Green Synthesis of Co-based Nanoparticles from Bean Shell Extract and Determination of the Effect of Their Activity on Sodium Borohydride Hydrolysis

doi:10.21203/rs.3.rs-4155908/v1

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Green Synthesis of Co-based Nanoparticles from Bean Shell Extract and Determination of the Effect of Their Activity on Sodium Borohydride Hydrolysis

https://doi.org/10.21203/rs.3.rs-4155908/v1

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The study investigated the effect of Co(0) and Co-B NPs catalysts synthesized using the method of green synthesis on the NaBH₄ hydrolysis process. The investigation of the structure and surface morphology of the catalysts was evaluated using XRD, SEM-EDX, TEM, FTIR, and XPS analyses. The effect of catalysts on NaBH₄ was examined at different catalyst amounts, different NaBH₄/NaOH concentrations, and different temperature values. At 30 °C solution temperature, the highest HGR and TOF values for Co(0) NPs catalyst were determined as 7326 mL.min^-1.g^-1_cat and 8572 mL.min^-1.g^-1_cat, respectively. Under the same conditions, HGR and TOF values for Co-B NPs catalyst were determined as 12524 mL.min^-1.g^-1_cat and 15189 mL.min^-1.g^-1_cat, respectively. The calculated activation energies for Co(0) and Co-B NPs catalysts are 37.68 kJmol^-1 and 21.28 kJmol^-1, respectively. Enthalpy and entropy values were calculated as 4.43 kJmol^-1 and 44 Jmol^-1K^-1 for Co(0) and 23.16 kJmol^-1 and 98 Jmol^-1K^-1 for Co-B NPs, using Eyring's equation, respectively. The catalysts showed a small decrease in activity over five cycles, indicating that they have potential for reusability.

Green synthesis

Co-B NPs

hydrolysis

kinetics

mechanism

Given the exponential expansion of the economy, the severity of the energy crisis, and the prevailing environmental challenges, there is an increasing emphasis on the investigation of renewable energy sources owing to their cost-effectiveness and environmentally sustainable attributes[1]. The fact that hydrogen has a high energy density, a high energy conversion efficiency, and emits very little greenhouse gas pollution makes it a viable energy source alternative to fossil fuels[2, 3]. Hydrogen is considered a sustainable fuel source that can be produced through multiple methods, and its generation rate can be controlled through various techniques at room temperature. However, NaBH₄, a component of metal boron hydride, is favored as a hydrogen source due to its high stability and lack of flammability [4–6]. The following equation demonstrates that spontaneous hydrolysis of NaBH₄ may take place when conditions are at room temperature and pressure:

NaBH₄ + (2 + x) H₂O→NaBO₂.xH₂O + 4H₂ (1)

The hydrolysis of NaBH₄ has been extensively studied using various catalysts, including noble metals [7, 8] and metal alloys[9]. While noble metal catalysts exhibit outstanding catalytic activity, their high cost and scarce availability limit their utility. In recent years, researchers have focused on the use of non-noble metal catalysts, specifically cobalt-based catalysts, such as pure cobalt [10], cobalt oxides [11], boron based [12], and bimetallic Co-based [13], as they are relatively abundant, inexpensive, and exhibit high catalytic activity. Subsequent studies have revealed that the addition of elements such Ni [14, 15], Cu [2], Ce, Mo, La [16] and P [17] to the Co-B catalyst system significantly improves its catalytic activity, which is attributed to the synergistic effects between the chemical elements that constitute the catalyst. The study conducted by Guo et al.[18] describes the synthesis of a hollow Co-Ni-B nanosphere catalyst through the utilization of a liquid phase reduction method and electro-displacement reaction, resulting in a catalyst with an impressive hydrogen generation rate and demonstrated excellent durability. Another study has reported that the Co-Ce-B catalyst, supported on a variety of materials, exhibits high catalytic activity for both the hydrolysis and alcoholysis of NaBH₄ [19]. According to a study by Zhuang et al. (2013), the Co-Mo-B catalyst was found to exhibit high catalytic activity for the decomposition of NaBH₄ in ethylene glycol solution, resulting in a rapid reaction kinetics and high yield [20]. Some studies also indicate that the four-component catalyst has an advantageous hydrolysis feature [5, 21]. Nanomaterials containing nanoparticles (NPs) with at least one size smaller than 100 nm are of great interest in various industries. To be more specific, metal nanoparticles are essential for improving both chemical and physical characteristics [22], and they are extensively used in a variety of platforms including sensing [23], optical [24], and catalytic research[25]. Nowadays, NPs are produced using a variety of physical and chemical processes, including spray pyrolysis, thermal decomposition, co-precipitation, sol-gel, and hydrothermal methods[26]. However, these techniques also use dangerous chemicals and their costs are high. Thus, the creation of more ecologically friendly and sustainable synthesis techniques is required [27]. Agglomeration of nanoparticles during a process reduces their catalytic activity and makes them difficult to reuse[28].

