Selective photocatalytic oxidation of 3-pyridinemethanol on platinized acid/base modified TiO2

Article abstract of DOI:10.1039/d1cy00569c

TiO₂catalysts, modified with acidic or alkaline solutions and then platinized, were used for the partial photocatalytic oxidation of 3-pyridinemethanol to 3-pyridinemethanal and vitamin B₃under environmentally friendly conditions. The reaction took place in water under UVA light and air oxygen. Catalysts were characterized by TEM, photoluminescence, DRIFT-IR, Raman, DRS, XPS, and photocurrent measurements. The photocatalytic activity results show that Pt loading of untreated samples leads to a significant activity improvement (hence product yield) as much as acid and alkaline treatments do. Moreover, the alkaline treated TiO₂samples exhibit a further increase in activity after loading with Pt. Pt acts as an electron scavenger promoting electron transfer from the TiO₂conduction band, consequently boosting the photogenerated pair numbers available for the reactive process. Photocurrent measurements show that the TiO₂photocatalysts' active sites increase significantly after platinization and alkaline/acid treatment. The treated and/or Pt loaded catalysts showed good thermal stability (at least up to 400 °C).

Full text of DOI:10.1039/d1cy00569c

Catalysis
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Selective photocatalytic oxidation of
-pyridinemethanol on platinized acid/base
modified TiO2†
3
Cite this: Catal. Sci. Technol., 2021,
11, 4549
Sedat Yurdakal, * Sıdıka Çetinkaya, Vincenzo Augugliaro, *b
a
a
cd
e
f
cd
Giovanni Palmisano, Jacinto Sá,
Erik Lewin
and Corrado Garlisi‡
TiO
2
catalysts, modified with acidic or alkaline solutions and then platinized, were used for the partial
under
photocatalytic oxidation of 3-pyridinemethanol to 3-pyridinemethanal and vitamin
B
3
environmentally friendly conditions. The reaction took place in water under UVA light and air oxygen.
Catalysts were characterized by TEM, photoluminescence, DRIFT-IR, Raman, DRS, XPS, and photocurrent
measurements. The photocatalytic activity results show that Pt loading of untreated samples leads to a
significant activity improvement (hence product yield) as much as acid and alkaline treatments do.
Moreover, the alkaline treated TiO2 samples exhibit a further increase in activity after loading with Pt. Pt
Received 31st March 2021,
Accepted 12th May 2021
acts as an electron scavenger promoting electron transfer from the TiO
boosting the photogenerated pair numbers available for the reactive process. Photocurrent measurements
show that the TiO photocatalysts' active sites increase significantly after platinization and alkaline/acid
treatment. The treated and/or Pt loaded catalysts showed good thermal stability (at least up to 400 °C).
2
conduction band, consequently
DOI: 10.1039/d1cy00569c
2
rsc.li/catalysis
1
–5
materials.
Despite its unique properties, there is
performance in
1
Introduction
continuous effort to improve TiO
2
TiO is a low cost and non-toxic wide bandgap semiconductor
with high photoactivity and photocorrosion resistance. It is
used in many different research areas such as photocatalysis,
photocatalysis, including its photo-response to visible light,
process selectivity and activity.
2
1
–3
6
–10
11–16
The photocatalytic
or photoelectrocatalytic
activity
photovoltaics,
photoelectrocatalysis
and
self-cleaning
of TiO was improved by treatment with different acids such
2
as HCl, HF, HNO , and H SO . After such treatments,
3
2
4
negligible differences were observed in the diffraction
2
−
−
patterns. Physical adsorption of SO4 and F , however, was
a
Kimya Bölümü, Fen-Edebiyat Fakültesi, Afyon Kocatepe Üniversitesi, Ahmet
Necdet Sezer Kampüsü, 03200 Afyonkarahisar, Turkey.
10,11
ascertained after H SO and HF treatments.
2
4
2
Only a few studies focused on TiO modification through
alkaline treatment. Eskandarloo and co-workers investigated
E-mail: syurdakal@aku.edu.tr, sidikacetinkayaa@gmail.com
b
“
Schiavello-Grillone” Photocatalysis Group, Dipartimento di Energia, Ingegneria
dell'Informazione e Modelli Matematici (DEIM), Università degli Studi di Palermo,
Viale delle Scienze (ed. 6), 90128 Palermo, Italy.
both NaOH treatment and Cu loading effects on TiO
2
1
7
photocatalytic activity for phenazopyridine degradation. The
combined effects of TiO2 treatment with NaOH and H O
E-mail: vincenzo.augugliaro@unipa.it
c
2 2
Department of Chemical Engineering, Khalifa University of Science and
were studied in the photocatalytic degradation of anionic
sulforhodamine B molecules at low pH (2.27) under visible
Technology, P.O. Box 127788, Abu Dhabi, United Arab Emirates.
