Sacrificial carbonaceous coating over alumina supported Ni-MoS2 catalyst for hydrodesulfurization

Article abstract of DOI:10.1039/c9ra00884e

Recent results have evidenced that carbon plays an important role in stabilizing the structure of the active phase in catalysts. In this work, carbon-coated alumina was prepared by applying polydopamine (PDA) as a sacrificial carbon source to modify the surface properties of γ-alumina, which then was used as a support to prepare supported NiMo catalysts for hydrodesulfurization (HDS) of dibenzothiophene (DBT). NiMo/Al₂O₃ catalysts exhibited limited hydrodesulfurization performances due to their strong metal-support interaction. Herein, we report an unexpected phenomenon that sacrificial carbon layers can be constructed on the surface of the Al₂O₃ support from the carbonization of polydopamine (PDA) and mediated the interaction between the active site and support. Through the removal of carbon layers and sulfidation, the resulting NiMo catalysts exhibit excellent performance for HDS reaction of dibenzothiophene (DBT), which is associated with adequate loading of residual carbon species, leading to an enhanced amount of active species under sulfidation conditions. Moreover, the facile synthetic strategy can be extended to the stabilization of the active phase on a broad range of supports, providing a general approach for improving the metal-support interaction supported nanocatalysts.

Full text of DOI:10.1039/c9ra00884e

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Sacrificial carbonaceous coating over alumina
supported Ni–MoS2 catalyst for
Cite this: RSC Adv., 2019, 9, 11951
hydrodesulfurization†
a
Yingrui Xu,a Pengyun Li,a Shenghua Yuan,b Baokuan Sui,b Weikun Lai,
*
a
Xiaodong Yi
and Weiping Fanga
*
Recent results have evidenced that carbon plays an important role in stabilizing the structure of the active
phase in catalysts. In this work, carbon-coated alumina was prepared by applying polydopamine (PDA) as
a sacrificial carbon source to modify the surface properties of g-alumina, which then was used as
a support to prepare supported NiMo catalysts for hydrodesulfurization (HDS) of dibenzothiophene
(DBT). NiMo/Al2O3 catalysts exhibited limited hydrodesulfurization performances due to their strong
metal-support interaction. Herein, we report an unexpected phenomenon that sacrificial carbon layers
can be constructed on the surface of the Al2O3 support from the carbonization of polydopamine (PDA)
and mediated the interaction between the active site and support. Through the removal of carbon layers
and sulfidation, the resulting NiMo catalysts exhibit excellent performance for HDS reaction of
dibenzothiophene (DBT), which is associated with adequate loading of residual carbon species, leading
to an enhanced amount of active species under sulfidation conditions. Moreover, the facile synthetic
strategy can be extended to the stabilization of the active phase on a broad range of supports, providing
a general approach for improving the metal-support interaction supported nanocatalysts.
Received 1st February 2019
Accepted 29th March 2019
DOI: 10.1039/c9ra00884e
rsc.li/rsc-advances
loading on the oxide support with high surface area.5–7
Considering its tunable and stable mechanical characteristics
Introduction
and low cost, alumina has been the most commonly applied
support for HDS.8–10 However, the conventional alumina
interact strongly with metal oxide precursors to form species
that are hard to be reduced or sulded.11–13
The hydrotreating processes (HDT) has been one of the most
important processes in petroleum industry due to the
increasing demand for high-quality ultra-clean fuel production
from heavy crude oils and the stringent regulations on envi-
ronmental issues. Among all the contaminants in gasoline or
diesel oil, sulfur-containing compounds such as thiophene,
benzothiophene (BT), dibenzothiophenes (DBT), and dime-
thyldibenzothiophenes (DMDBT), which generate SOX gases,
must be degraded or removed in order to prevent air pollution.
