Ni2P has shown very promising catalytic activity as a hydrotreating catalyst, but the exact structure of the actual active phase is not understood. The present work has systematically studied the structures and energetics of possible surface compositions of the (001) surface of Ni2P (i.e., the Ni3P2 plane) at hydrotreating conditions using density functional theory calculations. By comparing the energetics of surfaces with adsorbed H2S, -SH, and atomic sulfur and the surfaces with phosphorous replaced by sulfur as a function of concentration and atomic location on the (001) surface, we have identified a stable phosphosulfide surface consistent with a surface stoichiometry of Ni3PS. This surface composition is energetically more stable than bulk Ni2P or Ni3S2 at hydrotreating conditions. To provide additional confirmation of the proposed surface structure, we calculated CO absorbance frequencies on the Ni3P2 and Ni3PS surfaces as a function of surface CO coverage, and found that they are in excellent agreement with previously published experimental infrared spectroscopy data. These calculations, combined with literature experimental observations, indicate that the surface with 50% phosphorus replaced by sulfur and some atomic sulfur deposited on the three-fold hollow sites is an accurate representation for the actual active phase, or the so-called "phosphosulfide" surface, of Ni2P at typical hydrotreating conditions. (c) 2006 Elsevier Inc. All rights reserved.