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Article

The Dependence of NiMo/Cu Catalyst Composition on Its Catalytic Activity in Sodium Borohydride Hydrolysis Reactions

by
Zita Sukackienė
,
Gitana Valeckytė
,
Virginija Kepenienė
*,
Irena Stalnionienė
,
Vitalija Jasulaitiene
,
Jūratė Vaičiūnienė
,
Loreta Tamašauskaitė-Tamašiūnaitė
,
Vidas Pakštas
and
Eugenijus Norkus
*
Center for Physical Sciences and Technology, Saulėtekio Ave. 3, LT-10257 Vilnius, Lithuania
*
Authors to whom correspondence should be addressed.
Materials 2024, 17(17), 4353; https://doi.org/10.3390/ma17174353
Submission received: 6 August 2024 / Revised: 30 August 2024 / Accepted: 1 September 2024 / Published: 3 September 2024
(This article belongs to the Section Catalytic Materials)
Browse Figures
Versions Notes

Abstract

:
The production of high-purity hydrogen from hydrogen storage materials with further direct use of generated hydrogen in fuel cells is still a relevant research field. For this purpose, nickel-molybdenum-plated copper catalysts (NiMo/Cu), comprising between 1 and 20 wt.% molybdenum, as catalytic materials for hydrogen generation, were prepared using a low-cost, straightforward electroless metal deposition method by using citrate plating baths containing Ni2+–Mo6+ ions as a metal source and morpholine borane as a reducing agent. The catalytic activity of the prepared NiMo/Cu catalysts toward alkaline sodium borohydride (NaBH4) hydrolysis increased with the increase in the content of molybdenum present in the catalysts. The hydrogen generation rate of 6.48 L min−1 gcat−1 was achieved by employing NiMo/Cu comprising 20 wt.% at a temperature of 343 K and a calculated activation energy of 60.49 kJ mol−1 with remarkable stability, retaining 94% of its initial catalytic activity for NaBH4 hydrolysis following the completion of the fifth cycle. The synergetic effect between nickel and molybdenum, in addition to the formation of solid-state solutions between metals, promoted the hydrogen generation reaction.

