NO reduction over nanostructure M-Cu/ZSM-5 (M: Cr,Mn, Co and Fe) bimetallic catalysts and optimization of catalyst preparation by RSM

The selective catalytic reduction (SCR) of NO with NH₃ in the presence of oxygen over a series of H-ZSM-5 supported transition metal oxides (Co, Mn, Cr, Cu and Fe) was investigated. Among them, Cu/ZSM-5 nanocatalyst was found to be the most promising catalyst based on activity. The modification of Cu/ZSM-5 by adding different transition metals (Co, Mn, Cr and Fe) to improve the efficiency of NO conversion was studied. The results indicated that the Fe–Cu/ZSM-5 bimetallic nanocatalyst was the highest active catalyst for NO conversion (67% at 250 °C and 93% at 300 °C). Response surface methodology (RSM) involving central composite design (CCD) was employed to evaluate and optimize Fe–Cu/ZSM-5 preparation parameters (Fe loading, calcinations temperature, and impregnation temperature) in SCR of NO at 250 °C. The optimum condition for maximum NO conversion was estimated at 4.2 wt.% Fe loading, calcinations temperature of 577 °C and impregnation temperature of 43.5 °C. Under these condition, experimental NO conversion efficiency was 78.8%, which was close with the predicted value (79.4%).     

Selective Reduction of NO with CO Over Titania Supported Transition Metal Oxide Catalysts

A series of transition metal oxides promoted titania catalysts (MO _x /TiO₂; M = Cr, Mn, Fe, Ni, Cu) were prepared by wet impregnation method using dilute solutions of metal nitrate precursors. The catalytic activity of these materials was evaluated for the selective catalytic reduction (SCR) of NO with CO as reductant in the presence of excess oxygen (2 vol.%). Among various promoted oxides, the MnO _x /TiO₂ system showed very promising catalytic activity for NO + CO reaction, giving higher than 90% NO conversion over a wide temperature window and at high space velocity (GHSV) of 50,000 h⁻¹. It is remarkable to note that the catalytic activity increased with oxygen, up to 4 vol.%, under these conditions leading primarily to nitrogen. Our TPR studies revealed the presence of mixed oxidation states of manganese on the catalyst surface. Characterization results indicated that the surface manganese oxide phase and the redox properties of the catalyst play an important role in final catalytic activity.     

High activity and wide temperature window of Fe‐Cu‐SSZ‐13 in the selective catalytic reduction of NO with ammonia

Fe‐Cu‐SSZ‐13 catalysts were prepared by aqueous solution ion‐exchange method based on the one‐pot synthesized Cu‐SSZ‐13. The catalysts were applied to the selective catalytic reduction (SCR) of NO with NH₃ and characterized by the means of XRD, UV‐Vis, EPR, XPS, NH₃‐TPD, and so on. The selected Fe‐Cu‐SSZ‐13‐1 catalyst exhibited the high NO conversion (>90%) in the wide temperature range (225–625°C), which also showed good N₂ selectivity and excellent hydrothermal stability. The results of XPS showed that the Cu and Fe species were in the internal and outer parts of the SSZ‐13 crystals, respectively. The results of UV‐Vis and EPR indicated that the monomeric Cu²⁺ ions coordinated to three oxygen atoms on the six‐ring sites and Fe monomers are the real active species in the NH₃‐SCR reaction. Furthermore, the influence of intracrystalline mass‐transfer limitations on the Fe‐Cu‐SSZ‐13 catalysts is related to the location of active species in the SSZ‐13 crystals. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3825–3837, 2015     

