Search Results (44)

In wet flue gas desulfurization and denitrification processes, nitrite accumulation inhibits denitrification efficiency and induces secondary pollution due to its acidic disproportionation. This study developed a Mn-modified ZSM-5 zeolite catalyst, achieving efficient resource conversion of nitrite in nitrogen-containing wastewater through an O₃ + Mn/ZSM-5 catalytic system. Mn/ZSM-5 catalysts with varying SiO₂/Al₂O₃ ratios (prepared by wet impregnation) were characterized by BET, XRD, and XPS. Experimental results demonstrated that Mn/ZSM-5 (SiO₂/Al₂O₃ = 400) exhibited a larger specific surface area, enhanced adsorption capacity, abundant surface Mn³⁺/Mn⁴⁺ species, hydroxyl oxygen species, and chemisorbed oxygen, leading to superior oxidation capability and catalytic activity. Under the optimized conditions of reaction temperature = 40 °C, initial pH = 4, Mn/ZSM-5 dosage = 1 g/L, and O₃ concentration = 100 ppm, the $\mathrm{N}{\mathrm{O}}_2^{-}$ oxidation efficiency reached 94.33%. Repeated tests confirmed that the Mn/ZSM-5 catalyst exhibited excellent stability and wide operational adaptability. The synergistic effect between Mn species and the zeolite support significantly improved ozone utilization efficiency. The O₃ + Mn/ZSM-5 system required less ozone while maintaining high oxidation efficiency, demonstrating better cost-effectiveness. Mechanism studies revealed that the conversion pathway of $\mathrm{N}{\mathrm{O}}_2^{-}$ followed a dual-path catalytic mechanism combining direct ozonation and free radical chain reactions. Practical spray tests confirmed that coupling the Mn/ZSM-5 system with ozone oxidation flue gas denitrification achieved over 95% removal of liquid-phase $\mathrm{N}{\mathrm{O}}_2^{-}$ byproducts without compromising the synergistic removal efficiency of NO_x/SO₂. This study provided an efficient catalytic solution for industrial wastewater treatment and the resource utilization of flue gas denitrification byproducts. Full article

The efficient simultaneous removal of NO_x and CO from sintering flue gas under low-temperature conditions (110–180 °C) in iron and steel enterprises remains a significant challenge in the field of environmental catalysis. In this study, we present an innovative strategy to enhance the performance of CuSmTi catalysts through silver modification, yielding a bifunctional system capable of oxygen structure regulation and demonstrating superior activity for the combined NH₃-SCR and CO oxidation reactions under low-temperature, oxygen-rich conditions. The modified AgCuSmTi catalyst achieves complete NO conversion at 150 °C, representing a 50 °C reduction compared to the unmodified CuSmTi catalyst (T_100% = 200 °C). Moreover, the catalyst exhibits over 90% N₂ selectivity across a broad temperature range of 150–300 °C, while achieving full CO oxidation at 175 °C. A series of characterization techniques, including XRD, Raman spectroscopy, N₂ adsorption, XPS, and O₂-TPD, were employed to elucidate the Ag-Cu interaction. These modifications effectively optimize the surface physical structure, modulate the distribution of acid sites, increase the proportion of Lewis acid sites, and enhance the activity of lattice oxygen species. As a result, they effectively promote the adsorption and activation of reactants, as well as electron transfer between active species, thereby significantly enhancing the low-temperature performance of the catalyst. Furthermore, in situ DRIFTS investigations reveal the reaction mechanisms involved in NH₃-SCR and CO oxidation over the Ag-modified CuSmTi catalyst. The NH₃-SCR process predominantly follows the L-H mechanism, with partial contribution from the E-R mechanism, whereas CO oxidation proceeds via the MvK mechanism. This work demonstrates that Ag modification is an effective approach for enhancing the low-temperature performance of CuSmTi-based catalysts, offering a promising technical solution for the simultaneous control of NO_x and CO emissions in industrial flue gases. Full article

