Reactivity of V2O5&z.sbnd;WO3TiO2 de-NOx catalysts by transient methods

The reactivity of ternary V₂O₅&z.sbnd;WO₃TiO₂ de-NO_xing catalysts with compositions similar to those of commercial catalysts (WO₃ ca. 9% w/w, V₂O₅ < 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 V₂O₅ 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 V₂O₅&z.sbnd;WO₃TiO₂ 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.     

SO2 resistant antimony promoted V2O5/TiO2 catalyst for NH3-SCR of NOx at low temperatures

To get the low temperature sulfur resistant V₂O₅/TiO₂ catalysts quantum chemical calculation study was carried out. After selecting suitable promoters (Se, Sb, Cu, S, B, Bi, Pb and P), respective metal promoted V₂O₅/TiO₂ 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 V₂O₅/TiO₂ catalysts showed high catalytic activity for NH₃ selective catalytic reduction (NH₃-SCR) of NO_x carried at temperatures between 150 and 400 °C. The conversion efficiency followed in the order of Se > Sb > S > V₂O₅/TiO₂ > Cu but Se was excluded because of its high vapor pressure. An optimal 2 wt% ‘Sb’ loading was found over V₂O₅/TiO₂ for maximum NO_x conversion, which also showed high resistance to SO₂ in presence of water when compared to other metal promoters. In situ electrical conductivity measurement was carried out for Sb(2%)/V₂O₅/TiO₂ and compared with commercial W(10%)V₂O₅/TiO₂ catalyst. High electrical conductivity difference (ΔG) for Sb(2%)/V₂O₅/TiO₂ catalyst with temperature was observed. SO₂ deactivation experiments were carried out for Sb(2%)/V₂O₅/TiO₂ and W(10%)/V₂O₅/TiO₂ at a temperature of 230 °C for 90 h, resulted Sb(2%)/V₂O₅/TiO₂ 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%)/V₂O₅/TiO₂ than Sb(2%)/V₂O₅/TiO₂ catalyst.     

Ammonia activation over catalysts for the selective catalytic reduction of NOx and the selective catalytic oxidation of NH3. An FT-IR study

The adsorption and transformation of ammonia over V₂O₅, V₂O₅/TiO₂, V₂O₅-WO₃/TiO₂ and CuO/TiO₂ systems has been investigated by FT-IR spectroscopy. In all cases ammonia is first coordinated over Lewis acid sites and later undergoes hydrogen abstraction giving rise either to NH₂ amide species or to its dimeric form N₂H₄, hydrazine. Other species, tentatively identified as imide NH, nitroxyl HNO, nitrogen anions N₂⁻ and azide anions N₃⁻ are further observed over CuO/TiO₂. The comparison of the infrared spectra of the species arising from both NH₃ and N₂H₄ adsorbed over CuO/TiO₂ strongly suggest that N₂H₄ is an intermediate in NH₃ oxidation over this active selective catalytic reduction (SCR) and selective catalytic oxidation (SCO) catalysts. This implies that ammonia is activated in the form of NH₂ species for both SCR and SCO, and it can later dimerize. Ammonia protonation to ammonium ion is detected over V₂O₅-based systems, but not over CuO/TiO₂, in spite of the high SCR and SCO activity of this catalyst. Consequently Brönsted acidity is not necessary for the SCR activity.     

Catalytic activity and solid acidity of vanadium oxide thin layer loaded on TiO2, ZrO2, and SnO2

Vanadium oxide spread highly on TiO₂ (anatase, A) and SnO₂, and rather densely on TiO₂ (rutile, R) and ZrO₂ to make the monolayer in less than 4–5 V nm⁻². Profile of acid site of the monolayer was measured by temperature programmed desorption of ammonia, and its relation with the surface oxidation state was studied. The acid site density was high on the V₂O₅/TiO₂ (A) independent of the degree of oxidation. On the other hand, that of V₂O₅/TiO₂ (R) and V₂O₅/ZrO₂ depended on the oxidation state, and the high value of the concentration was observed on the oxidized one. The strength of acid site generated on the V₂O₅ monolayer on TiO₂ was as high as on the HZSM-5 zeolite. Turnover frequency (TOF) of propane conversion, and product selectivity were measured in propane oxidation. Among tested oxides, the V₂O₅/TiO₂ (A) showed the high TOF and selectivity to form propylene, while those loaded on TiO₂ (R) and ZrO₂ the small TOF and poor selectivity. Therefore, the reaction profile of activity and selectivity could be related with the extent of spreading and solid acidity. An idea of limit of the acid site density ca. 1.5 nm⁻² on the monolayer was elucidated.     

