Y Berro

S Gueddida

Y Bouizi

C Bellouard

El-E Bendeif

A Gansmuller

A Celzard

V Fierro

D Ihiawakrim

O Ersen

M Kassir

F El

Haj Hassan

S Lebegue

M Badawi

N Canilho

A Pasc

Imprinting isolated single iron atoms onto mesoporous silica by templating with metallosurfactants

Keywords: Single iron atom catalysts, mesoporous silica, metallosurfactants, Density Functional Theory, spin state, pair distribution function

Imprinting isolated single iron atoms onto mesoporous silica by templating with metallosurfactants

Introduction

Increasing the number of active sites of a material in order to enhance its catalytic activity is a continuous challenge [START_REF] Li | Ultrafine jagged platinum nanowires enable ultrahigh mass activity for the oxygen reduction reaction[END_REF][START_REF] Chen | Atomically Dispersed Iron-Nitrogen Species as Electrocatalysts for Bifunctional Oxygen Evolution and Reduction Reactions[END_REF][START_REF] Thomas | The advantages and future potential of single-site heterogeneous catalysts[END_REF]. This is commonly addressed by increasing the active surface area of catalytic particles and thus by decreasing their size. The most promising catalysts with the lowest size limit and thus, the highest atom efficiency are considered isolated single atom catalysts (SACs) [START_REF] Yang | Single-Atom Catalysts: A New Frontier in Heterogeneous Catalysis[END_REF][START_REF] Wang | Heterogeneous single-atom catalysis[END_REF][START_REF] Zhang | Single-Atom Catalysts: Emerging Multifunctional Materials in Heterogeneous Catalysis[END_REF][START_REF] Chen | Single-Atom Catalysts: Synthetic Strategies and Electrochemical Applications[END_REF][START_REF] Jones | Thermally stable singleatom platinum-on-ceria catalysts via atom trapping[END_REF][START_REF] Sun | Singleatom Catalysis Using Pt/Graphene Achieved through Atomic Layer Deposition[END_REF][START_REF] Yan | Single-Atom Pd 1 /Graphene Catalyst Achieved by Atomic Layer Deposition: Remarkable Performance in Selective Hydrogenation of 1,3-Butadiene[END_REF]. Several studies already demonstrated the efficiency of SACs compared to nanosized catalysts. For instance, in the water gas shift reaction [START_REF] Lin | Remarkable Performance of Ir 1 /FeO x Single-Atom Catalyst in Water Gas Shift Reaction[END_REF] catalyzed by Ir 1 /FeO x , single atoms promoted more than 70% of the catalytic activity, while subnano clusters and nanoparticles accounted for less than 30%. Pt-based SAC also proved to have high activity for the CO oxidation reaction [START_REF] Qiao | Preparation of highly effective ferric hydroxide supported noble metal catalysts for CO oxidations: From gold to palladium[END_REF][START_REF] Qiao | Singleatom catalysis of CO oxidation using Pt 1 /FeO x[END_REF]. This was correlated with the existence of vacant 5d orbitals of Pt ions, which reduce both the CO adsorption energy and the reaction activation barriers [START_REF] Qiao | Singleatom catalysis of CO oxidation using Pt 1 /FeO x[END_REF][START_REF] Li | Exploration of High-Performance Single-Atom Catalysts on Support M 1 /FeO x for CO Oxidation via Computational Study[END_REF]. Also Au-based SAC dispersed on FeO x nanocrystallites showed higher stability and sintering resistance compared to conventional Au nanostructures [START_REF] Qiao | Ultrastable singleatom gold catalysts with strong covalent metal-support interaction (CMSI)[END_REF][START_REF] Qiao | Highly Efficient Catalysis of Preferential Oxidation of CO in H 2 -Rich Stream by Gold Single-Atom Catalysts[END_REF].

Liu et al. [START_REF] Liu | Photochemical route for synthesizing atomically dispersed palladium catalysts[END_REF] succeeded to synthetize Pd/TiO 2 SACs, having nine times higher activity for C=C bond hydrogenation reaction than commercial Pd catalysts.

The main advancement of the SAC strategy was to reach similar or better catalytic activity with non-precious metal catalysts compared to expensive noble metals [START_REF] Yin | Single Cobalt Atoms with Precise N-Coordination as Superior Oxygen Reduction Reaction Catalysts[END_REF][START_REF] Chen | Isolated Single Iron Atoms Anchored on N-Doped Porous Carbon as an Efficient Electrocatalyst for the Oxygen Reduction Reaction[END_REF][START_REF] Yang | Unveiling the high-activity origin of single-atom iron catalysts for oxygen reduction reaction[END_REF][START_REF] Fei | General synthesis and definitive structural identification of MN4C4 single-atom catalysts with tunable electrocatalytic activities[END_REF][START_REF] Back | Single-atom catalysts for CO 2 electroreduction with significant activity and selectivity improvements[END_REF]. Thus, Pt catalysts could be replaced by Co [START_REF] Yin | Single Cobalt Atoms with Precise N-Coordination as Superior Oxygen Reduction Reaction Catalysts[END_REF] or Fe [START_REF] Chen | Isolated Single Iron Atoms Anchored on N-Doped Porous Carbon as an Efficient Electrocatalyst for the Oxygen Reduction Reaction[END_REF]-based SACs in the oxygen reduction reaction (ORR). The latter actually outperformed all ORR's reported catalysts to date, including commercial Pt/C. Despite the progress in this topic, one important challenge remaining is the preparation of SACs, with a tuned porosity and a controlled metal loading, while preventing agglomeration and migration of isolated atoms [START_REF] Liu | Photochemical route for synthesizing atomically dispersed palladium catalysts[END_REF][START_REF] Flytzani-Stephanopoulos | Atomically Dispersed Supported Metal Catalysts[END_REF]. In 2002, Tilley et al. [START_REF] Nozaki | Synthesis, Characterization, and Catalytic Performance of Single-Site Iron(III) Centers on the Surface of SBA-15 Silica[END_REF] proposed a new strategy for the preparation of single-site iron (III) centers, through grafting reactions of iron complex with pre-synthesized SBA-15, and proved their high selectivities as catalysts for oxidation of alkanes, alkenes, and arenes. More recently, Hock et al. [START_REF] Hu | Isolated Fe II on Silica As a Selective Propane Dehydrogenation Catalyst[END_REF] compared the catalytic performance for selective propane dehydrogenation of isolated Fe(II) synthesized via grafting on silica, iron oxide particles, and metallic nanoparticles. Herein, we show for the first time that micelles of ferrosurfactants can act as both porogen of the silica framework and as fine imprinters of iron atoms at the surface of mesopores, leading thus to the formation of mesoporous silica-supported isolated single iron atom catalysts (Fe/SiO 2 SACs) via direct one-step method. The interest in such catalysts comes from their wide applications as bifunctional metal-acidic catalysts for biomass upgrading [START_REF] Olcese | Gasphase hydrodeoxygenation of guaiacol over Fe/SiO 2 catalyst[END_REF], syngas production [START_REF] Wang | Synthesis, characterization and catalytic performances of Ce-SBA-15 supported nickel catalysts for methane dry reforming to hydrogen and syngas[END_REF],

