
==== Front
Sci Rep
Sci Rep
Scientific Reports
2045-2322
Nature Publishing Group UK London

71501
10.1038/s41598-024-71501-4
Article
Engineering of the ferrite-based support for enhanced performance of supported Pt, Pd, Ru, and Rh catalysts in hydrogen generation from NaBH4 hydrolysis
Mirshafiee Faezeh
Rezaei Mehran mrezaei@iust.ac.ir

https://ror.org/01jw2p796 grid.411748.f 0000 0001 0387 0587 School of Chemical, Petroleum and Gas Engineering, Iran University of Science and Technology (IUST), Tehran, Iran
6 9 2024
6 9 2024
2024
14 2081822 4 2024
28 8 2024
© The Author(s) 2024
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.
A series of M/NiCo-Ferrite (M: Pt, Pd, Ru, and Rh) nanoparticles were successfully synthesized, through a facile sol–gel auto-combustion followed by impregnation-reduction approach, as a catalyst for hydrogen generation from hydrolysis of NaBH4. All synthesized samples were characterized by XRD, N2 adsorption–desorption method, ICP-OES, FE-SEM, and EDX analysis. Compared to the other samples, it was observed that the Rh/NiCo-Ferrite sample exhibited higher particle distribution and surface area. To evaluate the hydrogen generation rate, the hydrolysis was carried out at a temperature of 35 °C, with an aqueous solution containing 5 wt.% NaBH4 and 3 wt.% NaOH. The experimental findings indicate that the Rh/NiCo-Ferrite sample exhibited a superior rate of hydrogen generation, with an average value of 11,667 mL/min.gcat, compared to the other samples studied. Enhanced catalytic properties may be responsible for its high activity. In addition, the activation energy of hydrolysis of sodium borohydride over the Rh/NiCo-Ferrite sample was 54.5 kJ/mol which is lower than the activation energy of many Ferrite-based catalysts. Moreover, the re-usability test of the Rh/NiCo-Ferrite sample denoted a decline in the catalytic activity after 4 recycling experiments due to the alterations in morphology and the reduction in the quantity of active phase.

Keywords

Hydrogen
Hydrolysis
Ferrite
Active support
Nobble metals
Subject terms

Environmental sciences
Energy science and technology
Materials science
Nanoscience and technology
Iran National Science Foundation (INSF)4013287 issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

The decrease in the reserves of fossil fuels such as coal, oil, and natural gas, as well as the increase in environmental pollution in recent years, has become a crisis in the field of energy. Therefore, searching for alternative technologies that can produce energy from renewable sources has become very important. Hydrogen, the lightest chemical element and the most abundant substance in the world holds immense potential as a promising clean and renewable energy carrier1. The potential of hydrogen as an environmentally friendly fuel source is driving research and innovation to establish a green hydrogen economy2. Hydrogen can be used in various fields such as transportation, electricity generation, etc. However, the production of hydrogen with an appropriate method, transportation, and storage are challenges that must be solved before this energy becomes widespread. Thus, to develop a hydrogen-based society, scientists searched for clean and safe methods for hydrogen production like using hydrogen storage materials. It is crucial to develop appropriate hydrogen storage material, with high hydrogen storage capacity and a controllable release rate, such as borohydride materials3. Alkali metals like K, Na, and Li have an excellent capacity to develop borohydride4. Among these, sodium borohydride (NaBH4) seems to be more suitable than others for reasons such as high hydrogen storage, low molecular weight, non-toxicity, stability in an alkaline environment, regeneration capacity, etc.5. Borohydride hydrolysis can be used to produce hydrogen for Proton Exchange Membrane Fuel Cells (PEMFC), which can power a range of electronic devices such as vehicles, smartphones, and tablets. The produced hydrogen is pure and does not contain catalyst poisons for the fuel cells6. Generally, hydrogen stored in sodium borohydride is released by thermolysis or hydrolysis, which hydrolysis is a more effective method7. However, spontaneous hydrolysis is very slow and hydrogen production efficiency is low, so it is necessary to use a catalyst to improve these cases8. Many catalysts were introduced for NaBH4 hydrolysis, with noble metals like Ru9, Pd10, Pt11, etc. being the most effective ones. But these catalysts due to their high surface energy usually face the problem of agglomeration that leads to a decrease in surface area. To address these challenges, it is recommended that metal nanoparticles be immobilized on a suitable carrier material12. Van der Waals interactions, electron transfers, and adhesion interactions between the support and active phases increase the stability of the catalysts, on the other hand, they provide a large and uniform distribution of active phases13.

