Blaise Dumat

Lorraine Montel

Léa Pinon

Pascal Matton

Laurent Cattiaux

Jacques Fattaccioli
email: jacques.fattaccioli@ens.fr

Jean-Maurice Mallet
email: jean-maurice.mallet@ens.fr

Mannose-coated Fluorescent Lipid Microparticles for Specific Cellular Targeting and Internalization via Glycoreceptor-Induced Phagocytosis

Keywords: oil-in-water emulsion droplets, fluorescent glycolipids, phagocytosis, carbohydrateslectins interactions, mannose receptor

In this work we report on the development of mannose-coated fluorescent lipid microparticles to study the role of C-type lectin membrane receptor in phagocytosis. The micrometric droplets of soybean oil in water emulsion were functionalized with a tailor-made fluorescent mannolipid. The amphiphilic ligand was built from a mannose unit, a lipid C11 spacer and a naphthalimide fluorophore. The droplets functionalization was monitored by fluorescence microscopy as well as the interaction with concanavalin A which was used as a model lectin in vitro. The use of a monovalent ligand on the surface of emulsion droplets yielded particles with an affinity approximately 40 times higher than that of free mannose. In cellulo, the coated droplets were shown to be specifically internalized by macrophages in a receptor-dependent phagocytic pathway. The naked droplets on the other hand display very little internalization due to their low immunogenicity. This work thus brings evidence that C-type lectin membrane receptors may act as phagocytic receptors. The droplets functionalization with the tailored amphiphilic fluorescent ligand and the droplets functionalization also provides insights into the development of organic fluorescent particles that may prove useful for developing targeted imaging and delivery tools.

Introduction

Phagocytosis is the process by which cells internalize objects larger than 0.5 microns. Whereas unicellular organisms use phagocytosis to capture and eat preys, in multicellular organisms it represents a fundamental part of innate immunity and organ homeostasis. Innate immunity relies on a specialized subset of cells, the phagocytes, which patrol the organism, identify, ingest and eliminate pathogens. Among them, macrophages are versatile cells residing in tissues that are able to scavenge worn-out cells and participate to the activation of the adaptive immune response.

Phagocytosis by macrophages is triggered by the binding of the target to specific receptors present at the surface of the phagocyte. Several receptors have been identified so far, each involving different signaling pathways and ingestion mechanisms. Opsonic receptors rely on the recognition of the pathogen by immunoglobulin or complement proteins. In the case of the Fcγ receptorsmediated phagocytosis, antigens present at the surface of the target are bound by specific soluble immunoglobulins (IgGs). [START_REF] Flannagan | The Cell Biology of Phagocytosis[END_REF] Fc regions of those IgGs are then actively recognized by the Fcγ receptors at the surface of the phagocyte, which form clusters and trigger the internalization. [START_REF] Flannagan | Dynamic Macrophage "Probing" Is Required for the Efficient Capture of Phagocytic Targets[END_REF][START_REF] Ben | Phagocytosis of Immunoglobulin-Coated Emulsion Droplets[END_REF][START_REF] Headley | Chimeric Antigen Receptors That Trigger Phagocytosis[END_REF] Engulfment then occurs by an actin-driven membrane extension and closure of a phagocytic cup around the foreign body to create a specific degradative compartment: the phagosome. [START_REF] Swanson | Shaping Cups into Phagosomes and Macropinosomes[END_REF][START_REF] Swanson | Cell Biology: The Extraordinary Phagosome[END_REF] While FcγR-mediated phagocytosis relies on specific recognition by IgG during the secondary immune response, Pattern Recognition Receptors recognize conserved pathogen-associated molecular patterns (PAMPs) on the surface of bacteria and yeasts to ensure innate immunity. [START_REF] Mcgreal | Ligand Recognition by Antigen-Presenting Cell C-Type Lectin Receptors[END_REF] Carbohydrate receptors like the Mannose Receptor (MR) family or DC-SIGN are C-type lectins that bind glycoconjugates present in cell walls in a Ca 2+ dependent manner.

MR is present in macrophages and targets polysaccharide patterns with terminal mannose, fucose and N-Acetylglucosamine (GlcNAc). [START_REF] Martinez-Pomares | The Mannose Receptor[END_REF] DC-SIGN is a receptor primarily expressed in dendritic cells but also in some sub-populations of macrophages that targets terminal mannose groups like mannans. [START_REF] Mcgreal | Ligand Recognition by Antigen-Presenting Cell C-Type Lectin Receptors[END_REF][START_REF] Hardison | C-Type Lectin Receptors Orchestrate Antifungal Immunity[END_REF] Their role in phagocytosis, and especially their ability to trigger it, is unclear as of yet.

They have been identified in phagosomes [START_REF] Tailleux | DC-SIGN Induction in Alveolar Macrophages Defines Privileged Target Host Cells for Mycobacteria in Patients with Tuberculosis[END_REF][START_REF] Cambi | Dendritic Cell Interaction with Candida Albicans Critically Depends on N-Linked Mannan[END_REF] and shown to participate in the binding and internalization of targets. [START_REF] Sung | Yeast Mannans Inhibit Binding and Phagocytosis of Zymosan by Mouse Peritoneal Macrophages[END_REF][START_REF] Serrano-Gómez | Dendritic Cell-Specific Intercellular Adhesion Molecule 3-Grabbing Nonintegrin Mediates Binding and Internalization of Aspergillus Fumigatus Conidia by Dendritic Cells and Macrophages[END_REF][START_REF] Ezekowitz | Molecular Characterization of the Human Macrophage Mannose Receptor: Demonstration of Multiple Carbohydrate Recognition-like Domains and Phagocytosis of Yeasts in Cos-1 Cells[END_REF] However, neither could induce phagocytosis in non-phagocytic cell lines, and whether they can trigger phagocytosis on their own or merely cooperate with other receptors to do so remains an open question. [START_REF] Cabec | The Human Macrophage Mannose Receptor Is Not a Professional Phagocytic Receptor[END_REF] Macrophage activity can be detected by small molecular fluorescent probes reporting on pH acidification or ROS production inside the phagosome. [START_REF] Yu | A Lysosome-Targetable and Two-Photon Fluorescent Probe for Monitoring Endogenous and Exogenous Nitric Oxide in Living Cells[END_REF][START_REF] Fernández | Smart Fluorescent Probes for Imaging Macrophage Activity[END_REF][START_REF] Vázquez-Romero | Multicomponent Reactions for de Novo Synthesis of BODIPY Probes: In Vivo Imaging of Phagocytic Macrophages[END_REF][START_REF] Kenmoku | Development of a Highly Specific Rhodamine-Based Fluorescence Probe for Hypochlorous Acid and Its Application to Real-Time Imaging of Phagocytosis[END_REF] Such probes however only report indirectly on processes posterior to the internalization. The adhesion and internalization steps of phagocytosis involve large three dimensional biological objects (cells, apoptotic bodies, bacteria) and can only be probed directly by large micrometric particulate biosensors. Nanoparticles and nanostructures have become platforms of choice to build up biosensors or delivery systems incorporating several probes, ligands, or cargo within the same object. [START_REF] Wolfbeis | An Overview of Nanoparticles Commonly Used in Fluorescent Bioimaging[END_REF][START_REF] Howes | Colloidal Nanoparticles as Advanced Biological Sensors[END_REF][START_REF] Faraji | Nanoparticles in Cellular Drug Delivery[END_REF][START_REF] Hu | Mannose-Functionalized Nanoscaffolds for Targeted Delivery in Biomedical Applications[END_REF] Although less used, microparticles can be made in similar fashion and come in solid (e.g. metallic particles, silica or polystyrene beads) or liquid form (e.g. liposomes, emulsions). Liquid particles have the distinct advantage of exhibiting a fluid interface similar to that of cellular lipid bilayers and they can reproduce the mobility of interfacial ligands and receptors during phagocytosis. To that end, liposomes are by far the most used liquid particles in biology, with numerous examples of cell receptor recognition by glycosylated structures. [START_REF] Mauceri | Recognition of Concanavalin a by Cationic Glucosylated Liposomes[END_REF][START_REF] Perche | Enhancement of Dendritic Cells Transfection in Vivo and of Vaccination against B16F10 Melanoma with Mannosylated Histidylated Lipopolyplexes Loaded with Tumor Antigen Messenger RNA[END_REF][START_REF] Espuelas | Influence of Ligand Valency on the Targeting of Immature Human Dendritic Cells by Mannosylated Liposomes[END_REF] Alternatively, oil-in-water (O/W) emulsions made out of vegetable oil are easily accessible and biocompatible particles. They have been used since half a century as colloidal drug carriers for various therapeutic applications and they can be fabricated with a narrow size distribution ranging from a few tenth of nanometers to several hundreds of microns. [START_REF] Tamilvanan | Oil-in-Water Lipid Emulsions: Implications for Parenteral and Ocular Delivering Systems[END_REF][START_REF] Leal-Calderon | Emulsion Science: Basic Principles[END_REF] Carefully functionalized with biologically-relevant adhesive molecules, they are able to specifically interact with cells [START_REF] Bourouina | Biomimetic Droplets for Artificial Engagement of Living Cell Surface Receptors: The Specific Case of the T-Cell[END_REF] and can be used as model particles for cell adhesion modeling. [START_REF] Fattaccioli | Specific Wetting Probed with Biomimetic Emulsion Droplets[END_REF][START_REF] Pontani | Biomimetic Emulsions Reveal the Effect of Mechanical Forces on Cell-Cell Adhesion[END_REF][START_REF] Bourouina | Formation of Specific Receptor-Ligand Bonds between Liquid Interfaces[END_REF] These properties make them good candidates to build up targeted platforms for imaging or therapeutic applications. Yet, turning emulsion droplets into biosensors requires controlled chemical surface functionalization with appropriate reporters and targeting moieties and such examples are still scarce. [START_REF] Zhang | Interfacial Bacsioconjugation on Emulsion Droplet for Biosensors[END_REF] We report herein on the development of fluorescent lipid particles targeted towards receptors from the lectin family based on micrometric O/W emulsion droplets functionalized with a tailor-made glycosylated fluorescent ligand. We have synthesized a fluorescent mannolipid consisting of a naphthalimide fluorophore, a C-11 spacer and a mannose and, after careful characterization of its optical properties, the glycolipid was used to functionalize the surface of micrometric oil-in-water emulsion droplets. We show that the resulting particles interact selectively in vitro with a model lectin protein and that they are specifically internalized by macrophages via glycoreceptor-mediated phagocytosis.

