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1.
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
2.
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
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.
The catalytic properties of cobalt containing ZSM-5 zeolites prepared by various methods were compared. TPR, XRD, N 2-BET, XPS, FTIR and UV–vis spectroscopy were used for characterizing the samples. Well-dispersed cobalt oxide-like species and isolated Co 2+ ions in charge compensation positions were found in the zeolite. Catalysts prepared using a single step cation exchange method showed high activity for N 2O decomposition in a temperature range 300–550°C, in the presence of 0–5% O 2, and high stability in the presence of 10% H 2O to the feed. UV–vis spectra and TPR experiments indicated the presence of some cobalt oxides, not detected by DRX, in a Co-ZSM-5 catalyst containing 3.76 wt% Co, prepared by a solid-state reaction procedure. The N 2O conversion over this catalyst was strongly affected by addition of both O 2 and H 2O to the feed. 相似文献
5.
Direct decomposition of nitrous oxide (N 2O) on K-doped Co 3O 4 catalysts was examined. The K-doped Co 3O 4 catalyst showed a high activity even in the presence of water. In the durability test of the K-doped Co 3O 4 catalyst, the activity was maintained at least for 12 h. It was found that the activity of the K-doped Co 3O 4 catalyst strongly depended on the amount of K in the catalyst. In order to reveal the role of the K component on the catalytic activity, the catalyst was characterized by XRD, XPS, TPR and TPD. The results suggested that regeneration of the Co 2+ species from the Co 3+ species formed by oxidation of Co 2+ with the oxygen atoms formed by N 2O decomposition was promoted by the addition of K to the Co 3O 4 catalyst. 相似文献
6.
通过液相离子交换法对H-beta分子筛进行改性得到Fe/beta分子筛,并应用于催化N2O直接分解反应,考察加入HNO3后的体系pH值对Fe/beta催化性能的影响。采用N2物理吸附-脱附、XRD、IR、DR UV-Vis、NH3-TPD和ICP-OES等对Fe/beta分子筛进行分析表征。结果表明,溶液pH值降低过程中催化剂的结晶度、比表面积、孔容及Fe3+含量呈现先增加后减少。溶液pH为2.0时所制备的Fe/beta分子筛催化N2O完全分解温度明显低于pH为2.6和1.0时制备的Fe/beta催化剂,显示了较好的催化活性。 相似文献
7.
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
8.
Catalytic performance of Sn/Al 2O 3 catalysts prepared by impregnation (IM) and sol–gel (SG) method for selective catalytic reduction of NO x by propene under lean burn condition were investigated. The physical properties of catalyst were characterized by BET, XRD, XPS and TPD. The results showed that NO 2 had higher reactivity than NO to nitrogen, the maximum NO conversion was 82% on the 5% Sn/Al 2O 3 (SG) catalyst, and the maximum NO 2 conversion reached nearly 100% around 425 °C. Such a temperature of maximum NO conversion was in accordance with those of NO x desorption accompanied with O 2 around 450 °C. The activity of NO reduction was enhanced remarkably by the presence of H 2O and SO 2 at low temperature, and the temperature window was also broadened in the presence of H 2O and SO 2, however the NO x desorption and NO conversion decreased sharply on the 300 ppm SO 2 treated catalyst, the catalytic activity was inhibited by the presence of SO 2 due to formation of sulfate species (SO 42−) on the catalysts. The presence of oxygen played an essential role in NO reduction, and the activity of the 5% Sn/Al 2O 3 (SG) was not decreased in the presence of large oxygen. 相似文献
9.
SO 2, which is an air pollutant causing acid rain and smog, can be converted into elemental sulfur in direct sulfur recovery process (DSRP). SO 2 reduction was performed over catalyst in DSRP. In this study, SnO 2-ZrO 2 catalysts were prepared by a co-precipitation method, and CO and coal gas, which contains H 2, CO, CO 2 and H 2O, were used as reductants. The reactivity profile of the SO 2 reduction over the catalysts was investigated at the various reaction conditions as follows: reaction temperature of 300–550 °C, space velocity of 5000–30,000 cm 3/g -cat. h, [reductant]/[SO 2] molar ratio of 1.0–4.0 and Sn/Zr molar ratio of SnO 2-ZrO 2 catalysts 0/1, 2/8, 3/5, 5/5, 2/1, 3/1, 4/1 and 1/0. SnO 2-ZrO 2 (Sn/Zr = 2/1) catalyst showed the best performance for the SO 2 reduction in DSRP on the basis of our experimental results. The optimized reaction temperature and space velocity were 325 °C and 10,000 cm 3/g -cat. h, respectively. The optimal molar ratio of [reductant]/[SO 2] varied with the reductants, that is, 2.0 for CO and 2.5 for coal gas. SO 2 conversion of 98% and sulfur yield of 78% were achieved with the coal gas. 相似文献
10.
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
11.
