Bulk hydrotreating MonW12-nS2 catalysts based on SiMonW12-n heteropolyacids prepared by alumina elimination method

Article abstract of DOI:10.1016/j.cattod.2020.07.018

A series of unsupported mono- and bimetallic Mo_nW_12-nS₂ catalysts were synthesized by alumina elimination from supported Mo_nW_12-nS₂/Al₂O₃ samples using acid etching. Alumina supported catalysts have been in turn prepared by using monometallic H₄SiMo₁₂O₄₀ and H₄SiW₁₂O₄₀ heteropolyacids (HPAs), their mixture with Mo/W atomic ratio equal to 1/11 and 3/9, and mixed bimetallic H₄SiMo₁W₁₁O₄₀ and H₄SiMo₃W₉O₄₀ HPAs. All catalysts were characterized by N₂ adsorption, temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), time-of-flight secondary ion mass spectrometry (ToF-SIMS), extended X-ray absorption fine structure (EXAFS) spectroscopy and powder X-ray diffraction (XRD) and their performance were evaluated in simultaneous hydrodesulfurization (HDS) of dibenzothiophene (DBT) and hydrogenation (HYD) of naphthalene. The etching process led to a successful removal of all the support and of the partially sulfided species, with sulfidation degrees of both Mo and W above 90 % on the final bulk solids. The active phase also underwent a rearrangement, as higher average length and stacking were measured on the bulk catalysts than on the original supported ones. Mixed MoWS₂ phase was evidenced in all solids, prepared from mixed HPAs (Mo_nW_12-nS₂) or from the mixture of monometallic HPAs (RefMo_nW_12-nS₂), by EXAFS and ToF-SIMS, with however a larger quantity on the MoW solids. It seems that the mixed MoWS₂ phase observed on the supported MoW catalysts is maintained through the etching process, while on RefMo_nW_12-nS₂ the mixed phase, observed in a much lesser extent in the corresponding supported catalyst, could result from the aggregation of the monometallic slabs. Mo_nW_12-nS₂ catalysts were found more effective than the monometallic catalysts and than the corresponding RefMo_nW_12-nS₂, in both dibenzothiophene hydrodesulfurization and naphthalene hydrogenation, which was related to the presence of the mixed phase maintained through the etching of the support.

Full text of DOI:10.1016/j.cattod.2020.07.018

Journal Pre-proof
Bulk hydrotreating MonW12−nS2 catalysts based on SiMonW12−n
heteropolyacids prepared by alumina elimination method
A. Kokliukhin (Investigation)<ce:contributor-role>Writing- original
draft), M. Nikulshina (Validation) (Writing - original draft), A.
Mozhaev (Investigation), C. Lancelot (Investigation) (Writing -
review and editing), C. Lamonier (Conceptualization), N. Nuns
(Investigation), P. Blanchard (Validation), A. Bugaev (Investigation)
(Methodology), P. Nikulshin
(Conceptualization)<ce:contributor-role>Writing- review and editing)
PII:
S0920-5861(20)30481-8
DOI:
https://doi.org/10.1016/j.cattod.2020.07.018
CATTOD 13013
Reference:
To appear in:
Catalysis Today
Received Date:
Revised Date:
Accepted Date:
30 January 2020
4 June 2020
20 July 2020
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Bulk hydrotreating MonW12-nS2 catalysts based on SiMonW12-n heteropolyacids prepared by
alumina elimination method
A. Kokliukhina,b,c, M. Nikulshinaa, A. Mozhaeva,c, C. Lancelotb, C. Lamonierb, N. Nunsd, P.
Blanchardb, A. Bugaeve,f, P. Nikulshina,c,g*
a Samara State Technical University, 244 Molodogvardiyskaya St., 443100, Samara, Russia
b Univ. Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois, UMR 8181 – UCCS – Unité de
Catalyse et Chimie du Solide, F-59000 Lille, France
c Gubkin Russian State University of Oil and Gas, Leninskiy Prospect 65, 119991, Moscow,
Russia
d Université Lille, CNRS, INRA, Centrale Lille, ENSCL, Univ. Artois, FR 2638 - IMEC - Institut
Michel-Eugène Chevreul, F-59000 Lille, France
e The Smart Materials Research Institute, Southern Federal University, Sladkova 178/24,
344090, Rostov-on-Don, Russia
f Southern Scientific Centre, Russian Academy of Sciences, Chekhova 41, 344006, Rostov-on-
Don, Russia
g All-Russia Research Institute of Oil Refining, 6 Aviamotornaya st., Moscow, 111116, Russia
*Corresponding author. Tel/Fax: +7 846 2423580
E-mail: p.a.nikulshin@gmail.com (Pavel Nikulshin)
Graphical abstract
1

Highlights:
 MonW12-nS2 catalysts were synthesized by HF etching of alumina supported
catalysts
 HF etching and resulfidation leads to enlargement of the active phase
particles.
 Mo-W interactions were detected in EXAFS and ToF-SIMS of MonW12-nS2 catalysts.
 Mo3W9S2 has the highest activity in DBT HDS and naphthalene HYD
 SiMo3W9HPA as precursor is more preferable for the formation of a mixed
MoWS2
Abstract
A series of unsupported mono- and bimetallic MonW12-nS2 catalysts were synthesized by
alumina elimination from supported MonW12-nS2/Al2O3 samples using acid etching. Alumina
supported catalysts have been in turn prepared by using monometallic H4SiMo12O40 and
H4SiW12O40 heteropolyacids (HPAs), their mixture with Mo/W atomic ratio equal to 1/11 and 3/9,
and mixed bimetallic H4SiMo1W11O40 and H4SiMo3W9O40 HPAs. All catalysts were characterized
by N2 adsorption, temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy
(XPS), high-resolution transmission electron microscopy (HRTEM), time-of-flight secondary ion
2

