NO Removal Performance Research Articles

RGO/Fe-doped g-C3N4 visible-light driven photocatalyst with improved NO removal performance

Semiconductor photocatalytic technology has become a new green way to control NOX emissions. In order to remove NOX effectively, a multi-stage electron transport channel was constructed to improve the photocatalytic activity of graphitic carbon nitride (g-C3N4), which was realized by doping Fe ions and compounding with reductive graphene oxide (rGO) simultaneously. The results of NO removal test showed that the as-prepared rGO/Fe-doped g-C3N4 visible-light driven composite photocatalyst exhibited significantly improved photocatalytic activity, and the optimum mass ratio of rGO to Fe was 2:3. This was attributed to the positive synergistic effect of Fe and rGO and the formation of a multi-stage electron transport channel between rGO and protonated Fe-doped g-C3N4 (Fe-C3N4), which further accelerated the separation and transfer of photo-induced carriers, broadened the visible light response range and enhanced the light absorption intensity. This work develops a g-C3N4-based visible-light photocatalyst that can effectively remove NO and provides a new perspective for the modification of photocatalyst.

Directional electron delivery and enhanced reactants activation enable efficient photocatalytic air purification on amorphous carbon nitride co-functionalized with O/La

Graphitic carbon nitride (g-C3N4) is an intriguing and rising visible light photocatalyst. However, g-C3N4 still suffers from random charge carriers transfer in planes and low efficiency in reactants activation, which leads to unsatisfactory photocatalytic efficiency. Herein, these formidable challenges are addressed via a new strategy of O/La co-functionalization in amorphous carbon nitride (C N-O La), leading to the formation of electronic channels for directional electron delivery in the interlayers, the generation of localized electrons for enhanced reactants activation and thus highly boosted photocatalytic performance for NO removal. With a closely combined experimental and theoretical approach, we have revealed that the breakage of in-plane hydrogen bonds between strands of polymeric melon units with CO32− could promote the amorphization of g-C3N4 and increase the visible light absorption ability. A directional electron delivery pathway (L2 → La → L1→ O) is proposed based on results of charge difference distribution and experimental observation. The electron localization could directly activate the O2 and NO molecules and dramatically promote the production of activated species (e.g. O2- and OH radicals) and thus enhancing the photocatalytic efficiency. With the optimized electronic structure, the photocatalytic NO removal ratio of C N-O La is significantly increased from 35.8% of pristine g-C3N4 to 50.4% and it is stable for recycled runs. The in situ FT-IR spectra, in combination with ESR spectra and DFT calculations, unravel the conversion pathway of photocatalytic NO oxidation on C N-O La where some important reaction intermediates are discovered. This work could provide a fascinating modification strategy for carbon nitride and new insights into mechanistic understanding of 2D layered photocatalysts.

Facile large-scale synthesis of Ce[sbnd]Mn composites by redox-precipitation and its superior low-temperature performance for NO removal

A novel redox precipitation route was developed for the preparation of CeMn composite oxides, where KMnO4 and Ce(NO3)3 were adopted as precursors. The samples were tested for selective catalytic reduction (SCR) of NO by NH3 at low temperature, and characterized using X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), field emission scanning electron microscope (FESEM), transmission electron microscope (TEM), thermogravimetric analysis (TG), hydrogen temperature-programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS) and in situ diffuse reflectance infrared transform spectroscopy (in situ DRIFT). This novel method produced samples with large specific surface area, low crystallinity, high oxidation degree and superior performances in low-temperature NH3-SCR. Ce3Mn containing almost 100% Mn4+ ion on its surface exhibited superior low-temperature reducibility, reaching up to above 95% NO conversion almost throughout the entire testing range of 353–453 K. The surface concentration of Mn4+, the strong synergistic effects between MnO2 and CeO2-x and the large surface area played a key role for its excellent catalytic activity for the complete reduction of nitrogen oxides. A primary Eley-Rideal (E-R) reaction mechanism for Ce3Mn catalyst was proposed with Lewis acid sites involved in the SCR reaction.