Graphical Abstract

1. Introduction

Hydrogen (H2) became the first potential alternative to fossil fuels and has emerged as a promising and pivotal alternative energy carrier [1]. The primary application of hydrogen as a fuel was started in the aerospace industry, with subsequent research and experimentation conducted in vehicles. In addition, extensive research was conducted on the use of hydrogen as a fuel in fuel cells [2]. Nowadays, the global demand for hydrogen reaches 95 Mt per year (2022), with a nearly 3% increase year on year [3]. However, the majority of hydrogen is produced from natural gas (62%), coal (19%), or naphtha reforming (18%). Nevertheless, these methods inevitably lead to the emission of CO2 and thus to the greenhouse effect, which is why research into new, alternative approaches to the production of low-emission hydrogen is relevant today [4,5,6]. Since H2 does not exist in its molecular form in nature, consequently, a variety of techniques, including electrochemical, thermochemical, photochemical, photocatalytic, and photoelectrochemical processes, have been subjected to extensive investigation and improvement on a global scale with regard to their potential for hydrogen production [7]. Furthermore, hydrogen fuel cells have already been implemented in various sectors, including aviation, electrical power generation, and automotive technology [1,7,8]. However, hydrogen is frequently employed as a compressed gas, which presents a number of challenges, including purification, transportation, and storage. Therefore, research has been focused on the development of solid-state hydrogen storage materials and direct hydrogen generation reactions [9,10]. The environmentally friendly chemical boron hydrides (NaBH4 (10.8 wt.%), LiBH4 (18 wt.%), NH3-BH3 (19.6 wt.%), etc.) have been considered promising for hydrogen storage and use in portable fuel cells due to their high H2 capacity [11,12]. Sodium borohydride (NaBH4) has been identified as a highly suitable material for hydrogen storage due to a number of favorable characteristics, including its high hydrogen content, cost-effectiveness, environmental compatibility, stability in alkaline solutions, non-flammability, and renewability [13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41]. The production of hydrogen from NaBH4 hydrolysis is described by the following reaction (1), as outlined in reference [9,21,25,26,27,28,29,30]:
NaBH4 + 2H2O → 4H2 + NaBO2, ΔH° = −216.7 kJ mol−1
The efficiency of the borohydride hydrolysis reaction is contingent upon two primary factors: the activation energy of the reaction and the quantity of hydrogen released. In contact with water, the NaBH4 starts producing H2; however, the H2 gas rate is very low and the water-NaBH4 solution is unstable. Raising the pH by adding NaOH stabilizes the NaBH4 solution. Therefore, the pivotal factor for the production of highly purified hydrogen through NaBH4 hydrolysis is the discovery of selective catalysts that could accelerate the generation of H2 in alkaline solution [42]. The development of efficient catalysts plays a pivotal role in the conversion and utilization of clean energy [10]. A number of noble metals, including Pt, Ru, Pd, and Rh, have been identified as highly effective catalysts for accelerating the hydrolysis of NaBH4 due to their favorable performance, stability, and tolerance to deactivation [15,17,19,20,26]. Furthermore, the dispersion of nano-sized noble metals on carbon results in the catalyst having an enhanced electroactive surface area, which in turn exhibits an improved hydrogen generation rate [43,44,45]. However, the high cost, low abundance, and rapid surface poisoning of these metals have prompted the development of new non-noble catalysts with activity comparable to that of noble metals [40,46,47,48,49]. Among the metals under consideration, those belonging to the transition metal family, including Ni, Co, Fe, Mn, Cr, or Mo, have attracted the attention of researchers [9,13,18,21,22,23,24,31,32,33,34,35,36,37,39,41,47,50,51]. In this group of non-noble metal catalysts, nickel has been selected due to its advantageous characteristics, including catalytic activity, ferromagnetic properties, and reduced operating costs in recycling [52,53]. Moreover, alloyed transition metals create a synergistic effect to enhance catalytic activity [54,55]. Ni–Mo composites show higher electrocatalytic activity than pure Ni due to the improved intrinsic activity of the material. This is explained by the change in electron density in the d-shell when Ni is alloyed with Mo [54,55]. A variety of techniques have been documented in the scientific literature for the production of pure metal films or a few component coatings of these metals in combination with one another. A. Didehban et al. successfully synthesized bimetallic NiCo on magnetic supports using a heterogeneous deposition-precipitation (HDP) method. Furthermore, the activation energy of the sodium borohydride hydrolysis reaction was established to be 63.27 kJ mol−1 when employing the Co-Ni/MSAC catalyst, as reported in reference [32]. Z. Liang et al. proposed the NiB/NiFe2O4 catalyst, prepared via the impregnation chemical reduction method [37]. The NiB/NiFe2O4 catalyst exhibited an activation energy of 72.52 kJ mol−1 for the sodium borohydride hydrolysis reaction. H. Guo et al. presented a method for the preparation of Ag-Ni core and shell nanoparticles, which involved the thermal decomposition of nickel and silver salts using a one-pot seed growth approach. The activation energy for hydrolysis of NaBH4 catalyzed by this substance was determined to be 57.62 kJ mol−1 [38]. The authors posit that the increased catalytic activity of the combined materials can be attributed to the synergistic properties that are induced by the intimate contact and interaction between the different components [32,37,38].
This paper presents an investigation of the catalytic activity of Ni-Mo catalysts towards the hydrolysis reaction of NaBH4 to produce hydrogen fuel. Non-noble catalysts were constructed by selecting the combination of Ni and Mo coatings on the Cu substrate, which was chosen as a substrate because it provides a stable base. In addition, it is easy to deposit the identical Ni coatings on the Cu surface. Molybdenum was chosen to increase the efficiency of the NaBH4 hydrolysis reaction. It should be noted that the Ni sublayer is also required to deposit molybdenum. The NiMo(1)/Cu, NiMo(5)/Cu, NiMo(10)/Cu, NiMo(15)/Cu, and NiMo(20)/Cu catalysts were prepared with varying Mo content (1, 5, 10, 15, and 20 wt.%, respectively) by a simple and cost-effective approach—electroless metal plating using morpholine borane as a reducing agent. The catalytic activity of the NiMo/Cu catalysts was examined in the generation of hydrogen from alkaline NaBH4 solutions during hydrolysis at varying temperatures. Furthermore, the structure of the prepared catalysts was characterized using a range of analytical techniques, including inductively coupled plasma optical emission spectroscopy (ICP-OES), field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD).

2. Materials and Methods

2.1. Chemicals

Copper sheet (Cu, 99.8% purity) and sodium molybdate dihydrate (Na2MoO4∙2H2O, 99.5%) were purchased from Sigma-Aldrich (Saint Luise, MO, USA). Sulphuric acid (H2SO4, 96%), hydrochloric acid (HCl, 35–38%), nickel sulfate hexahydrate (NiSO4∙6H2O, 98%), morpholine borane (C4H8ONH∙BH3, 97%), palladium chloride (PdCl2, 99.95%), sodium citrate (Na3C6H5O7, 99%), and sodium hydroxide (NaOH, 98.8%) were purchased from Chempur Company (Karlsruhe, Germany). All chemicals were of analytical grade and were used directly without further purification.

