André L A Marinho

Fabio S Toniolo

Fabio B Noronha

Florence Epron

Daniel Duprez

Nicolas Bion
email: nicolas.bion@univ-poitiers.fr

Highly active and stable Ni dispersed on mesoporous CeO 2 -Al 2 O 3 catalysts for production of syngas by dry reforming of methane

Keywords: EISA, Biogas, Dry Reforming of Methane, Ceria, Nickel, Mesoporous Material

Ni-based mesoporous mixed CeO2-Al2O3 oxide catalysts prepared by one pot Evaporation Induced Self Assembly (EISA) were tested in dry reforming of methane. The textural, structural and physicochemical properties of the catalysts were studied by N2 adsorption-desorption, TEM-EDS, XRD and elemental analysis. The mobility of oxygen in the structure and the redox properties were investigated by OSCC measurements, TPR and in situ DRIFTS. Finally, the post-reaction samples were analyzed by TGA, RAMAN and TEM. The calcined catalysts prepared by EISA method present mesoporous structures with highly dispersed Ni in the form of NiAl2O4 spinel clusters. After reduction, small size metallic Ni particles are observed (< 5 nm). The presence of Ce in the structure interacting strongly with Al2O3 promotes the oxygen mobility and acts as sites for CO2 adsorption, increasing the activity of the catalyst and promoting the carbon removal mechanism. In absence of Ce in the mesoporous alumina support, the activity for carbon gasification is limited and C filaments accumulate inside the reactor. The same behavior occurs with CeO2-Al2O3 oxide prepared by EISA method when Ni is post-impregnated because of the presence of isolated larger Ni particles which promotes the decomposition of CH4. Therefore, Ni-based CeO2-Al2O3 catalysts prepared by one pot EISA method exhibited high activity and stability for the dry reforming of methane thanks to both properties which are the confinement of Ni particles and the inhibition of carbon deposition.

Introduction

A large number of investigations originated from the necessity to replace the fossil fuel energy by the renewable sources of energy in order to limit the emission of greenhouse gases. Among these sustainable sources of energy, biogas, a first generation biofuel [START_REF] Naik | Production of first and second generation biofuels: A comprehensive review[END_REF] obtained by anaerobic digestion of biomass attracted much attention in recent years. It is composed of CH4, CO2 and other components like H2S, N2 and H2O [START_REF] Yang | Progress and perspectives in converting biogas to transportation fuels[END_REF] in small amounts and can be directly used as biofuel. Its conversion to high value-added products, like syngas or pure hydrogen by reforming reactions is also of major interest since it consists in the conversion of two greenhouse gases in valuable products. Then, biogas reforming has been extensively studied by many authors in the last years [START_REF] Charisiou | Syngas production via the biogas dry reforming reaction over nickel supported on modified with CeO2 and/or La2O3 alumina catalysts[END_REF][START_REF] Kathiraser | Highly active and coke resistant Ni/SiO2 catalysts for oxidative reforming of model biogas: Effect of low ceria loading[END_REF][START_REF] Bobadilla | Monitoring the Reaction Mechanism in Model Biogas Reforming by In Situ Transient and Steady-State DRIFTS Measurements[END_REF].

The main industrial process to produce syngas is the steam reforming of methane [START_REF] Boyano | Conventional and advanced exergoenvironmental analysis of a steam methane reforming reactor for hydrogen production[END_REF][START_REF] Rostrup-Nielsen | Hydrogen and synthesis gas by steamand CO2 reforming[END_REF].

However, the dry reforming of methane (DRM) appears more interesting when biogas is used as raw bioresource because it does not require the removal of CO2. DRM is a highly endothermic reaction which occurs above 700 °C and converts CH4 and CO2 to syngas (Eq. 1), but side reactions occur in the reactor as CH4 decomposition (Eq. 2), reverse Boudouard reaction (Eq. 3) and reverse water-gas shift reaction (Eq. 4) [START_REF] Yang | Progress and perspectives in converting biogas to transportation fuels[END_REF]. CH4 decomposition reaction is responsible for the carbon formation in the reactor and the RWGS for the decrease of the H2/CO molar ratio in the syngas.

CH4 + CO2 ↔ 2 CO + 2H2

(1)

CH4 ↔ C + 2H2 (2) C + CO2 ↔ 2 CO (3) CO2 + H2 ↔ CO + H2O (4)
The use of catalysts is required to lower the activation energy for this reaction and the main challenge of this technology is the development of a catalyst resistant to carbon formation.

Ni-based catalysts have been studied [START_REF] Damyanova | Ni-based catalysts for reforming of methane with CO2[END_REF][START_REF] Pompeo | Study of Ni and Pt catalysts supported on α-Al2O3 and ZrO2 applied in methane reforming with CO2[END_REF][START_REF] Zhang | Insight into the effects of the oxygen species over Ni/ZrO2 catalyst surface on methane reforming with carbon dioxide[END_REF][START_REF] Li | Methane reforming to synthesis gas over Ni catalysts modified with noble metals[END_REF] due the low cost and high activity of nickel, but this metal is prone to coke formation by promoting CH4 decomposition. Rostrup-Nielsen et al. [START_REF] Rostrup-Nielsen | Mechanisms of carbon formation on nickel-containing catalysts[END_REF] observed that the rate of CH4 decomposition is strongly affected by the Ni particle size and is favored in large Ni size. Therefore, the limitation of the Ni particle sintering process is crucial to prevent the carbon formation during the DRM at high temperature. The catalyst design enables the control of the metal particle size for instance by embedded the metal particle in the oxide matrix [START_REF] Marinho | Embedded Ni nanoparticles in CeZrO2 as stable catalyst for dry reforming of methane[END_REF] or by generating porosity in which the metal particle can be confined [START_REF] Fang | Methane Dry Reforming over Coke-Resistant Mesoporous Ni-Al2O3 Catalysts Prepared by Evaporation-Induced Self-Assembly Method[END_REF].

Mesoporous materials with high surface area and well defined pores have shown interesting results to disperse the metal into the structure. The use of mesoporous oxides like SBA-15 [START_REF] Wang | A comparison study on methane dry reforming with carbon dioxide over LaNiO3 perovskite catalysts supported on mesoporous SBA-15, MCM-41 and silica carrier[END_REF], Al2O3 [START_REF] Li | A facile route to ordered mesoporousalumina-supported catalysts, and their catalytic activities for CO oxidation[END_REF] and MCM-41 [START_REF] Wang | A comparison study on methane dry reforming with carbon dioxide over LaNiO3 perovskite catalysts supported on mesoporous SBA-15, MCM-41 and silica carrier[END_REF] has been reported to promote high dispersion of metal. The use of a one-step method to synthetize Ni-based catalysts allows the homogeneous dispersion of Ni with strong metal-support interaction. Evaporation Induced Self-Assembly (EISA) method has been employed by some authors [START_REF] Ma | Combined methane dry reforming and methane partial oxidization for syngas production over high dispersion Ni based mesoporous catalyst[END_REF][START_REF] Xu | CO2 reforming of CH4 over rare earth elements functionalized mesoporous Ni-Ln (Ln = Ce, La, Sm, Pr)-Al-O composite oxides[END_REF][START_REF] Grosso | Fundamentals of Mesostructuring Through Evaporation-Induced Self-Assembly[END_REF][START_REF] Kuemmel | Thermally Stable Nanocrystalline γ-Alumina Layers with Highly Ordered 3D Mesoporosity[END_REF] to generate very well ordered mesoporous material. Fang et al. [START_REF] Fang | Methane Dry Reforming over Coke-Resistant Mesoporous Ni-Al2O3 Catalysts Prepared by Evaporation-Induced Self-Assembly Method[END_REF] prepared Ni-Al2O3 catalysts by EISA method and observed the formation of the mesoporous structuration in which Ni is highly dispersed due the formation of small NiAl2O4 crystallite. After reduction, Ni metallic particles were smaller than the ones obtained with impregnated catalysts. It resulted in a decrease of the carbon formation for the Ni-Al2O3 prepared by the one step method.

Besides the control of Ni particle size, the support can also play a key role. A support favoring the presence of oxygen species at the surface of the catalysts may promote the oxidation of the carbon formed on the metal. The oxygen vacancies in the material promote the dissociation of CO2 into CO and O atoms on the surface [START_REF] Faria | Hydrogen production through CO2 reforming of methane over Ni/CeZrO2/Al2O3 catalysts[END_REF]. The balance between the rate of carbon gasification and the rate of methane decomposition determines the stability of the catalyst [START_REF] Stagg-Williams | CO2 Reforming of CH4 over Pt/ZrO2 Catalysts Promoted with La and Ce Oxides[END_REF]. CeO2 presents high oxygen capacity (OSC), which is related to the Ce 4+ /Ce 3+ redox couple associated to the generation of oxygen vacancies. Many researchers [START_REF] Stagg-Williams | CO2 Reforming of CH4 over Pt/ZrO2 Catalysts Promoted with La and Ce Oxides[END_REF][START_REF] Marinho | Steam reforming of ethanol over Ni-based catalysts obtained from LaNiO3 and LaNiO3/CeSiO2 perovskitetype oxides for the production of hydrogen[END_REF][START_REF] Liu | Dry Reforming of Methane on a Highly-Active Ni-CeO2 Catalyst: Effects of Metal-Support Interactions on C-H Bond Breaking[END_REF] reported its utilization as a catalyst support to promote the carbon removal mechanism. Fonseca et al. [START_REF] Fonseca | Unexpected redox behavior of large surface alumina containing highly dispersed ceria nanoclusters[END_REF] have synthetized CeO2-Al2O3 mesoporous materials by EISA method (with 6 to 35 wt% Ce) and reported the high redox properties of these mixed oxides above 400°C. However, above 20 wt% of Ce the mesoporosity was lost. Wang et al. [START_REF] Wang | One-pot Synthesis of Ordered Mesoporous NiCeAl Oxide Catalysts and a Study of Their Performance in Methane Dry Reforming[END_REF] prepared the NiCeAl catalyst with low Ce/Al ratio by also using EISA method, and studied its catalytic performances in DRM. Catalyst with high surface area and high dispersion of Ni on the support were obtained A brief study was finally carried out by in-situ DRIFTS to investigate the mechanism of carbon removal.

Experimental

Synthesis

The mesoporous catalysts were prepared using the slightly modified EISA method described by Yuan et al. [START_REF] Yuan | Facile Synthesis for Ordered Mesoporous γ-Aluminas with High Thermal Stability[END_REF]. 5 g of the surfactant Pluronic 123 (Aldrich) was dissolved in 100 mL of ethanol (99.9 %, Fluka) in a beaker and maintained under stirring for 1 hour. Then, 6 mL of HNO3 (90 wt%, Sigma-Aldrich) was added to the solution, followed by the addition of aluminum isopropoxide (Aldrich), cerium acetate (Aldrich) and nickel acetate (Aldrich). The solution was maintained under stirring for 5 hours. The evaporation of the solvent was carried out in an oven at 60 °C for 5 days and the samples were calcined in a muffle furnace at 800 °C for 4 hours with a ramp of 1 °C/min. The concentration of metals in the solution was fixed at 0.5 mol/L and a Ce/(Ce + Al) molar ratio equal to 0.1 was used as well as two Ni loadings (5 and 10 wt.%). The amount of Ce corresponds to 20 wt% of CeO2. In addition, one sample without cerium was prepared according to the same protocol. The prepared samples were denoted as follows: CeAl, 10Ni-Al, 10Ni-CeAl and 5Ni-CeAl.

