Pt/TiO2 photocatalysts deposited on commercial support for photocatalytic reduction of CO2

Article abstract of DOI:10.1016/j.jphotochem.2018.04.012

Pt/TiO₂ (PC500) photocatalysts with different amount of Pt (0.5–3.0 wt.%) deposited on commercial support (Al₂O₃ foam) were characterized in detail by X-ray powder diffraction, nitrogen physisorption measurement, UV–vis di

Full text of DOI:10.1016/j.jphotochem.2018.04.012

Accepted Manuscript
Title: Pt/TiO photocatalysts deposited on commercial support
2
for photocatalytic reduction of CO2
Authors: Minoo Tasbihi, Kamila Ko cˇ ´ı , Miroslava
Edelmannov a´ , Ivana Troppov a´ , Martin Reli, Reinhard
Schom a¨ cker
PII:
S1010-6030(17)31640-4
DOI:
Reference:
https://doi.org/10.1016/j.jphotochem.2018.04.012
JPC 11227
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
15-11-2017
23-2-2018
6-4-2018
Please cite this article as: Minoo Tasbihi, Kamila Ko cˇ ´ı , Miroslava Edelmannov a´ , Ivana
Troppov a´ , Martin Reli, Reinhard Schom a¨ cker, Pt/TiO2 photocatalysts deposited on
commercial support for photocatalytic reduction of CO2, Journal of Photochemistry
and Photobiology A: Chemistry https://doi.org/10.1016/j.jphotochem.2018.04.012
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Pt/TiO photocatalysts deposited on commercial support for photocatalytic
2
reduction of CO
2
a*
b
b
b
b
Minoo Tasbihi , Kamila Kočí , Miroslava Edelmannová , Ivana Troppová , Martin Reli , Reinhard
Schomäckera
aDepartment of Chemistry, Technische Universität Berlin, Straße des 17. Juni 124, 10623-Berlin,
Germany
bInstitute of Environmental Technology, VŠB-Technical University of Ostrava, 17. Listopadu 15,
Ostrava – Poruba 708 33, Czech Republic
*
Corresponding author: Tel.: +49 30 314 25644; Fax: +49 30 314 79552.
Email address: minoo.tasbihi@tu-berlin.de
Graphical abstract
Highlights





Pt/TiO
Pt/TiO
2
2
(0.5-3.0 wt.%) samples were prepared by deposition–precipitation method.
photocatalysts deposited on commercial support – Al foam.
photocatalytic reduction in gas phase
2
O
3
The photocatalysts were examined for CO
2
Deposition on the foam significantly increased the product yield.
Enhanced photocatalytic activity of photocatalyst can be attributed to the low electron-hole
recombination.

Abstract
Pt/TiO
commercial support (Al
2
(PC500) photocatalysts with different amount of Pt (0.5-3.0 wt.%) deposited on
foam) were characterized in detail by X-ray powder diffraction,
2
O
3
nitrogen physisorption measurement, UV-Vis diffuse reflectance spectroscopy, inductively coupled
plasma optical emission spectrometry (ICP-OES), photoelectrochemical measurements and tested
for CO
reaction product was methane; however, hydrogen and carbon monoxide were also detected. The
highest yields of CH , H and CO were achieved in the presence of 1.5 wt.% Pt/TiO . Base on the
performed experiments, it was suggested that the key role in the CO photocatalytic reduction plays
2
photocatalytic reduction and compared with commercial anatase TiO
2
PC500. The main
4
2
2
2
the potentials of electrons and holes within the electronic structure of photocatalyst, which were
markedly affected by the Pt loading in such a way that it improved the transformation efficiency of
CO
2
to methane. Significantly higher yields of CO photocatalytic reduction were achieved in the
2
presence of photocatalysts deposited on the commercial support compared to the powder
photocatalysts. These findings confirmed the great importance of the supports surface areas on
photocatalyst activity in photocatalytic reaction.
Keywords: CO
2
reduction, CH
4
, Photocatalysis, Pt/TiO
2
, Al
2
3
O support, Deposition-precipitation
1
. Introduction
Human activities since the beginning of the Industrial Revolution (taken as the year 1750) have
produced a 40% increase in the atmospheric concentration of carbon dioxide, from 280 ppm in 1750
to 400 ppm in 2015 [1]. This increase has occurred despite the uptake of a large portion of the
emissions by various natural "sinks" involved in the carbon cycle. Anthropogenic carbon dioxide
(
CO
fuels, principally coal, oil, and natural gas, along with deforestation, and soil erosion. The
possibility of converting the CO into clean fuel would provide an innovative solution for both the
2
) emissions (i.e., emissions produced by human activities) come from the combustion of fossil
2
future shortage of fossil fuels and the global warming problem [1,2].
The novelties and advances in the photocatalysis technology have introduced this method as a one
of the promising methods which not only can remove CO but can also convert it into energy-
2

