Reaction intermediate over mordenite-type zeolite catalysts for no reduction by hydrocarbons

An infrared spectroscopic study has been made over mordenite-type zeolite catalysts prepared by the ion exchanging method to observe a surface species during the selective reduction of NO by hydrocarbons with and without H₂O. The strong absorptions at 2,274 and 2,325 cm_-1 were observed over HM and CuHM as well as over CuNZA catalysts, respectively after the reaction without H₂O, regardless of the types of reductant employed. It may be attributed to the isocyanate (-NCO) species formed on the catalyst surface which may be one of the most probable reaction intermediates for this reaction system. When H₂O was added to the feed gas stream, its formation on the synthetic mordenite catalysts such as HM and CuHM was significantly suppressed, but not for CuNZA catalyst It agrees well with the fact that CuNZA catalyst exhibits a strong water tolerance for this reaction system. It also reveals that the formation of the -NCO species on the catalyst surface plays a crucial role for the maintenance of NO removal activity when H₂O exists in the feed gas stream.     

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.     

Selective reduction of nitric oxide with propene over platinum-group based catalysts: Studies of surface species and catalytic activity

The reduction of nitric oxide by propene in the presence of oxygen over platinum-group metals supported on TiO₂, ZnO, ZrO₂, and Al₂O₃ has been investigated by combined diffuse reflectance FT-IR spectroscopy and catalytic activity studies under flow reaction conditions at 523–673 K and atmospheric pressure. The catalytic activity for the selective reduction of nitric oxide and the intensity of the IR bands due to reaction species depended strongly on the nature of the support, type of supported metal, reaction time and temperature. The main surface species detectable by IR were adsorbed hydrocarbons (2900–3080 cm⁻¹), isocyanate (2180, and 2232–2254 cm⁻¹), cyanide (2125 cm⁻¹), nitrosonium (1901 cm⁻¹), CO₂ (2343–2357 cm⁻¹), CO (2058 cm⁻¹) and carbonate (1300–1650 cm⁻¹) species. In the case of rhodium containing catalysts, when supported on Al₂O₃, they exhibited both the highest concentration of surface species and the highest activity for nitric oxide reduction and selectivity to nitrogen. The catalytic activity and the IR intensities of the nitrosonium and isocyanate bands increased with reaction temperature, reached their maximum between 570 and 620 K, and then decreased at higher temperatures. The IR band intensities due to nitrogen containing surface species were found to be strongly correlated to the activity for nitric oxide conversion and only slightly related to the selectivity to dinitrogen.     

Selective catalytic reduction of NOx with propene over double wash-coat monolith catalysts

Selective catalytic reduction (SCR) among the approaches for alleviating NOx emission was much attracted. Zeolites have the advantage for adsorption of propene, and noble metal catalyst has the advantage for oxidation of NO to NO₂. Pt (or Au)/Al₂O₃ (or SiO₂) were used as the lower layer of double wash-coated monolith catalysts. Zeolites (H-mordenite or ZSM-5) were coated as the upper layer. The catalytic performance of the double wash-coated catalyst was, remarkably, improved. Also, temperature window was shifted to lower temperature and broadened. It was known that the combined noble metal monolith catalyst with zeolite was very effective in removing NOx by SCR with hydrocarbons.     

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.     

Selective catalytic reduction of nitrogen oxides by ammonia on iron oxide catalysts

In this study, new Fe₂O₃ based materials are developed for the selective catalytic reduction (SCR) of NO_x by NH₃ in diesel exhaust. As a result of the catalyst screening, performed in a synthetic model exhaust, ZrO₂ is considered to be the most effective carrier for Fe₂O₃. The modification of the Fe₂O₃/ZrO₂ system with tungsten leads to drastic increase of SCR performance as well as pronounced thermal stability. These results show that tungsten acts as bifunctional component. The highest catalytic activity is observed for ZrO₂ that is coated with 1.4 mol% Fe₂O₃ and 7.0 mol% WO₃ (1.4Fe/7.0W/Zr). By the use of this catalyst quantitative conversion of NO_x is obtained between 285 and 430 °C with selective formation of N₂. Here, the turnover frequency of NO_x per Fe atom is found to be 35 × 10⁻⁵ s⁻¹ that indicates a high catalytic performance. The SCR activity of the 1.4Fe/7.0W/Zr material is decreased in the presence of H₂O and CO₂, whereas it is increased by NO₂.Temperature programmed reduction by H₂ (HTPR) analyses show that the Fe sites of the 1.4Fe/7.0W/Zr catalyst are mainly in the form of crystalline Fe₂O₃, whereby relatively small oxide entities are also present. The strongly aggregated Fe₂O₃ species are associated with the presence of the promoter tungsten. Based upon stationary catalytic examinations as well as diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) studies we postulate an Eley Rideal type mechanism for SCR on 1.4Fe/7.0W/Zr catalyst. The mechanistic model includes a redox cycle of the active Fe sites. As first reaction step, we assume dissociative adsorption of NH₃ that leads to partial reduction of the iron as well as to production of very reactive amide surface species. These amide intermediates are supposed to react with gaseous NO to form N₂ and H₂O. In the final step, the reduced Fe sites be regenerated by oxidation with O₂. As a side reaction of SCR, imide species, originated from decomposition of amide, are oxidized by NO₂ or O₂ into NO.     

