Matching Relationship Between Carbon Material and Pd Precursor

Article abstract of DOI:10.1007/s10562-018-2630-y

The matching relationship between carbon material and Pd precursor was investigated by constructing Pd@C catalysts with four carbon materials (mesoporous carbon, activated carbon, N-doped carbon and O-doped carbon) and three Pd precursors (PdCl₂

Full text of DOI:10.1007/s10562-018-2630-y

Catalysis Letters
https://doi.org/10.1007/s10562-018-2630-y
Matching Relationship Between Carbon Material and Pd Precursor
Xiang Zhang1 · Yan Du2 · Hong Jiang1 · Yefei Liu1 · Rizhi Chen1
Received: 10 September 2018 / Accepted: 2 December 2018
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
The matching relationship between carbon material and Pd precursor was investigated by constructing Pd@C catalysts with
four carbon materials (mesoporous carbon, activated carbon, N-doped carbon and O-doped carbon) and three Pd precursors
(PdCl2, Pd(C2H3O2)2 and Pd(NO3)2) and evaluating their catalytic performance in the phenol hydrogenation to cyclohex-
anone. The Pd precursor or the carbon material has no obvious influence on the cyclohexanone selectivity, but strongly affects
the catalytic activity. The Pd@C prepared via PdCl2 shows good performance among all tested catalysts due to higher Pd
content and better Pd dispersion. Conversely, although Pd(NO3)2 is easily adsorbed by carbon carriers, the catalytic activity
is poor due to the worse Pd dispersion. The Pd(C2H3O2)2 adsorption is very sensitive to the surface properties of carbon,
and the N-doping can enhance the binding force between carbon and Pd2+, leading to higher Pd content and better Pd dis-
persion, thereby enhanced catalytic activity. This work would provide valuable references for the selection of Pd precursor
for a given support.
Graphical Abstract
Keywords Carbon · Pd precursor · Matching relationship · Phenol hydrogenation · Cyclohexanone
1 Introduction
Catalysts play a key role in the chemical industry by speed-
ing up the reaction progress and improving the product
selectivity. The homogeneous catalysts have limited indus-
trial applications owing to the difficult and costly catalysts
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X. Zhang et al.
separation and recovery [1, 2]. Many attentions have been
focused on the heterogeneous catalysts (such as supported
catalysts), due to easy removal of the catalysts from the reac-
tion mixture and permission of a continuous operation mode
[3, 4]. For heterogeneous catalysts, the interaction between
active metal and support affects significantly the catalytic
properties. Simultaneously, the support plays an important
role in product distribution [5]. Therefore, it is significantly
important to select suitable supports for heterogeneous
catalysts.
higher catalytic activities as compared to Pd(NO3)2, because
the presence of Cl ions led to an obvious increase in the
number of intermediates/sites of CO hydrogenation. The
literature showed that the selection of Pd precursor had a
significant effect on the structure and catalytic performance
of immobilized Pd NPs. However, no knowledge on how
to select a suitable Pd precursor for a given support was
achieved. Therefore, it is necessary to investigate the match-
ing relationship between Pd precursors and supports.
In this work, Pd@C catalysts were designed and con-
structed using two kinds of carbon materials (MC and AC)
and three Pd precursors (PdCl2, Pd(C2H3O2)2 and Pd(NO3)2).
XRD, ICP, BET, X-ray photoelectron spectrometry (XPS),
TEM, TG and other technologies were used to characterize
the microstructure of the supports and the Pd@C catalysts.
The phenol hydrogenation to cyclohexanone was used as
a model system to test the catalytic performance of Pd@C
catalysts, and the matching relationship between carbon
materials and Pd precursors was evaluated. Furthermore,
nitrogen-containing groups and oxygen-containing groups
were introduced into carbon materials, respectively, and the
influence of the groups in carbon materials on the selection
of Pd precursor was investigated.
Carbon materials, such as mesoporous carbon (MC) and
activated carbon (AC), have been considered as good sup-
ports for heterogeneous catalysts due to large specific surface
area, superior chemical and thermal stability [6]. In addition,
it is easy to modify the carbon materials to change the cata-
lytic performance, such as introducing nitrogen-containing
or oxygen-containing groups [5, 7–18]. The introduction of
nitrogen-containing groups not only enhances the binding of
the metal with the support and improves the metal dispersion
[9–11], but also forms a basic position in the carbon support,
conducive to the adsorption of acidic molecules, like phe-
nolic compounds [19, 20]. Introducing oxygen-containing
groups can adjust the hydrophilicity or hydrophobicity of the
carbon materials, and remove the impurities and increase the
number of defects for anchoring the metal particles [16, 17].
