Reaction intermediate/product-induced segregation in cobalt–copper as the catalyst for hydrogen generation from the hydrolysis of sodium borohydride

Abstract

Cobalt is the most attractive catalyst for hydrogen generation from the hydrolysis of sodium borohydride, NaBH₄, but its potential is further improved when it is combined with an inactive element like copper. Accordingly, several cobalt–copper catalysts (Co_xCu_1−x, with x as a mole ratio equal to 0, 0.1, 0.25, 0.5, 0.75, 0.9 or 1) were prepared. Under our conditions, Co_0.9Cu_0.1 shows the best performance, being able to complete H₂ evolution in <4 min (vs. <7 min for Co). However, Co_0.9Cu_0.1 is not as stable as expected; after the first cycle, the catalytic activity in terms of the H₂ generation rate halves, and then remains quite constant for additional cycles (up to five under our conditions). XPS measurements show that the surface composition of Co_0.9Cu_0.1 is subject to changes during hydrolysis; the anti-segregation of copper concomitantly takes place with the segregation of cobalt. This is explained through the occurrence of borate-induced segregation, favored due to the well-known strong affinity of cobalt for borate species. In other words, the catalytic activity of cobalt can be improved through combination with copper but, under our conditions, it cannot be stabilized. This is evidenced and discussed in detail herein.

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Introduction

Sodium borohydride, NaBH₄, in alkaline aqueous solution is a liquid-state hydrogen carrier. Since the early 2000s, it has been shown to be an attractive option in the field of hydrogen generation, mainly due to the fact that it generates four molecules of hydrogen under ambient conditions through hydrolysis:^1,2

NaBH₄ + 4H₂O → NaB(OH)₄ + 4H₂ (1)

The as-generated hydrogen is generally assumed to be pure and the by-product is a borate, precisely sodium tetrahydroxyborate, NaB(OH)₄. Conversions of 100% and tunable hydrogen generation rates can be easily achieved in the presence of an accelerator, generally a metal-based catalyst, or sometimes an acid.³ Actually, the catalysis of the hydrolysis of NaBH₄ has been much investigated within the past decade.^1–3

Cobalt is the most attractive metal to be used as the main element of catalysts for the hydrolysis of NaBH₄. It is cheaper and more abundant than noble metals like platinum and ruthenium, and, more interestingly, it can be as active as these noble metals. The catalytic activity of cobalt can be tuned through nano-/micro-sizing, anchoring onto supports like alumina or carbon, adding dopants like boron or phosphorus, and alloying with another transition metal.⁴ However, the role of cobalt in the catalytic reaction and the nature of the catalytically active sites remain unclear; for more details, the reader is invited to go through two reviews dedicated to cobalt in the hydrolysis of NaBH₄.^5,6

One of the weaknesses of cobalt is that it deactivates because of surface passivation, due to the strong adsorption of borates.⁷ Less adsorption may be expected when modifying the electronic structure of cobalt through alloying with a transition metal,⁸ especially an element with low catalytic activity for the hydrolysis of NaBH₄, like copper. To our knowledge, the effect of copper on the stability (or durability) of cobalt over multiple cycles has not been demonstrated yet, even though there are few reports that mainly show improved catalytic activity; this beneficial effect is explained through the prevention of catalytic particle agglomeration or the increase in the electron density of cobalt.^9–11 With another liquid-state hydrogen carrier, i.e., aqueous ammonia borane, NH₃BH₃, cobalt–copper was found to have better stability than pure cobalt. No significant activity loss was observed for cobalt–copper films over five consecutive cycles and this was explained in terms of the synergetic effect of the binary components.¹² Similar results were reported for cobalt–copper nanoparticles encapsulated in the pores of a metal–organic framework and the catalyst stability over five hydrolysis cycles was attributed to geometric effects.¹³ For cobalt–copper nanoparticles supported on carbon or silica, after ten catalytic reuses, a 5–7% activity decrease was found without agglomeration and leaching throughout the runs.^14,15 However, in one report, cobalt–copper was found to be unstable over multiple cycles. For binary nanoparticles made of 80 mol% cobalt and supported on hierarchically porous carbon, the hydrogen generation rate gradually decreased until it was divided by a factor of about 2 after four cycles. According to the authors, this was likely caused by the oxidation of the catalytically active sites during the hydrolysis of NH₃BH₃.¹⁶ In our opinion, this is unlikely since an aqueous solution of NH₃BH₃ is a reducing medium; oxidation might take place post-hydrolysis.

