Olivier Beyssac
email: olivier.beyssac@upmc.fr

New Trends in Raman Spectroscopy: From High-Resolution Geochemistry to Planetary Exploration

Keywords: mineralogy, SERS/TERS, time-resolved Raman, luminescence, Mars exploration

This article reviews nonconventional Raman spectroscopy techniques and discusses actual and future applications of these techniques in the Earth and planetary sciences. Time-resolved spectroscopy opens new ways to limit or exploit luminescence effects, whereas techniques based on coherent anti-Stokes Raman scattering or surfaceenhanced Raman spectroscopy allow the Raman signal to be considerably enhanced even down to very high spatial resolutions. In addition, compact portable Raman spectrometers are now routinely used out of the laboratory and are even integrated to two rovers going to Mars in the near future.

INTRODUCTION

Raman spectroscopy is a valuable, commonly-used, technique in the Earth and planetary sciences. Scientists from this community have been pioneers in using some of the major developments in Raman spectroscopy, such as Raman microprobes, portable Raman instruments, and the technique of Raman hyperspectral mapping.

Two obstacles to users are that 1) the Raman signal is inherently weak from most samples studied in the Earth and planetary sciences, and 2) the Raman signal is often parasitized/weakened by other signals, such as luminescence. Therefore, scientists have continuously explored various ways to make the Raman signal stronger and easier to detect. There are alternative ways of exciting/probing the Raman effect, which were imagined back in the 1970s, such as surface-and tip-enhanced Raman spectroscopy (SERS and TERS, respectively), coherent anti-Stokes Raman scattering (CARS), and time-resolved Raman (TRR) spectroscopy. First developed on customized homemade instruments, some of these techniques are now commercially available and widely used (e.g., SERS, TERS), while others are still under development/application in specific laboratories (TRR, CARS). But these techniques are now slowly emerging in the Earth and planetary sciences. In parallel, progress on the Raman spectrometer hardware, including miniaturization, make the spectrometer a more versatile instrument that can even be deployed out of the laboratory, from museums to deep oceans and as part of Mars rover scientific payloads. This article will provide a glimpse into these new and diverse trends in Raman spectroscopy.

RECONCILING LUMINESCENCE AND RAMAN

In a Raman experiment, the incident laser beam excites the targeted system into a virtual energy state. Scattering will be essentially elastic, but coupling between the laser's energy and the system's vibrations may trigger the creation and/or the absorption of phonons that produce the Stokes and anti-Stokes Raman effects by inelastic scattering (see Fig. 1 in Pasteris and Beyssac 2020 this issue). When excited by the laser, a sample's structural defects and chemical impurities may contribute to the total signal by emitting a strong background (due to small grain sizes or irregular sample surface) and/or a luminescence signal (due to the presence of organics or other emission centers). Very often, these signals are strong and can overwhelm the weak Raman signal: for instance, luminescence involves electronic transitions and generally complex emission signals. Some emission centers can generate discrete, narrow bands of luminescence in the spectrum, which can be confused with Raman peaks [START_REF] Lenz | Laser-induced REE 3+ photoluminescence of selected accessory minerals -an "advantageous artefact" in Raman spectroscopy[END_REF]. Luminescence is often seen as a limitation for Rama n spectroscopy because it generates an intense background that may interfere with the detection of the Raman signal. However, in many cases, the associated luminescence actually carries rich information, so providing a useful signal complementary to Raman [START_REF] Gaft | Modern Luminescence Spectroscopy of Minerals and Materials[END_REF]. Note that "luminescence" is here used as a generic term to describe all kinds of photoluminescence, whatever the mechanism of light emission (e.g., fluorescence, luminescence, or phosphorescence) (see [START_REF] Gaft | Modern Luminescence Spectroscopy of Minerals and Materials[END_REF] for further details).

Tuning Laser Wavelength

Many natural samples exhibit an inherent background, which often overwhelms the Raman signal. This may be due to rough surfaces or small grain sizes on the same order of magnitude as the laser wavelength. This is not recommended for conventional Raman analysis because the laser will interact with the surface rather than the volume of the particles and/or may even be diffracted by them for sizes smaller than the laser wavelength. Laser light will be scattered within samples having a small grain size and be returned to the spectrometer. Although most current Raman instruments have a single-stage monochromator that yields high luminosity, there is poor rejection efficiency of parasitic light responsible for the background. Another possible issue is luminescence, which is due to the radiative relaxation of electronic excitation in molecules (see Fig. 2 in Pasteris and Beyssac 2020 this issue). By itself, luminescence is a spectroscopic technique used in various fields of the Earth sciences [START_REF] Gaft | Modern Luminescence Spectroscopy of Minerals and Materials[END_REF] and in disciplines such as biomedicine, with its use of fluorescence spectroscopy [START_REF] Lakowicz | Principles of Fluorescence Spectroscopy[END_REF]. Each emission center has a specific absorption spectrum of incident light. After photons are absorbed, the system relaxes and energy is released as an emission spectrum specific to the element of interest in the host matrix. This emission spectrum is the signal that may be observed in the spectral window monitored for Raman spectroscopy. Importantly, the Raman shift corresponds to an energy absorbed and released by the sample, specifically referenced to the energy of the laser excitation. Thus, as the excitation wavelength/energy of the laser is varied, Raman peaks move in the light spectrum as viewed with respect to absolute wavenumber coordinates. In contrast, the positions of luminescence emission bands are constant with respect to absolute wavenumbers , whatever the exciting laser wavelength: they may or may not appear in the spectral window considered for Raman spectroscopy (FIG. 1).

Most commercial instruments offer several excitation wavelengths, from deep ultraviolet (UV) to near-infrared (IR). Basically, the intensity of the Raman signal is proportional to 1/ 4 , where  is the wavelength of the incident laser beam, but various constraints have to be considered when selecting a laser wavelength for Raman spectroscopy [START_REF] Dubessy | Instrumentation in Raman spectroscopy: elementary theory and practice[END_REF]Pasteris and Beyssac 2020 this issue). FIGURE 1 depicts the laser-dependent Raman spectral windows and scattering efficiency from the deep UV to near-IR, as well as the region where most luminescence is detected (visible to near-IR). Ultraviolet light is highly energetic and generally efficiently absorbed. Due to strong coupling, as well as scattering, the Raman signal is actually very strong in the UV. Luminescence is also likely excited in the sample, but it does not overlap and, therefore, does not interfere with the Raman signal. This is because the (Stokes) Raman spectral window is compressed in absolute wavelength in the UV and is shifted to lower wavelengths with respect to luminescence (FIG. 1).

The drawback is that UV light requires special optics and detectors, as well as avoidance of the effects of UV light (e.g., photo-oxidation) on the sample target. A specific use of UV Raman is resonance Raman: by tuning the laser wavelength (e.g., with a dye laser), it may be possible to match its energy with that of an absorption band of the sample, corresponding to an electronic transition (see FIG. 2

in Pasteris

and Beyssac 2020 this issue). This will specifically enhance (by a theoretical factor of up to 10 6 ) the intensity of the Raman peak associated with the electronic transition probed. This technique works well for studying organic compounds [START_REF] Steele | Geoscience meets biology: Raman spectroscopy in geobiology and biomineralization[END_REF] this issue) but is rarely used in the Earth sciences for minerals [START_REF] Tarcea | UV Raman spectroscopy-a technique for biological and mineralogical in situ planetary studies[END_REF]).

On the other side of the visible light spectrum, near-IR is less strongly absorbed than UV. Therefore, the excitation of luminescence is weaker, and the Raman spectral window is shifted to yet longer wavelengths, making the luminescence not visible.

But, due to the 1/ 4 scattering effect, near-IR light does not efficiently excite the Raman effect. In this region, the Raman signal is actually weak and most easily detected for targets with high Raman scattering cross-sections at this wavelength (e.g., carbonates, phosphates, sulfates). However, there is a specific signal-handling use of near-IR excitation called Fourier transform (FT) Raman, which is generally implemented with a 1,064-nm laser and a sensitive single-element, near-IR detector (e.g., InGaAs): this is because a charge-coupled device (CCD) has a very low detection efficiency at this long wavelength. Whereas a dispersive Raman system uses a grating to disperse light according to wavelength, an interferometer is used for FT Raman. It converts the Raman signal into an interferogram and allows the collection of the entire Raman spectrum simultaneously on the detector [START_REF] Parker | A review of the theory of Fourier-transform Raman spectroscopy[END_REF]. The drawbacks are that the Raman signal is inherently very weak, the spatial resolution is low compared to that of visible light, and the spectral domain is restricted by the sample size due to the diffraction limit of the long excitation wavelength. Therefore, this technique is rarely used in the Earth sciences. It is used in polymer, medical, or forensic sciences, where molecules are highly luminescent and have high Raman scattering cross-sections.

For all these reasons, a good compromise in the Earth sciences is generally to use visible light to excite the sample, typically in the range 450-650 nm. These wavelengths are easy to handle instrumentally, they enable good spatial resolution, and they efficiently excite the Raman effect [START_REF] Dubessy | Instrumentation in Raman spectroscopy: elementary theory and practice[END_REF].

Time-Resolved Spectroscopy

Another way to separate the Raman signal from luminescence is to play with the fact that these two processes have very different lifetimes. Thus, we can use time-resolved spectroscopy. "Lifetime" corresponds to the time spent by an emission center (sometimes called an activator) in the excited state before it returns to the ground state and emits a photon. A lifetime includes both absorption (nearly immediate) and relaxation (variable duration). Relaxation from virtual vibrational levels is immediate for Raman, typically in the range of 10 -14 seconds. On the other hand, relaxation from excited electronic levels is more sluggish, and the lifetime of luminescence covers a wide time range from the nanosecond in organic fluorescence [START_REF] Lakowicz | Principles of Fluorescence Spectroscopy[END_REF] to milliseconds or even seconds in minerals (e.g., Cr 3+ or REE 3+ ) [START_REF] Gaft | Modern Luminescence Spectroscopy of Minerals and Materials[END_REF].