bearing products such as methane (CH
4
), methanol (CH
3
OH), formaldehyde (HCHO), carbon
) [3,4]. Up to now, titania (TiO ) has
monoxide (CO), formic acid (HCOOH) and ethylene (C
2
H
4
2
been the most explored semiconductor for photocatalytic application due to its outstanding chemical
and thermal stability. However, it has large band gap and relatively high recombination rate of
photoinduced electron-hole pairs thus led to low photocatalytic activity [5].
As a result, it is significant impact to overcome these limitations and to improve the electron-hole
separation efficiency and light utilization ability of TiO
2
. Surface modification of TiO through
2
noble metal deposition is widely applied by researches [6,7]. Introduction of co-catalyst such as
noble metals like Pt, Pd, Rh, Ag, Cu and Au onto the surface of titania act as electron scavengers
and therefore inhibit electron-hole recombination and proved to improve the photocatalytic
performance [8]. Generally Pt [9], Pd [10] and Au [11] can lead to photocatalytic reduction of CO
towards methane production and Pt indicate that is the most effective co-catalyst to utilize the
photogenerated electron towards CO reduction [12].
2
2
In detail, the Fermi level of the metal is located energetically below the conduction band (CB) of
titania and by loading of these metals on titania, Schottky barrier can form at the interface between
titania and metals. The Schottky barrier acts as electron traps which prevents migration of excited
electrons to semiconductor and thus preventing the recombination [13].
Ishitani et al [14] first systematically studied the photocatalytic reduction of CO
metal-deposited TiO and found that depositing of metals (Pd, Rh, Pt, Au, Cu O) on TiO
greatly boost their photocatalytic activities for CO reduction to CH (in decreasing order) [14]. In
2
on a series of
2
2
2
can
2
4
other hand, the loading content of the metal is also important factor since high content of metal will
induce faster electron-hole recombination and deactivation of the photocatalyst soon [15]. For
example, Anpo et al. found that in case of Pt/TiO
2
, the yield of CH increased remarkably when the
4
amount of Pt was increased (0.1 to 1.0 wt%), but the addition of excess Pt was undesirable for an
efficient reaction [16].
2
+
Xiong et al. modified TiO
method for photocatalytic reduction of CO
in a continuous flow reactor. H , CH and CO were found to be the main products. The introduction
of Pt nanoparticles and Pt ions effectively enhanced the production of H and CH , but did not
have obvious impact on the production of CO. The maximum yields of H and CH under UV light
were 2763.1 and 264.5 µmol.gcat-1, and the maximum CH
yield under visible light irradiation was
38.6 µmol.gcat-1, which were all obtained by the mass ration of Pt to 0.3%. It was believed that the
2
nanoparticles by Pt ions and Pt nanoparticles through simple sol-gel
2
and water vapor under UV and visible light irradiation
2
4
0
2+
2
4
2
4
4
1
2
+
0
enhanced activity of Pt -Pt /TiO
2
catalyst may be due to the low recombination rate of
photogenerated electron-hole pairs caused by Pt nanoparticles and the strong visible light
2
+
absorption, as well as the high surface area induced by Pt ions [17]. Qiu-ye et al. improved the

photocatalytic activity of anatase TiO
2
nanosheet porous films by Pt nanoparticles. It was found that
the loading of Pt expanded the light absorption ability of the porous film and improved the
transformation efficiency of CO
2
to methane. The conversion rate of CO
2
to methane on Pt/TiO
2
-1
-2
film reached to 20.51 ppm. h ⋅cm [18]. Zhang et al. [15] investigated photocatalytic reduction of
CO
to 400 °C and 30 MPa beyond the critical point of water. CO, CH
products and HCOOH and HCHO were minor products at any temperatures and pressures. The
unexpected increase in the rates of CO, CH , HCOOH and HCHO was observed as the temperature
was elevated from 300 °C to 400 °C at a constant pressure of 30 MPa. This was attributed to the
formation of uniform single phase mixture of water and CO under supercritical condition, leading
to the enhanced efficiency of CO reduction. In contrast, the yield of H produced by water
2
by Pt-loaded TiO
2
in the mixture of CO
2
and H
2
O as a function of temperature and pressure up
4
and H were evolved as major
2
4
2
2
2
reduction showed a monotonous increase with temperature because of the thermal acceleration of
photocatalytic reaction. It is suggested that the control of the phase state of reaction medium has a
key role in the determination of conversion efficiency for the photocatalytic reduction of CO
water [15]. Xie et al. studied the effect of Ag, Au, Rh, Pd, Pt as a co-catalysts on photocatalytic
behaviors of TiO with an emphasis on the selectivity of photogenerated electrons for photocatalytic
CO reduction in the presence of H O. It was showed that the rate of CH formation increased in the
2
with
2
2
2
4
sequence of Ag < Rh < Au < Pd < Pt, corresponding well to the increase in the efficiency of
electron−hole separation. It was pointed out that Pt is the most effective cocatalyst to extract
photogenerated electrons for CO
presence of Pt [19] .
2
reduction and the selectivity of CH was the highest in the
4
In the literature, there are reports of many other additives used to enhance the photocatalytic
performance of a semiconductor. They can also promote the adsorption of specific reactants, taking
advantage of their large specific surface area, as well as extend the spectral range of light absorption
as photosensitizers [20]. In generally, using supported heterogeneous photocatalysts is often
beneficial as it can improve stability of active components and also their dispersion in the support
because of its high surface area.
In this study, Pt/TiO
loadings of 0.5, 1.0, 1.5 and 3.0 wt.% and deposited on commercial support (Al
The photocatalytic activity of as-prepared photocatalysts was investigated towards photocatalytic
reduction of CO . The relation between the structural, optical and photoelectrochemical properties
and photocatalytic activity was studied in details.
2
photocatalysts were prepared by a deposition-precipitation method with Pt
2
O
3
foam)
2
2
2
. Experimental
.1. Photocatalyst preparation

