Gérard Delahay

Selective catalytic reduction of NO by NH 3 on cerium modified faujasite zeolite prepared from aluminum scraps and industrial metasilicate

Keywords: Aluminum Scraps, Ce-Y, NH 3 -SCR, Hydrothermal stability

This work was devoted to the study of the selective catalytic reduction of NO by NH 3 on calcined and hydrothermal treated cerium loaded zeolite catalysts. The parent faujasite zeolite Na-F (Si/Al = 1.32 and S BET = 749 m 2 /g) used as support for the preparation of the catalysts was obtained from industrial sodium metasilicate and aluminum scraps. As expected, the NO conversion increased with increasing the percentage of cerium in the structure of the faujasite zeolite. Total NO conversion into N 2 was reached at 400°C at a space velocity of 250,000 h -1 .

The high conversion is due to the redox shift between Ce 3+ / Ce 4+ and the strong acid sites related to the rare earth present in the framework that is the key in SCR of NO process.

Moreover, the highest loaded cerium catalyst retained high almost its activity after thermal hydrotreatment at 850°C. This higher loading is desirable for both activity and stability provided that two stages of preparation are used to put the Ce ions in the sodalite cages.

Introduction

The nitrogen oxide NO is a harmful gas which is produced during combustion of fossil fuels at high temperatures. It is essentially issued from mobile or stationary sources by oxidation of the nitrogen of the air. The major sources that produce NO are the fossil fuels like coal in the electrical power plants or fuels in the engines of gasoline and diesel cars [START_REF] Brandenberger | The state of the art in selective catalytic reduction of NOxby ammonia using metal-exchanged zeolite catalysts[END_REF]. NO is harmful to health and environment; it contributes to death and serious respiratory illness like asthma, chronic bronchitis [START_REF] Epa | Overview of the human health and environmental effects of power generation: focus on sulfur dioxide, nitrogen oxides, and mercury[END_REF], [START_REF] Gao | A Review on Selective Catalytic Reduction of NOx by NH3 over Mn-Based Catalysts at Low Temperatures: Catalysts, Mechanisms[END_REF]. Furthermore, NO acidifies surface water (acid rain) reducing biodiversity and killing fishes. It is also damage forest ecosystem and contribute to the decreaseofvisibility by the formation of the ''smog''. For these reasons, the emissions of NO must be limited.

The NO molecule is thermodynamically unstable. Nevertheless, it does not decompose because of its high activation energy (364kJ/mol). Thus, a catalyst must be used to facilitate its decomposition and reduced it to N 2 and water [START_REF] Ille | L'u s t l[END_REF][START_REF] Roy | Catalysis for NOxabatement[END_REF]. The oxygen produced from the decomposition of NOor the gaseous mixture strongly adsorbed on the active sites of catalyst which causes their poisoning [START_REF] Roy | Catalysis for NOxabatement[END_REF]. To avoid this inhibiting effect on the transformation rate of NO, it is necessary to work in the presence of reducing agents like H 2 , NH 3 , hydrocarbon (HC) [START_REF] König | Combined removal of particulate matter and nitrogen oxides from the exhaust gas of small-scale biomass combustion[END_REF][START_REF] Moreno-González | Evidence of a Cu2+-Alkane Interaction in Cu-Zeolite Catalysts Crucial for the Selective Catalytic Reduction of NOx with Hydrocarbons[END_REF].The most common technology in the elimination of NO is the selective catalytic reduction of NO by NH 3 (NH 3 -SCR). Two main types of catalysts are commercially used for this technology: V 2 O 5-WO 3 /TiO 2 and Cu,Fe-zeolites. For the former type of catalyst, vanadium, which is toxic sublimes at high at high temperature. Moreover, it shows high oxidation of SO 2 to SO 3 which leads to decrease the activity and selectivity of NO x at 400°C [START_REF] You | Influence of Mn/Ce ratio on the physicochemical properties and catalytic performance of graphene supported MnOx-CeO2 oxides for NH3-SCR at low temperature[END_REF][START_REF] Yao | Selective catalytic reduction of NOx by NH3 over CeO2 supported on TiO2: Comparison of anatase, brookite, and rutile[END_REF][START_REF] Li | Ultra-low loading of copper modified TiO2/CeO2 catalysts for lowtemperature selective catalytic reduction of NO by NH3[END_REF]. For these reasons, copper and iron exchanged zeolites have been developed and are preferred for mobile Diesel sources [START_REF] Grzybek | Characterization of Co and Fe-MCM-56 catalysts for NH3-SCR and N2O decomposition: An in situ FTIR study[END_REF][START_REF] Ruggeri | Modelling the Hydrothermal Ageing of a Fe-Zeo-lite Catalyst for Automotive NH 3 -SCR Applications[END_REF][START_REF] Wang | Zeolite structure effects on Cu active center, SCR performance and stability of Cu-zeolite catalysts[END_REF]. In the last decades, the cerium-catalysts were investigated in the NH 3 -SCR of NO because of its redox property (Ce 3+ to Ce 4+ ) that promotes the activation of reactants on the surface of the catalysts [START_REF] Zhao | Simultaneous removal of elemental mercury and NO in simulated flue gas over V2O5/ZrO2-CeO2 catalyst[END_REF][START_REF] Wang | Rare-Earth-Doped Pt/Ba/Ce0. 6Zr0. 4O2-Al2O3 for NOx Storage and Reduction: The Effect of Rare-Earth Doping on Efficiency and Stability[END_REF]. The redox shift between Ce 4+ and Ce 3+ of CeO 2 can promote NO oxidation to NO 2 , which is beneficial to the NH 3 -SCR reaction [START_REF] Guo | Journal of Industrial and Engineering Chemistry Effect of preparation methods on the performance of CeO 2 / Al 2 O 3 catalysts for selective catalytic reduction of NO with NH 3[END_REF]. As such, CeO 2 has been widely used as key component of selective catalytic reduction catalyst for NO x removal such as CeO 2 /TiO 2 [START_REF] Yao | Morphology and Crystal-Plane Effects of CeO2 on TiO2/CeO2 Catalysts during NH3-SCR Reaction[END_REF], CeO 2 /Al 2 O 3 [START_REF] Jin | Praseodymium Oxide Modified CeO 2 / Al 2 O 3 Catalyst for Selective Catalytic Reduction of NO by NH 3[END_REF], CeO 2 /zeolite [START_REF] Ingole | A review on Selective Catalytic Reduction technique for diesel engine exhaust after treatment[END_REF], Mn-Ce/TiO 2 [START_REF] Boningari | Influence of elevated surface texture hydrated titania on Ce-doped Mn / TiO 2 catalysts for the low-temperature SCR of NO x under oxygen-rich conditions[END_REF]. It had been reported that ceria based catalyst has great resistance to SO 2 and H 2 O poisoning in the NH 3 -SCR reaction [21]. Wang et al. [START_REF] Wang | E ff ect of Ceria Precursor on the Physicochemical and Catalytic Properties of Mn -W / CeO 2 Nanocatalysts for NH 3 SCR at Low Temperature[END_REF]investigated the influence of cerium precursor in the NH 3 -SCR of NO at low temperature. They found that the catalyst prepared from Ce(NO 3 ) 3 .6H 2 O precursor (Ce-NO-R) with nanorods morphology exhibits higher NO x conversion and superior SO 2 /H 2 O resistance. Zhang et al. [START_REF] Zhang | Applied Surface Science In situ DRIFTS investigation of NH 3 -SCR reaction over CeO 2 / zirconium phosphate catalyst[END_REF] studied the ceria loaded zirconium phosphate catalysts in NH 3 -SCR. They found that the addition of 20 wt.% ceria to zirconium phosphate catalyst exhibited more than 98% NO x conversion and 98% N 2 selectivity in a wide temperature window of 250-425 °C [START_REF] Zhang | Applied Surface Science In situ DRIFTS investigation of NH 3 -SCR reaction over CeO 2 / zirconium phosphate catalyst[END_REF]. Carja et al. [START_REF] Carja | Mn-Ce/ZSM5 as a new superior catalyst for NO reduction with NH3[END_REF] studied the reduction of NO by NH 3 on Mn-Ce/ZMS-5 catalysts which present NO conversion about 75-100% in the temperature range of 244-550 °C. The work reported by Qi et al. [START_REF] Qi | Performance and kinetics study for low-temperature SCR of NO with NH3 over MnOx-CeO2 catalyst[END_REF] demonstrated a good NO conversion at 120°C of MnOx-CeO 2 catalyst. Wu et al. [START_REF] Wu | Effect of ceria doping on SO2 resistance of Mn/TiO2 for selective catalytic reduction of NO with NH3 at low temperature[END_REF] showed that the introduction of cerium on MnO x /TiO 2 catalyst leads to steady NO conversion of 84 % in the presence of SO 2 for 6.5hours in the reaction stream. Compared to MnO x /TiO 2 catalyst, there is a largely decrease of NO conversion to 30 %.

In this work, the reduction of NO by NH 3 on fresh and hydro-treated cerium-Faujasite zeolite catalysts was investigated. The parent zeolite Na-F (Si/Al = 1.32 and S BET = 749 m 2 /g) used as support for the preparation of catalysts was obtained from industrial sodium metasilicate and aluminum scraps. Moreover a large part of this study was devoted to the characterization of fresh and hydro-treated cerium-Faujasite zeolite catalysts by different techniques (XRD, FTIR, BET, UV-vis and H 2 -TPR)

Experimental

Materials

Industrial sodium metasilicate (Na 2 SiO 3 .5H 2 O, PQ CORPORATION) was used as silicon source for the preparation of the FAU zeolite. The aluminum source's was aluminum scraps collected from metal manufacturing industry in the region of Sfax(Al= 99.77% ± 0.02, Fe= 0.207± 0.006 and Cu= 0.008±0.001).The other chemical products are sodium hydroxide pellets (NaOH, Sigma Aldrich 99% purity) and cerium (III) chloride heptahydrate CeCl 3 .7H 2 O (Sigma Aldrich 99, 99%).

