Photocatalytic Removal Of Nitric Oxide Research Articles

Controlling Monomer Feeding Rate to Achieve Highly Crystalline Covalent Triazine Frameworks.

The synthesis of highly crystalline covalent triazine frameworks (CTFs) with ultrastrong covalent bonds (aromatic CN) from the triazine linkage presents a great challenge to synthetic chemists. Herein, the synthesis of highly crystalline CTFs via directly controlling the monomer feeding rate is reported. By tuning the feeding rate of monomers, the crystallization process can be readily governed in a controlled manner in an open system. The sample of CTF-HUST-HC1 with abundant exposed {001} crystal facets has the better crystallinity and thus is selected to study the effect of high crystallinity on photoelectric properties. Owing to the better separation of photogenerated electron-hole pairs and charge transfer, the obtained highly ordered CTF-HUST-HC1 has superior performance in the photocatalytic removal of nitric oxide (NO) than its lesser crystalline counterparts and g-C3 N4 .

SnO2/graphene quantum dots composited photocatalyst for efficient nitric oxide oxidation under visible light

In the present work, we have prepared tin oxide (SnO2)/graphene quantum dots (GQDs) composites and applied them for photocatalytic removal of nitric oxide (NO). In contrast to SnO2 alone, SnO2/GQDs composite has exhibited a remarkably enhanced activity under both full spectrum and visible light illumination. The crystal structure, morphology and surface state of the composite were studied by X-ray diffraction, transmission electron microscopy, Fourier-transformed infrared spectroscopy and X-ray photoelectron spectroscopy. Moreover, diffraction and reflectance spectra and photoluminescence spectra together with the photoelectrochemical tests show that the presence of GQDs in the composite could promote the visible light response as well as charge separation efficiency of the system. This makes SnO2/GQDs composite generate more active species (O2− and OH) for NO oxidation, as evidenced from the electron paramagnetic resonance measurements. This study hence could be referenced for future construction of efficient photocatalytic systems with wide bandgap oxides.

Influence of inorganic additives on the photocatalytic removal of nitric oxide and on the charge carrier dynamics of TiO2 powders

Building materials employing TiO2 as photocatalyst usually contain several additives. The interplay of the additives and the photocatalytic NO degradation has been rarely investigated, although it is of utmost importance for the design of new highly active materials. Hence, in the present study the effects of such additives (BaSO4, CaCO3, and Na2CO3) on the photocatalytic activity and on the charge carrier dynamics have been evaluated. The degradation of nitric oxide (NO) was studied employing a standardized photocatalytic activity test (ISO 22197-1) and the charge carrier kinetics were observed by transient absorption spectroscopy. TiO2-additive powder samples were obtained by grinding pure anatase TiO2 with a single additive with varying volume composition. Overall, the obtained apparent quantum yields of the samples correlated well with the charge carrier concentration. Moreover, BaSO4 has proven its chemically inert character. However, the optical properties of the powder samples containing BaSO4 lead to relatively high photonic efficiencies even with low TiO2 content. TiO2-BaSO4 samples with only 25% TiO2 showed nearly the same quantum yield for the NO degradation as the pure photocatalyst. The reason for the high photocatalytic activity is the strong absorption of UV-light caused by the scattering inside the powder samples. The charge carrier kinetics of TiO2 after the addition of Na2CO3 and CaCO3 revealed two different chemical effects of the additives. Na2CO3 reduced the apparent quantum yield of the TiO2 samples due to the fast recombination of charge carriers (identified by a fractal kinetics fit). On the other hand, the presence of CaCO3 had a beneficial influence on the stabilization of trapped charge carriers in TiO2.

<italic>In situ</italic> FT-IR investigation on visible light photocatalytic NO oxidation mechanism with (BiO)<sub>2</sub>CO<sub>3</sub> and N-doped (BiO)<sub>2</sub>CO<sub>3</sub> hiararchical microspheres

