NOx Molecules Research Articles

Remarkable improvement in the NOx storage ability of Pd/SSZ-13 zeolite PNA via a facile pretreatment strategy

A facile strategy was employed to pretreat Pd/SSZ-13 zeolite obtained via the initial-impregnated method under a microwave field, aiming to greatly enhancing the NOx storage-release ability. By investigating the effect of microwave treatment parameters (power, atmosphere and time) on the improvement of PNA performance, the optimal conditions were determined (175 W, 10 % O2/N2, 15 min). Under these conditions, the Pd/SSZ-13 exhibited an enhanced NOx adsorption and desorption amount to 2.60 and 2.38 times, respectively, compared to the untreated sample after three cycles of NO-TPD. Deeper insights into the variations in Pd/SSZ-13 properties induced by the microwave radiation were clarified through a series of characterizations. The micropore surface area of Pd/SSZ-13 zeolite activated by microwave increased by 3.91 %, and the micropore volume also improved by 3.85 %, which might facilitate the adhesion of more active Pd species to the materials surface, and aid in the dispersion of PdOx clusters into the zeolite channels. After microwave activation, the average particle size of Pd decreased from 10.09 nm to 7.08 nm. Particularly, the content of Pd(II) species such as Pd2+, [Pd(OH)]+ and PdO as active NOx adsorption sites enhanced from 66.68 % to 68.93 %. Additionally, the microwave treatment not only increased the number of Lewis and Brønsted acid sites, but also strengthened the interaction between NOx molecules and the zeolite skeleton. The above reasons collectively promoted the NOx storage-release properties of Pd/SSZ-13 zeolite, showing that the microwave pretreatment is a highly efficient strategy for improving the performance of PNA materials.

Highly selective sensing of toxic NOx gases for environmental monitoring using Ru-doped single walled TiO2 nanotube: A density functional theory study

Applying density functional theory simulations, in this study, we examine the sensing properties of Ru-TNT surface towards NOx molecules such as NO and NO2. The obtained results show that Ru doping creates a large-spin density over the Ru-TNT surface, which hints at an increase in the adsorption energy of NOx molecules. The electronic properties of the Ru-TNT surface are significantly altered after NO and NO2 molecule adsorption, as proven by the larger change in the energy gap values. Moreover, the selectivity study indicates that the Ru-TNT is capable of selectively sensing the NOx molecules in the presence of other gases, e.g., CO2, CO, NH3 and H2O. Using an external positive electric field can resist the strong anchoring of NOx molecules over the Ru-TNT surface, suggesting that material recovery is feasible. Thus, the Ru-TNT structure can be applied as a selective, sensitive, and promising ambient temperature gas sensor for NOx molecules.

Mechanistic insight and first principle analysis of cation-inverted zinc ferrite nanostructure: A paradigm for ppb-level room temperature NOx sensor

Herein, we adopted a new paradigm for developing a high-performance gas sensor by leveraging the mixed spinel ZnFe2O4 structure (mZFO) to enhance the adsorption of NOx molecules. Material characterization reveals the formation of the mZFO due to the cation inversion in lattice sites. The estimated value of the inversion degree is observed to shift from 0.78 to 0.39 with an increase in the calcination temperature. The mZFO nanoparticles calcined at 500 °C show exceptional sensing performance due to their suitable grain size (∼2 times Debye length), neck diameter, and surface area. The sensing studies conducted at various NOx concentrations indicate that the sensor can detect ppb level of NOx with a detection limit of about 9 ppb at room temperature. The detailed sensing mechanism is elucidated based on the density functional theory calculations (DFT) and Bader charge analysis. The outstanding sensor performance is attributed to the formation of a mixed spinel structure, wherein the adsorption energy of NOx (∼-0.6 eV) in the presence of surface adsorbed oxygen is higher than that of the normal spinel structure (∼-0.1 eV). Furthermore, the sensor exhibited a fast response and recovery times (7 and 92 s at 800 ppb NO2), excellent stability, and selectivity. The practical suitability of the mZFO sensor was studied by analyzing the vehicle exhaust emissions. We strongly believe this work would pave a novel approach to developing a high-potential gas sensor by modifying the cation distributions in the spinel ferrites.

