Martyna Przydacz

Marcin Jędrzejczyk

Jacek Rogowski

Dris Ihiawakrim

Nicolas Keller

Agnieszka M Ruppert
email: agnieszka.ruppert@p.lodz.pl

TiO2 supported non-noble Ni-Fe catalysts for the high yield production of 2,5-dimethylfuran biofuel

Keywords: Sustainable furanic biofuel, 2,5-dimethylfuran, 5-hydroxymethylfurfural, Ni-Fe/TiO2 catalysts

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

Introduction

Lignocellulosic biomass is an easily available and abundant valuable source of various platform molecules such as 5-hydroxymethylfurfural (HMF), the so-called "sleeping giant" of biobased intermediate chemicals [START_REF] Xu | Recent advances on solid acid catalyic systems for production of 5-Hydroxymethylfurfural from biomass derivatives[END_REF]. HMF can be conveniently catalytically transformed by hydrogenation into a span of industrially-viable products, among which 2,5-dimethylfuran (DMF) has been given a significant attention due to its potential application as biofuel or fuel additive. Compared to known benchmarks like ethanol, DMF has higher energy density and boiling point as well as octane number. It benefits at the same time from of lower volatility and positively is not miscible with water. Consequently, selective production of DMF from HMF with high yield is of great interest [START_REF] Mäki-Arvela | Catalytic Hydrogenation/Hydrogenolysis of 5-Hydroxymethylfurfural to 2,5-Dimethylfuran[END_REF][START_REF] Le | Production of HMF and DMF biofuel from carbohydrates through catalytic pathways as a sustainable strategy for the future energy sector[END_REF][START_REF] Esteves | Effect of support on selective 5-hydroxymethylfurfural hydrogenation towards 2,5dimethylfuran over copńper catalysts[END_REF][START_REF] Wang | Tuning dual active sites of Cu/CoCeOx catalysts for efficient catalytic transfer hydrogenation of 5-hydroxymethylfurfural to biofuel 2,5dimethylfuran[END_REF][START_REF] Guo | Selective hydrogenolysis of 5-hydroxymethylfurfural to produce biofuel 2, 5-dimethylfuran over Ni/ZSM-5 catalysts[END_REF][START_REF] Wang | Catalytic hydrogenolysis of biomass-derived 5hydroxymethylfurfural to biofuel 2, 5-dimethylfuran[END_REF].

Two competitive hydrogenation pathways of HMF to DMF are shown in Scheme 1, denoted as path A and B. Path A starts with the hydrodeoxygenation (hydroxy group removal) of HMF that yields to 5-methylfurfural . Subsequent reduction of carbonyl moiety produces 5-methylfurfurylalcohol (5-MFA) which in turn undergoes another hydrodeoxygenation leading to DMF as targeted product. Alternatively, 5-MFA may be dearomatized via furan ring hydrogenation giving 5-methyltetrahydrofuran alcohol (5-MTHFA). Both DMF and 5-MTHFA may be further hydrogenated to 2,5-dimethyltetrahydrofuran (DMTHF). In contrast, path B begins with the reduction of the aldehyde group of HMF giving 2,5-bis(hydroxymethyl)furan (BHMF). BHMF may be hydrodeoxygenated to 5-MFA and subsequently follow the same reaction pattern as described for Path A or be saturated to 2,5-bis(hydroxymethyl)tetrahydrofuran (BHMTHF) with furan ring hydrogenation. Although further hydrogenation of BHMTHF is also possible, it is not widely observed [START_REF] Przydacz | Solvothermal hydrodeoxygenation of hydroxymethylfurfural derived from biomass towards added value chemicals on Ni/TiO2 catalysts[END_REF][START_REF] Brzezińska | Self-tuned properties of CuZnO catalysts for hydroxymethylfurfural hydrodeoxygenation towards dimethylfuran production[END_REF].

Considering the span of parallel and consecutive reactions, and consequently the number of possible HMF hydrogenation products, the catalyst choice is of crucial importance. As regards of the DMF production, the catalyst should be tailored with focus on the selectivity to remove both aldehyde and hydroxy groups, without saturating the aromatic ring.

Currently used systems that are often based on noble metals give undesired overhydrogenation products like BHMTHF and DMTHF, [START_REF] Fulignati | Insight into the hydrogenation of pure and crude HMF to furan diols using Ru/C as catalyst[END_REF][START_REF] Luo | Base metal-Pt alloys: A general route to high selectivity and stability in the production of biofuels from HMF[END_REF] or even cracking reactions, e.g. with the formation of furans and the opening of furan ring [START_REF] Luo | Base metal-Pt alloys: A general route to high selectivity and stability in the production of biofuels from HMF[END_REF]. These in turn may cause the catalyst deactivation due to the deposition of carbon impurities [START_REF] Luo | Base metal-Pt alloys: A general route to high selectivity and stability in the production of biofuels from HMF[END_REF][START_REF] Requies | Hydrogenolysis of 5-Hydroxymethylfurfural To Produce 2,5-Dimethylfuran over ZrO2 Supported Cu and RuCu Catalysts[END_REF].

Scheme 1. Reaction pathway of HMF hydrodeoxygenation

Luo et al. demonstrated that carbon supported Pt, Ir, Pd and Ru catalysts can give overhydrogenated products when processing the reaction for too-extended time, with in turn a lowering of the DMF yield. The highest DMF yield obtained for 10%Ir/C and 10%Pd/C slightly overruns 50 % [START_REF] Luo | Comparison of HMF hydrodeoxygenation over different metal catalysts in a continuous flow reactor[END_REF]. Priecel et al. showed also the excessive activity of 5%Ru/C and 5%Ru/CNT, with a yield to the unwanted DMTHF reaching 50% and 65% after 2 h and 4 h of HMF hydrogenation, respectively [START_REF] Priecel | Fast Catalytic Hydrogenation of 2,5-Hydroxymethylfurfural to 2,5-Dimethylfuran with Ruthenium on Carbon Nanotubes[END_REF]. Moreover it was proved that monometallic Ru favours the undesired breaking of C-C bonds instead of the hydrodeoxygenation reaction [START_REF] Tan | Different Product Distributions and Mechanistic Aspects of the Hydrodeoxygenation of m-Cresol over Platinum and Ruthenium Catalysts[END_REF].

Similarly, application of Raney Ni in HMF hydrogenation at 100°C did not prevent the production of over-hydrogenated BHMTHF. Unfortunately, in this case of non-supported catalyst, possibilities to improve the selectivity and to mitigate the share of over-hydrogenation side-products are very limited [START_REF] Kong | Switchable synthesis of 2,5dimethylfuran and 2,5-dihydroxymethyltetrahydrofuran from 5-hydroxymethylfurfural over Raney Ni catalyst[END_REF].

The activity and selectivity of nickel-based supported catalysts strongly depend on the support properties. Metal oxides, well-known for their ability to enhance the adsorption of the C=O group of HMF on the Lewis acid sites of the catalyst, are supports of choice [START_REF] Chimentão | Selective hydrodeoxygenation of biomass derived 5-hydroxymethylfurfural over silica supported iridium catalysts[END_REF].

To address over-hydrogenation [START_REF] Kong | Switchable synthesis of 2,5dimethylfuran and 2,5-dihydroxymethyltetrahydrofuran from 5-hydroxymethylfurfural over Raney Ni catalyst[END_REF], decarbonylation [START_REF] Mäki-Arvela | Catalytic Hydrogenation/Hydrogenolysis of 5-Hydroxymethylfurfural to 2,5-Dimethylfuran[END_REF][START_REF] Duarte | Hydrodeoxygenation of 5-Hydroxymethylfurfural over Alumina-Supported Catalysts in Aqueous Medium[END_REF] and ring opening [START_REF] Zhu | Selective Hydrodeoxygenation of 5-Hydroxymethylfurfural to 2,5-Dimethylfuran over Ni Supported on Zirconium Phosphate Catalysts[END_REF] issues associated to monometallic catalysts, adding a second metal is a worth strategy to modulate the catalyst properties. In contrast to the alleviation of the activity of noble metals, the use of bimetallic catalysts based on non-noble metals like Fe, Co, Ni or Cu is a more economicallyreasonable approach [START_REF] Wang | Insights into the interfacial effects in Cu-Co/CeOx catalysts on hydrogenolysis of 5-hydroxymethylfurfural to biofuel 2,5dimethylfuran[END_REF]. Among them, Co is the most expensive metal, while Cu remains hard to disperse on metal oxide supports, what causes a low catalytic activity. On the other hand, Nibased bimetallic catalysts have already been successfully employed in various hydrogenation reactions, including the transformation of HMF to DMF, and their highly-appealing activity is attributed to specific properties originating from synergistic or bifunctional effects that are not observed in their monometallic counterparts. Indeed, Seemala et al. obtained a high yield to DMF at 200°C after 8 h of reaction over a Cu-Ni/TiO2 catalyst. Interestingly, the formation of a core@shell-like structure by metal segregation with a Cu-enriched surface and a Ni core interacting with the TiO2 support was pointed out as being responsible for reaching high selectivity and high catalytic performance.

Likewise, Fe-containing materials were presented as promising catalysts in several hydroconversion reactions of organic molecules [START_REF] Shi | Ni promotion by Fe: What benefits for catalytic hydrogenation?[END_REF]. In particular, the much higher activity of a Ni-Fe catalyst supported on silica compared to its Ni-Cu counterpart for a similar Ni:Me ratio of 3:1 was first reported by Kumbar et al. in 1992 in the selective hydrogenation of acetophenone and benzonitrile [START_REF] Kumbhar | Geometric and electronic effects in silica supported bimetallic nickel-copper and nickel-iron catalysts for liquidphase hydrogenation of acetophenone and benzonitrile[END_REF]. Recently, Shi et al. indicated that heterogeneities in the structure of reduced Ni-Fe nanoparticles are key to understand the catalytic behaviour of SiO2 supported bimetallic Ni-Fe systems in the liquid-phase hydroconversion of furfural into furfuryl alcohol [START_REF] Shi | Probing the core and surface composition of nanoalloy to rationalize its selectivity: Study of Ni-Fe/SiO2 catalysts for liquid-phase hydrogenation[END_REF]. However, in spite of multiple works reporting on the successful application of Ni-Fe catalysts, it must be said that barely a few works explains the role of Ni-Fe interactions on the catalytic activity of supported Ni-Fe bimetallic systems for the hydrogenation of HMF [START_REF] Przydacz | Highly Efficient Production of DMF from Biomass-Derived HMF on Recyclable[END_REF][START_REF] Yu | Robust and recyclable nonprecious bimetallic nanoparticles on carbon nanotubes for the hydrogenation and hydrogenolysis of 5-hydroxymethylfurfural[END_REF]. Yu et al. showed that even a small addition of Fe to Ni catalyst positively affected the selectivity of the HMF hydrogenation to BHMF and mitigated the formation of undesired by-products, while the highest yield to BHMF was achieved on a 10 wt.% Ni50Fe50/CNT catalyst [START_REF] Yu | Robust and recyclable nonprecious bimetallic nanoparticles on carbon nanotubes for the hydrogenation and hydrogenolysis of 5-hydroxymethylfurfural[END_REF]. We recently showed also the beneficial effect of the formation of a Ni-Fe alloy for the selective cleavage of the C-O bond. We proved that the introduction of Fe to a high surface area TiO2 supported Ni catalyst not only can improve the selectivity of the HMF hydrogenation to DMF, but also helps to improve the catalyst stability [START_REF] Przydacz | Highly Efficient Production of DMF from Biomass-Derived HMF on Recyclable[END_REF]. The 5:5 Ni:Fe ratio was the optimal ratio allowing the highest selectivity towards DMF to be achieved. This proof of principle study showed that further investigation remains necessary for getting better knowledge on the keyparameters driving the optimal physicochemical properties of the catalysts, and how they can tune the catalytic activity, particularly taking into account the very versatile and tunable character of Fe-Ni systems. Therefore, our work aims at investigating to which extent the performance of TiO2 supported Ni-Fe bimetallic catalysts in the HMF hydrogenation to DMF can be influenced by the preparation method, and notably by the temperature of the catalyst reduction. Key-factors responsible for the catalyst performances were derived and related to the main physicochemical properties of the bimetallic Ni-Fe/TiO2 catalysts. Particularly the structure and the alloy composition were found to determine the HMF conversion and to tune the yield to the reaction products.

