H Draouil

L Alvarez

J Causse

V Flaud

M A Zaïbi

J L Bantignies

M Oueslati

J Cambedouzou

Copper hexacyanoferrate (CuHCF) functionalized SWCNTs for the solidliquid selective extraction of cesium

Single-walled carbon nanotubes (SWCNTs) are functionalized with copper hexacyanoferrate (CuHCF) nanoparticles to prepare solid substrates for the sorption of cesium ions (Cs + ) from liquid outflows. The high mechanical resistance and large electrical conductivity of SWCNTs are associated to the ability of CuHCF nanoparticles to selectively complex Cs + ions in order to achieve membrane-like buckypapers presenting high loading capacity of cesium. The materials are thoroughly characterized using electron microscopy, Raman scattering, X-ray photoelectron spectroscopy and thermogravimetric analyses. Cs sorption isotherms are plotted after having measured the Cs + concentration by liquid phase ionic chromatography in the solution before and after exposure to the materials. It is found that the total sorption capacity of the material reaches 230 mg.g -1 , and that about one third of the sorbed Cs (80 mg.g -1 ) is selectively complexed in the CuHCF nanoparticles grafted on SWCNTs. These high values open interesting outlooks in the integration of such materials in devices for the controlled sorption and desorption of these ions.

Introduction

Radiotoxic elements constitute a source of pollution from the nuclear industry, for which it is important to anticipate effective processes. Among the elements of high radiotoxicity, 137 Cs constitutes a priority since it possesses a long half-life of more than 30 years and it decays to 137m Ba presenting a high gamma emission of 0.662 MeV. It has furthermore been released in former nuclear accidents and it has become an urgency to provide perennial solutions for treating water contaminated by radioactive cesium ions (Cs + ). Up to now, several materials have been proposed to achieve this application. However, the frequent coexistence of other ions in much larger concentration than Cs + in contaminated water makes the use of selective sorbents necessary in order to achieve efficient cleaning. Among the most promising materials envisioned for this application, powders of analogous of Prussian blue have been shown to selectively uptake large quantities of Cs + ions [START_REF] Jang | Removal of Radioactive Cesium Using Prussian Blue Magnetic Nanoparticles[END_REF][START_REF] Zhang | Removal of cesium from low-level radioactive wastewaters using magnetic potassium titanium hexacyanoferrate[END_REF]. However, this material tends to form sludge when hydrated, therefore complicating the process and the waste elimination.

Copper hexacyanoferrate functionalized single-walled carbon nanotubes for selective cesium extraction

Extraction on solid support is an interesting option because it allows treating large quantities of effluent by continuous processes, and to trap radioactive substances into more easily manipulable solid matrices. Attempts to graft Prussian blue analogues on solid supports such as mesoporous silica were made in the recent years [START_REF] Causse | Facile onepot synthesis of copper hexacyanoferrate nanoparticle functionalized silica monolths for the selective entrapment of 137 Cs[END_REF][START_REF] Delchet | Extraction of radioactive cesium using innovative functionalized porous materials[END_REF], and showed that it was possible to benefit from the sorption capacity of Prussian blue and, at the same time, to get a material easy to handle. However, silica based materials present an inherent weakness in their chemical composition: they are intolerant to basic medium, which restricts their application field. Singlewalled carbon nanotubes (SWCNTs) based membranes seem to be effective materials for this application as they combine several advantages. First, they often present large specific surface areas, considering that they can uptake important amounts of cesium. Moreover, the atomic structure of SWCNTs is chemically stable and resistant even to aggressive environments such as acidic or basic media.

As a matter of facts, it turned out from the literature that carbon nanotubes are good candidates for the sorption of toxic chemical elements and of many heavy metal ions such as Cs + , Cd 2+ , Pb 2+ , Sr 2+ [START_REF] Yavari | Adsorption of cesium(I) drom aqueous solution using oxidized multiwall carbon nanotube[END_REF][START_REF] Vipin | Sodium cobalt hexacyanoferrate encapsulated in alginate vesicle with CNT for both cesium and strontium removal[END_REF][START_REF] Vipin | Removal of Cs + and Sr2 + from water using MWCNT reinforced Zeolite-A beads[END_REF][START_REF] Krawczyk | Multiwalled carbon nanotubes as solid sorbent in dispersive micro solid-phase extraction for sequential determination of cadmium and lead in water samples[END_REF][START_REF] Kaper | Multi-method analysis of functionalized single-walled carbon nanotubes for cesium liquid-solid extraction[END_REF]. Yavari et al [START_REF] Yavari | Adsorption of cesium(I) drom aqueous solution using oxidized multiwall carbon nanotube[END_REF] first oxidized multi walled carbon nanotubes (MWCNTs) by nitric acid. They successfully grown functional groups such as carboxylic, lactone and phenol on MWCNTs surface. They reported that the amount of Cs + uptaken by functionalized MWCNTs depends upon aqueous solution parameters such as ionic strength, pH, and concentration of Cs + . The highest adsorption of Cs + by functionalized MWCNTs was found at higher pH, lower ionic strength and lower initial concentration of Cs + in aqueous solution. A. K. Vinin et al [START_REF] Vipin | Sodium cobalt hexacyanoferrate encapsulated in alginate vesicle with CNT for both cesium and strontium removal[END_REF] studied the removal of Cs + and Sr 2+ from aqueous solution by MWCNTs dispersed in sodium cobalt hexacyanoferrate (CoHCF). They reported that the presence of MWCNTs enhances the adsorption of Cs + and Sr 2+ on CoHCF. Recently, the same authors [START_REF] Vipin | Removal of Cs + and Sr2 + from water using MWCNT reinforced Zeolite-A beads[END_REF] have exchanged CoHFC by zeolite-A, which is encapsulated into MWCNTs. They have found that the quantity of removed Cs + decreased, but that the Sr 2+ extraction was favored. In order to further improve the performance of these materials for ionic sorption, it is interesting to use the known and mastered chemistry of carbon nanotubes (CNTs) in order to functionalize them with molecules or nanoparticles able to specifically capture the targeted ions. Some works have been carried out in this view using MWCNTs as support for CuHCF-NPs [START_REF] Li | In site formation and growth of Prussian blue nanoparticles anchored to multiwalled carbon nanotubes with poly(4vinylpyridine) linker by layer-by-layer assembly[END_REF][START_REF] Lee | Sorption of cesium ions from aqueous solutions by multi-walled carbon nanotubes functionalized with copper ferrocyanide[END_REF], and a Cs + sorption capacity of about 150 mg.g -1 was found for this substrate. Provided that SWCNTs possess a higher specific surface area than MWCNTs, they can be considered as a promising solid support for functionalization with molecules able to capture cations. Our group recently demonstrated that it is possible to graft dimethyl-acetylene dicarboxylate (DMAD) [START_REF] Kaper | Multi-method analysis of functionalized single-walled carbon nanotubes for cesium liquid-solid extraction[END_REF] and other organic functions [START_REF] Nicolle | Observing and predicting the preferential functionalization of metallic or semiconducting single-walled carbon nanotubes[END_REF] by a simple protocol based on Diels-Alder (DA) cycloadditions. In the case of DMAD grafted SWCNTs, we reported a high capacity of Cs + capture (250 mg of Cs per gram of grafted SWCNTs) [START_REF] Kaper | Multi-method analysis of functionalized single-walled carbon nanotubes for cesium liquid-solid extraction[END_REF]. These molecules are however not selective for Cs + sorption, and we therefore considered functionalization by selective moieties.

In this work, we present an efficient method to functionalize SWCNTs with copper hexacyanoferrate nanoparticles (CuHCF-NPs). We indeed aim to combine the ability to selectively capture Cs + ions of HCFs with the exceptional mechanical and electronic properties of SWCNT to elaborate efficient and reusable devices for the removal of large quantities of Cs + . We use a simple and original protocol based on two steps: first the functionalization of SWCNTs with propargylamine C3H5N1 (PRG), which is a primary amine used as a bridge between the SWCNT and CuHCF-NPs. The second step is the growth of CuHCF nanoparticles from the grafted PRG. This phase is based on two reactions: firstly the addition of copper nitrate Cu(NO3)2 and hence the addition of potassium ferrocyanide K4Fe(CN)6. These insertions are repeated for three cycles. We therefore present the characterization of the functionalized samples after each step of the elaboration procedure using different characterization techniques. The functionalization rate is quantified by thermogravimetric analysis (TGA) combined with spectroscopic analyzes including Raman spectroscopy and X-ray photoemission spectroscopy (XPS). High resolution transmission electron microscopy (HRTEM) is used to follow the evolution of the grafted nanoparticles growth. Finally, we detail the Cs + sorption capacity with both raw SWCNTs and CuHCF-NPs functionalized SWCNTs. Based on measurements of Cs + concentration before and after exposure to CuHCF-NPs functionalized SWCNTs by liquid phase ionic chromatography (LPIC), we obtained Cs + sorption isotherms showing a total Cs + sorption capacity of about 230 mg.g -1 , and a selective sorption capacity of about 80mg of Cs + per gram of grafted SWCNTs. This removal capacity compares to the highest efficiencies measured on other substrates, which is very encouraging in the perspective of developing reversible devices for the decontamination of liquids.