Fisher-Tropsch synthesis [START_REF] Kim | SBA-15-Supported Iron Catalysts for Fischer-Tropsch Production of Diesel Fuel[END_REF][START_REF] Prieto | Cobalt supported on morphologically tailored SBA-15 mesostructures: The impact of pore length on metal dispersion and catalytic activity in the Fischer-Tropsch synthesis[END_REF][START_REF] Cano | Effect of the activation atmosphere on the activity of Fe catalysts supported on SBA-15 in the Fischer-Tropsch Synthesis[END_REF], Friedel-Craft alkylation [START_REF] Sun | Highly dispersed iron oxide nanoclusters supported on ordered mesoporous SBA-15: A very active catalyst for Friedel-Crafts alkylations[END_REF], or oxidation reactions [START_REF] Nozaki | Synthesis, Characterization, and Catalytic Performance of Single-Site Iron(III) Centers on the Surface of SBA-15 Silica[END_REF][START_REF] Karimi | Highly Efficient Aerobic Oxidation of Alcohols Using a Recoverable Catalyst: The Role of Mesoporous Channels of SBA-15 in Stabilizing Palladium Nanoparticles[END_REF][START_REF] Liu | Au-Cu Alloy nanoparticles confined in SBA-15 as a highly efficient catalyst for CO oxidation[END_REF][START_REF] Rajabi | Efficient and Highly Selective Aqueous Oxidation of Sulfides to Sulfoxides at Room Temperature Catalysed by Supported Iron Oxide Nanoparticles on SBA-15[END_REF].

Common synthesis methods of Fe@SiO 2 involve either the salt impregnation of a presynthesized silica support with an iron salt solution [START_REF] Huang | Preparation, Characterization, and Application of Magnetic Fe-SBA-15 Mesoporous Silica Molecular Sieves[END_REF] or the co-precipitation method [START_REF] Li | Direct synthesis of highly ordered Fe-SBA-15 mesoporous materials under weak acidic conditions[END_REF]. However, those routes suffer from the formation of large nanoparticles aggregates and the interference of metal precursors during the polycondensation of the silica framework. Campelo et al. [START_REF] Campelo | Sustainable Preparation of Supported Metal Nanoparticles and Their Applications in Catalysis[END_REF] compared different synthesis methods and showed that the co-precipitation of silica and metal precursors directed by hydrothermal sol-gel synthesis gives a better dispersion of metal species on the silica surface than the salt impregnation of a pre-synthesized silica support. However, the metal precursors interfere with the polycondensation of the silica framework, resulting in undesirable altered structural morphology [START_REF] Barau | A Simple and Efficient Route to Active and Dispersed Silica Supported Palladium Nanoparticles[END_REF]. Improvement of nanoparticles distribution in a mesoporous silica matrix could be reached by using self-assembled surfactant phases, such as micelles or liquid crystals [START_REF] King | The preparation by true liquid crystal templating of mesoporous silicates containing nanoparticulate metals[END_REF][START_REF] King | The synthesis of mesoporous silicates containing bimetallic nanoparticles and magnetic properties of PtCo nanoparticles in silica[END_REF][START_REF] Liu | Highly active mesostructured silica hosted silver catalysts for CO oxidation using the one-pot synthesis approach[END_REF]. Among them, our group reported ferrosurfactants which are able to form either micelles [START_REF] Kim | Nanoparticle-free magnetic mesoporous silica with magnetoresponsive surfactants[END_REF], vesicles [START_REF] Kim | Spin State As a Probe of Vesicle Self-Assembly[END_REF] or to stabilize solid lipid nanoparticles. The latter, dispersed in a micellar solution of a non-ionic surfactant, P123, could lead to highly dispersed iron oxide nanoparticles within a mesoporous-macroporous silica matrix. The ferrosurfactant acted as the only source of iron and thus triggered the formation of nanoparticles within the macropores [START_REF] Kim | Metallo-Solid Lipid Nanoparticles as Colloidal Tools for Meso-Macroporous Supported Catalysts[END_REF].

More recently, Yang et al. [START_REF] Yang | Unveiling the high-activity origin of single-atom iron catalysts for oxygen reduction reaction[END_REF] also proposed a non-ionic surfactant (F127 ® )-assisted method for single iron atoms catalyst synthesis enabling the dispersion of iron species and therefore exhibiting a higher catalytic activity for ORR application.

Motivated by these pioneering works, the present study aims to elaborate mesoporous silica materials with supported isolated single iron atoms, Fe@silica SACs, through a sol-gel self-assembling mechanism from mixed micelles of non-ionic and metallosurfactants. The non-ionic surfactant template (here the Pluronic P123 ® ) was chosen to control the structural and textural properties of the silica framework (high surfaceactive area, pore volume, pore size) as it was demonstrated by XRD and N 2 sorption isotherms. More important, the metallosurfactant allows controlling the iron loading and the homogeneous distribution of iron atoms and lack of clustering as it was demonstrated by elemental analysis, STEM mapping, Pair Distribution Function (PDF)

analysis, MAS-NMR and magnetic measurements.

Materials and methods

Chemicals

Tetramethylorthosilicate (TMOS), Pluronic® P123, and hexadecyltrimethylammonium bromide (CTAB) were purchased from Sigma-Aldrich. Iron(III) chloride (FeCl 3 ) was purchased from Alfa Aesar. All reagents were used without further purification.

Cetylrimethylammoniumtrichloromonobromoferrate

(CTAF, C 16 H 33 N(CH 3 ) 3+ , FeCl 3 Br -) was synthesized as previously reported [START_REF] Kim | Nanoparticle-free magnetic mesoporous silica with magnetoresponsive surfactants[END_REF][START_REF] Kim | Spin State As a Probe of Vesicle Self-Assembly[END_REF][START_REF] Kim | Metallo-Solid Lipid Nanoparticles as Colloidal Tools for Meso-Macroporous Supported Catalysts[END_REF].

