g-C3N4 Decorated TiO2 Nanotube Ordered Thin Films as Cathodic Electrodes for the Selective Reduction of Oxalic Acid

A new value chain in carbon dioxide utilization is the production of high-added-value C2 products from CO2 via oxalic acid as intermediate. We report here the use of novel g-C3N4 decorated TiO2 nanotube ordered thin films as cathodic electrodes for the selective reduction of oxalic acid to high-added-value chemicals such as glyoxylic acid and glycolic acid. These electrodes have been tested in a three-electrode electrocatalytic cell, using 0.2 M Na2SO4 as electrolyte, at applied potentials of -1.1 and -1.3 V vs Ag/AgCl. The cathode side of the continuous electrocatalytic cell is feed with a solution of 0.03 M oxalic acid (OX). Glycolic acid (GC) and glyoxylic acid (GO) have been obtained as the main reaction products. g-C3N4 was deposited on the pristine TiO2 nanotube ordered array thin film (on a metallic Ti substrate acting as conductive element) by a modified chemical vapor deposition technique, varying the experimental parameters to obtain a series of materials. The pristine TiO2 nanotubes gives faradaic efficiency (FE) to GO and GC of 24%, and 34%, respectively, while after decoration of TiO2 nanotubes with g-C3N4 a large change in FEs is observed, depending on the specific characteristics of deposition, while the current density remains nearly constant. The FE to GC reaches at the best a value of 76% while the FE to GO is lowered to 12%. The preliminary interpretation of this drastic modification of the behaviour is related to the effect of g-C3N4 induced on the properties of TiO2 nanostructured film.