A knowledge-based design of catalysts requires the determination of so-called structure-activity-relationships. Such correlations between the catalytic activity and its properties on the microscopic and macroscopic scales help to identify new routes for improving state-of-the-art and to develop new generations of catalytic systems. However, in particular at the microscopic level, certain properties of the catalyst, like for example the oxidation state and the morphology of noble metal nanoparticles, can be influenced by the temperature and the atmosphere [1, 2]. As a consequence, it is mandatory to obtain such information under working conditions (“operando”).

This accounts in particular for automotive exhaust gas catalysts [3-5]. In the present study, ceria supported catalysts were investigated for application as diesel oxidation catalysts. In contrast to catalysts not supported on ceria, their activity strongly depended on the pre-conditioning. As previously observed, the activity could be significantly enhanced with respect to low temperature CO and hydrocarbon oxidation, by short reducing pulses (lean/rich-treatment) at moderate temperatures [6]. Since such treatments could be exploited in a car from the technological point of view it is of high interest to understand the origin and the mechanism of this activation step. Hence, a comprehensive approach was applied appreciating especially the importance of a catalyst characterization in relevant conditions (pressure and temperature). In this regard in situ environmental transmission electron microscopy (ETEM) was applied together with operando quick-scanning X-ray absorption spectroscopy (QEXAFS).
According to the QEXAFS data obtained on Pt/CeO₂/Al₂O₃ the low temperature activity of the catalyst was found to be improved (Figure 1), after the Pt oxidation state got reduced  effectively during a lean/rich treatment (Figure 2), e.g. by using CO as reductant. After activation, however, the catalyst got deactivated again during the subsequent light-off experiment, if treated at too high temperature, accompanied by Pt oxidation. Complementary  ETEM studies offered a closer look on the processes on the catalyst surface, e.g. of a reductively treated Pt/CeO₂ catalyst in an oxidizing atmosphere at elevated temperatures (10 mbar O₂, 400 °C). Prominent structural changes were found depending on the reaction atmosphere. Upon treatment similar to those where XAS evidenced deactivation of the catalyst and Pt oxidation, the disappearance of most of the smallest Pt particles was observed (Figure 3) and a lower number of larger particles were found to remain. The disapperance of small Pt particles could represent a possible deactivation pathway due to redispersion of Pt atoms at elevated temperatures. Hence, ETEM experiments confirmed the strong structural dynamics at the microscopic level of the catalyst, which probably represent the origin of the catalysts activation and deactivation and will be further exploited in future.

References
[1] Weckhuysen B M, In situ Spectroscopy of Catalysts, American Scientific Publishers (2004) 255-270
[2] Grunwaldt J-D, Wagner J B and Dunin-Borkowski R E, ChemCatChem 5 (2013) 62-80
[3] Deutschmann O and Grunwaldt J-D, Chem. Ing.Tech. 85 (2013) 595-617
[4] Doronkin D E, Casapu M, Günter T, Müller O, Frahm R and Grunwaldt J-D, J.Phys.Chem.C. 118 (2014) 10204-10212
[5] Gänzler A M, Casapu M, Boubnov A, Müller O, Conrad S, Lichtenberg H, Frahm R and Grunwaldt J-D, J. Catal. 328 (2015) 216-224
[6] Hoyer R, Schuler A, Franoschek S, Pauly T R and Jeske G, WO 2013/149881 A1, October 22, 2014

[7] The authors thank the BMBF-ANR project ORCA (Oxidation-Reduction-Catalyst, BMBF-19U15014B, ANR-14-CE22-0011-02) for financial support, the synchrotron beamlines SuperXAS (SLS, Villigen), ROCK (SOLEIL, Paris) and XAS (ANKA, Karlsruhe) for beamtime and the CLYM for access to the Ly-EtTEM.

Figures:

Figure 1: Light-off experiment after: lean conditioning at 500°C (a) and lean/rich conditioning at 250°C using C3H6 (b), H2 (c) and CO (d).

Figure 2: Respective XANES-spectra before the light-off experiment after: lean conditioning at 500°C and lean/rich conditioning at 250°C using C3H6, H2 and CO.

Figure 3: ETEM images of prereduced Pt/CeO2 at 400°C in high vacuum (left) and in 10mbar O2 30 min after adding gas.

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EMC Abstracts - https://emc-proceedings.com/abstract/using-in-situ-environmental-transmission-microscopy-and-operando-x-ray-absorption-spectroscopy-to-investigate-ceria-based-diesel-oxidation-catalysts/