Synthesis of Cu2(OH)3Cl as facile and effective Fenton catalysts for mineralizing aromatic contaminants: Combination of σ-Cu-ligand and self-redox property

Article abstract of DOI:10.1016/j.apcata.2021.118055

Cu₂(OH)₃Cl was synthesized as catalysts for mineralizing aromatic organics via Fenton reactions. Both Cu⁺ and Cu²⁺ are present in γ-Cu₂(OH)₃Cl, α-Cu₂(OH)₃Cl and CuOHCl. Among them, γ-Cu₂(OH)₃Cl with the highest Cu⁺ percentage is the most active. Under relatively mild conditions, 88.7 % phenol, more than 85 % bisphenol A, benzoic acid, 2-chlorophenol and salicylic acid and around 79 % aniline are mineralized at their ambient pH. Compared to the reference catalysts CuSO₄, CuO and Cu₂O, γ-Cu₂(OH)₃Cl shows better mineralization ability, HO^[rad] production and H₂O₂ utilization efficiency. Cu²⁺ can form σ-Cu-ligand complexes with phenolic hydroxyls in contaminants. Such complexes are readily attacked by H₂O₂, causing the electron transfer from aromatic rings to Cu²⁺ and the formation of HO^[rad]. Moreover, γ-Cu₂(OH)₃Cl shows self-redox property when induced by H₂O₂, initiating the electron transfer from H₂O₂ to Cu²⁺ and thus reducing Cu²⁺ into Cu⁺. These effects could accelerate Cu⁺ regeneration and improve H₂O₂ utilization efficiency.

Full text of DOI:10.1016/j.apcata.2021.118055

Applied Catalysis A, General 614 (2021) 118055
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Applied Catalysis A, General
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Synthesis of Cu2(OH)3Cl as facile and effective Fenton catalysts for
mineralizing aromatic contaminants: Combination of
self-redox property
σ-Cu-ligand and
Hao Wang*, Zeng Zhang, Hongyou Yin, Yan Wu
School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu, Sichuan Province, 610500, China
A R T I C L E I N F O
A B S T R A C T
Cu2(OH)3Cl was synthesized as catalysts for mineralizing aromatic organics via Fenton reactions. Both Cu+ and
Keywords:
Cu2+ are present in γ-Cu2(OH)3Cl,
α
-Cu2(OH)3Cl and CuOHCl. Among them, γ-Cu2(OH)3Cl with the highest Cu+
Copper hydroxychlorides
Fenton catalysts
percentage is the most active. Under relatively mild conditions, 88.7 % phenol, more than 85 % bisphenol A,
benzoic acid, 2-chlorophenol and salicylic acid and around 79 % aniline are mineralized at their ambient pH.
σ
-Cu-ligand
Self-redox
Compared to the reference catalysts CuSO4, CuO and Cu2O, γ-Cu2(OH)3Cl shows better mineralization ability,
•
HO production and H2O2 utilization efficiency. Cu2+ can form
σ
-Cu-ligand complexes with phenolic hydroxyls in
contaminants. Such complexes are readily attacked by H2O2, causing the electron transfer from aromatic rings to
•
Cu2+ and the formation of HO . Moreover, γ-Cu2(OH)3Cl shows self-redox property when induced by H2O2,
initiating the electron transfer from H2O2 to Cu2+ and thus reducing Cu2+ into Cu+. These effects could accel-
erate Cu+ regeneration and improve H2O2 utilization efficiency.
1. Introduction
condition [2]. Therefore, Cu-based catalysts can degrade organics in
neutral or near-neutral solutions, removing the complex
Aromatic-containing organics are present in a great variety of in-
dustrial wastewaters, particularly in oil industry. Most of them are
hardly to be completely eliminated by traditional water treatment
techniques and more toxic by-products may be produced during the
treatment process [1]. Advanced oxidation processes (AOPs) have
shown great potential in degrading refractory organics in water. Among
all AOPs, Fenton reactions have been widely used, in which the powerful
pre-acidification and post treatment of effluents. Additionally, Cu2+
complexes with organic degradation intermediates are easily decom-
•
posed by HO [2]. Lyu et al. [1,3,4] reported that Cu2+ in mesoporous
Cu-doped metal oxides or CuTiAl dandelion-like silica nanospheres
could form σ-Cu-ligands with phenolic hydroxyls in contaminants. These
ligands could interact with H2O2 to enhance the reduction of Cu2+ into
•
Cu+ and generate more HO . In recent five years, Cu-based heteroge-
•
oxidant hydroxyl radical (HO ) is generated through the decomposition
neous Fenton catalysts have attracted widespread attentions, which is
well reviewed by Li et al. most recently [5]. A lot of Cu-containing ox-
ides, hydroxides, composites and Cu-supported catalysts were prepared
for degrading organics via Fenton reactions, such as CuO [6], Cu2O [7],
CuFe2O4 [8], γ-Cu-Al2O3-Bi12O15Cl6 [9], CuNiFe layered double hy-
droxide (LDH) [10], CuMgFe LDH [11], CuNiSn LDH [12], Cu/TUD-1
[13] and Cu2O-Cu/C [14]. However, the preparation of Cu-based cata-
lysts generally requires organic templates [13], multiple metal doping
[9], porous support, thermal treatment at high temperatures [8] and/or
complex procedures [15]. Moreover, low valent metal active sites are
normally regenerated via the reaction between H2O2 and high valent
•
of H2O2 catalyzed by Fe2+ ion. HO can completely degrade or miner-
alize organic pollutants into non-toxic products like CO2, H2O and
inorganic salts. Heterogeneous Fenton catalysts can offer the advantages
of easy separation from effluents and reuse ability. In the last years there
have been increased attentions on developing heterogeneous catalysts
for Fenton reactions.
Cu can react with H2O2 more readily and rapidly when compared to
the conventional Fe Fenton catalysts. The activation of H2O2 by Cu+ to
•
generate HO (Eq. (1)) is 2–3 order of magnitude faster than that by Fe2+
[2]. Moreover, Cu-based Fenton system can work over a broader pH
range, unlike Fe-based Fenton system working only in the more acidic
•
metal (Eq. (2)), whose rate constant is much lower than that of the HO
* Corresponding author at: School of Chemistry and Chemical Engineering, Southwest Petroleum University, Xindu Avenue 8#, Xindu District, Chengdu 610500,
Sichuan Province, China.
