A porphyrin cobalt(II) complex linked to a TiO2/BiVO4 nanocomposite: Alcohol oxidation using nanohybrid materials as a photocatalyst: Via a mechanism approach

Article abstract of DOI:10.1039/c7nj04008c

Herein, a new nanohybrid catalyst was synthesized via covalently linking a tetraacetate porphyrin cobalt(ii) complex to a TiO₂/BiVO₄ nanocomposite, and its photocatalytic efficiency towards alcohol oxidation under visible light irradiation was monitored by GC. This nanohybrid material was implemented as a photocatalyst and characterized by the following physicochemical techniques: FE-SEM imaging, EDS, XRD, DRS, BET analysis, and FTIR spectroscopy. Additionally, its role as a photocatalyst was evaluated using a mechanistic approach under aerobic conditions by changing several parameters such as solvent, substrate, oxidant, amount of catalyst, reaction temperature, and ratio of oxidant to substrate. The photocatalytic activity of this nanohybrid catalyst, with an average size of 24 nm, increased almost twice as compared to that of the TiO₂/BiVO₄ nanocomposite from 35% to 85% conversion, whereas the selectivity was 99%. A plausible mechanism was proposed. In fact, the cobalt porphyrin complex as a light sensitizer improves the photocatalytic activity through impregnation onto the TiO₂ surface. The photocatalyst was reused several times without significant loss of its activity. Thus, this nanohybrid robust catalyst has excellent advantages such as facile synthesis, high activity and selectivity, and use of O₂ and other environmentally friendly oxidants under mild conditions towards aldehyde production.

Full text of DOI:10.1039/c7nj04008c

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Porphyrin cobalt(II) Complex linked to TiO2/BiVO4 nanocomposite: Alcohol oxidation
using nanohybrid materials as implement photocatalyst with mechanism approach
Parvin Askari, Sajjad Mohebbi*
Department of Chemistry, University of Kurdistan, Sanandaj, IRAN. POBox 66179-416.
sajadmohebi@yahoo.com, smohebbi@uok.ac.ir
Abstract
A new nanohybrid catalyst by covalent linking of tetraacetate porphyrin cobalt(II)
complex to TiO2/BiVO4 nanocomposite was synthesised and its photocatalytic efficiency
toward alcohol oxidation under visible light irradiation monitored by GC. This nanohybrid
materials as implement photocatalyst was characterized by physicochemical techniques FEꢀ
SEM imaging, EDS, XRD, DRS, BET and FTꢀIR spectroscopy. Also its role as photocatalyst
evaluated with mechanismetic approach in aerobic condition by changing several parameters
such as solvent, substrate, oxidant, catalysts amount, reaction temperature and ratio of
oxidant to substrate. The photocatalytic activity of this nanohybrid catalyst with average size
24 nm increased almost twice compared to TiO2/BiVO4 nanocomposite from 35% to 85%
conversion while the selectivity was 99%. A plausible mechanism was proposed. In fact, the
cobalt porphyrin complex as a light sensitizer improves the photocatalytic activity by
impregnation onto the TiO2 surface. The photocatalyst was reused for several times without
significant loss of activity. So, this nanohybrid robust catalyst has excellent advantages such
as facile synthesis, high activity and selectivity using O2 and other environmental friendly
oxidants in mild condition toward aldehyde production.
Keywords: Photocatalyst, nanohybrid materials, aerobic oxidation, nanocomposite.
1. Introduction
Semiconductor oxides are employed in the field of photocatalyst due to their specific
physiochemical and optical properties.1,2 In a photocatalytic process, generation of electronꢀ
hole pairs h+eꢀ by incident irradiation on semiconductor materials is required.3 The potential
of these excitons have capable of achieving the reduction and oxidation of the chemical
reagents.3 This bring semiconductors useful for a wide range of applications such as air
purification, CO2 reduction, effective degradation of organic pollutants in water or air, and
hydrogen production through photocatalytic waterꢀsplitting processes.4,5 Due to wide band
gap of the current well known photocatalysts such as TiO2, only very narrow fraction of the