Time-resolved spectroscopy uses a pulsed excitation signal synchronized with a gated detector. The main idea is that the Raman signal is excited immediately by and will last only during the excitation pulse, while the luminescence will be excited during the pulse but will continue to decay after the pulse. Using a short(-time) gate that matches the laser pulse in the time domain and that is perfectly synchronized with it, allows rejection of most of the luminescence signal and maximum collection of the Raman signal (FIG. 2). Time-resolved spectroscopy further allows collection of only the luminescence signal, by opening the detector gate just after the laser pulse.

The Raman signal is by then absent, because its excitation has relaxed, while the luminescence is still present. Using advanced instruments with perfect synchronization, it is possible to explore luminescence with various decay periods by using smart combinations of delays (moment the detector gate opens with respect to the laser pulse) and open-gate duration. Lastly, this technique allows proper characterization of the time decay of luminescence. A post pulse detector gate with appropriate opening duration is set to various delay times (gate opening after laser pulse), and the integrated intensity of the signal is plotted versus the delay time. These data give further information on the nature of the electronic transition and, more practically, on the identification of the emission center [START_REF] Lakowicz | Principles of Fluorescence Spectroscopy[END_REF][START_REF] Gaft | Modern Luminescence Spectroscopy of Minerals and Materials[END_REF].

Experimentally, instruments for time-resolved spectroscopy are mostly customized homemade instruments, although some manufacturers now propose time-resolved instruments for specific applications. In most cases, these setups use an intensified charge-coupled device (ICCD) with a photocathode for the gating (down to the nanosecond) and a photon intensifier to enhance the signal on the CCD. Alternatives are single-photon avalanche diode detectors [START_REF] Blacksberg | Miniaturized time-resolved Raman spectrometer for planetary science based on a fast single photon avalanche diode detector array[END_REF] or a picosecond Kerr gating [START_REF] Matousek | Fluorescence background suppression in Raman spectroscopy using combined Kerr gated and shifted excitation Raman difference techniques[END_REF]. For the excitation, pulsed lasers are used with pulse durations generally in the nanosecond range, which is similar to that achievable for ICCD gating. Choice of laser wavelengths obeys the same constraints as for continuous-wave Raman (see above). In the Earth and planetary sciences, several instrumental setups have been developed for standoff remote analysis by collimating the laser on the sample surface and collecting light through a telescope. The motivation here is to refine instruments for the field, including planetary exploration (see below), and to reject the daylight contribution by time-resolution. Some very nice results have been obtained with these systems, such as retrieving clean Raman spectra from highly luminescent minerals or performing Raman spectroscopy at distances of up to several tens of meters on large crystals/rocks [START_REF] Sharma | A combined remote Raman and LIBS instrument for characterizing minerals with 532 nm laser excitation[END_REF].

HIGH-RESOLUTION GEOCHEMISTRY WITH COHERENT OR ENHANCED RAMAN SPECTROSCOPY

New scientific questions drive the need to observe at ever higher spatial resolution and/or detect ever smaller amounts of a given molecule. The Raman signal is, however, inherently weak, especially for individual components in complex targets, which may be solid, fluid, mineral, or organic. Therefore, various strategies have been deployed to make the Raman signal stronger for spectroscopy and for imaging.

Coherent Raman Spectroscopy

Coherent Raman scattering includes an ensemble of techniques based on a moleculespecific, nonlinear, optical interaction sensitive to Raman vibrational resonances.

These techniques generally rely on the interaction of two laser beams: a pumped laser and a Stokes laser, for which their frequency difference matches a resonant vibrational Raman frequency of the molecule of interest. The techniques mainly differ in the detection of the Raman signal: the homodyne technique like coherent anti-Stokes Raman scattering (CARS), which directly detects the anti-Stokes signal, versus the heterodyne technique, such as stimulated Raman scattering (SRS), which detects the transferred modulation of either the pumped or the Stokes beam [START_REF] Houle | Rapid 3D chemical-specific imaging of minerals using stimulated Raman scattering (SRS) microscopy[END_REF]). These techniques are well developed in biomedical science to characterize, for example, lipid-rich structures using the C-H stretching vibration. Although such techniques require complex experimental setups and advanced expertise in spectroscopy, they have been applied successfully in the Earth sciences. An outstanding example is the characterization and 3-D imaging of fluid inclusions by combining CARS with a laser scanning confocal microscope [START_REF] Burruss | Unraveling the complexity of deep gas accumulations with three-dimensional multimodal CARS microscopy[END_REF]. In this case, the strong resonant character of molecules like CH4, CO2, H2O, or even hydrocarbons is used to generate 3-D images of the inclusions. The high spatial resolution and superb quality of these images open new doors for investigating not only the volumetric relations of the gases in the inclusions but also the microtextural features of the inclusions within the microfractures of the host mineral. Other applications of CARS for the microtextural imaging of carbonates in microfossils or sulfate inclusions in a calcite crystal are presented and discussed by [START_REF] Houle | Rapid 3D chemical-specific imaging of minerals using stimulated Raman scattering (SRS) microscopy[END_REF].

SERS and TERS

Surface-enhanced Raman spectroscopy (SERS) offers perhaps the most spectacular amplification of the Raman signal. Although the first observations of the physical effects involved in SERS were made in the 1970s, the technique really took off during the early 21 st century . Many fields now routinely use SERS, from material sciences to biomedicine [START_REF] Cialla | Surface-enhanced Raman spectroscopy (SERS): progress and trends[END_REF]), but still only rarely in the Earth sciences. Basically, SERS analyzes molecules adsorbed onto a specifically prepared metal surface (FIG. 3).

This metal surface should have plasmonic properties: at the interface between the metal and a dielectric material (something nonconductive, such as air), the incident light from the laser interacts with the free electrons of the metal to create a resonant interaction, which produces oscillations of the electrons. At the metal's surface this triggers the formation of electron density waves called plasmons. Electromagnetic enhancement of the Raman signal results from the amplification of the light by the excitation of localized plasmon resonances, which theoretically can reach factors of 10 10 -10 11 , but more practically 10 6 , depending on the sample and the supporting plasmonic material. There is also a chemical enhancement, not yet completely understood, but probably due to charge transfer mechanisms in which the laser excitation wavelength is resonant with the electronic states of the metal-molecule charge transfer. Theoretically, chemical enhancement can reach up to a factor of 10 3 . A complete description of SERS is given by [START_REF] Aroca | Surface Enhanced Vibrational Spectroscopy[END_REF].

Practically, SERS can be done with any conventional Raman microspectrometer that has good confocality and a high collection efficiency: the enhancement is localized within the very close vicinity of the substrate surface and rapidly decreases outward. The key aspect is the choice of the plasmonic substrate, which should have a peak of plasmon resonance in accordance with the laser wavelength. Typically, metals like Cu, Au, or Ag are used as substrates with laser wavelengths in most of the visible and near-IR region. In addition, these metals yield good interaction with many adsorbed molecules. Considerable work has been done to look for new SERS substrates (e.g., graphene, semiconductors, quantum dots) and/or to optimize the configuration of the substrate (shape, surface properties) to maximize the signal enhancement. Tip-enhanced Raman spectroscopy (TERS) is, in a way, derived from SERS, but the electromagnetic field enhancement is restricted to a sharp metallic tip irradiated by the laser light (FIG. 3). The sample is moved very close to the metal tip, which creates a localized region of SERS enhancement. The main advantage is the excellent and accurate spatial resolution reached by TERS, which allows structural and chemical characterization at the nanometer scale. Practically, TERS is performed by combining a Raman spectrometer with a scanning tunneling microscope (STM) or an atomic force microscope (AFM). Tip-enhanced Raman spectroscopy holds fantastic potential for analyzing and mapping at the nanometer scale, although further knowledge is required for precise estimation of the region of enhancement to properly assess the intrinsic resolution of the method and its real enhancement efficiency.

The SERS/TERS methods are now routinely and successfully used across a range of scientific disciplines-material sciences, biochemistry, biosensing, catalysis, electrochemistry, and forensic sciences-for which characterizing small amounts (almost at the single-molecule level) of organic compounds is critical. There are many fewer applications in the Earth sciences, although SERS/TERS may be used as an indicator of surface properties, thereby making it relevant for some specific mineral investigations or for the detection of organic molecules on mineral surfaces [START_REF] Mernagh | Using infrared and Raman spectroscopy to analyze gas-solid reactions[END_REF]). In the environmental sciences, SERS has been used to detect and characterize organic or inorganic aqueous or airborne contaminants, as well as to detect microbes or nanomaterials in environmental samples [START_REF] Halvorson | Surface-enhanced Raman spectroscopy (SERS) for environmental analyses[END_REF].

TAKING RAMAN OUT OF THE LABORATORY

Usually, a sample is carried to the laboratory where it is prepared (e.g., thin section, crushing) and then analyzed with instruments in an optimized environment. But sometimes it may be preferable to take a Raman instrument out of the lab to analyze a sample in the field. The reasons are multiple and diverse, from exploration geology in the field to analysis of priceless art or gem artifacts, which cannot be taken out of museum cabinets: this latter enables the study of nontransportable, fragile, and also large archeological objects, such as statues. There are not so many analytical techniques that are nondestructive and that can be deployed out of the lab routinely. Raman spectroscopy is one of the few, another being the complementary technique of X-ray fluorescence, which provides chemical information. Taking instruments out of the lab raises numerous challenges in terms of portability, mechanical reliability, resistance to environment, and energy autonomy. Fortunately, these have been progressively overcome for Raman spectroscopy.