SO2- and H2O-tolerant catalytic reduction of NOx at low temperatures through shielding ceria-based catalyst from poisoning: An efficient protection mechanism

Improving SO2/H2O resistance at low temperatures is a key challenge for industrial applications of NH3-SCR catalysts. Herein, a novel catalyst with remarkable low-temperature NH3-SCR activity and strong SO2/H2O resistance was designed by loading MoOx protective layer over porous kit-CeO2. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) coupled with basic characterization techniques were utilized to further elucidate the underlying mechanisms, and a dynamic balance between the deposition and decomposition of ammonia bisulfate was captured over porous kit-CeO2. Highly dispersed MoOx species pre-adsorbed with NH3 significantly curtailed the adsorption of NOx molecules, and the catalytic reaction over MoOx/kit-CeO2 primarily followed by the Eley-Rideal mechanism. At 250 °C, in the presence of SO2 and H2O, the Ce-O-Mo pairing site was almost unaffected and the super SO2- and H2O-tolerant property originated from the facilitated decomposition of ammonium bisulfate (ABS) and the shielding effect of Mo protective layer on SO2/H2O. This study provided an important guidance to the application of ceria-based catalysts for environmental control within low-temperature operations in non-power industries such as glass, steel, cement manufacturing, etc.

Substorm Driven Chorus Waves: Decay Timescales and Implications for Pulsating Aurora

AbstractEnergetic electron precipitation (EEP) associated with pulsating aurora can transfer greater than 30 keV electrons from the outer radiation belt region into the upper atmosphere and can deplete atmospheric ozone via collisions that produce NOx and HOx molecules. Our knowledge of exactly how EEP occurs is incomplete. Previous studies have shown that pitch angle scattering between electrons and lower‐band chorus waves can cause pulsating aurora associated with EEP and that substorms play an important role. In this work, we quantify the timescale of chorus wave decay following substorms and compare that to previously determined timescales. We find that the chorus decay e‐folding time varies based on magnetic local time (MLT), magnetic latitude, and wave frequency. The shortest timescales occur for lower‐band chorus in the 21 to 9 MLT region and compares, within uncertainty, to the energetic pulsating aurora timescale of Troyer et al. (2022, https://doi.org/10.3389/fspas.2022.1032552) for energetic pulsating aurora. We are able to further support this connection by modeling our findings in a quasi‐linear diffusion simulation. These results provide observations of how chorus waves behave after substorms and add additional statistical evidence linking energetic pulsating aurora to substorm driven lower‐band chorus waves.

Black phosphorene with Au modification: Oxynitride remover and hydrogen sensor

Effective detecting toxic gases and the clean energy hydrogen is urgently necessary in industrial production and our daily life. Here, the sensing properties of Au-adsorbed/doped black phosphorene monolayer towards six different gases (CO, H2, H2S, NO, NO2, and SO2) are theoretically investigated through density functional theory in combination with the non-equilibrium Green’s function approach. The calculated adsorption energy, the charge transfer, the electron localization function, the band structure, and the density of states reveal that the Au-adsorbed/doped black phosphorene exhibits sensitive chemisorption to NOx gases, and, surprisingly, the Au-doped black phosphorene exhibits sensitive physisorption towards H2 molecules. The calculated recovery time and current–voltage curves show that the Au-adsorbed/doped black phosphorene device acts as an ultra-sensitive (71%–100% sensitivity) remover of NOx molecules, and, importantly, the Au-doped black phosphorene sensor achieves a highly sensitive (85% sensitivity), selective, and reusable (0.01 ns recovery time) detection of H2 at room temperature. Our results provide a theoretical basis for the potential applications of the Au-adsorbed/doped black phosphorene in gas-sensing, especially in the desirable NOx remover and H2 sensor.

Combinations of essential oils as effective green stabilizers for single base propellants

AbstractNitrocellulose‐based propellants undergo natural (and inevitable) autocatalytic degradation reactions. The presence of a stabilizer is essential to avoid and reduce the degradation reactions by capturing the NOx molecules. Traditional stabilizers, such as diphenylamine, and ethylcentralite, react with NOx through their amine and/or amide groups, leading to n‐nitroso groups, which are known to have carcinogenic, mutagenic and reprotoxic properties. Molecules that are not prone to form by‐products containing nitrosamines have been studied as substitutes, such as guaiacol and curcumin. However, the aromatic rings on these molecules have their acidity augmented when capturing the NOx groups and, paradoxically, increasing hydrolysis rates. In order to find a solution for the problem, we evaluated the use of essential oils containing terpenes and/or phenolic compounds to introduce a buffering capacity. Several propellant formulations with those compounds were subjected to traditional stability tests (German, Bergman–Junk and Storage tests), Pressure Vacuum Stability Test and Heat Flow Microcalorimetry. Finally, industrial batches were produced for the evaluation of real (ballistic) performance. Results indicated that the usage of essential oils increases stability properties not only through a sum of effects, but also in a synergistic way. Ballistic results were also adequate.

