Photocatalytic degradation of methyl orange mediated by a silica coated nanomagnet porphyrin hybrid

Article abstract of DOI:10.1016/j.jorganchem.2021.121751

The photocatalytic activity of a silica coated nanomagnet porphyrin hybrid (NPH) and of the corresponding porphyrin precursors (H₂P and ZnP) was evaluated in the degradation of the methyl orange dye (MO) under visible light irradiation. The cat

Full text of DOI:10.1016/j.jorganchem.2021.121751

Journal of Organometallic Chemistry 938 (2021) 121751
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Photocatalytic degradation of methyl orange mediated by a silica
coated nanomagnet porphyrin hybrid
Kelly A.D.F. Castroa,#,∗, João M.M. Rodriguesb,#,∗, M. Amparo F. Faustinoa, João P.C. Tomé c,
José A.S. Cavaleiroa, Maria da Graça P.M.S. Nevesa,∗, Mário M.Q. Simõesa,∗
a LAQV-REQUIMTE, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal
b CICECO - Aveiro Institute of Materials, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal
c Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
The photocatalytic activity of a silica coated nanomagnet porphyrin hybrid (NPH) and of the correspond-
ing porphyrin precursors (H2P and ZnP) was evaluated in the degradation of the methyl orange dye (MO)
under visible light irradiation. The catalytic degradation of MO was performed under air, in the absence
and in the presence of aqueous hydrogen peroxide. The results show that the hybrid NPH was the most
effective photocatalyst causing the total degradation of MO after 270 min of white light irradiation (150
mW cm−2) in the presence of hydrogen peroxide. The remarkable photocatalytic activity of this NPH,
associated with the possibility of its reuse, makes this material a promising photocatalyst.
Received 31 January 2021
Accepted 15 February 2021
Available online 17 February 2021
Keywords:
Porphyrin
Silica nanomagnetic particles
Photocatalysis
methyl orange
Degradation
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
approaches based on catalytic processes appear as important al-
ternatives in the remediation of effluents. In fact, the fundamen-
Environmental pollution is one of the major and most urgent
problems of the modern world requiring a constant and special
attention from the scientific community. In particular, wastewater
from dyeing industries has become exceptionally worrisome, since
the presence of dyes can inhibit sunlight penetration and, conse-
quently, might reduce the photosynthetic process [1,2]. Following
these inputs, the scientific community is contributing with the de-
velopment of novel and economically sustainable methodologies
for the detoxification of textile wastewaters, aiming to follow the
recommended quality criteria and to close the water cycle [3,4].
The large-scale use of dyes results in the production of large
volumes of pollutants, which are often discarded without any pre-
vious treatment. Therefore, the development of efficient method-
ologies in order to minimize the environmental impact caused by
industrial effluents is of great interest [5]. Different methodolo-
gies have been developed for the degradation and/or scavenging
of those dyes, especially azo dyes (the most common synthetic
dyes, ca 70% wt.). These methodologies include adsorption, bio-
logical oxidation, membrane filtration, ozonation, oxidation using
UV/H2O2, UV/TiO2, UV or visible light and catalysis [3,6–9]. The
tal pillar of green chemistry is catalysis, generally resulting in sig-
nificant gains in terms of overall efficiency of a chemical reaction.
Porphyrins and analogues are among the catalysts with recognized
efficacy for azo dyes (photo)degradation [1,5,10].
Metalloporphyrins and analogues have already proved their ef-
ficiency as catalysts, namely for oxidative reactions [11–20]. Por-
phyrins are also efficient producers of reactive oxygen species
(ROS) in the presence of light, such as singlet oxygen (1O2) [21–
24], thus inducing many photocatalytic reactions [25,26]. The reac-
tion of ROS with the target pollutants is an alternative method for
removing organic pollutants [27,28]. From the successful history of
photocatalysts, the special attention given to porphyrins and ana-
logues by the scientific community is due to their strong absorp-
tion bands in the visible region, versatile chemical structures, and
facile tuning of the electronic properties. Besides, the insertion of
diverse metals into the core of the macrocycle can modulate the
catalytic activity associated with the porphyrins’ structure. Addi-
tionally, the development of heterogeneous catalysts based on por-
phyrins represents an important challenge for a sustainable devel-
opment.
