Investigations of soot combustion on Yttria-Stabilized Zirconia by Environmental Transmission Electron Microscopy (ETEM)
Résumé
Investigations of soot combustion on Yttria-Stabilized Zirconia by Environmental Transmission Electron Microscopy (ETEM) M. Aouine1, T. Epicier1,2, E. Obeid1, M. Tsampas1, A. Serve1, K. Pajot3, P. Vernoux1* 1Université de Lyon, Institut de Recherches sur la Catalyse et lEnvironnement de Lyon, UMR 5256, CNRS, Université C. Bernard Lyon 1, 2 avenue A. Einstein, 69626 Villeurbanne, France 2Université de Lyon, MATEIS, UMR 5510, CNRS, INSA de Lyon, 69621 Villeurbanne Cedex, France 3PSA PEUGEOT CITROËN, Centre technique de Vélizy, Route de Gisy 78943 Vélizy-Villacoublay, France *corresponding author:philippe.vernoux@ircelyon.univ-lyon1.fr Introduction Diesel Particulate Filters (DPFs) equip all Diesel light vehicles in Europe to remove toxic Particulate Matter (PM). They present high filtering efficiency (>99%) but must be periodically regenerated due to soot particles accumulation. In addition, from 2014, EURO 6 standards require the utilization of a NOx catalytic after-treatment device [1], preferentially placed between the Diesel Oxidation Catalyst (DOC) and the DPF [1]. Therefore, NO2 cannot anymore be utilized as an oxidant for soot combustion. This makes crucial the development of effective and durable catalysts for soot combustion with oxygen. We have recently reported that Yttria-stabilized Zirconia (YSZ), a pure oxygen ion ceramic conductor without any redox property, can continuously oxidize soot with oxygen in Diesel exhaust conditions [2]. It was proposed that the ignition of the soot oxidation on YSZ involves a fuel-cell-type electrochemical mechanism at the nanometric scale. Electrochemical oxidation of the soot could occur at the soot/YSZ interface while oxygen electrochemical reaction takes place at the triple phase boundary (tpb) soot/gas/YSZ. To get further insights of this mechanism, Environmental Transmission Electron Microscopy (ETEM) was used to in-situ follow the soot combustion at the interface with YSZ. Materials and Methods In situ, environmental experiments were performed in the Ly-EtTEM microscope, a last generation ETEM (TITAN 80-300 kV from FEITM) equipped with an aberration corrector. Pure oxygen was introduced owing to the high precision leaking valves at low pressure, up to typically 3 mbar. Samples prepared as a mixture of soot and YSZ particles (as described below) were deposited on titania grids with and without supporting film. These grids were subsequently mounted on a GatanTM heating stage in inconel; the temperature was raised up to typically 550°C. The microscope was operated at 80 and 300 kV. Soot was obtained from a mini Combustion Aerosol Standard (miniCAST, Jing Ltd. Switzerland) soot generator. The soot particles were produced with a mini-CAST burner from a propane/air flame. This mini-CAST soot shows an EC (Elemental Carbon) /TC (Total Carbon) ratio of ~0.95, closed to that reported in the literature for real Diesel soots [3]. Yttria-stabilized Zirconia (YSZ) powder, containing 8 mole % of Yttria from TOSOH, (ZrO2)0.92(Y2O3)0.08, was used as purchased. Collected soot and YSZ powder were mixed with a weight ratio of 1:4 and crushed for 20 minutes in a mortar in order to improve the soot/YSZ agglomerate contact, conventionally denoted as tight contact mode. Significance These in-situ ETEM observations confirm that bulk YSZ oxygens are the active species for soot oxidation at the soot/YSZ interface. Acknowledgements We acknowledge the CLYM (Centre Lyonnais de Microscopie, www.clym.fr) for its guidance in the Ly-EtTEM project which was financially supported by the CNRS, the Région Rhône-Alpes, the GrandLyon and the French Ministry of Research and Higher Education. The authors are grateful to the ANR (Agence National de la Recherche) for funding PIREP2 project (N° ANR-2010-VPTT 006-01) and ADEME (Agence de lEnvironnement et de la Maîtrise de lEnergie) for the PhD grants of E. Obeid and A. Serve. References 1B.A.A.L. van Setten, M. Makkee, J.A. Moulijn, Catalysis Reviews 2001, 43 (4), 489. 2.E. Obeid, L. Lizarraga, M.N. Tsampas, A. Cordier, A. Boréave, M.C. Steil, P. Vernoux, Journal of Catalysis 2014, 309, 87. 3.T. Ferge, E. Karg, A. Schroppel, K.R. Coffee, H.J. Tobias, M. Frank, E.E Gard, R. Zimmermann, Environmental Science & Technology 2006, 40 (10), 3327.