Revealing local variations in nanoparticle size distributions in supported catalysts: a generic TEM specimen preparation method

Journal article, 2015

The specimen preparation method is crucial for how much information can be gained from transmission electron microscopy (TEM) studies of supported nanoparticle catalysts. The aim of this work is to develop a method that allows for observation of size and location of nanoparticles deposited on a porous oxide support material. A bimetallic Pt-Pd/Al2O3 catalyst in powder form was embedded in acrylic resin and lift-out specimens were extracted using combined focused ion beam/scanning electron microscopy (FIB/SEM). These specimens allow for a cross-section view across individual oxide support particles, including the unaltered near surface region of these particles. A site-dependent size distribution of Pt-Pd nanoparticles was revealed along the radial direction of the support particles by scanning transmission electron microscopy (STEM) imaging. The developed specimen preparation method enables obtaining information about the spatial distribution of nanoparticles in complex support structures which commonly is a challenge in heterogeneous catalysis. Lay Description Catalysis is important in our everyday lives, whether it is for the production of chemicals and food or for cleaning the exhaust gas of cars, ships and power plants. Therefore it is crucial to get a better understanding of the structure and properties of the catalysts involved, which often are in the form of nanoparticles. Electron microscopy has proven to be a powerful tool to investigate these nanoparticles, and especially transmission electron microscopy (TEM) has been used to investigate the smallest structures, down to single atoms. Although the TEM is very capable of imaging even the smallest structures, the specimens in question need to be very thin, of the order 100 nanometres, which can make the specimen preparation demanding, since the nanoparticles are mostly supported on much larger porous alumina particles. We have developed a new specimen preparation method using a focused ion beam, which allows obtaining TEM specimens of these catalysts in a very controlled manner, compared to previous procedures which involved crushing the catalyst powder in a mortar to obtain small pieces. With these specimens, we could in the TEM analysis investigate the spatial distribution of nanoparticles within the alumina support structure, and we observed a different nanoparticle size distribution for different parts of the support, namely the outer edge and the interior. This was never before observed using specimens obtained from the conventional crushing method.