E-mail address: wanghaoswpu@hotmail.com (H. Wang).
https://doi.org/10.1016/j.apcata.2021.118055
Received 4 December 2020; Received in revised form 13 February 2021; Accepted 14 February 2021
Available online 21 February 2021
0926-860X/© 2021 Elsevier B.V. All rights reserved.

H. Wang et al.
Applied Catalysis A, General 614 (2021) 118055
generation reaction (Eq. (1)), which is the rate-limiting step of Fenton
reactions [16] and hence causes the insufficient efficiency of Fenton
catalysts for practical applications. Therefore, the development of a
facile, low-cost and mild method for synthesizing effective Cu-based
Fenton catalysts is still of considerable importance [2].
temperature.
Thermogravimetric (TG) and differential scanning calorimetry (DSC)
were carried out on a STA-449F3 thermal analyzer (Netzsch, Germany).
About 10 mg sample was loaded in an alumina crucible and heated at a
rate of 10 ◦C⋅min–1 from 40 to 750 ◦C under an argon flow of 60
mL⋅min–1
.
Cu+ + H2O2→Cu2+ + HO • + OH– k1 = 1.0 × 104M-1s-1
(1)
Scanning electron microscope (SEM) images were obtained on an
EVO MA-15 microscope (Zeiss, Germany). Prior to measurement, the
sample was dispersed in cyclohexane by ultrasound and then dropped on
the slide followed by removing the cyclohexane solvent by dry air.
Transmission electron microscopy (TEM) images were taken on a
Libra 200 (Zeiss, Germany) electron microscope operated at 200 kV.
Prior to measurement, the sample was dispersed in an ethanol solution
by ultrasound for 15 min and then dropped on a copper grid coated with
a carbon film.
Cu2+ + H2O2→Cu+ + HOO • +H+ k2 = 4.6 × 102M-1s-1
(2)
Copper hydroxychloride (Cu2(OH)3Cl) is a tribasic copper salt with
layered structure composed by edge-sharing HO–Cu–Cl octahedra [17,
18]. Cu2(OH)3Cl can be prepared by the precipitation between CuCl2
and alkaline. It generally occurs in several polymorphs including ata-
camite (α-Cu2(OH)3Cl), paratacamite (γ-Cu2(OH)3Cl) and bollackite
[19,20]. Among them, botallackite is the most unstable and can be
readily transformed into paratacamite and/or atacamite [20]. The
sheets of Cu2(OH)3Cl is made of two distinct octahedra [18].
Electron paramagnetic resonance (EPR) measurements were per-
formed on an EMX Plus spectrometer (Bruker, Germany), which was
operated with modulation amplitude of 1 G, modulation frequency at
100 kHz and microwave power of 15.89 mW. Before each measurement,
0.12 mL H2O2 (30 %, w/w) and 0.02 g catalyst were added into 20 mL
aqueous solution containing 100 mg⋅L–1 phenol. After stirring for 5 min,
α
-Cu2(OH)3Cl consists of Cu(OH)4Cl2 and Cu(OH)5Cl octahedra with a
ratio of 1:1 while γ-Cu2(OH)3Cl consists of Cu(OH)4Cl2 and Cu(OH)6
octahedra with a ratio of 3:1 [20]. Cu2(OH)3Cl polymorphs have been
the subject of extensive studies due to their interesting magnetic prop-
erties [18]. In addition, Cu2(OH)3Cl can also serve as catalyst for syn-
thesizing diethyl carbonate [21,22], nanomaterial for H2 storage [23],
adsorbent for removing methylene blue (MLB) from aqueous solution
[17], and precursor for thermal decomposition to synthesize nano-
structured CuO [24,25]. However, there are few reports on its applica-
tion as Fenton catalysts for degrading organic contaminants. Herein, a
series of Cu2(OH)3Cl were conveniently synthesized and used as het-
erogeneous catalysts to mineralize phenol and other aromatic contam-
inants which are typical organics in wastewater from oil industry.
Moreover, the catalytic mechanism was investigated.
1 mL aliquots filtered by a Nafin membrane of 0.22 μm was collected and
immediately injected into 10
μ
L aqueous solution containing 250
•
mmol⋅L–1 5,5-dimethyl-1-pyrroline N-oxide (DMPO) to form DMPO-HO
adduct. The mixture was then transferred into a capillary to carry out
EPR detection. A blank experiment was performed similar to the above
with the absence of catalyst. All experiments were performed under
room temperature.
X-ray photoelectron spectra (XPS) were performed on an Escalab
250Xi spectrometer (Thermo Fisher, USA) using Al Kα radiation. The C
1s peak at 284.8 eV was used as the internal standard to compensate for
the sample charging. The corresponding spectra were fitted by using the
XPSPEAK software.
2. Experimental
Raman spectra were obtained on an In Via Raman Microscope system
(Renishaw, UK) with an excitation line of 514.5 nm and a power of 0.17
mW.
2.1. Samples preparation
The resistivities of samples were measured on a ST-2258C multi-
function digital four-probe tester (Jingge, China). Before measurement,
the samples were pressed into pallets with around 0.5 mm thickness.
The content of copper ion in solution was determined by inductively
coupled plasma atomic emission spectroscopy (ICP-AES) on an Optima
7300 V spectrometer (PerkinElmer, USA).
Cu2(OH)3Cl was synthesized by a hydrothermal crystallization
method. First, a NaOH (0.75 mol⋅L–1) solution and a CuCl2 solution (0.5
mol⋅L–1) were added dropwise and simultaneously into a three-neck
flask under vigorous stirring at room temperature. Second, the suspen-
sion was further stirred for 30 min and then transferred into an auto-
clave followed by aging at various temperatures for a certain time.
Third, the solid products were collected by filtration, washing with
2.3. Catalytic activity test
water until neutral pH and drying at 80 ◦C for 8 h. Pure
α-Cu2(OH)3Cl
was synthesized via a precipitation method [26]. CaCO3 (0.005 mol)
was added into 200 mL CuCl2 (0.25 mol⋅L–1) solution under vigorous
stirring at room temperature for 24 h. The solid product was collected by
the same filtration, washing and drying procedures as above. CuOHCl
was prepared by a solvent-free method [27]. CuCl2 (0.01 mol) and CuO
(0.005 mol) powders were mechanically mixed in an agate mortar under
ambient conditions. Subsequently, the mixture was transferred to in a
muffle furnace and calcined at 250 ◦C for 24 h. All chemicals are AR
grade and purchased from Kelong Co. Ltd. (China). They are used as
received.
