V Pakštas

V Jasulaitienė

Bastien Leger

Jolanta Rousseau

Anne Ponchel

Eric Monflier

E Norkus

V Kepenienė

R Stagniūnaitė

L Tamašauskaitė-Tamašiūnaitė

B Léger

Co 3 O 4 /C and Au supported Co 3 O 4 /C nanocomposites -peculiarities of fabrication and application towards oxygen reduction reaction

Keywords: Stagniūnaitė, L. Tamašauskaitė-Tamašiūnaitė, cobalt oxide, impregnation, microwave irradiation, oxygen reduction

    Co3O4/C and Au supported Co3O4/C nanocomposites -Peculiarities of fabrication and application towards oxygen reduction reaction V. Kepenienė, R.

Introduction

The oxygen reduction reaction (ORR) is one of the two reactions occurring in the fuel cells where ORR takes place on the cathode side. Generally, the oxygen reduction reaction can occur via 3 different pathways: the 1e -, 2e -and 4e -mechanisms [START_REF] Song | Electrocatalytic Oxygen Reduction Reaction[END_REF][START_REF] Wang | AgSn intermetallics as highly selective and active oxygen reduction electrocatalysts in membraneless alkaline fuel cells[END_REF]. The reduction pathways can be selected depending on the application. The 1e -reduction pathway is of importance in the exploration of the ORR mechanism, 2e -reduction pathway is used for H 2 O 2 production, however, in the fuel cell processes, the 4e -pathway is highly preferred, which means direct oxygen reduction to water. Meanwhile, when ORR occurs via the 2e -pathway in the fuel cell, the hydroperoxide anion (HO 2 -) is formed which leads to a decrease in efficiency and damage of the membrane [START_REF] Liu | Electricity generation from banana peels in an alkaline fuel cell with a Cu 2 O-Cu modified activated carbon cathode[END_REF][START_REF] Zhang | Exploiting the waste heat from an alkaline fuel cell via electrochemical cycles[END_REF][START_REF] Yu | Reduced graphene oxide supported TiO 2 as high performance catalysts for oxygen reduction reaction[END_REF].

Although, Pt-based catalysts are considered to be the best catalysts for the oxygen reduction reaction in fuel cells, the scarcity and high cost of Pt greatly limit their commercialization and application. In the past decades, researches have directed much effort toward replacing Pt-based catalysts with other metals or metal oxides. Cobalt oxides or hybrid materials consisting of Co 3 O 4 deposited on different carbon (carbon black, graphene, nitrogen-doped graphene) surfaces were proposed for oxygen reduction in fuel cells because of their high electrocatalytic activity and superior stability to Pt in alkaline media [START_REF] Stagniūnaitė | N-doped carbon material modified with cobalt nanoparticles as catalyst for oxygen reduction[END_REF][START_REF] Tan | Perfectly ordered mesoporous iron-nitrogen doped carbon as highly efficient catalyst for oxygen reduction reaction in both alkaline and acidic electrolytes[END_REF][START_REF] Tian | Novel preparation of N-doped carbon spheres with excellent oxygen reduction reaction catalytic activity[END_REF][START_REF] Bonnefont | Challenges in the understanding oxygen reduction electrocatalysis on transition metal oxides[END_REF][START_REF] Uhlig | Electrochemical characterization of temperature dependence of plasma-treated cobalt-oxide catalyst for oxygen reduction reaction in alkaline media[END_REF][START_REF] Uskaikar | Electrocatalytic reduction of oxygen on Co 3 O 4 : Effects of processing method[END_REF][START_REF] Wang | Facile synthesis of three-dimensional porous nitrogen doped carbon supported Co 3 O 4 for oxygen reduction reaction and oxygen evolution reaction[END_REF][START_REF] Kim | The plasma-assisted formation of Ag@Co 3 O 4 core-shell hybrid nanocrystals for oxygen reduction reaction[END_REF][START_REF] Wang | Carbon supported MnO x -Co 3 O 4 as cathode catalyst for oxygen reduction reaction in alkaline media[END_REF]. S.-M. Kim et al. investigated the activity of Ag@Co 3 O 4 core-shell hybrid nanocrystals towards ORR in a 0.1 M NaOH solution [START_REF] Kim | The plasma-assisted formation of Ag@Co 3 O 4 core-shell hybrid nanocrystals for oxygen reduction reaction[END_REF]. They found that the Ag@Co 3 O 4 catalyst showed a 5.2 and 2.6-fold increase in activity as compared to that of bare Ag and AgCo catalysts, respectively. Moreover, the enhanced catalytic performance could be attributed to the strong chemical interaction and electronic coupling effects between the Ag and Co 3 O 4 nanocrystals and to the combination of electronic and geometric effects, which facilitates the O-O bond breaking and desorption for the oxygen reduction [START_REF] Kim | The plasma-assisted formation of Ag@Co 3 O 4 core-shell hybrid nanocrystals for oxygen reduction reaction[END_REF]. Y. Wang et al. also investigated the activity of spinel structure material Co 3 O 4 [START_REF] Wang | Carbon supported MnO x -Co 3 O 4 as cathode catalyst for oxygen reduction reaction in alkaline media[END_REF]. The authors compared the catalytic activity of Co 3 O 4 /C, MnO x Co 3 O 4 /C with that of commercial Pt/C catalysts and showed that Co 3 O 4 /C and MnO x Co 3 O 4 /C catalysts can be promising materials for ORR with the onset potentials of 0.834 and 0.874 V, respectively, as compared to the commercial Pt catalyst with the onset potential of 0.911 V [START_REF] Wang | Carbon supported MnO x -Co 3 O 4 as cathode catalyst for oxygen reduction reaction in alkaline media[END_REF]. Combining gold with cobalt oxides it has been reported as an effective way to increase the activity of catalysts for oxygen reduction, oxygen evolution or CO oxidation reactions due to the synergistic effect between gold and cobalt oxide [START_REF] Gamboa-Rosales | Effect of Au in Au-Co 3 O 4 /CeO 2 catalyst during oxygen-enhanced water gas shift[END_REF][START_REF] Sivakumar | Ternary nickel-cobalt-manganese spinel oxide nanoparticles as heterogeneous electrocatalysts for oxygen evolution and oxygen reduction reaction[END_REF][START_REF] Qu | Boosting the electrocatalytic performance of Pt, Pd and Au embedded within mesoporous cobalt oxide for oxygen evolution reaction[END_REF].

Moreover, combination of metals (Au, Ag or Pd) with cobalt oxide increases the surface area of catalyst, which shortens the reaction path from electrolyte to electrode surface. In addition to this, composite M-Co 3 O 4 (M-Au, Ag, Pd) catalysts were shown to be more active and stable in an alkaline solution, holding a more positive potential, thereby constituting promising electrocatalytic materials [START_REF] Hu | Facile synthesis of PdO-doped Co 3 O 4 nanoparticles as an efficient bifunctional oxygen electrocatalyst[END_REF][START_REF] Li | CuO nanowire@Co 3 O 4 ultrathin nanosheet core-shell arrays: An effective catalyst for oxygen evolution reaction[END_REF][START_REF] Trunov | Analysis of oxygen reduction reaction pathways on Co 3 O 4 , NiCo 2 O 4 , Co 3 O 4 -Li 2 O, NiO, NiO-Li 2 O, Pt, and Au electrodes in alkaline medium[END_REF][START_REF] Pletcher | Comparison of the spinels Co 3 O 4 and NiCo 2 O 4 as bifunctional oxygen catalysts in alkaline media[END_REF].

Cobalt or cobalt oxide nanoparticles as well as gold nanoparticles can be reduced using various reducing agents. Commonly used reducing agents for gold nanoparticles include sodium borohydride, sodium citrate, ascorbic acid and alcohols [START_REF] Kepenienė | Investigation of graphene supported platinum-cobalt nanocomposites as electrocatalysts for ethanol oxidation[END_REF][START_REF] Upskuvienė | Comparison of electrocatalytic activity for glucose electrooxidation of gold nanoparticles fabricated by different methods[END_REF][START_REF] Oliveira | A helpful method for controlled synthesis of monodisperse gold nanoparticles through response surface modeling[END_REF]. Co or cobalt oxides nanoparticles can also be reduced by polyols or borohydride from cobalt salt solutions or by the decomposition of an organometallic [START_REF] Puntes | Synthesis, structural order and magnetic behavior of selfassembled ε-Co nanocrystal arrays[END_REF][START_REF] Zhao | A simple method to prepare uniform cobalt nanoparticles[END_REF][START_REF] Wu | Interaction of fatty acid monolayers with cobalt nanoparticles[END_REF]. However, lately, biopolymers have been used as the most appropriate alternatives and environment friendly reducers as these are regenerative, cost effective and easily available. Among the various biopolymers, β-cyclodextrin (β-CD) could be chosen as multifunctional agent because of its adsorption properties and affinity to the carbon surface [START_REF] Kania | Cyclodextrins adsorbed onto activated carbons: Preparation, characterization, and effect on the dispersibility of the particles in water[END_REF][START_REF] Cassez | Unexpected multi-functional effects of methylated cyclodextrins in palladium charcoal-catalyzed Suzuki-Miyaura reaction[END_REF][START_REF] Cassez | Chemically modified cyclodextrins adsorbed on Pd/C Particles: new opportunities to generate highly Chemo-and stereoselective catalysts for Heck reaction[END_REF], ability of complexing metal salt such as cobalt nitrate to favor the Co dispersion on the substrate [START_REF] Bai | Effects of b-cyclodextrin introduction to zirconia supported-cobalt oxide catalysts: From molecule-ion associations to complete oxidation of formaldehyde[END_REF][START_REF] Bai | Cyclodextrin-cobalt (II) molecule-ion pairs as precursors to active Co 3 O 4 /ZrO 2 catalysts for the complete oxidation of formaldehyde: influence of the cobalt source[END_REF], its reducing properties for Au nanoparticles deposition where CD can act as a reducing agent in alkaline conditions [START_REF] Menuel | Cyclodextrins as effective additives in AuNPs-catalyzed reduction of nitrobenzene derivatives in a ball-mill[END_REF][START_REF] Pestovsky | Synthesis of gold nanoparticles by tetrachloroaurate reduction with cyclodextrins[END_REF] or as stabilizing agent [START_REF] Suárez-Cerda | A comparative study of the effect of α-, β-, and γ-cyclodextrins as stabilizing agents in the synthesis of silver nanoparticles using a green chemistry method[END_REF][START_REF] Suárez-Cerda | A green synthesis of copper nanoparticles using native cyclodextrins as stabilizing agents[END_REF]

Experimental

Chemicals

HAuCl 4 xH 2 O wt. 49% Au, Co(NO 3 ) 2 6H 2 O, carbon powder (99.99%) were purchased from Alfa-Aesar and Sigma-Aldrich Supplies. Pt/C wt. 46.4% Pt (TEC10E50E) was purchased from Tanaka Kikinzoku Kogyo K.K. Supplier. β-cyclodextrin was supplied by Roquette Freres (Lestrem, France). H 2 SO 4 (96%), NaOH (98.8%), NH 3 (28%), ethanol, ethylene glycol and acetone were purchased from Chempur Company. All chemicals were of analytical grade. Ultrapure water with the resistivity of 18.2 M cm -1 was used to prepare all the solutions.

Preparation of catalysts.

