Promotion of FFCA formation through the use of highly controlled Ru nanostructures as catalysts for the electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF)

The electrochemical oxidation of 5-hydroxymethylfurfural (HMF), a key biomass-derived platform molecule, offers a sustainable pathway to value-added chemicals. While ruthenium-based catalysts have been investigated for driving the complete oxidation of HMF to 2,5-furandicarboxylic acid (FDCA), the selective formation of partially oxidised intermediates, like 5-formyl-2-furancarboxylic acid (FFCA), a product with different functional groups, remains largely unexplored in alkaline media. In particular, the influence of catalyst crystallinity and defect density on selective FFCA electrosynthesis is still unclear. Here, we successfully synthesised and characterised two shape-controlled Ru nanostructures with distinct crystallinity. The first comprises Ru hourglass nanoparticles, monocrystalline and highly uniform, and the second consists of polymorphous nanoparticles, characterised by a combination of multiple Ru nanocrystals of uneven shapes. After supporting both materials on carbon, we evaluated their performance for HMF electrooxidation in alkaline electrolyte, comparing them to commercial RuC. Comprehensive structural and electrochemical characterisation enabled the correlation of morphology and crystallinity with catalytic behaviour. Both the hourglasses and polymorphous particles significantly outperformed the commercial RuC, achieving >95% HMF conversion with 66% selectivity toward FFCA and resulting in the first example of second-order kinetics for Ru-catalysed HMF electrooxidation. Stability tests show that while the polymorphous particles exhibited higher intrinsic activity, consistent with their defect-rich, polycrystalline structure, they suffered from faster deactivation. In contrast, the monocrystalline hourglass nanoparticles provided enhanced stability while maintaining high selectivity. These findings demonstrate that crystallinity engineering is a key strategy for balancing activity and durability in Ru-catalysed HMF electrooxidation in alkaline media.