Numerous support materials, including metal oxides14, carbon-based materials15, clays16, polymers17, etc. have been employed for the immobilization of metal nanoparticles in this context. It is worth noting that, in the majority of catalysts utilized for this process, a metal component was employed as the active species, while the support material was largely inactive in facilitating the hydrolysis reaction. Consequently, the development of an active support material capable of enhancing the activity of the metal component even at low loadings would be of significant value. The spinel ferrites with the general formula AFe2O4 (A = Co, Ni, Cu, Mg, Mn, Zn, etc.) are a kind of nanomaterials that are used for a variety of applications due to their suitable properties, such as low-price, high adsorption capacity, low dielectric constant, high magnetic properties, etc.18. Among various spinel ferrites, cobalt ferrites, CoFe2O4, are one of the most explored nanoparticles employed in various applications including microwave devices, data storage, drug delivery, catalyst, water treatment, lithium-ion rechargeable batteries, etc.19. Recently, cobalt ferrite was also used as a support in the sodium borohydride hydrolysis process. The study by Wang et al.20 explored the use of cobalt ferrite supported by various noble metal nanoparticles as catalysts for the hydrolysis of sodium borohydride. They found that Ru/CoFe2O4 exhibited the highest catalytic activity, with a turnover frequency of 421 molH2/min molcat. This suggests that the Ru/CoFe2O4 catalyst was able to produce a large amount of hydrogen gas per unit time per unit mass of the catalyst. Abdelsalam et al.21 investigated the hydrogen generation facilitated by the Ag/CoFe2O4–CNT nanocatalyst. The results demonstrated a noteworthy hydrogen generation rate (HGR) of 320 mL/min.gcat, coupled with a low activation energy of 14.7 kJ/mol. Furthermore, the Ag/CoFe2O4–CNT catalyst exhibited remarkable stability, retaining its activity throughout seven successive recycles.

Using alloy materials, due to the synergetic effect through electron transfer between metals, creates new catalytic properties22. Thus, we have chosen to explore the impact of this concern by synthesizing a composite ferrite material through the incorporation of cobalt with another metal. Within the spectrum of metallic species that can be used for this goal, nickel showed a suitable consistency with cobalt in the hydrolysis of sodium borohydride23. Other studies also showed a notable interaction between cobalt and nickel during a sodium borohydride hydrolysis reaction24. However, no research has used nickel and cobalt combined ferrite for sodium borohydride hydrolysis. Meanwhile, nickel cobalt-ferrite, in addition to the simultaneous use of two metals, nickel, and cobalt, which show very good binary properties in the sodium borohydride hydrolysis process, has a magnetic nature, which makes it easier to recover from the reaction medium. This aspect holds significant importance, particularly in the case of noble metals. Inspired by the preceding discussion, herein we synthesized a series of M/Nickel–Cobalt Ferrite (M: Pt, Pd, Ru, and Rh) nanoparticles as the catalyst for the hydrolysis of sodium borohydride. Through this research, the application of this spinel-based catalyst will be developed not only in the hydrolysis process but also in other processes like water treatment and semiconductor technology.

Experimental

Chemicals

In this study, several chemical reagents were employed, namely ferric nitrate nonahydrate (Fe(NO3)3·9H2O, Merck), Cobalt(II) Nitrate (Co(NO3)2·6H2O, Merck), Nickel(II) Nitrate (Ni(NO3)2·6H2O, Merck), palladium (II) Nitrate (Pd (No3)2, Merck), Platinum (II) Nitrate (Pt (No3)2, Merck), Ruthenium (III) nitrosyl nitrate (Ru(NO3)3, Merck), Rhodium (III) nitrate (Rh(NO3)3, Merck), NH3(ammonia, Nanotech), C6H8O7 (citric acid, Sigma-Aldrich), Sodium borohydride (NaBH4, Sigma-Aldrich 98%), and sodium hydroxide (NaOH, Merck). These chemicals were of analytical grade and were utilized in their received form.