E-mail: giovanni.palmisano@ku.ac.ae
d
18
Research and Innovation on CO
2
and H
2
(RICH) Center, Khalifa University of
irradiation. TiO2 was treated in 8 M NaOH and 100 mM
Science and Technology, P.O. Box 127788, Abu Dhabi, United Arab Emirates
H O aqueous solutions at 50 °C for 24 h. Nevertheless, these
2
2
e
Department of Chemistry - Ångström Laboratory, Physical Chemistry,
17,18
studies
focused on the synergic effect of both treatments,
Ångströmlaboratoriet, Lägerhyddsvägen 1, 751 20 UPPSALA, Sweden.
E-mail: jacinto.sa@kemi.uu.se
thus not shedding light on the potential benefits brought
about by only the alkaline treatment. NaOH treated and Pt
f
Department of Chemistry - Ångström Laboratory, Inorganic Chemistry,
Ångströmlaboratoriet, Lägerhyddsvägen 1, 751 20 UPPSALA, Sweden.
E-mail: erik.lewin@kemi.uu.se
2
loaded TiO samples were analysed for catalytic (and not
photocatalytic) oxidation of formaldehyde at room
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1cy00569c
Current affiliation: Material Research and Technology Department (MRT),
19
temperature by Nie and co-workers, yet the NaOH treated
samples did not show any catalytic activity. All of these
‡
1
7–19
studies,
however, did not target the application of (photo)
Luxembourg Institute of Science and Technology (LIST), 41, rue du Brill,
Belvaux, L-4422, Luxembourg, corradogarlisi@hotmail.it.
catalysts in selective synthetic reactions.
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In a previous study, we investigated the NaOH treatment
of BDH and Merck TiO2 catalysts for selective
Pt-loading was achieved by suspending 0.75 g of catalyst
in a solution containing 112.5 mL of water and 37.5 mL of
ethanol; a suitable amount of the Pt source (H PtCl ) was
2 6
2
0
3
-pyridinemethanol and 4-methoxybenzyl alcohol oxidation.
The modified TiO2 catalyst showed an amorphous titania
layer localized on the surface of anatase crystals, a higher
specific surface area, and lower crystallinity compared to
added to obtain a Pt content of 0.5% with respect to the TiO2
nominal amount. This Pt percentage was optimized using a
2
3
procedure reported in one of our previous studies. The
TiO2 suspension was ultrasonicated for 15 minutes. Pt
photodeposition was achieved in a Pyrex batch photoreactor
(cylindrical, 150 mL), where the suspension was magnetically
stirred to guarantee homogeneous Pt loading. A 250 W
medium pressure Hg lamp (Honle UVA hand model)
irradiated the photoreactor from the outside. The irradiation
energy reaching the suspension, measured by using a
untreated TiO . We have also investigated HCl treatment of
2
8
TiO2 catalysts for 4-nitrophenol degradation. Similarly, the
acidic treatment changed the titania structural and textural
properties and increased photocatalytic activity. This activity
increase may be ascribed to the presence of disordered TiO2
layers grown on the surface anatase crystals. HCl treatment
resulted in chlorine incorporation on amorphous regions of
the samples, followed by the formation of Zundel-like
structures. The interaction between the H-bonding of anatase
bridging hydroxyls and those structures boosts the acidic
−
2
radiometer (Delta Ohm, DO 9721), was ca. 39.2 mW cm (in
−
2
the 315–400 nm range) and 150 mW cm (in the 400–1050
nm range). During photodeposition, the temperature in the
reactor was kept constant (ca. 30 °C) by circulating water
through a Pyrex thimble surrounding it. The suspension was
continuously bubbled with nitrogen starting 15 minutes prior
to switching the lamp on. The gas flow, the irradiation and
the stirring were maintained for 2 hours. After that, the
platinized catalyst was separated by decantation. It was then
neutralized by dialysis using a polymeric membrane and,
finally, dried with the same procedure used for the acid or
base treated catalysts.
character of surface hydroxyl groups and, hence, the TiO
photocatalytic activity.
2
8
The present work presents a systematic investigation of
the effects of acidic or alkaline treatment on TiO structural
2
and activity properties. Moreover, the additional effect of
sample platinization was assessed. Two commercial anatase
2
(Merck and BDH TiO ) photocatalysts were treated in 1 M
HCl or NaOH aqueous solutions at a temperature of 100 °C.
Pt was loaded on both treated and untreated TiO samples by
2
photoreduction of chloroplatinic acid. The TiO
2
samples were
2
The treated photocatalysts are named with the TiO brand,
characterized by TEM, photoluminescence, DRIFT-IR, Raman,
DRS, XPS, and photocurrent measurements. Finally, they
were tested for the selective photocatalytic oxidation of
followed by the name of the acid or base used in the
treatment. For instance, Pt–Merck–HCl indicates the TiO2
(Merck brand) sample refluxed with 1 M HCl aqueous
solution at 100 °C (for 8 h) and then platinized.