mass spectrometry (ToF-SIMS), extended X-ray absorption fine structure (EXAFS) spectroscopy
and powder X-ray diffraction (XRD) and their performance were evaluated in simultaneous
hydrodesulfurization (HDS) of dibenzothiophene (DBT) and hydrogenation (HYD) of
naphthalene. The etching process led to a successful removal of all the support and of the partially
sulfided species, with sulfidation degrees of both Mo and W above 90% on the final bulk solids.
The active phase also underwent a rearrangement, as higher average length and stacking were
measured on the bulk catalysts than on the original supported ones. Mixed MoWS2 phase was
evidenced in all solids, prepared from mixed HPAs (MonW12-nS2) or from the mixture of
monometallic HPAs (RefMonW12-nS2), by EXAFS and ToF-SIMS, with however a larger quantity
on the MoW solids. It seems that the mixed MoWS2 phase observed on the supported MoW
catalysts is maintained through the etching process, while on RefMonW12-nS2 the mixed phase,
observed in a much lesser extent in the corresponding supported catalyst, could result from the
aggregation of the monometallic slabs. MonW12-nS2 catalysts were found more effective than the
monometallic catalysts and than the corresponding RefMonW12-nS2, in both dibenzothiophene
hydrodesulfurization and naphthalene hydrogenation, which was related to the presence of the
mixed phase maintained through the etching of the support.
Keywords: Hydrodesulfurization; Heteropolyanion; MoWS2 catalysts; Unsupported catalyst,
DBT, Naphthalene, EXAFS.
1. Introduction
Due to stricter environmental standards, the oil industry is under pressure to provide clean
fuels. Therefore, in recent years, special attention has been focused on improving the activity and
stability of hydrotreating catalysts for petroleum fractions. Mixed NiMoW sulfides based on
alumina [1-4], silica [5] or mesostructured silica [6] demonstrated excellent catalytic activity in
hydrodesulfurization (HDS) of sulfur compounds. The superiority of mixed NiMoW systems over
traditional bimetallic Ni(Co)Mo(W) catalysts was attributed to the synergistic effect when using
3

Ni, Mo and W. With the density functional theory (DFT) calculations, it was found that NiMoWS
catalysts have a more optimal metal-sulfur bond energy compared to NiMoS and NiWS catalysts
[2].
The use of Ni(Co)MoWS systems, in which both tungsten and molybdenum atoms are
simultaneously present, allowed increasing HDS and HYD activities [1-3,7-10]. Thomazeau et al.
[2] reported that the formation of mixed MoWS2 crystallites is possible only from a precursor
which contains both closely related metals in the structure at once. The structure of the mixed
active phase is greatly influenced by sulfidation conditions. Previously, for unpromoted catalysts
based on mixed H4SiMo3W9O40 heteropolyacid (HPA), we found that mixed MoWS2 active
species with a core-shell structure, in which smaller islands of Mo were surrounded by W atoms,
were formed in the gas-phase sulfidation, while in the liquid-phase sulfidation a structure with
random distribution of molybdenum and tungsten atoms was formed, as visualized by HAADF
[11]. Moreover, it was found that the use of a mixture of two monometallic H4SiMo12O40 and
H4SiW12O40 HPAs led to the preferential formation of corresponding monometallic MoS2 and WS2
particles. In the co-hydrotreatment of DBT and naphthalene, catalysts with an ordered core-shell
structure of the MoWS2 active phase had highest rate constants for both HDS and HYD reactions.
In addition, the possibility of using bulk Mo(W) sulfide catalysts, which do not contain a
support, in hydrotreatment processes on stationary catalyst beds is also being investigated. The
concentration of active phase in these catalysts can reach 80-100%. With the same composition of
the active phase, the activity of bulk catalysts in hydroprocessing can be 1.5–1.7 times higher than
that of their supported analogs. Thus, industrial bulk NiMoW NEBULA catalysts have higher
catalytic activity compared to traditional alumina supported catalysts. The increase in catalytic
activity can be explained by the formation of highly active trimetallic NiMoW sulfides [12]. In the
middle of last year, the ExxonMobil jointly with Albemarle proposed a new catalyst, Celestia™,
the successor of the NEBULA catalyst. Industrial implementation makes promising the
development of new catalytic systems based on bulk mixed NiMoW sulfides.
4

Recently, due to the development of technologies for deep hydroconversion of heavy oil
residues in three-phase suspension-type reactors (slurry-reactors) [13-16], in which nanoscale
(Ni)Mo(W)S2 particles are formed in situ [14-16], the interest to bulk catalysts based on transition
metal sulfides, has been increasing.
Various methods for preparing bulk catalysts such as comaceration [17], homogeneous
sulfide precipitation [18], thiosalt decomposition [19], hydrothermal [20-21] and solvothermal
[22-23] syntheses, as well as the method of fluoric acid (HF) etching of the substrate of supported
catalysts [24-25] have been described. Previously, we reported that unsupported catalysts
synthesized via etching of alumina support, exhibited higher catalytic properties compared to those
prepared by other methods [26], due to high dispersion of active sulfide particles and good
accessibility to active sites.
Summarizing the above, it seems appropriate to combine the method of HF etching of a
support and the use of mixed MoW oxide precursors to produce mixed bulk catalysts. That will
allow purposefully creating catalysts with a given composition of mixed MoWS2 particles. In the
present work, mixed MoWS bulk catalysts were synthesized by acid etching of alumina support
from supported sulfide catalysts based on mixed H4SiMo1W11O40 and H4SiMo3W9O40 HPAs.
Catalysts prepared from mixture of two monometallic H4SiMo12O40 and H4SiW12O40 HPAs with
the same Mo/W molar ratio as in corresponding mixed HPAs were used as reference samples. The
effects of catalyst composition on catalytic performance were studied in the hydrotreating
reactions of DBT and naphthalene. Prepared bulk catalysts were characterized by methods such as
nitrogen physisorption, extended X-ray absorption fine stricture spectroscopy (EXAFS),
transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), time-of-flight
secondary ion mass spectrometry (ToF-SIMS) and powder X-ray diffraction (XRD). Moreover, a
comparison was made with their alumina supported counterparts in order to reveal the effect of
the support.
5

