Catalytic removal of gaseous pollutant NO using CO: Catalyst structure and reaction mechanism

Carbon monoxide (CO) has recently been considered an ideal reducing agent to replace NH3 in selective catalytic reduction of NOx (NH3-SCR). This shift is particularly relevant in diesel engines, coal-fired industry, the iron and steel industry, of which generate substantial amounts of CO due to incomplete combustion. Developing high-performance catalysts remain a critical challenge for commercializing this technology. The active sites on catalyst surface play a crucial role in the various microscopic reaction steps of this reaction. This work provides a comprehensive overview and insights into the reaction mechanism of active sites on transition metal- and noble metal-based catalysts, including the types of intermediates and active sites, as well as the conversion mechanism of active molecules or atoms. In addition, the effects of factors such as O2, SO2, and alkali metals, on NO reduction by CO were discussed, and the prospects for catalyst design are proposed. It is hoped to provide theoretical guidance for the rational design of efficient CO selective catalytic denitration materials based on the structure-activity relations.

Strong metal oxide-zeolite interactions during selective catalytic reduction of nitrogen oxides

In response to the stricter EU VII emission standards and the "150 ℃ challenge", selective catalytic reduction by ammonia (NH3-SCR) catalysts for motor vehicles are required to achieve high NO conversion below 200 °C. Compounding metal oxides with zeolites is an important strategy to design the low-temperature SCR catalysts. Here, we original prepared Cu-SSZ-13 @ MnGdOx (Cu-Z @ MGO), which achieved over 90% NO conversion and 95% N2 selectivity at 150 ℃. It has been demonstrated that a uniform mesoporous loaded layer of MGO grows on Cu-Z, and a recrystallization zone appears at the MGO-Cu-Z interface. We discover that the excellent low-temperature SCR activity derives from the strong metal oxide-zeolite interaction (SMZI) effects. The SMZI effects cause the anchor and high dispersion of MGO on the surface of Cu-Z. Driven by the SMZI effects, the Mn3+/Mn4+ redox cycle ensures the low and medium temperature-SCR activity and the Cu2+/Cu+ redox cycle guarantees the medium and high temperature-SCR activity. The introduction of MGO improves the reaction activity of -NH2 species adsorbed at Mn sites at 150 ℃, achieving a cycle of reduction and oxidation reactions at low temperatures. This strategy of inducing SMZI effects of metal oxides and zeolites paves a way for development of high-performance catalysts.

Interface sites on vanadia-based catalysts are highly active for NOx removal under realistic conditions

TiO2-supported V2O5 catalysts are commonly used in NOx reduction with ammonia due to their robust catalytic performance. Over these catalysts, it is generally considered that the active species are mainly derived from the vanadia species rather than the intrinsic structure of V-O-Ti entities, namely the interface sites. To reveal the role of V-O-Ti entities in NH3-SCR, herein, we prepared TiO2/V2O5 catalysts and demonstrated that V-O-Ti entities were more active for NOx reduction under wet conditions than the V sites (V=O) working alone. On the V-O-Ti entities, kinetic measurements and first principles calculations revealed that NH3 activation exhibited a much lower energy barrier than that on V=O sites. Under wet conditions, the V-O-Ti interface significantly inhibited the transformation of V=O to V-OH sites thus benefiting NH3 activation. Under wet conditions, meanwhile, the migration of NH4+ from Ti site neighboring the V-O-Ti interface to Ti site of the V-O-Ti interface was exothermic; thus, V-O-Ti entities together with neighboring Ti sites could serve as channels linking NH3 pool and active centers for activation of NH4+. This finding reveals that the V-O-Ti interface sites on V-based catalysts play a crucial role in NOx removal under realistic conditions, providing a new perspective on NH3-SCR mechanism.

