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.     

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 nitric oxide with ammonia at low temperatures

Selective catalytic reduction of nitric oxide with ammonia in synthetic low temperature flue gases has been investigated on a commercially available precious metal catalyst, NO_xCAT 920 LT^TM. It has been found that this catalyst is capable of achieving up to 90% conversion at temperatures below 300°C and low space velocities (12 000 h⁻¹), even in the presence of 20 ppm sulfur dioxide. The ideal ammonia concentration to reduce slip and achieve maximum conversion seems to be a stoichiometric match between ammonia concentration and nitric oxide concentration. A dual site model is proposed to explain the selectivity dependence on the presence of water vapor or sulfur dioxide.     

SELECTIVE CATALYTIC REDUCTION OF NITRIC OXIDE BY AMMONIA ON PILLARED CLAY CATALYSTS: POSSIBLE MECHANISM AND PROMOTERS

The delaminated Fe₂0₃-pillared clay shows high activities for selective catalytic reduction (SCR) of NO by NH₂. Temperature program desorption (TPD) studies show that large amounts of NO., are adsorbed on the pillared clay catalyst at the SCR reaction temperatures (i.e. near 400°C). This result indicates that a Langmuir-Hinshelwood type mechanism (for reaction between chemisorbed NO, and NH, on the surface to form N₂) is operative for the pillared clay catalyst, which is in contrast to the SCR reaction on the commercial vanadia-based catalysts. The SCR activities for the delaminated Fe₂0₃-pillared clay catalyst are higher than that of a commercial-type V₂O₅ + WO₃/TiO₂ catalyst under SO₂ + H₂0 free conditions, but became lower in the presence of SO₂ +l H₂0. However, when promoted by doping 1-3% Cr₂0₃, the pillared clay catalyst exhibits higher SCR activities than the commercial-type catalyst in the presence of S0₂ + H₂O at all practical SCR reaction temperatures     

Selective catalytic reduction of nitric oxide by ammonia over Cu-exchanged Cuban natural zeolites

The catalytic selective reduction of NO over Cu-exchanged natural zeolites (mordenite (MP) and clinoptilolite (HC)) from Cuba using NH₃ as reducing agent and in the presence of excess oxygen was studied. Cu(II)-exchanged zeolites are very active catalysts, with conversions of NO of 95%, a high selectivity to N₂ at low temperatures, and exhibiting good water tolerance. The chemical state of the Cu(II) in exchanged zeolites was characterized by H₂-TPR and XPS. Cu(II)-exchanged clinoptilolite underwent a severe deactivation in the presence of SO₂. However, Cu(II)-exchanged mordenite not only maintained its catalytic activity, but even showed a slight improvement after 20 h of reaction in the presence of 100 ppm of SO₂.     

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.     

Selective catalytic reduction of nitric oxide by propane over vanadia-titania aerogels

An investigation has been carried out of the effect of vanadia loading on the activity and selectivity ofV₂O₅TiO₂ aerogel catalysts, prepared by a two-step procedure, for the reduction of NO by propane. The structure of catalysts have been characterized by laser Raman spectroscopy and XRD measurements. At vanadia loading levels below ca. 4.4 wt%, the vanadia is present in the form of coordinated polymeric species, whereas crystallites of V₂O₅ are formed at higher vanadia contents. At this critical level of 4.4 wt% V₂O₅, the kinetic measurements showed also a maximum in the activity per mass of catalyst which very likely indicated that the coordinated polymeric surface species are more active than crystalline V₂O₅. The selectivity towards the formation of dinitrogen decreased as the loading increased, presumably because of the formation of larger polymeric species and V₂O₅ crystallites, below and above the critical loading level, respectively. For the reduction of NO by propane, titania supported vanadia aerogel catalysts are significantly more active, per mass of catalyst, and more selective towards N₂ formation than conventionalV₂O₅TiO₂ and V₂O₅Al₂O₃ aerogel catalysts, at vanadia loading levels below ca. 11 wt%.     

