Aimery Auxéméry

Brigitte Botello Frias

Ekaterina Smal

Katarzyna Dziadek

Gilles Philippot

Piotr Legutko

Michail Simonov

Sébastien Thomas

Andrzej Adamski

Vladislav Sadykov

Aimery Auxemery

Sebastien Thomas

Ksenia Parkhomenko

Anne-Cécile Roger
email: annececile.roger@unistra.fr

Cyril Aymonier
email: cyril.aymonier@icmcb.cnrs.fr

Continuous supercritical solvothermal preparation of nanostructured ceria-zirconia as supports for dry methane reforming catalysts

Keywords: Dry reforming of methane, carbon dioxide valorization, nanocatalysts, nickel, mixed oxides, ceria-zirconia, supercritical solvothermal synthesis, flow synthesis

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

Introduction

For more than 20 years ceria and zirconia have remained of vital scientific and industrial interests in many domains. Optimizing ceria preparation is easy because it can exist thermodynamically only in the regular cubic structure with a Fm-3m space group, while zirconia is a much more complex material to be mastered as it exhibits quite rich polymorphism, including the existence of monoclinic, tetragonal and cubic phases. Indeed, stabilization of pure zirconia is not only temperature and pressure dependent, as reported in many works [START_REF] Block | Pressure-Temperature Phase Diagram of Zirconia[END_REF][START_REF] Kisi | Crystal Structures of Zirconia Phases and their Inter-Relation[END_REF], but is also impurity and grain-size dependent [START_REF] Dippel | Towards atomistic understanding of polymorphism in the solvothermal synthesis of ZrO2 nanoparticles[END_REF][START_REF] Shukla | Mechanisms of room temperature metastable tetragonal phase stabilisation in zirconia[END_REF][START_REF] Becker | Critical Size of Crystalline ZrO2 Nanoparticles Synthesized in Near-and Supercritical Water and Supercritical Isopropyl Alcohol[END_REF]. The high oxygen storage capacity (OSC) of ceria [START_REF] Priya | Optimization of Ceria-Zirconia Solid Solution based on OSC Measurement by Cyclic Heating Process[END_REF][START_REF] Kim | Characteristics in oxygen storage capacity of ceriazirconia mixed oxides prepared by continuous hydrothermal synthesis in supercritical water[END_REF] allows many interesting applications such as: CO2 capture/release processes [START_REF] Slostowski | CeO2 nanopowders as solid sorbents for efficient CO2 capture/release processes[END_REF], three-way catalysis (TWC) [START_REF] Ozawa | Core-shell type ceria zirconia support for platinum and rhodium three way catalysts[END_REF][START_REF] Kašpar | Use of CeO2-based oxides in the three-way catalysis[END_REF] etc. However, when used as a catalyst, ceria faces a high risk of sintering at high operating temperatures [START_REF] Priya | Optimization of Ceria-Zirconia Solid Solution based on OSC Measurement by Cyclic Heating Process[END_REF] and the incorporation of doping elements such as zirconium has proven to be helpful to increase bulk oxygen mobility and to prevent ceria from sintering at temperatures around 700-750 °C [START_REF] Crozier | In situ environmental TEM studies of dynamic changes in cerium-based oxides nanoparticles during redox processes[END_REF]. Linking improvement of ceria and zirconia properties to environmentally friendly syntheses techniques is proposed in this work by using the continuous supercritical synthesis as an alternative to conventional methods [START_REF] Tsang | Anticipatory life-cycle assessment of supercritical fluid synthesis of barium strontium titanate nanoparticles[END_REF][START_REF] Tsang | Supercritical fluid flow synthesis to support sustainable production of engineered nanomaterials: case study of titanium dioxide nanoparticles[END_REF].

Among others, the main solvents used to produce continuously materials in supercritical fluid are carbon dioxide, water, alcohols or mixtures of them [START_REF] Aymonier | Playing with solvents in supercritical conditions and the associated technologies for advanced materials by design[END_REF]. The uniform conditions rapidly attained within the supercritical solvent combined to the high supersaturation conditions lead to the formation of highly crystallized nanosized oxide particles, exhibiting a very narrow grain size distribution [START_REF] Aymonier | Playing with solvents in supercritical conditions and the associated technologies for advanced materials by design[END_REF]. Hence, this technique has been extensively used to successfully synthesize various metal oxides with fine control over chemical and phase compositions, size, morphology and structure [START_REF] Dippel | Towards atomistic understanding of polymorphism in the solvothermal synthesis of ZrO2 nanoparticles[END_REF][START_REF] Slostowski | CeO2 Nanocrystals from Supercritical Alcohols: New Opportunities for Versatile Functionalizations?[END_REF][START_REF] Philippot | Coupling in situ synchrotron radiation with ex situ spectroscopy characterizations to study the formation of Ba1-xSrxTiO3 nanoparticles in supercritical fluids[END_REF][START_REF] Taguchi | Growth Mechanism and Surface Chemical Characteristics of Dicarboxylic Acid-Modified CeO2 Nanocrystals Produced in Supercritical Water: Tailor-Made Water-Soluble CeO2 Nanocrystals[END_REF][START_REF] Philippot | Continuous BaTi1-yZryO3 (0 ≤ y ≤ 1) nanocrystals synthesis in supercritical fluids for nanostructured lead-free ferroelectric ceramics[END_REF][START_REF] Hayashi | Hydrothermal Synthesis of Metal Oxide Nanoparticles in Supercritical Water[END_REF][START_REF] Wang | Versatility of one-pot, single-step synthetic approach for spherical porous (metal) oxide nanoparticles using supercritical alcohols[END_REF].

Improvement of catalytic properties of both ZrO2 and CeO2 systems through cationic substitution, respectively with Ce n+ (n = 3, 4) and Zr 4+ cations, has been largely studied at intermediate temperatures by many research groups in both supercritical water and alcoholic solutions [START_REF] Dippel | Towards atomistic understanding of polymorphism in the solvothermal synthesis of ZrO2 nanoparticles[END_REF][START_REF] Becker | Critical Size of Crystalline ZrO2 Nanoparticles Synthesized in Near-and Supercritical Water and Supercritical Isopropyl Alcohol[END_REF][START_REF] Taguchi | Growth Mechanism and Surface Chemical Characteristics of Dicarboxylic Acid-Modified CeO2 Nanocrystals Produced in Supercritical Water: Tailor-Made Water-Soluble CeO2 Nanocrystals[END_REF][START_REF] Hayashi | Hydrothermal Synthesis of Metal Oxide Nanoparticles in Supercritical Water[END_REF][START_REF] Wang | Versatility of one-pot, single-step synthetic approach for spherical porous (metal) oxide nanoparticles using supercritical alcohols[END_REF][START_REF] Slostowski | Synthesis of cerium oxide-based nanostructures in near-and supercritical fluids[END_REF][START_REF] Giroire | Tuning surface grafting density of CeO2 nanocrystals with near-and supercritical solvent characteristics[END_REF][START_REF] Slostowski | Near-and Supercritical Alcohols as Solvents and Surface Modifiers for the Continuous Synthesis of Cerium Oxide Nanoparticles[END_REF].

