Influence of Pt Particle Size on the Activity of Pt/AC Catalyst in Selective Oxidation of Glycerol to Lactic Acid

Article abstract of DOI:10.1007/s10562-017-2016-6

Abstract: The size effect of Pt particles on selective oxidation of glycerol to lactic acid was investigated over 1.0?wt% Pt/activated carbon catalysts in base solutions. A series of Pt/activated carbon catalysts with Pt particle size ranging from 10.2 to 3.8?nm were prepared by simply verifying the deposition–precipitation temperature from 0 to 80 °C, which were characterized by XRD, CO pulse chemisorption, H₂–O₂ titration, TEM and zeta potential analyses. The Pt particle size showed evident impact on the pseudo turnover frequency of the catalyst, with the optimal activity achieved over medium-sized Pt particles. The lactic acid selectivity was less obviously affected by the Pt particle size. Meanwhile, the Pt particle size effect would be altered when factors including base type changed. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-017-2016-6

Catal Lett
DOI 10.1007/s10562-017-2016-6
Influence of Pt Particle Size on the Activity of Pt/AC Catalyst
in Selective Oxidation of Glycerol to Lactic Acid
Chen Zhang1,3 · Tao Wang1 · Yunjie Ding1,2
Received: 13 December 2016 / Accepted: 2 March 2017
© Springer Science+Business Media New York 2017
Abstract The size effect of Pt particles on selective
oxidation of glycerol to lactic acid was investigated over
1.0 wt% Pt/activated carbon catalysts in base solutions. A
series of Pt/activated carbon catalysts with Pt particle size
ranging from 10.2 to 3.8 nm were prepared by simply veri-
fying the deposition–precipitation temperature from 0 to
80°C, which were characterized by XRD, CO pulse chem-
isorption, H2–O2 titration, TEM and zeta potential analyses.
The Pt particle size showed evident impact on the pseudo
turnover frequency of the catalyst, with the optimal activ-
ity achieved over medium-sized Pt particles. The lactic acid
selectivity was less obviously affected by the Pt particle
size. Meanwhile, the Pt particle size effect would be altered
when factors including base type changed.
Graphical Abstract
Keywords Glycerol · Aerobic oxidation · Platinum ·
Particle size · Lactic acid
1 Introduction
As a major by-product of biodiesel manufacture, glycerol
is recognized as a valuable platform molecular due to its
triple hydroxyl groups. Under different strategies, high-val-
ued derivatives such as lactic acid (LA), propanediol, glyc-
eric acid (GLYA) could be obtained [1–3]. Because of the
extensive applications of LA in fields like food industry,
cosmetics and pharmaceuticals, a surge of research efforts
have been focused on LA production from glycerol [4–6].
Considering that noble metals like Au, Pt and Pd are fre-
quently adopted as active components in LA production,
the investigation of particle size effect on LA production
from glycerol is worth exploring to improve their utiliza-
tion efficiency.
* Tao Wang
wangtao@dicp.ac.cn
* Yunjie Ding
dyj@dicp.ac.cn
1
Dalian National Laboratory for Clean Energy, Dalian
Institute of Chemical Physics, Chinese Academy of Sciences,
Dalian 116023, People’s Republic of China
2
State Key Laboratory of Catalysis, Dalian Institute
Among the many strategies for the preparation of adjust-
able particle size of metals on catalyst supports, the manip-
ulation of metal particle size is often associated with the
of Chemical Physics, Chinese Academy of Sciences,
Dalian 116023, People’s Republic of China
3
University of Chinese Academy of Sciences, Beijing 100049,
People’s Republic of China
Vol.:(0123456789)
1 3

C. Zhang et al.
introduction of new chemicals (e.g., various precursors,
supports and modifiers) or is studied at different metal
loadings [7–10], which may bring interference to the inves-
tigation of particle size effect. One alternative strategy is
the adjustment of thermal treatment conditions, such as
reduction temperature and duration time [11, 12]. For
example, the Ru cluster size can be tuned from 4 to 12 nm
as H2 reduction temperature increased from 250 to 700°C
[11]. In this work, deposition–precipitation method was
adopted for catalyst preparation, and the Pt precursor was
reduced and deposited by NaBH4 in aqueous solution. Like
the above H2-reduction process, the variation in aqueous
reduction temperature might also generate Pt particles with
tunable size.
