Electrochemical reduction of acetonitrile to ethylamine

The study addresses the challenge of selectively producing primary amines, which are key building blocks in organic synthesis, from nitriles. Conventional thermal hydrogenation of acetonitrile to ethylamine operates at high temperature and pressure and often suffers from poor selectivity due to formation of secondary and tertiary amines. The research question is whether electrochemical reduction of acetonitrile at ambient conditions can selectively produce ethylamine with high Faradaic efficiency and industrially relevant rates, and what catalyst properties and operating conditions enable such selectivity. The work explores catalyst screening with emphasis on Cu, evaluates electrolyte pH and acetonitrile concentration effects, establishes reaction orders and onset potentials, assesses stability, and uses in situ mass spectrometry and DFT to elucidate mechanisms underpinning selectivity. This approach connects renewable electricity to primary amine synthesis, offering a pathway to decarbonize chemical manufacturing.

Prior literature highlights the importance and difficulty of selective nitrile hydrogenation to primary amines using thermal catalysis, with byproduct formation (secondary/tertiary amines) being a major issue (refs. 8–19, 33–34). Supported metal catalysts such as Ni, Pd, Pt, and Co-based systems have been used for acetonitrile hydrogenation with varying selectivity and conditions. Electrochemical studies of acetonitrile on Pt electrodes have focused on adsorption and (electro)reactivity (refs. 25–31), but selective electroreduction to primary amines under aqueous conditions and at high rates has been less explored. Insights from CO2 electroreduction show alkaline electrolytes suppress hydrogen evolution and influence selectivity via local pH effects (refs. 35–36), suggesting electrolyte engineering could benefit nitrile electroreduction. This work builds on those insights by systematically screening catalysts and electrolytes, and by coupling in situ mass spectrometry with DFT to rationalize activity and selectivity trends.

Catalyst preparation: Cu nanoparticles (25 nm) and Cu microparticles (0.5–1.5 μm) were sourced commercially. Oxide-derived Cu (OD-Cu) was made by electrochemically depositing a Cu2O film on porous carbon paper, then reducing at 10 mA cm−2 in 1 M NaOH for 15 min. Additional catalysts included Ag (20 nm), Bi (80 nm), In (80 nm), Sn (<150 nm), Pd/C (5 wt%), and Pt/C (5 wt%). For a Cu nanoparticle electrode, 25 mg Cu NPs were dispersed in 3 mL isopropanol with 20 μL of 5 wt% Nafion binder, sonicated (20 min), and drop-cast onto 2.5 cm2 carbon paper at a loading of 0.5 mg cm−2. Characterization: SEM/EDX (Auriga 60) and XPS (Thermo K-Alpha; adventitious C 1s = 284.5 eV) were used to analyze morphology and surface composition. Electrochemical surface area (ECSA) was determined by double-layer capacitance via CV in Ar-saturated 0.1 M HClO4 at multiple scan rates. Electrochemical setup: A two-compartment microfluidic flow cell with a Nafion 211 membrane separated cathode (acetonitrile reduction) and anode (oxygen evolution). Typical catholyte: 8 wt% acetonitrile in 1 M NaOH; anolyte: 1 M NaOH. Peristaltic pumps controlled flow; the catholyte effluent passed through a gas–liquid separator and was analyzed by GC (HayeSep D and Mol Sieve 5 Å; TCD and FID; Ar carrier gas). Liquid products were quantified by 1H NMR (Bruker AVIII 600 MHz) using D2O with DMSO internal standard and water suppression. Electrochemical measurements used an Autolab PG128N in a three-electrode configuration with Ag/AgCl reference, nickel foam anode (basic media) or IrO2/Ti (acidic). iR-corrected potentials were converted to RHE: E(RHE) = E(Ag/AgCl) + 0.210 V + 0.0591×pH + iR. Chronopotentiometry and LSV were employed; each data point was stabilized for 600 s before GC injection; liquid sampling for 300 s. In situ product detection: Flow electrolyzer mass spectrometry (FEMS) collected gaseous/volatile products via a hydrophobic PTFE membrane-covered PEEK capillary placed ~10–100 μm from the cathode surface. Products were analyzed with a quadrupole MS (ionization at 70 eV). Computations: Spin-polarized DFT (VASP) at the GGA level (PAW-PW91). Low-index (111) slabs of Cu, Ni, Pt (4-layer, 3×3) with ~15 Å vacuum were used; bottom two layers fixed; 400 eV plane-wave cutoff; 3×3×1 k-point mesh. Binding energies computed as BE = E(slab+ads) − E(slab) − E(ads). Gibbs free energies: G = E + ZPE − TS (298 K). Free energy diagrams for acetonitrile reduction and HER were constructed using the computational hydrogen electrode model.

