Photocatalytic degradation of dyes by mononuclear copper(II) complexes from bis-(2-pyridylmethyl)amine NNN-derivative ligands

Article abstract of DOI:10.1016/j.ica.2020.119924

Photocatalytic degradation of organic pollutant dyes under ultraviolet radiation has emerged as an efficient wastewater treatment. This work describes the application of four mononuclear copper(II) complexes coordinated to NNN ligands: bis-(2-pyridylmethyl)amine (BMPA), N-methylpropanoate-N,N-bis-(2-pyridylmethyl)amine (MPBMPA), N-propanoate-N,N-bis-(2-pyridylmethyl)amine (PBMPA) and N-propanamide-N,N-bis-(2-pyridylmethyl)amine (PABMPA); in the photocatalytic degradation of different dyes: methyl orange (MO), methylene blue (MB), crystal violet (CV), Congo red (CR) and Rhodamine B (RhB). The reactions were carried out under a UV lamp of 250 W, where 100percent of degradation was achieved in 90 min for all complexes using hydrogen peroxide as oxidant. Kinetic experiments were carried out to investigate the photodegradation of the dyes under a UV lamp of 24 W. The reactions followed a zero-order model in relation to the dye, showing that its concentration did not play a significant role in the photocatalysis. The reaction order in relation to hydrogen peroxide varied from 0 to 0.8, from low to high concentrations of oxidant. The light intensity and the intrinsic catalytic activity of the complexes are the most important features for the dye photodegradation pathway.

Full text of DOI:10.1016/j.ica.2020.119924

Inorganica Chimica Acta 512 (2020) 119924
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Photocatalytic degradation of dyes by mononuclear copper(II) complexes
from bis-(2-pyridylmethyl)amine NNN-derivative ligands
⁎
Samira S.F. Carvalho, Ana Carolina C. Rodrigues, Juliana F. Lima, Nakédia M.F. Carvalho
Universidade do Estado do Rio de Janeiro, Instituto de Química, Rua São Francisco Xavier, 524, Edifício Haroldo Lisboa da Cunha, IQ, Room 312a, Maracanã, 20550-013
Rio de Janeiro, RJ, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords:
Photocatalytic degradation of organic pollutant dyes under ultraviolet radiation has emerged as an efficient
wastewater treatment. This work describes the application of four mononuclear copper(II) complexes co-
ordinated to NNN ligands: bis-(2-pyridylmethyl)amine (BMPA), N-methylpropanoate-N,N-bis-(2-pyridylmethyl)
amine (MPBMPA), N-propanoate-N,N-bis-(2-pyridylmethyl)amine (PBMPA) and N-propanamide-N,N-bis-(2-
pyridylmethyl)amine (PABMPA); in the photocatalytic degradation of different dyes: methyl orange (MO),
methylene blue (MB), crystal violet (CV), Congo red (CR) and Rhodamine B (RhB). The reactions were carried
out under a UV lamp of 250 W, where 100% of degradation was achieved in 90 min for all complexes using
hydrogen peroxide as oxidant. Kinetic experiments were carried out to investigate the photodegradation of the
dyes under a UV lamp of 24 W. The reactions followed a zero-order model in relation to the dye, showing that its
concentration did not play a significant role in the photocatalysis. The reaction order in relation to hydrogen
peroxide varied from 0 to 0.8, from low to high concentrations of oxidant. The light intensity and the intrinsic
catalytic activity of the complexes are the most important features for the dye photodegradation pathway.
Photo-Fenton
Copper(II) complexes
Methyl orange
Methylene blue
Crystal violet
Congo red
Rhodamine B
Dye degradation
Kinetics
1. Introduction
sort of organic pollutants of different structures and stability [7].
Nevertheless, compounds containing cobalt, manganese, ruthenium,
Environmental pollution caused by organic compounds is an im-
cadmium, silver and copper have also been investigated as Fenton-like
catalysts [6,8–11].
portant issue concerning the future well-being of the global society. The
textile industry is responsible for 17–20% of the pollution caused by the
discard of untreated wastewater into water bodies [1–3], representing a
longstanding problem that causes environmental degradation, aesthetic
issues, and toxicity to aquatic and human lives [4–5]. Dyes are complex
and chemically stable molecules, design to last long, therefore resistant
to harsh conditions such as sunlight, water, soap, bleach and per-
spiration. The conventional degradation methods are not always ef-
fective and can release traces of these compounds in rivers and lakes
In the last decade, copper(II) complexes have been emerging as
photo-Fenton catalysts for dye oxidative degradation. Complexes with
salen-based ligand was tested in the photodegradation of Rhodamine B
under visible light [12]. Copper(II) 1D and 3D frameworks with dif-
ferent ligands have been described in the degradation of different or-
ganic dyes under visible light irradiation [13–19]. Although a number
of works have reported the efficiency of metal complexes for dye de-
gradation, they are selective or studied for only one or two dyes. Fur-
thermore, more detailed investigation about the mechanism of com-
plex-mediated photocatalysis are still needed. Many publications
described a first-order kinetics in relation to the dye, however, in many
cases the data are not of good quality with very few points to assign
reliably the correct kinetic model. Recent publication by Ollis [20] has
shown that for heterogeneous photocatalysis the most suitable reaction
model is zero-order in relation to the reactant, however, mis-
interpretation of the data has led many author to assign it as first-order.
Coordination compounds can absorb UV or visible light and assist
catalytic process [15], what is advantageous for photo-Fenton system
[
4,6]. Apart of being non-biodegradable, dyes and its products can be
carcinogenic, teratogenic and present acute toxicity [1], hence, new
methodologies to treat these organic pollutants efficiently before dis-
posal is mandatory for a cleaner and sustainable chemical industry.
