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Effects of alumina powder characteristics on H2 and H2O2 production yields in γ-radiolysis of water and 0.4 M H2SO4 aqueous solution
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 3 2 7 2 e1 3 2 7 7
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Effects of alumina powder characteristics on H2 and H2O2 production yields in g-radiolysis of water and 0.4 M H2SO4 aqueous solution Reiji Yamada*, Yuta Kumagai Research Group for Radiochemistry, Nuclear Science and Engineering Directorate, Japan Atomic Energy Agency, Tokai-mura, Naka-gun, Ibaraki-ken 319-1195, Japan
article info
abstract
Article history:
The effects of powder characteristics on H2 and H2O2 productions in 60Co g-radiolysis were
Received 20 April 2012
studied in pure water and in 0.4 M H2SO4 aqueous solutions containing alumina powders.
Received in revised form
In 0.4 M H2SO4 solution, the H2 yields strongly depended on alumina structures and
14 June 2012
decreased in the order of a > q > g-alumina, although the specific surface areas increased
Accepted 18 June 2012
as a < q < g. The yields increased with increasing specific surface area when compared
Available online 15 July 2012
among a-alumina. In pure water, similar dependence was observed but not as strong as that for 0.4 M H2SO4 solution. The H2O2 yields were strongly decreased by adding the
Keywords:
alumina powders in both water and 0.4 M H2SO4 aqueous solution, although the amounts
Hydrogen production
of decrease were almost neither correlated with specific surface areas nor structures. The
Radiation chemistry
enhancing H2 production was discussed in terms of the electron supply from alumina to
g-Radiolysis
aqueous solution as well as the adsorption of OH radicals on alumina surfaces.
Alumina
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen production due to g-radiolysis in heterogeneous systems consisting of a mixture of oxides and aqueous solutions has attracted researchers’ interest because the enhancement of hydrogen production occurs by mixing oxides into aqueous solutions. These heterogeneous systems give a good opportunity to study a new aspect of radiation chemistry from a viewpoint of the interface between oxide and aqueous solution, which could play an important role in the emission of electrons from oxide surface to aqueous solution as well as the adsorption of radicals and products produced by g-radiation on oxide surface. For investigating this new aspect, we have employed aqueous sulfuric acid solutions and reported much more enhancement of hydrogen production yields in the aqueous sulfuric acid solutions than in pure water when added with oxide powders, such as Al2O3,
SiO2, ZrO2, and TiO2 [1]. We recently reported that the H2O2 concentration in g-irradiated solutions strongly decreased and the final product H2 correspondingly increased when Al2O3 powder was added in pure water or in 0.4 M H2SO4 aqueous solution [2]. It is interesting to study how the oxide characteristics influence H2 and H2O2 production in g-irradiated aqueous solutions because the electron emission and the adsorption of radicals can be affected by the oxide characteristics. To the best of the authors’ knowledge, the detailed reports regarding the oxide characteristic effects on H2 production are only few. Some limited data about the dependence of oxide surface area on H2 production yields are available for water vapor adsorbed on ZrO2 powder surface [3], water containing TiO2 powder [4], and SiO2 powder [5]. For the study of powder characteristics on g-radiolysis, aluminum oxide is a suitable material because it has many types of alumina powders to have various crystal
* Corresponding author. Tel.: þ81 29 282 5403; fax: þ81 29 282 6556. E-mail address: [email protected] (R. Yamada). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.06.062
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 3 2 7 2 e1 3 2 7 7
structures and specific surface areas [6]. In the previous reports, we only used a-alumina as a representative of aluminum oxides [1,2]. Therefore, in this study, we have selected three typical types of alumina structures, a, q, and gpowders and studied the structure and surface area dependences on H2 and H2O2 production yields in pure water and in 0.4 M H2SO4 aqueous solution.
2.
Experiment
2.1.
Materials
The alumina powders used in this study are a-, q-, and galumina, with crystal structures of trigonal (corundum type), monoclinic, and tetragonal system (spinel type), respectively. These alumina powders are generally formed from thermal decomposition of alumina monohydroxide, boehmite (AlOOH or Al2O3∙H2O), at >1150 C for a, 1050e1150 C for q, and 500850 C for g-alumina [6]. We used two series of high purity and fine alumina powders fabricated from different starting materials: first ones, AKP series (AKP-3000 and AKP-50) supplied from Sumitomo Chemical Co., Ltd. Japan, which were fabricated from aluminum alkoxide, and second ones, TAIMICRON TM series (TM-DAR, TM-100D, and TM-300D) supplied from Taimei Chemicals Co., Ltd. Japan, which were fabricated from NH3AlCO3(OH)2. The characteristics of these powders such as sintering temperatures, crystal systems, purities, specific surface areas, average powder sizes, and bulk densities are listed in Table 1. These alumina powders were used as received. The 0.5 g alumina powder and 2 ml purified water or 0.4 M aqueous H2SO4 solution were encapsulated in a 5 ml vial. The air space in the vial was not converted into N2 gas or Ar gas prior to g-irradiation. The purified water was obtained by distillation and deionization, with an electric conductance nominally at <0.05 mS. Any OH scavengers were not used for H2 measurement as described in the previous paper [2].
2.2.
Measurement of hydrogen production
During g-irradiation, H2 gas was produced in the liquid of an irradiated vial and then released to the air space of the vial. For
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the measurement of H2 gas after irradiation, a 0.2 ml air was sampled from the air space through the sealed cap of the vial by using gas-tight syringe and injected into the gas chromatography [1]. The calibration of the H2 signal of gas chromatography was independently performed using standard gases composed of a mixture of argon and hydrogen, and the total number of H2 molecules released in the air space was evaluated by taking the total volume of the air space into consideration. The H2 gas measurements were carried out in one or two days after irradiation because of the sample transportation to the measurement site. We checked that this delay did not affect the results of the gas chromatography measurement by using samples contained with certain amounts of H2 gas.
