Enhanced NO Oxidation Research Articles

Performance, mechanism, and kinetics of NO removal using ligand-enhanced UV-Fenton driven by O2 from flue gas

Wet free radical advanced oxidation technology such as Fenton reaction is thought to be a very efficient method to remove NO from flue gas, but it requires the consumption of additional oxidants to achieve high removal efficiency. For this purpose, a ligand-enhanced UV-Fenton system without additional oxidants was proposed for NO removal. Three common chelating ligands, EDTA, NTA, and Citrate, were selected to promote UV-Fenton reaction for NO removal. The NO removal performance, mechanism, and kinetics of three different ligand-enhanced photochemical Fenton systems including EDTA/UV/Fe(II), NTA/UV/Fe(II), and Citrate/UV/Fe(II), were studied. Results showed that ligand addition can greatly promote a NO removal in the UV-Fenton system, in the following order of efficiency: EDTA > NTA > Citrate. The mechanism research indicated that these ligands not only enhanced NO oxidation removal by promoting the generation of •OH, but also provided NO complexation removal, which synergized oxidative denitrification and complexation denitrification in the same reaction system, greatly promoting wet NO removal. NO from simulated flue gas was mainly converted into NO3−, NO2−, NH4+, N2, and N2O through synergistic complexation-oxidation. The optimum ratio of ligand to Fe(II) was 1:1 for EDTA and NTA and 3:1 for citrate. NO removal efficiency was increased and then decreased with the increase in pH value or O2 concentration. The low temperature was found to be favorable for NO removal. Furthermore, the kinetics demonstrated that the L/UV/Fe(II) system reaction was a pseudo-first-order process involving nitric oxide. Finally, compared with other Fenton and Fenton-like denitrification systems, EDTA(NTA)-enhanced UV-Fenton system had shown great priority in terms of removal performance, absorbent cost, and waste disposal.

Distinct effects of Na on Cu-based Beta zeolites for the selective catalytic reduction of NOx with NH3

The distinct impact of Na on the NH3-SCR activity of Cu-based Beta zeolites was explored by incorporating Cu onto Na and H-type Beta zeolites via ion-exchange and impregnation methods. The characterization results revealed that an appropriate amount of Na facilitated the exchange of Cu ions into the Beta zeolites, leading to enhanced acidity and NOx adsorption properties. The CuNa-Beta catalysts obtained through ion-exchange, exhibited decreased NH3 oxidation activity at elevated temperatures while demonstrated enhanced NO oxidation activity at lower temperatures, resulting in the improved NOx conversion. However, when the Cu loading was conducted by the impregnation method, Na remained in the Cu/Beta zeolites caused the transformation of Cu to CuOx, resulting in the inferior NH3-SCR performance. These findings demonstrated the significant influence of co-cation Na and preparation methods on Cu locations and NH3-SCR performance of Cu-based Beta zeolites.

Mn-N/C interfacial bridge accelerating electrons transfer of MnCO3/α-Ni(OH)2 to promote surface adsorbates activation for favorable photocatalytic NO oxidation

Photocatalytic technology has been proven to be an effective strategy for addressing NOx pollution. However, the lack of adsorbates activation ability of photocatalysts results in the formation of secondary toxic by-products. Herein, MnCO3/α-Ni(OH)2 composite photocatalyst was developed through one-pot hydrothermal method. The introduction of MnCO3 not only improves visible light absorption, but also generates new Mn-C and Mn-N interfacial bridge structures between the exposed Mn atoms of MnCO3 and the C/N functional groups of α-Ni(OH)2. This unique structure speeds up the migration of photogenerated electrons and holes, promoting surface-adsorbed O2 and NO co-activation. Photocatalytic activity test shows that MnCO3/α-Ni(OH)2 has enhanced NO oxidation activity without producing harmful NO2 by-products. This research offers a fresh perspective on the surface adsorbates activation to prevent secondary toxic by-product generation.

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.

