Structural and catalytic properties of mono- and bimetallic nickel–copper nanoparticles derived from MgNi(Cu)Al-LDHs under reductive conditions

Accepted Manuscript Title: Structural and Catalytic Properties of Mono- and Bimetallic Nickel-Copper Nanoparticles Derived from MgNi(Cu)Al-LDHs under Reductive Conditions Author: Brindusa Dragoi Adrian Ungureanu Alexandru Chirieac Carmen Ciotonea Constantin Rudolf Sebastien Royer Emil Dumitriu PII: DOI: Reference:

S0926-860X(14)00711-X http://dx.doi.org/doi:10.1016/j.apcata.2014.11.016 APCATA 15109

To appear in:

Applied Catalysis A: General

Received date: Revised date: Accepted date:

8-9-2014 7-11-2014 11-11-2014

Please cite this article as: B. Dragoi, A. Ungureanu, A. Chirieac, C. Ciotonea, C. Rudolf, S. Royer, E. Dumitriu, Structural and Catalytic Properties of Mono- and Bimetallic Nickel-Copper Nanoparticles Derived from MgNi(Cu)AlLDHs under Reductive Conditions, Applied Catalysis A, General (2014), http://dx.doi.org/10.1016/j.apcata.2014.11.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Highlights Highly dispersed (bi)metallic NiCu NPs derived from as-synthesized MgNi(Cu)Al-LDHs;

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Extensive characterization of (bi)metallic NPs by in-situ XRD, TEM, N2O titration;

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(Bi)metallic NiCu NPs outstandingly active in hydrogenation of cinnamaldehyde;

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Cu-MgAl-LDH is extremely active even after mild reduction at 150 °C.

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Structural and Catalytic Properties of Mono- and Bimetallic Nickel-Copper Nanoparticles Derived from MgNi(Cu)Al-LDHs under Reductive Conditions Rudolf,a Sebastien Royer,b Emil Dumitriua a

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Brindusa Dragoi,a* Adrian Ungureanu,a Alexandru Chirieac,a Carmen Ciotonea,a, b Constantin “Gheorghe Asachi” Technical University of Iasi, Faculty of Chemical Engineering and

Université de Poitiers, CNRS UMR 7285, IC2MP, 4 Rue Michel Brunet, 86022 Poitiers Cedex, FRANCE

Corresponding Author, Tel: +40-232 278683, E-mail address: [email protected];

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*

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Environmental Protection, 73 Prof. D. Mangeron Bvd., 700050 Iasi, ROMANIA

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ABSTRACT Herein, a way to generate mono- and bi-metallic copper and/or nickel nanoparticles by the direct reduction of Mg(Ni)CuAl layered double hydroxides (LDHs) precursors is reported. Cu and Ni with various molar ratios (0:1, 1:4, 1:1, 4:1 and 1:0) were

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substituted in the MgAl brucite-like sheets during synthesis. Studies on the evolution of LDH

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structures under H2 reduction of as-synthesized samples at different temperatures (30 - 550 °C) followed by in-situ XRD evidenced the high reducibility of copper cations. The metallic phases

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generated during reduction of LDHs were dependent on the chemical composition of the sample. The corresponding TEM images display small and very well dispersed metallic nanoparticles. A positive effect of nickel cations on the dispersion and thermostability of metallic copper phase was also observed, especially with increasing Ni-content. For instance, metallic particle sizes of 6.5 and 3.3 nm were calculated for the NPs generated by reduction at 500 °C of MgCuAl and MgNiCuAl (Ni:Cu = 1:1), respectively. Outstanding results for the hydrogenation of cinnamaldehyde in liquid phase were obtained on Cu-rich materials that are able to generate metallic active sites even after reduction at temperature as low as 150 °C. Due to the difficulty to detect the metallic phases generated after reduction at 150 °C by usual techniques (e.g., TPR, insitu XRD and TEM) Cu0 was put in evidence by the dissociative chemisorption of N2O. 2

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1. INTRODUCTION Due to their unique electronic structure and very high surface to bulk atoms ratio, highly

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dispersed metal nanoparticles (NPs) represent the fundamental for the preparation of many catalysts involved in the energy production and storage technologies, pollution prevention, fine

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chemicals synthesis, etc. The catalytic activity and selectivity displayed by NPs strongly depend on their composition, size and shape, the nature of the support, the metal-support interactions, the

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preparation method, and so on. Substantial progresses were made for the understanding of the properties of monometallic NPs, while the bimetallic NPs still require constant efforts to figure out the metal-metal and metal-support interactions and how they are influenced by different

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factors. Various preparative approaches were reported for the supported metallic NPs, among them impregnation and precipitation, either at constant or variable pH, being the most used

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methods [1]. These methods involve the deposition of the metal precursors from the liquid phase on a solid support, with subsequently thermochemical treatments to generate metallic nanoparticles. The size and shape of these generated NPs depend on the chemical nature and

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porosity of the support, the chemical composition of the NPs (in particular, the case of

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bicomponent NPs) as well as the thermal treatments applied. Increasing dispersion by decreasing the particle size has drastic effects on the catalytic

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activity. Copper NPs used as catalysts in hydrogenation reactions are suitable examples to illustrate such effects. While bulk copper is catalytically inactive, the decrease of the copper NPs size below 10 nm results in a high amount of crystalline faces with highly uncoordinated surface atoms exposed and copper turns into active catalyst in the hydrogenation. The control of the size and thermostability of copper NPs is very challenging task because of the high mobility of copper cations resulting in a significant sintering even at low temperatures [2]. Although different strategies of synthesis were proposed, such as the control of the calcination conditions [3], the drying at ambient temperature [4], the promoting effect of a second metal [5, 6], the formation of the super-stoichiometric aluminates [7] etc., this lack of Cu NPs stability is far to be solved. Here, we show the possibility to prepare mono- and bimetallic NPs of copper and/or nickel by the direct reduction of MgNiCuAl layered double hydroxides (LDHs) precursors. LDHs are chosen as starting materials because their structure allows the accommodation of

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different cation in the lamellar sheets, with atomic scale homogeneity, and thus, providing a high degree of loading and dispersion of the precursors of the metallic catalytic active phases [8-10]. Usually, LDH-derived supported metal catalysts are obtained in two steps: (i) the calcination of LDH under air flow to generate homogeneous mixed oxides and (ii) the reduction of the

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homogeneous mixed oxides under flow of hydrogen to generate the corresponding metallic sites. Upon the thermal treatment and depending on the selected temperature, the lamellar structure is

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finally lost, but the local environment of cations remains unchanged.

The structural and textural modifications of LDH taking place during the thermal

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treatment are usually evaluated by ex-situ analyses such as XRD and nitrogen physisorption. Under such conditions, it is however difficult to clearly establish the structural transformation occurring during thermal treatment since only initial (as-synthesized LDHs, lamellar precursor)

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and final (calcined/decomposed LDHs, mixed oxides) states of these materials are examined; the phase transformation between these two states are unfortunately rarely examined. A first attempt

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to deeply investigate the structural changes of the carbonate MgAl LDH during the thermal treatments was made by Hibino et al. [11]. They activated the LDHs at different temperatures

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(from 350 to 1000 °C), and after cooling, they analyzed the materials by XRD and infrared spectroscopy. However, this approach is not very convenient because of the partial

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reconstruction of LDH structure in the presence of humidity by well-known “memory effect”, 500 °C.

