Double-Ligand [Fe/PNP/PP 3 ] and Their Hybrids [Fe/SiO 2 @PNP/PP 3 ] as Catalysts for H 2 -Production from HCOOH

: Two types of iron-based catalysts, [Fe/SiO 2 @ iPro PNP/PP3] and [Fe/SiO 2 @ tBu PNP/PP3], for the dehydrogenation of formic acid (FADH), were synthesized. These catalysts were developed using a double-ligand approach combining a PNP ligand and a PP3 ligand, demonstrating functionality without the need for additional cocatalysts or additives. Furthermore, hybrid catalysts [Fe/SiO 2 @ iPro PNP/PP3] and [Fe/SiO 2 @ tBu PNP/PP3] were created by covalently grafting PNP ligands onto SiO 2 particles. The hybrid [Fe/SiO 2 @ iPro PNP/PP3] exhibited enhanced recyclability, with turnover numbers (TONs) exceeding 74,000. In situ ATR-FTIR and UV-Vis spectroscopies were used to monitor the structure and dynamics of the catalysts under catalytic conditions, revealing the formation of active catalysts through the involvement of all components: [Fe (metal)/PNP (first ligand)/PP3 (second ligand)/FA (substrate)], which are crucial to FADH catalysis. An Arrhenius study revealed that the hybrid [Fe/SiO 2 @ iPro PNP/PP3] had a lower activation energy (E a = 42.5 kJ/mol) compared to its homogeneous counterpart (E a = 48.2 kJ/mol), indicating superior catalytic performance. Conversely, [Fe/SiO 2 @ tBu PNP/PP3] showed an increased activation energy (E a = 48.3 kJ/mol) compared to its homogeneous form (E a = 46.4 kJ/mol). This study discusses the differing roles of tBu PNP and iPro PNP in catalyst configuration, highlighting the potential of double-ligand catalysts to enhance the performance and recyclability of PNP ligands in FADH, offering significant implications for the development of efficient and reusable catalytic systems.


Introduction
Due to its high energy per mass content (120 MJ kg −1 ), H 2 is attractive for energy technologies [1].However, its low volumetric energy density (0.0108 MJ L −1 ) [2] mandates storage and transportation under high pressure (i.e., 100-700 bar) or as a liquid at −253 • C.This necessity makes H 2 storage and transportation a standalone task requiring further investigation [3].Within this context, the scientific community is shifting toward in situ H 2 production, either through water splitting [4], ammonia cracking [5], or the dehydrogenation of organic molecules such as methanol, formaldehyde, and formic acid, which can serve as liquid organic hydrogen carriers (LOHCs) [6,7].Among LOHCs, formic acid (FA) has gained considerable interest due to its high volumetric energy intensity (6.36 MJ L −1 ), low toxicity, and very low flammability [8].
The pioneering works of Beller [9,10], Himeda [11,12], and Laurenczy [13][14][15] reported efficient H 2 production from FA catalyzed by systems based on metal complexes of iridium, ruthenium, or iron combined with organic ligands containing P-, N-, and O-heteroatoms.PNP pincer-type ligands, which often surround the metal and provide high electron density, maintain a reductive character and act as σ-electron donors and/or π-electron acceptors within the metal complex structure [16].The involvement of phosphine moieties attempts Energies 2024, 17,3934 2 of 19 to influence transition metal hydrides by controlling steric hindrance and M-H bond characteristics [17].
One of the first catalysts using Ir and a pincer PNP-type ligand was reported in 2011 by Tanaka et al., where the ligand bears a [2,6-(ECH 2 )C 6 H 3 ]− structure with E being iso-propyl (iPro) groups.This Ir(III) trihydride catalyst achieved highly efficient dehydrogenation of 5 mmol HCOOH and trimethylamine-base, providing a TON of 2000 and TOF of 120,000 h −1 [18], at 80 • C in tert-butanol.The first PNP-type catalyst with a non-noble metal was [Fe(tBuPNP)(H) 2 (CO)], which was reported in 2013 by Milstein et al. using 2,6-di(di-tert-butylphosphinomethyl)pyridine as the PNP ligand.This system was efficient and stable in dioxane with NEt 3 at 40 • C, dehydrogenating 1 mol of formic acid in 10 days and achieving a TON value of 100,000 and 416 h −1 TOF [19].
In 2014, Pidko et al. demonstrated that the Ru-hydride-PNP complex (PNP)Ru(H)Cl(CO), with an aromatic PNP ligand with tert-butyl groups, was very active in the cycling catalytic operation of CO 2 to formate, where FA dehydrogenation provided TON = 326,500, TOF = 257,000 h −1 , with NHex 3 as a base at 90 • C in DMF [20].In the same year, Bielinski et al. published a FeII(H)(iPrPNP)(CO) complex, achieving one of the highest values of TON (983,642) and TOF (196,728 h −1 ) within one hour at 80 • C in dioxane in the presence of a Lewis acid, specifically LiBF 4 .The use of LiBF 4 boosted the catalytic performance, as the same system without the additive yielded a TON of 182 in 48 h [21].A system similar to the one used by Bielinski et al., with MnI instead of FeII in 1,4-dioxane at 80 • C, exhibited low performance for FA dehydrogenation, yielding a TON of only 190 after 14 h of operation.Interestingly, the use of LiBF 4 here had a negative impact, reducing the TON values from 190 to 19 for the same operation time [22].
A Ru-PN 3 P catalyst, where the ligand contained a dearomatized pyridine moiety and an imine arm, with Ru coordinated by 1N-and 2P-atoms, was able to work under continuous HCOOH feed (14.5 mL for 150 h) yielding TON > 10 6 in DMSO at 90 • C, with NEt 3 as an additive [23].Additionally, the corresponding Mn(PN 3 P)(CO) 2 catalyst published by the same group in 2023 achieved TON = 15,200 and 1416 mL of [H 2 +CO 2 ] gases [24].
Based on a 2,6-diaminopyridine scaffold, the corresponding Fe(II)-hydrido-carbonyl catalyst performed quite well in the dehydrogenation of 0.01 mol FA in THF with a base, giving a TON value of 10,000 [25].In 2019, Tondreau et al. combined a tBuPNNOP chelate ligand to form the (tBuPNNOP)Mn(CO) 2 catalyst, which, operating with 1 mL FA in chlorobenzene at 80 • C and NEt 3 as an additive, gave TOF = 8500 h −1 [26].In 2021, the group of Milstein introduced a Ru-9H-acridine pincer catalyst with extraordinary ability for the dehydrogenation of neat FA, with a total TON of 1,700,000 and demonstrated activity for continuous operation over 19 days, decomposing a total of 1.2 L of FA [27].
The hydrido complex of Ru with iProPNP, [Ru(H)(Cl)(CO)(HN{CH 2 CH 2 P(iPr) 2 } 2 )], was evaluated at different pH values for FA decomposition, providing the highest activity at pH = 4.5 with 1693 TON and 1912 h −1 TOF within 3 h of operation [28].By replacing the H-N group with Me-N, the system's performance was enhanced, resulting in a TON of 6801 and a TOF of 2598 h −1 ; however, the corresponding catalysts with Fe and Mn did not adopt this trend [28].
Marketwise, the cost of H 2 production via in situ catalytic H 2 production from LOHCs is a key factor, in conjunction with the efficiency and safety of the process.A recent cost analysis of such systems [32] revealed that the ligand significantly contributes to the final cost [32].Therefore, developing efficient and recyclable heterogeneous molecular catalysts based on low-cost ligands becomes mandatory.
So far, a brief review of the key progress in developing heterogeneous catalysts for FADH highlights that heterogeneous catalysts are a promising technology.For example, successful immobilization of the Ru/mTTPTS complex onto the MCM-41 surface [14] or immobilization of an IrCp* complex on a covalent triazine framework [33].The Ru catalyst in [14] was active in 20 consecutive cycles with a TON of 71,000, while in [33], an IrCp*-bipyridine complex achieved 1,060,000 TONs under continuous FA feed for 40 h.
Our research group engineered two hybrid catalysts, RPPh 2 @SiO 2 and polyRPhphos@SiO 2 [34], by grafting two phosphines, triphenylphosphine, and tris [2-(diphenylphosphine)ethyl]phosphine, onto the surface of SiO 2 particles.These systems exhibited totals of 176,000 and 65,000 TONs, respectively, under a continuous HCOOH feed [34].In the same context as hybrid materials, a heterogeneous catalyst produced by grafting imidazole-based ligands onto the surface of SiO 2 , which, with the incorporation of Fe II and a polyphosphine, showed significant performance and reusability [35].
A key finding was that using PP 3 as an additional ligand in FeL and RuL catalysts [16] significantly boosts the FADH efficiency.This 'Double-Ligand' technology in FeL/PP 3 and RuL/PP 3 benefits from the high efficiency and stability of the catalyst.
Thus, in this study, we aimed to address the limitations of current catalytic systems by developing a more efficient and straightforward approach.Traditional catalytic systems often require the synthesis of expensive and complex metal hydride structures before the catalytic procedure, which is both time-consuming and resource-intensive.These systems typically rely on pre-synthesized metal complexes with hydrides and necessitate the use of additives to boost performance.Our approach, however, synthesizes the catalyst in situ without the need for any performance-enhancing additives.This innovative method not only simplifies the synthesis process but also enhances the recyclability and efficiency of the catalysts in heterogeneous systems.By eliminating the challenges associated with pre-synthesized complexes and additives, our research provides a significant improvement in catalytic synthesis, addressing the existing research gap and offering a viable alternative for sustainable H 2 production.Characterization Techniques: Fourier Transform-Infrared spectra were recorded using an FT-IR Nicolet IS5 system equipped with an OMNIC software package 9.2.86 from 450 cm −1 to 4000 cm −1 , with 2 cm −1 resolution.Solid-state NMR spectra for 13 C, 31 P, and 29 Si were recorded in a 400 MHz magnet using cross-polarization (CP-MAS) at a spinning rate of 14,000 Hz with proton decoupling.The parameters used were as follows: acquisition time of 0.053 s, recycle delay of 0.5 s, contact time of 2 s, and 10,000 scans.