In the present article, cobalt–copper binary particles were targeted for use in the hydrolysis of NaBH₄. Cobalt was doped with copper and the content of the latter was tuned to get the best catalytic activity (i.e., a total conversion of 100% and the highest hydrogen generation rate) over one hydrolysis cycle. Under our conditions, cobalt–copper with a theoretical content of 10% copper showed the best performance, and consequently was tested over five cycles. A loss of catalytic activity occurred, especially in terms of the hydrogen generation rate. The selected catalyst was then characterized in order to better understand the reason(s) for deactivation. This is reported and discussed in detail hereafter.

Experimental

Cobalt nitrate hexahydrate, Co(NO₃)₂·6H₂O (Sigma-Aldrich), copper chloride dehydrate, CuCl₂·2H₂O (Sigma-Aldrich), sodium borohydride, NaBH₄ (Acros Organics), L-ascorbic acid, C₆H₈O₆ (Sigma-Aldrich), and sodium hydroxide, NaOH (Sigma-Aldrich), were used as received. Sodium borohydride was stored and handled in an argon-filled glove box (MBraun M200B, O₂ < 0.1 ppm, H₂O < 0.1 ppm). Ultrapure water (Millipore milli-Q, resistivity > 18.2 MΩ cm) was used.

The cobalt–copper Co_xCu_1−x (x = 0, 0.1, 0.25, 0.5, 0.75, 0.9, or 1, with x as a mole ratio) catalysts were prepared according to a procedure inspired by references.^17,18 For every synthesis, 15 mg of Co_xCu_1−x was the targeted yield. Typically, in a 100 mL two-neck round-bottom flask, the required masses of one or two of the salts Co(NO₃)₂·2H₂O and/or CuCl₂·6H₂O were solubilized in 10 mL of water containing C₆H₈O₆ (with a mole ratio of C₆H₈O₆/(Co + Cu) of 4). The solution was kept under stirring, and ultrasonicated for 5 min. Then, 5 mL of an aqueous solution of NaBH₄ (with a mole ratio of NaBH₄/(Co + Cu) of 2) was added dropwise and under stirring. The solution was kept under stirring for 10 min. A black suspension of particles was formed. The catalyst was finally recovered after centrifugation (3500 rpm, 15 min), washing (3 times with water and ethanol) and drying (80 °C, 2 h).

The Co_xCu_1−x catalysts were screened for the hydrolysis of NaBH₄. The hydrogen evolution experiments were performed as follows. In a glassy tube-like reactor, 15 mg of Co_xCu_1−x and 1 mL of water were introduced and ultrasonicated for 10 min. The reactor was immersed in an oil bath kept at a constant temperature (e.g., 40 °C for the screening tests, and 30–60 °C for the kinetic studies) and connected to a colored-water filled inverted burette. To start the hydrogen generation from the hydrolysis of NaBH₄, i.e., the catalytic reaction, 1 mL of alkaline solution (NaOH, 0.5 M) of NaBH₄ (120 mg) was injected into the reactor. Hydrogen evolution was maintained for 60 min and was stopped even if hydrolysis was not complete (this was the case for two catalysts, Cu and Co_0.1Cu_0.9; cf. sub-section 3.1). Under our conditions, the mole ratio of H₂O/NaBH₄ was 17.5 and the weight ratio of NaBH₄/Co_xCu_1−x was 8. At the beginning of the work, the temperature for the screening tests was set at 40 °C to allow fast successive experiments. For the stability experiments (with 5 successive cycles; details are given in sub-section 3.2), similar experimental conditions were used but, on the basis of the results of the kinetic study, the temperature was 30 °C. Through decreasing the hydrolysis temperatures, slower kinetics were targeted, because deactivation (poisoning), if any, is more pronounced at lower temperatures (slower adsorption/desorption rates onto the catalyst surface).