Synthesis of catalysts

To synthesize cobalt Ferrite, CoFe2O4, first stoichiometric ratio of cobalt nitrate and iron nitrate (CoFe=0.5) was dissolved in 70 mL of deionized water and mixed for 10 min at ambient temperature. Subsequently, citric acid was added to the solution in a one-to-one molar ratio (CA to Fe3+ + Co2+) and mixed for an additional 5 min. To neutralize the acidic environment of the solution, ammonia was introduced until the pH reached 7. Concurrently, the temperature was elevated to 100 °C until a gel-like mixture was formed. As the heating continues, spontaneous combustion occurs and within a few seconds, a brittle and porous substance is formed, which is the desired ferrite. The high pressure of generated gases during exit causes the breaking of large particles. The resulting product, which occupies a lot of space, was powdered using a mortar and calcined at 600 °C for 3 h. For the synthesis of nickel cobalt ferrite, Ni0.2Co0.8Fe2O4, the synthesis protocol is similar to that described above, with the exception that the raw materials employed in the first step include nickel nitrate in addition to cobalt nitrate and iron nitrate. These two samples are denoted as Co-Ferrite and NiCo-Ferrite, respectively. To load noble metals, a certain amount of noble metal solution and Ni0.2Co0.8Fe2O4 support with a metal content of 5 wt.% were dissolved in 50 mL of deionized water. Following an aging period of 30 min, the suspension was transferred to an oven to allow for water evaporation, after which the solid product, M/NiCo-Ferrite (M: Pt, Pd, Ru, and Rh), was collected. The synthesis procedures were replicated twice to ensure the reproducibility of the results.

Catalytic activity tests

The hydrogen generation rate was determined by conducting the hydrolysis reaction in a temperature-controlled glass reactor at 35 °C. Specifically, a two-neck round bottom flask containing 12 mg of catalyst was employed, and 3 mL of 5 wt.% NaBH4 and 3 wt.% NaOH aqueous solution was subsequently injected, resulting in an instance release of hydrogen. The volume of generated hydrogen was quantified by monitoring the displacement of water level in Erlenmeyer. Figure 1 shows a simple scheme of the relevant experimental setup used for this measurement. To determine the effect of reaction conditions on the hydrolysis reaction using the optimum catalysts, the operational conditions including the amount of catalyst (9–18 mg), NaBH4 (3–9 wt.%), and NaOH (1–4 wt.%) concentration, and reaction temperature (25–40 °C), were changed in the ranges, while other parameters are maintained at a fixed level. Following the reaction, the catalyst was separated via magnet and subjected to thorough washing with water, after which it was dried at 100 °C overnight and reused in a series of catalytic runs, under identical experimental conditions as those utilized previously.Fig. 1 An experimental setup used for generating H2 from hydrolysis of NaBH4.

Characterization

The XRD spectrum of synthesized catalysts was investigated by Bourevestnik (model DRON-8, Russian) equipment. Morphology analysis by scanning electron microscopy (FESEM) measurements was recorded on a TESCAN-MIRA3 (Czech) instrument. The elemental distribution was examined through the utilization of energy-dispersive X-ray (EDX) analysis. The surface area and the pore size distribution of prepared samples were determined based on the physical adsorption/desorption of nitrogen on the surface of the samples in the relative pressure range from 0.05 to 0.99 using the Belsorp mini II (Japan) instrument. The ICP analyses were carried out by a Perkin-Elmer optima 7300 DV spectrometer.

Results and discussions

Characterization of the synthesized catalyst

XRD analysis

The XRD spectrum of the synthesized samples is depicted in Fig. 2. The characteristic peaks at 2θ = 30.19, 35.57, 43.17, 57.14, and 62.74° are indicative of the formation of Co-Ferrite (JCPDS 77-0426), and confirm the crystalline structure of the catalyst. These diffraction peaks can be indexed to the (220), (311), (400), (511), and (440) crystal planes of nanoparticles. Comparing Fig. 2a and b, we can see that the introduction of nickel did not change the diffraction pattern of the Co-Ferrite structure. Literature review has demonstrated that all ferrites exhibit identical diffraction patterns, with only minor variations in peak position25.Fig. 2 XRD patterns of (a) Co-Ferrite, (b) NiCo-Ferrite, (c) Pt/NiCo-Ferrite, (d) Pd/NiCo-Ferrite, (e) Ru/NiCo-Ferrite, and (f) Rh/NiCo-Ferrite.