Pd nanoparticles (NPs) are widely used in hydrogenation
reactions due to the significant ability to activate H2 [19, 21].
Notably, the precursors of Pd are various, such as PdCl2,
Pd(C2H3O2)2 and Pd(NO3)2 [7, 8, 22–34], and the catalytic
performance of Pd based catalysts using different Pd precur-
sors is often quite different [23–34]. For example, Shen et al.
[25] studied the preparation of Pd/CeO2 catalysts from three
Pd sources (PdCl2, Pd(C2H3O2)2 and Pd(NO3)2) and applied
them to the methanol synthesis through CO hydrogenation.
They pointed out that the interaction between Pd NPs and
CeO2 of Pd–Cl and Pd–Ac (using PdCl2 and Pd(C2H3O2)2 as
Pd precursor, respectively) catalysts was stronger than that
of Pd–NO catalyst (using Pd(NO3)2 as Pd precursor), making
the Pd NPs disperse better and smaller in size, thereby higher
catalytic activities. And the Pd–Cl exhibited higher activity
than Pd–Ac at higher temperature. Borkowski et al. [26] pre-
pared two Pd catalysts by loading PdCl2 and Pd(OAc)2 on
cyclohexyldiamine-modified glycidyl methacrylate polymer
(GMA-CHDA) and applied in the Suzuki–Miyaura reaction.
They found that Pd(OAc)2 was reduced completely after
reaction, forming Pd(0) NPs with a diameter of 3–5 nm. In
contrast, PdCl2 was reduced only in ca. 50%, producing the
big Pd aggregates with lower catalytic activity. Ali et al. [24]
used three supports (SiO2, SiO2–Al2O3 and Al2O3) and two
Pd precursors (PdCl2 and Pd(NO3)2) to prepared catalysts
applied in CO hydrogenation. The results suggested that the
support and the Pd precursor did not affect the Pd disper-
sion. However, the catalysts synthesized by PdCl2 exhibited
2 Experimental
2.1 Chemicals
All the chemicals were used as received: AC (Jiangsu Zhuxi
AC Co., Ltd., China), MC (Nanjing JCNANO Technology
Co., Ltd., China), palladium acetate (Pd(OAc)2) and pal-
ladium chloride (PdCl2) (Sin-Platinum Metals Co., Ltd,
China), Palladium nitrate (Pd(NO3)2·2H2O) (Shanghai
Macklin Biochemical Co., Ltd., China), chlorhydric acid
(HCl), dicyandiamide (C2H4N4) and anhydrous sodium
carbonate (Na2CO3) (Shanghai Linfeng Chemical Reagent
Co., Ltd., China), nitric acid (HNO3), glacial acetic acid
(CH3COOH), phenol (C6H5OH), ethyl alcohol absolute
(C2H5OH), methanol (CH3OH) and citric acid monohy-
drate (C6H8O7·H2O) (Sinopharm Chemical Reagent Co.,
Ltd, China).
2.2 Catalyst Preparation
2.2.1 Preparation of N‑Doped Carbon
The N-doped carbon was synthesized according to the lit-
erature [35]. Firstly, 0.132 g of citric acid monohydrate
and 0.067 g of dicyandiamide were dispersed in 30 mL of
methanol with agitating. 0.4 g of MC was introduced into the
above solution and the mixture was continuously agitated for
12 h. The black powders were achieved after removing the
1 3

Matching Relationship Between Carbon Material and Pd Precursor
methanol with rotation evaporation, and dried at 60 °C over-
night followed by calcined under Ar atmosphere at 550 °C
for 4 h, and then naturally cooled to room temperature. The
produced solid is named MC–N; similarly, the name of AC
after N-doping is AC–N.
O
(cyclohexanone)
OH
OH
+H2
OH
2.2.2 Preparation of O‑Doped Carbon
+2H2
+
H
2
0.4 g MC was dispersed into aqueous solution of nitric acid
(30 wt%), and the mixture was refluxed at 60 °C for 4 h.
