Supported palladium catalysts are ubiquitous in organic synthesis and in chemical transformations where liquid environments are required. These reaction environments contain solvents and often adjunct chemicals in non-negligible amounts, e.g. bases. Little information is available about how transition metals behave in their presence. Here, we use quick EXAFS to show that an ethanol/water solvent reduces PdO nano-particles and to demonstrate that bases may also influence profoundly the catalyst structure. The 5wt% Pd/Al2O3 catalyst in the as received form comprised very small (&lt; 20 atoms) Pd0 particles in contact with O from the support (EXAFS, TEM). The same sample but calcined at 500°C exposed only PdO particles of ca. 3-4 nm. Exposure to an ethanol/water solvent flow while heating caused PdO reduction in both samples. While the as received sample reduced at 50°C, the calcined sample required 70°C for reduction showing that bulk PdO reduction occurs under these conditions but is more difficult. Reduction also depends on the position within the sample bed:3 a reduction gradient moves from the inlet towards the bed outlet. In the presence of additional 0.1 M K2CO3 the Pd K-edge jump was seen to change along the catalyst bed over time indicating mobility of Pd0 species induced by the presence of the base. Preliminary TEM analysis then demonstrates that K2CO3 causes detachment of metallic particles from the support and that are observed “free” on the TEM grid after this treatment: such detached particles were not observed in absence of the base. This work shows that liquid solvents and dissolved bases are not innocent reaction spectators. Heterogeneous catalysts can be strongly affected by the presence of a liquid environment thus potentially influencing the reaction output.

QuickEXAFS study of Pd leaching from solid catalysts in liquid environment

256th National Meeting (2018)

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