Photo-reduction of CO2 by vis light on polythiophene-ZSM-5 zeolite hybrid photo-catalyst

Article abstract of DOI:10.3390/molecules24050992

A new hybrid photo-catalyst based on ZSM-5 zeolite suitable for reduction of carbon dioxide was synthesized. The photo-catalyst was prepared by oxidative polymerization of thiophene with FeCl₃ in the presence of ZSM-5 with participation of ultrasound. The synthesized photo-catalyst strongly absorbs light radiation up to approx. 650 nm, with the absorption edge in the NIR region. Reactive radicals were generated by VIS light irradiation in an aqueous suspension consisting of the photo-catalyst with CO₂. Formic acid and acetic acid were generated as the main products of the CO₂ reduction. EPR spin trapping technique was applied to identify the reactive radical intermediates. In this work, the mechanism of product formation is also discussed.

Full text of DOI:10.3390/molecules24050992

molecules
Article
Photo-Reduction of CO2 by VIS Light on
Polythiophene-ZSM-5 Zeolite Hybrid Photo-Catalyst
1
Jana Kianicˇka , Gabriel Cík 1,*, František Šeršenˇ 2,*, Ivan Špánik 3, Robert Sokolík 2
ˇ
and Juraj Filo 2,
*
1
Department of Environmental Engineering, Faculty of Chemical and Food Technology, Slovak University of
Technology in Bratislava, Radlinského 9, SK-812 37 Bratislava, Slovakia; jana@kianicka.name
Institute of Chemistry, Faculty of Natural Science, Comenius University in Bratislava, Ilkovicˇova 6,
SK-842 15 Bratislava, Slovakia; robert.sokolik@uniba.sk
2
3
Department of Analytical Chemistry, Faculty of Chemical and Food Technology, Slovak University of
Technology in Bratislava, Radlinského 9, SK-812 37 Bratislava, Slovakia; ivan.spanik@stuba.sk
ˇ
*
Correspondence: gabriel.cik@stuba.sk (G.C.); f.sersen@gmail.com (F.Š.); juraj.filo@uniba.sk (J.F.)
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Received: 25 January 2019; Accepted: 6 March 2019; Published: 12 March 2019
Abstract: A new hybrid photo-catalyst based on ZSM-5 zeolite suitable for reduction of carbon
dioxide was synthesized. The photo-catalyst was prepared by oxidative polymerization of thiophene
with FeCl3 in the presence of ZSM-5 with participation of ultrasound. The synthesized photo-catalyst
strongly absorbs light radiation up to approx. 650 nm, with the absorption edge in the NIR region.
Reactive radicals were generated by VIS light irradiation in an aqueous suspension consisting of the
photo-catalyst with CO2. Formic acid and acetic acid were generated as the main products of the CO2
reduction. EPR spin trapping technique was applied to identify the reactive radical intermediates. In
this work, the mechanism of product formation is also discussed.
Keywords: carbon dioxide; VIS light photo-catalysis; polythiophene; ZSM-5; spin-trapping
1. Introduction
The concentration of one of the most important greenhouse gases, carbon dioxide, is steadily
increasing. According to the NOAA (National Centers for Environmental Information) [
1] the CO2
concentration rises approximately 2 ppm every year. In March 2014, the concentration of carbon
dioxide broke the 400 ppm milestone [2]. This negative situation results in climate changes and global
warming. The average temperature in August 2015 was 0.88 ◦C above the 20th century average of
15.6 ◦C and overcame the 136-year record [1].
Decreasing CO2 emission from the atmosphere through pre- and/or post-combustion and through
capturing and sequestration is energy demanding, thus uneconomical [3]. On the other hand, the
light-driven reduction of carbon dioxide is one of the most promising solutions for the possibility of
storing solar energy, since CO2 is reduced at relatively low temperature and atmospheric pressure to
give added value compounds [3,4]. Looking for an environmental-friendly photo-catalyst for effective
reduction of CO2 with H2O is the most challenging goal of many studies today. There are many
thoroughly researched inorganic semiconducting materials such as TiO2, CdS, ZnO, WO3, GaP and
others [5]. Inoue et al. [6] in 1979 demonstrated for the first time the photo-catalytic reduction of CO2
with water on TiO2 to give formic acid, formaldehyde, methanol, and methane as the main products.
