Influence of Thermal Activation of Titania on Photoreactivity of Pt/TiO2 in Hydrogen Production

Article abstract of DOI:10.1007/s10562-020-03321-w

Abstract: A series of Pt/TiO₂ photocatalysts was prepared by impregnation of fresh and thermal-activated titania (commercial Evonik Aeroxide P25 TiO₂) with an aqueous solution of H₂PtCl₆ followed by reduction in

Full text of DOI:10.1007/s10562-020-03321-w

Catalysis Letters
https://doi.org/10.1007/s10562-020-03321-w
Infuence of Thermal Activation of Titania on Photoreactivity of Pt/TiO2
in Hydrogen Production
Anna Yu. Kurenkova1 · Anna M. Kremneva1 · Andrey A. Saraev1 · Vadim Murzin2 · Ekaterina A. Kozlova1 ·
Vasily V. Kaichev1
Received: 28 April 2020 / Accepted: 14 July 2020
© Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
A series of Pt/TiO2 photocatalysts was prepared by impregnation of fresh and thermal-activated titania (commercial Evonik
Aeroxide P25 TiO2) with an aqueous solution of H2PtCl6 followed by reduction in an aqueous solution of NaBH4. The
thermal activation was performed by annealing in air. The photocatalytic activity of the Pt/TiO2 catalysts was measured
for the hydrogen production from a mixture of glycerol under UV radiation. It was found that the activation at 300–600 °C
provides an increase in the photoreactivity of resulting Pt/TiO2 photocatalysts in the production of hydrogen while its struc-
tural and textural properties do not change. This efect is due to formation of cationic vacancies that limits fast electron–hole
recombination.
Graphic Abstract
Keywords Photocatalysis · XPS · NEXAFS · XRD · Nanoparticles
1 Introduction
the direct splitting of water into H2 and O2 but also by the
photocatalytic reforming of organic compounds using solar
energy [2–4]. The latter process may also represent an efec-
tive way for the abatement of organic pollutants in waste-
water. Although the detailed mechanism of photocatalytic
reactions is not clear yet, it is commonly agreed that the
primary reactions responsible for the photocatalytic efect
are interfacial redox reactions of electrons and holes that
are generated when the semiconductor catalyst absorbs
light. As compared to other semiconductor photocatalysts,
titanium dioxide has shown the most promise for practical
applications because of its high photoreactivity (the pho-
tonic efciency can achieve 10% [5]), good stability, low
cost, and environmental friendliness. However, pure TiO2
exhibits insufcient photocatalytic activity due to the fast
Photocatalytic production of hydrogen has been actively
studied since 1972, when Fujishima and Honda discovered
photocatalytic splitting of water on TiO2 [1]. These stud-
ies have shown that hydrogen can be obtained not only by
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-020-03321-w) contains
supplementary material, which is available to authorized users.
* Vasily V. Kaichev
vvk@catalysis.ru
1
Boreskov Institute of Catalysis, 630090 Novosibirsk, Russia
2
Deutsches Elektronen-Synchrotron DESY, 22603 Hamburg,
Germany
Vol.:(0123456789)
1 3

A. Y. Kurenkova et al.
electron–hole recombination. To overcome this drawback,
the separation of the electron–hole pairs in a semiconductor
should be provided. This efect can be obtained, for example,
by deposition of platinum and other noble metals. Platinum
nanoparticles are very efective traps for electrons due to the
formation of the Schottky barrier at the metal–semiconduc-
tor contact, thereby increasing the lifetime of electron–hole
pairs [6]. As a result, the addition of even 1 wt% Pt leads to
an increase in the photocatalytic activity by several times
[7]. Another facile technique to increase the activity of TiO2
is its thermal treatment [8, 9]. According to the literature [9],
the annealing of titania may increase the rate of decomposi-
tion of methylene blue by approximately four times.
in a programmable electric furnace. This heat treatment
was performed for approximately 5 h; the thermal profle
included the slow heating from room temperature to the
desired temperature during 1 h with a constant heating rate,
the temperature holding with an accuracy of 10 °C for 3 h,
and the spontaneous cooling after turning of the furnace.
