ᐩᒣ኱ᏛỈ⣲ྠ఩య⛉Ꮫ◊✲ࢭࣥࢱ࣮◊✲ሗ࿌33㸸㸪

5HVHDUFK1RWH
&(&( ゐ፹⏝␯Ỉᛶࢩࣜ࢝ࣅ࣮ࢬࡢ⣽Ꮝᵓ㐀ཬࡧỈ⵨Ẽ྾╔≉ᛶ⏣ཱྀ ᫂ 1䠈ᮡᒣ㈗ᙪ 2䠈᳃⏣ὒᖹ 2䠈⏣୰ᑗ⿱ 3䠈ྂ⸨೺ྖ 4䠈᐀ീ೺୕ 51ᐩᒣ኱ᏛỈ⣲ྠ఩య⛉Ꮫ◊✲䝉䞁䝍䞊㻌 ᐩᒣᕷ஬⚟ 
ྡྂᒇ኱Ꮫ኱Ꮫ㝔ᕤᏛ◊✲⛉ྡྂᒇᕷ༓✀༊୙⪁⏫᰾⼥ྜ⛉Ꮫ◊✲ᡤᒱ㜧┴ᅵᒱᕷୗ▼⏫㸱㸰㸰Ѹ㸴஑ᕞ኱Ꮫ኱Ꮫ㝔ᕤᏛ◊✲㝔 ⚟ᒸᕷす༊ඖᒸ㸵㸲㸲␒ᆅ⛅⏣኱ᏛᕤᏛ㈨※Ꮫ㒊 ⛅⏣ᕷᡭᙧᏛᅬ⏫㸯㸫㸯Porosity and water vapor sorption property of new hydrophobic silica beads
for CECE catalyst supportAkira Taguchi1, Takahiko Sugiyama2, Yohei Morita2, Masahiro Tanaka3, Kenji Kotoh4,
Kenzo Munakata5
1

Hydrogen Isotope Research Center, University of Toyama
Gofuku 3190, Toyama 930-8555

2

Graduate School of Engineering and School of Engineering, Nagoya University
Furo-cho, Chikusa-ku, Nagoya 464-8603
3

National Institute for Fusion Science
Oroshi-Cho 322-6, Toki 509-5292

4

Graduate School of Engineering, Kyushu University
Motooka Nishi-ku 744, Fukuoka 819-0395

⏣ཱྀ࣭᫂ᮡᒣ㈗ᙪ࣭᳃⏣ὒᖹ࣭⏣୰ᑗ⿱࣭ྂ⸨೺ྖ࣭᐀ീ೺୕

5

Department Faculty of Engineering and Resource Science, Akita University
Tegata gakuen-machi 1-1, Akita 010-8502(Received December 17, 2013; May 23, 2014)$EVWUDFWThe porosity and water vapor sorption property of commercially available hydrophobic SiO2 beads were
investigated. The hydrophobic SiO2 beads, the surface of which has been modified by trimethylsilyl
functional groups, with a surface area of 70.7 m2/g, a mesopore diameter of about 36 nm and a mesopore
volume of about 0.91 cm3/g showed lower water vapor sorption property as compared to the unmodified SiO2
beads; the amount of monolayer water adsorbed were estimated to be 2.94×10-3 and 5.12×10-3 g(H2O)/g(adsorbent)
for hydrophobic and unmodified SiO2 beads, respectively. The evaluation of the heat of water vapor sorption
suggests that the suppression of water cluster formation by trimethylsilyl groups is attributed to thehydrophobic property.


