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1.
TiO2-SiO2 with various compositions prepared by the coprecipitation method and vanadia loaded on TiO2-SiO2 were investigated with respect to their physico-chemical characteristics and catalytic behavior in SCR of NO by NH3 and in the undesired oxidation of SO2 to SO3, using BET, XRD, XPS, NH3-TPD, acidity measurement by the titration method and activity test. TiO2-SiO2, compared with pure TiO2, 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% V2O5. The SCR activity and N2 selectivity are found to be more excellent over vanadia loaded on TiO2-SiO2 with 10–20 mol% of SiO2 than over that on pure TiO2, and this is considered to be associated with highly dispersed vanadia on the supports and large amounts of NH3 adsorbed on the catalysts. With increasing SiO2 content, the remarkable activity decrease in the oxidation of SO2 to SO3, favorable for industrial SCR catalysts, was also observed, strongly depending on the existence of vanadium species of the oxidation state close to V4+ on TiO2-SiO2, while V5+ exists on TiO2, according to XPS. It is concluded that vanadia loaded on Ti-rich TiO2-SiO2 with low SiO2 content is suitable as SCR catalysts for sulfur-containing exhaust gases due to showing not only the excellent de-NOx activity but also the low SO2 oxidation performance.  相似文献   

2.
The influence of ammonia and nitric oxide oxidation on the selective catalytic reduction (SCR) of NO by ammonia with copper/nickel and vanadium oxide catalysts, supported on titania or alumina have been investigated, paying special attention to N2O formation. In the SCR reaction, the VTi catalyst had a higher activity than VAl at low temperatures, while the CuNiAl catalyst had a higher activity than CuNiTi. A linear relationship between the reaction rate of ammonia oxidation and the initial reduction temperature of the catalysts obtained by H2-TPR showed that the formation rate of NH species in copper/nickel catalysts would be higher than in vanadia catalysts. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed that copper/nickel catalysts presented ammonia coordinated on Lewis acid sites, whereas ammonium ion adsorbed on Brønsted acid sites dominated on vanadia catalysts. The NO oxidation experiments revealed that copper/nickel catalysts had an increase of the NO2 and N2O concentrations with the temperature. NO could be adsorbed on copper/nickel catalysts and the NO2 intermediate species could play an important role in the reaction mechanism. It was suggested that the presence of adsorbed NO2 species could be related to the N2O formation.  相似文献   

3.
The SCR of NO and NO decomposition were investigated over a V–W–O/Ti(Sn)O2 catalyst on a Cr–Al steel monolith. The conversions of NO and NH3 over the reduced and oxidised catalysts were determined. The higher conversion of NO than of NH3 was observed in SCR over the reduced catalyst and very close conversions of both substrates were found over the oxidised one. The increase of the pre-reduction temperature was found to cause an increase in catalyst activity and its stability in direct NO decomposition. The surface tungsten cations substituted for vanadium ones in vanadia-like active species are considered to be responsible for the direct NO decomposition. The results of DFT calculations for the 10-pyramidal clusters: V10O31H12 (V–V) and V9WO31H12 (V–W) modelling (0 0 1) surfaces of vanadia and WO3–V2O5 solid solution (s.s.) active species, respectively, show that preferable conditions for NO adsorption exist on W sites of s.s. species and that reduction causes an increase in their ability for electron back donation to the adsorbed molecule. Electron back donation is believed to be responsible for the electron structure reorganisation in the adsorbed NO molecule resulting in its decomposition. The high selectivity of NO decomposition to dinitrogen was considered to be connected with the formation of the tungsten nitrosyl complexes solely via the W–N bond.  相似文献   

4.
The behavior of V=O band over V2O5 crystallite during NH3 adsorption and SCR reaction was characterized by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and the results are correlated with the reactivity in NH3 oxidation and SCR reaction. It is found that the decrease of V=O band intensity is due either to the reduction of V2O5 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 NH3 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   

5.
The selective catalytic reduction of NO+NO2 (NOx) 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/NOx and NO/NO2 inlet ratios has been studied. High NOx 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 DeNOx activity, with no detectable ammonia slip and a low N2O formation when NH3/NOx 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.  相似文献   

6.
In situ Raman spectroscopy was used for studying the ternary 2% CrO3–6% V2O5/TiO2 catalyst, for which a synergistic effect between vanadia and chromia leads to enhanced catalytic performance for the selective catalytic reduction (SCR) of NO with NH3. The structural properties of this catalyst were studied under NH3/NO/O2/N2/SO2/H2O atmospheres at temperatures up to 400 °C and major structural interactions between the surface chromia and vanadia species are observed. The effects of oxygen, ammonia, water vapor and sulfur dioxide presence on the in situ Raman spectra are presented and discussed.  相似文献   

