A highly efficient and environmentally-friendly oxidation process is always desirable for air purification. This study reported a novel carbon quantum dots (CQDs)/ZnFe2O4 composite photocatalyst for the first time through a facile hydrothermal process. The CQDs/ZnFe2O4 (15 vol %) composite demonstrates stronger transient photocurrent response, approximately 8 times higher than that of ZnFe2O4, indicating superior transfer efficiency of photogenerated electrons and separation efficiency of photogenerated electron–hole pairs. Compared with pristine ZnFe2O4 nanoparticles, CQDs/ZnFe2O4 displayed enhanced photocatalytic activities on gaseous NOx removal and high selectivity for nitrate formation under visible light (λ > 420 nm) irradiation. Electron spin resonance analysis and a series of radical-trapping experiments showed that the reactive species contributing to NO elimination were ·O2– and ·OH radicals. The possible mechanisms were proposed regarding how CQDs improve the photocatalytic performance of ZnFe2O4. The CQDs are believed to act as an electron reservoir and transporter as well as a powerful energy-transfer component during the photocatalysis processes over CQDs/ZnFe2O4 samples. Furthermore, the toxicity assessment authenticated good biocompatibility and low cytotoxity of CQDs/ZnFe2O4. The results of this study indicate that CQDs/ZnFe2O4 is a promising photocatalyst for air purification.
Since the pioneering work on polychlorinated biphenyl photodegradation by Carey in 1976, photocatalytic technology has emerged as a promising and sustainable strategy to overcome the significant challenges posed by energy crisis and environmental pollution.In photocatalysis, sunlight, which is an inexhaustible source of energy, is utilized to generate strongly active species on the surface of the photocatalyst for triggering photo-redox reactions toward the successful removal of environmental pollutants, or for water splitting.The photocatalytic performance is related to the photoabsorption, photoinduced carrier separation, and redox ability of the semiconductor employed as the photocatalyst.Apart from traditional and noble metal oxide semiconductors such as P25, bismuth-based compounds, and Pt-based compounds, 2D g-C3N4 is now identified to have enormous potential in photocatalysis owing to the special π-π conjugated bond in its structure.However, some inherent drawbacks of the conventional g-C3N4, including the insufficient visible-light absorption ability, fast recombination of photogenerated electron-hole pairs, and low quantum efficiency, decrease its photocatalytic activity and limit its application.To date, various strategies such as heterojunction fabrication, special morphology design, and element doping have been adopted to tune the physicochemical properties of g-C3N4.Recent studies have highlighted the potential of defect engineering for boosting the light harvesting, charge separation, and adsorption efficiency of g-C3N4 by tailoring the local surface microstructure, electronic structure, and carrier concentration.In this review, we summarize cutting-edge achievements related to g-C3N4 modified with classified non-external-caused defects (carbon vacancies, nitrogen vacancies, etc.) and external-caused defects (doping and functionalization) for optimizing the photocatalytic performance in water splitting, removal of contaminants in the gas phase and wastewater, nitrogen fixation, etc.The distinctive roles of various defects in the g-C3N4 skeleton in the photocatalytic process are also summarized.Moreover, the practical application of 2D g-C3N4 in air pollution control is highlighted.Finally, the ongoing challenges and perspectives of defective g-C3N4 are presented.The overarching aim of this article is to provide a useful scaffold for future research and application studies on defect-modulated g-C3N4.
Abstract. High contribution of secondary organic aerosol to the loading of fine particle pollution in China highlights the roles of volatile organic compounds oxidation. Therein, particulate active metallic oxides in dust, like TiO2 and Fe ions, were proposed to influence the photochemical reactions of ambient VOCs. A case study was conducted at an urban site within Xi'an, northwestern China, to investigate the origin and transformation of VOCs during a windblown dust-to-haze pollution episode, and the assumption that dust would enhance the oxidation of VOCs was verified. Local vehicle exhaust (24.76 %) and biomass burning (18.37 %) were found to be the two largest contributors to ambient VOCs. In the dust pollution period, sharp decrease of VOCs loading and aging of their components were observed. Simultaneously, the secondary oxygenated VOCs fraction (i.e., methylglyoxal) increased. Source strength, physical dispersion, and regional transport were eliminated from the major factor for the variation of ambient VOCs. In another aspect, about 2 and 3 times increase of the loading of Iron (Fe) and titanium (Ti) was found in the airborne particle, together with fast decrease of trans-/cis-2-butene ratios which demonstrated that dust can accelerate the oxidation of ambient VOCs and formation of SOA precursors.