Despite development of techniques such as adsorption oxida-
tive desulfurization, adsorptive desulfurization and bio-
desulfurization, catalytic hydrodesulfurization has been proved
as the most effective method for industrial application.1–4
Over the past decades, the traditional HDS catalysts con-
sisted of Ni (Co)–Mo (W) pre-sulded to form metal sulde (WS2
or MoS2,main component) coexisting with promoters (Ni or Co)
Towards this, various new support materials are introduced
for HDS catalysts. Those carbon-based materials, such as active
carbon or graphene with hierarchical structures and high
surface area, seem to be promising support materials in
hydrotreating reactions of heavy crude oil.10,14,15 The varied
functional groups on the carbon materials could be tailored by
oxidizing agents during the preparing process, which is very
different from the surface chemistry of alumina.16–19 As
a consequence, catalysts prepared with carbon as support
(active carbon or graphene) were more active than their
alumina-supported counterparts.20–22 However, the signicant
disadvantage of carbon supports for low packing density limits
their application in commercial HDT catalyst formulations.23
As the dispersion of the catalytic metal-sulde is difficult to
enhance due to the strong polarity and the limited surface area
of the alumina support,10,24–26 modication of alumina surfaces
to acquire proper acidic properties or appropriate metal-
support interactions were frequently applied in preparing
HDS catalysts with higher activity. In some other cases, inter-
actions between the loaded metal and support (carbon mate-
rials) may cause serious sintering problems under reaction
aNational Engineering Laboratory for Green Chemical Productions of
Alcohols-Ethers-Esters, College of Chemistry and Chemical Engineering, Xiamen
University, Xiamen 361005, China. E-mail: xdyi@xmu.edu.cn; laiweikun@xmu.edu.
cn
bSINOPEC Dallian Research Institute of Petroleum and Petrochemicals, Dalian
116045, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra00884e
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conditions.17 In order to properly alter the interaction between
active component and supporting material, alumina supported
systems with carbon11,27 were found to be more preferred to
apply in certain reaction systems. Thus the surface chemistry of
carbon-based materials, which might be tailored by oxidizing
agents and eventually affect the active phase on the catalyst, has
been investigated by many authors.16,17,19 Thus the conception
of adding carbon in tailoring surface of alumina as support
seems an appealing practice in HDS catalysts.
Previous study found polydopamine (PDA) lm was able to
develop over a wide variety types of material surfaces of complex
shape via spontaneous oxidative self-polymerization of dopa-
mine in aqueous solution.26,28 On account of the versatile
coating strategy, dopamine exhibits expansive potential as
building block for manipulating the coating layers over the
support materials. In this work, the surface properties of
commercial alumina were tuned by carbon-layers made from
pre-loaded polydopamine. The accumulation of PDA layer could
Preparation of HDS catalysts
The HDS catalysts were prepared by loading the active compo-
nents Ni and Mo on the alumina@carbon supports via simul-
taneous incipient wetness impregnation method. The Ni and
Mo precursors ((NH4)6Mo7O24$24H2O, 0.1168 g and Ni(NO3)2-
$6H2O, 0.1472 g) were mixed and well dispersed in deionized
water. Aer impregnation over 0.85 g support, the catalyst
precursors (NiMoAl@PDA-x and NiMoAl@C-1) were dried at
ꢁ
ꢁ
120 C overnight and calcined in owing air at 500 C for 2 h.
The nal loading amount is 3.0 wt% of NiO and 12.0 wt% of
MoO3 for each catalyst. The catalysts with Ni Mo loading over
Al@PDA-x and Al@C-1 were denoted as NiMoAl-x and NiMoAl-
1#, respectively. The commercial alumina loaded catalyst was
denoted as NiMoAl-0.
Characterization
effectively promote the reduction and suldation degree of The nitrogen adsorption–desorption measurements of the
subsequently loaded active phase. The polydopamine layers composite were performed on a Micromeritics Tristar 3020
tend to form thin shells over the alumina surface, creating adsorption automatic instrument at ꢀ196 ꢁC. The samples were
a complete interval between active phase and support to avoid pretreated at 300 ꢁC for 3 h in vacuum. The specic surface area
strong interactions. Meanwhile, the carbon shell was easily and pore volume of the catalysts were calculated from nitrogen
removed by post-thermal treatment under owing air, leaving adsorption–desorption isotherms using the Brunauer–Emmett–
the well-established metal-support interfaces undestroyed. As Teller (BET) and Barrett–Joyner–Halenda (BJH) methods.
