Near-Infrared-Driven Selective Photocatalytic Removal of Ammonia Based on Valence Band Recognition of an α-MnO2/N-Doped Graphene Hybrid Catalyst.

Near-infrared (NIR)-response photocatalysts are desired to make use of 44% NIR solar irradiation. A flower-like α-MnO2/N-doped graphene (NG) hybrid catalyst was synthesized and characterized by X-ray diffraction spectroscopy, transmission electron microscopy, Raman spectroscopy, UV-vis-NIR diffuse reflectance spectroscopy, and X-ray photoelectron spectroscopy. The flower-like material of α-MnO2/NG was oval-shaped with the semi major axis of 140 nm and semi minor axis of 95 nm and the petal thickness of 3.5-8.0 nm. The indirect band gap was measured to be 1.16 eV, which is very close to 0.909 eV estimated by the first-principles calculation. The band gap can harvest NIR irradiation to 1069 nm. The coupling of α-MnO2 with NG sheets to form α-MnO2/NG can significantly extend the spectrum response up to 1722 nm, improving dramatically the photocatalytic activity. The experimental results displayed that the α-MnO2/NG hybrid catalyst can recognize ammonia in methyl orange (MO)-ammonia, rhodamine B (RHB)-ammonia, and humic acid-ammonia mixed solutions and selectively degrade ammonia. The degradation ratio of ammonia reached over 93.0% upon NIR light irradiation in the mixed solutions, while those of MO, RHB, and humic acid were only 9.7, 9.4, and 15.7%, respectively. The products formed during the photocatalytic process were followed with ion chromatography, gas chromatography, and electrochemistry. The formed nitrogen gas has been identified during the photocatalytic process. A valence band recognition model was suggested based on the selective degradation of ammonia via α-MnO2/NG.