共查询到20条相似文献,搜索用时 31 毫秒
1.
The effect of oxygen concentration on the pulse and steady-state selective catalytic reduction (SCR) of NO with C 3H 6 over CuO/γ-Al 2O 3 has been studied by infrared spectroscopy (IR) coupled with mass spectroscopy studies. IR studies revealed that the pulse SCR occurred via (i) the oxidation of Cu 0/Cu + to Cu 2+ by NO and O 2, (ii) the co-adsorption of NO/NO 2/O 2 to produce Cu 2+(NO 3−) 2, and (iii) the reaction of Cu 2+(NO 3−) 2 with C 3H 6 to produce N 2, CO 2, and H 2O. Increasing the O 2/NO ratio from 25.0 to 83.4 promotes the formation of NO 2 from gas phase oxidation of NO, resulting in a reactant mixture of NO/NO 2/O 2. This reactant mixture allows the formation of Cu 2+(NO 3−) 2 and its reaction with the C 3H 6 to occur at a higher rate with a higher selectivity toward N 2 than the low O 2/NO flow. Both the high and low O 2/NO steady-state SCR reactions follow the same pathway, proceeding via adsorbed C 3H 7---NO 2, C 3H 7---ONO, CH 3COO −, Cu 0---CN, and Cu +---NCO intermediates toward N 2, CO 2, and H 2O products. High O 2 concentration in the high O 2/NO SCR accelerates both the formation and destruction of adsorbates, resulting in their intensities similar to the low O 2/NO SCR at 523–698 K. High O 2 concentration in the reactant mixture resulted in a higher rate of destruction of the intermediates than low O 2 concentration at temperatures above 723 K. 相似文献
2.
Pulse reaction method and in situ IR spectroscopy were used to characterize the active oxygen species for oxidative coupling of methane (OCM) over SrF 2/Nd 2O 3 catalyst. It was found that OCM activity of the catalyst was very low in the absence of gas phase oxygen, which indicated that lattice oxygen species contributed little to the yield of C 2 hydrocarbons. IR band of superoxide species (O 2−) was detected on the O 2-preadsorbed SrF 2/Nd 2O 3. The substitution of 18O 2 isotope for 16O 2 caused the IR band of O 2− at 1128 cm −1 to shift to lower wavenumbers (1094 and 1062 cm −1), consistent with the assignment of the spectra to the O 2− species. A good correlation between the rate of disappearance of surface O 2− and the rate of formation of gas phase C 2H 4 was observed upon interaction of CH 4 with O 2-preadsorbed catalyst at 700 °C. The O 2− species was also observed on the catalyst under working condition. These results suggest that O 2− species is the active oxygen species for OCM reaction on SrF 2/Nd 2O 3 catalyst. 相似文献
3.
The adsorption and coadsorption of selective catalytic reduction (SCR) reactants and reaction products on CuZSM-5-37 containing 11 wt.-% CuO have been studied by FTIR spectroscopy. The catalyst surface is characterized by both weak acidity and weak basicity as revealed by testing with probe molecules (CO 2, NH 3, H 2O). NO 2 adsorption results in formation of different kinds of nitrates. The same species are formed when NO is coadsorbed with oxygen at 180°C. NO adsorption at ambient temperature also leads to formation of nitrates as well as of Cu 2+NO species. In the presence of oxygen the latter are converted according to the scheme: NO → N 2O 3 → N 2O 4 → NO 2 → NO 3. It is concluded that the surface nitrates are important intermediates in the SCR process. They are thermally stable and resistant towards interaction with CO 2, N 2, O 2, and are only slightly affected by H 2O and NO. However, they posses a high oxidation ability and are fully reduced by propane at 180°C. It is concluded that one of the most important roles of oxygen in SCR by hydrocarbons is to convert NO x into highly active surface nitrates. 相似文献
4.