There are two main approaches in terms of instrumentation: 1) moving and adapting a conventional, but mobile, laboratory instrument; or 2) using a dedicated portable instrument. The first approach is possible with most commercial instruments but is limited to in-room experiments. Microscopes may still be used, but generally an optical fiber is used to transport the excitation laser and so enable Raman collection directly on the sample without moving it. At the end of the optical fiber, various kinds of optical heads may be adapted, such as microscope objectives or lenses with longer focal distances. Such an approach is used successfully in museums for studying pigments in paintings or analyzing gems on precious artifacts [START_REF] Vandenabeele | A decade of Raman spectroscopy in art and archaeology[END_REF][START_REF] Bersani | Methodological evolutions of Raman spectroscopy in art and archaeology[END_REF]. Such a configuration is also used on large instrumental facilities, such as a synchrotron, where Raman spectroscopy may be used to complement other techniques based on X-rays. Portable Raman instruments have existed commercially for nearly two decades, and there is now a large number of offerings with various collection configurations and excitation wavelengths [START_REF] Jehlička | Comparison of seven portable Raman spectrometers: beryl as a case study[END_REF]. In recent years, transmission spectrometers (those using transmissive gratings), gained in popularity, because they offer good spectral resolution and make easier the design of compact instruments.

With instruments going to Mars (see below), one specific application has been the design of Raman spectrometers for deep-ocean exploration [START_REF] Pasteris | Raman spectroscopy in the deep ocean: successes and challenges[END_REF]). Such instruments have been deployed thanks to remotely operated vehicles providing energy, data flow, and accurate positioning on targets of interest. These deep-ocean instruments are used to identify, and even quantify, the solutes present in seawater and to detect and characterize complex gas hydrates present in the deepocean. Future applications may include biological investigations based on Raman and luminescence signals. A first attempt to characterize the pigments present in a red fish was successful, although the story does not tell how the fish felt after being shot by the green laser (Zhang et al. 2011).

RAMAN IS GOING TO MARS!

Raman spectrometers have been selected for the scientific payload of two rovers going to Mars in the near future. Here is further recognition for the technique, which is long overdue on Mars: a scheduled Raman instrument was removed from the scientific payload of the now legendary Mars Exploration rovers Spirit and Opportunity in 2004. The two next missions, NASA's Perseverance rover and ESA/Roscosmos Exomars, have the same main scientific objective, which is to identify any trace evidence of ancient life on the surface of this planet. Raman spectroscopy is expected to play a crucial role in the detection and characterization of possible organics, but also to characterize minerals that may have been involved in biomineralization and fossilization processes, such as carbonates, phosphates, and some oxides (see Steele et al. 2020 this issue). More generally, Raman instruments are expected to provide structural and mineralogical information about Martian materials. Interestingly, three instruments will be deployed with very different strategies: the SuperCam and SHERLOC (Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals) instruments onboard the Perseverance rover and the RLS (Raman Laser Spectrometer) instrument onboard the Exomars rover (named Rosalind Franklin). Of course, these instruments represent an advanced engineering challenge in terms of compactness (in general they weigh only a few kilograms) and their resistance to extreme conditions (e.g., mechanical shocks, large thermal variations, and dust).

The Rosalind Franklin rover will carry a complete suite of scientific instruments to fully characterize the samples in situ. The selected landing site is Oxia Planum, a Noachian (Archean equivalent) clay-rich, vast, lowland plain near the equator. This rover is supposed to drill down to two meters through the Martian rocks and to crush the samples for advanced characterization. The objective is to characterize pristine rocks (via drilling) that were not affected by the intense UV and cosmogenic irradiation that affects the upper centimeters of the Martian surface, because such irradiation can be detrimental for organics. The RLS instrument is part of the Pasteur payload [also including the micrOmega infrared spectrometer and MOMA (Mars organic molecule analyzer) mass-spectrometer instruments] located in the body of the rover. Protected from the external environment, the RLS will operate in the dark to analyze powders carried by a carousel after the rocks are crushed. The instrument comprises a continuous laser (Nd:yttrium-aluminum-garnet at 532 nm), an optical head focusing the laser to a 50-m spot size and that collects the signal, a compact transmission spectrometer, and a CCD detector. The instrument should be able to characterize most minerals, including hydrated ones, as well as organics [START_REF] Rull | The Raman Laser Spectrometer for the ExoMars Rover mission to Mars[END_REF].

The Perseverance rover will carry a full suite of scientific instruments for in situ characterization of the Martian rocks. This rover is also equipped with a cache system and is supposed to assemble 20 samples that will be collected by a future Mars mission and return these samples to Earth [START_REF] Williford | The NASA Mars 2020 Rover Mission and the search for extraterrestrial life[END_REF]). Mars2020 will land in the Jezero Crater, which is a Noachian crater basin at the western edge of the Isidis Basin. It has been characterized as fluvial/deltaic sediment deposited into a circumneutral/low-salinity (i.e., potentially habitable) paleolake. The SHERLOC instrument is located on the arm turret of the rover and is associated with the PIXL (planetary instrument for X-ray lithochemistry) instrument, which uses X-ray fluorescence for chemical characterization to complement the information provided by SHERLOC.

These two instruments will work close to the sample after mechanical abrasion of the target to remove any dust and alteration layers (e.g., varnishes). The SHERLOC instrument is a deep-UV (laser excitation at 248.6 nm) Raman spectrometer optimized to detect, analyze, and map the distribution of organics in the targeted rock [START_REF] Wj | Deep UV Raman spectroscopy for planetary exploration: the search for in situ organics[END_REF]. The poor penetration of the UV laser limits excitation to only the sample's surface, but low irradiance of the sample can be maintained because of the (1) high efficiency of Raman scattering in the UV (FIG. 1), and (2) possible resonance enhancement effects for organics with aromatic or conjugated bonds. The key advantage here is that the Raman spectral window is free of luminescence. The SHERLOC instrument will be combined with a high-resolution context imager called

Watson.

The second Raman instrument onboard Perseverance is SuperCam (FIG. 4). This is a multifunctional spectroscopy instrument, a "Swiss Army-knife instrument", built by a USA-France-Spain consortium. The design is largely inherited from that of the ChemCam instrument onboard the Curiosity rover, which has been successfully analyzing the surface of Mars since 2012. SuperCam will remotely analyze the surface using laser-induced breakdown spectroscopy (LIBS), time-resolved Raman and luminescence spectroscopy, and visible-infrared (VISIR) spectroscopy. A color remote micro-imager (RMI) will provide context images and textural detail, and a microphone will record the LIBS plasma shock waves and the sounds of Mars [START_REF] Wiens | The SuperCam remote sensing instrument suite for the Mars 2020 Rover: a preview[END_REF]. For the Raman analyses, SuperCam faces two main technical challenges (compared to laboratory instruments): 1) remote analysis in daylight and timeresolved spectroscopy; 2) it will have to work on Mars. To do this, SuperCam combines a unit on top of the rover mast that can be operated in all directions and that includes a telescope and a pulsed laser (nanosecond pulses at 532 nm). SuperCam has a body unit that includes a transmission spectrometer and an ICCD in the rover body, and the two instruments are connected by an optical fiber and cables [START_REF] Wiens | The SuperCam remote sensing instrument suite for the Mars 2020 Rover: a preview[END_REF]. Time-resolution down to 100 ns on the ICCD allows for rejection of the ambient light continuously entering the telescope. It avoids unwanted luminescence, but can also exploit luminescence for the possible detection of trace elements (such as the rare earth elements) not achievable by LIBS. SuperCam is expected to be the "eye" of the rover by shooting rocks at distances of up to several meters to provide quantitative chemical (LIBS) and qualitative mineralogical (Raman and VISIR) information. Owing to its relative simplicity and speed of operation, SuperCam is supposed to guide the rover's displacement, to provide key information for the operation of other scientific instruments, and to document the geological context for selection of the sites to be sampled for Mars sample return.

SUMMARY

Conventional Raman spectroscopy is now routinely used in Earth science institutions (e.g., Chou and Wang 2017) and new developments are progressively emerging in this field to enhance the inherently weak Raman signal and/or to work at higher spatial resolutions. Raman mapping has also undergone a revolution and now can be done easily and it can complement other imaging techniques based on X-ray fluorescence [START_REF] Nasdala | Application of Raman-based images in the Earth sciences[END_REF]. "Outdoor" applications, including Mars exploration, clearly open new avenues for in situ mineralogy and geochemistry but face numerous complex technological challenges to capture the rich, yet subtle and weak, Raman signal.

FIGURE CAPTIONS FIGURE 1

Raman spectral windows for excitation at various laser wavelengths. For each laser, the dark bar on the left side of each color-shaded box represents the laser wavelength; two schematic Raman peaks at 500 and 1,000 Δcm -1 are depicted within each box to show a 2,000 cm -1 spectral window. Also shown are the general range of luminescence emissions ("luminescence/organic fluorescence" dark grey bar in middle of figure ) (LEFT) Schematic illustrations of the synchronization between the laser pulse and the intensified charge-coupled device (ICCD) gate to catch pulse-synchronous Ramanonly spectra (green; middle graph), or slightly post-pulse luminescence only (dashed pale blue line for long lifetime; dashed dark blue line for short lifetime; lower graph), or both (upper graph). (RIGHT) Total continuous-wave (CW) excitation Raman spectra, Raman spectra (green curve, peaks highlighted in yellow), and luminescence (here, due to REE 3+ in the zircon sample) spectra (red curve).

  and the relative efficiency of Raman scattering as a function of wavelength (red curve in upper part of figure). Abbreviation: a.u. = arbitrary units; IR = infrared. Equation in top grey box indicates how the intensity of the Raman shift is related to the exciting light wavelength. MODIFIED AFTER PANCZER ET AL. (2012).