The Co 3 O 4 /C and AuCo 3 O 4 /C catalysts were prepared using three different approaches: (a) impregnation (IMP), (b) impregnation plus microwave irradiation (IMP+MW) and (c) microwave irradiation (MW). According to our previous investigations [START_REF] Bai | Effects of b-cyclodextrin introduction to zirconia supported-cobalt oxide catalysts: From molecule-ion associations to complete oxidation of formaldehyde[END_REF][START_REF] Bai | Cyclodextrin-cobalt (II) molecule-ion pairs as precursors to active Co 3 O 4 /ZrO 2 catalysts for the complete oxidation of formaldehyde: influence of the cobalt source[END_REF], the molar ratio of cobalt and cyclodextrin equal to 1 : 0.1 was chosen. In this case, the gold amount in the Au- Using the impregnation approach (a) for fabrication of catalysts, both the reaction mixtures were stirred with a magnetic stirrer at a temperature of 25 ºC for 2h. Then the pH of solution was raised up to 9 by adding concentrated NH 3 with further stirring for 1h. The mixtures obtained were filtered and then desiccated at a temperature of 80 ºC for 4h. The materials were named as Co/C IMP and AuCo 3 O 4 /C IMP .

When the catalysts were prepared by the impregnation plus microwave irradiation approach (b), the procedure was performed under the same conditions as those previously described, except that after stirring the mixtures obtained were put into a microwave reactor Monowave 300 For the fabrication of catalyst using the microwave irradiation approach (c), the both reaction mixtures with pH equal to 9 were put directly into the microwave reactor without the impregnation period. The synthesis was carried out at a temperature of 170°C for 1h. The synthesized catalyst was filtered and desiccated at a temperature of 80 ºC for 4h. The materials were named as Co 3 O 4 /C MW and AuCo 3 O 4 /C MW .

Characterization of catalysts.

To estimate the shape and size of catalyst particles a transmission electron microscope Tecnai G2 F20 X-TWIN equipped with an EDAX spectrometer with an r-TEM detector was used. For microscopic examinations, 10 mg of sample was first sonicated in 1 ml of ethanol for 1 h and then deposited on the Cu grid covered with a continuous carbon film.

The composition of the obtained Co 3 O 4 /C and AuCo 3 O 4 /C was determined by means of Xray photoelectron spectroscopy (XPS) using an "ESCALABMKII" spectrometer (VG Scientific, UK) equipped with an Al Kα X-ray radiation source (1486.6 eV) operated at a fixed pass energy of 20 eV.

The morphology and composition of the fabricated catalysts were characterized using a SEM/FIB workstation Helios Nanolab 650 with an energy dispersive X-ray (EDX) spectrometer

INCA Energy 350 X-Max 20.
The Au and Co metal loadings were estimated from ICP-OES measurements by recording the ICP optical emission spectra using an ICP optical emission spectrometer Optima 7000DV (Perkin Elmer).

In order to evaluate the phases and lattice parameters of the synthesized catalysts, X-ray diffraction patterns were recorded using a D8 diffractometer (Bruker AXS, Germany, 2003) with Cu Kα radiation using a Ni/graphite monochromator. A step-scan mode was used in the 2-theta range from 10° to 80° with a step length of 0.02° and a counting time of 1 s per step. Phase identification was performed using the software package PDXL (Rigaku) and the ICDD powder diffraction data base PDF-4+ (2018 release).

Electrochemical measurements.

Electrocatalytic properties of the synthesized catalysts were evaluated using an "AutoLab PGSTAT100" electrochemical workstation with a rotating disk electrode (RDE) system and software Nova. A three-electrode conventional cell was used for the measurements. The glassy carbon RDE with a geometric area of 0.07 cm 2 was used as the working electrode. A Pt sheet was used as a counter electrode and an Ag/AgCl/KCl electrode was used as reference. The

ESA (cm 2 ) = Q (µC) / 400 (µC cm -2 ) ( 1
)
where 400 µC cm -2 is the charge required to reduce a monolayer of AuO adsorbed on the surface of the electrode.

The Koutecky-Levich (K-L) equations (2-4) were used for calculation of electron transfer numbers per oxygen molecule (n) in ORR:

j -1 = j k -1 + j d -1
(2)

j d = 0.62nFD 2/3 Co 2 ʋ -1/6 ω 1/2 (3) j -1 = j k -1 + ω -1/2 × A (4)
where, j, j k and j d are the measured current density, kinetic current density and the diffusion limiting current density (mA cm -2 ), respectively; n is the number of electrons transferred in the reaction; F is the Faraday constant; D is the diffusion coefficient of the reactant (1.9 x 10 -5 cm 2 s - 1 ); Co 2 is the concentration of the reactant in the bulk electrolyte (1.1 x 10 -6 mol L -1 ); ʋ is the kinetic viscosity of the electrolyte (1.13 x 10 -2 cm 2 s -1 ); ω is the rotation rate; A is the slope of the linear plot of j -1 versus ω -1/2 (K-L plot) [START_REF] Paulus | Oxygen reduction on a high-surface area Pt/Vulcan carbon catalyst: a thin-film rotating ring-disk electrode study[END_REF].

Results and discussions

In this study, the influence of fabrication conditions of the carbon supported Co The size of Au and Co 3 O 4 nanoparticles was also evaluated from TEM images (Fig. 3).

Referring to the images it might be assumed that Au particles grow on the surface of Co in agreement with the reported data of pure Co 3 O 4 crystals [START_REF] Yao | Xu Co 3 O 4 nanowires as high capacity anode materials for lithium ion batteries[END_REF][START_REF] Jin | Facile synthesis of monodisperse Co3O4 mesoporous microdisks as an anode material for lithium ion batteries[END_REF]. The Co 2p 3/2 binding energy peaks origina ted at approximately 782.4 eV and 784.6 may be corresponded to Co 2+ in the CoO phase [START_REF] Carvalho | Development of Co 3 [Co(CN) 6 ] 2 /Fe 3 O 4 Bifunctional Nanocompositefor Clinical Sensor Applications[END_REF]42]. It should be noted that a significant amount of oxygen species are also included in all the catalysts. The binding energy peaks at 529.8 -531.5 eV in the O1s spectra were assigned to the lattice oxygen species [START_REF] Hsieh | Luminescence mechanism of ZnO thin film investigated by XPS measurement[END_REF]. Moreover, the one at 533.4 eV could be attributed to the Co-O-C bond [START_REF] Zhu | Efficient activation of persulfate by Fe3O4@b-cyclodextrin nanocomposite for removal of bisphenol A[END_REF][START_REF] Zeng | Chen Nitrogen-Doped Porous Co 3 O 4 /Graphene Nanocomposite for Advanced Lithium-Ion Batteries Nanomaterials[END_REF]. For O1s XPS spectra, the higher BE peaks located at about 532-535

eV is usually attributed to the presence of surface of adsorbed O 2 , H 2 O, and CO 2 (Fig. 4 .0 V in the cathodic direction to 0.2 V vs. RHE at a scan rate of 5 mV s -1 . The rotation speed was varied from 1200 to 2500 rpm. According to the theory, at potential values between 0.7 and 0.9 V vs. RHE the ORR response is dominated by the kinetics of electrocatalysts [START_REF] Shi | Oxygen reduction reaction of Fe-Polyaniline/Carbon Nanotube and Pt/C catalysts in alkali media[END_REF].

Electrochemical measurements showed that all investigated catalysts exhibited typical ORR curves, but different onset potentials (Fig. 7 a-d). The onset potential for each catalyst estimated from measurements data was equal to 0.83, 0.83, 0.88 and 0.85 V for the Co Electrocatalytic activity of as prepared catalyst for ORR was also expressed by mass and specific activities of the catalysts at a potential value of 0.8 V at a rotation speed of 2500 rpm and are given in Fig. 8 (c,d). In order to present mass activity, oxygen reduction current densities were normalized by the Au or Co loadings (mA mg Au/Co -1 ) for each catalyst (Fig. 8 c).

To express specific activities oxygen reduction current densities were normalized by electrochemically active surface areas of Au for each AuCo 3 O 4 /C catalyst (mA cm -2 ) (Fig. 8 d).

Reviewing the results, it was noticed that the mass activity calculated by the Au loading is 3-5 After LSV measurements at a rotation speed from 1200 to 2500 rpm, the Koutecky-Levich plots were obtained at potentials from 0.4 to 0.6 V vs. RHE. Fig. 9 presents K-L plots of the investigated catalysts in an O 2 -saturated 0.1 M NaOH solution. In general, the K-L curves show a linear relationship between j -1 and ω -1/2 indicating the first-order dependence of the ORR kinetics at different potentials (0.4 -0.6 V). The non-zero intercepts of the extrapolated K-L lines show that the process of O 2 reduction is not entirely under the diffusion control even at more negative potentials. According to the K-L equations (2-4), the calculated number of electrons transferred (n) was varied from ca. 2.51 to 3.51 at more positive potentials (E > 0.6 V) and ca 3.94 at more negative potentials depending on the catalyst used (Fig. 9, the insets).

  . All these features prompted us to use β-CD in the preparation of Co 3 O 4 /C and AuCo 3 O 4 /C catalysts. This work presents the Co 3 O 4 /C and AuCo 3 O 4 /C catalysts prepared using three different approaches: impregnation (IMP), impregnation plus microwave irradiation (IMP+MW) and direct microwave irradiation (MW). The composition, morphology and structure of the prepared catalysts were carefully investigated using different materials characterization methods such as X-ray Diffraction, Field Emission Scanning Electron Microscopy (FESEM), Transmission Electron Microscopy (TEM) and Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). The electrocatalytic properties of the Co 3 O 4 /C and AuCo 3 O 4 /C catalysts towards the reduction of oxygen in an alkaline medium were investigated using the rotating disk electrode method. It has been determined, that the electrocatalytic parameters such as the onset potential and current density of the synthesized Co 3 O 4 /C and AuCo 3 O 4 /C towards the oxygen reduction reaction in an alkaline medium were quite close to the theoretical potential values between 0.7 and 0.9 V vs. RHE. The electrocatalytic activity of the as-prepared Co 3 O 4 /C and AuCo 3 O 4 /C catalysts depends on the preparation method used.

(

  Anton Paar). The reduction of Au and Co 3 O 4 nanoparticles was carried out at a temperature of 170°C for 1h. Then, the mixtures obtained were filtered and desiccated at a temperature of 80 ºC for 4h. The materials were named as Co 3 O 4 /C IMP+MW and AuCo 3 O 4 /C IMP+MW .

  catalyst layers were obtained according to the following steps: at first, the required amounts of the investigated catalysts were dispersed ultrasonically for 1 hour in a 0.1 ml of 2% polyvinylidenefluoride in an N-methyl-2-pyrrolydone solution. Then, 5 μl of the prepared suspension mixture was pipetted onto the polished surface of a glassy carbon electrode and dried in an oven at a temperature of 80 ºC for 4 h. The linear sweep voltammetry curves (LSVs) were recorded in a O 2 saturated 0.1 M NaOH solution in the electrode potential range from 1 to 0.4 V in the cathodic direction at a scan rate of 5 mV s -1 at a temperature of 25 o C, varying the rotation speed from 1200 to 2500 rpm. The electrolyte was saturated with O 2 for 15 min prior to each ORR test. Chronoamperommetric curves for ORR were recorded at a potential of 0.55 V in a O 2 saturated 0.1 M NaOH solution for 1900 s. The electrode potential is quoted versus the reversible hydrogen electrode (RHE). The presented current densities are normalized with respect to the geometric area of the electrode. The electrochemically active surface areas (ESAs) of Au in the prepared different AuCo 3 O 4 /C catalysts were determined from the cyclic voltammograms (CVs) recorded in a deaerated 0.5 M H 2 SO 4 solution at a sweep rate of 50 mV s -1 . The ESAs values of Au in the AuCo 3 O 4 /C catalysts were determined from the CVs by integrating the area of the peak associated with the reduction of AuO (Q) with a charge density of 400 C cm -2 [37]:

Fig. 1 .

