E Courjon

F Bassignot

G Ulliac

Thierry Laroche

J Garcia

D Gachon

Petit

S Queste

J-P Romand

S Ballandras
email: sylvain.ballandras@femto-st.fr

R Petit

An acoustic waveguide based on doubly-bonded silicon/PPT/silicon structures

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.

INTRODUCTION

Periodically Poled Transducers (PPTs) have been investigated recently as an alternative to classical inter-digital transducers for the excitation and detection of guided acoustic waves. The structure simply consists of two electrically conductive media deposited on each side of a periodically poled ferroelectric layer. This structure has been implemented using Z-cut of LiNbO 3 [START_REF] Courjon | Pure longitudinal plate mode excited by poled domains transducers on LiNbO3[END_REF] and has been associated with silicon to manufacture waveguides for radio-frequency (RF) applications. The excitation and detection principle suppresses the need for zero-defect lithography processes as the domain poling is not prevented by partially resolved resist lines and finally the top electrodes does not require any periodical structuring. However, this solution still shares with standard Surface Acoustic Wave (SAW) devices the need for packaging preventing any surface pollution which may here simply degrade the wave propagation (the electrical excitation actually is robust to surface pollution by principle). The best solution to allow for applying low cost packaging techniques to acousticwave-based devices consists of exciting waves trapped within a guiding structure in the bulk of implemented materials. This gives rise to the concept of Interfacial or Boundary Acoustic Waves. Many groups have been investigating the possibility to develop such solutions using standard inter-digited transducers. They must overcome the difficulty of processing machined surfaces, loosing the flatness qualities generally used for wafer bounding techniques. They may also used thick layer deposition approaches, then loosing the interest of single crystals, still the best solution for high quality factor acoustic wave resonators.

In this work, we have considered the possibility to combine the wave guiding within the bulk with our periodically poled transducers to benefit of all the pointed advantages of both approaches. First of all, we explain the periodically poled transducers fabrication principle and we present experimental results on lithium niobate (LiNbO 3 ) and lithium tantalate (LiTaO 3 ) wafers. After that, we present theoretical analysis results achieved using our finite element/boundary element simulation tool, allowing for the simulation of such material combinations used to guide ultrasonic waves. We have pointed out the existence of elliptically polarized guided modes for several material combinations and technological parameters. We also have identified the possibility to excite partially guided longitudinally polarized waves. theoretical analysis results of the PPT operation obtained, using our finite and boundary element simulation tool. Using the analysis of the transducer dispersion properties providing the optimum operating points and the temperature coefficient of frequency (TCF) analysis, we have chosen the best combination Si/PPT/Si to fabricate test vehicles. Poled LiNbO 3 wafers are first bonded onto silicon using Au/Au compression, then the resulting stack is metalized again to process a second wafer bonding on silicon as previously. These devices did exhibit a period of 50 μm (domain width 25 μm), yielding operating frequencies near 80 MHz. As predicted, we measure both contributions of elliptic and longitudinal modes near the expected contributions. II.

ADVANTAGES, FABRICATION AND CHARACTERIZATION OF PERIODICALLY POLED TRANSDUCERS

A. Advantages

PPTs have two principal advantages compared to standard periodic inter-digital electrode transducer (IDTs) deposited atop piezoelectric materials. The first one is the robustness of the excitation versus defects or surface contamination (no passivation is required to avoid short circuit as in the case of standard IDTs) and the second one is the possibility to excite waves exhibiting a wavelength equal to the poling period, contrary to standard IDTs for which the wavelength is twice the mechanical period of the grating (Bragg condition). It can also be noted that comparably to film bulk acoustic resonators (FBARs), it can operate with large excitation powers as the electrodes are separated by a bulk material layer. We have shown in [START_REF] Courjon | Fabrication of periodically poled domains transducers on LiNbO3[END_REF] the capability of the structure to excite elliptically and longitudinally polarized plate waves (Lamb modes). The PPT is fundamentally based on a periodically poled piezoelectric medium. Each side of this medium is metalized in order to obtain a capacitive dipole in which elastic waves can be excited by phase construction. Such a periodically poled structure can be advantageously achieved on ferroelectric materials like PZT because of the rather small value of its coercive electric field (the absolute value of the electric field above which the spontaneous polarization can be inverted) or LiNbO 3 and LiTaO 3 .

B. Fabrication

As previously presented [START_REF] Courjon | Pure longitudinal plate mode excited by poled domains transducers on LiNbO3[END_REF][START_REF] Courjon | Fabrication of periodically poled domains transducers on LiNbO3[END_REF], we have performed our experiments on a dedicated poling bench which can control the poling of thick (500 µm) optical quality Z-cut LiNbO 3 plates. It mainly consists of a high voltage amplifier used to submit the wafer to an electric field strong enough to invert the negative poling of the material. The coercive electric field of the lithium niobate is 21 kV/mm, consequently, the voltage needed to invert the domains is approximately 11 kV. The domains to be poled are defined using a photo-resist pattern on top of a plate surface. We have used a mask with several devices of 50 µm poling period. The plate is held in a Polymethyl methacrylate (PMMA) mounting by means of two O-ring joints which create two cavities full filled by a saturated lithium chloride solution used as a liquid electrode (as it is shown in the scheme of fig. 1). Figure 1.

Principle of the domain inversion bench

The high poling voltage is applied to the plate following the sequence shown in fig. 2. This sequence is designed to favor the domain nucleation, to stabilize the inverted domains (i.e. to avoid back-switching of the domains) and to avoid electrical breakdowns [START_REF] Ballandras | A novel surface wave transducer based on periodically poled piezoelectric domain[END_REF]. The poling process is monitored thanks to the measurement of the electric current crossing the wafer during the sequence. The signature of a successful domain inversion corresponds to a voltage dropping while a current discharge occurs simultaneously (see fig. 2). The poling can be easily controlled by a simple optical post-observation, as it generates a contrast at the edge of the poled domains (see fig. 3). Domain inversion monitoring: probed high voltage applied on the wafer (green curve, poling sequence) and resulting crossing current (red curve). Voltage and current gain are respectively 1V/5000V and 1V/5mA. 1 st step : the applied electric field is smaller than the coercive limit, to prepare the sample for the nucleation step. 2 nd step : the applied field overcomes the coercive limit, yielding nucleation and consequently poling (this step lies between the 2 dashed cursors). 3 rd step : linear decrease of the field to end-up the nucleation (self-termination step, exponential decrease of the current). 4 th step : stabilization of the poling by applying a field smaller than the coercive limit Figure 3.

Optical verification of the poling efficiency, case of a 50 µm period device, evidence of contrast changes between Z+ (native polarization) and Z-(switched polarization) domains. The evidence of contrast modification in the vicinity of the poled domain walls at the poled strip edge (the whole plate is Z+ oriented, the poled strips are Z-oriented) may e due to diffusion effects

C. Experimental results compared with theoretical analysis

We demonstrate the concept validity (Si/PPT(p=50 µm)/Si) at about 80 MHz. To characterize periodically poled transducers, we metalized each side of the transducer (sides Z + and Z -) with an aluminum layer. After that, the devices are characterized using a network analyzer. One side of the device is connected to the signal whereas the second one is linked to the ground. We obtain a response (fig. 4) with numerous parasitic bulk wave residuals but two strong contributions are pointed out at 76 and 130 MHz respectively. We compare the experimental results with the theoretical analysis using our finite element/boundary element simulation tool. The first value can be easily related to the asymptotic limit emphasized for the very first Lamb wave contribution (i.e. Rayleigh wave velocity) and the second one approaches the longitudinal bulk wave velocity along X axis. In this part, we first present analysis results achieved using our finite element/boundary element simulation tool [START_REF] Ballandras | A mixed finite element/boundary element approach to simulate complex guided elastic wave periodic transducers[END_REF], allowing for the simulation of such material combinations. After that, the combination of material Si/PPT/Si is chosen (easiest and cheapest to fabricate) and the dispersion properties of this transducer are studied providing the best operating points.

A. Analysis of several material combinations

The purpose of this analysis is to study influences on wave guidance and velocity. For that, we study two principal cases using LiNbO 3 and LiTaO 3 as ferroelectric material and we modify the guidance material (silicon, sapphire or diamond). Theoretical analysis results of these structures are obtained by means of our finite and boundary element simulation tool. Figures 5 and6 show the theoretical admittance of LiNbO 3 and LiTaO 3 PPT respectively (p=50 µm). Theoretical admittance of a 50 µm period LiTaO3 PPT for 30 or 60 µm thick for different guiding substrates There are two main differences between LiNbO 3 and LiTaO 3 transducers. The first one is a better spectrum purity for the LiTaO 3 configuration in comparison with the LiNbO 3 configuration. The second one concerns the Rayleigh wave velocity of LiTaO 3 (3500 m.s -1 ) which is lower than the one of LiNbO 3 (3800 m.s -1 ), that is why we observe the same acoustic waves for both materials and for each guiding substrates but at a lower frequency for the LiTaO 3 . The guidance conditions also depend on the ratio between the ferroelectric material thickness and the transducer wavelength (h/p). The elliptic mode actually is guided from a ratio h/p equal to 1. The structure diamond/PPT/diamond is the most guiding configuration thanks to the high propagation velocities in the diamond (12500 m.s -1 for the Surface Skimming Bulk Wave (SSBW)). In fact, the quasi-longitudinal mode is excited and guided at a velocity around 6500 m.s -1 . The last structure consisting of a silicon/PPT/silicon stuck presents interesting guidance properties for a ratio (h/p) smaller than 1, this parameter is treated in the next part. In this structure, the Guiding layer

Guiding layer

LiNbO3 transducer elliptic mode is mainly excited at about 5100 m.s -1 whereas the longitudinal mode at about 7300 m.s -1 is not guided, as the SSBW velocity in the silicon is near 5800 m.s -1 . This configuration is retained for the rest of the study because the elliptic mode is well guided, the fabrication process is quite simple and cheaper than structures based on other guiding materials.

B. Analysis of dispersion properties

We propose to closely investigate how to efficiently derive the dispersion curves and then to show at least for the elliptic mode of the waveguide the influence of the PPT geometry and the operating frequency on the mode properties. In that matter, we have considered the general shape of the transducer symbolized in fig. 7. We assume here a propagation between two silicon layers but the approach can be generalized to any guiding substrate. The period is arbitrarily fixed to 10 µm (i.e. width of each LiNbO 3 domain set to 5 µm, poling ratio=0.5).

Figure 7.

Scheme of PPT principle and generic shape for dispersion curve derivation

In Figure 8, we have plotted the theoretical admittance (conductance and susceptance) versus frequency for a thickness/period (h/p) ratio equal to 0.1, in order to identify the main contribution corresponding to an elliptically shaped mode.

Figure 8.

Harmonic admittance of the structure plotted in fig. 7 for a thickness/period ratio equal to 0.1

Once the mode identified, we follow the evolution of its resonance frequency and electromagnetic coupling coefficient for h/p ratios ranging from 0.1 to 10, is plotted on fig.9. There is no ambiguity concerning the elliptic polarization of the studied mode because there is no possible interaction with other modes (as often observed for Lamb waves for instance). This operating point is of very particular importance as it conditions the possibility to improve the design reliability by choosing operation condition poorly affected by technology, which is a key point for any practical exploitation of the device. The next section shows the fabrication of a device Si/PPT/Si.

IV.

DOUBLE BONDING SI/PPT/SI : FABRICATION,

CHARACTERIZATION AND TEMPERATURE SENSIBILITY

The concept has been experimentally checked first using a thick structure consisting of a Silicon(380 µm thick)/LiNbO 3 PPT (500 µm)/Silicon (380 µm) material stack.