Among the several biological, physical and chemical methods to
mitigate the damage caused by the dyes, the Advanced Oxidation
Processes (AOPs) have been highlighted [6]. For instance, Fenton and
photo-Fenton systems rely on the use of safe and eco-friendly transition
metal catalysts, traditionally iron(II) salts, and hydrogen peroxide to
generate the highly oxidant hydroxyl radical that can mineralize a wide
⁎
Corresponding author.
E-mail address: nakedia@uerj.br (N.M.F. Carvalho).
https://doi.org/10.1016/j.ica.2020.119924
Received 12 June 2020; Received in revised form 22 July 2020; Accepted 22 July 2020
Available online 25 July 2020
0020-1693/ © 2020 Elsevier B.V. All rights reserved.

S.S.F. Carvalho, et al.
Inorganica Chimica Acta 512 (2020) 119924
instrument from Bruker Daltonics, at capillary voltage of 4.0 kV in
−
4
−1
positive ion polarity, at 1.0 × 10 mol L methanol:water solution.
−3
−1
Conductimetric measurements of the complexes at 1.0 × 10 mol L
methanol solution were carried out in Gehaka CG 1800 instrument,
with constant cell of 1.0 cm . Complex characterization and dye de-
gradation were acquired by solution UV–VIS spectroscopy, recorded on
a diode-array Agilent 8453 spectrophotometer.
−
1
2.2. Synthesis of the complexes
The ligands BMPA, MPBMPA, PBMPA and PABMPA were synthe-
sized according to published procedures [25,26], as shown in Scheme
S1.
Fig. 1. Photo-Fenton mechanism for copper(II) complexes [17]. (LMCT: Ligand
to Metal Charge Transfer. L = Ligand, L’= Oxidized Ligand).
because of the presence of both organic ligand and transition metal.
Different ligand to metal charge transfers (LMCT) can occur and the
oxidized metal species can also interact with H O to form reactive
2 2
oxygen or free radical species [16,17,21]. In the presence of UV or
visible radiation Cu(II) complexes absorb photons leading to the for-
mation of oxidized ligand (L’) and Cu(I). The L’ can interact directly
2
.2.1. Complex [Cu(BMPA)Cl
The complex dichloro[bis-(2-pyridylmethyl)amine]copper(II) was
2
]
obtained following the published procedure [24]. A solution of the li-
gand BMPA (1.0 mmol; 0.199 g, in 5.0 cm of ethanol) was added to a
solution of CuCl
light blue precipitate was immediately obtained, which was filtered and
washed with ethanol (41% of yield).
3
3
2
⋅2H
2
O (0.5 mmol, 0.852 g, in 5.0 cm of ethanol) and a
with oxidizing pollutants (dyes), thereafter, regenerating the reduced
•
ligand form (L). Copper(I) reacts with H
2
O
2
to produce OH radicals. In
−1
FTIR (KBr, cm ): 3436, 3068, 2910, 2890, 1606, 1573, 1481,
parallel, independently of light irradiation, Cu(II) can react with H
2
O
2
+
1
2
435, 775. ESI-(+)-MS/Q-TOF: m/z 631.0008 [Cu
2
(BMPA)
2
Cl
3
]
, m/z
•−
and •OH
to generate O
2
2
. Fig. 1 simplifies the photo-Fenton me-
+
+
97.0184 [Cu(BMPA)Cl] 3
, m/z 261.0393 [Cu(BMPA)] . UV–VIS
chanism of the catalyst based on copper(II) complexes. Other non-
photoassisted charge transfer reactions can also be involved, for in-
stance Cu(II) complexes can interact with peroxide to form inter-
mediate species such as (L)Cu(II)–OOH or (L)Cu(I)-superoxide. The
reactive species formed are strongly oxidizing and can degrade organic
dyes. Furthermore, copper is stable in a larger range of pH compared to
the traditional iron Fenton catalysts, and deactivation of the catalyst by
hydroxide/oxide precipitation is less critical [17].
−1
−1
3
(
(
methanol, λ[nm] (ε[dm mol
cm ])): 251 (8.9 × 10 ), 671
2
−1
2
−1
1.1 × 10 ). Λ
M
(methanol) = 88.4 Ω
cm mol
.
2
.2.2. Complex [Cu(MPBMPA)Cl](ClO
The complex perchlorate of chloro[N-methylpropanoate-N,N-bis-(2-
4
)
pyridylmethyl)amine]copper(II) was obtained by the addition of a solu-
tion of the ligand MPBMPA (0.88 mmol, 0.151 g, in 5.0 cm of ethanol)
to a solution of CuCl
The new solution was stirred for 30 min and no precipitate was formed.
Then, 1.76 mmol of NaClO was added (0.247 g), the solution was left
3
3
2
⋅2H
2
O (1.76 mmol, 0.50 g, in 5.0 cm of ethanol).
Metal transition coordination compounds with the versatile NNN
ligand bis-(2-pyridylmethyl)amine (BMPA) have been applied in cata-
lytic oxidation reactions [22,23]. The complex [Cu(BMPA)Cl ] was
2
4
to rest around 5 °C, after which a light blue precipitate was formed. The
successfully described as catalyst in the oxidation of cyclohexane with
high conversions, using hydrogen peroxide as oxidant at mild condi-
tions [24], being a promising catalyst for oxidative degradation of or-
ganic dyes as well. This work aims to investigate four mononuclear
copper(II) complexes based on BMPA and derivative ligands: N-me-
thylpropanoate-N,N-bis-(2-pyridylmethyl)amine (MPBMPA), N-pro-
panoate-N,N-bis-(2-pyridylmethyl)amine (PBMPA) and N-propana-
mide-N,N-bis-(2-pyridylmethyl)amine (PABMPA), in the degradation of
dyes of different structures by photo-Fenton, namely methyl orange
solid was filtered and washed with ethanol (41% of yield).