2.2. Fabrication of Catalysts

Scheme 1 shows the electroless plating process of NiMo coatings on the Cu sheets using morpholine borane as a reducing agent in citrate solutions.
Briefly, the copper sheets intended for Ni-Mo deposition were prepared in the following manner: initially, Cu sheets in size of 1 × 1 cm were treated with a calcium magnesium oxide comprising between 50% and 100% of the material, otherwise known as “Vienna Lime” (Kremer Pigments GmbH & Co. KG, Aichstetten, Germany). They were then rinsed with deionized water and kept in a 10% HCl solution at room temperature for one minute, during which time any inorganic impurities were removed. They were subsequently washed and dried. Subsequently, the pre-treated Cu sheet was activated with Pd(II) ions through immersion in a 0.5 g L−1 PdCl2 solution for a period of 5 s. Following this, the sheet was rinsed with deionized water and placed in an electroless plating solution. The electroless deposition of Ni-Mo was conducted on Cu sheets using the plating solutions outlined in Table 1, with morpholine borane (MB) employed as the reducing agent.
It can be observed that the composition of the plating solution remained unvaried, with the exception of the concentration of Na2MoO4·2H2O. As a consequence, catalysts with a Mo content of 1−20 wt.% were prepared, comprising the following compositions: NiMo(1)/Cu, NiMo(5)/Cu, NiMo(10)/Cu, NiMo(15)/Cu, and NiMo(20)/Cu. In all cases, the pH of the plating solutions was 7 (as determined at room temperature), the temperature of the deposition bath was 50 °C, and the deposition time was 12 min for all catalysts under investigation.

2.3. Characterization of Catalysts

The morphology of the prepared NiMo/Cu catalysts was investigated via scanning electron microscopy (SEM) using an SEM workstation SEM TM 4000 Plus (HITACHI) with an energy dispersive X-ray (EDX) spectrometer. The metal loadings were determined via inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis. The ICP-OES spectra were recorded using an Optima 7000DV spectrometer (Perkin Elmer, Waltham, MA, USA) at wavelengths of λNi—231.604 nm and λMo—202.031 nm.
X-ray photoelectron spectroscopy (XPS) was employed to characterize a series of NiMo catalysts, utilizing a Kratos AXIS Supra+ spectrometer (Kratos Analytical, Manchester, UK, 219) with monochromatic Al Kα (1486.6 eV) X-ray radiation, powered at 225 W. The base pressure in the analysis chamber was less than 1 × 10−9 mbar and a low electron flood gun was used as a charge neutralizer. The survey spectra for each sample were recorded at a pass energy of 160 eV with a 1 eV energy step and high-resolution spectra (pass energy—20 eV, in 0.1 eV steps) over individual element peaks. The binding energy scale was calibrated by setting the adventitious carbon peak at 284.8 eV. The XPS data were converted to VAMAS format and processed using Avantage software version 5.9922 (Thermo Scientific, East Grinstead, UK).
The XRD patterns of the studied catalysts were obtained using an X-ray diffractometer Smart-Lab (Rigaku, Japan, 2011) equipped with an X-ray tube with a 9 kW rotating Cu anode. The measurements were conducted using the Bragg–Brentano geometry with a graphite monochromator on the diffracted beam and a step scan mode with a step size of 0.02 (in 2-theta scale) and a counting time of 1 s per step. The measurements were conducted in the 2-theta range of 10–75°. Phase identification was performed using the software package PDXL version 1.8.0.3 (Rigaku, Japan) and the ICDD powder diffraction database PDF4+ (2023 release).

2.4. Hydrolysis Measurements of NaBH4

A thermostatically controlled, hermetically sealed flask with an outlet linked to the MilliGascounter was employed to collect the produced hydrogen gas from a solution comprising 15 mL of 5 wt.% NaBH4 + 0.4 wt.% NaOH. The volume of the released hydrogen catalyzed by the prepared NiMo/Cu catalysts was measured using a MilliGascounter (Type MGC-1 V3.2 PMMA, Ritter, Bochum, Germany) connected to a personal computer. For all measurements, the prepared catalysts were placed in an alkaline sodium borohydride solution of the designated temperature and stirred with a magnetic stirrer. The hydrogen generation rate (HGR) was measured at a working solution within the temperature range of 303 to 343 K in order to determine the activation energy of the reaction. The activation energy (Ea) is defined as the minimum energy content required for a reacting species to carry out a given reaction. In general, higher reaction activity is associated with lower “Ea” values. The activation energy of the hydrolysis reaction of NaBH4 was calculated by using the Arrhenius equation based on the ln(k) versus 1/T dependence, as outlined in Equation (2):
ln k = ln A − Ea/(RT)
where A—the pre-exponential factor, Ea—the activation energy (J), R—the general gas constant (8.314 J mol−1 K−1), and k—the reaction coefficient.

