In situ spectroscopic investigation of CO accumulation and poisoning on Pd black surfaces in concentrated HCOOH

Attenuated total reflection-infrared (ATR-IR) spectroscopy is extended to investigate the surface poisoning species in the processes of (electro)chemical decomposition of formic acid (FA) on a state-of-the-art commercial Pd black catalyst in 5 M FA solution. During the FA decomposition under different potential settings including the open circuit potential (OCP, ca. 0.06 V vs. RHE), the constant potential 0.4 V (vs. RHE) and the scanned potentials between 0.1 and 0.5 V (vs. RHE), CO is clearly confirmed as a surface poisoning species with its vibrational frequencies located over ∼1845 to 2016 cm−1, featuring different CO bonding configurations (including the triple-, bridge- and linear-bonded CO species) on Pd black surfaces. COad coverage increases with increasing operation time and decreasing operation potential. Once formed, COad can only be removed at a much higher oxidation potential, corresponding to the reactivation of the Pd black surfaces. The present results provide a molecular level insight into an important aspect of the deactivation issue for a real Pd nanocatalyst in a practical FA concentration relevant to the anode operations of direct formic acid fuel cells (DFAFCs).

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► ATR-IR spectroscopy is used to study surface poisoning of Pd black in 5 M HCOOH.
► CO accumulation and removal on Pd black surfaces is clarified at molecular level.
► Lower potential favors CO accumulation on Pd black surfaces.
► Removal of as-formed CO reactivates Pd black surfaces for HCOOH electro-oxidation.