−1
FTIR (KBr, cm ): 3443, 3079, 2955, 2923, 1711, 1612, 1447,
1
283, 1105, 775. ESI-(+)-MS/Q-TOF: m/z 383.046 [Cu
+
+
(
MPBMPA)Cl]3 , m/z 348.0849 [Cu(MPBMPA)] . UV–VIS (methanol,
−1 −1 3 1
λ[nm] (ε[dm mol
cm ])): 257 (9.1 × 10 ), 678 (9.6 × 10 ). Λ
M
−1
2
−1
(
methanol) = 116.4 Ω
cm mol
.
2.2.3. Complex [Cu(PBMPA)](ClO
4
)
The complex perchlorate of [N-propanoate-N,N-bis-(2-pyr-
(
MO), methylene blue (MB), crystal violet (CV), Congo red (CR) and
idylmethyl)amine]copper(II) was obtained by the addition of a solution
Rhodamine B (RhB). The studied ligands are strong Lewis bases and can
favor the Cu(I)/Cu(II) redox cycle through LMCT, during the photo-
catalysis. Besides, ancillaries functional groups as carboxylate, methyl
ester and amide, can tune the complex catalytic activity, and the un-
saturated coordination of the proposed complexes can induce decom-
position of hydrogen peroxide and enhance the catalytic performance
3
of the ligand PBMPA (1.0 mmol, 0.278 g, in 5.0 cm of methanol) to a
3
solution of Cu(ClO
4
)
2
⋅6H
2
O (1.0 mmol, 0.371 g, in 5.0 cm of me-
thanol). The new mixture was stirred for 3 h and a blue precipitate was
formed (79% of yield).
−1
FTIR (KBr, cm ): 3430, 3088, 2923, 1612, 1572, 1500, 1441,
3
−1
−1
1
2
mol
316, 1092, 762. UV–VIS (methanol, λ[nm] (ε[dm mol
cm ])):
[
17]. Lastly, the kinetics of the dye photodegradation were investigated
4
1
−1
2
63 (1.4 × 10 ), 690 (8.1 × 10 ). Λ
M
(methanol) = 88.9 Ω
cm
in situ, in order to provide information about the reaction mechanism.
−1
.
2. Experimental
2.2.4. Complex [Cu(PABMPA)Cl]Cl
The complex chloride of chloro[N-propanamide-N,N-bis-(2-pyr-
idylmethyl)amine]copper(II) was synthesized by the addition of a solu-
2
.1. Materials and methods
3
tion of the ligand PABMPA (1.0 mmol, 0.273 g, in 5.0 cm of ethanol) to
3
All chemicals are of reagent grade and were used without further
a solution of CuCl
2
⋅2H
2
O (1.0 mmol, 0.173 g, in 5.0 cm of ethanol).
purification. Hydrogen peroxide (30% aqueous solution) from Sigma-
The new mixture was stirred for 30 min and a light blue precipitate was
Aldrich was titrated by the iodometric method and the determined
formed (28% of yield).
FTIR (KBr, cm ): 3440, 3274, 3075, 3037, 2922, 1655, 1610,
1437, 765, 644. ESI-(+)-MS/Q-TOF: m/z 368.0616 [Cu(PABMPA)Cl]
−
1
−1
concentration was 9.23 mol L
.
+
FTIR analyses were acquired in a Nicolet 6700, Thermo Scientific
spectrophotometer, in KBr pellets for the complex characterization or as
film over KBr window in the case of the ligands. The complexes were
analyzed by ESI-(+)-MS/Q-TOF mass spectrometry in a MicrOTOF-Q II
;
+
3
m/z 356,1094 [Cu(PABMPA)] . UV–VIS (methanol, λ[nm] (ε[dm
−
1
−1
3
1
mol
cm ])): 257 (9.2 × 10 ), 680 (8.9 × 10 ). Λ
M
(me-
−
1
2
−1
thanol) = 174.4 Ω
cm mol
.
2

S.S.F. Carvalho, et al.
Inorganica Chimica Acta 512 (2020) 119924
2.3. Photo-Fenton dye degradation tests
Individual stock solutions of the dyes methyl orange (MO), methy-
lene blue (MB), crystal violet (CV), Congo red (CR) and Rhodamine B
−
4
−1
(
RhB), were prepared in distilled water at 5.0 × 10
mol L
.
−
4
−1
−1
In a typical experiment, 300 µL of aqueous 5.0 × 10
mol L
−
4
stock solution of a given dye, 200 µL of aqueous 5.0 × 10
complex stock solution, 10 µL of aqueous H 9.23 mol L
solution and 2.49 mL of H
.0 mL, were sequentially mixed in a glass capped vial. The resulting
m1ol L
−
2
O
2
stock
2
O to complete a total reaction volume of
3
−
5
-1
initial concentrations were: [dye] = 5.0 × 10
plex] = 3.3 × 10 mol L ; [H
mol L ; [com-
−
5
−1
−2
−1
2 2
O ] = 3.1 × 10 mol L , resulting
in 1:1.5:940 complex:dye:H molar ratio. The reaction was carried
2 2
O
out under magnetic stirring, at 25 °C, and at preset time intervals
UV–VIS spectra were collected. To maintain the homogeneity in the
photon flux distribution [27], a vapor Hg lamp (250 W) to simulate UV
radiation was fixed 25 cm from the sample (Scheme S3(A)). Control
tests were performed with no light irradiation to confirm that the
complexes do not have any catalytic effect in the dark.
To determine the reaction order in relation to dye and hydrogen
peroxide, initial concentration was varied separately: 100 – 300 µL of
−
4
−1
aqueous 5.0 × 10
mol L
– 5.0 × 10
stock solution of methyl orange:
−
5
−5
−1
[
H
MO] = 1.7 × 10
mol L ; 10 – 500 µL of aqueous
Fig. 2. Proposed structures of the copper(II) complexes.
