Abstract: The different structures of PdOH2, PdOH, and PdO were optimized by the relativistic effective core potential (RECP) and the density functional method B3LYP. The results showed that Pd and H2O of stable Pd-OH2 molecule were not in the same plane, and the bond length (R_PdO) was 0.2283 nm. For PdOH molecule, the ground state was 2A', the structure parameters RPdO, ROH, and ∠PdOH were 0.1965 nm, 0.0968 nm, and 110.186°, respectively; The
ground state of PdO was 3Π, and the bond length was 0.1858 nm. Furthermore, the thermodynamic functions of PdOH2, PdOH, and PdO were also calculated according to electronic-vibration approximation, and based on this results, the relationship of the equilibrium pressures with temperatures was obtained. From the results, it was deduced that the poisoning effect of H2O was due to the adsorption of O and OH dissociated from H2O on palladium surface.

Key words: PdOH2, PdOH, PdO, Thermodynamic function, Dissociative adsorption, Poisoning