After screening, two samples of the best Co_xCu_1−x catalyst, respectively in fresh and used states, were characterized using X-ray diffraction (XRD; X'Pert Pro diffractometer, using copper Kα radiation with λ = 1.5406 Å; the patterns were analyzed with the help of the software X'Pert HighScore), scanning electron microscopy (SEM; Hitachi S4800 microscope), and X-ray photoelectron spectroscopy (XPS; ESCALAB 250 from Thermo Electron with a monochromatic source, Al Kα ray at 1486.6 eV; with an analyzed surface of 400 μm diameter; the binding energies (BE) of all core levels are referenced to the C–C of C 1s carbon at 284.8 eV).

Results and discussion

Screening of the Co_xCu_1−x catalysts

The hydrogen evolution curves obtained from the Co_xCu_1−x catalysts are reported in Fig. 1. For clarity, the curves of Co, Co_0.9Cu_0.1, Co_0.75Cu_0.25 and Co_0.5Cu_0.5 and those of Co, Co_0.25Cu_0.75, Co_0.1Cu_0.9 and Cu are plotted on separate graphs. Under our experimental conditions, the catalyst made of pure Cu is not active in the hydrolysis of NaBH₄, whereas pure Co is active, with a total conversion of 100% reached within 7 min. The hydrogen evolution curve of Co is typical, with an induction period (slow hydrogen generation rate) at the beginning of hydrolysis due to surface activation (through the reduction of oxidized cobalt species), followed by fast hydrolysis (ca. 250 mL of H₂ evolved in 4 min). These observations are consistent with the catalytic features of these pure metals, as reported in previous reports.^1–6

Fig. 1 Screening of Co, Co_0.9Cu_0.1, Co_0.75Cu_0.25, Co_0.5Cu_0.5, Co_0.25Cu_0.75, Co_0.1Cu_0.9 and Cu: hydrogen evolution curves from the hydrolysis of NaBH₄ performed at 40 °C (15 mg of catalyst, 120 mg of NaBH₄, 1 mL of alkaline water, [NaOH] = 0.5 M). For clarity, two graphs with different time scales are proposed.

The addition of cobalt to copper leads to more active catalytic particles. At 30 °C, Co_0.1Cu_0.9 is negligibly active, with ca. 15 mL of H₂ generated in 60 min, but Co_0.25Cu_0.75 is much more active, with the reaction completed in 30 min. With a further increase in the Co content, better catalytic results were obtained. With Co_0.5Cu_0.5 and Co_0.75Cu_0.25, the hydrolysis reactions are completed within 9 and 7 min, respectively. The best result can be observed for Co_0.9Cu_0.1, with 4 min being enough to generate the 4 moles of H₂ per mole of NaBH₄. This is even better than pure Co. Unlike for Co, there is no induction period with these bimetallic catalysts.^5,6

The previous results confirm that combining cobalt with copper leads to synergetic effects, via interactions between both metals, and thus to better reactivity and catalytic activity.^9–16 Electronic and/or geometric effects could rationalize such reactivity changes. For example, theoretical calculations have predicted an up-shift in the d band center of cobalt in the presence of copper (i.e., a change in the transition state energy) as well as strong segregation of the latter (i.e., surface enrichment by copper).⁸ It then seems likely that copper dilutes cobalt with the formation of more small cobalt-based active sites, and copper allows for better reducibility of the oxidized surface cobalt. The consequences are a very short/negligible induction period and faster hydrogen generation kinetics.