After cooling, the black solid was obtained by centrifuga-
tion, repeatedly washed with deionized water and ethanol
to neutral, then dried at 60 °C for 12 h. The resulting mate-
rial is denoted as MC–O; similarly, the name of AC after
O-doping is AC–O.
(phenol)
(1-hydroxycyclohexene)
(cyclohexanol)
Scheme 1 Reaction pathways for phenol hydrogenation
2.3 Loading of Pd NPs
of phenol, 20 mL of water and 0.2 g of Pd@C catalyst were
loaded into the reactor in order. The reactor was evacuated
and filled by a hydrogen-filled balloon (1 bar), and then
placed in an oil bath (80 °C). After 75 min, the catalyst was
separated from the reaction mixture by centrifugation, and
the products were analyzed by GC [7].
The Pd@MC catalysts were prepared by a wet impregnation
route. Briefly, the MC was dispersed into aqueous solution
of PdCl2, Pd(OAc)2 or Pd(NO3)2 containing 2 wt% Pd with
respect to MC. Na2CO3 was slowly added to the above solu-
tion at room temperature with stirring until the pH value
of the mixture reached neutral, then continued stirring for
12 h. Centrifugation was used to remove the filtrate and the
obtained black solid was repeatedly washed by deionized
water and ethanol to neutral, then dried at 60 °C for 12 h.
These obtained catalysts are designed as Pd@MC–Cl, Pd@
MC–Ac and Pd@MC–NO, respectively. Similarly, for AC
supports, the corresponding catalysts are marked as Pd@
AC–Cl, Pd@AC–Ac and Pd@AC–NO, respectively.
3 Results and Discussion
3.1 Matching Relationship Between MC and Pd
Precursor
From Fig. 1a, the selectivity of cyclohexanone is basically
the same, indicating that the Pd precursors have no signifi-
cant influence on the cyclohexanone selectivity, in line with
the literature [28]. However, the conversions of three Pd@
MC catalysts are quite different. The catalytic activity of
Pd@MC–Cl (conversion: 73.63%) is obviously higher than
Pd@MC–NO (conversion: 15.6%) and Pd@MC–Ac (con-
version: 3.33%), indicating PdCl2 is the most suitable Pd
precursor for MC. ICP, XRD and TEM were performed to
investigate the role of Pd precursor.
2.4 Characterization of Catalysts
A Rigaku Miniflex 600 diffractometer was used to analyze
the crystal structure. Morphology and microstructure were
investigated by transmission electron microscopy (TEM,
JEOL JEM 2100). The samples used for XRD and TEM
characterization were the recovered catalysts after the
hydrogenation of phenol. Textural properties were charac-
terized by using an ASAP 2020 analyzer. The pore volume
and surface area were achieved by the BJH and BET for-
mulas, respectively. The element composition and chemi-
cal state were investigated by XPS operating at the Thermo
ESCALAB 250 instrument. The Pd content was measured
by ICP-AES (Optima 2000DV). Thermal gravity analysis
was carried out on a NETZSCH STA449F3 under nitrogen
atmosphere.
The Pd loadings of Pd@MC–Cl and Pd@MC–NO are
higher, about 1.8 wt% closed to the theoretical Pd con-
tent of 2 wt% (Table 1). However, the Pd loading of Pd@
MC–Ac is lower, only 0.37 wt%, suggesting that MC has a
strong adsorption capacity for PdCl2 and Pd(NO3)2, but poor
adsorption capacity for Pd(C2H3O2)2. To intuitively observe
the difference in adsorption performance of Pd precursors by
MC materials, a set of adsorption experiments was designed
(Fig. 2). The solution color of three Pd precursors is yel-
low before wet impregnation, and the filtrate of PdCl2 or
Pd(NO3)2 solution turns colorless after wet impregnation.
But the filtrate of Pd(C2H3O2)2 solution still keeps yel-
low, further illustrating the poor adsorption performance
of Pd(C2H3O2)2 by MC. The phenomena may be caused
2.5 Catalytic Tests
The phenol hydrogenation to cyclohexanone (Scheme 1)
was performed in a 50 mL Schlenk flask. Typically, 0.1 g
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X. Zhang et al.
