Nitrogen Oxides Storage Research Articles

Utilization of Reactive Nitrogen Compounds for Nitrogen Circular Economy.

Nitrogen oxides (NOx) should be purified according to environmental regulations, being restricted increasingly year by year. A wide variety of denitration technologies, such as selective catalytic reduction (SCR) of NOx to nitrogen (N2) and NOx storage reduction (NSR) to N2 by injecting reducing agents like ammonia (NH3), has so far been developed practically. Sophisticated catalytic approaches are perhaps mandatory for the sustainability in energy including complete purification of NOx. As one of the solutions to overcome problems for environment and resource simultaneously, this concept article focuses on the utilization of reactive nitrogen (Nr) compounds, mainly NOx, for encouraging an opening to consider nitrogen circular economy. For the recycling of NOx via NH3, a challenging but rational catalytic technology can be proposed by an alternate switching the inlet gas between NOx containing oxidative gas and H2 containing reductive one without an operation to change the reaction temperature. Considering the reactivity of NOx higher than that of N2, this kind of NOx to NH3 (NTA) process is promising for synthesizing NH3, being valuable not only as fertilizer but also as fuel in near future.

Dielectric Properties of Materials Used for Microwave-Based NOx Gas Dosimeters.

Nitrogen oxides (NOx), primarily generated from combustion processes, pose significant health and environmental risks. To improve the coordination of measures against excessive NOx emissions, it is necessary to effectively monitor ambient NOx concentrations, which requires the development of precise and cost-efficient detection methods. This study focuses on developing a microwave- or radio frequency (RF)-based gas dosimeter for NOx detection and addresses the optimization of the dosimeter design by examining the dielectric properties of LTCC-based (Low-Temperature Co-fired Ceramics) sensor substrates and barium-based NOx storage materials. The measurements taken utilizing the Microwave Cavity Perturbation (MCP) method revealed that these materials exhibit more pronounced changes in dielectric losses when storing NOx at elevated temperatures. Consequently, operating such a dosimeter at high temperatures (above 300 °C) is recommended to maximize the sensor signal. To evaluate their high-temperature applicability, LTCC substrates were analyzed by measuring their dielectric losses at temperatures up to 600 °C. In terms of NOx storage materials, coating barium on high-surface-area alumina resolved issues related to limited NOx adsorption in pure barium carbonate powders. Additionally, the adsorption of both NO and NO2 was enabled by the application of a platinum catalyst. The change in dielectric losses, which provides the main signal for an RF-based gas dosimeter, only depends on the stored amount of NOx and not on the specific type of nitrogen oxide. Although the change in dielectric losses increases with the temperature, the maximum storage capacity of the material decreases significantly. In addition, at temperatures above 350 °C, NOx is mostly weakly bound, so it will desorb in the absence of NOx. Therefore, in the future development of a reliable RF-based NOx dosimeter, the trade-off between the sensor signal strength and adsorption behavior must be addressed.

Electric field-assisted NSR process for lean NOx reduction at low temperatures.

Lean-burn engines are gaining attention for their lower CO2 emissions, higher thermal efficiency, and improved fuel economy compared to traditional combustion engines. However, they present some difficulty for reducing nitrogen oxides (NOx) because of residual oxygen. To address this difficulty, NOx storage reduction (NSR) system, which combines noble metals and NOx adsorbents, is developed as a viable approach. But it requires cyclic operation, which adversely affects fuel efficiency. A novel approach proposed in this work is electric field-assisted lean NOx reduction, which applies an electric field to the NSR catalyst during lean conditions. This innovation uses surplus vehicle electricity for exhaust purification, enhances hydrogen transfer, and improves NOx reduction, even at low temperatures. Tests with a 3 wt% Pt-16 wt% BaO/CeO2 catalyst demonstrate markedly higher NOx conversion to N2 (13.1% vs. 2.9% without an electric field). This process is effective with extended electric field exposure, doubling the conversion rate. Electric field-assisted lean NOx reduction, by improving NSR technology, can enhance NOx conversion efficiency, reduce emissions, and optimize fuel efficiency in lean-burn engines.