RESULTS AND DISCUSSION

Design and synthesis of the fluorescence mannolipid. Lectins are a promising class of biomarkers (for bacteria or cancer cells detection for instance) and a number of synthetic ligands have been reported, [START_REF] Cecioni | Glycomimetics versus Multivalent Glycoconjugates for the Design of High Affinity Lectin Ligands[END_REF] some of them being fluorescent for targeted imaging of cell receptors. [START_REF] He | Fluorescent Glycoprobes: A Sweet Addition for Improved Sensing[END_REF][START_REF] He | Lightening Up Membrane Receptors with Fluorescent Molecular Probes and Supramolecular Materials[END_REF] Lectins however bind high-order glycan structures (such as mannans for DC-SIGN) while the affinity for monosaccharide units is low. [START_REF] Swaminathan | Role of Water in the Specific Binding of Mannose and Mannooligosaccharides to Concanavalin A[END_REF] The development of synthetic ligands targeting glycoreceptors with high affinity is thus a very demanding and time-consuming endeavor to produce dendrimers or oligomers. [START_REF] Mangold | Quinoxalinone Inhibitors of the Lectin DC-SIGN[END_REF][START_REF] Cattiaux | New Branched Amino Acids for High Affinity Dendrimeric DC-SIGN Ligands[END_REF] The aim of this project is to take advantage of the ligand mobility at the surface of the droplet, which can lead to the clustering of multiple ligands upon interaction with a multivalent receptor. It has been shown for instance on mannosylated liposomes that divalent ligands were as efficient for the recognition of dendritic cells than tetravalent ligands and that above a certain surface concentrations, monovalent ligand also yield good interactions. [START_REF] Espuelas | Influence of Ligand Valency on the Targeting of Immature Human Dendritic Cells by Mannosylated Liposomes[END_REF] To take advantage of the clustering effect, we have thus developed a monovalent mannolipid which is simpler and more costeffective to synthesize and is expected in a supramolecular context to exhibit a high affinity for the target receptors. To efficiently coat the surface of the droplets, the sugar moiety had to be tethered to a lipophilic anchor (Figure 1). Moreover, in order to characterize the functionalization of the droplets as well as to monitor their cellular fate using fluorescence microscopy, we chose to incorporate a fluorophore moiety. The mannolipid 1 was thus built from three parts: 11-azido undecanol as a lipophilic spacer, sugar block 6 and alkyne-functionalized piperidyl naphthalimide fluorophore 4 (Scheme 1). The naphthalimide fluorescent scaffold was selected for its photostability and ease of functionalization: 4-bromo-1,8-naphthalic anhydride commercial derivative can be easily and selectively derivatized via the bromine and anhydride groups. Moreover, naphthalimide derivatives have been extensively characterized and their photophysical properties are well understood. [START_REF] Saha | Influence of the Structure of the Amino Group and Polarity of the Medium on the Photophysical Behavior of 4-Amino-1,8-Naphthalimide Derivatives[END_REF][START_REF] Liu | Aziridinyl Fluorophores Demonstrate Bright Fluorescence and Superior Photostability by Effectively Inhibiting Twisted Intramolecular Charge Transfer[END_REF] The introduction of an amino group at the 4 position creates a push-pull structures and endows the naphthalimide with a solvatochromic and environment-sensitive fluorescence emission particularly suited for biological imaging. A major advantage of push-pull probes is their low fluorescence in water and polar solvent which make them particularly apt to probe hydrophobic environments or in our case to track O/W emulsion droplets. The choice of a cyclic amine substituent in position 4 is aimed at inhibiting twisted intramolecular charge transfer (TICT) commonly observed with dialkylamino electron donating group. Inhibition of TICT results in higher brightness and superior photostability. [START_REF] Liu | Aziridinyl Fluorophores Demonstrate Bright Fluorescence and Superior Photostability by Effectively Inhibiting Twisted Intramolecular Charge Transfer[END_REF] Piperidine was chosen since more strained cyclic amines with three or four carbon rings result in high fluorescence quantum yields in water which is not a desired property in this work.

Scheme 1: Synthesis of the mannolipid 1

Glycoside 7 was synthesized using glycosylation of 11-azido-undecanol with donor 6 using the odorless 2-methyl-5 terbutyl thiophenol strategy. The presence of the benzoate group in position 2 which participates in the glycosylation mechanism yields the α anomer (Carbon NMR: δ(Canomeric) = 99.7 ppm). Compound 7 was deprotected using sodium methoxide to give 8 which then reacted with 7 in a copper-catalyzed azide-alkyne cycloaddition to afford the mannolipid 1 (scheme 1). This highyielding convergent synthesis using widely applicable chemical functionalities (alcohol and azide/alkyne) offers the possibility to independently change any of the three components (sugar, spacer, fluorophore) of the molecule in future developments.

Charaterization of the fluorescent mannolipid. We first characterized the optical properties of the fluorescent mannolipid in water, in ethanol and in the soybean oil used for the fabrication of the emulsion droplets (Figure 2). Intermediate 4 was also studied in ethanol and in soybean oil (figure S1). The results are summarized in table 1. Naphthalimide 4 displays an intense and broad absorption band with a positive solvatochromism characteristic of an intramolecular charge transfer (ICT) band, due to the dipolar structure of the chromophore. In ethanol it emits a weak fluorescence signal centered on 545 nm, with a very large stokes shift. In soybean oil, the fluorescence signal is blue-shifted and much brighter with a structured emission spectrum showing a maximum at 495 nm and a shoulder peak around 530 nm. Both emission peaks have similar excitation spectra and thus result from an excited-state phenomenon. Dipolar chromophore usually emit from a relaxed excited state resulting from stabilizing interactions of the solvent molecules with the excited state which displays a stronger dipolar moment than the ground state. We assume that, due to the oil viscosity, the relaxation rate is in the same range as the radiative deactivation rate and thus the relaxed excited state coexists with the blue-shifted locally excited (LE) state giving two emission peaks. (a) In water, the cmc is too low and there is a poor linearity between the fluorescence and absorption of 1; the value of ΦF is thus a rough estimate given only for comparison purposes.

The optical properties of the mannolipid 1 in ethanol are essentially identical to those of intermediate 4. On the other hand, significant differences are observed in water or in soybean oil.

Compared to intermediate 4, the mannolipid displays a strong hypochromism in soybean oil but the quantum yield remains essentially identical as well as the structured emission spectrum (Figure 2).

In water the absorptivity of 1 is similar to that in oil and although it cannot be compared to 4, it suggests a similar hypochromism. The fluorescence intensity does not vary linearly with the absorption and the estimated quantum yield in water is also quite high for such a push-pull probe in water (Figure S2). [START_REF] Liu | Aziridinyl Fluorophores Demonstrate Bright Fluorescence and Superior Photostability by Effectively Inhibiting Twisted Intramolecular Charge Transfer[END_REF] These properties can arise from solubility issues like aggregation or from the formation of self-assembled structures due to the amphiphilic character of the molecule. We have thus investigated the formation of micelles in water by fluorescence spectroscopy (see figure 2B). Two linear regimes were identified, corresponding to the solute and micellar states of the mannolipid with a cmc of 3.3 µM. The micellar state is more emissive, which explains the fact that the measured quantum yield is higher than expected. A similar experiment in soybean oil reveals no signs of self-assembly and the mannolipid is soluble in oil at least up to 50 µM (Figure S3).

Overall, the fluorescent mannolipid displays a low fluorescence emission in polar solvents such as ethanol and water and a high brightness in the soybean oil used for fabricating the droplets. It is poorly soluble in water and well soluble in oil up to 50 µM at least. These results show that compound 1 is well-suited for functionalizing O/W emulsion droplets with high contrast: should an excess of mannolipid remain in the bulk phase, it is expected to yield a low fluorescence signal. We next studied the functionalization of emulsion droplets by the mannolipid in order to create multivalent mannosylated fluorescent microparticles able to specifically target mannose receptors such as MR or DC-SIGN at the cell surface.

Functionalization of droplets.

To characterize the ability of the mannolipid to functionalize the O/W emulsion, we studied its insertion on the surface of micrometric oil droplets of 10 µm diameter by fluorescence microscopy. The droplets were functionalized by the amphiphilic ligand using a recently reported procedure that allows rapid and efficient coating of O/W emulsion. [START_REF] Pinon | Kinetically Enhanced Fabrication of Homogeneous Biomimetic and Functional Emulsion Droplets[END_REF] While most surface coating protocols rely on the in-situ fabrication of the emulsion with the functional ligand, our protocol allows functionalizing the droplets after the emulsification. Briefly, the droplets can be fabricated in a Couette emulsifier with the desired size [START_REF] Mason | Emulsification in Viscoelastic Media[END_REF] and then functionalized by being put in the presence of the ligand in a polar cosolvent (here 4% v/v DMSO) during 30 minutes. [START_REF] Pinon | Kinetically Enhanced Fabrication of Homogeneous Biomimetic and Functional Emulsion Droplets[END_REF] It is thus possible to rapidly functionalize the droplets at various concentrations in mannolipid and to perform a thorough study of the surface coating with a single batch of droplets. The droplets were put in presence of increasing concentrations of mannolipid from 20 to 400 µM in the buffer. Since molar concentrations are a poor description for surface functionalization, we have converted the concentrations into surface equivalents assuming a surface excess of 50 Å 2 for the mannolipid as was previously reported for linear glycolipid with a single sugar unit and comparable chain lengths. [START_REF] Sekhar | A Systematic Surface Studies on the Glycolipids to Understand the Surface Adsorption Behavior[END_REF] We worked with 2.3x10 7 droplets of 10 µm diameter, which corresponds to 7.2x10 9 µm 2 of total droplets surface. In these conditions, 1 surface equivalent corresponds to 100 µM of mannolipid in a 250 µL working volume. S1 for detailed nonlinear curve fitting results). The two x-axes give the mannolipid concentration in micromolar as well as the corresponding surface equivalents (see text for explanation).