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solar radiation in UV light region could be used for excitation of electrons from conduction
band to valence band of TiO2.6 Therefore, to utilize the visible region of the solar spectrum
for photocatalytic phenomenon, tailoring the optical properties of titania is indispensable.7 In
this regard, TiO2 could be modified by metal doping, nonꢀmetal doping, ion implantation and
surface sensitization by organic dyes.8,9
Usually, the metal oxides have shown high level of the valence band at around 3.0 V that
causes a significant energy loss. But, the BiVO4 as an attractive semiconductor in visible light
region has relatively high conduction band edge position with band gape (Eg) in range of
2.4−2.5 eV.10,11 The reason could be related to composed of d orbitals level of the transition
metal oxides comparing to 6s orbital of Bi in the VB of monoclinic BiVO4, which maintain
the CB edge level decreasing to the hydrogen reduction potential as well. Through three
crystalline forms of BiVO4, monoclinic BiVO4 with band gap 2.4 eV shows higher
photocatalytic activities than tetragonal BiVO4 with band gap 3.1 eV. So, the visibleꢀlight
driven efficiency of photocatalyst is influenced by its crystal form and photophysical
properties.12
Hence, metalloporphyrin complexes could be considered as efficient sensitizers to harvest
visible light on the surface of TiO2 because of their structural analogues to chlorophyll in
plant photosynthesis.13 Due to an extensive delocalized π electron system, porphyrins have
high absorption extension coefficient in the visible region.14–16 Also, porphyrin with excellent
photophysical properties have small singletꢀtriplet splitting, high quantum yield for
intersystem crossing and long triplet state lifetime.13
Upon extending research in this field, mesoꢀtetra(4ꢀcarboxyphenyl)porphyrinato cobalt(II)
(CoTCPP) on TiO2/BiVO4 was successfully prepared by a facile method17 and was used as
efficient photocatalyst under visible light irradiation toward alcohol oxidation in mild
condition.
2. Experimental
2.1. Materials
4ꢀformylmethylbenzoate, nitrobenzene, pyrrole, propionic acid, potassium hydroxide,
hydrochloric acid, dimethylformamide, tetrahydrofuran, titanium (IV) isopropoxide,
NH4VO3, Bi(NO3)3.5H2O and alcohols including benzyl alcohol, 4ꢀcholorobenzyl alcohol, 4ꢀ

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methoxybenzyl alcohol and 4ꢀnitrobenzyl alcohol were purchased from Merck and Sigmaꢀ
Aldrich suppliers and used as received without further purification. Pyrrole was distillated.
2.2. Measurements
The field emission scanning electron microscopy (FESEM) images were recorded on a
TSCAN, Sꢀ300000 microscope. Energyꢀdispersive Xꢀray spectroscopy (EDS, TSCAN) was
used for the elemental analysis of nanomaterials. FTꢀIR spectra was recorded on a Bruker
VERTEX 80 v model spectrometer (KBr pellets). Xꢀray diffraction (XRD) patterns of the
samples were recorded on a HollandꢀPhilips Xꢀray powder diffractometer using Cu Kα
radiation (λ= 0.1542 nm) with scattering angles of 10° < 2θ < 80°, operating at 40 keV, and a
cathode current of 20 mA. UV–Vis diffuse reflectance spectra (DRS) of the samples were
recorded at room temperature with an Avaspecꢀ2048ꢀTEC spectrophotometer. Ultravioletꢀ
visible spectra (UVꢀVis) were measured on the Varian Cary 100 UVꢀVis spectrophotometer
in range of 350ꢀ800 nm. The photoluminescence (PL) spectra recorded by a Cary Eclipse
fluorescence spectrophotometer (Varian, Inc.) with a xenon discharge lamp with a
wavelength of 325 nm at room temperature. Nitrogen adsorptionꢀdesorption isotherms were
measured with an adsorption instrument (Tristar II 3020 Micromeritics) at 77 K to determine the
BrunauerꢀEmmettꢀTeller (BET) surface area and pore size distribution. Photocatalytic reactions
were carried out under visible light irradiation of 7 W at 550 nm. The reaction progress was
monitored by GC–MS, Agilent 7890A with both detectors FID and Mass Agilent 597530.
2.3. Preparation of Cobalt(II)-meso-tetra (4-carboxyphenyl) porphyrin (Co-TCPP)
In the first, mesoꢀtetrakis (4ꢀcarboxymethylphenyl) porphyrin (H2TCPP) was prepared
using reported synthetic methodologies as follow.18 To a mixture of 4ꢀformylmethylbenzoate
(20 mmol, 3.42 g), propionic acid (70 mL) and nitrobenzene (15 mL), 20 mmol distillated
pyrrole (1.42 mL) was added. The mixture was refluxed under 125ꢀ130 °C condition for 3.5 h
and cooled down. Lately, the purple crystals of the H2TCPP was filtrated and washed with
distilled water and dried in oven at 70 °C. In the second step, the tetracarboxylic acid was
obtained by the hydrolysis of 4ꢀcarboxymethylphenyl porphyrin with 10% aqueous potassium
hydroxide in tetrahydrofuran/ethanol (1:1) as solvent. The mesoꢀtetrakis (4ꢀcarboxyphenyl)
porphyrin (TCPP) as a purple powder also resulted from Acidification (pH = 2) of an aqueous
suspension of the porphyrin tetrapotassium salt with 1 M HCl and subsequent filtration. UVꢀ
Vis: 419 (Soret band), 516, 551, 596, 650 (Q bands). FTꢀIR (cm−1): 1725 (str., C=O), 3500
(str., OH) and 3300 (str., NꢀH). In final step, The TCPP was metallated by Co2+ using

