Search

Ni : organosilicon polymer coordination complex to in situ trap nanoscale Ni particles as OER pre-catalysts in a high SSA polymer-derived Si–C–O–N(H) support.

A polyol-assisted solvothermal route is used to synthesize Ni x Fe y nanoalloys supported on a highly electron conductive 2D transition metal Mo2CT x MXene. Structural, morphological and chemical characteristics of the materials are determined using several physicochemical techniques. The MXene support allows not only the formation of a nanostructured metallic Ni x Fe y nanoalloys, but also favors the interfacial charge transfer for the oxygen evolution reaction (OER). The Ni x Fe y @Mo2CT x material with a Ni/Fe ratio of 2.66 leads to the outstanding activity for the OER with an amazingly low Tafel slope value of 34 mV dec−1 and a current density of 10 mA.cm−2 at a potential of only 1.50 V vs. reversible hydrogen electrode (RHE). In situ Raman experiments show that β-NiOOH formed by oxidation of the nanoalloys under positive scan, likely containing a very small amount of Fe, is the active phase for the OER. This material exhibits also an excellent stability over 168 h in a 5 M KOH electrolyte. Transmission electron microscopy -electron energy-loss spectroscopy analyses after 100 voltammetric cycles between 0.2 and 1.55 V vs. RHE evidence for the first time that the MXene support is not fully oxidized in the first cycle. Also, oxyhydroxide layer formed in the OER potential region at the surface of the Ni x Fe y nanoparticles can be reversibly reduced.

MXenes are a young family of two-dimensional transition metal carbides, nitrides, and carbonitrides with highly controllable structure, composition, and surface chemistry to adjust for target applications. Here, we demonstrate the modifications of two-dimensional MXenes by low-energy ion implantation, leading to the incorporation of Mn ions in Ti

Summary Since their discovery in 2011, MXenes have garnered worldwide interest. Given their 2D structure, surface, or termination, chemistries play a vital role in most applications. X-ray photoelectron spectroscopy (XPS) is one of the most common characterization tools for quantifying surface terminations and overall chemistry. Herein, we critically review the XPS fitting models proposed for Ti3C2Tz MXene in the literature and make the case that they are at best incomplete and at worst contradictory. We propose a new fitting algorithm based on all the data obtained from previously published studies and propose a new method for quantifying the surface terminations in Ti3C2Tz. In our approach, we assign the Ti 2p peak at 455.1 eV to the C–Ti–O\O\O, and the peaks at 456.0, 457.0, 457.9, and 459.6 eV are assigned to C–Ti–O\O\F, C–Ti–O\F\F, C–Ti–F\F\F, and TiO2-xF2x, respectively. The first four represent possible Ti atom terminations; the last is an oxyfluoride. The C 1s peak at 282 eV, ascribed to C atoms surrounded by 6 Ti atoms, is so universal that it can almost be used as a reference.

MXenes are a young family of two-dimensional transition metal carbides, nitrides, and carbonitrides with highly controllable structure, composition, and surface chemistry to adjust for target applications. Here, we demonstrate the modifications of two-dimensional MXenes by low-energy ion implantation, leading to the incorporation of Mn ions in Ti3C2Tx (where Tx is a surface termination) thin films. Damage and structural defects caused by the implantation process are characterized at different depths by XPS on Ti 2p core-level spectra, by ToF-SIMS, and with electron energy loss spectroscopy analyses. Results show that the ion-induced alteration of the damage tolerant Ti3C2Tx layer is due to defect formation at both Ti and C sites, thereby promoting the functionalization of these sites with oxygen groups. This work contributes to the inspiring approach of tailoring 2D MXene structure and properties through doping and defect formation by low-energy ion implantation to expand their practical applications.

International audience; MXenes are a new, and growing, family of 2D materials with very promising properties for a wide variety of applications. Obtained from the etching of MAX phases, numerous properties can be targeted thanks to the chemical richness of the precursors. Herein, we highlight how etching agents govern surface chemistries of Ti 3 C 2 T x , the most widely studied MXene to date. By combining characterization tools such as X-ray diffraction, X-ray photoelectron, Raman and electron energy loss spectroscopies, scanning and transmission electron microscopies and a surface sensitive electrochemical reaction-the hydrogen evolution reaction, HER-we clearly demonstrate that the etching agent (HF, LiF/HCl or FeF 3 /HCl) strongly modifies the nature of surface terminal groups (F, OH and/or O), oxidation sensitivity, delamination ability, nature of the inserted species, interstratification, concentration of defects and size of flakes. Beyond showing how using these different characterization tools to analyze MXenes, this work highlights that the MXene synthesis routes can influence targeted applications.

