VOLUME 105, NUMBER 26, JULY 5, 2001
© Copyright 2001 by the American Chemical Society
LETTERS New Photocatalyst Group for Water Decomposition of RuO2-Loaded p-Block Metal (In, Sn, and Sb) Oxides with d10 Configuration J. Sato, N. Saito, H. Nishiyama, and Y. Inoue* Department of Chemistry, Nagaoka UniVersity of Technology, Nagaoka 940-2188, Japan ReceiVed: March 1, 2001; In Final Form: May 2, 2001
For the decomposition of pure water to hydrogen and oxygen, MIn2O4 (M ) Ca, Sr), Sr2SnO4, and NaSbO3 were found to make stable photocatalysts when combined with RuO2. The metal oxides are composed of the d10 p-block metal ions (In3+, Sn4+, and Sb5+), and it is demonstrated that the p-block metal oxides with d10 configuration form a new photocatalyst group different from the conventional metal oxide photocatalyst group consisting of the octahedrally coordinated d0 transition-metal ions such as Ti4+, Zr4+, Nb5+, and Ta5+.
In view of the current importance of hydrogen as a clean chemical source, the discovery of new kinds of the photocatalysts for water decomposition is among very important issues. Extensive research has so far been performed, but stable photocatalysts found in the past two decades for the overall splitting of pure water to produce hydrogen and oxygen are confined to the transition metal involving Ti4+, Zr4+, Nb5+, and Ta5+; SrTiO3 1, A2Ti6O13 (A ) Na, K, Rb),2,3 BaTi4O9,4,5 A2La2Ti3O10 (A) K, Rb, Cs),6,7 ZrO2,8 A4Nb6O17(A ) K, Rb),9 Sr2Nb2O7,10 ATaO3 (A ) Na, K),11,12 MTa2O6 (M ) Ca, Sr, Ba),13,14 and Sr2Ta2O710 were active when combined with NiO or RuO2 as a promoter. It should be noted that these metal oxides are composed of the octahderally coordinated d0 transition-metal ions. No stable photocatalysts with electronic configuration other than d0 have been known. It will be greatly beneficial to demonstrate that metal oxides having other electronic structures are useful for photoassisted water decomposition. The fundamental steps for water decomposition by metal oxide photocatalysts are (i) the generation of photoexcited charges, (ii) the separation of the charges without recombination, and (iii) transfer to metal oxide surfaces where the reduction and the oxidation of adsorbed species take place. The first and second step are strongly associated with the electronic structures * To whom correspondence should be addressed.
of the metal oxides, whereas the promoters, usually fine particles loaded on the metal oxide surfaces, are used to accelerate the third step. In a recent development of photocatalyts for water decomposition, a target has been placed on transition-metal oxides with complicated crystal structures in an attempt to promote the first and second step on the basis of the role of their structures in photexcitation and charge separation. A layer structure A4Nb6O17 (A ) K, Rb) consisting of macropolyanion sheets of niobates (N6O174-) has anisotropy along the stacking direction and alternatively arranged two different types of the layers. The fine Ni particle as a promoter was intercalated in one of the layers at which H2 evolution was accelerated, whereas the other layer produced O29. The layer structures were shown to have the functions of promoting the first and second step in the photocatalysis. In tunnel structural BaTi4O9 and A2Ti6O13 (A ) Na, K, Rb), the TiO6 octahedra of the metal oxides were so heavily distorted that the position of Ti4+ deviated from the center of gravity of surrounding the sixoxygen ion.15-17 The out-of-center Ti ions generated a dipole moment that worked as local fields in the interior of TiO6 octahedra. The fields had significant effects on the formation of photoexcited charges. In fact, a stable radical was observed by the EPR signal when irradiated with UV light at 77 K in the presence of gaseous molecules such as Ar, He, O2, and H2.18,19 A good correlation existed between the photocatalytic activity
10.1021/jp010794j CCC: $20.00 © 2001 American Chemical Society Published on Web 06/13/2001
6062 J. Phys. Chem. B, Vol. 105, No. 26, 2001
Figure 1. Evolution of H2 and O2 from pure water on 1 wt % RuO2loaded CaIn2O4 (a), RuO2-loaded Sr2SnO4 (b), and RuO2-loaded NaSbO3 (c). Xe lamp irradiation for a and Hg-Xe lamp irradiation for b and c.
