Generation Of OH Radicals Research Articles

Analysis of the combustion mechanism of diesel surrogate fuel under CO2/O2 atmosphere

Closed-cycle diesel engine (CCDE) can effectively control CO2 emissions and reduce the greenhouse gas effect, but diesel fuel combustion under an atmosphere with CO2 has the problem of misfire and unclear mechanism. In order to study the CO2 impact on diesel combustion under the CO2/O2 atmosphere, a quantum chemical (QC) model of diesel surrogate fuel (DSF) is proposed, which takes into account the CO2 pyrolysis effect (CO2⇆CO+12O2) and the new reaction of OH radical generation from pyrolysis products (CO+O2+H∙→CO2+OH∙). Firstly, the active sites of the DSF molecules are predicted by average local ionization energy (ALIE) and electrostatic potential (ESP). New chemical reaction paths are obtained using ab initio theory and density functional theory combined with thermodynamic combination algorithm. The QC model is then established by sensitivity analysis and simplification of the mechanism and computational fluid dynamics (CFD) simulation of constant volume combustion chamber (CVCC.) Secondly, the CVCC experiment bench and visualization system are set up. The flame propagation at different times (1.5 ms, 1.9 ms, 2.3 ms, 2.7 ms, 3.1 ms and 3.5 ms) are captured by high-speed cameras under four different CO2 concentrations (air condition, 50%CO2 + 50%O2, 43%CO2 + 57%O2 and 35%CO2 + 65%O2). Finally, the comparison between experiment and model shows that the QC model is suitable for studying combustion characteristics of diesel fuel under CO2/O2 atmosphere. Under maximum CO2 concentration (50%CO2 + 50%O2), The maximum errors of flame combustion limit length (FCLL) and Flame longitudinal section area (FLSA) are 8.54% and 4.94%, respectively. In addition, the CO2 pyrolysis reaction and the new reaction of generating OH radicals indicate that the CO2 chemical effect can promote the combustion of diesel fuel under CO2/O2 atmosphere.

Disclosing the Origin of Transition Metal Oxides as Peroxidase (and Catalase) Mimetics.

Since Fe3O4 was reported to mimic horseradish peroxidase (HRP) with comparable activity (2007), countless peroxidase nanozymes have been developed for a wide range of applications from biological detection assays to disease diagnosis and biomedicine development. However, researchers have recently argued that Fe3O4 has no peroxidase activity because surface Fe(III) cannot oxidize tetramethylbenzidine (TMB) in the absence of H2O2 (cf. HRP). This motivated us to investigate the origin of transition metal oxides as peroxidase mimetics. The redox between their surface Mn+ (oxidation) and H2O2 (reduction) was found to be the key step generating OH radicals, which oxidize not only TMB for color change but other H2O2 to produce HO2 radicals for Mn+ regeneration. This mechanism involving free OH and HO2 radicals is distinct from that of HRP with a radical retained on the Fe-porphyrin ring. Most importantly, it also explains the origin of their catalase-like activity (i.e., the decomposition of H2O2 into H2O and O2). Because the production of OH radicals is the rate-limiting step, the poor activity of Fe3O4 can be attributed to the slow redox of Fe(II) with H2O2, which is two orders of magnitude slower than the most active Cu(I) among common transition metal oxides. We further tested glutathione (GSH) detection on the basis of its peroxidase-like activity to highlight the importance of understanding the mechanism when selecting materials with high performance.

Generation of reactive species by naturally sucked air in the Ar plasma jet

In this study, we used an Ar plasma jet to enhance reactive oxygen and nitrogen species (RONS). Atmospheric air was mixed directly with the working gas through the hole of the quartz tube in the discharge region. The electrical, optical, and chemical characteristics of air-mixed Ar plasma via a hole and pure Ar plasma without a hole were compared and analyzed. The hydroxyl (OH) radicals, singlet oxygen (O), and N2 second positive system (SPS) produced in the gas phase were significantly higher in the air-mixed Ar plasma than that in the pure Ar plasma. In addition, the generation of OH radicals was higher in the gas and liquid phases. Furthermore, the concentrations of nitrogen dioxide (NO2) in air, hydrogen peroxide (H2O2), and nitrite (NO2−) the in liquid phase were measured and were found to be higher in the air-mixed Ar plasma than that in pure Ar plasma. The naturally sucked atmospheric air through the drilled hole could be utilized to enhance the RONS by keeping similar power consumption with pure Ar plasma in the gas and liquid phases at a low cost compared to the discharge conditions of other noble gases. This study will be beneficial in the commercialization of non-thermal plasma jets which could be relevant in biomedical and other non-thermal plasma-related applications.