Dual mid-infrared wavelength Faraday rotation spectroscopy NOx sensor based on NdFeB ring magnet array

Faraday rotation spectroscopy (FRS) exploits the magneto-optical Faraday rotation effect to achieve highly sensitive detection of paramagnetic molecules. Most of the currently reported FRS sensors suffer from high power consumption, while enabling detection of only a single component. In this work, a novel dual mid-infrared wavelength FRS sensor is proposed that enables the simultaneous detection of two paramagnetic molecules (nitrogen oxides, NOx = NO + NO2) in a single absorption cell. A neodymium-iron-boron (NdFeB) ring magnet array was designed to replace the solenoid coil to provide a stable, power-free magnetic field. The magnetic induction line distribution characteristics around the permanent magnets were analyzed based on the finite element method. Two mid-infrared quantum cascade lasers were used to detect the strongest fundamental ν2 rotational-vibrational band of NO (1875.81 cm−1) and the ν3 fundamental band of NO2 (1613.25 cm−1), respectively. A dual-wavelength Herriott cell (DWHC) is coaxially combined with a NdFeB ring magnet array to amplify the interaction of the laser beam with paramagnetic NOx molecules in an axial magnetic field. This low-power FRS NOx sensor achieves minimum detection limits of 0.58 ppb and 0.95 ppb for NO2 and NO, respectively, over an optical length of 23.7 m.

Adsorption Behaviors of Small Molecules on Two-Dimensional Penta-NiN2 Layers: Implications for NO and NO2 Gas Sensors

Penta-NiN2 is a two-dimensional layered material with pentagonal atomic arrangement and has recently been manufactured in experiments ( ACS Nano 2021, 15, 8, 13539). We study systematically the adsorption of CO, CO2, NO, NO2, NH3, and CH4 on penta-NiN2 nanosheet and its effects on electronic properties employing the first-principles method combined with the nonequilibrium Green’s function. Excellent sensing performance of penta-NiN2 is predicted for the NOx (x = 1, 2) molecules based on energetic, geometric, and electric analyses. The adsorption of NO and NO2 gas molecules may convert penta-NiN2 from a nonmagnetic semiconductor to magnetic metal. A current enhancement around 800% can be observed in penta-NiN2 connected to a circuit after its adsorption of NO. The unique selectivity and sensitivity to adsorption suggest that penta-NiN2 may become a widely used gas sensor for NO and NO2.

Investigating sensitivity of ozone to emission reductions in the New York City (NYC) metropolitan and downwind areas

Ozone sensitivity to emission changes in urban environments were examined using WRF and CAMx simulations for the New York City metropolitan area (NYC) with data from the 2018 Long Island Sound Tropospheric Ozone Study measurements. The WRF simulation showed large positive biases of windspeed over entire domain, but those were significantly reduced with observational nudging. The base CAMx simulation showed large positive bias for NO2 concentrations in NYC, which was corrected after NOx emission adjustment (22% reduction) based on previous work. The improved modeling platform demonstrated reliable performance skills for regulatory applications. Pollution transport from NYC to the downwind region were well captured by the model. Simulated ozone concentration increases in response to NOx emission reductions within NYC proximity suggested this area to be a VOC-limited ozone formation regime. Reducing anthropogenic emissions by 50% for all pollutants reduced the number of simulated ozone exceedances from 18 to 9 days for NYC. Emission reductions were even more effective for air quality improvements in the downwind region with ozone exceedances decreasing from 20 to 7 days. Source apportionment showed mobile and industrial solvent emission sectors to be contributing more to ozone exceedances than the electrical generating units (EGU) sector in both NYC and the downwind region. The largest air quality improvement appeared to be associated with VOC emission reductions from the industrial solvent sector. All source sectors yielded higher ozone production efficiency when emissions were reduced by 50%, suggesting remaining emissions can be more potent in producing ozone per molecule of NOx.

