Selective electrocatalytic hydrogenation of phenol to value-added products using PtRu electrodes: closing carbon balance by capturing gas-phase cyclohexane

Electrocatalytic hydrogenation (ECH) of aqueous phenol to cyclohexanone and cyclohexanol provides a sustainable strategy for simultaneous pollutant remediation and the synthesis of high-value chemicals. However, in both previous reports and our preliminary experiments, the liquidphase product distributions often suffer from incomplete carbon balance that could not be explained by volatilization, adsorption, membrane crossover, or analytical error. Motivated by this imbalance, a sealed H-cell equipped with a gas-absorption trap was implemented to capture volatile products. A bimetallic PtRu electrode supported on carbon cloth, prepared by cyclic electrodeposition, was then evaluated under ambient conditions. With gas capture, cyclohexane was identified as a co-product with cyclohexanone and cyclohexanol, accounting for the previously “missing” carbon. The PtRu electrode exhibited a superior phenol conversion of 98.9% and a high faradaic efficiency (FE) of 59.5%, with product selectivity of ∼32% cyclohexane, restoring the overall carbon balance to >95%. In situ FT-IR spectroscopy revealed the dynamic changes of substances during the phenol hydrogenation process, including the attenuation of aromatic C=C and phenolic C–O bands, along with the growth of C=O/O–H features, which is consistent with stepwise hydrogenation. Density functional theory calculations indicated that the synergistic effect between Pt and Ru simultaneously enhanced the capture of phenol molecules and promoted electron transfer between electrode and surface-bound phenol, facilitating hydrogenation and subsequent C–O removal. This work reconciles the long-standing selectivity/carbon-balance gap in phenol ECH and provides a practical protocol for accurate product quantification and resource-oriented management of phenolic wastewater.