Direct Conversion of Ethanol to Ethyl Acetate by Dynamic Polyoxometalate/Carbon Nanohorn Electrocatalytic Interfaces

The electrosynthesis of ethyl acetate (EtOAc) by oxidative esterification of aqueous (up to 20% water) ethanol (EtOH) is performed by employing the tetraruthenate polyoxometalate [Ru₄(μ-O)₄(μ-OH)₂(H₂O)₄(γ-SiW₁₀O₃₆)₂]^10- (Ru₄POM) electrocatalyst with carbon nanohorns (CNHs) as a heterogeneous support. This strategy involves a voltage-gated electro-adsorption of Ru₄POM on CNH-modified glassy carbon anodes with favorable interfacial dynamics, maintained under electrocatalytic conditions. These conditions are reached through the continuous reconstruction of the organic/inorganic interface (catch-and-release), as probed by converging thermal, microscopic, and electrochemical analyses. In fact, control experiments reveal that both pristine and N-doped CNHs display a Ru₄POM loading in the range 13-18 nmol mg^-1 with a remarkable ∼100 mV onset potential anticipation and current enhancement in the range 400-700% compared to the homogeneous conditions. By adopting the "catch-and-release" protocol, electro-esterification of aqueous EtOH features long-term stability of the productive current in the mA range (J_chrono ≈ 2 mA cm^-2 at +1.2 V vs Ag/AgCl probed up to 18 h), with Faradaic efficiencies, FE_EtOAc, of >90%. This effect is attributed to the crucial role of CNHs hydrophobicity to control hydrolysis equilibria, thus outperforming the solution-phase behavior, which levels off at FE_EtOAc < 60%.