Experimental section

Sample preparation

SWCNTs were purchased from Carbon Solution, Inc.(P2-SWCNTs). The diameter distribution is 1.4±0.2nm). The carbon purity of our SWCNTs is higher than 90 wt.% and the metal (Ni,Y) content is between 4-8 wt.%. Propargylamine (PRG), Cu(NO3)2 and K4Fe(CN)6 reactants were purchased from Sigma-Aldrich (99%) and used without further purification. The functionalization of SWCNTs by CuHCF-NPs was realized following two steps:

1-Functionalization with propargylamine

In the first step, we start from a mixture containing 60mg of SWCNTs with 25ml of pure PRG.

To optimally disperse SWCNTs in PRG, the mixture is ultrasonicated for 15min following the procedure described elsewhere [START_REF] Kaper | Multi-method analysis of functionalized single-walled carbon nanotubes for cesium liquid-solid extraction[END_REF]. The mixture is then transferred into a teflon-lined steel autoclave and heated to 100°C during 24 hours in order to allow the Diels Alder reaction between PRG and SWCNTs (Fig. 1a). This mixture is filtered and then washed with acetone in order to remove PRG molecules that did not covalently bound to SWCNTs. It is finally dried at room temperature. The recovered sample consists of a easy-to-handle buckypaper of grafted SWNCTs.

2-Cu-HFC-NPs growth

This step is based on two reactions, following the procedure described in a previous work [START_REF] Grandjean | Effect of the chemical nature of different transition metal ferrocyanides to entrap Cs[END_REF].

The first step features the addition of a copper nitrate Cu(NO3)2 solution in order to attach a Cu atom to the PRG. This Cu atom will serve as a nucleation center for the further growth of the CuHCF-NP. After careful rinsing with acetone, the second step consists in the addition of a potassium ferrocyanide K4Fe(CN)6 solution aiming at initiating the growth of the first cell of the CuHCF-NP, as depicted in Fig. 1b. Both solutions are prepared with a concentration of 10 - 2 M. The reactions are performed at room temperature, under simple agitation, and repeated for three cycles. The first cycle lasts 2*24 hours, the second one lasts 2*12 hours and the third one lasts 2*8 hours. The solution is then filtered, and the recovered sample is carefully washed with acetone, and dried at room temperature for 48 hours. When immersed in a solution containing Cs + ions, the K + anions are expected to be replaced by the Cs + anions initially in solution. This mechanism of ionic exchange is depicted in Fig. 1.c.

Material characterization

The chemical composition of the materials is monitored by X-ray photoelectron spectroscopy (XPS) on an ESCALAB 250 (Thermo Electron). The X-ray excitation is provided by a monochromatic Al Kα (1486.6eV) source. Analyzed surface has a 400 µm diameter. All XPS spectra were treated using AVANTAGE software. The background signal was removed using the Shirley [START_REF] Shirley | High-Resolution X-Ray Photoemission Spectrum of the Valence Bands of Gold[END_REF] method. Surface atomic concentrations are determined from photoelectron peaks areas using the atomic sensitivity factors reported by Scofield [START_REF] Scofield | Hartree-Slater subshell photoionization cross-sections at 1254 and 1487 eV[END_REF]. During the measurements the base pressure in the analysis chamber is around 8x10

Cesium sorption tests

Cs + sorption measurements are carried out in batch mode on both raw SWCNTs and on CuHCF-NPs functionalized SWCNTs according to the following protocol: 10mg of a buckypaper of carbon nanotubes are stirred with 20ml of a cesium nitrate solution using different concentrations (0.1mM, 0.5mM, 2mM, 4mM and 8 mM) for 24h. Carbon nanotubes are recovered by filtration using a filter paper with particle retention between 5 and 13 m and thoroughly washed using Milli-Q water. The solid samples are then dried for further analyses, while the liquid phase is analyzed by LPIC. The LPIC measurements of the solutions before and after exposure to carbon nanotubes allowed us to plot the sorption isotherm of Cs + .

Results and Discussion

In order to characterize the structure of SWCNTs after functionalization with CuHCF-NPs, we performed a multi-method analysis of the sample before and after the functionalization protocol. HRTEM images of pristine SWCNTs are shown in Fig. 2.a at low magnification. Clean bundles of SWCNTs are seen in the sample. On the higher magnification image shown on Figure 2.b, the bundles appear slightly distorted, probably due to the acceleration voltage of 200kV, but their surface have still a clean aspect. The length of these bundles is estimated to be superior to 1m. After functionalization, the TEM image (Fig. 2.c) shows that the sample becomes decorated by some particles presenting a typical size of a few nanometers and distributed throughout the nanotube bundle. Some of these particles appear darker (black squares in Fig. 2.c), however the majority of the NPs appears in light grey (red squares in Fig. 2.c) and are likely to be CuHCF-NPs. The difference in contrast could be due to the distance of the objects with regard to the focalization plane of electrons. In order to prove the chemical nature of the NPs, we performed complementary characterizations that are provided below.

High Resolution Transmission Electron Microscopy

Thermogravimetric analysis

The rate of PRG and CuHCF grafted onto the SWCNTs surface was determined by TGA measurements. TGA curves of raw SWCNTs, SWCNTS_PRG and SWCNTs_CuHCF are shown in Fig. 3.a, b and c, respectively. The weight loss observed below 100°C in raw SWCNTs (about 2wt. %) is related to the desorption of water or ungrafted molecules from SWCNTs surface. A more important weight loss is then observed in the temperature region between 250°C and 600°C, almost in the same proportion for both SWCNTs_PRG and SWCNTs_CuHCF. This temperature range corresponds to the elimination of PRG moieties [START_REF] Nicolle | Observing and predicting the preferential functionalization of metallic or semiconducting single-walled carbon nanotubes[END_REF]. Above 600°C, SWCNTs_PRG shows the same weight loss than pristine SWCNTs. However, a supplemental weight loss is observed in the case of SWCNTs_CuHCF on a large temperature range, which could be related to the thermal degradation of the organic (C) and inorganic (Cu, Fe, K and N) moieties found in CuHCF nanoparticles [START_REF] Lee | Sorption of cesium ions from aqueous solutions by multi-walled carbon nanotubes functionalized with copper ferrocyanide[END_REF]. Considering that the amount of PRG molecules and CuHCF-NPs grafted on carbon nanotubes is calculated based on the differential weight loss with regards to pristine SWCNTs, the PRG functionalization rate is estimated to 12wt. %, which corresponds to 1 PRG grafted molecule per 33 carbon atoms, and the proportion of CuHCF_NPs is estimated to about 15wt. %. These obtained grafting rates are in the same order as for several recently reported functionalized carbon nanotubes [START_REF] Kaper | Multi-method analysis of functionalized single-walled carbon nanotubes for cesium liquid-solid extraction[END_REF][START_REF] Vigolo | Evidence of sidewall covalent functionalization of single-walled carbon nanotubes and its advantages for composite processing[END_REF][START_REF] Munirasu | Functionalization of carbon materials using the Diels-Alder reaction[END_REF].

Raman spectroscopy

Raman spectra were recorded at room temperature on raw SWCNTs as well as on modified samples. In order to check the homogeneity of the functionalization, all treated samples were analyzed by Raman spectroscopy on 10 different regions. Very similar signals were found on each area, confirming the homogeneity of PRG and CuHCF_NPs grafting onto the SWCNTs surface.

For Raman analyses measured with an excitation wavelength of 633 nm, the spectra of functionalized samples shows distinct changes from that of raw SWCNTs (Fig. 4). The signals originating from SWCNTs Radial Breathing Modes (RBM) appear between 100 and 200 cm -1 depending on the tube diameter. We observe a strong decrease of the RBM intensity of modified samples (RBM band in Fig. 4). These changes are typical of chemical alteration of SWCNTs, and we indeed believe that the presence of PRG and CuHCF_NPs bound on SWCNTs surface impacts the normal breathing of the tubes. Similar effects have been reported by S. Hussain et al [START_REF] Hussain | Spectroscopic Investigation of Modified Single Wall Carbon Nanotube (SWCNTs)[END_REF], who have investigated modified SWCNTs and observed that all RBM intensities of treated samples decreased and disappeared, in relation with the duration of the acid treatment inflicted to SWCNTs. Also, R. Liu [START_REF] Liu | The Functionalization of Carbon Nanotubes[END_REF] found that 6 hours of sonication of SWCNTs causes a decrease of high wavenumber RBM intensities and explain it by the destruction of the tubes of small diameter or reduction in resonance enrichment of functionalized SWCNTs.

In the higher wavenumber area near ~1590cm -1 , we observe the tangential G modes of SWCNTs (G band in Fig. 4). With a 633nm excitation wavelength for Raman analyses we observe a Breit-Wigner-Fano (BWF) lineshape at around 1540cm -1 originating from the electron-phonon coupling in metallic nanotubes [START_REF] Piscanec | Optical phonons in carbon nanotubes: Kohn anomalies, Peierls distortions, and dynamic effects[END_REF]. The BWF signal completely disappears for SWCNTs_PRG as well as for SWCNTs_CuHCF. This disappearance indicates the loss of the metallicity of SWCNTs due to effective grafting [START_REF] Paillet | Vanishing of the Breit-Wigner-Fano component in individual single-wall carbon nanotubes[END_REF] leading to a weakening of the coupling. For the 532nm and 488nm excitation wavelengths (Fig. 5 and6), the resonance allows to probe the semiconductor nanotubes. Concerning the RBM region, a decrease of the peaks intensity is observed similarly to what has been described with the 633 nm excitation wavelength. This indicates that semiconducting tubes are also efficiently grafted. The G bands of the SWCNTs_PRG and SWCNTs_CuHCF evolve the same way whatever the excitation energy (G band in Fig. 5 and6). More precisely, the G band of SWCNTs_PRG shows a small shift of about 0.5 cm -1 towards low frequencies, which is attributed to the force constant modification between the carbon-carbon bonds following the PRG functionalization, which gives rise to an uniaxial or torsional strain [START_REF] Liu | Raman spectroscopy of strained single-walled carbon nanotubes[END_REF]. The G band of the SWCNTs_CuHCF sample exhibits a shift of about 2 cm -1 towards high frequencies attributed to a p-type doping brought by the HCF nanoparticles [START_REF] Forment-Aliaga | Strong p-Type Doping of Individual Carbon Nanotubes by Prussian Blue Functionalization[END_REF].