Recently, magnetite nanoparticles (Fe3O4) have originated
a
great interest due to their physicochemical characteris-
∗
Corresponding authors.
tics. Fe3O4 with different structures has been used in differ-
ent areas, such as environmental technology, nanotechnology,
medicine and catalysis [29–33]. In addition to their exceptional
E-mail addresses: kadfc16@gmail.com (K.A.D.F. Castro), jrodrigues@ua.pt (J.M.M.
Rodrigues), gneves@ua.pt (M.d.G.P.M.S. Neves), msimoes@ua.pt (M.M.Q. Simões).
#
These two authors contributed equally to this work.
https://doi.org/10.1016/j.jorganchem.2021.121751
0022-328X/© 2021 Elsevier B.V. All rights reserved.

K.A.D.F. Castro, J.M.M. Rodrigues, M. Amparo F. Faustino et al.
Journal of Organometallic Chemistry 938 (2021) 121751
variety of applications, magnetite nanoparticles have the ad-
vantage of their possible reuse. For instance, Qin and co-
workers studied Fe3O4/TiO2 magnetic nanoparticles for pho-
tocatalytic degradation of phenol [34]. In another study, it
was reported the synthesis and characterization of magnetic
nanoparticles functionalized with [5,15-bis(phenyl)-10,20-bis(4-
methoxycarbonylphenyl)porphyrin]platinum(II) and the photocat-
alytic activity of the hybrid was tested using 2,4,6-trichlorophenol
as the target pollutant [35]. More recently, magnetite nanoparticles
with different morphologies (cubic and spherical ones) decorated
with the 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin showed to
be able to generate 1O2 under ultraviolet irradiation. Their study
as photocatalysts towards bisphenol A (BPA) disclosed degradation
values of 64% (cubic-shaped) and of 90% (spherical shape) in the
presence of hydrogen peroxide (H2O2) [36]. Additionally, these ma-
terials retained their catalytic features for at least three catalytic
cycles.
with distilled water. The organic phase was dried (Na2SO4)
and the solvent was evaporated under reduced pressure. The
ZnP was obtained in quantitative yield by crystallization in
dichloromethane/hexane.
H2P: Yield: 19%; 1H NMR (DMSO-d6): δ -3.13 (s, 2H, NH), 2.37
(s, 9H, Ts-CH3), 3.18 (q, J = 6.2 Hz, 6H, CH2), 3.64 (q, J = 6.2 Hz, 6H,
CH2), 6.47 (s, 3H, NH-PhF4), 7.47 (d, J = 8.2 Hz, 6H, Ts-o-H), 7.81 (d,
J = 8.2 Hz, 6H, Ts-m-H), 7.90 (t, J = 6.1 Hz, 3H, NH-Ts), 9.24-9.33
(m, 8H, H-β) ppm. 19 F NMR (DMSO-d6): δ -163.08 to -162.97 (m,
2F, o-F), -166.67 (d, J = 18.4 Hz, 6F, Ts-o-F), -177.52 (t, J = 22.4 Hz,
1F,p-F), -183.67 to -183.77 (m, 6F, Ts-m-F), -186.43 to -186.25 (m,
2F, m-F). UV-Vis (CHCl3), λmax: (log ε): 421 (5.36), 511 (4.26), 548
(3.64), 586 (3.86), 653 (3.42). UV-Vis (DMSO), λmax: 424, 512, 548,
586, 648 nm. HRMS-ESI: Calculated for C71H50F17 N10O6S3 [M+H]+
1557.2805; found: 1557.2587.
ZnP: 1H NMR (DMSO-d6): δ 2.36 (s, 9H, Ts-CH3), 3.16 (q, J = 6.0
Hz, 6H, CH2), 3.61 (q, J = 6.0 Hz, 6H, CH2), 6.39 (s, 3H, NH-PhF4),
7.47 (d, J = 8.2 Hz, 6H, Ts-o-H), 7.81 (d, J = 8.2 Hz, 6H, Ts-m-H),
7.91 (t, J = 6.0 Hz, 3H, NH-Ts), 9.07-9.16 (m, 8H, β-H). 19 F NMR
(DMSO-d6): δ -162.78 to -162.87 (m, 2F, o-F), -166.22 (d, J = 17.8
Hz, 6F, Ts-o-F), -178.23 (t, J = 22.4 Hz, 1F, p-F), -184.06 (dd, J = 17.8
and 10.1 Hz, 6F, Ts-m-F), -186.73 to -186.92 (m, 2F, m-F). UV-Vis
(CHCl3), λmax (log ε): 418 (5.24), 513 (4.24), 580 (3.43) nm. UV-
Vis (DMSO), λmax: 420, 520, 586 nm. MS (MALDI TOF/TOF) (m/z):
1620.0 [M+H]+.