Experimental

Materials and methods

TiO2 support (PC500) was delivered by Millenium-Crystal (France). 5-Hydroxymethylfurfural (98%) was purchased from Fluorochem Ltd (United Kingdom) and metal precursors Ni(NO3)2 • 6•H2O (98%) and Fe(NO3)3•9 H2O (99 %) were supplied by Chempur (Poland). 1,4-Dioxane (99%) used as solvent was provided by Merck (Germany).

Catalyst preparation

Bimetallic catalysts based on nickel and iron were prepared with a nominal content of each metal of 5% wt. relative to the TiO2 support, and labelled as NiFe (S1), NiFe (S2), NiFe (P) and NiFe (C).

The NiFe (S1) catalyst was prepared by a two-step wet impregnation method. First, an appropriate amount of metal precursor Ni(NO3)2•6 H2O was dissolved in distilled water. After addition of TiO2 PC 500, the mixture was kept for 24 h. Excess of solvent was removed using rotative evaporator and the solid residue was dried at 120°C for 2 h and further calcined at 500°C for 5 h under air flow. Second, Fe(NO3)3•9H2O was dissolved in distilled water and calcined NiO/TiO2 was added and mixed. After 24 h, the solvent was removed using a rotative evaporator, the solid residue was dried at 120°C for 2 h and the catalyst was further calcined at 500°C for 5 h. Finally, the catalyst was reduced at 500°C for 1 h under H2 flow.

The NiFe catalyst (S2) was prepared in a similar way than NiFe (S1), except that iron was deposited first.

The NiFe (P) catalyst was prepared by the precipitation method. First Fe(NO3)3•9H2O was dissolved in distilled water and mixed with TiO2 PC 500 under stirring. After dropwise addition of 5.5 mL of 25% NH3 (aq.) at room temperature, the slurry was heat to 85°C and stirred for 24 h. The mixture was cooled down to room temperature, and the solid material was separated from the solution by filtration and washed with distilled water until neutral pH. The sample was further dried at 100°C for 24 h and calcinated at 400°C for 5 h under air flow. Next, Ni(NO3)2•6 H2O was introduced using the wet impregnation method as for other catalysts.

Finally, the catalyst was reduced at 500°C for 1 h under H2 flow directly before reaction.

The NiFe (C) catalysts were prepared by the wet co-impregnation method. Both Fe(NO3)3•9H2O and Ni(NO3)2•6 H2O precursors were dissolved in distilled water and TiO2 PC 500 was added. The mixture was kept under thorough stirring for 24 h. The water solvent was further removed using a rotative evaporator and the solid residue was dried in 120°C for 2 h. After calcination at 500°C for 5 h under air flow, the catalyst was reduced at a temperature ranging from 200°C to 600°C for 1 h under H2 flow directly before reaction.

For comparison, Ni and Fe monometallic catalysts were prepared by a one-step wet impregnation method in a similar way than NiFe (S1) or NiFe(S2), with a final reduction temperature of 500°C.

Independently of the catalyst, the oxidation step was performed with a heating rate of 5°C/min and a 20 cm 3 /min flow, while a heating rate of 15°C/min and a 50 cm 3 /min flow were used for the final reduction step. In all cases, the catalysts after reduction were cooled down to room temperature under H2 flow before being rapidly transferred to the reactor.

Characterization techniques

Inductively coupled plasma optical emission spectroscopy (ICP-OES) was carried out on an Optima 7000 DV spectrometer (Perkin Elmer), after a microwave-assisted acidic dissolution in aqua regia at 185°C under autogenic pressure. Independently on the preparation method, real Fe and Ni weight contents were measured at 5.0% ± 0.2%." X-ray diffraction (XRD) measurements were collected using a PANalyticalX'Pert Pro MPD diffractometer. The X-ray source was a copper long fine focus X-ray diffraction tube operating at 40 kV and 30 mA. Data were collected in the 5-90° 2θ range with 0.0167° step.

Crystalline phases were identified by references to the ICDD PDF-2 (version 2004) database.

Surface area and porosimetry measurements were carried out on ASAP2020

Micromeritics using N2 as adsorbant at -196°C, with a prior outgassing under vacuum at 200°C for 3 h in order to desorb the impurities or moisture.

Temperature-programmed desorption of NH3 (NH3-TPD) was used to assess the acidity of the catalysts. In a typical TPD protocol, the reduced sample was placed in a quartz flow reactor and heated at 500°C under He flow for 1 h to remove impurities from the surface.

Adsorption of NH3 was carried out at 100°C for 15 min. Physically-adsorbed NH3 molecules were removed by treating the sample with He for 15 min at 100°C before the TPD experiment was carried out from room temperature to 500°C with a 25°C min -1 temperature ramp (TCD detector).

Temperature-programmed reduction (TPR) was used for examining the catalyst reducibility, and performed on the AMI1 system from Altamira Instruments, USA, equipped with TCD. Before the measurements, the reduced catalysts were heated at 300°C for 30 min in a mixture of 2 vol.% O2 and 98 vol.% Ar at a space velocity W/F = 1.11×10 -5 g/h cm 3 (10°C/min heating rate). TPR profiles were recorded from 35°C up to 800°C, with a heating rate of 7°C/min, using a mixture of 5 vol.% H2 and 95 vol.% Ar at a similar space velocity. Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) measurements were performed using an ION-TOF GmbH instrument (TOF-SIMS IV) equipped with a 25 kV pulsed Bi + primary ion gun in the static mode. The samples were fixed to the sample holder by double sided adhesive tape. The analyzed area of the sample surface was 500 µm x 500 µm. During the analysis, a pulsed low-energy electron flood gun was used for charge neutralization.

CO-adsorbed FTIR spectra were registered on a Nicolett 6700 spectrometer equipped with MCT detector and a diffuse reflection chamber. Prior to exposure to CO, the samples were reduced at an appropriate temperature in 5% H2 in Ar for 1 h, then cooled to room temperature in an Ar flow to remove the adsorbed hydrogen and the background spectra were collected. CO adsorption was carried out for 30 min at 5 bar pressure of CO (5 vol.%) in Ar. Each spectrum was recorded with a resolution of 4 cm -1 performing 64 scans.

Transmission electron microscopy (TEM) mapping characterization was performed in a JEOL 2100F corrected Scanning Transmission Electron Microscope (STEM) equipped with a energy-dispersive X-ray (EDX) detector. The microscope was operated at 200 kV acceleration voltage.

Catalytic tests

The catalysts were tested in the 5-hydoxymethylfurfural (5-HMF) hydrogenation. The activity tests were performed in a 60 mL stainless-steel batch autoclave (Premex, Switzerland).

The reactions were carried out with 1 g of 5-HMF, 0.15 g of catalyst and 30 mL of 1,4-dioxane.

The reactor was flushed twice with hydrogen to remove air and further pressurized with hydrogen to 50 bar at room temperature. The reaction was performed at 220°C for 1 h with a stirring rate of 650 rpm. Afterwards, the reactor was cooled down to room temperature and the reaction mixture was centrifuged for 5 min at 3500 rpm to separate the catalysts from the liquid sample.

Reaction products were analysed using a GC instrument (Agilent 7820 A) equipped with a FID detector and high polarity wax column Agilent J&W CP-Wax 52 CB. Essential parameters were the following: injection volume 1.0 µL, inlet temperature 300°C, detector temperature 300°C, split flow 163.69 mL/min, column flow 1.6369 mL/min (N2). The initial column temperature was 50°C (5 min) with a temperature rise of 12°C/min and a final temperature of 200°C (20 min). The concentration of each compound in the product mixture was determined using calibration curves of pure compounds solution.

The activity of the catalysts were expressed in terms of HMF conversion and of reaction yields to given products, calculated as follows:

!"#$%&'("# = # *+, --# *+, / # *+, - • 100% 4(%56 7 = # 7 # *+, - • 100%
where n(HMF)i and n(HMF)r are the number of moles of HMF molecules, before and after test, respectively, and n(p) is the number of moles of a given product within the reaction mixture.

Results

Characterization of the TiO2 supported mono and NiFe bimetallic catalysts

The XRD patterns of the bimetallic nickel-iron catalysts are shown in Figure 1 and their main physico-chemical properties are listed in Table 1. The characteristic signature of the TiO2 38-0419) [START_REF] Lu | Hierarchical FeNi3 assemblies with caltrop-like architectures: Synthesis, formation mechanism and magnetic properties[END_REF][START_REF] Yu | Boosting the Oxygen Evolution Reaction by Controllably Constructing FeNi3/C Nanorods[END_REF][START_REF] Zheng | Tunable Electromagnetic and Microwave Absorption Properties of Magnetic FeNi3 Alloys[END_REF]. It should be noted that the body-centred cubic (bcc) phase of the NiFe alloy was not observed in our conditions, in agreement with Wojcieszak et al. on NiFe/SiO2 systems [START_REF] Shi | Probing the core and surface composition of nanoalloy to rationalize its selectivity: Study of Ni-Fe/SiO2 catalysts for liquid-phase hydrogenation[END_REF]. In NiFe alloys, the bcc phase is usually reported to be stable with small Ni concentrations [START_REF] Niu | Ab initio phase stability of some cubic phases of ordered Ni-Fe alloys at high temperatures and pressures[END_REF]. In comparison to the bare un-treated TiO2 support with a specific surface area of 322 m 2 /g, the NiFe bimetallic supported catalysts display lower surface areas of about 110 m 2 /g, 80 m 2 /g and 13 m 2 /g for a reduction temperature of 200-400°C, 500°C and 600°C, independently of the preparation method. For the TiO2 PC500 support with a mean crystallite size of 7 nm, it is known that the specific surface area and the pore volume of the catalysts are directly related to the mean size of the TiO2 crystallites, that is increasing with the increase in the temperature of the thermal treatments. This was visualized by the gradual sharpening of the main diffraction peak of the anatase phase of the TiO2 support, that corresponded to an increase in the mean anatase crystallite size up to 14 nm and 24 nm at 500°C and 600°C, respectively. This was associated to a decrease in the pore volume too.

