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1.
A La–Sr–Cu–O–S system with K 2NiF 4 perovskite-type structure has been studied as a novel SO x-resistant combustion catalyst. The XRD result implied that sulfur is incorporated into the structure as non-sulfate-type cations. An introduction of sulfur with highly positive valence (S 6+ or S 4+) into the lattice requires the charge compensation by decreasing the oxidation number of Cu. This is accompanied by the creation of more reducible Cu species, which would achieve the light-off of catalytic C 3H 6 oxidation at lower temperatures. More important feature of sulfur-containing compounds is that the catalytic C 3H 6 oxidation was significantly accelerated by addition of SO 2 to the gas feed. The catalytic performance for the oxidation of C 3H 6 and CO and the reduction of NO was finally evaluated in a simulated automotive exhaust in the presence of SO 2. 相似文献
2.
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
3.
The catalytic oxidation of sulfur dioxide to sulfur trioxide over several binary (M xO y/TiO 2) and ternary (V 2O 5/M XO Y/TiO 2) supported metal oxide catalysts was systematically investigated. The supported metal oxide components were essentially 100% dispersed as surface metal oxide species, as confirmed by Raman spectroscopy characterization. The sulfur dioxide oxidation turnover frequencies of the binary catalysts were all within an order of magnitude (V 2O 5/TiO 2>Fe 2O 3/TiO 2>Re 2O 7/TiO 2 CrO 3/TiO 2 Nb 2O 5/TiO 2>MoO 3/TiO 2 WO 3/TiO 2). An exception was the K 2O/TiO 2 catalysts, which is essentially inactive for sulfur dioxide oxidation. With the exception of K 2O, all of the surface metal oxide species present in the ternary catalysts (i.e., oxides of V, Fe, Re, Cr, Nb, Mo and W) can undergo redox cycles and oxidize SO 2 to SO 3. The turnover frequency for sulfur dioxide oxidation over all of these catalysts is approximately the same at both low and high surface coverages. This indicates that the mechanism of sulfur dioxide oxidation is not sensitive to the coordination of the surface metal oxide species. A comparison of the activities of the ternary catalysts with the corresponding binary catalysts suggests that the surface vanadium oxide and the additive surface metal oxide redox sites act independently without synergistic interactions. The V 2O 5/K 2O/TiO 2 catalyst showed a dramatic reduction in the catalytic activity in comparison to the unpromoted V 2O 5/TiO 2 catalyst. The ability of K 2O to significantly retard the redox potential of the surface vanadia species is primarily responsible for the lower catalytic activity of the ternary catalytic system. The fundamental insights generated from this research can potentially assist in the molecular design of the air pollution control catalysts: (1) the development of catalysts for low temperature oxidation of SO 2 to SO 3 during sulfuric acid manufacture (2) the design of efficient SCR DeNO x catalysts with minimal SO 2 oxidation activity and (3) improvements in additives for the simultaneous oxidation/sorption of sulfur oxides in petroleum refinery operations. 相似文献
4.
The activity of NO x storage-reduction (NSR) catalysts is greatly reduced by sulfur poisoning, caused by the SO 2 present in the exhaust stream. Desorption of sulfur species from poisoned NSR catalysts occurs at temperatures in excess of 600 °C using reducing atmospheres and conventional heating. In this work, microwave (MW) heating has been used to promote desulfurization of poisoned NSR catalysts. The experiments were carried out by heating the catalyst with MW radiation and using hydrogen as the reducing gas. Desorption of H 2S at 200 °C was observed. Desorption at even lower temperatures (150 °C) was observed when water was introduced to the system. In the presence of water, sulfur species desorbed as both H 2S and SO 2. An overall reduction of sulfur species of about 60% was obtained. The use of MW heating proves to be an efficient way to achieve regeneration of poisoned NSR catalysts. 相似文献
5.
Pd-only three-way catalysts prepared by the sol–gel method were investigated by the three-way catalytic performance test with a simulated exhaust gas in a continuous U-tube quartz reactor at a gas hourly space velocity of 72 000 h −1. The catalysts were characterized with XRD, XPS, BET surface area and pore volume. The activity and thermal stability of the Pd–Al 2O 3 catalyst prepared at pH 10 were superior to those at pH 4 during hydrolysis and condensation, which could be explained by the anchoring effect. Zr and V were found to be good promoters for the enhancement of the thermal stability and SO 2 resistance, respectively. Optimally formulated catalyst, Pd(1)–V(2)–Zr(10)–Al 2O 3, was thermally stable up to 900 oC and showed a much more improved low-temperature activity and excellent SO 2 resistance. 相似文献
6.