a result, the HDS activity of supported catalyst is positively Powder X-ray diffraction (XRD) characterization was carried out
affected. Consequently, discussions were carried out based on on a Rigaku Ultima IV automatic powder diffractometer oper-
the role of air-annealed carbon coating in affecting the HDS ated at 40 kV and 30 mA using Cu Ka (l ¼ 0.15406 nm) mon-
activity trends of NiMo/alumina catalysts.
ochromatized radiation with a scan speed of 10ꢁ minꢀ1. The
JCPDS le database was used for peak identication. To eval-
uate the combustion reactions, thermogravimetric analysis
(TGA) of the precursor before the combustion occurred was
conducted ꢀi1n owing air up to 800 ꢁC at a heating rate of
10 ꢁC min using a simultaneous TGA (SDT Q600) thermal
analyzer. Temperature Programmed Oxidation (TPO) was
carried out on a Micromeritics AutoChem II 2920 instrument.
The catalysts (100 mg) were pretreated in owing helium at
400 ꢁC for 1 h before cooled down to 30 ꢁC. Then the sample was
subjected to oxidation test from 30 to 800 ꢁC at a rate of
10 ꢁC minꢀ1 in a 5% O2/He stream (20 mL minꢀ1). The
temperature programmed reduction (TPR) of the sample was
performed over a homemade GC-TPR apparatus. For each test,
50 mg catalyst was pretreatꢁed in owing Ar stream at 300 ꢁC for
1 h and then cooled to 50 C. Subsequently, a 5% H2/Ar atmo-
sphere was altered and the catalyst sample was heated to 900 ꢁC
with a ramping rate of 10 ꢁC minꢀ1. The morphology of the
samples was acquired by a Sigma Zeiss scanning electron
microscope (SEM) at 5–15 kV. The regular transmission electron
microscopy (TEM) images by a JEM-1400 at 100 kV, and high-
resolution transmission electron microscopy (HRTEM) images
of the samples were obtained using a Jeol JEM-2100F eld
emission electron microscope operating at 200 kV. Aer ultra-
sonically dispersed in ethanol, the testing samples were
prepared by dropping the dispersed suspensions onto carbon-
Experimental sections
Materials
All of these reagents were purchased from Sinopharm Chemical
Reagent Co. and were used without further purication, except
that the dibenzothiophene (DBT) was purchased from Sigma-
Aldrich. In addition, the commercial alumina support used as
a control was obtained from Fushun Research Institute of
Petroleum and Petrochemicals, SINOPEC, Fushun, China.
Preparation of PDA coated g-alumina support
The tris(hydroxymethyl)methyl aminomethane buffer solution
(10 mM) with PDA (x mg mLꢀ1) was adjusted to pH ¼ 8.5 with
NaOH. Then the buffer solution with 5 g alumina powder was
transferred to a beaker under stirred conditions for 24 h at room
temperature. The product was collected and washed twice with
deionized water by centrifugation. Then the product was dried
ꢁ
at 120 C overnight to generate Al@PDA-x samples.
Preparation of carbon coated g-alumina support
The carbon-coated supports (denoted as Al@C-1) were prepared coated copper grids.
with Al@PDA-1 samples subjected to annealing treatment at The MoS2 dispersion over the sulded catalysts was further
ꢀ1
ꢁ
ꢁ
500 C in N2 ow for 2 h at a heating rate of 5 C min
.
determined by HRTEM measurement. The average slab length
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(L) and stacking number (N) were calculated based on the (the catalysts NiMoAl-x and NiMoAl-1# were generated from
ꢀ
ꢀ
following expressions:
NiMoAl@PDA-x and NiMoAl@C-1), respectively (Scheme 1). TG
and TPO technique were used to study the thermal decompo-
sition of the carbon-containing precursors (Ni and Mo species
loaded over uncalcined PDA or carbon-coated alumina) with
different PDA content and determine the annealing conditions
from the calcination procedure of oxide catalysts. Notably, the
NiMoAl-0 catalyst shows a slight decrease in mass, which
corresponds to the combustion process includes the dihydrox-
ylation of alumina (before 140 ꢁC) and the decomposition of Ni
and Mo precursors (140–600 ꢁC). For the Ni and Mo loaded
catalysts with PDA precursors, the thermal decomposition can
,
x
x
X
X
L ¼
xili
xi;
(1)
(2)
i¼1
i¼1
,
y
y
X
X
N ¼
yinNi
yi;
i¼1
i¼1
where li represents for the length of slab while Ni is the layer
number of slab i, xi represents for the slab number with length
of li, and yi stands for the number of slabs with stacking
numbers of ni. Usually, the average fraction of Mo atoms located
on the edge of the crystallites (denoted as fMo) is determined to
indicate the dispersion of reactive surface on the active phase.
