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1.
The activity and excellent selectivity (>90%) of γ-Al 2O 3-supported Ni for the selective catalytic oxidation (SCO) of NH 3 to N 2 with excess O 2 has been shown by microreactor studies. Further studies of the mechanism involved in this reaction have been carried out using TPD, TPO, TPReaction as well as DRIFTS. N 2H 4 and NO have been used to model the intermediates of the SCO mechanism (direct formation of N 2 via the recombination of two NH x species) and of the in situ SCR mechanism (two-step formation of N 2 via the reduction of an in situ produced NO species by a NH x species), respectively. Two IR absorption bands appear during the TPO of NH 3 in the temperature range of N 2 formation and have been assigned to stable bidentate nitrate surface species. This represents strong evidence that under the present conditions, formation of N 2 occurs via the in situ SCR mechanism. This also explains the sudden “NO jump” observed on various systems once the temperature is high enough to activate 50% of the NH 3 molecules fed to the catalyst. The fact that NO and NH 3 are able to react to give N 2 at low temperature (from 100°C) confirms that activation of NH 3 is the limiting step. In contrast, no evidence has been found to support the possibility of the SCO mechanism. 相似文献
2.
简述了NH 3和NO在催化剂表面吸附、转化活化和反应历程及H 2O和SO 2对以上反应行为的影响。分析表明,NH 3氧化脱氢进而与NO反应是决定NH 3反应性和最终产物的关键。NO以气态(Eley-Rideal机理)或硝基类物质等吸附态(Langmuir-Hinshelwood机理)形式参与选择催化还原(SCR)反应。提高催化剂酸性和氧化还原循环性能,利于NH 3和NO吸附和转化及相互间反应。高温时,H 2O影响轻微,而SO 2增强催化剂酸性,提高脱硝活性。低温时,H 2O和SO 2抑制NO吸附和转化活化,导致硫铵盐累积和活性位转变为硫酸盐使催化剂失活。因此,提高抗H 2O、抗SO 2性能是低温脱硝催化剂研发的重要方向。而发展在线升温等再生工艺以解决硝酸盐或含硫化合物导致的失活问题,对保障低温脱硝系统长期稳定运行具有重要意义。 相似文献
3.
Combined effect of H 2O and SO 2 on V 2O 5/AC the activity of catalyst for selective catalytic reduction (SCR) of NO with NH 3 at lower temperatures was studied. In the absence of SO 2, H 2O inhibits the catalytic activity, which may be attributed to competitive adsorption of H 2O and reactants (NO and/or NH 3). Although SO 2 promotes the SCR activity of the V 2O 5/AC catalyst in the absence of H 2O, it speeds the deactivation of the catalyst in the presence of H 2O. The dual effect of SO 2 is attributed to the SO 42− formed on the catalyst surface, which stays as ammonium-sulfate salts on the catalyst surface. In the absence of H 2O, a small amount of ammonium-sulfate salts deposits on the surface of the catalyst, which promote the SCR activity; in the presence of H 2O, however, the deposition rate of ammonium-sulfate salts is much greater, which results in blocking of the catalyst pores and deactivates the catalyst. Decreasing V 2O 5 loading decreases the deactivation rate of the catalyst. The catalyst can be used stably at a space velocity of 9000 h −1 and temperature of 250 °C. 相似文献
4.
A novel multiwalled carbon nanotube (CNTs) supported vanadium catalyst was prepared. The structure of catalyst prepared was characterized by TEM, BET, FTIR, XRD and temperature-programmed desorption (TPD) methods. The results indicated that vanadium particles were highly dispersed on the wall of carbon nanotubes. The V 2O 5/CNT catalysts showed good activities in the SCR of NO with a temperature range of 373–523 K. The Lewis acid sites on the surface of V 2O 5/CNT are the active sites for the selective catalytic reduction (SCR) of NO with NH 3 at low temperatures. It was suggested that the reaction path might involve the adsorbed NH 3 species reacted with NO from gaseous phase and as well as the adsorbed NO 2 species. The diameter of CNTs showed positive effect on the activities of the catalysts. Under the reaction conditions of 463 K, 0.1 Mpa, NH 3/NO = 1, GHSV = 35,000 h −1, and V 2O 5 loading of 2.35 wt%, the outer diameter of CNTs of 60–100 nm, the NO conversion was 92%. 相似文献
5.
