Visible light-assisted reduction of CO2 into formaldehyde by heteroleptic ruthenium metal complex–TiO2 hybrids in an aqueous medium

Abstract

The photocatalytic reduction of CO₂ with its simultaneous functionalization is a profound journey to achieve under an ambient condition. In the current research, precedence exists for the formation of HCHO, HCOOH, CO, CH₄, and CH₃OH after the reduction of CO₂ under suitable conditions. In this progression, HCHO is considered to be a reactive molecule, which occurs in the photocatalysis under suitable condition observed in the photocatalytic process. Herein, we report CO₂ reduction to formaldehyde via heterogeneous photocatalysis in an aqueous medium at pH 7. The as-synthesized hybrid photocatalyst is capable of being active under visible light (λ > 420 nm) by utilizing the heteroleptic ruthenium metal complex over TiO₂ nanoparticles via covalent interactions. The major reaction product was identified as formaldehyde, while trace amounts of CO and CH₄ were also detected in the presence of triethanolamine (TEOA) as a sacrificial donor. The maximum turnover number (720) for HCHO was obtained based on the metal complex used over the surface after 5 h visible light irradiation. Furthermore, formaldehyde (in situ) was utilized for the reaction with primary amines (aniline, 4-aminobenzoic acid) to form the corresponding imines under visible light. Directed by mechanistic studies, the results indicate for the first time that the C₁ reduced product of CO₂ in a heterogeneous medium can be utilized for synthesis of useful products.