Search Results (345)

Three MnCeTiO_x catalysts with the same composition were prepared by conventional co-precipitation (MCT-C), reverse co-precipitation (MCT-R), and parallel co-precipitation (MCT-P), respectively, and their low-temperature SCR performance for de-NO_x was evaluated. The textural and structural properties, surface acidity, redox capacity, and reaction mechanism of the catalysts were investigated by a series of characterizations including N₂ adsorption and desorption, XRD, SEM, XPS, H₂-TPR, NH₃-TPD, NO-TPD, and in situ DRIFTs. The results revealed that the most excellent catalytic performance was achieved on MCT-R, and more than 90% NO_x conversion can be obtained at 100–300 °C under a high GHSV of 80,000 mL/(g_cat·H). Furthermore, MCT-R possessed optimal tolerance to H₂O and SO₂ poisoning. The excellent catalytic performance of MCT-R can be attributed to its larger BET specific surface area; higher contents of Mn⁴⁺, Ce³⁺, and adsorbed oxygen species; and more adsorption capacity for NH₃ and NO. Moreover, in situ DRIFTs results indicated that the NH₃-SCR reaction follows simultaneously the Langmuir–Hinshelwood and Eley–Rideal mechanisms at 100 °C. By adjusting the adding mode during the co-precipitation process, excellent low-temperature de-NO_x activity of MCT-R can be obtained simply and conveniently, which is of great practical value for the preparation of a MnCeTiO_x catalyst for denitrification. Full article

Selective catalytic reduction of nitrogen oxides (NO_x) with ammonia (NH₃-SCR) has been implemented in response to the regulation of NO_x emissions from stationary and mobile sources above 300 °C. However, the development of NH₃-SCR catalysts active at low temperatures below 200 °C is still needed to improve the energy efficiency and to cope with various fuels. In this review article, recent reports on low-temperature NH₃-SCR catalysts are systematically summarized. The redox property as well as the surface acidity are two main factors that affect the catalytic activity. The strong redox property is beneficial for the low-temperature NH₃-SCR activity but is responsible for N₂O formation. The multiple electron transfer system is more plausible for controlling redox properties. H₂O and SO_x, which are often found with NO_x in flue gas, have a detrimental effect on NH₃-SCR activity, especially at low temperatures. The competitive adsorption of H₂O can be minimized by enhancing the hydrophobic property of the catalyst. Various strategies to improve the resistance to SO_x poisoning are also discussed. Full article

In view of the flue gas characteristics of cement kilns in China, the development of low-temperature denitrification catalysts with excellent anti-poisoning performance has important theoretical and practical significance. In this work, a series of MnCeO_x@TiO₂ and tourmaline-containing MnCeO_x@TiO₂-T catalysts was prepared using a chemical pre-deposition method. It was found that the MnCeO_x@TiO₂-T2 catalyst (containing 2% tourmaline) exhibited the best low-temperature NH₃-selective catalytic reduction (NH₃-SCR) performance, yielding 100% NO_x conversion at 110 °C and above. When 100–300 ppm SO₂ and 10 vol.% H₂O were introduced to the reaction, the NO_x conversion of the MnCeO_x@TiO₂-T2 catalyst was still higher than 90% at 170 °C, indicating good anti-poisoning performance. The addition of appropriate amounts of tourmaline can not only preferably expose the active {001} facets of TiO₂ but also introduce the acidic SiO₂ and Al₂O₃ components and increase the content of Mn⁴⁺ and O_α on the surface of the catalyst, all of which contribute to the enhancement of reaction activity of NH₃-SCR and anti-poisoning performance. However, excess amounts of tourmaline led to the formation of dense surface of catalysts that suppressed the exposure of catalytic active sites, giving rise to the decrease in catalytic activity and anti-poisoning capability. Through an in situ DRIFTS study, it was found that the addition of appropriate amounts of tourmaline increased the number of Brønsted acid sites on the catalyst surface, which suppressed the adsorption of SO₂ and thus inhibited the deposition of NH₄HSO₄ and (NH₄)₂HSO₄ on the surface of the catalyst, thereby improving the NH₃-SCR performance and anti-poisoning ability of the catalyst. Full article

