Photocatalytic NO Oxidation Research Articles

NiCoP/g-C3N4 Schottky heterojunctions towards efficient photocatalytic NO oxidation

NiCoP/g-C3N4 Schottky heterojunctions were created by a two-step synthesis for enhanced NO oxidation. NiCo-layered double hydroxides (LDH) were grown on superior thin graphic carbon nitride (g-C3N4) nanosheets by a controlling chemical co-precipitation method. A phosphating process at 300 °C resulted in the transformation in situ of NiCo-LDH into NiCoP composite nanoparticles to create Schottky heterojunctions. Using optimized conditions, heterojunction sample 15-NiCoP/CN revealed excellent photocatalytic NO oxidation with a removal rate of 78%. Photogenerated electrons were efficiently separated and transferred because of Schottky barrier, interfacial charge transport channel, and enhanced light absorption efficiency. Photogenerated electrons reacted with adsorbed O2 to generate superoxide radicals (·O2¯) as crucial active species of NO oxidation. The test indicated that the concentration of·O2¯ for sample 15-NiCoP/CN reached 7.1 × 10−5 M which was 6.5 times higher than that of g-C3N4 after light irradiation. In addition, low NO2 generation and mineralization rate (92%) suggested that NO was converted into NO3- or NO2- rather than NO2 to mitigate secondary pollution. This result supplied a novel approach to explore the application of transition metal phosphides and g-C3N4 in photocatalytic NO oxidation.

Stabilize the oxygen vacancies in Bi2SiO5 for durable photocatalysis via altering local electronic structure with phosphate dopant

Oxygen vacancies in catalysts can bring new properties, especially the enhanced catalytic efficiencies that are absent in their pristine counterparts. However, the deactivation of oxygen vacancies inevitably limits the durability of catalytic performance. Herein, a model of photocatalytic NO oxidation over Bi2SiO5 catalyst is selected to unravel the pivotal role of dopants in enhancing the long-term stability of oxygen vacancies. The introduction of phosphate dopants could stabilize oxygen vacancies via turning the local electronic structure. The phosphate dopants evoke the enhanced electron redistribution and cause the charge compensation to unsaturated coordination atoms around the oxygen vacancies. The unique electronic structure modulated by dopants could facilitate the adsorption and activation of H2O and O2, impeding those atoms from filling into oxygen vacancies. This work provides an innovative route for rationally altering the local electronic structure of catalysts toward efficient and sustainable photocatalytic applications.

In-situ formation of Au nanoparticles with surface plasmon resonance confined in the framework of Cu ions doped NH2-MIL-125(Ti) to enhance photocatalytic hydrogen production and NO removal

A novel bifunctional photocatalyst Au@NML-(Cu/Ti) was fabricated to improve the photocatalytic performance of hydrogen production and NO removal, in which the Cu ion was first doped into the titanium oxide clusters of NH2-MIL-125(Ti) (NML) via a convenient method, and uniformly dispersed gold nanoparticles with surface plasmon resonance effect in-situ generated under the constraint of NML-(Cu/Ti) framework. The introduction of copper ions reduced the band gap of NML and accelerated the charge separation rate. Moreover, the NML-(Cu/Ti) framework could prevent the Au nanoparticles agglomeration as far as possible via confinement effect to obtain highly dispersed gold nanoparticles in very small size, which can effectively improve the atom utilization efficiency for enhancing photocatalytic performance. The photocatalytic hydrogen production of Au@NML-(Cu/Ti) was 5193.4 μ mol·g−1, which was 11.8 times that of the original NML. Meanwhile, the NO removal rate of Au@NML-(Cu/Ti) was about 25.6% higher than that of the pristine NML. Moreover, the photocatalytic NO oxidation process was also monitored by in-situ DRIFTS, which indicates Au@NML-(Cu/Ti) could realize the effective oxidation of NO. These results showed that the synergistic combination of central metal cluster modification of MOFs with surface plasma resonance provided a feasible strategy for improving photocatalytic hydrogen production and NO removal.

