Photocatalytic NOx Research Articles

Photocatalytic NOx degradation performance of TiO2-nanofiber-spray-coated foam composite according to saturated conditions

We investigated the photocatalytic NOx degradation performance of foam composites using various TiO2 application methods and saturated conditions. TiO2 powder (TP) and TiO2 nanofiber (TF) were used. TF was fabricated using polyvinyl pyrrolidone, ethanol, acetylacetone, and titanium 4-isopropoxide via electrospinning. TF mixed with an inorganic coating material was sprayed on cementitious materials once. The NOx degradation performance of the TF-spray-coated specimen was higher than that of specimens with TP. The NOx removal performance of the form composite decreased with increasing specimen water content. The TF-spray-coated foam composite with activated carbon yielded the highest NOx removal performance.

100% N2O inhibition in photocatalytic NOx reduction by carbon particles over Bi2WO6/TiO2 Z-scheme heterojunctions

The chemically inert property of nitrous oxide (N2O) has made it difficult to be reduced to benign dinitrogen during (photo) selective catalytic reduction (SCR) of NOx. The introduction of oxygen vacancies and BrØnsted acid sites to thermal catalysts is found to effectively inhibit N2O formation. However, the efficiency is still far from satisfactory and the inhibition of N2O in photocatalytic selective reduction of NOx has not been investigated yet. Herein, 100% inhibition of N2O formation during photo-SCR of NOx with carbon particles is successfully achieved via the fabrication of Bi2WO6/TiO2 Z-scheme heterojunction photocatalyst. The completely inhibited N2O formation is attributed to the rapid charge separation and stronger N2O adsorption on the (101) plane of TiO2 according to the density functional theory calculations and experimental results. The presence of water vapor prolongs the reaction time for NOx photo-reduction of carbon particles but has no significant effect on carbon oxidation rate. Introduction of heterostructure interface effectively accelerates photo-induced charge carrier separation and transfer, thereby enhancing photocatalytic efficiency for NOx reduction with carbon particles. The optimized 0.2 Bi2WO6/TiO2 (molar ratio) composite achieves the highest formal electron/photon quantum efficiency of 0.036, which is 3.3 times that of TiO2. The results provide an alternative perspective on the catalyst fabrication via the simple heterojunction materials for toxic byproduct control in the field of pollutants removal.

2D-2D heterostructured composites Bi4O5Br2-SnS2 with boosted photocatalytic NOx abatement

A desirable photocatalytic system ought to feature efficient NOx removal and retardance of toxic NO2 generation. In this investigation, a series of novel two-denominational (2D)-2D composites Bi4O5Br2-SnS2 (X BS) were successfully fabricated through a simple mechanical ball-milling route and characterized systematically. The coexistence of both expected components was confirmed. 2D morphologies of both components were stacked in face-to-face manner and thus formed heterojunction structures, verified by SEM and TEM patterns. The visible-light absorption was enhanced in composites caused by the shrinkage of band gaps. With microstructural and optical merits, these binary composites showed reinforced photocatalytic abatement of NOx and attenuated generation of toxic NO2 under visible light, compared with bare Bi4O5Br2. Successive recycling catalytic experiments revealed the satisfactory reusability and structural stability of achieved composites. Ultimately, a reasonable type-II model was deduced to describe the migration and transfer of interface carriers based on experimental and analytical results. Such work designed and provided various bismuth-based composites constructed by the simple mechanical ball-milling for controlling and treating nitrogen oxides at ppb level in the atmospheric environment.

Ultra-stable type-II heterojunctions Bi4O5I2/FeVO4 of reinforced photocatalytic NOx removal abilities in visible light

In this investigation, a sequence of binary composites Bi4O5I2/FeVO4 (BF) were prepared through a facile solvothermal route and were systematically characterized by various analytical techniques. As verified from HRTEM images and XPS analyses, these composites were comprised of both expected components in heterojunction structures. Under visible light, as-prepared composites displayed reinforced photocatalytic performance over NOx removal at the ppb level in comparison to both bare components. Particularly, the best candidate BF2.5 possessed NOx removal around 40% with a selectivity for nitrate/nitrite species approaching 100%, which surpassed those by Bi4O5I2 and FeVO4, mainly attributing to favorable physicochemical merits such as the enhanced visible-light harvesting, effective transfer and separation of charge carriers and further boosted generation of reactive radicals, and the suitable composition in morphological harmony. These composites were quite robust and maintained photocatalytic performance well even up to five successive runs without surface cleaning. Eventually, an elementary photocatalysis mechanism of these composites was deduced in a type-II manner by entrapping experiments, ESR analyses, and band structural estimation.