FIGURE 2

 2 FIGURE 2 Time-resolved Raman and luminescence spectroscopy of a zircon grain.

FIGURE 3

 3 FIGURE 3 (A) Schematic illustration of the surfaced-enhanced Raman spectroscopy (SERS) technique; the region of enhanced Raman is located at the surface of metal nanoparticles. (B) Schematic illustration of the tip-enhanced Raman spectroscopy (TERS) technique; the region of enhanced Raman is located at the edge of the metal tip.

FIGURE 4

 4 FIGURE 4 Schematic diagram of the SuperCam instrument on board the NASA Mars 2020 mission. (UPPER) The mast unit, containing the laser, the telescope, the remote micro-imager (RMI) and the infrared (IR) spectrometer. (LOWER) The body unit is connected to the mast unit by an optical fiber and contains the ultraviolet (UV), violet (Vio) and transmission (trans.) spectrometers and detectors [demux = demultiplexer]. The instrument has a complete set of calibration targets for each technique, including a diamond for Raman. MODIFIED AFTER WIENS ET AL. (2016).

  Figure 1

Figure 3 Figure 4

 34 Figure 3

ACKNOWLEDGMENTS

Thanks are due to Terry Mernagh, Shiv Sharma, and Dominique De Ligny for insightful reviews, to Roger Wiens for reading a preliminary version of the manuscript and to Michel Gauthier for numerous spectroscopy discussions. Sincere thanks to guest editor Jill Pasteris for extremely patient and constructive handling of the manuscript. Funding from CNRS and Sorbonne Université is acknowledged.


Magdalena Brzezińska

Nicolas Keller

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

Magdalena Brzezinska

Self-tuned properties of CuZnO catalysts

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Introduction

Fossil fuel depletion puts us in front of a challenge of finding an alternative source of both fuels and chemicals. 1 Among all the candidates, lignocellulosic biomass seems to be one of the most promising sustainable source of raw carbon material, thanks to its abundance and the lack of competition with food production. Lignocellulosic biomass is therefore considered as a renewable source of platform molecules that have numerous potential applications in future biorefinery, chemicals and materials sectors. 2 In this frame, 5-hydroxymethylfurfural (HMF) is a very valuable platform molecule, that can be obtained via multi-step process initiated first by the acidic hydrolysis of lignocellulose. Thanks to its high functionalization, with both hydroxyl and carbonyl groups in the structure, HMF can be further catalytically transformed for in fine obtaining many industriallyrelevant products of both oxidation and reduction reactions. 3,4 Scheme 1 illustrated the HMF hydrodeoxygenation reaction pathway, with in the first step the HMF hydrogenation (pathway A) with the formation of 2,5-bishydroxymethylfuran (BHMF) or its hydrodeoxygenation (pathway B) with the formation of 5-methylfurfural (5-MF). In the next step, BHMF undergoes hydro-dehydration reaction into 5-methylfurfuryl alcohol (5-MFA), while 5-MFA may also be obtained by hydrogenation of 5-MF. Next, 5-MFA undergoes hydrodehydration with formation of 2,5-dimethylfuran (DMF) which can be further hydrogenated to 2,5-dimethyltetrahydrofuran (DMTHF). DMTHF may also be obtained via the hydrodeoxygenation of 2,5-bishydroxymethyl-tetrahydrofuran (BHMTHF), resulting from the hydrogenation of the C=C bonds in BHMF. Among many valuable HMF-derived chemicals, 2,5-bishydroxymethylfuran (BHMF) and 2,5-dimethylfuran (DMF) gained a lot of interest. While BHMF can be potentially used as a substrate for biopolymer production, as it shows similar properties to crude oil-derived poly(ethylene terephthalate) and can successfully replace it. 5,6 DMF is thought to be used as a biofuel (notably as cold-start fuel) 7 or as a biofuel additive without any change in the engine construction, thanks to physicochemical properties similar to those of gasoline and high octane number. 8 Most of the works is mainly devoted to the synthesis of DMF through HMF hydrodeoxygenation, that was widely achieved with the use of catalysts based on noble metals such as Pd, Pt, Rh and Ru. 9,10 However, the restrictive use of additional acids such as H2SO4, HCl or acetic acid for promoting the dehydration step during the HMF conversion and the high price of noble metals are clearly the driving force behind the search for active, selective and stable noble metal-free catalysts for HMF conversion. Up to now, catalytic systems based on Ni, Co, Fe and Cu were mainly investigated in the mentioned process 9,10,[12][13][14] and it was shown that non-noble metals can be as active and selective as their noble metal counterparts, while being more sustainable and allowing an easier selectivity control. Lilga et al. studied cobalt-based catalysts and evidenced that Raney Co and Co/SiO2 allowed similar performance than the Pt/Al2O3 catalyst to be achieved under similar reaction conditions, with the absence of any DMF production and BHMF as the main product obtained. 15 The Palkovits's group recently highlighted that higher HMF conversions were achieved on carbon supported nickel catalysts using ether solvents such as THF or MTHF when compared to alcoholic solvents; however the catalysts suffered from deactivation, while very high hydrogen pressure of 100 bar and high nickel loadings were unfortunately necessary to reach high DMF yield. 16 Although alumina supported nickel catalyst could substitute its palladium-based counterpart when water was used as solventsince both catalysts exhibited similar activity and selectivity profiles, Duarte et al. indicated that they both suffered from the absence of any DMF formation, while hydrogenation and dehydration products of HMF were observed like 5-MFA and furfuryl alcohol. The authors also evidenced that high selectivity to 5-MF and 5-MFA intermediates was observed on the copper-based counterpart catalyst, although the catalyst suffered from a moderate activity. 17 As far as copper catalysts are concerned, copper chromitean industrial hydrogenation catalyst -was one of the first catalysts studied in the HMF hydrodeoxygenation reaction thanks to the pioneering work by Dumesic group, that developed a process in which a sugar solution was directly converted into DMF with high selectivity (67%). Although the deactivation with time observed for the catalyst was overcome by performing a regeneration under hydrogen flow, both the high Cu loading and the high toxicity of chromium remained highly detrimental to the process and strategies for designing suitable catalysts still need to be implemented. 18 Due to the high affinity of Cu for the hydrogenation of carbonyl groups, copper-based catalysts are considered as promising systems for the HMF hydrodeoxygenation reaction. 19 In addition the support itself needs to exhibit appropriate acidity in order to obtain DMF with high selectivity, and many studies highlighted the key-role played by the support in the Cu catalyst behavior. Alumina was investigated as Cu catalyst support due to its high acidity. However, although the Cu/Al2O3 catalyst allowed DMF to be obtained as the main reaction product after 140 min of test conducted at 175°C under 20.7 bar of H2, the catalyst suffered from deactivation, that resulted from the inhibition of the catalyst active sites through water molecule adsorption. 20 The Luque's group investigated Al-SBA15 aluminosilicate materials as Cu catalyst support and reported partial HMF dehydration into 5-methyl-2-furfuryl alcohol with a 60% selectivity for short time reactions, due to the presence of acid centers and the use of microwave irradiation. Although higher selectivity was achieved through the incorporation of Zn into the structure of the support, that lowered the overall acidity of the system which inhibited proceeding side reactions, only traces of DMF were observed in the reaction mixture. 21 Zhang et al. recently compared the behavior of highly loaded Cu catalysts (~20%) supported on different oxides (Al2O3, ZrO2, CeO2, ZnO). On one hand, both Al2O3 and ZnO supports allowed the highest DMF yields to be obtained, as a result of the formation of the smallest Cu crystallite size among all tested supports. 22 On another hand, a high acidity and a better utilization of the internal hydrogen generated from methanol in this study were reported as important factors in the case of Cu/Al2O3 and Cu/ZnO catalysts, respectively. However, the reaction required to operate at relatively high temperature (240°C) and for long reaction times (>6h). Furthermore, the catalysts suffered from a poor stability with the highest Cu leaching being obtained for the ZnO supported catalyst.

The activity of different commercially available copperbased nanopowders (Cu, CuO, CuFe2O4, CuZnFe2O4, CuZn) was investigated by the Bart's group, among which the CuZn catalyst showed the highest activity and selectivity towards DMF when the reaction was performed in ethanol at 220°C under 30 bar H2 for 6 h. 23 This good catalytic performance was explained by the simultaneous presence of Zn 2+ cations which were identified as dehydration centers, and of Cu 0 acting as HMF hydrogenation centers. Further, relatively small-size CuZn particle for a nano composite catalyst (<150nm) was beneficial for obtaining high DMF yield. Although the catalyst exhibited a good stability with time in the reaction mixture, with no evidenced leaching despite the high Cu loading, the reaction selectivity was strongly reduced to achieve a level as low as 17% for the fourth cycle. In agreement with the dual nature of the CuZn catalyst surface developed in Bart's work, Zhu et al. optimized the Cu content of the catalyst at 54 wt.% of Cu for achieving full HMF conversion with high selectivity towards DMF when the reaction was performed at 220°C under 15 bar of H2 for longer than 5 h. 19 However, despite the use of a very high copper content, the CuZn catalysts turned out to be completely unstable as a function of test cycling and were consequently suffering from a low HMF conversion of 30% at a 10% selectivity to DMF after one single run.

The aim of this work is to investigate the key-factors responsible for high activity of CuZnO catalysts in the HMF hydrodeoxygenation reaction, by studying the influence of the preparation method of the catalysts on its catalytic properties.

A photoassisted method at room temperature was compared to a classical wet impregnation one with final temperature treatment. We showed that the choice of the preparation method and the chemical state of the Cu nanoparticles strongly influenced the catalyst activity, and allowed the HMF hydrodeoxygenation selectivity to be tuned. Performing an oxidative treatment as intrinsic part of the catalyst cycling protocol was reported to be crucial for both the performance and the stability of the catalysts.

