Particle Size Effects in Au/TiO₂ Catalysts for the Oxidation of 5-Hydroxymethylfurfural

Gold nanoparticles are great catalysts for selective hydrogenation and oxidation reactions, such as for the oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid, an important reaction in the production of bio-based polymers and an excellent model reaction to evaluate alcohol and aldehyde oxidation performance. The gold particle size is crucial for catalytic applications, but its role in HMF and other liquid-phase oxidation reactions remains insufficiently understood. We report gold particle sizes between 2 and 20 nm within a series of Au/TiO₂ catalysts, made by deposition precipitation with varying heat treatment conditions and precipitation agents. Gold particles larger than 3 nm exhibit good stability, in contrast to even smaller particles that suffer from particle growth, which we attributed to increased solubility probability. The oxidation rate of the alcohol and aldehyde group of HMF depends on the gold particle size, as oxidation rates of both groups decreased with increasing gold particle size. Our 4 nm Au/TiO₂ catalyst outperformed catalysts containing larger gold particle sizes with 2.5 times faster HMF oxidation and 2 times faster oxidation rate of the intermediate 5-hydroxymethyl-2-furancarboxylic acid. Altogether, this paper contributes to the field of gold catalysts in selective oxidation reactions, leading to more rational catalyst design.