Composite nanoarchitectonics with TiO2 nanocrystals and superior thin Ti3C2Tx nanosheets towards efficient NO removal

Accordion-like Ti₃C₂T_x MXene supplied a possibility to construct two-dimensional composites with novel performance. In this paper, few-layered Ti₃C₂T_x MXene was created via a chemical etching strategy. The oxidation in-situ using a powerful alkaline solution resulted successfully in TiO₂ nanocrystals grown on Ti₃C₂T_x nanosheets. The alkaline treatment adjusted terminations of the Ti₃C₂T_x MXene and controlled the oxidation degree by changing temperature. The ratio of Ti₃C₂T_x and TiO₂ was finally optimized. Because of Ti₃C₂T_x nanosheets with well conductivity and excellent light absorption as well as TiO₂ nanocrystal arrays on Ti₃C₂T_x nanosheets with a high specific surface area and more active sites, TiO₂/Ti₃C₂T_x composites revealed excellent photocatalystic activity, especially for NO removal. The improvement of separation and transfer efficiency of phootogenerated carriers is ascribed to the microstructure of TiO₂/Ti₃C₂T_x composites. The composite sample synthesized at 75 °C revealed the best NO removal efficiency, in which 70% of NO was removed at a concentration of 600 ppb. This study offers a new thought for preparing high performance MXene-based photocatalysts.

Balancing acid and redox sites of phosphorylated CeO2 catalysts for NOx reduction: The promoting and inhibiting mechanism of phosphorus

The role of phosphorus in metal oxide catalysts is still controversial. The precise tuning of the acidic and redox properties of metal oxide catalysts for the selective catalytic reduction in NO_x using NH₃ is also a great challenge. Herein, CeO₂ catalysts with different degrees of phosphorylation were used to study the balance between the acidity and redox property by promoting and inhibiting effects of phosphorus. CeO₂ catalysts phosphorylated with lower phosphorus content (5 wt%) exhibited superior NO_x reduction performance with above 90% NO_x conversion during 240-420 °C due to the balanced acidity and reducibility derived from the highest content of Brønsted acid sites on PO₄^3- to adsorb NH₃ and surface adsorbed oxygen species. Plenty of PO₃^- over CeO₂ catalysts phosphorylated with the higher phosphorus content (≥ 10 wt%) significantly disrupted the balance between the acidity and the redox property due to the reduced acid/redox sites, which resulted in the less active NO_x species. The mechanism of different structural phosphorus species (PO₄^3- and PO₃^-) in promoting or inhibiting the NO_x reduction over CeO₂ catalysts was revealed. This work provides a novel method for qualitative and quantitative study of the relationship between acidity/redox property and activity of catalysts for NO_x reduction.

Improvement of sulfur and water resistance with Fe-modified S-MnCoCe/Ti/Si catalyst for low-temperature selective catalytic reduction of NO with NH3

The low-temperature SCR of NO_x by NH₃ is restricted in application since the catalysts is easily poisoned by sulfur and water. The Fe modified Mn-Co-Ce/TiO₂/SiO₂ catalysts synthesized via impregnation method and sulfating were evaluated for low-temperature NH₃-SCR in the presence of SO₂ and H₂O. The calcination temperature and loading amounts of Mn, Fe, Co and Ce were optimized. Adding of Fe into S-MnCoCe/Ti/Si played an important role in resistance to sulfur and water poisoning. The optimal calcination temperature was 380 °C and the optical mass loading of the catalyst was 10% of Mn, 10% of Fe, 1% of Co and 4% of Ce. The optimal S-MnFeCoCe/Ti/Si catalyst maintained high NO_x conversion of 93% at reaction temperature of 160 °C in the presence of 50 ppm SO₂ and 10 vol% H₂O. The catalytic activity did not continue to fall after two times of repeated used in the temperature range of 100-200 °C, indicating its excellent sulfur and water durability and stability in the presence of SO₂ and H₂O. The interaction between MnO_x and FeO_x enhanced sulfur and water durability rather than other bi-metal interactions. Furthermore, the mechanism of Fe improving resistance to SO₂ and H₂O was discussed.

