Ni-Silicides nanoparticles as substitute for noble metals for hydrogenation of nitrobenzene to p-Aminophenol in sulfuric acid

Article abstract of DOI:10.1016/j.apcata.2016.04.013

The hydrogenation of nitrobenzene (NB) in acid medium is regarded as the most promising route to prepare p-aminophenol (PAP). The noble metals such as Pt have been reported as commercial catalysts but base metals are inactive due to strong acids needed in the reaction medium. Therefore, Ni-based catalyst has been attempted using solid acids, however, the results are not attractive. In this work, in order to improve the acid-resistance of Ni in sulfuric acid, we insert Si into the Ni crystal structure to form Ni-silicides. The as-prepared supported Ni-silicides nanoparticles exhibit excellent acid-resistance and perform high activity and selectivity of the entitled reaction. NiSi₂ nanoparticles supported on acid-treated active carbon are highly active and selective to convert NB to PAP in sulfuric acid. The XPS measurements show that the Ni in the Ni-silicides donates electrons to silicon, but the Ni valence is still far lower than nickel oxide, which causes its stability in acids and the activity to hydrogen activation is partially reserved. Thus, the catalysts exhibit excellent stability in alloy structure, morphology, and catalytic performance in reuses, suggesting the Ni-silicides nanoparticles as the promising substitute for precious metal for PAP manufactures.

Full text of DOI:10.1016/j.apcata.2016.04.013

Applied Catalysis A: General 520 (2016) 151–156
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Ni-Silicides nanoparticles as substitute for noble metals for
hydrogenation of nitrobenzene to p-Aminophenol in sulfuric acid
Zhen Donga, Tao Wanga, Jie Zhaoa, Teng Fua, Xuefeng Guoa, Luming Penga, Bin Zhaoa,
Nianhua Xuea, Weiping Dinga,∗, Zaiku Xieb,∗
a Key Lab of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China
b Shanghai Research Institute of Petrochemical Technology, Shanghai 201208, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The hydrogenation of nitrobenzene (NB) in acid medium is regarded as the most promising route to pre-
pare p-aminophenol (PAP). The noble metals such as Pt have been reported as commercial catalysts but
base metals are inactive due to strong acids needed in the reaction medium. Therefore, Ni-based cata-
lyst has been attempted using solid acids, however, the results are not attractive. In this work, in order
to improve the acid-resistance of Ni in sulfuric acid, we insert Si into the Ni crystal structure to form
Ni-silicides. The as-prepared supported Ni-silicides nanoparticles exhibit excellent acid-resistance and
perform high activity and selectivity of the entitled reaction. NiSi2 nanoparticles supported on acid-
treated active carbon are highly active and selective to convert NB to PAP in sulfuric acid. The XPS
measurements show that the Ni in the Ni-silicides donates electrons to silicon, but the Ni valence is
still far lower than nickel oxide, which causes its stability in acids and the activity to hydrogen activation
is partially reserved. Thus, the catalysts exhibit excellent stability in alloy structure, morphology, and
catalytic performance in reuses, suggesting the Ni-silicides nanoparticles as the promising substitute for
precious metal for PAP manufactures.
Received 28 January 2016
Received in revised form 31 March 2016
Accepted 14 April 2016
Available online 16 April 2016
Keywords:
Ni-silicides nanoparticles
High selectivity
Acid-resistance
Hydrogenation
p-aminophenol
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
mineral aqueous acid by solid acid to eliminate the corrosion. How-
ever, the yield of PAP is not attractive [7,8]. In addition, noble
p-Aminophenol (PAP) is an important intermediate and being
widely used in the manufacturers of medical drugs e.g. paracetamol
and acetaminophen, anticorrosion agents, and dyeing agents [1–3].
Commonly, PAP is produced from phenol, p-chloronitrobenzene
or p-nitrophenol, which has several drawbacks and suffers a poor
overall yield [4,5]. The hydrogenation of nitrobenzene (NB) to
PAP using Pt/C catalyst in acid medium is regarded as the most
promising route owing to low economical feature and high quality
product [6]. The reaction involves partially hydrogenation of NB to
phenylhydroxylamine (PHA), followed by Bamberger’s rearrange-
ment in situ to PAP in the presence of mineral acid. Aniline becomes
the main side product through the overhydrogenation of NB [2,3].