Fig. 1 Phenol hydrogenation performance of a Pd@MC, b Pd@MC–N, c Pd@MC–O
Table 1 Pd contents of Pd@MC
by the bigger and more complicated structure (trinuclear
molecular complex [Pd3(CH3COO)6] [36, 37] or polymeric
complex catena-[Pd(CH3COO)2]n composed of linear chains
[38, 39]) of Pd(C2H3O2)2. The minimal Pd content in Pd@
MC–Ac catalyst is consistent with the lowest catalytic activ-
ity (Fig. 1a). But the lower catalytic activity of Pd@MC–NO
catalyst cannot match with the higher Pd content (Table 1),
which may be caused by the difference in Pd dispersion.
In Fig. 3a, for Pd@MC–Cl and Pd@MC–NO, besides
the C peak at 23.8° [9], other three peaks can be observed at
2θ of ~40.1°, 46.6° and 68.0°, corresponding to the (111),
(200) and (220) planes of crystalline Pd, respectively [9,
Samples
Pd (wt%)
catalysts
Pd@MC–Cl
Pd@MC–Ac
Pd@MC–NO
Pd@MC–N–Cl
Pd@MC–N–Ac
Pd@MC–N–NO
Pd@MC–O–Cl
Pd@MC–O–Ac
Pd@MC–O–NO
1.77
0.37
1.81
1.46
1.01
1.66
1.65
0.56
1.34
Fig. 2 Pictures of the solutions before and after wet impregnation: a, a’ PdCl2, b, b’ Pd(C2H3O2)2, c, c’ Pd(NO3)2
Fig. 3 XRD patterns of a Pd@MC, b Pd@MC–N, c Pd@MC–O
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Matching Relationship Between Carbon Material and Pd Precursor
40]. But there is obvious difference in the peak intensity.
The intensity of Pd peaks for Pd@MC–Cl is obviously
lower than Pd@MC–NO irrespective of the similar Pd
loading (Table 1), which suggests that the Pd dispersion in
Pd@MC–Cl is better than Pd@MC–NO. It also has been
reported that PdCl2 tends to have good dispersibility on the
carrier, while Pd(NO3)2 has poor dispersibility due to strong
intermolecular forces [23, 25, 28]. The Pd peaks for Pd@
MC–Ac cannot be observed because of the less content of Pd
(Table 1). The TEM images show that the Pd dispersion in
Pd@MC–Cl is better than Pd@MC–NO (Fig. 4), in consist-
ent with the XRD patterns (Fig. 3a) and the catalytic activity
(Fig. 1a). Notably, the Pd dispersion in Pd@MC–Ac is very
worse, indicating that the MC not only has weak adsorption
ability for Pd(C2H3O2)2, but also has weak binding force
with Pd(C2H3O2)2, thereby worse Pd dispersion and poor
catalytic activity (Fig. 1a). This phenomenon is different
from the literature, and Pd(C2H3O2)2 often exhibits good dis-
persibility on other carriers like Al2O3, MgO, GMA-CHDA,
etc. [26, 28].
the Pd@MC catalysts (Fig. 1a). ICP and XRD were per-
formed to explain the mechanism.
The Pd contents in Pd@MC–N–Cl and Pd@
MC–N–NO catalysts are slightly decreased compared to
Pd@MC–Cl and Pd@MC–NO catalysts (Table 1), which
should be caused by the decrease in specific surface area
after N-doping (Fig. S1 and Table S1) [5]. In addition,
there is no obvious difference in the Pd dispersion by com-
paring Fig. 3a, b. However, the N-containing groups can
improve the phenol adsorption [19, 20], which can offset
the negative effect of the decrease in Pd content and make
the catalytic activity increase (Fig. 1a, b). The Pd content
in Pd@MC–N–Ac is significantly increased from 0.37 to
1.01 wt% after N-doping, possibly due to the enhanced
binding force between MC–N with Pd2+ [5, 9, 11, 42].
The increase in Pd content combined with the improve-
ment in phenol adsorption should be the major reasons for
the significant enhancement in catalytic activity of Pd@
MC–N–Ac (Fig. 1b). The results confirm that the N-con-
taining groups in MC have great effect on the adsorption
ability for Pd(C2H3O2)2, and Pd(C2H3O2)2 may be a good
Pd precursor for N-doped MC materials.
In order to further investigate the effect of groups (such
as N-containing and O-containing groups) on the selection
of Pd precursor for MC, MC materials were modified by
N-doping and O-doping (Fig. S2 and Table S2).