Pt/TiO photocatalysts containing different platinum loading were prepared by a deposition–
2
precipitation method, according to the method developed by Haruta and co-workers [21]. In this
procedure, different solutions were prepared, containing the appropriate amounts of the Pt precursor
(
H
2
PtCl
.0 wt.% of Pt in the final material. The pH of the solutions was adjusted to 9 by addition of a 0.1 M
solution of NaOH. Once the pH value was stable, 2 g of commercial TiO PC500
Millennium/CristalACTivTM) were added under vigorous stirring. The deposition–precipitation
6
.H
2
O, 8 wt.% in H
2
O, Sigma-Aldrich) in 300 mL of milli-Q water to reach 0.5, 1.0, 1.5 and
3
2
(
°
procedure was done at 70 C, maintaining the pH constant during 2 h, and then the suspension was
stirred overnight. The suspension was then washed and dried at 80 °C under vacuum condition
overnight. The resulted photocatalysts were labeled as x wt.% Pt/TiO
the nominal platinum loading and TiO PC500 , respectively.
The prepared photocatalysts were deposited on commercial support VUKOPOR (Lanik Ltd., Czech
2
, where x and TiO indicate
2
2
Republic). VUKOPOR is Al
2
O
3
foam with parameters: apparent density of ceramic body = 2.35
3
2
g/cm , specific surface area 0.6 cm /g, porosity of ceramic body = 33 %. The utilized foam had
diameter = 7 cm, height = 1 cm and weight 17.5 g. The photocatalyst was deposited on the foam as
follows: photocatalyst was suspended in 10 ml of distilled water and this suspension was applied to
the foam, which was previously and after application dried in the oven at 100° C for 1 h.
2
.2. Photocatalyst characterization
The crystalline structure and crystalline phase and of synthesized photocatalysts powders were
examined by X-ray powder diffraction (XRD) technique. The XRD patterns were obtained by
Rigaku SmartLab diffractometer (Rigaku, Japan) with detector D/teX Ultra 250 using Co Kα
radiation in the 2θ range of 5°-90° with a step size of 0.01°. XRD patterns were analyzed using
LeBail method (software PDXL2) to refine lattice parameters of the anatase. Background of the
patterns was determined using B-Spline function, peak shapes were modeled with a pseudo-Voigt
function accounting for a peak asymmetry due to axial divergence. The crystallite size of
photocatalysts was measured by Scherer equation using the strongest reflection peak (101).
Nitrogen physisorption at 77 K was performed on a 3Flex automated volumetric apparatus
(Micromeritics Instruments, USA) after degassing of materials at 50 °C for more than 120 h under
vacuum below 1 Torr. Degassing at low temperature was applied to remove physisorbed water, but
having no influence on the porous morphology of the developed materials. The specific surface
area, SBET, was calculated according to the classical Brunauer–Emmett–Teller (BET) theory for the
0
p/p range of 0.05–0.30 [22]. The net pore volume, Vnet, was determined from the nitrogen
0
adsorption isotherm at maximum p/p (~0.99). The pore-size distribution was evaluated from the

adsorption branch of the nitrogen adsorption-desorption isotherm by the Barrett–Joyner–Halenda
BJH) method [23] using the de Boer standard isotherm and assuming cylindrical pore geometry.
(
The concentration of Pt on the catalyst support was measured by inductively coupled plasma optical
emission spectrometry (ICP-OES, Varian Inc., USA). To dissolve the platinum, a microwave
method (Discover SP-D, CEM, USA) was used. Calibration of the setup was done with a
commercial Pt standard solution.
The UV-Vis DRS spectra of the prepared photocatalysts were recorded using a UV-2600
spectrophotometer with an ISR-2600 integrating sphere attachment (Shimadzu Scientific Co.,
Japan) from 200 to 800 nm. The interval sampling was set to 1 nm and width of the slit was set to 2
nm. Barium sulphate (BaSO
4
) was used as a reference material. Reflectance was recalculated using
the Kubelka–Munk transformation of the original diffuse reflectance spectra vs. photon energy
(Tauc plot) [24]:
(
1−ꢀꢁ)2
ꢂ.ꢀꢁ
퐹(푅 ) =
∞
where R
∞
is the measured reflectance (R =Rsample/Rstandard). The obtained spectra were transformed
∞
to the dependencies (퐹(푅 ) ∗ E)1 of against hν in order to determine the values of the energy of
/ꢂ
∞
indirect absorption edges.
Morphology of TiO
microscopy (SEM) (Hitachi SEM type SU8030 microscope operated at an acceleration voltage of
0 kV and a probe current of 15 pA) and transmission electron microscopy (TEM) (TECNAI G²20,
2
and 1.5 wt.% Pt/TiO photocatalysts were obtained by scanning electron
2
1
FEI company, USA operated at 200 kV, with LaB6 electron emitter and EDX r-TEM SUTW-
detector (EDAX Inc., USA)).
Photoelectrochemical measurements were carried out using a photoelectric spectrometer equipped
by a 150 W Xe lamp and potentiostat (Instytut Fotonowy, Poland). The three electrodes setup was
used for the recording of the photocurrent responses. Saturated Ag/AgCl and platinum wire were
used as reference and counter electrodes, respectively. The working electrode was prepared as
follows: small amount of photocatalyst was suspended in deionized water, deposited on the ITO
(
indium tin oxide) foil and dried. The 0.1 M KNO solution was used as electrolyte. The
3
photocurrent responses were recorded in the range of 240-450 nm (10 nm step) and in the potential
range of -0.2-1.0 V (0.1 V step). The electrolyte solution was purged by argon before the
measurement and during the measurement as well in order to ensure oxygen free environment.
2
.3. Photocatalytic reduction of CO
The photocatalytic reduction of CO
volume 348 ml) with a quartz window on the top of reactor (Figure 1). As a light source, the UV 8
2
2
proceeded in home-made batch stainless-steel photoreactor
(

