Julien Grand

Siddulu Naidu Talapaneni

Hristiyan A Aleksandrov

Georgi N Vayssilov

Svetlana Mintova

Hydrophobic Tungsten Containing MFI-type Zeolite Films for Exhaust Gas Detection

Keywords: Zeolites nanocrystals, adsorption, thin films, IR spectroscopy, hydrophobicity, exhaust gas

The assembly of highly hydrophobic nanosized tungsten containing MFI type zeolite nanocrystals (W-MFI) in films, and their further use for selective exhaust gas (CO, CO2, NO and NO2) detection was investigated by operando IR spectroscopy. Due to the hydrophobic nature and the presence of tungsten in the framework, the W-MFI films showed excellent sorption capacity towards all analytes, in comparison to the pure silica (Si-MFI) film. The high sensitivity of the W-MFI film towards low concentration of CO2 and NO2 (1-3 ppm) was demonstrated. In addition, the interactions between the analytes and zeolite films have been studied by quantum chemical calculations modeling of the W centers based on the density functional theory method.

Introduction

The ingrowing demand of environmental sensing data urges the development of selective chemical sensors. Over the past decades, there is an increasing demand for the fabrication of highly efficient sensors that are able selectively to detect CO, CO2 and NOx pollutant gases at ppm levels, which are continuously produced from combustion engines. [START_REF] Wales | Gas Sensing Using Porous Materials for Automotive Applications[END_REF][START_REF] Wetchakun | Semiconducting Metal Oxides as Sensors for Environmentally Hazardous Gases[END_REF] Issues concerned for the manufacturing of these type of sensors include materials sensitivity, selectivity, reproducibility, reliability, long-term stability and applicability to in-situ and real time measurements. [START_REF] Liu | A Survey on Gas Sensing Technology[END_REF] Moreover, these functionalities of the sensors should be preserved in the harsh environment conditions including high temperature, variation of humidity, variation of temperature, pressure and concentration of highly acidic compounds that are common for automotive exhausts.

Microporous molecular sieves have been widely considered for selective sensor applications due to their well-defined pore structure, particle dimensions and morphology in line with high surface area, sorption capacity and catalytic activity. [START_REF] Zheng | Exploitation of Unique Properties of Zeolites in the Development of Gas Sensors[END_REF][START_REF] Korotcenkov | Handbook for Gas Sensor Materials -Properties, Advantages and Shortcomings for Applications[END_REF] However, the slow diffusion of the analytes in the micropores of the micrometer-sized molecular sieves is not in ad equation with the real-time response requirements. Nanosized zeolites have been designed and prepared to leave behind these challenges. Variety of zeolite nanostructures and diverse depositing techniques on various supports were reported. [START_REF] Mintova | Nanosized Zeolites: Quo Vadis ?[END_REF][START_REF] Valtchev | Porous Nanosized Particles: Preparation, Properties, and Applications[END_REF][START_REF] Naydenova | Hybrid Sensors Fabricated by Inkjet Printing and Holographic Patterning[END_REF] As active layer of sensor devices, zeolite nanocrystals assembled as films are very favorable due to their high thermal stability, chemical resistance, enhanced specific surface area and decreased diffusion path lengths which leads to more sensitive tracking and real-time response to emissions. [START_REF] Zheng | Exploitation of Unique Properties of Zeolites in the Development of Gas Sensors[END_REF] They can be deposited in thin-to-thick films using different approaches such as direct crystallization, seed growth, spin-and dip-coating, and sol-gel technique. The film thicknesses also influence the sorption capacity and diffusion rate, while the main features of zeolite nanocrystals (hydrophobicity, adsorptive properties and their interaction with the analytes) determine their application for various sensors. Further properties have been improved by the confinement of desired metal clusters in the nanosized zeolite matrix. [START_REF] Talapaneni | Nanosized Sn-MFI Zeolite for Selective Detection of Exhaust Gases[END_REF][START_REF] Dubbe | The Effect of Platinum Clusters in the Zeolite Micropores of a Zeolite-Based Potentiometric Hydrocarbon Gas Sensor[END_REF][START_REF] Thomas | In Situ Infrared Molecular Detection Using Palladium-Containing Zeolite Films[END_REF] However, the size and location control, as well as the hydrophilic nature of the clusters seem to be difficult to handle. [START_REF] Fukumura | Molecular Nano Dynamics[END_REF][START_REF] Okumura | Generation of the Active Pd Cluster Catalyst in the Suzuki-Miyaura Reactions: Effect of the Activation with H2 Studied by Means of Quick XAFS[END_REF] Aluminum free zeolites, namely metallosilicates, have been widely considered to overcome these issues. [START_REF] Blasco | Unseeded Synthesis of Al-Free Ti-Β Zeolite in Fluoride Medium: A Hydrophobic Selective Oxidation Catalyst[END_REF][START_REF] Pinto | Slow Release of NO by Microporous Titanosilicate ETS-4[END_REF][START_REF] Corma | Sn-zeolite Beta as a Heterogeneous Chemoselective Catalyst for Baeyer-Villiger Oxidations[END_REF] Among the application of different metal oxides, the tungsten oxides have been found to be promising for the detection of NOx gases. [START_REF] Kim | Gas sensing characteristics of WO3 nanoplates prepared by acidification method[END_REF][START_REF] Qin | Microstructure Characterization and NO2-sensing Properties of Tungsten Oxide Nanostructures[END_REF][START_REF] Varsani | Zeolite-Modified WO3 Gas Sensors -Enhanced detection of NO2[END_REF] Although small sized tungsten oxide based materials display high sensitivities toward NOx, the greatest limitation is due to their poor selectivity.

Several groups showed that the selectivity for CO and NOx gases by tungsten oxide can be improved by modification with porous silicon supports. [START_REF] Yan | Room Temperature NO2-Sensing Properties of WO3 Nanoparticles/Porous Silicon[END_REF] Very recently, our group has demonstrated a novel approach for the incorporation of tungsten in the framework of MFI-type zeolite affording silanol-free nanocrystals, which are highly hydrophobic in nature. [START_REF] Grand | One-pot Synthesis of Silanol-free Nanosized MFI Zeolite[END_REF][START_REF] Mintova | Method for the Preparation of Defectfree Nanosized Synthetic Zeolite Materials[END_REF] Thus, the incorporation of tungsten in the nanosized zeolite framework is expected not only to provide materials with enhanced sensitivity toward exhaust gas but also to improve their hydrophobicity and thus their sorption capacity, as competition towards water sorption will be avoided.

Herein, we describe the preparation of tungsten containing MFI type zeolite nanocrystals (W-MFI) assembled in thin-to-thick films (600-1600 nm). The influence of the metal doping, the hydrophobicity and the film thicknesses on their performance for the selective detection of exhaust such as carbon monoxide, carbon dioxide, nitrogen oxide and nitrogen dioxide in the presence of water has been evaluated.

Experimental part

Preparation of W-MFI and Si-MFI zeolite coating suspensions

The stable coating suspensions of discrete nanosized W-MFI and Si-MFI type zeolites were prepared from colloidal precursors under hydrothermal synthesis conditions. [START_REF] Grand | One-pot Synthesis of Silanol-free Nanosized MFI Zeolite[END_REF] The as-prepared precursor suspensions for W-MFI and Si-MFI zeolites were aged at room temperature for 24 h prior further hydrothermal treatment at 90 °C for 12 h. After the hydrothermal treatment, the zeolite nanoparticles were purified by three steps centrifugation (20000 rpm, 30 min) using distilled water and finally stabilized in coating suspension with a solid content of 1 wt. % using absolute ethanol (99.9 %, Aldrich) as a solvent.

Zeolite thin films assembly

The crystalline MFI type nanosized zeolites were assembled in thin films on silicon wafers by spin-on process. The silicon wafers (100) with a dimension of 10 × 10 mm 2 were pre-cleaned with ethanol and acetone and dried with air gun prior deposition of the films. The MFI coating suspensions with a solid content of 1 wt. % in ethanol were filtrated through syringe filters (450 nm). The procedure involves: (i) deposition of a smooth layer consisting of 5 wt. % polyvinylpyrrolidone (PVP, average molecular weight ~ 10000, Sigma-Aldrich) in absolute ethanol by spin coating (acceleration: 2000 rpm s -1 , 3000 rpm for 20 s) (Supporting Information, SI Figure S1), followed by (ii) deposition of one or several MFI zeolite layers by spin coating (acceleration: 2000 rpm s -1 , 500-3000 rpm for 20 s). After each coating step, the films were dried at 80 °C for 20 min in order to improve the mechanical stability. Variation of the spin-coating speed and the number of layers afforded films of various thicknesses (600-1600 nm). The films were calcined at 550 °C for 2 h to remove the binder layer (PVP) and the template used in the MFI zeolite synthesis process (tetrapropylammonium hydroxide, TPAOH). Finally, the W-MFI and Si-MFI films of 600 nm (sample W-MFI-600) and 1600 nm (sample W-MFI-1600) thicknesses were used for detection of exhaust gases.

Characterization of zeolites films

The zeolite nanoparticles as Si-MFI and W-MFI powders were fully described in a previous paper. [START_REF] Grand | One-pot Synthesis of Silanol-free Nanosized MFI Zeolite[END_REF] The results from nitrogen sorption measurements of both materials are presented in Table S1 (see SI). The homogeneity features and thicknesses of the prepared MFI-type films were monitored by a field emission scanning electron microscope (FE-SEM TESCAN Mira). The crystalline structure of the films was studied with X-ray diffraction (XRD) using a PANalytical X'Pert Pro diffractometer in Debye-Scherrer geometry with Cu Ka radiation (l = 1.5418 Å). The thermal stability of the PVP binder was measured by thermogravimetric analysis (TGA) using a SETSYS evolution instrument; measurements in air atmosphere with a ramp heating of 5° C/min were carried out.

Detection of CO, CO2, NO and NO2: operando IR study

The detection of CO, CO2, NO and NO2 with zeolite films was followed by operando IR spectroscopy using a reactor cell and a gas flow system described in a previous paper. [START_REF] Thomas | In Situ Infrared Molecular Detection Using Palladium-Containing Zeolite Films[END_REF] The experiments were performed at 25 °C under Argon (Ar) flow and atmospheric pressure. Prior measurements, the zeolite films were activated at 300 °C for 6 h (ramping heating of 1 °C/min) under Ar with a flow of 20 cm 3 /min. The analytes (CO, CO2, NO and NO2) with a concentration ranging from 1 to 100 ppm in Ar with a total flow of 20 cm 3 /min were sent to the IR reactor cell.

All experiments were performed in the presence of water with a concentration of 100 ppm coming from the Ar carrier gas. The IR spectra were collected in a continuous mode (32 scans/spectrum) with a Bruker Tensor 27 spectrometer equipped with a DTGS detector. The first spectrum was recorded after 30 seconds; the waiting time between each scan was 1 minute. After the measurements, the zeolite films were regenerated following the activation process described above. The entire process including activation of samples (removal of adsorbed water and contaminants at 300 °C) and controlled adsorption and desorption of pure analytes (CO, CO2, NO and NO2) in subsequent cycles were performed in the operando IR cell in order to ensure identical conditions for the measurements.

