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MICROSCOPIE:RX+CARE:ECI2D:MEME+GFE:MAO:FBS:LBU:CGE:TEP:SLO:PVE; International audience; This study explores the dynamic properties of Pt/CeO2 catalysts to develop active catalytic converters for diesel exhausts. It is now well established that single Pt atoms can be stabilised on ceria surface defects under oxidising conditions. However, their catalytic activity is rather poor. A reducing treatment is required to build more active Pt nanoparticles. A Pt/CeO2 catalyst, containing mainly atomically dispersed Pt species, was exposed to reduction steps either at 250 or 500 °C to build Pt nanoparticles. Their redispersion after oxidising treatments in oxygen at three temperatures (room temperature, 250 °C and 500 °C) as well as in a simulated diesel exhaust gas was deeply investigated using different characterisation techniques such as high angle annular dark field scanning transmission electron microscopy, in situ Raman spectroscopy, in situ X-ray diffraction, X-ray photoelectron spectroscopy and H2-temperature-programmed reduction. The arrangement of close packed Pt clusters is a permanent event at low temperatures (

MICROSCOPIE:RX+ING+TEG:ATU:LBU:FBS:YSC:FRM; National audience; The direct decomposition of NOx at moderate temperatures is a challenge over noble metal based catalysts because of the poisoning of the metal surface by oxygen. Au is inactive for NO direct decomposition, but displays oxidation resistance. The objective of this work is to obtain durable activity for NO direct decomposition by combining inert Au with an active metal such as Pd, which is known to deactivate. Pd-Au/SiO2 nanoparticles were prepared using a surface redox method. TEM-EDX analyses showed that most particles (3-24 nm in size) contained both metals, though the composition was not homogeneous. In situ XRPD analyses initially displayed poorly crystalline Au, Pd and possibly a Pd-Au alloy phase. Calcination in air yielded PdO and a Au-rich alloy phase, whose composition (Au0.8Pd0.2) seems to be the limiting composition under oxidizing conditions. Under H2, β-PdHx (at 25°C) or metallic Pd (with a thermal expansion at 450°C) were obtained besides Pd-rich alloyed phases. After removing H2 pressure, PdHx decomposed into metallic Pd. The Au-modification of the parent Pd/SiO2 yielded a stable activity. The post-reaction XRPD pattern showed a significant fraction of metallic Pd and alloyed phases in addition to PdO.

RX+ING+TEG:FRM:ATU:YSC:FBS; International audience; NOx direct decomposition over noble metals is thermodynamically favored and does not require reducing agents. However, the challenge is the poisoning of the noble metal surface by oxygen. On the other hand, Au is inactive for NO direct decomposition, but is one of the metals most resistant against oxidation. Our objective is to obtain a durable catalyst for NO direct decomposition by combining inert Au with an active metal such as Pd, which deactivates when used alone. Silica-supported Pd, Au and Pd-Au nanoparticles were prepared using earlier recipes. TEM-EDX analysis of the bimetallic sample showed that most of the particles contained both metals. In situ XRD analyses carried out at various temperatures under H2 evidenced the formation of Pd-Au alloyed phases. Alloyed phases were also present and stable under air at 500 °C, though Pd content decreased. The direct decomposition of NO to N2 and N2O was studied at 450°C. Au/SiO2 is inactive. Interestingly, the Pd-Au was more active than the parent Pd sample. Pd-Au also exhibited a stable activity with time, in contrast to the parent Pd material. Detailed product analyses showed that O release was higher for the Pd-Au catalyst, confirming that Au prevented Pd deactivation by facilitating O desorption.