Materials
Peak correction was performed using a standard reference and field adjustment (adamantane).Thermogravimetric analysis (TGA-DTA) was performed using a SETARAM TGA 92 analyzer, (SETARAM Instrumentation, Caluire-et-Cuire, France) with a heat rate of 10 • C/min from 25 • C to 800 • C and a flow rate of 20 mL/min for the oxygen carrier gas.E h measurements were performed using an ORP electrochemical electrode combined with a Pt ring vs.Standard Hydrogen Electrode (SHE) [35] from Metrohm (Herisau, Switzerland), and the measurements were performed in situ during the catalytic procedure.Attenuated Total Reflection-FTIR (ATR-FTIR) experiments were recorded in situ in the region of 400-2000 cm −1 by an Agilent spectrometer (Agilent Technologies, Santa Clara, CA, USA), including a ZnSe-attenuated total reflection accessory.UV-Vis spectra were recorded on a Hitachi spectrophotometer, U2900 (Hitachi High-Tech Corporation, Tokyo, Japan) operating in the range λ = 250-800 nm.Liquid samples were measured in a 3 mL quartz cuvette (1 cm optical path).
ATR-FTIR measurement procedure: The ATR-FTIR procedure was carried out in situ using approximately 5 mg of ground KBr as the solid matrix.Two drops of each liquid sample (PNPs, PC, and FA) or the catalytic solutions were then added and allowed to absorb for 5 min.After absorption, the samples were pressed with an ATR-FTIR diamond head and measured.Solid samples, such as PP3 and SiO 2 , were measured directly without the addition of KBr.
Solution Potential (E h ) measurements: The reaction was carried out in a wide-neck glass tube to accommodate the redox-electrode while stirring.The amounts of reagents used and the adopted conditions were strictly similar to those for catalysis experiments.E h was continuously recorded after each addition of any reagent.Amounts of reagents for Fe catalytic system: 5 mL PC with 2 mL formic acid, 7.5 µmol [Fe II (BF 4 ) 2 × 6H 2 O], different amounts of PNP and SiO 2 @PNPs ligands, 7.5 µmol PP3.
Catalytic procedure: Catalytic reactions were carried out in a thermostatic doublewall reactor with the insertion of argon gas for the first 10 min and continuous stirring at 85 • C (±2 • C).The reactor was directly linked to a GC system (Shimadzu GC-2014 Gas Chromatograph with a Thermal Conductivity Detector, GC-TCD, Shimadzu Corporation, Kyoto, Japan) and a Carboxen-1000 column (Supelco, Bellefonte, PA, USA) for the analysis and identification of the produced gases.The total volume of the evolved gases was measured using a manual gas burette.In a typical catalytic experiment, 7.5 µmol Fe(BF 4 ) 2 × 6H 2 O were dispersed in 5 mL of PC and 2 mL of formic acid, followed by the addition of different ratios of the PNP ligand.The reaction took place for 10 min and then 7.5 µmol of P(CH 2 CH 2 PPh 2 ) 3 was inserted into the mixture.Without the use of PP3 the dehydrogenation of formic acid did not occur, meaning that the presence of PP3 is necessary.In all catalytic experiments, a molar ratio of [metal:PP3] = [1:1] was used.For the continuous operation catalytic experiments, the addition of formic acid was 1 mL when the produced gases reached around 1.2 Lt (1 mL of formic acid produces 1.2 Lt of gases).All catalytic data presented in this study were obtained from the average of at least three sets of experiments, with a standard error of 5%.Recycling experiments: To recover the catalytic materials, the reaction mixture was centrifuged at 6000 rpm for 10 min.The materials were separated from the solution, washed with methanol three times, and then dried into a dry pistol at 80 • C under vacuum.In the recycling sets of experiments, the same procedure as for a typical catalytic test described above was adopted; however, instead of new amounts of metals and PNP-based materials, the recovered solid was added.Addition of NaBH 4 : The NaBH 4 was added, where it is mentioned, after the generation of the catalytic system [metal/PNP/PP3] or [metal/SiO 2 @PNPs/PP3] as an aqueous solution.In the case of NaBH 4 addition, to ensure that the released gases were derived from FA dehydrogenation, the gas volume was measured 10 min after this addition.TONs and TOFs are calculated using the equations in our previous publication by our group [16,36] and are shown in the Supporting Information as Equations (S1) and (S2).
Synthesis and Characterization of the immobilized SiO 2 @ iPro PNP and SiO 2 @ tBu PNP Energies 2024, 17, 3934 5 of 19 Synthesis: In a 50 mL flask with 10 mL of methanol, 1.2 mmol of a PNP ligand and 1.1 mmol of 3-(chloropropyl)-trimethoxysilane were added and refluxed for 48 h.Then, 250 mg of dried SiO 2 particles were added to the solution, and the suspension was refluxed for 24 h.The obtained hybrid material was washed three times with methanol and was dried under a vacuum at 80 • C for 24 h (Scheme 1).
Energies 2024, 17, x FOR PEER REVIEW 5 of 20 Synthesis and Characterization of the immobilized SiO2@ iPro PNP and SiO2@ tBu PNP Synthesis: In a 50 mL flask with 10 mL of methanol, 1.2 mmol of a PNP ligand and 1.1 mmol of 3-(chloropropyl)-trimethoxysilane were added and refluxed for 48 h.Then, 250 mg of dried SiO2 particles were added to the solution, and the suspension was refluxed for 24 h.The obtained hybrid material was washed three times with methanol and was dried under a vacuum at 80 °C for 24 h (Scheme 1).Scheme 1.Molecular structures of iPro PNP, tBu PNP, SiO2@ iPro PNP, and SiO2@ tBu PNP.
