Charles Paleo
email: charles.paleo@univ-lyon1.fr

Fabrice Wilhelm

Philippe Ohresser

Edwige Otero

Andre Dias

Nora M Dempsey

Olivier Boisron

Clément Albin

Véronique Dupuis

Damien Le Roy

Magnetic hardness of graded interface exchange spring L10-FePt films with Co nanoinclusions

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

I. INTRODUCTION

Current spintronic technologies for non-volatile memory involves a recording layer with an anisotropy high enough to avoid loss due to thermal instability at room temperature. In this case, both soft ferromagnetic writing and reading head, and a high energy product spinning motor are required. The discovery of Giant Magnetoresistance (GMR) [START_REF] Baibich | Giant magnetoresistance of (001)fe/(001)cr magnetic superlattices[END_REF] allowed the transition from longitudinal to perpendicular recording, leading to higher storage densities [START_REF] Hartmann | Magnetic Multilayers and Giant Magnetoresistance: Fundamentals and Industrial Applications[END_REF], but requires an even greater anisotropy from the recording layer compared to longitudinal recording, as the anisotropy field must withstand the demagnetizing field [START_REF] Coey | Magnetism and Magnetic Materials[END_REF].

The anisotropy energy of a magnetic grain is E a = K u × V . As the volume V of the grain decreases, the anisotropy K u needs to be increased to ensure magnetization stability at room temperature. However, writing becomes harder and therefore more energy-consuming [START_REF] Vopson | Multiferroic composites for magnetic data storage beyond the super-paramagnetic limit[END_REF]. To allow easier writing, one solution could be to add a softer layer on top of the stable media which serves to initiate the reversal which then propagates to the harder layer by exchanged-coupling (exchange-coupled composite, ECC) at the interface [START_REF] Victora | Composite media for perpendicular magnetic recording[END_REF][START_REF] Chen | Fept-c graded media for ultra-high density magnetic recording[END_REF][START_REF] Skomski | Nucleation and wall motion in graded media[END_REF]. For this nucleation layer, oriented grains with an easy axis at 45 • can be used, lowering by half the required field for reversal, according to the Stoner-Wohlfarth model [START_REF] Gao | Magnetic recording configuration for densities beyond 1 tb/in 2 and data rates beyond 1 gb/s[END_REF][START_REF] Varvaro | Ultrahigh-density magnetic recording: storage materials and media designs[END_REF]. Graded media can also be used, softer near the writing head, the coercivity increasing with depth [START_REF] Coey | Magnetism and Magnetic Materials[END_REF][START_REF] Goll | Experimental realization of graded l10-fept/fe composite media with perpendicular magnetization[END_REF]. In both cases, the minimal energy needed for the reversal is equal to that needed to create the domain wall in the hard phase [START_REF] Suess | Multilayer exchange spring media for magnetic recording[END_REF][START_REF] Goncharov | Recording simulations on graded media for area densities of up to 1tbit/in.2[END_REF]. Studies on graded material has already been carried out on CoPt [START_REF] Mitsuzuka | Magnetic properties of copt/ co hard/soft stacked dot arrays[END_REF] and FePt [START_REF] Goll | Experimental realization of graded l10-fept/fe composite media with perpendicular magnetization[END_REF][START_REF] Wang | Fabrication and characterization of fept exchange coupled composite and graded bit patterned media[END_REF] where a decrease in coercivity of up to 60% has been observed, but highlight the need of precise characterization of both structure and magnetism.

In this context, nanocomposite benchmark systems where the microstructure should be controlled at the nanometer scale, are expected to give interesting insights. In this work, we produce FePt films integrating face-centered-cubic (fcc) Co nanoclusters (Co-NCs), using a mass selected cluster beam deposition (MS-LECBD) technique, combined with an in-situ electron beam evaporator. From atomic force microscopy (AFM), we verified the Gaussian-like profile of the cluster beam, as expected for MS-LECBD using laser vaporization [START_REF] Alayan | Application of a static quadrupole deviator to the deposition of size-selected cluster ions from a laser vaporization source[END_REF]. Grown films are compositionally graded in Co-NCs, which allows us to screen the nanocomposite magnetic properties as a function of the soft inclusion concentration. Within the combinatorial approach [START_REF] Green | Applications of high throughput (combinatorial) methodologies to electronic, magnetic, optical, and energy-related materials[END_REF][START_REF] Zambano | Dependence of exchange coupling interaction on micromagnetic constants in hard/soft magnetic bilayer systems[END_REF], several magnetic scanning probe techniques have been developed for high throughput studies, including superconducting quantum interference device (SQUID)-based microscopy [START_REF] Finkler | Nano-sized squid-on-tip for scan-ning probe microscopy[END_REF][START_REF] Shperber | Scanning squid microscopy in a cryogen-free cooler[END_REF] or Hall microscopy [START_REF] Roy | Fabrication and characterization of polymer membranes with integrated arrays of high performance micro-magnets[END_REF][START_REF] Oral | Real-time scanning hall probe microscopy[END_REF]. Here we used a scanning magneto-optical Kerr effect (MOKE) system with a microsecond pulsed magnetic field source capable of producing fields up to 10 T [START_REF] Dias | Preparation and characterisation of compositionally graded smco films[END_REF]. In particular, we studied the evolution of the coercive field with the volume fraction of Co-NCs and compared it with alloyed films of equivalent compositions and determined specific atomic magnetic moments as a function of the soft magnetic inclusion composition from X-ray magnetic circular dichroism (XMCD).

II. EXPERIMENTAL METHOD

Co-NCs are produced by gas-phase condensation, in a MS-LECBD system using laser vaporization, and deposited in a soft-landing regime [START_REF] Dupuis | Intrinsic magnetic properties of bimetallic nanoparticles elaborated by cluster beam deposition[END_REF]. Our experimental set-up integrates an in-situ electron beam evaporator with four crucibles to embed the NCs in any hosting material. In this study, the deposition incident angles on the substrate, both for the cluster beam and the atoms from the evaporator, are of 45 • (Fig. 1a).

Fig. 1b shows a transmission electron microscopy (TEM) image of the produced Co-NCs supported on an amorphous carbon membrane, and Fig. 1d the corresponding analysis of the cluster size distribution. In this work, Co-NCs are selected in mass, leading to a Gaussian size distribution centered around 7.9 nm in diameter with a 15% dispersion. In our previous work, we showed that such as-produced Co-NCs crystallize in a fcc structure, and form truncated octahedra exhibiting the co-existence of (111) and (100) facets [START_REF] Jamet | Magnetic anisotropy of a single cobalt nanocluster[END_REF], as expected from the Wulff model and represented on the schematic view of Fig. 1c.

All Co-NC depositions are made with the same settings for the clusters source and the electrostatic deviator, in order to ensure the same density profile of the cluster beam. We prepared specific samples to thoroughly calibrate the Co-NCs deposition profile by means of atomic force microscopy (AFM). Co-NCs were deposited onto on a silicon substrate with a native SiO 2 layer. Prior to deposition, Au/Ti marks for X and Y coordinates along two orthogonal directions of the plane were patterned by photolithography and lift-off techniques. AFM images of 1 µm 2 were then recorded at every millimeter along the X and Y directions. Fig. 2a shows the reconstructed car-tography of the density of clusters. Note that this was repeated once again, during another run of deposition, at several weeks interval, to validate the calibration, and further confirmed by superimposing the Co X-rays Absorption Spectroscopy (XAS) signal mapping prior to Extended X-Ray Absorption Fine Structure (EXAFS) and XMCD local measurements.

