Synergistic structural and surface promotion of monometallic (Pt) TWCs: Effectiveness and thermal aging tolerance

摘要

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 20wt% of Ce_(0.8)La_(0.2)O_(1.9) (structural promoter) and sodium in the range 5-15 wt% (surface promoter). The catalytic performance of the as prepared doubly-promoted Pt(Na)/Al2O3-(Ce_(0.8)La_(0.2)O_(1.9)) 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-(Ce_(0.8)La_(0.2)O_(1.9)) 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.