Bruno Ameduri
email: bruno.ameduri@enscm.fr

Fluoropolymers: a Special Class of Per-and polyfluoroalkyl Substances (PFASs) Essential for our Daily Life

Keywords: fluoropolymers, per-and polyfluoroalkyl substances, polymerization aids, regulators, surfactants

Per-or polyfluoroalkyl substances (PFASs) are valuable compounds which fall into two main families: i) the low molar-mass PFASs (including fluorinated telomers, perfluorocarboxylic acids, oligomers and surfactants) which are toxic, persistent, bioaccumulable, mobile and are under severe restricted (several products are already banned) and drastic regulations and ii) polymeric ones. Among the latter, polymers bearing fluorinated side moieties have also undergone such above issues. But, fluoropolymers (FPs) are niche specialty bio-inert and safe polymers, stable thermally, chemically and against UV and aging.

They display outstanding properties, their molar masses being able to reach several million g.mol -1 . They fulfil the Polymer of Low Concern criteria and are not prone to face such above hazardous issues. These (co)polymers can be used in many High Tech applications (e.g., Energy, electronics, aerospace, automotive and biomedical industries). All these PFASs have recently been under heavy pressure from regulators, press and media. However, the main concerns linked to the industrial productions of FPs have been addressed showing that these high value added polymers must be separated from conventional PFASs and are irreplaceable materials.

Introduction

Fluorine chemistry has been undergoing another severe attack, launched more than one decade ago. Media have been quite active to denounce the bad effects of fluorinated products while many plaintiffs have sued fluorochemical manufacturers. Formerly, the context of chlorofluorocarbons (CFCs) was severe and suitable alternative solutions were found with hydrogenofluorocarbons (HFCs) [START_REF] Molina | Stratospheric sink for chlorofluoromethanes: chlorine atomcatalysed destruction of ozone[END_REF][START_REF] Mclinden | Evolution of Refrigerants[END_REF]. Then, another major concern dealt with the fluorosis, a common disorder, characterised by hypomineralisation of tooth enamel caused by ingestion of excessive fluoride during enamel formation [START_REF] Buzalaf | Fluoride intake of children: considerations for dental caries and dental fluorosis[END_REF][START_REF] Iheozor-Ejiofor | Water fluoridation for the prevention of dental caries[END_REF]. The recent years have introduced a new battle against per-or polyfluoroalkyl substances (PFASs) for which the regulations will be stringent in these coming years [START_REF]Toward a new comprehensive global database of per-and polyfluoroalkyl substances (PFASs)[END_REF][6]. Various fluorochemicals are already banned and continuous procedure from competent authorities are pursuing their pressure to impose drastic legislations [6][7].

Containing multiple C-F bonds, per-and poly-fluoroalkyl substances (PFAS) represent a group of artificial/ anthropogenic chemicals with unique hydrophobic/lipophobic physicochemical properties and quite different physical, chemical, and biological properties [START_REF] Abunada | An Overview of Per-and Polyfluoroalkyl Substances (PFAS) in the Environment: Source, Fate, Risk and Regulations[END_REF][START_REF] Kwiatkowski | Scientific Basis for Managing PFAS as a Chemical Class[END_REF][START_REF] Buck | Perfluoroalkyl and Polyfluoroalkyl Substances in the Environment:Terminology, Classification, and Origins[END_REF]. These are valuable compounds, having found many applications in different areas [6,[START_REF] Kwiatkowski | Scientific Basis for Managing PFAS as a Chemical Class[END_REF][START_REF] Buck | Perfluoroalkyl and Polyfluoroalkyl Substances in the Environment:Terminology, Classification, and Origins[END_REF][START_REF] Ji | Challenges of aqueous per-and polyfluoroalkyl substances (PFASs) and their foreseeable removal strategies[END_REF][START_REF] Glüge | Information Requirements under the Essential-Use Concept: PFAS Case Studies[END_REF].

From the Organization for Economic Co-operation and Development (OECD) [START_REF]Toward a new comprehensive global database of per-and polyfluoroalkyl substances (PFASs)[END_REF], the number of PFAS molecules are >4700 PFASs while it reaches more than 9000 compounds according to the US Environment Protection Agency [13].

Applications of PFASs but….

A peculiar variety of PFASs deals with fluorinated surfactants. These surface active products [START_REF] Kissa | Fluorinated Surfactants and Repellents, Second Edition[END_REF] have been the best ones since their surface tension and surface properties are much better than those of HC surfactants [START_REF] Kissa | Fluorinated Surfactants and Repellents, Second Edition[END_REF][START_REF] Kostov | Original Fluorinated Surfactants Potentially Non-Bioaccumulable[END_REF][START_REF] Peshoria | Short-chain and long-chain fluorosurfactants in firefighting foam: a review[END_REF][START_REF] Peshoria | Understanding Aqueous Film-forming Foam Components[END_REF]. They have been involved in more than 200 applications (e.g., food packaging, paints, firefighting foams, cosmetics, electroplatings, photoacid generators, semiconductors and as polymerization aids in the emulsion radical polymerization of fluorinated alkenes [6, [START_REF] Glüge | Information Requirements under the Essential-Use Concept: PFAS Case Studies[END_REF][START_REF]PFAS Technical and Regulatory Guidance Document and Fact Sheets PFAS-1[END_REF]). However, the subject of issues deals with their toxicity, persistence [START_REF] Cousins | Why is high persistence alone a major cause of concern?[END_REF][START_REF] Scheringer | Stories of Global Chemical Pollution: Will We Ever Understand Environmental Persistence?[END_REF], bioaccumulation, water solubility and mobility [6,[START_REF] Abunada | An Overview of Per-and Polyfluoroalkyl Substances (PFAS) in the Environment: Source, Fate, Risk and Regulations[END_REF][START_REF] Buck | Perfluoroalkyl and Polyfluoroalkyl Substances in the Environment:Terminology, Classification, and Origins[END_REF][START_REF] Ji | Challenges of aqueous per-and polyfluoroalkyl substances (PFASs) and their foreseeable removal strategies[END_REF][START_REF] Cousins | The high persistence of PFAS is sufficient for their management as a chemical class[END_REF][START_REF] Anderson | Grouping of PFAS for human health risk assessment: Findings from an independent panel of experts[END_REF]. Indeed, the modern means of detection have been so powerful that several tenth of ppb [START_REF] Buck | Perfluoroalkyl and polyfluoroalkyl substances in the environment: Terminology, classification, and origins[END_REF] (or even hundreds of ppt) can be detected (just to recall that one ppb is a drop of water in an Olympic swimming pool!). As a matter of fact, to the best available Science, the Food and Drug Administration (FDA) has no evidence that PFASs, at levels found in the samplings, induce some human health issues [START_REF] Carlson | Systematic Evidence Map for Over One Hundred and Fifty Per-and Polyfluoroalkyl Substances (PFAS)[END_REF].

Actually, persistent organic pollutant (POP) regulation arising from the Stockholm Convention has forbidden perfluorooctane sulfonic acid (PFOS) since 2009, perfluorooctanoic acid (PFOA) since July 2020 and perfluorohexanesulfonic acid (PFHxS) since June 2022 while restrictions on perfluoroalkane carboxylic acids (PFCAs) are coming too [25] while the dossier is still under evaluation [26]. Furthermore, the restrictions imposed by the POP regulations may also consider substances as themselves or when they act as components of items, or incorporated in formulations above certain thresholds.

