Kinetics of the photocatalytic water-splitting reaction on TiO₂ and Pt/tiO₂ studied by time-resolved infrared absorption spectroscopy

The present account describes how the water-splitting reaction on TiO₂ (P-25) and Pt/TiO₂ catalysts is traced by time-resolved infrared (IR) absorption spectroscopy. The ac-coupled amplification of the IR signal allows detection of transient absorbance-change as small as 10^-6 with a time-resolution of 50 ns. The TiO₂ or Pt/TiO₂ catalyst irradiated by a 355 nm UV-pulse at time = 0 presents a transient IR absorption which monotonically increases in intensity with decreasing wavenumber from 3000 to 1000 cm^-1. Photogenerated electrons trapped in shallow mid-gap states are proposed to cause the absorption. The decay kinetics of the electrons traced by monitoring the IR absorbance at 2000 cm^-1 is sensitive to vapor atmospheres. The electrons recombine with the complementary holes along a multi-exponential rate law in the catalysts placed in a vacuum. Dioxygen arriving from the gas-phase captures the electrons at delay time of 10-100 μs following the UV irradiation. In the presence of water vapor, holes in the TiO₂ catalyst are captured by an adsorbed reactant (probably hydroxyl species) within 2 μs and the recombination is obstructed thereafter. The excess electrons cannot transfer to another adsorbate to be reduced (probably proton) and hence reduce the catalyst itself. On the Pt/TiO₂ catalyst exposed to water, the electron transfer takes place at 10 μs or later, following the hole transfer completed within 2 μs. The hole transfer on Pt/TiO₂ is insensitive to the pressure of water vapor whereas the rate of the electron transfer is enhanced with the increasing pressure at 1-10 Torr. Transient response of an adsorbate vibration is also observed. The O-H stretching band of an adsorbed hydroxyl species at 3677 cm^-1 thermally shifted to the low-wavenumber side, when the TiO₂ catalyst is irradiated by the UV-pulse.