The Co x @(FePt) 1-x nanocomposite films are deposited at room temperature in an ultra-high vacuum (UHV) chamber (base pressure of 10 -10 mbar), on a silicon substrate with a native SiO 2 layer. They are obtained by sequential deposition of Co-NCs, and atomic bilayers of Fe (1.2 nm in thickness) and Pt (1.4 nm in thickness), repeated six times in total and subsequently annealed at 700C for 20 min in order to obtain the L1 0 -FePt hard magnetic phase [START_REF] Zeng | Orientation-controlled nonepitaxial l10 copt and fept films[END_REF][START_REF] Paleo | Interplay between local structure and magnetic properties of graded exchange-coupled co@fept nanocomposite films[END_REF]. Throughout the deposition area, the surface coverage of the Co-NCs in each layer is of about 21%. The clusters' flux in the deposition chamber is measured using a Faraday cage before and after each cluster layer. Due to the incidence angle of 45 • , the deposition areas of the NCs' beam and the atoms from the evaporator form ellipses, with a long axis to short axis ratio of √ 2. The atoms from the evaporator cover a larger area of the substrate than the NCs, characterized by a long axis of 20 mm and 8 mm, respectively.

III. RESULTS AND ANALYSIS

A. Compositionally-graded Co@FePt NC films

The Co-NCs' deposition leads to the formation of an ellipse with a long axis to short axis ratio of 1.35, slightly less than √ 2 (Fig. 2a). The maximum density is found to be off centered within the ellipse, the concentration being nearly symmetric with respect to the major axis of the ellipse and highly asymmetric along the Y direction. As a result, considering the deposition profile along the Y axis at X = 0, the density gradient is about six times larger when going downward as compared to in the upward direction. Along this line, the variation of density is nearly linear (see Fig. 2b) between a maximum density of 1100 NCs/µm 2 down to 0 over a distance of 8 mm from the center.

The footprint of the Co-NCs' beam on the substrate is significantly smaller than the FePt atomic beam deposition area. Besides, the thickness and the magnetic properties of the FePt deposit, studied by AFM, X-ray reflectivity and scanning MOKE were found to be even over an area that is 6 times larger than the region containing Co nanoclusters (see Supporting Information [28] Fig. S1). Therefore, a single deposition contains a large region of homogeneous Co-free FePt film. Fig. S2 gathers the structural and magnetic properties of the FePt reference region and how the chemical ordering scales with the hard-magnetic properties of the FePt compound. The formation of the L1 0 -FePt phase is accompanied by an increase of the coercive field, up to 1.5 T. It is worth men- tioning here that tapping-AFM observations of the film's surface revealed a smooth surface of 1 nm roughness (not shown).

Based on the thoroughly calibrated deposition of the Co nanoinclusions, the deposition parameters were set to obtain a compositionally-graded Co x @(FePt) 1-x film with x varying from 0 to 50% (atomic proportions), over 8 mm.

B. Magnetic characterization

Local hysteresis loops were measured using a scanning polar MOKE system [START_REF] Dias | Preparation and characterisation of compositionally graded smco films[END_REF]. The system is equipped with a bipolar pulsed current source coupled with a Cu-coil of inner (outer) diameter ∼3 (10) mm. The length of individual field pulses is of the order of 16 µs, and the delay between positive and negative field pulses is about 10 ms. The hollow center of the coil allows the incident and reflected light beams to pass through. The film under measurement is scanned in-plane below the coil using an X-Y stage (minimum step size = 2 µm) and the coil to sample distance is set to 100 µm. Out-of-plane magnetization curves were measured every 500 µm in the X and Y directions across the film's surface. Fig. 3a shows the reconstructed map of the coercive field. The colorscale shows the value of coercive magnetic field and the variation of the coercive field matches with the footprint of the Co nanoinclusions deposition. Combining the density of deposited Co-NCs and the coercive field value leads to the average dashed curve of Fig. 3b. The symbols correspond to the results over a line at X = 1 and Y starting at 0, as schematized in Fig. 3c. Moreover, scanning XMCD measurements have also been carried out (on the DEIMOS beamline at SOLEIL synchrotron [START_REF] Ohresser | Deimos: A beamline dedicated to dichroism measurements in the 350-2500 ev energy range[END_REF][START_REF] Joly | Fast continuous energy scan with dynamic coupling of the monochromator and undulator at the deimos beamline[END_REF]) in total electron yield (TEY) and fluorescence to probe element-specific magnetic properties at the L 2,3 edges of Fe and Co. The sample was scanned every 500 µm along the long axis of the Co cluster deposition area to analyze the evolution of the atomic magnetic moments with the Co content. The magnetic moment reversal recorded at the L 3 edge of Fe and Co (Fig. 4) shows a clear single magnetic phase behavior over the whole range of Co-NCs content (0 to 50%), indicating a strong coupling between Co and Fe, in these graded-interface nanocomposite films. The decay of the coercive field with the Co content appears to be linear in XMCD while it decreases faster on SMOKE analysis (shown on Fig. 3b), probably as an effect of the probed depth: around 5 nm for XMCD TEY measurements [START_REF] Nakajima | Electronyield saturation effects in l -edge x-ray magnetic circular dichroism spectra of fe, co, and ni[END_REF][START_REF] Chen | Experimental confirmation of the x-ray magnetic circular dichroism sum rules for iron and cobalt[END_REF], 20 nm for MOKE [START_REF] Dias | Preparation and characterisation of compositionally graded smco films[END_REF]. Therefore, while MOKE probes the entire thickness of the sample, roughly a quarter of the sample is analyzed in XMCD. XAS spectra were recorded at room temperature, under an external field of 5 T parallel to the incident Xray beam and perpendicularly to the sample plane (90 • ). XMCD spectra on Fe L 2,3 peaks show a systematic increase of intensity when the Co content increases, as expected for higher amount of Co-Fe bonds (getting closer to Fe 65 Co 35 ) (Fig. 5) [START_REF] Coey | Magnetism and Magnetic Materials[END_REF].

Sum rules [START_REF] Thole | X-ray circular dichroism as a probe of orbital magnetization[END_REF][START_REF] Carra | Xray circular dichroism and local magnetic fields[END_REF] were applied to quantify the orbital and effective spin moments of Co and Fe on TEY measurements. The spectra were normalized and the continuum has been subtracted from the XAS for each polarization. The number of holes used in calculation is 3.7 for Fe (reported value for L1 0 -FePt [START_REF] Galanakis | Calculated magnetic properties of low-dimensional systems: the aucu-and aucu3-type ferromagnets[END_REF]) and 2.42 for Co (fcc bulk value calculated using the fully relativistic Dirac linear muffin-tin orbital band-structure method, [START_REF] Perlov | Py-lmto, a spinpolarized relativistic linear muffin-tin orbitals package for electronic structure calculations, PY-LMTO[END_REF]). The magnetic dipole term, which cannot be neglected in highly anisotropic phases such as L1 0 -FePt here [START_REF] Sakamoto | Effects of cobalt substitution in l 1 0 -(fe,co)pt thin films[END_REF], has not been removed. Therefore, only the effective spin moment is being shown and compared to other effective spin moments found in the literature [35, [START_REF] Dupuis | Magnetic moments in chemically ordered mass-selected copt and fept clusters[END_REF][START_REF] Zahlenwerte | Funktionen aus Naturwissenschaften und Technik: Neue Serie: = Numerical data and functional relationships in science and technology: new series[END_REF][START_REF]Metallic Alloys: Experimental and Theoretical Perspectives[END_REF][START_REF] Šipr | Influence of composition, many-body effects, spin-orbit coupling, and disorder on magnetism of co-pt solid-state systems[END_REF] (denoted µ S (Fe) and µ S (Co) for Fe and Co, respectively). The results are displayed as a function of the Co content on Fig. 6 (Fig. S4). To our knowledge, only a few works, like Sakamoto et al. [START_REF] Sakamoto | Effects of cobalt substitution in l 1 0 -(fe,co)pt thin films[END_REF], were reported on the element-specific magnetic moments of the ternary FeCoPt alloy, shown as a comparison in Fig. S3.

