Tailoring of the structure of Fe-cationic species in Fe-ZSM-5 by distribution of Al atoms in the framework for N2O decomposition and NH3-SCR-NOx

Enhancing Water Tolerance and N2 Selectivity in NH3-SCR Catalysts by Protecting Mn Oxide Nanoparticles in a Silicalite-1 Layer

Mn-based catalysts are promising candidates for eliminating harmful nitrogen oxides (NOx) via selective catalytic reduction with ammonia (NH3-SCR) due to their inherent strong redox abilities. However, poor water tolerance and low N2 selectivity are still the main limitations for practical applications. Herein, we succeeded in preparing an active catalyst for NH3-SCR with improved water tolerance and N2 selectivity based on protecting MnOx with a secondary growth of a hydrophobic silicalite-1. This protection suppressed catalyst deactivation by water adsorption. Interestingly, impregnating MnOx on MesoTS-1 followed by silicalite-1 protection allowed for a higher dispersion of MnOx species, thus increasing the concentration of acid sites. Consequently, the level of N2O formation is decreased. These improvements resulted in a broader operating temperature of NOx conversion and a modification of the NH3-SCR mechanism. Diffuse reflectance infrared Fourier transform spectroscopy analysis revealed that unprotected Mn/MesoTS-1 mainly followed the Eley-Rideal mechanism, while Mn/MesoTS-1@S1 followed both Langmuir-Hinshelwood and Eley-Rideal mechanisms.

Nature and Redox Properties of Iron Sites in Zeolites Revealed by Mössbauer Spectroscopy

Iron-containing zeolite-based catalysts play a pivotal role in environmental processes aimed at mitigating the release of harmful greenhouse gases, such as nitrous oxide (N2O) and methane (CH4). Despite the rich iron chemistry in zeolites, only a fraction of iron species that exhibit an open coordination sphere and possess the ability for electron transfer are responsible for activating reagents. In addition, the splitting of molecular oxygen is facilitated by bare iron cations embedded in zeolitic matrices. Mössbauer spectroscopy is the ideal tool for investigating the valency and geometry of iron species in zeolites because it leaves no iron forms silent and provides insights into in-situ processes. This review is dedicated to the utilization of Mössbauer spectroscopy to elucidate the nature of the extra-framework iron centers in ferrierite (FER), beta-structured (*BEA), and ZSM-5 zeolite (MFI) zeolites, which are active in N2O decomposition and CH4 oxidation through using the active oxygen derived from N2O and O2. In this work, a structured summary of the Mössbauer parameters established over the last two decades is presented, characterizing the specific iron active centers and intermediates formed upon iron's interaction with N2O/O2 and CH4. Additionally, the impact of preparation methods, iron loading, and the long-term stability on iron speciation and its redox behavior under reaction conditions is discussed.

Zeolite Synthesis in the Presence of Metallosiloxanes for the Quantitative Encapsulation of Metal Species for the Selective Catalytic Reduction (SCR) of NOx

Metal encapsulation in zeolitic materials through one-pot hydrothermal synthesis (HTS) is an attractive technique to prepare zeolites with a high metal dispersion. Due to its simplicity and the excellent catalytic performance observed for several catalytic systems, this method has gained a great deal of attention over the last few years. While most studies apply synthetic methods involving different organic ligands to stabilize the metal under synthesis conditions, here we report the use of metallosiloxanes as an alternative metal precursor. Metallosiloxanes can be synthesized from simple and cost-affordable chemicals and, when used in combination with zeolite building blocks under standard synthesis conditions, lead to quantitative metal loading and high dispersion. Thanks to the structural analogy of siloxane with TEOS, the synthesis gel stabilizes by forming siloxane bridges that prevent metal precipitation and clustering. When focusing on Fe-encapsulation, we demonstrate that Fe-MFI zeolites obtained by this method exhibit high catalytic activity in the NH3 -mediated selective catalytic reduction (SCR) of NOx along with a good H2 O/SO2 tolerance. This synthetic approach opens a new synthetic route for the encapsulation of transition metals within zeolite structures.