TGA-DTA analysis: The loading of PNP ligands onto the surface of the SiO2 particles was calculated from thermogravimetric analysis (see thermograms in Figures S3 and S4).From 250 °C to 500 °C a complete combustion of the carbonaceous backbone of the PNP ligands occurs, followed by two exothermic peaks at 300 and 500 °C.The calculated (%) loading based on the mass loss was found to be 9.5% (17 mmol iPro PNP/g of material) for SiO2@ iPro PNP and 9.7% (22 mmol tBu PNP/gr of material) for SiO2@ tBu PNP.
Solid-State NMR: Alongside FT-IR spectroscopy, an ss-NMR study was conducted by running 13 C, 31 P, and 29 Si spectra to further ensure the successful immobilization of the PNP ligands onto the surface of the SiO2 particles. 13C spectrum of SiO2@ iPro PNP is shown in the Supplementary Material in Figure S5, where, for convenience, the structure of the grafted molecule is shown on the left side.The peak at 9.46 ppm corresponds to the methyl carbon atoms (g) of the iso-propyl groups, while those at 16.14 and 21.59 ppm are assigned to methylene C atoms (b) and (a), respectively, of the linker propyl-group.The sharp peak at 26.46 ppm corresponds to the methylene carbons (e) connected directly to P atoms, while that at 45.50 ppm to the carbons (f) Scheme 1.Molecular structures of iPro PNP, tBu PNP, SiO 2 @ iPro PNP, and SiO 2 @ tBu PNP.
TGA-DTA analysis: The loading of PNP ligands onto the surface of the SiO 2 particles was calculated from thermogravimetric analysis (see thermograms in Figures S3 and S4).From 250 • C to 500 • C a complete combustion of the carbonaceous backbone of the PNP ligands occurs, followed by two exothermic peaks at 300 and 500 • C. The calculated (%) loading based on the mass loss was found to be 9.5% (17 mmol iPro PNP/g of material) for SiO 2 @ iPro PNP and 9.7% (22 mmol tBu PNP/gr of material) for SiO 2 @ tBu PNP.
Solid-State NMR: Alongside FT-IR spectroscopy, an ss-NMR study was conducted by running 13 C, 31 P, and 29 Si spectra to further ensure the successful immobilization of the PNP ligands onto the surface of the SiO 2 particles. 13C spectrum of SiO 2 @ iPro PNP is shown in the Supplementary Material in Figure S5, where, for convenience, the structure of the grafted molecule is shown on the left side.The peak at 9.46 ppm corresponds to the methyl carbon atoms (g) of the iso-propyl groups, while those at 16.14 and 21.59 ppm are assigned to methylene C atoms (b) and (a), respectively, of the linker propyl-group.The sharp peak at 26.46 ppm corresponds to the methylene carbons (e) connected directly to P atoms, while that at 45.50 ppm to the carbons (f) of the iso-propyl groups, which are also connected directly to P atoms.Finally, the peaks observed at 50.41 and 59.46 ppm are attributed to the methylene carbons (d) and (c), respectively, which are both bonded with the N atom of the iPro PNP ligand [22,38].
Accordingly, the 13 C spectrum of SiO 2 @ tBu PNP and the molecular structure of the grafted tBu PNP functionality are shown in Figure S6.The peak at 9.15 ppm corresponds to the methylene carbon atoms (e), which are connected directly to P atoms, while those at 16.21 and 30.04 ppm are assigned to methylene C atoms (b) and (a), respectively, of the linker propyl-group.The sharp peak at 26.32 ppm corresponds to the methyl carbons (g) of the tert-butyl groups, while the (f) carbons of the tert-butyl groups, which are connected to P atoms, appear at 45.53 ppm.Finally, the peaks observed at 49.47 and 59.41 ppm are attributed to the methylene carbons (d) and (c), respectively, which are both linked to the N atom of the tBu PNP ligand [22,38].
The 31 P spectra of SiO 2 @ iPro PNP and SiO 2 @ tBu PNP are presented in Figures S7 and S8, respectively.The intense and sharp peak at 41.68 ppm for SiO 2 @ iPro PNP (Figure S7) and at 71.41 ppm for SiO 2 @ tBu PNP (Figure S8) were assigned to the two chemically equivalent phosphorus atoms of the PNP ligands.The broad and low to medium-intensity peaks appeared at 35.84 and 64.39 ppm in the spectrum of SiO 2 @ iPro PNP and at 47.52 and 41.61 ppm in the SiO 2 @ tBu PNP spectrum are attributed to chemically non-equivalent phosphorus atoms of the PNP ligands; the observed chemical shifts could be a result of different interactions of the two phosphorus atoms due to spatial limitations derived from the grafting of the PNP functionalities on the silica support [22,38].
The 29 Si spectra of the hybrid materials SiO 2 @ iPro PNP and SiO 2 @ tBu PNP are presented in Figures S9 and S10, respectively.Both show two sets of bands centered at −100 and −60 ppm, which correspond to Q n and T n type of Si atoms.In the 29 Si spectrum of SiO 2 @ iPro PNP , the intense peak at −102.0 ppm is assigned to Q 3 atoms, the second highest peak at −110.8 ppm to Q 4 species, and a shoulder appearing at −92.0 ppm corresponds to Q 2 Si atoms; these siloxane-type Si are derived from the silica support used for grafting [39,40].The corresponding peaks of SiO 2 @ tBu PNP rose at −101.3, −110.7 and −90.4 ppm, respectively.The T n species originated from Si atoms of organo-silane alkoxy precursor and indicates the degree of co-polymerization with the silica surface; the T 3 bears 3 siloxane bonds and represents a species that has undergone full co-polymerization.In the 29 Si spectrum of SiO 2 @ iPro PNP , the T 2 peak at −58.9 ppm is the highest of the T n set peaks, with T 3 at −67.2 ppm being the second highest; the shoulder at −50.8 ppm is assigned to the T 1 species.The set of T n peaks for SiO 2 @ tBu PNP shows a similar profile, and the corresponding peaks appear at −57.9, −66,4, and −49,8 ppm, respectively.These data further confirm that the grafting process was successful, resulting in a strong attachment of the PNP functionalities on the silica surface via the formation of 2 or 3 siloxane bonds [39,40].