Other PFASs are also on the way to be restricted and environmental officials from a number of European countries have already announced a plan to restrict all PFASs by 2030 [26] while more than a dozen of American states have unveiled legislation which includes a ban on PFASs in the coming years. Thus, many efforts have been developed to synthesize alternatives to escape from PFOA and FPOS substituted by other PFAS (except in China where PFOA is still in use as well as others bearing shorter chain lengths as polymerization aids). Industrially produced alternatives are ammonium salt of hexafluoropropylene oxide dimer acid (Gen®X) and 4,8-dioxa-3H-perfluorononanoate (Adona®), developed by Chemours and Dyneon, respectively, but these products also display bioaccumulable, persistent, toxic and mobile (BPTM) issues [START_REF] Buck | Perfluoroalkyl and Polyfluoroalkyl Substances in the Environment:Terminology, Classification, and Origins[END_REF][START_REF] Qian | Surface Properties and Biological Effects of Fluoroether Surfactants[END_REF].

How to categorize PFASs?

Other PFASs, which are not necessary surfactants, fall into two main families [START_REF] Mueller | Naming Conventions and Physical and Chemical Properties of Per-and Polyfluoroalkyl Substances (PFAS)[END_REF]: i) the low molar-mass PFASs (including fluorinated telomers, perfluorocarboxylic acids and surfactants) which are BPTM. The second category encompasses polymeric ones especially polymers bearing fluorinated side chain [as poly(fluoroacrylate)s [START_REF] Yao | Fluorinated poly(meth)acrylate: Synthesis and properties[END_REF][START_REF] Van Der Veen | The effect of weathering on per-and polyfluoroalkyl substances (PFASs) from durable water repellent (DWR) clothing[END_REF], poly(oxetane)s, poly(fluorostyrene)s], perfluoropolyethers (PFPE) [START_REF] Friesen | Outstanding telechelic perfluoropolyalkylethers and applications therefrom[END_REF] and fluoropolymers (FPs).

Fluoropolymers

Among this last family, FPs, containing C-F bonds in the backbone, are niche specialty, bioinert and safe polymers as well as stable thermally, chemically and against UV and ageing. In addition, they display outstanding properties (high stability against temperature, chemicals, ignition, UV-light irradiation, and other unique characteristics [START_REF] Drobny | Technology of Fluoropolymers[END_REF][START_REF] Smith | Encyclopedia of Fluoropolymers Science and Technologies[END_REF][START_REF] Gardiner | Fluoropolymers: origin, production, and industrial and commercial applications[END_REF][START_REF] Dams | Industrial Aspect of Fluorinated Oligomers and Polymers[END_REF][START_REF] Puts | Polytetrafluoroethylene: synthesis and characterization of the original extreme polymer[END_REF][START_REF]Fascinating Fluoropolymers and their Applications[END_REF][START_REF] Ameduri | The Promising Future of Fluoropolymers[END_REF][START_REF] Sales | Fluoropolymers: The Safe Science That Society Needs[END_REF][START_REF]Fluoropolymer Market Overview[END_REF][START_REF] Roina | ePTFE functionalization for medical applications[END_REF][START_REF] Roina | ePTFE-based biomedical devices: An overview of surgical efficiency[END_REF][43]), and their molar masses can reach several million g.mol -1 [START_REF] Puts | Polytetrafluoroethylene: synthesis and characterization of the original extreme polymer[END_REF][START_REF] Sales | Fluoropolymers: The Safe Science That Society Needs[END_REF]. Their relevant combinations of properties are not matched by any of the alternative hydrocarbon polymers, hence making them so unique and valuable.

FPs can be used in many High Tech applications [START_REF] Drobny | Technology of Fluoropolymers[END_REF][START_REF] Smith | Encyclopedia of Fluoropolymers Science and Technologies[END_REF][START_REF] Gardiner | Fluoropolymers: origin, production, and industrial and commercial applications[END_REF][START_REF] Dams | Industrial Aspect of Fluorinated Oligomers and Polymers[END_REF][START_REF] Puts | Polytetrafluoroethylene: synthesis and characterization of the original extreme polymer[END_REF][START_REF]Fascinating Fluoropolymers and their Applications[END_REF][START_REF] Ameduri | The Promising Future of Fluoropolymers[END_REF][START_REF] Sales | Fluoropolymers: The Safe Science That Society Needs[END_REF][START_REF]Fluoropolymer Market Overview[END_REF][START_REF] Roina | ePTFE functionalization for medical applications[END_REF][START_REF] Roina | ePTFE-based biomedical devices: An overview of surgical efficiency[END_REF][43] (e.g., Energy, electronics aerospace and automotive industries, chemical process industry, transport, telecommunications, food and water treatment, pharmaceutical and medical devices [START_REF] Roina | ePTFE functionalization for medical applications[END_REF][START_REF] Roina | ePTFE-based biomedical devices: An overview of surgical efficiency[END_REF]). In addition, major actors working on the energetic transition are taking FPs more and more into account (fuel cell membranes, binders for Lithium ion batteries, backsheets for PVs, and electroactive devices such as actuators or artificial muscles as key-examples).

FPs should only be involved in those uses which are required for health, safety, or are critical for the functioning of society and no alternatives are available [START_REF] Glüge | Information Requirements under the Essential-Use Concept: PFAS Case Studies[END_REF]. Moreover, among all plastics, FPs represent a small tonnage since the production reached 320,300 tons in 2018 (compared to >400 million tons for all polymers), i.e. less than 0.1 % of conventional plastics [START_REF] Sales | Fluoropolymers: The Safe Science That Society Needs[END_REF][START_REF]Fluoropolymer Market Overview[END_REF][START_REF] Yli-Rantala | Fluorinated polymers in a low carbon, circular and toxic-free economy[END_REF] while their growth rate is 6-8% [6,[START_REF]Fluoropolymer Market Overview[END_REF].

Advantageously, most FPs used today fulfill the 13 Polymer of low concern criteria [START_REF] Henry | A Critical Review of the Application of Polymer of Low Concern and Regulatory Criteria to Fluoropolymers[END_REF][START_REF] Lohmann | Are Fluoropolymers Really of Low Concern for Human and Environmental Health and Separate from Other PFAS?[END_REF][START_REF] Korzeniowski | Critical Review of the Application of Polymer of Low Concern Regulatory Criteria to Fluoropolymers II: Fluoroplastics and Fluoroelastomers[END_REF] considered by OECD: polymer composition, molecular weight, percentage of oligomer, electrical charge, low molecular weight leachables, water/lipid solubility, Reactive Functional Groups, Functional Group Equivalent Weight, particle size, polymer stability, thermal stability, abiotic stability, and biotic stability. Furthermore, they are non-toxic; non-bio accumulative; insoluble in water and thus non-mobile; thermally, chemically and biologically stable; durable; and are not considered as Substances of Very High Concern (SVHC). While they are persistent, the lack of degradability during normal conditions of utilization and at their end of life make such specialty polymers totally inert and not harmful to human health or to the environment. These features also contribute to such PLC criteria.

