Chorkendorff, Ib

Dahl, S.

Helveg, S.

Johnson, E.

Molenbroeck, A. M.

Simonsen, Søren Bredmose

Online Research Database In Technology

                       General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.  • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal   If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.     Downloaded from orbit.dtu.dk on: Dec 20, 2017Catalytic Soot Oxidation studied by ETEMSimonsen, Søren Bredmose; Dahl, S.; Johnson, E.; Molenbroeck, A. M.; Chorkendorff, Ib; Helveg, S.Publication date:2009Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Simonsen, S. B., Dahl, S., Johnson, E., Molenbroeck, A. M., Chorkendorff, I., & Helveg, S. (2009). Catalytic SootOxidation studied by ETEM. Abstract from 9th European Congress on Catalysis, Salamanca, Spain.  Catalytic Soot Oxidation studied by ETEM  S.B. Simonsen1*, S. Dahl1 , E. Johnson2, A.M. Molenbroek1, I. Chorkendorff3 and S. Helveg1 1Haldor Topsøe A/S,Nymøllevej 55, DK-2800 Kgs. Lyngby, (Denmark) 2Nano-Science Center, University of Copenhagen, DK-2100 Copenhagen (Denmark) 3 CINF, Technical University of Denmark, DK-2800 Kgs. Lyngby (Denmark) *sbrs @topsoe.dk  Introduction Focus on challenges in diesel emission exhaust reduction (e.g. catalyst sintering and removal of soot) is increasing. Here environmental transmission electron microscopy (ETEM) is used for monitoring such processes with focus on soot oxidation.  An attractive approach for effective soot removal includes the introduction of filters on the diesel-driven vehicles and the fictionalization of the filters for catalytic soot oxidation [1,2]. Ceria-based materials are widely adopted for this purpose. It is generally accepted that the redox properties of ceria are crucial to the catalytic effect, but the detailed reaction mechanism and the location of the catalytic active sites are still being debated.  For instance, it has been proposed that the reaction occurs at the soot-ceria interface [3] and that the reaction occurs through spill-over of oxygen from ceria to reaction centers located at the soot surfaces [2-4].  Averaging techniques, such as temperature-programmed oxidation and thermo-gravimetric analysis, have mainly been used to study the soot oxidation reaction, e.g. [2-5]. Although these studies provide significant insight, previous work on metal-catalyzed gasification of graphite suggests that ETEM may be a beneficial complement, because the technique allows direct observations at the carbon-catalyst interface to be obtained in situ during reaction [6]. In this contribution we present an ETEM study of the reaction mechanisms and kinetics involved in ceria-catalyzed soot oxidation related to diesel engine emission control [7].   Materials and Methods ETEM has become a powerful tool in heterogeneous catalysis due to its ability to directly monitor catalysts in situ during exposure to reactive gases at elevated temperatures. The present experiments were performed using a CM300 FEG ETEM [8], or a Titan 80-300 image aberration-corrected ETEM, from FEI Company. To model soot deposited in a diesel particulate filter, a carbon black (CB) powder of particles with a diameter of about 30nm (Printex U, Degussa GmbH) was mechanically mixed with a CeO2 catalyst powder (or with an α-Al2O3 reference powder) with a volume ratio 1:10.  In the ETEM, the CB-oxide samples were exposed to ca. 2 mbar O2 and a temperature in the interval 300-600°C. The samples were monitored by acquisition of time-lapsed ETEM image series (movies) using a low-light and fast-scan CCD camera. The electron beam current density was kept below 0.16A/cm2, which was sufficiently low to avoid influence of the electron beam on the CB oxidation kinetics.    Results and Discussion The ETEM movies provide direct observations of soot particles in contact with catalytically active CeO2 (or inert Al2O3) during exposure to an oxidizing gas environment. The results show that the catalytic oxidation reaction involves processes confined to the soot-CeO2 interface region, and that the catalytic reaction surprisingly results in motion of soot agglomerates toward the catalyst surface (fig. 1), which acts to continuously re-establish the soot-CeO2 interface in the course of the oxidation process.           The observed reaction dynamics is found to consistently explain observations from ex situ oxidation experiments. Moreover, a quantitative analysis of ETEM movies obtained at different reaction temperatures demonstrates that the apparent activation energy of CeO2-catalyzed soot oxidation can be measured with ETEM for a more well-defined tight physical contact situation and is quantitatively in good agreement with previous kinetic studies, e.g. [3].    Significance         The first ETEM observations of the ceria-catalyzed soot oxidation reveal that the reaction mechanism involves reaction centers located near the soot-ceria interface.   Figure 1: A time-lapsed ETEM image series of soot in contact with a CeO2 catalyst acquired during the exposure to 2 mbar O2 at 550ºC. The time interval between the images is ~2min. Scale bar, 90nm. The figure is adapted from [7].   References  [1] van Setten, B.A.A.L., Makkee, and M., Moulijn., J.A., Catal. Rev. 43 (2001), 489  [2] Konstandopoulos, A.G., Papaionannou, E., Zarvalis, D.,  Skopa, S., Baltzopoulou, P., Kladopoulou, E., and Kostoglou, M., SAE Tech. Paper no. 2005-01-0670 (2005). [3] Dernaika, B., and Uner, D.A., Appl. Catal. B 40 (2003), 219 [4 ]Krishna, K., Bueno-López, Makkee, M., and Moulijn, M., Appl. Catal. B 75 (2007), 189 [5] Darcy, P., Costa, P., da Mellottee, H., Trichard, J.M., and Djega-Mariadasou, G., Catal. Today 119 (2007), 52 [6] Baker, R.T.K., France, J. A., Rouse, L., and Waite, R.J., J. Catal. 41 (1976), 22 [7] Simonsen, S.B., Dahl, S., Johnson, E., and Helveg, S., J. Catal. 255 (2008), 1 [8] Hansen, P.L., Helveg, S., and Datye, A.K., Adv. Catal. 50 (2006), 77  a b c soot CeO2 

Catalytic Soot Oxidation studied by ETEM

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Catalytic Soot Oxidation studied by ETEM

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