Preparation of silica materials

Silica materials were prepared by sol-gel process, through the cooperative templating mechanism, from micellar solutions of P123 (M w = 5800 g•mol -1 ) and CTAF (M w = 465.55 g•mol -1 ), and TMOS (M w = 152.22 g•mol -1 ) as inorganic precursor. The micellar solutions were prepared by adding 200 mg of P123 and an appropriate amount of CTAF (18, 36, 90, 181 mg) in 10 mL of 1M HCl, to reach the CTAF/P123 molar ratio r of 1, 2, 5 and 10, respectively. In a typical procedure, 310 mg of TMOS were added to those micellar solutions to obtain a P123/TMOS molar ratio of 0.017. The mixture was stirred at room temperature for 30 min before being transferred into a Teflon bottle and placed in a stainless-steel autoclave at 373 K for 48 h. The resulting hybrid product was then filtered through a paper filter on a funnel and dried in air for 24 h. The surfactant can be removed by Soxhlet extraction with ethanol to lead to pristine silica (P@SiO 2 (r)) or by calcination, to remove the organic part and to lead to ferrisilicates (Fe@SiO 2 (r)). The programming of the calcination procedure was set as follows: (step 1) increasing temperature from room temperature (RT) to 373 K (2K•min -1 )

and held for 1 h; (step 2) heating ramp from 373 K to 623 K (2K•min -1 ) and held for 1 h; and

(step 3) heating ramp from 623 K to 823 K (2K•min -1 ) and held for 3 h. The yield of the solgel process was 80% while the one of the iron loading in ferrisilicates was evaluated to 25%, except for Fe@SiO 2 [START_REF] Yan | Single-Atom Pd 1 /Graphene Catalyst Achieved by Atomic Layer Deposition: Remarkable Performance in Selective Hydrogenation of 1,3-Butadiene[END_REF], only 7%.

Characterization methods

Nitrogen sorption measurements

The textural properties of the synthesized materials after calcination were determined from the nitrogen adsorption/desorption isotherms obtained at 77 K (p/p 0 increasing from 10 -7 to 0.99 and decreasing from 0.99 to 0.10) using an automatic Micromeritics ASAP 2020 instrument. The specific surface area was determined using two methods, the BET method [START_REF] Brunauer | Adsorption of Gases in Multimolecular Layers[END_REF][START_REF] Rouquerol | Adsorption by powders and porous solids: principles, methodology, and applications[END_REF] and the NLDFT method [START_REF] Tarazona | Phase equilibria of fluid interfaces and confined fluids: Non-local versus local density functionals[END_REF]. The total pore volume was determined from the nitrogen adsorption at a relative pressure of 0.97, while the mesopore volume V meso was calculated by the difference between the total pore volume and the micro-pore volume calculated using the NLDFT method. The pores size distributions in the mesopore range were determined by the BarretJoynerHalenda (BJH) method with the KrukJaroniecSayari (KJS) correction to the desorption branch of the nitrogen isotherms [START_REF] Kruk | Application of Large Pore MCM-41 Molecular Sieves To Improve Pore Size Analysis Using Nitrogen Adsorption Measurements[END_REF][START_REF] Kruk | Characterization of the Porous Structure of SBA-15[END_REF]. Pore diameters corresponding to peaks in the latter PSDs were labelled D BJH .

X-ray diffraction

The measurements were performed using a Panalytical X'Pert Pro diffractometer e uipped with a Cu tu e, a e incident-eam monochromator 5406 and an X'Celerator detector. Small-angle X-ray scattering (SAXS) measurements were collected using 0.02 rad Soller slits, 1/16° fixed divergence and anti-scatter slits The X'Celerator detector was used as "scanning line detector D " with 0 5 8° active length Data collection was carried out in the scattering angle range 0.8-12° with a 0.0167° step over 60 min.

Elemental analysis

Si and Fe chemical analysis were performed using an Inductively Coupled Plasma Atomic Emis-sion Spectroscopy (ICP-AES).

Transmission electron microscopy and element mapping

Transmission electron microscopy (TEM) analyses were performed using a JEOL 2100 F electron microscope operating at 200 kV and equipped with a probe Cs corrector. The "HAADF-STEM" images were ac uired in scanning TEM STEM using a high angle annular dark field detector (HAADF). The elemental maps were acquired by Energy Dispersive X-Ray (EDX) Spectroscopy in STEM mode using a JEOL Silicon Drift Detector (DrySD60GV: sensor size 60 mm 2 ) with a solid angle of approximately 0.5 sr.

Total scattering X-ray measurements and PDF analysis

The total scattering X-ray measurements were performed at the High energy Beamline for

Buried-interface Structures and Materials Processing ID31 of the European Synchrotron

Radiation Facility (ESRF). The data-sets were collected using a large area, high energy single-photon counting detector, Pilatus3 2M CdTe and a high energy monochromatic beam 68 5 keV, 0 8099 Sample powder was loaded into a mm diameter Kapton capillary tube. The raw two-dimensional data-sets were azimuthally integrated and converted to onedimensional intensity versus 2θ using PyFAI software package [START_REF] Kieffer | PyFAI, a versatile library for azimuthal regrouping[END_REF]. The total scattering measurements have been carried out under the same experimental conditions for all the studied samples (Fe@SiO 2 (1), Fe@SiO 2 (2), Fe@SiO 2 [START_REF] Wang | Heterogeneous single-atom catalysis[END_REF] and Fe@SiO 2 [START_REF] Yan | Single-Atom Pd 1 /Graphene Catalyst Achieved by Atomic Layer Deposition: Remarkable Performance in Selective Hydrogenation of 1,3-Butadiene[END_REF]). The collected data were then corrected for experimental effects (absorption, multiple scattering, polarization, Compton scattering and Laue diffuse scattering) and the scattering signal from the air and the experimental set up was measured independently under the exact same conditions as the samples and subtracted as a background in the data reduction procedure. The data were truncated at a finite maximum value of Q max = 18 Å -1 beyond which the signal-tonoise ratio became unfavorable. All the differential experimental atomic pair distribution functions (d-PDF) were obtained by using the PDFgetX3 program [START_REF] Juhás | PDFgetX3 : a rapid and highly automatable program for processing powder diffraction data into total scattering pair distribution functions[END_REF] and following a procedure previously reported [START_REF] Hsieh | Structure and dynamics of guest molecules confined in a mesoporous silica matrix: Complementary NMR and PDF characterisation[END_REF][START_REF] Bendeif | Structure determination of molecular nanocomposites by combining pair distribution function analysis and solid-state NMR[END_REF].

