Cathodic Hydroxide Ions Induce Tetrose Formation during Glycolaldehyde Electroreduction to Alcohols: A Potential CO2-to-Carbohydrate Pathway.

The electrochemical synthesis of organic compounds from CO2 can potentially alleviate climate change by hampering the atmospheric accumulation of greenhouse gases. The production of carbohydrates from CO2 reduction will have promising applications for the manufacturing of valuable, multi-carbon compounds that are traditionally produced from the petrochemical or agricultural industries. In this work, we analyzed the copper-catalyzed electrochemical reduction of glycolaldehyde, a commonly observed trace CO2RR product that has been previously proposed as an intermediate for alcohol formation. We determine that glycolaldehyde is not the main intermediate on polycrystalline copper-based electrocatalysts that selectively produce ethanol. In an unbuffered electrolyte, the cathodic hydroxide ions produced induce the coupling of glycolaldehyde to tetroses in the solution phase, yielding a maximum glycolaldehyde-to-sugar conversion of 47.2% under ambient conditions. Using in situ infrared spectroscopy coupled with density functional theory (DFT) calculations, we show that glycolaldehyde reduction to alcohols proceeds via adsorption of its enol tautomer, η2(C,C)─CHOH═CHOH. Our findings not only shed light on the C2 alcohol formation pathways during CO2RR, but also imply that a CO2 electrolyzer can potentially produce C4 carbohydrates via CO2 reduction to glycolaldehyde followed by C─C coupling in the solution phase, with only a high local pH needed to drive the tetrose formation step.