HUANG et al.

Abstract

In spite of the numerous advances in the development of H₂ and O₂ evolutions upon water splitting, the separation of H₂ from O₂ still remains a severe challenge. Herein, the novel dual-functional nanocatalysts Pd/carbon nanosphere (CNS), obtained via immobilization of ultrafine Pd nanoparticles onto CNS, are developed and employed for both selective H₂ generation from HCOOH dehydrogenation and O₂ evolution from H₂O₂ decomposition. In these reactions, the highest activities for Pd/CNS-800 (i.e., calcinated at 800°C) are 2478 h⁻¹ and 993 min⁻¹ for H₂ and O₂ evolution, respectively. The highly efficient and selective “on–off” switch for selective H₂ generation from HCOOH is successfully realized by pH adjustment. This novel and highly efficient nanocatalyst Pd/CNS-800 not only provides new approaches for the promising application of HCOOH and H₂O₂ as economic and safe H₂ and O₂ carriers, respectively, for fuel cells, but also promotes the development of “on-off” switch for on-demand H₂ evolution.

INTRODUCTION

Since the H₂–O₂ fuel cell was first discovered by Grove in 1839,¹ it has been widely deemed as the most potential portable and auxiliary power generator because of its merits of green, simple structure, wide operating temperature range, high specific energy, and high energy conversion efficiency. Although H₂ and O₂ are easily produced by water electrolysis,^2–5 it is difficult to find ideal catalysts for this reaction and to separate H₂ from O₂ with membranes. Therefore, it is highly desirable to explore efficient and economic methods for H₂ and O₂ evolutions, separately.^6–10

The exploration of efficient and safe production, storage, and transportation (especially in long term) of H₂, an ultra-low density and awfully low-boiling point gas,^11–15 is a serious challenge.^16–20 Consequently, numerous inorganic and organic compounds have been proposed as hydrogen carriers, such as methanol,²¹ ammonia,^22,23 methane,²⁴ ammonia borane,^3,25–29 hydrazine hydrate,³⁰ dimethylaminoborane,³¹ sodium borohydride,^3,32–37 tetrahydroxydi-boron,^38–41 tetramethyldisiloxane,⁴² hydrazine borane,⁴³ and formic acid (FA).^44–46 Among them, FA, the main product of biomass manufacture by hydrolysis or oxidation of cellulose with high yields,^47–50 has become one of the most attractive hydrogen carriers due to its excellent hydrogen content (4.4 wt%), high volumetric hydrogen storage density of 53 g/L, nontoxicity, ease of portability, regeneration from CO₂ hydrogenation, and liquid stability at room temperature. As a result, a large variety of heterogeneous and homogeneous catalytic systems, such as those involving Pd, Au, and Pt, have been continuously designed and developed for selective H₂ generation from HCOOH (Equation 1),⁵¹ meanwhile suppressing CO production from HCOOH dehydration.⁵² [Image Omitted. See PDF]

In 2022, the Zhang et al.⁵³ first designed a green and facile strategy to synthesize the catalyst Pd–WO_x/(P)NPCC via the immobilization of Pd–WO_x nanoheterojunctions onto phosphate-mediated N-doped porous carbon cages (NPCC); these authors used this catalyst for H₂ production upon FA dehydrogenation. Remarkably, there are only a few available reports concerning the useful “on–off” switch for on-demand H₂ generation from HCOOH, which hence remains a serious challenge.^54,55 In 2020, a light-active binuclear iridium catalyst was successfully employed as a light “on–off” switch for on-demand H₂ generation from HCOOH by the Sofue et al.⁵⁶ The H₂ production was started by light irradiation, and it was suspended by switching the light off.

