Nitric Oxide Removal Efficiency Research Articles

Oxygen Vacancy Sites Enable Efficient Photocatalytic Oxidation of Nitric Oxide: The Role of W/Mo Valence Transition in Bi2W(Mo)O6‐x

AbstractOxygen vacancy (Ov) sites play a critical role in the activation and deep oxidation of nitric oxide (NO). However, controlling the concentration and type of Ov remains a significant challenge. In this study, Bi2W(Mo)O6‐x is investigated as a model system and demonstrates that increasing the concentration of Ov substantially enhances the efficiency of air NO removal. Increasing the Ov concentrations in Bi2WO6‐x and Bi2MoO6‐x improves NO removal efficiency ≈12‐ and 11‐fold, respectively, compared to their low‐Ov counterparts. This enhancement is attributed to improved adsorption and activation of NO/O2 molecules, better separation and transfer of photogenerated carriers, and increased visible light absorption. Notably, Bi2WO6‐x remains highly stable over ten recycling tests for continuous air NO deep photooxidation, while Bi2MoO6‐x shows a 43.5% decrease in efficiency after ten runs. This sustained performance is attributed to stable Ovs without changes in metal ion valence, unlike Bi2MoO6‐x, where instability arises from the reduction of Mo6+ to Mo4+. In situ DRIFTS reveals possible pathways for the deep photooxidation of NO to nitrate (NO3−). This study provides valuable insights into designing high‐performance, durable catalysts by effectively controlling Ov concentration and type, paving the way for efficient photocatalytic air purification technologies.

Built-in electric field mediated S-scheme charge migration and Co-N4(II) sites in cobalt phthalocyanine/MIL-68(In)-NH2 heterojunction for boosting photocatalytic nitric oxide oxidation

The efficiency of photocatalytic Nitric Oxide(NO) oxidation is limited by the lack of oxygen(O2) active sites and poor charge carrier separation. To address this challenge, we developed a molecular Cobalt Phthalocyanine modified MIL-68(In)–NH2 photocatalyst with a robust Built-in electric field(BIEF). In the 2 % CoPc-MIN sample, the BIEF strength is increased by 3.54 times and 5.83 times compared to pristine CoPc and MIL-68(In)–NH2, respectively. This BIEF facilitates the efficient S-scheme charge transfer, thereby enhancing photogenerated carrier separation. Additionally, the Co-N4(II) sites in CoPc can effectively trap the separated photoexcited electrons in the S-scheme system. In addition, the Co-N4(II) sites can also serve as active sites for O2 adsorption and activation, promoting the generation of superoxide radical (O2–), thereby driving the direct conversion of NO to nitrate(NO3–). Consequently, the 2 % CoPc-MIN sample exhibits a remarkable photocatalytic NO removal efficiency of 79.37 % while effectively suppressing the formation of harmful by-product nitrogen dioxide(NO2) to below 3.5 ppb. This study provides a feasible strategy for designing high-efficiency O2 activation photocatalysts for NO oxidation.

Engineering the defect distribution via boron doping in amorphous TiO2 for robust photocatalytic NO removal

Nitric oxide (NO) pollutant can threaten the regional environment and human health. Solar-powered photocatalytic oxidation method is desirable for NO removal (ppb level) at ambient conditions, but suffers from the unsatisfactory activity and poor selectivity due to the sluggish molecular oxygen activation. Herein, we demonstrate that efficient photocatalytic NO removal can be realized by modulating the defects distribution of amorphous TiO2 via boron doping. Theoretical calculations and experimental results reveal that H3BO3 modification can refine the structure of surface oxygen vacancies, promoting electron-rich molecular oxygen activation to •OH. Meanwhile, bulk-phase oxygen defects can be filled by free-state B atoms, accelerating the electron transfer via Ti-O-B bonding. Homogeneous boron-doped amorphous TiO2 achieves about 5 times higher NO removal efficiency (∼50%) than that of amorphous TiO2 (∼9%) without releasing remarkable NO2 (selectivity, ∼97%). This study provides a state-of-the-art approach for robust photocatalytic NO removal by engineering the defect distribution.