Maximizing Active Fe Species in ZSM-5 Zeolite Using Organic-Template-Free Synthesis for Efficient Selective Methane Oxidation

The selective oxidation of CH₄ in the aqueous phase to produce valuable chemicals has attracted considerable attention due to its mild reaction conditions and simple process. As the most widely studied catalyst for this reaction, Fe-ZSM-5 demonstrates high intrinsic activity and selectivity; however, Fe-ZSM-5 prepared using conventional methods has a limited number of active Fe sites, resulting in low CH₄ conversion per unit mass of the catalyst. This study reports a facile organic-template-free synthesis strategy that enables the incorporation of more Fe into the zeolite framework with a higher dispersion degree compared to conventional synthesis methods. Because framework Fe incorporated in this way is more readily transformed into isolated extra-framework Fe species under thermal treatment, the overall effect is that Fe-ZSM-5 prepared using this method (Fe-HZ5-TF) has 3 times as many catalytically active sites as conventional Fe-ZSM-5. When used for the selective oxidation of CH₄ with 0.5 M H₂O₂ at 75 °C, Fe-HZ5-TF produced a high C₁ oxygenate yield of 109.4 mmol g_cat^-1 h^-1 (a HCOOH selectivity of 91.1%), surpassing other catalysts reported to date. Spectroscopic characterization and density functional theory calculations revealed that the active sites in Fe-HZ5-TF are mononuclear Fe species in the form of [(H₂O)₃Fe(IV)═O]²⁺ bound to Al pairs in the zeolite framework. This differs from conventional Fe-ZSM-5, where binuclear Fe acts as the active site. Analysis of the catalyst and product evolution during the reaction suggests a radical-driven pathway to explain CH₄ activation at the mononuclear Fe site and subsequent conversion to C₁ oxygenates.

In situ deposition of 0D CeO2 quantum dots on Fe2O3-containing solid waste NH3-SCR catalyst: Enhancing redox and NH3 adsorption ability

As NO_x has been turning into a crucial environmental problem, NH₃-SCR technology with relatively simple device, reliable operation and low secondary pollution, has become a widely used commercial and mature de-nitration technology. However, some weaknesses restricted the further application of commercialized V₂O₅-WO₃/TiO₂ NH₃-SCR catalysts, while Fe₂O₃-based catalysts have received much attention due to their high thermal stability, passable N₂ selectivity and low cost. In this study, Fe₂O₃-containing solid waste derived from Zn extraction process of electric arc furnace dust was exploited as the base material for catalyst preparing. Owing to the complementary and synergistic effect of CeO₂ and Fe₂O₃, 0D CeO₂ quantum dots (CeQDs) with fully-exposed active sites, large specific surface area, and rapid charge transfer have been introduced and deposited onto Fe₂O₃-containing solid waste nanorods. The in-situ deposition of CeQDs led to the admirable enhancement in NH₃-SCR catalytic activity, N₂ selectivity and SO₂ tolerance of the extremely low-cost Fe₂O₃ catalyst. Comprehensive characterizations and DFT calculations describing the adsorption of O₂ and NH₃ were applied to analyze the catalyst structure and further investigate the detailed relationship between structural properties and activity as well as reaction mechanism. This work provides new insights for the high-value utilization of iron-containing solid waste and a practical reference for boosting the performance of NH₃-SCR catalysts by introducing quantum dots.

DFT Study on the Combined Catalytic Removal of N2O, NO, and NO2 over Binuclear Cu-ZSM-5

The large amount of nitrogen oxides (N2O, NO, NO2, etc.) contained in the flue gas of industrial adipic acid production will seriously damage the environment. A designed binuclear Cu-ZSM-5 catalyst can be applied to decompose N2O and reduce NO and NO2, purifying the air environment. Using the density functional theory method, the catalytic decomposition mechanisms of N2O, NOX-NH3-SCR, and NOX-assisted N2O decomposition is simulated over the Cu-ZSM-5 model. The results indicate that N2O can be catalytically decomposed over the binuclear Cu active site in the sinusoidal channel. The speed-limiting step is the second N2O molecule activation process. After the decomposition of the first N2O molecule, a stable extra-frame [Cu-O-Cu]2+ structure will generate. The subsequent discussion proved that the NOX-NH3-SCR reaction can be realized over the [Cu-O-Cu]2+ active site. In addition, it proved that the decomposition reaction of NO and NO2 can be carried out over the [Cu-O-Cu]2+ active site, and NO can greatly reduce the energy barrier for the conversion of the active site from [Cu-O-Cu]2+ to the binuclear Cu form, while NO2 can be slightly reduced. Through discussion, it is found that the binuclear Cu-ZSM-5 can realize the combined removal of N2O and NOX from adipic acid flue gas, hoping to provide a theoretical basis for the development of a dual-functional catalyst.