The degradation of phenol and other aromatic contaminants via
Fenton reactions was used to test the activities of catalysts. A certain
amount of catalyst was added into 100 mL phenol solution with a con-
centration of 100 mg⋅L–1 which is the common level in refinery waste-
water [28]. The suspension was stirred at a given temperature for a
given time. After that, the liquid was collected by filtration. This
investigation focuses on mineralization ability of catalysts, therefore,
the total organic carbon (TOC) values of feeds and products were
measured on a TOC-VCPH analyzer (Shimadzu, Japan). All TOC values
were measured three times to obtain an average value. For testing the
reuse ability, the catalysts after reaction were filtrated, washed with
water for several times and then dried at 100 ◦C under vacuum for 6 h.
To determine the active species during the Fenton reactions,
quenching experiments were performed by using tertbutyl alcohol
2.2. Characterization techniques
X-ray powder diffraction (XRD) patterns were collected on a X’PERT
Pro diffractometer (PANalytical, Netherlands) equipped with an X’Cel-
•
•
(TBA) and p-benzoquinone (BQ) as scavenger for HO and HOO ,
respectively [29]. Before adding H2O2, TBA or BQ was added into the
reaction system. Taking into account the TOC value affected by scav-
enger, high-performance liquid chromatography (HPLC, Shimadzu,
Japan) was used to analyze the concentration of phenol and the detailed
measurement procedure was reported previously [12].
erator detector, using Cu Kα and operating at 40 kV and 40 mA. The
scanning was performed in the 2θ range from 5 to 70◦ with a step of
0.02◦ and a counting time of 12 s each step.
Fourier transform infrared (FT-IR) spectra were acquired by a
WQ520 spectrophotometer (Beifen, China) in the range of 4000 to 400
cm–1 with a resolution of 2 cm–1, using KBr disk method at room
•
The production of HO formed during Fenton reactions was
2

H. Wang et al.
Applied Catalysis A, General 614 (2021) 118055
determined by fluorescence (FL) method using benzoic acid (BA) as
groups, corresponding to two types of ꢀ OH environments with different
atomic distances (dO–H) and hydrogen bond angles (θO–H⋅⋅⋅Cl) [17]. In
addition, the bands at 1633, 1461 and 1381 cm–1 may be due to the
residual water in the KBr discs and/or the deformation modes of
dangling ꢀ OH groups on the surface [35]. The bands at 989, 920 and
836 cm–1 can be assigned to the deforming modes of Cu–O–H in copper
hydroxychlorides [17,35]. TG and DSC curves of this sample are shown
in Fig. 3b. The decomposition of γ-Cu2(OH)3Cl consists of two mass loss
steps. One below 380 ◦C with an endothermal peak centered at 290 ◦C is
attributed to the dehydration. The other between 400 and 700 ◦C with
an endothermal peak centered at 456 ◦C can be attributed to the loss of
halogen and the formation of CuO [25].
•
probe, because BA can readily react with HO to form highly fluorescent
hydrobenzoic acids [30]. BA instead of phenol was added into the re-
action system containing H2O2 and catalysts. To avoid the deep degra-
dation of BA, the molar ratio of BA to H2O2 is set at 4.6 which is around
70 times higher than the stoichiometric molar ratio (0.067) for the
completely degradation of BA [31]. 1 mL reaction solution was collected
at various time intervals by a syringe with a 0.22 μm filter for removing
catalysts [32]. Its fluorescent intensity was monitored at 407 nm with
excitation at 300 nm on a LS-55 luminescence spectrometer (Perki-
nElmer, USA) [30]. The concentration of H2O2 in the reaction solution
was determined by the colorimetric DPD-POD method with a UV-1800
ultraviolet-visible spectrometer (Shimadzu, Japan) [33]. All chemicals
are used as received and the water is deionized and CO2 free.
SEM micrographs of γ-Cu2(OH)3Cl are shown in Fig. 4. It shows the
sample is constructed by stacked sheets [24]. TEM micrographs (Fig. 5)
also exhibit typical aggregated platelets morphology. MOHX materials,
where M is a metal such as Cu, Zn or Al, OH is a hydroxyl group, and X is
a halogen or oxygen, consist of thin sheets exhibiting strong intralayer
covalent and/or ionic bonding and weak interlayer hydrogen bonding,
therefore, they are considered as an H-bonded 2D layered material [35].
Cudennec et al. [36] indicated that divalent metal hydroxychlorides
have structures related to brucite type. The crystal structure of
γ-Cu2(OH)3Cl is shown in Fig. S1.
3. Results and discussion
3.1. Synthesis and characterization of copper hydroxychlorides
A series of copper hydroxychlorides were synthesized with various
molar ratios of CuCl2 to NaOH (Cu2+: OH–) under crystallization tem-
perature of 80 ◦C and time of 12 h. Their XRD patterns are shown in
Fig. 1. At Cu2+: OH– of 1:0.1, 1:0.5 and 1:1, both γ-Cu2(OH)3Cl (PDF 19-
Besides γ-Cu2(OH)3Cl and
α-Cu2(OH)3Cl, CuOHCl in rhombohedral
0389) and
α-Cu2(OH)3Cl (PDF 25-0269) are formed [20]. Herein, a
is also regarded as a crystal habit of copper hydroxychlorides [21,36].
strong reflection at 16.1◦ and two weak reflections at 17.6◦ and 31.6◦ are
Herein, γ-Cu2(OH)3Cl, α-Cu2(OH)3Cl and CuOHCl were synthesized and
assigned to
flections of
α
-Cu2(OH)3Cl. With the further addition of OHꢀ , the re-
the XRD patterns are shown in Fig. 6. Their reflections are well matched
α
-Cu2(OH)3Cl vanish and γ-Cu2(OH)3Cl dominates the
with JCPDS reference files of γ-Cu2(OH)3Cl (PDF 19-0389),
samples. It has been reported that the formation of
α-Cu2(OH)3Cl and
α-Cu2(OH)3Cl (PDF 25-0269) and CuOHCl (PDF 23-1063), respectively.
