Acid- and base-treated Fe3+-TiO2-pillared clays for selective catalytic reduction of NO by NH3

Fe³⁺-ion-exchanged delaminated pillared clays (PILCs) have been found previously to be more active than the vanadia-based catalysts for selective catalytic reduction (SCR) of NO by NH₃. The effects of acid treatment of the clay (before pillaring) and base treatments of the TiO₂-PILC (before ion exchange) on the activities of the Fe–TiO₂-PILC catalysts were studied. It was found that the acid treatment increased the activity (by 33%), but the base treatments decreased the activity (although they increased the cation exchange capacity of the pillared clay and, hence, the Fe content). The activities of the catalysts were directly related to their surface Brønsted acidities as identified by FT-IR of chemisorbed NH₃.     

The existence of dual Cu site involved in the selective catalytic reduction of NO with propene on Cu/ZSM-5

In order to establish the role of surface species in the selective catalytic reduction (SCR), in situ IR studies were carried out using a DRIFT (diffuse reflectance infrared Fourier transform) cell in gas mixtures of various C₃H₆/NO ratios containing excess oxygen. The location and mobility of Cu ions were investigated by recording the relevant bands of CO adsorbed on Cu/ZSM-5. The nitro species coordinated on Cu²⁺ and the -NCO surface complex as possible intermediates were observed in the reduction of NO with propene on Cu/ZSM-5 between 350 and 400°C. The reactivities of these species toward NO, O₂ and propene were examined. The nitro species can react with propene very rapidly to form N₂ without the formation of NCO species. NCO also reacts with NO₂ and/ or NO at 350°C. IR spectra of CO adsorbed on cuprous ions show that two kinds of Cu ions, which are responsible for the activation of NO and propene respectively, exist on Cu/ZSM-5. From these results, a dual site mechanism involving nitro species and -NCO species as intermediates is suggested.     

Selective catalytic reduction of NO by NH3 on Cu (II) ion-exchanged offretite prepared by different methods

The influence of the preparation method on the NO–SCR by NH₃ activity of Cu-OFF catalysts has been studied on presence of oxygen. The catalysts structure and the nature of copper species have been investigated by XRD, ²⁷Al MAS NMR, UV-visible spectroscopy and H₂-TPR. Among the examined preparation methods, the ionic exchange in aqueous solution gives the highest efficient catalyst in NO-SCR. It is shown that Cu cationic species are formed in large extent with this method despite the presence of minor amounts of small oxide clusters.     

Fe-ZSM-5 for Selective Catalytic Reduction of NO with NH3: A Comparative Study of Different Preparation Techniques

Fe-ZSM-5 are prepared by using four different techniques: conventional aqueous ion-exchange (CA), improved aqueous ion-exchange (IA), solid-state ion-exchange (SS) and chemical vapor ion-exchange (CV). All of the catalysts show very high activities for selective catalytic reduction (SCR) of NO with ammonia. However, the activities are different and follow the sequence of Fe-ZSM-5 (IA) > Fe-ZSM-5 (CA), Fe-ZSM-5 (SS) > Fe-ZSM-5 (CV). ESR results indicate that Fe³⁺ ions with tetrahedral coordination are the active sites for the SCR reaction.     

富氧条件Co/H-ZSM-5催化剂上CH4选择催化还原NO的研究

采用离子交换法制备了一系列具有不同硅铝比和不同Co负载量的Co/H-ZSM-5催化剂样品。富氧条件下考察了硅铝比、Co负载量、空速、O₂浓度及酸位对催化剂选择催化还原活性的影响。并对其进行了XRD、BET、H₂-TPR和DRS-UV-vis等表征。催化结果表明，催化剂的催化活性随Co负载量的增加而增加，随硅铝比的增加而减少；NO转化率随着空速的增加而降低。O₂体积分数为2%时，NO达最大转化率。表征结果表明，Co²⁺为活性中心，酸中心的存在对催化活性有一定的促进作用。     