Preparation of the zeolitc support

Firstly, NaOH pellets were dissolved in the required quantity of distilled water. Then the alkaline solution was divided to equal volumes in polypropylene bottles. The desired quantity of aluminum scraps was dissolved in one half of hydroxide solution volume and then filtered to remove the impurities. The silicon sources was added to the other half of hydroxide solution volume and mixed in capped bottle until clear. In order to obtain the aluminosilicate hydrogel, the silicon solution was poured to the aluminum solution quickly. The obtained thickly gel was stirred during a period of 10 min and then was aged for 14 h at room temperature (RT). Finally, the aged gel was heated at 80°C for 6 and 24hours. After the crystallization was achieved, the obtained solids were filtered, washed with distilled water until pH around 9, and then dried at 80°C for 24h.

Preparation of the catalysts

Cerium-zeolite faujasite (Ce-F) catalysts were prepared by ionic exchange method in aqueous solution by varying the concentration of cerium chloride: 0.02 M; 0.01 M and 0.005M. The required amount of cerium chloride was dissolved in 50mL of distilled water and then 1g of Na-F zeolite were added to the solution and mixed for 5 h at 50 °C. The slurry was then centrifugated, washed three times with distilled water to remove the non-exchanged cerium and adsorbed chlorides on the surface of the zeolite. The recovered solids were dried for 24h at 80 °C and then calcined for 1 hour at 450 °C. The obtained catalysts were called Ce(x)-F, with x the cerium amount in wt%.

Another catalyst called Ce(20)-F C was also prepared using 0.02M of cerium chloride solution. Nevertheless, after drying at 80 °C, the powder was calcined for 1 hour at 750 °C to ensure the siting of cerium in the sodalite cages. Finally, it was exchanged another time with 0.02M of cerium chloride solution and then calcined for 1 hour at 450 °C.

Characterization and analysis

The obtained samples were characterized by a variety of conventional techniques. Powder Xray diffraction (XRD) patterns were obtained using D8 ADVANCE BRUKER 40 Kv with Cu Kα radiation. The diffraction pattern was collected in the 2θ range of 5-50° at a scan speed of 0.02°. The prepared zeolite was compared with a commercial Na-Y zeolite (Aldrich, cat. no 33,444-8, lot 06402LR, Si/Al=2.5). Infrared spectra were scanned from 400 to 4000 cm -1 using the FourierTransform infrared model Perkin Elmer spectrum BX spectrometer. The scanning electron microscopy (SEM) images were taken on a Hitachi S 4800 operated at 5Kv. The BET surface areas (S BET ) and pore size distributions were determined by nitrogen adsorption-desorption on an ASAP2020 instrument (Micromeritics Instrument Corporation, GA. Prior to these measurements, the samples were degassed at 250 •C in vacuum.Thermogravimetric analysis (TGA) experiments were carried out on Perkin Elmer Simultaneous Thermal Analyser STA 6000 under anair at a flow rate of 100 cm 3 min -1 . The temperature increased from 30 to 900 °C at a rate of 10 °C/min. H 2 -TPR studies were conducted on a MicromeriticsAutochem 2910 instrument. 80 mg of oven dried sample was placed in one arm of a U-shaped quartz tube on quartz wool. Prior to TPR, the catalysts samples were pretreated with 20% O 2 /N 2 at 550 •C for 30 min (10°/min) to remove the water present, then are cooled in air at 50 ° C. The TPR analysis was carried out in a reducing mixture consisting of 3% H 2 /Ar and heated from 50 to 800°C (10 °C/min). UV/Vis diffuse reflectance spectra were collected by Lambda 40 Perkin spectrometer.

The NH 3 -SCR of NO catalytic test was performed in temperature programmed surface reaction (TPSR, 5°C/min)) using a flow reactor operating at atmospheric pressure with a space velocity of 250,000 h -1 . The catalyst sample (24 mg) was putted in quartz reactor positioned between two layers of inert quartz wool. The reactant gas composition was adjusted to 1000ppm NO, 1000ppm NH 3 , 8 % O 2 , 3.5 % H 2 O and balance with helium.

Hydrothermal treatment of catalysts was carried out in quartz fixed-bed microreactor.

However, 1 g of catalyst was placed in the reactor and heated to the desired temperature in flowing 20% O 2 /He. Then water is injected by circulating the airflow through a bubbler containing deionized water at 47 °C (10% H 2 O vapour) during 4 hours. All gas lines were heated to prevent water condensation. Four different hydrothermal temperatures were used: 700°C (ht 700), 800°C (ht 800), 850°C (ht 850) and 900°C (ht 900). The resulting solids were labeled Cu(x)-F ht XXX). Figure 1 shows the XRD patterns of the obtained solids at 80°C for 6h(Na-F 6h ) and 24h(Na-F 24h ). After 6 h of crystallization, one can see fine diffraction peaks characteristic of the FAU zeolite. The increase of the crystallization time to 24 h, there is an increase of the intensity of the peaks.

Results and discussions

In table 2, we reported the chemical analysis of the two samples Na-F 6h and 24 h Na-F 24h performed by EDS in five different spots. The Si/Al molar ratios of the two samples are comparable; Si/Al= 1.27 for Na-F 6h and Si/Al = 1.32 for 24 h Na-F 24h [START_REF] Treacy | Collection of simulated XRD powder patterns for zeolites fifth (5th) revised edition[END_REF]. Table1 : Chemical analysis of Na-F 6h, Na-F 24h samples performed with EDS. The SEM micrographs of Na-F24h (A1, A2) were displayed in Figure 2. The micrographs showed some crystals fused together forming agglomerate particles. Moreover, the particle size of isolated crystals was less 1µm. It can be seen from the image that the catalyst shows perfect degree of crystallization which is in accordance with the XRD results ( Figure 1). In the rest of the work we use the sample Na-F 24h and it will be named Na-F. The FTIR spectrum of Na-F zeolite was reported in Erreur ! Nous n'avons pas trouvé la source du renvoi. We found at 560 cm -1 the characteristic band related to the double six ring (D6R) in the faujasite zeolites [START_REF] Zhan | Control of particle size and surface properties of crystals of NaX zeolite[END_REF]. The bands at 666 cm -1 and 957cm -1 can be attributed to TO 4 symmetric and asymmetric stretching vibration modes of Si-O, respectively. The peak at 447 cm -1 was attributed to T-O bending mode. The peak at 745 cm -1 is characteristic for symmetric stretching vibrations of external linkages [START_REF] Zhan | Phase and morphology control of LTA/FAU zeolites by adding trace amounts of inorganic ions[END_REF]. The peak at 1646 cm -1 can be assigned to the bending mode of physically adsorbed water. According to Flanigen et al. [START_REF] Flanigen | Infrared Structural Studies of Zeolite Frameworks[END_REF],

Samples

the T-O band near 455 cm -1 for the zeolite FAU type Y contains a high-frequency shoulder which is not present in zeolite FAU type X. Also, the 406 cm -1 pore opening band present in X zeolite is not found in Y zeolite.

Figure 3 : FTIR spectrum of Na-F zeolite. dispersed into the zeolite porosity [START_REF] Flanigen | Infrared Structural Studies of Zeolite Frameworks[END_REF][START_REF] Guzman | On the formation of the acid sites in lanthanum exchanged X zeolites used for isobutane/cis-2butene alkylation[END_REF]. In general, with calcination at high temperature (>300 °C), cerium ions located in the supercages starts to migrate to the sodalite cage and are stabilized by the framework oxygens [START_REF] Du | Effect of cation location on the hydrothermal stability of rare earth-exchanged Y zeolites[END_REF]. All catalysts have a light yellow color, this observation indicates the presence of cerium tetravalent [START_REF] Lee | Calcination of cerium ( Ill ) exchanged Y zeolite[END_REF].

3.2.Characterisationof the prepared catalysts

On the other hand, the crystallinity of the catalysts decreased as the percentage of cerium increased. This feature can be explained by the removal of Al atoms from the framework without the destruction of the structure [START_REF] Le | Investigation on the mechanism of adsorption and desorption behavior in cerium ions modified Y-type zeolite and improved hydrocarbons conversion[END_REF] due to the siting of cerium in the sodalite cages. It indicated that cerium had a strong interaction with the matrix of the zeolite [START_REF] Shanqing | Different Influences of Lanthanum and Cerium on Stability of Y Zeolite and Their DFT Calculations[END_REF]. From this analysis we demonstrate that the two catalysts have the same morphology of the zeolite Na-F (Figure 2) with nanoscale crystals. Nevertheless, it was observed an agglomeration which is due to the interconnection of small particles, but the cerium particles is not observed in this magnification because they presented in a small amount.

