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 liquid-phase 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.

Spin-trapping agents-based electron paramagnetic resonance (EPR) is still widely used to detect hydroxyl radicals (•OH) in engineered environmental systems. Conventionally, the “four-line peak” of DMPO/•OH (1:2:2:1) was considered the gold standard for the presence of •OH, and signal intensity was occasionally applied to quantify •OH concentrations. Based on chemical reaction networks and reaction rate constants, we established a network dynamics model to quantitatively determine the concentrations of •OH and SO₄^•−. For example, when persulfate (S₂O₈^2–, 100 mmol/L, final pH = 3.33) was activated by FeS₂ (100 g/L), SO₄^•− concentration was 2.66 × 10⁻¹⁰ mol/L, 6 orders of magnitude higher than that of •OH (2.67 × 10⁻¹⁶ mol/L), while the concentration of DMPO/SO₄^•− (3.14 × 10⁻¹¹ mol/L) was 7 orders of magnitude lower than that of DMPO/•OH (2.34 × 10⁻⁴ mol/L). These results were validated by EPR. Our study revealed that 81.1%−81.5% of DMPO/•OH is derived from DMPO/SO₄^•− hydrolysis and only 18.5%−18.9% is from direct capture of •OH, questioning the reliability of detecting •OH based on the appearance of the “four-line peak”. Our study underscores the necessity of considering all the transformations among radicals and their adducts during EPR analysis, which also provides a direct and effective method for detecting other radicals with extremely short half-lives in other heterogeneous persulfate systems. The high sensitivity of SO₄^•− and •OH to pH also provides an avenue to regulate the generation of reactive species.