Improvement of FP production in more environmental friendly processes

Nowadays, FP manufacturers are aware of the potentially negative aspects of PFAS polymerization aids currently used to replace PFOA. The regulatory push from various authorities is helping the industry to swiftly move towards sustainable technologies and many efforts from these producers have been developed according two strategies: i) to change their process in eliminating fluorinated side-products such as low molar massoligomers, residual reactants and other volatiles. By improving abatement techniques and PFAS surfactant recovery in the manufacture of FPs, these processes allowed the reduction of 99% of fluorosurfactant emissions since the 1990s [START_REF]Risk Management Options Analysis[END_REF] while recent studies have brought it as high as 99.99% [START_REF] Distefano | Thermal destruction of PFAS during full-scale reactivation of PFAS laden granular activated carbon[END_REF]. Abundant literature is available in relation to the control and treatment of emitted PFAS from industrial processes [START_REF] Ji | Challenges of aqueous per-and polyfluoroalkyl substances (PFASs) and their foreseeable removal strategies[END_REF][START_REF]PFAS Technical and Regulatory Guidance Document and Fact Sheets PFAS-1[END_REF][START_REF] Liu | Microbial degradation of polyfluoroalkyl chemicals in the environment: A review[END_REF] while still many studies are still ongoing [START_REF] Trang | Low-temperature mineralization of perfluorocarboxylic acids[END_REF].

ii) to produce FPs free from fluorinated polymerization aids (FPAs) in high molar masses, without any by-products as well as matching the PLC criteria. Indeed, much progress has been extended in the last decade where several FP manufacturers have modified their production using NFPAs (or surfactants). An overview [START_REF] Ameduri | Developments in fluoropolymer manufacturing technology to remove PFAS as polymerisation aids[END_REF] on the technology based on NFPAs over the past decade highlights that major innovations and efforts have been taken by the whole industry (such as 3M, AGC, Arkema, Chemours, Daikin, Gujarat Fluorochemicals and Solvay) in filing multiple patents which claim such a NFPA technology. Indeed, in 2022, various FP manufacturers announced the discontinuation of use of FPAs in their manufacturing [START_REF] Ameduri | Developments in fluoropolymer manufacturing technology to remove PFAS as polymerisation aids[END_REF].

Therefore, the legislation process should be focused on the use of FPAs and their emissions.

In this regard, a regulatory decision tree can be suggested as below [START_REF] Ameduri | Developments in fluoropolymer manufacturing technology to remove PFAS as polymerisation aids[END_REF] (Figure 2), linked to manufacturing FPs in presence or absence of such surfactants (with process abatement to get a PFAS concentration lower than 25 ppb), essential use criteria (in terms of safety, performance, health), improvement of the process, alternatives. Hence, FPs produced without any FPAs should be exempted for all uses across all industries including consumer applications as they raise no risk to Environment and to mammal and human health.

Recycling, Elimination, Reuse, End of Life of FPs

While usages of the FPs have widely spread, waste treatment techniques cannot fulfil their increasing demand. Presently, incineration (83.5%; ca. 20.4 kt) is a way to treat these polymer wastes but, hydrogen fluoride gas, generated during the treatment, seriously damages refractory bricks of incinerators. Moreover, most remaining FP wastes are disposed in landfill (13.1%; 3.1 kt) [START_REF] Yli-Rantala | Fluorinated polymers in a low carbon, circular and toxic-free economy[END_REF], although unfilled PTFE wastes from manufactures are reused in ram extrusion applications after cleaning [START_REF] Ebnesajjad | Introduction to Fluoropolymers: Materials, Technology, and Applications[END_REF] and milling or they are decomposed into low molar mass-PTFE by heating, and reused as micro powder blend with other materials [START_REF] Ameduri | Recycling and End of life assessment of Fluoropolymers: Recent Developments, Challenges and Future Trends[END_REF].

Recycling FPs represents 3.4% (0.8 kt) only. In this tendency, various companies have already found solutions to depolymerize, recycle ("close the loop" even on pilot plant) or reuse FPs [START_REF] Ameduri | Recycling and End of life assessment of Fluoropolymers: Recent Developments, Challenges and Future Trends[END_REF][START_REF] Schlipf | Closing the recycling Loop. Up-Cycling of end-of-life fluoroplastics[END_REF][START_REF] Aleksandrov | Waste incineration of polytetrafluoroethylene (PTFE) to evaluate potential formation of per-and poly-fluorinated alkyl substances (PFAS) in flue gas[END_REF].

Interestingly, Hori's team [START_REF] Hori | Decomposition of fluoropolymers by their mineralization in subcritical water[END_REF] reported an effective method to allow complete defluorination of FPs (including PVDF, as well as copolymers of VDF and of TFE, as perfluorosulfonic acid membranes) to F -ions, by means of superheated water (or "pressurized" hot water at ca. 100-

°C

). This water is recognized as environmentally benign in waste management because it enables to transform hazardous chemicals into harmless products or to generate value-added compounds [START_REF] Hori | Decomposition of fluoropolymers by their mineralization in subcritical water[END_REF]. The procedure may also occur in presence of either potassium permanganate as oxidizing reactant or alkali reagents (NaOH or KOH) [START_REF] Hamaura | Efficient fluoride recovery from poly(vinylidene fluoride), poly(vinylidene fluoride-co-hexafluoropropylene)[END_REF]. The carbon content was decomposed into CO 2 in the gas phase whereas HCO 3 -was produced into the reaction solution. In the presence of Ca(OH) 2 , the released fluoride anions led to the formation of "artificial fluorspar" (CaF 2 ) as a well-known source of the Fluorine Chemistry process, hence closing the loop as an attractive technology.

Conclusions

PFASs have recently been under heavy pressure from US and EU regulators and increasingly forecast by press and media because of environmental concerns and BPTM issues. Among

Figure 1 :

 1 Figure 1: Summary of PFAS categorisations (readapted from ITRC [28].)

Figure 2 :

 2 Figure 2: Regulatory decision tree for fluoropolymers (reproduced with permission from [52]).

  them, FPs are specialty polymers of high molar-masses, are water insoluble, non-mobile, nonbioaccumulative and non-toxic, and do not release any toxic side products and oligomers under conventional uses. Hence, most FPs (>96% of global production) fulfil the conditions set out by the OECD to meet the definition of Polymers of Low Concern and do not fall into the category of most harmful chemicals. More recently, the main concerns have been addressed, both in using NFPAs and controlling >99 % PFAS emissions linked to appropriate abatement technology of the industrial productions of FPs. As a result, FPs produced with NFPA technology has led to a commercial success and further motivates the industry to work harder in this direction. Their applications deal with health, safety, performance and functioning of the society. Furthermore, alternatives to FPs, which should be assessed based on the criteria such as performance, accessibility, resource efficiency, waste generation, cost, availability, stability and lifetime, do not exist. In addition, various solutions are searched for recycling, reuse, circular economy and life cycle analysis. For example, mineralization of FPs generating F -ions would contribute toward closing the loop of the Fluorine element. Finally, the Fluoropolymer industry in Europe [59] has evaluated the contribution made by the FP manufacturers in terms of revenue, investment and employments, while many more significant benefits are generated along the value chain via the use of FPs in various critical applications. These high value added polymers must be separated from conventional PFASs and are essential for everyday life.

Bruno Ameduri -Institut Charles Gerhardt, Université Montpellier, CNRS, ENSCM, 34293

Montpellier, France Bruno Ameduri: ORCID 0000-0003-4217-6664

ACKNWLEDGMENTS

The author thanks coworkers cited in the list of references below and the French Fluorine Network (GIS).

