In-situ characterizations of Pt/CeO2 during lean/rich sequences

MICROSCOPIE+ATARI:CARE:ECI2D+GFE:MAO:FCA:CGE:SLO:PVE; International audience; Due to European regulation on car exhausts, NOx emissions must be cut down. NOxTrap system stores NOx during lean phase as nitrates and during short rich phase triggered by pulses of gasoil, nitrates are decomposed and NOx reduced into N2. Our objective is to combine DOC (Diesel Oxidation Catalyst) and NOx storage-reduction in one catalytic system. This study reports the effect of redox rich/lean cycles on the catalytic activity of a model catalyst, Pt/CeO2. This activation step provokes a huge improvement of the catalytic performances for hydrocarbon and CO oxidation. Raman and ETEM experiments indicate a modification of platinum nanoparticles (NPs) dispersion on ceria at both micrometric and nanometric scales.1. ScopeEuropean 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, such as for diesel engines. NOxTrap system works in cyclic gas-composition conditions. During the first step (lean phase), NOxTrap catalyst stores emitted NOx as nitrates until the surface reaches the saturation threshold. Then a pulse of fuel post-injection triggers the short second step (rich phase), during which the reducing conditions lead to nitrates decomposition, release as well as subsequent NOx reduction into N2 and surface regeneration. 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, especially Pt, even at high temperature due to the strong interaction between Pt and CeO2.1 At low temperature (below 200 °C) its NOx storage capacity is higher than that of barium oxide.2 Our objective is to combine the oxidation function (DOC) and the NOx reduction ability into a single catalytic system through a fine NOxTrap catalyst formulation tuning. This study aims to understand the effect of redox rich/lean cycles at 250°C on the catalytic activity of a model catalyst, Pt/CeO2 (1wt% Pt) through in-situ characterizations. 2. Results and discussionCeria provided by Solvay Special Chem. Company was impregnated with platinum (1 wt %) by Umicore and then calcined 2 h at 500 °C. Catalytic performances show an important shift to lower temperatures of CO and C3H¬6 conversion after 1 h of cycles between lean (90 sec, 10% O2, 500 ppm NO, 1000 ppm CO, 500 ppm C3H6, 10 %H2O in He) and rich (30 sec, 2% CO in He) at 250 °C (figure 1) which simulates the NOxTrap process. Similar activation steps were achieved for lean/rich sequences up to 500°C. The origin of this activation was studied by in-situ Raman spectroscopy and Environmental Transmission Electron Microscopy (ETEM, FEI TITAN ETEM G2 80-300 kV and DENS Solution heating sampled holder) after rich/lean sequences at 500°C. Ceria defect band and platinum oxide dispersion was assessed by Raman spectroscopy mapping at the micrometric level (200 µm x 200 µm). Platinum oxides re-dispersion and increase of lattice defect in ceria were both highlighted after lean/rich sequences (Figure 2). ETEM was complementary used to in-situ follow the Pt NPs dispersion on ceria during lean (O2, 10 mBar)/rich (H2, 10 mBar) sequences at 500°C. Several areas of the catalyst were observed to achieve statistical data on the Pt NPs size distribution (Figure 3). Under O2, we have observed the disappearance of most of the small Pt NPs while the consecutive rich sequence evidenced their reappearance but not necessary at the same position. The comparison of the Pt NPs size distribution highlights the impact of lean/rich treatments on the Pt/CeO2 interactions. 3. ConclusionsIn-situ characterizations of a Pt/CeO2 catalyst by using Raman spectroscopy and Environmental TEM have clearly shown that rich/lean sequences can strongly modify the Pt NPs distribution on the support both at the micrometric and nanometric levels. Such a redox sequence seems to optimize the Pt dispersion on the ceria and consecutively improve the catalytic properties.Figure 2 : Comparison of the intensity distributions during mapping of (A) the PtOx band area at 690 cm-1 and (B) the lattice defect band area at 250 cm-1 at initial oxidized state (blue) and after two cycles of reduction ( 10% H2 – 500 °C – 1 h ) and oxidation (20 % O2 – 500 °C – 1 h) (orange). Figure 3 : ETEM pictures of one catalyst area during lean/rich sequences at 500 °C. White arrows indicate relevant NPs modifications.AcknowledgmentFrench 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. R. Hoyer, A. Schuler, S. Franoschek, T.R. Pauly, G. Jeske, WO 2013/149881 A1, October 22, 20142. E. Rohart, V. Bellière-Baca, K. Yokota, V. Harlé, C. Pitois, Top. Catal. 2007, 42-43, 71–75.