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literature procedures.19 0.1 mmol of TCPP and 0.5 mmol cobalt acetate tetrahydrate was
dissolved in 20 ml of DMF. The mixture was refluxed for 2 h then DMF was removed by
distillation and the CoTCPP precipitated in water. Finally, the porphyrin was filtered and
dried at room temperature. UVꢀVis (nm): 430 (Soret band), 555, 594 (Q bands). FTꢀIR
(cm−1): 1712 (str., C=O), 3456 (str., OH).
2.4. Preparation of Co-TCPP@TiO2/BiVO4 nanohybrid
2.4.1. Preparation of TiO2
4.9 ml of titanium (IV) isopropoxide was peptized in 58 ml of biꢀdistilled water. Follow,
0.419 ml of concentrated HNO3 at 313 K was added. The process takes 24 h. The resulted sol
was consequently basified to reach pH 7 using 2 M NaOH. The resulting white suspension
was left stirring for 30 min. During hydrothermal procedure, the sol was placed in an oven
dryer using Teflonꢀlined stainless steel autoclave at 453 K for 24 h. Finally, product was
separated by centrifugation, rinsed with biꢀdistilled water, ethyl alcohol and dried. These
processes are conducted for 12 h under a condition of 373 K for 2 h.20
2.4.2. Preparation of BiVO4
5 mmol of NH4VO3 in 2 ml of 2 M NaOH was added to a solution of 5 mmol of
Bi(NO3)3.5H2O in 2 ml of 4 M HNO3 to form an intense orange suspension of BiVO4
precursor. The suspension was basified to pH 7 using 2 M NaOH to obtain a yellow
suspension. During the hydrothermal procedure, the suspension was stirred for 30 min and
placed in a convection oven using a Teflonꢀlined stainless steel autoclave under a condition
of 453 K for 24 h. The yellow solid product was separated by centrifuge, rinsed with biꢀ
distilled water, EtOH and expose into air for 12 h. Finally, BiVO4 was dried at 373 K for 2
h.20
2.4.3. Preparation of TiO2/ BiVO4 nanocomposite
The TiO2/BiVO4 composite was prepared by hydrothermal process with weight ratio 20%
of BiVO4 to 80% of TiO2. A titania sol of Ti(OiPr)4, as mentioned earlier, was added to
BiVO4 precursor to reach the desired weight ratios. Afterwards, the whole mixture was
basified to pH 7, stirred for 30 min and heated under a condition of 453 K for 24 h. The final
solids were washed with biꢀdistilled water and EtOH and left in air for 12 h at room
temperature consequently was dried at 373 K for 2 h.20

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2.4.4. Preparation of Co-TCPP@TiO2/BiVO4 nanohybrid
The linkage of CoꢀTCPP to the TiO2/BiVO4 was done as follow: To a solution of 1 mg
CoꢀTCPP in 15 mL methanol, 100 mg TiO2/BiVO4 nanoparticles was added. The resulting
suspension was stirred and refluxed at 40 °C for 8 h. In this way, the porphyrin was linked on
the surface of TiO2 nanoparticles due to the interaction between the phenoxy of porphyrin
and the hydroxyl groups on the surface of the TiO2 nanoparticles.
2.5. General procedures for photocatalytic process
Photocatalytic reactions were carried out in a Pyrex glass cell while aerated by narrow
nuzzle which equipped with the water circulation to keep the reaction temperature constantly.
The LED lamp 7 W was applied as light source. In each catalytic reaction cycle, 10 mg of
catalyst was dispersed in 1.5 ml solution of 0.1 mmol aromatic alcohol and sonicated for 30
seconds. Resulted suspension was transferred into the reaction cell under continuous stirring.
The solution was placed in the dark condition for 20 min to insure equilibrium between
adsorption and desorption. Then, the cell was irradiated by visible light (λ = 550 nm) through
a LED lamp for about 2 h. Reaction progress was monitored by GC by periodic sampling.
After the reaction was completed, the catalyst was separated by centrifuge, and washed with
acetonitrile for possible future reusing.
3. Result and discussion
3.1. Characterization of Co-TCPP@TiO2/BiVO4 nanohybrid
CoꢀTCPP was fixed on the surface of TiO2/BiVO4 nanocomposite through esterification
reaction of carboxylic group of porphyrin with hydroxyl group of nanocomposite in mild
condition. The obtained CoꢀTCPP@TiO2/BiVO4 nanohybrid was characterized.
The size and morphology of TiO2 and BiVO4 nanoparticles, TiO2/BiVO4 nanocomposite
and CoꢀTCPP@TiO2/BiVO4 nanohybrid were determined by scanning electron microscopy
(Figure 1). The SEM images of nanoparticles showed that the synthesized products were
monodisperse with narrow size distributions. Also, the average size grew from 8 nm for TiO2
to 24 nm for CoꢀTCPP@TiO2/BiVO4 nanohybrid. These results are in agreement with XRD.
In fact, TiO2 nanoparticles grow on the surface of the BiVO4 and increase the size of TiO2
nanoparticles. After modified TiO2/BiVO4 nanocomposite with metalloporphyrin, not only
the size of nanoparticles was increased but also, the morphology of the nanohybrid was
changed (Figure 1gꢀh). In this case, by highꢀmagnification SEM images (Figure 1e)

