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.     

In situ FT-IR study of NH3 formation during the reduction of NOx with propane on H/Cu-ZSM-5 in excess oxygen

IR experiments under flow of NO and propane on H/Cu-ZSM-5 evidence at 623 K the appearance of bands at 2248, 2157 and 2047 cm⁻¹ tentatively assigned through the use of ¹⁵NO to nitrile, carbonyl (CO---Cu⁺) and isocyano species respectively. Addition of O₂ suggests conversion of isocyano to isocyanato species (2208 cm⁻¹) which by hydrolysis leads to NH₃ formation, revealed by IR bands at 3366, 3290, 3192 and 1610 cm⁻¹.     

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。     

Microbial reduction of sulfur dioxide and nitric oxide

Two process concepts have been developed for a microbial contribution to the problem of flue gas desulfurization and NO_x removal. We have demonstrated that the sulfate-reducing bacterium Desulfovibrio desulfuricans can be grown in a mixed culture with fermentative heterotrophs in a medium in which glucose served as the only carbon source. Beneficial cross-feeding resulted in vigorous growth of D. desulfuricans, which used SO₂(g) as a terminal electron acceptor, with complete reduction of SO₂ to H₂S in 1–2 s of contact time. We have proposed that the concentrated SO₂ stream, obtained from regeneration of the sorbent in regenerable processes for flue gas desulfurization, could be split with two-thirds of the SO₂ reduced to H₂S by contact with a culture of sulfate-reducing bacteria. The resulting H₂S could then be combined with the remaining SO₂ and used as feed to a Claus reactor to produce elemental sulfur. However, the use of glucose as an electron donor in microbial SO₂ reducing cultures would be prohibitively expensive. Therefore, if microbial reduction of SO₂ is to be economically viable, less expensive electron donors must be found. Consequently, we have evaluated the use of municipal sewage sludge and elemental hydrogen as carbon and/or energy sources for SO₂ reducing cultures. Heat and alkali pretreated sewage sludge has been successfully used as a carbon and energy source to support SO₂ reduction in a continuous, anaerobic mixed culture containing D. desulfuricans. The culture operated for nine months with complete reduction of SO₂ and H₂S. Another sulfate-reducing bacterium, Desulfotomaculum orientis, has also been grown in batch cultures on a feed of SO₂, H₂ and CO₂. Complete reduction of SO₂ to H₂S was observed with gas-liquid contact times of 1–2 s. We have also demonstrated that the facultative anaerobe and chemoautotroph, Thiobacillus denitrificans, can be cultured anoxically in batch reactors using NO(g) as a terminal electron acceptor with reduction to elemental nitrogen. We have proposed that the concentrated stream of NO_x, as obtained from certain regenerable processes for flue gas desulfurization and NO_x removal, could be converted to elemental nitrogen for disposal by contact with a culture T. denitrificans. Two heterotrophic bacteria have also been identified which may be grown in batch cultures with succinate or heat and alkali pretreated sewage sludge as carbon and energy sources and NO as a terminal electron acceptor. These are Paracoccus denitrificans and Pseudomonas denitrificans.     

Heterogeneous reduction of nitric oxide on synthetic coal chars

Model compounds, with a controlled heteroatoms content and well-defined functionalities, were used to study the release of nitrogen compounds from char combustion. In the present work, the mechanisms involved in NO-char heterogeneous reduction were studied with a synthetic coal (SC) char as carbon source. Another synthetic char (SN) without any nitrogen in its composition was also employed in these studies. Temperature programmed reduction (TPR) tests with a gas mixture of 400 ppm NO in argon and with isotopically labelled nitric oxide, ¹⁵NO (500 ppm ¹⁵NO in argon), were carried out. The gases produced were quantitatively determined by means of MS and FTIR analysers.Under the conditions of this work the main products of the NO-C reaction were found to be N₂ and CO₂. The main path of reaction involves the formation of surface nitrogen compounds that afterwards react with nitrogen from the reactive gas to form N₂. It was observed that fuel-N also participates in the overall heterogeneous reduction reaction, although to a lesser extent.     

Catalytic reduction of nitrogen oxides with methane in the presence of excess oxygen

We discovered a family of catalysts that can effectively reduce NO_x with methane in the presence of excess oxygen. This new catalytic chemistry offers an alternative means for controlling NO_x emissions. Complete reduction of nitric oxide was obtained at 400°C over a Co-ZSM-5 catalyst. The presence of oxygen in the feed greatly enhances the nitric oxide reduction activity on Co-ZSM-5, and the nitric oxide conversion is strongly related to the inlet methane level. On the other hand, Cu-ZSM-5, which is a unique catalyst for the direct nitric oxide decomposition, is a poor catalyst for nitric oxide reduction by methane in the presence of excess of oxygen.     

