Journal of Nuclear and Radiochemical Sciences, Vol. 6, No. 2, pp. 131-134, 2005

ArticlesHydrogen Gas Evolution from Water Included in a Silica Gel Cavity and on Metal Oxides
with γ -Ray Irradiation
Y. Maeda,*, a Y. Kawana,a K. Kawamura,a S. Hayami,a S. Sugihara,a and T. Okaiba

Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
Department of Applied Quantum Physics and Nuclear Engineering, Faculty of Engineering, Kyushu University,
Hakozaki, Higashi-ku, Fukuoka 812-8581, Japanb

Received: April 4, 2005; In Final Form: July 19, 2005
Hydrogen gas evolution from water induced with γ−ray irradiation of silica gels and metal oxides dispersed inwater is studied by examining the hydrogen gas evolved. Hydrogen gas is detected by gas chromatography after60
Co γ−ray irradiation of the sample solution in a closed vial at room temperature. Water adsorbed on the surfaceof metal oxides shows more evolution of hydrogen gas than pure water. Hydrogen gas evolution is maximized in
the silica gels when the size of a silica gel cavity is about 2 nm, being on the order of spur size. The most active
surface toward the decomposition of water upon γ−ray irradiation is that of SiO2 dried at 100 ˚C after sample
preparation.

1. Introduction
The radiolysis of water is one of the most important factors
affecting the chemical condition of the coolant in nuclear reactors, and the G−values of water decomposition products andrate constants of the atomic reactions for these species are
required for a thorough analysis of the process. To date, there
is much data for the G−values and rate constants only atambient temperature. In nuclear reactors, fast neutrons show
smaller net−water decomposition than γ−rays.1 Radiolysis due
to β–decay of tritium–bearing wastes causes evolution of tritiated gas. Such hydrogen evolution is also an important
problem in the packaging of radioactive waste in cement orasphalt. In these cases, the radiolysis of water has been studied
from the standpoint of the depression of γ−ray decompositionof water, and the amount of hydrogen evolution has been generally evaluated with respect to the G(H2)−value for pure water.
Furthermore, hydrogen (H 2) is expected to become a majorenergy source in the future. Current steam reforming of hydrocarbons produces large amounts of carbon dioxide that is a
cause of global warming, and the substrate supplies are finite.
Safe and economically competitive H2 technologies should be
developed to meet future energy needs. Since the study of theso–called Honda and Fujishima Effect was published,2 manystudies on hydrogen evolution using light have been carriedout.3 This reaction mechanism consists of a photocatalytic
reaction which is caused by photosensitized oxidation andreduction due to electrons and hole pairs generated in an illuminated n–type semiconductor. Kato et al. report effective andcontinuous evolution of hydrogen gas from water with lightirradiation using nano particles of NaTaO3.4 They succeeded in
producing hydrogen gas at a rate of 19.8 mmol h−1 for longer
than 400 h. Hydrogen can be produced from γ−ray radiolysis
of water, similar to photocatalysis with ultraviolet or visible
light. Kohn suggested that enhanced decomposition wasobserved in the radiolysis of molecular substances in theadsorbed state.5 Therefore, an understanding of the radiolysisof adsorbed water becomes important for evaluating hydrogengas evolution due to γ−ray radiolysis of water. Of note is thepossibility of using radiation emitted by the unstable fission*Corresponding author. E-mail: y.maescc@mbox.nc.kyushu-u.ac.jp.
FAX: +81-92-642-2588.