Solid-state nuclear magnetic resonance

Solid-state NMR spectra were acquired on an AVANCE III Bruker spectrometer operating at 14 T ( 1 H NMR at 600 MHz). A Bruker 4 mm MAS probe was used for the experiment with MAS frequencies of 12.5 kHz. For 29 Si experiments, the (rf)-field strength applied for the 90° pulse was set to 50 kHz for a duration of 5 μs During ac uisition, SPINAL-64 1 H heteronuclear decoupling was applied at an rf-field strength of 80 kHz, with a pulse length of 5 8 μs [START_REF] Fung | An Improved Broadband Decoupling Sequence for Liquid Crystals and Solids[END_REF]. Optimized saturation recovery experiments were used for T 1 relaxation measurements with a list of 13 recovery delays. For 29 Si experiments a recovery delay of 300s was used for accumulating 256 scans. Regarding the evolution of 29 Si spinning sidebands, a recovery delay of 0.5s was used for accumulating 512 scans. A recovery delay of 3s was used for all 1 H experiments. The 1 H and 29 Si chemical shifts were reported relative to tetramethylsilane. The DMfit program was used for spectral deconvolution [START_REF] Massiot | Modelling one-and two-dimensional solid-state NMR spectra: Modelling 1D and 2D solid-state NMR spectra[END_REF]. Mathematica 11.2 software was used to fit the saturation-recovery T 1 relaxation curves and to perform the corresponding statistical analysis. Before NMR measurements, filled NMR rotors have been extensively dried for 72 h, at 323 K, with a turbo-molecular vacuum pump at pressure below 10 -4 mbar.

Magnetic measurements

Magnetic measurements were performed with a SQUID-VSM magnetometer (from Quantum Design) used in the dc-mode. Samples were introduced in a gelatin capsule placed in a plastic straw. Both temperature (from 2K to 400 K) and field (up to 7 T) dependences of magnetization were investigated.

Density functional theory calculations

Periodic spin polarized density functional theory (DFT) calculations were performed using the VASP package [START_REF] Kresse | Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set[END_REF] by means of the projector augmented wave (PAW) method [START_REF] Blöchl | Projector augmented-wave method[END_REF]. The GGA+U [START_REF] Perdew | Generalized Gradient Approximation Made Simple[END_REF][START_REF] Anisimov | First-principles calculations of the electronic structure and spectra of strongly correlated systems: the LDA + U method[END_REF][START_REF] Bengone | Implementation of the projector augmented-wave LDA+U method: Application to the electronic structure of NiO[END_REF] method was used for the exchange and correlation potential, in order to take into account the strong electron-electron interaction due to the iron localized d shell. The values of the parameters U and J were set to 3 eV and 0.9 eV, respectively [START_REF] Gueddida | Calculated impact of ferromagnetic substrate on the spin crossover in a Fe(1,10-phenanthroline) 2 (NCS) 2 molecule[END_REF]. Dispersion effects were considered by means of the DFT-D2 correction by Grimme et al. [START_REF] Grimme | Semiempirical GGA-type density functional constructed with a long-range dispersion correction[END_REF], as implemented in VASP [START_REF] Bučko | Improved Description of the Structure of Molecular and Layered Crystals: Ab Initio DFT Calculations with van der Waals Corrections[END_REF].

The interaction between a single iron atom and amorphous silica-surfaces was investigated by considering three different silanol densities: 7.

Results and discussion

Synthesis of silica materials

Silica materials were prepared through the self-assembling mechanism (Scheme 1) by adding a silica precursor (tetramethoxylsilane, TMOS) to a mixed micellar solution of a non-ionic surfactant, the Pluronic P123, and a metallosurfactant, CTAF, at various molar ratio, r (of 1, 2, 5, 10 where r = n CTAF /n P123 ). All syntheses were performed at pH 1 to prevent precipitation of iron hydroxides. Therefore, the expected silica materials would be positively charged (SiOH 2 + ) and would need a cationic porogen to drive the silica-surfactant interaction through the anionic counterion (SiOH 2 + /FeCl 3 Br -/CTA + ) and to favor the imprinting of iron centers into the matrix.

The hydrothermal reaction conditions are similar to the one generally used for the synthesis of SBA-15 (hydrothermal treatment under acidic catalysis, for 48 h at 373 K) [START_REF] Zhao | Triblock Copolymer Syntheses of Mesoporous Silica with Periodic 50 to 300 Angstrom Pores[END_REF][START_REF] Zhao | Morphological Control of Highly Ordered Mesoporous Silica SBA-15[END_REF]. Indeed, under these reaction conditions and in the absence of CTAF, the pristine material exhibits hexagonally ordered mesopores of around 8 nm diameter and a surface area (A BET ) and pore volume of 846 m 2 •g -1 and of 0.984 cm 3 •g -1 , respectively, as it was determined from XRD and N 2 adsorption measurements (Figure SI1). These values are in agreement with a typical SBA-15 silica material. [START_REF] Zhao | Morphological Control of Highly Ordered Mesoporous Silica SBA-15[END_REF] By adding CTAF, mixed micelles are formed. This affects both the meso-structure and the textural properties of the resulting materials, as reported in Table 1.

Scheme 1. Schematic representation of the sol-gel synthesis of silica materials through the self-assembling mechanism based on mixed micelles of metallosurfactant CTAF and pluronic P123.

Structural and textural characterization of Fe@SiO 2 (r) materials

From the XRD data (Figure 1A), it can be first seen that the mesopore ordering of the materials decreases with the increase in CTAF content. At low r values (r= 1 and 2), the materials exhibit three reflection peaks located respectively at 9.4, 5.5 and 4.7 nm for Fe@SiO 2 (1) and at 8.2, 4.9, and 4.4 nm for Fe@SiO 2 [START_REF] Chen | Atomically Dispersed Iron-Nitrogen Species as Electrocatalysts for Bifunctional Oxygen Evolution and Reduction Reactions[END_REF]. Their relative positions are 1, and 2, which can be attributed to the (100), ( 110) and (200) reflections of a hexagonal structure. According to Bragg's law, the unit cell dimension (a 0 ) can be calculated and was found to be equal to 10.9 nm for Fe@SiO 2 (1) and 9.5 nm for Fe@SiO 2 [START_REF] Chen | Atomically Dispersed Iron-Nitrogen Species as Electrocatalysts for Bifunctional Oxygen Evolution and Reduction Reactions[END_REF]. This decrease of the cell unit could be due to the silica wall thickness and/or pore diameter. The hexagonal ordering of the pores was further confirmed by transmission electron microscopy (TEM) micrographs showing honeycomb like arrangements (i.e. Fe@SiO 2 (1) in Figure 2). In addition to those hexagonally organized mesopores of about 8 nm in diameter and 3 nm in wall thickness, the micrographs also show the coexistence of larger, unorganized pores, of about 12 nm in diameter.

