Catalytic performance of nitrogen-doped activated carbon supported Pd catalyst for hydrodechlorination of 2,4-dichlorophenol or chloropentafluoroethane

Article abstract of DOI:10.3390/molecules24040674

Nitrogen-doped activated carbon (N-AC) obtained through the thermal treatment of a mixture of HNO₃-pretreated activated carbon (AC) and urea under N₂ atmosphere at 600 ^?C was used as the carrier of Pd catalyst for both liquid-phase hydrodechlorination of 2,4-dichlorophenol (2,4-DCP) and gas-phase hydrodechlorination of chloropentafluoroethane (R-115). The effects of nitrogen doping on the dispersion and stability of Pd, atomic ratio of Pd/Pd²⁺ on the surface of the catalyzer, the catalyst’s hydrodechlorination activity, as well as the stability of N species in two different reaction systems were investigated. Our results suggest that, despite no improvement in the dispersion of Pd, nitrogen doping may significantly raise the atomic ratio of Pd/Pd²⁺ on the catalyst surface, with a value of 1.2 on Pd/AC but 2.2 on Pd/N-AC. Three types of N species, namely graphitic, pyridinic, and pyrrolic nitrogen, were observed on the surface of Pd/N-AC, and graphitic nitrogen was stable in both liquid-phase hydrodechlorination of 2,4-DCP and gas-phase hydrodechlorination of R-115, with pyridinic and pyrrolic nitrogen being unstable during gas-phase hydrodechlorination of R-115. As a result, the average size of Pd nanocrystals on Pd/N-AC was almost kept unchanged after liquid-phase hydrodechlorination of 2,4-DCP, whereas crystal growth of Pd was clearly observed on Pd/N-AC after gas-phase hydrodechlorination of R-115. The activity test revealed that Pd/N-AC exhibited a much better performance than Pd/AC in liquid-phase hydrodechlorination of 2,4-DCP, probably due to the enhanced stability of Pd exposed to the environment resulting from nitrogen doping as suggested by the higher atomic ratio of Pd/Pd²⁺ on the catalyst surface. In the gas-phase hydrodechlorination of R-115, however, a more rapid deactivation phenomenon occurred on Pd/N-AC than on Pd/AC despite a higher activity initially observed on Pd/N-AC, hinting that the stability of pyridinic and pyrrolic nitrogen plays an important role in the determination of catalytic performance of Pd/N-AC.

Full text of DOI:10.3390/molecules24040674

molecules
Article
Catalytic Performance of Nitrogen-Doped Activated
Carbon Supported Pd Catalyst for
Hydrodechlorination of 2,4-Dichlorophenol
or Chloropentafluoroethane
Haodong Tang, Bin Xu, Meng Xiang, Xinxin Chen, Yao Wang and Zongjian Liu *
Institute of Industrial Catalysis, College of Chemical Engineering, Zhejiang University of Technology,
Hangzhou 300014, China; tanghd@zjut.edu.cn (H.T.); 2111601033@zjut.edu.cn (B.X.);
2
*
111601241@zjut.edu.cn (M.X.); 2111701032@zjut.edu.cn (X.C.); 2111801033@zjut.edu.cn (Y.W.)