Evaluation of the Solution Potential, E h , of the Catalytic Process
Before assessing H 2 production via FADH, the solution potential profile of the catalytic systems was monitored.As shown recently, in situ monitoring of the solution potential E h [41] offers a convenient and reliable tool to frame the physicochemical characteristics of the catalytic system under study.In this context, the E h -profiles of homogeneous [Fe II / iPro PNP/PP3], [Fe II / tBu PNP/PP3], and heterogeneous [SiO 2 @ iPro PNP/Fe II /PP3], and [SiO 2 @ tBu PNP/Fe II /PP3] systems were monitored using the optimal [PNP:Fe] ratio [1:1:1].The experimental Eh-data are presented in Figure 1.As seen, when all the reagents have been added, the solution potential Eh reaches a negative value of −49 mV to −67 mV, indicating the formation of an active environment for FA dehydrogenation [41].positive values (refer to data in Figure S11 in the Supplementary Information), and gas production was minimal.

Spectroscopic study of the catalytic systems
UV-Vis Spectroscopy: The in situ formation of the active [Fe II / iPro PNP/PP3] and [Fe II / tBu PNP/PP3] catalysts was confirmed using UV-Vis spectroscopy, as shown in Figure 2. Subsequently, spontaneous generation of bubbles was observed.It is important to note that, in the absence of PP3, the Eh of the catalytic reactions based on the [Fe II / iPro PNP], [Fe II / tBu PNP], [SiO 2 @ iPro PNP/Fe II ], and [SiO 2 @ tBu PNP/Fe II ] systems consistently remained at positive values (refer to data in Figure S11 in the Supplementary Information), and gas production was minimal.

Spectroscopic study of the catalytic systems
UV-Vis Spectroscopy: The in situ formation of the active [Fe II / iPro PNP/PP3] and [Fe II / tBu PNP/PP3] catalysts was confirmed using UV-Vis spectroscopy, as shown in Figure 2. The UV-Vis spectra of [Fe II / iPro PNP] and [Fe II / tBu PNP] present analogous profiles.The intense bands observed at 290 and 343 nm, as well as the low-intensity peaks at 410 and 470 nm, are attributed to the Fe 2 ⁺ oxidative species and the Metal-to-Ligand Charge Transfer (MLCT) transitions due to the interaction of Fe 2 ⁺ with the PNP ligands (d→πPNP) [31].In both systems, after the addition of the PP3 ligand, a band with a maximum at 500 nm is observed, which is assigned to the MLCT transition derived from the interaction of Fe 2 ⁺ with PP3 (d→πPP3) [16].This clearly indicates the formation of tertiary [Fe II / iPro PNP/PP3] and [Fe II / tBu PNP/PP3] systems during FADH catalysis.The mediumintensity band with a maximum at 390 nm, which also appeared after the addition of PP3 in both systems, indicates the generation of active Fe-H species directly involved in this type of catalysis and further supports the reactivity of the studied systems [42].Thus, UV-Vis spectroscopy was crucial in understanding the electronic interactions between the Fe 2 ⁺ ions and the ligands, as well as the evolution of the metal coordination environment during catalysis by identifying intermediate species.
The ATR-FTIR spectrum of FA in PC (Figure S12a) is characterized by bands at 1787 cm −1 , 1394 cm −1 , and 1195 cm −1 , corresponding to the stretching of the C=O bond, bending of the O-H bond, and stretching of the C-O bond, respectively.Additionally, the bands at 1115 cm −1 and 1048 cm −1 correspond to the stretching of the asymmetric C-O-C bond in PC.In Figure S12b, the bands at 1480 cm −1 , 735 cm −1 , and 692 cm −1 correspond to the stretching of the C=C bonds in aromatic structures and the stretching of the P-C bonds, respectively, in the PP3 ligand [35,43].The ATR-FTIR spectra of the hybrid materials SiO2@ iPro PNP and SiO2@ tBu PNP (Figure S12c,d) are dominated by the bands at 1054 cm −1 and 450 cm −1 , which correspond to the stretching of the Si-O-Si bonds, while the band at The UV-Vis spectra of [Fe II / iPro PNP] and [Fe II / tBu PNP] present analogous profiles.The intense bands observed at 290 and 343 nm, as well as the low-intensity peaks at 410 and 470 nm, are attributed to the Fe 2+ oxidative species and the Metal-to-Ligand Charge Transfer (MLCT) transitions due to the interaction of Fe 2+ with the PNP ligands (d→π PNP ) [31].In both systems, after the addition of the PP3 ligand, a band with a maximum at 500 nm is observed, which is assigned to the MLCT transition derived from the interaction of Fe 2+ with PP3 (d→π PP3 ) [16].This clearly indicates the formation of tertiary [Fe II / iPro PNP/PP3] and [Fe II / tBu PNP/PP3] systems during FADH catalysis.The medium-intensity band with a maximum at 390 nm, which also appeared after the addition of PP3 in both systems, indicates the generation of active Fe-H species directly involved in this type of catalysis and further supports the reactivity of the studied systems [42].Thus, UV-Vis spectroscopy was crucial in understanding the electronic interactions between the Fe 2+ ions and the ligands, as well as the evolution of the metal coordination environment during catalysis by identifying intermediate species.
The ATR-FTIR spectrum of FA in PC (Figure S12a) is characterized by bands at 1787 cm −1 , 1394 cm −1 , and 1195 cm −1 , corresponding to the stretching of the C=O bond, bending of the O-H bond, and stretching of the C-O bond, respectively.Additionally, the bands at 1115 cm −1 and 1048 cm −1 correspond to the stretching of the asymmetric C-O-C bond in PC.In Figure S12b, the bands at 1480 cm −1 , 735 cm −1 , and 692 cm −1 correspond to the stretching of the C=C bonds in aromatic structures and the stretching of the P-C bonds, respectively, in the PP3 ligand [35,43].The ATR-FTIR spectra of the hybrid materials SiO 2 @ iPro PNP and SiO 2 @ tBu PNP (Figure S12c,d and 450 cm −1 , which correspond to the stretching of the Si-O-Si bonds, while the band at 795 cm −1 refers to the stretching of the Si-OH bonds.More detailed information concerning the FT-IR characterization of these materials is provided in Figures S1 and S2. Figure S13 presents the ATR-FTIR spectra of [Fe II / iPro PNP] (Figure S13a), [Fe II / tBu PNP] (Figure S13b), [Fe II /SiO 2 @ iPro PNP] (Figure S13c), and [Fe II /SiO 2 @ tBu PNP] (Figure S13d) in PC in the presence of FA.The bands at 510 cm −1 , 450 cm −1 , and 417 cm −1 are assigned to the stretching of Fe-P and Fe-N bonds, respectively, confirming the formation of the Fe-PNP complexes in all cases.The peak at 1380 cm −1 (Figure S13a,c) is attributed to the iso-propyl groups of iPro PNP [44], and the peak at 1462 cm −1 (Figure S13b,d) is attributed to the tert-butyl groups of the tBu PNP ligands [45].The vibrations of the v(Si-O-Si) bonds from the SiO 2 matrix (Figure S12c,d) overlap with the strong bands from the stretching of the v(C-O-C) bond derived from the PC solvent (Figure S13c,d).Additionally, two other bands located at 1585-1581 cm −1 and 1508-1513 cm −1 in all spectra are attributed to formate vibrations, indicative of the type of bonding of the HCOO −1 anion, i.e., η 1 -O 2 CH and η 2 -O 2 CH, respectively [35,43].These data provide direct evidence that the complex [Fe/PNP/FA] has been formed in all cases.However, it should be noted that in the absence of PP3, the catalytic data show zero catalytic FADH activity.This indicates that [Fe/PNP/FA] without PP3 fails to generate an appropriate reduced environment for FADH catalysis, as evidenced by the positive E h values measured herein, as discussed above.
Overall, the ATR-FTIR spectra provide insights into the chemical bonding and structural changes occurring in the catalysts.The characteristic bands observed confirmed the ATR-FTIR spectra provide insights into the chemical bonding and structural changes occurring in the catalysts.The characteristic bands observed confirmed the successful coordination of the PNP and PP3 ligands with the Fe 2+ center, thus the formation of the active catalysts as well as the involvement of the formic substrate.