A study of nitrogen dioxide (NO2) periodicity over the United Arab Emirates using wavelet analysis

NO2 and nitric oxide (NO) are the most reactive gases in the atmosphere. The interaction of NOx molecules with oxygen, water and other chemicals leads to the formation of acid rain. The presence of NO2 in the air affects human health and forms a photochemical smog. In this study, we utilize wavelet analysis, namely, the Morlet wavelet, which is a type of continuous wavelet transform, to conduct a spectral analysis of the periodicity of nitrogen dioxide (NO2). The study is conducted using data from 14 weather stations located in diverse geographic areas of the United Arab Emirates (UAE) over a period of two years (2019 and 2020). We explain and relate the significance of human activities to the concentration level of NO2, particularly considering the effect of the COVID-19 lockdown to the periodicity of NO2. The results show that NO2 concentrations in desert areas such as Liwa and Al Quaa were unaffected by the lockdown period (April–July 2020) resulting from the COVID-19 pandemic. The other stations in the urban areas of Abu Dhabi city, Al Dhafra and Al Ain, showed a reduction in NO2 during the lockdown. NO2 is more highly concentrated during winter seasons than during other seasons. The periodicity of NO2 lasted from a few days up to 16 days in most regions. However, some stations located in the Al Dhafra region, such as Al Ruwais and the Gayathi School stations, exhibited a longer period of more than 32 days with a 0.05 significance test. In the Abu Dhabi region, NO2 lasted between 64 and 128 days at the Al Mafraq station. The correlation between the NO2 concentration across several ground stations was studied using wavelet coherence.

Contributions of Washcoat Components in Different Configurations to the NOX and Oxygen Storage Performance of LNT Catalysts

In addition to SCR systems, lean NOX traps (LNTs) are also used for exhaust aftertreatment of lean burn internal combustion engines to sustainably reduce NOX emissions. Modern LNTs consist of different functional compounds to maximize the performance during NOX storage and regeneration. Based on the material analysis of a serial production LNT, PGM loaded BaO, Al2O3, MgAl2O4, and CeO2 were identified as the main base materials. In this paper, the NOX storage capacity (NSC) of these compounds is investigated both as single catalysts and as physical mixtures to identify possible synergistic effects. Therefore, commercially available support materials were loaded with Platinum and tested in granular form under realistic conditions. To optimize the performance by reducing the diffusion pathways for NOX molecules during storage, PGM, BaO, and Ceria were combined in a composite by the incipient wetness impregnation of alumina. As a result, the temperature dependent NSC of the commercial LNT could be reached with the Pt/Rh/Ba10Ce25/Al2O3 infiltration composite, while reducing the oxygen storage capacity by about 45%. Without the additional Rhodium coating, the low-temperature NSC was insufficient, highlighting the important contribution of this precious metal to the overall performance of LNTs.

Observation-based analysis of ozone production sensitivity for two persistent ozone episodes in Guangdong, China

Abstract. An observation-based method (OBM) is developed to investigate the sensitivity of ozone formation to precursors during two persistent elevated ozone episodes observed at 77 stations in Guangdong. Average OH concentrations derived at the 77 stations between 08:00 and 13:00 local time stay within a narrow range of 2.5×106 to 5.5×106 cm−3 with a weak dependence on the NOx. These values are in good agreement with OH values observed at a rural station in the Pearl River Delta (PRD). They also agree well with a box model constrained by the ambient conditions observed during the two episodes. The OBM has been used to evaluate the ozone production efficiency, ε(NOx or volatile organic compound, VOC), defined as the number of O3 molecules produced per molecule of NOx (or VOC) oxidized. Average values of ε(NOx) and ε(VOC) determined by the OBM are 3.0 and 2.1 ppb ppb−1, respectively, and both compared well with values in previous studies. Approximately 67 % of the station days exhibit ozone formation sensitivity to NOx, and approximately 20 % of the station days are in the transitional regime sensitive to both NOx and VOC, and only approximately 13 % of the station days are sensitive to VOC. These results are in semi-quantitative agreement with the ozone formation sensitivity calculated by the box model constrained by ambient conditions observed during the two episodes. However, our OBM results differ from those of most previous investigations, which suggested that limiting the emission of VOC rather than NOx would be more effective in reducing ozone reduction in Guangdong.