铜系低温选择性催化还原脱硝催化剂的研究进展

选择性催化还原（SCR）技术已广泛应用在燃煤电站烟气脱硝技术中,开发低温高活性、高抗中毒性能的催化剂体系已经成为国内外学者的研究重点。Cu系催化剂由于具有良好的脱硝性能及水热稳定性,得到了广泛的研究和关注。本文综述了近年来活性组分Cu负载在TiO₂、Al₂O₃、碳基材料和分子筛等材料上的研究进展;重点分析了Cu系催化剂低温SCR反应机理,主要包括Eley-Rideal （E-R）机理和Langmuir-Hinshelwood （L-H）机理,同时分析了SCR反应的两个必然过程：吸附（NH₃吸附和NO_x吸附）和反应;简要地介绍了Cu系催化剂的抗水抗硫中毒性能研究现状以及反应机理,同时介绍了碱金属中毒、飞灰和催化剂烧结对催化剂失活的影响,结合生命周期分析SCR脱硝系统还原剂氨和尿素对NO排放的影响。在此基础上展望了未来铜系催化剂的研究方向：采用新型方式对催化剂进行改性、进一步采用表征和模拟技术研究催化体系的反应机理、优化锅炉和催化剂设计减轻催化剂失活以及研究适用于其他还原剂条件的高选择性催化剂。     

Selective Catalytic Reduction of NO by Ammonia over Raney‐Ni Supported Cu‐ZSM–5 I. Catalyst Activity and Stability

Composite materials containing Raney Ni and Cu‐ZSM‐5 are highly active catalysts for the selective catalytic reduction (SCR) of NO by NH₃. Their catalytic properties were studied with particular attention to the influence of moisture and SO₂ in the feed, and to effects of catalyst shaping operations. Composite materials (16–20 wt‐% zeolite) were prepared by mixing the components, with different degree of segregation in the resulting pressed particles, or by growing ZSM‐5 crystallites on the surface of leached Raney Ni, which were then exchanged with Cu ions. Catalytic tests were performed with 1000 ppm NO, 1000 ppm NH₃, 2 % O₂ in He, at 3–6.5 · 10⁵ h^–1 (related to zeolite component). With physical mixtures, the catalytic behaviour strongly depended on the mixing strategy, particles containing both Ni and zeolite being inferior to mixed Ni‐only and zeolite‐only particles. The SCR activity was promoted by 2 % H₂O in the feed, SO₂ (200 ppm) was a moderate poison at low temperatures, but indifferent or slightly promoting at high temperatures. A catalyst prepared from ZSM‐5 grown on Raney Ni, which was ranked intermediate in dry feed, was promoted to excellent performance in H₂O and SO₂ containing feed at T > 700 K and was stable for 38 h at 845 K. The results suggest that SCR catalysts containing highly active zeolites should be produced avoiding shaping operations e.g. by use of zeolite crystallites grown on wire packings.     

V2O5-MoO3/TiO2 催化剂的NOx选择性催化还原及SO2氧化活性

采用浸渍法以TiO₂为载体制备V₂O₅-MoO₃/TiO₂ 选择性催化还原催化剂,研究V₂O₅和MoO₃负载量对于催化剂选择性催化还原反应及SO₂氧化活性的影响,并考察氧含量、氨氮物质的量比和反应空速对3%V₂O₅-6%MoO₃/TiO₂催化剂选择性催化还原脱硝活性的影响。结果表明,随着催化剂中V₂O₅负载质量分数增加,V₂O₅-MoO₃/TiO₂ 催化剂的选择性催化还原活性和SO₂氧化活性均呈上升趋势。MoO₃的负载对催化剂的SO₂氧化活性有明显抑制作用。MoO₃负载质量分数超过9%,制备的催化剂既保持较高的低温选择性催化还原活性,又使选择性催化还原反应中的SO₂转化率小于1%。     

Study of M-ZSM-5 nanocatalysts (M:Cu,Mn, Fe,Co...) for selective catalytic reduction of NO with NH3: Process optimization by Taguchi method

