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Visible-Light Photocatalytic Ozonation Using Graphitic C3N4 Catalysts: A Hydroxyl Radical Manufacturer for Wastewater Treatment
journal contribution
posted on 2020-03-11, 15:33 authored by Jiadong Xiao, Yongbing Xie, Jabor Rabeah, Angelika Brückner, Hongbin CaoConspectusPhotocatalytic ozonation (light/O3/photocatalyst), an
independent advanced oxidation process (AOP) proposed in 1996, has
demonstrated over the past two decades its robust oxidation capacity
and potential for practical wastewater treatment using sunlight and
air (source of ozone). However, its development is restricted by two
main issues: (i) a lack of breakthrough catalysts working under visible
light (42–43% of sunlight in energy) as well as ambiguous property–activity
relationships and (ii) unclear fundamental reasons underlying its
high yield of hydroxyl radicals (•OH). In this Account,
we summarize our substantial contributions to solving these issues,
including (i) new-generation graphitic carbon nitride (g-C3N4) catalysts with excellent performance for photocatalytic
ozonation under visible light, (ii) mechanisms of charge carrier transfer
and reactive oxygen species (ROS) evolution, (iii) property–activity
relationships, and (iv) chemical and working stabilities of g-C3N4 catalysts. On this basis, the principles/directions
for future catalyst design/optimization are discussed, and a new concept
of integrating solar photocatalytic ozonation with catalytic ozonation
in one plant for continuous treatment of wastewater regardless of
sunlight availability is proposed.The story starts from our
finding that bulk/nanosheet/nanoporous
g-C3N4 triggers a strong synergy between visible
light (vis) and ozone, causing efficient mineralization of a wide
variety of organic pollutants. Taking bulk g-C3N4 as an example, photocatalytic ozonation (vis/O3/g-C3N4) causes the mineralization of oxalic acid (a
model pollutant) at a rate 95.8 times higher than the sum of photocatalytic
oxidation (vis/O2/g-C3N4) and ozonation.
To unravel this synergism, we developed a method based on in situ
electron paramagnetic resonance (EPR) spectroscopy coupled with an
online spin trapping technique for monitoring under realistic aqueous
conditions the generation and transfer of photoinduced charge carriers
and their reaction with dissolved O3/O2 to form
ROS. The presence of only 2.1 mol % O3 in the inlet O2 gas stream can trap 1–2 times more conduction band
electrons than pure O2 and shifts the reaction pathway
from inefficient three-electron reduction of O2 (O2 → •O2– → HO2• → H2O2 → •OH) to more efficient one-electron
reduction of O3 (O3 → •O3– → HO3• → •OH), thereby increasing the yield of •OH by a factor of 17. Next, we confirmed band structure
as a decisive factor for catalytic performance and established a new
concept for resolving this relationship, involving “the number
of reactive charge carriers”. An optimum balance between the
number and reducing ability of photoinduced electrons, which depends
on the interplay between the band gap and the conduction band edge
potential, is a key property for highly active g-C3N4 catalysts. Furthermore, we demonstrated that g-C3N4 is chemically stable toward O3 and •O2– but that •OH can tear and oxidize its heptazine units to form cyameluric acid
and further release nitrates into the aqueous environment. Fortunately, •OH usually attacks organic pollutants in wastewater
in preference to g-C3N4, thus preserving the
working stability of g-C3N4 and the steady operation
of photocatalytic ozonation. This AOP, which serves as an in situ •OH manufacturer, would be of interest to a broad chemistry
world since •OH radicals are active species not
only for environmental applications but also for organic synthesis,
polymerization, zeolite synthesis, and protein footprinting.
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EPRWastewater Treatment ConspectusPhotocatalytic ozonationAOPVisible-Light Photocatalytic Ozonationform cyameluric acidOHconduction band edgeHOrate 95.8 timesGraphitic C 3 N 4 CatalystsROSreactive oxygen speciesHydroxyl Radical Manufacturercharge carrier transferinlet O 2 gas streamO 2O 3bulk g-C 3 N 4g-C 3 N 4 catalystsphotocatalytic ozonationconduction band electronsphotoinduced charge carriersg-C 3 N 4new-generation graphitic carbon nitride
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