Synergistic effect of Mo2N and Pt for promoted selective hydrogenation of cinnamaldehyde over Pt-Mo2N/SBA-15

Article abstract of DOI:10.1039/c5cy01654a

The introduction of a suitable co-catalyst is a promising way to improve the catalytic efficiency of Pt catalysts. Herein, SBA-15 modified with well-dispersed Mo₂N nanoparticles (NPs) is synthesized by anchoring phosphomolybdic acid (HPM) on aminated SBA-15 (SBA-15-NH₂) primarily, followed by nitridation treatment under NH₃. After loading of Pt, the Pt-Mo₂N/SBA-15 catalyst with close-contact Pt and Mo₂N NPs can be formed. The intensive interaction of Pt and Mo₂N benefited from the close contact is verified by XRD and XPS tests. As a result, the ternary Pt-Mo₂N/SBA-15 catalysts have shown superior performance to Pt/SBA-15 catalysts for the selective hydrogenation of cinnamaldehyde to cinnamyl alcohol. The TOF over the 1 wt% Pt-Mo₂N/SBA-15 catalyst is close to that over the 3 wt% Pt/SBA-15 catalyst, but its selectivity to cinnamyl alcohol (COL) is 2.5 times that of 3 wt% Pt/SBA-15. The promoted activity of Pt-Mo₂N/SBA-15 is mainly attributed to the synergistic effect of Pt and Mo₂N NPs, which can cause the full development of the catalytic ability of Pt, thus reducing the Pt usage, through increasing the dispersion of Pt NPs and the fraction of Pt⁰ species in the catalyst.

Full text of DOI:10.1039/c5cy01654a

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Synergistic effect of Mo2N and Pt for promoted
selective hydrogenation of cinnamaldehyde over
Pt–Mo2N/SBA-15†
Cite this: DOI: 10.1039/c5cy01654a
Dong Wang,ab Yujun Zhu, a Chungui Tian,a Lei Wang,a Wei Zhou,a Yongli Dong,a
*
a
a
a
a
*
Qing Han, Yunfei Liu, Fulong Yuan and Honggang Fu
The introduction of a suitable co-catalyst is a promising way to improve the catalytic efficiency of Pt cata-
lysts. Herein, SBA-15 modified with well-dispersed Mo2N nanoparticles (NPs) is synthesized by anchoring
phosphomolybdic acid (HPM) on aminated SBA-15 (SBA-15-NH2) primarily, followed by nitridation treat-
ment under NH3. After loading of Pt, the Pt–Mo2N/SBA-15 catalyst with close-contact Pt and Mo2N NPs
can be formed. The intensive interaction of Pt and Mo2N benefited from the close contact is verified by
XRD and XPS tests. As a result, the ternary Pt–Mo2N/SBA-15 catalysts have shown superior performance to
Pt/SBA-15 catalysts for the selective hydrogenation of cinnamaldehyde to cinnamyl alcohol. The TOF over
the 1 wt% Pt–Mo2N/SBA-15 catalyst is close to that over the 3 wt% Pt/SBA-15 catalyst, but its selectivity to
cinnamyl alcohol (COL) is 2.5 times that of 3 wt% Pt/SBA-15. The promoted activity of Pt–Mo2N/SBA-15 is
mainly attributed to the synergistic effect of Pt and Mo2N NPs, which can cause the full development of
the catalytic ability of Pt, thus reducing the Pt usage, through increasing the dispersion of Pt NPs and the
fraction of Pt0 species in the catalyst.
Received 29th September 2015,
Accepted 11th November 2015
DOI: 10.1039/c5cy01654a
www.rsc.org/catalysis
practical application, it is very essential to decrease the usage
of Pt due to its high cost and scarcity. Many methods have
1. Introduction
Selective hydrogenation of α,β-unsaturated aldehydes has
attracted much attention in recent years because the products
are widely used in pharmaceuticals, perfumes and flavors.1
One of the main challenges is to improve the reduction of
CO to produce unsaturated alcohols because the hydroge-
nation of the CC bond is thermodynamically favoured.2–4
Selective hydrogenation of cinnamaldehyde (CAL) is a repre-
sentative example of these reactions. The hydrogenation of
CAL proceeds via first hydrogenation of CC and CO dou-
ble bonds to hydrocinnamaldehyde (HALD) and cinnamyl
alcohol (COL), respectively, followed by total hydrogenation of
these products to hydrocinnamyl alcohol (HALC) (Scheme 1).
It's well known that noble metals (Pt, Pd, Au, Rh and Ru) are
much more active in the hydrogenation of CAL.5–8 Among
them, Pt-based catalysts have been the focus for a long time
due to their excellent catalytic performance in the hydrogena-
tion of carbonyl compounds.9–13 However, for large-scale
been developed to enhance the performance of Pt-based cata-
lysts by tuning the species, shape or size of the platinum NPs.
Special supports may lead to high catalytic performances as
compared with conventional supports because of the specific
interactions between the support and Pt nanoparticles, such
as carbon nanotubes14,15 and graphene.16 Meanwhile, addi-
tion of a metal co-catalyst (oxide) to supported Pt catalysts is
a promising route. Typically, the use of a second metal (Fe,
Co)15 or metal oxides (TiO2,17 ZnO (ref. 18)) to promote the
catalytic activity of Pt has been demonstrated, which is
ascribed to the electron transfer between the promoters and
the metallic Pt.
Recently, transition metal nitrides and carbides have
received special attention in the catalysis field including
a Key Laboratory of Functional Inorganic Material Chemistry, Ministry of
Education of the People's Republic of China, Heilongjiang University, Harbin
150080, PR China. E-mail: yujunzhu@hlju.edu.cn, fuhg@vip.sina.com;
Fax: (+86) 451 8666 1259; Tel: (+86) 451 8660 9115
b School of Chemical and Environmental Engineering, Hanshan Normal University,
Chaozhou 521041, PR China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5cy01654a
Scheme 1 Reaction pathways for cinnamaldehyde hydrogenation.
