NO Removal Ratio Research Articles

A universal in situ lattice sulfidation synthesis method of metal sulfides/TiO2 heterostructures and its application in photocatalytic NO removal

It is a challenge to synthesize well-defined metal sulfides/TiO2 (MSx/TiO2) heterostructures because of the lattice mismatch between MSx and TiO2. In this work, an in situ lattice sulfidation synthesis method of MSx/TiO2 was developed. Under hydrothermal conditions, metal atoms in metal-doped TiO2 (MTiO2) reacted with S species to generate MSx in situ on TiO2 surface. This method is a universal preparation method and 8 kinds of MSx/TiO2 heterostructures (M = Mn, Fe, Co, Ni, Cu, Zn, Mo and Cd) were synthesized. And taking CdS/TiO2 as an example due to its highest photocatalytic activity, we can conclude that CdS grew tightly and dispersed uniformly on TiO2 surface. The combination with CdS improved the photocatalytic NO removal ratio of TiO2 greatly (>510 nm, from 2% to 34%; >400 nm, from 10% to 39%), which was high compared with other works.

Guest Editorial: Solar Energy Utilization

Guest Editorial: Solar Energy Utilization

Oxygen-vacancy-rich Ag/Bi5O7Br nanosheets enable improved photocatalytic NO removal and oxygen evolution under visible light exposure

Photocatalytic NO removal is a green and sustainable alternative to the conventional thermocatalysis in the conversion of NO to nitrates. However, the efficiency of photocatalytic NO removal is restricted by weak NO adsorption and high charge recombination on photocatalyst. Herein, we report on one-step synthesis of Ag/Bi5O7Br nanosheets with rich oxygen vacancies (OVs) by a facile liquid phase reduction method. Under visible light irradiation on oxygen-vacancy-rich Ag/Bi5O7Br for 50 min the photocatalytic NO removal ratio is up to 64.65%, which is about 1.6 times higher than that by using pristine Bi5O7Br. The average oxygen production rate is 823 μmol·g−1·h−1, which is nearly 10 times higher than that of Bi5O7Br. Density functional theory (DFT) calculations reveal that OVs incorporation and plasmonic Ag can synergistically strengthen NO adsorption on Bi5O7Br. This work highlights the great potential of defects and plasmonic metals on synergistic enhancement in photocatalytic NO removal and oxygen evolution.

Construction of Highly Dispersed Ni Sites on N‐rich Carbon Nitride for Enhanced Photocatalytic NO Removal

AbstractIntelligent active site engineering to harness photogenerated charge carriers and motivate surface reactions is at the core of catalysis. Herein, highly dispersed Ni sites are planted on C3N5, an N‐rich carbon nitride, by a facial two‐step annealing method to construct a Ni‐C3N5 material. The incorporation of Ni sites can significantly enhance the e–/h+ separation efficiency of C3N5 under light irradiation and promote the activation of O2 to produce reactive oxygen species. Compared with pristine C3N5 (with NO removal ratio of ≈35%), the as‐prepared 0.1‐ or 0.25‐Ni‐C3N5 material can remove ≈54% continuous‐flowing NO (initial concentration: 600 ppb) quickly in less than 25 min under white LED light irradiation. The long‐term photocatalytic performance demonstrates that the catalyst is stable without obvious attenuation in activity. The findings of the trapping experiments and in situ diffuse reflectance infrared fourier transform spectroscopy tests suggest that •O2− may mainly convert NO into NO2, while •OH and 1O2 play a significant role in the NO2 to NO3– conversion reaction. The present work brings new insights into the design of active sites on semiconductor photocatalytic materials for the treatment of NOx species.

