Ion-exchanged pillared clays for selective catalytic reduction of NO by ethylene in the presence of oxygen

Ion-exchanged pillared clays (PILCs) were studied as catalysts for selective catalytic reduction (SCR) of NO by ethylene. Three most important pillared clays, Al₂O₃-PILC (or Al-PILC), ZrO₂-PILC (or Zr-PILC) and TiO₂-PILC (or Ti-PILC), were synthesized. Cation exchanges were performed to prepare the following catalysts: Cu–Ti-PILC, Cu–Al-PILC, Cu–Zr-PILC, Cu–Al–Laponite, Fe–Ti-PILC, Ce–Ti-PILC, Ce–Ti-PILC, Co–Ti-PILC, Ag–Ti-PILC and Ga–Ti-PILC. Cu–Ti-PILC showed the highest activities at temperatures below 370°C, while Cu–Al-PILC was most active at above 370°C, and both catalysts were substantially more active than Cu-ZSM-5. No detectable N₂O was formed by all of these catalysts. H₂O and SO₂ only slightly deactivated the SCR activity of Cu–Ti-PILC, whereas severe deactivation was observed for Cu-ZSM-5. The catalytic activity of Cu–Ti-PILC was found to depend on the method and amount of copper loading. The catalytic activity increased with copper content until it reached 245% ion-exchange. The doping of 0.5 wt% Ce₂O₃ on Cu–Ti-PILC increased the activities from 10% to 30% while 1.0 wt% of Ce₂O₃ decreased the activity of Cu–Ti-PILC due to pore plugging. Cu–Ti-PILC was found to be an excellent catalyst for NO SCR by NH₃, but inactive when CH₄ was used as the reducing agent. Subjecting the Cu–Ti-PILC catalyst to 5% H₂0 and 50 ppm SO₂ at 700°C for 2 h only slightly decreased its activity. TPR results showed that the overexchanged (245%) PILC sample contained Cu²⁺, Cu⁺ and CuO. The TPR temperatures for the Cu–Ti-PILC were substantially lower than that for Cu-ZSM-5, indicating easier redox on the PILC catalyst and hence higher SCR activity.     

Characterization and activity of novel copper-containing catalysts for selective catalytic reduction of NO with NH3

Cu/Mg/Al mixed oxides (CuO = 4.0–12.5 wt%), obtained by calcination of hydrotalcite-type (HT) anionic clays, were investigated in the selective catalytic reduction (SCR) of NO by NH₃, either in the absence or presence of oxygen, and their behaviours were compared with that of a CuO-supported catalyst (CuO = 10.0 wt%), prepared by incipient wetness impregnation of a Mg/Al mixed oxide also obtained by calcination of an HT precursor. XRD analysis, UV-visible-NIR diffuse reflectance spectra and temperature-programmed reduction analyses showed the formation, in the mixed oxide catalysts obtained from HT precursors, mainly of octahedrally coordinated Cu²⁺ ions, more strongly stabilized than Cu-containing species in the supported catalyst, although the latter showed a lower percentage of reduction. The presence of well dispersed Cu²⁺ ions improved the catalytic performances, although similar behaviours were observed for all catalysts in the absence of oxygen. On the contrary, when the mixture with excess oxygen was fed, very interesting catalytic performances were obtained for the catalyst richest in copper (CuO = 12.5 wt%). This catalyst exhibited a behaviour comparable to that of a commercial V₂O₅–WO₃TiO₂ catalyst, without any deactivation phenomena after four consecutive cycles and following 8 h of time-on-stream at 653 K. Decreasing the copper content or increasing the calcination time and temperature led to considerably poorer performances and catalytic behaviours similar to that of the CuO-supported catalyst, due to the side-reaction of NH₃ combustion on the free Mg/Al mixed oxide surface.     

Selective catalytic reduction of NO by ammonia with fly ash catalyst

This paper investigates the selective catalytic reduction (SCR) of NO with NH₃ using fly ash catalyst. The catalytically active elements investigated here included Fe, Cu, Ni and V. The results indicated that fly ash, after pre-treatment, can be reasonably used as the SCR catalyst support to remove NO from flue gas. Cu gave the highest catalytic activity and NO conversion, compared with Fe, Ni and V. In the pre-treatment process, the nitric acid treatment and drying temperatures for the fly ash particles had little effect on the NO conversion. However, the calcination temperature had an important effect on the catalyst preparation process.     

