{Fe²⁺–imidazole} catalyst grafted on magnetic {Fe@Graphitized C} nanoparticles: a robust hybrid–catalyst for H₂ production from HCOOH

Abstract

A novel hybrid-catalyst was synthesized via covalent grafting of a molecular catalyst {Fe²⁺–Imidazole} on magnetic core–shell {Fe@Graphitized C, Fe@GC} nanoparticles synthesized via anoxic flame spray pyrolysis (A-FSP). Transmission electron microscopy (TEM) shows that A-FSP produces a fine C-shell consisting of few graphitized layers, where {Fe²⁺–Imidazole} is covalently grafted. The hybrid catalyst {Fe²⁺–Imidazole}@{Fe@GC} demonstrated highly efficient H₂ production from formic acid (HCOOH) at near-ambient conditions of P = 1 atm and T = 80 °C, yielding >37 Liters of high-purity H₂ and a high turnover-number (TON) of >203 000. The {Fe²⁺–Imidazole}@{Fe@GC} catalyst could be easily and efficiently recovered magnetically and reused for at least 12 catalytic cycles, demonstrating significant durability and reusability. Raman, FT-IR, TEM and XRD confirmed the preservation of key structural features of the hybrid catalyst, despite prolonged exposure to reaction conditions. We attribute these beneficial characteristics to the robustness of the FSP-made nanographitized layers and the enhanced efficiency of the {Fe-Imidazole} catalyst when interfaced with nanocarbon.

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1. Introduction

Hydrogen is a promising clean energy carrier owing to its high energy density (143 MJ kg⁻¹) and environmentally benign combustion, producing only water as a byproduct.^1–3 Among various hydrogen storage strategies, liquid organic hydrogen carriers (LOHCs), such as formic acid (HCOOH, FA), have gained significant attention owing to their efficient hydrogen release, low decomposition temperature, and minimal CO formation. These attributes make FA a viable candidate for practical hydrogen storage and utilization.^4–8

Thus far, the catalytic dehydrogenation of FA has been extensively studied, leading to the development of efficient homogeneous and heterogeneous molecular catalysts. Specifically, these include noble metal catalysts, e.g. iridium (Ir),⁹ platinum (Pt),¹⁰ and ruthenium (Ru),¹¹ which demonstrated high efficiency in the presence of suitable organic ligands; thus, currently, there is convergent evidence that ligands incorporating nitrogen (N) or phosphorus (P) play a crucial role in stabilizing active sites and modulating electron density, thereby enhancing catalytic activity and longevity.^12–15 However, the high cost, limited availability, and environmental concerns associated with noble metals have prompted efforts to explore non-noble metal alternatives, such as iron (Fe),¹⁶ cobalt (Co),¹⁷ and nickel (Ni).¹⁸ In this context, our laboratory has made significant advancements by designing highly efficient Fe-based catalytic systems, e.g. including a [FePP₃Lγ]/H₂N@SiO₂ catalyst, which achieved a TON of 29 372 and a reaction rate of >100 mL min⁻¹.¹⁹

With the aim of achieving industrial application, heterogeneous molecular catalysis has emerged as a viable alternative to homogeneous catalysis as it offers improved catalyst stability, reusability, and ease of separation from reaction mixtures. This approach involves immobilizing homogeneous catalysts onto solid supports such as silica (SiO₂),²⁰ carbon matrices^21,22 (e.g. activated carbon and carbon nanotubes), and metal–organic frameworks (MOFs).²³ These materials provide high chemical stability, large surface areas, and uniform metal dispersion, which can contribute to enhanced catalytic performance and durability.^24,25 In this context, we have demonstrated various promising options for Fe catalysts grafted on SiO₂.^19,26 Recently, by immobilizing imidazole onto silica via Si–O–Si bonds, we developed a novel catalyst [Fe(II)/imidazole/PP₃], which exhibited a high TON of ∼23 000 and a TOF of >5500 h⁻¹. However, as we have shown through careful analysis, prolonged acidic exposure, as required for reusability, degraded the stability of this catalyst via the C–O–C bond cleavage of the silane-spacer, reducing the catalyst reusability.²⁶ Subsequently, we have explored the option of replacing the SiO₂ matrix with a carbon matrix towards the aimed durability. Carbon materials are highly advantageous for catalytic applications due to their large surface area, porosity, chemical and thermal stability, as well as their versatility in surface functionalization, which allows for the tailoring of their properties to optimize catalytic performance.^27,28 Surface functionalization enables the introduction of active groups that strengthen the interaction between catalytic metal centers and the carbon support, thereby facilitating putative synergistic effects that can enhance overall catalytic performance.^29–31 For instance, electron transfer between carbon and the metal catalyst can improve both stability and catalytic efficiency.³² The mechanical strength of carbon-based materials also plays a crucial role in maintaining the structural integrity of catalysts, allowing them to endure operational stresses and regeneration processes. This durability is vital for ensuring catalyst recyclability, enabling repeated use across multiple cycles without substantial loss in performance.^30,33,34

