Abstract
The impact of the aging environment on the size, morphology and distribution of Pt nanoparticles supported on Al2O3 and on their NO oxidation activity is studied. To this end, the fresh catalyst Pt/Al2O3 (Pt/Al/F) is aged under different aging environments mimicking the aging process of a real diesel oxidation catalyst (DOC) including phosphorus (P) poison as a chemical contaminant. These catalysts are characterized by N2-physisorption, CO chemisorption, XRD, solid state 27Al MAS NMR, HAADF-STEM, HR-TEM, EDX analyses and CO-DRIFTS. The characterization results reveal that the thermal aging in air at 800°C leads to a heterogeneous size and spatial distribution of Pt nanoparticles in the catalyst (Pt/Al/O). The morphology is mainly limited to truncated cubic structures that are dominated with (100) crystal facets. Differently, aging in a lean diesel exhaust environment (Pt/Al/R) restricts the extensive Pt particle growth, size distribution to narrow and morphology predominantly to cuboctahedral that contains both (111) and (100) planes, though the former tends to dominate the surface. P seems to control both the growth of Pt particles and the morphology that is mainly limited to spherical irrespective of the aging environment (P/Pt/Al/O or P/Pt/Al/R). The normalized (per surface Pt atoms) forward rate constant of NO oxidation and the corresponding activation energy are determined for the differently treated catalysts and compared them with those reported in previous relevant studies. From these results it is evident that the reaction is indeed structure sensitive. The catalyst treated in lean diesel exhaust environment (Pt/Al/R) presents the best activity followed by the fresh Pt/Al/F and thermally Pt/Al/O aged catalysts. The difference in the NO oxidation activity of the catalysts is attributed to the morphology of Pt nanoparticles. These results correlate very well with the real DOC monolith that was aged on a heavy duty small truck for 250h under different driving profiles.
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