Direct dual-Z-scheme PANI/Ag2O/Cu2O heterojunction with broad absorption range for photocatalytic degradation of methylene blue

The PANI/Ag2O/Cu2O (PAC) direct dual-Z-scheme heterojunction was synthesized by ultrasonic-assisted wet impregnation method. The heterojunction utilizes broad spectrum form UV, visible to NIR region. The structural characterization and chemical composition was explored by series of instrumental techniques. A significant photodegradation rate of 275 × 10−4 min−1 against methylene blue (MB) were observed on addition of H2O2 at pH 8. H2O2 addition intensify the •OH radicals generation that photodegraded nearly 98% of MB in 40 min. Further, trapping experiment and ESR technique validate the active role of •OH, •O2− radicals and holes during photocatalytic process. Several critical factors governing the rate of photodegradation such as pH, concentration H2O2, MB and photocatalyst were also analysed. The degradation mechanism and the intermediate formed were established by performing HPLC and LCMS analysis. Lastly, the direction of charge migration, slower rate of recombination, and effective separation of charge carrier were explicit by XPS analysis and electrochemical study. Based on the obtained results, work function of independent photocatalysts and the alignment of energy bands, a plausible direct dual-Z-scheme charge migration were proposed.

Influence of the Oxide gas on thermal decomposing explosion of methyl nitrite based on instantaneous flame characterization

The decomposing explosion behavior and characteristics of methyl nitrite (MN) were investigated by analyzing the instantaneous flame of MN decomposition explosion in this work. The lowest explosion limit of MN decreases with the addition of nitric oxide, nitrous oxide and oxygen. The ternary diagrams of the explosive range for “methyl nitrite/nitrogen/oxide gas” mixtures at 40℃ and 100 kPa were acquired. The transient explosion process was characterized by high-speed video image and emission spectrum at 40℃ and 40 kPa. The results demonstrate that the decomposing explosion mainly includes dehydrogenation and oxidation of the carbonaceous species, reduction of nitric oxide and conversion of the OH radical, which can be divided into two reaction stages. The first stage is transient and lasts less than 50 ms, basically involving the generation of nitric oxide, methanol and formaldehyde, and most reactions of dehydrogenation and nitric oxide reduction. The second stage lasts more than 300 ms, and develops from the cool flame to the yellow flame, mainly involving the combustion reactions of hydrogen and carbon monoxide with the OH radical. The type and concentration of oxide gases affect the generation of key species HNO and OH radicals which in turn have an important impact on the evolution of the explosion.

Experimental study on the influence of H 2 O vapor and O 2 for desulfurization and denitrification of the diesel exhaust in marine application by non‐thermal plasma

To improve the pollutant removal efficiency by non-thermal plasma (NTP), the effect of H2O vapor and O2 on removal efficiency of NO and SO2 as well as the reduction of CO was investigated in H2O/O2/SO2/NO/C3H6/CO2/N2 system. To analyse the reaction mechanism, the effects of H2O vapor and O2 on the emission intensity of O ( 3 p 5 P → 3 s 5 S 2 0 ) and O H ( A 2 ∑ + → X 2 ∏ ) were investigated. The experimental results show that the increase of H2O vapour (0%–9.8%) promotes the generation of OH radicals, increases the removal efficiency of NO from 18.9% to 57.3%, decreases the energy per NO removed from 449.0 to 148.1 eV/NO, increases SO2 removal efficiency from 4.8% to 35.3%, decreases the energy per SO2 removed decreases from 2784.0 to 582.9 eV/SO2, and reduces the generation of CO from 460 to 229 ppm. In the range of 0%–15%, the increase of O2 content promotes the formation of O radicals, increases the removal efficiency of NO from 8.5% to 54.2%, and decreases the energy per NO removed from 994.3 to 156.4 eV/NO. In the range of 0%–5%, increasing O2 content promotes SO2 removal efficiency from 8.7% to 24.8%, and decreases the energy per SO2 removed from 2485.7 to 876.6 eV/SO2. However, CO generation increases from 200 to 350 ppm with the increase of O2 in 0%–5% due to the incomplete oxidation of C3H6. In the range of 5%–15%, the increase of O2 produces more O radicals, decreases the removal efficiency of SO2 from 24.8% to 23.5% and the generates of CO from 350 to 311 ppm. This study is helpful for improving the efficiency of NTP for desulfurization and denitrification, while reducing the by-product CO.