 1 Fig. 1. XRD patterns of the catalysts synthesized under different conditions: a) Co/C IMP (pattern

Fig. 2 .

 2 It is seen that Co 3 O 4 nanoparticles are precipitated in the form of small cauliflower-like structure on the surface of carbon in the Co 3 O 4 /C IMP+MW (Fig. 2 a) and Co 3 O 4 /C MW (Fig. S1 b) catalysts. The SEM image of Co/C IMP shows bigger and angular nanoparticles (Fig. S1 a). SEM images of AuCo 3 O 4 /C catalysts (Fig. 2 b, c, d) depict that spherical Au nanoparticles are grown on the surface of Co 3 O 4 /C. Moreover, Co 3 O 4 particles are finer and more complete settle down on the carbon surface, when the both Au and Co metal precursors are used for the synthesis of catalyst.

  3 O 4 /C. It has been determined that Co/C IMP prepared using the impregnation procedure exhibits some clusters of ca. 150-300 nm in size (Fig. S2 a) and contains Co according to the EDX analysis (Fig. S2 b). Meanwhile, in the AuCo 3 O 4 /C IMP catalyst (Fig. 3b) Au nanoparticles of ca. 15 nm in size were obtained. The Co 3 O 4 nanoparticles look more like clusters and are not separate. Using the microwave irradiation method after impregnation for the preparation of catalysts, Co 3 O 4 nanoparticles of ca. 20 nm in size for Co 3 O 4 /C IMP+MW were obtained (Fig. 3a). AuCo 3 O 4 /C IMP+MW contained Co 3 O 4 nanoparticles of ca. 9 nm and Au of ca. 41 nm in size (Fig. 3c).

Fig. 2 . 4 and

 24 Fig. 2. SEM images of the Co 3 O 4 /C IMP+MW , AuCo 3 O 4 /C IMP , AuCo 3 O 4 /C IMP+MW and

Fig. 3 .

 3 Fig. 3. TEM images of the a) Co 3 O 4 /C IMP+MW , b) AuCo 3 O 4 /C IMP , c) AuCo 3 O 4 /C IMP+MW and d)

Fig. 4 . 2 )Fig. 5 . 2 )Fig. 6 .

 42526 Fig. 4. XPS spectra of Co2p (a-d) and O1s (e-h) for the Co 3 O 4 /C IMP+MW and AuCo 3 O 4 /C

Fig. 8a presentsFig. 7 .

 7 Fig. 8a presents the comparison of electrocatalytic activity of the investigated catalysts with

Fig. 8 .

 8 Fig. 8. Comparison of LSV curves recorded on the Co 3 O 4 /C IMP+MW , all three AuCo 3 O 4 /C and

For the Co 3 Fig. 9 .

 39 Fig. 9. Koutecky-Levich plots of the Co 3 O 4 /C IMT+MW and AuCo 3 O 4 /C catalysts at different

The Co 3 O 4 /

 34 C and AuCo 3 O 4 /C catalysts have been prepared using the impregnation and rapid microwave irradiation approaches. Combination of these two methods has been established to be the best method for preparation of the carbon supported Co 3 O 4 and AuCo 3 O 4 catalysts. The

  Co 3 O 4 /C IMP+MW (pattern 2) and Co 3 O 4 /C MW (pattern 3) catalysts. Characteristic peaks corresponding to Co 3 O 4 (ICDD # 00-042-1467) were recorded for the catalysts prepared using the impregnation combined with the microwave irradiation and direct microwave approaches (Fig. 1a, pattern 2, 3). The size of Co 3 O 4 nanocrystallites was determined using Scherrer equation and values of full width at half maximum (FWHM) of Co 3 O 4 XRD peaks. Co 3 O 4 nanocrystallites were of ca. 30 nm in size in both Co 3 O 4 /C IMP+MW and Co 3 O 4 /C MW catalysts. The XRD pattern for Co/C IMP prepared by the impregnation approach does not show peaks corresponding to Co 3 O 4 (Fig. 1a, pattern 1). The presence of several diffraction peaks of low intensity at 2ϴ = 11.79, 23.70 and 34.88º were assigned to Co 3 (C 4 O 4 ) 2 (OH) 2 (ICDD # 04-020-4355) (Co/C IMP ). It could be, that the mild conditions without heating or microwave irradiation does not allow the formation of crystallized CoOx or Co(OH)Ox particles. As a result, amorphous particles could not be detected by XRD. More details on Co/C IMP and Co 3 O 4 /C MW properties are presented in the supplementary data.Figure 1 b presents patterns of AuCo 3 O 4 /C IPM , AuCo 3 O 4 /C IMP+MW and AuCo 3 O 4 /C MW . As evidenced by the data, the patterns of all AuCo 3 O 4 /C catalysts prepared by different approaches (Fig. 1b) reveal peaks corresponding to Au (ICDD # 00-004-7840) and Co 3 O 4 . According to the XRD data, the structure of all the AuCo 3 O 4 /C catalysts was quite similar (Fig. 1b). It was found that the AuCo 3 O 4 /C IPM , AuCo 3 O 4 /C IMP+MW and AuCo 3 O 4 /C MW catalysts consists of smaller Co 3 O 4 crystallites of ca. 8-9 nm in size and they are almost 4 fold smaller if compare with the Co 3 O 4 /C IMP+MW and Co 3 O 4 /C MW catalysts. Because the Au diffraction peaks of (111), (200) and (220) planes overlap with those of (311), (222) and (400) planes of Co 3 O 4 , the size of Au crystallites was qualified using the graphical Halder-Wagner method implemented in the PDXL software package (Rigaku). The estimated size of Au crystallites in the AuCo 3 O 4 /C IPM , AuCo 3 O 4 /C IMP+MW and AuCo 3 O 4 /C MW catalysts was equal to ca. 10 nm. The XRD lattice parameters and particular size of Au and Co 3 O 4 crystallites are given in theTable 1. Theoretic parameter of cubic gold lattice constant A is equal to 4.078 Å, whereas the one for Co 3 O 4 is equal to 8.08 Å. As seen from the data in Table 1, there are no changes in the lattice parameters for Au and Co 3 O 4 . Therefore, it can be assumed that solid solutions do not form in the synthesized AuCo 3 O 4 /C catalysts. However, the size of Co 3 O 4 crystallites in the AuCo 3 O 4 /C catalysts is approximately 4 times smaller as compared with those in the pure Co 3 O 4 /C catalysts.

	Intensity a.u.
	20	40	60

3 O 4 and AuCo 3 O 4 catalysts on their electrocatalytic activities towards ORR has been investigated. The Co 3 O 4 /C and AuCo 3 O 4 /C catalysts were prepared using three different approaches, i.e. the impregnation, impregnation combining with microwave irradiation and microwave irradiation

methods. The structural composition of catalysts was examined by X-ray diffraction. Figure

1a

presents the XRD patterns of the synthesized Co/C IMP (pattern 1),

Table 1 .

 1 XRD characteristics of the Co 3 O 4 /C and AuCo 3 O 4 /C catalysts: lattice constants and particle size. Representative SEM images of the Co 3 O 4 /C and AuCo 3 O 4 /C catalysts prepared by different approaches are shown in

	Catalysts	a, Å Au / Co 3 O 4	Crystalite size, nm Au / Co 3 O 4
	Co 3 O 4 /C IMP Co 3 O 4 /C IMP+MW Co 3 O 4 /C MW	not detected -/ 8.094 -/ 8.084	not detected -/ 29.2  0.8 -/ 30.1  0.5
	AuCo 3 O 4 /C IMP AuCo 3 O 4 /C IMP+MW	-4.0786 / 8.085 -4.0787 / 8.084	11.4  0.5 / 8.8  0.6 10.4  0.4 / 7.8  0.7
	AuCo 3 O 4 /C MW	-4.0781 / 8.082	10.4  0.5 / 8.2  0.5

  e-h, S4 c, d)[START_REF] Hsieh | Luminescence mechanism of ZnO thin film investigated by XPS measurement[END_REF]. According to the XPS data, the dominated Co in the AuCo 3 O 4 /C catalysts is the Co 3 O 4 phase. In the case of Co 3 O 4 /C catalysts, the main phase of Co is corresponded to CoO. The XPS spectra of the AuCo 3 O 4 /C catalysts showed peaks at 83.7 and 87.3 eV with a difference of 3.7 eV corresponds to the 4f 7/2 and 4f 5/2 of Au, respectively, which corresponds to metallic Au (spectra not shown). The Au and Co loadings in the synthesized catalysts were determined by ICP-OES. According to the synthesis parameters, the Au loadings were 0.184, 0.214 and 0.231 mg Au cm -2 in the AuCo 3 O 4 /C IMP , AuCo 3 O 4 /C IMP-MW and AuCo 3 O 4 /C MW catalysts, respectively. The amount of Co was 0.300 mg cm -2 in the synthesized Co 3 O 4 /C IMP+MW catalyst and 0.300, 0.365 and 0.394 mg Co cm -2 in the AuCo 3 O 4 /C IMP , AuCo 3 O 4 /C IMP+MW and AuCo 3 O 4 /C MW catalysts, respectively. The electrochemically active surface areas of Au in the prepared different AuCo 3 O 4 /C catalysts were determined from the CVs recorded in a deaerated 0.5 M H 2 SO 4 solution at a sweep rate of 50 mV s -1 (Fig. 5). The ESAs values of Au in all the AuCo 3 O 4 /C catalysts were calculated by integrating the area of the peak associated with the reduction of AuO monolayer [37]. The obtained ESAs values were 0.21, 0.13 and 0.08 cm 2 in the synthesized AuCo 3 O 4 /C IMP , AuCo 3 O 4 /C IMP+MW and AuCo 3 O 4 /C MW catalysts, respectively.

	Intensity (a.u.)	781.7 780.2	Co 3 O 4/ C IMP+MW 2p 1/2 Co2p 783.1 2p 3/2 784.6	531.6 530.3	532.8 534.0 535.2	O1s
		2p 3/2
	Intensity (a.u.)	779.8	781.3 783.0	2p 1/2	529.9	531.5	533.1
	Intensity (a.u.)	780.3	2p 3/2 781.8 783.7	2p 1/2	531.3 529.9	532.5 533.68
	Intensity (a.u.)	780.0	2p 3/2 781.6 783.6	2p 1/2	531.1 529.8	532.4 533.8
		770 780 790 800 810	525 530 535 540 545
		Binding Energy (eV)	Binding Energy (eV)

Table 2 .