2.3.
g-Ray irradiation
For g-irradiation, a 60Co g-source at JAEA-Takasaki was used at ambient temperature. During irradiation, the samples were statically positioned without any artificial stirring for mixing alumina powders and solutions. For absorbed dose measurements, the cellulose triacetate (CTA) film of Fuji FTR-125 was used [7]. The measured dose rates were 9e14 kGy/h and the irradiation time was 1 h. Here, 1 kGy is equivalent to 1 J absorbed in a mass of 1 g. The measured doses were simply applied for the absorbed doses in irradiated aqueous solutions without any corrections for the components of sulfuric acid and alumina because the difference in mass energy absorption coefficients among substances of H2O, H2SO4, and Al2O3 does not exceed 15% [8], and the absorbed dose ratio of water to CTA film was estimated to be about 1.07 [9] as described in the previous papers [1,2]. Thus, we simply regarded the absorbed dose of 1 kGy as the absorbed energy of 1 J in 1 ml irradiated solution.
2.4.
H2O2 measurement
The H2O2 concentrations in irradiated solutions were measured using the Ghormely tri-iodide method [10,11] in which I ion of iodide reagent added to irradiated solutions was oxidized to I 3 ion by H2O2 in the solutions. The concenion were determined using absorption trations of I 3
Table 1 e Characteristics of aluminum oxide powders.
Sintering temperature ( C) Crystal system Purity Specific surface area (m2/g) Average powder size (mm)d Bulk density (g/ml) a b c d e f
a-alumina. q-alumina. g-alumina. Median diameter. Lightly packed. Heavily packed.
AKP-3000
AKP-50
TM-DAR
TM-100D
TM-300D
>1150 Trigonala >99.99% 4.3 0.59 0.38 e-0.81f
>1150 Trigonala >99.99% 10.2 0.37 0.8e1.2
>1150 Trigonala 99.99% 13.8 0.17 0.9e1.0
1050e1150 Monoclinicb 99.99% 129 0.40 0.4e0.6
500e850 Tetragonalc 99.99% 220 0.21 0.4e0.6
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spectroscopy from the absorbance at 350 nm, which is the wavelength of the maximum absorption for I 3 . The top portion of the solutions, which did not contain alumina powder was sampled and used for the determination of H2O2 concentrations. The sampling was carried out without difficulty because the alumina powder was deposited on the bottom of the vial because of no usage of stirrer. The calibration of the absorbance at 350 nm to the H2O2 concentration was carried out using the samples with known H2O2 concentrations in pure water and in aqueous 0.4 M H2SO4 solution. The linearity of the calibration curves was good for both pure water and 0.4 M H2SO4 aqueous solution. The detail of H2O2 measurement was described in the previous paper [2].
3.
Results and discussion
Intensity of Gas Chromatography (arbitary unit)
Fist, we checked that hydrogen production occurred only from the radiolysis of aqueous solution and confirmed that H2 gas was neither detected when alumina powder alone was irradiated with g-rays (Fig. 1(a)) nor detected when alumina powder was added to 0.4 M H2SO4 aqueous solution without girradiation (Fig. 1(b)). As an example of H2 production from girradiated aqueous solution, Fig. 1(c) shows clear H2 peak in the case of 0.4 M H2SO4 aqueous solution containing alumina powder. The result of Fig. 1(a) indicates that the alumina powders used in this study were not contaminated with hydrocarbons, and the result of Fig. 1(b) indicates that metal ions were not dissolved in 0.4 M H2SO4 aqueous solution because those hydrocarbons and metal ions could be the sources to produce H2 gas because of the decomposition of hydrocarbons by g-irradiation and because of the formation of metal oxides from metal ions if they are dissolved from alumina powder into 0.4 M H2SO4 aqueous solution. In addition, the result of Fig. 1(a) implies that the amounts of adsorbed water on as-received alumina powders should be
H
N
2
Al2O3+ 0.4 M H2SO4
2
O
AKP-3000 : α, 4.3 m2/g
2
with γ-irradiation
c
Pure Water
AKP-50 : α, 10.2 m2/g TM-DAR : α, 13.8 m2/g
Al2O3+ 0.4 M H2SO4 without γ-irradiation
b
TM-100D : θ, 129 m2/g TM-300D : γ, 220 m2/g
Al2O3alone with γ-irradiation
a 2
negligible because adsorbed water on some oxides could lead to considerable H2 production by water radiolysis [3,12]. As described in Experimental, hydrogen gas, which was first produced in the aqueous solution and then released to the air space, was detected in the air space of the irradiated vial. Therefore, the amount of hydrogen gas obtained here was the end result of H2 production in the homogenous stage after many chemical reactions took place among radicals and molecules, such as H, OH, H2, and H2O2 that were produced at the primary stage of g-irradiation [8,13]. We already checked that the amount of released H2 was proportional to the irradiated volume of the aqueous solution and to the absorbed dose in our experimental conditions as shown in the previous paper (see Fig. 2 of Ref. 1). We therefore show the results of H2 production as the final product H2 yields, namely H2 molecules per aqueous solution volume per absorbed dose (mmol H2 L1 kGy1). For the conversion of the above H2 yields to Gvalues expressed in units of molecules produced per 100 eV of energy absorbed, we used the energy absorbed in aqueous solution alone, similarly as the previous studies [1,2] so that 100 mmol H2 L1 kGy1 is equivalent to 0.965 H2 molecules/ 100 eV without any corrections for the components of alumina and sulfuric acid [1,2]. Fig. 2 shows the final product H2 yields obtained for pure water containing a-, q-, and g-alumina powders with different specific surface areas. The result of pure water alone is also included for comparison. In general, the H2 yields apparently increased with increasing specific surface area of the alumina powders. This trend of the results is qualitatively similar to the result of the nanoparticle of TiO2 [4] and the results of silica gels and TiO2 [5]. Thus, the crystal structures apparently gave a minor effect on the H2 yields for pure water. We will discuss later on this matter. Fig. 3 shows the final product H2 yields obtained for 0.4 M H2SO4 aqueous solutions containing a-, q-, and g-alumina powders with different specific surface areas. The result of 0.4 M H2SO4 aqueous solution alone is also included for comparison. Contrary to the case of pure water, the H2 yields strongly depend on the crystal structures of the TM series supplied from the same vendor, that is, the yields
Without Alumina
3
4
5 6 Retention Time (min)
7
8
Fig. 1 e Typical examples of measured intensities of gas chromatography as a function of retention time: (a) alumina powder alone irradiated with 60Co g-rays, (b) 0.4 M H2SO4 aqueous solution containing alumina powder without g-irradiation, and (c) 0.4 M H2SO4 aqueous solution containing alumina powder irradiated with 60Co g-rays.