A. Fabrication

The fabrication of PPT-based waveguides consists in bonding a silicon wafer on each side of the periodically poled wafer. In that purpose, the lithium niobate wafer is bonded on a (100) 3" silicon wafer using a wafer bonding technique developed in our group based on a metal-metal adhesion at room temperature promoted by a high pressure applied to the material stack (fig. 10). Connection via are etched in each silicon substrate by Deep Reactive Ion Etching (DRIE) in order to reach the buried metallic electrodes. After that, the structure is diced to obtain 5 separated devices. This plot points out the good agreement between both results. We find two main contributions corresponding to the elliptical mode and the longitudinal mode. We can notice that the experimental admittance increases gradually. The main cause seems to be the wire length of the connection between the device and the support. This proves the feasibility of LiNbO 3based Silicon/PPT/Silicon devices. We also have measured the experimental admittance of the longitudinally polarized wave at different temperatures (between 278 and 323 K) of the test vehicle. Figure 14 shows the comparison between the theoretical and the experimental Temperature Coefficient of Frequency (TCF). We obtain the same order of magnitude at around -80 ppm/K. We can notice that the TCF is sensibly the same than the one of isolated LiNbO 3 devices.

V. CONCLUSION

We have proposed a first analysis of waveguides using different material combinations. We can mainly notice that acoustic waves in a LiNbO 3 structure are guided more rapidly and with less spectrum purity than the ones in a LiTaO 3 structure. An analysis concerning the dispersive behavior of PPT-based waveguide has been also achieved. These analysis are performed using FEA-BEM calculations as it is one of the only approach capable to accurately address this problem. We have demonstrated the feasibility of acoustic waveguide using periodically poled transducer on LiNbO 3 Z-cut bonded between two silicon wafers. The devices have been found to operate according to the theoretical predictions, allowing for a reliable analysis of their operation. The TCF of these devices is relatively high but a future work consisting in thinning the ferroelectric material will enable to reduce it considerably.

Figure 2 .

 2 Figure 2.

Figure 4 .

 4 Figure 4. Theory/experiment admittance of the 50 µm period PPT device. Two strong waves are visible, respectively at 76 and 130 MHz

  Figure 5.(a) Scheme of the PPT structure (b) Theoretical admittance of a 50 µm period LiNbO3 PPT for 30 or 60 µm thick for different guiding substrates

Figure 9 .

 9 Figure 9.Evolution of the elliptic mode synchronism frequency (red curve) and electromagnetic coupling (green curve) for small values of h/p ratio (ranging from 0.1 to 10 in our calculation)

Figure 10 .

 10 Figure 10. Double bonding of a lithium niobate wafer (center of the photograph) with silicon wafer (top and bottom) using room temperature Au/Au bonding process The process flowchart is reported in fig.11, showing the different implemented coatings and the technological sequence.

Figure 11 .

 11 Figure 11.

Figure 12 .

 12 Figure 12.

Figure 13 .

 13 Figure 13.

Figure 14 .

 14 Figure 14.

ACKNOWLEDGMENT

This work was supported by the french DGA (Délégation Générale pour l'Armement) under grant#07-34-020.


Amel Cydric Ghogia

Bruno F Machado

Simon Cayez

Ange Nzihou

Philippe Serp
email: philippe.serp@ensiacet.fr

Katerina Soulantica

Doan Pham
email: doan.phamminh@mines-albi.fr

Beyond confinement effects in Fischer-Tropsch Co/CNT catalysts

Keywords: Cobalt catalysts, Carbon nanotubes, Fischer-Tropsch synthesis, Confinement, Spillover

Fischer-Tropsch catalyst preparation parameters, such as cobalt precursor and impregnation solvent, determine structure and in fine catalytic performances of Co-based catalysts. Co particles of 3-7 nm mean size were prepared by incipient wetness impregnation on surface-oxidized CNT. In this work the crucial impact of the chemistry operating between the Co precursor and the support during catalyst preparation on the catalyst properties is demonstrated and the influence of CNT surface chemistry on catalyst characteristics has been assessed. Very good correlations were found between: i) the reduction degree of the catalysts and the concentration of the Co(OOC-) surface species; ii) the Co hcp /Co fcc ratio and the amount of CO-releasing groups on the support surface; iii) the extent of hydrogen spillover and the concentration of quinone surface groups. The initial catalytic activity could only be correlated to a combination of catalyst features: percentage of Co hcp , Co confinement, hydrogen spillover, and surprisingly the inverse of the Co initial particle size. No significant differences in S C5+ selectivity were observed, in accordance with comparable final Co particle size. Finally, the stability of the catalyst can be correlated with the initial Co particle size and the percentage of Co hcp in the catalyst. These results confirm the significant impact of the precursor/support couple on the structure and performance of Co/CNT catalysts, and reveal for the first time an inverse correlation between Co particle size and activity, which is attributed to the confined Co hcp phase.

Introduction

The Fischer-Tropsch synthesis (FTS) is a catalytic process enabling the conversion of syngas (CO/H 2 ) into higher hydrocarbons, which can be upgraded into liquid fuels. Today, this process becomes particularly attractive due to the stringency of environmental regulations, the decrease of fossil fuel reserves, the development of biomass derived syngas technologies as well as the ever increasing awareness of citizens on the utilization and benefits of sustainable fuels [START_REF] Sun | Beyond Cars: Fischer-Tropsch Synthesis for Non-Automotive Applications[END_REF]. To conduct FTS at low temperatures (LTFT, 220-260°C) [START_REF] Dry | The Fischer-Tropsch process: 1950-2000[END_REF], cobalt-based catalysts are particularly interesting since they are more active and stable than the iron-based ones, and much cheaper than the ruthenium ones [START_REF] Qi | A mini review of cobalt-based nanocatalyst in Fischer-Tropsch synthesis[END_REF][START_REF] Gholami | Recent advances in Fischer-Tropsch synthesis using cobalt-based catalysts: a review on supports, promoters, and reactors[END_REF][START_REF] Gholami | The influence of catalyst factors for sustainable production of hydrocarbons via Fischer-Tropsch synthesis[END_REF]. Oxide supports such as SiO 2 , Al 2 O 3 or TiO 2 are generally employed to stabilize cobalt particles, but they present the drawback of undesirable strong cobalt-support interactions that lead to the formation of non-reducible and inactive mixed metal oxides [START_REF] Gholami | Recent advances in Fischer-Tropsch synthesis using cobalt-based catalysts: a review on supports, promoters, and reactors[END_REF]. Such detrimental interactions can be circumvented by using carbon materials as support modifier [START_REF] Van De Loosdrecht | Carbon coated supports for cobalt based fischer-tropsch catalysts[END_REF][START_REF] Liu | Hydrothermal carbon-coated TiO 2 as support for Co-based catalyst in Fischer-Tropsch synthesis[END_REF] or directly as support [START_REF] Ghogia | Cobalt catalysts on carbonbased materials for Fischer-Tropsch synthesis: a review[END_REF], since their interaction with cobalt is weaker, which facilitates metal reduction. Additionally, the good thermal conductivity of some sp 2 carbon materials, such as carbon nanotubes (CNT) enables to limit methane formation and catalyst deactivation, by avoiding the formation of hot spots resulting from the high exothermicity of the FT reaction.

The principal identified Co/C catalyst features influencing the catalyst performances are: i) the Co particle size; ii) the Co reducibility; iii) the cobalt phase (fcc or hcp); and iv) the confinement of cobalt particles (particularly in CNT) [START_REF] Ghogia | Cobalt catalysts on carbonbased materials for Fischer-Tropsch synthesis: a review[END_REF]. Recently, we also reported that hydrogen spillover from the cobalt particles to the carbon support can also contribute to the enhancement of Co/C catalytic activity [START_REF] Ghogia | Hydrogen Spillover in the Fischer-Tropsch Synthesis on Carbon-supported Cobalt Catalysts[END_REF]. All these features, which are fixed either during catalyst preparation of during catalysis, can in turn be dictated by specific properties of the carbon support, such as the strength of the metal-support interaction (MSI), the support textural properties, its surface chemistry, its thermal stability and its conductivity [START_REF] Gerber | A theory/experience description of support effects in carbonsupported catalysts[END_REF]. Among the different methods for Co/C catalyst preparation, the incipient wetness impregnation (IWI) is by far the most popular since: i) it is relatively fast and inexpensive, and ii) small particle size and narrow particle distribution are usually obtained. Among the different carbon supports investigated for FTS, oxidized CNT have been particularly studied since they offer the additional possibility of confinement of the active phase [START_REF] Ghogia | Cobalt catalysts on carbonbased materials for Fischer-Tropsch synthesis: a review[END_REF][START_REF] Gerber | A theory/experience description of support effects in carbonsupported catalysts[END_REF]. Furthermore, it was demonstrated that oxidized CNT show catalytic activity towards CO hydro-deoxygenation reaction to yield long chain hydrocarbons similar to FTS [START_REF] Sims | Hydro-deoxygenation of CO on functionalized carbon nanotubes for liquid fuels production[END_REF].

Another important parameter to consider during catalyst preparation is the choice of the cobalt precursor and of the impregnating solvent. The decomposition of the cobalt precursor is a crucial step in Co-based catalyst preparation, and a slow decomposition rate favors small Co particle size [START_REF] Ghogia | Cobalt catalysts on carbonbased materials for Fischer-Tropsch synthesis: a review[END_REF]. Cobalt acetate and cobalt nitrate precursors are the most commonly used.

However, the data available in the literature are insufficient to explain the effect of cobalt precursors in the case of Co/C catalysts. Xiong et al. [START_REF] Xiong | Correlating the preparation and performance of cobalt catalysts supported on carbon nanotubes and carbon spheres in the Fischer-Tropsch synthesis[END_REF] prepared Co N /CNT and Co A /CNT (15 wt.%) catalysts using cobalt nitrate (Co N ) and cobalt acetate (Co A ) precursors by the IWI method. The Co N /CNT catalyst (d Co = 3.9 nm) was slightly more active than the Co A /CNT catalyst (d Co = 4.4 nm) in FTS. However, in another study [START_REF] Honsho | Fischer-Tropsch synthesis over Coloaded oxidized diamond catalyst[END_REF], Co catalysts prepared on activated carbon or oxidized diamonds as supports from cobalt acetate, were more active than those prepared from cobalt nitrate. These contradictory results could be related to the nature of the supports. Additionally, the solvent used during the catalyst preparation allows not only dissolving the precursor and promoting its contact with the support, but also controlling the final catalyst structure. The interaction between the cobalt precursor and the support depends on the solvent surface tension, which controls support wettability [START_REF] Ghogia | Cobalt catalysts on carbonbased materials for Fischer-Tropsch synthesis: a review[END_REF]. De Jong and coworkers compared water and ethanol as solvents for the impregnation of CNT with cobalt nitrate [START_REF] Eschemann | Effect of support surface treatment on the synthesis, structure, and performance of Co/CNT Fischer-Tropsch catalysts[END_REF].

The catalytic results showed that the catalysts prepared in ethanol (Co N-E /CNT) presented a superior cobalt-weight based activity over those prepared from an aqueous solution (Co N- W /CNT), because the cobalt particles had grown less during reduction and FTS, leading to higher specific metal surface areas. However, in most of these studies, not all the features influencing the catalyst performances have been considered for the rationalization of the catalyst activity.

