Alanine Formation in a Zero-Gap Flow Cell and the Role of TiO2/Ti Electrocatalysts.

The electrochemical synthesis of α-amino acids at room temperature and pressure is a sustainable alternative to conventional methods like microbial fermentation and Strecker synthesis. A custom-built zero-gap flow electrolyzer was used to study the electrosynthesis of alanine via the electrocatalytic reductive amination (ERA) of the corresponding biomassderivable α-keto acid precursor - pyruvic acid (PA), and hydroxylamine (NH2OH) at very low pH. Non-toxic, abundant, and easy to prepare TiO2/Ti electrocatalysts were utilized as the cathode. Three TiO2/Ti felt electrodes with different oxide thicknesses were prepared and their characterization results were correlated with their respective electrochemical performance in terms of Faradaic efficiency η, and partial current density |¯j|. Cyclic voltammetry indicated a different electrocatalytic reduction process on hydrothermally treated electrodes, compared to thermally oxidized ones. Hydrothermally treated electrodes were also found to have the thickest porous anatase TiO2 layer and achieved 50-75% alanine conversion efficiencies. Optimization showed that the cell potential, reactant flow rate and the PA:NH2OH ratio were crucial parameters in determining the conversion efficiency. η and |¯j| were found to significantly decrease when an excess of NH2OH is used and, an optimal alanine η of 75% was achieved at 2.0 V applied cell potential and 10 mL/h reactant flow rate.