The first dynamic atomic resolution environmental scanning transmission electron microscope (ESTEM) study of the nanoparticle sintering of a model Cu system is reported. ESTEM confers the advantage of a Z-contrast dependence in industrially representative conditions to provide enhanced visibility of sub-nanometer particles when compared with TEM. The importance of this is demonstrated by the significant enhancement in the Ostwald ripening rate of model Cu nanoparticles in the presence of 3 Pa hydrogen, an effect that is independent of the substrates studied. Temperatures of 400–550 °C are shown to switch the operating regime of the rate-limiting mechanism. Cu catalysts are used for methanol synthesis and hydrocarbon-conversion processes for fuel cells, and the importance of observing these catalysts in their working states is demonstrated. Unique ESTEM observations of Ostwald ripening are combined with kinetic models to improve the technical understanding of catalyst deactivation mechanisms.

Understanding the dynamic evolution of structural changes in catalysts at the atomic level under controlled reaction conditions is one of the most challenging areas in heterogeneous catalysis. Here we present aberration-corrected environmental TEM at the atomic level and electron diffraction of the reduction of model Co₃O₄ catalysts in H₂ to directly observe the dynamic phase evolution in the reduction process. New insights into the reduction of Co₃O₄ to the intermediate CoO include the formation of an advancing atomic scale interface between Co₃O₄ and CoO regions. The interface penetrates further into the Co₃O₄ crystal, with the CoO regions replacing the previous Co₃O₄ structure, with increasing reduction. The reduction to CoO proceeds at approximately 200 °C at rounded edges containing atomic steps on the surfaces. The interfaces are observed in crystals larger than approximately 15 nm in size but not in smaller crystals, which indicates the rapid reduction of smaller nanoparticles. The most dramatic changes are observed at 350 °C in larger crystals of size 50 nm.

We present studies of the structure and stability of catalytic gold nanoparticles through in situ aberration-corrected electron microscopy to investigate the effect of heating on the nature of the identified atomic active sites. Low coordination surface atoms are replaced by atomically clean surface facets through local rearrangements to minimise surface energy. The associated movement of surface atoms is proposed to directly precede Ostwald ripening. Expansive surface strain resulting from inherently strained structures, such as the decahedra, is shown to diminish with increasing particle size and the associated elastic energy is reduced through a shifting of the disclination axis towards the particle surface. At elevated temperatures a reduction in surface energy anisotropy may lead to energetically favourable morphologies with minimal intrinsic strain. Such processes will act as structural deactivation mechanisms, resulting in a loss of active sites without any necessary associated loss of surface area or change in particle size through traditional sintering mechanisms. Considerations of the active site stability and the particle size according to the reaction conditions are described.

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 γ-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.