Chemical effect of water addition on the ammonia combustion reaction

• Influence of water addition on ammonia combustion reaction are studied via ReaxFF; • Water addition is shown to promote the overall ammonia oxidation; • H 2 O enhances the conversion between NO and HNO at the middle stage of oxidation; • The number of NOx molecules decreases with reduction of O 2 concentration in O 2 /H 2 O environment. Ammonia has become an emerging fuel. However, its combustion product NOx is polluting, and it is necessary to explore its conversion process. To determine the effect of water addition on ammonia oxidation and NOx, reaction molecular dynamics simulations are performed to study the chemical reactions of ammonia. Ammonia oxidation is studied in O 2 /N 2 and O 2 /H 2 O environments in a determining density and O 2 /NH 3 ratio equal under high temperature conditions. This work focuses on the initial reaction path of ammonia combustion established through chemical trajectory analysis. The ammonia oxidation behavior, the distribution of the main products, and the corresponding time evolution behavior on the atomic scale are also studied. Finally, the interconversion processes of molecules containing N elements in the ammonia oxidation products are analyzed. Results show that when N 2 molecules are replaced by H 2 O molecules, the formation of HNO increases with the net flux of the reactions: N H + O H → H 2 N O , H 2 N O → H + H N O . We also investigate the effect of different O 2 concentrations. An increase of NO and HNO molecules with O 2 concentration mainly from the reactions: O 2 + H → O + O H , H 3 N O + O H → H 2 N O + H 2 O , H 2 N O → H + H N O and H N O + O H → H 2 O + N O . In addition, the conversion affected by water addition between NO and HNO occurs mainly in the middle time of the oxidation, and H 2 O enhances the severity of the oxidation with the number of OH radicals. Water addition provides a new way to control NOx emissions in ammonia combustion, that is, converting it into other H-containing compounds.

Effect of a Plasma Burner on NOx Reduction and Catalyst Regeneration in a Marine SCR System

The problem of environmental pollution by the combustion of fossil fuels in diesel engines, to which NOx emission is a dominant culprit, has accelerated global environmental pollution and global and local health problems such as lung disease, cancer, and acid rain. Among various De-NOx technologies, SCR (Selective Catalytic Reduction) systems are known to be the most effective technology for actively responding to environmental regulations set by the IMO (International Maritime Organization) in marine diesel applications. The ammonia mixes with the exhaust gas and reacts with the NOx molecules on the catalyst surface to form harmless N2 and H2O. However, since the denitrification efficiency of NOx can be rapidly changed depending on the operating temperature from 250 °C to 350 °C at 0.1% sur contents of the catalyst used in the SCR, a device capable of controlling the exhaust gas temperature is essential for the normal operation of the catalyst. In addition, when the catalyst is exposed to SOx in a low exhaust gas temperature environment, the catalyst is unable to reduce the oxidation reaction of the catalyst, thereby remarkably lowering the De-NOx efficiency. However, if the exhaust gas temperature is set to a high temperature of 360–410 °C, the poisoned catalyst can be regenerated through a reduction process, so that a burner capable of producing a high temperature condition is essential. In this study, a plasma burner system was applied to control the exhaust gas temperature, improving the De-NOx efficiency from the engine and regenerating catalysts from PM (Particulate Matter), SOOT and ABS (ammonia bisulfate), i.e., catalyst poisoning. Through the burner system, the optimum De-NOx performance was experimentally investigated by controlling the temperature to the operating region of the catalyst, and it was shown that the regeneration efficiency in each high temperature (360/410 °C) environment was about 95% or more as compared with the initial performance. From the results of this study, it can be concluded that this technology can positively contribute to the enhancement of catalyst durability and De-NOx performance.

Two-dimensional chromium phosphorus monolayer based gas sensors to detect NOx: A first-principles study

The successive realization of two-dimensional (2D) material sensors to detect diverse gas molecules motivates us to take the first-principles calculations to investigate the adsorption of typical gases molecules (such as NO, NO2, NH3, SO2, H2O, CO2 and CO) on a 2D chromium phosphorus (CrP) monolayer. The adsorption energies are calculated after determining the optimum adsorption location and orientation of these molecules on the CrP monolayer surface. The results display that there is appropriate adsorption strength and obvious charge transfer between the 2D CrP monolayer and NOx (x = 1,2) molecules, indicating that the CrP monolayer is preferable adsorption ability towards NOx. The current–voltage (I-V) curves of the NOx-adsorbed CrP monolayer are further investigated by the nonequilibrium Green's function (NEGF) method. Before and after adsorption of NOx on the CrP monolayer, the transport characteristics show apparent response with a significant change of I-V curves, indicating that the CrP monolayer is a candidate material for high selectivity and sensitivity NOx sensor. These results demonstrate that the CrP monolayer has excellent sensing performance, close to or even better than other 2D materials like phosphorene and Ga-doped graphene, which makes it a potential gas sensor for NOx.