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which can occur for the homogeneous mixed oxides obtained by thermal treatment treated below To overcome this shortcoming, in situ investigations by XRD, infrared spectroscopy and/or other analytical techniques allowing determinations of the morphostructural changes taking place in the LDHs materials during thermal processes seems to be more suitable. Kanezaki et al.[12] studied the modification of the layered structure of three carbonate LDHs by in situ XRD in the temperature range from 30 to 1000 °C. They delimited three temperature domains at which significant phase transformations occur in the structure (30 - 180 °C, 200 - 380 °C, and 400 1000 °C). The first interval corresponds to the crystalline lamellar structure when the characteristic diffraction peaks of LDHs are observed. In the second one, the sharp diffraction peaks corresponding to LDH phases progressively disappear suggesting the collapse of LDH lamellar structure, while in the third region, the metal oxides and spinel phases, issued from

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the LDH structure collapse, are detected. Kloprogge and Frost [13] examined, by in situ

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DRIFTS and TGA/DTA analyses, the evolution of the CO32- and HO- groups as well as the vibrations of the framework bonds during calcination of different CO32--LDHs (i.e., MgAl, NiAl and CoAl). For the three studied materials, they proposed the formation of a mixture of metal oxides and spinel phases starting from 350 °C. Yang et al.[14] used several in situ techniques

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such as XRD, DRIFTS and TG/MS to investigate the MgAl-LDH structure when submitted to thermal treatment from room temperature up to 560 °C under Ar atmosphere. Unlike Kanezaki et

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al. [12], theses authors proposed five temperature ranges associated to structural changes arising in the LDH structure. Accordingly, they observed the preservation of the layered structure up to

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190 °C. The only change observed consists in the removal of interlayer water molecules that causes the decreasing of the basal space from 7.5 to 6.6 Å. Spinel phase formation was not identified in the materials at higher temperatures (580 °C), but only a mixture of MgO and Al2O3

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was detected. Recently, Zhang et al.[15] investigated, by in situ XRD, the thermal stability of three LHDs-CO32- containing at least two divalent cations in the brucite-like sheets (i.e.,

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CuZnAl, CuZnFeAl and CuZnMnFeAl). They observed different thermal stabilities of the LDHs, as well as the formation of different crystalline oxide phases upon thermal treatment, depending

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on the LDH composition, temperature and calcination atmosphere (air or nitrogen). All these investigations provide information on the thermal transformations occurring in

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the LDH structure when they are transformed to homogeneous mixed oxides under air. However, in view of the catalytic applications, in particular for the hydrogenation reactions requiring easily

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accessible metallic active sites, the in-situ investigation of the changes taking place in the LDH structure during the thermal treatments under H2 flow are compulsory. Accordingly, the subsequent discussion is related to the morfostructural changes of Mg(Ni)CuAl-LDH during the reduction under H2 in the temperature interval ranging from 30 to 550 °C. As an alternative to this classical approach, we reported earlier the preparation of supported copper LDH catalysts by a mild thermal treatment of LDH precursors at 150 oC under reductive conditions (i.e., CuNiZnAl, CuNiMgAl, CuCoZnAl), which show outstanding activity in the hydrogenation of cinnamaldehyde (CNA) [16-19]. However, in-depth investigations are compulsory to shed more light on the complex phenomena such as composition- and temperature-dependent phase transition and cation reduction taking place upon direct reduction of copper- and/or nickel-containing layered double hydroxides. A better description of their

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structural properties with regard to catalytic applications is strongly required to enable the

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design of efficient LDH-derived catalysts. Therefore, MgNiCuAl LDHs were thermally treated under reductive gas flow (hydrogen) and the morphostructural evolution of these multicomponent LDH materials, as well as the nature and size of the metallic active sites, are assessed by in situ XRD and TEM analyses. Additional information on the reducibility of copper

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and nickel cations substituted in the brucite-like sheets, the interaction between the two cations and their interaction with the MgAl matrix were investigated by TPR analyses. A special

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attention was given to the monometallic copper samples derived from MgCuAl-LDH reduced at different temperatures, ranging from 150 to 500 °C. Due to the difficulty to identify the metallic

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phases generated after reduction at temperature as low as 150 °C by conventional techniques such as TPR, in-situ XRD and TEM, the surface Cu0 was evaluated by the dissociative adsorption of N2O. Finally, the catalytic properties of the mono- and bimetallic catalysts,

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obtained via direct reduction of LDHs, were evaluated for the hydrogenation of cinnamaldehyde, highlighting the formation of highly dispersed metallic phases. The effect of the reduction

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temperature on the catalytic performances of materials was also studied. The hydrogenation of cinnamaldehyde was chosen as test catalytic reaction due to: (i) its relevance to produce fine

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chemicals (i.e., cinnamyl alcohol, hydrocinnamaldehyde, and hydrocinnamylalcohol) and (ii) its [20, 21].

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2. EXPERIMENTAL

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sensitivity to nature of metallic active sites as well as their electronic and geometric properties

2.1. Materials

All chemicals were used as received without any supplementary purification: magnesium nitrate (Mg(NO3)2·6H2O, 99%, Sigma-Aldrich), aluminium nitrate (Al(NO3)3·9H2O, 98%, Sigma-Aldrich), nickel nitrate (Ni(NO3)2·6H2O, 98%, Sigma-Aldrich) and copper nitrate (Cu(NO3)2·3H2O, 98%, Sigma-Aldrich), sodium hydroxide (NaOH) sodium carbonate (Na2CO3 99.8%, Merck).

For catalytic runs, the chemicals were also used as purchased: trans-

cinnamaldehyde (C6H5CH=CHCHO, 98%, Merck) as reagent and propylene carbonate (C4H6O3, 99%, Sigma-Aldrich) as solvent. LDHs with different Ni:Cu ratios but with the M2+/M3+ = 2 were synthesized by coprecipitation under low suprasaturation method at 25 °C. A solution containing metal

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nitrates ([Mg2+] + [Ni2+] + [Cu2+] + [Al3+] = 1.5 M) and an alkaline solution ([NaOH] + [Na2CO3] = 2.4 M) were simultaneously added in a three-necked round-bottomed flask fitted with a pH electrode and thermometer. The suspension was vigorously stirred and maintained at the desired pH (~ 8) by adjusting the relative flow rate of the alkaline solution while the flow rate

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of the precursor solution was kept constant. The final suspension was aged under stirring at room temperature for 12 h. The precipitate was filtered, washed two times with carbonate solution in

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order to remove nitrate ions from the interlayers, and finally repeatedly washed with deionized water to remove residual sodium. The resulting precipitate was dried at 40 °C for 24 h. These

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materials are denoted as as-synthesized LDH materials in the following section. 2.2. Characterization of LDHs

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Chemical composition (Mg, Cu, Ni and Al) of the samples was performed on a Perkin sequential scanning inductively coupled plasma optical emission spectrometer (Optima 2000

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DV). Before analysis, a known amount of sample was dissolved in a diluted HCl solution and then heated under microwave until complete dissolution. Nitrogen physisorption was carried out on an Autosorb 1-MP instrument from

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Quantachrome at -196 °C. Surface area, pore volume and pores size distribution were obtained

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from the corresponding isotherms using the conventional calculation algorithm such as BET, de Boer and BJH.