Interfacial engineering in 3D/2D and 1D/2D bismuth ferrite (BiFeO3)/Graphene oxide nanocomposites for the enhanced photocatalytic activities under sunlight

3D-particulate and 1D-fiber structures of multiferroic bismuth ferrite (BiFeO3/BFO) and their composites with 2D-graphene oxide (GO) have been developed to exploit the different scheme of interfacial engineering as 3D/2D and 1D/2D systems. Particulates and fibers of BFO were developed via sol-gel and electrospinning fabrication approaches respectively and their integration with GO was performed via the ultrasonic-assisted chemical reduction process. The crystalline and phase formation of BiFeO3 and GO was confirmed from the XRD patterns obtained. The electron microscopic images revealed the characteristic integration of 3D particulates (with average size of 100 nm) and 1D fibers (with diameter of ~150 nm and few μm length) onto the 2D GO layers (thickness of ~27 nm). XPS analysis revealed that the BFO nanostructures have been integrated onto the GO through chemisorptions process, where it indicated that the ultrasonic process engineers the interface through the chemical modification of the surface of these 3D/2D and 1D/2D nanostructures. The photophysical studies such as the impedance and photocurrent measurements showed that the charge separation and recombination resistance is significantly enhanced in the system, which can directly be attributed to the effective interfacial engineering in the developed hetero-morphological composites. The degradation studies against a model pollutant Rhodamine B revealed that the developed nanocomposites exhibit superior photocatalytic activity via the effective generation of OH radicals as confirmed by the radical analysis studies (100% degradation in 150 and 90 min for 15% GO/BFO particulate and fiber composites, respectively). The developed system also demonstrated excellent photocatalytic recyclability, indicated their enhanced stability.

Enhanced-oxidation of sulfanilamide in groundwater using combination of calcium peroxide and pyrite

Fenton reaction using hydrogen peroxide (H2O2) has been widely applied to achieve the in-situ chemical oxidation of contaminated soil and groundwater. However, injecting and transporting H2O2 to a contaminated zone consumes the chemical through reactions with other substances and self-decomposition. Additionally, Fe(II), an activator for the Fenton reaction, scavenges hydroxyl radicals, greatly reducing its activity. Therefore, this study proposes a novel oxidation system combining calcium peroxide (CaO2) and pyrite for the degradation of oxidizable contaminants in groundwater. CaO2 is an oxygen releasing compound, and pyrite is a natural mineral that provides Fe(II). The individual applications of CaO2 and pyrite cannot generate OH radicals and oxidize the target pollutant, sulfanilamide. However, the combination of pyrite and CaO2 oxidized well sulfanilamide even in mild pH and 1.0 wt% of pyrite. Moreover, H2O2 and OH radicals are the dominant oxidants in the reaction. A speciation analysis shows the oxidation of pyrite in this combined system. Furthermore, this system oxidized 80% of 0.1 mM sulfanilamide, whereas only 30% was oxidized by conventional Fenton reaction, indicating that this combined system is effective and applicable to remediate groundwater. This study provides an alternative oxidation process to achieve in-situ chemical oxidation.