In recent years, there has been a growing interest in finding alternative methods for preparing NPs, with a particular focus on the use of green chemistry. One approach that has garnered significant research attention is the utilization of plant extracts [11]. Plant extracts contain a variety of bioactive compounds that can act as reducing agents, facilitating the reduction of metal ions to form NPs [29, 30]. Furthermore, these compounds also can act as capping agents, which helps to prevent agglomeration of the targeted NPs during the synthesis process [31, 32]. Rheum ribes plant develops naturally in Turkey [33]. Some plant species are cultivated for their therapeutic properties. The plant is rich in vitamins A, B1, B2, and E, which prevent cancer. Moreover, the Polygonaceae family, to which the Rheum ribes plant belongs, contains antioxidants [34] [35]. Rheum ribes plant is mixed with nature as waste. Therefore, Co(0) and Co-B nanocatalysts were synthesized by using the extract of this plant, and sodium borohydride hydrolysis was applied for the first time in this study. In this study, Co NPs and Co-B NPs catalysts were prepared by green synthesis method from irradiance extract by chemical precipitation reduction method and used in NaBH₄ hydrolysis. It was determined that Co NPs and Co-B NPs prepared with a new and environmentally friendly method have high activity in NaBH₄ hydrolysis. Hydrolysis of NaBH₄ n. degree and Langmuir Hinshelwood kinetic model kinetics were determined. TOF and TON of the hydrolysis of NaBH₄ were determined.

2.1. Materials

The rheum ribes were purchased in a market in Siirt, which is located in Turkey. The following chemicals were purchased from Merck: sodium hydroxide (NaOH), sodium borohydride (NaBH₄), and cobalt (II) chloride hexahydrate (CoCl₂.6H₂O).

2.2. Preparation of the extract

The Rheum ribes were ground, dried, and then sieved. 25 g of the sieved Rheum ribes and 250 mL of distilled water were added to a 500 mL flask. The mixture was then allowed to boil under reflux for 1 hour. After the obtained extract was cooled to room temperature, it was separated from the environment by filtration. To be employed in the green synthesis of Co(0) and Co-B NPs, the extracted material was stored in a closed container in a refrigerator at 4°C [32].

2.3. Green synthesis of nanoparticle catalysts

Chemical reduction precipitation and green synthesis were both used to prepare nanoparticle catalysts. Synthesis of Co-B NPs: After adding 0.01 mol of CoCl₂.6H₂O into 50 ml of extract, it was dissolved and left in an ice bath, and 0.05 mol of NaBH₄ dissolved in 50 ml of distilled water was added dropwise to the solution medium for catalyst reduction. The solution temperature was kept in the range of 0–5 ºC during the reduction of the catalyst [36]. After the reduction was completed, filtration separated the synthesised Co-B NPs from the environment. The obtained Co-B NPs catalyst was dried at 110 ºC nitrogen environment for 3 hours.

Synthesis of Co nanoparticles; After adding 0.01 mol of CoCI₂.6H₂O into 50 mL of extract, it was dissolved, and 50 ml of NaBH₄ dissolved in 50 ml of pure water was added dropwise to the solution medium for catalyst reduction. The reduction of the catalyst was carried out at room temperature. After the reduction was completed, the synthesized Co(0) NPs were separated from the environment by filtration. The obtained Co nanoparticle catalyst was dried in a nitrogen environment at 110 ºC for 3 hours.

2.4. Sodium borohydride hydrolysis of Co(0) NPs and Co-B NPs catalysts

The amount of hydrogen gas released in the hydrolysis experiments of sodium borohydride was measured by using catalytic hydrolysis measurements with Co(0) and Co-B NPs catalysts as catalysts, respectively. To conduct the experiments, 10 mL of distilled water and 25 mg of catalysts were each added separately. The process of displacement of water allowed the hydrogen gas produced during the reaction to finally collect in an upside-down burette after passing through a tube. Hydrogen generation was determined by monitoring the change in water levels in the burettes throughout the experiment. The present study investigates the effects of varying concentrations of NaOH and NaBH₄, as well as different catalyst amounts and temperatures, on the activities observed during sodium borohydride hydrolysis experiments.

2.5. Characterization of Co(0) and Co-B NPs catalysts

Analyses using transmission electron microscopy (TEM), X-ray diffraction (XRD), X-Ray photoelectron spectroscopy (XPS), infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and elemental analysis (EDX) were performed on catalysts manufactured using the green synthesis approach.

3.1. Characterization

Figure 1 shows the outcome of the XRD examination done on Co(0) and Co-B NPs catalysts to assess their phase purity and crystal structure.