X-Ray Photoemission Spectroscopy

XPS is an efficient technique to check the presence of organic groups, to identify the grafted molecules on the surface of SWCNTs and to reveal if they are covalently bound to the carbon walls. On the XPS survey of raw SWCNTs (Fig. 7.a), we identify, together with the expected carbon species around 285 eV, oxygen (around 530 eV), arising either from contamination and/or surface defects. The binding energy (BE) values for carbon and oxygen groups were attributed based on the reported literature [START_REF] Ling | The effect of different order of purification treatment on the purity of multiwalled carbon nanotubes[END_REF][START_REF] Ramirez-Calera | Solvent-free functionalization of fullerene C60 and pristine multi-walled carbon nanotubes with aromatic amines[END_REF]. On the PRG functionalized SWCNTs survey (Fig. 7.b), in addition to carbon and oxygen, the presence of nitrogen is well identified, related to the presence of PRG grafted molecules. The growth of copper hexacyanoferrate-NPs K2CuFe(CN)6 is accompanied by identification of new components (Fig. 7.c) associated to the presence of the new elements which are potassium, copper and iron.

To better understand the nature of chemical bondings in the raw and functionalized samples, we performed a high resolution measurements. In the following, the number of contributions within the C1s peaks is kept constant, independently of the functionalization steps. Chemical modifications are thus evidenced by changes in peak FWHM. Comparing the C1s core level of raw SWCNTs with the PRG functionalized sample (Fig. 8.a and 8.b), we clearly observe modifications around 286.05eV (related to C-O or C-N bonds) [START_REF] Ramirez-Calera | Solvent-free functionalization of fullerene C60 and pristine multi-walled carbon nanotubes with aromatic amines[END_REF] especially in terms of FWHM (from 1 eV for raw SWCNTs to FWHM = 1.3 eV for SWCNTs_PRG) and atomic content. This evolution is accompanied with a decrease of sp2 bonds at 284,4eV and an increase of sp3 bonds at 285 eV. Indeed, the functionalization of SWCNTs with PRG is expected to result in an increase of the C-N bond contribution in XPS spectra, as well as in a simultaneous increase of the sp3 contribution and decrease of the sp2 contribution. The latter effect is linked to the cycloaddition mechanism between SWCNT and PRG. It appears here that the C-N bond contribution increase is larger than the intensity changes related to C-C sp2 and sp3 bonds. This observation could be explained by the fact that a part of PRG adsorbed onto SWCNTs despite the acetone washing after grafting. The typical BE for nitrogen atoms in cyanide groups in hexacyanoferrate can be found between 397.5eV and 400eV [START_REF] Datta | In situ FTIR and XPS studies of the hexacyanoferrate redox system[END_REF][START_REF] Malik | Formation of ultra-thin prussian blue layer on carbon steel that promotes adherence of hybrid polypyrrole based protective coating[END_REF][START_REF] Naumkin | NIST X-ray Photoelectron Spectroscopy Database[END_REF]. Figures 9.a and 9.b display the high resolution N1s spectra of the SWCNTs_PRG and SWCNTs_CuHCF samples. The most affected N1s signal following the growth of CuHCF_NPs is at 397.6 eV, which shows a drastic intensity increase, so that we assign this signal to C≡N bonds in CuHCF_NPs. Thus, the nitrogen N1s is deconvoluted into four peaks involving C≡N (397.6 eV), C-NH2 (399eV), N-C=O (400.2eV) and N + (401.8eV) bonds [START_REF] Jia-Lin | Functionalized graphene oxide with ethlenediamine and 1, 6-hexanediamine[END_REF][START_REF] Ren | Raman, and XPS analysis during phosphate, nitrate and Cr(VI) removal by amine cross-linking biosorbent[END_REF].

In the Cu2p and Fe2p XPS core level spectra of CuHFC functionalized SWCNT (Fig. 9.c and d), we identify the presence of a bond at 932.7eV corresponding to Cu2p, and a bond at 708.31eV attributed to Fe2p [START_REF] Biesinger | Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Sc, Ti, V, Cu and Zn[END_REF]. Biesinger et al [START_REF] Biesinger | Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Sc, Ti, V, Cu and Zn[END_REF] have analysed different types of copper samples such as Cu metal and Cu2O and found the same deconvolution profile of Cu2p as in our sample. They have attributed the bond at lower energy to Cu + and the one at higher energy to Cu 2+ . In these compounds, Cu2+ is generally expected [START_REF] Wessells | Copper hexacyanoferrate battery electrodes with long cycle life and high power[END_REF][START_REF] Lisowska-Oleksiak | Organic-inorganic composites consisted of poly(3,4-ethylenedioxythiophene) and Prussian Blue analogues[END_REF] but a reversible solid-state reduction to Cu + is sometimes reported [START_REF] Oktawian | Electrochemical identity of copper hexacyanoferrate in the solid-state: evidence for the presence and redox activity of both iron and copper ionic sites[END_REF].

It is worth mentioning that the ratio Cu + /Cu 2+ is strongly dependent on the area under investigation in our samples. As we do not believe in a sample heterogeneity, we assume a reduction under x-ray beam, leading to a transformation of Cu 2+ into Cu + . This point will be accurately studied in the next investigations.

Cs sorption batch studies

The SWCNTs are used as CuHCF functionalized substrates for cesium capture. To evaluate the rate of Cs + sorption on raw and functionalized SWCNTs, we performed sorption experiments by exposing the samples to CsNO3 solution. We have firstly characterized samples exposed to the cesium solution by XPS analysis in order to reveal the presence of cesium and to check the evolution of the sorption capacity versus Cs + concentration. The full surveys of unexposed CuHCF functionalized SWCNTs and of CuHCF functionalized SWCNTs after exposure to cesium nitrate solutions of different concentrations are presented in Fig. 10. The signal of the Cs3p3/2 and Cs3d5/2 are detected respectively at BE ~997eV and ~724eV (Fig. 10). The intensity of each of these peaks increases with the increase of the cesium nitrate concentration (inset in Fig. 10). On the other hand, we note, as expected, the disappearance of the signal coming from potassium upon cesium exposure, which is in accordance with the exchange between Cs + and K + ions initially contained in CuHCF_NPs. In order to go further in the understanding of the sorption mechanism in this sample, we performed LPIC analyses of the solution before and after exposure to SWCNTs_CuHCF.

We measured the sorption isotherm of both raw and CuHCF functionalized SWCNTs samples from LPIC of the solutions, using the sorption capacity Φ 𝑎𝑑𝑠 (mg.g -1 ) defined as:

Φ 𝑎𝑑𝑠 = (𝐶 𝑖 -𝐶 𝑓 ) 𝑉 𝑚
Ci is the initial mass concentration (mg.L -1 ) of Cs + in the CsNO3 solution. Cf is the final mass concentration (mg.L -1 ) of Cs + in the CsNO3 solution, V is the volume of the CsNO3 solution (L) and m is the substrate mass (g). The sorption isotherm of Cs + on raw and CuHCF functionalized SWCNTs are illustrated in Fig. 11. Both raw and CuHCF functionalized SWCNTs show an increasing sorption capacity with Cs + concentration, though it is always lower for raw SWCNTs (up to 150 mg.g -1 for raw SWCNTs and to 230 mg.g -1 for CuHCF_SWCNTs). A coarse way of evaluating the proportion of Cs + ions complexed in CuHCF_NPs consists in subtracting the sorption isotherm of raw SWCNTs to that of SWCNTs_CuHCF. Therefore, the subtraction result (squares in Fig. 11) should correspond to the Cs selective part of the sorption isotherm. We found that the selective sorption of Cs reaches a plateau at a solution concentration of 4mM, whose value is of about 80mg.g -1 .