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Diffuse reflectance UV–visible (DR UV–vis) spectra were recorded on a Shimadzu UV2450 spectrometer equipped with an integrating sphere unit (ISR-2200). The spectra were collected in the range between 190 – 800 nm and using BaSO4 as the reflectance standard. In situ powder XRD patterns were recorded on a Bruker D8 ADVANCE X-ray diffractometer equipped with a VANTEC-1 detector, using a CuKα radiation (λ = 1.54184 Å) as X-ray source. The powder samples were placed on a kanthal filament (FeCrAl) cavity. The data were collected in the 2θ range from 15 to 70° with a step of 0.05° (step time of 2s). Phase identification was made by comparison with ICDD database. The diffractograms were recorded after in situ reduction from 30 to 500 °C under a flow of 3 vol.% H2 in He (30 mL min-1). The data were collected 3 times at each temperature. Infrared spectra (FT-IR) were collected on a Scimitar FTS 2000 (Digilab) between -1

-1

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400 and 4000 cm , 50 acquisitions per spectrum and a resolution of 4 cm . Samples were

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used as pellets formed by a mixture of 99 mg KBr and 1 mg of solid. Thermogravimetric analyses (TG) were performed on SDTQ600 instrument. Samples were analysed in the temperature range from 30 to 800 °C with a heating rate of 10 °C min-1. All measurements were performed under air flowing.

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Temperature programmed reduction (TPR) experiments were performed on an Autochem chemisorption analyser from Micromeritics, equipped with TCD and MS (Omnistar, Pfeiffer)

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detectors. About 100 mg of as-synthesized LDH were inserted in a U-shape microreactor. Before each TPR run, the catalyst was activated at 120 °C for 4 h under Ar flow (30 mL min-1). After

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cooling to 50 °C, H2 containing flow was stabilized (30 mL min-1, 3 vol.% H2 in Ar) and the TPR was performed from 50 to 800 °C with a temperature ramp of 2 °C min-1. The experimental TPR profiles have been fitted by Gaussian peak functions to determine the reduction maxima.

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High Resolution Transmission Electronic Microscopy (HR-TEM) was performed on a JEOL 2100 instrument operated at 200 kV with a LaB6 source and equipped with a Gatan Ultra

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scan camera. The images were recorded for fresh and reduced samples at different temperatures (150, 350 and 500 °). The reduction step was carried out as in similar conditions as for the

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thermal treatment before the catalytic test (section 2.3).

The dissociative N2O adsorption method was performed on a ChemBET Pulsar TRP/TPD

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apparatus from Quantachrome. In a typical experiment, the ~30 mg sample was placed in a Ushaped quartz reactor and was pre-treated under Ar flow (40 cm3 min−1) for 1 h at the required

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temperature (i.e., 150 and 500 °C), followed by cooling to room temperature. The reduction was performed by increasing the temperature either to 150 or 500 °C with a ramp of 5 °C min−1 under a 5% H2/Ar flow (50 cm3 min−1) (TPR1) and kept for 2 h at the reduction temperature. Then, the sample was cooled to 60 ± 5 °C under Ar flow and was exposed to a N2O flow for 0.5 h (30 cm3 min−1). The accessible Cu0 surface species was oxidize to Cu2O by N2O. Finally, a second TPR run (the same conditions as for TPR1) was carried out in order to reduce the Cu2O to metallic Cu (TPR2). The copper dispersion was calculated by using the following formula: D(%) = (2A2/A1)*100, where A2 and A1 stand for the area of the reduction peaks obtained in TPR2 and TPR1, respectively. 2.3. Catalytic test: hydrogenation of trans-cinnamaldehdye For each test, the as-synthesized catalyst (previously dried at constant temperature of

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40 °C) was reduced under hydrogen flow (1 L h-1) at different temperatures (i.e., 150, 350 and 500 °C) for 10 h (heating rate of 6 °C min-1 up to the reduction temperature). The catalytic tests were carried out at atmospheric pressure in a three neck glass reactor equipped with reflux condenser and magnetic stirrer (constant stirring of 900 rpm) under the following conditions: 1

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mL of reagent (7.94 × 10−3 mol), 25 mL of propylene carbonate (PC) as solvent, 0.265 g of catalyst, hydrogen flow of 1 L h-1, and constant reaction temperature of 150 °C. Samples were

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periodically taken off and analysed by GC with an HP 5890 series gas-chromatograph, which is equipped with a DB-5 capillary column and a flame ionization detector. The identification of the

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reaction products was achieved from the retention times of pure compounds and occasionally by GC-MS (Agilent 6890N system equipped with an Agilent 5973 MSD detector and a DB-5-ms column). The conversion of cinnamaldehyde and selectivity in the different hydrogenation

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products were calculated by taking into account the FID response factors for each compounds.

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3. RESULTS AND DISCUSSION

3.1. Physico-chemical properties of as-synthesized LDHs

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The as-synthesized multicomponent LDHs included in the present work were firstly

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investigated by routine physico-chemical analyses such as ICP, N2 physisorption, DR UV-vis, FT-IR, and TGA. The main results are shown in Table 1. The detailed discussion on these results

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is available in the supporting information.

Table 1.

3.2. Investigation of the structural transformations of LDHs-derived materials under hydrogen 3.2.1. In situ XRD and TEM after reduction at different temperatures The in situ XRD patterns of MgNiCuAl LDH materials treated under H2 at different temperatures are displayed in Fig. 1. As a first observation, it can be seen that brucite-like layers containing nickel cations are more stable than copper containing LDH, traces of the LDH structure being identified at even higher temperatures (i.e., 250 °C). This technique was also used in order to study the nature of the metallic phases formed after reduction. To achieve

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this purpose, the temperature was raised up from 30 to 550 °C during the experiments. The XRD patterns registered for the as-synthesized samples show the typical reflections of LDH crystalline phases, which are indexed in a hexagonal lattice with R-3m rhombohedral symmetry. On the other hand, differences between the as-synthesized samples with diverse compositions can be

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noticed. Therefore, as the amount of copper in the sample is higher, the diffraction peaks are sharper and narrower suggesting improved crystallinity (larger particles). To some extent, these

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results are in contradiction with those of Rives and Kannan [22] that obtained CuNiAl LDH materials with poor crystallinity when copper was in high amount in the sample. The 2θ value of

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the first peak of the doublet at 60 – 62 ° is used to calculate the “a” parameter of the elemental cell, which gives the distance between two neighbouring atoms in the brucite-like layers (a = ). All values are listed in Table 2. The incorporation of copper caused a

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2d(110);

slight increase of this parameter, in accordance with the larger ionic radius of Cu2+ (87 pm)

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relative to Ni2+ (83 pm) in an octahedral environment [23].

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Figure 1.

By subtracting the thickness of the brucite layers (tbrucite = 4.8 Å [8]) from the values of c’

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(c’ = d003, Table 2), the thickness of the interlayer space (tinterlayer, Table 2) was calculated. On the basis of these values, it was assumed that only the carbonate is the counter-ion incorporated in the interlayer space [8].

When the reduction temperature was increased up to 150 °C, different behaviours were observed depending on the sample composition. For the samples containing only nickel (Ni) or rich in nickel (Ni4Cu1), the registered diffractograms are very similar to those recorded at 30 °C. However, a slight shift of the (003) peak at high 2θ values, with minor decrease of the intensity and broadening of the diffraction peaks, are noticed. Consequently, the structural parameters remain very close to those of the as-synthesized samples (Table 2). Higher instability of the framework was observed for the Ni1Cu4 and Cu samples, and it can be correlated with the increasing amount of copper in the sample, while for the samples with higher amount of nickel (i.e., Ni, Ni4Cu1, and Ni1Cu1), the stability of the framework was much improved.