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traditional catalysis and electrocatalysis.19–30 For example,
the nitrides and carbides of molybdenum (Mo2N, Mo2C) pos-
sess similar catalytic properties to Pt-group metals,31 which
should be due to the fact that the incorporation of nitrogen
from the synergistic effect between Pt and Mo2N. The method
developed here provides a new strategy for increasing the
selectivity to unsaturated alcohols with low usage of Pt.
or carbon atoms into the lattice of Mo atoms extends the 2. Experimental sections
Mo–Mo bond distance and shifts of the d-band centre to a
value below the original Fermi level. In traditional catalysis,
2.1. Chemicals
Keggin-type phosphomolybdic acid (H3PMo12O40, HPM),
most of the attention has been focused on reactions involving
hydrogen activation/transfer, and efficacies of some metal
nitrides parallel to those of precious metals have also gained
attention,25,26,32–34 although direct substitution of these
noble metals is often lacking. What's more, based on our ear-
lier works,35–39 the experimental and theoretical studies indi-
cated that carbides (WC, Mo2C) and nitrides (WN) are effec-
tive co-catalysts of precious metals (Pt, Pd) facilitating the
electro-oxidation of fuel molecules via a “synergistic action”
mechanism. However, there is little report on the application
of transition metal nitrides in selective hydrogenation so far.
Therefore, we wonder whether the synergistic effect between
Pt and Mo2N could also be effective for enhancing the selec-
tivity to COL at high conversion of CAL with less consump-
tion of Pt.
Generally, the obtained metal nitrides have wide size dis-
tributions in the range of micrometers,40–43 which is a disad-
vantage to the possibility of Pt being in contact with Mo2N.
So, the key is to explore an effective method to obtain
supported Mo2N with a small size. Fortunately, in our previ-
ous work, small-sized and well-dispersed WN NPs of 2–3 nm
on GO-derived graphene were synthesized successfully by
using heteropolyacid H4SiW12O40 clusters as a W source.39
Thus, another heteropolyacid cluster, the Keggin-type
phosphomolybdic acid (H3PMo12O40, HPM), would be a suit-
able precursor for the synthesis of small-sized Mo2N NPs in
view of its small size, close to the nanometer level, and stable
structure.44 Besides, SBA-15, as a kind of ordered mesoporous
material, has a high specific surface area and suitable pore
structure, which could facilitate the dispersion of Mo2N. SBA-
15 is always inert in selective catalysis on account of the weak
acidity resulting from the silanol groups located on the pore
walls. Thus, the inactive siliceous properties of ordered meso-
porous SBA-15 make it an ideal support for investigating the
interaction between Pt and Mo2N with less interruption by
metal–support interaction.45
H2PtCl6 and 3-ammonia propyl triethoxy silane (NH2IJCH2)3-
SiIJOC2H5)3, APTES) were purchased from Sinopharm Chemi-
cal Reagent Co., Ltd. Triblock polyIJethylene oxide)-b-
polyIJpropylene oxide)-b-polyIJethylene oxide) copolymer
Pluronic P123 (Mw = 5800, EO20PO70EO20), 5 wt% Pt/C hydro-
genation catalyst and CAL were purchased from Aldrich. Iso-
propanol, tetraethyl orthosilicate (TEOS) and ethanol were
purchased from Tianjin Kermel Chemical Reagent Co., Ltd.
Silica gel was purchased from Aladdin. All regents were used
as received without further purification.
2.2. Catalyst preparation
SBA-15 was synthesized according to the method of Stucky
and coworkers.46 Typically, 4.0 g of Pluronic P123 was
dissolved in 120 mL of 2 mol L−1 hydrochloric acid solution
and 30 mL of water at around 30 °C followed by the addition
of 8.5 g of TEOS. After being stirred at 40 °C for 24 h, the
mixture was transferred into an autoclave and aged at 100 °C
for 24 h. Then, the white precipitate was collected by filtra-
tion and dried at 80 °C. The product was obtained by remov-
ing the template at 550 °C for 5 h.
SBA-15 modified with amine groups (SBA-15-NH2) was pre-
pared as follows: 0.5 g of SBA-15 was added to 2.0 mL of
APTES diluted with 2.0 mL of ethanol. After stirring for 24 h,
the excess APTES was removed by washing with ethanol and
centrifugation. Finally, the obtained solid was dried at 60 °C
for 12 h.
HPM/SBA-15-NH2 was synthesized through
a solvent-
evaporation-induced impregnation and vacuum-assisted pro-
cess. In a typical procedure, the as-prepared 0.5 g SBA-15-NH2
was placed in a suction bottle, and then the bottle was sealed
with a dropping funnel containing 0.15 g of HPM in a 10 mL
mixed solution of ethanol and water. After the suction bottle
had been maintained under vacuum for 30 min, the HPM
solution was added dropwise, so that the HPM solution
would aid full infusion into the pore of SBA-15-NH2. Then,
the suspension was transferred into a 50 mL beaker and
stirred at room temperature (20–30 °C) until removal of the
solvent. After washing with ethanol and drying at 60 °C for 4
h, the precursor HPM/SBA-15-NH2 was heated to 600 °C
inside a tubular furnace with a heating rate of 3 °C min−1
under a N2 atmosphere. Then, when the temperature reached
600 °C, N2 was cut off and the sample was exposed to pure
NH3 (80 ml min−1) for 3 h at 600 °C. After slowly cooling to
room temperature, the black powder Mo2N/SBA-15 was
obtained.
Encouraged by the above considerations, in this study,
SBA-15 modified with Mo2N (Mo2N/SBA-15) was synthesized
by anchoring HPM clusters on aminated SBA-15, followed by
nitridation under an NH3 atmosphere. Then, the ternary Pt–
Mo2N/SBA-15 sample was obtained by the reduction of the Pt
precursor on the Mo2N/SBA-15 composite. Meanwhile, the
catalytic performance was investigated over the ternary Pt–
Mo2N/SBA-15 sample for the selective hydrogenation of CAL.