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revelation could be seen the leafꢀlike porphyrin have radials growing on the surfaces of the
nanocomposite.
Fig. 1 Lowꢀ and highꢀmagnification FESEM images of the TiO2 (a, b), BiVO4 (c, d),
TiO2/BiVO4 (e, f) and CoꢀTCPP@TiO2/BiVO4 (g, h).
The composition of TiO2, BiVO4, TiO2/BiVO4 and porphyrinꢀanchored to TiO2/BiVO4
nanoparticles were confirmed by energy dispersive spectroscopy (EDS) (Figure 2aꢀd).
According to EDS analysis, the TiO2 nanoparticles only consists Ti and O elements. Also,
only Bi, V, and O elements was found in BiVO4 nanocomposite. The molar ratios was
reported on the base of EDS data sheet on the base of Kα and Mα in EDS, the exact weight
percentage (W%) 16.52% and 65.90% was obtained for V and Bi, respectively that are equal
to 1.03 mole ratio of V to Bi. Anyway, the Bi:V:O atomic molar ratio 1:1:4 was obtained,
which indicated stoichiometric product BiVO4. The EDS analyses of the nanocomposite of
TiO2/BiVO4 (Figure 2c) were also performed to determine the Ti, Bi, V and O as major
elements for TiO2/BiVO4 compounds. Besides, the figure 2d shows the elements in modified
nanocomposite. It was clearly confirmed the nanohybrid composition as containing Ti, O, Bi,
V, N, C and Co.

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Fig. 2 EDS spectrum of the TiO2 (a), BiVO4 (b), TiO2/BiVO4 (c) and CoꢀTCPP@TiO2/BiVO4 (d).
The FTꢀIR of the samples was shown in supporting information, (Figure S1). In TiO2 as
prepared sample, the broad band at 3700 to 3000 cmꢀ1 and a band at 1639 cmꢀ1 were assigned
to stretching of hydroxyl groups. A band between 784 and 560 cmꢀ1 was assigned to the TiꢀO
vibration. The symmetric and asymmetric stretching vibrations of V–O was appeared at 700ꢀ
850 cmꢀ1, and a band at 520 is due to the bending vibration of Bi–O bond in BiVO4. For the
TiO2/BiVO4 nanocomposite, the V–O stretching vibrations shifted to 708 cmꢀ1 and 816 cmꢀ1.
Another band at 576 cmꢀ1 could be assigned to the bending vibration of Bi–O bond. Anyway,
the vibration bands in the IR spectra of TiO2/BiVO4 confirm existence both of Bi–O and V–O
bonds with some small shifts. This indicates a successful bonding between BiVO4 and TiO2
in the mentioned hydrothermal process. The existence of COꢀTCPP in nanohybrid was
proved by comparing FTꢀIR, EDS and DRS of nanohybrid and nanocomposite. The FTꢀIR
spectra of the CoꢀTPCC@TiO2/BiVO4 nanohybrid clearly confirm the esterification of
hydroxyl groups on the surface of nanocomposite with carboxylate group of porphyrin.
Appearing new bands at 1616 cmꢀ1 and 1390 cmꢀ1, also weakness of bands at 3421 cmꢀ1 could

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be arguments toward covalence link between two moieties CoꢀTPCCꢀCOOH and
HOꢀ
TiO2/BiVO4. The weakness of band at 3421 cmꢀ1 is due to decreasing the number of hydroxyl
groups on the surface of TiO2/BiVO4 upon anchoring CoꢀTCPP on TiO2/BiVO4. In addition
to FTꢀIR, strong stacking of porphyrin to surface of nanocomposite could be count as another
argument to covalence bonding of carboxylate to hydroxyl groups on the surface of
nanocomposite. Because, leaching of CoꢀTCPP was not observed after several washing of
nanohybrid with the solvent. Both of washed nanohybrid and work up solvent was tested by
IR, UVꢀVis and EDS.
Figure 3 shows the XRD patterns of TiO2, BiVO4, TiO2/BiVO4 and Coꢀ
TCPP@TiO2/BiVO4. Comparing the XRD patterns from the JCPD reference files, the
diffraction peaks are in agreement with a Anatase and Rutile structure (JCPD=04ꢀ0477 and
JCPD=77ꢀ0444 respectively). The diffraction peaks corresponded to (1 0 1), (0 0 4), (2 0 0),
(1 0 5), (2 0 4), (1 1 6) and (2 1 5) planes of crystalline TiO2 by Anatase structure and (1 1 0),
(1 0 1) and (1 1 1) Rutile phase of crystalline TiO2. Figure 3b shows the XRD patterns of
BiVO4. The diffraction peaks corresponded to (1 0 1), (1 1 2), (0 0 4), (2 0 0), (2 1 1), (1 0 5),
(1 2 3), (2 2 0), (1 1 6), (1 3 2), (2 2 4), (3 0 5) and (2 0 8) planes of crystalline BiVO4 by
Tetragonal structure and (1 0 3), (1 1 0), (0 0 8), (1 1 6) and (2 1 3) Tetragonal phase of
crystalline Bi2VO5.5. The XRD pattern of the TiO2/BiVO4 composite materials (figure 3c)
reveals the formation of the Anatase phase of TiO2. This trend is followed by the 20Biꢀ80Ti
sample which the phase attributable to BiVO4 is not observed. Also, the Xꢀray pattern of Coꢀ
TCPP@TiO2/BiVO4 was corresponded to TiO2/BiVO4 nanocomposite that means the
structure of core nanocomposite remained stationary and robust (figure 3d)

