Electrochemical Biomass Valorization on Gold-Metal Oxide Nanoscale Heterojunctions Enables Investigation of both Catalyst and Reaction Dynamics with Operando Surface-Enhanced Raman Spectroscopy

The electrochemical oxidation of biomass platforms such as 5-hydroxymethylfurfural (HMF) to value-added chemicals is an emerging clean technology. However, mechanistic knowledge of this reaction in an electrochemical context is still lacking and operando studies are even more rare. In this work, we utilize core-shell gold-metal oxide nanostructures which enable operando surface-enhanced Raman spectroelectrochemical studies to simultaneously visualize catalyst material transformation and surface reaction intermediates under an applied voltage. As a case study, we show how the transformation of NiOOH from ~1-2 nm amorphous Ni layers facilitates the onset of HMF oxidation to 2,5-furandicarboxylic acid (FDCA), which is attained with near 100% Faradaic efficiency in 1M KOH. In contrast to the case in 1M KOH, NiOOH formation is suppressed, and consequently HMF oxidation is sluggish 10 mM KOH, even at high potentials. Operando Raman experiments elucidate how surface adsorption and interaction dictates product selectivity and how the surface intermediates evolve with applied potential. We further extend our methodology to investigate NiFe, Co, Fe, and CoFe catalysts and demonstrate that high water oxidation activity is not necessarily correlated with excellent HMF oxidation performance and highlight catalytic factors important for this reaction such as reactant-surface interactions and catalysts’ physical and electronic structure.