Enhanced photocatalytic reduction reaction over Bi³⁺-TiO₂ nanoparticles in presence of formic acid as a hole scavenger

A series of Bi³⁺-doped TiO₂ (Bi³⁺-TiO₂) catalysts with a doping concentration up to 2 wt% were prepared by a sol-gel method. The prepared photocatalysts were characterized by different means to determine their chemical composition, surface structure and light absorption properties. The photocatalytic activity of different Bi³⁺-TiO₂ catalysts was evaluated in the photocatalytic reduction of nitrate in aqueous solution under UV illumination. In the experiments, formic acid was used as a hole scavenger to enhance the photocatalytic reduction reaction. The experiments demonstrated that nitrate was effectively degraded in aqueous Bi³⁺-TiO₂ suspension by more than 83% within 150 min, while the pH of the solution increased from 3.19 to 5.83 due to the consumption of formic acid. The experimental results indicate that the presence of Bi³⁺ in TiO₂ catalysts substantially enhances the photocatalytic reaction of nitrate reduction. It was found that the optimal dosage of 1.5 wt% Bi³⁺ in TiO₂ achieved the fastest reaction of nitrate reduction under the experimental condition. Bismuth ions deposit on the TiO₂ surface behaves as sites where electrons accumulate. Better separation of electrons and holes on the modified TiO₂ surface allows more efficient channeling of the charge carriers into useful reduction and oxidation reactions rather than recombination reactions. Two intermediate products of nitrite and ammonia during the reaction were also monitored to explore the possible mechanisms of photoluminescence quenching and photocatalytic reduction in the context of donor-acceptor interaction with electron trapping centers.