Explanation of the Non-Linear Electrical Behavior of a Resistive NOx Dosimeter By Operando DRIFT Spectroscopy

Introduction To detect low NOx concentrations is still a focus for air-quality monitoring. The direct detection of the total dose of NOx is beneficial since the air pollution limits are given as hourly and annual mean values. A gas dosimeter for direct detection of the dose of NOx was presented in [1,2,3]. In the sorption phase, NOx molecules are adsorbed on the functional film and the electrical resistance changes. During this phase, the correlation between NOx dose and resistance change is linear until a certain loading state is reached. As soon as the characteristic curve gets non-linear, described often as saturation of the NOx adsorption [1], sensor regeneration is necessary. During thermal regeneration, the sorbed NOx species is desorbed from the surface, and a new sorption cycle can begin.To further understand the sensing mechanism and the effects of non-linearity, NOx dosimeters are investigated by operando DRIFT spectroscopy [4]. The electrical sensor signal and the formation of adsorbed species are investigated operando on the dosimeter. The non-linear behavior of the electrical properties occurring at a certain sensor signal are investigated at various film thicknesses [5] and are combined with the sorption properties. Materials and Methods Potassium-permanganate impregnated on lanthanum-stabilized alumina powder (K/Mn-La-Al2O3) [1] serves as the functional material. It is applied by screen-printing on an alumina substrate equipped with interdigitated Au electrodes and fired at 650 °C. Different layer thicknesses of the functional films are achieved by several screen-printing steps. The sensors are mounted in a gas purgeable operando DRIFTS cell (Fig. 1) [4] and heated by an integrated platinum heating structure to 350 °C. They are exposed to synthetic air (20 % O2 and 2 % humidity in N2 balance). NOx concentrations (ppb-range) are added and validated by a chemiluminescence detector (CLD). The sensor resistance is derived by impedance spectroscopy (f = 200 Hz, U eff = 200 mV) from an R||C equivalent circuit. The sensor signal is defined as the relative resistance change (R-R 0/R 0), with R 0 being the resistance in base gas and R the resistance during NOx exposure. Simultaneous DRIFT spectra are obtained and the absorbance -log(I/I 0), with the intensity I 0 in base gas and the intensity I during NOx adsorption, is calculated. The sensors are regenerated by heating up to 650 °C. Results and Discussion An operando DRIFTS result during exposure to three peaks of 50 ppb NOx with NOx pauses in-between is shown in Fig. 2. The NOx dose in ppm∙s is derived from the integration of the CLD NOx signal. The dose increases linearly during NOx exposure and remains constant during dosing pauses. The sensor signal shows the same behavior: ΔR/R 0 increases linearly when 50 ppb NOx are present and stays constant during NOx absence. The calculated absorbance shows a clear peak at a wave number of 1370 cm-1 occurring during NOx exposure. It can be attributed to the nitrate formation on potassium sites of K/Mn-La-Al2O3 and is characteristic for the sorption phase [4]. The calculated peak area A E of this nitrate peak is shown in Fig. 2, too. A E and therefore the amount of formed nitrates increases linearly during NOx addition and remains constant during NOx pauses. The electrical properties of the dosimeter can be related directly to the NOx dose and to the formed nitrates. One important point is that all curves increase linearly during NOx exposure and no saturation effects can be observed for the sensor signal and the nitrate formation. In Fig. 3, the sensor is exposed to three peaks of 390 ppb NOx. ΔR/R 0 gets non-linear after reaching 40 %. The sensor seems to saturate. The slope during NOx exposure is non-linear and the signal in NOx pauses decreases. In contrast, A E behaves almost linear. It is assumed that the electrically observed non-linearity cannot be explained by a saturation of the adsorption capability of K/Mn-La-Al2O3. It seems that the NOx adsorption capability is not exhausted, but the resistance shows a saturation effect. Conclusions The results show that the sensor behaves like a dosimeter with linear characteristics between NOx dose, sensor signal and formed nitrate species until a certain sensor signal of around 40 % is reached. At higher relative resistance changes, the electrical signal becomes non-linear, whereas the adsorption capability itself, i.e. the nitrate formation, remains linear and the NOx adsorption on K/Mn-La-Al2O3 is not saturated. This behavior will be investigated by operando DRIFTS in dependence on the film thickness.

