In situ DRIFTS study of photocatalytic CO ₂ reduction under UV irradiation

Photocatalytic reduction of CO ₂ on TiO ₂ and Cu/TiO ₂ photocatalysts was studied by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) under UV irradiation. The photocatalysts were prepared by sol-gel method via controlled hydrolysis of titanium (IV) butoxide. Copper precursor was loaded onto TiO ₂ during sol-gel procedure. A large amount of adsorbed H ₂O and surface OH groups was detected at 25°C on the TiO ₂ photocatalyst after being treated at 500°C under air stream. Carbonate and bicarbonate were formed rapidly due to the reaction of CO ₂ with oxygen-vacancy and OH groups, respectively, on TiO ₂ surface upon CO ₂ adsorption. The IR spectra indicated that, under UV irradiation, gas-phase CO ₂ further combined with oxygen-vacancy and OH groups to produce more carbonate or bicarbonate. The weak signals of reaction intermediates were found on the IR spectra, which were due to the slow photocatalytic CO ₂ reduction on photocatalysts. Photogenerated electrons merge with H ⁺ ions to form H atoms, which progressively reduce CO ₂ to form formic acid, dioxymethylene, formaldehyde and methoxy as observed in the IR spectra. The well-dispersed Cu, acting as the active site significantly increases the amount of formaldehyde and dioxymethylene, thus promotes the photoactivity of CO ₂ reduction on Cu/TiO ₂. A possible mechanism of the photocatalytic CO ₂ reduction is proposed based on these intermediates and products on the photocatalysts.