Preparation of Ni-Mo2C/carbon catalysts and their stability in the HDS of dibenzothiophene

Article abstract of DOI:10.1016/j.apcata.2017.04.008

The HDS activity and stability of Ni-Mo₂C/AC catalysts, prepared by carbothermal hydrogen reduction (CHR) at different temperatures and with different Ni:Mo ratios, is reported. The highest HDS activity occurred for catalysts with Ni:Mo ratios

Full text of DOI:10.1016/j.apcata.2017.04.008

Accepted Manuscript
Title: Preparation of Ni-Mo C/Carbon Catalysts and their
2
Stability in the HDS of Dibenzothiophene
Authors: Haiyan Wang, Shida Liu, Rubenthran Govindarajan,
Kevin J. Smith
PII:
S0926-860X(17)30164-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2017.04.008
APCATA 16206
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
6-2-2017
31-3-2017
14-4-2017
Please cite this article as: Haiyan Wang, Shida Liu, Rubenthran
Govindarajan, Kevin J.Smith, Preparation of Ni-Mo2C/Carbon Catalysts
and their Stability in the HDS of Dibenzothiophene, Applied Catalysis A,
Generalhttp://dx.doi.org/10.1016/j.apcata.2017.04.008
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

Preparation of Ni-Mo
Dibenzothiophene
2
C/Carbon Catalysts and their Stability in the HDS of
Haiyan Wang, Shida Liu, Rubenthran Govindarajan, Kevin J. Smith*
Department of Chemical and Biological Engineering, University of British Columbia, 2360 East
Mall, Vancouver, B.C., V6T 1Z3, Canada.
*Corresponding author. Tel.: +1 6048223601; Fax: +1 6048226003.
E-mail address: kjs@mail.ubc.ca (K. J. Smith)
Graphical Abstract:
1
/ 48

Highlights:

Carbothermal hydrogen reduction (CHR) temperature decreased to 550 C by Ni
addition in Ni-Mo C/carbon catalyst synthesis.
2



Ni addition increases S uptake during HDS, resulting in the rapid formation of Mo
MoS core-shell structures.
2
C-
2
The core-shell Mo
dibenzothiophene.
2
C-MoS structures promote direct desulfurization (DDS) of
2
HDS activity of Mo C enhanced with Ni addition up to Ni:Mo < 0.38.
2
Abstract
The HDS activity and stability of Ni-Mo C/AC catalysts, prepared by carbothermal
2
hydrogen reduction (CHR) at different temperatures and with different Ni:Mo ratios, is reported.
The highest HDS activity occurred for catalysts with Ni:Mo ratios of 0.38 and 0.19, when
prepared at the relatively low CHR temperature of 550 and 600 C, respectively. Partial
sulfidation of the Ni-Mo
dibenzothiophene (DBT) at 350 C and 2.1 MPa H
Ni content, resulting in the formation of Mo C-MoS
2
C/AC catalysts occurred rapidly upon reaction in 2 wt%
2
. Uptake of S was enhanced with increased
2
2
core-shell structures. The stack height of
2
/ 48

the MoS
2
shell increased with increased Ni content and promoted the direct desulfurization of
DBT.
Keywords: Molybdenum carbide; Promoter; Hydrodesulfurization (HDS); Carbothermal
hydrogen reduction; Dibenzothiophene
Introduction
Hydrodesulfurization (HDS) [1] is an important processing step in heavy oil upgrading that
uses NiMoS/Al
O
2 3
or CoMoS/Al
2
O catalysts to facilitate S removal from heteroaromatic
3
compounds present in the heavy oil [2, 3]. Novel catalysts, such as metal carbides and nitrides,
have also been investigated as potential hydrotreating catalysts over the past several decades [4-
6
], in part because of their noble metal-like catalytic properties [7-10]. Sajkowski and Oyama
11] reported that a Mo C/Al catalyst had twice the HDS activity of a conventional MoS
catalyst when measured at 633 K and 13.0 MPa H using a gas oil with 116 ppm S. The
comparison was made based on the number of active sites as measured by CO uptake. Similarly,
Da Coasta et al. [12] reported that a Mo C/Al catalyst doped with P was more active than an
industrial hydrotreating catalyst for aromatics hydrogenation, HDS and hydrodenitrogenation
[
2
2
O
3
2
2
2
2
O
3
3
/ 48

(HDN) when measured at industrial conditions (613 K, 3 MPa) using a gasoil with 47 ppm S
content.
One of the difficulties encountered when applying Mo
unstable because of surface sulfidation, even in the presence of low concentrations of S (< 0.1
wt%). Aegerter et al. [13] proposed that a thin layer of highly dispersed MoS is formed on the
surface of Mo C or Mo N particles during thiophene HDS, as evidenced by IR and TPD-CO
characterization of the catalysts. The authors proposed that Mo C and Mo N particles serve as
rigid substrates for a sulfided Mo phase, exposing a large number of co-ordinatively unsaturated
cus) Mo sites, which results in increased activity of the catalyst (by 50%), compared to a
conventional MoS catalyst. Da Costa et al. [14] also reported that a Mo C catalyst became more
active than a Co-Mo/Al commercial catalyst for the HDS of 4,6-dimethyldibenzothiophene
4,6-DMDBT) with low levels of S in the feed (<135 ppm). Oyama et al. [15] suggested the
formation of a Mo-carbosulfide active site to explain product selectivities observed during the
simultaneous conversion of sulfur, oxygen and aromatic compounds over Mo C, compared to the
2
C catalysts for HDS is that they are
2
2
2
2
2
(
2
2
2
O
3
(
2
product selectivities measured with a single reactant, especially for the case of cumene
hydrogenation. In another study, Brito and coworkers [16] presulfided a NiMo carbide catalyst
prior to HDS and claimed that a Ni-Mo-S phase was responsible for the measured HDS activity.
Guangzhou et al. [17] reported on NiMo carbide and Mo
2
C catalyst performance for the HDS of
dibenzothiophene (DBT) and pointed out that Ni addition increased the Mo
2
C activity by 57 %,
4
/ 48