Advancements in low-temperature NH3-SCR of NOx using Ba-based catalysts: a critical review of preparation, mechanisms, and challenges.

Presently, selective catalytic reduction (SCR), with either carbon monoxide, urea, hydrocarbons, hydrogen, or ammonia as a reductant, has become a nitrogen oxide (NOx) removal technology (NOx conversion) of many catalytic companies and diesel engine exhaust gas. Although, there exists a serious threat of low-temperature limitations. So far, certain scientists have shown that barium-based (Ba-based) catalysts have the potential to be highly effective at SCR of NOx at low temperatures when ammonia is used as the reducing agent. The process of NOx storage and reduction which alternate SCR is known as the Lean NOx trap. Herein, we give the condensed advancements and production of the catalysts that involve BaO in low-temperature NH3-SCR of NOx, the advantages of BaO catalysts compared to the recently hot electrocatalysis, the stability of BaO catalyst materials, and the condensed advancements and production of the catalysts that involve BaO in low-temperature NH3-SCR of NOx. These catalysts are viewed in the light of their preparation method, particulate, and posture in mixed oxides. Also, the characteristic features of Ba-based catalysts are carefully considered and briefed under the following areas: preparation method and precursor, crystallinity, calcination temperature, morphology, acid sites, the specific surface area for reaction, redox property, and activation energy of catalysts. More to these are the discussions on Eley-Rideal [E-R] and Langmuir-Hinshelwood [L-H] mechanisms, the H2O/SO2 and O2 permissiveness, and the NH3-SCR reaction mechanism over Ba-based catalysts highlighting their possible effects. Finally, we proposed the prospect and the likely future research plan for the low-temperature NH3-SCR of NOx.

On the Suitability of NOx-Storage-Catalysts for Hydrogen Internal Combustion Engines and a Radio Frequency-Based NOx Loading Monitoring

Hydrogen combustion engines can contribute to CO2-free mobility. However, they produce NOx emissions, albeit only to an extremely small extent when operated very leanly. One approach to reduce these emissions even further is to use exhaust gas aftertreatment systems like NOx storage catalysts (NSC). So far, they have mainly been used in diesel or gasoline applications. This contribution shows that under conditions such as those prevailing in hydrogen engines, the NSC can achieve not only a higher storage capacity for nitrogen oxides (NOx) but also a higher conversion. To ensure permanently high conversion rates, the amount of stored NOx has to be monitored permanently to prevent NOx breakthroughs. Conventional NOx sensors may not be accurate enough due to the very low NOx emissions. The functionality of the radio frequency (RF) sensor, which enables a direct determination of the NOx loading, is demonstrated for operation under hydrogen conditions. Furthermore, the influence of rich exhaust gas on the RF signal, which is relevant for a correct NOx loading determination during regeneration, is analyzed.

Identification of Active Ba Species on TiO2Photocatalyst for NOxTrapping

Photocatalytic nitrogen oxides (NOx) trapping is a new approach to reduce NOx emission from automobiles. The modification of TiO2 with Ba (Ba-TiO2) improves the NOx storage capacity (NSC) of TiO2. X-ray absorption spectroscopy shows that Ba-TiO2 consists of isolated Ba species and bulky Ba(NO3)2. Diffused reflectance infrared Fourier transform spectroscopy reveals that isolated Ba species affect the NOx trapping ability on neighboring Ti sites and enhance the photocatalytic NSC on Ba-TiO2.

Density functional theory study of NOx adsorption on alkaline earth metal oxide and transition metal surfaces

Since the emissions of nitrogen oxides (NOx) from automobiles cause air pollution, NOx storage-reduction (NSR) catalyst has been used to convert the NOx into harmless components such as N2 through the reduction of NOx. In this study, to provide fundamental understanding of key elementary steps of NSR, we established an extensive database for the adsorption properties of NO and NO2 on a wide range of metal and metal oxide surfaces. Our results show that the amount of charge transfer between NOx and surface is closely related to the molecular adsorption strength of NOx, and it changes the molecular stability of NOx on the surfaces by enlarging the inner bond length of N-O. Understanding the adsorption energy of the molecules or atoms that would participate in the reaction can be important to predict the ability of NOx storage and conversion in NSR. This study provides a useful insight for designing metals or metal oxides for NSR catalyst.