The results showed that the ternary Pt–Mo2N/SBA-15 sample
was superior to Pt/SBA-15 in selective hydrogenation of CAL
to COL. The excellent performance of the Pt–Mo2N/SBA-15
catalyst could be attributed to the enhancement of Pt disper-
sion and the increase in the amount of Pt0 species, resulting
The 3 wt% Pt–Mo2N/SBA-15 sample was formed by a
sodium borohydride (NaBH4) method according to
a
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procedure described earlier.47 Typically, 0.3 g of Mo2N/SBA-15
composite was impregnated with 2.4 g of ethanol solution of
H2PtCl6 (19.3 mmol L−1). After 6 h of stirring, the slurry was
evaporated to remove the excess solvent, followed by drying
at 120 °C overnight. Then, the catalyst precursors were
reduced in an aqueous solution of NaBH4. The 1 wt% Pt–
Mo2N/SBA-15 and 2 wt% Pt–Mo2N/SBA-15 samples were also
prepared by the same method. As references, Pt supported on
SBA-15 with different Pt contents (1 wt%, 2 wt% and 3 wt%)
and silica gel with a 3 wt% Pt content were also prepared
under similar conditions, which were denoted as 1 wt%
Pt/SBA-15, 2 wt% Pt/SBA-15, 3 wt% Pt/SBA-15 and 3 wt%
Pt/SiO2, respectively.
products was performed via calibration curves using benzyl
alcohol as an internal standard. The products were further
identified by GC-MS (Agilent 6890/5973N).
3. Results and discussion
3.1. Synthesis and characterization of Pt–Mo2N/SBA-15
The Pt–Mo2N/SBA-15 sample was prepared by the four steps
shown in Scheme 2: firstly, the positively charged APTES
reacts with the surface Si–OH of SBA-15 to form SBA-15-NH2
with a positive surface charge (step 1). The Fourier transform
infrared (FT-IR) spectra of SBA-15-NH2 (Fig. S1†) show that
the functional groups (–NH2) are grafted on the surface of
mesoporous silica SBA-15, and the content of APTES on SBA-
15-NH2 is about 16.2 wt% as clarified by the
thermogravimetric test (Fig. S2†). Secondly, the well-
dispersed Mo2N modified SBA-15 (Mo2N/SBA-15) was synthe-
sized by anchoring HPM on SBA-15-NH2 (step 2, Fig. S3†),
followed by nitridation under an NH3 atmosphere (step 3).
The Mo2N loading was about 22 wt% for the Mo2N/SBA-15
sample based on ICP-OES. It was further confirmed by X-ray
fluorescence spectrometry (XRF) analysis (Table S1†). Finally,
the Pt–Mo2N/SBA-15 catalyst was obtained by controlling the
growth of Pt NPs on Mo2N/SBA-15 using a NaBH4 method
(step 4). A series of characterizations were carried out for the
Pt–Mo2N/SBA-15 samples.
The Mo2N/SBA-15 and 3 wt% Pt–Mo2N/SBA-15 samples
were investigated by low-angle XRD (Fig. 1). SBA-15 and 3
wt% Pt/SBA-15 were also tested under the same conditions as
the references. All the samples exhibited the typical (100),
(110), and (200) diffraction peaks of SBA-15 in the region of
2θ = 0.5–2°, characteristic of a highly ordered hexagonal
mesostructure, which remained intact during post-grafting.
In contrast to SBA-15 (Fig. 1a), the diffraction peaks of the
Mo2N/SBA-15 (Fig. 1b) and 3 wt% Pt–Mo2N/SBA-15 (Fig. 1c)
samples shifted to a high-angle region and the intensity of
the (100) reflection peak was reduced, indicating that some
Mo2N and Pt NPs were scattered inside the pore channels of
SBA-15.
2.3. Catalyst characterization
The X-ray diffraction (XRD) patterns were obtained by using a
Bruker D8 Advance X-ray diffractometer with Cu Kα (λ =
1.5418 Å) radiation (40 kV, 40 mA). The scan range of the
wide-angle XRD was 10–80°. As for the small-angle XRD, the
scan range was 0.5–5°, and the scanning rate was 1° min−1.
The properties of the porous structures were derived from
N2 adsorption–desorption isotherm measurement at −196 °C
with a Micromeritics Tristar II surface area and porosity ana-
lyzer for mesoporous materials. In each case, the catalyst
(more than 100 mg) was degassed under vacuum at 150 °C
for 5 h before measurement. The pore size distribution and
surface area were determined from desorption isotherms by
the BJH and BET methods, respectively.
The morphology and structure of the samples were ana-
lyzed by a JEM-2100 transmission electron microscope (TEM)
with an acceleration voltage of 200 kV.
X-ray photoelectron spectroscopy (XPS) was performed by
using a Kratos-AXIS ULTRA DLD with an Al Kα radiation
source. Binding energies (BE) are referenced to the C (1s)
binding energy of carbon taken to be 284.7 eV.
The content of platinum and molybdenum in the Pt–
Mo2N/SBA-15 catalysts was determined by inductively coupled
plasma-optical emission spectrometry (ICP-OES), which was
performed by using a PerkinElmer Optima 7000DV analyzer.
Before measurement, each sample was dissolved in a diluted
HF and chloroazotic acid solution.
The content of molybdenum in Mo2N/SBA-15 was also
analyzed by X-ray fluorescence spectrometry (XRF) using a
Bruker S4 Explorer instrument.
The textural properties of the samples were investigated
by N2 adsorption–desorption isotherm measurement. As
shown in Fig. 2A, the isotherms of the Mo2N/SBA-15, 3 wt%
Pt–Mo2N/SBA-15 and 3 wt% Pt/SBA-15 samples exhibited
2.4. Selective hydrogenation of cinnamaldehyde
The catalyst (50 mg) was degassed at 150 °C for 2 h in pure
N2. Then, the catalyst was mixed with 1.00 g of CAL and 30
mL of isopropanol. The mixture was subsequently transferred
to a 100 mL autoclave. After flushing again three times with
N2 at 5 bar and three times with H2 at 5 bar, the reaction was
allowed to proceed at 80 °C under 10 bar of H2 for 2 h.
The products were analyzed by GC (Agilent 7820 A) with a
flame ionization detector (FID) and a HP-5 capillary column
(30 m × 0.32 mm × 0.25 mm). Quantitative analysis of the
Scheme
sample.