Effects of Cu doping on electrochemical NOx removal by La0.8Sr0.2MnO3 perovskites

Electrochemical removal of nitrogen oxides (NO_x) by perovskite electrodes is a promising method due to its low cost, simple operation and no secondary pollution. In this study, a series of La_0.8Sr_0.2Mn_1-xCu_xO₃ (x = 0, 0.05, 0.1 and 0.15) perovskites are fabricated as the improved electrodes of solid electrolyte cells (SECs) for NO_x removal and the effects of Cu doping are investigated systematacially. Multiple characterization methods are carried out to analyze the physicochemical properties of perovskites firstly. Then the performances of cells based on various perovskites are evaluated by the measurements of electrochemical properties and NO_x conversions. The results show that the Cu-doped electrode has more surface oxygen vacancies and a better redox property, thus having a higher NO_x conversion and smaller polarization resistance. The electrode based on La_0.8Sr_0.2Mn_0.9Cu_0.1O₃ has the maximum 70.8% NO_x conversion and the lowest 36.3 Ω cm² R_p value in the atmosphere of 1000 ppm NO at 700 °C. First-principle calculation reveals that the Cu-doped electrode is easier to form surface oxygen vacancy, while the surface oxygen vacancy plays an important role on electron transfer between electrode and NO_x molecule. This study not only provides a new strategy to enhance the electrode performance for NO_x removal in SECs but reveals the fundamental effect of Cu doping on the properties of La_0.8Sr_0.2MnO₃ perovskites.

Estimating NOx removal capacity of urban trees using stable isotope method: A case study of Beijing, China

It is widely recognized that green infrastructures in urban ecosystems provides important ecosystem services, including air purification. The potential absorption of nitrogen oxides (NO_x) by urban trees has not been fully quantified, although it is important for air pollution mitigation and the well-being of urban residents. In this study, four common tree species (Sophora japonica L., Fraxinus chinensis Roxb., Populus tomentosa Carrière, Sabina chinensis (L.)) in Beijing, China, were studied. The dual stable isotopes (¹⁵N and ¹⁸O) and a Bayesian isotope mixing model were applied to estimate the sources contributions of potential nitrogen sources to the roadside trees based on leaf and soil sampling in urban regions. The following order of sources contributions was determined: soil > dry deposition > traffic-related NO_x. The capacity of urban trees for NO_x removal in the city was estimated using a remote sensing and GIS approach, and the removal capacity was found to range from 0.79 to 1.11 g m^-2 a^-1 across administrative regions, indicating that 1304 tons of NO_x could be potentially removed by urban trees in 2019. Our finding qualified the potential NO_x removal by urban trees in terms of atmospheric pollution mitigation, highlighting the role of green infrastructure in air purification, which should be taken into account by stakeholders to manage green infrastructure as the basis of a nature-based approach.

Electrochemical removal of NOx by La0.8Sr0.2Mn1-xNixO3 electrodes in solid electrolyte cells: Role of Ni substitution

Electrochemical removal of nitrogen oxides (NO_x) by solid electrolyte cells (SECs) is a promising technology due to no required reductant. Herein, a series of La_0.8Sr_0.2Mn_1-xNi_xO₃ (0 ≤ x ≤ 0.5) perovskites were first synthesized and utilized as the electrode materials of SECs. The role of Ni substitution in electrode performance and NO_x reduction mechanism were revealed by various experimental characterization and first-principle calculations. The results indicate that the moderate Ni substitution (x ≤ 0.3) increased the NO_x conversion of electrodes while reduced the polarization resistance. The further investigation shows that this improvement was attributed to the more surface oxygen vacancies, better reducibility and higher Mn⁴⁺ proportion of the Ni-substituted perovskites. The electrochemical impedance spectroscopy (EIS) shows that these changes facilitated the NO_x adsorption and dissociation processes on the electrode. According to first-principle calculations, the Ni-substituted perovskite had a lower formation energy of surface oxygen vacancy, while the NO molecule adsorbed on defect surface gained more electrons thus was easier to be reduced and dissociated. Finally, the electrode performance at different operating temperatures and the operational stability were verified.

