Hydrodechlorination of chlorophenols at low temperature over highly defective Pd catalyst

Article abstract of DOI:10.1016/j.catcom.2013.06.014

Hydrodechlorination of chlorophenols at lower temperature was investigated over Pd catalyst supported on mesoporous silica-carbon nanocomposites (Pd/MSC). Characterization by XRD, HRTEM and H₂-TPD has shown Pd nanoparticles of 7 nm with saw-edged GB at nanocrystal joining areas and edge steps. The catalyst exhibits high activity for hydrodechlorination of chlorophenols at 258-313 K under normal hydrogen pressure. Using triethylamine as a probe combining with further theoretical calculation, it reveals that the HDC of chlorophenols at low temperature mainly occurs on Pd defects, meanwhile reactive H species plays a direct role when the reactant adsorbs on Pd via the formation of eithei π- or σ-complex.

Full text of DOI:10.1016/j.catcom.2013.06.014

Hydrodechlorination of chlorophenols at low temperature over highly defec-
tive Pd catalyst
Ling Cheng, Zhonghao Jin, Xingyi Wang
PII:
DOI:
Reference:
S1566-7367(13)00219-7
doi: 10.1016/j.catcom.2013.06.014
CATCOM 3535
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
3 May 2013
13 June 2013
13 June 2013
Please cite this article as: Ling Cheng, Zhonghao Jin, Xingyi Wang, Hydrodechlori-
nation of chlorophenols at low temperature over highly defective Pd catalyst, Catalysis
Communications (2013), doi: 10.1016/j.catcom.2013.06.014
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ACCEPTED MANUSCRIPT
Hydrodechlorination of chlorophenols at low temperature over highly
defective Pd catalyst
Ling Cheng, Zhonghao Jin, Xingyi Wang*
Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, East
China University of Science and Technology, Shanghai 200237, China.
Abstract
Hydrodechlorination of chlorophenols at lower temperature was investigated over Pd catalyst
supported on mesoporous silica-carbon nanocomposites (Pd/MSC). Characterization by XRD,
HRTEM and H2-TPD has shown Pd nanoparticles of 7 nm with saw-edged GB at nanocrystal
joining areas and edge steps. The catalyst exhibits high activity for Hydrodechlorination of
chlorophenols at 258-313 K under normal hydrogen pressure. Using triethylamine as a probe
combining with further theoretical calculation, it reveals that the HDC of chlorophenols at low
temperature mainly occurs on Pd defects, meanwhile reactive H species plays a direct role when
the reactant adsorbs on Pd via the formation of σ-complex.
Keywords: Hydrodechlorination; Chlorophenols; Pd catalyst; DFT calculation.
Corresponding author. Tel: +86-21-64253372, Fax: +86-21-64253372, E-mail:
wangxy@ecust.edu.cn (X.Y. Wang)
Address: P.O. 396, East China University of Science and Technology, 130 Meilonglu,
Shanghai 200237, P.R.China

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1. Introduction
Supported Pd catalysts are now receiving more and more attention for their catalytic activity
in treatment of wastewaters containing chlorinated organic pollutants, especially their applications
in the hydrodechlorination (HDC) [1]. The catalytic activity of inorganic ionic crystals in many
organic reactions is determined by low-coordinated surface ions (LCSI) [2]. Areas of atomic
disorder formed at the interface of crystal grains-grain boundaries (GB)-were theoretically
predicted to be a source of LCSI active sites in solid catalysts [3]. The stabilization of atomic
disorder in these areas has been further confirmed by more accurate density functional theory
(DFT) calculations [4-6] and by experimental observations [4,7-10]. The low-coordinated surface
Pd atoms (LCSPA) can locate at the edges and vertexes of Pd with small size and present high
catalytic activity in hydrogenation. Though high activity of nanoparticle Pd catalysts for HDC was
observed generally at 273 K or higher [11,12], in fact, the behavior of LCSPA for HDC at lower
temperature can be less available. Therefore, the activity of LCSPA at low temperature is much
worth of further investigation. However, experimental implementation of LCSPA in GB areas was
hampered by the lack of techniques for controlled formation of GB in high-surface-area
nanocrystalline Pd particle. Mesoporous silica-carbon nanocomposite material (MSC) [13,14]
with hybrid nature of carbon and silica as well as other unique properties is considered here as a
desired carrier to search for a novel method for increasing the content of LCSPA for Pd catalysts.
In previous work, Pd/MSC with 3.1 nm Pd particles presented high activity at 258 K for towards
the hydrodechlorination of chlorophenols [15], while this work confirms that high activity of
Pd/MSC is ascribed to highly defective Pd species, regardless of the size of Pd particles.