Experimental and computational study on removal of nitric oxide using NH2-MIL-125: Revisiting removal mechanism

The NH2-MIL-125 (NM-125) or modified NM-125 materials are used for photocatalytic removal of nitric oxide (NO). Most researchers attributed few NO2 emission during NO photocatalysis to high selectivity towards the formation of nitrate ions due to the neglect of the principle of electroneutrality and pore structure of NM-125. This may cause misleading photocatalytic mechanism of NO removal. Herein, this work proposes a new mechanism behind NO removal using NM-125 based on experimental investigations and theoretical calculations. First, experimentally, the highest NO removal efficiency of 88.54 % with negligible NO2 emission occurs at the anhydrous condition. In addition, in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements verify the presence and accumulation of NO2 as product during photocatalytic conversion of NO. Furthermore, density functional theory (DFT) calculations and grand canonical Monte Carlo (GCMC) computations demonstrate that there exist strong interactions between NO2 and -NH2 group within pores. Few NO2 is detected during experiments is probably due to its subsequent adsorption in the pores of NM-125. The new NO removal mechanism including photocatalytic conversion NO into NO2 and subsequent NO2 adsorption in the pores of NM-125 is thus proposed. This work provides a deeper insight into photocatalytic NO removal over NM-125 and its variants.

Nitric oxide removal by synergistic photooxidation and adsorption using Pd1/g-C3N4-UiO-66-NH2 P-A heterojunction

Simultaneous avoidance of secondary air pollution and catalyst deactivation remains a challenge for existing photocatalytic removal of nitric oxide (NO). Herein, a photocatalysis-adsorption (P-A) heterojunction is synthesized for eco-friendly NO removal by facilely grinding Pd single atoms (Pd1) anchored g-C3N4 (48CN/Pd) with UiO-66-NH2 (UION). Under visible-light irradiation, the 40UION-CN/Pd composite shows 93.91 % of NO removal efficiency with only 2.73 % of NO2 emissions and negligible catalyst deactivation. The DeNOx index increases remarkably from −0.66 for 48CN/Pd to 0.86 for 40UION-CN/Pd. Furthermore, the synthesized P-A heterojunction efficiently removes NO from real flue gases produced by a domestic coal-fired stove and from SO2-rich flue gases. The mechanism exploration based on experimental investigations and density functional theory (DFT) calculations reveals that UION captures the NO2 selectively produced by 48CN/Pd, simultaneously suppressing NO2 emissions and refreshing active sites for NO photooxidation. Moreover, the results of X-ray photoelectron spectroscopy (XPS), charge density difference calculation, energy-resolved distribution of electron traps (ERDT) and in-situ Kelvin probe force microscopy (KPFM) reveal that the built-in electric field and Type-II heterojunction formed between 48CN/Pd and UION promote the separation of photogenerated carriers, enhancing the catalytic activity of the composite. This work provides an innovative strategy to avoid NO2 emission and catalyst deactivation during photocatalytic NO removal.

Removal of NO at low concentrations from polluted air in semi-closed environments by activated biochars from renewables feedstocks

Efficient measures are urgently required in large cities for nitric oxide (NO) elimination from air in urban semi-closed environments (parking lots and tunnels), characterized by low NO concentrations (<10 ppmv) and temperatures. One of the most promising abatement alternatives is the NO oxidation to NO2, which can be further easily captured in an alkali solution or over a porous solid. However, most of the research devoted to this topic is focused on the elimination of NO from fuel exhaust gases, with high NO concentrations (400–2000 ppmv). In this work, sustainable and low-cost activated biochars of different origin and having very different ash contents were employed in NO removal at very low concentrations. Thus, low ash content forestry (oak woodchips, OAK) and high ash content from agriculture (oilseed rape straw, OSR) biochars were subjected to physical activation with CO2 at 900 °C (OAK550-A900CO2 and OSR700-A900CO2, respectively). The NO removal performance tests of such activated carbons were carried out at different experimental conditions: i.e., temperature, relative humidity (0–50 vol% RH), NO-containing gas (N2 or air), amount of activated carbon, and NO concentration, to assess how the activated biochar properties influence their NO removal capacity. The sample OSR700-A900CO2 contained a higher population of oxygen surface functionalities, which might play an important role in the NO removal efficiency in dry conditions since they could assist NO oxidation on carbon active sites when used above room temperature (50–75 °C). However, at room temperature (25 °C), the presence of narrow micropore size distribution at 6 Å became a more relevant property, since it facilitates an intimate contact between NO and O2. Accordingly, the activated biochar from OAK was much more efficient, achieving complete removal of NO from air flow (dry or with 50 vol% RH) at 25 °C during 400 min of testing, making it an ideal candidate as biofilter for purifying air streams of semi-closed spaces contaminated with low concentrations of NO.

