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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.
Cu-ZSM-5 and Cu-AlTS-1 catalysts were prepared by solid state ion exchange and studied in DeNO x reactions. A NO 3 type surface complex was found to be an active intermediate in the decomposition of NO and N 2O. Copper was oxidized to Cu 2+ in the decomposition reactions. Oscillations at full N 2O conversion were observed in the gas phase O 2 concentration, without any change in the N 2 concentration. The oscillation was synchronized by gas phase NO formed from the NO 3 complex. The same complex seems to be an active intermediate also in NO selective catalytic reduction (SCR) by methane, whereas carbonaceous deposits play a role in NO SCR by propane. TPD reveals that only 10–20% of the total copper in the zeolites participates in the catalytic cycles. 相似文献
3.
The catalytic behavior in N 2O reduction by propane in the presence of O 2, H 2O and SO 2 of Fe/ZSM-5 catalysts prepared by ion exchange and chemical vapour deposition (CVD) is reported. The catalyst prepared by CVD shows a lower dependence of the rate of selective N 2O reduction on the decrease in C 3H 8 to N 2O ratio in the feed and a higher resistance to deactivation by SO 2 in accelerated durability tests with high SO 2 concentration (500 ppm). This catalyst shows stable catalytic behavior in the presence of SO 2 for more than 600 h of time-on-stream. Characterization of the catalysts by UV–VIS–NIR diffuse reflectance indicates that the poor performances of the sample prepared by ion exchange could be related to the presence of highly clustered Fe 3+ species, in this catalyst. On the other hand, Fe 2O 3 particles are not present in the sample prepared by CVD while mainly isolated Fe 3+ ions and iron-oxide nanoclusters are present. 相似文献
4.
The catalytic decomposition of acrylonitrile (AN) over Cu-ZSM-5 prepared with various Cu loadings was investigated. AN conversion, during which the nitrogen atoms in AN were mainly converted to N 2, increased as Cu loading increased. N 2 selectivities as high as 90–95% were attained. X-ray diffraction measurements (XRD) and temperature-programmed reduction by H 2 (H 2-TPR) showed the existence of bulk CuO in Cu-ZSM-5 with a Cu loading of 6.4 wt% and the existence of highly dispersed CuO in Cu-ZSM-5 with a Cu loading of 3.3 wt%. Electron spin resonance measurements revealed that Cu-ZSM-5 contains three forms of isolated Cu 2+ ions (square-planar, square-pyramidal, and distorted square-pyramidal). The H 2-TPR results suggested that in Cu-ZSM-5 with a Cu loading of 2.9 wt% and below, Cu + existed even after oxidizing pretreatment. The activity of AN decomposition over Cu/SiO 2 suggested that CuO could form N 2, but, independent of the CuO dispersion, nitrogen oxides (NO x) were formed above 350 °C. Cu + and the square-pyramidal and distorted square-pyramidal forms of Cu 2+ showed low activity for AN decomposition. Temperature-programmed desorption of NH 3 suggested that N 2 formation from NH 3 proceeded on Cu 2+, resulting in the formation of Cu +. The Cu + ions were oxidized to Cu 2+ at around 300 °C. Thus, high N 2 selectivity over Cu-ZSM-5 with a wide range of temperature was probably attained by the reaction over the square-planar Cu 2+, which can be reversibly reduced and oxidized. 相似文献
5.
A series of CeO 2 promoted cobalt spinel catalysts were prepared by the co-precipitation method and tested for the decomposition of nitrous oxide (N 2O). Addition of CeO 2 to Co 3O 4 led to an improvement in the catalytic activity for N 2O decomposition. The catalyst was most active when the molar ratio of Ce/Co was around 0.05. Complete N 2O conversion could be attained over the CoCe0.05 catalyst below 400 °C even in the presence of O 2, H 2O or NO. Methods of XRD, FE-SEM, BET, XPS, H 2-TPR and O 2-TPD were used to characterize these catalysts. The analytical results indicated that the addition of CeO 2 could increase the surface area of Co 3O 4, and then improve the reduction of Co 3+ to Co 2+ by facilitating the desorption of adsorbed oxygen species, which is the rate-determining step of the N 2O decomposition over cobalt spinel catalyst. We conclude that these effects, caused by the addition of CeO 2, are responsible for the enhancement of catalytic activity of Co 3O 4. 相似文献
6.