 2 3 O 4 /C IMP-MW , AuCo 3 O 4 /C IMP , AuCo 3 O 4 /C IMP+MW and AuCo 3 O 4 /C MW catalysts, respectively (Fig. 7 a-d). Undoubtedly, the onset potential equal to 0.75 V for Co/C IMP and Co 3 O 4 /C MW catalysts is too low for ORR, though LSV curves closely resembled those presented in literature (Fig. S5 b, d), therefore, further these catalysts were not investigated. Comparison of onset potentials of the proposed Co 3 O 4 /C and AuCo3O4/C catalysts with other reported in literature is given in the As it is evident from the data in Table 2, the as-prepared Co 3 O 4 /C and AuCo3O4/C catalysts present similar or even higher onset potentials.

Table 2 .

 2 The comparison of onset potentials of various catalysts.

	Catalysts	Electrolyte	Onset potential,	References
			(V vs RHE)
	Co3O4/CIMP-MW	0.1 M NaOH	0.830	this work
	AuCo3O4/CIMP	0.1 M NaOH	0.830	this work
	AuCo3O4/CIMP+MW	0.1 M NaOH	0.880	this work
	AuCo3O4/CMW	0.1 M NaOH	0.850	this work
	Co 3 O 4 /C	1 M KOH	0.834	[14]
	Co 3 O 4	0.1 M KOH	0.710	[18]
	Co 3 O 4 /C	1 M KOH	0.783	[47]
	Co 3 O 4 /CNTs	0.1 M KOH	0.720	[48]
	Co 3 O 4 /rmGO	0.1 M KOH	0.880	[49]

highest electocatalytic activity towards oxygen reduction reaction in an alkaline media exhibits the AuCo 3 O 4 /C IMP+MW catalyst as compared with that of the Co 3 O 4 /C IMP+MW catalyst and AuCo 3 O 4 /C IMP and AuCo 3 O 4 /C MW catalysts, which were prepared using the separate impregnation and microwave irradiation methods. Our prepared catalysts can be further investigated as a promising cathode material for alkaline fuel cells.


Micky Tackx
email: tackx@cict.fr

Fre´de´ric Aze´mar

Ste´phanie Bouleˆtreau

Niels De Pauw

Kees Bakker

Benoit Sautour

Ste´phane Gasparini

Karline Soetaert

Stefan Van Damme

Patrick Meire

Zooplankton in the Schelde estuary, Belgium and the Netherlands: long-term trends in spring populations

Keywords: Schelde estuary, zooplankton, long-term

A compilation of available data in between 1967 and 2002 on spring zooplankton abundance was made for the brackish and the freshwater zone of the Schelde estuary. The general picture is a significant increase of 1-2 orders of magnitude in abundance for Rotifera, Copepoda and Branchiopoda (mainly Cladocera) in the freshwater zone, while zooplankton abundance in the brackishwater zone remained more constant. Possible natural and management related causes for this increase in zooplankton abundance are briefly discussed.

The Schelde estuary is one of the few remaining estuaries with an extensive salt-, brackish-and freshwater tidal reach in Europe (Fig. 1). Located in an area with high population density, the Schelde was considered as one of the most eutrophic systems in the world during the second half of the 20th century [START_REF] Heip | Biota and abiotic environnement in the Westerschelde estuary[END_REF]. A gradual improvement is however observed since the mid-seventies as a result of substantial emission reduction efforts throughout the watershed [START_REF] Van Damme | De waterkwaliteit van de Zeeschelde: evolutie in de voorbije dertig jaar[END_REF]. By their pelagic life-style and their requirement for oxygen, zooplankton organisms are strongly affected by the water quality.

In this paper, we look at the long-term changes in the mesozooplankton populations in the Schelde estuary. We have combined abundance data on mesozooplankton in the Schelde obtained during 1967-1969[START_REF] De Pauw | On the distribution of Eurytemora affinis (Poppe) (Copepoda) in the Western Scheldt estuary. Ver-ha¨ndlungen -Internationale Vereinigung fu¨r[END_REF][START_REF] De Pauw | Bijdrage tot de kennis van milieu en plankton in het Westerschelde estuarium[END_REF][START_REF] Bakker | Comparison of plankton assemblages of identical salinity ranges in estuarine tidal and stagnant environments[END_REF]), 1989-1991[START_REF] Soetaert | Spatial and temporal patterns of the zooplankton in the Westerschelde[END_REF]) and 1996-2000(Tackx et al., 2003 and unpublished results of the ongoing 'Onderzoek Milieu -effecten Sigmaplan (OMES) project). Two zones were considered : the brackish water zone (at 57.5-71.5 km from the mouth) and the freshwater zone (78.5-155 km) (Fig. 1; [START_REF] Van Damme | Spatial and temporal patterns of water quality along the estuarine salinity gradient of the Scheldt estuary (Belgium and The Nethelands): results of an integrated monitoring approach[END_REF]. For each spring month (February-May), the average abundance of each taxon was calculated from all available data within the zone. The number of stations per zone varied between 1 and 12 (minimum 3 in most cases). During 1989During -1991, 200 , 200 ls of water was pumped at 2.5 m below surface, at 2.5 m above bottom and at mid-depth and filtered through a 50 lm sieve and the three samples were pooled. In all other cases, 50 ls taken from the surface was filtered through a 50 lm net. Samples were stored in 4% formaline and abundance analysed under binocular microscope. Taxa were grouped as Rotifera, Copepoda (copepodites and adults) and Branchiopoda. Significance of trends in abundance with time were tested by Spearmann rank at p < 0.5, and differences in abundance between time periods by Mann-Whitney at p < 0.05. Oxygen concentrations for the same zones and months were obtained from essentially the same sources as the zooplankton data, but not always sampled at the same location and at the same time as zooplankton.

During the study period, rotifer abundance (Fig. 2a) in the brackish water zone increased between 1967-1969 and 1989-1991 to remain rather stable afterwards. In the freshwater zone, it increased steadily over two orders of magnitude during the study period (Fig. 2b). Copepod abundance in the brackish zone (Fig. 2c) increased between 1967-1969 and 1989-1991, to decrease slightly afterwards. In the freshwater zone (Fig. 2d), copepod abundance increased over two orders of magnitude during the study period. This different tendency between the two zones can be explained by the fact that the dominant spring species Eurytemora affinis has shifted its bulk population from the brackish to the freshwater zone since the mid-nineties (Appeltans et al., 2003). In the brackish water zone, Branchiopoda (Fig 2e) (essentially Cladocera) were scarce and occurred in fluctuating abundance throughout the study period. The same was true for the freshwater zone (Fig. 2f) until 1991, but from 1996 onwards, the population reached a higher and steadier abundance. The quasi total absence of Copepoda and Branchiopoda from the freshwater zone during 1989-1991 was probably due to the fact that the only freshwater station sampled during this period was Antwerpen (78.5 km). This station was characterised by poor water quality conditions at that time, which caused a paucity of planktonic organisms [START_REF] Soetaert | Spatial and temporal patterns of the zooplankton in the Westerschelde[END_REF].

The general increase in zooplankton abundance since the 60s in the freshwater zone is likely to be a reflection of changed environmental conditions. Despite global warming, water temperature in the freshwater zone does not show a trend during the period considered (OMES database). Restoration efforts are more likely to be at the origin of this zooplankton development. A complete overview of management strategies and their consequences regarding the Schelde estuary is given by Van [START_REF] Van Damme | De waterkwaliteit van de Zeeschelde: evolutie in de voorbije dertig jaar[END_REF][START_REF] Van Damme | Spatial and temporal patterns of water quality along the estuarine salinity gradient of the Scheldt estuary (Belgium and The Nethelands): results of an integrated monitoring approach[END_REF]. The general decrease in pollution levels and especially the increase in oxygen concentration occurring mainly in the freshwater zone from the 90ties onwards (Fig. 2e,f) can be considered an important factor in relation to this zooplankton development. Multivariate analysis using the more the consistent dataset between zooplankton and environmental variables since the OMES monitoring campaigns (1986) could further elucidate this hypothesis (Aze´mar et al., in preparation).

Figure 1 .

 1 Figure 1. Positioning (inset) and map of the Schelde estuary showing the brackish and the fresh water zone. Numbers are distance in km from mouth at Vlissingen of stations used for monitoring in the OMES project.

Figure 2 .

 2 Figure 2. Spring (February-May) mean abundance (bars) and maximum abundance (line with diamonds) of Rotifera (a), Copepoda (c) and Branchiopoda (e) observed in the brackish zone and of Rotifera (b), Copepoda (d) and Branchiopoda (f) observed in the freshwater zone of the Schelde estuary from 1967 till 2002. Note the logarithmic scale. Shaded area in Fig. 2e, f Percentage oxygen saturation. See text for further explanation.


Samahe Sadjadi

Maryam Akbari

Bastien Léger

Eric Monflier

Majid M Heravi

Eggplant-derived biochar-halloysite nanocomposite as supports of Pd nanoparticles for the catalytic hydrogenation of nitroarenes in presence of cyclodextrin

Keywords: Halloysite, Biochar, Pd nanoparticles, Hydrogenation, Cyclodextrin

A novel halloysite-hydrochar nanocomposite has been prepared and applied for the immobilization of Pd NPs to furnish an efficient catalyst for the hydrogenation of nitroarenes. It was confirmed that the use of catalytic amount of -cyclodextrin (-CD) could improve the yield of the reaction significantly. With the aim of investigation of the effect of combination of Hal and Char, Char surface modification and the way of use of -CD on the catalytic activity, several control catalysts were prepared and their catalytic activities were compared with that of the catalyst. It was confirmed that the use of Hal-Char as a support was more effective than the use of each component individually. Moreover, the use of -CD in its free form was more efficient than incorporating it to the framework of the catalyst or as a capping agent. It was also found that Char in its unmodified form was more efficient than modified ones. To justify the results, a precise study was carried out by comparing the average Pd particle size and loading of each samples. It was confirmed that the Pd particle size and dispersion effectively affected the catalytic activity. Additionally, -CD amount was a key factor for achieving high catalytic activity.