It is very interesting to remark that this value compares very well with the value obtained on the basis of XPS measurements. The fact that only the Cs fraction contained in the CuHCF_NPs is detected by XPS could be related to the degasing needed by the latter technique, which could be responsible for the removal of Cs ions bound to the SWCNT surface, and not intercalated inside CuHCF_NPs. This sorption capacity can be compared with that of other materials dedicated to Cs removal from aqueous effluents. MWCNTs-reinforced zeolite-A beads have been developed for the removal of cesium and strontium from water and were studied in a recent work showing that the maximum theoretical sorption capacity for cesium was 113mg.g -1 [START_REF] Vipin | Removal of Cs + and Sr2 + from water using MWCNT reinforced Zeolite-A beads[END_REF]. Oxidized MWCNTs were also used for cesium capture and it have been demonstrated that they can sorb a maximum value of about 10mg.g -1 . This low sorption capacity is related to their multiwall structure that limits their specific surface and therefore reduces the sorption capacity [START_REF] Yavari | Adsorption of cesium(I) drom aqueous solution using oxidized multiwall carbon nanotube[END_REF]. Other studies showed that SWCNTs buckypapers can be functionalized with DMAD and are able to sorb about 250mg.g -1 , but DMAD molecules are not selective of Cs [START_REF] Kaper | Multi-method analysis of functionalized single-walled carbon nanotubes for cesium liquid-solid extraction[END_REF]. The sorption capacity found for SWCNTs_CuHCF substrates is therefore very encouraging. For pure CuHCF, a maximum Cs sorption capacity of 420 mg.g -1 has been determined [START_REF] Omura | Cs + trapping in size-controlled nanopaves of hexacyanoferrates[END_REF]. Provided that the mass content of CuHCF_NPs is 15 wt.%, we can conclude that all available sites in our CuHCF_NPs are occupied by Cs + ions. This is in very good agreement with the disappearance of the K peak in the XPS survey.

Conclusion

We reported on the elaboration and characterization of new sorbent materials for the solidliquid extraction of Cs + ions. Propargylamine molecules are first grafted onto the SWCNT surface and act as a linker for the growth of CuHCF nanoparticles. A high grafting rate of PRG (1 molecule for 30 carbon atoms) results in 15 wt.% of CuHCF nanoparticles in the end of the elaboration process. Based on a careful analysis combining XPS of the materials after Cs exposure, and LPIC of the solutions before and after exposure, it was possible to determine a total sorption capacity of 230 mg.g -1 of Cs + ions, among which 80 mg.g -1 are likely to be selectively incorporated inside CuHCF_NPs. These results compare nicely with existing solid substrates dedicated to Cs capture, and open interesting perspectives towards their integration inside electrodriven devices for the controlled sorption/desorption of Cs + ions.

Figure 1 :

 1 Figure 1: Schematic representations of a) SWCNT functionalized with propargylamine, b) SWCNT functionalized with a HCF-NP, and c) Exchange mechanism between K + and Cs + ion.

Figure 2 :

 2 Figure 2: HRTEM images of raw SWCNT at low and high magnification (a and b, respectively) and SWCNTs-CuHCF (c). Black squares and red squares magnify additional features with regards to raw SWCNTs.

Figure 3 :

 3 Figure 3: TGA measured under argon atmosphere of a) raw SWCNT; b) SWCNTs_PRG and c) SWCNTs_CuHCF

Figure 4 :

 4 Figure 4: Raman spectrum (=633nm) of a) Raw SWCNTs, b) SWCNTs_PRG, and c) SWCNTs_CuHCF. The curves have been shifted in height for clarity.

Figure 5 :

 5 Figure 5: Raman spectrum (=488nm) of a) Raw SWCNTs, b) SWCNTs_PRG, and c) SWCNTs_CuHCF. The curves have been shifted in height for clarity.

Figure 6 :

 6 Figure 6: Raman spectrum (=532nm) of a) Raw SWCNTs, b) SWCNTs_PRG, and c) SWCNTs_CuHCF. The curves have been shifted in height for clarity.

Figure 7 :

 7 Figure 7: XPS surveys of a) Raw SWCNTs, b) SWCNTs_PRG and c) SWCNTs_CuHFC,

For

  CuHCF functionnalized SWCNTs (figure8.c), the signal around 286 eV related to C-N bonds decreases compared to SWCNTs_PRG from ~14 % to 10.6 %. Considering that the sample is washed again after the total functionalization, i.e. after the growth of hexacyanoferrate nanoparticles, PRG-CuHCF particles not covalently bound to SWCNTs could have been removed during this final washing step.

Figure 8 :

 8 Figure 8: XPS C1s core level spectra of a) raw SWCNTs, b) SWCNTs_PRG and c) SWCNTs_CuHCF.

Figure 9 :

 9 Figure 9: XPS N1s core level spectra of a) SWCNTs_PRG and b) SWCNTs_CuHCF c) XPS Cu2p core level spectra of CuHCF_SWCNTs, d) XPS Fe2pcore level spectra of CuHCF_SWCNTs

Figure 10 :

 10 Figure 10: XPS surveys of a) SWCNTs_CuHCF, b) SWCNTs_CuHCF_Cs [0.1mM], c) SWCNTs_CuHCF_Cs [2mM], and d)SWCNTs_CuHCF_Cs [4mM] (inset: XPS of Cs3d core levels).

Figure 11 :

 11 Figure 11: Sorption isotherm of Cs + ions on: raw SWCNTs and SWCNTs_CuHCF. Cs + ions selectively sorbed inside CuHCF_NPs.

  -10 Torr. Binding energies (BE) of all core levels are referred to the C=C of C1s carbon at 284.4eV. The functionalization rate is deduced by thermogravimetric analyses (TGA), which are performed on a Setaram SETSYS evolution TGA/TDA 16 under Argon atmosphere with a heating rate of 5°C/min. Raman scattering experiments are carried out on a Horiba Jobin Yvon LabRAM Aramis spectrometer. Two excitation laser wavelengths are used: (i) = 633nm with a spot size of 1m, a x100 objective, the recording time is set equal to 2X30s and power is kept at 610W, (ii) = 532nm with a spot size of 1m, a x50 objective, the power is kept at 475W and the recording time is set equal to 2X50s. Other Raman experiments are performed using a T64000 Spectrometer Raman Jobin Yvon, where the excitation laser wavelength is 488nm with a x50 objective, the power is kept at 620W and the recording time is set equal to 2X40s.

Transmission electron microscopy (TEM) is performed on a JEOL 2200FS. Samples are deposited on 400-mesh holey carbon-coated copper grids. Liquid phase ionic chromatography (LPIC) is performed using an ICS-5000-Dionex instrument with 4 scans for each sample.

table 1

 1

	):

Table 1 :

 1 Elemental ID and Quantification of C1s core level of raw SWCNTs_ according to the XPS measurement

Table 2 :

 2 Elemental ID and Quantification of C1s core level of SWCNTs_PRG_ according to the XPS measurements

		BE (eV)	FWHM (eV)	Assignment	Atomic %	Ref
	Peak 1	283.3	0.9	Vacancies	1.1	[25-26]
	Peak 2	284.4	0.9	C=C	64.1	[24-25]
	Peak 3	285.3	1.1	C-C/C-H	16.1	[24-25]
	Peak 4	286.2	1.2	C-O	10.6	[24-25]
	Peak 5	287.4	1.5	C=O	3.4	[24-25]
	Peak 6	289.2	2.6	COOH	4.7	[24-25]

Table 3 :

 3 Elemental ID and Quantification of C1s core level of SWCNTs_CuHCF_ according to the XPS measurements

Acknowledgements

The authors acknowledge financial support from XXX and the assistance from the Balard platform in Montpellier. Support financier: CEA, Université de Tunis et L2C Discussions scientifiques et assistance technique : Raman : Alban Jonchere, David Maurin LPIC : Cyrielle Rey MET : Henri Pierre Brau et Xavier Le Goff


Yasmine M Hajar

Laureline Treps

Carine Michel

Elena A Baranova

Stephan N Steinmann
email: stephan.steinmann@ens-lyon.fr

Theoretical insight into the origin of the electrochemical promotion of ethylene oxidation on ruthenium oxide

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

Introduction

Metal-support interaction (MSI) and alkali metal doping are two methods that tune a catalyst performance through the modification of the surface's electronic structure, and concomitantly, the binding energy of the chemisorbed species [START_REF] Boudart | Kinetics of heterogeneous catalytic reactions[END_REF][START_REF] Campbell | Catalysis at Surfaces[END_REF] . Alternatively, based on the work of Wagner [START_REF] Wagner | [END_REF] , Vayenas and co-workers progressed in working on the effect of an electrochemical supply or removal of ions to or from a catalyst deposited on solid ionically-conducting electrolytes, e.g., Y2O3doped ZrO2 (YSZ, O 2-conductor), BaZr0.8Y0.2O3-δ (BZY , H + conductor), β"-Al2O3 (K + conductor), etc. [4][5][6][START_REF] Wieckowski | Catalysis and Electrocatalysis at Nanoparticle Surfaces[END_REF] . They demonstrated a reversible alteration of catalytic activity and in some cases selectivity when an electric current or potential is applied [START_REF] Vayenas | Electrochemical Activation of Catalysis: Please do not adjust margins Promotion, Electrochemical Promotion, and Metal-Support Interactions[END_REF][START_REF] Vayenas | [END_REF][10] . This phenomenon, called Electrochemical Promotion of Catalysis (EPOC), or Non-Faradaic Electrochemical Modification of Catalysis (NEMCA), has been shown to be functionally equivalent to MSI effect between a dispersed catalyst phase and conducting supports (TiO2, CeO2, ZrO2, etc.) but is operationally more controlled, as it allows an in-operando modification of the electronic properties of catalysts [START_REF] Vayenas | Electrochemical Activation of Catalysis: Please do not adjust margins Promotion, Electrochemical Promotion, and Metal-Support Interactions[END_REF] . Our recent paper has demonstrated the functional equivalency between EPOC and MSI as we have compared the promotive effect of supporting nanoparticles on metal oxides to electrochemically promoting the nanoparticles. The rates increase due to EPOC and supporting the catalysts on active metal oxides were similar in the case of platinum and ruthenium catalysts 11 . Furthermore, many highly dispersed electrochemically promoted systems have been established, rendering the practical application of EPOC more attractive 11,12,[START_REF] Katsaounis | [END_REF][22][START_REF] Vernoux | Recent advances in electrochemical promotion of catalysis[END_REF][START_REF] González-Cobos | [END_REF][13][14][15][16][17][18][19][START_REF] Dole | Handbook of Nanoelectrochemistry[END_REF] . Experimentally, the origin of EPOC is mostly explained by the electrochemically induced promoters (see Figure 1), which have been substantiated by many in-situ and ex-situ techniques: x-ray photoelectron spectroscopy (XPS) 25,26 , in-situ XPS 27 , temperatureprogrammed desorption (TPD) 28,29 , scanning tunneling microscopy (STM), photoemission spectroscopy (PES) 30 , in-situ AC impedance spectroscopy 31,32 , and isotopic exchange 33 . When applying an electrochemical potential across a solid electrolyte on which a metallic catalyst is adsorbed, ions originating from the solid electrolyte, O δ-in case of YSZ, are (partially) discharged at the three-phase boundary and migrate to the gas exposed catalyst surface to formally form a double layer O δ--δ + (δ + is the mirror charge in the conductor for preserving the neutrality). Figure 1 demonstrates the oxygen migration in the case of a continuous catalyst film similar to what has been used in previous papers [34][35][36] . It has been determined that in the presence of oxygen ions there is a oneto-one correlation between the applied potential (UWR) and the change in work function (ΔΦ) at the metal-gas interface in a wide range of temperatures 6,37 (Eq. 1): eΔUWR = ΔΦ Eq. 1