MICROSCOPIE:RX+ATARI:CARE:ECI2D+MAO:FBS:FCA:CGE:SLO:PVE; International audience; Nowadays, design of new catalysts has to take into account scarcity of sensitive elements especially noble metals. When decreasing metal loading over a support to address this scarcity, new types of catalytic sites (single atoms, subnanometric clusters) can be stabilized [1]. Such highly dispersed sites have to be more efficient as their number is limited at low loading. Furthermore, the interface between such highly dispersed sites and the support plays a key role since it determines their thermodynamic stability for a given atmosphere as well as their electronic and catalytic properties. We have recently demonstrated the dynamic nature of Pt NPs supported on ceria under reducing/oxidizing sequences at temperatures below 500 °C [2]. This concept of dynamic catalysts can be used to tailor and stabilize more efficient catalytic sites by applying appropriate treatments.This work aims to get new insights into the impact of redox sequences on the Pt/CeO2 interactions for tailoring DOC catalysts. A Pt/CeO2 catalyst containing single-atom Pt was prepared and exposed to model redox sequences in H2/O2 at mild temperatures. The catalyst was reduced either at 250 °C or 500 °C to get different kinetics of the dynamic Pt NPs construction. Then, it has been re-oxidized at three different temperatures (RT, 250 °C, 500 °C) to modulate the redispersion rate. Combined characterization techniques were implemented such as HAADF STEM microscopy, in-situ Raman spectroscopy, in-situ XRD, XPS, CO-FTIR and H2-TPR to deeply characterize the catalyst after redox sequences, in connection with its catalytic activity for CO, NO and propene oxidation in a simulated lean exhaust containing water.Stabilization of raft-like Pt clusters of ca 1 nm diameter in strong interaction with CeO2 was evidenced after redox sequences at 250 °C without Pt incorporation into the CeO2 bulk. These particular sites appeared an intermediate state between single atoms and 3D particles with suitable properties for CO adsorption. They were essentially observed after the 250 °C/RT sequence for which CO was converted at low temperature. Furthermore, the 250 °C/RT sequence was shown to strongly improve CeO2 reducibility and to stabilize new sites containing Pt2+, Ce4+ cations and peroxo species that could be located at the Pt raft/CeO2 and involved in the reaction mechanism. Finally, a new methodology using in situ Raman mapping showed that dynamics between Pt single atoms and 3D particles take place at the microscale, i.e. at the powder grain scale. [1] J. Jones, H. Xiong, A.T. DeLaRiva, E.J. Peterson, H. Pham, S.R. Challa, G. Qi, S. Oh, M.H. Wiebenga, X.I. Pereira Hernández, Y. Wang, A.K. Datye, Science, 2016, 353, 6295.[2] A.M. Gänzler, M., Casapu, P. Vernoux, S. Loridant, F.J.C.S. Aires, T. Epicier, B. Betz, R. Hoyer, J.-D. Grunwaldt, Angew. Chem. 56 (2017) 13078-13082.

RX+ECI2D+JMI:YAI:FBS; National audience; Magnetite samples have been synthesized and studied as cycling materials in a chemical loop process for H2 production from ethanol and water. Surface and structural changes during the process were characterized by various techniques such as X-ray diffraction, XPS, and Mössbauer spectroscopy to evidence the cycling process and understand the material deactivation.

RX+ECI2D+OVO:FBS:JMI; International audience; Several M-modified iron oxides of the spinel family have proven to be effective electron and O2- vectors for the production of hydrogen in the chemical-loop reforming of bio-alcohols. The present work is specifically focused on investigation of ethanol anaerobic decomposition over spinel oxides, which results in significant structural changes of the oxygen carrier material itself and corresponds to a first step of the chemicalloop reforming process. In particular, a series of binary/ternary M-modified ferrospinels were prepared by a co-precipitation method and tested in terms of both redox properties and intrinsic catalytic activity in addition to a complex ex situ study that encompasses the solid-state chemistry investigations of the fresh and reduced oxygen carrier materials. It was found that Co/Cu incorporation facilitates total/partial oxidation of ethanol, giving rise to high yields of H-2, COx, and H2O; whereas Mn incorporation predominantly favored dehydrogenation and condensation reactions, leading to the formation of acetaldehyde and acetone. In addition, the incorporation of Mn contributed to significantly reduce the amount of coke formed; however, it caused a lower intrinsic reducibility, which was explained by the formation of a thermodynamically stable and hardly reducible layer of MnxFeyO solid solution.