although there was no conclusion drawn regarding the effect of S on the metal carbides during
HDS. The transformation of Mo C catalysts in the presence of S-compounds remains somewhat
2
unclear and the application of these catalysts for HDS in high S concentrations (as is the case for
gas oils derived from residue oils, for example) has not been reported.
Several methods have been developed to synthesize Mo
unsupported catalysts or as Mo C dispersed on high surface area Al
18, 19]. The most common synthesis method uses temperature-programmed reduction (TPR) of
metal oxides in mixtures of H and a hydrocarbon such as CH [20] or C [21]. However, the
2
C catalysts, either as bulk,
2
O
2 3
[13] and carbon supports
[
2
4
H
2 6
resulting low surface area and carbon residue on the catalyst surface means that the catalysts
have relatively low activity. An alternative preparation method uses carbothermal hydrogen
reduction (CHR), employing a carbon support and pure H
2
to generate the metal carbide at high
C, as first
o
reduction temperatures (700-800 C). This method yields nanoparticles of Mo
2
demonstrated by Mordenti et al. [22] and Liang et al. [23] and also reported previously by the
authors [24]. Thus, CHR is a relatively facile method to synthesize carbon supported Mo
2
C
catalysts, in which the carbon support may also suppress coke formation during residue oil
upgrading [25]. However, CHR has a limited capacity to achieve complete
reduction/carburization of the catalyst precursor, even at high reduction temperatures. For
example, in previous work [24], CHR of ammonium heptamolybdate impregnated activated
charcoal (AC) yielded a mixed Mo
2 x
C/MoO C
y
phase with a relatively high portion of Mo C
2
5
/ 48

o
(
~50%) obtained at 700 C. However, almost half of the carbon support was hydrogenated
mostly to CH ) during the CHR process, which decreased the support area available to achieve
C.
(
4
high dispersion of the Mo
2
One potential option to increase the Mo
the synthesis temperature required for the formation of the metal carbide. Liang et al. [26]
reported that the presence of Co decreased the CHR temperature for Mo C formation to 600 C,
at a Co/Mo molar ratio of 1.0. They concluded that the Co increased the formation of CH
2
C dispersion is to add a second metal to decrease
o
2
x
species during CHR, facilitating carburization of the Mo. Similar effects have been reported in
nd
the synthesis of W
has also been shown to provide synergistic effects in the case of Co added to Mo
for the chemo-selective reduction of nitroarenes, with the Co-Mo C/AC catalyst having
significantly higher activity than the Mo C/AC alone [29]. Moreover, the bimetallic carbides
2
C by CHR using Ni as the promoter [27, 28]. The presence of the 2 metal
2
C when used
2
2
formed in the presence of Mo and other transition metals [30, 31] such as Fe, Co and Ni, show
effective catalytic activity in hydrotreating reactions.
The present study reports on the CHR synthesis of Ni-Mo
AC) and their application to the HDS of DBT at high S concentrations (~ 3500 ppm) that mimic
the S content of heavy oil [1]. New data are reported that capture the effect of Ni content on the
formation of the Ni-Mo C catalysts and their resulting HDS catalytic activity. In particular, the
2
C supported on activated carbon
(
2
6
/ 48

changes in catalyst morphology that occur during reaction, with the formation of a core-shell
MoS -Mo C structure, is reported for the first time. The structural changes are related to the
stability of the Ni-Mo C/AC and the Mo C/AC catalysts.
2
2
2
2
2. Experimental
2
.1 Catalyst Preparation
2
Activated charcoal (AC: 1025 m /g, pore volume 0.85 mL/g) was impregnated with a
solution of ammonium heptamolybdate (AHM) in acetone (10 %) and H O (90 %) and rotary
2
evaporated under vacuum to remove all the solution. The resulting solid was aged in air for 4 h
o
before drying at 110 C overnight to yield the precursor. The precursor was then calcined in N
2
at
o
3
00 C for 5 h to obtain the oxide states of Mo, Ni or bimetallic NiMo. The calcined precursor
(ca. 1.8 g) was placed in a U-tube reactor and subsequently converted to the Ni-Mo
2
C/AC
catalyst by CHR in a continuous H
2
flow (200 mL (STP)/min), while increasing temperature
o
o
o
from room temperature to 500 C at 10 C /min, followed by increasing temperature at 1 C/min
o
to 650 C and holding the final temperature for 90 min before quenching to room temperature in
N
2
[24]. This catalyst (designated as 10%Mo
catalysts, prepared similarly but with successive impregnations of the AC with solutions of AHM
and Ni(NO , respectively, to obtain precursors with different Ni:Mo ratios. After calcination,
the precursors were converted to Ni-Mo C/AC by CHR as described above but with final
2
C/AC-650) was compared with Ni-Mo
2
C/AC
)
3 2
2
7
/ 48

o
o
reduction temperatures of 550 C or 600 C. The resulting Ni-Mo
2
C catalysts with different
C-y, where x represents the Ni:Mo atomic
C/AC-650 and Ni-Mo C/AC
Ni:Mo ratio (from 0.02-0.76) are designated as Ni
x
Mo
2
o
ratio and y represents the reduction temperature ( C). The 10% Mo
2
2
catalysts, used in the batch reactor to determine reaction kinetics, were loaded into the batch
reactor without passivation. The catalysts were transferred from the preparation U-tube reactor to
the batch reactor in a glove bag under a N blanket to ensure minimal air exposure. For the
2
catalyst stability tests conducted in the trickle-bed reactor and for the characterization of the
fresh catalysts (XRD, BET), the catalyst samples were first passivated in a flow of 1 vol % O
at room temperature for 2 h prior to the analysis and a mild pre-reduction of the passivated
2
/N
2
catalysts was done prior to the reaction test. Carbon supported MoS
herein as a reference to compare with the used Ni-Mo C/AC catalysts. The MoS
C/AC precursor followed by ex-situ presulfiding in a 100 mL
2
2
(MoS /AC) was also used
2
2
/AC precursor
was prepared similarly to the Mo
2
o
decalin for 3 h at 350 C under approximately 10 vol%H
2
S/H
2
generated from CS (with further
2
details provided in the Supporting Information).
2
.2 Catalyst Characterization
Elemental analysis of the catalysts was conducted by inductively coupled plasma atomic
o
emission spectroscopy (ICP-OES). All samples were acid digested in aqua regia at 110 C to
extract all metals from the support, prior to the analysis. The adsorbed S on the used catalysts
8
/ 48

