Highly
active nonpromoted W-based hydrotreating catalysts are prepared
through a molecular approach with a control of each step. This approach
yields WS2 crystallites exhibiting hexagonal 2D morphology,
which have been characterized by combining XPS and HR HAADF-STEM techniques
and ab initio molecular modeling. The first step is the impregnation
of a well-defined precursor, [W(OEt)5]2, grafted
onto partially dehydroxylated amorphous silica–alumina (ASA)
and characterized at the molecular level by spectroscopic techniques
(NMR and IR). The use of increasing amounts of W precursor reveals
the formation of (i) a layer of tungsten surface species grafted on
the surface and (ii) layers of more mobile adsorbed species loosely
bonded to the surface. Sulfidation of these materials provides WS2 supported on ASA, which shows unprecedented lower sulfidation
temperatures down to ambient temperature and improved activity by
comparison with conventional references (polyoxometalate route). In
addition, these improved activities are explained not only by a better
level of sulfidation but also by the 2D hexagonal-like morphology
of WS2 crystallites (revealed by HR HAADF-STEM), in contrast
to a truncated triangle-like morphology for conventional samples.
This molecular approach thus opens new avenues to understand and improve
the performances of hydrotreating catalysts.
Changes in the two‐dimensional (2D) morphology of nonpromoted WS2 and Ni‐promoted WS2 (NiWS) catalytically active nanolayers supported on amorphous silica‐alumina (ASA) are revealed by aberration‐corrected scanning transmission electron microscopy and rationalized by density functional theory calculations. The presence of the Ni atoms modifies the triangular shape of nonpromoted WS2/ASA nanolayers into a more hexagonal one for NiWS/ASA by stabilizing the S edge more strongly than the W edge. The higher catalytic activity induced by the addition of the promoter confirms the relevance of such a morphology effect for industrial catalysts.
K. Szeto from C2P2-LCOMS. ETH Zürich, CPE Lyon and UCBL are also acknowledged for their scientific support and the access to the process and characterization facilities. ASSOCIATED CONTENT Supporting Information. Experimental and computational details; in-situ IR and solid-state NMR spectra are available free of charge on the ACS Publications website.
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