RX+CARE:ECI2D+GFE:AWS:FBS:SLO:CGE:PVE; International audience; 1. IntroductionEnvironmental regulations on NOx emissions of mobile sources are becoming increasingly demanding. The NOx storage-reduction (NSR) catalytic technology is one of the solution lean-burn engines. NSR catalysts work in cyclic gas-composition conditions. During the first step (lean phase), NSR materials store emitted NOx as nitrates until the surface reaches the saturation threshold. A pulse of fuel post-injection then triggers the short second step (rich phase), during which the reducing conditions lead to nitrate decomposition and release as well as subsequent NOx reduction into N2 and surface regeneration. Ceria is an interesting material since it plays a double function first as a high specific surface area support [1] for platinum group metal (PGM) dispersion and secondly as a NOx trap at low temperature until 350°C. Below 200°C, the NOx storage capacity of ceria is higher than that of barium which is commonly used as a storing agent [2]. In addition, ceria can limit the thermal sintering of Pt nanoparticles [3]. This study aims to deeply characterize the impact of the nature of dopant (Zr and Pr) on the redox properties of ceria-based oxides.2. Experimental/methodologyDifferent compositions of commercial ceria-based oxides were provided by the Solvay Special Chem Company : CeO2, Ce0.49Zr0.51O2 (CZ)) and Ce0,80Pr0,20O2 (CP20). All samples have been calcined at 800°C for 2 h. In-situ X-Ray Diffraction measurements were carried out in an atmosphere-controlled Anton Paar XRK 900 reactor chamber, either under air, N2 or H2, using a Versatile Panalytical X’Pert Pro MPD Diffractometer equipped with a diffracted beam graphite monochromator (Cu Kα radiation) and a 1-dimensional multistrip detector (X’Celerator). Diffractograms were collected at several temperatures from 25 to 750°C and crystallographic parameters were determined by the Rietveld method. Surface and bulk reducibility was characterized by Temperature-Programmed Reduction (TPR) in H2 from room temperature up to 850°C. Oxygen-Temperature-Programmed Desorption (TPD) experiments were also performed to quantify the surface reactivity of the different oxides toward oxygen. Oxygen was adsorbed at 500°C while the desorption was followed in He from RT up to 800°C with an heating ramp of 10°C/min. In-situ Raman spectroscopy was used to determine structural/electronic defects and surface oxygen species of the samples after reducing/oxidizing treatments. Spectra were recorded with a LabRam HR Raman spectrometer (Horiba-Jobin Yvon) at low temperature after oxidation or reduction at 525°C. Three exciting wavelengths were used (514, 633 and 785 nm) and the laser spot was focused using a ×50 long working distance objective. The in-situ Raman spectra were recorded using a THMS600 cell (Linkam) between -196°C and 525°C under 10 % H2-N2, N2 and 10% O2-N2.3. Results and discussionXRD patterns have evidenced the cubic structure (Fm-3m) for CeO2 and CP20 while CZ alsocontains a t’ tetragonal phase. The lattice parameters were extracted from in-situ patterns inair, N2 and H2 as a function of the temperature. The increase of the lattice parameter withrespect to the one measured in air at a given temperature was used to estimate the bulkreducibility. Figure 1a clearly shows that CP20 is reduced in pure H2 from 300°C, instead of400°C for CZ and above 700°C for CeO2. These results are in good agreement with TPRexperiments which evidenced that the surface reduction of doped ceria is promoted by theinsertion of Pr. By the same way during O2-TPD experiments, a low O2 desorption peakbelow 400°C was only observed on CP20. In addition, the capacity of the ceria oxide tochemisorb oxygen was strongly improved in the presence of Pr.Beyond confirming the XRD structural characterization, Raman mappings evidenced a highhomogeneity at the micrometer scale. Furthermore, structural defects and oxygen specieswere observed with this technique. In particular, Raman spectra of CP20 oxide highlighted adefects band at 570 cm-1 (Figure 1B) which can be explained by a resonance effect involvinga particular defect stabilized at the oxidized state. The reduction of cations contained in suchdefect modifies its electronic properties inhibiting the resonance effect. Concerning CZsupport, a typical band of Ce3+ was visible after reduction whereas peroxo species wereobserved after re-oxydation contrarily to the other solids.Figure 1. (A) Variation of the fluorite lattice parameter as a function of temperature in pure H2 (B) Evidence of a newdefects band enhanced by Raman Resonance effect for CP20.4. ConclusionStructure, surface and bulk properties of three different ceria oxides have been deeplyinvestigated in both oxidized and reduced states. The role of the dopants, i.e. Pr and Zr, onthe redox properties in cycling lean/rich conditions encountered in NSR processes will bediscussed.5. References[1] Z. Say, E.I. Vovk, V.I. Bukhtiyarov, E. Ozensoy, Appl. Catal. B Environ. 142-143 (2013) 89–100.[2] E. Rohart, V. Bellière-Baca, K. Yokota, V. Harlé, C. Pitois, Top. Catal. 42-43 (2007) 71–75.[3] Y. Nagai, T. Hirabayashi, K. Dohmae, N. Takagi, T. Minami, H. Shinjoh, S. Matsumoto, J. Catal. 242 (2006) 103–109.