< Table a The mean crystallite size for TiO2 defined as the average size of the coherently diffracting domains, was determined by applying the Scherrer equation to the (101) peak of anatase at 25.7°, with the usual assumption of spherical crystallites taking into account the intrinsic broadening of the peaks due to the instrumentation.

NH3-TPD was used to determine the acidity of the catalysts, summarized in Table 1. The acidity of both monometallic catalysts stays in the same range with a slightly higher acidity for the Fe catalyst, that can be explained by its stronger oxyphilic character and affinity to the Oatom [START_REF] Shi | Ni promotion by Fe: What benefits for catalytic hydrogenation?[END_REF][START_REF] Han | The synergistic effect between Ni sites and Ni-Fe alloy sites on hydrodeoxygenation of lignin-derived phenols[END_REF]. The interaction between Fe and O atoms of support can lead to a partial positive charging of the Fe atoms that in turn favour NH3 adsorption and enhances the surface acidity [START_REF] Yi | Plasma-assisted ammonia decomposition over Fe-Ni alloy catalysts for COx-Free hydrogen[END_REF]. As regards of the monometallic counterparts, the strongly higher acidity of bimetallic NiFe catalysts reveals the key-role played by the NiFe alloy in interaction with the TiO2 support, that creates additional (novel) metallic surface sites acting as centers for NH3 adsorption. However, the results showed that the acidity of the bimetallic catalysts reduced at 500°C depends on its synthesis method, the lowest and the highest acidity being obtained for the NiFe (S2) and NiFe (S1) catalysts, at 475 µmol/g and 670 µmol/g, respectively.

The number of acidic centers on the surface of the bimetallic NiFe catalysts depends also on its reduction temperature. The highest acidity was observed for the catalyst NiFe (C) reduced at the low temperature of 200°C. This could be explained by the residual presence of metal oxides on the catalyst surface, that are known to act as additional Lewis acid sites next to the acid sites of titania [START_REF] Belokopytov | An infrared study of the Surface Properties of Metal Oxides 2. The Interaction of Ammonia with the Surface of Fe2O3, ZnO, MoO3, and V2O5[END_REF][START_REF] Auroux | Microcalorimetric study of the acidity and basicity of metal oxide surfaces[END_REF]. The acidity of the surface decreases with increasing the reduction temperature to 400°C due to the gradual reduction of nickel and iron oxides into metallic phases.

The acidity of the catalyst reduced at 500°C was slightly higher -590 µmol/g. The significantly lower acidity of the catalyst reduced at 600°C is caused probably by the decrease of the surface area of the support and the increased size of the metal particles.

The reducibility of the NiFe catalysts prepared using different methods was studied by TPR measurements. Figure 2 shows the reduction profiles recorded on the bimetallic catalysts as well as on their monometallic counterparts. They are characterized by the presence of a single reduction peak for all investigated catalysts, that however differs in terms of temperature range (peak width) and maximum. The reduction of the monometallic Ni catalyst takes place in one step with a maximum of hydrogen consumption at ca. 425°C that indicates a relatively strong interaction between the metal and the support surface [START_REF] Liu | Enhanced low-temperature activity of CO2 methanation over highly-dispersed Ni/TiO2 catalyst[END_REF][START_REF] Lin | High performance of Ir-promoted Ni/TiO2 catalyst toward the selective hydrogenation of cinnamaldehyde[END_REF]. By contrast, monometallic Fe catalyst displays a single reduction peak with a maximum around 340°C assigned to the twostep iron oxides reduction, as typically observed for low-loaded iron catalysts [START_REF] Ibrahim | Influence of support type and metal loading in methane decomposition over iron catalyst for hydrogen production[END_REF]. As far as the bimetallic Ni-Fe catalysts were concerned, the reduction profiles displayed a main peak ranging from 300°C to 500°C attributed to the reduction of oxide phases of both metals. They evidenced a shift towards lower temperatures of the maximum of hydrogen consumption, at 415°C, 405°C, 385°C and 390°C for NiFe (S1), NiFe (S2), NiFe (P) and NiFe (C)-500 catalysts, respectively. This suggests that, whatever the preparation methods, both metal species are in close contact which affects their reducibility [START_REF] Yu | Robust and recyclable nonprecious bimetallic nanoparticles on carbon nanotubes for the hydrogenation and hydrogenolysis of 5-hydroxymethylfurfural[END_REF]. In an interdependent manner, the presence of Fe can induce the reduction of nickel oxide, while the presence of Ni can inhibit the reduction of iron oxide [START_REF] Huynh | Promoting effect of Fe on supported Ni catalysts in CO2 methanation by in situ DRIFTS and DFT study[END_REF]. Moreover, the diffusion of Fe atoms into the Ni crystallites during the formation of the alloy structure is proposed to weaken the interaction of the host metal with the support, what in turn may also facilitate the reducibility of Ni species. It must be noted that among all bimetallic catalysts, the sharpest reduction peak in a narrow temperature range was observed for the Ni-Fe(C)-500 sample. This might evidence the strongest synergy between both metals during the reduction of the oxides, able to impact in an homogeneous way on the reduction temperature of both species. FTIR spectra of CO adsorbed on the NiFe bimetallic catalysts were recorded to characterize the effect of both the reduction temperature and the preparation method on the qualitative and quantitative change of adsorption sites on the surface of the bimetallic catalysts (Figure 3).

Under given conditions, it must be noted that CO does not adsorb at the surface of Fe species, so that all the bands are attributed to CO adsorbed on nickel species [START_REF] Przydacz | Highly Efficient Production of DMF from Biomass-Derived HMF on Recyclable[END_REF].

Typically, in the 2000-2100 cm -1 range, the bands reflect the linear adsorption of CO on metallic nickel crystallites. In particular, the bands at 2084 cm -1 , 2056 cm -1 and 2025 cm -1 are assigned to CO linearly-adsorbed on highly dispersed metal crystallites, to physically-adsorbed CO and to the linear adsorption of CO on moderately-dispersed metal, respectively [START_REF] Mihaylov | Formation of Ni(CO)4 during the interaction between CO and silica-supported nickel catalyst: An FTIR spectroscopic study[END_REF][START_REF] Bartholomew | The stoichiometry of hydrogen and carbon monoxide chemisorption on alumina-and silica-supported nickel[END_REF].

First it is worth noting that both parameters investigated did not significantly influence the nature of the surface sites, as similar bands were observed (eg. no appearance of lowerwavenumber bands assigned to bridged CO adsorption, no shift in wavenumbers). Fig. 3A shows that the preparation method has an important effect on the availability of Ni at the surface of the bimetallic NiFe catalysts. Indeed, in terms of band intensity, the following ranking was The influence of both the preparation method and the reduction temperature on the normalized intensity of selected ions identified on the catalyst surface is shown in Fig. 4.

Interestingly a similar normalized intensity was recorded for Ni + and Fe + in the case of NiFe(S1)

and NiFe (C)-500 catalysts in contrast to both NiFe(S2) and NiFe(P) catalysts for which Fe was predominantly exposed on the surface in comparison to Ni, in a larger extent in the case of the NiFe(P) catalyst. The reduction temperature had as well a prime importance on the relative presence of Ni and Fe species at the surface. Here, considering the accuracy of the measurements, it can be noted that in the 200-500°C temperature range, similar Ni + /Fe + ratios were observed, while increasing the reduction temperature to 600°C led to a drastic change of the surface composition, with a pronounced increase in the Fe presence at the catalyst surface.

Catalytic activity

Table 2 shows the activity of the bimetallic NiFe catalysts in the HMF hydrodeoxygenation, expressed in terms of HMF conversion and yields to the different products. Regardless of the preparation method, a high conversion of the HMF substrate (over 94 %) was obtained with each catalyst, and almost no ring hydrogenation products (BHMTHF, MTHFA, DMTHF) were observed. By contrast, the catalysts strongly differentiate in terms of distribution of the reaction products. The highest yield towards DMF was obtained on NiFe (C)-500 and NiFe (S1) catalysts, at 71% and 61%, respectively. Both NiFe(S2) and NiFe (P) catalysts displayed far lower DMF yields of 23% and 20%, respectively, with a more homogeneous distribution towards the main products observed, namely 5-MFA, BHMF and DMF.

< Table 2 >

Table 2. Influence of the preparation method on the activity of the NiFe catalysts reduced at 500°C.

Catalysts HMF conversion [%]

Product Next, the influence of the reduction temperature of the NiFe(C) prepared by the wet coimpregnation method on the catalyst activity is shown in Table 3. The catalyst treated at the temperature of 200°C showed a very low activity, with a HMF conversion of 11% and no DMF formation. This could be explained by the residual presence of metal oxides on the catalyst surface. A significant increase in activity was observed after reduction of the catalyst within the 300-500°C temperature range. Reactions with the catalysts reduced at 300°C and 400°C allowed complete HMF substrate conversion to be achieved, together with a similar DMF yield of 69%. The highest performance was shown by the catalyst reduced at 500°C, namely a nearlyfull HMF conversion of 96%, and the highest DMF yield of 71%. By contrast, reducing the catalyst at the higher temperature of 600°C caused a drastic decrease in the activity of the catalyst, with a low HMF conversion of 42%, and a very low DMF yield of 5%.

< Table 3 >

Discussion

The activity of the TiO2 supported NiFe bimetallic catalysts in the hydrogenation of HMF into DMF was investigated, and several key factors with significant influence on the reaction have been identified by studying the influence of both the preparation method and the reduction temperature. The results showed that the structure of the NiFe bimetallic particles strongly impacted the catalytic behaviour. To this end, additional TEM analysis with EDX elementary mapping and line scan profile has been conducted on both NiFe(C)-500 and NiFe(C)-600 catalysts, as they displayed respectively the highest and the lowest DMF yield among the series of catalysts (Figure 5).

First, the catalysts cannot be considered sensu stricto as supported catalysts, with dispersed metallic nanoparticles supported on a TiO2 host, due to similar particle sizes between the anatase crystallites and the metallic phase. In-depth characterization of these systems is challenging, as they are known to show a poor Z-contrast, that prevents from a doubtless identification of phases by TEM imaging [START_REF] Shi | Probing the core and surface composition of nanoalloy to rationalize its selectivity: Study of Ni-Fe/SiO2 catalysts for liquid-phase hydrogenation[END_REF]. However, as regards of the NiFe(C)-500 catalyst, the EDX mapping and the line scan profile confirmed the formation of NiFe alloy structures with a core/shell structure (Fig. 5a-e), with a Ni-rich core, and a majority of Fe as Fe-rich phases or nanoparticles composing the shell. In agreement with recent reference works on bimetallic systems [START_REF] Palazzolo | The delicate balance of phase speciation in bimetallic nickel cobalt nanoparticles[END_REF][START_REF] Nguyen | Bimetallic Phosphide (Ni,Cu)2P Nanoparticles by Inward Phosphorus Migration and Outward Copper Migration[END_REF], larger red areas in comparison to green ones (Fig. 5a vs. 5b) , and the typical EDX line profile with only Fe visible at both sides (Fig. 5e), were observed. The partial (nonperfect) nature of the core-shell structure is suggested as red areas corresponding to Ni atoms remain visible on the overlay image (Fig. 5c). This is in good agreement with both CO adsorbed-FTIR and ToF-SIMS analyses that showed the clear availability of Ni species at the surface of the NiFe alloy.

