Nanostructural Studies of Fresh and Road-Aged Practical Pt/SiO2 and Pt-Pd/Al2O3 Diesel Oxidation Catalysts by using Aberration-Corrected (Scanning) Transmission Electron Microscopy

We report direct observations of nanostructural and compositional evolution in complex technological diesel oxidation nanocatalysts (DOCs) by using aberration-corrected (scanning) transmission electron microscopy (AC-(S)TEM) at the atomic level and nano-beam electron diffraction (NBED). Two representative practical DOC systems with varying metal loading on different supports have been employed in the study. Nanostructural and compositional variations in fresh and road-aged Pt nanocatalysts supported on SiO2 spheres (referred to as Pt-only DOCs) and bimetallic Pt-Pd nanocatalysts on ?-Al2O3 (bimetallic DOCs) are quantified by inductively coupled plasma (ICP) analysis, ion chromatography, X-ray photoelectron spectroscopy and X-ray diffraction. Pt nanocatalysts on silica spheres are shown to maintain their spherical morphologies with stepped surfaces of low symmetry atomic planes. Nanoscale clusters and spherical nanoparticles are present in the fresh bimetallic DOCs. By using AC high-angle annular dark-field (HAADF) STEM, we elucidate atomic, structural and morphological changes in the aged nanocatalysts and discuss their behaviour. Notably, AC-HAADF-STEM and energy dispersive X-ray (EDX) spectroscopy of the aged bimetallic DOC have shown that a majority of the nanoparticles remain spherical and only a minority of the particles show larger faceted particles with core-shell-like structures. The atomic level structural changes of the nanocatalysts on the two supports indicate that the metal loading, dispersion, initial nanoparticle size and the support play key roles in the stability and aging behaviour of the DOCs. The retention of spherical particles is important as they are believed to be more active for the oxidation of CO in the diesel engine exhaust. The results have important implications in the design of novel technological DOCs.