With the rapid development of industry, air pollution has become a focused issue worldwide. As one of the major air pollutants, nitric oxide (NO) would lead to serious atmospheric problems such as acid rain, haze, and photochemical smog. Photocatalysis, as a green and effective technology, has become one of the most promising technologies for solving air pollution problems. As one of the typical photocatalyst, (BiO)2CO3 consists of alternating layers of [Bi2O2]2+ and CO32− groups and has good optical absorption property, emerging as an attractive photocatalyst. In this work, (BiO)2CO3 and in situ N-doped (BiO)2CO3 hierarchical microspheres were fabricated by a one-pot hydrothermal method and applied to photocatalytic NO removal. The as-prepared samples were characterized by XRD, SEM, TEM, XPS, UV-vis DRS, PL and ESR. The photocatalytic NO oxidation process was monitored by in situ DRIFTS. The results indicated that the addition of urea has significant impact on (BiO)2CO3. The changed exposed facet influenced the morphological structure, band gap has narrowed and electronic-hole recombination rate of the as-prepared N doped (BiO)2CO3 has decreased. There are a lot of reports on photocatalytic NO oxidation, and the photocatalytic reaction mechanism for NO purification has been proposed for some photocatalysts with ·O2− and ·OH radicals as the main reactive species and nitrates as the final products. However, little is known about the reaction intermediates during photocatalysis. To reveal the reaction mechanism of photocatalytic NO oxidation, in situ DRIFTS investigation was applied to probe the reaction process. The photocatalytic NO oxidation mechanism was revealed based on the in situ DRIFTS and ESR results. With N doping, the reaction process and the free radicals which participated during the photocatalystic reaction process become different from the pure (BiO)2CO3 sample. For BOC, ·O2− is main active radical, but for NBOC, the active radical changed to ·OH. A new intermediate of NO+ was discovered during photocatalysis, which increased the efficiency of the reaction. Because of the N doping, the electronic structure of (BiO)2CO3 has been broken, which leads to the formation of more oxygen vacancy. The present work could provide new perspectives for advancing the photocatalysis efficiency, offer a new insight into the photocatalytic NO oxidation process and promote large-scale environmental applications of high-performance photocatalysts.

Switching of semiconducting behavior from n-type to p-type induced high photocatalytic NO removal activity in g-C3N4

The utilization of photocatalytic technology to remove air pollutants has attracted global interest. However, it still suffers from low removal activities under visible light irradiation. In this study, we demonstrated that the switching of the semiconducting behavior from n-type to p-type can efficiently improve the photocatalytic activity of g-C3N4 for nitric oxide (NO) removal about 3.5 times. This is due to that such switching could change the majority of carriers in g-C3N4 from electrons to holes. Interestingly, the photocatalytic removal of NO in both n-type and p-type g-C3N4 is proceeded via hole oxidation. More importantly, p-type g-C3N4 displays strong stability in both photocatalytic performance and crystal structures. This study provides a new strategy to improve the photocatalytic activity of semiconductors for air pollution removal.

Enhanced visible-light photo-oxidation of nitric oxide using bismuth-coupled graphitic carbon nitride composite heterostructures

Pure bismuth (Bi) metal-modified graphitic carbon nitride (g-C3N4) composites (Bi-CN) with a pomegranate-like structure were prepared by an in situ method. The Bi-CN composites were used as photocatalysts for the oxidation of nitric oxide (NO) under visible-light irradiation. The inclusion of pure Bi metal in the g-C3N4 layers markedly improved the light absorption of the Bi-CN composites from the ultraviolet to the near-infrared region because of the typical surface plasmon resonance of Bi metal. The separation and transfer of photogenerated charge carriers were greatly accelerated by the presence of built-in Mott–Schottky effects at the interface between Bi metal and g-C3N4. As a result, the Bi-CN composite photocatalysts exhibited considerably enhanced efficiency in the photocatalytic removal of NO compared with that of Bi metal or g-C3N4 alone. The pomegranate-like structure of the Bi-CN composites and an explanation for their improved photocatalytic activity were proposed. This work not only provides a design for highly efficient g-C3N4-based photocatalysts through modification with Bi metal, but also offers new insights into the mechanistic understanding of g-C3N4-based photocatalysis.

Removal of Nitric Oxide through Visible Light Photocatalysis by g-C3N4 Modified with Perylene Imides

For photocatalytic removal of nitric oxide (NO), two major issues need to be addressed: incomplete oxidation of NO and deactivation of the photocatalyst. In this study, we aimed to solve these two problems by constructing an all-solid-state Z-scheme heterojunction (PI-g-C3N4) consisting of g-C3N4 surface modified with perylene imides (PI). PI-g-C3N4 exhibits significant enhancement in photocatalytic activity (in comparison to pristine g-C3N4) when examined for NO removal. More importantly, the Z-scheme charge separation within PI-g-C3N4 populates electrons and holes into the increased energy levels, thereby enabling direct reduction of O2 to H2O2 and direct oxidation of NO to NO2. H2O2 can further oxidize NO2 to NO3– ion at a different location (via diffusion), thus alleviating the deactivation of the catalyst. The results presented may shed light on the design of visible photocatalysts with tunable reactivity for application in solar energy conversion and environmental sustainability.