INTRODUCTION

Ever-increasing environmental concerns have resulted in much attention to the development of environmentally benign heterogeneous catalysts. In this line, (bio)Char as a carbonaceous product of the carbonization of biomass offers significant environmental merits. Chars are used extensively for fuels production, soil remediation, waste management, greenhouse gas reduction, absorbent, catalysis and supercapacitors. [START_REF] Zhang | Ionic liquid-functionalized biochar sulfonic acid as a biomimetic catalyst for hydrolysis of cellulose and bamboo under microwave irradiation[END_REF][START_REF] Cai | Titanium dioxide-coated biochar composites as adsorptive and photocatalytic degradation materials for the removal of aqueous organic pollutants[END_REF][START_REF] Huang | Effective removal of Cr(VI) using -cyclodextrin-chitosan modified biochars with adsorption/reduction bifuctional roles[END_REF][START_REF] Liu | Enhancement of As(V) adsorption from aqueous solution by a magnetic chitosan/biochar composite[END_REF][START_REF] Li | In situ preparation of biochar coated silica material from rice husk[END_REF] Hydrothermal carbonization is one of the most common methods for the preparation of Chars and the Chars produced from a hydrothermal process are often called hydrochar. The Chars can be applied in their plain form or activated form to improve their properties and increasing their efficiencies. Moreover, they can be used with other materials to furnish composites with diverse ranges of applications. [START_REF] Zhang | Ionic liquid-functionalized biochar sulfonic acid as a biomimetic catalyst for hydrolysis of cellulose and bamboo under microwave irradiation[END_REF][START_REF] Cai | Titanium dioxide-coated biochar composites as adsorptive and photocatalytic degradation materials for the removal of aqueous organic pollutants[END_REF][START_REF] Huang | Effective removal of Cr(VI) using -cyclodextrin-chitosan modified biochars with adsorption/reduction bifuctional roles[END_REF][START_REF] Liu | Enhancement of As(V) adsorption from aqueous solution by a magnetic chitosan/biochar composite[END_REF][START_REF] Li | In situ preparation of biochar coated silica material from rice husk[END_REF] One of the natural inorganic materials with tubular morphology is halloysite, Hal, (Al2Si2O5(OH)4•nH2O) that is a dioctahedral 1:1 layered aluminosilicate with properties similar to those of kaolin. The Hal tubes with inner diameters of 10-30 nm and outer diameters of 50-70 nm have different electric charge and chemical composition on each surface. [START_REF] Massaro | Halloysite nanotubes as support for metal-based catalysts[END_REF] These physical and chemical features along with biocompatibility and high thermal and mechanical stability resulted in the extensive researches for disclosing the utility of Hal in diverse range of applications [START_REF] Glotov | Mesoporous Metal Catalysts Templated on Clay Nanotubes[END_REF] , ranging from catalysis to drug delivery. [START_REF] Cai | Titanium dioxide-coated biochar composites as adsorptive and photocatalytic degradation materials for the removal of aqueous organic pollutants[END_REF][START_REF] Glotov | Mesoporous Metal Catalysts Templated on Clay Nanotubes[END_REF][START_REF] Zhang | Palladium nanoparticles deposited on silanized halloysite nanotubes: synthesis, characterization and enhanced catalytic property[END_REF][START_REF] Ahmad | Biochar as a sorbent for contaminant management in soil and water: a review[END_REF][START_REF] Dmitry | Halloysite Nanotubes as Biomimetic Nanoreactor[END_REF][START_REF] Abdullayev | Halloysite Tubes as Nanocontainers for Anticorrosion Coating with Benzotriazole[END_REF][START_REF] Yah | Selective Modification of Halloysite Lumen with Octadecylphosphonic Acid: New Inorganic Tubular Micelle[END_REF][START_REF] Lvov | Halloysite Clay Nanotubes for Loading and Sustained Release of Functional Compounds[END_REF][START_REF] Vinokurov | Core/shell ruthenium -halloysite nanocatalysts for hydrogenation of phenol[END_REF][START_REF] Ma | Halloysite Nanotubes: Green Nanomaterial for Functional Organic-Inorganic Nanohybrids[END_REF] Hal can be successfully used with different materials such as carbon and polymeric compounds. [START_REF] Massaro | Design of PNIPAAM Covalently Grafted on Halloysite Nanotubes as Support for Metal-Based Catalysts[END_REF][START_REF] Yah | Biomimetic Dopamine Derivative for Selective Polymer Modification of Halloysite Nanotube Lumen[END_REF] Use of poly/oligo saccharides for the catalysis is a well-established topic. Among oligosaccharides, cyclodextrins (CDs) that are cyclic oligosaccharides have received much attention. The unique chemical and physical properties of CDs such as their cone-shape structure with hydrophobic interior space and hydrophilic exterior, bio-compatibility and non-toxicity as well as their availability and relatively low cost render them promising candidates for various applications, ranging from catalysis to drug delivery systems. [START_REF] Hapiot | Recent breakthroughs in aqueous cyclodextrin-assisted supramolecular catalysis[END_REF][START_REF] Hapiot | Unconventional Approaches Involving Cyclodextrin-Based, Self-Assembly-Driven Processes for the Conversion of Organic Substrates in Aqueous Biphasic Catalysis[END_REF][START_REF] Prochowicz | Interactions of Native Cyclodextrins with Metal Ions and Inorganic Nanoparticles: Fertile Landscape for Chemistry and Materials Science[END_REF] CD can exert its catalytic activity through formation of inclusion complex with the substrate and serving as a molecular transfer shuttle. [START_REF] Hapiot | Recent breakthroughs in aqueous cyclodextrin-assisted supramolecular catalysis[END_REF][START_REF] Hapiot | Unconventional Approaches Involving Cyclodextrin-Based, Self-Assembly-Driven Processes for the Conversion of Organic Substrates in Aqueous Biphasic Catalysis[END_REF][START_REF] Prochowicz | Interactions of Native Cyclodextrins with Metal Ions and Inorganic Nanoparticles: Fertile Landscape for Chemistry and Materials Science[END_REF] Moreover, CD can be considered as a potent capping agent for the synthesis of nanoparticles with catalytic utilities.

Considering the importance of anilines in industry, development of efficient protocols for the synthesis of anilines through hydrogenation of nitrobenzenes has been focused. In this context, diverse range of heterogeneous catalysts have been developed. [START_REF] Bouchenafa-Saib | Effect of oxidant treatment of date pit active carbons used as Pd supports in catalytic hydrogenation of nitrobenzene[END_REF][START_REF] Yu | Study on the nitrobenzene hydrogenation over a Pd-B/SiO2 amorphous catalyst[END_REF][START_REF] Giordano | Preparation of rhodium catalysts supported on carbon nanotubes by a surface mediated organometallic reaction[END_REF][START_REF] Li | Nitrobenzene hydrogenation with carbon nanotubesupported platinum catalyst under mild conditions[END_REF][START_REF] Arora | 4-Nitrophenol reduction catalysed by Au-Ag bimetallic nanoparticles supported on LDH: Homogeneous vs. heterogeneous catalysis[END_REF] Despite considerable achievements, some challenging issues such as high loading of catalyst, long reaction time and high temperatures remained unsolved.

In continuation of our research on the development of heterogeneous catalysts, [START_REF] Sadjadi | Combination of carbon nanotube and cyclodextrin nanosponge chemistry to develop a heterogeneous Pd-based catalyst for ligand and copper free C-C coupling reactions[END_REF] recently, we have focused our research on the Hal-based catalysts. [START_REF] Sadjadi | Pd@HNTs-CDNS-g-C3N4: a novel heterogeneous catalyst for promoting ligand and copper-free Sonogashira and Heck coupling reactions, benefits from halloysite and cyclodextrin chemistry and g-C3N4 contribution to suppress Pd leaching[END_REF] In this line, we have recently reported the high performance of the Hal-carbon nanocomposites as catalyst supports. [START_REF] Sadjadi | Pd nanoparticles immobilized on the polydopamine decorated halloysite nanotubes hybridized with N-doped porous carbon monolayer: A versatile catalyst for promoting Pd catalyzed reactions[END_REF] Encouraged by those promising results, herein we design a novel nanocomposite, Hal-Char, composed of Hal and hydrochar and utilized it as a support for the immobilization of Pd nanoparticles. The resulting catalytic system, Pd@Hal-Char, was applied for promoting the hydrogenation of nitroarenes in the presence of -CD. A precise study was carried out to investigate the contribution of composite components to the catalysis. Moreover, the role of -CD in the catalysis and the effect of using it in its free form and incorporated in the backbone of the catalyst on the catalytic activity were scrutinized. Additionally, using other control catalysts, the effect of surface modification of Char was investigated. To justify the results and find a relationship between the catalytic performance and the structural features, the average Pd particle size and Pd loading of all samples were obtained and compared. Finally, the recyclability of the catalyst and Pd leaching were examined.

EXPERIMENTAL SECTION

Materials. Green head of eggplant was selected as an abundant domestic biomass source for the synthesis of hydrochar. In this line, the heads of eggplants were washed, dried in oven at 120 °C and powdered prior to use. All chemicals and reagents used for the synthesis of the catalyst and performing the hydrogenation reaction, including nitrobenzene, 1-nitronaphtalene, 4nitroacetophenone, Hal, N-methylimidazole, (3-chloropropyl) trimethoxysilane, -CD, Pd(OAc)2, sodium borohydride, toluene, ethanol and sulfuric acid were of analytical grade, purchased from Sigma-Aldrich and used without further purification.

Characterization. The catalyst as well as the control samples were characterized using ICP-AES, TGA, XRD, BET, FTIR, ZP and TEM. ICP analyses were accomplished by using ICP analyzer (Varian, Vista-pro). The sample preparation for ICP analysis was accomplished according to the previous reports. [START_REF] Bahri-Laleh | Pd immobilized on dendrimer decorated halloysite clay: Computational and experimental study on the effect of dendrimer generation, Pd valance and incorporation of terminal functionality on the catalytic activity[END_REF] Typically, a known amount of the catalyst was digested in the concentrated acidic solution. Thermogravimetric analyses (TGA) were carried out using METTLER TOLEDO thermogravimetric analysis apparatus with heating rate of 10°C.min -1 under N2 atmosphere over the range of 50 to 800 °C. XRD patterns of the catalyst and the control supports were recorded using a Siemens, D5000. Cu Kα radiation from a sealed tube. BET analyses were performed using a Belsorp Mini II instrument. The degassing process of the samples was carried out at 100 °C for 3h. FTIR spectra were recorded by employing PERKIN-ELMER-Spectrum 65 instrument.

Transmission Electron Microscopy (TEM) was performed on a Tecnai Microscope (200 kV). In detail, the heterogeneous catalysts, after into powder, were deposited onto a carbon coated copper grid. Metal nanoparticle size distributions have been determined from the measurement of around 200 nanoparticles found in arbitrarily chosen area of the images using the program SCION Image. Zeta potential measurements were carried out in water suspension using a Malvern Zetasizer Nano ZS at a controlled temperature (25±0.1°C). The -potential corresponds to the potential difference between the dispersion medium and the electrical double layer of the fluid attached to the dispersed particles. The measurements are based on a Laser Doppler electrophoretic mobility of the heterogeneous catalyst via the Helmholtz-Smoluchowski equation  = (η/ε)μe, where μe is defined as the ratio between the velocity of the heterogeneous catalyst and the magnitude under the applied electric field, η is the viscosity of the suspending liquid and ε stands for the dielectric conductivity of water. The heterogeneous catalysts were analyzed without any previous treatment. All the samples were analyzed in triplicate using the DTS Software from Malvern Instruments to acquire the phase plot and the zeta potential distribution.

Synthesis:

Synthesis of Char. In a typical procedure, a mixture of green head of eggplant (10 g), H2SO4 (0.5 ml) and distilled water (100 ml) was prepared in an autoclave and then heated at 200 °C for 15 h. After cooling to the room temperature, the black solid was collected by filtration, washed with distilled water and dried at 100 °C for 24 h. According to CHN analysis, the percent of N, C and H was calculated to be 1.62, 45.76 and 5.22%. Noteworthy, the study of the effect of the used amount of green head of eggplant on the content of the Char of the final Hal-Char nanocomposite showed that there was not a linear relationship between the amount of the used green head of eggplant and the Char content on the final nanocomposite. More precisely, TG analysis of various Hal-Char samples prepared by use of different ratio of Hal: head of eggplant (use of Hal: Head of eggplant ratio of 1:2, 1:4 and 1: 8) revealed (Figure S1) that increasing the head of eggplant from 2 to 4 g did not increase the Char content on the resulting Hal-Char nanocomposite, while further increase to 8 g led to the increase of the Char content from 20 wt. % to 26 wt.% on the Hal-Char nanocomposite. As the catalyst prepared by use of 1:4 ratio of Hal: head of eggplant showed the best catalytic results, this ratio was selected as the optimum amount for the preparation of all the control samples.