−
1
−2
2
O
2
9.23 mol L
stock solution: [H
2
O
2
] = 3.1 × 10
– 1.5 mol
−1
L
.
as donor atom [31]. In the free ligand MPBMPA, C]O appears at
The experiment was set up to allow the kinetic investigation and
−
1
−1
1
737 cm and CeO at 1243 cm , while in the complex these bands
simultaneous collection of UV–VIS data, therefore mild conditions such
as distance and potency of the light source were employed recombined.
The kinetic experiments were conducted in a capped cuvette inside the
UV–VIS spectrometer by in situ photocatalysis conducted under UV light
−1
appear at 1711 and 1283 cm , respectively, what can suggest that the
methyl ester group is interacting weakly with the copper(II) center.
Therefore, it is proposed a cationic five coordinated complex with one
chloride ligand coordinated to the copper(II) center. This proposed
structure is different from the published single crystal X-ray data for
(
external Black light 24 W from Philips) fixed 12 cm from the cuvette,
under stirring at 25 °C (Scheme S3(B)). Both photocatalytic system was
kept closed by a physical barrier to avoid dissipation and leakage of
harmful light. The lamp of the spectrophotometer was turned on only
during the electronic spectrum acquisition.
[
Cu(MPBMPA)Cl
2
], which adopted a square pyramidal geometry and
presented two chloride ions as ligands. Besides, it presented an un-
−1
2
changed ν(CO )as at 1731 cm in relation to the free ligand, since the
ester group appears uncoordinated [32]. The structure is also confirmed
by the species [Cu(MPBMPA)Cl]+ at m/z 383.046 in the ESI-(+)-MS/
Q-TOF spectrum (Fig. S6).
The MO decolorization was calculated using the following equation:
Abs
0
− Abs
t
⎡
⎤
Degradation(%) =
× 100
Complex [Cu(PBMPA)](ClO
4
) was prepared with Cu(ClO
, ionic ClO
(Fig. S3) and by the electrical conductivity
typical of 1:1 electrolyte. The carboxylate ν(CO as and ν(CO bands
4 2
) since no
⎢
⎣
Abs
⎥
⎦
0
(1)
−
product was obtained from the synthesis with CuCl
2
4
was
−1
confirmed at 1092 cm
2
)
2 s
)
3
. Results and discussion
can give information about the coordination mode to the copper(II)
center. In the free ionic ligand the bands appear at 1597 and
3
.1. Characterization of the complexes
−1
−1
1
384 cm , resulting in Δν(CO
2
) = 213 cm . In the complex a higher
−
1
value for Δν(CO
2
) = 1645 − 1311 = 334 cm
indicates a mono-
Four mononuclear copper(II) complexes were synthesized (Scheme
dentate coordination mode [33]. The similar complex [Cu(PBMPA)Cl]
showed a pentacoordinated square pyramidal geometry, with the
chloride ion coordinated to copper(II) [32]. In the present synthesis, a
solvent molecule could be occupying the fifth position of the co-
ordination sphere.
S2) and characterized by FTIR, electronic spectroscopy in UV–VIS re-
gion, ESI-(+)-MS/Q-TOF and conductimetry, as presented in the
Supporting Information. The proposed structures are presented in
Fig. 2.
2
The complex [Cu(BMPA)Cl ] adopts a five-coordinated distorted
Finally, for the complex [Cu(PABMPA)Cl]Cl it is proposed as a five
coordinated structure, with the coordination of the amide oxygen to the
copper center. This is confirmed by the amide C]O band shift from
square-pyramidal geometry, with one elongated Cu-Cl bond, according
to published single crystal X-ray structure [28,29]. Relatively high
electric conductivity (Table S1) for a neutral complex confirms the
lability and week coordination of the chloride ligand [30]. ESI-(+)-MS/
Q-TOF spectrum (Fig. S5) shows peaks related to dimeric species in
solution [Cu
−1
1677 in the free ligand to 1650 cm
in the complex (Fig. S4). The
electrical conductivity is compatible with a 2:1 electrolyte, probably
due to the lability of the chloride ion. Similar coordination mode was
+
2
(BMPA)
2
Cl
3
]
at m/z 631.0008 and to monomeric species
+
2 4
observed for the iron(III) complex [Fe(PABMPA)Cl ](ClO ) with the
[
Cu(BMPA)Cl] at m/z 297.0184.
In the synthesis of [Cu(MPBMPA)Cl](ClO
chlorate was necessary to form a precipitate, ionic ClO
−1
C]O band at 1653 cm [25], and for the copper(II) complex with the
analogous ligand N-alanineamide-N,N-bis-(2-pyridylmethyl)amine
4
) the counter-ion per-
−
4
was con-
(Fig. S2) and by the
−
1
4
(dpgs) [Cu(dpgs)Cl]ClO that also presented square pyramidal geo-
metry with coordination by the amide oxygen [34].
firmed by the FTIR large band at 1093 cm
electrical conductivity typical of a 1:1 electrolyte (Table S1) [30]. The
FTIR band of the ester group can provide information about the co-
ordination to the copper(II) center, where a negative shift of the car-
bonyl stretching frequency and a positive shift of the CeO stretching
frequency have been taken as evidence for the carbonyl oxygen acting
For all complexes (Figs. S1–S4, Table S2), electronic UV–VIS
spectra showed three major bands: a stronger band around 270 nm
assigned to π → π* intraligand charge transfer, a band centered at
300 nm assigned to metal-to-ligand charge transfer, and a weaker band
3

S.S.F. Carvalho, et al.
