Photocatalytic properties of M_2Ti_6O_(13) (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

摘要

The photocatalytic properties of RuO_2-dispersed M_2Ti_6O_(13) (M = Na, K, Rb, Cs) were investigated as to the photoexcitation ability of the hexatitanates and the role of the dispersed RuO_2. 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 H_2, He, N_2, O_2 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 TiO_6 octahedra forming the rectangular tunnels of M_2Ti_6O_(13) (M = Na, K, Rb). RuO_2 was dispersed on the hexatitanates by impregnation with either RuCl_3(aq) solution or Ru_3(CO)_(12) in tetrahydrofuran and the subsequent oxidation in the temperature range 573-823 K. The photocatalytic activity for water decomposition was higher for RuO_2/M_2Ti_6O_(13) (M = Na, K, Rb) photocatalysts prepared using Ru_3(CO)_(12) that for RuCl_3. High resolution transmission electron microscopic images showed that uniformly distributed smaller RuO_2 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 M_2Ti_6O_(13).