W Hg lamp (peak intensity at 254 nm wavelength; Ultra-Violet Products Inc.) was used and was
located over the quartz window of reactor. A pressure sensor (Greisinger, GMSD 3.5 BAE) for
controlling the pressure was connected to the reactor throughout the experiment. The photocatalytic
reaction was prepared by two different ways: a) 0.1 g photocatalyst was added to the bottom of the
reactor or b) the photocatalytic reactor was filled by 10 ml of distilled water and by foam coated
with 0.1 g of the same photocatalyst.
Before the start of the photocatalytic reaction, the reactor was sealed and purged with CO . The
2
gaseous samples were discontinuously taken using gastight syringe (Hamilton Co., Reno, USA).
The samples were taken discontinuously using gastight syringe (Hamilton Co., Reno, USA) and
immediately analyzed on gas chromatograph (Shimadzu Tracera GC-2010Plus) equipped with BID
(
barrier discharge detector). The measurements of each sample have been reproducibly repeated.
Three different blank tests were performed to ensure that products (methane and carbon monoxide)
have been formed from CO . The first blank was UV-irradiation of CO without the photocatalyst,
the second was in the dark with the photocatalyst and CO under the same experimental conditions
2
2
2
and the third was over the illuminated photocatalyst in the absence of CO
monoxide were detected in the above blank tests.
2
. No methane and carbon
Figure 1. The top-view photo of photoreactor for the photocatalytic reduction of CO with
2
investigated photocatalyst deposited on commercial support under UV light irradiation.
3
3
. Results and discussion
.1. Structural, textural and surface characteristics of the photocatalyst
The XRD patterns of TiO
of TiO and Pt/TiO are composed of anatase (JCPDS No. 21-1272) as indicated by the peaks
emerging with (1 0 1), (1 0 3), (0 0 4), (1 1 2), (2 0 0), (1 0 5), (2 1 1), (2 0 4), (1 1 6) and (2 2 0). In
detail TiO contains broad reflection of anatase peaks while by loading Pt, the crystalline phase of
titania is growing gradually and the intensity of peaks increased by increasing the amount of Pt.
2
and Pt/TiO
2
photocatalysts are shown in Figure 2. The crystalline phase
2
2
2

This behavior might be due to the ultrafine dispersion of Pt nanoparticles on the surface of titania
and also may be due to the integrated of Pt nanopartcles inside the titania structure (Figure 2). This
result is in the agreement with [9,17,25]. The calculated crystallite size, the lattice parameters and
cell volume of investigated photocatalysts are listed in Table 1. As it is summarized in Table 1, the
crystallographic axis a and c and the cell volume were increased by increasing of Pt content. Based
4
+
4+
on these results and due to almost similar ionic radii Ti and Pt , Pt ions are replacing some Ti
ions in the crystal lattice points [26,27]. As no Pt or Pt compound peaks were detected (Figure 2),
it can be summarized that Pt ions are incorporated into the crystal structure of anatase titania.
The crystallite size of the anatase is relatively stable and varies from 16.1 to 16.7 nm, see Table 1.
In detail, the crystallite size of TiO
2
increases to 16.7 nm by increasing the amount of Pt to 1.5 wt.%
and then decreases to 16.1 nm for 3.0 wt.% Pt/TiO
2
[28].
TiO2
0
1
1
3
.5 wt.% Pt/TiO2
.0 wt.% Pt/TiO2
.5 wt.% Pt/TiO2
.0 wt.% Pt/TiO2
2
0
30
40
50
60
70
80
90
2
Theta ( ° )
Figure 2. XRD patterns of investigated photocatalysts.
The UV-Vis DRS reflectance spectra of investigated photocatalyst are shown in Figure 3. Figure 3a
shows the reflectance spectra of samples. As expected, the addition of Pt influenced the absorption
properties of TiO . As it is shown the reflectance spectrum shifted toward the visible region by
2
increasing the Pt loading (Figure 3a). The corresponding values of band-gap energies were
determined by using the Tauc plot (Figure 3b) and the calculated value are listed in Table 2. The
band-gap energies of Pt/TiO
2
photocatalysts are 3.47-3.44 eV while for PC500 TiO is 3.5 eV. Pt
2
doping caused changes between TiO
has no significant changes [28].
2
and Pt/TiO
2
photocatalysts but the calculated band gap energy
(a)
(b)

Figure 3. UV-Vis DRS reflectance spectra of investigated photocatalyst; (a) the reflectance spectra
of TiO and Pt/TiO photocatalysts, (b) Tauc plot of samples for indirect band-gap determination.
2
2
The basic textural properties of the investigated photocatalysts are summarized in Table 2 and
2
2
Figure 4. The specific surface area of photocatalysts varied from 96 m /g to 286 m /g. TiO
2
2
2
exhibited high specific surface area of 286 m /g while the specific surface area decreases to 96 m
-1
g with 1.5 wt.% Pt loading. This behavior could be caused by the fast growth of particle size [15].
Another reason could be the blockage or collapse of small pores, whose loss can be seen in the
nitrogen sorption isotherms for Pt/TiO
2
photocatalysts in comparison to the TiO
2
.
2
-1
The specific surface area then increases to 102 m g for 3.0 wt.% Pt loading. The prepared Pt/TiO
2
photocatalysts have comparable surface area and very similar pore volumes as it is listed in Table 2
and shown in Figure 4 b.
The actual amount of Pt was measured by ICP–OES (Table 2). As it can been seen, it is lower than
the nominal one, which can be due to the various steps during the photocatalyst preparation. In
details, the amount of Pt in the samples with 1.5 wt.% and 3 wt.% of Pt loading is almost similar
with the theoretical amount of Pt. However, in case of samples with 0.5 wt% and 1 wt% of Pt, the
Pt amount is almost half of the theoretical amount.

(a)
(b)
2
2
1
1
50
0.5
TiO2
TiO2
0
1
1
.5 wt.%/TiO2
.0 wt.%/TiO2
.5 wt.%/TiO2
0.5 wt.%/TiO2
1.0 wt.%/TiO2
1.5 wt.%/TiO2
3.0 wt.%/TiO2
00
50
00
0.4
0.3
0.2
0.1
0.0
3.0 wt.%/TiO2
5
0
0
0
.0
0.2
0.4
0.6
0.8
1.0
0
10
20
30
40
50
60
70
80
90 100
0
p/p (-)
d (nm)
Figure 4. (a) The measured nitrogen adsorption-desorption isotherms and (b) evaluated pore-size
distributions of investigated photocatalysts.
The morphology and structure of TiO
images) and Figure 6 (SEM images) which clearly show the different morphology and structure of
TiO and Pt/TiO photocatalysts. It is essential to mention that the 1.5 wt.% Pt/TiO photocatalyst
was chosen as it shows the highest activity for photocatalytic CO reduction.
From the 1.5 wt.% Pt/TiO (Figure 5), the lattice fringes of TiO nanoparticles are clear and confirm
the crystal form of anatase for TiO according to the interplanner spacing [17,29]. Interestingly, no
Pt nanoparticles can be seen in the TEM images which can be due to the integration of ultrafine Pt
2
and 1.5 wt.% Pt/TiO
2
are represented in Figure 5 (TEM
2
2
2
2
2
2
2
nanoparticles inside the titania structure which is accordance with the N
XRD results (Figure 2 and Table 1).
2
sorption results [30] and