Recently, the O₂ evolution upon hydrogen peroxide (H₂O₂) decomposition (Equation 2) aroused wide attention due to its widespread applications in miniaturized spacecraft, submarines, hydrogen fuel cells, and so forth.^57–59 [Image Omitted. See PDF]

As part of our recent studies on the “on–off” switch for H₂ generation,^60–63 herein we report the design and synthesis of efficient Pd/carbon nanosphere (CNS) nanocatalysts by immobilization of ultrafine Pd nanoparticles (NPs) onto the surface of CNS, for the selective H₂ generation from FA dehydrogenation. In particular, highly uniform CNSs were obtained by a solution-based process and pyrolysis treatment using 3-aminophenol and formaldehyde precursors.⁶⁴ Then, the optional nanocatalyst Pd/CNS-800, for which Pd/CNS was heated at 800°C, presented the highest catalytic activity for H₂ generation from HCOOH, with the highest turnover frequency (TOF) of 2478 h⁻¹ and 100% H₂ selectivity. Next, we minutely investigated the chemical kinetics of the nanocomposite, Pd/CNS-800, CO₂ capture experiment, tandem reactions, catalyst recycling experiment, and “on–off” switch for H₂ production upon FA dehydrogenation. In addition, the nanocomposite Pd/CNS-800 was also successfully employed as an efficient catalyst for O₂ evolution upon H₂O₂ decomposition.

RESULTS AND DISCUSSION

First, the CNS precursor was synthesized by polymerization of 3-aminophenol and formaldehyde in the presence of ammonium hydroxide at 30°C (Scheme 1).⁶⁴ Then, the CNSs formed upon the addition of acetone, which was used to dissolve the interior part. Next, CNS-700, CNS-800, and CNS-900 were prepared upon carbonization of the CNSs for 6 h at 700°C, 800°C, and 900°C, respectively. Finally, the Pd nanohybrids Pd/CNS-700, Pd/CNS-800, and Pd/CNS-900, respectively, were obtained upon dissolving K₂PdCl₄ and the corresponding CNS in deionized water, followed by fast reduction using NaBH₄ at 30°C.

Scheme 1. Synthesis of the nanocatalyst Pd/CNS-800. CNS, carbon nanosphere.

To study the nanostructures and morphologies of CNS, CNS-800, Pd/CNS-700, Pd/CNS-800, and Pd/CNS-900, their transmission electron microscope (TEM) images were recorded and are represented in Figure 1. Figure 1A shows that the CNSs present a uniform nanosphere structure with an average size of 194 nm (Figure S1). After carbonization, the uniform nanosphere structure was preserved in CNS-700 (173 nm; Figures S2 and S3), CNS-800 (164 nm; Figure 1B and Figure S4), and CNS-900 (155 nm; Figures S5 and S6), and the size of the CNSs decreased with the increased temperature of calcination. As displayed in Figure 1C–F, the Pd NPs are homogeneously distributed on the surface of the CNSs. Pd/CNS-800 (3.1 nm; Figure S7) is slightly smaller than Pd/CNS-700 (4.5 nm; Figure S8) and Pd/CNS-900 (5.8 nm; Figure S9). Then, Fourier-transform infrared spectroscopy (FTIR), Raman spectra, and X-ray powder diffraction (XRD) were further employed to investigate the microscopic structures of Pd/CNS-700, Pd/CNS-800, and Pd/CNS-900. In the FTIR spectrum of the CNSs (Figure 2A), the absorption peaks at 620, 708 and 875 cm⁻¹, 1109 and 1185 cm⁻¹, 1446 and 1509 cm⁻¹, 1622 cm⁻¹, 2854 and 2924 cm⁻¹, and 3362 cm⁻¹ correspond to the stretching vibrations of C–H, C–O–C, C═C, –NH₂, –CH₂–, and –OH, respectively.⁶⁵ These absorption peaks, except for the –OH group, were comparatively weaker in Pd/CNS-700, Pd/CNS-800, and Pd/CNS-900, indicating that the surface functional groups were carbonized at high temperature. As shown in Figure 2B, the strong characteristic peaks centered at around 1602 and 1342 cm⁻¹ are attributed to the graphitic carbon (G-band) and disordered carbon (D-band), respectively.⁶⁶ The values of I_G/I_D were 0.89, 0.92, and 1.0 for Pd/CNS-700, Pd/CNS-800, and Pd/CNS-900, respectively, indicating that the graphitic carbon was more readily obtained at higher calcination temperature. In Figure 2C, the diffraction peak at 2ϴ = 24° corresponds to the d-spacing of graphitic carbon (002) (JCPDS Card No. 75-0444),⁶⁷ and meanwhile, Pd (111), Pd (200), and Pd (220) were recorded in Pd/CNS-700, Pd/CNS-800, and Pd/CNS-900 (JCPDS Card No. 87-00643),⁶⁸ suggesting that the Pd NPs had been immobilized on the CNS surface. As a consequence, the comparison of H₂ generation from FA dehydrogenation with n_FA/n_SF of 1:3 catalyzed by Pd/CNS-700, Pd/CNS-800, and Pd/CNS-900 was investigated and shown in Figure 3A.

