Characterization of the active phase in NiW/gamma-Al2O3 catalysts in various stages of sulfidation with FTIR(NO) and XPS

The sulfidation of oxidic mixed metal catalysts is an important step in the activation of these catalyst precursors for hydrotreating reactions. The s ulfidation of NiW/gamma -Al2O3 catalysts is especially interesting since it is possible to study intermediate stages of sulfidation as separate, stabl e phases. By carrying out a systematic study, a detailed picture was obtain ed of the sulfiding reactions and the species formed. The major techniques used were FTIR(NO) and semiquantitative quasi in situ XPS. A reference cata lyst was obtained by extraction of Ni from the oxidic catalysts. Four types of Ni are present in the oxidic NiW/gamma -Al2O3 catalysts, viz., Ni in a surface aluminate, Ni in a mixed oxide with W,Ni in a mixed oxide with W an d Al, and bulk Ni aluminate. No separate Ni oxide phase is present. It was found that even in dried NiW/gamma -Al2O3 catalysts, a significant part of Ni strongly interacts with either W or the support and is present in subsur face positions. This subsurface Ni species migrates to the surface upon sul fidation below 373 K. Sulfidation below 473 K shows the development of a Ni sulfide species, which is in close interaction with an oxidic or partially sulfided W6+ phase. In addition, part of the W phase can be sulfided at lo w temperature to form WS3 and it was established that no W4+ species are fo rmed below 600 K. XPS indicated that sulfidation above 600 K results in a d istinct change in the chemical environment of Ni ions, which was assigned t o the formation of the so-called NiWS phase. Remarkably, FTIR(NO) showed th at after sulfidation above 700 K, the gas-phase-exposed W4+ sites become la rgely inaccessible for NO. This is interpreted as a full decoration of WS2 edges, probably with Ni sulfide, which occurs at a significantly higher tem perature than the initial formation of NiWS. (C) 2000 Academic Press.