at 200 kV. The samples were pretreated by ultrasonic dis-
persion in ethanol. CO pulse chemisorption and H2–O2
titration (HOT) were performed on an Altamira Instruments
AMI-300 with a thermal conductivity detection device. CO
pulse chemisorption was generally operated on a 150 mg
sample in a quartz U-tube. Samples were reduced by 10%
H2/Ar at 200°C for 1 h. Then samples were cooled to 30°C
in He atmosphere. Afterwards, samples were exposed to
pulses of 10% CO/He at 30°C. A CO/Pt stoichiometry of 1
was used for calculations [14]. The HOT experiments were
similar. After 10% H2/Ar reduction of samples at 200°C for
1 h and He post flush, turn on the 10% O2/He pulse chem-
isorption at 30°C. Afterwards, switch to Ar flush for 1 h
before another round of 10% H2/Ar pulses. A H2/Pt stoichi-
ometry of 1.5 was used for Pt diameter calculations [15].
Zeta potentials of AC at different temperatures (20–60°C)
were measured on a Malvern Zetasizer ZS90. The AC sup-
port was ball-milled to less than 1 μm and suspended in
water before test, and the test was performed on the same
AC suspension to avoid influence of AC concentration
variation.
Herein, a series of 1.0 wt% Pt/AC catalysts with differ-
ent Pt particle sizes were prepared by changing the prep-
aration temperature from 0 to 80°C, and the particle size
influence on glycerol oxidation to LA was explored under
mild conditions. During a 4-run recycling experiment, the
Pt/AC catalyst maintained stable Pt particle size and cata-
lytic performance. Characterizations including XRD, CO
pulse chemisorption, H2–O2 titration, TEM and zeta poten-
tial analyses were performed to gain insight of the catalyst
structure–activity relationship.
2.3 Glycerol Oxidation Reaction
The glycerol oxidation reaction was typically performed
in a 100 mL three-neck flask under ambient pressure in an
oil bath, with 25 g (10 wt%) of glycerol solution, 1.90 g of
LiOH·H2O (purity 90%) and 0.25 g of catalyst being added.
When temperature was stable, turn on the oxygen flow
(100 mL/min) and the magnetic stirrer (800 r/min). After
the reaction, the aqueous solution was filtrated, neutralized
and diluted. Then the products were analyzed by an Agi-
lent 1100 high-performance liquid chromatography with a
refractive index detector and an Alltech OA-1000 column
at 80°C. The products were identified by the retention time
of pure chemicals. The reaction conversion and product
selectivity were calculated by the same method reported
previously [13].
2 Experimental
2.1 Catalyst Preparation
Deposition–precipitation method was adopted during
the Pt/AC catalyst preparation [13]. Typically, 3 g of AC
(Tangshan United Carbon) was suspended in 300 mL of
deionized water under continuous stirring, and then 0.03 g
Pt in form of H2PtCl6 solution was added slowly. The pro-
cess was carried out in a water bath in order to control the
preparation temperature from 0 (in an ice-water mixture) to
80°C. After 0.5 h, excess amount of NaBH4 solution was
introduced dropwise. Then the mixed solution was kept stir-
ring for 6 h. Afterwards, the black precipitates were washed
thoroughly with deionized water until Cl− was undetected
by AgNO3, and were separated by filtration before drying
in an oven at 120°C overnight. Each of the Pt/AC catalysts
was denoted with a number representing the preparation
temperature in Celsius degree, e.g. Pt/AC-20 indicates that
the catalyst was prepared at 20°C.
3 Results and Discussion
3.1 Catalyst Characterization
3.1.1 XRD Analyses of Pt/AC Catalysts
The XRD patterns of a series of Pt/AC catalysts prepared at
temperatures ranging from 0 to 80°C are shown in Fig. 1.
The wide diffraction peaks at 24.0° and 43.5° are ascribed
to activated carbon support [16]. Other diffraction peaks
at 39.8°, 46.2°, 67.5° and 81.3° are attributed to the (111),
(200), (220), and (311) lattice planes of metallic platinum
(JCPDS, 00-004-0802), respectively. As Fig. 1 shows,
2.2 Catalyst Characterization
X-ray diffraction (XRD) analysis of catalysts was per-
formed on a PANalytical X’Pert PRO diffractometer with
Cu Kα1 radiation at 40 kV and 40 mA. TEM analysis was
conducted with a JEM-2100 electron microscope operating
1 3

Influence of Pt Particle Size on the Activity of Pt/AC Catalyst in Selective Oxidation of Glycerol…
gradually higher metal dispersion and smaller Pt particle
Activated carbon
Pt
size on AC support.