Selective catalytic reduction (SCR) stands out as a pivotal method for curbing NO_x emissions from flue gas. The support, crucially, for SCR efficacy, loads and interacts with the active components within the catalyst. The catalysts could be amplified by the denitration performance of the catalyst by enhancements in support pore structure, acidity, and mechanical robustness. These improvements ensure efficient interaction between the support and active materials, thereby optimizing the structure and property of the catalysts. TiO₂ is the most commonly used support of the NH₃-SCR catalyst. The catalyst with TiO₂ support has poor thermal stability and a narrow temperature range, which can be improved. This paper reviews the research progress on the effects of various aspects of TiO₂ support on the NH₃-SCR catalyst’s performance, focusing on the TiO₂ crystal type, TiO₂ crystal surface, different TiO₂ structures, TiO₂ support preparation methods, and the effects of TiO₂-X composite support on the NH₃-SCR catalyst’s performance. The reaction mechanism, denitrification performance, and anti-SO₂/H₂O poisoning performance and mechanism of TiO₂ support with different characteristics were described. At the same time, the development trend of the NH₃-SCR catalyst using TiO₂ as the support is prospected. It is hoped that this work can provide optimization ideas for SCR catalyst research. Full article

The formation of soot and NOx in ammonia/ethylene flames with varying ammonia ratios was investigated through experimental and numerical analysis. The spatial distribution of the soot volume fraction and NOx concentrations along the flame central line were measured, and the mechanism of soot and NOx formation during ammonia/ethylene co-combustion was analyzed using CHEMKIN 17.0. The experimental results indicated that the soot volume fraction decreases with an increase in ammonia ratio, with the soot peak concentration occurring in the upper region of the flame. The distribution of NOx is complex. In the initial part of the flame, a higher concentration of NOx is generated, and the lower the ammonia ratio, the higher the concentration of NOx. As the combustion process progresses, the concentration of NOx initially decreases and then subsequently increases rapidly, with higher ammonia ratios leading to higher concentrations of NOx. The addition of ammonia results in a decrease in CH₃, C₂H₂, and C₃H₃, and an increase in CN concentration. This leads to a transformation of carbon atoms within the combustion system, reducing the available carbon for soot formation and suppressing its generation. A higher ammonia ratio increases the likelihood that NH₃ will be oxidized to N₂, as well as increasing the probability that any generated NO will undergo reduction to N₂ through the action of the free radicals NH₂ and NH. Full article

Aerosol acidity is a critical factor affecting atmospheric chemistry. Here, we present a study on annual, monthly, and daily variations in PM_2.5 pH in Shanghai during 2010–2020. With the effective control of SO₂ emissions, the NO₂/SO₂ ratio increased from 1.26 in 2010 to 5.07 in 2020 and the NO₃⁻/SO₄²⁻ ratio increased from 0.68 to 1.49. Aerosol pH decreased from 3.27 in 2010 to 2.93 in 2020, regardless of great achievement in reducing industrial SO₂ and NOx emissions. These findings suggest that aerosol acidity might not be significantly reduced in response to the control of SO₂ and NOx emissions. The monthly variation in pH values exhibited a V-shape trend, mainly attributable to aerosol compositions and temperature. Atmospheric NH₃ plays the decisive role in buffering particle acidity, whereas Ca²⁺ and K⁺ are important acidity buffers, and the distinct pH decline during 2010–2016 was associated with the reduction of Ca²⁺ and K⁺ while both temperature and SO₄²⁻ were important drivers in winter. Sensitivity tests show that pH increases with the increasing relative humidity in summer while it is not sensitive to relative humidity in winter due to proportional increases in ${\mathrm{H}}_{\mathrm{a}\mathrm{i}\mathrm{r}}^{+}$ and aerosol liquid water content (ALWC). Our results suggest that reducing NOx emissions in Shanghai will not significantly affect PM_2.5 acidity in winter. Full article