Figure 5. TEM images of TiO
2
and 1.5 wt.% Pt/TiO .
2
Furthermore, the SEM images of TiO
2
presented in Figure 6 show the homogeneous porous
structure of TiO nanoparticles with certain roughness on the surface while by loading 1.5 wt.% Pt,
2
the photocatalyst reveals larger and well-shaped titania nanoparticles (Figure 6) which is in the
agreement with the XRD results (Figure 2, Table 1). It is noteworthy to mention that EDX of 1.5
wt% Pt/TiO
2
was done, but no Pt particles could be found. Even though the the presence of 1.4
wt.% Pt was confirmed by ICP –OES (Table 2).

Figure 6. SEM images of TiO
2
and 1.5 wt.% Pt/TiO .
2
The spectrophotoelectric measurements were carried out in order to evaluate the photocurrent
generation of all prepared photocatalysts under various wavelengths and at fixed potential of 1.0 V
(Figure 7). The photocurrent responses below 320 nm are not very distinguished due to device
limitations. Nevertheless, it is clearly visible each photocatalyst immediately generate photocurrent
right after irradiation from 320 nm. The rapid photocurrent generation is attributed to the initial
excitation of electrons to conduction bands. There is also recorded slower generation of
photocurrent during the irradiation. This slow photocurrent rise has been attributed to photo-excited
states which exhibit a short lifetime and probability for a lattice relaxation at the surface, in which
deep unknown centers form when shallow donors convert into deep donors [31]. After closing the
shutter (irradiation interruption) the current generation rapidly decreases. Although, not instantly,
but the decrease of current is slower, which means the recombination of electrons and holes is not
immediate. The results from the photocurrent measurements predict higher photocatalytic activity
of pristine TiO under 320 nm and higher. Not only its photocurrent generation is the highest but
2
also the decrease of photocurrent after switching off the light is the slowest pointing out slower
recombination of charge carriers.

0
0
0
0
0
0
.25
.20
.15
.10
.05
.00
TiO2
0
1
1
3
.5 wt.% Pt/TiO2
.0 wt.% Pt/TiO2
.5 wt.% Pt/TiO2
.0 wt.% Pt/TiO2
2
80
300
320
340
360
380
400
420
440
Wavelength (nm)
Figure 7. Current generation in the presence of investigated photocatalysts at 1.0 V vs. Ag/AgCl
electrode.
3
. 2. Photocatalytic reduction of CO
Figure 8 shows the yields comparison of two different applications of photocatalyst in the reactor. It
is obvious that the deposition of 1.5 wt.% Pt/TiO photocatalyst on the foam has increased the
yields of all products of photocatalytic reduction of CO . Hydrogen yields increased more than
2
2
2
twenty times, methane yields more than thirteen times and carbon monoxide yields increased
twenty-one times. A significant increase in yields was due to the increase in the surface on which
the photocatalyst was coated. When the photocatalyst was applied to the bottom of the reactor, an
2
utilizable area was only 42 cm . Whereas when the photocatalysts was deposited on the foam (a
2
2
specific surface area is 0.56 m /g) the utilizable area was 9 m . For this reason, a larger number of
photons could be used to generate a pair of electron -hole, which subsequently led to an increased
yield of CO
2
photocatalytic reduction. These findings confirmed the great importance of the
supports surface areas on photocatalyst activity in photocatalytic reaction.

3
3
2
2
1
1
5
0
5
0
5
0
5
0
1
1
.5 wt.% Pt/TiO2
.5 wt.% Pt/TiO2 on
foam
0
3
6
9
12
15
18
Time (h)
a)
2
0
8
6
4
2
0
8
6
4
2
0
1
.5 wt.% Pt/TiO2
1
1
1
1
1
1.5 wt.% Pt/TiO2 on
foam
0
3
6
9
12
15
18
Time (h)
b)
7
00
1
1
.5 wt.% Pt/TiO2
6
5
4
3
2
1
00
00
00
00
00
00
0
.5 wt.% Pt/TiO2 on
foam
0
3
6
9
12
15
18
Time (h)

c)
Figure 8. Time dependence of a) methane, b) carbon monoxide and c) hydrogen yields over 1.5
wt.% Pt/TiO
2
photocatalyst, which was applied in two ways in the CO photocatalytic reduction
2
(related to the weight of photocatalyst).
Figure 9 shows the production rate of the main reaction products versus irradiation time. As it is
shown, by increasing amount of Pt from 0.47 wt.% (1.0 wt.% Pt/TiO photocatalysts) to 1.4 wt.%
1.5 wt.% Pt/TiO photocatalysts) the rates of CH , CO and H increase. A further increase in the Pt
real loading to 2.2 wt.% (3.0 wt.% Pt/TiO photocatalysts) led to decrease of all rates of products.
This behavior can be explain to the fact that the electron generated on TiO can be extracted by a Pt
particle on the surface and on the other side the distance between the hole remaining on TiO and
2
(
2
4
2
2
2
2
the Pt particle may become shorter if the Pt loading becomes higher and thus this could increase the
possibility of recombination of electrons and holes [32]. Therefore, a decrease in the photocatalytic
activity of photocatalysts was observed at higher Pt loading (3.0 wt.% Pt/TiO
which was also confirmed by low photocurrent generation.
2
photocatalysts)
1
1
1
1
6
4
2
0
8
6
4
2
0
TiO2
0
1
1
3
.5 wt. % Pt/TiO2
wt. % Pt/TiO2
.5 wt. % Pt/TiO2
wt. % Pt/TiO2
0
3
6
9
12
Time (h)
a)