A series of different transition metals (V, Co, Cr, Mn, Fe, Ni, Cu and Zn) promoted H-ZSM-5 catalysts were prepared by impregnation method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The catalytic activity of these catalysts was evaluated for the selective catalytic reduction (SCR) of NO with NH₃ as reductant in the presence of oxygen. The results revealed that the catalytic activity of Cu-ZSM-5 nanocatalyst for NO conversion to N₂ was 80% at 300 ℃, which was the best among various promoted metals. Design of experiments (DOEs) with Taguchi method was employed to optimize NH₃-SCR process parameters such as NH₃/NO ratio, O₂ concentration, and gas hourly space velocity (GHSV) over Cu-ZSM-5 nanocatalyst at 250 and 300 ℃. Results showed that the most important parameter in NH₃-SCR of NO is O₂ concentration; followed by NH₃/NO ratio and GHSV has little importance. The NO conversion to N₂ of 63.1% and 94.86%was observed at 250 ℃ and 300 ℃, respectively under the obtained optimum conditions.     

Speciation of major and selected trace elements in IGCC fly ash

The speciation of Ga, Ge, Ni, V, S and Fe in fly ash from IGCC power plant were investigated for possible further extraction process by combining conventional mineral and chemical analysis, leaching tests, wet sequential extraction, Mössbauer and XAFS spectroscopies. The results shown that Ge occurs mainly as water-soluble species, GeS and/or GeS₂ and hexagonal GeO₂. Ga is present as an oxide, Ni occurs mainly as nickeline (NiAs), with minor proportions of Ni arsenates and vanadium as V(III) with minor amounts of V(IV) in the aluminosilicate glass matrix. Pyrrhotite and wurtzite-sphalerite are sulfide species containing Fe and Zn, but an important fraction of iron is also present in the aluminosilicate glass. These clear differences between the speciation of the above elements in this material and those reported for fly ash from conventional PC combustion.     

Selective catalytic reduction of NO with NH3 at low temperatures over iron and manganese oxides supported on mesoporous silica

A series of catalysts of iron–manganese oxide supported on mesoporous silica (MPS) with different Mn/Fe ratio were studied for low-temperature selective catalytic reduction (SCR) of NO with ammonia in the presence of excess oxygen. Effects of amounts of iron–manganese oxide and calcination temperatures on NO conversion were also investigated. It was found that the Mn–Fe/MPS with Mn/Fe = 1 at the calcination temperature of 673 K showed the highest activity. The results showed that this catalyst yielded 99.1% NO conversion at 433 K at a space velocity of 20,000 h⁻¹. H₂O has no adverse impact on the activity when the SCR reaction temperature is above 413 K. In addition, the SCR activity was suppressed gradually in the presence of SO₂ and H₂O, while such effect was reversible after heating treatment.     

Ce-Fe/ZSM-5催化剂用于燃机烟气SCR脱硝的性能

采用浸渍法制备了一系列Ce?Fe/ZSM-5催化剂,基于燃机烟气工况研究了Fe含量及Ce掺杂量对Fe/ZSM-5催化剂的中高温脱硝性能的影响,并结合一系列表征技术对其物化性能进行研究。结果表明,Fe含量为4wt%时,Fe/ZSM-5催化剂在550℃下NOx转化率为77.11%。掺杂Ce后Ce?Fe/ZSM-5催化剂的高温脱硝效果明显提升,Ce负载量为1wt%时,550℃时NOx转化率仍保持95.92%,催化剂有优异的中高温催化活性,比Fe4/ZSM-5提高了18.81%。同时改变烟气中的NO2和O2含量,发现NO2和O2浓度增加均可提高催化剂的脱硝性能,水热老化测试表明Ce1?Fe4/ZSM-5催化剂具有优异的水热稳定性,分别在10vol% H2O, 600℃和10vol% H2O, 800℃条件下老化后,在450?550℃内NOx转化率保持约90%。适量掺杂Ce后催化剂表面Lewis酸含量及强度增强,表面吸附氧比例增大,Ce与Fe元素间的协同作用提高了催化剂的高温氧化还原能力,提升了高温活性,因此促进了中高温条件下SCR反应的进行。     

Synthesis of carbon nanotubes by catalytic conversion of methane: Competition between active components of catalyst