Standard catalytic H 2 -production
Based on the data obtained from E h mapping, UV-Vis, and ATR-FTIR data, a standard FA dehydrogenation experiment was conducted in propylene carbonate (PC) at 85 • C with 2 mL FA (48 mmol), catalyzed by Fe II / iPro PNP/PP3, Fe II /SiO 2 @ iPro PNP/PP3, Fe II / tBu PNP/PP3, or Fe II /SiO 2 @ tBu PNP/PP3.The catalytic performance, as demonstrated by H 2 production from FA, directly correlates with the electronic and structural properties of the catalysts observed in the UV-Vis and ATR-FTIR analyses.The volume of gases produced over time, as presented in Figure 4, confirmed the catalytic reactivity of the studied systems.All catalytic gaseous products were subjected to a quality check by GC-TCD, and a typical chromatograph is shown in Supporting Information Figure S15.The only detected products were H 2 and CO 2 without any trace of CO, which makes the present catalytic systems suitable for use in fuel cell technology.
The catalysis proceeds under the same conditions and follows the protocol described above, using 2 mL of FA at the beginning of the reaction.However, when 1200 mL of gases Catalytic conditions: 5 mL of PC and 2 mL of FA at 85 • C. 7.5 µmol of Fe II (BF 4 ) 2 •6H 2 O reacted with 7.5 µmol of iPro PNP, SiO 2 @ iPro PNP (40 mg), tBu PNP, or SiO 2 @ tBu PNP (53 mg).The reaction commenced after the addition of 7.5 µmol of PP3.
More specifically, the homogeneous [Fe II / iPro PNP/PP3] generated over 2300 mL of gases in 50 min, with a TON of 6406 and a TOF value of 7718 h −1 .[Fe/ tBu PNP/PP3] produced 2260 mL of gases in 60 min, resulting in a TON of 6242 h −1 .For comparison, the homogeneous [Fe II /L g /PP3], first reported by our research group [14], achieved a TOF value of 5208 h −1 , significantly lower than the [Fe II / tBu PNP/PP3] reported here.Interestingly, the heterogeneous [Fe II /SiO 2 @ iPro PNP/PP3] proved to be more efficient than its homogeneous counterpart, achieving a TON of 6460 within 40 min and a TOF of 9787 h −1 , the highest among all systems studied herein.Finally, [Fe II /SiO 2 @ tBu PNP/PP3] achieved a TON of 6297 h −1 , slightly higher than the TON of the corresponding homogeneous [Fe II / tBu PNP/PP3] system.
The catalysis proceeds under the same conditions and follows the protocol described above, using 2 mL of FA at the beginning of the reaction.However, when 1200 mL of gases are released, a volume approximately estimated to be produced from the decomposition of 1 mL of FA, an additional 1 mL of FA is added.Under this setup, the catalysis driven by the present systems continues for 4 h and consumes 8-10 mL of FA.The gas production from FA over time catalyzed by tertiary [Fe II / iPro PNP/PP3], [Fe II / tBu PNP/PP3], [Fe II /SiO 2 @ iPro PNP/PP3], and [Fe II /SiO 2 @ tBu PNP/PP3] is shown in Figure 5.For comparison, the efficiency of [Fe II /PP3] under the same experimental conditions is detailed in Figure S14b in the Supplementary Material and represented by the gray lines in Figure 5a,b.
Under continuous operation, the homogeneous [Fe II / iPro PNP/PP3] (Figure 5a, red line) and [Fe II / tBu PNP/PP3] (Figure 5b, green line) converted 9 and 10 mL of FA (see Table S1 of the Supplementary Material) and produced 8120 mL and 8770 mL of gases at rates of 45 and 53 mL/min, respectively.In terms of TONs, they achieved 22,135 and 23,907 with TOF values of 5533 h −1 and 5976 h −1 , respectively.Both [Fe II / iPro PNP/PP3] and [Fe II / tBu PNP/PP3] exhibited better performance than the reference homogeneous [Fe/PP3] system, which provided 7680 mL of gases from 9 mL of FA at a rate of 40 mL/min, resulting in a TON of 20,936 and a TOF value of 5234 h −1 .The enhanced reactivity of the tertiary systems compared to the binary [Fe/PP3], which does not include any PNP molecule as a second ligand, is due to the 'double ligand' beneficial effect [33].This effect arises from the second σ-donor ligand, which further electronically enriches the metal center and enhances its catalytic behavior.
are released, a volume approximately estimated to be produced from the decomposition of 1 mL of FA, an additional 1 mL of FA is added.Under this setup, the catalysis driven by the present systems continues for 4 h and consumes 8-10 mL of FA.The gas production from FA over time catalyzed by tertiary [Fe II / iPro PNP/PP3], [Fe II / tBu PNP/PP3], [Fe II /SiO2@ iPro PNP/PP3], and [Fe II /SiO2@ tBu PNP/PP3] is shown in Figure 5.For comparison, the efficiency of [Fe II /PP3] under the same experimental conditions is detailed in Figure S14b in the Supplementary Material and represented by the gray lines in Figure 5a,b.S1 of the Supplementary Material) and produced 8120 mL and 8770 mL of gases at rates of 45 and 53 mL/min, respectively.In terms of TONs, they achieved 22,135 and 23,907 with TOF values of 5533 h −1 and 5976 h −1 , respectively.Both [Fe II / iPro PNP/PP3] and [Fe II / tBu PNP/PP3] exhibited better performance than the reference homogeneous [Fe/PP3] system, which provided 7680 mL of gases from 9 mL of FA at a rate of 40 mL/min, resulting in a TON of 20,936 and a TOF value of 5234 h −1 .The enhanced reactivity of the tertiary systems compared to the binary [Fe/PP3], which does not include any PNP molecule as a second ligand, is due to the 'double ligand' beneficial effect [33].This effect arises from the second σ-donor ligand, which further electronically enriches the metal center and enhances its catalytic behavior.
The heterogeneous [Fe II /SiO2@ tBu PNP/PP3] system (Figure 5b, orange line) showed slightly better catalytic performance than the corresponding homogeneous [Fe II / tBu PNP/PP3] system (Figure 5b, green line) during the first 2 h of catalysis.However, it showed a delay, ultimately providing 7860 mL of gases at an average rate of 44 mL/min, with a TON of 21,427 and a TOF value of 5356 h −1 .In contrast, under continuous FA feeding, the heterogeneous [Fe II /SiO2@ iPro PNP/PP3] (Figure 5a, blue line) performed much better than the homogeneous [Fe II / iPro PNP/PP3] (Figure 5a, red line) and was by far the best catalytic system studied herein.It produced 12,010 mL of gases within 220 min at a rate of 65 mL/min from the decomposition of 11 mL of FA, achieving a TON of 35,079 with a TOF of 9643 h −1 .The heterogeneous [Fe II /SiO 2 @ tBu PNP/PP3] system (Figure 5b, orange line) showed slightly better catalytic performance than the corresponding homogeneous [Fe II / tBu PNP/PP3] system (Figure 5b, green line) during the first 2 h of catalysis.However, it showed a delay, ultimately providing 7860 mL of gases at an average rate of 44 mL/min, with a TON of 21,427 and a TOF value of 5356 h −1 .In contrast, under continuous FA feeding, the heterogeneous [Fe II /SiO 2 @ iPro PNP/PP3] (Figure 5a, blue line) performed much better than the homogeneous [Fe II / iPro PNP/PP3] (Figure 5a, red line) and was by far the best catalytic system studied herein.It produced 12,010 mL of gases within 220 min at a rate of 65 mL/min from the decomposition of 11 mL of FA, achieving a TON of 35,079 with a TOF of 9643 h −1 .
Indeed, during the continuous operation experiment, gas production ceased after an adequate amount of FA had been added.This could be because the FA used contained 2.5% water.This small amount of water accumulates in the catalytic mixture over time, causing the catalyst to slowly deactivate and lose its efficiency.The amount of active catalyst is 7.5 µmol, and even this small amount of water is sufficient to impact its performance.Molecular catalysts, like the ones we used, do not operate at maximum efficiency in the presence of water, leading to incomplete catalysis of the FA.In homogeneous catalytic systems, the suppressive impact of water is not easily overcome [36].Nevertheless, as we demonstrate below, current heterogeneous systems offer an economical solution to mitigate this inhibitory effect.That is, when catalytic H 2 production ceased, the solid catalyst was centrifuged, rinsed, and reused, leading to continuous H 2 production from new quantities of added FA (Figure 6).This indicates that the drop in catalytic performance after the dehydrogenation of approximately 11 mL of FA was not due to irreversible catalyst damage but could be remedied through a simple washing and drying process.