was determined using a Perkin-Elmer 2400 series II CHNS/O Analyzer. The combustion
o
temperature was set at 975 C.
The Brunnauer-Emmett-Teller (BET) surface area, pore volume and pore size of the
calcined Ni-Mo
2
C precursors, passivated Ni-Mo
2
C and used catalysts (following HDS reaction)
o
were determined from N
2
adsorption/desorption isotherms measured at -196 C using a
Micromeritics ASAP 2020 analyzer. The used catalysts were washed with acetone several times
before analysis to remove the decalin solvent used in the reaction. About 0.1 g of the sample was
o
degassed at 200 C (100 µm Hg) for 4 h prior to the measurement. The BET surface area was
calculated from the adsorption isotherm in the relative pressure range (P/P
pore volume (Vtotal) was obtained based on the total N adsorption measured at P/P
micropore volume (Vmicro) was obtained by the t-plot method using data measured in the range of
P/P = 0.18-0.50. The mesopore volume was calculated from the desorption isotherm and the
average pore diameter was calculated assuming a cylindrical pore such that dpore= 4Vtotal/SBET
o
) of 0.1-0.6. The total
2
o
= 0.99. The
o
.
A Bruker D8 Focus (0-20, LynxEye detector) diffractometer was used to identify the bulk
phases of the synthesized passivated catalysts using a Co Kα (λ=1.789 Å) radiation source. The
o
instrument was operated at 35 kV and 40 mA in a scan range of 2ϴ = 5-90 with a step size of
o
0
.04 and scan speed of 0.5 s/step. After background subtraction, phase identification was
9
/ 48

processed using standard software. The XRD of the fresh and used catalysts were compared to
identify bulk phase changes of the catalysts during reaction.
X-ray photoelectron spectroscopy (XPS) analysis was conducted using a Leybold Max200
spectrometer for surface analysis of the used catalysts. Mg Kα was used as the photon source
generated at 1253.6 eV. For a survey scan, the pass energy was set at 192 eV; for a narrow scan,
it was set at 48 eV. The C 1s peak was taken as the reference peak at 284.5 eV to account for
charging effects. Exposure of the samples to the atmosphere prior to analysis was minimized to
less than 2 minutes.
Transmission electron microscope (TEM) images of the supported catalysts were generated
using a (JEOL) JEM 2200FS electron microscope operated at 200 kV, with a 1.9 Å point-to-
point resolution. Used catalysts were analyzed to identify the particle size distribution and
characteristic slabs of the synthesized/formed species. The samples were prepared by dispersion
in ethanol and then sonicated for several minutes. A drop of the suspension was placed on a 230
mesh copper grid coated with Formvar-Carbon film and then dried in a vacuum chamber. The
clusters/particles identified in the TEM images were measured and counted to yield a size
distribution that was fitted to a lognormal distribution to determine the average size of the
cluster/particle. The number of formed MoS layers was also counted based on the TEM images
2
10 / 48

and the stacking degree (N) of MoS
2
was calculated as ꢀ = ∑ꢂꢃꢄ..ꢅ ꢁ ꢀ / ∑
ꢂꢃꢄ..ꢅ
ꢁꢂ, where n is
i
ꢂ
ꢂ
the number of stacks with N layers.
i
A time of flight secondary ion mass spectrometer (TOF-SIMS; a PHI TRIFT V instrument)
was used in the static SIMS mode to analyse the used catalysts and to identify changes in catalyst
surface properties (with or without Ni as a promoter) following the HDS reaction. TOF-SIMS
yields both elemental and molecular ions, and hence it can provide detailed information on the
upper layer of the used catalysts. The catalyst samples were sputtered by a primary ion beam of
+
Au before the spectra were collected, both in the positive and negative mode. For each analysis
11
2
the delivered ion dose was 6.3×10 ion/cm , and the total sputtering time for each spectrum was
approximately 1800 s. The detected mass range was within 0-1850 amu and triplicate spectra
were collected at three different points across the prepared samples with a raster size of 400.0
µm ×400.0 µm as representative sample areas.
2
.3 Catalytic Tests for Hydrodesulfurization
The HDS of dibenzothiophene (DBT) was carried out in a 300 mL stirred-batch reactor
(
Autoclave Engineers) to assess the catalytic activities of the as-prepared Ni-Mo
catalysts with different Ni:Mo ratios (Ni:Mo = 0 ~ 0.76). The experiments were operated at 350
oC with an initial H
pressure of 2.1 MPa with ca. 2.0 wt% of DBT (0.35 wt% S) as the reactant
in 100 mL decalin. Dissolution of DBT in decalin was assisted by sonication, which also
2
C/AC fresh
2
11 / 48

