In situ photoacoustic FTIR studies on photocatalytic oxidation of 2-propanol over titanium(IV) oxide

Article abstract of DOI:10.1016/j.catcom.2016.04.017

Adsorption and photocatalytic reaction of 2-propanol over titanium(IV) oxide (TiO₂) were studied by Fourier transform infrared spectroscopy (FTIR) using a photoacoustic (PA) technique. At first, the validity of the FTIR system using a home-made PA cell was confirmed by comparison with conventional transmission FTIR. The spectrum of TiO₂ with 2-propanol showed different propoxy peaks from those of gaseous 2-propanol as a result of chemisorption on TiO₂. Finally, in situ FTIR observation of the photocatalytic oxidation of 2-propanol was carried out. Under ultraviolet irradiation, PA spectra changed due to complete oxidation of 2-propanol through acetone as an intermediate.

Full text of DOI:10.1016/j.catcom.2016.04.017

Catalysis Communications 83 (2016) 1–4
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
In situ photoacoustic FTIR studies on photocatalytic oxidation of
2-propanol over titanium(IV) oxide
b
Naoya Murakami a,b,c, , Nobunori Koga
⁎
a
Department of Applied Chemistry, Faculty of Engineering, Kyushu Institute of Technology, 1-1 Sensuicho, Tobata, Kitakyushu 804-8550, Japan
Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu-ku, Kitakyushu 808-0196, Japan
Research Center for Eco-fitting Technology, Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu-ku, Kitakyushu 808-0196, Japan
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Adsorption and photocatalytic reaction of 2-propanol over titanium(IV) oxide (TiO2) were studied by Fourier
transform infrared spectroscopy (FTIR) using a photoacoustic (PA) technique. At first, the validity of the FTIR
system using a home-made PA cell was confirmed by comparison with conventional transmission FTIR. The
spectrum of TiO2 with 2-propanol showed different propoxy peaks from those of gaseous 2-propanol as a result
of chemisorption on TiO2. Finally, in situ FTIR observation of the photocatalytic oxidation of 2-propanol was
carried out. Under ultraviolet irradiation, PA spectra changed due to complete oxidation of 2-propanol through
acetone as an intermediate.
Received 12 February 2016
Received in revised form 16 April 2016
Accepted 24 April 2016
Available online 26 April 2016
Keywords:
Photoacoustic spectroscopy
FTIR
© 2016 Elsevier B.V. All rights reserved.
Photocatalytic
Titanium(IV) oxide
1. Introduction
and in-situ FTIR measurements have been studied [6–9]. For FTIR mea-
surement of semiconductor particles, a special set-up has often been
Environmental remediation using a semiconductor photocatalyst
such as titanium(IV) oxide (TiO2) particles has attracted much attention
because photocatalytic materials enable the removal of hazardous ma-
terials in an environmentally friendly way by utilization of light energy
in a redox reaction [1,2]. Various photocatalytic evaluation methods
for practical applications, e.g. water remediation, air purification and
self-cleaning effect, have been proposed and the evaluation method
for of the photocatalytic performance has been standardized [3]. The re-
moval of volatile organic compounds (VOC) is one of the conventional
evaluation methods. Usually, gaseous products that are generated by
photocatalytic oxidation of VOC are evaluated by analysis of a certain
volume of sampling gas with gas chromatography, and carbon dioxide
(CO2), which is the final product of oxidation of an organic compound,
is conventionally estimated as photocatalytic activity in most cases [3].
However, some intermediate products that have a high boiling point
and strong absorption on the photocatalyst surface are not detected
by this method. In such cases, ex situ qualitative analysis should be
carried out after the reaction [4,5].
used to estimate photoabsorption because strongly scattering materials
cannot be accurately evaluated by conventional transmission. Recently,
the attenuated total reflection (ATR) technique has been widely used
for FTIR measurements to observe a catalytic interface during illumina-
tion [10–13]. Although the ATR technique is suitable for investigating
solid–liquid interfaces of particle, compared to the diffuse reflectance
(DR) technique, little information on the gas phase over a catalyst is
obtained. Moreover, particle size must be below the penetration depth
of light because the sample is photoirradiated from the opposite side
to the solid–liquid interface that is observed by the ATR technique.
Photoacoustic (PA) technique is one of possible methods for in situ
observation of a photocatalytic reaction over semiconductor nanoparti-
cles without any pretreatment in the same way as a normal photocata-
lytic reaction, and PA technique is applicable to even opaque and
strongly scattering solid materials because photoabsorption is detected
by photothermal waves [14,15]. Photoacoustic Fourier transform infra-
red spectroscopy (FTIR-PAS) has been studied for photochemical
reaction [16], solid-gas reaction [17] and comparison with DRS and
ATR [18], but there have been no studies on in situ observation of a
photocatalytic reaction over semiconductor particle. We have used
PAS for in situ observation of a photocatalytic reaction, and trivalent
titanium species [19,20] and heat of an exothermic reaction [21] were
detected with photocatalytic reaction over TiO2 powder. In the present
study, we established an FTIR-PAS system using a home-made PA cell
Fourier transform infrared spectroscopy (FTIR) is a possible method
for observation of compounds adsorbed on the photocatalyst surface,
⁎
Corresponding author at: Department of Applied Chemistry, Faculty of Engineering,
Kyushu Institute of Technology, 1-1 Sensuicho, Tobata, Kitakyushu 804-8550, Japan.
E-mail address: murakami@life.kyutech.ac.jp (N. Murakami).
http://dx.doi.org/10.1016/j.catcom.2016.04.017
1566-7367/© 2016 Elsevier B.V. All rights reserved.