International audience; Two-dimensional (2D) carbides and/or nitrides (so-called MXenes) are among the latest and largest family of 2D materials. Due to their 2D nature and their unique properties of hydrophilicity, good metallic conductivity, and structural diversity, these materials are intensively studied for sensing applications or as supports for nanomaterials toward, e.g., plasmonics, catalytic, or energy storage applications. For these potential usages, the extent to which the electronic properties of MXene sheets are modified upon functionalization or intercalation is critical, and an optimized nondestructive probing of the interaction between MXene layers and functionalization is important to be determined. Here, these issues are addressed using a combination of first-principles simulations and electron energy loss spectroscopy (EELS) experiments performed at the nanoscale on Ti3C2Tx and Ti2CTx MXene multilayers, where T denotes the surface functionalization groups. Based on a detailed analysis of the carbon and surface group K edge fine structure, we show that the C-K edge is an ideal marker for surface-induced electronic structure modifications in the Tin+1Cn conducting core. These results highlight how a nanometer-scale impurity can very locally interact with a Ti3C2Tx multilayer and modify its electronic structure. This approach allows discrimination between the surface and core alteration of the Ti3C2Tx layers. Finally, the higher sensitivity to surface states of the Tin+1Cn conducting core in Ti2CTx as compared to Ti3C2Tx is discussed. We expect these results to offer an approach for understanding MXenes’ behavior and especially characterize their interactions with other nanomaterials when used in composites

International audience; The development of reliable electrolyzers is closely related to the development of a cost-effective highly active and stable electrocatalysts for the oxygen evolution reaction (OER). Herein, a simple method is used to synthesize a non-noble metal-based electrocatalyst for OER by synergistically coupling a catalytically active cobalt layered double hydroxide (Co-LDH) with a highly electrically conducting 2D transition metal carbide, Ti3C2Tx MXene. The synergy between these two bidimensional materials (Co-LDH and Ti3C2Tx), evidenced by coupling electron energy loss spectroscopy and density functional theory simulations, results in superior electrocatalytic properties and makes possible having an excellent and stable oxygen evolution electrocatalyst. Moreover, the oxidative-sensitive MXene structure is preserved during the synthesis of the composite and the formation of a well recovering Co-LDH phase avoids the irreversible oxidation of MXene at high potential values, which may affect its conductivity. With an overpotential of approximate to 330 mV at a current density of 10 mA cm(-2) the catalyst exhibits a higher catalytic activity toward OER than commercial IrO2 catalysts.