Effective nitric oxide removal from flue gas using UV/H2O2 solution catalyzed by Fe3O4@FeEDTA

AbstractBACKGROUNDHow to control NOx emission in flue gas economically and efficiently is the focus in the field of energy and environment in the world. In this work, Fe3O4‐magnetic microspheres were employed as the matrix with loading FeEDTA (where EDTA is ethylenediaminetetraacetic acid) to prepare magnetic denitrification absorbent Fe3O4@FeEDTA. The performance of nitric oxide (NO) removal using Fe3O4@FeEDTA dispersed in H2O2 solution with ultraviolet (UV) was first investigated.RESULTSThe results showed that Fe3O4@FeEDTA could be used as a good catalytic oxidation denitrification agent. Increasing H2O2 concentration and O2 concentration could promote NO absorption. NO removal efficiency increased as pH value increased from 1.5 to 3.0, then decreased when pH continued to increase. The elevated temperature was conducive to NO removal. CO2 hardly affected the removal of NO, while SO2 with high concentration was not conducive to NO absorption. In addition, the NO removal mechanism was preliminarily analyzed with X‐ray photoelectron spectrometry, electron spin resonance and ion chromatography. The mechanism study indicated that NO removal by Fe3O4@FeEDTA/H2O2 was a catalysis–complexation–oxidation–absorption process, in which •OH was determined to be the main oxidizing species, and NO captured by Fe(II)EDTA was mainly oxidized by •OH to NO3−.CONCLUSIONThe catalytic activity of Fe3O4@FeEDTA could maintain stability during the repeat experiment, realizing the recycling and reuse of Fe3O4@FeEDTA. The UV‐Fe3O4@FeEDTA‐activated H2O2 advanced oxidation technology is suitable for wet denitrification. © 2023 Society of Chemical Industry (SCI).

BiOI-SnO2 Heterojunction Design to Boost Visible-Light-Driven Photocatalytic NO Purification.

The efficient, stable, and selective photocatalytic conversion of nitric oxide (NO) into harmless products such as nitrate (NO3-) is greatly desired but remains an enormous challenge. In this work, a series of BiOI/SnO2 heterojunctions (denoted as X%B-S, where X% is the mass portion of BiOI compared with the mass of SnO2) were synthesized for the efficient transformation of NO into harmless NO3-. The best performance was achieved by the 30%B-S catalyst, whose NO removal efficiency was 96.3% and 47.2% higher than that of 15%B-S and 75%B-S, respectively. Moreover, 30%B-S also exhibited good stability and recyclability. This enhanced performance was mainly caused by the heterojunction structure, which facilitated charge transport and electron-hole separation. Under visible light irradiation, the electrons gathered in SnO2 transformed O2 to ·O2- and ·OH, while the holes generated in BiOI oxidized H2O to produce ·OH. The abundantly generated ·OH, ·O2-, and 1O2 species effectively converted NO to NO- and NO2-, thus promoting the oxidation of NO to NO3-. Overall, the heterojunction formation between p-type BiOI and n-type SnO2 significantly reduced the recombination of photo-induced electron-hole pairs and promoted the photocatalytic activity. This work reveals the critical role of heterojunctions during photocatalytic degradation and provides some insight into NO removal.

A Novel Method Based on Hydrodynamic Cavitation for Improving Nitric Oxide Removal Performance of NaClO2.