The influence of NO on the adsorption and desorption of NO 2 on BaO/TiO 2 has been studied under lean conditions. The adsorption of NO 2 involves the disproportionation of NO 2 into an adsorbed nitrate species and NO released to the gas phase with a 3:1 ratio, Three different nitrate species form on the catalyst: surface nitrates on the TiO 2 support, surface nitrates on BaO, and bulk barium nitrate. The stability of the three species in different gas feeds was investigated by temperature-programmed desorption (TPD). The reverse reaction of the NO2 disproportionation has also been observed. If NO is added to the feed, nitrates previously formed on the sorbent will decompose into NO2. Therefore, the above chemical equation should be considered as an equilibrium reaction. Applying this finding to the NOx storage and reduction catalyst means that NO probably reacts with the previously formed nitrates yielding NO2 as an intermediate product. This NO2 is subsequently reduced by the reducing agents (hydrocarbons and CO) present during the regeneration period. 相似文献
5.
NH 3 stored on zeolites in the form of NH 4+ ions easily reacts with NO to N 2 in the presence of O 2 at temperatures <373 K under dry conditions. Wet conditions require a modification of the catalyst system. It is shown that MnO 2 deposited on the external surface of zeolite Y by precipitation considerably enhances the NO x conversion by zeolite fixed NH 4+ ions in the presence of water at 400–430 K. Particle-size analysis, temperature-programmed reduction, textural characterization, chemical analysis, ESR and XRD gave a subtle picture of the MnO 2 phase structure. The MnO 2 is a non-stoichiometric, amorphous phase that contains minor amounts of Mn 2+ ions. It loses O 2 upon inert heating up to 873 K, but does not crystallize or sinter. The phase is reducible by H 2 in two stages via intermediate formation of Mn 3O 4. The manufacture of extrudates preserving stored NH 4+ ions for NO x reduction is described. It was found that MnO 2 can oxidize NO by bulk oxygen. This enables the reduction of NO to N 2 by the zeolitic NH 4+ ions without gas-phase oxygen for limited time periods. The composite catalyst retains storage capacity for both, oxygen and NH 4+ ions despite the presence of moisture and allows short-term reduction of NO without gaseous O 2 or additional reductants. The catalyst is likewise suitable for steady-state DeNO x operation at higher space velocities if gaseous NH 3 is permanently supplied. 相似文献
6.
The interactions between Pd/TiO 2 catalyst and the reactants and potential reaction intermediates present during aqueous nitrate reduction, including NO 3−, NO 2− and NO in the presence of H 2 and H 2O were studied by infrared spectroscopy. Adsorbed forms of NO, nitrite and nitrate could all be detected in the presence of water. In the presence of water/H 2, nitrate was the most stable surface species followed by nitrite and then highly reactive NO, suggesting that the reduction of nitrate to nitrite is the rate-limiting step. High concentrations of adsorbed nitrite appear to be linked to the detection of gaseous N 2O while the formation of ammonia is related to reactions on the Pd surface and the extent of formation is linked to high levels of adsorbed NO in addition to the surface hydrogen availability and the presence of water. 相似文献
7.
We have obtained mass spectra of negative ions produced by rays in artificial air at atmospheric pressure (N 2: 80%, O 2: 20%, H 2O: 20–1500 ppm, CO 2: 0.2–300 ppm, NO, NO 2 0.02 ppm). We observed two main categories: hydrates built on simple ions (O 2−, O 3−, OH −, CO 3−, CO 4−, HCO 3−, NO 2−, NO 3−), hydrates built on complex ions (NO x−, HNO γ, HCO 3−HNO x, x = 2,3; Y = 2, 3). For high values of hygrometry, CO 2 content and ageing time (5 msec) we observe the disappearance of O 2−, O 3−, OH − hydrates whereas the major part of the spectrum consists of complex ions. 相似文献
8.
The reaction mechanism of the reduction of NO by propene over Pd-based catalysts was studied by FTIR spectroscopy. It was observed that the reaction between NO and propene most probably goes via isocyanate (2256–2230 cm −1), nitrate (1310–1250 cm −1) and acetate (1560 and 1460 cm −1) intermediates formation. Other possible intermediates such as partially oxidized hydrocarbons, NO 2, and formates were also detected. The reaction between nitrates and acetates or carbonates reduced nitrates to N 2 and oxidized carbon compounds to CO 2. In situ DRIFT provides quick and rather easily elucidated data from adsorbed compounds and reaction intermediates on the catalyst surface. The activity experiments were carried out to find out the possible reaction mechanism and furthermore the kinetic equation for NO reduction by propene. 相似文献
9.