0
50 100 150 Final product H yield / μmol H2 L-1 kGy-1
200
2
Fig. 2 e Final product H2 yields in 60Co g-radiolysis of pure water containing a-, q-, and g-alumina powder whose amounts were 0.5 g in 2 ml solution, namely average 20 wt %. Specific surface areas of the powders are noted beside the horizontal bars. The H2 yield of 100 mmol LL1 kGyL1 is equivalent to 0.965 H2 molecules/100 eV.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 3 2 7 2 e1 3 2 7 7
AKP-3000 : α, 4.3 m2/g AKP-50 : α, 10.2 m2/g TM-DAR : α, 13.8 m2/g TM-100D : θ, 129 m2/g TM-300D : γ, 220 m2/g 0.4 M H2SO
Without Alumina 0
50 100 150 Final product H yield / μmol H2 L-1 kGy-1 2
200
Fig. 3 e Final product H2 yields in 60Co g-radiolysis of 0.4 M H2SO4 aqueous solutions containing a-, q-, and g-alumina powder whose amounts were 0.5 g in 2 ml solution, namely average 20 wt%. Specific surface areas of the powders are noted inside the horizontal bars. The H2 yield of 100 mmol LL1 kGyL1 is equivalent to 0.965 H2 molecules/ 100 eV.
decreased in the order of a > q > g-alumina, although those specific surface areas increased as a < q < g. When compared among the alumina powders having the same crystal structure, namely, a-alumina, the yields increased with the specific surface area of the a-alumina. For increasing the H2 production in the g-radiolysis of water, the increase of the concentration of hydrated electron (e aq ) in irradiated water is important because H2 production occurs from the following reactions [8,14]: 2e aq /H2 þ 2OH
e aq þ H/H2 þ OH
and
For acidic aqueous solution such as H2SO4, the following reactions are dominant [1,2,8]:
13275
surface [2]. Fig. 4 shows that the final product H2O2 yields (mmol H2O2 L1 kGy1), which were obtained by that the H2O2 concentrations in irradiated solutions were divided by absorbed doses. As shown in Fig. 4, the a-, q-, and g-alumina powders used in this study strongly decreased the final product H2O2 yields compared with those yields in pure water alone and in 0.4 M H2SO4 aqueous solution alone. Contrary to the clear dependences of H2 yields on crystal structures and specific surface areas, those dependences of the H2O2 yields were not clear in pure water with alumina and in 0.4 M H2SO4 aqueous solution with alumina. Here, we note that H2O2 production in 0.4 M H2SO4 alone was nearly one-order higher than that in pure water alone, which is correlated to higher H2 production in 0.4 M H2SO4 alone than in pure water alone because H2O2 production is the oxidant counterpart of H2 production in the radiolysis of aqueous solution [16]. The results of H2 yields of Figs. 2 and 3 as a function of specific surface area are shown in Fig. 5. This figure reveals that the H2 yields were affected by the crystal structures from the fact that the increase of H2 yields with specific surface area was retarded by changing crystal structures from a- to q- and to g-alumina. Compared with the stronger retardation in 0.4 M H2SO4 aqueous solution, the weaker retardation in pure water seems to lead to the different features of the specific surface area ddependences. It is still unclear why the retardation in 0.4 M H2SO4 is stronger than that in pure water. Nevertheless, it can be said that the alumina in 0.4 M H2SO4 aqueous solution could exhibit the surface characteristics related to the H2 production more directly compared with that in pure water. These phenomena are probably related to the different reaction processes of H2 production, that is, through H for 0.4 M H2SO4 aqueous solution and through e aq for pure water, respectively. These differences make the reaction processes for the end results of H2 production in 0.4 M H2SO4 aqueous solution simpler than those in pure water. The different
þ e or Hþ /H and H þ H/H2 aq þ H3 O Deceasing the concentrations of OH radical and H2O2 molecule in irradiated water can also contribute to the increase of H2 production because the following reactions are retarded, and eventually, H2 production is increased [8,14]: OH þ H2 /H þ H2 O; H2 O2 þ H/OH þ H2 O;
OH þ H/H2 O;
OH þ e aq /OH
H2 O2 þ e aq /OH þ OH
In addition to the above reactions, the following reaction also contributes to the reduction of OH radicals in the case of H2SO4 aqueous solution [14,15], OH þ HSO 4 /SO4 þ H2 O
In the previous report [2], a-alumina powder (AKP-50) decreased H2O2 concentrations in irradiated water as well as in irradiated 0.4 M H2SO4 aqueous solution and correlatively increased the H2 yields. The decrease of H2O2 concentration was brought by not only the decrease of H2O2 molecules but also the decrease of OH radicals because the latter is highly expected from the adsorption of OH radicals on the alumina
Fig. 4 e Final product H2O2 yields in 60Co g-radiolysis of pure water containing a-, q-, and g-alumina powder of 20 wt% as well as in 0.4 M H2SO4 aqueous solution containing a, q, and g-alumina powder of 20 wt%. The H2O2 yields were determined from the H2O2 concentrations after 60 Co g-irradiation. The yields in pure water are multiplied by 10.