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Nevertheless, it is worthy to mention that the typical reflections of the LDHs are still detected after the thermal treatment at 150 °C. These results are different from those early reported in the literature for CuMgAl-LDHs [24]. Indeed, taking into account the (003) reflection, the brucitelike structure was observed to be significantly affected even at 100 °C, whereas at higher

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temperatures (i.e., > 150 °C), the layered structure completely collapsed [24]. The improved stability of the Ni1Cu4- and Cu-samples presented in our work could be explained by: (i) the

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lower amount of copper introduced in the brucite-like layers and (ii) the presence of nickel in the Ni1Cu4-sample that can act as a stabilizer of the structure.

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On the basis of these results, it can be assumed that, after reduction at 150 °C (note that this temperature is one of the temperatures at which the samples were reduced in order to obtain catalysts for the hydrogenation of CNA), the structural changes detectable by XRD consists in

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the fragmentation of the larger LDH particles into smaller particles, (especially for the copper rich samples) and narrowing of the interlayer space because of the removal of physisorbed water,

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as schematized in Fig. 2. This interpretation is confirmed by the TEM images collected after reduction at 150 °C under H2 flow (vide infra). At the same time, the easily accessible cations

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placed in the corners and on the edges of the particles are susceptible to be reduced by hydrogen giving rise to highly dispersed metallic active phases (as it will be further confirmed by the

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catalytic tests results) deposited on the reminiscent LDH crystals (i.e., Mo/LDH composites). However, due to the high dispersity of these metal phases, they cannot be detected neither in the

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diffractograms nor on the TEM images collected after reduction at 150 °C, even for the samples containing only copper cations in the brucite-like sheets, which proved to be outstandingly active in the hydrogenation reactions (vide infra).

Figure 2.

Table 2.

After reduction at 250 °C, the recorded diffractograms still show the typical

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reflections of the hydrotalcite phase, but with much less intensities and broadening

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compared with those recorded at 150 °C (Fig. 1). Even Cu- and Ni1Cu4-samples, the two less stable materials, still show the LDHs typical reflections. For these two Cu rich materials, the decrease of the interlayer space is however, more pronounced than for the Ni rich samples. As observed in Table 2, the “c” parameter decreased with the increase of the reduction temperature.

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It is known that this parameter, which provides information on the thickness of the galleries, is associated to the size and orientation of the interlayer anion and strength of the electrostatic

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forces between the interlayer anion and the layers [25]. The decrease of this parameter could be related to both the gradual removal of the interlayer water molecules and change in the

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orientation of the carbonate ions, which results in higher electrostatic interaction between the layer and interlayer (as discussed in the FT-IR section, Fig. S3).

It is obvious that the insertion of copper in the brucite-like layers favours of a faster

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removal of the interlayer water, as well as an increase in electrostatic interactions between carbonate ions and the brucite-like sheets. The particular behaviour of copper rich hydrotalcites

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is already known and more details on this subject can be found in Kannan’s work on copper in various environments such as CuCoAl, CuNiAl and CuMgAl hydrotalcite like materials [24].

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When the reduction temperature is further increased at 350 °C, additional considerable phase transformations are observed for all samples irrespective of the material composition. The

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typical reflections of hydrotalcite-like structure are no longer detected in the diffractograms (Fig. 1). Although at these two reduction temperatures, i.e. 250 and 350 °C, the reduction of the

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cations (especially copper) takes place, the diffractograms recorded for the samples with low amount of copper do not contain any reflection characteristic to the metallic phases, Cu0 or Ni0. However, as the content of copper increases, a broad and poorly defined peak is observed at 2θ ~ 43.4 °, which is attributed to the diffraction of the (111) plane of Cu0 (ICDD 04-0836). The intensity this peak is observed to increase with the content in copper. Moreover, for Ni1Cu4- and Cu-samples, a second reflection at 2θ ~ 50.5 ° was also observed after reduction at 350 °C, which is attributed to the diffraction of the (200) plane of metallic copper. As concerns the samples with high amount of Ni, no reflection attributable to metallic Ni0 can be observed in the diffractograms, even after reduction at 350 °C. After reduction at T ≥ 450 °C, a very broad and low intense reflection was observed at 2θ ~ 44.5 ° (ICDD 04-0850). This reflection is attributed to the diffraction of the (111) plane of Ni0. For the Ni1Cu1- sample, the broad band at 2θ ~

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50 ° could be assigned to the diffraction of (200) planes of Cu-Ni alloys [26, 27].

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Unfortunately, the poor definition of the recorded signal and its overlapping by kanthal signals (signal of XRD support) make not reliable the application of the Scherrer equation to evaluate the size of the generated metallic particles. Finally, although the samples were thermally treated until 550 °C, no visible signals of

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spinel phases can be observed, irrespective of the material composition. This result is considered as a positive feature in relation to the reducibility of copper and nickel, knowing that they are

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hardly reducible when spinel phase is formed, i.e. reduction occurring above 700 °C [28].

The morphostructural properties of the LDHs materials before and after reduction under

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hydrogen at different temperatures (i.e., 150, 350, and 500 °C) were investigated by TEM analysis. Representative images are displayed in Fig. 4. As a first remark, typical morphology

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with hexagonal plate-shaped particles, whose sizes increase with the increasing of copper amount in the sample, as indicated by XRD results, is observed for the fresh sample, i.e., not

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reduced. After reduction at 150 °C, no significant change in the morphology of the samples is observed, excepting the Cu-sample for which some morphological changes can be observed because of the Jahn-Teller effect affecting the copper in the octahedral coordination. This

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observation is in line with XRD interpretation. Likewise, it is not possible to distinguish metallic

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particles in these images because they are too small or in too low content to be detected. The reduction of the sample at 350 °C produced total or partial damage at morphostructural level of

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the samples, depending on their composition.

Figure 4.

Therefore, the plate-shaped particles are still observed for Ni-sample, but their thickness is smaller than for the unreduced sample, while metallic particles are hardly observed. Such observation explains the absence of diffraction peaks corresponding to the metallic phases in the diffractograms collected at 350 °C (Fig. 1). For Cu-sample, only traces of the initial plate-shaped particles can be observed. Instead, small and very well dispersed copper nanoparticles, those average size is ~ 2.6 nm, are homogeneously distributed throughout the sample. For the sample containing both copper and nickel (Ni1Cu1-sample), the TEM images show an intermediate situation between Cu- and Ni-samples. The plate-shaped particles are

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better distinguished in comparison with the Cu-sample, while the metallic particles generated upon reduction are smaller, with an average size of ~ 2.1 nm. This decrease in size could be explained by the positive effect of nickel on the copper dispersion. This effect was previously observed for copper-nickel nanoparticles deposited on SBA-15 [5, 29]. The reduction at 500 °C

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generated discernable metallic nanoparticles for Ni-sample that are very well dispersed throughout the sample. The measured average size is ~ 4.7 nm. In the case of Cu-sample, an

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increase in the average size is observed for the Cu0 particles (from 2.6 to 6.5 nm) due to the thermal sintering upon the increase of the reduction temperature. Similar effect is observed for

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the Ni1Cu1-sample. However, for this last sample, the increase is only of 1.2 nm suggesting a

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better resistance to sintering of the copper-nickel NPs in the LDH environment.

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3.2.2. Temperature programmed reduction

The reducibility of the as-synthesized LDH materials was studied by H2-TPR and the results are shown in Fig. 3. For the sake of clarity, the TPR profiles were shifted along the y-axis.