For comparison, the catalyst denoted as 10Ni/CeAl was prepared by incipient-wetness impregnation of the CeAl support prepared as described above. In this case, the Ni precursor was nickel nitrate (Aldrich) and the impregnation process occurred in ethanol solution. The sample dried in an oven overnight and calcined at 800 °C for 4 hours, 1 °C/min.

Characterization

The chemical composition of the samples was determined by ICP-OES with a PerkinElmer Optima 2000 DV instrument. The samples were totally mineralized using a mixture of HNO3 and HCl. The measurements of Oxygen Storage Complete Capacity (OSCC) were carried out in an atmospheric quartz fixed-bed reactor placed in a furnace connected to a Porapak column and a TCD detector. The method described elsewhere [START_REF] Duprez | Oxygen storage and mobility on model three-way catalysts[END_REF] consists in a pretreatment of 20 mg of catalyst under He (30 mL/min) from room temperature to 500 °C, with a ramp of 2 °C/min. At this temperature, the sample is completely oxidized by 10 O2 pulses. After that, 10 pulses of CO are injected up to the maximum reduction of the sample and a series of O2 pulses to reoxidize the oxide solid. The consumption of CO and O2 as well as the production of CO2 are determined from the area of corresponding peaks obtained with the TCD detector. The OSCC values are calculated from the CO2 production instead of CO uptake, due the possibility to form carbonate species during the CO pulses. The OSCC is expressed in µmol CO2.g -1 . The same procedure was made twice and a mean value of OSCC was retained.

In order to characterize the adsorbed species on the catalysts during the reaction of dry registered in the range of 1200-4000 cm -1 by collecting 30 scans with a resolution of 4 cm -1 .

RAMAN spectroscopy was used to characterize the carbon depositions post-reaction (see Supporting Information).

Catalytic Test

The DRM tests were carried out in a quartz fix-bed reactor at atmospheric pressure. The catalyst was mixed with SiC to ensure a better thermal control in the reactor (SiC/catalyst = 1.5).

In order to compare the reaction under similar conversion, the mass of catalyst introduced in the reactor was 20 and 30 mg for 10 wt%Ni and 5 wt%Ni, respectively. The pressure of the reactor was controlled online by a pressure sensor (Keller PR21S, 0-5 bar) to verify the absence of pressure drop. The sample was reduced in situ with pure H2 (30 mL/min) from room temperature up to 800 °C for 1 hour, with a ramp of 10 °C/min. After the reduction, the reactor was purged with N2 (30 mL/min). The feed composition was 45 mL/min CH4 and 45 mL/min CO2, without dilution. The outlet gas is analyzed online in 2 analysis zone: the zone 1 consists in a Varian GC equipped with 3 columns (13X Molecular Sieve, Porapak Q and 5A Molecular Sieve) to separate all the gases, and FID and TCD detectors, using He as carrier gas. In this zone CH4, CO2 and CO are analyzed. The zone 2 concerns the hydrogen analysis in a Varian GC equipped with 5A Molecular Sieve column, using argon as carrier gas, and a TCD detector.

The calculations for the quantification of CH4 conversion (XCH4), CO2 conversion (XCO2) and H2/CO molar ratio are described in Eq. 5, Eq. 6 and Eq. 7 respectively where Fx is the molar flow of component x. The molar flow was obtained by an external calibration of each gas, using the total flowrate analyzed online by a flowmeter (Ritter) installed in the outlet gas position.

X CH 4 = (F CH 4 ) in -(F CH 4 ) out (F CH 4 ) in x 100 (5)
X CO 2 = (F CO 2 ) in -(F CO 2 ) out (F CO 2 ) in x 100 (6)
H 2 CO = (F H 2 ) out (F CO ) out (7)
After reaction, the reactor was by-passed to conserve the carbon deposits formed during reaction. Differential Thermogravimetric Analysis (DTA) was performed in a TA SDT-Q600

instrument. The variations in the weight of the post-reaction samples (20mg) were monitored increasing the temperature under mixture of 10 % O2/He (100 mL/min) from room temperature up to 1000 °C.

Results and Discussion

3.1-Catalyst characterization

Table 1 reports The structure of the various samples was characterized by performing XRD and TEM analyses. The XRD at low angle (Fig. S1) does not show any peak characteristic from hexagonal mesoporosity while N2 physisorption isotherms suggested a rather narrow pore size distribution. Therefore, it seems that the porosity of the material is not well organized, as it can be confirmed by TEM of the calcined 10Ni-CeAl sample (Fig. 2). observed for all the catalysts prepared by EISA method or impregnation. This result suggests that ceria nanoparticles are highly dispersed as nanoclusters on the large alumina surface as observed in our previous work [START_REF] Fonseca | Unexpected redox behavior of large surface alumina containing highly dispersed ceria nanoclusters[END_REF].  In order to investigate the reduction behavior of the samples, in situ XRD was performed during reduction with diluted hydrogen for 5Ni-CeAl, 10Ni-CeAl and 10Ni/CeAl samples (Fig. 4). All the XRD patterns show peaks at 2θ = 51.5° and 75.6° related to Kanthal owned to the equipment. For 5Ni-CeAl, the structure remains completely amorphous from 25 and 600 °C. The spinel phase has been commonly used to control the Ni sintering during reduction at high temperature, providing a high metal dispersion on the catalyst [START_REF] Xu | Bound-state Ni speciesa superior form in Ni-based catalyst for CH4/CO2 reforming[END_REF]. Karam et al. [START_REF] Karam | Nanostructured Nickel Aluminate as a Key Intermediate for the Production of Highly Dispersed and Stable Nickel Nanoparticles Supported within Mesoporous Alumina for Dry Reforming of Methane[END_REF] synthetized unsupported NiAl2O4 by EISA method. After reduction at 800 °C, they observed the formation of Ni 0 nanoparticles dispersed over γ-Al2O3 with very low crystallite size in order of 7 nm. The diffractogram of calcined 10Ni/CeAl sample also exhibited the main peak ascribed to CeO2, at 2 = 33.2 (Fig. 4), which is likely due to its segregation after Ni impregnation and calcination since the CeAl sample did not display this feature (Fig. 3b). However, the intensity of this line starts to decrease when the sample is heated at 200 °C and it is no longer observed after reduction at 800 °C. According to the literature [START_REF] Marinho | Embedded Ni nanoparticles in CeZrO2 as stable catalyst for dry reforming of methane[END_REF][START_REF] Da Fonseca | Study of the effect of Gd-doping ceria on the performance of Pt/GdCeO2/Al2O3 catalysts for the dry reforming of methane[END_REF], the reduction of ceria-based materials leads to the shift of the lines characteristic to CeO2 to lower angles due to the conversion of Ce 4+ to Ce 3+ species but the intensities of these lines do not change. Therefore, what is causing the disappearance of the CeO2 lines during reduction?

                Intensity (a.u.) 2 (°) Al 2 O 3 NiAl 2 O 4 Ni/CeO 2 Al 2 O 3 (a) NiAl 2 O 4
Intensity (a.u.) 2 (°) CeAl 5Ni-CeAl 10Ni-CeAl 10Ni/CeAl 10Ni-Al (b)        Al 2 O 3 NiAl 2 O 4
In order to investigate the ceria reduction behavior, a Ni/CeO2 reference sample was analyzed by in situ XRD during reduction (Figure S2). It is noticed a shift of the lines corresponding to CeO2 to lower 2θ values, indicating an increase in the ceria lattice parameter due to the formation of Ce 3+ species. In addition, the intensities of these peaks increase as a result of the growth of ceria crystallite size, However, the lines corresponding to ceria phase are always observed during the reduction process instead of disappearing as it was observed for the 10Ni/CeAl catalyst.

We have previously prepared CexAl samples with different Ce contents (6.0; 11.0; 20.8; 29.3 and 35.3 wt.% Ce) by EISA method [START_REF] Fonseca | Unexpected redox behavior of large surface alumina containing highly dispersed ceria nanoclusters[END_REF]. The diffractograms of the calcined samples did not show the lines corresponding to alumina and the lines typical of CeO2 were only detected for the samples containing 29.3 and 35.3 wt.% Ce. For CexAl containing 6.0; 11.0; 20.8 wt.% of Ce, the CeO2 lines were only detected when the samples were heated under air at 1000 °C. This result indicates that CexAl samples prepared by EISA method contain very small and well dispersed CeO2 nanoclusters. The reduction of the sample with 29.3 wt.% Ce was followed by in situ XRD. No lines were detected on the diffractograms up to 800 °C but the lines characteristic of CeAlO3 appeared only after heating at 1000 °C. TPR and in situ XANES experiments showed that Ce 4+ is completely reduced to Ce 3+ between 400 -500 o C, while a commercial CeO2 samples exhibited a much lower reduction degree (10%). Based on Isotopic Exchange and solid-state NMR experiments, it was proposed a reduction mechanism involving the reaction between the ceria nanoclusters and alumina with formation of perovskite-like CeAlO3 (Eq. 8).

2 CeO2 + H2 + Al2O3  2 CeAlO3 + H2O (8)
Luisetto et al. [START_REF] Luisetto | Ni/CeO2-Al2O3 catalysts for the dry reforming of methane: The effect of CeAlO3 content and nickel crystallite size on catalytic activity and coke resistance[END_REF] also observed the disappearance of the lines characteristic of CeO2 as well as the presence of the line attributed to CeAlO3 after reduction at 1073 K of a NiCeAl sample prepared by the sol-gel method. This result was attributed to the reduction of CeO2 to Ce2O3, which reacted with alumina to form the CeAlO3 phase (Eqs. 9 and 10). Shyu et al. [START_REF] Shyu | Surface characterization of alumina-supported ceria[END_REF] proposed that small crystallite of CeO2 can be transformed into CeAlO3 at reduction temperatures higher than 600 o C, whereas the conversion of large CeO2 particles to CeAlO3 occurs above 800 o C.

In our work, the decrease in the intensity of the line characteristic of CeO2 on the diffractogram of 10Ni/CeAl catalyst during reduction suggests that Ce 3+ species, once they are formed, diffuses into the alumina lattice, forming a CeAlO3 phase that it is not crystalline and, then it is not detected by XRD. For the other catalysts, CeO2 is highly dispersed over alumina and then, their lines are not detected on the diffractograms. However, the formation of CeAlO3

is expected for all catalysts prepared by EISA method.

TEM images of reduced 10Ni-CeAl catalyst (Fig. 5 The reducibility of the samples was studied by TPR and the profiles are shown in Fig. 6.