Selective catalytic reduction of nitric oxide with ethane and methane on some metal exchanged ZSM-5 zeolites

The selective reduction of nitric oxide by methane or ethane, in the presence and in the absence of a large excess of oxygen, has been investigated on Cu/ZSM-5, Co/ZSM-5, Rh/ZSM-5 and Pt/ZSM-5 catalysts over a wide range of temperatures. It has been found that the maximum nitric oxide conversion is higher with ethane than with methane and the temperature of this maximum is lower with ethane. In the absence of oxygen the order of activity is Rh/ZSM-5>Pt/ZSM-5>Co/ZSM-5> Cu/ZSM-5 with the Cu/ZSM-5 being essentially inactive, while in the presence of oxygen the order is: Rh/ZSM-5>Co/ZSM-5>Cu/ZSM-5> Pt/ZSM-5 when ethane is used as reductant and: Rh/ZSM-5>Co/ZSM-5> Cu/ZSM-5>Pt/ZSM-5 when methane is used. The effect of the oxygen content has been investigated for the Co/ZSM-5 catalyst. It has been found that with a small quantity of oxygen the catalytic activity decreases markedly; with higher oxygen content the activity of the catalyst rises again. It appears that two different reaction schemes may be operative, one in the absence of oxygen the other in the presence of oxygen. It is concluded that neither carbonaceous deposits, nor nitrogen dioxide formation in the gas phase are important in the reaction mechanism on metal-containing zeolites. It is proposed that the reaction is essentially a redox process in which decomposition of nitric oxide occurs on reduced metallic or metal ion sites (the relative activity of each of these depending on the choice of metal), leading to the formation of gaseous nitrogen and adsorbed oxygen, followed by the removal of the adsorbed oxygen by the hydrocarbon, thus recreating the active centres.     

Dinitrogen formation over low-exchanged Cu-ZSM-5 in the selective reduction of NO by propane

The role of gaseous NO and C₃H₈ has been studied over low-exchanged Cu-ZSM-5 zeolite employing TPD, FTIR and pulse technique with the alternate introduction of NO or C₃H₈ onto the catalyst surface. The rate of the N₂ formation is directly proportional to the content of gaseous NO and the surface coverage with 2-nitrosopropane. There was no formation of N₂ during interaction of gaseous C₃H₈ with NO adsorbates. However, 2-nitrosopropane and its isomer acetone oxime were also formed in this reaction. This revised version was published online in November 2006 with corrections to the Cover Date.     

NO2 formation over Cu-ZSM-5 and the selective catalytic reduction of NO

The extent of the selective catalytic reduction (SCR) of nitric oxide to dinitrogen in the presence of excess oxygen is enhanced by the oxygen on several zeolite-based catalysts and using different reductants. When the catalyst is Cu-ZSM-5 and the reductant is a hydrocarbon, an NO₂ intermediate has been suggested by several investigators. This work shows that at short residence times, with excess reductant and in the absence of oxygen, the NO₂ itself is reduced only back to NO. Thus, for the selective reduction of NO₂ to N₂ (N-pairing) strongly oxidizing conditions are required, same as for the complete reduction of NO. In the presence of excess oxygen the activity of Cu-ZSM-5 in the NO + O₂ reaction to form NO₂ parallels the SCR in every respect. It is higher over Cu-ZSM-5 than on Cu/Al₂O₃ or on H-ZSM-5. The coppercontaining zeolite is also active in the decomposition of NO₂ back to NO and O₂ while the other catalysts are much less active. The inhibiting effect of water on the NO + O₂ catalytic reaction is also parallel to the effect on SCR. This evidence strengthens the notion of an NO₂ intermediate.     

Effects of the Si/Al atomic ratio on the activity of Cu-ZSM-5 catalysts for nitric oxide decomposition

A review of literature data for nitric oxide decomposition over Cu-ZSM-5 catalysts leads us to conclude that the turnover frequency depends on the Si/Al atomic ratio in a way opposite to the trend suggested by Iwamoto and co-workers and considered correct by Shelef in a recent letter published in this Journal. In particular we show that the turnover frequency increases with the number of Al atoms per unit cell (i.e. decreasing the Si/Al atomic ratio). This result suggests that the most active sites for NO decomposition over Cu-ZSM-5 catalysts may contain two close copper ions.     