The production and decay of singlet molecular oxygen ( 1O 2) in TiO 2 photocatalysis were investigated by monitoring its phosphorescence under various reaction conditions. First, the effects of additives such as KBr, KSCN, KI, H 2O 2, and ethanol on the amount of 1O 2 produced by photo excitation of P25 TiO 2 were measured. The same additives were employed to investigate the effect on the amount of O 2− produced. Comparison between the effects on 1O 2 and O 2− suggested that 1O 2 is formed by the electron transfer mechanism, the reduction of molecular oxygens to O 2− by photogenerated electrons and the subsequent oxidation of O 2− to 1O 2 by photogenerated holes. The formation of 1O 2 decreased at pH < 5 and pH > 11, indicating that the intermediate O 2− is stabilized at the terminal OH site of the TiO 2 surface in the pH range of 5 < pH < 11. Eighteen commercially available TiO 2 photocatalysts were compared on the formation of 1O 2 and O 2− in an aqueous suspension system. The formation of 1O 2 was increased with decreasing size of TiO 2 particles, indicating that a large specific surface area causes a higher possibility of reduction producing O 2− and then a large amount of 1O 2 is formed. The difference in the crystal phase (rutile and anatase) did not affect the formation of 1O 2. 相似文献
10.
In the present work, the initial stages of the photocatalytic oxidation of toluene over two commercial TiO 2 powders were investigated at the molecular level by ESR spectroscopy. UV-irradiation of the TiO 2 samples, which present different phase composition and surface area, gave rise to several oxygenated radicals like O −, O 2− and O 3−, as well as Ti 3+ centers. The proportion of these species generated depends on the gaseous environment (vacuum or oxygen), and the structural and morphological characteristic of the TiO 2 samples. In contrast, co-adsorption of toluene and oxygen on any of the TiO 2 samples studied yields upon UV illumination slightly different ESR signals, which have been assigned to the formation of benzylperoxy radicals, Ph-CH 2OO, adsorbed on the semiconductor. Such species are only detected when TiO 2 is exposed to toluene–oxygen mixtures enriched in the organic, and under these conditions neither of the other oxygenated radicals is formed. The reasons for this behavior are discussed on the basis of the mechanism for the photocatalytic oxidation of toluene. 相似文献
11.
On an anodic alumina supported silver catalyst with a low Ag loading (1.68 wt.%), NO x (NO/He, NO/O 2/He, NO 2/He) adsorption measurements and NO x-temperature programmed decomposition (TPD)/temperature programmed surface-reaction (TPSR) measurements in different gas streams (He, C 3H 6/He, C 3H 6/O 2/He) were conducted to investigate the formation, consumption and reactivity of surface adsorbed NO x species. During NO adsorption, no noticeable uptake of NO was detected. Introducing oxygen greatly improved the formation of ads-NOx species. A greater quantity of surface nitrate species was found after NO2 adsorption, accompanied with gaseous NO release. The result of TPSR demonstrates the surface nitrate species can be effectively and preferentially reduced by propene. When introducing oxygen into the propene gas stream of TPSR test, the significantly increased amount of reacted nitrate undoubtedly shows the importance of oxygen in activating propene. The pathway for the selective reduction of NOx in the presence of excess oxygen is proposed to pass through the selective reduction of the adsorbed nitrate species with the activated propene. The enhanced NOx conversion when replacing NO with NO2 was attributed to the stronger NOx adsorption capacity and oxidation ability of NO2, than those for NO. With increasing oxygen concentration, the difference between NO and NO2 would gradually decrease, and finally disappear in a high excess of oxygen. 相似文献
12.