2. Experimental
2.1. Catalyst preparation
A series of bulk MonW12-nS2 hydrotreating catalysts was synthesized by HF etching of
alumina from MonW12-n/Al2O3 catalysts. First, supported samples were prepared by the incipient
wetness method via impregnation of 훾-Al2O3 extrudates with aqueous solutions of H4SiMonW12-
nO40 HPAs [4,9-11]. These mixed HPAs are derived from the Keggin-type polyoxometallate
H4SiW12O40, consisting of a regular SiO4 tetrahedron surrounded by 12 WO6 octaedra, which are
connected by shared edges to form trimetallic W3O13 groups joined together by their vertices.
Replacement of one or 3 W atoms by Mo led to the formation of α-H4SiMo1W11O40 and β-
H4SiMo3W9O40 HPAs, the structure and synthesis methodology of which were described earlier
by Nikulshina et al. [9]. Two catalysts were synthesized using mixed Keggin-type α-
H4SiMo1W11O40 and β-H4SiMo3W9O40 HPAs (named as Mo1W11S2 and Mo3W9S2, respectively),
another two were based on monometallic SiMo12 and SiW12 HPAs (named as MoS2 and WS2,
respectively), and finally two more bimetallic MoW reference samples were synthesized from a
mixture of monometallic HPAs with Mo/W ratios of 1/11 and 3/9 (denoted RefMo1W11S2 and
RefMo3W9S2, respectively). The oxidic dried samples were sulfided in a fixed-bed reactor at 400
°C for 4 h in a stream of 10 vol. % of H2S in H2 under atmospheric pressure. Detailed
characterization of the prepared supported catalysts was described in [9,11]. After sulfidation, the
samples were treated by a solution of hydrofluoric acid. A typical HF etching process was carried
out as follows [24]: 10 g of sulfided MonW12-n/Al2O3 catalyst, 215 g of 45 wt. % HF, and 537 ml
of deionized water were added to a plastic beaker. Required amounts of HF were calculated to
have a molar ratio HF/ Al2O3 of 60. This mixture was kept with magnetic stirring at 40 ºC for 4 h
to form a black suspension. The solid material was then filtered, washed with 50 ml of deionized
water and acetone, dried in rotary evaporator under vacuum. The chemical compositions of the
prepared catalysts are given in Table 1. Content of carbon and sulfur was measured using 2400
Series II CHNS elemental analyzer. Amount of metals was determined using EDX-7000P XRF
6

analyzer. Prior the characterization of the active phase of the catalysts and evaluation of their
catalytic activities, the solids were sulfided according to the method described above.
2.2. Characterization of the catalysts
2.2.1. Textural properties of catalysts
Textural characteristics of the alumina supported sulfides and solids after HF etching were
determined by N2 physisorption at 77 K on a Quantochrome Autosorb-1 adsorption porosimeter.
The samples were outgassed under vacuum at 623 K for 6 h prior to the adsorption. Specific
surface areas were calculated using the BET method at a relative partial pressure Р/Р0 = 0.05–0.3.
Total pore volumes were determined by a desorption curve using the BJH model at a relative
partial pressure Р/Р0 = 0.99. Pore size distributions were calculated using DFT analysis.
2.2.2. TPR
H2-TPR of the sulfided samples was carried out on a TPDRO 1100 apparatus with the use
of a thermal conductivity detector. Analysis was held in a mixture of H2 and N2 (5 vol. % of H2)
under the following conditions: volume flow rate of 25 ml/min, temperature range from room
temperature to 900 ºC, heating rate of 10 ºC /min.
2.2.3. HRTEM
The morphology of the prepared samples was investigated by means of HRTEM. HRTEM
measurements were performed using a Tecnai G2 20 microscope with a 0.14 nm lattice-fringe
resolution and an accelerating voltage of 200 kV. For statistical analysis, slab length and layer
stacking of 400-600 MonW12-nS2 crystallites were determined for each catalyst.
̅
The average length (퐿) was calculated as an arithmetic mean and corresponds to the average
dimension of the projection of the slabs parallel to the electron beam [26]. The number of slabs
N
per stack was determined to obtain the average stacking degree ( ):
7

n N

i
i
N  i1..t
,
(1)
n

i
i1..t
where ni is the number of stacks with Ni slabs.
The dispersion of Mo(W)S2 species was evaluated statistically by dividing the total number
of Mo(W) atoms on the edge by the total number of Mo(W) atoms using crystallite sizes
determined from HRTEM, which is a common method for determining metal sulfides dispersion
for supported [27,28] and bulk [29,30] catalysts. It is assumed that during sulfidation ideal
hexagonal particles are formed.
2.2.4. Powder XRD
Powder XRD data collection was performed on an ARLX’TRA diffractometer with Cu Kα
emission (λ = 1.54056 Å) operating at 43 kV and 38 mA and recorded peaks were identified using
standard JCPDS files. The approximate crystallite dimensions of the MoS2 slabs were calculated
using the Debye-Scherrer relation [31]:
푘
∙휆
002
퐷002
=
(2)
훽
∙cos휃
002
where 퐷002 is the mean size of ordered (crystalline) domains (Å) along the stacking direction; λ is
the X-ray wavelength; θ is the Bragg angle; β002 (or FWHM) is the line broadening at half
maximum intensity; k002 is the dimensionless shape factor. The shape factor k002 depends on the
crystal shape and is close to 0.9 for WS2 [32] and to 0.76 for MoS2 [31]. The average number of
layers N was calculated using the equation N = D002/6.17, where 6.17 Å corresponds to the value
of the interlayer spacing in the 2H-WS2 structure.
The crystallite size along the basal direction was calculated using the Debye-Scherrer
equation (2) applied to the broadening of the diffraction peak (110). The same calculation method
was previously used by de la Rosa et al. [31] for MoS2 prepared by HF acid etching of the support.
The peak (110) is not affected by imperfect stacking or bending/folding of layers [31,33].
However, Liang et al. [33] found that the shape factor k110 depends on the β110 angular line width,
8