2
Procedure for the synthesis of the Pt–Mo2N/SBA-15
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parameters, such as the BET specific surface area and cumu-
lative pore volume, are listed in Table 1, and the pore size
distributions are displayed in Fig. 2B. The BET surface areas
of SBA-15 and Mo2N/SBA-15 were 758 and 370 m2 g−1, respec-
tively. The cumulative pore volumes for the SBA-15 and
Mo2N/SBA-15 samples were 1.05 and 0.68 cm3 g−1. The meso-
porous size distribution in Fig. 2B showed that the pore
diameters of the above two samples were 9.4 and 8.0 nm. It's
worth noting that the specific surface area, pore volume and
pore size of the Mo2N/SBA-15 sample decreased compared
with those of the SBA-15 host, because the introduced Mo2N
occupied the mesopore space leading to the decrease in pore
width. Moreover, the ordered channel of SBA-15 was not
destroyed or blocked by the Mo2N NPs, which indicates that
Mo2N may be well dispersed on the channel surface of SBA-
15. For the 3 wt% Pt–Mo2N/SBA-15 sample, the correspond-
ing physicochemical parameters were further reduced. For
the 3 wt% Pt–Mo2N/SBA-15 sample, the BET surface area,
cumulative pore volume and pore size distribution decreased
to 350 m2 g−1, 0.59 cm3 g−1 and 7.5 nm, respectively,
suggesting that Pt loaded on the Mo2N/SBA-15 composite also
occupied the surface and channel of the SBA-15 support.
The TEM and elemental micrographs were obtained to
investigate the morphology, metal distribution and particle
size of the Mo2N/SBA-15 and 3 wt% Pt–Mo2N/SBA-15 samples,
as displayed in Fig. 3. The TEM images of Mo2N/SBA-15
(Fig. 3a), 3 wt% Pt–Mo2N/SBA-15 (Fig. 3d) and 3 wt% Pt/SBA-
15 (Fig. 3h) showed a highly ordered 2-D hexagonal meso-
structure, the same as that of pure silica SBA-15 (Fig. S4†),
which were in good agreement with the results of the small-
angle XRD and N2 adsorption–desorption isotherm measure-
ments. From Fig. 3a, it can be seen that the small Mo2N NPs
were highly dispersed on the SBA-15 support with uniform
size (<d> = 3.8 0.1 nm). It was also confirmed by the corre-
sponding elemental micrograph for Si, Mo and N (Fig. 3b).
The formation of the small-sized Mo2N by our strategy
should be attributed to the following factors: the high surface
area and the restriction effect resulting from the mesoporous
structure of the SBA-15 support, the small size and stable
structure of the HPM precursor, and the anchoring function
of APTES to HPM clusters. The ethoxy groups of APTES could
react with the surface Si–OH of SBA-15 to form SBA-15-NH2.
This leaves the functional groups (–NH2) available on the sur-
face. The –NH2 groups on SBA-15-NH2 can combine with the
proton to form a cation (–NH3)+, and the HPM cluster can
easily ionize to form a discrete, negatively charged hetero-
polyanion [PO4IJMo3O9)4]3−. Thus, this anion can combine
with the –NH2 groups on SBA-15-NH2 to obtain the HPM/SBA-
15-NH2 complex by electrostatic interaction. Followed by the
nitridation in pure NH3, the small-sized Mo2N NPs on SBA-15
were gained without aggregation due to the intensive interac-
tion of HPM clusters with SBA-15-NH2. After loading of Pt, the
TEM image of 3 wt% Pt–Mo2N/SBA-15 (Fig. 3d) showed high
metal dispersion, but the particle size (<d> = 8.2 0.3 nm)
was larger than that of the Mo2N/SBA-15 composite. Although
it is difficult to distinguish Pt from Mo2N due to their similar
Fig. 1 Low-angle XRD patterns of (a) SBA-15, (b) Mo2N/SBA-15, (c) 3
wt% Pt–Mo2N/SBA-15 and (d) 3 wt% Pt/SBA-15.
Fig.
2 (A) Nitrogen adsorption–desorption isotherms and (B) the
corresponding pore size distributions calculated from the desorption
isotherms using the BJH method for (a) SBA-15, (b) Mo2N/SBA-15, (c) 3
wt% Pt–Mo2N/SBA-15 and (d) 3 wt% Pt/SBA-15 samples.
representative type IV curves with an obvious hysteresis loop
at a relative pressure (P/P0) of 0.60–0.75 according to the
IUPAC classification. It is very similar to that of pure silica
SBA-15, having a narrow pore size distribution of the cylindri-
cal channels.46 The main part of the hysteresis loops
remained in the same relative pressure range. The P/P0 posi-
tion of the inflection points is clearly related to the diameter
in the mesoporous range, and the sharpness of the step indi-
cates the uniformity of the mesoporous size distribution.
However, the desorption branch extends to a lower relative
pressure for the Mo2N/SBA-15 and 3 wt% Pt–Mo2N/SBA-15
samples, suggesting a partial loss of structural organization
and the formation of some narrower pores by the introduc-
tion of Mo2N and Pt one by one. Meanwhile, the porosity
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Table 1 Relevant physicochemical parameters of the different samples
b
c
SBETa (m2 g−1
)
Vporea (cm3 g−1
)
Pore sizea (nm)
dXRD (nm)
dTEM (nm)
Pt loadingd (wt%)
¯
¯
Sample
SBA-15
Mo2N/SBA-15
3 wt% Pt–Mo2N/SBA-15
3 wt% Pt/SBA-15
3 wt% Pt–Mo2N/SBA-15-re
758
370
350
625
390
1.05
0.68
0.59
0.96
0.62
9.4
8.0
7.5
8.5
7.5
—
—
3.8
8.2
10.8
20.7
—
—
2.95
2.89
2.80
4.2/Mo2N
5.4/Pt
8.3/Pt
6.2/Pt
a
b
Derived from N2 adsorption–desorption isotherms. Average particle sizes calculated from XRD patterns of Mo2N or Pt based on Scherrer's
c
d
equation. Average particle sizes calculated from at least 100 individual crystallites in TEM images. The loading content of Pt determined by
ICP-OES. e 3 wt% Pt–Mo2N/SBA-15 sample after four consecutive catalytic runs.