Evaluation of H2 Influence on the Evolution Mechanism of NO Storage and Reduction over Pt–Ba–Ce/γ-Al2O3 Catalysts

Abstract In this investigation, Pt–Ba–Ce/γ-Al2O3 catalysts were prepared by incipient wetness impregnation and experiments were performed to evaluate the influence of H2 on the evolution mechanism of nitrogen oxides (NOx) storage and reduction (NSR). The physical and chemical properties of the Pt–Ba–Ce/γ-Al2O3 catalysts were studied using a combination of characterization techniques, which showed that PtOx, CeO2, and BaCO3, whose peaks were observed in X-ray diffraction (XRD) spectra, dispersed well on the γ-Al2O3, as shown by transmission electron microscope (TEM), and that the difference between Ce3+ and Ce4+, as detected by X-ray photoelectron spectroscopy (XPS), facilitated the migration of active oxygen over the catalyst. In the process of a complete NSR experiment, the NOx storage capability was greatly enhanced in the temperature range of 250–350 °C, and reached a maximum value of 315.3 μmol·gcat−1 at 350 °C, which was ascribed to the increase in NO2 yield. In a lean and rich cycling experiment, the results showed that NOx storage efficiency and conversion were increased when the time of H2 exposure (i.e., 30, 45, and 60 s) was extended. The maximum NOx conversion of the catalyst reached 83.5% when the duration of the lean and rich phases was 240 and 60 s, respectively. The results revealed that increasing the content of H2 by an appropriate amount was favorable to the NSR mechanism due to increased decomposition of nitrate or nitrite, and the refreshing of trapping sites for the next cycle of NSR.

NOx Storage and Reduction Coupled with Selective Catalytic Reduction for NOx Removal in Light‐Duty Vehicles

AbstractThere are two alternative, main technologies for removal of nitrogen oxides (NOx) from vehicle exhaust gases, namely, selective catalytic reduction (SCR) and NOx storage and reduction (NSR). The SCR technology consists of using ammonia (NH3), produced by hydrolysis of urea that is stored into an on‐board tank in the vehicle, to reduce NOx selectively to nitrogen (N2). In the NSR strategy, the conversion of NOx into N2 occurs through a two‐step cyclic operation. During the fuel‐lean stage, the NOx is trapped on the catalyst; then, the engine is switched to a fuel‐rich condition, under which NOx is released and reduced preferentially to N2, although some NH3 can also be produced. The concept of coupling NSR with SCR is based on tuning the operation of the NSR catalyst to produce a controlled amount of NH3, which is stored on the SCR catalyst during the fuel‐rich stage and used to reduce the remaining NOx on the SCR catalyst, placed downstream of the NSR catalyst in a sequential NSR+SCR configuration. Alternatively, the same concept applies to the dual‐layer architecture, which comprises a SCR layer deposited on top of the NSR layer in a single monolith. The internal generation of NH3 in the NSR catalyst avoids the need of external supply from the on‐board tank, but also achieves impressive NOx‐to‐N2 efficiency, apparently allowing zero‐level pollutant emission.

Evaluation of the real-time de-NOx performance characteristics of a LNT-equipped Euro-6 diesel passenger car with various vehicle emissions certification cycles

Advanced nitrogen oxides (NOx) after-treatment systems for diesel cars are effectively reducing tailpipe NOx emissions in laboratory test cycles; however, some de-NOx systems showed limitation for passing environmental standards during real-world driving conditions. In this study, the NOx concentration of a lean NOx trap (LNT)-equipped diesel engine were investigated over various vehicle certification cycles. The LNT performance was compared to the new European driving cycle (NEDC), world-harmonized light-duty vehicle test cycle (WLTC), federal test procedure (FTP)-75, highway fuel economy test (HWFET), and US06. The real-time NOx concentration behaviors were tracked using NOx sensors at the engine-out and downstream of the LNT to determine the NOx storage and regeneration phase. The NOx conversion efficiencies were 36.3–71.7% of which reflecting the mode severity and cycle duration of the diesel engine. The tailpipe NOx emissions were 0.059 g/km during the NEDC which was within the Euro-6 emissions regulations. The NOx emissions in the WLTC, FTP-75, and US06 modes were approximately 1.9, 1.5, and 6.6 times higher than the NEDC due to the higher frequency of LNT purge and higher engine-out NOx formation. During the diesel particulate filter (DPF) regenerating stage in WLTC mode, tailpipe NOx emissions substantially increased by more than 8.8-fold. The exhaust gas recirculation (EGR) supply and lambda control scheme were closely related with strong NOx increment at de-NOx and de-PM processes.

