Ni EXAFS studies of the Ni$z.sbnd;Mo$z.sbnd;S structure in carbon-supported and alumina-supported Ni$z.sbnd;Mo catalysts

Sulfided NiS/C, NiMoS/C, and NiMoS/Al2O3 catalysts were prepared by pore volume impregnation with a solution containing nitrilotriacetic acid, which gives almost exclusively the NiMoS structure. The catalysts were studied by means of in situ EXAFS measurements at the Ni K-Edge. The Ni in the carbon-supported Ni catalysts was present in a Ni3S2-like phase. In the alumina-supported and carbon-supported NiMoS catalysts no bulk Ni sulfide was detected up to a Ni/Mo ratio of about 0.5, and all Ni was present at the NiMoS structure, which is the active phase in hydrodesulfurization. In this structure the Ni atoms are situated in a square pyramid of five S atoms at 2.22 Å, and have Mo neighbors at 2.85 Å. This indicates that the Ni atoms are situated on top of the S4 squares at the MoS2 edges in the Mo plane, in millerite-type Ni sites. A NiNi coordination at 3.2 Å was observed at low Ni/Mo ratios, indicating that part of the Ni atoms are present at neighboring sites on the MoS2 edge. In order to examine the influence of the H2S partial pressure on the Ni structure, a NiMoS/Al2O3 catalyst was exposed to several flushing procedures after sulfidation. When the catalyst was flushed with a mixture of 2% H2S or less in H2, the NiS coordination number decreased, indicating that the Ni atoms become uncovered and are made available for catalysis.