Dual Promotional Effects of TiO2-Decorated Acid-Treated MnO x Octahedral Molecular Sieve Catalysts for Alkali-Resistant Reduction of NO x

A comprehensive review of deactivation and modification of selective catalytic reaction catalysts installed in cement kilns

After the ultralow emission transformation of coal-fired power plants, cement production became China's leading industrial emission source of nitrogen oxides. Flue gas dust contents at the outlet of cement kiln preheaters were as high as 80-100 g/m3, and the calcium oxide content in the dust exceeded 60%. Commercial V2O5(-WO3)/TiO2 catalysts suitable for coal-fired flue gas suffer from alkaline earth metal Ca poisoning of cement kiln flue gas. Recent studies have also identified the poisoning of cement kiln selective catalytic reaction (SCR) catalysts by the heavy metals lead and thallium. Investigation of the poisoning process is the primary basis for analyzing the catalytic lifetime. This review summarizes and analyzes the SCR catalytic mechanism and chronicles the research progress concerning this poisoning mechanism. Based on the catalytic and toxification mechanisms, it can be inferred that improving the anti-poisoning performance of a catalyst enhances its acidity, surface redox performance-active catalytic sites, and shell layer protection. The data provide support in guiding engineering practice and reducing operating costs of SCR plants. Finally, future research directions for SCR denitrification catalysts in the cement industry are discussed. This study provides critical support for the development and optimization of poisoning-resistant SCR denitrification catalysts.

Unlocking Mixed-Metal Oxides Active Centers via Acidity Regulation for K&SO2 Poisoning Resistance: Self-Detoxification Mechanism of Zeolite-Confined deNOx Catalysts

Selective catalytic reduction of nitrogen oxides (NOx) with ammonia (NH3-SCR) is an efficient NOx reduction strategy, while the denitrification (deNOx) catalysts suffer from serious deactivation due to the coexistence of multiple poisoning substances, such as alkali metal (e.g., K), SO2, etc., in industrial flue gases. It is essential to understand the interaction among various poisons and their effects on the deNOx process. Herein, the ZSM-5 zeolite-confined MnSmOx mixed (MnSmOx@ZSM-5) catalyst exhibited better deNOx performance after the poisoning of K, SO2, and/or K&SO2 than the MnSmOx and MnSmOx/ZSM-5 catalysts, the deNOx activity of which at high temperature (H-T) increased significantly (>90% NOx conversion in the range of 220-480 °C). It has been demonstrated that K would occupy both redox and acidic sites, which severely reduced the reactivity of MnSmOx/ZSM-5 catalysts. The most important, K element is preferentially deposited at -OH on the surface of ZSM-5 carrier due to the electrostatic attraction (-O-K). As for the K&SO2 poisoning catalyst, SO2 preferred to be combined with the surface-deposited K (-O-K-SO2ads) according to XPS and density functional theory (DFT) results, the poisoned active sites by K would be released. The K migration behavior was induced by SO2 over K-poisoned MnSmOx@ZSM-5 catalysts, and the balance of surface redox and acidic site was regulated, like a synergistic promoter, which led to K-poisoning buffering and activity recovery. This work contributes to the understanding of the self-detoxification interaction between alkali metals (e.g., K) and SO2 on deNOx catalysts and provides a novel strategy for the adaptive use of one poisoning substance to counter another for practical NOx reduction.

Insight into the Alkali Resistance Mechanism of FeMoTiOx Catalysts for NH3 Selective Catalytic Reduction of NO: Self-Defense Effects of MoOx for Alkali Capture

The deactivation of selective catalytic reduction (SCR) catalysts caused by alkali metal poisoning remains an insurmountable challenge. In this study, we examined the impact of Na poisoning on the performance of Fe and Mo co-doped TiO2 (FeaMobTiOx) catalysts in the SCR reaction and revealed the related alkali resistance mechanism. On the obtained Fe1Mo2.6TiOx catalyst, the synergistic catalytic effect of uniformly dispersed FeOx and MoOx species leads to remarkable catalytic activity, with over 90% NO conversion achieved in a wide temperature range of 210-410 °C. During the Na poisoning process, Na ions predominantly adsorb on the MoOx species, which exhibit stronger alkali resistance, effectively safeguarding the FeOx species. This preferential adsorption minimizes the negative effect of Na poisoning on Fe1Mo2.6TiOx. Moreover, Na poisoning has little influence on the Eley-Rideal reaction pathway involving adsorbed NHx reacting with gaseous NOx. After Na poisoning, the Lewis acid sites were deteriorated, while the abundant Brønsted acid sites ensured sufficient NHx adsorption. As a benefit from the self-defense effects of active MoOx species for alkali capture, FeaMobTiOx exhibits exceptional alkali resistance in the SCR reaction. This research provides valuable insights for the design of highly efficient and alkali-resistant SCR catalysts.

