Unexpected Catalytic Influence of Atmospheric Pollutants on the Formation of Environmentally Persistent Free Radicals

Abstract

Environmentally persistent free radicals (EPFRs) have been recognized as harmful and persistent environmental pollutants. In polluted regions, many acidic and basic atmospheric pollutants, which are present at high concentrations, may influence the extent of the formation of EPFRs. In the present paper, density functional theory (DFT) and ab-initio molecular dynamics (AIMD) calculations were performed to investigate the formation mechanisms of EPFRs with the influence of the acidic pollutants sulfuric acid (SA), nitric acid (NA), organic acid (OA), and the basic pollutants, ammonia (A), dimethylamine (DMA) on α‑Al2O3 (0001) surface. Results indicate that both acidic and basic pollutants can enhance the formation of EPFRs by acting as “bridge” or “semi-bridge” roles by proceeding via a barrierless process. Acidic pollutants enhance the formation of EPFRs by first transferring its hydrogen atom to the α‑Al2O3 surface, which subsequently react with phenol to form an EPFR. In contrast, basic pollutants enhance the formation of EPFRs by first abstracting a hydrogen atom from phenol to form a phenoxy EPFR, which eventually interact with the α‑Al2O3 surface. These new mechanistic insights will inform in understanding the abundant EPFRs in polluted regions with high mass concentrations of acidic and basic pollutants.

Keywords: Environmentally persistent free radicals (EPFRs), α-Al2O3(0001), Acidic pollutants, Basic pollutants, Hydrogen atom transfer