3. Results and Discussion

3.1. Coatings, Microstructure and Morphology Studies

The primary objective of this study was to develop an effective catalyst for the generation of hydrogen from an alkaline sodium borohydride solution using a straightforward methodology. Consequently, two-component NiMo coatings with varying Mo concentrations were deposited on Cu sheets via a straightforward electroless deposition process, employing morpholine borane as the reducing agent in a citrate solution. Table 2 presents the elemental composition of the prepared NiMo(1)/Cu, NiMo(5)/Cu, NiMo(10)/Cu, NiMo(15)/Cu and NiMo(20)/Cu catalysts. The molybdenum content was varied within the range of 1 to 20 wt.% molybdenum. Accordingly, the amount of nickel in the coatings was in the range of 99 wt.% to 80 wt.%. The deposition time—12 min—was observed as the optimal cause. It is important to highlight that upon further augmentation of the molybdenum precursor concentration in the plating bath, the reaction does not start to occur. It could be caused by the Mo and its compounds adsorbing on the catalytically active surface and preventing chemical precipitation. Molybdenum itself is not catalytically active in the morpholine borane oxidation reaction [10,11,12,13,14]. Consequently, it was not feasible to deposit coatings with a greater amount of molybdenum than 20 wt.%. Moreover, higher Mo content in Ni–Mo composites could deteriorate the corrosion resistance due to surface roughness and grain refinement [56]. It was observed that the deposition rate decreased from 3.65 to 3.1 µm h−1 during the preparation of catalysts containing 1 and 5 wt.% Mo, respectively. Additionally, the total content of Ni and Mo decreased from 652.03 to 555.55 µg cm−2 depending on the content of Mo. It is known that the oxidation of morpholine by borane is catalyzed by nickel. However, when nickel is co-located with molybdenum, the catalytically active Ni centers are covered by Mo, resulting in a decrease in the reaction rate. Conversely, when the molybdenum content in the coating reached 10−20 wt.%, the deposition rate remained constant at approximately 2.75 µm h−1. Moreover, the general Ni and Mo contents in the coatings exhibit minimal variation, remaining within the range of 473.31 to 490.35 µg cm−2 (Table 2). The findings confirm that an increase in the molybdenum content of the catalyst results in a decline in the electroless deposition rate, which ultimately leads to the cessation of the reaction at higher concentrations of Mo6+. However, nickel–molybdenum alloys have been demonstrated to be effective catalysts for the hydrolysis of borohydrides.
The morphology and particle size of the prepared NiMo/Cu catalysts were assessed through the utilization of scanning electron microscopy (SEM) images. Figure 1 shows the SEM images of the prepared NiMo(1)/Cu, NiMo(5)/Cu, NiMo(10)/Cu, NiMo(15)/Cu, and NiMo(20)/Cu catalysts. As illustrated in Figure 1, the surface morphology of the prepared catalysts is observed to be compact, smooth, and free of cracks, exhibiting distinctive multi-layered cauliflower-like structures. Furthermore, the SEM images demonstrate that all of the prepared coatings are composed of particles of varying dimensions that coalesce to form oval agglomerates (Figure 1a–e). It is believed that an increased number of agglomerates on the surface will result in an enhanced active surface area, thereby improving the efficacy of the catalysts. The estimated average size range of the agglomerates on the surface is 0.286 to 2.143 μm.

3.2. XPS Analysis of the Prepared Catalysts

The electronic state and surface interaction between the atoms in coatings were investigated via XPS spectra analysis in order to gain insight into the promoting effect of Mo on the catalytic activity. Figure 2 reports the X-ray photoelectron spectra of the Ni 2p (a–e), Mo 3d (f–j), and O 1s (k–o) electron levels in the NiMo(1)/Cu), NiMo(5)/Cu, NiMo(10)/Cu, NiMo(15)/Cu, and NiMo(20)/Cu catalysts. As can be observed in Figure 2a–e, the two intense peaks in the Ni 2p spectra at binding energies of 852.7 ± 0.5 eV and 870.0 ± 0.5 eV with a split spin-orbit component of Δmetal = 17.3 eV were assigned to Ni0 2p3/2 and Ni0 2p1/2, respectively [57,58]. The lower intensity Ni 2p3/2 binding energy peaks originating at 853.2–855.5 eV were assigned to Ni2+, as reported in previous studies [57] (Figure 2a–c).
In the case of Mo 3d XPS data, the doublet peaks originating at 228.1 ± 0.4 eV and 231.3 ± 0.4 eV for Mo 3d5/2 and Mo 3d3/2 transitions, respectively, demonstrated the presence of Mo0 metallic phase in all samples [58]. The binding energy peaks at 229.2 ± 0.6 and 232.1 ± 0.4 eV for Mo 3d5/2 and Mo 3d3/2 transitions, respectively, may be assumed to be Mo4+ and indicated that apart from Mo0, Mo in the NiMo coatings existed as MoO2 [58]. Moreover, in all cases, the spin-orbital splitting transition peaks in the O 1s XPS spectra, with the binding energy peaks at 530.7–531.7 eV, correspond to the lattice oxygen species, while the dominant peaks at 532.0 ± 0.5 eV may be assumed to be hydroxides and defect oxides [57,58,59]. Higher binding energy peaks located at approximately 533.0 ± 0.7 eV are usually attributed to the presence of the surface of adsorbed O2, H2O, and CO2 (Figure 2) [59].