and this correlation between β110 and k110 was taken into account. According to the experimental
angular line widths, the shape factor k110 was equal to 1.49.
2.2.5. XPS
XPS experiments were performed on a KRATOS Axis Ultra spectrometer with a
monochromatic Al Kα source (hν = 1486.6 eV, 150 W). The binding energy (BE) values were
referred to C1s at 284.8 eV to account for charging effects [34]. The spectrum decomposition was
carried out using Casa XPS software after applying a Shirley background subtraction and using
Gaussian–Lorentzian (30/70) parameters. S2p, Mo3d, W4f, C1s and O1s spectra were collected.
The relative concentrations of each species Mo6+, Mo5+ (MoSxOy), Mo4+ (MoS2) on Mo3d region
(analogically for W4f) were determined for all sulfided catalysts. The relative amounts of metal
fractions were determined according to previous works [11, 35, 36].
2.2.6. EXAFS spectroscopy
The Mo K-edge and W L1,2,3 -edges EXAFS spectra were collected at BM31 beamline of the
ESRF (Grenoble, France) [37]. The sample powder was diluted with boron nitride, pressed into a
pellet and sealed with Kapton tape. All spectra were recorded at room temperature in transmission
mode with simultaneous collection of molybdenum and tungsten metal foils for energy calibration.
The energy was selected with a double-crystal Si (111) monochromator in the continuous scanning
mode. EXAFS data analysis (normalization, background removal, energy alignment, extraction of
χ(k) signal and Fourier analysis) was performed in the Demeter software package [38], FEFF6 [39]
was used to calculate theoretical phases and amplitudes. The following structural parameters were
set as variables during the fit: interatomic distances (R), coordination numbers (N), Debye-Waller
factors (σ²) for Mo‒Mo, Mo‒S, Mo‒W (and W‒Mo), W‒W, and W‒S scattering paths. To reduce
the number of variables, R and σ² for W‒Mo path were set equal to the corresponding values of
푁
Mo‒W path. The equation 푁W−Mo
=
Mo−W was also applied, based on the stoichiometric ratio of
3
the two metals. Two energy shifts (ΔE0) were used for all paths at Mo K- and L3-edges The fit was
9

performed in R-space in 1.2 - 3.4 Å range using a multiple k-weighted data (1,2,3). The Fourier-
transformation ranges for Mo K-edge and W L3-edge were set to ΔkK = 3.5-15.0 Å−1, ΔkL3 = 4.3-
2
16.3 Å−1, respectively. Amplitude reduction factors (S0 ) were obtained by fitting bulk MoS2 and
WS2 references. For direct comparison, the same fitting strategy was applied for the analysis of
the data previously collected for the supported catalysts [9].
2.2.7. ToF-SIMS measurements
ToF-SIMS measurements were performed with a TOF.SIMS 5 spectrometer (ION-TOF
GmbH Germany) equipped with a bismuth liquid metal ion gun (LMIG). The compacted samples
+
were bombarded with pulsed Bi3 primary ion beam (25 keV, 0.25 pA) rastered over a 100 × 100
m2 surface area. With 30 scans and 128x128 pixels, the total primary ion dose does not amount up
to 1012 ions/cm2 ensuring static conditions. Charge effects due to primary ion beam were
compensated by means of a 20 eV pulsed electron flood gun. Cycle time was fixed at 150 µs in
order to detect secondary molecular ions up to 2000 m/z. The mass resolution (m/∆m) measured
-
on our spectra was about 4000 at m/z = 143 for MoO3 . This good mass resolution allowed us to
identify high m/z ionic fragments by their exact mass and the attribution could be confirmed most
of the time by the simulated isotopic pattern.
2.3. Catalytic performances
The HDT activity tests were performed in a bench-scale flow reactor at 320 ºC, 3.0 MPa
total pressure of hydrogen, with a liquid hourly space velocity (LHSV) of 40 h−1 and a 500 NL/L
volume ratio of hydrogen to feed. Pre-sulfided MonW12-nS2 catalysts (0.2 g) were diluted with 0.6
cm3 of low-surface-area carborundum (0.2–0.4 mm) and placed in the center of the reactor with
an internal diameter of 0.8 cm. A toluene solution of DBT (Aldrich, 1500 ppm of S), naphthalene
(Aldrich, 3wt. %) and hexadecane (as an internal standard, 1 wt. %) was used as a model feedstock.
The liquid product compositions of the samples collected every hour were determined using a
Crystall-5000 Gas Chromatograph equipped with a 30 m OV-101 column. The reaction products
were identified by matching retention times with those of commercially available standards and
10

by GC/MS analysis using a Finnigan Trace DSQ. All catalysts exhibited stable performance,
achieving a steady state after 7 – 10 h.
The rate constants of the pseudo-first-order reactions of the DBT HDS and naphthalene HYD
were determined using the following equations:
FNaph
W
F
kHDS   DBT ln(1 xDBT
)
and
(3)
kHYD  
ln(1 xNaph ),
W
kHDS
kHYD
where
and
are the pseudo-first-order reaction constants for the DBT HDS and
xDBT
naphthalene HYD (mol g−1 h−1), respectively,
and
are the conversions (%) of DBT, and
xNaph
are the reactant flows in moles (mol h−1) and W is the
F
FNaph
naphthalene, respectively, DBT and
weight of the catalyst (g).
The HDS products from DBT included biphenyl (BP) via the direct desulfurization (DDS)
pathway, as well as cyclohexylbenzene (CHB) and dicyclohexyl (DCH) from the HYD pathway.
Only traces of hydrogenated tetrahydro- and hexahydrodibenzothiophenes were observed. The
HYD/DDS selectivity was calculated according to the reaction network for DBT HDS:
kHYD CCHB CDCH
SHYD/DDS