Photocatalytic NOx Removal in Bismuth-Oxyhalide (BiOX, X = I, Cl) Cement-Based Materials Exposed to Outdoor Conditions

Cement-based materials modified with 3D BiOX (X = I, Cl) microspheres at different percentages (1, 5 and 10% by weight of the cement binder) were prepared to investigate the durability of the photocatalytic NOx removal under outdoor conditions. Weathering—corresponding to a period of 13 months outdoors—was studied in terms of NO removal efficiency under visible and UVA light irradiation for BiOI and BiOCl mortars, respectively. Following this period, the samples were protected from the environment for four years, and NOx removal and selectivity to nitrates were assessed. BiOI and BiOCl mortar samples were initially photocatalytically active; NOx removal performance increased as BiOX content increased. There was good photocatalyst dispersion, and compressive strength was not significantly impacted. The BiOI mortars had nearly completely lost their activity after 5 years from casting, whereas mortars containing 10% BiOCl had maintained about 7% of initial performance. The results suggest that mortar deactivation is due to surface dirt and nitrates accumulation from NOx oxidation on the surface rather than carbonation. An internal self-deactivation mechanism that affects BiOI in mortar matrix has also been postulated.

Fluid dynamics and mass transfer in curved reactors: A CFD study on Dean flow effects

The effect of Dean flow on the performance of a curved channel reactor in terms of fluid flow and mass transfer characteristics was numerically evaluated. Laminar multi-component gas flow considering the photocatalytic NOx degradation as model reaction was investigated using a validated three-dimensional computational fluid dynamics (CFD) model. The curved channel device was compared with a conventional straight channel reactor, considering the same catalytic area and cross-section. The results showed that the velocity profile was distorted from its original symmetrical shape, and the maximum velocities shifted towards the outer-side wall for De > 120.1, due to the influence of centrifugal acceleration in the radial direction. The curved channel reactor resulted in higher mass transfer coefficients compared to the straight channel device. The higher mass transfer enhancement factor (γ ~ 1.8) was obtained for AR = 1 and Re = 725.5. However, this condition imposed a pressure drop 3 times higher the straight channel device. The reactor’s performance was also represented by an association of ideal reactors (PFR and CSTR). The NO conversion in the curved channel device was equivalent to the behavior of a CSTR (n = 1.5) due to the Dean flow contributions favoring the mixing. Additional analyzes regarding the influence of the radius of curvature were carried out in a U-shaped reactor. Finally, an analysis of dimensionless numbers (P é clet and Damköhler) corroborated the results found numerically. Therefore, this study can provide insights into the design and analysis guidelines for curved channel reactors.

One-step fluorine-free synthesis of delaminated, OH-terminated Ti3C2: High photocatalytic NOx storage selectivity enabled by coupling TiO2 and Ti3C2-OH

Difficulties associated with MXene fabrication typified by an environmentally hazardous fluorine-based etching process followed by delamination and fluorine reduction steps create obstacles against the expansion of this outstanding material and its application in air purification. Herein, we demonstrate a one-step hydrothermal process that involves alkali (NaOH)-assisted aluminum etching in the presence of a delaminating agent (hydrazine) for synthesis of delaminated, OH-terminated MXene (Ti3C2-OH). The process does not require the use of fluorine-containing compounds and produces Ti3C2-OH sheets without the need for post-synthesis treatment. Ti3C2-OH shows a strong synergistic effect as a co-catalyst when coupled with TiO2 during photocatalytic oxidation of NOx. Excellent NOx storage selectivity (91 %) and a positive DeNOx index (+ 0.30) were achieved at NO conversion of 54 %. The photocatalytic activity of the hybrid shows high sensitivity to the surface chemistry of the co-catalyst as Ti3C2-OH outperforms delaminated, F-terminated Ti3C2 (fabricated via the traditional process), which shows low NOx storage selectivity (65 %) and a negative DeNOx index (−0.05) at NO conversion of 47 %. We envision that these findings will benefit ongoing efforts aimed at developing effective alternative methods for MXene synthesis and provide a pathway for rational design of efficient NOx abatement photocatalysts.