γ-Cu2(OH)3Cl phases is closely related to the relative concentration of
OH– and Cu2+ [20]. The more basic environment accelerates the
-Cu2(OH)3Cl into γ-Cu2(OH)3Cl [26]. When Cu2+
:
3.2. Phenol mineralization activity of copper hydroxychlorides
recrystallization of
α
OH– increases to 1:1.8, a weak reflection at 35.6◦ assigned to CuO (PDF
48-1548) is observed, due to the excess alkaline [34]. Pure γ-Cu2(OH)3Cl
can be obtained at Cu2+: OH– of 1:1.5, at which the effects of crystalli-
zation temperature and time on the structure of γ-Cu2(OH)3Cl were
investigated. The XRD patterns are shown in Fig. 2a and b. No impurities
are observed for all samples but the crystallinity varies significantly with
the change in temperature and time. The most ordered structure is
achieved after crystallization at 80 ◦C for 18 h. This sample was chosen
for the following characterization and activity test.
Copper hydroxychlorides with different crystal structures
(γ-Cu2(OH)3Cl, α-Cu2(OH)3Cl and CuOHCl) are used as Fenton catalysts
to mineralize phenol. Their TOC removals are listed in Table 1. Among
them, γ-Cu2(OH)3Cl shows the highest TOC removal. Moreover, in order
to investigate the effect of crystallinity degree of γ-Cu2(OH)3Cl on its
activity, samples with various crystallinity degree achieved by changing
the crystallization time were chosen as catalysts. The relative crystal-
linity degree was roughly estimated by the ratio of the peak intensity of
(021) plane to that of γ-Cu2(OH)3Cl crystallized for 18 h which has the
highest crystallinity among samples [37]. Their crystallinity degrees and
TOC removals are listed in Table 1. A similar trend in TOC removals and
The FTIR spectrum of γ-Cu2(OH)3Cl is given in Fig. 3a. The bands
around 3448 and 3357 cm–1 are attributed to the stretching of hydroxyl
Fig. 1. XRD patterns of copper hydroxychlorides synthesized with various Cu2+:OHꢀ molar ratios.
3

H. Wang et al.
Applied Catalysis A, General 614 (2021) 118055
Fig. 2. XRD patterns of γ-Cu2(OH)3Cl synthesized with various crystallization time (a) and temperatures (b).
Fig. 3. FTIR spectrum (a) and TG and DSC curves (b) of γ-Cu2(OH)3Cl.
Fig. 4. The representative SEM micrographs of γ-Cu2(OH)3Cl.
crystallinity degrees was observed. The catalyst with the highest crys-
tallinity exhibits the most active activity, because the well crystallized
catalyst has a relatively compact and arranged structure and thus could
facilitate faster and easier electron transfer during Fenton reactions
[38].
The reaction conditions of phenol mineralization over γ-Cu2(OH)3Cl
were further optimized. The effects of catalysts dosage, H2O2 dosage,
initial pH, reaction temperature and time on TOC removal were listed in
4

H. Wang et al.
Applied Catalysis A, General 614 (2021) 118055
Fig. 5. The representative TEM images of γ-Cu2(OH)3Cl.
constant at H2O2 dosage above 53 mmol⋅L–1. It is found that the TOC
removal reaches 78.4 % even at the H2O2 dosage of 18 mmol⋅L–1 which
is close to the theoretical H2O2 dosage (around 15 mmol⋅L–1) for
completely mineralizing phenol. Fenton reactions are normally sensitive
to the pH of solution and are preferable at the lower pH value, because
•
the formation of HO is facilitated in more acidic solution [2]. Although
the TOC removal decreases with the increase of pH value, this catalyst
still can mineralize 86.8 % phenol under the ambient pH of phenol feed
(6.4). Moreover, around 86 % TOC removal can be retained even at pH
of 8, indicating that increasing the pH to alkaline values seems little
inhibition on the activity of catalyst. This suggests such catalyst can
work over a broader pH range. It is found that extending reaction time at
elevated temperature promotes the mineralization of phenol. For prac-
tical applications, the optimal reaction conditions are catalyst dosage
0.4 g⋅L–1, H2O2 dosage 53 mmol⋅L–1, initial pH 6.4, reaction temperature
50 ◦C and time 1.5 h, under which 88.7 % phenol can be mineralized. All
catalysts mentioned below were evaluated under these conditions
except where noted.
•
For the Fenton reaction over γ-Cu2(OH)3Cl, HO is suggested to be the
primary active species. To confirm that, TBA and BQ were used as
•
•
scavenger of HO and HOO , respectively. The reactions were performed
under the optimal conditions as mentioned above. The removals of
phenol before and after adding different amounts of scavenger are
shown in Fig. S2. TBA significantly inhibits the degradation of phenol, as
confirmed by the fact that the removal of phenol decreases to 6.6 % at
the molar ratio of TBA to H2O2 of 1:2. On the contrary, the addition of
BQ exhibits little effect on the removal of phenol. This demonstrates that
Fig. 6. XRD patterns of γ-Cu2(OH)3Cl, α-Cu2(OH)3Cl and CuOHCl.
Table 2. Only 2.5 % phenol is mineralized without catalysts. The
introduction of γ-Cu2(OH)3Cl significantly improves the TOC removal to
above 80 %. At the catalyst dosage of 0.4 g⋅L–1, the TOC removal reaches
the maximum. Further addition of catalyst leads to the decline of ac-
•
•
tivity, because the excessive metal species could scavenge HO radicals
HO is the main active specie responsible for removing phenol.
[39]. When H2O2 is absent, nearly no mineralization is observed. The
increase in H2O2 amount can promote the TOC removal which is nearly
Table 1
TOC removals of copper hydroxychlorides with various structures and crystallinities. Reaction conditions: H2O2 dosage 70 mmol⋅Lꢀ 1, catalyst dosage 0.4 g⋅Lꢀ 1, 50 ◦C,
1 h and ambient pH 6.4, 100 mg⋅Lꢀ 1 phenol solution.
Catalysts
TOC removal (%)
Catalysts
Crystallization time (h)
Relative crystallization degree (%)
TOC removal (%)
γ-Cu2(OH)3Cl
87.1
85.3
82.5
/
6
71.4
81.3
100
85.9
86.7
87.1
84.6
α
-Cu2(OH)3Cl
12
18
24
γ-Cu2(OH)3Cl
CuOHCl
/
48.1
5

H. Wang et al.
Applied Catalysis A, General 614 (2021) 118055
Table 2
TOC removals over γ-Cu2(OH)3Cl under various reaction conditions.