Following our interest in developing new materials for
environmental remediation, in this publication we report
the synthesis and characterization of core-shell magnetite-
silica nanoparticles functionalized with the Zn(II) complex
of
a
porphyrin obtained from the reaction of 5,10,15,20-
tetrakis(pentafluorophenyl)porphyrin with tosylethylenediamine.
Furthermore, the efficacy of this material to act as photocatalyst
was evaluated under white light irradiation (380-750 nm) using
methyl orange dye (MO) as the target pollutant. The studies
were performed under air in the absence and in the presence of
aqueous H2O2.
2.3. Preparation of the Nanomagnet Porphyrin Hybrid (NPH) material
2. Experimental
The Nanomagnet Porphyrin Hybrid (NPH) material was pre-
pared according to the procedure described in the literature [39].
Briefly, the magnetic cores (magnetite) were synthesized by the co-
precipitation method under basic conditions using ammonium hy-
droxide. Then, the cores after being coated with silica using the
silicic acid approach were maintained under stirring for 24 h in
the presence of (3-aminopropyl)triethoxysilane (APTS) in order to
obtain the desired nanoparticles functionalized with aminopropyl
chains. The magnetic aminopropyl silica nanoparticles (Si-NP) were
filtered, washed several times with ethanol, followed by magnetic
decantation purification. Subsequently, the immobilization of the
porphyrin ZnP into Si-NP was performed. For this, a previously
prepared ethanol suspension of the Si-NP [39] (13.5 mL, corre-
sponding to 250 mg of Si-NP) were filtered through a polyamide
membrane, washed several times with DMSO and re-suspended in
DMSO (6 mL). A solution of ZnP (20.0 mg, 12.3 μmol) in DMSO (2
mL) was added to the previous Si-NP suspension and the result-
ing mixture was stirred for 24 h at 160°C (Scheme 1). The immo-
bilization of ZnP was easily monitored by thin-layer chromatog-
raphy since the disappearance of the spot corresponding to ZnP
was accompanied by the concomitant appearance of a spot corre-
sponding to the NPH material at the application point. The insol-
uble material with a violet color was washed several times with
appropriate solvents: firstly, dichloromethane and then a mixture
of dichloromethane/methanol (90:10) until the Soret band of the
ZnP was no longer detected through UV-Vis in the rinsing sol-
vent. The quantification of the non-immobilized ZnP present in
the washing solvents allowed to calculate the ZnP loading in the
material (based on the ε value of the Soret band of ZnP). In the
washing process, the hybrid material NPH in DMSO was firstly de-
canted by recurring to a magnet and then filtered under vacuum,
using a polyamide membrane on a Büchner funnel. The NPH ma-
terial was re-suspended and kept in dry DMSO (25 mL), making a
stock solution of the NPH photocatalyst for the photocatalytic ac-
tivity assays. NPH was characterized by UV-Vis and fluorescence
spectroscopy.
2.1. Reagents and Equipment
All chemicals used in this study were purchased from Sigma-
Aldrich or Merck and were of analytical grade.
Electronic spectra (UV-Vis) were obtained on a Shimadzu UV-
2501PC spectrophotometer, in the 350-800 nm range. The fluores-
cence emission spectra were recorded in DMF in 1 × 1 cm quartz
optical cells at 298 K under normal atmospheric conditions on a
computer-controlled Horiba Jobin Yvon FluoroMax-3 spectrofluo-
rimeter. The widths of both excitation and emission slits were set
at 2.0 nm. 1H and 19 F NMR spectra were recorded on a Bruker
Avance AMX 300 spectrometer at 300.13 MHz and 282.38 MHz,
respectively. Deuterated dimethylsulfoxide was used as solvent and
TMS (δ = 0 ppm) as the internal standard; chemical shifts are re-
ported in ppm (δ) and coupling constants (J) are given in Hz. Mass
spectra were acquired on an Applied Biosystems Analyzer mass
spectrometer (MALDI TOF/TOF). HRMS-ESI spectra were recorded
on VG AutoSpec-M spectrometer.