The sample CeAl presents a broad peak between 400 and 680 °C. According to the literature, Ce-based compounds normally present a first reduction peak at 400 °C, corresponding to the reduction of surface ceria to Ce2O3 while a second peak above 800 °C is attributed to the reduction of Ce 4+ to Ce 3+ in the bulk phase [START_REF] Lafaye | Impact of cerium-based support oxides in catalytic wet air oxidation: Conflicting role of redox and acid-base properties[END_REF][START_REF] Yao | Ceria in automotive exhaust catalysts I. Oxygen storage[END_REF]. In our work, CeAl sample contains only ceria surface species. The 5Ni-CeAl and 10Ni-CeAl catalysts prepared by the one-pot method show two main reduction peaks; a broad one between 250 and 500 °C and an intense peak with a maximum at around 800°C. The 10Ni/CeAl sample, prepared in two steps, presents the same TPR profile, except that the second reduction peak is shifted to higher temperature, with a maximum at 860°C. For these 3 samples, the first reduction region could be attributed to the surface ceria reduction, as observed for the CeAl sample, or to the reduction of bulk NiO species [START_REF] Faria | Hydrogen production through CO2 reforming of methane over Ni/CeZrO2/Al2O3 catalysts[END_REF].

However, 10Ni-Al does not present any peak in this temperature range, which suggests the absence of isolated NiO particles in the Ni-based catalysts whatever the mode of incorporation of Ni, in one step (Ni-CeAl) or in two steps (Ni/CeAl), or the composition of the support (CeAl or Al). Therefore, the broad peak between 250 and 500 °C can be mainly attributed to surface ceria reduction. The second peak around 800 °C observed for all Ni-based catalysts is ascribed in the literature to the reduction of NiAl2O4 spinel phase, where Ni is in strong interaction with the support [START_REF] Zieliński | Morphology of nickel/alumina catalysts[END_REF]. Furthermore, some authors also attributed this TPR peak to the reduction of Ce 4+ to Ce 3+ and the formation of CeAlO3 perovskite phase above 827 °C in the TPR profile [START_REF] Luisetto | Ni/CeO2-Al2O3 catalysts for the dry reforming of methane: The effect of CeAlO3 content and nickel crystallite size on catalytic activity and coke resistance[END_REF]. However, the support CeAl does not presented this peak at high temperature and we can assume that this peak can be correlated only to NiAl2O4 reduction. The same reduction peak was observed by Ma et al. [START_REF] Ma | Combined methane dry reforming and methane partial oxidization for syngas production over high dispersion Ni based mesoporous catalyst[END_REF] in their study about Ni-Al2O3 catalysts prepared by EISA method and calcined at 700 °C, and it was attributed to the reduction of Ni in spinel structure of alumina.

They also confirmed the absence of bulk NiO phase because of the absence of peak at lower temperature in the TPR profile, which was confirmed also by XRD.

It is reported in the literature [START_REF] Molina | α-Alumina-Supported Nickel Catalysts Prepared from Nickel Acetylacetonate: A TPR Study[END_REF] are not completely reduced during the TPR, with a reduction degree around 50%. Our previous work [START_REF] Fonseca | Unexpected redox behavior of large surface alumina containing highly dispersed ceria nanoclusters[END_REF] showed that CexAl materials calcined at low temperature (400 °C), containing

CeAlO3 pseudo phase displayed around 100 % of reduction of ceria below 500°C. Therefore, the calcination at high temperature strengthens the interaction between Ce and Al in the framework, decreasing its reducibility. CO at this temperature. The same assumption will be adopted for the samples 5Ni-CeAl and 10Ni-CeAl, considering that all the O atoms are released from CeO2 structure. The presence of Ce in the structure increases the OSCC value due its reducibility, as observed for the all catalysts as well as for the CeAl support. Furthermore, the addition of Ni in the structure slightly increases the OSCC value. The insertion of Ni into alumina framework led to an increase in the oxygen mobility of oxygen species around Ce atoms, favoring the reduction of Ce 4+ to Ce 3+ . The higher mobility of oxygen in the Ni-based samples prepared by EISA method is in agreement with the shift to lower temperature in the Ce reduction observed during TPR, generating more oxygen vacancies [START_REF] Campbell | Oxygen Vacancies and Catalysis on Ceria Surfaces[END_REF]. During the re-oxidation step it was not formed CO2, which is an indication of the low basicity of these samples, characteristic of CeAl material, as shown in a previous study [START_REF] Fonseca | Unexpected redox behavior of large surface alumina containing highly dispersed ceria nanoclusters[END_REF].

3.2-DRM reaction

The curves of the CH4 and CO2 conversions and H2/CO molar ratio during the DRM reaction are presented in Fig. 7. 10Ni-Al and 5Ni-CeAl catalysts exhibited low initial CH4 and CO2 conversions that significantly increased during the first 2 h of TOS and then they levelled off. For 10Ni-CeAl and 10Ni/CeAl catalysts, the CH4 and CO2 conversions remained constant since the beginning of reaction. However, the test was interrupted after 24 h of TOS for 10Ni/CeAl catalyst due to the significant increase in the pressure drop on the reactor. The H2/CO molar ratio after 72 h of TOS was between 0.70 -0.85 for all catalysts. The H2/CO molar ratio lower than 1.0 and the conversion of CO2 higher than the conversion of CH4 are due to the occurrence of the reverse water gas shift reaction (RWGS) (Eq. 4).

The induction period observed for the 10Ni-Al and 5Ni-CeAl catalysts, and to a lesser extent for 10Ni-CeAl, has been reported in the literature for some Ni-based catalysts with low basicity [START_REF] Takanabe | Influence of reduction temperature on the catalytic behavior of Co/TiO2 catalysts for CH4/CO2 reforming and its relation with titania bulk crystal structure[END_REF]. Faria et al. [START_REF] Faria | Hydrogen production through CO2 reforming of methane over Ni/CeZrO2/Al2O3 catalysts[END_REF] But the partially oxidized Ni particles were reduced again by the syngas produced during the reaction, increasing the conversion of reactants. In our work, it is also possible to observe the low H2/CO at the beginning of the reaction for these catalysts and its increase together with the enhancement in CH4 conversion after few minutes of time on stream.

3.3-Characterization of post-reaction catalysts

The main drawback on the development of catalysts for the dry reforming of methane is the deactivation by carbon formation. Therefore, it is very important to evaluate the carbon formed after the catalytic test. The mechanism of formation and growth of carbon filaments at the metal surface of Ni-based catalysts is well known [START_REF] Rostrup-Nielsen | Mechanisms of carbon formation on nickel-containing catalysts[END_REF]. The source of this carbon is methane and its dehydrogenation on the surface of the metallic particle, which is the first step of the DRM mechanism. Carbon might be oxidized by O atoms produced by the CO2 dissociation at the metal-support interface or it can migrate into the particle structure, leading to the growth of carbon filament. The analysis the deposited on the samples by thermogravimetric analysis after reaction and the corresponding DTG curves are presented in Fig. 8. The 5Ni-CeAl and 10Ni-CeAl catalysts do not present any peak corresponding to the oxidation of carbon species. The 10Ni/CeAl sample displays two peaks centered at 490 and 616 °C while the 10Ni-Al sample has only one centered at 640 °C. According to literature [START_REF] Sánchez-Sánchez | Ethanol steam reforming over Ni/MxOy-Al2O3 (M=Ce, La, Zr and Mg) catalysts: Influence of support on the hydrogen production[END_REF][START_REF] De Lima | Evaluation of the performance of Ni/La2O3 catalyst prepared from LaNiO3 perovskite-type oxides for the production of hydrogen through steam reforming and oxidative steam reforming of ethanol[END_REF], the peak at 490 °C ascribed to amorphous carbon structure and the peaks above 600 °C are correlated to filamentous carbon with single or multiple walls (SWCNT and MWCNT). The characterization by RAMAN spectroscopy of the spent catalyst confirms the formation of MWCNT with low degree of crystallinity with bands at 1350 and 1580 cm -1 assigned to D and G bands respectively (Fig. S3) [START_REF] Rabelo-Neto | CO2 reforming of methane over supported LaNiO3 perovskitetype oxides[END_REF]. The rate of carbon formation for the catalysts during DRM at 800 °C for 72 h of TOS are listed on Table 4. Ni/0Ce-AlO 800 °C, CH4:CO2 = 1:1 3.7 [START_REF] Chen | High carbon-resistance Ni/CeAlO3-Al2O3 catalyst for CH4/CO2 reforming[END_REF] Ni/05CeAlO 800 °C, CH4:CO2 = 1:1 2.8 [START_REF] Chen | High carbon-resistance Ni/CeAlO3-Al2O3 catalyst for CH4/CO2 reforming[END_REF] Ni/10CeAlO 800 °C, CH4:CO2 = 1:1 1.8 [START_REF] Chen | High carbon-resistance Ni/CeAlO3-Al2O3 catalyst for CH4/CO2 reforming[END_REF] Ni/15CeAlO 800 °C, CH4:CO2 = 1:1 1.2 [START_REF] Chen | High carbon-resistance Ni/CeAlO3-Al2O3 catalyst for CH4/CO2 reforming[END_REF] Ni/CeO2 800 °C, CH4:CO2 = 1:1 9.7 [START_REF] Marinho | Embedded Ni nanoparticles in CeZrO2 as stable catalyst for dry reforming of methane[END_REF] Ni@CeO2 800 °C, CH4:CO2 = 1:1 1.6 [START_REF] Marinho | Embedded Ni nanoparticles in CeZrO2 as stable catalyst for dry reforming of methane[END_REF] Ni@CeZrO2 800 °C, CH4:CO2 = 1:1 0.0 [START_REF] Marinho | Embedded Ni nanoparticles in CeZrO2 as stable catalyst for dry reforming of methane[END_REF] LaNiO3 800 °C, CH4:CO2 = 1:1 27.0 [START_REF] Rabelo-Neto | CO2 reforming of methane over supported LaNiO3 perovskitetype oxides[END_REF] LaNiO3/SiCeO2 800 °C, CH4:CO2 = 1:1 0.3 [START_REF] Rabelo-Neto | CO2 reforming of methane over supported LaNiO3 perovskitetype oxides[END_REF] NiCu/Ce0.9Gd0.1O2 800 °C, CH4:CO2 = 1:1 12.2 [START_REF] Bonura | Ceria-gadolinia supported NiCu catalyst: A suitable system for dry reforming of biogas to feed a solid oxide fuel cell (SOFC)[END_REF] Ni/CeO2 800 °C, CH4:CO2 = 1:1 4.8 [START_REF] Luisetto | Dry reforming of methane over Ni supported on doped CeO2: New insight on the role of dopants for CO2 activation[END_REF] Ni/CZ100 700 °C, CH4:CO2 = 1:1 0.4 [START_REF] Kambolis | Ni/CeO2-ZrO2 catalysts for the dry reforming of methane[END_REF] Ni/CZ75 700 °C, CH4:CO2 = 1:1 3.5 [START_REF] Kambolis | Ni/CeO2-ZrO2 catalysts for the dry reforming of methane[END_REF] Ni/CZ44 700 °C, CH4:CO2 = 1:1

Ni/CZ28 700 °C, CH4:CO2 = 1:1 0.7 [START_REF] Kambolis | Ni/CeO2-ZrO2 catalysts for the dry reforming of methane[END_REF] Fig. 9 shows the TEM images of the post-reaction 10Ni-CeAl, 10Ni-Al and 10Ni/CeAl catalysts. No carbon filaments are observed on the TEM images of 10Ni-CeAl catalyst, in agreement with TG analysis. For this sample, the average Ni particle size is around 10 nm, which indicates a slight increase in comparison to the reduced sample (4-5 nm according to the XRD and TEM results). On the contrary, the images of 10Ni-Al and 10Ni/CeAl catalysts after DRM reaction reveals high carbon formation, with the presence of many carbon filaments. Due to the presence of Ni on the extremity of the carbon filaments (Fig. 9-D and 9-F), these catalysts remained quite active during the reaction until blockage of reactor. The Ni particle size is higher than the ones observed for the 10Ni-CeAl catalyst after three days on stream (around 20 nm for both catalysts).

