Surface-dependent sulfidation and orientation of MoS2 slabs on alumina-supported model hydrodesulfurization catalysts

Abstract

The constant improvement of hydrotreating (HDT) catalysts, driven by industrial and environmental needs, requires a better understanding of the interactions between the oxide support (mostly alumina) and the MoS 2 active phase. Hence, this work addresses the supportdependent genesis of MoS 2 on four planar, single crystal -Al 2 O 3 surfaces with different crystal orientations (C (0001), R , M and A). In contrast to classical surface science techniques, which often rely on UHV-type deposition methods, the Mo is introduced by impregnation from an aqueous solution, in order to mimic the standard incipient wetness impregnation. Comparison between different preparation routes, impregnation vs. equilibrium adsorption (selective adsorption), is also considered. AFM, XAS, TEM and XPS show that the -Al 2 O 3 orientation has a clear impact on the strength of metal-support interactions at the oxide state with consequences on the sulfidation, size, stacking and orientation of MoS 2 slabs. Aggregation of molybdenum oxide particles is observed on the C (0001) plane suggesting weak metal-support interactions leading to high sulfidation degree with large slabs. Conversely, the presence of well-dispersed individual oxide particles on the R plane implies stronger metal-support interactions leading to a low sulfidation degree and shorter MoS 2 slabs. Both A and M facets, of similar crystallographic structure, display an intermediate behaviors in terms of sulfidation rate and MoS 2 size in line with intermediary metal-support interactions. Polarization-dependent Grazing-Incidence-EXAFS experiments as well as HR HAADF-STEM analysis allow us to demonstrate a surface-dependent orientation of MoS 2 slabs. A predominant basal bonding is suggested on the C (0001) plane in agreement with the existence of weak metal-support interactions. Conversely, a random orientation (edge and basal-bonding) is observed for the other planes. Generalization of these conclusions to industrial catalysts is proposed based on the comparison of the surface structure of the various model -Al 2 O 3 orientations used in this work and the predominantly exposed -Al 2 O 3 surfaces ((110), (100) and (111)).

Recent Insights in Transition Metal Sulfide Hydrodesulfurization Catalysts for the Production of Ultra Low Sulfur Diesel: A Short Review

“…Early reports mentioned that the catalytic performance of MoS 2 -based catalysts strongly depends on their morphology but also on the orientation, since slabs may be bonded either by the edge or by the basal planes [27] depending on the support. Usually, over gamma alumina, the preferential bonding is by the basal planes (111) and (100) since these display relatively weak and intermediate interactions with MoS 2 structures respectively as Bara et al recently showed [28]. By contrary, the plane (110) presented highly dispersed and oriented oxide particles with strong metal support interaction, the authors associated this plane with small and weakly stacked MoS 2 slabs and a very low sulfidation degree.…”

Surface‐Dependent Activation of Model α‐Al2O3‐Supported P‐Doped Hydrotreating Catalysts Prepared by Spin Coating

“…At ransition from edge to basal-bondedMoS 2 was detected on the (100) plane at 773 Ksulfidation temperature, suggesting that the chemistry at the interface is not the sole determinant factor on the orientation of the MoS 2 slabs and the outcome of the molecular-scale descriptiono fa ctive phase-support interactions appearsm ultifactorial. [34] Moreover,t he precise nature of alumina thin films is still am atter of debate since Kresse et al [35] showedw ith the help of DFT that their structure and stoichiometry can be very differentf rom g-Al 2 O 3 .B ara et al [24] circumvented this issue by using a-Al 2 O 3 waferso fv arious orientationsi no rder to mimic the surfacec hemistryo fg-Al 2 O 3 . [24,25] Four different a-Al 2 O 3 orientations were used (A(112 0), C(0001), M(101 0), and R(11 02)) and their hydroxylated surface structures were comparedt o the speciation of surface hydroxyls (Al-OH)o ng-Al 2 O 3 .E xtensive experimental and modeling effort has been devoted to the description of the sorption sites on the a -Al 2 O 3 C(0001) plane, with predominantly OH groups bridging two 6-fold coordinated aluminums( Al 6c -m 2 -OH) that are also presento nt he (111)f acet of g-Al 2 O 3 .T he structure of the A(112 0), M(101 0) and R(11 02) planesh ave been less studied and therefore their surfaces tructure is still am atter of debate.…”

Hierarchical Structure of NiMo Hydrodesulfurization Catalysts Determined by Ptychographic X‐Ray Computed Tomography

“…Said activation is achieved either through gas-phase or liquidphase sulphidation at temperatures of roughly 350 8C, yielding molybdenum disulphide (MoS 2 ) type particles or slabs. [5] These particles are on average smaller than 10 nm in diameter and may contain Co or Ni atoms at the particle edge positions. The latter are considered to be the primary active sites in the catalytic desulfurization reaction.…”