ZSM-5分子筛催化剂上氨选择性催化还原NO_x的研究进展

氨选择性催化还原(NH_3-SCR)是控制NO_x排放的有效手段,对Fe-ZSM-5、Cu-ZSM-5、Mn-ZSM-5及多金属负载的ZSM-5分子筛催化剂在NH_3-SCR脱除NO_x中的性能及影响因素进行总结,并展望ZSM-5分子筛在NO_x脱除中的发展方向。     

Ammonia activation over catalysts for the selective catalytic reduction of NOx and the selective catalytic oxidation of NH3. An FT-IR study

The adsorption and transformation of ammonia over V₂O₅, V₂O₅/TiO₂, V₂O₅-WO₃/TiO₂ and CuO/TiO₂ systems has been investigated by FT-IR spectroscopy. In all cases ammonia is first coordinated over Lewis acid sites and later undergoes hydrogen abstraction giving rise either to NH₂ amide species or to its dimeric form N₂H₄, hydrazine. Other species, tentatively identified as imide NH, nitroxyl HNO, nitrogen anions N₂⁻ and azide anions N₃⁻ are further observed over CuO/TiO₂. The comparison of the infrared spectra of the species arising from both NH₃ and N₂H₄ adsorbed over CuO/TiO₂ strongly suggest that N₂H₄ is an intermediate in NH₃ oxidation over this active selective catalytic reduction (SCR) and selective catalytic oxidation (SCO) catalysts. This implies that ammonia is activated in the form of NH₂ species for both SCR and SCO, and it can later dimerize. Ammonia protonation to ammonium ion is detected over V₂O₅-based systems, but not over CuO/TiO₂, in spite of the high SCR and SCO activity of this catalyst. Consequently Brönsted acidity is not necessary for the SCR activity.     

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.     

Selective catalytic reduction of NO and NO2 at low temperatures

The fast SCR reaction using equimolar amounts of NO and NO₂ is a powerful means to enhance the NO_x conversion over a given SCR catalyst. NO₂ fractions in excess of 50% of total NO_x should be avoided because the reaction with NO₂ only is slower than the standard SCR reaction.

At temperatures below 200 °C, due to its negative temperature coefficient, the ammonium nitrate reaction gets increasingly important. Half of each NH₃ and NO₂ react to form dinitrogen and water in analogy to a typical SCR reaction. The other half of NH₃ and NO₂ form ammonium nitrate in close analogy to a NO_x storage-reduction catalyst. Ammonium nitrate tends to deposit in solid or liquid form in the pores of the catalyst and this will lead to its temporary deactivation.

The various reactions have been studied experimentally in the temperature range 150–450 °C for various NO₂/NO_x ratios. The fate of the deposited ammonium nitrate during a later reheating of the catalyst has also been investigated. In the absence of NO, the thermal decomposition yields mainly ammonia and nitric acid. If NO is present, its reaction with nitric acid on the catalyst will cause the formation of NO₂.     

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.     

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.     

A comparative study of Ag/Al2O3 and Cu/Al2O3 catalysts for the selective catalytic reduction of NO by C3H6

The selective catalytic reduction (SCR) of NO by C₃H₆ in excess oxygen was evaluated and compared over Ag/Al₂O₃ and Cu/Al₂O₃ catalysts. Ag/Al₂O₃ showed a high activity for NO reduction. However, Cu/Al₂O₃ showed a high activity for C₃H₆ oxidation. The partial oxidation of C₃H₆ gave surface enolic species and acetate species on the Ag/Al₂O₃, but only an acetate species was clearly observed on the Cu/Al₂O₃. The enolic species is a more active intermediate towards NO + O₂ to yield—NCO species than the acetate species on the Ag/Al₂O₃ catalyst. The Ag and Cu metal loadings and phase changes on Al₂O₃ support can affect the activity and selectivity of Ag/Al₂O₃ and Cu/Al₂O₃ catalysts, but the formation of enolic species is the main reason why the activity of the Ag/Al₂O₃ catalyst for NO reduction is higher than that of the Cu/Al₂O₃ catalyst.     

