Electron microscope studies of platinum/alumina reforming catalysts

Platinum/alumina catalysts were examined following ageing-regeneration cycles typical of industrial use and after extreme treatments in "oxidising" or "reducing" environments. Details of particle morphology and the statistics of particle size distribution (PSD) were obtained by high-resolution electron microscopy, with complementary data from hydrogen chemisorption measurements. It was found that catalyst sintering during a simulated industrial cycle was minimal, whereas "reducing" or "oxidising" treatments at high temperature (600 °C) caused loss of accessible Pt surface area, as measured by chemisorption: the PSDs and particle geometries, however, then differed greatly, despite similar chemisorptive capacities. Conversely, it was also observed that no appreciable particle growth resulted from severe reducing treatment despite a reduced chemisorptive capacity. After 24 days at 600 °C the catalyst metal remained as discrete single crystals or twinned Pt particles. No redispersion by oxide spreading was observed after oxidation or "chloriding" treatments; instead, the clustering of Pt metal crystallites to form continuous agglomerates, apparently confined in size and shape only by the three-dimensional geometry of the alumina support, seemed to be commonplace. The irregular morphology of these agglomerates was revealed by stereo-imaging, and hydrogen chemisorption measurements confirmed their high surface areas.