Nickel sulfide crystals in Ni-Mo and Ni-W catalysts: Eye-catching inactive feature or an active phase in its own right?

Ni sulfide (NiSx) crystals dominate the TEM images and STEM-EDX elemental maps of used Ni-Mo and Ni-W hydrotreating catalysts. Crystal sizes ranging from about 5 to over 100 nm in diameter are observed not only in severely deactivated catalysts after use in commercial reactors, but also in highly active catalysts obtained after relatively short testing regimes. Depending on Ni-S stoichiometry and reaction conditions, NiSX crystals have different dimensions, shapes and crystal structures. Many of the NiSx crystals are decorated by multiple layers of MoS2/WS2. Although the activity of bulk Ni sulfide is generally deemed to be negligible, this does raise the question if these crystals play a role in catalysis after all. The number and percentage of accessible Ni sites in different catalysts can be estimated by combining the results of (i) the dispersion measurements on the MoS2/WS2 phase, (ii) the dispersion measurements on the visualized NiSx crystals and (iii) the composition of the catalyst matrix surrounding the NiSx crystals as derived from the phase analysis. The calculations based on the apparent Ni to Mo/W ratio of the catalyst matrix show that the catalyst matrix contains much more Ni than what can be accommodated on the MoS2/WS2 edges. We assume that this excess Ni is present as small non-visualized NiSx crystals mixed in the catalyst matrix. The fraction of Ni atoms on the surface of NiSx crystals can be calculated using the parameters of the millerite structure. The calculations carried out for NiMo/Al2O3 and NiW bulk catalysts show that the NiSx crystals could account for 19% and 48% of the active Ni sites, respectively. Despite this relatively high number of accessible Ni sites on the surface of NiSx crystals, no positive effect of a Ni/W ratio higher than 0.25 on the catalyst HDS and HDN activity was observed for a series of NiW bulk catalysts with variable Ni/W ratio. This suggests that, despite their spectacular presence in the TEM micrographs, the NiSX crystals do not significantly contribute to the HDS and HDN activity. (C) 2016 Elsevier B.V. All rights reserved.