A new approach for the effective removal of NOx from flue gas by using an integrated system of oxidation-absorption-biological reduction

A new process of NO_x removal from flue gas, using an integrated system of oxidation-absorption-biological reduction (OABR), is introduced. The experimental results show that increasing the NO_x oxidation ratio in flue gas can effectively improve the NO_x removal efficiency of the OABR system. The NO_x removal efficiency could reach 98.8% with 0.02 M NaHCO₃ as the chemical absorbent and under the condition of the optimal NO_x oxidation ratio of 50%. During stable operation, the OABR system could maintain a high NO_x removal efficiency (above 94%) under the following conditions: 1-8 vol% (10⁴-8 × 10⁴ ppmv) O₂, 200-800 ppmv NO_x, 0.5-1.5 L/min gas flow rate and 100-800 ppmv SO₂. The nitrogen equilibrium results showed that about 59% of the nitrogen in the inlet NO_x were transformed to N₂ through microbial denitrification, 37% of the nitrogen were converted to biological nitrogen for microbial growth, and only 1.1% of the nitrogen remained in the liquid phase. This new approach has an excellent NO_x removal performance and great potential for industrial application.

Facile synthesis and characterization of anatase TiO2/g-CN composites for enhanced photoactivity under UV-visible spectrum

For the purpose of atmospheric NO removal, anatase TiO₂/g-CN photocatalytic composites were prepared by using a facile template-free calcination route in atmospheric conditions. Considerably fiscal NP400 and laboratory-grade melamine were used as the precursor of the composites. Additionally, samples were prepared with different wt. ratios of TiO₂ and melamine by using two distinct calcination temperatures (550 °C/600 °C). The morphological attributes of the composites were assessed with X-ray diffraction, scanning and transmission electron microscopy, infrared spectroscopy, and X-ray photoelectron spectroscopy. Additionally, the optical traits were evaluated and compared using UV-visible diffuse reflectance spectroscopy and photoluminescence analysis. Finally, the photodegradation potentials for atmospheric NO by using the as-prepared composites were assessed under both UV and visible light irradiation. All the composites showed superior NO oxidation compared to NP400 and bulk g-CN. For the composites prepared by using the calcination temperature of 550 °C, the maximum NO removal was observed when the NP400 to melamine ratio was 1:2, irrespective of the utilized light irradiation type. Whereas for increased calcination temperature (600 °C), the maximum NO removal was observed at the precursor mix ratio of 1:3 (NP400:melamine). Successfully narrowed energy bandgaps were perceived in the as-prepared composites. Moreover, a subsequent drop in NO₂ generation during NO oxidation was observed under both UV and visible light irradiation. Interestingly, higher calcination temperature during the synthesis of the catalysts has shown a significant drop in NO₂ generation during the photodegradation of NO.

Boosting the Alkali/Heavy Metal Poisoning Resistance for NO Removal by Using Iron-Titanium Pillared Montmorillonite Catalysts

It is still a big challge to improve the alkali and heavy metal resistance of deNO_x catalysts for selective catalytic reduction (SCR) of NO_x with NH₃. In this study, a novel catalyst developed by pillaring montmorillonite with iron and titanium (Fe-Ti-MMT) was proposed. It is quite interesting that high resistance to alkaline and heavy metals has been demonstrated by using Fe-Ti-MMT catalysts. It has been demonstrated that the specific pillaring synthesis procedure and further addition of the Ti pillared sites greatly contributed to the wide active temperature window and enhanced the resistance to alkali and heavy metal. The higher ratio of active Fe²⁺ species, more active acid sites, and enhanced ammonia adsorption indicated the remarkable activity as well as K and Pb resistance. Moreover, the K and Pb poisons would promote the generation of active adsorbed NO_x species on the Fe-Ti-MMT but induce the formation of stable inactive ones on that of Fe-MMT, which greatly tuned the reaction pathways and improved the reaction rate for Ti modified Fe pillared MMT catalysts. The strategy of incorporating Ti into the Fe pillared MMT catalysts strongly provides a novel inspiration for keeping excellent NH₃-SCR performance in the presence of alkali/heavy metal for NO_x removal.