Research progress in the SO2resistance of the catalysts for selective catalytic reduction of NOx

The selective catalytic reduction(SCR)of NOxwith NH3has been proven to be an efficient technology for NOx conversion to N2.However,the catalysts used for SCR usually suffer from the problem of sulfur poisoning which seriously limits their practical application.This review summarized sulfur poisoning mechanisms of various SCR deNOxcatalysts and strategies to reduce deactivation caused by SO2such as doping metals,control-ling the structures and morphologies of the catalysts,and selecting appropriate supports.The methods and procedures of catalysts preparation and the reaction conditions also have effect on SO2-resistance of the catalysts. Several novel catalyst systems that exhibited good SO2resistance are also introduced.This paper could provide guidance for the development of highly efficient sulfur-tolerant deNOxcatalysts.     

杂多酸改性V-Mo/Ti-W催化剂的宽温SCR脱硝性能

采用等体积浸渍法制备了一系列Keggin型杂多酸改性的V-Mo/Ti-W催化剂,并通过X射线衍射（XRD）、BET比表面分析（BET）、NH₃程序升温脱附（NH₃-TPD）、H₂程序升温还原（H₂-TPR）、X射线光电子能谱（XPS）和傅里叶变换红外光谱（FTIR）等表征方法对催化剂物化性质进行了表征分析。各项表征分析结果表明,杂多酸改性后的HPAs-V-Mo/Ti-W催化剂相比于改性前的催化剂,平均孔径尺寸更大,可抑制硫酸氢铵沉积,以提高催化剂抗硫性能;与V-Mo/Ti-W样品相比,改性后的催化剂具有更强的NH₃吸附能力、更好的还原性能和更多的化学吸附氧物种,从而表现出更优异的脱硝性能和N₂选择性。杂多酸改性后的催化剂在200~380℃温度范围内、150000h^-1高空速、高浓度SO₂和H₂O存在的模拟烟气中表现出高 NH₃-SCR活性、优异的抗SO₂性能,并保持几乎100%的N₂选择性,可作为燃煤电厂深度调峰的宽温催化剂,具备良好的工业应用价值。     

Selective reduction of NO by NH3 over manganese–cerium mixed oxides: Relation between adsorption, redox and catalytic behavior

Manganese–cerium mixed oxide catalysts with different molar ratio Mn/(Mn + Ce) (0, 0.25, 0.50, 0.75, 1) were prepared by citric acid method and investigated concerning their adsorption behavior, redox properties and behavior in the selective catalytic reduction of NO_x by NH₃. The studies based on pulse thermal analysis combined with mass spectroscopy and FT-IR spectroscopy uncovered a clear correlation between the dependence of these properties and the mixed oxide composition. Highest activity to nitrogen formation was found for catalysts with a molar ratio Mn/(Mn + Ce) of 0.25, whereas the activity was much lower for the pure constituent oxides. Measurements of adsorption uptake of reactants, NO_x (NO, NO₂) and NH₃, and reducibility showed similar dependence on the mixed oxide composition indicating a clear correlation of these properties with catalytic activity. The adsorption studies indicated that NO_x and NH₃ are adsorbed on separate sites. Consecutive adsorption measurements of the reactants showed similar uptakes as separate measurements indicating that there was no interference between adsorbed reactants. Mechanistic investigations by changing the sequence of admittance of reactants (NO_x, NH₃) indicated that at 100–150 °C nitrogen formation follows an Eley–Rideal type mechanism, where adsorbed ammonia reacts with NO_x in the gas phase, whereas adsorbed NO_x showed no significant reactivity under conditions used.     