AUTHOR INFORMATION


Manon Fraulob

Siyuan Pang

Sophie Le Cann

Romain Vayron

Mathilde Laurent-Brocq

Iwona Jasiuk

Guillaume Haïat

Biomechanical characterisation of the bone-implant interface with nanoindentation and Raman spectroscopy

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

Introduction

Implants are commonly used in dental and orthopedic surgery. However, failure rates remain high, because of a lack of primary stability (immediately after surgery) or of secondary stability (a few weeks/months after surgery). To anticipate implant failure and achieve a long-term osseointegration, the biomechanical properties of the bone-implant interface need to be better characterized [1]. Local biomechanical and structural properties of newly formed bone tissue around the implant surface were investigated using nanoindentation [2] and Raman spectroscopy and the results were compared to mature bone.

Methods

Sample: An in vivo bone-implant model has been developed using coin-shaped Ti6Al4V implant (Ø=5mm, H=3mm) surrounded with PTFE to create a 700 µm gap between the implant surface and mature bone tissue (Fig 1) [3]. After 10 weeks of osseointegration in a New Zealand white rabbit femur cortex, the femur was harvested with implant in place, embedded into PMMA and horizontally cut in the middle of the implant to expose the bone-implant interface. Raman spectroscopy: Lines with a length of 400 µm were scanned with infrared light (wavelength 785 nm) in mature and newly formed bone tissue (Fig 1). Spectra were averaged and analyzed to extract mineral and collagen molecular compositions as well as bone mineralization information. Nanoindentation: 3 µm-depth indents were realized on the same lines as for Raman spectroscopy. Values of the elastic modulus were extracted from the loading curves and averaged for each line and each region of interest.

Results

The mean value of the elastic modulus measured by nanoindentation was equal to 13.9 ± 1.9 GPa (respectively 16.8 ± 1.2 GPa) in newly formed (respectively mature) bone tissue. Raman spectra obtained in mature and newly formed bone tissue exhibit different patterns (Fig 2). An increase in mineral-tocollagen and carbonate-to-mineral ratios was measured between newly formed bone and mature bone, respectively from 14 ± 1.5 to 18 ± 2.5 and from 0.17 ± 0.01 to 0.19 ± 0.02, whereas the mineral crystallinity decreased from 0.060 ± 0.002 to 0.058 ± 0.001 a.u.

Discussion

This in vivo coin-shaped implant model is well adapted to study newly formed bone properties as it enables to easily distinguish newly formed bone tissue from mature bone tissue, which is not the case when using implants employed clinically [4]. The lower elastic moduli [2], mineral-to-collagen and carbonate-to-mineral ratios measured in newly formed bone tissue suggest that it is less mineralized than mature bone tissue. Further studies will investigate more samples at different time points to confirm the results and investigate the evolution of the bone properties during healing.

Figure 1 :

 1 Figure 1: 2D image with bone colored in red and PMMA in yellow. Nanoindentation and Raman measurements were realized in newly formed bone (a, b, c) and mature bone tissue (d).

Figure 2 :

 2 Figure 2: Raman spectra in newly formed bone (red) and mature bone (black).

Acknowledgements

This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement No 682001, project ERC Consolidator Grant 2015 BoneImplant), and under the Marie Sklodowska-Curie grant agreement No 797764.


S Selvakumar

N Nuns

M Trentesaux

V S Batra

J.-M Giraudon

J.-F Lamonier

Reaction of formaldehyde over birnessite catalyst : a combined XPS and ToF-SIMS study

Keywords: Formaldehyde, Birnessite, X-Ray Photoelectron Spectroscopy, Time of Flight Secondary Ion Mass Spectrometry

Birnessite with very high surface area (>180 m 2 .g -1 ) has been prepared by oxidation of Mn(NO3)2 with H2O2 in KOH solution. The catalytic performance of this free noble metal based material for the formaldehyde (HCHO) selective conversion into CO2 is excellent. Therefore this birnessite material has been selected for X-ray photoelectron spectroscopy (XPS) analysis in combination with time-of-flight secondary ion mass spectroscopy (ToF-SIMS) study to understand the mechanistic interaction between adsorbate and adsorbent. Thermo-desorption of formaldehyde saturated birnessite has been conducted under argon atmosphere, using a catalysis cell allowing the monitoring of the birnessite surface modification. XPS study shows (i) the partial oxidation of formaldehyde at room temperature through the formate species formation and manganese species reduction and (ii) the generation of carbonate species with temperature. ToF-SIMS analyses gave more insight in the kind of cations from birnessite interacting with adsorbed molecules: formate ions interact with manganese and potassium ions while carbonate ions interact only with potassium ions. Formate oxidation takes place on Mn ions to give COx(g) species while formate ions readily decompose on K + sites at higher temperature.

Introduction

Indoor emission of formaldehyde (HCHO) has been subject of recent consideration from many governments around the world because HCHO impacts adversely the health of people exposed to this pollutant. The catalytic complete oxidation is regarded as the most promising technology to convert this molecule into harmless species (H2O, CO2) [START_REF] Torres | Formaldehyde: Catalytic Oxidation as a Promising Soft[END_REF]. Supported noble metal based materials are very efficient for this reaction since formaldehyde can be converted at ambient temperature [START_REF] Zhang | Catalytic performance and mechanism of a Pt/TiO2 catalyst for the oxidation of formaldehyde at room temperature[END_REF][START_REF] Chen | Complete oxidation of formaldehyde at ambient temperature over γ-Al2O3 supported Au catalyst[END_REF][START_REF] Xu | Efficient Removal of Formaldehyde by Nanosized Gold on Well-Defined CeO2 Nanorods at Room Temperature[END_REF][START_REF] Zhang | Alkali-Metal-Promoted Pt/TiO2 Opens a More Efficient Pathway to Formaldehyde Oxidation at Ambient Temperatures[END_REF][START_REF] Yan | Enhanced formaldehyde oxidation on CeO2/AlOOHsupported Pt catalyst at room temperature[END_REF]. Among the noble metals tested (Pt, Pd, Rh, Au), platinum is the most active phase [START_REF] Zhang | A comparative study of TiO2 supported noble metal catalysts for the oxidation of formaldehyde at room temperature[END_REF]. In particular titania supported platinum doped with Na is shown as the most promising catalytic system for the formaldehyde oxidation [START_REF] Zhang | Alkali-Metal-Promoted Pt/TiO2 Opens a More Efficient Pathway to Formaldehyde Oxidation at Ambient Temperatures[END_REF]. However owing to the high price and limited resources of noble metals, a wide application of these catalysts is highly restricted.

Noble metal free catalysts are proposed to potentially replace noble metal compositions [START_REF] Royer | Perovskites as Substitutes of Noble Metals for Heterogeneous Catalysis: Dream or Reality[END_REF].