The reduction of nitrogen oxides (NO_x), critical pollutants from stationary to mobile sources, mainly relies on the selective catalytic reduction (NH₃-SCR) method, employing ammonia to reduce NO_x into nitrogen and water. However, conventional catalysts, while effective, pose both environmental and operational challenges. This study investigates ceria-zirconia-supported molybdenum-based catalysts, exploring the effects of zirconium doping and different catalyst synthesis techniques, i.e., co-precipitation and impregnation. The catalytic performance of the differently prepared samples was significantly influenced by the molybdenum incorporation method and the zirconium content within the ceria-zirconia support. Co-precipitation at higher temperatures resulted in catalysts with better structural attributes but slightly lower catalytic activity compared to those prepared via impregnation. Optimal NO_x reduction (close to 100%) was observed at a 15 mol% zirconium doping level when using the impregnation method. Full article

Decarbonizing maritime transport hinges on transitioning oil-fueled ships (98.4% of the fleet) to renewable and low-carbon fuel types. This shift is crucial for meeting the greenhouse gas (GHG) reduction targets set by the IMO and the EU, with the aim of achieving climate neutrality by 2050. Ammonia, which does not contain carbon atoms that generate CO₂, is considered one of the effective solutions for decarbonization in the medium and long term. However, the concurrent increase in nitrogen oxide (NO_x) emissions during the ammonia combustion cycle, subject to strict regulation by the MARPOL 73/78 convention, necessitates implementing solutions to reduce them through optimizing the combustion cycle. This publication presents a numerical study on the optimization of diesel and ammonia injection phases in a ship’s medium-speed engine, Wartsila 6L46. The study investigates the exhaust gas emissions and combustion cycle parameters through a high-pressure injection strategy. At an identified 7° CAD injection phase distance between diesel and ammonia, along with an optimal dual-fuel start of injection 10° CAD before TDC, a reduction of 47% in greenhouse gas emissions (GHG = CO₂ + CH₄ + N₂O) was achieved compared to the diesel combustion cycle. This result aligns with the GHG reduction target set by both the IMO and the EU for 2030. Additionally, during the investigation of the thermodynamic combustion characteristics of the cycle, a comparative reduction in NO_x of 4.6% was realized. This reduction is linked to the DeNO_x process, where the decrease in NO_x is offset by an increase in N₂O. However, the optimized ammonia combustion cycle results in significant emissions of unburnt NH₃, reaching 1.5 g/kWh. In summary, optimizing the combustion cycle of dual ammonia and diesel fuel is essential for achieving efficient and reliable engine performance. Balancing combustion efficiency with emission levels of greenhouse gases, unburned NH₃, and NO_x is crucial. For the Wartsila 6L46 marine diesel engine, the recommended injection phasing is A710/D717, with a 7° CAD between injection phases. Full article

While selective catalytic reduction (SCR) has long been indispensable for nitrogen oxide (NO_x) removal, optimizing its performance remains a significant challenge. This study investigates the combined effects of structural and intake parameters on SCR performance, an aspect often overlooked in previous research. This paper innovatively developed a three-dimensional SCR channel model and employed response surface methodology to conduct an in-depth analysis of multiple key factors. This multidimensional, multi-method approach enables a more comprehensive understanding of SCR system mechanics. Through target optimization, we achieved a simultaneous improvement in three critical indicators: the NO_x conversion rate, pressure drop, and ammonia slip. It is worth noting that the NO_x conversion rate has been optimized from 17.07% to 98.25%, the pressure drop has been increased from 3454.62 Pa to 2558.74 Pa, and the NH₃ slip has been transformed from 122.26 ppm to 17.49 ppm. These results not only advance the theoretical understanding of SCR technology but also provide valuable design insights for practical applications. Our findings pave the way for the development of more efficient and environmentally friendly SCR systems, potentially revolutionizing NO_x control in various industries. Full article