It can be seen from Fig. 1 that there is no significant peak for both Co(0) and Co-B NPs. It is seen that Co(0) and Co-B NPs have an amorphous structure. The possible reason for the amorphous nature of the catalyst is due to the boron metal in the environment [37]. Many unsaturated regions can improve the catalytic performance of amorphous catalysts [38]. The SEM analysis gives information about the morphological structure of the synthesized nanoparticles, and therefore, the surface morphology of the green synthesis Co (0) and Co-B nanoparticle catalyst was examined using SEM, and the result of the analysis is given in Fig. 2.

The surface of the Co(0) NPs catalyst resembles a spongy, fibrous structure, and it can be seen from Fig. 2(a) that small Co(0) NPs nanoparticles form on the surface. From Fig. 2(b), it can be seen that the Co-B NPs catalyst is made of small spherules and is nanostructured. It can be seen from Fig. 2 that Co(0) NPs and Co-B NPs catalysts are porous. It can be seen from Fig. 2 that Co(0) NPs and Co-B NPs catalysts agglomerate. It is due to the high surface charge of agglomeration [32]. Agglomeration of a catalyst increases the activity of the catalyst [39]. Şahin et al. stated that the Co catalyst they prepared to use in NaBH₄ hydrolysis was agglomerated [40]. Quantitative and qualitative analysis and homogeneity of green synthesis Co(0) and Co-B NPs catalysts were investigated by EDX analysis, and the results are given in Fig. 3.

It is clearly seen in Fig. 3 that Co(0) and Co-B NP catalysts were successfully produced by green synthesis. The C element in the structure of the synthesized catalysts originates from the extract. It is seen in Fig. 3 in the form of three different spectra that Co(0) metal is in the structure of both catalysts. The spectrum value of metal B is low compared to other metals (0.17 KeV), and therefore it is not very obvious. Figure 3 shows that both Co-B NPs and Co(0) NPs catalysts were reduced and synthesized in pure form. Wang et al. stated that B atoms are electron-deficient and Co atoms are electron-rich. Therefore, electron transfer between catalyst and reactant is easy, which is beneficial in the hydrolysis process of NaBH₄, which will increase the rate of hydrolysis [41]. The results given in the hydrolysis section also support this. Figure 4 shows the result of the FTIR study done in the region of 4000 − 400 cm^− 1 to identify the structure of the Co(0) and Co-B NPs catalyst.

The graphical representation in Fig. 4 indicates the existence of OH- in the peaks of 3350–3700 cm^− 1, which can be attributed to the water in the structure and the extract. The presence of C-O in the extract structure, caused by phenolic groups, is indicated by the peak of 2230–2320 cm^− 1[42]. The peak at 1270 cm^− 1 corresponds to the C-N stretching vibrations of aromatic amines [43]. The stretching vibrations of cobalt and oxygen are responsible for the peak at 1550 cm-1, which shows that cobalt forms bonds with the oxygen in the extract structure[44]. The peaks at 550 cm^− 1, 850 cm^− 1, and 1020 cm^− 1 are due to boron lattice vibration [45]. To determine the morphological and structural properties of green synthesis Co(0) and Co-B NPs catalysts, the TEM analysis was performed, and the images obtained are given in Fig. 5.

The visual representation in Fig. 5(a) illustrates that the Co(0) nanoparticles exist in both layered and agglomerated forms. The SEM images revealed that the Co(0) nanoparticles were in an agglomerated state. Figure 5(a) illustrates that the Co(0) nanoparticles exhibit a diameter distribution ranging from 30–70 nm. As shown in Fig. 5(b), the structural properties of Co-B NPs, such as agglomerated platelets and tiny spherical forms, are in line with the results of the SEM examination. Figure 5(b) shows that the sizes of Co-B nanoparticles range from 40 to 80 nm in diameter. Aggregation appeared in Co(0) and Co-B nanoparticles, which is attributed to their high surface energies. Agglomeration was also determined from the SEM image results. The nanoparticles exhibit both hydrophilic and hydrophobic properties, which are seen concurrently in the black and white regions. The presence of white dots and rings in the dark background is compatible with the polycrystalline structure[46].

The elemental content and states of Co(0) and Co-B NPs were examined using XPS analysis; the findings are shown in Figs. 6 and 7.

Figure 6 displays high resolution X-ray photoelectron spectroscopy (XPS) spectra of the cobalt (Co) and boron (B) regions in the Co-B catalyst, wherein solely the Co, B, and C elements were detected. Two bands were fitted to fit the Co 2p3/2 spectra. The identification of Co is attributed to the band observed at 780.1 eV. The binding energy values of the Cobalt species exhibited comparability with the standard values of 778.3 eV, and negligible drift was detected [47]. The two bands in the B 1s spectra are ascribed to elemental boron at ~ 187.5 eV and B³⁺ at ~ 190.0 eV. In Co-B, the boron oxide bond energy was greater than the expected value of 187.1 eV. According to the literature, this was caused by the electron-donating properties of boron oxide on Co species[48].