At higher r values (r= 5 and 10), the materials exhibit a broad peak located at 7.8 nm for Fe@SiO 2 (5) and at 6.6 nm for Fe@SiO 2 [START_REF] Yan | Single-Atom Pd 1 /Graphene Catalyst Achieved by Atomic Layer Deposition: Remarkable Performance in Selective Hydrogenation of 1,3-Butadiene[END_REF]. This indicates the formation of a wormhole-like structures. The loss of ordering might be due to the increase of the total surfactant n P123+CTAF /n TMOS molar ratio (R) with the increase of r values. As a matter of fact, in order to control the iron/silica ratio, only the P123/TMOS molar ratio was maintained constant, without adjusting R (n P123+CTAF /n TMOS ) as a function of r (n CTAF /n P123 ). Yet, it is communally accepted that the parameter R is essential in getting well-defined architectures and it strongly depends on the nature of the surfactant. For instance, hexagonally ordered materials could be obtained with solely P123 at R=0.017 and with solely CTAF at R=0.17. Another important parameter influencing the morphology and the pore size of the resulting material is the size and nature of the micelles used as template. The iron ions from CTAF might induce the complexation of ethylene oxide units of P123 and thus the shrinkage of the mixed micelles and the reduction of the silica/surfactant interaction. Obviously, when increasing the CTAF content, the repetition distance decreases, indicating a decrease of the pore size and/or of the silica wall thickness. Further insights in the pore size distribution (PSD) were obtained from nitrogen adsorption/desorption isotherms. Except Fe@SiO 2 [START_REF] Yan | Single-Atom Pd 1 /Graphene Catalyst Achieved by Atomic Layer Deposition: Remarkable Performance in Selective Hydrogenation of 1,3-Butadiene[END_REF], which exhibits a type IV isotherm with one hysteresis loop, characteristic of unimodal mesoporous materials, all materials rather exhibit a type II isotherms with two main steps desorption branch indicating the coexistence of mesopores of different sizes (Figure 1B). On all isotherms, it can be observed that the value of the relative pressure, at which the first capillary condensation occurs is almost the same, between 0.4 and 0.5. Since the p/p 0 position of the inflection point is related to the pore diameter, according to Kelvin's equation, this observation suggests that all materials have mesopores of about 5 nm in diameter. As a matter of fact, Fe@SiO 2 [START_REF] Yan | Single-Atom Pd 1 /Graphene Catalyst Achieved by Atomic Layer Deposition: Remarkable Performance in Selective Hydrogenation of 1,3-Butadiene[END_REF] has a unimodal distribution of mesopores centered on 5 nm, as shown by the pore diameter distribution obtained by applying the BJH method to the nitrogen desorption isotherms (D BJH , Figure 1C). The second inflection point appears for the other materials at a p/p 0 value comprised between 0.7 and 0.8, suggesting larger pores. For Fe@SiO 2 (1) and Fe@SiO 2 (2), three N 2 desorption steps can be identified at the following p/p 0 intervals: between 0.50 and 0.75, between 0.75 and 0.80, and between 0.80 and 0.99, which indicate the existence of mesopores within three ranges of pore diameters. For the Fe@SiO 2 (5) material, one can observe two zones N 2 desorption steps, at p/p 0 between 0.50 and 0.80, corresponding to the porosity peak around 5 nm, and at p/p 0 between 0.80 and 0.99, corresponding to larger pores (8 < D BJH < 16 nm), where dV•dD BJH -1 is around 0.05 cm 3 •g -1 •nm -1 . The Fe@SiO 2 [START_REF] Yan | Single-Atom Pd 1 /Graphene Catalyst Achieved by Atomic Layer Deposition: Remarkable Performance in Selective Hydrogenation of 1,3-Butadiene[END_REF] sample shows one N 2 adsorption zone (0.50 < p/p 0 < 0.80) corresponding to the 5 nm pore diameter peak, with no adsorption for p/p 0 above 0.80. The pore sizes of all materials are wider than the one obtained by using only CTAF as template, without adding the non-ionic P123 surfactant (of 2.4 nm). [START_REF] Kim | Nanoparticle-free magnetic mesoporous silica with magnetoresponsive surfactants[END_REF] So is the wall thickness, ε, as it was calculated y the su traction of the pore size D BJH ) from the interlayer distance (d Bragg ) for the wormlike materials and from the cell unit for the hexagonal ordered ones. The surface areas are similar for the hexagonally ordered materials Fe@SiO 2 (1) and Fe@SiO 2 (2) on the one hand, and for the wormlike materials Fe@SiO 2 [START_REF] Wang | Heterogeneous single-atom catalysis[END_REF] and Fe@SiO 2 [START_REF] Yan | Single-Atom Pd 1 /Graphene Catalyst Achieved by Atomic Layer Deposition: Remarkable Performance in Selective Hydrogenation of 1,3-Butadiene[END_REF] on the other hand. All materials have high surface areas, A BET and S NLDFT , and total pore volumes V 0.97 (Table 1). A sharp increase, of almost 200 m 2 •g -1 , can be noticed between the two series of materials.

Therefore, Fe@SiO 2 [START_REF] Wang | Heterogeneous single-atom catalysis[END_REF] and Fe@SiO 2 (10) might be considered more attractive for catalytic applications, such as in hydrodeoxygenation reactions for example.

Chemical characterization of Fe@SiO 2 (r) materials

The attenuated total reflectance infrared ATR-IR spectra of Fe@SiO2(r) materials confirms the formation of the silica framework during the sol-gel process and the removal of the organic part of the surfactant after calcination. Characteristic peaks of the stretching and bending vibrations of Si-O-Si, Si-O and Si-OH bonds are present in the 900-1100 cm -1 region, while no peaks of CH 2 moieties appear between 2800 and 2900 cm -1 (Figure SI2 left). The ATR spectra also show the presence of OH groups at 1633 cm -1 and at 3200-3700 cm -1 that were assigned to silanol groups and physisorbed water, respectively. Quantitative assessment of silanol densities on the surface of those materials was obtained from BET and TGA analysis (Table 1 and SI3 for the calculation method).

All materials exhibit silanols density values higher than for the similarly synthesized SBA-15like silica, with no CTAF (around 2.50 OH/nm 2 ). This may be due to the existence of iron The condensation degree of silica in Fe@SiO 2 (r) materials could be assessed from 29 Si solidstate NMR. Relative proportions of Q 2 (SiO 2 (OH) 2 silanols), Q 3 (SiO 3 (OH) silanols), and Q 4 (SiO 4 bonds) units were determined after deconvolution of the peaks at -91, -100 and -110 ppm, respectively and all materials exhibit the same condensation degree Q 2 :Q 3 :Q 4 of 4:24:72 (+/-2%). Nevertheless, despite the absence of additional 29 Si signals, evidence is found for the presence of paramagnetic Fe. Indeed, the very high magnetic moment of unpaired electrons, located at paramagnetic centers, affects the NMR properties of coupled spins in different ways [START_REF] Bakhmutov | Strategies for Solid-State NMR Studies of Materials: From Diamagnetic to Paramagnetic Porous Solids[END_REF]. Nuclei that are chemically bound, or in the first coordination shell of paramagnetic iron cannot be observed due to extreme paramagnetic broadening. However, above this cutoff radius, nuclear spins become observable despite considerable paramagnetic broadening [START_REF] Kim | The degree of polymerization and structural disorder in (Mg,Fe)SiO3 glasses and melts: Insights from high-resolution 29 Si and 17 O solid-state NMR[END_REF].