dark points in Fig. 3d, it can be seen that the Pt NPs would
also be highly dispersed on the Mo2N/SBA-15 composite with-
out agglomeration as shown in the corresponding element
maps in Fig. 3g. Furthermore, the HRTEM image of the 3 wt%
Pt–Mo2N/SBA-15 sample in Fig. 3e exhibited the small-sized
particles in close contact with each other on SBA-15. The larger
particles were typical Mo2N with a size of about 8.0 nm,
whereas the relatively small ones located around the Mo2N
grains can be recognized as Pt particles with the (111) crystal
plane. A similar nanostructure was also observed in different
regions of the 3 wt% Pt–Mo2N/SBA-15 sample (Fig. S5†),
which demonstrates that this contact of Pt with Mo2N is com-
mon in the sample. The particle size of Pt in the 3 wt% Pt–
Mo2N/SBA-15 sample was about 5–6 nm which was consistent
with the value of 5.4 nm calculated from XRD data by
Scherrer's equation (Table 1). Thus, the images reveal the
coexistence of Pt and Mo2N in the 3 wt% Pt–Mo2N/SBA-15
sample. Significantly, it shows distinctly the intimate contact
of Pt with Mo2N, which could lead to an intensive interaction
between them. Besides, in the case of 3 wt% Pt/SBA-15, it
exhibited polydispersity with average particle sizes of about
12.6 nm (Fig. 3i); even some much larger Pt aggregations (25
nm) could be seen, implying the sintering or aggregation of
Pt NPs on SBA-15. Chen and Zhu et al.48,49 reported that the
Pt particles of Pt/SBA-15 prepared by an impregnation method
were severely sintered and showed a particle size of about 18
nm, in agreement with our results. The observations demon-
strate that the dispersion of Pt NPs could be controlled effec-
tively by the addition of homogeneous Mo2N on the surface of
SBA-15. The results also reflect the intensive interaction
between Mo2N and Pt NPs owing to their similar electronic
structures,25,31 which may improve the properties of Pt in the
catalysis field. In our earlier works, this similar effect was also
observed for Pt-WC (WN) nanostructures on graphene, and
the DFT calculations revealed that Pt particles preferentially
grow on WC rather than on graphene when Pt attaches to the
pre-existing WC/graphene hybrid.39,50 Thus, according to the
above results, it's reasonable to conclude that the Pt NPs pref-
erentially grow on the highly dispersed Mo2N grains anchored
on SBA-15 in view of the similar structure and properties of
Mo2N and WC (WN).39,50 Therefore, the universal particles in
3 wt% Pt–Mo2N/SBA-15 became much larger than those of the
Mo2N/SBA-15 composite. This indicates that the Mo2N NPs
help to disperse Pt NPs and effectively prevent agglomeration
of Pt NPs.
The samples were also characterized using wide-angle
XRD. As shown in Fig. 4, all samples displayed a diffuse peak
of amorphous SiO2 around 22.5°. For Mo2N/SBA-15 (Fig. 4b),
it exhibited characteristic diffraction lines of γ-Mo2N
(JCPDS#25-1366) at 36.9 and 43.6°, although they were broad
diffraction peaks. The grain size of Mo2N NPs on the Mo2N/
SBA-15 sample was about 4.2 nm according to Scherrer's
equation, consistent with the observation from the TEM
image (<d> = 3.8 0.1 nm), which indicates that Mo2N NPs
are well dispersed on the SBA-15 support. Meanwhile, the dif-
fraction peaks of the 3 wt% Pt–Mo2N/SBA-15 sample (Fig. 4c)
located at 39.7 and 45.9° can be identified as the (111) and
(200) reflections of Pt with a face-centered cubic structure.
However, the peaks belonging to Mo2N disappeared for
Fig. 3 (a) TEM image, (b) element maps and (c) size distribution of
particles for the Mo2N/SBA-15 sample; (d) TEM image, (e) HRTEM
image, (f) size distribution of the particles and (g) element maps of the
3 wt% Pt–Mo2N/SBA-15 sample; (h) TEM image and (i) size distribution
of particles for the 3 wt% Pt/SBA-15 sample. Panels (c), (f) and (i) are
the size histogram of the particle distribution and the average size of
the particles determined by statistical analysis of the TEM images; at
least 100 individual crystallites were analyzed.
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Fig. 4 Wide-angle XRD patterns of (a) SBA-15, (b) Mo2N/SBA-15, (c) 3
wt% Pt–Mo2N/SBA-15 and (d) 3 wt% Pt/SBA-15.
Fig. 5 Pt 4f XPS spectra of (a) 3 wt% Pt–Mo2N/SBA-15 and (b) 3 wt%
Pt/SBA-15.
3 wt% Pt–Mo2N/SBA-15. The results are in line with other
reports.23,51–53 It can be explained by the intensive interaction
of Pt with Mo2N NPs and the epitaxial growth of Pt on Mo2N,
due to their similar electronic structures. In addition, by
comparing the wide-angle XRD pattern of 3 wt% Pt–Mo2N/
SBA-15 (Fig. 4c) with that of 3 wt% Pt/SBA-15 (Fig. 4d), it can
be seen that the intensity of the diffraction peak of Pt (111)
at 39.7° is much weaker than that of 3 wt% Pt/SBA-15,
although their Pt loading amount was similar (2.95 wt% and
2.89 wt%, respectively, determined by ICP-OES and shown in
Table 1). Based on the calculation using Scherrer's equation,
the Pt particle size of the 3 wt% Pt/SBA-15 sample (8.3 nm)
was larger than that of 3 wt% Pt–Mo2N/SBA-15 (5.4 nm),
which implies that the addition of Mo2N is able to improve
the dispersion of Pt on the SBA-15 support. The results are
consistent with the TEM images.