A common issue when loading particles with fluorescent dyes is the self-quenching observed when the dyes are in close contact with one another. [START_REF] Reisch | Fluorescent Polymer Nanoparticles Based on Dyes: Seeking Brighter Tools for Bioimaging[END_REF] Here, interestingly, we do not observe any extinction of the fluorescence signal when increasing the amount of mannolipid on the particles even at the concentration where the surface is theoretically packed at maximum capacity and above. The mean fluorescence intensity plotted against the concentration of mannolipid follows a classical binding curve that can be fitted to a Langmuir isotherm, with a dissociation constant KD = 0.88 ± 0.10 surface eq. (Figure 4).

These results show that the emulsion droplets are efficiently coated by the fluorescent mannolipid without impacting its fluorescent properties, thus affording bright and stable fluorescent particles.

The functionalized droplets can be stored at least for 24 hours in the fridge without apparent modification of their size and functionalization and they are photostable under epifluorescence illumination.

Interaction with concanavalin A. The selective recognition of a mannose-specific receptor by the coated droplets was assessed in vitro with concanavalin A (ConA). ConA is a tetravalent lectin of the same sub-type (C-type lectins, active only upon calcium binding) as the Mannose Receptor or DC-SIGN. [START_REF] Martinez-Pomares | The Mannose Receptor[END_REF] The Mannose Receptor, DC-SIGN and ConA bind α-D-mannosyl (amongst other glycosylated structures) residues in terminal positions of large structures like glycoproteins or oligosaccharides. ConA can also interact with isolated D-mannose albeit with a much lower affinity, with reported Kd values of 360 µM for α-D-mannopyranoside or 131 µM for methyl-α-Dmannopyranoside. [START_REF] Swaminathan | Role of Water in the Specific Binding of Mannose and Mannooligosaccharides to Concanavalin A[END_REF][START_REF] Chervenak | Calorimetric Analysis of the Binding of Lectins with Overlapping Carbohydrate-Binding Ligand Specificities[END_REF] ConA was thus chosen as a model since it is an easily accessible and wellstudied lectin with similar properties as the target cellular membrane receptors.

Mannose-coated droplets were incubated with Alexa647-conjugated ConA (0.24 µM) for 30 minutes and, after washing the excess protein, observed under the microscope with two different excitation wavelengths: DAPI channel (387/11 nm excitation filter) for the naphthalimide (mannolipid) and Cy5 channel (650/13 nm excitation filter) for Alexa 647 (ConA). Microscopy images show a strong fluorescent intensity on the surface of the droplets in both channels evidencing the binding of ConA to the mannosylated droplets (Figure 5). We assessed the selectivity of the binding by performing negative control experiments with noncoated (naked) droplets and competitive binding assays with D-mannose and D-galactose on droplets coated with 0.6 eq. (i.e. 58 µM) of mannolipid (Figure 6A). Despite the presence of nonspecific binding on naked droplets, there is a specific interaction between ConA and mannosecoated droplets, since the binding can be inhibited by increasing concentrations of D-mannose, a competing ligand, but not by galactose which does not bind to ConA. A control binding experiment with Alexa 647 labeled peanut agglutinin (PNA) a galactose lectin was also performed at two different surface coating concentration, 1.2 and 6 surface equivalents. On the contrary to ConA, no binding is observed with PNA apart from a low non-specific binding that do not vary with the surface coating (Figure S4). To measure the affinity of the mannose-coated particles, we have performed a competitive binding assay in the presence of free α-D-mannose, which was able to completely inhibit the interaction with an IC50 of 1.92 mM (Figure 6B). Nonlinear curve fitting yielded an apparent dissociation constant of 2.3 mM. If we consider simple model where the two ligand compete at the same binding site, we can calculate the Kd between the mannose-coated droplets and ConA. [START_REF] Hulme | Ligand Binding Assays at Equilibrium: Validation and Interpretation[END_REF] Considering the reported Kd of 360 µM for mannose, [START_REF] Swaminathan | Role of Water in the Specific Binding of Mannose and Mannooligosaccharides to Concanavalin A[END_REF] we find a Kd of 9 µM for the droplets coated with 0.6 surface equivalent (see SI for curve fitting and equations, Table S2).

In liposomes, the deformable surface can hinder the recognition and requires a careful design of the ligand. It has been shown in vitro and in cellulo that the recognition of glycosylated liposomes by lectins is highly dependent on the state of phase and composition of the lipid bilayers and that the ligands have to be distanced from the surface. [START_REF] Engel | Influence of Spacer Length on the Agglutination of Glycolipid-Incorporated Liposomes by ConA as Model Membrane[END_REF][START_REF] Jeong | Effect of Space Length of Mannose Ligand on Uptake of Mannosylated Liposome in RAW 264.7 Cells: In Vitro and in Vivo Studies[END_REF][START_REF] Villalva | Influence of the State of Phase of Lipid Bilayer on the Exposure of Glucose Residues on the Surface of Liposomes[END_REF] In our design, the targeting moiety is readily exposed at the surface of the droplets without any spacing linker and still enables efficient recognition. This is in line with earlier findings that the stiffer surface of O/W droplets facilitates target recognition compared to liposomes where the ligand needs to be distanced from the surface. [START_REF] Yeeprae | Effect of Mannose Density on Mannose Receptor-Mediated Cellular Uptake of Mannosylated O/W Emulsions by Macrophages[END_REF][START_REF] Zhang | Janus Emulsions for the Detection of Bacteria[END_REF] Overall, we thus show that O/W emulsions are highly convenient platforms to build-up targeted particles in well-defined size and we confirm the initial rationale that the organization of monovalent sugars onto a liquid interface yields high affinity and specific lectin ligands with an affinity between ConA and the mannose-coated droplets 40 times higher than that of free mannose.

Phagocytosis of mannolipid-coated droplets. The phagocytic uptake of mannolipid-coated 8 μm diameter oil droplets at two surface densities (0.1 and 1 surface equivalent) was assessed and compared to the uptake of uncoated droplets, using a method described previously. [START_REF] Montel | A Multiparametric and High-Throughput Assay to Quantify the Influence of Target Size on Phagocytosis[END_REF] As can be seen on Figure 7A, mannolipid-coated droplets were internalized, however, fluorescence from the mannolipid can be seen in all macrophages regardless of the internalization of droplets. An average of 1.3 saturated droplets/cell were internalized, a number similar to the uptake of saturated IgGcoated 8 μm diameter droplets by the same cells. [START_REF] Montel | A Multiparametric and High-Throughput Assay to Quantify the Influence of Target Size on Phagocytosis[END_REF] Unsaturated droplets were less internalized, with a phagocytic index of 0.3 droplet/cell, and the uptake of uncoated droplets was significantly lower than both coated droplet assays, under 0.05 droplet/cell, as summarized on To assess whether soluble mannolipid can activate the macrophages and indirectly increase the phagocytic uptake of droplets, cells were exposed to soluble mannolipid (72 μM) for 10 minutes.

They were then rinsed and presented with uncoated droplets, and their uptake compared with unstimulated cells presented with coated droplets. Cells exposed to soluble mannolipid exhibited similar low levels of uptake as non-stimulated cells towards uncoated droplets (around 0.01 droplet per cell, Figure 8B), demonstrating that the presence of mannolipids on the surface of the droplets is required for internalizing the targets. Previous experiments showed that soluble D-mannose and mannans can impede the phagocytosis of zymosan particles by saturating cell receptors. [START_REF] Sung | Yeast Mannans Inhibit Binding and Phagocytosis of Zymosan by Mouse Peritoneal Macrophages[END_REF] We performed the same kind of experiments with mannolipid-coated droplets. While 50 mM of D-Mannose had no discernable effect on the uptake of droplets, the addition of 10 mg/ml of yeast mannans efficiently reduced the uptake of mannolipid-coated droplets by 35 %. The lack of inhibition by monovalent D-mannose can easily be explained by the low affinity between CLRs and free mannose (Kd of 2.3 mM reported for DC-SIGN), [START_REF] Feinberg | Multiple Modes of Binding Enhance the Affinity of DC-SIGN for High Mannose N-Linked Glycans Found on Viral Glycoproteins[END_REF] but interestingly, D-mannose and mannans showed enhanced inhibition when presented together, reducing the phagocytic index by 70 %.

Our results show that the surface functionalization with mannose units specifically triggers the phagocytosis of micrometric emulsion droplets in a receptor-dependent process. In earlier studies aimed at studying the involvement of mannose receptors in phagocytosis, the activation of the Mannose Receptor and DC-SIGN was usually performed with yeasts [START_REF] Cambi | Dendritic Cell Interaction with Candida Albicans Critically Depends on N-Linked Mannan[END_REF][START_REF] Ezekowitz | Molecular Characterization of the Human Macrophage Mannose Receptor : Demonstration of Multiple Carbohydrate Recognition-like Domains and Phagocytosis of Yeasts in Cos-1 Cells[END_REF] , bacteria [START_REF] Tailleux | DC-SIGN Induction in Alveolar Macrophages Defines Privileged Target Host Cells for Mycobacteria in Patients with Tuberculosis[END_REF][START_REF] Ackerman | High-Throughput Assay to Determine the Phagocytic Activity of Clinical Antibody Samples[END_REF] or zymosan particles [START_REF] Sung | Yeast Mannans Inhibit Binding and Phagocytosis of Zymosan by Mouse Peritoneal Macrophages[END_REF] . The downside of such experiments is that other proteins at the surface of the particles can activate alternative phagocytic pathways simultaneously while our results show that uncoated O/W emulsion droplets display very little unspecific internalization.

CONCLUSION

We have developed a supramolecular platform composed of a fluorescent mannolipid adsorbed on emulsion droplets that can be used to form targeted lipid particles able to interact with lectin with high affinity and specificity.

The functionalized microparticles were used to activate and study the mannose receptor-mediated phagocytosis in macrophages. The phagocytosis of the microparticles was specific and receptordependent as shown by inhibition by mannans and by the negligible unspecific internalization.

Compared to an alternative approach using targeted emulsions packed with lipophilic dyes, 57

Figure 1 .

