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 (Pd 5 P 2 , PdP 2 ), the Pd 5 P 2 phosphide being prevailing. On the nanoparticle surface, palladium is present as a phosphide (BE (Pd3d 5∕2 ) = 336.2 eV; BE (P2p 3∕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 PdP 2 nanoparticles, H 3 PO 3 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 PdP 2 . It is assumed that formation of small palladium clusters on the surface of the Pd 5 P 2 nanoparticles ensures the high selectivity of the Pd–P catalyst in the o-CNB hydrogenation.