Hydrodechlorination (HDC) catalyzed by palladium has important applications in the disposal of chlorinated organic pollutants (COPS), yet the associated catalytic sites and the underlying reaction mechanism have not been clearly revealed. Here, we use Pd nanocrystals (Pd NCs) as the probe catalyst to investigate the HDC catalytic sites as well as the reaction mechanism in aqueous solution with 4-chlorophenol as the model COP. It was found that the numbers of edge and corner atoms (defect atoms) on cubic Pd NCs with varying sizes (6.3-18.4 nm) showed linear correlation with the HDC activity, suggesting that these defect atoms are the main catalytic sites for HDC. The production of chloride ion, the absence of Ullmann coupling product in the absence of H-2, and the positive effect of electron-withdrawing substituents on HDC demonstrate that the HDC should include an oxidative addition process that follows the nucleophilic aromatic substitution. (C) 2018 Elsevier Inc. All rights reserved.