Photocatalytic NO removal on BiOI surface: The change from nonselective oxidation to selective oxidation

Numerous publications have investigated nitric oxide (NO) removal through semiconductor photocatalytic technology. However, few reports are available on the products and mechanisms of the photocatalytic NO removal process. In this study, BiOI hollow microspheres are synthesized for the photocatalytic removal of NO under the visible light irradiation. Results show that NO removal product and NO removal mechanism could interfere with each other. NO removal process could be changed from nonselective oxidation to selective oxidation as the irradiation time increases. Meanwhile, the product of NO removal could be changed from nitrate (NO3−) to nitrogen dioxide (NO2). These interesting changes were attributed to the generated NO3−, which was produced from the reactions between NO and OH. The generated NO3− could inhibit OH generation, thus leading to a change in the NO removal products and NO removal mechanism. This study can improve our understanding of NO removal on the photocatalyst surface and serve as a guide in using photocatalysts for NO removal.

Efficient photocatalytic removal of nitric oxide with hydrothermal synthesized Na0.5Bi0.5TiO3 nanotubes

In this study, Na0.5Bi0.5TiO3 nanotubes were synthesized with a facile hydrothermal method using TiO2 P25 (Degussa) and bismuth citrate (BiC5H6O7) as precursors in concentrated NaOH and ammonia alkali solution. The samples were systematically characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The as-prepared Na0.5Bi0.5TiO3 nanotubes exhibited superior activity for photocatalytic removal of gaseous nitric oxide (NO) over TiO2 P25 (Degussa) under simulated solar-light irradiation, the NO removal rate can reach as high as ca. 200ppb·min−1 over the Na0.5Bi0.5TiO3 nanotubes in a continuous reactor with an initial NO concentration of 400ppb. The intrinsic hollow-nanotube structure of the Na0.5Bi0.5TiO3 photocatalysts contributes to its superior activity under simulated solar light. This work provides a facile route to prepare Na0.5Bi0.5TiO3 nanotubes and suggests that the Na0.5Bi0.5TiO3 nanotubes are ideal candidates for efficient removal of nitric oxide in indoor/outdoor air.

Photocatalytic removal of nitric oxide by multi-walled carbon nanotubes-supported TiO2

Multi-walled carbon nanotubes (MWCNTs) coated with TiO 2 were prepared by a sol-gel method and characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and UV-Vis diffuse reflectance spectroscopy. It was found that MWCNTs/TiO 2 consisted mainly of anatase phase. The introduction of MWCNTs inhibited TiO 2 grain growth. Moreover, the adsorption edge of MWCNTs/TiO 2 shifted toward long-wavelength region compared with bare TiO 2 . The photocatalytic ability of MWCNTs/TiO 2 for NO removal was experimentally investigated in a fixed-bed reactor. The results showed that a lower initial NO concentration was conducive to NO removal. A negative effect of SO 2 on denitration was observed. In contrast, O 2 and H 2 O played a promotional role in the photocatalytic denitration. At optimal conditions (73 mg/m 3 NO, 8% O 2 , 5% H 2 O), NO removal efficiency of 46% was achieved. In addition, the reaction mechanism of denitration was proposed. MWCNTs provide another transfer pathway for photogenerated electrons, assisting in electron and hole separation. This electron-scavenging character of MWCNTs causes MWCNTs/TiO 2 to perform better in photocatalytic denitration than bare TiO 2 .

Photocatalytic removal of nitric oxide by Bi2Mo3O12 prepared by co-precipitation method

Bi2Mo3O12 oxide was prepared by co-precipitation method at 250°C. The successful formation of the oxide was determined by X-ray powder diffraction (XRD) and its physical properties were studied using scanning electron microscopy (SEM), diffuse reflectance spectroscopy (DRS) and adsorption–desorption N2 isotherms (BET). Bi2Mo3O12 sample was composed of agglomerates of primary particles with faces and edges well defined. The oxide exhibited excellent performance for the removal of nitric oxide (NO) from air, reaching a percentage remove of 88.9% in steady state under UVA light irradiation. The synergetic effect between Bi2Mo3O12 and Bi2MoO6 phases was also investigated for this reaction. The results indicated that Bi2MoO6 is less active than Bi2Mo3O12 for the photocatalytic removal of NO under same experimental conditions.