Catalyst
dosage
H2O2 dosage
Initial
pH
Reaction
temperature
(oC)
Reaction
time (h)
TOC
(mmol⋅Lꢀ 1
)
removal
(%)
(g⋅Lꢀ 1
)
0
70
70
70
70
70
0
6.4
6.4
6.4
6.4
6.4
6.4
6.4
6.4
6.4
6.4
4
50
50
50
50
50
50
50
50
50
50
50
50
50
50
20
30
40
60
50
50
50
1
2.5
0.2
0.4
0.6
0.8
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
1
82.8
87.1
84.1
83.8
0
1
1
1
1
18
35
53
88
53
53
53
53
53
53
53
53
53
53
53
1
78.4
82.2
86.8
87.2
89.2
88.5
85.9
85.8
21.1
64.0
83.7
89.6
85.5
88.7
89.2
1
1
1
1
5
1
7
1
8
1
6.4
6.4
6.4
6.4
6.4
6.4
6.4
1
1
1
Fig. 7. The reaction time dependence of the FL emission intensity in the sys-
tems of (a) γ-Cu2(OH)3Cl-H2O2-BA, (b) CuSO4-H2O2-BA, (c) CuO-H2O2-BA, (d)
Cu2O-H2O2-BA, and (e) H2O2-BA. Reaction conditions: BA concentration 500
mg⋅Lꢀ 1, H2O2 dosage 0.88 mmol⋅Lꢀ 1, catalyst dosage 0.4 g⋅Lꢀ 1 and ambient
pH 3.3.
1
0.5
1.5
2
3.3. Comparison with reference catalysts
Three commonly used Cu-containing Fenton catalysts were chosen as
references, one is a homogeneous catalyst CuSO4, the others are het-
erogeneous catalysts CuO and Cu2O. They are purchased from Kelong
Co. Ltd. (China).The XRD patterns of CuO and Cu2O are shown in
Fig. S3. Their phenol mineralization activities are listed in Table 3. All
reference catalysts show poorer mineralization ability than
•
γ-Cu2(OH)3Cl. HO is essential to Fenton catalytic activity. Therefore,
•
the production of HO during the reaction was determined by FL method
using BA as probe. The FL intensity varies directly with the amount of
•
HO [29]. Fig. 7 shows the FL intensity variation with the reaction time
in aqueous solution over γ-Cu2(OH)3Cl and reference catalysts. The FL
intensity is very low with the absence of catalyst while it increases
considerably after introducing catalysts, because catalysts accelerate the
•
formation of HO . Moreover, the FL intensity at each time interval fol-
lows the order of γ-Cu2(OH)3Cl>CuSO4>CuO>Cu2O, in agreement with
that of TOC removal. The steepest slope of plot over γ-Cu2(OH)3Cl in-
•
dicates that it can produce HO more rapidly than the reference catalysts
•
Fig. 8. DMPO trapped EPR spectra in the system of (a) γ-Cu2(OH)3Cl-H2O2-
phenol, (b) CuSO4-H2O2-phenol, (c) CuO-H2O2-phenol, (d) Cu2O-H2O2-phenol
and (e) H2O2-phenol.
[40]. In general, the rate of HO generation over solid Fenton catalysts is
still lower than that for catalyst in aqueous solution by 2–3 magnitudes
•
[41]. Herein, γ-Cu2(OH)3Cl exhibits higher HO generation rate than
CuSO4.
•
concentration generated by each catalyst [31]. EPR results are consis-
DMPO trapped EPR was used to further detect the production of HO .
•
•
tent with the above BA probe results, suggesting that more HO can be
The EPR spectra of HO generated from γ-Cu2(OH)3Cl and reference
generated over γ-Cu2(OH)3Cl.
catalysts are shown in Fig. 8. For the blank experiment without catalyst,
no EPR signal is observed. The introduction of catalysts leads to the
During Fenton reactions, H2O2 could be converted into O2 or less
active radicals, which reduces the utilization efficiency of H2O2 for
contaminants removal [42]. Therefore, the utilization efficiency of H2O2
is defined as the ratio of the stoichiometric H2O2 consumption
(Δ[H2O2]S) for the mineralization of phenol to the actual H2O2 con-
sumption (Δ[H2O2]A) in Fenton reactions [1]. Based on the TOC
removal, the amounts of the mineralized contaminants can be obtained,
and thus the value of (Δ[H2O2]S) was calculated. In addition, the actual
H2O2 consumption (Δ[H2O2]A) was obtained by the concentration of
H2O2 which was determined using a DPD-POD method. Herein, the
H2O2 utilization efficiencies over catalysts are calculated and listed in
Table 3. It is in the order of γ-Cu2(OH)3Cl > CuO > Cu2O > CuSO4. More
than 80 % H2O2 utilization efficiency is achieved over γ-Cu2(OH)3Cl
even at the ending of reaction, which is much higher than those of the
reference catalysts. However, the H2O2 utilization efficiency of the most
reported Cu-based systems was often in the range of 10–60 % [1].
•
appearance of 4-fold characteristic peaks of DMPO-HO adduct with an
intensity ratio of 1:2:2:1 [29]. Their intensities follow the order of
γ-Cu2(OH)3Cl>CuSO4>CuO>Cu2O. Due to the identical experiment
•
conditions, the intensity of EPR signal is dependent on the HO
Table 3
TOC removal and H2O2 utilization efficiency over catalysts. Reaction conditions:
H2O2 dosage 53 mmol⋅Lꢀ 1, catalyst dosage 0.4 g⋅L–1, 50 ◦C, 1.5 h and pH 6.4,
100 mg⋅Lꢀ 1 phenol solution.