The effective spin moment at Fe edges increases continuously with the Co content, from 2.35µ B in the Co-free area up to 2.76µ B in the Co richest area (50% Co). The absolute value reaches the higher range of reported values for chemically ordered L1 0 -FePt, or Fe 3 Pt [START_REF] Galanakis | Calculated magnetic properties of low-dimensional systems: the aucu-and aucu3-type ferromagnets[END_REF]. However, the large dispersion of the absolute values found in the literature show its high sensitivity to the chemical and structural order [START_REF] Galanakis | Calculated magnetic properties of low-dimensional systems: the aucu-and aucu3-type ferromagnets[END_REF][START_REF] Dupuis | Magnetic moments in chemically ordered mass-selected copt and fept clusters[END_REF]. The proximity of Fe atoms to Co favors the increase of the Fe atomic moment, as predicted by the rigid band model of Slater-Pauling, and experimentally measured on B2 FeCo [START_REF] Maclaren | Electronic structure, exchange interactions, and curie temperature of feco[END_REF]).

The increase of the Fe spin moment is nearly linear but displays a slight change of slope at around 35% Co. In the Fe-Pt alloy, the structural transition from tetragonal L1 0 to face-centered-cubic L1 2 is expected to occur between 65% and 68% Fe, with potential intermediate A1 solid solution face-centered-cubic [START_REF] Smithells | Smithells metals reference book[END_REF]. However, given that the orbital moment of Fe is greater in the L1 0 phase than in the L1 2 phase, one could expect that this structural change comes with a drastic decrease in the orbital moment when the Co content increases. But our findings show instead a nearly constant µ L /µ S ratio, suggesting a similar trend for both µ L and µ S . In a recent study of the chemical structure of Co@FePt nanocomposite films studied by EXAFS [START_REF] Paleo | Interplay between local structure and magnetic properties of graded exchange-coupled co@fept nanocomposite films[END_REF], analysis showed that Co-rich regions are well described by fcc (Co+Fe) 3 Pt chemical environment.

As opposed to µ S (Fe), µ S (Co) decreases linearly when the Co content increases. Interestingly, the slope is similar to the one that goes from the µ S (Co) measured in CoPt [START_REF] Galanakis | Calculated magnetic properties of low-dimensional systems: the aucu-and aucu3-type ferromagnets[END_REF][START_REF] Dupuis | Magnetic moments in chemically ordered mass-selected copt and fept clusters[END_REF][START_REF] Šipr | Influence of composition, many-body effects, spin-orbit coupling, and disorder on magnetism of co-pt solid-state systems[END_REF] to the one in Co (hcp) [START_REF] Chen | Experimental confirmation of the x-ray magnetic circular dichroism sum rules for iron and cobalt[END_REF][START_REF] Zahlenwerte | Funktionen aus Naturwissenschaften und Technik: Neue Serie: = Numerical data and functional relationships in science and technology: new series[END_REF] (slope better viewed on Fig. S5). This is in good agreement with the persistence of Co-rich regions after annealing that are larger when the Co content increases. The absolute value of µ S (Co) is systematically higher (ranging from 1.99µ B at high Co content to 2.08 µ B at lower concentration) as compared to 1.98µ B in disordered CoPt [START_REF] Dupuis | Magnetic moments in chemically ordered mass-selected copt and fept clusters[END_REF] or 1.62µ B in bulk hcp Co [START_REF] Chen | Experimental confirmation of the x-ray magnetic circular dichroism sum rules for iron and cobalt[END_REF][START_REF] Zahlenwerte | Funktionen aus Naturwissenschaften und Technik: Neue Serie: = Numerical data and functional relationships in science and technology: new series[END_REF]. However, the high values of µ S (Co) must be taken with caution as the number of holes in NCs often differs from the one of the bulk [START_REF] Basch | The electronic structure of small nickel atom clusters[END_REF][START_REF] Luis | Tuning the magnetic anisotropy of co nanoparticles by metal capping[END_REF]. Moreover, surface-induced anisotropy in clusters can sometimes lead to either a lift [START_REF] Eastham | Enhanced magnetocrystalline anisotropy in deposited cobalt clusters[END_REF] or a reinforcement [START_REF] Eastham | Quenching of ferromagnetism in cobalt clusters embedded in copper[END_REF] of the orbital moment quenching typically found in 3d transition-metals, depending on the cluster size and interaction with the matrix. Contrary to µ L (Fe) which increases, we found that µ L (Co) remains nearly constant, at 0.21µ B , over the whole range of concentration. Like in Fe-Pt alloy, it is established that the proximity effect of Co and Pt on the Co magnetic moment highly depends on the structure. An increase in magnetic moment is expected for distorted FeCo alloys [START_REF] Burkert | Giant magnetic anisotropy in tetragonal feco alloys[END_REF]. In our samples, the tetragonal L1 0 matrix could have induced a distortion of the Co-rich inclusion. The experimental work of Ueno et al. on ultra-thin Co layers sandwiched in between Pt layers showed that as the fraction of Co-Pt bonds with respect to Co-Co increases, the spin and orbital moments decrease while the ratio µ L /µ S is constant [START_REF] Ueno | Enhanced orbital magnetic mo-ments in magnetic heterostructures with interface perpendicular magnetic anisotropy[END_REF]. In contrast, density functional theory-generalized gradient approximation (DFT-GGA) computations on small CoPt alloyed clusters by Hu et al. [START_REF] Hu | Structural and magnetic properties of copt clusters[END_REF] revealed a systematic increase of the Co magnetic moment as the number of Co-Pt bonds increases.

The ratio µ L /µ S at both Fe and Co edges does not vary with the overall composition, showing a similar variation of µ S and µ L for each element. Its absolute value does not depend on the number of holes [START_REF] Thole | X-ray circular dichroism as a probe of orbital magnetization[END_REF][START_REF] Carra | Xray circular dichroism and local magnetic fields[END_REF], and remains in between the L1 0 phase and the L1 2 phase for Fe, which could suggest a mixture of the two structures. As for Co, the ratio µ L /µ S is in the high range of reported values in Co-Pt alloy. The high value of µ L could be of structural origin in the graded interface between the Co inclusions and the matrix.

Samples were probed at two different angles, at 90 • (the X-ray beam is perpendicular to the sample plane, and the electric field lies in the plane of the sample) and at 30 • (with thus a non-zero out-of-plane component of the electric field). The results are summarized in Table I for Fe and Table II for Co, in order to investigate the magnetocrystalline anisotropy [START_REF] Bruno | Tight-binding approach to the orbital magnetic moment and magnetocrystalline anisotropy of transitionmetal monolayers[END_REF][START_REF] Stöhr | Exploring the microscopic origin of magnetic anisotropies with x-ray magnetic circular dichroism (xmcd) spectroscopy[END_REF].

At the Fe edges, µ L is systematically larger in the plane of the sample. This is even stronger in the regions that contain Co. At the Co edges, in contrast, µ L is slightly smaller in-plane, as it increases by around 10% when the electric field is at 30 • out of the plane. Note that asdeposited Co nanoclusters are face-centered cubic and are randomly oriented. Our findings would translate in a film plane that is easy for µ L (Fe) but hard for µ L (Co). However, several effects qualify this conclusion about the anisotropy of the orbital moment. Indeed, spin-orbit coupling and interface roughness between hetero-atoms may lead to an in-plane spin reorientation, overshadowing the magnetocrystalline anisotropy. In particular, it has been shown that the presence of intermixing may lead to the orbital anisotropy not being proportional to magnetocrystalline anisotropy [START_REF] Bruno | Magnetic surface anisotropy of transition metal ultrathin films[END_REF][START_REF] Andersson | Influence of ligand states on the relationship between orbital moment and magnetocrystalline anisotropy[END_REF]. Besides, it is now well established that TEY measurements suffer from saturation effect which results in a recorded signal not proportional to the cross-section, especially at grazing incidence, and this effect is more pronounced for Fe than for Co atoms [START_REF] Nakajima | Electronyield saturation effects in l -edge x-ray magnetic circular dichroism spectra of fe, co, and ni[END_REF].