Under Magic Angle Spinning (MAS) sample rotation, this broadening gives rise to intense sideband patterns in the MAS NMR spectra, which are separated from the isotropic signal by multiples of the rotation frequency [START_REF] Duer | Solid-state NMR spectroscopy: principles and applications[END_REF]. Since the gradual signal loss and broadening occur simultaneously, the boundary between observable and non-observable spins is somewhat diffuse (~3.3Å) [START_REF] Hazen | High-pressure crystal chemistry of andradite and pyrope; revised procedures for high-pressure diffraction experiments[END_REF][START_REF] Armbruster | Single-crystal X-ray structure study of synthetic pyrope almandine garnets at 100 and 293 K[END_REF], nevertheless it is possible to distinguish between short range effects, inducing signal loss, and medium range effects up to the nanometer scale, inducing line broadening and spin lattice R 1 relaxation. Because of the low Fe concentration in the materials studied here, no short-range effects like the decrease in signal intensity or selective loss of pore surface (Q 2 /Q 3 ) sites with Fe loading could be established. Indeed, even for Fe@SiO 2 [START_REF] Yan | Single-Atom Pd 1 /Graphene Catalyst Achieved by Atomic Layer Deposition: Remarkable Performance in Selective Hydrogenation of 1,3-Butadiene[END_REF], the Fe content remains below 2 wt.% (Table 2). Nevertheless, medium-range effects such as an increase of sideband pattern intensity is indeed observed for 29 Si and 1 H NMR spectra when the CTAF/P123 ratio increases (Figure 3 A,B). This qualitative result is a consequence of the presence of paramagnetic species and indicates an increase of the average spatial proximity between paramagnetic iron spins and the diamagnetic neighbors, at the subnanometric scale as determined by the pair distribution function analysis (PDF).

Additionally, the study of spin lattice R 1 relaxation also shows the existence of medium-range paramagnetic effects. Indeed, the 29 Si and 1 H characteristic T 1 relaxation times are low compared to standard SBA-15 type material and therefore semi-quantitative information is best obtained by fitting the peak areas (M) obtained from saturation recovery experiments to a modified "stretched exponential" e uation of characteristic time T' with β < being the peak area for the longest delay spectrum and a constant equal to 1 if corresponds to the fully relaxed signal [START_REF] Tse | Nuclear Spin-Lattice Relaxation Via Paramagnetic Centers Without Spin Diffusion[END_REF][START_REF] Hartman | Silicon-29 MAS NMR of the aluminosilicate mineral kyanite: residual dipolar coupling to aluminum-27 and nonexponential spin-lattice relaxation[END_REF].

While spin diffusion commonly induces relaxation of all spins at a same rate β , our data is est fitted for β fixed at 0 5 (Figure SI3) which is typical for relaxation dominated through space dipolar coupling with unpaired electronic spins χ 2 is two times lower than for β This result is consistent with a homogeneous iron distribution on the nanometer scale. 1 H T 1 relaxation also shows non-exponential behavior and relaxation curves are best fitted with a higher value of β = 0.74. Despite the fact that NMR samples have been extensively dried to remove 1 H from adsorbed water molecules, 1 H spin diffusion relaxation is still active and this value of β is typical when mixed relaxation mechanisms act on the spins [START_REF] Hartman | Heterogeneities in sol-gel-derived paramagnetics-doped forsterites and willemites -Electron microprobe analysis and stretched-exponential 29 Si MAS NMR spin-lattice relaxation studies[END_REF]. As seen on This trend suggests an increase in the number of paramagnetic iron sites within the materials.

Characterization of single atom Fe@SiO 2 (r) materials

A first quantitative assessment of the iron loading, obtained from both ICP-AES elementary analysis, is found to be about 0.56, 0.98, 2.31, and 2.64 wt.% for Fe@SiO 2 (1), Fe@SiO 2 (2), Fe@SiO 2 [START_REF] Wang | Heterogeneous single-atom catalysis[END_REF], and Fe@SiO 2 [START_REF] Yan | Single-Atom Pd 1 /Graphene Catalyst Achieved by Atomic Layer Deposition: Remarkable Performance in Selective Hydrogenation of 1,3-Butadiene[END_REF], respectively, as compared to Si (see Table 2 for the iron loading values compared to SiO 2 ). Those results were further confirmed by the elemental mapping obtained by scanning transmission electron microscopy (STEM). The Fe content in these materials, as compared to the Si one, is found to be about 0.8, 1.1, 2.5, and 2.7 wt.% for Fe@SiO 2 (1), Fe@SiO 2 (2), Fe@SiO 2 [START_REF] Wang | Heterogeneous single-atom catalysis[END_REF], and Fe@SiO 2 [START_REF] Yan | Single-Atom Pd 1 /Graphene Catalyst Achieved by Atomic Layer Deposition: Remarkable Performance in Selective Hydrogenation of 1,3-Butadiene[END_REF], respectively. Moreover, as it can be seen in Figure 4B,C and SI4, the micrographs show a homogeneous dispersion of iron atoms within the silica matrix, without formation of any agglomerates.

UV-Vis diffuse reflectance spectra (Figure SI2 right) allowed us to determine the coordination of iron in the materials. The absorption band at 235 nm, which can be assigned to the charge transfer of pπ-dπ transition etween oxygen and iron in the ferrisilicate material, suggests tetrahedral iron atoms, insulated from each other. In addition, the absence of any absorbance band above 350 nm in the spectrum reveals the absence of octahedral iron ions, and consequently the absence of iron oxide nanoparticles in the Fe@SiO 2 materials [START_REF] Li | Preparation of mesoporous ferrisilicate with high content of framework iron by pH-modification method and its catalytic performance[END_REF].

The absence of nanoparticles was also confirmed by the analysis of the experimental pair distribution function derived from the corresponding total scattering measurements of the materials (Figure 4A). From the PDF diagrams, one can clearly observe well-defined features up to about 7 Å. The rapid damping of PDF oscillations beyond this distance indicates that only isolated subnanometer iron clusters might be formed and are uniformly distributed throughout the samples, with no aggregation. Larger nanoparticles would have resulted a spatial extent of atomic correlations in the PDF diagram.

It is obvious that the intensities of the PDF peaks corresponding to the contributions of the Fe atoms increase as the molar ratio r (nCTAF/nP123) increases. The first two PDF peaks located in the r range of 2.0-2.6 Å are characteristic of Fe-O distances, while the PDF peaks located in the range of 3-6 Å can be assigned to the interactions of the iron atoms with the silica pore walls or to Fe-Fe correlations.