丙烯选择还原一氧化氮反应铜基交联黏土催化剂的研究

采用聚合羟基阳离子合成交联蒙脱土(PILC),经SO2-4改性,制备了应用于C3H6选择还原NO的铜基交联黏土催化剂。考察了交联剂种类、Cu担载量及水蒸气存在对催化剂性能的影响,并采用DTA、XRD对PILC进行表征。研究发现,Al-PILC较Zr-PILC具有较好的热稳定性;350℃时Cu/Al-PILC(Cu质量分数为3%)上NO转化率达52.0%;由于金属氧化物交联柱表面的疏水特性,Cu/Al-PILC较Cu/ZSM-5具有较强的抗水蒸气能力,10%水蒸气存在仅使NO最大转化率下降了13.7%,NO和C3H6转化曲线较不含水蒸气时向高温方向移动。     

Influence of cocations on catalytic activity of copper ion-exchanged ZSM-5 zeolite for reduction of nitric oxide with ethene in the presence of oxygen

The coexistence of alkaline earth (Ca and Sr) and transition metal (Fe, Co and Ni) cations with Cu ions in ZSM-5 zeolite is effective in the promotion of the maximum activity and the expansion of the active temperature range for the reduction of nitric oxide with ethene in the presence of excess oxygen.     

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。     

Selective catalytic reduction of NO by NH3 on titanium pillared montmorillonite

The acidity of a titanium pillared montmorillonite (Ti-PILC) has been modified by two methods of sulfation. The acidic properties of these pillared clays have been studied by temperature-programmed ammonia desorption and FTIR analysis of pyridine adsorption. The catalytic activity of these titanium pillared clays in the selective catalytic reduction (SCR) of NO by NH₃ with or without SO₂ has been investigated. For the three titanium pillared clays, a high resistance to SO₂ presence has been observed.     

Pillared clay and zirconia-based monolithic catalysts for selective catalytic reduction of nitric oxide by methane

New monolithic catalysts based on zirconia and pillared clays (PILC) have been studied for NO_x removal by CH₄ in the presence of oxygen. A comparative study of the influence of ZrO₂ from various commercial sources for the system Pd–ZrO₂ and the effect of the noble metal chosen for the system NM–PILC was carried out, trying to correlate the catalytic activity with the physico-chemical properties of these catalysts. The obtained results indicate that structure and surface acidity of the support plays an important role on the selectivity to NO_x reduction, although properties such as the surface area or pore volume could also determine the overall activity of the monolithic catalysts.     

Selective catalytic reduction of NO with C2H4 over Cu/ZSM-5: Influences of oxygen partial pressure and incorporated rhodia

Catalytic activities of copper-exchanged ZSM-zeolites for the reduction of nitric oxide to nitrogen using ethylene as reducing agent in the presence or absence of oxygen are compared with those of Rh-ZSM-5 zeolites and binary Cu/Rh-ZSM-5 catalysts at 623–823 K. Copper ZSM-5 systems with high copper loadings (> 2.1) are shown to be much more active in the presence of oxygen, in marked contrast to Rh ZSM-5 catalysts which were more active in its absence. Binary ZSM-5 catalysts containing both copper and rhodium maintained intermediate activity in both conditions. A sample of Cu ZSM-5 containing only 1.8% copper also evidenced some activity in both conditions, which could be understood on the basis of catalytic action by copper ions within the zeolitic framework, allied to inherent Bronsted acidity of the latter. Comparative catalytic data are presented for the oxygen partial pressure dependence of the conversion to nitrogen over all catalysts at 673 K. These delineate the interplay between selective catalytic reduction (SCR) of NO by C₂H₄ and ethylene oxidation by dioxygen. Techniques utilised to characterise the catalysts include: powder XRD, TPR in H₂, and TPD of O₂.     

Decomposition of nitric oxide and its reduction by CO over superconducting and related cuprate catalysts

Catalytic activities of five non-conducting and three superconducting cuprates were measured for the decomposition of NO and the reduction of NO by CO. The concentration of the reactants and the space velocities approximate the conditions of automotive catalysts. In contrast to earlier results, obtained at 20 to 30 times higher partial pressures of NO and 20 to 100 times lower space velocities, none of the catalysts, including five perovskite-like cuprates, showed significant activity for the decomposition of NO at reaction temperatures up to 700 °C. All catalysts were fairly active for the reduction of NO. At temperatures above about 400 °C on the perovskite-like cuprates YBa₂Cu₃O_7-x and Ba₂CuO_3.5-x , the rates for NO reduction were higher than on CuO. All catalysts lost activity for NO reduction in the presence of oxygen (oxidizing conditions).     

