Preparation of porous niobium oxide by the exfoliation of K 4 Nb 6 O 17 and its photocatalytic activity
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Preparation of porous niobium oxide by the exfoliation of K4 Nb6 O17 and its photocatalytic activity Ryu Abe Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan
Kiyoaki Shinohara and Akira Tanaka Nikon Corp., 1-10-1 Asamizodai, Sagamihara 228, Japan
Michikazu Hara, Junko N. Kondo, and Kazunari Domena) Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan (Received 17 September 1997; accepted 5 January 1998)
A new porous material was prepared from a layered compound, K4 Nb6 O17 , through the exfoliation of its layers. A composite of the niobate sheets and MgO particles were obtained by precipitating the exfoliated two-dimensional niobate sheets with MgO fine particles. Porous niobium oxide was obtained by removal of the MgO particles from the composite after thermal treatment. It had a large surface area and showed higher photocatalytic activity than the original H+yK4 Nb6 O17 for H2 evolution from various aqueous alcohol solutions.
I. INTRODUCTION
K4 Nb6 O17 is known as a typical photocatalyst for decomposition of water into H2 and O2 .1–6 As shown in Fig. 1, K4 Nb6 O17 consists of octahedral units of NbO6 which form a two-dimensional layered structure via bridging oxygen atoms.7,8 The layers are negatively charged, and K+ ions exist between the layers to compensate for the negative charges of the layers. There exist two types of interlayer (interlayer I and II) alternately, which are effectively utilized as reaction sites. The high photocatalytic activity of K4 Nb6 O17 compared with other “bulk type” photocatalysts is a typical consequence. However, this catalyst has some problems for application to other reaction systems. One is a poor reactivity for bulky reactants. Because of the well-ordered stacking of niobate sheets, it is difficult for bulky reactants to approach to the active sites at the interlayer space. Therefore, the efficient photocatalysis of K4 Nb6 O17 is limited to small molecules such as H2 O and MeOH, and an obvious decrease in photoreactivity was found for bulky reactants. Therefore, we attempted a synthesis of porous catalyst from K4 Nb6 O17 by means of the exfoliation of niobate sheets. Recently, it has been reported that several classes of layered compound can be exfoliated by some soft-chemical treatment. K4 Nb6 O17 also can be exfoliated with tetra-n-butylammonium hydroxide (TBA+ OH– ) as reported by Mallouk et al.9,10 The exfoliated niobate sheets could be precipitated by addition of cations such as H+ .11 We attempted to use a)
Address correspondence to this author. J. Mater. Res., Vol. 13, No. 4, Apr 1998
inorganic fine particles instead of cations when they were precipitated in order to make porous space among exfoliated niobate sheets. II. EXPERIMENT
K4 Nb6 O17 was prepared according to the previous report.8 The proton exchange of K4 Nb6 O17 was carried out in 0.5 molyl of aqueous sulfuric acid
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