Final Product H2 yield / μmol H2 L-1 kGy-1
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200 0.4 M H2SO4 Water 150
α-alumina 100
θ -alumina γ -alumina
50
0
0
50
100
150
200
250
Specific surface area / m2 g-1
Fig. 5 e Final product H2 yields in 60Co g-radiolysis of pure water and 0.4 M H2SO4 aqueous solution containing a-, q-, and g-alumina powder of 20 wt% as a function of the specific surface area of the alumina powder.
affinities of H atom and e aq to oxide surface may also contribute to the phenomena. Here, it is worth noting that the order of H2 yields, a > q > g, is the same as the order of sintering temperatures, as shown in Table 1. We consider that the alumina powder sintered at higher temperature would have higher emission of excited and secondary electrons from the alumina surface, which could lead to higher H2 production [3]. Those higher emission as well as less defects for trapping electrons could be expected for more stable and less imperfect alumina structure that are produced at higher sintering temperatures. The energy and electron transfer processes from oxide to water were discussed in terms of several models including exited and secondary electrons [3,17e19], and the pathways of energetic electrons to prehydrated and hydrated electrons were also discussed [20]. The crystal structure dependence of the H2 formation was reported in the case of radiolysis of water with ZrO2 powder [21]. In the previous paper [2], the importance of the reduction of OH radical and H2O2 molecule concentrations in g-irradiated aqueous solutions was pointed out for H2 production. In addition to the reduction, this study reveals that the effect of the crystal structures of alumina, which could affect the electron transport and supply from the inside of the alumina to the surface, is strong for the H2 production. When alumina of the same crystal system is used, H2 production increases with the specific surface area, which can increase spaciously the emission of electrons as well as the adsorption of OH radicals. As mentioned that H2O2 production is the oxidant counterpart of H2 production in radiolysis of aqueous solution, the electrons emitted from the alumina surface should be involved in both H2 and H2O2 productions. In addition, the reactions of exited electrons on the surface are important because electron transfer might be able to cross over the interface between water and solid surface. We could observe the clear structure dependence of H2 production, but its dependence was not clear for H2O2 production because H2O2 yields were strongly decreased by the addition of any alumina
powders. Therefore, the radiation-induced reactions of H2O2 at different surface structures are still needed to be studied for understanding the role of alumina surfaces. The decomposition of H2O2 by g-irradiation in the presence of alumina of the different structures will provide a new insight into radiationinduced phenomena at the interface. In future studies, the energy and number distributions of electrons excited on each oxide surface as well as emitted from each oxide to aqueous solutions may be necessary for better understanding the roles of excited electrons and secondary electrons in the H2 and H2O2 productions because the different energy of electrons originated from the different oxide structures would affect differently those productions in g-irradiated aqueous solutions. The evaluation methodology for those distributions should be assessed from the above viewpoint.
4.
Summary
The dependence of alumina powder structures on H2 and H2O2 productions induced by 60Co g-radiolysis was studied in the case of pure water and of 0.4 M H2SO4 aqueous solution containing alumina powders. The H2 yields in 0.4 M H2SO4 aqueous solution were strongly dependent on alumina structures, that is, the H2 yields decreased in the order of a > q > g-alumina. This order is the same as the order of the sintering temperatures, which indicates that more stable and less imperfect alumina powder sintered at higher temperature gave higher H2 yields. When compared within a-alumina, H2 yields increased with the specific surface area. In the case of pure water, similar structure dependence was observed, but it was rather weak compared with 0.4 M H2SO4 aqueous solution so that the H2 production yields in pure water appear to increase slightly with the increasing specific surface area of a-, q-, and g-alumina alumina. The H2O2 yields were drastically decreased by adding the alumina powders in both pure water and 0.4 M H2SO4, although they were not clearly dependent on either specific surface areas or structures. These results suggest that the enhancement of H2 production would be related to the structure-dependent electron supply from the alumina to the aqueous solution in addition to the adsorption of OH radicals on alumina surfaces.
Acknowledgments This work was supported by the Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research.
references
[1] Yamada R, Nagaishi R, Hatano Y, Yoshida Z. Hydrogen production in the g-radiolysis of aqueous sulfuric acid solutions containing Al2O3, SiO2, TiO2 or ZrO2 fine particles. Int Hydrogen Energy 2008;33:929e36. [2] Yamada R, Kumagai Y, Nagaishi R. Effect of alumina on the enhancement of hydrogen production and the reduction of hydrogen peroxide in the g-radiolysis of pure water and 0.4
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M H2SO4 aqueous solution. Int Hydrogen Energy 2011;36: 11646e53. Petrik NG, Alexandrov AB, Vall AI. Interfacial energy transfer during gamma radiolysis of water on the surface of ZrO2 and other oxides. J Phys Chem B 2001;105:5935e44. Seino S, Yamamoto TA, Fujimoto R, Hashimoto K, Katsura M, Okuda S, et al. Enhancement of hydrogen evolution yield from water dispersing nanoparticles irradiated with gammaray. J Nucl Sci and Technol 2001;38:633e6. Maeda Y, Kawana Y, Kawamura K, Hayama S, Sugihara S, Okai T. Hydrogen gas evolution from water included in a silica gel cavity and on metal oxides with g-ray irradiation. J Nucl Radiochem Sci 2005;6:131e4. MacZura G, Goodboy KP, Koenig JJ. Aluminum oxide (Alumina). In: Grayson M, editor. “Encyclopedia of Glass, Ceramics, and Cement”, encyclopedia REPRINT series. John Wily & Sons; 1985. p. 14e40. Tanaka R, Mitomo S, Tamura N. Effects of temperature, relative humidity and dose rate on the sensitivity of cellulose triacetate dosimeters to electrons and g-rays. Int J Appl Radiat Isot 1984;35:875e81. Spinks JWT, Woods RJ. An introduction to radiation chemistry. New York: John Wiley & Sons, Inc; 1990. Tanaka R, Mitomo S, Sunaga H, Matsuda K, Tamura N. Manual of CTA dose meter; 1982. JAERI-M 82e033 (in Japanese). Allen AO, Hochanadel CJ, Ghormley JA, Davis TW. Decomposition of water and aqueous solutions under mixed neutron and gamma radiation. J Phys Chem 1952;56:575e86. Hochanadel CJ. Effects of cobalt g-radiation on water and aqueous solutions. J Phys Chem 1952;56:587e94.