In this work, we introduce the advantages of combining alkoxide and nitrate precursors in a supercritical ethanol/water mixture for the supercritical sol gel like synthesis of the CeO2-ZrO2 system (CexZr1-xO2; 0 ≤ x ≤ 0.75) towards dedicated catalytic purposes. The choice of a 0.29:0.71 ethanol to water molar ratio and 400 °C and 25 MPa as operating conditions is based on previous research works on the synthesis of highly crystallized and narrow sized metal oxide nanoparticles with fine control over structure and morphology [START_REF] Reveron | One-pot synthesis of well-crystallized and pure barium titanate nanoparticles in supercriticals fluids[END_REF]. The nanostructured CexZr1-xO2 systems were subsequently used as supports for nickel to obtain catalysts for dry reforming of methane (DRM) reactions for the comparative study of our synthesized nanostructured supports and a reference CexZr1-xO2 (x = 0.50) system prepared via conventional methods.

In DRM reactions, the critical parameters are large coking of the catalysts causing a rapid loss of activity, and sintering of the active metal at the high reaction temperatures needed for complete CO2 and CH4 conversions. To overcome the coking formation drawback, different options are possible. Addition of oxygen in the feed to perform the oxidative DRM reduces drastically the coking of the catalyst but in the same time causes the deactivation due to the reoxidation of the active metals in the catalyst [START_REF] Shen | Ni/MgAl2O4 catalyst for low-temperature oxidative dry methane reforming with CO2[END_REF]; very fine reaction parameters control is necessary as well as the kinetic analysis is recommended for the process optimization and reactor design. The use of supports with high oxygen storage capacity is another option to overcome the deactivation of catalysts during DRM. Modified fluorite-like supports, doublelayered hydroxides, hydrotalcites, nanocatalysts and especially mixed oxides as CexZr1-xO2 materials are believed to be promising supports for good Ni dispersion and coke control in DRM [START_REF] Świrk | Yttrium promoted Ni-based double-layered hydroxides for dry methane reforming[END_REF][START_REF] Debek | Ceria Promotion over Ni-Containing Hydrotalcite-Derived Catalysts for CO2 Methane Reforming[END_REF].

For this study the metallic nickel was supported via wet impregnation on the calcined nanostructured CexZr1-xO2 (0 ≤ x ≤ 0.75) supports prepared by continuous supercritical solvothermal method. The catalysts were fully characterized prior and post DRM tests and results were compared to those obtained on the reference Ce0.50Zr0.50O2 catalyst support (provided by Solvay) prepared via a conventional coprecipitation method.

Material and methods

Preparation of catalysts

Precursors

CexZr1-xO2 (x = 0.00; 0.25; 0.50; 0.75) nanostructured supports were prepared with the supercritical fluid method starting from the following precursors: ammonium cerium (IV) nitrate hexahydrate (99.99% Sigma-Aldrich) Ce(NH4)2(NO3)6.6H2O and zirconium propoxide solution, 70 wt.% in 1-propanol (Sigma-Aldrich) Zr(OCH2CH2CH3)4. Precursors were used as received and were diluted in absolute ethanol (reagent grade, AtlanticLabo). Ni(NO3)2.6H2O (Sigma-Aldrich, ≥98.5%) was used as precursor of nickel active phase.

Supercritical fluid synthesis system and protocol

In order to achieve a total cation concentration of 10 mM for each sample of the CexZr1-xO2 series (x = 0.00; 0.25; 0.50; 0.75), proper amounts of precursors were dissolved in absolute ethanol inside a glovebox under controlled argon atmosphere. Subsequently, the precursor solution was stirred until full dissolution of precursors and placed outside the glove box. A 2 mL min -1 argon flow was fed through the solution to avoid exposure to air during the syntheses. All synthetic cycles were performed at T = 400 °C and p = 25 MPa. In these operating conditions, the ethanol / water mixture with a 0.29/0.71 molar ratio was well above its critical coordinates (Tc = 283.55 °C, pc = 11.69 MPa, NIST Standard Reference Database). The experimental custom-built, flow reactor used for the supercritical synthesis of the CexZr1-xO2 nanostructured support series is shown in Figure 1. The stainless-steel reactor used was equipped with two injection lines. The first injection line was used to feed the reactor with the precursor solution dissolved in absolute ethanol while the second injection line was used to flow preheated de-ionized water at 225 °C. At the mixing point between the two injection lines, the temperature was set to 150 °C prior entering the reactor (p= 25 MPa, T = 400 °C) where the crystallization of nanoparticles occurred under supercritical conditions. The residence time inside the reactor was 30 seconds.

Right after the reactor is located a cold zone to quench the nanoparticle growth. The nanoparticle solution is then recovered in suspension after the pressure regulating valve at ambient conditions then centrifuged at 10000 rpm for 20 minutes. The CeO2-ZrO2 nanoparticles are then separated from the supernatant and left to dry at ambient conditions. Fresh CexZr1-xO2 samples will be hereafter referred to as ZrO2, Ce0.25Zr0.75O2, Ce0.50Zr0.50O2 and Ce0.75Zr0.25O2 corresponding to x = 0.00; 0.25; 0.50 and 0.75, respectively (Table 1).

Fresh samples were calcined at 800 °C for 6 h in air with a heating rate of 4 °C min -1 . Calcined samples will be referred hereafter to as ZrO2-800, Ce0.25Zr0.75O2-800, Ce0.50Zr0.50O2-800 and Ce0.75Zr0.25O2-800 (Table 1).

A nickel loading of 10 wt% (theoretical) was selected. Nickel was loaded on all the calcined CexZr1-xO2 support samples using classical wet impregnation method. The proper amount of Ni(NO3)2. 6H2O salt was dissolved in a small quantity of distilled water, and then 0.15-0.20 g of CexZr1-xO2-800 powder was added. The mixture was stirred for 1 h, afterwards, water was partly evaporated at 60-70 °C, then dried completely in the oven during 8 h at 100 °C. Finally, the materials were calcined at 800 °C for 6 h in air with a heating rate of 4 °C min -1 . Nickelcontaining samples will be referred to as Ni/ZrO2, Ni/Ce0.25Zr0.75O2, Ni/Ce0.50Zr0.50O2 and Ni/Ce0.75Zr0.25O2 (Table 1).

Additionally, one Ce0.50Zr0.50O2 support from Solvay, referred as Ce0.50Zr0.50O2-ind, was chosen as reference to be compared with the synthesized nanostructured Ce0.50Zr0.50O2-800 material. This sample was prepared by a traditional coprecipitation synthesis and was chosen for comparison of the supercritical solvothermal preparation and non-supercritical solvothermal preparation on the example of one composition. The reference sample underwent the same thermal treatment as the other samples described abovecalcination step, followed by wet impregnation with nickel precursor and final calcination at 800 °C for 6 h in air with a heating rate of 4 °C min -1 . Nanocrystals parameter refinements were performed using the FullProf_Suite © software with the Thompson-Cox-Hastings model to describe the peak profiles. In the purpose of this study, refined parameters were the scale factor for each different phase, the instrumental zero, unit cell parameters, Gaussian and Lorentzian contributions to the total peak broadening, and the microstructural strain parameter. The background, based on an eight-position background file generated with FullProf_Suite © , was also refined.

Raman spectroscopy

Raman spectra were collected at room temperature on a Thermo Scientific DXR Raman spectrometer using an argon ion source (λexc = 532 nm) in a backscattering geometry. Focus was made using a x10 magnification objective with a laser spot of approximately 3 µm 2 . A resolution of 3 cm -1 was defined over the 150-3500 cm -1 domain. The data were collected and integrated using the OMNIC software.

Scanning electron microscopy (SEM)

Scanning electron microscopy (SEM) measurements were conducted on an electron microscope ZEISS GEMINI SEM 500 with a resolution of 1.2 nm at 500 V and 1.1 nm at 1 kV, equipped with an Inlens secondary electron (SE) detector and a SE2 detector.