The mechanism of Ce-MCM-41 catalyzed peroxone reaction into •OH and •O2− radicals for enhanced NO oxidation

A series of Cerium-doped MCM-41 were prepared by hydrothermal method and used in peroxone process (O3/H2O2) for low-temperature NO oxidation. The experimental results showed that the NO oxidation efficiency could reach 93% for 3Ce-MCM-41 (molar ratio Ce/Si = 3%) under the molar ratio of H2O2/NO = 2.8 and O3/NO = 0.5. The addition of SO2 was conducive to the improvement of NO oxidation efficiency, which was assigned to SO2 improving the desorption of the intermediate product NO2 and promoting the progress of NO oxidation reaction. The hydroxyl and superoxide radical scavenger tert‑butanol and p-Quinone confirmed that O3/H2O2/3Ce-MCM-41 followed the •OH and •O2− radicals co-oxidation mechanism. Afterwards, the characterized results exhibited that Ce was incorporated into the framework of MCM-41 and presented in tetrahedral coordination, which enhanced Lewis acid sites over Ce-MCM-41 samples. Besides, the presence of Ce3+/Ce4+ promoted electron transfer. Meanwhile, the high surface area of 3Ce-MCM-41 (819 m2•g−1) was benefited to reactants transfer. Thus, O3/H2O2/3Ce-MCM-41 process achieved a high NO oxidation efficiency.

Visible light activation of photocatalysts formed from the heterojunction of sludge-generated TiO2 and g-CN towards NO removal

The feasibility of preparing TiO2/g-CN heterojunction from Ti-incorporated dried dye wastewater sludge is explored in this study. Two reaction routes of composite formation were evaluated. In the initial approach, one-step calcination of dried sludge and melamine mixture @600 °C was carried out. Detailed morphological and chemical characterizations showed that the one-step calcination route did not create TiO2/g-CN composites; instead, only N-doped anatase TiO2 composites were formed. Moreover, due to the non-uniform composition of organic content in the dried sludge, it was not easy to control the N doping level by varying melamine content (0–100%) in the precursor mix. However, successful formation of anatase TiO2 and g-CN was observed when a two-step calcination route was followed, i.e., via synthesis of anatase TiO2 from dried sludge, and later development of heterojunction by calcining (@550 °C) the TiO2 and melamine mixture. X-ray diffraction along with infrared and X-ray photoelectron spectroscopy verified the effective heterojunction. In addition, maximum atmospheric NO removal under UV and visible light were observed for the prepared composite when the melamine content in the precursor mixture was 70%. After 1 h of UV and visible light irradiation, the best TiO2/g-CN composite removed 25.71% and 13.50% of NO, respectively. Optical characterization suggested that the enhanced NO oxidation under UV/visible light was due to the bandgap narrowing and diminished photogenerated electron-hole recombination.

The superior performance of CoMnOx catalyst with ball-flowerlike structure for low-temperature selective catalytic reduction of NOx by NH3

A novel ball-flowerlike catalyst (CoMnOx-BF) was developed by hydrothermal method for the selective catalytic reduction (SCR) of NOx by NH3. Besides, a counterpart sample (CoMnOx) was prepared by coprecipitation way for comparison. The activity test results demonstrated that CoMnOx-BF catalyst exhibited admirable SCR performance and N2 selectivity in a broad temperature range of 150–350 °C, along with an excellent resistance to SO2 and high durability. Based on characterization experiments, it could be found that the superior surface area resulted from suppressed crystallization, the enrichment of surface labile oxygen and Mn4+, the stronger surface acidity and redox ability were all beneficial to the NH3-SCR performance of CoMnOx-BF catalyst. Additionally, the enhanced NO oxidation and more activated ad-reactants on catalyst surface could also conduce to the outstanding SCR activity of CoMnOx-BF at low temperature.

Catalytic Oxidation of NO over LaCo1−xBxO3 (B = Mn, Ni) Perovskites for Nitric Acid Production