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2. Experimental
2.1. Catalyst preparation
The ordered MSC composed of silica and carbon with the C/SiO2 ratio of 3.82, MC
(mesoporous carbon) and MS (mesoporous silicate) were synthesized by the previously reported
methods [13,14], and impregnated in 0.48 M H2PdCl4 aqueous solution for 24 h at 298 K. The
resulted mixtures were dried at 353 K for 8 h in a vacuum oven after an ultrasonic treatment for
60 min. The resulting dry solids were reduced under the flow of H2 (30 ml min-1) at 473 K for 3 h
in a tubular furnace, and finally, the Pd catalysts supported on MSC, MC and MS were obtained.
The loadings of Pd on Pd/MSC, Pd/MC and Pd/MS were determined by ICP to be 3.2, 3.6 and 3.4
wt%, respectively.
2.2. Catalysts Characterization
The powder X-ray diffraction patterns (XRD) of the samples were recorded on a Rigaku
D/Max-rC powder diffractometer using Cu Kα radiation (40 kV and 100 mA). The diffractograms
were recorded within the 2θ range of 10 to 80o with a 2θ step size of 0.01o and a step time of 10 s.
The nitrogen adsorption and desorption isotherms were measured at 77 K on an ASAP 2400
system in static measurement mode. TEM/HRTEM analyses were carried out using a JEM-2010F
operated by an accelerating voltage of 200 kV. H2-Temperature-programmed desorption (H2-TPD)
was conducted in the N2 flow at 20 K min-1 from 293 to 873 K with a final isothermal hold of 60
min. Based on TCD calibrations and analysis of the effluent gas, the TPD profiles recorded in this
paper can be attributed solely to H2 uptake or release.
2.3 DFT calculation
Calculations have been performed with the generalized gradient approximation [16] using

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the PWScf code [17]. Electron-ion interactions were described by ultrasoft pseudopotentials [18],
with electrons from H 1s, C 2s, 2p, N 2s, 2p, O 2s, 2p, Cl 3s, 3p, and Pd 4s, 4p, 4d, 5s shells
explicitly included in the calculations. Plane-wave basis set cutoffs for the smooth part of the
wave functions and the augmented density were 25 and 200 Ry, respectively. The Pd(111)
(Pd(211)) surface was modeled as a periodic slab with 4 (12) layers, and the vacuum between
slabs is ~10 Å. A (2×2) ((2×2)) surface cell with corresponding 2×2×1 (2×2×1) k-point mesh was
used for Pd(111) (Pd(211)) surface. Accordingly, the adsorbate coverage evaluated with reference
to the number of surface Pd sites (surface Pd sites on the step edge) was high enough (1/16 and
1/8 ML at Pd(111) and Pd(211) surfaces, respectively) to exclude any co-adsorption effect. The
molecules were adsorbed on one side of the slab only and during structural optimization, the
adsorbates and all the atoms of slabs, except those in the bottom layer, were allowed to move
(force threshold is 0.05 eV/Å). In this work, the adsorption energy of molecule was estimated by
using the following equation:
Ead = -[Emol/surf﹣Emol–Esurf
]
Emol/surf is the total energy of the interaction of molecule-Pd system, Emol and Esurf are the
total energies of an isolated molecule in gas phase and the clean Pd surface, respectively.
2.4 Activity test
In a typical reaction procedure, Pd/MSC (0.10 g, 3.2 wt.%) was added to a stirred solution
composed of 4-CP (0.648 g, 5 mmol), Et3N (0.511 g, 5 mmol, 1.0 equivalence (equiv.) vs. the
number of chlorine atoms) and 50 ml methanol in a modified 100 ml three-necked flask with a
jacket connected to a cooling recirculator (Scientz DL-3020) at 258 K. The flask which contained
the reaction mixture was purged 6 times with hydrogen in order to remove air completely before