We present a systematic study of the NH 3-SCR reactivity over a commercial V 2O 5–WO 3/TiO 2 catalyst in a wide range of temperatures and NO/NO 2 feed ratios, which cover (and exceed) those of interest for industrial applications to the aftertreatment of exhaust gases from diesel vehicles. The experiments confirm that the best deNO x efficiency is achieved with a 1/1 NO/NO 2 feed ratio. The main reactions prevailing at the different operating conditions have been identified, and an overall reaction scheme is herein proposed. Particular attention has been paid to the role of ammonium nitrate, which forms rapidly at low temperatures and with excess NO2, determining a lower N2 selectivity of the deNOx process. Data are presented which show that the chemistry of the NO/NO2–NH3 reacting system can be fully interpreted according to a mechanism which involves: (i) dimerization/disproportion of NO2 and reaction with NH3 and water to give ammonium nitrite and ammonium nitrate; (ii) reduction of ammonium nitrate by NO to ammonium nitrite; (iii) decomposition of ammonium nitrite to nitrogen. Such a scheme explains the peculiar deNOx reactivity at low temperature in the presence of NO2, the optimal stoichiometry (NO/NO2 = 1/1), and the observed selectivities to all the major N-containing products (N2, NH4NO3, HNO3, N2O). It also provides the basis for the development of a mechanistic kinetic model of the NO/NO2–NH3 SCR reacting system. 相似文献
6.
The behavior of V=O band over V 2O 5 crystallite during NH 3 adsorption and SCR reaction was characterized by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and the results are correlated with the reactivity in NH 3 oxidation and SCR reaction. It is found that the decrease of V=O band intensity is due either to the reduction of V 2O 5 surface and/or to the adsorption of ammonia. The 70% intensity of original V=O band is preserved up to 573 K under the conditions of SCR reaction. The vanadium oxidation state is about +4.4. When the temperature reached 673 K, almost all the V=O band was recovered. From these results, it can be suggested that the decrease of the apparent SCR activity due to the increase of NO amount through NH 3 oxidation above 673 K be attributed to the increase of two neighboring V=O sites, which favor the NO formation in ammonia oxidation. 相似文献
7.
An IR and microcalorimetric study of adsorption and in situ reaction at room temperature of NO, O 2, NO 2 and NH 3 provides evidence of the involvement of surface nitrate species in this reaction. On the basis of this new experimental evidence a mechanism is proposed for the low temperature SCR reaction of NO by NH 3 over titanium silicalites (TS-1). 相似文献
8.
As part of a fundamental and applied work on the development of an unsteady mathematical model of the NH 3-selective catalytic reduction (SCR) process for design and control of integrated after-treatment systems of heavy-duty engines, we present herein a transient kinetic analysis of the standard SCR NO + NH 3 system which provides new insight in the catalytic kinetics and mechanism prevailing at low temperatures. Based on kinetic runs performed over a commercial powdered V 2O 5–WO 3–TiO 2 catalyst in the 175–450 °C T-range feeding NH 3 and NO (1000 ppm) in the presence of H 2O (1–10%, v/v) and O 2 (2–6%, v/v), an original dual-site modified redox rate law is derived which effectively accounts for NH 3 inhibition effects observed during transient reactive experiments at T < 250 °C. We also demonstrate that implementation of the novel modified redox kinetics into a fully predictive 1D + 1D model of SCR monolith reactors can significantly improve simulations of SCR transient runs at different scales, including engine test bench experiments over full-scale SCR honeycomb catalysts. 相似文献
9.