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The reduction of cations in the as-synthesized LDHs is difficult to follow because it proceeds in

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parallel with the thermal decomposition of the LDH structure. Therefore, the TPR maxima shown in Fig. 3 do not essentially correspond to the reduction of cations from the initial

Ac ce p

crystalline LDH structures, but rather to the reduction of cations from the intermediate structures resulting from LDH structure decomposition, which are amorphous as nicely shown by the in situ XRD patterns (Fig. 1, above 350 °C). Figure 3.

Previous experiments have shown that Mg2+ and Al3+ cations are not reducible in the experimental conditions of TPR, given no H2 consumption for the MgAl matrix over the TPR temperature range [30]. As concerns the reduction of the compensating anions (i.e., NO3- and CO32-), the following particularities should be mentioned: the reduction of NO3- to NO takes place at 300 - 350 °C but no nitrate traces were detected in the samples by FT-IR spectroscopy while CO32- is removed as CO2. It can be therefore concluded that all peaks

14

Page 14 of 35

in the TPR profiles are attributed to the reduction of the Cu2+ and Ni2+ cations into their metallic counterparts. The sample containing only copper exhibits a main peak with a maximum at 356 °C. This hydrogen consumption is attributed to the reduction of Cu2+ → Cu0. This is quite surprising

ip t

because TPR performed on CuZnAl(Zr)-mixed oxides, for instance, indicated reduction temperatures in the range 200-300 °C for copper species [25]. This high temperature of reduction

cr

(i.e., 356 °C) observed here could be explained by a strong interaction between copper cations and the matrix formed by magnesium and aluminium. Similar observation was made by Rebours

us

et al.[31] for Ni-Al mixed oxides derived from the calcination of the corresponding LDH precursors. They observed that aluminium has a strong effect on the thermal stability of nickel oxide and therefore, a surface enrichment in aluminium is seemed to be responsible for the

an

resistance to thermal sintering of the nickel oxide. On the other side, it can be observed that the reduction peak is not really symmetric. A small delay on the ascending part of the peak can be

M

observed, and is attributed to the Jahn-Teller effect associated with Cu2+ cations existing in a distorted octahedral symmetry, as also observed by DR UV-vis spectroscopy. Rives and Kannan

d

[15] made a similar observation for copper rich NiCuAl LDH for which the TPR peak has a similar asymmetric shape. It can be concluded that the peak with the maximum at 356 °C is due

te

to the reduction of copper cations with two different octahedral symmetries. The TPR profile of the sample containing only nickel as reducible cations shows only one

Ac ce p

broad peak centred at 661 °C. This hydrogen consumption is associated to nickel cations accepting two electrons to get metallic nickel. This high reduction temperature indicates a strong interaction of nickel with LDH matrix or the presence of amourphous spinel (not detected by XRD).

For the samples containing both cations, Ni2+ and Cu2+, in different ratios, the TPR profiles contain at least two peaks. The first one is centred at 346 °C for Ni1Cu4 and Ni1Cu1, and 374 °C for Ni4Cu1 while the second one is observed at 490 °C for Ni1Cu4, 460 °C for Ni1Cu1 and 634 °C for Ni4Cu1. The low temperatures peaks are attributed to the reduction of Cu2+ cations, whereas those at high temperatures are ascribed to the reduction of Ni2+ cations. It can be seen that the reducibility of the transition metals is improved when both cations are present in the structure, especially for Ni2+, the corresponding reduction temperatures being shifted to lower

15

temperatures (Fig. 3). This suggests a high metal dispersion and a synergistic interaction

Page 15 of 35

between the two cations in close proximity to each other, as it was previously observed for copper and nickel oxides highly dispersed on silica, zeolites, carbon or alumina [2, 32-35]. However, particular behaviours can be noticed, depending on the sample composition. Therefore, for the Ni1Cu4-sample, the reduction of copper cations occurs at 346 °C, at a

ip t

temperature 10 °C below than for the monometallic sample, while Ni2+ cations are reduced at 490 °C, at a temperature 171 °C lower in comparison with the Ni-sample. For the Ni4Cu1-

cr

sample, a small delay in the reduction of copper cations was observed, more exactly with 18 °C in comparison with the Cu-sample, while for Ni2+, the reduction occurs at 634 °C (only 27 °C

us

lower in comparison with the monometallic Ni-sample. Interestingly, the lowest temperature for the reduction of Ni2+ was measured over the Ni1Cu1-sample (i.e., 460 °C), which means 201 °C lower than the temperature required for the reduction of Ni2+ in Ni-sample. The copper reduction

an

in Ni1Cu1-sample occurs as the same temperature (i.e., 346 °C) as for copper in Cu1Ni4-sample. According to these results, it can be assumed that at Ni:Cu atomic ratio close to 1, the synergic

3.2.3. Dissociative adsorption of N2O

M

interactions between copper and nickel are strongest.

d

The dissociative adsorption of N2O on copper is an approach to evaluate the degree of

te

dispersion of surface metallic copper, considering that bulk copper is not titratable by N2O [36]. It is documented that below 100 °C no other surface entity participates and N2O reacts only with

Ac ce p

the accessible copper atoms according to the equation: N2O + 2Cu(s) → N2 + Cu2O(s)

(1),

where s denotes surface atoms [36, 37]. Moreover, a comprehensive study on the measurement of accessible copper surface by N2O adsorption clearly showed that the variation of the adsorption temperature in the range from 25 to 90 °C does not influence the amount of adsorbed N2O and therefore, the measured surface area does not suffer significant changes in this range of temperature, while the N2O partial pressure influences only the required time for the molecule breakthrough [38]. On the basis of these studies, the Cu-sample was treated at 60 °C with pure N2O for 0.5 h for the measurements. Since the catalytic performance of Cu-sample in the hydrogenation of cinnamaldehyde after reduction at different temperatures (150, 350, and 500 °C) was evaluated, the use of this method was thought as a direct evidence of the generation of metallic copper by reduction at 150 °C, on one hand. On the other hand, the

16

Page 16 of 35

method was used to measure the dispersion of the metallic copper after reduction at 500 °C. In all experiments, the runs were labelled as follows: TPR1 denotes the reduction under H2 before N2O treatment, while TPR2 denotes the recorded TPR after N2O treatment. In Fig. 5, the TPR profiles for the experiments performed at 150 and 500 °C are displayed.

ip t

In the case of the experiment performed at 150 °C, the reduction of the first run (TPR1), was carried out under thermo-programmed conditions from 50 to 150 °C, followed by an

cr

isothermal step of 2 h in order to allow of a part of the cationic copper (Cu2+) to be reduced to zero-valent copper, Cu0. This runs was not registered. The TPR2 profile shows a maximum

us

centred at ~ 330 °C and a broad shoulder at ~ 270 °C According to the equation (1) and considering that only a part of copper is reduced at 150 °C, it is thus normal to find out two reduction temperatures in the TPR2 profile. The temperature corresponding to the shoulder is

an

attributed to the reduction of Cu+ to Cu0 while the maximum at ~ 330°c is associated to the

M

reduction of Cu2+ that was not reduced in the first run (Fig. 3). Figure 5.

d

The increasing of the reduction temperature increases the amount of metallic copper.

te

Indeed, the TPR2 profile in the experiment performed at 500 °C displays a reduction peak centred at 200 °C, whose area is much higher than for the corresponding peak from the TPR2 in

Ac ce p

the experiment performed at 150 °C; for this last experiment, a continuous reduction from 200 to 270 °C of Cu+ species is observed. This difference could be explained by the structural changes taking place in the lamellar structure of the samples, especially at 500 °C when a higher amount of copper become more accessible to hydrogen and therefore, its reduction is easier to be performed. Additionally, the experiment carried out at 500 °C provided information of the dispersion/accessibility of/to copper nanoparticles. By applying the equation provided in the experimental part, a copper dispersion of 73.7 % was calculated, meaning that almost ¾ of copper atoms are accessible. This high value can be explained by the high amount of copper that can be statistically incorporated in the brucite like-sheets. This result is in well agreement with the results from TEM analysis. Indeed, the image recorded for the Cu-samples reduced at 500 °C (Fig. 4), shows many metallic NPs homogeneously distributed throughout all sample. 17

Page 17 of 35

3.3. Catalytic performances of the LDH-derived materials During the hydrogenation of cinnamaldehyde, three different products can be obtained (Scheme 1): unsaturated alcohol (CNOL), saturated aldehyde (HCNA) and saturated alcohol (HCNOL). Although all three hydrogenated products have industrial applications, the

ip t

unsaturated alcohol is usually the most demanded product. This is a very challenging situation since it is much easier to hydrogenate the C=C group, which is thermodynamically favoured,

an

Scheme 1.

us

cr

rather than C=O group.