EPR study of electrochemical lithium intercalation in V2O5 cathodes

The evolution of the cathode material of Li/V₂O₅ cells upon lithium intercalation is studied by Electron Paramagnetic Resonance (EPR) spectroscopy. Below one lithium per V₂O₅(x < 1), where the electrochemical process is completely reversible, V^IV ions of the - and δ-Li_x V₂O₅ phases are detected by EPR. Within this intercalation range, the shape of the EPR signal is dominated by the strong exchange interactions between magnetically concentrated V^IV ions of the Li_x V₂O₅ phases. The intercalation range x > 1 is characterized by a new EPR signal attributed to magnetically diluted vanadyl ions VO²⁺. It is shown that these species do not belong to the Li_x V₂O₅ matrix, but most probably originate from the liberation of surface vanadyl ions during the irreversible transformation of δ-Li_x V₂O₅ into γ-Li_x V₂O₅. On the basis of these results, a mechanism to explain the partial irreversibility of the electrochemical processes is proposed. A third EPR signal is also observed for x 1. This signal is attributed to an electron-hole defect in Li_x V₂O₅, originating from a local charge compensation of the vanadyl vacancies.     

Factors controlling the selectivity of V2O5 supported catalysts in the oxidative dehydrogenation of propane

The surface properties of a series of V₂O₅ catalysts supported on different oxides (Al₂O₃, H–Na/Y zeolite, MgO, SiO₂, TiO₂ and ZrO₂) were investigated by transmission electron microscopy and FTIR spectroscopy augmented by CO and NH₃ adsorption. In the case of the V₂O₅/SiO₂ system TEM images evidenced the presence of V₂O₅ crystallites, whereas such segregated phase was not observed for the other samples. VO_x species resulted widely spread on the surface of Al₂O₃, H–Na/Y zeolite, MgO and SiO₂, whereas on TiO₂ and ZrO₂ they are assembled in a layer covering almost completely the support. Furthermore, evidences for the presence in this layer of V–OH Brønsted acid sites close to the active centres were found. It is proposed that propene molecules primarily produced by oxydehydrogenation of propane can be adsorbed on this acid centres and then undergo an overoxidation by reaction with redox centres in the neighbourhood. This features could account for the low selectivity of V₂O₅/TiO₂ and V₂O₅/ZrO₂ catalysts.     

制备方法对V_2O_5-WO_3/TiO_2选择性催化还原催化剂性能的影响

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。     

Thermal behaviour, redox properties, and catalytic performance of alkali-promoted V2O5 catalysts in the selective oxidation of p-methoxytoluene: a comparative study

Alkali-promoted V₂O₅ catalysts M–V₂O₅ (M=Li, K, Cs) were synthesised by impregnation of V₂O₅ with alkali sulphate solution. Pure V₂O₅ was used for comparison. X-ray diffraction, spectroscopic (FTIR), and thermoanalytical methods (STA/MS) have been used to characterise the phase composition, the adsorption properties, and the reducibility of the catalysts. The catalytic performance was proved using the oxidation of p-methoxytoluene (PMT) to p-methoxybenzaldehyde (PMBA) as test reaction. The surface acidity is lowered, but the reducibility is enhanced with increasing size and basic properties of the alkali cation. This leads to an increased adduct (PMT) adsorption and decreased product (PMBA) adsorption in the order V₂O₅2O₅K–V₂O₅2O₅. Consequently, the catalytic performance is improved in the same way. The formation of bronze phases at relative low temperatures in the case of K– and Cs–V₂O₅ stabilise V⁴⁺ oxidation state and improve the redox properties and consequently the catalytic results. The admixture of the non-reactive pyridine enhances the aldehyde selectivity by further lowering of the surface acidity. Additionally, pyridinium cations generated during catalytic reaction and incorporated into the formed alkali bronze phases stabilise these structures.     

