Microwave-Induced Deep Catalytic Oxidation of NO Using Molecular-Sieve-Supported Oxygen-Vacancy-Enriched Fe-Mn...

A novel microwave (MW) catalytic oxidation denitrification method was developed, which can deeply oxidize NO into nitrate/nitrite with little NO₂ yield. A molecular-sieve-supported oxygen-vacancy-enriched Fe₂O₃-MnO₂ catalyst (Ov-Fe-Mn@MOS) was fabricated. Physicochemical properties of the catalyst were revealed by various characterization methods. MW irradiation was superior to the conventional heating method in NO oxidation (90.5 vs 70.6%), and MW empowered the catalyst with excellent low-temperature activity (100-200 °C) and good resistance to H₂O and SO₂. Ion chromatography analysis demonstrated that the amount of nitrate/nitrite accounted for over 90.0% of the N products, but the main product gradually varied from nitrate to nitrite as the reaction proceeded because of the switching of the main reaction path of NO removal. Mechanism analyses clarified that NO oxidation was a non-radical catalytic reaction: (i) the chemisorbed NO on ≡Mn(IV) reacted with O₂* to produce nitrate and (ii) the excited NO* due to MW irradiation reacted with the active O* generated from Ov···O₂ to form nitrite. Density functional theory calculations combined with electron paramagnetic resonance tests revealed the promotional effects of Fe₂O₃ in (i) boosting the Ov's quantity; (ii) facilitating O₂ adsorption; (iii) increasing the nitrite formation; and (iv) alleviating the suppression of SO₂.