Selective catalytic reduction of NO with propane over CoZSM-5 containing alkaline earth cations

Selective catalytic reduction (SCR) of nitric oxide (NO) with propane is studied over CoZSM-5 catalysts with a series of exchanged cobalt concentrations (0.9-7.5 wt.-%). The overall activity for SCR of NO is found to increase linearly with the cobalt content in the range below the maximum exchange capacity (CoAl= 0.5). However, when the cobalt loading exceeds the exchange capacity of the zeolite, viz.CoAl 0.5, the combustion of propane is favored significantly, resulting in a decrease of the NO conversion. The presence of excess Co²⁺ in zeolite appears to bring about the marked falls in adsorption of NO. In this case cobalt oxide particles are presumed to form, which promote the oxidation of propane. Nevertheless, the addition of alkaline-earth metal cations (Ba, Ca) resulted in the suppression of propane oxidation over CoZSM-5, and improved the NO conversion dramatically.     

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.     

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 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 catalytic reduction of nitric oxide by ammonia on Fe-promoted active carbon

Selective catalytic reduction of nitric oxide by ammonia on Fe³⁺-promoted active carbons was investigated. The catalysts were prepared by the impregnation of active carbon (N/m) preoxidised with concentrated nitric acid at different temperatures. The amount of oxygen-containing surface groups on the supports was determined by infrared spectroscopy and X-ray photoelectron spectroscopy (XPS) while the distribution of active material was investigated using XPS. Catalytic activity of Fe³⁺-active carbon systems depended on the degree of the oxidation of the supports and pretreatment of the catalysts (drying, calcination in helium). The presence of oxygen in the reaction mixture enhanced the nitric oxide conversion. The catalysts showed a long-term stability.     

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 nitric oxide by ethylene in the presence of oxygen over Cu ion-exchanged pillared clays

Cu²⁺ ion-exchanged pillared clays are substantially more active than Cu²⁺-ZSM-5 for selective catalytic reduction (SCR) of NO by hydrocarbons. More importantly, H₂O (or SO₂) has only mild effects on their activities. First results on Cu²⁺-exchanged TiO₂-pillared montmorillonite were reported by this laboratory (Yang and Li, Ref. [1]), that showed overall activities two to four times higher than Cu²⁺-ZSM-5.

A delaminated pillared clay was subjected to Cu²⁺ ion-exchange and studied for SCR by C₂H₄ in this work. The Cu²⁺ ion-exchanged delaminated Al₂O₃-pillared clay yielded substantially higher SCR rates than both Cu²⁺-exchanged TiO₂-pillared clay and Cu²⁺-ZSM-5 at temperatures above 400°C. The peak NO conversion was 90% at 550°C and at a space velocity of 15,000 h⁻¹ (with O₂ = 2%). The peak temperature decreased as the concentration of O₂ was increased. The macroporosity in the delaminated pillared clay was partially responsible for its higher peak temperatures (than that for laminated pillared clays). At 1000 ppm each for NO and C₂H₄, the NO conversion peaked at 2% O₂ for all temperatures. H₂O and SO₂ caused only mild deactivation, likely due to competitive adsorption (of SO₂ on Cu²⁺ sites and H₂O on acid sites). The high activity of Cu²⁺-exchanged Al₂O₃-pillared clay was due to a unique combination of the redox property of the Cu²⁺ sites and the strong Lewis acidity of the pillared clay. The suggested mechanism involved NO chemisorption (in the presence of O₂) on Cu²⁺OAl³⁺-on the pillars, and C₂H₄ activation on the Lewis acid sites to form an oxygenated species.     

Pd/Al2O3催化剂上CH4选择性还原NO的研究

考察了Pd/Al₂O₃、In/Al₂O₃和Co/Al₂O₃对甲烷选择性还原NO的催化活性。结果表明，采用浸渍法制备的Pd/Al₂O₃、In/Al₂O₃和Co/Al₂O₃三种催化剂，在有氧气氛下，用CH₄作还原剂催化还原NO时，Pd/Al₂O₃催化剂的活性最佳，热稳定性好，在550 ℃，用CH₄选择还原NO，Pd/Al₂O₃催化剂表现出较强的催化能力，NO的转化率达到100%。在高空速实验中，该催化剂亦表现出较高的活性，其活性顺序为Pd/Al₂O₃>In/Al₂O₃>Co/Al₂O₃。实验研究了助催化剂、氧含量以及空速对Pd/Al₂O₃催化剂活性的影响。     

Effect of operating conditions on the reduction of nitrous oxide by propane over a Fe-zeolite monolith

Fluidised bed combustion is an important source of nitrous oxide emissions. The influence of different operating parameters, such as catalyst volume, temperature, gas hourly space velocity, and hydrocarbon addition, on the activity, selectivity, and poisoning tolerance of a Fe-ZSM-5 monolith for the nitrous oxide selective catalytic reduction, has been investigated under realistic conditions, at bench scale.