IV. CONCLUSION

To conclude, compositionnally-graded Co x @FePt 1-x nanocomposite films (x varying from 0 to 50%) were prepared by co-deposition from two independent MS-LECBD and e-beam evaporation techniques. We used combined local characterization techniques to thoroughly analyze the dependence of the magnetic properties on the concentration of Co nanoinclusions. Interestingly, the decay of coercivity is found to be substantially smaller in the nanocomposite films as compared to the alloyed configuration, as expected in a two-phase exchange spring magnet. Fe and Co magnetic moments were quantified using XMCD for the first time in such compositionnally-graded Co x @FePt 1-x nanocomposite films, which showed significant variations over the studied range of composition. Fe spin and orbital moments are found to increase with the Co content. Oppositely, Co spin and orbital moments, which remain relatively high as compared to any references, decrease. These unprecedented experimental findings could be of great interest for the design of semi-hard nanocomposite films with a fine tune of magnetization reversal.

FIG. 1 .

 1 FIG. 1. (a) Schematic view of the sample preparation. (b) Transmission Electron Microscopy image of an assembly of Co nanoclusters. (c) schematic view of a fcc-Co truncated octahedron of the mean size [23]. (d) Statistical analysis of the Co nanoclusters' diameter.

FIG. 2 .

 2 FIG. 2. Co-NCs' deposition profile. (a) Reconstructed map of the Co-NCs' density on a surface, from AFM observations. For this, images of area of 1 × 1 µm 2 were made. (b) Conversion of the measured AFM Co-NCs density and the Co concentration in the nanocomposite film. The blue dashed line is a linear guide to the eye. (c) Schematic profile of the compositionally-graded Co NCs on a Si substrate.

FIG. 3 .

 3 FIG.3. Coercive field mapping over the clusters' deposition area of the Co@FePt nanocomposite film, from scanning MOKE with measured hysteresis cycles superimposed. (b) Deduced compositional dependence of the nanocomposite coercive field (dashed is third order polynomial interpolation). (c) Schematic profile of the compositionally-graded Cox@(FePt)1-x film, as deposited.

FIG. 4 .

 4 FIG. 4.Coercive field at the Fe and Co L3-edges as a function of Co-NCs content.

FIG. 5 .

 5 Photon energy (eV)

TABLE I .

 I Orbital, spin moment and ratio of Fe atoms at different Co content at 90 • (X-ray beam perpendicular to the sample plane) and 30 • . Moments in µB/at.

	(Fe edges)	µL	µL	µS	µS	µL/µS µL/µS
	Co %	(30 • ) (90 • ) (30 • ) (90 • )	(30 • )	(90 • )
	0	0.05	0.13	2.52	2.36	0.020	0.055
	25	0.12	0.15	2.59	2.51	0.046	0.060
	50	0.12	0.17	2.74	2.74	0.044	0.067

TABLE II .

 II Orbital, spin moment and ratio of Co atoms at different Co content at 90 • and 30 • . Moments in µB/at.

	(Co edges)	µL	µL	µS	µS	µL/µS µL/µS
	Co %	(30 • ) (90 • ) (30 • ) (90 • ) (30 • )	(90 • )
	25	0.23	0.21	2.31	2.08	0.10	0.10
	50	0.25	0.22	2.13	2.00	0.12	0.11

ACKNOWLEDGMENTS

This work is supported by the ANR through the collaborative project SHAMAN (ANR-16-CE09-0019), and the doctoral school ED PHAST. The authors thanks Rémy Fulcrand and Agnès Piednoir, from the ILM laboratory for fruitful discussion and technical support in the cleanroom and AFM experiments. XAS and XMCD experiments were performed on the DEIMOS beamline at SOLEIL Synchrotron, France. We are grateful to the SOLEIL staff for smoothly running the facility.


Alexandre Bartz

Juliette Janes

Laurent Romary

Philippe Gambette

Rachel Bawden

Pedro Ortiz Suarez

Benoît Sagot

Simon Gabay

Expanding the content model of annotationBlock

Linguistic annotation benefits from ISO specifications such as the Morphosyntactic Annotation Framework (MAF), whose recommendations have been added to the TEI P5 1 (ISO-24611 2012; [START_REF] Stührenberg | The TEI and Current Standards for Structuring Linguistic Data[END_REF]. Relying on feature structures (cf. ex. 1), these recommendations have however not been fully integrated into the TEI stand-off annotation model [START_REF] Bański | Wake up, standOff![END_REF]) and, for instance, it is currently impossible to encode feature structures within the listAnnotation and annotationBlock elements. <seg> <w xml:id="s1w1">lõg</w> <w xml:id="s1w2">tems</w> </seg> <spanGrp type="wordForm">   </spanGrp> <fs xml:id="s1fs1"> <f name="lemma"> <string>long</string> </f> </fs> <fs xml:id="s1fs2"> <f name="lemma"> <string>temps</string> </f> </fs> Example 1: TEI encoding following the MAF 1 https://tei-c.org/release//doc/tei-p5-doc/en/html/FS.html.

1

With the multiplication of annotation tools, the case is becoming more complex and is no longer limited to low-level linguistic annotation (lemma, POS, etc.) using the fs element. For instance, it is becoming more and more common for medievalists [START_REF] Stutzmann | Paléographie statistique pour décrire, identifier, dater... Normaliser pour coopérer et aller plus loin ?[END_REF]) and modernists [START_REF] Gabay | Traduction automatique pour la normalisation du français du XVII e siècle[END_REF] working on normalisation tasks to encode various levels of transcription (lõg tems → long tems) or offer a version which is fully aligned with contemporary spellings (lõg tems → longtemps). New elements, such as reg, would therefore be extremely useful (cf. Ex. 2). <seg corresp="#s1"> <w xml:id="s1w1">lõg</w> <w xml:id="s1w2">tems</w> </seg> […] <spanGrp type="wordForm">   </spanGrp> <reg type="reg" xml:id="s1reg1">long</reg> <reg type="reg" xml:id="s1reg2">tems</reg> […] <spanGrp type="formNorm">  </spanGrp> <reg type="norm" xml:id="s1norm1">longtemps</reg> Example 2: Annotation of linguistic normalisation Sadly, in the case of multiple levels of embedded stand-off data, the current version of the guidelines promotes a multiplication of standOff elements with a semantically inappropriate seg to store the necessary annotation (cf. Ex. 3).

<standOff type="linguistic"> <seg corresp="#s1"> <spanGrp>[…]</spanGrp> <fs>[…]</fs> </seg> </standOff> <standOff type="norm"> <seg corresp="#s1"> <spanGrp>[…]</spanGrp> <reg>[…]</reg> </seg> </standOff>
Example 3: Annotation of linguistic normalisation Using the data of the E-ditiones project, we will make a case for a more appropriate use of standOff with one listAnnotation per annotation type and an extended version of the annotationBlock content model (cf. Ex. 4). This model, also an ISO recommendation (ISO-24624 2016), has originally been created to identify the reference features needed to transcribe spoken resources that are anchored on a single reference timeline, but also for integrating mechanisms to encompass most usual transcription conventions. It therefore needs to include not only the fs element for reference annotation, but also additional information about editorial transcription such as normalisation (reg) or any other philological intervention on the text (cf. the model.pPart.transcriptional class: add, corr, damage, del, handShift, mod, orig, redo, reg, restore, retrace, secl, sic, supplied, surplus, unclear, undo).

<TEI> <text>
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Data

Scripts and data are available at https://github.com/e-ditiones/Annotator.