Inorganica Chimica Acta 512 (2020) 119924
Fig. 3. Structure of the studied dyes.
around 670 nm from d-d transition.
oxygen species responsible for the dye degradation. Finally, the system
composed by catalyst, H and UV light was evaluated and the system
2 2
O
allowed a total photodegradation of methyl orange, revealing a pro-
mising result and the effect of the catalyst in the photodegradation. The
results are summarized in Table 1.
In order to understand the effect of pure Cu(II) ions and free ligand
in the reported homogeneous photocatalysis, the previous system was
3
.2. Photo-Fenton dye degradation tests
The mononuclear Cu(II) complexes prepared were investigated as
catalysts in the photodegradation of five dyes of different structures:
methyl orange (MO), methylene blue (MB), crystal violet (CV), Congo
red (CR) and Rhodamine B (RhB) (Fig. 3). The reactions were carried
out in water, due to the high solubility of the complexes, at 1:1.5:940 of
repeated either replacing [Cu(MPBMPA)Cl](ClO
CuCl , or by the free ligand MPBMPA. The system containing only li-
gand, H and UV reached a degradation of 76% of MO. Considering
that the reaction medium contains reactive oxygen species such as OH,
from the H photolysis induced by UV irradiation, the ligand can be
4
) by a Cu(II) salt,
2
2 2
O
complex:dye:H
reactor equipped with UV lamp (250 W) (Scheme S3).
Initially, the influence of the presence of UV light, H
on dye degradation was evaluated. The tests were carried out using [Cu
MPBMPA)Cl](ClO ) as catalyst and methyl orange as dye. Also, CuCl
and pure ligand MPBMPA (L) were tested to study the impact of free Cu
2 2
O molar ratio, at 25 °C, in a homemade photocatalytic
•
2 2
O
2 2
O and catalyst
oxidized by these species or by photon absorption and the oxidized li-
gand can interact directly with MO improving dye degradation.
Furthermore, the system formed by only Cu(II) salt, H O and UV light
2 2
showed 100% of degradation, indicating that the photodegradation of
the dye is also catalyzed by the copper(II) salt. In addition to the per-
(
4
2
(
II) ions and free ligand on the photocatalysis.
Fig. 4(a) shows that the photodegradation reaction only induced by
formance of the radicals generated by H
2
O
2
+ UV, Cu(II) ions also react
[17].
The system formed by Cu(II) salt + ligand + H
evaluated and MO degradation of 98% was achieved. In comparison to
the isolated complex [Cu(MPBMPA)Cl](ClO ) + H + UV that also
2 2
UV light without catalyst or H O , does not lead to the degradation
process within the studied reaction time. Posteriorly, the catalyst was
added to the medium and no change was observed. Therefore, the
following tests were performed with H
evaluated. Firstly, the degradation test was performed only in the
presence of catalyst and H under dark and the dye absorbance
spectrum did not show any evidence of degradation. A test was also
carried out by adding only H into the reaction medium and sub-
•
•−
•
with H
2
O
2
forming OH, O
2
and OH
2
2 2
O + UV was also
2 2
O and three new systems were
4
2 2
O
showed 100% of MO degradation, when Cu(II) and ligand are mixed in
situ they can interact readily and allow LMCT to take place. These re-
sults indicate that either isolated complex or in situ formed complex can
contribute to the photocatalysis and act efficiently in the degradation of
the studied pollutants. Although both complex and salt lead to 100% of
2 2
O
2 2
O
mitting the system to UV irradiation and 38% of dye degradation was
observed after 90 min, due to light induced formation of reactive
CuCl +L+UV+H O
(c) 100
(a) 100
2
2
2
(b) 100
CuCl +UV+H O
2
2
2
Cat+UV+H O
2
2
80
60
40
20
0
L+UV+H O2
80
60
40
20
0
80
2
Cat+H O2
2
UV+H O2
60
2
Cat+UV
UV
MO
MB
CV
CR
RhB
[
[
[
[
Cu(BMPA)Cl ]
40
20
0
2
Cu(MPBMPA)Cl](ClO )
4
Cu(PBMPA)](ClO )
4
Cu(PABMPA)Cl]Cl
0
30
60
90
0
30
60
90
0
30
60
90
Time (min)
Time (min)
Time (min)
Fig. 4. Photodegradation of methyl orange at complex:dye:H
ClO ) as catalyst; (b) by different Cu(II) complexes; (c) of different dyes by [Cu(BMPA)Cl
legend, the reader is referred to the web version of this article.)
2
O
2
molar ratio of 1:1.5:940, 25 °C, UV light (250 W): (a) in different conditions with [Cu(MPBMPA)Cl]
(
4
2
] as catalyst. (For interpretation of the references to colour in this figure
4

S.S.F. Carvalho, et al.
Inorganica Chimica Acta 512 (2020) 119924
Table 1
Table 2
Degradation percentage and kinetic data of photodegradation of dyes.
a
Photodegradation percentage of methyl orange at 90 min catalyzed by [Cu
(
4
MPBMPA)Cl](ClO ).
Catalyst
Dye [H
2
O
2
]
Degradation90min(%)
k
L
obs(μmol
−1
Conditionsa
Degradation90min (%)
(mol L−1)
min 1)b
−
UV
6.5
38
UV + H
Catalyst + UV
Catalyst + H
Catalyst + UV + H
CuCl + UV + H
Ligand + UV + H
CuCl + Ligand + UV + H
2
O
2
[Cu(BMPA)Cl
2
]
MO
MO
MO
MO
MO
MO
MO
MO
MO
MO
MO
MO
MO
CV
0.031
0.062
0.154
0.31
0.37
0.46
0.62
0.92
1.23
1.54
1.54
1.54
1.54
1.54
45
51
67
79
100
100
95
96
99
0.274
0.286
0.425
0.561
1.106
1.311
1.256
1.735
1.894
2.565
2.962
2.630
2.835
3.969
7.012
0.201
3.933
6.5
2.5
100
100
76
2
O
2
2
O
2
2
2
O
2
2 2
O
2
2
O
2
98
a
10−5 mol L−1
MO at 5.0
×
;
Catalyst: [Cu(MPBMPA)Cl](ClO
4
)
at
−
5
−1
−2
mol L−1 and UV light (250 W);
98
100
90
3
.3 × 10
Ligand: MPBMPA at 3.3 × 10
mol L ; H O at 3.1 × 10
2 2
−
5
−1
[Cu(MPBMPA)Cl
2
]
4
)
mol L
.