3
-pyridinemethanol
under
environmentally
friendly
conditions. Thermal stability and cyclic tests of some
samples were also carried out. Pyridine-3-carboxylic acid
In order to test the thermal stability of the HCl treated
BDH sample, aliquots of the catalyst were calcined at 400,
700 and 1000 °C in a furnace (Protherm, PLF-110/10 model)
through a ramp of 3 °C per min and maintaining the final
temperature for 3 h. They were named as BDH–HCl–x in
which x indicates the calcination temperature. The Pt–BDH–
HCl and Pt–BDH–NaOH catalysts were also thermally tested
at 400 °C by using the same procedure.
3
(vitamin B ) is one of the potential 3-pyridinemethanol
partial oxidation products. This compound, whose world
production amounts to ca. 35 000 tons per year, is generally
2
1,22
used to prevent and treat the pellagra disease.
The
industrial production of vitamin B3 takes place at high
pressure by oxidising picolinic isomers on vanadia–titania–
zirconia oxide supported catalysts in nitric acid,
2
1
permanganate or chromic acid. In this context, production
of vitamin B via a photo-process would circumvent the need
2.2 Characterization
3
for high-pressure processes and avoid the use of toxic
oxidants.
TEM (transmission electron microscopy) analyses were
performed to study the photocatalysts' morphology and
identify amorphous parts by using a Tecnai G2 transmission
electron microscope at 200 kV. Initially, the catalyst samples
were suspended in 2-propanol and ultrasonicated for 5
minutes. 2 μL of the suspension were dropped on a formvar/
carbon 300 mesh Cu grid (Tedpella) and left to dry
completely at room temperature.
2
Experimental
2
.1 Photocatalyst preparation
2
.0 g of BDH or Merck TiO catalyst were placed in a Pyrex
2
balloon, connected to a Graham condenser and containing
2
0
5
00 mL of 1 M HCl or NaOH aqueous solution. Then, the
titania suspension was refluxed under magnetic stirring at
00 °C for 8 h. The suspension was thus decanted for 16 h at
XPS (X-ray photoelectron spectroscopy) was performed by
using an Ulvac-Phi Quantera II instrument with
1
monochromatic Al Kα radiation. TiO samples were attached
2
room temperature to let the catalyst settle down. The
powders were then neutralized by dialysis using a polymeric
membrane with deionized water and finally dried at 60 °C
through a rotary evaporator (Heidolph model M) at 120 rpm.
to carbon tape and floated electrically by using microscope
slides. Measurements were carried out under constant
+
neutralization using low energy electrons and Ar ions
2
4
because of the catalysts' isolation feature. Survey scans and
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details of selected core levels were obtained from the
catalysts' surface. The adventitious carbon was chosen as a
charge reference with the C 1s peak position at 284.8 eV. The
sample composition was estimated by using sensitivity
factors provided by the instrument manufacturer.
was magnetically stirred and in contact with the atmosphere.
The reactor was irradiated from the top by four fluorescent
lamps (Philips, 8 W), with the main emission peak centred at
365 nm. The average value of the radiation energy reaching
the surface of the suspension was 2.1 mW cm in the 315–
400 nm wavelength range. All photocatalytic runs were
performed at room temperature and neutral pH.
−
2
The characteristic molecular vibrations of the
photocatalysts' surface groups were investigated by DRIFT-IR
using a Bruker VERTEX 80/80v with an MCT detector cooled
by liquid nitrogen. 512 consecutive scans were acquired in
The initial concentration of 3-pyridinemethanol was 0.50
−
1
mM and the photocatalyst amount was 0.20 g L for all
photocatalytic experiments. Before starting the runs, the
suspension was kept for 30 min under stirring in the dark
and at room temperature in order to reach the
thermodynamic equilibrium, and then the lamps were turned
on.
−
1
absorbance mode for each spectrum at a 4 cm resolution in
−
1
the 4000–400 cm wavenumber range.
Raman spectroscopy measurements using a Witec Alpha
3
00R equipment allowed investigating the possible
occurrence of shifts due to oxygen vacancies on the samples.
The excitation wavelength and the laser power were 532 nm
and 75 mW, respectively. Two hundred consecutive scans for
each spectrum were obtained over an extended a range of
The suspension in contact with the atmosphere allowed
oxygen to continuously diffuse into the liquid. The mixing
helps with O2 absorption. The contact surface area is also
high to permit easy oxygen absorption; therefore, we
assumed that the oxygen concentration is constant during
the run and it can be considered as included in the kinetic
constant.
During the photocatalytic runs, samples were withdrawn
from the suspension and analyzed after filtration through a
0.45 μm syringe filter (HA, Millipore).
−
1
1
00–900 cm with a 1 s integration time.