Cherif Larabi

Cuirong Chen

Nicolas Merle

Marc O Charlin

Kai C Szeto

Aimery De Mallmann

Anass Benayad

Karima B T Meziane

Akim Kaddouri

Hai P Nguyen
email: hai.p.nguyen@toyota-europe.com

Mostafa Taoufik
email: mostafa.taoufik@univ-lyon1.fr

Well-defined surface tungstenocarbyne complex through the reaction of [W

Keywords: DeNOx, Single site catalyst, Ceria, Tungsten, XAFS

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Significant worldwide increase of stationary thermal power plants and mobile combustions engines culminates with the production of large quantity of carbon dioxide and nitrogen oxides (NOx). NOx contributes to ecosystem damage, serious health issues and worsens the climate changes, through the greenhouse effect, acid rains and ozone destruction. [1] Therefore, the emission level legislation of this pollutant has been drastically going down. [2,3] The NOx emissions were minimized through improvements such as exhaust gas recirculation, homogeneous charge compression ignition technologies and optimized injection systems as well as improved air control. [4] These reducing measures would lower the emission of NOx albeit with the net increase of the particulate matter (PM) amount and unburned hydrocarbons. As the PM and NOx emission are totally dependent, the declining of the former trigger the rising of the latter and viceversa. Thus, to improve the combustion engine efficiency and minimize fuel consumption, it is desirable to operate at a lean fuel mixture, this is typically the case, for diesel engine. [5] Under these conditions, in addition to a complete combustion product (CO2 and H2O), a significant amount of NOx is produced. Therefore, exhaust gas after-treatment systems are necessary in order to meet the stringent harmful emission limits. Catalysis has made impressive development in the field of NOx reduction, since the beginning of the eighties, by the implementation of the three way catalyst systems. However, the diesel exhaust composition brings new challenges that mobilized industries, researchers and authorities to meet many improvement and innovative measures. Most effective and useful method for NOx removal without compromising the engine performances is the selective catalytic reduction (SCR) assisted with reducing agents (NH3 or hydrocarbon). [6,7] The most efficient approach is SCR using ammonia (NH3-SCR), this process leads to high NOx conversions at fairly low temperatures and in large temperature ranges. [8] Initially, this technology was set up in power plants and industrial installation for many years ago, but right now, they are extensively adopted to heavyduty and light-duty trucks as well as to locomotives and ships.

A wide range of SCR catalysts have been developed. The most commercially viable and utilized catalysts are TiO2 supported V2O5 associated to WO3 or MoO3 materials. [9,10] Though the catalytic activity of this class of materials are acceptable, they suffer however from lots of weaknesses such as low selectivity, narrow operation temperature range, poor deNOx activity at low temperature and beyond all, another central environmental issue due to the releasing of deleterious and toxic VOx species. [11] It has been a non-stop story since then, and several efficient catalysts based on transition (W, Nb, Mo, Zr, Ta,…) [12] and rare earth metals (Ce, Y,..) [13] have been well described. In particular, relevant studies on new catalytic systems as WO3-CeO2, Nb2O5-CeO2, MnOx-WO3-CeO2, Cu-Zeolite and Fe-zeolite have brought important improvement in the field of NOx reduction. [13][14][15] Due to the oxygen buffering capacity and redox properties of the ceria, it is widely used in these processes. [16] The major drawbacks of pure ceria relate to low thermal resistance and weak acidity. Hence, in order to improve these properties, new ceria based catalysts have been developed the use of ceria doped with other rare-earth or transition metal oxides such as ZrO2, TiO2 and their modification with WO3 which improves their redox properties, thermal stability as well as surface acidity. [13,16] Recently, through several mechanistic studies, it was found that the catalytic activity depends not only on the acidic and redox properties of the material, [9,17] but also on the metal-support interactions. [18] In particular, the nature of the supported tungsten species is determining factor. The reported Conventional catalysts are normally prepared by uncontrolled impregnation, resulting in different species, including isolated surface tungsten sites, WxOy clusters, amorphous WO3 and Ce2(WO4)3. [19] Indeed, those phases are all observed by any characterization technique and this will thereby complicate the surface species identification and the drawing of structure-activity relationship.

It has been proposed that single-site heterogeneous catalysts with a controlled coordination sphere of the metal are highly effective for the selective reduction of NOx. [20] The key step is the formation of isolated supported metal species which improves the metal dispersion, increases the metal-support interactions. Therefore, a powerful approach known as surface organometallic chemistry (SOMC) can be applied in order to prepare such single-site species. This methodology involves the controlled grafting of a suitable organometallic precursor on a support, creating a firmly bonded surface organometallic fragment. Previous studies have shown that catalytic materials prepared through this approach can lead to an enhanced activity and facilitate further mechanistic studies. [21] Moreover, great efforts have been devoted toward achieving high-performance catalysts via a chemical design. It is believed that the missing link for the use of catalysts in automotive applications is related to increasing the amount of nanosized and atomic-scale catalysts without impinging on their size, structure, shape, and interactions. This can be carefully monitored by SOMC. [START_REF] Beniya | Towards dense single-atom catalysts for future automotive applications[END_REF] However commercial SCR catalysts are loaded with high amounts of WO3, [23,24] in order to ensure a higher stability and a sufficient acidity, thus increasing the NH3 adsorption strength which improves the activity and also the selectivity by inhibiting the oxidation of NH3. [17] This phenomenon is also found in the case of industrial metathesis catalysts, where a high amount of metal such as tungsten, ca. 10 wt%, is required to reach high catalytic activities while only a low fraction of metal sites is active. [25] And then, based on studies that unraveled the structure of the active species, [26][27][28] surface organometallic chemistry has led to the development of well-defined highly active catalysts that contain only a low amount of metal. [29][30][31][32][33] On the other hand, surface organometallic chemistry on oxidizing support and for deNOx applications is rare. We hereby report the first example of a highly active deNOx catalyst with low loading prepared by the grafting a Schrock type tungsten complex on ceria along with its characterization.

Results and discussion

2.1. Grafting of W(CtBu)(CH2tBu)3 on the ceria (CeO2-200) surface via SOMC approach. SOMC methodology was not previously investigated on a ceria support for the grafting of this tungsten complex. Prior to the catalysts preparation, the ceria support (CeO2) was characterized. The IR spectrum of ceria dehydroxylated at 200 °C pictured in Figure 1 shows four (O-H) vibration bands attributed to different structures of cerium hydroxyl on the surface (terminal and bridging O-H), as reported in the literature (Figure S1). [34,35] The intensity of the band at 3714 cm -1 associated with isolated OH groups is weak and the spectrum is rather dominated by the broad signal centered at 3630 cm -1 attributed to bridged hydroxyl groups. In addition a large band centered at 3527 cm -1 corresponds to a residual cerium oxy-hydroxide phase located within the pores. [36] To achieve the grafting and the functionalization of surface hydroxides under optimum conditions, it is desirable to know their amount. Among different quantification methods, the chemical titration using Al(iBu)3 to react with a highly reactive organometallic complex, such as Al(iBu)3 has shown to be reliable. [37] This latter reacts in pentane quantitatively with surface hydroxyl groups by releasing one equivalent of isobutane per OH (Figure S2). The quantification of isobutane by GC shows that CeO2-200 contains 0.7 mmol OH.g -1 .

The grafting reaction of W(CtBu)(CH2tBu)3 on CeO2-200 is carried out in pentane at room temperature during 4 h. Elemental analysis of this material showed the presence of 3.3 wt% W, which correspond to ca. 0.18 mmolW.g -1 . The evaluation of the amount of carbon revealed the presence of 2.16 wt% C (1.79 mmolC.g -1 ), which gives a C/W ratio of 9.95. Furthermore, a quantitative GC analysis of the gas released during the grafting process, shows the formation of 0.38 mmol of neopentane, ca. 1.9 tBuCH3 per W. This corresponds to a partial consumption of surface hydroxyls (ca. 55 % of the initial surface O-H groups). Overall, the data are consistent with the formation of bipodal surface species bearing two ligands, by a protonolysis with surface hydroxyls leading to a concomitant release of ca. two neopentane per tungsten atom grafted, as highlighted in Scheme 1.