7.
V.A. Kondratenko  M. Baerns   《Catalysis Today》2007,121(3-4):210-216
An effect of oxygen species formed from O2, N2O and NO on the selectivity of the catalytic oxidation of ammonia was studied over a polycrystalline Pt catalyst using the temporal analysis of products (TAP) reactor. The transient experiments were performed in the temperature range between 773 and 1073 K in a sequential pulse mode with a time interval of 0.2 s between the pulses of the oxidant (O2, N2O and NO) and NH3. In contrast to adsorbed oxygen species formed from NO, those from O2 and N2O reacted with ammonia yielding NO. It is suggested that the difference between these oxidising agents may be related to the different active sites for dissociation of O2, N2O and NO, where oxygen species of various Pt-O strength are formed. Weaker bound oxygen species, which are active for NO formation, originate from O2 and N2O rather than from NO. These species may be of bi-atomic nature.  相似文献   

8.
浸渍法制备15% MnOx/5% WO3/TiO2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NH3-SCR脱硝反应机理,并推测反应路径。结果表明,催化剂的NH3-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NH3吸附是还原剂的主要来源,Brønsted酸位吸附的NH4+随温度上升参与反应的比例略有提高。NH3的吸附活化是整个反应的控制步骤,吸附态NH3更易与NO2发生反应,NO与催化剂表面的相互作用明显弱于NO2。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NH3的吸附和活化,O2存在条件下促进NH3-SCR反应进行,阻止NO在催化剂表面形成双齿硝酸盐。NO与NH3在催化剂表面存在吸附竞争,NO的吸附作用强于NH3,温度达到100℃后吸附的NH3方可大量活化并与NOx发生进一步反应。  相似文献   

9.
Nitric oxide and nitric dioxide compounds (NOx) present in stack gases from nitric acid plants are usually eliminated by selective catalytic reduction (SCR) with ammonia. In this process, small quantities of nitrous oxide (N2O) are produced. This undesirable molecule has a high greenhouse gas potential and a long lifetime in the atmosphere, where it can contribute to stratospheric ozone depletion. The influence of catalyst composition and some operating variables were evaluated in terms of N2O formation, using V2O5/TiO2 catalysts. High vanadia catalyst loading, nitric oxide inlet concentration and reaction temperature increase the generation of this undesirable compound. The results suggest that adsorbed ammonia not only reacts with NO via SCR, but also with small quantities of oxygen activated by the presence of NO. The mechanism proposed for N2O generation at low temperature is based on the formation of surface V–ON species which may be produced by the partial oxidation of dissociatively adsorbed ammonia species with NO + O2 (eventually NO2). When these active sites are in close proximity they can interact to form an N2O molecule. This mechanism seems to be affected by changes in the active site density produced by increasing the catalyst vanadia loading.  相似文献   

10.
The reaction pathways of N2 and N2O formation in the direct decomposition and reduction of NO by NH3 were investigated over a polycrystalline Pt catalyst between 323 and 973 K by transient experiments using the temporal analysis of products (TAP-2) reactor. The interaction between nitric oxide and ammonia was studied in the sequential pulse mode applying 15NO. Differently labelled nitrogen and nitrous oxide molecules were detected. In both, direct NO decomposition and NH3–NO interaction, N2O formation was most marked between 573 and 673 K, whereas N2 formation dominated at higher temperatures. An unusual interruption of nitrogen formation in the 15NO pulse at 473 K was caused by an inhibiting effect of adsorbed NO species. The detailed analysis of the product distribution at this temperature clearly indicates different reaction pathways leading to the product formation. Nitrogen formation occurs via recombination of nitrogen atoms formed by dissociation of nitric oxide or/and complete dehydrogenation of ammonia. N2O is formed via recombination of adsorbed NO molecules. Additionally, both products are formed via interactions between adsorbed ammonia fragments and nitric oxide.  相似文献   