The Pt/TiO2 photocatalysts were prepared by impregnation
of fresh or calcinated titania with an aqueous solution of
H2PtCl6 (Reakhim, Russia, 98%) followed by reduction in
an aqueous solution of NaBH4 (Acros Organics, 98%) at
room temperature [7]. The photocatalysts were referred to
as “T300”, “T400”, “T500”, “T600”, “T700”, and “T800”,
respectively. The photocatalyst prepared without the ther-
mal activation was referred to as “RT”. Platinum content
was approximately 1 wt% in all the photocatalysts. All the
chemical reagents used in the experiments, excluding dis-
tilled water, were obtained from commercial sources as
guaranteed-grade reagents and were used without further
purifcation and treatment.
Despite the simplicity of the latter approach, the reasons
for the observed increase in activity are still under debate.
Moreover, conficting points of view are given in the litera-
ture. For example, Sugapriya et al. [8] studied the efect of
annealing of TiO2 nanoparticles and found that their photo-
catalytic performance is improved due to the transformation
of amorphous titanium dioxide to anatase at 450 °C. Fass-
ier et al. [9] concluded that the crystallite size is the most
important controlling factor for photocatalytic activity rather
than the powder specifc surface area or the anatase/rutile
polymorph ratio. In contrast, Su et al. [10] found strong
infuence of the anatase/rutile ratio on the photoreactivity
in the oxidation of methylene blue under UV radiation.
Herein we present a new insight into the thermal acti-
vation of TiO2-based photocatalysts. To elucidate the ori-
gin of this efect we prepared Pt/TiO2 photocatalysts using
fresh and thermal-activated Evonik Aeroxide P25 TiO2 and
checked their photoreactivity in the hydrogen production.
We chose the photocatalytic production of hydrogen from
an aqueous solution of glycerol as a case photocatalytic reac-
tion due to its practical importance. Indeed, glycerol is one
of biomass-derived materials, and it is produced in large
amounts as a by-product in the transesterifcation of veg-
etable oils into biodiesel fuels [4]. Besides, photoreforming
of glycerol can provide the yield of seven moles of hydro-
gen from one mole of glycerol under mild conditions that is
made this process suitable for industrial applications.
3 Methods
The photocatalysts were examined by UV–vis spectroscopy,
X-ray photoelectron spectroscopy (XPS), X-ray absorption
near edge structure (XANES) spectroscopy, and X-ray dif-
fraction (XRD) and N2 adsorption techniques. The difuse
refectance UV–vis spectra were obtained using a Shimadzu
UV-2501 PC spectrophotometer with an ISR-240A difuse
refectance unit. The specifc surface area (SSA) was calcu-
lated by the Brunauer–Emmett–Teller method using nitrogen
adsorption isotherms measured at liquid nitrogen tempera-
tures with an automatic Micromeritics ASAP 2400 sorptom-
eter. XRD patterns were recorded on a Bruker D8 Advance
difractometer in the 2θ range from 20° to 80° using the Cu
Kα radiation. The mean sizes of crystallites in the samples
were estimated from the full width at half maximum of cor-
responding peaks using the Scherrer formula. The phase
composition of the photocatalysts was quantitatively ana-
lyzed using the Rietveld refnement method.
The XPS study was performed using an X-ray photoelec-
tron spectrometer (SPECS Surface Nano Analysis GmbH,
Germany) equipped with a PHOIBOS-150 hemispherical
electron energy analyzer, a XR-50 M X-ray source, and an
ellipsoidal crystal monochromator FOCUS-500. The core-
level spectra were obtained under ultrahigh vacuum condi-
tions using the monochromatic Al Kα radiation. The charge
correction was performed by setting the Ti2p3/2 peak at
459.0 eV [12]. In this case, the main peak in the C1s spectra
was observed at 285.2 0.1 eV. Relative concentrations of
elements were determined from the integral intensities of
the core-level spectra using the cross sections according to
Scofeld [13]. For detailed analysis, the spectra were ftted
2 Experimental
2.1 Materials
To prepare the photocatalysts, we used commercial Evonik
Aeroxide P25 (formerly known as Degussa P25) titanium
dioxide synthesized via fame pyrolysis of TiCl4. Fresh
Aeroxide consists of multiphase TiO2 nanoparticles con-
taining anatase, rutile, and a small amount of amorphous
TiO2 [11]. The thermal activation of titania was performed
by calcination in air at 300, 400, 500, 600, 700, or 800 °C
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Infuence of Thermal Activation of Titania on Photoreactivity of Pt/TiO2 in Hydrogen…
into several peaks after the background subtraction by the
Shirley method. The ftting procedure was performed using
the CasaXPS software. The line shapes of the peaks were
approximated by the multiplication of Gaussian and Lor-
entzian functions.