Figure 1. TEM images of (A) CNS, (B) CNS-800, (C) Pd/CNS-700, (D,E) Pd/CNS-800, and (F) Pd/CNS-900. CNS, carbon nanosphere; TEM, transmission electron microscope.

Figure 2. (A) FTIR, (B) Raman spectra, and (C) XRD of Pd/CNS-700, 800 and 900. CNS, carbon nanosphere; FTIR, Fourier-transform infrared spectroscopy; XRD, X-ray diffraction.

Pd/CNS-800 exhibited a higher catalytic performance, with the superior TOF of 2478 mol(H₂) mol_Pd⁻¹ h⁻¹ and 100% H₂ selectivity than Pd/CNS-700 (949 h⁻¹) and Pd/CNS-900 (703 h⁻¹) in H₂ generation due to the smallest size of the PdNPs in Pd/CNS-800. Then, the H₂ generations were carried out under different molar ratios FA/SF with the optimal Pd/CNS-800 nanocatalyst. As shown in Figure 3B, the H₂ production rate increased a lot when sodium formate (SF) was added to the FA solution. Moreover, 0.02 mmol of Pd with various amounts of CNS-800 (from 20 to 50 mg) were also tested for H₂ generation upon FA dehydrogenation. Figure S10 shows that the amount of CNS-800 has a great influence on the catalytic properties. In particular, 40 mg of CNS-800 provided the highest TOF (2478 h⁻¹) in FA dehydrogenation. Hence, Pd/CNS-800 and n_FA/n_SF of 1:3 were chosen as the optimal conditions for further chemical kinetics, CO₂ capture experiment, dehydrogenation-hydrogenation tandem reaction, catalyst recycling experiment, and the “on–off” switch of H₂ production.

Figure 3. Comparison of H2 generation from FA dehydrogenation with nFA/nSF of 1:3 catalyzed (A) by Pd/CNS-700, Pd/CNS-800, and Pd/CNS-900; (B) by 2 mol % Pd/CNS-800 with nFA/nSF = 1:0, 1:1, 1:2, 1:3, and 1:4 at 60°C. CNS, carbon nanosphere; FA, HCOOH; SF, HCOONa.

To search why Pd/CNS-800 was so efficient, its Brunauer–Emmett–Teller (BET), high-resolution TEM (HRTEM), high-angle annular dark-field scanning transmission electron microscope, energy dispersive X-ray spectrometry (EDX), and X-ray photoelectron spectroscopy (XPS) were further investigated. Figure S11 suggests that Pd/CNS-800 presents a characteristic mesoporous structure. The BET surface area, total pore volume, and mean pore diameter of Pd/CNS-800 are 703.14 m²/g, 0.59 cm³/g, and 3.34 nm, respectively.

Inductively coupled plasma-optical emission spectrometry indicated that the Pd NPs were mostly loaded onto CNS-800 (4.4 wt% Pd), which was just a little less than the theoretical value (5.05 wt%). As displayed in Figure S12, the crystal lattice spacing of 0.224 nm, distinctly observed in the HRTEM of Pd/CNS-800, corresponds to Pd (111).⁶⁹ As shown in Figure 4A–E, EDX mapping shows that Pd is uniformly distributed on the surface of CNS-800, and meanwhile, the CNSs are doped with N atoms. In Figure 4F, the coexistence of Pd, C, and N elements in Pd/CNS-800 is also verified by the EDX sum spectrum. Then, the EDX element compositional line scanning profiles of a Pd NP (yellow line in Figure 5A) were recorded and are shown in Figure 5B. It is clear that Pd NPs were embedded into the CNS. This finding further confirms that ultrafine Pd NPs have been successfully immobilized onto the surface of the CNS. In addition, the electronic effects and chemical states of Pd/CNS-800 have been analyzed by XPS in Figure 6. As displayed in Figure 6A, Pd, C, N, and O signals emerged in the full XPS spectrum of Pd/CNS-800. As shown in Figure 6B, the Pd 3d spectrum of Pd/CNS-800 with four peaks located at 336.2 and 341.5 eV and 337.7 and 343.2 eV are assigned to Pd (0) resp. Pd (II),⁷⁰ suggesting that PdNPs have been partly oxidized to Pd (II) by air. In Figure 6C, the C 1s spectrum of Pd/CNS-800 was deconvoluted into four peaks located at 290, 286.8, 285.4, and 284.6 eV fitting to O–C═O, C═O, C–N, and C–C/C═C, respectively. The N 1s spectrum of Pd/CNS-800 with three peaks centered at 404.6 401.0, and 398.5 eV corresponded to graphitic N, pyrrolic N resp. pyridinic N. In summary, these results show that as-obtained CNS-800 possesses a uniform mesoporous nanosphere structure with abundant surface –OH, –NH₂ and graphitic, pyrrolic and pyridinic N groups, which are favorable for Pd NP stabilization (Figure 6D).

