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• The influences of electronic metal-support interactions (EMSI) on the catalytic hydrogenation of ethylene over Pd n /CeO2 (n = 4, 7, 10) have been studied. • The charge redistribution on the Pd cluster creates 2 typical sites: IF sites located at the interface between metal and support and T sites located at the topmost of the cluster. • IF sites show extremely higher activity for H 2 dissociation and T sites show better activity for ethylene hydrogenation. • At high H coverage, both the kinetics and thermodynamics of the hydrogenation reaction would be significantly promoted. • The two types of sites should collaborate synergically to promote the hydrogenation process. The catalytic activity of supported catalyst is closely related to the electronic metal-support interactions (EMSI). In this paper, first principles density functional theory study was conducted to understand the EMSI between Pd clusters (Pd 4 , Pd 7 , Pd 10) and CeO 2 substrate. When Pd was loaded on CeO 2 surface, the electrons of Pd would transfer to the CeO 2 substrate via Pd-O bonding interactions. Moreover, the charge redistribution on the Pd cluster leads to significant polarization of electron density. Consequently, the Pd atoms located at the metal-support interface (IF site) are positively charged, while the Pd atoms at the topmost (T site) are negatively charged. The H 2 dissociation and ethylene hydrogenation at these sites were calculated with both low and high hydrogen coverage. It was found that the two types of sites should collaborate synergically to promote the hydrogenation of ethylene. In particular, the IF sites exhibit relatively high activity for H 2 dissociation, while the T sites show superior catalytic efficiency for ethylene hydrogenation. Therefore, the EMSI induced charge polarization can tune the electronic structure and charge state of supported metal catalysts, which further determines the performance of different region on the catalyst. [Display omitted] [ABSTRACT FROM AUTHOR]