11.
G. Ramis  Li Yi  G. Busca 《Catalysis Today》1996,28(4):1528-380
The adsorption and transformation of ammonia over V2O5, V2O5/TiO2, V2O5-WO3/TiO2 and CuO/TiO2 systems has been investigated by FT-IR spectroscopy. In all cases ammonia is first coordinated over Lewis acid sites and later undergoes hydrogen abstraction giving rise either to NH2 amide species or to its dimeric form N2H4, hydrazine. Other species, tentatively identified as imide NH, nitroxyl HNO, nitrogen anions N2 and azide anions N3 are further observed over CuO/TiO2. The comparison of the infrared spectra of the species arising from both NH3 and N2H4 adsorbed over CuO/TiO2 strongly suggest that N2H4 is an intermediate in NH3 oxidation over this active selective catalytic reduction (SCR) and selective catalytic oxidation (SCO) catalysts. This implies that ammonia is activated in the form of NH2 species for both SCR and SCO, and it can later dimerize. Ammonia protonation to ammonium ion is detected over V2O5-based systems, but not over CuO/TiO2, in spite of the high SCR and SCO activity of this catalyst. Consequently Brönsted acidity is not necessary for the SCR activity.  相似文献   

12.
束航  张玉华  范红梅  张亚平  杨林军 《化工学报》2015,66(11):4460-4468
采用工业用V2O5-WO3/TiO2催化剂,基于傅里叶原位红外光谱(in situ FT-IR)技术考察了SCR脱硝过程中催化剂表面NH4HSO4的生成与分解特性。结果表明:在V2O5-WO3/TiO2催化剂表面ABS的生成可由催化剂V═O基团上Lewis酸上配位吸附活化态的NH3在O2环境中与SO2反应生成,也可由SO2与催化剂表面反应生成的吸附态金属硫酸盐中间物VOSO4与气态NH3直接反应生成;NO能通过与NH4HSO4中的NH4+直接反应来降低NH4HSO4降解的温度窗口,促进其在催化剂表面的分解行为,NO的脱除与NH4HSO4的生成是相互抑制关系;NH4HSO4本身的负载量影响其分解与挥发行为。  相似文献   

13.
The activity and excellent selectivity (>90%) of γ-Al2O3-supported Ni for the selective catalytic oxidation (SCO) of NH3 to N2 with excess O2 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. N2H4 and NO have been used to model the intermediates of the SCO mechanism (direct formation of N2 via the recombination of two NHx species) and of the in situ SCR mechanism (two-step formation of N2 via the reduction of an in situ produced NO species by a NHx species), respectively. Two IR absorption bands appear during the TPO of NH3 in the temperature range of N2 formation and have been assigned to stable bidentate nitrate surface species. This represents strong evidence that under the present conditions, formation of N2 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 NH3 molecules fed to the catalyst. The fact that NO and NH3 are able to react to give N2 at low temperature (from 100°C) confirms that activation of NH3 is the limiting step. In contrast, no evidence has been found to support the possibility of the SCO mechanism.  相似文献   

14.
The kinetics of the reaction of NO, N2O and CO2 with activated carbon without catalyst and impregnated with a precursor salt of vanadium (ammonium monovanadate) was investigated. The conversion of NO, N2O and CO2 was studied (450–900°C) using a TGA apparatus and a fixed bed reactor. The reactor effluents were analysed using a GC/MS on line. The addition of vanadium increased carbon reactivity and adsorption at lower temperatures. For NO and N2O conversion the main products obtained were N2, N2O, CO and CO2 but for CO2 conversion only CO was detected. In situ XRD was a useful tool for interpreting catalyst behaviour and identifying phases present during reaction conditions. The catalytic effect of vanadium can be explained by the occurrence of redox processes in which the catalyst is reduced to lower oxidation states such as V2O5/V6O13.  相似文献   

15.
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 V2O5/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 V2O5/CNT are the active sites for the selective catalytic reduction (SCR) of NO with NH3 at low temperatures. It was suggested that the reaction path might involve the adsorbed NH3 species reacted with NO from gaseous phase and as well as the adsorbed NO2 species. The diameter of CNTs showed positive effect on the activities of the catalysts. Under the reaction conditions of 463 K, 0.1 Mpa, NH3/NO = 1, GHSV = 35,000 h−1, and V2O5 loading of 2.35 wt%, the outer diameter of CNTs of 60–100 nm, the NO conversion was 92%.  相似文献   

16.
A series of CeO2 promoted cobalt spinel catalysts were prepared by the co-precipitation method and tested for the decomposition of nitrous oxide (N2O). Addition of CeO2 to Co3O4 led to an improvement in the catalytic activity for N2O decomposition. The catalyst was most active when the molar ratio of Ce/Co was around 0.05. Complete N2O conversion could be attained over the CoCe0.05 catalyst below 400 °C even in the presence of O2, H2O or NO. Methods of XRD, FE-SEM, BET, XPS, H2-TPR and O2-TPD were used to characterize these catalysts. The analytical results indicated that the addition of CeO2 could increase the surface area of Co3O4, and then improve the reduction of Co3+ to Co2+ by facilitating the desorption of adsorbed oxygen species, which is the rate-determining step of the N2O decomposition over cobalt spinel catalyst. We conclude that these effects, caused by the addition of CeO2, are responsible for the enhancement of catalytic activity of Co3O4.  相似文献   