Correspondence: zjliu@zjut.edu.cn; Tel.: +86-571-88320412
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀ
ꢁꢂꢃꢄꢅꢆ
Received: 25 December 2018; Accepted: 12 February 2019; Published: 14 February 2019
Abstract: Nitrogen-doped activated carbon (N-AC) obtained through the thermal treatment of a
◦
mixture of HNO -pretreated activated carbon (AC) and urea under N atmosphere at 600 C was used
3
2
as the carrier of Pd catalyst for both liquid-phase hydrodechlorination of 2,4-dichlorophenol (2,4-DCP)
and gas-phase hydrodechlorination of chloropentafluoroethane (R-115). The effects of nitrogen
doping on the dispersion and stability of Pd, atomic ratio of Pd/Pd2+ on the surface of the catalyzer,
the catalyst’s hydrodechlorination activity, as well as the stability of N species in two different reaction
systems were investigated. Our results suggest that, despite no improvement in the dispersion of Pd,
2+
nitrogen doping may significantly raise the atomic ratio of Pd/Pd on the catalyst surface, with a
value of 1.2 on Pd/AC but 2.2 on Pd/N-AC. Three types of N species, namely graphitic, pyridinic,
and pyrrolic nitrogen, were observed on the surface of Pd/N-AC, and graphitic nitrogen was stable in
both liquid-phase hydrodechlorination of 2,4-DCP and gas-phase hydrodechlorination of R-115, with
pyridinic and pyrrolic nitrogen being unstable during gas-phase hydrodechlorination of R-115. As a
result, the average size of Pd nanocrystals on Pd/N-AC was almost kept unchanged after liquid-phase
hydrodechlorination of 2,4-DCP, whereas crystal growth of Pd was clearly observed on Pd/N-AC after
gas-phase hydrodechlorination of R-115. The activity test revealed that Pd/N-AC exhibited a much
better performance than Pd/AC in liquid-phase hydrodechlorination of 2,4-DCP, probably due to the
enhanced stability of Pd exposed to the environment resulting from nitrogen doping as suggested
by the higher atomic ratio of Pd/Pd2 on the catalyst surface. In the gas-phase hydrodechlorination
of R-115, however, a more rapid deactivation phenomenon occurred on Pd/N-AC than on Pd/AC
despite a higher activity initially observed on Pd/N-AC, hinting that the stability of pyridinic and
pyrrolic nitrogen plays an important role in the determination of catalytic performance of Pd/N-AC.
+
Keywords: activated carbon; hydrodechlorination; nitrogen doping; Pd
1
. Introduction
Chlorinated organics are among the most significant and widespread harmful materials in the
environment. They have been found in a variety of environmental situations, such as water, air,
and soil. For instance, chlorophenols, a class of persistent pollutants that are toxic and resistant to
environmental degradation through chemical, biological, and photolytic processes, have become
an important component of industrial wastewater due to their extensive uses as raw materials
or intermediates to produce pesticides, dyes, and leather [
1–3]. Another well-known example is
chlorofluorocarbons, many of which have been widely used as refrigerants, aerosol propellants,
Molecules 2019, 24, 674; doi:10.3390/molecules24040674
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Molecules 2019, 24, 674
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or solvents. However, they are scientifically linked to the depletion of the ozone layer and
thus pose a serious threat to the global climate [ ]. Since these chlorinated organics are very
4,5
difficult to destroy by incineration, new technologies must be developed to recycle or destroy
them. Catalytic hydrodechlorination has been considered as a promising treatment technique for
both chlorophenols [
hydrodechlorination offers a nondestructive way to dispose of chlorophenols and thus allows a
recovery of the treatment product (e.g., phenol or cyclohexanone) [ ]. In the case of treating
chlorofluorocarbons, gas-phase catalytic hydrodechlorination can transform chlorofluorocarbons
into hydrofluorocarbons [10 12], which can serve as a replacement for chlorofluorocarbons and are
6–10] and chlorofluorocarbons [11–14]. For example, liquid-phase catalytic
6,7
–
significantly less harmful to the environment.
Supported Pd catalysts are commonly used in the hydrodechlorination process due to their high
activity and carbon materials, e.g., activated carbons (AC) are often selected as the supports because
of their high specific surface area and excellent stability. During reactions, however, Pd tends to
aggregate on the surface of carbon carriers and thus loses its catalytic activity, probably due to the
relatively weak interaction between carbon and metal active components [14,15]. It has been reported
that nitrogen doping may not only change the physical and chemical properties of carbon carriers but
may also enhance the interaction between support and metal active components, and thus improve the
activity and stability of the catalyst [15
carbon supports can greatly improve the catalytic performance of Pd catalysts in liquid-phase catalytic
hydrodechlorination [ 10 17 19]. Zhou et al. reported that nitrogen-doped graphene-supported Pd
catalysts exhibited much higher catalytic activities for hydrodechlorination of 2,4-dichlorophenol
2,4-DCP) in comparison with Pd/graphene [17]. Baeza et al. found that nitrogen doping can
greatly improve the performance of carbon-supported Pd catalysts in the hydrodechlorination of
-chlorophenol to phenol [18]. In our group, we observed that nitrogen-doped mesoporous carbon
,16]. Indeed, many studies have shown that nitrogen doping of
8
– , –
(
4
prepared through a one-step method was a more effective Pd support than un-doped mesoporous
carbon in the hydrodechlorination of 2,4-DCP [20]. Although much work has been done on liquid-phase
catalytic hydrodechlorination, there is little work on the use of nitrogen-doped carbon supports in the
gas-phase catalytic hydrodechlorination. In particular, to the best of our knowledge, a comparative study
on the use of nitrogen-doped activated carbon (N-AC) in both liquid-phase catalytic hydrodechlorination
and gas-phase catalytic hydrodechlorination is still missing.