Improvements to these systems were made because the recombination of the photo-induced pair
electron-hole was very fast and the catalyst absorbed only a small fraction of light in the UV region of
the solar spectrum (no absorption was observed in the visible region) [
7]. These disadvantages can be
overcome by doping of semiconductors with different metal, cations and ions (Fe3+, Mo5+, Ru3+, Os3+
,
Molecules 2019, 24, 992; doi:10.3390/molecules24050992
www.mdpi.com/journal/molecules

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Re5+, V4+, Rh3+, anions from C, N, F, P, S); coupling with other semiconducting oxides (Cu2O, PbS,
CdS, CdSe) and sensitization by dye molecules (erythrosin B, eosin). The band gap can be reduced by
suitable modification and the photo-reactivity for both oxidation and reduction can be increased [7].
The photo-catalytic activity of the TiO2 semiconductor can drastically increase as the particle
sizes became smaller than 10 nm due to charge carrier mobility [8]. Crystalline microporous
aluminosilicates—zeolites—with cages and channels of molecular dimensions are very suitable for this
purpose. Cavities create conditions for stabilization of metastable species, providing photochemical
routes that are typically unfavorable in other media. In the building units of TO4 (T = Al, Si)
aluminum causes a charge imbalance which is balanced by the extra framework of the ion-exchangeable
cations. Due to their properties, zeolites are widely used in environmental, consumer, chemical, and
petrochemical industries [9]. In order to further reduce the particle sizes of TiO2 photo-catalysts, highly
dispersed titanium oxide clusters were prepared in microporous zeolite Y and molecular sieves. UV
activation of the TiO2-mesoporous zeolites, Ti-MCM-48 and Ti-MCM-41 leads to the photo-catalytic
reduction of CO2 and H2O and formation of CH3OH and CH4 as main products. Dispersed titanium
oxide clusters confined in zeolite micropores showed significantly higher activity in CO2 reduction
than the powdered TiO2 catalyst [10].
Another possibility for charge transport is use of organic dyes or organic semiconductors, which
often possess better optical, electronic, chemical, and structural properties compared to inorganic
semiconductors. Polythiophene (PT) is a very important part of conjugated polymers, where sp2pz
hybridization causes there to be one unpaired electron in the carbon atom and thereby possesses
properties of semiconductors or metals [11]. PT without side chains is neither soluble nor fusible and
thus PT has been widely used in chemical and optical sensors, light-emitting diodes and displays,
photovoltaic devices, and solar cells since 1980 [12]. In our earlier work, we prepared a heterogeneous
photo-catalytic system (thiophene oligomers—ZSM-5 zeolite) suitable for photolytic degradation
of organic pollutants such as 4-chlorophenol by visible light [13]. Inspired by the polythiophene
properties, here we are interested in the preparation of environmental-friendly photo-catalysts suitable
for photo-reduction of CO2 into value added compounds. The goal of this study was the preparation
of the photo-catalyst Na-ZSM-5-PT by chemical oxidative polymerization of thiophene for CO2
photo-reduction. The synthesized photo-catalyst was characterized by UV-Vis spectroscopy, FT-IR
spectroscopy, SEM (scanning electron microscopy), Mössbauer spectroscopy, and thermal gravimetric
analysis. The photo-catalytic activity of the material was evaluated. Finally, a mechanism for the
photo-catalytical reduction of CO2 was suggested.