In the removal of nitric oxide (NO) by sodium chlorite (NaClO2), the NaClO2 concentration is usually increased, and an alkaline absorbent is added to improve the NO removal efficiency. However, this increases the cost of denitrification. This study is the first to use hydrodynamic cavitation (HC) combined with NaClO2 for wet denitrification. Under optimal experimental conditions, when 3.0 L of NaClO2 with a concentration of 1.00 mmol/L was used to treat NO (concentration: 1000 ppmv and flow rate: 1.0 L/min), 100% of nitrogen oxides (NOx) could be removed in 8.22 min. Furthermore, the NO removal efficiency remained at 100% over the next 6.92 min. Furthermore, the formation of ClO2 by NaClO2 is affected by pH. The initial NOx removal efficiency was 84.8-54.8% for initial pH = 4.00-7.00. The initial NOx removal efficiency increases as the initial pH decreases. When the initial pH was 3.50, the initial NOx removal efficiency reached 100% under the synergistic effect of HC. Therefore, this method enhances the oxidation capacity of NaClO2 through HC, realizes high-efficiency denitrification with low NaClO2 concentration (1.00 mmol/L), and has better practicability for the treatment of NOx from ships.

Microwave-Assisted Rapid Substitution of Ti for Zr to Produce Bimetallic (Zr/Ti)UiO-66-NH2 with Congenetic "Shell-Core" Structure for Enhancing Photocatalytic Removal of Nitric Oxide.

Efficient nitric oxide (NO) removal without nitrogendioxide(NO2 )emission is desired for the control of air pollution. Herein, a series of (Zr/Ti)UiO-66-NH2 with congenetic shell-core structure, denoted as Ti-UION, are rapidly synthesized by microwave-assisted post-synthetic modification for NO removal. The optimal Ti-UION (i.e., 2.5Ti-UION) exhibits the highest activity of 80.74% without NO2 emission with moisture, which is 21.65% greater than that of the UiO-66-NH2 . The NO removal efficiency of 2.5Ti-UION further increases to 95.92% without photocatalyst deactivation under an anhydrous condition. This is because selectively produced NO2 in photocatalysis is completely adsorbed into micropores, refreshing active sites for subsequent reaction. In addition, the enhanced photocatalytic activity after Ti substitution is due to the presence of Ti electron acceptor, the potential difference between the shell and core of Ti-UION crystal, and the high conductivity of TiO units. Additionally, the improved adsorption of gas molecules not only favors NO oxidation, but also avoids the emission of NO2 . This work provides a feasible strategy for rapid metal substitution in metal-organic frameworks and insights into enhanced NO photodegradation.

Oxidation absorption of nitric oxide from flue gas using biochar-activated peroxydisulfate technology

Biochar-activated peroxydisulfate oxidation technology has received more and more attention due to its low metal ion leakage rate and low solid waste post-treatment requirement. In this paper, rice straw biochar-activated peroxydisulfate oxidation technology is used for the first time to oxidize nitric oxide (NO) from flue gas through inducing free radicals. The mechanism of NO oxidation removal is mainly explored. The main factors affecting NO removal, removal products and free radicals are also systematically studied. Results show that the main product of NO oxidation removal is NO3− and no extra by-product is formed. ·OH is identified as the first important free radical for oxidizing NO, while SO4−· is determined to be the second important free radical for oxidizing NO. Peroxydisulfate oxidation only plays a minor supplementary role in NO removal. Increasing rice straw biochar mass concentration and operation temperature promote NO removal. Increasing NO concentration and solution pH value are found to be harmful to NO removal. Increasing peroxydisulfate concentration first increases and then decreases NO removal efficiency. Under optimized process parameters, the maximum NO removal efficiency reaches 95.2%.

Single Pd atoms anchored graphitic carbon nitride for highly selective and stable photocatalysis of nitric oxide