Modeling of CO, CO2, NO and NO2 adsorption on W-MFI zeolite

Quantum chemical calculations based on the density functional theory method were carried out using periodic plane wave basis for the valence electrons with a cutoff energy of 400 eV. The calculations were performed with the Vienna ab initio simulation package (VASP) [START_REF] Kresse | Ab initio Molecular-Dynamics Simulation of the Liquid-Metal-Amorphous-Semiconductor Transition in Germanium[END_REF][START_REF] Kresse | Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set[END_REF] using the gradient corrected PW91 exchange-correlation functional [START_REF] Perdew | Accurate and Simple Analytic Representation of the Electron-Gas Correlation Energy[END_REF] and ultrasoft pseudopotentials. [START_REF] Vanderbilt | Soft Self-Consistent Pseudopotentials an a Generalized Eigenvalue Formalism[END_REF][START_REF] Kresse | Norm-Conserving and Ultrasoft Pseudopotentials for First-Row and Transition Elements[END_REF] The large size of the unit cell (see below) allowed us to sample the Brillouin zone using only the Γ point. [START_REF] Jeanvoine | Brønsted Acid Sites in HSAPO-34 and Chabazite: An Ab Initio Structural Study[END_REF] An orthorhombic unit cell of the MFI type zeolite framework was employed, which consists of 96 T atoms with dimensions: a = 20.271 Å, b = 19.925 Å, and c = 13.332 Å. [START_REF] Baerlocher | Database of Zeolite Structures[END_REF] During the geometry optimization, all atoms of MFI structure as well as the adsorbate species were allowed to relax until the force on each atom becomes less than 2´10 -4 eV/pm. The vibrational frequencies of the CO and CO2 adsorbate species were simulated and compared with the experimental data. They were calculated via a numerical normal mode analysis in which the Hessian elements were approximated as finite differences of analytical gradients, displacing each atomic center by 1.5 pm either way along each Cartesian direction.

Results and discussion

General characterization of zeolite powders and films

The W-MFI zeolite crystals used herein are silanol-free and hydrophobic (See SI, Figure S2 and previous work [START_REF] Grand | One-pot Synthesis of Silanol-free Nanosized MFI Zeolite[END_REF] ) in nature meaning that no silanol groups are present and thus guarantee the film stability. Therefore, to ensure the preparation of smooth and homogeneous films with high mechanical stability, a polymeric PVP binder was used as first layer deposited on the pre-cleaned silicon wafers. Previously, the PVP has been used as wetting agent for the preparation of metal oxides thin films and nanowires without influencing their structure and morphology. [START_REF] Enculescu | Influence of Polyvinylpyrolidone as an Additive in Electrochemical Preparation Of ZnO Nanowires and Nanostructured Thin Films[END_REF] Moreover, the PVP was reported for efficient preparation of crack-free TiO2 films having improved activity after PVP burnout. [START_REF] Tanaka | Improved Photocatalytic Activity of Zeolite-and Silica-Incorporated TiO2 Film[END_REF] Therefore a thin PVP layer was used as a binder between the silicon wafer and the zeolite nanocrystals by enhancing their polar interactions and improving the wetting effect at the silicon wafer surface (Figure 1a). The PVP was easily removed during the calcination process at 550 °C as the TGA defines its decomposition at 425 °C (See SI, Figure S3). The nanosized zeolite crystals (Si-MFI and W-MFI) dispersed in ethanol were deposited on silicon wafers by spin coating method. The homogeneity and thickness of the Si-MFI and W-MFI films were confirmed by FE-SEM as shown in Figure 1. Variation of the spin-coating speed and the number of layers afforded films of various thicknesses. After calcination, well-packed zeolite nanocrystals in the films covering entirely and homogeneously the silicon wafers were obtained. Si-MFI deposited as a reference thin film has been prepared comparable with the W-MFI one (Figure 1b and1c). The highest thickness holding an homogeneous and stable zeolite film has been measured as 1.6 µm (Figure 1d). No traces of the PVP were observed after calcination (See SI, Figure S4). In addition, the X-ray diffraction pattern showed the high crystallinity and purity of the MFI nanocrystals assembled in the films (See SI, Figure S5). The XRD pattern for the thicker film was analogous to the corresponding pattern recorded from the MFI powder sample with random crystal orientation; the peak at 33 °2q corresponds to a signal of the silicon wafer.

In the following experiments, zeolite films with thicknesses of 600 nm (W-MFI-600) and 1600 nm (W-MFI-1600) have been used.

Water adsorption of zeolite powders and films

The water adsorption on both zeolite powders and films (W-MFI and Si-MFI) has been studied using TG and operando IR spectroscopy (Figure 2). As expected the W-MFI zeolite crystals without silanol groups exhibit high hydrophobicity (See SI, Figure S2 and previous work [START_REF] Grand | One-pot Synthesis of Silanol-free Nanosized MFI Zeolite[END_REF] ).

Calcined Si-MFI and W-MFI powder samples were kept during 7 consecutive days at relative humidity (RH) of 70%. The total water adsorbed in the two samples has been evaluated using TG analysis (Figure 2a). It is clearly observed that even the pure silica zeolite (Si-MFI) contains about 4 % water. The weight loss below 150 °C corresponds to water adsorbed in the channels and on the surface of the Si-MFI zeolite. Moreover, with decreasing the size of the Si-MFI zeolite crystals, a substantial increase of the external surface area and silanol-defect sites are observed as shown by N2 sorption and in situ IR studies (Figure S6). While the W-MFI zeolite shows less than 0.2 % weight loss, which is in the range of the measurement error (±0.1 %). In parallel, the capacity of activated W-MFI-1600 and Si-MFI-1600 zeolite films (300 °C, 6 h) towards water vapor (100 ppm concentration in argon flow) was followed by operando IR spectroscopy (Figure 2b). The FTIR spectroscopy data is in agreement with the results obtained for zeolite powders (Figure 2a). The Si-MFI film absorbed 27 times more water than the W-MFI film calculated from the bands areas at 3500 and 1600 cm -1 . Based on the above results, it is expected that the sensitivity towards the exhaust gases (NO, NO2, CO and CO2) in the presence of water would not be disturbed in the case of the W-MFI films.

Effect of the film thickness on detection of NO and NO2

The response of W-MFI zeolite films (W-MFI-600 and W-MFI-1600) toward low concentration of NO and NO2 analytes (1-5 ppm) is depicted in Figure 3. Detailed kinetics data is provided in SI (Figures S7 andS8). As general trend, for both W-MFI zeolite films (W-MFI-600 and W-MFI-1600) the area of the IR bands in the range 1650 -1480 cm -1 , corresponding to monodentate and bidentate nitrites/nitro compounds, [START_REF] Hadjiivanov | Use of Overtones and Combination Modes for the Identification of Surface NOx Anionic Species by IR Spectroscopy[END_REF][START_REF] Hadjiivanov | Identification of Neutral and Charged NxOy Surface Species by IR Spectroscopy[END_REF] increased with increasing of the analyte concentration and exposure time.

At low concentration (1 ppm), the W-MFI-600 film shows faster response in comparison to the W-MFI-1600 which is explained with faster diffusion in the thinner film. Indeed, both analytes (Figure 3) were effectively and distinctly detected by the W-MFI-600 film after 30 and 150 seconds. On the contrary, low responses were observed using the W-MFI-1600 film regardless of the time of exposure, meaning that 1 ppm concentration is too low to afford a reliable response using the W-MFI-1600 film.

At slightly higher concentration (3-5 ppm), both films exhibits clear response towards both analytes. However, at 5 ppm of NO, the responses after 30 and 150 seconds of the WMFI-600 film are mingled which means that the saturation appears in the first minute (Figure 3a). This phenomenon is not observed with the W-MFI-1600 film (Figure 3a). It appears that at 3 ppm NO2, (Figure 3b) a remarkable increase in the response with the WMFI-1600 film with exposure time is observed. This film shows higher response compared to the one obtained with the WMFI-600 film.

This implies that at concentration of analytes of 3-5 ppm, the sorption capacity overcomes the limiting low diffusion rate observed at 1 ppm. In addition, the low sensitivity of the thicker film is rapidly overcome by its high sorption capacity. The highest sensitivity of the W-MFI-1600 film towards NOx in comparison to the Si-MFI-1600 film is clearly demonstrated (Figure 4a). The Si-MFI-1600 film shows no readable response towards the NO analyte whereas the W-MFI-1600 reached its saturation level at 30 ppm. In addition, a clear response is observed at 5 ppm (Figure 4b). Concerning the NO2 sorption, the saturation level is reached at around 30 ppm with Si-MFI-1600 film, while for the W-MFI-1600 film is higher than 75 ppm (Figure 4a).

The change of the IR bands at 1380-1590 cm -1 corresponding to chemisorbed carbonates and the one centered at 2290-2390 cm -1 corresponding to physisorbed carbonates are followed at different gas concentrations (Figure 5). [START_REF] Kock | In Situ FT-IR Spectroscopic Study of CO2 and CO Adsorption on Y2O3, ZrO2, and Yttria-Stabilized ZrO2[END_REF] The data related to 1 ppm CO and CO2 are not reported here since no readable responses were observed with both Si-and W-MFI films. The W-MFI-1600 film shows excellent selectivity and sensitivity toward CO2 in comparison with the Si-MFI-1600 film and in comparison with the CO (Figure 5a). Indeed, the W-MFI-1600 film was saturated at higher concentrations (around 50 ppm) with both CO and CO2 gas. Whereas the Si-MFI-1600 showed no clear response to CO in the concentrations range used, as well as no clear response for CO2 (20-30 ppm) (Figure 5a). On the contrary, the first response of the W-MFI-1600 film at 3 ppm CO2 and at 5-10 ppm CO were measured (Figure 5b).

Modeling of the NO, NO2, CO and CO2 adsorption on the W-MFI zeolite

The adsorption of NO, NO2, CO and CO2 analytes have been modeled on the most stable species W VI =O which were found to exist in the W-MFI zeolite nanocrystals as reported in our previous work (Figure 6). [START_REF] Grand | One-pot Synthesis of Silanol-free Nanosized MFI Zeolite[END_REF] W VI =O -NO W VI =O -NO2

W VI =O -CO W VI =O -CO2 Adsorption of NO molecule to W VI =O species in the W-MFI does not lead to the formation of adsorption complex as the guest molecule does not interact directly with the W VI =O center.

Concerning the NO2 analyte, the formation of nitrate species was obtained when NO2 is adsorbed by the W VI =O center of the W-MFI zeolite. Its structure is bidentate, and the third oxygen center from the nitrate is not coordinated to a cationic center. Similarly to NO, CO molecule does not interact with W VI =O species, while in the case of CO2 adsorption carbonate species are formed at the W VI =O moiety. In the obtained structure the carbonate is bound in bidendate fashion to the tungsten and the third oxygen center from the carbonate is not coordinated to a cationic center.

Using the notation of surface carbonates, introduced by Vayssilov et al. [START_REF] Vayssilov | Reassignment of the Vibrational Spectra of Carbonates, Formates, and Related Surface Species on Ceria: A Combined Density Functional and Infrared Spectroscopy Investigation[END_REF] , those carbonate species can be assigned as 1.10 type. The theoretical results combined with the experimental clearly show that the introduction of a WO center in the zeolite framework has distinct effects on the improved sorption behavior of the W-MFI zeolite compared to the Si-MFI zeolite. Indeed, the formation of bidentate species, such nitrates or carbonates, is only favored by the W VI =O center, which is not present in the Si-MFI zeolite. Otherwise, when the analyte is monodentate (CO and NO), no interaction with the W center could be found. However, the sorption capacity is improved for the W-MFI. It means that the highly hydrophobic silanol-free structure obtained by the introduction of W has a positive effect on the sorption capacity of the MFI type zeolite. Indeed, the absence of water due to the absence of silanols in the W-MFI zeolite leads to an increase capacity for NO and CO analytes on the W-MFI zeolites. This non-competitive water/analytes sorption could be applied to every type of analytes different from water. The range and limitations of this phenomenon is under investigation.