Water freezing is a crucial physical phenomenon. The process of ice formation, and the estimation of the ice nucleation rate also have important applications. However, until now, the experimental phenomenon of rapid freezing of water in nanoseconds has not been fully explained theoretically, and the physics underlying the experimental phenomenon has still not been revealed. In this work, combining classical nucleation theory with Mie theory, a kinetic model is developed that reproduces for the first time the experimental phenomenon of decreasing transmissivity. The process of ice formation (nucleation, growth and engulfment) has been revealed. In the process of theoretical derivation, the Zel'dovich-Frenkel (ZF) equation is developed, indicating a limit to the phase transition driving force |Δμ|/(kBT) ≤ 1. By analyzing the experimental and simulation results, it is suggested that the change in the transparency of the sample may be caused by the ice/vacuum interface scattering. In addition, during the rapid phase transition, it was found that the phase transition continues to occur even after phase fraction normalization. Finally, the approximate formula between the nucleation rate and sample transparency is given. This formula can predict the change of sample transparency during phase transition and provides a way to measure the nucleation rate. The results presented here give an insight into the phase transition kinetics, and the methodology may also work for the phase transitions of other materials.
Formation and decay of formaldehyde oxides (CH2OO) affect the complete oxidation of formaldehyde. However, the speciation and reactivity of CH2OO are poorly understood because of its extremely fast kinetics and indirect measurements. Herein, three isomers of CH2OO (i.e., main formic acid, small dioxirane, and minor CH2OO Criegee) were in situ determined and confirmed as primary intermediates of the room-temperature catalytic oxidation of formaldehyde with two reference catalysts, that is, TiO2/MnOx–CeO2 and Pt/MnOx–CeO2. CH2OO Criegee is quite reactive, whereas formic acid and dioxirane have longer lifetimes. The production, stabilization, and removal of the three intermediates are preferentially performed at high humidity, matching well with the decay rate of CH2OO at approximately 6.6 × 103 s–1 in humid feed gas faster than 4.0 × 103 s–1 in dry feed. By contrast, given that a thinner water/TiO2 interface was well-defined in TiO2/MnOx–CeO2, fewer reductions in the active sites and catalytic activity were found when humidity was decreased. Furthermore, lethal intermediates mostly captured at the TiO2/MnOx–CeO2 surface suppressed the toxic off-gas emissions. This study provides practical insights into the rational design and selectivity enhancement of a reliable catalytic process for indoor air purification under unfavorable ambient conditions.
Abstract Solid atmospheric particulates can act as heterogeneous drivers for gas loss and particle aging during haze episodes. Observational and experimental evidence reveals an unidentified competitive mechanism involving transition metal ions (TMIs) that catalyze the heterogeneous oxidation of isoprene. Hydroxyl radicals (OH) were generated through the reaction of singlet oxygen (O( 1 D)) with molecular water at the surface of earth‐abundant manganese (Mn) nanoparticles. The energy threshold for OH production was minimized to 213 kJ mol −1 in the presence of alkali K + ions, significantly lower than the 392 kJ mol −1 required for ozone photolysis. The rapid loss of isoprene (1.60 × 10 −2 s −1 ) for the particulate mixtures resulted in the formation of approximately 70% C 1 –C 4 carbonyl oligomers via interfacial binding modes, which promoted particle growth. This contrasts with the higher yields of C 5 products typically observed in gas‐phase reactions of isoprene with OH radicals. The findings could enhance the understanding of severe haze formation, particularly under complex air pollution conditions.
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