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MICROSCOPIE+ATARI:CARE:ING+FCA:MAO:FRM:LMA:PGE:PVE; International audience; Ceria is a rather important material for applications in catalysis both as a catalyst and as a catalyst support. Although widely studied by TEM (Transmission Electron Microscopy) and despite an exhaustive literature on the structure of reactive facets of CeO2 correlated to its catalytic mechanisms, the temporal evolution of the atomic surface structure exposed to realistic redox conditions together with its possible influence on the supported metallic phases remains elusive. In order to contribute to a better understanding of the latter we present several studies performed within realistic gas environments in a dedicated FEI Titan 80-300 aberration-corrected environmental TEM. Firstly, we provide a direct visualization and quantification of the cationic mobility on (100) surfaces of CeO2 nanocubes in different controlled environments and achieve control of the surface dynamics under exposure to realistic gas atmospheres. In a second step we present the morphological, structural and chemical changes on metallic catalysts supported on ceria during redox cycles through two examples: (i) Ir/CeO2 used in the steam reforming of methane; we observed changes in size and morphology of Ir nanoparticles associated with modification of its catalytic behavior that could be associated with the existence of a metastable iridium sesquioxide phase never observed before. (ii) Pt/CeO2 used as a diesel oxidation catalyst; we evidenced a dynamic structural behavior of Pt nanoparticles on the ceria surface under reducing/oxidizing conditions at moderate temperatures where redispersion occurs in oxidizing atmospheres whereas Pt nanoparticles reform under reducing conditions leading to a protocol to control Pt particle formation.

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.

MICROSCOPIE+ATARI:CARE:ECI2D+GFE:MAO:FCA:CGE:SLO:PVE; International audience; The shape, size and electronic properties of Pt nanoparticles supported on ceria have been tailored during model redox sequences based on successive H2 (reduction) and O2 (oxidation) 1 hour treatments at moderate temperatures (250 °C and 500 °C). Raft shaped Pt nanoparticles of around 1 nm size are produced on the surface, in strong interaction with ceria, then boosting the oxygen transfer from the oxide towards Pt. In situ techniques establish the presence of Pt cations in an intermediate oxidation state, probably Pt2+, in interaction with peroxo oxygen species.