Compensation or Aggravation: Pb and SO2 Copoisoning Effects over Ceria-Based Catalysts for NOx Reduction

Severe catalyst deactivation caused by multiple poisons, including heavy metals and SO₂, remains an obstinate issue for the selective catalytic reduction (SCR) of NO_x by NH₃. The copoisoning effects of heavy metals and SO₂ are still unclear and irreconcilable. Herein, the unanticipated differential compensated or aggravated Pb and SO₂ copoisoning effects over ceria-based catalysts for NO_x reduction was originally unraveled. It was demonstrated that Pb and SO₂ exhibited a compensated copoisoning effect over the CeO₂/TiO₂ (CT) catalyst with sole active CeO₂ sites but an aggravated copoisoning effect over the CeO₂-WO₃/TiO₂ (CWT) catalyst with dual active CeO₂ sites and acidic WO₃ sites. Furthermore, it was uniquely revealed that Pb preferred bonding with CeO₂ among CT while further being combined with SO₂ to form PbSO₄ after copoisoning, which released the poisoned active CeO₂ sites and rendered the copoisoned CT catalyst a recovered reactivity. In comparison, Pb and SO₂ would poison acidic WO₃ sites and active CeO₂ sites, respectively, resulting in a seriously degraded reactivity of the copoisoned CWT catalyst. Therefore, this work thoroughly illustrates the internal mechanism of differential compensated or aggravated deactivation effects for Pb and SO₂ copoisoning over CT and CWT catalysts and provides effective solutions to design ceria-based SCR catalysts with remarkable copoisoning resistance for the coexistence of heavy metals and SO₂.

Progress of selective catalytic reduction denitrification catalysts at wide temperature in carbon neutralization

With the looming goal of carbon neutrality and increasingly stringent environmental protection policies, gas purification in coal-fired power plants is becoming more and more intense. To achieve the NOx emission standard when coal-fired power plants are operating at full load, wide-temperature denitrification catalysts that can operate for a long time in the range of 260–420°C are worthy of study. This review focuses on the research progress and deactivation mechanism of selective catalytic reduction (SCR) denitration catalysts applied to a wide temperature range. With the increasing application of SCR catalysts, it also means that a large amount of spent catalysts is generated every year due to deactivation. Therefore, it is necessary to recycle the wide temperature SCR denitration catalyst. The challenges faced by wide-temperature SCR denitration catalysts are summarized by comparing their regeneration processes. Finally, its future development is prospected.

Low-Temperature SCR Catalyst Development and Industrial Applications in China

In recent years, low-temperature SCR (Selective Catalytic Reduction) denitrification technology has been popularized in non-power industries and has played an important role in the control of industrial flue gas NOx emissions in China. Currently, the most commonly used catalysts in industry are V2O5-WO3(MoO3)/TiO2, MnO2-based catalysts, CeO2-based catalysts, MnO2-CeO2 catalysts and zeolite SCR catalysts. The flue gas emitted during industrial combustion usually contains SO2, moisture and alkali metals, which can affect the service life of SCR catalysts. This paper summarizes the mechanism of catalyst poisoning and aims to reduce the negative effect of NH4HSO4 on the activity of the SCR catalyst at low temperatures in industrial applications. It also presents the outstanding achievements of domestic companies in denitrification in the non-power industry in recent years. Much progress has been made in the research and application of low-temperature NH3-SCR, and with the renewed demand for deeper NOx treatments, new technologies with lower energy consumption and more functions need to be developed.

Alkali-Resistant Catalytic Reduction of NOx by Using Ce-O-B Alkali-Capture Sites

Reducing the poisoning effect arising from alkali metals over catalysts for selective catalytic reduction (SCR) of NO_x by NH₃ is still an urgent issue to be solved. Herein, alkali-resistant NO_x reduction over B-doped CeO₂/TiO₂ catalysts (Ce-B/TiO₂) with Ce-O-B alkali-capture sites was originally demonstrated. It was noted that boron was confirmed to be doped into the lattice of CeO₂ to form the Ce-O-B structure. In this way, more active Ce(III) species and oxygen vacancies were generated from B-doped CeO₂, thus accelerating the redox cycle and enhancing the adsorption/activation of NO. Gratifyingly, the created Ce-O-B sites as alkali-capture sites could be effectively combined with K and release the poisoned Ce active sites, which maintained efficient NH₃ and NO adsorption/activation over K poisoned Ce-B/TiO₂. This work paves a way for designing highly efficient and alkali-resistant SCR catalysts in both academic and industrial fields.