Ni(OH)2 clusters decorated NiTiO3 Z-scheme heterojunctions with strong O2 activation and robust interfacial built-in electric field for efficient NO photo-oxidation

Photocatalytic oxidation offers a viable method for removing low-concentration NO from the atmosphere. However, poor oxygen adsorption-activation capacity and high re-combination rate of electrons and holes limit the photocatalytic NO oxidation activity. To over-come these problems, Ni(OH)2 clusters decorated NiTiO3 Z-scheme heterojunctions are constructed by a precipitation method. The tight interfacial contacts and built-in electric field between the (002) crystal facet of Ni(OH)2 and (012) crystal facet of NiTiO3 promote a Z-scheme charge transfer mechanism. This Z-scheme mechanism will enhance the charge-transfer driving force and result in high separation efficiency of the charges, thus leading to an enhanced photocatalytic performance. Importantly, introducing Ni(OH)2 is considered a promising strategy to enhance the adsorption ability of the O2, NO and NO2. This might help to increase O2 activation efficiency and speed up NO deep oxidation, leading to high NO removal efficiency and low NO2 selectivity. Benefiting from the above excellent properties, the photocatalytic NO oxidation efficiency of Ni(OH)2/NiTiO3 composites is enhanced and the formation of more toxic NO2 is inhibited compared with that of NiTiO3. This work offers a promising and effective route to design efficient NiTiO3-based photocatalysts in the application of environmental remediation.

Constructing Oxygen Vacancies on Bi2MoO6 Nanosheets by Aqueous Ammonia Etching with Enhanced Photocatalytic NO Oxidation Performance

Constructing Oxygen Vacancies on Bi₂MoO₆ Nanosheets by Aqueous Ammonia Etching with Enhanced Photocatalytic NO Oxidation Performance

Study on the effective removal of NO by light-driven N-BiOCl

In this paper, an N-doped BiOCl photocatalyst was prepared and systematically characterized. The results showed that N doping does not change the crystal phase of BiOCl but changes the surface morphology of BiOCl nanosheets and increases specific surface area. N-doped BiOCl broadens the light absorption region, narrows the band gap and promotes the separation of the photoexcited carriers. N could be adsorbed on the Bi or to replace Cl atom in the BiOCl lattice, causing the generation of intermediate energy levels in the band gap of N-BiOCl including N 2p-Cl 2p hybrid states, N 2p-O 2p hybrid states or N 2p state. N-BiOCl achieves about 60% removal efficiency of NO under all-light irradiation and exhibits favorable stability and recyclability. The presence of some H2O in the reaction system promotes the photodegradation of NO, while O2 greatly affects the efficiency of NO removal. Moreover, N doping promotes the formation of •O2− and •OH on N-BiOCl photocatalyst under all-light irradiation, resulting in the enhancement of photocatalytic oxidation of NO to NO3–. NO could be adsorbed on the surface of N-BiOCl or oxidized into NO2– under darkness and further oxidized into NO3– under all-light irradiation. However, the accumulation of the final product on the catalyst surface results in a decrease in photoactivity.

Insight into the mechanism of deep NO photo-oxidation by bismuth tantalate with oxygen vacancies

Deeply photocatalytic oxidation of nitrogen oxides is still difficult to achieve, mainly limited by few intrinsic active sites and inefficient carrier separation of photocatalysts. Accordingly, we develop a simple room temperature tactic to introduce oxygen vacancies (OVs) into Bi3TaO7 (BTO). Based on solid experimental and DFT theoretical supports, we explore the mechanism of NO removal over OVs decorated BTO (OVs-BTO). OVs can not only alter the distribution of local electrons to result in the formation of a fast charge transfer channel between OVs and the adjacent Ta atoms, which improves the transport rate of photogenerated carriers; but also function as active sites to adsorb small molecules (NO, O2 and H2O), which being activated and positively drive the NO oxidation reaction. In order to investigate a possible reaction path, a combination of in-situ DRIFTS and simulated Gibbs free energy reveals that the intermediate products of OVs-BTO are helpful to promote the deep oxidation of NO to NO3-, while pristine BTO is more likely to produce NO2 intermediate toxic by-products, which greatly hinders the deep photocatalytic oxidation of NO. This work provides insights into the role of OVs in photocatalysts, and also points out a guideline for the mechanism of semiconductor photocatalysts in eliminating gaseous pollutants.

New insights into the removal of nitric oxide using UiO-66-NH2: Synergistic photooxidation and subsequent adsorption