To improve the deep sulfation of alumina support, the inertness material activated carbon was used as an alternative support for copper/cerium catalysts to remove SO 2 from incineration flue gas which contained other air pollutants such as NO X, CO, CO 2, HCl, carbon particulates, and heavy metal vapor. During the 473–820 K, the AC support showed no retention of SO 2. However, the metal Pb composed in the flue gas exhibited the toxic characterization to M/AC catalysts, which was due to the outer orbitals of d subshell all paired. 相似文献
7.
Cerium–zirconium solid solutions were prepared and characterized to determine the effects of SO 2/redox exposure on the microstructure of the crystallites. The mixed oxides were prepared via co-precipitation of cerium(IV) ammonium hydroxide and zirconium oxynitrate using ammonium hydroxide. The oxides were characterized prior to and after SO 2 exposure to discern the effects of temperature and SO 2 on crystallite properties. The samples were treated at 673 and 1073 K with a flow of 100 ppm SO 2/balance N 2 with a concomitant redox pulse of 5% H 2/balance N 2 and 5% O 2/balance N 2 on 10 s intervals. The cubic crystalline structure (CaF 2) was observed and maintained for compositions ranging from 100 to 25 at.% cerium without indication of a separate tetragonal phase. While the cubic structure was maintained, the addition of zirconium in the cubic lattice reduced the cubic lattice parameter ( a0) and the crystallite grain size. Under identical redox conditions and temperatures, exposure to SO 2 resulted in smaller grain sizes, as calculated by X-ray diffraction (XRD) and confirmed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). In addition, higher exposure temperatures resulted in larger crystallite grain sizes without altering the cubic lattice parameter. Auger electron spectroscopy (AES) confirmed the absence of surface sulfur species, indicating sulfur dioxide alters the microstructure of the crystallites under redox conditions without the formation of cerium–zirconium chemisorbed sulfur species. 相似文献
8.
MnO x and Sm–Mn catalysts were prepared with the coprecipitation method, and they showed excellent activities and sulfur resistances for the selective catalytic reduction of NO x by NH 3 between 50 and 300 °C in the presence of excess oxygen. 0.10Sm–Mn catalyst indicated better catalytic activity and sulfur resistance. Additionally, the Sm doping led to multi-aspect impacts on the phases, morphology structures, gas adsorption, reactions process, and specific surface areas. Therefore, it significantly enhances the NO conversion, N 2 selectivity, and sulfur resistance. Based on various experimental characterization results, the reaction mechanism of catalysts and the effect of SO 2 on the reaction process about the catalysts were extensively explored. For 0.10Sm–Mn catalyst, manganese sulfate and sulfur ammonium cannot be generated broadly under the influence of SO 2 and the amount of surface adsorbed oxygen. The Bronsted acid sites strengthen significantly due to the addition of SO 2, enhancing the sulfur resistance of the 0.10Sm–Mn catalyst. 相似文献
9.
The storage of SO 2 in manganese based materials was investigated in flow reactor experiments. Manganese oxide precipitated with ammonia and hydrogen peroxide stored about 76 wt.% of SO 2 at a high diffusion rate into the bulk. Doping with potassium increases the SO 2 storage rate substantially at 200 °C, but has an only minor effect at 400 °C. Kinetic studies showed that the storage of SO 2 in pure and potassium doped manganese oxide is controlled by the kinetics of the sulfate formation reaction on the catalyst surface up to complete sulfation, whereas the storage on manganese cerium mixed oxide is limited by internal diffusion of the formed sulfate. The sulfate formation reaction was found to be first order with respect to both SO 2 and manganese oxide. For the potassium doped catalyst sulfur was found to be bound on manganese sites being transferred to potassium afterwards. 相似文献
10.
NH_3选择性催化还原NOx技术的关键是催化剂,其中,理解催化剂的硫中毒机制是获取抗硫中毒催化剂的有效方法。借助原位拉曼和红外表征考察氧化态、还原态CeO_2催化剂在二氧化硫、氧气处理气氛中体相和表面硫酸盐以及氧缺陷的变化。NH_3-SCR催化反应结果表明,硫酸化还原态CeO_2具有较优的脱硝性能,这主要来源于还原态CeO_2上的缺陷,有利于氧化SO_2,产生表面硫酸盐,增强催化剂酸性,同时抑制体相硫酸盐的生成,促进NH_3-SCR反应的进行。 相似文献
11.