In the present study, the influence of cobalt precursor (nitrate or acetate) and the impregnation solvent (water or ethanol) on the structure of Co/CNT catalysts prepared by IWI is studied. We intend for the first time at drawing structure/performances correlations considering all the features known to influence the catalyst performances, that is: Co particle confinement, Co particle size, Co reducibility, Co crystallographic phase, and hydrogen spillover.

Experimental section

Catalyst preparation

The CNT were synthesized at 650 °C by catalytic chemical vapor deposition in a fluidized bed reactor using ethylene (C 2 H 4 ) as a carbon source and an AlFeCoO 4 catalyst [START_REF] Contreras | Effect of mesoporous carbon support nature and pretreatments on palladium loading, dispersion and apparent catalytic activity in hydrogenation of myrcene[END_REF]. The CNT were purified using an aqueous solution (50 vol% H 2 SO 4 ) under reflux at 140 °C for 3 h. The purified CNT were further oxidized with HNO 3 under reflux at 140 °C for 3 h. The oxidized CNT contain 1.3 % w/w of catalyst residue (TGA analysis, Figure S1) consisting in FeCo nanoparticles, which are encapsulated in carbon, thus, not catalyzing the FTS (EDX analysis, Figure S1).

Material characterization

Dynamic Vapor Sorption (DVS) was used to measure the sorption of water and ethanol on the CNT at 25 °C, using a device form Surface Measurement Systems. The temperature of the chamber and the nitrogen gas flow were set at 25 °C and 500 mL.min -1 , respectively. 0.1 g of sample was used for the measurements. The isothermal stages were: i) drying level, circulation of dry nitrogen gas for 60 min; ii) 10% RH (relative humidity) from 0% to 90% RH; and iii) 95% RH. The water or ethanol content at equilibrium was determined according to the relative humidity as follows (Eq. 1):

(Eq. 1)

𝑔 𝑎𝑑𝑠 𝑔 -1 𝐶𝑁𝑇 = ( 𝑚 𝑆 𝑚 𝑝 )
Where m s is the mass of the solvent adsorbed on the CNT, and m p is the mass of dry CNT.

A JEOL JEM-1011 microscope equipped with a tungsten thermionic electron source and with an acceleration voltage of 100 kV was used for routine TEM observations. A JEOL JEM 2100F-EDS microscope with an acceleration voltage of 200 kV was used for high-resolution transmission electron microscopy (HRTEM) observations. The particle size distribution of fresh and spent catalysts was determined by TEM. At least 300 Co particles were measured for each sample. For TEM analyses, catalyst samples were prepared by ultrasound-assisted dispersion in pure ethanol, and the suspensions were dropped onto a collodion carbon covered copper film. The filling yield was calculated by counting the particles deposited inside and on the outer surface of 40 different CNT (about 300 Co particles). The yield (Eq. 2) was calculated as follows [START_REF] Nguyen | Confinement of metal nanoparticles in carbon nanotubes[END_REF]:

(Eq. 2)

Yield = 2 ( 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒𝑠 𝑖𝑛𝑠𝑖𝑑𝑒 𝑇𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒𝑠 × 100 ) -100
The specific surface area, and pore size distribution were determined from N 2 adsorption/desorption isotherms at -196 °C by using a Micromeritics instrument. Prior to analysis, all samples were degassed under vacuum at 120 °C for 6 h.

The crystalline structure and the crystallite size of the fresh catalysts were determined by X-ray diffraction (XRD). Before analysis, the catalysts were reduced under 40%H 2 /Ar flow for 2 h at 350 °C with a heating rate of 5 °C.min -1 . The standard XRD measurements were performed on a Bragg-Brentano configuration (θ-θ) EMPYREAN diffractometer equipped with a cobalt anode fed at 35 kV and 45 mA. The radiation generated by the anode has an average wavelength (K α ) λ = 1.79 Å. The analysis was carried with an angular domain 2θ between 10 and 120°, a step of 0.07 °/s and an acquisition time of 200 s. For the in-situ XRD, the heating oven was mounted on the diffractometer, followed by the introduction of the sample into the heating chamber. Using a gas composition of 5%H 2 in N 2 , the furnace chamber was purged at 1 bar for four vacuum-5%H 2 /N 2 cycles. The catalyst was progressively reduced (from Co 3 O 4 to Co 0 ) in the furnace under a 5%H 2 /N 2 flow from 30 °C to 800 °C with a heating rate of 5 °C. min -1 and diffractograms were recorded at various temperatures. The Co crystallite size of the fresh catalysts was determined using the Scherrer equation [START_REF] Schanke | Study of Ptpromoted cobalt CO hydrogenation catalysts[END_REF] and the HighScore software, while the crystalline structure of the spent catalysts was determined by MAUD software. Rietveld refinement was also applied to quantify the different crystalline phases.

The thermogravimetric analysis was performed with a TGA/DSC Shimadzu apparatus. The measurements were carried out from 25 to 350 °C under nitrogen flow of 20 mL.min -1 with a heating rate of 2 °C.min -1 , with a plateau time of 3 h at 350°C.

The X-ray photoelectron spectroscopy (XPS) was performed using a monochromatized Al K α (hν = 1486.6 eV) source on a ThermoScientific K α system. The X-ray spot size was about 400 µm. The pass energy was fixed at 30 eV with a step of 0.1 eV for core levels and 160 eV for surveys (step 1eV). The spectrometer energy calibration was done using the Au 4f 7/2 (83.9 ± 0.1 eV) and Cu 2p 3/2 (932.8 ± 0.1 eV) photoelectron lines. XPS spectra were recorded in direct mode N(Ec) and the background signal was removed using the Shirley method [START_REF] Repoux | Comparison of background removal methods for XPS[END_REF].

Magnetic measurements were performed using a Quantum Design Vibrating Sample Magnetometer (VSM) at 300 K and 5 K and field up to 5 T [START_REF] Verelst | Synthesis and characterization of CoO, Co 3 O 4 , and mixed Co/CoO nanoparticules[END_REF]. This technique makes it possible to determine the saturation magnetization (M s , Eq. 3) of a sample of catalyst under the effect of a magnetic field and to deduce its reducibility knowing the saturation magnetization of the bulk cobalt (M s bulk, equal to 163 emu g Co -1 ).

𝑀 𝑠 𝑠𝑎𝑚𝑝𝑙𝑒 (𝑒𝑚𝑢 𝑔 -1 𝐶𝑜 ) = 𝑀 𝑠 (𝑉𝑆𝑀) 𝑚 𝑐𝑎𝑡 × %𝐶𝑜 (𝐸𝑞. 3)
With M s (VSM) the magnetization obtained by the equipment, m cat the mass of the catalyst and % Co the cobalt content obtained by ICP. This allows the determination of the reduction degree of cobalt (Eq. 4).

𝑅𝑒𝑑𝑢𝑐𝑡𝑖𝑜𝑛 𝑑𝑒𝑔𝑟𝑒𝑒 (%) = 𝑀 𝑠 𝑠𝑎𝑚𝑝𝑙𝑒 𝑀 𝑠 𝑏𝑢𝑙𝑘 × 100 (𝐸𝑞. 4)

The measurements were performed on reduced catalysts. Before analysis, the catalysts were reduced in 40%H 2 /Ar flow for 8 h at 350 °C with a heating rate of 5 °C.min -1 . Standard VSM (Vibrating Sample Magnetometry) capsules were filled with an exactly weighted amount (a few mg) of sample and sealed. To avoid any accidental oxidation, the samples were prepared in a glovebox, and transferred to the cryostat in a Schlenk vessel. The VSM capsule containing the sample is rapidly introduced into the VSM in order to avoid air exposure. For each catalyst, two measurements were performed. A first hysteresis cycle was performed at 300 K (27 °C) and the field-dependent magnetization was recorded. A second field-dependent magnetization was recorded after having cooled down to 5 K (-268 °C) while the 5 T magnetic field was applied.

Temperature-programmed reduction (TPR) profiles of the calcined catalysts were recorded with a Micromeritics AutoChem 2920 Analyzer in the temperature range of 50-1000 °C, in order to determine the reducible species present in the catalyst and to reveal the temperature at which the reduction occurs. 100 mg of the sample was placed in a quartz reactor and reduced by a 5%H 2 /N 2 gas mixture with a flow rate of 25 mL.min -1 and a heating rate of 10 °C.min -1 .

Prior to the analysis, the samples were purged under argon at 120 °C for 1 h.

The cobalt loading of the catalysts was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) after mineralization of the samples in a mixture of HNO 3 /HCl at 190 °C in an inert autoclave for one day.

Hydrogen temperature programmed desorption coupled with mass spectroscopy (H 2 -TPD-MS) (Altamira Instruments AMI-300 device) was used in order to characterize the nature of oxygen surface groups after HNO 3 treatment, to elucidate the nature of hydrogen species during the desorption step, and to determine the average active particle size of metal in supported Cocatalysts. First, the catalyst was purged with argon at 120 °C for 1 hour in order to clean the surface of the sample. Then, the system was cooled to 50 °C followed by a reduction at 350 °C for 6 h at 5 °C.min -1 under 5%H 2 /N 2 flow and then cooled down to 100 °C under hydrogen flow. The H 2 chemisorption took place at 100 °C. Then the flow of hydrogen was switched to helium until 50 °C. H 2 -TPD-MS was finally performed (10 °C.min -1 ramp until 1050 °C) to quantify the amounts of released H 2 , CO, and CO 2 . The H 2 peak surface (400-800 °C) was used to determine the cobalt dispersion (Eq. 5) and average particle size (Eq. 6) by the following formula:

(Eq. 5)

D(%) = 𝑉 𝑚 × 𝑀 × 𝐹 𝑉 𝑚𝑜𝑙 × %𝑀𝑒 × 100
Where V m is the irreversibly chemisorbed gas (cm 3 , STP per gram of catalyst), M is the molar mass of the metal (g per mol of metal), V mol is the standard molar volume of the adsorbate (cm 3 STP per mol), % Me is the mass fraction of metal, and F is the stoichiometry factor (number of gas molecules per atom of metal) [START_REF] Aznárez | Performance of palladium and platinum supported on alumina pillared clays in the catalytic combustion of propene[END_REF].

(Eq. 6) 𝑑 𝐶𝑜 (nm) = 96 𝐷%
Where d Co (nm) is the average particle size.

Fischer Tropsch synthesis

FT synthesis was performed in a fixed-bed reactor composed of a stainless-steel tube of 33 cm length and 14 mm inner diameter. The reactor was loaded with 36 g of α-Al 2 O 3 followed by a mixture of 2 g of catalyst and 8 g of inert SiC (125-250 µm) in order to well dissipate the heat released from the reaction, and finally 20 g α-Al 2 O 3 and a quartz wool layer. A thermocouple was placed at the center of the catalyst bed to control the reaction temperature. The reactor was heated from ambient temperature to 120 °C with a heating rate of 1 °C.min -1 under a flow of argon (100 mL.min -1 ) for 1 hour. Then, the catalyst was reduced in situ under a 40%H 2 /Ar (100 mL.min -1 ) flow at 350 °C for 8h with a heating rate of 5 °C.min -1 . After the reduction, the FT reaction was carried out at 220 °C and 20 bar for 94 h. The inlet gas mixture was composed of 52% (v/v) H 2 , 25% (v/v) CO and 23% (v/v) Ar. The condensable products were recovered using a hot condenser (150 °C) followed by a cold condenser (4 °C) and analyzed by GC-SIMDIST (SHIMAZU) equipped with a flame ionization detector (FID). The non-condensable gaseous products recovered after the cold condenser were analyzed by µ-GC (A 3000 model, Agilent) equipped with a thermal conductivity detector (TCD). The catalytic tests were repeated twice.

