Phosphorus-Modified Palladium Hydrogenation Catalysts: An Electron-Microscopy Study.

Size and phase composition of the Pd-P catalyst, which is highly active in the hydrogenation of unsaturated compounds have been established using X-ray powder diffraction (XRD) analysis, high resolution transmission electron microscopy (HRTEM) technique in combination with energy-dispersive X-ray (EDX) analysis and selected area electron diffraction (SAED) analysis. The reduction of Pd(acac)₂ with hydrogen in the presence of white phosphorus (P/Pd = 0.3) in N,N-dimethylformamide (DMF) at 80 °C results in formation of Pd-enriched polycrystalline nanoparticles with a predominant diameter of 5.7 nm (C(Pd) = 1 mmol·L⁻¹) and 5.1 nm (C( Pd) = 5 mmol · L⁻¹). The polycrystallites consist of palladium phosphides (Pd5P2, Pd3P0.8 and palladium crystallites. It is found that phase composition of the Pd-P catalyst is changed at the stage of Pd-P sample preparation for XRD analysis. It is established that palladium phosphide Pd6P detected by XRD technique is a product of the solid-phase reaction between Pd crystallites and Pd₅P₂, Pd₃P(0.8) palladium phosphides at the stage of high-temperature sample preparation (400 °C). The mechanism of low-temperature synthesis of palladium phosphides was proposed.