Characterization of Potential Dependent Fouling of Cu Electrodes during Electrochemical Hydrogenation and Hydrogenolysis of Furfural in Acid

Abstract

Furfural (FF) is a platform molecule derived from biomass, that can be electrochemically reduced to produce furfuryl alcohol (FA), a furanic polymer intermediate, and 2-methylfuran (MF), a fuel candidate. On Cu in acidic media, it has been shown that FA and MF are produced via parallel reactions as opposed to a series reaction. Our previous work investigated the reactions for FA and MF production electrochemically with FF concentrations between 10 and 120 mM, showing positive order with respect to FF at low concentrations and zero order rates with respect to FF at higher concentrations. This finding suggesting non-competitive Langmuir-Hinshelwood mechanisms at concentrations of FF between 10 and 120 mM in pH 0 and 1 acidic solutions. At 200 mM concentrations of FF, we have identified fouling on the Cu surface. At moderate potentials, the fouling takes the form of a polymer material, whereas at more negative reduction potentials the fouling takes the form of graphitic coking. Understanding how fouling occurs on Cu electrodes is important in achieving progress towards mitigating or reducing the impact of fouling through reactor setup and catalysts design.In this work, conditions that promote fouling were selected so that the fouling of Cu electrodes could be studied. The conditions were chronoamperometry at -560 or -700 mV RHE, 200 mM FF, 0.5M H2SO4, 20:80 vol% acetonitrile:water, for three hours durations. The Cu electrodes were reused for 3 experiments to further promote fouling and study the impact on the electrochemical performance. We showed that after reuse at -560 mV RHE, a polymeric material formed on the Cu electrode, while when reused at -700 mV RHE the electrode was instead covered in a coke like material. FTIR spectroscopy showed that the polymer formed at -560 mV resembled poly(furfuryl alcohol) while the coking substance showed broad absorbance, expected of coke like materials. To better understand the coke material formed at -700 mV, Raman spectroscopy was used. The material resembled graphitic disorganized carbon, with a D/G band intensity ratio of 0.652.