The evolution of CNA conversion with reaction time for three catalyst compositions (i.e.,

M

Cu, Ni1Cu1 and Ni) is presented in Fig. 6. The results show that the direct reduction of the assynthesized LDH precursors generates very active metallic nanocomposite catalysts. However,

te

can be observed.

d

depending on the material composition and the reduction temperature, variations in the activities

Ac ce p

Figure 6.

Therefore, Cu-sample reduced at 150 °C is extremely active, the total conversion of the CNA taking place in only 180 min of reaction. Instead, the overall activity slightly decreased when the reduction temperature is increased to 350 °C, the CNA being completely converted in 250 min. The reduction at 500 °C was followed by a new decrease in the overall activity. These results are very interesting because they highlight that a copper-based catalytic precursor such as MgCuAl-LDH can be reduced at low temperature (i.e., 150 °C) in order to generate active sites that present high catalytic activity, in well agreement with the dissociative adsorption of N2O, which clearly show the reduction of a part of Cu2+ to Cu0 at this temperature. As it was shown above, when the lamellar structure is subjected to mild reduction at 150 °C, the fully accessible copper cations placed at the corners and on the edges of the crystallites are firstly reduced, and the resulting metallic copper nanoparticles are supported on the brucite-like sheets

18

Page 18 of 35

without any sintering phenomenon (Fig. 7). This is explained by the low temperatures of preparation and treatments before test (for example the Tammann temperature of Cu0 is 405 oC [39]). At higher temperatures of reduction (i.e., 350, 500 °C) copper particles increase in size upon thermal sintering (as clearly indicated by XRD and TEM after reduction), the number of

ip t

metallic surface active sites decreases and therefore, the catalytic activity became lower (Fig. 6). The catalytic behaviour of copper catalysts derived from LDH structures was previously study in

cr

the liquid phase hydrogenation of cinnamaldehyde [7]. However, in that work, CuAl and ZnCuAl LDHs were firstly calcined to mixed oxides and then reduced under hydrogen at 200 °C

us

to get the metallic copper. Although the reaction was performed at 10 bar of H2 and 120 °C, the resulted catalysts manifested significantly lower catalytic activity, expressed as conversion of CNA, than those reported in our contribution for which the catalytic activity was measured at

an

atmospheric pressure. Thus, a conversion of ~ 90% CNA after ~ 350 min of reaction was observed for ZnCuAl sample, while ~ 80 % after ~ 430 min of reaction for CuAl. This difference

M

in the catalytic behaviours reported herein vs that reported by Marchi et al.[7] could be explained by a first thermal treatment applied to the LDH materials under air, i.e. calcination. The

d

structural modifications occurring during this step go along with the formation of CuO dispersed on a super-stroichiometric zinc aluminate spinel. Additionally, the formation of CuAl2O4 spinel

te

cannot be excluded. If CuO is easily reduced to zero-valent copper (T < 300 °C), the reduction of Cu2+ in spinel phase require high temperature of reduction (T > 700 °C) [40]. It is evident that in

Ac ce p

this case, only a part of copper is reduced, the number of active sites is lower, and therefore, the catalyst is less efficient. The direct reduction of the LDH avoids the formation of hardly reducing spinel phases and a much higher amount of copper became active in the hydrogenation reaction even at low temperature of reduction. For the Ni1Cu1-sample, the evolution of catalytic activity with the reduction temperature is different that for the Cu-sample. Thus, the reduction of the Ni1Cu1 catalyst at 150 °C leads to a less active catalyst (conversion of 60 % in 360 min of reaction) because nickel remains as cationic species and only copper is responsible for the catalytic activity. However, due to the lower amount of copper in this sample, the total number of active metallic sites is lower and the catalytic activity decreases. The increase in the reduction temperature at 350 °C improved the catalytic activity, suggesting that a higher amount of available metal sites are generated

19

because a part of nickel is also reduced at this temperature. After reduction at 500 °C,

Page 19 of 35

although the total amount of Ni2+ cations is reduced due to the synergic interaction between copper and nickel as discussed in TPR section, the catalytic activity is the same as for the sample reduced at 350 °C. This result could be explained by the minor thermal sintering of metallic particles at this temperature as shown by TEM analysis (the particle size increases from 2.1. to

ip t

3.3 nm after reduction at 350 and 550 °C, respectively) and/or the formation of coppersegregated CuNi alloys, these last crystalline phases being identified by in situ XRD.

cr

The sample containing only nickel as reducible cations does not show any relevant catalytic activity for cinnamaldehyde hydrogenation neither for reduction at 150 nor 350 °C

us

because of the absence of nickel reduction at these two temperatures. A conversion of CNA of only 18 % in 360 min after reduction at 350 °C was obtained. Although TPR indicates that only a small part of Ni2+ is reduced at 500 °C, this temperature was enough to get a very active catalyst

an

(Fig. 6). It is worth mentioning that the activity of the Ni-sample reduced at 500 °C is comparable with that displayed by the Cu-sample reduced at 150 °C. These results clearly show

M

that the preparation of cost-effective catalysts based on copper containing LDHs, able to generate highly dispersed metallic active phases even at low temperatures by direct reduction of the

te

the hydrogenation of CNA.

d

lamellar structure, is an economic alternative to the expensive noble metals, normally used for

Ac ce p

Figure 7.

Fig. 7 illustrates the selectivity to CNOL as a function of solid composition and reduction temperature, which are two important factors affecting the selectivity to CNOL. For Ni-sample reduced at 150 and 350 °C, the selectivity to CNOL is close to zero. After reduction at 500 °C, few active sites able to hydrogenate the C=O bond are generated, leading to a selectivity to CNOL of 3.5 % for a CNA conversion of 20 %. However, for higher conversion levels (60 %), the selectivity to CNOL decreased to 1.8 %. This is a typical behaviour of nickel on which the adsorption of CNA molecule mainly occurs via C=C bond, with formation of HCNA as main partially hydrogenated product (for instance, > 90 % for an interval of conversion ranging from 17 to 95 %). The introduction of copper besides nickel in the MgAl matrix (Ni1Cu1-sample)

20

drastically changed the chemoselectivity of the catalyst. After reduction at 150 and 350 °C,