Photoluminescence emission spectra (PL) were taken
using a Perkin Elmer LS 55 fluorescence spectrometer
equipped with a front surface sample holder into which
powder samples were placed. The chosen operative
parameters for PL spectra were: excitation wavelength, 300
−
1
nm; scanning speed, 200 nm min ; excitation slit width, 5
nm; and emission slit, 10 nm.
UV-vis diffuse reflectance (DRS) spectra were acquired in
the 200–800 nm range using a UV-vis spectrophotometer
2.4 Analytical techniques
A high performance liquid chromatograph (HPLC, Shimadzu,
Prominence LC-20A model) equipped with an SPD-M20A
photodiode array detector and Phenomenex Synergi 4 μm
Hydro-RP 80A column allowed the assessment of the species
produced during the photocatalytic runs. The mobile phase
was a mixture of methanol (40%) in water and its flow rate
(
Shimadzu UV-2600) and BaSO as reference.
4
The samples' photocurrent measurements were carried
out by using a potentiostat–galvanostat (CompactStat model,
Ivium) with a three-electrode system in which the TiO2
catalyst, deposited on fluorine doped tin oxide (FTO), was the
working electrode, Pt was the counter one and Ag/AgCl (3.0
M KCl) was the reference one. The preparation of the working
electrode was achieved by suspending 4 mg of sample in 4
mL of 2-propanol and treating in an ultrasound batch for 15
minutes. The FTO glass surface (1 cm × 2.5 cm part) was
covered with 10 μL of each suspension using a micropipette
and then waiting until dried. This deposition was reiterated
3
−1
was 0.2 cm min . The column's working temperature was
0 °C. Total organic carbon (TOC) was measured using a
4
Shimadzu TOC-LCPN model to quantify the extent of
substrate total oxidation.
3
Results and discussion
3.1 Characterization part
1
5 times to build up enough catalyst (150 μg) on the FTO
surface.
Transient photocurrents were recorded in a 50 mL beaker
containing Na SO (0.1 M) as an electrolyte. The side of the
The XRD patterns of all the samples exhibit the peaks
expected for anatase reflections (JCPDS n. 78-2486); the
patterns are not reported here.
2
0
2
4
glass slides covered with TiO
fluorescent lamps (Philips, 8 W) with the main emission peak
centred at 365 nm. The irradiation energy reaching the
2
samples was irradiated using 3
In the Pt-loaded catalysts, metal nanoparticles are clearly
identifiable as dark spots in TEM images (Fig. 1 and S1†).
The spots are not uniformly distributed over crystalline
titania nanoparticles and they are arranged into clusters with
different shapes and sizes up to 10 nm. As shown in the red
circles in Fig. 1, Pt particles present a lattice spacing of ca.
−
2
supported semiconductor was ca. 2 mW cm (measured in
the 315–400 nm range). Photocurrent transient curves were
recorded at 1.0 V vs. Ag/AgCl (3 M KCl) and pH 7. Linear
sweep voltammograms were acquired at a scan rate of 10 mV
0.224 nm, which was assigned to the (111) plane of metallic
−
1
25,26
s .
Pt.
Treated BDH and Merck showed an amorphous layer
8
covering the anatase surface. Notably, the Pt clusters are
dispersed exclusively over anatase crystals, while the
amorphous layer's surface appears to be free from Pt
particles. This feature may be attributed to the permeation of
H PtCl through the amorphous layer, which is facilitated by
2
.3 Photocatalytic activity setup and procedure
A cylindrical beaker (volume: 250 mL; diameter: 6.7 cm) was
used as a photoreactor. The aqueous suspension (150 mL)
2
6
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Fig. 1 TEM micrographs of selected platinized samples.
the latter's highly porous structure. Therefore, the liquid
solution can easily reach the crystalline core of the particles
where the reduction of Pt ions to metallic platinum is more
likely to take place. Moreover, the lack of Pt on the
2
of small particles and/or electron donation from Pt to TiO .
Since TEM shows that the average Pt particle size is about 5
nm, the shift is most likely related to interfacial electron
donation that is consistent with the changes observed from
UV-vis and Raman.
amorphous layers of TiO
2
provides evidence that
photogenerated electron–hole pairs cannot reach these
surface layers, confirming that the amorphous portion of the
nanoparticles is not able to produce metallic Pt via
photoreduction of the corresponding cation.
UV-vis absorbance spectra of the pure and platinized BDH
and Merck samples are reported in Fig. 2a and b,
respectively. For all the samples, the marked increase in the
absorption at wavelengths below 400 nm can be attributed to
the indirect band-to-band transition of anatase (∼3.2 eV).
Table 1 shows the results of the XPS analysis. The C1s
signal failed to correct the charging effect; therefore, the O1s
signal for bulk was used for this purpose. No peak
The Pt-loaded catalysts show
a significant absorbance
enhancement in the visible range (400–800 nm). The stronger
broad background absorption can be ascribed to the darker
colour of the catalyst following Pt loading, rather than to a
localized surface plasmon resonance (LSPR) effect, which is
usually manifested through the emergence of absorption
3
+
attributable to Ti was observed; consequently, all signals
4
+
were very well fitted with a single component related to Ti .