A review of Mn-based catalysts for low-temperature NH3-SCR: NOx removal and H2O/SO2 resistance

The development of high-efficiency catalysts is the key to the low-temperature NH₃-SCR technology. The introduction of SO₂ and H₂O will lead to poisoning and deactivation of the catalysts, which severely limits the development and application of NH₃-SCR technology. This review introduces the necessity of NO_x removal, explains the mechanisms of H₂O and SO₂ poisoning on NH₃-SCR catalysts, highlights the Mn-based catalysts of different active metals and supports and their resistance to H₂O and SO₂, and analyses the relationship between metal modification, selection of support and preparation method, morphology and structure design and SO₂/H₂O resistance. Given the current problems, this review points out the future research focus of Mn-based catalysts and also puts forward corresponding countermeasures to solve the existing problems.

MnO2 -Based Materials for Environmental Applications

Manganese dioxide (MnO₂ ) is a promising photo-thermo-electric-responsive semiconductor material for environmental applications, owing to its various favorable properties. However, the unsatisfactory environmental purification efficiency of this material has limited its further applications. Fortunately, in the last few years, significant efforts have been undertaken for improving the environmental purification efficiency of this material and understanding its underlying mechanism. Here, the aim is to summarize the recent experimental and computational research progress in the modification of MnO₂ single species by morphology control, structure construction, facet engineering, and element doping. Moreover, the design and fabrication of MnO₂ -based composites via the construction of homojunctions and MnO₂ /semiconductor/conductor binary/ternary heterojunctions is discussed. Their applications in environmental purification systems, either as an adsorbent material for removing heavy metals, dyes, and microwave (MW) pollution, or as a thermal catalyst, photocatalyst, and electrocatalyst for the degradation of pollutants (water and gas, organic and inorganic) are also highlighted. Finally, the research gaps are summarized and a perspective on the challenges and the direction of future research in nanostructured MnO₂ -based materials in the field of environmental applications is presented. Therefore, basic guidance for rational design and fabrication of high-efficiency MnO₂ -based materials for comprehensive environmental applications is provided.

Formation of Fly Ash Catalysts and Selection of a Matrix Binder and Its Application in Denitration

The development of high-efficiency and low-cost new catalysts is an extremely attractive topic. In this study, two different matrix bentonite-modified fly ash catalysts were successfully prepared, and the compressive strength of the catalyst was studied by using unsaturated dynamic and static triaxial technology. The axial compressive strength of FC (fly ash catalysts added with Ca-based bentonite) was greater than that of FN (fly ash catalysts added with Na-based bentonite). The catalyst reached 978 kPa. The prepared catalyst was characterized by X-ray diffraction analysis, Fourier transform infrared spectroscopy, scanning electron microscopy, and specific surface area analysis (BET) of the catalyst. In addition, denitration performance of different catalysts was explored, and the reaction conditions were optimized. The results demonstrate that when the mixing ratio of fly ash and calcium-based bentonite in the FC is 4:1, the compressive strength is relatively high, and the denitration rate reaches about 82%.

Self-Protected CeO2-SnO2@SO42-/TiO2 Catalysts with Extraordinary Resistance to Alkali and Heavy Metals for NOx Reduction

Reducing the poisoning effect of alkali and heavy metals over ammonia selective catalytic reduction (NH₃-SCR) catalysts is still an intractable issue, as the presence of K and Pb in fly ash greatly hampers their catalytic activity by impairing the acidity and affecting the redox properties of the catalysts, leading to the reduction in the lifetime of SCR catalysts. To address this issue, we propose a novel self-protected antipoisoning mechanism by designing SO₄^2-/TiO₂ superacid supported CeO₂-SnO₂ catalysts. Owing to the synergistic effect between CeO₂ and SnO₂ and the strong acidity originating from the SO₄^2-/TiO₂ superacid, the catalysts show superior catalytic activity over a wide temperature range (240-510 °C). Moreover, when K or/and Pb are deposited on SO₄^2-/TiO₂ catalysts, the bond effect between SO₄^2- and Ti-O would be broken so that the sulfate in the bulk of SO₄^2-/TiO₂ superacid support would be induced to migrate to the surface to bond with K and Pb, thus prohibiting poisons from attacking the Ce-Sn active sites, and significantly boosting the resistance. Hopefully, this novel self-protection mechanism derived from the migration of sulfate in the SO₄^2-/TiO₂ superacid to resist alkali and heavy metals provides a new avenue for designing novel catalysts with outstanding resistance to alkali and heavy metals.