To investigate the surface electronic state of Pt species
and the interaction between Pt and Mo2N NPs, the 3 wt% Pt–
Mo2N/SBA-15 sample was further analyzed by XPS. The Pt 4f
XPS spectra of the 3 wt% Pt/SBA-15 and 3 wt% Pt–Mo2N/SBA-
15 samples are shown in Fig. 5, together with their
deconvolution obtained by fitting Gaussian peaks after Shir-
ley background subtraction. In Fig. 5a, the Pt 4f7/2 binding
energy (BE) of 3 wt% Pt–Mo2N/SBA-15 at 70.8 eV was
assigned to platinum in the metallic state (Pt0); the Pt 4f7/2
peak at 72.5 eV could be attributed to the Pt2+ species.15,54
For the 3 wt% Pt/SBA-15 sample, it can be observed that there
were three Pt 4f7/2 peaks located at 70.5, 72.0 and 74.8 eV,
which were ascribed to the Pt0, Pt2+ and Pt4+ species, respec-
tively. As far as the Mo 3d spectrum of Mo2N/SBA-15 (Fig. 6a)
is concerned, the peaks of Mo 3d were located at 229.5, 233.2
and 236.2 eV. The peak at 229.5 eV was typical of Moδ+ (2 < δ
< 4) and assigned to Mo2N species; the BE values of 233.2
and 236.2 eV were typical of Mo4+ and Mo6+, and ascribed to
Mo oxynitrides.55,56 The corresponding Mo 3d BE values for
3 wt% Pt–Mo2N/SBA-15 (Fig. 6b) were 229.2, 232.6 and
235.5 eV. This indicates that Mo2N and Mo oxynitrides coex-
ist on the surface of these samples. Interestingly, the peak of
Mo from Mo–N was located at 229.5 eV for Mo2N/SBA-15;
however, it was at 229.2 eV for 3 wt% Pt–Mo2N/SBA-15. It is
noticed that there is a negative shift relative to that of Mo2N/
SBA-15. This shift should be attributed to the electron trans-
fer between Pt and Mo2N via their intimate contact. This
transfer is also reflected by comparing the Pt XPS of 3 wt%
Pt–Mo2N/SBA-15 with that of 3 wt% Pt/SBA-15. As shown in
Table 2, the Pt 4f7/2 peaks of Pt0 and Pt2+ species for 3 wt%
Pt/SBA-15 were located at 70.5 and 72.0 eV, respectively. How-
ever, the Pt 4f7/2 BE values of the metallic Pt0 and Pt2+ species
for 3 wt% Pt–Mo2N/SBA-15 were 70.8 and 72.5 eV, respec-
tively, having positive shifts of about 0.3 and 0.5 eV to higher
energy than that for 3 wt% Pt/SBA-15. Similar results were
also found in our previous work.36,38,39 This shift also indi-
cates the acquisition of the electron of Mo2N after Pt growth.
The above results confirm the intensive interaction between
Pt and Mo2N combined with the TEM and XRD results for
3 wt% Pt–Mo2N/SBA-15.
On the other hand, in terms of the quantitation of the sur-
face Pt species, the fractions of the Pt0 and Pt2+ species in
3 wt% Pt–Mo2N/SBA-15 were estimated to be 71% and 29%,
respectively (Table 2). Attractively, the amounts of the Pt0,
Fig. 6 Mo 3d XPS spectra of (a) Mo2N/SBA-15 and (b) 3 wt% Pt–Mo2N/
SBA-15.
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Table 2 XPS data of the Pt 4f levels for the different samples
intensive interaction between Pt and Mo2N. It results in the
improvement of the dispersion of Pt. Moreover, the XPS
results illustrate the electron transfer from Pt to Mo2N and
the increase in the amount of Pt0 species on the surface of
Pt species BE
(eV)
Content
of Pt
species
(%)
Pt
Sample
species
4f7/2
4f5/2
3
wt% Pt–Mo2N/SBA-15, which further confirms the
3 wt% Pt–Mo2N/SBA-15
Pt0
70.8
72.5
70.5
72.0
74.8
71.0
72.5
75.2
74.1
76.0
73.8
75.5
78.1
74.4
76.0
78.5
71
29
53
32
15
62
25
13
interaction.
Pt2+
Pt0
3 wt% Pt/SBA-15
Pt2+
Pt4+
Pt0
3.2. Catalytic performance
3 wt% Pt–Mo2N/SBA-15-r
The catalytic performance was evaluated in the liquid-phase
selective hydrogenation of CAL over the 3 wt% Pt–Mo2N/SBA-
15 catalyst. The 3 wt% Pt/SBA-15 and Mo2N/SBA-15 catalysts
were also tested as references. Table 3 displays the main
results. It can be seen that the Mo2N/SBA-15 catalyst showed
negligible activity (5.8% conversion of CAL) and selectivity to
COL (9.4%), and CAL was mainly converted to side products
(90.6%), which indicates that Mo2N possesses much lower
catalytic activity for selective hydrogenation of CAL. The
3 wt% Pt/SBA-15 catalyst exhibited 47.7% conversion of CAL
and 29.6% selectivity to COL, while the selectivities to HALD,
HALC and other side products were 21.2%, 18.1% and
31.1%, respectively. In the case of the 3 wt% Pt–Mo2N/SBA-15
catalyst, the conversion of CAL attained was 85.8% and the
selectivity to COL was 78.0%; the selectivity to HCAL, HCOL
and other side products were 7.9%, 10.7% and 3.4%, respec-
tively. Therefore, it's clear that Mo2N could markedly enhance
the activity of Pt NPs on 3 wt% Pt–Mo2N/SBA-15 for the con-
version of CAL. Compared with other catalysts including the
3 wt% Pt/SiO2 and 5 wt% Pt/C commercial catalysts (Table 3),
the 3 wt% Pt–Mo2N/SBA-15 catalyst exhibits the best activity.
