Characterization of the Active Phase in NiW/γ-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 sulfidation of NiW/γ-Al2O3 catalysts is especially interesting since it is possible to study intermediate stages of sulfidation as separate, stable phases. By carrying out a systematic study, a detailed picture was obtained of the sulfiding reactions and the species formed. The major techniques used were FTIR(NO) and semiquantitative quasi in situ XPS. A reference catalyst was obtained by extraction of Ni from the oxidic catalysts. Four types of Ni are present in the oxidic NiW/γ-Al2O3 catalysts, viz., Ni in a surface aluminate, Ni in a mixed oxide with W, Ni in a mixed oxide with W and Al, and bulk Ni aluminate. No separate Ni oxide phase is present. It was found that even in dried NiW/γ-Al2O3 catalysts, a significant part of Ni strongly interacts with either W or the support and is present in subsurface positions. This subsurface Ni species migrates to the surface upon sulfidation 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 low temperature to form WS3 and it was established that no W4+ species are formed below 600 K. XPS indicated that sulfidation above 600 K results in a distinct change in the chemical environment of Ni ions, which was assigned to the formation of the so-called NiWS phase. Remarkably, FTIR(NO) showed that after sulfidation above 700 K, the gas-phase-exposed W4+ sites become largely inaccessible for NO. This is interpreted as a full decoration of WS2 edges, probably with Ni sulfide, which occurs at a significantly higher temperature than the initial formation of NiWS.