Beyond Direct Two-Electron Oxidation: High-Potential Formaldehyde Oxidation on Palladium Still Generates Hydrogen...

Electrochemical oxidation is typically regarded as incompatible with hydrogen intermediate formation, particularly at highly positive potentials where surface hydrogen species are expected to be rapidly oxidized. Formaldehyde oxidation reaction (FOR) on palladium represents a prominent example: low-potential FOR has been associated with hydrogen generation, whereas high-potential FOR is commonly interpreted as a direct two-electron oxidation to formate that bypasses hydrogen intermediates. Here we challenge this conventional view using a Janus palladium membrane electrode (J-Pdm) that spatially decouples hydrogen generation, absorption, permeation, and detection. By monitoring the hydrogen oxidation reaction (HOR) current on the electrochemically isolated detection side of J-Pdm, we directly quantify hydrogen atoms generated during FOR under alkaline conditions. Unexpectedly, permeated hydrogen remains detectable over a broad FOR potential window up to 1.5 V vs RHE (reversible hydrogen electrode), demonstrating that high-potential FOR on Pd is not a hydrogen-free oxidation process. Coulometric measurements confirm hydrogen uptake within the Pd lattice, with H/Pd ratios reaching up to 0.57. Spectroscopic and computational results support a sequential hydrogen-generating oxidation mechanism in which formaldehyde-derived intermediates undergo favorable C-H activation, while the resulting hydrogen atoms compete among absorption into the Pd lattice, recombination to H2, and oxidation to protons. FOR-generated hydrogen atoms can be utilized for styrene hydrogenation in an organic phase, establishing anodic hydrogenation as a viable electrosynthetic strategy. These findings provide new mechanistic understanding of formaldehyde oxidation on palladium and also demonstrate J-Pdm as a powerful platform for detecting and utilizing transient hydrogen intermediates in anodic reactions.