Pd–H Species on Electrode Stabilized by Solvent Co-Adsorption: Observation by Operando IR Spectroscopy

On-top hydrogen on a Pd cathode (Pd–H) was observed for the first time by operando infrared (IR) spectroscopy at 2030 cm –1, being stabilized by co-adsorbed solvent ( n -alkane) molecules. A proton-exchange membrane (PEM) reactor with Pt/C (anode) and Pd/C (cathode) catalysts was modified to transmit an IR beam for operando observation. Pd–H species was observed when (1) H + was supplied to the cathodic chamber through the PEM, (2) solvent was flowed at the cathode, and (3) an appropriate bias was applied to the cathode. Because atomic H on Pd surfaces interacts with multiple Pd atoms, on-top Pd–H is proposed to be stabilized by co-adsorbed solvent molecules. Spectra of the solvent were deformed when Pd–H species were present: the peak intensity increased and nonfundamental bands at 2800–2500 cm –1 became pronounced. The intensification of these bands was caused by resonance with free-electron absorption bands in the IR region.