Conclusions

The assembly of hydrophobic W-MFI and Si-MFI type zeolite nanocrystals in thin films for selective detection of exhaust gases (CO, CO2, NO, NOx) was carried out. The zeolite nanocrystals were spin-coated on silicon wafers using a polymer as a binder in order to increase the mechanical stability of the films. Besides, a multi-step deposition of the zeolite nanocrystals was applied to afford homogeneous films with thicknesses from 600 to 1600 nm with high thermal and mechanical stability. Zeolite films with a thickness 600 and 1600 nm films were further used for detection of low concentrations of exhaust gases (1-100 ppm of CO, CO2, NO and NO2) in the presence of water (100 ppm).

The fastest detection of the exhaust gases was measured with the thinner film (600 nm), however, the thicker films (1600 nm) showed higher sorption capacity. ASSOCIATED CONTENT

Supporting information.

The supporting information file includes: Polyvinylpyrrolidone structure (Figure S1); Silanol-free zeolite nanocrystals evidenced by [START_REF] Baerlocher | Database of Zeolite Structures[END_REF] Si NMR and FTIR spectroscopies (Figure S2); TG and dTG analysis of PVP polymer (Figure S3); PVP removal by calcination process evidenced by FTIR spectroscopy (Figure S4); XRD patterns of W-MFI zeolite as bulk and as thin film (Figure S5); N2 sorption and FTIR spectra of nanosized and micronsized Si-MFI zeolites (Figure S6); Kinetic evolution of NO analyte adsorption on W-MFI films (Figure S7); Kinetic evolution of NO2 analyte

Figure 1 .

 1 Figure 1. FE-SEM side-view images of (a) as-deposited PVP binder layer (100 nm), (b) calcined

Figure 2 .

 2 Figure 2. (a) Water content in calcined W-MFI (black) and Si-MFI (red) powder samples stored

Figure 3 .

 3 Figure 3. Responses of activated zeolite films W-MFI-600 (black) and W-MFI-1600 (red)

Figure 4 .

 4 Figure 4. Detection of NO with W-MFI-1600 (black) and Si-MFI-1600 (blue), and NO2 with

Figure 5 .

 5 Figure 5. Detection of CO with W-MFI-1600 (black) and Si-MFI-1600 (blue) films and CO2

Figure 6 .

 6 Figure 6. Optimized structures for NO, NO2, CO and CO2 adsorption at W VI =O site of the W-

  The fast detection is explained with the fast diffusion within the thin films in comparison to the thick one. The saturation of the W-MFI zeolite films towards all analytes is reached after 2 minutes at 30 ppm concentration. High sensitivity of the W-MFI zeolite films compared to pure Si-MFI films towards very low concentrations (1-30 ppm) of exhaust gases in the presence of 100 ppm of water was demonstrated. The highly hydrophobic nanosized zeolites with incorporated W in the framework (W-MFI) deposited in thin films demonstrate high molecular recognition capacity toward low concentrations of exhaust gases. It has been shown that the W-MFI films can detect low concentration of NO2 and CO2 (1-3 ppm). The highest sensitivity and sorption capacity of W-MFI film was observed towards NO2 exposure (1 ppm).

adsorption on W-MFI films (Figure S8); Kinetic responses of W-MFI zeolite films exposed to CO and CO2 gas (Figure S9).

We acknowledge the financial support from the C²-MTM project « EMERGENTS » Region of Normandy and Marie Lozier for technical support. H.A.A. acknowledges financial support by the Bulgarian Science Fund (Project DFNI-T02/20).

A similar trend has been observed when both films were exposed to CO and CO2 analytes. The IR bands at 1380-1590 cm -1 corresponding to chemisorbed carbonates and the one centered at 2290-2390 cm -1 corresponding to physisorbed carbonates were measured (See SI Figures S9a andS9b). The W-MFI-1600 film shows high response towards analytes with a concentration of 3-5 ppm compared to the W-MFI-600 film. Therefore the W-MFI-1600 film was further used for the follow up experiments; the concentration of the analytes was increased up to 100 ppm.

Detection of exhaust gases using zeolite films: operando IR study

The detection of exhaust gases (NO, NO2, CO and CO2 with concentrations 1 -100 ppm) in argon flow by W-MFI-1600 and Si-MFI-1600 films were followed by operando IR spectroscopy (SI, Figure S10). Spectra were recorded until a saturation at each concentration was reached.

Importantly, all experiments were performed in the presence of water (100 ppm from the Ar flow) to prove that the enhanced hydrophobicity has a significant effect on sorption capacity and sensitivity of the zeolite films. The integrated IR bands at saturation level attributed to the adsorption of the corresponding species on the Si-MFI-1600 and W-MFI-1600 films are following a Langmuir type evolution with the increase of the analytes concentrations (Figure 4). The change of the IR bands in the range 1650 -1480 cm -1 for both NO and NO2 analytes on W-MFI-1600 film, corresponding to monodentate and bidentate nitrites/nitro compounds, [START_REF] Hadjiivanov | Use of Overtones and Combination Modes for the Identification of Surface NOx Anionic Species by IR Spectroscopy[END_REF][START_REF] Hadjiivanov | Identification of Neutral and Charged NxOy Surface Species by IR Spectroscopy[END_REF] respectively are followed as a function of gas concentrations.

AUTHOR INFORMATION

Corresponding author *Email: Svetlana.mintova@ensicaen.fr ACKNOWLEDGMENT


Michelangelo Polisi

Julien Grand

Rossella Arletti

Nicolas Barrier

Sarah Komaty

Moussa Zaarour

Svetlana Mintova

Giovanna Vezzalin

Giovanna Vezzalini

CO 2 Adsorption/Desorption in FAU Zeolite Nanocrystals: In Situ Synchrotron X-ray Powder Diffraction and in Situ FTIR Spectroscopic Study

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

Introduction

The continuous increase of carbon dioxide in the atmosphere is one of the main causes of climate change [START_REF] Yang | Progress in carbon dioxide separation and capture: a review[END_REF] . As long as the global energetic is satisfied by the combustion of carbon-based fossil fuels, serious environmental problems, such as reducing the CO2 emission, must be faced with efficient solutions.

Different technologies have been examined in the last years in order to capture and separate CO2 from flue gases. CO2 capture in solid systems provides a way to permanently store, easily transport, and possibly extract carbon dioxide, skipping the technical and environmental difficulties involved in large scale direct stocking into the Earth's depths.

Several studies have explored methods of separation using materials that can operate via weak physisorption interactions or strong chemisorption processes, such as activated carbon materials, polymers, calcium oxides, microporous and mesoporous materials [START_REF] Morris | Gas storage in nanoporous materials[END_REF][START_REF] Choi | Adsorbent materials for carbon dioxide capture from large anthropogenic point sources[END_REF] . Reactions, characterized by CO2 trapping via open structure solid state systems (for instance microporous materials such as zeolites and MOF), allow to tune CO2-equilibrium in a given environment, and represent an efficient way of low-energy-storage/release of carbon dioxide.

Zeolites are appropriate candidates for selective adsorption and separation of carbon dioxide [START_REF] Siriwardane | Adsorption of CO2 on Molecular Sieves and Activated Carbon[END_REF] , not only due to their high adsorption capacity and their particularly low price [START_REF] Choi | Adsorbent materials for carbon dioxide capture from large anthropogenic point sources[END_REF][START_REF] Zukal | Adsorption of carbon dioxide on high-silica zeolites with different framework topology[END_REF] , but also because of their properties (i.e. crystal size, pore architecture, chemical composition, nature of extra-framework cations). In addition, the properties of zeolites, influencing their sorption performance, can be tuned systematically during synthesis process.

Particular attention has been paid to faujasite type zeolites (FAU framework type [START_REF] Baerlocher | Atlas of Zeolite Framework Types sixth[END_REF] ), which are considered as possible candidates for CO2 separation process. The adsorption performances of FAU type zeolites, either zeolite X or Y, were investigated with different methods. Lee et al. [START_REF] Lee | Adsorption equilibria of CO2 on zeolite 13X and zeolite X/activated carbon composite[END_REF] and Lu et al. [START_REF] Lu | Comparative study of CO2 capture by carbon nanotubes, activated carbons, and zeolites[END_REF] found that zeolite 13X possesses higher adsorption capacity for CO2, and a more favorable adsorption, with respect to activated carbon composites. Siriwardane et al. [START_REF] Siriwardane | Adsorption of CO2 on zeolites at moderate temperatures[END_REF] report that 13X zeolite shows preferential adsorption for CO2 from a gas mixture representing coal combustion compared to other commercial zeolites (4A, 5A, APG-II and WE-G 592). Also computational studies [START_REF] Maurin | Adsorption mechanism of carbon dioxide in faujasites: grand canonical Monte Carlo Simulations and microcalorimetry measurements[END_REF][START_REF] Plant | Molecular dynamics simulation of the cation motion upon adsorption of CO2 in faujasite zeolite systems[END_REF] indicate zeolites X and Y as the most promising adsorbents for CO2 storage. Recently, experimental structural studies have been carried out to investigate the adsorption properties of zeolite Na-Y by both neutron [START_REF] Wong-Ng | Investigation of NaY Zeolite with adsorbed CO2 by neutron powder diffraction[END_REF] and synchrotron X-ray powder diffraction. [START_REF] Arletti | Evidence for the formation of stable CO2 hydrates in zeolite Na -Y: Structural characterization by synchrotron X-ray powder diffraction[END_REF] Both studies indicated that the cations and water molecules distribution in the pores of zeolites strongly affects the CO2 adsorption/desorption dynamics.

In order to exploit the material functionalities and to design tailor-made materials, it is of paramount importance to unravel the mechanisms that underlie the CO2 trapping phenomena by understanding the host-guest interactions. Moreover, the interest in nanosized zeolites strongly increased in the last few years owing to the potential variation of their properties as a function of the size [START_REF] Mintova | Nanosized microporous crystals: emerging applications[END_REF] . The advantages of the use of nanosized zeolites lie in their unique properties such as tunable crystal size, chemical composition, larger external surface areas, reduced diffusion path lengths and more accessible active sites compared to micron-sized zeolite crystals. On the other hand the size of zeolite plays a major role when it comes to biomedical applications. In this work, the CO2 adsorption/desorption mechanism in two nanosized zeolite samples with FAU framework type having different Si/Al ratios, cations, and particle size distribution is investigated. Combined in situ synchrotron high resolution X-ray powder diffraction (XRPD) and in situ FTIR spectroscopy experiments were carried out to unravel the host-guest and guest-guest interactions governing the CO2 adsorption/desorption mechanism.

Experimental

Na-X and Na-Y zeolites synthesis

Materials

The following initial reagents were used for preparation of the precursor mixtures: Al powder (Al, 325 mesh, 99.5 %, Alfa Aesar); Aluminum hydroxide (Al(OH)3, Sigma-Aldrich); Sodium aluminate (Al2O3 ~ 50-56 wt. %, Na2O ~ 40-45 wt. %, Sigma-Aldrich); Sodium hydroxide (NaOH, Sigma-Aldrich, 97 %); Colloidal silica (SiO2, Ludox-HS 30, 30 wt. % SiO2, pH=9.8, Sigma ??Aldrich); Sodium silicate (SiO2 ~ 26.5 wt. %; Na2O, ~10 wt. %, Aldrich).The zeolite samples were prepared following a procedure recently published by our group. 15 Na-X. The nanosized zeolite Na-X was synthesized from a clear precursor suspension. The initial reactants were mixed to prepare two initial solutions denoted A and B according to the following procedure:

Solution A was prepared by dissolving 2.5 g of NaOH in 3 g dd H2O followed by slow addition of 0.297 g alumina powder (325 mesh, 99.5 %); Solution B was prepared by mixing 10 g colloidal silica (Ludox-HS 30, 30 wt. % SiO2, pH=9.8) with 1.1 g NaOH and 1 g dd H2O. Solution A was added drop wise under vigorously stirring to the solution B (solution B was kept in ice during the mixing).