removed dissolved air from the prepared liquid feed. The catalyst was added to obtain 0.13-0.14
metal/100 mL liquid in the slurry. The fresh catalyst and O-free reactant liquid were placed in the
reactor sequentially, under a N blanket using a glove bag. The reactor was then sealed and
flushed with N to remove residual air from the reactor. After a leak test, ultra-high purity H
was used to first flush and then fill and pressurize the reactor to the desired level, following
g
2
2
2
which the reactor was heated to a pre-determined temperature within 35 mins. After the reaction
period (up to 5 h) the reactor was cooled to room temperature and de-pressurized. Liquid
samples were recovered periodically during the reaction and analyzed using a Shimadzu (QP-
2010-S) GC/MS and RTX5 30 m ×0.25 mm capillary column based on an internal calibration
method. The internal standard used in this study was diphenylmethane (DPM). Overall, the
activity data reported herein were measured with a carbon balance ≥ 94% and several
experiments were repeated to quantify the error in the activity data.
A trickle bed reactor (length 50 cm, hot zone 30 cm with an internal diameter of 8.64 mm)
operated in a down-flow mode was also used to compare the stability of the Ni0.19Mo C/AC-600
2
o
and 10 % Mo
2
C/AC-650 catalysts. The experiments were operated at 310 C at a constant
pressure of 4.1 MPa. The liquid feed, consisting of ca. 2.0 wt% DBT in decalin, was fed to the
reactor by means of a high-pressure piston pump (Gilson model 307). The H flow was
controlled by a mass flow controller (Brooks 5850TR). All the experiments were operated with a
2
-1
H /feed volumetric ratio of 600 and a liquid-hourly space velocity (LHSV) of 8 h . The liquid
2
12 / 48

product was collected periodically from a condenser placed after the reactor exit and held at
room temperature. Finally, all samples were analyzed using gas chromatography (Shimadzu GC)
by a capillary column (SHRXI-5MS, 15 m x 0.25 mm x 0.25 um) and a FID detector. Before
performing the activity test, the passivated catalyst (ca. 0.77 g, 2.5 mL) was activated in-situ
o
under H
2
flow at 400 C for 2 h to remove the passivation layer from the catalyst surface.
3. Results
3
.1 Catalyst Characterization
3
.1.1 XRD Analysis
XRD analysis of the calcined Ni-Mo C/AC catalyst precursors showed an amorphous
2
structure, indicating a high dispersion of metals on the support (Figure S1, Supplementary
Information). Figure 1 presents the XRD patterns of the Ni-Mo
2
C/AC catalysts with Ni:Mo
o
ratios of 0-0.76, and prepared at different CHR temperatures (Figure 1(a): 550 C; Figure 2(b):
o
6
00 C). Included at the top of both Figure 1 (a) and (b) are the spectra for 10%Mo
2
C/AC-750,
o
the unpromoted Mo
2
C prepared at a reduction temperature of 750 C to show the positions of β-
Mo C (PDF card # 00-036-0787). Mo
2
2
C is detected in all the diffractograms of Figure 1, with
diffraction peaks at 2θ = 40.12º, 44.21º, 46.14º, 61.35º, 72.87º, 82.97º, 86.58º and 89.51º and no
o
other bulk phases of MoO
x
present in the samples. At a reduction temperature of 550 C, the
peak intensities of Mo
2
C increased with increased Ni:Mo ratio in the range of 0.02-0.44 and the
13 / 48

o
same phenomenon occurred at a reduction temperature of 600 C, but for a narrower Ni:Mo ratio
of 0.02 to 0.19. Figure 1(b) shows that when Ni:Mo ratio is ˃ 0.19, the intensities of the Mo
reflections do not increase, however other phases are detected. The peaks at 51.28 ºand 60.23 º
2
C
are assigned to metallic Ni (PDF card # 00-004-0850) and the separation of Ni from the Mo C
2
o
can be observed when the Ni:Mo ratio is ˃ 0.38 at both reduction temperatures of 550 C and
o
6
00 C. A clear phase separation of Ni from the Mo
2
C was also reported by Puello-Polo et al.
[31] and Santillán et al. [30] when synthesizing the catalysts using the same metal salts as those
used herein. The Ni-Mo bimetallic carbide (Ni Mo , PDF card # 01-089-4883) was detected
6
6
C
2
with diffraction angles 2θ = 47.76º, 50.51ºand 53.40ºwhen the Ni:Mo ratio reached 0.38 at
both reduction temperatures, whereas nickel carbide was not detected in any of the samples.
Analysis of the XRD data for the Ni
decrease in the Mo C lattice parameters with increased Ni content (Table 1). The diffraction
angles also shifted to higher diffraction angles for the Ni0.19Mo C/AC-600 compared to the
unmodified catalyst (10%Mo C/AC750), implying some incorporation of Ni into the β-Mo
crystal structure. Thus, the interaction between Ni and Mo C occurs at low Ni loadings. The
lattice parameters at higher Ni contents were not determined because of Ni phase separation and
x
Mo
2
C/AC-600 catalysts ( 0≤x≤0.19 ) yields a small
2
2
2
2
C
2
the crystallite sizes are not calculated from the XRD data because small particles of Mo
sensitive and re-oxidize during analysis.
2
C are air
14 / 48

3
.1.2 Catalyst Composition and BET Analysis
The measured Ni:Mo ratios of the catalysts are reported in Table 2. The carbon loss (burn
off%) from the AC support that occurred during CHR (Table 2) increased with increased Ni:Mo
ratio and increased reduction temperature. During CHR, the AC support reacts with the H
2
,
mostly forming CH , as determined by GC-FID analysis of the product gases (Figure 2). The
4
o
maximum concentration of CH
4
in the product gas from the CHR was ~2.0 mol% at 650 C for
concentration was
significantly higher (up to ~ 6 mol%), even at 600 C. Figure 2(b) shows that the maximum in
the CH content of the CHR product gas increased with increased Ni content. Furthermore, the
CH formation profiles are different between the unpromoted and Ni- promoted Mo
former case, the CH
remains approximately constant during the hold period; whereas, for the Ni-Mo
CH concentration decreases significantly above the highest reduction temperature of 600 C.
The burn-off mass (carbon loss) for the unprompted Mo
similar to the Ni0.19Mo C/AC-550 (reduced at 550 C), clearly indicating that the presence of Ni
accelerates the carbon loss at low temperature, which reduces the carburization time required for
the unpromoted Mo
2
C (Figure 2 (a)), whereas, in the presence of Ni, the CH
4
o
4
4
2
C. In the
o
o
o
4
content doubles every 50 C in the temperature interval 550 C~650 C and
2
C precursors, the
o
4
o
2
C reduced at 650 C was ~ 18 wt%,
2
the Mo precursor. Also note that the burn-off mass is well correlated with CH
CHR (Figure S2, Supporting Information).
4
formation during
15 / 48

