Photocatalytic titanates

Recently, we have shown that the combination of barium tetratitanate, BaTi40g and sodium hexatitanate, NagTigOis, with ruthenium oxides leads to active photocatalysts for water decomposition[1,2]. The unique feature of these photocatalysts is that no reduction of the titanates is required to be activated this is intrinsically different from conventional photocatalysts using TIO2 which are often heat-treated in a reducing atmosphere. Such different photocatalytic characteristics suggest that efficiency for the separation of photoexcited charges (a pair of electrons and holes) which is the most important step in photocatalysis is... 

Since BaTi40g and Na2TieOi3 have a pentagonal prism tunnel and a rectangular tunnel structure, respectively, we have paid attention to the role of the tunnel structures in photocatalysis. A barium titanate, Ba4Tii303o, in a series of Ba-Ti titanates was chosen as a representative with non-tunnel structure, and we have compared the photocatalytic activity and the ability for the production of surface radical species with uv irradiation between the tunnel and the non-tunnel titanates. 

The extension of this approach to artificial leaves based on titanates, niobates, tantalates, metal nitrides and phosphides, metal sulfides, and other transition metal oxides appears possible and useful in order to enhance the photocatalytic efficiency. In addition, the construction of multicomponent systems such as Ti02-CdS or MoS2-CdSe for overall water splitting could also lead to further improvements. This... 

Domen, K., Kudo, A., Onishi, T., Kosugi, N., and Kuroda, H., Photocatalytic decomposition of water into hydrogen and oxygen over nickel (II) oxide-strontium titanate (SrTiOj) powder. 1. Structure of the catalysts, ]. Phys. Chem., 90, 292,1986. 

Inoue, Y., Kubokawa, T., and Sato, K., Photocatalytic activity of alkali-metal titanates combined with Ru in decomposition of water, /. Phys. Chem., 95, 4059,1991. 

Kohno, M., Kaneko, T., Ogura, S., Sato, K., Inoue, Y. 1998. Dispersion of ruthenium oxide on barium titanates (BasTij,0. , Ba.,Tij303, BaTi.,0, and BajTi,Oj, ) and photocatalytic activity for water decomposition. J Chem Soc Faraday Trans 94 89-94. 

Wagner FT, Somerjai GA (1980) Photocatalytic and Photoelectrochemical hydrogen production on strontium titanate single crystals. J Am Chem Soc 102 5494-5502... 

Fig. 15.11 A. model of photocatalytic active sites of Ru02-loading BaTi409 and M2Ti6013(M = Na,K,Rb) titanates for water decomposition.

In the development of meta oxiae photocata-ysts with high and stable photocatalytic activity for water decomposition, the establishment of a correlation betweer photocatalytically active sites and metal oxide structures is desirable. In particular, it is important to see how the local structures of metal oxides are associated with the essential steps such as photoexcitation, the transfei of excited charges to the surface, and reduction/oxidation of adsorbec reactants. This chapter deals with photolysis of water by titanates with tunnel structures. The roles of tunnel-related local structures ir the photocatalysis and of Ru02 promoters loaded on the titanates are presented. 

Fig. 15.3 Comparison of photocatalytic activity on Ru02/barium titanates of (a)Ba6Tii70, (b)Ba4Tiu03o, (c) BaTi409 and (d) Ba2Ti9020.

Photocatalytic and Photoelectrochemical Hydrogen Production on Strontium Titanate Single Crystals. Both metal-free and platinized samples studied in aqueous alkaline electrolytes or in the presence of NaOH-coated crystals. 391... 

Apart from oxides such as TiO2, ZnO, and ZrO2, sulfides (typically CdS), nio-bates, tantalates, titanates, and other compounds have been tested as photocatalytic materials for splitting water. Composites (Kida et al., 2003 Rincon et al., 2007) and porous materials (Velaski et al., 2006) have been introduced as electrode materials in photovoltaic cells in the last years. 

Nano-composite photocatalysts, CuOx-TiOa, were synthesized from Cu(OAc)2-intercalated fibrous layered titanates by thermal decomposition in different atmospheres (N2, air, and H2). The structural characterization using XRD, UV-vis, XPS, and SEM implied that the composite of partially reduced CuOx and anatase-t)q3e Ti02 in a waffle-like texture would be a reason for the excellent photocatalytic activity for H2 production from CH3OH/H2O mbttures. 

Structure and photocatalytic property of Cu-titanate nano-composites... 

Table 4 shows the photocatalytic activity of CuOx supported on the pristine samples of layered titanates. The loading (26 wt% Cu) corresponds to the Cu/Ti ratio (0.33) for the composite prepared by the intercalation process. The two pristine layered titanates showed very low activity without loading CuOx. In contrast, the activity of the CuOx-loaded samples was quite different the protonated phase produced ca. 18-times higher rate of Ha evolution. Copper oxides in these materials were deposited only on the surface of the fibrous crystals. However, the protonated phase was decomposed to produce anatase-type TiOa as in the case of the intercalated composite (Fig. 5). This result also supports that the formation of anatase is essential for the photocatalytic activity. 

On this basis, the chapter will discuss examples of materials containing perov-skites and related oxides with photocatalytic activity. More specifically, the structural and morphological characteristics of titanates, niobates, tantalates, ferrites, aluminates, and transition metal-lanthanide mbced oxides will be evaluated in a close relation with their photocatalytic performances. 

Perovskite structures of rare earth titanate oxides are of considerable interest in recent years [63,64], Among different rare earth metals, lanthanum was the most investigated. In order to accommodate the excess oxygen, the replacement of divalent cations such as Ca or Sr in general perovskite materials by a tri-valent cation such as La " resulted in excess electrons and slabs of a distorted perovskite structure. With the highly donor-doped (110) perovskite, the separation of electrons and holes is more facile than other semiconductor materials because of a narrower depletion layer and a more drastic band bending. In this way the activity of the photocatalytic processes were improved. As obviously, to suppress the recombination of the generated electrons and holes is an important condition [63,64]. 

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