Temperature Dependence of Photocatalytic Water Splitting under Visible Light Irradiation over Ir- and Sb-Codoped SrTiO₃:Al

Temperature dependence of photocatalytic water splitting over CrOx/Rh and CoOOH cocatalyst-coloaded SrTiO3:Al codoped with Ir and Sb single-particulate photocatalysts (CrOx/Rh/SrTiO3:Ir,Sb,Al/CoOOH) under visible light irradiation was investigated. When the reaction temperature was increased from 280 to 340 K, the activity became low, and degradation became significant. Moreover, the activity was partially revived with lowering reaction temperature from 340 to 290 K. These results suggest that reversible and irreversible negative factors worked with rising reaction temperature. The temperature dependence was caused by the working of the negative factors such as the irreversible degradation of photocatalyst · cocatalyst and the reversible increase of recombination probability of photogenerated carriers more than the positive factor such as increasing the rate of surface reaction. X-ray photoelectron spectra revealed that detachment and/or aggregation of CoOOH loaded on SrTiO3:Ir,Sb,Al brought the irreversible degradation of the photocatalyst material at 340 K. In contrast to it, the change of a CrOx shell in the CrOx/Rh cocatalyst was not observed, suggesting that the reverse reactions to form water from evolved H2 and O2 and to reduce O2 were not enhanced even though reaction temperature was risen. On the other hand, the photoluminescence intensity of Eu3+ as a guest doped to SrTiO3:Ir,Sb,Al as a host due to band gap excitation of SrTiO3 became low with rising reaction temperature, suggesting that the recombination probability as a nonradiative deactivation of photogenerated carriers in the host of SrTiO3 increased. Therefore, it was suggested that the negative factors such as degradation of a CoOOH cocatalyst and increasing recombination probability of photogenerated carriers caused the decrease of the photocatalytic water-splitting activity of CrOx/Rh/SrTiO3:Ir,Sb,Al/CoOOH.