2. Results and Discussion
2.1. Characterization of Hybrid Photo-Catalyst Na-ZSM-5-PT
An accompanying feature in the preparation of photo-catalysts, after the addition of thiophene,
was a color change from the light brown (from FeCl3) aqueous suspension and Na-ZSM-5 to brown
until black. Color changes also occurred in the prepared photo-catalysts, from the original white
to brown to black shades in hybrid systems based on the Na-ZSM-5 zeolite (See Figures S1–S3 in
supporting information). In our previous work [14], we found that by irradiation of zeolite ZSM-5,
whose crystals were on the surface covered by a layer of organic semiconductor (polythiophene),
with visible light resulted in the production of reactive forms of oxygen (ROS) which effectively kills
pathogenic microorganisms in an aquatic environment. By using the EPR (electron paramagnetic
resonance) spin trapping technique and applying 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spin
trapping agent we demonstrated that the effective “killing” particle in the microorganisms was a
hydroxyl radical, detected as •DMPO–OH spin-adduct under the given experimental conditions.
Furthermore, polythiophene, generated by the oxidative polymerization of thiophene with FeCl3,
was not imposed evenly on the crystal surface, but produced non-uniform polymer agglomerates,
which greatly distorted the “uniform surface” of the zeolite crystals. To prevent this effect to the greatest

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extent possible, we performed the polymerization of thiophene with participation of ultrasonic stirring.
It has been shown that this technique also leads to effective polymerization, in which case the crystals of
ZSM-5 were uniformly coated by the formed polythiophene across the whole surface. Figure 1a shows
the surface of the hybrid catalyst, prepared by oxidative polymerization, but the reaction mixture
was effective stirred only with a magnetic stirrer. Figure 1b shows a SEM image of the synthesized
hybrid photo-catalyst, obtained by the ultrasound participation (the reaction flask was immersed in
◦
a water bath with the ultrasound; bath temperature was stabilized at 50
±
2 C). The comparison
of the two images clearly shows that the effect of the ultrasonic stirring is reflected in a uniformly
cascade-deposition of thin polymer layers across the whole surface of the zeolite crystal. It can be
assumed that the photo-catalyst prepared by the ultrasound participation ensures a higher specific
surface area and thus a more efficient supply of oxygen and reactive substrate molecules (CO2) to the
crystal surface of the photo-catalyst.
Figure 1. SEM images of the synthesized photo-catalysts Na-ZSM5-PT: (
(b) with ultrasound participation.
a) without the use of ultrasound
Figure 2 shows the TG/DTA curves of pure Na-ZSM-5 and the synthesized hybrid photo-catalyst
Na-ZSM-5-PT. From the TGA curves it can be seen that in both systems, there was only a slight weight
◦
loss up to 100 C, which may be attributed to the evaporation of the bound water or evaporation of the
residual thiophene monomer and its lower oligomers in the hybrid photo-catalyst, respectively. The
◦
weight loss in Na-ZSM-5-PT is at 100 C ca. 3 wt%. According to the DTA curves, oxidative degradation
◦
of the polymer in the oxygen atmosphere is observable at 350 C. Two exothermic maximums at about
475 ◦C and 490 ◦C can be registered on the DTA curve of hybrid Na-ZSM-5-PT. Both of these peaks
are of course subject to the thermal decomposition process of the present organic materials in the
zeolite struct◦ure. While the first peak (475 ◦C) is clearly related to the oxidation of polythiophene, the
second (490 C) is more likely associated with the thermal oxidation reactions of the remaining organic
materials (low concentrations) which are always added to the reaction mixture for the synthesis of
ZSM-5. These materials fulfill the function of structure directing agents—SDAs (“templates”)—in
the formation of a concrete zeolite structure [15]. This idea is also supported by the fact that at an
approximately equal temperature (490 ◦C) the maximum on the DTA curve of non-modified zeolite
also appears.
Based on these results, we can conclude that “regularly deposited” layer of organic semiconductor
(PT) on the surface of the crystal Na-ZSM-5 provides sufficient thermal stability during photocatalyzed
reactions with the catalyst Na-ZSM-5-PT.