采用溶胶-凝胶法制备了一系列TiO 2、TiO 2-Al 2O 3(TiAl)、MnO 2/TiO 2(MnTi)和MnO 2/TiO 2-Al 2O 3(MnTiAl)样品,在固定床实验装置上研究了MnTi和MnTiAl催化剂的脱硝、脱汞性能,并对相应的样品进行了BET、XRD、H 2-TPR、XPS表征分析。表征结果表明,Al 2O 3掺入TiO 2后能极大提高载体的比表面积,提升催化剂氧化还原性能,且有利于高价态锰离子(Mn 3+和Mn 4+)和化学吸附氧(O*)在催化剂表面富集。固定床实验结果表明,在反应温度范围内,MnTiAl催化剂脱硝、脱汞性能均优于MnTi催化剂,MnTiAl催化剂在200℃时脱硝、脱汞效率分别高达88.5%和96.1%。MnTiAl脱除烟气Hg 0过程中,将Hg 0氧化为Hg 2+的同时,催化剂表面Mn 3+、Mn 4+和O*浓度均被消耗,同时烟气中的O 2能将催化剂表面较低价态的锰离子(Mn 2+和Mn 3+)重新氧化为高价态锰离子(Mn 3+和Mn 4+),并且能补充催化剂表面的化学吸附氧(O*),进而实现催化剂催化氧化Hg 0过程。 相似文献
13.
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
14.
This paper presents an investigation on the NO oxidation properties of perovskite oxides. La 1−xCe xCoO 3 ( x = 0, 0.05, 0.1, 0.2, 0.3, 0.4) perovskite-type oxides were synthesized through a citrate method and characterized by XRD, BET and XPS. The catalytic activities were enhanced significantly with Ce substitution, and achieved the best when x was 0.2, but decreased at higher x values. The performed characterizations reveal that the adsorbed oxygen on the surface plays an important role in the oxidation of NO into NO 2. The surface compounds after the co-adsorption of NO and O 2 at room temperature, were investigated by DRIFTS and TPD experiments. Three species: the bridging nitrate, the hyponitrites and the monodentate nitrate, were formed on the surface. The order of thermal stabilities was as follows: monodentate > hyponitrite > bridging. Among them, only the monodentate nitrate which decomposed at above 300 °C, would desorb NO 2 into the gas phase. When Ce was added, the temperature of monodentate nitrate desorption became low and the adsorption of the other two species decreased. This might be related to the oxidation state of Co on the surface. Analysis by synthesizing the characterization results and catalytic activity data shows that large amounts of adsorbed oxygen, small amount of inactive compounds on the surface and low NO 2 desorption temperature are favorable for the oxidation of NO. 相似文献
15.
A combined spectroscopic and catalytic study of the NO reactivity on microporous aluminophosphates, with chabasite-related structure, CoAPO-34, CuAPO-34 and CuAPSO-34, is reported. NO and CO adsorption were monitored by FTIR spectroscopy, and revealed that Co 2+/Co 3+ and Cu +/Cu 2+ redox couples, the sites responsible for the catalytic activity, are present in these catalysts. CoAPO-34 catalysts showed exceptionally high performances in the oxidation of NO to NO 2, and poor activity in other DeNO x reactions. Copper-based aluminophosphates and silico-aluminophosphates, besides good performances in the NO oxidation to NO 2, showed good activity in the N 2O decomposition even in the presence of oxygen or water in the feed. The presence of silicon has beneficial effects both on the thermal and hydrothermal stability of the zeolitic structure, as well as on the catalytic performances of the metal-aluminophosphates. 相似文献
16.
The activity of several catalysts are studied in the soot combustion reaction using air and NO/air as oxidising agents. Over Al 2O 3-supported catalysts NO (g) is a promoter for the combustion reaction with the extent of promotion depending on the Na loading. Over these catalysts SO 42− poisons this promotion by preventing NO oxidation through a site blocking mechanism. SiO 2 is unable to adsorb NO or catalyse its oxidation and over SiO 2-supported Na catalysts NO (g) inhibits the combustion reaction. This is ascribed to a competition between NO and O 2. Over Fe-ZSM-5 catalysts the presence of a NO x trapping component does not increase the combustion of soot in the presence of NO (g) and it is proposed that this previously reported effect is only seen under continuous NO x trap operation as NO 2 is periodically released during regeneration and thus available for soot combustion. Experiments during which the [NO] (g) is varied show that CO, rather than an adsorbed carbonyl-like intermediate, is formed upon reaction between NO 2 (the proposed oxygen carrier) and soot. 相似文献
17.