In this work, we present a comparative study on the use of N-AC supported Pd catalysts in
both liquid-phase catalytic hydrodechlorination of 2,4-DCP (see Scheme 1a) and gas-phase catalytic
hydrodechlorination of chloropentafluoroethane (R-115) (see Scheme 1b). The present work mainly
focuses on two aspects: (i) the stability of N species in two different reaction systems and (ii) the effects
of nitrogen doping on the dispersion and stability of Pd, atomic ratio of Pd/Pd2+ on the surface of the
catalyst, and the catalyst’s hydrodechlorination activity.
OH
OH
OH
Cl
Pd
Catalyst
Cl
H
H2
HCl
Cl
H
H
(
a)
Pd
Catalyst
F
F
F
F
F
HCl
F
F
H2
F
F
F
Cl
H
(
b)
Two hydrodechlorination reactions:
b) hydrodechlorination of R-115.
Scheme 1.
(
(a) hydrodechlorination of 2,4-DCP and

Molecules 2019, 24, 674
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2
. Results and Discussion
2
.1. Catalyst Characterization
Nitrogen was introduced into AC through the thermal treatment of a mixture of AC and urea.
In order to determine whether N species were present on the surface of catalyst, the as-prepared catalyst
was characterized by X-ray photoelectron spectroscopy (XPS). The wide scan XPS spectrum of the
Pd/N-AC catalyst indicates the presence of N on the surface of Pd/N-AC catalyst (see Figure S1 in the
Supported Materials). Figure 1a presents the N1s XPS spectrum of Pd/N-AC catalyst. For comparison,
the N1s XPS spectra of spent Pd/N-AC catalysts are also shown in Figure 1. As indicated by the
decomposed peaks, three types of N were identified: pyridinic, pyrrolic, and graphitic N. Pyridinic
N bonded with two C atoms as a member of a hexagon, pyrrolic N bonded with two C atoms by
forming a pentagon, and graphitic-N substituted for the C atom in graphene layer by bonding to
2
three C atoms in a sp configuration [21]. The total content of N on the surface of Pd/N-AC catalyst
determined by XPS is about 5.3 wt.%, with pyridinic N being about 2.5 wt.%, pyrrolic N being about
2
.0 wt.%, and graphitic N being about 0.8 wt.%, respectively (see black bars in Figure 1d). Despite
the successful incorporation of nitrogen into AC, nitrogen doping seemed not to have a significant
effect on the pore volume and surface area of the catalyst. The total pore volume determined by N
2
3
3
adsorption–desorption isotherms is 0.42 m /g for Pd/AC or 0.41 m /g for Pd/N-AC, with the surface
area obtained through Brunauer–Emmett–Teller (BET)method being about 840 m /g for Pd/AC or
2
2
8
25 m /g for Pd/N-AC.