Nitrogen oxides (NO_x), harmful pollutants primarily from fossil fuel combustion, pose significant environmental and health risks. Among mitigation technologies, NH₃-SCR is widely adopted due to its high efficiency and industrial viability. MnO₂-based catalysts, particularly α-MnO₂, have gained attention for low-temperature NH₃-SCR owing to their redox properties, low-temperature activity, and environmental compatibility. In this study, α-MnO₂ catalysts with tunable oxygen vacancy concentrations were synthesized by varying calcination atmospheres. Compared to α-MnO₂-Air, the oxygen vacancy-rich α-MnO₂-N₂ exhibited stronger acidity, enhanced redox properties, and superior NH₃/NO adsorption and activation, achieving 98% NO conversion at 125–250 °C. Oxygen vacancies promoted NH₃ adsorption on Lewis/Brønsted acid sites, facilitating -NH₂ intermediate formation, while enhancing NO oxidation to reactive nitrates. In situ DRIFTS revealed a dual E-R and L-H reaction pathway, with oxygen vacancies crucial for NO activation, intermediate formation, and N₂ generation. These findings underscore the importance of oxygen vacancy engineering in optimizing Mn-based SCR catalysts. Full article

Nitrogen oxides (NOx) and chlorinated volatile organic compounds (CVOCs) are major environmental pollutants, posing severe risks to human health and ecosystems. Traditional single-component catalysts often fail to remove both pollutants efficiently, making synergistic catalytic technologies a critical research focus. In this study, a mesoporous HPW-CS-Ce-Ti oxide catalyst, modified with H₃PW₁₂O₄₀ (HPW) and chitosan (CS), was synthesized via self-assembly. The optimized 10HPW-CS-Ce_0.3-Ti catalyst achieved nearly 100% NO conversion at 167–288 °C and a T₉₀ of 291 °C for CVOC conversion, demonstrating superior dual-pollutant removal. HPW and chitosan facilitated mesoporous structure formation, enhancing mass transfer and active site availability. HPW doping also modulated the Ce⁴⁺/Ce³⁺ ratio, boosting redox capacity and surface-active oxygen species, while increasing acidity to promote NH₃ and CVOC adsorption. This study presents a novel catalyst and synthesis method with significant potential for environmental protection and human health. Full article

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

Sludge alkaline fermentation liquid (AFL) is a potential carbon source for biological denitrification. However, its effectiveness is limited due to the presence of nutrients and heavy metals. In this study, acid-modified sepiolite (MSEP) was used to extract short-chain fatty acids (SCFAs) from AFL under optimized conditions and then with the prepared MSEP-AFL as a carbon source for denitrification. The optimal condition with an MSEP dosage of 1.96 g/L and pH 7.93 at 30 °C was obtained based on single-factor experiments and response surface methodology (RSM). Carbon balance revealed that 96.2% of the SCFAs, including 43.7% acetate and 23.5% propionic acid, was retained in the MSEP, demonstrating its high selectivity. The adsorption process followed the pseudo-second-order kinetic and Langmuir isothermal model, indicating dominant physical adsorption on the surface or in the fiber pores. This was further supported by the changes in the morphological features and surface properties of the MSEP. In the batch nitrate utilization experiments, the prepared MSEP-AFL was proven to be efficient as a carbon source, with a nitrate removal efficiency of 88.7% and a specific denitrification rate of 8.2 mg NOx-N/g VSS·h, which was 22% higher than that of the AFL. This was due to the establishment of a delicate “release–utilization” balance. These findings contribute to our understanding of the use of AFL for denitrification. 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

Here, we successfully synthesized Sr-doped perovskite-type oxides of La_1−xSr_xCo_1−λO_3−δ, “LSX” (x = 0, 0.1, 0.3, 0.5, 0.7), using the glycine-assisted solution combustion method. The effect of strontium doping on the catalyst structure, NO to NO₂ conversion, NO_x adsorption and storage, and NO_x reduction performance were investigated. The physicochemical properties of the catalysts were studied by XRD, SEM-EDS, N₂ adsorption–desorption, FTIR, H₂-TPR, O₂-TPD, and XPS techniques. The NSR performance of LaCoO₃ perovskite was improved after Sr doping. Specifically, the perovskite with 50% of Sr doping (LS5 sample) exhibited excellent NO_x storage capacity within a wide temperature range (200–400 °C), and excellent stability after hydrothermal and sulfur poisoning. It also displayed the highest NO_x adsorption–storage capacity (NAC: 1889 μmol/g; NSC: 1048 μmol/g) at 300 °C. This superior performance of the LS5 catalyst can be attributed to its superior reducibility, better NO oxidation capacity, increased surface Co²⁺ concentration, and, in particular, its generation of more oxygen vacancies. FTIR results further revealed that the LSX catalysts primarily store NO_x through the “nitrate route”. During the lean–rich cycle tests, we observed an average NO_x conversion rate of over 50% in the temperature range of 200–300 °C, with a maximum conversion rate of 61% achieved at 250 °C. Full article