Selective catalytic reduction (SCR) of NO with methane in the presence of excess oxygen has been investigated over a series of Mn-loaded sulfated zirconia (SZ) catalysts. It was found that the Mn/SZ with a metal loading of 2–3 wt.% exhibited high activity for the NO reduction, and the maximum NO conversion over the Mn/SZ catalyst was higher than that over Mn/HZSM-5. NH 3–TPD results of the catalysts showed that the sulfation process of the supports resulted in the generation of strong acid sites, which is essential for the SCR of NO with methane. On the other hand, the N 2 adsorption and the H 2–TPR of the catalysts demonstrated that the presence of the SO 42− species promoted the dispersion of the metal species and made the Mn species less reducible. Such an increased dispersion of metal species suppressed the combustion reaction of CH 4 by O 2 and increased the selectivity towards NO. The Mn/SZ catalysts prepared by different methods exhibited similar activities in the SCR of NO with methane, indicating the importance of SO 42−. The most attractive feature of the Mn/SZ catalysts was that they were more tolerant to water and SO 2 poisoning than Mn/HZSM-5 catalysts and exhibited higher reversibility after removal of SO 2. 相似文献
12.
A series of cobalt–cerium mixed oxide catalysts (Co 3O 4–CeO 2) with a Ce/Co molar ratio of 0.05 were prepared by co-precipitation (with K 2CO 3 and KOH as the respective precipitant), impregnation, citrate, and direct evaporation methods and then tested for the catalytic decomposition of N 2O. XRD, BET, XPS, O 2-TPD and H 2-TPR methods were used to characterize the catalysts. Catalysts with a trace amount of residual K exhibited higher catalytic activities than those without. The presence of appropriate amount of K in Co 3O 4–CeO 2 may improve the redox property of Co 3O 4, which is important for the decomposition of N 2O. When the amount of K was constant, the surface area became the most important factor for the reaction. The co-precipitation-prepared catalyst with K 2CO 3 as precipitant exhibited the best catalytic performance because of the presence of ca. 2 mol% residual K and the high surface area. We also discussed the rate-determining step of the N 2O decomposition reaction over these Co 3O 4–CeO 2 catalysts. 相似文献
13.
The selective catalytic reduction of NO by H 2 under strongly oxidizing conditions (H 2-SCR) in the low-temperature range of 100–200 °C has been studied over Pt supported on a series of metal oxides (e.g., La 2O 3, MgO, Y 2O 3, CaO, CeO 2, TiO 2, SiO 2 and MgO-CeO 2). The Pt/MgO and Pt/CeO 2 solids showed the best catalytic behavior with respect to N 2 yield and the widest temperature window of operation compared with the other single metal oxide-supported Pt solids. An optimum 50 wt% MgO-50wt% CeO 2 support composition and 0.3 wt% Pt loading (in the 0.1–2.0 wt% range) were found in terms of specific reaction rate of N 2 production (mols N 2/g cat s). High NO conversions (70–95%) and N 2 selectivities (80–85%) were also obtained in the 100–200 °C range at a GHSV of 80,000 h −1 with the lowest 0.1 wt% Pt loading and using a feed stream of 0.25 vol% NO, 1 vol% H 2, 5 vol% O 2 and He as balance gas. Addition of 5 vol% H 2O in the latter feed stream had a positive influence on the catalytic performance and practically no effect on the stability of the 0.1 wt% Pt/MgO-CeO 2 during 24 h on reaction stream. Moreover, the latter catalytic system exhibited a high stability in the presence of 25–40 ppm SO 2 in the feed stream following a given support pretreatment. N 2 selectivity values in the 80–85% range were obtained over the 0.1 wt% Pt/MgO-CeO 2 catalyst in the 100–200 °C range in the presence of water and SO 2 in the feed stream. The above-mentioned results led to the obtainment of patents for the commercial exploitation of Pt/MgO-CeO 2 catalyst towards a new NO x control technology in the low-temperature range of 100–200 °C using H 2 as reducing agent. Temperature-programmed desorption (TPD) of NO, and transient titration of the adsorbed surface intermediate NO x species with H 2 experiments, following reaction, have revealed important information towards the understanding of basic mechanistic issues of the present catalytic system (e.g., surface coverage, number and location of active NO x intermediate species, NO x spillover). 相似文献
14.
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
15.
以Co(NO_3)_2·6H_2O为钴源,K_2CO_3为沉淀剂,采用沉淀法制备Co_3O_4催化剂,用于催化N_2O直接分解反应。利用N_2~-物理吸附、XRD、FT-IR、TEM、TPR和ICP等对其进行表征,考察沉淀方式对Co_3O_4催化剂结构及其催化性能的影响。结果表明,沉淀方式对制备的Co_3O_4催化剂织构性质、物相组成和晶粒尺寸等影响不大,但显著影响其K残留量和还原性能,进而决定催化剂直接催化分解N_2O的催化性能。反加法制得的催化剂中K残留量为1.43%,明显高于正加法,同时催化剂中Co~(3+)较正加法更易还原,因而表现出更高的催化性能。在空速10 000 h~(-1)和N_2O体积分数0.1%的条件下,反加法制备的催化剂可在280℃催化N_2O完全分解,较正加法低20℃。 相似文献
16.