Based on the figures provided, there are no visible peaks associated with noble metals in the X-ray diffraction patterns of the supported samples. This may be due to the weight percentage of these metals being below the detection limit of the X-ray diffraction device. Apart from these instances, no other peaks were observed, suggesting that the synthesized catalysts were pure and the synthesis process was successful.

FESEM analysis

Microscopic images of the surface of the synthesized samples are presented in Fig. 3. FESEM images rely on the interaction of electrons with the sample surface to provide valuable information about the surface morphology26. It is evident from the figure that most particles exhibit a nearly spherical or polyhedral morphology which is placed next to each other, forming irregular nanosized shapes. These nanoparticles, providing more active sites, can lead to better performance in the hydrolysis process27. After the addition of nickel to the Co-Ferrite carrier, the surface of the samples appeared slightly rougher, possibly due to the formation of grains or a new layer on the surface28. Upon the introduction of noble metals onto the NiCo-Ferrite support, the particles showed a slight dispersion, at the same time samples preserved their general shape and the structure of the samples remained unchanged. These results confirm that the introduction of noble metals did not change the employed support. Furthermore, it is reasonable to say that the Rh/NiCO-Ferrite sample exhibits a more homogeneous particle distribution compared to the other samples.Fig. 3 FESEM results of (a) Co-Ferrite, (b) NiCo-Ferrite, (c) Pt/NiCo-Ferrite, (d) Pd/NiCo-Ferrite, (e) Ru/NiCo-Ferrite, and (f) Rh/NiCo-Ferrite.

N2 adsorption–desorption analysis

Table 1 summarizes the results of the nitrogen adsorption/desorption analysis conducted on the synthesized samples. It is evident that the introduction of nickel metal in the cobalt ferrite carrier structure led to an increase in the surface area and total pore volume. However, loading noble metals onto the NiCo-Ferrite support yielded varying results, with the Rh/NiCo-Ferrite sample exhibiting the highest surface area and the Ru/NiCo-Ferrite sample displaying the smallest surface area. It is likely that the decrease in surface area of the Pt/NiCo-Ferrite, Pd/NiCo-Ferrite, and Ru/NiCo-Ferrite samples compare with NiCo-Ferrite can be attributed to the blockage of the catalyst pores caused by the added metal particles29. Additionally, there was a reduction in the pore volume observed after the noble metals were loaded onto the NiCo-Ferrite carrier.Table 1 Pore characteristic of synthesized samples.

Sample	SBETa	Vtotalb	M.P.Dc	
Co-Fer-Reduction	26.72	0.181	27.09	
Ni0.2Co0.8-Ferrite	46.33	0.217	18.78	
Pt/ NiCo-Ferrite	32.71	0.099	12.13	
Pd/ NiCo-Ferrite	35.98	0.179	19.90	
Ru/ NiCo-Ferrite	31.23	0.162	20.80	
Rh/ NiCo-Ferrite	46.45	0.142	12.26	
aSurface area with BET_Method (m2/g).

bVolume adsorbed at p/p0 = 0.99 (mL/g).

cMean pore diameter (nm).

In Fig. 4, we can observe the nitrogen adsorption/desorption isotherms and pore size distribution curves of the synthesized catalysts. In general, the adsorption line showed a gradual increase in all isotherms until high relative pressures, after which the desorption line slowly moved to the moderate p/p0. According to Fig. 4a, at higher relative pressures, isotherms Type III and IV were observed, indicating the existence of meso and macropores. BJH analysis shown in Fig. 4b confirmed the presence of these pores, with the largest pore size distribution found in the Ru/NiCo-Ferrite sample.Fig. 4 N2 adsorption–desorption isotherms (a), and BJH pore size distribution curves (b) of the samples.

ICP-OES analysis

ICP-OES was utilized to compare the experimental and theoretical quantities of Co, Ni, and Rh metals in the synthesized catalysts. The samples were initially prepared by dissolving in hydrochloric acid and nitric acid, and then the metal amounts were determined using an ICP device. As shown in Table 2, the measured weight percentage for the elements closely matched the values employed during synthesis, demonstrating the accuracy and success of the material synthesis.Table 2 ICP-OES of synthesized samples.