Replacing alkaline for alkaline-earth metal hydroxide in the synthesis gel during the synthesis of siliceous SSZ-13 zeolite (Si/Al~10) yields SSZ-13 with novel, advantageous properties. Its NH₄-form ion-exchanges higher amount of isolated divalent M(II) ions than the conventional one: this is the consequence of an increased number of Al pairs in the structure induced by the +2 charge of Sr(II) cations in the synthesis gel that force two charge-compensating AlO₄⁻ motives to reside closer together. We characterize the +2 state of Co(II) ions in these materials with infra-red spectroscopy and X-ray absorption spectroscopy measurements and show their utility for NOx pollutant adsorption from ambient air: the ones derived from SSZ-13 with higher Al pair content contain more isolated cobalt(II) and, thus, perform better as ambient-air NOx adsorbers. Notably, Co(II)/SSZ-13 with an increased number of Al pairs is significantly more hydrothermally stable than its NaOH-derived analogue. Loading Pd(II) into Co-SSZ-13(Sr) produces an active NOx adsorber (PNA) material that can be used for NOx adsorption from simulated diesel engine exhaust. The critical issue for these applications is hydrothermal stability of Pd-zeolites. Pd/SSZ-13 synthesized in the presence of Sr(OH)₂ does not lose its PNA capacity after extremely harsh aging at 850 and 900 °C (10 h in 10% H₂O/air flow) and loses only ~55% capacity after hydrothermal aging at 930 °C. This can be extended to other divalent metals for catalytic applications, such as copper: we show that Cu/SSZ-13 catalyst can survive hydrothermal aging at 920 °C without losing its catalytic properties, metal dispersion and crystalline structure. Thus, we provide a new, simple, and scalable strategy for making remarkably (hydro)thermally stable metal-zeolite materials/catalysts with a number of useful applications. Full article

Cu-CHA zeolites have proven to be effective for NO_x reduction, but a drawback in using CHA zeolites is the cost associated with using expensive organic structure-directing agents. To overcome this drawback, we are reporting here the synthesis of Cu-CHA zeolite catalysts in both their NH₄-form as well as K-form that do not require the use of organic structure-directing agents. After comprehensive characterization by XRF, XRD, ²⁷Al NMR spectroscopy, FE-SEM, SEM/EDS, N₂-adsorption/desorption, NH₃-TPD, H₂-TPR, and XPS, the zeolite catalysts were tested for NO_x conversion by NH₃-selective catalytic reduction (NH₃-SCR). Cu-NH₄-CHA zeolite catalysts exhibited remarkable activity and thermal stability over a wide temperature window, outperforming their counterpart K-forms. Among the NH₄-forms of CHA zeolite catalysts, the 0.1 M Cu-NH₄-CHA showed the best catalytic performance, achieving 50% NO_x conversion at a temperature as low as 192 °C, and reaching full conversion of NO_x at 261 °C. These Cu-based CHA zeolite catalysts are promising thanks to their environmentally friendly synthesis and offer the opportunity of maximizing DeNO_x strategies in applications for NO_x pollution abatement. Full article

At present, passive NO_x adsorbers (PNAs) represent one of the most effective technologies for addressing NO_x emissions from diesel engines during cold-start periods. Conventional PNAs, which primarily consist of noble metals (such as Pt, Pd, and Ag) loaded on metal oxides or zeolites, share the common drawback of high production costs. Consequently, developing low-cost PNAs with outstanding NO_x storage performance remains a significant challenge. In this study, a series of CuxBa5Ce adsorbents were synthesized using the impregnation method, and a monolithic adsorbent was employed to evaluate NO_x storage and release performance. Techniques such as XRD, UV-Vis DRs, H₂-TPR, XPS, and in situ DRIFTs confirmed the crucial roles of Cu and Ba in NO_x storage and release. Specifically, the incorporation of Cu into CeO₂ enhanced NO_x storage performance. Moreover, in the Cu3Ba5Ce adsorbent, the addition of Ba not only introduced new storage sites and altered the stability of NO_x adsorption species but also helped prevent the aggregation of CuO, thereby prolonging the complete NO_x storage duration and satisfying desorption temperature requirements. The Cu3Ba5Ce adsorbent exhibited the most favorable NO_x storage performance, including a complete NO_x storage time of 135 s and a NO_x storage efficiency exceeding 50% at 80 °C over a 10 min period. While PNAs loaded with noble metals, such as Pd/CeO₂ and Pt/CeO₂, exhibited NO_x storage efficiencies below 50% after adsorbing for 5 min at 80 °C. Therefore, this research offered a crucial strategy for developing non-noble-metal-loaded, Ce-based PNAs. Full article