Nitric acid (HNO3) is an important building block in the chemical industry. Industrial production takes place via the Ostwald process, where oxidation of NO to NO2 is one of the three chemical steps. The reaction is carried out as a homogeneous gas phase reaction. Introducing a catalyst for this reaction can lead to significant process intensification. A series of LaCo1−xMnxO3 (x = 0, 0.25, 0.5 and 1) and LaCo1−yNiyO3 (y = 0, 0.25, 0.50, 0.75 and 1) were synthesized by a sol-gel method and characterized using N2 adsorption, ex situ XRD, in situ XRD, SEM and TPR. All samples had low surface areas; between 8 and 12 m2/g. The formation of perovskites was confirmed by XRD. The crystallite size decreased linearly with the degree of substitution of Mn/Ni for partially doped samples. NO oxidation activity was tested using a feed (10% NO and 6% O2) that partly simulated nitric acid plant conditions. Amongst the undoped perovskites, LaCoO3 had the highest activity; with a conversion level of 24.9% at 350 °C; followed by LaNiO3 and LaMnO3. Substitution of LaCoO3 with 25% mol % Ni or Mn was found to be the optimum degree of substitution leading to an enhanced NO oxidation activity. The results showed that perovskites are promising catalysts for NO oxidation at industrial conditions.

Boosting photocatalytic oxidation on graphitic carbon nitride for efficient photocatalysis by heterojunction with graphitic carbon units

Graphitic carbon nitride has been considered as a promising metal-free visible light photocatalyst for air pollutants oxidation due to its suitable band-gap energy and higher conduction band edge. Herein, we have developed a facile approach for dramatically downwards shifting band edge positions of carbon nitride up by about 1 eV via in-plane heterojunction with graphitic carbon units to enhance the oxidation capability of the electron holes generated from the valence band. The graphitic carbon units in junction with tri-s-triazine domains were clearly observed and its in-plane hybridization with carbon nitride was formed during the copolymerization using melamine with a small amount of m-phenylenediamine as the precursors. The direct intralayer junction between the tri-s-triazine and the graphitic carbon domain, essentially different with interlayer junction reported in literature, is able to shift downwards the band edge positions via merging electron density of states of carbon nitride with that of graphitic carbon, and thus would be beneficial for separation of photoexcited charge carriers and generation of hydroxyl radicals for the oxidation of pollutants. The hybrid photocatalyst prepared with a small quantity (less than 1%) of m-phenylenediamine and melamine as precursors has shown much enhanced NO oxidation to final products (NO2− and NO3−) and increased NO removal 10% than the one from melamine only.

Promoting diesel soot combustion efficiency by tailoring the shapes and crystal facets of nanoscale Mn3O4

Various shapes (hexagonal nanoplate, octahedral, and nanoparticle) of nanoscale Mn3O4 have been successfully prepared via facile hydrothermal and co-precipitation methods, respectively, and evaluated for their performance on diesel soot combustion. The catalytic performance of hexagonal nanoplates Mn3O4 (Mn3O4-HNS) is distinguished from the samples with other shapes by its superior activity on soot combustion with Tm of 407.7 °C and SmCO2 of 99.1% at the gas hourly space velocity of 9990 h−1 with the feed composition of 2500 ppm NO/5 vol.%O2/N2 under loose contact mode. The physicochemical properties of Mn3O4 were systematically examined by different characterization techniques, including XRD, FTIR, BET, H2-TPR, FE-SEM, HR-TEM, XPS, soot-TPR, and NO/NO + O2-TPSR. The different crystal shapes controlled by the preparation method are found to influence the extent of exposed Mn3O4 crystal facet, as determined by HR-TEM, which, in turn, was found to correlate with the catalytic performance. The kinetic study of soot combustion over these samples further revealed that the Ea obtained from three samples ranked as the order of (112) < (101) < (220) facets in reverse order of activity data, confirming the crystal-facet dependent reactivity. In view of the difference in catalytic activity and their physicochemical properties as estimated by various techniques, the superior catalytic performance of Mn3O4-HNS for diesel soot combustion was originated from its exposed (112) facet in association with its good lower temperature reducibility, abundant surface Mn4+ and active oxygen species, and the enhanced NO oxidation capability. Furthermore, the best performing Mn3O4-HNS displayed the good stability through recycling test. Our work may provide an important insight on the design strategies to develop the high-efficient soot combustion catalysts under the working conditions of the diesel engines.