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[12] LaVerne JA, Tandon L. H2 production in the radiolysis of water on UO2 and other oxides. J Phys Chem B 2003;107: 13623e8. [13] Buxton GV. The radiation chemistry of liquid water: principles and applications in Chapter 12. In: Mozumder A, Hatano Y, editors. Charged particles and photon interactions with matter. New York: Marcel Dekker; 2004. [14] Buxton GV, Greenstock CL, Helman WP, Ross AB. Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals in aqueous solution. Phys Chem Ref Data 1988;17(2):513e769. [15] Lesigne B, Ferradini C, Pucheault J. Pulse radiolysis study of the direct effect on sulfuric acid. J Phys Chem 1973;77: 2156e8. [16] Cae¨r SL, Renault JP, Mialocq J-C. Hydrogen peroxide formation in the radiolysis of hydrated nanoporous glasses: a low and high dose rate study. Chem Phy Lett 2007;450: 91e5. [17] LaVerne JA, Tandon L. H2 production in the radiolysis of water on CeO2 and ZrO2. J Phys Chem B 2002;106:380e6. [18] Schatz T, Cook AR, Meisel D. Charge transfer across the silica nanoparticle/water interface. J Phs Chem B 1998;102: 7225e30. [19] Dimitrijevic NM, Henglein AH, Meisel D. Charge separation across the silica nanoparticle/water interface. J Phys Chem B 1999;103:7073e6. [20] LaVerne JA, Pimblott SM. New mechanism for H2 formation in water. J Phys Chem A 2000;104:9820e2. [21] LaVerne JA. H2 formation from the radiolysis of liquid water with zirconia. J Phys Chem B 2005;109:5395e7.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Effects of alumina powder characteristics on H2 and H2O2 production yields in g-radiolysis of water and 0.4 M H2SO4 aqueous solution Reiji Yamada*, Yuta Kumagai Research Group for Radiochemistry, Nuclear Science and Engineering Directorate, Japan Atomic Energy Agency, Tokai-mura, Naka-gun, Ibaraki-ken 319-1195, Japan
article info
abstract
Article history:
The effects of powder characteristics on H2 and H2O2 productions in 60Co g-radiolysis were
Received 20 April 2012
studied in pure water and in 0.4 M H2SO4 aqueous solutions containing alumina powders.
Received in revised form
In 0.4 M H2SO4 solution, the H2 yields strongly depended on alumina structures and
14 June 2012
decreased in the order of a > q > g-alumina, although the specific surface areas increased
Accepted 18 June 2012
as a < q < g. The yields increased with increasing specific surface area when compared
Available online 15 July 2012
among a-alumina. In pure water, similar dependence was observed but not as strong as that for 0.4 M H2SO4 solution. The H2O2 yields were strongly decreased by adding the
Keywords:
alumina powders in both water and 0.4 M H2SO4 aqueous solution, although the amounts
Hydrogen production
of decrease were almost neither correlated with specific surface areas nor structures. The
Radiation chemistry
enhancing H2 production was discussed in terms of the electron supply from alumina to
g-Radiolysis
aqueous solution as well as the adsorption of OH radicals on alumina surfaces.
Alumina
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen production due to g-radiolysis in heterogeneous systems consisting of a mixture of oxides and aqueous solutions has attracted researchers’ interest because the enhancement of hydrogen production occurs by mixing oxides into aqueous solutions. These heterogeneous systems give a good opportunity to study a new aspect of radiation chemistry from a viewpoint of the interface between oxide and aqueous solution, which could play an important role in the emission of electrons from oxide surface to aqueous solution as well as the adsorption of radicals and products produced by g-radiation on oxide surface. For investigating this new aspect, we have employed aqueous sulfuric acid solutions and reported much more enhancement of hydrogen production yields in the aqueous sulfuric acid solutions than in pure water when added with oxide powders, such as Al2O3,
SiO2, ZrO2, and TiO2 [1]. We recently reported that the H2O2 concentration in g-irradiated solutions strongly decreased and the final product H2 correspondingly increased when Al2O3 powder was added in pure water or in 0.4 M H2SO4 aqueous solution [2]. It is interesting to study how the oxide characteristics influence H2 and H2O2 production in g-irradiated aqueous solutions because the electron emission and the adsorption of radicals can be affected by the oxide characteristics. To the best of the authors’ knowledge, the detailed reports regarding the oxide characteristic effects on H2 production are only few. Some limited data about the dependence of oxide surface area on H2 production yields are available for water vapor adsorbed on ZrO2 powder surface [3], water containing TiO2 powder [4], and SiO2 powder [5]. For the study of powder characteristics on g-radiolysis, aluminum oxide is a suitable material because it has many types of alumina powders to have various crystal
* Corresponding author. Tel.: þ81 29 282 5403; fax: þ81 29 282 6556. E-mail address: [email protected] (R. Yamada). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.06.062
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structures and specific surface areas [6]. In the previous reports, we only used a-alumina as a representative of aluminum oxides [1,2]. Therefore, in this study, we have selected three typical types of alumina structures, a, q, and gpowders and studied the structure and surface area dependences on H2 and H2O2 production yields in pure water and in 0.4 M H2SO4 aqueous solution.
2.
Experiment
2.1.
