Mechanistic study on electro-oxidation of 5-hydroxymethylfurfural and water molecules via operando surface-enhanced Raman spectroscopy coupled with an Fe3+ probe

Electrocatalytic oxidation of biomass such as 5-hydroxymethylfurfural (HMF) is a green route to produce value-added chemicals. However, in-depth understanding of the electrocatalytic process is still lacking. Herein, the mechanism of HMF electro-oxidation was studied by operando surface-enhanced Raman spectroscopy (SERS) coupled with an Fe3+ probe. A multicomponent platform consisting of titanium plates integrated with a gold film and uniform nickel, cobalt and copper nanoparticles (NPs) was employed as an electrode as well as a SERS substrate, enabling operando SERS study on the electrocatalytic process with simultaneous evaluation on catalytic activity and visualization on the variations of active sites. Fe3+ was intentionally added into the electrolyte as a probe to correlate the nature of active sites with catalytic activity and product selectivity in virtue of its strong interaction with the active sites of the electrocatalysts. Electrodes integrated with Ni and Cu NPs exhibited excellent HMF oxidation performance but their activity were suppressed by adding 1 ppm Fe3+. Conversely, the activity towards water oxidation was significantly enhanced by Fe3+ addition. Fe3+ inhibits the oxidation of Ni2+ to Ni3+ and promotes the formation of more CuO and CuIII. CuIII is better active sites for water oxidation. The presence of Fe3+ also inhibits the transformation of Co3+ to Co4+, but it has no pronounced effects on HMF oxidation performance. A highest 2,5-furandicarboxylic acid yield (94.0%) and a maximum HMF conversion (98%) are obtained from the electrode integrated with Cu NPs at a potential of 1.47 V. HMF oxidation on Cu NPs follows the aldehyde oxidation pathway, which is not affected by Fe3+ addition. This work could afford a feasible route to explore mechanisms of electrocatalytic processes.