Selective oxidation of glycerol with oxygen in base-free solution over N-doped-carbon-supported Sb?PtSb2 hybrid

Article abstract of DOI:10.1016/S1872-2067(19)63476-5

Selective oxidation of glycerol with molecular oxygen in base-free aqueous solutions has become a hot topic, as the rapidly increasing production of biodiesel is creating a surplus of glycerol. In this work, an N-doped-carbon-supported core-shell structured Sb?PtSb₂ hybrid catalyst was prepared via a facile synthesis route, in which a mixture of glucose, melamine, and SbCl₃ (Sb-NC) was pyrolyzed, then impregnated with Pt by immersion in an aqueous solution of H₂PtCl₆, and further treated in hydrogen flow. Characterization of the catalyst products indicated that introducing SbCl₃ can increase the surface area of the binary glucose + melamine pyrolyzed support (NC), and Sb?PtSb₂ hybrids could be formed on the surface of an Sb-NC support during hydrogen treatment at 700 °C. It was found that the Sb?PtSb₂/NC catalyst was more active for the selective oxidation of glycerol in a base-free aqueous solution than Sb-free NC-supported Pt (Pt/NC). Further characterization also indicated that the promising performance of Sb?PtSb₂/NC might be attributed to its enhanced oxygen activation.

Full text of DOI:10.1016/S1872-2067(19)63476-5

Chinese Journal of Catalysis 41 (2020) 494–502
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Selective oxidation of glycerol with oxygen in base-free solution
over N-doped-carbon-supported Sb@PtSb hybrid
2
Lihua Yang , Tianqu He , Chujun Lai , Ping Chen , Zhaoyin Hou a,b,*
a
a
a
a
a
Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Department of Chemistry, Zhejiang University, Hangzhou 310028, Zhejiang,
China
b
Center of Chemistry for Frontier Technologies, Department of Chemistry, Zhejiang University, Hangzhou 310028, Zhejiang, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Selective oxidation of glycerol with molecular oxygen in base-free aqueous solutions has become a
hot topic, as the rapidly increasing production of biodiesel is creating a surplus of glycerol. In this
Received 4 July 2019
Accepted 6 August 2019
Published 5 March 2020
work, an N-doped-carbon-supported core-shell structured Sb@PtSb
via a facile synthesis route, in which a mixture of glucose, melamine, and SbCl
lyzed, then impregnated with Pt by immersion in an aqueous solution of H PtCl
in hydrogen flow. Characterization of the catalyst products indicated that introducing SbCl
increase the surface area of the binary glucose + melamine pyrolyzed support (NC), and Sb@PtSb
hybrids could be formed on the surface of an Sb-NC support during hydrogen treatment at 700 °C. It
was found that the Sb@PtSb /NC catalyst was more active for the selective oxidation of glycerol in a
base-free aqueous solution than Sb-free NC-supported Pt (Pt/NC). Further characterization also
2
hybrid catalyst was prepared
(Sb-NC) was pyro-
, and further treated
can
3
2
6
3
Keywords:
Glycerol
Oxidation
Base-free solution
2
2
Sb@PtSb
2
2
indicated that the promising performance of Sb@PtSb /NC might be attributed to its enhanced
Alloy
oxygen activation.
©
2020, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1
.
Introduction
[3–7]. Among these processes, selective oxidation of glycerol
with molecular oxygen over heterogenous catalysts to form
dihydroxyacetone (DHA), glyceric acid (GLYA), glyceraldehyde
(GLYHD), hydroxypyruvic acid (HPYA), and lactic acid has be-
come a hot topic both in academic research and industrial ap-
plications [8–13]. Based on previously published works, it was
generally concluded that the oxidation of glycerol with molec-
ular oxygen is a structure-sensitive process, in which the reac-
tion mechanism and product distribution vary significantly
with different conditions (pH, temperature, and substrate to
metal ratio) [11–22].
Biodiesel is a renewable, biodegradable fuel manufactured
domestically from vegetable oils, animal fats, or recycled res-
taurant grease. It can be used in pure form or it may be blended
with petroleum diesel at any concentration in most injection
pump diesel engines. Glycerol is formed as a byproduct during
the production of biodiesel, and the amount of glycerol pro-
duced has increased quickly with the rapid expansion of bio-
diesel applications [1,2]. It was estimated that the production of
glycerol in 2020 will be six times higher than demand [2].