Cooperative effect induced by the mixing of Na-ZSM-5 and Pd/H3PW12O40/SiO2 in the selective catalytic reduction of NO with aromatic hydrocarbons

Pd was loaded on the dispersed H₃PW₁₂O₄₀ (HPW) over the SiO₂ surface, and the catalyst was applied to the selective reduction of NO with aromatic hydrocarbons. The catalyst exhibited high activity in the NO reduction when branched aromatic hydrocarbons, such as toluene and xylene, were used as reductants. The catalytic activity of Pd/HPW/SiO₂ was improved remarkably by physically mixing it with Na-ZSM-5. From the temperature programmed desorption (TPD) of toluene and the analysis of the products, it was inferred that the activity was enhanced when Pd/HPW/SiO₂ and Na-ZSM-5 were mixed. In other words, aromatic hydrocarbons were partially oxidized to yield oxygenated hydrocarbons, e.g., benzaldehyde and phthalic anhydride, over Pd/Na-ZSM-5; in this reaction, a part of Pd migrated from Pd/HPW/SiO₂ to Na-ZSM-5 during the course of the physical mixing procedure. Subsequently, the oxygenated hydrocarbons reacted with NO entrapped with HPW over Pd to yield N₂.     

Cobalt catalysts in selective catalytic reduction of NO by methane

Impregnated cobalt-containing catalysts are studied in the selective catalytic reduction of NO by methane. The active component of all the impregnated cobalt-containing catalysts is Co₃O₄. The role of O₂ seems to maintain the surface stoichiometry of Co₃O₄. The main reason of decrease of catalytic activity of samples based on MgO, SiO₂, Al₂O₃ in selective catalytic reduction of NO is due to oxide-oxide interaction promoted by water. Catalysts based on montmorillonite (HMM) are stable in the presence of water. All the carriers can be placed by the capability for oxide–oxide interaction in the following order: SiO₂ ≫ MgO > Al₂O₃ ≫ HMM. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

NOx选择催化还原用Au/海泡石催化剂的研究

以天然海泡石为原料,对其进行酸改性后作为催化剂载体。采用浸渍法制备了用于CO选择催化还原NOx的负载型Au/海泡石催化剂。设计正交实验对海泡石酸改性条件进行了研究,对催化剂催化还原NOx的活性进行了评价。实验结果表明,该催化剂对于NOx催化还原反应有一定活性。对酸改性前后的海泡石及制得的催化剂进行了TGA、IR和XRD分析,证实海泡石是一种较好的催化剂载体。     

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。     

Molybdenum-titanium oxide catalysts: the influence of the preparation conditions on their activity for the selective catalytic reduction of NO by NH3

MoO₃/TiO₂ catalysts of varying molybdenum content were prepared at various pHs and concentrations of the impregnating solution using the equilibrium deposition filtration (EDF) method. Moreover, a catalyst corresponding to the EDF one with the maximum Mo loading was prepared using the conventional non-dry impregnation (NDI) method. The above catalysts were characterized using X-ray powder analysis, diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy, and tested for the selective catalytic reduction of NO by NH₃ in the temperature range 250–450 °C. It was found that the application of EDF results in an improved MoO₃/TiO₂ catalyst exhibiting higher activity than the corresponding sample prepared by the conventional NDI method. The catalytic activity correlates well with the concentration of the Mo species strongly interacting with the anatase surface. The concentration of the above species is maximized when the EDF method is employed to prepare the catalysts. This is especially, so when low pH and Mo concentration of impregnating solution are used.     

Selective catalytic reduction of NO by propene in excess oxygen on Pt- and Rh-supported alumina catalysts

A comparative study of Pt/alumina and Rh/alumina catalysts was performed for the selective catalytic reduction of NO with C₃H₆ in the presence of excess oxygen. Pt/alumina was more active for NO reduction at lower temperatures compared to Rh/alumina. However, the latter exhibited clearly superior performance in terms of selectivity to N₂. This makes Rh/alumina a more suitable catalyst for the selective catalytic reduction of NO under excess oxygen conditions. Detailed kinetic studies of the SCR of NO were performed on Pt/alumina and Rh/alumina to obtain low-temperature kinetic expressions for NO reduction and C₃H₆ oxidation in the presence and absence of NO. Qualitative similarities yet quantitative differences in these kinetic expressions appear to indicate the existence of two partially similar mechanistic schemes. One is based on the indirect participation of the reductant through reduction of the active sites, followed by dissociative adsorption of NO on reduced sites (applicable for Pt/alumina). The other is based on the direct participation of the reductant (apparently by its partial oxidation) in forming an activated intermediate species, followed by its interaction with activated NO (applicable for Rh/alumina).