Figure 4. (A) HAADF-STEM image; (B) combined (Pd, N, and C), (C) Pd, (D) C, and (E) N EDX compositional mapping; (F) sum spectrum of Pd/CNS-800. CNS, carbon nanosphere; EDX, energy dispersive X-ray spectrometry; HAADF-STEM, high-angle annular dark-field scanning transmission electron microscope.

Figure 5. (A) STEM image of Pd/CNS-800; (B) EDX spectrum of Pd/CNS-800 indicating the distributions of Pd along line scan outline. CNS, carbon nanosphere; EDX, energy dispersive X-ray spectrometry; STEM, scanning transmission electron microscope.

Figure 6. XPS of (A) full spectrum, (B) Pd 3d, (C) C 1s, and (D) N 1s in Pd/CNS-800. CNS, carbon nanosphere; XPS, X-ray photoelectron spectroscopy.

Detailed physical characterizations confirm that ultrafine Pd NPs have been successfully immobilized onto the surface of CNSs in Pd/CNS-800. It seems that graphitic C or N in CNS-800 serves as an electron donor to Pd atoms, increasing their surface electron density, which facilitates the cleavage of the C–H bond in FA dehydrogenation.

In this study, chemical kinetics (including initial FA concentration, Pd/CNS-800 dosage, and reaction temperature) of FA dehydrogenation were studied (Figure 7). First, the dehydrogenation of FA was performed with different initial FA amount ranging from 0.5 to 1.25 mmol. A linear relationship between the H₂ generation rate and initial FA concentration, where the slope is 0.93, is shown in Figure 7A. Then, the dehydrogenation of FA was conducted in the presence of various Pd/CNS-800 catalyst dosages ranging from 1.0 to 2.5 mol% (Figure 7B).

Figure 7. Plots of produced gas volume versus time for H2 generation from FA dehydrogenation under (A) various concentrations of FA, (B) different amounts of catalyst, and (C) various reaction temperatures; (D) comparison of H2 generation from FA dehydrogenation catalyzed by Pd/CNS-800 and Pd/C; (E) H2 generation with and without NaOH trap; (F) GC spectra of the generated gas mixture from FA over Pd/CNS-800 at 60°C. CNS, carbon nanosphere; FA, HCOOH; GC, gas chromatograms.