2
N. Murakami, N. Koga / Catalysis Communications 83 (2016) 1–4
for in situ observation, and we compared the results with those obtain-
ed by conventional transmission. We also detected IR adsorption attrib-
utable to 2-propanol over TiO2 powder using the PA technique.
Finally, in situ FTIR observation of photocatalytic 2-propanol oxidation
was studied, which is the first study for the FTIR measurement
conducted in the same way as a normal photocatalytic reaction.
2. Experimental
2.1. FTIR-PAS measurements
For FTIR-PAS measurements, a home-made PA cell composed
of
a duralumin body with an inner volume of ca. 0.2 mL
(8 mmΦ × 4 mmH), a calcium fluoride (CaF2) window and two valves
for gas exchange was used. Digital photograph of a home-made PA
cell is shown in supplementary Fig. S1. An FTIR spectrometer (Nicolet,
iS 10) was used as interference IR sources. The digital PA signal acquired
by a digital MEMS microphone (Invensense, ADMP441) buried in the
cell was recorded by using a PC equipped with digital I/O devices. The
interferrogram acquired by analog conversion of the digital PA signal
was Fourier-transformed with the Happ-Genzel window function, and
the PA spectra were obtained by normalizing with carbon black powder
as a reference to compensate wavenumber-dependent light intensity.
Fig. 1. (a) FTIR spectra of polystyrene standard obtained by transmission and (b) PA
spectrum of polystyrene foam.
3.2. PA spectra of TiO2 powder adsorbed with 2-propanol
Figs. 2 and S3 show PA spectra of TiO2 powder in an oxygen atmo-
sphere, gaseous 2-propanol and TiO2 powder in an oxygen atmosphere
containing 2-propanol. Characteristic peaks at 2981, 1461, 1381, 1251,
1154 and 1072 cm−1 attributed to ν(CH), δas(CH3), δs(CH3), δ(OH),
ν(CO) and ν(CC) of 2-propanol [9,22] were observed in the PA spectrum
of TiO2 in an atmosphere containing 2-propanol, and the spectrum was
slightly different from that of gaseous 2-propanol. Xu et al. observed
peaks at 1362, 1166 and 1124 cm−1 attributed to ν(CO), νas(CH3) and
νs(CH3) of the chemisorbed 2-propoxide species, respectively [7].
Actually, peaks in this region of the difference spectrum (Fig. 2d)
were observed in the present study. A small peak in the range of
3100–3500 cm−1 was also observed, and it is presumably attributed
to ν(O-H) of water formed during the reaction process of 2-propanol
with the OH group on the TiO2 surface [7,9]. These results indicate
that not only gaseous 2-propanol but also adsorbed 2-propanol on
TiO2 was detected by the PA technique.
2.2. Adsorption of 2-propanol on TiO2 powder
Commercially available TiO2 powder supplied by Showa Denko
(Super-Titania F6 A) was used without any pretreatment and placed
in the PA cell. The atmosphere was controlled by a flow of oxygen
with or without 2-propanol vapor, and FTIR measurements were carried
out after shutting off the valves, i.e., in a closed system at room
temperature.
2.3. Photocatalytic oxidation of 2-propanol over a TiO2 film
TiO2 powder (Super-Titania F6 A) was uniformly spread on a glass
plate (6.0 mmΦ × 0.09 mmt) by the squeegee method, and the plate
was placed in the PA cell. Oxygen containing gaseous 2-propanol
(0.66 mmol L−1) was injected into the cell using a gastight syringe,
and measurements were carried out in the closed system at room tem-
perature. Ultraviolet (UV) irradiation was performed through a window
on the top of the cell using a light-emitting diode (Nichia NCSU033B,
emitting around 365 nm, 10.5 mW cm−2) after 2-propanol had reached
an adsorption equilibrium. The concentrations of 2-propanol, acetone
and CO2 were estimated by FTIR measurements using the PA technique.
3. Results and discussion
3.1. Confirmation of the validity of FTIR-PAS measurements
Fig. S2 shows a power spectrum of the light source obtained by
conventional FTIR measurement and a non-normalized PA spectrum
of carbon black powder. It is known that PA intensity is in principle
proportional to the absorption, but PA intensity is saturated in the
range of strong absorption and depends only on intensity of light source,
as seen in PA spectrum of carbon black powder [14]. Therefore, it is rea-
sonable that the spectra were in agreement, though a slight difference
was observed in the lower wavenumber region (b1200 cm−1) due to
transmittance loss of the CaF2 window of the PA cell.
Fig. 1 shows FTIR spectra of polystyrene standard obtained by
transmission and PA spectrum of polystyrene foam. Although the peak
intensity ratios were slightly different, the peak positions were the
same. These results are reasonable since polystyrene foam is a mixture
of polystyrene and gas. Moreover, the results showed that the PA tech-
nique can be used to evaluate an opaque material such as polystyrene
foam without pretreatment.
Fig. 2. PA spectra of (a) TiO2 powder in an oxygen atmosphere, (b) gaseous 2-propanol
and (c) TiO2 powder in an oxygen atmosphere containing 2-propanol, and
(d) difference spectra of (c)-(b).