Technologies for the nitrogen oxides reduction from flue gas: A review

The required energy of the global industry is mostly generated from fossil fuel sources, such as natural gas, gasoline, diesel, oil, and coal. Nitrogen oxides are one of the main air pollutants that are produced from the combustion of fossil fuels in stationary and mobile sources. Development of new technologies to decrease the NO_x emission from exhaust gases is essential due to the harmful effect of NO_x on the environment and human health. Compared with pre-combustion and combustion methods (with <50% NO_x removal efficiency), the post-combustion methods with higher efficiency (above 80%) have attracted more attention in NO_x elimination. This review describes the currently used technologies of NO_x abatement. Different available post-combustion methods of NO_x removal, including selective catalytic reduction (using different types of reducing reagents, including ammonia, hydrogen, hydrocarbons, and carbon monoxide), selective noncatalytic reduction, wet scrubbing, adsorption, electron beam, nonthermal plasma, and electrochemical reduction of NO_x, are discussed.

Biological removal of nitrogen oxides by microalgae, a promising strategy from nitrogen oxides to protein production

Nitrogen oxides (NO_x) are the components of fossil flue gases that give rise to serious environmental and health hazards. Among the available techniques for NO_x removal, microalgae-based biological removal of NO_x (BioDeNO_x) is a promising and competent technology with eco-friendly path of low energy and low-cost solution for the pollution. In this review article, current biological technologies including bacteria-based and microalgae-related BioDeNO_x are discussed. Comparing to direct BioDeNO_x approach, indirect BioDeNO_x by microalgae is more promising since it is more stable, reliable and efficient. By transforming inorganic nitrogen nutrients to organic nitrogen, microalgae can potentially play an important role in converting NO_x into high-value added products. The microalgae-based BioDeNO_x process displays an attractive prospect for flue gas treatment to reduce environmental NO_x pollution and potentially supply protein products, establishing an efficient circular-economy strategy.

Fast heterogeneous loss of N2O5 leads to significant nighttime NOx removal and nitrate aerosol formation at a coastal background environment of southern China

Nitrate radical (NO₃) and dinitrogen pentoxide (N₂O₅) play crucial roles in the nocturnal atmosphere. To quantify their impacts, we deployed a thermal-dissociation chemical ionization mass spectrometry (TD-CIMS), to measure their concentration, as well as ClNO₂ at a coastal background site in the southern of China during the late autumn of 2012. Moderate levels of NO₃, N₂O₅ and high concentration of ClNO₂ were observed during the study period, indicating active NO_x-O₃ chemistry in the region. Distinct features of NO₃, N₂O₅ and ClNO₂ mixing ratios were observed in different airmasses. Further analysis revealed that the N₂O₅ heterogeneous reaction was the dominant loss of N₂O₅ and NO₃, which showed higher loss rate compared to that in other coastal sites. Especially, the N₂O₅ loss rates could reach up to 0.0139 s^-1 when airmasses went across the sea. The fast heterogeneous loss of N₂O₅ led to rapid NO_x loss which could be comparable to the daytime process through NO₂ oxidization by OH, and on the other hand, to rapid nitrate aerosol formation. In summary, our results revealed that the N₂O₅ hydrolysis could play significant roles in regulating the air quality by reducing NO_x but forming nitrate aerosols.

