Mechanistic Origin of the 2e– CO2 Reduction Selectivity of Single-Atom Catalysts via Kinetic Bifurcation of *COOH

Abstract

Electrochemical two-electron CO2 reduction (2e– CO2RR) to carbon monoxide (CO) and formic acid (HCOOH) on single-atom catalysts (SACs) has long been rationalized by independent reaction pathways initiated from distinct CO2 adsorption modes. Here, employing constant-potential ab initio molecular dynamics (AIMD) simulations combined with the slow-growth enhanced sampling, we demonstrate that product selectivity across diverse SACs is governed by the kinetic bifurcation of a common *COOH intermediate. We uncover that the initial CO2 activation proceeds via cation-assisted chemisorption even on electron-deficient metal centers such as Sb and Sn. The downstream product branching originates from a kinetic competition between CO and HCOOH formation, which effectively explains experimental selectivity. Beyond reproducing empirical trends, our findings establish *COOH bifurcation kinetics as a key governing principle for 2e– CO2RR, providing mechanistic insights for the rational design of selective SACs.