There is a positive correlation (slope: 1.0) between the H₂ evolution rate and Pd/CNS-800 catalyst dosages. The FA dehydrogenation is first-order kinetics for both initial FA concentration and Pd/CNS-800 catalyst dosage. Finally, FA dehydrogenation was carried out at various reaction temperatures (from 303 to 333 K) in Figure 7C. The H₂ generation rate increases with the increase of the reaction temperature, and the TOF value increases from 413 to 2478 mol(H₂) mol_Pd⁻¹ h⁻¹ (Figure S13). The apparent activation energy (E_a) of Pd/CNS-800 in FA dehydrogenation is 39.4 kJ/mol, as calculated using the Arrhenius equation. In Figure 7D, Pd/CNS-800 shows the highest TOF of 2478 h⁻¹ in FA dehydrogenation, while the TOF of commercial Pd/C was only 915 h⁻¹. Then, XPS of Pd/C has also been measured for comparison with Pd/CNS-800. As shown in Figure S14, there is a slight shift in the Pd 3d of Pd/CNS-800 as compared to that of Pd/C, indicating that charge transfer from N sites to Pd occurs via orbital hybridization between N and Pd.^71–73 It appears that graphitic C or N in CNS-800 is serving as an electron donor to the Pd atom and increases its surface electron density, facilitating the cleavage of the C–H bond upon FA dehydrogenation. To test the selectivity of H₂ production upon the FA dehydrogenation, the gas mixture (H₂ and CO₂) released from HCOOH dehydrogenation flowed into an NaOH solution for the selective absorption and removal of CO₂. Figure 7E indicates that the volume of generated gas had almost fallen by half with the NaOH trap, showing that the gas released from HCOOH dehydrogenation was a mixture of CO₂ and H₂ with a mole ratio of 1:1. As shown in Figure 7F, the gas chromatograms (GCs) confirmed that the gas mixture released from HCOOH dehydrogenation was CO₂ and H₂, with a mole ratio of nearly 1:1. The CO₂ capture experiment and GC results verified that CO-free H₂ generation upon HCOOH dehydrogenation catalyzed by Pd/CNS-800 was successfully designed and developed for fuel cell power systems.

The recycling tests of the Pd/CNS-800 catalyst for H₂ production upon the FA dehydrogenation are illustrated in Figure S15. After H₂ production was finished, the Pd/CNS-800 catalyst was centrifuged, filtered, washed, and recycled for the next run with another fresh FA–SF aqueous solution. The heterogeneous Pd/CNS-800 catalyst was successfully recycled at least five runs without any significant decrease in activity. Then, the morphology and structure of the fifth recycled Pd/CNS-800 catalyst were measured by XRD, FTIR, TEM, and XPS. As shown in Figures S16 and S17, the fifth recycled Pd/CNS-800 catalyst remained with same structure as the fresh catalyst, which is demonstrated by XRD and FTIR. In Figure S18, the mean size of the fifth recycled PdNPs only grew from 3.10 to 3.68 nm (Figure S19). As displayed in Figure S20, it is clear that only a very small amount Pd (0) of Pd/CNS-800 catalyst have been oxidized into Pd (II) in the lifetime of HCOOH exposure. These results suggest that Pd/CNS-800 is an efficient, heterogeneous, and durable catalyst in FA dehydrogenation.

The practical application of H₂ generation upon HCOOH dehydrogenation is not only for efficient production, long-term storage, and safe and economic transportation of H₂ gas but also for its in situ utilization in the dehydrogenation–hydrogenation tandem reaction.⁷⁴ As demonstrated in Scheme 2, the tandem reaction of norbornene hydrogenation with the generated H₂ upon HCOOH dehydrogenation was performed in a dual-chamber reactor. The desired tandem reaction product, bicyclo[2.2.1]heptane, was obtained in 99% yield (Figure S21), demonstrating H₂ evolution upon HCOOH dehydrogenation.

Scheme 2. Tandem reaction for norbornene hydrogenation. CNS, carbon nanosphere; FA, HCOOH.

A mechanistic proposal of Pd/CNS-800 catalyzed HCOOH dehydrogenation, derived from the literature,^75,76 is shown in Figure 8. Originally, the HCOOH molecule is adsorbed on the Pd NP surface of Pd/CNS-800 and protonates it, forming a Pd–H bond noted H* (I). Subsequently, CO₂* is generated by β-hydride elimination of coordinated formate (II, III). Finally, H₂ is released by the reductive elimination of two hydride ligands H* from the Pd NP surface.

Figure 8. Mechanism of H2 production from FA dehydrogenation. CNS, carbon nanosphere; FA, HCOOH.

To avoid the safety risks and high cost in the process of long-term storage and transportation of H₂ gas, it is still a serious challenge to design and develop an efficient “on–off” switch for on-demand H₂ generation from HCOOH. As illustrated in Figure 9, H₂ generation from HCOOH dehydrogenation over the catalyst Pd/CNS-800 was successfully switched “on” and “off” by pH adjustment. To be specific, the H₂ generation was immediately suspended by adding NaOH solution because FA had been turned into SF. Then, the H₂ generation was reactivated by adding an H₂SO₄ solution by reason of FA reconstruction. However, there is a gradual decline in the H₂ generation rate after each “on–off” switch due to the dilution of the solution.^77–79

Figure 9. “On–off” switch of H2 production from HCOOH dehydrogenation. Reaction conditions: 2 mmol of HCOOH, 6 mmol of HCOONa, and 0.01 mmol of Pd/CNS-800 in water (3 mL) at 60°C. CNS, carbon nanosphere.

