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Heterogeneous dual-atom catalysts (DACs) in transition metal–nitrogen–carbon (M–N–C) frameworks demonstrate high tunability for electrochemical CO2 reduction, yet the mechanistic origin of their metal composition-dependent selectivity remains elusive. While conventional transition-metal (TM)-based DACs predominantly yield CO, the strategic incorporation of main-group metals (MMs) enables a product switch to formic acid, as exemplified by NiSn DACs. Through operando-modeling constant-potential DFT and microkinetic analysis of 16 TMMM combinations (TM = Mn/Fe/Co/Ni; MM = In/Sn/Sb/Bi), we unravel an adsorption duality mechanism governing selectivity. Under working conditions, the in situ transition from chemisorbed CO2 to physisorbed configurations steers the reaction pathway toward the *OCHO intermediate rather than the *COOH. This dynamic reconfiguration originates from asymmetric charge accumulation at dual-metal centers, quantified through our proposed charge aggregation intensity (CAI) descriptor. The CAI-directed screening identifies NiSb DAC as a promising candidate for HCOOH production. Our work establishes the atomic-scale design principle linking interfacial charge accumulation to dynamic adsorption evolution in DACs, providing a reliable framework for targeted CO2-to-chemical conversion.
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