Bulk and supported transition metal oxides were found to be the low-cost promising materials for low-temperature formaldehyde oxidation [START_REF] Averlant | Mesoporous Silica-Confined Manganese Oxide Nanoparticles as Highly Efficient Catalysts for the Low-Temperature Elimination of Formaldehyde[END_REF][START_REF] Quiroz | Total Oxidation of Formaldehyde over MnOx CeO2 Catalysts: The Effect of Acid Treatment[END_REF][START_REF] Bai | Effects of β-cyclodextrin introduction to zirconia supported-cobalt oxide catalysts: from molecule-ion associations to complete oxidation of formaldehyde[END_REF][START_REF] Liu | Effect of Mn substitution on the promoted formaldehyde oxidation over spinel ferrite: Catalyst characterization, performance and reaction mechanism[END_REF]. Sekine et al. [START_REF] Sekine | Removal of formaldehyde from indoor air by passive type aircleaning materials[END_REF] found that manganese dioxide (MnO2) was the most effective transition metal oxides for the formaldehyde catalytic removal. The nanostructured MnO2/cellulose composites showed excellent catalytic performance in HCHO oxidation [START_REF] Zhou | Facile In-Situ Synthesis of Manganese Dioxide Nanosheets on Cellulose Fibers and their Application in Oxidative Decomposition of Formaldehyde[END_REF]. The obtained pure manganese oxide structure and morphology in their study greatly influenced the catalytic performance for HCHO oxidation.

Testing different one-dimensional tunnel-type manganese oxide, Chen et al. [START_REF] Chen | Tunnel structure effect of manganese oxides in complete oxidation of formaldehyde[END_REF] showed that cryptomelane structure presents much higher catalytic activity in HCHO oxidation than pyrolusite and todorokite.

Birnessite-type manganese oxides are also very active for low-temperature oxidation of formaldehyde [START_REF] Wang | Room-Temperature Oxidation of Formaldehyde by Layered Manganese Oxide : Effect of Water[END_REF][START_REF] Tian | Highly active manganese oxide catalysts for low-temperature oxidation of formaldehyde[END_REF][START_REF] Chen | Self-Assembly of Novel Mesoporous Manganese Oxide Nanostructures and Their Application in Oxidative Decomposition of Formaldehyde[END_REF][START_REF] Zhou | Control over the morphology and structure of manganese oxide by tuning reaction conditions and catalytic performance for formaldehyde oxidation[END_REF][START_REF] Wang | Layered manganese oxides for formaldehyde oxidation at room temperature : the effect of interlayer cations[END_REF][START_REF] Li | Birnessite-Type Manganese Oxide on Granular Activated Carbon for Formaldehyde Removal at Room Temperature[END_REF]. Birnessite-type manganese oxides are layered structured materials with edge-shared MnO6 octahedra forming negatively charged layers, and cations and water molecules are located between the manganese oxide layers. The general formula is Ax(Mn 4+ ,Mn 3+ )2O4, yH2O where A is H + or metal cation (K + , Na + ). A very recent research showed that room-temperature oxidation of formaldehyde is possible using layered manganese oxide, i.e. birnessite as active phase [START_REF] Wang | Room-Temperature Oxidation of Formaldehyde by Layered Manganese Oxide : Effect of Water[END_REF]. The authors claim that the formaldehyde conversion was related to the water content in the birnessite. Formate and carbonate species were seen as intermediate products. Remarkably surface carbonate poisoning was suppressed in humid air indicating that water likely assists the carbonate transformation into CO2. Studying the effects of interlayer cations [START_REF] Wang | Layered manganese oxides for formaldehyde oxidation at room temperature : the effect of interlayer cations[END_REF] and manganese vacancy [START_REF] Wang | The effect of manganese vacancy in birnessite-type MnO2 on room-temperature oxidation of formaldehyde in air[END_REF], Wang et al. highlighted the role of potassium ions enhancing the activity of surface oxygen and resulting in the continuous and complete oxidation of HCHO at room temperature.

The synthesis of manganese based materials with novel morphologies and high surface areas have become an attractive research field. Indeed for the formaldehyde removal at very low concentrations, a hybrid system combining selective formaldehyde adsorption followed by its selective destruction through catalytic oxidation can be an attractive alternative to solve the problem of formaldehyde concentrations at indoor level [START_REF] Labaki | Removal of toluene over NaX zeolite exchanged with Cu2+[END_REF]. The birnessite-type manganese oxides can be prepared by redox precipitation, sol-gel, and hydrothermal processes. The most common and the oldest synthesis protocol is based on the oxidation of Mn (II) by bubbling gaseous oxygen O2 in strongly basic media. The second synthetic route uses the reduction of Mn (VII) with several reducing agents such as Mn (II) [START_REF] Luo | Double-Aging Method for Preparation of Stabilized Na-Buserite and transformations to Todorokites Incorporated with Various Metal[END_REF], H2O [START_REF] Chen | Hydrothermal Synthesis and Characterisation of KxMnO2.yH2O[END_REF][START_REF] Kim | Preparation of Layered MnO2 via Thermal Decomposition of KMnO4 and its Electrochemical Characterizations[END_REF], sugars [START_REF] Ching | Sol-Gel Synthesis of Birnessite from KMnO4 and Simple Sugars[END_REF], organic acids [START_REF] Bach | Birnessite Manganese Dioxide Synthesized via a Sol-gel Process : A New Rechargeable Cathodic Material For Lithium Batteries[END_REF] or organic solvents [START_REF] Ma | Syntheses of Birnessites Using Alcohols as Reducing Reagents: Effects of Synthesis Parameters on the Formation of Birnessites[END_REF]. An original route for birnessite preparation has been proposed by Eren et al. [START_REF] Eren | Performance of birnessite-type manganese oxide in the thermal-catalytic degradation of polyamide 6[END_REF] based on the oxidation of Mn(NO3)2 with H2O2 in KOH solution.

However the textural properties of birnessite prepared in this condition is not specified by the authors. In this study we used this synthesis protocol with the objective of reaching a large specific surface area for birnessite material.

Surface characterization of heterogeneous catalysts is usually done over as-made sample with the objectives to study the dispersion and the nature of the phases formed as well as the metalmetal or metal-support interactions. In addition the monitoring of the evolution of various adsorbed surface species on the catalyst with probe molecules can be useful to identify intermediate products using a combination XPS-FTIR [START_REF] Schwartz | Reaction of acetonitrile with the silicon (001) surface : A combined XPS and FTIR study[END_REF] or XPS-TDS [START_REF] Yuan | Surface Chemistry of Formaldehyde on Rutile TiO2(110) Surface: photocatalysis vs Thermal-catalysis[END_REF][START_REF] Habermehl-Cwirzen | XPS, TDS, WF and LEED results[END_REF]. Moreover ToF-SIMS in combination with XPS has been successfully implemented in order to investigate the mechanism of catalyst deactivation in oxidation of chlorinated volatile organic compound as well as the role of water in the performances of a post-plasma catalytic process [START_REF] Bertinchamps | Elucidation of deactivation or resistance mechanisms of CrOx, VOx and MnOx supported phases in the total oxidation of chlorobenzene via ToF-SIMS and XPS analyses[END_REF][START_REF] Nuns | A combined ToF-SIMS and XPS study for the elucidation of the role of water in the performances of a Post-Plasma Process using LaMnO3+δ as catalyst in the total oxidation of trichloroethylene[END_REF]. XPS analysis allows an easier quantitative approach, while ToF-SIMS analysis gives molecular information with higher surface sensitivity [START_REF] Weng | Advances in the surface characterization of heterogeneous catalyst using ToF-SIMS[END_REF].

In this paper, we have investigated the reaction of formaldehyde over K-birnessite surface using a spectroscopic multi-technic approach based on a combined XPS and ToF-SIMS analyses. The paper is devoted to the study of (i) surface composition over K-birnessite after formaldehyde exposure at 25°C and (ii) surface composition change through the thermo-desorption process.