The intense peak of 282.3 eV in Fig. 7 is an indication of the bond between the carbon in the structure of Rheum ribes extract, which is used in the green synthesis of Co(0) nanoparticle catalyst, and cobalt. Khassin et al. found similar results in their study [49]. The element cobalt exhibits two distinct peaks at 779.1 eV and 794.8 eV. The presence of cobalt metal is responsible for the occurrence of these two peaks. The observed peak at 779.1 eV can be attributed to the process of cobalt reduction through the use of hydrogen [50]. The TEM and XRD results also support the XPS results of Co(0) and Co-B nanoparticles.

3.2. Hydrolysis of sodium borohydride in the presence of Co and Co-B NPs catalysts

3.2.1. Effected of concentration of NaOH

NaOH is usually employed as an inhibitor to prevent the self-hydrolysis of NaBH₄ aqueous solution, which is necessary for optimal preservation of NaBH₄ solution. The hydrogen generation rates of Co(0) and Co-B NPs catalysts were examined at 30°C in solutions with 2.5 wt.% NaBH₄ concentration and the catalyst at 25 mg (Fig. 8). As the NaOH concentration increased from 0 wt.% to 2.5 wt.%, the hydrogen generation rate increased significantly. As the NaOH concentration increased from 2.5 wt.% to 5.0 wt.%, the hydrogen generation rate decreased significantly. According to the results obtained, when the NaOH concentration for the Co(0) NPs catalyst was changed by 0 wt.%, 1 wt.%, 2.5 wt.%, and 5 wt.%, the corresponding hydrogen generation rates were calculated as 758.08, 2804.84, 7325.06, and 4878.04 mL.min^− 1.g^− 1_cat, respectively.

Under the same conditions and concentrations, the computed hydrogen generation rates for the Co-B NPs catalyst are 3189.76, 5201.60, 9758.40, and 5048.80 mL.min^− 1.g^− 1_cat, respectively. The high concentrations of NaOH inhibit the hydrolysis of NaBH₄. An increase in the activation of BH₄⁻ and the number of OH⁻ ions can be a result of increasing the concentration of NaOH from 0% to 2.5 wt.% [51, 52]. The rate of hydrogen generation decreased when NaOH concentrations reached over 2.5 wt.%. The increased viscosity of the NaBH₄ solution may have contributed to this outcome by slowing the diffusion rate of the BH₄⁻ ions and allowing the OH⁻ ions to occupy the active sites [53]. Another explanation for this circumstance is that a high concentration of NaOH elevates the pH of the solution, causing OH⁻ ions to play an inhibitory function and a reduction in the concentration of proton ions [53, 54]. The catalytic activity is negatively impacted by this gradual ionization of NaBH₄ [55, 56]. In addition, a high concentration of NaOH may be responsible for encouraging the creation of Co-B particles of a larger size, which in turn reduces the catalytic reactivity of the catalysts [57]. The hydrolysis of sodium borohydride is highly dependent on the concentration of NaOH in the solution. The effect of NaOH concentration on hydrogen release from NaBH₄ hydrolysis was controversial for different catalysts. In the study of Özdemir et al. on the Co-B catalyst, the steady-state hydrogen generation volume ratio increases significantly by increasing the NaOH concentration from 5 to 10 wt.%. However, he reported that the concentration of NaOH gradually decreases as it rises above 10 wt.%, and this may be due to the presence of NaOH, which promotes the generation of active cobalt boride components up to 10 wt.%, and because of the increased stability, sodium borohydride prevents the hydrolysis reaction [58, 59]. The Co-Ni-B catalyst study by Fang et al. stated that the proper concentration of OH⁻ in the solution can improve the catalyst dispersion, and the OH⁻ ligated on the catalyst surface can promote the cleavage of O-H in H₂O. However, the high OH⁻ concentration slows the solution mobility, decreasing the hydrogen generation rate [14, 60, 61]. Hsueh et al. prepared a polymer template-Ru catalyst and observed that the hydrogen generation rate decreases gradually with NaOH concentration increasing from 1–15% [62]. Similar patterns may be seen, indicating that the hydrolysis rates for the Co(0) and the Co-B catalysts are essentially identical. For Co(0) and Co-B NPs catalysts, all hydrogen was released from the solution at one-minute time intervals for all NaOH concentrations. Despite this, it is clear that the hydrolysis of NaBH₄ in the presence of Co(0) proceeds at a slower rate when compared to the hydrolysis of Co-B NPs. Thus, it can be concluded that the rate of hydrolysis largely depends on the generation method of the catalysts.

