Direct Evidence for Carbon Monoxide-to-Dioxin Conversion via Fly-Ash-Catalyzed Sequential Reactions in Incineration Flue Gas

Abstract

The formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in industrial thermal processes is traditionally attributed to solid carbon precursors via de novo synthesis. Carbon monoxide (CO), despite serving as an operational indicator of incomplete combustion, has not been directly implicated as a molecular carbon source for dioxin generation. Here, we provide definitive experimental and mechanistic evidence that CO can be catalytically converted into dioxins and dioxin-like compounds through a sequential reaction network mediated by fly ash. Using fixed-bed reactors packed with real fly ash, we isolate CO, H2O, and HCl as minimal reactants to reconstruct flue-gas chemistry. Transition metal species (Cu, Fe) in fly ash catalyze CO conversion via a coupled water–gas shift and Fischer–Tropsch-type process, generating a wide spectrum of aliphatic and aromatic hydrocarbons. Concurrently, fly ash mediates HCl oxidation to active Cl2, enabling chlorination of these intermediates into chlorobenzenes and chlorophenols, which subsequently evolve into polychlorinated biphenyls (PCBs), polychlorinated naphthalenes (PCNs), and ultimately PCDD/Fs. Notably, Cl4–8DD/F congeners account for >90% of the formed PCDD/Fs, consistent with de novo synthesis signatures. This work establishes a new mechanistic paradigm for dioxin generation and highlights CO emission control as a critical lever for mitigating toxic pollutant generation.