Effects of microwaves on selective oxidation of toluene to benzoic acid over a V2O5/TiO2 system

Two series of catalysts, V₂O₅/TiO₂ and modified V₂O₅/TiO₂, were prepared with a conventional impregnation method. They were tested in the selective oxidation of toluene to benzoic acid under microwave irradiation. The reaction conditions were optimized over V₂O₅/TiO₂. It was found that in the microwave catalytic process the optimum reactor bed temperature of the titled reaction decreases to 500 K (600 K in the conventional process). The modification of V₂O₅/TiO₂ with MoO₃, WO₃, Nb₂O₅ or Ta₂O₅, which has no negative influence on the reaction in the conventional catalytic process, can greatly promote the catalytic activities in the microwave process, leading to a high yield of benzoic acid (41%). The effects of microwave electromagnetic field on the catalysts are discussed.     

Surface characteristics and activity in selective oxidation of o-xylene of supported V2O5 catalysts prepared by standard impregnation and atomic layer deposition

Alumina-, silica-, and titania-supported vanadium oxide systems with V₂O₅ loadings ranging from 3 to 12 wt.%, corresponding to 0.02–0.09 V/(Al,Si,Ti) atomic ratios, were prepared by atomic layer deposition (ALD) and compared with the corresponding impregnated catalysts. The surface acidic properties of the supports and catalysts were investigated using ammonia adsorption microcalorimetry to determine the number and strength of the surface acid sites. Deposition of V₂O₅ on alumina and titania supports gave rise to catalysts with lower amounts of acid sites than the respective supports, while for the samples prepared on silica, an increase of the number of acid sites was observed after V₂O₅ deposition. As a common trend, the surface acid strength was greater for the ALD catalysts than for the impregnated ones, suggesting a stronger interaction of the VO species with the support centers, which act as electron attractor centers creating Lewis-like vanadium species. Redox cycles were performed, involving temperature programmed reduction (TPR) analyses separated by an oxidation treatment (TPO). The results evidenced the good reversibility of the redox behavior of the vanadium centers in every case, while significant differences were observed when comparing the temperatures of reduction (T_max). Lower T_max values were observed for the better dispersed vanadia catalysts. After reduction, the V centers had a final formal average oxidation state corresponding to +3 or less (+2.5 to +2). The reactivity of the vanadia systems was examined by measuring their performance for the oxidation of o-xylene to phthalic anhydride. Activity tests indicated the superior selectivity of the V₂O₅ systems based on the more acidic supports (Al₂O₃ and TiO₂). The nature of the support governed the activity, and the more concentrated catalysts gave rise to improved selectivity to phthalic anhydride.     

载体对V_2O_5-MoO_3/TiO_2催化剂表面性质及其选择性催化还原脱硝性能的影响

采用比表面积分别为101.86 m2·g-1(A)、86.37 m2·g-1(B)和7.78(C)m2·g-1(C)的TiO_2载体,通过分步浸渍法制备V2O5-Mo O3/TiO_2(A,B,C)选择性催化还原脱硝催化剂。在空速为10 000 h-1和氨氮体积比1.0条件下,以TiO_2(A)与TiO_2(B)为载体制备的催化剂脱硝活性在反应温度窗口(350~450)℃超过90%,且具有良好的高温抗硫中毒性能和相对较小的氨气氧化率。而以TiO_2(C)为载体制备的脱硝催化剂活性温度窗口窄,在350℃时获得的最高脱硝活性仅为73%,且对NH3的氧化作用较强。利用X射线衍射、低温N2吸附-脱附、紫外-可见漫反射光谱、H2程序升温还原和NH3程序升温脱附等对载体和催化剂进行表征。结果表明,活性组分V2O5在载体TiO_2(A)上分散性良好,主要以孤立态钒氧物种形式存在,因此,以TiO_2(A)为载体制备的催化剂比表面积、氧化还原性和表面酸性等性能更优。     

TiO2-SiO2 and V2O5/TiO2-SiO2 catalyst: Physico-chemical characteristics and catalytic behavior in selective catalytic reduction of NO by NH3

TiO₂-SiO₂ with various compositions prepared by the coprecipitation method and vanadia loaded on TiO₂-SiO₂ were investigated with respect to their physico-chemical characteristics and catalytic behavior in SCR of NO by NH₃ and in the undesired oxidation of SO₂ to SO₃, using BET, XRD, XPS, NH₃-TPD, acidity measurement by the titration method and activity test. TiO₂-SiO₂, compared with pure TiO₂, 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% V₂O₅. The SCR activity and N₂ selectivity are found to be more excellent over vanadia loaded on TiO₂-SiO₂ with 10–20 mol% of SiO₂ than over that on pure TiO₂, and this is considered to be associated with highly dispersed vanadia on the supports and large amounts of NH₃ adsorbed on the catalysts. With increasing SiO₂ content, the remarkable activity decrease in the oxidation of SO₂ to SO₃, favorable for industrial SCR catalysts, was also observed, strongly depending on the existence of vanadium species of the oxidation state close to V⁴⁺ on TiO₂-SiO₂, while V⁵⁺ exists on TiO₂, according to XPS. It is concluded that vanadia loaded on Ti-rich TiO₂-SiO₂ with low SiO₂ 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 SO₂ oxidation performance.     