(4)
kDDS
CBP
where CCHB, CBCH and CBP are the concentrations (mol. %) of CHB, DCH and BP in the reaction
products, respectively.
The turnover frequencies (TOF, s−1) normalized on edge sites of MonW12-nS2 slabs for the
HDS of DBT, HYD of naphthalene allowed us to get more complete understanding of the catalytic
properties of the active phase species. TOF values were calculated using the following equations:
FDBT  xDBT
FNaph  xNaph
and
,
(5)
TOFHDS

TOFHYD

С
С






С
СMoS
2






WS2
MoS2
WS2
W 

 D3600
W 

 D3600
ArW ArMo
ArW ArMo
are the reactant flows (mol h−1),
FDBT
xDBT
and
FNaph
xNaph
are the conversions (%) of
where
and
С
С
MoS2
DBT and naphthalene, respectively; W is the weight of the catalyst (g);
and
are the
11
WS2

effective content of W and Mo, respectively, in MonW12-nS2 species (wt. %); D is the dispersion
ArW
ArMo
are the standard atomic weights of tungsten (183.9 g/mol)
of MonW12-nS2 species;
and
and molybdenum (95.9 g/mol), respectively.
3. Results
3.1. Textural properties of catalysts
The textural properties of the prepared samples are summarized in Table 1. Sulfided
alumina-based precursors displayed surface areas in the range of 164 to 218 m2/g and pore volumes
around 0.53 cm3/g. The alumina removal resulted in a decrease of the surface area to 3-14 m2/g
and pore volume to 0.01-0.04 cm3/g. These significant changes in the textural properties are related
to the total removal of the porous support. X-ray fluorescence and XPS analysis confirmed the
absence of aluminum and silicon in the solids after etching procedure. According to Fig. 1S, all
materials exhibit adsorption isotherms similar to type I with virtually a non-porous structure. These
results are in agreement with those previously reported for bulk catalysts [40-43].
3.2. TPR analysis
Fig. 1 shows the H2-TPR profiles of the unsupported MonW12-nS2 catalysts and of their
references. The reduction of the MoS2 sampleproceeded in the region 100-400 ºC with a maximum
at 303 ºC. The TPR profiles of WS2 displayed two reduction regions: 250-400 ºC (Tmax=342 ºC)
and 400-600 ºC (Tmax=461 ºC). The appearance of a peak in the high-temperature region for WS2
can be attributed to stronger W‒S bond than Mo‒S [44]. All bimetallic catalysts also exhibited
two main reduction peaks. According to the literature [45-47], the first low-temperature peak
corresponds to surface weakly bonded sulfur. Afanasiev et al. [46] believe that after the reduction
of surface sulfur, coordinative unsaturated sites (CUS) are formed, which in turn are responsible
for the reactions that occur on the surface of the catalyst. This allows to indirectly estimate the
possible number of active sites. Lower reduction temperatures of mixed HPA based catalysts are
12

observed compared to that of their corresponding references: 334 and 378°C for Mo1W11S2 and its
reference, 319 and 339°C for Mo3W9S2 and its reference for the first reduction peak. This can be
related to a decrease in the strength of the metal-sulfur bond due to the formation of mixed MoWS2
particles with a more optimal bond energy value [48]. As the molybdenum loading increased, the
reducibility of the catalysts was enhanced as indicated by the decrease in the reduction temperature
(342°C for WS2, 334°C for Mo1W11S2, 319°C for Mo3W9S2) and the increase in H2 consumption
(230 mmol/mol for WS2, 390 mmol/mol for Mo1W11S2, 640 mmol/mol for Mo3W9S2, Table 2).
Furthermore, the H2 consumption was higher for the samples based on MoW HPAs than for their
analogs prepared from a mixture of separate HPAs (390 and 367 mmol/mol for Mo1W11S2 and its
Ref, 640 and 450 mmol/mol for Mo3W9S2 and its Ref), which indicates that a larger amount of
active sites is present in the MonW12-nS2 catalysts.
The maxima temperature of the first reduction peak was lower in the supported catalysts
than in the corresponding bulk ones after HF etching. H2 consumption of all tungsten-containing
catalysts was higher than the values of corresponding alumina supported samples (Table 2), which
can be related to a larger amount of active sites through increasing the number of Mo(W)S2 species.
3.3. Morphology of the active phase
Morphology of MonW12-nS2 particles was investigated by HRTEM. Fig. 2 shows
representative TEM images taken for bulk samples. Typical fringes of Mo(W)S2 crystallites with
6.1 Å interplanar distances were observed on micrographs of all sulfided catalysts. Average length
and stacking of bulk MonW12-nS2 catalysts are shown in Table 3. Monometallic WS2 catalyst had
the highest average length (6.7 nm) and stacking (3.5) among all the prepared samples. MoS2
species of pure molybdenum catalyst were shorter (5.2 nm) and a little less stacked than those of
pure tungsten disulfide one. Addition of molybdenum atoms in the content of the catalysts resulted
in a decrease in size of sulfided slabs. Both bimetallic reference catalysts had almost the same
average length despite different Mo contents, while the particles size decreased with the increase
of Mo amount in mixed HPAs based catalysts. All bimetallic samples had an average stacking
13