Experimental Support synthesis

ZnO was synthesized using precipitation synthesis method in which 1.75 g of zinc(II) acetate dihydrate(Zn(OAc)2•2H2O, Sigma-Aldrich, Saint Louis, MO, USA, ACS reagent, ≥98%) and 0.84 g of sodium carbonate (Na2CO3, Sigma-Aldrich, 99.5%) were Please do not adjust margins Please do not adjust margins dissolved under stirring in 50 mL of distilled water, respectively. Both aqueous solutions were mixed, and the obtained precipitate was aged at room temperature in the mother liquor for 24 h under continuous stirring. The suspension was further centrifuged for 30 min at 3500 rpm, and finally washed and filtrated under vacuum with distilled water. The resulting powder was dried at 100°C for 12 h and subsequently calcined at a temperature of 300°C for 2 h with a 10°C/min heating rate.

Catalyst preparation

CuZnO catalyst prepared via wet impregnation method. The catalyst was prepared with a 10 wt.% metal content using an aqueous solution of Cu(NO3)2 (Chempur, pure) with a concentration of 5000 mg/l. The ZnO support was impregnated by the metallic salt solution and the catalyst suspension was aged for 24 h at room temperature, before the excess of solvent was evaporated at 100°C using a magnetic stirrer. The sample was dried at 100°C for 1 h, before to be calcined in flowing air at 300°C for 2 h (heating rate of 10°C/min). After cooling down to room temperature, the catalyst was further reduced in hydrogen flow (60 mL/min) for 1 h at a temperature within the 200-550°C range with a heating rate of 10°C/min. The non-reduced and reduced catalysts were labeled as CuZnO(I) and CuZnO(I)Tred, respectively, depending on the reduction temperature.

Photon-assisted preparation of CuZnO catalysts. The catalyst was performed by irradiating with solar light an aqueous suspension of the ZnO support containing Cu(NO3)2 as copper precursor. The irradiation was provided by an Atlas Suntest XLS+ reaction chamber equipped with a Xenon arc lamp adjusted to a 500 W/m 2 irradiance (with a 30 W/m 2 UV-A content) (320-800 nm wavelength range, ICH Q1B guidelines). The amount of copper precursor used was adjusted to have a Cu content of 10 wt % in the catalyst.

The dissolution of 38 mg of Cu(II) nitrate trihydrate (Sigma-Aldrich, p.a) was performed in water at room temperature, before 90 mg of ZnO was dispersed under stirring in 100 ml of copper solution in a beaker-type glass reactor at a 0.38 g/l concentration. Prior to irradiation, the suspension was stirred in the dark for 30 min at 60°C to ensure the establishment of the adsorption/desorption equilibrium, before the photon-assisted synthesis was performed under stirring under solar light for 2 h. The synthesis was monitored by UV-vis spectrophotometry by following the disappearance of the absorption peak at λ=800 nm. The catalyst was washed and filtrated under vacuum several times with distilled water, and finally dried at 100°C for 1 h. The as-prepared sample was then labelled as CuZnO(P).

Catalytic test

5-hydroxymethylfurfural hydrodeoxygenation. In a typical 5hydroxymethylfurfural hydrodeoxygenation experiment, 1 g of HMF, 0.3 g of a catalyst and 30 ml of dioxane were combined in a 60 ml Hastelloy autoclave. The reactor was pressurized with hydrogen to 30 bar and the temperature was maintained at 220°C for 5 h. After the reaction time, the autoclave was cooled down and the remaining pressure was released. The obtained reaction mixture was then centrifuged to separate the catalyst from the solution. The products were further analysed with an Agilent 7820A GC instrument equipped with a flame ionization detector and a CP-Wax 52 CB capillary column.

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

𝐶𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 = 𝑛 *+, --𝑛 *+, / 𝑛 *+, - • 100% 𝑌𝑖𝑒𝑙𝑑 7 = 𝑛 7 𝑛 *+, - • 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.

For assessing the catalyst recycling behavior, the catalyst was recovered from the reaction media by centrifugation, with a subsequent washing with deionized water and final drying step.

Catalyst characterization

X-ray diffraction (XRD) measurements were collected using a D8 Advance Bruker diffractometer operating in a ϴ/ϴ mode. The Xray source was a Cu long fine focus X-ray diffraction tube operating at 40 kV and 40 mA, with a Ka1 radiation at l=1.5406 Å. Data were collected in the 20-85° 2ϴ range with a 0.0263° scan step and a 0.6 s step time. Some patterns have been investigated by Rietveld refinement with the Fullprof software for which the modified Thompson-Cox-Hasting function was chosen to generate the line shape of the diffraction peaks. Instrumental broadening has been previously determined by measuring the scattering from corundum (NIST standard SRM 1976b). 24,25 Temperature-programmed reduction (TPR) was performed on AMI1 system from Altamira Instruments, USA, equipped with a thermal conductivity detector and was used for examining the reducibility of catalysts. All the catalysts prepared through the wet impregnation method were calcined at 300°C (with a 10°C/min heating rate) 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 , before the 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.

Scanning electron microscopy (SEM) was performed in secondary electron mode on a JEOL JSM-6700 F FEG microscope.

Transmission electron microscopy (TEM) was performed using a JEOL 2100F with a point resolution of 0.2 nm. A Mo grid was used for performing EDS analysis of Cu-ZnO samples. The interplanar spacings were calculated using ImageJ software.

Secondary ion mass spectra were recorded with a time-offlight secondary ion mass spectrometer (ToF-SIMS IV) manufactured by ION-TOF GmbH, Germany, equipped with a 25 kV pulsed Bi 3+ primary ion gun working in the static mode. This journal is © The Royal Society of Chemistry 20xx

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The measurements were performed in the static mode. The samples were pressed into pellets and then fixed to the sample holder by double-sided tape. The analyzed area of the sample surface was 500 µm × 500 µm. During analysis, a pulsed lowenergy electron flood gun was used for charge neutralization. The experiments were repeated three times for each sample.

Acetone-adsorbed FTIR measurements were carried out with Nicolet 6700 FTIR spectrometer equipped with MCT detector made by Thermo Scientific, using DRIFT cell. Before measurement, the samples were treated following the same procedure used to prepare the catalysts. After cooling to 40°C in Ar flow, acetone sorption was first performed for 20 min and the chamber was further purged with Ar flow for 20 min to get rid of gaseous acetone. All spectra were recorded with a time interval of 2 min in a wavenumber range of 1800-1600 cm -1 , with a scan number of 64, and a spectral resolution set at 4 cm -1 , and using a 20 ml/min gas flow.

X-ray Photoelectron Spectroscopy (XPS) characterization was performed on a ThermoVGMultilabESCA3000 spectrometer (Al Kα anode at hλ=1486.6 eV). The energy shift due to electrostatic charging was subtracted using the adventious sp 2 carbon C1s band at 284.6 eV. Contributions with Doniach-Sunjic shape 26 and a 'S-shaped' Shirley type background 27 were used.

Measurements of Cu and Zn content in the postreaction mixture were performed with Flame Atomic Absorption Spectrometry (FAAS) using Unicam Solaar M6 Atomic Absorption spectrometer.

Results

Catalytic activity

Influence of pretreatment. Table 1 shows the influence of the reduction pretreatment conditions on the performances of the CuZnO catalysts prepared via the impregnation method. The CuZnO(I) catalyst pre-reduced at the high temperature of 550°C exhibited a marginal HMF conversion.

Interestingly by lowering the reduction temperature from 550°C to 200°C, the HMF conversion gradually increases up to 41%, accompanied by a slight increase in the BHMF selectivity. The non-reduced CuZnO(I) catalyst allowed to reach almost full HMF conversion (99%), together with a high selectivity to BHMF. As a result, the highest yield to BHMF of 93% was obtained on the oxidized CuZnO(I) catalyst.

Those observations are in contrast with the literature concerning Cu catalysts supported on various oxides such as Al2O3, CeO2, TiO2 or ZnO used for HMF conversion, for which a pre-reduction step at high temperature is usually required. 19,20,22,28,29 Interestingly, the CuZnO catalyst without any pre-reduction treatment was more active than the Cu/Al2O3 catalyst, that is actually considered as the reference benchmark catalyst, 22 and that showed a conversion as low as 21% with only 15% yield to BHMF in our reaction conditions.

Table 1 Influence of the reduction pretreatment conditions on the activity of CuZnO catalysts in HMF hydrodeoxygenation

Influence of reaction time. The activity profile was influenced by the reaction time for the CuZnO(P) catalyst (Table 2). Although a high HMF conversion of 93% was reached already for 1 h of reaction, the DMF yield remained only moderate at 48%. Extending the reaction time allowed to reach a much higher DMF yield of nearly 80%, giving traces of intermediate reaction products like BHMF and 5-MFA and a very low yield to further C=C hydrogenation product DMTHF (9%).

The situation was fully different in the case of the CuZnO(I) catalyst. Already almost full conversion was reached after only 0.5 h of reaction giving a full selectivity to BHMF. BHMF is obtained via C=O bond hydrogenation from HMF and, although it is an intermediate product in the DMF production, the reaction selectivity to BHMF remained however constant for the 5 h of the reaction. This contrasts with the CuZnO(P) catalyst, for which both the C=O hydrogenation and the subsequent C-O hydrogenolysis occurred, leading to the conversion of BHMF to MFA and DMF. Reaction conditions: 220°C; 5 h; 0.3 g of catalyst; 1 g of HMF; 30 ml of dioxane; 30 bar of hydrogen. (*) Some by-products in small amounts were detected but not identified. In addition to the accuracy of the measurements at low conversion, this can explain the slight mismatch in the closure of the carbon balance.

a A reference Cu/Al2O3 benchmark catalyst was prepared via wet impregnation with a 10 wt.% metal content using an aqueous solution of Cu(NO3)2. The γ-Al2O3 (Fluka) support was impregnated by the metallic salt solution and the catalyst suspension was aged for 24 h at room temperature, before the excess of solvent was evaporated using rotary evaporator. The sample was dried at 100°C for 1 h, before to be calcined in flowing air at 300°C for 2 h. After cooling down to room temperature, the catalyst was further reduced in hydrogen flow for 1 h at 300°C.