8
7
6
5
4
3
2
1
0
TiO2
0
1
1
3
.5 wt. % Pt/TiO2
wt. % Pt/TiO2
.5 wt. % Pt/TiO2
wt. % Pt/TiO2
0
3
6
9
12
Time (h)
b)
3
00
2
2
1
1
50
00
50
00
TiO2
0
1
1
3
.5 wt. % Pt/TiO2
wt. % Pt/TiO2
.5 wt. % Pt/TiO2
wt. % Pt/TiO2
5
0
0
0
3
6
9
12
Time (h)
c)
Figure 9. Time dependence of (a) methane (b) carbon monoxide and (c) hydrogen yields over
photocatalysts deposited on foam in the CO
photocatalyst).
2
photocatalytic reduction (related to the weight of
Figure 10 shows the rates of all the products after 12 h of irradiation. TiO
formation of CH together with H resulting from the parallel water decomposition which
competing with CO reduction for conduction band electrons. By loading of 0.26 wt.% Pt (0.5 wt.
Pt/TiO photocatalyst) on TiO , the amount of H increases while the production rates of CH
2
led mainly to the
4
2
2
%
2
2
2
4

and CO did not change. This behavior could be expected taking into account the catalytic properties
of Pt for water decomposition [28,33]. On the other hand, loading of 0.26 wt.% Pt (0.5 wt. %
Pt/TiO
As it is shown in Figure 10, the increase of real Pt content from 0.26 wt.% to 0.47 wt.% (0.5 wt. %
Pt/TiO and 1.0 wt. % Pt/TiO photocatalyst, respectively) the rate of H and CO slightly increase
while the production rates of CH decreases. As it is presented in Figure 10, the crystallite size of
.0 wt.% Pt/TiO slightly increases and this can led to increase the possibility of recombination rate
of excited electron and hole on the surface of the catalysts and resulted on decreasing the number of
the available electron for CH production. All these changes in presence photocatalysts with small
real Pt contents (0.26 and 0.47 wt. %) were not significant. On the other hand, the significant
2
photocatalyst) cause increasing the crystallite size and declining the specific surface area.
2
2
2
4
1
2
4
production rates of all products especially H
loading (1.4 wt. %).
2
were observed with higher increasing of the real Pt
This means that loading of 1.40 wt.% Pt (1.5 wt. % Pt/TiO
of electron density on the catalyst surface and thus, increases the photocatalytic reduction of CO
parallel with water decomposition. By increasing more Pt content to 2.20 wt.% Pt (3.0 wt. %
Pt/TiO photocatalyst), the rates of H and CO drastically decrease while the CH formation rate
slightly decrease while the crystallite size did not change however the specific surface area
2
photocatalyst) enhances the enrichment
2
in
2
2
4
2
-1
increases to 102 m g . These results are with an agreement with the research results observed by
Ambrožová et. al. [34] and Xie et. al. [12]. They supposed that if the Pt loading is higher the
electron formed on TiO
2
can be extracted to a Pt particle and the distance between the hole on TiO
2
and the Pt particle may become shorter. It could increase the occasion of electrons and holes
recombination. For that reason, a decrease in the photocatalytic activity of photocatalysts was
observed at the higher Pt loading (3 wt. % Pt/TiO
photocurrent generation (Figure 7).
2
). This result was also confirmed by low

20
18
16
14
12
10
8
6
4
2
0
H2/20
CH4
CO
TiO2
0.5 wt.% Pt/TiO2 1 wt.% Pt/TiO2 1.5 wt.% Pt/TiO2 3 wt.% Pt/TiO2
Figure 10. Dependence of the product yields (after 12 h of irradiation) in the presence of
investigated photocatalysts.
4
. Conclusion
Pt/TiO photocatalysts with various Pt loading of 0.5, 1.0, 1.5 and 3.0 wt.% were synthesized by
deposition-precipitation method, deposited on the commercial support (Al foam) and
investigated for photocatalytic reduction of CO . The surface area of TiO decreases by loading of
2
2
O
3
2
2
Pt while its crystallite size increases. The amount of actual Pt which was measured by ICP
measurements is lower than the nominal value that can be due to the preparation and drying
procedure. H
actual loading from 0.26 to 0.46 wt.% (0.5 wt.% Pt/TiO
not caused significantly changes in formed products in comparison with pure TiO
actual loading of 1.40 wt. % (1.5 wt.% Pt/TiO ) effectively enhanced the production rates while
further increase of Pt content decrease the reaction rate. The highest yields of all products of CO
photocatalytic reduction were achieved in the presence of 1.5 wt.% Pt/TiO . Thus, it can be
2
, CH
4
and CO were found to be the main reaction products. The introduction of Pt
and 1.0 wt.% Pt/TiO , respectively) did
. The higher Pt
2
2
2
2
2
2
concluded that, in this study, the optimum actual amount of Pt loading is 1.4 wt.%. The enhanced
photocatalytic activity of photocatalyst can be attributed to the low-recombination rate of
photogenerated electron-hole pairs caused by present of Pt on the titania surface. Significantly
higher (approximately 20 times) yields of CO
2
photocatalytic reduction were achieved in the
presence of photocatalysts deposited on the commercial support compared to the powder