3.3. XRD Analysis of the Prepared Catalysts

It should be noted that the XRD analysis revealed highly comparable crystallographic patterns across all of the prepared catalysts (Figure 3). As can be seen, two intense diffraction peaks at 2-theta = 43.4 and 50.5° are observed in all profiles and are assigned to the (111) and (200) crystallographic planes of crystalline Cu from the Cu sheet (JCPDS No. 04-0836) [60]. No distinct sharp peak is observed for the Ni and Mo, indicating a low level of crystallinity where the size of the crystalites does not exceed 5 nm. As the concentration of Mo in the catalyst rises, the nickel lattice may undergo tensile strain, and the Ni lattice may deform. However, the XRD patterns of NiMo/Cu catalysts show a broad and diffuse peak centered at about 44–51°; therefore the lattice parameters of this compound cannot be accurately estimated. The obtained broad and diffuse Ni-Mo(x) peak could be attributed to the amorphous compounds of Ni and Mo or to the nickel-molybdenum solid-state liquid formation [55,61,62].

3.4. Catalysts Activity toward NaBH4 Hydrolysis

The catalytic performances of the prepared NiMo/Cu catalysts toward NaBH4 hydrolysis were evaluated in the solution containing 5 wt.% NaBH4 + 0.4 wt.% NaOH in the temperature range of 303–343 K. Figure 4 shows the temperature-time dependent volume of H2 generated (mL) from the NaBH4 solution obtained on the NiMo(1)/Cu (a), NiMo(5)/Cu (b), NiMo(10)/Cu (c), NiMo(15)/Cu (d). and NiMo(20)/Cu (e) catalysts at different temperatures. As can be seen from this figure, the hydrogen generation rate (HGR) increases in all cases as the temperature increases from 303 to 343 K. The summarized data for the HGR are given in Table 3. The data show that all the catalysts studied exhibited catalytic performance toward the hydrolysis of NaBH4. However, when the catalysts are compared, the results clearly show that the catalyst with the highest content of molybdenum exhibits the most effective catalytic activity toward the NaBH4 hydrolysis reaction, ranging from 0.4 to 6.48 L min−1 gcat−1 at the temperature of 303–343 K, respectively. The results showed that increasing the temperature of the operating bath from 303 to 343 K resulted in a 22-fold increase in the rate of hydrogen generation. Figure 4a′–e′ shows the Arrhenius plots for all of the catalysts investigated. The rate of hydrogen gas released during the reaction (k) was calculated using the known amount of hydrogen produced after 30 min at each temperature. The activation energy of the electrochemical process (Ea, kJ mol−1) was then calculated using the Arrhenius Equation (2). It was observed that the activation energy of the reaction increased with increasing Mo content in the catalyst (Table 3). The lowest Ea value of 54.26 kJ mol−1 was obtained for the NiMo(1)/Cu catalyst (Figure 4a′); however, the lowest hydrogen generation rate of 0.23–2.72 L min−1 gcat−1 was also observed for this catalyst in the temperature range of 303–343 K (Table 3). Although the NiMo(1)/Cu catalyst required the least amount of energy to initiate the reaction, the highest amount of released hydrogen was obtained with the NiMo(20)/Cu catalyst, although its activation energy was 60.49 kJ mol−1. As the temperature was increased from 303 to 343 K, the HGR for the NiMo(20)/Cu catalyst was calculated to range from 0.40 to 6.48 L min−1 gcat−1. In general, all of the investigated catalysts, NiMo(1)/Cu, NiMo(5)/Cu, NiMo(10)/Cu, NiMo(15)/Cu, and NiMo(20)/Cu, exhibited catalytic activity for sodium borohydride hydrolysis; however, the NiMo(20)/Cu catalyst demonstrated 19%, 13%, 33%, and 43%, higher catalytic efficiency for hydrogen generation than NiMo(15)/Cu, NiMo(10)/Cu, NiMo(5)/Cu, and NiMo(1)/Cu, respectively, at the lowest temperature (303 K) of the working bath.
When the working bath was maintained at 343 K, the NiMo(20)/Cu catalyst exhibited markedly enhanced catalytic efficiencies for hydrogen generation, with values 13%, 16%, 39%, and 58% higher than those observed for the NiMo(15)/Cu, NiMo(10)/Cu, NiMo(5)/Cu, and NiMo(1)/Cu catalysts, respectively. It is noteworthy that the higher molybdenum content of approximately 10–20 wt.% in the NiMo/Cu catalysts results in a higher hydrogen generation rate. The data obtained demonstrate that the successful interstitial insertion of larger amounts of Mo into nickel coatings resulted in a notable enhancement in catalytic activity toward H2 generation. It has been demonstrated that a low activation energy does not necessarily result in a high value of released hydrogen. This phenomenon may be attributed to the enhanced synergistic effect of the metals present in the catalysts. It has been observed that transition-metal-based binary alloys exhibit notable catalytic activity, which is primarily attributed to their distinctive characteristics, including a high concentration of unsaturated sites and an amorphous structure with short-range order, long-range disorder, and chemical stability [33,41,63]. The activation energies of the NiMo/Cu catalysts are among the most active compounds recently reported for non-noble metal electrocatalysts for borohydride hydrolysis, as shown in Table 4.
The Ea values obtained for all of the investigated catalysts in the range of 54.26 to 60.49 kJ mol−1 are comparable to those reported for other materials, including CoMoB/C [39], Ni–Ru/50WX8 [38], Ni–Fe–B [33], and Ag-Ni core-shell [40]. The values obtained for the investigated catalysts are higher than those for materials such as Co-Ni/MSAC (63.27 kJ mol−1) [32], CoFeMo/Ni (66.80 kJ mol−1) [41], bulk Ni catalysts (67.90 kJ mol−1 and 72.52 kJ mol−1) [27,31], nickel-cobalt (alloys) (68.84 kJ mol−1) [50], or CoFeMo/Ni (66.8 kJ mol−1) [41]. Nevertheless, noble metals such as Pt, Ru, and Ag have been demonstrated to reduce the activation energy of the borohydride hydrolysis reaction (see Table 4, [38,40]). However, we present a non-noble, inexpensive NiMo composite with a comparable efficiency for hydrogen generation from an alkaline sodium borohydride solution.
The investigation revealed that the NiMo(20)/Cu catalyst exhibited the highest efficiency in catalyzing the NaBH4 hydrolysis reaction, resulting in the highest H2 generation volume. Consequently, a stability and reusability test was conducted on the NiMo(20)/Cu catalyst, given its pivotal role in the practical application of the hydrogen generation system. A stability test was conducted by reusing the NiMo(20)/Cu catalyst in five cycles of hydrogen generation in a 5 wt.% NaBH4 and 5 wt.% NaOH solution at 343 K. At the end of each cycle, the catalyst was washed several times with deionized water to remove any deposited sodium metaborate, which could potentially lead to clogging of the active centers, and then dried and reused. Figure 5 illustrates that the catalytic activity of the NiMo(20)/Cu catalyst remains largely unaltered.
The NiMo(20)/Cu catalyst exhibited a remarkable retention of 94% of its initial catalytic activity for NaBH4 hydrolysis at the end of the fifth cycle. The slight decline in activity may be attributed to the precipitation of a minute quantity of sodium metaborate byproduct on the catalyst surface. This phenomenon results in a reduction in the number of active centers, which ultimately leads to a decline in the catalyst’s activity. Another significant factor contributing to the observed decline in catalyst activity is the surface oxidation of nickel-based catalysts, which results in deactivation. Therefore, the loss of catalytic activity centers, surface oxidation, or the loss of catalytic NiMo(20)/Cu nanoparticles during separation and post-run washing also contributed to a reduction in the hydrogen generation rate.
In conclusion, the favorable activation energies obtained in the present work on the proposed NiMo(1)/Cu, NiMo(5)/Cu, NiMo(10)/Cu, NiMo(15)/Cu, and NiMo(20)/Cu catalysts, which lead to an improvement in catalytic hydrolysis performance, can be attributed to the distinctive characteristics of the surface and the synergistic effects of Ni and Mo.