2.2. Preparation of porphyrins H2P and ZnP
The porphyrins were prepared according to the following steps:
i) The precursor 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin
[H2(TPFPP)] was synthesized by condensation of pyrrole with
pentafluorobenzaldehyde under acidic conditions [11,37].
ii) The tri-substituted free-base porphyrin (H2P) was obtained by
structural modification of [H2(TPFPP)] in the presence of the
nucleophile N-tosylethylenediamine, as described in the litera-
ture [38].
iii) The Zn(II) complex of H2P (ZnP) was obtained by adding
zinc(II) acetate (42.1 mg, 0.32 mmol) to a solution of H2P (50.0
mg) in dichloromethane/methanol (2:1, 15 mL) and the result-
ing mixture was refluxed at 60 °C for 1 h (Scheme 1). After
cooling to room temperature, the reaction mixture was washed
2

K.A.D.F. Castro, J.M.M. Rodrigues, M. Amparo F. Faustino et al.
Journal of Organometallic Chemistry 938 (2021) 121751
Scheme 1. Synthetic procedure used to obtain the Nanomagnet Porphyrin Hybrid (NPH) and the structure of methyl orange (MO) used as azo dye model.
A stock solution of the dye ([MO] = 8.26 × 10−2 mol L−1) was
2.4. Singlet oxygen generation
prepared in milli-Q water. To a 4 mL quartz cuvette, 3.0 mL of wa-
Stock solutions of each porphyrin (H2P and ZnP) and of 1,3-
diphenylisobenzofuran (DPiBF) in DMF at 0.1 mM and 10 mM, re-
spectively, were prepared. The reaction mixture containing DPiBF
(50 μM) and a solution of each photocatalyst (0.5 μM) in DMF was
irradiated in a quartz cell (3 mL), under magnetic stirring at an ir-
radiance of 10 mW cm−2 with a homemade LEDs array. The LEDs
array is composed of a matrix of 5 × 5 LEDs making a total of
25 light sources with an emission peak centered at 654 nm and
ter, 200 μL of MO dye solution and the adequate volume of stock
solution of the catalyst (H2P or ZnP in DMSO) were added to ob-
tain a final concentration of 0.5 μM. For the reactions carried out in
the presence of hydrogen peroxide, 100 μL of aqueous H2O2 (30%,
w/w) were added. In the case of the heterogeneous photocatalyst
(NPH), the solution was stirred in the dark (30 min) before irra-
diation in order to obtain an equilibrium point of initial physical
adsorption of MO over the surface of the photocatalyst. All photo-
catalytic experiments were accomplished under similar conditions.
The photocatalytic performance was monitored indirectly by relat-
ing the decrease in the absorbance of MO at 464 nm in solution
with its degradation. The photocatalytic reactions in the presence
of hydrogen peroxide were also performed in the presence of man-
nitol. The production of •OH radicals was investigated indirectly
by degradation of terephthalic acid (TA) [40]. The reaction mix-
ture containing TA (1 mM) and a solution of each photocatalyst
(0.5 μM H2P, 0.5 μM ZnP and 5.0 μM of ZnP in NPH in PBS solu-
tions (pH = 7.0) with 1% DMSO and 100 μL of aqueous H2O2 (30%,
w/w) was irradiated in a quartz cell (4 mL) with white light at
an irradiance of 150 mW cm−2. The reaction mixture was stirred
in the dark (30 min) before irradiation. The TA degradation was
monitored by measuring the emission at 450 nm (λexc = 315 nm)
for 60 min of irradiation. Dark controls were included.
a bandwidth at half maximum of
20 nm. During the irradia-
tion period the solutions were stirred at ambient temperature. The
DPiBF degradation was monitored by measuring the absorbance
decrease at 415 nm at irradiation intervals of 1 min. The percent-
age of decay of DPiBF absorption is related with the production
of singlet oxygen. The quantification was achieved by the differ-
ence between the initial and the final absorbance at 415 nm over
a given irradiation time. The same strategy was adopted for NPH,
but the photocatalyst concentrations used were 0.5, 1.0, 2.0, and
5.0 μM. The results obtained were compared with those obtained
in the presence of H2TPP (5,10,15,20-tetraphenylporphyrin) and in
the absence of any porphyrin (negative control) under similar irra-
diation conditions.
2.5. Photocatalytic activity
The photocatalytic activity of H2P, ZnP and NPH was evaluated
in aqueous solutions using methyl orange (MO) as model substrate
and under visible light irradiation, which was performed with a
halogen 500 W lamp at an irradiance of 150 mW cm−2. This light
source was positioned 10 cm away from the batch reactor and the
temperature was kept at ca 20 °C under stirring. The reactions
were performed under air and in the presence of aqueous H2O2.