Materials
The alumina powders used in this study are a-, q-, and galumina, with crystal structures of trigonal (corundum type), monoclinic, and tetragonal system (spinel type), respectively. These alumina powders are generally formed from thermal decomposition of alumina monohydroxide, boehmite (AlOOH or Al2O3∙H2O), at >1150 C for a, 1050e1150 C for q, and 500850 C for g-alumina [6]. We used two series of high purity and fine alumina powders fabricated from different starting materials: first ones, AKP series (AKP-3000 and AKP-50) supplied from Sumitomo Chemical Co., Ltd. Japan, which were fabricated from aluminum alkoxide, and second ones, TAIMICRON TM series (TM-DAR, TM-100D, and TM-300D) supplied from Taimei Chemicals Co., Ltd. Japan, which were fabricated from NH3AlCO3(OH)2. The characteristics of these powders such as sintering temperatures, crystal systems, purities, specific surface areas, average powder sizes, and bulk densities are listed in Table 1. These alumina powders were used as received. The 0.5 g alumina powder and 2 ml purified water or 0.4 M aqueous H2SO4 solution were encapsulated in a 5 ml vial. The air space in the vial was not converted into N2 gas or Ar gas prior to g-irradiation. The purified water was obtained by distillation and deionization, with an electric conductance nominally at <0.05 mS. Any OH scavengers were not used for H2 measurement as described in the previous paper [2].
2.2.
Measurement of hydrogen production
During g-irradiation, H2 gas was produced in the liquid of an irradiated vial and then released to the air space of the vial. For
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the measurement of H2 gas after irradiation, a 0.2 ml air was sampled from the air space through the sealed cap of the vial by using gas-tight syringe and injected into the gas chromatography [1]. The calibration of the H2 signal of gas chromatography was independently performed using standard gases composed of a mixture of argon and hydrogen, and the total number of H2 molecules released in the air space was evaluated by taking the total volume of the air space into consideration. The H2 gas measurements were carried out in one or two days after irradiation because of the sample transportation to the measurement site. We checked that this delay did not affect the results of the gas chromatography measurement by using samples contained with certain amounts of H2 gas.
2.3.
g-Ray irradiation
For g-irradiation, a 60Co g-source at JAEA-Takasaki was used at ambient temperature. During irradiation, the samples were statically positioned without any artificial stirring for mixing alumina powders and solutions. For absorbed dose measurements, the cellulose triacetate (CTA) film of Fuji FTR-125 was used [7]. The measured dose rates were 9e14 kGy/h and the irradiation time was 1 h. Here, 1 kGy is equivalent to 1 J absorbed in a mass of 1 g. The measured doses were simply applied for the absorbed doses in irradiated aqueous solutions without any corrections for the components of sulfuric acid and alumina because the difference in mass energy absorption coefficients among substances of H2O, H2SO4, and Al2O3 does not exceed 15% [8], and the absorbed dose ratio of water to CTA film was estimated to be about 1.07 [9] as described in the previous papers [1,2]. Thus, we simply regarded the absorbed dose of 1 kGy as the absorbed energy of 1 J in 1 ml irradiated solution.
2.4.
H2O2 measurement
The H2O2 concentrations in irradiated solutions were measured using the Ghormely tri-iodide method [10,11] in which I ion of iodide reagent added to irradiated solutions was oxidized to I 3 ion by H2O2 in the solutions. The concenion were determined using absorption trations of I 3
Table 1 e Characteristics of aluminum oxide powders.
Sintering temperature ( C) Crystal system Purity Specific surface area (m2/g) Average powder size (mm)d Bulk density (g/ml) a b c d e f
a-alumina. q-alumina. g-alumina. Median diameter. Lightly packed. Heavily packed.
AKP-3000
AKP-50
TM-DAR
TM-100D
TM-300D
>1150 Trigonala >99.99% 4.3 0.59 0.38 e-0.81f
>1150 Trigonala >99.99% 10.2 0.37 0.8e1.2
>1150 Trigonala 99.99% 13.8 0.17 0.9e1.0
1050e1150 Monoclinicb 99.99% 129 0.40 0.4e0.6
500e850 Tetragonalc 99.99% 220 0.21 0.4e0.6
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spectroscopy from the absorbance at 350 nm, which is the wavelength of the maximum absorption for I 3 . The top portion of the solutions, which did not contain alumina powder was sampled and used for the determination of H2O2 concentrations. The sampling was carried out without difficulty because the alumina powder was deposited on the bottom of the vial because of no usage of stirrer. The calibration of the absorbance at 350 nm to the H2O2 concentration was carried out using the samples with known H2O2 concentrations in pure water and in aqueous 0.4 M H2SO4 solution. The linearity of the calibration curves was good for both pure water and 0.4 M H2SO4 aqueous solution. The detail of H2O2 measurement was described in the previous paper [2].
3.