In addition, the catalytic performance of the Pd/CNS-800 nanocatalyst has also been compared with recent reports for selective H₂ generation from FA dehydrogenation.^{49,73,80–92} Figure 10 shows that the catalytic activity of Pd/CNS-800 nanocatalyst is fairly acceptable, with the TOF of 2478 h⁻¹.

Figure 10. Comparison of recent literature with Pd/CNS-800 for selective H2 generation from FA dehydrogenation. CNS, carbon nanosphere; FA, HCOOH.

In this study, the Pd/CNS-800 catalyst was also used for O₂ evolution upon the decomposition of H₂O₂. The chemical kinetics (including initial H₂O₂ concentration, Pd/CNS-800 dosage, and reaction temperature) of O₂ evolution were investigated (Figure 11A–C). These results showed that the H₂O₂ decomposition was first-order kinetics for all three initial H₂O₂ concentrations, Pd/CNS-800 catalyst dosage, and reaction temperature. The E_a value of Pd/CNS-800 in H₂O₂ decomposition is 37.6 kJ/mol. In addition, Pd/CNS-800 exhibited superior catalytic activity, with TOF of 993 mol(H₂) mol_Pd⁻¹ min⁻¹, compared to commercial Pd/C in O₂ evolution from H₂O₂ decomposition (Figure 11D).

Figure 11. Plots of produced O2 volume versus time for O2 evolution upon the decomposition of H2O2: (A) various concentrations of H2O2, (B) different amounts of catalyst, and (C) various reaction temperatures; (D) comparison of O2 generation catalyzed by Pd/CNS-800 and Pd/C. CNS, carbon nanosphere.

CONLUSION

In summary, novel, efficient, and multifunctional Pd/CNS nanocatalysts were designed and developed, via the immobilization of ultrafine Pd NPs onto the surface of CNSs, for selective H₂ generation from FA dehydrogenation and O₂ evolution from H₂O₂ decomposition. Then, the optimal nanocatalyst Pd/CNS-800 presented the highest catalytic activity in both H₂ generation (2478 $${\mathrm{mol}}_{{{\rm{H}}}_{2}}$$ mol_Pd⁻¹ h⁻¹) from HCOOH and O₂ evolution (993 $${\mathrm{mol}}_{{{\rm{O}}}_{2}}$$ mol_Pd⁻¹ min⁻¹) from H₂O₂. The CO₂ capture experiment, tandem reaction, and GC results confirmed that CO-free H₂ generation upon HCOOH dehydrogenation catalyzed by the Pd/CNS-800 nanocomposite was successfully developed for fuel cell power systems. It appears that graphitic C or N in CNS-800 serves as an electron donor to the Pd atom and increases its surface electron density, facilitating the cleavage of the C–H bond in FA dehydrogenation.

A highly efficient and selective “on–off” switch for the selective H₂ generation from HCOOH dehydrogenation was successfully realized by pH adjustment. The highly efficient nanocatalyst Pd/CNS-800 developed herein should possess the promising application of HCOOH and H₂O₂ as an economic and safe H₂ resp. O₂ carrier for the fuel cell. In addition, it could be useful for a number of other catalytic reactions.

ACKNOWLEDGMENTS

Financial support from the National Natural Science Foundation of China (Grant No. 21805166), the 111 Project of China (Grant No. D20015), The outstanding young and middle-aged science and technology innovation teams, Ministry of Education, Hubei province, China (T2020004), Foundation of Science and Technology Bureau of Yichang City (A21-3-012), and the University of Bordeaux and the Centre National de la Recherche Scientifique (CNRS), is gratefully acknowledged; thanks to eceshi (www.eceshi.com) for the transmission electron microscope test and Yuan Zhou from Shiyanjia Lab (www.shiyanjia.com) for the gas chromatogram analysis.

CONFLICTS OF INTEREST

The authors declare no conflicts of interest.