Both in the absence or in the presence of poisons, such as H₂O, NO, and SO₂, the optimisation of operating conditions gives rise to a broadening of the temperature window for N₂O reduction, making it more compatible with real application conditions, with a simultaneous reduction in hydrocarbon fugitive emission, resulting in an environmental friendly process.

Excessively high reaction temperatures seem to be needed to obtain an acceptable level of N₂O decomposition. On the contrary, high N₂O reduction conversions are obtained, even in the presence of poisons and at relatively low temperatures, which is the preferred situation in the processes of pollutants removal from stationary combustion sources.

The optimum value of C₃H₈/N₂O ratio to be used for reducing N₂O over the catalyst system seems to be about the unity, since higher N₂O and C₃H₈ conversions and lower hydrocarbon unwanted emissions are attained, with a low consume of propane as selective reductant.     

Selective catalytic reduction of nitric oxide over vanadia grafted on titania. Influence of vanadia loading on structural and catalytic properties of catalysts

The selective catalytic reduction (SCR) of NO by NH₃ has been studied over vanadia/ titania catalysts prepared by selective immobilization of vanadyl alkoxide species on two structurally different titania supports. The loading of vanadia was varied from 1.8 to 7.5 ,mol V⁵⁺ per m² surface area. Comparative kinetic measurements at 150 °C show that the NO turnover frequencies increase by more than an order of magnitude when the vanadia loading is increased from 1.8 to 3 mol V⁵⁺/m². In the region of lower SCR activity, i.e. at lower coverages ( 2 mol V⁵⁺/m²), small clusters and ribbons of vanadia are detected in the Raman spectra, whereas at loadings where maximum NO turnovers are achieved ( 3 mol V⁵⁺/m²) the prevalent vanadia species are well-developed two-dimensional vanadia layers bound to titania.     

Formation of active phases in MoVTeNb oxide catalysts for ammoxidation of propane

The method of drying, heat evaporation or spray drying, of the aqueous suspension of starting chemicals has a pronounced effect on the phase composition of the final MoVTeNb catalyst, which ultimately influences the catalytic properties in propane ammoxidation reaction. The sample synthesized by spray drying is active and selective; it contains two main crystalline phases, orthorhombic M1 and hexagonal M2. The activity of the sample prepared by heat evaporation is low. This sample does not contain the active M1 phase and consists of hexagonal M2, TeMo₅O₁₆, and Mo_5−x(V/Nb)_xO₁₄ phases. The different mechanisms of phase composition formation in the samples synthesized by heat evaporation or spray drying arise from the different chemical nature of corresponding solid precursors.     

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.     

Oxidative dehydrogenation of propane over vanadium oxide based catalysts Effect of support and alkali promoter

The oxidative dehydrogenation of propane was investigated using vanadia type catalysts supported on Al₂O₃, TiO₂, ZrO₂ and MgO. The promotion of V₂O₅/Al₂O₃ catalyst with alkali metals (Li, Na, K) was also attempted. Evaluation of temperature programmed reduction patterns showed that the reducibility of V species is affected by the support acid–base character. The catalytic activity is favored by the V reducibility of the catalyst as it was confirmed from runs conducted at 450–550°C. V₂O₅/TiO₂ catalyst exhibits the highest activity in oxydehydrogenation of propane. The support’s nature also affects the selectivity to propene; V₂O₅ supported on Al₂O₃ catalyst exhibits the highest selectivity. Reaction studies showed that addition of alkali metals decreases the catalytic activity in the order non-doped>Li>Na>K. Propene selectivity significantly increases in the presence of doped catalysts.     

