Nitrogen Oxide Abatement Research Articles

Innovative In/H-SSZ-39 Catalysts: An Exploration in NOx Reduction via CH4-SCR

Nitrogen oxides (NOx), pivotal atmospheric pollutants, significantly threaten the environment and human health. The CH4-SCR process, leveraging the abundance and accessibility have methane, emerges as a promising avenue for NOx abatement. Previous studies have demonstrated that zeolite support with twelve-membered ring (12-MR) and five-membered ring (5-MR) structures are susceptible to framework collapse in the presence of H2O, leading to catalyst deactivation. Consequently, there is a necessity to explore novel zeolites with enhanced hydrothermal stability for application in CH4-SCR processes. This research introduced for the first time an investigation into a novel In/H-SSZ-39 catalyst, which was synthesized via ion exchange and meticulously optimized for preparation conditions, including calcination temperature and In ions concentration, and reaction conditions, including CH4/NO ratio, O2 concentration, H2O content, and Gas Hourly Space Velocity (GHSV). Furthermore, long-term operation tests and stability tests were conducted on the In/H-SSZ-39 catalyst. In addition, a series of characterizations were conducted to delve into the reasons behind how preparation conditions influence catalytic activity, as well as to investigate the changes in physicochemical properties during the reaction process.

Field deployable laser sensor for calibration-free and real-time measurement of NOx from stationary sources

Nitrogen oxides (NOx) are common pollutants emitted from stationary sources, such as industrial boilers and power plants, with significant adverse impacts on the environment and biosphere. The abatement of NOx from anthropic industrial processes requires reliable and sensitive monitoring systems. To address this challenge, we report a sensitive, robust, and compact quantum cascade laser (QCL)-based sensor to measure NO and NO2 emissions from industrial exhausts. This sensor uses two quantum cascade lasers, which take advantage of the intense absorption spectra of NO at 1929 cm−1 and NO2 at 1599 cm−1. A high-temperature multipass cell is designed to withstand harsh environments, which increases the light path length. The NOx concentrations are directly determined from the wavelength modulation spectroscopic model without the need for calibration. We conducted several laboratory experiments to evaluate the sensor's performance, with a focus on accuracy, sensitivity and selectivity. In the end, we showcased the field deployment of our sensor by continuously measuring NOx concentrations in a 2.8-MW gas-fired industrial boiler.

NOx Abatement by a TiO2-Based Coating under Real-Life Conditions and Laboratory-Scale Durability Assessment

In urban environments, various pollutants generated by road traffic, human, and industrial activities degrade outdoor and indoor air quality. Among these pollutants, nitrogen oxides (NOx) are subject to air quality regulations designed to protect human health and the environment. It is therefore crucial to keep their concentration as low as possible. Advanced oxidation processes are a practical choice for the degradation of NOx; among them, heterogeneous photocatalysis has proven to be a viable route. However, while the efficiency of this process has been widely demonstrated on a laboratory scale, it is still the subject of debate for real-life applications. The purpose of this study was to present a new field experiment on the application of a photocatalytic coating to outdoor walls. Air quality monitoring stations were used to evaluate the NOx concentration reduction instead of the chemiluminescent analyzer, in order to increase the number of sampling points. Statistical analysis was carried out to interpret the results. Density probability functions were plotted and showed a positive impact of the coating, leading to lower NOx concentrations. This work was completed by a laboratory-scale assessment of the coating’s durability using abrasion, QUV, and immersion/drying tests. The air depollution capacity of the chosen coating was significantly reduced after QUV testing.