Solvothermal growth of {0 0 1} exposed anatase nanosheets and their ability to mineralize organic pollutants. The effect of alcohol type and content on the nucleation and growth of TiO2 nanostructures

Herein, the series of {001} exposed anatase nanosheets from HF-assisted solvothermal growth synthesis were obtained. The two-dimensional TiO2 were characterized, including both bulk (XRD, DR-UV–Vis, Mott-Schottky) and surface characteristics (N2 sorption, XPS, SEM) with experimental results compiled with Density Functional Theory (DFT) calculations. The effect of alcohol amount and type was studied, demonstrating the crucial role of nucleation rate on the final material properties, which was connected with the possible ligand exchange between F− and ROH in the [TiF6]2− octahedra, leading to further TiO2 condensation through an ether elimination reaction. Highly active 2D nanoparticles were easily obtained for different reaction times when a moderate amount of n-butanol or n-hexanol was introduced to the synthesis. The photoactivity of 2D TiO2 with exposed {001} facet strongly depended on the photocatalytic available surface area with an increased amount of ortho–hydroxyphenol (catechol) formed as aby-product. Meanwhile, the rise of n-butanol or n-hexanol content during solvothermal reaction or changing it to ethanol allowed to form smaller and more crystalline anatase particles, but their activity was hindered. For these samples, the slow phenol disappearance and no catechol formation occurred due to (i) alower amount of adsorbed fluorine, (ii) band position shifting towards lower values, and (iii) the surface presence of Ti3+, which prevented the generation of OH radicals. It was also found that changing the nucleation rate by modifying the reaction environment influences the fluorine ions distribution, determining the final properties of 2D structure photocatalyst.

Extended visible light driven photocatalytic hydrogen generation by electron induction from g-C3N4 nanosheets to ZnO through the proper heterojunction

Abstract The alarming energy crises has forced the scientific community to work for sustainable energy modules to meet energy requirements. As for this, ZnO/g-C3N4 nanocomposites with proper heterojunction were fabricated by coupling a proper amount of ZnO with 2D graphitic carbon nitride (g-C3N4) nanosheets and the obtained nanocomposites were applied for photocatalytic hydrogen generation from water under visible light illumination (λ > 420 nm). The morphologies and the hydrogen generation performance of fabricated photocatalysts were characterized in detail. Results showed that the optimized 5ZnO/g-C3N4 nanocomposite produced 70 µmol hydrogen gas in 1 h compare to 8 µmol by pure g-C3N4 under identical illumination conditions in the presence of methanol without the addition of cocatalyst. The much improved photoactivities of the nanocomposites were attributed to the enhanced charge separation through the heterojunction as confirmed from photoluminescence study, capacity of the fabricated samples for •OH radical generation and steady state surface photovoltage spectroscopic (SS-SPS) measurements. We believe that this work would help to fabricate low cost and effective visible light driven photocatalyst for energy production.

Combinatorial Analysis of Sparse Experiments on Photocatalytic Performance of Cement Composites: A Route toward Optimizing Multifunctional Materials for Water Purification.

Blending TiO2 and cement to create photocatalytic composites holds promise for low-cost, durable water treatment. However, the efficiency of such composites hinges on cross-effects of several parameters such as cement composition, type of photocatalyst, and microstructure, which are poorly understood and require extensive combinatorial tests to discern. Here, we report a new combinatorial data science approach to understand the influence of various photocatalytic cement composites based on limited datasets. Using P25 nanoparticles and submicron-sized anatase as representative TiO2 photocatalysts and methyl orange and 1,4-dioxane as target organic pollutants, we demonstrate that the cement composition is a more influential factor on photocatalytic activity than the cement microstructure and TiO2 type and particle size. Among the various cement constituents, belite and ferrite had strong inverse correlation with photocatalytic activity, while natural rutile had a positive correlation, which suggests optimization opportunities by manipulating the cement composition. These results were discerned by screening 7806 combinatorial functions that capture cross-effects of multiple compositional phases and obtaining correlation scores. We also report •OH radical generation, cement aging effects, TiO2 leaching, and strategies to regenerate photocatalytic surfaces for reuse. This work provides several nonintuitive correlations and insights on the effect of cement composition and structure on performance, thus advancing our knowledge on development of scalable photocatalytic materials for drinking water treatment in rural and resource-limited areas.

