Article pubs.acs.org/JPCC
Hydrogen Production by γ‑Ray Irradiation from Different Types of Zeolites in Aqueous Solution Yuta Kumagai,*,† Atsushi Kimura,‡ Mitsumasa Taguchi,‡ and Masayuki Watanabe† †
Nuclear Science and Engineering Center, Japan Atomic Energy Agency, 2-4 Shirane Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan ‡ Quantum Beam Science Center, Japan Atomic Energy Agency, 1233 Watanuki-machi, Takasaki-shi, Gunma 370-1292, Japan S Supporting Information *
ABSTRACT: Dihydrogen (H2) production via irradiation of zeolite−water mixtures was studied to investigate the effect of zeolites on the reaction process for H2 production. Four types of zeolites were comparatively examined under anoxic and aerated conditions. High yields of H2 were observed for the zeolites with high aluminum contents, A-type and X-type zeolites, compared to the Y-type and mordenite-type zeolites. The difference in the H2 yields was considerable at weight fractions of water below 50 wt % under anoxic conditions. The experimental results demonstrated a reaction pathway for H2 production that has different yields between the four zeolites. The H2 yields were compared with chemical analyses of the zeolites, and the comparison suggested that the extraframework aluminum species in the zeolites are involved in the reaction pathway for H2 production. On the other hand, under aerated conditions, the H2 yields from the zeolites were lower than under anoxic conditions probably due to the H2O2 that was produced by water radiolysis. Moreover, the difference between the zeolites in H2 yields was suppressed at water fractions above 50 wt %. The comparable H2 yields suggested yet another reaction pathway for H2 production that is less affected by the structure and composition of the zeolites.
1. INTRODUCTION Dihydrogen (H2) production is fundamentally important as an intrinsic product for understanding water decomposition and subsequent radical reactions. The decomposition of water by ionizing radiation has been extensively studied owing to its importance in the nuclear industry. The research on water radiolysis has produced an adequate understanding of the decomposition of water and aqueous solutions1−3 and has further progressed to broader interests concerning the interactions between ionizing radiation and materials, which are individually connected to specific applications. One of these subjects is radiolysis in solid/liquid systems, where an understanding of interfacial phenomena is essential.4,5 Ionization and excitation by radiation stimulate energy/charge transfer across the solid/liquid interface to decompose water molecules and the interface can mediate the reactions of electrons and radicals.6−16 The H2 production, in principle, reflects these interfacial phenomena. This study focused on zeolite/water mixtures. Zeolites are crystalline aluminosilicates that have nanometer-scale porous frameworks and exchangeable cations in extraframework positions. The radiolysis of zeolite/water systems also has a specific importance in the application of zeolites as ionexchange adsorbents in treating radioactively contaminated water.17−19 During the treatment and storage of spent zeolite adsorbents, it has been found that after the treatment water radiolysis produces H2. Appropriate estimation and control of © 2017 American Chemical Society
H2 are indispensable for the safe operation of the treatment process. Previous studies dealing with the radiolysis of zeolites with adsorbed water agree to the point that irradiation of the solid matrices of the zeolites induces decomposition of water in the pores resulting in H2 production.20−24 The energy/charge transfer from the zeolites was confirmed by pulse radiolysis studies of excess electrons in hydrated zeolites.25−27 The pulse radiolysis studies concluded that generation of excess electrons hydrated in the zeolite pores occurred at yields greater than hydrated electron (e−aq) during the radiolysis of bulk water. The high yields of e−aq would qualitatively explain the H2 production by the irradiation of hydrated zeolites because e−aq is a main precursor of H2 in water radiolysis.28,29 However, understanding radiation-induced reactions in zeolites is not sufficient to estimate the yield of H2 because the reaction scheme of H2 production is not well-defined.30 The production of H2 must be dependent on the yield of the reaction process from the precursors. However, the reaction scheme has been assumed to be similar to the spur reaction process in water radiolysis, where H2 is produced by the reaction between the products of water radiolysis, such as e−aq. Previous studies of the radiation-induced reactions did not focus on the effects of Received: April 14, 2017 Revised: August 1, 2017 Published: August 3, 2017 18525
DOI: 10.1021/acs.jpcc.7b03532 J. Phys. Chem. C 2017, 121, 18525−18533
Article
The Journal of Physical Chemistry C
Figure 1. Structures of the zeolites: (a) A-type zeolite (framework type code: LTA), (b) X- and Y-type zeolites (framework type code: FAU), and (c) mordenite-type zeolite (framework type code: MOR). The illustrations show wireframes connecting the positions of Si or Al, and oxygen atoms are left out for clarity.
2. EXPERIMENTAL SECTION 2.1. Materials. The zeolites used in this study were synthetic zeolites with sodium (Na+) as extraframework cation in the A-type (powder