燃煤电厂废旧SCR脱硝催化剂中TiO2载体的回收与再利用

以燃煤电厂废弃SCR脱硝催化剂为研究对象,考察了浸出时间、温度、液固比和NaOH浓度对碱浸过程Al、Si、V、W、Ti等元素浸出率以及浸出渣比表面积、孔容的影响。结果表明,碱浸的最优工艺条件为反应时间180 min、温度160℃、液固比15 ml·g^-1、NaOH浓度2.5 mol·L^-1。在最优工艺条件下得到的浸出渣主要成分为锐钛矿型TiO₂,经稀硫酸洗涤后作为载体,通过负载与新鲜催化剂相同含量的WO₃和V₂O₅,制备了V₂O₅-WO₃/TiO₂催化剂。对催化剂进行脱硝活性评价,结果表明,再生载体制备的催化剂脱硝活性恢复至新鲜催化剂水平,300℃时NO转化率达到97.8%,且具有良好的抗硫抗水性。     

An IR study of thermally stable V2O5-WO3 -TiO2 SCR catalysts modified with silica and rare-earths (Ce, Tb, Er)

The catalytic activity of silica-free and silica-modified rare earth (Ce, Tb, Er) containing V₂O₅-WO₃ -TiO₂ catalysts in the selective catalytic reduction of NO by ammonia has been investigated as a function of ageing temperatures. The adsorption of ammonia on the catalysts and the behavior of their surface hydroxy groups and of bulk vibrations has also been studied by IR spectroscopy. Rare earths slightly decrease the catalytic activity of catalysts in a fresh state, and this has been attributed to the perturbation, observed by IR, of the vanadyl groups with a likely lowering of their Lewis acidity. However, rare earths (in particular Tb and Er) increase strongly the catalytic activity of catalysts after ageing. Silica only does not seem to have a positive effect on thermal stability and activity when vanadium is present. It has been concluded that rare earths strongly increase the thermal resistance of the catalysts and inhibit rutilization and surface area loss because they do not penetrate the anatase bulk while tend to cover the external surface. In addition the negative action of free vanadium on phase stability is decreased due to formation of rare earth vanadates.     

Effect of promoters including tungsten and barium on the thermal stability of V2O5/sulfated TiO2 catalyst for NO reduction by NH3

The effect of tungsten and barium on the thermal stability of V₂O₅/TiO₂ catalyst for NO reduction by NH₃ was examined over a fixed bed flow reactor system. The activity of V₂O₅/sulfated TiO₂ 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 V₂O₅ catalyst supported on sulfated TiO₂. 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 V₂O₅ and of anatase into rutile phase of TiO₂. However, the NO removal activity of V₂O₅/sulfated TiO₂ 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 BaSO₄ on the catalyst surface at high reaction temperature of 600 °C. The addition of SO₂ to the feed gas stream could partly restore the NO removal activity of thermally aged V₂O₅/sulfated TiO₂ catalyst containing barium.     

V_2O_5-WO_3/TiO_2催化剂的制备及其烟气脱硝性能

以活性成分负载量、负载顺序和焙烧温度等关键制备参数因素进行正交实验设计制备了V_2O_5-WO_3/TiO_2催化剂,对其进行XRD和TPR表征,并在自行设计搭建的SCR烟气脱硝实验平台上评价其(300~390)℃的SCR脱硝性能。结果表明,活性成分钒和钨绝大多数以非晶态形式存在于载体表面,且具有良好的分散性;主要活性成分V_2O_5负载量越高,脱硝率越高;400℃焙烧温度可以形成催化反应所需的晶相,且维持催化剂较高的比表面积;催化剂低温活性和高温活性是由表面富集和各种成分之间相互作用共同产生的结果,活性组分与载体之间的相互作用对315℃低温脱硝活性影响明显,以先钒后钨负载顺序为宜,表面富集对390℃高温脱硝活性起主要作用,以钒钨同时负载或先钒后钨负载顺序较好;随着m(WO_3)∶m(V_2O_5)的增加,在7.5∶1处催化剂的脱硝率升至最高,随后迅速下降,WO_3负载质量分数以6%为宜。在优化条件V_2O_5负载质量分数0.8%、WO_3负载质量分数6%、先钒后钨负载和400℃焙烧温度下制备了催化剂并进行脱硝性能验证,315℃低温脱硝活性达到69.56%。     