3.2.2. Effect of concentration of NaBH₄

The concentration of NaBH₄ plays an important role in the hydrolysis of sodium borohydride. In the current study, NaBH₄ was studied at different molar concentrations (1%, 2.5%, 5%, and 7.5 wt.%) for the catalytic hydrolysis reaction of NaBH₄. NaOH concentrations and catalyst amounts were kept constant at 2.5 wt.% and 25 mg, respectively. The effects of different sodium borohydride concentrations on Co(0) and Co-B NPs catalysts are shown in Fig. 9. It can be seen from the figure that the hydrogen generation volume increases as the NaBH₄ concentration increases. The volume of hydrogen that was created in the amount of 2.5 wt.% NaBH₄ in the Co(0) NPs catalyst was finished in 3.5 minutes with 660 mL, while it was measured that the amount of hydrogen that was produced in the Co-B NPs catalyst was 720 mL in a shorter period of just 3 minutes. In the hydrolysis of NaBH₄, when the amount of NaBH₄ increased from 1 wt.% to 7.5 wt.% with the use of the Co(0) NPs catalyst, the hydrogen generation rate increased from 4873 to 11035 mL.min^− 1.g^− 1_cat. It was determined that the hydrogen generation rate increased from 7520 to 18598 mL.min^− 1.g^− 1_cat with the use of the Co-B NPs catalyst under the same conditions. That is, the hydrogen generation rate increases significantly as the NaBH₄ concentration increases, indicating that the NaBH₄ concentration affects the hydrolysis catalyzed by Co(0) and Co-B NPs catalysts. Similar results have been observed by various researchers. A high concentration of NaBH₄ leads to an increase in the viscosity and alkalinity of the reaction solution. The increase in viscosity and pH plays an important role in transport resistance. Therefore, this limits the mass transport of borohydride to the surface of the catalyst. Also, an increase in the pH of concentrated solutions leads to a decrease in hydrogen formation, increasing stability. Similar results were found by Dönmez et al., who found a moderate correlation between the NaBH₄ concentration and the hydrogen generation rate as a result of the formation of NaBO₂, a hydrolytic by-product of NaBH₄, which is absorbed on the surface of the catalyst and occludes its active sites [63]. The preceding data demonstrate that NaBH₄ concentration in the reactant solution affects the hydrolysis rate. The formation of sodium borate (NaBO₂) as a reaction product results in the creation of a barrier at the reaction interface, thus impeding the contact between the reactant solution and the catalyst. The amount of sodium borohydride utilized as a substrate directly relates to the formation of the borate that is produced [45, 64]. NaBH₄ is projected to release all of its H₂ completely and promptly in practical use. Because of this, the process of producing hydrogen by catalytic hydrolysis has to make use of the right concentration of NaBH₄ in order to be considered environmentally friendly and to provide energy that can be utilized sustainably. Based on these results, the optimal amount of NaBH₄ for producing hydrogen through NaBH₄ hydrolysis is 2.5 wt.%.

3.2.3. Effect of the amount of catalyst

The rate of hydrolysis of NaBH₄ is mainly affected by the amount of catalyst, so the effect of different catalyst amounts was also studied. The results obtained to analyze the effect of the amounts of Co(0) and Co-B NPs catalysts (10, 25, 35, and 50 mg) on the hydrogen generation rate show that the self-hydrolysis of NaBH₄ happens slowly. Because of this, the experiment cannot produce 100% hydrogen yield without a catalyst [63]. Figure 10 illustrates the connection between the rate of hydrogen generation and the quantity of catalyst added for both catalysts under identical circumstances, demonstrating that the time to complete the reaction decreases rapidly as the amount of catalyst added increases. In addition, the rate of the NaBH₄ hydrolysis reaction shows good linear agreement with the amount of catalyst added. An important observation was that increasing the amount of catalyst from 10 mg to 25 mg showed a significant difference in the hydrogen generation rate, whereas increasing the amount of catalyst from 25 mg to 50 mg did not show a significant difference in the hydrogen generation rate and exhibited similar behavior. However, the hydrogen generation rate in the hydrolysis of sodium borohydride using 50 mg Co(0) and Co-B NPs catalysts was calculated as 5700 mL.min^− 1.g^− 1_cat and 9992 mL.min^− 1.g^− 1_cat, respectively. It was determined that the Co-B catalyst gave a better result than the Co(0) NPs catalyst. Thus, it can be assumed that the hydrogen generation activity is closely related to the amount of catalyst, that is, it can be controlled by varying catalyst loading. Therefore, to achieve better catalytic performance using as little catalyst as possible, 25 mg of Co(0) and Co-B NPs catalysts were used in subsequent experiments.