photocatalysts. The findings confirmed the great importance of the supports surface areas on
photocatalyst activity in photocatalytic reaction.
Acknowledgment
This project was funded by the Federal Ministry of Education and Research of Germany under the"
CO
2
Plus funding measure - Use of CO
2
to broaden the raw material basis "under the grant number
0
33RC003 and by the International Postdoc Initiative (IPODI) of the European Union and also
supported of the project LO1208 “TEWEP” from Ministry of Education, Youth and Sports of
Czech Republic. We also thank LANIK company for providing foams and Dip. Ing. Sören Selve,
Mr. Jan Simke and Dip. Ing. Ulrich Gernert for providing TEM and SEM images.
References
[
1] T.J. Blasing, Recent Greenhouse Gas Concentrations., (2009) 1–5.
doi:10.3334/CDIAC/atg.032.
[
2] O. Ola, M.M. Maroto-Valer, Transition metal oxide based TiO
induced CO photoreduction, Appl. Catal. A Gen. 502 (2015) 114–121.
doi:10.1016/j.apcata.2015.06.007.
2
nanoparticles for visible light
2
[
[
[
[
[
[
3] D.Y.C. Leung, G. Caramanna, M.M. Maroto-Valer, An overview of current status of carbon
dioxide capture and storage technologies, Renew. Sustain. Energy Rev. 39 (2014) 426–443.
doi:10.1016/j.rser.2014.07.093.
4] L. Liu, Y. Li, Understanding the reaction mechanism of photocatalytic reduction of CO
2
with
H
2
O on TiO -based photocatalysts: A review, Aerosol Air Qual. Res. 14 (2014) 453–469.
2
doi:10.4209/aaqr.2013.06.0186.
5] J. Low, B. Cheng, J. Yu, Surface modification and enhanced photocatalytic CO reduction
2
performance of TiO
doi:10.1016/j.apsusc.2016.09.093.
6] X. Li, J. Wen, J. Low, Y. Fang, J. Yu, Design and fabrication of semiconductor photocatalyst
2
: a review, Appl. Surf. Sci. 392 (2017) 658–686.
for photocatalytic reduction of CO
doi:10.1007/s40843-014-0003-1.
2
to solar fuel, Sci. China Mater. 57 (2014) 70-100.
7] S.N. Habisreutinger, L. Schmidt-Mende, J.K. Stolarczyk, Photocatalytic reduction of CO
TiO and other semiconductors, Angew. Chemie - Int. Ed. 52 (2013) 7372–7408.
2
on
2
doi:10.1002/anie.201207199.
8] H. Abdullah, M.M.R. Khan, H.R. Ong, Z. Yaakob, Modified TiO
2
photocatalyst for CO
2
photocatalytic reduction: An overview, J. CO
doi:10.1016/j.jcou.2017.08.004.
2
Util. 22 (2017) 15–32.

[
9] Z. Xiong, Z. Lei, X. Chen, B. Gong, Y. Zhao, J. Zhang, C. Zheng, J.C.S. Wu, CO
photocatalytic reduction over Pt deposited TiO nanocrystals with coexposed {101} and
001} facets: Effect of deposition method and Pt precursors, Catal. Commun. 96 (2017) 1–5.
2
2
{
doi:10.1016/j.catcom.2017.03.013.
[
[
[
[
[
10] R. Camarillo, S. Tostón, F. Martínez, C. Jiménez, J. Rincón, Enhancing the photocatalytic
reduction of CO
photocatalysts, J. Supercrit. Fluids. 123 (2017) 18–27. doi:10.1016/j.supflu.2016.12.010.
11] M. Tahir, Synergistic effect in MMT-dispersed Au/TiO monolithic nanocatalyst for
plasmon-absorption and metallic interband transitions dynamic CO photo-reduction to CO,
Appl. Catal. B Environ. 219 (2017) 329–343. doi:10.1016/j.apcatb.2017.07.062.
12] S. Xie, Y. Wang, Q. Zhang, W. Deng, Y. Wang, MgO- and Pt-promoted TiO as an efficient
2
through engineering of catalysts with high pressure technology: Pd/TiO
2
2
2
2
photocatalyst for the preferential reduction of carbon dioxide in the presence of water,
Catalysis. ACS Catal. 4 (2014) 3644–3653. doi:10.1021/cs500648p.
13] O. Ola, M.M. Maroto-Valer, Review of material design and reactor engineering on TiO
2
photocatalysis for CO
2
reduction, J. Photochem. Photobiol. C Photochem. Rev. 24 (2015)
1
6–42. doi:10.1016/j.jphotochemrev.2015.06.001.
14] O. Ishitani, C. Inoue, Y. Suzuki, T. Ibusuki, O. Ishitani|, C. Inoue, Y. Suzuki, T. Ibusuki,
Photocatalytic reduction of carbon dioxide to methane and acetic acid by an aqueous
suspension of metal-deposited TiO
doi:10.1016/1010-6030(93)80023-3.
15] Q.H. Zhang, W.D. Han, Y.J. Hong, J.G. Yu, Photocatalytic reduction of CO
Pt-loaded TiO catalyst, Catal. Today. 148 (2009) 335–340.
doi:10.1016/j.cattod.2009.07.081.
16] M. Anpo, H. Yamashita, Y. Ichihashi, S. Ehara, Photocatalytic reduction of CO
various titanium oxide catalysts, J. Electroanal. Chem. 396 (1995) 21–26. doi:10.1016/0022-
728(95)04141-A.
17] Z. Xiong, H. Wang, N. Xu, H. Li, B. Fang, Y. Zhao, J. Zhang, C. Zheng, Photocatalytic
2
, J. Photochem. Photobiol. A Chem. 72 (1993) 269–271.
[
[
[
2
with H O on
2
2
2
with H 0 on
2
0
2
+
0
reduction of CO
2
on Pt –Pt /TiO
2
nanoparticles under UV/Vis light irradiation: A
combination of Pt doping and Pt nanoparticles deposition, Int. J. Hydrogen Energy. 40
2015) 10049–10062. doi:10.1016/j.ijhydene.2015.06.075.
18] L. Qiu-ye, Z. Lan-lan, L. Chen, C. Yu-hui, W. Xiao-dong, Y. Jian-jun, Photocatalytic
Reduction of CO to Methane on Pt/TiO Nanosheet Porous Film, Adv. Condens. Matter
2
+
(
[
[
2
2
Phys. 2014 (2014) 1–6. doi:10.1155/2014/316589.
19] S. Xie, Q. Zhang, G. Liu, Y. Wang, Photocatalytic and photoelectrocatalytic reduction of
CO using heterogeneous catalysts with controlled nanostructures, Chem. Commun. 52
2