From this original approach a more complete understanding of the surface chemistry of formaldehyde on K-birnessite surface is expected. Indeed there is no study reporting clear evidence of kind of sites involve in the formation and the transformation of intermediates produced at the surface after formaldehyde adsorption.

Experimental methods

Synthesis of material

Manganese based material was prepared at room temperature by oxidation of Mn(NO3)2 with H2O2 in KOH solution [START_REF] Eren | Performance of birnessite-type manganese oxide in the thermal-catalytic degradation of polyamide 6[END_REF]. An aqueous basic solution of H2O2 was first prepared as follows: 200 mL of KOH (0.6 mol.L -1 ) was mixed with 200 mL of 3% H2O2. This mixture was then added dropwise to 100 mL of Mn(NO3)2.xH2O solution (0.2 mol.L -1 ) at room temperature. After complete precipitation the final reaction mixture was stirred at room temperature for 2 hours and kept at the same temperature for 24 hours ageing. The final suspension was filtered and washed with distilled water for several times and dried at 100°C overnight. The above synthesized material was calcined at different temperature (300°C, 450°C and 600°C) for 3 hours under air flow.

In the following while the catalytic tests were performed with the three calcined samples, the formaldehyde adsorption and thermo-desorption studies were conducted over the selected birnessite material, i. e. material calcined at 300°C and labelled KB from here onwards.

Catalytic activity test

The formaldehyde catalytic oxidation was carried out in a fixed bed continuous flow microreactor (i.d. 10 mm) loaded with 200 mg of catalyst. Gaseous formaldehyde was generated from solid paraformaldehyde through a permeation tube placed in a permeation chamber (Dynacalibrator, VICI Metronics, Inc.) which was maintained at a constant temperature (100°C). The reaction mixture containing 120 ppm HCOH, 20 vol.% O2 was balanced by He (total flow rate =100 mL min -1 ). Experiments were conducted by saturating the catalyst with HCHO at 25°C. Once saturation was complete, the reaction temperature was increased from room temperature to 200°C using a heating rate of 1°C min -1 and the data were collected during the heating run. The effluent gas from the reactor containing formaldehyde, oxygen and carbon dioxide was analyzed on-line by a Varian CP 4900 MicroGC chromatograph equipped with a thermal conductivity detector.

Adsorption of formaldehyde

The adsorption of HCHO on KB was performed at 25°C by passing the carrier gas (Ar) through the Dynacalibrator® permeation chamber (100°C). Before the adsorption step, the KB material was pre-treated with argon at 150°C to remove water and surface impurities. Surface saturation of the material was followed by mass spectrometry (Omnistar, GSD 301-Pfeiffer). The birnessite was then purged at 25°C with argon until the physisorbed formaldehyde was removed (2 hours). The obtained material was labelled as KB-F25.

Thermo-desorption of formaldehyde saturated birnessite

The formaldehyde saturated birnessite KB-F25 was placed into a catalysis cell connected to a radial chamber (P = 10 -8 mbar) allowing the monitoring of the birnessite surface by successively two surface analysis techniques, X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (ToF-SIMS). The thermo-desorption experiments were conducted in argon atmosphere (30 mL.min -1 ) using the catalysis cell, from 25°C to 200°C (2°C.min -1 ). The maximum temperature was maintained for 15 min. Before being introduced into the radial chamber the sample was cooled down to 25°C and degassed to reduce pressure to 10 -6 mbar. The solid was labelled KB-FX, where X represents the maximum temperature to which the solid was heated before spectroscopic analysis.

X-Ray Photoelectron Spectroscopy (XPS)

XPS experiments were performed using an AXIS Ultra DLD Kratos spectrometer equipped with a monochromatized aluminum source (Al Kα = 1486.7 eV) and charge compensation gun.

Adventitious carbon contamination is commonly used as a charge reference for XPS spectra and all binding energies were referenced by setting the C-C component to a binding energy of 284.5 eV. The XPS photopeaks with multiple components were resolved by a peak-fitting program assuming a mixed Gaussian (70%) / Lorentzian (30%) peak shape using the software supplied by CasaXPS. Semi-quantitative analysis accounted for a linear background subtraction. The evolution of the surface atomic percentage of the birnessite material upon formaldehyde adsorption and heating is given in Table S1.

Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS)

ToF-SIMS data were acquired using a ToF-SIMS 5 spectrometer (ION-TOF GmbH Germany) equipped with a bismuth liquid metal ion gun (LMIG). The samples were bombarded with pulsed Bi 3+ primary ion beam (25 keV, 0.25 pA) rastered over a 500 m × 500 m surface area.

With a data acquisition of 100 s, the total fluence does not amount up to 10 12 ions/cm 2 ensuring static conditions. Charge effects were compensated by means of a 20 eV pulsed electron flood gun. Data were collected over a mass range m/z = 0-200 for both positive and negative secondary ions. The fragments were identified by their exact mass, coupled with the appropriate intensities for the expected isotope pattern.

Results and discussion

Characterization and catalytic performance of Mn containing material

Table 1 shows the structural and textural properties obtained for the sample calcined at different temperatures. The birnessite type structure with relative broad diffraction peaks is only observed for sample calcined at 300°C (Figure S1). For calcination temperature of 450°C the α-MnO2 cryptomelane structure is obtained while at 600°C a mixture containing the cubic bixbyite structure α-Mn2O3 and α-MnO2 cryptomelane structure is observed (Figure S1). The average crystallite size (Dc) calculated from the Scherrer equation increases with the calcination temperature in agreement with the decrease of BET surface area (SBET) and total pore volume (Vp) and increase of pore diameter (Dp) (Table 1). The different catalysts have been tested in the complete oxidation of formaldehyde. In order to mimic the formaldehyde adsorption phenomena which will be discussed in the next section, experiments were conducted by saturating first the catalyst with HCHO vapor at 25°C. The evolution of formaldehyde and carbon dioxide amounts is shown (Figure 1) as a function of temperature in the 25-200°C range in the presence of the catalyst previously calcined at 300°C, 450°C or 600°C. Irrespective of the calcination temperature, HCHO desorption occurs between 30°C and 100°C with a maximum value at around 50°C. This value is in good agreement with that observed by Quiroz et al. over MnOx material during HCHO TPD [START_REF] Quiroz | Total Oxidation of Formaldehyde over MnOx CeO2 Catalysts: The Effect of Acid Treatment[END_REF]. Carbon dioxide formation also takes place in a single broad peak, between 50°C and 130°C, with a maximum at 100°C for sample calcined at 300°C and at 110°C for samples calcined at 450°C and 600°C.

