In-situ characterizations of Pt/CeO2 during lean/rich sequences.
Résumé
European environmental regulations on NOx emissions of mobile sources are becoming more and more severe. NOxTrap catalyst is one of the solution to reduce NOx into N2 under lean-burn conditions. NOxTrap catalysts are commonly composed of Pt for its oxidative properties during lean phases; Rh to reduce NOx during rich phases; an alkaline compound as barium oxide and an oxide support. Ceria offers a high specific surface area support for metal dispersion due to the strong interaction between Pt and CeO2[1]. At low temperature (below 200 °C) its NOx storing capacity is higher than barium, the commonly used storing agent.[2] This study shows in-situ characterizations used to understand the impact of rich/lean cycle of NOxTrap system on the oxidative function of a model catalyst Pt/CeO2 (1wt% Pt).
Catalytic performances show an important shift to lower temperatures of CO and C3H6 conversion after
1 h of cycles between lean and rich at 250 °C which simulates the NOxTrap process. The origin of this activation was studied by in-situ characterization. Micrometric scale ceria defect band and platinum oxide dispersion was assessed by Raman mapping. Platinum oxides redispersion and increase of lattice defect in ceria were both highlighted using in-situ redox cycle. In-situ observations during reductive and oxidant periods at nanometric scale were assessed by a FEI TITAN ETEM G2 80-300 kV. A slight redispersion of Pt NPs was noticed comparing initial reduce state and after oxidation and reduction treatment. The redox activation step at 250 °C seems to optimize the Pt dispersion on the ceria and consecutively increases the catalytic activity during oxidative step. We showed the effectiveness of this process to reduce the efficient temperature of DOC catalyst.
Acknowledgement. French 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 Compagny for material contribution and the CLYM for access to the Ly-EtTEM.
References:
[1] R. Hoyer, A. Schuler, S. Franoschek, T.R. Pauly, G. Jeske, WO 2013/149881 A1, October 22, 2014
[2] E. Rohart, V. Bellière-Baca, K. Yokota, V. Harlé, C. Pitois, Top. Catal. 2007, 42-43, 71–75.