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. 0.06V vs. RHE), the constant potential 0.4V (vs. RHE) and the scanned potentials between 0.1 and 0.5V (vs. RHE), CO is clearly confirmed as a surface poisoning species with its vibrational frequencies located over similar to 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). (C) 2011 Elsevier B.V. All rights reserved.