Therefore, the density of this double layer and subsequently the work function of the catalyst are in-operando controlled by an electrochemical potential application. The change of work function leads to catalytic rate and selectivity alternation due to Please do not adjust margins Please do not adjust margins the change in the adsorption properties of reactants and/or intermediates. Since the coverage effect is experimentally detectable, most of EPOC is rationalized in terms of an increased/decreased availability of the promoter on the surface, suggesting a direct implication of the promoter.

However, these experiments do not directly observe the reactive surface species and the promoter might just generate an electrostatic field, which itself impacts the kinetics of the reaction 38,39 . Therefore, computational mechanistic studies are complementary to the experimental characterizations. Early theoretical modeling of EPOC was performed using clusters of metallic atoms to model the Cl adsorption on Cu and O adsorption on Pt surface 40,41 . In the case of co-adsorption of Cl and benzene on the surface of Cu, Pacchioni et al. showed that Cl induces an increase in the Cu work function and therefore an increase in the adsorption energy of the benzene product resulting in an inhibition of the acetylene trimerization reaction 40 . In the case of O adsorption on Pt, the authors drew a correlation between the increase in the catalyst-electrode potential and corresponding work function and the decrease in the experimental peak temperature of adsorbed oxygen 41 . However, they showed subsequently that the oxygen desorption temperature is not directly correlated to the oxygen desorption energy, concluding that the effect of the change in work function on the bonding of adsorbed molecules (reactants and products) depends on the nature of the surface's chemical bond 40 . Later, Leiva et al., have investigated the effect of the positive charge of Na on the work function of the Pt surface and compared it to when oxygen was jointly present 42 .

In more recent theoretical modeling studies, the effect of external and internal electric field on the activity of catalysts was investigated with correspondence to solid oxide fuel and electrolysis cells 38,39,[42][43][44][45][46] . A cluster model and hybrid density functional theory (DFT) calculations were used by González et al. to simulate the electric field effect on the adsorption of CO on Cu, Rh and their bimetallics 44 . They concluded that the CO bond increases in strength with a positive electric field. Chen et al. used a dipole layer to model the electric field effect on the Ni/YSZ interface 43 . They found that a positive potential increases the activation energy of oxygen vacancy formation, therefore reducing the deactivation mechanism of the catalystelectrode at the three-phase boundary. In another work, the activation energy of C-H dissociation in methane was found to increase as a function of the field strength resulting in a decrease of the methane steam reforming reaction 38 .

Herein, we rely on the model reaction of ethylene oxidation 31,32,[34][35][36][47][48][49][50] to demonstrate, using ab-initio computations, the effect of electrochemical potential on the surface reactivity. We will elucidate on the one hand the impact of the electrochemical potential on the surface state compared to thermal catalysis and on the other hand, the impact of the potential generated electric field on the reactivity of ethylene and oxygen activation. To be more precise, the most stable surface oxygen termination is determined as a function of the electrochemical potential. For a given surface state, the adsorption and dissociation energies of gaseous ethylene and oxygen are dependent on the potential-generated electric field, which is modeled through modification of the total surface charge.

Theory and Computational Method

Slab model and computational details

Plane wave DFT calculations were performed using projector augmented wave pseudopotentials provided in VASP (Vienna Ab-initio Simulation Package) 51,52 . The energy evaluation exploited the generalized gradient approximation (GGA) with a Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional together with the density-dependent dispersion correction dDsC 53,54 . Additional computations at the Heyd-Scuseria-Ernzerhof (HSE) level of theory of the work function of the three main surfaces of RuO2 show that the PBE work function is in excellent agreement with experimental values (5.8 to 6.6 V) 55 , while the HSE results are 0.5 V higher compared to PBE and reach 7.1 V for the most oxygenated surface (Figure S17). Nevertheless, even the 0.5 V difference between HSE and PBE is small enough to ensure that the qualitative mechanistic insight gained in this study does not significantly depend on the level of theory.

In the bulk rutile structure of RuO2, the Ru atoms are six-fold coordinated to oxygen atoms, while O atoms are coordinated to three Ru atoms in a planar sp 2 hybridization 56 . The surface Please do not adjust margins Please do not adjust margins energies of the bulk-truncated, bridging-oxygen-terminated RuO2 (110), RuO2 (100), and RuO2 (101) surfaces were determined by previous DFT calculations and the (110) facet shown to be the most stable one and thus prevailing for polycrystalline RuO2 57 . On the RuO2 (110) surface, a weakly bound oxygen species can be found to adsorb on-top (O ot ) of the otherwise five-fold coordinatively unsaturated Ru (5f-cus-Ru), while stronger bridging oxygen (O br ) atoms are found on the six-fold coordinatively saturated Ru sites (6f-cus-Ru) 58 .

The slab used to model RuO2 (110) rutile 59 film is made out of three layers of Ru with intermediate O resulting in a ~10 Å thick slab. The intermediate layer is fixed, but all other atoms are allowed to relax during the computation until the forces are less than 0.03 eV/Å. A vacuum spacing of 15 Å was included to reduce the periodic interaction in the surface normal direction. The effect of vacuum spacing is shown in Figure S 9. In terms of unit size, a 6.2058 x 6.2897 x 25.1387 Å 3 cell with a corresponding 6 x 6 x 1 Monkhorst-Pack k-point mesh is used. The highest frequency of the plane wave basis set corresponds to a cut-off energy of 550 eV (Figure S 10) and the electronic energy is converged to reach a threshold of 10 -6 eV. Isolated molecules are optimized using the same parameters at the gamma point using a cubic box of (8000 Å) [START_REF] Wagner | [END_REF] . The electrochemical promotion effect on the catalyst slab was modeled by adding the linearized Poisson-Boltzmann equation 60,61 to the Kohn-Sham equations using the VASPsol module 62 . This method, also known as surface charging 63 or grandcanonical DFT [64][65][66][67] , has been validated for the adsorption of pyridine on Au (111) by Steinmann et al. 68 . The implicit solvation model places a quantum-mechanical solute in a cavity surrounded by a continuum dielectric description of the solvent and counter-charge distribution, allowing to mimic the effective electrochemical double layer 69,70 . Such a surface polarization by modifying the surface charge also introduces an electric field as analyzed by us in a previous work 71 . The use of symmetric slabs together with the linearized Poisson-Boltzmann equation allows to unambiguously reference the Fermi-level to the potential far away from the surface, assumed to correspond to vacuum. The effect of the addition of charges on the electrochemical potential (i.e., work function) for different systems is shown in Figure S 11. The diffuse counter-charge added through the linearized Poisson-Boltzmann equation to neutralize the system is a proxy to model the effect of smoothly varying oxygen coverages, in analogy to our previous work on the indirect effects of bases in heterogeneous catalysis 71,72 . In EPOC there is no solvent present, just an effective double layer generated, in our case, by the varying amount of oxygen species. Hence, we have carefully assessed the influence of the relative permittivity of the "solvent" and the cavity size (Figure S 12 and S13). Both have been found to have a negligible effect on the surface free energies of the three main surface terminations (2O br /2O ot , 2O br /2Ru, and 2Ru/2Ru). We conclude, therefore, that the obtained results do not significantly depend on the choice of technical parameters that have no clear meaning in EPOC and that the surface charging in combination with the linearized Poisson-Boltzmann equation is a convenient and efficient computational model to investigate EPOC.

To find the minimum energy pathway between adsorption of ethylene and its consecutive dissociation, transition state computation was performed using the Nudged Elastic Band method in combination with Dimer method and Quasi-Newton optimization. The obtained structure was verified to yield a single imaginary frequency consistent with the reaction path under study using vibrational frequency analysis.