A novel TiO 2/Al 2O 3/cordierite honeycomb-supported V 2O 5–MoO 3–WO 3 monolithic catalyst was studied for the selective reduction of NO with NH 3. The effects of reaction temperature, space velocity, NH 3/NO ratio and oxygen content on SCR activity were evaluated. Two other V 2O 5–MoO 3–WO 3 monolithic catalysts supported on Al 2O 3/cordierite honeycomb or TiO 2/cordierite honeycomb support, two types of pellet catalysts supported on TiO 2/Al 2O 3 or Al 2O 3, as well as three types of pellet catalysts V 2O 5–MoO 3–WO 3–Al 2O 3 and V 2O 5–MoO 3–WO 3–TiO 2 were tested for comparison. The experiment results show that this catalyst has a higher catalytic activity for SCR with comparison to others. The results of characterization show, the preparation method of this catalyst can give rise to a higher BET surface area and pore volume, which is strongly related with the highly active performance of this catalyst. At the same time, the function of the combined carrier of TiO 2/Al 2O 3 cannot be excluded. 相似文献
10.
TiO 2-SiO 2 with various compositions prepared by the coprecipitation method and vanadia loaded on TiO 2-SiO 2 were investigated with respect to their physico-chemical characteristics and catalytic behavior in SCR of NO by NH 3 and in the undesired oxidation of SO 2 to SO 3, using BET, XRD, XPS, NH 3-TPD, acidity measurement by the titration method and activity test. TiO 2-SiO 2, compared with pure TiO 2, exhibits a remarkably stronger acidity, a higher BET surface area, a lower crystallinity of anatase titania and results in allowing a good thermal stability and a higher vanadia dispersion on the support up to high loadings of 15 wt% V 2O 5. The SCR activity and N 2 selectivity are found to be more excellent over vanadia loaded on TiO 2-SiO 2 with 10–20 mol% of SiO 2 than over that on pure TiO 2, and this is considered to be associated with highly dispersed vanadia on the supports and large amounts of NH 3 adsorbed on the catalysts. With increasing SiO 2 content, the remarkable activity decrease in the oxidation of SO 2 to SO 3, favorable for industrial SCR catalysts, was also observed, strongly depending on the existence of vanadium species of the oxidation state close to V 4+ on TiO 2-SiO 2, while V 5+ exists on TiO 2, according to XPS. It is concluded that vanadia loaded on Ti-rich TiO 2-SiO 2 with low SiO 2 content is suitable as SCR catalysts for sulfur-containing exhaust gases due to showing not only the excellent de-NO x activity but also the low SO 2 oxidation performance. 相似文献
11.
A series of B-doped V 2O 5/TiO 2 catalysts has been prepared the by sol-gel and impregnation methods to investigate the influence of B-doping on the selective catalytic reduction (SCR) of NO x with NH 3. X-ray diffraction, Brunauer-Emmett-Teller specific surface area, scanning electron microscope, X-ray photoelectron spectroscopy, temperature-programmed reduction of H 2 and temperature-programmed desorption of NH 3 technology were used to study the effect of the B-doping on the structure and NH 3-SCR activity of V 2O 5/TiO 2 catalysts. The experimental results demonstrated that the introduction of B not only improved the low-temperature SCR activity of the catalysts, but also broadened the activity temperature window. The best SCR activity in the entire test temperature range is obtained for VTiB 2.0 with 2.0% doping amount of B and the NO x conversion rate is up to 94.3% at 210 ℃. The crystal phase, specific surface area, valence state reducibility and surface acidity of the active components for the as-prepared catalysts are significantly affected by the B-doping, resulting in an improved NH 3-SCR performance. These results suggest that the V 2O 5/TiO 2 catalysts with an appropriate B content afford good candidates for SCR in the low temperature window. 相似文献
12.
The fast SCR reaction using equimolar amounts of NO and NO 2 is a powerful means to enhance the NO x conversion over a given SCR catalyst. NO 2 fractions in excess of 50% of total NO x should be avoided because the reaction with NO 2 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 NOx 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/NOx 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. 相似文献
13.