Erreur ! Nous n'avons pas trouvé la source du renvoi. presents the nitrogen adsorption-desorption isotherms of the samples Na-F, Ce (5)-F, Ce (10)-F, Ce (20)-F and Ce (20)-F C . A typical type I adsorption isotherm is obtained in each case, indicating that the solids are microporous according to the classification ofIUPAC [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF]. The presence of hysteresis loop from 0.66 to 0.98 arises indicated the presence of mesoporous from the packing of zeolite nanocrystals [START_REF] Komaty | A facile route toward the increase of oxygen content in nanosized zeolite by insertion of cerium and fluorinated compounds[END_REF]. In Erreur ! Nous n'avons pas trouvé la source du renvoi. were reported the BET surface area S BET external surface, micropore volume and micropore surface area of the samples Na- The TGA/DTGA profiles of three samples were displayed in Figure 6. The Na-F, Ce(20)-F and Ce(20)-F C show a mass loss at the range of temperature (30-400°C) corresponding to water adsorbed on the surface and that present in the zeolite channels contributing to 21%, 25%, and 18% of their total masses, respectively. 4f. The electronic transitions from oxygen to cerium need more energy for a hexa-coordinated than tetra-coordinated Ce 4+ . So, the band at 310 nm is due to the presence of one type of well-dispersed Ce 4+ species probably in a tetra-coordinated environment [START_REF] Laha | Cerium containing mcm-41type mesoporous materials and their acidic and redox catalytic properties[END_REF]. For the pure ceria, the profile showed two peaks at 256 and 347nm. According to the literature [START_REF] Guo | UV and visible Raman studies of oxygen vacancies in rare-earth-doped ceria[END_REF] pure ceria presents three peaks at 255, 285 and 340nm. The first peak corresponds to O 2-to Ce 3+ charge transfer. Whereas, the other two peaks may be attributed to the O 2-to Ce 4+ charge transfer (285nm) and interband (340nm) transition. zeolite and the second one can be related to isolated cerium species [START_REF] Moreira | The effect of cerium introduction on vanadium-USY catalysts[END_REF]. With the two steps preparation including the high temperature of calcinations, Ce ions may enter are in the S I sites coordinated with three oxygen atoms of the zeolitic framework and have substituted the sodium in the sodalite cages. Ce 4+ ions located in the sodalites are more difficult to be reduced. For that reason, the H 2 consumption is refer to the oxide existing in the surface and in the supercage of zeolite [START_REF] Li | Tuning of acidity in CeY catalytic cracking catalysts by controlling the migration of Ce in the ion exchange step through valence changes[END_REF]. 3. It can be seen that Ce(20)-F catalyst is more active and has the lowest light-off temperature than the other catalysts.

Furthermore, with the increase of cerium content for Ce(x)-F catalysts there was a decrease of the T50 and an increase of the NO conversions.

As said before, withthe increase of cerium content there was an increase of NO conversion in the whole temperature range. Literature reported that the high activity cerium catalysts in theNH 3 -SCR [START_REF] Online | A DFT + U study of NO evolution at reduced CeO 2 (110) †[END_REF] is due to the redox property of Ce 4+ /Ce 3+ couple.

Indeed, in the presence of oxygen, Ce 3+ oxidized toCe 4+ which oxidizesNO to NO 2 , in low temperature region, which is among the key steps in theNH 3 -SCR. The cerium have a redox property like the others transition metal, but the cerium have orbital f which make the distribution of catalytic sites, like oxygen vacancies, especially in the case of surface redox reactions [START_REF] Online | A DFT + U study of NO evolution at reduced CeO 2 (110) †[END_REF], e.g. bonding with molecules like nitric oxides [START_REF] Ito | Selective reduction of NO , with ammonia over cerium-exchanged mordenite[END_REF].Also the orbital 4f of Ce 4+ cation, in the surface, has the capacity to store and release reversibly or selectively electrons, so this property can have an impact in the stabilization of adsorbed reactants and induced their reactions. So the presence of sufficient oxygen vacancies on the surface is the key to initiate the co-adsorption of NO and the later reaction for the formation of N 2 [START_REF] Online | A DFT + U study of NO evolution at reduced CeO 2 (110) †[END_REF]. Table 3: The light of temperatures, the maximum NO conversions and the related temperatures of prepared catalysts.

Hydrothermal stability

An hydrothermal treatment at high temperature is usually used for simulating the ageing of the NH 3 -SCR catalysts for the automotive applications. To study the effect of hydrothermal treatment, two sets of experiments were performed by aging all catalysts at 700 and 800°C for ), all aged catalysts show different degrees of activity loss at high temperatures compared with fresh catalysts. However, for the catalyst with highest cerium content Ce(20)-F C ht700 shows a slight degradation of NO conversion throughout the temperature range. . In table 4, we reported the NO conversions of fresh and aged catalysts at 500°C. From these results, it is noted that when the percentage of cerium increases the catalyst remains stable after hydrothermal treatment and has a good NO conversion. Indeed, the Ce(20)-Fht700 catalyst presents 96 % of NO conversion at 500 °C compared to Ce(5)-Fht700 which presents only 22 %.

On the other hand, for the hydrotreated catalysts at 800°C, one can observe that the increase of the aging temperature has a remarkable influence on the catalytic performance. For the two catalysts Ce(5)-Fht800

and Ce(10)-Fht800,NO conversions drastically decreased to16 % and 13%, respectively. 20)-F C ht 900) the catalytic activity was decreased to 12%. This reduction in the conversion is due to the destruction of the catalyst structure and the formation of cerium oxide at 28° and 47 ° as shown by the XRD ( Figure 13).

The slight decrease of NO conversions for the catalysts hydrotreated at 700 °C, 800 °C and 850 °C can be related to the presence of cerium cations into the sodalite cages of the zeolite Na-F. Actually, during the preparation of the catalyst Ce (20)-F C , the latter was calcined at higher temperature (750 °C) compared to the other catalysts (450 °C). The use of this higher calcination temperature permits to the cerium cations to migrate from the supercage to the sodalite cage and to form bridges with the oxygen framework which stabilize the zeolites tructure [START_REF] Akah | Application of rare earths in fluid catalytic cracking : A review[END_REF][START_REF] Du | Effect of cation location on the hydrothermal stability of rare earth-exchanged Y zeolites[END_REF]. Y. Shu et al. [START_REF] Akah | Application of rare earths in fluid catalytic cracking : A review[END_REF] reported that the hydrothermal treatment of zeolite catalysts at high temperatures (500-800°C), provokes the dealumination of the framework especially for those having low Si/Al ratio. So, the aluminum atoms migrate from their positions and settle down in zeolite channels and cages. To moderate the deactivation of the zeolite catalysts rare earth cations like cerium was added to the formulation. The addition of cerium prevented the framework dealumination, and preserved the acid sites, which greatly ameliorated the hydrothermal stability.

Conclusion

In this study, FAU zeolite has been prepared using cheap starting materials; aluminum scrap and industrial sodium metasilicate with hydrogel molar composition

3. 1 .

 1 Characterization of the prepared zeolitesErreur ! Nous n'avons pas trouvé la source du renvoi.The molar composition of the starting gel used to study the effect of crystallization time is the following: 5.19 Na 2 O: Al 2 O 3 :2.9 SiO 2 :182.76 H 2 O. After aging for 14 hours at room temperature, the gel was crystallized at 80°C for 6 and 24 hours.

Figure 1 :

 1 Figure 1 : XRD of Na-F 6h , Na-F 24h prepared at 80°Cfor 6 and 24 hours, respectively.

2 Figure 2 :

 22 Figure 2 : SEM of (A1, A2) Na-F, (B1, B2) Ce (5)-F, and (C1, C2) Ce (20)-F c .

Figure 4

 4 Figure4showed the XRD patterns of the carrier Na-F and the Ce-exchanged catalysts Ce(5)-

Figure 4 :

 4 Figure 4 : XRD of Na-F, Ce (5)-F, Ce (10)-F, Ce (20)-F and Ce (20)-F C

F, Ce ( 5 )

 5 -F, Ce (10)-F, Ce (20)-F and Ce (20)-F C . The BET total surface area (S BET ) has changed under the condition of different Ce percentages from low to high with respect to microporous materials of Na-F zeolite itself, which possess rich specific surface area (S BET = 749m 2 /g). The introduction of the rare earth cations causes a decrease in the micropore surface area and volume. It is attributed to the existence of the cerium cations or extraframework aluminum species in the cages or channels blocking the pore structure[START_REF] Liu | A potential substitute for CeY zeolite used in fluid catalytic cracking process[END_REF].

Figure 5 :

 5 Figure 5 : BET of Na-F, Ce (5)-F, Ce (10)-F, Ce(20)-F and Ce(20)-F C .

Figure 6 :Figure 7 .

 67 Figure 6 : TGA/DTGA of Na-F, Ce(20)-F and Ce(20)-F C .

Figure 7 : 2 (Figure 8 .

 728 Figure 7 : UV-vis spectroscopy of Ce (5)-F, Ce (10)-F, Ce (20)-F, Ce (20)-F C and CeO 2 .

Figure 8 :Figure 9 .

 89 Figure 8 : H 2 -TPR of Ce (5)-F, Ce (10)-F and Ce (20)-F C and pure CeO 2 .

Figure 9 :

 9 Figure 9 :The NO conversion of Fresh catalysts Ce (5)-F, Ce (10)-F,Ce (20)-F, Ce (20)-F C and Na-F support.

4Figure 10 :

 10 Figure 10 : Hydrothermal treatment of Ce(5)-F, Ce(10)-F, Ce(20)-F and Ce(20)-FC at

Furthermore, the

  catalyst Ce(20)-Fht800 presents only 81% of conversion at 500°C. But for the Ce(20)-F C ht800 the conversion remains stable (96% of NO conversion) at 500°C. The above results indicate that Ce(20)-F C ht800 is more stable than other catalysts during the hydrothermal treatment. Because of its hydrothermal stability the catalyst Ce(20)-F C ht800 was tested after undergoing higher hydrothermal treatment temperature, 850 and 900 ° C, for 4 hours.

Figure 10 :

 10 Figure 10 : Hydrothermal treatment of Ce(5)-F, Ce(10)-F, Ce(20)-F and Ce(20)-F C at 700°C for 4hours.

Figure 11 :

 11 Figure 11 :Hydrothermal treatment of Ce(5)-F, Ce(10)-F, Ce(20)-F and Ce(20)-F C at 800°C

Figure 12

 12 Figure 12 :The NO conversion of catalysts Ce (20)-F C before and after

Figure 13 :

 13 Figure 13 : XRD of catalyst Ce (20)-F C before and after hydrothermal treatment 4h at 700, 800, 850 and 900°C.

5 . 19 Na 2 O : Al 2 O 3 : 2 . 9 SiO 2 :

 519223292 182.76 H 2 O. The obtained zeolite after hydrothermal synthesis at 80 °C for 24 h, has been used as carrier to prepare cerium exchanged catalysts with different cerium content.The NO conversion increased with increasing the percentage of cerium in the framework of zeolite. The best catalytic activities is concerned to the catalysts Ce(20)-F and Ce(20)-F C catalyst. It should be underlined that, after hydrothermal treatment of the catalysts, Ce(20)-F C at 850°C for 4 hours, NO conversion remains stable whereas for Ce(20)-F prepared by a classic way, a noticeable loss of activity is already observed after hydrothermal treatment at 800°C. The two steps procedure, allowing the siting of Ce ions into the sodalite cages of the faujasite greatly improves the hydrothermal stability of the catalysts. Contrarly to copper faujasite, Ce-FAU catalysts shows hydrothermal stability comparable to other small pore zeolithe structures. The increase of the hydrothermal treatment temperature to 900°C leads to a drastic decrease of NO conversion to 12%. This behavior can be related to the destruction of structure of zeolite and the appearance of CeO 2 peaks as shown by XRD technique and is generally observed for the other zeolite structures.