After CHR, the shape of the N
2
adsorption isotherms were similar to the AC support (Figure
S3, Supporting Information). The surface area of the catalysts prepared at a CHR temperature of
o
o
5
50 C did not show a significant change with increased Ni content; whereas, at 600 C, the
surface area decreased with increased Ni content (Table 3 and Figure S4, Supporting
Information). Moreover, both catalysts showed a general trend of increased pore size with
increased Ni content and the increase was due to an increase in mesopore volume (Figure S4).
3
.2 Catalyst Characterization after Reaction
All the catalysts were characterized after the HDS reaction to identify potential changes in
the catalyst chemical and physical properties. In addition, the MoS
reported here as a comparator for the used Ni-Mo C/AC catalyst properties. The catalysts were
separated by high speed centrifuge from the slurry recovered from the batch reactor. The solid
2
/AC catalyst properties are
2
fraction was then washed in acetone and dried overnight under a N
2
blanket in a glove bag. Prior
o
to XPS and TEM analysis, the dried samples were degassed in vacuum at 200 C for 4 h. The
transfer of the samples to the vacuum chamber (XPS, TEM) was done within 5 min to minimize
exposure to air.
3
.2.1 XRD and BET Analysis
XRD analysis of the used catalysts (Figure S5, Supporting Information) showed no change
in the Mo
2
C bulk structure after reaction, indicating that the Mo C crystallites are resistant to
2
16 / 48

bulk sulfidation under the chosen reaction conditions, consistent with the observation made by
Aegerter et al. [13]. There were no peaks that could be assigned to S compounds in the XRD
pattern of the used catalysts either.
The BET analysis data (Table 3) generally indicate that after the HDS reaction, there was a
significant loss in catalyst surface area, whereas the average pore size and mesopore volume
fraction increased, implying that mostly micropores were blocked during reaction.
3
.2.2 XPS and CHNS Analysis
The effect of increased Ni:Mo ratio on the XPS spectra of the used catalysts is shown in
Table 4 and Figure 3. The elemental surface compositions clearly show the presence of S on the
catalyst surface after the HDS of DBT, consistent with data reported by Sajkowski and Oyama
o
[11]. As illustrated in Table 4, the used Ni-Mo
2
C catalysts reduced at 550 C have higher S
o
content than the same catalysts reduced at 600 C. In addition, the Ni-Mo
2
C catalysts all have
higher S content after reaction than the Mo C catalyst with no Ni addition.
2
Apart from a clear increase in S content on the catalyst surface with increased Ni addition,
the Mo 3d XPS spectra of Figure 3 (a) and (c) show an obvious shift from lower B.E. to higher
B.E. as Ni content increases, indicative of the formation of Mo-S surface species. The Mo 3d
shift to higher B.E. on the Ni-Mo
2
C is greater than that of the Mo C/AC, indicating more
2
sulfidation of the Mo in the presence of the Ni.
17 / 48

Figure 4 compares the Mo 3d and S 2p XPS spectra of the Ni0.19Mo
reaction for different periods to that of the MoS /AC catalyst. The S 2p peak at B.E.= 162.2 eV
confirms the presence of Mo-S surface species, consistent with the results from the Mo 3d
spectra. In addition, it is noted that the Mo 3d B.E. of the Ni-Mo C catalyst surface is relatively
stable after 1 h of reaction and clearly different from that of the MoS /AC surface. Table 5 also
2
C/AC-550 catalyst after
2
2
2
shows that the S species accumulate during reaction since the S content (i.e. the atomic ratio of S
to Mo or S wt%) increased with increased reaction time for all catalysts. The S content (wt%)
measured by CHNS and reported in the last column of Table 5, does not show a clear difference
between the two catalysts. However, the S wt% calculated based on the XPS data give clear
differences between these two catalysts, showing a strong ability of S adsorption in the presence
of Ni. For the 10%Mo
2
C/AC-650 catalyst, the S:Mo ratio increased from 0.30 to 0.59, whereas
for the Ni-Mo
catalyst.
2
C it is increased from 0.69 to 0.99, indicating a faster sulfidation of the Ni-Mo C
2
3
.2.3 TOF-SIMS Analysis
The negative ion mass spectrum of Mo, sulfur or oxysulfide (oxide) spectra were recorded
during sputtering from a catalyst sample area of 400 ×400 µm. Several interesting Mo
characteristic fragments containing oxygen, sulfur or both were detected in the used catalysts,
-
-
S- or MoS
such as MoS or MoO
2
(m/z=130), MoOS or MoO
3
(m/z=146), MoO
2
2
(m/z=162),
MoO
3
S- (m/z=178) and MoO
S
2 2
(m/z=194). The presence of sulfur indicates that the surface of
18 / 48