ERI and SSZ-13 were subjected to post-synthetic treatments (depending on the zeolite topology) to create micro-/mesoporous materials. The results in terms of NH₃-SCR-DeNO_x show that the applied treatments improved the catalytic activity of the Cu-containing ERI-based materials; however, the NO conversion did not vary for the different materials treated with NaOH or NaOH/HNO₃. For the micro-/mesoporous Cu-containing SSZ-13, a lower NO conversion in NH₃-SCR-DeNO_x was observed. Thus, our findings challenge the current paradigm of enhanced activity of micro-/mesoporous catalysts in NH₃-SCR-DeNO_x. The modification of the supports results in the presence of different amounts and kinds of copper species (especially isolated Cu²⁺ and aggregated Cu species) in the case of ERI- and SSZ-13-based samples. The present copper species further differentiate the formation of reactive reaction intermediates. Our studies show that besides the μ-η²,η²-peroxo dicopper(II) complexes (verified by in situ DR UV-Vis spectroscopy), copper nitrates (evidenced by in situ FT-IR spectroscopy) also act as reactive intermediates in these catalytic systems. Full article

To investigate the control mechanisms of NOx precursors and the synergistic effects of composite catalysts during proline pyrolysis, a systematic series of experiments was conducted utilizing composite catalysts with varying Fe-Ca ratios. Product distribution analysis was employed to elucidate the catalysts’ mechanisms in reducing NOx precursor emissions. The synergistic interactions between Fe and Ca were quantitatively assessed through comparative theoretical and experimental release calculations. The results indicate that an increase in the Fe content in the catalyst led to a rise in amine concentrations from 0.9% to 2.95%, implying that Fe facilitates the generation of amine-N through ring-opening and substitution reactions. When the Fe to Ca ratio was balanced at 1:1, nitrogen predominantly participated in the formation of purines via cyclization and substitution reactions. Additionally, all composite catalysts exhibited a suppressive effect on the release of NOx precursors, attributed to their significant enhancement of solid product retention. Fe-Ca composite catalyst synergistically inhibits the release of gaseous nitrogen. Notably, the strongest synergistic effect was observed with a 1:3 Fe to Ca ratio, which reduced the release of NH₃ by 38.7% and HCN by 53.6% during proline pyrolysis. This study offers valuable insights into the control of NOx precursors and the optimization of nitrogen-rich biomass pyrolysis processes. Full article

The maritime industry needs sustainable, low-emission fuels to reduce the environmental impact. Ammonia is one of the most promising alternative fuels because it can be produced from renewable energy, such as wind and solar. Furthermore, ammonia combustion does not emit carbon. This review article covers the advantages and disadvantages of using ammonia as a sustainable marine fuel. We start by discussing the regulations and environmental concerns of the shipping sector, which is responsible for around 2% to 3% of global energy-related CO₂ emissions. These emissions may increase as the maritime industry grows at a compound annual growth rate of 4.33%. Next, we analyze the use of ammonia as a fuel in detail, which presents several challenges. These challenges include the high price of ammonia compared to other fossil fuels, the low reactivity and high toxicity of ammonia, NOx, and N₂O emissions resulting from incomplete combustion, an inefficient process, and NH₃ slipping. However, we emphasize how to overcome these challenges. We discuss techniques to reduce NOx and N₂O emissions, co-combustion to improve reactivity, waste heat recovery strategies, the regulatory framework, and safety conditions. Finally, we address the market trends and challenges of using ammonia as a sustainable marine fuel. Full article