and the radical concentration, thus indicating that the distorted TiO6 octahedra of the tunnel structures promoted the first and second steps.20 It can be summarized that the recent work on photocatalysts for water decomposition has focused on the geometric structural features of the transition-metal oxides. Putting emphasis on the electronic structures of metal oxides, we have examined p-block metal (In, Sn, and Sb) oxides and discovered that MIn2O4 (M ) Ca, Sr), Sr2SnO4, and NaSbO3 were active for the overall splitting of water when combined with RuO2. In the synthesis of MIn2O4 (M ) Ca, Sr, Ba), the coprecipitate was first prepared: alkaline metal or alkaline earth metal nitrates and indium nitrate were dissolved in a water-ethanol mixture, to which an oxalic acid ethanol solution was added. The precipitate was aged at 353 K, dried at 393 K, and calcined at 1423 K for 16 h. For the preparation of Sr2SnO4, a 2:1 molar ratio mixture of SrCO3 and SnO2 was calcined at 1373 K for 16 h. NaSbO3 was prepared by calcination of a 1:1 molar mixture of Na2 (CO3) and Sb2O5 in air at 1173 K for 16 h. The formation of the metal oxides was confirmed by their X-ray diffraction patterns. The p-block metal oxides thus prepared were impregnated up to incipient wetness with ruthenium carbonyl complex, Ru3 (CO)12, in tetrahydrofuran, dried at 353 K and oxidized to produce RuO2 in air at 673 K for 5 h. The photocatalytic reaction was carried out in a gas circulation reaction system. About 250 mg of powder photocatalysts was placed in a quartz reaction cell filled with ca. 20 cm3 of distilled and ion-exchanged pure water. Ar gas of 13.3 kPa was circulated with a piston pump during the reaction. The powder photocatalysts were dispersed in the water by the stirring of Ar gas bubbling and illuminated by an outer Xe or Hg-Xe lamp usually operated at 400 and 200 W, respectively. The evolved gases were analyzed by an on-line gas chromatograph. Figure 1 shows the evolution of hydrogen and oxygen from pure water on 1 wt % RuO2-loaded CaIn2O4, Sr2SnO4, and NaSbO3. For all three samples, UV irradiation produced H2 and O2 from the initial stage and their products increased with increasing time. In the repeated run on RuO2-loaded CaIn2O4, the production of H2 and O2 occurred in a similar manner and no deterioration of the photocatalytic activity was observed. For RuO2-loaded NaSbO3, the evolution of H2 was somewhat larger in the initial stage, but H2 and O2 increased linearly with irradiation time in later reaction time. For these metal oxides, little production of H2 and O2 was observed in the absence of RuO2.
Letters
Figure 2. Photocatalytic activity of 1 wt % RuO2-loaded MIn2O4 (M ) Ca, Sr, Ba) under Xe lamp irradiation.
Figure 3. UV diffuse reflectance spectra of MIn2O4 (M ) Sr, Ba).
Figure 2 shows the photocatalytic activity of 1 wt % RuO2loaded MIn2O4 (M ) Ca, Sr, Ba) with different alkaline earth metal atoms. RuO2-loaded SrIn2O4 showed the production of both H2 and O2, although its activity was smaller by a factor of 4.3 than that of RuO2-loaded CaIn2O4, whereas RuO2-loaded BaIn2O4 exhibited little activity. This indicates that the origin of the active sites is similar in CaIn2O4 and SrIn2O4 but intrinsically different from that of BaIn2O4. Under similar reaction conditions, the photocatalytic activity of RuO2-loaded CaIn2O4 was approximately one-half that of RuO2-loaded BaTi4O9 and similar to that of RuO2-loaded Na2Ti6O13, which are the representative d0 metal oxide photocatalysts.4,19 Figure 3 shows the UV diffuse reflectance spectra of SrIn2O4 and BaIn2O4. Light absorption of SrIn2O4 began at around 430 nm and attained the maximum level at 300 nm. This absorption spectrum was nearly the same as that of CaIn2O4. On the other hand, the onset absorption wavelength of BaIn2O4 was around 430 nm and main absorption occurred at 400 nm, reaching the maximum level at 350 nm. The absorption characteristics were different between SrIn2O4 and BaIn2O4. SrIn2O4 has an orthorhombic structure21 with a unit cell of a ) 0.9809, b ) 1.1449, and c ) 0.3265 nm, whereas BaIn2O4 has a monoclinic
Letters structure22 with a ) 1.4432, b ) 0.5833, and c ) 2.0792 nm and β ) 110.02°. This indicates that the different structures between them are responsible for differences in absorption characteristics and photocatalytic activity. In a comparison of the active metal oxides for the overall splitting of water, one conventional group belongs to the transition-metal oxides with octahedrally coordinated d0 configuration, whereas the new other one belongs to the p-block metal oxides with octahedrally coordinated d10 configuration. It is interesting to note that both electronic structures of completely empty and filled d orbitals are associated with the generation of photocatalytic activity. This indicates that the d orbitals play an important role in photoexcitaion and photocatalytic activity. In the d0 transition-metal oxides, the conduction bands are formed by the empty d orbitals, whereas the valence bands are formed by the oxygen 2p orbitals. It appears that the absence of the d electrons near the valence bands leads to efficient electron transfer from the oxygen 2p bands to the conduction levels. On the other hand, the p block metal oxides are the oxides of post-transition metals, and an increase in the nuclear charges causes a stabilization of the d shell to an extent to which the d orbitals are regarded as core levels. The valence bands are composed essentially of the oxygen 2p orbitals, as with the d0 transition-metal oxides. The d orbital levels are so deep that the inner d electrons have little influence on electron transfer from the oxygen 2p bands to the conduction levels. Thus, no involvement of the d electrons in photoexcitation is similar in the d0 transition and d10 p-block metal oxides. Although it is needed to extend research to other d10 p-block metal oxides and, furthermore, to reveal the mechanism of photocatalysis based on their detailed electronic and geometric structures, the new concept can be established that octahedrally coordinated d10 p-block metal oxides are active for water decomposition. The present results are encouraging for the establishment of a new group of photocatalysts for the overall splitting of water.
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