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Fig. 3 XRD patterns of (a) TiO2, (b) BiVO4, (c) TiO2/BiVO4 and (d) CoꢀTCPP@TiO2/BiVO4.
Also, the specific surface area was measured from BET isotherms, and the pore size distribution
diagram was obtained using BJH approach. Consequently, the value of BET surface area related to
TiO2, TiO2/BiVO4 and CoꢀTCPP@TiO2/BiVO4 were 17.56, 50.73 and 32.04 m2/g, respectively.
So, significant difference exists between the BET surface areas of nanocomposite in compared with
nanohybride. This remarkable diversity attributes to surface porosity changing and probable
morphology changing of nanostructure during modification of surface of nanocomposite with Coꢀ
TCPP.
The optical properties of the asꢀsynthesized nanoparticles were evaluated by diffuse
reflectance UVꢀVis spectroscopy. The band gap energy (Eg) was determined from the Tauc’s
plots by the equation αhν = A (hν−Eg)n/2, where α, h, υ, Eg and A are absorption coefficient
near the absorption edge, Planck's constant, light frequency, band gap energy, a constant,
respectively. Also n=1 is indirect band gap TiO221 and n=4 is direct band gap BiVO4.22,23 The
band gap of TiO2, BiVO4, TiO2/BiVO4 nanocomposite and CoꢀTCPP@TiO2/BiVO4
nanohybrid as a key factor of photocatalytic activity24 were measured 2.95, 2.24, 2.54 and
2.57 eV, respectively. The results showed the band gap of TiO2/BiVO4 nanocomposite and
CoꢀTCPP@TiO2/BiVO4 nanohybrid photocatalyst are lower than pure TiO2, which implied

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the red shift of absorption light in visible light region. In contrast with the TiO2/BiVO4, a
qualitative red shift to higher wavelength was observed in the absorption edge of nanohybrid,
whose absorption edge extended to visible wavelength of 482 nm.
The electronic absorption bands of cobalt porphyrin solution relevant the Soret band at 430
nm and Q bands at 555, 594 nm. While in solid state, DRS of nanohybrid Coꢀ
TCPP@TiO2/BiVO4 shows two peaks at 390 nm and 482 nm (figure 4b). These red shifts are
due to electronic correlation between CoꢀTCPP to TiO2 which could be assigned to the
transition of a1u(π)–e*g(π) (Soret band) and a2u(π)–e*g(π) (Q band) in CoꢀTCPP, respectivly.25
The increment in Qꢀband absorption is strong argument for successfully linking of the
porphyrin on the surface of TiO2/BiVO4 nanocomposite.
Fig. 4 Diffuse reflectance spectroscopy analysis of (a) TiO2, (b) BiVO4, (c) TiO2/BiVO4 and (d) Coꢀ
TCPP@TiO2/ BiVO4 and the plot of transferred Kubelka–Munk versus energy of the light absorbed
by the catalyst (inset: a) TiO2, (inset: b) BiVO4, (inset: c) TiO2/BiVO4 and (inset: d) CoꢀTCPP@TiO2/
BiVO4.

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The photoluminescence (PL) test of the photocatalysts could reveal the existence of
surface defects, oxygen vacancies, migration, the transference, and recombination processes
of the photogenerated electron and hole pairs on the samples.26,27 Herein, the roomꢀ
temperature PL spectra were measured in air with the excitation wavelength of 325 nm. PL
emission intensity and photocatalytic performances are strongly correlated in reverse
direction.28,29 The lower PL intensity usually indicates the lower charge recombination of
electron and hole pairs due to the high separation efficiency of photoinduced charge, thus
cause the higher photocatalytic activity. Figure 5 shows PL spectra of TiO2, BiVO4,
TiO2/BiVO4 and CoꢀTCPP@TiO2/BiVO4. It was observed that the TiO2/BiVO4 and Coꢀ
TCPP@TiO2/BiVO4 exhibited much lower emission intensity than bare TiO2 and BiVO4.
Therefore, irradiative charge recombination in the TiO2/BiVO4 and CoꢀTCPP@TiO2/BiVO4
should be decreased because the PL intensity was decreased. So, electron transference will be
facile between two components of BiVO4/TiO2 nanocomposite and follow it, the
photocatalytic activity of CoꢀTCPP@TiO2/BiVO4 should be enhanced.
1200
1000
800
600
400
200
0
200
400
600
800
Wavelength (nm)
Fig. 5 Photoluminescent spectra of the asꢀprepared TiO2, BiVO4, TiO2/BiVO4 composite and Coꢀ
TCPP@TiO2/BiVO4 nanohybrid excited by photons at 325 nm.