Catalysts
TOC removal (%)
Utilization efficiency of H2O2 (%)
γ-Cu2(OH)3Cl
CuSO4
88.7
85.4
82.5
79.7
81.2
55.3
67.7
62.6
CuO
Cu2O
6

H. Wang et al.
Applied Catalysis A, General 614 (2021) 118055
Notably, the homogeneous catalyst CuSO4 shows the poorer H2O2 uti-
lization efficiency than three heterogeneous catalysts. The inefficient
decomposition of H2O2 into H2O and O2 predominates the homogeneous
Fenton reaction due to the high H2O2 concentration which exceeds the
organics concentration and thus accelerates the inefficient decomposi-
tion of H2O2, whereas the adsorption of organic molecules on the surface
of solid catalysts leads to a significant increase in the local organics
concentration in the reaction layers, allowing better H2O2 utilization
efficiency in the heterogeneous Fenton reaction [43].
3.4. Stability of γ-Cu2(OH)3Cl
γ-Cu2(OH)3Cl catalyst was recovered and reused to mineralize
phenol for several times. The TOC removals are shown in Fig. 9. After 7
recycle runs, the catalyst keeps a TOC removal around 87.7 %. The XRD
pattern of the recovered catalyst (Fig. S4) shows no noticeable change in
phase composition with respect to the fresh catalyst. In addition, the Cu
ion content in solution after reaction was detected by ICP-AES. It is
found that Cu ions concentration released in solution is 1.8 mg⋅Lꢀ 1 after
Fig. 10. TOC removals of other aromatic contaminants over γ-Cu2(OH)3Cl
catalyst.
7 runs. The leaching of Cu is lower than the EU directives (<2 mg⋅L–1
)
but higher than the USA regulations (<1.3 mg⋅L–1) [3]. It indicates that
metal leaching should be further controlled. The amount of leached
metal ions is dependent on the contact time between the catalyst and the
reactive solution [44], though γ-Cu2(OH)3Cl is relatively stable in acidic
range [45]. The inhibition of metal leaching may be achieved by
bonding γ-Cu2(OH)3Cl with porous supports [1,3]. The corresponding
research is in progress.
Reaction conditions: Organics concentration 100 mg⋅Lꢀ 1, H2O2 dosage 53
mmol⋅Lꢀ 1, catalyst dosage 0.4 g⋅Lꢀ 1, 50 ◦C, 1.5 h and ambient pH.
preferably under acidic solution [46,48]. These results further demon-
strate γ-Cu2(OH)3Cl as effective Fenton catalysts for mineralizing con-
taminants with aromatic ring. Moreover, MLB and amoxicillin (AM)
which are seldom detected in wastewater from oil industry but
frequently occurred in urban wastewater are also used to test the
mineralization ability of γ-Cu2(OH)3Cl. The concentrations of MLB and
AM are 100 mg⋅L–1 and the reactions were performed under the same
conditions as those mentioned in Fig. 10. Noted that the ambient pH
value is 6.4 for MLB and 5.9 for AM. It is found that the TOC removals of
MLB and AM are 35.6 % and 63.4 %, respectively. The relatively lower
mineralization degrees of MLB and AM are attributed to the strong
3.5. Mineralization of other aromatic organics
Beside phenol, bisphenol A (BPA), benzoic acid (BA), 2-chlorophenol
(2-CP), salicylic acid (SA) and aniline (AN) are commonly occurring and
hardly degradable aromatic contaminants in wastewater from oil in-
dustry. They are also used to test the activity of γ-Cu2(OH)3Cl. It is noted
that the ambient pH values of the feeds are 6.5 (BPA), 4.5 (BA), 5.7 (2-
CP), 5.2 (SA) and 7.2 (AN), respectively. The results are shown in
Fig. 10. More than 85 % TOC removals can be achieved for BPA, BA, 2-
CP and SA, whereas the TOC removal of AN is 78.9 %, which might be
attributed to the higher basicity of AN. It is reported that AN degrada-
tion via Fenton reactions prefers under pH of 2–5 and the degradation
can be significantly depressed at pH above 6 [46]. Xue et al. [47] found
the rate constant of AN degradation over zero-valent iron Fenton cata-
lyst decreased by 100 times as the pH increased from 2 to 5 and no
obvious degradation was observed at pH of 7. This is probably because
the degradation of AN firstly undergoes the loss of NH+4 which is
•
scavenge of HO by their intermediates [31].
3.6. Possible catalytic mechanism of γ-Cu2(OH)3Cl
On the basis of above results, the possible catalytic mechanism of
γ-Cu2(OH)3Cl is discussed.
First, Cu+ species in catalysts play an important role on Fenton ac-
tivity. Three copper hydroxychlorides with different crystal structures
(γ-Cu2(OH)3Cl, α-Cu2(OH)3Cl and CuOHCl) were characterized by XPS.
Their Cu 2p3/2 XPS spectra are shown in Fig. 11 and the Cu+ percentages
Fig. 9. Variation of TOC removal in recycle runs over γ-Cu2(OH)3Cl catalyst.
Fig. 11. Cu 2p3/2 spectra of γ-Cu2(OH)3Cl, α-Cu2(OH)3Cl and CuOHCl.
7

H. Wang et al.
Applied Catalysis A, General 614 (2021) 118055
calculated by XPS deconvolution are listed in Table 4. Each spectrum
can be deconvolved into four peaks. The peaks around 933.9 and 935.5
eV can be attributed to Cu 2p3/2 of Cu+ and Cu2+, respectively [49]. The
other two peaks around 941.4 and 943.7 eV are attributed to the
shake-up satellite lines of Cu2+ [50]. It indicates that both Cu+ and Cu2+
are present on copper hydroxychlorides, due to the weak stability of the
lattices
[34].
Cu+
percentage
is
in
the
order
of
γ-Cu2(OH)3Cl>
α
-Cu2(OH)3Cl>CuOHCl, in consistent with the order of
•
catalytic activity. Cu+ ions can react with H2O2 to generate HO and thus
dominate the mineralization [9]. The DMPO trapped EPR spectrum in
the system of γ-Cu2(OH)3Cl and H2O2 (Fig. S5) clearly confirms the
•
generation of HO .
(3)
Fig. 12. Raman spectra of (a) fresh γ-Cu2(OH)3Cl and (b) phenol-
absorbed γ-Cu2(OH)3Cl.
(4)
Cu+nCu2+2ꢀ n(OH)3Cl + H2O2→Cu+m Cu2+2-m (OH)3Cl + HOO • +H+,
spectra of phenol and γ-Cu2(OH)3Cl adsorbing phenol are shown in
Fig. 13. After interacting with γ-Cu2(OH)3Cl, the binding energy of O 1s
shifts from 532.0 eV to 532.4 eV, which is caused by the formation of
Cu–O–C between Cu and phenolic hydroxyls [53]. Furthermore, after
Fenton reaction for 5 min and 90 min, the catalysts were collected by
filtration, washing and drying and then characterized by XPS. Their XPS
spectra are given in Fig. 14 and the Cu+ percentages are listed in Table 4.