Recycling of the catalytic materials
The enhanced reactivity of [Fe II /SiO2@ iPro PNP/PP3] under continuous catalytic operation and its heterogeneous nature prompted the recovery of the solid material from the catalysis medium for reuse in a second catalytic process with a continuous FA feed.The solid was recovered from the reaction mixture by centrifugation, washed with methanol (MeOH), and dried.In the recycling experiments, the same protocol as for the continuous operation process was adopted; however, instead of new amounts of the Fe precursor and SiO2@ iPro PNP or SiO2@ tBu PNP, the recovered solids were used.The [Fe II /SiO2@ tBu PNP] was not able to perform after the first use; however, the [Fe II /SiO2@ iPro PNP] operated for a second time after the addition of a new dose of PP3, producing 6620 mL of gas from decomposing 8 mL of FA at an average rate of 36 mL/min (Figure 6, violet line), with a TON of 23,822 and a TOF value of 6438 h −1 (Table S1).The [Fe II /SiO2@ iPro PNP] catalyst was recovered for a second time, washed, dried, and reused in a catalytic process under continuous FA feed for a third time.Its performance in terms of gas production vs. time The enhanced reactivity of [Fe II /SiO 2 @ iPro PNP/PP3] under continuous catalytic operation and its heterogeneous nature prompted the recovery of the solid material from the catalysis medium for reuse in a second catalytic process with a continuous FA feed.The solid was recovered from the reaction mixture by centrifugation, washed with methanol (MeOH), and dried.
In the recycling experiments, the same protocol as for the continuous operation process was adopted; however, instead of new amounts of the Fe precursor and SiO 2 @ iPro PNP or SiO 2 @ tBu PNP, the recovered solids were used.The [Fe II /SiO 2 @ tBu PNP] was not able to perform after the first use; however, the [Fe II /SiO 2 @ iPro PNP] operated for a second time after the addition of a new dose of PP3, producing 6620 mL of gas from decomposing 8 mL of FA at an average rate of 36 mL/min (Figure 6, violet line), with a TON of 23,822 and a TOF value of 6438 h −1 (Table S1).The [Fe II /SiO 2 @ iPro PNP] catalyst was recovered for a second time, washed, dried, and reused in a catalytic process under continuous FA feed for a third time.Its performance in terms of gas production vs. time is shown in Figure 6 (cyan line).Although it remained active, the observed reactivity was significantly reduced (for details, see Table S1).After the third use, it was no longer able to release gas.This is mainly attributed to the reduction in the amount of solid catalyst because, for practical reasons, its recuperation from the reaction mixture via centrifugation is not fully successful, leading to a significant loss of the catalytic material.Despite this, the recycling experiments highlight the ability of the SiO 2 @ iPro PNP hybrid material to form stable and durable [Fe II /SiO 2 @ iPro PNP] units capable of operating in FADH under continuous FA feed and to be recovered and reused in new processes.Within this framework, [Fe II /SiO 2 @ iPro PNP], associated with PP3, operated three times in total, producing 21,890 mL of gas from the decomposition of 23 mL of FA and achieving a TON of 74,451.
As demonstrated hereafter, the high reactivity of [Fe II /SiO 2 @ iPro PNP/PP3] has a thermodynamic basis, as proven by Arrhenius analysis.