In this study, we examine the interaction of N 2O with TiO 2(1 1 0) in an effort to better understand the conversion of NO x species to N 2 over TiO 2-based catalysts. The TiO 2(1 1 0) surface was chosen as a model system because this material is commonly used as a support and because oxygen vacancies on this surface are perhaps the best available models for the role of electronic defects in catalysis. Annealing TiO 2(1 1 0) in vacuum at high temperature (above about 800 K) generates oxygen vacancy sites that are associated with reduced surface cations (Ti 3+ sites) and that are easily quantified using temperature programmed desorption (TPD) of water. Using TPD, X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS), we found that the majority of N 2O molecules adsorbed at 90 K on TiO 2(1 1 0) are weakly held and desorb from the surface at 130 K. However, a small fraction of the N 2O molecules exposed to TiO 2(1 1 0) at 90 K decompose to N 2 via one of two channels, both of which are vacancy-mediated. One channel occurs at 90 K, and results in N 2 ejection from the surface and vacancy oxidation. We propose that this channel involves N 2O molecules bound at vacancies with the O-end of the molecule in the vacancy. The second channel results from an adsorbed state of N 2O that decomposes at 170 K to liberate N 2 in the gas phase and deposit oxygen adatoms at non-defect Ti 4+ sites. The presence of these O adatoms is clearly evident in subsequent water TPD measurements. We propose that this channel involves N 2O molecules that are bound at vacancies with the N-end of the molecule in the vacancy, which permits the O-end of the molecule to interact with an adjacent Ti 4+ site. The partitioning between these two channels is roughly 1:1 for adsorption at 90 K, but neither is observed to occur for moderate N 2O exposures at temperatures above 200 K. EELS data indicate that vacancies readily transfer charge to N 2O at 90 K, and this charge transfer facilitates N 2O decomposition. Based on these results, it appears that the decomposition of N 2O to N 2 requires trapping of the molecule at vacancies and that the lifetime of the N 2O–vacancy interaction may be key to the conversion of N 2O to N 2. 相似文献
7.
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
8.
Using a local density functional calculation, we investigate the adsorption energies, geometries and electronic structures of single Cu 2+ and Fe 2+ ions on a Si(100) surface. The adsorption energy results reveal that both ions are stably adsorbed in a hollow site; from this site, Cu 2+ is located at 0.51 Å and Fe 2+ is located at 0.41 Å from the surface, respectively. The adsorption energy of Cu 2+ is about 1.5 times larger than that of Fe 2+. This means that Cu 2+ is strongly adsorbed on the silicon surface. From the analysis of spin density, we find that Cu 2+strongly attracts the electrons of the silicon surface, because spin density is delocalized in the silicon backbone. As a result, the 3d orbital of Cu 2+ becomes more similar to the closed-shell state compared to that of Fe 2+. This means that the core electronic state of Cu 2+ is closer to the neutral atom, which is qualitatively confirmed by XPS measurements. 相似文献
9.
The zeolites with MEL structure were synthesized via the hydrothermal method and the zeolites-supported catalysts, such as Cu 2+, Ga 3+, Co 3+, Ce 2+ and VO 2+/zeolites, were prepared by the incipient wetness impregnation. The structures of the synthesized zeolites were characterized by techniques of XRD, FT-IR, SIMS, 29Si and 27Al MAS NMR. The selective catalytic reduction (SCR) of NO by ammonia was carried out with a glass reactor under a downstream flow. The synthesized TS-2 showed no significant DeNO x activity, instead of catalyzing the ammonia oxidation at a high temperature. Furthermore, the catalytic activity of TS2 zeolite can be effectively modified and tuned up through incorporating second metal ion such as Fe 3+, Co 3+, and Al 3+ into the framework (i.e., [Fe,Ti]Z11, [Co,Ti]Z11, and [Al,Ti]Z11). Among the synthesized bimetallosilicates, the [Fe,Ti]Z11 zeolite is the most active catalyst for the SCR DeNO x with ammonia; the NO conversion and the N 2 yield reach around 80%. In addition, impregnating the metal ions on TS2 or bimetallosilicates is also a very effective way to improve the SCR DeNO x activity. Ga 3+/[Fe40,Ti40]Z11 and Co 3+/[Fe40,Ti40]Z11 are the most active catalysts and show a potential for the practical applications. 相似文献
10.
The decomposition of N 2O, and the catalytic reduction by NH 3 of N 2O and N 2O + NO, have been studied on Fe-BEA, -ZSM-5 and -FER catalysts. These catalysts were prepared by classical ion exchange and characterized by TPR after various activation treatments. Fe-FER is the most active material in the catalytic decomposition because “oxo-species” reducible at low temperature, appearing upon interaction of Fe II-zeolite with N 2O (-oxygen), are formed in largest amounts with this material. The decomposition of N 2O is promoted by addition of NH 3, and even more with NH 3 + NO in the case of Fe-FER and -BEA. It is proposed that the NO-promoted reduction of N 2O originated from the fast surface reaction between -oxygen O * and NO * to yield NO 2*, which in turn reacts immediately with NH 3. 相似文献
11.