The Ti K-edge XANES spectra of the Pt/TiO2 photo-
catalysts and reference anatase and rutile samples were
obtained at the P65 beamline at the synchrotron radiation
facility PETRA III (DESY, Hamburg) [14]. A double-crystal
fxed-exit monochromator based on Si(111) single-crystals
designed by Oxford Ltd. was used. The monochromator was
cooled to liquid nitrogen temperature in order to reduce lat-
tice strain by the heat-load impinging on the frst crystal.
XANES spectra were measured in the fuorescence detection
mode using a passivated implanted planar silicon (PIPS)
Fig. 1 Photocatalytic production of hydrogen from an aqueous solu-
detector. The XANES spectra were analyzed with the Dem-
eter (IFEFFIT) program package [15].
tion of glycerol on Pt/TiO2 under UV radiation depending on the cal-
cination temperature
The photocatalytic activity of the Pt/TiO2 catalysts was
measured for the hydrogen production from a mixture of
glycerol and water in a laboratory-made batch reactor. The
experiments were performed at atmospheric pressure and at
room temperature. The volume of the reactor was 125 cm3.
Before the experiment, 50 mg of a photocatalyst was sus-
pended in the solution, which contained 2.8 mL of glycerol,
97.2 mL of water, and 200 mg of NaOH. The reactor was
loaded by this suspension and then was purged with Ar for
30 min. After that, the suspension was illuminated using a
30 W LED lamp with a wavelength of 380 nm (the emission
spectrum is present in Fig. S1). The procedure for determin-
ing the rate of hydrogen production is described in detail in
Supporting Information.
To refne the activation efects, morphology, chemistry,
and phase composition of the photocatalysts were studied.
Table 1 shows the main properties of the fresh and thermal-
activated Pt/TiO2 photocatalysts. All the samples exhibit
well developed XRD patterns (Fig. S2), which match well
with anatase, rutile, and metallic platinum.
No changes in the structural and textural properties were
observed after the calcination at temperatures below 600 °C.
According to the Rietveld refnement, the ratio of anatase
to rutile (A/R) in the uncalcined photocatalysts and in the
Pt/TiO2 photocatalysts calcined at 300, 400, and 500 °C is
85/15. The mean size of crystallites (D) determined by the
Scherrer formula for anatase and platinum is also constant,
while the mean size of rutile crystallites increases slightly
with the calcination temperature. The specifc surface area
and the pore volume V are 52–55 m2 g−1 and 0.43–0.50
cm3 g−1, respectively; some deviation observed in these
values is due to experimental errors. All these parameters
drastically change after the calcination at 700 and 800 °C
due to rutile formation. Indeed, it is well known that tita-
nium dioxide, the only naturally occurring oxide of titanium
at atmospheric pressure, exhibits three polymorphs: rutile,
anatase, and brookite [17]. While rutile is the stable phase,
both anatase and brookite are metastable. In the photocata-
lysts under study, brookite was not detected. At temperature
above 600 °C, anatase starts to transform to rutile and after
the calcination at 800 °C, no anatase was detected by XRD.
This process is accompanied by a strong decrease in the
specifc surface area and in the pore volume, whereas the
mean size of rutile crystallites increases from 32 to 125 nm
(Table 1).
