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目

論

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文
C3N4 光触媒によるヨウ化水素水溶液からの水素生
成

水素により制御可能な開閉器の実験的検証

室温近傍での真空蒸留に伴うトリチウム水の濃度変化………………

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Original
H. HAGIWARA, S. IDA, T. ISHIHARA
Hydrogen Production on C3N4 Photocatalyst from Hydrogen Iodide Aqueous Solution ………

1

S. AKAMARU, M. MURAI, M. HARA
Experimental study of a hydrogen-controllable switch of electric circuits ………………………

11

M. HARA, K. KOBAYASHI, S. AKAMARU, M. NAKAYAMA, M. SHOJI,
Y. OSHIMI, O. MACHIDA, T. YASUMATSU
Changes in the concentration of tritiated water under vacuum distillation
at around ambient temperature ………………………………………………………………………

19

Note
A. TAGUCHI, T. NAKAMORI, Y. YONEYAMA
Synthesis and Structural Change of CHA Type Zeolitein the Repeated Seed-Growth Synthesis………………………………………………………………

29

富山大学研究推進機構水素同位体科学研究センター研究報告 37：1－9，2017.

論文C3N4 光触媒によるヨウ化水素水溶液からの水素生成萩原英久 1)、伊田 進太郎 2)、石原

達己 3,4)1)富山大学 研究推進機構 水素同位体科学研究センター〒930-8555 富山市五福 3190
2)熊本大学大学院 先端科学研究部〒860-8555 熊本市中央区黒髪 2-39-1
3)九州大学 カーボンニュートラル・エネルギー国際研究所〒819-0395 福岡市西区元岡 744
4) 九州大学大学院 工学研究院 応用化学部門〒819-0395 福岡市西区元岡 744

Hydrogen Production on a C3N4 Photocatalyst from a Hydrogen Iodide Aqueous
Solution
Hidehisa Hagiwara,1) Shintaro Ida,2) Tatsumi Ishihara3,4)
1) Hydrogen Isotope Research Center, Organization for Promotion of Research,
University of Toyama, Gofuku 3190, Toyama, 930-8555, Japan
2) Faculty of Advanced Science and Technology, Kumamoto University
Kurokami 2-39-1, Chuo-ku Kumamoto, 860-8555, Japan
3) International Institute for Carbon-Neutral Energy Research, Kyushu University
Motooka 744, Nishi-ku, Fukuoka, 819-0395, Japan
4) Department of Applied Chemistry, Faculty of Engineering, Kyushu University
Motooka 744, Nishi-ku, Fukuoka, 819-0395, Japan(Received December 25, 2017; accepted June 22, 2018)
Abstract
Photocatalytic hydrogen production on graphitic carbon nitride (g-C3N4) from a hydrogen iodide (HI)aqueous solution was investigated with respect to light energy conversion. The photoabsorption and surface1

萩原英久・伊田進太郎・石原達己

area of g-C3N4 depended strongly on the preparation temperature. The highest photocatalytic activity for HI
decomposition was obtained with g-C3N4 prepared at 773 K, which had high photoabsorption capacity. This
study revealed that the activity for I2 formation on the g-C3N4 surface could be improved to achieve efficient
HI decomposition with stoichiometric H2 and I2 formation.

1. Introduction
Hydrogen is considered to be a clean next-generation energy carrier, as it can beproduced from various sources and used to generate electrical or thermal energy without CO2
emission. However, most commercial hydrogen gases are produced from fossil fuels by steam
reforming with CO2 emission [1]. Therefore, hydrogen is currently not a clean energy source.
To overcome this situation, various methods of producing hydrogen by water decomposition
using renewable energy sources have been studied [2]. Among these, the solar IS (iodine-sulfur)
process is a relatively new hydrogen production method [3].
The IS process is a thermochemical water splitting cycle, and the solar IS process
employs concentrated solar thermal energy as a heat source [4]. This process comprises threechemical reactions as shown below:

2HI → H2 + I2 (g)(723 K)(1)

H2SO4 (g) → SO2 (g) + H2O + 1/2O2(1123 K)

(2)

SO2 (g) + I2 + 2H2O → 2HI + H2SO4 (aq)(373 K)(3)

The IS process results in complete thermal decomposition of water into H2 and O2 at hightemperature. This process was expected to improve HI decomposition for practical use because
the low HI decomposition ratio (ca. 20% at 673 K) limits its total efficiency.
To overcome this issue, photocatalytic HI decomposition was investigated in this study.
If HI decomposition occurs on photocatalysts under UV or visible light, and sulfuric acid2

C3N4 光触媒によるヨウ化水素水溶液からの水素生成

decomposition is induced by infrared heating in sunlight, the conversion efficiency of the solar
IS process would improve. While several studies have been published on photocatalytic HI
decomposition [5,6], the photocatalytic activity of graphitic carbon nitride (g-C3N4) for HI
decomposition has not yet been investigated. Graphitic carbon nitride is an organic
semiconductor photocatalyst, which was reported in 2009, that shows activity for organic
pollutants and water decomposition [7]. In this study, g-C3N4 was prepared from melamine atdifferent temperatures, and the photocatalytic activity for HI decomposition was evaluated.