Synthesis of Hal-Char composite. Inspired by the previous reports, [START_REF] Cai | Titanium dioxide-coated biochar composites as adsorptive and photocatalytic degradation materials for the removal of aqueous organic pollutants[END_REF][START_REF] Ruan | Facile preparation of rosin-based biochar coated bentonite for supporting -Fe2O3 nanoparticles and its application for Cr(VI) adsorption[END_REF][START_REF] Jin | Carbon hybridized halloysite nanotubes for high-performance storage capacities[END_REF] Hal-Char nano composite was prepared. Typically, a mixture of Hal (1 g), green head of eggplant (4 g), H2SO4 (0.5 ml) in distilled water (100 ml) was prepared in an autoclave. The resulting mixture was then heated to 200 °C for 15 h. Upon completion of the reaction, the resulting dark grey precipitate was separated, washed with distilled water and dried at 100 C.

Noteworthy, the control Hal-Char samples with different Char contents were synthesized similarly, except 2 and 8 g green head of eggplant were applied per 1 g Hal.

Synthesis of Hal-Char-IL. Initially, the IL was prepared according to the previously reported procedure with slight modification. [START_REF] Zhang | Ionic liquid-functionalized biochar sulfonic acid as a biomimetic catalyst for hydrolysis of cellulose and bamboo under microwave irradiation[END_REF] Briefly, N-methylimidazole (1 mol) was mixed with (3-chloropropyl)trimethoxysilane (1 mol) in solvent-free condition at 95 °C overnight. Upon completion of the reaction, the mixture was cooled, washed with diethyl ether and dried under vacuum at 40 °C. Subsequently, IL-modified Hal-Char was prepared as follows: 1.5 g of Hal-Char was added to a solution of IL (1.5 g) and triethylamine (5 ml) in a mixture of water and ethanol as a solvent (50 ml, 1:1). Then, the resulting mixture was refluxed at 95 °C for 48 h. At the end of the reaction, the as-prepared Hal-Char-IL was washed with distilled water and ethanol several times and dried at 100 C.

Synthesis of Hal-Char-SO3H. The modification of Hal-Char by sulfuric acid was performed according to the previous reports with slight modification. [START_REF] Kang | Preparation of biomass hydrochar derived sulfonated catalysts and their catalytic effects for 5-hydroxymethylfurfural production[END_REF] Typically, 1 g of Hal-Char was added to 10 mL H2SO4 (98%) and mixed at 100 C for 15 h. After that, the acid-treated composite was collected, washed with deionized water several times and dried in oven at 100 C for 12 h. Synthesis of Hal-Char-CD. Typically, 20 ml of NaOH (10 M) was added into a solution of 1.5 g of Hal-Char in ethanol: water (50 ml, 1:1). Subsequently, 1.5 g of tosylated -CD (-CD -OTs) was added to the previous mixture. The latter was then refluxed at 90 °C overnight. Finally, the modified Hal-Char was collected by filtration and dried at 100 °C for 24 h. Synthesis of Pd@Hal-Char. Immobilization of palladium nanoparticles was achieved through wet impregnation procedure. Briefly, 1.2 g of Hal-Char was added to 30 mL toluene and sonicated for 30 min to furnish a well-dispersed Hal-Char suspension. Then, a solution of 0.09 mmol of Pd(OAc)2 in 12 mL toluene was added dropwise to the Hal-Char suspension. The mixture was then stirred at room temperature for 2 h. To reduce palladium salt and furnish Pd(0) nanoparticle, a solution of NaBH4 in H2O (10 mL, 0.2 N) was added into the suspension in a dropwise manner and the resulting mixture was stirred for 2 h. Finally, the precipitate was filtered, washed three times with MeOH and toluene and dried in oven at 100 °C for 12 h. The general synthetic procedure of the catalyst is shown in Figure 1.

Synthesis of Pd@Hal, Pd@Char, Pd@Hal-Char-IL, Pd@Hal-Char-CD, Pd@Hal-Char-SO3H, Pd-CD@Hal-Char. The procedure for the immobilization of Pd nanoparticles was similar to the method used for the synthesis of Pd@Hal-Char, except different supports were used instead of Hal-Char. Noteworthy, for the synthesis of Pd-CD@Hal-Char, -CD was used as a capping agent in the course of Pd immobilization.

Pd@CD was also achieved by reduction of Pd salt in the presence of -CD in the aqueous media. In Figure 1, the schematic procedures for the synthesis of the catalysts are depicted. General procedure for the hydrogenation of nitro compounds. A typical procedure used for the hydrogenation of nitro compounds was as follows: nitro compound (1 mmol), the catalyst (0.03 mol %), the catalytic amount of β-CD (0.004 mmol) and deionized water (2 mL) were placed into a reactor at room temperature. In the course of hydrogenation reaction, hydrogen (1 bar) was purged into the flask and the mixture was stirred vigorously (800 rpm). Upon completion of the reaction (monitored by TLC), the catalyst was filtered, washed with H2O and dried at 100 °C overnight for reusing in the consecutive reaction runs. On the other hand, the product was isolated by evaporation of the solvent and identified by comparing its melting/boiling point and FTIR spectrum with that of the authentic sample.

RESULTS AND DISCUSSION

The FTIR spectra of Char, Hal-Char, Hal-Char-IL, Hal-Char-CD and Hal-Char-SO3H are depicted in Figure 2. The FT-IR spectrum of Char showed five main absorption bands at around 3426 cm - 1 (-OH), 2923 cm -1 (-CH), 1696 cm -1 (-C=O), 1583 cm -1 (-C=C) and 1284 cm -1 (-C-O). The observation of these bands can confirm that Char contains various functional groups.

Hal-Char spectrum exhibited the characteristic bands of Char. Moreover, an additional band can be observed at 1081 cm -1 that can be assigned to the Si-O stretching and confirming incorporation of Hal in the structure of the hybrid system. Comparing the FTIR spectrum of the catalyst with that of pristine Hal (to avoid complexity, the FTIR spectrum of Hal is presented in SI, Figure S2) showed that some of the characteristic bands of pristine Hal were missing in the FTIR spectrum of Hal-Char. This observation that is in good agreement with other reports [START_REF] Abdullayev | Enlargement of Halloysite Clay Nanotube Lumen by Selective Etching of Aluminum Oxide[END_REF] can be assigned to the acid treatment in the course of formation of Hal-Char, i.e. use of sulfuric acid. In more detail, in the FTIR spectrum of Hal-Char the bands at 3691 cm -1 and 3624 cm -1 that are representative of inner hydroxyl stretching vibrations of Hal aluminol groups are less pronounced, implying the removal of alumina layer of Hal. Moreover, the characteristic band of Si-O-Al perpendicular stretching (754 cm -1 ) is disappearing.

The FTIR spectrum of Hal-Char-IL showed the characteristic bands of Hal-Char composite.

Furthermore, the characteristic bands of IL, i.e. the bands at 1631 cm -1 (-C=N) and 1576 cm -1 (-C-N) overlapped with the bands of Hal. As depicted, the FTIR spectrum of Hal-Char-SO3H is distinguished from the other samples. In this FTIR spectrum, the bands at 3691 cm -1 and 3624 cm - 1 are completely disappeared and a broad band at that region can be observed, indicating the effecting Hal etching and the formation of amorphous spheroidal nanoparticles of SiO2. [START_REF] White | The stability of halloysite nanotubes in acidic and alkaline aqueous suspensions[END_REF] another proof for more effective Hal etching is increase of the intensity of the band at1081 cm -1 that is indicative of silica nanoparticles [START_REF] Abdullayev | Enlargement of Halloysite Clay Nanotube Lumen by Selective Etching of Aluminum Oxide[END_REF] . Moreover, the band at 1345 cm -1 can be attributed to -S=O functionality. The FTIR spectrum of Hal-Char-CD exhibited the characteristic bands of Hal-Char composite. The characteristic bands of -CD (-OH, and -CH2) were overlapped with those of Hal-Char. Notably, FTIR spectra cannot solely confirm the structures of the catalysts. Hence, more characterizations were performed to verify the formation and structures of the catalysts. Next, the textural properties of the as-synthesized supports and the effect of surface treatment of Char were determined from nitrogen physisorption analysis. The BET specific surface areas of all samples were obtained and compared in Table 1. As tabulated, the specific surface area of Hal-Char composite was 20 m 2 .g -1 (Table 1, entry 3), that is lower than bare Hal (51 m 2 .g -1 ) and almost similar to that of Char (22 m 2 .g -1 ). This observation can imply that Char could cover the Hal surface. Moreover, it can be seen that incorporation of IL and -CD could decrease this value to 11 m 2 .g -1 and 19 m 2 .g -1 respectively, indicating that the surface of the composite was covered with these functionalities. In the case of Hal-Char-SO3H, it can be seen that the acid treatment led to the increase of the specific surface area to 36 m 2 .g -1 . This observation is in good accordance with the literature, in which it is declared that the acid treatment of Hal could result in the increase of the specific surface area. [START_REF] White | The stability of halloysite nanotubes in acidic and alkaline aqueous suspensions[END_REF] Noteworthy, nitrogen adsorption-desorption isotherm of Hal-Char (Figure S3) showed the type II isotherms with H3 hysteresis loops. [START_REF] Yuan | Functionalization of Halloysite Clay Nanotubes by Grafting with γ-Aminopropyltriethoxysilane[END_REF] Table 1. Textural properties of the prepared materials Entry Catalyst Support SBET (m 2 .g -1 ) a The prepared catalyst supports were also characterized via XRD analysis (Figure 4). The XRD pattern of Char exhibited a sharp band (002) at 25° and a broad band (ɣ) around 15°. According to the literature, [START_REF] Manoj | Structural characterization of graphene layers in various Indian coals by X-Ray Diffraction technique[END_REF] the (002) band indicates the graphite-like structures (crystalline carbon), while the other band is due to the saturated structures such as aliphatic side chains, attached to the edge of the crystalline carbon. [START_REF] Manoj | Structural characterization of graphene layers in various Indian coals by X-Ray Diffraction technique[END_REF] The The XRD analysis was also exploited for the characterization of Pd@Hal-Char catalyst (Figure 5).

As shown, the XRD pattern of the catalyst is similar to that that Hal-Char (to provide better comparison, the XRD pattern of Hal-Char is merged in Figure 5) and the characteristic bands of Pd nanoparticles are not observable. According to previous reports, this issue can be assigned to the low loading of Pd, as was also confirmed by ICP analysis, and its high dispersion, as detected in TEM images. 40 The nitrogen adsorption-desorption isotherm of the Pd@Hal-Char catalyst was also recorded (Figure 6) and compared with that of Hal-Char catalyst (Figure S3). The comparison showed that both samples exhibited the same type of isotherm (type II isotherms with H3 hysteresis loops).

Comparison of the specific surface area of two samples revealed that upon immobilization of Pd nanoparticles, the specific surface area reduced to 15 m 2 .g -1 , confirming that Pd nanoparticles are located on the surface of the support. To further characterize Pd@Hal-Char, it was subjected to the FTIR and TG analyses (Figures 7 and8). As shown in Figure 7, the FTIR spectrum of the catalyst is similar to that of Hal-Char. To study the dispersion, the morphology and the average size of Pd nanoparticles in the catalysts, the TEM analysis of these catalysts has been performed at low and high magnification (Figure 9 and Figure S6). To elucidate how use of Hal, Char and the modified Hal-Char supports affect the distribution and average Pd particle size, the TEM images of Pd@Hal-Char and all of the control catalysts were recorded (Figures 9 and Figure S6) and compared with that of the catalyst.