2
2
The loaded Pt on the TiO2 surface was in metallic form.
The signal was shifted with respect to bulk platinum by ca.
.3 eV, plus it has a large asymmetry related to the presence
2
7
0
bands in the visible region. Moreover, the platinized TiO
2
samples treated with an acid or base exhibit a slightly lower
absorbance in the visible region compared to the untreated
ones. This drop is more significant after alkaline treatment,
especially for the BDH catalysts. This may be due to the
different crystallinities, which decreased in the BDH (81 to
2
Table 1 XPS analysis results of the TiO samples
2−
4+
0
[
Bulk O
]
Amount [OH] Amount [Ti
]
[Pt ]
Sample
(eV)
(%)
(eV) (%)
(eV) (eV)
61%) samples following NaOH treatment, much more than
BDH
530.6
530.6
530.6
530.6
530.6
60
71
70
61
78
75
532.0 40
531.6 29
532.2 30
532.0 39
531.8 22
531.9 25
459.5
459.4
459.4
459.5 71.4
459.4 71.4
459.4 71.4
—
—
—
the Merck samples (85 to 79%) as reported in our previous
2
0
BDH–HCl
BDH–NaOH
Pt–BDH
Pt–BDH–HCl
Pt–BDH–NaOH 530.6
work. These results show that Pt loading is more effective
for the surface of the untreated TiO samples with respect to
that of the base/acid treated ones. On the other hand, both
acid and base treatment for un-platinized TiO did not result
2
2
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Fig. 2 UV-vis absorbance spectra of the pristine and platinized Merck (a) and BDH (b) samples.
g
Fig. 3 Raman spectra of the BDH and Pt–BDH samples. The inset of the figure shows the enlarged view of the E peak.
in any significant change in the absorption properties, as
reported elsewhere.
Eg signal of anatase is of particular interest since its
shifts can be correlated to the presence of oxygen
2
0
The Raman spectra of the BDH and Pt–BDH catalysts
see Fig. 3) highlight the typical bands characteristic of
vacancies in the TiO
samples, Pt-loading, in addition to determining
broadening of the anatase peaks and a marked decrease
in their intensity, leads to a significant shift of the E
peak to higher wavenumbers, as shown in Fig. 3. This
feature presumably arises from the perturbation of the Ti–
O–Ti symmetry with the generation of oxygen vacancies
causing lattice contractions and thus the shift and
broadening of Raman modes.
2
lattice. For both BDH and Merck
(
a
the anatase crystal phase of TiO . Eg modes arise from
2
the symmetric stretching vibration of O–Ti–O, the B1g
peak is due to the symmetric bending vibration of O–Ti–O
and the A1g peak is due to asymmetric bending vibration
g
2
8
of O–Ti–O. It is worth mentioning that both acid and
alkaline treatments did not bring about any noticeable
2
0
shift of the Raman peaks (spectra not shown). The main
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Fig. 4 DRIFT-IR spectra of the pristine and platinized Merck (a) and BDH samples (b).
FT-IR spectra of the photocatalysts are shown in Fig. 4.
treatments introduce a large amount of OH groups on the
photocatalyst surface. In this case, hydroxyl groups are
directly introduced from the sodium hydroxide solution on
the catalyst surface. On the other hand, the growth of the
water complex bands in the acid-treated catalysts can be
Two broad bands can be observed in the 3800–2800 and
−
1
1
800–800 cm ranges. The former absorption, including also
−
1
the narrow line at 3710 cm , is produced by the overlap of
the bands due to stretching vibration modes of hydrogen
−
−
bonded OH groups and of water molecules coordinated to
ascribed to the incorporation of Cl ions into the amorphous
4
+
surface Ti , whereas the latter absorption band embraces
titania chains, which replace lightly bound oxygen ions and
−
1
8
the bending vibration mode of these (1625 cm ) and the Ti–
O–Ti bridging stretching mode in the crystal below 1200
act as new adsorption sites for water complexes. The
photodeposition of Pt on TiO2 particles results in a small
decrease of the water and hydroxyl bands compared to the
−
1 8,29
cm .
The spectra confirm that the HCl and NaOH
Fig. 5 Photoluminescence emission spectra of the pure, platinized and platinized HCl/NaOH modified BDH (a) and Merck (b) samples.
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bare catalysts. Two small peaks appeared in the 2990–2900
−
1
and 2935–2840 cm
ranges owing to symmetric and
asymmetric CH stretching, respectively (Fig. 4). These peaks
2
were attributed to residual organics on the catalyst surface,
mainly derived from ethanol used in the Pt photodeposition
process or to hydrocarbon traces in the air.