17.
The adsorption of HCN on, its catalytic oxidation with 6% O2 over 0.5% Pt/Al2O3, and the subsequent oxidation of strongly bound chemisorbed species upon heating were investigated. The observed N-containing products were N2O, NO and NO2, and some residual adsorbed N-containing species were oxidized to NO and NO2 during subsequent temperature programmed oxidation. Because N-atom balance could not be obtained after accounting for the quantities of each of these product species, we propose that N2 and was formed. Both the HCN conversion and the selectivity towards different N-containing products depend strongly on the reaction temperature and the composition of the reactant gas mixture. In particular, total HCN conversion reaches 95% above 250 °C. Furthermore, the temperature of maximum HCN conversion to N2O is located between 200 and 250 °C, while raising the reaction temperature increases the proportion of NOx in the products. The co-feeding of H2O and C3H6 had little, if any effect on the total HCN conversion, but C3H6 addition did increase the conversion to NO and decrease the conversion to NO2, perhaps due to the competing presence of adsorbed fragments of reductive C3H6. Evidence is also presented that introduction of NO and NO2 into the reactant gas mixture resulted in additional reaction pathways between these NOx species and HCN that provide for lean-NOx reduction coincident with HCN oxidation.  相似文献   

18.
V2O5 supported on sulfated TiO2 catalyst was investigated by using Raman and infrared spectroscopies to examine the surface structure of vanadia and the hydroxyl groups of titania along with the sulfate species on the catalyst surface. The surface structure of vanadia plays a critical role, particularly for the reduction of NO by NH3. The polymeric vanadate species on the catalyst surface is the active reaction site for this reaction system. The surface sulfate species enhanced the formation of the polymeric vanadate by reducing the available surface area of the catalyst. The formation of the polymeric vanadate species on the catalyst surface also depends on the number of hydroxyl groups on the support. Both the sulfate and the vanadate species strongly interacted with the hydroxyl groups on titania. The fewer the number of the hydroxyl sites on the catalyst surface became by increasing the calcination temperatures, the more the polymeric vanadate species formed. A model was proposed to elucidate the progressive alteration of the surface structure of vanadia by the amounts of V2O5 loadings and the sulfate species on the catalyst surface.  相似文献   

19.
The catalytic reduction of NOx 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 O2 concentration. The rate of the selective catalytic reduction of NO with ammonia may be considerably enhanced by converting part of the NO into NO2. DRIFT measurements have shown that NH3 and NO2 are adsorbed on the catalyst surface on the contrary of NO. The experiments have shown that the decrease in N2 selectivity of the SCR reaction is mainly due to the SCO of ammonia and to the formation of nitrous oxide.  相似文献   

20.
Vanadia-silica aerogels, containing 10 to 30 wt% V2O5, and a xerogel were prepared from vanadium(V) oxide triisopropoxide and vanadium (III) acetylacetonate (V(III)acac) precursors using the solution-sol-gel method and different drying processes, including conventional evaporative and high-temperature and low-temperature supercritical drying. The behavior of these mixed oxides in the selective catalytic reduction of NO by NH3 was tested and compared to that of other vanadia-silica and vanadia-titania catalysts. The structural and catalytic properties of the sol-gel derived vanadia-silica mixed oxides were found to be mainly influenced by the drying method, the vanadia content and the vanadia precursor used. For a particular vanadia content (10 wt%), low-temperature supercritical drying and evaporative drying resulted in significantly higher vanadia dispersion than high-temperature supercritical drying, which led to crystalline V2O5. Turnover frequencies for SCR at temperatures T < 475K were highest for low-temperature aerogels containing well-dispersed vanadium oxide species. Exposing these catalysts to higher temperatures under SCR conditions resulted in agglomeration/redispersion phenomena and at temperatures T > 550K best catalytic behavior was observed with vanadia-silica mixed oxides for which Raman spectroscopy indicated the presence of crystalline V2O5, as was the case for aerogels obtained by high-temperature supercritical drying and the low-temperature aerogel with the highest vanadia content (30 wt%).  相似文献   

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