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being stirred vigorously (1000 rpm) under ordinary hydrogen atmosphere. In the case of the
stepwise addition of Et3N, a given amount of Et3N was added into the reaction mixture at the
set-up of reaction and given intervals, and the total amount of Et3N added was 1.0 equiv.. The
products and reactants were analyzed by GC (Perkin Elmer Claus 500 equipped with an HP-5
column). The turnover frequency (TOF) is estimated from the number of molecules transformed
per surface metal atom per minute.
3. Results and discussion
3.1. Characterization
All supports and Pd catalysts which were reduced under the flow of H2 (30 ml min-1) at 473
K for 3 h exhibit representative of small-angle XRD patterns of the two dimensional (2-D)
hexagonal mesostructure (Fig. 1), namely, one strong diffraction peak (10) at 2θ=0.5-1.5° and two
weak peaks (11 and 20) at larger angles. For Pd catalysts, several peaks at 2θ of 40.1, 46.5, and
68.0o are observed (Fig. 1 insert), assignable to the 111, 200, and 220 reflections of the
face-centered cubic (fcc) Pd lattice (JCPDS, NO. 05-0681). For Pd catalysts under study, the Pd
loading was determined by ICP to be 3.2-3.6 wt% (Table 1). TEM results (Fig. 2(A), 2(C) and
2(E)) show that mesostructure maintains perfect after supporting Pd. HRTEM (Fig. 2(B), 2(D) and
2(F)) in conjunction with powder XRD and XPS (Supporting Information, SI) confirms the
formation of Pd0 nano-particles with fcc Pd lattice. The ultrasonic treatment followed by H2
reduction led to Pd nanocrystal aggregates through displacement of atoms [19,20], shift of atomic
planes and formation of saw-edged GB at nanocrystals joining areas (Fig. 2B). The creation of
areas with atomic disorder due to mutual penetration of surface Pd atoms into the bulk of
neighboring Pd nanocrystals by diffusion in opposite directions implies that more distorted zones

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should be formed. It can be expected that a large fraction of LCSPA on the surface are formed at
saw-edged GB at nanocrystal joining areas on the average 7 nm size Pd particles (Table 1),
although it is difficult to estimate the fraction. As reported, Pd2+ ions with high dispersion on
silica of MSC may be reduced sonochemically [19,20]. This phenomenon can be interpreted in
terms of the electrochemical theory of adsorption of electrolytes [21,22]. Relationship between
the nature of adsorption of these precursors and the metal state in the target catalysts has been
revealed. XPS confirmed that 34.1% Pd2+ ions were reduced sonochemically (SI). When the
temperature is raised to 398 K, Pd2+ ions anchored on carbon species of MSC are reduced, so that
the densification of Pd atoms on Pd particles occurs. The Pd/MC and Pd/MS catalysts present Pd
nanocrystal mean diameters of 2.0 and 3.6 nm, respectively (Table 1). H2-TPD indicates that H2
desorption starts at 600 K on Pd/MSC, and the peak temperature is higher than 873 K while H2
desorption appears on Pd/MC with maximum at 813 K (SI), and on Pd/MS with maximum at 770
K. Obviously, a stronger adsorption of H species occurs on Pd/MSC. Considering the fact that the
H2 desorption from MS and MSC was not detected until 773K (SI) and the amount of H2
desorption from Pd/MSC and Pd/MS exceeds that of the chemisorbed hydrogen by a factor of up
to 94 and 41 (Table 1), it can be deduced that a larger amount of H2 desorption from Pd/MS and
Pd/MSC can be ascribed to spillover hydrogen [23]. Triethylamine (Et3N)-TPD indicates that the
desorption peak after 573 K on Pd/MSC is much stronger than those on Pd/MC and Pd/MS (SI),
while Py-IR can not show significant acid on these three catalysts. Moreover, Et3N desorption was
almost not found on MSC and MC. These results imply that strong adsorption of Et3N can be
ascribed to the interaction of a pair of electrons in N of Et3N with low-coordinated Pd atoms.
3.2 DFT calculation