Stable co-production of olefins and aromatics from ethane over Co2+-exchanged HZSM-5 zeolite

Olefins and aromatics can be stably co-produced from ethane over a Co-exchanged HZSM-5 catalyst in which isolated Co(ii) species are anchored at Brønsted acid sites and active for efficient ethane dehydrogenation.

Air pollution and cause-specific mortality: A comparative study of urban and rural areas in China

Air pollution increases the risks of all-cause mortality, cardiovascular mortality and respiratory mortality across China. However, the urban-rural differences in the associations between air pollution and mortality have not been clearly identified. In this study, a distributed lag nonlinear model was used to examine whether the air pollutants-mortality associations vary between urban and rural areas. Then, we used logistic regression analyses to evaluate the air pollutants-mortality relations. Also, generalized additive models were simulated to evaluate the nonlinear curves. Our results showed that the relative risks of air pollution-related mortality were generally higher in rural areas, where PM_2.5 pollution was the dominant factor (p-value < 0.05). Mortality risks for all-cause, cardiovascular and respiratory will increase when average annual PM_2.5 concentrations exceed approximately 38 μg/m³, 41 μg/m³ and 41 μg/m³, respectively, all of which exceed the annual Grade II standards. In urban areas, PM_10-2.5 and NO₂ were associated with mortality (p-value < 0.05). We proposed some area-specific strategies for controlling the NO₂ pollution and PM_10-2.5 pollution in urban areas and the PM_2.5 pollution in rural areas to eliminate the gaps. Our findings identify that rural residents are more sensitive to air pollution than urban residents in China, and this result challenges previous assumptions about the more adverse effects of urbanization on residents' health in developing countries.

Assessing NO2-related health effects by non-linear and linear methods on a national level

Exposure to NO₂ pollution has a significant adverse effect on residents' health. However, few studies have assessed the health effects associated with NO₂ pollution. Compared with PM_2.5 pollution, the harmfulness of NO₂ pollution has not been quantitatively studied or clearly identified. In this study, we assessed the NO₂ exposure-related health effects by non-linear and linear methods, taking advantage of online monitoring and survey data. We also assessed the economic cost of NO₂ pollution in 338 cities in China. Our results showed that the average annual concentration of NO₂ in the top fifteen cities with more than ten million permanent residents (except for Shenzhen, in the Guangdong province) exceeded the annual Grade II standards (40 μg/m³). The estimated national NO₂-related all-cause mortality for non-linear and linear methods were 388.5 × 10³ (95% CI: 198.1 × 10³-748.2 × 10³) and 374.1 × 10³ (95% CI: 194.3 × 10³-695.9 × 10³), respectively. The total calculated national economic cost was about 28.8 billion US$ (95% CI: 14.7-55.4) in 2016. In addition, the comparison results showed that the harm caused by PM_2.5 pollution was about four times that of NO₂ pollution. Our statistics contribute to the limited research on NO₂ pollution's effects on health and the economy in China.

Low-temperature selective catalytic reduction of N2O by CO over Fe-ZSM-5 catalysts in the presence of O2

Nitrous oxide (N₂O) is an important ozone-depletion substance and greenhouse gas. Selective catalytic reduction (SCR) of N₂O by CO is considered an effective method for N₂O elimination. However, O₂ exhibited a significant inhibition effect on the catalytic performance of N₂O reduction by CO. A series of iron-based catalysts were prepared to investigate the effect of O₂ in SCR of N₂O by CO. The Fe-Z-pH2 (Fe-ZSM-5 ion-exchanged under pH of 2) catalyst manifested superior activity at low temperature and excellent O₂ resistance in N₂O reduction process. The characterization results from UV-vis DR spectra and XPS indicated that α-sites are the main active sites in Fe-Z-pH2, and they were inert to O₂ but highly active to N₂O. It could be concluded that the competition effect between N₂O and O₂ was very important over different catalysts. O₂ is more prevalent over α-Fe₂O₃ catalyst, while N₂O dominates over Fe-Z-pH2 catalyst. Moreover, in the presence of O₂, Fe-Z-pH2 exhibited better performance for N₂O removal than non-noble mixed oxide catalysts, which might broaden the application of low-temperature SCR of N₂O by CO.