Na (wt. %) Al (wt.%) Si (wt. %) Si/Al (mol/mol) Na-F 6h Na-F 24h

	19.62	35.26	46.74	1.27
	21.07	33.95	46.97	1.32

Table 2 :

 2

	Catalyts	S BET (m 2 /g)	S external (m 2 /g) V micropore (cm 3 /g)	Micropore
					area (m 2 /g)
	Na-F	749	121	0.28	628

BET surface area, external surface, micropore volume and micropore surface area of the samples Na-F, Ce (5)-F, Ce (10)-F, Ce (20)-F and Ce (20)-F C .

Table 4 :

 4 NO conversionsat 500°C of fresh and aged catalysts at 700 °C and 800 °C.

	Catalysts	NO Conv. Fresh	NO Conv after H.T	NO Conv.After H.T
		catalysts (%)	700°C (%)	800°C (%)
	Ce(5)-F	55	22	16
	Ce(10)-F	88	67	13
	Ce(20)-F	100	89	81
	Ce(20)-F C	100	96	96


Zouhaier Ksibi1

Abdelhamid Ghorbel1

Gérard Delahay2

Ag/ZrO2 and Ag/Fe-ZrO2 catalysts for the low temperature total oxidation of toluene in the presence of water vapor Rimeh Ismail1 • Jihène

Keywords: Aerogel supports, Ag/ZrO2, Ag/Fe-ZrO2, Toluene oxidation, Water vapor

New mesoporous and well-structured Ag-based catalysts (xAg/ZrO2 and xAg/Fe-ZrO2 with x = 2 wt%) have been investigatedin the total oxidation of toluene with water vapor. The results showed that the catalytic activity of systems increases, in the 350-550 °C temperature range, following this order: ZrO2< Fe-ZrO2 < Ag/ZrO2 ≈ Ag/Fe-ZrO2. Nevertheless, it was revealed that Ag/Fe-ZrO2 is the most active solid for the low temperature total oxidation of toluene (< 350 °C). The highest low temperature activity of this new catalyst can be related to the presence of more reactive surface oxygen, new active redox and acidic sites at Ag/Fe-ZrO2 surface, most probably generated by the simultaneous presence of Ag and Fe species.

Introduction

Volatile organic compounds (VOCs), like aldehydes, alcohols, ketones, aromatic, halogenated and sulfur containing compounds, which can be emitted from automobile industries, petroleum refineries, textile manufacturers, chemical industries, solvents processes, fuel combustions, pharmaceuticalplants, decomposition in the biosphere and biomass, etc., remain one of the major sources of air pollution [1,2].Therefore, it is necessary to control and reduce the emissions of these compounds. Destructive methods (catalysis oxidation and biofiltration) and recovery methods (condensation, adsorption, absorption, and membrane separation) have been developed for VOCs removal [3]. Among the available technologies, the catalytic oxidation is most popular and effective because of its versatility of eliminating a range of organic pollutants under mild operating conditions [1]. On the other hand, this technique is relatively more environmentally friendly due to its low temperature operation and the formation of less dioxins and noxious products [1]. Divers catalytic systems containing noble or transition metals (e.g., Pd, Pt, Mn, Fe, etc.) have been investigated and demonstrated a high catalytic performances for the destruction of various VOCs (e.g., propane, carbon monoxide, methane, toluene, etc.) [1][2][3][4][5][6][7][8][9][10][11]. In recent years, silver-based materials have been extensively investigated and recognized as suitable catalysts for the low temperatures oxidation of VOCs [6,12]. These catalysts bring about a special interest due to a number of valuable features of Ag including its ability to activate molecular oxygen, homogeneous distribution of metal particles/clustersover various supports [e.g., TiO2, ZrO2, SiO2, CeO2, MnxOy, Si3N4, Fe2O3, etc.] and a synergistic action with a large number of promoters and modifiers (noble and transition metals, halogencontaining compounds, etc.) [12]. Recently, the activity exploration focused on various Ag supported catalysts has attracted increasing interest in the oxidation of toluene which is known as one of the most targeted components among VOCs [6,13,14]. In this framework, Qu et al. [13] have showed that the addition of Ag greatly enhances the reactivity of Mn/SBA-15 for toluene oxidation. The authors have reported that the strong interactions between Ag and Mn species promotes both the reducibility of catalysts and the formation of abundant active lattice oxygen, thus increasing the catalytic activity. On the other hand, Zhang et al. [6] have studied the effect of Ag loading on the structure and performance of the Ag nanoparticles supported on UiO-66 catalysts for toluene oxidation. They have concluded that the increase of the Ag amount affects the UiO-66 structure and the 10Ag-U catalyst showed excellent catalytic performance due to higher lattice oxygen and surface Ag 0 content. Furthermore, Lu et al. [14 ] have studied the removal of toluene by plasma-catalytic oxidation (non-thermal plasma (NTP)) over Ag-modified FeOx / SBA-15 solids. Their results revealed that the incorporation of FeOx and AgOx significantly enhances the toluene removal efficiency and CO2 selectivity of the NTP systems. The authors have reported that the silver particles tended tocover the catalyst surface in the form of Ag and Ag2 O, which improves the dispersion of FeOx and lengthened the residual time of toluene in the NTPcatalytic system, there by promoting the reaction of toluene with oxygen on the catalyst surface increasing toluene oxidation and catalyst stability. To the best of our knowledge, up to date, no studies related to the total oxidation of toluene (C6 H5 -CH3 ) over Ag and Fe supported on ZrO2 solid, has been described in the literature. Therefore, we reported in this work the preparation and characterization of new Ag/ZrO2 , Fe-ZrO2 , and Ag/Fe-ZrO2 catalytic systems for the total oxidation of toluene in the presence of water vapor. ZrO2 and Fe-ZrO2 catalytic supports were prepared by the one step sol-gel procedure which represent one of the low cost techniques that can easily be applied to large area deposition [15 , 16 ]. This method allows the facile synthesis of stable metal oxides with better purity and homogeneity at ambient conditions [15 ]. It gives high chemical homogeneity, provides good dispersion nanoparticles, well-controlled composition, and useful for the applications where large area coatings are required [15 ]. Silver was deposited at ZrO2 and Fe-ZrO2 surface by the impregnation method. The mutual relationship between the physicochemical properties and catalytic activity in the toluene oxidation of Ag/ZrO2 , Fe-ZrO2 , and Ag/Fe-ZrO2 samples has been discussed.

Experimental Catalysts preparation

ZrO2 and Fe-ZrO2 aerogel supports were prepared by the sol-gel method using a similar procedure to that reported in our previous work for TiO2 [17]. In fact, the requisite quantities of Zirconium (IV) butoxide (Zr(OC4H9)4, Sigmaaldrich, Purity (p) = 80 %) and Ethyl acetoacetate (C6H10O3, Fluka, Purity (p) > 99.5% Sigma-aldrich, p = 80%) were dissolved in anhydrous ethanol (C2H6O, Aldrich, p = 99.8%) and mixed under stirring at room temperature. Then, a dilute solution of HNO3 (0.1 M) was gradually added with a suitable molar ratio nH2O/nZr = 10 to control hydrolysis. Finally, the mixture was kept under continuous stirring at room temperature until gelification. For Fe-ZrO2 solid, an appropriate amount of iron(III) nitrate (Fe(NO3)3,6H2O), Sigma-aldrich, p ≥ 99.95%) was introduced in the mixture before hydrolysis. The resulting gel were dried in autoclave (at T = 243 °C and P = 63 bar) to form the aerogel supports (ZrO2 , Fe-ZrO2 ) which were calcined at 550 °C for 3 h under O2 flow (30 cm3 /min). The Ag/ZrO2 and Ag/Fe-ZrO2 catalysts, containing a theoretical loading 2 wt% of Ag, were prepared by the standard impregnation method. Hence, the required quantity of AgNO3 (Sigma-Aldrich, p ≥ 99.0% was dissolved in water ( H2O) and the obtained clear solution was mixed under stirring with ZrO2 or Fe-ZrO2 powder to obtain Ag/ZrO2 or Ag/Fe-ZrO2 sample which was dried at 100 °C for 12 h and subsequently calcined at 550 °C for 3 h under O2 flow (30 cm3 /min).