This can be attributed to the improvement of the dispersion
of Pt NPs and the intensive interaction between Pt and Mo2N
revealed by TEM and XRD characterizations. As we know,
small nanoparticle size and high dispersion of metals are
beneficial to high activity in the selective hydrogenation.57,58
The smaller the nanoparticles, the more the active sites for Pt
to absorb the reactants.15 What's more, the main product
over the 3 wt% Pt–Mo2N/SBA-15 catalyst was COL via CO
bond hydrogenation with more than 78.0% selectivity, which
Pt2+
Pt4+
Pt2+ and Pt4+ species in 3 wt% Pt/SBA-15 without Mo2N NPs
were about 53%, 32% and 15%. It's notable that the amount
of Pt0 species in 3 wt% Pt–Mo2N/SBA-15 is more than that in
3 wt% Pt/SBA-15, and almost no Pt4+ species is observed in
the 3 wt% Pt–Mo2N/SBA-15 sample. This is due to the fact
that the presence of nitrides (carbides) could largely improve
the crystallinity of Pt NPs because of the intensive interaction
of nitrides (carbides) with Pt NPs.39,50
Briefly, the highly ordered mesoporous structure is kept
for SBA-15 with the loading of Mo2N and Pt confirmed by the
low-angle XRD, N2 adsorption–desorption isotherms and
TEM results. Furthermore, the TEM images and elemental
micrograph show that the Mo2N and Pt NPs are well dis-
persed on SBA-15, but after loading of Pt, the particles grow
much larger for the 3 wt% Pt–Mo2N/SBA-15 sample. And the
HRTEM images show that there is an intimate contact
between Pt and Mo2N grains, which suggests that the Pt NPs
prefer to grow on the highly dispersed Mo2N grains. In addi-
tion, the dispersion of Pt is better on the 3 wt% Pt–Mo2N/
SBA-15 sample than that on the 3 wt% Pt/SBA-15 sample. It's
also clarified by the wide-angle XRD results. For the 3 wt%
Pt–Mo2N/SBA-15 sample, the diffraction peaks of γ-Mo2N dis-
appear with the introduction of Pt to Mo2N/SBA-15, and the
intensity of the Pt peak becomes much weaker than that of
3 wt% Pt/SBA-15, which also demonstrates that there is an
Table 3 Hydrogenation of cinnamaldehyde over different catalystsa
TOFb
Selectivity (%)
Conversion
(%)
Sample
(h−1
)
COL
HALD
HALC
Othersc
Mo2N/SBA-15
5.8
47.7
14.1
5.5
85.8
70.8
16.3
36.8
90.0
—
9.4
29.6
35.2
46.0
78.0
76.9
80.2
27.2
40.6
0
0
18.1
9.8
20.9
10.7
8.4
8.0
12.7
38.4
90.6
31.1
20.6
2.4
3.4
8.0
0
39.6
9.8
3 wt% Pt/SBA-15
2 wt% Pt/SBA-15
1 wt% Pt/SBA-15
3 wt% Pt–Mo2N/SBA-15
2 wt% Pt–Mo2N/SBA-15
1 wt% Pt–Mo2N/SBA-15
3 wt% Pt/SiO2
238
102
80
423
521
239
224
328
21.2
33.8
30.7
7.9
6.7
11.8
20.5
11.2
5 wt% Pt/C
a
b
Reaction conditions: 50 mg of the catalyst, 1.00 g of CAL, 30 mL of isopropanol, 10 bar H2, 80 °C, 2 h. Turnover frequency (TOF) = [moles
c
of cinnamaldehyde reacted]/[(moles of Pt loading) × (reaction time)]. Includes 1-(3-propoxyprop-1-enyl)benzene, cinnamyl formate, cinnamic
acid, benzyl, cinnamate, 4,4-diphenylcyclohexa-1,5-dienyl acetate, and other condensation products that could be identified by GC-MS because
of their large molecular weights.
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was much higher than that of the 3 wt% Pt/SBA-15, 3 wt% Pt/
SiO2 and commercial 5 wt% Pt/C catalysts. Consequently, it
is evident that the addition of Mo2N also could promote the
selectivity for the hydrogenation of CAL to COL. The
enhanced performance of the 3 wt% Pt–Mo2N/SBA-15 catalyst
is even superior to the reported Pt supported on new carbo-
naceous materials under similar reaction conditions.14–16,47 It
is found that the cinnamaldehyde adsorption through its
CO bond becomes more favorable with the increase of the
metallic character of the Pt clusters, and the large fraction of
Pt4+ atoms is responsible for the high-molecular-weight sec-
ondary products obtained by experiments and DFT calcula-
tions.47 In this work, the interaction between Pt and Mo2N
increases the fraction of Pt0 species and lessens the fraction
of Pt4+ species on the surface of 3 wt% Pt–Mo2N/SBA-15.
Therefore, it suggests that the high selectivity to the COL
product over the 3 wt% Pt–Mo2N/SBA-15 catalyst is attributed
to the higher amounts of Pt0 species on its surface. In other
words, Mo2N could be used as a highly effective co-catalyst of
Pt NPs for the liquid-phase hydrogenation of CAL to COL.
The catalytic performances were also investigated over the
Pt–Mo2N/SBA-15 catalysts with different Pt contents (Table 3).
As shown in Table 3, the TOF was 239 h−1 for 1 wt% Pt–
Mo2N/SBA-15 with 16.3% conversion of CAL. However, when
the loading of Pt increased to 2 times (2 wt%), the conversion
of CAL for 2 wt% Pt–Mo2N/SBA-15 increased to 70.8%, 4.3 times
that for 1 wt% Pt–Mo2N/SBA-15. Thus, the TOF for 2 wt%
Pt–Mo2N/SBA-15 (521 h−1) is higher than that for 1 wt%
Pt–Mo2N/SBA-15 (239 h−1). Furthermore, for 3 wt% Pt–Mo2N/
SBA-15, the conversion of CAL slightly increased to 85.8%.
Then, the TOF decreased to 423 h−1 for 3 wt% Pt–Mo2N/SBA-15.
This demonstrates that the Pt NPs on the 2 wt% Pt–Mo2N/
SBA-15 catalyst were the most active in the conversion of CAL
due to the synergistic effect of Pt and Mo2N NPs, which leads
to the sharp increase in the CAL conversion and TOF.
Moreover, comparing the binary Pt/SBA-15 with the ternary
Pt–Mo2N/SBA-15 with the same Pt content, the ternary ones
displayed more outstanding catalytic performance in the con-
version of CAL and the selectivity to COL than the binary
ones. When the Pt content decreased from 3 wt% to 1 wt%
for the Pt/SBA-15 catalyst, the TOF decreased from 238 to
80 h−1. However, the TOF over 1 wt% Pt–Mo2N/SBA-15 could
attain 239 h−1, which was close to that over the 3 wt% Pt/SBA-15
catalyst. Simultaneously, the selectivity to COL over 1 wt% Pt–
Mo2N/SBA-15 was 80.2%, 2.5 times that over 3 wt% Pt/SBA-15.