3.4-Reaction mechanism for DRM

With the aim of better understand the mechanism of reaction, in-situ DRIFTS experiments were performed under alternatively flowing CH4, CO2, CH4 and CH4/CO2. Fig. 10 displays the spectra collected for 10Ni-Al catalyst obtained after subtraction of the background spectrum. Under CH4 atmosphere, the spectrum presents a very intense band at 3010 cm -1 assigned to gas-phase CH4. After 10 min of TOS a band at 2060 cm -1 assigned to the stretching vibration mode of linear CO species adsorbed on metallic Ni [START_REF] Kitla | Infrared Studies on Bimetallic Copper/Nickel Catalysts Supported on Zirconia and Ceria/Zirconia[END_REF][START_REF] Németh | Impregnated Ni/ZrO2 and Pt/ZrO2 catalysts in dry reforming of methane: Activity tests in excess methane and mechanistic studies with labeled 13 CO2[END_REF][START_REF] Das | Silica-Ceria sandwiched Ni core-shell catalyst for low temperature dry reforming of biogas: Coke resistance and mechanistic insights[END_REF] appears, corresponding to the oxidation of C produced by the decomposition of CH4 with the oxygen coming from the support. Bands with negative intensities are observed in the 3600-3800 cm -1 region, corresponding to the hydroxyl groups of alumina [START_REF] Ferreira-Aparicio | Mechanistic aspects of the dry reforming of methane over ruthenium catalysts[END_REF][START_REF] Amenomiya | Infrared spectroscopy of C 18 O2 on alumina[END_REF]. It means that there was a consumption of hydroxyl groups concomitant with the formation of CO adsorbed on the metal. Ferreira-Aparicio [START_REF] Ferreira-Aparicio | Mechanistic aspects of the dry reforming of methane over ruthenium catalysts[END_REF] studied the DRM mechanism over Ru/Al2O3 catalysts and they also observed the participation of OH groups from alumina on the oxidation of carbon formed during the CH4 decomposition. The hydroxyl groups diffuse towards the metal particle promoting the oxidation of carbonaceous species and generating oxygen vacancies on the surface. The steps are demonstrated in the equations 11 -14.

Switching the gas to CO2, there is the appearance of a very intense band at 2360 cm -1 due to gas-phase CO2. Furthermore, the adsorbed CO is rapidly desorbed and gives gas-phase CO (double bands centered at 2143 cm -1 ), which disappears during time on stream while bands at 1573 and 1517 cm -1 , assigned to asymmetric νC=O stretching mode of carbonate species (CO3 2-) appears [START_REF] Ferreira-Aparicio | Mechanistic aspects of the dry reforming of methane over ruthenium catalysts[END_REF]. The 3600-3800 cm -1 region is complicated to analyze due the influence of some bands attributed to gas-phase CO2, but there is no regeneration of OH group with time, indicating the difficulty of the support to store O from gas phase and to fill in the oxygen vacancies previously generated. Switching again to CH4, there is CO formation only in gas phase and the bands in OH region, showing the consumption of carbonates species. The release of oxygen to the metal generates oxygen vacancies on the support, which is occupied by the oxygen from carbonate species. The carbonate species acts as O supply in the absence of CO2.

The participation of this species was also observed by O'Connor et al [START_REF] O'connor | An In-situ DRIFTS Study of the Mechanism of the CO2 Reforming of CH4 over a Pt/ZrO2 Catalyst[END_REF]. Finally, in the presence of the two gases, CH4 and CO2, it is possible to observe CH4, CO2 and CO in gas phase. It is important to note the absence of carbonate species and the negative bands in OH region under DRM conditions, indicating a surface with deficiency of lattice oxygen species.

Then, there is a high consumption of oxygen by carbon gasification in this catalyst and the unbalance between this reaction and the methane decomposition explains the high carbon formation observed for this catalyst. The steps of carbon removal mechanism can be described by the equations (11 to 18 the bands corresponding to carbonate species are still present, which was not the case for 10Ni-Al. This result indicates that 10Ni-CeAl catalyst has a higher amount of oxygen species on its surface, in the form of CO3 The DRIFTS spectra of the 10Ni/CeAl catalyst in the presence of CH4 do not show the bands corresponding to adsorbed CO (Fig. 12-A). It has been reported that the CO heat of adsorption is lower on large Ni metal particles than on the small ones [START_REF] Sh | Size and Support Effects for CO Adsorption on Gold Model Catalysts[END_REF]. Consequently, the CO is more easily desorbed than for catalysts prepared by EISA method due its larger Ni crystallite size, as observed by XRD. Under DRM condition (Fig. 12-D), the catalyst exhibits a spectra very similar to the one observed for 10Ni-Al, with no bands attributed to carbonate species. Then, less oxygen species are available on the surface to promote the carbon oxidation.

CO2 (g) + O 2-⇌ 2 CO3 2- (17)
Therefore, the unbalance between carbon gasification and carbon formation rates is responsible for the high formation of carbon over this catalyst during the catalytic test. The in-situ DRIFTS

shows that the addition of Ni and Ce by EISA method improves the amount of oxygen species on the surface of the 10Ni-CeAl catalyst under DRM condition and avoids catalyst deactivation. The mechanism observed in the in-situ DRIFTS experiments is in agreement with the bi-functional mechanism described in the literature by many authors for dry reforming of methane [START_REF] Ferreira-Aparicio | Mechanistic aspects of the dry reforming of methane over ruthenium catalysts[END_REF][START_REF] O'connor | An In-situ DRIFTS Study of the Mechanism of the CO2 Reforming of CH4 over a Pt/ZrO2 Catalyst[END_REF][START_REF] Wei | Isotopic and kinetic assessment of the mechanism of reactions of CH4 with CO2 or H2O to form synthesis gas and carbon on nickel catalysts[END_REF][START_REF] Bitter | Mono and Bifunctional Pathways of CO2/CH4 Reforming over Pt and Rh Based Catalysts[END_REF]. The CH4 adsorption occurs at the surface of metallic Ni particle, producing hydrogen atoms and reactive carbon species. The rate of this reaction is extremely affected by the Ni particle size and the formation of carbon species is favored on large Ni particles [START_REF] Kim | Effect of metal particle size on coking during CO2 reforming of CH4 over Ni-alumina aerogel catalysts[END_REF]. The carbon can be oxidized by the oxygen atoms present at the metal-support interface, producing CO. The oxygen vacancy generated is replenished by the oxygen from the CO2 dissociative adsorption, promoting the mechanism of carbon removal from the metallic surface. Furthermore, CO2 also adsorbs on the basic sites of the support in the form of carbonate species, which also acts like source of oxygen for the regeneration of oxygen vacancies.

However, if the support does not release enough oxygen to the metal particle, the carbon species polymerize to less active carbon, to the grow of carbon filaments. Therefore, it is important to have a balance between the rate of methane decomposition on the metal and the rate of carbon gasification at the metal/support interface.

3.5-Effect of control of Ni particle size and oxygen mobility on catalyst stability

The control of Ni particle size plays an important role on the prevention of carbon deposition and catalyst stability during DRM reaction [START_REF] Rostrup-Nielsen | Mechanisms of carbon formation on nickel-containing catalysts[END_REF][START_REF] Kim | Effect of metal particle size on coking during CO2 reforming of CH4 over Ni-alumina aerogel catalysts[END_REF][START_REF] Padi | Coke-free methane dry reforming over nano-sized NiO-CeO2 solid solution after exsolution[END_REF]. Therefore, different catalyst preparation methods have been proposed in the literature to produce small and stable metal particles [START_REF] Wang | One-pot Synthesis of Ordered Mesoporous NiCeAl Oxide Catalysts and a Study of Their Performance in Methane Dry Reforming[END_REF][START_REF] Padi | Coke-free methane dry reforming over nano-sized NiO-CeO2 solid solution after exsolution[END_REF][START_REF] Huang | Synthesis of a Highly Active and Stable Nickel-Embedded Alumina Catalyst for Methane Dry Reforming: On the Confinement Effects of Alumina Shells for Nickel Nanoparticles[END_REF][START_REF] Liu | Catalysis by Supported Single Metal Atoms[END_REF]. The encapsulated structures, for example core-shell and mesoporous catalysts, have been extensively studied to develop stable catalysts for DRM [START_REF] Bian | Sandwich-Like Silica@Ni@Silica Multicore-Shell Catalyst for the Low-Temperature Dry Reforming of Methane: Confinement Effect Against Carbon Formation[END_REF][START_REF] Das | Silica-Ceria sandwiched Ni core-shell catalyst for low temperature dry reforming of biogas: Coke resistance and mechanistic insights[END_REF]. In this case, the metal is confined into support structure which inhibits metal sintering and suppress under oxidizing and reducing environments, respectively. The CeAlO3 perovskite-type phase was proposed to play a key role to avoid carbon formation during the dry reforming of methane [START_REF] Luisetto | Ni/CeO2-Al2O3 catalysts for the dry reforming of methane: The effect of CeAlO3 content and nickel crystallite size on catalytic activity and coke resistance[END_REF]. Then, CeO2 carries out the oxidation of CHx species formed on the metal surface, restoring and maintaining the redox cycle active. In the present study, the 10Ni-Al catalyst, which does not have any oxygen mobility in its structure, as shown by the OSCC experiments at 500 °C, is not able to prevent carbon deposition. DRIFTS experiments showed that, at high temperature, the hydroxyl groups can oxidize the carbon formed but the adsorption of CO2 is not favored, decreasing the presence of oxygen species at the surface and consequently high carbon formation, as showed in TEM images. The presence of CeO2 promotes oxygen mobility in the catalyst, which is responsible to oxidize the carbon formed on the Ni surface. Therefore, the Ni-based catalysts prepared by EISA method combine the two characteristics necessary for both high activity and resistance to deactivation by carbon formation. Because of the high resistance to coke formation, its application in other reforming process, such as steam reforming of methane, can be interesting in order to maximize the yield for H2 production, with potential to application in pilot plant scale.

  and a good resistance to carbon deposition was observed at 700°C. The synthesis of mesoporous materials with high substitution of Al by Ce and Ni is more complicated and it is the subject of the present study.The aim of this study is the preparation of Ni supported on mesoporous CeO2-Al2O3 catalysts by one pot EISA method with high surface area and high Ni dispersion and accessibility for the production of syngas through DRM at high temperature. Two specific parameters have been deeply investigated: the influence of the confinement effect of the one step EISA method compared with the post-impregnation of the Ni; the role of the Ce introduction in the mixed oxide on the carbon gasification properties. N2 physisorption was performed to study the surface area and porosity. The structure was analyzed by Transmission Electronic Microscopy (TEM) and X-ray diffraction (XRD). In-situ X-ray diffraction (XRD) and Temperature-Programmed Reduction (TPR) were used to follow the reducibility of NiAl2O4. The carbon formed was analyzed by Thermogravimetric analysis (TGA) and TEM.

