Identification of Active Sites over Metal-Free Carbon Catalysts for Flue Gas Desulfurization

Carbon-based catalysts have been extensively used for flue gas desulfurization (FGD) and have exerted great importance in controlling SO₂ emissions over the past decades. However, many fundamental details about the nature of the active sites and desulfurization mechanism still remain unclear. Here, we reported the experimental and theoretical identifications of active sites in FGD on carbon catalysts. Temperature-programmed decomposition allowed us to modulate the number of oxygen functional groups on carbon catalysts and to establish its correlation with desulfurization activity. Selective passivation further demonstrated that the ketonic carbonyl (C═O) groups are the intrinsic active sites for FGD reaction. Combined with transient response experiments, quasi-in situ X-ray photoelectron spectroscopy, and density functional theory simulations, it was revealed that desulfurization reaction on carbon catalysts mainly proceeded via the Langmuir-Hinshelwood mechanism, during which the nucleophilic ketonic C═O groups served as active sites for chemically absorbing SO₂ and their adjacent sp²-hybridized carbon atoms dissociatively activated O₂. It also turned out that the formation of H₂SO₄ is the reaction barrier step. The output of this study should not only advance the understanding of desulfurization at the atomic scale but also provide a general guideline for the rational design of efficient carbon catalysts for FGD.