5HVHDUFKQRWHThe combined electrolysis catalytic exchange (CECE) method is a practical process for theenrichment of heavy water and the extraction of tritium from light and heavy water mixtures.[13] From the development of the CECE process and afterwards, much effort has been devoted
to improve the catalytic activity, design of the catalyst bed, and operation parameters.[1,4,5]
One of the CECE catalysts commercially available at the present is the styrene-divinylbenzene
copolymer supported Pt catalyst which has been known as the Kogel catalyst.[1,3-5] Although
the superior activity and life time with satisfactory operating results of the Kogel catalyst havebeen well known, one problem is the difficulty in its mass production and hence a high price.
Therefore, the development of a new preparation method with improved activity is still aninteresting task.
From these backgrounds, we started our attempt at preparing a new CECE catalyst fromcommercially available, relatively cheap materials such as SiO2, Al2O3 and activated carbon.
Among the several physical and chemical properties required for a CECE catalyst support, suchas porosity, water resistance, and hardness, one of the most important properties ishydrophobicity.[1,4,5] Although the hydrophobic/hydrophilic property is difficult to define
quantitatively [6], it has been known that hydrophilic CECE catalysts easily cause pore blocking
due to the condensation of water vapor, resulting in the decrease in catalytic activity.[1,4,5] We&(&(ゐ፹⏝␯Ỉᛶࢩࣜ࢝ࣅ࣮ࢬࡢ⣽Ꮝᵓ㐀ཬࡧỈ⵨Ẽ྾╔≉ᛶhave chosen hydrophobic SiO2 beads (Fuji
Silysia Chemical Ltd., Aichi), denoted as
SiO2B, for use as the catalyst support in a new
CECE catalyst system. In this note, we report
some fundamental properties of SiO2B,
especially focusing on water vapor sorption
properties in addition to other pore structuralproperties. As a reference material, SiO2
beads (CARiACT-Q50, Fuji Silysia, denoted )LJ SEM image of SiO2B. Inset: Appearance ofas SiO2BH2O) were used in this study. This SiO2B and SiO2BH2O.
SiO2BH2O was the parent material for SiO2B,
which had trimethylsilyl functional groups
grafted on the surface (grafting density of 1.2– 1.8 groups/nm2 in catalog). It should be
noted

that

SiO2BH2O

also

shows

hydrophobicity, since it is composed of puresilica [6], which excludes Al3+ or related
counter-cations of Na+ or Ca2+ as is the casewith hydrophilic silica gel or zeolite )LJ N2 sorption isotherm of SiO2B and SiO2BH2O.
Inset: Barrett-Joyner-Halenda (BJH) pore size
molecular sieves.

distribution of SiO2B and SiO2BH2O evaluated from

Fig. 1 shows the SEM image of SiO2B the desorption branch. Symbols are circle for SiO2B
and the optical photo of SiO2B and SiO2BH2O.

and triangle for SiO2BH2O, respectively.

The color of these beads was white and the diameter was about 2.4 – 4.1 mm (Fig.1 inset). The
SEM measurements (JSM-6701F, JEOL) revealed that SiO2B possessed textural pores with thediameter of several tens of nanometers. These pores existed as continuous wormhole-likestructures in a SiO2 framework.
Nitrogen sorption isotherms were measured by using Autosorb-1MP (Quantachrome) at 196 ºC. The samples were evacuated previously at 200 ºC for more than 12 h. Brunauer–
Emmett–Teller surface area (S.A.) were found to be 70.7 and 76.2 m2/g for SiO2B and SiO2BH2O,
respectively. Both SiO2B and SiO2BH2O showed a type V isotherm as shown in Fig. 2 [7]; thesorption capacity was small in low and middle P/P0 ranges, and then suddenly increased at
higher P/P0. This suggested that unrestricted monolayer-multilayer adsorption could occur.[7,8]Indeed, the micropore volumes (Vpmicro) below a P/P0 of 0.205, corresponding to the pore⏣ཱྀ࣭᫂ᮡᒣ㈗ᙪ࣭᳃⏣ὒᖹ࣭⏣୰ᑗ⿱࣭ྂ⸨೺ྖ࣭᐀ീ೺୕

Table 1 Summary of sorption study of SiO2B and SiO2BH2O.

Vpmicro Vpmeso[cm3/g] [cm3/g]

Dp
[nm]

Vpmeso Vpmacro[cm3/g] [cm3/g]

H2O vapor sorption
25 ºC (BET constant)wmC[][g(H2O)/g]

0.030
0.034

36
37

0.909
0.934

2.94 u 10-3
5.12 u 10-3

N2 sorption
Dp
S.A.
[m2/g] [nm]SiO2B
SiO2BH2O

70.7
76.2

49.0
63.3

Hg porosimetry

0.176
0.187

0.059
0.050

7.67
5.36

diameter of 2.1 nm, were 0.030 and 0.034
cm3/g for SiO2B and SiO2BH2O, respectively(Table 1). On the other hand, the pore volume
in the region between 0.205 and 0.957,
corresponding to the mesopore diameter of
2.1 to 47 nm (Vpmeso), were found to be 0.176
and 0.187 cm3/g, respectively, and larger than
Vpmicro. The Barrett-Joyner-Halenda (BJH)
pore size distribution curve (desorption)LJ  Pore size distribution of (circle) SiO2B andbranch) also revealed that most of the pores (triangle) SiO2BH2O obtained by mercury porosimetry.
had sizes in the range of 30 – 60 nm (Fig. 2, Inset: cumulative intrusion of Hg.