the catalyst has been changed during the HDS reaction with the formation of Mo-O-S or Mo-S
bonds on the catalyst surface. In order to eliminate possible variations between samples, the
normalized ion intensity yield (i.e. relative intensity, RI) is used to make comparisons. The
distribution of identified Mo oxysulfide species (MoO
x
S
y
) is represented by the abundance of
each Mo oxysulfide species divided by the abundance of all MoO
S
x y
ions (RI’). The identified
molecular fragments and their relative abundance are reported in Table 6. These data confirm the
adsorption of S on the catalyst surface during reaction and indicate that in the presence of Ni, the
amount of MoS surface species is almost 2x’s higher than that on the unpromoted Mo
Ni:Mo=0), consistent with the XPS analysis results.
2
C
(
3
.2.4 TEM Analysis
o
The TEM micrographs of the used Ni-Mo C/AC catalysts reduced at 600 C with various
2
Ni:Mo ratios are presented in Figure 5 and 6. The formation of core-shell like structures is
clearly visible, suggesting that a Mo C core is surrounded by MoS generated during the HDS
reaction. The size of the core-shell structure increases from 9.7 nm to 20.2 nm as Ni content
increases, as shown by the lognormal distribution data of Figure 5. The Mo C-MoS clusters
become agglomerated when the Ni:Mo ratio ˃ 0.44. At low Ni content, such as Ni0.02Mo C/AC-
00, the contrast between particle and support is low and it is difficult to quantify the particle
2
2
2
2
2
6
size from the TEM micrograph. For the 10%Mo
2
C/AC-650, the characteristic slabs of MoS are
2
19 / 48

difficult to identify as shown in Figure S6. Note also that the number of MoS
2
layers formed
around the Mo C core increases with increased Ni content, as reported in Figure 6.
2
3
.3 Catalyst Performance in HDS of DBT
Dibenzothiophene (DBT) was chosen as the model reactant to determine the activity of the
o
prepared catalysts. The results of the activity tests at 350 C for Ni-Mo
2
C/AC catalysts, prepared
at different reduction temperatures and Ni:Mo ratios are presented in terms of DBT conversion in
Table S1 (Supporting Information). The mass of catalyst used for each activity measurement
varied to account for the AC mass loss during the various CHR conditions, however, the total
Mo+Ni mass remained relatively constant (~0.14 gmetal/100 mLfeed) for all experiments. The data
of Table S1 (Supporting Information) show that the DBT conversion on Ni-Mo C/AC reduced at
2
o
5
50 C was generally higher than the DBT conversion measured on the unpromoted Mo
2
C/AC
o
catalyst and higher than the DBT conversion measured on the Ni-Mo
2
C/AC reduced at 600 C,
provided the Ni:Mo ratio is ＜ 0.44. The data clearly show improved activity of the Mo
2
C in the
presence of Ni. The conversion over the Ni-Mo C/AC-550 catalyst increased with increased Ni
2
content for Ni:Mo ratios of 0-0.38, but decreased when the ratio increased above 0.38. The
decreased conversion is likely due to the Ni phase separation that occurs at high Ni content and
higher carbon loss (burn-off%) above a Ni:Mo ratio of =0.38 that may also decrease Mo
dispersion. Similar observations with respect to increased HDS activity of P-doped Ni-
20 / 48

Mo
2
C/Al
2
O catalysts with increased Ni content, were reported by Sundaramurthy et al. [32],
3
using gas oils derived from Athabasca bitumen.
Kinetic analysis of the batch reactor data is complicated by the fact that the Mo C-based
2
catalysts undergo sulfidation during reaction and consequently, their activity and selectivity may
change with reaction time in the batch reactor. As shown in Table 5, the S content of the
catalysts (with or without Ni addition) increases with reaction time. However, the difference in
the amount of sulfur adsorbed on the 10%Mo
2
C/AC-650 and Ni0.19Mo C/AC-600 is constant at
2
~0.27 At% independent of reaction time (Table 5, column 3). This indicates that Ni has the
st
ability to increase S adsorption but the effect of the Ni occurs rapidly within the 1 hour of
reaction. The continued adsorption of S beyond this period is a consequence of the carbon
support. Furthermore, Figure 4 shows that there is no visible difference with respect to the Mo
st
oxidation state of the catalyst after the 1 hour of the reaction. To confirm the stabilization of the
catalysts, Figure 7 reports the DBT conversion as a function of time-on-stream for the Mo C and
2
o
the Ni0.19Mo
2
C/AC-600 catalysts measured at 310 C and 4.1 MPa H
2
pressure and a LHSV of 8
-1
h in the downflow, fixed-bed reactor. A rapid decrease in DBT conversion occurs within the
first 60 mins of operation beyond which both the DBT conversion and product selectivities
stabilize. Continued operation beyond 5000 min showed that the activity of the Ni0.19Mo
00 catalyst remained stable, whereas the DBT conversion over the Mo C/AC showed a small
decline. Hence, to minimize these catalyst stabilization effects on the kinetic analysis of the
2
C/AC-
6
2
21 / 48

batch reactor data, only data collected after 1 h operation are included in the analysis. Hence, the
kinetic analysis is performed on the partly sulfided, but stabilized Mo C-based catalysts.
2
The kinetics of the HDS of DBT on all the synthesized catalysts were determined assuming
1st-order reactions and a reaction pathway illustrated in Figure S7 (Supporting Information) [33].
Direct desulfurization of DBT (the DDS pathway) to form biphenyl (BPh) competes with a
hydrogenation pathway (the HYD pathway) in which DBT is converted to1,2,3,4-
tetrahydrodibenzothiphene (THDBT) before S-removal to yield cyclohexylbenzene (CHB). The
kinetic parameters of this simplified reaction scheme were estimated using a Levenberg-
Marquardt nonlinear regression methodology to solve the relevant ODEs (Equations 1-4) as
below.
ꢆꢇꢈꢉꢊ
=
−ꢋꢄꢌꢍꢎꢏ − ꢋ ꢌ
(1)
(2)
(3)
(4)
ꢐ
ꢍꢎꢏ
ꢆꢅ
ꢆꢇꢊꢑꢈꢉꢊ
ꢆꢅ
=
ꢋ ꢌ
− ꢋ ꢌ
ꢍꢎꢏ ꢒ ꢏꢓꢍꢎꢏ
ꢄ
ꢆꢇꢉꢔꢕ
=
ꢋꢐꢌꢍꢎꢏ
ꢆꢅ
ꢆꢇꢖꢑꢉ
=
ꢋꢒꢌꢏꢓꢍꢎꢏ
ꢆꢅ
st
The 1 -order estimated kinetic rate constants k
j
, j=1-3, are reported in Table 7 and the
model fit to the data is shown in Figure S8 and Figure S9, for the CHR reduction temperatures of
o
o
5
50 C and 600 C, respectively, based only on the data obtained after a reaction time > 1 h. The
22 / 48