3. Results and Discussion
3.1. Synthesis and characterization of the photocatalysts
The synthetic methodology used in the preparation of the ho-
mogeneous and the heterogeneous photocatalysts is summarized
in Scheme 1. The porphyrin derivative H2P was prepared by a
3

K.A.D.F. Castro, J.M.M. Rodrigues, M. Amparo F. Faustino et al.
Journal of Organometallic Chemistry 938 (2021) 121751
Fig. 2. Emission spectra of Si-NP and NPH in DMSO solution.
cles (Si-NP) did not present bands in the region of 400 nm (Figure
S6).
The presence of ZnP in the hybrid was also confirmed by the
emission spectrum obtained of the NPH material in DMSO solu-
tion upon excitation at 410 nm (Fig. 2). The strong emission band
observed in the red region at ca. 594 nm [λ > Qx(0,0)] can only be
due to immobilized ZnP since the Si-NP did not present any emis-
sion band upon 410 nm excitation. Similar results were observed
for other porphyrin derivatives when immobilized [42].
Fig. 1. UV-Vis spectra of H2P, ZnP and NPH in DMSO.
controlled nucleophilic substitution of three of the p-fluorine
atoms in H2(TPFPP) with N-tosylethylenediamine, according to the
procedure reported previously by our group [38]. The subsequent
reaction of H2P with zinc(II) acetate in dichloromethane:methanol
(2:1) at 60 °C afforded the corresponding ZnP complex (Scheme 1).
The structural confirmation of both derivatives H2P and ZnP
was performed by recurring to adequate spectroscopic techniques
namely 1H NMR, mass spectrometry (Figures S1-S5 in the Support-
ing information) and UV-Vis spectroscopy.
3.2. Singlet oxygen generation
The ability of photocatalysts to generate ROS, namely singlet
•
oxygen (1O2), superoxide anion radical (O2 −), hydrogen peroxide
(H2O2) and hydroxyl radical (•OH) is considered a crucial feature
regarding their efficiency in photocatalysis [43]. In general, when
porphyrin derivatives are used as photocatalysts, 1O2 is the ma-
jor ROS involved [25]. The production of 1O2 was assessed by an
indirect chemical method using 1,3-diphenylisobenzofuran (DPiBF)
as a probe. As other furans, this compound is able to react as a
diene in a [4+2] process with 1O2 as the dienophile affording the
colorless o-dibenzoylbenzene; with this quencher, exclusively 1O2
is detected [44].
The covalent immobilization of ZnP onto the core-shell mag-
netite silica nanoparticles (Si-NP) functionalized with amino
groups occurred via nucleophilic substitution of the p-fluorine
atom at the C6F5 group and was performed in DMSO at 160 °C for
24 h; the functionalized nanoparticles Si-NP were obtained by re-
acting the magnetic cores covered with silica with APTS according
with literature procedures [39,41]. After the immobilization step,
the obtained violet nanoparticles were thoroughly washed with
appropriate solvents affording the desired NPH, the ZnP loading in
the NPH found was 3.37 × 10−4 mol g−1. This loading was deter-
mined by quantifying the amount of the non-immobilized ZnP by
UV-Vis in the rinsing solutions. Then, the NPH was re-suspended
in DMSO and the success of the immobilization was confirmed by
UV-Vis and fluorescence spectroscopy.
So, the photodegradation of DPiBF mediated by H2P, ZnP, NPH
and also by H2TPP (used as reference) was qualitatively assessed
by monitoring its absorbance decay at 415 nm as a function of the
irradiation time with red light (654
20 nm). The results in Fig. 3
show that the absorbance of DPiBF at 415 nm decreased in the
presence of all porphyrin derivatives under irradiation as a func-
tion of time through a 1st kinetic order. Comparing the photode-
composition slope promoted by each porphyrin is observed that
H2P was the best 1O2 generator, being even better than the ref-
erence (H2TPP), followed by ZnP and NPH. For NPH, a significant
increase of 1O2 production was observed when its concentration
was increased from 0.5 to 5 μM.
This observation was important in order to establish the
amount of material required for the photocatalytic reactions. This
reduction in 1O2 production is in accordance with other studies in-
volving silica nanoparticles or others nanoplatforms where the 1O2
production by the porphyrin immobilized is significantly reduced
[45].
