Identification of catalytic activity descriptors for selective 5-hydroxymethyl furfural electrooxidation to 2,5-furandicarboxylic acid†
Several transition metal oxides and hydroxides have been reported as active electrocatalysts towards the oxidation of 5-hydroxymethyl furfural (HMF), an important biomass-derived compound for downstream sustainable chemical and transport fuel production. However, few studies have investigated the activity descriptors influencing the reactivity and selectivity of these materials towards HMF electrooxidation (HMFOR). Herein, Co, Ni and Cu-based Prussian blue analogue (PBA) electrocatalysts were systematically investigated to identify the intrinsic electronic contributions of these redox species towards HMF electrooxidation activity. Cu-phase PBAs exhibited the highest faradaic efficiency of 97.4%, followed by Co-phase (90.5%) and Ni-phase (82.6%) PBAs towards generating 2,5-furandicarboxylic acid (FDCA) at 1.42 V vs. RHE in 1.0 M KOH. This activity trend is found to be influenced by the amount of nucleophilic OH* species expressed on the electrocatalyst's surface. A higher expression of these surface species results in a lower activation energy barrier for the rate limiting step, leading to increased selectivity towards FDCA. Analysis of the bulk electronic properties shows that a strong M–O bond covalency and high d-orbital occupancy contribute to this high expression of electrophilic sites. These findings establish a basis for rationalising the origins of FDCA selectivity in HMFOR catalysts, which importantly eschews using catalysts with high oxygen evolution reaction (OER) activity as a prerequisite for high HMFOR performance.