Switching on photocatalytic NO oxidation and proton reduction of NH2-MIL-125(Ti) by convenient linker defect engineering

Photocatalysis technology has been widely adopted to abate typical air pollutants. Nevertheless, developing photocatalysts aimed at improving photocatalytic efficiency is a challenge. Herein, the linker-defect NH2-MIL-125(Ti) photocatalyst was synthesized through a convenient one-step heating-stirring method (just adjusting multiple temperatures) to firstly realize efficient photocatalytic performances of NO removal and hydrogen evolution. The optimal sample (named 65-NMIL) with a linker-defect content of 32.08% exhibited a NO removal ratio of 65.49%, which was 37.57% higher than that of pristine NH2-MIL-125(Ti), and displayed better H2-production activity. Through ESR, it was confirmed that 65-NMIL can generate more •O2- and •OH under visible light, and the radical trapping experiment further proved that •O2- played a more important role in photocatalytic activity. Moreover, the photocatalytic NO oxidation process was also monitored by in situ DRIFTS, it was found that the defective samples could promote the oxidation of NO and intermediates to the final product (NO3-). On the basis of the above-mentioned photocatalytic experimental results and characterization, a possible mechanism or pathway was proposed and illustrated. This work can provide a new strategy for the subsequent defect engineering for photocatalytic MOFs materials to further solve environmental and energy crises.

InP quantum dots on g-C3N4 nanosheets to promote molecular oxygen activation under visible light

Largely limited by the high dissociation energy of the OO bond, the photocatalytic molecular oxygen activation is highly challenged, which restrains the application of photocatalytic oxidation technology for atmospheric pollutants removal. Herein, we design and fabricate the InP QDs/g-C3N4 compounds. The introduction of InP QDs promotes the charge transfer within the interface resulting in the effective separation of photo-generated carriers. Furthermore, InP QDs greatly facilitates the activation of molecular oxygen and promote the formation of O2− under visible-light illumination. These conclusions are identified by experimental and calculation results. Hence, NO can be combined with the O2− to form OONO intermediate to direct conversion into NO3−. As a result, the NO removal ratio of g-C3N4 has a onefold increase after InP QDs loaded and the generation of NO2 is effectively inhibited. This work may provide a strategy to design highly efficient materials for molecular oxygen activation.

Facile synthesis of Bi2O3 nanocrystals for photocatalytic NO oxidation and its conversion pathway via in situ DRIFTS

In this work, the two different polymorphs of Bi2O3, including α-Bi2O3 and β-Bi2O3, were fabricated by the post-calcined the precursor at 300 and 500 °C respectively. The pathway of photocatalytic NO oxidation over the as-prepared photocatalyst (β-Bi2O3) was explored. The results showed that NO removal ratio on as-synthesis samples followed the order: β-Bi2O3 (32 %) > the precursor (13 %) > α-Bi2O3 (5 %). The ESR and in situ DRIFTS demonstrated that the superoxide (O2−), as the main radical, can react with the absorbed NO on the surface of β-Bi2O3 to generate the final products NO3− and NO2−. Therefore, such findings reveal the possible photocatalytic pathway of NO oxidation on β-Bi2O3 based on in situ DRIFTS and give insight into understanding the polymorphism of Bi2O3 in efficient photocatalytic NO oxidation under the visible light.

Noble-metal-free cobaloxime coupled with metal-organic frameworks NH2-MIL-125: A novel bifunctional photocatalyst for photocatalytic NO removal and H2 evolution under visible light irradiation

The novel bifunctional NH2-MIL-125/Co(dmgH)2 composite catalysts with several different Co(dmgH)2 contents that can simultaneously achieve photocatalytic NO removal and hydrogen production were first prepared by a simple and convenient method. The corresponding physical and chemical properties of the composite catalysts were characterized by SEM, XRD, ESR, in situ DRIFTS, etc. The characterization results indicated that the noble-metal-free Co(dmgH)2, which was much cheaper and more available than most noble-metals such as Pt, could be an effective co-catalyst to accelerate the separation of photogenerated electron-hole pairs, further eventually enhancing the photocatalytic efficiency. Under visible-light irradiation for half an hour, the NO removal ratio of NH2-MIL-125/Co(dmgH)2 (3 wt%) increased by 22.7 % compared with the pristine NH2-MIL-125 without Co(dmgH)2 loading. In addition, it was found that Eosin Y dye-sensitized NH2-MIL-125/Co(dmgH)2 (3 wt%) was capable of promoting a hydrogen generation rate of 2195 μmol g−1 h-1 under visible light, which was 12.6 times greater than the original NH2-MIL-125. This strategy was expected as an available way to fabricate noble-metal-free molecular complexes with metal-organic frameworks (MOFs) to enhance the photocatalytic NO removal and hydrogen production performance simultaneously.