EPR spectroscopic characterization of DeNOx and SO2 oxidation catalysts and model systems

The industrial SO₂ oxidation catalyst VK69 deactivates at around 440°C in a 10% SO₂, 11% O₂, 79% N₂ gas mixture. In situ EPR measurements show that the deactivation is caused by the precipitation of V(IV) compounds. DeNO_x catalysts based on V₂O₅/TiO₂, the TiO₂ 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.     

V2O5 catalysts supported on Al2O3–SiO2 mixed oxide: V, H MAS solid-state NMR, DRIFTS and methanol oxidation studies

Alumina–silica mixed oxide, synthesized by the sol–gel technique, was used as a support for dispersing and stabilizing the active vanadia phase. The catalysts were characterized employing ⁵¹V and ¹H solid-state MAS NMR, diffuse reflectance FT-IR, BET surface area measurements. The partial oxidation activities of the catalysts were tested using methanol oxidation as a model reaction. ⁵¹V solid-state NMR studies on the calcined catalysts showed the peaks corresponding to the presence of both tetrahedral and distorted octahedral vanadia species at low vanadia loadings and with an increase in V₂O₅ content, the ⁵¹V chemical shifts corresponding to amorphous V₂O₅ like phases were observed. DRIFTS studies of the catalysts indicated the vibrations corresponding tetrahedral vanadia species at low and medium loadings and at high V₂O₅ contents the vibrations corresponding V=O bonds of V₂O₅ agglomerates were observed. The V/Al–Si catalysts exhibited high selectivity for the dehydration product dimethyl ether in the methanol partial oxidation studies showing the predominance of the acidic nature of the alumina–silica support over the redox properties of the active vanadia phase.     

V2O5-WO3-MoO3/TiO2催化剂用于NH3选择性还原减排NOx的性能（英文）

The V2O5-WO3-MoO3/TiO2 honeycomb catalyst was prepared with industrial grade chemicals. The structural and physico-chemical properties were analyzed with X-ray diffraction (XRD), scanning electron micrograph (SEM) and mercury porosimetry. The NOx conversion and durability were investigated on a pilot plant test set under the actual operational conditions of a coal fired boiler. The catalyst monolith had good formability with mass percentage of V:W:Mo:TiO2 :fiber glass= 1:4.5:4.5:72:18. Vanadium, tungsten and molybdenum species were highly dispersed on anatase TiO2 without causing the transformation of anatase TiO2 to rutile by calcining under a current of air at 450℃ for 4.5 h, but there were some degrees of crystal distortion. The catalyst particle sizes were almost uniform with close pile-up and the pore structure was regular with complete macro-pore formation and large specific surface area. The NOx conversion was sensitive to temperature but nearly insensitive to NH3 . The catalyst showed strong adaptability to NOx concentration with activity above 80% in the range of 615-1640 mg·m-3 . Within the range of 720-8640 h continuous operation, the NOx conversion dropped at a rate of about 1% reduction per 600 h.     

Improvement of response characteristics of TiO2 humidity sensors by simultaneous addition of Li2O and V2O5

Porous TiO₂ ceramics were prepared by adding various amounts of Li₂O and V₂O₅ and the humidity sensitivity of the resulting ceramics was investigated by means of electrical measurements, scanning electron microscopy and mercury intrusion porosimetry. Simultaneous addition of Li₂O and V₂O₅ to TiO₂ enabled sintering at temperatures as low as 700 °C and also decreased the impedance of the ceramics. Furthermore, in the ceramics including these additives simultaneously, excellent humidity sensitivity as well as good response characteristics were observed. The microstructures of these ceramics depended on the firing temperature and the amount and ratio of Li₂O/V₂O₅, and optimum humidity sensitivity was observed for the sample including both 0.25 mol% Li₂O and 0.75 mol% V₂O₅ fired at 700 °C. These results indicated that the humidity sensitivity and its response characteristics were closely related to the microstructure, and that improving the uniformity of microstructure is important for improving humidity sensitivity and its response characteristics.