Photocatalytic degradation of ibuprofen on S-doped BiOBr

Doping heterogeneous atoms into BiOX is recognized as an effective method to improve its photocatalytic activity. Here, S-doped BiOBr (S–BiOBr) was synthesized via a solvothermal method in the absence of water, which is supposed to substitute O as S2− in the lattice. This material is firstly used for the visible-light-driven degradation of ibuprofen, a model anti-inflammatory drug. The degradation efficiency of S–BiOBr is much higher than that of pure BiOBr. The degradation kinetic constant for S–BiOBr (2.48 × 10−2 min−1) is about 3 times as high as that of pure BiOBr (0.83 × 10−2 min−1). It is found that S-doping tunes the band structure of BiOBr, leading to a narrower band gap and thus higher utilization efficiency of visible light. The degradation of ibuprofen on S–BiOBr can be attributed to the generation of H2O2 and OH radicals. OH radical plays a synergistic role along with holes in the photocatalytic degradation process, which is supposed to be better than the reported single hole- or superoxide-dominant reaction. This work reveals a previously unrecognized and more efficient method for the degradation of organic contaminants on BiOBr.

Co-existing siderite alleviates the Fe(II) oxidation-induced inactivation of Fe(III)-reducing bacteria.

Abiotic Fe (II) oxidation widely occurs in the natural subsurface environment and engineered dynamic processes, which possibly impacts the growth of indigenous microbes. As previously discovered, the oxidation of aqueous Fe2+ at neutral pH effectively inactivates iron-reducing bacteria Shewanella oneidensis strain MR-1 (MR-1). Herein, the impacts of co-existing iron mineral on the oxidation of aqueous Fe2+ and the subsequent disinfection activity on MR-1 were investigated with siderite selected as a representative iron mineral in the subsurface environment. The oxidation rate of aqueous Fe2+ and the amount of generated OH radical increased as the content of siderite increased, while the MR-1 inactivation was alleviated. An initial concentration of 2.0 × 106 CFU/mL MR-1 was inactivated by about 2.7 orders of magnitude during oxidation of 0.2 mM FeSO4 alone for 30 min, which was reduced to only about 0.6 orders of magnitude in the presence of 4.3 mM co-existing siderite. ROS scavenging results confirmed that the OH radical generated in the bulk solution was not the leading role for the inactivation of MR-1. Morphological changes of the cells observed by SEM demonstrated that the disruption of the cell membrane was alleviated by siderite, which was further supported by the XRD and FTIR spectra. The underlying mechanism was proposed to be the reduced contact time of Fe2+ and MR-1 cells due to the accelerated oxidation. This work provides new insights into the disinfection behavior of heterogeneous Fe (II) oxidation on iron cycling bacterial in the natural environment.

Ignition delay time measurements and kinetic modeling of methane/diesel mixtures at elevated pressures

Natural gas/diesel dual-fuel (DF) combustion technology has attracted substantial attention in terms of thermal-efficiency improvement and emission reduction in advanced compression ignition engines. As known, autoignition behavior of DF mixtures plays a significant role in controlling the ignition timing and combustion performance in engines. In the present study, ignition delay time (IDT) measurements of methane/diesel mixtures with three varying diesel substitution ratios (denoted as DSR30, DSR50, and DSR70) were conducted on both a heated rapid compression machine and a heated shock tube under wide-range conditions (T = 640–1450 K, p = 6–20 bar, ϕ = 0.7, 1.0, and 2.0 in ‘air’ mixtures). Experimental results show that DF blends exhibit typical two-stage autoignition characteristics along with the negative temperature coefficient (NTC) response at the temperature region currently investigated, owing to the addition of high-reactivity diesel fuel. Moreover, both the total and first-stage IDTs decrease with the rise of pressure and equivalence ratio as well as diesel substitution ratio. Additionally, a crossover of IDTs occurs at a high temperature (~1400 K) for varying equivalence ratios, and there exists a non-linear promoting effect of diesel contents on IDTs. The simulation results performed using a detailed chemical kinetic mechanism (POLIMI_1412) in conjunction with a well-validated tri-component diesel surrogate show generally good agreement with the experimental results at the current test conditions. Furthermore, species evolution assisted with brute-force sensitivity analysis was carried out to further understand the autoignition chemistry of the DF mixture, particularly the chemical interplay between methane and diesel during the low-temperature ignition processes. It is found that methane hardly generates OH radicals, which are mainly produced via the low-temperature oxidation pathways of diesel fuel. The competition between methane and diesel for OH radicals inhibits the consumption of diesel while promotes the depletion of methane, resulting in an inhibiting impact on the overall reactivity of the reaction system. What's more, the experimental data reported herein provide a foundation for the development of DF kinetic models with high accuracy and robustness.