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To better understand the experimental results, we have also performed a systematic density
functional theory (DFT) calculations [16-18] of the adsorption of 4-CP (Fig. 3(A)-3(H)), Et3N and
4-tert-butyl-chlorobenzene (4-t-BCB) (SI) on terrace and step sites of Pd(111) surface (modeled
by Pd(111) and Pd(211) surfaces, respectively) [24,25]. The adsorption energies of chlorophenols
via the formation of π-complex with the aromatic ring are greater than the ones via the formation
of σ-complex with the C-Cl (Table 2). On Pd(111) and Pd(211), the energies of 4-CP adsorption
via π-complex are 0.72 and 0.89 eV, respectively, while the ones of Et3N via the formation of
σ-complex, 0.48 and 0.84 eV, respectively. It can be expected that 4-CP should adsorb on Pd
atoms generally in the form of π-complex. As seen in the case of Pd/MS, the interaction of Et3N
with Pd(111) is too weak to compete for active sites with 4-CP via the formation of π-complex.
The inhibition by Et3N adsorption occurs mainly on Pd defects. Therefore, it is understandable
that the TOF of 4-CP on Pd/MSC and Pd/MC with considerable defective positions will increase
to a large extent when the Et3N content is decreased through the stepwise addition into the
reaction liquor.
3.3. HDC of chlorophenols
The catalytic activity derived from reaction profiles in Fig. 4 is expressed as the turnover
frequency (TOF) as shown in Table 1. Here, the TOF means the number of molecules transformed
per surface Pd atom and per minute (SI). Et3N was used either as a base to combine with HCl
formed during HDC or as a probe to investigate the active sites on Pd catalysts at low temperature.
With adding 1.0 equiv. Et3N one batch, the TOF for 4-chlorophenol (4-CP) at 258 K over Pd/MSC
is 12.2 min-1, exceeding the one on Pd/MC (0.9 min-1) or Pd/MS (1.7 min-1). Phenol is the only
product, and no formation of other species is found. On the assumption that the solubility of H2

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has no significant change within the range of 258-278 K, the temperature dependence of the initial
reaction rate can be used to generate apparent activation energies. It is interesting to find that the
associated Ea value of HDC of 4-CP is 20.9 kJ mol-1 on Pd/MSC, markedly lower than the value
(40.9 kJ mol-1) obtained on Pd/MC (SI). When the reduction temperature of Pd/MSC was
increased from 523 K to 673 K, the TOF dropped to 4.3 min-1, due to the decrease of LCSPA
resulted from aggregation of Pd particles.
For the recycling study, the HDC of 4-CP was performed under the same reaction conditions
as described above. After five successive runs were tested, the TOF on used Pd/MSC is 11.8 min-1,
similar to that on the fresh one, and the Pd content is 3.1 wt.%. A low Pd leaching amount (less
than 0.3 % of Pd content of catalyst) is detected by ICP analysis of the catalyst-free liquor. And
0.2% Cl species on Pd/MSC used for five times can be detected by XPS. A low amount of Cl
deposition is ascribed to the presence of Et3N which can combine readily with HCl formed during
HDC.
When Et3N is added with 0.25 equiv. per 60 min or 0.5 equiv. each batch, the HDC of 4-CP
on Pd/MSC is much promoted when Et3N amount is sufficient to neutralize HCl formed during
HDC (Fig. 4). In these tests, the addition of Et3N by two steps is more effective, and the TOF
increases up to 26.2 min-1 (Table 1). Meanwhile the corresponding Ea was 20.3 kJ mol-1 within the
range of 258-278 K (SI), almost as same as the value with one batch addition of Et3N. This fact
implies that the active sites for the HDC of 4-CP in both cases are identical, or in other words, the
intrinsic activity of Pd/MSC is not modified. Interaction of Et3N with active sites only leads to the
decrease in the number of the sites available for reactants. For Pd/MC, the stepwise addition of
Et3N increases the TOF of 4-CP to 2.1 min-1 (Table 1). On Pd/MS catalyst, HDC of 4-CP is