Assuming the MoS2 slabs were perfect hexagons, the value of fMo
ꢁ
be divided into three regions, which are located in 40–140 C,
ꢁ
ꢁ
140–400 C, and 400–700 C, respectively. As shown in Fig. 1a,
the weight loss at 40–140 ꢁC is ascribed to the removal of
physical-adsorbed moisture,29 and the abrupt weight loss
is dened with the following eqn (3)
ꢁ
between 140 and 400 C during the calcination is attributed to
,
t
t
the carbonization of PDA coating and loss of dehydroxylation of
alumina30 while process at 400–700 ꢁC to the combustion of
carbon29 and Ni–Mo species.30 The decomposition process of
PDA continues until the temperature reaches 900 ꢁC. The
weight losses in the 140–400 ꢁC, and 400–700 ꢁC region
increased with increase of PDA content in the buffer solution,
while the NiMoAl@PDA-1 and NiMoAl@C-1 exhibit TG proles
with similar weight losses (Table S1†).
X
X
ꢀ
ꢁ
fMo
¼
ð6ni ꢀ 6Þ
3ni2 ꢀ 3ni þ 1 ;
(3)
i¼1
i¼1
where the ni (determined by the length of MoS2 slabs, L ¼ 3.2
˚
(2ni ꢀ 1) A) represents the number of Mo atoms located along
one edge of single MoS2 nanoslab, and t represents the total
number of MoS2 slabs observed from TEM images.
To investigate the residual carbon content aer calcination
in air, the TG patterns of the calcined catalyst were shown in
Fig. 1b. The two degradation steps lie in 40–450 ꢁC and 450–
Catalytic performance evaluation
The dibenzothiophene hydrodesulfurization activity of the
catalysts was evaluated in a high-pressure xed-bed continuous-
ow stainless steel reactor, using 2.0 wt% DBT in cyclohexane as
a model compound. For each run, 100 mg catalyst (20–40 mesh
particle size), which was diluted with 400 mg quart pellets, was
presulded at 400 ꢁC at a ow rate of 40 mL minꢀ1 of 15% H2S/
H2 under atmospheric pressure. The tests were stabilized at
250 ꢁC with a total pressure of 1.5 MPa, a liquid hour space
velocity (LHSV) of 4 hꢀ1 and an H2/oil ratio of 600/1 (v/v). The
liquid product was collected and analyzed by a gas chromato-
graph (GC-9560) equipped with FID detector and SE-30 capillary
column. The reaction products were identied by matching
their retention times with those of the commercial standards
together with GC/MS analysis using a Finnigan Trace DSQ.
ꢁ
ꢁ
700 C. The weight loss in 40–450 C were attributable to the
removal of physically adsorbed water, the dihydroxylation of
alumina and partial decomposition of the residual carbon
ꢁ
content. The amount of weight loss in the region of 40–450 C
for the NiMoAl-x catalysts were greater than the NiMoAl-
0 sample. The slightly decreased weight in the region of 450–
700 ꢁC belongs to the decomposition of coke species which were
relatively difficult to degrade. Aer annealing in owing air, all
the catalysts represent little carbon residuals, which are proved
by the results of TG and TPO analysis. This result clearly
demonstrates that the dopamine self-assembly strategy affords
a good carbon coating on the Al2O3 surface, subsequently
Results and discussion
The commercial alumina applied in preparing Al2O3@C mate-
rials were small grains with irregular shape. Aer 24 h of stir-
ring in a Tris buffer solution with dopamine constituent, the
alumina was covered by polydopamine assemblies (denoted as
Al@PDA-x) without any visible modication in the particle size,
underlining the advantage of a dopamine-based synthetic
strategy for constructing shell architectures. When subjected to
a thermal treatment at 500 ꢁC in N2 atmosphere, the PDA layers
were converted in situ to form robust carbon shells (Al@C). The
catalysts were prepared by loading Ni and Mo species directly
over commercial alumina, Al@PDA-x or Al@C-1 support and
then annealed in owing air. The catalyst precursors before
Scheme 1 Schematic illustration for the formation process of the
calcination are denoted as NiMoAl@PDA-x and NiMoAl@C-1 catalysts.