The quantity of HCHO and CO2 released is proportional to the specific surface area of the material. The highest SSA birnessite material shows the adsorption of formaldehyde in high quantity at 25°C. It can be suggested that the formation of surface oxidation products such as formate species [START_REF] Quiroz | Total Oxidation of Formaldehyde over MnOx CeO2 Catalysts: The Effect of Acid Treatment[END_REF] which decompose into gaseous CO2 takes place since the amount of CO2 is much higher than that expected through the oxidation of 120 ppm of formaldehyde. It is also observed that, irrespective of the calcination temperature, HCHO is completely converted at 100°C and H2O and CO2 are the only products released. Compared with published results, birnessite material synthesized in this study presents comparable catalytic performances for the formaldehyde selective conversion into CO2 (HCHO conversion of 100% at 100°C [START_REF] Tian | Highly active manganese oxide catalysts for low-temperature oxidation of formaldehyde[END_REF]), or even slightly higher (HCHO conversion of 61% at 100°C [START_REF] Zhou | Control over the morphology and structure of manganese oxide by tuning reaction conditions and catalytic performance for formaldehyde oxidation[END_REF]. The preparation of high-specific-surface-area birnessite material can be useful in a two-step adsorption-oxidation process for the formaldehyde removal at very low concentrations [START_REF] Labaki | Removal of toluene over NaX zeolite exchanged with Cu2+[END_REF].

Therefore the material calcined at 300°C (labelled KB) showing remarkable textural properties has been selected for XPS -ToF-SIMS analyses to obtain information on the mechanism of formaldehyde activation and transformation on the birnessite surface.

Study of formaldehyde reaction by XPS

Formaldehyde adsorption over birnessite material

The reactivity of formaldehyde at 25°C with KB sample was first monitored. The atomic percentage of carbon at the surface of as-synthetized birnessite corresponds to 3% (Table S1). After HCHO adsorption onto the birnessite surface at 25°C (KB-F25), this percentage increases significantly (15%) indicating that carbon species are retained on the surface (Table S1). The C 1s core level envelope presents an important contribution at BE of 288.2 eV (CF). This BE value is lower than that observed for carboxylate species (CCB). The observed binding energy value at 288.2 eV for KB-F25 is close to that observed for adsorbed formaldehyde on rutile TiO2 (110) surface [START_REF] Yuan | Surface Chemistry of Formaldehyde on Rutile TiO2(110) Surface: photocatalysis vs Thermal-catalysis[END_REF][START_REF] Yuan | Photocatalytic Cross-Coupling of Methanol and Formaldehyde on a Rutile TiO2(110) Surface[END_REF]. Based on the report of adsorption of HCHO on rutile TiO2 (110) surface, it can be suggested that formaldehyde is also associatively adsorbed on the birnessite surface. However C1s BE should be of the same magnitude for both formate and formaldehyde species, assuming the molecules are bonded end-on through the oxygen atom. Since formation of adsorbed formate species is often proposed as a stable intermediates in the oxidation of formaldehyde [START_REF] Quiroz | Total Oxidation of Formaldehyde over MnOx CeO2 Catalysts: The Effect of Acid Treatment[END_REF], the dissociation of formaldehyde into formate (HCOO -) and (H + ) species can also occur. Therefore CF contribution could arise from both formaldehyde and formate species formation on the birnessite surface.

The partial oxidation of formaldehyde into formate is confirmed by the change in Mn average oxidation state (AOS). XPS spectra of the core levels of Mn 3s are plotted in Figure 4. Mn3s photopeak has two multiplet split components due to the coupling of non-ionized 3s electron with 3d valence-band electrons. The two maxima in the binding energy have been evaluated fitting two components in Mn 3s envelope using CasaXPS software. The magnitude of peak splitting (ΔEs) depends on the Mn AOS value, determined from the correlation between the binding energies of the doublet separation of Mn 3s (Es) and the AOS, AOS = 8.956 -1.13(Es) as proposed by Galakhov et al. [START_REF] Galakhov | Mn 3s exchange splitting in mixed-valence manganites[END_REF]. The value of Mn AOS decreases from 3.8 to 3.0 after HCHO adsorption, showing evidence of the reduction of manganese species. This extent of Mn reduction can be readily correlated to the oxidation of the adsorbed formaldehyde into formate species at 25°C.

Before HCHO adsorption, the as-synthetized birnessite (KB) surface shows one main contribution to O 1s signal at 529 eV which can be unambiguously attributed to lattice oxygen (OL) (Figure S2) [START_REF] Zhang | Effect of aspect ratio and surface defects on the photocatalytic activity of ZnO nanorods[END_REF]. Indeed oxygen from metal oxides appears at a significantly lower BE compared to most other oxygen species. After HCHO adsorption (KB-F25), the appearance of second photopeak at higher BE, 531.5 eV is observed along with the photopeak observed for KB sample (Figure S2). This new photopeak (OF) can be assigned to O-C and O=C arising from adsorbed formate and formaldehyde species. K 2p region is composed of a doublet with BE contributions at 292 eV and 294.7 eV which are respectively assigned to K 2p3/2 and K 2p1/2 of potassium lines (Figure S3). These values are in good agreement with those reported over K-birnessite material by Thenuwara et al. [START_REF] Thenuwara | Intercalation of Cobalt into the Interlayer of Birnessite Improves Oxygen Evolution Catalysis[END_REF]. No clear modification of the K 2p envelope is seen after HCHO adsorption. Figure 6 shows the evolution of the intensity of the two carbon components (CF and CCB) as a function of temperature together with the evolution of the average oxidation number of manganese. The XPS peak intensity has been normalized, dividing the carbon component intensity by the Mn 2p photopeak intensity. From 50 to 100°C, the CF component intensity decrease can be explained by the formaldehyde desorption without Mn AOS value change.

From 100°C to 175°C, the decrease in intensity of CF component fits well with the decrease in Mn AON. Consequently the formate species are readily oxidized by oxygen species from the birnessite. At 200°C the birnessite is probably reduced into Mn3O4 in accordance with the Mn AON of 2.6. The oxidation process leads to the formation of carbonates species. However in order to explain the incomplete carbon balance (Figure 6), it can be suggested that there is release of gaseous COx species during this process. and Mn average oxidation number as a function of temperature Alkali metal carbonates are easily formed on oxide surfaces [START_REF] Scchukarev | XPS Study of Group IA Carbonates[END_REF]. Therefore the formation of K2CO3 can be proposed owing to the presence of K + ions located between the manganese oxide layers in the birnessite structure. In order to assess this assumption, a curve fitting of the K 2p line into two components has been done for KB-F125, KB-F150 and KB-F175 samples. The results of K 2p spectra curve fitting are shown in Figure 7 and Table 2. The K 2p3/2 BE at 292.4 eV (KO) is assigned to potassium species residing between the Mn based sheets [START_REF] Kim | Synthesis of colloidal MnO2 with a sheet-like structure by one-pot plasma discharge in permanganate aqueous solution[END_REF], while the K 2p3/2 component at higher BE (293.1-293.3 eV) arises from potassium with carbonate surrounding (KCB). The KCB/CCB atomic ratio is close to 2, in very good agreement with the formation of K2CO3 species on the surface detected from 125°C as seen onwards (Table 2). The formation of carbonates species is not obvious from the O 1s spectrum because of the 2.5 Nevertheless for KB-F175 sample, the O 1s component at high BE is shifted to lower value (-0.5 eV) in comparison with that observed for KB-F samples heated at lower temperature. This could be explained by the disappearance of formate species and the exclusive presence of carbonates at 175°C. S4). The use of isotopic patterns for the potassium ( 39 K and 41 K with respective abundance of 93% and 7%) reinforced the ion assignment. Characteristic secondary ions from birnessite (KB) were assigned and reported in Table S2. Secondary ions relative to potassium are only detected in positive mode while Mn fragment ions are detected in both polarities. Secondary ions containing both K and Mn cations are not detected in the m/z range studied. Figures 8 and9 show the negative and positive ToF-SIMS spectra obtained before and after HCHO adsorption on the as-synthetized birnessite surface. For the as-made sample (KB) beyond m/z = 50 in the negative and positive spectra the following most intense peaks are observed (ion, m/z value):