MXenes are single or few-layered two-dimensional (2D) transition metals carbides and/or nitrides successfully synthesized in 2011, with chemical composition Mn+1XnTx, where M stands for an early transition metal, X is C and/or N, Tx are a surface terminations and n = 1, 2, or 3 [1]. These materials are considered as promising candidates in many applications such as energy storage, sensing, catalysis, and optoelectronics. For instance, MXenes combine high conductivity and hydrophilicity making them interesting candidates for applications involving aqueous media and requiring good electronic properties or easy processing. Moreover, they exhibit a wide chemical variability thanks to their parent MAX phases precursors, a 150-plus member family of layered ternary carbides or nitrides with chemical composition Mn+1AXn (where the A-element mainly belong to the columns 13 and 14 of the periodic table). This offers the unique opportunity to design the MXenes on demand, with considerably different physicochemical properties for target applications [2-4]. Tuning the MXene properties is also possible through the manipulation of their surface termination groups T (i.e., T = −F, −OH, −O, or −Cl). The crucial role of the surface groups has recently triggered many studies aiming at controlling and characterizing the MXene surface structure/chemistry mainly by using different etching protocols [5]. However, the nature of the surface groups, inherited from the A element exfoliation step of the MAX phases by chemical reactions, is hardly tunable by this mean except by changing the etching protocole which might have significant impact on the quality of the synthetized MXenes. Here, we demonstrate that medium energy ion implantation can be used to modify the MXene surface functionalization, through the controlled formation of structural defects and the incorporation of foreign species in Ti3C2Tx thin films fabricated by spin coating (Fig.1 a-b). Defects created by Mn implantation in a wide fluence range (1x1014-5x1016 cm-2) are characterized at different depths by XPS on Ti2p core-level spectra, ToF-SIMS and EELS analyses. For the latest, we use an original approach that we have developed [6], allowing an estimation of the level of damage in the material by studying the C-K edge. Our results show that the 60 keV Mn implantation leads to successful incorporation of Mn in the Ti3C2 thin film. This is accompanied by the structural distortion of the Ti-C layers as evidenced by EELS experiments (Fig. 1c), preferential Ti sputtering as compared to C without any significant amorphization or phase transformation up to a fluence of 1x1016 cm-2. However, for the highest fluence studied (5x1016 cm-2), complete amorphization was observed. For the subthreshold amorphization fluence, the study of the fine structure of the O K-edge and of the electron diffraction pattern suggest the formation of extended defects in the Ti layers including the occupation of the defective Ti sites by oxygen, leading to local environment for the oxygen atoms like graphene oxide. Finally, our results show that the number of defects can be directly and in a reproductible way tunned by the implantation fluence. [1] M. Naguib, M. Kurtoglu, V. Presser, J. Lu, J. Niu, M. Heon, L. Hultman, Y. Gogotsi, M.W. Barsoum, Adv. Mater. 2011, 23, 4248−4253 [2] Y. Gogotsi, B. Anasori, ACS Nano 2019, 13, 8491−8494 [3] Q. Tao, M. Dahlqvist, J. Lu, S. Kota, R. Meshkian, J. Halim, J. Palisaitis, L. Hultman, M. W. Barsoum, P. O. Persson, J. Rosen, Nat. Commun. 2017, 8, No. 14949. [4] B. Anasori, C. Shi, E.J. Moon, Y. Xie, C.A. Voigt, P. R. Kent, S.J. May, S.J. Billinge, M.W. Barsoum, Y. Gogotsi, Nanoscale Horiz. 2016, 1, 227−234 [5] M. Benchakar, L. Loupias, C. Garnero, T. Bilyk, C. Morais, C. Canaff, N. Guignard, S. Morisset, H. Pazniak, S. Hurand, et al. Appl. Surf. Sci. 2020, 530, No. 147209 [6] T. Bilyk, M. Benchakar, M. Bugnet, L. Loupias, P. Chartier, H. Pazniak, M.-L. David, A. Habrioux, S. Celerier, J. Pacaud, and V. Mauchamp, J. Phys. Chem. C (to be published, https://dx.doi.org/10.1021/acs.jpcc.0c06798) Figure 1

The synthesis of Ti3C2Tx (and Ti2CTx alike) MXenes is performed with a new etching environment (FeF3/HCl). Significant differences in terms of surface functionalization (including the insertion of iron cations between the MXene sheets), morphology, nature of impurities, water intercalation and reactivity are obtained in comparison with samples prepared by conventional HF or LiF/HCl etching methods. In particular, the new synthesis method allows modifying the reactivity of Ti3C2vs. water with improved selectivity towards the formation of TiO2 anatase nanoparticles on the MXene sheets whereas rutile TiO2 nanoparticles are favoured by the LiF/HCl method. The role of the Fen+ and Al3+ cations inherent to this synthesis process in the inhibition of the anatase to rutile transformation is shown to be a key parameter. In addition, the MXene oxidation rate in water is also increased when the material is prepared with FeF3/HCl compared to the LiF/HCl method owing to the surface modification of the MXene during the etching step. This oxidation rate can easily be tuned by adjusting the duration of immersion. This work expands the number of etching methods to form MXenes while offering new opportunities in the rationalized design of MXene@TiO2 nanocomposite structures for future applications.

International audience; The MXenes, among which Ti 3 C 2 T x is the most studied, are a large family of two-dimensional materials with proven potential in a variety of application fields (e.g., energy storage and conversion, water purification, electromagnetic interference shielding, humidity sensor, etc.). For most of these applications, the properties of MXenes depend, at least partly, on their water sorption ability and on the induced structural swelling, which is commonly considered a stepwise process, like in claylike materials. In the present study, we rather evidence the systematic coexistence of different hydrates in MXene interstratified crystals. Hydration heterogeneity and related structure disorder are described from the quantitative analysis of X-ray diffraction data. This specific methodological approach allows disentangling the complex interstratification and rationalizing the prediction of MXene electrical properties. The widespread use of this approach paves the way for a systematic and thorough determination of MXene structure, including order−disorder, and thus for grasping the influence of structural disorder (hydration heterogeneity) on a large number of MXene physical properties (e.g., optical transparency, capacitance). Deciphering this complex structural disorder is also essential in the design of new MXene-based materials for a variety of applications (supercapacitors, batteries, water treatment, etc.).