Nanometer perovskite-type oxides La 1−xSr xMO 3−δ (M = Co, Mn; x = 0, 0.4) have been prepared using the citric acid complexing-hydrothermal-coupled method and characterized by means of techniques, such as X-ray diffraction (XRD), BET, high-resolution scanning electron microscopy (HRSEM), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and temperature-programmed reduction (TPR). The catalytic performance of these nanoperovskites in the combustion of ethylacetate (EA) has also been evaluated. The XRD results indicate that all the samples possessed single-phase rhombohedral crystal structures. The surface areas of these nanomaterials ranged from 20 to 33 m 2 g −1, the achievement of such high surface areas are due to the uniform morphology with the typical particle size of 40–80 nm (as can be clearly seen in their HRSEM images) that were derived with the citric acid complexing-hydrothermally coupled strategy. The XPS results demonstrate the presence of Mn 4+ and Mn 3+ in La 1−xSr xMnO 3−δ and Co 3+ and Co 2+ in La 1−xSr xCoO 3−δ, Sr substitution induced the rises in Mn 4+ and Co 3+ concentrations; adsorbed oxygen species (O −, O 2−, or O 22−) were detected on the catalyst surfaces. The O 2-TPD profiles indicate that Sr doping increased desorption of the adsorbed oxygen and lattice oxygen species at low temperatures. The H 2-TPR results reveal that the nanoperovskite catalysts could be reduced at much lower temperatures (<240 °C) after Sr doping. It is observed that under the conditions of EA concentration = 1000 ppm, EA/oxygen molar ratio = 1/400, and space velocity = 20,000 h −1, the catalytic activity (as reflected by the temperature ( T100%) for EA complete conversion) increased in the order of LaCoO 2.91 ( T100% = 230 °C) ≈ LaMnO 3.12 ( T100% = 235 °C) < La 0.6Sr 0.4MnO 3.02 ( T100% = 190 °C) < La 0.6Sr 0.4CoO 2.78 ( T100% = 175 °C); furthermore, there were no formation of partially oxidized by-products over these catalysts. Based on the above results, we conclude that the excellent catalytic performance is associated with the high surface areas, good redox properties (derived from higher Mn 4+/Mn 3+ and Co 3+/Co 2+ ratios), and rich lattice defects of the nanostructured La 1−xSr xMO 3−δ materials. 相似文献
18.
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
19.
Photocatalysis of a hollandite compound K xGa xSn 8−xO 16 ( x = ca. 1.8) was examined for the reduction of nitrate ion with a reducing agent of methanol in water under UV irradiation. Hollandites have a characteristic one-dimensional tunnel structure. The hollandite powder, which was prepared by the sol–gel method and unloaded with any additives like metals, was used as the photocatalyst and its photocatalytic reaction was analyzed quantitatively by using ion chromatography and on-line mass spectrometry, and its reaction mechanism was analyzed by in-situ FT-IR. The hollandite photocatalyst showed a significant activity for the formation of N 2 from NO 3−. The nitrate was reduced to N 2 and NO 2−, while the reducing agent methanol was partly oxidized to change to formic acid. The conversion of NO 3−was proportional to the yields of N 2, NO 2−, and HCOO −. The present photocatalyzed decomposition of NO 3− to N 2 would be a useful photocatalysis for the environmental protection of water. 相似文献
20.
The NO-H 2-O 2 reaction was studied over supported bimetallic catalysts, Pt-Mo and Pt-W, which were prepared by coexchange of hydrotalcite-like Mg-Al double layered hydroxides by Pt(NO 2) 42−, MoO 42−, and/or WO 42− and subsequent heating at 600 °C in H 2. The Pt–Mo interaction could obviously be seen when the catalyst after reduction treatment was exposed to a mixture of NO and H 2 in the absence of O 2. The Pt-HT catalyst showed the almost complete NO conversion at 70 °C, whereas the Pt-Mo-HT showed a negligible conversion. Upon exposure to O 2, however, Pt-Mo-HT exhibited the NO conversion at the lowest temperature of ≥30 °C, compared to ≥60 °C required for Pt-HT. EXAFS/XANES, XPS and IR results suggested that the role of Mo is very sensitive to the oxidation state, i.e., oxidized Mo species residing in Pt particles are postulated to retard the oxidative adsorption of NO as NO 3 and promote the catalytic conversion of NO to N 2O at low temperatures. 相似文献
|