By contrast, the higher reduction temperature of 600°C caused a redispersion and homogenisation of the Fe atoms within the NiFe alloy, as no core-shell structure was evidenced on the EDX mapping and the line scan profile (Fig. 5f-j). The temperature probably allowed surface diffusion of the Fe atoms of Fe-rich shell, in agreement with the lower availability of Ni species evidenced by CO adsorbed-FTIR and ToF-SIMS. It also allowed enhanced diffusion to the bulk of the alloy, in agreement with the Fe-enrichment of the Ni core observed by XRD. Our earlier works showed that the formation of a NiFe alloy influences both activity and selectivity in the HMF hydrogenation over bimetallic Ni-Fe catalysts [START_REF] Kumbhar | Geometric and electronic effects in silica supported bimetallic nickel-copper and nickel-iron catalysts for liquidphase hydrogenation of acetophenone and benzonitrile[END_REF]. In the present work, we are depicting further that the structure of the NiFe alloy is of prime importance, in particular the surface availability of Fe and Ni species, and impacts the behaviour of the catalyst.

We suggest that the discrepancy between the bulk and the surface composition of the NiFe alloy is a key-feature driving the catalytic performances. Indeed, the core/shell structure of the alloy was identified as a Fe-containing Ni-rich phase with fcc structure [START_REF] Kong | Rational design of Ni-based catalysts derived from hydrotalcite for selective hydrogenation of 5hydroxymethylfurfural[END_REF]. The higher diffusion coefficient for Fe in comparison to Ni allows the incorporation of Fe into the bulk structure of Ni. Dynamic changes of the alloy composition were observed especially with the increase of the reduction temperature of the catalysts where more Fe atoms were incorporated into the bulk structure [START_REF] Hajalilou | Nanostructured soft magnetic materials synthesized via mechanical alloying: a review[END_REF]. However, it should be mentioned that the Fe-enriched core remained Ni-rich, and the bcc phase of the NiFe alloy that is characteristic of the Fe-rich alloy was not observed in our case.

The composition of the surface of the NiFe alloy differs strongly from its bulk one, and in particular with a drastic change observed between samples reduced at 500°C and 600°C. The The lowering of the Fe:Ni ratio can also be related to the way hydrogen is chemisorbed.

Indeed, hydrogen chemisorption occurs preferentially on Ni rather than on Fe [START_REF] Kim | Cooperativity and dynamics increase the performance of NiFe dry reforming catalysts[END_REF], what provides in consequence much stronger SMSI effect on Ni. This might result in the migration of titania on Ni, and the consequent reduction of the accessibility of Ni. In particular, that effect could be pronounced at the high temperature of 600°C, for which the SMSI effect is stronger, as well as for both FeNi(S2) and FeNi(P) catalysts, for which Ni was introduced in the last step.

Ni is known to be very active in the hydrogenation of HMF, but however not very selective [START_REF] Przydacz | Solvothermal hydrodeoxygenation of hydroxymethylfurfural derived from biomass towards added value chemicals on Ni/TiO2 catalysts[END_REF][START_REF] Shi | Probing the core and surface composition of nanoalloy to rationalize its selectivity: Study of Ni-Fe/SiO2 catalysts for liquid-phase hydrogenation[END_REF]. The positive impact of Fe addition may be explained by the change of HMF adsorption geometry on the NiFe bimetallic system compared to the monometallic Ni catalyst. We propose that the adsorption of HMF occurs on monometallic Ni through both furan ring and carbonyl groups, and on the NiFe alloy only through the carbonyl group, thus disfavouring the hydrogenation of the aromatic ring that would lower strongly the selectivity to DMF. This was recently suggested by Chen et al. in the case of the hydrogenation of furfural [START_REF] Yu | Theoretical and experimental studies of the adsorption geometry and reaction pathways of furfural over FeNi bimetallic model surfaces and supported catalysts[END_REF]. In that case, the Ni atoms are responsible for the hydrogenation and the Fe atoms are beneficial for the repulsion of the furan ring. In consequence, achieving high selectivity to DMF with a high HMF conversion on Ni-Fe catalysts would require the presence of metallic Ni sites available at the surface and maintained dispersed thanks to Fe-containing surface phases, that favour the creation of Ni-Fe interfaces and prevent from the hydrogenation of the ring.

Conclusions

We demonstrated the ability of earth-abundant non-noble bimetallic NiFe alloy supported on TiO2 to be active and selective catalysts for the high yield production of 2,5-dimethylfuran from the biomass-derived 5-hydroxymethylfurfural platform molecule, as a green biofuel competitive to benchmarks like ethanol. Using TiO2 PC500 as host support, we studied the influence of both the preparation method and the reduction temperature on the catalyst performances, and we showed that they both strongly impacted the catalyst behaviour and in turn the yield to DMF. Dynamic changes of the alloy properties were observed especially with the increase of the reduction temperature of the catalysts where more Fe atoms were incorporated into the bulk structure.

The catalyst performance is directly influenced by the structure of the NiFe alloy, with a key-feature relying on the discrepancy between its bulk and surface composition. Among the series of catalysts studied, the catalyst prepared by co-impregnation of both metal precursors and reduced at 500°C led to the highest yield to DMF of 71%, with a nearly full HMF conversion of 96%. It features a core/shell structure with an Fe-containing Ni-rich core surrounded/decorated by a partial/unperfect Fe-rich shell. This surface structure and composition allow for the availability of highly active Ni domains at the alloy surface, maintained dispersed by the exposure of Fe-rich surface phases, and is proposed to promote the HMF adsorption through the carbonyl group, while preventing from the further hydrogenation of the aromatic furan ring to maintain high selectivity. By contrast, the availability of surface Ni species is strongly lowered by the migration/redispersion of Fe surface species and the resulting loss of the core/shell structure when the catalyst is reduced at a too high temperature, what in turn leads to lower HMF conversion.

  anatase support was observed, and all the support reflexes were indexed in the I41/amd tetragonal unit cell of the anatase TiO2 polymorph, notably at 2θ = 25.3°, 37.8°, 48.1°, 53.9°, 55.1°, 62.7°, 68.8°, 70.3°, 75.0°. No peaks suggesting the presence of metallic or oxide phases of iron or nickel were detected. By contrast, the diffraction peaks observed within the regions 43-44° and 50-51° in 2θ were assigned respectively to the (111) and (200) reflections of Nirich Ni-Fe solid solution (NiFe alloy) with face-centered-cubic (fcc) structure (JCPDS card no.

  The low-angle shift observed in comparison to the characteristic 2θ value of metallic Ni (111) (2θ = 44.4° -JCPDS 04-0850) is also reported to evidence the formation of the Ni-rich NiFe alloy[START_REF] Putro | Selective hydrogenation of unsaturated carbonyls by Ni-Fe-based alloy catalysts[END_REF]. The structure of the NiFe alloy strongly depends on both Fe and Ni contents, that are determining the extent of the shift of the reflex position. The peak assigned to the NiFe alloy in the 43-44° region is gradually shifted towards lower angles with the increase in the catalyst reduction temperature from 300°C to 600°C, the shift at 2θ = 43.8° being maximum after reduction at 600°C. This is related to the dynamic change of the alloy composition. The diffusion of iron atoms into the host nickel crystallites causes an increase in the lattice parameters due to the larger radius of Fe in comparison to Ni.[START_REF] Hajalilou | Nanostructured soft magnetic materials synthesized via mechanical alloying: a review[END_REF][START_REF] Ebrahimi | Structure and properties of electrodeposited nanocrystalline FCC Ni-Fe alloys[END_REF][START_REF] Mu | Iron-Doped Nickel Oxide Nanocrystals Alkaline Water Splitting[END_REF]. Thus, the shift of the NiFe alloy peak towards lower angles suggested an enrichment of the alloy composition with Fe. This enrichment in iron of the NiFe alloy was accompanied by a slight increase in the crystallite size from 9 nm at 300°C to 15 nm at 600°C. When it comes to the catalysts prepared using different methods, the peak assigned to the Ni-rich NiFe alloy is also visible at 43.8° for both NiFe(S1) and NiFe(S2) catalysts with a similar average crystallite size. A slight shift towards higher 2θ angles is noticeable for the NiFe(P) catalyst, which suggests a lower Fe enrichment in comparison to other bimetallic samples.

Figure 1 .

 1 Figure 1. Powder XRD patterns of TiO2 supported NiFe bimetallic catalysts, (A) influence of

Figure 2 .

 2 Figure 2. TPR profiles of the TiO2 supported mono and bimetallic catalysts: a) NiFe (S1); b)

  observed: NiFe (S1) > NiFe (C)-500 and NiFe (S2) > NiFe(P). In particular the availability of Ni for the catalysts prepared by successive impregnation is related to the impregnation sequence, as Ni was introduced last in the case of the NiFe (S1) catalyst, and first for the NiFe (S2) counterpart.

Figure 3 .

 3 Figure 3. CO-adsorbed FTIR spectra of TiO2 supported NiFe bimetallic catalysts. (a) influence

Figure 4 .

 4 Figure 4. Normalized intensity of selected ions calculated on the basis of ToF-SIMS spectra

Table 3 .

 3 220°C; 1 h; 0.15 g of catalyst; 1 g of HMF; 30 ml of dioxane; 30 bar of hydrogen Influence of the reduction temperature on the activity of the NiFe(conditions: 220°C; 1 h; 0.15 g of catalyst; 1 g of HMF; 30 ml of dioxane; 30 bar of hydrogen

Figure 5 .

 5 Figure 5. EDX elementary mapping and line scan profile for the NiFe(C) catalysts reduced at 500°C (top, a-e) and 600°C (bottom, f-j). (a,f) Fe-

  partial or unperfect nature of the Fe shell surrounding the Fe-enriched Ni core observed at 500°C for the most efficient catalyst can be seen as a Fe-rich surface phase or as Fe nanoparticles decorating the Ni-rich core. In both cases, this surface structure and composition allow for the availability of Ni species at the catalyst surface, as evidenced by both CO adsorbed-FTIR and ToF-SIMS analyses. Similar observation was pointed out by Wojcieszak at al. where asymmetrical Fe-rich shell was developed around Ni particles in the case of Ni:Fe/SiO2 catalysts[START_REF] Shi | Probing the core and surface composition of nanoalloy to rationalize its selectivity: Study of Ni-Fe/SiO2 catalysts for liquid-phase hydrogenation[END_REF].By contrast, the absence of real core/shell structure at 600°C resulted simultaneously from the Fe-enrichment of the core and from the migration/redispersion of Fe surface species. The redistribution of surface Fe caused a drastic lowering of the availability of surface Ni species, as visualized by the lower adsorption of CO by FTIR and the strong increase in the Fe:Ni surface ratio in the top-surface layer by ToF-SIMS. This higher Fe:Ni ratio is also observed for NiFe(S2) and NiFe(P) prepared by other methods samples and reduced at 500°C. In those three cases, the high Fe:Ni surface ratio with low Ni accessibility/availability also evidenced by FTIR was associated to low DMF yields (23%, 20% and 5% for NiFe(S2), NiFe(P) and NiFe(C)-600 catalysts, respectively) in comparison to the NiFe(C)-500 catalyst.