Decomposition of naproxen by plasma in liquid process with TiO2 photocatslysts and hydrogen peroxide

Naproxen (NPX), one of the representative non-steroidal anti-inflammatory drug (NSAID) ingredients, was decomposed by plasma in liquid process (PiLP). Strongly oxidized species generated in the plasma field of the PiLP, such as OH radicals, were confirmed by optical emission spectroscopy Increasing the operation parameters (pulse width, frequency and applied voltage) of the power supply promoted plasma field generation and OH radical generation, and affected the NPX decomposition rate. Although the NPX decomposition reaction rate was improved by up to 18–30% by adding TiO2 photocatalyst powder and H2O2 to PiLP, but the optimal addition amount should be determined considering the plasma generation and scavenger effects. A decomposition pathway was proposed, in which NPX was mineralized into CO2 and H2O through five intermediates mainly by decarboxylation, demethylation, hydroxylation, and dehydration reactions via hydroxyl radicals.

Visual and colorimetric detection of uric acid in human serum and urine using chitosan stabilized gold nanoparticles

In this paper, the chitosan stabilized gold nanoparticles was developed as a catalyzed 3,3′,5,5′-tetramethylbenzidine - H2O2 system for visual and colorimetric detection of uric acid. In theory, uric acid can be catalyzed by uricase to produce H2O2, and the chitosan stabilized gold nanoparticles were successfully synthesized by chemical reduction and showed an intrinsic peroxidase-like activity. The chitosan stabilized gold nanoparticles could effectively catalyze decomposition of H2O2 to generate OH radicals. The peroxidase substrate 3,3′,5,5′-tetramethylbenzidine is oxidized in the presence of OH radicals, resulting in a visible color change (from colorless to blue). This convenient, novel and sensitive method could be used for detection of uric acid based on the chitosan stabilized gold nanoparticles, with a linear range from 0.1 to 30 μM, and a limit of detection (S/N ratio of 3) as low as 0.04 μM. Furthermore, the detection method has been successfully applied to human serum and urine samples by visual observation. The recoveries of uric acid in human serum and urine were in the range of 96.1–103.1% and 95.2–97.7%, respectively. These results revealed that the visual colorimetric method may provide an alternative to clinical medicine and daily monitoring.

Enhanced heterogeneous Fenton-like degradation of refractory organic contaminants over Cu doped (Mg,Ni)(Fe,Al)2O4 synthesized from laterite nickel ore

The heterogeneous Fenton-like catalyst (Mg,Cu,Ni)(Fe,Al)2O4 was synthesized via a coprecipitation method using laterite nickel ore leaching solution as raw material. The effects of CuCl2·2H2O addition and calcination temperature on the microstructures and degradation properties of the obtained products were investigated. Results showed that higher calcination temperature could promote the migration of Cu2+ ions from CuO to the spinel ferrite lattice and occupied octahedral sites. The degradation efficiencies (η) of various types of low-concentration dyes and tetracycline were higher than 95%, which was mainly due to the accelerated generation of OH radicals by the synergistic effect of Fe3+ and Cu2+ ions in octahedral sites of the formed (Mg,Cu,Ni)(Fe,Al)2O4. Moreover, after five consecutive degradation cycles, the η of RhB was still close to 100%, TOC removal efficiency was maintained around 40% and the concentrations of metallic ions in degraded solutions were all lower than the national effluent discharge standard (GB8978-1996), confirming the as-obtained (Mg,Cu,Ni)(Fe,Al)2O4 was an eco-friendly heterogeneous Fenton-like catalyst with excellent stability and reusability. This study may provide an effective reference for large scale preparing efficient heterogeneous Fenton-like catalysts from natural minerals in treating the wastewater contaminated by refractory organics.