products contained in spent nuclear fuel elements.
Nakashima and Tachikawa irradiated three types of silica gelin tritiated water vapor and reported that energy transfer fromthe silica gel to the water plays an important role in the decomposition of water.6 They reported about 40 times larger G(H2)at p = 0.05 for tritiated water than for pure water. Here, p is
W H 2 O /W S G O H , where W H 2 O and W S G O H are the weight ofphysisorbed HTO(gas) and the weight of silica gel with surfacehydroxyl(gas), respectively. Sunaryo et al. studied the γ−radiolysis of water with 60Co γ−rays.1 The method is based on the
combination of three kinds of aqueous solution containing: (a)2 × 10−3 mol kg−1 NaNO2; (b) 10−3 mol kg−1 acetone + 10−2 mol
kg −1 methanol; (c) 10 −3 mol kg −1 HClO 4 + 10 −2 mol kg −1
methanol. The values of GH2, GH, Ge−, GOH, and GH2O2 , for the
γ−radiolysis of water were determined to be 0.45, 0.63, 2.75,
3.12, and 0.58, respectively. In addition, the values for fastneutron radiolysis of water were determined at room temperature. The rate constants for the reactions 2(· CH 2 OH) →(CH2OH)2, 2(·CH2OH) → CH2O + CH 3OH, and ·CH2OH +H2O2 → CH2O + H2O + ·OH were determined to be 2.25 × 109,
1.5 × 108, and 5 × 104 M−1s−1, respectively. This reveals thatthe fast neutron radiolysis brought smaller radical yields, larger
molecular yields and smaller net water decomposition due tothe high LET of the fast neutrons. Wada et al. began a photoelectrochemical study which used γ−radiation instead of visible
light to generate electrons and hole pairs, and reported thathydrogen gas evolution was about 60 times that of the blankwhen TiO2 Ru (deposited on the surface) was used as a catalyst.7 However, G(H2) for the blank is reported to be 0.012,
which is not in accord with the accepted value, 0.45.
Yamamoto et al. used nanoparticles of TiO2 and α– and γ−
Al2O3, and their surface–treated nanoparticles, and reported
that hydrogen gas evolution induced by γ−ray irradiation of α–and γ–Al2O3 in aqueous solution was much higher, 7 − 8 times
that of the background.8 Cecal et al. reported the enhancementof hydrogen gas evolution in γ–ray irradiated water containing
ZrO2, TiO2, BeO, and SiO2.9 Jung et al. studied the effect of
the addition of methanol or EDTA on hydrogen gas evolution
in the γ−ray irradiation of water containing TiO2 and reported
that the addition of EDTA increased hydrogen gas evolution.10
Comparison of the activities of the catalysts described above
for hydrogen gas evolution is difficult because the data are
obtained with different ratios of catalyst weight/water volume© 2005 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 11/07/2005

Maeda132 J. Nucl. Radiochem. Sci., Vol. 6, No. 2, 2005
and different irradiation conditions. In this paper the activities
of nanoparticles of SiO2 and TiO2 for hydrogen gas evolution
are examined to develop the high yields of hydrogen via γ−rayirradiation of water. Here, G−values are defined as the numberof species formed or destroyed per 100 eV of energy absorbed.
The symbol G with a subscript (such as GH2 ) shows the primaryyield, while a G with parentheses (such as G(H2)) indicates theexperimentally evaluated yields of the product.

in the free space in the vial was also analyzed by gas chromatography (Shimadzu; GC−8A, TCD). Molecular sieve(13X, 60/80) was packed in a column (diameter 3 mm × length
2.5 m) and Ar gas was used as the carrier. Injection temperature was 90 ˚C and column temperature, 120 ˚C. The amountof hydrogen gas dissolved in water was negligible in comparison with the amount accumulated. The G(H 2 )−values ofhydrogen gas production from water were obtained as theaverage value of three measurements.

2. Experimental
Three types of silica gels (Fuji Davison) varying in diameterof cavity, e.g.– Silica gel A (diameter of cavity: 20 Å), B(40Å), and ID(150 Å), were used. Three types of nano particles of
SiO2 were also prepared, using different drying temperatures.
To a mixed solution of 14 mL of 25% ammonia solution and
20 mL of ethanol, 10 mL of TEOS (tetraethylorthosilicate) was
slowly poured. The precipitates were washed with ethanol
several times and a portion was dried at 100 ˚C for 2 h (thissample is abbreviated as SiO2 100 ˚C). Second portion wasdried at 150 ˚C for 2 h (SiO2 150 ˚C) and another at 230 ˚C for
2 h (SiO2 230 ˚C).
Two types of nano particles of TiO2 were prepared, alsousing different drying temperatures. Titanium tetraisoperoxide(TTIP) was used as a starting material for the TiO2 nano particles and the coupling agent (isopropyltris(dioctyl pyrophosphate)titanate) was supplied from Ajinomoto Fine Techno. Co.
To a mixed solution of 10 mL of TTIP and 50 mL of ethanol
was slowly poured a mixed solution of 1.1 mL of conc. HCl
and 30 mL of ethanol. The mixture was allowed to standovernight, and a pale yellow liquid was obtained after evaporating with the coupling agent at 80 ˚C. After one day, theliquid was washed with methanol and hexane, centrifuged,
washed with hexane, and dried to obtain a powder of TiO2. A
part of the powder was dried at 500 ˚C for 2 h (TiO2 500 ˚C).
The surface area of the particles was measured via the BET
method. The particle sizes were estimated to be 400 − 500 nm
for SiO2 and 40 − 50 nm for TiO2 based on the surface area andthe assumption that the particles are in cubic form.
About 200 mg of the particles were dispersed in degassed
water(about 300 mg) in a 3 mL vial under nitrogen atmosphere.
γ−Ray irradiation was performed at room temperature with a60
Co γ−ray source at Kyushu University. The dose rate of theγ−ray was set to 374 Gy h−1 and irradiation time was about 30
h. No detectable leakage of hydrogen gas out of the vials was
confirmed by an independent method: A defined volume of
mixed gas (hydrogen/air) was sealed into an unirradiated vial
for a week, and then the contents were evaluated using gas
chromatography. After irradiation, gas which had accumulated