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Please do not adjust margins Recycling of CuZnO catalysts. Taking into account the influence of the reduction pretreatment on the CuZnO(I) catalyst performances and the reductive nature of the reaction media, it was worth investigating the cycling behavior of the studied CuZnO catalysts (Table 3). First, the direct reuse of the CuZnO(I) catalyst led to a drastic drop of the activity, almost no HMF conversion (6%) being observed for the second test. However, when the catalyst was additionally calcined at 300°C after the first test, a high HMF conversion was maintained at 95%, while a change in the product distribution was observed, with a decrease in the BHMF yield from 91% to 31% together with an increase in the DMF yield from 0% to 34%. This catalyst change with the increase in the final product of the HMF hydrodeoxygenation reaction may indicate that activation of the catalyst occurred. This activity trend was amplified when performing further consecutive cycling with intermediate oxidation step, with the maintain of a HMF conversion within the 96-100% range. At the 3rd cycle, a further change in the product distribution was noted, with a complete disappearance of the BHMF product and a strong increase in the DMF yield till a stable level around 80% ± 5% for several test cycles.

In order to investigate the processes occurring during the cycling procedure, the CuZnO(I) catalyst was firstly reduced under hydrogen at 300°C for activating the CuZnO catalysts, and subsequently oxidized in similar conditions than those used in the cycling tests. The results obtained on the CuZnO(I)-(red)(ox) catalyst showed that only a slight activation occurred, what indicates that other factors plays a key role in the activation of CuZnO(I) catalysts during the cycling tests.

When it comes to the CuZnO(P) catalyst, it displayed a very high activity with full HMF conversion and DMF as the main hydrodeoxygenation product with a DMF yield of 79%. Similarly as in the case of the CuZnO(I) catalyst, a direct reuse of the CuZnO(P) catalyst resulted in a decrease in the activity, but in a less pronounced way, with the maintain of a HMF conversion of 90%, while the DMF yield decreased down to 43% and the BHMF yield increased to 26%. Performing an intermediate calcination at 300°C after test allowed a total HMF conversion to be achieved for several cycles, with a high DMF yields of 92% and 94%. This activity dropped only with the 4th reuse giving BHMF and DMF as main reaction products with yields of 47% and 41%, respectively.

Characterization of fresh CuZnO catalysts

The XRD patterns of the bare ZnO support and of the CuZnO catalysts prepared through the photon-assisted method and the wet impregnation method with different reduction pretreatments are shown in Figure 1, and the mean crystallite size of both the copper containing phases and the ZnO support are shown in Table 4. They displayed the diffraction peaks characteristic of ZnO crystallized in the hexagonal wurtzite structure and corresponding to the diffraction of the (100), (002), ( 101), ( 102), (110), and (103) planes for the most intense peaks (P63mc space group, JCPDS 00-036-1451). 30 In addition to the reflexes from the ZnO support, the XRD patterns of both CuZnO(P) and CuZnO(I) catalysts exhibited the diffraction peaks attributed to the (002), ( 111), (-202), ( 020), ( 202), (-113), (-222) and ( 222) planes of monoclinic Cu (II) O nanoparticles (JCPDS Card 01-089-5895). While the as-synthesized ZnO support had a mean crystallite size of 10 nm, the implementation of both preparation methods led to an increase in the mean crystallite size of the ZnO support to 20 nm for the CuZnO(P) catalyst, and in a larger extent to 42 nm for the CuZnO(I) counterpart. As a result, CuZnO(I) and CuZnO(P) catalysts had a specific surface area of 25 m 2 /g and 33 m 2 /g, respectively, while the bare support exhibited a surface area of 68 m 2 /g. For both freshly prepared catalysts, a mean CuO crystallite size of 10 nm was observed for CuZnO(P), whereas a significantly larger mean size of 23 nm was obtained for CuZnO(I), as a result of the final calcination step of the catalyst at 300°C with exothermic decomposition of the precursor salt. Independently of the preparation method, the CuZnO catalyst can be considered as a CuO-ZnO composite catalyst, since the CuO The nature of the Cu phase progressively turned into metallic Cu with the increase in the reduction temperature, so that both Cu (II) O and Cu 0 phases were observed for the catalyst treated at 200°C -with a mean crystallite size of 13 and 22 nm respectively, while only reflexes corresponding to the diffraction of the (111), ( 200) and (220) planes of Cu 0 (JCPDS 01-085-1326) were observed for the CuZnO(I)300 catalyst -with no size change.

A further increase in the reduction temperature resulted in the formation of intermetallic CuZn compound, with the presence of additional reflexes in 2q at 42.4°, 43.5° and 49.2°at 550°C attributed to the Cu0.2Zn0.8 alloy phase, 31 with a mean crystallite size of 32 nm. the intermediate temperature of 400°C, the observed reflexes might be attributed to an intermetallic CuZn phase with a different composition. By contrast, the mean ZnO crystallite size was not affected by the reduction step under hydrogen and remained within the 42-46 nm range.

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Characterization of used and post-reaction treated catalysts.

Figure 2 and Table 5 show the influence of the 1st catalyst cycling on the XRD patterns of both CuZnO catalysts and on the derived mean crystallite sizes. Besides the reflexes of the ZnO support, the pattern of the used CuZnO(I) evidenced the reflexes of the metallic Cu phase, with a mean crystallite size of 28 nm, characterizing the bulk reduction of CuO into metallic Cu during the catalytic reaction under pressured hydrogen. The further recalcination of the used CuZnO(I) resulted in the reoxidation of the metallic copper into both CuO and Cu2O phase, with a smaller mean crystallite size of 11 nm and 12 nm, respectively. When it comes to the CuZnO(P) catalyst, the reaction induced a reduction of the pristine monoclinic CuO crystallites into cubic Cu2O crystallites (JCPDS Card 00-005-0667) with a mean crystallite size of 41 nm, whereas a broad peak corresponding to metallic Cu was observed. After the calcination treatment of the used CuZnO(P) catalyst, the both Cu2O and CuO crystallites were observed, with a mean crystallite size of 15 nm and 12 nm. The mean crystallite size of the ZnO was not affected by the treatments applied, considering the accuracy of the measurements.

In addition, Rietvelt refinement of diffractograms showed that no incorporation of copper ions within the zinc oxide support lattice occurred, neither during the catalyst preparation step, nor during the reaction at 220°C under 30 bar of hydrogen or the intermediate re-oxidation step after test, regardless of the synthesis method used (Table S1).

The CuZnO(I) catalyst shows higher temperature stability and the particle size stays nearly stable during the reaction. The reoxidation step allows to reach smaller particle size that is then kept during the further oxidation cycles (Table S2). By contrast, the CuZnO(P) catalyst is more prone to changes as the initial photoassisted synthesis treatment was performed at room temperature. In the presence of reductive reaction conditions, larger Cu2O nanoparticles are formed, which redispersed to the initial size during their reoxidation step forming back CuO. Figure 3 and4 shows SEM and TEM images, respectively, of selected CuZnO(I) and CuZnO(P) catalysts during the test cycling procedure. The SEM images indicate that the overall morphology of the ZnO-based materials was not modified by the Cu nanoparticle synthesis processes compared to the bare support (Figure S1), except a more marked increase in the mean ZnO particle size in the case of the CuZnO(I) samples compared to the CuZnO(P) counterpart. The electronic similarity between Cu and Zn did not enable to discriminate between both oxide particles and mainly surface topographic contrast was observed. However, overall EDS analysis evidenced the presence of copper within the catalysts. Only imaging in the back-scattered mode enabled to visualize the distribution of the metallic Cu nanoparticles in the catalysts after test, as shown in the case of the used CuZnO(P). Figure 4a,b evidences the general morphology of the ZnO support material prepared through the carbonate precipitation method, with interplanar spacings of 0.28 nm and 0.25 nm consistent with the (100) and (101) planes of hexagonal wurtzite ZnO crystallites, 32 and with crystallite sizes in agreement with those derived from the XRD patterns. When it comes to the CuZnO catalysts, whatever the applied treatments, the similarities of the interplanar spacings corresponding to the main planes of ZnO and of both CuO or Cu2O crystallites did not allow the specific identification and location of the phases in the different CuZnO catalysts that consisted of oxide crystallites in very close contact. However, overall EDS analysis and mapping STEM images confirmed the presence of copper in both samples (Figure S2). The TEM images of both fresh catalysts confirmed as well that the growth of ZnO crystallites was more By contrast, the images recorded on the used catalysts reveal the presence of metallic Cu nanoparticles in very close contact with the ZnO crystallites, as evidenced in Figure 4f for instance in the case of the catalyst prepared via the impregnation method, with the interplanar spacing of 0.21 nm consistent with the (111) planes of metallic copper (JCPDS No. 01-085-1326).

When it comes to the used catalysts after the reoxidation step, the difficulty to visualize the Cu based phase within the CuZnO samples confirmed the bulk reoxidation of the metallic Cu nanoparticles observed by XRD (not shown).