Methodology for Simultaneous Analysis of Photocatalytic deNOx Products

The ISO standard 22197-1:2016 used for the evaluation of the photocatalytic nitric oxide removal has a main drawback, which allows only the decrease of nitric oxide to be determined specifically. The remaining amount, expressed as “NO2”, is considered as a sum of HNO3, HONO NO2, and other nitrogen-containing species, which can be potentially formed during the photocatalytic reaction. Therefore, we developed a new methodology combining our custom-made analyzers, which can accurately determine the true NO2 and HONO species, with the conventional NO one. Their function was validated via a photocatalytic experiment in which 100 ppbv of either NO or NO2 dispersed in air passed over (3 L min−1) an Aeroxide© TiO2 P25 surface. The gas-phase analysis was complemented with the spectrophotometric determination of nitrates (NO3−) and/or nitrites (NO2−) deposited on the P25 layer. Importantly, an almost perfect mass balance (94%) of the photocatalytic NOx abatement was achieved. The use of custom-made analyzers enables to obtain (i) no interference, (ii) high sensitivity, (iii) good linearity in the relevant concentration range, (iv) rapid response, and (v) long-term stability. Therefore, our approach enables to reveal the reaction complexity and is highly recommended for the photocatalytic NOx testing.

Singlet Oxygen and Mobile Hydroxyl Radicals Co-operating on Gas-Solid Catalytic Reaction Interfaces for Deeply Oxidizing NOx.

Learning from the important role of porphyrin-based chromophores in natural photosynthesis, a bionic photocatalytic system based on tetrakis (4-carboxyphenyl) porphyrin-coupled TiO2 was designed for photo-induced treating low-concentration NOx indoor gas (550 parts per billion), achieving a high NO removal rate of 91% and a long stability under visible-light (λ ≥ 420 nm) irradiation. Besides the great contribution of the conventional •O2- reactive species, a synergic effect between a singlet oxygen (1O2) and mobile hydroxyl radicals (•OHf) was first illustrated for removing NOx indoor gas (1O2 + 2NO → 2NO2, NO2 + •OHf → HNO3), inhibiting the production of the byproducts of NO2. This work is helpful for understanding the surface mechanism of photocatalytic NOx oxidation and provides a new perspective for the development of highly efficient air purification systems.

Oxygen vacancy configuration in confined BiVO4-Bi2S3 heterostructures promotes photocatalytic oxidation of NO

Oxygen vacancy configuration is a promising means for tuning the electronic structure and determining the adsorption/desorption mode of the reactants in photocatalytic NOx oxidation. We report here that the formation of oxygen vacancies (OVs) can activate free NO and O2 molecules via a changed oxidation route, leading to fast conversion of NO into NO3-. The boosted NO oxidation reaction was conducted on novel OVs-induced BiVO4-Bi2S3 heterojunctions with the shape of double-deck hollow nanospheres assembled by a hard-templating treatment, followed by an annealing process. Abundant OVs were incorporated into the lattice of the as-synthesized heterojunctions, which significantly decreased the recombination level of the photo-exited carriers, leading to the promotion of the exchange of adsorbed ambiance oxygen and lattice oxygen, and the formation of reactive oxygen species and the final highly-efficient removal of NO under visible-light illumination. This work verifies the role of defective and hierarchical interface configuration on catalytic NOx oxidation and provides a viable methodology for the discovery of advanced photocatalysts via surface engineering.

