Palladium, supported crystallite size effect

Addition of 1-5% Re increased the activity of Pd for the hydrogenation of nitro-compounds, e.g., nitrobenzene but more Re had an adverse effect due to an increase in palladium crystallite size alumina was a better support than MgO, TiOj. ... 

The blocking effect provides an easy explanation for recent data [5] on properties of the Pd/C catalysts which were prepared from different precursors. Palladium crystallites with a broad distribution in size (2-20 nm) were seen by electron microscopy after reduction of supported palladium chloride in alkaline liquid phase and medium-sized palladium particles (1-4 nm) upon thermal decomposition of Pd(0) complex with dlbenzylldenaoetone. Nevertheless, both catalysts appeared approximately twice as active as the catalyst vftilch was prepared via reduction of adsorbed palladium chloride in hydrogen and contained palladium particles of 1-2 nm in size. Because those were Sibunlt-based catalysts in Ref. 5, the results show possibility for the blocking effect to be displayed in the case of mesoporous siqpports as well. 

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