Transmission electron microscopy (TEM) and Electron dispersive energy (EDS)

Particle size, morphology of powders and compositions were examined using a high resolution transmission electron microscope (HR-TEM FEG JEOL 2200FS) equipped with a highresolution camera. The accelerating voltage used was 200keV and Digital Micrograph software was used to acquire the data.

Electron dispersive energy spectroscopy was performed during transmission electron microscopy analysis by probing Lα = 2.042 and 4.839 keV for zirconium and cerium, respectively, and a probe current of 2.562 nA. For each sample, 15 scans of 270 s were performed on three different zones in order to determine the composition and homogeneity of the samples. Digital micrograph software was used to interpret the collected images. Powders were dispersed in ethanol and ultrasonicated for better dispersion prior to be placed on 200 mesh carbon-supported copper grids.

Nitrogen adsorption-desorption (BET method)

Nitrogen adsorption-desorption measurements were performed at -196 °C on a Micromeritics ASAP 2420 apparatus. Specific surface areas were calculated using the Brunauer-Emmet-Teller (BET) method. Samples were previously outgased at 200 °C for 6 h to remove the adsorbed moisture/ air components.

Metallic surface area measurements by hydrogen adsorption

Prior to analysis the samples were reduced at 450 °C in a flow of 10 vol.% of H2 in argon, total flow 50 mL min -1 , heating rate 10 °C min -1 ; after 6 h of the hold time the samples were purged with pure argon and cooled down till 50 °C. After the temperature stabilization hydrogen was adsorbed by pulses with intermediate argon purge. The nickel metallic surface area was calculated using the estimation of the adsorbed hydrogen from the mathematical analysis of the pulses. The Micromeritics AutoChem II 2920 apparatus was used for these measurements.

Temperature programmed oxidation (TPO)

Temperature programmed oxidation coupled with thermogravimetric analysis (TPO-TGA) was used to quantify the amount of carbonaceous deposits on the Ni-containing catalysts after the catalytic tests. The temperature was ramped at a rate of 10 °C•min -1 from room temperature to 900 °C, while a mixture of 20% oxygen in nitrogen (synthetic air purchased from SOL France) was passed through the catalyst at 1 atm. Quantification of carbon deposition was possible by monitoring the weight loss by on-line thermogravimetric analysis (TGA) on the Q 5000IR equipment from TA Instruments.

Catalytic tests

The catalytic activity measurements were performed in a I-shape fixed bed quartz reactor with an inner diameter of 6 mm. The catalyst bed was placed between two pieces of quartz wool with gas flows pointing downwards. Prior to methane dry reforming reaction, 10 mg of the catalyst powder mixed with 30 mg of β-SiC (provided by SICAT, particles size 0.125-0.150 mm) were reduced under a 5 NmLmin -1 flow of pure H2 at 450 °C for 6 h. After the pretreatment, a feed mixture consisting of CH4:CO2:N2:Ar = 1:1:0.5:4.5 (molar) was introduced into the reactor, N2 served as an internal standard. The total flow rate was equal to 70 NmLmin -1

(GHSV 420 Lh -1 gcat -1 (STP)). The reforming tests were studied with an increase of temperature from 550 to 800 °C with a stabilization of 0.5 h every 50 °C. Isothermal experiments at 700 °C were also performed to study the stability of the catalysts. Gas bottles of CH4, CO2, H2, N2 and Ar were purchased from Air Liquide and SOL France. Gas flows were regulated by Brooks SLA 5850S mass flow controllers connected to Brooks 0254 secondary electronics.

Temperature was regulated by a PID controller. Water was collected in a trap cooled with the Huber TC40E thermostat. Effluent gases were analyzed online by an Inficon 3000 microchromatograph equipped with Molsieve 5 Å and Poraplot Q columns and TCD detectors.

A test with pure β-SiC did not show any conversion of methane neither of CO2 at all studied temperatures.

Results and discussion

Materials characterization

Firstly, all fresh samples were characterized before further calcination and impregnation with an aqueous solution of nickel nitrate.

Normalized X-ray diffraction patterns of fresh CexZr1-xO2 (0 ≤ x ≤ 0.75) nanopowders are presented in Figure 2. The diffraction pattern of ZrO2 exhibits a mixture of both monoclinic (P21/c) and tetragonal (P42/nmc) crystal structures (Figure 2a). The inset of Figure 2 Moreover, we can observe as expected a peak shift to smaller 2θ as the Ce content increases.

Indeed, the cerium atomic radius (185 pm) being bigger than the zirconium one (155 pm), when increasing the cerium content in the solid solution, the lattice gets bigger leading to a peak shift to smaller 2θ. Refined lattice parameters and phase ratios for each CexZr1-xO2 nanopowders support, obtained from FullProf_suite © , are listed in Table 2, along with the crystallographic information files. According to Rietveld refinements, ZrO2 nanopowder is composed of 34 vol.% of monoclinic phase and 66 vol.% of the tetragonal one. The average crystallite sizes S1, in supplementary information [START_REF] Keramidas | Raman scattering from CaxZr1-xO2-x阳x, a system with massive point defects[END_REF][START_REF] Keramidas | Raman Scattering Study of the Crystallization and Phase Transformations of ZrO2[END_REF][START_REF] Hirata | Infrared and Raman Spectroscopic Studies of ZrO2 Polymorphs Doped with Y2O3 or CeO2[END_REF][START_REF] Weber | Raman study of CeO2: Second-order scattering, lattice dynamics, and particle-size effects[END_REF][START_REF] Reddy | Raman and X-ray Photoelectron Spectroscopy Study of CeO2-ZrO2 and V2O5/CeO2-ZrO2 Catalysts[END_REF][START_REF] Michel | Study by Raman spectroscopy of orderdisorder phenomena occurring in some binary oxides with fluorite-related structures[END_REF]. A structure difference is clearly visible between powders ZrO2 (Figure 4a) and Ce0.25Zr0.75O2

(Figure 4b). While the former contains a mixture of both tetragonal and monoclinic phases, the latter exhibits mainly the tetragonal one. The difference is less visible between Ce0.50Zr0. 50O2

(Figure 4c) and Ce0.75Zr0.25O2 (Figure 4d), the main difference being the quasi absence of signal at ≈ 330, 630 and 835 cm -1 for the latter. For ZrO2 (Figure 4a), out of the 18 theoretical active Raman modes expected for monoclinic ZrO2 (9Ag + 9Bg) [START_REF] Hirata | Infrared and Raman Spectroscopic Studies of ZrO2 Polymorphs Doped with Y2O3 or CeO2[END_REF], only 13 modes are visible: 7 Ag modes located at: 172, 184, 304, 340, 552, 623 and 637 cm -1 , and 6 Bg modes are located at 215, 327, 376, 496, 531 and 608 cm -1 . These mostly strong and well-separated peaks observed in the 150-650 cm -1 domain are sufficiently well defined to assert that they correspond to the monoclinic phase. However, 5 Eg active modes located at 266, 408 and 415 cm -1 , 1 Ag at 473 cm -1 and 1 B1g mode at 583 cm -1 correspond to the tetragonal phase contained in ZrO2. For the Ce0.25Zr0.75O2 nanopowder (Figure 4b) active modes are visible at 232 (B1g), 311 (B1g), 456 (Eg), 472 (Eg), 608 (A1g) and 835 (Eg) cm -1 corresponding to the tetragonal ZrO2 (A1g + 2B1g + 3Eg). These band broadenings, resulting from cationic substitution in ZrO2 by Ce n+ (n = 3, 4), reveal the presence of a locally slightly disordered phase due to the strain imposed by larger cerium cations. For the Ce0.50Zr0.50O2 powder (Figure 4c), 4 main broad bands are visible at 345, 462, 625 and 835 cm -1 attributable to a tetragonal phase. Raman spectrum of the Ce0.75Zr0.25O2 powder (Figure 4d), on the other hand, tends to display the cubic CeO2 structure (Fm-3m). For a pure cubic structure (Figure 4e), there is only the triply degenerate T2g mode (A1g + Eg) at 461 cm -1 , corresponding to a symmetric breathing of oxygen ions octahedrally coordinated to the metal cation, which remains active. In the case of Ce0.75Zr0.25O2, we can see that two other modes (630 and 835 cm -1 ) are weak but still actives, meaning that the structure is not purely cubic.