Catalytic tests

The toluene oxidation tests were carried out in a fixed bed reactor using 20 mg of catalyst which was packed between the layers of a quartz wool. The catalyst bed temperature was monitored and controlled by a temperature controller equipped with two thermocouples inserted at the catalyst reactor bed and furnace. The toluene concentration was adjusted by passing an appropriate He flow through a saturator, kept at 0.7 °C, containing toluene. A total flow rate of 50 cm3 /min gas mixture containing 0.1 vol% toluene (purity; p > 99.7%), 3.4 vol% H2 O (p > 99.5%), 8 vol% O2 (p > 99.9%) and balance with He (p > 99.9%) were used. The equivalent gas hourly space velocity (GHSV) obtained was of 60,000 h-1 . The catalytic activity was evaluated in terms of toluene conversion to CO2 ,in the 200-550 °C temperature range, with a ramp of 6 °C/min. Exit gases were monitored continuously by sampling on line to a quadrupole mass gas spectrometer Pfeiffer Vacuum equipped with

Catalysts characterization

Textural properties of the samples were determined by N2-physisorpton at 77 K using a Micromeritics ASAP 2000 instrument. The samples were previously degassed for 5 h at 250 °C. Structural features were investigated by a Brüker AXS D8 diffractometer with CuKα radiation (λ = 1.5406 Ǻ) and the spectra were collected using a step size of 0.02° in the range 20° and 70°. JCPDS powder diffraction files were used for phase identification. Redox behaviors of solids were analyzed by temperature-programmed reduction technique (H2-TPR) with an AUTOCHEM 2910 (Micromeritics). Prior to H2-TPR measurements, the sample (80 mg) was pre-treated under 5 vol% O2 in He (30 cm3/min) at 550 °C (ramp 10 °C min-1) for 30 min. After being cooled down to 50 °C in the same atmosphere, the sample was flushed with He (30 cm3/min) then exposed to a flow containing 5 vol% H2 in Ar (30 cm3/min) and heated between 50 °C and 800 °C with a heating rate of 10°C/min. Acidity of samples was examined by NH3-TPD using an AUTOCHEM 2910 (Micromeritics). The solid was pre-treated at 550 °C (ramp 10 °C/min) during 2 h, under air flow (30 cm3/min). Then, it was exposed to 5 vol% NH3 in He (flow rate = 30 cm3/min) for 30 min to be flushed after with He (30 cm3/ min) during 30 min to remove the physisorbed ammonia. Finally, the NH3 desorption was done in helium flow (30 cm3/ min) from 100 to 550 °C (the heating rate was 10 °C/min).

Results and discussion

The samples (ZrO2, Fe-ZrO2, Ag/ZrO2 and Ag/Fe-ZrO2) were tested in the toluene (C6H5-CH3) oxidation in the presence of water vapor (3.4 vol%). The collected results (C6H5-CH3 conversion and CO2 concentration as temperature reaction function) are shown in Figs. 1A,B. The temperatures corresponding to 10%, 50%, and 90% of toluene conversions (denoted as T10, T50, and T90) are used to evaluate and compare the catalytic activities of solids.

It is well established that the mechanism of VOCs oxidation over transition metal oxides follows a Mars-Van Krevelen mechanism, which is known as redox mechanism, involving two successive steps in terms of the cyclic reaction. In the first step, oxygen vacancies on the catalyst surface were reduced as they react with the organic molecules. In the second step, the pre-formed reduced site regenerated immediately through the consumption of gaseous oxygen or the transfer of oxygen atoms from the bulk to the surface [18][19][20]. Therefore, the oxygen carrier capacities (adsorbed and lattice oxygen species) and reducibility of catalyst determined its reactivity in the oxidation of VOCs [18][19][20][21]. Noting that the surface acidity of catalyst also play a key role in the oxidation reaction since it could facilitate both the sorption of VOC at the catalyst surface and the breaking of carbon-carbon bonds which promote the CO2 production [22].

As it can be seen in Fig. 1, the ZrO2 support exhibits the lowest C6H5-CH3 conversions to CO2 (a maximum of 50% at 550 °C) due to the low reactivity of its acidic sites in this reaction and the absence of redox ones. However, the addition of iron or silver species significantly enhances the activity for the toluene oxidation. Higher C6H5-CH3 conversions to CO2 are obtained in the 350-550 °C temperature range over Ag/ZrO2 (near 100%) compared to that observed in the case of the aerogel Fe-ZrO2 (a maximum of 95% at 550 °C). This result demonstrates that the actives sites (most probably acidic and redox ones) created by silver species are more reactive in the toluene oxidation than those generated by iron ones. The temperatures at which toluene conversion reaches 10%, 50%, and 90%, over Fe-ZrO2, Ag/ZrO2 and Ag/Fe-ZrO2 catalysts, are indicated in Table 1.

The catalytic activity test demonstrates that Ag/ZrO2 and Ag/Fe-ZrO2, which were found more active than Fe-ZrO2 (Fig. 1A,B), exhibit approximately a similar oxidation activity at high temperatures (100% of toluene conversion for T > 400 °C). However, the simultaneous presence of Ag and Fe leads to the most active Ag/Fe-ZrO2 catalyst in the low temperatures range, therefore, T10 are 349 °C, 341 °C and 250 °C over Fe-ZrO2, Ag/ZrO2 and Ag/Fe-ZrO2, respectively. Based on this result, we can suggest the existence of Ag-Fe interactions, which promote the dispersion of silver and iron active species, facilitates the mobility and activation of oxygen, and enhance the reactivity of acidic and redox sites, thereby increasing the low temperature activity of Ag/Fe-ZrO2 catalyst in the total oxidation of toluene. This is in perfect agreement with the result already obtained by Lu et al. [14] for the non-thermal plasma oxidation of toluene. In fact, the authors have demonstrated that 2%FeOx-1%AgOy/SBA-15 catalyst exhibits higher stability, toluene removal efficiency and CO2 selectivity than 3%FeOx/SBA-15 system. Their results revealed that silver, existing in the form of Ag particles and Ag2O on the catalyst surface, adsorb toluene and promote the production of hydroxygroups, thus improving the reductive activity of FeOx. The authors have concluded that the interaction between Ag and Fe species could increases the catalyst activity and consequently improves the performance of the plasma-catalytic degradation of toluene. Meanwhile, Scirè et al. [23] reported that Ag could weaken the Fe-O bond, increase the mobility of the lattice oxygen, and thus improve the VOCs oxidation. By contrast, Rioseco et al. [24] have concluded that no increase in the catalytic activity for the complete combustion of toluene was observed upon low Ag substitution in LaFeO3 perovskite.

Table 1 Conversions of toluene at different temperatures over thecatalysts Table 2 Catalytic activity of silver-based catalysts in thetoluene oxidation

It is worthy to note that, the efficiency obtained over the new Ag/Fe-ZrO2 catalyst in the low temperature total oxidation of toluene (particularly T10) is better compared to some literature data (Table 2) since the reaction was performed inthis work with water vapor (3.4 vol%), at higher gas hourly space velocity (GHSV = 60,000 h-1) and with a low amount of catalyst (m = 20 mg).

The results of the N2 physisorption analysis, given in Table 3, reveal that all the samples exhibit after calcination at 550 °C developed texture (high surface area (SBET ≥ 76m2/g) and total pore volume (VPT ≥ 0.24 cm3/g)) demonstrating a good thermal stability of catalysts. Nevertheless, it should be mentioned that a slight decrease of the SBET and VPT accompanied by an increase of the pore diameter of ZrO2 is observed after Fe and/or Ag addition. This might be attributed, on one hand, to the partial obstruction of some pores of solids by the supported species [26], and on the other hand, to the increase of the crystallite size of zirconia after its modification by iron and/or silver species [27]. Similar results have been obtained by Dastan et al. [27] upon heat treatment of TiO2 NP's system. The authors have explained the decrease of the surface area and pore volume of this later solid, with the increase of its pore diameter, upon heat treatment, to the increase in the crystallite size and the collapse of pore structure of TiO2 NP's at higher annealing temperature.

Table 3 Textural properties of samples

Based on the N2 adsorption-desorption isotherms and BJH pore size distribution curves of solids (shown in Figs. 2 and3, respectively) it can be concluded that all the samples are mesoporous systems (isotherms type IV according to the IUPAC classification) with monomodal (case of ZrO2 and Ag/ZrO2: hysteresis loops type H2) or binodal (for Fe-ZrO2 and Ag/Fe-ZrO2: hysteresis loops type H2) porous distribution. It should be also mentioned that the increase of zirconia pores size after the iron and/or silver addition could indicate the existence of divers interactions between ZrO2 and the supported species which probably affects the texture of solids as previously suggested for various aerogels catalysts [17]. Markedly, no reflections associated with metallic iron or its oxide phases are found. This implies that Fe species are highly dispersed on ZrO2 surface. However, a weak diffraction line corresponding to the metallic silver was detected at around 2θ = 38.1° (111) for Ag/ZrO2 sample revealing the presence of a trace amount of metallic Ag (PDF 65-2871). This result suggests the existence of Ag-Fe interactions which contribute to the high dispersion of silver species at the catalyst surface.

The crystallites size of pure and modified zirconia is calculated using the Scherrer's formula [28][29][30] and the obtained values are 10.6, 18, 11.8, and 19.8 nm for ZrO2, Fe-ZrO2, Ag/ZrO2, and Ag/Fe-ZrO2, respectively. Based on this result, it can be concluded that all the samples are characterized by a nanometer size and the addition of Ag and/or Fe contributes to the crystal growth of zirconia.

H2-temperature-programmed reduction (H2-TPR analysis) was performed to study the redox properties of materials and the results are illustrated in Fig. 5. For the ZrO2 solid, no H2 consumption peaks are detected indicating that Zr4+ is a nonreducible cation [31]. For the aerogel Fe-ZrO2, two main H2-consumption peaks are appeared in the experimental temperature range; the peak at low temperatures (centered at T ≈ 150 °C) could be attributed to the reduction of Fe2O3 to Fe3O4 and the other one, observed at higher temperatures (centered at T ≈ 350 °C), can be associated with the Fe2O3 → Fe3O4 → Fe 0 transformations [32]. Similarly, a new H2-consumption peaks are observed at low temperatures (< 300 °C) in the H2-TPR profile of Ag/ZrO2 and Ag/Fe-ZrO2 catalyst and can be ascribed to the reduction of both silver and iron species [33,34]. It is noteworthy that after the addition of Ag to Fe-ZrO2 solid, the reduction peaks of Fe species was shifted to the lower temperatures indicating an enhancement of the iron species reducibility, must probably due to their interactions with silver which is consistent with the XRD results. Similar results, suggesting the improvement of FeOx reducibility by silver addition, have been previously obtained by Lu et al. [14] for 2%FeOx-1%AgOy/SBA-15 system. The total acidity of solids was evaluated using the temperature-programmed desorption of ammonia (NH3-TPD) and the results are presented in Fig. 6. The ZrO2 and Fe-ZrO2 aerogel supports show a broad NH3 desorption peak between 100 and 400 °C corresponding to NH3 desorbed from weak and medium strong acid sites [35]. The intensity of this former peak is higher for the solid containing Fe compared to that characterizing pure zirconia. This indicates that the presence of iron increases the total acidity of the support and contributes to the creation of new strong acid sites (NH3 desorbed at T > 400 °C). The addition of silver affects the total acidity of ZrO2 and Fe-ZrO2 solids and leads also to the creating of new strong acid sites (desorption of NH3 at T > 400 °C), especially, at Ag/Fe-ZrO2 surface.