Numerous studies have explored metal nanoparticles supported on carbon matrices, with Pd/C catalysts gaining significant attention due to their high catalytic activity, ease of synthesis, and recyclability.^22,35–39 However, challenges, such as metal nanoparticle agglomeration, which leads to reduced active surface area and adsorption of toxic byproducts, limit their long-term efficiency.⁴⁰ Additionally, the high cost of palladium necessitates the development of more cost-effective and sustainable alternatives.⁴¹

To overcome these challenges, we have developed⁴³ a cost-effective {Fe²⁺–imidazole} catalyst, which, when immobilised on activated carbon, exemplifies a highly efficient hybrid catalyst achieving a remarkable TON >353 000 over 8 cycles.⁴² Herein, we further advanced this technology by grafting the {Fe²⁺–imidazole} catalyst onto nanographitized carbon tightly associated with the magnetic Fe-core, hereafter codenamed {Fe@Graphitized C, Fe@GC}. Thus, we have produced a novel nanohybrid {Fe²⁺–imidazole}@{Fe@GC}; see Fig. 1. This approach aimed to achieve a triple gain: [i] high H₂ production, [ii] enhanced reusability and [iii] easy recovery via magnetic collection.

Fig. 1 The synthesis pathway of the {Fe²⁺–imidazole}@{Fe@GC} hybrid: (a) the {Fe@GC} synthesized in situ via anoxic flame spray pyrolysis (A-FSP); (b) TEM images show the formation of fine-coating of few-graphitized layers around the magnetic Fe-core; (c) {Fe@GC} functionalized by covalent grafting of [Fe²⁺–imidazole] complexes via sol–gel chemistry; (d) the catalytic {Fe²⁺–imidazole}@{Fe@GC} material was evaluated in formic acid dehydrogenation (FADH) for 12 repetitive sequential cycles; after each cycle, the catalyst was directly and easily recovered by a magnet due to its magnetic Fe-core.

Previous works have shown encouraging results, indicating that the chemical stability of graphitized carbon can enhance durability and reusability, making it highly promising for catalysis, environmental remediation, and energy conversion.^43–46 Herein, the synthesis of {Fe@Graphitized C} particles was performed using an anoxic flame spray pyrolysis (A-FSP) process.^47–51

This enabled the production of high-quality {Fe@Graphitized C, Fe@GC} particles, in which a nano-thin coating of a few graphitized layers hermetically encapsulates the Fe core; see Fig. 1. Then, we covalently grafted a {Fe²⁺–imidazole} catalyst onto {Fe@GC} to produce stable {Fe²⁺–imidazole}@{Fe@GC} hybrids.

Specific aims of the present work include: [i] to develop hybrid {Fe²⁺–imidazole}@{Fe@GC} materials using A-FSP and grafting technologies, [ii] to validate and optimize their efficiency for H₂ production via HCOOH dehydrogenation at near ambient conditions, [iii] to study their recovery/reusability in multiple cycles of H₂ production and [iv] to understand the key-role of the graphitized-nanocarbon on the performance and stability of the hybrid {Fe²⁺–imidazole}@{Fe@GC}.

2. Experimental methods

2.1 Instrumentation

2.1.1 Powder X-ray diffraction. Powder X-ray diffraction (pXRD) analysis was conducted at ambient temperature utilizing a Bruker D8 Advance diffractometer. The instrument was equipped with Cu Kα radiation (λ = 1.5406 Å) and a secondary monochromator, operating at 36 kV and 36 mA. Data were acquired over a 2θ range of 10° to 80°.

2.1.2 N₂ porosimetry. The N₂ adsorption–desorption isotherms were measured at 77 K on a NOVAtouch LX2 Quantachrome porosimeter. Prior to the measurements, the samples were degassed at 80 °C for 16 h under a vacuum. The specific surface area (SSA) was determined using the Brunauer–Emmett–Teller (BET) method for adsorption and desorption data points. The specific surface area was found using adsorption data points in the relative pressure P/P₀ range of 0.1–0.3. While the density functional theory (DFT) method was used to determine the pore radius using absorption data points from 0.35 to 0.99 P/P₀, the total pore volume was obtained at the 0.99 P/P₀ points.

2.1.3 Raman spectroscopy. Raman spectra were collected using a HORIBA XploRA PLUS Raman microscope integrated with an Olympus BX41 optical microscope. Prior to measurements, each powdered sample was pressed into a pellet by gently compressing it between two glass plates. A 1 mW laser power was used, which was experimentally optimized to ensure that the crystal phase remained unaffected during the analysis. A 785 nm diode laser served as the excitation source, with the beam focused through the microscope. Spectral acquisition was performed under the following conditions: an exposure time of 10 seconds per scan, with 30 accumulations.

2.1.4 Thermogravimetric analysis (TG-TDA). The organic content in our catalytic materials was determined through thermogravimetric analysis (TGA) using a SHIMADZU DTG-60 instrument coupled with a TA-60 WS analyzer. Approximately 30 mg of the sample was placed in a platinum crucible positioned on one arm of the thermobalance, while an empty platinum crucible served as a reference on the opposite arm. The analysis was conducted within a temperature range of 20 to 700 °C, with a heating rate of 10 °C min⁻¹, under a synthetic air atmosphere.

