Synthesis of shape-controlled metal nanoparticles for 5-hydroxymethylfurfural (HMF) electrochemical oxidation

This thesis explores the use of shape-controlled synthesis of cobalt (Co) and ruthenium (Ru) nanoparticles as a strategy to enhance their catalytic activity and selectivity toward the electrooxidation of 5-hydroxymethylfurfural (HMF). In this thesis control over nanoparticle synthesis is employed as a tool to advance the understanding and applicability of Co- and Ru-based catalysts for the electrochemical oxidation of HMF. The aim is to elucidate how shape and composition influence catalytic performance, under the premise that only through a deeper understanding of fundamental structure–function relationships can highly effective and efficient catalysts be developed to ultimately support a smooth sustainable transition. HMF is a key platform chemical derived from biomass, and its oxidation products are gaining increasing industrial relevance as precursors for the synthesis of polymers, fuels, and fine chemicals. In particular, the electrochemical oxidation of HMF offers a sustainable and efficient route to synthesise value-added compounds such as 2,5-furandicarboxylic acid (FDCA), a precursor for the synthesis of polyethylene furanoate (PEF) which is a promising bio-based alternative to polyethylene terephthalate (PET). While HMF oxidation was traditionally carried out thermocatalytically, in the past two decades, electrochemical oxidation has emerged as a more environmentally friendly alternative. With this approach, the reaction can be performed under standard temperature and pressure conditions, potentially reducing its carbon footprint. Additionally, as HMF oxidation occurs at the anode, this reaction can be coupled with reductive processes occurring in the same potential range, as the hydrogen evolution reaction (HER), obtaining two useful products at once. Lastly, the engineering of nanomaterials as electrocatalysts has shown promising results in achieving selectivity towards specific intermediates over the fully oxidised product FDCA, thus further expanding the potential application of this process. For these reasons, numerous catalytic systems have been investigated to both elucidate the reaction mechanism and enhance the efficiency of HMF electrooxidation. Both earth-abundant metals (Co, Cu, Ni, Fe, Mn, Mo, V) and precious metals (Ru, Pt, Au) have been explored as active catalysts for this process. Each metal displays a characteristic activity and selectivity toward the various oxidation products, determined by the potential at which the reaction occurs and the specific chemistry of the material, which in turn promote distinct mechanistic pathways. In particular, Co-based materials are a catalyst of choice for this reaction, as they boast high catalytic activity and selectivity toward FDCA, in addition to being earth-abundant and ecologically and economically sustainable. However, their application is often limited by sluggish reaction kinetics and their high selectivity towards FDCA means that accessing the intermediate oxidation products on Co catalysts can be difficult. Ru nanomaterials, on the other hand, have demonstrated good catalytic activity and selectivity towards other oxidation products of HMF, such as 2,5-furandicarboxaldehyde (DFF). However, Ru being a noble metal, their application could be limited by high costs and availability, making it pivotal to obtain Ru materials with exceptional catalytic properties, that can overcome the applicability challenges. In this thesis, the synthesis of shape-controlled nanoparticles was utilised to produce branched Co nanostructures that could help overcome the kinetic challenges associated with Co-based nanocatalysts for the HMF electrooxidation. Specifically, Co branches were grown on both spherical gold (Au) and cubic copper (Cu) seeds. It was observed that retaining a metallic, highly conductive Co core within the branches mitigated the poor conductivity issues characteristic of Co oxide materials, enhancing electron transfer and hence improving the reaction kinetics. Nonetheless, HMF upgrading is a complex process, which involves multiple parallel and consecutive reactions, potentially occurring on distinct active sites of the catalyst surface, but fundamental and systematic studies on the intrinsic kinetics of HMF oxidation remain scarce. To address this knowledge gap, two different active metals were exposed on the same branched nanocatalyst, namely Co and Cu, to investigate their effect on the mechanism and the kinetics of the reaction. It was found that the presence of both metals at the surface can favour a “cascade-like” mechanism, which positively affects the activity and kinetics of the electrooxidation of HMF, when compared to the use of the single metals, Co and Cu. The work on Co catalysts was successful in overcoming the kinetic limitations to produce faster catalysts for HMF electrooxidation to the fully oxidised FDCA. However, the challenge of accessing less oxidised products, other than FDCA, still remained. For this reason, both shape-controlled Ru hourglasses and Ru polymorphous nanoparticles were synthesised and used as catalysts for this reaction. With these catalysts, the selectivity of the electrooxidation of HMF was successfully tuned and the first ever selective electrooxidation of HMF to the less oxidised 5-formyl-2-furancarboxylic acid (FFCA) on Ru catalysts was demonstrated.