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4. Study of photocatalytic activity
The photocatalytic activity and efficiency of the nanohybrid catalyst Coꢀ
TCPP@TiO2/BiVO4 was studied toward oxidation of alcohols and compared with
nanocomposite TiO2/BiVO4. Photocatalytic reactions were run in a water bath and irradiated
using a LED light source with wavelength 550 nm. The reaction progress was monitored by
GC. The oxidation reaction condition was optimized by investigation on the influence of
amount of catalyst, temperature, solvent, oxidant and reactant/oxidant mole ratio. Summary
of result have inserted in tables 1 and 2.
To find optimum condition, the catalytic oxidation reaction was run in different
temperature and reaction progress was monitored during 2 h in acetonitrile. At 70 °C the
yield of oxidation process was reached to 80% while at 25 °C and 55 °C yield was 40% and
60%, respectively. The yield of the reaction was decreased at higher temperature than 70 °C
due to deformation of porphyrin part of catalyst. Conversely, at low temperature, energy does
not available enough to activate the catalyst then results in low benzyl alcohol conversion.
So, the optimize temperature 70 °C was chosen for further reactions because catalyst shows
higher efficiency and shorter reaction time. It is noticeable that the selectivity for aldehyde
production remains at 99%. Therefore, CoꢀTCPP@TiO2/BiVO4 is suggested as a highly
selective catalyst for the oxidation of alcohols.
Table 1 The effect of various parameters on the catalytic oxidation of benzyl alcohol to benzaldehyde
by CoꢀTCPP@TiO2/BiVO4.
Entry Solvent
Temp. (°C)
Time (min)
Oxidant Selectivity (%) Yield (%)
1
2
3
4
5
6
7
8
CH3CN
CH3CN
CH3CN
DMF
CH2Cl2
DMF/ CH2Cl2
H2O
CH3CN
CH3CN
CH3CN
CH3CN
25
55
70
70
70
70
70
25
40
50
70
120
120
120
120
120
120
120
120
120
120
120
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
H2O2
H2O2
H2O2
O2
>99
>99
>99
>99
>99
>99
ꢀ
>99
>99
>99
ꢀ
40
60
80
65
40
50
<1
20
30
40
<1
9
10
11
Reaction condition: 10 mg catalyst, 0.1 mmol benzyl alcohol, 1.5 ml solvent, 1.5 mmol oxidant.

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Table 2 Photocatalytic oxidation of benzyl alcohol under visible light irradiation. a
Catalyst
amount (mg)
Time
(min)
60
Selectivity
(%)
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
ꢀ
Yield
(%)
85
50
20
25
25
5
10
10
35
25
<1
<1
Entry Catalyst
Oxidant
1
2
3
4
CoꢀTCPP@TiO2/BiVO4 10
CoꢀTCPP@TiO2/BiVO4 10
CoꢀTCPP@TiO2/BiVO4
TBHP
H2O2
O2
O2
O2
O2
O2
O2
60
5
120
120
120
120
120
120
60
60
120
120
CoꢀTCPP@TiO2/BiVO4 10
CoꢀTCPP@TiO2/BiVO4 15
5
6
7
8
9
10
11
12b
TiO2/BiVO4
TiO2/BiVO4
TiO2/BiVO4
TiO2/BiVO4
TiO2/BiVO4
No catalyst
5
10
15
10
10
ꢀ
TBHP
H2O2
O2
CoꢀTCPP@TiO2/BiVO4 10
O2
ꢀ
aReaction condition: benzyl alcohol (0.1 mmol), acetonitrile (1.5 ml), reaction temperature 70 °C, oxidant (1.5
mmol), in the case of H2O2 the reaction temperature was 50 °C. H2O2 was added in several times but all of
TBHP was added once
bResult in dark.
In continue several solvents with different polarities were used in response of the oxidation
of alcohols. Under the optimum reaction conditions, 10 mg catalyst and 1.5 mmol of oxidant
were used in different solvents such as, acetonitrile (CH3CN), water, dimethylformamide
(DMF) and dichloromethane (CH2Cl2). The results confirmed that the best yield was obtained
in acetonitrile solvent. The CH2Cl2 had the lowest conversion that should be associated with
the lower boiling point of the solvent. Meanwhile, only benzaldehyde was detected as single
product in all solvents. By comparing the result in table 1, acetonitrile should be selected as
the best solvent. Solvent has several effect on reactions, including its role on energy transfer
to avoid local overheating, mass transfer and dispersion of materials, polarization and
inductive effects, activity and stability of the metal type.30 However, this result could be
described as faster mass transfer to/from the surface of CoꢀTCPP@TiO2/BiVO4 during
catalytic reaction. The results revealed that the solvent significantly influenced the adsorption
of reagents and the diffusion of products during the oxidation.
To optimize the substrate to oxidant ratio, different molar ratios of benzyl alcohol to
aqueous TBHP at 1:1, 1:1.5 and 1:2 molar ratios were applied (Figure 6). The highest
conversion percent was obtained at 1:1.5 molar ratio. Further increasing of oxidant was not
affected the conversion percent. However, exceeding the TBHP caused some decreasing of
conversion due to the generation of tertꢀbutyl alcohol, which act as a competitive inhibitor in
the oxidation process.31,32