3.1.2 TEM of Pt/AC‑20 and Pt/AC‑70 Catalysts
Pt/AC-0
The TEM images of Pt/AC-20 and Pt/AC-70 catalysts are
presented in Fig. 2 as typical examples. The Pt particle
sizes of the two samples exhibit obvious difference. In addi-
tion, their Pt particle distributions were statistically meas-
ured from the TEM images by counting over 150 particles,
with the average diameter of Pt/AC-20 at 4.8 1.0 nm and
Pt/AC-70 at 2.5 1.0 nm (Fig. 2a2, b2), respectively. There-
fore, elevating preparation temperature of Pt/AC catalysts
would favor the dispersion of Pt, which is in good agree-
ment with XRD observations (Fig. 1).
Pt/AC-20
Pt/AC-30
Pt/AC-40
Pt/AC-60
Pt/AC-70
Pt/AC-80
10
20
30
40
50
60
70
80
90
2θ (°)
3.1.3 Chemisorption Analyses of Pt/AC Catalysts
Fig. 1 XRD patterns of Pt/AC catalysts prepared at different tem-
peratures. Characteristic diffraction peaks of Pt were marked by black
triangles
In order to get a more thorough picture of the Pt particle
sizes of the series of Pt/AC catalysts, CO pulse chem-
isorption was applied to calculate their respective average
Pt nanoparticle size, and HOT analyses were performed
as corroboration (Table 1). In line with XRD and TEM
elevated preparation temperature leads to the gradual
weakening of Pt diffraction peak at 39.8°, suggesting the
a1
b1
55
55
a2
b2
50
50
Pt/AC-70
Pt/AC-20
45
45
40
d = 2.5 1.0 nm
d = 4.8 1.0 nm
40
35
30
25
20
15
10
5
35
30
25
20
15
10
5
0
0
0-1
1-2
2-3
3-4
4-5
5-6
6-7
7-8
8-9
0-1
1-2
2-3
3-4
4-5
5-6
6-7
7-8
8-9
Particle size (nm)
Particle size (nm)
Fig. 2 TEM images and Pt particle size distributions of a Pt/AC-20 and b Pt/AC-70 catalysts
1 3

C. Zhang et al.
Table 1 The influence of preparation temperature on Pt particle size
1.0
0.8
0.6
0.4
0.2
0.0
100
Catalyst
D (nm)a
ζb
80
60
40
20
CO-pulse
HOT
(mV)
c
Pt/AC-0
10.2
8.8
7.9
4.8
4.1
4.0
3.8
12.6
8.2
6.4
4.3
3.6
3.8
3.6
–
Pt/AC-20
Pt/AC-30
Pt/AC-40
Pt/AC-60
Pt/AC-70
Pt/AC-80
−17.0 5.2
−11.6 2.5
−8.8 1.5
−2.8 0.1
c
–
–
c
0
aD represents the average Pt particle size calculated from the CO
pulse chemisorption data based on hemisphere model
0
20
40
60
80
Preparation Temperature (°C)
bζ represents the zeta potential of AC suspension corresponding to
each preparation temperature
Fig. 3 TOF and LA selectivity of Pt/AC catalysts in LiOH solution
at glycerol conversion of 20–30%. Reaction conditions: t=90°C,
cOut of the detection limitation
p=0.1 MPa,
nLiOH:nGlycerol =1.5:1, FO2 =100 mL/min, stirring
speed=800 r/min. TOF was calculated based on the dispersion of Pt
obtained from the CO pulse chemisorption data
analyses, a steady decrease of Pt particle size from 10.2
to 3.8 nm was observed as the preparation temperature
increased from 0 to 80°C (Table 1, CO-pulse data). We
assumed that this size-temperature correlation might be
caused by the interfacial charge variation of the AC suspen-
sion. As shown in Table 1, the zeta potential of AC suspen-
sion shifted from around −17 5.2 mV to −2.8 0.1 mV as
temperature increased from 20 to 60°C, i.e., the amount of
negative charge on the suspended AC support decreased as
temperature increased. As a result, the electrostatic adsorp-
tion of Pt4+ cations might be weaker or less per adsorption
site, which would favor the dispersion of Pt4+ [17, 18].