Conclusions

In this work, a highly crystalline and mesoporous ZrO2, Fe-ZrO2, Ag/ZrO2, and Ag/Fe-ZrO2 catalytic systems were prepared for the total oxidation of toluene in the presence of water vapor. It was shown that the presence of Fe or Ag stabilizes the tetragonal phase of ZrO2 and leads to the creation of new redox and strong acid sites at zirconia surface. On the other hand, it was revealed that the coexistence of Ag and Fe species promotes both the acidity and reducibility of catalysts and contributes to the formation of more reactive oxygen which is beneficial for the low temperature toluene oxidation. Ag/Fe-ZrO2 was found to be the most active catalyst in the low temperature total oxidation of toluene (T10 = 250 °C) and exhibits approximately similar catalytic performances to that obtained over Ag/ZrO2 at higher temperatures (350-550 °C). Interestingly, 100% toluene conversion to essentially CO2 is obtained in 400-550 °C temperature range over the new Ag/Fe-ZrO2 catalyst.

Compliance with ethical standards

Conflict of interest The authors declare that there is no conflict of interest regarding the publication of this article.

  Faraday and SEM detectors (0-200 amu). The toluene conversion (%) is expressed as: where [Toluene]in and [Toluene]out are the inlet and outlet concentrations of toluene, respectively. The concentration of CO2 (ppm) was calculated as given below: where [I ]0 , [I ]7000 and [I ]t are the intensity of CO2 signals at inlet, outlet and at any reaction temperature (t ), respectively. 7000 ppm is the inlet concentration of CO2 .

Fig. 1 A

 1 Fig. 1 A Toluene conversion and B CO2 concentration (ppm) over (a) ZrO2, (b) Fe-ZrO2, (c) Ag/ZrO2, and (d) Ag/Fe-ZrO2 samples

Fig. 2

 2 Fig. 2 N2 adsorption-desorption isotherms of (a) ZrO2, (b) Fe-ZrO2, (c) Ag/ZrO2, and (d) Ag/Fe-ZrO2 samples

Fig. 4

 4 Fig. 4 XRD patterns of (a) ZrO2, (b) Fe-ZrO2, (c) Ag/ZrO2, and (d) Ag/Fe-ZrO2 samples

Fig. 5

 5 Fig. 5 H2-TPR of (a) ZrO2, (b) Fe-ZrO2, (c) Ag/ZrO2, and (d) Ag/Fe-ZrO2 samples

Fig. 6

 6 Fig. 6 NH3-TPD of (a) ZrO2, (b) Fe-ZrO2, (c) Ag/ZrO2, and (d) Ag/Fe-ZrO2 samples


J Long

V A Khomchenko

E Mamontova

J.-M Thibaud

J Rouquette

M Beaudhuin

D Granier

R A S Ferreira

L D Carlos

B Donnadieu

M. S C Henriques

J A Paixão

Y Guari

J Larionova

Room temperature magnetoelectric coupling in a molecular ferroelectric Ytterbium(III) complex

Magnetoelectrical (ME) materials combine magnetic and electric polarizabilities in the same phase. Such systems offer a basis for developing high-density data storage, spintronic or low consumption devices owing to the possibility to trigger one property by the other. Such objectives require a strong interaction between the constitutive properties, a criterion which is rarely met in classical inorganic ME materials at room temperature. We provide evidence of a strong ME coupling in a paramagnetic ferroelectric lanthanide coordination complex with magnetostrictive phenomenon. The properties of this molecular material show it may be competitive to inorganic magnetoelectrics.

One Sentence Summary:

The room temperature magnetoelectric control of ferroelectric domains was demonstrated in a Yb 3+ molecule-based material.

A ferroelectric material exhibits a permanent electrical polarization that can be switched by an electric field [START_REF] Han | Towards the Development of Flexible Non-Volatile Memories[END_REF][START_REF] Hong | Emerging Non-Volatile Memories[END_REF]. Such electro-active materials find a wide range of applications including temperature sensing, data-storage, piezoelectric devices, and electro-optics. The association of electrical and magnetic polarizabilities gives rise to exciting multifunctional systems denoted as magnetoelectrics (ME) which describes the influence of a magnetic (electric) field on the polarization (magnetization) allowing the constitutive properties to be simultaneously triggered [START_REF] Eerenstein | Multiferroic and magnetoelectric materials[END_REF][START_REF] Wang | Multiferroicity: the coupling between magnetic and polarization orders[END_REF]. For computing, modifying the polarization/magnetization using low-magnitude magnetic/electric field may reduce the energy needed and speed up non-volatile memory devices' processing's rate [START_REF] Eerenstein | Multiferroic and magnetoelectric materials[END_REF][START_REF] Scott | Data storage: Multiferroic memories[END_REF][START_REF] Mandal | Designing switchable polarization and magnetization at room temperature in an oxide[END_REF]. Conventional approaches to designing single-phase ME are widely based on inorganic materials, such as oxides or fluorides [START_REF] Mandal | Designing switchable polarization and magnetization at room temperature in an oxide[END_REF][START_REF] Fiebig | The evolution of multiferroics[END_REF][START_REF] Scott | Multiferroic magnetoelectric fluorides: why are there so many magnetic ferroelectrics?[END_REF]. However, if the origins of ferroelectricity and magnetism are associated with different carriers, we should expect only a moderate ME coupling [START_REF] Cheong | Multiferroics: a magnetic twist for ferroelectricity[END_REF]. For instance, only a few examples exist of ME coupling at room temperature [START_REF] Mandal | Designing switchable polarization and magnetization at room temperature in an oxide[END_REF][START_REF] Pitcher | Tilt engineering of spontaneous polarization and magnetization above 300 K in a bulk layered perovskite[END_REF][START_REF] Kitagawa | Low-field magnetoelectric effect at room temperature[END_REF]. In contrast, large ME coupling has been found in strain mediated multi-phase materials, such as composites or multi-layers, by combining piezoelectricity with the magnetostrictive effect [START_REF] Eerenstein | Multiferroic and magnetoelectric materials[END_REF].

Molecular materials exhibit numerous advantages in comparison to inorganic ones, such as structural diversity, soft chemistry routes, environmentally friendly processing and shaping, optical transparency, and light density [START_REF] Bruce | Molecular Materials[END_REF]. For these reasons, molecular ferroelectrics [START_REF] Hang | Metal-organic complex ferroelectrics[END_REF][START_REF] Zhang | Ferroelectric Metal-Organic Frameworks[END_REF] are often considered as an alternative towards traditional metal oxides [START_REF] Horiuchi | Above-room-temperature ferroelectricity in a single-component molecular crystal[END_REF][START_REF] Horiuchi | Organic ferroelectrics[END_REF]. Even though molecular materials exhibit magnetic ordering temperature below room temperature, important ME coupling may be expected by the association of paramagnetism and ferroelectricity (Fig. S1) [START_REF] Fiebig | The evolution of multiferroics[END_REF] since both these properties could involve the same chemical element. Although ME coupling in non-ferroelectric molecular materials has been reported (17,[START_REF] Fittipaldi | Electric field modulation of magnetic exchange in molecular helices[END_REF], the interaction between magnetism and ferroelectricity is not extensively studied [START_REF] Jain | Multiferroic Behavior Associated with an Order-Disorder Hydrogen Bonding Transition in Metal-Organic Frameworks (MOFs) with the Perovskite ABX3 Architecture[END_REF][START_REF] Kagawa | Ferroelectricity in a one-dimensional organic quantum magnet[END_REF][START_REF] Xu | Coexistence of Magnetic and Electric Orderings in the Metal-Formate Frameworks of[END_REF][START_REF] Cai | Above-Room-Temperature Magnetodielectric Coupling in a Possible Molecule-Based Multiferroic: Triethylmethylammonium Tetrabromoferrate(III)[END_REF][START_REF] Wang | Magnetoelectric coupling in the paramagnetic state of a metal-organic framework[END_REF][START_REF] Rodríguez-Velamazán | Magnetically-induced ferroelectricity in the (ND4)2[FeCl5(D2O)] molecular compound[END_REF][START_REF] Naka | Magnetoelectric effect in organic molecular solids[END_REF][START_REF] Ueda | Room-temperature magnetoelectric effect in a chiral smectic liquid crystal[END_REF]. The ferroelectric ordering temperatures are typically lower than room temperature for the few examples that exist. Moreover, the modification of the polarization/magnetization by applying magnetic/electric fields, which remains also challenging in pure inorganic materials, requires large magnitude operating magnetic field [START_REF] Evans | Magnetic switching of ferroelectric domains at room temperature in multiferroic PZTFT[END_REF].