The

  N2 physisorption technique was used to measure the specific surface area and pore size distribution of each material. The experiment was carried out on TRISTAR 3000 Micromeritics equipment at -196 °C. Approximately 100 mg of catalyst was used for each measurement and the samples were degassed under vacuum overnight at 250 °C. The specific surface was calculated by BET equation and the pore size distribution was calculated by BJH method. The structure and Ni particle size of the catalyst 10Ni-CeAl was evaluated by Transmission Electron Microscopy (TEM) in a Jeol 2100 UHR apparatus, with 0.19 and 0.14 nm punctual and linear resolution, equipped with a LaB6 filament. The range of X-rays emitted from the samples upon electron impact was 0-20 keV. The in situ X-ray diffraction was performed in a Bruker D8 Advance X-ray powder diffractometer operated at 40 kV and 40 mA, using CoKα radiation (λ = 1.790307 Å), equipped with a Position Sensitive Detector, VANTEC-1, kβ filter (Ni), used in scanning mode. The 2θ range analyzed was 10 to 80°, step of 0.05°, step time equal to 2 s. The flow of reduction was composed of 10 % H2/He and the XRD patterns were collected at different temperatures (room temperature, 200 °C, 600 °C, 800 °C and room temperature after reduction). Experiments of temperature-programmed reduction (TPR) were performed in a setup equipped with thermal conductivity detector. The samples (400 mg) were pre-treated in-situ under He at 200 °C for 1 hour and cooled down to room temperature. The TPR experiment was carried out from room temperature up to 1000 °C under a 10 % H2/Ar mixture (30 mL/min) and heating ramp equal to 10 °C/min.

  the chemical composition and textural properties of each sample. The Ni and Ce contents are close to the nominal values. Fig.1(A) shows the nitrogen adsorptiondesorption curves of the samples after calcination. The isotherms are type IV according to the IUPAC classification, characteristic of a mesoporous material, with H1-type hysteresis corresponding to cylindrical pores. Some information on the organization of these pores is given in Fig.1(B), with narrow distribution of pores for all catalysts. The smallest pore size is obtained for 10Ni-Al sample (distribution between 4 and 6 nm) while the highest pore size and distribution are observed for 10Ni-CeAl (between 4 and 10 nm). The comparison between10Ni-Al and 10Ni/CeAl catalysts show that the addition of Ce in the catalyst leads to higher pore size and wider pore distribution. On the contrary, despite of the narrow pore distribution of 10Ni/CeAl, its surface area and pore volume decreased in comparison to CeAl. These result show that the incipient-wetness impregnation of Ni probably causes the blockage of some pores.

Figure 1 :

 1 Figure 1: (A) N2 adsorption-desorption curves and (B) distribution of pore size obtained by

Figure 2 :

 2 Figure 2 : TEM images of the calcined 10Ni-CeAl catalyst.

Figure 3 :

 3 Figure 3: Diffractograms of (a) Al2O3 and references (Ni/CeO2 and NiAl2O4), (b) CeAl, 5Ni-CeAl, 10Ni-CeAl, 10Ni/CeAl and 10Ni-Al calcined samples.

A

  peak at 2θ = 62° characteristic of metallic Ni phase (ICSD 98-004-1508) appears at 800 °C. The Ni crystallite size after reduction at 800 °C was 5.4 nm. For 10Ni-CeAl and 10Ni/CeAl catalysts, the intensity of the lines characteristics of γ-Al2O3 and/or NiAl2O4 decreases and they are shifted to higher 2θ position, when the catalysts are heated under H2 at 800 °C. Furthermore, it is also observed the appearance of the diffraction line attributed to metallic Ni at 2 = 62°. These results suggest the reduction of NiAl2O4 phase, producing metallic Ni and Al2O3. The calculated Ni crystallite size for 10Ni-CeAl catalyst is 4.1 nm. The presence of NiAl2O4 spinel could prevent the sintering of Ni even after reduction at high temperature. The spinel structure is partially transformed into highly dispersed Ni 0 on Al2O3 during reduction. 10Ni/CeAl catalyst has larger Ni crystallite size (11.3 nm) compared to 10Ni-CeAl, which is a clear effect of the preparation method on the control of the growth of Ni crystallite size at high reduction temperature. However, regardless of the mode of introduction of Ni in the CeAl support prepared by EISA method, NiAl2O4 spinel structure is formed upon the calcination process at 800°C.

Figure 4 :

 4 Figure 4: In situ X-ray diffraction patterns of 5Ni-CeAl, 10Ni-CeAl and 10Ni/CeAl during the

  For the sample prepared by the citric acid method, the diffractograms of the calcined and reduced NiCeAl sample did not exhibit the lines characteristic of CeO2 or CeAlO3 phases. This result was also observed in our work for 5Ni-CeAl and 10Ni-CeAl catalysts. .[START_REF] Chen | High carbon-resistance Ni/CeAlO3-Al2O3 catalyst for CH4/CO2 reforming[END_REF] prepared 10 wt.%Ni/xCeO2/Al2O3 catalysts containing different ceria content (x = 0, 5, 10, 15 wt.% CeO2) by impregnation of γ-Al2O3 support with a solution of Ce(NO3)3•6H2O followed by calcination at 450 °C for 2h. Then, the samples were impregnated with aqueous solution of Ni(NO3)2•6H2O and calcined at 500 °C for 5h. in situ XRD showed the decrease in the intensity of the lines characteristic of CeO2 and the appearance of the lines corresponding to metallic Ni when the samples were heated at 600 °C. Increasing the reduction temperature continuously decreased the CeO2 lines, whereas the intensities of the metallic Ni lines increased. At 900 °C, the lines typical of CeO2 completely disappeared and new lines corresponding to CeAlO3 were detected, indicating that CeO2 was reduced to CeAlO3.

  ) show high number of black spots corresponding to Ni metallic formed after reduction. The Ni particles are homogeneously distributed on the support, with particle sizes around 5 nm, in accordance with values obtained by XRD. The high dispersion of Ni results from the reduction of NiAl2O4, as observed from the XRD experiments, which prevents sintering at high temperature.

Figure 5 :

 5 Figure 5: TEM images from the structure of the 10Ni-CeAl catalyst after reduction at 800 °C.

Figure 6 :

 6 Figure 6: TPR profiles of the catalysts and the support CeAl.

  that the formation of NiAl2O4 in the Ni/Al2O3 catalysts may occur when the catalyst is calcined above 600 °C, by the reaction between the Ni precursor and the surface Al2O3 oxides. Scheffer et al.[START_REF] Scheffer | Temperature-programmed reduction of NiOWO3/Al2O3 Hydrodesulphurization catalysts[END_REF] studied the effect of the calcination temperature of Ni/Al2O3 catalysts on the Ni The increase of the calcination temperature from 400 °C to 900 °C shifts the main reduction peak from low temperature, centered at 450 °C, to higher temperature, above 750 °C. The higher calcination temperature favors the diffusion of Ni cations inside the support, making Ni oxide more hardly reducible. Morris et al.[START_REF] Morris | Ordered Mesoporous Alumina-Supported Metal Oxides[END_REF] observed by using27 Al MAS NMR the replacement of tetrahedrally coordinated Al by Ni, forming NiAl2O4 in the samples with 10 wt% of Ni, showing the high solubility of Ni into the Al2O3 framework. They also did not observe the presence of NiO phase in their catalysts. Then, the TPR results of the present study suggest that the EISA method does not favor the formation of bulk NiO species on the support. In contrast the Ni would be incorporated in the Al2O3 framework. Now, if one compares the TPR profiles of the Ni/CeAl and Ni-CeAl samples to the one of CeAl, it can be seen that the peak ascribed to ceria surface reduction shifts to lower temperature when Ni is present. This supposes that Ni 2+ diffused to alumina framework could weaken the interaction between Ce and Al and then ceria can easily reduce in this range of temperature. In order to obtain the reduction degrees of Ce and Ni, the values of H2 consumption and the theoretical values for each reduction are listed on the Table 2. The H2 consumption of Ce and Ni was calculated assuming at first approximation the reduction of CeO2 to Ce2O3 below 600 °C and NiAl2O4 to Ni 0 above 800 °C. The results obtained suggests that CeO2 and NiAl2O4

  investigated the changes in Ni phase for Ni/Al2O3 catalyst by XRD. After exposition to a CO2 stream, the diffractogram reveal the appearance of the lines characteristics of NiO phase and the lines attributed to Ni 0 phase are no longer detected. Rabelo-Neto et al.[START_REF] Rabelo-Neto | CO2 reforming of methane over supported LaNiO3 perovskitetype oxides[END_REF] carried out in situ XPS under dry reforming conditions using LaNiO3 precursor catalyst. They observed a decrease in the intensity of the peak characteristic of metallic Ni at 66.6 eV during the first 120 min of time of stream, which suggests the oxidation of metallic Ni particles. Then, the intensity of this peak increased, indicating that NiO is reduced by the H2 produced in the DRM reaction. These results demonstrated that CO2 can oxidize the surface of Ni particles when the catalysts are initially contacted with the feed, resulting in a loss of activity.