Carbon supported noble metal catalysts, especially Pt/C, are consid-
ered as the best catalyst for this reaction, whereas the high price and
the inevitable loss of precious metal by corrosion limit its further
application. Therefore, several attempts have been made to replace
metals suffering from low selectivity to PAP have been reported
due to significant excessive hydrogenation of PHA to form aniline.
Alternative of cheap and abundant base metals such as Ni to Pt or
other noble metals was tried [9–11]. Considering transition metals
always suffer from the acid corrosion, further modification to form
novel materials with excellent acid-resistance is increasingly nec-
essary. Binary alloys of Ni-silicides coating films was reported to
have good corrosion resistance in the aqueous solutions of dilute
acid solution [12,13]. To date, metal silicides have attracted grown
attentions in many fields, including complementary metal-oxide-
semiconductor devices [14,15], hydrogen evolution reaction [16],
photoelectrochemical water splitting [17,18], thin film coatings
[13,19], and especially some catalytic applications. For example,
Nuzzo et al. reported the synthesis of metal silicides and their high
activity and selectivity for the competitive dehydrogenation and
hydrogenolysis of cyclohexane [20]. Recently, Baiker et al. revealed
that amorphous Pd81Si19 exhibited a more than 50 times higher TOF
than a conventional Pd/SiO2 catalyst toward the semihydrogena-
tion of a propargylic alcohol using “supercritical” CO2 as solvent
[21]. Liang et al. reported silicon modified Reney-Ni presented
∗
Corresponding authors.
E-mail addresses: dingwp@nju.edu.cn (W. Ding), xzk@sinopec.com (Z. Xie).
http://dx.doi.org/10.1016/j.apcata.2016.04.013
0926-860X/© 2016 Elsevier B.V. All rights reserved.

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Z. Dong et al. / Applied Catalysis A: General 520 (2016) 151–156
Fig. 1. Results of catalytic experiments. A) Catalyst supported on Al2O3, and the inset is Ni-Si-C/Al2O3 with different carbon contents. B) Support influences on catalytic
performance of 6Ni/Support, 6Ni-Si/Support, and Ni-Si-C/Support. Reaction conditions: 60 mg catalyst, 80 mg nitrobenzene, 20 mL 1.5 mol/L H2SO4 solution, 1 MPa H2, 393 K,
1 h C) Catalytic activity of yNi-Si/AC6 at various reaction time. D) The recycle experiment results of 20Ni-Si/AC6. The experiment was conducted at the conditions: 60 mg
catalyst, 80 mg nitrobenzene, 20 mL 1.5 mol/L H2SO4 solution, 1 MPa H2, 393 K, 2 h. Reaction 10* to 12* used 10 mL 3.0 mol/L H2SO4 solution and 10 mL ethanol as solvent.
Catalyst was filtrated, washed, and dried at 353 K before next use.
Table 1
Comparison of catalysis performances on reported catalysts used for hydrogenation of NB to PAP.
Reaction conditions
Catalysis performance
Catalyst
Loading
(%)
Ref.
