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1.
The sulphur tolerance and thermal stability of a 2 wt% Ag/γ-Al 2O 3 catalyst was investigated for the H 2-promoted SCR of NO x with octane and toluene. The aged catalyst was characterised by XRD and EXAFS analysis. It was found that the effect of ageing was a function of the gas mix and temperature of ageing. At high temperatures (800 °C) the catalyst deactivated regardless of the reaction mix. EXAFS analysis showed that this was associated with the Ag particles on the surface of the catalyst becoming more ordered. At 600 and 700 °C, the deactivating effect of ageing was much less pronounced for the catalyst in the H 2-promoted octane-SCR reaction and ageing at 600 °C resulted in an enhancement in activity for the reaction in the absence of H 2. For the toluene + H 2-SCR reaction the catalyst deactivated at each ageing temperature. The effect of addition of low levels of sulphur (1 ppm SO 2) to the feed was very much dependent on the reaction temperature. There was little deactivation of the catalyst at low temperatures (≤235 °C), severe deactivation at intermediate temperatures (305 and 400 °C) and activation of the catalyst at high temperatures (>500 °C). The results can be explained by the activity of the catalyst for the oxidation of SO 2 to SO 3 and the relative stability of silver and aluminium sulphates. The catalyst could be almost fully regenerated by a combination of heating and the presence of hydrogen in the regeneration mix. The catalyst could not be regenerated in the absence of hydrogen. 相似文献
2.
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
3.
The catalytic behavior of the V-M/AC (M=W, Mo, Zr, and Sn) catalysts were studied for the NO reduction with ammonia at low temperatures, especially in the presence of SO 2. The presence of the metal oxides does not increase the V 2O 5/AC activity but decreases it. Except V-Mo/AC, the other catalysts are promoted by SO 2 at 250°C, especially for V-Sn/AC. However, the promoting effect of SO 2 is gradually depressed by catalyst deactivation. Changes in catalyst preparation method can improve the catalyst stability in short-term but cannot completely prevent the catalyst from a long-term deactivation. Mechanisms of the promoting effect and the deactivation of V-Sn/AC catalyst by SO 2 were studied using Fourier transform infrared spectroscopy (FT-IR) spectra and measurement of catalyst surface area and pore volume. The results showed that both the SO 2 promotion and deactivation are associated with the formation of sulfate species on the catalyst surface. In the initial period of the selective catalytic reduction (SCR) reaction in the presence of SO 2, the formed sulfate species provide new acid sites to enhance ammonia adsorption and thus the catalytic activity. However, as the SCR reaction proceeds, excess amount of sulfate species and then ammonium-sulfate salts are formed which is stabilized by the presence of tin oxide, resulting in gradual plugging of the pore structures and the catalyst deactivation. 相似文献
4.
A novel activated carbon-supported vanadium oxide catalyst was studied for SCR of NO with NH 3 at low temperatures (100 – 250°C). The effects of reaction temperature, preparation conditions and SO 2 on SCR activity were evaluated. The results show that this catalyst has a high catalytic activity for NO–NH 3–O 2 reaction at low temperatures. Preoxidation of the calcined catalyst helps improve catalytic activity. V 2O 5 loading, other than calcination temperature, gives a significant influence on the activity. SO 2 in the flue gas does not de-activate the catalyst but improves it. A stability test of more than 260 h shows that the catalyst is highly active and stable in the presence of SO 2. 相似文献
5.
The effect of tungsten and barium on the thermal stability of V 2O 5/TiO 2 catalyst for NO reduction by NH 3 was examined over a fixed bed flow reactor system. The activity of V 2O 5/sulfated TiO 2 catalyst gradually decreased with respect to the thermal aging time at 600 °C. The addition of tungsten to the catalyst surface significantly enhanced the thermal stability of V 2O 5 catalyst supported on sulfated TiO 2. On the basis of Raman and XRD measurements, the tungsten on the catalyst surface was identified as suppressing the progressive transformation of monomeric vanadyl species into crystalline V 2O 5 and of anatase into rutile phase of TiO 2. However, the NO removal activity of V 2O 5/sulfated TiO 2 catalyst including barium markedly decreased after a short aging time, 6 h at 600 °C. This may be due to the transformation of vanadium species to inactive V–O–Ba compound by the interaction with BaO which was formed by the decomposition of BaSO 4 on the catalyst surface at high reaction temperature of 600 °C. The addition of SO 2 to the feed gas stream could partly restore the NO removal activity of thermally aged V 2O 5/sulfated TiO 2 catalyst containing barium. 相似文献
6.