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。     

The selective non-catalytic reduction of nitric oxide using ammonia at up to 15% oxygen

Selective non-catalytic reduction of nitric oxide (NO) using ammonia was studied with up to 15% (by volume) oxygen at 102 kPa. The experiments were conducted in an electrically heated laminar-flow, quartz reactor using mixtures of N₂, O₂, NO, and CO to simulate exhaust gas. The base case condition included 330 ppmv of NO, 495 ppmv of NH₃, and 15% O₂. At a reactor temperature of 1050 K, 77% of the NO was removed. For a lower oxygen concentration of 1%, the NO removal was as high as 98% at 1100 K. The degraded performance at high oxygen concentrations is attributed to increases in the oxidation reactions. A major result of this work was the quantification of the amount of N₂O in the treated gases. For the base case conditions, 21 ppmv of N₂O was measured for a reactor temperature of 1075 K. Increasing the ratio of NH₃ to NO (by increasing the NH₃ concentration) increased the maximum NO removal and decreased the temperature at which this level of NO removal was achieved. For the higher NH₃ concentrations, however, the N₂O concentration increased to as high as 54 ppmv. The oxidation products of ammonia (in the absence of NO) for these conditions were found to include first N₂O beginning at 900 K and then NO beginning at 1050 K. Comparisons between these experimental results and predictions from the Miller and Bowman (1989) model indicate that further enhancements of the model may be necessary to incorporate the features of high oxygen conditions.     

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.     

Determination of active palladium species in ZSM-5 zeolite for selective reduction of nitric oxide with methane

The active site in ZSM-5 zeolite-supported palladium, which shows the catalytic activity for NO reduction with methane as a reducing agent, has been investigated qualitatively and quantitatively by means of NO chemisorption and NaCl titration, comparing with PdO supported on silica. Palladium species in 0.4 wt.% Pd loaded H-ZSM-5 can adsorb NO equimolarly after calcination at 773 K, and almost all the NO was desorbed at around 673 K, while the palladium species on PdO/SiO₂ hardly adsorbed NO. The palladium species in Pd(0.4)/H-ZSM-5 are ion-exchangeable with Na⁺ in NaCl solution, indicating that they exist in a cationic state of an isolated Pd²⁺. This method for quantitative analysis of the isolated Pd²⁺ cations is named as ‘NaCl titration’. The amount of the isolated Pd²⁺ cationic species increased with increasing palladium content on Pd/H-ZSM-5, and PdO co-existed above 1 wt.%. The amount of the isolated Pd²⁺ cation was unchanged after the reaction of NO₂–CH₄, NO₂–CH₄–O₂, or CH₄–O₂ at 673 K, while the adsorbed amount of NO per the Pd²⁺ as determined by NO-TPD decreased after the NO₂–CH₄–O₂ reaction. It was found by NaCl titration that the catalytic activity of Pd/H-ZSM-5 for NO₂–CH₄–O₂ reaction increased with increasing amount of the isolated Pd²⁺ cationic species up to 0.7 wt.%, while the increase in the amount of PdO led to decrease in selectivity towards NO₂ reduction. The palladium species that are active and selective for NO reduction with CH₄ will be proposed.     

Studies of the selective reduction of nitric oxide by hydrocarbons

The selective reduction of nitric oxide with isobutane in the presence and absence of oxygen has been studied over Cu-ZSM-5-14-114 and compared to the simple decomposition reaction. In the presence of sufficient oxygen, complete combustion of the hydrocarbon was observed, whereas in its absence, acid catalysis accompanied by dehydrogenation associated with coking occurred. The effect of adding nitric oxide to the HC/He stream in the absence of oxygen was small; the conversion to nitrogen was little affected by the hydrocarbon. When oxygen was added, however, the decomposition was complete at 573 K even at high space velocities (e.g., SVH 10⁴) even though the combustion was not. Isobutane (an alkane) was more effective than C₃H₆ (an alkene) for the selective reduction; this is not generally the case. The presence of excess oxygen inhibited aging of the catalyst.     

Improved lean deNOx performance of Cu-ZSM-5 through alternative synthesis conditions for ZSM-5

A series of ZSM-5 samples with similar SiO₂/Al₂O₃ ratio was synthesised using different aluminum sources and by including varying amounts of calcium hydroxide during the synthesis. The samples were subsequently ion-exchanged with Cu-ions, and evaluated with respect to deNOx activity in the absence and presence of water. The choice of aluminum source and the presence of calcium hydroxide during the zeolite synthesis affected the lean deNOx activity of Cu-ZSM-5.     