3.2.4. Effected of Temperature

Hydrolysis of NaBH₄ was carried out under standard conditions (stabilized with 2.5 wt.% NaBH₄ and 2.5 wt.% NaOH) using 25 mg of Co(0) and Co-B NPs catalysts synthesized from Rheum ribes shell extracts at a temperature range of 30–60°C. Figure 11 shows the effect of reaction temperature on the hydrogen generation rate. Higher reaction temperatures were determined to increase the rate of hydrogen generation from NaBH₄ hydrolysis. In the use of the Co(0) catalyst, the hydrogen generation rate increased sharply as the temperature increased from 30 to 60°C. The reaction completion time decreased from 3.5 minutes to 1 minute for 680 mL of hydrogen volume, and the hydrogen generation rate increased from 7520 to 29290 mL.min^− 1.g^− 1_cat (Fig. 11a). With the use of the Co-B NPs catalyst, the hydrogen generation rate increased when the temperature was increased from 30 to 60°C, but it was not seen to increase as sharply as with the Co(0) NPs catalyst. The reaction time for a 720 mL hydrogen volume decreased from 3 minutes to 1.5 minutes with a Co-B NPs catalyst, and the hydrogen generation rate increased from 10374 to 23228 mL.min^− 1.g^− 1_cat (Fig. 11b). In addition, it was found that the rate of hydrogen generation increased when the temperature was increased from 30 to 60°C. The results suggest that the hydrolysis of the alkaline NaBH₄ solution exhibits heightened sensitivity to variations in reaction temperature, particularly at lower temperatures.

3.2.5. Study of catalyst kinetics

The rate of hydrogen production increases with higher temperatures, and there exists a non-linear correlation between the quantity of hydrogen created and the duration of the reaction at any specific temperature. This observation defined that the hydrolysis of NaBH₄ exhibits a reaction order of n.

nth-order kinetic model

Based on the nth-order kinetics, the reaction rate per unit volume can be defined as [65]

-${r}_{A}=-\frac{dCA}{dt}=k{C}^{n}A$ (2)

The following equations are produced by performing the operations of separating and integrating:

$$-\underset{{CA}_{0}}{\overset{CA}{\int }}\frac{{dC}_{A}}{{{C}_{A}}^{n}}=k\underset{0}{\overset{t}{\int }}dt$$

$$\frac{1}{\left(1-n\right)}\left(\frac{1}{{C}_{A}^{n-1}}-\frac{1}{{C}_{Ao}^{n-1}}\right)=kt (n\ne 1)$$

As a result, the nth order may be found using the slope of the line as a function of time from the Eq. (4) graph.

Langmuir-Hinshelwood kinetic model

In this model, zero-order and first-order Langmuir adsorption kinetics are combined[65–67].

$$\frac{{d}_{CA}}{dt}=-{r}_{CA}=-k\frac{{K}_{a}{C}_{A}}{1+{K}_{a}{C}_{a}}$$

$$\left({C}_{Ao}-{C}_{A}\right)+\frac{1}{{K}_{a}}\text{ln}\left(\frac{{C}_{Ao}}{{C}_{A}}\right)=kt$$

k = Aexp(-Ea/RT) (7)

lnk = lnA-(Ea/R).(1/T) (8)

The Arrhenius plot of 1/T versus ln (k) for the nth-order is shown in Figs. 12a and 12b. The activation energies for Co(0) and Co-B catalysts in the aqueous solution of NaBH₄ were 37.68 kJ.mol − 1 and 21.28 kJ.mol − 1, respectively. The Langmuir–Hinshelwood kinetic model (Eqs. (5), (6), (7), and (8)) was then applied to all of the experiments at temperature. The logarithm of k was plotted as a function of 1/T (Figirus 12a-b) to calculate EA over 30–60 ºC. The activation energies for Co(0) and Co-B catalysts were 38.0 kJ.mol − 1 and 17.61 kJ.mol − 1, respectively. In both models, the activation energies were close to each other. We can state that the Langmauer-Hiswold kinetics support the notion that the hydrolysis of sodium borohydride is nth-order kinetic model.

In addition, enthalpy and entropy values for Co(0) and Co-B NPs were also calculated using Eyring's equation (Eq. 9).

ln rate/T=-∆H/RT+∆S/R (9)

The values of ΔH = 4.43 kJmol^− 1and ΔS = 44 Jmol^− 1.K^− 1 were determined for the Co(0) NPs catalyst by using the slope and intercept of the Eyring plot of ln (rate/T) versus 1/T. Similarly, the values of ΔH = 23.16 kJmol^− 1and ΔS = 98 Jmol^− 1.K^− 1 were computed for the Co-B NPs catalyst.

The Langmuir-Hinshelwood model [65, 67, 68] is extensively used in the field of chemical kinetics to enhance comprehension of the variations in reaction order (n) and activation energy (E_a) associated with chemical processes. Based on the aforementioned facts, the proposed explanation of the catalytic hydrolysis of NaBH₄ may be attributed to the Langmuir-Hinshelwood type mechanism. Consideration is given to the adsorption of both reactants, namely NaBH₄ and H₂O, on the catalyst. Figure 13 illustrates the ionization process of NaBH₄.