The stability under different atmospheres (dry or wet nitrogen or air, HF, reaction medium) and the catalytic performances of BaF 2 -LaF 3 based catalysts for the fluorination of 2-choropyridine by Cl/F exchange with HF as fluorinating agent were investigated. Thermal decomposition of metal trifluoroacetate and precipitation from nitrate precursor methods were used as synthesis methods. Depending on the synthesis method and the La/La + Ba molar ratio (x), an intimate mixture of BaF 2 and LaF 3 metal fluorides or a Ba 1−x La x F 2+x mixed metal fluoride with high specific surface area were formed. In comparison with BaF 2 , the mixed metal fluorides led to higher specific surface areas and prevented the sintering phenomenon observed under wet atmosphere which led to a significant decrease of the specific surface area. Moreover, Ba 1−x La x F 2+x catalysts had not evolved under the fluorination reaction contrary to BaF 2 for which a stable BaF 2−x Cl x material (with x close to 0.8) was formed by reaction with the by-product HCl. The turn over frequency (TOF) was similar for all barium based catalysts for the fluorination of 2-chloropyridine. Nevertheless, among the studied materials, Ba 0.5 La 0.5 F 2.5 was the most promising catalyst for the transformation of 2-chloropyridine due to high activity (related to its high specific surface area and its moderate strength of Lewis acidity) and high stability.

International audience; The alkylation of sulfur-containing compounds (benzothiophene and thiophene) with 2-methyl-1-pentene was investigated over MgF2-supported heteropoly acid (H3PW12O40: HPW) as catalysts under mild conditions (85 degrees C, atmospheric pressure). The magnesium fluoride used as support was prepared by a sal-gel method and exhibited a high specific surface area (186 m(2) g(-1)). Several solids containing various amounts of HPW (in the range of 5 wt% to 46 wt%) were prepared by incipient wetness impregnation and fully characterized (ICP-OES, XRD, N-2 adsorption, TEM, IR, adsorption of pyridine followed by IR, F-19 and P-31 MAS NMR). For all solids, it was observed that the Keggin structure of HPW was preserved and that the heteropoly acid was finely dispersed on the MgF2 support. A significant modification of the textural properties of the sample containing the higher amount of HPW (46 wt%) was observed and attributed to a pore blocking phenomenon by HPW. As expected, the HPW loading in the range of 5-30 wt% allowed an increase of the Bronsted acidity. Whatever the catalyst, benzothiophene was only transformed by mono-alkylation, whereas the olefin underwent alkylation, oligomerization and isomerization, the latter being largely predominant and yielded 2-methyl-2-pentene. The increase of the HPW loading until 30 wt% led to an increase of the catalytic activity, the selectivity being not modified. In addition, a significant deactivation of catalysts containing 30 wt% and 40 wt% of HPW was observed and attributed to a strong adsorption of the alkylated products. Thiophene presented a poor reactivity through alkylation compared to benzothiophene.

The role of the surface groups T (T = OH, O or F) in the chemical bonding in two-dimensional Ti

International audience; The selective fluorination by successive Cl/F exchanges of the dichloromethylbenzene, was studied in the presence of HF as the fluorinating agent. The influence of the presence of a catalyst or a basic solvent (such as dioxane, pyridine, tributylphosphate) in order to favour the fluorination was investigated. In mild conditions, (50 °C and after 1 h of reaction), the fluorination of the dichloromethylbenzene was observed and the polymerization reaction inhibited. The best results were obtained in the presence of the HF-dioxane system and antimony trifluoride as catalyst.

International audience; The addition of zinc in low amount to chromium based catalyst supported over partially fluorinated alumina has a positive effect for the fluorination reaction of CF3CH2Cl in the presence of dioxygen in order to prevent the catalyst deactivation. However, under these operating conditions, the Deacon reaction by reaction with HCl produced by Cl/F exchanges could be involved. The formation of various by-products was observed corresponding to the addition of HCl or Cl2 into halogenated double bonds.

The role of the surface groups T (T = OH, O or F) in the chemical bonding in two-dimensional Ti3C2Tx MXene is directly evidenced combining electron energy-loss spectroscopy in a transmission electron microscope and simulations based on density functional theory. By focusing on the 1s core electrons excitations of the C and (F, O) atoms, the site projected electronic structure is resolved. The Electron Energy-Loss Near Edge Structures (ELNES) at the C-K edge are shown to be sensitive to the chemical nature and the location of the T-groups on the MXene's surface and thereby allow for the characterization of the MXene's functionalization on the nanometre scale. In addition, the ELNES at the C and F-K edges are shown to be determined by the hybridizations of these atoms with the Ti d bands: these edges are thus relevant probes of the Ti d density of states close to the Fermi level which is of particular interest since it drives most of the Ti3C2Tx electronic properties. Finally, the crucial role in the MXene's functionalization of the etchant used for its synthesis is evidenced by locally determining the [O]/[F] concentration ratio using the corresponding K edges. This ratio is shown to be drastically increased from 1.4 to 3.5 when using HF or LiF/HCl respectively.