Temporal dynamics of SO2 and NOX pollution and contributions of driving forces in urban areas in China

SO₂ and NO_X pollution have significantly reduced the air quality in China in past decades. Haze and acid rain have negatively affected the health of animals, plants, and human beings. Documented studies have shown that air pollution is influenced by multiple socioeconomic driving forces. However, the relative contributions of these driving forces are not well understood. In this study, using the structural equation model (SEM), we quantified the contributing effects of various forces driving air pollution in 2015 in prefecture-level cities of China. Our results showed that there has been significant control of SO₂ pollution in the past 20 years. The annual average SO₂ concentration has dropped from 83 μg/m³ in 1996 to 21 μg/m³ in 2015, while the annual average NO_X concentration has increased from 47 μg/m³ in 1996 to 58 μg/m³ in 2015. We evaluated data on the annual average concentrations of SO₂, which in some cities may mask the differences of SO₂ concentrations between different months. Hence, SO₂ pollution should continue to be controlled in accordance with existing policies and regulations. However, we suggest that NO_X should become the new focus of air pollution prevention and treatment. The SEM results showed that industrial scale, city size, and residents' activities have a significant impact on NO_X pollution. Among these, industrial scale had the highest contribution. The findings from our study can provide a theoretical basis for the formulation of NO_X pollution control policy in China.

FeOx/Al2O3 catalysts for high-temperature decomposition of N2O under conditions of NH3 oxidation in nitric acid production

The δ and θ Al₂O₃ phases well stabilized Fe(iii) in T_d or O_h coordination, which were identified as the active species in high temperature decomposition of N₂O in a complex gas mixture produced by oxidation of ammonia.

One-pot construction of Fe/ZSM-5 zeolites for the selective catalytic reduction of nitrogen oxides by ammonia

A simple one-step hydrothermal approach is developed to construct Fe/ZSM-5 zeolites for the selective catalytic reduction of nitrogen oxides by ammonia.

Deep insight into the structure–activity relationship of Nb modified SnO2–CeO2 catalysts for low-temperature selective catalytic reduction of NO by NH3

The structure–activity relationship of Nb modified SnO₂–CeO₂ catalysts was investigated for selective catalytic reduction of NO by NH₃.

Selective Transformation of Various Nitrogen-Containing Exhaust Gases toward N2 over Zeolite Catalysts

In this review we focus on the catalytic removal of a series of N-containing exhaust gases with various valences, including nitriles (HCN, CH3CN, and C2H3CN), ammonia (NH3), nitrous oxide (N2O), and nitric oxides (NO(x)), which can cause some serious environmental problems, such as acid rain, haze weather, global warming, and even death. The zeolite catalysts with high internal surface areas, uniform pore systems, considerable ion-exchange capabilities, and satisfactory thermal stabilities are herein addressed for the corresponding depollution processes. The sources and toxicities of these pollutants are introduced. The important physicochemical properties of zeolite catalysts, including shape selectivity, surface area, acidity, and redox ability, are described in detail. The catalytic combustion of nitriles and ammonia, the direct catalytic decomposition of N2O, and the selective catalytic reduction and direct catalytic decomposition of NO are systematically discussed, involving the catalytic behaviors as well as mechanism studies based on spectroscopic and kinetic approaches and molecular simulations. Finally, concluding remarks and perspectives are given. In the present work, emphasis is placed on the structure-performance relationship with an aim to design an ideal zeolite-based catalyst for the effective elimination of harmful N-containing compounds.

Al-free Fe-beta as a robust catalyst for selective reduction of nitric oxide by ammonia

Al-free Fe-beta prepared via a post-synthesis solid-state metallation route is established as an active and durable catalyst for NH₃-SCR.

Characterization of Fe2+ ions in Fe,H/SSZ-13 zeolites: FTIR spectroscopy of CO and NO probe molecules

FTIR spectra of adsorbed NO and CO were used to characterize Fe²⁺ ions in different cationic positions in Fe,H/SSZ-13 zeolites.

A reusable material with high performance for removing NO at room temperature: performance, mechanism and kinetics

Removing NO from the air with a reusable material at room temperature is challenging.