IL-assisted synthesis of defect-rich polyaniline/NH2-MIL-125 nanohybrids with strengthened interfacial contact for ultra-fast photocatalytic degradation of acetaldehyde under high humidity

A novel strategy and design for ionic liquid (IL) assisted in-situ grafting of polyaniline (PANI) oligomers in NH2-MIL-125 was proposed to synthesize the defect-rich IL-PANI/NMIL(Ti) nanohybrids with an intimate interface for the superior photodegradation of CH3CHO. PANI was dissolved into small oligomers with the aid of IL [EMIM]BF4 and was composited to NH2-MIL-125 by a strong interfacial connection through Ti-N bonds (XPS) with high dispersion and large surface area (BET = 1125 m2/g). Characterizations results showed that smart grafting of PANI-oligomers generated abundant defects (3.4-times, ESR) and intensified Lewis acid sites (1.2-times, CO-DRIFT) in IL-PANI/NMIL(Ti) compared to NH2-MIL-125. It promoted adsorptive interactions for aldehyde VOCs capturing, which in-turn increased the adsorption rate (ka) of CH3CHO. Smart grafting also improved the light absorption of IL-PANI/NMIL(Ti) from 500 to 800 nm, reduced band gap (1.7 eV), increased photo-current density (2.2-folds of pure MOF), enhanced O2− and OH radicals generation and improved e−/h+ mobility. As a result, IL-PANI/NMIL(Ti) showed 11.4 and 4.2 times higher ka (0.262 min−1) of CH3CHO photodegradation kinetics than PANI and NH2-MIL-125, respectively, at 75% relative humidity (RH). The ka of CH3CHO degradation was steadily increased with increasing humidity (<80 RH%), indicating that H2O from humid air was beneficial for CH3CHO degradation. Furthermore, IL-PANI/NMIL(Ti) exhibited the excellent recycling stability compared to NH2-MIL-125, as only a 5% decrease in photocatalytic activity was observed after five cycling runs. Considering the excellent photostability and ultrafast photodegradation of CH3CHO, the current study opens up new perspectives in fabricating MOFs for a wide-range of practical applications.

Crystal phase-dependent generation of mobile OH radicals on TiO2: Revisiting the photocatalytic oxidation mechanism of anatase and rutile

Titanium dioxide has been the most popular environmental photocatalyst of which role critically depends on the generation of OH radicals. In particular, the mobile free OH racial (OHf) generation and the subsequent diffusion from the surface are critical in achieving the mineralization of non-adsorbing substrates by extending the reaction zone from the surface to the solution bulk. Here the origin of the crystalline phase-dependent generation of OHf was investigated using tetramethylammonium (TMA) cation as a main probe compound for OHf in a UV/TiO2 photocatalytic system. We found a clear evidence that the mobile free OH radical is generated through a reductive conversion of dissolved O2 on anatase only (O2 → H2O2 → OHf). The surface trapped holes are not involved in OHf formation, but lead to the generation of surface-bound OH radical (OHs) on both anatase and rutile. The generation of OHf is favorable on anatase because more H2O2 are evolved (via dioxygen reduction) and adsorbed on the anatase surface. Rutile showed little sign of OHf formation. The generation of 18O-labelled p-hydroxybenzoic acid on anatase only (not rutile) from benzoic acid oxidation under 18O2-saturated condition provides a solid evidence that the OHf generation mechanism on anatase involves the reductive pathway. Better understanding of OHf production pathway in photocatalysis will provide a new insight leading to an engineering solution for how the production of OHf can be maximized, which is critically important in achieving the efficient photocatalytic oxidation of various pollutants.

Improved removal of Congo Red from wastewater by low-rank coal using micro and nanobubbles

The influences of micro and nanobubbles (MNBs) on the adsorption of Congo Red (CR) on low-rank coal (LRC) were investigated. The size of microbubbles and nanobubbles diameters ranged from 2 to 600 μm and 50–200 nm, respectively. MNBs simultaneously increased adsorption capacity and rate of CR on LRC; the maximum adsorption rates achieved with the use of MNBs were 2 times of that without MNBs for both Wanli and Mengdong coal. Without MNBs, it took 25 min for Wanli coal to achieve the same residual CR concentration that Wanli with MNBs achieved in 1 min. The MNBs mainly reduced the CR concentration through the generation of OH radicals and improvement of its adsorption on LRC. MNBs were easy to attached to LRC particles, so they could act as “porter” to carry CR to LRC. The high concentration of CR-carrying MNBs increased CR/LRC contact probability to accelerate the adsorption of CR. The mechanism by which MNBs assisted adsorption provides a new method of removing organic pollutants in less time and with lower cost, which holds great potential in a wide range of applications.