In the past decade, much effort has been devoted to devel-
oping processes to convert glycerol into valuable products
The use of carbon-supported Pt catalysts has been widely
reported in the base-free aqueous liquid-phase oxidation of
*
Corresponding author. E-mai: zyhou@zju.edu.cn
This work was financially supported by the National Natural Science Foundation of China (21773206 and 21473155) and Natural Science Foundation
of Zhejiang Province (L12B03001).
DOI: S1872-2067(19)63476-5 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 41, No. 3, March 2020

Lihua Yang et al. / Chinese Journal of Catalysis 41 (2020) 494–502
495
glycerol [23,24]. But the deactivation of nano-sized Pt catalysts
has often been reported during the selective oxidation of alco-
hols due to the weaker interactions between Pt and the carbon
support, over-oxidation of surface Pt nanoparticles (NPs),
and/or the strong adsorption of organic carboxylic acids
rate of 5 °C/min) under a nitrogen flow of 20 mL/min. The
obtained black powder was treated in 50 mL HCl (37 wt.%)
under stirring for 6.0 h to remove the soluble species. The ac-
id-treated solid sample was then filtered out, washed thor-
oughly with distilled water, and dried in vacuum at 60 °C over-
night. The prepared sample is denoted as Sb/NC. Secondly,
Sb/NC (1 g) was dispersed in a 7-mL aqueous solution of
[23–25].
Recently, it was recommended that adding and/or alloying
Pt with another metal is an effective approach to preventing the
deactivation of Pt NPs. For example, Au- [15,26–28], Bi-
H
2
PtCl (0.01 g Pt/mL) under stirring, and further treated in an
6
ultrasonic bath for 30 minutes. This suspension was then dried
at 60 °C, and the dried black powder was treated at 700 °C un-
der hydrogen flow for 1 h. The final catalyst is denoted as
Sb@PtSb /NC. The N-free Sb@Pt/C catalyst was prepared by
following the same procedures as above without the addition of
melamine in the feed.
[8,9,29], Ru- [30], Sn- [31,32], Ni- [33], Co- [34] and Sb- [35,36]
promoted or alloyed Pt catalysts have been used for selective
oxidation in the liquid phase. At the same time, it was also re-
ported that N atoms in the framework of the carbon support
can donate electrons to Pt, thereby enhancing the interactions
between the support and the metal, and thus improving the
dispersion, activity, and stability of Pt, and/or generating new
active sites [24,37–39].
Previous work in our laboratory showed that a PtSb alloy
supported on multi-walled carbon nanotubes (MWCNTs) was
highly active for the selective oxidation of glycerol in base-free
aqueous solutions and that a PtSb alloy can hinder the cleavage
of C–C bonds [35]. Recent works published by other groups
have also confirmed that a Pt-Sb alloy was both more active
and selective for the formation of DHA [36,40]. But the fabrica-
2
2.1.2. Pt/NC
Specific amounts of glucose and melamine (with a mass ra-
tio of 1:1.5, in order to ensure the same N content in both
Pt/NC and Sb@PtSb /NC) were added and mixed manually in a
ceramic mortar for 1 h, and then the mixture was dried in vac-
uum at 80 °C overnight. After that, the mixture was transferred
to a quartz furnace and heated at 700 °C for 4 h (with a heating
rate of 5 °C/min) under a nitrogen flow of 20 mL/min. The
prepared sample is denoted as NC. Secondly, NC (1 g) was dis-
2
tion of PtSb alloy NPs on MWCNTs is
a
tedious,
persed in a 7-mL aqueous solution of H
2
PtCl (0.01 g Pt/mL)
6
time-consuming, and dangerous process [41], and commercial
MWCNT supports are expensive.
under stirring. The suspension was then treated in an ultra-
sonic wave bath for 30 minutes. After that, the suspension was
dried at 60 °C, and the dried black powder was further treated
at 700 °C under hydrogen flow for 1 h. The final catalyst is de-
noted as Pt/NC. N-free Pt/C catalyst was prepared by following
the same procedures used to produce Pt/NC without the addi-
tion of melamine in the feed.
In this work, we report a facile preparation method for the
direct fabrication of Sb@PtSb
ture on N-doped carbon via simple pyrolysis of a mixture of
glucose, melamine, and SbCl , according to the reaction scheme
shown in Fig. 1. Furthermore, this Sb@PtSb /NC catalyst was
2
hybrids with a core-shell struc-
3
2
characterized and tested for the selective oxidation of glycerol
in base-free aqueous solutions.