NO2 formation and its effect on the selective catalytic reduction of NO over Co/ZSM-5

Catalytic performance of Co/ZSM-5 with different metal loadings and of HZSM-5 was compared in the NO + O₂, C₃H₈ + O₂, and NO + C₃H₈ + O₂ reactions. It was found that Co/ZSM-5 catalysts containing only isolated cobalt ions in cationic positions are inactive in NO₂ formation. To achieve appreciable NO conversion in the SCR process over these catalysts higher reaction temperatures are required. These results make it possible to suggest that NO₂ formation is not a prerequisite for the SCR of NO with hydrocarbons over Co/ZSM-5. With increasing Co loading, however, Co/ZSM-5 begins to exhibit activity in NO₂ formation. This is explained by the formation of cobalt oxide particles on the zeolite carrier, which are active in the NO₂ formation. Increase in NO₂ formation strongly enhances catalytic activity in SCR of NO at lower reaction temperatures. Comparison of the C₃H₈ conversion in the C₃H₈ + O₂ and C₃H₈ + O₂ + NO reactions provides evidence that NO₂ activates hydrocarbon molecules resulting in the formation of the reaction intermediates of the SCR process.On leave from N.D. Zelinskii Institute of Organic Chemistry, Leninskii Pr. 47, Moscow, Russia.     

MnOx/CeO2 catalysts for the low-temperature selective catalytic reduction of NO with NH3

CeO₂–nanorod support was synthesized by hydrothermal method and different manganese oxides (MnO, MnO_2, and Mn₂O₃) were impregnated over support by the wet-impregnation forming MnO/CeO₂-NR, MnO₂/CeO₂-NRm and Mn₂O₃/CeO₂-NR. The physico-chemical properties of the as-prepared catalysts were analyzed using x-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area, x-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning electron microscope–energy-dispersive x-ray spectroscopy (SEM–EDX), hydrogen-temperature-programmed reduction (H₂-TPR), and Raman spectroscopy. These catalysts were further analyzed for NO reduction using NH₃ as a reducing gas in the temperature range of 50 to 450°C. The results confirmed that MnO₂/CeO₂-NR gave the maximum NO conversion (65%) and N₂ selectivity (89%) among all catalysts. Further, MnO₂/CeO₂-NR catalyst was studied for the effect of MnO₂ loading and more than 90% NO conversion and N₂ selectivity were obtained in the temperature range of 250 to 300°C.     

Wire-mesh honeycomb catalysts for selective catalytic reduction of NO with NH3

Both flat and corrugated wire mesh sheets were coated with aluminum powder by using electrophoretic deposition (EPD) method. Controlled thermal sintering of coated samples yielded uniform porous aluminum layer with a thickness of 100 μm that was attached firmly on the wire meshes. Subsequent controlled calcination formed a finite thickness of Al₂O₃ layer on the outer surface of each deposited aluminum particles, which resulted in the formation of Al₂O₃/Al double-layered composite particles that were attached firmly on the wire surface to form a certain thickness of porous layer. A rectangular-shaped wire-mesh honeycomb (WMH) module with triangular-shaped channels was manufactured by packing alternately the flat sheet and corrugated sheet of the Al₂O₃/Al-coated wire meshes. This WMH was further coated with V₂O₅-MoO₃-WO₃ catalyst by wash-coating method to be applied for the selective catalytic reduction (SCR) of NO with NH₃. With an optimized catalyst loading of 16 wt%, WMH catalyst module shows more than 90% NO conversion at 240 °C and almost complete NO conversion at temperatures higher than 300 °C at GHSV 5,000 h⁻¹. When compared with conventional ceramic honeycomb catalyst, WMH catalyst gives NO conversion higher by 20% due to reduced mass transfer resistance by the existence of three dimensional opening holes in WMH.     