Then all the fresh nanopowders were calcined at 800 °C following by wet impregnation with 10 wt.% of Ni. Table 3 represents the summary of the results of textural and structural properties of the CexZr1-xO2 fresh supports, of calcined materials and of the final catalysts impregnated with 10 wt.% of Ni. The fresh CexZr1-xO2 samples exhibit satisfactorily high specific surface area, SBET is in the range of 140-160 m 2 g -1 , which is quite high for the ceria-zirconia mixed oxide prepared by other synthetic routes. The addition of CeO2 into zirconia leads to a creation of mixed oxides accompanied with an increase in specific surface area passing via a maximal value in the case of stoichiometric Ce0.50Zr0.50O2 material. As expected, after the calcination step the specific surface area of all investigated samples decreases. It is worthy to note, that the mixed CeO2-ZrO2 binary materials keep much higher specific surface areas (SBET 60-78 m 2 g - 1 ) than those determined for undoped zirconia (SBET 30 m 2 g -1 ). The same tendency is observed for the total pore volume of the studied samples as well (Table 3). The calcination and impregnation steps induce a decrease in the total pore volume (Vp) for all the samples. For ceria-rich Ce0.75Zr0.25O2 sample, the total Vp decreases from 0.22 for the fresh support to 0.14 cm 3 g -1 after calcination at 800 °C and further impregnation. These two steps, calcination and impregnation, also induce an increase in the pore diameter (Dp), probably indicating the sintering of small initial nanocrystals and blocking of the small pores with supported NiO particles, so the pore diameter in the range of 10-20 nm might be simply attributed to the intraparticle porosity. The crystalline domain sizes of the calcined supports and those of the supported NiO particles were calculated from XRD data using Scherrer equation (Table 3). The XRD patterns of calcined and impregnated samples are shown in Supplementary material (Figure S2). In the case of NiO-containing samples, the crystalline domain sizes determined for samples based on mixed ceria-zirconia supports increase from 5-6 nm (Table 2) to 11 nm (Table 3), whatever the composition. The structure of the ZrO2 materials underwent distinct changes during calcination step at 800 °C and further calcination after Ni impregnation: the increase in the crystalline domain sizes from 5 to 22 nm and 22 to 25 nm indicates more important sintering of the initial nanocrystals comparing to the mixed ceria-zirconia samples (Table 3). The crystalline domain size calculated from XRD results of the mixed oxide supports (10-11 nm, Table 3) is smaller than that determined for pure ZrO2 (22-25 nm), indicating that the presence of CeO2 is desired to stabilize the structure of CexZr1-xO2 materials after calcination and after impregnation with nickel.

Figure 5 presents the SEM images of the Ce0.75Zr0.25O2 materials. It can be observed that the light, flake-like morphology characteristic of the fresh support (Figure 5a) turned after calcination step into a granulated structure (Figure 5b and5c). It is consistent with the previous determined crystalline domain sizes and is additionally confirmed by a decrease of the specific surface area from 146 m 2 g -1 for Ce0.75Zr0.25O2 down to 60 m 2 g -1 for Ce0.75Zr0.25O2-800 and down to 33 m 2 g -1 for Ni/Ce0.75Zr0.25O2.

The mean size of the crystalline domains of NiO ranges between 17 nm for Ni/Ce0.50Zr0.50O2

and 24 nm for Ni/ZrO2. An industrial ceria-zirconia mixed support was used to prepare a reference nickel-containing catalyst Ni/Ce0.50Zr0.50O2-ind of the same composition as Ni/Ce0.50Zr0.50O2. The characterization results of this reference support and its corresponding catalyst are reported in (35 and 17 nm, respectively). This is probably due to the stronger metal-support interaction in case of the use of the nanostructured support supposing much better metal dispersion and higher metallic surface area. The H2 adsorption results allowed to calculate these values: nickel metallic surface area for nanostructured material with the support prepared by continuous supercritical solvothermal method is 5 times higher than the nickel metallic surface area for the sample with the reference support prepared by conventional coprecipitation method (5.6 and 1.2 m 2 g -1 , respectively, Table 5).

100 nm 100 nm 100 nm

Catalytic activity results

The results of the catalytic activity of the pre-reduced Ni/CexZr1-xO2 catalysts in the reaction of dry reforming of methane are summarized in Table 4 for different reaction temperatures. It is noticeable that the sample based on ZrO2 nanoparticles as support leads to similar conversions of CO2 and CH4 than those exhibited by the samples based on mixed CeO2-ZrO2 oxides. It is however very difficult to catch the differences in catalytic behavior between Ni/Ce0.25Zr0.75O2, Ni/Ce0.50Zr0.50O2 and Ni/Ce0.75Zr0.25O2 and make a clear conclusion. All the catalysts prepared by supercritical solvothermal method seem to exhibit a good behavior towards deactivation since the conversions obtained at the cooling step are very close to those exhibited in the heating step. As an example, at 700 °C, the CO2 and CH4 conversions for Ni/Ce0.50Zr0.50O2 in the heating step are 61 and 36%, respectively, and 59 and 34%, respectively, when cooling down.

At the contrary, Ni/Ce0.50Zr0.50O2-ind, despite a good catalytic behavior in the heating step, presents much lower conversions in the cooling step. At 700 °C, the CO2 and CH4 conversions for Ni/Ce0.50Zr0.50O2-ind in the heating step are 60 and 49%, respectively, and 33 and 20%, respectively, when cooling down. It's a point to a bad resistance of this catalyst to deactivation under the reaction conditions. The same tendency is noticeable for the H2/CO ratio, which may serve as an indicator of the catalyst selectivity. Better understanding of the catalytic behavior of those materials may be achieved by analyzing the isothermal catalytic tests at 700 °C that give an insight into the sample stability (Figure 6). Once again the activity of all the catalysts prepared by continuous supercritical solvothermal method appears very similar (Figure 6). The slope of deactivation vs time on stream is identical for all samples, the CO2 conversion decreased from 49-52 to 46-50% over 50 hours. To have a better indication on the catalytic activities longer time-on-stream tests or the use of harsher conditions of DRM are needed. Anyway it can be seen that the deactivation is much more pronounced for the sample with the reference support, Ni/Ce0.50Zr0.50O2-ind: the CO2 For more convincing comparison, the turnover frequency (TOF, s -1 ) was calculated for the initial activity point of the two samples with the same composition, Ni/Ce0.50Zr0.50O2 and Ni/Ce0.50Zr0.50O2-ind, using the experimentally determined metallic nickel surface (Table 5).

The nanostructured material shows a TOF 7 times higher comparing to the value obtained for Ni/Ce0.50Zr0.50O2-ind sample, 4.7 and 0.7 s -1 , respectively. This indicates very high intrinsic activity of metallic nickel sites dispersed on CeO2-ZrO2 synthesized via continuous supercritical solvothermal route.