2.2. Catalyst characterization
2
.
Experimental
The real content of Sb and Pt in the prepared catalysts were
measured using the following procedures. A sample (0.5 g) was
treated in air from 25 to 900 °C (with a heating rate of 5
2
2
.1. Catalyst preparation
°C/min), and then the residual solids were dissolved with 50
.1.1. Sb@PtSb
2
/NC
mL of aqua regia. Metal ions in the resulting solution were de-
tected via inductively coupled plasma-atomic emission spec-
troscopy (ICP, Plasma-Spec-II spectrometer), and these data
are summarized in Table 1.
Specific amounts of glucose, melamine, and SbCl
3
(with a
mass ration of 6:6:1) were added and mixed manually in a ce-
ramic mortar for 1 h, and then the mixture was dried in vacu-
um at 80 °C overnight. After that, the mixture was transferred
to a quartz furnace and heated at 700 °C for 4 h (with a heating
The texture and structure of all samples were characterized
by N
adsorption measurements at –196 °C in a static volumet-
2
ric apparatus (Micromeritics ASAP2020). Samples were first
treated at 250 °C for 4 h under high vacuum. The specific sur-
face area was calculated using the Brunner-Emmet-Teller
(
BET) method and pore size distributions were calculated us-
ing the Barrett-Joyner-Halenda (BJH) method. X-ray diffraction
XRD) was performed on RIGAKU D/MAX 2550/PC diffractom-
eter using Cu K radiation at 40 kV and 100 mA. Diffraction data
(
α
was recorded using continuous scanning at 0.02°/s and a step
size of 0.02°. Scanning electron microscope (SEM) images were
obtained with a Zeiss Sigma field-emission SEM (Model 8100).
Transmission electron microscopy (TEM, JEOL-2010 F) was
used to observe the morphologies and dimensions of catalysts
Fig. 1. Schematic illustration of the synthesis of catalysts.

496
Lihua Yang et al. / Chinese Journal of Catalysis 41 (2020) 494–502
Table 1
The textural structure and composition of catalysts.
a
b
Surface area
Pore volume
(cm /g)
ICP loading (wt.%)
XPS composition (at.%)
Catalyst
2
3
(m /g)
Pt
Sb
0
N
0
1.76
0
O
C
Pt/C
Pt/NC
Sb@Pt/C
115
144
264
275
0.065
0.078
0.159
0.200
4.41
5.64
5.79
6.03
20.87
21.93
22.00
19.15
78.82
75.92
74.96
75.46
0
9.93
11.03
Sb@PtSb
2
/NC
1.77
a
b
Detected via ICP analysis. Detected via XPS.
at an accelerating voltage of 200 kV. X-ray photoelectron spec-
tra (XPS) were recorded on a Perkin-Elmer PHI ESCA System,
The X-ray source was a Mg standard anode 146 (1253.6 eV)
operated at 12 kV and 300 W.
mm, Agilent). The analysis procedures are described in detail in
our previous work [42]. The selectivity of each product was
calculated as: (mmol of product in reaction mixture) × (the
number of carbon atoms in product) / ((initial mmol of glycerol
− mmol of glycerol left) × 3) × 100%.
Oxygen adsorption and activation were measured over the-
se catalysts via temperature programmed desorption of O
-TPD). The catalyst precursor was first pretreated at 700 °C
in a purified H flow of 30 mL/min for 1 h, and then the reactor
was cooled to 50 °C in a H atmosphere. Then, the synthesized
catalyst was purged in a purified Ar flow of 30 mL/min at 50 °C
for 1 h. After that, the catalyst was exposed to 10% O /Ar for 30
min, and then purged with Ar for 60 min at 50 °C in order to
eliminate any physically adsorbed O . The desorption of ad-
sorbed oxygen was conducted by ramping the catalyst temper-
ature to 600 °C at a rate of 10 °C/min, and O (m/e = 32) was
2
(O
2
3. Results and discussion
2
2
3.1. Characterization
2
Fig. 2 shows the N
tributions of Pt/C, Pt/NC, Sb@Pt/C, and Sb@PtSb
2
adsorption isotherms and pore size dis-
/NC. All cat-
2
2
alysts were found to exhibit type I pattern isotherms, according
to the International Union of Pure and Applied Chemistry
(IUPAC) classification, which suggested that they were typical
microporous materials. On the basis of adsorption data, the
2
detected in the effluent and recorded as a function of tempera-
ture using a quadrupole mass spectrometer (Omni Star TM,
GSD301, Switzerland).
calculated BET surface areas of Pt/C, Pt/NC, Sb@Pt/C, and
2
2
Sb@PtSb /NC were 115, 144, 264, and 275 m /g, respectively.