Fresh Pd/N-AC catalyst
Spent Pd/N-AC catalyst in
hydrodechlorination of R-115
410 408 406 404 402 400 398 396 394 392
410 408 406 404 402 400 398 396 394 392
Binding Energy (eV)
Binding Energy (eV)
(
a)
(b)
Spent Pd/N-AC catalyst in
hydrodechlorination of 2,4-DCP
Fresh Pd/N-AC
Spent Pd/N-AC (2,4-DCP)
Spent Pd/N-AC (R-115)
3
2
1
0
Pyridinic N
Pyrrolic N
Graphitic N
408 406 404 402 400 398 396 394 392
Binding Energy (eV)
Type of N species
(
c)
(d)
Figure 1. N1s XPS spectra (a–c) and the content of N of different types (d) of fresh Pd/N-AC catalyst
and spent Pd/N-AC catalysts in hydrodechlorination of R-115 and 2,4-DCP. The raw data are shown
by the black curve, and the fitting curve is indicated by the red curve. The decomposed peaks are
presented by the blue (pyridinic N), green (pyrrolic N), and purple curves (graphitic N).

Molecules 2019, 24, 674
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The loading of Pd determined by elemental analysis is about 2.95 wt.% for Pd/AC or 2.97 wt.%
for Pd/N-AC. To elucidate the effect of nitrogen-doping on the size of Pd catalyst, the microstructures
of both Pd/AC and Pd/N-AC was investigated by TEM. Figure 2 presents the TEM images of the
as-prepared Pd/AC and Pd/N-AC catalysts, along with the corresponding Pd size distribution of two
catalysts. To our surprise, the result showed that nitrogen doping cannot improve the dispersion of
Pd, with the average Pd size of Pd/AC being 2.8 nm and that of Pd/N-AC being 4.3 nm. The XRD
measurement also suggested that the crystal size of Pd on Pd/AC is smaller than that on Pd/N-AC
(
Figure 3). It has been reported that when carbon supports are doped with nitrogen, the spin density
of the supported Pd decreases remarkably with its d band center moving to a deeper energy level,
thus leading to an improvement in the oxidation resistance of Pd [22]. To determine whether such an
effect exists in the Pd/N-AC catalyst, Pd 3d XPS spectra of both Pd/AC and Pd/N-AC were recorded
2
+
(
Figure 4). An analysis of these XPS spectra indicated that the atomic ratio of Pd/Pd on the catalyst
2+
surface is about 1.2 on Pd/AC or 2.2 on Pd/N-AC. Since Pd can easily be reduced to Pd under H2
atmosphere at 200 C, we speculated that the ionic Pd species observed should be attributed to the
◦
oxidation of Pd when the catalysts were exposed to air. This inference is supported by the Cl 2p spectra
of both Pd/AC and Pd/N-AC (see Figure 4c,d), where no peak in the energy range of 190–210 eV could
be observed. The O1s XPS spectra indicated that the presence of O on the surface of the catalyst (the
amount of O on the surface of the fresh and spent catalysts ranges from about 4 to 8 atm.%, see Table S1
in the Supported Materials), which mainly comes from the contribution of the support (namely AC)
and adsorbed O-containing materials. These observations suggest that despite no improvement in the
dispersion of Pd, nitrogen doping can enhance the stability of Pd exposed to the environment and thus
raise the atomic ratio of Pd/Pd2 on the catalyst surface.
+
(a)
(b)
2
1
1
0
6
2
8
4
0
20
16
12
8
Average size = 2.7 nm
Average size = 4.3 nm
4
0
0
1
2
3
4
5
6
7
8
0
1
2
3
4
5
6
7
8
Particle size / nm
c)
Particle size / nm
(
(d)
Figure 2. TEM images of (a) Pd/AC and (b) Pd/N-AC, and Pd size distribution of Pd/AC (c) and
(
d) Pd/N-AC.

Molecules 2019, 24, 674
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C
Spent
Pd/N-AC
(R-115)
6
4
2
0
Pd
C Pd
Spent Pd/AC (R-115)
Spent
Pd/N-AC
（2,4-DCP)
Spent
Pd/AC
Fresh
Pd/N-AC
Spent Pd/N-AC (R-115
Spent Pd/AC (2,4-DCP)
(R-115)
Spent
Pd/AC
Fresh
Pd/AC
（
2,4-DCP)
Spent Pd/N-AC (2,4-DCP)
Fresh Pd/AC
Fresh Pd/N-AC
10
20 30 40 50 60 70 80
Two theta (degree)
Catalysts
(
a) (b)
Figure 3.