The catalytic performances were calculated using the following equations: CO conversion ( , %, Eq. 7):

𝑋 𝐶𝑂 𝑋 𝐶𝑂 = 𝐹 𝐶𝑂 ,𝑖 -𝐹 𝐶𝑂, 𝑜𝑢𝑡
𝐹 𝐶𝑂, 𝑖 (𝐸𝑞.7)

Where: = molar flow rate [mol.s -1 ] of CO fed to the reactor (mol.s -1 ); = molar 𝐹 𝐶𝑂 ,𝑖 𝐹 𝐶𝑂 ,𝑜𝑢𝑡 flow rate [mol.s -1 ] of CO at the reactor outlet (mol.s -1 ) CTY = Cobalt-Time-Yield (mol CO .mol Co -1 .s -1 , Eq. 8):

CTY = 𝐹 𝐶𝑂 ,𝑖 × 𝑋 𝐶𝑂 × 𝑀 𝐶𝑜 𝑚 𝑐𝑎𝑡 (𝐸𝑞.8)
Where: = molar flow rate [mol.s -1 ] of CO fed to the reactor (mol.s -1 ); = mass of 𝐹 𝐶𝑂 ,𝑖 𝑚 𝑐𝑎𝑡 catalyst (g); = molar mass of Co (g.mol -1 ). 𝑀 𝐶𝑜 Turn-over frequency (TOF, s -1 , Eq. 9): reactor outlet [START_REF] Scalbert | Development of an innovative XRD-DRIFTS prototype allowing operando characterizations during Fischer-Tropsch synthesis over cobalt-based catalysts under representative conditions[END_REF].

TOF = 𝐶𝑇𝑌 × 𝑀 𝐶𝑜 𝐷 𝐶𝑜 × 𝑥 𝐶𝑜 (𝐸𝑞.

Results and discussion

Catalyst characterization and influence of the support on catalyst features

Nitric acid oxidized CNT (220 m 2 .g -1 , d ext. = 12.6 nm and d int. = 4.5 nm) were used to support the cobalt active phase. The chemical and textural properties of this carbon support are given in Table S1. IWI was performed to load 15% Co w/w in water or ethanol for the nitrate precursor yielding Co N-W /CNT and Co N-E /CNT catalysts, respectively. For the acetate precursor (same metal loading), the IWI was performed in water and ethanol/water mixtures (for solubility reasons), yielding Co A-W /CNT and Co A-EW /CNT catalysts, respectively. ICP-OES analyses of the calcined catalysts (Table 1) showed cobalt loadings of 17.8 (Co N-W /CNT), 16.0 (Co N-E /CNT), [START_REF] Nguyen | Confinement of metal nanoparticles in carbon nanotubes[END_REF].0 (Co A-W /CNT), and 17.5 wt% (Co A-EW /CNT). Catalyst characterization was performed on calcined, freshly reduced (350 °C) and spent catalysts. First, we focused on the size of the cobalt particles, their possible confinement, their crystallographic phase (hcp/fcc), and their reduction degree (Co 0 /Co n+ ). Then, we have assessed the influence of CNT surface chemistry and H 2 spillover. Finally, the catalytic performances were correlated to catalyst properties (Co particle size, reducibility, phase, confinement and hydrogen spillover).

Cobalt particle size, confinement, crystallographic phase and composition.

The mean cobalt particle size of the reduced catalyst after exposure to air was estimated by TEM (Fig. 1) and XRD line broadening analysis (Fig. S2, Table 1). The TEM analyses show relatively small cobalt particles of 3-6 nm with smaller particles obtained for the acetate precursor for a given solvent, and in ethanol for a given precursor. The Co mean particle size follows the order Co N-W /CNT (6.2 nm) > Co N-E /CNT (5.6 nm) > Co A-W /CNT (4.6 nm) > Co A- EW /CNT (3.2 nm). In addition, HRTEM observations show that a significant number of Co particles are confined in the inner cavity of CNT (Fig. S3 and Table 1), particularly in the case of the ex-nitrate catalysts. Whatever the catalyst, the Co mean particle size is smaller for the confined particles than for those deposited on the external surface (Fig. S4). The filling yield, calculated according to a simple relation [START_REF] Nguyen | Confinement of metal nanoparticles in carbon nanotubes[END_REF], follows the order Co N-E /CNT 5.6 nm (74 %) > Co N-W /CNT 6.2 nm (57 %)

> Co A-W /CNT 4.6 nm (27 %) > Co A-EW /CNT 3.2 nm (16 %).
Cobalt filling yields of 10-30%

have been previously reported for highly loaded Co/CNT catalysts (10-15% w/w) prepared from cobalt nitrate on HNO 3 oxidized open-ended CNT [START_REF] Xiong | Correlating the preparation and performance of cobalt catalysts supported on carbon nanotubes and carbon spheres in the Fischer-Tropsch synthesis[END_REF][START_REF] Akbarzadeh | Effect of cobalt catalyst confinement in carbon nanotubes support on Fischer-Tropsch synthesis performance[END_REF]. For the cobalt acetate precursor, a small portion of particles was reported to be encapsulated in Co A /CNT catalysts [START_REF] Narasimharao | Cobalt oxide supported multi wall carbon nanotube catalysts for hydrogen production via sodium borohydride hydrolysis[END_REF], but the amount has not been quantified. The lower filling yield and higher dispersion obtained for the ex-acetate catalysts could be related to the ability of acetate ions to deprotonate surface -OH groups [START_REF] Liu | Surface complexes of acetate on edge surfaces of 2:1 type phyllosilicate: Insights from density functional theory calculation[END_REF][START_REF] Ahmad | Calorimetric screening of co-operative effects in adsorption of Co(II) on γ-alumina surface in the presence of Co-complexing anions in aqueous solution[END_REF] (phenolic or carboxylic in the case of CNT) with concomitant acetic acid formation and thus to favor cobalt grafting and dispersion on the newly created surface groups on the external surface of CNT.

The smaller Co mean particle size (for both ex-nitrate and ex-acetate catalysts), and the higher filling yield (for ex-nitrate catalyst) obtained in ethanol can be rationalized by solvent surface tension considerations. Indeed, higher filling yield [START_REF] Nguyen | Confinement of metal nanoparticles in carbon nanotubes[END_REF][START_REF] Tessonnier | Selective deposition of metal nanoparticles inside or outside multiwalled carbon nanotubes[END_REF], and better metal dispersion (controlled during the drying step) [START_REF] Eschemann | Effect of support surface treatment on the synthesis, structure, and performance of Co/CNT Fischer-Tropsch catalysts[END_REF] for CNT-supported metal catalysts were reported for Comparing the adsorption of water and ethanol vapors on CNT, the initial relative pressure of the isotherm depends on the polarity of the adsorbents; and at low pressures, the adsorption of ethanol on CNT is higher than that of water. These results demonstrate that the exploitation and interpretation of impregnating solvent adsorption isotherms by measuring surface tensions is an interesting discriminant analysis to not only to rationalize the results obtained in terms of filling yield and metal dispersion, but also guide the choice of impregnation solvent for IWI.

Here, the use of ethanol, a solvent with low surface tension, allows a better wetting of the CNT surface, and results in higher filling yield and cobalt dispersion. ) [START_REF] Schanke | Study of Ptpromoted cobalt CO hydrogenation catalysts[END_REF]. The mean XRD particle size was in very good agreement with the one determined by TEM (Table 1).

The evolution of the Co crystallographic phases was monitored by in-situ XRD during the reduction of the calcined catalysts (Figure 2). At 30 °C, all catalysts show the presence of the S8). These analyses confirm the absence of such species on the investigated catalysts. The reduction degree of Co catalysts can also be determined by magnetometry at room temperature [START_REF] Wolf | Role of CO in the waterinduced formation of cobalt oxide in a high conversion Fischer-Tropsch environment[END_REF]. CoO being non-magnetic it does not contribute to the magnetization of the sample. Thus, a decrease in the Ms as compared to the bulk Co magnetization value (163 emu.g Co -1 ) [START_REF] Jaumann | The synthesis of superparamagnetic cobalt nanoparticles encapsulated in carbon through high-pressure CVD[END_REF] means that part of the Co present in the sample is oxidized. The total Co in the sample being known from ICP, the CoO amount can be calculated. Therefore, the reduction degree of the catalysts after reduction in 40%H 2 /Ar flow for 8 h at 350 °C min -1 was determined by magnetic measurements without exposing the samples to the air. The sample magnetization versus the intensity of the applied magnetic field was recorded at 300 K (Fig. S9 and Table 1). 1). The reducibility of the catalysts was studied by temperature-programmed reduction. Figure S10 shows °C), which fits well with the data reported in the literature (220-280 °C) [START_REF] Phaahlamohlaka | Effects of Co and Ru intimacy in Fischer-Tropsch catalysts using hollow carbon sphere supports: assessment of the hydrogen spillover processes[END_REF][START_REF] Akbarzadeh | Effect of manganese on Co-Mn/CNT bimetallic catalyst performance in Fischer-Tropsch reaction[END_REF]. The lower reduction temperature observed for the first peak of the ex-nitrate catalysts could be explained by a confinement effect [START_REF] Akbarzadeh | Effect of cobalt catalyst confinement in carbon nanotubes support on Fischer-Tropsch synthesis performance[END_REF][START_REF] Chen | Effect of confinement in carbon nanotubes on the activity of Fischer-Tropsch iron catalyst[END_REF]. Considering the width of peaks II and III, it seems hazardous to comment on the differences observed in Figure S10 between the four profiles. However, neither in-situ XRD nor these TPR experiments point to an easier complete reduction of the exnitrate catalysts compared to the ex-acetate ones, which could be in part due to a possible confinement effect [START_REF] Chen | Effect of confinement in carbon nanotubes on the activity of Fischer-Tropsch iron catalyst[END_REF], even if the absence of confinement effects on the reduction temperature of Co/CNT catalysts has been reported [START_REF] Zhu | Synthesis and catalysis of location-specific cobalt nanoparticles supported by multiwall carbon nanotubes for Fischer-Tropsch synthesis[END_REF]. It is also worth noting the smaller cobalt particles present on the Co A-W /CNT and Co A-EW /CNT catalysts that are the first to be completely reduced (Fig. 2). Thus, besides confinement effects, the Co particle size and the strength of the MSI should also be considered to rationalize the easier reduction (and reduction degree) of the exacetate catalysts (vide infra).