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120
100
80
60
40
20
0
1:1
1:1.5
1:2
0
20
40
60
80
100
120
Time (min)
Fig. 6 Oxidation of benzyl alcohol with different molar ratios of benzyl alcohol/TBHP.
Table 3 shows the summarized results of the oxidation reaction of primary alcohols in the
presence of CoꢀTCPP@TiO2/BiVO4 in optimized conditions. The smooth changes in the
yield of the reaction should be due to the effect of the electronꢀdonating and electronꢀ
withdrawing properties of functional groups on the benzene ring. Anyway, the selectivity was
remained about 99%.
Table 3 Catalytic activity of CoꢀTCPP@TiO2/BiVO4 nanohybrid in optimized conditions. a
Selectivity
%
yield
%
Entry
Substrate
Product
HO
HO
O
O
1
>99
>99
>99
80
75
70
2
3
Cl
Cl
HO
O
4
>99
60
NO2
NO2
a Reaction condition: Substrate (0.1 mmol), Amount of catalyst (10 mg), TBHP (1.5 mmol), reaction
temperature 70 °C, acetonitrile (1.5 ml).

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In order to understand the catalytic process, different oxidant such as molecular oxygen
(O2), hydrogen peroxide (H2O2) and tꢀbutyl hydroperoxide (TBHP), as an organic
hydroperoxides were considered. Among these oxidants, the TBHP has shown higher
performance than H2O2 and O2 in both dark and under visible light irradiation. The concise of
result are shown in table 2. However, in spite of inactivity of the molecular oxygen in dark
condition, it acts as a moderate oxidant under visible light irradiation, which could be
explained by consideration of mechanism of this reaction. This noticeable finding could be
described by understanding the role of each component in photocatalytic process.
4.1. Stability and reusability of catalyst
The performance of catalyst was studied by reusing the recycled catalyst toward benzyl
alcohol oxidation. To do so, the used catalyst was filtered, washed with acetonitrile, and
utilized for the new catalytic oxidation reaction in the same condition. The results revealed
that the catalyst was being active even after eight consecutive usages without losing
significant photocatalytic activity and selectivity (Fig. 7). Anyway, a little decreasing of
activity should be due to missing some ultrafine particles during washing process in each
step.
Also, XRD and FTꢀIR analysis were performed before and after reaction for study the
stability of the CoꢀTCPP@TiO2/BiVO4 catalyst (Figure 8 and S2). Actually, no changes was
observed in the FTꢀIR spectra and XRD patterns of the fresh and recycled catalyst, which
corresponds to state stability of CoꢀTCPP@TiO2/BiVO4 structure in the oxidation reaction
process.
Meanwhile, the UV–Vis spectra (Figure 9) of the reaction solution after filtration did not
show any absorption peaks related to CoꢀTCPP, which confirm that the CoꢀTCPP leaching
not happen during the oxidation reactions.

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100
90
80
70
60
50
40
30
20
10
0
1
2
3
4
5
6
7
8
Reaction Runs
Fig. 7. Number of recycling of the catalyst.
Fig. 8 XRD patterns of the CoꢀTCPP@TiO2/BiVO4 nanohybrid, after (a) and before (b) catalytic
reaction.