[
[
Cu(PBMPA)](ClO
Cu(PABMPA)Cl]Cl
96
MO photodegradation after 90 min, Fig. 4(a) indicates that the pho-
tocatalysis in the presence of [Cu(MPBMPA)Cl](ClO or
CuCl + MPBMPA allow higher rate of photodegradation. After few
minutes, for [Cu(MPBMPA)Cl](ClO ) and Cu(II) + Ligand photo-
[Cu(BMPA)Cl2]
100
100
88
4
)
[Cu(BMPA)Cl
2
2
2
]
]
]
RhB 1.54
CR
MB
[
[
Cu(BMPA)Cl
Cu(BMPA)Cl
1.54
1.54
2
100
4
degradation higher than 50% was reached, favored by the LMCT. Ki-
netic studies reported below enable full discussions.
Fig. 4(b) shows the results of MO degradation for all four Cu(II)
complexes reported, which presented good photodegradation ability
with complete degradation of the dye within 90 min. For [Cu
a
Catalyst at 3.3 × 10−5 mol L−1; dye at 5.0 × 10−5 mol L−1; UV light
(
24 W).
b
Zero-order rate constant.
molar ratio). The band at λ
After addition of catalyst and H O , a considerable red-shift to
2 2
max
= 464 nm is assigned to the azo bond.
(
BMPA)Cl
2
], [Cu(PBMPA)](ClO
4
) and [Cu(PABMPA)Cl]Cl, 100% of MO
is already decomposed in 60 min of irradiation.
In order to prove the efficiency of the copper(II) complexes in the
2
photocatalysis of different dyes, the catalyst [Cu(BMPA)Cl ] was em-
ployed (Fig. 4(c)). MO, MB, CV and RB dyes suffered total degradation
after 60 min, however, for CR 90 min were necessary to achieve total
degradation, probably because of the presence of two azo bonds and
higher molecular weight. These results show the wide range of per-
formance of the copper(II) complexes, degrading dyes with different
structures and classes.
λ
max
= 495 nm was observed, suggesting subtle structural changes in
MO probably due to interaction with the Cu(II) complex. The decay of
the azo band over time indicates the destruction of the chromophore by
the cleavage of the azo bond and disruption of conjugated π-bonds in
the molecule, leading to colorless oxidized products [4,6].
Fig. 5(b) shows the time trace for the decay of MO absorbance at
λ
max
2 2
= 495 nm, for different concentrations of H O . In a first moment,
experiments catalyzed by [Cu(BMPA)Cl ] were carried out using the
2
−1
same H O2 concentration as with the vapor Hg lamp (0.031 mol L ),
2
however, only 45% of degradation was achieved in 90 min due to the
2 2
lower radiation intensity of the 24 W lamp. So, the H O concentration
was increased until complete degradation was achieved, as summarized
in Table 2.
Concerning the kinetic study of the photodegradation, eq. (2) shows
a proposed rate law for the reaction.
3
.3. Kinetic experiments of dye photodegradation
In order to get information about the reaction mechanism of the dye
photodegradation, kinetic experiments were carried out in water, at
5 °C. To set up an experiment where UV–VIS spectra were recorded at
2
every 1 min for 90 min, an UV lamp of lower intensity (24 W) was
coupled to the spectrophotometer allowing the cuvette to be irradiated
during the whole reaction time (Scheme S3(B)).
d[dye]
dt
a
2 2
O
]b
−
= v = k [dye] [H
(2)
Fig. 5(a) shows typical UV–VIS spectra acquired during the de-
At excess of H
2
O
2
, the equation can be simplified and the effect of
gradation of the azo dye methyl orange, catalyzed by [Cu(BMPA)Cl
with H O at 15.4 × 10 mol L (1:1.5:45000 of complex:dye:H O
2 2 2 2
2
],
the dye concentration in the reaction rate can be isolated (eq. (3) and
(4)).
−1
−1
Fig. 5. (a) UV–VIS spectra of MO photodegradation by [Cu(BMPA)Cl
2
], H
2
O
2
and UV light (24 W). Dashed line: pure MO. (b) Time trace at λmax = 495 nm at
−1
2 2
different concentrations of H O from 0.031 to 1.54 mol L
(see Table 2).
5

S.S.F. Carvalho, et al.
Inorganica Chimica Acta 512 (2020) 119924
apparent first-order rate constants that diminish with increasing re-
actant concentration, what is an evidence of the wrong assignment.
Ollis showed that such studies are the result of intrinsic zero-order data
plotted on a semilog graph, and involves zero-order rate limitation by
d[dye]
dt
a
−
= v = kobs [dye]
(3)
(4)
where,
k
obs = k [H
2
O
2
]b
reactant saturation, electron transfer to O , oxygen mass transfer, or
2
light supply. Besides, the competition for oxidant between the dye and
As shown in Fig. 5(b), at lower concentrations of oxidant, but still at
its oxidized intermediates, would exhibit an apparent first-order be-
havior over time, even while the initial rate is zero-order within the
concentration range studied.