Table 1 . Physicochemical properties of the NiFe bimetallic catalysts.

 1

	Catalyst	BET Surface area [m 2 /g]	Total pore volume [cm 3 /g]	NiFe alloy mean cristallite size [nm]	Anatase TiO2 mean crystallite size [nm] a	Acidity [µmol/g]
	TiO2	322	0.46	-	7	-
	Ni	-	-	-	-	130
	Fe	-	-	-	-	180
	NiFe (S1)	80	0.26	10	14	670
	NiFe (S2)	78	0.26	10	14	475
	NiFe (P)	81	0.31	14	14	530
	NiFe (C)-200	110	0.34	-	11	739
	NiFe (C)-300	108	0.36	9	12	553
	NiFe (C)-400	109	0.35	11	12	460
	NiFe (C)-500	81	0.26	12	14	590
	NiFe (C)-600	13	0.14	15	24	220

1 >

Acknowledgements

The authors gratefully acknowledge that this work was financially supported by a grant from the National Center of Science (NCN) in Krakow OPUS-LAP (2020/39/I/ST4/02039). D. Ihiawakrim (IPCMS) is thanked for performing TEM characterizations.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. All authors read and approved the final manuscript.

CReDiT


C Royer

S Bernard

O Beyssac

E Balan

O Forni

Michel Gauthier

M Morand

Y Garino

P Rosier

Impact of UV radiation on the Raman and infrared spectral signatures of sulfates, phosphates and carbonates: implications for Mars exploration

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

that prevailed on Mars during the Noachian [START_REF] Vaniman | Magnesium sulphate salts and the history of water on Mars[END_REF][START_REF] Bibring | Mars Surface Diversity as Revealed by the OMEGA/Mars Express Observations[END_REF][START_REF] Poulet | Phyllosilicates on Mars and implications for early martian climate[END_REF][START_REF] Mustard | Hydrated silicate minerals on Mars observed by the Mars Reconnaissance Orbiter CRISM instrument[END_REF][START_REF] Murchie | A synthesis of Martian aqueous mineralogy after 1 Mars year of observations from the Mars Reconnaissance Orbiter[END_REF][START_REF] Carter | Detection of Hydrated Silicates in Crustal Outcrops in the Northern Plains of Mars[END_REF]. Alternatively, other minerals like phosphates, which may crystallize during magmatic processes, may provide crucial information on the volatile content of the deep planet. Their identification and the determination of their water content may thus shed new light on the early geologic and climatic history of Mars.

Although infrared (IR) spectroscopy has long been used from orbit only with CRISM or OMEGA to identify minerals on Mars [START_REF] Bibring | Mars Surface Diversity as Revealed by the OMEGA/Mars Express Observations[END_REF][START_REF] Poulet | Phyllosilicates on Mars and implications for early martian climate[END_REF][START_REF] Mustard | Hydrated silicate minerals on Mars observed by the Mars Reconnaissance Orbiter CRISM instrument[END_REF][START_REF] Murchie | A synthesis of Martian aqueous mineralogy after 1 Mars year of observations from the Mars Reconnaissance Orbiter[END_REF][START_REF] Carter | Detection of Hydrated Silicates in Crustal Outcrops in the Northern Plains of Mars[END_REF][START_REF] Ehlmann | Subsurface water and clay mineral formation during the early history of Mars[END_REF], IR data can now be collected from the ground using the SuperCam instrument aboard Perseverance [START_REF] Maurice | Mars 2020 SuperCam Bundle[END_REF][START_REF] Maurice | Mars 2020 SuperCam Bundle[END_REF][START_REF] Fouchet | The SuperCam infrared spectrometer for the perseverance rover of the Mars2020 mission[END_REF]. Furthermore, IR data can be directly confronted to Raman spectroscopic data collected using SuperCam (Wiens et al., 2022;[START_REF] Clavé | Carbonate Detection with SuperCam in Igneous Rocks on the floor of Jezero Crater, Mars[END_REF][START_REF] Beyssac | Petrological traverse of the olivine cumulate Séítah formation at Jezero crater, Mars : A perspective from SuperCam onboard Perseverance[END_REF] or SHERLOC (Bhartia et al., 2021;Scheller et al., 2022a). A similar strategy, i.e., allowing both IR and Raman data to be collected on the same targets, has been adopted by ESA for Rosalind Franklin, the rover of the ExoMars mission, which will be equipped with two IR instruments (ISEM [START_REF] Korablev | The Atmospheric Chemistry Suite (ACS) of Three Spectrometers for the ExoMars 2016 Trace Gas Orbiter[END_REF]MicrOmega Bibring et al. 2017), and one Raman instrument (RLS [START_REF] Rull | The Raman Laser Spectrometer for the ExoMars Rover Mission to Mars[END_REF]. The combined use of IR and Raman spectroscopies at the surface of Mars is expected to allow identification of minerals and quantification of their water content. However, as is the case for organic molecules at the surface of Mars [START_REF] Fornaro | UV irradiation of biomarkers adsorbed on minerals under Martian-like conditions: Hints for life detection on Mars[END_REF][START_REF] Fox | Radiolysis of Macromolecular Organic Material in Mars-Relevant Mineral Matrices[END_REF][START_REF] Megevand | Impact of UV Radiation on the Raman Signal of Cystine: Implications for the Detection of S-rich Organics on Mars[END_REF], these minerals have been bombarded by ionizing radiation. In fact, in the absence of a global magnetic field, the thin CO 2 atmosphere of Mars absorbs most X-rays and far-UV radiation, but lets mid-and near-UV photons in the range 190 -410 nm, gamma rays, solar energetic protons, and galactic cosmic rays reach the surface [START_REF] Patel | Ultraviolet radiation on the surface of Mars and the Beagle 2 UV sensor[END_REF][START_REF] Dartnell | Modelling the surface and subsurface Martian radiation environment: Implications for astrobiology[END_REF]Hassler et al., 2014). Even though UV photons do not penetrate that much below the surface [START_REF] Carrier | Attenuation of Ultraviolet Radiation in Rocks and Minerals: Implications for Mars Science[END_REF], they can interact with minerals exposed at the surface, altering their composition via radiolysis and their structure via the incorporation of electronic defects. Such alteration may modify their spectral responses for techniques sensitive to the crystal structure, chemical nature and hydration signature (H 2 O and OH), such as Raman and infrared spectroscopies [START_REF] Dartnell | Modelling the surface and subsurface Martian radiation environment: Implications for astrobiology[END_REF]Hassler et al., 2014). This may lead to misinterpretations of the collected signals. Worse, most mineral calibration targets having served to characterize instrumental responses are terrestrial analogues which have never experienced the levels of UV radiation bombarding the surface of Mars [START_REF] Manrique | SuperCam Calibration Targets: Design and Development[END_REF]. Experimentally constraining in the laboratory the impact of UV radiation on the structure, water content and spectral signatures of hydrated and non-hydrated minerals is therefore critical to properly interpret the data collected on Mars.

The evolution of the IR spectral signatures of sulfates, phosphates or carbonates experimentally exposed to simulated Mars surface UV radiation conditions has been investigated by a rather limited number of authors [START_REF] Bish | Stability of hydrous minerals on the martian surface[END_REF][START_REF] Cloutis | Stability of hydrated minerals on Mars[END_REF][START_REF] Cloutis | Spectral reflectance properties of minerals exposed to simulated Mars surface conditions[END_REF][START_REF] Poitras | Mars analog minerals' spectral reflectance characteristics under Martian surface conditions[END_REF][START_REF] Turenne | Spectral reflectance properties of minerals exposed to martian surface conditions: Implications for spectroscopy-based mineral detection on Mars[END_REF]). An even lower number of studies have been dedicated to the evolution of their Raman spectral signatures [START_REF] Bonales | Dehydration rate of the glycine-MgSO 4 •5H 2 O complex and the stability of glycine expelled from the complex by in situ Raman spectroscopy under Mars-relevant conditions[END_REF]. Of note, extrapolating laboratory results to natural settings remains difficult, in particular because geological timescales cannot be replicated in the laboratory, questioning the intrinsic relevance of laboratory experiments. Yet, the instruments onboard Perseverance do not only measure rocks having been exposed to UV radiation for millions or billions of years. In fact, Perseverance carries an abrasion tool allowing the exposure of a patch of 5 cm diameter of pristine rock initially lying 2 to 16 mm below the surface [START_REF] Moeller | The Sampling and Caching Subsystem (SCS) for the Scientific Exploration of Jezero Crater by the Mars 2020 Perseverance Rover[END_REF], to be studied using spectroscopic tools within a few Martian Sols. This justifies experimental investigations, even over short timescales. Following a previous study dedicated to the impact of UV radiation on organic molecules [START_REF] Megevand | Impact of UV Radiation on the Raman Signal of Cystine: Implications for the Detection of S-rich Organics on Mars[END_REF], we have therefore conducted laboratory experiments to document the impact of UV radiation on the water content and structure of mineral phases of major interest in the current exploration of Mars.

Here, we first investigated the evolution of the Raman and IR spectroscopic signatures of initially more or less hydrated sulfates (Ca, Na, Mg, Fe, and Al-rich sulfates) during an experimental exposure to UV radiation within a Martian chamber at a temperature relevant for the rocks at the surface of Mars. Sulfates are of particular interest in the context of the Mars2020 mission as they are considered as having a high biopreservation potential [START_REF] Scheller | First-Results from the Perseverance SHERLOC Investigation: Aqueous Alteration Processes and Implications for Organic Geochemistry in Jezero Crater[END_REF], and they may also result from the interaction of water with rocks. We also conducted similar experiments for a slightly longer duration using phosphates (Ca, Na and Mg-rich phosphates) and carbonates (Ca, Na and Mg-rich carbonates) since these minerals are also of high interest for astrobiology [START_REF] Mcmahon | A Field Guide to Finding Fossils on Mars[END_REF] and more generally for geology. The Raman spectra of these minerals were collected during the experiments in situ within the Martian chamber, while their IR spectra were collected at the end of experiments, once the irradiated samples were removed from the Martian chamber.

