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Insight into the formation of the active phases in supported NiW hydrotreating catalysts

Paper ID Volume ID Publish Year Pages File Format Full-Text
43912 45995 2007 17 PDF Available
Title
Insight into the formation of the active phases in supported NiW hydrotreating catalysts
Abstract

The sulfidation mechanism of supported NiW hydrotreating catalysts is described in detail based on literature data and experiments for carbon- and alumina-supported NiW catalysts. It is more difficult to convert tungsten oxides to WS2 than molybdenum oxides to MoS2 and therefore typical sulfidation procedures tend to result in oxysulfidic tungsten phases (WOxSy) next to the layered WS2 phase. The former phase appears to stabilize rather dispersed Ni sulfide particles (NiS–WOxSy) which is found to be a precursor to the ‘Ni–W–S’ phase: Mössbauer spectroscopy clearly shows that the NiS particles redisperse over the WS2 edges when partly oxidic tungsten phases transform to WS2 slabs. As a consequence of very slow W sulfidation, the Ni promoter ion can end up in two phases, i.e. ‘Ni–W–S’-type and in NiS–WOxSy phases, this in contrast to Co(Ni)Mo catalysts that mainly contain ‘Co(Ni)–Mo–S’ phases after typical sulfidation procedures. The NiS–WOxSy phase is known to perform well in liquid-phase hydrodesulfurization (HDS) reactions whereas the ‘Ni–W–S’ phase is most active for gas-phase HDS. The relative speciation of these two phases for alumina-supported NiW can be controlled by parameters as the calcination temperature and the sulfidation temperature and pressure. Notably, elevated sulfidation pressures are preferred over elevated sulfidation temperatures because in the latter case the WS2 slabs tend to sinter and crystallize which negatively affects the performance. This work shows the existence of a strong support effect for carbon- and alumina-supported NiW. Similar trends were found for amorphous-silica–alumina-supported NiW which is consistent with the finding that the sulfidation mechanism is similar to that of NiW/Al2O3.Even when WS2 sulfidation is completed, as for instance found for a carbon-supported NiW catalyst at 673 K, a significant fraction of Ni is not involved in the ‘Ni–W–S’ phase. Chelating agents improve the edge occupation in NiW catalysts significantly by retarding Ni sulfidation. The result is a more efficient formation of the ‘Ni–W–S’ phase and optimized catalysts have a significantly higher activity than conventional ones.

Keywords
Hydrodesulfurization; NiWS; Sulfidation; Support effect
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Insight into the formation of the active phases in supported NiW hydrotreating catalysts
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Publisher
Database: Elsevier - ScienceDirect
Journal: Applied Catalysis A: General - Volume 322, 16 April 2007, Pages 16–32
Authors
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Subjects
Physical Sciences and Engineering Chemical Engineering Catalysis
Get Full-Text Now
Don't Miss Today's Special Offer
Price was $35.95
You save - $31
Price after discount Only $4.95
100% Money Back Guarantee
Full-text PDF Download
Online Support
Any Questions? feel free to contact us