Nature of active sites in Ni₂P hydrotreating catalysts as probed by iron substitution

A series of NiFeP/SiO₂ catalysts with different Ni:Fe molar ratios (1:0, 3:1, 1:1, 1:3, 0:1) was investigated for the hydrodesulfurization (HDS) of 4,6-dimethyldibenzothiophene. The Fe component is a good probe for active sites because Ni₂P and Fe₂P adopt the same hexagonal crystal structure, yet Fe₂P is completely inactive for HDS. X-ray diffraction analysis and FTIR spectroscopy of adsorbed CO indicated the formation homologous alloys. At 3.1 MPa and 613 K (340 °C) the activity of the alloys was similar to that of N₂P, which was very high. There was also unprecedented selectivity toward direct desulfurization (DDS). A reconstruction of the NiFe phase occurred to expose more Ni sites, likely driven by the formation of surface Ni-S bonds as observed by EXAFS. The analysis showed that Ni(2) pyramidal sites responsible for hydrogenation were largely replaced by Fe. This left behind Ni(1) tetrahedral sites which favor DDS and explains the reactivity results