Sample	Co (wt.%)	Ni (wt.%)	Rh (wt.%)	
Co-Ferrite	26.5	–	–	
Co-Ferrite (exp)	24.8	–	–	
NiCo-Ferrite	21	5	–	
NiCo-Ferrite (exp)	20.73	4.76	–	
Rh/NiCo-Ferrite	19.95	4.75	5	
Rh/NiCo-Ferrite (exp)	18.99	4.61	4.39	

Catalytic activity tests

The catalytic performance of the synthesized samples in the hydrolysis of sodium borohydride was evaluated via quantification of the hydrogen generated during the reaction. Figure 5 illustrates the hydrogen generation trend over time on stream, for the two cobalt ferrites and NiCo-ferrites catalysts. Notably, the calcined cobalt ferrite did not yield any hydrogen production. Research has demonstrated that the metallic cobalt species serve as the active sites upon which borohydride ions are adsorbed during the hydrolysis reaction29. Therefore, to generate these metallic cobalt species, the synthesized cobalt ferrite was subjected to chemical reduction with sodium borohydride, as opposed to calcination. This reduction process was conducted using a molar ratio of NaBH4/Co(II) = 5, thereby facilitating the conversion of cobalt species into a metallic state. As evidenced by the results, the sample subjected to reduction displayed a substantial generation rate of hydrogen, highlighting the potential of this approach in the production of active supports for sodium borohydride hydrolysis. It is noteworthy that most of the supports utilized in this process are not catalytically active, instead serving solely as sites for active phase distribution.Fig. 5 Hydrogen generation curves of the NaBH4 hydrolysis over different samples(5 wt.%NaBH4, 3 wt.%NaOH, 35 °C, and 12 mg catalyst).

In addition, Fig. 5 demonstrated that a significant increase in the rate of hydrogen generation has been achieved through the coupling of cobalt metal with 20 wt.% nickel, resulting in the formation of a nickel–cobalt combined ferrite. This composite material exhibits a higher level of activity compared to cobalt ferrite as a support. Moreover, our previous investigation has revealed that the addition of nickel to cobalt during the sodium borohydride hydrolysis process results in a significant improvement in performance due to the favorable electron interaction between the two metals. Given these findings, the present study employed nickel–cobalt composite ferrite as an active support for the deposition of noble metals.

Figure 6 shows a significant rise in hydrogen production with just 5% of precious metals added to the NiCo-Ferrite support. It is obvious that different metal catalysts supported on NiCo-Ferrite have almost the same performance. The observation suggests that these metals may share certain structural similarities that result in similar catalytic performance in this reaction. The metals Pt, Pd, Ru, and Rh belong to the same group in the periodic table, and they all have similar electronic structures30. This similarity in electronic configurations can lead to comparable catalytic behavior, as the availability of d-orbitals for bonding and interaction with reactants may be similar across these metals. In addition, these metals may exhibit similar binding energies with reactant molecules such as BH4- and H2O31. This can result in comparable adsorption strengths, leading to similar catalytic activities. Notably, Rh/NiCo-Ferrite exhibited the highest hydrogen generation rate, with a rate of 11,667 mL/min.gcat. Furthermore, the kinetics of most reactions on these samples were observed to be significantly enhanced, as evidenced by a steeper induction time. Specifically, the Rh/NiCo-Ferrite sample produced 235 mL of hydrogen in just two minutes. These findings are consistent with the existing literature, which highlights the favorable impact of noble metal incorporation on catalytic activity32–34.Fig. 6 Hydrogen generation curves of the NiCo-ferrite supported samples (5 wt.%NaBH4, 3 wt.%NaOH, 35 °C, and 12 mg catalyst).

Effect of different operational conditions on the catalyst activity

Catalyst amount effect

The impact of varying the quantity of Rh/NiCo-Ferrite catalyst on the rate of hydrogen generation during the sodium borohydride hydrolysis process is illustrated in Fig. 7. Notably, as the catalyst loading is increased from 9 to 18 mg, a corresponding enhancement in hydrogen generation is observed, accompanied by a decrease in reaction completion time. This outcome can be attributed to the increased exposure of active sites to sodium borohydride upon increasing the quantity of catalyst employed, resulting in a more robust reaction progression35.Fig. 7 The effect of catalyst amount in the range of 9–18 mg on the volume of hydrogen generation.