N. Murakami, N. Koga / Catalysis Communications 83 (2016) 1–4
3
Fig. 3. PA spectra of a TiO2 film in an oxygen atmosphere containing 0.66 mmol L−1 of 2-propanol under UV irradiation.
3.3. Photocatalytic oxidation of 2-propanol over a TiO2 film
in a micro reactor because gas sampling is not required for product
analysis.
In situ observation of photocatalytic oxidation of 2-propanol over
a TiO2 film was carried out using FTIR-PAS. Fig. 3 shows PA spectra
of TiO2 in an oxygen atmosphere containing 0.66 mmol L−1 of 2-
propanol. Characteristic peaks at 2981, 1388, 1250, 1154 and
1070 cm−1 attributed to 2-propanol were observed in the PA spectrum
at 0 min. Under UV irradiation, intensities of these peaks decreased,
while peaks at 1737, 1365 and 1216 cm−1 attributed to ν(CO),
δs(CH3) and ν(CC) of acetone [9] appeared and they disappeared with
longer photoirradiation. It is well known that acetone is generated
as an intermediate product during photocatalytic oxidation of 2-
propanol [23,24]. Therefore, these peaks are attributed to acetone gen-
erated by oxidation of acetone, as a result of 2-propanol oxidation. The
assignment of these peaks is reasonable according to results shown in
Section 3.2 and literature values [7,9]. On the other hand, the peak
in the range of 2300–2380 cm−1, which is attributed to CO2, monoton-
ically increased with increase in irradiation time.
4. Conclusion
We have shown the validity of FTIR-PAS measurement and applied it
to observation of a photocatalytic reaction. FTIR-PAS measurements of
TiO2 powder showed that PA spectra were reflected by not only gaseous
2-propanol but also 2-propanol adsorbed on TiO2. FTIR-PAS measure-
ment of TiO2 under UV irradiation in the presence of 2-propanol is appli-
cable to in situ observation of a photocatalytic reaction, and the present
study is a first step for the development of in situ observation system.
Acknowledgements
This work was supported by Grant-in-Aid of Young Scientists
(B) (24750204) implemented by the Ministry of Education, Culture,
Sports, Science and Technology (MEXT).
Fig. 4 shows the time courses of concentrations of 2-propanol,
acetone and CO2 in the PA cell with photocatalytic reaction over TiO2.
The concentrations of 2-propanol, acetone and CO2 were determined
by the peak intensities at 2981, 1737 and 2359 cm−1, respectively, of
the PA spectra and standard curve for semi-quantitative determination.
The time course showed a representative kinetic curve for the consecu-
tive reaction. With longer photoirradiation, CO2 reached saturation at
2.0 mmol L−1, suggesting total decomposition of 2-propanol to CO2.
These results indicate that normal photocatalytic oxidation of 2-
propanol proceeds in the PA cell as observed in a photocatalytic reactor,
and this technique can therefore be applied to observation of a reaction
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2016.04.017.
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Fig. 4. Time courses of concentrations of 2-propanol, acetone and CO2 in the PA cell under
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