Ba、Co共掺MnOx复合氧化物低温选择性催化还原NO研究

利用柠檬酸络合法制备了一系列Ba、Co掺入MnO_x脱硝催化剂,研究了Ba、Co掺入对MnO_x低温NH₃-SCR性能的影响。测试结果表明：Ba单独掺入时抑制了MnO_x的催化性能,Co单独掺入时促进了MnO_x的催化性能;而当Ba、Co共掺时,催化剂性能出现了最大的提升,其中3BaMnCoO_x表现出了优异的脱硝活性,反应温度高于180℃时其脱硝性能在99%以上,且表现出了良好的抗水硫性能。采用XRD、SEM、NO-TPD、NH₃-TPD、H₂-TPR和NO吸附原位红外等表征手段对催化剂进行了表征。结果表明,Co掺杂后形成了MnCo₂O_4.5固溶体,赋予了催化剂优异的氧化还原性能;Ba掺杂为催化剂提供了新的NO吸附位点,导致了新的活性硝酸盐吸附物种形成,显著促进了催化剂的NO吸附性能。这些特性使得BaMnCoO_x低温脱硝催化剂展现出了优异的脱硝性能。     

Steam tolerance of Fe/ZSM-5 catalyst for the selective catalytic reduction of NOx

This article reports the effects of steam on the activity and stability of Fe/ZSM-5 for the selective catalytic reduction of NO withiso-butane. When the feed contained 10% of H₂O, the de-NO_x activity was maintained if the temperature was above the maximum conversion temperature. However, when the temperature was below the maximum conversion temperature, the catalytic activity decreased. The effect of high temperature steam treatment on the stability was also examined. After the steam treatment, the activity of Fe/ZSM-5 decreased due to the dealumination of ZSM-5 and the migration of Fe ion isolated in the ion exchange site to form ferromagnetic iron agglomerate. The physicochemical properties of the fresh and deactivated catalysts were monitored by ESR,²⁷Al MAS NMR, XPS, XRD, TPR and FT-IR spectroscopy. This paper is dedicated to Professor Wha Young Lee on the occasion of his retirement from Seoul National University.     

Cu(x)/TiO2催化剂NH3-SCR低温脱硝性能

以硝酸铜和二氧化钛为原料,采用浸渍法制备Cu(x)/TiO_2(x为以TiO_2载体质量为基准的铜负载量,下同)催化剂。运用XRD、XPS、NO-TPD、H_2-TPR等对催化剂进行了表征,在微型固定床反应器中评价了Cu(x)/TiO_2催化剂在以NH_3为还原剂的选择性催化还原NO反应(NH_3-SCR)中的脱硝活性。结果表明,铜物种以Cu_2O和Cu O的形式共存于TiO_2载体上;铜负载量影响催化剂的脱硝性能;Cu(6)/TiO_2催化剂(6代表铜的负载量为6%)表现出较好的氧化还原性和对反应物NO的吸附-脱附能力,低温脱硝活性较好,NO转化率达到85%和95%时对应反应温度T85和T95分别为195和218℃,NO转化率大于95%的活性窗口温度为218～270℃,宽度为52℃。     

Low-temperature selective catalytic reduction of NO with NH3 based on MnOx-CeOx/ACFN

MnO _x -CeO _x /ACFN were prepared by the impregnation method and used as catalyst for selective catalytic reduction of NO with NH3 at 80°C-150°C. The catalyst was characterized by N₂-BET, scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). The fraction of the mesopore and the oxygen functional groups on the surface of activated carbon fiber (ACF) increased after the treatment with nitric acid, which was favorable to improve the catalytic activities of MnO _x -CeO _x /ACFN. The experimental results show that the conversion of NO is nearly 100% in the range 100°C-150°C under the optimal preparation conditions of MnO _x -CeO _x /ACFN. In addition, the effects of a series of performance parameters, including initial NH₃ concentration, NO concentration and O₂ concentration, on the conversion of NO were studied. __________ Translated from Chemical Industry and Engineering Progress, 2007, 27(1): 87–91 [译自: 化工进展]     

Effect of transition metals addition on the catalyst of manganese/titania for low-temperature selective catalytic reduction of nitric oxide with ammonia