2.1.5 Transmission electron microscopy (TEM). The morphology, nanostructure, and chemical composition of the nanoparticles were examined using an FEI Titan 80–300 STEM microscope at a 300 kV accelerating voltage. STEM images were obtained with a beam convergence semiangle of 21.5 mrad.

2.1.6 Scanning electron microscopy (SEM). Images were obtained using a Thermo Fisher Pharos Phenom G2 field-emission scanning electron microscope (FE-SEM) (Waltham, MA, USA) operating under high-vacuum conditions (0.1 Pa) with an accelerating voltage of 15 kV. Both backscattered and secondary electron detectors were utilized at a 20 : 80 ratio. Prior to imaging, the samples were coated with a 5 nm layer of chromium using a Quantum Design Q150T Plus sputter coater (Darmstadt, Germany). Additionally, energy-dispersive spectroscopy (EDS) analysis was performed using the same instrument to investigate the surface chemical composition of the nanocatalysts.

2.1.7 Solution potential. The solution potential (E_h) was continuously monitored in situ throughout the experiment using a combined platinum (Pt) ring electrode (Metrohm 6.0451.100). This electrode was directly integrated into the reactor system to ensure real-time measurement of the reaction potential within the solution.

2.1.8 Fourier-transform infrared spectroscopy (FTIR). FTIR spectra were recorded using a Nicolet IS5 system equipped with OMNIC FTIR Software version 9.2.86. The spectra were collected over the range of 4000 to 400 cm⁻¹, with a resolution of 2 cm⁻¹ and an average of 100 scans.

2.2 Synthesis of core–shell {Fe@Graphitized C, Fe@GC} by anoxic flame spray pyrolysis (A-FSP)

The {Fe@GC} particles were synthesized using our FSP set-up described earlier^50,52 with a modification to create an oxygen-lean atmosphere. Specifically, the flame was hermetically enclosed with a quartz tube to prevent ambient air entrainment; see Fig. 1. In addition, an N₂ sheath was used to control the anoxicity of the combustion atmosphere.

The Fe precursor solution was prepared by dissolving 0.4 M iron naphthenate (Alfa Aesar, 38% in mineral spirits, 6% Fe) in xylene (Sigma-Aldrich, purity >97%). Based on systematic screening, the optimal A-FSP conditions employed were as follows: the flame was enclosed using a 40 cm quartz tube, with no gap between the tube and the burner. The solution was fed through a capillary at 2 mL L⁻¹ and dispersed by 2 L min⁻¹ O₂ (Linde, purity >99%), respectively, into a stoichiometric, self-sustaining oxygen–methane (2.5 L min⁻¹, 1.5 L min⁻¹) pilot flame to initiate combustion. The pressure drop at the nozzle tip was set at 2 bar, and an additional 20 L min⁻¹ sheath N₂ was used. The product powder was collected using a vacuum pump (Busch V40) on a glass microfiber filter (Albet Labscience GF6 257 mm in diameter). The burner-to-filter distance (BFD) was fixed at 69 cm. The nanomaterials were collected and stored in glass vials under an inert argon atmosphere until use. This material is listed as Fe@C-prist in Table 1.

Table 1 Structural and surface characteristics of pristine, post-calcined and post-functionalized {Fe²⁺–imidazole}@{Fe@GC} hybrid

Material	Material configuration	Phase composition (%) (±1)/crystallite size (nm) (±0.5)					d BET (nm)	SSA (m^2 g^−1) (±1)	Pore volume (cm^3 g^−1) (±0.005)
		Magnetite Fe3O4	Wustite FeO	Martensite C0.2Fe1.8	Iron Fe^0	Cementite Fe3C
Fe@C-prist	FSP-made core@shell Fe@carbon (pristine)	24/24	29/28	22/9	—	25/26	12	83	0.199
Fe@C-calc	Fe@C-prist calcined at 600 °C for 24 h under Varigon H5	32/15	5/10	3/8	53/38	7/17	6	157	0.379
Fe@C-graf	Fe@C-calc grafted with Fe^2+–imidazole via silane	29/14	4/23	6/8	49/55	12/20	7	101	0.358
Fe@C-aftercat	Fe@C-graf after 12 cycles of FADH catalysis	27/25	4/22	4/7	41/53	24/24	10	84	0.218

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2.2.1 Post-FSP treatment. Post-FSP treatment of {Fe@GC} nanoparticles was conducted to investigate and optimize the phase composition, particle size, and carbon graphitization. A calcination protocol was performed at T = 600 °C for 24 h under a reducing Varigon H5 gas (95% Ar + 5% H₂) to enhance the graphitization of carbon by facilitating the removal of oxygen-containing functional groups, thereby promoting a higher degree of structural ordering. Additionally, the reducing conditions influenced the phase evolution of iron, stabilizing metallic Fe or promoting the formation of FeC phases as verified by XRD results; see Fig. 2a. The so-obtained material is codenamed Fe@C-calc, as listed in Table 1.

Fig. 2 (a) XRD patterns of all the studied materials along with the JCPDS cards of the dominant phases. (b–e) STEM (top) and HRTEM (bottom) images of (b) Fe@C-prist, (c) Fe@C-calc, (d) Fe@C-graf and (e) Fe@C-aftercat (12 uses).