inset), which was consistent with SEM measurements. The pore diameter (Dp) was found to be
49.0 and 63.3 nm for SiO2B and SiO2BH2O, respectively. However, these high P/P0 region is
excluded from the applicability of the N2 sorption study [8]. Therefore, we investigated the poresize distribution using mercury porosimetry (AutoPore IV 9510, Micromeritics). Fig. 3 shows
the pore size distribution curves of SiO2B and SiO2BH2O and the cumulative intrusion of Hg(inset). It is clearly seen that both SiO2B and SiO2BH2O possess mesopores with the pore
diameter of 36 and 37 nm, respectively, which are smaller than the ones from the BJH pore size
distribution [8]. The pore volumes of the mesopore (Vpmeso, 2.0 – 50 nm) and macropore(Vpmacro, 50  414 nm) regions were listed in Table 1. It was revealed that both SiO2B andSiO2BH2O were mesopore-rich materials. This finding is consistent with the SEM image (Fig.
1). From these data, we have determined the S.A., Dp and Vpmeso of SiO2B were 70.7 m2/g, 36
nm, and 0.909 cm3/g, respectively.
Water vapor sorption isotherms were measured with Hydrosorb1000 (Quantachrome). The
samples were previously heated in vacuum at 200 ºC for more than 12 h. Water vapor sorption
isotherms (25 ºC) are shown in Fig. 4 and corresponding BET plots are shown in Fig. 4 inset.
The adsorption capacity close to saturation was about 0.50 mmol/g for SiO2B, while it was

&(&(ゐ፹⏝␯Ỉᛶࢩࣜ࢝ࣅ࣮ࢬࡢ⣽Ꮝᵓ㐀ཬࡧỈ⵨Ẽ྾╔≉ᛶmore than 1.94 mmol/g for SiO2BH2O. For
SiO2BH2O, the amount of water vapor
increased gradually at P/P0 < 0.7, and theamount adsorbed steeply increased at P/P0
larger than 0.8. Such a steep increase can be
attributed to the formation of water clusters as
will

discuss

below.

Also,

substantial

hysteresis was observed in the desorption
branch of SiO2BH2O. The presence of )LJ  Water vapor sorption isotherm of SiO2B andhysteresis suggests an interaction between the SiO2BH2O. Inset; BET plots by water sorption.

Symbols are circle for SiO2B and triangle foradsorbent (SiO2BH2O) and water molecules. SiO2BH2O, respectively.
The silica surface mainly consists of siloxane
bridges (Si-O-Si) and fewer silanol groups.[6,9] These surface siloxane bridges are hydrolyzed
by adsorbed water molecules to form silanols, which are considered to cause the decrease inwater molecules desorbed.
On the other hand, for SiO2B, the amount of water vapor adsorption is limited; the water
adsorption ability was lower than that of SiO2BH2O over the whole range of P/P0, and the steep
increase at high P/P0 was absent. The amount of water adsorbed at P/P0 =0.99 was less than
one-fourth of that for SiO2BH2O. These findings demonstrated the hydrophobicity of SiO2B as
expected. The decrease in water adsorption property was also confirmed by the significant
reduction of the hysteresis loop in the desorption branch. These data clearly shows thehydrophobic property of SiO2B.
The surface property on water vapor sorption was evaluated using the BET plots (Fig. 4
inset). The amount of monolayer water adsorbed (wm) can be estimated to be 2.94×10-3 and5.12×10-3 (g(H2O)/g(adsorbent)) for SiO2B and SiO2BH2O, corresponding to 0.163 and 0.284 mmol/g,
respectively (Table 1). The BET constant (C) of SiO2B was slightly larger than that of SiO2BH2O,
suggesting an interaction in this early stage. By using the surface area (S.A.) evaluated from N2
sorption study (Table 1) and Avogadro’s number (NA), the number of water molecule per unit
surface area (Nwater) can be calculated from the following equation:
 ൈ



 ൌ ͳͺǤͲͳͷൈǤǤ

The Nwater for SiO2B and SiO2BH2O was 1.39 and 2.24 (molecules/nm2), respectively. Assuming
the geometrical closest packing of H2O molecules (0.125 nm2/molecule), corresponding to 8
molecules/nm2, it seems that the surface coverage by water molecules was low.