The influence of the preparation method and of the treatment applied (fresh, used and used catalyst after reoxidation step) on the surface state of the CuZnO catalysts is shown in Figure 5 and S3. The Zn 2p orbital XPS spectra exhibit a single contribution, with the doublet related to Zn 2p3/2 and 2p1/2 spin-orbit components of Zn 2+ (Zn-O) surface species, at 1021.1 and 1044.2 eV, in agreement with the literature. 35 While the same surface species were observed on the CuZnO(I) catalyst, it was worth noting the presence of an additional doublet contribution at 1022.8 and 1045.9 eV for the CuZnO(P) catalyst. This higher energy contribution was attributed to Zn 2+ in a different chemical environment, and was assigned to Zn-O-Cu bonds. Indeed, in an electrostatic model, the higher electronegativity of Cu compared to Zn resulted in a O ® Cu electron transfer in a Zn-O-Cu configuration, that would consequently lead to shift the electronic density of the Zn-O bond towards the oxygen atoms, therefore increasing the electron deficiency of the Zn atoms. This partial substitution of Zn 2+ by Cu 2+ might occur only at the surface of the CuZnO catalyst prepared through the photoassisted synthesis, since ZnO is prone to surface photocorrosion under irradiation. 32,36,37 33,34 In the case of the CuZnO(I) catalyst, no difference was observed in terms of Zn surface species neither directly after the reaction on the used catalyst, nor after the reoxidation of the used catalyst. By contrast, the spectra recorded on the used CuZnO(P) catalyst revealed the appearance of a new doublet contribution at 1019.8 and 1042.9 eV. This lower energy contribution was attributed to Zn d+ species (ZnOx) that may be formed at the interface with copper under the highly reductive reaction conditions. The presence of partially oxidized Zn d+ species was observed by Behrens et al. at the defective Cu surface of a high-performance Cu/ZnO/Al2O3 catalyst for methanol synthesis as a result of a dynamic SMSI effect with reduction of ZnO particles leading to partial coverage of the metal particles with ZnOx. 38 This additional partially oxidized contribution was removed during the subsequent reoxidation step of the used CuZnO(P) catalyst. Whatever the preparation method, the Cu 2p orbital XPS spectra shows the presence Cu 2+ species at the surface of the fresh catalysts, with a broad doublet contribution (main orbital peak at 932.5 eV) and the usual broad and badly-resolved satellite features of Cu 2+ species (Figure S3). The thinner doublet contribution observed on the used catalysts was in agreement with the formation of metallic copper observed by XRD analysis, although XPS does not allow to discriminate between metallic copper and Cu + species. Further, the peak asymmetry indicated that surface oxidation of the copper species occurred. The surface of the used reduced catalysts turned back to Cu 2+ during the reoxidation step of both used catalysts, with the observation of broader and slightly shifted peaks.

The reducibility of the catalysts

The reducibility of the CuZnO catalysts (fresh and after reoxidation treatment) was investigated by TPR measurements. The TPR profile of the fresh CuZnO(I) catalyst presented in Figure 6A shows a broad reduction peak with several contributions within the 180-400°C temperature region and a very small effect at 400-650°C. This complex TPR profile results, first from the two-step reduction of the Cu II species in CuO to Cu I and subsequently to Cu 0 39 and from the presence of different types of species differing in terms of size, dispersion, and interaction with the support. The low temperature small peak (180-220 o C) results probably from amorphous, small size, weekly interacting with the support Cu species. 40,41 The main effect (180-400°C) is related with the step-wise reduction of CuO that is also directly affected by the presence of crystalline CuOx nanoparticles of different sizes, additionally interacting with the support differently -the stronger the interaction, the higher the reduction temperature. 39,42,43 Final reduction effect at high temperatures in the range of 400-700°C can be connected with the reduction of Cu 2+ species which are in strong interaction with the support, with further CuZn alloy formation. 44 After recalcination of the used catalyst, the TPR profile is shifted to lower temperature in comparison to the fresh CuZnO(I), and only one reduction peak is visible, with a maximum at 230°C. It may be related to a redistribution of smaller CuO crystallites. 45 It shows clearly that the Cu species are more susceptible to be reduced.

The TPR profiles of the CuZnO(P) catalysts prepared through the photon-assisted method are presented in Figure 6B. TPR profile of the fresh CuZnO(P) shows two reduction effects, in the 125-290°C and 400-760°C temperature range. The first asymmetric peak with a maximum at 210°C strongly shifted to low temperature when compared to the fresh CuZnO(I) catalyst, may be assigned to the two-step reduction of CuO, but in this case with much smaller and more uniform crystallite size. The second effect at high temperature, as in the case of the CuZnO(I) catalysts, may be related with the reduction of Cu 2+ in strong interaction with the support.

After oxidation of the used CuZnO(P) catalyst, a slight change of the position of the reduction peak in comparison to fresh catalyst is visible which can occur due to the thermal reduction treatment. However, the changes of the reduction temperature are not so much pronounced like for the CuZnO(I) counterpart and the reduction temperature maximum for the catalyst after the reoxidation step remains in the same range as for the CuZnO(I) counterpart after the similar cycling treatment.

FTIR analysis of the acetone adsorption on the catalyst surface

The first step of the HMF hydrodeoxygenation reaction requires carbonyl group adsorption. Therefore, acetone was chosen as a probe molecule and its adsorption was monitored by FTIR study (Figure 7). The main, broad band at 1710 cm -1 can be assigned to the carbonyl group vibrations of acetone 46 and the small band located in the range of 1680-1670 cm -1 can be assigned to mesityl oxide carbonyl group vibrations. 47 Acetone adsorption allows to investigate formation of the acid centres of the catalyst due to the formation of the mesityl oxide. Mesityl oxide originates from aldol condensation of two adsorbed acetone molecules via dehydration reaction which occurs on acid centers. 48,49 The highest acetone adsorption is visible for the CuZnO(I) catalyst, however after reaction no characteristic band at 1710 cm -1 is observed any more. It can be related with the formation of Cu 0 . Additionally it cannot be fully excluded that active sites are partially blocked by carbon deposit formed during the test.

After recalcination of CuZnO(I) much smaller band is observed, additionally a small, broad band at 1680 cm -1 appears, which can be assigned to the carbonyl group vibrations This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins of the mesityl oxide. Therefore it can be assumed that during the postreaction treatment of the CuZnO(I) catalyst, formation of the acidic centres occurs. In case of the CuZnO(P) catalyst prepared with photon-assisted deposition method, the acetone adsorption is significantly smaller than in case of CuZnO(I). This might be related with stronger CuO-ZnO interaction and therefore and presence of the Cu-O-Zn bonds due to the partial dissolution of Cu 2+ ions in the ZnO lattice, which may influence of the acetone adsorption. What is more, interaction of copper oxide with ZnO, increases the electron density of Cu by charge transfer from ZnO with formation of copper species with lower positive charge Cu (2-δ)+ . This effect is enhanced for low Cu content catalyst and for the small CuO particle size as in the case of CuZnO(P). 50 Interestingly, for the CuZnO(P) the band of mesityl oxide C=O is present also for the fresh catalyst. This can be related with the presence of Zn-O-Cu sites at the surface of the catalyst, as putted forward by the XPS study, which can serve as Lewis acid center, allowing dehydration of the adsorbed molecule. Although after reaction the decrease of the intensity of the acetone C=O group vibration band is observed, in contrast to the CuZnO(I), acetone adsorption still occurs, what may be also related with the presence of Zn-O-Cu sites (although in a lower extent) and/or with the formation during the reaction of partially oxidized Zn d+ centers or of Cu2O species. Further calcination of the used catalyst causes an increase in the amount of adsorbed acetone. What is more, increase of formation of mesityl oxide was also noted, what suggests, that further formation of the acid sites occurred.

Catalyst stability characterization

In order to investigate the stability of investigated catalyst FAAS measurements of the reaction liquid and ToF-SIMS measurements of the catalyst surface were performed. FAAS measurements revealed that in case of the catalyst prepared via wet impregnation method no leaching of the Cu 2+ or Zn 2+ occurs, whereas in case of the catalyst prepared via photonassisted deposition method only slight leaching of both copper and zinc ions take place, with ions release smaller than 0.3%wt of the initial amount in the catalyst. Therefore, leaching cannot be considered as a reason for catalyst deactivation during recycling tests. ToF-SIMS analysis of the catalyst surface was performed to further investigate the deactivation phenomena of the catalysts (Table 6). Table 6. The intensity ratios of selected ions calculated on the basis of the ToF-SIMS mass spectra collected from the surface of CuZnO(P) and CuZnO(I) catalysts after tests

In the case of the CuZnO(I) catalysts, both C + /Cu + and C + /Zn + ratios are gradually increasing from around 0.1 for the fresh catalyst (that corresponds to the usual background ratio) up to 0.5 for C + /Cu + and 1.5 for C + /Zn + for the catalyst after the 3 rd test cycle. Similar observations can be made for the CuZnO(P) catalyst, where both C + /Cu + and C + /Zn + ratios are increasing from 0.086 and 0.161 values up to 1.430 and 3.120, respectively. Therefore, the results indicated that carbon deposition may be a reason for the catalyst deactivation which occurs during the recycling tests.

Discussion

We believe that the presence of Cu n+ is necessary in the initial reaction step for facilitating the pre-adsorption of HMF on the surface via the C=O bond, since it has been reported that both Cu 2+ and Cu + centers have a pivotal role as far as C=O bond is concerned. 51 The Cu n+ sites at the catalyst surface can act as electrophilic or Lewis acidic centers for polarizing the C=O bond via the lone electron pair in oxygen, with as consequence an enhancement of the C=O adsorption and of the activity. 52 This enhancement was noticed in several processes which require a C=O bond hydrogenation, such as the hydrogenation of dimethyloxalate to ethanol 53 or the hydrogenation of adipic acid to 1,6-hexanediol. 54 Although direct relationship between the number of Cu + or Cu 2+ centers and the activity is only scarcely observed, 54 the role of Cu n+ ions is often related to the stabilization of reaction intermediates, like the methoxy and acyl species in the methyl acetate hydrogenation 53 or in LA hydrogenation. 55 In our case, the surface of CuZnO(I) exposing Cu 2+ sites was characterized by a high adsorption, whereas the high HMF conversion is reached already after 0.5 h of reaction. The same catalyst is fully reduced after the reaction cycle and thus did not exhibit any Please do not adjust margins Please do not adjust margins adsorption, and consequently no HMF conversion in the reaction is obtained when the catalyst is reused as-it with no intermediate treatment. This contrasts with the CuZnO(P) catalyst, which is not fully reduced after reaction -with the presence of the Cu2O phase in addition to the metallic copper -and therefore allows HMF adsorption via the C=O bond to occur. As a result, HMF conversion can further proceed on the used catalyst during the next test cycle without any intermediate treatment.