 1 Figure 1. Structure of the mannolipid and expected positioning at the oil/water interface. A fluorescent dye is attached to the lipophilic anchor.

Figure 2 .

 2 Figure 2. (A) Spectral properties of the mannolipid in ethanol, water and soybean oil. Normalized absorption and fluorescence emission spectra. (B) Fluorescence intensity as a function of concentration in water.

Figure 3 .

 3 Figure 3. (A) Transmission, epifluorescence and overlay microscopy images (40X objective) of 10 µm emulsion droplets functionalized with 1 surface equivalent of mannolipid. (B) Size distribution of the droplets measured using fluorescence images and Gaussian fit (red curve). (C) Plot of the fluorescence intensity profile along the dashed lines shown in (A).

Figure

  Figure 3A shows pictures of droplets functionalized with 1 surface equivalent. The droplets display a fluorescence emission excited in the DAPI channel (387 ± 11 nm BP excitation filter) evidencing the successful surface coating by the mannolipid. The fluorescence images were used to analyze the particles and measure the size distribution of the droplets (Figure 3B). A Gaussian fit yielded an average size of 10.3 ± 2.1 µm. The droplets are thus fairly monodispersed. The plot of the intensity profile along the diameter of a droplet (Figure 3C) shows a slightly higher intensity on the edges of the droplets showing that the mannolipid indeed inserts on the surface as expected. The low contrast is expected from the wide-field collection of the emitted light.

Figure 4 :

 4 Figure 4: Titration of the mannolipid adsorption on the surface of 10 µm droplets. Error bars represents the standard deviation to the mean intensity measured on 5 wide-field microscopy images for each datapoint. Red line is a fit of the titration curve to a Langmuir adsorption isotherm (see Supplementary Materials TableS1for detailed nonlinear curve fitting results). The two x-axes give the mannolipid concentration in micromolar as well as the corresponding surface equivalents (see text for explanation).

Figure 5 .

 5 Figure 5. Pseudo-colored epifluorescence microscopy images (40X objective) of emulsion droplets coated with 1.2 surface equivalents of mannolipid and incubated with 0.24 µM of ConA. Green channel (naphthalimide fluorescence) red channel (Alexa 647 fluorescence of labeled ConA).

Figure 6 .

 6 Figure 6. (A) Binding selectivity: Average intensity measured for droplets coated with 0.6 eq. of mannolipid incubated with 0.48 µM of ConA and in the presence of free sugars. Control with naked droplets. (B) Competitive binding assay: Titration of ConA labeled with Alexa 647 (0.48 µM) with mannose in the presence of droplets functionalized with 0.6 surface equivalent of mannolipid 1 (i.e. 58 µM of mannolipid). Average of two independent experiments.

Figure 8A.

  Onlive imaging, mannolipids formed clusters at the contact site with cells, as seen on the images and plots presented in Figure7B-D (see also supplementary movie). Live phagocytic assay presenting coated droplets to macrophages showed that the mannolipid is internalized from the solution, without any direct contact between the cell and the droplets required for the transfer. The same phenomenon occurred when soluble mannolipid was injected in the culture medium. In both cases, the internalization of mannolipid takes less than 2 minutes, as seen on SI FigureS5.

Figure 7 .

 7 Figure 7. (A) Confocal image of macrophages presented with mannolipid-coated oil droplets (1 surface eq.). F-actin in red, mannolipid in green, nucleus in blue. Mannolipid is present in equal quantity inside all macrophages, independently of the uptake of droplets. (B-C) Live confocal image of mannolipid-coated droplets (0.1 eq, in green) in contact with macrophages (SiRactin 100 nM in magenta). (D) Plot profile of intensity along the white line, respectively, from the cell contact toward the exterior. Clusters are formed at cell contact.

Figure 8 .

 8 Figure 8. (A) Phagocytic index for 8 μm diameter mannolipid-coated droplets (1 eq, 0.1 eq and uncoated) internalized by RAW264.7 macrophages for 45 min (B) Relative phagocytic index for coated (1 eq) and uncoated droplets by macrophages exposed to soluble D-mannose, mannans or mannolipid

  our work underlines the importance of developing fluorescent surface ligands: the fluorescent mannolipid developed herein allows a facile characterization of the surface functionalization by fluorescence microscopy, and also to visualize the fate of the ligands and in particular their clustering at the interface during cellular adhesion. The mannosylated emulsion droplets developed herein bring supporting evidence of the ability of glycoreceptors to trigger phagocytosis independently of other known phagocytic receptors. Future developments are needed to isolate and identify specific receptors involved in the process. In particular, the inhibition by mannans the synergistic effect observed in combination with mannose calls for continued investigations. This specific internalization pathway can be used for the targeting and delivery of lipophilic dyes towards phagocytic cells of the immune system. Other fluorescent ligands with different sugars may also be envisioned to target additional receptors. While micrometric particles are needed to study phagocytosis, this work also opens the possibility to develop nanometric targeted fluorescent emulsions for more diverse targeting and bioimaging applications. PHAGODROP). PM, JF, JMM acknowledge funding from the PSL -Chimie doctoral program. LP acknowledge funding from Sorbonne Université IPV doctoral program.

Table 1 :

 1 Spectroscopic data of 1 and 4 in various solvents. λabs: peak absorption wavelength, ε:

	Solvent	Molecule	λabs	ε	λem	Δλ	ΦF	Brightness

molar absorption coefficient, λem: peak emission wavelength (sh: shoulder peak), Δλ: stokes shift in nm, ΦF: fluorescence quantum yield.

Acknowledgements. We thank the staff of the cleanroom of the Institut Pierre-Gilles de Gennes for their help and support all throughout the project.

DECLARATIONS

All authors have read and approved the final manuscript.


Caroline Norsica
email: caroline.norsic@univ-poitiers.fr

Jean-Michel Tatibouëta
email: jean.michel.tatibouet@univ-poitiers.fr

Catherine Batiot-Dupeyrata
email: catherine.batiot.dupeyrat@univ-poitiers.fr

Elodie Fourréa
email: elodie.fourre@univ-poitiers.fr

Non thermal plasma assisted catalysis of methanol oxidation on Mn, Ce and Cu oxides supported on -Al2O3

Keywords: Non thermal plasma, methanol, oxidation, metal oxide, TPR, ozone Highlights

A non thermal plasma reactor coupled with different metal oxide based catalysts (MnO2, CeO2, CuO, MnO2-CeO2, MnO2-CuO supported on Al2O3) has been developed for the decomposition of methanol at low concentration (25-150 ppm) and low residence time (0.36 s) in an air flow. The geometry of the reactor developed in this study led to the formation of a surface plasma that presented high energetic efficiency since 75 % methanol conversion was reached with a concentration of 25 ppm of methanol at an energy density of 20 J.L -1 in a single plasma configuration. The methanol conversion was improved when a catalyst was placed beyond the discharge. Apart from CO2, secondary products were identified as formaldehyde, methyl formate, methylal, CO and ozone. The influence of the methanol concentration, energy density and metal oxide catalyst composition on the methanol conversion, by-products selectivities and ozone formation were studied. The best methanol conversion and lower ozone residual concentration were obtained with bimetallic oxides MnO2/CuO/Al2O3 and MnO2/CeO2/Al2O3. A kinetic model and reaction mechanisms were proposed for methanol decomposition without catalyst, building on the work carried out in diverse publications.

•

A plasma reactor with an elaborated configuration was developed • Single plasma led to methanol conversion of 75 % at 25 ppm initial concentration • MnO2/CeO2/ Al2O3 and MnO2/CuO/Al2O3 catalysts presented the best activities

•

The reactor configuration and catalyst support may be extrapolated to larger scale

Introduction

Considering the prevailing problematics regarding air quality (and consecutive impact on health), global warming and rising worldwide demand in energy, the use of viable technologies allowing air purification at lowest energy costs appears as an undeniable solution to overcome these environmental issues. Within the variety of accessible new technologies, non thermal atmospheric plasma (NTAP) is now recognized as an efficient, low cost process which has led to a large amount of research on numerous applications (energy, depollution, biomedical, surface functionalization, to name a few), published work and reviews [START_REF] Bogaerts | Gas discharge plasmas and their applications[END_REF][START_REF] Attri | Utility of plasma: a new road from physics to chemistry[END_REF][START_REF] Tendero | Atmospheric pressure plasmas: A review[END_REF][START_REF] Kogelschatz | Dielectric-barrier Discharges: Their History, Discharge Physics, and Industrial Applications Plasma[END_REF][START_REF] Kogelschatz | From ozone generators to flat television screens: history and future potential of dielectric-barrier discharges[END_REF][START_REF] Van Durme | Combining non thermal plasma with heterogeneous catalysis in waste gas treatment: A review[END_REF][START_REF] Hyun-Ha | Non thermal Plasma Processing for Air-Pollution Control: A Historical Review, Current Issues, and Future Prospects[END_REF][START_REF] Vandenbroucke | Non thermal plasmas for noncatalytic and catalytic VOC abatement[END_REF].