Herein, it is further proved that the intensive interaction
between Pt and Mo2N is beneficial to the promotion of the
catalytic performance of Pt, and Mo2N as a co-catalyst could
lessen the usage of Pt for the selective hydrogenation of CAL
to some extent. However, this effect of Mo2N on Pt is closely
related to the particle size of Mo2N. As references, large
Mo2N NPs supported on SBA-15 were also prepared by
impregnating SBA-15 without APTES modification as the sup-
port (denoted as Mo2N/SBA-15-1), and after loading 3 wt% Pt
on Mo2N/SBA-15-1, the catalyst was denoted as Pt–Mo2N/SBA-
15-1. The TEM images of Mo2N/SBA-15-1 (Fig. S6†) showed
that the Mo2N NPs were about 20 nm in size with worse uni-
formity than those in Mo2N/SBA-15 prepared by using SBA-
15-NH2 as the support (Fig. 3a). The XRD patterns of Mo2N/
SBA-15-1 (Fig. S7†) showed that the γ-Mo2N phase was
formed, and its size was about 7.5 nm according to Scherrer's
equation. Besides, the peaks of the MoO2 were observed,
indicating the important role of APTES in obtaining the
phase-pure and small-sized Mo2N NPs. The conversion of
CAL over 3 wt% Pt–Mo2N/SBA-15-1 was 53.5%, and the selec-
tivity to COL was 47.2% (Fig. S8†). This indicates that it's
essential to support small-sized Mo2N NPs on SBA-15 as the
promoter, because there is more exposed surface of Mo2N
NPs for Pt NPs to be in contact with. Thus, the intensive
interaction between them could be fully exploited.
The stability of 3 wt% Pt–Mo2N/SBA-15 was also examined
based on its excellent catalytic activity for the selective hydro-
genation of CAL. The catalyst was reused directly without any
treatment after precipitation and separation from the reac-
tion solution. It is noteworthy that the 3 wt% Pt–Mo2N/SBA-
15 catalyst presented relative stabilization under the reaction
conditions; a slight decrease in the conversion and selectivity
appeared after the catalyst was reused three times, and the
conversion of CAL and the selectivity to COL still reached
about 80% and 70%, respectively (Fig. 7). However, after four
catalytic cycles, the conversion and selectivity decreased to
60% and 50%, respectively. Obviously, the reusability of the
3 wt% Pt–Mo2N/SBA-15 catalyst is superior to that of the
3 wt% Pt/SiO2 catalyst (Fig. S9†), but it's slightly inferior to
the 5 wt% Pt/C commercial catalyst (Fig. S10†) that displayed
low selectivity to COL. In order to investigate the reason for
the decrease in activity, the 3 wt% Pt–Mo2N/SBA-15 catalyst
after four cycles was characterized by ICP-OES, XRD, XPS and
TEM, and the results are shown in Table 1 and Fig. 8. The
ICP-OES analysis showed that the Pt content was about
2.80 wt% for the 3 wt% Pt–Mo2N/SBA-15 catalyst after four
cycles, which indicates that there is slight leaching of Pt in
the process compared to the fresh 3 wt% Pt–Mo2N/SBA-15
catalyst (2.95 wt%). Thus, the slight leaching of Pt should not
Fig. 7 Stability test for 3 wt% Pt–Mo2N/SBA-15 shown as performance
versus number of recycling runs. Reaction conditions: 50 mg of 3 wt%
Pt–Mo2N/SBA-15, 1.00 g of CAL, 30 mL of isopropanol, 10 bar of H2,
80 °C, 2 h.
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which resulted in the difference of Pt species on the surface
from the Pt/SBA-15 catalyst and the electron transfer between
Pt and Mo2N. The Pt–Mo2N/SBA-15 catalyst exhibited a
remarkably enhanced catalytic performance for the liquid-
phase hydrogenation of cinnamaldehyde. The selectivity to
COL over the 1 wt% Pt–Mo2N/SBA-15 catalyst is 2.5 times that
over the 3 wt% Pt/SBA-15 catalyst at the same TOF. It illus-
trates that Mo2N could be a new kind of co-catalyst of Pt pro-
moting its catalytic efficiency in selective hydrogenation. This
promotion can be ascribed to the synergistic effect between
Pt and Mo2N: the first one, the presence of Mo2N NPs favours
the dispersion of the Pt NPs; the second one, the Mo2N NPs
increase the amount of Pt0 species on the surface of the Pt–
Mo2N/SBA-15 catalyst. The present study opens up a new way
for preparing robust Pt-based catalysts for a wide range of
interesting and industrial reactions.
Acknowledgements
Fig. 8 (a) Wide-angle XRD patterns, (b) Pt 4f XPS spectra, (c) TEM
image and (d) particle size distribution of the 3 wt% Pt–Mo2N/SBA-15
catalyst after four catalytic cycles.
We gratefully acknowledge the support to this research by the
National Natural Science Foundation of China (21371053),
Application Technology Research and Development Projects
in Harbin (2013AE4BW051), Natural Sciences Fund of Hei-
longjiang Province (B2015009), Postdoctoral Science-research
Developmental Foundation of Heilongjiang Province of China
(LBH-Q12022), Innovative Research Team in Heilongjiang
University (Hdtd2010-10) and Program for Innovative
Research Team in University (IRT-1237), and China Postdoc-
toral Science Special Foundation (No. 2015 T80374). We are
also thankful for the project sponsored by the Scientific
Research Foundation for the Returned Overseas Chinese
Scholars, State Education Ministry (2013-1792).
be the main reason for the decrease in the activity. According
to the XRD result of 3 wt% Pt–Mo2N/SBA-15 after recycling
runs (Fig. 8a), the intensity of Pt (111) and (200) diffraction
peaks became stronger and sharper, as well as the appear-
ance of Pt (220) diffraction peak at 67.8°. Moreover, the TEM
image of the 3 wt% Pt–Mo2N/SBA-15 catalyst after four cycles
showed that there was a remarkable particle aggregation on
the catalyst surface (Fig. 8c), and the average particle size
increased from 8.2 nm for the fresh 3 wt% Pt–Mo2N/SBA-15
catalyst to 20.7 nm after the fourth catalytic cycle. In addi-
tion, as shown in Fig. 8b and Table 2, the other significant
changes were the presence of Pt species on the surface of the
used catalyst, the fraction of Pt0 species decreased from 71%
to 62%, and the Pt4+ species appeared (15%) that contributed
to the increase in the high-molecular-weight secondary prod-
ucts. Thus, it's clear that the reduced amount of Pt0 species
means the reduction of the possibility for CAL adsorption
through its CO bond, which isn't favourable to the genera-
tion of COL, and the increase in the amount of Pt4+ species is
beneficial to the increase of the secondary products. As a con-
sequence, the main reason for the decrease in activity for the
3 wt% Pt–Mo2N/SBA-15 catalyst can be attributed to the
aggregation of particles and the reduced amount of Pt0 spe-
cies on the surface. It also confirmed that the Pt0 species play
a key role in the selectivity to COL in the selective hydrogena-
tion of cinnamaldehyde.
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