UiO-66 variants are widely used for photocatalysis and adsorption, but few studies reported their combined effects in one process. This paper reports that the mechanism behind NO removal using UiO-66-NH2 includes photocatalytic oxidation and subsequent adsorption. First, the UiO-66-NH2 was selected out of UiO-66-X variants (X = H, NH2, NO2, OH, Br, Br2) because of its high NO removal efficiency (i.e., 80.54%), which is 8.55–64.95 times higher than those of other UiO-66 variants. Then, the NO reaction pathway and degradation mechanisms are proposed based on the experimental results and theoretical calculations. In an anhydrous environment, the NO removal efficiency increases from 80.54% at 40–60% of humidity to 96.23%. Moreover, NO2 emissions and catalyst deactivate are not observed. These findings indicate that the photocatalytic NO degradation includes the photocatalytic oxidation of NO into NO2 on the surface of UiO-66-NH2 and subsequent NO2 adsorption in micropores. The reason is that the formation of NOx- ions without H2O is theoretically impossible according to the principle of electroneutrality. In-situ DRIFTS also confirms the mechanism. Furthermore, density functional theory (DFT) and grand canonical Monte Carlo (GCMC) simulations were carried out to understand the improved NO removal caused by synergistic photocatalysis and adsorption. In summary, this work proposes a new mechanism for NO removal that combines the photocatalytic oxidation and adsorption capability of UiO-66-NH2, the new mechanism provides a new strategy to further improve the NO removal efficiency of UiO-66-NH2 and a new way of inhibiting the deactivation.

Efficient NO removal and photocatalysis mechanism over Bi-metal@Bi2O2[BO2(OH)] with oxygen vacancies

Photocatalysis technology prevails as a feasible option for air pollution control, in which high-efficiency charge separation and effective pollutant activation are the crucial issues. Here, this work designed Bi-metal@ Bi2O2[BO2(OH)] with oxygen vacancies (OVs) catalyst for photocatalytic oxidation of NO under visible light, to shed light on the above two processes. Experimental characterizations and density functional theory (DFT) calculations reveal that a unique electron transfer covalent loop（[Bi2O2]2+ → Bi-metal → O2−）can be formed during the reaction to guide the directional transfer of carriers, significantly improving the charge separation efficiency and the yield of active oxygen species. Simultaneously, the defect levels served by OVs also play a part. During the NO purification process, in-situ DRIFTS assisted with DFT calculations reveal that Bi metals could be functioned as electron donors to activate NO molecules and form NO−, a key intermediate. This induces a new reaction path of NO → NO− → NO3− to achieve the harmless conversion of NO, effectively restraining the generation of noxious intermediates (NO2, N2O4). It is expected that this study would inspire the design of more artful photocatalysts for effective charge transfer and safe pollutants purification.

Crystal-facet and microstructure engineering in ZnO for photocatalytic NO oxidation

Photocatalysis is believed to be an important way of reducing NO pollutant in air and the facet engineering of semiconducting oxides could enhance the efficiency of the photocatalysis. ZnO nanoparticles with different exposed crystalline facets were successfully synthesized using a hydrothermal method and their photocatalytic degradation towards NO was investigated. The crystals from ZnCl2 precursor were hexagonal mesoporous ones with exposed (0002) facet, while those from zinc acetate were in the form of flakes or wheat ears with enhanced exposure of (101(−)1) facet. Calcination in air imparted an enhanced the textural coefficient of the orientated facets as well as the oxygen defects. The nanocrystals with enhanced (0002) facet and lower flat-band energy did better in photoelectrochemical water-oxidation than those with exposed (101(−)1) facet that showed superior photocatalytic activity (approaching 76.7 ± 0.6% under 365 nm photons) for NO oxidation. According to theoretical calculations, (101(−)1) facet with O termination showed much higher affinity to NO molecules than other configurations, and the oxygen vacancy in ZnO played an minor role in the photocatalytic oxidation of NO. A high quantum efficiency approaching 97.5 ± 1.4% under 275 nm photons was obtained for the ZnO crystals from zinc acetate with mixed (0002) and (101(−)1) facets. This research explores the special characteristics of ZnO with different exposed facets and is important for the future design of highly efficient photocatalyst for hazardous material removal.

Visible light-driven deep oxidation of NO and its durability over Fe doped BaSnO3: The NO+ intermediates mechanism and the storage capacity of Ba ions

A Fe doped BaSnO3 (Fe-BaSnO3) photocatalyst was synthesized by a decomposition method of the peroxo-precursor. The Fe-BaSnO3 not only exhibited a great activity and selectivity in NO photocatalytic oxidation under visible light but also showed an excellent performance in terms of durability. The experimental results showed that doping Fe ions into BaSnO3 produced more oxygen vacancies (OVs) and visible light further promoted the formation of OVs, which was beneficial to the formation of reactive oxygen species (ROS). The Fe ions benefited to convert NO to a high-energy NO+, which was easily deep-oxidized into nitrate. Moreover, the alkaline earth metal ions (Ba2+) could store the nitrate or nitrite generated by the NO oxidation, resulting in exposing the active sites at Fe ions and then maintaining the durable oxidizing NO over Fe-BaSnO3. This work may shed light on a novel strategy for developing high-efficiency photocatalysts for durable deep oxidation of NOx.