Figure 7 :

 7 Figure 7: CH4 conversion, CO2 conversion and H2/CO molar ratio during the dry reforming

Figure 8 :Table 4 :

 84 Figure 8: DTG curves obtained from the post-reaction samples analyzed by TGA

Figure 9 :

 9 Figure 9: TEM images of the 10Ni-CeAl (A, B), 10Ni-Al (C,D) and 10Ni/CeAl (E, F) post-

CO3 2 - 18 )Figure 10 :

 21810 Figure 10: DRIFTS spectra for 10Ni-Al at 750 °C after; (A) flow of CH4; (B) flow of CO2; (C)

Figure 11 :

 11 Figure 11: DRIFTS spectra for 10Ni-CeAl at 750 °C after; (A) flow of CH4; (B) flow of CO2;

Figure 12 :

 12 Figure 12: DRIFTS spectra for 10Ni/CeAl at 750 °C after; (A) flow of CH4; (B) flow of CO2;

  carbon deposition. Recently, our group have developed embedded-Ni catalyst in CeO2 and CeZrO2 supports which exhibited high resistance against Ni sintering, as well as an enhanced enhanced catalytic stability, due the better dispersion of Ni and the inhibition of the formation of encapsulated carbon species. The high interaction between Ni-O-Ce prevents further Ni sintering at DRM condition. Luisetto et al.[32] studied the influence of Ce addition on the performance of Ni/Al2O3 catalyst prepared by different synthesis methods for DRM reaction. The amount of carbon accumulated on the catalysts after DRM at 1073 K from 1000 to 60 mg of carbon/g of catalyst when the Ni crystallite size decreased from 22.6 to 5.8 nm. The catalyst with the highest Ni dispersion exhibited also the highest amount of Ce 3+ species determined by XPS. According to the authors, the combination of small Ni crystallite size and high amount of CeAlO3 favored the lower carbon formation on the NiCeAl catalyst during DRM reaction. The formation of CeAlO3 was also observed by Chein e Fung [68] with the addition of 5wt% of Ce in Ni/Al2O3 catalyst for DRM reaction. The authors associated the carbon suppression to the role of CeAlO3 in the catalyst activity, promoting the redox pair Ce 4+ -Ce 3+ during carbon removal mechanism. Chen et al.[33] compared the performance of 10 wt.%Ni/xCeO2/Al2O3 catalysts containing different ceria content (x = 0, 5, 10, 15 wt.% CeO2) for the DRM at 800 o C. The rate of carbon deposition on the catalysts after 250 h of TOS varied from 3,68 mg of carbon/g of catalyst/h (Ni/Al2O3) to 1,16 mg of carbon/g of catalyst/h (Ni/15CeO2/Al2O3). TEM images revealed that the Ni particle size was approximately the same for all catalysts reduced below 900 o C (at around 10 nm). Therefore, the authors ruled out the effect of Ni dispersion on the formation of carbon. The authors proposed that CeAlO3 formed during reduction reacts with CO2, producing CeO2 and CO (Eqs. 19 and 20). Prakash et al.[69] also demonstrated the reversible redox transformation between CeAlO3 and CeO2/Al2O3 oxides at 700 °C and 750 °C

Table 1 :

 1 Chemical composition and textural properties of the catalysts.

							Mean Pore
			CeO2		SBET (m 2 .g -	Pore Volume
	Sample	Ni wt%		Ce/(Ce+Al)			Diameter
			wt%		1 )	(cm 3 .g -1 )
							(nm)
	CeAl	---	21.7	0.10	196	0.47	7.1
	5Ni-CeAl	5.1	21.9	0.11	221	0.44	5.8
	10Ni-CeAl	9.4	17.5	0.09	208	0.50	7.3
	10Ni/CeAl	12.6	18.3	0.10	144	0.35	7.1
	10Ni-Al	9.2	---	---	232	0.42	5.2

Table 2 :

 2 H2 consumption, theorical consumption and reduction degree for Ce and Ni from the

	catalysts.
				Theoretical H2	Reduction
		H2 consumption (µmol.g -1 )
	Sample			consumption (µmol.g -1 )	degree (%)
		Below 600 °C Above 800 °C	Ce	Ni	Ce	Ni
	CeAl	370.0	---	630.4*	---	59	---
	5Ni-CeAl	359.4	513.2	636.2*	861.3**	56	60
	10Ni-CeAl	208.7	931.4	508.4*	1571.6**	41	59
	10Ni/CeAl	243.3	1253.4	531.6*	2118.8**	46	59
	10Ni-Al	---	837.8	---	1537.9**	---	54
	* 2 CeO2 + H2  Ce2O3 + H2O	** NiAl2O4 + H2  Ni + Al2O3 + H2O

Results of the OSCC measurements, performed to obtain more information about the redox properties of the catalysts, are presented in the

Table 3 .

 3 CO can be oxidized by the O from the CeO2 phase or NiO phase. The oxygen released from CeO2 is of major importance for the DRM reaction because it can react with the carbon formed during CH4 decomposition, avoiding the catalyst deactivation by carbon deposition. It should be mentioned that during the OSCC measurements the 10Ni-Al sample does not exhibit any CO2 production. It is explained by the absence of NiO phase in this sample and the inability of NiAl2O4 phase to directly oxidize

Table 3 :

 3 OSCC at 500 °C for calcined CeAl, 5Ni-CeAl, 10Ni-CeAl and 10Ni/CeAl samples.

	Catalyst	OSCC (µmol CO2.g -1 )
	CeAl	273
	5Ni-CeAl	348
	10Ni-CeAl	309
	10Ni/CeAl	323
	10Ni-Al	0

  2-and hydroxyl groups. These carbonate species donate oxygen and promotes the carbon removal mechanism. Therefore, the correct balance between methane decomposition and carbon gasification explains the absence of carbon deposits during the DRM over 10Ni-CeAl catalyst.

		0.010
												(A)			0.5
																	(B)
		0.008
															0.4
	Kubelka-Munk	0.004 0.006 0.002	3760	3707	3600	10 min				2053			Kubelka-Munk	0.3 0.2 0.1		10 1	min min	1566	1527
						1 min										2143
		0.000								2143				0.0
		4000		3600		3200	2800	2400	2000	1600	1200
															4000	3600	3200	2800	2400	2000	1600	1200
								Wavenumber (cm -1 )							Wavenumber	(cm -1 )
												(A)
		0.2												0.5
												(C)				(D)
														0.4
	Kubelka-Munk	0.1			10 1	min min						Kubelka-Munk	0.3 0.2	10 min	1568
		0.0												0.1
			3760	3683										0.0	1 min
										2143
																	2143
		4000		3600		3200	2800	2400	2000	1600	1200		4000	3600	3200	2800	2400	2000	1600	1200
							Wavenumber	(cm -1 )							Wavenumber (cm -1 )

Table 4

 4 summarizes the rate of carbon deposition during DRM over different catalysts from the literature. It is clear that the Ni-based catalysts containing ceria prepared by EISA method of our work exhibits lower carbon formation rates than other catalysts from the literature.

	2 CeAlO3 + CO2  Al2O3 + 2CeO2 + CO

oxygen mobility due to the high interaction between Ni and the support, suppressing carbon formation [START_REF] Marinho | Embedded Ni nanoparticles in CeZrO2 as stable catalyst for dry reforming of methane[END_REF].

The core-shell catalysts have been studied for DRM reaction, however its complexity and the use of high-value added reagents for synthesis methods, has stimulated the search for new synthesis procedures simpler to replicate. Xiang et al. [START_REF] Xiang | One-Pot Synthesis of Ordered Mesoporous NiSiAl Oxides for Catalyzing CO2 Reforming of CH4[END_REF] have synthesized NiSiAl samples by EISA method and the catalyst was stable during 100 hours at 700 °C for the DRM reaction with low carbon formation, showing the absence of any sintering process.

In the present work, the Ni particle size effectively affected the carbon deposition rate.

The EISA method allowed to synthetize very well dispersed Ni particles on the high surface area support (5Ni-CeAl and 10Ni-CeAl catalysts). The NiAl2O4 spinel phase precursor, as observed by XRD and TPR, produced small Ni crystallite size during reduction at 800 °C, regardless of the Ni loading, whereas larger particles were formed on the catalysts prepared by the incipient-wetness impregnation method. The smaller Ni crystallite size could control the rate of methane decomposition and avoid carbon formation. However, carbon deposition was still observed on the 10Ni-Al catalyst also prepared by EISA method, which indicates that the presence of ceria is fundamental for catalyst stability [START_REF] Laosiripojana | Synthesis gas production from dry reforming of methane over CeO2 doped Ni/Al2O3: Influence of the doping ceria on the resistance toward carbon formation[END_REF]. The addition of Ce increases oxygen storage capacity and base properties of the support, leading to an enhanced carbon resistance [START_REF] Stroud | Chemical CO2 recycling via dry and bi reforming of methane using Ni-Sn/Al2O3 and Ni-Sn/CeO2-Al2O3 catalysts[END_REF]. In this work, the addition of Ce to prepare the CeAl mixed oxide by EISA method does not contribute to modify the acid-base properties of the support (Fig. S4).

Conclusions

A series of Ni-based catalysts prepared by EISA method was tested in the dry reforming reaction. It was possible to produce materials with high surface area and well defined mesopores. The synthesis conditions favored the formation of NiAl2O4 spinel phase with very well dispersed Ni particles on the support, which inhibits the sintering process at high temperature. The presence of Ce interacting strongly with Al2O3 promotes the oxygen mobility in the material, responsible for the carbon removal mechanism, and enhances the activity of the catalyst. The combination of small Ni crystallite and oxygen mobility could suppress carbon formation due the control of the rates of both CH4 decomposition and carbon gasification. These results indicate that the EISA method is appropriate to produce Ni dispersed catalysts with high surface area and oxygen mobility for application in reforming reactions.

Conflicts of interest

The authors do not have any conflicts to declare.

Acknowledgements

The authors thank CNPq (Conselho Nacional de Desenvolvimento Científico e European Union (ERDF) and Région Nouvelle Aquitaine region are also gratefully acknowledged for their financial support.


A Jouan

G Singh

E Lesne

D C Vaz

M Bibes

A Barthélémy

C Ulysse

D Stornaiuolo

M Salluzzo

S Hurand

J Lesueur

C Feuillet-Palma

N Bergeal

Quantized conductance in

published or not. The documents may come

Electric-field effect control of two-dimensional electron gases (2-DEG) has enabled the exploration of nanoscale electron quantum transport in semiconductors. Beyond these classical materials, transition metal-oxide-based structures have d-electronic states favoring the emergence of novel quantum orders absent in conventional semiconductors. In this context, the LaAlO 3 /SrTiO 3 interface that combines gate-tunable superconductivity and sizeable spin-orbit coupling is emerging as a promising platform to realize topological superconductivity. However, the fabrication of nanodevices in which the electronic properties of this oxide interface can be controlled at the nanoscale by field-effect remains a scientific and technological challenge. Here, we demonstrate the quantization of conductance in a ballistic quantum point contact (QPC), formed by electrostatic confinement of the LaAlO 3 /SrTiO 3 2-DEG with a split-gate. Through finite source-drain voltage, we perform a comprehensive spectroscopic investigation of the 3d energy levels inside the QPC, which can be regarded as a spectrometer able to probe Majorana states in an oxide 2-DEG.