There are no changes in the selection of Pd precursor for
MC–O in comparison with MC (Fig. 1a, c), and PdCl2 is
still the optimal Pd precursor. However, the catalytic activi-
ties of Pd@MC–O catalysts are all significantly decreased
compared to Pd@MC catalysts (Fig. 1a). ICP and XRD were
used to confirm the reasons.
Compared to Pd@MC catalysts (Fig. 1a), the catalytic
activities of Pd@MC–N catalysts are all significantly
increased (Fig. 1b). That’s because the introduction of
nitrogen-containing groups can increase the binding
force between the Pd NPs and the support [9, 11], thereby
increasing the Pd dispersion. In addition, the introduction
of nitrogen-containing groups can also form strong basic
sites in the carbon support [19, 41], which can signifi-
cantly increase the adsorption of phenol [19, 20], leading
to the increase in catalytic activity. The catalytic activi-
ties of three Pd@MC–N catalysts follow the order: Pd@
MC–N–Cl>Pd@MC–N–Ac>Pd@MC–N–NO, indicating
PdCl2 is still the most suitable Pd precursor for MC after
N-doping. However, compared to Pd@MC–Ac, the cata-
lytic activity of Pd@MC–N–Ac is significantly improved
from 3.33 to 82.78%, larger than Pd@MC–N–NO, unlike
With respect to Pd@MC–O–Cl or Pd@MC–O–NO, the
Pd loading is reduced (Table 1), owing to the reduction in
the specific surface area of MC–O (Fig. S1 and Table S1).
No obvious difference in the Pd dispersion after O-doping
is observed (Fig. 3a, c). Additionally, the introduction of
O-containing groups increases the hydrophilicity of MC
(Fig. S4), leading to the preferential adsorption of water by
MC–O, thereby inhibited adsorption of phenol [14]. These
aspects make the catalytic activity of Pd@MC–O–Cl or
Pd@MC–O–NO decrease (Fig. 1c). For Pd@MC–O–Ac,
the Pd loading is increased, possibly because of the formed
defects on the carbon surface by HNO3 treatment [16, 17].
Fig. 4 TEM images of a Pd@MC–Cl, b Pd@MC–Ac, c Pd@MC–NO
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X. Zhang et al.
Table 2 Pd contents of Pd@AC
But the O-doping inhibits the phenol adsorption, resulting
in the reduced catalytic activity (Fig. 1c).
Samples
Pd (wt%)
catalysts
Pd@AC–Cl
Pd@AC–Ac
Pd@AC–NO
Pd@AC–N–Cl
Pd@AC–N–Ac
Pd@AC–N–NO
Pd@AC–O–Cl
Pd@AC–O–Ac
Pd@AC–O–NO
1.53
1.09
1.56
1.62
1.17
1.99
1.31
0.79
1.63
The above results indicate that PdCl2 is the most suitable
Pd precursor for MC materials, regardless of the introduction
of the groups (N-containing or O-containing). The choice of
Pd(NO3)2 as a palladium precursor for MC is not suitable.
Similar phenomena were observed on other supports includ-
ing Al2O3, CeO2, SiO2, MgO, etc. [23–25, 28]. Pd(C2H3O2)2
is an optional precursor for N-doped MC because the N-dop-
ing can improve the adsorption of Pd(C2H3O2)2 on the MC.
3.2 Matching Relationship Between AC and Pd
Precursor
oxygen-containing groups (C–O, C=O and COOH) but also
nitrogen-containing groups (Pyridine N and Graphite N) on
the AC surface. Thus, the abundant groups on the AC sur-
face may enhance its binding force with Pd2+, leading to
the increased adsorption capacity for Pd(C2H3O2)2, thereby
higher Pd content (Table 2). But the ICP results cannot be
coincided with the catalytic activity (Fig. 5a). For example,
Pd@AC–NO has higher Pd content, but exhibits the worst
catalytic activity. In contrast, Pd@AC–Ac with lower Pd
content has a good catalytic performance, indicating the Pd
content is not a key factor in determining the catalytic activ-
ity of Pd@AC. According to the characterization results of
Pd@MC catalysts, the Pd dispersion may be a dominant
factor.