Materials and Methods

Selected minerals and general strategy

Five sulfates (exhibiting different hydration states), three phosphates (exhibiting different hydration states) and three carbonates (exhibiting different hydration and hydroxylation states) were selected for the present study (cf Table 1). Besides for the hydromagnesite (i.e., the Mg-rich hydrocarbonate for which a natural single crystal was simply taped on a Scanning Electron Microscope stub), we used a mechanical press to prepare pellets of compressed samples (several mg of each mineral were pressed at 2 tons onto a stub of 1 cm in diameter, leading to a final thickness of about 1 mm). Irradiation experiments were conducted in a dedicated Martian chamber built by the Cellule Projet @ IMPMC (Fig. 1) [START_REF] Megevand | Impact of UV Radiation on the Raman Signal of Cystine: Implications for the Detection of S-rich Organics on Mars[END_REF]. In this chamber, samples are exposed to UV radiation at 0°C under a primary vacuum (< 1 mbar). A Martian-like CO 2 atmosphere was not introduced into the chamber to prevent absorption of UV by CO 2 and contamination from air. Previous studies have shown that a few mbars of CO 2 have no influence on the UV induced degradation of amino acids [START_REF] Ten Kate | The effects of Martian near surface conditions on the photochemistry of amino acids[END_REF], nor on the UV induced dehydration of sulfates [START_REF] Bonales | Dehydration rate of the glycine-MgSO 4 •5H 2 O complex and the stability of glycine expelled from the complex by in situ Raman spectroscopy under Mars-relevant conditions[END_REF]. Of note, all the samples have been introduced into the Martian chamber and exposed to the pressure and temperature conditions of the experiments for 15 hours before being exposed to UV radiation. Such exposition to low pressure and temperature conditions did not have any effect on the degree of hydration of the samples investigated here as verified using Raman spectroscopy directly performed in the chamber.

Samples were then exposed to UV produced by a 150 W arc lamp equipped with a high pressure Xenon bulb (UXL-150SP -LOT-ORIEL). Note that this is the exact same lamp which the Mars Organic Molecule Irradiation and Evolution (MOMIE) setup relies on [START_REF] Stalport | Investigating the Photostability of Carboxylic Acids Exposed to Mars Surface Ultraviolet Radiation Conditions[END_REF][START_REF] Stalport | The Photochemistry on Space Station (PSS) Experiment: Organic Matter under Mars-like Surface UV Radiation Conditions in Low Earth Orbit[END_REF][START_REF] Poch | Chemical evolution of organic molecules under Mars-like UV radiation conditions simulated in the laboratory with the "Mars organic molecule irradiation and evolution" (MOMIE) setup[END_REF][START_REF] Poch | Laboratory insights into the chemical and kinetic evolution of several organic molecules under simulated Mars surface UV radiation conditions[END_REF] and which was used to document the impact of UV radiation on cystine [START_REF] Megevand | Impact of UV Radiation on the Raman Signal of Cystine: Implications for the Detection of S-rich Organics on Mars[END_REF]. This lamp delivers a UV spectrum (190 -400 nm) with a pattern similar to that of the Martian surface radiation spectrum [START_REF] Patel | Ultraviolet radiation on the surface of Mars and the Beagle 2 UV sensor[END_REF][START_REF] Dartnell | Modelling the surface and subsurface Martian radiation environment: Implications for astrobiology[END_REF]Hassler et al., 2014), and a flux of the same order of magnitude as the one undergone by the Martian surface, simulating well true Martian conditions. Note that a DUV Grade Fused Silica window allowed rejecting most of the IR radiation (down to 2.89 µm), mitigating a potential heating of the samples by the UV lamp although the 400 nm -2.89 µm range is still available for radiant heating (Fig. 2). Raman spectra of starting and irradiated materials were measured directly in the Martian chamber (from 200 to 2000 cm -1 and from 3000 to 4000 cm -1 ), that is, under the conditions of the experiments. The Raman spectra of sulfate samples were measured before and after 30 and 270 min of exposure to UV radiation, while those of phosphate and carbonate samples were measured before and after 5, 30, 270 and 1500 min of exposure to UV. At the end of the irradiation experiments (i.e., after 270 minutes for sulfates and after 1500 minutes for phosphates and carbonates), samples were removed from the chamber and measured with diffuse reflectance IR spectroscopy (over the 1-2.75 µm range). Note that the contribution of the volume of irradiated materials to the measured signals differed from one technique to another as the total volume probed is different for Raman and IR spectroscopies (and rather difficult to estimate), challenging the direct comparison between the data.

Time resolved Raman and IR reflectance

Following [START_REF] Fau | Pulsed laser-induced heating of mineral phases: Implications for laser-induced breakdown spectroscopy combined with Raman spectroscopy[END_REF][START_REF] Fau | Time-resolved Raman and luminescence spectroscopy of synthetic REE-doped hydroxylapatites and natural apatites[END_REF] and [START_REF] Megevand | Impact of UV Radiation on the Raman Signal of Cystine: Implications for the Detection of S-rich Organics on Mars[END_REF], we relied on a customized time-resolved Raman spectrometer built by the Cellule Projet at IMPMC to collect the Raman spectra of the samples investigated. Within the Martian chamber used here, samples are at a distance of 8 meters from the telescope, mimicking the configuration of remote Raman measurements conducted on Mars using SuperCam. Here, the fine control and synchronization of both time delay and gating time of the camera allows sub-nanosecond time resolution experiments: we collected Raman signals using a 2 ns ICCD gate centered on the 1.2 ns laser pulse. Filters are used to limit the laser irradiance at the surface of samples below 10 10 W/m 2 to prevent any laser-induced damage [START_REF] Fau | Pulsed laser-induced heating of mineral phases: Implications for laser-induced breakdown spectroscopy combined with Raman spectroscopy[END_REF]. In the remote setting, the laser is collimated at the sample surface 8 m from the telescope Schmidt plate on a spot of ∼ 6 mm diameter, and the Raman signal is collected by a conventional Schmidt-Cassegrain telescope (Celestron-C8 202 mm diameter Schmidt plate) on a spot of ∼ 5 mm diameter co-aligned with the laser spot. After the telescope, a notch filter cuts off the Rayleigh scattering at ∼ 90 cm -1 and the signal is collected by an optical fiber and sent into the modified Czerny-Turner spectrometer coupled with the intensified ICCD camera. This spectrometer has three motorized gratings that can be selected depending on the spectral window and spectral resolution requested. Here, we used a 600 gr/mm grating yielding a spectral resolution of about 10-12 cm -1 , i.e., spectral resolution similar to that of the SuperCam instrument. To maximize the signal-to-noise ratio (SNR), we accumulated signals collected over 10 6 laser shots. We also collected the luminescence signal with no Raman contribution using a 100 µs ICCD gate opened just after the laser pulse [START_REF] Fau | Pulsed laser-induced heating of mineral phases: Implications for laser-induced breakdown spectroscopy combined with Raman spectroscopy[END_REF][START_REF] Fau | Time-resolved Raman and luminescence spectroscopy of synthetic REE-doped hydroxylapatites and natural apatites[END_REF][START_REF] Megevand | Impact of UV Radiation on the Raman Signal of Cystine: Implications for the Detection of S-rich Organics on Mars[END_REF].

IR reflectance data were collected with a commercial Fourier transform spectrometer Nicolet 6700 FTIR operating at IMPMC over the 1 to 2.7 µm spectral range with a spectral resolution of 1 cm -1 , allowing for the detection of features related to hydration (H 2 O vibrations) and structure (CO 3 2-and SO 4 2-). The acquired spectra were divided by a blank (a silver mirror) before any further processing.

After being removed from the Martian chamber and prior to IR analysis, samples were conserved in a dry environment at low temperature in a cold room.

Results

Starting materials

The Raman and IR spectra of sulfates, phosphates and carbonates are extensively described in the literature (cf for instance [START_REF] Wang | Sulfates on Mars: A systematic Raman spectroscopic study of hydration states of magnesium sulfates[END_REF][START_REF] Buzgar | The Raman Study of Certain Sulfates. Analele Stiintifice Ale Universitatii[END_REF] For all these minerals, the most intense Raman peak is due to the ν 1 symmetric stretching mode of SO 4 , PO 4 and CO 3 and located at ∼ 1010 cm -1 , ∼ 960 cm -1 and ∼ 1085 cm -1 for sulfates, phosphates and carbonates respectively. Additional peaks due to these molecular groups are the doubly degenerate ν 2 bending mode at ∼ 450 cm -1 and ∼ 430 cm -1 for sulfates and phosphates respectively (the ν 2 vibration peak is not observed in our carbonate samples), the triply degenerate antisymmetric ν 3 stretching mode at ∼ 1160 cm -1 , ∼ 1060 cm -1 and ∼ 1470 cm -1 , and the triply degenerate ν 4 bending mode at ∼ 640 cm -1 , ∼ 580 cm -1 and ∼ 720 cm -1 , always for sulfates, phosphates and carbonates respectively. Last, some lattice modes may be detected in these phases at lower wavenumbers. When present, H 2 O contributes a broad band at ∼ 3400-3600 cm -1 , while HO -contributes one or several sharp bands in the range ∼ 3500-3700 cm -1 (Fig. 3a,b,c).

The near infrared spectra of these materials show numerous absorption bands attributed to different overtones of the stretching (ν 1 and ν 3 ) and bending (ν 2 and ν 4 ) vibrations of the structural anion (SO 4 2-, CO 3 2-or PO 4 3-) combined with the vibrations of water present in the crystal or adsorbed on its surface. In sulfates, we find similar spectra with absorptions at 1.2, 1.5 (triply degenerate in particular in CaSO 4 •2 H 2 O), 1.75, 1.92 and 2.25 µm. The carbonate samples show a wider range of spectra, with the main absorption bands related to structural water in Na 2 CO 3 and to CO 3 in CaCO 3 (at 2.34 and 2.53 µm). Hydromagnesite shows in addition an absorption band around 1.4 µm assigned to the first overtone of the OH stretch and around 1.9 µm corresponding to the combination of stretching and bending of H 2 O. Finally the phosphates present few spectral signatures in the infrared, except a band at about 1.9 µm related to the stretching and bending vibrations of water (Fig. 3d, e, f).

Evolution of sulfates with increasing irradiation

Luminescence

The luminescence signal (collected using a 100 µs ICCD gate opened just after the laser pulse) strongly increases with increasing irradiation duration (Fig. 4). The increase of this luminescence roughly follows a first-order logarithmic law with time (Fig. 4f), highlighting that UV radiation has had the most impact at the beginning of the experiments. This luminescence seems to have a short lifetime, since most of it is not rejected even when using a 2 ns ICCD gate centered on the 1.2 ns laser pulse to collect the signal. Because of this strong luminescence contribution, the Raman spectra collected in the present study have all been normalized to the total signal received by the spectrometer, to discuss the evolution of the Raman signal relative to the background contribution (Fig. 5).

Raman signal

The Raman signal of sulfates (i.e., the Raman peaks of the signal collected using a 2 ns ICCD gate centered on the 1.2 ns laser pulse) decreases with increasing irradiation duration relative to the background contribution as shown when data are normalized to the total signal received by the spectrometer (Fig. 5). Irradiation seems to be responsible for a modification of the background shape (i.e., the slope of the background has increased with increasing irradiation duration). After (Fig. 5f), highlighting, as for the luminescence, that the impact of UV radiation was maximum at the beginning of the experiments.

Raman-based water detection

The specific sub-peak structures in their Raman water bands at about 3400 cm -1 provide direct information on the hydration states of sulfates [START_REF] Wang | Sulfates on Mars: A systematic Raman spectroscopic study of hydration states of magnesium sulfates[END_REF] highly hydrated (Al 2 SO 4 • 18 H 2 O). Besides for the Na-rich sulfate, the exposure of sulfates to UV irradiation appears to be responsible for the disappearance of the peaks related to the structural water they initially contain. In fact, with the exception of the Ca-rich sulfate, all the initially hydrated sulfates investigated do not anymore show Raman hydration feature at about 3400 cm -1 (Fig. 5).