The inhibition effect of H 2O on V 2O 5/AC catalyst for NO reduction with NH 3 is studied at temperatures up to 250 °C through TPD, elemental analyses, temperature-programmed surface reaction (TPSR) and FT-IR analyses. The results show that H 2O does not reduce NO and NH 3 adsorption on V 2O 5/AC catalyst surface, but promotes NH 3 adsorption due to increases in Brønsted acid sites. Many kinds of NH 3 forms present on the catalyst surface, but only NH 4+ on Brønsted acid sites and a small portion of NH 3 on Lewis acid sites are reactive with NO at 250 °C or below, and most of the NH 3 on Lewis acid sites does not react with NO, regardless the presence of H 2O in the feed gas. H 2O inhibits the SCR reaction between the NH 3 on the Lewis acid sites and NO, and the inhibition effect increases with increasing H 2O content. The inhibition effect is reversible and H 2O does not poison the V 2O 5/AC catalyst. 相似文献
14.
采用密度泛函理论(DFT)方法研究了NO和NH 3在完整和有缺陷的γ-Al 2O 3(110)表面吸附与SCR(选择催化还原)反应特性。研究表明,NO在完整的(110)表面的吸附作用较弱,而NH 3分子的吸附作用较强,NH 3分子在Al原子顶位可形成稳定吸附。反应路径研究结果表明完整的(110)表面上SCR反应的决速步为-NH 2NO基团的分解,反应的最大能垒为235.75 kJ·mol -1。对于产生氧空穴的有缺陷(110)表面,NO和NH 3均可稳定吸附,NH 3在吸附过程中可直接裂解成NH 2和H。另外,SCR反应在有缺陷(110)表面的最大能垒明显较低,说明氧空穴的存在促进了SCR脱硝反应的进行。 相似文献
15.
The catalytic reduction of NO x in the typical operation temperatures and oxygen concentrations of diesel engines has been studied in the presence of V3W9Ti in a tubular flow reactor. The results have shown that the selective catalytic reduction is strongly affected by the oxygen concentration in low temperature range (150–275 °C). At higher temperatures, the reaction becomes independent of the O 2 concentration. The rate of the selective catalytic reduction of NO with ammonia may be considerably enhanced by converting part of the NO into NO 2. DRIFT measurements have shown that NH 3 and NO 2 are adsorbed on the catalyst surface on the contrary of NO. The experiments have shown that the decrease in N 2 selectivity of the SCR reaction is mainly due to the SCO of ammonia and to the formation of nitrous oxide. 相似文献
16.
对国内某1000MW燃煤发电机组失活选择性催化还原(SCR)催化剂进行CeO 2改性再生。对再生前后样品进行N 2吸附-脱附、扫描电子显微镜(SEM)、X射线荧光光谱(XRF)、傅里叶变换红外光谱(FTIR)对比表征分析。在自制固定床反应系统上对CeO 2改性再生催化剂(CeReCat)进行Hg 0氧化性能测试,同时研究了SO 2、H 2O、NO和NH 3对Hg 0氧化性能的影响。结果表明,CeO 2改性再生方法可有效清洗失活SCR催化剂表面杂质,恢复催化剂表面活性位点和孔隙结构,可使Ce、V两种活性元素得到有效负载。CeO 2改性后的样品Hg 0氧化性能显著提升,3.0 CeReCat具有最佳Hg 0的氧化效率。此外,烟气中加入600μL/L SO 2后,3.0 CeReCat仍具有高达74.4%的Hg 0氧化效率,抗SO 2性能较好。烟气中的NO可轻微促进Hg 0的氧化。由于竞争吸附作用,烟气中的H 2O和NH 3会抑制Hg 0的氧化。CeO 2改性再生催化剂置于SCR系统下层时,由于烟气NH 3浓度较低而具有较高Hg 0氧化效率,具有良好的应用前景。 相似文献
17.