Study of the spent catalysts

In order to better understand the behavior of the samples under isothermal catalytic conditions at 700 °C, two additional characterizations were performed: XRD of the spent samples and carbon deposit analysis by TPO-TGA.

X-ray diffraction patterns of the spent samples are shown in Figure 7 as well as in the supplementary material in Figure S2 for the comparison with the catalysts before catalytic tests. The TPO-TGA profiles are shown in Figure 8. There were several mass losses observed which may be divided into two groups: 1) low temperature loss at temperatures in the range 350-650°C, may be attributed to the "reactive" carbon in forms of fibers or nanotubes, that may participate in the overall reaction mechanism (for example, in the Boudouard reaction); 2) high temperature loss at temperatures in the range 650-850 °C, may correspond to the graphitic carbon, the main cause of deactivation as it covers the metallic nickel particles and reduces the number of active sites [START_REF] Dudder | The role of carbonaceous deposits in the activity and stability of Ni-based catalysts applied in the dry reforming of methane[END_REF].



In order to compare the selectivity of the catalysts to the coke formation, the carbon formation rate (CFR) was calculated in mmol of carbon formed per gram of catalyst per hour (Table 5).

Additionally, the selectivity of the catalytic materials to carbon formation was calculated in mmol of carbon per mol of converted carbon during the reaction; the results are collected in Table 5. This analysis permits to compare the catalytic behavior and more precisely the resistance to carbon formation of the prepared materials in the DRM conditions. The catalyst based on reference support accumulated the largest quantity of carbon deposit, TPO-TGA results confirm the XRD analysis of this sample after catalytic test. Among the mixed oxides nanostructured supports the Ce-rich material Ni/Ce0.75Zr0.25O2 shows the highest CFR. The presence of larger quantity of Zr allows decreasing the CFR, but when using pure ZrO2 as catalyst support still much more coke is formed. It was reported by Boucard et al [START_REF] Boucard | Beneficial use of CeO2 nanocatalyst for black liquor conversion under sub and supercritical conditions[END_REF] that nanostructured CeO2, prepared in supercritical water, may help to reduce the coke formation rate and increase the production of H2 in the hydrothermal conversion of biomass. This effect of nano-CeO2 is probably due to its ability of water splitting and formation of active oxygen on the catalyst surface. Oxygen storage capacity (OSC) of the CeO2 with different structures, cubic morphology and irregular-shaped, is much higher for the nanostructured nanoparticles with a small size and active tetragonal (101) facets [START_REF] Zhang | Extra-low temperature oxygen storage capacity of CeO2 nanocrystals with cubic facets[END_REF][START_REF] Dejhosseini | Catalytic Cracking Reaction of Heavy Oil in the Presence of Cerium Oxide Nanoparticles in Supercritical Water[END_REF]. It is usually attributed to the larger exposed surface area in the smaller nanocrystals and indicates that the oxygen molecules involved in the oxygen storage/release process are located on the surface of CeO2 [START_REF] Kim | Oxygen nonstoichiometry of nanosized ceria powder[END_REF]. Addition of the ZrO2 to CeO2 and formation of the mixed oxide nanostructure helps increase the specific surface area and to stabilize the tetragonal structure of CeO2.

Comparing the CFR of the ceria-zirconia materials after DRM catalytic test (Table 5) allows making a conclusion that an optimal Ce/Zr ratio in the support is needed for improving the resistance to coke of the catalyst. The optimal catalyst which forms less coke and more importantly less graphitic coke than the other samples is the Ni/Ce0.50Zr0.50O2 material (stoichiometric Ce/Zr ratio). In comparison the reference sample of the same composition showed the highest CFR that indicates clearly the advantages of the proposed synthesis in continuous supercritical media regarding the formation of CeO2-ZrO2 nanostructured supports with high OSC (was not measured due to the very small quantity of nanostructured samples).

A correlation between the crystalline domain size of the ceria-zirconia support and the supported NiO, Ce/Zr ratio in the support from one side and the catalytic activity in DRM and coke resistance from the other side may be done. The smallest crystalline domain size of NiO 17 nm as the precursor of nickel metallic active sites was possible to achieve using the mixed oxide support of stoichiometric ratio of ceria and zirconia that offered the best metal-support interaction. Based on the current study, it can be stated that, the smaller is the crystalline domain size of the mixed oxide support, the better performance in DRM may be expected as well as better resistance to coking. The nanostructured supports were obtained due to the specific conditions of the synthesis in supercritical medium. The nanostructured CexZr1-xO2 materials are found to be promising supports not only for nickel but for other active metals dispersion and for coke control in the reactions where coking is abundant.

Conclusions

In this work a family of nanostructured CexZr1-xO2 (0 ≤ x ≤ 0.75) materials was prepared by continuous supercritical solvothermal method. They were calcined at 800 °C and then impregnated with 10 wt.% of nickel. All the prepared materials as well as one reference sample with the same composition were characterized; Ni-containing materials were studied in catalysis of DRM with characterization of spent materials after tests.

Fresh supports CexZr1-xO2 before calcination step have shown the formation of ceria-zirconia mixed oxides. After calcination and impregnation steps in the sample ZrO2 without incorporation of ceria the crystalline domain size was observed around 22-25 nm. The incorporation of ceria helps to stabilize the ceria-zirconia mixed oxides with much smaller crystalline domain size around 10-11 nm; it also improves the activity in DRM and decreased the formation of carbon deposit during the catalytic tests. The carbon deposit analysis after DRM tests revealed that the carbon deposit rate was more important for the ZrO2 sample and much more important for the reference sample. The optimal composition of the nanostructured support is Ce0.50Zr0.50O2, with the stoichiometric Ce/Zr ratio, which shows a higher activity in DRM and lower coke formation rate in comparison to the reference sample with the same composition. The continuous supercritical solvothermal method allows to prepare nanostructured mixed oxides materials that could be promising supports for different active metals dispersion and for coke control in the reactions where coking may be abundant.

Figure 1 .

 1 Figure 1. Scheme of the set-up used for synthesizing the CexZr1-xO2 (0≤x≤0.75) nanoparticles (HPP=high pressure pump, tR=reaction time, PRV=pressure regulating valve).

  (A.C.S.) calculated from XRD patterns and based on Rietveld refinements are for monoclinic and tetragonal ZrO2 5.4 and 5.8 nm, respectively. A.C.S. of tetragonal Ce0.25Zr0.75O2, Ce0.50Zr0.50O2 and Ce0.75Zr0.25O2 powders are 4.9, 5.3 and 5.0 nm, respectively.

Figure 2 .Table 2 .

 22 Figure 2. Powder X-ray diffraction patterns of the fresh samples a) ZrO2, b) Ce0.25Zr0.75O2, c) Ce0.50Zr0.50O2 and d) Ce0.75Zr0.25O2. # and * denote Bragg reflexions for the monoclinic and tetragonal structures of ZrO2, respectively.

Figure 3 .

 3 Figure 3. TEM images, particle size distributions and EDS contrast images (in blue and red Ce and Zr centers, respectively) of fresh a) ZrO2, b) Ce0.25Zr0.75O2, c) Ce0.50Zr0.50O2 and d) Ce0.75Zr0.25O2. On the top right is displayed a zoom of an individual crystal of ZrO2.

Figure 4 .

 4 Figure 4. Raman spectra of fresh a) ZrO2, b) Ce0.25Zr0.75O2, c) Ce0.50Zr0.50O2, d) Ce0.75Zr0.25O2 and e) CeO2 powders. # and * denotes Raman active modes for the monoclinic and tetragonal structures, respectively.

  diameter; Vp = pore volume, D = size of crystalline domain calculated from XRD results; sup =support;-= not applicable

Figure 5 .