Arrhenius analysis
A thermodynamic study was conducted on the [Fe II / iPro PNP/PP3], [Fe II / tBu PNP/PP3], [Fe II /SiO 2 @ iPro PNP/PP3], and [Fe II /SiO 2 @ tBu PNP/PP3] systems.The gas production over time from FADH catalysis driven by each system at different temperatures (85 • C, 75 • C, 65 • C, and 55 • C) was measured and is presented in Figure 7.The corresponding TOF values were applied to the Arrhenius equation lnTOF = − E a R × 1 T + c (where E a repre-sents the activation energy in J/mol, T is the temperature in K, and R = 8.314 J/K•mol), to create the Arrhenius plots.Figure 7a,c shows the gas production over time at different temperatures for the [Fe II / iPro PNP/PP3] and [Fe II /SiO 2 @ iPro PNP/PP3] systems, respectively.The corresponding Arrhenius plots are shown in Figure 7b,d.Based on the linear Arrhenius fits, the activation energies for [Fe/ iPro PNP/PP3] and [Fe II /SiO 2 @ iPro PNP/PP3] were calculated to be E a = 48.2± 0.1 kJ/mol and E a = 42.5 ± 0.1 kJ/mol, respectively, elucidating the higher catalytic activity of the latter system.The gas production vs. time from FADH at different temperatures catalyzed by [Fe II / tBu PNP/PP3] and [Fe II /SiO2@ tBu PNP/PP3] is shown in Figure 8a,c, respectively.Figure 8 b,d present the corresponding Arrhenius plots, where the lines fit to the data parameterized by the Arrhenius equation.This analysis estimated the activation energy, which is found to be Ea = 46.4± 0.09 kJ/mol for [Fe II / tBu PNP/PP3] and Ea = 48.3± 0.08 kJ/mol for [Fe II /SiO2@ tBu PNP/PP3].[Fe II / tBu PNP/PP3] exhibits a slightly lower Ea compared to that of [Fe II /SiO2@ tBu PNP/PP3], which could be attributed to the spatial restrictions of the immobilized tBu PNP with the bulky t-butyl groups on the silica surface.
The Ea values of the homogeneous [Fe II / iPro PNP/PP3] and [Fe II / tBu PNP/PP3], as well as the heterogeneous [Fe II /SiO2@ iPro PNP/PP3] and [Fe II /SiO2@ tBu PNP/PP3], found herein, are within the range of 40-60 kJ/mol, similar to those reported for analogous catalytic systems active in FADH catalysis [28,34].However, it is highlighted that the low Ea of the heterogeneous [Fe II /SiO2@ iPro PNP/PP3], which is 42.5 ± 0.1 kJ/mol, is the lowest among the catalysts studied herein.This lower Ea explains the higher catalytic activity of [Fe II /SiO2@ iPro PNP/PP3] on a physicochemical basis.The gas production vs. time from FADH at different temperatures catalyzed by [Fe II / tBu PNP/PP3] and [Fe II /SiO 2 @ tBu PNP/PP3] is shown in Figure 8a,c, respectively.Figure 8b,d present the corresponding Arrhenius plots, where the lines fit to the data parameterized by the Arrhenius equation.This analysis estimated the activation energy, which is found to be E a = 46.4± 0.09 kJ/mol for [Fe II / tBu PNP/PP3] and E a = 48.3± 0.08 kJ/mol for [Fe II /SiO 2 @ tBu PNP/PP3].[Fe II / tBu PNP/PP3] exhibits a slightly lower E a compared to that of [Fe II /SiO 2 @ tBu PNP/PP3], which could be attributed to the spatial restrictions of the immobilized tBu PNP with the bulky t-butyl groups on the silica surface.
the heterogeneous [Fe II /SiO2@ iPro PNP/PP3] and [Fe II /SiO2@ tBu PNP/PP3], found herein, are within the range of 40-60 kJ/mol, similar to those reported for analogous catalytic systems active in FADH catalysis [28,34].However, it is highlighted that the low Ea of the heterogeneous [Fe II /SiO2@ iPro PNP/PP3], which is 42.5 ± 0.1 kJ/mol, is the lowest among the catalysts studied herein.This lower Ea explains the higher catalytic activity of [Fe II /SiO2@ iPro PNP/PP3] on a physicochemical basis.

Comparison with the literature
Table 1 presents the activities of molecular metal catalysts involving iPro PNP and tBu PNP ligands for FADH, discussed in terms of TONs, TOFs, operational time, and the The E a values of the homogeneous [Fe II / iPro PNP/PP3] and [Fe II / tBu PNP/PP3], as well as the heterogeneous [Fe II /SiO 2 @ iPro PNP/PP3] and [Fe II /SiO 2 @ tBu PNP/PP3], found herein, are within the range of 40-60 kJ/mol, similar to those reported for analogous catalytic systems active in FADH catalysis [28,34].However, it is highlighted that the low E a of the heterogeneous [Fe II /SiO 2 @ iPro PNP/PP3], which is 42.5 ± 0.1 kJ/mol, is the lowest among the catalysts studied herein.This lower E a explains the higher catalytic activity of [Fe II /SiO 2 @ iPro PNP/PP3] on a physicochemical basis.

