NaBH4 hydrolysis (Fig. 5c). 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.
Table 2
Comparison of hydrogen evolution rates of SP and SP-PA particles.
S. No | Catalyst | Hydrogen evolution rate (mL min− 1 g− 1 of catalyst) |
1 | SP | 133.3 |
2 | SP-PA | 762.4 |
Effect of NaBH4 concentration on NaBH4 hydrolysis
In order to determine the effect of concentration of NaBH4 concentration on the hydrogen production rate at SP-PA catalyst, the NaBH4 hydrolysis was studied at different concentrations of NaBH4 concentration by keeping the amount of catalyst as constant and the results are shown in Fig. 6. As the concentration of NaBH4 increases from 2–15% (0.54 M to 4.054 M), the rate of hydrogen generation was increased (Table S2). The plot of ln k versus the NaBH4 concentration is linear with a slope of 0.44 indicating the deviation of NaBH4 hydrolysis from zero order kinetics. The increase in viscosity that limits the mass transfer, the accumulation of sodium metaborate by-product on catalyst surface, and increase in pH that deactivates the acidic functionality of the catalyst are the probable cause for the deviation of NaBH4 hydrolysis from zero order kinetics.
Effect of catalyst concentration on NaBH4 hydrolysis
Further, the effect of catalyst dosage on NaBH4 hydrolysis was evaluated by the hydrolysis of 2% solution of NaBH4 using SP-PA catalyst (5, 10, 15 and 20 mg) at room temperature. Figure 7 shows the variation of volume of hydrogen generation with the different dosage 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 mg to 10 mg which might be due to the increase in active surface area while increasing the dosage of SP-PA catalyst (Table S3). 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 catalytic surface.
Effect of Temperature on the hydrolysis NaBH4 at SP-PA catalyst
Since SP-PA particles enhanced NaBH4 hydrolysis for the hydrogen generation, the further studies were carried out to understand the kinetics of NaBH4 hydrolysis at SP-PA particles. In order to obtain the activation energy for NaBH4 hydrolysis, the reaction was carried out at various temperature (298, 308, 318, 328 and 338 K) using 5 mg SP-PA particles. Figure 8 displays the effect of temperature towards the hydrolysis of NaBH4 at SP-PA catalyst. As the temperature increases, the hydrogen evolution rate is also increased indicating the temperature dependence of NaBH4 hydrolysis at SP-PA catalyst.
The hydrogen production rate obtained at various temperature towards the hydrolysis of NaBH4 was summarized in Table S4. Arrhenius plot is used to calculate the activation energy of NaBH4 hydrolysis at SP-PA catalyst (Fig. 9). While plotting log K with 1000/T, a straight line was obtained with a slope of -1.5625. From the slope value, activated energy of SP-PA catalyst towards NaBH4 hydrolysis was calculated and it is found to be 29.92 kJ mol− 1. The activation energy of NaBH4 hydrolysis at SP-PA particles was compared with values reported in the literature (Table 3). The appreciable less 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 NaBH4. In addition, the collision factor or the pre-exponential factor was obtained from the intercept of Arrhenius plot (Fig. 9). The collision factor at SP-PA catalyst is 1.41 ×108 towards NaBH4 hydrolysis.