A simple seed-mediated growth method for the synthesis of highly morphology controlled CrOx/Ce0.2Zr0.8O2 catalysts and their enhanced NO oxidation

A series of highly oriented CrOx/Ce0.2Zr0.8O2 nanoparticles were synthesized via a simple seed-mediated growth approach. The relationship between the catalytic oxidation capacity and morphology was evaluated by the oxidative denitrification (2NO+O2→2NO2). The results indicate that the addition of different seed helps to form the dendritic shape of CrOx/Ce0.2Zr0.8O2 catalysts. Among them, CCZ4-C3N4 (the g-C3N4 seed-mediated approach of CrOx/Ce0.2Zr0.8O2 catalysts) exhibited the best catalytic activity for the oxidation of NO, which reached 57.47% at 300°C. The g-C3N4 seed makes Ce0.2Zr0.8O2 growing along the (111) face of itself, which can benefit for the surface enriched with Ce and Cr exposed. The exposed enriched surface structure promotes the generation of Ce3+ and Cr6+ on the surface, which enhances the NO oxidation capacity. The good dispersion of Cr species on the surface also leads to the high catalytic performance.

The enhanced performance of MnOx catalyst for NH3-SCR reaction by the modification with Eu

A series of MnEuOx catalysts were prepared by the co-precipitation method and used for selective catalytic reduction of NOx with NH3. It was found that the modification of MnOx catalyst by Eu with a Eu/Mn molar ratio of 0.1 could greatly enhance its SCR activity and broaden its operating temperature window, accompanied by good resistance to SO2 and H2O. From the characterization results of N2 adsorption, XRD, XPS, H2-TPR, it could be concluded that the larger surface area resulted from the inhibited crystallization of MnOx, the enrichment of surface Mn4+ and chemisorbed oxygen, and the higher reducibility all made a contribution to the high SCR activity of MnEuOx-0.1 catalyst. Furthermore, the results of in situ DRIFTS study revealed the NH3-SCR reactions over MnOx and MnEuOx-0.1 were mainly controlled by L-H mechanism (<150°C) and E-R mechanism (>150°C) respectively, and the contribution of E–R mechanism became more important with increasing reaction temperature. The enhanced NO oxidation and NH3 adsorption should be mainly contribute to the distinguished SCR performance of MnEuOx catalyst in the lower temperature range (<150°C) and higher temperature range (>150°C) respectively.

Photocatalytic removal of NO from coal-fired flue gas by H2O2/UV reaction over TiS catalyst

This study aimed at investigating the photocatalytic oxidation removal of NO from the flue gas of coal-fired boilers by H2O2/UV (UV is ultraviolet radiation) reaction over TiS catalyst. The effects of different catalysts and gas composition on the oxidation of NO were investigated. We found that the S doped titania (TiO2) catalyst could clearly improve the NO oxidation, compared with TiO2 and no catalyst in the H2O2/UV system. In addition, the H2O2 and UV also played an important role in the NO oxidation. The NO oxidative product was the stable nitrate. Analysis of catalysts' characterization by XRD, TEM, XPS, UV–vis and DFT calculations showed that the S (S6+) doped TiO2 catalyst included the heterostructure of Ti1−xSxOx+2 compounds and TiO2. The S doping increased the VB potential for the TiS catalyst. We designed different tests of PL, LSV, EPR liquid UV–vis analysis-PCBA and RhB to confirm the mechanism of enhanced NO oxidation over TiS catalyst. It was found that the reactions in the VB were important to the NO oxidation over the TiS(TiO2)+UV+H2O2 catalytic system. Due to this reason, enhanced NO oxidation was achieved over the TiS catalyst.

Mechanistic Aspects of deNOx Processing over TiO2 Supported Co–Mn Oxide Catalysts: Structure–Activity Relationships and In Situ DRIFTs Analysis

Anatase TiO2-supported manganese and cobalt oxide catalysts with different Co/Mn molar ratios were synthesized by a conventional impregnation method and used for selective catalytic reduction (SCR) of NOx with NH3. The catalysts were characterized by N2 adsorption/desorption, X-ray diffraction, X-ray photoelectron spectroscopy, and temperature-programmed desorption with NH3 and NOx. Characterization of the catalyst confirmed that by using Co3O4 over Mn/TiO2, we enhanced NO oxidation ability. From in situ diffuse reflectance infrared transform spectroscopy (DRIFTs) analysis of desorption and the transient reaction, we concluded that the addition of Co could remarkably lower the activation energy of NOx chemisorption on the catalyst surface. In addition, low-temperature SCR activity mainly results from a “fast SCR” reaction. We observed four NOx species (bidentate nitrates, gaseous NO2, linear nitrites, and monodentate nitrites) on the surface of Mn/TiO2 and Co–Mn/TiO2 catalysts that all participated in the...