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Figure 2. TG/DTA analysis of pure zeolite (Na-ZSM-5) and the hybrid system Na-ZSM-5-PT.
UV–VIS spectrum of the photo-catalyst is shown in Figure 3a. The curve illustrates that the
maximum of the absorption band is at wavelength ca. 480 nm. Pure PT has a maximum at 476 nm [16].
The record also shows that on the base polymer backbone there are present cation radicals (PT•+) and
anion radicals (PT•−) (polarons) [17] shifting the absorption into the infrared region. Figure 3b shows
the NIR spectrum of heterogeneous system. The absorption in the red and near-infrared region is
proof of the formation of dications (bipolarons) during the polymerization [18].
Figure 3. UV–VIS reflectance spectrum (displayed in absorbance) of synthesized photo-catalyst in the
wavelength range 200–1100 nm (a) and a NIR spectrum in the range 900–2000 nm (b).
The photo-catalyst was characterized also by FT-IR spectroscopy. Spectra (See Figures S4 and S5
in supporting information) were recorded before and after photo-reaction to monitor the photo-catalyst
stability. No significant changes were observed.
The potential presence of residual Fe3+ and/or Fe2+ in a prepared catalyst was verified by
X-ray microanalysis and Mössbauer spectroscopy. The Mössbauer spectrum of Fe in the sample of
photo-catalyst obtained after thorough dedoping (insulating) by ammonium hydroxide and washing
with deionised water, is shown in Figure 4a. Due to the wide dispersion of points, the sample of
hybrid photo-catalyst contains only negligible amounts of Fe after dedoping. According to the X-ray
microanalysis the sample contained 0.08 wt% Fe and 3.1 wt% S (not shown). Figure 4b shows the
Mössbauer spectrum of the hybrid catalyst sample obtained after re-doping with FeCl3 enriched with

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radioisotope 57Fe. From the quadruple coupling constant (
∆
= 0.61
±
0.02 mm s−1) and from the
values of isomer shift (
δ
= 00:17
±
0.01 mm s−1) it can be stated that the Fe in both samples is mostly
present in the oxidation state Fe3+. From the fact that the sample after dedoping contains only a
negligible amount of Fe and it was not possible to identify the presence of Fe2+, it can be concluded
that the almost immeasurable amount of Fe cannot be crucially involved in the photoionization of the
photochemical reactions that ultimately lead to a reduction of CO2.
Figure 4. Mössbauer spectra: (
a) dedoped hybrid system Na-ZSM-5-PT and (b) re-doped Na-ZSM-5-PT
with FeCl3 enriched with radioisotope 57Fe.
2.2. Detection of Radical Intermediates and Products of CO2 Photo-Reduction in the Aquatic Environment;
Photo-Reduction Mechanism
In our previous work [19] we studied the formation mechanism of the gaseous products of the
CO2 reduction in the system of ZSM-5, in which channels oligomers of thiophene were incorporated.
The aqueous suspension was saturated with CO2 before irradiation; the produced gas phase was
continuously collected during the radiation. The presence of gaseous compounds was analyzed by gas
chromatography. In the gas mixture only the presence of H2 was confirmed. Any organic substance
(e.g., methane), that were expected, were not present. We also found that the intensity of the EPR
signal (cation radical, polaron) was increased with the irradiation time, which can be attributed to the
fact that the excitation stimulated the release of electrons which subsequently let to growth of the PT
polarons concentration. The released electron then took part in reactions resulting in the formation of
H2. The presence of other gaseous products of the CO2 photo-reduction was not confirmed.
In our experiments, blank reactions were performed to ensure no hydrocarbon production due to
the photo-reduction of CO2 and to eliminate the surrounding interference. The first blank experiment
was illuminated without the catalyst, the second one was in the dark with the catalyst and CO2 under
the same experimental conditions, and the third one was illuminated photo-catalyst in the absence of
CO2. No hydrocarbons were detected in the blank tests.