Efficient, stable, and selective photocatalytic conversion of nitric oxide (NO) into nitrogen dioxide (NO 2 ) is highly desirable but remains a big challenge. We prepare single atom catalyst (SAC) by anchoring single Pd atoms onto graphitic carbon nitride (CNPd) via chemical impregnation followed by calcination. The prepared CNPd SAC is confirmed by aberration-correction high-angle-annular-dark field scanning transmission electron microscopy and X-ray absorption fine structure spectroscopy. The synthesized SAC outperforms its competitors reported earlier for photocatalytic removal of NO under visible light and simulated sunlight irradiation in terms of selectivity and stability. The SAC maintains a NO removal efficiency of 83% and an instantaneous selectivity for NO 2 of 92.8% over 117.6 h under simulated sunlight with the inlet NO concentration of 12 ppm. This duration is about 23.5 times the longest duration tested for other catalysts in the literature. The experimental results and density functional theory (DFT) calculations reveal that single Pd atom promotes the photocatalytic degradation of NO. Moreover, the DFT calculations prove that the nitrate ions that accumulate on the surface of the SAC can react with NO to produce NO 2 . This reaction enhances the selectivity for NO 2 and stability of the SAC. The CNPd catalyst exhibits high photocatalytic activity, selectivity and stability for the removal of NO under visible light. • The CNPd SACs exhibit stable and high photocatalytic removal of NO. • The CNPd SACs show ultra-high selectivity for the conversion of NO into NO 2 which can be easily addressed by wet scrubbing. • NO removal efficiency of CNPd SACs remain 83% after solar irradiation over 117.6 h. • DFT calculations reveal the mechanism of photocatalytic over CNPd SACs. • DFT calculations explain the reason for selective conversion of NO to NO 2 .

Rapid fabrication of MgO@g-C3N4 heterojunctions for photocatalytic nitric oxide removal.

Nitric oxide (NO) is an air pollutant impacting the environment, human health, and other biotas. Among the technologies to treat NO pollution, photocatalytic oxidation under visible light is considered an effective means. This study describes photocatalytic oxidation to degrade NO under visible light with the support of a photocatalyst. MgO@g-C3N4 heterojunction photocatalysts were synthesized by one-step pyrolysis of MgO and urea at 550 °C for two hours. The photocatalytic NO removal efficiency of the MgO@g-C3N4 heterojunctions was significantly improved and reached a maximum value of 75.4% under visible light irradiation. Differential reflectance spectroscopy (DRS) was used to determine the optical properties and bandgap energies of the material. The bandgap of the material decreases with increasing amounts of MgO. The photoluminescence spectra indicate that the recombination of electron-hole pairs is hindered by doping MgO onto g-C3N4. Also, NO conversion, DeNOx index, apparent quantum efficiency, trapping tests, and electron spin resonance measurements were carried out to understand the photocatalytic mechanism of the materials. The high reusability of the MgO@g-C3N4 heterojunction was shown by a five-cycle recycling test. This study provides a simple way to synthesize photocatalytic heterojunction materials with high reusability and the potential of heterojunction photocatalysts in the field of environmental remediation.

High Gravity-Enhanced In Situ Goethite-Catalyzed Alkaline H2O2 Systems for Nitric Oxide Removal in a Rotating Packed Bed: Mass-Transfer and Reaction Mechanism

It is a challenge to maintain Fenton’s oxidizability under alkaline conditions to efficiently oxidize nitric oxide (NO). Here, we find that an efficient catalyst goethite (α-FeOOH) to generate •OH is formed in an alkaline Fenton system and propose an in situ reactive oxygen species (ROS, hydroxyl radical (•OH) and hydroperoxyl anion (−OOH)) utilization mechanism in the high-gravity-enhanced α-FeOOH-catalyzed oxidation of NO within a rotating packed bed (RPB). A mathematical model is derived to elucidate the mechanism and evaluate the synergistic effect of ROS. The simulation results show that the generation and utilization sites of •OH gradually overlap in the liquid film with the increase in rotating speed. Additionally, the formation and properties of α-FeOOH are characterized to show its high dispersity in the RPB. Based on the mechanism, an in situ HiGee-AOP NO removal process is developed. Experimentally, the NO removal efficiency of the process can exceed 95% under optimal conditions.