Finally, it is noteworthy that the presence of the double peak between 2.0 and 2.6 Å indicates two related Fe environments. Further insights on the Fe oxidation, spin states and on Fe-Fe interactions were gathered from magnetic measurements. The magnetization field dependences of the Fe@SiO 2 (r) measured at 10K after a first cooling at 15K•min -1 are plotted in Figure 5. These measurements have been subtracted from the diamagnetic contribution of sample holder and pristine P@SiO 2 (r) silica, as described in SI, and have been scaled with respect to Fe content using the ICP-AES elementary analysis results. They can be fitted by a sum of two Brillouin functions as follow:

where is the only fitted parameter giving the proportion of the high spin S=5/2, and are the saturation magnetization of Fe 3+ in the high spin and low spin states, respectively ( and ), and are the corresponding Brillouin functions (S=5/2, 1/2).

The fit with the aforementioned equation has been performed for samples exhibiting a Curie temperature dependence under 79.6 kA/m above 10K (see Figure SI5). Despite only one parameter is fitted, a very good agreement is obtained between the simple model and the measurements. The fit is not changing while taking into account an additional contribution of Fe 2+ (S=2), confirming thus the presence of solely Fe 3+ ions. No ferromagnetic contribution is detectable at this scale. The high spin fractions deduced from the fits are reported in Table 2. They are in good agreement with those deduced from the Curie constant (see Figure SI5). No fit with the Brillouin equation has been performed for the Fe@SiO 2 [START_REF] Wang | Heterogeneous single-atom catalysis[END_REF] sample since the temperature dependence of the magnetization deviates from the Curie law below 15 K (see figure SI6). This deviation is attributed to spin crossover towards low spin with decreasing temperature, which occurs below 10 K for other samples (Figure SI7).

Nevertheless, the high spin fraction reported in Table 2 is stable with temperature above 10 K (or 15 K for Fe@SiO 2 (5)) and is then a relevant parameter characterizing the Fe sites in the silica pores. It decreases as the Fe content increases, concomitantly with the decrease of the pore size. Thus, beyond the sharing of Fe spin state according to Si sites or pore sizes, the consistent quantitative analysis in terms of Curie law or Brillouin function without interaction between Fe spins (no Curie-Weiss temperature) supports the assumption of isolated Fe atomic centers that are well dispersed within the materials. [START_REF] Wang | Heterogeneous single-atom catalysis[END_REF] as it deviates from the Curie law below 15K. For this sample, the fit with the Curie law has been performed in the 15-30 K.

DFT calculations

Several structural configurations of the Fe/SiO 2 system were calculated in order to determine the most favorable adsorption position (top vs bridge, Figure 6) as well as the spin state, The Fe atom was adsorbed on the silica surface through its oxygen atoms by removing the corresponding hydrogen atoms. Figure 6 shows the single iron atom adsorbed on the typical silica surface with a silanol coverage equal to 4.6 OH per nm 2 at top (left panel) and bridge (right panel) positions. For the top positions, the HS state magnetic moment of iron is 4 97 μB S 5/2 while the one of the LS state is 00 μB S /2 , which correspond to Fe 3+ .

However, the HS and LS state magnetic moments of the iron in the bridge positions are found to e 4 00 μB S 2 and 0 00 μB S 0 , respectively, which are the HS and LS states of Fe 2+ . It is noteworthy that, for the various silica surfaces, the calculations show that the adsorption of the iron atom at top positions has always the lowest total energy. Thus, the Fe 3+ oxidation state is the most favorable whatever the silanols density, in agreement with experimental findings where no Fe 2+ contribution is observed. Nevertheless, the calculations show that, in the absence of geometrical constraints, the adsorption of the single iron atom on various amorphous silica-surface stabilizes the HS state. This result is in contrast with magnetic results where a contribution of low spins had to be considered. Such low spin state is however not surprising in presence of porous materials where strains are expected.

Therefore, additional DFT calculations were performed starting from optimized top positions at high and low spin, while taking into account geometrical constraints. The contraction of the geometries along the x and y directions has been set to mimic the fact that the silica surface is not perfectly planar but slightly curved which is modeled in our simulations by a pressure.

The choice of a contraction value of 2% has been done as a follow: the calculations with an applied pressure smaller than 2% does not show a significant effect on the studied properties.

Also, pressures exceeding 2% are probably unrealistic and could give rise to unphysical phenomena. For high silanols densities (7.2 and 4.6 OH/nm²), our calculations (Table SI3) confirm the coexistence of LS and HS Fe 3+ species determined from magnetic measurements (Table 2). For low silanols densities (3.3 OH/nm²), the LS and HS calculations converged to the same magnetic state (LS with a magnetic moment of 1 µB). This result is in line with the experimental trend observed in Table 2 showing that the relative amount of LS Fe 3+ increase when decreasing the silanol density. In conclusion, DFT calculations confirmed the experimental results obtained from magnetic measurements, indicating a mixture of HS and LS Fe 3+ species.

Conclusion

In this study, we demonstrated that single iron atoms-supported materials (Fe/SiO 2) can be prepared by hydrothermal synthesis using mixed non-ionic/metallosurfactants as templates.

The non-ionic Pluronic P123 surfactant allows controlling the structural and textural properties of the silica framework while the ferrosurfactant acts as a metal donor, in addition to its role of porogen. The iron loading in the final materials depends on the molar ratio of surfactants and can be accurately tuned, up to 1wt% (0.15 Fe atoms/nm 2 ). Pair distribution function analysis, STEM mapping and magnetic measurements demonstrated the absence of any particle or cluster. Experimental data showed that the Fe species are Fe 3+ , in agreement with the DFT calculations. Those materials proved a good distribution of iron atoms on the silica surface, with increased number of active sites, which might find straightforward applications in catalysis [START_REF] Chen | Atomically Dispersed Iron-Nitrogen Species as Electrocatalysts for Bifunctional Oxygen Evolution and Reduction Reactions[END_REF][START_REF] Thomas | The advantages and future potential of single-site heterogeneous catalysts[END_REF][START_REF] Yang | Single-Atom Catalysts: A New Frontier in Heterogeneous Catalysis[END_REF][START_REF] Wang | Heterogeneous single-atom catalysis[END_REF][START_REF] Zhang | Single-Atom Catalysts: Emerging Multifunctional Materials in Heterogeneous Catalysis[END_REF][START_REF] Chen | Single-Atom Catalysts: Synthetic Strategies and Electrochemical Applications[END_REF].

  repeated slabs were separated by more than 20 Å of vacuum in the z direction. A plane-wave basis set with a kinetic energy cutoff of 450 eV was used to ensure the convergence of our calculations Due to the large size of the supercell, only the Γ point was used to sample the Brillouin zone. The total energy converged within 10 -6 eV. For the atomic relaxation, the single iron atom and the atoms of the first layer of the silica-surface were relaxed by nullifying the forces on the atoms with a precision of 0.03 eV/Å and the other layers were kept fixed.

Figure 1 .