Catalytic growth of carbon nanotubes from methane was studied as competitive catalytic process in situ of both oxidative (partial oxidation) and non-oxidative (pyrolysis) conversion. Ni and Fe may act as either competing or cooperative catalyst components in the process of carbon nanotubes growth. The competition between Ni and Fe in the process of carbon nanotube growth on Ni-Fe based stainless steel alloy during partial oxidation of methane results in suppression of Ni catalytic activity in favor of Fe. The discrimination is so strong that iron is segregated from Ni-Fe based stainless steel alloy leaving characteristic Ni-enriched corrosion caverns. Interaction between Ni and Fe during non-oxidative conversion, in contrast to the oxidative conversion, leads to cooperative effect; the activity of bimetallic catalyst increases as compared with monometallic one. Depending on particular catalyst bed composition, the nanotubes of various morphologies may occur. In particular, the use of perovskite-type catalyst for partial oxidation of methane leads to formation of “olive-branch”-like peculiar carbon nanostructures.     

Effect of Cu doping on the SCR activity of CeO2 catalyst prepared by citric acid method

CeO₂–CuO catalyst prepared by citric acid method was investigated for selective catalytic reduction of NO with NH₃. The activity of the CeO₂ catalyst was enhanced about 8–27% in the temperature range of 125–225 °C at a space velocity of 28,000 h⁻¹ by the addition of Cu. It was found that the state of Cu species had great impact on the SCR performance of CeO₂–CuO catalyst. Cu²⁺ can enhance the low temperature activity of SCR reaction, while CuO would promote NH₃ oxidation before SCR reaction at high temperature, which would cause the decrease of its high temperature SCR activity.     

Synthesis and characterization of novel solid base catalyst from fly ash

A new type of solid base catalyst was synthesized by chemical and thermal activation of fly ash, collected from Thermal Super Power Station situated in Kota, Rajasthan, India. The chemical activation was carried out by 50 wt.% NaOH followed by thermal activation at 450 °C. The modified physiochemical property of solid base fly ash (SBFA) was determined by X-ray diffraction, FT-IR spectroscopy, Scanning Electron Microscopy, N₂ adsorption-desorption studies and Flame Atomic Absorption Spectrophotometry. The results reveal that the catalyst is nano-crystalline in nature with crystallite size 11 nm and particle size in the range 840 nm to 6.95 μm. The surface basicity and therefore, catalytic activity in SBFA was originated by increased hydroxyl content as compared to fly ash, suggesting that the catalyst possess higher surface active sites. The basicity of the catalyst was measured by liquid phase, solvent free, single step condensation of benzaldehyde with cyclohexanone giving higher conversion (>70%) and selectivity (>80%) of desired product α,α′-dibenzylidenecyclohexanone. This excellent conversion shows that the catalyst has sufficient basic sites both on the surface and in the bulk, responsible for the catalytic activity. Furthermore, this catalyst may replace conventional environmentally hazardous homogeneous liquid bases making an ecofriendly; solvent free, solid base catalyzed process. The application of fly ash to synthesize a solid base catalyst finds a noble way to utilize this abundant waste material.     

Mesoporous Fe-containing ZSM-5 zeolite single crystal catalysts for selective catalytic reduction of nitric oxide by ammonia

Mesoporous and conventional Fe-containing ZSM-5 catalysts (0.5–8 wt% Fe) were prepared using a simple impregnation method and tested in NO selective catalytic reduction (SCR) with NH₃. It was found that mesoporous Fe-ZSM-5 catalysts exhibit higher SCR activities than comparable conventional catalysts. Furthermore, conventional Fe-ZSM-5 catalysts have maximum activity at ~2.5 wt% Fe while for the mesoporous system, optimal NO conversion is obtained for the catalysts with ~6 wt % Fe.     