The interplay between superconductivity and spin-orbit coupling (SOC) is at the center of an intensive research effort since this unique combination can promote topological superconductivity, an exotic electronic state with remarkable chiral properties [START_REF] Alicea | New directions in the pursuit of Majorana fermions in solid state systems[END_REF]. In particular, topological superconductors are predicted to be suitable hosts for Majorana zero energy modes, which could be used to encode and manipulate non-local quantum information, opening new perspectives for the realization of "fault tolerant" quantum computation technology [START_REF] Stern | Topological Quantum Computation-From Basic Concepts to First Experiments[END_REF][START_REF] Das Sarma | Majorana zero modes and topological quantum computation[END_REF]. While most of experiments arXiv:1903.12134v1 [cond-mat.mes-hall] 28 Mar 2019 in this field focus on hybrid devices made of a semiconductor nanowires coupled to conventional superconductors [START_REF] Mourik | Signatures of Majorana Fermions in Hybrid Superconductor-Semiconductor Nanowire Devices[END_REF][START_REF] Finck | Anomalous Modulation of a Zero-Bias Peak in a Hybrid Nanowire-Superconductor Device[END_REF][START_REF] Churchill | Superconductornanowire devices from tunneling to the multichannel regime: Zero-bias oscillations and magnetoconductance crossover[END_REF][START_REF] Zhang | Quantized Majorana conductance[END_REF], oxide heterostructures offer a new material-based approach to this question.

Transition metal oxides present a wide variety of electronic properties, from superconductivity to magnetism that, combined in artificial heterostructures with atomically sharp interface, provide a unique opportunity for the generation of novel emergent properties and functionalities [START_REF] Hwang | Emergent phenomena at oxide interfaces[END_REF]. The LaAlO 3 /SrTiO 3 interface, discovered by Ohtomo and Hwang in 2004, is a prime example [START_REF] Ohtomo | A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface[END_REF]. While both materials are wide-gap band insulators, growing a thin layer of LaAlO 3 on top of SrTiO 3 leads to the formation of a high mobility 2-DEG at the interface, whose carrier density can be tuned by means of top-, side-and back-gating [START_REF] Caviglia | Electric field control of the LaAlO3 SrTiO3 interface ground state[END_REF][START_REF] Hurand | Field-effect control of superconductivity and Rashba spin-orbit coupling in top-gated LaAlO3/SrTiO3 devices[END_REF][START_REF] Stornaiuolo | Weak localization and spin-orbit interaction in side-gate field effect devices at the LaAlO3/SrTiO3 interface[END_REF]. Beyond a certain filling threshold, likely related to a Lifshitz transition in the band structure [START_REF] Singh | Competition between electron pairing and phase coherence in superconducting interfaces[END_REF][START_REF] Valentinis | Modulation of the superconducting critical temperature due to quantum confinement at the LaAlO3/SrTiO3 interface[END_REF], the interface exhibits superconductivity with a gate-tunable critical temperature. The breaking of translational symmetry in the interfacial quantum well also generates a spin-orbit coupling of Rashba type, whose strength can be tuned by gate voltage [START_REF] Caviglia | Tunable Rashba Spin-Orbit Interaction at Oxide Interfaces[END_REF][START_REF] Ben Shalom | Tuning Spin-Orbit Coupling and Superconductivity at the SrTiO3/LaAlO3 Interface: A Magnetotransport Study[END_REF][START_REF] Singh | Effect of disorder on superconductivity and Rashba spin-orbit coupling in LaAlO3/SrTiO3 interfaces[END_REF][START_REF] Lesne | Highly efficient and tunable spin-to-charge conversion through Rashba coupling at oxide interfaces[END_REF]. Superconductivity and Rashba spin-orbit coupling, which remarkably coexist in LaAlO 3 /SrTiO 3 , are the two main ingredients required to generate superconducting topological phases [START_REF] Alicea | New directions in the pursuit of Majorana fermions in solid state systems[END_REF][START_REF] Fu | Superconducting Proximity Effect and Majorana Fermions at the Surface of a Topological Insulator[END_REF][START_REF] Stornaiuolo | Signatures of unconventional superconductivity in the LaAlO3/SrTiO3 two-dimensional system[END_REF][START_REF] Kuerten | In-gap states in superconducting LaAlO3/SrTiO3 interfaces observed by tunneling spectroscopy[END_REF]. The LaAlO 3 /SrTiO 3 interface could therefore serve as platform to fabricate devices in which these two properties could be tuned at the relevant scales using a set of nano-gates.

In this article, we report on the fabrication of a QPC at a LaAlO 3 /SrTiO 3 interface, a prototypical mesoscopic device made by the deposition of a metallic split-gate on a 2-DEG [START_REF] Van Wees | Quantized conductance of point contacts in a two-dimensional electron gas[END_REF]. The observation of well-defined plateaus in the conductance of the device, quantized in integer values of 2e 2 /h (where e is the electron charge and h is the Planck constant), indicates ballistic transport involving a limited number of conduction channels which are spin degenerate. By measuring non-linear transport at finite source-drain voltage, we perform a spectroscopy of the 3d-levels and extract the characteristic energies of the confinement potential. Under a magnetic field, we observe the formation of spin-polarized subbands and determine the Landé g-factor, whose value of order of 1 strongly differs from that of the free electrons. The demonstration of quantized conductance in LaAlO 3 /SrTiO 3 QPC is the first step towards the realization of a spectrometer able to provide distinctive signatures of topological superconductivity in oxide 2-DEGs [23,[START_REF] Beenakker | Search for Majorana Fermions in Superconductors[END_REF].

In a QPC, quantization of conductance is observed when the width of the constriction is comparable with the Fermi wavelength λ F = 2πh √ 2mE F [START_REF] Van Wees | Quantized conductance of point contacts in a two-dimensional electron gas[END_REF]. In addition, the length, L, of the channel between the reservoirs must be smaller than or comparable to the elastic mean free path l e for ballistic transport to take place. QPCs are nowadays routinely fabricated in semiconducting heterostructures such as AlGaAs/GaAs ones in which the very low carrier density (∼10 11 cm -2 ) results in large values of λ F ( 50 nm) [START_REF] Rossler | Transport properties of clean quantum point contacts[END_REF]. The extreme cleanliness of these heterostructures also results in a large mean free path that can exceed several micrometers at low temperatures [START_REF] Rossler | Transport properties of clean quantum point contacts[END_REF]. In contrast, the 2-DEG at the LaAlO 3 /SrTiO 3 interface involves a higher carrier density (∼10 13 cm -2 ) and has a reduced l e (∼ 10-50 nm) imposing stronger constraints on the practical realization of such devices. In this system, the conduction band is formed by coupling of the t 2g orbitals (d xy , d xz and d yz orbitals) at neighbouring Ti lattice sites through the 2p orbitals of the oxygen atoms. Under strong quantum confinement in the direction perpendicular to the interface, the degeneracy of the t 2g bands is lifted, resulting in a rich and complex band structure with discrete 2D states separated by typical energies of tens of meV [START_REF] Salluzzo | Orbital Reconstruction and the Two-Dimensional Electron Gas at the LaAlO3/SrTiO3 Interface[END_REF][START_REF] Berner | Direct k-Space Mapping of the Electronic Structure in an Oxide-Oxide Interface[END_REF]. Whilst multiband transport occurs at high carrier doping, in the depleted regime, only the lowest d xy energy bands are filled. Figure 1 shows a schematic view of the QPC which is formed by applying a negative voltage on the split-gate to electrostatically deplete the 2-DEG underneath. We used the LaAlO 2) by taking a ratio ωy/ωx 2.5.

the two fingers at the centre of the split-gate is W = 25 nm, which is comparable to the Fermi wavelength of the 2-DEG. Near the bottleneck of the constriction, the split-gate imposes a smoothly varying confining potential that can be modeled by a saddle form (Fig. 1d) [START_REF] Buttiker | Quantized transmission of a saddle-point constriction[END_REF] V (x, y)

= V 0 - 1 2 mω 2 x x 2 + 1 2 mω 2 y y 2 (1)
where V 0 is the potential at the centre of the QPC, m is the electron mass and the frequencies ω x and ω y define the curvatures of the potential in the two directions. Because the potential along the y direction is that of an harmonic oscillator, the lowest energy band of the 2-DEG is split into several 1D subbands. The number of conduction modes involved in the transport, which is related to the width of the constriction, is controlled by the split-gate voltage V SG . For a given value of V SG , channels corresponding to subbands of energies E n = V 0 + (n + 1/2)hω y , which are smaller than E F , are open and participate to the conduction through the constriction. In the framework of the Landauer-Buttiker formalism, the contribution of each channel to the conductance of the QPC is given by its transmission T n [START_REF] Buttiker | Quantized transmission of a saddle-point constriction[END_REF]:

T n (E) = 2 1 + e -π n (2)
where n = 2(E -E n )/hω x . The total conductance is the sum of the conductance of each of the channels, G = n T n G 0 , where G 0 = 2e 2 h is the quantum of conductance for spin degenerate subbands.

After fabrication, the sample was anchored to the mixing chamber of a dilution refrigerator and connected to coaxial lines that include low pass LC filters and copper powder filters at the lowest temperature stage. The differential conductance, G(V sd )=dI/dV sd , was measured with a small ac excitation current using standard lock-in techniques in a four-terminal configuration. After cooling the sample, the back-gate voltage was first swept to a maximum value (V BG = 15 V) to ensure that no hysteresis would take place upon further gating [START_REF] Biscaras | Limit of the electrostatic doping in two-dimensional electron gases of LaXO3(X = Al, Ti)/SrTiO3[END_REF]. During this forming step, the 2-DEG and the split gate are both kept at the ground (V SG = 0 V). The back-gate voltage was then reduced to 8.1 V to strongly deplete the 2-DEG, and the differential conductance, G, of the nanodevice was measured as a function of the dc source-drain voltage V sd and split-gate voltage V SG . For all the data presented in the following, the resistance of the reservoirs measured with the channel fully open (V SG = 0.5 V) was subtracted to obtain the intrinsic conductance of the QPC. Figure 2 shows the evolution of the conductance at V sd = 0 V for four different temperatures. At V SG = -0.2 V the QPC is pinched off. Plateaus corresponding to the quantized values of the conductance in steps of G 0 = 2e 2 /h appear when the split-gate voltage is increased, which indicates that ballistic transport involving spin-degenerate bands is taking place in the QPC. As seen from the fit in Fig. 2 inset, this behaviour is well described by the Landauer-Buttiker formalism (Eq. ( 2)) [START_REF] Buttiker | Quantized transmission of a saddle-point constriction[END_REF]. Oscillations of the conductance can be observed on the plateaus, which are likely related to interference and resonance effects associated with the geometry of the constriction [START_REF] Kirczenow | Theory of the conductance of ballistic quantum channels[END_REF]. An increase in temperature modifies the Fermi distribution and consequently the occupation of electronic states at the Fermi energy. This leads to a thermal broadening of the conductance steps between plateaus and, in addition, to a suppression of oscillations with temperature. The maximum number of conduction modes is related to the Fermi wavelength, n max 2W λ F . As seen in Figure 2, a maximum of three plateaus can be identified in this gate range, corresponding to λ F 15 nm. A clear quantization of the conductance, such as that observed here, only occurs when ω y > ω x . This is confirmed by the fit shown in Fig. 2 inset, which provides a ratio of ω y /ω x 2.5. Previous studies have reported the fabrication of split-gate devices in LaAlO 3 /SrTiO 3 but without achieving the quantization regime [START_REF] Thierschmann | Transport regimes of a split gate superconducting quantum point contact in the two-dimensional LaAlO3/SrTiO3 superfluid[END_REF].