(
a) XRD patterns of fresh and spent catalysts and (b) crystal size of Pd of different catalysts
calculated based on the Scherrer’s equation along <111> direction.
346 344 342 340 338 336 334 332 330
346 344 342 340 338 336 334 332 330
Binding Energy (eV)
Binding Energy (eV)
(
a)
(b)
Fresh Pd/N-AC
Fresh Pd/N-AC
Fresh Pd/AC
Fresh Pd/AC
190
195
200
205
210
525
530
535
540
545
Binding Energy (eV)
c)
Figure 4. Pd 3d XPS spectra of fresh (
of two catalysts: ( ) Cl 2p and (
Binding Energy (eV)
(
(d)
a
) Pd/AC and (b) Pd/N-AC catalysts. Cl 2p and O 1s XPS spectra
) O1s. The raw data are shown by the black curve, and the fitting
curve is indicated by the red curve. The decomposed peaks are presented by the blue (Pd ) and green
c
d
0
2
+
(
Pd ) curves.
2
.2. Catalytic Performance
The catalytic performances of Pd/AC and Pd/N-AC catalysts in the hydrodechlorination of
,4-DCP to phenol are shown in Figure 5. Under our experimental conditions, there was no deep
2

Molecules 2019, 24, 674
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hydrogenation product (e.g., cyclohexanone) and only 2-chlorophenol (2-CP) was identified as the
dechlorinated by-product over all three catalysts. The absence of 4-chlorophenol can be ascribed to the
steric hindrance, by which the para-substituted Cl was easily attacked by the active H [23]. Therefore,
the possible pathway of hydrodechlorination of 2,4-DCP in our case was 2,4-DCP
→
2-CP
→
phenol.
This implies that prolonging the reaction duration favors the formation of phenol, which is consistent
with the findings from Figure 4 that show that with an increasing reaction time the concentration of
phenol increases monotonously. From Figure 4, we also observed that the incorporation of nitrogen
into AC can greatly improve the performance of Pd catalysts. For example, the conversions of 2,4-DCP
at a reaction time of 60 min over Pd/AC and Pd/N-AC catalysts are, respectively, about 29% and 99%.
However, a positive doping effect cannot be observed in the gas-phase hydrodechlorination of R-115.
As shown in Figure 6, while the selectivity to pentafluoroethane (R-125) over Pd/AC or Pd/N-AC
catalyst has no obvious change during time on stream, deactivation can be observed on both catalysts.
Despite the initial conversion of R-115 over Pd/N-AC being slightly higher than over Pd/AC, the
former deactivated more rapidly than the latter. After 9 h on stream, for example, the conversion of
R-115 was reduced to about 48% over Pd/N-AC and 39% over Pd/AC. These experimental results
suggest that the incorporation of nitrogen into AC cannot improve the performance of Pd catalysts in
the gas-phase hydrodechlorination of R-115.
18
16
14
12
10
8
6
4
2
0
1
8
6
2
,4-DCP
Phenol
-Chlorophenol
1
2
14
1
2
0
8
6
4
2
0
2,4-DCP
Phenol
1
2-Chlorophenol
0
20
40
60
80 100 120 140 160
0
20
40
60
80
100
Time (min)
a)
Time (min)
(
(b)
Figure 5. The concentrations of 2,4-DCP, 2-Chlorophenol and phenol as a function of reaction time
over (a) Pd/AC and (b) Pd/N-AC.
