Synergistic structural and surface promotion of monometallic (Pt) TWCs: Effectiveness and thermal aging tolerance
Highly effective and thermally stable monometallic (Pt) three way converters (TWCs) with very low Pt loading (0.5 wt%) were developed through synergistic structural and surface promotion by modifying the TWC washcoat with 20 wt% of Ce0.8La0.2O1.9 (structural promoter) and sodium in the range 5–15 wt% (surface promoter). The catalytic performance of the as prepared doubly-promoted Pt(Na)/Al2O3-(Ce0.8La0.2O1.9) catalytic systems, in both “fresh” and “aged” (at 900 °C) states, was studied and compared with that of a commercial bimetallic (Pt/Rh) TWC. The catalytic measurements were performed on coated cordierite monoliths, the whole concept simulating real automotive exhaust conditions at the stoichiometric point. An extensive characterization study, involving thermo-gravimetric analysis (TGA/DTA), scanning and transmission electron microscopy (SEM, TEM), X-ray diffraction (XRD), N2 porosimetry and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), was undertaken in order to gain insight and correlate the structural/textural characteristics of these novel catalytic systems with their performance and substantial thermal aging tolerance. Optimum behavior in terms of both catalytic efficacy in simulated TWC conditions and thermal durability, even in comparison with the commercial bimetallic TWC with ∼4.5-fold higher precious metal loading, was demonstrated by the doubly promoted Pt(Na)/Al2O3-(Ce0.8La0.2O1.9) sample with the higher sodium loading of 15 wt% Na. The formation of β″-alumina during aging treatment, providing intimate alkali–noble metal contact and therefore preserving their electronic interactions, suggests the mechanism for the enhanced thermal durability and the origin of the remained after aging alkali action.
Graphical abstractHighly effective and thermally stable monometallic (Pt) three way converters were developed by means of synergistic structural (by Ce0.8La0.2O1.9) and surface (by Na) promotion. Their remarkable aging tolerance can be rationalized by the in situ formation of β/β″-alumina phases which preserves alkali–noble metal interactions.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Synergy between structural and surface-induced promotion in TWCs is demonstrated. ► The synergy provides very effective and thermally stable doubly-promoted catalysts. ► The remarkable aging tolerance is related with the in situ formation of β″-alumina. ► The preserved promotion after aging overcomes losses due to textural degradation.
Journal: Applied Catalysis B: Environmental - Volume 106, Issues 1–2, 21 July 2011, Pages 228–241