⏣ཱྀ࣭᫂ᮡᒣ㈗ᙪ࣭᳃⏣ὒᖹ࣭⏣୰ᑗ⿱࣭ྂ⸨೺ྖ࣭᐀ീ೺୕

The differential heat of water vapor
adsorption was shown in Fig. 5. For SiO2B, ahigh heat evolution (about 60 kJ/mol) was
seen in the initial stage of water adsorption,and then it decreased to less than 44 kJ/mol,which is the heat of liquefaction of water [10],at the adsorption amount of 0.05 mmol/g.
This large exothermic effect is probably due
to the strong interaction between water and )LJ Differential heat of adsorption of water vapor
unreacted silanol groups (Si-OH), which were on (circle) SiO2B and (triangle) SiO2BH2O.
left during the grafting of bulky trimethylsilane.[9,11,12] The heat of adsorption reached aminimum at around 0.2 - 0.3 mmol/g, which is close to the value of wm (0.163). Then, the heatof adsorption slightly increased with an increase in the adsorption amount. However, theformation of water clusters was not detectable (below 44 kJ/mol), obviously preventing theadditional adsorption of water molecules to grow the water clusters. On the other hand, for
SiO2BH2O, the heat of adsorption was low at the initial stage, indicating less interaction between
siloxane and water molecules and hence suggesting the hydrophobic character of thesurface.[6,9] The minimum value of the heat of adsorption was observed at the adsorption
amount of around 0.20  0.30 mmol/g, which is close to the wm (0.284 mmol/g). Then, thereleased heat increased gradually as the adsorption proceeded, attaining about 44 kJ/mol at theadsorption amount of about 1.65 mmol/g. These findings support the idea that after formationof monolayers, additional water molecules coordinate to the water molecules to form water
clusters in the pores. The hydrolysis of siloxane bonds to generate silanol groups probably takes
place during water uptake, which may explain the large hysteresis in the desorption isotherm.
An FT-IR study may help us obtain more detailed understanding of the hydrolysis process, but
it would be out of the scope of this paper.
In conclusion, we have investigated the porosity and water vapor sorption properties oftrimethylsilane grafted hydrophobic silica beads (SiO2B), which we have chosen as a catalyst
support for CECE reaction. The low water vapor sorption capacity and restricted interaction forwater cluster formation of this SiO2B demonstrated a desired feature for a catalyst support:
prevention of pore blocking due to water condensation. Preparation and characterization of Ptloaded catalysts and the CECE reaction activity of resultant catalysts will be reported elsewhere.

&(&(ゐ፹⏝␯Ỉᛶࢩࣜ࢝ࣅ࣮ࢬࡢ⣽Ꮝᵓ㐀ཬࡧỈ⵨Ẽ྾╔≉ᛶ$FNQRZOHGJHPHQWThe authors thank to Fuji Silysia Chemical Ltd., Aichi, for their kindly supply of SiO2 beads.
This work is performed with the support and under the auspices of the NIFS Collaboration
Research program (NIFS13KOBA029).
5HIHUHQFHV[1] J. P. Butler, Sep. Sci. Technol.  (1980) 371.
[2] G. Vsaru, “Tritium Isotope Separation”, CRC press, Boca Raton, Florida, (1993).
[3] A. Matsushima, T. Haneda, S. Hayashi, S. Kiyota, JNC Technical Review (Saikuru Kiko
Giho), No.20s1 (2003) 73, (in Japanese).
[4] T. Sugiyama, Y. Asakura, T. Uda, Y. Abe, T. Shiozaki, Y. Enokida, I. Yamamoto, J. Nucl.
Sci. Technol.  (2004) 696.
[5] T. Sugiyama, E. Suzuki, M. Tanaka, I. Yamamoto, Fus. Sci. Technol.  (2011) 1323.
[6] E.-P. Ng, S. Mintova, Micropor. Mesopor. Mat.  (2008) 1.
[7] K. S. W. Sing, Pure Appl. Chem.  (1982) 2201.
[8] S. Lowell, J. E. Shields, M. A. Thomas, M. Thommes, “Characterization of Porous Solids
and Powders: Surface Area, Pore Size and Density”, Kluwer Academic Publishers,
Dordrecht, The Netherlands (2004).
[9] L. T. Zhuravlev, Colloids Surf. A.  (2000) 1.
[10] D. R. Lide, H. V. Kehiaian, “CRC Handbook of Thermophysical and Thermochemical
Data”, CRC press, Boca Raton, Florida, (1994).
[11] X. S. Zhao, G. Q. Lu, J. Phys. Chem. B,  (1998) 1556.
[12] R. Anwander, I. Nagl, M. Widenmeyer, G. Engelhardt, O. Groeger, C. Palm, T. Röser, J.
Phys. Chem. B,  (2000) 3532.