Although the Cu n+ sites are required for the first step, namely the HMF adsorption on the catalysts via the C=O bond, they are however not the active sites for the hydrogenation process. Indeed Cu 0 is formed in situ in the process, and its role is related to the dissociation of H2 and to the reduction of the C=O bond, so that a cooperative effect associated to the presence of metallic and Cu n+ sites is necessary. This is in agreement with the literature reports concerning other hydrogenation processes. 56 The HMF adsorption via the C=O bond and the subsequent efficient reduction of the C=O bond enable to get high yield to BHMF, while the further reduction towards DMF implies C-O hydrogenolysis to take place, for which the presence of both Cu 0 and acid sites is required.

Oxygen vacancies in ZnO that act as Lewis acid sites can be formed at the ZnO-CuO interfacial sites, due to the existence of stronger interactions between ZnO and Cu species. 57 The increased synergy within ZnOx-CuOx phases was already proposed as an important factor promoting the activity, particularly in the case of methanol synthesis. Valant et al. showed recently that the activity of the Cu-ZnO catalyst in the methanol synthesis is directly correlated to the number of contact points between Cu species and ZnO nanoparticles. 58 From the other hand, Behrens et al. proposed that the Cu-ZnO synergy was related to the presence of Zn δ+ at the Cu surface (steps/defects), which might result from a strong Cu-ZnO interaction allowing a partial coverage of the Cu surface with ZnOx under reducing conditions. This could result in the increase in the binding strength of intermediates and in the decrease of reaction barriers in CO hydrogenation. 38 This may explain why the reaction goes forward towards further hydrogenation products like DMF in the case of the CuZnO(P) catalyst, whereas by contrast the reaction is stopped on the CuZnO(I) catalyst despite the latter shows a slightly higher activity. Lewis acid sites were formed on CuZnO(P) during the photo-assisted synthesis procedure as ZnO is prone to surface photocorrosion under irradiation. Their presence was confirmed by the formation of mesityl oxide during the IR study with acetone as probe molecule. Their formation was noticed also for the used CuZnO(I) catalyst after reoxidation treatment and this directly resulted in pushing the selectivity of the reaction. Additionally the most active catalyst -CuZnO(P) -that allows to reach high DMF yield had small size Cu particles (~10 nm), that probably facilitates the phase contact within the CuOx-ZnO material.

Finally we showed that the catalysts can be further activated during the reaction, and thanks to that, that the reaction selectivity can be tuned. The possibility of tuning the reaction selectivity was shown already in this reaction, however only by changing the reaction conditions, like by elevating the temperature or the pressure. Indeed, mild reaction conditions allowed the formation of BHMF, whereas DMF was the main product in harsh conditions. 19 This was however followed by a fast deactivation of the catalyst already in the second reaction cycle.

In our case, the high activity obtained was proposed to result from the formation of Lewis acid sites (Cu-ZnO interfacial sites), as well as from the migration and the redispersion of Cu particles that can occur during both the reduction and the reoxidation treatments. 59,60

Conclusions

In this work, we showed that robust 10%CuZnO catalysts allow both BHMF or DMF products to be obtained with high yields from HMF. The selectivity was proposed to depend on the presence of acid sites at the catalyst surface, that might probably correspond to ZnOx-CuO and/or Zn-O-Cu interfacial sites. Those sites can be formed during the photo-assisted synthesis of the CuZnO catalyst, or can be created in situ in the highly reductive reaction environment. The catalyst prepared at room temperature through the photoassisted method possesses acid sites, what enable the direct DMF formation. By contrast, those acid sites were only generated during the reaction on the catalyst prepared via the classical wet impregnation method. What is more, we have concluded that high HMF conversion is possible when Cu n+ and Cu o sites are present, as Cu n+ allows for HMF adsorption via the C=O bond, while Cu o allows for H2 dissociation and reduction.

Before subsequent reaction cycles, re-oxidation of the CuZnO catalyst needs to be performed, what can lead to the redispersion of Cu particles. This oxidative treatment allows to obtain active and selective catalyst for several reaction cycles. By contrast, performing the reduction of the CuZnO catalyst at high temperature (300-550°C) before the reaction, can result in the CuZn alloy formation and only marginal HMF conversion can be obtained. This journal is © The Royal Society of Chemistry 20xx

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  conditions: 220°C; 0.3 g of catalyst; 1 g of HMF; 30 ml of dioxane; 30 bar of hydrogen.

Figure 1 .

 1 Figure 1. X-Ray Diffraction patterns of the ZnO support, the CuZnO(P) catalyst and the CuZnO(I) catalyst with different reduction pretreatments. The crystallographic planes indicated correspond to the diffraction planes of the ZnO support.

*

  defined as the mean size of the coherently-diffracting domains, derived from the Scherrer equation using the classical assumption of spherical crystallites. The fullwidth at half maximum of the diffraction peaks of ZnO (100), Cu (111), Cu2O (200) and CuO (111) was used for the estimation.

Figure 2 .

 2 Figure 2. X-Ray Diffraction patterns of the bare ZnO support and both CuZnO(P) and CuZnO(I) catalysts, fresh, after test and after test followed by intermediate calcination at 300°C. The crystallographic planes indicated correspond to the diffraction planes of the ZnO support

Figure 3 .

 3 Figure 3. SEM images of both CuZnO(I) and CuZnO(P) catalysts as a function of the test cycle : fresh, used and used/reoxidized. The right (b) image was taken in the backscattered mode for evidencing in a more pronounced way the contrast between metallic Cu and ZnO.

Figure 4 .

 4 Figure 4. TEM images of (a,b) the ZnO support, (c,d) the fresh CuZnO(P) catalyst, (e) the fresh CuZnO(I) catalyst, and (f) the used CuZnO(I) catalyst. The (101) and (100) planes of ZnO have interplanar distances of 0.25 nm and 0.28 nm (JCPDS No.00-036-1451), while the (111) and (002) planes of the CuO phase have 0.23 nm and 0.25 nm interplanar distances (JCPDS No. 01-089-5895), and the (111) planes of the Cu2O phase have 0.25 nm interplanar distances, respectively (JCPDS No. 00-005-0667)33,34

Figure 5 .

 5 Figure 5. Influence of the CuZnO catalyst preparation method and of the treatment applied (fresh, used and used catalyst after reoxidation step) on the Zn 2p3/2 and 2p1/2 orbital region. (a) photoassisted method ; (b) impregnation method.

Figure 6 .

 6 Figure 6. TPR profiles of a) the CuZnO(I) and b) the CuZnO(P) catalysts, fresh and after reoxidation treatment.

Figure 7 .

 7 Figure 7. Acetone-adsorbed FTIR profiles for CuZnO(I) and CuZnO(P) catalysts according to the recycling treatment

Table 2

 2 Influence of the reaction time and catalyst preparation method on the activity of CuZnO catalysts in HMF hydrodeoxygenation

Table 4

 4 Main properties of the CuZnO catalysts derived from XRD patterns

Table 3

 3 Activity and test-cycling behavior in HMF hydrodeoxygenation of CuZnO catalysts.

		HMF conv. [%]				Product yield [%]
	Catalyst		5-MF	BHMF	5-MFA	DMF	BHMTHF	DMTHF
	CuZnO(I)	99	2	91	2	0	0	0
	CuZnO(I)-r2	6	3	0	3	0	0	0
	CuZnO(I)-r2(ox)	95	7	31	7	34	8	1
	CuZnO(I)-r3(ox)	100	0	0	0	85	0	15
	CuZnO(I)-r4(ox)	96	3	11	5	72	0	4
	CuZnO(I)-r5(ox)	99	2	9	4	77	2	6
	CuZnO(I)-(red)(ox) a	99	5	81	10	2	1	0
	CuZnO(P)	100	0	5	6	79	1	9
	CuZnO(P)-r2	90	7	26	10	43	1	4
	CuZnO(P)-r2(ox)	100	0	0	1	94	0	5
	CuZnO(P)-r3(ox)	100	1	0	3	92	0	4
	CuZnO(P)-r4(ox)	100	2	47	9	41	1	0
	CuZnO(P)-r5(ox)	93	5	47	12	42	0	1

Reaction conditions: 220°C; 5 h; 0.3 g of catalyst; 1 g of HMF; 30 ml of dioxane; 30 bar of hydrogen. Nomenclature: eg. the CuZnO(I)-r2 and CuZnO(I)-r2(ox) catalysts corresponds to the 2 nd test of the CuZnO(I) catalyst without and with an intermediate oxidation treatment at 300°C, respectively; a CuZnO(I)-(red)(ox) catalyst corresponds to the 1 st test of the CuZnO(I) catalyst after a reduction treatment under hydrogen at 300°C followed by an oxidation treatment at 300°C.

Table 5 .

 5 Main physicochemical properties derived from XRD patterns of both CuZnO(I) and CuZnO(P) catalysts, fresh, after test and after test followed by intermediate calcination at 300°C.

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Acknowledgements

The authors gratefully acknowledge that this work was financially supported by a grant from the National Center of Science (NCN) in Krakow (Poland) (2016/22/E/ST4/00550). This research was co-funded by the EU Erasmus+ program. T. Dintzer and V. Papaefthimiou (ICPEES) are thanked for performing SEM and XPS analysis, respectively. C. Lefèvre (IPCMS Strasbourg) and W. Maniukiewicz (LUT) are thanked for contributing to XRD.

Conflicts of interest

There are no conflicts to declare.