For FTS tests, the catalysts were reduced at 350 °C for 8 h under an H 2 /Ar atmosphere. We followed this reduction by TPD/MS analyses (350 °C, H 2 /N 2 , 6 h) to probe the nature of the evolved gases (Fig. 3). The analysis of these spectra is very informative about the complex surface chemistry that takes place during the reduction of the catalyst. First, the amount of water evolved is significantly higher for the ex-acetate catalysts, in accordance with the higher reduction degree determined by in-situ XRD and VSM experiments. Water evolution occurs in three steps: at 100-150 °C, at 220-270 °C, and at 300-350 °C. The first peak that corresponds to the desorption of physisorbed water is logically more intense for the Co N-W /CNT, which has been prepared in water. This catalyst also presents a larger peak area of CO+CO 2 releasing groups (Table S3). The second peak at 220-270 °C should be mainly associated to the reduction of Co 3 O 4 to CoO. Additionally, in this temperature range, CO 2 evolution also occurs by the catalytic decomposition of the carboxylic groups of the support (vide infra Fig. 4a), which can also produce water [START_REF] Rivera-Cárcamo | Stabilization of metal single atoms on carbon and TiO 2 supports for CO 2 hydrogenation: the importance of regulating charge transfer[END_REF]. Finally, the more intense last peak (300-350 °C) is associated with the reduction of CoO to Co 0 . In this temperature range, the evolution of CO 2 /CO is also observed (Fig. 3), which should in principle arise from the catalytic decomposition of carboxylic anhydrides [START_REF] Figueiredo | Characterization of active sites on carbon catalysts[END_REF], with the concomitant formation of methane. The formation of methane in this temperature range should be associated to the CO methanation activity of the cobalt phase, which also produces water [START_REF] Castillo | The consequences of surface heterogeneity of cobalt nanoparticles on the kinetics of CO methanation[END_REF][START_REF] Wang | Effect of vanadium promotion on activated carbon-supported cobalt catalysts in Fischer -Tropsch synthesis[END_REF]. From ICP analyses.

XPS analyses were performed in order to probe the MSI. Figure S11 shows the Co 2p spectra normalized on the C 1s peak for the four calcined catalysts after air exposure. The Co/C atomic ratio (Table 2) is higher for the ex-acetate catalysts, which contain lower number of confined nanoparticles and present smaller mean particle sizes than those of ex-nitrate catalysts. Distinct peaks (Table 2) stemming from 2p 3/2 at 779.5-781.1 eV and 2p with those obtained by XRD on these same calcined samples (not shown), for which the ratio was also around 0.5. particle size, present a stronger interaction with the supports, which can explain their lower degree of reduction (vide supra).

In order to obtain further information on the reason of these different interactions, which lead to differences in the reducibility of cobalt, and in order to explain the different Co hcp /Co fcc ratios observed, the surface chemistry of the support was examined.

Effect of carbon surface chemistry on catalyst features and hydrogen spillover

The surface chemistry of the carbon support can influence the final features of the supported catalyst in a number of ways [START_REF] Serp | CHAPTER 1 -Carbon (Nano) materials for Catalyst[END_REF][START_REF] Z-Scheme | Functionalization of porous carbons for catalytic applications[END_REF]. Indeed, metal dispersion [START_REF] Peng | Effect of carbon surface functional groups on the synthesis of Ru/C catalysts for supercritical water gasification[END_REF][START_REF] Kang | Palladium Catalysts supported on activated carbone with different textural and surface chemical properties[END_REF], MSI [START_REF] Román-Martínez | Metal-support interaction in Pt/C catalysts. Influence of the support surface chemistry and the metal precursor[END_REF][START_REF] Zhang | Probing the metal-support interaction in carbon-supported catalysts by using electron microscopy[END_REF], metal location [START_REF] Nguyen | Confinement of metal nanoparticles in carbon nanotubes[END_REF][START_REF] Serp | Catalysis in carbon nanotubes[END_REF], and H-spillover [START_REF] Ghogia | Hydrogen Spillover in the Fischer-Tropsch Synthesis on Carbon-supported Cobalt Catalysts[END_REF][START_REF] Dibandjo | Hydrogen storage in hybrid nanostructured carbon/palladium materials: Influence of particle size and surface chemistry[END_REF] have been reported to be influenced by the surface chemistry of the carbon support. The carbon surface being rather inert, it is generally accepted that it is necessary to functionalize it in order to disperse and stabilize the metallic phase [START_REF] Serp | Carbon. In Comprehensive Inorganic Chemistry II (Second Edition)[END_REF].

Nevertheless, questions exist on the stability of the functional surface groups during catalyst preparation, and particularly during the calcination and reduction steps, generally carried out at high temperatures. The chemistry that can take place during these key steps of catalyst preparation between the metal precursor and the support surface is crucial for the determination of catalyst properties, complex and most of the times insufficiently discussed.

The thermal decomposition of cobalt acetate under an inert atmosphere has been studied via thermogravimetric analysis, in-situ XRD, and FTIR [START_REF] Wanjun | Mechanism of thermal decomposition of cobalt acetate tetrahydrate[END_REF]. The volatile products formed during the decomposition are water vapor, acetic acid, ethenone (ketene), acetone, and CO 2 . In the case of cobalt nitrate decomposition under an inert atmosphere, water vapor and various nitrogen oxides (NO, NO 2 , N 2 O 4 and N 2 O 5 ) are the produced volatile products [START_REF] Ehrhardt | Thermal decomposition of cobalt nitrato compounds: Preparation of anhydrous cobalt(II)nitrate and its characterisation by Infrared and Raman spectra[END_REF]. In our study, TGA analyses were performed with the dried Co N-W /CNT and Co A-W /CNT catalysts under N 2 (Figure S12). The temperature program was the same as the one followed for catalyst calcination (N 2 , 20 °C to 350 °C at 10 °C.min -1 followed by an isotherm of 3 h at 350 °C). The reaction starts at lower temperature for the Co N-W /CNT catalyst and finishes faster for the Co A-W /CNT catalyst.

The weight losses of the two samples are very different. It is only 3.3 % for Co A-W /CNT and 22.1 % for Co N-W /CNT. The low weight loss observed for the Co A-W /CNT catalyst could be explained by the loss of acetic acid (formed by deprotonation of surface -OH groups by acetate ions) during the impregnation or drying steps. Another explanation could be a reaction between the decomposition products and the carbon surface leading to its functionalization. The important weight loss observed for the Co N-W /CNT catalyst (resulting mainly from nitrogen oxide) can have a pronounced influence on the CNT surface chemistry. Indeed, it is known that the (catalyzed) oxidation of carbon by NO x can contribute to the formation of oxygen functional groups and even to carbon gasification [START_REF] Aarna | A review of the kinetics of the nitric oxide-carbon reaction[END_REF][START_REF] Stanmore | Oxidation of carbon by NO x , with particular reference to NO2 and N2O[END_REF][START_REF] Chen | Efficient functionalization of multi-walled carbon nanotubes by nitrogen dioxide[END_REF].

TPD-MS analysis is a common method for the characterization of the oxygen-containing surface groups of carbon materials, which decompose releasing CO and/or CO 2 and in some cases H 2 O at different temperatures (Table S4) [START_REF] Figueiredo | Characterization of active sites on carbon catalysts[END_REF][START_REF] Z-Scheme | Functionalization of porous carbons for catalytic applications[END_REF]. The evolution of CO 2 , CO and H 2 during TPD-MS analyses under helium for the four reduced catalysts provide valuable information on Co reducibility, Co crystal phase and H 2 spillover, respectively, and they are shown on Figure 4. The analyses were performed after the in-situ reduction of the calcined catalysts under the same conditions than the ones used during the FTS tests (20 °C to 350 °C at 10°C.min -1 under inert/H 2 mixture followed by an isotherm of 6 h. at 350 °C, Figure 3) to probe the surface chemistry. It is worth pointing out that nitro groups decompose at about 280 °C, resulting in the release of NO 2 [START_REF] Salame | Surface chemistry of activated carbons: Combining the results of temperature-programmed desorption, Boehm, and potentiometric titrations[END_REF]; thus, such groups are not present on the catalyst surface.

Looking at the CO 2 and CO spectra, it is clear that the catalyst surface chemistry drastically changes during catalyst preparation. The CNT support contains carboxylic (CO 2 at 217/291 °C), carboxylic anhydride (CO 2 and CO at 437 °C), lactone (CO 2 at 576 °C), phenol (CO at 717 °C) and carbonyl/quinone (CO at 882 °C) surface groups (see Tables S5 for the results of the deconvolution of CO 2 and CO profiles of CNT using a multiple Gaussian function). As expected, considering their decomposition temperature, the carboxylic groups (Figure 4a)

disappear almost completely upon catalyst preparation at 350 °C, and a Co(OOC-) interface is created for the four catalysts, associated to a narrow and intense peak characteristic of a catalytic decomposition between 427 and 448 °C, depending on the catalyst. This phenomenon has already been reported for Ru/CNT [START_REF] Machado | Understanding the surface chemistry of carbon nanotubes: Toward a rational design of Ru nanocatalysts[END_REF] and Pd/CNT [START_REF] Rivera-Cárcamo | Control of the single atom/nanoparticle ratio in Pd/C catalysts to optimize the cooperative hydrogenation of alkenes[END_REF] catalysts prepared on oxidized CNT.

It is also obvious from Figure 4a,b that new functionalities are created on the ex-nitrate catalysts associated to a significant CO 2 evolution at 515-530 °C and a CO evolution at 530-555 °C. The appearance of these new surface groups during catalyst preparation and their influence on catalyst properties are discussed below.

CO 2 releasing groups on catalyst surface and their influence on cobalt reducibility.

The CO 2 -releasing groups on the catalyst surface (Figure 4a, and Figure S13 and Tables S3 for CO 2 deconvolution profiles of the Co/CNT catalysts) are of three main types: i) residual carboxylic groups decomposing at ~160 (peak#1) and ~380-420 °C (peak#2); ii) Co(OOC-)

interface groups (peak#3) decomposing at ~440 °C; and iii) groups decomposing at ~450-490 (peak#4) and ~490-530 °C (peak#5). This third type of groups is particularly present for the exnitrate catalysts. They could be created by (catalyzed) oxidation with nitrogen oxides, and considering the narrow peaks, they most likely decompose catalytically over a temperature range, which could correspond to carboxylic anhydrides or lactones. The decomposition of carboxylic anhydride also produces CO, and CO is indeed produced (Figure 4b and Figure S14) at similar temperatures (450-460 °C). In addition to carboxylic acids, carboxylic anhydrides and lactones have been reported to form during carbon oxidation with gaseous nitric acid (a complex mixture of HNO 3 , O 2 , H 2 O, and various nitrogen oxides) [START_REF] Xia | A highly efficient gas-phase route for the oxygen functionalization of carbon nanotubes based on nitric acid vapor[END_REF][START_REF] Xu | Gaseous nitric acid activated graphite felts as hierarchical metal-free catalyst for selective oxidation of H 2 S[END_REF]. Since in our case, the formation/decomposition of these groups can be cobalt-assisted, it is difficult to make a rigorous assignment. Nevertheless, considering their decomposition temperature, we propose that peak#4 corresponds to anhydrides, possibly coordinated to cobalt, and peak#5 corresponds to lactone surface groups. The peak#3 associated to the cobalt-support interface (Co(OOC-) species) can be related to the strength of the MSI, and consequently to cobalt reducibility. On CNT-supported catalysts, it has been reported that the presence of lateral surface-carboxylate ligands on metallic particles significantly contributes to strengthen the MSI [START_REF] Machado | Understanding the surface chemistry of carbon nanotubes: Toward a rational design of Ru nanocatalysts[END_REF]. It is clear from

Figure 5a that the cobalt-surface carboxylate interface is more developed for the ex-nitrate catalysts than for the ex-acetate ones, even if these latter present a smaller Co mean particle size. It is thus reasonable to propose that the Co particles of the ex-nitrate catalysts should interact stronger and should be more difficult to reduce than the ones of the ex-acetate catalysts.

In fact, a very good correlation was found between the concentration of the Co(OOC-) species (Tables S3) and the reduction degree of the catalysts (Figure 5a). The cobalt nitrate decomposition during the impregnation/calcination step contributes to CNT oxidation, and to the creation of a strong MSI, limiting further cobalt reduction. Thus, the chemical reactions taking place (or not) between the metal precursor and the carbon support during catalyst preparation can affect the MSI.