Restrictions of nitric oxide electrocatalytic decomposition over perovskite cathode in presence of oxygen: Oxygen surface exchange and diffusion

Nitric oxide (NO) abatement from engine exhaust is of great significance to alleviate air pollution and haze. Compared with the traditional selective catalytic reduction (SCR) technology, electrocatalytic decomposition of NO simplifies the reductant supply system and therefore avoids secondary pollution. In this study, typical perovskite La0.6Sr0.4CoxFe1-xO3-δ (LSCF) infiltrated by different dosages of nano ceria Ce0.9Gd0.1O1.9 (GDC) was used as composite cathodes, in order to explore the critical factors to restrain NO conversion in excess of O2. The results show that electron as reactive species transfers among NO, ABO3-type cathode and oxygen vacancy. The maximum of NO removal efficiency can reach 96.27 % in absence of O2 and up to 80.55 % in presence of 1% O2 in case of LSCF infiltrated by moderate dosages LSCF-GDC(2), which is superior to those of LSCF, LSCF-GDC(4) and LSM-GDC(nano) composite cathode. Compared to oxygen storage capacity (OSC) caused by the infiltration of nano ceria, higher surface oxygen exchange coefficient (kδ) and chemical diffusion coefficient (Dchem) lead to the significant decrease in polarization resistance (Rp), and consequently to the enhancement of NO removal in presence of O2. No matter what kind of oxygen deriving from oxygen reduction reaction (ORR) and NO reduction reaction (NORR), GDC infiltration into LSCF improves oxygen transport property and however, the property of cathode in ORR is dominant over in NORR in presence of O2. Moderate GDC loading has the highest oxygen transport kinetics, and oxygen surface exchange is faster than chemical diffusion, due to lower activation energy. Over loading of GDC with greater ohmic resistance (Rs) inversely influences the NO removal.

Adjustable Underwater Gas Transportation Using Bioinspired Superhydrophobic Elastic String

Dynamic and precise manipulation of the gas flow in a liquid environment through a facile and reliable approach is of great importance for directional gas transportation and multiphase chemical reactions. In this research, elastic superhydrophobic strings were prepared by a one-step, non-fluorinated dip-coating strategy. The surface-treatment string demonstrated a good superaerophilicity underwater. By simply elongating or shortening superaerophilic strings, the gas flux underwater was precisely manipulated in a gas-siphon underwater experiment. The result reveals that a large strain of the treated string induces a low gas flow, and a rope woven with more strings results in a larger range of gas flow regulation. The elastic superhydrophobic/superaerophilic string was utilized to adjust the reaction time of carbon dioxide and sodium hydroxide aqueous solution successfully. Furthermore, in a wet oxidation experiment for treating simulated flue gas composed of nitric oxide (NO), nitrogen and oxygen, superhydrophobic and stretched strings with a strain of 200% demonstrated a 7.9% higher NO removal efficiency than that of untreated strings. Interestingly, NO removal efficiency can be regulated by mechanical stretching of gas-conducting strings. We believe that this facile and low-cost approach provides a valid method of on-demand manipulation of the gas flow for underwater gas transportation.

Superoxide anion and singlet oxygen dominated faster photocatalytic elimination of nitric oxide over defective bismuth molybdates heterojunctions

Establishing an ideal photocatalytic system with efficient reactive oxygen species (ROS) generation has been regarded as the linchpin for realizing efficient nitric oxide (NO) removal and unveiling the ROS-mediated mechanism. In this work, a novel oxygen-deficient 0D/1D Bi3.64Mo0.36O6.55/Bi2MoO6 heterojunctions (BMO-12-H) were successfully synthesized under the enlightenment of clarified crystal growth mechanism of bismuth molybdates. Because of the synergies between defect-engineering and heterojunction-construction, BMO-12-H demonstrated improved photoelectrochemical properties and O2 adsorption capacity, which in turn facilitated the ROS generation and conversion. The enhancement of •O2– and 1O2 endowed BMO-12-H with strengthened NO removal efficiency (59%) with a rate constant of 12.6*10-2 min−1. A conceivable NO removal mechanism dominated by •O2– and 1O2 was proposed and verified based on the theoretical calculations and in-situ infrared spectroscopy tests, where hazardous NO was oxidized following two different exothermic pathways: the •O2–-induced NO → NO3– process and the 1O2-induced NO → NO2 → NO3– process. This work offers a basic guideline for accelerating ROS generation by integrating defect-engineering and heterojunction-construction, and provides new insights into the mechanism of efficient NO removal dominated by •O2– and 1O2.