The UV-Vis spectra of H2P, ZnP and of the NPH suspension
in DMSO are displayed in Fig. 1. The non-immobilized porphyrin
derivatives H2P and ZnP show the typical Soret band at 424 and
420 nm, respectively, accompanied by the less intense Q bands at
512, 548, 586, and 648 nm for H2P and at 520 and 586 nm for
ZnP; this decrease in the number of the Q bands after metalation
is related to the alteration in the micro symmetry of the porphyrin
macrocycle. In the magnetic hybrid NPH the presence of ZnP com-
plex was promptly confirmed by the appearance of the Soret band
at 419 nm and of the corresponding Q-bands at 515 and 580 nm.
These features validated the success of the immobilization and the
UV-Vis spectrum profile is similar to that obtained for the non-
immobilized ZnP in solution. Additionally, the UV-Vis spectra were
also acquired in a mixture of DMSO:H2O and are shown in the SI
(Figure S6). As it is expected the spectrum of the silica nanoparti-
The photostability and the solubility of the catalyst in solu-
tion are important for homogeneous catalysis. The photostability
of H2P, ZnP and NPH were investigated by UV-Vis, monitoring the
absorbance of the corresponding Soret band after different times of
4

K.A.D.F. Castro, J.M.M. Rodrigues, M. Amparo F. Faustino et al.
Journal of Organometallic Chemistry 938 (2021) 121751
Fig. 3. Time dependent photodecomposition of DPiBF (50 μM) in DMF upon irradiation with red light (654
20 nm) in the absence (DPiBF) or in the presence of H2P, ZnP
and NPH (0.5 μM for H2P, ZnP and H2TPP and 0.5, 1.0, 2.0 and 5.0 μM of ZnP on NPH). H2TPP was used as reference. Note: Small bars are overlapped by the symbols.
Fig. 4. UV-Vis spectroscopy study of H2P, ZnP and NPH at a concentration of 5.0 μM in DMSO/water before and after white light irradiation at an irradiance of 150 mW
cm−2 at different irradiation times (0 - 240 min).
irradiation (Fig. 4). In all cases, a good photostability was observed
under the photocatalytic studies conditions.
in the reaction mixture at time zero and At is the MO absorbance
at an established time (t).
The catalysts H2P, ZnP and NPH showed significant differences
(p < 0.05) in terms of dye degradation efficiency when the irradi-
ations were performed without H2O2 (Fig. 4). NPH was the most
efficient photocatalyst causing a reduction of 54% in MO dye con-
centration after 240 min of irradiation when compared with H2P
and ZnP which attained a maximum of ca 12% under the same ir-
radiation period. In addition, the catalytic reactions were carried
out in the absence of white light irradiation (dark conditions), and
the MO degradation was not observed in the presence of none of
the photocatalysts (H2P, ZnP and NPH) under dark conditions, as
well as for both controls (dark and light).
3.3. Photocatalytic activity
The photocatalytic activity of H2P, ZnP and NPH was evaluated
using MO as an azo dye model due to its resistance to environ-
mental degradation. The experiments were performed under white
light irradiation (380-750 nm) under air, and in the absence or in
the presence of aqueous H2O2 as oxidant. The degradation of MO
in the presence of H2P, ZnP and NPH was monitored by measur-
ing the decay of MO absorbance band at 464 nm. The photocat-
alytic efficiency of the different materials was expressed using the
following equation: (A0-At)/A0, where A0 is the absorbance of MO
The results obtained when the experiments were repeated in
the presence of H2O2 are summarized in Figs. 6 and 7, in the dark
5

K.A.D.F. Castro, J.M.M. Rodrigues, M. Amparo F. Faustino et al.
Journal of Organometallic Chemistry 938 (2021) 121751
Fig. 5. Photocatalytic degradation of methyl orange (MO) under air at different photoreaction times under light irradiation (150 mW cm−2): in the presence of 0.5 μM of
H2P (red shape) and ZnP (green shape); 5.0 μM of ZnP in NPH (purple shape) and in the absence of catalyst (Control, blue shape). The absorption of MO was monitored at
464 nm. Note: Small bars are overlapped by the symbols (n = 3).