CO releasing groups on catalyst surface and their influence on the produced cobalt phase.

The CO-releasing groups on the catalyst surface (Figure 4b and Figure S13 and Tables S3) are

of five types: i) the carboxylic anhydrides, which are the less abundant decomposing at ~510-540 °C (peak#1); ii) phenol groups decomposing at ~525-550 °C (peak#2), iii) ether groups decomposing at ~650-680 °C (peak#3) [START_REF] Shen | Surface chemical functional groups modification of porous carbon, recent patents[END_REF], iv) carbonyl/quinone groups decomposing at ~800-820 °C (peak#4), and v) low amount of groups decomposing at ~930-950 °C (peak#5) that could correspond to chromene or pyrone [START_REF] Pereira | Adsorption of dyes on activated carbons: Influence of surface chemical groups[END_REF][START_REF] Shafeeyan | A review on surface modification of activated carbon for carbon dioxide adsorption[END_REF][START_REF] Zielke | Surface-oxidized carbon fibers: I. Surface structure and chemistry[END_REF]. The decomposition of the cobalt nitrate precursor during catalyst preparation induces the formation of large amounts of phenolic groups, starting releasing CO at around 450 °C. These groups could be formed directly, or by catalytic reduction (either on cobalt particles or via the H-spillover) of transient carboxylic/anhydride groups (Figure 3) [START_REF] Kundu | Thermal stability and reducibility of oxygencontaining functional groups on multiwalled carbon nanotube surfaces: a quantitative high-resolution XPS and TPD/TPR study[END_REF]. The formation of phenol groups has been reported to occur during the impregnation/drying steps of nitrate salts on activated carbon [START_REF] Barroso-Bogeat | Activated carbon surface chemistry: Changes upon impregnation with Al(III), Fe(III) and Zn(II)-metal oxide catalyst precursors from NO 3 -aqueous solutions[END_REF]. The release of CO even during the reduction step of the catalyst at 350 °C (Figure 3) is interesting. It is known that the presence of CO during the reduction of cobalt FTS catalysts can favor the formation of Co hcp through the transient formation and decomposition of cobalt carbide (Co 2 C) [START_REF] Ducreux | Microstructure of supported cobalt Fischer-Tropsch catalysts[END_REF][START_REF] Kwak | Enhanced catalytic activity of cobalt catalysts for Fischer-Tropsch synthesis via carburization and hydrogenation and its application to regeneration[END_REF][START_REF] Patanou | The impact of sequential H 2 -CO-H 2 activation treatment on the structure and performance of cobalt based catalysts for the Fischer-Tropsch synthesis[END_REF]. Indeed, the in-situ XRD experiments (5 °C.min -1 from 30 to 800 °C with plateau of 22 min. at each temperature, Figure 2) have shown that the four catalysts reduced at 350 °C present very different Co hcp /Co fcc ratios.

Even if the Co 2 C transformation to Co hcp is generally performed sequentially, it has been reported that the Co hcp metallic phase is produced under H 2 +CO atmosphere in the case of Co/CNF catalysts [START_REF] De | Development of hexagonal closed-packed cobalt nanoparticles stable at high temperature[END_REF]. It is thus reasonable to propose that the concomitant presence of CO and H 2 in the gas phase during the reduction step should contribute to an increase of the Co hcp /Co fcc ratio. Thus, the high Co hcp /Co fcc ratio measured for the ex-nitrate catalysts could be associated to the higher amount of CO-releasing groups in these materials. Figure 5b shows the excellent correlation between the amount of CO-releasing groups and the Co hcp /Co fcc ratio. This offers real perspectives to control the Co hcp /Co fcc ratio in Co/C catalysts by controlling the gaseous atmosphere during the in situ reduction of the catalysts.

Quinone groups and their influence on hydrogen spillover.

H-spillover is a potential handle to enhance hydrogen storage in CNT, and such enhancement has been often reported for cobalt-based supported materials [START_REF] Chen | Enhancement of hydrogen spillover onto carbon nanotubes with defect feature[END_REF][START_REF] Rather | Hydrogen uptake of cobalt and copper oxide-multiwalled carbon nanotube composites[END_REF][START_REF] Aghababaei | Optimizing the conditions of multiwalled carbon nanotubes surface activation and loading metal nanoparticles for enhanced hydrogen storage[END_REF]. In order to assess the Hspillover, H 2 -TPD-MS analyses of the catalysts (Figure 4c) are very informative. No peak related to H 2 desorption was observed for the CNT support. The H 2 desorption profiles of all the catalysts show two main large peaks. The first peak between 400 and 800 °C is due to H 2 release from the Co particles [START_REF] Ghogia | Hydrogen Spillover in the Fischer-Tropsch Synthesis on Carbon-supported Cobalt Catalysts[END_REF]. However, if we consider the H 2 quantities released (Table 3)

for the calculation of the metal particle size (Co N-W /CNT = 2.0 nm; Co N-E /CNT = 1.3 nm; Co A- W /CNT = 2.2 nm; and Co A-EW /CNT = 2.3 nm), it is evident that H 2 amounts are too high to correspond only to the Co adsorbed H 2 . Particularly, the Co N-E /CNT catalyst, which presents the larger mean particle size and the lower reduction degree releases the highest amount of H 2 .

Consequently, this peak should also integrate the hydrogen desorbed via reverse H-spillover [START_REF] Lachawiec | Reverse spillover of hydrogen on carbon-based nanomaterials: evidence of recombination using isotopic exchange[END_REF], which is the migration of H atoms from the support onto the cobalt particles, where they recombine into H 2 that desorbs [START_REF] Ghogia | Hydrogen Spillover in the Fischer-Tropsch Synthesis on Carbon-supported Cobalt Catalysts[END_REF]. Considering the fact that the mean cobalt particle sizes as determined by TEM and XRD are in good agreement (Table 1), we used the TEM values and the cobalt reduction degree (from in-situ XRD) to estimate what should be the theoretical amount of H 2 that should desorb from these particles in the absence of reverse H-spillover (Table 3). Then, we used these latter values to estimate the H 2 evolution linked to reverse Hspillover. It is also interesting to note that H 2 desorption was observed at 350 °C during the TPD-MS analyses performed during the catalyst reduction step (Figure S14). This desorption peak was particularly intense for the Co N-E /CNT catalyst. In that case, H 2 desorption should be mainly associated to reverse H-spillover, since the peak was of very low intensity or even absent for the other catalysts.

The second peak of the H 2 -TPD-MS analyses of the catalysts (absent in the case of the original support) that starts at ~ 800 °C is associated to the decomposition (without the help of the supported metal) of stable C-H functionalities, which have been created by H-spillover from the metallic particles [START_REF] Bhowmick | Hydrogen spillover in Ptsingle-walled carbon nanotube composites: Formation of stable C-H bonds[END_REF]. From the values reported on Table 3, the H-spillover is more pronounced on the Co N-E /CNT catalyst. This result can be rationalized by considering the fact that on carbon-supported catalysts, the hydrogen spillover is enhanced on supports presenting a high amount of surface oxygen groups [START_REF] Gerber | A theory/experience description of support effects in carbonsupported catalysts[END_REF], and particularly quinone groups [START_REF] Wang | Enhanced hydrogen spillover on carbon surfaces modified by oxygen plasma[END_REF]. Since quinone groups cannot be distinguished from other carbonyl groups by TPD, we rely on XPS analyses performed on the calcined catalysts to determine their concentration. The deconvoluted O 1s XPS spectra of the catalysts, presented on Figure S15, and Table S6, show the results of these deconvolutions (the quantification of each type of surface species was made base on the FWHM of the peaks) [START_REF] Contreras | Effect of mesoporous carbon support nature and pretreatments on palladium loading, dispersion and apparent catalytic activity in hydrogenation of myrcene[END_REF]. The O 1s region of the four catalysts is composed of three components, corresponding to quinone surface groups (~ 529.8 eV), C=O containing surface groups (~ 531.3 eV) and C-O surface groups (~ 532.9 eV) [START_REF] Fan | Hydrothermal synthesis and activation of graphene-incorporated nitrogen-rich carbon composite for high-performance supercapacitors[END_REF][START_REF] Díez | Enhanced reduction of graphene oxide by high-pressure hydrothermal treatment[END_REF]. Figure 5c shows the correlation between the amount of quinone groups on the support and the total amount of H 2 desorbed from spillover. Obviously, a higher concentration in quinone groups favors the H-spillover. Even if the Co N-E /CNT catalyst presents a higher H-spillover, it is worth mentioning that these species are not reactive enough to contribute to a high degree of reduction of this catalyst, this latter being dictated mainly by the strength of the MSI.

The most relevant findings on our analysis on the role of carbon surface chemistry and Hspillover on the final features of the reduced Co/CNT catalysts are resumed on Scheme 1.

Scheme 1. Most relevant phenomena occurring during Co/CNT catalyst reduction.

CNT surface oxidation by nitric acid creates various functional groups. The carboxylic groups contribute to cobalt particle stabilization, and a strong cobalt-carboxylate interface contributes to a decrease of the reduction degree of cobalt. These groups can decompose to CO 2 or be reduced to phenol groups (either by the cobalt or by the spilled over H* species). The carboxylic anhydrides decompose at relatively low temperature to produce CO and CO 2 . The evolved CO, which could also arise from phenol group decomposition, is consumed together with H 2 either to produce Co hcp , or to produce methane by methanation (either by the cobalt or by the spilled over H* species). The quinone groups constitute efficient shuttles for hydrogen spillover on the CNT surface. All these phenomena are particularly present in the case of the ex-nitrate catalysts, for which surface oxidation occurs during the calcination step due to nitrogen oxide evolution.

After having demonstrated the impact of the chemistry operating during catalyst elaboration on the catalysts features it is important to rationalize these effects in the context of the FTS.

Structure/performance correlations

Catalytic Performances in FTS. Figure 6a shows the CTY evolution (mol CO .mol Co -1 .s -1 ) as a function of time-on-stream (TOS, h) for the four catalysts during FTS at 20 bar, 220 °C, H 2 /CO = 2 v/v, GHSV = 1100-2300 mL.g -1 .h -1 . The average deactivation parameter (r i /r 54h , ratio of the initial rate to the rate at steady state (after 54 h TOS), as well as the average sintering parameter of Co particles (d calculated based on mean particle size obtained by TEM of the fresh and spent catalysts) are also presented on this figure. The lower catalytic performances obtained in this work as compared to those reported by de Jong et al. [START_REF] Eschemann | Effect of support surface treatment on the synthesis, structure, and performance of Co/CNT Fischer-Tropsch catalysts[END_REF][START_REF] Van Deelen | Cobalt nanocrystals on carbon nanotubes in the Fischer-Tropsch synthesis: Impact of support oxidation[END_REF] for 9%Co/CNT catalysts in microreactors under similar conditions can be explained by the use of a microchannel FT reactor [START_REF] Chabot | A mathematical modeling of catalytic milli-fixed bed reactor for Fischer-Tropsch synthesis: Influence of tube diameter on Fischer Tropsch selectivity and thermal behavior[END_REF]. The results from the catalytic tests revealed significant differences in the performances (activity, selectivity and stability) of the studied catalysts (Figure 6 and Table 4). For the exnitrate cobalt catalysts, higher activities were found for the systems impregnated with ethanol, as reported in the work of de Jong et al. [START_REF] Eschemann | Effect of support surface treatment on the synthesis, structure, and performance of Co/CNT Fischer-Tropsch catalysts[END_REF] In the case of the ex-acetate catalysts, the catalyst prepared in water performed significantly better than the one prepared in the ethanol/water mixture. Overall, the activity order follows: ). c) Final CTY (in 10 -4 mol CO .mol Co -1 .s -1 ). d) Initial TOF. e) Initial TOF after correction on the basis of the reduction degree. f) Final TOF. g) TEM particle size of spent catalysts. .