With the rapid development of industrialization, the emission of nitrogen oxides (NO_x) has become a global environmental issue. Uranium is the primary fuel used in nuclear power generation. However, the production of uranium, typically based on the uranyl nitrate method, usually generates large amounts of nitrogen oxides, particularly NO₂, with concentrations in the exhaust gas exceeding 10,000 ppm. High concentrations of nitrogen dioxide are also produced during silver electrolysis processing and the treatment of waste electrolyte solutions. Traditional V-W/TiO₂ NH₃-SCR catalysts typically exhibit high catalytic activity at temperatures ranging from 300 to 400 °C, under conditions of low NO_x concentrations and high gas hourly space velocity. However, their performance is not satisfying when reducing high concentrations of NO₂. This study aims to optimize the traditional V-W/TiO₂ catalysts to enhance their catalytic activity under conditions of high NO₂ concentrations (10,000 ppm) and a wide temperature range (200–400 °C). On the basis of 3 wt% Mo/TiO₂, various loadings of V₂O₅ were selected, and their catalytic activities were tested. Subsequently, the optimal ratios of active component vanadium and additive molybdenum were explored. Simultaneously, doping with WO₃ for modification was selected in the V-Mo/TiO₂ catalyst, followed by activity testing under the same conditions. The results show that: the NO_x conversion rates of all five catalysts increase with temperature at range of 200–400 °C. Excessive loading of MoO₃ decreased the catalytic performance, with 5 wt% being the optimal loading. The addition of WO₃ significantly enhanced the low-temperature activity of the catalysts. When the loadings of WO₃ and MoO₃ were both 3 wt%, the catalyst exhibited the best denitrification performance, achieving a NO_x conversion rate of 98.8% at 250 °C. This catalyst demonstrates excellent catalytic activity in reducing very high concentration (10,000 ppm) NO₂, at a wider temperature range, expanding the temperature range by 50% compared to conventional SCR catalysts. Characterization techniques including BET, XRD, XPS, H₂-TPR, and NH₃-TPD were employed to further study the evolution of the catalyst, and the promotional mechanisms are explored. The results revealed that the proportion of chemisorbed oxygen (O_α) increased in the WO₃-modified catalyst, exhibiting lower V reduction temperatures, which are favorable for low-temperature denitrification activity. NH₃-TPD experiments showed that compared to MoO_x species, surface WO_x species could provide more acidic sites, resulting in stronger surface acidity of the catalyst. Full article

Due to high greenhouse gas emissions, countries worldwide are stepping up their emission reduction efforts, and the global demand for new, carbon-free fuels is growing. Ammonia (NH₃) fuels are popular due to their high production volume, high energy efficiency, ease of storage and transportation, and increased application in power equipment. However, their physical characteristics (e.g., unstable combustion, slow flame speed, and difficult ignition) limit their use in power equipment. Based on the structural properties of the power equipment, NH₃ fuel application and emissions characteristics were analyzed in detail. Combustion of NH₃ fuels and reduction measures for NO_x emissions (spark plug ignition, compression ignition, and gas turbines) were analyzed from various aspects of operating conditions (e.g., mixed fuel, fuel-to-exhaust ratio, and equivalence ratio), structure and strategy (e.g., number of spark plugs, compression ratio (CR), fuel injection, and ignition mode), and auxiliary combustion techniques (e.g., preheating, humidification, exhaust gas recirculation, and secondary air supply). The performance of various NH₃ fuel cell (FC) types was analyzed, with a focus on the maximum power achievable for different electrolyte systems. Additionally, the application and NO_x emissions of indirect NH₃ FCs were evaluated under flame and catalytic combustion conditions. The system efficiency of providing heat sources by burning pure NH₃, anode tail gas, and NH₃ decomposition gas was also compared. Based on a comprehensive literature review, the key factors influencing the performance and emissions of NH₃-powered equipment were identified. The challenges and limitations of NH₃-powered equipment were summarized, and potential strategies for improving efficiency and reducing emissions were proposed. These findings provide valuable insights for the future development and application of NH₃ FCs. Full article

In this paper, a detailed mechanism is discussed for two processes: deNOx and deN₂O. An FAU catalyst was used for the reaction with Cu-Fe bimetallic adsorbates represented by a dimer with bridged oxygen. Partial hydration of the metal centres in the dimer was considered. Ab initio calculations based on the density functional theory were used. The electron parameters of the structures obtained were also analysed. Visualisation of the orbitals of selected structures and their interpretations are presented. The presented research allowed a closer look at the mechanisms of processes that are very common in the automotive and chemical industries. Based on theoretical modelling, it was possible to propose the most efficient catalyst that could find potential application in industry–this is the FAU catalyst with a Cu-O-Fe bimetallic dimer with a hydrated copper centre. The essential result of our research is the improvement in the energetics of the reaction mechanism by the presence of an OH group, which will influence the way NO and NH₃ molecules react with each other in the deNOx process depending on the industrial conditions of the process. Our theoretical results suggest also how to proceed with the dosage of NO and N₂O during the industrial process to increase the desired reaction effect. Full article