Therefore, when the NaBH4 was added to reduce the Pt4+
cations, the particle size would be bigger at low tempera-
ture and smaller at high temperature.
AC-70 and Pt/AC-80 catalysts might be due to their simi-
lar Pt particle sizes (Table 1). In both experimental and
theoretical literatures, the Pt (111) planes were indicated as
the most active sites in glycerol oxidation reaction [1, 21].
Meanwhile, the Pt (111) planes were also related with the
activation of O2, side reactions, and the adsorption of prod-
ucts [21, 22]. Therefore, proper exposure of Pt (111) planes
would maximize the catalyst performance.
In terms of product distribution, the Pt/AC catalysts gen-
erally showed a LA selectivity of less than 60% when glyc-
erol conversion was low (at 20–30%), and catalysts with
bigger Pt particle size showed slightly higher LA selectiv-
ity (as shown in Fig. 3). When reaction time was prolonged
to 6 h, the catalysts all displayed a LA selectivity at over
60%, a GLYA selectivity at over 10%, and the selectivity
of other products at lowerthan 10%. The difference in LA
selectivity among these catalysts indicates that the Pt parti-
cle size would slightly affect the LA selectivity in the initial
stage, but the influence would be minor with extended reac-
tion time.
3.2 Results of Glycerol Oxidation Reaction Over Pt/AC
Catalysts
Figure 3 and Table 2 shows the evaluation data of a series
of Pt/AC catalysts in glycerol oxidation reaction in the
presence of LiOH, which is previously examined as a desir-
able base for LA production [13]. As Fig. 3 shows, with the
increase of preparation temperature from 0 to 30°C (i.e.,
the decrease of Pt particle size from 10.2 to 7.9 nm), there
was a gradual elevation of catalysts’ pseudo TOF from 0.50
to 0.70 s−1. However, further increase in preparation tem-
perature (i.e., decrease in Pt particle size) resulted in a dra-
matic decline in TOF to less than 0.21 s−1.
In order to further explore the Pt particle size effect
under different basic conditions, six of the Pt/AC catalysts
were evaluated in the presence of KOH (as shown in Fig. 4;
Table 3). Similar to the variation trend in LiOH solution,
the highest TOF (0.63 s−1) was also obtained over the Pt/
AC-30 catalyst (average Pt particle size at 7.9 nm); the
catalysts prepared at 60–80°C were of similar particle size
and similar catalytic performance (as shown in Fig. 4).
Unlike the LiOH case, the glycerol conversion at 6 h
displayed a different order from their initial TOF order
(as shown in Table 3). Glycerol conversion is considered
to be related with both the transformation rate at each site
(i.e. the TOF of each site) and the number of active site.
In Table 2, the 6 h glycerol conversion variation ten-
dency is quite similar to that of TOF. These observations
indicated that there is an optimal Pt particle size for cata-
lysts under the current reaction conditions, in line with
Lei’s report over Pt/CNT and Lakshmanan’s work over Au/
CeO2 [19, 20]. Also, the similar activity of Pt/AC-60, Pt/
1 3

Influence of Pt Particle Size on the Activity of Pt/AC Catalyst in Selective Oxidation of Glycerol…
Table 2 Catalytic performance
Catalyst
Conversion
(%)
Selectivity (%)
of Pt/AC catalysts in glycerol
oxidation in LiOH solution
LAa
GLYA
TA
5.4
8.3
11.5
6.5
3.9
4.4
4.1
GA
OA
AA
FA
Pt/AC-0
85.2
96.5
100
90.9
72.6
72.3
72.7
64.9
63.9
65.1
62.2
62.9
62.2
60.5
15.4
11.8
10.1
14.0
18.4
17.7
15.9
5.3
6.6
6.4
7.8
6.1
7.3
8.7
1.8
2.5
3.9
2.3
2.0
2.1
2.4
0.8
1.1
1.5
1.4
1.2
1.8
2.0
5.4
5.7
1.5
5.8
5.5
4.5
6.3
Pt/AC-20
Pt/AC-30
Pt/AC-40
Pt/AC-60
Pt/AC-70
Pt/AC-80
Reaction conditions: t=90°C, p=0.1 MPa, nLiOH:nGlycerol =1.5:1, 6 h, FO2 =100 mL/min, stirring
speed=800 r/min
aLA lactic acid, GLYA glyceric acid, TA tartronic acid, GA glycolic acid, OA oxalic acid, AA acetic acid, FA
formic acid
factor for glycerol conversion is the transformation rate
1.0
0.8
0.6
0.4
0.2
0.0
100
per site; whereas in KOH solution, the dominant factor
is the number of active site. The gap in TOF between Pt/
80
AC-30 and Pt/AC-80 catalysts was narrower in KOH than
in LiOH solution, which confirms that the size effect on
60
transformation rate is more obvious in LiOH solution.