 5 Figure 5. SEM images of the fresh, calcined and impregnated with 10wt% of Ni materials (Ce0.75Zr0.25O2).

Figure 6 .

 6 Figure 6. Isothermal tests at 700 °C: CO2 and CH4 conversions vs time on stream.

  All the three samples based on mixed ceria-zirconia oxides prepared by supercritical solvothermal method show a little segregation of the pure CeO2 phase after 50 h of catalytic test at 700 °C. This might indicate that as-prepared nanostructured materials are not entirely stable under continuous high temperature treatment. Probably longer calcination step would be needed with slower heating rate in order to get better stabilization of the mixed oxide nanostructures before their use in DMR. Another explanation might consist in the possible slow loss of the structured water from zirconium polyhydroxo-oligomers which are formed in the presence of water during materials' synthesis and stabilized in the nano-structure. Probably, the supercritical synthesis without use of water but only pure alcohols could be a solution to avoid the phase segregation and to stabilize the mixed oxide nanostructure. Nevertheless, the presence of such segregation did not influence the catalytic activity in DRM. There is no visible graphitic carbon which reflection appears at 2Ɵ equal to ca. 26 °[START_REF] Debek | Ceria Promotion over Ni-Containing Hydrotalcite-Derived Catalysts for CO2 Methane Reforming[END_REF].It is noticed that the XRD pattern of Ni/Ce0.50Zr0.50O2-ind after test does not show any phase change of the mixed ceria-zirconia structure. But in the same time it shows the wide peak at 2Ɵ equal to ca. 26 ° indicating a large quantity of the formed graphitic carbon. This confirms the fast deactivation trend of this sample during time-on-stream DRM test.

Figure 7 .

 7 Figure 7. Diffraction patterns of the spent samples after isothermal catalytic tests at 700 °C.

Figure 8 .

 8 Figure 8. TPO-TGA profiles of spent samples after isothermal catalytic tests at 700 °C.

Table 1 .

 1 List of synthesized samples

	Fresh supports	Calcined supports	Corresponding catalysts
	ZrO2	ZrO2-800	Ni/ZrO2
	Ce0.25Zr0.75O2	Ce0.25Zr0.75O2-800	Ni/Ce0.25Zr0.75O2
	Ce0.50Zr0.50O2	Ce0.50Zr0.50O2-800	Ni/Ce0.50Zr0.50O2
	Ce0.75Zr0.25O2	Ce0.75Zr0.25O2-800	Ni/Ce0.75Zr0.25O2
	-	Ce0.50Zr0.50O2-ind	Ni/Ce0.50Zr0.50O2-ind
	2.2. Materials characterization

2.2.1. Powder X-ray diffraction (XRD)

	For fresh supports, room temperature powder X-ray diffraction (XRD) patterns were recorded
	on a PANalytical diffractometer with the Bragg-Brentano θ-2θ geometry, over the 10 < 2θ <
	130° range, with an angular step of 0.0167° and a step time of 0.6 s. The diffractometer was
	equipped with a secondary monochromator and the CuKα1 = 1.5418 Å radiation was generated
	at 45 kV and 40 mA. For both calcined supports and the corresponding catalysts, X-ray
	diffraction patterns were recorded on a Brucker D8 Advanced Powder Diffraction System with
	a VANTEC detector side and Ni filtered CuKα1 radiation, over 2θ range 20-80 ° using a step
	size of 0.01 ° and a step time of 0.2 s. Prior to measurements, powders were ground and sieved
	using a 40 µm sieve.

Table 3 .

 3 Textural and structural properties of the materials

Table 3

 3

. Comparing Ni/Ce0.50Zr0.50O2-ind and Ni/Ce0.50Zr0.50O2, it clearly appears that despite similar BET surfaces (36 and 42 m 2 g -1 , respectively), similar crystalline domain sizes of the support (10 and 11 nm, respectively) the crystalline domain size of NiO is twice bigger for industrial-based catalyst compared to nanostructured catalyst prepared in supercritical conditions

Table 4 .

 4 Summary of the catalytic results as function of temperature at heating and cooling steps.

Table 5 .

 5 Catalytic properties of the impregnated materials.

	Catalyst	SNi,	TOF,*	CFR,	Coke selectivity,
		m 2 g -1	s -1	mmolC gcat -1 h -1	mmolC mol -1 Cconv
	Ni/ZrO2	-	-	0.45	0.20
	Ni/Ce0.25Zr0.75O2	-	-	0.34	0.16
	Ni/Ce0.50Zr0.50O2	1.2	4.7	0.08	0.038
	Ni/Ce0.75Zr0.25O2	-	-	0.13	0.058
	Ni/Ce0.50Zr0.50O2-ind	5.6	0.7	3.43	1.4
		-= not applicable, CFR=carbon formation rate, * = at the initial time (1h)

Acknowledgements

The authors acknowledge the European ERANet RUS Plus program for financial support in the frames of the project "NiCe -Design of Ni/ceria-zirconia nanomaterials by continuous synthesis in supercritical medium: tuning catalytic properties". This work was performed as a part of the large international collaboration GDRI "Catalytic valorization of biomass" supported by CNRS in France and RFBR in Russia. The Polish authors thank for the financial support the National Centre for Research and Development in the framework of NiCe ERANet RUS Plus -S&T project (5/RUSPLUS -S&T/2016). Katarzyna Dziadek acknowledges the Erasmus+ Program for the financial support. Aimery Auxemery acknowledges the Innovation Fund Denmark in the frames of the GCAM project for the financial support. The authors from ICMCB acknowledge CNRS, the University of Bordeaux and Bordeaux INP and also the "Région Nouvelle-Aquitaine" for their financial support. The authors thank Dr Thierry Dintzer for fruitful discussion on the SEM results.


Sylvain Fleury
email: sylvain.fleury@ensam.eu

Aurélien Agnes

Rishi Vanukuru

Emma Goumillout

Nicolas Delcombel

Simon Richir

Aurélien Agnès

Studying the Effects of Visual Movement on Creativity

Keywords: Perceptive Movement, Divergent Creativity, Virtual Reality, Creativity Stimulation, Digital Tools for Creativity

teaching and research institutions in France or abroad, or from public or private research centers.

Introduction

Numerous writers and philosophers have put forth and supported the idea that walking makes thinking easier. It is common to observe people who cannot make a phone call without walking back and forth. One explanation for this is that walking speeds up the heartbeat, causing more blood and oxygen to flow to muscles and organs, including the brain. Walking is also said to be a way of connecting the rhythm of our body to our mental state.We do not make a conscious effort when we walk, therefore leaving the mind available for more creative thinking.

The question of the effects of body postures and physical activity on intellectual performance of different kinds has long been the subject of studies in the field of management and organization of work spaces. Non-sedentary organisation of work spaces, i.e. the absence of offices allocated to a particular employee, has two main consequences. The first one is the decrease of territoriality, leading employees to make better use of t he whole premises and move around more. The second effect is an increase in the level of physiological arousal that promotes intellectual work [START_REF] Knight | Get up, stand up: The effects of a non-sedentary workspace on information elaboration and group performance[END_REF].