NaBH4 concentration effect

Figure 8 illustrates the impact of varying sodium borohydride concentration on the quantity of hydrogen generated through the sodium borohydride hydrolysis process. Notably, an initial increase in hydrogen generation is observed upon elevating the sodium borohydride concentration from 3 to 7 wt.%. This phenomenon can be attributed to the increased contact between borohydride molecules and the active sites located on the catalyst's surface. However, beyond a concentration of 7 wt.%, the production of hydrogen decreases, likely due to the saturation of the catalyst's active sites by BH4- as well as an increase in solution viscosity. These findings are consistent with the existing literature on the subject36.Fig. 8 The effect of NaBH4 concentration on the volume of hydrogen generation.

Effect of NaOH concentration

Figure 9 depicts the impact of various concentrations of sodium hydroxide (1, 2, 3, and 4 wt.% relative to the weight of the solvent) on the volume of hydrogen generated. The maximum hydrogen generation rate increased as the NaOH concentration rose from 1 to 3 wt.%. The increase in hydroxide (OH−) ions in the reaction medium can enhance the electronic properties of the catalyst, which is beneficial for hydrogen evolution37. However, after further expanding the sodium hydroxide concentration, the gas produced did not change significantly and may have decreased slightly. As a result, the concentration of 3 wt.% was deemed to be the best concentration. The reason that the H2 generation rate did not increase with a higher concentration of sodium hydroxide can be attributed to the disturbance of the alkaline balance of the reaction medium as well as the occupation of the catalyst surface by the excess hydroxide (OH−) anions coming from the NaOH, instead of the BH4− anions, which hinders the adsorption and activation of BH4− on the catalyst, thereby decreasing the rate of hydrogen generation38.Fig. 9 The effect of sodium hydroxide concentration on the volume of hydrogen generation.

Temperature effect

The results of the Fig. 10 demonstrate the impact of temperature on the rate of hydrogen generation during the sodium borohydride hydrolysis process. Notably, a positive correlation is observed between hydrogen generation rate and temperature, whereby an increase in temperature leads to an elevated hydrogen generation rate. This phenomenon, compatible with other studies39, can be attributed to the greater reactivity and availability of molecules at higher temperatures which facilitates and enhances the hydrogen generation process.Fig. 10 The effect of temperature on the volume of hydrogen generation.

Kinetic study

The catalyst's performance evaluation often involves the determination of the activation energy of the underlying reaction, given the catalyst's primary function of reducing this energy barrier. A lower activation energy leads to reduced energy requirements for initiating the reaction, which correspondingly results in an elevated reaction rate. The conversion of temperature effect data in Fig. 10 into an Arrhenius plot, which entails plotting the Ln(r) diagram relative to 1/T as per Eqs. (1–3), allows for the straightforward calculation of activation energy through the slope of the resulting graph.1 r=k.CNaBH4a.CNaOHb.CCatc

2 k=k0.exp(-EaR.T)

3 Ln(r)=Ln(k0)+aLnCNaBH4+bLnCNaOH+cLnCCat-EaR.T

The aforementioned equations involve several parameters, wherein 'r' denotes the hydrogen generation rate, 'k0' represents the constant factor related to the particle collision frequency, 'Ea' signifies the activation energy, 'R' denotes the ideal gas constant, and 'T' corresponds to the reaction temperature.

The logarithmic graph presented in Fig. 11 illustrates the hydrogen generation rate plotted against the inverse of temperature. The slope of the curve, obtained via curve fitting, corresponds to the activation energy of the catalyst, yielding a value of 54.5 kJ/mol. This value is noteworthy as it is lower than that of other catalysts based on spinel ferrites40, thus establishing a favorable characteristic of the catalyst under consideration.Fig. 11 Arrhenius diagram obtained from the data in Fig. 10.