4 Results and Discussion
Aeroxide is cheap and extremely photoactive in many pho-
tocatalytic reactions, and as a result has become almost the
“gold standard” in semiconductor photochemistry research
[16]. However, bare Aeroxide exhibits a very low activity
in photoreforming of glycerol (0.15 μmol H2 min−1 for the
uncalcinated sample). The platinum deposition leads to an
increase in activity by more than an order of magnitude. The
results of photocatalytic tests of the fresh and thermal-acti-
vated platinized samples are presented in Fig. 1. One can see
that the thermal treatment increases photoreactivity of Pt/
TiO2 in the hydrogen production. An exception is the sample
calcined at 800 °C, which exhibits a very low activity. The
maximum activity, 3.6 mmol H2 min−1, or 4.3 mmol g−1 h−1
is observed for the sample calcined at 500 °C, while the
further increase of the calcination temperature leads to a
decrease in the photoactivity. The highest quantum ef-
ciency is 1.2% (λ = 380 nm). This value is quite good for
the hydrogen production from a glycerol solution (Table S1).
The XRD data are confrmed by UV–vis spectroscopy.
The UV–vis difuse refectance spectra of the fresh and
thermal-activated photocatalysts are presented in Fig. 2. All
1 3

A. Y. Kurenkova et al.
Table 1 Properties of fresh and thermal-activated Pt/TiO2 photocatalysts
Sample
Da of
Da of rutile Da of Pt (nm)
A/R from XRD
A/R from
XANES
SSA (m2 g−1
)
Vb (cm3 g–1)
[O]/[Ti]
from XPS
TiO2
anatase (nm) (nm)
RT
19
19
19
19
19
29
–
30
30
31
31
32
74
125
4.6
4.4
4.6
4.5
4.3
4.4
5.3
85/15
85/15
85/15
85/15
84/16
14/86
0/100
89/11
90/10
86/14
85/15
87/13
22/78
0/100
55
55
53
52
55
19
10
0.48
0.50
0.43
0.49
0.52
0.074
0.035
2.07
2.16
2.26
2.42
2.41
2.69
2.43
T300
T400
T500
T600
T700
T800
aCrystalline size calculated using the Scherrer equation
bPore volume determined by low temperature adsorption of N2
that XANES are sensitive not only to the oxidation state of
specifc atoms but also to its local chemical environment.
The Ti K-edge XANES spectra of the Pt/TiO2 photocata-
lysts are presented in Fig. 3a in comparison to the spectra
of bulk anatase and bulk rutile. One can see that the spec-
tra of anatase and rutile difer signifcantly. The spectrum
of anatase contains two strong peaks at 4987 and 5003 eV,
while the spectrum of rutile contains three strong peaks at
4987, 4992, and 5004 eV. This is in full agreement with the
literature [20]. The spectra of the fresh Pt/TiO2 photocatalyst
and the Pt/TiO2 photocatalysts calcined at 300–500 °C are
similar to the spectrum of anatase, whereas the spectra of the
Pt/TiO2 photocatalysts calcined at 700–800 °C are similar to
the spectrum of rutile.
The simplest approach in the XANES data-processing is
a linear combination ftting (LCF) [21]. In the LCF method,
the X-ray absorption spectrum is modelled by the least-
squares ftting using a linear combination of known spe-
cies to ft an unknown spectrum. In our case, the Ti K-edge
XANES spectra of the photocatalysts were well approxi-
mated by a linear combination of the spectra of anatase and
rutile. The approximation of the XANES spectrum of the
T700 photocatalyst by a linear combination of spectra of
anatase and rutile is shown in Fig. 3b.
Fig. 2 Difuse refectance spectra of fresh and activated Pt/TiO2 pho-
tocatalysts
the spectra contain a single steep absorption edge around
400 nm corresponding to its bandgap. The spectra of the
photocatalysts calcined at 700 and 800 °C show a red shift,
indicating formation of rutile. Indeed, the optical properties
of titanium dioxide depend on its crystal structure. In par-
ticular, the optical bandgap of anatase is 3.23 eV (384 nm),
whereas that of rutile is 3.02 eV (410 nm) [18].
The anatase/rutile ratios determined by the LCF method
are presented in Table 1. One can see that XANES gives a
higher anatase content in the photocatalysts than XRD does.