Fig. 5 shows the PL spectra of the neat TiO , platinized
2
TiO2 and platinized HCl/NaOH modified TiO2 (BDH and
Merck) samples. No significant difference was observed
between the untreated and acid- or base-treated catalysts
2
0
(
their spectra are not shown). The same consideration
applies to the Pt–TiO and Pt–TiO –HCl/NaOH spectra (see
Fig. 5). The main emission signal was localized at ca. 420–
25 nm for all the catalysts, and it is produced by the band-
2
2
Fig. 7 Photocurrent measurements of Pt–BDH–NaOH in the presence
4
(a) and in the absence (b) of 0.5 mM 3-pyridinemethanol. [Na
100 mM. Applied potential: 1.0 V vs. Ag/AgCl (3 M KCl). pH 7.
2 4
SO ]
=
to-band transition energy generated by the migration of
electrons from states in conduction bands back to states in
the TiO valence band. Less energetic signals, observable at
2
higher wavelengths (in the 450–550 nm range) were
attributed to excitonic PL derived from recombination events
that take place at defective sites of the crystals and are
recurrent in nanostructured materials. The lower PL intensity
in the platinized samples, as shown in Fig. 5, may be
attributed to the role of Pt in suppressing photocarrier
recombination. Indeed, platinum works as an electron
scavenger on the TiO2 surface, where electrons are
applying a constant voltage of 1.0 V vs. Ag/AgCl (Fig. 7). In
this case, higher current values were obtained.
Fig. 8 shows the linear sweep voltammograms of the Pt–
BDH–NaOH catalyst in the presence (Fig. 8a and c)/absence
(Fig. 8b and d) of 3-pyridinemethanol in the voltage range
−0.20 V to 1.50 V. By increasing the voltage, the current values
also increased linearly. However, the relationship between
bias and current is a logarithmic function above ca. 1.2 V;
the current increases to ca. 40 μA at 1.5 V from ca. 5 μA at 1.2
V. Higher current intensities were obtained in the presence of
3-pyridinemethanol than in the absence of the substrate both
under UVA or dark conditions. In the dark, a negligible
current was present till a potential of 1.2 V and, after that
voltage, it also increased logarithmically, similar to what was
obtained under UVA.
2
transferred from the TiO conduction band to Pt clusters,
thus enhancing the number of electron–hole pairs available
for the photocatalytic process.
Fig. 6 reports the photocurrent measurements obtained at
1
2
.0 V vs. Ag/AgCl with the Pt loaded TiO catalysts. The
photocurrents measured in the Pt-loaded HCl or NaOH treated
samples were about twice higher than that of the platinized
BDH (Pt-BDH) catalyst. Photoactivity tests (see section 3.2)
support these results; both platinized and acid/alkaline treated
catalysts showed a much higher photocatalytic activity than
those subjected to only the platinization treatment.
3.2 Photocatalytic activity
Table 2 summarizes the selective photocatalytic oxidation
results of 3-pyridinemethanol to 3-pyridinemethanal and
Photocurrent measurements of Pt–BDH–NaOH were also
performed in the presence of 3-pyridinemethanol upon
vitamin B using the pristine, Pt loaded, HCl/NaOH treated
3
and Pt–HCl/NaOH treated BDH and Merck photocatalysts.
Fig. 8 Current–potential profiles of Pt–BDH–NaOH in the presence (a
Fig. 6 Photocurrent measurements of the Pt–BDH–HCl (a), Pt–BDH–
and c) and in the absence (b and d) of 0.5 mM 3-pyridinemethanol.
NaOH (b), and Pt–BDH (c) catalysts. [Na
potential: 1.0 V vs. Ag/AgCl (3 M KCl). pH 7.
2
SO
4
] =100 mM. Applied
2 4
The curves c and d were obtained in the dark. [Na SO ] =100 mM.
Reference electrode: Ag/AgCl (3 M KCl). pH 7. Scan rate: 10 mV/s.
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Table 2 Selective photocatalytic oxidation results of 0.5 mM 3-pyridinemethanol by using the Pt loaded HCl or NaOH treated BDH and Merck TiO
photocatalysts in water under UVA at pH 7. Un-loaded Merck and BDH were used for comparison
2
3
-Pyridinemethanal
Vitamin B
[%]
3
selectivity
selectivity [%]
6
a
−
r
0
× 10
X
[%]
1h
X
[%]
3h
t
1/2
SCO2/6
−1
Catalyst
(mM m h
)
X = 15%
X = 50%
X = 15%
X = 50%
[min]
[%] X3h
Ref.