Scheme 1: Schematic illustration of the grafting of W(≡CtBu)(CH2tBu)3 on CeO2-200

The textural properties of CeO2-200 and W(≡CtBu)(CH2tBu)3/CeO2-200 were examined by nitrogen adsorption-desorption isotherm measurements, the physisorption isotherms and the pore size distribution are depicted in Figure S3. As shown, the shape of these material isotherms correspond to type V according to IUPAC classification and display an H2-type hysteresis loop characteristic of capillary condensation between the aggregates constituting the solid. The initial increase in adsorption capacity at low relative pressure is due to monolayer adsorption. The upward deviation in the range of P/P0 = 0.5-0.7 for the support and the catalyst is associated with progressive filling of the space between the aggregates of particles. The neat ceria has a specific surface area of ca. 205 ± 10 m 2 .g -1 . It was found that the surface area was slightly decreased by a functionalization with the transition metal by reducing the value to 190 ± 10 m²/g. The pore volumes also decreased from 0.24 ± 0.01 to 0.21 ± 0.01 cm 3 .g -1 .

Figure 1 DRIFT spectrum of a) ceria dehydroxylated at 200 °C; b) after the grafting of W(CtBu)(CH2tBu)3.

The grafting reaction was monitored by DRIFT spectroscopy (Figure 1). After surface functionalization of ceria surface by W(≡CtBu)(CH2tBu)3, the isolated (CeO-H) band at 3712 cm -1 disappeared. The new bands appearing in the 3100-2850 cm -1 and 1620-1400 cm -1 ranges are characteristic of aliphatic (C-H), and δ(C-H) vibrations of the perhydrocarbyl ligands coordinated to surface tungsten. Noteworthy, a band at 2120 cm -1 , which is absent in the neat ceria sample (Figure 1a) is observed (Figure 1b). This signal has already been found and ascribed to the forbidden 2F5/2→2F7/2 electronic transition of the subsurface Ce 3+ (due to the partial reduction of ceria). [38] Moreover, the DRIFT spectrum of the resulting material (Figure 1b) shows a partial consumption of the other OH vibration bands, located between 3700 and 3600 cm -1 , while a new broad band appears, resulting from the interaction of some OH groups with tungsten alkyl ligands. The evolution of the signals in this spectral region with the grafting has already been described for the reaction of W(≡CtBu)(CH2tBu)3 with alumina dehydroxylated at 500 °C, where only terminal tetrahedral Al sites react completely. [39] The X-ray diffraction analyses depicted in Figure S4, revealed that the samples exhibited the same diffraction peaks at 28.5°, 33.1°, 47.5°, 56.4°, 59.1°, 69.7° and 79.1° characteristic of the cubic fluorite structure of CeO2. [40] This observation suggests that the functionalization did not affect the crystalline structure of ceria. From the diffraction pattern, the mean size of microcrystals could be evaluated, since it is related to diffraction peaks broadening by Scherrer's equation. The average crystal sizes thus estimated, as summarized in Table S1, indicated that the ceria particles have the tendency to agglomerate during the grafting of the tungsten complex, which corroborates the slight lowering in the surface area. 1 H MAS solid-state NMR spectrum of the resulting material depicted in Figure S5a, shows large signals at -3 and 0.1 ppm, tentatively attributed to the methyl of neopentyl and neopentylidyne ligands respectively. The 1 H NMR is less informative due to a broadening and shifting of the signal positions due to the presence of paramagnetic species (Ce +3 ) on the catalyst. [41,42] Moreover, from a 30% 13 C labelled molecular complex 13 C CP MAS NMR data highlighted in Figure S5b shows an intense and broad signal centered at 26 ppm assigned to the methyl of the t-Bu fragments and a broad, albeit weak signal at 80 ppm which can be assigned to the methylene carbons of the neopentyl fragment (-CH2tBu). A weak signal at 287 ppm can be tentatively attributed to the quaternary carbons of carbyne ligands. The broadening of the resonances and their shift to higher field is presumably due to paramagnetic Ce 3+ ions in trigonal and cubic sites present in the ceria support as already identified. [41,42] X-ray photoelectron spectroscopy analyses were carried out in order to provide more information on the oxidation state of the elements composing the sample: (W, Ce, O). Figure S6 shows the representative XPS spectra for the Ce 3d, O 1s and W 4f area. Generally, ten features are found in the Ce 3d region due to the pairs of spin orbit doublet as depicted in Figures S6 a andb. Six peaks are labelled v, v″, v‴ (3d5/2) and u, u″, u‴ (3d3/2), associated to Ce +4 and u 0 , u', v 0 , v' contribution of Ce +3 (3d 10 4f 1 ), as described in the literature. [43,44] The total fraction of Ce 3+ were estimated by taking the fitted Ce 3+ peaks area to the total deconvoluted spectra (%Ce 3+ = (Ce 3+ /(Ce 4+ + Ce 3+ ) and (%Ce 4+ = 100 -%Ce 3+ ). [45] The concentration of Ce 3+ in the thermally treated CeO2 is evaluated to be ca. 32%, with respect to the total amount of Ce, thus the main oxidation state of ceria is Ce 4+ (68%). Noteworthy, after the grafting of the organometallic complex, the amount of Ce 3+ has slightly risen to 34%, suggesting the presence of more surface oxygen vacancies. [46] It is widely reported that concomitant presence of Ce 3+ and Ce 4+ offered an oxygen buffering capacity and redox properties that can promote the NOx dissociation. [47,48] O 1s spectra of CeO2 and W(CtBu)(CH2tBu)3/CeO2 are compared and pictured in Figure S6 c andd. The peaks are quite large, leading to two binding energy contributions for O 2-: 531 eV and a shoulder at 532.5 eV, respectively assigned to lattice oxygen (O 2-) of CeO2 (denoted Oβ) [47] and to surface oxygen (denoted as Oα) such as (O -) in defect oxide or OH, [46] respectively. The relative amounts of Oα calculated (Oα/(Oα+Oβ)) were 45% for neat CeO2 and 37% for W(CtBu)(CH2tBu)3/CeO2-200. The partial suppression of the Oα can be attributed to their interaction with the grafted W. [19] The chemisorbed oxygen may promote the oxidation of NO to NO2, inducing the "Fast SCR", and result in higher overall SCR catalytic activity. [49] But also can promote high temperature oxidation of NH3 and affect the selectivity. [18] Thus, a suitable amount of Oα is desired in order to increase the reaction rate but not too much since it might affect the selectivity. [18] Thus, an optimal amount of Oα is desirable in order to increase the reaction rate by not too much since it might affect the selectivity. [50] The XPS analysis was also used to investigate the oxidation state of tungsten loaded on the support. The W4f signal is depicted in Figure S6 e, it shows the presence of two signals attributed to W 4f5/2 and 4f7/2 at 37.5 and 35.3 eV respectively, after deconvolution. These values are characteristic of W (+VI) for the grafted complex. The W4f peaks overlap with the Ce 5 s peaks, as showed in Figure S6f, making discrepancies for the curve deconvolution, nevertheless, these results are consistent with the presence of only W(+VI) but maybe with some light structure heterogeneities. Electron paramagnetic resonance (EPR) spectroscopy of the W(CtBu)(CH2tBu)3/CeO2-200 catalyst depicted in Figure S7 did not show any signal of isolated W(V) ions expected with g factor between 1.39 and 1.85. [51] This suggest that all tungsten atoms are present in oxidation state of W(VI).