Alkali and Phosphorus Resistant Zeolite-like Catalysts for NOx Reduction by NH3

At present, the deactivation of selective catalytic reduction (SCR) catalysts caused by the coexistence of alkali metal and phosphorus (P) remains an urgent problem and lacks corresponding strategies against catalyst poisoning. Herein, a novel zeolite-like Ce-Si₅Al₂O_x catalyst derived from an ultrasmall nanozeolite EMT precursor was synthesized without organic templates at ambient temperature. This catalyst was able to maintain above 95% NO_x conversion in the 270-540 °C temperature range. Moreover, 1 wt % potassium (K) and 5 wt % P loading had no influence on the SCR performance of the Ce-Si₅Al₂O_x catalyst at 300-480 °C. It was demonstrated that cerium (Ce) was highly dispersed in the amorphous aluminum (Al) silicate derived from EMT zeolites and expressed high catalytic performance. Besides, a large number of acid sites were reserved to absorb ammonia allowing effective participation in the SCR reaction and capturing alkali metals, thus improving the SCR performance and K resistance. Additionally, the strong interaction between Ce and aluminosilicate decreased cerium phosphate production, preventing deactivation of the catalysts. Thus, this novel low-cost zeolite-like Ce-Si₅Al₂O_x catalyst with a highly active ion-exchanged metal phase and abundant surface acid sites paves a way for designing new efficient and poisoning-resistant SCR catalysts for practical applications.

Cu-Mn-Ce ternary oxide catalyst coupled with KOH sorbent for air pollution control in confined space

For air pollution control in confined space such as submarine and spacecraft, copper-manganese-cerium ternary oxide catalysts coupled with KOH sorbent were synthesized through the wet impregnation method, solid-state impregnation method A and B, and wet/solid-state impregnation method. The samples were tested for CO and CO₂ removal dynamically and isothermally from 30 °C to 150 °C using two fixed bed reactors, and then characterized by XRD, nitrogen adsorption and desorption, and FE-SEM/EDS. The results showed that all the coupled CuMnCe/KOHs were able to catalyze CO and capture the produced CO₂ in situ. While the coupling treatments affected the CO oxidation and CO₂ absorption performance of the samples significantly and differently. Among all samples, CuMnCe/KOH-WSI with the large KOH bulk phase exhibited the outstanding CO catalytic activity and CO₂ sorption efficiency, higher than the uncoupled CuMnCe/KOH. While for CuMnCe/KOH-WI and CuMnCe/KOH-SI-I samples demonstrating high-dispersed KOH species in the catalyst, the addition of the sorbent could inhibit the catalyst activity due to the occupation of the surface site and pore structure. Furtherly, the effect of the temperature was varied for CO conversion and CO₂ capture performances of the sample, while they achieved an optimization balance at 150 °C for CuMnCe/KOH-WSI.

Alkali-Poisoning-Resistant Fe2O3/MoO3/TiO2 Catalyst for the Selective Reduction of NO by NH3: The Role of the MoO3 Safety Buffer in Protecting Surface Active Sites

The exhaust gas contains harmful products, including fuel-additive elements such as compounds of sodium, which cause dramatic catalyst deactivation of catalysts during selective catalytic reduction (SCR) of NO with NH₃. There is an increasing demand to synthesize alkali-poisoning-resistant catalysts for industrial NH₃-SCR applications. In this study, the as-synthesized Fe₂O₃/MoO₃/TiO₂ exhibits a high degree of resistance toward Na₂SO₄ poisoning during the NH₃-SCR reaction. With 500 μmol g^-1 Na⁺ poisoning, Fe₂O₃/MoO₃/TiO₂ showed approximately 95% (or more) of its original activity throughout the entire temperature rage. Even with 700 μmol g^-1 Na⁺ poisoning, Fe₂O₃/MoO₃/TiO₂ still performed well. The 500 and 700 μmol g^-1 Na⁺ loadings dictate that, on average, SCR catalysts could be exposed to alkali-rich and highly dusty environments for more than 14 000 and 20 000 h, respectively. The layered MoO₃ building block is used as a binding buffer and sandwiched between the active phase and TiO₂ support to provide sufficiently stable binding sites for Na₂SO₄ poison and to present alkali blocking of the surface active phase. Our findings provide useful information regarding the use of MoO₃ as a safety buffer for developing functional NH₃-SCR catalysts with enhanced alkali-poisoning-resistant performance and long lifetimes.