RX+CARE:ECI2D+GFE:AWS:FBS:SLO:CGE:PVE; International audience; Environmental regulations on NOx emissions of mobile sources are becoming increasingly demanding. The NOx storage-reduction (NSR) catalytic technology is one of the solution lean-burn engines. NSR catalysts work in cyclic gas-composition conditions. During the first step (lean phase), NSR materials store emitted NOx as nitrates until the surface reaches the saturation threshold. A pulse of fuel post-injection then triggers the short second step (rich phase), during which the reducing conditions lead to nitrate decomposition and release as well as subsequent NOx reduction into N2 and surface regeneration. Ceria is an interesting material since it plays a double function first as a high specific surface area support1 for platinum group metal (PGM) dispersion and secondly as a NOx trap at low temperature until 350°C. Below 200°C, the NOx storage capacity of ceria is higher than that of barium which is commonly used as a storing agent2. In addition, ceria can limit the thermal sintering of Pt nanoparticles3. This study aims to deeply characterize the impact of the nature of dopant (Zr and Pr) on the redox properties of ceria-based oxides.

International audience; This article addresses the main results of the PEPS (PEllet Photonic Sensor) project, whcich aims at developing a new gas sensing transducer via a technological breakthrough: the combination of photonics (insensitive to external electromagnetic disturbances) and catalysis (reversibility, limited energy consumption). Indeed, catalytic reactions are often exothermic and this heat can modify the properties of optical devices. The experimental studies performed during the PEPS project highlighted the rapid and reversible response at room temperature of catalytic powders towards different concentrations of H2 in air. The thermal and optical properties of the materials used for the integrated photonic component have also been studied. These results have been exploited for the design of the photonic transducers. The feasibility of the physical transduction principle has been demonstrated by developing different prototypes based on Bragg gratings and Multimode Interference Components.

RX+CARE+MTS:CZA:ABO:FBS:PVE; International audience; La0.6Sr0.4BO3 type-perovskites (with B = Fe, Mn or Ti) were synthesized by a complex route from the thermal decomposition of the chelated nitrate precursors, as soot oxidation catalysts under low oxygen partial pressures. The nature of the B-site cation modifies the morphology, the structural symmetry, the charge compensation mechanism and the redox properties. The generation of oxygen vacancies is predominant in LSF while an increase in the oxidation state of B cations occurs in LSM and LST, respectively. A direct link was established between the reducibility of the perovskites and their ability to oxidize soot. Fe and Mn-based perovskites exhibit similar catalytic behaviors, superior to LET, which are attributed to the presence of oxygen vacancies in LSF and Mn4+ cations in ISM. This latter redox property is particularly suitable to oxidize soot at low oxygen partial pressures, as encountered in gasoline direct injection engine exhausts. (C) 2014 Elsevier B.V. All rights reserved.

Magnetite samples were synthesized and studied as the cycling material of a chemical loop process for hydrogen production from ethanol and water used as reducing and oxidizing species, respectively. Surface and structural changes during the process were characterized by various techniques such as X-ray diffraction, X-ray photoelectron, and Mossbauer spectroscopy in order to evidence the real cycling process and understand the cause of the material deactivation so that it can be suppressed or minimized for an industrial application. We found that the complete recovery of the initial cycling material was possible, but that a slow accumulation of coke took place over time under cycling conditions. Indeed, this deposited coke corresponds to only a part of the coke formed, since water makes partial re-oxidation possible. A third step to burn the coke left over by the air will thus have to be periodically added for a sustainable industrial process, unless a cycling material and/or certain conditions capable of either totally preventing the formation of coke or leading to the formation of coke that is not oxidized by water are found.