In the current article, the effect of Si/Al ratio on the NO_x adsorption and storage capacity over Pd/Beta with 1 wt% Pd loading was investigated. The XRD, ²⁷Al NMR and ²⁹Si NMR measurements were used to determine the structure of Pd/Beta zeolites. XAFS, XPS, CO-DRIFT, TEM and H₂-TPR were used to identify the Pd species. The results showed that the NO_x adsorption and storage capacity on Pd/Beta zeolites gradually decreased with the increase of Si/Al ratio. Pd/Beta-Si (Si-rich, Si/Al~260) rarely has NO_x adsorption and storage capacity, while Pd/Beta-Al (Al-rich, Si/Al~6) and Pd/Beta-C (Common, Si/Al~25) exhibit excellent NO_x adsorption and storage capacity and suitable desorption temperature. Pd/Beta-C has slightly lower desorption temperature compared to Pd/Beta-Al. The NO_x adsorption and storage capacity increased for Pd/Beta-Al and Pd/Beta-C by hydrothermal aging treatment, while the NO_x adsorption and storage capacity on Pd/Beta-Si had no change. Full article

Adsorption is a potential technology that is expected to meet NO_x ultra-low emission standards and achieve the recovery of NO₂. In this study, the adsorption/desorption behavior of NO_x with competitive gases (e.g., H₂O(g) and CO₂) was studied on MFI zeolites with different Si/Al ratios and under different relative humidity (0~90% RH). Sample characterization of self-synthesizing zeolites was conducted by means of X-ray diffraction, Ar adsorption-desorption, and field emission scanning electron microscopy. The results showed that low-silica HZSM-5(35) showed the highest NO_x adsorption capacity of 297.8 μmol/g (RH = 0) and 35.4 μmol/g (RH = 90%) compared to that of other adsorbents, and the efficiency loss factor of NO_x adsorption capacity at 90%RH ranged from 85.3% to 88.1%. A water-resistance strategy was proposed for NO_x multicomponent competitive adsorption combined with dynamic breakthrough tests and static water vapor adsorption. The presence of 14% O₂ and lower adsorption temperature (25 °C) favored NO_x adsorption, while higher CO₂ concentrations (~10.5%) had less effect. The roll-up factor (η) was positively correlated with lower Si/Al ratios and higher H₂O(g) concentrations. Unlike Silicalite-1, HZSM-5(35) exhibited an acceptable industrial desorption temperature window of NO₂ (255~265 °C). This paper aims to provide a theoretical guideline for the rational selection of NO_x adsorbents for practical applications. Full article

CeO₂ nanoparticle-loaded MnO₂ nanoflowers, prepared by a hydrothermal method followed by an adsorption-calcination technique, were utilized for selective catalytic reduction (SCR) of NO_x with NH₃ at low temperatures. The effects of Ce/Mn ratio and thermal calcination temperature on the NH₃–SCR activity of the CeO₂-MnO₂ nanocomposites were studied comprehensively. The as-prepared CeO₂-MnO₂ catalysts show high NO_x reduction efficiency in the temperature range of 150–300 °C, with a complete NO_x conversion at 200 °C for the optimal sample. The excellent NH₃–SCR performance could be ascribed to high surface area, intimate contact, and strong synergistic interaction between CeO₂ nanoparticles and MnO₂ nanoflowers of the well-designed composite catalyst. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) characterizations evidence that the SCR reaction on the surface of the CeO₂-MnO₂ nanocomposites mainly follows the Langmuir–Hinshelwood (L-H) mechanism. Our work provides useful guidance for the development of composite oxide-based low temperature NH₃–SCR catalysts. Full article