Ab-initio atomistic thermodynamics including the electrochemical potential

Thermodynamic stability of a surface can be derived by comparing a multitude of surface structures and their corresponding surface free energy (SFE) 𝛾"𝑇, 𝑃 & ' ( (Eq. 2). The surface with the lowest SFE is the most stable structure. Following the work of Reuter et al. 73 , two main variables that are required for the calculation of RuO2 (110) surface free energy are temperature (T) and oxygen partial pressure

(𝑃 & ' ). 𝛾"𝑇, 𝑃 & ' , 𝑁 ,-, 𝑁 & ( = 1 2𝑆 2 𝐸 ,-& ' 4567 ( 𝑁 ,-, 𝑁 & ) -𝑁 ,-𝐸 ,-& ' 7-5: +(2 𝑁 ,--𝑁 & ) 1 2 <𝜇 & ' > "𝑇, 𝑃 & ' > ( + 𝑅𝑇𝑙𝑛 𝑃 & ' 𝑃 > B C Eq. 2
Where 2S serves to normalize 𝛾 by the two surfaces S of the symmetrical unit cell. The Gibb's energy of the bulk is taken equal to the electronic energy computed for the primitive cell of the bulk 𝐸 ,-& ' 7-5: , while the free energy of the surface is approximated by the electronic energy 𝐸 ,-& ' 4567 . NRu and NO are the numbers of Ru and O atoms in the supercell, while 𝑃 > is the standard pressure. The determination of the standard chemical potential of oxygen 𝜇 & ' > and the detailed derivations are given in section 4 of the ESI. In equation 2, the oxygen supply is gaseous, whereas, in EPOC conditions, the oxygen is supplied in its ionic form from the oxygen-conducting electrolyte material as follows:

𝑂 EF (𝑌𝑆𝑍) + 𝑅𝑢𝑂 E → 𝑂@𝑅𝑢𝑂 E + 2𝑒 F Eq. 3

It has been established that the equilibrium potential of SOE with respect to vacuum is 5.14 V 74 , whereas that of SHE is 4.44 V 75 . Therefore the equilibrium potential of SHE with respect to SOE is equal to -0.7 V.

As shown in the ESI, the chemical potential of oxygen anions 𝜇 & 'M is linked with the chemical potential of oxygen from the gas phase and the applied potential, so that the surface free energy becomes: Please do not adjust margins Please do not adjust margins

𝛾"𝑇, 𝑃 & ' , 𝑁 ,-, 𝑁 & , 𝑈 6O>PQ/STU ( = 𝟏 𝟐𝑺 ⎩ ⎪ ⎪ ⎨ ⎪ ⎪ ⎧ 𝑬 𝑹𝒖𝑶 𝟐 𝒔𝒍𝒂𝒃 "𝑵 𝑹𝒖 ,

Adsorption energy calculation including electrochemical potential

The adsorption energy on a symmetrical RuO2 surface adsorbing one C2H4 from each side is expressed by: 𝑬 𝒂𝒅𝒔 = 𝟏 𝟐 {𝑬 𝟐𝑪 𝟐 𝑯 𝟒 /𝒔𝒖𝒓𝒇 -"𝟐 × 𝑬 𝑪 𝟐 𝑯 𝟒 + 𝑬 𝒔𝒖𝒓𝒇 (• Eq. 5

Where 𝐸 E' ' T ƒ /4-"… is the energy of the state containing two symmetrically adsorbed C2H4, 𝐸 4-"… is the energy of the bare surface, and 𝐸 ' ' T ƒ is the energy of an isolated C2H4 molecule in the gas phase. From the equation above, a more negative value for the adsorption energy indicates a more stable adsorbed C2H4 on the RuO2 (110) surface. The adsorption is performed symmetrically on both surfaces to avoid spurious dipole interactions. The C2H4 coverage corresponded to 25 % of the Ru density on the surface.

The presence of an electrochemical potential is likely to affect the electronic energy of the surface and of the ethylene adsorbed on the surface, resulting in a change of the adsorption energy of molecule as follows:

𝑬 𝒂𝒅𝒔 (𝑼 𝒂𝒏𝒐𝒅𝒆/𝑺𝑯𝑬 ) = 𝟏 𝟐 † 𝑬 𝟐𝑪 𝟐 𝑯 𝟒 /𝒔𝒖𝒓𝒇 "𝑼 𝒂𝒏𝒐𝒅𝒆/𝑺𝑯𝑬 ( - ‡𝟐 × 𝑬 𝑪 𝟐 𝑯 𝟒 (𝒈) + 𝑬 𝒔𝒖𝒓𝒇 (𝑼 𝒂𝒏𝒐𝒅𝒆/𝑺𝑯𝑬 )‰ Š Eq. 6
As explained in section 2.1, the VASPsol module allows the modification of the surface charge (and thus the work function, i.e., the electrochemical potential) of the slab and thus mimicking the effect of the effective double layer. Performing geometry optimisations at various charges, we can fit the relation between the electronic energy and the electrochemical potential for the bare slab, 𝑬 𝒔𝒖𝒓𝒇 , and for the one containing two symmetrically adsorbed C2H4, 𝑬 𝟐𝑪 𝟐 𝑯 𝟒 /𝒔𝒖𝒓𝒇 . Then, the dependence of the adsorption energy upon the electrochemical potential is trivially derived. The procedure is illustrated in Figure S15 and is analogously applied for all adsorbed species, including transition states, as illustrated in Figure S16.

Experimental

Synthesis of RuO2 nanoparticles

Ruthenium oxide (RuOx) nanoparticles were synthesized using the polyol synthesis method 76 . In short: 0.105 g of Ruthenium (III) chloride (Sigma Aldrich Ò ) precursor salt was dissolved in 15 mL of ethylene glycol (Fisher Scientific Ò ) containing 0.25 M tetra-methyl ammonia hydroxide (TMAH) (Sigma Aldrich Ò ). The solution was pre-mixed for an hour in a round-bottom flask then heated up to 160°C under reflux and cooled down directly after the temperature was reached. The initial pH of the synthesis solution was 12 and dropped to 7 after the reflux indicating nanoparticles formation 76 .

Physiochemical characterization

The Scanning Transmission Electron Microscopy (STEM) micrographs of ruthenium oxide nanoparticles were performed using FEI Titan3 80 -300 microscope operated at 300 kV and equipped with an energy dispersive X-ray (EDX) spectrometer. Annular Dark Field (ADF) images were taken using a Fishione detector to provide a contrast between the nanoparticles and the support at convergence and collection angles of 17 and 60 mrad, respectively. The RuO2 colloidal solution was diluted and sonicated in ethanol then deposited dropwise on a 200 mesh copper grid coated with a lacy carbon support film (Ted Pella). ImageJ Ò software was used to find the distribution of the ruthenium oxides RuOx nanoparticles size. Figure 2a shows a representative STEM of the free-standing nanoparticles of ruthenium oxide. The ADF image represents the nanoparticles as bright spots on the darker background. The particle size distribution is shown in the histogram in Figure 2b and the average particle size was calculated to be 0.9 nm.

Electrochemical cell and reactor

Colloidal solution of ruthenium oxide nanoparticles was deposited 10 µL at a time over the yttria-stabilized zirconia (YSZ) solid electrolyte (1.9 cm 2 ), with intermittent drying at 100 o C. The final metal loading was 0.1 mg of RuOx covering 1 cm 2 surface area of the YSZ electrolyte. On the other side of the electrolyte, the catalytically inert gold counter electrode (CE) and reference electrode (RE) were deposited using commercial gold paste (Gwent group) prior to the deposition of RuOx catalyst. The surface area of CE and RE was 1 and 0.2 cm 2 , respectively. Catalytic measurements were carried out at atmospheric pressure in the single-chamber capsule described in our previous work such as all electrodes are under the same gaseous conditions 77,78 . The working electrode side of the electrolyte was pressed against a gold mesh (1 cm 2 ) that served as a current collector, while the counter and reference electrodes were pressed directly against gold wires 79 . Two type K thermocouples (Omega Ò ) were placed in the vicinity of the Please do not adjust margins Please do not adjust margins electrochemical cell, one for temperature control and one for data acquisition. A schematic of the system is shown in Figure S1 and S2. The reaction gases were ethylene (Linde, 0.5% C2H4 in He), oxygen (Linde, 20% O2 in He), and helium (Linde, 99.999% He) as a carrier gas. The total flow rate was constant at 100 mL min -1 . Gas composition was varied using MKS, 1259-C and 1261-C series flow meters and detected using a non-dispersive infrared (NDIR) CO2 gas analyzer (Horiba, VA-3000). A potentiostat-galvanostat (Arbin Instruments Ò , MSTAT) was connected to the electrodes to apply constant electric current or potential on the electrochemical cell (Figure S2).

Results and discussion

Electrochemical promotion of RuOx for complete ethylene oxidation reaction

Figure 3a shows the evolution of the reaction rate of the RuO2 catalyst during a potentiostatic step of 0.5 V. At t=0.5 h, the constant potential was applied and resulted in a 30% increase in the reaction rate, reaching a new constant value over a duration of two hours. At potential interruption (t=2.5h), the reaction rate decreased to its initial steady-state value within an hour. The ratio of the closed-circuit rate r over the open-circuit rate ro is denoted as the rate enhancement ratio, 𝜌 (Eq. 7).

