Parameter Effects and Reaction Pathways of Photoreduction of CO2 over TiO2/SO42– Photocatalyst

The feasibility of applying a modified acidic photocatalyst (TiO2/SO42–) to reduce carbon dioxide was investigated. The photocatalytic reduction of CO2 was conducted in a bench-scale batch photocatalytic reactor. Three near-UV black lamps with a maximal spectrum wavelength of 365 nm were assembled on the top of the reactor to provide an average irradiation intensity of 2.0 mW/cm2. The TiO2/SO42– photocatalyst was prepared by a modified sol–gel process and coated on stainless steel substrates for the reduction of CO2. Experimental parameters such as reductants, the initial CO2 concentration, and the reaction temperature were investigated. The results indicated that the highest photoreduction rate of CO2 was observed using H2 as a reductant over TiO2/SO42–. The major gaseous products from CO2 photoreduction were carbon monoxide and methane, while other minor products of ethene and ethane were also detected. The photoreduction rate of CO2 was increased with initial CO2 concentration and reaction temperature, which promoted the formation of products. Furthermore, the FT-IR spectra showed that formic acid, methanol, carbonate ions, formaldehyde, and methyl formate formed on the surface of TiO2/SO42– photocatalyst. Two reaction pathways of CO2 photoreduction over TiO2/SO42– were proposed. One reaction pathway described the formation of gaseous products CO, CH4, C2H4, and C2H6. The other reaction pathway formed CO3ads2–, CH3OHads, HCOOads−, HCOOHads, HCOHads, and HCOOCH3ads on the surface of the TiO2/SO42– photocatalyst.