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1.
The adsorption of ammonia on three different monolithic SCR catalysts was investigated experimentally. Two of the examined monoliths were coated cordierite honeycombs, containing either V 2O 5–WO 3/TiO 2 or V 2O 5/TiO 2 as active components. The third sample was a commercially available extruded monolithic honeycomb consisting of V 2O 5–WO 3/TiO 2, and was tested for comparison. Different transient response methods and a modified temperature programmed desorption (TPD) method were used to investigate the ammonia adsorption in the presence of water (5%) and oxygen (10%). Under typical SCR conditions (stoichiometric ratio NH 3:NO<1), the ammonia coverage of the WO 3-free catalyst sample was significantly lower. Ammonia is more strongly adsorbed on the WO 3-containing catalyst. The extruded catalyst adsorbed substantially larger amounts of ammonia especially at low temperatures, leading to a slow response under transient conditions. 相似文献
2.
The unsteady-state kinetics of the selective catalytic reduction (SCR) of NO with NH 3 is studied over V 2O 5–WO 3/TiO 2 model catalysts by means of the transient response method. NH 3 strongly adsorbs onto the catalyst surface whereas NO does not adsorb appreciably. A dynamic mathematical model based on a Temkin-type desorption process for NH 3 and a SCR reaction rate with a complex dependence on the ammonia surface coverage is well suited to represent the data. 相似文献
3.
As part of a fundamental and applied work on the development of an unsteady mathematical model of the NH 3-selective catalytic reduction (SCR) process for design and control of integrated after-treatment systems of heavy-duty engines, we present herein a transient kinetic analysis of the standard SCR NO + NH 3 system which provides new insight in the catalytic kinetics and mechanism prevailing at low temperatures. Based on kinetic runs performed over a commercial powdered V 2O 5–WO 3–TiO 2 catalyst in the 175–450 °C T-range feeding NH 3 and NO (1000 ppm) in the presence of H 2O (1–10%, v/v) and O 2 (2–6%, v/v), an original dual-site modified redox rate law is derived which effectively accounts for NH 3 inhibition effects observed during transient reactive experiments at T < 250 °C. We also demonstrate that implementation of the novel modified redox kinetics into a fully predictive 1D + 1D model of SCR monolith reactors can significantly improve simulations of SCR transient runs at different scales, including engine test bench experiments over full-scale SCR honeycomb catalysts. 相似文献
4.
In the off-gases of internal combustion engines running with oxygen excess, non-thermal plasmas (NTPs) have an oxidative potential, which results in an effective conversion of NO to NO 2. In combination with appropriate catalysts and ammonia (NH 3-SCR) or hydrocarbons (HC-SCR) as a reducing agent, this can be utilized to reduce nitric oxides (NO and NO 2) synergistically to molecular nitrogen. The combination of SCR and cold plasma enhanced the overall reaction rate and allowed an effective removal of NOX at low temperatures. Using NH3 as a reducing agent, NOX was converted to N2 on zeolites or NH3-SCR catalysts like V2O5–WO3/TiO2 at temperatures as low as 100–200 °C. Significant synergetic effects of plasma and catalyst treatment were observed both for NH3 stored by ion exchange on the zeolite and for continuous NH3 supply. Certain modifications of Al2O3 and ZrO2 have been found to be effective as catalysts in the plasma-assisted HC-SCR in oxygen excess. With an energy supply of about 30 eV/NO-molecule, 500 ppm NO was reduced by more than half at a temperature of 300 °C and a space velocity of 20 000 h−1 at the catalyst. The synergistic combinations of NTP and both NH3- and HC-SCR have been verified under real diesel engine exhaust conditions. 相似文献
5.