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the calcination in air, compared with NiMoAl-x catalyst.
According to literature, the thermal decomposition of carbon
species under air ow could be mainly attributed to the removal
ꢁ
of structural carbon at ca. 350 C and the pyrolysis of support
carbon at ca. 550 ꢁC.31 However, no shoulder peak at ca. 350 ꢁC
was attributable to the decomposition of structural carbon in
the catalysts, indicating no detectable structural carbon species
were formed aer annealing under air atmosphere. The exis-
tence of support carbon was conrmed by the peaks centred at
400–600 ꢁC. The results evidence that pre-coated PDA can
generate support carbon that would suppress crystallite growth
of active phase, probably owing to the sufficient interaction to
efficiently suppress the interactions between active phase and
the support. Moreover, with increased PDA content, the
decomposition peak of carbon residuals at ca. 550 ꢁC shied to
lower temperature region, and the PDA coating on the alumina
surface (NiMoAl@PDA-1) was easier to decompose than carbon
coating (NiMoAl@C-1).
Aer coated with PDA or carbon coating, g-alumina was
conrmed as the predominant phase without signicant phase
conversion (Fig. 3). Aer been calcined under owing air and
subsequently suldation, and both of the resulting oxide and
sulde catalysts (NiMoAl-x and NiMoAl-1#) only exhibit XRD
patterns with broad diffraction peaks at 45.8ꢁ and 66.8ꢁ, in
accordance with the characteristic patterns of g-alumina. No
diffraction peaks attributed to Ni and Mo phases, such as MoS2,
NiSx, NiMoO4, NiAlO4, Al2(MoO4)3, etc., could be observed
Fig. 1 TG curves of (a) the catalysts precursors with carbon content
and (b) the corresponding catalysts after annealing under air flow at
500 ꢁC.
underlining the unique structural benets of the PDA-derived
architecture for mediating the surface properties of the support.
The TPO results in Fig. 2 show that the NiMoAl@C-1 and
NiMoAl@PDA-1 samples exhibited the detection of signicant
CO2 and CO formation by the MS signal (m/z ¼ 44, 28) during
Fig. 2 TPO patterns of the catalysts and precursors (NiMoAl@C-1 and
NiMoAl@PDA-1).
Fig. 3 XRD patterns of the oxide (a) and sulfide (b) NiMoAl-x catalysts.
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during any synthetic step, or aer suldation (Fig. S1†). This
observation conrmed the high dispersion of Ni and Mo species
over the surface of the supports.
Previous studies show that the deposition kinetics of dopa-
mine does not depend markedly on the surface chemistry of the
substrate, and the thickness of lms correlate closely to the
dopamine content in tris buffer solutions.32 The surface area
and porosities of the samples were investigated by N2 adsorp-
tion–desorption analysis, and all the samples exhibit similar
type IV isotherms with HI hysteresis loops (Fig. S2†). The
detailed texture parameters, including the surface areas from
BET analysis and the pore volumes from the BJH method are
nearly unchanged in reference to the commercial alumina
(Table 1), which indicate the addition of PDA coating caused
minimum effect on the texture properties of the alumina before
and aer PDA modication. In contrast, the NiMoAl@PDA-1
and NiMoAl@C-1 catalysts with carbonous shells that were
not yet deprived, the surface area and average pore diameter of
these two samples were signicantly reduced due to the thin
lm assembly on the surfaces of Al2O3 to reduce accumulated
pores by the introduced carbon deposition. These character-
izations conclusively demonstrate that the existence of
polydopamine-derived carbon. Furthermore, as the deposition
kinetics of dopamine does not depend markedly on the surface
chemistry of the substrate, which demonstrates the promising
strategy for the engineering of supported Ni–Mo nanocatalysts.