From screening, Co_0.9Cu_0.1 and Co were selected and additional hydrogen evolution experiments were performed using both catalysts at 30, 50 and 60 °C, while keeping the other experimental conditions identical. The curves allowed for the determination of the hydrogen generation rates (denoted as r), which were used along with the Arrhenius equation to calculate the respective apparent activation energy values. The results are shown in Fig. 2 and 3. They were found to be 16.5 and 43 kJ mol⁻¹ for Co_0.9Cu_0.1 and Co, respectively, indicating the beneficial effect of copper when combined with cobalt. A direct comparison with data available in the open literature is generally irrelevant because of discrepancies in the experimental conditions, but it may be indicative to report few examples. With respect to Co, the energy of 43 kJ mol⁻¹ favorably compares with the values generally reported for cobalt-based catalysts (40–60 kJ mol⁻¹).⁴ However, the energy found for Co_0.9Cu_0.1 is low in comparison to the data available in the open literature;⁴ similar values were otherwise reported for the hydrolysis of NH₃BH₃ using bimetallic FeCo nano-alloys.¹⁹

Fig. 2 Co_0.9Cu_0.1: (left) hydrogen evolution curves from the hydrolysis of NaBH₄ performed at 30, 40, 50 and 60 °C (15 mg of catalyst, 120 mg of NaBH₄, 1 mL of alkaline water, [NaOH] = 0.5 M); (right) determination of the apparent activation energy through plotting ln(r) as a function of 1/T (Arrhenius equation).

Fig. 3 Co: (left) hydrogen evolution curves from the hydrolysis of NaBH₄ performed at 30, 40, 50 and 60 °C (15 mg of catalyst, 120 mg of NaBH₄, 1 mL of alkaline water, [NaOH] = 0.5 M); (right) determination of the apparent activation energy through plotting ln(r) as a function of 1/T (Arrhenius equation).

Stability of Co_0.9Cu_0.1 over 5 cycles

In the first approach, the stability of Co_0.9Cu_0.1 over 5 cycles at 30 °C was studied, where after each cycle the catalyst was kept in the spent fuel (10 min) and then a new batch of NaBH₄ (120 mg in 1 mL of alkaline solution) was added. The results are reported in Fig. 4. The catalytic activity of Co_0.9Cu_0.1 is gradually degraded. The total conversion reaches 89% after the 5^th cycle, and the hydrogen generation rate decreases from 78.5 to 18 mL min⁻¹. Likely explanations may be catalyst deactivation from surface poisoning (the adsorption of borate by-products),^5–7 or agglomeration of the metallic particles.^9–11 Another explanation may be changes in solution features (a higher volume, more and more borate by-products, increased viscosity, etc.); it is known that such variations may negatively impact the hydrolysis reaction.^1–3,20

Fig. 4 Stability of Co_0.9Cu_0.1 over 5 hydrolysis cycles at 30 °C: after each cycle the catalyst was kept in the spent fuel (10 min) and then a new batch of NaBH₄ (120 mg in 1 mL of alkaline solution) was added. For clarity, the successive curves are plotted every 30 min (x axis).

In a second approach, the stability of Co_0.9Cu_0.1 over 5 cycles at 30 °C was studied, where after each cycle the catalyst was extracted from the spent fuel (using centrifugation), washed with water and ethanol (to remove most of the surface-adsorbed by-products),⁷ and dried (80 °C, 1 h). Such an experimental approach is preferable to avoid catalytic activity degradation due to changes in the features of the hydrolysis medium. The results are reported in Fig. 5. The catalyst is clearly stable in terms of total conversion, with 100% conversion after each cycle. As the solution features were identical for each cycle in this second approach, it can reasonably be concluded that the decrease in the total conversion over the series of tests in the first approach was due to changes in solution features, like the increasing amount of borates in the hydrolysis medium.