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inhibited slightly by Et3N.
To identify the activity of Pd defects, these TOF values are normalized to the number of
surface defect Pd atoms (TOFd). As the result without the inhibition by Et3N, the TOFd values of
4-CP on Pd/MSC and Pd/MC are then calculated to be 26.3 and 4-7 min-1, respectively (SI). The
decrease in TOFd with the decrease in nanoparticle size simply reflects that there is some factor(s)
governing the reaction. Considering a large difference in the ability to H-spillover on these two Pd
catalysts, it can be deduced that the reactive H species play a critical role in the HDC of
chlorophenol via the formation of π-complex.
In order to investigate the activity of Pd defects on Pd/MSC for HDC of the reactants via the
formation of σ-complex, 4-t-BCB is chosen as a reactant, as it adsorbs on the active site only via
the formation of σ-complex because of a bulky tert-butyl group on the aromatic ring. The
adsorption energies of 4-t-BCB via the formation of σ-complex with the C-Cl on Pd(111) and
Pd(211) are 0.11 and 0.15 eV, respectively (Table 2), indicating that 4-t-BCB on Pd(111) can only
be activated to a small extent. Therefore, the fact that low conversion of 4-t-BCB was observed on
Pd/MS is due to its having a small fraction of Pd defects. However, the situation is quite different
for Pd/MSC, for TOFd is up to 12.2 min-1, lower than the one of 4-CP, due to π-complex with
aromatic ring being more reactive. But for Pd/MC, TOF and TOFd are only 1.3 and 4.7 min-1,
respectively. The significant deference in TOFd for the HDC of 4-t-BCB on Pd/MSC and Pd/MC
indicates that reactive H species plays a critical role when the reactant adsorbs on Pd via the
formation of σ-complex. Though the mode of adding Et3N varied, the conversion of 4-t-BCB as a
function of time kept constant on all catalysts, indicating that the inhibition by Et3N decreases to a
larger extent, due to a large difference of adsorption energies between 4-t-BCB and Et3N.

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In conclusion, a combination of XRD, HRTEM and H2-TPD has shown that there are Pd
nanoparticles of 7 nm with saw-edged GB at nanocrystal joining areas and edge steps on hybrid
support of carbon and silica. Et3N as a probe combining with theoretical calculation informs that
the HDC of chlorophenols at low temperature mainly occurs on Pd defects. This work can be
helpful for the design Pd catalysts used in HDC at low temperature.
Acknowledgement
We would like to acknowledge the financial support from National Basic Research Program
of China (No. 2010CB732300, 2011AA03A406) and National Natural Science Foundation of
China (No. 21277047) and Commission of Science and Technology of Shanghai Municipality
(11JC1402900).
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Captions
Table 1 The properties and the activities of catalysts for HDC of chlorophenols with the different modes of
adding Et3N
Table 2 Calculated values of adsorption energies for absorbents on Pd(111) and Pd(211) and Pd-Cl distances
Fig. 1. XRD patterns of supports and Pd catalysts.
Fig. 2. TEM images of Pd/MSC (A), Pd/MC (C) and Pd/MS (E), and HRTEM images of Pd/MSC (B),
Pd/MC (D) and Pd/MS (F).
Fig. 3. Calculated structures of 4-CP on Pd(111) ((A) top and (B) side views of σ adsorption, (C) top and (D)
side views of π adsorption) and Pd(211) ((E) top and (F) side views of σ adsorption, (G) top and (H) side
views of π adsorption) surfaces; the H atoms are purple, the C atoms are grey, the O atoms are red and the
Cl atoms are green.
Fig. 4. The effect of the manners of Et3N addition on the HDC of 4-CP at 258 K. Reaction condition: 5
mmol 4-CP; methanol 50 ml; Pd catalysts 0.1 g.