The reaction pathways of N 2 and N 2O formation in the direct decomposition and reduction of NO by NH 3 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 NH 3–NO interaction, N 2O formation was most marked between 573 and 673 K, whereas N 2 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. N 2O 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
13.
采用实验方法研究了低成本环境友好型添加剂抗坏血酸(AA)对Fe 2+/H 2O 2体系氧化NO气体及其对体系内H 2O 2分解的影响,分析了AA对体系氧化NO能力及H 2O 2分解的影响机制。研究结果表明:AA通过加速Fe 3+向Fe 2+的转化而促进Fe 2+/H 2O 2体系对NO的氧化。[AA] 0:[Fe 2+] 0对体系氧化NO的能力及H 2O 2的分解具有重要影响。综合考虑NO氧化脱除量及H 2O 2消耗量,合理的[AA] 0:[Fe 2+] 0为1/3~1/2。AA的分次添加方式可大幅度提升体系氧化NO气体的能力。研究结果可望为发展基于H 2O 2为氧化剂的烟气NO绿色氧化技术提供理论基础。 相似文献
14.
Conversion of CH 4, C 2H 6, C 3H 8, benzene and their binary mixtures over H-NaZSM-5 catalyst in the presence of N 2O was studied. It was found that under experimental conditions methane alkylates benzene to give toluene and xylenes. Acidity of the catalyst had no effect on the reactivity of active oxygen formed from N 2O towards methane and benzene, but affected their secondary transformation. Acidic samples favored the reaction of aromatic ring methylation with methane whereas deep oxidation of CH 4 prevailed on NaHZSM-5. Based on the relative reactivities and 13C label distribution in the products of 13CH 4+C 6H 6+N 2O feed conversion, the scheme of hydrocarbon transformation was proposed. 相似文献
15.
Fe/ZSM-5 catalysts with high Fe loading (Fe/Al1) have been prepared by sublimation of FeCl 3 onto H-ZSM-5 samples of different Si/Al ratios. They catalyze NO x reduction with hydrocarbons in an excess of O 2 and H 2O. TPR shows that the Fe in the zeolite cavities is different from Fe 2O 3 particles. Naked Fe 3+ ions are absent; oxo-ions, which are equally well reducible by CO and H 2, prevail. A minority of the Fe complexes lose oxygen upon mere heating to 500°C; some of the reduced sites are reoxidized only by N 2O. The population of oxo-complexes that lose oxygen by heating depends on the Si/Al ratio, this dependence is in qualitative agreement with the model of (2+) charged binuclear ions [HO–Fe–O–Fe–OH] 2+. Upon reacting with NO, the bridging O atom is transferred and NO 2 is formed. This step is not rate limiting for active catalysts with high Al/Si ratio and high Fe loading, but it becomes critical with zeolites of low Al/Si ratio. 相似文献
16.
采用共沉淀法和沉淀浸渍法制备了纳米氧化铈-二氧化硅(CeO 2-SiO 2)介孔材料吸附剂,主要考察了其对水中铜离子(Cu 2+)的吸附行为。通过X射线衍射(XRD)、扫描电镜(SEM)和氮吸附(BET)等手段对合成的介孔材料进行了性能表征,并通过静态吸附实验分析了溶液pH、溶液初始金属离子质量浓度、吸附剂用量、吸附时间等条件对介孔材料吸附Cu 2+性能的影响。结果表明:共沉淀法制备的纳米CeO 2-SiO 2介孔材料对Cu 2+的去除效果较沉淀浸渍法要好;当溶液pH=7.0时CeO 2-SiO 2介孔材料对Cu 2+的吸附效果最好,20 min时基本达到吸附平衡;溶液初始Cu 2+浓度增大Cu 2+去除率降低,Cu 2+累计吸附量增大;随着吸附剂用量增加Cu 2+去除率增大,当CeO 2-SiO 2吸附剂用量为0.15 g/L时对Cu 2+的去除率趋于稳定;CeO 2-SiO 2吸附剂对不同金属离子吸附性能由大到小的顺序为Cu 2+、Fe 2+、Mn 2+,该吸附过程均符合准二级动力学模型。 相似文献
17.