The resulting clear suspension was aged 24 h at room temperature, and then the water content was adjusted to obtained a colloidal precursor suspension with the following composition: 10 SiO2: 1.1 Al2O3: 9 Na2O: 50 H2O. The hydrothermal crystallization was conducted at 50 °C for 24 h. After crystallization, the suspensions were purified by high-speed centrifugation (20000 rpm, 100 min).

The resulting solid was washed with doubly distilled water until pH 7.5 is reached. The nanosized crystals were freeze-dried to prevent their irreversible agglomeration.

Na-Y . The nanosized Na-Y zeolite was synthesized from a clear precursor suspension. The initial reactants were mixed to prepare two initial solutions denoted as A and B according to the following procedure: Solution A was prepared by dissolving 0.65 g of sodium aluminate in 3 g double distilled water (dd H2O); Solution B was prepared by mixing of 13 g colloidal silica (Ludox-HS 30, 30 wt. % SiO2, pH=9.8) with 2.5 g NaOH and 0.98 g (dd) H2O. Solution A was added drop wise under vigorously stirring to the solution B (solution B was kept in ice during the mixing). The resulting clear suspension was aged four days at room temperature. A colloidal precursor suspension with the following composition was obtained: 10 SiO2: 0.7 Al2O3: 6.5 Na2O: 122 H2O. The hydrothermal crystallization was conducted at 100 °C for 2 h. The crystalline product was purified and dried as described for zeolite Na-X.

Characterization of Na-X and Na-Y zeolites

Dynamic Light Scattering analysis (DLS):

The size of the zeolite nanoparticles was measured by a Malvern Zetasizer Nano instrument using a backscattering geometry (scattering angle of 173°, He-Ne laser with a 3 mW output power at a wavelength of 632.8 nm). The DLS analyses were performed on samples in water suspensions with a solid concentration of 1 wt. %.

Transmission Electron Microscopy (TEM) imaging:

The crystal size, morphology and crystallinity of zeolites were determined by a transmission electron microscopy (TEM) using a Titan 80-300 operating at 300 kV.

Inductively Coupled Plasma (ICP) analysis:

The chemical composition of the zeolites was determined by inductively coupled plasma optical emission spectroscopy using a Varian ICP-OES 720-ES; the chemical composition of nanosized zeolites are shown in Table 1.

N2 adsorption analysis:

The porosity of the samples was measured using a Micrometrics ASAP 2020 volumetric adsorption analyzer. Samples were degassed at 275 °C under vacuum overnight prior to the measurement. The external surface area and micropore volume were estimated by -plot method using Silica-1000 (22.1 m 2 g -1 assumed) as a reference. The micropore and mesopore size distributions of solids were estimated by the Nonlocal Density Functional Theory (NLDFT) and Barret-Joyner-Halenda (BJH) on the desorption branch methods, respectively.

Thermogravimetric analysis:

The moisture content and stability of samples were investigated using a SETSYS instrument (SETARAM) analyzer (heating rate of 5 °C min -1 under 40 ml.min -1 flow of air).

2.2.6

In situ FTIR study using CO2 as a probe molecule: The adsorption of CO2 on Na-X and Na-Y samples was investigated using FTIR spectroscopy. Powders were pressed (~10 7 Pa) into selfsupported discs (2 cm 2 area, ~20 mg) and placed in an IR cell equipped with KBr windows. IR spectra were recorded using a Nicolet 6700 IR spectrometer with a MCT detector and an extended-KBr beam splitter was used. Spectra were recorded in the 400-5500 cm -1 range with a resolution of 4 cm -1 and 128 scans were collected for each spectrum. A movable quartz sample holder allowed placing the self-supported discs in the infrared beam, for recording spectra, and moving it into a furnace at the top of the cell for thermal treatment. The cell was connected to a high vacuum line with a reachable pressure of ~10 -9 Bar. The samples were activated in a two-steps process: (i) 100 °C for 0.5 h to desorb most the physisorbed water, and (ii) 250 °C for 2 h, to remove any chemisorbed water. Both steps were performed under secondary vacuum (~5•10 -9 Bar). Small CO2 doses were introduced into the FTIR cell at room temperature and kept in equilibrium for 15 minutes. High vacuum desorption was then conducted at room temperature (RT) for 15 minutes, followed by desorption at 250 °C for 15 minutes.

In situ X-Ray Powder Diffraction (XRPD) study:

Synchrotron XRPD experiments were performed on the high resolution beamline ID22 at ESRF (Grenoble -France) with a fixed wavelength of 0.3999 Å. The two samples Na-Y and Na-X were packed in 0.7 quartz capillaries and mounted on a support connected to a gas-rig. The samples were heated in-situ using a hot-air blower with a heating rate of 3 °C/min from room temperature (RT) to 250°C and evacuated nominally to 5 10 -3 Torr by means of a turbo vacuum pump to favor water release. Once dehydrated, the samples were kept at RT. CO2 was sent into the capillaries (maximum pressure of 1.85 and 1.5 bar for Na-Y and Na-X, respectively) and equilibrated for 15 minutes. Then the samples were pumped 10 minutes to test the CO2 persistence in zeolite porosities.

Diffraction patterns were recorded at different times with a high-resolution multi-analyzer, composed of nine analyzer crystals, in the 2θ range 0-26° (data collection time: 37 minutes). For both samples data collections were performed as follows: (1) at RT (samples named Na-X-RT, Na-Y-RT);

(2) at 250° C (samples named Na-X-D-HT, Na-Y-D-HT); (3) at RT after dehydration (samples named Na-X-D-RT, Na-Y-D-RT); (4) after CO2 loading (samples named Na-X-CO2 , Na-Y-CO2); (5) at RT after pumping (samples named Na-X-AP, Na-Y-AP).

Structural Refinements:

All structure refinements of Na-X and Na-Y samples were performed with Rietveld method using GSAS package 16 with EXPGUI interface 17 . To locate the loaded CO2 molecules in both zeolite samples, the starting stages of refinement were carried out in the space group (s.g.) Fd-3, using the framework coordinates published by Su and co-workers [START_REF] Su | Location of Na+ Ions in Fully Dehydrated Na+-saturated Zeolite Y (FAU, Si/Al = 1.56)[END_REF] . Sodium cations, water and CO2 molecules were found by difference Fourier maps. After the localization of the extra-framework species, the refinement suggested a higher symmetry for the Na-Y zeolite. For this sample, the Fd-3m s.g. and the model of Olson and co-workers [START_REF] Olson | Reinvestigation of the crystal structure of the zeolite hydrated NaX[END_REF] were used for all refinements.

Lattice parameters and refinement details are presented in Table S1, Observed and calculated pattern after Rietveld refinement are reported in Figure S1a-h. The Bragg peak profiles were modeled by a pseudo-Voigt function [START_REF] Thomson | Rietveld refinement of Debye-Scherrer synchrotron X-Ray data from Al2O3[END_REF] , and peak intensity cut-off was set to 0.01 of the peak maximum. The background curve was fitted by a Chebyshew polynomial with a variable number of coefficient from 15 to 30. The 2θ-zero shift, the overall scale factor and the unit cell parameters were refined accurately in each cycle. Soft constraints were imposed on the tetrahedral T-O lengths (1.63 Å), and on the C-O lengths of the CO2 molecules (1.18 Å) and the tolerance value was set at 0.03. The isotropic thermal displacement parameters of the framework oxygen atoms were constrained to the same value. For samples Na-X-RT and Na-Y-RT also the thermal displacement parameters of the water molecules were refined constrained to the same value.

Results and Discussion

Zeolites characterization

TEM images show the high crystallinity of both samples. The Na-X and Na-Y zeolites present uniform and highly crystalline particles with average size of 20 nm and 150 nm, respectively (Figure 1a,b). The homogeneous zeolite nanocrystals exhibit a typical octahedral morphology with fully crystalline and well-developed faces, which become more pronounced in the case of Na-Y. The DLS curves for Na-X and Na-Y samples confirm the narrow, monomodal particle size distribution of the crystals (Figure 1c). The back-scattering of light produced from these samples was strong enough to be monitored as a function of the particle numbers. The cumulant analysis gives an average particle diameter, which is located between the minimum and maximum particle sizes present in the samples (Figure 1c). The value of the polydispersity index for both measurements is 0.05, which shows that the samples contain particles with a very narrow particle size distribution. These DLS results are consistent with the TEM measurements (Figure 1a,b). The chemical composition of the samples determined by ICP analysis are presented in Table 1. The Si/Al ratios for nanosized Na-X and Na-Y samples are 1.24 and 2.54 wt. %, respectively. The amount of water in both samples was determined by thermogravimetric analysis (TG).

The TG/dTG results of the samples are shown in Figure 2. The dTG curve of Na-X zeolite shows two main mass losses at 100 °C and 146 °C corresponding to water adsorbed in the channels and on the surface of the zeolite nanoparticles and differently bonded, contributing to 25 wt. % of its total mass loss. The Na-Y zeolite shows only one peak at 102 °C, contributing to 19 wt.% loss of its total mass. Na-Y: Na59.5(Al55.9Si136.1O384) . 167 H2O (Si/Al = 2.43)

In situ XRPD characterization of nanosized zeolites

Structural features of hydrated nanosized zeolites

The in situ XRPD measurements at RT and the corresponding Rietveld refinement allowed to obtain a snapshot of the structure features of both samples before dehydration and CO2 adsorption/desorption cycling. In particular, the position of the extra-framework species (cations and H2O molecules), which plays an important role in the CO2 sorption mechanism, was determined in both samples.

In the Na-X sample, four crystallographic independent Na sites were identified: Na2, Na2', Na3, Na6 (labelled after Olson [START_REF] Olson | Reinvestigation of the crystal structure of the zeolite hydrated NaX[END_REF] ) (Figure 3a). Na2 and Na2' sites are in the sodalite cages and correspond to site I and I' according to the conventional classification [START_REF] Frising | Extraframework cation distributions in X and Y faujasite zeolites: A review[END_REF] . Na3 site is located at the center of the hexagonal window between the sodalite cage and the supercage and corresponds to site II [START_REF] Frising | Extraframework cation distributions in X and Y faujasite zeolites: A review[END_REF] . Site Na6 is in the supercage near the site III'b [START_REF] Su | Location of Na+ Ions in Fully Dehydrated Na+-saturated Zeolite Y (FAU, Si/Al = 1.56)[END_REF] . The total amount of sodium is 96.5 atoms p.u.c., which is in good agreement with the ICP analysis (94.5 atoms p.u.c.). Six crystallographic independent water sites were found, accounting for a total of 250 H2O molecules, which is in good agreement with 252 molecules p.u.c. determined by TG analysis. Five water molecules are located in the supercage, coordinated to the Na6 cations or to the framework oxygen atoms, and one in the sodalite cage coordinated to Na2' cations (for structure details see Tables S2 andS3).

The same Na sites were found in the Na-Y sample, with the exclusion of site Na6 located in the supercage (Figure 3). The sodium total amount is 55.4 atoms p.u.c., which is in good agreement with the ICP results (59.5 Na p.u.c.). Five positions, accounting for 235 water molecules p.u.c., were located in the supercage: two of them coordinated to framework oxygen atoms and one coordinated to the Na4 cations (Tables S2 andS3). While the water content calculated by the Rietveld refinement for the Na-X is similar to that determined by the TG analysis, in the case of Na-Y, a higher amount of water was obtained by the refinement. The difference could be related to the hydrophilicity of the sample, leading to different hydration degrees in relation to the environment humidity.