A kinetic model for ammonia selective catalytic reduction over Cu-ZSM-5

Kinetic modeling, in combination with flow reactor experiments, was used in this study for simulating NH₃ selective catalytic reduction (SCR) of NO_x over Cu-ZSM-5. First the mass-transfer in the wash-coat was examined experimentally, by using two monoliths: one with 11 wt.% wash-coat and the other sample with 23 wt.% wash-coat. When the ratio between the total flow rate and the wash-coat amount was kept constant similar results for NO_x conversion and NH₃ slip were obtained, indicating no significant mass-transfer limitations in the wash-coat layer. A broad range of experimental conditions was used when developing the model: ammonia temperature programmed desorption (TPD), NH₃ oxidation, NO oxidation, and NH₃ SCR experiments with different NO-to-NO₂ ratios. 5% water was used in all experiments, since water affects the amount of ammonia stored and also the activity of the catalyst. The kinetic model contains seven reaction steps including these for: ammonia adsorption and desorption, NH₃ oxidation, NO oxidation, standard SCR (NO + O₂ + NH₃), rapid SCR (NO + NO₂ + NH₃), NO₂ SCR (NO₂ + NH₃) and N₂O formation. The model describes all experiments well. The kinetic parameters and 95% linearized confidence regions are given in the paper. The model was validated with six experiments not included in the kinetic parameter estimation. The ammonia concentration was varied from 200 up to 800 ppm using NO only as a NO_x source in the first experiment and 50% NO and 50% NO₂ in the second experiment. The model was also validated with transient experiments at 175 and 350 °C where the NO and NH₃ concentrations were varied stepwise with a duration of 2 min for each step. In addition, two short transient experiments were simulated where the NO₂ and NO levels as well as NO₂-to-NO_x ratio were varied. The model could describe all validation experiments very well.     

选择性催化还原(SCR)法脱除NOx的研究进展

本文介绍了用作NOx选择催化还原的碳氢化物、氨气、尿素等几种还原剂和活性炭、分子筛、金属氧化物、堇青石蜂窝陶瓷、复合载体等几类载体催化剂的研究进展情况，并阐述了各自的优缺点，综述．了尿素作为还原剂，复合载体的催化剂将是脱除氮氧化物的研究热点。     

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.     

Selectivity-determining role of C3H8/NO ratio in the reduction of nitric oxide by propane in presence of oxygen over ZSM5 zeolites

The reaction of (NO + C₃H₈ + O₂) can result in selective formation of NO₂ over H-ZSM5, Cu,H-ZSM5, Ag,H-ZSM5, and Li,H-ZSM5 catalysts when the concentrations of NO and O₂ are 0.1 and 9%, SV > 60,000 h⁻¹ (typical for automotive exhausts), and C₃H₈/NO > 1. Despite stoichiometric excess of reductant hydrocarbon below this limit, the in situ formed NO₂ does not react with C₃H₈, thus conversion of NO to N₂ is negligible. NO can be reduced by C₃H₈ selectively to N₂ only when C₃H₈/NO ≧ 1. Contrary to many suggestions the reaction temperature, concentration of oxygen, space velocity, and type of exchange ions have minor influence on the selectivity for N₂. These parameters affect the rates of reactions (NO + ₂), (C₃H₈ + NO_x) and (C₃H₈ + O₂), therefore they also affect the production of N₂ in the HC-SCR process, but only when the ratio of C₃H₈/NO permits. The metal-exchanged zeolites were prepared in situ by solid-state ion exchange from H-ZSM5. Despite the low degree of copper exchange (63%), Cu,H-ZSM5 produces substantially more N₂ than H-ZSM5, Ag,H-ZSM5, or Li,H-ZSM5. However, the selectivity for N₂ is lowest over Cu,H-ZSM5, which also produces considerable NO₂ in the reaction of (NO + C₃H₈ + O₂) even at C₃H₈/NO ≧ 1. Contrary to prior findings, the catalytic activity of Cu,H-ZSM5 for the oxidation of NO by O₂ to NO₂ in absence of hydrocarbon was comparable to that of H-ZSM5 at high space velocities (2.3 l g⁻¹ min⁻¹). By replacing 30 and 40% of the protons of H-ZSM5 by Ag⁺ and Li⁺ ions in Ag,H-ZSM5 and Li,H-ZSM5, respectively, the catalytic activity for this reaction becomes negligible at temperatures ≧100°C. Some mechanistic consequences of these experimental observations are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.