Nickel sulfide crystals in Ni-Mo and Ni-W catalysts: Eye-catching inactive feature or an active phase in its own right?
| Autor: | Jaap A. Bergwerff, Jaap N. Louwen, Xuanqi Li, Joachim Loos, Sonja Eijsbouts, Leo Woning |
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| Rok vydání: | 2017 |
| Předmět: |
chemistry.chemical_classification
Materials science Nickel sulfide Sulfide 02 engineering and technology General Chemistry Crystal structure engineering.material 010402 general chemistry 021001 nanoscience & nanotechnology 01 natural sciences Catalysis 0104 chemical sciences Crystal chemistry.chemical_compound Crystallography chemistry engineering 0210 nano-technology Hydrodesulfurization Millerite Stoichiometry |
| Zdroj: | Catalysis Today. 292:38-50 |
| ISSN: | 0920-5861 |
| DOI: | 10.1016/j.cattod.2016.08.028 |
| Popis: | Ni sulfide (NiS x ) 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, NiS x crystals have different dimensions, shapes and crystal structures. Many of the NiS x crystals are decorated by multiple layers of MoS 2 /WS 2 . 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 MoS 2 /WS 2 phase, (ii) the dispersion measurements on the visualized NiS x crystals and (iii) the composition of the catalyst matrix surrounding the NiS x 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 MoS 2 /WS 2 edges. We assume that this excess Ni is present as small non-visualized NiS x crystals mixed in the catalyst matrix. The fraction of Ni atoms on the surface of NiS x crystals can be calculated using the parameters of the millerite structure. The calculations carried out for NiMo/Al 2 O 3 and NiW bulk catalysts show that the NiS x 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 NiS x 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 NiS x crystals do not significantly contribute to the HDS and HDN activity. |
| Databáze: | OpenAIRE |
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