Structural features of dehydrated nanosized FAU zeolites

The removal of water is a fundamental step for the use of FAU zeolites as adsorbents, since most of the porosity is occupied by H2O molecules, thus hindering the penetration of other species under ambient condition. The HT treatment at 250°C seems to be not completely effective in dehydrating the Na-X sample since the structural refinement shows the presence of about 38 residual H2O molecules. On the contrary, sample Na-Y was successfully dehydrated under the HT treatment at 250°C. Once the samples were brought back to RT conditions, due to their strong hydrophilic character, several H2O molecules were re-adsorbed and 126 and 35 H2O molecules p.u.c. were located in samples Na-X-D-RT and Na-Y-D-RT, respectively, (Table S2 and Scheme 1).

Structural features of nanosized FAU zeolites after adsorption of CO2

The adsorption of CO2 by the Na-X and Na-Y samples was followed by in situ XRPD. The host-guest and guest-guest interactions are discussed in this section. The intrinsic limit of the X-ray diffraction analysis, i.e., the determination of an average structure for the material under study, has to be taken into account

In the supercage of the Na-X sample, as a consequence of the interaction between the adsorbed CO2 and the residual H2O molecules, bi-dentate bicarbonate groups labelled CO3-1 (Figure 3c, Tables S2 andS3) were observed. The sodium cations hosted in the supercage (Na6 site) are not shielded by the framework and thus can be responsible for the CO2 polarization. The preferential formation of bicarbonate groups, associated to the presence of cations in this site (site III according to Ref. [START_REF] Frising | Extraframework cation distributions in X and Y faujasite zeolites: A review[END_REF] ), has been already discussed in Ref. [START_REF] Jacobs | Surface probing of synthetic faujasites by adsorption of carbon dioxide. Part 1.-Infra-red study of carbon dioxide adsorbed on Na-Ca-Y and Na-Mg-Y zeolites[END_REF] . One of the oxygens forming the CO3 group (site O13, Table S2) corresponds to the H2O molecule W8 in the dehydrated phase at room temperature (Na-X-D-RT).

The presence of water during the CO2 adsorption has been demonstrated to favor the formation of bicarbonates species via hydroxyl group formation [START_REF] Bonenfant | Advances in principal factors influencing carbon dioxide adsorption on zeolites[END_REF] . From the inspection of Figure 3c, one can argue that the geometry of group CO3-1, does not perfectly fit that of a bicarbonate group (i.e. with the angle O-C-O = 120°). This could be an artefact derived from the average structure determined by Xray diffraction and can be attributed to the presence, in the same crystallographic position, of a second configuration consisting of a CO2 linear molecule and a water molecule, alternatively present (Figure 4). Assuming the alternative existence of CO3, CO2 and H2O it is impossible to establish the relative amount of the three species. However, assuming the only presence of the bicarbonate, the total number of bicarbonate groups would be 29 (i.e. 3.6 molecules in each supercage). The distribution of the bi-dentate bicarbonate groups, bonded to Na cations, present in one supercage is shown in Figure 5. During the CO2 adsorption, additional 35 H2O molecules are delivered to the Na-X zeolite sample accounting for a total of 138 re-adsorbed H2O molecules; the additional water probably came from the CO2 bottle or from the gas lines.

In the Na-Y zeolite only CO2 molecules are hosted in the supercage, accounting for a total amount of 66 molecules. All the physisorbed CO2 occupy three different positions as shown in Figures 3d and6: (1) CO2-2 (see Tables S2 andS3 for structural details), accounting for 3d and6). Even in the Na-Y zeolite further water molecules are adsorbed during the delivery of the CO2 and as a whole 55 water molecules were found (not shown in Figure 3). Figure 6 shows a possible distribution of the CO2 molecules, Na cations and W3 in the supercage taking into account their occupancy factors, distances and steric hindrance. Two crystallographic equivalent CO2 clusters are located -formed by one CO2-2, two CO2-3 and one CO2-4 molecules-coordinated to Na3 cations and W3 via CO2-2 and CO2-4 molecules. Each cluster is in weak interaction with that hosted in the neighboring supercage via the CO2-3 molecule sited in the 12 membered ring (Figure 7). The presence of these large clusters probably hinders further CO2 adsorption.

The Na3 -CO2 distances found for CO2-2 and CO2-4 are consistent with the linearly coordinated molecules as reported in Ref. [START_REF] Maurin | Adsorption mechanism of carbon dioxide in faujasites: grand canonical Monte Carlo Simulations and microcalorimetry measurements[END_REF] . On the contrary, CO2-3 is not coordinating cations, but shows weak interactions with framework hydroxyl groups, as confirmed by the in situ FTIR results,

where a band at 3742 cm -1 , characteristic of silanol groups, is present in the activated zeolite (SI, Figure S2). Despite the presence of H2O molecules and cations coordinating CO2 molecules, the presence of carbonates is not detected. This probably because Na3, contrariwise to what observed for Na6 in Na-X, is well compensated by framework oxygen atoms and thus is less effective in the CO2 polarization.

Structural features of nanosized FAU zeolites after desorption of CO2

The persistence of CO2 in the zeolite samples is strongly dependent on the host-guest interactions occurring during the absorption. In our experiments the desorption mechanism of CO2

was studied under vacuum. The Na-X-CO2 sample is stable even under vacuum (5 x 10 -3 Torr): from 29 CO3 bi-dentate bicarbonate groups "nominally" adsorbed (see discussion above), about 24 remain in the pores even after pumping. On the contrary, in the Na-Y-CO2 sample, the pumping seems to be more effective than in the Na-X-CO2. The CO2 content decreases more than a half, i.e., about 31 CO2 molecules p.u.c. from the original 66 still remain in the zeolite porosities. This is consistent with the fact that in the Na-Y-CO2 sample only physisorbed CO2 molecules are present.

In situ FTIR characterization of nanosized FAU zeolites

In order to obtain more information about the adsorption mechanism of CO2 in both Na-X and Na-Y zeolites, the CO2 adsorption (at RT) and desorption (at RT and 250 °C) were monitored by in situ FTIR spectroscopy (Figures 8 and9). In both cases, the absence of the band at 1627 cm -1 in the IR spectra of the activated samples counts for the successful removal of water. Nevertheless, considering the results from the TG for sample Na-X (Figure 2), we may expect the presence of some residual water in the channels of zeolite at the activation temperature used (250° C).

Upon exposing Na-X to excess of CO2, new sets of bands appear which were not present in the IR profile of the activated sample (Figure 8, Table 2). The new band at 2352 cm -1 corresponds to CO2 physically adsorbed on the zeolite, the broadening of the feature is explained by the presence of excess CO2 in the gas phase (non-adsorbed form). As a consequence of the high CO2 content, the [START_REF] Arletti | Evidence for the formation of stable CO2 hydrates in zeolite Na -Y: Structural characterization by synchrotron X-ray powder diffraction[END_REF] C isotope (natural abundance) in CO2 is detected at 2285 cm -1 . On the other hand, bands arising between 1707 and 1366 cm -1 are attributed to the chemisorbed CO2. These correspond to different types of carbonates due to the participation of framework oxygen atoms (Table 2 and Figure 8). These results are consistent with the report by Busca 24 where bands are attributed to perturbed CO2, bicarbonates, mono-and bi-dentate carbonates. Upon subjecting the samples to high vacuum desorption (~5•10 -9

Bar) at room temperature, the band corresponding to physisorbed CO2 (and CO2 in gas phase) diminished drastically, while those corresponding to the chemisorbed CO2 remained unchanged with the exception of the bands at 1484 and 1435 cm -1 (monodentate carbonates species) that slight increase. Such feature can result from the reaction of physisorbed CO2 during evacuation. A quasi total desorption for the CO2 at 250 °C was achieved; the spectrum highly resembles that of the activated sample.

Figure 8. FTIR spectra of nanosized Na-X zeolite activated at 250 °C (black), after adsorption of 0.01 Bar CO2 (red), under secondary vacuum (~5•10 -9 Bar) at room temperature (blue), and under secondary vacuum (~5•10 -9 Bar) and at 250 °C (green).

Table 2. FTIR bands and the corresponding assignments recorded for the Na-X nanosized zeolite under adsorption of 0.01 Bar CO2. The adsorption of CO2 on the Na-Y zeolite induced the formation of a band at 2354 cm -1 corresponding to the physisorbed CO2 together with a set of bands related to chemisorbed CO2 observed between 1750 and 1300 cm -1 (Figure 9, Table 3). Unexpectedly, signals of bicarbonates were detected at 1681 and 1364 cm -1 despite the activation at 250 °C which is enough to eliminate the water from the sample as evidence by TGA. One possible explanation for their existence is the reaction of incoming CO2 with residual water (not deteced by IR) available in the zeolite channels or with the residual acid sites detected at 1653cm -1 . A desorption behavior similar to Na-X was recorded for Na-Y at RT: the band corresponding to physisorbed CO2 disappeared completely, chemisorbed CO2 (perturbed CO2 and bicarbonates) decreased significantly, while the bands corresponding to monodentate carbonates and residual acidity increased. As shown in Figure 9 (green curve), an increase of the temperature up to 250 °C was required to almost fully desorb the chemisorbed CO2. The latest spectrum (green) is very similar to that collected from the activated Na-Y (black), demonstrating the reversibility of CO2 sorption within the Na-Y nanosized zeolite with minor residual acidity maintained.

Bands (cm

, Table 3. FTIR bands and the corresponding assignments recorded for the Na-Y nanosized zeolite under adsorption of 0.01 Bar of CO2.

Bands (cm

Discussion

Nanosized Na-Y and Na-X zeolites show a different behavior in adsorbing/desorbing of CO2.

The main differences are related to the amount of CO2 adsorbed and to their interaction with the zeolite framework and extra-framework species.

In this work, for the first time, the coupling of two complementary techniques (XRPD and IR) offers the opportunity to probe the CO2 adsorption/desorption mechanisms at different levels (i.e.

average structure by XRPD vs local structure by IR). In fact, the results for Na-Y XRPD are consistent with the presence of physisorbed CO2 molecules only, while IR results show even the presence of mono dentate carbonates formed after framework oxygen interaction and bicarbonates probably formed due to the interactions with residual H2O that unintentionally reentered the zeolite channel or residual acidic sites. The chemisorbed CO2 species are probably present in very low amount that cannot be detected by X-ray diffraction. Regarding the nanosized Na-X zeolite, both physisorbed and chemisorbed species were detected by both the XRPD and IR techniques. In particular, XRPD indicates the presence of bicarbonate only, while IR detected even mono and bidentate carbonates.

The interpretation of the results relative to desorption deserves particular attention. From the spectroscopic investigations, it appears that, while the physisorbed species are removed from the zeolites channel by pumping under secondary vacuum (~5•10 -9 Bar) at room temperature, carbonates and bicarbonates remain in the pores. While the carbonates are completely removed from the pores under heating at 250 °C. On the contrary from the XRPD results it appears that both chemisorbed and physisorbed species are only partially removed from the porosity. This is due to different vacuum conditions of the two devices (~5•10 -9 Bar for IR experiment vs ~6•10 -7 Bar for XRPD one).