The Cu+ percentage increases from 36.7 % for the fresh catalyst to 45.5
% after reaction for 5 min and it recovers to 36.8 % after the ending of
reaction. This suggested that the chelated Cu2+ is reduced to Cu+ due to
the reaction between ligands and H2O2, leading to the electron transfer
from aromatic rings to Cu2+ as Eq. (4). With the proceeding of reaction,
the ligands and organics are completely degraded accompanying with
the recovery of catalyst. Furthermore, DMPO trapped EPR spectra of the
system containing γ-Cu2(OH)3Cl and H2O2 with the presence and
absence of phenol were measured, as shown in Fig. S5. After introducing
n < m
(5)
Second, the complex of contaminants with γ-Cu2(OH)3Cl can
enhance the Fenton catalytic efficiency. Upon Fenton reactions, aro-
matic contaminants normally degrade via the formation of in-
termediates with phenolic hydroxyls [15,46]. The phenolic hydroxyls
are favorable to interact with the surface Cu2+ of γ-Cu2(OH)3Cl to form
σ
-Cu-ligand complexes [1,5]. H2O2 can directly attack such complexes
•
to form HO and HO-adduct radicals. The latter could discharge an
electron to Cu2+ in the complexes due to the electron transfer from
π of
the aromatic ring to Cu2+, resulting in both the reduction of Cu2+ to Cu+
and the hydroxylation of aromatic ring [9,51]. The above processes are
summarized in Eqs. (3) and (4). To confirm that, Raman and XPS were
used to characterize the catalyst. 0.04 g γ-Cu2(OH)3Cl was soaked in
phenol solution (100 mg⋅L–1) followed by filtration, washing with water
and drying at 60 ◦C for 8 h. Such sample and fresh γ-Cu2(OH)3Cl were
characterized by Raman spectroscopy as shown in Fig. 12. For
γ-Cu2(OH)3Cl adsorbing phenol, besides a series of bands assigned to
γ-Cu2(OH)3Cl (360, 422, 511 and 574 cm–1 attributed to Cu–O–Cu
stretching; 801, 896, 927 and 973 cm–1 attributed Cu–O–H deformation
[35]) and a band around 1596 cm–1 assigned to bending vibration of
H2O [52], an additional weak band at 1334 cm–1 is observed, which is
•
phenol, the EPR signal of HO increases. This indicates that the
complexation between phenol and catalyst could enhance the genera-
•
tion of HO [9]. Such complexes may offer at least three advantages.
First, the electron transfer in the complexes could accelerate the
reduction of Cu2+ into Cu+, leading to the fast regeneration of Cu+
active sites [54]. It may break the rate limitation step of conventional
–
associated with the C H bending vibration of aromatic ring breathing
which can be contributed to the Cu-ligand complexes [1]. Moreover,
another additional band at 1640 cm–1 in the spectrum of
phenol-adsorbed γ-Cu2(OH)3Cl can be assigned to the aromatic ring
stretch of the substituted aromatic species [1]. In addition, O 1s XPS
Table 4
Binding energies of Cu 2p3/2 and Cu+ percentage of catalysts.
Binding energy (eV)
Percentage of
Cu+
Cu2+
Cu2+
sat.
Cu2+
sat.
Samples
Cu+ (%)a
CuOHCl
933.9
934.1
933.9
934.0
934.1
933.7
935.2
935.3
935.6
935.9
935.6
935.7
941.3
941.6
941.4
941.6
941.7
941.3
943.6
943.9
943.7
943.6
944.1
943.7
32.9
33.8
36.7
45.5
36.8
42.2
α
-Cu2(OH)3Cl
γ-Cu2(OH)3Cl
γ-Cu2(OH)3Cl-5 min
γ-Cu2(OH)3Cl-90 min
γ-Cu2(OH)3Cl+H2O2-
20 min
γ-Cu2(OH)3Cl+H2O2-
933.7
935.4
941.4
943.8
42.9
40 min
a
Peak area of Cu+ in % of the total Cu 2p3/2 area.
Fig. 13. O 1s XPS spectra of (a) phenol and (b) phenol-absorbed γ-Cu2(OH)3Cl.
8

H. Wang et al.
Applied Catalysis A, General 614 (2021) 118055
Table 5
Room temperature conductivities of catalysts before and after adsorbing H2O2.
Conductivity (×10ꢀ 5Ω⋅cmꢀ 1
)
Catalysts
Pristine
H2O2-adsorbed
γ-Cu2(OH)3Cl
CuO
1.20
4.86
4.69
3.37
93.52
4.82
4.74
6.81
Cu2O
FeOCl
suggested that the chorine in γ-Cu2(OH)3Cl might change the reduction
potential of Cu ions and serve as a bridge to facilitate the electron
transfer between Cu2+ and H2O2, leading to the fast reduction of Cu2+
and the cleavage of H2O2 [55,64]. However, the comprehensive insight
into the role of chlorine is still required.
Based on the above results, the catalytic mechanism over
γ-Cu2(OH)3Cl is proposed and shown in Scheme 1. An in-situ partial
reduction of Cu2+ in γ-Cu2(OH)3Cl into Cu+ occurs due to the electron
transfer from H2O2 to γ-Cu2(OH)3Cl induced by their interaction. Cu+
•
ions act as primary active sites to react with H2O2 to generate HO and be
Fig. 14. XPS spectra of Cu 2p3/2 of (a) fresh γ-Cu2(OH)3Cl, (b) γ-Cu2(OH)3Cl
after reaction for 5 min, (c) γ-Cu2(OH)3Cl after reaction for 90 min, (d)
γ-Cu2(OH)3Cl adsorbing H2O2 for 20 min, (e) γ-Cu2(OH)3Cl adsorbing H2O2 for
40 min.
converted into Cu2+. Simultaneously,
σ-Cu-ligand complexes are formed
via the ligands between Cu2+ and phenolic hydroxyls. H2O2 can directly
attack such complexes and lead to the rapid electron transfer from the
aromatic ring to Cu2+, which results in the reduction of Cu2+ into Cu+,
•
Fenton catalysts relying on the slow reaction between Cu2+ and H2O2
(Eq. (2)). Second, the interaction between complexes and H2O2 could
the hydroxylation of phenol and the generation of additional HO .