Co N-E /CNT > Co A-W /CNT ≈ Co N-W /CNT > Co A- EW /CNT.
Regarding the selectivity to higher hydrocarbons, similar C 5+ selectivity's were found for all the catalysts (80-82%), which could be related to similar particle size measured after catalysis for all catalysts (8-10 nm, Table 4). However, the chain growth probability factor (α) was higher for ex-nitrate catalysts (0.85-0.88) than for ex-acetate ones (0.61-0.74).

As far as stability is concerned, the average deactivation parameter follows the order: Co

Structure/performance relationships.

Classical descriptors of Co-based FTS catalyst performances include cobalt particle size [START_REF] Bezemer | Cobalt particle size effects in the Fischer-Tropsch reaction studied with carbon nanofiber supported catalysts[END_REF],

crystallographic phase [START_REF] Pour | Effects of metallic cobalt crystal phase on catalytic activity of cobalt catalysts supported on carbon nanotubes in Fischer-Tropsch synthesis[END_REF], interaction with the support and reducibility [START_REF] Yang | The correlation of interfacialinteraction and catalytic performance of N-doped mesoporous carbon cupported cobalt nanoparticles for Fischer-Tropsch synthesis[END_REF][START_REF] Xiong | Cobalt catalysts supported on a micro-coil carbon in Fischer-Tropsch synthesis: A comparison with CNTs and CNFs[END_REF], and confinement [START_REF] Akbarzadeh | Effect of cobalt catalyst confinement in carbon nanotubes support on Fischer-Tropsch synthesis performance[END_REF][START_REF] Xie | Effect of catalyst confinement and pore size on Fischer-Tropsch synthesis over cobalt supported on carbon nanotubes[END_REF]. We have shown that these features of the Co/CNT catalysts are intimately linked to the surface chemistry of the support. In the case of Co/CNT catalysts, the confinement effects are often presented as a determining factor for catalyst activity. Thus, a high TOF should be obtained with a Co/CNT catalyst presenting a high filling yield, a high proportion of Co hcp , a high degree of reduction, a high spillover, and a Co particle size > 6-8 nm [START_REF] Bezemer | Cobalt particle size effects in the Fischer-Tropsch reaction studied with carbon nanofiber supported catalysts[END_REF][START_REF] Den Breejen | On the origin of the cobalt particle size effects in Fischer-Tropsch catalysis[END_REF]. It is clear from the data reported in Table 1 and Figure 7a, which resumes the main catalyst features on a 0-1 scale, that none of the prepared catalysts presents maximum values over all the criteria considered. Two catalysts with very different profiles can be distinguished. The first one is the Co N-E /CNT catalyst, which even if it presents the lowest reduction degree, is very well positioned for as far as all the other criteria are concerned, and presents the highest TOF. The second one is the Co A-EW /CNT catalyst, which exhibits a high degree of reduction, but is poorly positioned for all other criteria, and leads to the lowest TOF value.

As most of the catalyst features reported on Figure 7a are related to the fresh catalysts, we rely on the values of the initial TOF (calculated with initial Co particle size) corrected from the cobalt reduction degree (TOF corr. , Table 4) to draw structure/activity correlations. We tried to correlate the different features of the catalysts shown on Figure 7a, both independently or in combination with the TOF values. As expected [START_REF] Ghogia | Cobalt catalysts on carbonbased materials for Fischer-Tropsch synthesis: a review[END_REF], poor (or no) correlations exist between the TOF and each of the parameters considered independently (Figure S16). Only the cobalt filling yield can be roughly related to the TOF corr. (coefficient of determination R² = 0.8012). We tried also correlations with only two or three parameters. When using two parameters (Figure S17), the best correlation (R² = 0.9877) included also the cobalt filling yield and surprisingly the inverse of the cobalt initial particle size. A correlation was also found including the cobalt filling yield and the H-spillover (R² = 0.9554). When using three parameters (Figure S18), the best correlation (R² = 0.9906) included the three previously considered catalyst features: the cobalt filling yield, inverse of the cobalt initial particle size and H-spillover. The best correlation (R² = 0.9943, Figure 7b) is the one obtained by combining four parameters: the percentage of Co hcp , the filling yield, the H-spillover amount, and the inverse of the cobalt initial particle size. It is not surprising to integrate in such correlation the Co hcp , the confinement degree and the extent of H-spillover, since it has been shown that all these features positively affect the catalytic activity. The fact that the TOF should increase at lower particle size is surprising, and in fact goes against what has been shown in the literature for Co particles for which the TOF is increased with particle size up to about 6-10 nm, after which it remains stable [START_REF] Bezemer | Cobalt particle size effects in the Fischer-Tropsch reaction studied with carbon nanofiber supported catalysts[END_REF][START_REF] Den Breejen | On the origin of the cobalt particle size effects in Fischer-Tropsch catalysis[END_REF]. It is nevertheless necessary to integrate a decisive element, namely that this general tendency reported for cobalt in the literature does not concern confined metallic particles. Thus, the trend observed in our work may be linked to the fact that the most active particles being those that are confined, the evolution of the TOF with their size (in the 3-7 nm range) does not follow the previously established rule. The reason of the lower TOF generally reported on small unconfined Co particles (< 6-8 nm) is the strong adsorption of CO on edge/corner sites, as well as a lower intrinsic activity at the terraces [START_REF] Den Breejen | On the origin of the cobalt particle size effects in Fischer-Tropsch catalysis[END_REF]. This could not be the case for confined Co hcp particles. Indeed, although CO adsorption is stronger on Co hcp particles than on Co fcc ones [START_REF] Liu | Crystallographic dependence of CO activation on cobalt catalysts: HCP versus FCC[END_REF],

the Co hcp particles are more active. Additionally, different trends in TOF according to cobalt particle size were reported in the case of Co fcc and Co hcp particles [START_REF] Ghampson | Effects of pore diameter on particle size, phase, and turnover frequency in mesoporous silica supported cobalt Fischer-Tropsch catalysts[END_REF], and to the best of our knowledge, no clear correlation between TOF and particle size has been reported up to now in the literature for Co hcp in FTS. As far as confined particles are concerned, it is noteworthy that confinement can lead to downshifted d-band states, and consequently to weaker adsorption energy of adsorbates as CO [START_REF] Xiao | Toward fundamentals of confined catalysis in carbon nanotubes[END_REF]. It seems reasonable that a modification of the energy landscape in a confined environment and the fact that a large fraction of our catalysts contain

Co hcp are at the origin of the correlation found. It is the combined effects of a smaller initial cobalt particle size and a higher hydrogen spillover that can explain the higher initial TOF (corrected from the cobalt reduction degree) measured for the Co A-W /CNT catalyst as compared to the Co N-W /CNT one.

In FTS, the catalyst stability is impacted by several factors such as metal sintering (coalescence and Ostwald ripening), cobalt oxidation, and carbon deposition, which negatively affect the rate [START_REF] Saib | Fundamental understanding of deactivation and regeneration of cobalt Fischer-Tropsch synthesis catalysts[END_REF][START_REF] Rytter | Deactivation and regeneration of commercial type Fischer-Tropsch Co-catalysts -A mini-review[END_REF], or carbide formation, CO reduction, and restructuring of the surface, which have a positive influence [START_REF] Khorashadizadeh | Modeling the kinetics of cobalt Fischer-Tropsch catalyst deactivation trends through an innovative modified Weibull distribution[END_REF]. In the case of Co/CNT catalysts, cobalt re-oxidation, cobalt-support interactions, wax accumulation and sintering have been identified as the main sources of catalyst deactivation [START_REF] Chernyak | Structural evolution, stability, deactivation and regeneration of Fischer-Tropsch cobaltbased catalysts supported on carbon nanotubes[END_REF][START_REF] Nakhaei | Prediction of cobalt particle size during catalyst deactivation in Fischer-Tropsch synthesis[END_REF][START_REF] Trépanier | Deactivation behavior of carbon nanotubes supported cobalt catalysts in Fischer-Tropsch synthesis[END_REF]. In our study, the catalyst stability is expressed as the average deactivation parameter (r i /r 54h ). We tried to correlate the different features of the catalysts shown on Figure 7a, both independently or in combination with the deactivation parameter. The best correlation (Figure 7c) was obtained by combining the initial particle size and the percentage of Co hcp in the catalysts. This good correlation could be tentatively rationalized by the fact that sintering [START_REF] Park | Fischer -Tropsch catalysts deposited with size-controlled Co 3 O 4 nanocrystals : Effect of Co particle size on catalytic activity and stability[END_REF] and cobalt oxidation [START_REF] Wolf | Size dependent stability of cobalt nanoparticles on silica under high conversion Fischer-Tropsch environment[END_REF] should be favored with small cobalt particles [START_REF] Tsakoumis | Deactivation of cobalt based Fischer-Tropsch catalysts: a review[END_REF]. Furthermore, the high stability of the Co hcp phase under FTS conditions has been reported [START_REF] Kwak | Enhanced catalytic activity of cobalt catalysts for Fischer-Tropsch synthesis via carburization and hydrogenation and its application to regeneration[END_REF][START_REF] Patanou | The impact of sequential H 2 -CO-H 2 activation treatment on the structure and performance of cobalt based catalysts for the Fischer-Tropsch synthesis[END_REF][START_REF] González-Carballo | Synthesis of cobalt nanodumbbells and their thermal stability under H 2 , H 2 /CO and O 2 atmospheres[END_REF][START_REF] Lyu | Supported cobalt nanoparticles with a single active phase for Fischer-Tropsch synthesis[END_REF], and some studies have pointed out that this phase is more stable under FTS conditions than the Co fcc one [START_REF] Gnanamani | Fischer-Tropsch synthesis: Activity of metallic phases of cobalt supported on silica[END_REF][START_REF] Harmel | hcp-Co nanowires grown on metallic foams as catalysts for the Fischer-Tropsch synthesis[END_REF]. Finally, for the ex-nitrate catalysts containing a significant amount of confined particles, we noticed a template effect of the CNT cavity, since the sintering was more pronounced for the Co particles located on the external surface of the CNT (Figure S19).

CONCLUSIONS

Oxidized CNT were used to support cobalt particles (3-7 nm). Co/CNT catalysts were prepared in two different solvents (water and ethanol) from two different precursors (nitrate and acetate).

The use of ethanol, which presents a lower surface tension than water, allowed reaching higher metal dispersion. The lower filling yield obtained for the ex-acetate catalysts could be related to the ability of acetate ions to interact with the oxidized CNT external surface via deprotonation of surface -OH groups. A rigorous characterization of the catalysts at different stages of their preparation shows that the surface chemistry, which operates between the cobalt precursor and the surface of the support, affects many characteristics of the final catalyst, such as: the metalsupport interaction, the Co hcp /Co fcc ratio and the extent of hydrogen spillover. Particularly, the oxygenated groups of the support, either present on the initial support or created during the preparation of the catalyst, strongly contribute to the catalyst features. The CO 2 releasing groups contribute to stabilize Co particles and reduce their reducibility, while the CO releasing groups contribute to increase the Co hcp /Co fcc ratio and hydrogen spillover (quinone groups). The prepared catalysts present very different features (Co reduction degree, confinement, crystal phase, particle size, H-spillover), and accordingly different activity and stability in FTS. The selectivity to higher hydrocarbons (S C5+ ) is similar for all the catalysts, which can be rationalized by a similar Co particle size for the spent catalysts. The differences in initial activity and stability have been rationalized considering catalyst structures. Interestingly, our correlation results show that the initial TOF increases for smaller particle size, which is in contrast to what is generally found for Co fcc /C catalysts. This has been tentatively interpreted by an increased contribution of the confinement effect of Co hcp particles. Finally, the stability of the catalyst is mainly affected by the initial Co particle size and the percentage of Co hcp .