International audience; Large range of catalytic systems can be obtained from metal fluorides and oxide hydroxide fluorides prepared by soft chemistry due to various properties, especially high specific surface area and tunable acid-base properties. This article highlights the promising properties of these systems for many heterogeneous catalytic processes which extends the catalogue of solid heterogeneous catalysts and supports for future industrial applications.

Large range of catalytic systems can be obtained from metal fluorides and oxide hydroxide fluorides prepared by soft chemistry due to various properties, especially high specific surface area and tunable acid–base properties. This article highlights the promising properties of these systems for many heterogeneous catalytic processes which extends the catalogue of solid heterogeneous catalysts and supports for future industrial applications.

Valence electron energy-loss (VEEL) spectroscopy in the transmission electron microscope is combined to ab initio calculations to investigate the dielectric properties of multilayer (ML) two-dimensional ${\mathrm{Ti}}_{3}{\mathrm{C}}_{2}{T}_{2}$ ($T\ensuremath{\equiv}\text{OH}$ or F) MXene. Besides evidencing important similarities between the ${\mathrm{ML}\ensuremath{-}\mathrm{Ti}}_{3}{\mathrm{C}}_{2}{T}_{2}$ and TiC valence electron gas behaviors, a clear interband transition characteristic of the most stable site of the $T$ functionalization groups is identified in the VEEL spectrum. This signature, highly dependent on the $T$-group localization on the surface, has a prominent effect on the optical properties of the ML, leading to $40%$ variations in the optical conductivity in the middle of the visible spectrum. Such a dependence could be of crucial interest for optical transparent thin films or sensing applications.

International audience; Several inorganic hydroxide fluorides MFn−x(OH)x with M being either aluminum, iron, or magnesium were synthesized using a sol–gel method. These mesoporous fluoride materials exhibited very high specific surface areas (between 60 and 370 m2 g−1) depending on the synthesis parameters used (nature of the metal, HF/Al ratio, and thermal treatment). It is shown that their acidic properties can also be adjusted by tuning these parameters. Indeed, in the case of aluminum hydroxide fluorides, bi-acidic catalysts containing both Brønsted and Lewis acid sites were obtained, the amount of acid sites and the Lewis/Bronsted ratio (in the range of 1.9–7.6) being very dependent on the synthesis parameters. For example, the increase of the calcination temperature of aluminum hydroxide fluorides led to a decrease of both Lewis and Brønsted acid site, but also an increase of the Lewis/Bronsted ratio. The most acid fluoride exhibited 542 μmol g−1 of Lewis acid sites and 291 μmol g−1 of Brønsted acid sites.These inorganic hydroxide fluorides were successively used as catalysts for the solvent-free acylation of 2-methylfuran by acetic anhydride under mild conditions (50 °C, atmospheric pressure) yielding selectively 2-acetyl-5-methylfuran. The activity of fluoride can be related to the amount of both Lewis and Brønsted acid sites. In general the higher these values, the greater the activity in acylation. A reaction mechanism was proposed involving a bi-acidic site which could be a Lewis acid site (unsaturated aluminum) in the vicinity of a Brønsted acid site.

The catalytic fluorination of 2-chloropyridine into 2-fluoropyridine by Cl/F exchange in presence of HF and a metal fluoride as catalyst was investigated. Crystallized metal fluorides (BaF2, MgF2, CaF2, ZnF2, LaF3 and CeF3) with significant specific surface area between 65 and 125 m2 g−1 were prepared by thermal decomposition of corresponding metal trifluoroacetates at 350 °C under nitrogen. These solids were stable under fluorination operating conditions involved in the formation of 2-fluoropyridine. An important gain in activity for the catalytic fluorination of 2-chloropyridine was obtained with these metal fluorides compared to corresponding oxides used as catalyst. BaF2 and MgF2 were the most active catalysts for the fluorination of 2-chloropyridine. The higher activities are in relation with the weak or moderate strength of Lewis acidity of the active sites, measured by CO adsorption followed by IR spectroscopy.