The GC chromatogram of CO2 reduction products in water obtained after 7.5 h light exposure is
shown in Figure 5. The aqueous suspension of the photo-catalyst in this case was bubbled through
with CO2 during the whole period of irradiation (2 mL min−1). The chromatogram shows the CO2
photo-reduction product identified by this technique as only one main product—acetic acid (retention
time 63.354 min). The presence of this substance was demonstrated using a standard and by measuring
the mass spectrum. Based on the knowledge about CO2 photo-reduction on TiO2 [20], we expected the

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presence of methanol and therefore we tried to identify methanol by the internal standard method.
The methanol gave a retention time of 7.449 min., which was not identified in the sample after 7.5 h
exposure. This observation can be explained by the fact that methane likely immediately enters fast,
subsequent radical reactions.
Figure 5. GC chromatogram of CO2 reduction product.
To identify other possible products, the isotachophoresis method was therefore used. Figure 6
shows the isotachophoresis analysis record of the catalyst Na-ZSM-5-PT aquatic suspension bubbled
with CO2 without and with visible light irradiation (240 min). The analysis of the sample shows,
that in the aqueous layer two substances can be identified. One was identified as formic acid with
RSH = 0.195 (relative step height) and the other as acetic acid with RSH = 0.492. After 240 min of
mol L−1 formic acid (12.6 mol g−1-cat)
exposure, in the reaction mixture there was to be found 0.038 µ µ
and 0.5 µmol L−1 acetic acid (166 µmol g−1-cat).
Figure 6. Analysis of CO2 photo-reduction products in water after 240 min by isotachophoresis:
) without irradiation; (
) 150 W (~60 W m−2) metal halogen lamp irradiation with visible light
(
a
b
irradiation (240 min). (1—formic acid, 2—acetic acid).
EPR spin trapping technique was applied for detection of paramagnetic intermediates. The spin
trapping agent DMPO was chosen to follow the formation of paramagnetic species generated upon

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light exposure. Figure 7 shows the EPR spectra of an aqueous suspension of Na-ZSM-5-PT (bubbled for
30 min with CO2 before irradiation) after 5 and 12 min irradiation by the halogen lamp. In the first phase
of irradiation only one EPR signal, characterized with spin Hamiltonian parameters AN = 1.501 mT
AH = 1.483 mT, g = 2.0058 was registered (Figure 7, Exp5). This signal belongs to •DMPO–OH spin
adduct [21]. During prolongation of time irradiation one can observe production of further radical
adducts (Figure 7, Exp.12). The EPR signal after 12 min irradiation consists of four radicals (R1 to
R4). By simulation of this spectrum using the EPR-WINSIM 2002, we identified the radical R1 with
g = 2.0058 and the hyperfine splitting constants AN = 2.51 mT and AH = 1.18 mT. On the basis of these
values, we cannot exactly determine the nature of this radical. We assume that it could be the radical
,
of •CR type. Another radical adduct •DMPO-R2 was identified with g = 2.0058
, AN = 1.55 mT, and
AH = 2.3 mT. According to the Spin Trap Database [22] the R2 radical could be •DMPO–CH3, whose
hyperfine splitting constants are in the range of AN = 1.53–1.656 mT and AH = 2.2–2.373 mT. The R2
•
radical could also be a DMPO adduct with radical CH2OH, for which the hyperfine spitting constants
are in the range of AN = 1.545 – 1.62 mT and AH = 2.25–2.3 mT
[
22]. Or R2 can be a DMPO adduct
•
with radical CH3 C(H)OH (AN = 1.58–1.588 mT and AH = 2.27–2.28 mT [22
–
25]. From the comparison
•
of the DMPO–R2 hyperfine splitting constants with the •above-mentioned literature data, we can
•
•
assume that this radical is either an adduct DMPO with CH3, CH2OH or CH3 C(H)OH radical.