Manganese–cerium mixed oxide catalysts with different molar ratio Mn/(Mn + Ce) (0, 0.25, 0.50, 0.75, 1) were prepared by citric acid method and investigated concerning their adsorption behavior, redox properties and behavior in the selective catalytic reduction of NO x by NH 3. The studies based on pulse thermal analysis combined with mass spectroscopy and FT-IR spectroscopy uncovered a clear correlation between the dependence of these properties and the mixed oxide composition. Highest activity to nitrogen formation was found for catalysts with a molar ratio Mn/(Mn + Ce) of 0.25, whereas the activity was much lower for the pure constituent oxides. Measurements of adsorption uptake of reactants, NO x (NO, NO 2) and NH 3, and reducibility showed similar dependence on the mixed oxide composition indicating a clear correlation of these properties with catalytic activity. The adsorption studies indicated that NO x and NH 3 are adsorbed on separate sites. Consecutive adsorption measurements of the reactants showed similar uptakes as separate measurements indicating that there was no interference between adsorbed reactants. Mechanistic investigations by changing the sequence of admittance of reactants (NO x, NH 3) indicated that at 100–150 °C nitrogen formation follows an Eley–Rideal type mechanism, where adsorbed ammonia reacts with NO x in the gas phase, whereas adsorbed NO x showed no significant reactivity under conditions used. 相似文献
12.
The presence of sulfur in automotive exhaust is known to be detrimental to lean-NO x traps as SO 2 is oxidized to SO 3 that competes with NO 2 for sites on the trap and is difficult to remove. In this study the effect of adding Cu to the prototypical Pt–BaO/γ-Al 2O 3 formulation on the system's tolerance for sulfur was investigated. It was found that in the absence of sulfur, Cu decreases the performance in terms of both NO x storage capacity and reduction of NO x to N 2 during regeneration. In the presence of SO 2, Cu provides a significant improvement in sulfur tolerance so that, after sulfur exposure, the storage capacity of the Cu-modified material can exceed that of the baseline material. The sulfur tolerance afforded by Cu is attributed to a moderation in the activity for SO 2 oxidation resulting from the formation of a Pt–Cu bimetallic phase. The propensity for NO oxidation is also modified, but to a lesser effect. Evidence for the bimetallic phase is provided by temperature-programmed reduction (TPR) and electron microscopy. The impact of SO 2 on the Cu-modified material is greater during the regenerative reduction cycle. In this case, the results suggest that sulfur blocks Pt and possibly Cu sites and that the sulfur is not removed by oxidation during the subsequent storage cycle. Hence, activity lost during the reduction cycle is not restored. In contrast, sulfur that blocks Pt sites on the baseline material during the reduction cycle is subsequently oxidized and desorbs from the Pt, restoring the activity. However, some of the resulting SO 3 reacts with the BaO to form BaSO 4, and there is a partial loss of storage capacity. 相似文献
13.
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
14.
In order to improve the poor regeneration properties of the ZnO–Al 2O 3 (ZA)sorbent with a high sulfur removing capacity and fast H 2S absorption rate, 5–10 wt.% of various additives such as iron (Fe 2O 3), cobalt (Co 3O 4), nickel (NiO) and cerium oxide (CeO 2) were added to the ZA sorbent. These sorbents were prepared by the co-precipitation method and their sulfur removing capacities and regeneration properties were measured in a fixed-bed reactor during multiple cycles at middle-temperature ranges between 480 and 580 °C. The sulfur removing capacities of these sorbents measured 0.17–0.20 g S/g sorbent, which corresponded to 80% of the theoretical value and the values were maintained to within 10 cycles. The poor regeneration property of the ZA sorbent, which needed a long time (600 min), was also improved by addition of these promoters. These sorbents were regenerated completely within 300 min. The additives such as Fe, Co and Ni formed their aluminates, which did not change into sulfide form during sulfidation. The additives/aluminates played an important role in transforming S in ZnS into SO 2. Cerium dioxide showed a similar role with such aluminates in the oxidation of sulfur. The catalytic roles of the promoters and the changes in the physical properties of sorbents, during multiple cyclic tests, are discussed. 相似文献
15.
Reactions between graphitic carbon and a reduced C–O–H fluid were investigated using a mixture of stearic acid C 18H 36O 2 and oxalic acid dihydrate C 2H 6O 6, as the fluid source at high pressure and temperature (HP–HT) of 7.7 GPa and 1500°C in a platinum sealed capsule. A reduced C–O–H fluid mainly composed of methane and water, was formed by the thermal decomposition of the fluid source before reaching the HP–HT condition. An exchange reaction between carbon and methane occurred and starting carbon was re-crystallized to flaky graphite crystals, but no diamond was formed for the duration up to 24 h. In the experiment for 48 h, octahedral diamond crystals of a few to a few tens of micrometers in size were observed along with the recrystallized graphite. These results show that reduced C–O–H fluid acts as a diamond forming catalyst, although a long incubation time is necessary for diamond formation. 相似文献
16.