Atomically dispersed magnesium enhancing reactive oxygen species generation over g-C3N4 nanosheets for efficient photocatalytic NO removal

Photoinduced reactive oxygen species (ROSs)-involving nitrogen oxide (NOx) abatement offers a green way to mitigate environmental air pollution. Developing cost-effective photocatalysts remains challenging yet crucial. Herein, an atomically dispersed Mg-modified g-C3N4 nanosheet photocatalyst (xMg-CN) with excellent photoactivity is developed. The 1.0Mg-CN sample delivers a high NO removal rate of 52.3% under visible light irradiation within 30 min, far surpassing the Mg-free analog (CN, 41.2%). Moreover, a suppressed NO2 by-product discharge can be achieved. The introduction of Mg single atoms (SAs) alters the charge density distribution on the triazine ring plane of g-C3N4, inducing the formation of a built-in electric field, which improves the photoexcited charge carrier separation. Additionally, Mg SAs contribute to the adsorption and activation of O2 molecules, resulting in enhanced ROSs production, as evidenced by the experiments and theoretical simulations. The findings shed light on the role of SAs in designing highly efficient g-C3N4-based photocatalysts for air purification.

Efficient visible-light-driven photocatalytic performance for nitrogen oxide abatement application in the air by a typical metal/semiconductor heterojunction

To improve the photocatalytic oxidation activity of SnO2 nanorods, we decorated Ag nanoparticles onto SnO2 nanorods via a photoreduction route at low temperature. The metal/semiconductor heterojunction based on Ag/SnO2 nanorods indicated a high photocatalytic activity towards NO gas removal in visible light region at the air conditions. Detailly, the NO photocatalytic removal performance of Ag/SnO2 heterojunctions are significantly enhanced, achieving 50.6 % after 30 min, approximately 1.9 times higher than that of SnO2 nanorods. In addition, the Ag/SnO2 heterojunctions have a lower NO2 gas conversion efficiency of 1.0 %, while pure SnO2 nanorods have a high NO2 conversion efficiency of 5.2 %. Moreover, photogenerated holes play a crucial role in the photocatalytic activity of NO gas removal.

Improving catalytic performance of Cu-SSZ-13 for NOx abatement via in-situ introduction of La and Ce from spent catalyst

Designing a cost-effective and fast fabrication Cu-SSZ-13 with improved selective catalytic reduction of NO by ammonia (NH3-SCR) performance remains an open question. Here we report that LaCe-SSZ-13 zeolites were synthesized from spent fluid catalytic cracking catalyst within 12 h. After ion-exchanging with CuSO4 solution, the obtained LaCeCu-SSZ-13 catalysts exhibit significantly higher catalytic activity and hydrothermal stability than conventional Cu-SSZ-13. Various characterizations reveal that La and Ce modification not only facilitates the disperse of isolated Cu2+ species, but also effectively mitigates dealumination and inhibits copper species aggregation after hydrothermal aging. In-situ diffuse reflection infrared Fourier transform spectrum analyses demonstrate that La and Ce modification can accelerate the adsorption capacity for NH3 and NOx and promote the decomposition of NH4NO3 intermediate species on the catalyst, benefiting the improvement of low-temperature activity. The combination of experiments and theoretical calculations further demonstrate that the most stable positions of La and Ce are on different eight-membered rings in SSZ-13, and then Cu2+ ions tend to enter eight-membered rings to enhance the NH3 adsorption capacity and decrease the H2O adsorption capacity, which results in higher low-temperature activity and hydrothermal stability than Cu-SSZ-13. Our work provides a feasible strategy for preparing the efficient rare-earth-modified Cu-zeolite based NH3-SCR catalyst for nitrogen oxide abatement.

New approach into NOx removal from flue gas by carbohydrazide

Selective non-catalytic reduction (SNCR) is a proven technique to reduce nitrogen oxide (NOx) emissions from stationary sources. The process utilizes NH3 or urea as a reducing agent, which is only effective in a narrow and high temperature range with the maximum NOx abatement falling with the increase of the O2 content. In large industrial boilers, the effectiveness of the deNOx process may be decreased with boiler load variations and excess O2. This study investigated carbohydrazide as a reagent to improve SNCR performance. NO reduction by carbohydrazide was experimentally investigated in a pilot-scale flow reactor. The effect of reaction temperature, molar ratio of NH2 to NO (normalized stoichiometric ratio, NSR) on the NO reduction was determined. Moreover, the promotion effect of carbohydrazide on the reactivity of NH3 and urea was also investigated. The experimental results indicated that NO reduction by carbohydrazide occurred in a relatively low temperature range of 650–850 °C, and the optimum reaction temperature was about 730 °C at NSR of 2.0, the O2 content of 11.5%–16.6% and the initial NOx concentration of 635 mg/m3. After adding a small amount of carbohydrazide to an NH3 or urea solution, the data showed that carbohydrazide could shift the temperature window of NH3 to the left, but had no positive effect on urea deNOx and the NO conversion efficiencies of NH3 deNOx. Consequently, carbohydrazide is a potential reducing agent for NOx, but was not adapted to improve the deNOx performance with NH3 and urea.