In order to verify whether PdCl2 is the most suitable Pd pre-
cursor for other carbon materials, the matching relationship
between AC and Pd precursor was also investigated.
Like Pd@MC catalysts (Fig. 1a), the three Pd@AC
catalysts exhibit significantly different catalytic activities
(Fig. 5a). The Pd@AC–Cl has the highest catalytic activity
(conversion: 74%), indicating PdCl2 is also the most suitable
precursor for AC. However, the catalytic activity of Pd@
AC–Ac (conversion: 62.11%) is higher than Pd@AC–NO
(conversion: 35.89%), which is not coincided with Pd@MC
catalysts (Fig. 1a). The reasons for the difference in catalytic
activities of Pd@AC catalysts were investigated as follows.
Like MC (Table 1), AC has a strong adsorption capac-
ity for PdCl2 and Pd(NO3)2 (Table 2). The adsorption
capacity for Pd(C2H3O2)2 by AC (1.09 wt%) is common
but stronger than the slight adsorption by MC (0.37 wt%),
indicating that the Pd(C2H3O2)2 adsorption is very sensitive
to the support properties and the AC surface is beneficial
for the Pd(C2H3O2)2 adsorption. The literature shows that
surface oxygen containing functionality of carbonaceous
support favors the Pd loading [16, 17]. Analogously, the
nitrogen-containing groups on the carbon surface also con-
tribute to the Pd loading by strengthening the dispersion
[9, 11]. According to the XPS results (Table S2 and Figs.
S2, S3), there is only one oxygen-containing group (C–O)
on the MC surface, while there are not only a plurality of
From Fig. 6a, the intensity of Pd peaks follows the order:
Pd@AC–NO>Pd@AC–Cl>Pd@AC–Ac, which indicates
the Pd dispersion in Pd@AC–NO is worse than Pd@AC–Cl.
The Pd peaks for Pd@AC–Ac cannot be observed, which
can be explained by the less Pd content or good Pd disper-
sion. The Pd NPs in Pd@AC–Cl disperses well (Fig. 7a),
and combined with higher Pd content can lead to the highest
catalytic activity (Fig. 5a). Very interestingly, compared to
Pd@MC–Ac (Fig. 4b), the Pd dispersion in Pd@AC–Ac can
be significantly improved (Fig. 7b), possibly because of the
enhanced binding force between AC and Pd2+ [9, 11, 16, 17].
Thus, despite of the lowest Pd content in Pd@AC–Ac, the
best Pd dispersion guarantees the higher catalytic activity.
Fig. 5 Phenol hydrogenation performance of a Pd@AC, b Pd@AC–N, c Pd@AC–O
1 3

Matching Relationship Between Carbon Material and Pd Precursor
Fig. 6 XRD patterns of a Pd@AC, b Pd@AC–N, c Pd@AC–O
Fig. 7 TEM images of a Pd@AC–Cl, b Pd@AC–Ac, c Pd@AC–NO
Like Pd@MC–NO (Fig. 4c), the worst Pd dispersion in
Pd@AC–NO (Fig. 7c) results in the lowest catalytic activ-
ity (Fig. 5a). These results confirm that the Pd dispersion is
a key factor in determining the catalytic activity of Pd@AC.
Similarly, AC materials were also modified by N-doping
and O-doping (Fig. S3 and Table S2) to investigate the effect
of groups on the selection of Pd precursor.
The increase in Pd content can make the catalytic activity
of Pd@AC–N enhance (Fig. 5b). However, there is no line
relation between the increase in Pd content and catalytic
activity. For Pd@AC–N–Ac, the increase in Pd content is
not high, but the better Pd dispersion (Fig. 6b) results in
the extraordinary catalytic performance. In contrast, with
respect to Pd@AC–N–NO, although the increase in Pd con-
tent is the highest, the poor Pd dispersion (Fig. 6b) offsets
the positive effect due to the increase in Pd content, thereby
lower increased degree of catalytic activity.
Like Pd@MC–N catalysts (Fig. 1b), the catalytic activi-
ties of Pd@AC–N catalysts are improved by N-doping
(Fig. 5b). However, the selection of suitable Pd precursor for
AC is changed after N-doping. The catalytic activity of Pd@
AC–N–Ac is significantly increased from 62.11 to 93.48%
and becomes the highest, indicating Pd(C2H3O2)2 is the most
suitable Pd precursor for AC after N-doping. ICP and XRD
were used to investigate the mechanism.