Surface modification to control the secondary pollution of photocatalytic nitric oxide removal over monolithic protonated g-C3N4/graphene oxide aerogel

Recently, photocatalytic NOx treatment has attracted great attention on account of the use of environmental-friendly and tremendous energy source. However, the difficult recovery of most reported powdery photocatalysts and the high generation rate of toxic NO2 byproduct limit its application. Here, we designed a novel monolithic protonated g-C3N4/graphene oxide aerogel through a direct frozen-drying method. A remarkable surface electric charge change of negative g-C3N4 to positive protonated g-C3N4 can be observed after the protonating treatment, which connects with negative graphene oxide nanosheets through the formation of strong electrostatic self-assembly to accelerate the photogenerated charge carriers transfer. Graphene oxide aerogel acts as a monolithic substrate, which provides abundant porous structure, enhanced visible-light absorption, and electrons transport pathway to improve photocatalytic activity. Importantly, the introduction of H atoms on the N atoms of p-C3N4 promotes the activation of oxygen atoms, thus improving the oxidization of NO2 to nitrate. As a result, protonated g-C3N4/graphene oxide aerogel reveals an excellent NO removal ratio (46.1%) and low NO2 generation (2.4%), demonstrating its excellent promising for air pollution purification. Our current work affords novel innovative insight for the construction of monolithic photocatalysts to control the secondary pollution for environmental remediation.

Synthesis of three-component C3N4/rGO/C-TiO2 photocatalyst with enhanced visible-light responsive photocatalytic deNOx activity

In order to realize high efficiency photocatalysis deNOx effect, a homogeneous three-component photocatalyst, C3N4/rGO/C-TiO2 was successfully synthesized by an in situ synthesis method. First, C3N4/rGO composite was prepared to increase the contact surface between them. Then, C-TiO2 was crystallized on the surface of the C3N4/rGO to obtain relatively dispersed C-TiO2 nanoparticles on the C3N4/rGO. The as-prepared photocatalyst was evaluated by NO removal ratio under weak visible and UV LED light irradiation. C3N4/rGO/C-TiO2 was compared with sample prepared by simple mixing of C3N4, rGO and C-TiO2 within a solution. As a result, sample prepared by in situ synthesis exhibited the highest photocatalytic activity, together with high apparent quantum efficiency. The transient absorption measurement clarified that the composite photocatalyst possessed longer carrier lifetime if compared to that of each component material, resulted to its higher photocatalytic activity. The three-component photocatalyst is thought to have shown enhancement in photocatalytic activity by Z-scheme mechanism with rGO as an electron mediator to improve the performance of Z-scheme.

Synergistic effects of crystal structure and oxygen vacancy on Bi2O3 polymorphs: intermediates activation, photocatalytic reaction efficiency, and conversion pathway