These results indicate that introducing N and/or Sb into the
precursor (by adding melamine and/or SbCl ) can increase the
2.3. Glycerol oxidation
3
surface area and pore volume of the prepared catalyst. In addi-
tion, the effect of introducing Sb to the carbon support can be
observed clearly in their typical SEM images. The outlines of
Pt/C and Pt/NC look like closely packed solid laminates (see
Glycerol oxidation was carried out in a 25-mL cus-
tom-designed stainless-steel autoclave with a glass inner layer,
which contained 5 mL of a 0.2 g/mL glycerol solution and 0.100
g of catalyst. The reactor was sealed, filled with 0.6 MPa of O
2
,
2
Fig. 3a–3d), while Sb@Pt/C and Sb@PtSb /NC exhibited po-
placed in water bath preheated to the required temperature,
and maintained at that temperature for a given time under
vigorous stirring with a magnetic stirrer. After the reaction, the
catalyst was removed by centrifugation and the aqueous solu-
tion was analyzed using a high-performance liquid chromato-
graph (HPLC, Agilent 1100 series) equipped with a refractive
index detector (RID) and a Zorbax SAX column (4.6 mm × 250
rous structures (see Fig. 3e–3h).
Fig. 4 compares the XRD patterns of Pt/C, Pt/NC, Sb@Pt/C
and Sb@PtSb /NC. It can be seen that the main diffraction
2
peaks of Pt were detected in Pt/C and Pt/NC, while the calcu-
lated Pt crystallite sizes based on the full-width at
half-maximum of the Pt(111) peak were 36.2 and 32.5 nm,
respectively. These results indicate that introducing N into the
A
d
c
300
200
100
0
B
0.004
0.002
0.000
d
c
b
a
b
a
1
10
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
Pore diameter (nm)
Fig. 2. (A) N
2
adsorption-desorption isotherms and (B) pore size distributions of (a) Pt/C, (b) Pt/NC, (c) Sb@Pt/C, and (d) Sb@PtSb /NC.
2

Lihua Yang et al. / Chinese Journal of Catalysis 41 (2020) 494–502
497
Sb
Pt
PtSb2
3
11
2
10
2
200
11
Pt Sb
5
2
20
10
003
3
21
4
20
511
1
d
c
012
1
41
1
50 202
143122
1
11
2
00
220
b
a
1
0
20
30
40
50
 ()
Fig. 4. XRD patterns of (a) Pt/C, (b) Pt/NC, (c) Sb@Pt/C, and (d)
60
70
80
2
Sb@PtSb /NC.
2
support [42]. At the same time, it is interesting to note that a
PtSb alloy (JCPDS 14-0141) was detected in the Sb@PtSb /NC
2
2
catalyst along with metallic Sb (JCPDS 35-0732), which means
that Pt can enter into the lattice of Sb. Peaks around 23.8° and
28.6° confirmed the existence of a separate Sb phase. On the
other hand, separate Sb and Pt phases and trace amounts of
5
Pt Sb alloy were detected in the XRD pattern of Sb@Pt/C.
Typical HRTEM images indicated that a layer of PtSb
2
alloy
formed on the surfaces of well-crystallized Sb particles in the
Sb@PtSb /NC catalyst (see Fig. 5), as the observed d-spacing
values of 0.325 nm and 0.350 nm correspond to the (200)
plane of PtSb alloy and the (101) plane of Sb (see Fig. 5b), re-
spectively. Line-scan EDS results further confirmed that Pt was
2
Fig. 3. SEM images of (a,b) Pt/C, (c,d) Pt/NC, (e,f) Sb@Pt/C, and (g,h)
Sb@PtSb /NC.
2
2
mainly located in the shell of the Sb@PtSb
2
hybrid (see Fig.
catalyst can improve the dispersion of Pt, which might be at-
tributed to the fact that N can increase the surface area of the
carbon support and induce defects in the carbon support,
strengthening the interactions between Pt and the carbon
5c–5f).