4. Conclusions

In this study, we successfully prepared NiMo/Cu catalysts with varied molybdenum content, ranging from 1 to 20 wt.%, using the low-cost, simple electroless metal plating method. This method employs morpholine borane as a reducing agent in citrate solutions. The principal findings are as follows:
It was found that the maximum amount of molybdenum, 20%, can be reached with further passivation of the catalytic surface. All of the investigated coatings were observed to be compact, devoid of any cracks or defects, and exhibited catalytic activity for the sodium hydrolysis reaction.
The hydrogen generation rate was found to be dependent on the molybdenum content in the catalysts and the temperature of the working solution. Hydrogen generation rates of between 2.72 and 6.48 L min−1 gcat−1 were achieved at a temperature of 343 K by employing the NiMo/Cu catalysts with varying molybdenum content, from 1 to 20 wt.%, respectively.
The activation energy of the borohydride hydrolysis reaction exhibited a slight increase, from 54.26 to 60.49 kJ mol−1, as the molybdenum content in the NiMo/Cu catalysts was increased from 1 to 20 wt%.
The deposition of two-component coatings combining Ni and Mo has been demonstrated to result in enhanced performance, specifically in terms of hydrogen generation rate and remarkable stability toward catalytic activity for NaBH4 hydrolysis. Consequently, these catalysts represent a promising avenue for hydrogen production from alkaline NaBH4 solutions.

Author Contributions

Conceptualization, Z.S. and G.V.; methodology, V.P., V.J. and J.V.; software, G.V., V.K. and Z.S.; validation, Z.S., L.T.-T. and E.N.; formal analysis, I.S.; investigation, G.V. and I.S.; data curation, Z.S.; writing—original draft preparation, V.K.; writing—review and editing, L.T.-T.; visualization, Z.S.; supervision, E.N. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

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Materials 17 04353 sch001
Scheme 1. The electroless plating process of NiMo coatings on the Cu sheets.
Scheme 1. The electroless plating process of NiMo coatings on the Cu sheets.
Materials 17 04353 sch001
Materials 17 04353 g001
Figure 1. SEM images of prepared (a) NiMo(1), (b) NiMo(5), (c) NiMo(10), (d) NiMo(15), and (e) NiMo(20) catalysts.
Figure 1. SEM images of prepared (a) NiMo(1), (b) NiMo(5), (c) NiMo(10), (d) NiMo(15), and (e) NiMo(20) catalysts.
Materials 17 04353 g001
Materials 17 04353 g002
Figure 2. High-resolution XPS spectra of Ni 2p (a–e), Mo 3d (f–j), and O 1s (k–o) for different NiMo/Cu catalysts.
Figure 2. High-resolution XPS spectra of Ni 2p (a–e), Mo 3d (f–j), and O 1s (k–o) for different NiMo/Cu catalysts.
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Materials 17 04353 g003
Figure 3. XRD patterns of the prepared Ni/Cu and NiMo/Cu catalysts.
Figure 3. XRD patterns of the prepared Ni/Cu and NiMo/Cu catalysts.
Materials 17 04353 g003
Materials 17 04353 g004
Figure 4. H2 generation from 15 mL 5 wt.% NaBH4 + 0.4 wt.% NaOH catalyzed by the NiMo(1)/Cu (a), NiMo(5)/Cu (b), NiMo(10)/Cu (c), NiMo(15)/Cu (d), and NiMo(20)/Cu (e) catalysts at different temperatures. (a′–e′) The corresponding Arrhenius plots.