Page 20 of 35

the catalyst was able to hydrogenate CNA to CNOL indicating a change in the adsorption mode of cinnamaldehyde on metallic sites. The selectivity to CNOL decreased significantly after reduction of the catalyst at 500 °C, whereas the selectivity to HCNA was improved (not shown). According to the TPR results, the species existing in the catalyst at 150, 350 and 500 °C are as

ip t

follows: Ni2+-Cu0 at 150 °C, Ni2+>Ni0-Cu0 at 350 °C, and Ni2+
cr

besides the metallic ones changes the adsorption mode of CNA resulting in favouring the C=O hydrogenation, in well agreement with the work of Marchi et al. [7]. Accordingly, when these

us

dual sites are generated on the catalyst surface, the cationic sites are responsible for the activation of C=O bond adsorbed on it, while the neighbouring metallic site will generate the hydrogen required for hydrogenation reaction. Following this reasoning, it is expected that the

an

increasing in the reduction temperature to diminish the amount of cationic sites and to generate higher content of metallic sites, increasing thus, the activity but lowering the selectivity to

M

CNOL. The sample reduced at 500 °C is consequently much less selective to CNOL. At higher conversion (i.e., 60 %), the selectivity to CNOL is lower than for a conversion of 20 % but the

d

selectivity evolution with temperature of reduction follows the same rules as at lower conversion. This confirms that the increase in the reduction temperature (i.e., 500 °C) continues to be

te

unfavourable for the formation of CNOL due to the decrease in cationic site content. The highest selectivity to CNOL (~ 30 % for a conversion of CNA of 20 %) was obtained

Ac ce p

on Cu-sample, irrespective of the reduction temperature. It is worthy of note that, although the overall activity of this sample gradually decreased with the reduction temperature, the selectivity to CNOL did not significantly change with the temperature, suggesting that the nature of the active sites responsible for the formation of CNOL does not depend on the reduction temperature. Additionally, a particle size effect on the selectivity is also excluded, at least for particle size below 7 nm. Therefore, the selectivity could be explained by electronic effects as a result of the highly dispersed state of copper, as in this particular case when the metallic sites are generated from the as-synthesized LDH structure under reductive conditions. It is worth mentioning that these catalytic results are in line with those already published by our group for copper-nickel or copper-chromium nanoparticles supported on mesoporous SBA-15 silica either by impregnation or precipitation [5, 6, 29]. All these investigations

21

Page 21 of 35

clearly indicated that very small and highly dispersed copper nanoparticles are outstanding catalysts for the hydrogenation of cinnamaldehyde. However, when copper lonely was supported on SBA-15, the materials did not show any catalytic activity but when a second transition metal was added to copper the catalyst show high activity. This behaviour was attributed to the effect

ip t

of stabilization exerted by the second metal on the copper nanoparticles [5, 6]. In the case of CuLDH material, for example, when copper is dispersed at atomically level in the brucite-like

cr

layers, it do not require a second transition metal for stabilization and thus this particular

us

structure allows the stabilisation and dispersion of copper active sites. CONCLUSIONS

The data presented herein disclose that the crystallinity and thermal stability of

an

MgNiCuAl-LDHs under reductive conditions strongly depends on the chemical composition (i.e., Ni:Cu molar ratio). Albeit the Ni-sample is less crystallized, the thermal stability of LDH

M

structure is high, traces of the lamellar structure being detected even after thermal treatment at 250 °C. The Cu-sample appears very well crystallized but the thermal stability of LDH under reductive conditions is low due to the Jahn-Teller phenomenon associated with copper. The

d

progressive introduction of nickel to copper in the MgAl brucite-like sheets results is less

te

crystallized LDH structures but with improved thermal stability. The concomitant presence of nickel and copper in the brucite-like layers was favourable for the reducible properties, as well.

Ac ce p

Therefore, improved reducibility of both cations was observed in the MgNiCuAl samples as compared with MgCuAl and MgNiAl samples. These results sustain a synergic interaction between these two cations in close proximity to each other. The reduction under H2 at different temperatures (ranging from 30 to 550 °C) of the as-synthesized samples generated highly dispersed metallic nanoparticles that were identified by in-situ XRD and TEM, and whose size depend on the reduction temperature and Ni:Cu ratio; a maximum average size of 6.5 nm was obtained for Cu-sample reduced at 500 °C. Due to the high sensitivity of the dissociative adsorption of N2O method, the formation of the metallic active copper after reduction at 150 °C in the Cu-sample was proven. It was interesting to observe that CuMgAl sample reduced at 150 °C exhibits the same catalytic activity as NiMgAl sample reduced at 500 °C. It can be finally concluded that the reduction at low temperature of the as-synthesized copper and coppernickel based layered double hydroxides, with MgAl as matrix, represents a valuable

22

Page 22 of 35

strategy to prepare hydrogenation catalysts because it is a simple route to obtain highly dispersed and stable metallic nanoparticles, while avoiding the time and energy consuming calcination

cr

ip t

step.

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Supplementary Information.

SI material provides results and discussion for N2 adsorption/desorption, DR UV-vis and FT-IR spectroscopies, and TG analysis for MgNiCuAl-LDHs and it is available free of charge via the

an

internet at ……...

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Acknowledgements

The authors acknowledge the financial support from the National Council of Romania under PNII-Idei Project No. 485/2009. Dr. Alexandra Sasu (Technical University “Gheorghe Asachi”

References

te

d

of Iasi) is acknowledged for performing some of catalytic tests.

Ac ce p

[1] G. Mul, J.A. Moulijn, in: G.A. Anderson, M.F. Garcia (Eds.) Supported Metals in Catalysis, Imperial College Press, London, 2005, pp. 1-29. [2] A. Carrero, J.A. Calles, A.J. Vizcaino, Appl. Catal. A: General 327 (2007) 82-94. [3] P. Munnik, M. Wolters, A. Gabrielsson, S.D. Pollington, G. Headdock, J.H. Bitter, P.E. de Jongh, K.P. de Jong, J. Phys. Chem. C 115 (2011) 14698-14706. [4] T. Toupance, M. Kermarec, C. Louis, J. Phys. Chem. B 104 (2000) 965-972. [5] A. Ungureanu, B. Dragoi, A. Chirieac, C. Ciotonea, S. Royer, D. Duprez, A.S. Mamede, E. Dumitriu, ACS Appl. Mater. Interfaces 5 (2013) 3010-3025. [6] B. Dragoi, A. Ungureanu, A. Chirieac, V. Hulea, S. Royer, E. Dumitriu, Catal. Sci. Technol. 3 (2013) 2319-2329. 23

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[7] A.J. Marchi, D.A. Gordo, A.F. Trasarti, C.R. Apesteguia, Appl. Catal. A: General 249 (2003) 53-67. [8] F. Cavani, F. Trifiro, A. Vaccari, Catal. Today 11 (1991) 173-301. [9] B.F. Sels, D.E. de Vos, P.A. Jacobs, Catal. Rev.43 (2001) 443-488.

ip t

[10] A. Vaccari, Appl. Clay Sci. 14 (1999) 161-198.

[11] T. Hibino, Y. Yamashita, K. Kosuge, A. Tsunashima, Clays Clay Miner. 43 (1995) 427-432.

cr

[12] E. Kanezaki, Solid State Ionics 106 (1998) 279-284. [13] R.T. Kloprogge, R.L. Frost, Appl. Catal. A: General 184 (1999) 61.

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[14] W. Yang, Y. Kim, P.K.T. Liu, M. Sahimi, T.T. Tsotsis, Chem. Eng. Sci. 57 (2002) 29452953.

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an

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Dumitriu, Environ. Eng. Manag. J. 9 (2010) 1203-1210;

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d

1561-1571.

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te

Environ. Eng. Manag. J. 11 (2012) 47-54.