Electrochemical Reduction of NO2

The use of diesel engines is becoming more widespread because their fuel economy is superior to and their emission levels of carbon monoxide and carbon dioxide are considerably lower than those of gasoline engines. However, the high-temperature combustion in diesel engines generates significant amounts of nitrogen oxides (NOx), which have harmful effects on the environment and human beings, and are limited by increasingly stringent government regulations worldwide. The O2-rich environment of diesel engine exhaust will deactivate the traditional three-way catalysts that work effectively in gasoline engine exhaust. Therefore, there is a great demand for new technology to control NOx emissions in diesel engine exhaust. The most extensively researched technologies in this area are currently selective catalytic reduction with ammonia (NH3-SCR) and NOx storage and reduction catalysts (NSR), both of which require a reducing agent, either from a secondary supply system or by switching the operation state of the engine between lean and rich conditions. One attractive alternative to these approaches is electrochemical NOx reduction using a solid state cell. Using this approach, NOxis reduced to nitrogen at the polarized cathode, thereby eliminating the need for the addition of reducing agents or changes in the operational state of the engine. At present, the main obstacle to the practical application of this technology is the achievement of high selectivity for NOx reduction in the presence of excess O2. Because competitive O2 reduction consumes substantial amount of current, the current efficiency or selectivity of the cell towards NOxreduction is genetally a few percent relative to the total current supplied to the cell. Extensive research effort has been put on finding suitable cathode materials or optimizing the cell structure to increase its selectivity. However, the highest current efficiency reported in the literature was below 20% in an oxidizing atmosphere using a multilayered electrochemical cell with Pt cathode [1]. In this study, we proposed a novel concept for the electochemical NOx reduction, which significantly improve the selectivity towards NOx reduction. According to our previous research, it is found that the electrochemical reduction of NOx is probably limited by the formation of intermediate NO2. Besides, NO2 is also reported to be an necessary precursor for the NOx trapping process over the NOx aborbents added in the electrochemical cell. NO is usually the dominant species of NOx (90-95%) in exhaust gases, but can be converted to NO2 efficiently by the catalysis of DOC. As the high efficiency of DOC for NO converstion to NO2 has already been well demonstrated, in this study, we focus on the reduction of NO2 on the electrochemical cell. We developed and characterised two typed of cells: one is a modify mulilayered cells with Pt and Ni/YSZ cathode with a K-Pt-Al2O3 adsorption layer, the other is a LSM/CGO symmetric cell infiltrated with BaO. We compare the NOx removal properties of the cells in the atmosphere of mainly NO2 with O2 with that in mainly NO with O2 and investigated the effect of temperature, voltage, and polarization method on the performance of cells. Under O2-rich conditions, a current efficiency as high as 50-65% is achieved with a NOx converstion of 50-70% on a modified multilayer cell with Pt and a 10% current efficiency with more than 30% NOx converstion is realized on a fully ceramic cell free of noble metal. The contribution will briefly outline, how the concept can be further implemented in automotive exhaust cleaning systems. [1] K. Hamamoto, Y. Fujishiro, M. Awano, J. Electrochem. Soc., 155 E109 (2008)

Surface Ba species effective for photoassisted NOx storage over Ba-modified TiO2 photocatalysts

Surface modification of a TiO2 photocatalyst with Ba species was investigated in photoassisted nitrogen oxide (NOx) storage under UV-light irradiation. The NOx storage capacity in the Ba-modified TiO2 photocatalyst was 1.4 times higher than that of the non-modified TiO2 photocatalyst. Structure and role of the surface Ba species on a TiO2 surface were characterized using temperature programmed reaction (TPR), diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), and transmission electron microscopy (TEM). The characterization results suggested that Ba–Ti mixed oxides with a two-dimensional layer and amorphous structure on a TiO2 surface were generated via decomposition of Ba(NO3)2 precursors by O2 pretreatment at 773K. Based on the results of the characterizations and the reactions, the generated Ba–Ti mixed oxides on a TiO2 surface stored NOx more densely than the TiO2 surface; therefore, the generation improved the activity in the photoassisted NOx storage under UV-light irradiation.

