Enhanced Visible-Light Nitride Photocatalyst For Air Remediation From NOx Pollutants

Emissions of NOx are one of the most noteworthy issues affecting all people on this planet.Most systems that have been used to reduce the risk of NOx gas and remove it from the air have drawbacks such as high cost,special operation conditions,and time consuming.So,researchers are seeking alternative methods other than conventional methods for NOx removal.The photocatalysis method adjusts with significant advantages,where it is eco-friendly and economical treatment innovation.The conventional photocatalysts most used for air decontamination are titanium oxide(Ti O₂),zinc oxide(Zn O),and Graphitic carbon nitride(g-C₃N₄or CN).The g-C₃N₄ is the most promising photocatalytic material,it has a proper bandgap of 2.7 e V,which enables g-C₃N₄to absorb visible light very efficiently,and also it has an appropriate conduction band(CB)position at-1.1 e V to reduce protons,g-C₃N₄is demonstrated to be able to achieve photocatalytic NOx removal.However,still pristine g-C₃N₄reveals a low efficiency for NOx elimination due to electrons-holes pair's fast recombination and low surface area.The impact impediments of photocatalysts can be minimized via several approaches such as doping technique,and also the photocatalysts material can be improved through the formation a good alignment between two semiconductors with different bandgap energy,which facilitates the process of movement of generated photoelectrons and reduce electron-hole pair recombination rather than the improvement in visible light absorbance.In the first work,we have developed a facile approach for dramatically downwards shifting band edge positions of carbon nitride up by about 1 e V via in-plane heterojunction with graphitic carbon units to enhance the oxidation capability of the electron holes generated from the valence band.The graphitic carbon units in junction with tri-s-triazine domains were clearly observed and its in-plane hybridization with carbon nitride was formed during the copolymerization using melamine with a small amount of m-phenylenediamine(MPD)as the precursors.The direct intralayer junction between the tri-s-triazine and the graphitic carbon domain,essentially different with interlayer junction reported in literature,is able to shift the band edge positions downwards via merging electron density of states of carbon nitride with that of graphitic carbon,and thus would be beneficial for separation of photoexcited charge carriers and generation of hydroxyl radicals for the oxidation of pollutants.The hybrid photocatalyst prepared with a small quantity(less than 1%)of MPD and melamine as precursors have shown much enhanced NO oxidation to final products(NO₂^-and NO₃^-)and increased NO removal 10%than the one from melamine only.In the second work,different ionic salts were mixed with the melamine precursor to prepare cation(Na⁺,K⁺,and NH₄⁺)or anion(SO₄^2-,S₂O₈^2-)incorporated CN photocatalyst for efficient NO removal.The combinatorial study of the six salt additives indicated that the thermal behavior of the salts is important to the melamine polymerization and the functional groups of resultant CN,though all of them are found to increase the surface area of the final photocatalyst.Dangling-C?N bonds and more surface moieties at higher oxidation state were found when the polymerization is mediated with Na⁺and K⁺(per)sulfate.The sodium and ammonia salts caused an upward shift of the the conduction band energy of the final photocatalyst,while K₂SO₄and K₂S₂O₈ induced a downward shift of the conduction band.The CN mediated with persulfate increased the NO removal efficiency by 20%than pristine CN(60%),while the CN mediated with sulfate reached an efficiency to near 100%.The samples mediated with persulfate salts showed higher selectivity for ionic species than those with sulfate salts.The residue of the salts in the final photocatalysts showed a minor effect on the initial NO removal but may be important in absorbing the produced proton in the longer run.In the third work,Dual-phase Nb₂O₅(N)/Ga N(O)photocatalyst with N doping in Nb₂O₅ and O in Ga N was synthesized via nitridation and re-oxidation(NRO)of Ga Nb O₄.The NRO process induced the pulverization of the particles and nanosized heterojunctions between the Nb₂O₅(N)and Ga N(O)phase.The utilization of Ga Nb O₄as a precursor enhances the nitridation process than pure gallium oxide while retards the oxidation of pure niobium nitride to give an improved doping composite with the desired bandgap around 2.0 e V with the indirect transition.The photocatalyst exhibited very high NO removal efficiency(nearly 100%)under visible-light and very high selectivity(89%)for the formation of ionic species rather than more toxic NO₂,which could be related to the intimate junction between the Ga N phase with high conduction band and Nb₂O₅ phase with low conduction bandgap for the simultaneous generation of·O₂?and·OH radicals,respectively.The NRO sheds light on the preparation of composite photocatalyst containing nitride and oxide.