Elucidating Acidic Electro-Oxidation Pathways of Furfural on Platinum

Electrocatalysis poses a number of possible advantages for the valorization of biomass-derived feedstocks, most critically its amenability to direct conversion in acidic aqueous media. Furanic biomass derivatives, such as furfural, are substrates with a number of value-added chemical outlets via partial oxidation, most notably furoic acid (FA), a potential precursor to 2,5-furandicarboxylic acid (FDCA). Pairing such partial oxidations with H2 evolution or other reduction reactions (e.g. CO2) in an electrochemical cell presents an opportunity to perform electrolysis at lowered voltages, while coproducing products that are more valuable than O2. Here, we have utilized differential reactor studies with online electrochemical mass spectrometry (OLEMS), as well as in situ infrared spectroscopy attenuated total reflectance-surface-enhanced infrared reflection-absorption spectroscopy (ATR-SEIRAS), to probe the oxidative reaction pathways of furfural on platinum catalysts in acidic electrolyte. We find furfural electro-oxidation selectivity to depend on potential, with the largest shifts corresponding to the transition of Pt to Pt-oxide. Below 1.2 VRHE, FA and 5-hydroxyfuroic acid are the primary products. At higher potential, selectivity shifts predominantly toward 5-hydroxy-furan-2(5H)-one (HFN), with the appearance of maleic acid (MA) as well. ATR-SEIRAS and OLEMS indicate that decomposition and overoxidation to CO2 occurs via decarbonylated or decarboxylated ring intermediates, while MA is less easily activated toward further oxidation once it is formed. Significant oxidative currents are only achieved at potentials where the surface is cleared of CO, which is derived from spontaneous decarbonylation of furfural on the metallic Pt surface. As potential is increased, selectivity to C4 and C5 oxygenates over CO2 is then promoted by a high steady-state surface coverage of organic intermediates that inhibit rapid adsorption and addition of oxygen from the discharge of water. Based on these findings, we propose a reaction pathway and directions for the design of more active and selective electrocatalysts.