According to the previous reports [START_REF] Abdullayev | Enlargement of Halloysite Clay Nanotube Lumen by Selective Etching of Aluminum Oxide[END_REF] , H2SO4 could dissolve alumina sheets in the structure of Hal. This phenomenon, i.e. Hal etching, that occurs through diffusion of hydrogen ions within Hal pores and following interactions with alumina, is dependent on the reaction time, temperature and H2SO4 concentration. Hence, it was expected that the morphologies of the catalysts were distinguished from pristine Hal and on the other hand, Pd@Hal-Char-SO3H that was prepared through harsher acidic condition differed from other catalysts. As expected, in the TEM images of Pd@Hal-Char, Pd-CD@Hal-Char, Pd@Hal-Char-CD, Pd@Hal-Char-IL, that were treated with more diluted acidic solution for shorter reaction time, the rodlike structure of Hal was observable. However, small SiO2 nanoparticles, derived from dealuminated Hal can also be detected. In the case of Pd@Hal-Char-SO3H that was treated with sulfuric acid twice, first in the step of Hal-Char Next, the activity of the Pd@Hal-Char catalyst in the hydrogenation of nitrobenzene and 1nitronaphthalene as two model substrates in the presence of H2 gas was investigated (Scheme 1).

Initially, the model reactions were carried out under mild and environmentally-benign condition, i.e. in the presence of 0.03 mol %Pd@Hal-Char at 50 °C in water as solvent and under 1 bar of hydrogen.

Scheme 1. Hydrogenation of nitroarene and 1-nitronaphthalene.

Under these conditions, aniline and naphthylamine with moderate yields were furnished respectively after 1 h. To increase the yields of the reactions and decrease the reaction time, the reaction variables, i.e. the solvent, the catalyst amount and the reaction temperature were optimized. It was indicated that at lower temperature (room temperature), the reaction could proceed to furnish the desired products with comparative yields. Hence, 25 °C was selected as the reaction temperature. Moreover, it was found that the reaction could be catalysed in the presence of lower amount of the catalyst, 0.03 mol %Pd@Hal-Char, and further increase of the catalyst content did not improve the yield of the reaction. Furthermore, water was found as the chosen solvent. Under the optimum reaction conditions, the model reactions led to the formation of aniline and naphthylamine in 75 % and 65% yields respectively after 1 h. To further increase the yields of the products, β-CD (0.004 mmol) was utilized as phase transfer agent along with the catalyst.

Interestingly, it was found that the use of the catalytic amount of CD increased the reaction yields significantly. Armed with the optimum reaction conditions, contribution of Hal, Char and -CD to the catalysis was scrutinized. First, to investigate whether combination of Hal and Char could affect the catalytic activity, two control catalysts, Pd@Hal, Pd@Char, were synthesized and their catalytic activities were compared with that of the catalyst (Table 2, entries 1 and 3). The precise comparison of the catalytic activity of the catalyst with that of Pd@Hal and Pd@Char showed that the catalytic activity decreased in the order of Pd@Hal-Char > Pd@Char > Pd@Hal, confirming that the use of Hal-Char nanocomposite as a catalyst support was more efficient than the use of each component individually. To elucidate the origin of this observation, the Pd loading (obtained from ICP) and the average Pd particle size (obtained from TEM) for these three samples were compared (Table 2, entries 1, 3 and 5). As tabulated, in all three samples the loading of Pd is almost similar.

More precisely, the Pd loading in the catalyst is slightly higher than those two other samples, while the Pd loading in Pd@Char was the lowest. Considering the average Pd particle size and the TEM images, it can be seen that in the case of Pd@Hal, the average Pd particle size was the largest (4.8 ± 1.3 nm) and the Pd nanoparticles were not well-dispersed on the support, while in Pd@Char and Pd@Hal-Char, the Pd nanoparticles were highly dispersed on the support and the average Pd particle sizes were almost similar and smaller than that of Pd@Hal. Considering the difference of the catalytic activities of Pd@Hal and Pd@Char that possess almost similar Pd loading but different Pd average particle sizes, it can be concluded that the Pd average size can effectively affect the catalytic activity. Moreover, it can be found that Char as a support can lead to the effective dispersion of the Pd nanoparticles compared to Hal. Comparing the catalytic activity of the catalyst and Pd@Char that has almost similar Pd loading and average Pd particle sizes, it can be postulated that the incorporation of Hal can improve the catalytic activity probably through the synergistic effect with Char. In order to better understand these different catalytic results, the zeta potential measurements of these catalysts dispersed into aqueous media have been carried out.

Indeed, zeta potential measurements are used to characterize the stability of colloidal suspensions into solvents. Then, it should be noticed that in our case, there is a correlation between the zeta potential value of the catalyst into water and the catalytic activity which could be related to a better dispersion. These measurements are summarized in Table S1 and Figure S8. The zeta potential value of the pristine Hal is about -32.9 mV. This value is commonly reported in the literature. [START_REF] Joo | Opening and blocking the inner-pores of halloysite[END_REF] The zeta potential values of Pd@Hal-Char, Pd@Char and Pd@Hal are -53.0 mV, -50.1 mV and -40.3 mV respectively, indicating that the colloidal dispersion stability is decreasing in the order Pd@Hal-Char > Pd@Char > Pd@Hal. Consequently, there is a good correlation between the colloidal suspension stability and the catalytic activity. Indeed, the better the colloidal dispersion stability, the lower the aggregation of the particles and consequently the better interactions between the substrate and the catalytic species would be. Next, to further confirm the role of -CD in the catalysis, the catalytic activities of the two aforementioned control catalysts Pd@Hal and Pd@Char in the presence and absence of -CD were examined. It was shown that (Table 2, entries 2 and 4) in all cases, the use of -CD as a co-catalyst could significantly increase the yields of the reactions. Notably, the amount of -CD is an important parameter and the use of lower quantity of -CD led to lower yields of the desired products. The contribution of -CD to the catalysis can be attributed to its capability of formation of inclusion complex with the hydrophobic substrate, nitroarene and 1-nitronaphthalene, and transfering them into the aqueous media. [START_REF] Hapiot | Recent breakthroughs in aqueous cyclodextrin-assisted supramolecular catalysis[END_REF] To elucidate whether the incorporation of -CD in the scaffold of the catalyst could affect the catalytic activity as effectively as -CD in its free form, a control catalyst, Pd@Hal-Char-CD, was synthesized and used as a catalyst for promoting the model reactions. Interestingly, it was found that (Table 2, entry 11) the incorporation of -CD in the structure of the catalyst was not as effective as the use of -CD in its free form and the use of -CD as a co-catalyst led to a catalytic system with superior catalytic activity. Comparing the Pd loading and the average Pd particle size of the catalyst with those of Pd@Hal-Char-CD, it can be found that the Pd loading in the catalyst was slightly higher than that of Pd@Hal-Char-CD, while, the average Pd particle size in the catalyst was smaller than that of Pd@Hal-Char-CD (4.9±1.7 nm). This observation further confirmed the role of the Pd nanoparticle size on the catalytic activity. On the other hand, the lower catalytic activity of Pd@Hal-Char-CD can be justified by considering the amount of -CD in the two reaction mixtures. As discussed in the characterization section, the amount of -CD in Pd@Hal-Char-CD sample is only 2 wt.%. Hence, the total content of -CD in the reaction mixture would be much lower than that of Pd@Hal-Char, in which 0.004 mmol β-CD was used as cocatalyst. This issue confirms the importance of the role of -CD as phase transfer agent. Even if the zeta potential value of Pd@Hal-Char-CD is higher than Pd@Hal-Char catalyst, it should be noticed that, considering the zeta potential distribution of Pd@Hal-Char-CD (Figure S8), there is an important modification of the total counts and a broadening of the zeta potential distribution shape during the triplicate analysis. Consequently, the exploitation of zeta potential measurement in the case of Pd@Hal-Char-CD is not so clear.

It is postulated that in the Pd@Hal-Char-CD, low content of -CD in the catalyst structure prevented it from acting properly as a phase transfer agent. To validate this assumption, a control reaction in the presence of Pd@Hal-Char-CD and -CD was carried out. It was found that the addition of -CD could significantly improve the yields of the desired products.

To investigate whether the use of -CD as a capping agent in the course of Pd immobilization could improve the catalytic activity, another control catalyst, Pd-CD@Hal-Char, was prepared and its catalytic activity was compared with that of Pd@Hal-Char. Lower catalytic activity of this sample (Table 2, entry 12) compared to that of the catalyst was observed. This difference is in correlation with zeta potential measurement and also colloidal suspension stability in water.

Indeed, the zeta potential value of Pd-CD@Hal-Char is about -32.2mV and also lower than Pd@Hal-Char zeta potential value. Moreover, the comparison of the structural features of Pd-CD@Hal-Char with that of the catalyst implied that the Pd loading in the former is higher than the catalyst, while Pd nanoparticles in both samples were well-dispersed and the average Pd particle sizes in both samples were almost similar. Considering these results, it was expected that Pd-CD@Hal-Char exhibited superior catalytic activity compared to Pd@Hal-Char. However, converse result was obtained and the catalytic activity of Pd-CD@Hal-Char was much lower than that of the catalyst. Similar to Pd@Hal-Char-CD, the lower catalytic activity of Pd-CD@Hal-Char can be attributed to the low amount of -CD in the sample that did not allow the effective phase transfer of the hydrophobic substrates. It is worth mentioning that the use of -CD in its free form along with Pd-CD@Hal-Char could remarkably increase the yield of the reaction, implying the crucial role of phase transfer of substrates during the catalytic process. Noteworthy, the investigation of the catalytic activity of Pd@CD as a homogeneous catalyst confirmed its inferior catalytic activity compared to that of Pd@Hal-Char (45 and 40% aniline and naphthylamine were respectively).

The effect of the surface modification of Char on the catalytic activity was also investigated. To this purpose, two control catalysts, Pd@Hal-Char-IL and Pd@Hal-Char-SO3H were prepared and their catalytic activities for promoting the model reactions in the presence and absence of -CD were compared with that of the catalyst. It was proved that (Table 2, entries 7-10) in these two cases, the use of -CD is essential for achieving higher yields. Moreover, it was found that the surface modification of Char with IL or -SO3H functionalities had detrimental effect on the catalytic activity. Moreover, the zeta potential values of Pd@H-C-IL and Pd@H-C-SO3H calculated to be -23.4 mV and -39.1 mv respectively, that were lower than that of Pd@Hal-Char.