Acid
Surfactant T(K)
P(MPa) Time(h) nNB/nmetal Conv.(%) PAP selectivity(%)
1
2
3
4
5
6
7
8
9
20Ni-Si/AC6
6Ni-Si/AC6
Pt/C
Pt/activated characol
Pt/C
PVP-PdCl2-0.5LaCl3
Au/TiO2
Pt/ZrO2
20
6
3
5
1.5
H2SO4
H2SO4
H2SO4
H2SO4
H2SO4
×
×
393
393
353
358
363
357
373
353
413
438
383
353
423
523
423
393
393
383
353
1
1
3
4
8
1.5
–
3
4
8
4
2
10
3
2
0.5
–
2
3
3
–
3
11
99
100
97
100
100
100
78
87
20
100
61
41
52
35
100
80
85
100
100
77
70
85
46
81
88
85
76
78
26
47
55
49
100
24
50
63
This work
×
√
0.7
0.5
0.07
0.1
1
15111
25000
15115
300
1078
38161
1000
4200
10875
29870
1753
–
[4]
[29]
[30]
[31]
[32]
[33]
[34]
[3]
[1]
[35]
[7]
[36]
[37]
[23]
[38]
[39]
[40]
√
√
8 × 10−3 mol Pd/L H2SO4
0.9
2
H2SO4
Zirconium sulphate
CO2
MgAPO-5
carbon
CF3COOH
S2O82−/ZrO2
ZSM-5
FSAPO-5
H2SO4
SO3H C/SBA-15
SAPO-5
×
×
2
Pt-Sn/Al2O3
0.04Pt0.07Sn
0.015Pt
3
3
0.1
1
–
4
20
×
√
0.2
0.4
0.4
0.1
0.4
0.1
0.9
1
10 Pb-Pt/MgAPO-5
11 Pt/C
√
12 Pt/C
×
√
13 Pt-S2O82−/ZrO2
14 Pt/ZSM-5
15 Pt/SiO2
×
√
–
12
9
5
16 CN/Ni/Al2O3
17 Ni/SiO2
18 NiCoB/SAPO-5
19 Ni-Pt/ZSM-5
×
√
0.8
0.4
2.8
√
×
90
10Ni1Pt
H2SO4
50:1 w/w 93
high selectivity to semi-hydrogenated products (2-butene-1,4-
diol) from hydrogenation of highly concentrated 2-butyne-1,4-diol
[22]. These results documented have shown that the silicon added
to form alloys with noble metals or transition metals modify the
catalysts in both stability and products selectivity. But their cat-
alytic property for hydrogenation under acid conditions has never
been reported.
nanoparticles exhibit superior acid-resistance under hydrother-
mally acid conditions with structural and catalytic stability in
reuses and show excellent catalytic properties for producing PAP
from hydrogenation of NB in harsh acidic medium.
2. Experimental
In this study, we report several Ni-silicides catalysts used for the
entitled reaction with different supports, nickel loadings, and car-
bon coating contents via a very simple preparation route. According
to characterizations, highly dispersed uniformly sized Ni-silicides
2.1. Support preparation
Rod-like ␥-Al2O3 was synthesized by hydrothermal method
[23]. Briefly, aluminum isopropoxide (Aladdin, 96 g) was added to

Z. Dong et al. / Applied Catalysis A: General 520 (2016) 151–156
153
Fig. 2. TEM and HRTEM images of A) 6Ni-Si-C/Al2O3 with carbon content of 2.88 wt.%; B) 20Ni-Si/AC6. A’) 6Ni-Si-C/Al2O3 after hydrogenation reaction in sulfuric acid for
1 h. B’) 20Ni-C/AC6 catalyst after 12 recycles. Reaction details are mentioned above.
Table 2
Atomic concentration according to XPS analysis.
Samplesa
Ni
Si
O
C
Al
Si:Nib
6Ni-Si/Al2O3
6Ni-Si-C/Al2O3C% = 2.88
20Ni-Si/AC6
1.25
0.63
0.83
0.80
8.36
5.49
4.66
4.47
59.29
53.16
15.75
15.76
9.29
21.81
25.22
–
6.69
8.71
5.61
5.59
15.50
78.76
78.97
Used 20Ni-Si/AC6
–
a
Used catalyst is after recycle experiment for 12 runs in harsh acidic environment at 393 K. Catalytic test details were mentioned at Section 2.
Atomic ratio of Si to Ni.
b
the mixture of oleic acid (Aladdin, 7.5 g) and NH3·H2O (Aladdin,
2.3. Characterization
6.2 mL) at 353 K and kept for 5 h. Then, the resultant was hydrother-
mal treated at 453 K for 48 h. After that, the powder was filtrated,
washed with ethanol and calcination at 823 K for 5 h to give the ␥-
Al2O3. Active carbon (denoted as AC0, Aladdin, 1.5 g) was treated
with sulfuric acid (1.5 mol/L, 35 mL) in a hydrothermal autoclave
at 393 K with stirring for a certain time and followed by wash, fil-
tration and drying, denoted as ACx, where x is the hours of acid
treatment.