Band-Gap Modulation for Enhancing NO Photocatalytic Oxidation over Hollow ZnCdS: A Combined Experimental and Theoretical Investigation

The photocatalytic treatment of NO at ppm-level concentrations for the environment and human health protection has attracted ever-increasing interest in academia and industry. Here, hollow ZnCdS nanocage catalysts with different Cd dopings were prepared and used to remove NO under visible light, and a high removal rate of 85% was achieved. Density functional theory (DFT) theoretical calculations and experiments showed that Cd ions could effectively modulate the band gap of ZnS into the visible-light-absorbing range (2.69 eV). Zn0.5Cd0.5S with a cavity structure has an excellent photoelectric response and low electron–hole recombination probability. A relaxation signal of up to 2.33 μs was detected in the transient absorption spectrum, indicating the long lifetime of the photoexcited carriers. Furthermore, the photocatalytic oxidation process of NO was dynamically monitored by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). It is found that NO adsorbed on the surface of Zn0.5Cd0.5S rapidly transformed into dimers (N2O2) under dark and facilitated NO conversion to nitrate via intermediates such as nitrite after illumination (N2O2 → NO → NO3–). The high activity and stability of Zn0.5Cd0.5S present a high potential for scale-up application of the catalyst for NO oxidation under visible light.

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.

Vacancy-Rich and Porous NiFe-Layered Double Hydroxide Ultrathin Nanosheets for Efficient Photocatalytic NO Oxidation and Storage.

An appealing strategy in the direction of circular chemistry and sustainable nitrogen exploitation is to efficiently convert NOx pollutants into low-toxic products and simultaneously provide crop plants with metabolic nitrogen. This study demonstrates that such a scenario can be realized by a defect- and morphology-coengineered Ni-Fe-layered double hydroxide (NiFe-LDH) comprising ultrathin nanosheets. Rich oxygen vacancies are introduced onto the NiFe-LDH surface, which facilitate charge carrier transfer and enable photocatalytic O2 activation into superoxide radicals (•O2-) under visible light. •O2- on NiFe-LDH thermodynamically oxidizes NO into nitrate with selectivity over 92%, thus suppressing dangerous NO2 emissions. By merit of abundant mesopores on NiFe-LDH ultrathin nanosheets bearing a high surface area (103.08 m2/g), nitrate can be readily stored without compromising the NO oxidation reactivity or selectivity for long-term usage. The nitrate species can be easily washed off the NiFe-LDH surface and then enriched in the liquid form as easy-to-use chemicals.

Study on Preparation of Ag/AgCl/ZIF-8 Composite and Photocatalytic NO Oxidation Performance

Study on Preparation of Ag/AgCl/ZIF-8 Composite and Photocatalytic NO Oxidation Performance

Earth‐Abundant CaCO3‐Based Photocatalyst for Enhanced ROS Production, Toxic By‐Product Suppression, and Efficient NO Removal

Photoinduced reactive oxygen species (ROS)‐based pollutant removal is one of the ideal solutions to achieve the conversion of solar energy into chemical energy and thus to address environmental pollution. Here, earth‐abundant CaCO3‐decorated g‐C3N4 (g‐C3N4 labeled as CN, CaCO3‐decorated g‐C3N4 sample labeled as CN‐CCO) has been constructed by a facile thermal polymerization method for safe and efficient photocatalytic NO removal. The decorated CaCO3 as “transit hub” extends the π bonds of CN to deviate from the planes and steers the random charge carriers, which thus provides extra active sites and expedites spatial charge separation to facilitate adsorption/activation of reactants and promote formation of ROS participating in the removal of pollutant. Furthermore, boosted generation of ROS regulates the photocatalytic NO oxidation pathway and thus increases the selectivity of products. NO prefers to be directly oxidized into final product (nitrate) rather than toxic intermediates (NO2), which is well demonstrated by theoretically simulated ROS‐based reaction pathways and experimental characterization. The present work promotes the degradation of pollutant and simultaneously suppresses the formation of toxic by‐product, which paves the way for ROS‐based pollutant removal.