Results and discussion
Intensity of Gas Chromatography (arbitary unit)
Fist, we checked that hydrogen production occurred only from the radiolysis of aqueous solution and confirmed that H2 gas was neither detected when alumina powder alone was irradiated with g-rays (Fig. 1(a)) nor detected when alumina powder was added to 0.4 M H2SO4 aqueous solution without girradiation (Fig. 1(b)). As an example of H2 production from girradiated aqueous solution, Fig. 1(c) shows clear H2 peak in the case of 0.4 M H2SO4 aqueous solution containing alumina powder. The result of Fig. 1(a) indicates that the alumina powders used in this study were not contaminated with hydrocarbons, and the result of Fig. 1(b) indicates that metal ions were not dissolved in 0.4 M H2SO4 aqueous solution because those hydrocarbons and metal ions could be the sources to produce H2 gas because of the decomposition of hydrocarbons by g-irradiation and because of the formation of metal oxides from metal ions if they are dissolved from alumina powder into 0.4 M H2SO4 aqueous solution. In addition, the result of Fig. 1(a) implies that the amounts of adsorbed water on as-received alumina powders should be
H
N
2
Al2O3+ 0.4 M H2SO4
2
O
AKP-3000 : α, 4.3 m2/g
2
with γ-irradiation
c
Pure Water
AKP-50 : α, 10.2 m2/g TM-DAR : α, 13.8 m2/g
Al2O3+ 0.4 M H2SO4 without γ-irradiation
b
TM-100D : θ, 129 m2/g TM-300D : γ, 220 m2/g
Al2O3alone with γ-irradiation
a 2
negligible because adsorbed water on some oxides could lead to considerable H2 production by water radiolysis [3,12]. As described in Experimental, hydrogen gas, which was first produced in the aqueous solution and then released to the air space, was detected in the air space of the irradiated vial. Therefore, the amount of hydrogen gas obtained here was the end result of H2 production in the homogenous stage after many chemical reactions took place among radicals and molecules, such as H, OH, H2, and H2O2 that were produced at the primary stage of g-irradiation [8,13]. We already checked that the amount of released H2 was proportional to the irradiated volume of the aqueous solution and to the absorbed dose in our experimental conditions as shown in the previous paper (see Fig. 2 of Ref. 1). We therefore show the results of H2 production as the final product H2 yields, namely H2 molecules per aqueous solution volume per absorbed dose (mmol H2 L1 kGy1). For the conversion of the above H2 yields to Gvalues expressed in units of molecules produced per 100 eV of energy absorbed, we used the energy absorbed in aqueous solution alone, similarly as the previous studies [1,2] so that 100 mmol H2 L1 kGy1 is equivalent to 0.965 H2 molecules/ 100 eV without any corrections for the components of alumina and sulfuric acid [1,2]. Fig. 2 shows the final product H2 yields obtained for pure water containing a-, q-, and g-alumina powders with different specific surface areas. The result of pure water alone is also included for comparison. In general, the H2 yields apparently increased with increasing specific surface area of the alumina powders. This trend of the results is qualitatively similar to the result of the nanoparticle of TiO2 [4] and the results of silica gels and TiO2 [5]. Thus, the crystal structures apparently gave a minor effect on the H2 yields for pure water. We will discuss later on this matter. Fig. 3 shows the final product H2 yields obtained for 0.4 M H2SO4 aqueous solutions containing a-, q-, and g-alumina powders with different specific surface areas. The result of 0.4 M H2SO4 aqueous solution alone is also included for comparison. Contrary to the case of pure water, the H2 yields strongly depend on the crystal structures of the TM series supplied from the same vendor, that is, the yields
Without Alumina
3
4
5 6 Retention Time (min)
7
8
Fig. 1 e Typical examples of measured intensities of gas chromatography as a function of retention time: (a) alumina powder alone irradiated with 60Co g-rays, (b) 0.4 M H2SO4 aqueous solution containing alumina powder without g-irradiation, and (c) 0.4 M H2SO4 aqueous solution containing alumina powder irradiated with 60Co g-rays.
0
50 100 150 Final product H yield / μmol H2 L-1 kGy-1
200
2
Fig. 2 e Final product H2 yields in 60Co g-radiolysis of pure water containing a-, q-, and g-alumina powder whose amounts were 0.5 g in 2 ml solution, namely average 20 wt %. Specific surface areas of the powders are noted beside the horizontal bars. The H2 yield of 100 mmol LL1 kGyL1 is equivalent to 0.965 H2 molecules/100 eV.
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AKP-3000 : α, 4.3 m2/g AKP-50 : α, 10.2 m2/g TM-DAR : α, 13.8 m2/g TM-100D : θ, 129 m2/g TM-300D : γ, 220 m2/g 0.4 M H2SO
Without Alumina 0
50 100 150 Final product H yield / μmol H2 L-1 kGy-1 2
200
Fig. 3 e Final product H2 yields in 60Co g-radiolysis of 0.4 M H2SO4 aqueous solutions containing a-, q-, and g-alumina powder whose amounts were 0.5 g in 2 ml solution, namely average 20 wt%. Specific surface areas of the powders are noted inside the horizontal bars. The H2 yield of 100 mmol LL1 kGyL1 is equivalent to 0.965 H2 molecules/ 100 eV.
decreased in the order of a > q > g-alumina, although those specific surface areas increased as a < q < g. When compared among the alumina powders having the same crystal structure, namely, a-alumina, the yields increased with the specific surface area of the a-alumina. For increasing the H2 production in the g-radiolysis of water, the increase of the concentration of hydrated electron (e aq ) in irradiated water is important because H2 production occurs from the following reactions [8,14]: 2e aq /H2 þ 2OH
e aq þ H/H2 þ OH
and
For acidic aqueous solution such as H2SO4, the following reactions are dominant [1,2,8]:
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surface [2]. Fig. 4 shows that the final product H2O2 yields (mmol H2O2 L1 kGy1), which were obtained by that the H2O2 concentrations in irradiated solutions were divided by absorbed doses. As shown in Fig. 4, the a-, q-, and g-alumina powders used in this study strongly decreased the final product H2O2 yields compared with those yields in pure water alone and in 0.4 M H2SO4 aqueous solution alone. Contrary to the clear dependences of H2 yields on crystal structures and specific surface areas, those dependences of the H2O2 yields were not clear in pure water with alumina and in 0.4 M H2SO4 aqueous solution with alumina. Here, we note that H2O2 production in 0.4 M H2SO4 alone was nearly one-order higher than that in pure water alone, which is correlated to higher H2 production in 0.4 M H2SO4 alone than in pure water alone because H2O2 production is the oxidant counterpart of H2 production in the radiolysis of aqueous solution [16]. The results of H2 yields of Figs. 2 and 3 as a function of specific surface area are shown in Fig. 5. This figure reveals that the H2 yields were affected by the crystal structures from the fact that the increase of H2 yields with specific surface area was retarded by changing crystal structures from a- to q- and to g-alumina. Compared with the stronger retardation in 0.4 M H2SO4 aqueous solution, the weaker retardation in pure water seems to lead to the different features of the specific surface area ddependences. It is still unclear why the retardation in 0.4 M H2SO4 is stronger than that in pure water. Nevertheless, it can be said that the alumina in 0.4 M H2SO4 aqueous solution could exhibit the surface characteristics related to the H2 production more directly compared with that in pure water. These phenomena are probably related to the different reaction processes of H2 production, that is, through H for 0.4 M H2SO4 aqueous solution and through e aq for pure water, respectively. These differences make the reaction processes for the end results of H2 production in 0.4 M H2SO4 aqueous solution simpler than those in pure water. The different