These results show that the design of high-performance FT catalysts remains challenging, and that an optimal combination of all catalyst features (not obtained in this study) requires a fine control of number of parameters related to the preparation of the catalytic system. Last but not least, in the structure-performance correlations presented in this study, the contribution of the different factors has not been modulated. This has not prevented excellent correlations to be obtained. Attributing different weight coefficients could also be sound, and also lead to different correlations. However, this would need the use of dedicated algorithms, which is outside the scope of the present article that aims, for the first time, at demonstrating that complex relations should be considered in order to rationalize the behavior of the Co based catalysts in FTS.

ASSOCIATED CONTENT Supporting Information. Additional catalyst characterization (TEM, HRTEM, TPD, XPS, TPR, TGA, VSM, adsorption and desorption isotherms of ethanol and water).

Figure 1 .

 1 Figure 1. TEM micrographs and Co particle size distribution of the four catalysts.

  solvents showing low surface tension. The ethanol surface tension at 20 °C is 22.4 mN.m -1 , whereas the one of water is 72.8 mN.m -1 . The isotherms of ethanol and water adsorption and desorption on CNT are shown in Fig. S5. The hydrophobic character of the CNT surface and the high polarity of the water molecules induces a shift in water adsorption towards higher relative pressures compared to ethanol vapors. The large hysteresis observed on the water isotherms can be tentatively correlated with the presence of an open CNT inner cavity.

Figure

  FigureS2shows the ex-situ X-ray diffraction patterns of the catalysts after reduction at 350 °C

  CoO and Co 3 O 4 phases. The presence of small amounts of CoO could be attributed to the autoreduction of Co 3 O 4 assisted by CO evolved from the decomposition of the support functional groups during the calcination step[START_REF] Park | Auto-Reduction Behavior of cobalt on graphitic carbon nitride coated alumina supports for Fischer -Tropsch synthesis[END_REF]. Co 3 O 4 is fully reduced to CoO at 300-450 °C. Then, CoO reduction progressively took place from 300 to 550 °C to form Co hcp and Co fcc . The carbon peak at 2θ = 30° significantly decreased at 700-800 °C due to cobalt-catalyzed carbon hydrogasification[START_REF] Trépanier | Synthesis of CNT-supported cobalt nanoparticle catalysts using a microemulsion technique: Role of nanoparticle size on reducibility, activity and selectivity in Fischer-Tropsch reactions[END_REF]. This phenomenon is more pronounced for the Co A-W /CNT and particularly for Co A-EW /CNT catalysts, due to their higher cobalt dispersion.

Figure 2

 2 Figure 2 also shows the quantitative evolution of the crystalline phases as a function of

Figure 2 .

 2 Figure 2. XRD patterns and evolution of the Co crystal phases during the in-situ reduction

  the TPR profiles of the calcined catalysts. Three broad peaks are observed at 250-320 °C (I), 370-460 °C (II) and 550 °C (III). The two first peaks (I and II) correspond to the typical two-step reduction of cobalt catalysts, from Co 3 O 4 to CoO, then CoO to metallic cobalt. The last peak (III) is attributed to the catalytic gasification of the carbon support [12]. The decomposition of CNT surface oxygen groups (accompanied by CO or CO 2 release), and further reduction by H 2 of carbon oxides to methane over Co particles, which is also possible in this temperature range [31]. A complete reduction of Co 3 O 4 consumes one hydrogen molecule for the first reduction process (Co 3 O 4 + H 2 → 3CoO + H 2 O), and three hydrogen molecules for the second (3CoO + 3H 2 → 3Co + 3H 2 O). It is thus expected that the reduction peak areas of Co 3 O 4 to CoO and CoO to Co will have a ratio of 1:3. As expected from the in-situ XRD experiments, the position of peak (I) is shifted to lower temperatures for the two ex-nitrate catalysts (260-290

Figure 3 .

 3 Figure 3. TPD-MS spectra of H 2 O, CH 4 , CO and CO 2 obtained during the catalyst reduction

Figure 4 .

 4 Figure 4. TPD/MS spectra of the catalysts and CNT support: a) CO 2 evolution; b) CO

Figure 5 .

 5 Figure 5. Correlation between: a) the cobalt reduction degree and the amount of CO 2 evolved

Figure 6 .

 6 Figure 6. FTS catalytic performances of the Co/CNT catalysts: a) CTY as a function of TOS;

Figure 7 .

 7 Figure 7. a) Main catalyst features and performances on a 0-1 scale; b) structure/activity

  Co(NO 3 ) 2 ,6H 2 O 99%, in 10 mL of water or ethanol) were introduced and sonicated for 20 min. Then, four cycles of 10 min sonication (one every hour) were performed under static vacuum in order to complete the impregnation. Finally, the solid was dried at 120 °C overnight and calcined under argon flow at 350 °C for 3 h with a heating rate of 2 °C.min -1 . The resulting

	The cobalt catalysts were prepared by incipient wetness impregnation (IWI) with a solution of
	either cobalt acetate (Co(CH 3 COO) 2 .4H 2 O 97%) or cobalt nitrate (Co(NO 3 ) 2 .6H 2 O 99%). A
	cobalt loading of 15% on the carbon support was targeted. First, 2 g of oxidized CNT were

dried at 120 °C for 1 h under dynamic vacuum. Then, the solution containing cobalt acetate (1.54 g Co(CH 3 COO) 2 .4H 2 O, in 10 mL of water or ethanol-water mixture) or cobalt nitrate (1.76 g catalysts were denoted Co N-W /CNT (N = nitrate; W = water), Co N-E /CNT (N = nitrate; E = ethanol), Co A-W /CNT (A = acetate; W = water), and Co A-EW /CNT (A = acetate; EW = ethanolwater mixture).

  [START_REF] Ghogia | Hydrogen Spillover in the Fischer-Tropsch Synthesis on Carbon-supported Cobalt Catalysts[END_REF]

	Where: after the catalytic tests. Co loading (g co .g cat -1 ); 𝑥 𝐶𝑜 = Selectivity into a given hydrocarbon containing carbon atoms ( , %, Eq. 10): = Co dispersion calculated based on the TEM Co size 𝐷 𝐶𝑜 𝑗 𝑖 𝑆 𝑖 𝑆 Where: = selectivity; = molar flow rate of the corresponding product quantified at the 𝑆 𝑖 𝐹 𝑗

𝑖 = 𝑖 × 𝐹 𝑗 𝐹 𝐶𝑂, 𝑖 -𝐹 𝐶𝑂, 𝑜𝑢𝑡

× 100 (𝐸𝑞.10)

Table S2

 S2

) did not evolve upon a prolongation of the isotherm to 8 h (as performed in-situ before the FTS tests). The degree of reduction follows the order:

Co A-W /CNT (88 %) > Co A-EW /CNT (83 %) > Co N-W /CNT (67 %) > Co N-E /CNT (49 %);

and the Co hcp /Co fcc ratio follow the order: Co N-W /CNT (8.28) > Co N-E /CNT (5.12) > Co A-W /CNT (1.93) > Co A-EW /CNT (0.69). The differences observed in the reduction degree and the Co hcp /Co fcc ratio can be rationalized by differences in the CNT surface chemistry of the different samples (vide infra Effect of carbon surface chemistry on catalyst features and hydrogen spillover). Since XRD cannot detect eventual very small particles/clusters, we also performed STEM/HAADF analyses on Co A-W /CNT and Co A-EW /CNT samples (Fig.

Table 1 .

 1 Summary of the results of catalyst characterization and catalyst activity in FTS.

	Reduction degree

Table 2 .

 2 Co/C atomic ratio, Co 3+ /(Co 2+ +Co 3+ ) ratio, and Co 2p binding energy (BE) of the catalysts.

	Co binding energy (eV)
	2p 3/2	2p 1/2

Table 3 .

 3 Results of the deconvolution of H 2 TPD spectra of Co/CNT catalysts. ) H 2 quantities released between 400 and 800 °C. b). Theoretical amount of H 2 that should desorb from Co particles (in the absence of reverse H-spillover) estimated from TEM particle size and cobalt reduction degree (from in-situ XRD). c) Amount of H 2 obtained by abstracting the values of column 2 from the values of column 2. d) Sum of H 2 quantities released between 800 and 1070 °C and H 2 from reverse H-spillover. e) Total H 2 quantities released between 400 and 1070 °C (sum of values of column 3 and column 5).

	Catalyst	H 2 peak (400-800 ºC) a)	H 2 from Co b)	H 2 from reverse H-spillover c)	Total spillover d)	H 2 total e)
				(mol.g -1 )
	Co N-W /CNT	700	343	357	1149	1493
	Co N-E /CNT	970	475	495	1575	2050
	Co A-W /CNT	590	322	268	1455	1777
	Co A-EW /CNT	620	546	74	1019	1565

a

Table 4 .

 4 FTS catalytic performances of the Co/CNT catalysts. ) Initial CO conversion. b) Initial CTY (in 10 -4 mol CO .mol Co -1 .s -1

	Catalyst	CO a) (%)	CTY i b) CTY f c)	TOF d) TOF corr. e) TOF f) (10 -3 .s -1 )	TEM g) (nm)	S CH4 (%)	S C2-C4 (%)	S C5+ (%)	O/P	 r i /r 54h	d (%)
	Co N-W /CNT	48	6.4	4.5	3.8	6.0	3.4	8	13	6.5	81 0.11 0.85 1.4	29
	Co N-E /CNT	87	13.8	6.9	7.2	14.8	6.5	10	14	5.3	80 0.05 0.88 1.8	78
	Co A-W /CNT	64	10.9	4.5	4.7	5.4	3.8	9	13	4.8	82 0.07 0.61	2	95
	Co A-EW /CNT 23	3.6	1.46	1.1	1.3	1.3	10	13	7.3	80 0.38 0.74 2.3 212

a

  A- EW /CNT > Co A-W /CNT > Co N-E /CNT > Co N-W /CNT; and the average sintering parameter follows the same order.These results show that the catalyst prepared from the nitrate precursor in ethanol is the most efficient, while that prepared from acetate precursor in ethanol/water mixture is the less efficient. The two other catalysts, Co A-W /CNT and Co N-W /CNT show similar performances in terms of activity, selectivity to C 5+ and stability at steady state, but the Co

N-W /CNT catalyst allows reaching a higher  value.

ACKNOWLEDGMENTS

The authors thank V. Collière (LCC), S. Patry (RAPSODEE), D. Marty (RAPSODEE), M. Ribeiro (RAPSODEE) and P. Bertorelle (RAPSODEE) for technical support.

Funding Sources

This research (CatFisch project) was funded by the "Région Ocitanie" and the University of Toulouse (France). B.F.M. acknowledges the exploratory project under the Fundação para a Ciência e a Tecnologia (FCT, Portugal) Investigator Programme (ref. IF/00301/2015) with financial support from FCT/MCTES, through national funds (PIDDAC).

AUTHOR INFORMATION

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Competing interests

The authors declare no competing interests.