Cobalt-promoted
and nonpromoted MoS2 nanolayers supported
on alumina are prepared and activated under various sulfidation (temperature/pressure
(T, P)) conditions which induce
the formation of nanolayers with two-dimensional (2D) morphology of
MoS2 tuned by the presence of the promoter and by the sulfidation
conditions. An unprecedented high selectivity is found for the CoMoS
nanolayers. The origin of this selectivity is explained by 2D morphology
effects quantified by high-resolution scanning transmission electron
microscopy in high-angle annular dark field mode (HR HAADF-STEM) and
density functional theory (DFT) calculations. A quantitative structure–selectivity
relationship is identified between the 2D shape index of CoMoS nanolayers
and their selectivity performances. This 2D shape index is determined
by statistical analysis of the CoMoS nanolayers identified after principal
component analysis processing of HR HAADF-STEM images. It is shown
that this shape index, reflecting the isotropic/anisotropic degree
of the nanolayers’ morphology, is directly linked to the nature
of active M- and S-edges exposed by the CoMoS nanolayers, as proposed
by DFT calculations. This 2D shape index may thus serve as a key descriptor
for the selectivity of the CoMoS nanolayers. The correlation is rationalized
by a simple kinetic modeling where hydrodesulfurization (HDS) and
hydrogenation (HYD) rate constants are parametrized as a function
of the S-edge/M-edge sites by using DFT-calculated descriptors. HR
HAADF-STEM also highlights the existence of nonequilibrium CoMoS layers
with more irregular 2D shapes, which can also be correlated to selectivity
through a specific shape descriptor. More generally, this study reveals
that the HDS/HYD selectivity can be controlled by the 2D shape driven
by the activation–sulfidation steps of the catalyst. It provides
a new approach for establishing a reliable methodology for the rational
design of highly selective nanocatalysts.
The structure of oxidic precursors of supported NiMo hydrodesulfurization catalysts has been investigated indepth by the combination of laser Raman spectroscopy and Xray absorption spectroscopy measured at the Mo and Ni K edges at the different stages of the preparation. The oxidic catalysts were prepared by incipient wetness impregnation of δ-alumina with a solution obtained by dissolving MoO 3 in H 2 O 2 and subsequently adding Ni(NO 3 ) 2 ·6H 2 O in this asprepared solution (8 wt % MoO 3 ; 2 wt % NiO). The formation of the 6-molybdoaluminate Anderson-type heteropolyanion (AlMo 6 O 24 H 6 ) 3− and of a mixture of oxo−hydroxo nickel species and bulk and/or surface NiAl-layered double hydroxide dispersed at the surface of the support has been identified after drying. Upon further thermal treatment at 723 K under dried air, calcined dehydrated catalyst is constituted of highly distorted isolated or partially condensed tetrahedral Mo units with terminal mono-oxo groups and of bulk and/or surface NiAl 2 O 4 -type and NiO-type species. After further exposure of the calcined catalyst to air moisture, a partial recovery of the Anderson-type molybdenum heteropolyanion and NiAl-layered double hydroxide species is evidenced by X-ray absorption spectroscopy. The nature and dispersion of active species formed after sulfidation under H 2 S/H 2 of the different oxidic catalysts (dried-NiMo, dehydrated-calcined NiMo, and calcined-NiMo samples) are finally discussed in light of the structure of the parent oxidic precursors.
For
the first time, the sulfidation process of a bimetallic NiMo
catalyst supported on alumina has been followed by combining time-resolved
laser Raman spectroscopy (LRS) and X-ray absorption spectroscopy (XAS)
quasi simultaneously at both Ni and Mo K edges. Multivariate data
analysis reveals that the thermal activation upon 15% H2S/H2 atmosphere of a dehydrated-calcined NiMo(VI) catalyst
involves (i) a 5-stepped mechanism with oxysulfided or fully sulfided
Mo intermediate species and (ii) a direct transformation of oxidic
nickel species into NiS
x
and NiMoS ones.
Complementary information extracted from LRS and Quick-XAS data permitted
to identify at the early stage of the sulfidation the trimeric Mo(V/VI)
oxysulfide species [Mo3(μ2O)4(μ2S)μ2{S2}(Ot)2(St)3] grafted to the support
surface, which is quickly transformed into the Mo(IV) intermediate
species [Mo3(μ3S)(μ2S)2μ2{S2}(Ot)2{S2}t]. Above 190 °C, the Mo(IV) second
intermediate is transformed into Mo(IV)S3, itself transformed
into the final Mo(IV)S2 at T > 220
°C.
Thanks to the unambiguous comparison of sulfidation kinetics for both
metals the incorporation of promoter into the extended sulfidic molybdenum-based
phase has been unprecedentedly related to the formation of the MoS3 intermediate species.
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