Selective catalytic reduction of NOx, by propene over copper-exchanged pillared clays

Copper-exchanged pillared clays were examined as an SCR catalyst for NO_x, removal by propene. Both micropores and mesopores were simultaneously developed by pillaring a bentonite with TiO₂. Therefore, TiO₂-pillared clay has about 8 to 9 times higher surface area and 3 times higher pore volume than the parent unpillared bentonite. The presence of water in the feed gas stream caused a small and reversible inhibition effect on NO removal activity of Cu/Ti-PILC. The water tolerance of Cu/Ti-PILC was higher than copper-exchanged zeolites such as CuHM and Cu/ZSM-5 due to its high hydrophobicity as confirmed by H₂O-TPD experiment. Copper-exchanged PILC was confirmed to be an active catalyst for NO_x, removal by propene. The addition of copper to TiO₂-pillared clay greatly enhanced the catalytic activity for NO removal. Cupric ions on Ti-PILC were active reaction sites for the present reaction system. The state of copper species on the surface of Ti-PILC varied with the content of copper and TiO₂. The catalyst having more easily reducible cupric ions showed maximum NO conversion at relatively lower reaction temperatures. It indicates that the redox behavior of cupric ions is directly related to NO removal mechanism. The redox property of cupric ions depended on the copper content and dehydration temperature of PILC.     

Selective catalytic reduction of NO with propene over CuMFI zeolites: dependence on Si/Al ratio and copper loading

Selective catalytic reduction of nitrogen oxide by propene in an oxidising atmosphere was studied on several CuMFI catalysts with different Si/Al ratios (11 Si/Al 100) and different copper loadings (between 0 and 5.5 wt.-%). From the results it was observed that the influence of zeolite Si/Al ratio on CuMFI catalytic activity for NO SCR by propene is dependent on the catalyst copper loading. Furthermore, the effect of catalyst copper loading on catalytic performance depended on the catalyst Si/Al ratio. The results also demonstrated that CuMFI catalysts with different Si/Al ratios and copper loadings, but with the same Cu/Al ratio and, therefore, the same copper exchange level have similar catalytic activity profiles for NO SCR.

It was further observed that not all Cu cations exchanged into MFI catalysts have equivalent catalytic activity for NO SCR, which made the existence of different copper environments on CuMFI catalysts evident, isolated Cu²⁺ ions being the most active species for NO SCR by propene.

Moreover, the results showed an improvement of the CuMFI catalytic activity at low temperatures by increasing the catalyst copper exchange level and, consequently, decreasing the number of Brönsted acid sites, which can be performed either by increasing the zeolite Si/Al ratio or copper loading.     

Effects of copper loading on NH<Subscript>3</Subscript>-SCR and NO oxidation over Cu impregnated CHA zeolite

Cu/CHA catalysts with various Cu loadings (0.5 wt%–6.0 wt%) were synthesized via incipient wetness impregnation. The catalysts were applied to the selective catalytic reduction (SCR) of NO with NH₃ and NO oxidation reaction. XRD and N₂ adsorption-desorption data showed that CHA structure was maintained with the incorporation of Cu, while specific surface areas decreased with increasing Cu loading. At intermediate Cu loading, 4 wt%, the highest NH₃-SCR activity was observed with ～98% N₂ selectivity from 150 °C to 300 °C. Small amounts of water, 2%, slightly increased NO conversion in addition to the remarkable N₂O and NO₂ reduction at high temperature. Water effects are attributed to the improved Cu ion reducibility and mobility. NO oxidation results provided no relation between NO₂ formation and SCR activity. Physicochemical properties, NO conversion, N₂ selectivity, and activation energy data showed that impregnated samples’ molecular structure and catalytic activity are comparable to the conventional ion-exchanged (IE) samples’ ones.     