Previous studies performed on this topic shed light on different factors that may influence the CO2 adsorption in zeolites, finding that in addition to the structural and chemical features of the zeolite samples, other aspects must be considered [START_REF] Bonenfant | Advances in principal factors influencing carbon dioxide adsorption on zeolites[END_REF] . In fact, the carbonates/bicarbonates formation plays a key role during the carbon dioxide adsorption, the presence of these species might decrease the accessibility of CO2 molecules limiting the adsorption and hindering the regeneration capacity in successive adsorption-desorption cycles [START_REF] Bonenfant | Advances in principal factors influencing carbon dioxide adsorption on zeolites[END_REF][24][START_REF] Angell | Infrared spectroscopic investigations of zeolites and adsorbed molecules. Part V. Carbon dioxide[END_REF] . The formation of carbonates, in most of the cases, is associated with the presence of cations in the electrostatically less-shielded type III site, located in the FAU supercage, present only in the Na-X zeolite [START_REF] Jacobs | Surface probing of synthetic faujasites by adsorption of carbon dioxide. Part 1.-Infra-red study of carbon dioxide adsorbed on Na-Ca-Y and Na-Mg-Y zeolites[END_REF] . In our study the presence of carbonates/bicarbonates was evidenced in both Na-X and Na-Y samples, even if in very different amount.

The influence of water in the adsorption of CO2 is of paramount importance, being water one of the key components in flue gas. The presence of H2O molecules in the zeolite cavities during the CO2 adsorption seems to favor the formation of bicarbonates species via hydroxyl group reaction [START_REF] Siriwardane | Adsorption of CO2 on Molecular Sieves and Activated Carbon[END_REF][START_REF] Bonenfant | Advances in principal factors influencing carbon dioxide adsorption on zeolites[END_REF] .

Different behaviors in relation to H2O content were reported. Brandani and co-workers [START_REF] Brandani | The Effect of Water on the Adsorption of CO2 and C3H8 on Type X Zeolites[END_REF] observed that even small amount of water can inhibit the CO2 adsorption on different cationic forms of zeolite X. Bertsch and co-workers [START_REF] Bertsch | An Infrared Specroscopic study of the adsorption of water and carbon dioxide by Linde molecular sieve X1[END_REF] suggest that the presence of small amount of pre-adsorbed water greatly accelerate the rate of carbon CO2 adsorption, presumably by catalyzing the chemisorption step.

In our study the presence of H2O molecules favors the formation of bicarbonates in nanosized Na-X zeolite. The small amount of bicarbonate found in the Na-Y should be due to the use of moist CO2.

Two recent papers investigated from the structural point of view the CO2 adsorption in micron-sized zeolite Y by neutron powder diffraction [START_REF] Wong-Ng | Investigation of NaY Zeolite with adsorbed CO2 by neutron powder diffraction[END_REF] and Synchrotron X-ray Powder diffraction [START_REF] Arletti | Evidence for the formation of stable CO2 hydrates in zeolite Na -Y: Structural characterization by synchrotron X-ray powder diffraction[END_REF] .

In both papers only physisorbed CO2 was found, which is in agreement with our work for the nanosized Na-Y. The discrepancies on the number of molecules measured is due to different pressure conditions used for the CO2 adsorption and to the different crystal size of the samples studied. In fact in the nanosized zeolites analyzed in this work favored the adsorption leading to an higher CO2 adsorption capacity.

Wong et al. [START_REF] Wong-Ng | Investigation of NaY Zeolite with adsorbed CO2 by neutron powder diffraction[END_REF] found two crystallographically independent CO2 sites, bonding the Na cations, in the supercage. Arletti and co-workers [START_REF] Arletti | Evidence for the formation of stable CO2 hydrates in zeolite Na -Y: Structural characterization by synchrotron X-ray powder diffraction[END_REF] found the evidence of tetrameric clusters of CO2 molecules connected by water bridges to the Na cations of adjacent FAU supercages. These clusters, even if placed in the same portion of the supercage, are different from that found in this work, consisting of three/five CO2 molecules. This could be dictated by the higher amount of molecules adsorbed in consequence of the higher CO2 pressure applied in the present work. In both papers, a CO2 molecule located in the supercage (O16-C15-O16 12 and CO2b 13 ) corresponds to the CO2-4 molecule of this supercages, almost corresponds to that reported in Ref. [START_REF] Wong-Ng | Investigation of NaY Zeolite with adsorbed CO2 by neutron powder diffraction[END_REF] (O12-C11-O13), but shifted of about 1.9 Å along the molecule axis.

Conclusions

The adsorption of CO2 in two FAU zeolite samples with nanosized dimensions, one corresponding to zeolite Na-X (Si/Al=1.24) and the other to Na-Y (Si/Al=2.54) was studied by X-ray powder diffraction and FTIR spectroscopy. In the case of nanosized Na-X, 29 CO2 molecules p.u.c., while in the nanosized Na-Y 66 molecules p.u.c. were adsorbed. The structural refinement results are in line with the FTIR spectroscopy analysis, representing the presence of physisorbed CO2 in both zeolites, while chemisorbed CO2 was not detected in the Na-Y by XRPD probably due to the low amount. In the case of Na-X, on the contrary, the presence of the Na6 cations in the supercage and of water molecules induces the formation of bidentate bicarbonate groups, identified by both XRPD and FTIR.

The release of adsorbed species under high vacuum treatment (5•10 -9 Bar) from the nanosized Na-Y zeolite, where the CO2 is mainly physisorbed, is almost complete. On the contrary, for the nanosized Na-X the vacuum treatment induces the desorption of physisorbed CO2 only, while the chemisorbed species remain trapped in the pores. Only high temperature treatment (above 300 °C)

effectively removes the carbonates.

The FAU zeolite with nanosized crystals are considered to be used for target CO2 delivery in biomedicine where the ratio of physisorbed and chemisorbed CO2 is of great importance.

Figure 1 .

 1 Figure 1. TEM images of as prepared (a) Na-X and (b) Na-Y samples, and (c) DLS curves of zeolite suspensions with a solid content of 1 wt. %.

Figure 2 .

 2 Figure 2. (a) Thermogravimetric (TG) and (b) differential thermogravimetric (dTG) curves of Na-X and Na-Y zeolite samples.

Figure 3 .

 3 Figure 3. Structures of the zeolite samples under different conditions: (a) Na-X-RT, (b) Na-Y-RT, (c) Na-X-CO2 (after CO2 adsorption), (d) Na-Y-CO2 (after CO2 adsorption), (e) Na-X-AP (after CO2 desorption), and (f) Na-Y-AP (after CO2 desorption); a, b: the Na sites are drawn according to their occupancy factors; c, d, e, f: only the Na sites in the supercage are shown. Water molecules are not shown for sake of clarity.

Scheme 1 .

 1 Scheme 1. Number of H2O molecules in as-prepared Na-X and Na-Y zeolites at room temperature (Na-X -RT, Na-Y-RT), at 250° C and under vacuum (5• 10 -3 Torr) (Na-X-D-HT, Na-Y-D-HT) and back to RT (Na-X-D-RT, Na-Y-D-RT) as determined by in situ XRPD and Rietveld refinement.

Figure 4 .

 4 Figure 4. Possible configurations of the CO3-1 group in Na-X-CO2 sample.

Figure 5 .

 5 Figure 5. Possible distribution of CO3 groups and Na6 cations in the supercage of the Na-X-CO2 sample, taking into account their occupancy factors and distances (only bonded Na6 are drawn).

Figure 6 .

 6 Figure 6. Possible distribution of CO2 molecules, Na cations and W3 molecule in the supercage of the Na-Y-CO2 sample (only Na3 and W3 bonded to the clusters are drawn). The labels C2, O21 and O22 represent the CO2-2 molecule, C3, O31 and O32 the CO2-3 molecule and C4, O41 and O42 the CO2-4 one.

Figure 7 .

 7 Figure 7. Interactions among CO2-2 and CO2-3 molecules in neighboring supercages in Na-Y-CO2 sample.

Figure 9 .

 9 Figure 9. FTIR spectra of nanosized Na-Y zeolite under activation at 250 °C (black), after adsorption of 0.01 Bar CO2 (red), under secondary vacuum (~5•10 -9 Bar) at room temperature (blue), and under secondary vacuum (~5•10 -9 Bar) and activation at 250 °C (green).

  addition, the position of CO2-3 molecule of this work, in the window between two

  The modifications of zeolite crystal size, from micron to nanoscale yielding biocompatible nanozeolites become of important research field. The target delivery of CO2 by nanozeolites is considered as an essential moderator in human physiology, controlling blood circulation and pH. (Ref 15: S. Komaty, C. Anfray, M. Zaarour, H. Awala, Valérie Ruaux, Samuel Valable, and Svetlana Mintova, "A Facile Route toward the Increase of Oxygen Content in Nanosized Zeolite by Insertion of Cerium and Fluorinated Compounds" Molecules (2018) 23, 37-40. Ref 16: A. Clément, D. Biao; S. Komaty, S. Mintova, S.

	Valable, "Acute toxicity of silver free and encapsulated in nanosized zeolite for eukaryotic cells"
	ACS Applied Materials & Interfaces (2017) 9, 13849-13854.; Ref. 17 : V. Georgieva, R. Retoux, V.
	Ruaux, V. Valtchev, S. Mintova "Detection of CO2 and O2 by iron loaded LTL zeolite films"
	Frontiers of Chemical Science and Engineering 12 (2018) 94-102.)

Table 1 .

 1 Chemical composition of nanosized Na-X and Na-Y zeolite samples determined by ICP.

	Sample	Si (wt. %)	Al (wt. %)	Na (wt. %)	Si/Al
	Na-X	19.5	15.8	14.5	1.24
	Na-Y	43.1	17.0	15.4	2.54

  An interesting observation lies on the existence of bidentate carbonate in Na-X, but not in Na-Y. The lower Si/Al ratio of the Na-X sample induces a closer proximity of Al within the framework and then of Na cations, and thus higher possibilities for the formation of bidentate species exist.

			Activated CO2	2 d vac RT	2 d vac HT
	2354	Physisorbed CO2	0	34.86	0.15	0
	1709	CO2 molecules perturbed	0	16.97	4.69	0.22
	1681 & 1364	Bicarbonates	0	16.92	8.43	0.53
	1653	Residual acidity	0	3.48	5.35	1.69
	1488 & 1429	Monodentate carbonates	0	1.94	6.33	0.83
	-1 )	Assignements		FTIR band area (a.u.)

Acknowledgment

Authors thank Dr. Carlotta Giacobbe and all the staff of ID22 beamline at ESRF (Grenoble, France), for their help during XRPD data collections.

Supplementary Material deposited:

Table S1. Lattice parameters and Rietveld refinement according factors for Na-X and Na-Y structures under the different conditions. Table S2. Atomic coordinates, occupancy factors and thermal parameters for Na-X (a) and Na-Y (b) structures under the different conditions. Table S3. Selected framework and extraframework distances for Na-X (a) and Na-Y (b) structures under the different conditions.


Dr Maxime Debost

Dr Paul B Klar

Dr Nicolas Barrier

Dr Edwin B Clatworthy

Dr Julien Grand

Dr Fabien Laine

Dr Petr Brázda

Dr Lukas Palatinus

Dr Nikolai Nesterenko

Dr Philippe Boullay

Dr Svetlana Mintova

Synthesis of discrete CHA zeolite nanocrystals without organic templates for selective CO 2 capture

Keywords: Nanozeolites, Template-free synthesis, CO2 adsorption, CO2 In-situ PXRD, precession-assisted 3D ED

Small-pore zeolites such as chabazite (CHA) are excellent candidates for the selective separation of CO2, however, the current synthesis involves several steps and the use of organic structuredirecting agent (OSDA), increasing their cost and energy requirements. Here we report the synthesis of small-pore zeolite crystals (aluminosilicate) with CHA-type framework structure by direct

synthesis in a colloidal suspension containing a mixture of inorganic cations only (Na + , K + and Cs + ). The location of CO2 molecules in the host structure was revealed by 3D electron diffraction (3D ED). The high sorption capacity for CO2 (3.8 mmol/g at 121 kPa), structural stability and regenerability of the discreate CHA zeolite nanocrystals is maintained for 10 consecutive cycles without any visible degradation. The CHA zeolite (Si/Al = 2) reaches an almost perfect CO2 storage capacity (8 CO2 per unit cell) and high selectivity (no CH4 was adsorbed).