Design of bimetallic catalyst with dual-functional Cu-Ce sites for synergistic NOX and toluene abatement

Nitrogen oxides (NOx) and volatile organic compounds (VOCs) coexist in the flue gases from coal-fired power plants and iron/steel smelting industry. In this study, a dual-site catalyst, composed of Cu-Ce-Ti mixed oxide, was screened for the synergistic elimination of NOx and VOCs. The catalyst showed a synergistic removal efficiency above 95% for both C7H8 and NO at 250 °C, and sustain the NOx and VOCs removal efficiencies above 90% even in the presence of 5 vol% H2O and 500 ppm SO2 at 300 °C. Mechanistic studies revealed that the Cu and Ce sites play individual roles in toluene oxidation and NOx reduction, respectively. The primarily occurred SCR reaction facilitated electron exchange through the redox cycling between Ce4+/Ce3+ and Cu2+/Cu+ pairs, resulting in a high abundance of Cu2+ sites being highly active in toluene oxidation. This study could provide new strategy for high-performance catalysts toward synergistic NOx and VOCs abatement.

Promoted NH3-SCR activity and hydrothermal stability of Cu-SSZ-50 catalyst synthesized by one-pot method

Nitrogen oxide (NOx) is one of the most critical contaminants in the air, and the control of NOx emission from diesel vehicles is very important. Cu-based small-pore zeolites have already been applied for NOx abatement on diesel vehicles. Among the small-pore zeolites, Cu-SSZ-50 catalysts with good NH3-SCR catalytic activity were believed to have potential for application. In this study, a one-pot synthesis method for Cu-SSZ-50 catalysts was developed for the first time, using the co-templates of Cu-TEPA and 2,6-dimethyl-N-methylpyridinium hydroxide. In this synthesis method, Cu-SSZ-50 with various Cu contents can be obtained by adjusting the amount of Cu-TEPA without the need for a further after-treatment process. The addition of Cu-TEPA affected the framework atoms and Cu species, and a lower Si/Al ratio and more SCR active Cu species were obtained. The synthesized catalyst with a Cu/Al ratio of 0.40 exhibited over 90% NOx conversion between 200 °C and 450 °C for the selective catalytic reduction of NOx with NH3 (NH3-SCR). Meanwhile, over 80% NOx conversion could be obtained from 250 °C to 450 °C after hydrothermal aging at 750 °C for 16 h. In addition, both L-H and E-R mechanisms were proven to exist for the one-pot-synthesized Cu-SSZ-50 by in situ DRIFTS experiments. The simple synthesis procedure, excellent catalytic activity and hydrothermal stability brighten the prospects for the application of Cu-SSZ-50.

Recent progress in novel zeolite catalysts for selective catalytic reduction of nitrogen oxides