In the presented results of homogeneous catalysis, a similar beha-
vior is taking place. The apparent first-order rate constants calculated
from the exponential fit of the experimental data are expressed in Table
S3 and also diminish linearly with increased initial concentration of MO
large excess in relation to the dye, it is clear by the shape of the curve
that the kinetics follows pseudo-zero order equation (eq. (5) and (6)),
where the concentration of the dye over time did not affect the reaction
rate.
d[dye]t
dt
−
= kobs
(5)
(6)
(
Fig. S24), therefore, this is further confirmation that the reaction does
[
dye]t = −kobs t
not follow 1st-order mechanism, but a disguised zero-order behavior
[20]. Silver(I) complexes at heterogeneous photodegradation condition
also presented zero-order kinetics [35].
To confirm the reaction order in relation to the dye, the MO ab-
sorbance decay with time was fit by the integrated equations for the
most common reaction orders: zero-, first- (eq. S1 - S2) and second-
In order to determine the reaction order b in relation to H
2 2
O ac-
order (eq. S3). Fig. 6(a) shows the fitting for the lowest H
2
O
2
con-
cording to eq. (4), the plot kobs versus [H ] is shown in Fig. 9(a), and
a linear relation is expected if b is 1. A slight deviation for the first-order
2 2
O
−1
2
centration tested (0.031 mol L ), where the R is high for both zero-
and first-order. Fig. 6(b) shows the linear plot for first-order, where the
model was observed since the linear regression gives a relatively poor
2
2
linear regression R is also reliable, although a deviation from the ex-
R
value of 0.937. The calculated average intrinsic first-order rate
−1
perimental data and a parabolic residual plot (Fig. S9) indicates that
this is not the most appropriated model to describe the system. The
same behavior was observed for the concentrations from 0.031 to
constant k is 1.464 min . To confirm the reaction order in relation to
, the logarithm was applied in both sides of eq. (6), leading to the
relation represented in eq. (7).
2 2
H O
−1
0
.31 mol L (Figs. S9 – S12). However, as the concentration of H
further increased (Figs. S13–S18), it is less evident that the data still
follows a zero-order model, as can be seen for the highest H con-
2 2
O
log (kobs) = log (k) + b. log ([H
2
O
2
]
0
)
(7)
2
O
2
Fig. 9(b) shows the plot of log(kobs) versus log([H
clear that two different values of reaction order were observed for b: 0
2
O
2
]
0
) where it is
centration tested in Fig. 7(a). After 40 min the dye was completely
consumed, and the shape of the curve could be misinterpreted as an
exponential decay of first-order reaction, but the linear plot (Fig. 7(b))
undoubtedly rules out the first-order model. In the high H O con-
2 2
centration cases, to find the zero-order kinetic constant rate, the data
was fitted only in the beginning of the reaction.
To provide further evidence of the reaction order, in a second set of
experiments the initial concentration of methyl orange was varied as
shown in Fig. 8(a). The Figs. S19–S23 show the fits for all MO con-
centrations tested. Table 3 presents the values of the observed pseudo-
zero order rate constants and Fig. 8(b) presents the log plot of kobs
versus MO concentration. Although a variation was observed in the
values from experimental error, an horizontal straight line with slope of
2 2 2 2
at low H O concentrations and 0.8 at high H O concentrations.
Probably at lower concentration of oxidant the light intensity plays the
dominant role in the dye photodegradation, but at higher concentra-
tions of oxidant the reaction rate is affected by its concentration. This
may be due to the catalase-like activity displayed by the complex, that
at low concentration of oxidant does not form enough hydroxyl radi-
2 2
cals. However, at higher concentration of H O , the formation of hy-
droxyl radicals starts to play an important effect in the reaction rate.
Evidence of catalase-like activity can be seen from the significant in-
tercept in Fig. 9(a), which has been associated with the parallel reac-
tion of H
Fig. 10(a) compares the quenching of the azo band by the different
copper(II) complexes, where [Cu(MPBMPA)Cl ] was the most efficient
with 100% of degradation and kobs = 2.962 μmol L min , followed
2 2
O disproportionation [37].
0.0015 confirms the assigned pseudo-zero order in relation to MO.
2
−1
−1
Ollis [20] have established based on experimental kinetic data, that
−
1
−1
for heterogeneous photocatalysis the most suitable reaction model in
relation to the reactant is zero-order. However, misinterpretation of the
data has led many author to assign it as first-order, because the semilog
plot of C(t) vs. time is usually linear. However, they often exhibit
closely by [Cu(PABMPA)Cl]Cl kobs = 2.835 μmol L
min
(Figs.
S25-S27). [Cu(BMPA)Cl ] and [Cu(PBMPA)](ClO ) presented slightly
2
4
−
1
−1
smaller rate constants, 2.565 and 2.630 μmol L min , respectively.
Intrinsic rate constants (Table 2) reflect the high catalytic activity of the
−
2
2 2 2
Fig. 6. (a) Kinetic fit of the time trace and linear plots of the UV–VIS band at λmax = 495 nm, from the degradation of MO by [Cu(BMPA)Cl ], H O (3.1 × 10
−
1
mol L ) and UV light (24 W). (b) Linear plot for first-order equation (eq. S2).
6

S.S.F. Carvalho, et al.
Inorganica Chimica Acta 512 (2020) 119924
2.0
1.5
1.0
0.5
0.0
(a)
(
b)
Experimental
0
1
2
2
1
st order ; R = 0.987
2
2
st order ; R = 0.860
-
-
2
Zero order ; R = 0.999
-3
-
4
0
20
40
60
80
100
0
20
40
60
80
100
Time (min)
Time (min)
−
1
2 2 2
Fig. 7. (a) Kinetic fit of the time trace and linear plots of the UV–VIS band at λmax = 495 nm, from the degradation of MO by [Cu(BMPA)Cl ], H O (15.4 × 10
−
1
mol L ) and UV light (24 W). (b) Linear plot for first-order equation (eq. S2).