Figure 4. H2 generation from 15 mL 5 wt.% NaBH4 + 0.4 wt.% NaOH catalyzed by the NiMo(1)/Cu (a), NiMo(5)/Cu (b), NiMo(10)/Cu (c), NiMo(15)/Cu (d), and NiMo(20)/Cu (e) catalysts at different temperatures. (a′–e′) The corresponding Arrhenius plots.
Materials 17 04353 g004
Materials 17 04353 g005
Figure 5. Reusability of the NiMo(20)/Cu catalyst after five cycles in 5 wt.% NaBH4 + 0.4 wt.% NaOH solution.
Figure 5. Reusability of the NiMo(20)/Cu catalyst after five cycles in 5 wt.% NaBH4 + 0.4 wt.% NaOH solution.
Materials 17 04353 g005
Table 1. Composition of the electroless plating solution and the deposition parameters of the catalysts.
Table 1. Composition of the electroless plating solution and the deposition parameters of the catalysts.
CatalystComposition of Plating Solution (mol L−1) and Plating Conditions
NiSO4Na3C6H5O7MBNa2MoO4pHT, °Ct, min
NiMo(1)/Cu0.10.040.20.000175012
NiMo(5)/Cu0.0005
NiMo(10)/Cu0.0008
NiMo(15)/Cu0.0010
NiMo(20)/Cu0.0012
Table 2. The elemental composition and metal loading of the prepared NiMo/Cu coatings analyzed via ICP-OES.
Table 2. The elemental composition and metal loading of the prepared NiMo/Cu coatings analyzed via ICP-OES.
CatalystElement, wt.%Element Loading, µg cm−2Deposition Rate, µm h−1
NiMoNiMoNi + Mo
NiMo(1)/Cu99.200.85646.505.525652.033.65
NiMo(5)/Cu94.605.41525.5030.05555.553.10
NiMo(10)/Cu89.9910.01425.9647.35473.312.65
NiMo(15)/Cu85.4614.54419.0571.30490.352.75
NiMo(20)/Cu80.3119.69391.1595.90487.052.75
Table 3. H2 generation rates and activation energies of the prepared catalysts.
Table 3. H2 generation rates and activation energies of the prepared catalysts.
CatalystEa, kJ mol−1T, Kv, mL min−1v, L min−1 gcat−1
NiMo(1)/Cu54.26303 K0.300.23
313 K0.540.42
323 K0.850.65
333 K2.051.57
343 K3.552.72
NiMo(5)/Cu57.98303 K0.300.27
313 K0.580.52
323 K0.900.81
333 K2.242.01
343 K4.403.96
NiMo(10)/Cu59.19303 K0.330.35
313 K0.660.69
323 K1.041.09
333 K2.552.69
343 K5.175.46
NiMo(15)/Cu58.35303 K0.320.32
313 K0.740.76
323 K1.231.25
333 K2.102.14
343 K5.565.67
NiMo(20)/Cu60.49303 K0.390.40
313 K0.780.81
323 K1.251.28
333 K3.353.44
343 K6.326.48
Table 4. Comparison of activation energy (Ea).
Table 4. Comparison of activation energy (Ea).
CatalystElectrolyteEa, kJ mol−1Reference
CoMoB/C5 wt.% NaBH4 + 7 wt.% NaOH51.00[39]
Ni–Ru/50WX85 wt.% NaBH4 + 5 wt.% NaOH52.73[40]
Ni–Fe–B5 wt.% NaBH4 + 4 wt.% NaOH57.00[33]
Ag-Ni core-shell10 wt.% NaBH4 + 10 wt.% NaOH57.62[38]
Co-Ni/MSAC5 wt.% NaBH4 + 5 wt.% NaOH63.27[32]
CoFeMo/Ni5 wt.% NaBH4 + 0.4 wt.% NaOH66.80[41]
Ni5 wt.% NaBH4 + 0.1 M NaOH67.90[31]
Ni-Co (Alloy)5 wt.% NaBH4 + 5 wt.% NaOH68.84[50]
Ni5 wt.% NaBH4 + 10 wt.% NaOH72.52[37]
NiMo(1)/Cu5 wt.% NaBH4 + 0.4 wt.% NaOH54.26this study
NiMo(5)/Cu5 wt.% NaBH4 + 0.4 wt.% NaOH57.98this study
NiMo(10)/Cu5 wt.% NaBH4 + 0.4 wt.% NaOH59.19this study
NiMo(15)/Cu5 wt.% NaBH4 + 0.4 wt.% NaOH58.35this study
NiMo(20)/Cu5 wt.% NaBH4 + 0.4 wt.% NaOH60.49this study
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Sukackienė, Z.; Valeckytė, G.; Kepenienė, V.; Stalnionienė, I.; Jasulaitiene, V.; Vaičiūnienė, J.; Tamašauskaitė-Tamašiūnaitė, L.; Pakštas, V.; Norkus, E. The Dependence of NiMo/Cu Catalyst Composition on Its Catalytic Activity in Sodium Borohydride Hydrolysis Reactions. Materials 2024, 17, 4353. https://doi.org/10.3390/ma17174353