Selective catalytic reduction (SCR) using zeolite catalysts is a key technology for the abatement of nitrogen oxides (NOx) from lean-burn heavy-duty vehicles. With the increasing stringency of emission regulations, researchers are now devoting more efforts to enhancing the efficiency and durability of zeolite catalysts. This paper intends to summarize the very recent (mainly in the last five years) advancements in the development and application of novel zeolites as SCR catalysts. Specifically, we first provide a detailed overview of the synthesis, structure-performance relationship, and potential application of small-pore zeolites with different topologies (CHA, AEI, LTA, AFX, SFW, KFI, ERI/EAB, LEV and RHO) as catalysts in SCR with NH3 as the reductant. Then, advances in the post-synthetic modifications of small-pore zeolites for NH3-SCR are reviewed, followed by an introduction of Cu-free small-pore zeolites as NH3-SCR catalysts. We also summarize the recent mechanistic understandings of NH3-SCR chemistry over small-pore zeolite catalysts, mainly over Cu-SSZ-13, using in situ or operando spectroscopy. The application of zeolites as catalysts in SCR with hydrocarbons (e.g., CH4, CH3OH) or H2 as the reductant is also briefly discussed. Finally, we discuss the potential of several novel small-pore zeolites, such as Cu-SSZ-39, for real application in NH3-SCR and the development of new SCR systems with minimized formation of byproducts, such as the greenhouse gas N2O and the toxic HCN.

Deactivation of Cu SCR catalysts based on small-pore SSZ-13 zeolites: A review

Increasingly stringent regulation towards nitrogen oxides (NOx) emission is foreseen across the world to mitigate their detrimental effects on environment and human health. Selective catalytic reduction (SCR) of NOx with NH3 is regarded as the most efficient technology for NOx abatement in both mobile and stationary sources. As the state-of-art SCR catalyst, transition metals supported on small-pore zeolites, especially Cu on chabazite SSZ-13 (Cu-SSZ-13), have been successfully applied to NOx abatement in road transport. Their application can be expanded to stationary sources due to the gradual phase-out of petrol/diesel vehicles worldwide between 2030 and 2035. Accordingly, the stability and activity of such catalysts need to be revisited and tuned to accommodate the complex operating conditions in various industries such as wider temperature windows and the more complex compositions of the exhausts. This review focuses on the poisoning and deactivation of zeolite-based SCR catalysts, especially Cu-SSZ-13. State-of-the-art of different mechanisms of deactivation (including hydrothermal ageing, sulphur poisoning, hydrocarbon poisoning and chemical poisoning) are discussed critically, attempting to establish the link between current zeolite-based SCR catalysts and the possible application demands in industries.

High-resolution regional emission inventory contributes to the evaluation of policy effectiveness: a case study in Jiangsu Province, China

Abstract. China has been conducting a series of actions on air quality improvement for the past decades, and air pollutant emissions have been changing swiftly across the country. Provinces are an important administrative unit for air quality management in China; thus a reliable provincial-level emission inventory for multiple years is essential for detecting the varying sources of pollution and evaluating the effectiveness of emission controls. In this study, we selected Jiangsu, one of the most developed provinces in China, and developed a high-resolution emission inventory of nine species for 2015–2019, with improved methodologies for different emission sectors, best available facility-level information on individual sources, and real-world emission measurements. Resulting from implementation of strict emission control measures, the anthropogenic emissions were estimated to have declined 53 %, 20 %, 7 %, 2 %, 10 %, 21 %, 16 %, 6 %, and 18 % for sulfur dioxide (SO2), nitrogen oxides (NOx), carbon monoxide (CO), non-methane volatile organic compounds (NMVOCs), ammonia (NH3), inhalable particulate matter (PM10), fine particulate matter (PM2.5), black carbon (BC), and organic carbon (OC) from 2015 to 2019, respectively. Larger abatement of SO2, NOx, and PM2.5 emissions was detected for the more developed region of southern Jiangsu. During the period from 2016 to 2019, the ratio of biogenic volatile organic compounds (BVOCs) to anthropogenic volatile organic compounds (AVOCs) exceeded 50 % in the month of July, indicating the importance of biogenic sources for summer O3 formation. Our estimates in annual emissions of NOx, NMVOCs, and NH3 were generally smaller than the national emission inventory, MEIC (the Multi-resolution Emission Inventory for China), but larger for primary particles. The discrepancies between studies resulted mainly from different methods of emission estimation (e.g., the procedure-based approach for AVOC emissions from key industries used in this work) and inconsistent information of emission source operation (e.g., the penetration and removal efficiencies of air pollution control devices). Regarding the different periods, more reduction of SO2 emissions was found between 2015 and 2017 and of NOx, AVOCs, and PM2.5 between 2017 and 2019. Among the selected 13 major measures, the ultra-low-emission retrofit in the power sector was the most important contributor to the reduced SO2 and NOx emissions (accounting for 38 % and 43 % of the emission abatement, respectively) for 2015–2017, but its effect became very limited afterwards as the retrofit had been commonly completed by 2017. Instead, extensive management of coal-fired boilers and the upgrade and renovation of non-electrical industry were the most important measures for 2017–2019, accounting collectively for 61 %, 49 %, and 57 % reduction of SO2, NOx, and PM2.5, respectively. Controls on key industrial sectors were the most effective for AVOC reduction in the two periods, while measures relating to other sources (transportation and solvent replacement) have become more important in recent years. Our provincial emission inventory was demonstrated to support high-resolution air quality modeling for multiple years. Through scenario setting and modeling, worsened meteorological conditions were found from 2015 to 2019 for PM2.5 and O3 pollution alleviation. However, the efforts on emission controls were identified to largely overcome the negative influence of meteorological variation. The changed anthropogenic emissions were estimated to contribute 4.3 and 5.5 µg m−3 of PM2.5 concentration reduction for 2015–2017 and 2017–2019, respectively. While O3 was elevated by 4.9 µg m−3 for 2015–2017, the changing emissions led to 3.1 µg m−3 of reduction for 2017–2019, partly (not fully though) offsetting the meteorology-driven growth. The analysis justified the validity of local emission control efforts on air quality improvement and provided a scientific basis to formulate air pollution prevention and control policies for other developed regions in China and worldwide.