magnitude of the rate constant for the total consumption of DBT (ktotal=k
indicates that the Ni0.38Mo C/AC-550 and Ni0.19Mo C/AC-600 have the highest activity among
all the catalysts. Figure 8 shows that the rate constants k and k increase with increased Ni
1
+k
2
) (Table 7)
2
2
1
2
addition up to a maximum value and then decrease at high Ni contents. The thermal reactions are
neglected since the thermal related conversions are very low (less than 5%).
The catalyst selectivity to the three main products of BPh, THDBT and CHB is illustrated in
Figure S10. Biphenyl was the main product resulting from direct hydrogenolysis of the C-S bond
of DBT, accounting for ˃80% of all products of the Mo
the Ni promoted catalysts, as shown from Figure S10 (a) and (d). These data imply that the DDS
reaction route is dominant and is the preferred route for S removal on these Mo C-based
catalysts. The data of Table 7, comparing the relative rates of DDS to HYD and reflected in the
/k , also suggests an increase in DDS selectivity with increased Ni/Mo ratio.
2
C/AC-650 catalyst, and even higher for
2
value of SDDS/HYD =k
2
1
The hydrogenated product CHB is mainly formed from the hydrogenation of THDBT and the
data show that the presence of Ni increases CHB selectivity (Figure S10 (b), (c), (d) and (e)).
4. Discussion
The addition of Ni is shown to decrease the CHR temperature required for Mo C formation
2
from the AHM/AC calcined precursor and this observation is in agreement with that reported
previously by Liang et al. [26]. In a previous study [24] the optimized reduction temperature for
23 / 48

o
Mo
2
C formation from AHM supported on AC was reported to be 650 C, whereas in the present
C formation is possible at significantly lower CHR temperature
550 C) with Ni addition. Furthermore, the amount of Ni is shown to be critical in terms of the
study, we demonstrate that Mo
2
o
(
phases generated by CHR and in terms of the catalyst DBT activity. At a reduction temperature
o
of 550 C, the formation of Mo
2
C was confirmed by XRD when the Ni:Mo ratio is low (0.09).
at relatively low temperature [34, 35], hydrogenation of the carbon to yield
can occur at low temperature. In addition, by activating H at relatively low temperature,
Mo oxide is more readily reduced which makes the subsequent conversion to Mo carbide or
oxycarbide more facile. As shown in Figure 2, the CH concentration generated in the product
gas from CHR of the Ni0.19Mo C/AC-600 catalyst is ~4x’s higher than that of the 10%Mo C/AC-
50 catalyst when the reduction temperature reaches 600 C. In general, the concentration of CH
Since Ni activates H
2
CH
4
2
4
2
2
o
6
4
generated during CHR increases as the reduction temperature increases and as Ni loading
increases. The XRD data show that as the amount of Ni increases, the intensity of the Mo
2
C
diffraction peaks increases. Figure 2 shows that the CH
about 600 C and Ni can lower this temperature. Hence, although temperature is the most
important factor determining the CH concentration measured during CHR, the Ni:Mo ratio also
4
generation rate increases rapidly above
4
plays an important role in the reduction. We conclude that although the presence of Ni decreases
the required reduction temperature for carbide formation, temperature remains the key factor in
the CHR synthesis of metal carbides. However, higher temperature results in a higher degree of
24 / 48

crystallization and higher carbon loss (burn-off) from the carbon support. The burn-off was 48%
o
for the Ni0.76Mo
2
C/AC-600 whereas at 550 C, the burn-off decreased significantly to only 28%.
If we compare different Ni:Mo ratios, the difference in mass loss between Ni:Mo ratios of 0.02
and 0.19 is much larger than the mass loss between 0.19 and 0.76, suggesting that the
effectiveness of Ni in increasing the CH
When the Ni:Mo ratio reaches 0.38 at both 550 C and 600 C, Ni
XRD, indicating that Ni has been incorporated into the Mo C structure during the CHR process.
When the Ni:Mo ratio is > 0.38, metallic Ni appears. Unlike Mo C, the formation of Ni Mo
4
concentration is reduced at high Ni concentrations.
o
o
6
Mo
6
C species appear in the
2
2
2
6
6
C
2
and Ni are mostly dependent on Ni content rather than CHR temperature. Thus, the formation of
the bimetallic carbide and Ni phase separation appears at the same Ni:Mo ratio despite
differences in CHR temperature. Table 3 and Figure S4 show that when the catalyst precursors
o
2
were reduced at 550 C, the surface areas were approximately constant at ~ 800 m /g for
o
different Ni:Mo ratios; whereas, at a CHR temperature of 600 C, there was a clear decrease in
surface area with increased Ni content. The differences are due to the fact that the pore widening
process that occurs during CHR, removes some pore walls between mircopores causing a loss in
o
o
surface area at 600 C; whereas, at 550 C the widening occurs, but since the pores are still
developing, the pore walls and the resulting surface area remain intact. In addition, the catalyst
pore size increased in the presence of Ni at both CHR temperatures, with most of the micropores
25 / 48