CQDs modified Co-BiOCl nanosheets with improved effective light absorption and charge separation for photocatalytic CIP degradation and NOX removal

In this work, the Co-BiOCl/CQDs photocatalyst, with excellent degradation performance of ciprofloxacin and NOX, was synthesized by one-step hydrothermal method. By quantifying the effective light absorption (Iabs), charge separation efficiency (ηsep) and charge injection efficiency (ηinj) of the photocatalyst, it could be obtained that Co-doping could enhance the charge separation, and CQDs could improve the effective light absorption efficiency, while the charge injection efficiency of Co-BiOCl/CQDs was similar to that of BiOCl, demonstrating that Co and CQDs could not enhance the charge injection, and strongly demonstrating that the enhancement in effective light absorption and charge separation were main factors for improving photocatalytic activity. Moreover, the photocatalytic mechanism of CIP and NOx under visible light was also proposed. This work offered a new insight for the effect of Co and CQDs in the photocatalysts of Co-doping or CQDs based.

Photochemical Transformation Pathways of Nitrates from Photocatalytic NOx Oxidation: Implications for Controlling Secondary Pollutants

Photochemical Transformation Pathways of Nitrates from Photocatalytic NOx Oxidation: Implications for Controlling Secondary Pollutants

High-concentration N-doped La2Ti2O7 nanocrystals: Effects of nano-structuration and doping sites on enhancing the photocatalytic activity

A series of N-doped La2Ti2O7 nanocrystal for photocatalytic NOx oxidation are successfully prepared by a two-steps process including hydrothermal reaction and heat treatment under NH3 flow. The effects of different nitrogen doping amounts on the structure, optical property and photocatalytic activity of the obtained N-doped La2Ti2O7 are investigated. The photocatalytic activity and stability of the N-doped La2Ti2O7 samples are evaluated through photocatalytic oxidation of NOx under UV and visible light irradiation. The nanosheet morphology of N-doped La2Ti2O7 is originated from La2Ti2O7 precursor synthesized by hydrothermal reaction. The addition of triethanolamine (TEA) in the hydrothermal reaction largely affects the morphology of La2Ti2O7 to form nanocrystal. The light absorption of La2Ti2O7 nanocrystal is cut off at 338 nm, while the N-doped La2Ti2O7 nanocrystals exhibit the extended light absorption up to 560 nm. It is found that the N doping into La2Ti2O7 enhances absorption in the visible light region to exhibit higher activity for photocatalytic oxidation of NO gas. The optimized N-doped La2Ti2O7 nanocrystal with 4.79 at% of nitrogen dopant possesses the best photocatalytic NOx oxidation activity under both ultraviolet and visible light irradiation compared with N-doped La2Ti2O7 samples obtained by hydrothermal reaction without TEA or flux synthesis of La2Ti2O7 and the subsequent treatment with NH3. The enhancement of photocatalytic activity should originate from simultaneous achievement of large specific surface area and strong visible light absorption due to the large amount of nitrogen dopants.

Photochemical reaction of NO2 on photoactive mineral dust: Mechanism and irradiation intensity dependence

Photochemical reaction of NO2 on semiconductor is of scientific significance in both photocatalytic NOx removal and nitrogenous aerosol formation. A fundamental understanding of the reaction mechanism and how irradiation intensity influences the product distribution and reaction kinetics remains uncertain. By means of DRIFTS (diffuse reflection infrared Fourier transform spectroscopy) and flow reactor system, we for the first time investigated the irradiation-intensity-dependence of this heterogeneous process. Surface nitrate and nitrite present opposite variations in response to light intensity. The formation rate of individual nitrogenous product is susceptible to the intensity variation of weak irradiation, while tends to be unsusceptible under strong irradiation. The phenomenon is explained by the balance between heterogeneous uptake of NO2 and photoinduced generation of reactive oxygen species varying with irradiation intensity. The highest yield of total nitrogenous products appears under weak irradiation, indicating the photolysis of nitrite and photoreduction of nitrate under strong irradiation. The experimental results suggest that irradiation intensity should be introduced into atmospheric models for its varied impacts on heterogeneous chemistry. Moreover, irradiation intensity should be considered as an important factor in the photocatalytic removal of NOx.