Promotional role of Co on Cu-SAPO-34 towards enhanced denitration performance and SO2 tolerance

It is well known that Cu-SAPO-34 catalyst has high activity for nitrogen oxides (NOx) abatement from diesel vehicle emission. However, its low sulfur resistance and hydrothermal stability still retard its reality application. Herein, a series of novel binary CuCo-SAPO-34 catalysts were designed and prepared through a simple one-pot method and applied for NOx selective catalytic reduction with ammonia. Compared with Cu-SAPO-34, the binary CuCo-SAPO-34 catalysts exhibited enhanced activity and SO2 tolerance. To investigate the effects of Co addition and the reaction mechanism, various characterizations including X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX) mapping, physical and chemical adsorption, the in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTs), etc., were adopted. The results reveal that the Co3O4 nanoclusters existing in CuCo-SAPO-34 could efficiently accelerate the oxidation of NO to NO2, and the addition of Co is conducive to improving its sulfur resistance. In addition, CuCo-SAPO-34 also displayed superior high- and low-temperature hydrothermal stability. The binary CuCo-SAPO-34 developed in this work may be a good candidate for Cu-SAPO-34 based catalysts in practical diesel vehicle emission control.

Experimentation and numerical modeling of SCR spray droplets pre and post impingement on a mixer plate

Selective Catalytic Reduction (SCR) systems are one of the leading techniques used in the after treatment systems of the diesel engines for the abatement of nitrogen oxides (NOx) emissions. Ammonia is used as reducing agent in the catalyst which is supplied in the mixing chamber in the form of Urea Water Solution (UWS - 32.5% urea solution) spray. After evaporation of water, urea droplets undergo a series of secondary reactions eventually leading to the formation of solid deposits on the walls of the system. This not only hinders the deNOx efficiency but also interferes with the overall after treatment system by creating back pressure. Therefore, a deep understanding of the factors and mechanisms of spray–wall interaction is a basic requirement to prevent any undesired decomposition phenomena that could lead to failure of the whole system.In the present paper, droplet diameter distributions are analyzed pre and post impingement events on a hot mixer plate surface, both experimentally and numerically. The work was performed using a commercial six-hole pressure driven injector in a test section of heated cross flow wind tunnel. Phase Doppler Anemometry (PDA) was used to measure the droplet kinetics prior and after the wall impingement on the mixer plate. Based on the data, numerical simulation was setup using a commercial CFD code, AVL FIRE and droplet characteristics as well as thermal response of the mixer plate analyzed against the experimental data.