将偏钒酸铵和羟丙基甲基纤维素搅拌混溶制备滴胶液,以自制的7孔圆柱蜂窝钛硅复合氧化物为载体,采用滴胶涂覆法制备了V 2O 5/(TiO 2-SiO 2)脱硝催化剂。通过X射线衍射(XRD)、程序升温还原(H 2-TPR)、比表面积分析(BET)和催化剂评价等方法对制备的脱硝催化剂进行分析表征。考察了五氧化二钒在钛硅复合氧化物载体上的分布、五氧化二钒涂覆量等对催化剂性能的影响,并就反应温度、反应空速、氨与一氧化氮物质的量比等条件对脱硝催化剂活性的影响进行了研究。实验结果表明,使用五氧化二钒涂覆量为1.2%(质量分数)的V 2O 5/(TiO 2-SiO 2)脱硝催化剂,在反应温度为320 ℃、空速为10 000 h -1、氨与一氧化氮物质的量比为1.2条件下进行脱硝活性实验,一氧化氮转化率达到93.0%。经与工业脱硝催化剂对比实验表明,制备的V 2O 5/(TiO 2-SiO 2)脱硝催化剂性能优良,并且降低了钒的使用量,节约了生产成本,减少了环境污染。 相似文献
6.
The behavior of fresh and aged EUROCAT V 2O 5–WO 3/TiO 2 SCR catalysts in the (I) oxidative dehydrogenation of light alkanes, and conversion of (ii) propan-2-ol, (iii) NO in the presence of propene and oxygen, (iv) propane and propene and (v) chloropropane is reported to analyse possible modifications of the catalyst properties during use and give a more comprehensive general picture of its surface and reactivity properties. 相似文献
7.
Texture, structure and composition of the fresh and the used EUROCAT oxide V 2O 5–WO 3/TiO 2 SCR catalysts have been investigated with aid of nitrogen adsorption/desorption at 77 K, mercury porosimetry, X-ray diffraction and chemical analyses. The BET specific surface area is around 47 and 46 m 2/g for the fresh and the used catalyst, respectively. Both samples are essentially mesoporous, with pore diameters centred in the range 10–20 nm according to the data derived from the nitrogen adsorption and around 40 nm according to the mercury porosimetry. The anatase form of TiO 2 and traces of orthorhombic V 2O 5 are the only crystalline phases identified in both materials. According to the chemical analysis there are no significant differences in the contents of the main constituting elements between the fresh and the used catalyst. Selective dissolution with NH 3 aq. reveals that in the used catalyst the amount of vanadium component not susceptible to the treatment increases. 相似文献
8.
The development of a catalytically active filter element for combined particle separation and NO x removal or VOC total oxidation, respectively, is presented. For NO x removal by selective catalytic reduction (SCR) a catalytic coating based on a TiO 2–V 2O 5–WO 3 catalyst system was developed on a ceramic filter element. Different TiO 2 sols of tailor-made mean particle size between 40 and 190 nm were prepared by the sol–gel process and used for the impregnation of filter element cylinders by the incipient wetness technique. The obtained TiO 2-impregnated sintered filter element cylinders exhibit BET surface areas in the range between 0.5 and 1.3 m 2/g. Selected TiO 2-impregnated filter element cylinders of high BET surface area were catalytically activated by impregnation with a V 2O 5 and WO 3 precursor solution. The obtained catalytic filter element cylinders show high SCR activity leading to 96% NO conversion at 300 °C, a filtration velocity of 2 cm/s and an NO inlet concentration of 500 vol.-ppm. The corresponding differential pressures fulfill the requirements for typical hot gas filtration applications. For VOC total oxidation, a TiO 2-impregnated filter element support was catalytically activated with a Pt/V 2O 5 system. Complete oxidation of propene with 100% selectivity to CO 2 was achieved at 300 °C, a filtration velocity of 2 cm/s and a propene inlet concentration of 300 vol.-ppm. 相似文献
9.
A characterization study on a practice-oriented V 2O 5/WO 3–TiO 2 SCR catalyst deactivated by Ca and K, respectively, was carried out using NH 3-TPD, DRIFT spectroscopy, and XPS as well as theoretical DFT calculations. It was found from NH 3-TPD experiments that strongly basic elements like K or Ca drastically affect the acidity of the catalysts. Detailed DRIFT spectroscopy experiments revealed that these poisoning agents mostly interact with the Brønsted acid sites of the V 2O 5 active phase, thus affecting the NH 3 adsorption. Moreover, these experiments also indicated that the V 5+ = O sites are much less reactive on the poisoned catalysts. XPS investigations of the O 1s binding energies showed that the oxygen atoms of the V 5+ = O sites are affected by the presence of the poisoning agents. Based on these results and on DFT calculations with model clusters of the vanadia surface, the poisoning mechanism is explained by the stabilization of the non atomic holes of the (0 1 0) V 2O 5 phase as a result of the deactivation element. Consequently, V–OH Brønsted acid sites and V 5+ = O sites are inhibited, which are both of crucial importance in the SCR process. The deactivation model also gives an explanation to the very low concentrations of potassium needed to deactivate the SCR catalyst, since one metal atom sitting on such a non-atomic hole site deactivates up to four active vanadium centers. 相似文献
10.