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Table 1 The properties and the activities of catalysts for HDC of chlorophenols with the different modes of adding
Et3N
Loadinga
H2 desorbed b
dPd
Ead
Samples
SBET
TOFe / min-1
DP
/ m2g-1
/ nm
/ wt.%
/ μmolg-1
/ nmc
/ KJmol-1
4-CP
4-t-BCB
Pd/MSC
15.9
345 (413)f
6.2 (6.5)
3.2
2307
7.0
11.1/(12.2)H
12.2/(22.2)L/ (26.3)H
0.9/ (2.1)L/ (2.0)H
/(15.3)H
Pd/MC
Pd/MS
523 (682)
324 (377)
3.1 (3.2)
6.9 (7.7)
3.6
3.4
173
2.0
3.6
39.5
---
1.3/ (1.4)L
0.9/ (0.9) L
2076
1.7/ (2.0)L
a
d
Determined by ICP; b determined by TPD; c calculated from H2 chemisorption data; the temperature dependence of
the TOF at the first ten minutes was used to generate apparent activation energies; e the values within parenthesis noted
with “L” was obtained by stepwise addition Et3N with amount of 0.25 equiv. per 60 min; the one noted with “H”,
f
obtained by stepwise addition Et3N with amount of 0.5 equiv. at 50 % conversion; the values within parenthesis for
SBET and Dp corresponding to the carriers.

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Table 2 Calculated values of adsorption energies for absorbents on Pd(111) and Pd(211) and Pd-Cl distances
Pd (111)
Pd-Cl distances / Å
Pd (211)
Pd-Cl distances / Å
Absorbent
Ead / eV
Ead / eV
σ
π
σ
π
3.34
---
---
3.299
σ
π
σ
π
4-CP
4-t-BCB
Et3N
0.07
0.11
0.48
0.09
0.72
---
---
3.09
2.59
---
0.24
0.15
0.84
0.22
0.89
---
---
2.60
2.92
---
3.99
---
---
CB
0.84
3.09
0.86
2.59
4.04

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Pd
Pd
Pd
Pd/MSC
Pd/MS
10
Pd/MC
10 20 30 40 50 60 70 80
2 / o
Pd/MC
Pd/MS
11
11
20
20
X6
X6
Pd/MSC
MC
MS
MSC
1
2
o 3
4
5
2 /
Fig. 1. XRD patterns of supports and Pd catalysts.

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Fig. 2. TEM images of Pd/MSC (A), Pd/MC (C) and Pd/MS (E), and HRTEM images of Pd/MSC (B),
Pd/MC (D) and Pd/MS (F).

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A
B
D
C
E
F
G
H
Fig. 3. Calculated structures of 4-CP on Pd(111) ((A) top and (B) side views of σ adsorption, (C) top and (D)
side views of π adsorption) and Pd(211) ((E) top and (F) side views of σ adsorption, (G) top and (H) side
views of π adsorption) surfaces; the H atoms are purple, the C atoms are grey, the O atoms are red and the
Cl atoms are green.

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Fig. 4. The effect of the manners of Et3N addition on the HDC of 4-CP at 258 K. Reaction condition: 5
mmol 4-CP; methanol 50 ml; Pd catalysts 0.1 g.

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Graphical abstract

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Research highlights
► Highly defective Pd catalyst/silica-carbon nanocomposites was prepared by impregnation ►
The catalyst exhibits high activity for hydrodechlorination of chlorophenols at 258 K. ► HDC of
chlorophenols at low temperature mainly occurs on Pd defects. ► H species plays a direct role
when the reactant adsorbs on Pd via the formation of σ-complex.