Within the diversity of non thermal plasma configurations [START_REF] Tendero | Atmospheric pressure plasmas: A review[END_REF], dielectric barrier discharge (DBD) plasma has reached a place of interest mainly due to its flexibility and energy efficiency. DBD plasma is an ionized gas that can be obtained by applying a high difference of potential between two electrodes isolated from each other by a dielectric. Under a gaseous atmosphere, electrons are accelerated by an electric field and collide with neutral molecules in the gas phase which generates highly active species (atoms, ions, electrons, free radicals, photons, etc.). In such a case (non-equilibrium plasma), the temperature of electrons is much higher than that of the gas so that the process can take place at near ambient temperature. It can be recalled that non thermal plasma first applications concerned the production of ozone and soon after water purification [START_REF] Kogelschatz | Dielectric-barrier Discharges: Their History, Discharge Physics, and Industrial Applications Plasma[END_REF][START_REF] Kogelschatz | From ozone generators to flat television screens: history and future potential of dielectric-barrier discharges[END_REF]. Consequently, for the purpose of improving air quality, research on gas treatment and volatile organic compounds (VOCs) removal were largely studied [START_REF] Van Durme | Combining non thermal plasma with heterogeneous catalysis in waste gas treatment: A review[END_REF][START_REF] Hyun-Ha | Non thermal Plasma Processing for Air-Pollution Control: A Historical Review, Current Issues, and Future Prospects[END_REF][START_REF] Vandenbroucke | Non thermal plasmas for noncatalytic and catalytic VOC abatement[END_REF][START_REF] Xiao | Non thermal plasmas for VOCs abatment[END_REF][START_REF] Sultana | Abatment of VOCs with alternate adsorption and plasma assisted regeneration[END_REF]. It was also demonstrated that the presence of a catalyst within a plasma discharge resulted in a synergistic effect and was improving VOCs and by-products conversion. Various publications and reviews give detailed reports on non thermal plasma (NTP) coupled to catalysis on VOCs removal such as trichloroethylene [START_REF] Vandenbroucke | Non thermal plasmas for noncatalytic and catalytic VOC abatement[END_REF][START_REF] Vandenbroucke | TCE abatement with a plasma-catalytic combined system using MnO2 as catalyst[END_REF][START_REF] Vandenbroucke | Combination of non thermal plasma and Pd/LaMnO3 for dilute trichloroethylene abatement[END_REF][START_REF] Nakagawa | Investigation of Humidity Effect on Atmospheric Plasma Decomposition of Toxic Gas With Direct Optical Measurement of OH Radicals[END_REF], benzene [START_REF] Vandenbroucke | Non thermal plasmas for noncatalytic and catalytic VOC abatement[END_REF][START_REF] Karuppiah | Catalytic non thermal plasma reactor for the abatement of low concentrations of benzene[END_REF][START_REF] Xu | Benzene Removal Using Non thermal Plasma with CuO/AC Catalyst: Reaction Condition Optimization and Decomposition Mechanism[END_REF][START_REF] Li | Post plasma-catalysis for VOCs degradation over different phase structure MnO2 catalysts[END_REF], toluene [START_REF] Vandenbroucke | Non thermal plasmas for noncatalytic and catalytic VOC abatement[END_REF][START_REF] Delagrange | Combination of a non thermal plasma and a catalyst for toluene removal from air: Manganese based oxide catalysts[END_REF][START_REF] Ch | Catalytic non thermal plasma reactor for abatement of toluene[END_REF], isopropanol [START_REF] Jarrige | Plasma-enhanced catalysis of propane and isopropyl alcohol at ambient temperature on a MnO2-based catalyst[END_REF][START_REF] Ch | Novel Catalytic Dielectric Barrier Discharge Reactor for Gas-Phase Abatement of Isopropanol[END_REF][START_REF] Sivachandiran | Isopropanol saturated TiO2 surface regeneration by non thermal plasma: Influence of air relative humidity[END_REF] and methanol [START_REF] Derakhshesh | Mechanism of methanol decomposition by non thermal plasma[END_REF][START_REF] Hsiao | Plasmaassisted decomposition of methanol and trichloroethylene in atmospheric pressure air streams by electrical discharge processing[END_REF][START_REF] Hoon Lee | Effect of catalyst deactivation on kinetics of plasma-catalysis for methanol decomposition[END_REF][START_REF] Zhu | Post-plasma catalytic removal of methanol over Mn-Ce catalysts in an atmospheric dielectric barrier discharge[END_REF]. In particular, a significant attention has been brought on manganese oxide based catalysts, owing to their capacities to store oxygen and activate ozone [START_REF] Vandenbroucke | TCE abatement with a plasma-catalytic combined system using MnO2 as catalyst[END_REF][START_REF] Li | Post plasma-catalysis for VOCs degradation over different phase structure MnO2 catalysts[END_REF][START_REF] Delagrange | Combination of a non thermal plasma and a catalyst for toluene removal from air: Manganese based oxide catalysts[END_REF][START_REF] Jarrige | Plasma-enhanced catalysis of propane and isopropyl alcohol at ambient temperature on a MnO2-based catalyst[END_REF][START_REF] Zhu | Post-plasma catalytic removal of methanol over Mn-Ce catalysts in an atmospheric dielectric barrier discharge[END_REF][START_REF] Rezaei | EXAFS and kinetic study of MnOx/-alumina in gas phase catalytic oxidation of toluene by ozone[END_REF]. It is also worthy to mention that plasma-catalytic reactions improved by ozone were vastly investigated. More specifically, the VOCs abatement on MnO2 and CeO2 oxides supported on a large panel of materials (Al2O3, TiO2, zeolites) have been studied [START_REF] Eliasson | ozone synthesis from oxygen in dielectric barrier discharges[END_REF][START_REF] Barakat | Oxidation of isopropanol and acetone adsorbed on TiO2 under plasma generated ozone flow: Gas phase and adsorbed species monitoring[END_REF][START_REF] Maciuca | Coupling Non thermal Plasma with Photocatalysis for Odorous Pollutants Removal[END_REF][START_REF] Oh | Effect of zeolite in surface discharge plasma on the decomposition of toluene[END_REF][START_REF] Maciuca | Synergetic effect by coupling photocatalysis with plasma for low VOCs concentration removal from air[END_REF][START_REF] Harling | The role of ozone in the plasmacatalytic destruction of environmental pollutants[END_REF].

In this paper, we report the abatement of methanol (considered as a model molecule) by non thermal plasma discharge processing with and without catalyst in post plasma configuration The advantage of such configuration is that it can be easily extrapolated on a larger scale with higher gas flows.

Analysis

Calculation of the plasma energy, Ep can be obtained by the Manley method [START_REF] Manley | The electronic characteristics of the ozonator discharge[END_REF] that allows the measurement of the charges, Q across the gas on a capacitance added on the circuit. The parameters that can be measured are the applied voltage, Ua, and the voltage, Udbd, at the capacitance (Cm) terminals. For this purpose, two high voltage probes (Lecroy PPE20KV)

were connected, one to the generator output and the second one at the capacitance terminals, respectively. In order to obtain the injected power over one period of time, Lissajous curve, that provides the transferred charge in the reactor, Q = Udbd x Cm as a function of the applied voltage, Ua, was plotted. The plasma power (P) was then calculated from the multiplication of the frequency (f, in Hz) by the integrated area of the Lissajous parallelogram.

The specific energy density (Ed in J.L -1 ) that links the discharge power to the flow rate (equation 1) is considered as a reference parameter to assess plasma energetic efficiency.

𝐸 𝑑 = 𝑃/𝐹 where F is the total flow rate in L.s -1 (1)

The identification and quantification of gas products, after their contact with the plasma and/or catalyst was determined by online gas chromatography (Perkin Elmer) equipped with a flame ionization detector (FID) and a methanizer. The methanol conversion (X, %) was calculated following:

𝑋 = [𝑀𝑒𝑂𝐻] 0 -[𝑀𝑒𝑂𝐻]
[𝑀𝑒𝑂𝐻] 0 × 100

Selectivities of CO, CO2 and by-products were calculated from the concentration of identified products, Xp. Calibration curves for each compounds, Xp, were carried out to determine products concentrations.

𝑆𝑒𝑙 𝑋 𝑝 (%) = [𝑋 𝑝 ].(𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝐶 𝑜𝑓 𝑋 𝑝 ) [𝑀𝑒𝑂𝐻] 0 -[𝑀𝑒𝑂𝐻]
× 100

The carbon balance, representative of the converted methanol into by-products, CO and CO2, was determined from the sum of the selectivities:

BC (%) = ∑ 𝑆𝑒𝑙 𝑋 𝑝
All the experiments were reproducible and a margin of error of ± 2 % was calculated. with a metal oxide loading of 5 wt.%. The impregnation was carried out under reduced pressure in a rotary evaporator during 4 h. Water was evaporated by keeping the system at 40

°C for 2 h. The solid was then dried overnight at 110 °C and then calcined under air, during 3 hours at 300 °C for manganese, 400 °C for copper and 600 °C for cerium oxides.

Mixed oxides of MnO2-CeO2/Al2O3 and MnO2-CuO/Al2O3 were prepared according to the same procedure as described above by successive impregnation. The target loading was close to 5 wt.% for each oxide.

The specific surface area of the samples was determined by the Brunauer-Emmett-Teller (BET) method from the nitrogen adsorption isotherms at -196 C in an automated Micromeritics Tristar 3000 apparatus after drying for 90 min at 350 C. The catalysts were characterized by hydrogen temperature programmed reduction (H2-TPR) using a Micromeritics Autochem 2910 instrument. Samples were pre-treated at 300 °C using high purity helium (50 mL.min -1 ) for 30 minutes. Then H2 (5 % in Argon) was used as reducing gas with a flow rate of 50 mL.min -1 . The catalyst was heated up to 600 °C with a temperature rate of 10 °C.min -1 . The metal oxide loadings were determined by inductively coupled plasma optical emission spectrometry (ICP-OES) using a Perkin Elmer Optima 2000 DV.

Results and discussion

Catalyst Characterization

The prepared catalysts were characterized by BET, ICP and H2-TPR. The specific surface area, before and after plasma treatment, and metal loading are summarized in table 1. The deposition of a metal oxide led to a slight decrease (up to 12 % for MnO2-CeO2/γ-Al2O3) of the surface area of the commercial alumina balls. The contact of the catalyst with longlived species exiting from the plasma reactor did not affect their surface areas, or only by a few percent (max 5 %).