To retard the sintering, a series of transition metals were added to the low-temperature SCR catalysts based on Mn/TiO₂, and activity of these catalysts was investigated. It was found that the transition metal had significant effects on the catalytic activity. With the addition of transition metals, more NO could be removed at lower temperature. The temperature of 90% NO conversion could decrease to 361 K by using Fe(0.1)–Mn(0.4)/TiO₂. The results of X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron diffraction spectra (EDS) indicated that manganese oxides and titania could be better dispersed in the catalyst, and higher catalytic activity was obtained. From X-ray photoelectron spectrum (XPS) it could be known that solid solution was formed among the transition metal, manganese oxides and titania. With the formation of this solid solution, the Brunauer–Emmett–Teller (BET) area and pore volume increased. Furthermore, the in situ diffuse reflectance infrared transform spectroscopy (DRIFT) results showed that by using these catalysts, more NO could be oxidized to NO₂ and nitrate, and then reacted with NH₃. Therefore, the catalytic activity was greatly improved by the addition of transition metals.     

Boosting low-temperature selective catalytic reduction of NO with NH3 of V2O5/TiO2 catalyst via B-doping

A series of B-doped V₂O₅/TiO₂ catalysts has been prepared the by sol-gel and impregnation methods to investigate the influence of B-doping on the selective catalytic reduction (SCR) of NO_x with NH₃. X-ray diffraction, Brunauer-Emmett-Teller specific surface area, scanning electron microscope, X-ray photoelectron spectroscopy, temperature-programmed reduction of H₂ and temperature-programmed desorption of NH₃ technology were used to study the effect of the B-doping on the structure and NH₃-SCR activity of V₂O₅/TiO₂ catalysts. The experimental results demonstrated that the introduction of B not only improved the low-temperature SCR activity of the catalysts, but also broadened the activity temperature window. The best SCR activity in the entire test temperature range is obtained for VTiB_2.0 with 2.0% doping amount of B and the NO_x conversion rate is up to 94.3% at 210 ℃. The crystal phase, specific surface area, valence state reducibility and surface acidity of the active components for the as-prepared catalysts are significantly affected by the B-doping, resulting in an improved NH₃-SCR performance. These results suggest that the V₂O₅/TiO₂ catalysts with an appropriate B content afford good candidates for SCR in the low temperature window.     

Effect of Co content on the catalytic activity of CoSiBEA zeolite in the selective catalytic reduction of NO with ethanol: Nature of the cobalt species

The effect of Co content on the catalytic activity of CoSiBEA zeolites in the selective catalytic reduction (SCR) of NO with ethanol is investigated. The Co_xSiBEA zeolites (x = 0.3, 0.7, 3.6 and 6.75 Co wt.%) are prepared by a two-step postsynthesis method which allows to control the introduction of cobalt into zeolite and thus to obtain catalysts with specific Co sites. The nature of the active sites is characterized by XRD, diffuse reflectance UV–vis, H₂-TPR and XPS.

The catalytic activity of Co_xSiBEA strongly depends on the nature and environment of Co species. Zeolites with isolated lattice tetrahedral Co(II) (Co_0.3SiBEA and Co_0.7SiBEA samples) are active in SCR of NO with ethanol with selectivity toward N₂ exceeding 85% for NO conversion from 20 to 70%. When additional isolated extra-lattice octahedral Co(II) species appear (Co_3.6SiBEA sample), the full oxidation of ethanol by dioxygen becomes a very important reaction pathway. In presence of additional cobalt oxides (Co_6.75SiBEA sample), the activity and selectivity toward N₂ substantially change and full oxidation of ethanol to CO₂ is the main reaction pathway and full NO oxidation also takes place in the temperature range 550–775 K. The lack of correlation between the activity in SCR of NO with ethanol and NO oxidation to NO₂ suggests that the two reactions are more competitive than sequential.     