Consequently, we consider that radical R2 can be a mixture of these three radicals. Radical R3 can be
unequivocally assigned to adduct DMPO with •OH radical. The values of g = 2.0058 and coupling
constants AN = 1.501 mT and AH = 1.483 mT are very close to the values reported in the literature for
such spin adducts [21] with values AN = 1.497 mT and AH = 1.477 mT. Radical R4 with AN = 1.4 mT is
probably a product of DMPO decomposition.
g = 2.0058
Exp5
R3
R3
R3
R2
R1
R3
R2
R4
R2
R1
R2
R4
R2
R1
R1
R2 R1
R1
R4
Exp12
R1
R2
R3
R4
332
334
336
338
340
magnetic induction [mT]
Figure 7. EPR spectra of Na-ZSM5-PT aqueous suspension (bubbled 30 min with CO2 before irradiation)
after 5 min (Exp5) and 12 min (Exp12) irradiation with a halogen lamp and bubbling with CO2 outside
of the resonator cavity together with the fitted spectra of spin adducts DMPO-R1, DMPO-R2, DMPO-R3,
and DMPO-R4.
Based on currently known facts about the mechanism of CO2 photo-reduction on TiO2
photo-catalyst [26–32] and our findings, it is possible, with respect to the registered transitional

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organic radicals and identified end products, to formulate a photo-reduction mechanism for the hybrid
photo-catalysts Na-ZSM-5-PT followed by the partial steps.
Our proposed mechanism (Scheme 1) is supported by several other authors [13,14,19,27,28] who
discovered the formation of radical intermediates in an aqueous medium in the presence of CO2. Based
on newly gained knowledge it can be stated that photochemical reduction of CO2 is initiated by the
release of an electron from the excited state of polythiophene.
Scheme 1. Proposed mechanism of formic acid (blue) and acetic acid formation (red) together with
detected radicals (orange).
•
The result of this step is the production of a hydrogen radical (H ) and an anion-radical of carbon
•−
dioxide (CO2 ). This anion-radical undergoes cascade reactions proposed in Scheme 1 to form the end
•
•
•
•
•
products (formic acid, acetic acid, etc.). Several radicals ( OH, CH3, CH2OH, CH3 C(H)OH, C , and
•
currently unidentified radical R1) occur on the surface of the photo-catalyst in the subsequent thermal
reactions. The mechanism of radical production in relation to the end product production based on
measured EPR spectra can be also explained on the basis of the alternative mechanism discussed
in previous works [27
,28], proposed for the CO2 photo-reduction in an aqueous suspension of TiO2.
According to the conclusions of these works, carbon radicals (C•, measured at low temperature) and

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atomic oxygen [O] are formed on the surface of the photo-catalyst by photo-reduction of CO2, in
accordance with the scheme: CO2 CO + O and CO
C• + O. A methyl radical (•CH3) can be
formed also by reaction of C• with a hydrogen radical (H•).
→
→
The probability, that the chemistry of formation of acetic acid and formic acid takes place according
to the proposed mechanism, is confirmed by the fact that, the temperature during the irradiation
increased from 22 ◦C and stabilized at 34
may be influenced by other factors: the catalyst/water ratio, CO2/H2O ratio, temperature, radiation
intensity, and others. Work in this direction is the subject of further studies.
±
1 ◦C. This indicates that the subsequent thermal reactions
3. Experimental
3.1. Preparation of Photo-Catalyst
Chemicals used for the photo-catalyst preparation, i.e., anhydrous iron (III) chloride (Fluka),
thiophene (Sigma-Aldrich), zeolite Na-ZSM-5 (VÚRUP co., Slovak Republic) and trichloromethane
(Merck) were of reagent grade and used without supplementary purification. CO2 (Messer
Austria GmBH, contains: CO2 99.995%, O2 < 10 vpm, N2 < 25 vpm, water vapor < 5 vpm,
carbohydrates < 1 vpm and CO < 1 vpm) was employed in photo-catalytical reduction tests.