around 3.0, which expectedly exceeded the values for conventional supported catalysts with 1-2
stacking layers [49,50]. Therefore, removing of the alumina support led to agglomeration of
Mo(W)S2 species and to reduction of their dispersion compared to initial supported ones.
3.4. XRD
The XRD patterns of the prepared MonW12-nS2 catalysts are given in Fig. 3. All samples
exhibited several well-resolved XRD peaks at 14 (002), 33 (100), 40 (103), and 59° (110) 2θ
corresponding to MoS2 (WS2) structure. It can be seen that the results of XRD do not allow to
identify a difference in the structure of prepared bimetallic catalysts due to slight shifts of the
reflections between MoS2 and WS2 phases. Average crystallite sizes were estimated by Debye-
Scherrer equation (Table 3). Monometallic references WS2 and MoS2 catalysts had the longest
and the shortest sulfide slabs, respectively, as observed by HRTEM. Average length and stacking
of crystallite in both mixed HPAs based samples and RefMo1W11S2 were equivalent and less than
those of WS2. In contrast, the values calculated for RefMo3W9S2 were close to MoS2 catalyst. Such
decrease of particles size indicated that the sample prepared from two HPAs may have separate
monometallic MoS2 and WS2 species. In addition, it should be noted that, on the one side, a slight
difference in the cell parameters of MoS2 and WS2 phases may also result in increasing the width
of XRD peaks, leading to an underestimated value of the particle size, and on the other side, TEM
results are weighted by the number of particles, while XRD is weighted by particle volume, i.e.
larger particles give more contribution.
3.5. Sulfidation of metals evaluated by XPS
Fig. 4 shows examples of decomposition of Mo 3d and W 4f XPS spectra of the sulfided
MonW12-nS2 catalysts. The Mo spectra exhibit a Mo 3d5/2 peak at about 229.0 eV characteristic of
MoS2 [4,34,35,51], the W 4f spectra contain 4f5/2 peak at about 32.4 eV, associated with WS2
[4,11,50,52].
14

Table 4 gives the metal fractions of molybdenum and tungsten species present on the surface
of the synthesized MonW12-nS2 catalysts. All studied samples had a high sulfidation degree of
metals (92 rel. % and more), corresponding to an increase of around 8% for Mo and more than
30% for W compared to supported catalysts [11]. That high amount of metals in the Mo(W)S2
phase and the near absence of oxide and oxysulfide species is the result of HF etching, where only
fully sulfided species were resistant. Surface Mo/W ratios were close to the bulk values determined
by XRF analysis due to homogeneous distribution of particles in bulk solids. Higher Mo/W atomic
ratios were detected in bulk sulfide solids compared to supported precursors. This result can be
explained by the higher percentage of tungsten oxide and oxysulfide species compared to
molybdenum ones contained in sulfided Mo(W)/Al2O3 catalysts [11], that were completely
dissolved during the HF treatment. The S/metal atomic ratios for all catalysts were close to the
theoretical value of 2 expected for MoS2/WS2.
3.6. EXAFS analysis
Fig. 5 compares the magnitude of k3-weighted Fourier transformed (FT) EXAFS data for
both unsupported and supported [9] MonW1-nS2 catalysts (solid and dashed lines, respectively).
The spectra of all catalysts exhibit two main peaks at ca. 2.0 Å and 2.9 Å (phase-uncorrected)
corresponding to metal-sulfur (M‒S) and metal-metal (M‒M) scatterings, respectively. The shape
of FT EXAFS spectra of the MonW1-nS2 catalysts depends on the starting oxide precursor. In both
Mo K and W L3-edge spectra, the highest peak intensity in the region of 3 Å (phase-uncorrected)
was observed for monometallic MoS2 and WS2 catalysts, while this intensity was lower in
bimetallic samples, especially in the case of the catalyst based on the mixed SiMo3W9 HPA. This
fact is indicative of the presence of two different metal-metal contributions (homo- and
heterometallic) in the second coordination shell. Indeed, the W‒W (Mo‒Mo) and W(Mo)‒Mo(W)
contributions have similar frequencies, but are out of phase and cancel each other, which leads to
a decrease in the amplitude of the resulting χ(k) and χ(R) signals as shown in Fig. S3. The same
effect leads to a high uncertainty in the determination of the total Mo‒M and W‒M coordination
15

푁
numbers, which was overcome by fixing 푁W−Mo
=
Mo−W, based on the stoichiometric ratio of
3
the two metals.
The obtained best-fit values are listed in Table 5. All catalysts have relatively high
sulfidation degree with NM-S ~ 5 – 6 depending on the sample. The second shell of monometallic
samples contains one M‒M contribution at 3.16 Å: Mo‒Mo in MoS2 (NMo-Mo = 3.7) and W‒W in
WS2 (NW-W = 4.4). The total M-M coordination numbers increase after elimination of the support
indicating a larger length of the slabs. The coordination numbers M‒M in bimetallic catalysts
depend on the initial precursor. At both Mo K- and W L3-edges, NM1-M2 (where M1 and M2
correspond to different types of metal atom) is higher in the mixed HPA based catalyst than in its
bimetallic reference, which evidences the formation of a bimetallic MoWS2 phase. Reference
bimetallic samples also contain a mixed M1-M2 contribution, but the corresponding coordination
numbers are very close to the experimental uncertainty. To highlight the structural changes
induced by the elimination of the support, the corresponding values for the supported catalyst [9]
analyzed using the same fitting model are reported in brackets.
3.7. ToF-SIMS analysis
Two zones were analyzed by ToF-SIMS on Mo1W11 and Mo3W9 as well as on their reference
counterparts RefMo1W11S2 and RefMo3W9S2. This technique allows to investigate possible
interaction between Mo and W, as it probes the top layers (1-3 nm) of the catalysts and gives
-
molecular information about surface and interfaces. MoxOySz- and WxOySz monometallic
fragments were indeed recorded on pure MoS2 and WS2 (Fig. 6). On the mixed catalysts, in
addition to these monometallic fragments, mixed fragments containing Mo and W are also
evidenced (Fig. 6 and Fig. S4). These fragments clearly show that a close interaction exists
between Mo and W in the bulk catalysts, indicating that the mixed MoWS2 phase evidenced in the
supported catalysts is maintained after etching of the support. The intensity of each identified
mixed oxide fragments was normalized in order to compare the four catalysts, as illustrated in Fig.
7. Normalization was performed using fragment intensity of 182WS-. If mixed fragments are
16