International audience; The fluorination of 2-chloropyridine to produce selectively 2-fluoropyrydine by Cl/F exchange at 300 or 380 °C with HF as the fluorinating agent and a metal oxide as precursor of catalyst was investigated. The influence of the activation step under HF gas on the structural, microstructural and physico-chemical properties of various oxides such as MgO, CeO2, Ce0.5Zr0.5O2, LaFeO3, Cr2O3, were studied. This activation step by HF led to the formation of the corresponding fluorides and/or oxyfluorides materials which were the active catalysts for the Cl/F exchanges. Moreover, this activation step modified strongly the initial properties of the starting materials. This corresponded to a strong decrease of the initial specific surface area and of the basic properties. In the same time, an increase of Lewis acid properties measured by FTIR with pyridine as probe molecule was noticed. MgO, which was initially the most basic oxide studied was also the most active for the fluorination of the 2-chloropyridine after its transformation into fluoride solid which was the active catalyst. This could be due to a lower strength Lewis acidity of the active sites formed by the activation step by HF directly correlated to the basic properties of the initial oxides. A mechanism of the 2-chloropyridine fluorination involving Lewis acid unsaturated metal site was proposed.

Selective trifluoromethylation of the corresponding chlorinated compounds is achieved by successive Cl/F exchange under mild conditions using HF as the fluorinating agent.

International audience; 2-fluoropyridine is selectively prepared by Cl/F exchange from 2-chloropyridine at 380 C with HF as the fluorinating agent and a metal oxide catalyst Various catalysts such as MgO CeO(2) Ce(0 5)Zr(0 5)O(2) LaFeO(3) and Cr(2)O(3) presenting various acid-base properties were compared MgO the most basic solid is the most active for the formation of 2-fluoropyridine

The Cl/F exchange of chlorinated hydrocarbons with HF was carried out in the gas phase (P = 1 atm. and T = 380 °C) on chromium catalysts doped with zinc. A promoting effect of zinc has been found for various chlorinated hydrocarbons. This positive effect was more significant for CF3CH2Cl than for CF3CHCl2 or CH2Cl2. CF3CH2Cl was less reactive than the two other reactants. The promoting effect observed for an atomic ratio Zn/(Zn + Cr) = 0.15 was higher with a low (Zn + Cr) content, which corresponds to a better dispersion of the different elements on the surface. The presence of zinc is supposed to promote the dispersion of chromium and to favour its fluorination to form the centers which are active in Cl/F exchange and were characterized in XPS by energies corresponding to the presence of “CrOF” and “Cr–F” species. Transformation of CF3CH2Cl (T = 380 °C, CF3CH2Cl/HF = 1/4) over Zn-promoted Cr/AlF3 catalysts. Effect of the Zn/(Zn + Cr) atomic ratio and of (Zn + Cr) total amount (figures on the curves) on the activity.

International audience; The selective fluorination by successive Cl/F exchanges of alpha alpha alpha-trichlorotoluene substituted or not by a chlorine atom was studied in the presence of HF as the fluorin long agent The influence of the presence of a catalyst or a basic solvent (such as dioxane pyridine tributylphosphate) in order to control the fluorination was also investigated In mild conditions (50 C and after 1 h of reaction) HF in excess was required in order to obtained the trifluoromethylation by Cl/F exchanges The presence of SbCl(5) in small amount activated the Cl/F exchanges and only a stoichiometric amount of HF was required whatever the chlorinated molecules Selective mono and difluorination could be obtained by using basic solvents (C) 2010 Elsevier B V All rights reserved

Pure ceria-zirconia mixed oxides Ce x Zr1−x O2 with high specific surface area were synthesized with a new epoxyde driven sol–gel route and characterized by thermal analysis, X-ray diffraction studies and transmission electron microscopy. This sol–gel method is cheap and uses only a few steps. The Ce x Zr1−x O2 mixed oxides were obtained in the range of 0 ≤ x ≤ 1 (except for x = 0.8) and crystallised at 350 °C after decomposition of the gels. This temperature is very low in comparison with the other methods. The studies of the influence of different synthesis parameters (concentration of the sol and decomposition temperature) allowed us to determine the conditions to obtain the best homogeneity in the gel to avoid the formation of a mixture of phases instead of mixed oxides. This approach leads to the synthesis of oxide with specific surface area above 100 m2 g−1. The elaboration of an ambigel could increase this value up to 195 m2 g−1 for x = 0.5. This sol–gel synthesis offers new perspectives for these oxides in several applications. Generally, these oxides are difficult to obtain pure in large range of composition at low-temperature and with high specific surface area by other methods.