The carbon modied alumina surface have shown valid
effect in tailoring the interactions between active phases and
support. Thus, temperature programmed reduction (TPR) of the
catalysts in oxidic state were carried out to study the reducibility
of metallic species loaded on the supports. As expected, the
catalysts prepared with carbon-coated alumina were more easily
reduced than the NiMoAl-0 under H2 ow according to the
corresponding TPR results (Fig. 4). All the catalysts exhibit
a principle reduction peak between 400 and 500 ꢁC, which is
related to the reduction of octahedral coordinated polymeric
Mo species (from Mo6+ to Mo4+).7 This reduction peak for all the
catalysts prepared with PDA coating or carbon coating shied to
lower temperature region, meaning that the pre-loaded PDA or
carbon shell is capable in weakening the interaction between
Mo species and alumina support. When the PDA content in
Fig. 4 H2-TPR patterns of the oxide catalysts.
buffer solution increased reaches to 1 mg mLꢀ1 (NiMoAl-1), the
rst reduction peak of the catalyst located at ca. 450 ꢁC, which is
lowest among all the catalysts. The peak position for NiMoAl-1#
was similar with NiMoAl-1, which was also located at lower
temperature region than the commercial catalyst. The weak
shoulder peak at 500–600 ꢁC, which could be attributed to
reduction of NiO, increased with increasing PDA content,
indicating the addition of PDA attribute to the dispersion of NiO
over the surface of alumina. The reduction peak located at 700–
900 ꢁC is related to a further reduction of Mo4+ to Mo0. In
contrast, without PDA introduced, the NiMoAl-0 catalysts show
strong high-temperature Mo reduction peaks, resulting in
strong metal-support interaction and diminished catalytic
performance. The result provided that the modied interaction
between the carbon and support, or improved the thermal
stability of active phases were retained aer a 500 ꢁC annealing
in air, which were similar with the result in previous study.33
The XPS spectra of the sulded catalysts were collected and
then decomposed in order to characterize the bonding state and
surface concentration of the active phase over the catalyst.
While no evidence in C 1s (Fig. S3†) shows proof for the
formation of NiMoC phase, it is well admitted that the
Table 1 Summary of the several typical properties of the catalysts
ꢀd
ꢀe
SBETa (m2 gꢀ1
)
Vb (cm3 gꢀ1
)
Dc (nm)
L
(nm)
N
fMo
f
Catalysts
Al2O3
NiMoAl-0
234
202
200
195
192
190
187
203
157
161
0.53
0.42
0.41
0.42
0.40
0.39
0.40
0.39
0.33
0.35
6.3
6.2
6.8
6.4
6.4
6.7
6.8
6.1
6.0
6.5
—
—
—
5.0
4.9
4.6
4.8
4.9
5.0
5.2
—
2.0
2.1
2.2
2.1
2.0
2.2
2.1
—
0.23
0.22
0.26
0.25
0.23
0.22
0.23
—
NiMoAl-0.5
NiMoAl-0.75
NiMoAl-1
NiMoAl-1.25
NiMoAl-1.5
NiMoAl-1#
NiMoAl@PDA-1
NiMoAl@C-1
—
—
—
a
b
c
d
e
BET surface area. Pore volume. Average pore diameter. Average length of MoS2 nanoslabs. Average stacking degree of MoS2 nanoslabs.
Average fraction of Mo atoms located on the edge of the MoS2 crystallites.
f
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Table 3 Catalyst performance of DBT over the sulfided catalystsa
suldation degree of Mo and the formation of NiMoS phase are
considered to be vital on determining their HDS performance.6
Therefore, the deconvolution results of Mo 3d and Ni 2p spectra
were well collected in Table 2. According previous studies, the
binding energy of located at 228.7 ꢂ 0.1 eV, 232.6 ꢂ 0.2 eV and
230.6 ꢂ 0.2 eV were well attributed to the disulde phase or
MoS2 (MoIV 3d5/2), the Mo oxide species (MoVId5/2) and an
intermediate phase (MoV 3d5/2, which can be identied as
MoOxSy species), respectively. The broad peak centred at ca.
