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

Mixed-metal variants (Ni1-xM'(x))(2)P (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 (Ni1-xCrx)(2)P, (Ni1-xFex)(2)P, and (Ni1-xCox)(2)P 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(3/2) XPS 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 (Ni0.8Cr0.2)(2)P, supporting the occurrence of charge transfer from Cr to Ni.