通过焙烧猪骨和鸡骨获得羟磷灰石(nHAP)载体,并采用浸渍法制备Co3O4/nHAP催化剂。采用XRD、N2物理吸附-脱附、FT-IR和H2-TPR等对催化剂进行表征,在连续流动微反装置上考察催化剂催化分解N2O的性能。结果表明,相比于鸡骨源Co3O4/nHAP催化剂,以猪骨源HAP为载体的催化剂因其较大的比表面积以及较小的Co3O4粒径尺寸,提供了更多的活性位点。特别是猪骨源Co3O4/nHAP催化剂中适量的K、Na等元素促进了Co^3+到Co^2+的还原,削弱了Co-O键,使催化剂的催化活性显著提高。 相似文献
17.
In this work, we investigated the activity and stability of Ag–alumina catalysts for the SCR of NO with methane in gas streams with a high concentration of SO 2, typical of coal-fired power plant flue gases. Ag–alumina catalysts were prepared by coprecipitation–gelation, and dilute nitric-acid solutions were used to remove weakly bound silver species from the surface of the as prepared catalysts after calcination. SO 2 has a severe inhibitory effect, essentially quenching the CH 4-SCR reaction on this type catalysts at temperatures <600 °C. SO 2 adsorbs strongly on the surface forming aluminum and silver sulfates that are not active for CH 4-SCR of NO x. Above 600 °C, however, the reaction takes place without catalyst deactivation even in the presence of 1000 ppm SO 2. The reaction light-off coincides with the onset of silver sulfate decomposition, indicating the critical role of silver in the reaction mechanism. SO 2 is reversibly adsorbed on silver above 600 °C. While alumina sites remain sulfated, this does not hinder the reaction. Sulfation of alumina only decreases the extent of adsoption of NO x, but adsorption of NO x is not the limiting step. Methane activation is the limiting step, hence the presence of sulfur-free Ag–O–Al species is a requirement for the reaction. Strong adsorption of SO 2 on Ag–alumina decreases the rates of the reaction, and increases the activation energies of both the reduction of NO to N 2 and the oxidation of CH 4, the latter more than the former. Our results indicate partial contribution of gas phase reactions to the formation of N 2 above 600 °C. H 2O does not inhibit the reaction at 625 °C, and the effect of co-addition of H 2O and SO 2 is totally reversible. 相似文献
18.
The role of La 2O 3 loading in Pd/Al 2O 3-La 2O 3 prepared by sol–gel on the catalytic properties in the NO reduction with H 2 was studied. The catalysts were characterized by N 2 physisorption, temperature-programmed reduction, differential thermal analysis, temperature-programmed oxidation and temperature-programmed desorption of NO. The physicochemical properties of Pd catalysts as well as the catalytic activity and selectivity are modified by La2O3 inclusion. The selectivity depends on the NO/H2 molar ratio (GHSV = 72,000 h−1) and the extent of interaction between Pd and La2O3. At NO/H2 = 0.5, the catalysts show high N2 selectivity (60–75%) at temperatures lower than 250 °C. For NO/H2 = 1, the N2 selectivity is almost 100% mainly for high temperatures, and even in the presence of 10% H2O vapor. The high N2 selectivity indicates a high capability of the catalysts to dissociate NO upon adsorption. This property is attributed to the creation of new adsorption sites through the formation of a surface PdOx phase interacting with La2O3. The formation of this phase is favored by the spreading of PdO promoted by La2O3. DTA shows that the phase transformation takes place at temperatures of 280–350 °C, while TPO indicates that this phase transformation is related to the oxidation process of PdO: in the case of Pd/Al2O3 the O2 uptake is consistent with the oxidation of PdO to PdO2, and when La2O3 is present the O2 uptake exceeds that amount (1.5 times). La2O3 in Pd catalysts promotes also the oxidation of Pd and dissociative adsorption of NO mainly at low temperatures (<250 °C) favoring the formation of N2. 相似文献
19.
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
20.
Various spinel-type catalysts AB 2O 4 (where A = Mg, Ca, Mn, Co, Ni, Cu, Cr, Fe, Zn and B = Cr, Fe, Co) were prepared and characterized by XRD, BET, TEM and FESEM-EDS. The performance of these catalysts towards the decomposition of N 2O to N 2 and O 2 was evaluated in a temperature programmed reaction (TPR) apparatus in the absence and the presence of oxygen. Spinel-type oxides containing Co at the B site were found to provide the best activity. The half conversion temperature of nitrous oxide over the MgCo 2O 4 catalyst was 440 °C and 470 °C in the absence and presence of oxygen, respectively (GHSV = 80,000 h −1). On the grounds of temperature programmed oxygen desorption (TPD) analyses as well as of reactive runs, the prevalent activity of the MgCo2O4 catalyst could be explained by its higher concentration of suprafacial, weakly chemisorbed oxygen species, whose related vacancies contribute actively to nitrous oxide catalytic decomposition. This indicates the way for the development of new, more active catalysts, possibly capable of delivering at low temperatures amounts of these oxygen species even higher than those characteristic of MgCo2O4. 相似文献
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