The SCR of NO and NO decomposition were investigated over a V–W–O/Ti(Sn)O 2 catalyst on a Cr–Al steel monolith. The conversions of NO and NH 3 over the reduced and oxidised catalysts were determined. The higher conversion of NO than of NH 3 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: V 10O 31H 12 (V–V) and V 9WO 31H 12 (V–W) modelling (0 0 1) surfaces of vanadia and WO 3–V 2O 5 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
11.
XPS study V 2O 5–WO 3/TiO 2 mixed oxide catalysts for Selective Catalytic Reduction (SCR) of NO x carried out by researchers from ten different laboratories shows good reproducibility of the chemical shift results. The binding energies of the corresponding core level spectra allow us to identify the chemical states of main elements as Ti(IV), V(V) and W(VI). No other oxidation states for these elements were observed both for fresh/used and for crushed/monolith samples. Discrepancy in quantitative data can be proposed to arise from the heterogeneity of their composition as a function of depth. This suggestion is confirmed by SIMS data and ion etching experiments which indicate surface location of V 2O 5 phase, as well as impurity ones, with respect to TiO 2 and WO 3 and their redistribution as result of catalyst operation. 相似文献
12.
采用共沉淀法、溶胶-凝胶法和浸渍法制备了V2O5质量分数4%和WO3质量分数6%的V2O5-WO3/TiO2选择性催化还原催化剂,对比了3种方法制备的催化剂选择性催化还原NO性能。采用X射线衍射、热重、N2吸附-脱附和程序升温还原等对制备的V2O5-WO3/Ti O2催化剂的结构和性质进行表征,结果表明,共沉淀法制备的V2O5-WO3/TiO2催化剂具有更好的选择性催化还原NO活性和更大的比表面积,影响共沉淀法选择性催化还原NO活性的主要因素是催化剂的热稳定性、表面羟基数量、比表面积、粒径分布以及V、W与Ti之间的内在作用。 相似文献
13.
Two different commercial SCR catalysts belonging to the V 2O 5–WO 3–TiO 2 system, and different alternative catalysts based on Mn, Fe, Cr, Al and Ti oxides have been tested in the conversion of VOCs in excess oxygen in a temperature range typical of the SCR process (500–700 K). Propane, propene, isopropanol, acetone, 2-chloropropane and 1,2-dichlorobenzene have been fed with excess oxygen and helium. The industrial catalysts are poorly active in the conversion of propane, giving mainly rise to propene by oxy-dehydrogenation. The conversion of propene is higher with CO as the predominant product. In any case, the oxidation activity depends on the vanadium content of the catalyst. Isopropanol is mainly converted into acetone and propene, while acetone is burnt predominantly to CO. Mn- and Fe- containing systems are definitely more active in the conversion of hydrocarbons and oxygenates, giving rise almost exclusively to CO 2. 2-Chloropropane is selectively dehydrochlorinated to propene and HCl starting from 350 K, propene being later burnt to CO on the industrial V 2O 5–WO 3–TiO 2 catalysts, whose combustion activity is, apparently, not affected by chlorine. On the contrary, chlorine strongly affects the behavior of Mn-based catalysts, that are active in the dehydrochlorination of 2-chloropropane, but are simultaneously deactivated with respect to their combustion catalytic activity. The conversion of 1,2-dichlorobenzene gives rise to important amounts of heavy products in our experimental conditions with relatively high reactant concentration. 相似文献
14.