2.3 Synthesis of the {Fe²⁺–imidazole}@{Fe@GC} hybrid

For the synthesis of {Fe²⁺–imidazole}@{Fe@GC}, 1 mmol of 3-chloropropyltrimethoxysilane was added to 10 mL of methanol (MeOH) containing 1.2 mmol of imidazole. The resulting mixture was stirred at 80 °C for 24 h. Subsequently, the reaction mixture was cooled to 40 °C, followed by the addition of 0.5 mmol of [Fe(BF₄)₂·6H₂O] (Sigma Aldrich) dissolved in 10 mL of MeOH, and stirring was continued for an additional 2 h. Thereafter, 0.4 g of FSP-made {Fe@C-calc} matrix was introduced into the reaction medium along with 5 mL of ethanol (EtOH), and the system was stirred at 60 °C for 24 h. The solid product was then collected, washed twice with 5 mL of MeOH and 5 mL of EtOH, and dried under vacuum at 70 °C using a drying pistol. The obtained hybrid material is depicted in Fig. 1 and is named as Fe@C-graf in Table 1.

2.4 Catalytic H₂ production process

The catalytic reactions were conducted in a double-walled reactor under an Ar atmosphere with continuous stirring, maintaining a stable temperature of 80 ± 1 °C using a heated magnetic stirrer. The reactor was directly connected to a gas chromatography (GC) system (Shimadzu GC-2014 Gas Chromatograph equipped with a thermal conductivity detector (GC-TCD) and a Carboxen-1000 column) for real-time gas analysis and identification. The total gas volume was determined using a manual gas burette.

In the catalytic reaction, 15 μmol of {Fe²⁺–imidazole}@{Fe@GC} was introduced into a 7 mL reaction mixture composed of 5 mL propylene carbonate (PC) [Merck] and 2 mL formic acid (FA) {Sigma-Aldrich, [97.5/2.5H₂O] (v/v)}. Following an incubation period of 30 minutes under continuous stirring, 7.5 μmol of PP₃ {tris[2-(diphenylphosphino)ethyl]phosphine, Sigma-Aldrich, purity of 98%}, was added to initiate gas evolution. The catalytic performance was evaluated using TONs and TOFs equations.^19,26,42,53 The continuous feeding process was sustained by the incremental addition of 2 mL of FA each time, and 1800 mL of gas (H₂ + CO₂) was produced until the reaction was completed.

3. Results

3.1 Characterization of materials

XRD analysis (Fig. 2a and S2 in ESI,†) shows that the dominant diffraction peaks of Fe@C-prist correspond to magnetite (Fe₃O₄) [JCPDS 19-0629], wüstite (FeO) [JCPDS 06-0615], martensite (C_0.2Fe_1.8) [JCPDS 44-1289] and cohenite (Fe₃C) [JCPDS 35-0772], confirming the presence of a core–shell structure. We have calculated the composition of each phase, and the crystallite sizes of these dominant phases are listed in Table 1. Additionally, a broad peak at 2θ = 26° was attributed to the graphitic carbon.⁴⁶ Upon calcination at 600 °C under H₂/Ar (5/95), a partial reduction of Fe₃O₄ and FeO occurred, leading to the formation of metallic iron (Fe⁰) [JCPDS 06-0696]. Simultaneously, an oxidation process resulted in the emergence of traces of luogufengite (ε-Fe₂O₃) [JCPDS 16-0653], while Fe₃O₄ and Fe₃C were retained, and additional carbide phases, such as Fe₇C₃ [JCPDS 89-2005], were detected, as shown in Fig. S2.† After grafting, the XRD profile of Fe@C-graf was unchanged (Fig. 2a), indicating that the Fe-phases were not affected by the grafting process. This was verified by HR-TEM, as shown in Fig. 2b and c, which show the Fe-core and graphitized-C coatings in the grafted Fe@C-graf, corresponding to the {Fe²⁺-imidazole}@{Fe@GC} hybrid.

Further analysis by SEM-EDX spectra, see Fig. S5 in ESI,† confirms the non-altered elemental arrangement in the Fe@C-calc and Fe@C-graf materials. TEM analysis provides critical insights into the structural and morphological evolution of the catalytic material under different stages of catalytic use/reuse. Specifically, Fig. 2b illustrates the TEM image of the pristine material (Fe@C-prist), showing the Fe core and its coating with nano-thin graphitized carbon. The TEM image reveals a well-formed graphitized shell that hermetically coats the Fe particle, forming a uniform and well-defined structure.

The presence of distinct lattice fringes confirms the crystallinity of the Fe core. Miller planes of three different d-spacings are resolved with d = 0.211 nm representing the cementite (2 1 0) lattice,⁵⁴d = 0.245 nm representing the wustite (1 1 1) phase⁵⁵ and d = 0.251 nm representing the magnetite (3 1 1) phase.⁵⁶Fig. 2c and d show the TEM images of Fe@C-calc and Fe@C-graf, respectively, demonstrating that after calcination and grafting, the magnetic Fe core encapsulated by nano-thin graphitized carbon remained intact.