The process involves two primary kinetic phases: the dissociative chemisorption of tetrahydroborate onto the metal atoms present on the surface, and the subsequent transfer of the negative charge from the Co–BH₃ species to the hydrogen atom of the Co–H moiety (I). The Co(0) species, functioning as the proton acceptor, reacts with the hydrogen generated during the dissociation of B-H and BH₃⁻ (II) compounds, respectively. then, the electron that was extracted from BH₃⁻ is then transferred to Co(0) for further processing (III). The cobalt(0) species has the ability to accept a hydrogen atom from the hydroxyl group (O-H) via the involvement of the coordinated electron (IV). The subsequent release of hydrogen gas is the consequence of the hydrogen being transported from the Co(0) into the system (V)[69, 70]. The same hydrolysis mechanism as that of sodium borohydride is valid for the Co-B NPs catalyst.

The turnover frequency (TOF) of Co(0) and Co-B nanoparticles generated by green synthesis is one of the most important parameters impacting the catalyst's performance when utilized in sodium borohydride hydrolysis. Eq. 10 is used to compute TOF[71].

$$TOF=\frac{{n}_{{H}_{2}}\left(mol\right)}{t\left(min.\right)x{n}_{Co}\left(mol\right)}$$

t is the entire reaction time in minutes, nH2 is the number of moles of hydrogen gas that was created, and nCo(0) is the number of moles of cobalt that was present in the catalyst. TOF values were calculated separately for Co(0) and Co-B NPs catalysts for each temperature according to Eq. 10. Figure 13 shows the comparison of the catalytic hydrolysis of NaBH₄ by Co(0) and Co-B NPs catalysts prepared at different temperatures. Figure 14 displays the HGR and TOF values of the green synthesis-produced Co(0) and Co-B NPs catalysts. At 30°C solution temperature, the highest HGR and TOF values for Co(0) NPs catalyst were determined as 7326 mL.min^− 1g^− 1_cat and 8572 mL.min^− 1g^− 1_cat, respectively. Under the same conditions, HGR and TOF values for Co-B NPs catalyst were determined as 12524 mL.min^− 1g^− 1_cat and 15189 mL.min^− 1g^− 1_cat, respectively.

As seen in Fig. 14, TOF values were higher than HGR values with increasing temperature. The results show that the activity of the Co(0) and Co-B NPs catalysts synthesized by green synthesis is very high.

Table 1 compares the TOF and activation energies values of the Co(0) and Co-B NPs catalysts synthesized in this study with those of other catalysts in the literature. Due to varying experimental circumstances, it is difficult to compare the Ea directly reported here with the values published in the literature for cobalt catalysts; nonetheless, this is the lowest value yet documented for cobalt-based catalysts. The high activity of the Co(0) and Co-B NPs catalysts created by the green synthesis method to hydrolyze sodium borohydride is shown by the TOF values in Table 1.

Table 1

Co-based catalysts for the hydrolysis of NaBH₄: A comparison of recent literature results
Catalyst	HGR (mL.min^− 1.g_cat⁻¹)	TOF (mL.min^− 1.g^− 1_cat)	Ea (kJ/mol)	Temp. ºC	ΔH (kJ/mol)	ΔS (mol/J.K)	Ref
PdO-Co3O4	5484.2	2900.7	52.0	25			[72]
Co0.45W0.55		8401	55.56	30			[73]
CoNP@ZIF-8		14023	62.9	30			[74]
Co − B NPs		26000		30			[75]
Co − B/Ni foam		11000	33	30			[76]
Co₂B		8500	77.9	25			[77]
p(AAc)-Co		14040	29.35	30	36.85	-157.88	[78]
Co(0)	7326	8572	37.68	30	4.43	44	This work
Co(B)	12524	15189	21.28	30	23.16	98	This work

3.2.6. Reusability tests of Co(0) and Co-B NPs catalysts

The reusability and stability of a catalyst have significant implications for its uses. The objective of this study was to examine the reusability of Co(0) and Co-B NPs catalysts, which were generated utilizing environmentally friendly synthesis techniques, by assessing their durability. Based on the data shown in Fig. 15, it can be seen that there was an almost negligible loss in the quantity of hydrogen generated after five consecutive cycles. In Fig. 15, it was seen that the percentage of reusability of the Co-B catalyst was higher than that of the Co(0) catalyst. It was tested at 98% in the Co-B catalyst and 88% in the Co(0) catalyst. Using an external magnet, the used catalyst was extracted from the by-product solution after the catalytic hydrolysis process. After that, the catalyst was cleaned with deionized water and put to use once again. Between tests, there was a minor decline nonetheless. The presence of metaborates on catalyst surfaces, which cause both blockage and toxicity, may help to explain this. Including metaborates is known to increase solution viscosity, which is detrimental to hydrogen production[79]. The data demonstrate that Co(0) and Co-B NPs have a highly stable structure and good activity, which qualify them for usage in a variety of practical applications.