Fe-modified MnOx/TiO2 as the SCR catalyst for simultaneous removal of NO and mercury from coal combustion flue gas

Fe-modified MnOx/TiO2 (Fe-MnOx/TiO2) was synthesized by a sol-gel method and employed as the catalyst of selective catalytic reduction (SCR) for removing NO and mercury simultaneously from coal combustion flue gas. The experimental results demonstrated that Fe doping modified both NO and Hg0 removal performance of the catalyst. The optimal temperature for NO conversion was 200–250 °C, exhibiting good low-temperature activity, and lower temperature was also beneficial for Hg0 removal. The promotion of HCl on Hg0 removal efficiency was enhanced and the inhibitions of SO2 and NH3 were impaired after Fe doping on the catalyst. Effects of O2, NO and H2O on Hg0 removal efficiencies over MnOx/TiO2 and Fe-MnOx/TiO2 were all similar with each other. NO conversion and Hg0 removal efficiency could reach 76.8% and 65.6%, respectively, in simulated coal-fired flue gas at 250 °C and be maintained for a relatively long time. The modification mechanism of the Fe doping was investigated by characterization methods including BET, XRD, H2-TPR and XPS. The results showed that the introduction of Fe improved the BET surface area, the dispersity of the metal oxides, the redox behavior and surface concentrations of chemisorbed oxygen and Mn4+ of the catalysts, which was in favor of the catalytic activity. Meanwhile, Mn4+ could be protected by the existence of Fe to some extent when exposed to SO2 so that the sulfur resistance of the catalyst was strengthened as well.

Highly enhanced visible-light photocatalytic NOx purification and conversion pathway on self-structurally modified g-C3N4 nanosheets

The unmodified graphitic carbon nitride (g-C3N4) suffers from low photocatalytic activity because of the unfavourable structure. In the present work, we reported a simple self-structural modification strategy to optimize the microstructure of g-C3N4 and obtained graphene-like g-C3N4 nanosheets with porous structure. In contrast to traditional thermal pyrolysis preparation of g-C3N4, the present thermal condensation was improved via pyrolysis of thiourea in an alumina crucible without a cover, followed by secondary heat treatment. The popcorn-like formation and layer-by-layer thermal exfoliation of graphene-like porous g-C3N4 was proposed to explain the formation mechanism. The photocatalytic removal performance of both NO and NO2 with the graphene-like porous g-C3N4 for was significantly enhanced by self-structural modification. Trapping experiments and in-situ diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) measurement were conducted to detect the active species during photocatalysis and the conversion pathway of g-C3N4 photocatalysis for NOx purification was revealed. The photocatalytic activity of graphene-like porous g-C3N4 was highly enhanced due to the improved charge separation and increased oxidation capacity of the O2− radicals and holes. This work could not only provide a novel self-structural modification for design of highly efficient photocatalysts, but also offer new insights into the mechanistic understanding of g-C3N4 photocatalysis.

Unraveling the Mechanisms of Visible Light Photocatalytic NO Purification on Earth-Abundant Insulator-Based Core-Shell Heterojunctions.

Earth-abundant insulators are seldom exploited as photocatalysts. In this work, we constructed a novel family of insulator-based heterojunctions and demonstrated their promising applications in photocatalytic NO purification, even under visible light irradiation. The heterojunction formed between the insulator SrCO3 and the photosensitizer BiOI, via a special SrCO3-BiOI core-shell structure, exhibits an enhanced visible light absorbance between 400-600 nm, and an unprecedentedly high photocatalytic NO removal performance. Further density functional theory (DFT) calculations and X-ray photoelectron spectroscopy (XPS) analysis revealed that the covalent interaction between the O 2p orbital of the insulator (SrCO3, n-type) and the Bi 6p orbital of photosensitizer (BiOI, p-type) can provide an electron transfer channel between SrCO3 and BiOI, allowing the transfer of the photoexcited electrons from the photosensitizer to the conduction band of insulator (confirmed by charge difference distribution analysis and time-resolved fluorescence spectroscopy). The •O2- and •OH radicals are the main reactive species in photocatalytic NO oxidation. A reaction pathway study based on both in situ FT-IR and molecular-level simulation of NO adsorption and transformation indicates that this heterojunction can efficiently transform NO to harmless nitrate products via the NO → NO+ and NO2+ → nitrate or nitrite routes. This work provides numerous opportunities to explore earth-abundant insulators as visible-light-driven photocatalysts, and also offers a new mechanistic understanding of the role of gas-phase photocatalysis in controlling air pollution.