Effect of preparation methods on the performance of CuFe-SSZ-13 catalysts for selective catalytic reduction of NOx with NH3

CuFe-SSZ-13 catalyst showed excellent performance in the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) for diesel engine exhaust purification. To investigate the effect of preparation methods on NH₃-SCR performance, Fe was loaded into one-pot synthesized Cu-SSZ-13 catalysts through solid-state ion-exchange (SSIE), homogeneous deposition precipitation (HDP) and liquid ion-exchange (IE), respectively. Three CuFe-SSZ-13 catalysts showed similar SO₂ resistance, which was better than that of Cu-SSZ-13. The improvement was attributed to the protection of Fe species. Hydrothermal stability of three CuFe-SSZ-13 catalysts was significantly different, which was attributed to the state of active species caused by different preparation methods. Compared with the other two catalysts, more active species existed inside the zeolite pores of CuFe-SSZ-13_SSIE. During hydrothermal aging, the aggregation of these active species in the pores caused the collapse of catalyst structure, ultimately leading to the deactivation of CuFe-SSZ-13_SSIE. In contrast, Fe species was dispersed better on the surface over CuFe-SSZ-13_IE, enhancing the hydrothermal stability of catalysts. Consequently, Fe loading effectively improved the resistance of SO₂ and H₂O over Cu-SSZ-13. For CuFe-SSZ-13, large amounts of active species located inside the zeolite pores are not beneficial for the hydrothermal stability.

Potential application of aerobic denitrifying bacterium Pseudomonas aeruginosa PCN-2 in nitrogen oxides (NOx) removal from flue gas

Conventional biological removal of nitrogen oxides (NOx) from flue gas has been severely restricted by the presence of oxygen. This paper presents an efficient alternative for NOx removal at varying oxygen levels using the newly isolated bacterial strain Pseudomonas aeruginosa PCN-2 which was capable of aerobic and anoxic denitrification. Interestingly, nitric oxide (NO), as the obligatory intermediate, was negligibly accumulated during nitrate and nitrite reduction. Moreover, normal nitrate reduction with decreasing NO accumulation was realized under O2 concentration ranging from 0 to 100%. Reverse transcription and real-time quantitative polymerase chain reaction (RT-qPCR) analysis revealed that high efficient NO removal was attributed to the coordinate regulation of gene expressions including napA (for periplasmic nitrate reductase), nirS (for cytochrome cd1 nitrite reductase) and cnorB (for NO reductase). Further batch experiments demonstrated the immobilized strain PCN-2 possessed high capability of removing NO and nitrogen dioxide (NO2) at O2 concentration of 0-10%. A biotrickling filter established with present strain achieved high NOx removal efficiencies of 91.94-96.74% at inlet NO concentration of 100-500ppm and O2 concentration of 0-10%, which implied promising potential applications in purifying NOx contaminated flue gas.

Air pollution control systems in WtE units: an overview

All WtE (waste-to-energy) plants, based on combustion or other thermal processes, need an efficient gas cleaning for compliance with legislative air emission standards. The development of gas cleaning technologies started along with environment protection regulations in the late 1960s. Modern APC (air pollution control) systems comprise multiple stages for the removal of fly ashes, inorganic and organic gases, heavy metals, and dioxins from the flue gas. The main technologies and devices used for abatement of the various pollutants are described and their basic principles, their peculiarities, and their application are discussed. Few systems for cleaning of synthesis gas from waste gasification plants are included. Examples of APC designs in full scale plants are shown and cautious prospects for the future development of APC systems are made.

NOx removal in chemical absorption-biological reduction integrated system: process rate and rate-limiting step

Biological reduction of Fe(III)EDTA is considered as the key step that limits the removal efficiency of the chemical absorption-biological reduction integrated system. In this study, the process rates of each reaction step under typical conditions (T=50°C, C FeII(EDTA)=1-5 mmol/L, CNO=0-500 ppm, CO2=1-10%, pH=7) were determined. Relevant kinetic constants including rate constants of absorption part and Michaelis-Menten kinetic constants of regeneration part were also obtained. On this basis, the theoretical process rates of each reaction step were predicted and compared in a steady state. The results confirmed that the removal rate of NO in this system is limited by the biological reduction of Fe(III)EDTA. Moreover, it indicated that increasing the concentration of total iron appropriately could enhance the bioreduction of Fe(III)EDTA.