In conclusion, the synthesis of Co(0) and Co-B nanoparticles was been achieved through a simple, environmentally sustainable, cost-effective, and effective method utilizing Rheum ribes. Characterization analyses show that the samples are composed of Co(0) and Co-B nanoparticles. The experimental observations of catalyzed hydrolysis of NaBH₄ demonstrated that the activity is sensitive to both the morphological alterations and the synthesis techniques of catalysts. The activation energies for the Co(0) and Co-B NPs catalysts were determined to be 37.68 kJ/mol and 21.28 kJ/mol, respectively, in the nth-order kinetic model. The findings from the activity results also indicated that the catalysts comprising Co(0) and Co-B catalysts have the potential for reusability, although with a little decline in activity seen across five cycles. For this reaction, it performed better than the reported values of many cobalt-based catalysts. Therefore, the developed preparation method is promising for producing active Co-based catalysts.

Ethical Approval and Consent to participate

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Human Ethics

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Consent for publication

Informed consent was obtained prior to performing the procedure, including permission for publication of all photographs and images included herein.

Availability of supporting data

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Competing interests

The authors declare no competing interests.

Funding

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Authors' contributions

Ömer Şahin: Supervision, investigation, visualization, writing—review & editing.

Orhan Baytar: Conceptualization, investigation, visualization, writing—review & editing.

Nasreddin GENLİ: İnvestigation, visualization, writing—review & editing.

Arzu Ekinci: Conceptualization, investigation, visualization, writing—review & editing.

Acknowledgments

I would like to thank the authors who contributed to the study.

Authors' information

Orhan BAYTAR, Department of Chemical Engineering, Faculty of Engineering, Siirt University, Siirt, Turkey. [email protected]

Ömer ŞAHİN, Istanbul Technical University, Faculty of Engineering, Department of Chemical Engineering, İstanbul, Turkey. omersahin@itü.edu.tr

Nasrettin GENLİ, Vocational School of Diyarbakir, Dicle University, Diyarbakir, Turkey. [email protected]

Arzu EKİNCİ, Siirt University, Faculty Of Health Sciences, Department Of Occupational Health And Safety, Siirt, Turkey. [email protected]

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You are reading this latest preprint version

Green Synthesis of Co-based Nanoparticles from Bean Shell Extract and Determination of the Effect of Their Activity on Sodium Borohydride Hydrolysis

Status:

Version 1

Abstract

Figures

1. Introduction

2. Material and Method

2.1. Materials

2.2. Preparation of the extract

2.3. Green synthesis of nanoparticle catalysts

2.4. Sodium borohydride hydrolysis of Co(0) NPs and Co-B NPs catalysts

2.5. Characterization of Co(0) and Co-B NPs catalysts

3. Results and Discussion

3.1. Characterization

3.2. Hydrolysis of sodium borohydride in the presence of Co and Co-B NPs catalysts

3.2.1. Effected of concentration of NaOH

3.2.2. Effect of concentration of NaBH₄

3.2.3. Effect of the amount of catalyst

3.2.4. Effected of Temperature

3.2.5. Study of catalyst kinetics

nth-order kinetic model

-\({r}_{A}=-\frac{dCA}{dt}=k{C}^{n}A\) (2)

Langmuir-Hinshelwood kinetic model

3.2.6. Reusability tests of Co(0) and Co-B NPs catalysts

Conclusion

Declarations

References

Additional Declarations

Status:

Version 1

Green Synthesis of Co-based Nanoparticles from Bean Shell Extract and Determination of the Effect of Their Activity on Sodium Borohydride Hydrolysis

Status:

Version 1

Abstract

Figures

1. Introduction

2. Material and Method

2.1. Materials

2.2. Preparation of the extract

2.3. Green synthesis of nanoparticle catalysts

2.4. Sodium borohydride hydrolysis of Co(0) NPs and Co-B NPs catalysts

2.5. Characterization of Co(0) and Co-B NPs catalysts

3. Results and Discussion

3.1. Characterization

3.2. Hydrolysis of sodium borohydride in the presence of Co and Co-B NPs catalysts

3.2.1. Effected of concentration of NaOH

3.2.2. Effect of concentration of NaBH4

3.2.3. Effect of the amount of catalyst

3.2.4. Effected of Temperature

3.2.5. Study of catalyst kinetics

nth-order kinetic model

-\({r}_{A}=-\frac{dCA}{dt}=k{C}^{n}A\) (2)

Langmuir-Hinshelwood kinetic model

3.2.6. Reusability tests of Co(0) and Co-B NPs catalysts

Conclusion

Declarations

References

Additional Declarations

Status:

Version 1

3.2.2. Effect of concentration of NaBH₄