This work unraveled the synergistic effects of crystal structure and oxygen vacancy on the photocatalytic activity of Bi2O3 polymorphs at an atomic level for the first time. The artificial oxygen vacancy is introduced into α-Bi2O3 and β-Bi2O3 via a facile method to engineer the band structures and transportation of carriers and redox reaction for highly enhanced photocatalysis. After the optimization, the photocatalytic NO removal ratio on defective β-Bi2O3 was increased from 25.2% to 52.0% under visible light irradiation. On defective α-Bi2O3, the NO removal ratio is just increased from 7.3% to 20.1%. The difference in the activity enhancement is associated with the different structure of crystal phase and oxygen vacancy. The density functional theory (DFT) calculation and experimental results confirm that the oxygen vacancy in α-Bi2O3 and β-Bi2O3 could promote the activation of reactants and intermediate as active centers. The crystal structure and oxygen vacancy could synergistically regulate the electrons transfer pathway. On defective β-Bi2O3 with tunnel structure, the reactants activation and charge transfer were more efficient than that on α-Bi2O3 with zigzag-type configuration because the defect structures on the surface of α-Bi2O3 and β-Bi2O3 were different. Moreover, the in situ FT-IR revealed the mechanisms of photocatalytic NO oxidation. The photocatalytic NO conversion pathway on α-Bi2O3 and β-Bi2O3 can be tuned by the different surface defect structures. This work could provide a novel strategy to regulate the photocatalytic activity and conversion pathway via the synergistic effects of crystal structure and oxygen vacancy.

Oxygen activation of noble-metal-free g-C3N4/α-Ni(OH)2 to control the toxic byproduct of photocatalytic nitric oxide removal

Photocatalytic NOx removal is regarded as a promising technique for air purification in recent years. However, NOx removal process usually accompanies with toxic NO2 generation, leading to serious secondary pollutant and limiting its practical application. Herein, a noble-metal-free g-C3N4/α-Ni(OH)2 photocatalyst was fabricated through hydrothermal method. The introduction of α-Ni(OH)2 microspheres combines with g-C3N4, resulting in an enlarged surface area from 35 to 95 m2/g, while the unique structure of α-Ni(OH)2 broadens visible-light response range and accelerates photogenerated charge carriers transfer. More importantly, the H atoms of α-Ni(OH)2 improve the adsorption and activation of O2 and oxygen atoms to promote NO and NO2 oxidized to nitrate. Consequently, g-C3N4/α-Ni(OH)2 shows enhanced NO removal ratio of 51.3% without any toxic NO2 generation, indicating its great potential for air purification. Our results provide fresh insight into the design of highly efficient and safe photocatalyst for toxic byproduct control in the application of environmental remediation.

Nitrogen defect structure and NO+ intermediate promoted photocatalytic NO removal on H2 treated g-C3N4

By heating g-C3N4 powder in the hydrogen atmosphere, nitrogen defects were introduced into the framework of g-C3N4 where the nitrogen atoms in g-C3N4 were reacted and partially removed with hydrogen. The effects of nitrogen defects on the electronic structure, optical properties, generation of reactive oxygen species and photocatalytic NO oxidation of g-C3N4 were investigated by combining experimental characterization and DFT theoretical calculations. The N defect is located at N2C sites and can be tuned by the H2 treating temperature. The obtained N defective g-C3N4 products possessed narrower bandgap adjusted by surface N defects and were able to promote the separation of photoexcited charge carries and produce reactive oxygen species more efficiently than pristine g-C3N4. The NO+ reaction intermediate was formed on the N defects sites and enabled an accelerated photocatalytic reaction that contributed to enhanced photocatalytic NO removal. The NO removal ratio on N defects g-C3N4 obtained at 600 °C (CH-H-600) was 2.6 times that of pristine g-C3N4 under visible light irradiation. The present work could provide new insights into the understanding of the role of N-defects in g-C3N4 and application of photocatalytic technology for efficient air purification.