obtained on all the samples, the intensity of the corresponding peaks is higher on the catalysts
prepared from mixed SiMonW12-n HPA compared to those prepared from a mixture of
monometallic HPAs, as observed on the corresponding supported catalysts.
3.8. Catalytic performances
Catalytic behavior of the synthesized MonW12-nS2 catalysts was evaluated in simultaneous
HDT of two model compounds, DBT and naphthalene (Table 6). The results indicated that, under
the chosen reaction conditions, substitution of one or three tungsten atoms by molybdenum in the
catalyst composition resulted in an increase in HDS as well HYD activities compared to
monometallic MoS2 and WS2 references, regardless of the precursor type. HDS activities in the
two catalysts with Mo/W ratio = 1/11 were close. However, the Mo3W9S2 catalyst prepared from
the mixed SiMo3W9 HPA has a twice-higher catalytic activity than its RefMo3W9S2 counterpart
prepared from mixture of two separate HPAs in both studied reactions (conversions of 50 and 29%
respectively in HDS and conversions of 35 and 18% respectively in HYD). These results clearly
show the advantage of using the mixed SiMo3W9 HPA as the starting oxidic precursor for the
synthesis of bimetallic bulk MoWS2 catalyst.
The detailed analysis of the reaction products shows that for all studied catalysts, the
preferential pathway in DBT HDS is pre-HYD. Selectivity HYD/DDS ratio of the prepared
catalysts ranges from 1.0 to 2.8.
4. Discussion
In order to gain a better understanding of the synergetic effect between molybdenum and
tungsten, the rate constants in HDS and HYD were calculated by additive way based on the results
obtained over monometallic MoS2 and WS2 references catalysts. Experimental values of the rate
constants in DBT HDS and naphthalene HYD over Mo1W11S2 and Mo3W9S2 surpass the
theoretical ones by 1.4 and 2.7 times, respectively. Catalytic activities of bimetallic reference
samples are also higher than predicted, however, the difference is smaller.
17

A comparison of catalytic properties of unsupported and alumina based catalysts shows that
alumina etching results in an increase of total HDS and HYD activity that can be attributed to the
absence of metal-support interaction and, as a consequence, raise of metal sulfidation degree (Fig.
8). It should be noted that in case of bulk catalysts, the WS2 sample based on monometallic SiW12
HPA demonstrated improved catalytic properties compared to monometallic MoS2 one. We
suggest that there are two possible reasons for this effect: (i) more significant concentration of
sulfide species (WS2) in W-based catalyst during support removing due to dissolution of low active
in catalysis oxide and oxysulfide particles contained in a larger amount in W-based catalyst
compared to Mo-based one; (ii) more covalent W‒S strength in bulk WS2 sample due to the
absence of oxysulfide species compared to supported WS2/Al2O3 analog.
HF acid treatment of alumina supported sulfide catalysts leads to an increase in the linear
size and average stacking number of sulfide particles, which is independently confirmed by
HRTEM, XRD and EXAFS data. The metal sulfidation degree was also raised especially for
tungsten, due to removal of non sulfided species. Moreover, the above results allow us to suppose
that the nature of starting oxidic precursor influences the active phase composition and properties.
We previously reported the formation of mixed MoWS2 particles with core-shell structure, with
Mo atoms predominantly located together in the core region, in Mo3W9/Al2O3 catalyst during gas
phase sulfidation [9,11]. This fact is confirmed by higher total Mo‒M coordination numbers in
both supported and unsupported catalysts, compared to those for W‒M contribution. Moreover,
the changes in coordination numbers after the elimination of the support (see Table 5) indicate that
the increase in Mo‒Mo coordination numbers is more significant than the increase in Mo‒W ones,
which means that the agglomeration of MoWS slabs preferentially occurs via Mo edges as
schematically illustrated in the Fig. 9a. It should be noted, that for unsupported catalyst the total
Mo‒M coordination numbers are close to 6 as in the bulk samples, suggesting that most of
molybdenum is located inside the slabs and not on the edges.
18

The preferential location of Mo-atoms in the core-region of the slabs is also confirmed by
Mo‒S coordination numbers close to 6. The lower values obtained for W‒S contribution can be
explained by the elongation of W‒S distances for two sulfur atoms located at M-edge, which may
lead to a partial antiphase of the resulting signals, which cannot be reproduced by using a single
M‒S scattering path. Together with a significant fraction of Mo‒W coordination, the obtained
results confirm the preservation of mixed MoWS2 active phase in the unsupported Mo3W9S2 mixed
HPA based catalysts suggesting the structure of the active phase after removal of the support.
The above correlates with the ToF-SIMS analysis. The higher amount of mixed sulfide
species in MoW HPA based catalyst explains its almost twice higher activity in DBT HDS and
naphthalene HYD compared to the bimetallic references. The small amount of mixed Mo‒W phase
observed in EXAFS for RefMo3W9S sample may be explained by partial connection of
monometallic MoS2 and WS2 slabs to bimetallic ones as schematically illustrated in Fig. 9b.
However, the efficiency of such particles is still lower, compared to those formed from the mixed
precursor. That assumption is in line with the calculated TOF values (Table 6) as well as HDS and
HYD rate constants which correlated with the relative W‒Mo coordination number (Fig. 10).
Comparing the sample within one supported or unsupported series of catalysts, it can be seen that
catalytic activity was increased with an increase of NW-Mo
.
5. Conclusions
We found that the HF etching of the alumina in MonW12-n/Al2O3 sulfided catalysts led to
successful removal of the support. Further interaction between the particles of the etched sulfide
active phase after resulfidation of bulk solids resulted to an increase in the average particle length
and stacking number, which is independently confirmed by HRTEM, XRD and EXAFS data. The
metal sulfidation degree was also raised especially for tungsten, due to removal of non sulfided
species. The presence of mixed MoWS2 slabs was confirmed by EXAFS and ToF-SIMS analysis.
A beneficial effect in the catalytic activity of bimetallic unsupported MoWS2 catalysts
compared to monometallic MoS2 and WS2 ones has been shown. Total DBT HDS and naphthalene
19

