2D & 3D in situ study of the calcination of Pd nanocatalysts supported on delta-Alumina in an Environmental Transmission Electron Microscope

The quality of metallic nanoparticles (NPs) used in heterogeneous catalysis relies through many aspects on their small size, on the homogeneity of their spatial distribution on their supports and on their ability to resist to sintering or coalescence. It is thus very important to quantify these parameters and understand the mechanisms controlling the growth of NPs during the genesis process of the catalyst. Whereas conventional Transmission Electron Microscopy (TEM) is currently used for these purposes, it most frequently remains a 'static' method where results are obtained in high vacuum and post mortem, i.e. after the typical drying, calcination and reduction steps without the possibility to follow directly the evolution of both NPs and supports during those treatments. Environmental TEM (ETEM) unlocks this blocking and allows elementary mechanisms, such as Ostwald Ripening and coalescence to be unravelled through direct in situ observations. We report here an ETEM study of the preparation of Pd-based narrow NPs, less than 5 nm in size, deposited on a d-alumina support. We focused on 3 main objectives: (i) quantifying the sizes of NPs at each preparation step performed in situ under environmental (i.e. respectively oxygen or air and hydrogen atmospheres at working temperatures) and comparing them to post mortem measurements; (ii) identify the oxidation state of the NPs through an in situ High Resolution imaging study of their crystallographic structure; (iii) explore the possibilities of environmental tri-dimensional (3D) studies by tilt series based Electron Tomography. This last item represents a challenging breakthrough in the characterization of nanocatalysts; it will be demonstrated that the use of modern instruments (microscope and accessories) allows tomographic acquisitions to be performed very fast, within a few minutes and even seconds, which opens the way to the 3D tracking of microstructures almost in real time during their evolution under gas and at high temperature.