Selective Oxidation of Glycerol into Formic Acid by Photogenerated Holes and Superoxide Radicals

Abstract

Photocatalysis is a promising technology for conversion of the glycerol into formic acid, but photocatalytic oxidation of C-C bonds in glycerol exhibits poor selectivity towards formic acid because the photogenerated radicals (e.g., hydroxyl radicals) further oxidize formic acid to CO₂ . In this work, a synergy of photogenerated holes and superoxide radicals that achieved the selective oxidation of glycerol into formic acid over the TiO₂ catalyst was revealed. The charge separation of pristine TiO₂ was improved with the aid of oxygen, which resulted in efficient hole oxidation of the C-C bonds in glycerol to formic acid. Surface active species were controlled to prevent being converted to hydroxyl radicals on TiO₂ by controlling the oxygen and water contents, which solved the problem of formic acid peroxidation without sophisticated catalyst modifications. Mechanism studies suggested that glyceraldehyde and glycolaldehyde were the intermediates to generate formic acid. This work provides a green and efficient approach to produce formic acid as a liquid hydrogen carrier from bio-based alcohols.

Keywords: formic acid; glycerol; hydroxyl radicals; photocatalysis; superoxide radicals.