TM/TiO 2/SiO 2 photocatalysts were prepared by the photodeposition method using transition metal salts (TM=Fe 3+, Co 2+, Ni 2+ and Cu 2+) as precursors and the surface bond-conjugated TiO 2/SiO 2 as supporter in N 2 atmosphere, and were characterized by XRD, XPS, UV-Vis diffuse reflection and zeta-potential. Their photocatalytic activities were evaluated using reactive brilliant red K-2G (K-2G) and cationic blue X-GRL (CBX) showing different adsorption behavior on the oxides. Fe, Cu supported TiO 2/SiO 2 can efficiently extend the light absorption to the visible region. XPS analysis verified that the introduction of transition metal lead to the changes of the electronic environmental of Ti cations and the zeta-potential of oxides. As a result, K-2G has higher adsorption on the modified TiO 2/SiO 2 than that on the baked one, while the adsorption of CBX has a little change on the both oxides. At the same time, for the photodegradation of K-2G, Fe 3+, Co 2+, Ni 2+-modified catalysts show that their photoactivities are 3.3–2.2 times higher than the bare one. On the contrast, all transition-metal-supported catalysts have no significant activity improvement except that Fe/TiO 2/SiO 2 shows 1.68 times higher activity for the photodegradation of CBX. The results indicate that the photoactivity could be increased in photodegradation of dyes by changing the performances of adsorption to dyes and absorption to light of photocatalyst. 相似文献
18.
A multi-component NO x-trap catalyst consisting of Pt and K supported on γ-Al 2O 3 was studied at 250 °C to determine the roles of the individual catalyst components, to identify the adsorbing species during the lean capture cycle, and to assess the effects of H 2O and CO 2 on NO x storage. The Al 2O 3 support was shown to have NO x trapping capability with and without Pt present (at 250 °C Pt/Al 2O 3 adsorbs 2.3 μmols NO x/m 2). NO x is primarily trapped on Al 2O 3 in the form of nitrates with monodentate, chelating and bridged forms apparent in Diffuse Reflectance mid-Infrared Fourier Transform Spectroscopy (DRIFTS) analysis. The addition of K to the catalyst increases the adsorption capacity to 6.2 μmols NO x/m 2, and the primary storage form on K is a free nitrate ion. Quantitative DRIFTS analysis shows that 12% of the nitrates on a Pt/K/Al 2O 3 catalyst are coordinated on the Al 2O 3 support at saturation. When 5% CO2 was included in a feed stream with 300 ppm NO and 12% O2, the amount of K-based nitrate storage decreased by 45% after 1 h on stream due to the competition of adsorbed free nitrates with carboxylates for adsorption sites. When 5% H2O was included in a feed stream with 300 ppm NO and 12% O2, the amount of K-based nitrate storage decreased by only 16% after 1 h, but the Al2O3-based nitrates decreased by 92%. Interestingly, with both 5% CO2 and 5% H2O in the feed, the total storage only decreased by 11%, as the hydroxyl groups generated on Al2O3 destabilized the K–CO2 bond; specifically, H2O mitigates the NOx storage capacity losses associated with carboxylate competition. 相似文献
19.
Cu 2+ ion-exchanged pillared clays are substantially more active than Cu 2+-ZSM-5 for selective catalytic reduction (SCR) of NO by hydrocarbons. More importantly, H 2O (or SO 2) has only mild effects on their activities. First results on Cu 2+-exchanged TiO 2-pillared montmorillonite were reported by this laboratory (Yang and Li, Ref. [1]), that showed overall activities two to four times higher than Cu 2+-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. 相似文献
20.
The catalytic reduction of N 2O by CH 4, CO, and their mixtures has been comparatively investigated over steam-activated FeZSM-5 zeolite. The influence of the molar feed ratio between N 2O and the reducing agents, the gas-hourly space velocity, and the presence of O 2 on the catalytic performance were studied in the temperature range of 475–850 K. The CH 4 is more efficient than CO for N 2O reduction, achieving the same degree of conversion at significantly lower temperatures. The apparent activation energy for N 2O reduction by CH 4 was very similar to that of direct N 2O decomposition (140 kJ mol −1), being much lower for the N 2O reduction by CO (60 kJ mol −1). This suggests that the reactions have a markedly different mechanism. Addition of CO using equimolar mixtures in the ternary N 2O + CH 4 + CO system did not affect the N 2O conversion with respect to the binary N 2O + CH 4 system, indicating that CO does not interfere in the low-temperature reduction of N 2O by CH 4. In the ternary system, CO contributed to N 2O reduction when methane was the limiting reactant. The conversion and selectivity of the reactions of N 2O with CH 4, CO, and their mixtures were not altered upon adding excess O 2 in the feed. 相似文献
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