Reusability test

The stability analysis in Fig. 12, conducted throughout four consecutive cycles, revealed a gradual decrease in the quantity of hydrogen production, albeit to a lesser extent than that observed during the second cycle. The factors commonly associated with the decline in catalyst activity encompass the formation of byproducts that obstruct catalytic active sites on the catalyst surface, dissolution of catalytic components in the alkaline environment of NaBH4, accumulation of active phases, loss of catalyst material during recycling, oxidation of the catalyst surface, and other related phenomena41.Fig. 12 Recycling performance of Rh/NiCo-Ferrite catalyst for 4 consecutive cycles.

Analysis of used catalyst

To ascertain the underlying cause of the observed decline in catalyst activity, various analyses were conducted on the used sample. The analytical results in Fig. 13 revealed that the surface properties of the deactivated catalyst have a different morphology from that of fresh Rh/NiCo-Ferrite sample suggesting the adsorption of impurities on the surface of catalyst and its degradation. The used catalyst was found to be in an amorphous agglomerated state, while the particles displayed a spherical morphology and a mean diameter of 80–100 nm in the fresh sample. Modifying the catalyst surface to enhance its stability and prevent impurities from adsorbing onto the active sites can be useful. This can be achieved through techniques such as surface functionalization, coating with protective layers, or introducing stabilizing agents.Fig. 13 FESEM analysis of Rh/NiCo-Ferrite catalyst (a) fresh, and (b) after 4 cycles.

Figure 14 depicts the EDX patterns of the fresh and used Rh/NiCo-Ferrite samples. As evident from the Fig. 14a, the intensity of the peaks attributed to the four metal components, namely rhodium, cobalt, nickel, and iron, experienced a decrease after four cycles of use, which is likely a result of washing. Additionally, the dissolution of catalytic components in the alkaline environment of sodium borohydride may contribute to this observation. To address the issues, optimizing the washing procedure and the pH of the reaction mixture to minimize the dissolution of catalytic components can improve the stability of the catalyst.Fig. 14 EDX analysis of Rh/NiCo-Ferrite catalyst (a) fresh, and (b) after 4 cycles.

The research compares the hydrogen generation potential of different noble metal-based catalysts based on their hydrogen generation rate and activation energy. The results in Table 3 indicate that the catalyst being studied has achieved a satisfactory outcome compared to the performance of the other mentioned catalysts.Table 3 The hydrogen generation properties of different nobble metal-based catalysts.

Catalyst	Temperature (°C)	Catalyst amount (mg)	NaBH4 concentration (wt.%)	HGR (mL/min.g)	Ea (kJ/mol)	References	
Ru-Co/C	35	100	10	3420	56.3	42	
Ru/LiCO2	30	20	10	3100	68.5	43	
Pt and Rh/ LiCoO2	30	0.011	0.046 g	7200	–	44	
Pd/MWCNT	22	10	835 μmol	23	62.66	45	
Ru/TiO2	25	50	1 M	100	62	46	
Ru/C	35	300	3.2 M	318	61.2	47	
Rh/NiCo-Ferrite	25–40	12	5	11,667	54.5	This work	

Conclusion

Here, a group of M/ Ni0.2Co0.8 Fe2O4 (M: Pt, Pd, Ru, and Rh) nanoparticles were synthesized to utilize them as effective catalysts for hydrogen generation via the hydrolysis of NaBH4. Based on the characterization results, it was observed that the Rh/NiCo-Ferrite catalyst exhibited a favorable characterization including an acceptable dispersion of Rh nanoparticles over the NiCo-Ferrite sample and a large surface area of 46.45 m2/g. As such, it is anticipated that this catalyst would demonstrate superior performance towards the hydrolysis of NaBH4, relative to the other catalysts investigated. Employing a temperature of 35 °C, a solution containing 5 wt.% NaBH4 and 3 wt.% NaOH, the hydrogen generation rate was up to 11,667 mL/min.gcat over Rh/ NiCo-Ferrite. In addition, based on the Arrhenius equation, the activation energy (Ea) of this reaction was 54.5 kJ/mol which is comparable to other ferrite-based catalysts.

Acknowledgements

This work is based upon research funded by Iran National Science Foundation (INSF) under project No. 4013287 and 97017638.

Author contributions

F.M.: Conceptualization, methodology, validation, investigation, writing—original draft. M.R.: Supervision, validation, funding, writing—review and editing.

Data availability

The datasets used and analyzed during the current study are available from the corresponding author upon reasonable request.

Competing interests

The authors declare no competing interests.

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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