𝝆 = 𝒓 𝒓 𝒐

Eq. 7

Another determining parameter of the EPOC effect is the apparent Faradaic efficiency Λ:

𝜦 = 𝜟𝒓 𝑰/𝒏𝑭 Eq. 8
Where Δr is the difference between the closed and open circuit reaction rate (Δr = rro), I is the applied current, n is the number of electrons transferred (2 for O 2-), F is the Faraday constant. The ρ and Λ values in Figure 3a are 1.23 and 6, respectively.

Figure 3b shows the reaction rate change under a negative galvanostatic step of -100 µA. The reaction rate of ethylene oxidation showed an enhancement in its value under the cathodic polarization as well reaching a 30% increase in the catalytic rate with Λ of -6. This increase in the rate under negative polarization was not detected previously except under fuel-rich conditions 34 . Therefore, this could be related to the contamination of previous catalysts with Na which inhibited any promotional effect at the cathodic counterpart 11 or due to the morphology obtained in the current sodium-free catalyst which has an average particle size below 1 nm. Finally, multiple step-change galvanostatic tests were carried under cathodic and anodic polarization and are summarized in Figure 3c showing the effect of the applied current on the catalytic rate increase and the corresponding measured catalyst-electrode This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins potential. The increase in the rate under cathodic and anodic polarization shows that the change in the work function in both directions result in an enhancement of the catalytic rate. However, anodic polarization had a stronger effect on the rate than cathodic polarization. A maximum L factor of 92 at an applied current of 0.5 µA was reached and the highest increase of the catalytic rate by 41% was found at +50 µA.

DFT modeling of RuO2 (110) rutile structure and ethylene adsorption

RuO2 (110) rutile structure

In the RuO2 rutile bulk 73,80 every metal atom is coordinated with six oxygens and every oxygen links to three metal neighbors. The oxygen bonds to Ru are not all equivalent and can be distinguished into four in-plane ones of 1.97 Å length and two orthogonal ones of 1.92 Å length. The nomenclature for the oxygen on the surface will be O ot for oxygen directly on top of Ru and O br for oxygen bridging between two ruthenium atoms. Ru 4,5,6f nomenclature will be used to refer to the number of coordination such that sixfold, 6f coordination is for Ru saturated with six bonds of oxygen, 5f is for Ru missing O ot coordination and 4f is for Ru missing two O br coordination. This nomenclature is similar to Reuter et al. 73 and differs from Over et al. 58 where 1f and 2f nomenclature was used for Ru coordinated with O ot and O br , respectively. As to the surface calculations, the representative unit cell used was chosen to be a nearly square p(2x2) super-cell with three layers of Ru in the z-direction. The middle layer was fixed depicting the bulk whereas the remaining ones were free to move

Prediction of RuO2 (110) termination

Under open-circuit conditions, the most stable surface termination was identified using the minimal surface free energy (SFE) approach. The oxygen chemical potential 𝜇 & ' effect on the SFE of RuO2 slab truncated at different oxygen positions was compared to that found in Reuter et al. as shown in Figure S 5 and the most stable surface is presented as a function of temperature and oxygen partial pressure. It was found that the surface termination with 2O br /2O ot was the most stable at the considered temperatures and oxygen partial pressures. Only at P(O2) as small as 10 -10 kPa, the 2O br /2Ru surface becomes the most stable above a temperature of 500 o C. The energy of a reduced surface (with 2Ru/2Ru termination) was unstable in comparison, which rules it out as a stable surface under open-circuit potential conditions. SFE calculation for RuO2 (110) that has a 2Ru/2O ot termination without O br atoms was compared as an alternative form of a stoichiometric surface. However, it was found to be less stable than the other surfaces under all conditions.

Effect of potential on surface free energy

The previous section showed that RuO2 (110) with 2O br /2O ot termination is the most stable structure under open-circuit conditions. In this section, we introduce the effect of the application of an electrochemical potential. The change in number of electrons along with a diffuse counter-charge layer added through the linearized Poisson-Boltzmann equation allows to mimic implicitly an intermediate effective double layer Please do not adjust margins Please do not adjust margins of oxygen ions. The resulting effective double layer affects the surface free energy and the RuO2 surface termination (Eq. 4). At a partial pressure of oxygen equal to 1 kPa (used as a lower limit for experimental conditions, see Figure S7 for partial pressure effect), the effect of temperature and electrode potential on the most stable surface structure is depicted in Figure 5.

At the experimental temperature of 350 o C, it can be seen that structure B (2O br /2O ot ) is the most stable surface termination in the electrode potential range of -0.5 to +0.15 V vs SOE. At the cathodic side, as the temperature increases, the oxygen chemical potential decreases, which results in stabilizing the surface with less oxygen. At 350 o C, the stable surface termination transitions from structure C (2O br /1O ot Ru) to structure D (2O br /2Ru) as the potential decreases. At the extreme cathodic polarization (under -1.2 V vs SOE), which is outside the experimental range of +/-0. 8 V vs SOE (-0.1 -1.5 V vs SHE) (see Figure 3), the 2Ru/2Ru surface termination becomes the most stable. At the positive side of the potential range, SFE calculations demonstrated that an additional structure with an extra oxygen on the O ot atom is more stable than 2O br /2O ot . At 350 °C, above a potential of 0.15 V vs SOE (0.85 V vs SHE), structure A (2O br /4O ot ) became the most stable. Calculation for an intermediate structure 2O br /3O ot showed that it was not more stable compared to 2O br /4O ot in the given potential range. These findings illustrate that as the potential value increased, the surface which is covered with a higher number of oxygen atoms becomes more stable, as shown in the surface phase diagram (Figure 5). This increase of oxygen concentration on the surface with increased potential conforms with the oxygen backspillover process that was discussed extensively in previous experimental and theoretical papers. Compared to the temperature and O2 pressure dependence discussed above, the surface state is more strongly dependent on the applied electrochemical potential. In general, this explains part of the EPOC effect, since tuning the electrochemical potential provides an efficient means to control the catalyst surface state, which in turn modifies the reactivity of the catalyst. Here, ethylene adsorption is studied on RuO2 (110) with the most stable 2O ot /2O br surface termination prior to polarization. σ-C2H4 molecule adsorption energy was calculated for three positions of C2H4: on the 2O ot , 2O br , and O ot O br . The adsorption energy under open-circuit potential (ocp) condition was the lowest when ethylene adsorbed on 2O ot in σ form (-2.9 eV), and the highest when adsorbed in π-form on O ot (-1.6 eV). σ-bond on 2O br and diagonally on O ot O br yields intermediate adsorption energies of -2.0 and -2.8 eV, respectively (shown in Figure S 17). Figure 6 shows the effect of the electrode potential on the stability of σ-C2H4 adsorption energy. It can be seen that the increase in the electrode potential (proportional to the surface work function) results in increasing (in absolute value) the heat of adsorption of σ-C2H4 rendering ethylene adsorption more stable than at ocp beyond a potential of 1.3 V vs SHE. This trend is paralleled by the increasing positive partial charge of the molecule (determined from Hirshfeld charge) as a function of potential (right axis of Figure 6). Thus, this figure confirms the EPOC mechanism 8 that the increase in the work function results in an increase in the heat of adsorption (produced thus negative) of ethylene and its electropositive behavior. This journal is © The Royal Society of Chemistry 20xx

Adsorption of ethylene on the surface

Please do not adjust margins Please do not adjust margins The effect of the potential was similar on the dissociation intermediates (dissociated C-H and dissociated C-C) (Figure 7), i.e., the adsorption energy of these intermediates is strengthened as a function of increasing potential.

DFT prediction of ethylene dissociation on 2O br /2O ot surface termination

Figure 8 shows the energy diagram computed using DFT for the C-C bond dissociation on the 2O br /2O ot RuO2 (110) surface termination. Our results (Figure 8b) demonstrate that the energy barrier for C-C bond dissociation decreases as a function of increasing applied potential and becomes smaller than under ocp conditions beyond a potential of 0.3 V vs SHE or -0.4 V vs SOE. The activation energy continues to decrease beyond this potential value, which demonstrates the advantageous effect of potential on the facilitation of the C-C bond dissociation step.

Similarly, the reaction energy decreased as a function of the electrode potential rendering the reaction more exothermic as a function of increasing potential.

DFT prediction of oxygen adsorption and dissociation on the 2O br /2Ru surface termination

The enhancement in the reaction rate of ethylene oxidation was also found to experimentally occur under negative potential as demonstrated in Figure 3. Since the surface state strongly depends on the electrochemical potential (Figure 5), we have hypothesized that under negative polarization it is the activation of O2 that is accelerated. Therefore, further computation was performed to investigate the effect of potential on the filling of the 2O ot vacancies by adsorbing gaseous O2. Note, that there are two sources of oxygen: The electrochemical one that determines the surface state and is source for the electron charges, and the gas-phase oxygen, which is the one for which we have computed the activation and adsorption energy. Since the electrochemical potential and the O2 gas-flow are fixed independently, the chemical potentials of these two oxygen sources are not equalized, i.e., the system is not in equilibrium. The oxygen required for the non-Faradaic C2H4 oxidation is taken from the gas phase. Therefore, it is a key point that EPOC has a non-equilibrium nature such that the reactivity of oxygen stemming from YSZ (Faradaic reaction) is different than the one from gas-phase (non-Faradaic reaction).