We can speculate that at least the fresh photocatalyst and the
samples calcined at 300 and 400 °C have this efect due to
the presence of amorphous TiO2 whose Ti K-edge XANES
spectrum is similar to the spectrum of anatase [20]. The cal-
cination at higher temperatures leads to the transformation of
amorphous TiO2 to anatase and further to rutile. As a result,
both XRD and XANES give the same anatase/rutile ratio
for the photocatalyst calcined at 500 °C. This fnding indi-
cated that the thermal activation at 500 °C leads to remove
amorphous TiO2 which positively afects the activity of the
Hence, our data indicate that the enhanced activity of the
Pt/TiO2 photocatalysts obtained via the thermal activation at
300–500 °C is not a result of changes in its texture proper-
ties or the anatase/rutile ratio. However, it is likely that the
methods used cannot detect some structural features of the
photocatalysts. For instance, Bickley et al. [19] suggested
that some individual anatase nanoparticles may be covered
by a thin overlayer of rutile and the photocatalytic activity of
this form of titanium dioxide may be greater than the activ-
ity of either pure crystalline phase. To check the presence
of an excess of rutile, undetectable by XRD, all the photo-
catalysts were investigated by XANES. It should be stressed
1 3

Infuence of Thermal Activation of Titania on Photoreactivity of Pt/TiO2 in Hydrogen…
Fig. 3 Normalized Ti K-edge XANES spectra of bulk anatase, bulk rutile, and the fresh and thermally activated Pt/TiO2 photocatalysts (a);
approximation of the XANES spectrum of Pt/TiO2-T700 by a linear combination of spectra of anatase and rutile (b)
resulting Pt/TiO2 photocatalyst as it provides a good contact
of the platinum nanoparticles and anatase necessary for the
formation of the Schottky barrier.
spin-orbital splitting of 3.33 eV (Fig. 4a), which is typical of
platinum in the metallic state [22]. For the fresh photocata-
lyst and the photocatalysts calcined at 300–600 °C the [Pt]/
[Ti] atomic ratio is in the range 0.015–0.017, which slightly
increases with the calcination temperature. The [Pt]/[Ti]
atomic ratio for the Pt/TiO2 photocatalysts calcined at 700
and 800 °C increases to 0.037 and 0.058, respectively. This
is in good agreement with the XRD data (Table 1), which
indicates that the dispersion of the platinum nanoparticles
does not change with the calcination temperature with the
Finally, we investigated the photocatalysts by XPS. The
Pt4f and Ti2p core-level spectra of the fresh and activated Pt/
TiO2 photocatalysts are presented in Fig. 4a, b. It is known
that the 4f level of platinum splits to the Pt4f7/2 and Pt4f5/2
sublevels due to spin-orbital interaction. The Pt4f spectra of
all the catalysts were approximated by one asymmetric dou-
blet with the Pt4f7/2 binding energy of 71.1 0.1 eV with the
Fig. 4 Normalized Pt4f (a) and Ti2p (b) core-level spectra of studied catalysts. The Pt4f spectra are normalized to the integrated intensity of cor-
responding Ti2p spectra
1 3

A. Y. Kurenkova et al.
exception of 800 °C when the mean size of nanoparticles
slightly increases because of low surface area of titania.
Since XPS is a surface-sensitive method, the increase in the
[Pt]/[Ti] atomic ratio for the T700 and T800 photocatalysts
is caused by a decrease in the specifc surface area.
some additional experiments and DFT calculations should
be performed to study in detail the formation of the cationic
vacancies in titania and their role in photocatalytic reactions.