Considering these results, the lower catalytic activities of Pd@H-C-IL and Pd@H-C-SO3H can be attributed to their lower dispersion into the aqueous phase. The comparison of the Pd loading in Pd@Hal-Char-IL with that of the catalyst proved the higher Pd content in Pd@Hal-Char-IL. This observation can be attributed to the role of IL in anchoring the Pd nanoparticles through providing electrostatic interactions. [START_REF] Moghaddam | Gold nanoparticles anchored onto the magnetic poly(ionic-liquid) polymer as robust and recoverable catalyst for reduction of Nitroarenes[END_REF] Although Pd nanoparticles are the main catalytic active sites in the hydrogenation reaction, it can be seen that this sample with higher Pd content lower catalytic activity than the catalyst that has lower Pd loading. This observation can be justified by considering the larger average Pd particle size in Pd@Hal-Char-IL (4.5± 1.1 nm) compared to that of the catalyst. In the case of Pd@Hal-Char-SO3H, the Pd loading and the average Pd particle size were almost similar to those of the catalyst, however, its catalytic activity was lower than that of the catalyst. This observation can be assigned to the dissolution of Hal in the course of acid treatment. This result can further confirm the role of Hal in the catalysis and the synergism between Hal and Char.

It was found that the Pd@Hal-Char catalyst could efficiently promote the hydrogenation of nitrobenzene and 1-nitronaphthalene that is a sterically demanding substrate. To further investigate the generality of this protocol and study whether the catalyst was selective toward the nitro functional group and could selectively hydrogenate -NO2 over -C=O, the hydrogenation of 4nitroacetophenone in the presence of Pd@Hal-Char and -CD was also carried out. It was demonstrated that under the reaction condition, the corresponding product could be achieved in quantitative conversion and yield (100%) and no alcohol was detected as by-product, confirming the selectivity of the catalyst.

One of the most important advantage of heterogeneous catalysts is their capability of recovery and recycling. In this context, the stability of Pd@Hal-Char upon recycling was examined. To this purpose, upon completion of the reaction, Pd@Hal-Char was filtered off, washed with water for several times and then dried at 100 °C and subjected to the next reaction run. The yield of the desired product after each recycling was obtained and compared with that of fresh Pd@Hal-Char (Figure 10). As illustrated in Figure 10, Pd@Hal-Char exhibited high recyclability up to 7 runs without significant loss of its catalytic activity. As shown in Figure 11, the FTIR spectrum of the recycled catalyst after seven runs showed the characteristic bands of the fresh Pd@Hal-Char. The similarity of the FTIR spectra of the fresh and recycled catalysts indicated the structural stability of Pd@Hal-Char upon recycling. Moreover, the FTIR spectrum of the recycled catalyst did not show additional bands, implying that the active surface of the catalyst was not covered with organic substrates. All of these characterizations clearly show the robustness of Pd@Hal-Char catalyst during the catalytic process. Transmittance (%) Wavenumber (cm -1 catalytic performance were studied. To this purpose, several control catalysts, including Pd@Hal, Pd@Char, Pd@Hal-Char-CD, Pd-CD@Hal-Char, Pd@CD, Pd@Hal-Char-IL, Pd@Hal-Char-SO3H were prepared and their catalytic activities were compared with that of the catalyst. Moreover, the catalytic activities of the catalysts in the absence and presence of -CD were investigated. It was confirmed that the use of -CD as phase transfer agent along with the catalyst is imperative to achieve high catalytic activity. Furthermore, comparison of the Pd particle average size and the loading of all the samples demonstrated that the content of -CD in the reaction mixture played an important role in the catalysis and in the Pd@Hal- Char-CD and Pd-CD@Hal-Char samples that -CD content was not adequate to allow effective formation of inclusion complex with hydrophobic substrates, low catalytic activities were observed. Moreover, the catalytic activities of Pd@Hal and Pd@Char were inferior to Pd@Hal-Char, indicating the synergism between Hal and Char. Regarding the surface modification of Char, it was found that incorporation of IL led to higher Pd loading but larger average particle size and consequently lower catalytic activity. In the case of acid treatment, the analyses confirmed that Hal could be structurally altered under acidic conditions. This issue had a detrimental effect on the catalytic activity.

Figure 1 .

 1 Figure 1. The schematic procedures for the synthesis of the catalysts.

Figure 2 .

 2 Figure 2. FTIR spectra of Char, Hal-Char, Hal-Char-IL, Hal-Char-CD and Hal-Char-SO3H.

  Hal-Char-IL Hal-Char-SO3HHal-Char-CD

  BET surface areas)To further characterize the supports and investigate the mass loading of Char, ionic liquid and cyclodextrin, thermogravimetric analysis of the supports was performed. As illustrated in Figure3, the thermogram of Char showed two weight loss stages, the first one below 200 °C is related to the loss of water and the next one at 580 °C can be assigned to the degradation of Char. The thermogram of Hal-Char exhibited three main weight losses, at about 150, 500 and 580 °C. The first weight loss stage is due to the loss of water in the structure of Hal and Char, the other weight losses are due to the dehydroxylation of the Hal matrix and decomposition of Char respectively.[START_REF] Polanský | Influence of dehydration on the dielectric and structural properties of organically modified montmorillonite and halloysite nanotubes[END_REF] To measure the content of char, the thermogram of Hal-Char was compared with that of the pristine Hal (to avoid complexity, the thermogram of Hal is presented in FigureS4). It was found that the content of Char was about 20 w/w%. Considering the thermogram of Hal-Char-IL, four weight loss stages over the range of 50-800 °C can be seen. The first stage at about 130 °C is due to the loss of water, the second one at about 300 °C is due to the IL decomposition[START_REF] Cao | Comprehensive Investigation on the Thermal Stability of 66 Ionic Liquids by Thermogravimetric Analysis[END_REF] and two others can be assigned to the decomposition of Char and dehydroxylation of the Hal. Comparing this thermogram with that of Hal-Char, the content of IL was estimated to be about 7 wt. %.Considering the thermogram of Hal-Char-CD and comparing it with that of Hal-Char, it can be found out that the content of CD on the Hal-Char-CD was very low (about 2 wt.%). In the case of Hal-Char-SO3H, the thermogram is different from other samples. Indeed, the first weight loss stage is occurred at 150 °C that is due to the loss of water and the two other ones at 330 and 550 °C are due to the degradation of organic moieties and Char. As discussed above, this observation can be attributed to the dissolution of inner surface of Hal and the formation of amorphous spheroidal nanoparticles of SiO2.[START_REF] White | The stability of halloysite nanotubes in acidic and alkaline aqueous suspensions[END_REF]

Figure 3 .

 3 Figure 3. The TG analyses of Char, Hal-Char, Hal-Char-IL, Hal-Char-CD and Hal-Char-SO3H.

  XRD pattern of Hal-Char exhibited the characteristic bands of Char and the bands at 2θ = 16.4°, 17.9°, 25.5°, 26.6°, 29.8°, 39.3°, 52.2° and 61.6°. The comparison of the XRD pattern of Hal-Char with that of pristine Hal (to avoid the complexity, the XRD pattern of pristine Hal is depicted in Figure S5 35, 39 ), it can be seen that two XRD patterns are distinguishable. More precisely, the intensities of the two first characteristic bands of Hal was decreased. According to the literature 33 , this observation can be due to the partial loss of the tube wall crystallinity. The comparison of the XRD patterns of the Hal-Char-IL, Hal-Char-CD with that of Hal-Char showed that the incorporation of ionic liquid and -CD did not induce significant change. More precisely, neither significant shift in the bands nor change of the interlayer distance was observed in the XRD patterns of these two samples compared to that of Hal-Char. Contrary to the mentioned samples, the XRD pattern of Hal-Char-SO3H indicated that the acid modification of Hal resulted in a significant change in the structure of the catalyst and the intensities of the characteristic bands of Hal reduced dramatically. According to the literature, 34 long term treatment of Hal with concentrated acid can lead to delamination in an acidic environment and formation of amorphous silica.

Figure 4 .

 4 Figure 4. XRD patterns of Char, Hal-Char, Hal-Char-IL, Hal-Char-CD and Hal-Char-SO3H.

Figure 5 .

 5 Figure 5. XRD patterns of Hal-Char and Pd@Hal-Char.

Figure 6 .

 6 Figure 6. Nitrogen adsorption-desorption isotherm of the Pd@Hal-Char catalyst.

Figure 7 . 1 )

 71 Figure 7. FTIR spectrum of Pd@Hal-Char.

Figure 8 .

 8 Figure 8. Thermogram of Pd@Hal-Char.

Figure 9 .

 9 Figure 9. a) TEM images at a magnification of 150K of Pd@Hal-Char and b) The TEM images

Figure 10 .

 10 Figure 10. Recyclability of Pd@Hal-Char for the hydrogenation reaction of nitrobenzene under

Figure 11 .

 11 Figure 11. FTIR spectra of the recycled Pd@Hal-Char catalyst, Blue: fresh and Red: recycled

Table 2 .

 2 The catalytic activity, the average Pd particle size and the loading of the catalytic

		samples. a
	Entry	Catalyst	Aniline Yield (%) b	Naphthylamine Yield (%) b	Pd loading (mmol.g -1 )	Pd NPs size (nm)	TOF for Aniline (h -1 )	TOF for Naphthylamine (h -1 )
	1	Pd@Hal	30	10	0.027	4.8±1.3	1297	307
	2	Pd@Hal + -CD c	65	60	0.027	4.8±1.3	2810	1844
	3	Pd@Char	40	30	0.026	3.6±1.1	1307	693
	4	Pd@Char + -CD	75	50	0.026	3.6±1.1	1951	1156
	5	Pd@Hal-Char	75	65	0.028	3.7±1	2435	1490
	6	Pd@Hal-Char + -CD	100	95	0.028	3.7±1	3226	2179
	7	Pd@Hal-Char-IL d	60	30	0.041	4.5±1.1	1612	570
	8	Pd@Hal-Char-IL + -CD	90	45	0.041	4.5±1.1	2406	834
	9	Pd@ Hal-Char-SO3H e	45	50	0.028	3.9±1	1524	1204
	10	Pd@ Hal-Char-SO3H + -CD	55	55	0.028	3.9±1	1863	1324
	11	Pd@ Hal-Char-CD	60	50	0.025	4.9±1.7	2662	1696
	12	Pd-CD@ Hal-Char	50	40	0.032	3.8±1	1451	825

a Reaction condition: Substrate (1 mmol), catalyst (0.03 mol%), -CD (0.004 mmol), H2O (2 mL), H2 (1 bar) at 25 C in 1 h. b Isolated yields c Use of β-CD in its free form d Trimethoxysilyl propyl imidazolium ionic liquid (IL) grafted on Hal-Char e Functionalized Hal-Char using H2SO4

ACKNOWLEDGMENTS

The TEM in Lille (France) are supported by the Conseil Régional du Nord-Pas de Calais and the European Regional Development Fund (ERDF). The authors are grateful to Dr. Ahmed Addad (UMET, University of Lille) for technical assistance in TEM measurements. This work has been supported by the Center for International Scientific Studies & Collaborations (CISSC) and French Embassy in Iran and Hubert Curien French-Iranian partnership "PHC GUNDISHAPUR 2018" n° 40870ZG. MMH and Akbari are also thankful to Iran National Science Foundation for the Individual given grant, No. 96010807. S. Sadjadi and Heravi appreciate Iran National Science Foundation for the Individual given grant, No. 97009384. ABBREVIATIONS Halloysite : Hal ; Hydrochar : Char ; Halloysite-hydrochar : Hal-Char ; -cyclodextrin : -CD ; Ionic liquid : IL

AUTHOR CONTRIBUTIONS

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

SYNOPSIS

Pd immobilized on halloysite-hydrochar nanocomposite has been prepared and applied for hydrogenation of nitroarenes in the presence of CD.