Fig. 5 Stability of Co_0.9Cu_0.1 over 5 hydrolysis cycles at 30 °C: after each cycle, the catalyst was extracted from the spent fuel (centrifugation), washed with water and ethanol, and dried at 80 °C for 1 h. For clarity, the successive curves are plotted every 15 min (x axis).

In Fig. 5, the activity of the catalyst and the hydrogen generation rate are affected: after the first cycle, it decreases from 78.5 to 38 mL min⁻¹, and then is almost constant for the other cycles (31–36 mL min⁻¹). In other words, the addition of copper does not prevent cobalt from losing catalytic activity over very few hydrolysis cycles under our experimental conditions. According to the existing literature,^5–7,9–11 this may be explained through surface-adsorbed borate by-products (inducing poisoning), or the agglomeration of the metallic particles. As discussed in the previous paragraph, changes in solution features are unlikely. There may also be other reasons. This is discussed hereafter.

Characterization of Co_0.9Cu_0.1

To better understand the reasons behind the aforementioned catalyst deactivation, two samples of Co_0.9Cu_0.1 were prepared in high amounts, to be characterized in the fresh and used states (denoted F–Co_0.9Cu_0.1 and U–Co_0.9Cu_0.1). The former state corresponds to the freshly prepared catalyst. The latter state corresponds to the catalyst after use in hydrolysis for 5 cycles; it was washed and dried before characterization. For both, SEM observations (Fig. 6) showed agglomerated particles, which could consist of metallic particles embedded in a matrix made of borates as reported in ref. 21.

Fig. 6 SEM images of F–Co_0.9Cu_0.1 (left) and U–Co_0.9Cu_0.1 (right), with the scale bar being 300 nm.

The XRD patterns are reported in Fig. 7. As reported for many cobalt-based catalysts reduced by a boron hydride (i.e., NaBH₄, and NH₃BH₃),^5,6,21 Co_0.9Cu_0.1, which is predominantly made of cobalt, is amorphous. With particular attention, two diffraction peaks of low intensity can be seen at the 2θ values of 43.4° and 50.5°; they may be ascribed to the (111) and (200) planes of metallic copper (ref. 01-085-1326).

Fig. 7 XRD patterns of F–Co_0.9Cu_0.1 (fresh state) and U–Co_0.9Cu_0.1 (used state). The broad peak at 2θ = 12–14° is due to the Kapton film we generally use to protect samples from air and moisture.

The fresh and used Co_0.9Cu_0.1 samples were analyzed using XPS (Fig. 8). For attribution, the binding energy (BE) values were compared to databases²² and selected articles from the open literature.^5,6,21 The presence of both metals was confirmed. The Co 2p binding energies (BEs) are typical of Co(II). The Co 2p3/2 BE of both F–Co_0.9Cu_0.1 and U–Co_0.9Cu_0.1 is 781.4 eV, and can be attributed to Co(OH)₂. The Cu 2p peaks can be ascribed to Cu or Cu₂O, which generally display similar spectra over the range of BEs from 930 to 960 eV.^22,23 Boron was found on the surface of both samples and the B 1s BE at 191.9 eV is attributed to boron oxides/borates (formed through the hydrolysis of NaBH₄ during the reduction and hydrolysis steps). Note that for such cobalt-based catalysts, the borates are the matrix that are embedded in and they imply the agglomeration of the metallic particles; this is illustrated in Fig. 9 for U–Co_0.9Cu_0.1.^5,6,21 The O 1s BE is 531.7 eV for both materials, consistent with B–O and Co–O environments. To sum up these XPS results, one can state that F–Co_0.9Cu_0.1 and U–Co_0.9Cu_0.1 mainly consist of Co(OH)₂, Cu and borates.

Fig. 8 XPS spectra of F–Co_0.9Cu_0.1 (blue lines) and U–Co_0.9Cu_0.1 (red lines), with focus on the regions of Co 2p, Cu 2p, B 1s and O 1s.