Influence of NO2 on the selective catalytic reduction of NO with ammonia over Fe-ZSM5

The influence of NO₂ on the selective catalytic reduction (SCR) of NO with ammonia was studied over Fe-ZSM5 coated on cordierite monolith. NO₂ in the feed drastically enhanced the NO_x removal efficiency (DeNOx) up to 600 °C, whereas the promoting effect was most pronounced at the low temperature end. The maximum activity was found for NO₂/NO_x = 50%, which is explained by the stoichiometry of the actual SCR reaction over Fe-ZSM5, requiring a NH₃:NO:NO₂ ratio of 2:1:1. In this context, it is a special feature of Fe-ZSM5 to keep this activity level almost up to NO₂/NO_x = 100%. The addition of NO₂ to the feed gas was always accompanied by the production of N₂O at lower and intermediate temperatures. The absence of N₂O at the high temperature end is explained by the N₂O decomposition and N₂O-SCR reaction. Water and oxygen influence the SCR reaction indirectly. Oxygen enhances the oxidation of NO to NO₂ and water suppresses the oxidation of NO to NO₂, which is an essential preceding step of the actual SCR reaction for NO₂/NO_x < 50%. DRIFT spectra of the catalyst under different pre-treatment and operating conditions suggest a common intermediate, from which the main product N₂ is formed with NO and the side-product N₂O by reaction with gas phase NO₂.     

Activity, selectivity and durability of VO–ZSM-5 catalysts for the selective catalytic reduction of NO by ammonia

ZSM-5 zeolite was loaded with vanadyl ions (VO²⁺) by treatment of Na–ZSM-5 with aqueous VOSO₄ solution at pH 1.5–2. The catalytic material was tested for the selective catalytic reduction of NO with ammonia at temperatures between 473 and 823 K and normal pressure using a feed of 1000 ppm NO, 1000 (or 1100) ppm NH₃ and 2% O₂ in He. The catalyst proved to be highly active, providing, e.g. initial NO conversions of >90% at 620 l g⁻¹ h⁻¹ (≈400 000 h⁻¹) and 723 K, and selective, providing nitrogen yields equal to NO conversion at equimolar feed in a wide temperature range and only minor N₂O formation at NH₃ excess. Admixture of SO₂ (200 ppm) resulted in an upward shift of the useful temperature range, but did not affect the catalytic behaviour at temperatures ≥623 K. No SO₂ conversion was noted at T ≤ 723 K and 450 l g⁻¹ h⁻¹. The poisoning effect of water (up to 4.5 vol%) was weak at temperatures between 623 and 773 K. VO-ZSM-5 catalysts are gradually deactivated already under dry conditions, probably by oxidation of the vanadyl ions into pentavalent V species. This deactivation is considerably accelerated in the presence of water.     

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%).     

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 NO by C2H2 over MoO3/HZSM-5

Molybdenum impregnated HZSM-5 zeolite catalysts with MoO₃ loading from 1 to 8 wt.% were studied in detail for the selective catalytic reduction (C₂H₂-SCR) of NO by acetylene. A 83.9% of NO could be removed by the reductant at 350 °C under 1600 ppm of NO, 800 ppm of C₂H₂ and 9.95% of O₂ in He over 2%MoO₃/HZSM-5 catalyst with a specific activity of in NO elimination and the competitiveness factor (c.f.) of 33.6% for the reductant. The NO elimination level and the c.f. value were ca. 3–4 times as high as those using methane or propene as reductant over the catalyst in the same reaction condition. About same reaction rate was estimated in NO oxidation as that in the NO reduction over each xMoO₃/HZSM-5 (x = 0–8%) catalyst, which confirms that NO₂ is a crucial intermediate for the aimed reaction over the catalysts. Appropriate amount of Mo incorporation to HZSM-5 considerably enhanced the title reaction, both by accelerating the intermediate formation and by strengthening the adsorption NO_x on the catalyst surface under the reaction conditions. Rather lower adsorption tendency of acetylene compared with propene on the catalysts explains the catalyst's steady performance in the C₂H₂-SCR of NO and rapid deactivation in the C₃H₆-SCR of NO.