Pd–P Hydrogenation Catalyst: Nanoparticle Nature and Surface Layer State

The Pd–P selective catalyst for liquid-phase hydrogenation of o-chloronitrobenzene (o-CNB) was obtained by the reduction of Pd(acac)2 with hydrogen at 80∘C in the presence of white phosphorus (P/Pd = 1) in N,N-dimethylformamide (DMF). It has been shown [(high-resolution transmission electron microscopy (HRTEM), energy dispersive X-ray spectroscopy (EDX), X-ray powder diffraction (XRD)] that such low-temperature synthesis of the Pd–P catalyst affords nanoparticles of palladium phosphides (Pd5P2, PdP2), the Pd5P2 phosphide being prevailing. On the nanoparticle surface, palladium is present as a phosphide (BE (Pd3d5∕2) = 336.2 eV; BE (P2p3∕2) = 130 eV) and as palladium clusters of ≈ 1 nm in diameter. The formation of the Pd–P catalyst proceeds through a number of stages: a redox process between Pd(acac)2 and white phosphorus affording mainly PdP2 nanoparticles, H3PO3 and acacH; next follows the reduction of unreacted Pd(acac)2 with hydrogen at 80∘C and the reaction of Pd(0) atoms with each other and with PdP2. It is assumed that formation of small palladium clusters on the surface of the Pd5P2 nanoparticles ensures the high selectivity of the Pd–P catalyst in the o-CNB hydrogenation.