Raman analysis further confirmed that introducing SbCl
into the mixture of glucose and melamine can induce defects in
the final catalyst (see Fig. 6), as the calculated intensity ratio of
3
c
e
5
0
60
Sb La1
Sb La1-Fitted
Pt Ma1
Sb La1
Sb La1-Fitted
Pt Ma1
Pt Ma1-Fitted f 50
d 40
Pt Ma1-Fitted
40
30
20
10
0
30
20
10
0
0
10 20 30 40 50 60 70
Position (nm)
0
10 20 30 40 50 60 70 80 90
Position (nm)
Fig. 5. TEM images and EDS spectra of Sb@PtSb /NC.
2

498
Lihua Yang et al. / Chinese Journal of Catalysis 41 (2020) 494–502
Sb
PtSb2
I /I =0.953
d
D
G
Pt
O
Sb
PtSb
2
I /I =0.915
c
D
G
I /I =0.934
Pt
D
G
b
a
b
a
I /I =0.863
D
G
500
1000
1500
2000
2500
-
1
Raman shift (cm )
100
200
300
Temperature ( C)
-TPD profiles of (a) Pt/NC and (b) Sb@PtSb
400
500
600
o
Fig. 6. Raman spectra of (a) Pt/C, (b) Pt/NC, (c) Sb@Pt/C, and (d)
Sb@PtSb /NC.
Fig. 7. O
2
2
/NC catalysts.
2
the D band to G band (I
D
/I
G
) increased from 0.863 (in Pt/C) to
to Pt, as the detected binding energy of the Pt 4f peaks shifted
from 71.4 and 74.7 eV (in Pt/C) to 71.0 and 74.3 eV (in Pt/NC),
and from 71.3 and 74.6 eV (in Sb@Pt/C) to 70.8 and 74.1 eV (in
0.915 (in Sb@Pt/C), 0.934 (in Pt/NC), and 0.953 (in
Sb@PtSb /NC). In addition, the introduction of N can also in-
2
crease the amount of carbon defects, as the ratio of the D band
to the G band (I /I ) of Pt/NC increased compared to that of
Pt/C. And these defects can bring additional active sites, which
encourages the oxidation reaction [29,43].
Sb@PtSb
confirmed the formation of a PtSb
the detected binding energy of Pt 4f in Sb@PtSb
within the range of values reported for PtSb in the literature
2
/NC) (see Fig. 8A and 8B). At the same time, XPS also
alloy in Sb@PtSb /NC, as
/NC falls well
D
G
2
2
2
2
Temperature-programmed desorption of O
Fig. 7) over Pt/NC and Sb@PtSb /NC catalysts indicated that
mainly atomic oxygen was adsorbed on the surface of both
catalysts [44,45], but the amount of desorbed O over
Sb@PtSb /NC was much higher than that over Pt/NC. The
above XRD analysis indicated that the crystalline structure of
the PtSb alloy consists of a Pa-3 unit cell structure with a di-
mension of 0.644 nm (see Fig. 4), while the crystalline structure
of elemental Pt consists of an Fm-3m unit cell structure with a
dimension of 0.392 nm. The enhanced activation of oxygen
2
(O
2
-TPD, see
[35]. Deconvolution of the N 1s spectra (see Fig. 8C) suggested
the formation of pyridinic nitrogen (N1, 398.5 eV), pyrrolic
nitrogen (N2, 400.1 eV), quaternary nitrogen (N3, 401.1 eV),
and pyridine N-oxide nitrogen (N4, 403.3 eV) in Pt/NC and
2
2
2
Sb@PtSb /NC [42]. It was reported that pyridinic N can reduce
2
the energy barrier for the adsorption of reactants on adjacent
carbon atoms and accelerate the rate-limiting first-electron
transfer process, resulting in a significant enhancement of cat-
alytic activity of the carbon surface [46,47]. Moreover, quater-
nary N in a graphene structure can bring about a non-uniform
electron distribution and a significant enhancement of the cat-
alytic activity of the carbon surface [48]. Introducing Sb and N
dopants into the catalyst can enhance the interactions between
Pt NPs and the carbon support, leading to an easier transfer of
electrons from N atoms to Pt NPs, and these electron-enriched
Pt particles are more active in the oxidation of glycerol.
