Density functional study on the heterogeneous oxidation of NO over α-Fe₂O₃ catalyst by H₂O₂: Effect of oxygen vacancy

Highlights: • NO and H₂O₂ adsorption on perfect and oxygen defect α-Fe₂O₃ (0 0 1) surface were studied by DFT calculations. • H₂O₂ shows high chemical reactivity for its adsorption on oxygen defect α-Fe₂O₃ (0 0 1) surface. • Oxygen vacancy plays an important role of the catalytic oxidation of NO by H₂O₂ over the α-Fe₂O₃ catalyst surfaces. • Mechanism of NO oxidation over α-Fe₂O₃ (0 0 1) surface by H₂O₂ was explained. - Abstract: Catalytic oxidation with H₂O₂ is a promising method for NOx emission control in coal-fired power plants. Hematite-based catalysts are attracting increased attention because of their surface redox reactivity. To elucidate the NO oxidation mechanism on α-Fe₂O₃ surfaces, density functional theory (DFT) calculations were conducted by investigating the adsorption characteristics of nitric oxide (NO) and hydrogen peroxide (H₂O₂) on perfect and oxygen defect α-Fe₂O₃ (0 0 1) surfaces. Results show that NO was molecularly adsorbed on two kinds of surfaces. H₂O₂ adsorption on perfect surface was also in a molecular form; however, H₂O₂ dissociation occurred on oxygen defect α-Fe₂O₃ (0 0 1) surface. The adsorption intensities of the two gas molecules in perfect α-Fe₂O₃ (0 0 1) surface followed the order NO > H₂O₂, and the opposite was true for the oxygen defect α-Fe₂O₃ (0 0 1). Oxygen vacancy remarkably enhanced the adsorption intensities of NO and H₂O₂ and promoted H₂O₂ decomposition on catalyst surface. As an oxidative product of NO, HNO₂ was synthesized when NO and H₂O₂ co-adsorbed on the oxygen defect α-Fe₂O₃ (0 0 1) surface. Analyses of Mulliken population, electron density difference, and partial density of states showed that H₂O₂ decomposition followed the Haber–Weiss mechanism. The trends of equilibrium constants suggested that NO adsorption on α-Fe₂O₃ (0 0 1) surface was more favorable at low than at high temperatures, whereas H₂O₂ adsorption was favorable between 375 and 450 K. These calculations results well agreed with the experimental ones and further elucidates the reaction mechanisms.