X-ray diffraction (XRD) patterns were recorded on a Phillips
X’Pro diffractometer (Phillips, Holland) using CuK␣ radiation
(40 kV, 35 mA). Thermogravimetric analysis (TGA) was conducted
on an STA 449C Thermal Star 300 instrument (Netzsch, Germany)
in air atmosphere with a heating rate of 10 K/min to 1073 K. Nitro-
gen adsorption-desorption isotherms was carried out at 77 K on
Quantachrome Autosorb-1 Surface Area and Pore Size Analyzer
(Quantachrome, U.S.). The specific surface area was determined by
Brunauer-Emmett-Teller (BET) method. The pore size distribution
was calculated from the corresponding desorption branch of N2
isotherm by density functional theory (DFT) for ACx and Barrett-
Joyner-Halenda (BJH) method for ␥-Al2O3. Transmission electron
microscopy (TEM) measurements were carried out with a JEOL
JEM-200CX instrument (JEOL, Japan) at an accelerating voltage of
200 kV. Dynamic NB contact angle test for catalyst was conducted
by using SL200B contact angle-measuring device (Kino, U.S.) at
room temperature [27,28]. The powder sample was first wetted
with ethanol and then pressed into sheet (0.1 cm thick). After dried
at 353 K, the contact angle was measured. Compositions on the
catalysts surface and their corresponding binding energies were
determined by PHI 5000 VersaProbe X-ray photoelectron spec-
troscopy (ULVCA-PHI, U.S.) equipped with a monochromatic Al
K␣ X-ray source under pressure of 10−7 mbar. XPS analysis was
conducted at room temperature. Binding energies were calibrated
using C1s peak (284.6 eV) as a reference.
2.2. Catalyst preparation
All catalysts were initially prepared by incipient impregnation
of nickel (II) nitrate hexahydrate (Aldrich) onto the above supports.
Firstly, the wet samples were dried at 393 K for 12 h and calcined at
723 K for 5 h to give NiO/Al2O3 and 673 K for 5 h in N2 for catalysts
loaded on carbon supports, respectively. Secondly, NiO/Al2O3 was
reduced by 5% H2 in N2 at 873 K for 1 h, and Ni/AC6 was treated by
5% H2 in N2 at 673 K for 1 h due to carbon-thermic reduction during
the calcination [24,25]. Thirdly, the sample was cooled to 623 K and
treated with 5% SiH4 in Ar (molar ratio of Ni to SiH4 equals 1:5) to
give Ni-silicides, denoted as yNi-Si/Support (y = 6, 10, 20) [26]. Note
here y stands for Ni content in supported nickel catalysts, which is
6 wt.%, 10 wt.%, or 20 wt.%, respectively. Finally, carbon coating was
conducted at 1073 K to give 6Ni-Si-C/Al2O3 with carbon contents
of 2.88 wt.% by adjusting molar ratio of Ni:CH4 to 1:45.

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Z. Dong et al. / Applied Catalysis A: General 520 (2016) 151–156
Fig. 3. XRD patterns of catalysts before and after reaction. A) 6Ni-Si-C/Al2O3 with different carbon contents; B) 20Ni-Si/AC6. Reaction details are mentioned above.
2.4. Catalytic test
presented in Fig. 1C. NB conversion firstly increases linearly with
the reaction time and closes to total conversion in 2 h for 20Ni-
Si/AC6. A little longer time is needed for the catalysts with lower
nickel loadings to convert NB completely. The selectivities to PAP
keep over 99%, indicating the low activity of the catalyst for exces-
sive hydrogenation of PHA to form side product. To further test the
robustness and stability of the 20Ni-Si/AC6, the recycle experiment
is carried out and the results are shown in Fig. 1D. It is very impres-
sive that 20Ni-Si/AC6 possesses unchanged catalytic activity in 12
runs. The decreases in conversion in the 7th, 8th, and 9th uses are
due to the increased polarity change of the support in acid and are
overcome by adding ethanol as co-solvent (also see the discussion
below).