Synthesis and Performance of Iron Oxide-Based Ceramsite in a Biotrickling Filter for Nitrogen Oxides Removal

A novel medium containing iron oxide-based porous ceramsite (modified ceramsite) and commercial ceramsite was used in two bench-scale biotrickling filter for nitrogen oxides removal to evaluate the performance of the modified ceramaite. In this study, we adopted the method of surface coating by choosing FeCl3·6H2O as modifier. Under the calcination time was 4 h at the temperature of 500°C, the modified filler presented excellent adsorption for microbial growth and the mass dry weight of biofilm reached a peak of 1.28 mg/g. Results showed that the density increased by 17% and porosity increased by about 15%, and the isoelectric point PI increased more than 4 times, while the surface pH value reduced to 3.46. The surface of pH is much lower than the isoelectric point, to ensure that its surface is electropositive, besides, the modified ceramsite had a more rough surface compared with ceramsite and changed the two-dimensional rough surface into three-dimensional surface. It only took 8 days for the biotrickling filter to start up with the modified ceramsite while the commercial ceramsite was about 22 days, which greatly shortened the start-up period. Packing with the modified filler, the NO removal performance stayed quite stable and efficient, the removal efficiency of NO did not apparently decrease under high inlet concentration of NO and basically maintained an average value of 92.8% during the whole operation period of treatment. While the commercial ceramsite presented an obvious removal decrease comparing with the modified, especially with the high NO inlet concentration of above 1600 mg/m3 and the removal efficiency was less than 80%. The iron oxide-based ceramsite proved to have the capability for improving the performance of the biofilter for NO removal. Furthermore, the fall off of the surface coating is not obvious under the experimental condition and the property of the coating is relatively stable in a long period of operation. Thus, our findings support the modified filler of iron oxide-based ceramsite as a material for use in filter media in in a biotrickling filter for pollutant gas removal.

Z-scheme CaIn2S4/Ag3PO4 nanocomposite with superior photocatalytic NO removal performance: fabrication, characterization and mechanistic study

A Z-scheme CaIn2S4/Ag3PO4 photocatalyst with superior photocatalytic NO removal performance was fabricated and characterized, and mechanistic analysis was also carried out.

Effects of temperature on NOx removal with Mn-Cu/ZSM5 catalysts assisted by plasma

Manganese-copper catalysts are well-known catalysts for selective catalytic reduction (SCR) of NOx. To evaluate the NOx removal in the combination system of catalysts and plasma, a series of Cu-Mn/ZSM-5 catalysts with varying amounts of manganese doping were synthesized by an incipient wetness impregnation method and used for SCR of NOx assisted by plasma at different temperatures. The results showed that the combination of catalyst and plasma promoted NO removal efficiency, the introduction of Mn on Cu/ZSM5 enhanced the catalyst activity, and 12Mn-12Cu/ZSM5 exhibited the best NO removal performance with about 90% NO removal efficiency at 25 °C. With a rise in temperature, the reduced electric field (E/N) increased, intensifying the discharge and generating more active radicals. In the absence of catalyst, an increase in the temperature had a negative effect on NO removal due to the reduction of the predominant oxidant O3. However, when the temperature increased in the combination system, NO removal efficiency decreased below 100 °C, while increased above 100 °C, which increased dramatically at low discharge power above 150 °C.

Visible-light-induced charge transfer pathway and photocatalysis mechanism on Bi semimetal@defective BiOBr hierarchical microspheres

Charge transfer pathway and catalysis mechanism are two major issues in a specific catalytic reaction process. To further probe these two aspects of photocatalytic NO oxidation to address the environmental problem, Bi metal@defective BiOBr hierarchical microspheres were fabricated and used as a visible light photocatalyst. The interfacial and surface properties of Bi metal@defective BiOBr were optimized to understand the SPR effect of Bi metal and the oxygen vacancies (OVs) formed in situ. It was found that the charge transfer pathway on Bi metal@defective BiOBr has been significantly changed from that on pristine BiOBr. The Bi semimetal could act both as a charge transfer bridge and as a hot electron donor. The OVs induced the formation of an intermediate level in the band structure of BiOBr and promote O2 activation and thus the generation of O2− species. Due to the synergistic effects of Bi metal and OVs, Bi metal@defective BiOBr demonstrated highly enhanced visible light photocatalytic performance for NO removal. The photocatalytic NO oxidation process has been monitored by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), which could reveal the reaction intermediate products accurately. On the basis of an investigation with in situ DRIFTS and the simulation of the electronic structure, a new photocatalysis mechanism involving Bi metal, OVs, and NO transformation was proposed. The perspectives on the charge transfer pathway and photocatalysis mechanism in the present work can be extended to other catalysts for tuning the interfacial properties and enhancing the photocatalytic performance to address environmental problems.