100
100
Pd/AC
90
80
70
60
50
40
30
Pd/N-AC
Pd/AC
Pd/N-AC
80
6
4
0
0
20
0
2
4
6
8
10
0
2
4
6
8
10
Time on stream(h)
a)
Time on stream(h)
(
(b)
Figure 6. Catalytic performance of hydrodechlorination of R-115 over Pd/AC and Pd/N-AC: (
conversion and (b) R-125 selectivity.
a) R-115
The above activity test indicates that the effect of nitrogen doping on the catalytic performance
seems to be different in two hydrodechlorination reaction systems. In our previous study on
hydrodechlorination of 2,4-DCP over Pt/MC catalyst [20], we found that during reaction the crystal
growth of Pd occurred and nitrogen doping could enhance the resistance to the growth of nanosized

Molecules 2019, 24, 674
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Pd. To determine whether a similar phenomenon could be observed on Pt/AC catalysts, we examined
the spent catalysts by TEM (Figure 7). In comparison with the results shown in Figure 2, it is clear that
the size of Pd particles in both Pt/AC and Pt/N-AC catalysts has no obvious change after reaction,
which is consistent with the measurement by XRD (see Figure 3). Thus, the better catalytic performance
observed on Pd/N-AC than on Pd/AC in the hydrodechlorination of 2,4-DCP cannot be attributed to
the better Pd dispersion and higher resistance to the growth of nanosized Pd under reaction conditions.
Additionally, despite the fact that after reaction, the Pd loading reduced to 2.74 wt.% on Pd/AC and
2.85 wt.% on Pd/N-AC, the loss of Pd during reaction was not responsible for the observed difference
in activity between the two catalysts because the lost Pd was still in the reactor and thus participated
in the reaction. Since the hydrodechlorination of 2,4-DCP was carried out in water, it was expected
that the following two aspects might be responsible for the positive effect on the performance of Pd
catalysts via nitrogen doping: (i) Nitrogen doping can improve resistance to oxidation of Pd in water
during liquid-phase hydrodechlorination of 2,4-DCP as suggested by the result that nitrogen doping
can significantly raise the atomic ratio of Pd/Pd2+ on the catalyst surface and (ii) The presence of
pyridinic and pyrrolic nitrogen species on the support surface may improve the hydrophilicity of
Pd/N-AC, which favors the reactant molecules in the water accessing the active sites in the pores via
capillary action and thus raises the catalytic performance of the catalysts.
(a)
(b)
2
2
1
1
4
0
6
2
8
4
0
24
20
16
12
8
4
0
Average size = 2.8 nm
Average size = 4.4 nm
0
1
2
3
4
5
6
7
8
0
1
2
3
4
5
6
7
8
Particle size / nm
c)
Particle size / nm
(
(d)
Figure 7. TEM images of (a) spent Pd/AC and (b) spent Pd/N-AC, and Pd size distribution of (c) spent
Pd/AC and (d) spent Pd/N-AC in the hydrodechlorination of 2,4-DCP.

Molecules 2019, 24, 674
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In the gas-phase hydrodechlorination of R-115, however, the TEM investigation suggests that
the crystal growth of Pd during reaction occurs on both Pt/AC and Pt/N-AC catalysts due to a high
reaction temperature (Figure 8). After 9 h time on stream the size of Pd particles was raised from
2.7 to 3.4 nm (about a 25.9% increase) over Pd/AC and from 4.3 to 5.9 nm (about a 37.2% increase)
over Pd/N-AC. Thermodynamically, the driving force for crystal growth is the minimization of free
energy in the system, and a high reaction temperature may promote the diffusion of Pd atoms or
clusters and thus favors the growth of nanosized Pd particles. XRD measurement also confirmed
the crystal growth of Pd during the reaction (Figure 3). Additionally, since there was only a slight
change in the Pd loading of two catalysts after the reactions were observed (2.85 wt.% on Pd/AC
and 2.82 wt.% on Pd/N-AC), the deactivation phenomenon cannot be attributed to the loss of Pd
during reaction. Therefore, the sintering of Pd particles should be the main reason for the deactivation
of two catalysts during time on stream, and the result that Pd/N-AC deactivates more rapidly than
Pd/AC can be attributed to a higher growth rate of Pd on Pd/N-AC. To understand the reason for the
higher growth rate of Pd observed on Pd/N-AC catalyst, the stability of N species during reaction
was investigated by XPS (Figure 1b,c). When compared with the fresh Pd/N-AC catalyst with about
2
.5 wt.% pyridinic N, 2.0 wt.% pyrrolic N, and 0.8 wt.% graphitic N (Figure 1b), the spent Pd/N-AC
catalyst for hydrodechlorination of R-115 possesses almost the same amount of graphitic nitrogen
about 0.8 wt.%) but much less pyridinic (about 0.8 wt.%) and pyrrolic nitrogen (about 0.7 wt.%).