Transformation of amorphous Bi2O3 to crystal Bi2O2CO3 on Bi nanospheres surface for photocatalytic NOx oxidation: Intensified hot-electron transfer and reactive oxygen species generation

• Amorphous Bi 2 O 3 shell on the surface of Bi nanospheres was transformed to crystal Bi 2 O 2 CO 3 . • Bi 2 O 2 CO 3 shell reduced the transmission resistance of hot electrons excited from Bi core. • The generation of reactive oxygen species were enhanced over Bi@crystal Bi 2 O 2 CO 3. • •O 2 − radicals were the major reactive oxidation species contributing to the oxidation of NO x . • Generation mechanisms of ROS were proposed over Bi@crystal Bi 2 O 2 CO 3 photocatalyst. The inevitable amorphous Bi 2 O 3 on the surface of plasmonic Bi nanospheres serving as the recombination center of hot electron-hole pairs, hindered the transfer of photo-generated hot electrons and production of reactive oxygen species (ROS) seriously. In this study, Bi@amorphous Bi 2 O 3 was transformed to Bi@crystal Bi 2 O 2 CO 3 by secondary hydrothermal reaction. Bi@Bi 2 O 2 CO 3 core–shell photocatalyst exhibited higher visible-light catalytic activity (34.1%) than Bi@Bi 2 O 3 did (12.3%) in terms of NO x removal. Photoelectrochemical, surface photovoltage spectroscopy spectra and Kelvin probe force microscopy measurement results indicate that Bi 2 O 2 CO 3 shell reduced the transmission resistance of hot electrons excited from Bi core. O 2 -TPD, trapping experiments and density functional theory further confirmed that the transferred hot electrons were a help for ROS generation and •O 2 − radicals were the major contributor. This study not only addresses the unsettled issue of surface amorphous Bi 2 O 3 , but also provides a facile method to transform the amorphous shell to crystal phase of core–shell photocatalyst and new insights into the hot electrons separation and the formation of main ROS over the surface crystal transition of plasmonic Bi.

Enhanced visible-light-induced photocatalytic NOx degradation over (Ti,C)-BiOBr/Ti3C2Tx MXene nanocomposites: Role of Ti and C doping

NOx gas pollutants have induced continuous and massive damage to our health and environment. Photocatalysts are regarded as an efficient approach for NOx degradation. Various strategies have been devoted to optimizing photocatalyst performance. In this work, a bismuth oxybromide (BiOBr) photocatalyst was successfully combined with 2D Ti3C2Tx MXene by the solvothermal treatment. It is also found that partial decomposition of Ti3C2Tx has led the in situ doping sources for BiOBr/Ti3C2Tx nanocomposite. The photocatalytic NOx purification activity of the constructed (Ti, C) co-doped BiOBr/Ti3C2Tx nanocomposites was evaluated under light illumination. The results showed that up to 61% of NOx gas has been decontaminated as short as 10 min, exceeding the pristine BiOBr and P25 titania photocatalysts. Stable performance was also achieved after several cycles, which makes (Ti,C)-BiOBr/Ti3C2Tx nanocomposites a great promise for the long term and repeated utilization. The present strategy enhanced the photoinduced electron-hole separation and transfer of the heterostructured (Ti,C)-BiOBr/Ti3C2Tx. The match band structure configuration between (Ti,C)-BiOBr and Ti3C2Tx MXene and the synergistic process between photogenerated charge carrier transfer and intermediate Ti and C levels is discussed to explain the improved photocatalytic mechanism.

Highly-efficient green photocatalytic cementitious materials with robust weathering resistance: From laboratory to application.

The combined use of nano-TiO2 with cementitious materials offers an environmentally-friendly way to combat the air pollution problem. However, a trade-off between a high efficiency and a robust weathering resistance has often to be made for most of the attempted nano-TiO2 incorporation methods. This paper developed a simple and effective "spraying" method to coat nano-TiO2 particles on the surface of concrete surface layers (CSL). The results showed that the NOx removal rate of the samples increased with an increase in both the concentrations of nano-TiO2 solutions and the number of times of the spraying action. And the conditions for preparation of the Spray AB (the CSL were first sprayed with the 30gL-1 TiO2-solution 20 times, followed by mechanical compaction, and for another 20 times after the compaction) were found to be optimal in terms of NOx removal performance and weathering resistance. The Spray AB was superior to the 5% TiO2-intermixed samples with respect to photocatalytic NOx removal ability. Compared with TiO2-dip-coated samples, the Spray AB samples had better and robust weathering resistance. A case study on the factory-fabricated green Eco-blocks (produced by the laboratory-developed spray method and the conventional intermix method) was performed. Examination and comparison on their respective photocatalytic NOx removal further verified the advantages of the spray method over the intermix method.