Removal of Nitrogen Oxide Based on Anammox through Fe(II)EDTA Absorption

Nitrogen oxides in flue gas cause considerable environmental and health problems. In this study, the NO removal through Fe(II)EDTA absorption based anaerobic ammonium oxidation (Anammox) reduction was first evaluated in batch tests. The removal efficiency and removal rate of NO reached 74.5%–85.4% and 1.62–15.48 μmol NO gVSS–1 h–1 with Fe(II)EDTA absorption, respectively. The optimal temperature and pH for Fe(II)EDTA absorption based Anammox reduction of NO was 30 °C and 7.0 in the range of temperature from 25 to 45 °C and pH from 6.5 to 8.0, respectively. The maximum specific substrate utilization rate and half-saturation constant of Fe(II)EDTA-NO were experimentally determined as 0.09 mmol N gVSS–1 h–1 and 1.338 mM, respectively. Then, a sequencing batch reactor (SBR1) performing Fe(II)EDTA absorption based Anammox reduction for NO removal was continuously operated for 35 days. The NO removal performance of SBR1 decreased gradually, and the NO removal efficiency and NO removal rate decreased from 86.65%...

Solvent-assisted synthesis of porous g-C3N4 with efficient visible-light photocatalytic performance for NO removal

Graphitic carbon nitride (g-C3N4) with efficient photocatalytic activity was synthesized through thermal polymerization of thiourea with the addition of water (CN-W) or ethanol (CN-E) at 550 °C for 2 h. The physicochemical properties of the g-C3N4 were investigated by X-ray diffraction, transmission electron microscopy, ultraviolet-visible spectroscopy, photoluminescence spectroscopy, diffuse-reflection spectroscopy, BET and BJH surface area characterization, and elemental analysis. The carbon content was found to have self-doped into the g-C3N4 matrix during the thermal polymerization of thiourea and ethanol. CN-W and CN-E showed considerably enhanced visible-light photocatalytic activity, with NO removal percentages of 37.2% and 48.3%, respectively. Compared with pure g-C3N4, both the short and long lifetimes of the charge carriers in CN-W and CN-E were found to be prolonged. The mechanism of improved visible-light photocatalytic activity was deduced. The present work may provide a facile route to optimize the microstructure of g-C3N4 photocatalysts for high-performance environmental and energy applications.

Optimization of wet denitration by dual oxidant (H2O2/S2O82 −) advanced oxidation process

A new type of wet denitration oxidant, which was composed of H2O2 and Na2S2O8, had been studied for its removal performance of NO from flue gas in a bubble column reactor. The interactive effects of pH, temperature and molar ratio of Na2S2O8/H2O2 on NO removal efficiency and NO absorption rate were investigated by a three-factor Box-Behnken design. The second order polynomial models were obtained to describe the relationship between the operation conditions and the responses. The results of analysis of variance such as the coefficient of determination, P values, adequate precision values and the lack of fit F-test implied the satisfactory adjustment of the quadratic models. The results of three dimensional plots showed that the interactive effect of temperature and pH had the most significant impact on these responses. The optimized parameters were successfully determined by BBD. Under the obtained optimum conditions of pH (a value of 11), temperature (50°C) and Na2S2O8/H2O2 molar ratio (a value of 0.005), NO removal efficiency of 78.68% and NO absorption rate of 7.58E-07mol/m2·s were observed respectively.

Multipollutant removal of Hg0/SO2/NO from simulated coal-combustion flue gases using metal oxide/mesoporous SiO2 composites

This study investigated the multipollutant control effectiveness for Hg0/SO2/NO using V, Mn, and Cu oxides/mesoporous SiO2 particulate composites. The presence of metal oxides decreased the specific surface area and pore volume and altered the surface morphology and sizes of the raw SiO2 particles. V4+/V5+, Mn4+, and Cu+/Cu2+ were the major valence states in the VOx, MnOx, and CuOx/SiO2 composites, respectively. Amorphous and highly dispersed metal oxides presenting in the SiO2 composites was confirmed by the X-ray diffraction characteristic peaks. Multipollutant control of Hg0 and NO using the metal oxide/SiO2 composites was achieved at both 150 and 350°C under the simulated coal-combustion flue gas condition. CuOx/SiO2 with >40% Hg0 removal at 350°C further implies that strong chemical bonds could form between Hg species and the surface groups of CuOx/SiO2. The metal oxide/SiO2 composites had up to 62% NO removal enhancement with injection of NH3 at 350°C, indicating the effectiveness of catalytic reduction of NO. The CuOx/SiO2 composite showed the greatest Hg0 and NO removal performance, which may stem from the presence of active CuOx in high content and its large surface area and mesoporosity.