Noble-Metal-Free NOx Storage over Ba-Modified TiO2 Photocatalysts under UV-Light Irradiation at Low Temperatures

Photoassisted nitrogen oxide (NOx) storage was investigated over the barium-modified titanium dioxide (Ba/TiO2) photocatalysts under UV-light irradiation at a high concentration of nitrogen monoxide (NO) of 200 ppm and a high gas hourly space velocity (GHSV) of 50 000 h–1. The NO removal efficiency was kept >99% during the 1020 s interval from the start of the reaction over the Ba/TiO2 photocatalyst. The temperature-programed desorption (TPD) measurement and X-ray diffraction (XRD) technique revealed that barium oxide (BaO) species functioned as a NOx storage material even at low temperatures, which result in improvement of the performance of the photoassisted NOx storage.

The Effect of Hydrogen on the Storage of NOx Over Silver, Platinum and Barium Containing NSR Catalysts

Addition of hydrogen significantly increases the amount of stored nitrogen oxides (NOx) over Pt/Ba/Al2O3 at 100 A degrees C and Ag/Al2O3 between 100 and 200 A degrees C. This increase is unique for hydrogen as reductant. A higher apparent NOx storage capacity is also observed by simultaneous reduction of NO in a selective catalytic reduction (SCR) reaction, where, however, the conversion in H-2-SCR is < 10 % in this study.

Multi-Scale Modeling Study of Barium Nitrate Reduction in NOx Traps

Nitrogen oxide (NOx) abatement from lean-burning engines is a major environmental challenge for the upcoming years. With increasing regulations in terms of NOx emission, only sophisticated engine aftertreatment techniques can meet the required emissions. Lean NOx Traps (LNT) are one of those few catalytic techniques that are potential candidates for a large-scale application. Their operation is based on alternating NOx storage cycles over a basic storage material with nitrate reducing cycles with an external reductant (CO, H2 and hydrocarbons). Because of the complex chemistry involved, modeling reactions taking place during storage-reduction cycling is still poorly understood. We address one of the key issues in kinetic modeling related to LNT regeneration that is the competition between direct and indirect reduction pathways. Recent experimental work has evidenced the formation of barium isocyanates but no relevant kinetic modeling has been done up to now. The present modelling study, based on multi-scale approaches (from DFT to kinetic modelling), shows that both direct (isocyanate) and indirect (decomposition) pathways may contribute significantly to nitrate reduction.

Global distribution of peroxyacetyl nitrate.

Nitrogen oxides (NOx) have a central role in the chemistry of the atmosphere, especially in key processes relating to ozone, hydroxyl-radical (OH) and acid formation. High reactivity of NOx (lifetime of 0.5-2 days) precludes hemispheric-scale transport and it has been proposed that non-methane hydrocarbons present in the troposphere can transform NOx into its organic forms principally as peroxyacetyl nitrate (PAN). PAN is highly stable in the colder regions of the middle and upper troposphere and can provide a mechanism for NOx storage and transport. Once transported, PAN and its homologues can easily release free NOx in warmer atmospheric conditions. PAN is probably ubiquitous and its concentrations could exceed those of NOx in clean tropospheric conditions. Here we present the first view of the global distribution of PAN based on extensive shipboard and aircraft measurements. PAN is more abundant in the Northern than in the Southern Hemisphere and in the continental than in the marine troposphere. In contrast to its behaviour in polluted atmospheres, PAN mixing ratios in winter greatly exceed those in summer. These measurements provide a basis for assessing the significance of PAN as a reservoir of NOx and for extending and validating reactive nitrogen chemistry theory in the troposphere.