5,5-Dimethyl-1-pyrroline-N-oxide (DMPO, Sigma-Aldrich) was distilled prior to application.
Catalyst Na-ZSM-5-PT was prepared by chemical oxidative polymerization of thiophene with
anhydrous FeCl3 in the presence of Na-ZSM-5. The used zeolite was of the following composition
(molar ratio): Na2O:CaO:Al2O3:SiO2 = 0.61:0.34:1.0:43.6. Na-ZSM-5 was treated in a trichloromethane
solution of FeCl3 (0.25 mol L−1) with ultrasonic stirring. After 1 hour, thiophene was added. After 4 h,
the product was filtered off, de-doped with NH4OH and washed with a sufficient amount of distilled
water, dried in air at 20 ◦C. Prior to removal of reactant residuals, the catalyst was dried in an oven at
105 ◦C for 2 h.
3.2. Characterisation of Photo-Catalyst
X-ray microanalysis and the morphology of photo-catalyst Na-ZSM-5-PT were conducted by
scanning electron microscopy with an JXA 840A instrument (JEOL) with an accelerating voltage of
15 kV.
For the study of the overall thermal behavior of photo-catalyst Na-ZSM-5-PT, a thermogravimetric
differential thermal analysis (TG-DTA) system (Derivatograph Q 1500D) was used. Sample (100 ± 4 mg
)
was heated from 30 ◦C up to 500 ◦C at a rate of 5 ◦C min−1
.
The diffuse reflectance spectra in the UV–VIS region were measured with the fiber optics
spectrophotometer system Ocean Optics consisting of an Hi-Res spectrometer HR 4000CG-UV-NIR,
◦
◦
UV–VIS light source DH-2000-BAL, and standard reflectance accessory with 45 /45 geometry. Spectra
were measured in the region 230–900 nm.
The diffuse reflectance spectra in the range 900–2600 nm were measured with the fiber optics
spectrophotometer system Ocean Optics consisting of spectrometer NIR256-2.5, VIS–NIR light source
◦
◦
HL-2000-FHSA and standard reflectance accessory with 45 /45 geometry. For each measurement, the
detector was calibrated to the blank polymer. In this way, changes in spectral reflectance due to ageing
of the substrate were largely excluded.
FTIR spectra were obtained using a Nicolet 6700 FT-IR (Thermo Scientific, USA) FTIR
spectrophotometer. The spectra were measured in the range of 600–4000 cm−1, sensitivity 4, 40 scans
repeated for each analysis. An ATR adapter with a diamond window was used for spectral
measurements using the principle of total reflectivity
The Mössbauer spectra were obtained with an Elron-Intertechnique SA-41 cryomagnetic
Mössbauer spectrometer. 57Co with an activity of about 20 mCi (1 GBq) and a Cr matrix was used
as a source. The range of the maximum rate was 10 mm s−1. The measured spectra were evaluated
according to the NORMOS program.

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3.3. Photocatalytical Experiment
The photo-catalytical reduction of CO2 was carried out in a glass reactor (150 mL) which contained
150 mg modified zeolite and 100 mL of purified water (pH = 5.6). The reactor was bubbled with CO2
(2 mL min−1) throughout the reaction. The suspension was magnetically stirred during the irradiation
by a 150 W metal halogen lamp (Tesla;
≈
60 W m−2) passed through a 7 cm water filter for 8 h. After the
reaction was finished, the solution was filtered and the filtrate was analyzed by gas chromatograp◦hy
(dynamic headspace GC-TOFMS, sorbent Tenax TA, column DB-FFAP, GC temperature program: 30 C
(20 min)—2 ◦C min−1—130 ◦C—10 ◦C min−1—220 ◦C) equipped with a mass detector. The presence
of acids in the sample was evaluated by isotachophoretic analysis (ITP) on Analyser ZKI–02 (Villa
Labeco Co., Spišská Nová Ves, Slovakia) using a column-coupling technique. The diameter of the
pre-separation column was 0.8 and 0.3 mm for the analytical column. Both columns were equipped
with a conductivity detector. Determination of ions was accomplished by the following electrolyte
system—solvent: leading A1: 10 mM histidine + HISCL, pH = 6.1, leading A2: 5 mM capric acid +
5 mM histidine.