Spatially resolving SCR reactions over a Fe/zeolite catalyst

There have been several recent reports regarding the spatial resolution of gas species in monolith-supported automobile exhaust catalysts, with examples including characterization of DOCs (diesel oxidation catalysts) and LNTs (lean NOx traps). However, spatially resolving gas concentrations in NH3-SCR (selective catalytic reduction) applications is limited due to the difficulty in ppm-level NH3 detection in the presence of percent levels of water and N2 using mass spectrometry. In this study, a gas-phase FT-IR analyzer was used for the first time to spatially resolve gas concentrations in a monolith-supported SCR catalyst. The reactions analyzed include standard SCR, fast SCR and SCR with NO2. The results show that the three SCR reactions proceed at significantly different rates, especially at temperatures below 300°C, and can be correlated to the amount of catalyst used. For example, the catalyst lengths needed to achieve 80% NOx conversion at 300°C are 2.4, 1.2 and 0.5cm for conditions that meet the standard SCR, NO2-SCR and fast SCR reaction stoichiometries, respectively. For the standard SCR reaction, kinetic analysis, and spatially resolved NO oxidation and SCR results consistently indicate that the rate-determining step is NO oxidation. NH3 has an inhibition effect, as it suppresses NO oxidation by competitive adsorption on the active sites. At 300°C, the outlet NOx conversion is not limited by the reaction kinetics, but by insufficient NH3 supply, since part of the NH3 is oxidized by O2. Compared with 300°C, higher NOx conversions are attained at 400 or 500°C, which is due to significantly enhanced NO oxidation, and the resulting increase in NH3 reacting with NOx via SCR rather than O2 via NH3 oxidation. For NO2-SCR, a considerable amount of N2O was formed at 250°C but decreased with increasing temperature. The decreased N2O is due to improved selectivity in the NO2-SCR to N2, as well as N2O decomposition at the back part of the catalyst at high temperature. Finally, different SCR reaction patterns were identified when testing with NO:NO2=3:1 and 1:3. For NO:NO2=3:1, the SCR reactions proceed in series, namely through the fast reaction first, followed by standard SCR. The fast SCR and NO2-SCR reactions proceed in parallel for NO:NO2=1:3. The results indicate that if NO2 is the limiting reactant, fast SCR dominates, but if excess NO2 is available, the NO2 SCR reaction can proceed in parallel.

NO x-assisted soot oxidation over K/CuCe catalyst

CeO 2 and CuO x -CeO 2 supported potassium catalysts were synthesized by wetness impregnation method. The catalysts were characterized by BET, NO-TPO, NO x -TPD and soot-TPO measurements. By the decoration of potassium and copper, the maximum soot combustion temperature of the ceria-based catalyst decreased to 338 and 379 °C in the presence and absence of NO under a loose contact mode, respectively. The pronouncedly enhanced NO oxidation ability by copper introduction and NO x storage capacity by potassium modification were especially important in the NO x -assisted soot oxidation reaction with the K/CuCe catalyst.

Rapid NO 2 formation in diluted petrol-fuelled engine exhaust—A source of NO 2 in winter smog episodes

Substantially enhanced rates of oxidation of nitric oxide have been found in petrol engine exhaust diluted with air in the dark. The main product of oxidation is nitrogen dioxide, but up to 20% of the oxidised NO can be in the form of other NO y species. Similarly enhanced NO oxidation rates are also found in the presence of petrol vapour. The kinetics of NO oxidation are close to zero order in NO, the rate being appreciably faster than the known thermal reaction 2NO + O 2 → 2NO 2. Attempts to isolate the chemical species responsible for this rate enhancement from petrol vapour and engine exhaust have led to identification of 1-methyl-1,3-cyclopentadiene and 1,3-cyclohexadiene. A mechanism has been developed in which addition of NO 2 to a conjugated diene causes initiation of a free-radical chain oxidation. A simple numerical model incorporating this process is capable of explaining the observed rate enhancements.