3. Results and Discussion
Absorption dose D is defined as
D = 0.877 × Dr × (µ / µair) × 100
where Dr is irradiation dose, µ mass absorption coefficient of aparticular material, and µair mass absorption coefficient of air.
The values of mass absorption coefficient µ are dependent on
the number of electrons constituting the material. The absorption of energy for a heterogeneous system is calculated fromthe weight average presented by the weight ratio of mass absorption coefficients for each element constituting the material.
Furthermore, the fact that the energy absorbed by the catalysts
may contribute to the decomposition of physisorbed/chemisorbed
water must be considered. That is, the energy absorbed by catalysts may be consumed more effectively in the formation ofhydrogen gas than the energy absorbed by bulk water. Energy
transfer phenomena have been observed in radiolysis of molecular silica gel11 and porous glass.12 It has been reported that
this process involves transfer of energy, positive charge, andelectrons, and that an electron or exciton is capable of long−range migration. For these reasons, it is difficult to evaluate
exact G(H2)−values in this experiment. Bond−dissociationenergies of molecular hydrogen and oxygen are 432 and 493.6
kJ mol−1, respectively, and that of H2O averages 458.9 kJ mol−1.
The G(H2)−values of hydrogen gas production from water(y)measured for the silica gels, SiO 2 , TiO 2 , and TiO 2 (500 ˚C)dispersed in water, are listed in Table 1. Figure 1 shows thesurface area dependence of the G(H2)−value. The experimental
G(H2)−value for pure water is 0.43, close to the reported value
0.45.1 The G(H2)−values listed in Table 1 should be considered raw data because the values are not corrected for thedifference in the γ−ray absorption coefficients of the metaloxides and for that in the γ−ray absorption coefficients between
the metal oxides and water. As discussed above, water
adsorbed on the catalysts may accept energy from the catalysts.
It is clear from Table 1 that the SiO2 and TiO2 have certain
effects on the enhancement of hydrogen gas evolution. The
plots of Figure 1 can be fit to y = 0.0012x + 0.424, where y is

TABLE 1: G–Values of Hydrogen Gas Production from Water for Metal Oxides Dispersed in Water
Catalyst
Silica gel A (20 Å)
Silica gel B (70 Å)
Silica gel ID (150 Å)
SiO2 (100 ˚C)
SiO2 (150 ˚C)
SiO2 (230 ˚C)
TiO2
TiO2
TiO2
TiO2 (500 ˚C)
TiO2 (500 ˚C)
TiO2 (500 ˚C)Surface Area/m2g−1

Dose/kGy

G(H2)−value

All H2/µL*

4.7 × 102
3.9 × 102
2.2 × 102874
6.6 × 10
6.6 × 10
6.6 × 10
1.2 × 10
1.2 × 10
1.2 × 10

34.6
34.6
34.6
18.2
17.9
24.3
8.99
8.99
8.99
8.99
8.99
8.99

1.0
0.8
0.6
2.9
2.8
1.5
0.88
0.86
1.05
0.58
0.52
0.52

24.9
21.2
16.4
37.1
35.7
26.5
9.82
10.93
14.03
6.42
6.54
6.99

* Experimental error is about 15%.

Remarks

pH 5 with HCl
pH11 with KOH
pH 5 with HCl
pH11 with KOH

J. Nucl. Radiochem. Sci., Vol. 6, No. 2, 2005 133

Hydrogen Gas Evolution from Water Included
12.2

1
0.9

y = 0.0012x + 0.424

1

y = 0.0064x + 0.463

0.8
0.7

0.6

G-values

G-values

0.8

0.4
0.2
0

0.6
0.5
0.4
0.3
0.2

0

50

100

150

200

250

300

350

400

450

500

Surface Area/m2 g -1

0.1
0Figure 1. Plots of hydrogen gas evolution vs. the surface areas of thesilica gels.