MICROSCOPIE+ATARI:CARE:ECI2D+GFE:MAO:FCA:CGE:SLO:PVE; International audience; 1.IntroductionThe tailoring of Pt based diesel oxidation catalysts is crucial to enhance the efficiency of the diesel vehicles after treatment. For instance, specific interactions between Pt and ceria can improve the thermal stability of Pt nanoparticles (NPs) in an oxidizing atmosphere as in diesel exhausts, through the formation of rigid Pt-O-Ce bonds.1 More recently, Jones et al.2 have evidenced stabilization of Pt single atoms and clusters on ceria surface steps, strongly suggesting the doping of ceria subsurface by Pt cations in good agreement with DFT studies3 which also predict Pt2+ cations incorporation between surface oxygen ions species of the support. We have recently demonstrated the dynamic nature of Pt NPs on ceria under reducing/oxidizing sequences at temperatures below 500 °C.4 Lean/rich pulses can be used to control the formation of Pt NPs and enhance their catalytic activity for CO oxidation.4 This study aims at getting new insights into the impact of redox sequences on the Pt and CeO2 interactions.2.Experimental/methodsCeria provided by Solvay Special Chem. Company was dry impregnated with Pt (0.9 wt %) and calcined 4 h at 500 °C. Before each characterization or catalytic test, the catalyst was systematically pretreated in O2 for 1 h at 500 °C to get a reference state. Model redox sequences were composed of a first reduction step of 1 h in 10% H2 followed by an oxidation one also of 1 h in 20% oxygen. They were performed at different temperatures: 500/500 °C (reduction/oxidation), 250/250 °C and 250 °C/RT. Catalytic performances were measured for CO and propylene oxidation in a lean mixture (10% O2, 1000 ppm CO, 500 ppm NO, 500 ppm C3H6, 10% H2O in He) that simulates diesel exhaust gas. Temperature-Programmed Reduction (TPR) experiments were conducted with a Inficon JPC400 mass spectrometer. STEM observations were performed with an Environmental Transmission Electron Microscope (Ly-ETEM, FEI TITAN ETEM operated at 300 kV). The spatial distribution of surface oxygen electrophilic species and PtOx clusters were assessed by Raman spectroscopy mapping at the micrometric level (200 µm x 200 µm) recorded at RT under oxygen flow. HRXANES was also implemented to in situ follow the oxidation degree of Ce cations in the catalyst.3.Results and discussionAs previously observed with rich/lean pulses,4 model redox sequences can achieve outstanding catalytic performances for CO and C3H6 oxidation, much higher than those recorded at the reference state. TPR experiments have shown that model redox sequences increase the reducibility of ceria. The H2 consumption only starts above 150 °C on the first TPR, suggesting the absence of Pt NPs able to chemisorb H2 below this temperature, as confirmed by STEM observations. Successive redox sequences (TPR 2 and TPR 3) promote the formation of Pt NPs in strong interaction with ceria as shown by the excellent reducibility of the support as early as the room temperature. The lower the temperature of the redox sequences, the greater the reducibility. Variations of the Raman band associated with PtOx at 690 cm-1 demonstrate that such species evolve upon redox sequences with stabilization of Pt cations in another oxidation state, probably Pt2+, as suggested by XPS measurements and DFT calculations.3 These Pt2+ cations are in interaction with peroxo oxygen species (Raman bands at 860 cm-1). No Ce3+ cations were detected both by electronic Raman spectroscopy and HR-XANES measurements, confirming that these oxygen peroxo species did not re-oxidized ceria but are stabilized on the surface.The shape of the Pt NPs was followed along the redox sequences. After the first reduction step, only raft shaped particles or single atoms are observed on the surface (Fig. 1c). This morphology is not modified after a subsequent oxidation step but the surface mean diameter of Pt rafts slightly increases from 1.0 to 1.3 nm, maybe due to Pt oxidation. Statistics on the ratio between the number of rafts and single atoms were tricky to estimate. However, CO chemisorption performed after each reduction step shows no significant modification of the Pt dispersion along the redox cycles.4.ConclusionsThis work showed that raft shaped Pt nanoparticles in closed interaction with ceria can be stabilized by model redox sequences at mild temperatures. This maximizes the Pt/ceria interface, promoting the oxygen transfer from the support towards Pt which is a key step in the CO oxidation mechanism. Pt cations are stabilized in an intermediate oxidation state, probably Pt2+, in interaction with peroxo oxygen species, confirming the high availability of oxygen species for oxidation reactions.AcknowledgementFrench National Research agency ‘Agence Nationale de la Recherche’ (ANR), project ORCA (ANR-14-CE22-0011-02) and German Federal Ministry for Economic Affairs and Energy (BMWi) are acknowledged for their financial support. The authors thank Solvay Special Chem Company for material contribution and the CLYM for access to the Ly-EtTEM.References[1]Y. Nagai, K. Dohmae, Y. Ikeda, N. Takagi, T. Tanabe, N. Hara, G. Guilera, S. Pascarelli, M.A. Newton, O. Kuno, H. Jiang, H. Shinjoh, S. Matsumoto, Angew. Chem., 2008, 47, 9303-9306.[2]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.[3]A. Bruix, Y. Lykhach, I. Matolínová, Armin Neitzel, T. Skála, N. Tsud, M. Vorokhta, V. Stetsovych, K. Ševčiková, J. Mysliveček, R. Fiala, M. Václavů, K.C. Prince, S. Bruyère, V. Potin, F. Illas, V. Matolin, J. Libuda, K.M. Neyman, Angew. Chem., 2014, 53, 10525-10530.[4]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., 2017, 56, 13078-13082.