In this work, a combinatorial methodology was developed for photodegradation catalyst screening. A fluorescence imaging detection system was designed for high throughput analysis, 1,6-hexamethylenediamine was used as the probe molecule for catalyst testing. The photodegradation activity of catalysts was evaluated by 1,6-hexamethylenediamine consumption during the photodegradation reaction. The methodology could provide reliable results. We found that pure TiO 2, ZrO 2, Nb 2O 5, MoO 3, and WO 3 did not show much activity for 1,6-hexamethylenediamine photodegradation under visible light. TiO 2 catalysts doped with different metal ions were tested. When TiO 2 was doped with Ta 2O 5, Nb 2O 5, V 2O 5, MoO 3, or WO 3, higher activity for photodegradation was observed. The doping of La 3+, Ba 2+, and Br − to TiO 2 did not improve the catalytic activities. When doping TiO 2 with Mn 2+, Cl −, Al 3+, Cu 2+, Fe 3+, Na +, Mg 2+, Li +, F −, Co 2+, or K +, catalytic activity was lower than that of pure TiO 2. After elaborate catalysts screening, we discovered new catalysts, such as 50–70% TiO 2/0–20% WO 3/20–40% VO 2.5 and 20–30% TiO 2/30–50% MoO 3/40–60% VO 2.5 as well as 30% WO 3/20% ZrO 2/50% NbO 2.5 (synthesized from ZrCl 4, NbCl 5, and (NH 4) 5H 5[H 2(WO 4) 6]·H 2O in ethanol solution or suspension) and 60–70% WO 3/Nb 2O 5 (synthesized from WCl 6 and NbCl 5 in ethanol solution). We observed that the catalytic activity is sensitive to preparation methods and catalyst specific surface areas. When P123 (HO(CH 2CH 2O) 20(CH 2CH(CH 3)O) 70(CH 2CH 2O) 20H, designated EO 20PO 70EO 20) was used as template to synthesize mesoporous materials, the mesoporous catalysts showed higher activity than regular catalytic materials. 相似文献
15.
The reactivity of ternary V 2O 5&z.sbnd;WO 3TiO 2 de-NO xing catalysts with compositions similar to those of commercial catalysts (WO 3 ca. 9% w/w, V 2O 5 < 2% w/w) is investigated by transient techniques (temperature programmed desorption, TPD; temperature programmed surface reaction, TPSR; and temperature programmed reaction, TPR). The results indicate that the reactivity of the ternary catalysts in the SCR reaction increases on increasing the vanadia loading, and that the ternary catalysts are more active than the corresponding binary vanadia-titania samples with the same V 2O 5 loading. Indeed the SCR reaction is monitored at lower temperatures and high NO conversions are also preserved at high temperatures. TPSR and TPR data show that at low temperatures the SCR reaction occurs via a redox mechanism that involves at first the participation of the catalyst lattice oxygen and then the reoxidation of the reduced sites by gas-phase oxygen. Based on TPSR and TPR data, the higher reactivity of the ternary catalysts has been related to their superior redox properties, in line with previous chemico-physical characterisation studies. The catalyst redox properties thus appear as a key-factor in controlling the reactivity of V 2O 5&z.sbnd;WO 3TiO 2 de-NO xing catalysts at low temperatures. The results also show that at high temperatures the surface acidity plays an important role in the adsorption and activation of ammonia. 相似文献
16.
Ag-based catalysts supported on various metal oxides, Al 2O 3, TiO 2, and TiO 2–Al 2O 3, were prepared by the sol–gel method. The effect of SO 2 on catalytic activity was investigated for NO reduction with propene under lean burn condition. The results showed the catalytic activities were greatly enhanced on Ag/TiO 2–Al 2O 3 in comparison to Ag/Al 2O 3 and Ag/TiO 2, especially in the low temperature region. Application of different characterization techniques revealed that the activity enhancement was correlated with the properties of the support material. Silver was highly dispersed over the amorphous system of TiO 2–Al 2O 3. NO 3− rather than NO 2− or NO x reacted with the carboxylate species to form CN or NCO. NO 2 was the predominant desorption species in the temperature programmed desorption (TPD) of NO on Ag/TiO 2–Al 2O 3. More amount of formate (HCOO −) and CN were generated on the Ag/TiO 2–Al 2O 3 catalyst than the Ag/Al 2O 3 catalyst, due to an increased number of Lewis acid sites. Sulfate species, resulted from SO 2 oxidation, played dual roles on catalytic activity. On aged samples, the slow decomposition of accumulated sulfate species on catalyst surface led to poor NO conversion due to the blockage of these species on active sites. On the other hand, catalytic activity was greatly enhanced in the low temperature region because of the enhanced intensity of Lewis acid site caused by the adsorbed sulfate species. The rate of sulfate accumulation on the Ag/TiO 2–Al 2O 3 system was relatively slow. As a consequence, the system showed superior capability for selective adsorption of NO and SO 2 toleration to the Ag/Al 2O 3 catalyst. 相似文献
17.