(20)
BDH
Pt–BDH
BDH–HCl
Pt–BDH–HCl
BDH–NaOH
Pt–BDH–NaOH
Merck
36.2
136
129
203
90.0
145
41.1
117
117
192
115
141
54
65
38
46
41
48
52
49
38
46
43
49
8
8
8
8
10
9
8
7
9
9
15
55
50
66
38
54
17
41
44
61
46
50
34
91
90
93
81
82
37
74
87
89
88
83
—
52
64
33
90
53
Low
3
32
21
22
8
Low
4
17
19
26
8
58
36
41
39
41
15
14
15
15
16
(20)
(20)
Pt–Merck
49
36
39
40
42
15
14
14
14
17
88
70
37
66
60
Merck–HCl
Pt–Merck–HCl
Merck–NaOH
Pt–Merck–NaOH
7
10
(20)
a
X = conversion; X1h and X3h = conversion after 1 or 3 h of irradiation. CO
2
selectivity values, divided by 6 for stoichiometric normalization,
were calculated for 3 h of irradiation (X3h).
Table 2 reports the initial reaction rate (−r ), the product
substrate conversion for 1 h irradiation increases from 15%
to 55% and from 17% to 41% for the BDH and Merck
catalysts, respectively. Platinizing of the acid/base treated
TiO2 samples also positively affects the reactivity; for
instance, the conversion values for 1 h irradiation are 66%
and 61% for the Pt–BDH–HCl and Pt–Merck–HCl samples,
respectively. The reactivity of the Pt–BDH–NaOH and Pt–
Merck–NaOH samples, however, was lower than that of the
0
selectivity for 15 and 50% conversion, the conversions for 1 h
(X1h) and 3 h (X3h) of reaction time, the half-life times (t1/2)
and CO2 selectivity after a 3 h reaction time. The initial
reaction rates have been calculated by the following
equation:
ꢀ
ꢁ
ꢀ
ꢁ
1
S dt
dn
V dC
S dt
ð−r0Þ ¼
−
¼
−
(1)
0
0
corresponding HCl-treated catalysts; the (−r
0
) values of the
−
4
−4
−1
where n denotes the number of 3-pyridinemethanol moles, t
is the irradiation time, S is the specific surface area of the
used catalyst, V is the suspension volume and C is the
former ones are 1.45 × 10 and 1.41 × 10 mM m h , while
−
4
−4
those of the latter ones are 2.03 × 10 and 1.92 × 10 mM m
−
1
h .
Fig. 9 shows the photocatalytic oxidation results of
3
-pyridinemethanol concentration. The (−r ) value is
0
independent of the surface area of the photocatalyst, and
hence it is used as a relevant parameter to assess its intrinsic
activity.
3-pyridinemethanol obtained using the BDH and Pt–BDH–
HCl samples. It is clear that the reactivity and product yield
5
improved significantly in the TiO catalyst modified by both
2
The photoreactivity results indicate that the acid/base
treatments lead to a significant activity increase with respect
acid treatment and Pt loading. The aldehyde amount reached
the maximum at 90 min, transforming into vitamin B whose
3
concentration increased steadily during all the reaction
(Fig. 9b).
8
,20
to the untreated samples.
For instance, the initial reaction
rate of Merck–HCl is almost 3 times higher than that of
−
5
−5
−1
untreated Merck (11.7 × 10 vs. 4.11 × 10 mM m h ).
Fig. 10 reports the (−r
0
) values of the acid or base treated
Owing to the fact that the second oxidation step of
and Pt loaded BDH and Merck TiO photocatalysts. The
2
3
-pyridinemethanol is vitamin B production, an increase in
results obtained from the untreated and un-platinized
samples are also provided for the sake of comparison.
Interestingly, the platinized (Pt–BDH and Pt–Merck) samples
showed a comparable reactivity to that of the acid/base
modified TiO (BDH–HCl and Merck–HCl) catalysts. Sample
2
modification, using both methods, platinization and acid/
base treatment simultaneously, improves the catalyst activity
3
the substrate conversion implies a boost in the vitamin B
3
concentration. Moreover, CO2 selectivity values are also
correlated to the conversion ones. Indeed, the extent of the
total oxidized substrate increased with irradiation time. As in
the case of acid or base treatments, platinizing of untreated
TiO2 significantly promoted the catalyst photoactivity; the
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Fig. 9 Photocatalytic results of oxidation of 3-pyridinemethanol (♦) to 3-pyridinemethanal (■) and vitamin B
3
(▲) vs. irradiation time by using the
3
BDH (a) and Pt–BDH–HCl (b) catalysts in water under UVA irradiation at pH 7. The 3-pyridinemethanal and vitamin B values are indicated by right
facing arrows.
markedly, suggesting a possible synergistic effect of these
two methods.
The platinized BDH samples were also heated at 400
°C for 3 h (Pt–BDH–HCl-400 and Pt–BDH–NaOH-400) to
check their thermal stability. This thermal treatment did
not affect the samples' photoreactivity in a significant
way (see Table 3), however the 3-pyridinemethanal
selectivity values increased by about 15–20%. In
agreement with the XPS results obtained with untreated
2
It is well known that TiO catalysts are highly stable and
1
–3
resistant to photocorrosion.