The classic posture in office work is a sitting one. Time spent sitting is associated with negative health effects, and this has been motivating more and more people to adopt standing workstations. At present, the effects of this type of arrangement on cognitive performance are not entirely clear. In a study by [START_REF] Finch | Taking a stand: the effects of standing desks on task performance and engagement[END_REF], standing had no effect on text comprehension or performance on a creative task compared to sitting. The only difference between the two conditions in this study was the commitment to the task. Participants performing these tasks while standing showed more interest and enthusiasm. In research by [START_REF] Gilson | Do sitting, standing, or treadmill desks impact psychobiological indicators of work productivity?[END_REF], participants were asked to perform cognitive tasks while sitting, standing with a higher desk, or standing while walking on a treadmill. The results indicate that standing allows for better regulation of stress levels (measured here by the cortisol level in saliva), and better allocation of attentional resources. Walking, on the other hand, does not make a difference on these variables compared to standing without movement in this study.

However, other studies show that similar to running and dancing (e.g. [START_REF] Gondola | The enhancement of creativity through long-and short-term exercise programs[END_REF][START_REF] Gondola | The effects of a single bout of aerobic dancing on selected tests of creativity[END_REF], walking can have positive effects on creativity [START_REF] Schaefer | Cognitive performance is improved while walking: Differences in cognitive-sensorimotor couplings between children and young adults[END_REF][START_REF] Weuve | Physical activity, including walking, and cognitive function in older women[END_REF]). These effects may be explained by a decrease in stress hormones in the brain [START_REF] Salmon | Effects of physical exercise on anxiety, depression, and sensitivity to stress: a unifying theory[END_REF], but also by a decrease in blood pressure and oxygen supply to the brain that facilitates mental associations (O'Mara, 2019). Zhou, Zhang, Hommel and Zhang (2017) compared divergent creative performance for people sitting, standing still, and walking. Standing led to better performance compared to sitting, and walking also improved performance in the study. The authors consider that more divergent thinking is displayed when the corresponding physical activity requires more control resources. Conversely, performance in some cognitive tasks like subtraction or counting phonemes in words tends to be degraded when they are conducted while walking [START_REF] Srygley | When does walking alter thinking? Age and task associated findings[END_REF].

In a recent study by [START_REF] Frith | Experimental effects of acute exercise and music listening on cognitive creativity[END_REF], participants were asked to perform divergent and convergent thinking tasks by walking on a treadmill, listening to music, or just sitting. Walking on the treadmill had no significant effect on the different indicators of divergent creativity, which is inconsistent with the study by Oppezzo and Schwartz (2014), but also with [START_REF] Abdullah | Shining (blue) light on creative ability[END_REF], for whom walking on the treadmill improves divergent but not convergent thinking. The main constant in the literature is the lack of effect of physical activity on convergent thinking. It seems that walking can have a positive effect on divergent creativity, although this does not seem to occur in all contexts. A particularly comprehensive study on the subject has been carried out by Oppezzo and Schwartz (2014). It shows that walking improves divergent thinking (measured by Guiford's alternate uses test; [START_REF] Guilford | Alternate uses. Form A[END_REF], but not convergent thinking (measured by Barron's symbolic equivalence test; [START_REF] Barron | Creativity and psychological health[END_REF]. Experimental sessions lasted a few minutes and the authors found that the positive effects of walking were maintained for the next few minutes even if the participant sat down. Furthermore, this research also shows that divergent thinking performance is higher for participants pushed in a wheelchair outside than for participants sitting in a stationary chair inside. However, it is not clear from the study whether this difference is due to being in motion and therefore seeing the scenery moving, or being in an outdoor environment. The effect of physical activity alone on divergent creativity is noted because the participants are indoors. Similarly, when the experience is replicated while walking outside, the same differences appear. Finally, an experiment was conducted to study the effect of sitting or standing and the effect of being indoors or outdoors, using another measure of creativity. In this experiment, all cases of walking had an effect on creativity, and being outside had an effect on the "novelty" of the ideas proposed. However, two aspects of this analysis are missing in order to provide a more complete understanding of the effect of physical activity. First, there was no comparison of the results between sitting outside and inside, and walking outside and inside. Indeed, the creativity scores all appear to be higher when walking outside than when walking inside. Secondly, the variable "dynamic outdoors -sitting" is significantly different from "sitting indoors", as the participants are still moving. Thus, they are not comparing two immobile positions, but two conditions in which there is no physical activity, without isolating the effect of movement as perceived by the participants. This perception of movement could be of two types: it could be the visual perception of movement, or it could be the physical sensation of being moved. In the present research, we have chosen to isolate specifically the effect of visually perceived movement on creativity.

This idea that visual movement can have an impact on certain cognitive components and more specifically creative capabilities has not been demonstrated in the literature to the best of our knowledge, although some research projects in the field of neuroscience do provide indications of the effect. The cerebellum is particularly known for its role in motor functions. However, it is also involved in visual perceptive treatments [START_REF] Baumann | Consensus paper: the role of the cerebellum in perceptual processes[END_REF], particularly visual perception of movement [START_REF] Nawrot | Motion perception deficits from midline cerebellar lesions in human[END_REF] and visual attention [START_REF] Brissenden | Cortico-cerebellar networks for visual attention and working memory[END_REF]. The cerebellum is also engaged in cognitive functions such as working memory [START_REF] Küper | Cerebellar fMRI activation increases with increasing working memory demands[END_REF]. Finally, the cerebellum is involved in various creativity tasks [START_REF] Levy | Rostral and caudal prefrontal contribution to creativity: A meta-analysis of functional imaging data[END_REF]. In particular, it is more engaged in so-called idea generation tasks, i.e. divergent creativity tasks, than in combination tasks, i.e. convergent creativity tasks. The pattern of these results is consistent with the results of Oppezzo and Schwartz (2014).

The objective of our study was to deepen the understanding of this effect of movement on creativity by specifically isolating the visual perceptual component of movement through the use of virtual reality. In addition, virtual reality technologies have been shown to be effective for ideation (e.g. [START_REF] Yang | Examining creativity through a virtual reality support system[END_REF][START_REF] Feeman | Exploration and evaluation of CAD modeling in virtual reality[END_REF][START_REF] Bourgeois-Bougrine | The expression of users' creative potential in virtual and real environments: An exploratory study[END_REF]. Although virtual reality is not a priori conducive to walking, it does allow us to create impressions of movement in the virtual environment. It is possible to create the illusion of movement by travelling through a virtual scene. To avoid the risks of motion sickness, the chosen scene in our experiment was a train cab with windows on each side, which allowed the scenery to move past the windows, giving the impression of moving forward without creating an inconsistency between the sensations of the body and perception. We hypothesize that in the study by Oppezzo and Schwartz (2014), the positive effect of the wheelchair on creativity comes from the fact that even without muscle activity, the brain receives visual information indicating that it is moving, and that this triggers cognitive effects relatively similar to those obtained by walking. This leads us to the hypothesis that in a virtual environment, participants may perform better when present in a moving environment (in our case a train) rather than a stationary one, even if they are physically not moving at the time.

Method

Participants 32 volunteers (26 men and 8 women) participated in the current study. The youngest was aged 14 years and the oldest 33 years. Their mean age was 22.1 years (SD = 2.98). All of them were students or interns involved in the field of virtual reality.

Material and procedure

Two creativity tests were used in this experiment: Guilford's Alternate Uses test (GAU) and the Remote Association Test (RAT). The GAU is a test of divergent creativity that consists of participants thinking of as many alternative uses of a given object as possible during a limited time period [START_REF] Guilford | Alternate uses. Form A[END_REF]. In our experiment, participants were given 5 minutes to think and speak out alternative uses of a common masonry brick. This test therefore leads to the collection of a set of ideas for each participant. Each set of ideas is then analyzed to be characterized on the basis of four criteria which constitute the scores. These scores for the GAU are originality (which evaluates whether the participant's ideas were unique when compared to other participants), fluency (the number of ideas generated), flexibility (the number of ideas belonging to different domains or categories) and elaboration (the amount of added detail given for each idea). The RAT is a test of convergent creativity where participants are presented with groups of three words, each of which is semantically related to a fourth word that participants must guess [START_REF] Mednick | Incubation of creative performance and specific associative priming[END_REF]. The RAT scores then correspond to the number of words guessed by the participants.