Highly dispersed MnOx–FeOx supported by silicalite-1 for the selective catalytic reduction of NOx with NH3 at low temperatures

MnO_x–FeO_x-Loaded silicalite-1 catalysts exhibit high NO_x conversion at low temperatures.

In situ decorated MOF-derived Mn–Fe oxides on Fe mesh as novel monolithic catalysts for NOx reduction

In situ decorated MOF-derived Mn–Fe oxides on Fe mesh were developed as novel monolithic catalysts for NO_x reduction.

Promotional effects of modified TiO2- and carbon-supported V2O5- and MnOx-based catalysts for the selective catalytic reduction of NOx: a review

This review presents the promotional effects of transition metal modification over TiO₂- and carbon-supported V₂O₅- and MnO_x-based SCR catalysts.

Enhancing Water Resistance of a Mn-Based Catalyst for Low Temperature Selective Catalytic Reduction Reaction by Modifying Super Hydrophobic Layers

OMS-2 catalysts exhibit excellent selective catalytic reduction (SCR) activity at low temperature but weak H₂O resistance restricts its industrial application. To remarkably improve the water resistance of Mn-based catalysts is a key technical problem. In this work, the H₂O endurance and self-cleaning properties of OMS-2 catalysts are remarkably improved by the facile process, construction of hydrophobic coating. The performance of the hydrophobic layer on the bulk OMS-2 catalyst surface could be effectively controlled by adjusting the polydimethylsiloxane (PDMS) vapor deposition temperature. It is discovered that the 200 °C catalyst obtained super hydrophobic properties and formed with a contact angle of 160.3°, which not only exhibited satisfactory NH₃-SCR activity at low temperatures (140-300 °C) but also dramatically improved H₂O endurance and self-cleaning performance. Moreover, the mechanism of improving H₂O resistance and stability of the 200 °C catalyst was investigated in detail through a series of characterizations. Although the SCR activity of the 200 °C catalyst decreased slightly because of the combination of some active species (O_α and Mn³⁺) with PDMS, the H₂O passivation of the active species was eliminated. The advantage of self-cleaning was confirmed by the analysis of surface species and simulation experiments, which could avoid the accumulation of intermediates on the surface and promote the stability of the OMS-2 catalyst for NH₃-SCR at low temperature. This method of constructing special coating might be a huge step to remarkably improve the H₂O endurance properties of the catalyst and provided a new concept for future industrial application.

Fabrication of Highly Dispersed Cu-Based Oxides as Desirable NH3-SCR Catalysts via Employing CNTs To Decorate the CuAl-Layered Double Hydroxides

In this study, three kinds of CuAl-LDO/CNT (LDO, layered double oxide) catalysts were prepared by the assembly of CNTs and CuAl-LDH (LDH, layered double hydroxides) as well as subsequently structural topological transformation. The effects of the assembly method on the surface structure property and the DeNO_x performance of the prepared samples were systematically investigated. It was found that three CuAl-LDO/CNT catalysts showed preferable NH₃-SCR catalytic performance compared with CuAl-LDO where the catalyst CuAl-LDO/CNTs(I) exhibited optimum NO_x conversion (>80%) and N₂ selectivity (>90%) within 180-300 °C. Such fine catalytic performance can be attributed to the proper surface acidity and redox ability of the catalyst, which might be correlated with the high dispersion of Cu-based active centers caused by the induced nucleation and effective separation action of LDH by carbon nanotubes. In addition, the outstanding H₂O and SO₂ resistance of the CuAl-LDO/CNTs(I) catalyst was also obtained because of the synergistic effect between CuAl-LDO and CNTs, which could greatly promote the activation and decomposition of ammonium sulfate at lower temperatures.

Enhancing low-temperature SCR de-NOx and alkali metal poisoning resistance of a 3Mn10Fe/Ni catalyst by adding Co

Alkaline K poisoned and Co-modified catalysts were prepared using Fe and Mn as active components, nickel foam as a carrier, and Co as a trace additive.

Low-valence or tetravalent cation doping of manganese oxide octahedral molecular sieve (K-OMS-2) materials for nitrogen oxide emission abatement

Substitutionally doped oxide catalysts with abundant oxygen vacancy defects (OVDs) can effectively improve the catalytic activity efficiency.