226.0 eV is assigned to the S 2s, which should be subtracted
from the total spectrum of Mo 3d.19,34 Compared with the
catalysts prepared with PDA, the decreased amount of MoVI and
MoV species of NiMoAl-0 shown in Table 3 can be indicating the
existence of Mo species that were difficult to reduced, which can
be ascribed to the strong interaction between active phase and
DBT
Rc
Product distributionb (%)
Conv.
(%)
(10ꢀ7 mol
Catalyst
BCH
CHB
BP
gꢀ1 sꢀ1
)
NiMoAl-0
1.8
2.0
1.8
1.6
1.8
1.9
2.2
10.2
14.6
14.8
10.4
10.1
12.0
13.5
88.0
83.4
83.4
88.0
88.1
86.1
84.3
74.2
79.3
78.9
88.5
82.4
83.7
85.3
5.8
6.2
6.1
6.9
6.4
6.5
6.6
NiMoAl-0.5
NiMoAl-0.75
NiMoAl-1
NiMoAl-1.25
NiMoAl-1.5
NiMoAl-1#
a
Reaction conditions: temperature 300 ꢁC, H2 pressure 2.0 MPa, LHSV
12 hꢀ1, H2/liquid (v/v) 600, feed 2 wt% DBT in decalin. BCH:
b
bicyclohexyl, CHB: cyclohexylbenzene, BP: biphenyl, DBT:
c
dibenzothiophene. Average specic rate (moles of DBT transformed
alumina support. The atomic fractions of MoS2 for the PDA or per second and per gram of catalyst).
carbon- modied catalyst are higher than the commercial
catalyst, while the NiMoAl-1 sample exhibit largest MoS2
constitution, which demonstrated the highest suldation
degree.
layers with a lattice distance for the (002) plane of ca. 0.61 nm
could be observed in all the HRTEM images (Fig. 6 and S3†).
The MoS2 dispersion over the sulded catalysts was also
determined by HRTEM measurement, and the statistic results
were summarized in Table 1. Compared with the commercial
catalyst, the NiMoAl-x catalysts exhibit slightly larger stacking
number (Ns) of MoS2 (Ls ¼ 4.8–5.2 nm, Ns ¼ 1.9–2.2, Table 1).
Specially, the NiMoAl-1 sample exhibits an adequate Ls value of
4.6 nm with the largest stacking number of 2.2. Therefore, the
NiMoAl-1 sample presents the highest calculated dispersion
among the relevant catalysts. That is, it possessed the largest
population of active edge sites among all the catalysts, indi-
cating the effect of residual carbon species over the dispersion
of active phases. Thus the occupation of Ni atoms on the edge
sites of MoS2 can be efficiently promoted, which favor the
formation of Ni–Mo–S active phases.34 This result clearly
demonstrates that the high MoS2 dispersion was obtained by
the strategy of thermally annealing Al2O3 enclosed within pol-
ydopamine formed carbon shells with NiMo precursor, which
would inuence the catalytic performance of the NiMo catalyst
in HDS reactions.
As shown in Fig. 5b, Ni species in the sulded catalysts can
be divided into three categories: the NiMoS phase located at
853.7 ꢂ 0.2 eV, the NiSx species located at 853.0 ꢂ 0.1 eV and the
unsulded NiO species centered at 856 ꢂ 0.2 eV. The remaining
two broad peaks are satellite lines of the corresponding Ni
species as a supplement. The fraction of NiMoS phase for all the
NiMoAl-x is higher than the commercial catalyst (NiMoAl-0),
while the NiMoAl-1 exhibit highest NiMoS proportion, indi-
cating that this catalyst possessed the largest number of active
sites, which were in good accordance with the results of atomic
fraction for MoIV phase. The relative higher fraction of NiMoS
phase demonstrated that the interaction between active phases
and support alumina were signicantly reduced, resulting in
catalysts with better suldation degree, which could lead to
promoted catalytic HDS performance. As expected, the active
phases over carbon-coated alumina annealing in owing air
atmosphere (NiMoAl-1#) exhibited enhanced NiMoS phase
fractions than the unmodied NiMoAl-0, which indicate that
the carbon layers were also efficient in optimising the interac-
tion between active phases and support, yet is slight inferior to
the PDA-coating (NiMoAl-1).