The pivotal effects of oxygen vacancy on Bi2MoO6: Promoted visible light photocatalytic activity and reaction mechanism

Bi2MoO6, a typical Bi-based photocatalyst, has received increasing interests and been widely applied in various fields. However, the visible light photocatalytic activity of Bi2MoO6 is still restricted by some obstacles, such as limited photo-response and low charge separation efficiency. In this work, we developed a facile method to introduce artificial oxygen vacancy into Bi2MoO6 microspheres, which could effectively address these problems and realize highly efficient visible light photocatalysis. The experimental and theoretical methods were combined to explore the effects of oxygen vacancy on the electronic structure, photocatalytic activity and the reaction mechanism toward NO removal. The results showed that the addition of NaBH4 during catalyst preparation induced the formation of oxygen vacancy in Bi2MoO6, which plays a significant role in extending the visible light absorption of Bi2MoO6. The visible light photocatalytic activity of Bi2MoO6 with oxygen vacancy was obviously enhanced with a NO removal ratio of 43.5%, in contrast to that of 25.0% with the pristine Bi2MoO6. This can be attributed to the oxygen vacancy that creates a defect energy level in the band gap of Bi2MoO6, thus facilitating the charge separation and transfer processes. Hence, more reactive radicals were generated and participated in the photocatalytic NO oxidation reaction. The in situ FT-IR was used to dynamically monitor the photocatalytic NO oxidation process. The reaction intermediates were observed and the adsorption-reaction mechanism was proposed. It was found that the reaction mechanism was unchanged by introducing the oxygen vacancy in Bi2MoO6. This work could provide new insights into the understanding of the oxygen vacancy in photocatalysis and gas-phase photocatalytic reaction mechanism.

A Bi/BiOI/(BiO)2CO3 heterostructure for enhanced photocatalytic NO removal under visible light

Narrow-band BiOI photocatalysts usually suffer from low photocatalysis efficiency under visible light exposure because of rapid charge recombination. In this work, to overcome this deficiency of photosensitive BiOI, oxygen vacancies, Bi particles, and Bi2O2CO3 were co-induced in BiOI via a facile in situ assembly method at room temperature using NaBH4 as the reducing agent. In the synthesized ternary Bi/BiOI/(BiO)2CO3, the oxygen vacancies, dual heterojunctions (i.e., Bi/BiOI and BiOI/(BiO)2CO3), and surface plasmon resonance effect of the Bi particles contributed to efficient electron-hole separation and an increase in charge carrier concentration, thus boosting the overall visible light photocatalysis efficiency. The as-prepared catalysts were applied for the removal of NO in concentrations of parts per billion from air in continuous air flow under visible light illumination. Bi/BiOI/(BiO)2CO3 exhibited a highly enhanced NO removal ratio of 50.7%, much higher than that of the pristine BiOI (1.2%). Density functional theory calculations and experimental results revealed that the Bi/BiOI/(BiO)2CO3 composites promoted the production of reactive oxygen species for photocatalytic NO oxidation. Thus, this work provides a new strategy to modify narrow-band semiconductors and explore other bismuth-containing heterostructured visible-light-driven photocatalysts.

Roles of N-Vacancies over Porous g-C3N4 Microtubes during Photocatalytic NO x Removal.

The development of catalysts that effectively activate target pollutants and promote their complete conversion is an admirable objective in the environmental photocatalysis field. In this work, graphitic carbon nitride (g-C3N4) microtubes with tunable N-vacancy concentrations were controllably fabricated using an in situ soft-chemical method. The morphological evolution of g-C3N4, from the bulk to the porous tubular architecture, is discussed in detail with the aid of time-resolved hydrothermal experiments. We found that the NO removal ratio and apparent reaction rate constant of the g-C3N4 microtubes were 1.8 and 2.6 times higher than those of pristine g-C3N4, respectively. By combining detailed experimental characterization and density functional theory calculations, the effects of N-vacancies in the g-C3N4 microtubes on O2 and NO adsorption activation, electron capture, and electronic structure were systematically investigated. These results demonstrate that surface N-vacancies act as specific sites for the adsorption activation of reactants and photoinduced electron capture, while enhancing the light-absorbing capability of g-C3N4. Moreover, the porous wall structures of the as-prepared g-C3N4 microtubes facilitate the diffusion of reactants, and their tubular architectures favor the oriented transfer of charge carriers. The intermediates formed during photocatalytic NO removal processes were identified by in situ diffuse reflectance infrared Fourier transform spectroscopy, and different reaction pathways over pristine and N-deficient g-C3N4 are proposed. This study provides a feasible strategy for air pollution control over g-C3N4 by introducing N-vacancy and porous tubular architecture simultaneously.