The LA selectivity of Pt/AC catalysts in KOH solution
40
was stable (at ca. 40%) when glycerol conversion was
maintained at 20–30% or the reaction time was 6 h (with
20
glycerol conversions generally over 60%). In other words,
the Pt particle size effect on LA selectivity was even less
obvious in KOH than in LiOH solution.
0
20
40
60
80
The difference in the Pt particle size effect under dif-
ferent basic conditions is speculated to be associated with
the reaction mechanism. LA production from glycerol
oxidation involves the dehydrogenation of glycerol to
intermediates (glyceraldehyde and dihydroxyacetone) and
intermediates’ dehydration and Cannizzaro rearrange-
ment to LA [1, 4, 20], which are catalyzed by the syn-
ergy effect between base and Pt [13, 21]. In the presence
of LiOH or KOH solution, the intermediate distributions
and the reaction rate at each step would be different, and
the influence of Pt particle size on each step may not be
the same.
Preparation Temperature (°C)
Fig. 4 TOF and LA selectivity of Pt/AC catalysts in KOH solution
at glycerol conversion of 20–30%. Reaction conditions: t=90°C,
p=0.1 MPa,
nKOH:nGlycerol =1.5:1, FO2 =100 mL/min, stirring
speed=800 r/min. TOF was calculated based on the dispersion of Pt
obtained from the CO pulse chemisorption data
With same Pt loading, Pt/AC catalyst (of same weight)
with bigger Pt particle size would have fewer active
sites. Then, we assumed that in LiOH solution, the major
Table 3 Catalytic performance
Catalyst
Conversion
(%)
Selectivity (%)
of Pt/AC catalysts in glycerol
oxidation in KOH solution
LA
GLYA
TA
GA
OA
AA
FA
Pt/AC-20
Pt/AC-30
Pt/AC-40
Pt/AC-60
Pt/AC-70
Pt/AC-80
60.6
64.1
63.2
69.9
73.6
72.7
38.0
42.9
42.8
44.7
46.2
44.6
21.3
22.7
19.0
16.2
19.2
15.6
5.9
6.9
4.4
4.2
4.4
4.3
11.9
14.7
10.8
11.4
11.8
13.4
2.7
1.9
2.6
2.6
2.0
2.8
3.6
3.6
4.9
4.8
5.1
5.2
16.5
7.3
15.5
16.2
11.4
14.1
Reaction conditions: t=90°C, p=0.1 MPa, nKOH:nGlycerol =1.5:1, 6 h, FO2 =100 mL/min, stirring
speed=800 r/min
1 3

C. Zhang et al.
The stability of the Pt nanoparticles was tested over the
Pt/AC-70 catalyst (Fig. 5). It exhibited quite stable cata-
lytic performance during the 4-run recycling experiments
in KOH solution, with glycerol conversion and LA selec-
tivity being kept at over 77% and 42%, respectively. The
TEM image (Fig. 6) of the spent-4th catalyst implies that
the Pt nanoparticles maintained good dispersion during the
recycling evaluations, which is essential for its practical
applications.
preparation temperature favors smaller Pt nanoparticle. The
Pt particle size effect on glycerol oxidation to LA has been
investigated over Pt/AC catalysts in both LiOH and KOH
solutions. In both cases, the Pt particle size shows evi-
dent impact on the catalyst activity (represented by pseudo
TOF), with the size effect more remarkable in LiOH solu-
tion and the optimized Pt particle size at 7.9 nm; how-
ever, the Pt particle size effect on LA selectivity was less
obvious.
Acknowledgements This work was supported by the National Nat-
ural Science Foundation of China (21176236).
4 Conclusions
A series of Pt/AC catalysts with tunable Pt particle size
were prepared by changing the preparation tempera-
ture during the deposition–precipitation process. Higher
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