In order to check if the HCl
treatments affect the catalyst stability, the treated samples
underwent thermal treatments at different temperatures
(until 1000 °C) for 3 h in air. These samples were used for
2
2
3
-pyridinemethanol partial oxidation. The reactivity results,
Degussa P25 and home-prepared TiO
uncalcined Pt-TiO samples, metallic Pt is present on
the TiO2 surface and, in the calcined ones, platinum is
2
catalysts, in the
0
reported in Table 3, highlight that the BDH–HCl samples did
not show a significant activity loss up to 700 °C, thus
indicating that these samples are very stable even at high
temperatures. As expected, extreme calcination temperature
2
0
2+
present in both metallic (Pt ) and cationic (Pt ) forms.
In conclusion, these stability results show that the
investigated TiO2 samples could also be used in
photo(thermo)catalytic processes.
(1000 °C) detrimentally affected the catalyst activity; indeed
BDH–HCl-1000 showed an even lower activity than untreated
BDH. Likely explanations of this drastic decrease are that at
Besides thermal stability, cyclic tests were also performed.
Our samples are powdered, and in a slurry system, it is
difficult to reuse them without losing a small amount.
1
000 °C, TiO particles sinter with a great decrease of active
2
surface area and that the amount of surface hydroxyl groups,
responsible for absorption, decreases. The thermal
O
2
2
However, TiO samples in powder form could be re-used by
stability of the NaOH treated samples up to 700 °C was
using a filtration system, centrifuge, or decantation. It is well
known and valid for all powdered nanoparticles that a small
quantity of TiO may be lost during the separation. However,
2
already investigated in our previous work and the sample was
2
0
stable until 400 °C.
in practical application, TiO is used in a supported form on
2
a suitable material to solve the separation process without
3
0
2
losing any TiO particles. On these grounds, in order to do
cyclic tests, the Pt–BDH–NaOH sample was deposited on an
FTO surface (Pt–BDH–NaOH/FTO) as described in section 2.2.
After that, the Pt–BDH–NaOH/FTO sample was used for
3-pyridinemethanol oxidation under UVA irradiation (see Fig.
S2†). The TiO amount deposited on FTO is very low (150 μg)
2
and then the irradiation time was increased to 20 h. In the
slurry system, the irradiation time was 3 h so that the
selected 20 h is equal to 6–7 cycles of a slurry run. Two
consecutive experiments showed almost the same substrate
conversion value (ca. 20%) thus suggesting that the prepared
catalyst is highly stable. Electrochemical characterization
(
photocurrent measurements) of the Pt–BDH–NaOH/FTO
Fig. 10 Initial reaction rate (−r
0
) values of the acid or base treated
samples. Data of the un-treated
sample before and after a photocatalytic run lasting 20 h has
and/or Pt loaded BDH and Merck TiO
2
and un-platinized samples are provided for the sake of comparison.
also been performed. As it may be noted in Fig. S3,† almost
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Table 3 Photocatalytic oxidation results of 3-pyridinemethanol (0.5 mM) under UV irradiation at pH 7: thermal treatment effect on the treated and/or
2
Pt loaded BDH TiO samples
3
-Pyridinemethanal
Vitamin B
[%]
3
selectivity
X = 50%
6
selectivity [%]
−
r
0
× 10
X
[%]
1
X
[%]
3
t
1/2
[min]
−1
Catalyst
[mM m h
]
X = 15%
X = 50%
X = 15%
BDH–HCl
129
129
121
38
46
43
73
46
58
48
62
36
38
38
8
11
9
9
8
9
10
11
14
15
14
50
47
43
6
66
64
54
55
90
86
78
15
93
92
82
81
64
65
78
BDH–HCl–400
BDH–HCl–700
BDH–HCl–1000
Pt–BDH–HCl
Pt–BDH–HCl–400
Pt–BDH–NaOH
203
203
145
144
41
51
41
47
15
16
16
20
33
34
53
52
Pt–BDH–NaOH–400
X = conversion; X1h and X3h = conversion after 1 or 3 h of irradiation.
the same photocurrent values were observed, with the last Conflicts of interest
four switch on–switch off measurements overlapping. All
There are no conflicts to declare.
previous results suggest that the prepared catalysts are
stable.
Acknowledgements
4
Conclusions
S. Yurdakal and S. Çetinkaya thank the University of Afyon
Kocatepe (BAP project no: 17.KARİYER.15) for financial
support.
The effect of Pt loading and acid/base treatment on TiO
2
catalysts was investigated by characterization methods and
performing photocatalytic 3-pyridinemethanol oxidation
under environmentally friendly conditions. The activity was
significantly increased by these treatments. Even though
selectivities were not particularly affected by these
treatments, product yields increased considerably due to
enhanced activity. The characterization of treated TiO2
catalysts revealed that the occurrence of disordered regions
on anatase crystal surfaces was more significant upon acid
and basic treatments. Pt was deposited on catalysts which
previously underwent an acid/basic treatment or not: Pt
clusters work as electron scavengers, collecting electrons
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