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

Abstract

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 3.1MPa and 613K (340°C) the activity of the alloys was similar to that of Ni2P, 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.