Photocatalytic properties of M2Ti6O13 (M=Na, K, Rb, Cs) with rectangular tunnel and layer structures: Behavior of a surface radical produced by UV irradiation and photocatalytic activity for water decomposition

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Shuji Ogura, Mitsuru Kohno, Kazunori Sato and Yasunobu Inoue


Abstract

The photocatalytic properties of RuO2-dispersed M2Ti6O13 (M=Na, K, Rb, Cs) were investigated as to the photoexcitation ability of the hexatitanates and the role of the dispersed RuO2. An EPR signal with g=2.020, g=2.018 and g=2.004 produced by UV irradiation of the hexatitanates at 77 K had a short life time in vacuum but remained stable upon exposure to gaseous molecules such as H2, He, N2, O2 and Ar. These gases brought about different stability and pressure dependence of the signal intensity, by which the radical is assigned to O- resulting from the surface lattice oxygen. The efficiency of the radical formation is associated with the presence of internal fields in the distorted TiO6 octahedra forming the rectangular tunnels of M2Ti6O13 (M=Na, K, Rb). RuO2 was dispersed on the hexatitanates by impregnation with either RuCl3(aq) solution or Ru3(CO)12 in tetrahydrofuran and the subsequent oxidation in the temperature range 573–823 K. The photocatalytic activity for water decomposition was higher for RuO2/M2Ti6O13 (M=Na, K, Rb) photocatalysts prepared using Ru3(CO)12 than for RuCl3. High resolution transmission electron microscopic images showed that uniformly distributed smaller RuO2 particles were formed for the former photocatalyst, which is considered to be responsible for the higher activity. The correlation between the concentration of the O- radical and photocatalytic activity is established, thus indicating that the O- radical is proposed to work as a hole site in the photocatalytic reaction, and the ability to separate photoexcited charges reflects the photocatalytic activity order of M2Ti6O13.


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