Fig. 8. (a) Time trace at λmax = 495 nm for different initial concentration of MO and (b) respective log plot (eq. (5)), from the photo-degradation of MO by [Cu
−
5
−1
−1
−1
(
BMPA)Cl
2
] (3.3 × 10
mol L ), H
2
O
2
(15.4 × 10
mol L ) and UV light (24 W).
Table 3
constant k, and the hydroxyl radical concentration in the case of liquid-
a
Degradation percentage and kinetic data of photodegradation of dyes.
phase homogeneous catalysis, which is dependent of the intensity of the
light [20]. The kinetic data were also well adjusted to zero-order for all
dyes (Figs. S28-S31). This may be the case of the studied dyes, which
(μmol L−1)
−1
min−1)b
[
MO]
o
kobs (μmol L
1
2
3
4
5
6.7
5.0
3.3
1.7
0.0
1.973
1.812
1.916
2.007
1.938
presented similar values for zero-order
obs = 2.565, CV: kobs = 3.969; MB: kobs = 3.933 μmol L min . In
the case of CR, two azo bonds are present and the kobs = 0.201 μmol
kobs (Table 2): MO:
−1
−1
k
−1
−1
L
min is roughly thirteen folds lower than the analogous MO that
presented only one azo bond. By other side, the dye RhB presented a
a
Catalyst at 3.3 × 10−5 mol L−1; dye at 5.0 × 10−5 mol L−1; UV
−1
−1
higher kobs = 7.012 μmol L
min
probably because of its fluor-
light (24 W).
escent property that favor the light absorption and subsequent de-
gradation by the catalyst. Comparing with the data at the higher in-
tensity vapor Hg lamp (250 W), the structure of the dye did not present
any effect on the photocatalytic activity. But in the case of the lower
intensity UV lamp (24 W), probably not enough OH⋅ radicals were
formed to degrade the two azo bonds of CR in 50 min.
b
Zero-order rate constant.
complexes. Since all of them presented similar structures, ligands and
charge transfers, similar behavior is expected. A correlation with redox
potentials measured in DMSO for related iron(III) complexes with the
same ligands showed that the complexes with the ligands MPBMPA and
PABMPA presented the lowest redox potentials [36], what can favor the
LMCT and consequently promoted the photocatalysis. By other side, the
iron(III) complexes with BMPA and PBMPA presented the most nega-
tive redox potentials and more energy will be necessary to perform the
LMCT.
Fig. 10(b) shows the degradation of the different dyes with the
2
complex [Cu(BMPA)Cl ], were it is possible to observe a complete de-
composition at 50 min, except for Congo red (CR) that achieved 88% of
degradation only after 90 min. According to the literature, the structure
of the dye does not affect appreciably the photocatalytic reaction rate,
but it is rather affected by the type of catalyst expressed in the rate
4. Conclusions
Herein, the photocatalytic degradation of dyes of different struc-
tures: methyl orange, methylene blue, crystal violet, Congo red and
Rhodamine B, promoted by copper(II) complexes with NNN derivative
ligands was investigated. All dyes were successfully degraded in less
than 90 min, under UV radiation provided by a vapor Hg lamp of
250 W, only in the presence of both complex and hydrogen peroxide as
oxidant. The ligand to metal charge transfer (LMCT) was associated
with the improvement in the catalytic degradation promoted by the
complexes, enhancing even further the dye degradation by oxygenated
7

S.S.F. Carvalho, et al.
Inorganica Chimica Acta 512 (2020) 119924
2 2 2
Fig. 9. (a) Zero-order rate constant versus H O concentration (eq. (4)) and (b) respective log plot (eq. (7)), from the degradation of MO by [Cu(BMPA)Cl ] and UV
light (24 W).
−
1
mol L−1) and UV light (24 W). (b) Photo-
Fig. 10. (a) Time trace at λmax = 495 nm for MO photo-degradation by different complexes, H
degradation of different dyes catalyzed by [Cu(BMPA)Cl ].
2
O
2
(15.4 × 10
2
radials formed from Cu(II) ion, H
2
O
2
and UV light. Kinetics experiments
Declaration of Competing Interest
carried out in situ with a UV light of lower radiation, unveiled an
unusual zero-order reaction in relation to the dye. The experimental
data were better fit by a zero-order equation and experiments with
variable initial concentration of dye showed that the zero-order rate
constant did not vary with concentration, supporting this proposal.
These results represent a new analysis of the kinetics of homogeneous
photocatalysis of organic dyes, in accordance with described analogous
heterogeneous photocatalysis. Moreover, a variable reaction order in
relation to hydrogen peroxide was observed, from 0 to 0.8 from low to
high concentrations of oxidant. It was also demonstrated that the effect
of the light and type of catalysts together with the charge transfer that
occurs between the metal and the ligands, plays the major role in de-
termining the path of the dye photodegradation.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgements
We acknowledge Conselho Nacional de Desenvolvimento Científico
e Tecnológico (BR) – CNPq (PQ-2/2018), Fundação de Amparo à
Pesquisa do Estado do Rio de Janeiro – FAPERJ (JCNE 2018: E-26/
203.023/2018) and Coordenação de Aperfeiçoamento de Pessoal de
Nível Superior – Brasil (CAPES) (PROAP and PhD Scholarship) for
supporting this project.
Appendix A. Supplementary data
CRediT authorship contribution statement
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.ica.2020.119924.
Samira S.F. Carvalho: Data curation, Formal analysis,
Investigation, Methodology, Validation. Ana Carolina C. Rodrigues:
Data curation, Formal analysis, Methodology. Juliana F. Lima:
Conceptualization, Formal analysis, Investigation, Methodology,
Writing - original draft, Writing - review & editing. Nakédia M.F.
Carvalho: Conceptualization, Formal analysis, Funding acquisition,
Investigation, Methodology, Project administration, Resources,
Supervision, Validation, Visualization, Writing - original draft, Writing -
review & editing.
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