2
over the Sb@PtSb
2
catalyst might be attributed to the for-
mation of a PtSb alloy shell and the enlarged distance between
2
Pt atoms, which are more favorable for the dissociative adsorp-
tion of oxygen (see Fig. 7). In other words, the formation of a
PtSb
2
alloy shell in the Sb@PtSb hybrid increased the space
2
between metal atoms, allowing the adsorption and surface
diffusion of more oxygen atoms in the catalyst.
The above characterizations confirmed that N-doped car-
XPS analysis provided evidence of electron transfer from N
bon-supported Sb@PtSb
2
hybrids with a core-shell structure
O 1s
N3
7
0.8
74.1
74.6
C 1s
Pt 4d
Sb 3d
N1
N2
A
C
B
d
N1
N3
Pt 4p
N2
7
1.3
N4
N 1s
c
N4
Pt 4f
d
c
b
7
1.0
74.3
74.7
75
b
a
7
1.4
b
a
a
0
200
400
600
70
80
85
395
400
405
410
Binding Energy (eV)
Binding Energy (eV)
Binding Energy (eV)
Fig. 8. (A) XPS spectra of (a) Pt/C, (b) Pt/NC, (c) Sb@Pt/C, and (d) Sb@PtSb
d) Sb@PtSb /NC; (C) The BE of the N 1s peak in (a) Pt/NC and (b) Sb@PtSb
2
/NC; (B) The BE of the Pt 4f peak in (a) Pt/C, (b) Pt/NC, (c) Sb@Pt/C, and
/NC.
(
2
2

Lihua Yang et al. / Chinese Journal of Catalysis 41 (2020) 494–502
499
could be fabricated in a facile manner via the direct pyrolysis of
100
3
a mixture of glucose, melamine, and SbCl , followed by im-
Conv.
pregnation with Pt.
DHA
8
6
4
0
0
0
GLYA
GLYCA
HPYA
3.2. Catalytic activity for selective oxidation of glycerol
Catalytic reaction results confirmed that this Sb@PtSb
2
/NC
catalyst was highly active for the selective oxidation of glycerol
in base-free aqueous solutions. As is summarized in Table 2,
Sb/NC was inactive for the oxidation of glycerol, which indi-
cates that Pt and/or PtSb provide the active sites for this reac-
tion. The detected conversion of glycerol over the monometallic
Pt/C catalyst was low (13.5%), and mainly GLYA (75.2%), DHA
20
0
2
0
2
4
6
8
Reaction time (h)
(
12.8%), and HPYA (5.6%) were detected. The conversion of
glycerol over Pt/NC improved significantly to 21.5% with
7.3% selectivity for GLYA. The higher activity of Pt/NC may be
Fig. 9. Time course of glycerol oxidation over Sb@PtSb /NC.
2
7
attributed to its higher surface area, increased amount of car-
bon defects, and higher dispersion of Pt, while electron dona-
tion from N to Pt can also accelerate the activation of molecular
oxygen and improve the reaction rate. When Sb was added to
the catalyst (Sb@Pt/C), the conversion of glycerol further in-
creased to 26.0% with a 73.3% selectivity for GLYA. It is inter-
esting to note that the detected conversion of glycerol jumped
7
0.2% within the first four hours, and then increased steadily
in the following time. The reaction rate slowed down with in-
creasing conversion, which might be caused by the strong ad-
sorption of the formed products. This phenomenon was dis-
cussed by Davis et al. in 2014 [23], and several works from
Zhou et al. also confirmed that strongly adsorbed intermediates
can decrease the reaction rate over Pt/AC(PO) catalysts[51,52].
When these adsorbed intermediates were washed off, the
to 65.3% over Sb@PtSb
/NC under the same reaction condi-
2
tions, and the selectivity for DHA increased to 39.2%. The cal-
culated TOF value on the basis of total Pt metal in
Sb@PtSb /NC catalyst could be recovered. At the same time, it
2
–
1
was found that the selectivity to DHA reached 56.9% at the
beginning of the reaction, and it decreased continuously with
time on stream. These results indicate that DHA is an important
intermediate of the reaction. The selectivity to GLYA increased
from 37.5% (at the beginning of the reaction) to 70.2% (at 4 h),
and then changed only slightly. On the other hand, the selectiv-
ity of HPYA and products derived from C–C bond cleavage
Sb@PtSb
2
/NC at a conversion of 10% reached 130.1 h , which
–1
–1
is higher than those of Pt/C (34.3 h ), Pt/NC (67.0 h ), and
Sb@Pt/NC (76.3 h ) [49,50]. The enhanced activity of
Sb@PtSb /NC could be attributed to its higher surface area (see
–1
2
Fig. 2 and Table 1) and increased amount of surface defects,
which promoted the adsorption and activation of glycerol and
oxygen. At the same time, the formation of a PtSb
Sb@PtSb /NC was also favorable for the adsorption and trans-
fer of oxygen (see the O -TPD in Fig. 7). It was also found that
the Sb@PtSb hybrid was more selective for the formation of
DHA, which might be attributed to the fact that the PtSb alloy
can suppress the further oxidation of DHA (see Fig. 9) [40].