EDS/EDX spectra obtained from TEM/STEM analysis in Fig. S6a† of the pristine sample display multiple iron peaks, indicating the dominance of Fe within the material. Additionally, the presence of oxygen suggests the formation of iron oxides, while the detection of carbon corroborates the presence of the protective carbon shell. Upon calcination, no significant morphological changes are observed (Fig. 2b); however, the iron core appears denser and exhibits enhanced crystallinity due to the removal of amorphous carbon during the process, while the integrity of the graphitized carbon shell remains preserved. For the functionalized sample (Fig. 2d), the core–shell structure is retained, yet an increased heterogeneity in contrast is observed. This variation suggests successful surface functionalization, potentially altering the electron density at the carbon interface. Additionally, STEM reveals partial nanoparticle agglomeration, though the core–shell structure remains unaffected. The EDS/EDX spectrum for Fe@C-calc (Fig. S6b in ESI†) is similar to those of Fe@C-prist, showing only peaks corresponding to C, Fe, and O, while the spectrum of Fe@C-graf (Fig. S6c†) exhibits additional peaks due to the presence of N and Si as result of imidazole grafting via Si–O-anchoring sites. These findings highlight the successful FSP contribution and the subsequent successful grafting process for the synthesis of {Fe@Carbon@Imidazole-Fe²⁺}, which preserves its structural robustness while maintaining its core–shell architecture, a key aspect of its potential catalytic applications.

For further investigation of the surface characteristics of the materials and the structural modifications occurring during synthesis, Raman and FT-IR spectroscopy were employed. Fig. 3 depicts the Raman spectra of our materials, exhibiting two characteristic peaks at 1313 cm⁻¹ and 1598 cm⁻¹. Specifically, the peak at 1598 cm⁻¹ corresponds to the graphitic G-band, which is indicative of sp²-hybridized carbon structures and represents the fundamental feature of graphite. On the other hand, the peak at 1313 cm⁻¹ is attributed to lattice defects, structural disorder, and low-symmetry configurations within the graphite framework.^57–59 These characteristic bands indicate the presence of sp² and sp³ carbons within the matrices in alignment with the observation of the I_D/I_G ratio, which serves as a key tool for detecting structural alterations in carbon-based materials.

Fig. 3 Raman spectra of (a) Fe@C-prist, (b) Fe@C-calc, (c) Fe@C-graf, and (d) Fe@C-aftercat (12 uses). The colored peaks correspond to the deconvolution of D and G subcomponents.

Furthermore, the peak observed at 593 cm⁻¹ in Fig. S4† is associated with wüstite (FeO). Additional peaks correspond to the oxidation products of wüstite, specifically hematite, at approximately 200 cm⁻¹ and 300 cm⁻¹ and magnetite at 682 cm⁻¹.^60,61

To focus on the two main distinct Raman peaks, i.e. G and D, which correspond to distinct vibration modes in carbon-based materials, their deconvolution using Gaussian fitting was performed (Fig. 3). The G band corresponds to the E_2g vibration of sp²-bonded carbon pairs, while the D band is linked to the A_1g mode, akin to an in-plane breathing vibration. D1 peak (1350 cm⁻¹) is associated with the presence of sp³ hybridized carbon atoms and edge defects. D2 (1200 cm⁻¹), D3 (1500 cm⁻¹) and D’ (1620 cm⁻¹) are additional defects related to peaks that are signs of amorphous carbon contribution and sp²-sp³ hybridization transitions or secondary disorder effects. G1 (1580 cm⁻¹) indicates the graphite like carbon structure, representing the in-plane vibration mode of graphitic domains.^62,63

As observed, in the three treatments of our sample (FSP-pristine, calcined, and grafted), the I_D/I_G ratios indicate a significant degree of structural disorder in the carbon matrix. This disorder arises from the flame spray pyrolysis process, where rapid cooling and combustion conditions influence the material's morphology,⁵⁰ as analysed recently for our FSP-made carbon-coated TiO₂.⁶⁴ Here, the I_D/I_G ratios further confirm the presence of both sp²-and sp³-hybridized carbon domains. Notice that the nano-thin graphitic carbon clearly resolved by TEM images (Fig. 2b–e), corresponds to the non-negligible D-band in Raman (Fig. 3), which originates from the local non-planar geometry of the graphitic carbon layers that bend to encapsulate the spherical magnetic Fe-particles. In addition, strains in the sp² carbon lattice and structural defects may break the perfect sp²-symmetry of graphite, rendering the -otherwise forbidden- D band, Raman-active.⁶⁵

Upon calcination at 600 °C under an Ar/H₂ atmosphere, the I_D/I_G ratio decreases to 3.1, indicating a partial reduction of amorphous carbon and increased sp²-graphitization. Following the immobilization of [Fe²⁺–imidazole] functionalities, the I_D/I_G ratio increased to 4.0, indicating that grafting introduces additional distortion and structural defects within the carbon framework. Overall, the present Raman data, together with TEM, confirm that the FSP-made Fe@GC particles retain their key structural features during calcination and optimization (Fe@C-calc) and grafting process (Fe@C-graf).