Table 3
Comparison of activation energies of various catalyst for the hydrolysis of NaBH4
Catalyst | Hydrogen evolution rate (mL min− 1 g− 1 of catalyst) | Activation energy (kJ mol− 1) | Reference |
MOF-derived carbon-confined ultrafine Co catalyst | 619 | 45.0 | (Xu et al. 2019) |
plasma synthesized Ni nanoparticles | 1000 | 69.76 | (Ghodke et al. 2020) |
CoB particles immobilized in carbon nanofibers | 473.3 | 47.7 | (Li et al. 2020) |
nano-bimetallic cobalt-nickel catalysts supported on magnetic substrates | 186 | 37.62 | (Didehban et al. 2020) |
Electrodeposited Co–Zn on Ni foam | 455 | 50.2 | (Wei et al. 2017) |
bimetallic Ni–Co nanoparticles on reduced graphene oxide | 1280 | 55.12 | (Chou et al. 2015) |
Alloy and core-shell supported Co-Ni bimetallic nano-catalysts | 658.8 | 50.0 | (Didehban et al. 2018) |
CuFe2O4/RGO catalyst | 622 | 33.95 | (Tang et al. 2016) |
self-supported cobalt oxide nanorod | 1940 | 59.84 | (Huang et al. 2016) |
Sulphonated silica Sulphonated silica / carbon | 768 914 | 24.74 18.97 | (Hayagreevan et al. 2021) |
SP-PA | 762.4 | 29.92 | This work |
Calculation of thermodynamic parameters
Erying plot is used to calculate the Thermodynamic parameters like \({\varDelta H}^{‡}\), \({\varDelta S}^{‡}\) and \({\varDelta G}^{‡}\) for the formation of activated complex in order to predict the mechanism of NaBH4 hydrolysis at SP-PA catalyst. Figure 10 shows the Erying plot of NaBH4 hydrolysis at SP-PA catalyst. \({\varDelta H}^{‡}\) is calculated from the slope of Erying plot and \({\varDelta H}^{‡}\) is calculated from the slope of Erying plot. The calculated \({\varDelta H}^{‡}\)and \({\varDelta S}^{‡}\) are 27.28 kJ mol− 1 and − 97.75 J K− 1, respectively for NaBH4 hydrolysis at SP-PA catalyst. The positive value of \({\varDelta H}^{‡}\)suggested that the formation of activated complex from the initial reactants and the catalyst is an endothermic reaction. The \({\varDelta G}^{‡}\) (free energy change of activated complex) was also calculated from the values of \({\varDelta H}^{‡}\) and \({\varDelta S}^{‡}\) obtained for NaBH4 hydrolysis at SP-PA catalyst by taking the average temperature (318 K) using the equation \({\varDelta G}^{‡}= {\varDelta H}^{‡}-T{\varDelta S}^{‡}\). The calculated \({\varDelta G}^{‡}\) for SP-PA catalyst for NaBH4 hydrolysis is 58.36 kJ mol− 1 suggesting that the formation of activated complex as a result of reaction between NaBH4, SP-PA particles and water is a non-spontaneous process. The obtained the kinetics and thermodynamic parameters of NaBH4 hydrolysis at SP-PA particles were summarized in Table 4.
Table 4
Kinetic and Thermodynamic parameters obtained for SP-PA particles.
Catalyst | Activation Energy (\({E}_{a}\)) (kJ mol− 1) | Pre-exponential Factor (A) | \({\varDelta H}^{‡}\) (kJ mol− 1) | \({\varDelta S}^{‡}\) (J K− 1) | \({\varDelta G}^{‡}\) (kJ mol− 1) |
SP-PA | 29.92 | 1.41 ×108 | 27.28 | -97.75 | 58.36 |
From the Table 4, it is noted that the difference in activation energy and enthalpy of activation is 2.64 kJ mol− 1 which is termed as the enthalpy of adsorption \({\varDelta H}_{ads}\). The positive \({\varDelta H}_{ads}\) depicts that the adsorption of borohydride ions and water molecules at SP-PA particles is an endothermic process.
Further, the sign of \({\varDelta S}^{‡}\) gives an idea about the mechanism of a reaction i.e. to say whether the reactants are associated or dissociated during the formation of activated complex. The negative value of \({\varDelta S}^{‡}\) suggests the 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 NaBH4 at SP-PA particles is schematically illustrated in scheme 3. Initially, the reactants BH4− ion and H2O are adsorbed on the surface of SP-PA particles. Then, hydride ion of BH4− takes up the acidic proton from phosphoric acid group of SP-PA particles to form molecular hydrogen. Then, B in BH4− abstracts OH− ion from the adsorbed H2O molecule at SP-PA particles to form BH3(OH)− and the proton from water combines with phosphate ions resulting in the degeneration of 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
In order to analyse the stability and durability of SP-PA catalyst towards the hydrolysis of NaBH4, the SP-PA particles were collected via centrifugation, digested with dil. HCl in order to restore the acidic phosphoric acid groups, washed with water, dried in oven and examined the hydrolysis of NaBH4 for four cycles (Fig. 11). 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 NaBH4 hydrolysis for four cycles with slight loss of catalytic activity.