Fig. 9 shows the time course of glycerol oxidation over
Sb@PtSb /NC at 60 °C in 0.2 g/mL glycerol aqueous solution. It
2
alloy in
(such as GLYCA) increased continuously. In this case, we think
2
that the primary product DHA and/or GLYA would further oxi-
dize to HPYA and GLYCA during the reaction. The proposed
reaction network is illustrated in Fig. 10.
That is, GLY was first oxidized to DHA or GLYHD, then
GLYHD was further oxidized to GLYA, and then GLYA could be
oxidized to GLYCA, OXA, or even to CO
was further oxidized to HPYA, and then HPYA could be oxidized
to GOX, OXA, or even to CO (see Fig. 10).
Fig. 11 shows the performance of the recycled
Sb@PtSb /NC catalyst for the selective oxidation of glycerol
under base-free conditions. The conversion of glycerol de-
creased from 65.3% to 43.1% after the catalyst was used five
times, while the selectivity of GLYA and DHA remained almost
2
2
2
2
. At the same time, DHA
2
can be seen that the conversion of glycerol increased quickly to
2
Table 2
a
The performance for selective oxidation of glycerol .
2
b
Product selectivity
c
(mol%)
Conv. TOF
Catalyst
–1
(
mol%) (h )
DHA GLYA GLYCA HPYA Others
Sb/NC
Pt/C
Pt/NC
Sb@Pt/C
0
–
0
0
0
0
0
0
2.5
0
0
13.5
21.5
26.0
34.3
67.0
76.3
12.8
13.2
14.5
75.2
77.3
73.3
51.8
5.6
4.3
3.8
2.3
6.4
5.2
8.4
4.2
OH
OH
OH
OH
HO
O
O
O
HO
O
O
3
4
5
O C O
7
9
OH
OH
OH
Sb@PtSb
2
/NC 65.3 130.1 39.2
Glyceraldehyde
GLYHD
Glyceric Acid
GLYA
Glycollic Acid
GLYCA
Oxalic Acid
OXA
1
Carbon Dioxide
CO2
a
Reaction conditions: catalyst loading of 0.100 g, 5 mL of 0.2 g/mL
OH
aqueous glycerol solution, 60 °C, 0.6 MPa O
2
, 3 h.
HO
OH
8
b
Calculated on the basis of total Pt at a conversion of 10%.
O
2
O
Glycerol
GLY
c
O
HO
O
Calculated as: (mmol of product in reaction mixture) × (the number of
6
O
HO
OH
OH
carbon atoms in product)/ ((initial mmol of glycerol − mmol of glycerol
left) × 3) × 100%. GLYA: glyceric acid; DHA: 1,3-dihydroxyacetone;
GLYCA: glycolic acid; HPYA: hydroxy-pyruvic acid.
OH
Dihydroxyacetone
DHA
Hydroxypyruvic Acid
HPYA
CO2 Glyoxalic Acid
GOX
Fig. 10. The glycerol oxidation network.

500
Lihua Yang et al. / Chinese Journal of Catalysis 41 (2020) 494–502
–1
cific activity based on the total amount of Pt reached 130.1 h .
80
60
40
20
0
80
60
40
20
0
Conv.