The FT-IR spectra depicted in Fig. 5a confirm the successful grafting in Fe@C-graf. Specifically, the FTIR spectrum exhibited a broad absorption band at 3426 cm⁻¹, attributed to O–H and N–H stretching vibrations.⁶⁶ Additionally, distinct peaks were identified at 3043 cm⁻¹ and 2933 cm⁻¹, corresponding to aliphatic C–H stretching vibrations.⁶⁷ The absorption band at 1633 cm⁻¹ was assigned to aromatic C=C stretching vibrations, while the peak at 1062 cm⁻¹ indicated the presence of C–O bonds. Finally, the characteristic Fe–O vibrations were observed at 580 cm⁻¹.

The textural properties of the core–shell nanoparticles were also evaluated using N₂ adsorption–desorption isotherms. Note that FSP-made nanoparticles generally do not have pores; hence, the vacuum space between nanoparticles is considered to be the theoretical porosity. As shown in the BET analysis (Fig. S3 in ESI†), the pristine material exhibited a specific surface area (SSA) of 83 m² g⁻¹, indicative of type IV isotherms.⁶⁸ Following calcination, the SSA increased to 157 m² g⁻¹, suggesting enhanced porosity due to the partial removal of amorphous carbon and structural reorganization, as corroborated by Raman spectroscopy (Fig. 3b). Further functionalization of the surface with [Fe²⁺–imidazole] resulted in a decrease in SSA to 101 m² g⁻¹, accompanied by a decrease in pore volume. Analysing the pore size and volume trends (Fig. 5c) reveals that post-calcination of Fe@C- calc leads to an increase in pore volume. Specifically, Fe@C-prist exhibits a total pore volume of 0.199 cm³ g⁻¹, which nearly doubles to 0.379 cm³ g⁻¹ after calcination. This change is attributed to the effects of calcination under reducing conditions (5% H₂/Ar), which enhance the graphitization of the carbon layers. Fig. 5c also shows pore sizes ranging from 4 nm to 60 nm, which could potentially enhance catalytic kinetics. After grafting, the pore volume remains stable at 0.358 cm³ g⁻¹, indicating structural integrity.

3.2 Catalytic H₂ production from HCOOH

The {Fe²⁺–imidazole}@{Fe@GC} hybrid was evaluated as a catalyst in formic acid dehydrogenation (FADH) in the presence of PP₃ in propylene carbonate. Fig. 4a shows that the catalyst was able to catalyze 8 mL of FA through continuous feeding, providing 6.4 L of gas with a reaction rate of 43 mL min⁻¹, TONs of 17 447 and TOFs of 4985 h⁻¹ (Table 2). According to the well-known reaction path,⁶⁹ the evolved gas consisted exclusively of H₂ : CO₂ {50% : 50%} (Fig. S8†). Herein, this was confirmed by online gas chromatography with a thermal conductivity detector (GC-TCD) and the analysis confirmed that it consisted exclusively of H₂ and CO₂ in a 1 : 1 ratio (Fig. S8†). The complete absence of CO, demonstrates the high selectivity of the Fe-imid catalyst.⁶⁹ For completeness, a control experiment using a simple mixture of the particles and the Fe ions [Fe@C/Fe²⁺/PP₃] yielded only 120 mL of gas (Fig. 4a), thus demonstrating that the covalent-grafting of the {Fe-imidazole} complex on the {Fe@GC} surface is of key-importance for the H₂-FADH by the {Fe²⁺–imidazole}@{Fe@GC} hybrid. All catalytic experiments were conducted in triplicate, yielding consistent outcomes within ±5% (not shown).

Fig. 4 (a) Catalytic H₂–FADH by the {Fe²⁺–imidazole/PP₃}@{Fe@GC} catalysts (black circles) or a simple mixture of Fe@GC + Fe/PP₃ (blue triangles). (b) Evaluation of catalyst {Fe²⁺–imidazole/PP₃}@{Fe@GC} reusability: gas production over time for up to 12 reuses. (c) Solution potential mapping (E_hvs. SHE) for {Fe²⁺–imidazole/PP₃}@{Fe@GC} (green bars) or simple mixture of Fe@GC + Fe/PP₃ (purple bars). (d) Turnover numbers (TONs) recorded across multiple catalytic cycles. (e) Turnover frequencies (TOFs) observed for each catalytic cycle. (f) Comparative E_h profile between the 1st and 12th catalytic use.

Table 2 Overall FADH catalytic data over 12th consecutive uses for the {Fe²⁺–imidazole}@{Fe@GC} material

Fe@C-graf material	Mass (mg)	Reaction time (min)	VH2 + CO2 (L)	Rate (ml L^−1)	TONs	TOFs (h^−1)	VFA (ml)
1st use	42	211	6.4	43	17 447	4985	8
2nd use	42	212	6.4	43	17 447	4988	8
3rd use	42	221	6.4	40	17 447	4846	8
4th use	40	254	6.4	36	17 447	4154	8
5th use	40	255	6.2	35	16 902	3977	8
6th use	39	209	6.5	44	17 720	5092	8
7th use	38	241	6.2	36	16 902	4204	8
8th use	38	244	6	38	16 357	4020	8
9th use	35	252	6.1	36	16 629	3959	8
10th use	35	251	6.1	36	16 629	3978	8
11th use	34	255	6	35	16 357	3849	8
12th use	34	245	5.8	38	15 811	3875	8
Total	—	2850	74,5	—	203 095	—	96