MnO2 -, 86.9; MnO3 -, 102.9; Mn2O4 -, 173.9, Mn + , 54.9; MnO + , 70.9; MnOH + , 71.9; K2O + , 93.9; K2OH + , 94.9; K2O2 + , 109.9. The ToF-SIMS spectra obtained for KB-F25 sample exhibit additional peaks. In the negative ToF-SIMS spectrum the following characteristic peaks (ion, m/z value) are found: CHO2 -, 45.0; MnCH3O2 -, 101.9; MnCH3O3 -, 117.9, MnC2H3O4 -, 145.9;

MnC3H3O6 -, 189.9. The most intense peak at 45 m/z is assigned to the presence of formate species, in agreement with XPS results. Interestingly, the ToF-SIMS study shows evidence of formate interaction with Mn, since MnCHyOz -fragment ions are clearly detected. In the positive spectrum, the following characteristic peaks (ion, m/z value) are found: K2 + , 77.9; K2H + , 78.9; K2CHO2 + , 123.9. The important increase in K2 + and K2H + ions intensity could arise from the presence of K2CO3 [START_REF] Mills | Highly Stable Novel Inorganic Hybrids[END_REF] after formaldehyde adsorption. This interpretation is supported by the detection of other positive ion fragments in the 175-180 m/z range assigned to K3CO3 + isotopic pattern (Figure S5). 125°C. This result tends to show carbonate formation at the birnessite surface, suggesting that part of gaseous COx reacts at the vicinity of potassium species. Moreover a temperature offset related to formate species transformation onto potassium is clearly seen as the intensity of K2CHO2 + starts to decrease at higher temperature (150°C). In the absence of possible redox mechanism with potassium site, a thermal decomposition of these species could be proposed. In summary, this spectroscopic multi-technic approach allows better insight into the adsorption/oxidation steps of formaldehyde and into the thermo-desorption of intermediate products in interaction with the K-birnessite. After formaldehyde adsorption/oxidation at 25°C, formate species were observed at the surface as main products of formaldehyde oxidation. The value of Mn average oxidation number (AON) decreases from 3.8 to 3 after formaldehyde exposure at ambient temperature. While XPS spectroscopy is able to show evidence of formate interaction with Mn species, ToF-SIMS analysis allows the conclusion that formate species are also present on K sites. Different reactivity of formate species has been shown depending on the type of adsorption site. On Mn site, formate species are converted into gaseous COx and the Mn AON decreases from 3 to 2.6 according to the transformation of birnessite into Mn3O4. The potassium sites are favorable to the carbonate formation and the transformation of formate species in interaction with K takes place at higher temperature through probably a thermal decomposition.

Figure 1 .

 1 Figure 1. Evolution of HCHO (straight line) and CO2 (dotted line) amounts in function of

Figure 2 .

 2 Figure 2. Possible surface species formed from the formaldehyde interaction

Figure 3 .

 3 Figure 3. C 1s XPS signals obtained for KB and KB-F25 samples

Figure 4 .

 4 Figure 4. Mn 3s XPS signals obtained for KB and KB-F25 samples

Figure 5 .Figure 6 .

 56 Figure 5. Evolution of the C1s XPS signal (straight lines) and components from curve fitting

  oxygen components coming from the formate and the carboxylate species.

Figure 7 .

 7 Figure 7. K 2p XPS signal obtained for KB-F125, KB-F150, KB-F175 samples (straight

Figure 8 .Figure 9 .

 89 Figure 8. Negative ToF-SIMS spectra of (a) KB and (b) KB-F25 samples

Figure 10 .

 10 Figure 10. Evolution of the intensity of MnCH3O2 -, K2CHO2 + and K3CO3 + fragments with temperature.

Table 1 .

 1 Textural and structural properties of samples calcined at different temperatures This result underscores the fact that the oxidation of Mn(NO3)2 with H2O2 in KOH solution is a suitable process for the preparation of material with very high surface area in comparison with those prepared by reduction of Mn (VII) with other reducing agents.

	Calcination	S BET	V p	D p	XRD phase	D c
	Temperature / °C	/m 2	.g -1	/ cm 3	.g -1	/nm		/nm
	300	184	0.69	14	Kx(Mn 4+ ,Mn 3+ )2O4	5
	450	93	0.62	25	α-MnO2	21
	600	42	0.44	44	α-MnO2 + α-Mn2O3	28

The BET surface area of the birnessite (184 m 2 .g -1 ) is much higher than those reported by Tian et al. (145 m 2 .g -1 ) [17], Chen et al. (70 m 2 .g -1 ) [18] and Zhou et al. (95 m 2 .g -1 ) [19] for crystallized K-type birnessite prepared through the reduction of KMnO4.

Table 2 .

 2 Binding energy (BE) and Full width at half maximum (FWHM) of the different components issue from the C 1s and K 2p3/2 curve fitting and KCB/CCB atomic

			ratio as a function of temperature treatment
			C 1s			K 2p3/2		Atomic
	sample	BE (eV)	FWHM (eV)	BE (eV)	FWHM (eV)	ratio
		CCB	CF	CCB	CF	KCB	KO	KCB	KO	KCB/CCB
	KB	289	-			-	292.3	-	1.4	-
	KB-F25	-	288.3		1.3	-	292.3	-	1.4	-
	KB-F50	-	288.3		1.3	-	292.3	-	1.4	-
	KB-F75	-	288.5		1.3	-	292.4	-	1.5	-
	KB-F100	-	288.5		1.3	-	292.4	-	1.5	-
	KB-F125 289.1 288.6 1.2	1.1	293.1	292.4	1.7	1.3	2.0
	KB-F150 289.1 288.5 1.2	1.1	293.2	292.4	1.8	1.3	2.3
	KB-F175 289.1	-	1.1		293.1	292.5	1.8	1.4	2.3
	KB-F200 289.1	-	1.1		293.3	292.5	1.7	1.5	2.0
	3.3

. Study of formaldehyde reaction by ToF-SIMS

	3.3.1. Formaldehyde adsorption over birnessite material
	ToF-SIMS is able, unlike XPS, to give molecular information about interface through the
	detection of molecular ions including elements of adsorbent (birnessite) and molecule(s) of
	adsorbate (formaldehyde/formate). The high mass resolution provided by ToF-SIMS analysis
	allowed successful discrimination of very close m/z values. For example the mass separation
	of Mn + (m/z = 54.938) and OK + (m/z = 54.959) ions has been possible (Figure

Acknowledgement

The authors thank the Chevreul Institute (FR 2638) for its help in the development of this work.

Chevreul Institute is supported by the « Ministère de l'Enseignement Supérieur et de la Recherche », the « Région Nord-Pas de calais » and the « Fonds Européen de Développement des Régions ». Support from Indo-French Centre for the Promotion of Advanced Research (IFCPAR) /Centre Franco-Indien pour la Promotion de La Recherche Avancée (CEFIPRA) is also gratefully acknowledged.

Supplementary Material

XRD data, XPS O 1s and K 2p signals before and after HCHO adsorption, XPS surface atomic percentage, ToF-SIMS data