2. Experimental
2.1. Photocatalyst Preparation and Characterization
All reagents were used without further purification. The g-C3N4 photocatalysts wereprepared by heating melamine (99.0%, Wako Pure Chemical Industries, Ltd., Japan) in air. The
melamine powder (5 g) was placed in an alumina crucible and calcined at a predetermined
temperature (723–973 K) in a muffle furnace. Platinum nanoparticles were employed as cocatalyst for the hydrogen evolution reaction. Pt was loaded on g-C3N4 using an evaporation todryness method with aqueous tetraammineplatinum(II) nitrate (Pt(NH3)4(NO3)2, 99.995%,
Sigma-Aldrich Co., USA). A mixture of C3N4 powder and Pt precursor was heated at 573 K for
2 h under hydrogen gas flow (50 ml min−1).
X-ray diffraction (XRD) patterns of the samples were obtained with an X-ray
diffractometer (RINT2000, Rigaku Corp., Japan) equipped with Cu K radiation. The
Brunauer-Emmett-Teller (BET) surface area was determined with an adsorption apparatus(BELSORP-mini, MicrotracBEL Corp., Japan). UV–Vis and IR diffuse reflectance spectra
were obtained with UV–Vis (UV-3600, Shimadzu Corp., Japan) and FT-IR (Nicolet 6700,
Thermo Fisher Scientific Inc., USA) spectrophotometers, respectively. X-ray photoelectron
spectroscopy (XPS) was performed with an X-ray photoelectron spectrometer (AXIS-165,3萩原英久・伊田進太郎・石原達己

Shimadzu Corp., Japan).

2.2. Photocatalytic Reaction
Photocatalytic HI decomposition was performed in a quartz reactor containing 10 mM
aqueous HI. The reactor was irradiated with a Xe lamp (2.0 W cm−2) with magnetic stirring.
The evolved gases were detected using a gas chromatograph (GC-8A, Shimadzu Corp., Japan)
with a thermal conductivity detector. After the photocatalytic reaction, the catalyst wascollected by centrifugation and separated from the reaction solution by decantation. The
amounts of triiodide (I3−) and iodide (I−) ions were determined by titration with aqueous
Na2S2O3 and AgNO3, respectively.

3. Results and Discussion
Figure 1 shows XRD patterns of carbon nitrides prepared at different temperatures. Mostof the diffraction peaks were assigned to g-C3N4, as reported previously [8]. While g-C3N4 was
obtained above 773 K, the sample calcined at 973 K was completely decomposed. The

Fig. 1 XRD patterns of synthesized g-C3N4 powders.

4

C3N4 光触媒によるヨウ化水素水溶液からの水素生成

Table 1 Weight change and specific surface area of samples calcined at different temperatures.
Calcination temp.

Weight of melamine

Weight of calcined sample

Specific surface area

/K

/g

/g

/m2 g−1

773

5.02

2.77

4.7

823

5.00

2.61

9.5

873

5.04

2.21

25.7

923

4.99

1.27

39.9

diffraction peaks of g-C3N4 became
sharper as the calcination temperature
increased, indicating that the crystallinityof g-C3N4 increased with the calcination
temperature.
Table 1 shows the weight of thesamples before and after calcination ateach temperature. The weight loss during

Fig. 2 Formation mechanism of g-C3N4 inthermal decomposition of melamine [9].

heat treatment was caused by sublimation of melamine and desorption of ammonia, which are
driven by both deamination and the formation of aromatic units [9], as shown in Fig. 2. These
reactions accelerate as the temperature increases. Therefore, melamine polymerization
increased with the calcination temperature, and the weight of the prepared sample decreased
due to deammoniation. These results were consistent with the XRD results. The specific surface
area of the g-C3N4 samples, obtained by nitrogen gas adsorption, is also summarized in Table
1. The surface area increased with the calcination temperature, and that of a sample calcined at
923 K was about 40 m2 g−1. Furthermore,hysteresis loops were observed in the adsorptiondesorption isotherms of all samples, which were identified as IUPAC type H3 [10]. It was thus
confirmed that the carbon nitrides prepared in this study were agglomerates of plate-likecrystals such as graphite.