Comparing with reported catalysts used in the mentioned reac-
tion, as listed in Table 1, it is easy to see that supported precious
metal catalysts especially Pt/C were fully studied at relative mild
synthetic conditions. Publications mainly focused on catalytic per-
formance or mechanism over noble metal based catalysts under
H2SO4 [4,29–31,34] and solid acid [1,3,7,33,36,37]. Beyond that,
Zhang et al. studied the catalytic hydrogenation performance in
pressurized CO2/H2O system over Pt-Sn/Al2O3 catalyst, however
the result is not attractive [34]. Tanielyan et al. reported Pt/C cat-
alyst under H2SO4 processed 100% conversion and 85% of PAP
selectivity [30]. In the last 20 years, base metal as alternative of
noble metal was studied. Due to acid corrosion, Ni-based catalysts
were typically used after modification or in solid acid system. Our
previous paper suggests CN encapsulated Ni/Al2O3 shows good
hydrogenation ability and excellent selectivity to PAP [23]. How-
ever, we found Ni-based catalysts derictly used with solid acid
show inattractive peformence especially PAP selectivity. Accord-
ingly, supported Ni-silicides (20Ni-Si/AC6 and 6Ni-Si/AC6) show
excellent seletivity to PAP via NB hydrogenation, and also good
hydrogennation ability under relative mild synthesis condition.
Catalytic performance was evaluated in a batch-type autoclave
reactor (Kemi, China). In a typical reaction, catalyst (60 mg) and
NB (Aladdin, 80 mg) were added into H2SO4 solution (1.5 mol/L,
20 mL) or mixed solution (10 mL ethanol + 10 mL 3 mol/L H2SO4).
The reactor was sealed and filled with H2 to a pressure of 1.0 MPa
at room temperature after purges. The reactor was then heated to
393 K by 2 K/min with stirring and hold for different time. After
cooling down the reactor to room temperature naturally and the
catalyst was separated, washed with ethanol, and dried at 353 K for
next application. The resulting solution was diluted to 100 mL using
ethanol and analyzed by high performance liquid chromatography
(Agilent HPLC 2016, U.S.) equipped with a C18 column and an UV
detector (254 nm). The eluent was methanol/H2O (volume ratio:
40/60).
3. Results and discussion
3.1. Catalytic test
The catalytic results of hydrogenation of NB over prepared cata-
lysts in related components and structures are shown in Fig. 1. The
concentrated proton would adsorb on the surfaces of 6Ni/Al2O3 and
etch the catalyst fast since no product is detected. While owing
to carbon encapsulation, nickel particles in 6Ni-C/Al2O3 are pro-
tected from etching and possess good activity under the acidic
conditions (Fig. S1). In contrast, 6Ni-Si/Al2O3 and 20Ni-Si/Al2O3
show improved performances. However, Al2O3 dissolves seriously
in above catalysts. Taking advantages of carbon coating, 6Ni-Si-
C/Al2O3 catalysts are found to be protected from corrosion and the
hydrogenation activity is influenced as well (inset Fig. 1A). With the
increase of the carbon content, the NB conversion firstly increases
and reaches the highest value. Then the NB conversion begins to
decrease when the carbon content grows larger. We speculate that
the volcano-shaped relationship between the NB hydrogenation
activity and carbon content depends on molar ratio of Ni:CH4: only
an appropriate ratio of Ni:CH4 can lead to the optimum carbon
layer thickness; too much carbon may reduce the Ni2Si content
in the final composites and even the active sites, and thus lower
the NB hydrogenation activity. Therefore, the optimization in both
the carbon layer thickness and the number of active sites by an
appropriate CH4 deposition can better facilitate the NB conversion.
Different with 6Ni/Al2O3, 6Ni/ACx catalysts exhibit comparable
performance with 6Ni-C/Al2O3 due to carbon layers piled on cata-
lysts particles. Ni-silicides catalysts show the improved activities.
