Electron- and hole-capture reactions on Pt/TiO₂ photocatalyst exposed to methanol vapor studied with time-resolved infrared absorption spectroscopy

The decay kinetics of photogenerated electrons in a Pt/TiO₂ catalyst has been studied by time-resolved infrared absorption (TR-IR) spectroscopy. The electron decay was drastically affected by exposure to methanol vapor, and the electron-hole pair recombination was prevented. A certain fraction of the electrons remained beyond 1 s and slowly decayed from ∼1-9 s. The prevented recombination was interpreted with the effective capture of the holes. The extent of the hole capture was insensitive to the pressure of methanol vapor. An irreversibly adsorbed methoxy groups was proposed to capture the holes on the basis of the steady-state FT-IR spectra of the catalyst exposed to methanol vapor of different pressures. However, the slow decay of the electrons exhibited a positive-order response to the methanol pressure. Undissociated methanol physisorbed and equilibrated with the gas phase, which was identified in the steady-state spectra, should play a key role in the electron capture process. Possible mechanisms of the process are discussed.