The sample with 3.3 wt% W was studied by X-ray absorption spectroscopy (Figure 2 and Table 1), in order to shed light on the structure of the supported species. For the first peak of the Fourier transform (right in Figure 2), two levels of coordinated light atoms back-scatterers could be evidenced at ca. 1.78 Å and 2.26 Å. Considering the level at 1.78 Å, two types of atoms, O and C were considered, oxygen coming from the ceria surface and carbon from a carbyne ligand. The parameters thus extracted from the fit of the EXAFS signal are in agreement with a bipodal structure, (-O)2W(CtBu)(CH2tBu), with ca. two oxygen atoms at 1.78(1) Å, ca. one carbon atom at 1.78(1) Å and another one at 2.26(3) Å, attributed most probably to carbon atoms of a neopentylidyne and a neopentyl ligands respectively. The W-O distance seems marginally short but tungsten has been found to be surrounded by around six oxygen atoms at 1.787(1) Å in a W0.2Ce0.8O2 mixed metal oxide. [52] Table 1: EXAFS parameters for the supported complex, W(≡CtBu)(CH2tBu)3/CeO2. (a) The errors generated by the EXAFS fitting program "RoundMidnight" are indicated between parentheses. Finally, W(RCtBu)(CH2tBu)3/CeO2-200 has been submitted for HRTEM study. The distribution of atomic tungsten on the surface of ceria was confirmed in an indirect manner by HRTEM and STEM analyses (Figure S8). Indeed, the combination of HRTEM and STEM with EDX analyses showed the homogeneity of the sample. No observable cluster or nanoparticles were observed even under extensive magnification (B1 nm), but at the same time, the EDX examination confirmed the presence of W.

Type of neighbor

The reactivity of W(CtBu)(CH2tBu)3 with CeO2 dehydroxylated at 200 °C has been investigated. The grafting occurs on Ce-OH groups by protonolysis, affording bipodal surface species, as suggested by quantification of released neopentane and elemental analysis. Further characterizations by DRIFT and solid state NMR confirm the presence of neopentyl fragments on the material. Tungsten, mainly at oxidation state VI has been revealed by XPS and EXAFS. XPS further provides quantitative information about the Ce 3+ /Ce 4+ ratio on the surface upon grafting. A more representative environment around tungsten on the surface is proposed by EXAFS data. Surprisingly, there is only one cerium atom being close to the tungsten center. The proposed model (Figure 3) fits well with the data and comprises bipodal tungsten surface species, as also indicated by elemental analysis.

Catalytic activity tests

The precatalyst W(CtBu)(CH2tBu)3/CeO2-200 was calcined under dry air at 500 °C for 16 h, allowing the removal of organic ligands (Figure S9) and affording catalyst 1 before evaluating the selective reduction of NOx in a continuous flow reactor. The temperature dependencies of the NOx reduction are depicted in Figure 4. This catalyst shows more than 95 % conversion of NOx between 280 and 400 °C (Figure 4, 1 st cycle). Indeed, a high activity at low temperature (200-250 °C) with a sharp increase of the conversion with temperature is observed. The selectivity for N2 is outstandingly high until 400°C (Figure 4). The fresh catalyst starts to oxidize NH3 from ca. 400 °C, resulting in a fast drop in the NO conversion and N2 selectivity. However, upon repeating SCR cycles, we observe that the same catalyst is getting more stable, selective and temperature resistant. Full NO conversion is kept until 550 °C. Such evolution indicates that the surface sites responsible for NH3 oxidation have been gradually inhibited.

After five catalytic cycle tests, the catalyst seems to be stable and converts more than 99 % NOx between 250 and 500°C. This suggest that the surface species occur under the catalytic conditions (O2, NH3, NO and high temperatures) to form stable and more active species.

Moreover, long term stability of the catalyst at 300 °C was also studied. The results depicted in Figure S10 indicate that the activity of the catalysts remains stable for more than 50 h, with a conversion of ca. 99 %. To extend the behavior of this catalysts for deNOx related reactions, two experiments with a separate (NO + O2) and (NH3 + O2) feed were performed. The result of NO and NH3 oxidation versus temperature, presented in Figure S11, showed that the oxidation of NO into NO2 occurs at low temperature, ca. 150 °C and increase linearly with the temperature to reach a pseudo-plateau between 280 and 380 °C before increasing gradually again. The first part of the curve can be explained by role of the surface oxygen available that can promote the oxidation of the NO into NO2 to reach a plateau when the oxygen are consumed. Then thermal oxidation can follow. In contrast, the oxidation of NH3 did not take place before the reaction temperature reaches 300 °C, where a sharp increase was observed for temperatures higher than 300 °C. This can explain the high activity observed for this SOMC catalysts where the oxidation of NO into NO2 is favored over NH3 oxidation, the NO2 can promote the fast SCR.

Additional catalysis cycles have been investigated in the presence of 5 vol% water (Figure 5), reflecting more realistic conditions in combustion engines. Although the activity in the first cycle is far lower, which can be explained by the strong adsorption of water on the surface, the same tendency upon multiple recycling tests remains the same. After each catalytic run, the activity improves. This better activity is due to a better W dispersion on the ceria where only isolated sites are obtained in the case of the catalyst prepared through SOMC approach, contrary to catalysts prepared by classical methods composed of a statistical distribution of surface species (Figure S12). Thus, the catalytic behavior is highly dependent on the preparation method. The well-controlled distribution of the W species on the surface of ceria prepared through the advanced surface organometallic functionalization could result in a higher concentration and strength of the Brønsted acids sites [19,46] or more presumably in the formation of WCe1-O2- clusters on the surface, highly active and more accessible to the reactants.

Conclusions

The present work describes the first example of a deNOx catalyst prepared by surface organometallic chemistry. The grafting of W(≡CtBu)(CH2tBu)3 on CeO2 dehydroxylated at 200 °C affords isolated bipodal species, as revealed by DRIFT, solid state NMR, elemental analysis, XPS and EXAFS. The fresh catalyst transforms NO and NH3 into N2 in a typical exhaust gas composition after calcination at 500 °C, with almost full conversion between 280 and 400 °C. Importantly, when multiple SCR cycles were studied, the catalytic performance increased, leading to a full conversion of NO in the 220 -500 °C temperature range, including that in the presence of 5 vol% water. Such an evolution is surprising, and is presumably due to an inhibition of NH3 oxidation by flawed sites of ceria. The good dispersion of tungsten may also increase the acidity and oxygen-buffering capacity of the material and thereby results in a more active SCR catalyst. Ongoing experiments using in-operando techniques (EXAFS and DRIFT) during the recycling should provide information on the evolution of the catalyst when increasing the number of SCR cycles and explain the higher activity by identification of different sites on the surface.

Experimental section

All syntheses were performed under pure and dry argon, using a standard Schlenk techniques and a glove box. Solvents were purified and dried according to standard procedures. C6D6 was distilled over Na/benzophenone and stored over 3Å molecular sieves.

Characterization methods

Elemental analyses were performed at Mikroanalytisches Labor Pascher. Gas-phase analyses were performed on a Hewlett-Packard 5890 series II gas chromatograph equipped with a flame ionization detector and KCl/Al2O3 column (30 m × 0.32 mm). Diffuse reflectance infrared spectra were collected in a Nicolet 6700 FT-IR spectrophotometer in 4 cm -1 resolution. An air-tight IR cell with CaF2 windows was applied and the final spectra comprise 64 scans. Solution NMR spectra were recorded on an Avance-300 Bruker spectrometer. All chemical shifts were measured relative to residual 1 H or 13 C resonances in the deuterated solvent: C6D6, 7.15 ppm for 1 H, 128 ppm for 13 C. 1 H and 13 C solid-state NMR spectra were recorded on Brüker Avance-500 spectrometers with a conventional doubleresonance 4 mm CP-MAS probe. The samples were introduced under argon in a zirconia rotor (4 mm), which was then tightly closed. In all experiments, the rotation frequency was set to 10 kHz. Chemical shifts were given with respect to tetramethylsilane (TMS) as external reference for 1 H and 13 C NMR. Xray diffraction was performed on a Bruker D8 diffractometer at Bragg-Bretano geometry. The instrument is equipped with Cu-tube ( = 1.5406 Å). The Electron paramagnetic resonance (EPR) continuous-wave (CW) measurements were performed through sealed quartz tube on X-Band Bruker ELEXSYS E500 spectrometer operating at 9.8 GHz. Spectra were recorded with the same modulation frequency (100 kHz), the same modulation of amplitude and the same power level for each sample. In order to avoid the saturation effect, the measurements were made at different microwave power levels (0.1-4 mW). The modulation of amplitude (1-5 G) was adjusted for the same reason. The lowtemperature measurements were made with cryogenic systems. Specific surface area and total pore volume of the materials were measured on ASAP 2020 (Micromeritics) surface area and porosimetry system. Extended X-ray absorption fine structure (EXAFS) spectra were acquired at ELETTRA, using XAFS beamline (experiment code: 20145422) [START_REF] Di Cicco | 14th International Conference on X-Ray Absorption Fine Structure[END_REF] at room temperature at the tungsten LIII-edge. A pair of Si(111) crystals were used as monochromator and the harmonics were rejected by a detuning of the second crystal. The spectra were recorded in the transmission mode between 9.9 and 11.43 keV. Three scans were collected for each sample. Each data set was collected simultaneously with a W metal foil reference (11206.7 eV), and was later aligned according to that reference (maximum in first derivative of the W foil first peak). The W supported sample was packaged within a nitrogen filled glovebox in a double air-tight sample holder equipped with kapton windows. The data analyses were carried out using the program "Athena" and the EXAFS fitting program "RoundMidnight", from the "MAX" package, using spherical waves. The program FEFF8 was used to calculate theoretical files for phases and amplitudes based on model clusters of atoms. The scale factor, S0 2 = 0.94, was evaluated from [W(≡CtBu)Np3] molecular complex diluted in BN and conditioned as a pellet (one carbon at 1.76(1) Å, with three carbon atoms at 2.10(1) Å in the first coordination sphere and one carbon atom at 3.25(3) Å and three carbon atoms at 3.34(3) Å). The refinements were carried out by fitting the structural parameters Ni, Ri, σi and the energy shift, ΔE0 (the same for all shells).