AMA Style

Sukackienė Z, Valeckytė G, Kepenienė V, Stalnionienė I, Jasulaitiene V, Vaičiūnienė J, Tamašauskaitė-Tamašiūnaitė L, Pakštas V, Norkus E. The Dependence of NiMo/Cu Catalyst Composition on Its Catalytic Activity in Sodium Borohydride Hydrolysis Reactions. Materials. 2024; 17(17):4353. https://doi.org/10.3390/ma17174353

Chicago/Turabian Style

Sukackienė, Zita, Gitana Valeckytė, Virginija Kepenienė, Irena Stalnionienė, Vitalija Jasulaitiene, Jūratė Vaičiūnienė, Loreta Tamašauskaitė-Tamašiūnaitė, Vidas Pakštas, and Eugenijus Norkus. 2024. "The Dependence of NiMo/Cu Catalyst Composition on Its Catalytic Activity in Sodium Borohydride Hydrolysis Reactions" Materials 17, no. 17: 4353. https://doi.org/10.3390/ma17174353

APA Style

Sukackienė, Z., Valeckytė, G., Kepenienė, V., Stalnionienė, I., Jasulaitiene, V., Vaičiūnienė, J., Tamašauskaitė-Tamašiūnaitė, L., Pakštas, V., & Norkus, E. (2024). The Dependence of NiMo/Cu Catalyst Composition on Its Catalytic Activity in Sodium Borohydride Hydrolysis Reactions. Materials, 17(17), 4353. https://doi.org/10.3390/ma17174353

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