We designed a chiral lanthanide complex exhibiting an above-room temperature ferroelectricity which, in association with a strong magnetostriction, gives rise to a remarkable ME coupling. This allowed tuning of the ferroelectric domains at the nanometric scale by applying a relatively low magnetic field at room temperature. Our molecular approach to design ME materials relied on the association of paramagnetic lanthanide ions, such as Yb 3+ , with a chiral diamagnetic zinc complex in order to favor the crystallization in one of the ten polar point groups compatible with ferroelectricity. We chose the Yb 3+ ion because it possesses a large total magnetic moment, which being oriented along a magnetic field can provide an anisotropic magnetostriction underlying the coupling between magnetic and structural subsystems. ) 2 ] (S,S-2). We found using single crystal X-ray diffraction that R,R-1

and S,S-2 are isostructural to the dysprosium analogue [START_REF] Long | A High-Temperature Molecular Ferroelectric Zn/Dy Complex Exhibiting Single-Ion-Magnet Behavior and Lanthanide Luminescence[END_REF]. The two enantiomers crystallized in the polar space group P2 1 with two independent homochiral dinuclear complexes within the asymmetric unit (Fig. 1, Fig. S2-S3). These two complexes exhibit differences in the crystallographic distances and geometries (Tables S1-S2) that gives rise to uncompensated dipolar moments along the b axis. We confirmed this by analyzing the ions' location in the unit cell (Table S3) [START_REF] Li | Anomalously rotary polarization discovered in homochiral organic ferroelectrics[END_REF][START_REF] You | Quinuclidinium salt ferroelectric thin-film with duodecuplerotational polarization-directions[END_REF][START_REF] Yang | Directional Intermolecular Interactions for Precise Molecular Design of a High-Tc Multiaxial Molecular Ferroelectric[END_REF].

We corroborated the enantiomeric nature of R,R-1 and S,S-2 using solid state circular dichroism (CD) (Fig. S4), with the mirror-symmetrical CD spectra showing Cotton effects of opposite signs at λ max = 261, 303 and 386 nm. The presence of the zinc complex acted also as a sensitizer towards lanthanide ions. Hence, the complexes R,R-1 and S,S-2 showed the typical Yb 3+ luminescence in the 925-1075 nm NIR region at both, room and low temperatures (Fig. S5-S11(32)). We verified the absence of solvent molecules with thermogravimetric analyzes (TGA) which also indicated that both enantiomers remain stable up to 550 K (Fig. S12). We also used single X-ray diffraction at 400 K and differential scanning calorimetry (DSC) to confirm the absence of a phase transition up to the decomposition temperature (Fig. S13). This demonstrated that the material remains crystallized in the P2 1 polar space group. We investigated the magnetic properties of R,R-1 and S,S-2 using SQUID magnetometry. We confirmed the paramagnetic behavior expected for a single Yb 3+ ion with a 2 F 7/2 ground state (J = 7/2, L = 3, S = 1/2) and a room temperature magnetic moment close to 4.32  B (Fig. S14). We also observed a typical slow relaxation of the magnetization at low temperature (Fig. S15-S20, Tables S4-S6(32)). Both enantiomers crystallized in the acentric space group P2 1 , giving a 2 point group. We confirmed ferroelectric behavior of R,R-1 (Fig. S21-S22 [START_REF]Materials and Methods are available as Supplementary Materials on Science Online[END_REF]) which also applies to S,S-2 because the ferroelectric properties of pairs of enantiomers are known to be identical [START_REF] Li | Molecule-Based Ferroelectric Thin Films: Mononuclear Lanthanide Enantiomers Displaying Room-Temperature Ferroelectric and Dielectric Properties[END_REF][START_REF] Sui | Ionic Ferroelectrics Based on Nickel Schiff Base Complexes[END_REF]. The absence of phase transition up to the decomposition of the material (550 K) indicates that the ferroelectric Curie point (T c ) shall be located at a higher temperature as observed, for instance, in such ferroelectrics as [START_REF] Zhang | Ferroelectric Metal-Organic Frameworks[END_REF]. We carried out piezoresponse force microscopy (PFM) measurements to investigate the polar behavior and ME coupling [START_REF]Materials and Methods are available as Supplementary Materials on Science Online[END_REF][START_REF] Elisabeth | Piezoresponse force microscopy (PFM)[END_REF]. We measured both the vertical and lateral responses, each of them consisted of amplitude and phase signals directly related to the magnitude of the polarization and to its orientation, respectively. This method provided sufficient spatial resolution for the detailed study of in-plane (IP, lateral PFM amplitude  phase) and out-of-plane (OOP, vertical PFM amplitude  phase) piezoelectric behavior at the nanoscale [START_REF] Elisabeth | Piezoresponse force microscopy (PFM)[END_REF][START_REF] Ivanov | Self-assembled diphenylalanine peptide microtubes covered by reduced graphene oxide/spiky nickel nanocomposite: An integrated nanobiomaterial for multifunctional applications[END_REF]. Because additional contributions having ionic transport or electrostatic nature could affect the PFM signals, we carried out the measurements taking into account the protocol described by Vaseduvan et al. [START_REF] Vasudevan | Ferroelectric or non-ferroelectric: Why so many materials exhibit "ferroelectricity" on the nanoscale[END_REF] and Balke et al. [START_REF] Balke | Differentiating Ferroelectric and Nonferroelectric Electromechanical Effects with Scanning Probe Microscopy[END_REF] (Fig. S23-S24, [START_REF]Materials and Methods are available as Supplementary Materials on Science Online[END_REF]). Hence, the PFM responses we observed reflected the polarization state of the material. We obtained a clear piezoresponse coming from several planes of R,R-1 single crystal (Fig. S25), but we detected the strongest simultaneous OOP and IP responses for the largest crystal facet accounting for the ( ) plane (Fig. S26). The atomic force microscopy (AFM) for the ( ) plane reveals a stripe-like morphology (Fig. 2A). We found a peculiar organization in the OOP and IP PFM response that demonstrated the presence of spontaneously polarized domains of opposite polarization states (shown in red and blue, Fig. 2B). Such stripes organization was previously observed in others molecular or metal oxides ferroelectrics such as BiFeO 3 [START_REF] Elisabeth | Piezoresponse force microscopy (PFM)[END_REF][START_REF] You | An organic-inorganic perovskite ferroelectric with large piezoelectric response[END_REF][START_REF] Fu | Diisopropylammonium Bromide Is a High-Temperature Molecular Ferroelectric Crystal[END_REF][START_REF] Ye | Metal-free three-dimensional perovskite ferroelectrics[END_REF]. The crystal symmetry predicts that the ferroelectric domain structure visualized by the IP and OOP components of the piezoresponse must be consistent. However, the resulting PFM response depends on the azimuthal angle between the scanning direction and the polar axis and contains different tensile and shear piezoelectric strain components (as described in the "PFM Measurements" subsection of the Supplementary Materials). This situation is reminiscent to that observed in the benchmark molecular ferroelectric diisopropylammonium bromide [START_REF] Fu | Diisopropylammonium Bromide Is a High-Temperature Molecular Ferroelectric Crystal[END_REF]. The number of polarization directions in the ferroelectric phase depends on the symmetry of the paraelectric phase according to the initial work of Aizu [START_REF] Aizu | Possible Species of Ferromagnetic, Ferroelectric, and Ferroelastic Crystals[END_REF] and recently applied to molecular ferroelectrics [START_REF] Hang | Metal-organic complex ferroelectrics[END_REF][START_REF] Shi | Symmetry breaking in molecular ferroelectrics[END_REF][START_REF] Tang | Multiaxial Molecular Ferroelectric Thin Films Bring Light to Practical Applications[END_REF]. Although the structure of the paraelectric phase cannot be examined (T c is higher than decomposition temperature), thus precluding the determination of the uniaxial or multiaxial character of the polarization, clear correlation between the IP and OOP responses can be observed in some other PFM experiments (Fig. S27).

[(CH 3 ) 4 N]HgCl 3 or [C(NH 2 ) 3 ][Al(H 2 O) 6 ](SO 4 ) 2
We investigated the polarization switchability using direct current (dc) bias voltage of ± 30 V applied to different areas (rectangulars in Fig. 2C,D). Within these areas, we achieved uniform (monodomain-like) polarization states, visible on both, IP and OOP images (Fig. 2D). Taking advantage of the absence of a phase transition, we performed PFM measurements at 450 K (Fig. S24(32)). We found a notable decrease in the electromechanical response, directly related to the polarization, which reflected the temperature dependent behavior for a ferroelectric material. The switchable character of the electric polarization was further demonstrated by applying opposite dc bias to generate box-in-box patterns that showed clear 180° phase contrast and domain walls (Fig. S28). Moreover, the piezoresponse hysteresis loops obtained by switching spectroscopy (SS)-PFM was found to exhibit a centered square-like shape with a 180° switching for the phase component (Fig. 3A). We also measured the typical butterfly loops of the displacement signal which confirmed the ferroelectric character [START_REF] Vasudevan | Ferroelectric or non-ferroelectric: Why so many materials exhibit "ferroelectricity" on the nanoscale[END_REF]. Previously switched areas (± 30 V) revealed a shift of the phase and amplitude component towards either positive or negative voltage, due to the formation of coherent remnant polarization states that caused a strong depolarization field (Fig. 3A). We determined a maximum local longitudinal piezoelectric coefficient at room temperature, which corresponded to a polarization of the order of magnitude of 10 µC.cm 2 . The theoretical spontaneous polarization which we estimated from the point charge model [START_REF] Li | Anomalously rotary polarization discovered in homochiral organic ferroelectrics[END_REF][START_REF] You | Quinuclidinium salt ferroelectric thin-film with duodecuplerotational polarization-directions[END_REF][START_REF] Yang | Directional Intermolecular Interactions for Precise Molecular Design of a High-Tc Multiaxial Molecular Ferroelectric[END_REF] along the b axis by considering only the metal ions and some ligand's atoms (Zn 2+ , Yb 3+ , O  and N + ) gives 3.32 µC.cm  ² along the [0 ] direction (see Table S3 (32)).