Materials

Ceria (CeO2) HAS-5 Actalys 922 from Solvay ("Rare Earth La Rochelle") was calcined for 16 h at 500 °C under a flow of dry air, then evacuated under vacuum at high temperature. After a re-hydration under an inert atmosphere, the ceria was partially dehydroxylated at 200 °C under a high vacuum (10 -5 mbar) for 15 h to give a yellow solid, noted as CeO2-200

Tungsten complexes (W(CtBu)(CH2tBu)3 [START_REF] Clark | [END_REF], as well as 30 % labelled W(*CtBu)(*CH2tBu)3) [36] were prepared according to the described procedures.

Catalysts preparation

SOMC catalyst (1)

A mixture of W(CtBu)(CH2tBu)3 (1.6 g, 1.2 mmol) and CeO2-200 (7 g) was stirred in pentane for 4 h, at room temperature. The W(≡CtBu)(CH2tBu)3/CeO2-200 material was washed three times with pentane via filtration-condensation cycles. Then, all volatiles were condensed into a 6 L vessel in order to quantify neopentane evolved during the grafting reaction. After evaporation of the solvent, the resulting grey powder was dried under vacuum (10 -5 mbar).

Conventional catalyst (2)

For comparison, the impregnated catalyst based on tungsten (W) supported on ceria was prepared by a conventional wet impregnation. In a typical reaction, at room temperature a water solution of ammonium metatungstate ((NH4)6H2W12O40, 100 mg in 5 mL of water) was added to 2 g of ceria. The suspension was stirred overnight at room temperature and then water was evacuated in a rotary evaporator at 50 °C. The material obtained was then calcined under dry air at 500 °C for 16 h. The elemental analysis indicated the presence of ca. 3.5 wt% W.

NH3-SCR Catalytic performance tests

The NH3-SCR catalytic evaluations were performed in a quartz fixed-bed reactor. A total gas flowrate of 300 mL.min -1 (hourly space velocity: 30000 h -1 ) at atmospheric pressure, typically composed of 300 ppm NO, 350 ppm NH3, 10 vol. % O2 and 3 % H2O (when used), was sent through 30 mg of W(≡CtBu)(CH2tBu)3/CeO2-200 catalyst diluted with 500 mg of silicon carbide, in order to ensure a volume of 1 cm 3 . The reactor equipped with an internal thermometer, was heated from room temperature to 600 °C with a heating rate of 10 °C.min -1 . The system was kept at 600 °C for 10 min before cooling down. Analyses of the gas concentrations of NO, NO2, N2O and NH3 were carried out using Antaris IGS FTIR (Thermo-Fischer) equipped with a gas cell of 200 mL. N2 selectivity was evaluated assuming that the outlet gas contains no other N-compounds than NO, NO2, N2O and NH3 (Scheme 1).

Scheme. 1:

Reactions occurring during the SCR of NOx with NH3.

The catalytic activity and N2 selectivity were calculated by the following equations: [19] 𝑁𝑂 𝑐𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 (%) =

[𝑁𝑂] 𝑖𝑛 -

Conflict of interest

There are no conflicts to declare.

Figure 2 W

 2 Figure 2 W LIII-edge k 2 -weighted EXAFS (left) and its Fourier transform (right) for W(≡CtBu)(CH2tBu)3/CeO2-200 material. Solid lines: experimental and dashed lines: spherical wave theory.

  Δk: [1.8 -14.5 Å -1 ] -ΔR: [0.5-4.0 Å]; S0 2 = 0.94; ΔE0 = 4.7 ± 1.2 eV (the same for all shells); Fit residue: ρ = 5.6 %; Quality factor: (Δ) 2 /ν = 2.36 (ν = 15 / 30).(b) Shell constrained to a parameter above.Moreover the two carbon back-scatterers are located at distances in the range of WC and W-C triple and single bonds respectively, as observed in [W(CCMe3)(=CHCMe3)(CH2CMe3)(dmpe)] molecular complex (1.785(8) Å for WCtBu and 2.258(9) Å for W-CH2tBu).[53] Similar parameters were obtained when fitting the k 3 χ(k) spectrum. The fit could be also improved by adding further layers of backscatters, with in particular two types of oxygen atoms at 2.69(2) and 2.94(2) Å and only ca. one cerium atom at 3.57(3) Å. The inclusion of tungsten as a second neighbor was not statistically validated. Therefore, this EXAFS study suggests a single-site structure ((-O)2W(CtBu)(CH2tBu)), as the one represented in Figure3, where the tungsten atom is in a pseudo-octahedral environment can be tentatively proposed (in the cerium oxide crystal,[54] the Ce-O bond distance is ca. 2.34 Å, and the shortest Ce --Ce distance is around 3.83 Å).

Figure 3 :

 3 Figure 3: Proposed structure for the species resulting from the grafting of W(CtBu)(CH2tBu)3 onto CeO2-200.

Figure 4

 4 Figure 4 NOx conversion and N2 selectivity over 1 in function of temperature during 7 cycles. Feed composition: 300 ppm NO, 350 ppm NH3 and 10 vol % O2 in He.

Figure 5

 5 Figure 5 NOx conversion and N2 selectivity over 1 as a function of temperature during 5 cycles. Feed composition: 300 ppm NO, 350 ppm NH3, 5 vol% H2O, and 10 vol% O2 in He.

  [𝑁𝑂] 𝑜𝑢𝑡 -[𝑁𝑂 2 ] 𝑜𝑢𝑡 [𝑁𝑂] 𝑖𝑛 × 100 𝑁 2 𝑠𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦 (%) = [𝑁𝐻 3 ] 𝑖𝑛 + [𝑁𝑂] 𝑖𝑛 -[𝑁𝑂 2 ] 𝑜𝑢𝑡 -2[𝑁 2 𝑂] 𝑜𝑢𝑡 -[𝑁𝐻 3 ] 𝑜𝑢𝑡 [𝑁𝐻 3 ] 𝑖𝑛 + [𝑁𝑂] 𝑖𝑛 × 100

Acknowledgements

The financial support of Toyota Motor Corporation is gratefully acknowledged. CC thanks CSC (China Scholarship Council) for financial support. At ELETTRA synchrotron light source, in Basovizza, Italy, we would like to thank Luca Olivi for his help during the recording of the X-Ray absorption spectra and for the use of the glove-box.