The inhibition effect of H 2O on V 2O 5/AC catalyst for NO reduction with NH 3 is studied at temperatures up to 250 °C through TPD, elemental analyses, temperature-programmed surface reaction (TPSR) and FT-IR analyses. The results show that H 2O does not reduce NO and NH 3 adsorption on V 2O 5/AC catalyst surface, but promotes NH 3 adsorption due to increases in Brønsted acid sites. Many kinds of NH 3 forms present on the catalyst surface, but only NH 4+ on Brønsted acid sites and a small portion of NH 3 on Lewis acid sites are reactive with NO at 250 °C or below, and most of the NH 3 on Lewis acid sites does not react with NO, regardless the presence of H 2O in the feed gas. H 2O inhibits the SCR reaction between the NH 3 on the Lewis acid sites and NO, and the inhibition effect increases with increasing H 2O content. The inhibition effect is reversible and H 2O does not poison the V 2O 5/AC catalyst. 相似文献
7.
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
8.
The deactivation of a commercial type V 2O 5-WO 3-TiO 2 monolith catalyst under biomass combustion was studied at a full-scale grate-fired power plant burning straw/wood using a slip stream pilot scale reactor. The aerosols in the flue gas consisted of a mixture of potassium chloride and sulphate. Three catalyst elements were exposed at 350 °C, and one element was exposed at 250 °C for comparison. The catalyst activity was measured in the reactor at the exposure temperature by addition of NH 3 and extra NO. The activity, in terms of a first-order rate constant, dropped by 52% after about 1140 h indicating a very fast deactivation compared to coal firing. It was also found that the reactor temperature was not of importance for the deactivation rate. SEM-EDX analysis showed that particle deposition and pore blocking contributed to the deactivation by decreasing the diffusion rate of NO and NH 3 into the catalyst. However, potassium also penetrated into the catalyst wall and the resulting average K/V ratio in the catalyst structure was high enough (about 0.3–0.5) for a significant chemical deactivation. Chemisorption studies carried out in situ showed that the amount of chemisorbed NH 3 on the catalyst decreased as a function of exposure time, which reveals that Brøndsted acid sites had reacted with potassium compounds and thereby rendered inactive. When washed by 0.5 M H 2SO 4 the regenerated catalyst regains a higher activity than that of the fresh catalyst at temperatures higher than 300 °C, but even though reactivation is possible, the deactivation rate appears too high for practical use of the SCR process in straw combustion. 相似文献
9.
The industrial SO 2 oxidation catalyst VK69 deactivates at around 440°C in a 10% SO 2, 11% O 2, 79% N 2 gas mixture. In situ EPR measurements show that the deactivation is caused by the precipitation of V(IV) compounds. DeNO x catalysts based on V 2O 5/TiO 2, the TiO 2 support, analytical grade anatase and transition metal-exchanged Al-PILCs (pillared clay) have been characterized by EPR spectroscopy and the catalytic activity of the catalysts monitored up to 500°C. Depending on the exchanged metal ion, a relatively large temperature range for the catalytic activity towards the SCR reaction was observed. 相似文献
10.
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
11.
To get the low temperature sulfur resistant V 2O 5/TiO 2 catalysts quantum chemical calculation study was carried out. After selecting suitable promoters (Se, Sb, Cu, S, B, Bi, Pb and P), respective metal promoted V 2O 5/TiO 2 catalysts were prepared by impregnation method and characterized by X-ray diffraction (XRD) and Brunner Emmett Teller surface area (BET-SA). Se, Sb, Cu, S promoted V 2O 5/TiO 2 catalysts showed high catalytic activity for NH 3 selective catalytic reduction (NH 3-SCR) of NO x carried at temperatures between 150 and 400 °C. The conversion efficiency followed in the order of Se > Sb > S > V 2O 5/TiO 2 > Cu but Se was excluded because of its high vapor pressure. An optimal 2 wt% ‘Sb’ loading was found over V 2O 5/TiO 2 for maximum NO x conversion, which also showed high resistance to SO 2 in presence of water when compared to other metal promoters. In situ electrical conductivity measurement was carried out for Sb(2%)/V 2O 5/TiO 2 and compared with commercial W(10%)V 2O 5/TiO 2 catalyst. High electrical conductivity difference (Δ G) for Sb(2%)/V 2O 5/TiO 2 catalyst with temperature was observed. SO 2 deactivation experiments were carried out for Sb(2%)/V 2O 5/TiO 2 and W(10%)/V 2O 5/TiO 2 at a temperature of 230 °C for 90 h, resulted Sb(2%)/V 2O 5/TiO 2 was efficient catalyst. BET-SA, X-ray photoelectron spectroscopy (XPS) and carbon, hydrogen, nitrogen and sulfur (CHNS) elemental analysis of spent catalysts well proved the presence of high ammonium sulfate salts over W(10%)/V 2O 5/TiO 2 than Sb(2%)/V 2O 5/TiO 2 catalyst. 相似文献
12.