We can rationalize this value which was weaker than the experimental one, by pointing out the complexity of the structure. Besides, we did not take into account the effect of additional molecular dipoles constituting the coordination complexes as well as covalency. Thus, we could compare the experimental value with Rochelle salt (0.25 µC.cm  ²) and was the same order of magnitude that the better performing molecular ferroelectrics [START_REF] Zhang | Ferroelectric Metal-Organic Frameworks[END_REF][START_REF] Horiuchi | Above-room-temperature ferroelectricity in a single-component molecular crystal[END_REF][START_REF] Fu | Diisopropylammonium Bromide Is a High-Temperature Molecular Ferroelectric Crystal[END_REF][START_REF] Ye | Metal-free three-dimensional perovskite ferroelectrics[END_REF]. The ferroelectric character of R,R-1 crystals suggests to us the possibility of a ME coupling at room temperature. We performed PFM measurements on the same single crystal in the presence of a dc magnetic field of ± 1 kOe applied along the ( ) plane. We observed the stripe-like morphology obtained by AFM on a new surface area (Fig. 4A). We found ferroelectric polarization redistribution with a low magnitude magnetic field of ± 1 kOe, similar to that produced by an electrical bias voltage (Fig. 4B,4C). The change of the polarization states and enhancement of the response make the effect easy to see. The ferroelectric domain's modification appears only in some parts of the region as it has been systematically observed in the rare examples of materials investigated by PFM [START_REF] Evans | Magnetic switching of ferroelectric domains at room temperature in multiferroic PZTFT[END_REF][START_REF] Xie | Multiferroic CoFe2O4-Pb(Zr0.52Ti0.48)O3 core-shell nanofibers and their magnetoelectric coupling[END_REF][START_REF] Miao | Magnetic-field-induced ferroelectric polarization reversal in magnetoelectric composites revealed by piezoresponse force microscopy[END_REF][START_REF] Henrichs | Multiferroic Clusters: A New Perspective for Relaxor-Type Room-Temperature Multiferroics[END_REF].

The typical magnitude of the magnetoelectric tensor component u d oop

where D is the thickness of the ( )-oriented crystal, is the change in vertical surface displacement induced by the change in lateral magnetic field , attains the order of 100 mV.Oe 1 .cm 1 .

Although this value reflects the polarization change for a given magnetic field, it greatly exceeded (by at least one order magnitude) those observed in the bulk BiFeO 3 multiferroics (ranging from 0.6 to 7 mV.Oe 1 .cm 1 ) (48) and was comparable with those of ferroelectric/ferro(ferri)magnetic composites with strain-mediated ME coupling [START_REF] Vaz | Magnetoelectric Coupling Effects in Multiferroic Complex Oxide Composite Structures[END_REF]. Additional confirmation of the change in responses comes from our in-situ PFM measurements performed on other crystal facets and with various magnetic fields (Fig. S29-S32( 32)).

We found additional evidence of the ME interaction from our SS-PFM local hysteresis loops measurements we performed under dc magnetic field. These loops were strongly affected by a magnetic field, specifically in their asymmetry, height, and in the coercive fields (Fig. 3B). Taking advantage of such ME coupling, we actuated variable polarization states via the dc electric and magnetic fields (Fig. 3C). Such feature may be suitable for multilevel polarization states devices designed for high-density data systems [START_REF] Lee | Multilevel Data Storage Memory Using Deterministic Polarization Control[END_REF]. Remarkably, this interaction occurs in the paramagnetic state at room temperature using a moderate magnetic field (1 kOe). The low magnetic field we used clearly contrasts with other molecule-based materials that required fields of several Tesla to induce a change in the pyroelectric currents [START_REF] Wang | Magnetoelectric coupling in the paramagnetic state of a metal-organic framework[END_REF]. Such room-temperature low-field switching is also quite rare in metal oxides [START_REF] Kitagawa | Low-field magnetoelectric effect at room temperature[END_REF][START_REF] Ueda | Room-temperature magnetoelectric effect in a chiral smectic liquid crystal[END_REF][START_REF] Evans | Magnetic switching of ferroelectric domains at room temperature in multiferroic PZTFT[END_REF].

The strong ME effect we observed is because the same chemical element, Yb 3+ , is implicated in the two functionalities. Lanthanide ions present a larger spin-orbit coupling with respect to transition metal ions in classical inorganic magnetic ferroelectrics. Applying a magnetic field to the material containing anisotropic Yb 3+ (non-zero orbital angular momentum, L = 3, spin-orbit coupling) should affect the crystal lattice producing magnetostriction. To confirm this, we investigated the magnetostriction, , at the microscopic and macroscopic levels. We measured the local surface displacement using contact AFM mode (no electric field applied) and evaluated it as a function of the applied magnetic field. We found a large room-temperature field-induced mechanical deformation (parastriction) (Fig. 3D). The displacement increased with magnetic field and reached about 10 4 (1 kOe). The magnetostriction value we found was consistent with the macroscopic data we obtained from single-crystal X-ray diffraction experiment we conducted under a magnetic field of about 0.8 kOe applied with a deviation angle of 10 ± 5° along the [0 ] direction and yielding  value of up to 1  10 3 (Table S7(32)). We confirmed this effect on a different single-crystal and by collecting three data sets to provide a statistical analysis (Table S8).

Reversing the direction of the magnetic field did not change the sign of the magnetostriction (Table S7), which was fully consistent with the expected quadratic behavior. This large magnetostriction was comparable to those observed in ytterbium-based paramagnets. This phenomenon had a single-ion character primarily, and the related deformation can be equivalent in magnitude with that characteristic of magnetically long-range ordered systems [START_REF] Bumagina | Magnetostriction in ionic rare earth paramagnets[END_REF]. Importantly, the association of magnetostriction and ferroelectricity engenders a pronounced ME coupling as observed in inorganic multi-phase ME materials (3). Thus, we propose that the ME interaction in our Yb 3+ -based ferroelectric complex originates from magnetoelastic coupling that corresponds to the magnetic field-induced deformation of the crystal lattice in the paramagnetic phase. Because R,R-1 exhibited simultaneously magnetostriction and ferroelectricity, applying a magnetic-field induced a mechanical strain via spin-lattice coupling that in turn influenced the polarization by affecting the overall dipole configuration. To support this, we made a comparative analysis of the crystal structures we obtained under two directions of the magnetic field with respect to the zero-field one. Differences in the metal-ligand distances (Table S9 andS10) for both Yb 3+ and Zn 2+ ions could be discerned that in turn affect the dipole order. Hence, the polarization values we calculated were found up to 10% greater or weaker (Table S3 and S11), depending on the orientation of the magnetic field, with respect to the zero-magnetic field value. Such results are in line with those obtained by PFM.

We have demonstrated room temperature magnetoelectric control of ferroelectric domains in a molecule-based material. Thus, in the Yb 3+ -based chiral compound R,R-1, the combination of ferroelectric behavior with a magnetostrictive effect generates a strong ME coupling we observed at room temperature and with a relatively low magnetic field. These properties are useful for practical device application including non-volatile memory where information would be stored as electrically detectable and controllable by Yb 3+ paramagnetism. More generally, such features appear particularly unique in single-phase materials and confirm that the genuine chemical design of multifunctional molecular materials may provide an alternative strategy to usual solid-state compounds for engineering ME devices.

Fig. 1 .

 1 Fig. 1. Crystal structures of R,R-1 and S,S-2. (A) Molecular structure of the dinuclear Zn 2+ /Yb 3+ complexes R,R-1 and S,S-2 and their enantiomeric relationship. Color code: orange Yb 3+ , light blue Zn 2+ , blue N, red O, grey C. Hydrogen atoms have been omitted for clarity. (B) View of the crystal packing arrangement of R,R-1 along the a axis emphasizing the two homochiral complexes. (C) Single-crystal facets assignment and view of the slice in the crystallographic ) plane.

Fig. 2 .

 2 Fig. 2. Scanning probe microscopy (SPM) measurements of the ( )-oriented R,R-1 single crystal: (A) AFM topography of the ( ) plane. (B) OOP (left) and IP (right) PFM responses without electrical bias. The color mapping reveals a peculiar organization demonstrating the presence of spontaneously polarized domains of opposite polarization states (shown in red and blue). (C) Magnified ( 3) part of the scanned area (marked as a square in the previous images) with the OOP (left) and IP (right) PFM responses at 0 V. (D) OOP (left) and IP (right) PFM responses measured for the same area after the direct current (dc) bias voltage (± 30 V) poling. (E) Schematic of the PFM experiment.

Fig. 3 .

 3 Fig. 3. SS-PFM hysteresis loops, multilevel states and magnetostriction. (A) SS-PFM hysteresis loops obtained for the virgin (0 V) and polished by dc bias voltage (± 30 V) areas: phase and displacement components as a function of the voltage. (B) SS-PFM hysteresis loops obtained at zero and under applied magnetic field of ± 1 kOe. (C) Normalized piezoresponse as a function of the magnetic field enlightening the six remanent polarization states that could be actuated by applying magnetic and/or electric fields. (D) Magnetostriction measured on a single crystal of R,R-1 at ± 1 kOe (averaged on 5 loops). The lines are guide for the eye.

Fig. 4 .

 4 Fig. 4. Room temperature PFM studies under magnetic field evidencing the ME coupling. (A) AFM topography of the ( )-plane for R,R-1. (B) OOP and IP PFM responses at H = 0 Oe. (C) OOP and IP PFM responses at H = 1 kOe evidencing the ME coupling as redistribution in the ferroelectric domains (change in colors) and increase in the electromechanical response. (D) Sketch illustrating the possible deformation of the individual complex under an applied magnetic field.

Acknowledgments. We thank J. Haines for fruitful discussions on crystallography. F. Salles is acknowledged for discussion on the point charge model. Funding: The authors are grateful to University of Montpellier (UM), Centre National de la Recherche Scientifique (CNRS), Plateforme d'Analyse et de Caractérisation (PAC) et Institut Charles Gerhardt de Montpellier (ICGM), Fundação para a Ciência e a Tecnologia (FCT) Portugal (R&D project No. PDTC/QUI-QUI/098098/2008-FCOMP-01-0124-FEDER-010785) and NoE FAME for financial support. M. S. I. is grateful to FCT for financial support through the project MATIS -Materiais e Tecnologias Industriais Sustentáveis (CENTRO-01-0145-FEDER-000014). V. A. K. is grateful to FCT for financial support through the FCT Investigator Programme (project IF/00819/2014/CP1223/CT0011). This work was partly supported by funds from FEDER (Programa Operacional Factores de Competitividade COMPETE)

Supplementary Materials:

Materials and Methods

Figs. S1-S32

Tables S1-S11

References (52-85)