 1 Figure 1. Textural properties of Fe@SiO 2 (r) materials (r = n CTAF /n P123 = 1, 2, 5, 10): (A) XRD patterns, (B) N 2 adsorption-desorption isotherms, and (C) pore size distribution.

Figure 2 .

 2 Figure 2. Transmission electron micrograph of Fe@SiO 2 (1).

  silica crosslinking during the polymerization, which may indicate the formation of Si-O H …Fe onds instead of Si-O-Si bonds.

Figure 3 .

 3 Figure 3. Evolution of 29 Si (A) and 1 H (B) MAS NMR spectra for increasing r values of Fe@SiO 2 (r) materials: (a) r=1; (b) r=2; (c) r=5 and (d) r=10. Spinning sidebands are marked y * and ν MAS =12.5kHz. Histograms showing the evolution of 29 Si (C) and 1 H D characteristic T' spin-lattice relaxation times for increasing r-values in Fe@SiO 2 (r) materials. Saturation recovery curves have een fitted to "stretched exponential" functions with β 0 5 for 29 Si and β 0 74 for 1 H. Error bars reflect the 95% confidence interval of the fits.

Figure 3 (

 3 Figure 3 (C,D) characteristic T' relaxation times decrease when CTAF/P 23 ratio increases

Figure 4 .

 4 Figure 4. Comparison of experimental atomic pair distribution functions (PDFs) in real space, G(r), for Fe@SiO 2 (r) materials and allocation of the main interatomic distances in the range 2 -7 Å (A). Relative elemental maps of Fe and Si obtained by EDX in the scanning TEM mode for (B) Fe@SiO 2 (1), (C) Fe@SiO 2 (10) (iron in green and silica in red). The scale bare corresponds to 10 nm.

Figure 5 .

 5 Figure 5. Field dependence behavior of the magnetization measured at 10K and corrected from the diamagnetic signal for Fe@SiO 2 (r) (r = 1, 2, 5, 10) samples. It is plotted as a function of the dimensionless parameter where is the Bohr magneton and the Boltzmann constant. The straight lines are fits by the sum of the two Brillouin functions corresponding to S = 5/2 and 1/2.

  without or under geometrical constraints (2% contraction of the silica surface along the x and y directions), as in mesopores. The calculations were firstly started from reasonable estimations at various adsorption positions (top, bridge) on surfaces with various silanol densities (SiO 2 -7.2, SiO 2 -4.6 and SiO 2 -3.3) [64,65]. For each configuration, two sets of calculations were performed for the Low-Spin (LS) and High-Spin (HS) states of iron.

Figure 6 .

 6 Figure 6. Panels showing the single iron atom adsorbed on amorphous silica surface with asilanol density of 4.6 OH/nm 2 at top (left) and bridge (right) positions, respectively. For the top adsorption, the equilibrium distance between the iron atom and the substrate is found to be 1.81 Å in HS and 1.78 Å in LS (left). For the bridge adsorption, in the HS state, the equilibrium iron-surface distances were found to be 1.99 and 1.85 Å for both oxygen atoms, whereas in the LS state, these distances became 1.85 and 1.81 Å, respectively (right).

  2, 4.6 and 3.3 OH/nm 2 [64,65]. The fully hydroxylated surface (SiO 2 -7.2) was represented by a supercell containing 402 atoms, and for SiO 2 -4.6 and SiO 2 -3.3, 384 and 375 atoms, respectively. In all the cases, the periodically

Table 1 .

 1 Textural properties of Fe@SiO 2 (r) materials (r = 1, 2, 5, 10) and pristine material, obtained in the absence of CTAF: interlayer distance/unit cell dimension for hexagonal networks d Bragg /a 0 , pore diameter D BJH , wall thickness ε, surface area A BET and S NLDFT , mesopores volume V 0.97 , and silanols density ρ Si-OH .

		d Bragg/ a 0	D BJH	ε	A BET	S NLDFT	V 0.97	ρ Si-OH
		(nm)	(nm)	(nm)	(m 2 •g -1 )	(m 2 •g -1 )	(cm 3 •g -1 )	(OH/nm 2 )
	Fe@SiO 2 (1)	9.4/10.9	4.8/7.3 /12.4	3.6	731	809	1.34	9.6
	Fe@SiO 2 (2)	8.2/9.5	5.0/6.6 /12.3	2.9	750	819	1.41	5.5
	Fe@SiO 2 (5)	7.8/-	4.9	2.9	957	974	1.43	3.8
	Fe@SiO 2 (10)	6.6/-	5.5	1.1	993	1013	1.34	4.0
	P@SiO 2	10/11.6	8	2	846	942	0.984	2.6

Table 2 .

 2 Fe content (wt.%) with respect to the total samples mass used to scale the magnetization. Fraction of Fe 3+ spins in the high spin state (5/2) deduced from the temperature dependence of magnetization (Curie law) or field dependence of magnetization measured at 10 K. No fit with Brillouin functions has been performed for Fe@SiO 2

		Fe loading
		(wt.%)
	Fe@SiO 2 (1)	0.23	0.72	0.71
	Fe@SiO 2 (2)	0.41	0.49	0.47
	Fe@SiO 2 (5)	0.97	0.31*	-
	Fe@SiO 2 (10)	1.08	0.25	0.24

*

Acknowledgements

Authors would like to thank S. Parant for technical assistance with spectroscopic measurements, C. Gardiennet and G. Kervern for useful discussions on NMR. Authors acknowledge the Center of Magnetism of the Institute Jean Lamour for magnetic measurements facilities and the X-ray diffraction platform PMD²X of the Institut Jean Barriol for SAXS measurements time. The authors acknowledge financial support from the "Mira elle+" project of the "Lorraine Université d'Excellence" Investissements d'avenir -ANR), CPER Enerbatin and PHC CEDRE Future Materials. SG, SL and MB also acknowledge financial support through the COMETE project (COnception in silico de Matériaux pour l'EnvironnemenT et l'Energie co-funded by the European Union under the program "FEDER-FSE Lorraine et Massif des Vosges 2014-2020". HPC resources have been provided by GENCI-CCRT (Grant and No. A0060910433).

1. Structural properties of pristine SBA-15-like material, obtained in the absence of CTAF; 2. Spectroscopic measurements (ATR and UV-Vis); 3. Thermogravimetric analysis; 4. NMR; 5.

Elemental cartography; 6. Magnetic measurements (substraction of the diamagnetic signal and low temperature spin crossover) and 7. DFT calculations.

Funding Sources

FEDER, LUE, ANR, CNRS Lebanon, Lebanese University.

Author Contributions

The manuscript was written through contributions of all authors.

Abbreviations

CTAF cetyltrimethylammoniumtrichloromonobromoferrate, DFT Density Functional Theory, STEM scanning transmission electron microscopy.

Supplementary data