This work investigates the effect of treatments under different CH 4-containing atmospheres on the reactivity of fresh and S-poisoned 2% w/w Pd/Al 2O 3/CeO 2 catalysts for methane combustion. Over the fresh catalyst the decomposition/reformation processes of PdO occurring during cycles of CH4-reducing/lean combustion pulses allowed the complete recovery of activity losses possibly associated with H2O poisoning which were observed during prolonged exposure under lean combustion conditions. The presence of CeO2 markedly enhances both the activity losses under lean combustion conditions and the rate of PdO reoxidation/reactivation upon Pd redox cycle. Under lean combustion conditions, regeneration of catalyst deactivated by exposure to SO2-containing atmosphere required very high temperatures (above 750 °C) in order to decompose stable sulphate species adsorbed on the support. Treatments consisting of alternate CH4-reducing/lean combustion pulses allowed a complete recovery of activity at much lower temperatures (550–600 °C) due to the reduction of sulphates by CH4 activated on the surface of Pd metal. A protecting role of CeO2 on Pd poisoning due either to exposure to SO2-containing atmosphere or to spill-back of support sulphates species was also evidenced. 相似文献
13.
The reforming of CH 4 with CO 2 over activated carbon- or silica-supported cobalt catalysts with and without added MgO as promoter has been studied over a range of temperatures (500–700°C). A significant effect of the MgO on catalyst efficiency was observed. The presence of MgO markedly reduces the carbon deposition on the surface of the catalyst and therefore, contributes to the stability of the catalyst. Based on temperature-programmed surface reaction experiments of chemisorbed CO 2, the role of MgO may be ascribed to the formation of strongly adsorbed CO 2 species over its surface. These CO 2 species can easily react with the surface carbon deposits under CO 2-reforming reaction conditions, preventing in this way catalyst deactivation. 相似文献
14.
The fast SCR reaction using equimolar amounts of NO and NO 2 is a powerful means to enhance the NO x conversion over a given SCR catalyst. NO 2 fractions in excess of 50% of total NO x should be avoided because the reaction with NO 2 only is slower than the standard SCR reaction. At temperatures below 200 °C, due to its negative temperature coefficient, the ammonium nitrate reaction gets increasingly important. Half of each NH3 and NO2 react to form dinitrogen and water in analogy to a typical SCR reaction. The other half of NH3 and NO2 form ammonium nitrate in close analogy to a NOx storage-reduction catalyst. Ammonium nitrate tends to deposit in solid or liquid form in the pores of the catalyst and this will lead to its temporary deactivation. The various reactions have been studied experimentally in the temperature range 150–450 °C for various NO2/NOx ratios. The fate of the deposited ammonium nitrate during a later reheating of the catalyst has also been investigated. In the absence of NO, the thermal decomposition yields mainly ammonia and nitric acid. If NO is present, its reaction with nitric acid on the catalyst will cause the formation of NO2. 相似文献
15.
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
16.