Like Pd@MC–O catalysts (Fig. 1c), the catalytic activi-
ties of Pd@AC–O catalysts are significantly decreased after
O-doping (Fig. 5c). However, the selection of Pd precur-
sor for AC–O in comparison with AC (Fig. 5a) is changed.
The catalytic activity of Pd@AC–O–Ac is lower than Pd@
AC–O–NO. To evaluate the reasons, the three Pd@AC–O
catalysts were characterized by ICP and XRD.
The Pd contents are all improved (Table 2), which is dif-
ferent from Pd@MC–N catalysts (Table 1). The increase
in Pd contents for Pd@AC–N catalysts can be explained as
follows. On one hand, the introduction of N can increase the
interaction between AC–N and Pd2+ [5, 9, 11, 42], leading
to higher adsorption of Pd precursor, thereby increased Pd
content. On the other hand, the N-doping makes the specific
surface area of AC reduce, but its specific surface area is sig-
nificantly larger than MC (Fig. S1 and Table S1), providing
enough loading sites to support the Pd NPs and inhibiting
the negative effect of the decrease in specific surface area.
The Pd contents in Pd@AC–O–Cl and Pd@AC–O–Ac
are decreased after O-doping (Table 2), which may be
caused by the following reasons. On one hand, the O-doping
makes the specific surface area of AC–O reduce (Fig. S1
and Table S1). On the other hand, the treatment with HNO3
decreases the basic N-containing groups (pyridine nitro-
gen and graphite nitrogen) in AC (Fig. S3), leading to the
decreased adsorption of PdCl2 and Pd(C2H3O2)2 [5], espe-
cially for Pd(C2H3O2)2. The Pd dispersion in Pd@AC–O–Cl
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X. Zhang et al.
or Pd@AC–O–Ac after O-doping does not significantly
changes (Fig. 6c). The reduction in Pd content, combined
with decreased phenol adsorption due to the change in the
hydrophilicity of AC (Fig. S4), will make the catalytic activ-
ity of Pd@AC–O–Cl or Pd@AC–O–Ac decrease (Fig. 5c).
Interestingly, the Pd content in Pd@AC–O–NO is slightly
increased, and the Pd dispersion is significantly improved
(Fig. 6c), which may be related with the change in surface
groups of AC after O-doping. A new O-containing group,
namely nitrogen oxide [43], is formed on the carbon surface
by the treatment with HNO3 (Fig. S3). The nitrogen oxide
on the AC–O surface is similar with the anion of Pd(NO3)2,
which may be conducive to the adsorption of Pd(NO3)2
and improving the binding force between AC–O and Pd2+.
Thus, improved Pd content and dispersion can be obtained.
However, the hydrophilicity of the O-doped AC is increased
(Fig. S4), inhibiting the phenol adsorption. Compared to
the improvement in Pd content and dispersion, the influence
of the decrease in phenol adsorption may be more obvi-
ous, leading to the reduction in catalytic activity of Pd@
AC–O–NO (Fig. 5c). At the same time, the improvement in
Pd content and dispersion can make lower reduced degree
of catalytic activity as compared to Pd@AC–O–Cl and Pd@
AC–O–Ac.
phenol adsorption. The work can aid the development of the
Pd@C catalysts with high catalytic performance.
Acknowledgements The financial supports from the National Key
R&D Program (Grant No. 2016YFB0301503), the National Natural
Science Foundation (Grant Nos. 21776127, 91534210) and the Jiangsu
Province Natural Science Foundation for Distinguished Young Scholars
(Grant No. BK20150044) of China are gratefully acknowledged.
Compliance with Ethical Standards
Conflict of interest All authors declare no conflicts of interest.
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4 Conclusions
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X. Zhang et al.
Affiliations
Xiang Zhang1 · Yan Du2 · Hong Jiang1 · Yefei Liu1 · Rizhi Chen1
1
* Yan Du
State Key Laboratory of Materials-Oriented Chemical
duyan@njtech.edu.cn
Engineering, Nanjing Tech University, Nanjing 210009,
People’s Republic of China
* Rizhi Chen
2
rizhichen@njtech.edu.cn
College of Environment, Nanjing Tech University,
Nanjing 210009, People’s Republic of China
1 3