Enhanced photocatalytic conversion of NOx with satisfactory selectivity of 3D-2D Bi4O5Br2-GO hierarchical structures via a facile microwave-assisted preparation

A novel series of 3D-2D Bi4O5Br2-GO hierarchical composites were fabricated via a facile and efficient microwave-assisted protocol and were systematically characterized by various analyses. In composites, both components of 3D hierarchical Bi4O5Br2 and 2D graphite-phased graphene oxide (GO) were verified in heterojunction structures by Raman, UV–Vis DRS, TEM, and elemental mapping spectra. Under visible light irradiation, the photocatalytic NOx removal by composites was obviously enhanced with satisfactory selectivity, though removal of NO was remained almost unchanged. Particularly, the best catalog Bi4O5Br2-GO1.6 possessed outstanding conversion rate for NOx increased by 18% and selectivity for nitrite and nitrate declined by 32% comparing to pure Bi4O5Br2, which was mainly relevant to enhanced visible-light harvesting, strengthened spatial separation of photoinduced charge carries, proper phase composition, and harmony of 3D-2D morphology. Moreover, these stable composites could be reused for several times without obvious activity loss. Basing upon entrapping experiments and analytical results, a plausible photocatalysis mechanism was eventually proposed.

Core‐crown Quantum Nanoplatelets with Favorable Type‐II Heterojunctions Boost Charge Separation and Photocatalytic NO Oxidation on TiO2

AbstractFunctionalization of TiO2 (P25) with oleic acid‐capped CdSe(core)/CdSeTe(crown) quantum‐well nanoplatelets (NPL) yielded remarkable activity and selectivity toward nitrate formation in photocatalytic NOx oxidation and storage (PHONOS) under both ultraviolet (UV‐A) and visible (VIS) light irradiation. In the NPL/P25 photocatalytic system, photocatalytic active sites responsible for the NO(g) photo‐oxidation and NO2 formation reside mostly on titania, while the main function of the NPL is associated with the photocatalytic conversion of the generated NO2 into the adsorbed NO3− species, significantly boosting selectivity toward NOx storage. Photocatalytic improvement in NOx oxidation and storage upon NPL functionalization of titania can also be associated with enhanced electron‐hole separation due to a favorable Type‐II heterojunction formation and photo‐induced electron transfer from the CdSeTe crown to the CdSe core of the quantum well system, where the trapped electrons in the CdSe core can later be transferred to titania. Re‐usability of NPL/P25 system was also demonstrated upon prolonged use of the photocatalyst, where NPL/P25 catalyst surpassed P25 benchmark in all tests.

Photocatalytic H2 Evolution, CO2 Reduction, and NOx Oxidation by Highly Exfoliated g-C3N4

g-C3N4, with specific surface area up to 513 m2/g, was prepared via three successive thermal treatments at 550 °C in air with gradual precursor mass decrease. The obtained bulk and exfoliated (1ex, 2ex and 3ex) g-C3N4 were characterized and tested as photocatalysts for H2 production, CO2 reduction and NOx oxidation. The exfoliated samples demonstrated graphene-like morphology with detached (2ex) and sponge-like framework (3ex) of layers. The surface area increased drastically from 20 m2/g (bulk) to 513 m2/g (3ex). The band gap (Eg) increased gradually from 2.70 to 3.04 eV. Superoxide radicals (·O2−) were mainly formed under UV and visible light. In comparison to the bulk, the exfoliated g-C3N4 demonstrated significant increase in H2 evolution (~6 times), CO2 reduction (~3 times) and NOx oxidation (~4 times) under UV light. Despite the Eg widening, the photocatalytic performance of the exfoliated g-C3N4 under visible light was improved too. The results were related to the large surface area and low e−-h+ recombination. The highly exfoliated g-C3N4 demonstrated selectivity towards H2 evolution reactions.