A 5 wt% CoO x/TiO 2 catalyst has been used to study the effect of calcination temperature on the activity of this catalyst for CO oxidation at 100 °C under a net oxidizing condition in a continuous flow type fixed-bed reactor system, and the catalyst samples have been characterized using TPD, XPS and XRD measurements. The catalyst after calcination at 450 °C gave highest activity for this low-temperature CO oxidation, and XPS measurements yielded that a 780.2-eV Co 2p 3/2 main peak appeared with this catalyst sample and this binding energy was similar to that measured with pure Co 3O 4. After calcination at 570 °C, the catalyst, which had possessed practically no activity in the oxidation reaction, gave a Co 2p 3/2 main structure peak at 781.3 eV which was very similar to those obtained for synthesized Co nTiO n+2 compounds (CoTiO 3 and Co 2TiO 4), and this catalyst sample had relatively negligible CO chemisorption as observed by TPD spectra. XRD peaks indicating only the formation of Co 3O 4 particles on titania surface were developed in the catalyst samples after calcination at temperatures ≥350 °C. Based on these characterization results, five types of Co species could be modeled to exist with the catalyst calcined at different temperatures. Among these surface Co species, the Type A clean Co 3O 4 particles were predominant on a sample of the catalyst after calcination at 450 °C and highly active for CO oxidation at 100 °C, and the calcination at 570 °C gave the Type B Co 3O 4 particles with complete Co nTiO n+2 overlayers inactive for this oxidation reaction. 相似文献
17.
Simultaneous dry removal of SO 2 and NO x from flue gas has been investigated using a powder-particle fluidized bed. In a process of flue gas desulfurization by use of solid sorbents such as FeO (dust from a steel plant) and CuO, the smaller the particle size of sorbents, the higher the expected SO 2 conversion. In a powder-particle fluidized bed (PPFB), fine particles less than 40 μm in diameter fed into the bed are fluidized with coarse particles. But only the fine particles are entrained from the bed, and their residence time in the bed is remarkably long. The reduction of NOx with NH3 in the fluidized bed is catalyzed by coarse particles or both coarse and fine particles. In this study, PPFB was applied to simultaneous dry SO2/NOx removal process, and several kinds of sorbents or catalysts were evaluated in a PPFB. Using the selected sorbents and catalysts, kinetic measurements were made in the temperature range of 300 to 600°C. SO2 removal efficiencies were affected by reaction temperature, sorbent/S ratio, and static bed height. NOx removal efficiencies in excess of 95% were achieved at NH3/NOx mole ratio of 1.0. When FeO was used as sorbent, SO2 conversion increased with increasing temperature and reached 80% at 600°C. 相似文献
18.
A novel regenerable Fe/activated coke (AC) desulfurizer prepared by impregnation of Fe(NO 3) 3 on an activated coke was investigated. Experiment results showed that at 200 °C the SO 2 adsorption capacity of the Fe/AC was higher than that of AC or Fe 2O 3. Temperature-programmed desorption (TPD) revealed that H 2SO 4 and Fe 2(SO 4) 3 were generated on the desulfurizer upon adsorption of SO 2. Effect of desulfurization temperature was also investigated which revealed that with increasing temperature from 150 to 250 °C, the SO 2 removal ability gradually increases. The used Fe/AC can be regenerated by NH 3 at 350 °C to directly form solid ammonium-sulfate salts. 相似文献
19.
Activity for hydrolysis of CCl 2F 2 (CFC12) on various metal sulfate was investigated. Zr(SO 4) 2 was found to be the most active while FeSO 4, Cr 2(SO 4) 3, Al 2(SO 4) 3, La 2(SO 4) 3 and Ce 2(SO 4) 3 had intermediate activity. MnSO 4, CoSO 4, and MgSO 4 showed low activity and SrSO 4, CaSO 4, and BaSO 4 had even less activity. The major carbon containing product was CO 2 and small amount of CClF 3 and CO were formed over several sulfates. The crystal structure of the sulfates was stable during decomposition of CCl 2F 2, and the conversion reached a steady state after initial decrease at 275 °C over Zr(SO 4) 2 catalyst. The concentration of surface hydroxyl was larger than that over AlPO 4-based catalysts and a reaction mechanism similar to that over AlPO 4-based catalysts was proposed. 相似文献
20.
采用V_2O_5/WO_3-TiO_2作为脱硝催化剂,考察活性组分V_2O_5和助剂WO_3负载量对催化剂脱硝活性和抗硫抗水性能的影响。结果表明,3%V_2O_5/x WO_3-TiO_2催化剂(x=3%、4%、5%、6%、7%、8%、9%、10%)上NOx转化率随着WO_3负载量增加而升高,催化剂反应温度窗口不断拓宽。单独通水蒸汽及同时通SO2和水蒸汽对催化剂的毒害作用均较强,表明H2O和NH3的竞争吸附是催化剂抗硫抗水性能较差的重要原因。SO_2与H_2O和NH_3反应生成亚硫酸铵盐和硫酸铵盐,导致催化剂孔隙堵塞,催化活性降低。 相似文献
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