In situ Raman spectroscopy was used for studying the ternary 2% CrO 3–6% V 2O 5/TiO 2 catalyst, for which a synergistic effect between vanadia and chromia leads to enhanced catalytic performance for the selective catalytic reduction (SCR) of NO with NH 3. The structural properties of this catalyst were studied under NH 3/NO/O 2/N 2/SO 2/H 2O 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
18.
利用溶胶-凝胶技术原位合成一系列不同V 2O 5担载量的V 2O 5/TiO 2催化剂,通过BET、XRD、NH 3-TPD及紫外-可见光等手段对催化剂进行表征。结果表明:制备的催化剂均具有介孔结构,V 2O 5在TiO 2表面高度分散,且存在3种典型的酸性位。通过选择性催化还原反应对V 2O 5/TiO 2催化剂进行活性评价,结果显示随着V 2O 5含量的增加,NO转化率大于75%的温度窗口向低温方向偏移,含10% (质量分数)V 2O 5的催化剂的NO转化率为80%的温度窗口最宽为200~450℃,240℃时20 h连续实验表现出稳定的抗硫抗水性能。结合紫外-可见光谱分析,揭示了钒掺杂所形成的单聚和低聚钒酸盐为催化剂的活性组分。 相似文献
19.
The fresh catalyst V 2O 5–WO 3/TiO 2 and catalyst used in SCR for 9000 h have been studied by the solid state 51V NMR spectroscopy in static and MAS conditions. According to 51V NMR in both samples the majority of vanadium sites are in a distorted octahedral environment similar to that in V 2O 5. There is a strong interaction between vanadium oxide and the support, but the concentration of vanadium atoms strongly bound to the surface is very small and can be detected only in MAS NMR experiments or after removing the excess of V 2O 5. There is no influence of WO 3 additives on the structure of the particles of V 2O 5, whereas the influence on the structure of strongly bounded V cannot be excluded. Combination of static (wide line) and MAS NMR techniques permit the characterization of not only the structure of the vanadium species but also small changes in their local environment. Hence these experiments show that there are some distortions of the local environment of vanadium sites of the vanadium oxide particles compared with the polycrystalline V 2O 5; treatment by SCR increases these distortions. 相似文献
20.
TiO 2–Al 2O 3 composite supports have been prepared by chemical vapor deposition (CVD) over γ-Al 2O 3 substrate, using TiCl 4 as the precursor. High dispersion of TiO 2 overlayer on the surface of Al 2O 3 has been obtained, and no cluster formation has been detected. The catalytic behavior of Mo supported on Al 2O 3, TiO 2 and TiO 2–Al 2O 3 composite has been investigated for the hydrodesulfurization (HDS) of dibenzothiophene (DBT) and methyl-substituted DBT derivatives. The conversion over the Mo catalysts supported on TiO 2–Al 2O 3 composite, in particular for the HDS of 4,6-dimethyldibenzothiophene (4,6-DMDBT) is much higher than that of conversion obtained over Mo catalyst supported on Al 2O 3. The ratio of the corresponding cyclohexylbenzenes/biphenyls is increased over Mo catalyst supported on TiO 2–Al 2O 3 composite support. This means that the reaction rate of prehydrogenation of an aromatic ring rather than the rate of hydrogenolysis of C–S bond cleavage is accelerated for the HDS of DBT derivatives. The Mo/TiO 2–Al 2O 3 catalyst leads to higher catalytic performance for deep HDS of gas oil. 相似文献
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