Nickel silicides particles in 6Ni-Si-C/ACx are supposed to be cov-
ered by thick carbon layers and therefore catalysts exhibit lower
activity than 6Ni-Si/ACx (Fig. 1B). Then, yNi-Si/AC6 catalysts with
different nickel loadings show excellent catalytic performances, as
3.2. Characterizations
Morphologies and structures of the fresh 6Ni-Si-C/Al2O3 cata-
lyst are studied by TEM as shown in Fig. 2A. It can be seen that
the synthesized ␥-Al2O3 shows a uniform rod-like structure with
diameters of about 20 nm. The appearance of the (121) plane of
Ni2Si is detected by HRTEM. It is noteworthy that lattice distance
of Ni (111) is 0.203 nm when measured by the same instrument,
which is close to that of Ni2Si (121). Therefore, Ni2Si is further
verified as combined with XRD and XPS characterizations bellow.
The corresponding HRTEM image reveals that a well-defined crys-
talline lattice with interfringe distance of 0.312 nm, which is in
good agreement with the interplanar distance of the (111) plane
of NiSi2. Ni2Si is observed as highly dispersed nanoparticles and
coated with carbon layers. After hydrogenation reaction for 1 h,
the Ni2Si nanoparticles are not influenced by the harsh acid envi-

Z. Dong et al. / Applied Catalysis A: General 520 (2016) 151–156
155
Fig. 4. High resolution XPS spectra: A) C 1s; B) O 1 s; C) Si 2 p; D) Ni 2 p of fresh catalysts and used catalysts including 6Ni-Si/Al2O3, 6Ni-Si-C/Al2O3 C% = 2.88, 20Ni-Si/AC6,
and 20Ni-Si/AC6 after recycle experiment in sulfuric acid at 393 K.
ronment, whereas ␥-Al2O3 is etched to some extent as shown in
Fig. 2A’. Increasing carbon coating content can protect the struc-
ture of catalyst from corrosion (Fig. S1). The AC support is treated
by sulfuric acid for several hours to increase its stability in cat-
alytic reaction and consequently the hydrophilicity is improved
due to the introduction of oxygenic groups, which favors the metal
dispersion (Fig. S2–S4 and Table S1) [41,42]. Correspondingly, the
fresh 20Ni-Si/AC6 catalyst possesses uniform and highly dispersed
nanoparticles, as shown in Fig. 2B. The corresponding HRTEM image
reveals that a well-defined crystalline lattice with interfringe dis-
tance of 0.312 nm, which is in good agreement with the interplanar
distance of the (111) plane of NiSi2. Fig. 2B’ depicts the structure
of 20Ni-Si/AC6 catalyst after 12 recycles. The nanoparticles are still
highly dispersed without aggregation or change in particle size dis-
tribution. The HRTEM image shows the (111) and (220) planes of
NiSi2, which further reveal the good stability and acid-resistance of
20Ni-Si/AC6.
XRD patterns of fresh 6Ni-Si-C/Al2O3 with different carbon con-
tents exhibit typical diffractions of ␥-Al2O3. Ni2Si, detected by TEM,
is proved to be highly dispersed on supports since its characteris-
tic diffractions are detected in used 6Ni-Si-C/Al2O3 where a large
amount of ␥-Al2O3 is etched by the acid solution as shown in Fig. 3A
[43]. The acid etch to ␥-Al2O3 can be relieved by carbon coating
since diffractions of ␥-Al2O3 are detected in 6Ni-Si-C/Al2O3 with
different carbon content of 3.91 wt.% (see TEM results in Fig. S1).
XRD measurements on 20Ni-Si/AC6 indicates the NiSi2 nanopar-
ticles can work well without crystal phase change in sulfuric acid
solution even after reuses of 12 runs, as depicted in Fig. 3. These
results suggest that both Ni2Si and NiSi2 nanoparticles show excel-
lent acid-resistance, see Fig. S5 for more details.