The H2-TPR profile of MnO2/Al2O3 consists of an asymmetric peak, with a maximum at 300 °C. The curve finishes with a shoulder starting above 350 °C (figure 3a). The fitting of the peak (figure 4) revealed the presence of a second step of reduction with a maximum at 395 °C. According to the literature [START_REF] Jarrige | Plasma-enhanced catalysis of propane and isopropyl alcohol at ambient temperature on a MnO2-based catalyst[END_REF][START_REF] Kapteijn | Alumina supported manganese oxide catalysts[END_REF][START_REF] Arena | Structure and redox properties of bulk and supported manganese oxide catalysts[END_REF][START_REF] Wang | Effects of precursors for manganeseloaded -Al2O3 catalysts on plasma catalytic removal of o-xylene[END_REF][START_REF] Trawczyński | Oxidation of ethanol over supported manganese catalysts -Effect of the carrier[END_REF][START_REF] Saqer | Catalytic oxidation of toluene over binary mixtures of copper, manganese and cerium oxides supported on -Al2O3[END_REF][START_REF] Radhakrishnan | Situ Laser Raman Spectroscopy and ab Initio Calculations[END_REF], the reduction of MnO2 to Mn2O3/Mn3O4 then MnO leads to the formation of 2 peaks in the H2-TPR profile. The hydrogen consumption value, measured in this case, is very close to the estimated one that indicates a complete reduction of the sample (table 2). Additionally, the manganese oxide phase depends on the nature of the precursor used during the catalyst preparation. At low loading (≤ 5 %), when a nitrate precursor is used, Mn 2+ ions are susceptible to diffuse on the surface and stabilize as a MnO2 phase during calcination. Finally, from EXAFS, Raman and ab initio calculations, it was suggested that the active centers of MnOx/-Al2O3 catalysts consisted of four coordinate mononuclear manganese species [START_REF] Radhakrishnan | Situ Laser Raman Spectroscopy and ab Initio Calculations[END_REF].

The H2-TPR profile of CeO2/Al2O3 is characterized by a low-intensity and broad consumption curve that starts above 300 °C and reach a maximum at 460 °C (figure 3a). CeO2 is known to be reduced at temperatures over 500 °C (700 °C for bulk ceria) and that below this temperature, only the reduction of the upmost layers of Ce 4+ occurs. The addition of a support decreases the reduction temperature, the value depending on its nature and on the cerium oxide dispersion. From literature data on ceria reducibility, the oxygen loss from TPR results [START_REF] Saqer | Catalytic oxidation of toluene over binary mixtures of copper, manganese and cerium oxides supported on -Al2O3[END_REF][START_REF] Yao | Ceria in Automotive Exhaust Catalysts Oxygen Storage[END_REF] corresponds approximately to one-electron transfer per CeO2, in accordance with the reduction of Ce 4+ to Ce 3+ .

As soon as CeO2 is deposited over alumina prior to MnO2, the reduction peak of MnO2 is shifted to a higher value: maximum at 320°C instead of 280°C without CeO2. It shows unambiguously that a much stronger interaction between manganese and the support exist in the presence of CeO2, suggesting a highly dispersed system.

Regarding CuO/Al2O3 containing sample (Fig. 3b), the H2-TPR present an asymmetric peak at 263 °C with a long shoulder that ends at 500 °C. The same peak fitting method as for MnO2

(Gaussian fitting with Origin software) was carried out and a second peak, less intense (1/3 of main peak area), at 360 °C was evidenced. The intense peak can be attributed to the reduction of CuOx species, probably highly dispersed and amorphous [START_REF] Saqer | Catalytic oxidation of toluene over binary mixtures of copper, manganese and cerium oxides supported on -Al2O3[END_REF] and the shoulder to a small amount of CuO crystallites. A complete reduction of the sample into metallic Cu was achieved as it is observed in table 2, from the calculated (10.9 cm 3 .g -1 ) and measured (10.2 cm 3 .g -1 ) hydrogen consumption required for the reduction. The addition of copper reduced slightly the reduction temperature of MnO2 from 300 to 272 °C (figure 3b) and increased the intensity of the resulted peak. According to Sager et al. [START_REF] Saqer | Catalytic oxidation of toluene over binary mixtures of copper, manganese and cerium oxides supported on -Al2O3[END_REF], the addition of copper oxide stabilizes the manganese oxide in the form of MnO2, which is then reduced at a lower temperature.

Methanol conversion in single plasma configuration

In the single plasma configuration (without catalyst), the methanol conversion increases from 44 % to 75 % with the decrease of the initial concentration of methanol from 150 to 50 ppm (figure 5a) at constant energy input (20 J.L -1 ). These results are in agreement with those from various authors such as Delagrange et al. [START_REF] Delagrange | Combination of a non thermal plasma and a catalyst for toluene removal from air: Manganese based oxide catalysts[END_REF] who observed similar behavior for toluene removal from air gas flow.

Non thermal plasma treatment of air leads to the formation of ozone as a main by product [START_REF] Eliasson | ozone synthesis from oxygen in dielectric barrier discharges[END_REF]. The ozone concentration, remaining unaltered whatever the methanol concentration, indicates that the conversion of methanol does not result from its oxidation by ozone. This behavior is in agreement with various published studies such as that of Harling et al. [START_REF] Harling | The role of ozone in the plasmacatalytic destruction of environmental pollutants[END_REF] who showed, in the case of toluene and cyclohexane, that ozone cannot directly decompose the The selectivity to CO2 does not reach 100 % whatever the methanol concentration. Secondary products were detected and identified as CO, formaldehyde (HCOH), methyl formate (HCOOCH3) and methylal (CH3OCH2OCH3). Surprisingly, the carbon balance is not completed for methanol initial concentrations higher than 50 ppm suggesting the formation of an undetected by-product which could be formic acid resulting from formaldehyde oxidation [START_REF] Tatibouët | Transient isotopic study of methanol oxidation on unsupported V2O5. Mechanism of methylal formation[END_REF] (see part 3.5).

Methanol conversion increases with the energy density input rise (figure 6a) whatever the methanol initial concentration. The conversion tendency is as expected [START_REF] Delagrange | Combination of a non thermal plasma and a catalyst for toluene removal from air: Manganese based oxide catalysts[END_REF][START_REF] Ch | Novel Catalytic Dielectric Barrier Discharge Reactor for Gas-Phase Abatement of Isopropanol[END_REF][START_REF] Kim | Decomposition of gas-phase benzene using plasma-driven catalyst (PDC) reactor packed with Ag/TiO2 catalyst[END_REF], the lower the methanol initial concentration, the higher the methanol conversion. Different authors [START_REF] Yan | From Chemical Kinetics to Streamer Corona Reactor and Voltage Pulse Generator[END_REF][START_REF] Rosocha | Advanced Oxidation and Reduction Processes in the Gas Phase Using Non thermal Plasmas[END_REF] explained this tendency via kinetic analysis and proposed that chemical reactions are governed by radical reactions in the plasma discharge.

At 25 ppm, the main products are carbon oxides whose selectivity increases with the energy density (figure 7a). Secondary products are detected with a majority of formaldehyde (25 %) that decreases to 15 % above 20 J.L -1 . In much less extent, methyl formate (4 %) and methylal (4 %) are generated. Methylal selectivity increases with the energy density (4 to 8 %) while the opposite behavior is observed for methyl formate (4 to 2 %). The same trend is observed at higher methanol initial concentration but with a lower formation of carbon oxides. These results were compared with published data on methanol, IPA and ethanol abatement with different configurations of non thermal plasma reactors. The list of references given in the table below (table 3) is non exhaustive but gives an overview of different systems aiming at the abatement of various pollutants. In all cases, the conversion results reported here are made without a catalyst. The DBD reactor tested in this study is as efficient, in terms of energy density and pollutant conversion, as some of the other systems published in the literature. According to several authors [START_REF] Sugasawa | Additive Effect of Water on the Decomposition of VOCs in non thermal Plasma[END_REF][START_REF] Sato | Analysis of a Methanol Decomposition Process by a Nonthermal Plasma Flow[END_REF][START_REF] Bundaleska | Hydrogen production from methanol reforming in microwave "tornado"-type plasma[END_REF][START_REF] Horvath | Influence of the Outer Electrode Material on Ozone Generation in Corona Discharges[END_REF] the methanol decomposition by non thermal plasma occurs mainly through radical reactions with formation of formaldehyde, methyl formate and methylal before complete oxidation to carbon oxides. It is important to notify that NOx was not detected at 20 J. L -1 .

Effect of the electrode nature

The effect of the electrode nature was evaluated on the methanol conversion and ozone production. We observed a slight increase, from 44 to 47 %, of the methanol conversion with stainless steel electrodes instead of copper electrodes (figure 8). In the same time, the ozone concentration decreased from 180 ppm to 166 ppm. It has been already proven that the nature of the electrode affects not only the plasma current-voltage characteristics but also the ozone concentration [START_REF] Horvath | Influence of the Outer Electrode Material on Ozone Generation in Corona Discharges[END_REF][START_REF] Souza-Corrêa | Ozone decay on stainless steel and sugarcane bagasse surfaces[END_REF][START_REF] Peyrous | Technological parameters that influence the production of ozone in A .D. C. Corona discharge[END_REF]. These authors emphasized the importance of the interaction of ozone with the electrodes and showed that stainless steel electrodes favored ozone decomposition while copper produced higher ozone yield in both positive and negative corona discharges. Where R is radical, C is pollutant, A, B, C are by-products and M are bulk gas compounds.

They suggested that at intermediate energy densities, that is when the radical concentration increases, then radical linear termination reaction (R3) becomes the dominant process and the decomposition of methanol follows the relation:

[𝐶] [𝐶 0 ] = 1 -𝑋 = exp - 𝐸 𝛽 hence 𝛽 = -𝐸 ln (1 -𝑋)
Where [C] and [C0] are the pollutant and initial pollutant concentrations, E is the energy density (J.L -1 ), X the conversion.  value (J.L -1 ) corresponds to the specific energy. They specified that the energy efficiency of the system may be evaluated by the  value. The smaller the energy cost, the smaller the  value and the higher the removal efficiency. We applied this model to our experimental data and plotted ln (1-X), as a function of the energy density (E) and this for 4 different concentrations. The  values, deduced from the slopes, were then plotted as a function of the square root of the concentration and we obtained a linear correlation between the two parameters:

𝛽 = 2.8√[𝐶 0 ] Thus, 𝑋 = 1 -𝑒𝑥𝑝 -𝐸 (2.8√[𝐶 0 ] ⁄ )
The theoretical calculations fit with the experimental results and are displayed on figure 9.

This model is in accordance with results published by Delagrange and al. [START_REF] Delagrange | Combination of a non thermal plasma and a catalyst for toluene removal from air: Manganese based oxide catalysts[END_REF] and with Yan and al. approximation: the lowest the concentration, the lowest the energy cost of the system and the highest the removal efficiency. ⁄ )

The model was extrapolated to higher energy densities for each concentration (figure 10).