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To further understand the physicochemical catalytic events, we monitored the solution potential (E_h). As shown,^{26,42,70–74} monitoring of E_h in FADH catalysis provides information on the evolution of the key-reducing components of the system, i.e., a negative E_h should be established beyond a threshold-value for successful H₂ production via FADH.⁷⁰ In this context, the E_h data in Fig. 4c demonstrate that the hybrid catalyst {Fe²⁺–imidazole/PP₃}@{Fe@GC} creates the necessary reducing E_h profile, suitable for fast FADH catalysis via the generation of key-determinant hydride species.⁷⁰

As a control, the E_h profile of the control experiment [simple mixture Fe@C + Fe²⁺/PP₃] (Fig. 4c, dark red bars) demonstrates that this system is unable to create the appropriate reducing environment, substantiating its minimal reactivity in FA dehydrogenation.

Overall, the present data demonstrate that [i] the {Fe²⁺–imidazole/PP₃}@{Fe@GC} hybrid is highly efficient in H₂ production via FADH at near ambient conditions; [ii] the key parameter of the mechanism is the successful generation of reducing equivalent, hydrides, in accordance with the established mechanism of the [Fe-imid/PP₃]catalyst. This proves that the grafted {Fe-imid} retains all its catalytic functionalities while the {Fe@GC} particle acts as a support that promotes the H₂-FADH process.

3.2.1 Recollection–recyclability of the magnetic catalyst. Here, we exploit the magnetic {Fe@GC} core as a means to easily recollect and reuse the {Fe@Carbon@Imidazole–Fe²⁺/PP₃} catalyst. Thus, after the initial use, the catalyst was magnetically separated, washed with methanol, dried, and re-used in a fresh mixture of [solvent + FA + PP₃], i.e., as in the first cycle. Under these conditions, the reaction successfully restarted up to 12 times; see Fig. 4b. The data, also shown in Table 2, demonstrate a stable H₂ production performance with minor variations. In this way, the cumulative gas production over 12 cycles totaled 74.5 L (H₂ + CO₂), corresponding to 37.25 L of pure H₂ and an overall TON of 203 095, with a total FA consumption of 96 mL. The TONs and TOFs achieved per cycle are shown in Fig. 4d and e, respectively. Additionally, E_hvs. SHE measurements for the 1st and 12th cycles revealed consistent electrochemical behavior (Fig. 4f), confirming that the catalyst did not undergo significant degradation or deactivation over repeated uses.

4. Discussion

The present data demonstrate [i] the {Fe²⁺–imidazole/PP₃}@{Fe@GC} is a promising catalyst for efficient H₂ production via FA dehydrogenation; [ii] the nanostructure ensures high recyclability, durability under repeated use, and significant FA-volume handling highlight its robustness under harsh conditions without performance deterioration; [iii] the magnetic-core allows easy recollection/reuse of the catalyst. These advantages provide a competitive edge compared to similar literature-reported catalytic systems,^26,75–78 further reinforcing its potential for practical applications in sustainable hydrogen production.

Hereafter, we have peered into the effect of prolonged re-use, specifically 12 times, on the physicochemical characteristics of the catalyst. The working hypothesis was that, in a robust system such as the present one, subtle changes that might otherwise be negligible become more discernible after 12 reuses.

4.1 Post catalytic characterization of {Fe²⁺–imidazole/PP₃}@ {Fe@GC} material after prolonged use

Elemental analysis (Table S4 in ESI†) indicates that after 12 reuses, the atomic concentrations of iron and imidazole remained unchanged, suggesting the integrity of their grafting despite multiple catalytic cycles. Additionally, the detection of P atoms indicates PP₃ adsorption during catalysis. TEM-EDS analysis (Fig. S6d in the ESI†) also confirmed the presence of iron, oxygen, and carbon, and phosphorus, further supporting the adsorption of PP₃ onto the catalyst surface during catalytic operation. As shown in the TEM image (Fig. 2e), after 12 reuses, the core–shell structure remains intact, and the core with d = 0.198 nm represents the α-Fe (1 1 0) phase. XRD data suggest progressive oxidation of Fe₃O₄, leading to the formation of maghemite (Fe₂O₃), while iron carbides (Fe₅C₂ and Fe₃C) remain structurally preserved (Fig. 2a). Additionally, the emergence of graftonite (Fe₃(PO₄)₂) was observed, which is likely attributed to the adsorption of the PP₃ additive, as corroborated by EDS. The observed shifts in the 2θ values suggest modifications in the electronic structure of the material, aligning with trends identified in FT-IR data,^79,80 as shown hereafter.