A Robust Mesoporous Al2 O3 -Based Nanocomposite Catalyst for Abundant NOx Storage with Rational Design of Pt and Ba Species.

The nanostructural design of heterogeneous catalysts has often been demanded for assessing synergetic effects, which should be developed further by using high-surface-area porous metal oxide supports. However, such opportunities have been undermined by the poor stability of ordered mesoporous structures. Herein, rational design is demonstrated to obtain nanocomposite catalysts showing improved NOx storage properties owing to the presence of Ba species over a well-designed mesoporous alumina (Al2 O3 ) support. It is found that Ba species are impregnated successfully only after the stabilization of the mesoporous structure by full crystallization of Al2 O3 frameworks to the γ-phase, with the formation of Pt nanoparticles coinciding with complete removal of organic components. All the insights during this synthetic procedure are essential for designing high-performance catalysts to purify and recover NOx molecules, and are applied for designing a variety of cutting-edge mesoporous nanocomposite catalysts.

Monolayer Janus Te2Se-based gas sensor to detect SO2 and NOx: a first-principles study.

In this study, the adsorption of gas molecules, such as O2, NH3, CO, CO2, H2O, NOx (x = 1, 2) and SO2, on Janus Te2Se monolayer has been investigated by means of density functional theory (DFT) calculations. We show that Janus Te2Se monolayer is preferable for SO2 and NOx molecules with suitable adsorption strength and apparent charge transfers. We further calculated the current-voltage (I-V) curves using the nonequilibrium Green's function (NEGF) method. The transport feature exhibits distinct responses with a dramatic change of I-V curves before and after NOx (SO2) adsorption on Janus Te2Se. Thus, we predict that Janus Te2Se could be a promising candidate for SO2 and NOx sensors with high selectivity and sensitivity. Moreover, the effect of strain on the gas/substrate adsorption systems was also studied, implying that the strained Janus Te2Se monolayer could enhance the sensitivity and selectivity to SO2 and NO2. The adsorbed SO2 and NO2 on Janus Te2Se could escape by releasing the applied strain, which indicates that the capture process is reversible. Our study widens the application of Janus Te2Se not only as piezoelectric materials, but also as a potential gas sensor or capturer of SO2 and NOx with high sensitivity and selectivity.

Non-catalytic NOx reduction for marine Diesel engine: an experimental approach

Recent legislation concerning air pollutant emissions from ships (Tier III) has considerably decreased NOx emission limits for recreational ships with less than 500 gross tonnages and greater than 24 m load line length (LLL), the so-called “large yachts”. For these yachts, which adopt multiple Diesel engines with maximum power ranging from 800 to 2000 kW, the new NOx limit is less than 2 g/kWh. To satisfy that limit, engine makers suggest the adoption of a classic SCR system with urea injection but this methodology, in addition to requiring a large space for installation, does not seem so appropriate for diesel engines of large yachts that normally operate at low power with exhaust temperatures often well below 300 °C, therefore not sufficient to allow the correct functioning of the SCR. The aforementioned circumstances have prompted Sanlorenzo Spa (La Spezia – Italy), one of the world’s leading companies in the production of pleasure boats, to collaborate with the University of Pisa in the study of alternative solutions to SCR systems. Recently there is a great interest in innovative NOx abatement systems, based on the possibility of absorbing NOx molecules into seawater through the utilization of appropriate aqueous solutions containing targeted saline oxidizing agents. The present study describes the first results of a research program aimed to identify, from one side, the best saline oxidizing agent for an aqueous NOx abatement and, from the other, the best washing methodology compatible with the propulsion layouts usually adopted on yachts. In the engine testing laboratories of the Department of Energy, Systems, Territory and Construction Engineering (DESTeC) of the University of Pisa, an experimental apparatus has been created to reproduce the real conditions of underwater engine exhausting. This device was combined with a small 1.2 L direct injection diesel engine. The first results showed a reduction of the NOx emissions of about 27% by using a gas washing with wet scrubber, followed by bubbling in water, using 0.2 molar NaClO saline solutions.