Quantum Chemical Insight into Species Competition in Electrochemical Upgrading of Bio-Oil

Bio-oil derived from lignocellulosic biomass represents an alternative to fossil fuels as an energy source and chemical feedstock. Fast pyrolysis of lignocellulosic biomass yields a rich mixture of oxygenated organic species with low energy density and low stability, which can be electrochemically upgraded to increase energy density and stability and produce value-added chemicals. Here, we focus on electrocatalytic hydrotreatment of furfural (FF) and 4-propylphenol (4-PP), two constituents of raw bio-oil. In a binary mixture, the presence of FF has been shown to inhibit the reduction of 4-PP. The underlying mechanisms of these competing reactions are probed using density functional theory (DFT) and experimental electrochemical deposition techniques. The binding energies of FF and 4-PP, as well as their respective reduction products and associated reaction thermodynamics, are examined comparatively on multiple facets of single-crystal platinum and ruthenium catalysts. FF is shown to bind competitively compared to 4-PP, particularly on Pt(111) and Ru(0001), and hydrogenation products such as tetrahydrofurfuryl alcohol (THFA) and 4-propylcyclohexanol (4-PCOL) were found to bind less strongly to the surfaces than the reactants. FF reduction side products, including dimers furoin (FUI) and hydrofuroin (HF), were also found to bind competitively with 4-PP. No thermodynamic barriers were observed for FF hydrogenation to THFA, whereas 4-PCOL formation is favored on Pt facets, and propylcyclohexane (PC) formation is favored on Ru. The presence of barriers to 4-PP hydrogenation indicates that FF hydrogenation is thermodynamically favored on Pt and Ru. Further, limiting potential calculations suggest that the FF reduction pathways are generally favorable around 0 V vs. RHE. In contrast, the 4-PP reduction pathways require a more cathodic bias, achieved at potentials less than -0.1 V vs. RHE. The adsorption behavior of FF-derived reduction products on polycrystalline Pt is experimentally validated using a Quartz Crystal Microbalance (QCM). Such experiments yield estimates of aqueous-phase free energies of adsorption, which are compared with computational results that are correlated to the aqueous phase. Overall, the DFT model results suggest that the observed inhibition of 4-PP by FF is due to competitive adsorption and favorable FF reaction thermodynamics.