From these calculations, an energy density of 45 J.L -1 is required to reach 95 % conversion at 25ppm initial methanol concentration. This value increases to 70 J.L -1 at 50 ppm, 80 J.L -1 at 100 ppm and 105 J.L -1 at 150 ppm. In order to minimize the energy consumption and make use of the ozone produced by the discharge, a catalyst was placed in post plasma configuration. Ozone is stable up to 250°C and a catalyst is necessary to trigger its decomposition. Ozone decomposes into active species that can undergo oxidation reactions at the surface of the catalyst.

Coupling non thermal plasma to catalysis

It is now well-admitted that the addition of a catalyst within the discharge or in the post plasma region is primordial to promote the pollutant conversion and selectivity to CO2 and The beneficial effect of the catalysts on methanol conversion is as follow:

Al2O3  CeO2/ Al2O3  CuO/ Al2O3 < MnO2/Al2O3 < MnO2-CeO2 /Al2O3 < MnO2-CuO /Al2O3
Selectivity to COx is only improved when CeO2 is supported on alumina, rising from 35 % (plasma alone) to 44 %. The MnO2 based catalysts show a slight decrease of COx selectivities (30-32 %) while the pure Al2O3, CuO and mixed CeO2-MnO2 selectivities remain unaltered (34-35 %). CeO2-MnO2/ Al2O3, CuO/Al2O3 and CeO2/Al2O3 catalysts increase the carbon balance up to 70 %, 73 % and 74 %, respectively. When MnO2-CuO is supported on alumina, the carbon balance does not differ much from single plasma treatment (60-61 %). A little improvement is observed with MnO2 /Al2O3 (65 %). An experiment has been performed with 4 g of MnO2-CeO2/Al2O3 catalyst in order to test the efficiency of the catalyst on the conversion of methanol. The methanol oxidation reaction was considerably enhanced.

Methanol conversion was total for initial concentrations of 100 and 50 ppm and reached 84 % at 150 ppm and the carbon balance was 100 % at any concentration. The selectivity to byproducts is shown in Table 4. At 50 ppm, the selectivity to CO and CO2 was complete, but 23 ppm of ozone was not converted. Compared to plasma alone the high value of methyl formate selectivity for an initial methanol concentration of 100 and 150 ppm strongly suggests that the undetected product is formic acid which could react on the catalyst surface with methanol to form methyl formate through esterification reaction.

Ozone consumption follows roughly the methanol conversion (figure 11) and reaches a maximum for MnO2-CuO/Al2O3 catalyst highlighting the role of ozone as oxidative species on a catalyst.

When looking in more details at the reaction, it was possible to differentiate the activity of the catalyst to secondary products. Methylal selectivity remained low and constant (2-4 %)

independently of the catalyst nature whereas methyl formate selectivity increases strongly along with ozone consumption. COx selectivity (44%) was the highest when cerium was present on the catalyst surface. If MnO2-CuO /Al2O3 showed the best ozone abatement and conversion rate, it also favors secondary products formation such as formaldehyde, methyl formate and CO. The increase of by-products selectivities as a function of the oxide nature revealed the catalyst potential to make use of ozone. We have observed that ozone generated by the plasma alone does not participate in the pollutant conversion by gas phase reaction. The presence of a catalyst enhances the methanol decomposition via ozone activation on its surface and its subsequent dissociation into atomic oxygen which then participates in the methanol oxidation reaction.

As observed by several authors [START_REF] Naydenov | Ozone decomposition and CO oxidation on CeO2[END_REF][START_REF] Bulanin | Infrared Study of Ozone Adsorption on CeO2[END_REF][START_REF] Dhandapani | Gas phase ozone decomposition catalysts[END_REF], ozone activation on CeO2 proceeds by ozone adsorption on Lewis acid site through an electron transfer from Ce 3+ to O3 leading to the formation of an instable intermediate ozonide species O3 -(Ce 3+ + O3  Ce 4+ + O3 -), stabilizing on basic surface sites, which rapidly forms surface peroxide and superoxide ions.

At low temperature, these species react with methanol or adsorbed methoxy groups to form formate species prior to CO/CO2.

The beneficial effect of MnO2 on ozone activation is very well-known and has been widely investigated [START_REF] Dhandapani | Gas phase ozone decomposition catalysts[END_REF][START_REF] Einega | Catalytic oxidation of benzene with ozone over aluminasupported manganese oxides[END_REF][START_REF] Futamura | Synergistic effect of silent discharge plasma and catalysts on benzene decomposition[END_REF][START_REF] Einaga | Oxidation behavior of cyclohexane on alumina-supported manganese oxides with ozone[END_REF][START_REF] Santos | The role of lattice oxygen on the activity of manganese oxides towards the oxidation of volatile organic compounds[END_REF][START_REF] Li | Mechanism of Ozone Decomposition on a Manganese Oxide Catalyst In Situ Raman Spectroscopy and Ab Initio Molecular Orbital Calculations[END_REF][START_REF] Wan | Removal of low-concentration formaldehyde in air by DC corona discharge plasma[END_REF][START_REF] Qian | Structure-activity relationship of CuO/MnO2 catalysts in CO oxidation[END_REF][START_REF] Fortunato | Spinel type oxide for low temperature CO oxidation generated by use of an ultrasonic aerosol pyrolysis process[END_REF][START_REF] Luo | In situ XRD, Raman, and TPR studies of CuO/Al2O3 catalysts for CO oxidation[END_REF][START_REF] Severino | Comparative study of alumina-supported CuO and CuCr2O4 as catalysts for CO oxidation[END_REF][START_REF] Thomas | An analysis of the first step of phenol adsorption-oxidation over coprecipitated Mn-Ce catalysts: A DRIFTS study[END_REF][START_REF] Einaga | Performance evaluation of a hybrid system comprising silent discharge plasma and manganese oxide catalysts for benzene decomposition[END_REF]. The interest comes from two major catalytic properties of MnO2: 1) MnO2 is an oxide presenting different oxidation states; 2) it has an ability to store oxygen.

The most reliable point regarding VOC decomposition by ozone concerns the active oxygen species, coming from surface stabilized ozone and responsible for the improved reactivity. In fact, adsorbed ozone species do not remain in molecular form and transform to oxygen species such as O -, O3 -and active species such as O ( 3 P), O ( 1 D) which show higher reactivity towards VOC abatement than does simply ozone. The active metal oxide sites appear to be Mn 4+ that is reduced back to Mn 2+ during desorption of oxygen active species [START_REF] Radhakrishnan | Situ Laser Raman Spectroscopy and ab Initio Calculations[END_REF][START_REF] Santos | The role of lattice oxygen on the activity of manganese oxides towards the oxidation of volatile organic compounds[END_REF]. The ozone decomposition mechanism [START_REF] Li | Mechanism of Ozone Decomposition on a Manganese Oxide Catalyst In Situ Raman Spectroscopy and Ab Initio Molecular Orbital Calculations[END_REF] proposed by Li and al. leads to the formation of peroxide and superoxide species.

O3 + M n+ → O2 + O -+ M (n+1)+ O -+ O3 → O2 + O2 - M = metal site
The formation of superoxide or peroxide with ionic character was also suggested by Oyama et al. [START_REF] Radhakrishnan | Situ Laser Raman Spectroscopy and ab Initio Calculations[END_REF][START_REF] Dhandapani | Gas phase ozone decomposition catalysts[END_REF]. They highlighted the importance of the catalyst nature (p-type oxides like MnO2

and CuO) on ozone decomposition. These active oxygen surface species can not only react with pollutant molecules at the catalyst surface as proposed by Futamura [START_REF] Futamura | Synergistic effect of silent discharge plasma and catalysts on benzene decomposition[END_REF], but also improve the selectivity to CO2 [START_REF] Einaga | Oxidation behavior of cyclohexane on alumina-supported manganese oxides with ozone[END_REF][START_REF] Santos | The role of lattice oxygen on the activity of manganese oxides towards the oxidation of volatile organic compounds[END_REF][START_REF] Li | Mechanism of Ozone Decomposition on a Manganese Oxide Catalyst In Situ Raman Spectroscopy and Ab Initio Molecular Orbital Calculations[END_REF][START_REF] Wan | Removal of low-concentration formaldehyde in air by DC corona discharge plasma[END_REF].

A synergetic effect was observed when CuO or CeO2 was deposited prior to MnO2 on alumina. Within the different reported mechanisms, the most recent studies on CuO mixed oxides implied that the improved reactivity towards methanol conversion and ozone abatement is due to reactions occurring at the interphase of the CuO with MnO2 [START_REF] Zhu | Post-plasma catalytic removal of methanol over Mn-Ce catalysts in an atmospheric dielectric barrier discharge[END_REF][START_REF] Qian | Structure-activity relationship of CuO/MnO2 catalysts in CO oxidation[END_REF][START_REF] Fortunato | Spinel type oxide for low temperature CO oxidation generated by use of an ultrasonic aerosol pyrolysis process[END_REF][START_REF] Luo | In situ XRD, Raman, and TPR studies of CuO/Al2O3 catalysts for CO oxidation[END_REF][START_REF] Severino | Comparative study of alumina-supported CuO and CuCr2O4 as catalysts for CO oxidation[END_REF].

The catalytic activity was found to be dependent on the perimeter size between CuO and MnO2. Not only the availability of oxygenated species, but also the couple Mn 3+ /Mn 4+ may participate in the reaction. In fact, it was suggested that the redox couple, Cu 2+ + Mn 3+ →Cu + + Mn 4+ formed on the surface of CuO-MnOx sample enhances the catalyst reactivity in oxidation reaction [START_REF] Qian | Structure-activity relationship of CuO/MnO2 catalysts in CO oxidation[END_REF]. TPR results showed that addition of copper oxide favors the MnO2 reduction and then may be responsible for the enhanced activity of MnO2/CuO/Al2O3 on the VOC conversion [START_REF] Saqer | Catalytic oxidation of toluene over binary mixtures of copper, manganese and cerium oxides supported on -Al2O3[END_REF].