A stable single-crystal Bi3NbO7 nanoplates superstructure for effective visible-light-driven photocatalytic removal of nitric oxide

In this study, bismuth niobate (Bi3NbO7) single-crystal nanoplates superstructure (BNS) was prepared via a facile hydrothermal route without adding any surfactants and templates by using bismuth citric and niobium pentoxide as precursors. The as-prepared products were systematically characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) nitrogen adsorption–desorption, X-ray photoelectron spectroscopy (XPS), and UV–vis diffuse reflectance spectrum (DRS). The characterization results revealed that the BNS nanostructures were self-assembled of single crystalline nanoplates under hydrothermal environment. These BNS exhibited excellent visible-light-driven (λ>420nm) photocatalytic performances for the removal of gaseous nitrogen monoxide (NO), the removal of NO reached 42.3% in 40min at the presence of BNS, which was much higher than those of C-doped TiO2 (25% of NO removal), the InVO4 hollow microspheres (25% of NO removal), as well as the BiOBr nanoplates microspheres (30% of NO removal). Close investigation indicated that plenty of pores existed in the aggregation of BNS superstructures, which could serve as efficient transport paths for NO molecules and harvesting of more light. Moreover, the BNS exhibited high stability during multiple runs of photocatalytic removal of NO due to their special superstructures. The study provides a facile method to synthesize BNS with high efficiency and high stability in the visible-light spectral range.

Silver Modified Degussa P25 for the Photocatalytic Removal of Nitric Oxide

A study of the photocatalytic behaviour of silver modified titanium dioxide materials for the decomposition and reduction of nitric oxide (NO) gas has been carried out. The effects of silver loading, calcination temperature, and reaction conditions have been investigated. Prepared photocatalysts were characterised using XRD, TEM, and XPS. A continuous flow reactor was used to determine the photocatalytic activity and selectivity of NO decomposition in the absence of oxygen as well as NO reduction using CO as the reducing agent, over the prepared photocatalysts. XRD and TEM analysis of the photocatalysts showed that crystalline silver nitrate particles were present on the titanium dioxide surface after calcination at temperatures of up to200∘C. The silver nitrate particles are thermally decomposed to form metallic silver clusters at higher temperatures. XPS analysis of the photocatalysts showed that for each of the temperatures used, bothAg+andAg0were present and that theAg0/Ag+ratio increased with increasing calcination temperature. The presence of metallic silver species on theTiO2surface dramatically increased the selectivity forN2formation of both decomposition and reduction reactions. When CO was present in the reaction gas, selectivities of over 90%were observed for all the Ag-TiO2photocatalysts that had been calcined at temperatures above200∘C. Unfortunately these high selectivities were at the expense of photocatalytic activity, with lower NO conversion rates than those achieved over unmodifiedTiO2photocatalysts.

Photocatalytic Reductions of Nitric Oxide in gas Phase and Nitrate ion in Water with Reducing Agents on Hollandite Catalyst

AbstractPhotoctalysis of a unique hollandite compound was investigated for the reductions of nitric oxide (NO) in gas phase and nitrate ion (NO3) in water with reducing agents under UV irradiation. Both NO and NO3 are hazardous chemicals to human. Recently, it was reported that titanium oxide (TiO2) photocatalyst perfomaed the oxidative decomposition of NO to NO3. However, an ideal photocatalytic removal of NO would be the reductive decomposition of NO to N2 and O2 without by-products. In this study, the surface activity of the unloaded hollandite, prepared by sol-gel method, was applied to the photocatalytic reaction. The present photocatalysis was quantitatively examined by using on-line mass spectrometry and ion chromatography, and the adsorption species on the hollandite surface was analyzed by in-situ IR spectroscopy. UV-ixradiated hollandite extu'bited high selectivity for the formation of N2 in the reductive decomposition of NO. Moreover, this catalyst also showed the reductive decomposition of NO3 to NO2 and N2 in water. In particular, the photocatalytic decomposition of NO3 would be a new pathway that has not been reported for popular photo-catalysts such as unloaded TiO2. Probably, this type compound is expected to have potentialities as a new type photocatalyst.