Bridging the g-C3N4 Interlayers for Enhanced Photocatalysis

Graphitic carbon nitride (g-C3N4) has been widely investigated and applied in photocatalysis and catalysis, but its performance is still unsatisfactory. Here, we demonstrated that K-doped g-C3N4 with a unique electronic structure possessed highly enhanced visible-light photocatalytic performance for NO removal, which was superior to Na-doped g-C3N4. DFT calculations revealed that K or Na doping can narrow the bandgap of g-C3N4. K atoms, intercalated into the g-C3N4 interlayer via bridging the layers, could decrease the electronic localization and extend the π conjugated system, whereas Na atoms tended to be doped into the CN planes and increased the in-planar electron density. On the basis of theoretical calculation results, we synthesized K-doped g-C3N4 and Na-doped g-C3N4 by a facile thermal polymerization method. Consistent with the theoretical prediction, it was found that K was intercalated into the space between the g-C3N4 layers. The K-intercalated g-C3N4 sample showed increased visible-light absor...

Iodide surface decoration: a facile and efficacious approach to modulating the band energy level of semiconductors for high-performance visible-light photocatalysis

We herein report a facile and general approach to modulating the band energy level of semiconductors for visible-light photocatalysis via iodide surface decoration. This strategy enables the wide-band-gap Bi2O2CO3 to possess a continuously tunable band gap and profoundly boosted visible-light photocatalytic performance for dye degradation and NO removal.

Sulfur-doping synchronously ameliorating band energy structure and charge separation achieving decent visible-light photocatalysis of Bi2O2CO3

Sulfur doping simultaneously endows the wide-band-gap Bi2O2CO3 promoted band energy structure and charge separation achieving enhanced visible-light photocatalytic performance for dye degradation and NO removal.

An Advanced Semimetal-Organic Bi Spheres-g-C3N4 Nanohybrid with SPR-Enhanced Visible-Light Photocatalytic Performance for NO Purification.

To achieve efficient photocatalytic air purification, we constructed an advanced semimetal-organic Bi spheres-g-C3N4 nanohybrid through the in-situ growth of Bi nanospheres on g-C3N4 nanosheets. This Bi-g-C3N4 compound exhibited an exceptionally high and stable visible-light photocatalytic performance for NO removal due to the surface plasmon resonance (SPR) endowed by Bi metal. The SPR property of Bi could conspicuously enhance the visible-light harvesting and the charge separation. The electromagnetic field distribution of Bi spheres involving SPR effect was simulated and reaches its maximum in close proximity to the Bi particle surface. When the Bi metal content was controlled at 25%, the corresponding Bi-g-C3N4 displayed outstanding photocatalytic capability and transcended those of other visible-light photocatalysts. The Bi-g-C3N4 exhibited a high structural stability under repeated photocatalytic runs. A new visible-light-induced SPR-based photocatalysis mechanism with Bi-g-C3N4 was proposed on the basis of the DMPO-ESR spin-trapping. The photoinduced electrons could transfer from g-C3N4 to the Bi metal, as revealed with time-resolved fluorescence spectra. The function of Bi semimetal as a plasmonic cocatalyst for boosting visible light photocatalysis was similar to that of noble metals, which demonstrated a great potential of utilizing the economically feasible Bi element as a substitute for noble metals for the advancement of photocatalysis efficiency.

Water-promoted low-concentration NO removal at room temperature by Mg-doped manganese oxides OMS-2

Nanostructured magnesium doped manganese oxides (Mg-SC-OMS-2) was prepared through doping Mg into OMS-2 using an ultrasound-assisted approach. This Mg-SC-OMS-2 material showed low crystallinity with a large surface area and extensive defects which are totally different from those synthesized by conventional method. Extraordinarily high NO removal performance was obtained. It retained 99% NO removal ratio for as long as 8h under extremely high space velocity of 120,000mL/g/h. More attractively, when water vapor was introduced, above 50% removal ratio of catalyst was maintained for as long as 250h. The high catalytic activity of Mg-SC-OMS-2 is associated with presence of redox pair Mn3+/Mn4+ and activated oxygen species at oxygen vacancies occurring via the pathway of Mars-van Krevelen mechanism.

Controlling interfacial contact and exposed facets for enhancing photocatalysis via 2D-2D heterostructures.

Herein, we report a facile strategy for the creation of 2D layered heterostructures with intimate interfacial contact and exposed reactive facets. The 2D layered heterostructures with intimate contact by sharing the interfacial oxygen atoms and exposed reactive facets endowed the as-prepared BiOIO3/BiOI nanostructures with highly enhanced visible photocatalytic performance for NO removal.