Highly efficient ethylene production via electrocatalytic hydrogenation of acetylene under mild conditions

Selective hydrogenation of acetylene to ethylene (HAE) is important for non-oil ethylene production but thermocatalytic HAE requires high temperatures (>200 °C) and elevated pressures (~5 bar), consumes significant H2, and suffers from over-hydrogenation to ethane. Electrocatalytic HAE (E-HAE) offers a more economical, energy-efficient, and environmentally friendly alternative by operating at room temperature and ambient pressure and using water as the hydrogen source powered by renewable electricity. This study aims to develop a highly efficient and selective E-HAE process using carbon-supported Cu microparticles, optimize catalyst structure (exposure of active facets) and electrode architecture (microporous gas diffusion layer) to enhance acetylene adsorption/hydrogenation versus HER, and elucidate the reaction mechanism governing selectivity via in situ spectroscopy and DFT.

Prior work on thermocatalytic acetylene hydrogenation spans various supports and metals (e.g., Pd, Ag, Ni, Au), but faces energy and selectivity challenges (refs. 4–12). Earlier electrochemical studies demonstrated acetylene reduction on Ag, Cu, polymer-modified, and Mo-complex electrodes with limited efficiency and often in closed systems (refs. 16–21). Recent advances explored electrocatalytic semi-hydrogenation with modified copper-based catalysts and room-temperature processes (refs. 13–15). However, achieving high Faradaic efficiency and suppressing HER remain key challenges, motivating the present approach combining tailored Cu facets, gas diffusion architecture, and potential control.

Catalyst synthesis: Cu microparticles (MPs) were synthesized via a solvothermal polyol route in ethylene glycol using CuSO4·5H2O precursors with PVP, NaCl, NaOH, and ascorbic acid; particles (~1 μm) were washed and stored in alcohol. CuO was obtained by calcining Cu MPs at 500 °C in 20% O2/Ar. Additional reference catalysts (MoS2 via autoclave synthesis; commercial Pt/C, Pd/C; TiO2 P25; activated carbon; Cu plate) were prepared as described. Characterization: Morphology and structure were examined by SEM, HAADF-STEM, TEM; phase by XRD. Surface area (BET) and hydrophobicity (contact angle) characterized the effect of the microporous gas diffusion layer (GDL). Electrochemically active surface area (ECSA) was measured by Pb underpotential deposition. Electrochemical setup and testing: Catalyst inks (4 mg catalyst with 5 wt% Nafion) were drip-coated on GDL-coated carbon paper (AvCarb GDS3250) shaped as 1×1 cm2 working area; H-type cell with Nafion-115 separator, Pt mesh counter, Hg/HgO reference (converted to RHE). Electrolyte: KOH (0.1–3 M), 25 °C. Acetylene was continuously bubbled; flow-controlled gas delivery. Linear sweep voltammetry (10 mV s−1) in Ar- vs C2H2-saturated electrolyte assessed onset and activity; rotating disk electrode (5 mm diameter, 2500 rpm) corroborated trends. Reactions at fixed potentials were run for 2 h prior to product analysis; 90% iR compensation used where indicated. Electrode architecture: Comparison of Cu on pristine carbon paper (Cu/CP) versus GDL-coated (Cu/GDL-CP). GDL increased hydrophobicity and BET surface area (from 0.37 to 4.55 m2 g−1), promoting gas–liquid–solid interfaces and mass transfer. Product analysis and metrics: Gas products quantified by GC with TCD (H2) and FID (hydrocarbons) using loop injection via a ten-port valve. Faradaic efficiencies (FE) calculated from GC peak areas, calibration factors, flow rate (4.8 mL min−1; 2.4 mL min−1 in tandem experiments), time (2 h), charge, and stoichiometry. Geometric current densities for acetylene consumption (jC2H2) and ethylene formation (jC2H4) computed from FE and total current density. Acetylene reaction rates per area derived from product formation rates. Liquid products checked by NMR (none detected). In situ spectroscopy: Cu K-edge XANES/EXAFS performed in a custom H-cell under Ar or C2H2 at OCP and at applied potentials (e.g., −0.6 V vs RHE) to probe oxidation state changes and structure. In situ Raman (637 nm excitation) probed adsorbed C2H2 species. In situ ATR-FTIR (0.1 M KOH) monitored surface intermediates across 0 to −0.9 V vs RHE. Computations: DFT (VASP) with PBE-GGA, PAW, D3 dispersion, 400 eV cutoff; 6×6 four-layer Cu(111)/(100)/(110) slabs with C2H2 coverages determined from adsorption free energies; water-network model for proton transfer; transition states via fixed bond length method and vibrational checks. Free energy corrections (ZPE, vibrational, entropy) applied; potential dependence via computational hydrogen electrode and a linear barrier shift with effective symmetry factor 0.5 to estimate barriers at −0.6 V vs RHE. Bader charge analysis and charge density difference visualized electron transfer.