XPS measurements are conducted to evaluate the details of
surface states of the catalysts. Fig. 4 provides the core-level XPS pat-
terns in the C 1 s, O 1 s, Si 2 p, and Ni 2 p regions. The analysis results
are summarized in Table 2. The C 1s XPS patterns of 20Ni-Si/AC6,
no matter fresh or used, depict similar peaks to 6Ni-Si/Al2O3. How-
ever, due to the influence of supports the full width half maxima of
the XPS peaks was quite different. The peaks at B. E. of 284.6, 285.5,
and 287.0 eV in C 1 s XPS pattern of 6Ni-Si-C/Al2O3 are attributed to
sp2-hybridized graphitic carbon, carbon atom bonded with oxygen,
and carbon component bonded with oxygen in Al2O3, respectively
[23,39,44]. The O 1 s XPS profiles can be fitted to a broad peak cor-
responding to the oxygen in Al2O3, adsorbed atomic oxygen, and
oxygen bonded with silicon.
the surfaces of supports since it can be detected on the surfaces of
AC6 with further SiH4 treatment (Fig. S6). The Ni 2p patterns taken
over 6Ni-Si/Al2O3 are similar to that of 6Ni-Si-C/Al2O3. The two
couples of Ni 2 p peaks appeared at ∼853.0 and ∼869.7 eV (Ni2Si)
and ∼856.3 and ∼873.9 eV (NiSi2) for the samples 6Ni-Si/Al2O3
and 6Ni-Si-C/Al2O3 indicate the mixed phases of Ni2Si and NiSi2
in these catalysts [46,47]. The carbon coating by methane treat-
ment increases the carbon concentration by 6.21% and also leads
to the destruction of crystal structure of NiSi2 and the formation
of new phase of Ni2Si, referring to Table 2 and Fig. 4D, which is in
agreement with XRD results.
Due to carbon encapsulation, concentration of Ni was decreased.
Si:Ni atomic ratio was changed provides an evidence of phase
changes of Ni-silicides. Note the full width half maxima of the XPS
peaks especially of Ni 2 p that was quite different was assumed
caused by low quality of original data. Using active carbon as sup-
port, the catalysts show different property. With higher nickel
loading, for the fresh and used 20Ni-Si/AC6 catalyst, Ni 2 p core-
level spectra are attributed to pure NiSi2. After 12 catalytic recycles,
no difference in atomic concentrations and Si:Ni atomic ratio is
observed in Table 2, while the B. E. of Ni 2 p core-level peaks
of 20Ni-Si/AC6 show negative shifts of 0.3 eV, as compared with
the catalysts before reaction, which may be caused by the sur-
face change of the support during reaction in sulfuric acid [41,42].
It is noteworthy that 6Ni-Si/AC0, 6Ni-Si/AC3, and 6Ni-Si/AC6 also
present similar Si:Ni atomic ratio with 20Ni-Si/AC6 and their B. E. of
Ni 2p3/2 peaks also show negative shifts when acid-treated active
carbons are used as supports, see Fig. S6 and Table S2. It could be
concluded that the Ni-silicides are mixture phases of NiSi2 and Ni2Si
with alumina as support but only NiSi2 on active carbon support.
The Ni in Ni-silicides donates electrons to silicon, but its valence is
still far lower than nickel oxide, which causes its stability in acids
and the activity to hydrogen activation is partially reserved.
Due to the catalytic test is performed with water as solvent,
dynamic NB contact angle of the as prepared catalysts were mea-
sured to evaluate their hydrophobicity, which was supposed to
have significant influences on catalytic performance (Fig. S7).
Combined with the results of catalytic test, it is found that the
hydrophobicity favors the catalysts contact with NB droplets and
further promotes their hydrogenation property.
Fig. 5 displays the NB contact angle for 6Ni-Si/AC6 after differ-
ent reaction time. The catalyst exhibits decreased hydrophobicity
and therefore increased hydrophilicity due to the extended oxy-
genic groups on the support surfaces after used in sulfuric acid
for long time. Furthermore, we found that the dispersion of 20Ni-
Si/AC6 became better when ethanol was added as co-solvent (Fig.
Fig. 4C displays the Si 2 p core-level peaks with two B. E. posi-
tions of ∼100.0 and ∼103.1 eV for Ni-Si alloy and silicon oxide
[13,45]. The silicon oxide is expected from the silicon deposited on

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