(2016) 35–59. doi:10.1039/C5CC07613G.
[
[
20] S. Bai, J. Jiang, Q. Zhang, Y. Xiong, Steering charge kinetics in photocatalysis: intersection
of materials syntheses, characterization techniques and theoretical simulations, Chem. Soc.
Rev. 44 (2015) 2893–2939. doi:10.1039/C5CS00064E.
21] M. Haruta, T. Kobayashi, H. Sano, N. Yamada, Novel gold catalysts for the oxidation of
carbon monoxide at a temperature far below 0 °C, Chem. Lett. 16 (1987) 405–408.
doi:10.1246/cl.1987.405.
[
[
22] K.S.W.S. S.J. Gregg, Adsorption, Surface Area and Porosity, United States, New York,
1
982..pdf, Academic Press INC. (LONDON) LTD, New York, 1982.
23] P.P.H. E. P. Barrett, L. G. Joyner, The determination of pore volume and area distributions in
porous substances. I. computations from nitrogen isotherms, J. Am. Chem. Soc. 73 (1951)
3
73–380. doi:10.1021/ja01145a126.
[
[
24] L. Yang, B. Kruse, Revised Kubelka–Munk theory I. theory and application, J. Opt. Soc.
Am. 21 (2004) 1933. doi:10.1364/JOSAA.21.001933.
25] J. Mao, L. Ye, K. Li, X. Zhang, J. Liu, T. Peng, L. Zan, Pt-loading reverses the
photocatalytic activity order of anatase TiO
2
{001} and {010} facets for photoreduction of
CO to CH , Appl. Catal. B Environ. 144 (2014) 855–862. doi:10.1016/j.apcatb.2013.08.027.
2
4
[
[
26] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomie distances in
halides and chaleogenides, Acta Crystallogr. Sect. A. 32 (1976) 751–767.
27] R.L. González, A.M. Torres-Huerta, M.A. Domínguez-Crespo, A. García-Murillo, R. Silva-
Rodrigo, R.L. González, Influence of phases content on Pt/TiO
2
, Pd/TiO catalysts for
2
degradation of 4-chlorophenol at room temperature, J. Nanomater. 2016 (n.d.).
doi:http://dx.doi.org/10.1155/2016/1805169.
[
28] O. Fontelles-Carceller, M.J. Muñoz-Batista, E. Rodríguez-Castellón, J.C. Conesa, M.
Fernández-García, A. Kubacka, Measuring and interpreting quantum efficiency for hydrogen
photo-production using Pt-titania catalysts, J. Catal. 347 (2017) 157–169.
doi:10.1016/j.jcat.2017.01.012.
[
[
[
29] M. Tasbihi, U. Lavrencic Stangar, U. Černigoj, J. Jirkovsky, S. Bakardjieva, N. Novak Tušar,
Photocatalytic oxidation of gaseous toluene on titania/mesoporous silica powders in a
fluidized-bed reactor, Catal. Today. 161 (2011) 181–188. doi:10.1016/j.cattod.2010.08.015.
30] J. Xing, Y.H. Li, H.B. Jiang, Y. Wang, H.G. Yang, The size and valence state effect of Pt on
photocatalytic H
2
evolution over platinized TiO photocatalyst, Int. J. Hydrogen Energy. 39
2
(2014) 1237–1242. doi:10.1016/j.ijhydene.2013.11.041.
31] J.C. Moore, C. V Thompson, A phenomenological model for the photocurrent transient
relaxation observed in ZnO-based photodetector devices, Sensors. 13 (2013) 9921–9940.

doi:10.3390/s130809921.
[
32] S. Xie, Y. Wang, Q. Zhang, W. Deng, Y. Wang, MgO- and Pt-promoted TiO as an efficient
2
photocatalyst for the preferential reduction of carbon dioxide in the presence of water, ACS
Catal. 4 (2014) 3644–3653. doi:10.1021/cs500648p.
[
[
33] K. Maeda, K. Domen, Photocatalytic Water Splitting: Recent Progress and Future
Challenges, (2010) 2655–2661. doi:10.1021/jz1007966.
34] N. Ambrožová, M. Reli, M. Šihor, P. Kuśtrowski, J.C.S. Wu, K. Kočí, Copper and platinum
doped titania for photocatalytic reduction of carbon dioxide, Appl. Surf. Sci. 430 (2017) 475-
4
87. doi:10.1016/j.apsusc.2017.06.307.

Tables
Table 1. Crystalline structural properties of investigated photocatalysts.
Crystallite size
Lattice parameters and cell volume
3
Photocatalyst
TiO
(nm )
a=b (nm)
c(nm)
V(nm )
2
8.8
0.37971
0.37862
0.37866
0.37873
0.37905
0.95171
0.95141
0.95100
0.95152
0.95229
13.7214
13.6384
13.6400
13.6485
13.6820
0
1
1
3
.5 wt.% Pt/TiO
.0 wt.% Pt/TiO
.5 wt.% Pt/TiO
.0 wt.% Pt/TiO
2
2
2
2
15.6
16.4
16.7
16.1
Table 2. The characterization properties of investigated photocatalysts.
Pt content
from
ICP-OES
Pore volume
Indirect band-
gap energy
(eV)
Surface area
(
V
net
)
2
-1
Photocatalyst
(S
BET) (m g )
3
-1
(
mm lig g )
(wt.%)
TiO
2
286
98
97
96
102
375
348
354
336
318
-
3.50
3.47
3.47
3.46
3.44
0
1
1
3
.5 wt.% Pt/TiO
.0 wt.% Pt/TiO
.5 wt.% Pt/TiO
.0 wt.% Pt/TiO
2
2
2
2
0.26
0.47
1.40
2.20