Fig. 6. Degradation of methyl orange (MO) under dark conditions at different reaction times: in the presence of aqueous H2O2 (0.25 mol L−1) and 0.5 μM of H2P (red bars);
0.5 μM of ZnP (green bars); 5.0 μM of ZnP in NPH (purple bars) and with no catalyst (Control, blue bars). The MO absorbance was monitored at 464 nm. Note: Small bars
are overlapped by the symbols. Note: Small bars are overlapped by the symbols (n = 3).
and after being irradiated, respectively. The results in show that in
the absence of light (dark conditions) and after 90 min of reaction
in the presence of any of the catalysts and H2O2 only 7% of MO
was oxidized. After 270 min of reaction this value reached 13% for
ZnP and NPH while for H2P the value remains almost constant (ca
8%) and was similar to that observed for the control assay (reac-
tion performed in the presence of H2O2 only). These results indi-
cate that there is no remarkable difference between the different
catalysts when dye degradation is performed in the presence of
H2O2 but in the absence of light.
The results in Fig. 6 show that the profile of these reactions
in the presence of aqueous H2O2 is totally different when carried
out under light irradiation. In fact, the action of light was particu-
larly relevant for improving the rate of oxidation mediated by NPH
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K.A.D.F. Castro, J.M.M. Rodrigues, M. Amparo F. Faustino et al.
Journal of Organometallic Chemistry 938 (2021) 121751
Fig. 7. Photocatalytic degradation of methyl orange (MO) in the presence of aqueous H2O2 (0.25 mol L−1) at different irradiation times (irradiation with white light at an
irradiance of 150 mW cm−2) in the presence of 0.5 μM of H2P (red bars); 0.5 μM of ZnP (green bars); 5.0 μM of ZnP in NPH (purple bars) and in the absence of any catalyst
(Control, blue shape). The MO absorbance was monitored at 464 nm. Note: Small bars are overlapped by the symbols. Note: Small bars are overlapped by the symbols
(n = 3).
Fig. 8. Photocatalytic degradation of methyl orange (MO) in the presence of aqueous H2O2 (0.25 mol L−1) and mannitol (0.1 mol L−1) at different irradiation times (irradiation
with white light at an irradiance of 150 mW cm−2) mediated by 0.5 μM of H2P (red bars), 0.5 μM of ZnP (green bars) and 5.0 μM of ZnP in NPH (purple bars). The blue bars
represent the control reaction (only hydrogen peroxide and mannitol). The band of MO was monitored at 464 nm. Note: Small bars are overlapped by the symbols (n = 3).
(100% after 270 min) and ZnP (75% after 270 min). When com-
pared with the photoreactions performed under light irradiation
but in absence of H2O2 (Fig. 5), an increment of ca 46% in catalytic
activity was achieved for NPH, 56% for ZnP and 12.5% for H2P. So,
the beneficial effect of H2O2 in MO photodegradation is obvious
when the results are compared with those obtained in its absence
(Figs. 5 and Fig. 7).
The photodegradation of MO in the presence of ZnP, NPH and
H2O2 showed to be time-dependent. This time dependence was
also observed for NPH in the absence of H2O2. In fact, a longer
contact time with the photocatalyst can facilitate the interaction
of MO with the oxidation promoting species. The pathway respon-
sible for MO photodegradation after photocatalyst activation by
white light in the presence of molecular oxygen (O2) can involve
7

K.A.D.F. Castro, J.M.M. Rodrigues, M. Amparo F. Faustino et al.
Journal of Organometallic Chemistry 938 (2021) 121751
Fig. 9. Emission spectra (λexc = 315 nm) obtained during the photolysis of TA (1.0 mM) at different irradiations times in the presence of H2O2 (0.25 mol L−1) and (A) 0.5
μM H2P, (B) 0.5 μM ZnP and (C) 5.0 μM of ZnP in NPH; all the irradiations were performed under white light at an irradiance of 150 mW cm−2 in PBS solutions (pH = 7.0)
with 1% DMSO. (D) Evolution of HTA formation during the photolysis of TA for 60 min in the presence of 0.5 μM H2P, 0.5 μM ZnP and 5.0 μM of ZnP in NPH.
ROS such as hydrogen peroxide, superoxide and hydroxyl radi-
cals (type I photochemical pathway) and/or singlet oxygen (pho-
tochemical pathway type II). Additionally, the decomposition of
H2O2 through a Fenton-like reaction can be facilitated in the pres-
ence of NPH affording hydroxyl radicals [46]. The results show that
the photocatalytic activity of H2P, ZnP and NPH cannot be justi-
fied only by their efficiency to produce 1O2 (H2P > ZnP > NPH)
and probably other highly reactive species such as hydroxyl radi-
cal (•OH) are also involved. In order to verify if the production of
•OH is also involved in the photocatalytic degradation of MO, the
reactions in the presence of H2O2 were carried out in the pres-
ence of mannitol, an effective radical scavenger for •OH (Fig. 8)
[47].
iron oxide surfaces resulting in lower concentration of the highly
fluorescent HTA (Fig. 9D). The fluorescence pattern observed dur-
ing the irradiation of TA in the presence of NPH (Fig. 9C) is due
to the interaction between the nanoparticles’ surface and TA. It is
worth to mention that in the absence of hydrogen peroxide, the
HTA signal is not observed.