0

20

40

60

80

Surface Area/m2 g -1

Figure 3. Plots of hydrogen gas evolution vs. the surface areas of the
TiO2.
4
3.5

y = 0.302x + 0.372

G-values

3
2.5
2
1.5
1
0.5
0

0

1

2

3

4

5

6

7

8

9

Surface Area/m2 g -1

Figure 2. Plots of hydrogen gas evolution vs. the surface areas of the
SiO2.

the G(H2)−value of hydrogen gas production of water and x thesurface area of the metal oxides. The plots show that hydrogengas evolves mainly on the surface of the silica gels and thatwater adsorbed on the surface of the metal oxides is more easily
decomposed by γ−ray irradiation than pure water. This conclusion is reliable even though the G(H2)−values are calculated on
the assumption that the γ−ray absorption coefficients of the
supported materials are equal to that of water. Nakashima andTachikawa studied hydrogen gas evolution for a system ofwater vapor on silica gel and reported that the volume ofhydrogen gas evolution can be expressed as a function of thesurface coverage and the concentration of surface hydroxyls.6
The liquid phase results here show that the volume of hydrogengas evolution can be expressed as a function of the surface
area, in accordance with the gas phase results. The size of thecavity of Silica gel A is 2 nm and that of the spur produced by γ−
ray irradiation is reported to be 2 nm.13 Therefore, the size ofthe cavity is similar to that of the reaction region for each interaction of a γ−ray with the materials.
Hydrogen gas evolution was measured for SiO2 dispersed inwater, and the G(H2)−values for hydrogen gas production ofwater (y) are plotted versus the surface area of the SiO2 (x) in
Figure 2. The plots can be approximated by the relation y =0.302x + 0.372. The coefficient for the SiO2, 0.302 is larger
than that for the silica gels. This fact supports the conclusionthat the surface of SiO2 is more active to the decomposition ofwater upon γ−ray irradiation than that of the silica gels. The
difference in the activity between the silica gels and SiO2 results
in a difference in surface hydroxyl concentration, a differencein surface structure, or in a difference in the physical properties
responsible for the surface structure. Which difference, however,is not clear from this experiment. The SiO2 (100 ˚C) has larger
surface area than the SiO2 (230 ˚C). Therefore the SiO2 (100˚C) is the most active among the varieties of SiO2. The reac-tivity of the SiO2 dispersed in water toward γ−ray irradiation
may be not explained by the same mechanism as that for thedecomposition of water by light because SiO2 is not reported tobe active in the decomposition of water by light. Under the
influence of γ−rays, the catalysts experience an energetically
activated state [H2O X]*, which produces ·H and ·OH radicals.
Finally, hydrogen is produced. This radical process is one ofthe probable reactions for hydrogen production. Here, theratios of hydrogen/oxygen were not evaluated because a trace
amount of oxygen remained in the vials.
The results for the TiO 2 dispersed in water are shown in
Figure 3 and the G(H2)−values can be approximated by y =0.0064x + 0.463. This result is similar to that for the silica
gels. Cecal et al.9 measured the hydrogen production yieldwith a total irradiation dose of 100 to1700 kGy: The rate ofhydrogen gas production in the case of TiO2 increases with thetotal irradiation doses. The activity for water decomposition of
TiO2 is less than that of SiO2 in the range less than 100 kGy,
and the difference obtained here between TiO2 and SiO2 for theactivities of water decomposition are in accordance with theirs.
The existence of Ti3+ produced by irradiation may be importantto the water decomposition although we do not have any
evidence to that effect.
To study the effect of the pH of the solution on hydrogen
evolution, TiO2 in water was irradiated in solutions of different
pH. For TiO2, slightly larger G(H2)−values were observed foralkaline solution (Table 1). Dainton and Watt have reported
that the G(H2)−values in γ−radiolysis of water (0.04 M aqueous
acrylamide solution) slightly increase with a decrease in the pHof solution.14 Therefore, the difference in the experimental data
here is within experimental errors if the effect of Cl− on γ−ray−decomposition is considered, although the interference of Cl−
in hydrogen gas evolution was not observed in a separate
experiment.
It is considered that in order to produce hydrogen gas withhigh yield, electron−hole pairs produced by γ–ray irradiation
should be separated rapidly, and the hydrogen gas produced
must rapidly leave the reaction site before the revers−reactions
with oxygen occurs.
4. Conclusion
SiO 2 catalysts enhanced the radiolytic decomposition ofwater and subsequent production of H2 by γ−ray irradiation.
Hydrogen gas evolution was maximized when the size of thecavity of the silica gels was about 2 nm, on the order of spur
size. The most active surface toward the radiolytic decomposition of water was that of SiO2 dried at 100 ˚C after preparation.

134 J. Nucl. Radiochem. Sci., Vol. 6, No. 2, 2005
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