FT-IR spectroscopy (Fig. 5b) revealed a broad absorption band at 3438 cm⁻¹, attributed to O–H and N–H stretching vibrations.⁶⁶ Additionally, characteristic peaks at 3043 cm⁻¹, 2933 cm⁻¹, 1427 cm⁻¹, and 1340 cm⁻¹ corresponded to aliphatic C–H stretching vibrations.⁶⁷ A distinct peak at 1791 cm⁻¹, assigned to C=O stretching,⁸¹ was detected exclusively in the post-catalysis samples (Fe@C-aftercat), indicating residual formic acid (HCOOH) despite their extensive washing. The 1579 cm⁻¹ band was attributed to C=C aromatic vibrations, with an additional N–H stretching mode overlapping in the same region.⁸² Of particular significance were the peaks at 1200 cm⁻¹ and 1054 cm⁻¹, which confirmed the presence of phosphine post-catalysis, corresponding to P=O and P–Ph vibrations, respectively. Furthermore, a peak at 700 cm⁻¹ was assigned to P–C stretching, derived most probably from PP₃ adsorbed on the catalyst surface. The peak at 1054 cm⁻¹ in the as-prepared catalyst spectrum was also associated with Si–O stretching, indicating the presence of Si-centres derived from the silane precursor used for [Fe²⁺–imidazole] grafting.^83–85 Subtle shifts in FT-IR peaks of the post-catalysis samples, compared to those of PP₃, suggest modifications in bond strength and electron density, likely due to interactions involving grafted [Fe²⁺–imidazole] and PP₃.^86,87 To further confirm PP₃ adsorption on the hybrid catalyst after 12 repetitive cycles, thermogravimetric analysis (TGA) was conducted on the residual solid. As illustrated in Fig. S1 in ESI,† organic mass loss increased from 14% for the unused catalyst (Fig. S1a†) to 34% (Fig. S1c†) for the after-catalysis sample in the temperature range of 200–550 °C, which is attributed to the decomposition of PP₃.

Fig. 5 FT-IR spectra of (a) a comparison of Fe@C-calc and Fe@C-graf and (b) comparison among Fe@C-graf, Fe@C-aftercat (1st use and 12th use), and PP₃. (c) Pore size distribution of all the hybrid materials using the DFT method.

Following catalytic recycling, the Raman spectrum of the post-catalysis material showed that the intensity of the I_D/I_G ratio decreased from 4.0 for the unused catalyst to 3.8, indicating an enhancement in crystallinity and a reduction in structural defects (see Fig. 3d). These findings are in agreement with our previous work, where a carbon matrix was restructured after FADH catalysis, decreasing its I_D/I_G ratio.⁴² Finally, BET analysis revealed a reduction in pore volume, with the specific surface area decreasing to 83 m² g⁻¹ for the recycled catalyst. This decrease is likely due to the adsorption of PP₃ on the carbon surface during the catalytic process, as evidenced by spectroscopies and TGA data.

In summary, the post-catalytic characterization confirms that the structural integrity of the graphitized core–shell catalyst remains largely preserved. A minor oxidation of the matrix occurred, i.e., by the acid HCOOH; however, its magnetic properties remain intact. The elemental profiles of iron and imidazole exhibit negligible variation after catalysis, underscoring the robustness and effectiveness of the catalyst despite the adsorption of PP₃. The catalyst maintained the integrity of both the graphitized core–shell carbon structure and the iron molecular catalytic centre while successfully operating in 12 cycles of FADH catalysis, demonstrating its multifunctional capability.

5. Conclusions

In this study, a novel magnetic graphitized core–shell catalyst, {Fe@Graphitized Carbon}@{Fe²⁺–Imidazole}, was successfully designed and synthesized using the FSP process and chemical grafting. Diligent control of anoxic FSP enabled the formation of ultrafine graphitized carbon around the magnetic Fe core. The active metal catalytic center, comprising Fe²⁺–imidazole, was grafted onto the particles via a straightforward synthesis approach utilizing a silane intermediate.

The so-synthesized {Fe@Graphitized Carbon}@{Fe²⁺–Imidazole} catalyst exhibited remarkable durability and recyclability in H₂ production via HCOOH, achieving TONs of 203 095 while consuming 96 mL of formic acid over 12 catalytic cycles. A detailed analysis using TEM, Raman, XRD, and FTIR verified that the catalyst retained its nano-thin graphitized carbon shell structure and magnetic properties throughout the process up to the 12th reuse.

Technology-wise, the present work demonstrates that flame spray pyrolysis (FSP) is a promising, scalable technology that may pave the way towards the – long-envisaged – transition to industrial production of versatile nanocatalytic materials for H₂ production via formic acid (HCOOH) dehydrogenation.

Data availability

The data supporting this study are not publicly available owing to commercial sensitivity, confidentiality restrictions and privacy concerns.

Author contributions

C. G. catalytic experiments, methodology, investigation, data analysis, writing–original draft, and writing–review & editing; C. D. materials synthesis via FSP, methodology, investigation, data analysis, writing–original draft, and writing–review & editing; S. S. TEM measurements; Y. D. conceptualization, data analysis, and writing–review & editing; M. L. conceptualization, data analysis, supervision, and writing–review & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The research project was supported by the Hellenic Foundation for Research and Innovation (H.F.R.I.) under the “2nd Call for H.F.R.I. Research Projects to support Faculty Members & Researchers” (Project Number 3832).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta03079j

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