These results suggest that the photocatalytic mechanism in-
volves mainly two ROS species, namely singlet oxygen and hy-
droxyl radical. At the end of the photocatalytic reaction using NPH,
when total degradation of MO was observed, fresh solutions of MO
and H2O2 were added to the reaction and subjected to the same
reaction conditions of the first use. Again, total degradation of MO
was observed after 240 min. Indeed, it was possible to reuse NPH
at least three times without any loss of catalyst activity.
In fact, the photocatalytic efficacy of ZnP and NPH was strongly
affected by the presence of mannitol confirming the role of •OH
radicals in MO photodegradation (Fig. 8). These results obtained
with hydrogen peroxide in the presence of mannitol are similar to
the ones obtained when the reactions were performed without hy-
drogen peroxide (see Fig. 5).
4. Conclusions
Terephthalic acid (TA) has been used to indirectly measure the
production of •OH by fluorescence. The •OH radical species re-
act with TA to yield an intensely fluorescent mono-hydroxylated
derivative (HTA). Fig. 9 shows the production of HTA in a phos-
phate buffer solution (PBS, pH = 7.0) with H2O2 in the presence
of H2P, ZnP and NPH under white light irradiation at different
periods of irradiation. Upon irradiation, hydroxyl radicals are pro-
duced, yielding a high HTA fluorescence signal. All the photocat-
alysts are able to generate •OH radical in the presence H2O2 and
under light, as indicated by the oxidation of the TA probe. Several
studies [46] have shown that the TA probe have strong affinity for
Core-shell magnetite-silica nanoparticles decorated with a por-
phyrin bearing meso-aryl groups with N-tosylethylenediamine
residues can be considered an excellent photocatalyst to be used
for dyes degradation. The NPH material turned out to be an ef-
fective photocatalyst, with higher photostability and the advantage
of possible reuse. Additionally, the photocatalytic activity was im-
proved with the addition of aqueous hydrogen peroxide. In this
case, two mechanisms (type I and type II) can be involved in the
MO photodegradation. The efficacy of this new photocatalytic ma-
terial in the photodegradation of other dyes will be evaluated in
future work.
8

K.A.D.F. Castro, J.M.M. Rodrigues, M. Amparo F. Faustino et al.
Journal of Organometallic Chemistry 938 (2021) 121751
Declaration of Competing Interest
[13] M.M.Q. Simões, I.C.M.S. Santos, Maria da Graça P.M.S. Neves, A.M.V. Cavaleiro,
J.A.S. CavaleiroA.J.L. Pombeiro (Ed.), Functionalization of sp2 and sp3 Carbon
Centers Catalyzed by Polyoxometalates and Metalloporphyrins, Advances in
Organometallic Chemistry and Catalysis (2013) 59–71.
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
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feiçoamento de Pessoal de Nível Superior). The authors also ac-
knowledge the University of Aveiro and FCT/MCTES (Fundação
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da Graça P.M.S. Neves, A.C. Tomé, S.L.H. Rebelo, A.M.V.M. Pereira, J.A.S. Cav-
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para
a Ciência e Tecnologia and Ministério da Ciência, Tec-
nologia e Ensino Superior) and European Union (COMPETE and
FEDER programs) for the financial support of UIDB/50006/2020
(LAQV-REQUIMTE), UIDB/50011/2020 & UIDP/50011/2020 (CICECO)
and UIDB/00100/2020 (CQE) research units and to the project
COP2P (PTDC/QUI-QOR/30771/2017), through national funds and,
when applicable, co-financed by FEDER–Operational Thematic Pro-
gram for Competitiveness and Internationalization-COMPETE 2020,
within the PT2020 Partnership Agreement. K. A. D. F. Castro thanks
CNPq for her post-doctoral scholarship (Process 201107/2014-7)
and J. M. M. Rodrigues acknowledges funding in the form of a
Junior Researcher Contract under the scope of the project COP2P
(PTDC/QUI-QOR/30771/2017).
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