Insights into the reaction mechanism for 5-hydroxymethylfurfural oxidation to FDCA on bimetallic Pd-Au nanoparticles

This work deals with the oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) in water using supported Pd-Au nanoparticles. The active phase composition was shown to be crucial for FDCA formation. Indeed, both Au and Pd monometallic nanoparticles formed 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) under the studied conditions; however, with Pd nanoparticles HMFCA was not further transformed, while Au and bimetallic Pd-Au systems both catalysed its oxidation to FDCA. The thermal treatment of Pd-Au catalysts considerably modified their catalytic activity, because Pd atoms migrated and concentrated onto the outer part of bimetallic nanoparticles. The resulting active phase morphology showed a different reaction path for FDCA formation compared to the untreated catalyst, with an important contribution of the Cannizzaro reaction. PVP-protected Pd-Au nanoparticles with different structures (either alloy or core-shell morphology) were synthesized and their reactivity tested in order to confirm the presence of different mechanisms for HMF oxidation, depending on whether the active phase preferentially exposes either Pd or Au atoms.