Molecular Insights into the Spontaneous Generation of CO ₂ in Reaction of CO and HOCl at Air–Water Interfaces

The final step for CO in fossil fuel combustion and tropospheric chemistry is its conversion to CO₂ via ·OH oxidation, which represents a major source of greenhouse gas emissions and has been extensively studied. However, the mechanisms for CO-to-CO₂ transformation beyond ·OH-mediated pathways in the troposphere remain poorly understood. Herein, we identify a novel atmospheric CO₂ generation pathway driven by a heterogeneous reaction between CO and HOCl at air-water interfaces, which operates independently of ·OH. Using <i>ab initio</i> molecular dynamics, we elucidate a stepwise mechanism: (i) coordination of the HO moiety of HOCl with the C atom of CO, forming a HOCO intermediate, followed by (ii) Cl atom transfer to the C center accompanied by simultaneous release of HCl and (iii) dissociation of the formed HCl. The reaction exhibits a remarkably low free-energy barrier (Δ<i>G</i>_TS = 13.6 kcal mol^-1) at approximately 300 K. More importantly, Δ<i>G</i>_TS decreases with decreasing temperature. When the temperature decreases to 243 K, the reaction is almost spontaneous (Δ<i>G</i>_TS = 1.7 kcal mol^-1). These findings have further important implications for understanding acid rain formation and O₃ depletion mechanisms.