3.4. EPR Spin Trapping Experiment
The EPR spectra were measured by means of EMX Plus X-band EPR spectrometer with a High
Sensitivity Probe-head (Bruker). An aqueous suspension of Na-ZSM-5-PT (1.5 mg mL−1, 400
µL)
mixed with 300
µL of DMPO solution (0.212 M) was saturated by CO2 using a mild gas stream. The
prepared system was photo-excited by the radiation of a 150 W metal halogen lamp through a 7 cm
water filter outside the EPR resonator. After 5 respective 12-minute exposure the suspension was
immediately transferred into the small quartz flat cell (WG 808-Q, Wilmad-LabGlass, optical cell length
0.045 cm) and EPR spectra were recorded.
4. Conclusions
A new hybrid photo-catalyst on the basis of a zeolite Na-ZSM-5 was synthesized. On the surface
of the crystals was an in situ deposited layer of organic polymer semiconductor—polythiophene.
Polymerization of thiophene was carried out by oxidative polymerization with FeCl3. To ensure the
“uniformity” of the crystal surface coverage, polymerization was conducted under ultrasonic stirring
participation. The photo-catalyst was applied to the photo-reduction of CO2 using VIS light. In this
way synthesized hybrid photo-catalyst showed high thermal stability in an oxygen environment. The
photo-catalyst is not soluble in any ordinary organic solvent. The advantage of such a photo-catalyst is
the fact that the polythiophene strongly absorbs light radiation up to ca 650 nm, with the absorption
edge encroaching into NIR, which allows the use of spectral favorable sources for CO2 reduction.
Analysis of the products resulting from the reduction of CO2 showed the presence of two products,
acetic acid and lesser concentrations of formic acid. This fact allows the use of the main product—the
acetic acid—for the preparation of ammonium acetate, which has widespread use in chemical practice,
particularly as a raw material for various syntheses and pH adjustment. In future, it may be possible
to use this method (e.g., by introducing nitrogen atom to the molecule through ammonium acetate)
for the production of specialty value added chemicals. The mechanism of reaction product formation
was suggested.
Supplementary Materials: The following are available online, Figure S1: Left: Pure Na-ZMS-5 before reaction.
Right: Photo-catalyst prepared without participation of ultrasound. Small amount of FeCl3 (~ 0.05 mol L−1
)
was present in reaction mixture, Figure S2: Left: Pure Na-ZMS-5 before reaction. Right: Photo-catalyst prepared
with participation of ultrasound. FeCl3 (0.25 mol L−1) was present in reaction mixture, Figure S3: Left: Pure
Na-ZMS-5 before reaction. Right: Photo-catalyst prepared with participation of ultrasound doped with FeCl3
(
0.55 mol L−1), Figure S4. FT-IR spectrum of photo-catalyst before reaction, Figure S5. FT-IR spectrum of
photo-catalyst begore reaction.
ˇ
ˇ
Author Contributions: J.K., G.C., F.Š., I.Š. J.F. and R.S. performed experiments. G.C., F.Š. and J.F. analyzed results
and wrote the manuscript. All authors contributed to the paper and approved the manuscript.

Molecules 2019, 24, 992
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Funding: This research was funded by scientific grant agency VEGA, grant number 1/0501/15 and by Comenius
University Science Park, ITMS: 26240220086.
Acknowledgments: The authors thank M. B. Miglierini for the Mössbauer spectra measuring.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compound NaZSM-5-PT is available from the author (G.C.).
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2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
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