The as-prepared catalysts were tested to evaluate their
hydrodesulfurization (HDS) performance, and the bulk sulfur-
containing molecular, dibenzothiophene (DBT) was selected
as model compound to evaluate the HDS performance of the
PDA-modied catalysts, in comparison with the commercial
High resolution transmission electron microscopy (HRTEM)
was carried out to further observe the morphology and structure
of the catalysts in sulded state. The randomly oriented MoS2
Table 2 Fractions of different Mo and Ni species in the sulfided NiMo catalysts from XPS analysis
MoIV
MoV
MoVI
NiMoS
BE (eV)
NiSx
NiII
Catalysts
BE (eV)
% atom
BE (eV)
% atom
BE (eV)
% atom
% atom
BE (eV)
% atom
BE (eV)
% atom
NiMoAl-0
228.8
228.7
228.7
228.7
228.6
228.7
228.7
80.5
82.2
83.0
84.2
82.7
82.4
82.5
230.4
230.4
230.6
230.4
230.5
230.6
230.6
7.9
8.5
5.2
7.3
7.5
8.5
6.2
232.7
232.6
232.6
232.8
232.8
232.6
232.6
11.6
9.3
11.8
8.5
9.8
9.1
853.7
853.8
853.6
853.7
883.7
853.7
853.6
65.0
68.8
71.4
69.6
67.1
64.9
70.6
853.1
853.0
853.1
853.0
853.0
853.0
853.0
4.5
7.2
4.2
5.4
6.2
7.5
5.2
856.0
856.2
856.1
856.1
856.2
856.2
856.2
30.5
24.0
24.4
25.0
26.7
26.6
24.2
NiMoAl-0.5
NiMoAl-0.75
NiMoAl-1
NiMoAl-1.25
NiMoAl-1.5
NiMoAl-1#
11.3
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Fig. 5 (a) Mo 3d and (b) Ni 2p XPS spectra of the sulfided catalysts.
catalysts (NiMoAl-0). The catalytic activities of the as-prepared combustion of the carbon shells, the HDS activity over the PDA
catalysts were evaluated under 300 ꢁC with an H2/liquid of or carbon coating catalysts were promoted compared with the
600/1 (v/v), a liquid hour space velocity (LHSV) of 12 hꢀ1 and commercial catalyst (NiMoAl-0), which clearly demonstrated the
a total pressure of 2.0 MPa. The conversion and detailed unique structural benets of the dopamine-derived shell
product selectivity were listed in Table 3. Previous studies architecture for optimizing the interaction between active phase
proved that the HDS reactions of DBT also occurs in two parallel and alumina support. In this respect, the strategy of thermally
paths: the direct desulfurization pathway (DDS pathway), where annealing Ni Mo catalysts over carbon shells generated from
biphenyl (BP) was produced; and the hydrogenation route (HYD polydopamine was successful in optimizing the properties and
pathway), where the cyclohexylbenzene (CBH) and dicyclohexyl activities over NiMo/Al2O3 catalysts. The difference in product
(DCH) were formed (Scheme S1†). As displayed in Table 3, all distribution for the catalysts were hardly observed as similar
the NiMoAl-x series catalysts exhibit higher HDS activity of DBT active phase were presented, indicating the loading of carbon-
under such reaction conditions. The HDS catalytic efficiencies residual coating caused minimum effect over the texture prop-
towards DBT over NiMoAl-x catalysts increased with increasing erties and the structure of active phase. Interestingly, the
PDA content and peaked at approximately 1 (x value in NiMoAl- MoNiAl-1# catalyst with removed carbon-coating also exhibits
x). However, further increasing the PDA content demonstrated promoted DBT conversion than the commercial catalyst, but
no positive changes in the catalytic efficiency. The result indi- was slightly lower than MoNiAl-1, which could be probably
cates that aer annealing in owing air at 500 ꢁC for attributed to a weak metal-support interaction and low specic
Fig. 6 Typical HRTEM images of the sulfided (a) NiMoAl-0 (b) NiMoAl-0.5 (c) NiMoAl-0.75 (d) NiMoAl-1 (e) NiMoAl-1.25 (f) NiMoAl-1.5.
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surface area.17 It is generally known that the catalytic behavior
essentially is determined by their structural features and
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