The development of affordable and energy-efficient materials for the chemical and petrochemical industry is crucial as this sector is the largest consumer of energy and the third largest direct emitter of greenhouse gases. [START_REF] Fischedick | Climate Change 2014: Mitigation of Climate Change, in Chapter 10 -Industry. IPCC Working Group III Contribution to AR5[END_REF][START_REF]Tracking Clean Energy Progress[END_REF] The demand for new materials for CO2 capture or separation from natural gas is of significant importance. New effective sorbents are expected to meet many important criteria such as high capacity, selectivity and stability, as well as recyclability and fast kinetics. [START_REF] Patel | Carbon Dioxide Capture Adsorbents: Chemistry and Methods[END_REF][START_REF] Bacariza | Tuning Zeolite Properties towards CO2 Methanation: An Overview[END_REF] Small-pore zeolites are attractive candidates because of the shape and size and of the pores, as well as the presence of extra-framework cations. [START_REF] Dusselier | Small-Pore Zeolites: Synthesis and Catalysis[END_REF] By blocking the pore entries, the extraframework cations selectively provide access for molecules to the zeolite channels and cages depending on the nature of the molecules. [START_REF] Li | Discriminative separation of gases by a 'molecular trapdoor' mechanism in chabazite zeolites[END_REF] A high selectivity towards CO2 over CH4 for small-pore zeolites with RHO, MER and CHA type framework has been observed, indicating that the cation gating effect permits the uptake of CO2 but not CH4. [START_REF] Georgieva | Triggered Gate Opening and Breathing Effects during Selective CO2 Adsorption by Merlinoite Zeolite[END_REF][START_REF] Palomino | Zeolite Rho: a highly selective adsorbent for CO2 /CH4 separation induced by a structural phase modification[END_REF] The framework of CHA type zeolites consists of double 6membered ring (D6R) and cha cage units, where the super cages (10 × 8 × 8 Å 3 ) are interconnected by a 3-dimensional pore system with 8-membered ring (8MR) entries (3.8 × 3.8 Å 2 ). [START_REF] Smith | Site Preferences in the Mixed Cation Zeolite, Li,Na-Chabazite: A Combined Solid-State NMR and Neutron Diffraction Study[END_REF] The selective adsorption of CO2 in CHA zeolites containing various cations (K + , Rb + , or Cs + ) have been investigated demonstrating that the cations controlling the adsorption of CO2 over CH4 are the ones located at the centre of the 8MRs. Their high separation ability was attributed to cation gating behaviour described as a molecular "trapdoor" mechanism. [START_REF] Du | An optimal trapdoor zeolite for exclusive admission of CO2 at industrial carbon capture operating temperatures[END_REF][START_REF] Shang | Determination of Composition Range for "Molecular Trapdoor" Effect in Chabazite Zeolite[END_REF][START_REF] Shang | Adsorption of CO2, N2, and CH4 in Cs-exchanged chabazite: A combination of van der Waals density functional theory calculations and experiment study[END_REF][START_REF] Li | Temperature-regulated guest admission and release in microporous materials[END_REF] Shang et al. [START_REF] Li | Discriminative separation of gases by a 'molecular trapdoor' mechanism in chabazite zeolites[END_REF] proposed that CO2 and CO interact strongly with the 8MR door-keeping cations (K + , Rb + , Cs + ) due to their respective quadrupole and dipole moment and higher polarizability as compared to N2 and CH4. This interaction displaces the door-keeping cations located at site SIII' (Figure 1a) from the 8MR, allowing the selective entry of molecules into the super cage of the CHA zeolite.

The cation displacement is temporary and reversible, thus the "weakly" interacting molecules such as CH4 and N2 are excluded while CO and CO2 are selectively adsorbed. Shang et al. [START_REF] Shang | Determination of Composition Range for "Molecular Trapdoor" Effect in Chabazite Zeolite[END_REF] determined that a CHA zeolite with an Si/Al ratio lower than 3 will display "trapdoor" behaviour for the selective CO2 adsorption from a mixture containing CH4 provided the extra-framework cations are located at the centre of the 8MRs.

So far, CHA type zeolites have been synthesised using organic structure-directing agents (OSDAs), [START_REF] Zones | Zeolite SSZ-13 and its method of preparation[END_REF] seeds [START_REF] Imai | Direct crystallization of CHA-type zeolite from amorphous aluminosilicate gel by seed-assisted method in the absence of organic-structure-directing agents[END_REF] or by recrystallization of H-FAU zeolite to K-CHA under alkaline treatment. [START_REF] Bourgogue | Process for the preparation of synthetic zeolites, and zeolites obtained by this process[END_REF] Previously, Liu et al. [START_REF] Liu | Synthesis of low-silica CHA zeolite chabazite in fluoride media without organic structural directing agents and zeolites[END_REF] reported the synthesis of OSDA-free pure CHA-type zeolite using NH4F. This method affords micron-sized crystals of 15-20 µm with a low amount of defects, however, the large size of the crystals affects their performance in gas adsorption [START_REF] Prodinger | Impact of chabazite SSZ-13 textural properties and chemical composition on CO2 adsorption applications[END_REF] and catalysis [START_REF] Li | Synthesis of nano-SSZ-13 and its application in the reaction of methanol to olefins[END_REF] due to diffusion limitations. Up to now, nanosized CHA zeolite has been obtained either by recrystallization of FAU-type zeolite synthesized with the assistance of an OSDA [START_REF] Takata | Nanosized CHA zeolites with high thermal and hydrothermal stability derived from the hydrothermal conversion of FAU zeolite[END_REF] or by ball milling. [START_REF] Anand | Downsizing the K-CHA zeolite by a postmilling-recrystallization method for enhanced base-catalytic performance[END_REF] We here report on the OSDA-free synthesis of a CHA type zeolite with excellent CO2 sorption capacity and selectivity. Nanosized CHA type zeolite crystals with a size below 200 nm and a Si/Al ratio of 2.0 are obtained by direct hydrothermal treatment of colloidal precursor suspensions containing a mixture of inorganic structure directing cations only (Na + , K + and Cs + ). The nanosized CHA crystals with plate-like morphology are shown in Figure 1b andc. The mixed inorganic cations were not only used to prepare the clear colloidal precursor suspensions, but also to prevent Ostwald ripening [22] during the crystallization process and facilitate the formation of nanosized CHA crystals. The use of the mixture of three inorganic cations (Na + , K + and Cs + ) is required, and their concentration was fine-tuned in order to synthesize nanosized CHA zeolite crystals free of impurities (Figure 1d, Table S1). The formation of secondary phases, such as RHO or FAU type zeolites, was avoided by optimizing the chemical composition of the colloidal precursor suspensions and the physical parameters controlling the synthesis (time of aging, speed of stirring, and synthesis temperature) (Table S1). The Le Bail refinement of the powder X-ray diffraction (PXRD) pattern (Figure 1d) using the space group R-3m confirmed that single phase CHA zeolite was synthesized. The refinement also revealed the presence of anisotropic peak broadening due to the plate-like shape of the crystallites and a certain degree of structural disorder.

The size of the CHA crystallites, as estimated by the Scherrer equation, [START_REF] Patterson | The Scherrer Formula for X-Ray Particle Size Determination[END_REF] is approximately 42 nm (thickness, along the c-axis) by 189 nm (width, in the ab-plane) which is in agreement with the HRTEM observations. Furthermore, the NMR spectroscopic results confirmed the formation of crystalline CHA zeolite. The 27 Al NMR spectrum contains only one peak at 57.8 ppm corresponding to tetrahedrally coordinated Al (Figure S1), while the [START_REF] Gemmi | 3D Electron Diffraction: The Nanocrystallography Revolution[END_REF] Si NMR spectrum contains peaks at -89.3 ppm, -93.6 ppm, -99.1 ppm, -104.6 ppm and -109.1 ppm corresponding to Q 0 (4Al), Q 1 (3Al), Q 2 (2Al), Q 3 (1Al) and Q 4 (0Al) silicon tetrahedrally coordinated, respectively. [START_REF] Kadja | Revisiting the seed-assisted synthesis of zeolites without organic structuredirecting agents: insights from the CHA case[END_REF] A Si/Al ratio of 2.0 was estimated according to the equation of Engelhardt and Michel using the [START_REF] Gemmi | 3D Electron Diffraction: The Nanocrystallography Revolution[END_REF] Si NMR data. [START_REF] Engelhardt | High-Resolution Solid-State NMR of Silicates and Zeolites[END_REF] 3a). Based on the TGA, the number of CO2 and H2O molecules per unit cell was estimated to be 8 and 28 for CO2-loaded and hydrated nanosized CHA zeolite sample, respectively (Table S2). In addition, TGA (Figure 3a) over 10 cycles of CO2 adsorption/desorption show that the sorption capacity of the nanosized CHA zeolite is fully preserved. The high stability and regenerability of the nanosized CHA zeolite is also confirmed by the absence of any changes in the PXRD patterns of the sample after 10 cycles (Figure S2). The nanosized CHA zeolite adsorbed 3.8 mmol/g CO2 at 121 kPa and 273 K (Figure S3) which is significantly higher than the Cs-CHA prepared by ion exchange from OSDA synthetized CHA (< 2.5 mmol/g at 253 K) [START_REF] Li | Discriminative separation of gases by a 'molecular trapdoor' mechanism in chabazite zeolites[END_REF] and similar to the pure K-CHA zeolite (> 4 mmol/g). This value is also comparable with the maximum CO2 uptake obtained in SSZ-13 (CHA type) zeolites. [START_REF] Hudson | Unconventional, highly selective CO2 adsorption in zeolite SSZ-13[END_REF][START_REF] Pham | Molecular Basis for the High CO2 Adsorption Capacity of Chabazite Zeolites[END_REF] After demonstrating the high CO2 adsorption capacity of the nanosized CHA type zeolite, the selectivity was investigated by studying the adsorption of CH4. The nanosized CHA zeolite demonstrated high selectivity for CO2 over CH4 as shown by both TGA (Figure 3) and in situ FTIR (Figure S4) experiments. The selective CO2 adsorption in the CHA zeolite is confirmed by the presence of a band at 2345 cm -1 corresponding to the physisorbed CO2, and bands in the range 1316-1680 cm -1 representing the chemisorbed CO2 (Figure S4a). No CH bands in the region 2700-2900 cm -1 corresponding to CH4 are present in the IR spectra (Figure S4b). In terms of the CO2 adsorption capacity and selectivity, the performance of the nanosized CHA rivals the best CHA zeolites synthesized using OSDA. [START_REF] Hudson | Unconventional, highly selective CO2 adsorption in zeolite SSZ-13[END_REF][START_REF] Pham | Molecular Basis for the High CO2 Adsorption Capacity of Chabazite Zeolites[END_REF] This goal is reached without the need for high-temperature calcination (above 500 ºC) to eliminate the OSDA and post-synthetic ion exchange to ensure CO2 selectivity. To gain further insight into the selective CO2 adsorption by the nanosized CHA, a single crystal structure investigation was performed on a CO2 loaded sample in order to obtain a complete structure determination including the localization of extra-framework cations and guest molecules (CO2). In the past, the fine structure of Chabazite was investigated by single crystal X-ray diffraction. [START_REF] Dent | Crystal structure of chabazite, a molecular sieve[END_REF] In our case, due to the small size of the CHA nanocrystals (less than 10 -2 µm 3 ), structure analysis based on precession-assisted 3D ED [START_REF] Gemmi | 3D Electron Diffraction: The Nanocrystallography Revolution[END_REF] was carried out. The measurements of six different nanosized single crystals were combined to obtain a high-quality data set (Table S3). The electrostatic potential map obtained by ab initio structure solution (charge flipping method [START_REF] Palatinus | Superflip -a computer program for the solution of crystal structures by charge flipping in arbitrary dimensions[END_REF] ) revealed two occupied extra-framework sites: Site SII above the D6R with oxygen distances of about 2.8 Å and Site SIII' at the center of the 8MR with oxygen distances between 3.2 and 3.5 Å (Figure 4). Taking dynamical diffraction effects into account, [START_REF] Palatinus | Structure refinement using precession electron diffraction tomography and dynamical diffraction: tests on experimental data[END_REF] The refinement results (more details in Supporting Information)