Sel.-GLYA
Sel.-DHA
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Graphical Abstract
Chin. J. Catal., 2020, 41: 494–502 doi: S1872-2067(19)63476-5
Selective oxidation of glycerol with oxygen in base-free solution
over N-doped-carbon-supported Sb@PtSb hybrid
2
Lihua Yang, Tianqu He, Chujun Lai, Ping Chen, Zhaoyin Hou *
Zhejiang University
An Sb@PtSb hybrid catalyst with a core-shell structure was fabri-
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氮掺杂碳负载Sb@PtSb 催化剂的制备及其在甘油选择性氧化反应中的应用
2
a
a
a
a
a,b,*
杨丽华 , 何天衢 , 赖楚钧 , 陈 平 , 侯昭胤
a
浙江大学化学系生物质化工教育部重点实验室, 浙江杭州310028
b
浙江大学化学系化学前瞻技术研究中心, 浙江杭州310028
摘要: 甘油是生物柴油生产过程中生成的副产物, 随着生物柴油产量的快速增长,甘油的量也迅速增加. 据估计, 到2020年
甘油的产量将比需求量高出6倍. 因此, 将过剩的甘油转化为其它更有价值的化学品具有重要意义.
在已经发表的文献中, 各种碳材料负载的Pt催化剂被广泛应用于液相中甘油的选择性氧化. 但是, 由于Pt纳米颗粒与
碳载体之间相互作用较弱, 因此Pt纳米颗粒易团聚和流失, 而且Pt的过度氧化和有机羧酸的强吸附也导致Pt催化剂失活.
最近的研究表明, 采用含氮的碳载体可以增强Pt与载体间的相互作用, 这种载体还有可能将电子转移给Pt, 从而有效提高Pt
的分散度、活性和稳定性. 与此同时, 引入其它金属如Co, Cu, Bi, Sb等与Pt形成合金也能有效改善催化剂的活性和稳定性.
我们在前期工作中曾经发现多壁碳纳米管(MWCNTs)负载的PtSb合金在甘油氧化反应中具有很高的活性和二羟基丙酮选
择性, 可以抑制C–C裂解, 并提高了催化剂的稳定性. 但是在MWCNTs上组装PtSb颗粒的过程繁琐且危险, 需要对载体进行
氧化(浓硝酸)、嫁接硫醇、浸渍金属、高温还原等, 同时MWCNTs的价格也较高.
本文采用简单的热解方法将氮和锑同时引入到碳基载体中, 并用此载体制备了具有核壳结构的Sb@PtSb
2
/NC催化剂.
o
首先将葡萄糖、三聚氰胺和SbCl
3
混合后在氮气中于700 C热解得到含Sb和N的多孔碳载体, 再通过浸渍还原法将Pt还原并
负载到该载体上即得到具有核壳结构的Sb@PtSb /NC催化剂. 该催化剂对催化甘油氧化具有较高的活性, 同时具有较好的
2
稳定性. 氮气吸附表征表明, 引入N和Sb都能提高载体的比表面积和孔体积, 其中Sb的引入使得催化剂表面形成了多孔结
2
构(SEM表征). XRD、TEM和EDS表征证明了具有核壳结构的Sb@PtSb 颗粒在载体表面上的生成. Raman光谱表明N和Sb
的引入增加了碳缺陷, 有可能带来新的活性位点. O -TPD表征表明Sb@PtSb /NC对氧的吸附量远高于Pt/NC, 这可能归因
2
2
2
于PtSb 合金中Pt-Sb金属间的原子间距增大, 有利于氧的吸附和表面扩散, 从而显著提高了催化剂活性. XPS表征表明了从

502
Lihua Yang et al. / Chinese Journal of Catalysis 41 (2020) 494–502
N到Pt的电子转移, 而这种富含电子的Pt具有更高的活性. 将制备的催化剂用于考评催化甘油氧化的活性, 发现相比于
o
Pt/NC, Sb@PtSb /NC催化剂催化甘油氧化具有显著增加的活性, 二羟基丙酮选择性也明显提高, 在60 C, 0.6 MPa O 气氛
2
2
下, 100 mg催化剂与5 mL 0.2 g/mL甘油水溶液反应3 h得到了65.3%的甘油转化率, 以及39.2%的二羟基丙酮选择性和51.8%
2
的甘油酸选择性. 这可能归因于载体比表面积的增加、更多的碳缺陷, 以及PtSb 合金的形成. 使用五次后的催化剂仍保持
较高的催化活性, 证实了该催化剂具有较好的稳定性.
关键词: 甘油; 氧化; 无碱溶液; Sb@PtSb
收稿日期: : 2019-07-04. 接受日期: 2019-08-06. 出版日期: 2020-03-05.
通讯联系人. 电子信箱: zyhou@zju.edu.cn
2
; 合金
*
基金来源: 国家自然科学基金(21773206, 21473155); 浙江省自然科学基金(L12B03001).
本文的电子版全文由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).