Research Progress on Sulfur Deactivation and Regeneration over Cu-CHA Zeolite Catalyst

Benefiting from the exceptional selective catalytic reduction of NOx with ammonia (NH3-SCR) activity, excellent N2 selectivity, and superior hydrothermal durability, the Cu2+-exchanged zeolite catalyst with a chabazite structure (Cu-CHA) has been considered the predominant SCR catalyst in nitrogen oxide (NOx) abatement. However, sulfur poisoning remains one of the most significant deterrents to the catalyst in real applications. This review summarizes the NH3-SCR reaction mechanism on Cu-CHA, including the active sites and the nature of hydrothermal aging resistance. On the basis of the NH3-SCR reaction mechanism, the review gives a comprehensive summary of sulfate species, sulfate loading, emitted gaseous composition, and the impact of exposure temperature/time on Cu-CHA. The nature of the regeneration of sulfated catalysts is also covered in this review. The review gives a valuable summary of new insights into the matching between the design of NH3-SCR activity and sulfur resistance, highlighting the opportunities and challenges presented by Cu-CHA. Guidance for future sulfur poisoning diagnosis, effective regeneration strategies, and a design for an efficient catalyst for the aftertreatment system (ATS) are proposed to minimize the deterioration of NOx abatement in the future. Finally, we call for more attention to be paid to the effects of PO43- and metal co-cations with sulfur in the ATS.

Photocatalytic NOx abatement: The effect of high air flow velocity

In the literature, the abatement of nitrogen oxide (NOx) emissions by photocatalysis is rarely carried out at real-world air flow velocities. Here, we investigated the performance of two commercial photocatalysts, Aeroxide® TiO 2 P25 (Evonik Industries, Germany) and FN NANO®2 (Advanced Materials-JTJ, Czech Republic), P25 containing 88 % anatase and 12 % rutile, while FN 2 moreover 13 % of a binder. The degradation of NOx pollutants (0.1 and 1.0 ppmv) at air flow velocities ranging from 0.02 to 0.7 m s−1 was tested, the photocatalytic efficiency being determined for various slit heights (5–25 mm) and rate of volume flow (1500–11 000 cm 3 min−1). The photocatalysts achieved substantial NO and NO2 abatement. Pollutant conversion decreased as the air flow velocity increased, with the highest conversion (80%) occurring at 0.1 m s−1. The NOx conversions were slightly higher for NO than for NO2, and significantly higher for the NO concentration of 0.1 ppmv. Slit height had a negligible effect, indicating a substantial degree of mixing in the direction perpendicular to the flow; consequently, the flow cannot be laminar in nature as the ISO standard (22197-1:2016) states. This finding is supported by the nanoindentation technique showing that the surface roughness contributed to the formation of vortexes and enhancement of the mass transport. To our best knowledge, this is the first study to test commercial photocatalysts under such a wide range of air velocities and, in doing so, it has identified considerable implications for outdoor air purification.

Attributing Increases in Ozone to Accelerated Oxidation of Volatile Organic Compounds at Reduced Nitrogen Oxides Concentrations.

Surface ozone (O3) is an important secondary pollutant affecting climate change and air quality in the atmosphere. Observations during the COVID-19 lockdown in urban China show that the co-abatement of nitrogen oxides (NOx) and volatile organic compounds (VOCs) caused winter ground-level O3 increases, but the chemical mechanisms involved are unclear. Here we report field observations in the Shanghai lockdown that reveals increasing photochemical formation of O3 from VOC oxidation with decreasing NOx. Analyses of the VOC profiles and NO/NO2 indicate that the O3 increases by the NOx reduction counteracted the O3 decreases through the VOC emission reduction in the VOC-limited region, and this may have been the main mechanism for this net O3 increase. The mechanism may have involved accelerated OH-HO2-RO2 radical cycling. The NOx reductions for increasing O3 production could explain why O3 increased from 2014 to 2020 in response to NOx emission reduction even as VOC emissions have essentially remained unchanged. Model simulations suggest that aggressive VOC abatement, particularly for alkenes and aromatics, should help reverse the long-term O3 increase under current NOx abatement conditions.