Z-Scheme 2D/2D Heterojunction of Black Phosphorus/Monolayer Bi2 WO6 Nanosheets with Enhanced Photocatalytic Activities.

Black phosphorus (BP), a star-shaped two-dimensional material, has attracted considerable attention owing to its unique chemical and physical properties. BP shows great potential in photocatalysis area because of its excellent optical properties; however, its applications in this field have been limited to date. Now, a Z-scheme heterojunction of 2D/2D BP/monolayer Bi2 WO6 (MBWO) is fabricated by a simple and effective method. The BP/MBWO heterojunction exhibits enhanced photocatalytic performance in photocatalytic water splitting to produce H2 and NO removal to purify air; the highest H2 evolution rate of BP/MBWO is 21042 μmol g-1 , is 9.15 times that of pristine MBWO and the NO removal ratio was as high as 67 %. A Z-scheme photocatalytic mechanism is proposed based on monitoring of . O2 - , . OH, NO2 , and NO3 - species in the reaction. This work broadens applications of BP and highlights its promise in the treatment of environmental pollution and renewable energy issues.

Z‐Scheme 2D/2D Heterojunction of Black Phosphorus/Monolayer Bi2WO6 Nanosheets with Enhanced Photocatalytic Activities

AbstractBlack phosphorus (BP), a star‐shaped two‐dimensional material, has attracted considerable attention owing to its unique chemical and physical properties. BP shows great potential in photocatalysis area because of its excellent optical properties; however, its applications in this field have been limited to date. Now, a Z‐scheme heterojunction of 2D/2D BP/monolayer Bi2WO6 (MBWO) is fabricated by a simple and effective method. The BP/MBWO heterojunction exhibits enhanced photocatalytic performance in photocatalytic water splitting to produce H2 and NO removal to purify air; the highest H2 evolution rate of BP/MBWO is 21042 μmol g−1, is 9.15 times that of pristine MBWO and the NO removal ratio was as high as 67 %. A Z‐scheme photocatalytic mechanism is proposed based on monitoring of .O2−, .OH, NO2, and NO3− species in the reaction. This work broadens applications of BP and highlights its promise in the treatment of environmental pollution and renewable energy issues.

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.

In-situ hydrothermal fabrication of Sr2FeTaO6/NaTaO3 heterojunction photocatalyst aimed at the effective promotion of electron-hole separation and visible-light absorption

The Sr2FeTaO6/NaTaO3 heterojunctions with the controllable molar ratio and space of heterojunction transitional zone were fabricated using in situ hydrothermal method by adjusting hydrothermal temperature and time. Comparing with the pure NaTaO3 nanosheets and Sr2FeTaO6 nanoparticles, the Sr2FeTaO6/ NaTaO3 heterojunctions exhibit higher photocatalytic performances, due to introducing narrow-bandgap Sr2FeTaO6 component, which could obviously enhance its absorbability in visible light region and separation efficiency of photo-generated electron-hole pairs. Under visible light irradiation, the optimal Sr2FeTaO6/NaTaO3 heterojunction, with the molar ratio of 0.53 and space of heterojunction transitional zone of 0.35 nm, shows the best photocatalytic activity with the NO removal ratio of 72% in 40 min, hydrogen evolution rate of 1334 μmol h−1 g−1 in 5 h, and apparent quantum efficiency (AQE) for hydrogen evolution of 38.8% at 420 nm. More interestingly, the space of heterojunction transitional zone, related to the interface bonding degree of heterojunction material, may not be the only factor which influences transferring and separating efficiency of photo-generated charge carriers, but it is certainly an important one, which provides a new cognitive perspective for constructing and understanding efficient heterostructure photocatalyst.