þ e or Hþ /H and H þ H/H2 aq þ H3 O Deceasing the concentrations of OH radical and H2O2 molecule in irradiated water can also contribute to the increase of H2 production because the following reactions are retarded, and eventually, H2 production is increased [8,14]: OH þ H2 /H þ H2 O; H2 O2 þ H/OH þ H2 O;
OH þ H/H2 O;
OH þ e aq /OH
H2 O2 þ e aq /OH þ OH
In addition to the above reactions, the following reaction also contributes to the reduction of OH radicals in the case of H2SO4 aqueous solution [14,15], OH þ HSO 4 /SO4 þ H2 O
In the previous report [2], a-alumina powder (AKP-50) decreased H2O2 concentrations in irradiated water as well as in irradiated 0.4 M H2SO4 aqueous solution and correlatively increased the H2 yields. The decrease of H2O2 concentration was brought by not only the decrease of H2O2 molecules but also the decrease of OH radicals because the latter is highly expected from the adsorption of OH radicals on the alumina
Fig. 4 e Final product H2O2 yields in 60Co g-radiolysis of pure water containing a-, q-, and g-alumina powder of 20 wt% as well as in 0.4 M H2SO4 aqueous solution containing a, q, and g-alumina powder of 20 wt%. The H2O2 yields were determined from the H2O2 concentrations after 60 Co g-irradiation. The yields in pure water are multiplied by 10.
Final Product H2 yield / μmol H2 L-1 kGy-1
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200 0.4 M H2SO4 Water 150
α-alumina 100
θ -alumina γ -alumina
50
0
0
50
100
150
200
250
Specific surface area / m2 g-1
Fig. 5 e Final product H2 yields in 60Co g-radiolysis of pure water and 0.4 M H2SO4 aqueous solution containing a-, q-, and g-alumina powder of 20 wt% as a function of the specific surface area of the alumina powder.
affinities of H atom and e aq to oxide surface may also contribute to the phenomena. Here, it is worth noting that the order of H2 yields, a > q > g, is the same as the order of sintering temperatures, as shown in Table 1. We consider that the alumina powder sintered at higher temperature would have higher emission of excited and secondary electrons from the alumina surface, which could lead to higher H2 production [3]. Those higher emission as well as less defects for trapping electrons could be expected for more stable and less imperfect alumina structure that are produced at higher sintering temperatures. The energy and electron transfer processes from oxide to water were discussed in terms of several models including exited and secondary electrons [3,17e19], and the pathways of energetic electrons to prehydrated and hydrated electrons were also discussed [20]. The crystal structure dependence of the H2 formation was reported in the case of radiolysis of water with ZrO2 powder [21]. In the previous paper [2], the importance of the reduction of OH radical and H2O2 molecule concentrations in g-irradiated aqueous solutions was pointed out for H2 production. In addition to the reduction, this study reveals that the effect of the crystal structures of alumina, which could affect the electron transport and supply from the inside of the alumina to the surface, is strong for the H2 production. When alumina of the same crystal system is used, H2 production increases with the specific surface area, which can increase spaciously the emission of electrons as well as the adsorption of OH radicals. As mentioned that H2O2 production is the oxidant counterpart of H2 production in radiolysis of aqueous solution, the electrons emitted from the alumina surface should be involved in both H2 and H2O2 productions. In addition, the reactions of exited electrons on the surface are important because electron transfer might be able to cross over the interface between water and solid surface. We could observe the clear structure dependence of H2 production, but its dependence was not clear for H2O2 production because H2O2 yields were strongly decreased by the addition of any alumina
powders. Therefore, the radiation-induced reactions of H2O2 at different surface structures are still needed to be studied for understanding the role of alumina surfaces. The decomposition of H2O2 by g-irradiation in the presence of alumina of the different structures will provide a new insight into radiationinduced phenomena at the interface. In future studies, the energy and number distributions of electrons excited on each oxide surface as well as emitted from each oxide to aqueous solutions may be necessary for better understanding the roles of excited electrons and secondary electrons in the H2 and H2O2 productions because the different energy of electrons originated from the different oxide structures would affect differently those productions in g-irradiated aqueous solutions. The evaluation methodology for those distributions should be assessed from the above viewpoint.
4.
Summary
The dependence of alumina powder structures on H2 and H2O2 productions induced by 60Co g-radiolysis was studied in the case of pure water and of 0.4 M H2SO4 aqueous solution containing alumina powders. The H2 yields in 0.4 M H2SO4 aqueous solution were strongly dependent on alumina structures, that is, the H2 yields decreased in the order of a > q > g-alumina. This order is the same as the order of the sintering temperatures, which indicates that more stable and less imperfect alumina powder sintered at higher temperature gave higher H2 yields. When compared within a-alumina, H2 yields increased with the specific surface area. In the case of pure water, similar structure dependence was observed, but it was rather weak compared with 0.4 M H2SO4 aqueous solution so that the H2 production yields in pure water appear to increase slightly with the increasing specific surface area of a-, q-, and g-alumina alumina. The H2O2 yields were drastically decreased by adding the alumina powders in both pure water and 0.4 M H2SO4, although they were not clearly dependent on either specific surface areas or structures. These results suggest that the enhancement of H2 production would be related to the structure-dependent electron supply from the alumina to the aqueous solution in addition to the adsorption of OH radicals on alumina surfaces.
Acknowledgments This work was supported by the Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research.
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