• Catalyst screening: Cu nanoparticles delivered the highest ethylamine Faradaic efficiency (FE) of 96% at −0.29 V vs RHE among tested metals (Pd, Pt, Ni, Bi, Sn, In, Ag, OD-Cu, Cu microparticles).
• Activity and selectivity on Cu: Under optimal conditions (1 M NaOH, 12 wt% acetonitrile), an ethylamine partial current density up to 846 mA cm−2 was achieved at −0.73 V vs RHE. With 2 M NaOH and 8 wt% acetonitrile, ethylamine partial current density reached 635 mA cm−2 at −0.73 V.
• Reaction order: j_ethylamine measured at −0.45 V showed a reaction order of ~0.91 with respect to acetonitrile concentration, indicating near first-order dependence.
• Electrolyte pH effects: Acidic (0.5 M H2SO4) and neutral (0.5 M Na2SO4) electrolytes favored hydrogen evolution and suppressed acetonitrile reduction. Alkaline electrolytes (0.1–2 M NaOH) enhanced ethylamine formation and suppressed HER; plotting vs SHE suggested pH-independence of ethylamine formation rate, implying the rate-determining step does not involve OH− and water is the likely proton source.
• Stability: In a flow cell at 100 mA cm−2 with 8 wt% acetonitrile in 1 M NaOH, Cu maintained a stable potential (~−0.46 V) and ethylamine FE >86% over 20 hours; minor disturbances were attributed to H2 bubble dynamics. Post-reaction SEM/XPS showed no major changes, though some catalyst loss from the carbon paper was observed.
• Product distribution (FEMS): At −0.5 V vs RHE, signals for water, acetonitrile, H2, and ethylamine dominated. Only trace diethylamine was detected; no triethylamine was observed. Onset potentials from LSV–FEMS: H2 at ~−0.21 V, ethylamine at ~−0.23 V, diethylamine at ~−0.32 V; triethylamine signal unchanged. Ethylamine forms at lower overpotential than diethylamine.
• DFT mechanism and selectivity: Free energy diagrams indicate the most favorable pathways differ by metal, with overall thermodynamics most favorable on Cu(111). The most difficult steps: Cu(111) adsorption of *CH3CN (ΔG ≈ +0.20 eV), Ni(111) reduction of *CH3CH2N (≈ +0.26 eV), Pt(111) desorption of *CH3CH2NH2 (≈ +0.45 eV). Nitrile reduction is more favorable than HER on Cu and Ni by ~0.16 eV and ~0.08 eV, respectively, whereas HER is more favorable on Pt by ~0.19 eV. Predicted and observed selectivity/activity trend: Cu > Ni > Pt for ethylamine FE; HER trend Pt > Ni > Cu.

The findings demonstrate that selective electrochemical reduction of acetonitrile to ethylamine is feasible at ambient conditions with high Faradaic efficiency and substantial partial current densities, addressing the longstanding selectivity limitations of thermal nitrile hydrogenation. Cu catalysts deliver superior selectivity because their intermediate binding strengths are moderate, which balances activation and desorption steps, as supported by DFT free-energy analyses. Operating in alkaline electrolytes suppresses competitive hydrogen evolution and enhances ethylamine formation; the observed pH independence of the rate (when referenced to SHE) suggests the rate-determining step is likely the initial activation of acetonitrile (single-electron reduction or proton-coupled electron transfer) with water as the proton source rather than OH− participation. In situ FEMS shows ethylamine forms at lower overpotential than diethylamine, consistent with a pathway where diethylamine arises from condensation of ethylamine with an imine intermediate. The high activity and stability in a flow electrolyzer, combined with mechanistic insights, indicate a viable electrified route to primary amines with improved selectivity over traditional thermocatalytic processes.

This work establishes an electrocatalytic route for converting acetonitrile to ethylamine at ambient conditions with high selectivity and high rates. Cu nanoparticle catalysts achieved up to 96% ethylamine FE at −0.29 V vs RHE and partial current densities up to 846 mA cm−2 under optimized alkaline conditions, with stable operation at 100 mA cm−2 for 20 hours. Mechanistic studies (FEMS and DFT) attribute the superior selectivity on Cu to moderate binding of key intermediates, while alkaline electrolytes mitigate HER. These advances provide an alternative, electrified pathway for primary amine synthesis, contributing to decarbonization of chemical manufacturing. Future work could focus on scaling and process integration, improving catalyst adhesion and durability in flow devices, exploring other nitrile substrates and catalyst compositions, and further optimizing membranes and electrolyte environments to enhance selectivity and reduce HER.

• The demonstrated performance and selectivity were obtained in alkaline aqueous electrolytes; activity was poor in acidic and neutral media due to dominant HER, which may limit applicability across different process conditions.
• Stability testing over 20 hours showed some loss of catalyst particles from the porous carbon paper, which may contribute to slight decreases in Faradaic efficiency and indicates a need to improve catalyst/support adhesion.
• The study focuses on acetonitrile as the substrate; generalization to other nitriles was not evaluated here.
• While FEMS detected only trace diethylamine and no triethylamine, trace byproduct formation indicates side reactions can occur under certain potentials.
• DFT calculations were performed on ideal (111) facets and may not fully capture the complexity of nanoparticle surfaces and dynamic electrochemical interfaces.