HYD activities were correlated with the Mo-W contribution. Among the prepared bimetallic
catalysts, the Mo3W9S2 sample synthesized using H4SiMo3W9O40 HPA as starting material
demonstrated the highest HDS and HYD activities. Together with a significant fraction of Mo‒W
coordination, the obtained results confirm the preservation of mixed MoWS2 active phase in
unsupported Mo3W9S2 mixed HPA based catalysts suggesting the structure of the active phase
after removal of the support.
Acknowledgments
Authors thank Russian Science Foundation for financial support of the investigation by
Grant No. 17-73-20386. Chevreul Institute (FR 2638), Ministère de l’Enseignement Supérieur et
de la Recherche, Région Nord – Pas de Calais and FEDER are acknowledged for supporting and
funding partially this work (TEM and ToF-SIMS).
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that
could have appeared to influence the work reported in this
CRediT author statement
A. Kokliukhin: Investigation, Writing- Original draft preparation; M. Nikulshina: Validation,
Writing - Original draft preparation; A. Mozhaev: Investigation; C. Lancelot: Investigation,
Writing - Reviewing and Editing; C. Lamonier: Conceptualization; N. Nuns: Investigation; P.
Blanchard: Validation; A. Bugaev: Investigation, Methodology; P. Nikulshin:
Conceptualization, Writing- Reviewing and Editing
20

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24

Captions for Tables
Table 1.
Composition and textural properties of prepared unsupported MonW12-nS2 catalysts.
Table 2.
Table 3.
Quantitative H2-TPR results of unsupported MonW12-nS2 catalysts.
Morphological characteristics of MonW12-nS2 active phase species calculated from
TEM micrographs and XRD.
Table 4.
Table 5.
Metal fractions measured by XPS for molybdenum and tungsten species present in
unsupported MonW12-nS2 catalysts.
Structural parameters obtained from the Fourier-analysis of Mo K- and W L3-edges
EXAFS spectra of unsupported MonW12-nS2 catalysts. The values in brackets
correspond to the supported catalysts. The values for W‒Mo scattering path given
without the uncertainty were linked to the values for Mo‒W path.
Table 6.
Catalytic properties of prepared unsupported MonW12-nS2 catalysts in HDT of a
mixture of DBT and naphthalene.
Captions for Figures
Figure 1. H2-TPR profiles for unsupported MonW12-nS2 catalysts.
Figure 2. HRTEM micrographs of unsupported MonW12-nS2 catalysts.
Figure 3. XRD patterns of unsupported MonW12-nS2 catalysts.
25

Figure 4. XPS Mo 3d and W 4f spectra recorded for unsupported MonW12-nS2 catalysts; in
blue: Mo(W)6+ oxide contributions; in pink: Mo(W)SxOy contributions; in gray:
Mo(W)S2 contributions (For interpretation of the references to color in this figure
legend, the reader should refer to the web version of the article).
Figure 5. Phase-uncorrected Fourier transformed Mo K- (a) and W L3- (b) edges k3-weighted
EXAFS data for unsupported monometallic (blue lines) and bimetallic (red lines)
catalysts. Green lines correspond to the sample prepared from the mixture of
monometallic HPAs. Dashed lines correspond to the data for similar supported
catalysts reported in [9]. Corresponding χ(k) signals are shown in Figure S2 of the
Supporting Information.
Figure 6. Mass spectra obtained by TOF-SIMS analysis of unsupported MoS2, WS2,
Mo3W9S2 and RefMo3W9S2 catalysts
Figure 7. Fragments contribution on unsupported MonW12-nS2 catalysts.
Figure 8. Rate constants of DBT HDS (a) and naphthalene HYD (b) over unsupported
MonW12-nS2 and supported MonW12-n/Al2O3 [9]catalysts.
Figure 9. The possible simplified model of mixed MoWS2 active particles in unsupported
Mo3W9S2 (a) and RefMo3W9S2 (b) catalysts.
Figure 10. Activity in DBT HDS (a) and naphthalene HYD (b) over unsupported MonW12-nS2
and supported MonW12-n/Al2O3 [9] catalysts depending on the W-Mo coordination
number.
26

Table 1
Composition and textural properties of prepared unsupported MonW12-nS2 catalysts.
Content in the catalyst (wt. %)
Mo/W atomic
Specific
surface area
(m2/g)
Pore volume
(cm3/g)
Average pore
diameter (nm)
Catalyst
S/(Mo+W)
ratio a
C
S
2.0
1.2
1.4
1.9
1.5
1.0
35.0
25.3
27.2
29.4
28.1
30.4
-
1.8
2.2
2.2
2.0
2.2
2.1
8 / 210 b
0.04 / 0.57 b
0.03 / 0.48
0.01 / 0.56
0.01 / 0.51
0.03 / 0.51
0.04 / 0.55
< 3.8 / 7.2 b
< 3.8 / 7.9
< 3.8 / 6.8
< 3.8 / 6.8
< 3.8 / 7.2
< 3.8 / 7.9
MoS2
-
14 / 164
5 / 218
5 / 179
10 / 182
3 / 190
WS2
0.15
0.55
0.20
0.60
Mo1W11S2
Mo3W9S2
RefMo1W11S2
RefMo3W9S2
a XRF analysis; b Values for supported sulfided MonW12-n/Al2O3 catalysts.
27

Table 2
Quantitative H2-TPR results of unsupported MonW12-nS2 catalysts
1st peak
H2 cons.
(mmol/molM)
303 / 343* 715 /1453*
2nd peak
H2 cons.
(ºC) (mmol/molM)
3rd peak
H2 cons.
(ºC) (mmol/molM)
Total H2
cons.
Catalyst
Tmax
Tmax
Tmax (ºC)
(mmol/molM)
MoS2
-
-
-
-
60
-
715
895
622
813
746
670
WS2
342 / 259
334 / 255
319 / 263
230 / 99
390 / 247
640 / 258
367 / 180
450 / 330
461
506
514
503
515
604
232
174
379
202
674
Mo1W11S2
Mo3W9S2
-
-
-
-
-
RefMo1W11S2 378 / 252
RefMo3W9S2 339 / 269
-
-
* Values for supported sulfided MonW12-n/Al2O3 catalysts.
28