Enhancing quantum efficiency at Ag/g-C3N4 interfaces for rapid removal of nitric oxide under visible light

Nitric oxide (NO) pollution presents a pressing issue in preserving the air quality for sustainable development. It is highly desirable to employ robust photocatalytic materials for addressing NO pollution in an energy-efficient fashion. Here, Ag/g-C3N4 heterojunctions are successfully fabricated via a photo-reduction approach. Ag nanoparticles are deposited on g-C3N4, having an average diameter of 8.45 nm. Upon incorporating Ag species, the energy band gap of g-C3N4 is reduced from 2.76 to 2.70 eV. Compared with g-C3N4, the photocatalytic NO removal efficiency of 20% Ag/g-C3N4 is increased to 80% because of the enhanced quantum efficiency (14 × 10−4%). After five recycling runs, the 20% Ag/g-C3N4 heterojunction still presents an excellent performance, representing a highly stable photocatalytic material. The critical contribution of •O2‾ and holes during the photocatalytic processes are also studied using radical trapping experiments. Our work offers a simple but scalable approach for generating high-performance photocatalytic materials, potentially enabling efficient removal of NO pollution without extensive energy consumption.

Nitric oxide and nitrite removal by partial denitrifying hollow-fiber membrane biofilm reactor coupled with nitrous oxide generation as energy recovery

ABSTRACT Nitrogen oxide (NOx) emissions cause significant impacts on the environment and must therefore be controlled even more stringently. This requires the development of cost-effective removal strategies which simultaneously create value-added by-products or energy from the waste. This study aims to treat gaseous nitric oxide (NO) by hollow-fibre membrane biofilm reactor (HFMBfR) in the presence of nitrite () and evaluate nitrous oxide (N2O) emissions formed as an intermediate product during the denitrification process. Accumulated N2O can be utilised in methane oxidation as an oxidant to produce energy. In the first stage of the study, the HFMBfR was operated by feeding only gaseous NO as the nitrogen source. During this period, the best performance was achieved with 92% NO removal efficiency (RE). In the second stage, both NO gas and were supplied to the system, and 91% NO and 99% reduction were achieved simultaneously with the maximum N2O generation of 386 ± 31 ppm. Lower influent carbon to nitrogen (C/N) ratios, such as 4.5 and 2.0, and higher −N loading rate of 158 mg N day−1 favoured N2O generation. An improved NO removal rate and N2O accumulation were seen with the increasing amount of in the medium. The 16S rDNA sequencing analysis revealed that Alicycliphilus denitrificans and Pseudomonas putida were the dominant species. The study shows that an HFMBfR can be successfully used to eliminate both and gaseous NO and simultaneously generate N2O by adjusting the system parameters such as C/N ratio, and loading.

Synthesis of MnO2/Mg-Al layered double hydroxide and evaluation of its NO-removal performance

The removal of toxic nitric oxide (NO) from exhaust gas is challenging and expensive. MnO2/Mg-Al LDH, a composite of manganese dioxide and Mg-Al layered double hydroxide, was synthesized via the rehydration method for potential application in NO removal. MnO2/Mg-Al LDH was found to be complexed with MnO2, pillared between the LDH layers and intercalated with CO32− and OH−. We investigated the performance of a MnO2/Mg-Al LDH treatment in removing NO from exhaust gas. The exhaust gas was circulated in a reaction tube filled with MnO2/Mg-Al LDH, CO3･Mg-Al LDH, or a MnO2 + CO3･Mg-Al LDH mixture, and the NO removal efficiency of the MnO2/Mg-Al LDH composite was compared with that of the other compounds. When NO gas (150 ppm, linear velocity 1.0 m/min) was distributed at 170 °C, treatment with the MnO2･Mg-Al LDH composite and CO3･Mg-Al LDH yielded removal rates of 83.9% and 0%, respectively. The significantly higher NO removal rate of MnO2/Mg-Al LDH was ascribed to the transformation of NO to NO2 via oxidation over MnO2. Further, the NO removal rate after 90 min through the application of a simple mixture of MnO2 and CO3･Mg-Al LDH (Mn: 4.2 wt%) was only 1.4%, suggesting that a synergistic effect was generated by the MnO2/Mg-Al LDH composite. Though NO removal by MnO2/Mg-Al LDH was reduced at temperatures of 140 °C and higher, the results suggest that the synthesized compound can be much more effective than those commonly used for NO removal from exhaust gas.