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Ac ce p

[21] P. Maki-Arvela, J. Hajek, T. Salmi, D. Y. Murzin, Appl. Catal., A, 2005, 292, 1-49. [22] V. Rives, S. Kannan, J. Mater. Chem. 10 (2000) 489-495. [23] www.webelements.com

[24] S. Kannan, V. Rives, H. Knozinger, J. Solid State Chem. 177 (2004) 319-331. [25] S. Velu, K. Suzuki, M. Okazaki, M.P. Kapoor, T. Osaki, F. Ohashi, J. Catal. 194 (2000) 373-384.

[26] J.H. Lin, P. Biswas, V.V. Guliants, S. Misture, Appl. Catal. A: General 387 (2010) 87-94. [27] J.A. Dalmon, G.A. Martin, J. Catal. 66 (1980) 214-221. [28] A.M. Becerra, A.E. Castro-Luna, J. Chil. Chem. Soc. 50 (2005) 465-469. [29] A. Ungureanu, B. Dragoi, A. Chirieac, S. Royer, D. Duprez, E. Dumitriu, J. Mater. Chem. 21 (2011) 12529-12541.

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[30] V. Rives, M.A. Ulibarri, A. Montero, Appl. Clay Sci. 10 (1995) 83-93.

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[31] B. Rebours, J.B. d'Espinose de la Caillerie, O. Clause, J. Am. Chem. Soc. 116 (1994) 17071717. [32] E. Asedegbega-Nieto, B. Bachiller-Baeza, A. Guerrero-Ruiz, I. Rodriguez-Ramos, Appl. Catal. A: General 300 (2006) 120-129.

ip t

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[34] J. Alvarez-Rodríguez, M. Cerro-Alarcón, A. Guerrero-Ruiz, I. Rodriguez-Ramos, A.

cr

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us

176-185.

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[38] G.C. Chinchen, C.M. Hay, H.D. Vandevell, K.C. Waugh, J. Catal. 103 (1987) 79-86. [39] S. Golunski, Platinum Metals Rev. 51 (2007) 162.

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te

d

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[40] W. Mista, M. Zawadzki, H. Grabowska, Res. Chem. Intermed. 29 (2003) 137.

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Page 25 of 35

ip t cr us

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Table 1. The main physico-chemical properties of as-made MgNiCuAl samples. Chemical composition Synthesis

Ni

Peak* (cm-1) 3460

Ni4Cu1

1632

Ni1Cu1

1362

Ni1Cu4 Cu

1000-400

SBET M2+/ (m2.g-1) (Mg+Al) (wt%) 1/0 1.20 64 3.86 1.24 75 1.00 1.29 124 0.25 1.36 198 0/1 1.46 104 DR UV-vis Wavelength# Band assignment (nm)

M

Ni/Cu (Molar ratio)

Band assignment

Stretching vibrations of physisorbed water on the brucite like-layers The bending mode vibrations of water molecules in the interlayer Antisymmetric stretching of interlayer carbonate

Ac ce p

Sample

1/0 3.69 0.92 0.23 0/1

M2+/ (Mg+Al) (wt%) 1.14 1.16 1.19 1.22 1.24 FT-IR

d

Ni Ni4Cu1 Ni1Cu1 Ni1Cu4 Cu

Ni/Cu (Molar ratio)

te

Sample

N2 physisorption

ICP

Hydrotalcite framework vibrations #

Vp (cm3.g-1) 0.38 0.45 0.63 0.93 0.77 TG TWL %

218

O2- → Mn+ charge transfer transition

39.25

770

Cu2+ in the octahedral symmetry

38.56

Isolated Cu2+

39.78

305

2+

740 375

Ni

in the octahedral symmetry

38.72

Isolated Ni2+

38.66

*Main bands found in all samples; Main wavelengths found in all samples; TWL – Total weight loss.

26

Page 26 of 35

ip t cr us

an

Table 2. Structural parameters of the MgNiCuAl-LDHs after reduction at different temperatures.

Ac ce p

Ni1Cu1

d

Ni4Cu1

te

Ni

a 3.048 3.042 3.039 3.057 3.039 3.034 3.059 3.050 3.033 3.063 3.043 3.030 3.068 3.045 3.021

Ni1Cu4

Cu

Structural parameters c c’ 22.28 7.43 21.97 7.32 20.65 6.88 23.80 7.93 21.78 7.26 20.47 6.82 23.73 7.91 21.35 7.12 20.47 6.82 23.39 7.80 20.47 6.82 20.08 6.69 23.60 7.87 20.03 6.68 19.80 6.60

M

Sample

Treduction tinter 2.63 2.52 2.08 3.13 2.46 2.02 3.11 2.32 2.02 3.00 2.02 1.89 3.07 1.88 1.80

(°C)

30 150 250 30 150 250 30 150 250 30 150 250 30 150 250

27

Page 27 of 35

ip t cr us an

Ac ce p

te

d

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Figure 1. In-situ XRD patterns for as-synthesized MgNiCuAl-LDHs after reduction at different temperatures.

28

Page 28 of 35

ip t

Ac ce p

te

d

M

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cr

Figure 2. Schematic representation of the structure of a LDH: (left) - as-synthesized form and (right) - after reduction at 150 °C under H2 flow (● = M0, metallic species).

29

Page 29 of 35

ip t cr us an M

Ac ce p

te

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Figure 3. TPR profiles of as-synthesized MgNiCuAl-LDHs.

30

Page 30 of 35

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Figure 4. TEM images for fresh and reduced samples (i.e., Cu, Ni and Ni1Cu1-samples) at different temperatures. Inserted graphs: particles size distribution and average particles size.

31

Page 31 of 35

o

TCD signal, a.u.

Cu

2+ Cu

Cu

o

ip t

+ Cu

TPR1

cr

(500)

TPR2 (500)

100

200

300

400

500

an

Temperature, oC

us

(150)

TPR2

Ac ce p

te

d

M

Figure 5. The reduction profiles recorded for Cu-sample: TPR2(150) – the TPR profile registered after the re-oxidation of the Cu0 with N2O in the experiment performed at 150 °C; TPR1(500) – the TPR profile registered before the re-oxidation of the Cu0 with N2O in the experiment performed at 500 °C; TPR2(500) – the TPR profile registered after the re-oxidation of the Cu0 with N2O in the experiment performed at 500 °C.

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ip t cr

us

Figure 6. Effect of the catalyst composition and reduction temperature on the catalytic activity of the metallic/LDH-derived materials. (Reaction conditions: Treaction = 150 °C; 1 mL of CNA; 25 mL of PC

Ac ce p

te

d

M

an

as solvent; 0.265 g of catalyst; agitation speed of 900 rpm; hydrogen flow rate of 1 L h-1).

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ip t cr us an

Figure 7. Effect of the catalyst composition and reduction temperature on the selectivity to CNOL of metallic/LDH-derived materials. (Reaction conditions: Treaction = 150 °C; 1 mL of CNA; 25 mL of

Ac ce p

te

d

M

PC as solvent; 0.265 g of catalyst; agitation speed of 900 rpm; hydrogen flow rate of 1 L·h-1).

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O

H

H

CNA cinnamaldehyde

CNOL cinnamyl alcohol

H2 catalyst

2

OH

H

H2 catalyst

H

CH2

1

H

3

H2 catalyst

O CH2 CH2 CH2 OH

CH2 CH2

HCNOL hydrocinnamyl alcohol

4 HCNA hydrocinnamaldehyde

H2 catalyst

ip t

H

Ac ce p

te

d

M

an

us

cr

Scheme 1. Reaction pathways for the hydrogenation of cinnamaldehyde.

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