indicate that the SII site is a split site occupied by Na (40%) and K (60%), whereas the SIII' site is partially occupied by 73.4(3)% Cs only (Table S4 and Figure S5). Despite having less than 1 Cs + cation per 8MR, this CHA zeolite exhibits an overall molecular "trapdoor" effect that prevents CH4 to be adsorbed. [START_REF] Shang | Determination of Composition Range for "Molecular Trapdoor" Effect in Chabazite Zeolite[END_REF] A remarkable feature illustrating the role of the Cs + cation in the "trapdoor" effect is that the mean atomic displacement parameter of the Cs + cation perpendicular to the 8MR door is very large (U^= 0.346 Ų) compared to the displacements parallel to the 8MR (U∥ = 0.019 and 0.030 Ų). Cs + cations are strongly displaced back and forth from the center of the 8MR site even at low temperature (T = 100 K). This is consistent with previous NMR measurements and DFT calculations on Cs-exchanged CHA where the Cs + cations were reported to demonstrate greater mobility in the presence of CO2. [START_REF] Li | Discriminative separation of gases by a 'molecular trapdoor' mechanism in chabazite zeolites[END_REF] The possibility to have such an atomic displacement induced by the electron beam can be disregarded as similar behavior is confirmed from Rietveld refinement based on the PXRD data recorded at room temperature (see Supporting Information). This tendency of the door-keeping cation to move away from its average position allows some molecules to go through and enter the supercage. After the completion and refinement of the host structure, the difference potential map revealed the position of the CO2 guest molecules within the cha supercage (Figure 5). Even though the individual atoms were not well discernible, the position of the CO2 molecule refined to reasonable values, and the refinement was stable with a minimum set of constraints on the molecule geometry (Tables S5 andS6). The CO2 molecules are approximately aligned with the long axis of the CHA unit cell and coordinate the extra-framework cations. The shortest host-guest contacts are 2.621(12) Å for CO2-K + and 3.105(9) Å for CO2-Cs + (Table S7). This is, to our knowledge, the first direct determination of the CO2 position in CHA zeolite with low Si/Al ratio. Notably, this position is different from any of the positions found in Li-, Na-and K-CHA zeolites using neutron powder diffraction data [START_REF] Smith | Site Preferences in the Mixed Cation Zeolite, Li,Na-Chabazite: A Combined Solid-State NMR and Neutron Diffraction Study[END_REF] , where CO2 is located predominantly in the center of the 8MR at site SIII'. Noteworthy, the refined CO2 position, the CO2-Cs + distance and the increased mobility of Cs due to the presence of guest CO2 are consistent with first principle calculations performed for low Si/Al ratio Cs-CHA zeolites. [START_REF] Shang | A density functional theory study for the adsorption of various gases on a caesium-exchanged trapdoor chabazite[END_REF] The position of the CO2 molecules was also confirmed by Rietveld Eight CO2 molecules per unit cell correspond to ~2.5 mmol/g, which is less than the measured sorption capacity at ambient pressure (Figure S3). We identified a similar discrepancy between the crystallographic results and sorption measurements in the work by Pham et al. [START_REF] Pham | Molecular Basis for the High CO2 Adsorption Capacity of Chabazite Zeolites[END_REF] A likely explanation is that crystallographic studies are only sensitive to CO2 molecules that are adsorbed inside the zeolite crystals, i.e. in the cages. However, CO2 is adsorbed in the mesopores between the CHA nanocrystals (textural mesopores) and at the surface of the crystals that increases the amount of the measured CO2. From single crystal diffraction data, [START_REF] Simon | Single-Crystal Refinement of the Structure of Carbon Dioxide[END_REF] the estimated volume needed by a single CO2 molecule in a solid is ~44.5 Å 3 .

Considering the CHA framework and extra-framework cations, the volume accessible to CO2 molecules is approximately 500 Å 3 per unit cell, which corresponds to a maximum of about 11 CO2 molecules per unit cell. About 73% of this hypothetical upper limit is reached in our nanosized CHA zeolite sample. Taking inter-atomic distances into account, any so far reported CO2 site in CHA [START_REF] Pham | Molecular Basis for the High CO2 Adsorption Capacity of Chabazite Zeolites[END_REF] does not allow to host more than 9 molecules per unit cell (see Supporting Information). This limit is obeyed in CHA zeolites with Si/Al = 6-12 reported by Pham et al. [START_REF] Pham | Molecular Basis for the High CO2 Adsorption Capacity of Chabazite Zeolites[END_REF] The refined number of molecules is ~8.2 for both Li-and Na-CHA, ~5.5 for pure Si-CHA (all three based on powder neutron diffraction at T = 10 K) and ~6.0 for K-CHA (based on PXRD).

However, the majority of the molecules in all these zeolites occupies the 8MR, rendering the desirable trapdoor effect impossible. Thus, with 8 CO2 per unit cell, the CHA zeolite of this work reaches an almost perfect CO2 storage capacity together with the excellent selectivity (no CH4 was adsorbed).

In summary, we report the direct and environmentally benign synthesis of nanosized CHA zeolite (Si/Al = 2) without the use of OSDAs. A mixture of three alkali cations, Cs + , K + and Na + , was used as an inorganic templating agent to direct the synthesis of the nanosized CHA zeolites, and in particular Cs + was employed as the primary cation responsible for the selective adsorption of CO2 over CH4. In addition to the improvement in the pore accessibility and the adsorption kinetics, the nanosized CHA zeolite demonstrated a high CO2 absorption capacity. Moreover, the Cs + cations improve the stability of the CHA structure, demonstrating recyclability under high temperature treatment and multi-cycle CO2 adsorption experiments.

This environmentally benign synthesis of CHA nanocrystals is energyefficient and more affordable than other methods reported so far in the open literature. Potentially scalable for industrial applications, the nanosized CHA zeolite is a highly promising material for applications in both CO2 capture and CO2/CH4 separation.

Figure 1 .

 1 Figure 1. Identification of nanosized CHA. a, CHA framework (SG: R-3m) projected along the b-axis showing the super cage (in light blue). Bonds between Si/Al (tetrahedral sites) and oxygens, D6R and 8MR units and sites for extra-framework cations (denoted S) are indicated for clarity. b, TEM image of nanosized CHA crystals synthesized without OSDAs in aqueous solution under hydrothermal synthesis conditions (90 °C for 8 h). Length of the scale bar is 200 nm. c, High-resolution TEM image of a crystallite looking along [1-10]. Length of the scale bar is 40 nm. Inset: Fourier transform from the area encircled in black. d, Le Bail refinement of the powder X-ray diffraction pattern of the assynthesized CHA nanosized crystals.

Figure 2 .

 2 Figure 2. In-situ PXRD study. CHA nanocrystals were subjected to adsorption of CO2 after activation (350 °C under N2): (a) schematic representation of the measurement cycles at different temperatures and atmospheres. The assynthesized CHA was activated under heating at 350 °C (Steps 1 to 2). Then the samples were measured at 350 °C under CO2 flow (40 mL.min -1 keeping the pressure at 1 bar) with decreasing the temperature until room temperature (Steps 3 to 4). The CHA was reactivated under heating at 350 °C to desorb the CO2 molecules (Steps 5 to 2'). More details are provided in the experimental section (method). b, Change of the diffraction intensity of 2-10 reflection at 12.8° (2θ) over the measurement cycle. c, Evolution of the unit-cell parameters a (disks) and c (squares) vs temperature and atmospheres.

Figure 3 .

 3 Figure 3. CO2 vs CH4 absorption. a, Thermogravimetric analysis (TGA) of nanosized CHA zeolite subjected to 10 consecutive adsorption/desorption cycles of CO2: temperature variation (black line), CO2 sorption (red rectangles) and mass variation (red line). b, mass variation upon CO2 adsorption (red line) and (c) CH4 adsorption (green line) on as-synthesized nanosized CHA. The CH4 flow was delayed and started at 25 °C instead of 350 °C for safety reasons.

  a structural model for the nanosized CHA zeolite was refined with an overall composition of Cs6.61(2)K3.6Na2.4(Si23.4Al12.6)O72(CO2)8, which is in agreement with the chemical composition Cs6.3(4)K3.6(2)Na2.5(5)(Si23.6(9)Al12.4(7))O72(CO2)8.0(3) determined by EDX combined with TGA.

Figure 4 .

 4 Figure 4. Structure solution. Electrostatic potential map obtained from ab initio structure solution using single nanocrystals PEDT datasets. Isosurface levels correspond to 3.5σ[V(r)]. a, Projection along c. Yellow, red and green transparent colours indicate framework, SII and SIII' sites, respectively. Tetrahedral sites (blue lines) and framework oxygen atoms (red lines) are included for clarity. b, Side view of eight membered ring (8MR) with elongated potential around SIII' site. c, Perpendicular view of 8MR. d, Side view of D6R with SII sites.

  refinement of PXRD data (Tables S8-S9 and FigureS8), complementing the ED results. These structural results are further supported by the superposition of the calculated micropore volume of the cha cage and the refined position of the CO2 molecule (Figure5).

Figure 5 .

 5 Figure 5. CO2 position in nanosized CHA zeolite probed by electron diffraction. a, Projection of the refined structure along the a direction showing the localization of CO2 molecules inside the super cage. The plotted atom size represents a probability sphere of 40% for the extra-framework cations and 10% for the CO2 molecules based on refined ADPs. b, Overlay of difference potential map (grey and yellow isosurface levels correspond to 2σ[ΔV(r)] and 3σ[ΔV(r)], respectively) and the refined position of the CO2 molecule. c, Expected potential map based on calculated structure factors. In (d) and (e), the positions of the CO2 molecules are superimposed on the void volume of the cha cage along a and c direction, respectively. The occupancy of each displayed CO2 molecule is 22.2%, i.e. on average only 2.67 of the 12 shown molecule sites are occupied.

We report on the synthesis of nanosized CHA type zeolite without organic structure-directing agents. The zeolite material exhibits excellent CO2 storage capacity, a high stability under high temperature treatment and selectivity towards CO2 over CH4, controlled by Cs+ in the host structure. The environmentally benign and energy-efficient synthesis of CHA nanocrystals is more affordable than other methods reported so far in the open literature.

Acknowledgements

Financial support from CARNOT ESP (project NZAMS-CO2), TOTAL and Industrial Chair ANR-TOTAL "Nanoclean Energy" is acknowledged, as well as from the Normandy Region through the RIN Recherche Program. The work at IoP Prague was performed withing CzechNanoLab Research Infrastructure supported by MEYS CR (LM2018110) using instruments of the ASTRA laboratory established within the Operation program Prague Competitiveness project CZ.2.16/3.1.00/24510 and supported by the Czech Science Foundation, project number 19-08032S.