Silicate-based oxides are of interest as electrolytes and electrode materials in intermediate-temperature solid-oxide fuel cells (SOFCs).[1–7] We have developed a sol–gel-based strategy for the production of mesoporous, nanostructured, singlephase La9.33Si6O26 apatite in an aerogel-type framework. Silicon alkoxides react with lanthanum(III) aqueous ions in alcohol solutions driven by a proton-scavenging agent, propylene oxide. Varying the means by which pore fluid is removed from the resultant lanthanum silicate gels yields three distinct pore–solid monolithic nanoarchitectures: aerogels, ambigels, and xerogels. The ambigel and aerogel nanoarchitectures exhibit high specific surface areas (from 285 to 408 m2 g–1), aperiodic through-connected networks of mesopores, and covalently bonded networks of non-agglomerated nanoparticles. Calcination at relatively mild temperatures for this oxide (800 °C) converts the amorphous gels to nanocrystalline apatite La9.33Si6O26. Although densified during calcination, the aerogel and ambigel nanoarchitectures retain porosity, high surface area, and limited particle agglomeration.

The aim of this paper is to advance in the field of the development of low-cost synthesis techniques easy to transfer at an industrial scale for electrolyte functional materials in intermediate temperature solid oxide fuel cells (IT-SOFC). We developed an original sol–gel route to synthesize La9.33Si6O26 oxides. This approach is straightforward, inexpensive, versatile and it produces pure powders with controlled size and morphology. Via sol–gel route, crystallization temperature of such phases are drastically reduced: 800 °C in comparison with 1500 °C in solid-state conventional routes. The dependance of the nature of the phases was studied by varying several parameters as the hydrolysis rate, the volumetric ratio of catalyst to TEOS and the silicon concentration. All these parameters affect the homogeneity of the gel and thus, the purity of the oxides obtained after heat treatment in air. These oxyapatite materials have particle size of about 100 nm after heat treatment at 1000 °C. The use of these powders for ceramic processing allows a decrease of the sintering temperature of more than 200 °C.

International audience; Solid oxide fuel cells have considerable interest in recent years, because of their high efficiency and environmentally friendly nature. Such systems required oxygen-conducting electrolytes and now the most common electrolyte is yttria stabilized zirconia (YSZ). This compound exhibits high oxide ion conductivity at elevated temperatures (850–1000 °C). However, this high working temperature causes problems in terms of materials selection and lifetime. One solution is to develop new oxide ions conductors exhibiting high oxide ion conductivity at intermediary temperatures (700–800 °C). Recent work has identified Ln10−xSi6O26±z (Ln = rare earths) as a good fast oxide ion conductor. Undoped and doped Ln10−xB6O26±z (B = Si or Ge) oxides are currently prepared by solid-state methods. In that work, we propose a sol–gel process to synthesize powders of La9.33Si6O26 type-silicated apatites. The main advantage is to decrease the crystallization temperature in ,comparison to the conventional methods, allowing the synthesis of reactive powders with nanometric particles size. These oxides are synthesized using silicon alkoxide and lanthanum nitride as precursors. In the litterature, no study refers to the synthesis of mixed oxides with silicon alcoxides. However, there are several studies on sol–gel synthesis of glasses with this precursor. In this study, several processing parameters have been investigated (the hydrolysis ratio, the concentration of metallic precursors in the sol and the role of organic compounds) in order to synthesize pure phases after the decomposition of the sols. Pure powders of La9.33Si6O26 type-silicated apatites are obtained at 800 °C. These powders were used to prepare ceramics. Several processing parameters as morphology of powders (agglomeration, particle sizes) and, heating profiles have been studied on the densification. Dense ceramics (90–95%) have been prepared at temperatures around 1400 °C. The used of sol–gel powders allow the decrease of the sintering temperature of about 200 °C.