Effect of the Presence of Ceria in the NSR Catalyst on the Hydrothermal Resistance and Global DeNOx Performance of Coupled LNT–SCR Systems

The global performance of coupled LNT–SCR systems, addressed to high NOx-to-N₂ conversion, minimal ammonia slip and null N₂O production, as well as the hydrothermal resistance of single NSR and SCR monolith catalysts and their coupling is discussed. Pt–Ba/Al₂O₃ and Pt–Ce–Ba/Al₂O₃ were washcoated on cordierite monoliths as NSR catalysts, and Cu/CHA was washcoated on similar monoliths as SCR catalysts. Both monoliths were coupled in two subsequent reactors to conform the LNT–SCR system. Previously to washcoating, the fresh powder catalysts and after severe hydrothermal aging were fully characterized by N₂ adsorption–desorption isotherms at 77 K, X-ray diffraction, NH₃ temperature-programmed desorption, and H₂ chemisorption to relate textural and chemical characteristics with the DeNO_x performance. The Cu/CHA catalyst shows an excellent hydrothermal resistance for the NH₃–SCR reaction. Incorporation of ceria to the model Pt–BaO/Al₂O₃is beneficial for the NO-to-NOx oxidation and NO₂ storage, improving NO conversion at low temperature and reducing the NH₃ slip. However, addition of ceria is detrimental for the hydrothermal resistance of the NSR catalyst. However, this detrimental effect is minimized when the NSR catalyst is coupled with the Cu/CHA monolith downstream of the NSR catalyst, achieving the coupled LNT–SCR device high NO conversion and minimal NH₃ slip with superior N₂ selectivity for an extended temperature windows, including as low as 220 °C, and maintaining performance even after severe hydrothermal aging.     

H2O和SO2对Mn-Fe/MPS催化剂用于NH3低温还原NO的影响

研究了H2O和SO2对Mn-Fe/MPS催化剂低温下选择性催化NH3还原(SCR)NO的影响. 结果表明,Mn-Fe/MPS催化剂具有良好的催化活性,在空速为20000 h-1、反应温度433 K时,NO的SCR转化率达99.1%. 在反应温度低于413 K时,水蒸汽(10%, j)在一定程度上降低了催化活性;超过433 K时,这种影响可完全消除,NO的SCR转化率达到97.8%以上. 低浓度SO2(100′10-6)存在条件下,443 K时催化效率仍可稳定在97.2%. 在水和SO2共存的情况下,生成的硫酸盐和亚硫酸盐沉积在催化剂表面导致催化剂逐渐失活,FT-IR测试也表明伴随SCR反应生成了硫酸铵. 提高反应温度可以延缓催化剂的失活. 此外还研究了不同活化温度对催化剂活性恢复的影响,结果表明,当活化温度达到773 K时,催化剂活性可以完全恢复. 本研究中的催化剂的综合性能优于目前文献报道的其他催化剂.     

Effect of Nickel as Dopant in Mn/TiO<Subscript>2</Subscript> Catalysts for the Low-Temperature Selective Reduction of NO with NH<Subscript>3</Subscript>

Nickel doped manganese oxide supported on titania materials were investigated for the low-temperature NH₃-SCR. For this purpose, a series of Ni modified Mn/TiO₂ catalysts were prepared and evaluated for the low-temperature SCR of NO with ammonia in the presence of excess oxygen. The catalytic performance of these materials was compared with respect to the nickel weight percentage in order to examine the correlation between physicochemical characteristics and reactivity of optimized materials. It was found that the 5% Mn–2% Ni/TiO₂ catalyst showed the highest activity and yielded 100% NO conversion at 200 °C. XRD results reveal highly dispersed manganese–nickel species on TiO₂ support for the Mn–Ni/TiO₂ catalysts. Our TPR data results suggested an increase in reducibility of manganese species in Mn–Ni/TiO₂ catalysts. The absence of the high-temperature (736 K) peak indicates that the dominant phase is MnO₂. This increase of reducibility and dominant MnO₂ phase seems to be the reason for the enhanced activity and time on stream patterns of nickel-promoted titania-supported manganese catalysts. BET results illustrate that the high NO conversion is strongly dependant on the specific surface area and pore volume of this catalyst. All the physicochemical techniques we used suggested that the composition of manganese and nickel oxides on the support surface is playing an important role for the enhancement of NO conversion and the prominent time on stream stability.