Catalytic reduction of nitric oxide on Pt and Rh catalysts supported on alumina and titania synthesized by the sol-gel method

Platinum and rhodium supported on alumina and titania were synthesized by the sol-gel method. Characterization and catalytic activity for the reduction of nitric oxide by carbon monoxide was performed. In sol-gel ‘sintered/reduced catalysts’ a redispersion of the metal phase and higher resistance to sintering was observed. On the other hand, in the impregnated ‘sintered/reduced catalysts’, an important sintering effect was observed. The sol-gel ‘reduced catalysts’ and ‘sintered/reduced catalysts’ showed higher activity than that of impregnated reference catalysts, mainly when the titania is the support. Moreover, sol-gel preparations are more selective to N₂, whereas impregnated reference catalysts are selective to N₂O. The increase in dispersion and high resistance to sintering on sol-gel ‘sintered/reduced catalysts’ was interpreted as a surface migration effect of the metal particles buried in alumina and titania gels.     

Selective reduction of nitric oxide by methanol and dimethyl ether over promoted alumina catalysts in excess oxygen

Selective catalytic reduction (SCR) activity for NO conversion to N₂ over γ-alumina, vanadia/alumina and molybdena/alumina catalysts has been investigated with methanol (MeOH) and dimethyl ether (DME) as reductants under lean conditions. Molybdena/alumina catalysts showed high efficiency for NO reduction with either reductant, especially at low temperature, which may involve surface formyl produced by oxidative dehydrogenation. Sulphated γ-alumina remains active for NO reduction with MeOH, while sulphated 5 wt.% MoO₃/Al₂O₃ remains active with both MeOH and DME over a broad temperature range.     

堇青石蜂窝陶瓷载CuO选择催化还原NO的研究

以堇青石蜂窝陶瓷(CC)为载体、CuO和不同助剂为活性组分，用浸渍法制备CuO/CC、CuO-NiO/CC和CuO-NiO-CeO₂/CC催化剂, 采用TPR、XRD和XPS等测试方法对催化剂进行表征。TPR结果表明，催化剂主要以Cu²⁺形式存在。采用程序升温和恒温法在固定床反应器常压条件下研究了以尿素作还原剂还原模拟汽车尾气中的NO。结果显示，CuO-NiO-CeO₂/CC催化剂在(150～350) ℃具有较高的活性，250 ℃时具有较高的转化率。     

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.     

Supramolecular assembly of porphyrin bound DNA and its catalytic behavior for nitric oxide reduction

A stable Fe(4-TMPyP)-DNA-PADDA (FePyDP) film was prepared on pyrolytic graphite electrode (PGE) through the supramolecular interaction between water-soluble iron(III) meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (Fe(4-TMPyP)) and DNA template, where PADDA (poly(acrylamide-co-diallyldimethylammonium chloride)) is employed as a co-immobilizing polymer. Electronic absorption spectral and quartz crystal microbalance measurements revealed that Fe(4-TMPyP) interacted with DNA to generate a species with the molar ratio of 1:5 for Fe(4-TMPyP):DNA phosphate. Cyclic voltammetry of FePyDP film showed a pair of stable and reversible peaks corresponding to Fe^III/Fe^II redox potential of −0.13 V versus Ag|AgCl in pH 7.4 PBS. The electron transfer was expected across the double-strand of DNA by an “electron tunneling” mechanism. The modified electrode displayed an excellent catalytic activity for NO reduction at −0.61 V versus Ag|AgCl. The catalytic current was enhanced at lower pH. Chronoamperometric experiments demonstrated a rapid response to the reduction of NO with a linear range from 0.1 to 90 μM. The detection limit was 30 nM at a signal-to-noise ratio of 3.     

Role of potassium on the structure and activity of alumina-supported vanadium oxide catalysts for propane oxidative dehydrogenation

The influence of potassium on the structure and properties of alumina-supported vanadium oxide catalysts has been studied by in situ Raman spectroscopy, temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), a probe reaction of acid/base–redox sites (methanol chemisorption) and tested in oxidative dehydrogenation (ODH) of propane. Potassium coordinates to surface vanadium oxide species altering its structure but does not form bulk compounds, possibly because the total V+K coverage does not reach the monolayer coverage on alumina. The interaction of K with V weakens the terminal V=O bond. K-doped alumina (KAl)-supported vanadia catalysts show lower acidity, a decrease of reducibility and a decrease of propane conversion values. These trends do not correspond with the changes in the terminal V=O bond energy. Thus, it appears that the terminal V=O bond of surface vanadium oxide species is not the active site for propane ODH, oxidation of methanol to formaldehyde and for the reduction of surface vanadium oxide species by hydrogen. Potassium also changes the acid–base characteristic of the system and decreases the acidic character of surface vanadia. This shift in the acid–base character to a more basic system must also account for the better selectivity in propane ODH due to a variation in the interaction between the intermediates and the surface.