(
In the hydrodechlorination of 2,4-DCP, however, the amounts of three types of N species on the
spent Pd/N-AC catalyst are very close to those on the fresh catalyst. These observations hint that
graphitic nitrogen is stable in both hydrodechlorination of 2,4-DCP and R-115, with pyridinic and
pyrrolic nitrogen being unstable during hydrodechlorination of R-115. Thus, we speculate that a higher
growth rate of Pd observed on Pd/N-AC catalyst should be related to the loss of surface nitrogen
(namely pyridinic and pyrrolic nitrogen), which may lead to the instability of Pd particles. As a result,
enhancing the stability of both pyridinic and pyrrolic nitrogen is crucial for the nitrogen-doped carbon
materials used in the gas-phase catalytic hydrodechlorination.
(a)
(b)
Figure 8. Cont.

Molecules 2019, 24, 674
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2
1
1
0
6
2
8
4
0
20
16
12
8
4
0
Average size = 3.4 nm
Average size = 5.9nm
0
1
2
3
4
5
6
7
8
0
1
2
3
4
5
6
7
8
Particle size / nm
c)
) spent Pd/AC, and (
Particle size / nm
(
(d)
Figure 8. TEM images of (
a
b
) spent Pd/N-AC, and Pd size distribution of (
c) spent
Pd/AC and (d) spent Pd/N-AC in the hydrodechlorination of R-115.
3
. Materials and Methods
3
.1. Materials and Chemicals
All chemicals were of analytical grade and used as received. Except for PdCl (purchased from
2
Sino-Platinum Metals Co. Ltd., Kunming, China) and nitric acid (purchased from Shanghai Lingfeng
Chemical Reagent Co., Ltd., Shanghai, China), all other reagents were obtained from Aladdin Chemical
Reagent Co., Ltd (Shanghai, China). Coconut shell AC was purchased from Hainan Coconut Shell
Active Carbon Plant (Hainan, China).
3
.2. Preparation of N-AC
Coconut shell AC was crushed and sieved to granules in the range of 40–60 mesh. These AC
◦
granules were then washed with deionized water and dried at 110 C for about 24 h. Thirty grams of
AC (40–60 mesh) was put into a flask containing 100 mL of 20% dilute nitric acid. The flask was then
◦
put in a water bath (90 C), and the AC-nitric acid mixture was refluxed for 5 h before cooling to room
temperature and filtering. Finally, the residue (namely AC) was washed with deionized water until
◦
neutral pH eluate was obtained and dried at 110 C for about 8 h.
The HNO -pretreated AC (2 g) was mixed with 2.5 g of urea and then put into a tubular furnace.
3
Before heating, N atmosphere (30 mL/min) was passed through the tubular furnace for 30 min to
2
◦
remove the air. After turning off the N flow, the HNO -pretreated AC was heated at 10 C/min to
00 C and then kept at this temperature for 3 h. Finally, N-AC was obtained after the tubular furnace
2
3
◦
6
was cooled to room temperature in a N atmosphere (30 mL/min).
2
3
.3. Preparation and Reduction of Supported Pd Catalysts
Supported Pd catalysts were prepared through a wet impregnation method. Briefly, 1.00 g support
was placed into a container, and then 2.50 mL of PdCl solution (20.31 g/L) was added dropwise to
2
◦
the support, followed by impregnation for 12 h. Supported Pd catalysts were reduced by H at 200 C
2
for 2 h before the hydrodechlorination activity test and characterization.
3
.4. Characterization
Elemental analysis was conducted on an elemental analyzer (Vario Macro cube, Germany). Surface
areas of the samples were measured on a Micrometrics ASAP 2020 (Micrometrics Instrument Co.,
◦
Norcross, GA) instrument. The samples were degassed under N flow at 200 C for 6 h prior to
2
◦
analysis at –196 C (77 K). X-ray diffraction (XRD) patterns of the samples were obtained using a
Rigaku D/max-RA powder diffraction-meter (Rigaku, Tokyo, Japan) equipped with Cu Ka radiation.