Selective catalytic reduction of nitric oxide with ammonia using MoO3/TiO2: Catalyst structure and activity

A series of titania supported MoO₃ catalysts (0–20 wt.-% MoO₃) were prepared by dry impregnation. The influence of the MoO₃ content on their catalytic performance for the selective catalytic reduction (SCR) of nitric oxide by ammonia in the presence of oxygen, as well as on their textural and structural properties has been studied. The samples were characterized by XRD, XPS, IR, and BET and porosimetry measurements. The coverage of the TiO₂ support by surface polymeric molybdenum species (where molybdenum is octahedrally coordinated) increases with the molybdenum loading. The formation of a layer of these interacting species on top of the titania surface is complete in the range 15–20 wt.-% MoO₃. The formation of crystallites of bulk MoO₃ starts before the completion of this surface layer (at around 10 wt.-% MoO₃) and increases progressively as the molybdenum loading increases from 10 to 20 wt.-% MoO₃. The SCR activity of the MoO₃/TiO₂ catalysts increases as the MoO₃ content increases to 15 wt.-% and then, for a further increase of the molybdenum loading, it slightly decreases. No specific influence of the molybdenum content on the resistance of catalysts towards SO₂ was observed; the same slight deactivation took place, when the SCR activity was measured in the presence of SO₂ in the feed, for all samples. Our results indicate that the octahedrally coordinated polymeric molybdenum surface species are mainly responsible for the exhibited SCR activity of the MoO₃/TiO₂ catalysts.     

Selective reduction of nitric oxide by methane on H-form zeolite catalysts in oxygen-rich atmosphere

Selective reduction of NO by CH₄ in the presence of excess oxygen was investigated using H-form zeolite catalysts. H-ZSM-5, H-ferrierite, and H-mordenite showed high catalytic activity and selectivity. On the contrary, H-USY and Al₂O₃ were not effective for this reaction. Both NO-CH4 and O₂-CH₄ reaction hardly proceeded on H-ZSM-5. Higher NO_x conversion was obtained in the NO₂-O₂-CH₄ and NO₂-CH₄ systems than in the NO-O₂-CH₄ system under high GHSV condition. It seemed that NO₂ plays an important role for selective reduction of NO by CH₄ on H-form zeolites.     

High activity of copper-boralite in the reduction of nitric oxide with propane / oxygen

The activity of ZSM-5 and boralite zeolites (Na, H and Cu forms) in the reduction of nitric oxide with propane / oxygen is compared. Copper ion-exchanged boralite is more active than a corresponding Cu-ZSM-5 sample with the same MFI structure, Si/M (M = Al or B) ratio and copper content, showing that in copper-exchanged samples Brønsted acid sites do not play a key role in the reaction mechanism, in contrast to that found for samples without the exchanged transition metal.     

On the mechanism of selective catalytic reduction of NO by propylene over Cu-Al-MCM-41

The selective catalytic reduction of NO by propene in the presence of excess oxygen over Cu-Al-MCM-41 catalyst has been studied by a number of catalytic techniques to characterize the structural, chemisorptive and reactive properties. The characterization using XRD and NMR revealed that the structure of Cu-Al-MCM-41 remained unchanged after the reaction. The active sites related to the reduction of NO and the reaction mechanism were explored based on the data of H₂ temperature programmed reduction (TPR), NO temperature programmed desorption (TPD), X-ray spectrometry (XPS) and in situ FT-IR. The results showed that a redox of Cu ions in Cu-Al-MCM-41 between monovalent and divalent states happened during the selective catalytic reduction of NO, and the reduction seemed to proceed via the intermediate of organic nitro compounds produced by the reaction of propylene adspecies and NO₂. In addition, the presence of oxygen is essential to the formation of NO₂ intermediate and to the cycle of active center between Cu²⁺ and Cu⁺. However, it also caused the deep oxidation of propylene, leading to the depletion in reducing agent at higher temperature.     

Effect of the nature and structure of pillared clays in the catalytic behaviour of supported manganese oxide

In the present work, the complete oxidation of acetone has been studied over a series of supported manganese oxide catalysts on the unpillared and the Al- and Zr-pillared forms of two natural clays, a montmorillonite and a saponite. A significant influence of the clay supports characteristics on the catalytic performance has been found. The following order of improving catalytic performance is established with respect to the pillars composition: Al-pillared clays < unpillared clays < Zr-pillared clays.     

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.