Ni EXAFS studies of the NiMoS structure in carbon-supported and alumina-supported NiMo catalysts

Abstract

Sulfided NiS/C, NiMoS/C, and NiMoS/Al 2O 3 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 Ni 3S 2-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 S 4 squares at the MoS 2 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 MoS 2 edge. In order to examine the influence of the H 2S partial pressure on the Ni structure, a NiMoS/Al 2O 3 catalyst was exposed to several flushing procedures after sulfidation. When the catalyst was flushed with a mixture of 2% H 2S or less in H 2, the NiS coordination number decreased, indicating that the Ni atoms become uncovered and are made available for catalysis.