The Pd contents in the catalysts were determined on a 7600 ultraviolet spectrophotometer. Transmission

Molecules 2019, 24, 674
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electron microscopy (TEM) investigation of the samples was conducted on a JEOL JEM-1200EX electron
microscope (JEOL Co., Tokyo, Japan). The average particle size of Pd particles was calculated by
Image J software. X-ray photoelectron spectroscopy (XPS) was performed on aKratos AXIS Ultra DLD
using a monochromatic Al Ka excitation source (1486.6 eV). The C 1s peak (284.6 eV) was used as the
internal standard.
3
.5. Catalyst Activity Test
The hydrodechlorination of R-115 was carried out at 450 C in a fixed-bed reactor with an inner
diameter of 15 mm. The loading of catalyst was 2 mL, the flow rates of R-115 and H were, respectively,
◦
2
−
1
5
and 15 mL/min, and the space velocity was 600 h . Before conducting online analysis by gas
chromatograph (Jiedo GC-1690), the hydrodechlorination product was washed by sodium hydroxide
solution and dried by anhydrous calcium chloride.
The hydrodechlorination of 2,4-DCP was conducted in a three-necked flask (250 mL) under
atmospheric pressure. The flask, which contains 0.10 g of catalyst suspended in 200 mL of 2,4-DCP
solution (containing 0.5 g 2,4-DCP) with a pH of 12 adjusted by 1.0 M NaOH, was placed in a water-bath
◦
(
SDC-6, Scientz Co, China) with a temperature of 25
±
0.5 C. The suspension was purged with a
N flow (50 mL min1) for 30 min, and then the N flow was switched to a H flow (180 mL/min)
2
2
2
under continuous vigorous stirring (850 rpm). Samples were taken at 30 min intervals and the catalyst
particles were removed by fast filtration. The concentrations of the reactant, intermediate, and product
in the filtrate were determined by a gas-phase chromatography with a flame ionization detector.
Prior to gas-phase chromatography analysis, the filtrate was neutralized using 0.1 M HCl solution.
4
. Conclusions
In summary, we have presented a comparative study on the use of N-AC supported Pd
catalysts in both liquid-phase catalytic hydrodechlorination of 2,4-DCP and gas-phase catalytic
hydrodechlorination of R-115. XPS investigation suggests that graphitic, pyridinic, and pyrrolic
nitrogen were observed on the surface of Pd/N-AC, and while the former is stable in both liquid-phase
hydrodechlorination of 2,4-DCP and gas-phase hydrodechlorination of R-115, the latter two are
unstable during gas-phase hydrodechlorination of R-115. As a result, the average size of Pd
nanocrystals on Pd/N-AC was almost unchanged after liquid-phase hydrodechlorination of 2,4-DCP,
whereas crystal growth of Pd was clearly observed on Pd/N-AC after gas-phase hydrodechlorination
of R-115. The activity test revealed that Pd/N-AC exhibits a better performance than Pd/AC in
liquid-phase hydrodechlorination of 2,4-DCP probably due to the enhanced stability of Pd exposed
to the environment resulting from nitrogen doping as suggested by the higher atomic ratio of
Pd/Pd2+ observed on the catalyst surface of Pd/N-AC. In the gas-phase hydrodechlorination of
R-115, however, a more rapid deactivation occurs on Pd/N-AC than on Pd/AC, hinting that the
stability of pyridinic and pyrrolic nitrogen plays an important role in the determination of catalytic
performance of Pd/N-AC.
Supplementary Materials: The Supplementary Materials are available online.
Author Contributions: H.T. and Z.L. conceived and designed the experiments; B.X. and M.X. performed the
experiments; X.C. and Y.W. helped with characterization of the samples; H.T. and Z.L. wrote the paper.
Funding: This research was supported by the National Natural Science Foundation of China (Grant No. 21776257).
Conflicts of Interest: The authors declare no conflict of interest.
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