Copper and iodine co-modified TiO ₂ nanoparticles for improved activity of CO ₂ photoreduction with water vapor

Copper and iodine co-modified TiO ₂ nanoparticles (Cu-I-TiO ₂) were synthesized through a combined hydrothermal and wet-impregnation process. The structures and properties of the catalysts were characterized by XRD, BET, SEM/EDX, XPS, and UV-vis diffuse reflectance spectroscopy. Iodine ions were doped in the TiO ₂ lattice by replacing Ti ⁴⁺ and, consequently, Ti ³⁺ was generated to balance the charge. Iodine doping reduced the TiO ₂ crystal size and was responsible for visible light absorption. Cu species were found to deposit on the surface of TiO ₂ and resulted in a slightly increased particle size. The activity of the Cu-I-TiO ₂ catalyst was investigated by the photocatalytic reduction of CO ₂ with water vapor, and CO was found to be the major reduction product with trace amounts of CH ₄ generated. Under UV-vis irradiation, the activity of the co-modified catalyst (Cu-I-TiO ₂) was higher than that of the single ion-modified catalysts (Cu-TiO ₂ or I-TiO ₂). Under visible light irradiation, the addition of Cu to I-TiO ₂ did not lead to significant improvements in CO ₂ reduction. Methyl chloride (CH ₃Cl) was detected as a reaction product when CuCl ₂ was used as the precursor in the synthesis, thus suggesting that methyl radicals are reaction intermediates. When CuCl ₂ was used as the Cu precursor, a three-fold increase in CO ₂ photoreduction activity was observed, as compared to when Cu(NO ₃) ₂ was used as the Cu precursor. These differences in activities were probably due to enhanced Cu dispersion and the hole-scavenging effects of the Cl ions. However, the formation of by-products (e.g., CH ₃Cl) may be undesirable.