Subsequently, the hydroxylated intermediates can be further mineral-
•
•
ized into CO2 and H2O by HO .
produce additional HO via Eq. (4) route. Third, the direct attacking of
The unique property of γ-Cu2(OH)3Cl make it a competent material
H2O2 to the complexes can prevent the inefficient decomposition of
as Fenton catalyst. Both Cu+ and Cu2+ are present in γ-Cu2(OH)3Cl. On
•
H2O2 [9] and the scavenging of HO by H2O2 [55]. As a result,
•
the one hand, Cu+ is primary active sites for generating HO . On the
•
γ-Cu2(OH)3Cl may produce more HO and improve H2O2 utilization
other hand, ligands formed between Cu2+ and phenolic hydroxyls could
accelerate the regeneration of Cu+ during the Fenton reaction [1].
Moreover, the facile self-redox ability of γ-Cu2(OH)3Cl caused by the
interaction with H2O2 could further promote the production and the
efficiency.
Third, γ-Cu2(OH)3Cl could interact with H2O2 to facilitate the in-situ
reduction of some Cu2+ into Cu+ by the electron transfer from H2O2 to
•
γ-Cu2(OH)3Cl, responsible for the generation of more HO . To confirm
•
regeneration of Cu+. As a result, γ-Cu2(OH)3Cl can generate more HO
that, 0.04 g fresh γ-Cu2(OH)3Cl was soaked in H2O2 solution (53
mmol⋅L–1) for 20 and 40 min, respectively. After that, the catalysts were
filtrated, washed, dried at 50 ◦C for 8 h and then characterized by XPS.
Their XPS spectra are shown in Fig. 14 and their Cu+ percentages are
listed in Table 4. The Cu+ percentage increases from 36.7 % of the fresh
catalyst to 42.2 % after adsorbing H2O2 for 20 min and 42.9 % for 40
min, confirming the reduction of Cu2+ into Cu+ (Eq. (5)). The Cu2+ ions
in γ-Cu2(OH)3Cl are susceptible to be reduced to Cu+ upon the inter-
action with H2O2, which is regarded as self-redox property [31,55].
FeOCl is also a lamellar material and exhibits self-redox property [31,
55]. γ-Cu2(OH)3Cl probably has the similar property with FeOCl. It is
reported that FeOCl can accommodate the intercalated electron donor
molecules and thus enable the charge transfer from the intercalant to
and utilize H2O2 more efficiently, resulting in superior mineralization
performance. However, for the reference catalysts, not all the above
positive effects can be achieved. Cu2+ and Cu+ are exclusively present in
CuO and Cu2O, respectively [65], leading to the lack of synergic effects
between Cu+ and Cu2+ redox pairs [29,66]. In addition, electron-rich Cu
centers (Cu+) could not form
σ-Cu-ligand with phenolic compounds
because organics are preferentially accumulated around the
electron-poor centers [9]. Therefore, Cu2+ prefers to form
σ-Cu-ligand
with phenolic hydroxyls [67]. Moreover, the absence of electron transfer
from H2O2 is proved by little change in the conductivities of CuO and
Cu2O before and after adsorbing H2O2 (Table 5). Herein, the higher
activity of CuO than Cu2O might be attributed to the lack of ligands in
the latter and their different structures: mono-crystalline for CuO and
poly-crystalline for Cu2O, as reported by Scuderi et al. [68]. So far, this is
just an initial work for γ-Cu2(OH)3Cl as Fenton catalysts, and the further
study on comprehensively understanding the structure-activity corre-
lation is in progress.
•
Fe3+ to generate Fe2+ [56,57]. Such character offers FeOCl rapid HO
generation rate and thus make it an effective Fenton catalyst [31,58,59].
By XPS, Yang et al. [31] demonstrated that the intercalation or reaction
of H2O2 with FeOCl could reduce partial Fe3+ in FeOCl into Fe2+ via the
electron transfer from H2O2 to Fe3+. Besides the layered structure, the
key for intercalation is the strong oxidizing host which can provide the
driving force for guest molecules intercalation [57]. It is reported that
the VB potentials of FeOCl and Cu2(OH)3Cl are 1.96 [59] and 3.5 eV
[34], respectively. The more positive VB potential suggests stronger
oxidation ability [60]. To further verify the electron transfer between
H2O2 and γ-Cu2(OH)3Cl, the resistivities of pristine γ-Cu2(OH)3Cl and
γ-Cu2(OH)3Cl adsorbing H2O2 for 40 min were measured at room tem-
perature to calculate their conductivities. The results are listed in
Table 5. Almost 78 times increase in conductivity after adsorbing H2O2
is observed, which can be attributed to the electrons added into the host
lattice upon oxidation of guest species [61,62]. A FeOCl sample was also
synthesized according to reference [63] and its XRD pattern is shown in
Fig. S6. After adsorbing H2O2 for 40 min, the conductivity of FeOCl was
enhanced by around 2 times, as listed in Table 5. In addition, it is also
4. Conclusions
γ-Cu2(OH)3Cl is a readily synthesized material in which both Cu2+
and Cu+ are present. Cu+ could act as major catalytic active sites for
Fenton degradation of phenol. During the Fenton reaction, phenolic
hydroxyls in organics can form
σ
-Cu-ligand complexes with Cu2+. Sub-
sequently, H2O2 can attack complexes to induce the electron transfer
from the aromatic ring to Cu2+, leading to the rapid regeneration of Cu+
•
and the generation of additional HO . Moreover, the self-redox property
of γ-Cu2(OH)3Cl induced by its interaction with H2O2 could initiate the
charge transfer from H2O2 to γ-Cu2(OH)3Cl and thus promote the partial
reduction of Cu2+ into active Cu+. Such positive effects may not only
break the limitation of the slow regeneration of Cu+ which is a rate-
9

H. Wang et al.
Applied Catalysis A, General 614 (2021) 118055
Scheme 1. Possible catalytic mechanism over γ-Cu2(OH)3Cl.
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Declaration of Competing Interest
The authors report no declarations of interest.
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Appendix A. Supplementary data
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