of the AC support enlarged into mesopores, which is beneficial for molecular diffusion of large
molecules such as DBT.
XRD analysis of the fresh and used catalysts showed that the bulk phase properties of the
catalysts were unchanged after reaction with DBT. However, CHNS and XPS analysis both
showed that S was present on the surface of the used catalysts. Note that the measured d-spacing
from the TEM is ~0.63 nm (Figure 6), indicating the formation of MoS
Mo C/AC catalysts [36]. A similar observation was reported by Zhang et al. [37] on a used
Ni Mo N catalyst after hydrogenation in the presence of 100 ppm of thiophene. As shown in
2
species on the used Ni-
2
2
3
Table 4 and Figure S11, the amount of S increased with increased Ni content of the catalyst,
suggesting that the Ni enhanced S adsorption on the metal sites. In addition, the TEM results
confirmed this conclusion showing an increase in the number of MoS
2
layers with increased Ni
o
content of the catalyst (Figure 6). Also note that the catalysts reduced at 550 C had higher S
o
content after use than those reduced at 600 C. These results are consistent with the data obtained
by CHNS analysis in which S content of the used catalysts is well correlated with the Ni:Mo
ratio. The difference in S content of the catalyst as measured by CHNS (bulk analysis) versus
XPS (surface analysis; reported in the last two columns of Table 4) can be accounted for by the S
adsorbed inside the pores of the carbon support, the carbon “sink effect” proposed by Laine et al.
[
38]. There is a ~ 1.0 wt% difference between the bulk and surface S content of the Ni-Mo C
2
o
catalysts reduced at 550 C; whereas, the difference is ~ 0.5 wt% for the Ni-Mo
2
C catalysts
26 / 48

o
reduced at 600 C. This difference is caused by the pore structure modification during CHR at
different reduction temperatures. Since many micropores are destroyed at higher reduction
o
temperature (600 C), the S adsorption capacity is decreased compared to the catalyst prepared at
the lower CHR temperature.
XRD showed that Ni incorporates with Mo species during the CHR process. The calculated
lattice parameters of β-Mo C decrease with increased Ni content before phase separation at high
2
Ni content, which implies that smaller Ni atoms randomly substitute for Mo atoms in the metal
carbide crystal structure. A similar conclusion was made by Wan and Leonard [39] in regards to
Fe modified Mo
catalyst surface after reaction with DBT. TEM analysis of the used Ni-Mo
Figure 5) clearly showed that the Mo C particle size increased with Ni content. The catalyst
metal carbide particle size is well correlated with Ni:Mo ratio as shown in Figure 9. Comparing
the 10%Mo C/AC-650 and Ni0.38Mo C/AC-600 catalysts with the same burn off shows an
increase in particle size from 5 nm for the 10%Mo C/AC-650 catalyst to 15 nm for the
C/AC-600 catalyst. Similar results were reported by Yao et al. [40] in their study of a
NiMoC catalyst and Leonard and Wan [39] noted that unlike Fe, which decreases the size of
Mo C particles, Ni will increase the size of Mo C. Figure 6 also shows that addition of Ni
increases the number of MoS layers in the MoS shell that surrounds the Mo C of the used
catalyst. Ni increases the stack number of MoS as has also been reported on NiMoS/Al
2
C. XPS shows that the addition of Ni increased the S adsorption on to the
2
C/AC-600 catalysts
(
2
2
2
2
Ni0.38Mo
2
2
2
2
2
2
2
2
O
3
27 / 48

catalysts by Lercher et al. [35]. The STEM mode selected area analysis of the used catalyst
shown in Figure 10, confirms that the Ni, Mo and S are closely bound in the used catalyst,
suggesting that Ni can enhance the S interaction with Mo.
The present study has shown that Ni addition has a significant impact on both the CHR
process and the S adsorption by the catalyst during the HDS reaction, and that both depend on
the Ni content of the catalyst. During reaction, Ni enhances the adsorption of S by the Mo to
form Ni-Mo-S on the outside of the Mo C particle, leading to the generation of a core-shell like
2
structure. None of the characterization data, including the TOF-SIMS analysis, identified the
presence of Mo carbo-sulfide structures, as proposed by Oyama et al. [15], and although they
were not present as bulk species, we cannot exclude the possibility that they exist on the catalyst
surface under reaction conditions.
The data of Table 7 and Figure S8 show that Ni added to the Mo
2
C catalyst increases DBT
conversion. A trend notable from Table 7 and Figure 8 is that k , representing catalytic ability to
2
perform DDS, increases to a maximum value with increased Ni content. When the Ni
concentration is relatively low, the added Ni is incorporated with Mo species into the final metal
carbide phase. Thus, in the HDS of DBT, increased Ni increases the Mo S stacking number and
2
according to the ‘rim-edge’ model of Chianelli and Daage [41], a higher stacking number yields
higher DDS selectivity. However, when the Ni concentration is above a critical value (Ni:Mo >
28 / 48

0
.38), the added Ni is not incorporated into the Ni-Mo carbide phase; on the contrary, more Ni
causes phase separation and enlarges the particle size leading to less active sites and eventually
lower activity. Also, available surface area is decreased by increased burn-off with increased Ni
content, resulting in decreased dispersion of active sites and reduced activity. Moreover, more
reduced single phase Ni contributes to the hydrogenation ability of the catalyst and thus, a
o
decrease in SDDS/HYD is observed. Note that the trend of SDDS/HYD versus Ni content at 600 C is
not very clear, due to the loss of surface area along with Ni addition affecting the catalytic
performance. For the above reasons, it is found that the best performing catalysts are Ni0.38Mo
C-600. They have a total kinetic constant for DBT conversion of 1.11 and 1.14
mL/gmetal.min along with a very high SDDS/HYD ratio of2.7 and 3.1, respectively.
2
C-
550 and Ni0.19Mo
2
5. Conclusions
Ni addition significantly reduces the temperature required for Mo
2
C generation by CHR,
o
with the presence of Mo
2
C confirmed by XRD at a CHR temperature of 550 C for a catalyst
o
with a Ni:Mo ratio of 0.09. This CHR temperature is about 100 C lower than that required to
form Mo
structure during the CHR process and TEM showed that increased Ni content increased the
Mo C particle size. However, for Ni:Mo > 0.38 phase separation occurred, decreasing the
catalyst activity and the direct desulfurization selectivity. The Ni also increased the rate of S
2
C in the absence of Ni. XRD and XPS show that Ni is incorporated into the Mo
2
C
2
29 / 48