Green synthesis of glycolic acid through the electrocatalytic reduction of oxalic acid over black TiO2: An experimental and theoretical study

Herein, we present the electrocatalytic four-electron hydrogenation of oxalic acid into glycolic acid using black TiO₂ as an electrocatalyst. Oxalic acid is an abundant compound found in several sources of organic waste. The results showed a high selectivity of black TiO₂ toward glycolic acid, with the formation of glyoxylic acid being the rate-limiting step (glyoxylic acid is the two-electron intermediate). The highest Faradaic efficiency (FE) of 69.6% ± 8.3% was achieved at 10.2 mA cm⁻² in 4 h of electrolysis using an H-type cell operated at room temperature, with 50.2% ± 3.8% of oxalic acid conversion (degradation kinetic constant k = 0.0042 ± 0.0001 min⁻¹), 58.8% ± 7.0% of reaction yield and 1.2 ± 0.18 g L⁻¹ of glycolic acid production. A theoretical model of black TiO₂ coming from anatase TiO₂ was implemented by introducing Ti³⁺ defects, which gave black TiO₂ the theoretical capability to easily transform oxalic acid into glycolic acid as experimentally observed. The reaction mechanism was supported and described in detail by density functional theory calculations, which revealed that surface Ti³⁺ states were the main catalytic sites. This is the first time that a detailed step-by-step mechanism at the atomic level has been proposed for this electrocatalytic reaction, which represents a valuable contribution to the understanding of this process of high energy/environmental interest. This is also the first time that black TiO₂ has been used as an electrocatalyst for this sustainable process.