The electrocatalytic glycerol oxidation reaction (GOR) represents a mild and sustainable pathway to produce value-added chemicals such as formate. However, the rational design and controlled synthesis of efficient catalysts to achieve highly selective C-C bond cleavage of glycerol towards formate remains a great challenge. Herein, atomic Pd doped Cu3N hollow nanocubes with Pd-Cu dual sites were prepared via the unique Kirkendall process, exhibiting superior electrocatalytic GOR activity and selectivity towards formate with a maximum Faraday efficiency of 91.6%. In situ spectroscopic characterization and theoretical calculations reveal that Pd atoms located in partial Cu substituent sites could facilitate the generation of active *OH and enhance the adsorption of glycerol molecule on adjacent Cu sites, while Pd atoms located in Cu3N unit center could effectively reduce the energy barrier of C-C bond cleavage for the rate determining step. Our work provides new insights into the development of well-defined dual-site catalysts to boost glycerol electrooxidation to formate.