The interaction of sulfur dioxide with a commercial NO x storage-reduction catalyst (NSR) has been investigated using in situ IR and X-ray absorption spectroscopy. Two pathways of catalyst deactivation by SO 2 were identified. Under lean conditions (exposure to SO 2 and O 2) at 350 °C the storage component forms barium sulfates, which transform from surface to hardly reducible bulk sulfate species. The irreversible blocking of the Ba sites led to a decrease in NO x storage capacity. Under fuel rich conditions (SO 2/C 3H 6) at 350–500 °C evidence for the formation of sulfides on the oxidation/reduction component (Pt) of the catalyst was found, which blocks the metal surface and thus hinders the further reduction of the sulfides. 相似文献
17.
SCR-deNO x reaction and SO 2–SO 3 oxidation tests were carried out by different research groups over fresh and used EUROCAT oxide samples in order to characterize the reactivity of the catalysts and to compare data obtained in several laboratories (Politecnico of Milan, Università of Salerno, ENEL of Milan, Boreskov Insitute of Catalysis). Data are presented which indicate that the used EUROCAT catalyst is slightly more active both in the deNOx reaction and SO2–SO3 oxidation than the fresh sample. An analyses of data collected over honeycomb catalysts by means of a 2D, single-channel model of the SCR monolith reactor has been performed to evaluate the intrinsic kinetic constant of the deNOx reaction; a satisfactory comparison has been obtained between estimation of the intrinsic kinetic constant and estimation of the intrinsic catalyst activity from data collected over powdered catalysts. A good agreement has been found in the experimental results collected in the different labs, both for the deNOx reaction and SO2–SO3 oxidation. 相似文献
18.
采用(NH 4) 2S溶液吸收净化高浓度SO 2烟气,得到(NH 4) 2S 2O 3和NH 4HSO 3的混合溶液并转移至高压反应釜中,控制反应条件,两种物质发生自氧化还原反应,生成硫磺和(NH 4) 2SO 4.实验考察了吸收SO 2过程和自氧化还原过程的影响条件,结果表明:在pH=3~7,SO 2气体流速300 ml·min -1,(NH 4) 2S浓度为0.2~1.2 mol·L -1,常温条件下,烟气中二氧化硫的吸收率达到99.8%以上,且无H 2S生成;在pH=2.5~3.0,温度为130℃条件下,反应进行1 h,硫磺收率达到95%以上,溶液经过蒸发结晶得到(NH 4) 2SO 4.用X射线衍射(XRD)和X射线荧光光谱(XRF)对硫磺和硫酸铵进行表征分析,结果表明:硫磺的纯度为99.14%,硫酸铵中氮元素含量为23.6%. 相似文献
19.
通过分步浸渍法制备了一系列以碱土金属或过渡金属为助剂,低钒含量的V-M x(SO 4) y-W/Ti及V-M xO y-W/Ti催化剂,对负载相同金属离子硫酸盐及氧化物的催化剂性质进行了对比考察。通过选择性催化还原(SCR)反应对催化剂进行活性评价,结果显示,负载某金属硫酸盐比相应氧化物助剂的催化剂活性好。负载钴离子的催化剂具有良好的低温活性,进而对负载钴离子的催化剂进行了抗硫抗水性能测试,并借助XRD、NH 3-TPD、H 2-TPR、TG、BET对负载不同金属硫酸盐及氧化物助剂的催化剂进行了表征,揭示了活性好的催化剂(如负载CoSO 4的催化剂)通常具有较多的表面酸及较好的低温还原能力。 相似文献
20.
The promotive effects of cerium oxide on commercial three-way catalysts (TWCs) for purification of motor exhaust gases have been widely investigated in recent years. This work shows the cooperative effects of CeO 2–Pd on the kinetics of CO oxidation over Pd/CeO 2–ZrO 2. Under reducing-to-moderately oxidizing conditions, a zero-order O 2 pressure dependence is found which can be interpreted on the basis of a mechanism involving a reaction between CO adsorbed on Pd and surface oxygen from the support. The high oxygen-exchange capability of the CeO 2–ZrO 2 support, as determined from temperature-programmed reduction/oxygen uptake measurements is suggested as being responsible for such a catalytic behavior. 相似文献
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