Oxygen vacancies in actiniae-like Nb2O5/Nb2C MXene heterojunction boosting visible light photocatalytic NO removal

Photocatalytic oxidation is a green and energy-saving technology for removing the low concentration NO in ambient atmosphere. High efficient and stable photocatalysts are essential for achieving excellent NO conversion effect. In this work, oxygen vacancy-rich Nb2O5/Nb2C heterojunction photocatalysts were prepared from Nb2C MXene by hydrothermal process and heat treatment in H2/Ar. The (001) facet-dominant Nb2O5 nanorods in-situ grown on Nb2C MXene formed an actiniae-like structure, which promotes the active site exposure and enlarges the reaction interface. Under visible light, the optimized photocatalyst exhibits excellent NO removal rates in the relative humidity range of 25–75%. Mechanism research shows that the oxygen vacancies greatly improve the photocatalytic NO oxidation capacity. Oxygen vacancies on (001) facet of Nb2O5 nanorods can boost the visible light absorption, effectively adsorb and activate the reactant gas. The separation of photogenerated carriers was synergistically enhanced by the oxygen vacancies and Nb2O5/Nb2C heterojunction.

Rational design of n-Bi12TiO20@p-BiOI core–shell heterojunction for boosting photocatalytic NO removal

Bismuth titanate (Bi12TiO20) with unique sillenite structure has been shown to be an excellent photocatalyst for environmental remediation. However, the narrow light-responsive range and rapid recombination of photoinduced electrons-holes limit the photocatalytic performance of Bi12TiO20. To overcome the limitations, a practical and feasibleway is to fabricate heterojunctions by combining Bi12TiO20 with suitable photocatalysts. Here, using a facile chemical precipitation method, a novel and hierarchical core–shell structure of n-Bi12TiO20@p-BiOI (BTO@BiOI) heterojunction was rationally designed and synthesized by loading BiOI nanosheets on BTO nanofibers. The constructed BTO@BiOI composites exhibited significant charge transfer ability due to the synergistic effects of the built-in electric field between BTO and BiOI as well as close interfacial contacts. In addition, the narrow bandgapcharacteristics of the BiOI led to wide light absorption ranges. Therefore, the BTO@BiOI heterojunction exhibited an improved photocatalytic performance under visible light irradiation. The NO removal efficiency of optimal BTO@BiOI was 45.7%, which was significantly higher compared tothat of pure BTO (3.6%) or BiOI (23.1%). Moreover, the cycling experiment revealed that BTO@BiOI composite has a good stability and reusability. The possible mechanism of photocatalytic NO oxidation over BTO@BiOI was investigated in detail.

Insight on photocatalytic oxidation of high concentration NO over BiOCl/Bi2WO6 under visible light

The exhaust gas from an SCR (Selective Catalytic Reduction) unit still has about 100 ppm NO. BiOX/Bi2WO6 catalysts were fabricated by a one-step ultrasound-assisted method and employed for photocatalytic oxidation of NO under visible light. The bandgap energy of BiOX (Cl, Br, I)/Bi2WO6 were estimated to be 2.65 eV, 2.35 eV, and 1.75 eV, respectively. Photocurrent density and EIS curves revealed that BiOCl/Bi2WO6 had the optimal charge transfer velocity. Simulated NO removal experiments of BiOX/Bi2WO6 catalysts were carried out in a fixed bed reactor, 20–30% NO (100 ppm inlet) removal rate was achieved under visible light while promoting by H2O vapor, and the maximum NO removal rate is up to 31% on BiOCl/Bi2WO6. The fitted L-H model revealed that NO oxidation followed two-centered adsorption, and scavenger experiments confirm that the main ROS (Reactive Oxygen Species) was •OH. In situ DRIFT spectra revealed that there were bridging nitrates, monodentate and bridging nitrites, and other intermediates formed on the BiOCl/Bi2WO6, these species can be scrubbed by seawater to regenerate the catalyst. The NO photocatalytic oxidation pathway was proposed. Good activity and stability of BiOCl/Bi2WO6 provide a new approach for scale-up application of NO oxidation under visible light.

Highly efficient photocatalytic NO removal and in situ DRIFTS investigation on SrSn(OH)6

A novel SrSn(OH)6 photocatalyst with large plate and particle size were synthesized via a facile chemical precipitation method. The photocatalytic activity of the SrSn(OH)6 was evaluated by the removal of NO at ppb level under UV light irradiation. Based on the ESR measurements, SrSn(OH)6 photocatalyst was found to have the ability to generate the main active species of O2•−, •OH and 1O2 during the photocatalytic process. Moreover, SrSn(OH)6 photocatalyst not only exhibits high photocatalytic activity for NO removal (79.6%), but also has good stability after five cycles. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to investigate the NOx transfer pathway and the intermediate products distribution during the adsorption and photocatalytic NO oxidation process. The present work not only provides an efficient material for air pollutants purification at room temperature but also in-depth understanding of the mechanism involved in the photocatalytic NO removal process.