Nickel sulfide catalysts specific surface

Adsorption methods may be used to provide information about the total surface area of a catalyst, the surface area of the phase carrying the active sites, or possibly even the type and number of active sites. The interaction between the adsorbate and the adsorbent may be chemical (chemisorption) or physical (phys-isorption) in nature and ideally should be a surface-specific interaction. It is necessary to be aware, however, that in some cases the interaction between the adsorbate and the adsorbent can lead to a chemical reaction in which more than just the surface layer of the adsorbent is involved. For example, when using oxidizing compounds as adsorbates (O2 or N2O) with metals such as copper or nickel or sulfides, subsurface oxidation may occur. 

Commercial heterogeneous HDS catalysts for refinery use consist, almost without exception, of nickel- and/or cobalt-promoted molybdenum oxide located on a high surface area (approx. 300 m g ) alumina or silica-alumina support. Cobalt and nickel promoters increase the catalytic activity, particularly towards thiophenes whether Co or Ni is used as a promoter depends on the specific function for which the catalyst should be optimal. The catalyst material is shaped into porous pellets, a few millimeters in size, and these pellets are loaded into the reactor, forming a catalyst bed of 30-200 m volume. During start-up of a freshly loaded reactor, the catalyst bed, which is in the oxidic form, is sulfided, typically by treatment with an oil feed which has been spiked with a reactive sulfur compound that readily generates H2S in situ. The oxidic precursor phases (non-stoichiometric CoMo or NiMo surface oxides) are thereby converted into sulfidic phases termed Co-Mo-S and Ni-Mo-S. The conversion from the oxidic phase to the sulfidic is accompanied by a reduction in Mo oxidation state from +6 to +4. 

---