The value of ω y characterizing the harmonic confinement potential can be determined experimentally by measuring the conductance at non-zero source-drain voltage [START_REF] Kouwenhoven | Nonlinear conductance of quantum point contacts[END_REF]33]. In Fig. 3a, we plot a series of conductance curves, G(V sd ) recorded for sequential steps in the split-gate voltage [33].

Large nonlinearities are observed for applied biases comparable to the subband energy spacing. For

V sd = 0 V, the chemical potentials in the source and drain, µ s and µ d respectively, are aligned and we observe an accumulation of curves corresponding to the plateaus at integer values of G 0 (e. g. points "A" and "B" in Fig. 3a). At finite source-drain voltage, the chemical potentials of the left and right reservoirs are shifted by an amount eV sd . When this energy exceeds the splitting between two subbands (eV sd > hω y ), the number of transport modes available for left-going and right-going electrons differs by one, which produces additional half plateaus at conductance values of (n+1/2)G 0 [START_REF] Kouwenhoven | Nonlinear conductance of quantum point contacts[END_REF][33][34]. This is seen at points labelled C and D in Fig. 3a, further confirming the quantization of the conductance. Although these additional plateaus are predicted to be midway between those given by Eq. ( 2), some deviations of the conductance from half-integer values are also expected in particular for small numbers of channels [34]. An asymmetry is observed in V sd which is likely due to an intrinsic asymmetry in the electrostatic potential defining the constriction. Non-linear transport can also be analysed by plotting the transconductance of the QPC, dG/dV SG (V sd ), corresponding to the derivative of G with respect to the split-gate voltage (Fig. 3b). In contrast to the series of conductance curves of Fig. 3a, which highlight the conductance plateaus at multiple values of G 0 , the transconductance emphasizes the transitions between the plateaus. A set of regularly spaced points (in yellow), indicating a maximum in dG/dV SG , are connected by lines of high transconductance to form a diamond-like structure centered on V sd = 0 V (see dashed line). Each diamond represents a well defined quantized plateau where the chemical potentials of the two reservoirs lie in the same subband. On the other hand, regions outside the diamonds describe the situation where µ s and µ d are aligned with different bands leading to conductance plateaus at (n + 1/2)G 0 . The crossing points (yellow) at V sd 0 V corresponds to split-gate voltages at which the bottom of a new subband is aligned with the source and drain chemical potentials (e.g. point "E"). The separation between two consecutive energy levels can be directly measured through the source drain voltage at the edges of the diamonds. At point "H" in Fig. 3b, we read hω y = eV sd ("H") 0.65 meV, which is consistent with the splitting between the d xy subbands due to lateral confinement in the QPC. Considering the ratio ω y /ω x 2.5 previously extracted from the fit in Fig. 2 inset, we obtain hω x 0.26 meV, which parametrizes the confinement potential of Eq. ( 1).

V SG (V) G 0 2G 0 3G 0 1/2G 0 1/2G 0 0G 0 5/2G 0 3/2G 0 3/2G 0 E k x µ L µ R E k x µ L µ R B A E k x µ L µ R D E k x µ L µ R E k x µ L µ R E G E k x µ L µ R E k x µ L µ R E k x µ L µ R F H E k x µ L µ R C -0.5 0 0.5 V sd (mV)
We now investigate the effect of a magnetic field B perpendicular to the 2-DEG plane, on the energy levels of the QPC. As seen in Figure 4a, the plateaus of conductance in the G(V SG ) curve are now quantized in half-integer values of G 0 , which indicates that the spin degeneracy is lifted.

Consequently, the diamond-like structure is duplicated in the transconductance map of Fig. 4c.

The Zeeman splitting between spin-polarized subbands can be directly measured by following the evolution of transconductance at V sd = 0 V as a function of the magnetic filed (Fig. 4a inset). The two spin bands are separated by an energy ∆E z = g * µ B B, where µ B is the Bohr magneton and g * is the Landé g-factor, which describes the susceptibility of the electronic spin states to the external field, and which fundamentally governs spin-transport characteristics, e.g. spin coherence [START_REF] Salis | Electrical control of spin coherence in semiconductor nanostructures[END_REF]. Note that because of the reduced electron mobility in this depleted regime (µ 100 cm 2 V -1 s -1 ), the magnetic field used in this study is too weak to generate Landau levels and form hybrid magneto-electric subbands [START_REF] Annadi | Quantized Ballistic Transport of Electrons and Electron Pairs in LaAlO3/SrTiO3 Nanowires[END_REF].

To access ∆E z and calculate the g-factor, we measure δV SG at 6 T (Fig. 4a inset) and convert it into an energy scale through the measurement of the splitting generated by the source drain bias dV SG dV sd as reported in Fig. 3b [START_REF] Martin | Enhanced Zeeman splitting in Ga0.25In0.75As quantum point contacts[END_REF] .

|g| = 1 µ B δ(∆E z ) δB = e µ B δV sd δV SG δV SG δB (3)
For the n = 2 conductance step we find g * = 0.85 ± 0.1. An estimate of the g-factor can also be obtained by fitting the conductance quantization curve of Fig. 4a, provided a Zeeman splitting term is introduced into Eq. ( 2). With this method, we obtain value g * = 0.9 ± 0.1, similar to that determined by the first method. It is consistent with values obtained by non-direct methods reported in the literature. For instance, in the low-doping regime, a g-factor between 0.5 and 1 was extracted from the analysis of the magneto-conductance in a weak-localization approach [START_REF] Caviglia | Tunable Rashba Spin-Orbit Interaction at Oxide Interfaces[END_REF]. A g-factor of 0.95, close to our value, was also used to reproduce the experimental temperature dependence of ARPES spectrum [START_REF] Cancellieri | Polaronic metal state at the LaAlO3/SrTiO3 interface[END_REF]. The reduction of the g-factor with respect to the free electrons one is not currently understood in at LaAlO 3 /SrTiO 3 interfaces but has often be attributed to the effect of spin-orbit coupling. Its low value could explain the high-value of the parallel superconducting critical magnetic field which has been found to exceed the Pauli limit if considering a conventional value of g 2 [START_REF] Herranz | Engineering two-dimensional superconductivity and Rashba spin-orbit coupling in LaAlO3/SrTiO3 quantum wells by selective orbital occupancy[END_REF][START_REF] Kim | Intrinsic spin-orbit coupling in superconducting δ-doped SrTiO3 heterostructures[END_REF].

In summary, we have demonstrated the realization of a QPC in the 2-DEG confined at the LaAlO 3 /SrTiO 3 interface, which exhibits a clear conductance quantization due to ballistic transport in a restricted number of conducting channels. The characteristics energy scales of the confinement potential extracted from non-linear transport are consistent with a d xy band transport. The measurement of the g-factor under a magnetic field performed on the n = 2 conductance step provides a value in the range 0.85-0.9 for the g-factor, which differs from that of free electrons. The conductance curve with plateaus at multiples of e 2 /h shown in Fig. 4a, is characteristic of the normal state of a QPC under a magnetic field. The demonstration of quantized conductance in oxide QPC is a first step towards experiments to study topological superconductivity in oxide 2-DEGs. Wimmer

3 FIG. 1 :

 31 FIG. 1: Schematic description of the Quantum Point Contact device. a) Hall Bar designed by the amorphous LaAlO3 technique and covered with a split-gate. b) SEM image of the QPC showing the central part of the split-gate (false color). The separation between the two sides of the split-gate is 25 nm, c) Scheme of the 3dxy orbitals localised on Ti ions, which form the lowest energy conducting band in the 2-DEG. d) Representation of the confining potential at the center of the split-gate described by Eq. (1). Within the harmonic approximation, the lowest 3dxy band is split into several sub-levels (n = 1,2,3..) separated by an energy hωy.

FIG. 2 :

 2 FIG. 2: Ballistic transport and conductance quantization. Differential conductance of the QPC as a function of splitgate voltage measured at V sd = 0 for different temperatures and a fixed back-gate voltage VBG = 8.1 V. Plateaus appear at integer value of G0 characteristic of ballistic transport involving a reduced number of conduction modes. Inset) Fit of the G(VSG) curve at T = 100 mK (dashed line) obtained with Eq. (2) by taking a ratio ωy/ωx 2.5.

FIG. 3 :

 3 FIG. 3: Spectroscopy of the QPC subbands a) Differential conductance of the QPC as a function of source-drain voltage for many values of the split-gate voltage in the range [-0.2 V, 0.1 V] and separated by an interval of 3 mV. Energy diagrams indicate the position of the chemical potentials in the left and right reservoirs corresponding to the different points labelled A,B,C and D. Integer (e. g. points A and B) and half-integer (e.g. points C and D) quantized conductance plateaus correspond to dark regions where curves accumulate. b) Transconductance dG/dVSG of the QPC (color scale) as a function of sourcedrain voltage and split-gate voltage. Dashed lines (guide for the eyes) highlight a diamond-like structure formed by joining light-colored points corresponding to maximum values of dG/dVSG. Diamonds centered around V sd =0 V represent integer conductance plateaus whereas external half-diamonds represent half-integer conductance plateaus. Energy diagrams show the position of the chemical potentials in the left and right reservoirs corresponding to the points labelled E, F, G, H and I on the color plot.

3 FIG. 4 :

 34 FIG. 4: Zeeman splitting and spin-polarized subbands under magnetic magnetic field. a) Conductance of the QPC at V sd = 0 V as a function of split-gate voltage VSG for B = 6 T (red line) fitted by Eq. (2) with an additional Zeeman term (dashed line). Inset) Transconductance dG/dVSG of the n = 2 step at V sd = 0 V as a function of split-gate voltage and magnetic field. b) Prototype device for the detection of Majorana states in LaAlO3/SrTiO3 interfaces with a QPC. An additional topgate is used to create, close to the QPC, a junction between a superconducting region and a normal one with strong spin-orbit interaction. Inset) Scheme for detection of Majorana states (taken from [23]). c) Transconductance dG/dVSG of the QPC (color scale) under a B= 6T magnetic field as a function of source-drain voltage and split-gate voltage. The diamond-like structure formed by joining light-colored points corresponding to maximum values of dG/dVSG. Diamonds centered around V sd =0 V represent conductance plateaus in half integer values of G0.

Acknowledgments

The authors acknowledge M. Aprili, A. Caviglia, A. Akhmerov, R. Citro, M. Grilli, S. Caprara and L. Benfatto for stimulating discussions. This work was supported by the French RENATECH network (French national nanofabrication platform), the Région Ile-de-France in the framework of CNano IdF, OXYMORE and Sesame programs, by CNRS through a PICS program and by the ANR JCJC (Nano-SO2DEG). The Authors acknowledge received funding from the project Quantox of QuantERA ERA-NET Cofund in Quantum Technologies (Grant Agreement N. 731473) implemented within the European Union's Horizon 2020 Program. The authors also acknowledge the COST project Nanoscale coherent hybrid devices for superconducting quantum technologies-Action CA16218.