- Cu microparticles expose predominantly Cu(100) and (110) facets (fcc), favoring acetylene adsorption/hydrogenation over HER. - Mass transfer enhancement via a microporous GDL substantially increases jC2H2 and jC2H4 compared with pristine carbon paper, despite similar ECSA; GDL suppresses HER FE, particularly below −0.6 V vs RHE. - Potential-dependent selectivity: at −0.3 V vs RHE, FE(C2H4) ~53% and FE(1,3-butadiene) ~47%. As potential becomes more negative, FE(C2H4) rises, peaking at 83.2% at −0.6 V with total current density 29 mA cm−2; 1,3-butadiene FE drops below 10%; ethane formation is fully avoided at potentials ≥ −0.6 V. At more negative potentials, H2 and deep-hydrogenated products (e.g., ethane) appear, reducing FE(C2H4). - Performance: maximum jC2H4 = 26.7 mA cm−2 at −0.7 V; acetylene consumption rate rC2H2 = 0.59 mmol cm−2 h−1 and jC2H2 = 26.7 mA cm−2 at −0.7 V with ~4.9% conversion. - Scalability: tandem cell operation (flow 2.4 mL min−1) increases acetylene conversion from 9.3% (single) to 17.6% (double) and 23.3% (triple), maintaining >80% ethylene selectivity and ~50% FE for ethylene with reaction rate ~0.45 mmol cm−2 h−1. - Stability: at −0.6 V, FE(C2H4) >70% maintained for >100 h with ~12 mA cm−2 current; activity loss attributed to polyacetylene deposition (Raman peaks at 1102 and 1492 cm−1). - Mechanism and spectroscopy: In situ Cu K-edge XANES shows higher binding energy upon C2H2 exposure, indicating electron transfer from Cu to adsorbed C2H2 (π* filling). In situ Raman detects *C2H2 on Cu (red-shifted C≡C band). ATR-FTIR reveals a key *C2H3 intermediate (negative band at 1594 cm−1) whose intensity increases with more negative potentials; *C2H2 band at ~1670 cm−1 observed; water desorption at 1630 cm−1. - DFT insights: Strong C2H2 adsorption on hollow sites of Cu(111)/(100)/(110) with significant charge transfer (up to 0.65 e− to adsorbate). Equilibrium coverages: 1.0 ML on (100) and (110), 0.5 ML on (111). Formation of surface *H is thermodynamically and kinetically unfavorable on C2H2-covered surfaces (ΔGads(*H): 0.56, 0.17, 0.49 eV on (100), (110), (111); proton-transfer barriers to form *H: ~0.81, 1.84, 0.97 eV), rationalizing suppressed HER. - Reaction pathway: E-HAE proceeds via electron-coupled proton transfer from water: *C2H2 + H2O + e− → *C2H3 + OH−; subsequent hydrogenation to *C2H4 and easy desorption to C2H4. Rate-limiting first hydrogenation barriers at 0 V: 0.79 eV (100), 0.86 eV (110), 1.24 eV (111). At −0.6 V, electrochemical barriers decrease, especially for the first step, enhancing ethylene selectivity relative to potential-independent C–C coupling pathways that form 1,3-butadiene. - Overall, the Cu MPs outperform tested and reported catalysts in FE(C2H4) and geometric current densities for E-HAE under the studied conditions.

The study demonstrates that tailoring Cu surface structure and electrode architecture enables preferential acetylene adsorption and hydrogenation while suppressing HER under mild, aqueous conditions. Electron transfer from Cu to adsorbed C2H2 strengthens selective adsorption, activates the triple bond, and directs the electron-coupled proton transfer mechanism, avoiding reliance on surface *H formation and thus limiting HER. Potential control modulates the kinetics of electrochemical steps, allowing enhancement of ethylene formation rates and suppression of both over-hydrogenation to ethane (avoided at potentials ≥ −0.6 V) and C–C coupling to 1,3-butadiene. The microporous GDL improves gas delivery and three-phase interfaces, significantly boosting jC2H2 and jC2H4. In situ spectroscopies corroborate adsorbate-induced electronic changes and identify key intermediates (*C2H3), while DFT quantifies coverage effects, barriers, and potential dependence that underpin the observed selectivity trends. Together, these findings address the central challenge of achieving high Faradaic efficiency against HER in E-HAE and indicate a viable, scalable route to efficient ethylene production using renewable electricity and water as the hydrogen source.

A highly efficient electrocatalytic acetylene-to-ethylene process was achieved at room temperature and ambient pressure using Cu microparticles on a GDL-supported electrode. By optimizing catalyst facets, employing a microporous GDL to enhance mass transfer, and tuning electrode potential, the system reached FE(C2H4) of 83.2% at −0.6 V with 29 mA cm−2 total current density, jC2H4 up to 26.7 mA cm−2 at −0.7 V, and stable operation exceeding 100 h. In situ spectroscopy and DFT reveal that electron transfer from Cu to adsorbed acetylene promotes selective adsorption and hydrogenation via electron-coupled proton transfer, suppressing HER and excessive hydrogenation. The approach surpasses prior reports and shows promise for scalable, green ethylene production. Future work could focus on mitigating polyacetylene deposition, further enhancing single-pass conversion (e.g., through reactor and flow optimization), and exploring facet engineering or alloying to lower barriers while maintaining selectivity.

- Single-pass acetylene conversion remains modest in a single cell (~4.9% at −0.7 V), requiring tandem cells to increase conversion (up to 23.3% in triple-cell configuration). - Long-term operation shows some activity loss, likely due to polyacetylene deposition that blocks active sites. - At more negative potentials than −0.6 V, HER and deeper hydrogenation (e.g., ethane formation) increase, reducing FE(C2H4), indicating a narrow optimal potential window. - Mechanistic and performance insights were obtained in alkaline electrolyte; translation to other pH regimes or practical gas-fed electrolyzers may require further validation.