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.     

Alumina- and titania-based monolithic catalysts for low temperature selective catalytic reduction of nitrogen oxides

The selective catalytic reduction of NO+NO₂ (NO_x) at low temperature (180–230°C) with ammonia has been investigated with copper-nickel and vanadium oxides supported on titania and alumina monoliths. The influence of the operating temperature, as well as NH₃/NO_x and NO/NO₂ inlet ratios has been studied. High NO_x conversions were obtained at operating conditions similar to those used in industrial scale units with all the catalysts. Reaction temperature, ammonia and nitrogen dioxide inlet concentration increased the N₂O formation with the copper-nickel catalysts, while no increase was observed with the vanadium catalysts. The vanadium-titania catalyst exhibited the highest DeNO_x activity, with no detectable ammonia slip and a low N₂O formation when NH₃/NO_x inlet ratio was kept below 0.8. TPR results of this catalyst with NO/NH₃/O₂, NO₂/NH₃/O₂ and NO/NO₂/NH₃/O₂ feed mixtures indicated that the presence of NO₂ as the only nitrogen oxide increases the quantity of adsorbed species, which seem to be responsible for N₂O formation. When NO was also present, N₂O formation was not observed.     

过渡金属/分子筛催化剂上选择性催化还原氮氧化物的研究进展

综述了近十年来过渡金属/分子筛催化剂上氨和碳氢化合物选择性催化还原NOx方面的研究进展。在NH3-SCR体系,着重介绍了铜基和铁基分子筛催化剂的研究状况,探讨了分子筛催化剂在该体系中的失活原因;在HC-SCR体系,总结了不同过渡金属、分子筛类型、还原剂、H2O和SO2等对催化剂活性的影响,探讨了目前比较公认的碳氢化合物选择性催化还原NOx的反应机理。最后展望了分子筛催化剂在选择性催化还原NOx领域今后的研究方向。     

Effect of oxygen on selective catalytic reduction of NO by NH3 over copper ion exchanged mordenite-type zeolite catalyst

The effect of oxygen on the selective catalytic reduction of NO by NH₃ was examined over a copper exchanged mordenite type zeolite catalyst. The catalytic activity for NO reduction by NH₃ in the presence of oxygen was at least one order of magnitude higher than that in the absence of oxygen and it was fully reversible with respect to the presence of oxygen in the feed gas stream. Based upon ESR, TPD, TPO and TPSR studies, the redox behavior of copper ions was closely related to the enhancement of NO removal activity by the introduction of oxygen to the feed gases.     

Ni/H-USY催化剂上甲烷选择性催化还原NOx的性能

考察了不同负载量的Ni/H-USY（ultra stable Y zeolite）催化剂在氧气含量5%情况下甲烷选择性催化还原氮氧化物性能,同时考察了添加微量Pd和Pt对15％(质量分数)Ni催化剂性能的影响.结果表明,催化剂的催化活性与活性组分的含量间存在密切关系,具有一个最佳的负载量.Pd、Pt的添加提高了氮氧化物的去除率,拓宽了反应活化温度的窗口,同时具有较强的抑制N₂O生成的能力.此外采用XRD、TPR等技术对Ni/H-USY体系催化剂的物相结构及氧化还原性能进行了研究,初步探讨了催化剂的催化活性与活性中心的大小与分布之间的关系.     