5萩原英久・伊田進太郎・石原達己

Figure3shows

UV–Visdiffuse reflectance spectra of the
preparedg-C3N4

samples.

The

absorption edge was 450 nm for the gC3N4 calcined at 773 K, and this shiftedto a shorter wavelength after heating at
a higher temperature. The band gap
energyofthesamplescanbeFig. 3 UV–Vis DR spectra of g-C3N4 samples.

determined from a plot of (F(R)hν)1/2
versus light energy (Fig. 3 inset), where R, h, and ν are the reflectance coefficient, Planck's
constant, and the light frequency. The optical band gaps of g-C3N4 calcined at 773, 823, 873,
and 923 K were estimated as 2.7, 2.75, 2.8, and 3.0 eV, respectively. As the prepared g-C3N4
samples can absorb visible light, they are more favorable for solar energy conversion than TiO2
photocatalysts.
Photocatalytic decomposition of HI on Pt/g-C3N4 was performed under Xe lamp
irradiation. The control experiments
showed that no detectable product wasformed in the absence of either thephotocatalysts or light irradiation. The
main products of the photocatalytic HI
decomposition were H2 and I3−, which
was produced by reaction between I2
and I−. As shown in Fig. 4, thephotocatalytic activity of the g-C3N4
depended

6

on

the

calcination

Fig. 4 Amounts of H2 and I3− formed on Pt/g-C3N4
photocatalysts after HI decomposition for 12 h.

C3N4 光触媒によるヨウ化水素水溶液からの水素生成

temperature.

The

g-C3N4

samplecalcined at 773 K showed the highesthydrogenproduction

duringphotocatalytic HI decomposition. Thiscan be explained by differences in thelight absorbed by the prepared g-C3N4
photocatalysts. However, the amountsofI2

formed

were

below

the

stoichiometric amounts in all cases. To

Fig. 5 XPS spectra of Pt/g-C3N4 prepared at 773 K (a)determine

before and (b) after HI decomposition. (c) NaI referencethereasonfornon-stoichiometric H2 and I2 (I3−) formation,

data.

XPS was performed. Figure 5 shows XPS spectra of the catalyst before and after photocatalytic
HI decomposition with NaI as a reference. Despite washing several times with pure water, I 3d
peaks were observed from the Pt/g-C3N4 catalyst after the reaction. This indicates the presenceof strongly adsorbed iodine on the g-C3N4. Liu et al. reported that products accumulated on ag-C3N4 photocatalyst and inhibited photocatalytic reactions [11]. For HI photodecomposition
on Pt/g-C3N4, no co-catalyst was used for I2 formation, while a Pt co-catalyst was used for H2
formation. Therefore, it appeared that I2 formation on the g-C3N4 catalyst surface proceeded
less easily than H2 formation. Since the surface areas of the g-C3N4 catalysts prepared at a highcalcination temperature tended to be large, it is believed that inhibition of the photocatalytic
reaction also increased, due to iodine accumulation. Consequently, it is considered that g-C3N4
calcined at 773 K with a high photoabsorption capacity and a small surface area showed thehighest photocatalytic activity for HI decomposition.

7

萩原英久・伊田進太郎・石原達己

4. Conclusions
Photocatalytic hydrogen production on Pt/g-C3N4 from aqueous HI was investigated in thisstudy. XRD, UV–Vis absorption spectra, and BET surface area measurements revealed that the
crystallinity, photoabsorption properties, and surface area of the prepared g-C3N4 strongly
depended on the calcination temperature. The highest photocatalytic activity for HI
decomposition was obtained with g-C3N4 prepared at 773 K, due to its high photoabsorption
capacity. Compared to H2 formation, the activity of the Pt/g-C3N4 surface for I2 formation
appeared low; thus, co-catalysts should be employed for I2 formation to improve thephotocatalytic activity of g-C3N4 for HI decomposition.

Acknowledgement
This research was partially supported by JSPS KAKENHI Grant-in-Aid for Specially
Promoted Research (JP16H06293), and Grant-in-Aid for Young Scientists (JP24686107). The
authors would like to thank Enago (www.enago.jp) for the English language review.

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C3N4 光触媒によるヨウ化水素水溶液からの水素生成[11] J. Liu, Y. Liu, N. Liu, Y. Han, X. Zhang, H. Huang, Y. Lifshitz, S.-T. Lee, J. Zhong, Z.
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