It can be seen in Figure 10 that the adsorption of a bonded oxygen molecule at ocp (dotted red line) is -1.43 eV and decreases to -2.1 eV when the O-O bond breaks (dotted green line). The adsorption energy of intact and dissociated dioxygen decreases with negative potential (decreasing work function) which forms an opposite trend compared to the adsorption energy of the electropositive ethylene. This is in agreement with empirical EPOC rules which state that a decrease in the work function results in an increase in the chemisorptive bond of electron acceptors, e.g., O2 and CO2 [START_REF] Vayenas | Electrochemical Activation of Catalysis: Please do not adjust margins Promotion, Electrochemical Promotion, and Metal-Support Interactions[END_REF] . The effect of potential on the adsorption of oxygen and ethylene is directly compared in Figure 1 to highlight the difference between an electron donor and an electron acceptor. The addition of electrons results in a decrease of the electrode potential (and proportionally a decrease of the work function). With this trend, the adsorption of oxygen can be seen to increase (in absolute value) compared to under ocp condition, becoming more stable below a potential value of 0.5 V vs SHE for "intact" O2 and 0.7 V vs SHE for dissociated O2 (Figure 10). Figure 11 shows the effect of applied potential on the binding energy of oxygen. Not only the adsorption energy but also the activation and reaction energies for oxygen dissociation decrease as a function of decreasing electrochemical potential. Thus, the dissociation of an oxygen molecule is facilitated when applying a negative potential. This decrease in the activation energy is in good agreement with the experimental cathodic reaction rate that showed a higher rate compared to under ocp condition.

The geometries for oxygen dissociation are given in

Experimental and DFT results comparison

In conclusion, the increase in the experimental catalytic rate as a function of positive potential can be explained by the facilitation of the C-C bond dissociation, whereas the increase in the catalytic rate as a function of negative polarization is due to the ease in the oxygen bond rupture. Those two potential effects can be correlated to the experimental data of Figure 3c, Furthermore, it should be noticed that the slope of the decreasing activation energy of C-C bond dissociation as a function of increasing potential is higher than that of the O-O bond dissociation as a function of decreasing potential value. This higher change in the activation energy at the positive potential is in agreement with the higher experimental increase in the catalytic rate under anodic potential application compared to the cathodic one. Interestingly enough, if overlaying Figure 7 and Figure 10, it can be seen that the adsorption energy of dissociated ethylene as a function of potential becomes smaller than that at ocp, above 0.7 V vs SHE while the adsorption energy of dissociated oxygen is smaller than that at ocp at potentials below 0.7 vs SHE. In summary, the atomistic origin of the EPOC effect on the ethylene oxidation rate on the surface of ruthenium oxide has therefore been identified to be due to the potential-dependent electric field at the catalyst surface, paralleling results of the effect of the addition of a base for formic acid decomposition 71 . This electric field complements the tuning of the surface state due to the backspillover.

Conclusions

For the first time, experimental results for the electrochemical promotion of ethylene oxidation on the surface of RuO2 (110) have been rationalized using Density Functional Theory (DFT) modeling, explicitly taking into account the effect of the applied electrochemical potential. Experimentally, we obtained an increase in the catalytic rate under anodic and cathodic potential for the ethylene oxidation over RuO2 nanoparticles. In order to elucidate the experimental findings, we have studied the adsorption and dissociation of σ-C2H4 on the 2O br /2O ot surface termination of RuO2 (110) surface. We found that C2H4 preferably adsorbs on the two O ot atoms via a σ-bonded structure and that the heat of adsorption produced increases with increasing potential. Concomitantly, the activation energy of the C-C bond dissociation was found to decrease as a function of potential. While under negative potential, the adsorption of oxygen is facilitated and the activation energy of the O-O dissociation decreased. The facilitation of the C-C bond dissociation under positive potential and of the O-O bond dissociation under negative potential, explain the experimentally observed catalytic rate increase under both anodic and cathodic polarization, respectively. Furthermore, these findings are supported by the fact that surface free energy calculation showed that the surface with higher coverage of oxygen became more stable as a function of increasing potential. This is in excellent agreement with the mechanism of oxygen ion back-spillover and the rules of electrochemical promotion of catalysis.

Figure 1 :

 1 Figure 1: A schematic representation of ruthenium electrode deposited on O 2-conducting YSZ solid electrolyte, showing the locations of the metal/electrolyte double layer [Oᵟ -δ+] and the effective double layer created at the metal/gas interface. All electrodes are under the same gaseous conditions. The potential-controlled ion migration (backspillover) and charge transfer affect the adsorption strength of electropositive ethylene and electronegative oxygen gas species (shown on the right).

Figure 2 :

 2 Figure 2: (a) STEM of ruthenium colloid and (b) corresponding particle size distribution. Scale bar is 10 nm.

Figure 3 :

 3 Figure 3: Transient rate response of RuO2 NPs to a step change of (a) an applied potential UWR = 1 V, (b) an applied constant current I=-100µA and (c) a summary of rate increase as a function of multiple galvanostatic applications. o.c.: open-circuit. Conditions: T = 350 o C, PC2H4 = 0.012 kPa, PO2=3 kPa. Total flow rate =100 ccm. The corresponding Faradaic efficiency (in black) and enhancement ratio (in red) on top of each current value are also shown in part (c).

  . The coordination of Ru atoms at top and bottom open surfaces changed coordination accordingly to the three surfaces being tested. The three common surface terminations are presented in Figure 4 (side and top views) such that (a) and (d) show a termination where all Ru have their saturated six-fold coordination due to the O br and O ot atoms, called 2O br /2O ot ; (b) and (e) present the stoichiometric RuO2 (110) terminated with 2O br /2Ru, and (c) and (f) show a termination that lacks oxygen and therefore its surface ends with 2Ru/2Ru which are fourfoldand fivefold-coordinatively unsaturated.

Figure 4 :

 4 Figure 4: Possible three terminating planes of RuO2 (110) surface; side (perspective) view of surface with a) 2O br /2O ot , b) 2O br /2Ru and c) 2Ru/2Ru termination; (d-f) are top view of the three surfaces. (Ru: large light spheres, O: red small spheres). Solid lines representing the cell boundary.

Figure 5 :

 5 Figure 5: Surface phase diagram of stable structure of RuO2 as a function of potential and temperature. P(O2)=1 kPa. Structures A to E are illustrated on the right with top and side view.

  Non-EPOC related DFT modeling of ethylene oxidation on ruthenium surface has been studied previously: In Lopez et al., adsorption of π-C2H4 directly on a free Ru atom was studied and its migration to form oxametallacycle (OMME) intermediate was shown. However, when the RuO2 surface has full coverage of oxygen, ethylene adsorption transitions from epoxide to the more stable O-C2H4-O form 81 . Only the C-H bond dissociation was studied in Liang et al., showing the facilitation of the C-H cleavage on a fully oxidized surface compared to a stoichiometric one 82 .

Figure 6 :

 6 Figure 6: C2H4 adsorption energy and its total charge as a function of potential on RuO2 (110) surface with 2O br /2O ot termination. Dotted line corresponds to ocp values.

Figure 7 :

 7 Figure 7: C2H4 and consecutive intermediates' adsorption energy as a function of potential on RuO2 (110) surface with 2O br /2O ot termination. Dotted line corresponds to ocp adsorption value, while the solid line corresponds to adsorption energy variation as a function of potential value.

Figure 8 :

 8 Figure 8: (a) Energy diagram and (b) corresponding activation and reaction energies as a function of potential for C-C bond dissociation over 2O br /2O ot surface termination of RuO2 (110).

Figure 9 :

 9 Figure 9: Structure of the initial, transition and final state of (C-C) bond dissociation on 2O br /2O ot RuO2 (110) surface.

Figure 10 :

 10 Figure 10: Energy diagram and corresponding activation and reaction energy as a function of energy for O-O bond dissociation over 2O br /2Ru surface termination of RuO2 (110).

  Figure11shows the effect of applied potential on the binding energy of oxygen. Not only the adsorption energy but also the activation and reaction energies for oxygen dissociation decrease as a function of decreasing electrochemical potential. Thus, the dissociation of an oxygen molecule is facilitated when applying a negative potential. This decrease in the activation energy is in good agreement with the experimental cathodic reaction rate that showed a higher rate compared to under ocp condition.The geometries for oxygen dissociation are given in Figure S 21, which shows the oxygen bond dissociation and its transition state. The distance between the two oxygen increased from an initial length of 1.4 Å to a final value of 3.1 Å, passing by 1.7 Å at the transition state.

Figure 11 :

 11 Figure 11: Energy diagram and corresponding activation and reaction energy as a function of potential for O-O bond dissociation over 2O br /2Ru surface termination of RuO2 (110).

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 3

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 7

Acknowledgements

The authors thank University of Ottawa, CNRS, and ENS Lyon for support of research conducted within the context of the International Associated Laboratory 'Fundamental catalysis for green chemistry: From well defined active sites to mechanistic explorations -FUNCAT. ' and Natural Sciences and Engineering Research Council of Canada (NSERC) for financial support. The authors acknowledge Dr M. Couillard from National Research Council (NRC), Canada for STEM measurements. The authors thank the SYSPROD project and AXELERA Pôle de Compétitivité for financial support (PSMN Data Center). The first author thanks NSERC for the Vanier Canada Graduate scholarship support.

Conflicts of interest

There are no conflicts to declare.