Regarding cerium oxide based catalysts, Zhu et al. have proposed a mechanism for methanol decomposition on Mn-Ce catalysts. Based on their studies and related published work [START_REF] Zhu | Post-plasma catalytic removal of methanol over Mn-Ce catalysts in an atmospheric dielectric barrier discharge[END_REF][START_REF] Wan | Removal of low-concentration formaldehyde in air by DC corona discharge plasma[END_REF][START_REF] Doornkamp | The universal character of the Mars and Van Krevelen mechanism[END_REF][START_REF] Finocchio | Characterization and hydrocarbon oxidation activity of coprecipitated mixed oxides Mn3O4/Al2O3[END_REF][START_REF] Phillips | Sequential Photooxidation of Methanol to Methyl Formate on TiO2(110)[END_REF], they suggested that Mn loading was the most important parameter affecting the abatement of methanol. The addition of CeO2, acting as an oxygen release /storage material, led to interactions between Mn and Ce oxides and a synergetic effect was observed, enhancing the oxygen mobility on the catalyst and allowing the deep oxidation of methanol. It is now well established that methanol activation proceeds via its methoxy form adsorbed on the metal oxide surface. Following this step, oxidation into formaldehyde occurs via dehydrogenation (-H elimination) [START_REF] Houtman | reaction of methanol on Rh(111) and Rh(111)-(2x2)O surfaces: spectroscopic identification of adsorbed methoxide and  1 -formaldehyde[END_REF][START_REF] Mullins | Adsorption and reaction of methanol on thin-film cerium oxide[END_REF][START_REF] Tatibouët | Methanol oxidation as a catalytic surface probe[END_REF]. This reaction is then closely followed by the formation of methyl formate and methylal via O* activation [73Erreur ! Source du renvoi introuvable.], with a fraction of these by-products being then desorbed into CO and CO2.

Therefore, the increased presence of formate ions on the surface, that are prone to react with methanol, explains the high methanol conversion and methyl formate selectivity, as observed here.

Conclusion

The abatement of methanol during non-thermal plasma discharge processing, under specific conditions implying a low residence time (0.36 s) and specific energy (20 J.L -1 ), led to the conversion of methanol up to 75 % at 25 ppm and 44 % at 150 ppm initial methanol concentration with 160 ppm ozone formation. The plasma reactor consisted in a surface DBD plasma with an innovative configuration that may be extrapolated to industrial scale. Byproducts were identified as formaldehyde, methyl formate, methylal, CO, CO2 and ozone.

Ozone oxidation properties were activated in the presence of a catalyst in the post discharge

( 1 .

 1 PPC). The experiments are carried out under specific conditions, implying a low residence time (0.36 s) and low methanol concentration(25-150 ppm). The plasma reactor consists in DBD surface plasma with an innovative configuration that may be extrapolated to industrial scale. The present reactor configuration is the result of optimized work that allows the treatment of high flow rates of pollutants with large ozone production. The results are presented in terms of methanol conversion, by-products formation and ozone concentration as a function of methanol concentration, catalyst nature and deposited energy. A kinetic model is presented and reaction mechanisms are proposed for methanol decomposition without catalyst. Non thermal plasma reactor characteristics and experimental setup A plasma reactor with a dielectric barrier discharge (DBD) configuration was developed for the decomposition of methanol. The DBD plasma is confined in a Plexiglas case of length 230 mm, width 100 mm and 50 mm height (figure1). A glass plate of 2 mm thickness (size: 90 x 230 mm) serves as the dielectric material. Copper electrodes are placed on both sides of the dielectric. One of the electrodes is made of a copper tape (Leicester England Advance) of size 50 mm 50 mm. The other one is a copper comb made of 5 equidistant branches (diameter 2 mm, 50 mm length). A small plexiglass casing, holding the dielectric and the copper comb, is placed in the reactor. It also aims at forcing the gas to flow in a 15 mm gap over the plasma discharge. Sinusoidal high voltage was applied to comb electrode, while the other (copper tape) was connected to ground. The discharge is initiated between the branches and, depending on the voltage input or frequency, spreads between the branches over the surface.

Figure 1 :

 1 Figure 1: non thermal plasma reactor

Figure 2 :

 2 Figure 2: Experimental setup including the evaporation system where liquid methanol is vaporized and

Figure 3

 3 Figure 3 presents the H2-TPR profiles of the five catalysts. H2-TPR is a commonly used

Figure 3 :

 3 Figure 3: H2-TPR profiles of the catalysts supported on -Al2O3. (A): MnO2 and CeO2 based catalysts;

Figure 4 :

 4 Figure 4: H2-TPR peak fitting of MnO2/Al2O3 catalyst (Gaussian fitting from Origin software)

Figure 5 : 1 Figure 6 :

 516 Figure 5: (a) Evolution of methanol conversion, COx selectivity (CO + CO2), carbon balance and

Figure 7 :

 7 Figure 7: Selectivities to by-products ( CO,  CO2,  formaldehyde,  methyl formate and  methylal) as a function of the energy density at (a) 25 ppm, (b) 50 ppm, (c) 100 ppm, (d) 150 ppm

Figure 8 :

 8 Figure 8: effect of electrode chemical nature on methanol conversion and ozone production

Figure 9 :

 9 Figure 9 : (a) Variation of the  value as a function of methanol concentration from the model

Figure 10 :

 10 Figure 10: Extrapolation curves of the model as a function of energy density

H2O.

  As mentioned above, MnO2 and CeO2 based catalysts have shown very interesting effects when coupled to a plasma discharge. The effect of these metal oxides supported on alumina on methanol conversion is detailed below. Additionally, CuO was deposited on these materials and its influence investigated. The experiments were carried out at an initial concentration of 150 ppm of methanol in a 5 L.min -1 flow rate at a constant energy density of 20 J.L -1 . 1 g of catalyst was placed in the reactor (figure2) and held between two pieces of quartz wool. The effects of different catalysts in a post discharge configuration were evaluated in terms of methanol conversion, carbon balance, by-products and ozone concentration. The results are displayed in figure11.

Figure 11 :

 11 Figure 11 : Evolution of methanol conversion, COx selectivity (CO + CO2), carbon balance and ozone concentration as a function of catalyst nature. Conditions: 150 ppm of methanol in a 5 L.min -1 air flow at a constant energy density of 20 J.L -1 and 1 g of catalyst.

Figure 12 :

 12 Figure 12 : Evolution of by-products selectivities, COx selectivity (CO + CO2) and ozone concentration as a function of catalyst nature. Conditions: 150 ppm of methanol in a 5 L.min -1 air flow at a constant energy density of 20 J.L -1 and 1 g of catalyst.

FiguresFigure 1 :Figure 2 :Figure 3 :

 123 Figures

Figure 4 :Figure 5 : 1 Figure 6 :

 4516 Figure 4: H2-TPR peak fitting of MnO2/Al2O3 catalyst (Gaussian fitting from Origin software)

Figure 7 :Figure 8 :Figure 10 :Figure 11 :Figure 12 :

 78101112 Figure 7: Selectivities in by products (CO, CO2, formaldehyde, methyl formate and methylal)

Table 1 :

 1 BET and ICP analysis of the different catalysts

	Catalyst	SBET (m 2 .g -1 )	SBET (m 2 .g -1 )	ICP (oxide wt.%)
			After reaction
	γ-Al2O3	202	202	-
	MnO2/γ-Al2O3	179	207	5
	CeO2/γ-Al2O3	178	172	4.7
	CuO/γ-Al2O3	197	181	3.9
	MnO2-CeO2/γ-Al2O3	177	178	4.5 (CeO2)
				3.8 (MnO2)
	MnO2-CuO/γ-Al2O3	186	168	3.7 (CuO)
				4 (MnO2)

Table 3 :

 3 Overview of different single NTAP systems.

	Plasma	Pollutant	gas	Flow rate	Residence	[C]	Conversion	Energy	Energy	Refs.
	type			(L.min -1 )	time (s)	(ppm)	(%)	density	efficiency
								(J.L -1 )	(kJ.mmol -1 )
	Volumic	methanol	air			400	50 at 120°C	230	27	[23]
	DBD						50 at 300°C	60	7	[23]
	Volumic	methanol	air	1	0.14	1500	60	560	15	[25]
	DBD
	Volumic	IPA	air	14	6	500	92	300	16	[19]
	DBD
	Surface	acetone	air	8	1.1	50	17	6	17	[29]
	DBD
	Surface	toluene	air	0.5	0.94	200	60	600	120	[30]
	DBD
	Surface	Isovaler-	air	10	0.9	50	39	5.4	7	[31]
	DBD	aldehyde
	Surface	toluene	air	5	0.05	200	70	310	53	[41]
	DBD	benzene					40	305	92	[46]
	Surface	methanol	air	5	0.36	50	60	20	16	This
	DBD									work

Table 4 :

 4 Summary of methanol conversion and by-products selectivities as a function of initial methanol concentration on 4g of MnO2-CeO2 /Al2O3 catalyst. Conditions: 5 L.min -1 air flow and constant energy density of 20 J.L -1 .

Table 2 :

 2 configuration. Three metal oxides, MnO2, CeO2, CuO supported on Al2O3, were studied. It appeared that bi-metallic oxides of MnO2/CeO2/ Al2O3 and MnO2/CuO/Al2O3 presented the best activities in terms of methanol conversion, carbon oxides selectivities and ozone utilization. When 4g of MnO2/CeO2/Al2O3 were used 100 % methanol conversion was obtained with 48 % and 52 % selectivity to CO and CO2, respectively in a 50 ppm pollutant flow. A synergetic effect was observed with CuO or CeO2 deposited prior to MnO2 on alumina and the improved reactivity towards methanol conversion and ozone use could be due to reactions occurring at the interphase of the CuO or CeO2 with MnO2. The mechanism could consist in the activation of methanol O-H bonds, forming adsorbed methoxy species on the surface. They decompose first into formaldehyde, then in methyl formate and methylal via O* activation, and finally into CO and CO2, being desorbed from the surface. Formic acid is produced from the hydrolysis of methyl formate and the increased presence of formate ions on the surface explains the high methanol conversion and methyl formate selectivity. H2-TPR of catalysts: Hydrogen consumption (theoretical and experimental)

Table 3 :

 3 Overview of different single NTAP systems.

Table 4 :

 4 Summary

Acknowledgment

The authors are thankful to the ANRT for the financial support.