Effects of metal substitution in transition-metal phosphides (Ni1−xM′x)2P (M′ = Cr, Fe, Co) studied by X-ray photoelectron and absorption spectroscopy

Abstract

Mixed-metal variants (Ni_1−xM′_x)₂P (M′ = Cr, Fe, Co; 0 ≤ x ≤ 1) of the binary phosphide Ni₂P, a well-known hydrodesulfurization and hydrodenitrogenation catalyst, have been examined by X-ray photoelectron (XPS) and absorption (XANES) spectroscopy. The P 2p_3/2 binding energies and K-edge absorption energies in (Ni_1−xCr_x)₂P, (Ni_1−xFe_x)₂P, and (Ni_1−xCo_x)₂P are lower than in Ni₂P, with the shifts becoming more pronounced with greater difference in electronegativity between P and Ni/M′. These shifts are influenced primarily by intraatomic effects. However, secondary interatomic effects are also implicated in the Ni 2p_3/2XPS spectra, which show enhanced satellite intensities, and in the Ni K- and L-edge absorption spectra, indicating a charge transfer from M′ to the more electronegative Ni atoms. This phenomenon was confirmed in the metal L-edge XANES spectra, which revealed lower Ni and higher M′ L-edge absorption energy as the degree of charge transfer increases. The Ni K-edge XANES spectra also show a pre-edge feature whose intensity decreases from Ni₂P to (Ni_0.8Cr_0.2)₂P, supporting the occurrence of charge transfer from Cr to Ni.