All the Ti2p spectra contain two sharp peaks corre-
sponding to the Ti2p3/2 and Ti2p1/2 sublevels (Fig. 4b). The
peaks have symmetric shape, which is typical of titanium
in the oxidized state. Taking into account the Ti2p3/2 bind-
ing energy of 459.0 eV and the spin-orbital splitting of
5.66 eV, we concluded that titanium is mainly in the Ti4+
state. No additional peaks were observed at lower binding
energy, which may be attributed to reduced titanium spe-
cies, such as Ti3+. According to the literature, the Ti2p3/2
binding energy for TiO2 is in the range of 458.7–459.2 eV,
whereas for titanium in the Ti3+ state, the binding energy is
between 456.2 and 457.4 eV [12]. This is an expected result
because the thermal activation was performed in air. The
formation of Ti3+ defects in TiO2, which is usually accom-
panied by the formation of oxygen vacancies, was detected
after the annealing of anatase in a reducing atmosphere, such
as vacuum or hydrogen [23]. At the same time, we found
that the calcination of TiO2 leads to an increase in the [O]/
[Ti] atomic ratio (Table 1). The fresh Pt/TiO2 photocata-
lyst is characterized by the [O]/[Ti] atomic ratio equal to
2.07, which is close to the stoichiometric value. This ratio
monotonously increases from 2.16 to 2.42 when the calcina-
tion temperature increases from 300 to 500 °C. This means
that the thermal activation leads to the formation of cationic
vacancies.
5 Conclusions
The Pt/TiO2 photocatalysts prepared by fresh and thermal-
activated Evonik Aeroxide P25 TiO2 were tested in the
production of hydrogen from aqueous solutions of glycerol
under UV radiation. It was found that the thermal activation
of titania in the temperature range between 300 and 600 °C
leads to an increase in the photoreactivity of resulting Pt/
TiO2 photocatalysts. The highest activity was obtained
after annealing at 500 °C. According to the XPS study this
efect is due to formation of cationic vacancies that limit
the fast electron–hole recombination. After annealing at
700–800 °C, anatase transforms irreversibly to rutile. The
rutile formation is accompanied by a strong decrease in the
specifc surface area, which negatively afects the rate of
hydrogen production. Hence, the thermal activation of tita-
nia at 500 °C can be used to improve the activity of titania-
based photocatalysts. In addition it was found that the ther-
mal activation at 500 °C leads to remove amorphous TiO2
which positively afects the activity of the resulting Pt/TiO2
photocatalyst as it provides a good contact of the platinum
nanoparticles and anatase necessary for the formation of the
Schottky barrier. Hence, both these efects, the creation of
cationic vacancies and transformation of amorphous TiO2 to
anatase under the thermal activation can provide enhanced
photocatalytic activity of Pt/TiO2 photocatalysts.
It should be noted that the formation of cationic vacancies
in titania is not a new efect. Recently, Ghosh and Namb-
issan [24] studied the annealing of anatase nanoparticles.
Using photoluminescence spectroscopy, they showed the
formation of vacancy-type defects such as oxygen vacancy
(VO), cation vacancy (VTi) or vacancy-combination defects.
The application of positron annihilation spectroscopy and
Coincident Doppler broadening spectroscopy made it pos-
sible to prove the formation of cation vacancy VTi and larger
vacancy-combination defects as divacancy VTi+O and triva-
cancy VTi+O+Ti. During the annealing, the concentration of
such defect-combinations initially increases to a maximum
at approximately 300 °C and then decreases at higher tem-
peratures due to gradual annealing. We can speculate that
the divacancy VTi+O and trivacancy VTi+O+Ti are also formed
during the thermal activation of our photocatalysts. Accord-
ing to XPS (Table 1) the concentration of cation vacan-
cies in our titania samples increases with the calcination
temperature excepting 800 °C when anatase transforms to
rutile in full. Being negatively charged, these defects within
TiO2 nanoparticles could play the role of hole traps, thereby
increasing the lifetime of electron–hole pairs in the semi-
conductor and increasing their photoreactivity. Certainly,
Acknowledgements This work was supported by Russian Science
Foundation (Grant #19–73-20020). The XPS and XRD experiments
were performed using facilities of the shared research center “National
center of investigation of catalysts” at Boreskov Institute of Catalysis.
The authors are grateful to S. Cherepanova for the XRD study and T.
Larina for the UV–vis measurements. The authors are also grateful to
the staf of DESY for their support during the beam time.
Compliance with Ethical Standards
Conflict of interest The authors declare that they have no confict of
interest.
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