Comparison with the literature
Table 1 presents the activities of molecular metal catalysts involving iPro PNP and tBu PNP ligands for FADH, discussed in terms of TONs, TOFs, operational time, and the need for additives.Currently, the PNP ligands used in FADH catalysis include iPro PNP, which are associated with Ir [18], Ru [28], Fe [19], and Mn [22] to form the corresponding metal complexes.In all these cases, the active catalysts are {hydrido}-and/or {carbonylo}-{ iPro PNP}-metal complexes, which require additives to boost their activity [19,28].Some of these catalysts have achieved high TON values along with long working times of up to 9.5 h and high TOF [21].To date, no references in the literature have been found on the use of tBu PNP as a ligand in FADH catalysis.In this research, Fe-catalysts are formed in situ by the association of [Fe/ iPro PNP/PP3] and [Fe/ tBu PNP/PP3].Both catalysts are immediately active in FADH catalysis without the need for an additive or additional ligation with hydrides and/or CO.They operate for 4 h and achieve TON values of 22,100 and 23,900, respectively.Covalent grafting of iPro PNP and tBu PNP on a silica surface results in hybrids SiO 2 @ iPro PNP and SiO 2 @ tBu PNP, which are used as modified scaffolds for catalysis.Their association with Fe and PP3 forms in situ the heterogeneous [Fe/SiO 2 @ iPro PNP/PP3] and [Fe/SiO 2 @ tBu PNP/PP3], which are active in FADH catalysis.Additionally, the [Fe/SiO 2 @ iPro PNP/PP3] catalyst is recyclable and demonstrates significant performance, achieving 74,450 TONs within 12 h of continuous operation.

Conclusions
This work explores the use of two PNP ligands, iPro PNP and tBu PNP, in doubleligand Fe-catalysts for the dehydrogenation of FA.A convenient in situ formation of the [Fe II / iPro PNP/PP3] and [Fe II / tBu PNP/PP3] catalysts is presented, which does not require additives or additional ligation with hydrides and/or CO for efficient catalysis.The synthesis of SiO 2 @ iPro PNP and SiO 2 @ tBu PNP materials via covalent immobilization on a silica surface has been achieved and demonstrated by advanced spectroscopic techniques.Accordingly, the in situ generation of [Fe II /SiO 2 @ iPro PNP/PP3] and [Fe II /SiO 2 @ tBu PNP/PP3] in catalysis has been proven, as well as their efficiency.Among the catalysts presented here, [Fe II /SiO 2 @ iPro PNP/PP3] exhibited remarkable catalytic behavior; it was recyclable, operating three times under a continuous feed of FA, producing a total of 10,945 H 2 gas and achieving a TON of 74,451.This reactivity has a thermodynamic basis, as proven by Arrhenius analysis, due to a low activation energy (E a ) of 42.5 ± 0.1 kJ/mol, which is the lowest among the others measured in this study.The development of efficient and recyclable catalysts can potentially lower the costs associated with H 2 production and storage, facilitating broader adoption in various industrial applications.Moreover, our catalysts' performance under mild conditions aligns with the industry's need for scalable and cost-effective solutions.By addressing the current challenges in H 2 production and offering viable alternatives, our work contributes to the advancement of new energy technologies and supports the transition to a more sustainable energy future.The commercial application of these catalysts could enhance H 2 's role as a clean energy carrier, promoting environmental benefits and industrial innovation.

Figure 2 .
Figure 2. UV-Vis spectra of [Fe II / iPro PNP] (red line) and [Fe II / tBu PNP] (green line) in PC and FA.The corresponding UV-Vis spectra of [Fe II / iPro PNP] and [Fe II / tBu PNP] after the addition of PP3 are depicted by blue and cyan lines, respectively.

Figure 2 .
Figure 2. UV-Vis spectra of [Fe II / iPro PNP] (red line) and [Fe II / tBu PNP] (green line) in PC and FA.The corresponding UV-Vis spectra of [Fe II / iPro PNP] and [Fe II / tBu PNP] after the addition of PP3 are depicted by blue and cyan lines, respectively.

Figure 5 .
Figure 5. Continuous operation of systems (a) [Fe II / iPro PNP/PP3] (red line), [Fe II /SiO2@ iPro PNP/PP3] (blue line), [Fe II /PP3] (gray line), (b) [Fe II / tBu PNP/PP3] (green line), [Fe II /SiO2@ tBu PNP] (orange line), and [Fe II /PP3] (gray line).The arrows indicate the addition of a new amount of FA (1 mL).Catalytic conditions: 5 mL PC and 2 mL FA at 85 °C.7.5 µmol Fe II (BF4)2.6H2Oreact with 7.5 µmol of iPro PNP, SiO2@ iPro PNP (40 mg), tBu PNP, or SiO2@ tBu PNP (53 mg).The reaction begins after the addition of 7.5 µmol of PP3.Under continuous operation, the homogeneous [Fe II / iPro PNP/PP3] (Figure 5a, red line) and [Fe II / tBu PNP/PP3] (Figure 5b, green line) converted 9 and 10 mL of FA (see TableS1of the Supplementary Material) and produced 8120 mL and 8770 mL of gases at rates of 45 and 53 mL/min, respectively.In terms of TONs, they achieved 22,135 and 23,907 with TOF values of 5533 h −1 and 5976 h −1 , respectively.Both [Fe II / iPro PNP/PP3] and [Fe II / tBu PNP/PP3] exhibited better performance than the reference homogeneous [Fe/PP3] system, which provided 7680 mL of gases from 9 mL of FA at a rate of 40 mL/min, resulting in a TON of 20,936 and a TOF value of 5234 h −1 .The enhanced reactivity of the tertiary systems compared to the binary [Fe/PP3], which does not include any PNP molecule as a second ligand, is due to the 'double ligand' beneficial effect[33].This effect arises from the second σ-donor ligand, which further electronically enriches the metal center and enhances its catalytic behavior.The heterogeneous [Fe II /SiO2@ tBu PNP/PP3] system (Figure5b, orange line) showed slightly better catalytic performance than the corresponding homogeneous [Fe II / tBu PNP/PP3] system (Figure5b, green line) during the first 2 h of catalysis.However, it showed a delay, ultimately providing 7860 mL of gases at an average rate of 44 mL/min, with a TON of 21,427 and a TOF value of 5356 h −1 .In contrast, under continuous FA feeding, the heterogeneous [Fe II /SiO2@ iPro PNP/PP3] (Figure5a, blue line) performed much better than the homogeneous [Fe II / iPro PNP/PP3] (Figure5a, red line) and was by far the best catalytic system studied herein.It produced 12,010 mL of gases within 220 min at a rate of 65 mL/min from the decomposition of 11 mL of FA, achieving a TON of 35,079 with a TOF of 9643 h −1 .

Author Contributions:
Conceptualization, M.L.; H 2 production, M.T.; ssNMR spectroscopy, M.T.; writing-original draft preparation, M.T. and M.L.; revision preparation, M.T., and M.S.; writingreview and editing, Y.D. and M.L.; supervision, M.L.All authors have read and agreed to the published version of the manuscript.Funding: 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 and Researchers" (Project Number: 3832).