This is also the case for the Ca-rich sulfate after 270 minutes of irradiation.

IR signal

At the end of the irradiation experiments (i.e., after 270 minutes of exposure to UV), the sulfates investigated were measured with IR spectroscopy and their spectra were compared to those of the starting materials (Fig. 6). The absorption bands corresponding to the stretching/bending vibrations of the H 2 O and OH groups, to various combinations of these modes and to overtones with SO 4 modes (e.g., at 2.217 and 2.268 µm in gypsum), can still be observed in the near-IR reflectance spectra of the sulfates having been exposed to UV radiation for 270 minutes. The observed enlargement of the absorption bands after exposure to UVs indicate the creation of electronic defects rather than dehydration. Of note, atmospheric water may have been adsorbed onto the samples during their storage at 0°C before NIR measurements despite the presence of a desiccant.

Evolution of phosphates and carbonates with increasing irradiation

The phosphates and carbonates investigated here were exposed to UV radiation conditions similar to those to which sulfates were exposed, but for a slightly longer duration. The luminescence and Raman signals of the phosphates and carbonates were collected after 5, 30, 270 and 1500 min of exposure to UV and compared to the spectra of the starting materials (Figs. 78). As observed for sulfates, the luminescence signals of phosphates and carbonates strongly increase with increasing irradiation duration, and their Raman signals decrease with increasing irradiation duration relative to the background contribution. Note that, as for sulfates, the Raman spectra of phosphates and carbonates have all been normalized to the total signal received by the spectrometer. Irradiation seems to be also responsible for a modification of the background shape (i.e., the slope of the background has increased with increasing irradiation duration). After 1500 minutes of irradiation, only the main peaks of phosphates and carbonates can still be observed (i.e., the ν 1 peaks at about 960 cm -1 and 1085 cm -1 , of PO irradiated phosphates and carbonates. As for sulfates, results of peak fitting reveal that the decrease of the Raman signal decreases exponentially as a function of the duration of irradiation (Figs. 7d -8d), highlighting, as the luminescence is exponential, that UV radiation has had the most impact at the beginning of the experiments.

Of note, the Raman signal of phosphates evolves a bit faster with experiment duration (peak decrease and background increase) than that of carbonates, suggesting a priori that phosphates are less resistant to UV than carbonates. But this may be related to the ratio of the volume effectively probed by Raman to the volume effectively affected by UV radiation. A higher contribution to the signal of a pristine volume of sample (i.e., a volume unaffected by UV) would artifactually indicate a higher resistance to UV. For instance. the hydromagnesite seems to be the most UV-resistant sample of all the samples investigated here, but it is also the only transparent monocrystal investigated (all the other samples were pressed into pellets). For instance, the volume of pristine hydromagnesite in fine contributing to the Raman spectra collected on this target may thus be a lot larger than the volumes of the pristine parts of the other samples contributing to the measured signals, thereby artifactually minimizing the effect of exposure to UV on the Raman signal. Most of the phosphates investigated here were nominally anhydrous, except for the magnesium phosphate, and thus exhibited no Raman water bands at about 3400 cm -1 and 3600 -3700 cm -1 for OH in hydromagnesite. Still, the negative slope of the background collected from 2500 to 3700 cm -1 on phosphates has changed, becoming less negative with increasing irradiation. Things seem different for carbonates: the signal collected on the Ca-carbonate, nominally anhydrous, has not changed during exposition to UV. In contrast, the exposure to UV has been responsible for the non detection of the Raman peaks related to structural water initially present in the Na and Mg-carbonates, without modifying the slope of the background collected from 2500 to 3700 cm -1 .

NIR data are consistent with Raman data. The near-IR spectra of phosphates have not changed that much during the experiments, but only a gentle absorption band related to hydration at 1.9 µm was observed in the spectra of the starting materials. The near-IR spectra of the Ca and Na-carbonates have not changed that much neither during exposure to UV, in contrast to that of the initially hydrated Mg-carbonate (hydromagnesite): the 1.9 µm band related to structural H 2 O turned shallower whereas the 1.45 µm, attributed to hydroxyl groups, remained the same, although enlarged, which suggests a possible dehydration of the sample induced by irradiation (or local heating) and/or a creation of electronic defects.

Discussion

Impact of exposure to UV radiation

Although their surface color have become a little more yellow under UV, the Raman spectra of the sulfates, phosphates and carbonates exposed to UV under Mars conditions still exhibit the main feature observed in the Raman spectra of the starting materials, i.e., the peak attributed to the ν 1 symmetric stretching modes. This shows that the minerals investigated here can withstand simulated Martian conditions (including UV radiation) for a certain time, although longer exposure to UV radiation may lead to the strong alteration of their structure at the surface, which will strongly affect both IR and Raman signals, as anticipated by earlier experimental studies [START_REF] Mukhin | Experimental simulations of the photodecomposition of carbonates and sulphates on Mars[END_REF]. The non detection of new peak indicate that no transitory phase nor new species have formed during the experiments.

Still, a short exposure to UV has been responsible for a strong increase of the luminescence signal of all the minerals investigated, and for a strong decrease of their Raman signal relative to the background contribution. As is the case for organics exposed to UV radiation under Mars conditions [START_REF] Megevand | Impact of UV Radiation on the Raman Signal of Cystine: Implications for the Detection of S-rich Organics on Mars[END_REF], such evolution could be caused by an increasing concentration of electronic defects contributing to the background. In fact, previous ESR (Electronic Spin Resonance) investigations have reported the creation of radical species in carbonates exposed to UV light [START_REF] Bartoll | Generation and conversion of electronic defects in calcium carbonates by UV/Vis light[END_REF][START_REF] Kabacińska | Nanoscale Effects of Radiation (UV, X-ray, and γ) on Calcite Surfaces: Implications for its Mechanical and Physico-Chemical Properties[END_REF][START_REF] Kabacińska | Generation of UV-induced radiation defects in calcite[END_REF]). An increase in the concentration of point defects may also explain the broadening of IR bands observed for all samples investigated here, as previously reported for a number of materials exposed to ionizing radiation (e.g., [START_REF] Fourdrin | Induced modifications of kaolinite under ionizing radiation: An infrared spectroscopic study[END_REF][START_REF] Megevand | Impact of UV Radiation on the Raman Signal of Cystine: Implications for the Detection of S-rich Organics on Mars[END_REF].

Of note, these defects do not seem to be luminescing centers: concentration quenching would decrease the lifetime of luminescence, as reported for both organic and inorganic materials (Chen and Knutson, 1988; [START_REF] Ju | Concentration quenching of persistent luminescence[END_REF][START_REF] Meza | Luminescence Concentration Quenching Mechanism in Gd2O3:Eu3+[END_REF][START_REF] Green | Solid state concentration quenching of organic fluorophores in PMMA[END_REF][START_REF] Fau | Time-resolved Raman and luminescence spectroscopy of synthetic REE-doped hydroxylapatites and natural apatites[END_REF]. But such concentration quenching does not seem to occur here since the lifetime of the luminescence signal does not change with increasing duration of exposure to UV radiation. The decrease of the Raman signal may be due to the radiolysis of chemical bonds or the formation of electronic point defects, the accumulation of radiation damages possibly leading to modifications of the chemical composition and periodicity of the mineral structure. Note that such a decrease may also be related to the strong increase of the background contribution, eventually obscuring the Raman signal.

Of note, all initially hydrated starting materials do not show anymore Raman features attributed to hydration at the end of the present experiments. Yet, neither [START_REF] Morris | The case against UV photostimulated oxidation of magnetite[END_REF] nor [START_REF] Yen | Stability of hydroxylated minerals on Mars: A study on the effects of exposure to ultraviolet radiation[END_REF] observed UV photodehydration, but [START_REF] Bonales | Dehydration rate of the glycine-MgSO 4 •5H 2 O complex and the stability of glycine expelled from the complex by in situ Raman spectroscopy under Mars-relevant conditions[END_REF] experimentally showed that exposure to UV enhances the kinetics of dehydration. Dehydration may be a direct consequence of the simulated Martian conditions. In fact, [START_REF] Bishop | Low-temperature and low atmospheric pressure infrared reflectance spectroscopy of Mars soil analog materials[END_REF] showed that minerals may dehydrate when only exposed to low partial pressure of H 2 O. Here, all the samples investigated have been exposed to the pressure and temperature conditions of the experiments for 15 hours before being exposed to UV radiation, and no dehydration was observed during this time. Dehydration may also result from an increase of the sample surface temperature under exposure to the lamp flux, as reported by [START_REF] Morris | The case against UV photostimulated oxidation of magnetite[END_REF] while relying on an IR-rich Xe(A) radiation responsible for a large increase (> 500°C) of the surface temperature of rather dark samples such as goethite and lepidocrocite. Yet, the rather low flux generated by the lamp used for the present study (∼ 100 times less intense than that of [START_REF] Morris | The case against UV photostimulated oxidation of magnetite[END_REF]) and the efficient filtering of radiations > 3 µm of the fused silica glass window (leading to a dose delivered to the surface of the samples as low as the Another pending question resides in the volume in fine affected by UV itself. During exposure to UV radiation, only the near surface of the samples investigated is penetrated by UV photons [START_REF] Fornaro | UV irradiation of biomarkers adsorbed on minerals under Martian-like conditions: Hints for life detection on Mars[END_REF][START_REF] Fox | Radiolysis of Macromolecular Organic Material in Mars-Relevant Mineral Matrices[END_REF]. However, the actual thickness irradiated can vary from one sample to another depending on their optical properties, as the volume of sample actually contributing the Raman and IR signals. Plus, it is very likely that a volume of pristine (non-irradiated) material contributes to each of the measurements reported here, and that this volume decreases with increasing duration of exposure to UV as a result of the reduction of the transparency, either due to a color change or a structural alteration. As a result, it remains difficult to properly quantify the effect of exposure to UV for the samples investigated here. Both the increase of the background, likely related to the increasing creation of defects in the irradiated volume, and the decrease of the Raman signal, likely compensated by the contribution of a volume of pristine material, may be artifactually minimized. The same is true for IR, although the respective volumes of pristine and irradiated materials contributing to the signals may be greater thanks to longer wavelength. Properly quantifying the impact of UV radiation of the spectral signatures of sulfates, phosphates and carbonates would require a dedicated sample preparation ensuring that the entirety of each sample has been interacted with UV photons.

At that point, a still pending difficulty would be to have a sufficient volume of sample to properly measure its spectral signature. In any case, it should be kept in mind that although no quantitative data can be provided here, the experimental setup adopted for the present study mimics pretty well Martian conditions in terms of UV radiation and signal collection.