Hydrogen is the most promising energy vector for the future energy grid implementation. It can be obtained from different methods of production. However, an eco-friendly hydrogen with a high purity can only be produced using water electrolysis. Furthermore, the design of low cost, active and stable electrodes is required for the development of large scale electrolysis systems. In this context, 2D materials are of upmost interest for the development of catalysts in reason of their high surface to volume ratio, conferring them unique properties far from those of their bulk counterparts. In this way, MXene family (discovered in 2011) is a good candidate. Their intrinsic properties (high electronic conductivity, hydrophilicity, versatile chemistry) reinforces the passion they arouse and make these materials as promising candidates for the design of efficient catalysts. In this work, several Ti3C2Tx MXenes were first synthesized using different etching agents and characterized in order to elucidate the hydration chemistry, composition, structure, surface chemistry and macroscopic properties. This MXene, which nevertheless exhibits a low catalytic performance toward hydrogen and oxygen evolution reactions, has been used as a support for cobalt-based layered double hydroxides. The presence of numerous terminal groups on the MXene surface allows obtaining a good dispersion of the active phase. In addition, MXene ensures the good electronic conductivity of the electrode which promotes the charge transfer. The structural properties of the resulting material (Co-LDH@ Ti3C2Tx) were studied and correlated to its good catalytic activity toward oxygen evolution reaction in alkaline medium. This performance could be associated to the electronic interaction occurring between the active phase and the MXene. Further, Mo2CTx MXenes were also synthesized, characterized and used as a precursor for the synthesis of a MoS2/Mo2CTx two-dimensional heterostructure. This innovative material was obtained by topotactic transformation (sulfurization) of Mo2CTx. This composite has proven to be an excellent catalyst toward hydrogen evolution reaction in alkaline medium. This high activity could be attributed to the intimate contact existing between the MoS2 and Mo2CTx phases on one hand, allowing an activation of the 2D sulfide basal plane and to the presence of weakly coordinated atoms on the other hand, allowing the water activation at low overpotentials.Thus, efficient and stable MXene-based catalysts have been developed for oxygen and hydrogen evolution reaction. The prospects for this work are numerous considering the chemistry richness of these new 2D materials in order to obtain more efficient electrodes.; L'hydrogène est le vecteur énergétique le plus prometteur pour l’implémentation de la grille énergétique du futur. Il peut être obtenu en employant diverses méthodes de production. Parmi tous ces procédés, l’électrolyse de l’eau est d’intérêt car elle permet d’obtenir directement du dihydrogène de grande pureté qui peut donc être utilisé au sein d’une pile à combustible H2/O2. Toutefois, l’élaboration d’électrodes peu coûteuses, actives et stables est nécessaire pour entreprendre le développement des électrolyseurs à grande échelle. Dans ce contexte, les matériaux 2D suscitent un engouement important pour l’élaboration de catalyseurs en raison de l’élévation du rapport entre atomes de surface et atomes de volume, qui leur confère des propriétés extrêmement différentes de celles de leurs analogues massifs. À ce titre, les MXènes (découverts en 2011) sont des matériaux particulièrement intéressants. Leurs caractéristiques intrinsèques (conductivité électronique élevée, hydrophilie, chimie versatile) renforcent encore l’intérêt qu’ils suscitent et font de ces matériaux des candidats de choix pour l’élaboration de matériaux catalytiques.Au cours de cette thèse les MXènes de type Ti3C2Tx ont tout d’abord été synthétisés à l’aide de différents milieux exfoliants, puis caractérisés dans le but d’en appréhender la chimie d’hydratation, la composition, la structure, la microstructure, la chimie de surface et les propriétés macroscopiques. Ce MXène, présentant néanmoins une faible activité pour les réactions de dégagement de dihydrogène et de dioxygène, a été utilisé comme support d’hydroxydes doubles lamellaires à base de cobalt. La présence de nombreux groupements terminaux à la surface du MXène permet d’obtenir une bonne dispersion de la phase active. De plus, le MXène assure la bonne conductivité électronique de l’électrode favorisant les transferts de charge. Les propriétés structurales du matériau résultant (Co-LDH@Ti3C2Tx) ont été étudiées et reliées à son excellente activité catalytique envers la réaction de dégagement de dioxygène en milieu alcalin. Cette performance a pu être associée au travers de l’étude de l’interaction électronique existant entre phase active et MXène. Suite à ces premiers travaux, des MXènes de type Mo2CTx ont aussi été synthétisés, caractérisés et utilisés en tant que précurseur pour la synthèse d’une hétérostructure bidimensionnelle de type MoS2/Mo2CTx. Ce matériau innovant fut obtenu par transformation topotactique (sulfuration) de Mo2CTx. Ce composite s’est avéré être un excellent catalyseur envers la réaction de dégagement de dihydrogène en milieu électrolytique alcalin. Cette augmentation d’activité a pu d’une part être attribuée au contact intime existant entre les phases MoS2 et Mo2CTx., permettant notamment une activation du plan basal du sulfure 2D et d’autre part à la présence d’atomes faiblement coordonnés permettant une activation de l’eau à de faibles surtensions.Ainsi, des catalyseurs à base de MXènes, performants et stables pour les deux réactions en jeu dans un électrolyseur alcalin, ont été élaborés. Les perspectives de ce travail afin d’obtenir des électrodes encore plus performantes sont nombreuses compte-tenu de la richesse de la chimie de ces nouveaux matériaux 2D.