Effect of substrate roughness on NOx removal of poly heptazine imides coated cement pastes exposed to washing and weathering

Developing photocatalytic coatings onto cementitious materials is an attractive approach to combat air pollution. However, the low photocatalysis durability of the photocatalytic cementitious materials is the main challenge for their large-scale implementation in aggressive outdoor environments. Here, a coating composed of poly heptazine imides (PHI) was applied onto hardened cement pastes with different substrate roughness for nitrogen oxides (NOx) abatement under artificial washing and natural weathering. The apparent appearance, micromorphology and chemical composition of the PHI-coated cement pastes were characterized via visual inspection, scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy and attenuated total reflectance–Fourier transform infrared analysis. It was found that increasing the substrate roughness improved the adhesion of PHI coating against the washing test, contributing to a higher photocatalytic efficiency of NOx removal at highly rough substrates after subjecting to the washing test. After the weathering tests, the NOx removal of PHI-coated cement pastes increased and then decreased slightly with the increasing substrate roughness; the highest photocatalytic efficiency was observed at a substrate roughness of 2.83 μm. The observations open a window for improving the durability of photocatalytic coatings applied onto cementitious materials under real-life conditions.

MnOx catalysts with different morphologies for low temperature synergistic removal of NOx and toluene: Structure–activity relationship and mutual inhibitory effects

The simultaneous catalytic removal of nitrogen oxides (NOx) and toluene, in an existing selective catalytic reduction (SCR) unit, has drawn much attention. Exploring the interaction effects between the catalytic oxidation of toluene and the reduction of NOx is crucial to ensure the successful design and development of catalysts for their simultaneous removal. MnOx nanorod and MnOx nanocuboid were synthesized to compare their catalytic performances for the simultaneous removal of NOx and toluene. The MnOx nanorod catalyst possessed greater catalytic activity than the MnOx nanocuboid catalyst and exhibited a greater than 85% catalytic activity in the simultaneous abatement of NOx and toluene at 275 °C. The MnOx nanorod catalyst had a higher proportion of Mn4+ and a better redox ability compared with the MnOx nanocuboid, resulting in an excellent catalytic activity. Thus, it was selected as a model catalyst to investigate the interaction effects. Compared with the separate NH3-SCR and toluene oxidation reactions, there was a mutual inhibitory effect between the SCR reaction and toluene oxidation in the simultaneous removal process over the model catalyst (MnOx nanorod). This mutual inhibitory effect was investigated systematically using TPD, GC-MS and in situ DRIFTs. Mutual inhibitory effect over the model catalyst mainly manifested as follows: 1) the formation of intermediate products in the oxidation process of toluene covered the active sites; 2) the competitive adsorption of NH3 and toluene occurred; and 3) the SCR reaction and toluene oxidation process competed for reactive oxygen species.

Political differences in air pollution abatement under the Clean Air Act

In this paper, we study how local politics affect state level implementation of a critical federal environmental policy, the Clean Air Act, in the electricity generation sector. The analysis focuses on the installation of capital intensive air pollution abatement technology in highly regulated “nonattainment” areas, which violate federal air quality standards. The primary research design uses a regression discontinuity in the vote share for Republican governors and event study analyses of close elections. In nonattainment areas, Republican gubernatorial control differentially decreases new air pollution abatement capital spending by 90% and the probability of installing the most effective nitrogen oxide abatement technology by nine percentage points, relative to attainment areas and the counterfactual of Democratic gubernatorial control. The health benefits from reduced nitrogen oxide emissions in nonattainment areas likely exceed the additional costs of new air pollution abatement technology at electric utilities. However, the estimated benefit-cost ratios are smaller than those from many other air pollution abatement policies and net benefits may be negative for technology that will operate for fewer than five years.