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Fig. 9 UVꢀVisible spectra of mixture reaction contain CoꢀTCPP@TiO2/ BiVO4 nanocatalyst after the
eighth recovery for test of leaching (inset: UVꢀvis spectra of CoꢀTCPP).
5. Plausible mechanism
To understand the primary active species in the aerobic oxidation reaction of benzyl
alcohol in the presence of CoꢀTCPP@TiO2/BiVO4 under visible light irradiation, a series of
controlled experiments was performed under argon atmosphere or in the presence of radical
scavenger compounds. In this research, benzoquinone (BQ) was used as a superoxide radical
•−
(O2 ) scavenger, tertꢀbutyl alcohol (tꢀBuOH) as scavenger for hydroxyl radicals, ammonium
oxalate (AO) as a photogenerated holes scavenger and silver nitrate as electrons scavenger.
As shown in Figure 10, converting alcohols to aldehydes in argon atmosphere is meaningless,
which this test confirmed that the oxygen was the main species in the oxidation of benzyl
alcohol. Increasing the BQ to the reaction does not have a significant effect on conversion of
benzyl alcohol with H2O2 and TBHP oxidants but significantly reduces the conversion of
alcohols to aldehydes with O2 oxidant, which confirms the essential role of O2 in the
oxidation of alcohols. A similar inhibitory effect was observed by AO reducing. The
conversion in this case represents the essential role of holes that created by light incident
during the reaction by three oxidants. On the other side, increasing tꢀBuOH (TBA) to the
reaction does not have a significant effect on reaction except on reaction by TBHP oxidant.
This confirms being ineffective of OH on the progress of the reaction. The reason to this may
associate with the reaction in acetonitrile solvent and solvent production, which do not let the
OH to be produced during the time of reaction.33 Based on the above test results, the species

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O2 and created holes were recognized as main species in the oxidation reaction of alcohol
with O2. Moreover, it is well understood that, the OH radicals are operating as nonꢀselective
agent in the progress of reactions and in the absence of such radicals; the reaction would be
selective.34
100
80
60
(a)
40
(b)
20
0
0
1
2
3
4
5
Irradiation time (h)
Fig. 10 The photocatalytic oxidation of benzyl alcohol by the CoꢀTCPP@TiO2/BiVO4 nanocatalyst in
present of oxygen (a) and argon (b).
According to control experiments conducted in this section, a possible mechanism in
converting aromatic alcohols to aldehydes was carried out according to the following:
The mechanism of dye sensitized TiO2 under visible light, involves the electron transition
1
from the ground state of porphyrin dye [Pp] to the singlet excited state [Pp]*.35 The
3
relaxation of the singlet excited state, generates the triplet excited state [Pp]* through a
1
process of intersystem crossing. In fact, the photogenerated electron transitions from [Pp]*
3
and [Pp]* could occur to conduction band of TiO2/BiVO4 nanocomposite. On the based of
DRS, UVꢀVis and cyclic voltammetry (CV) results, energy level diagrams were obtained
(Figure 11). In case, the CB and VB of BiVO4 was placed between the CB and the VB of
TiO2 (Schematic S3). So, electron capture by O2 from the conduction band of nanocomposite
•−
yield the superoxide radical anion (O2 ) which proceed oxidation of alcohols. Furthermore,
CoꢀTCPP plays as a sensitizer, while partial oxidation CoꢀTCPP by oxygen should not be
ignored that is unfavorable.36,37

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Fig. 11 Proposed photocatalytic mechanism path by molecular oxygen O2.
However, when the system irradiate under visible light in present of H2O2 as electron
acceptor, an electron (e−) promotes from the valence band into the conduction band of
nanocomposite and leaves a hole (h+) behind. So, the TiO2/BiVO4 nanocomposite will
activate to generate electronꢀhole pairs.38,39 In fact, the life time of positive hole increases by
prohibition of charge recombination by intersystem electron crossing to the vacant valance
band of excited CoꢀTCPP. This will conduct the oxidation of substrate through positive hole
(figure 12). Thus, in this case porphyrin plays as sensitizer and relay moiety.
Fig. 12 Proposed photocatalytic mechanism by molecular oxygen H2O2 oxidant.

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Conclusion
The novel CoꢀTCPP@TiO2/BiVO4 nanohybrid was prepared and used as efficient
photocatalyst in aerobic oxidation of alcohols to the corresponding aldehydes. The
conversion percent of oxidation reaction raised significantly up to 85 comparing to bare
TiO2/BiVO4 nanocomposite. Although, activity of this catalyst remain constantly upon
visible light irradiation in presence of TBHP as oxidant, but it shows higher photocatalytic
efficiency in present of molecular oxygen. It noticeable that no catalytic activity observed in
absent of light. Also, the catalyst was recovered and reused easily without significant loss of
activity. So, this nanohybrid photocatalyst could be considered as a great candidate in
transformation of functional groups in organic materials with advantages high efficiency,
facile procedure and high selectivity.
Supporting information
The FTꢀIR spectra of synthesis samples (figure S1ꢀS2) and energy bands diagram (Schematic
S3) are presented in the supporting information.
Acknowledgment
We gratefully acknowledge financial support from the Iranian National Science Foundation
(INSF). We also appreciate the University of Kurdistan.
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Novel visible light sensitized nanohybrid Co-TCPP@TiO2/BiVO4 was used as high efficient,
stable, reusable and robust photocatalyst using O2 toward aldehyde production.