Implications for the ongoing exploration of Mars

The present results have strong implications for the ongoing exploration of Mars. In fact, most of the surface of Mars is continuously exposed to UV radiation, and it has been the case for quite a long time. Therefore, one should not expect to detect pristine materials, except for measurements on freshly exposed surfaces, for which the timescale of analysis will be highly critical: the faster after excavation, the better. Even 24h of irradiation may be enough to impact the Raman and IR spectral signatures of any target. Therefore, measurements on abraded patches on Mars should be done at latest a Sol after the abrasion to prevent any radiation-induced modifications of the rocks investigated. As shown here, the identification of sulfates, phosphates and carbonates can be compromised if they experienced UV radiation for any given time. Sulfates have been widely detected on the Martian surface, either from orbit in Valles Marineris, Margaritifer Sinus, and Meridiani Planum [START_REF] Bishop | Spectral identification of hydrated sulfates on Mars and comparison with acidic environments on Earth[END_REF][START_REF] Vaniman | Magnesium sulphate salts and the history of water on Mars[END_REF][START_REF] Bibring | Mars Surface Diversity as Revealed by the OMEGA/Mars Express Observations[END_REF][START_REF] Gendrin | Sulfates in Martian Layered Terrains: The OMEGA/Mars Express View[END_REF][START_REF] Lane | Identifying the Phosphate and Ferric Sulfate Minerals in the Paso Robles Soils (Gusev Crater, Mars) Using an Integrated Spectral Approach[END_REF] or in situ by rovers again in Meridiani Planum, Endeavour Crater and Gale Crater (Squyres et al., 2004a,b;[START_REF] Crumpler | Context of ancient aqueous environments on Mars from in situ geologic mapping at Endeavour Crater[END_REF][START_REF] Rapin | Hydration state of calcium sulfates in Gale crater, Mars: Identification of bassanite veins[END_REF], and also at Jezero crater by Perseverance [START_REF] Clavé | Carbonate Detection with SuperCam in Igneous Rocks on the floor of Jezero Crater, Mars[END_REF]. A precise identification of the detected sulfate species has strong implications on their formation conditions (e.g. [START_REF] Rapin | Hydration state of calcium sulfates in Gale crater, Mars: Identification of bassanite veins[END_REF]. For instance, the detection of bassanite rather than that of anhydrite or gypsum, will be interpreted as indicating very specific local conditions in particular regarding water salinity [START_REF] Rapin | Hydration state of calcium sulfates in Gale crater, Mars: Identification of bassanite veins[END_REF]. The same is true for phosphates and carbonates [START_REF] Pollack | Thermal emission spectra of Mars (5.4-10.5 microns) -Evidence for sulfates, carbonates, and hydrates[END_REF][START_REF] Calvin | Hydrous carbonates on Mars?: Evidence from Mariner 6/7 infrared spectrometer and ground-based telescopic spectra[END_REF][START_REF] Clavé | Carbonate Detection with SuperCam in Igneous Rocks on the floor of Jezero Crater, Mars[END_REF]. But, as shown here, the modification of their spectral signatures due to UV radiation may bias their identification and/or the estimate of their abundances in rocks, which is directly based on the spectral responses of targets investigated. Similarly, their measured hydration states may not reflect their true/original hydration state. Because both the abundances and the hydration states of sulfates, phosphates and carbonates are critical for Martian paleoenvironmental reconstructions, the effects of exposure to UV have to be taken into account to prevent potentially significant biases.

The present study also highlights that luminescence signals have to be interpreted with special care.

A strong and short-lived luminescence signal resembling the ones collected here on irradiated sulfates, phosphates and carbonates may easily be misinterpreted as organic fluorescence. In fact, the very short lifetime of the intrinsic molecular fluorescence of organic compounds (e.g., nanoseconds) compared to most mineral luminescence (micro-to milliseconds), makes it supposedly pertinent to rely on luminescence signals to unambiguously identify organics, as done by [START_REF] Scheller | First-Results from the Perseverance SHERLOC Investigation: Aqueous Alteration Processes and Implications for Organic Geochemistry in Jezero Crater[END_REF]. However, the short-lived luminescence signals measured here have nothing to do with organics, even though organics may also exhibit such short-lived luminescence signals after exposure to UV [START_REF] Megevand | Impact of UV Radiation on the Raman Signal of Cystine: Implications for the Detection of S-rich Organics on Mars[END_REF]. On Mars, a solution to ensure interpretations could be to repeat any questionable measurement after a certain time, especially for data collected on freshly excavated surfaces. Obviously, the only proper way to interpret any data is to do it with caution, knowing that most of the answers will be provided by the samples which will be brought back to Earth by the upcoming MSR mission.

Conclusion

Altogether, although not quantitative, the present results evidence that exposure to UV has a significant effect on the Raman and IR spectral signatures of sulfates, phosphates and carbonates, which has, in turn, strong implications for the ongoing exploration of Mars. As a precaution, all the targets measured on Mars should be considered as having been exposed to UV radiation to some extent.

This will prevent any misinterpretation regarding the nature, abundances and water content of the mineral phases in presence. Of note, the potential impact of additional processes should also be investigated and taken into account for the proper interpretation of existing and future data. In fact, the (sub)surface of Mars is not only exposed to UV radiation, but is also bombarded by gamma-rays, solar energetic protons and galactic cosmic rays which are not absorbed by the thin CO 2 atmosphere and may interact with rocks lying at the surface [START_REF] Kminek | The effect of ionizing radiation on the preservation of amino acids on Mars[END_REF][START_REF] Dartnell | Modelling the surface and subsurface Martian radiation environment: Implications for astrobiology[END_REF]Hassler et al., 2014;[START_REF] Fox | Radiolysis of Macromolecular Organic Material in Mars-Relevant Mineral Matrices[END_REF]. The widespread presence of dust could also generate a strong background blurring Raman signals [START_REF] Maurice | Mars 2020 SuperCam Bundle[END_REF][START_REF] Maurice | Mars 2020 SuperCam Bundle[END_REF]. In addition, a number of other ageing processes likely act simultaneously at the surface of Mars, including oxidation [START_REF] Lasne | Oxidants at the Surface of Mars: A Review in Light of Recent Exploration Results[END_REF] and degradation induced by fluid circulation [START_REF] Viennet | Experimental reproduction of the martian weathering profiles argues for a dense Noachian CO2 atmosphere[END_REF], the impact of these processes on sulfates, phosphates and carbonates (as well as on phyllosilicates and organic compounds) remaining to be constrained to properly reconstruct Martian paleoenvironments and optimize the search for traces of life on Mars.

Figure 1 :Figure 2 :

 12 Figure 1: Top: CAD images of the chamber, its illumination device and the sample holder mechanism. Bottom: Picture of the vacuum chamber containing the samples (the 6 cylindrical pellets) seen from the Raman measurement side. The plate can translate in two degrees of freedom to center each pellet in the laser beam. The pierced plate in the foreground contains an illumination system to facilitate the positioning of the pellets.

Figure 3 :

 3 Figure 3: Raman (a, b, c) and IR (d, e, f) spectra of the studied minerals before irradiation, normalized and shifted for clarity. The attribution of their main features is indicated. More details about the attribution of the sulfates near-IR absorption features are given in Supplementary materials.

Figure 4 :

 4 Figure 4: a to e: Raw luminescence spectra of sulfate samples as a function of irradiation duration. f : Relative total integrated luminescence as a function of the irradiation duration for each sulfate sample.

Figure 5 :

 5 Figure 5: a to e: Raman spectra of the various sulfate minerals measured with a 2 ns gate. The spectra have been normalized by their total surface to highlight their relative variation. f : relative area of each major Raman scattering line as a function of the irradiation duration.

Figure 6 :

 6 Figure 6: Near-infrared spectra of sodium sulfate (a), calcium sulfate (b), magnesium sulfate (c) and aluminum sulfate (d).

Figure 7 :

 7 Figure 7: a to c: Raman spectra of the various carbonate minerals measured with a 2 ns gate. The spectra have been normalized by their total surface to highlight their relative variation. d: relative area of each major Raman scattering line as a function of the irradiation duration.

Figure 8 :

 8 Figure 8: a to c: Raman spectra of the various phosphate minerals measured with a 2 ns gate. The spectra have been normalized by their total surface to highlight their relative variation. d: relative area of each major Raman scattering line as a function of the irradiation duration.

Figure 9 :

 9 Figure 9: Near-infrared spectra of carbonates (a to c) and phosphates (d to f ).

  dose delivered by the Sun to the surface of Mars) likely prevented thermal dehydration, especially given that the temperature was regulated during the experiments. In fact, although the temperature of the chamber and of the sample holder may increase of 20 to 30°C under exposure to the lamp in the absence of thermal regulation, no such increase was observed during the experiments conducted under thermal regulation. Rather than indicating the dehydration of the samples initially hydrated, the total disappearance of the Raman water bands at about 3400 cm -1 of all the samples initially hydrated most likely result from the strong increase of the background which eventually masks the Raman signal over the 3000 to 4000 cm -1 spectral window. Still, clear differences exist between the water bands present in the near-IR spectra of the starting materials and those of the samples having been exposed to UV radiation, suggesting that dehydration, structural alteration, or both did occur during the experiments. It remains however difficult to directly relate Raman and IR observations since the volume actually probed by each technique is different. Worse, it is likely that the total volume of sample actually probed and contributing to the Raman or IR signals collected are different for the starting materials and for the samples exposed to UV. In fact, although slight, the change in color of the samples investigated likely reduces their transparency, thus limiting the penetration of the laser and incident light for IR. A partial structural alteration would also reduce the optical penetration depth and therefore the contribution of pristine materials to the total signals.

Table 1 :

 1 List of the studied samples.

	Name	Formula	Ref. Sigma-Aldrich/N°CAS
	Calcium sulfate dihydrate	CaSO 4 • 2 H 2 O	1.02161/10101-41-4
	Sodium sulfate	Na 2 SO 4	239313/7757-82-6
	Aluminum sulfate, octadecahydrate	Al 2 (SO 4 ) 3 • 18 H 2 O	227617/7784-31-8
	Magnesium sulfate	MgSO 4	M7506/7487-88-9
	Iron sulfate (III), hydrate	Fe 2 (SO 4 ) 3 • x H 2 O	307718/15244-10-7
	Calcium phosphate	Ca 3 (PO 4 ) 2	21218/7758-87-4
	Sodium phosphate	Na 3 PO 4	342483/7601-54-9
	Magnesium phosphate hydrate	Mg 3 (PO 4 ) 2 • x H 2 O	344702/53408-95-0
	Calcium carbonate	CaCO 3	10679-36/471-34-1
	Sodium carbonate, decahydrate	Na 2 CO 3 • 10 H 2 O	1.06391/6132-02-1
	Hydromagnesite	4 (MgCO 3 )Mg(OH) 2 • 4 H 2 O	Natural sample

Acknowledgments

We acknowledge the support of the IMPMC spectroscopy platform. We acknowledge financial support from CNRS INP (PI: O. Beyssac), the program Emergences Alliance Sorbonne Universiteť (Project MarsAtLab PI: S. Bernard), the program Convergence Sorbonne Université (Project Parimicrobial, PI: K. Benzerara), and from the Institut des Materiaux Sorbonne Universite IMat (Project Ageing on Mars -PI: S. Bernard).

Competing interests

The authors declare that they have no competing interests.

Open research

All the data supporting this study are available upon request.