Molecular Insights into the Spontaneous Generation of CO2 in Reaction of CO and HOCl at Air–Water Interfaces

Abstract

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 ab initio 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 (ΔG_TS = 13.6 kcal mol^–1) at approximately 300 K. More importantly, ΔG_TS decreases with decreasing temperature. When the temperature decreases to 243 K, the reaction is almost spontaneous (ΔG_TS = 1.7 kcal mol^–1). These findings have further important implications for understanding acid rain formation and O₃ depletion mechanisms.