Gallium oxide promoted zeolite catalysts for oxidehydrogenation of propane

Novel gallium-containing catalysts for oxidehydrogenation of propane, based on zeolite Beta, ZSM-5 and ferrierite, have been prepared and characterised by scanning electron microscopy, IR, MAS NMR and Raman spectroscopies. The catalytic properties of zeolitic matrixes with B, Al, and both ions at tetrahedral sites have been studied. Transformation of propane on pure zeolites and promoted with gallium (III) oxide depended on the structure of the matrix, its morphology and the type of cations occupying zeolite framework sites. Formation of new hydroxyl groups has been evidenced for some MFI zeolites promoted with Ga₂O₃.     

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%.     

Effects of the pretreatment of Cu-Y zeolite catalysts on the reduction of nitric oxide with ammonia

Experiments were conducted to study the effects of pretreating Cu-Y zeolite catalysts on the reduction of nitric oxide with ammonia. Changing the oxidation state and environment of the copper in Cu-Y changed the optimal reaction temperature of the NO---NH₃ reaction and the activity of this catalyst. At the optimal temperature, the activity of Cu-Y after dehydration was higher than that of hydrated Cu-Y. In addition, at the optimal temperature, the activity of Cu-Y after the NO---NH₃ reaction followed by oxidation or ammonia pretreatment followed by oxidation was higher than that of dehydrated Cu-Y. The activity of Cu-Y at the optimal temperature was decreased by reduction at higher temperature but increased by reduction at lower temperature. Furthermore, the activity of Cu-Y increased as the copper loading was increased. The optimal reaction temperatures of the NO---NH₃ reaction over various pretreated Cu-Y catalysts were 106, 126, 220 and 300°C, respectively.     

铜系催化剂选择性催化还原脱除NOx研究

以硅胶改性堇青石蜂窝陶瓷为载体,分别制备了以Cu-O、Cu-Ce-O和Cu-Ce-Mn-O为活性组分的催化剂。以CO(NH₂)₂为还原剂,在固定床反应器中进行选择性催化还原NO的研究。采用XRD、SEM和BET等测试方法对催化剂进行表征。结果表明,在温度(300～500) ℃,催化剂Cu-Ce-Mn-O/SiO₂/堇青石的活性优于催化剂Cu-O/SiO₂/堇青石和Cu-Ce-O/SiO₂/堇青石,反应温度为450 ℃、空速为8 000 h^-1时,Cu-Ce-Mn-O/SiO₂/堇青石催化剂催化还原NO的转化率可达到88%。     

Heterogeneous catalytic decomposition of nitrous oxide

An overview is given on the ongoing activities in the area of the decomposition of nitrous oxide, N₂O, over solid catalysts. These catalysts include metals, pure and mixed oxides, supported as well as unsupported, and zeolitic systems. The review covers aspects of the reaction mechanism and kinetics, focusing on the role of surface oxygen, the inhibition by molecular oxygen, water and other species, poisoning phenomena and practical developments.     

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).     

Selective reduction of nitrogen oxides with hydrocarbons over solid acid catalysts in oxygen-rich atmospheres

Highly selective reduction of nitrogen oxides to dinitrogen occurs to a high level in oxygen-rich atmospheres by using a small amount of propane as a reducing agent over alumina, silica-alumina, titania and zirconia catalyst. Judging from the data of activity and ammonia TPD measurement on a series of silica-alumina catalysts, acidity is suggested to be one of the main factors that determine catalytic activity.     

Effects of methane and oxygen on decomposition of nitrous oxide over metal oxide catalysts

Catalytic activities of various metal oxides for decomposition of nitrous oxide were compared in the presence and absence of methane and oxygen, and the general rule in the effects of the coexisting gases was discussed. The reaction rates of nitrous oxide were well correlated to the heat of formation of metal oxide, i.e., a V-shaped relationship with a minimum at −ΔH_f⁰ around 450 kJ (O mol)⁻¹ was observed in N₂O decomposition in an inert gas. In the case of metal oxides having the heat of formation lower than 450 kJ (O mol)⁻¹, CuO, Co₃O₄, NiO, Fe₂O₃, SnO₂, In₂O₃, Cr₂O₃, the activities were strongly affected by the presence of methane and oxygen. On the other hand, the activities of TiO₂, Al₂O₃, La₂O₃, MgO and CaO were almost independent. The reaction rate of nitrous oxide was significantly enhanced by methane. The promotion effect of methane was attributed to the reduction of nitrous oxide with methane: 4N₂O+CH₄→2N₂+CO₂+2H₂O. The activity was suppressed in the presence of oxygen on the metal oxides having lower heat of formation. On the basis of Langmuir–Hinshelwood mechanism, the effect of oxygen on nitrous oxide decomposition was rationalized with the strength of metal–oxygen bond.