简述了不同反应物组合在碳材料表面的行为特征,单组分NO可以形成吸附态的NO 2、二聚体(NO) 2、—NO 2或吡啶类的化合物;O 2存在时NO被吸附态的氧氧化成NO 2;NO、O 2和NH 3同时存在时,反应发生在吸附态的NH 3和吸附态的NO 2之间。着重详述了活性碳纤维(activated carbon fibers,ACF)催化剂上的选择性催化还原(selective catalytic reduction,SCR)NO的机理为:低温时以NH 3为还原剂的SCR(NH 3-SCR)遵循Langmuir-Hinshelwood机理,较高温度时NH 3-SCR 遵循Eley-Rideal机理;分析指出了催化剂孔结构特征和表面化学官能团是ACF能低温选择性催化还原NO的主要影响因素。 相似文献
18.
The delaminated Fe 20 3-pillared clay shows high activities for selective catalytic reduction (SCR) of NO by NH 2. 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 N 2) 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 Fe 20 3-pillared clay catalyst are higher than that of a commercial-type V 2O 5 + WO 3/TiO 2 catalyst under SO 2 + H 20 free conditions, but became lower in the presence of SO 2 +l H 20. However, when promoted by doping 1-3% Cr 20 3, the pillared clay catalyst exhibits higher SCR activities than the commercial-type catalyst in the presence of S0 2 + H 2O at all practical SCR reaction temperatures 相似文献
19.
浸渍法制备15% MnO x/5% WO 3/TiO 2低温脱硝催化剂,利用原位傅里叶变换红外(in situ FT-IR)设计包括多种吸附反应以及不同预处理方式的微观暂态试验与微观稳态试验,研究其NH 3-SCR脱硝反应机理,并推测反应路径。结果表明,催化剂的NH 3-SCR反应主要以Eley-Rideal机理方式进行,仅在一定温度条件下可以看到Langmuir-Hinshclwood反应路径。催化剂表面Lewis酸位的NH 3吸附是还原剂的主要来源,Brønsted酸位吸附的NH 4+随温度上升参与反应的比例略有提高。NH 3的吸附活化是整个反应的控制步骤,吸附态NH 3更易与NO 2发生反应,NO与催化剂表面的相互作用明显弱于NO 2。NO会在催化剂表面氧化活性中心形成大量双齿配位型硝酸盐,阻碍NH 3的吸附和活化,O 2存在条件下促进NH 3-SCR反应进行,阻止NO在催化剂表面形成双齿硝酸盐。NO与NH 3在催化剂表面存在吸附竞争,NO的吸附作用强于NH 3,温度达到100℃后吸附的NH 3方可大量活化并与NO x发生进一步反应。 相似文献
20.
以FeSO 4·7H 2O[Fe(NO 3) 3·9H 2O]为铁源,采用新型微波热解法制备γ-Fe 2O 3[a-Fe 2O 3]催化剂样品,通过XRD、N 2等温吸附-脱附、压汞法等实验手段对催化剂样品晶相、微观孔结构等进行表征;考察两种催化剂样品的NH 3-SCR脱硝性能,通过归一化处理得到两种催化剂在不同温度下的本征脱硝反应速率,同时对比研究了γ-Fe 2O 3与钒系催化剂的脱硝活性;研究氨氮比、氧浓度等运行参数对γ-Fe 2O 3催化剂NH 3-SCR脱硝性能的影响规律,并对其抗硫抗水性能进行考察.结果表明:采用新型微波热解法可得到纯度较高的γ-Fe 2O 3催化剂,其介孔分布合理且大孔数量丰富;同时γ-Fe 2O 3催化剂表现出优于a-Fe 2O 3催化剂的脱硝性能,400℃时最大NO x转化率达到96%,300、325、350℃下单位面积脱硝速率达到a-Fe 2O 3催化剂的3倍左右;γ-Fe 2O 3催化剂具备优良的抗硫抗水性能,其最佳氨氮比为1、最佳氧体积分数为3.5%. 相似文献
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