Fig. 9 SEM image of U–Co_0.9Cu_0.1, with the scale bar being 167 nm. The metallic particles (black) are embedded in a matrix made of borates.

The spectra focusing on the Cu 2p region also shows a peak at ca. 927 eV, belonging to Co 2s (Fig. 8). The relative intensity between the peaks of Cu 2p3/2 and Co 2s decreases after hydrolysis, suggesting more cobalt after hydrolysis. The surface atomic concentrations of the elements were determined from the photoelectron peaks areas, using the atomic sensitivity factors.²⁴ For the fresh sample, it was found that the formula is Co_0.92Cu_0.08, in good agreement with the target Co_0.9Cu_0.1. However, for the used sample, a decrease in the copper content was found, with a formula of Co_0.98Cu_0.02. This indicates a compositional change after reaction; the catalyst surface has been enriched with cobalt. Given that, unlike copper, cobalt is known to have strong affinity for the hydrolysis intermediates (hydroxyborate) and products (borates), the surface enrichment may be explained through borate-induced segregation.

The loss in catalytic activity of Co_0.9Cu_0.1 can thus be explained as follows. Because of the strong affinity of cobalt for hydroxyborates and borates, cobalt segregates and enriches the particle surfaces when put into contact with aqueous NaBH₄, whereas copper anti-segregates. Consequently, Co_0.9Cu_0.1 acts like pure Co, that is, the beneficial effect of copper is lost, at least partially, leading to deactivation over multiple cycles due to borate adsorption. In other words, the catalytic activity of cobalt can be improved through combination with copper, but under our conditions the catalytic activity of cobalt cannot be stabilized.

Conclusions

Cobalt is a good candidate for being the main element of catalysts for the hydrolysis of NaBH₄, but its potential is further improved when it is combined with copper. Under our conditions, we demonstrated that Co_0.9Cu_0.1 is more active than Co, with the hydrogen evolution being completed in less than 4 and 7 min, respectively. Furthermore, significantly different apparent activation energies were found (16.5 and 43 kJ mol⁻¹, respectively). These results confirm the beneficial effect of copper on the catalytic activity of cobalt. The addition of copper is also expected to improve the stability of cobalt through reducing or avoiding deactivation, generally due to strongly adsorbed borate by-products. This is in fact the principal expectation from copper. However, under our conditions, Co_0.9Cu_0.1 does not show stable activity over several hydrolysis cycles. The hydrogen generation rate decreases, being divided by a factor of 2 after the first hydrolysis. Like pure Co, Co_0.9Cu_0.1 deactivates when reused for the hydrolysis of NaBH₄. To explain the reason for such deactivation, Co_0.9Cu_0.1 was characterized in its fresh and used states. XPS measurements showed that the surface composition of Co_0.9Cu_0.1 is subject to change. Quantification of the surface elements showed a stoichiometry of Co_0.92Cu_0.08 for the fresh sample, but Co_0.98Cu_0.02 for the used one. It is shown, for the first time, that anti-segregation of copper takes place concomitantly with segregation of cobalt upon hydrolysis. These surface phenomena then modify the catalytic surface. The surface enrichment of cobalt is much more likely, due to borate-induced segregation, which is clearly detrimental to the synergetic effect achieved in the freshly-prepared catalyst. In conclusion, the catalytic activity of cobalt can be improved through combination with copper, but under our conditions it cannot be stabilized. Further efforts are required to stabilize cobalt-based multimetallic catalysts for the hydrolysis of NaBH₄. Alternatives to copper, the addition of a third element to Co_0.9Cu_0.1, and/or heat treatments may be a few of the first paths to be explored.

Acknowledgements

The authors acknowledge the University of Monastir for H. K.'s scholarship. H. K. acknowledges Mr Salem Ould-Amara (University of Montpellier) for his time and assistance.

Notes and references

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