MICROSCOPIE:RX+ECI2D:ING+FMO:MAO:FBS:JRO:LPI; International audience; PGM-CeO2 (PGM = Pt, Pd, Ir, Rh, Ru) catalysts were prepared by one-step solution combustion synthesis (SCS) and characterized by elemental analysis, N-2 volumetry, aberration-corrected HRTEM, X-ray diffraction, and CO-DRIFTS. The samples, consisting of 2-6 nm metal nanoparticles supported on mesoporous ceria, were tested in CO oxidation in absence and presence of hydrogen (PROX). To our knowledge, this work presents the first comparison of all the platinum-group metals (except Os) for these reactions in the same conditions. The as-prepared SCS catalysts are active in CO oxidation and a reducing treatment has no significant effect on their performances. While the best catalyst in H-2-free CO oxidation is Rh-CeO2, the addition of a high hydrogen excess decreases the Rh catalyst activity but enhances the CO oxidation rate on all other systems, including alumina-supported metals employed as reference catalysts. The resulting PROX turnover frequencies (Pt > Pd > Rh > Ir > Ru) follow the trends predicted by published density-functional-theory calculations considering the COad + O-ad elementary reaction as the rate-determining step. Pt-CeO2 is not only the most active but also the most selective catalyst, reaching near 100% CeO2 at low temperature (ca. 100 degrees C). The alumina-supported catalysts appeared less active than their ceria-supported counterparts in both reactions. The effect of heating/cooling cycles on the reaction kinetics was also investigated. Whereas the Pt, Pd and Ir ceria-supported catalysts were stable throughout PROX cycles, Rh and Ru ones exhibited apparently chaotic behaviors above ca. 200 degrees C, which are proposed to be induced by favorable CO dissociation and methanation pathways and/or variable metal oxidation states. (C) 2015 Elsevier B.V. All rights reserved.

MICROSCOPIE:RX:XPS+ATARI:ECI2D:ING+GPO:SLO:FBS:LBU:MAO:LMA:PGE:FMO:LPI; International audience; Mesoporous ceria loaded with 0.06-0.93 wt% iridium was synthesized in one step by the ambient air combustion of an aqueous solution of ceric ammonium nitrate, ammonium hexachloroiridate, and glycine fuel. The structural properties of the powders, and the influence of such parameters as Ir loading and thermochemical post-treatments, were investigated combining aberration-corrected HRTEM, SEM, in situ XRD, XPS, DRIFTS, and Raman spectroscopy. The materials, which appeared spongy at the micrometer scale, exhibited ca. 30 nm-sized ceria crystallites with a layered structure at the nanoscale. After reducing treatments, Ir nanoparticles anchored at the surface of ceria grains were identified, and their size (ca. 2 nm) did not evolve upon further heating at up to 900 degrees C. A detailed picture of the Ir-CeO2 interface could be established, with the presence of Irx(+)-O2--Ce3+ entities along with oxygen vacancies. The powders loaded with only 0.1 wt% Ir were successfully employed as catalysts for the production of hydrogen from methane and water in low-steam conditions at 750 degrees C. Due to their higher Ir dispersion and stronger Ir-CeO2 interaction, the combustion-synthesized materials outperformed their conventionally prepared counterparts in terms of activity and stability, making them promising as active catalytic layers for solid-oxide fuel cells integrating the gradual internal reforming concept.

Seed-assisted solution synthesis of hollow IF-MoS2 nanoparticles allows independent control of particles size and MoS2 slabs crystallinity. Variations of the reaction mixture composition influence the particle size in the range 50–150 nm. As demonstrated by Rietvelt refinement of the X-ray diffraction patterns, the sulfide crystallinity depends only on the post-treatment temperature (350–750 °C) and not on the particle size. The tribological properties of new MoS2 nanoparticles prepared by seed-assisted solution technique were investigated and showed a strong decrease in the friction coefficient and wear compared with base oil. Small particles of 50–60-nm size showed the best results. The particle size above 100 nm is deleterious for the lubrication properties since it hinders particles penetration into the contact zone. MoS2 slabs crystallinity had lesser influence on the lubrication efficiency. However, less-crystallized samples treated at 350 °C showed better lubrication, apparently because of easier exfoliation of the individual MoS2 slabs, leading to more efficient formation of tribofilm.