Participants had to successively perform the GAU and the RAT aloud while immersed in a virtual environment using a HTC Vive headset. This virtual environment was that of the interior of an empty train car (see Figure 1).

Figure 1. The experimental virtual environment from the participants' point of view

For 16 participants, the virtual environment was stationary, while for the remaining 16 participants, the train was moving, which was visually achieved by the moving of lights past the window (see Figure 2). In order to avoid any bias as a result of visual inspiration, no landscape was visible through the train's windows, as if the participants were in a tunnel. Thus, when the train was moving, the lights from inside the tunnel could be seen moving by. This train set was designed to represent a visual movement without movement of the user, while also avoiding motion sickness.

Figure 2. Three screeshots from the participants' point of view in the moving condition, with with the lights moving from front to back

Results

Table 1 shows the averages and standard deviations of the GAU and RAT sores under moving and non-moving conditions. We can see that empirically, the scores for all creativity criteria measured in the experiment are higher in the condition with movement than in the condition without movement. However, the significance of these differences needs to be assessed.

The homoscedasticity of the different variables has been tested using Levene tests. These tests revealed acceptable homoscedasticity for flexibility (F(1, 29)=.348, p=.560), fluency (F(1, 29)=.714, p=.405), elaboration (F(1, 29)=.092, p=.763), originality (F(1, 29)=2.592, p=.1182) and convergence (F(1, 29)=.249, p=.621). All the comparisons of the means have been made by conducting T-tests. These tests did not reveal any significant differences for originality (t(29)=-1.781, p=.087), fluency (t(29)=-1.536, p=.135), elaboration (t(29)=-.827, p=.415), or RAT scores (t(29)=-.703, p=.489). Nevertheless, a significant difference was revealed for flexibility (t(29)=-2.140, p=.041).

Discussion

The fact that that walking leads to an improvement in certain general cognitive performances [START_REF] Aspinall | The urban brain: analysing outdoor physical activity with mobile EEG[END_REF][START_REF] Erickson | Exercise training increases size of hippocampus and improves memory[END_REF][START_REF] Keinänen | Taking your mind for a walk: a qualitative investigation of walking and thinking among nine Norwegian academics[END_REF][START_REF] Kuo | Sensorimotor-conceptual integration in free walking enhances divergent thinking for young and older adults[END_REF][START_REF] Labonté-Lemoyne | The delayed effect of treadmill desk usage on recall and attention[END_REF] and divergent creativity in particular (Oppezzo & Schwartz, 2014), has been known for many years. This phenomenon of improved cognitive functioning related to walking is usually explained in terms of increased blood flow due to physical effort, which in turn has an impact on brain functioning [START_REF] Ferris | The effect of acute exercise on serum brain-derived neurotrophic factor levels and cognitive function[END_REF][START_REF] Hillman | Be smart, exercise your heart: exercise effects on brain and cognition[END_REF]. In the study by Oppezzo and Schwartz (2014), participants' divergent creativity was higher when pushed in a wheelchair than when they were immobile. This may suggest that being in motion, or more precisely perceiving oneself in motion, would be sufficient to generate an increase in creative performance. Thus, the objective of our study was to determine whether the perception of being in motion increases creativity, and more specifically to understand if the increase in creativity when walking is only due to physical activity or if there is also an additional effect of the visual perception of movement. The hypothesis was that participants immersed in a virtual reality train cab with a HMD would perform better in divergent creativity tasks when the train was moving than when it was stationary.

The results indicate that visual perception of movement does lead to a significant increase in performance on divergent creativity tasks. This difference concerns the criterion of flexibility, i.e. the range of ideas produced by participants. However, consistent with the study by Oppezzo and Schwartz (2014), the effect of visual movement on convergent creativity tasks is not significant. This pattern is also consistent with the results of [START_REF] Levy | Rostral and caudal prefrontal contribution to creativity: A meta-analysis of functional imaging data[END_REF] revealing that the cerebellum is particularly engaged in divergent creativity, but less so for convergent one. These results suggest that in the case of walking, physical activity is only one factor that leads to the stimulation of cognitive functions, and the perception of movement might be another important factor.

A recent study has shown links between visual behaviour and divergent creativity [START_REF] Kwon | Does Visual Fixation Affect Idea Fixation?[END_REF]. However, these were based on different views of the images related to the GAU test. Participants were shown single or multi-angle images of objects for which they had to imagine alternative uses. In this case, the different angles of view may have resulted in different visual behaviours/perceptions, but also different forms of inspiration. This situation is not really comparable to our study in which the GAU was purely verbal, with no visual representation of the objects that were to be thought of.

Recommendations can still be drawn from these results. Many tools for making virtual reality meetings and conference calls have been developed in recent years. One could imagine that these tools may incorporate an option for virtual meeting environments to give the impression of travel in order to encourage the creativity of the participants involved. Brainstormings and brainwritings are common methods used in companies (Nutzmann, Sauer, Vob & Bozkurt, 2019). It is possible to carry out electronic brainstormings which can for example be based on a 3D environment with avatars for each participant [START_REF] Buisine | Proteus vs. social identity effects on virtual brainstorming[END_REF] or on a digitized card game [START_REF] Lo | HexDeck: Gamification of Tangibles for Brainstorming. Consilence and Innovation in Design[END_REF]. These types of creativity workshops based on the use of a computer allow us to benefit from certain advantages, for example by manipulating social influence effects [START_REF] Hénaff | Applying the SIDE model to brainwriting: The impact of intergroup comparison and anonymity on creative performance[END_REF]. However, computers are rather designed to be used sitting down and therefore without physical movement. One can imagine integrating perceptual movement on the screen on the interface used for creativity.

The main limitation of our study concerns the measures of creativity used. Interpretations made on GAU scores representing divergent creativity and RAT scores representing convergent creativity should be considered with caution as these measures are not completely independent and pure [START_REF] Patterson | The experimental effects of acute walking on cognitive creativity performance[END_REF][START_REF] Cortes | Re-examining prominent measures of divergent and convergent creativity[END_REF]. In order to remove doubts about the measures, further studies involving several different types of divergence and convergence oriented tasks should be carried out in order to have a better understanding of the effects involved.

Furthermore, we have chosen the speed at which the train travels arbitrarily, but we do not know at this stage whether this is the optimal speed to support creativity. Future research could be conducted to identify the effects of different speeds of motion on creativity. It is also possible that non-rectilinear movement may have an impact. Other forms of perceptual movement that can be easily integrated into an environment, such as a water fountain or a pendulum, could also be tested.

Table 1 .

 1 Means and Standard Deviation (in brackets) for the four parameters of GAU and the scores of CRA

		Originality	Fluency	Flexibility	Elaboration	RAT scores
	Without movement	5.87 (3.98)	13.53 (5.74)	9.07 (4.68)	3.67 (3.66)	6.07 (2.28)
	With movement	9.31 (6.56)	16.88 (6.37)	12.81 (5.06)	4.67 (3.18)	6.56 (1.55)

Acknowledgement

We thank the Time to Concept Chair for providing the necessary resources for this research.