A study of the abatement of VOC over V2O5–WO3–TiO2 and alternative SCR catalysts

The conversion of C3 organic compounds (propane, propene, 1- and 2-propanol, allyl alcohol, propanal, acrolein, acetone and 1- and 2-chloropropane) in the presence of excess oxygen has been investigated over two V–W–TiO₂ commercial SCR catalysts differing in the V content and over Mn–TiO₂ alternative SCR catalysts. V–W–Ti catalysts show poor activity in the oxidation of hydrocarbons and oxygenates and give significant amounts of partial oxidation products. Moreover they give rise to CO in excess of CO₂. The sample higher in V is more active. Mn–TiO₂ is definitely more active in oxidation of hydrocarbons and oxygenates, and produces, at total conversion, CO₂ as the only detectable product.

V–W–Ti catalysts are very active in dehydrochlorination of the two 2-chloropropane isomers and retain the same oxidation activity also in the presence of HCl. On the contrary, Mn-based catalysts in the presence of chlorocarbons convert into dehydrochlorination catalysts but lose their catalytic activity in oxidation. V–W–Ti catalysts can be used in Cl-containing atmospheres while Mn–TiO₂ can be proposed for DeNO_x and VOC abatement in Cl-free atmospheres such as for diesel engine exhaust gas purification.     

Evaluation of V2O5–WO3–TiO2 and alternative SCR catalysts in the abatement of VOCs

Two different commercial SCR catalysts belonging to the V₂O₅–WO₃–TiO₂ 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 CO₂. 2-Chloropropane is selectively dehydrochlorinated to propene and HCl starting from 350 K, propene being later burnt to CO on the industrial V₂O₅–WO₃–TiO₂ 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.     

Sulfuric acid formation over ammonium sulfate loaded V2O5–WO3/TiO2 catalysts by DeNOx reaction with NOx

Operating the SCR DeNO_x reactor at temperatures below 200 °C results in a considerable saving in operating costs. Plant experience shows that on the catalysts in these second generation DeNO_x plants, even for flue gases with SO₂ concentration below 10 mg/m³, over 1–2 years operating time sizeable quantities of ammonium sulfates accumulate. Ammonium sulfates deposited on V₂O₅–WO₃/TiO₂ catalysts react with NO_x to nitrogen and sulfuric acid. Second-order rate constants of this reaction for temperatures of 170 °C have been derived. It could be shown that the sulfuric acid formed on the catalyst is displaced by water vapour and desorbs resulting in gas phase concentrations of up to 6.5 mg acid/m³ flue gas. Plant equipment downstream of the ammonium sulfate containing low temperature DeNO_x catalysts has to be protected against the corrosive action of the sulfuric acid in the flue gases leaving the DeNO_x reactor.     

Industrial features

The main physical and chemical features of the fresh and the used samples from the industrial supplier are given. The industrial catalytic conditions are given resulting in the so-called used catalyst after 9000 hours at 280°C in an industrial gas turbine.     

Catalytic activation of ceramic filter elements for combined particle separation, NOx removal and VOC total oxidation

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 TiO₂–V₂O₅–WO₃ catalyst system was developed on a ceramic filter element. Different TiO₂ 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 TiO₂-impregnated sintered filter element cylinders exhibit BET surface areas in the range between 0.5 and 1.3 m²/g. Selected TiO₂-impregnated filter element cylinders of high BET surface area were catalytically activated by impregnation with a V₂O₅ and WO₃ 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 TiO₂-impregnated filter element support was catalytically activated with a Pt/V₂O₅ system. Complete oxidation of propene with 100% selectivity to CO₂ was achieved at 300 °C, a filtration velocity of 2 cm/s and a propene inlet concentration of 300 vol.-ppm.     

制备条件对柴油车用V2O5-WO3/TiO2催化剂催化性能的影响

采用偏NH4VO3和TiO2为前驱体制备选择性催化还原(SCR)催化剂涂层所需浆料,分别以分步涂覆法和一步涂覆法将浆料浸涂在蜂窝陶瓷载体上,得到了整体涂覆式选择性催化还原催化剂.对草酸添加量、焙烧时间及V2O5含量对催化活性的影响进行了研究,结果表明,V2O5(4%)-WO3/TiO2(450℃,3 h)和V2O5(5...     

Fe2O3对V2O5-WO3/TiO2催化剂表面性质及其性能的影响

催化剂是选择性催化还原(SCR)脱硝技术的核心,研究Fe对钒钛系SCR催化剂脱硝活性及SO₂/SO₃转化率的影响具有重要意义。采用等体积浸渍法制备了不同Fe/V质量比的Fe₂O₃-V₂O₅-WO₃/TiO₂催化剂,并进行表征,研究Fe对钒钛系SCR催化剂脱硝活性及SO₂/SO₃转化率的影响,并讨论Fe对于钒钛系SCR催化剂表面性质的影响。结果表明,随着催化剂表面Fe₂O₃含量增加,催化剂的脱硝效率及二氧化硫氧化率均是先上升后下降,当Fe/V质量比为3.0时,催化剂的脱硝效率和二氧化硫氧化率均达到最大值91.78%、1.01%。XPS及H₂-TPR结果表明,随着Fe₂O₃含量增加,催化剂表面钒活性组分的相对含量及V⁴⁺/V⁵⁺比减小,催化剂表面吸附氧(O_α)浓度增加,催化剂的氧化能力增强。NO-TPD结果表明,随着Fe₂O₃含量增加,催化剂表面吸附NO的能力增强。     

Effective regeneration of thermally deactivated commercial V-W-Ti catalysts

An effective method for the regeneration of thermally deactivated commercial monolith SCR catalysts was investigated. Two types of regenerated solutions, namely NH₄Cl (1 mol/L) and dilute H₂SO₄ (0.5 mol/L), were employed to treat the used catalyst. The effects of temperature and the regeneration process on the structural and textural properties of the catalysts were determined by X-ray diffraction, scanning electron microscopy, N₂ adsorption/desorption, elemental analysis and Fourier transform infrared spectroscopy. The results suggest that the anatase phase of the used catalyst is maintained after exposure to high temperatures. Some of the catalytic activity was restored after regeneration. The catalyst regenerated by aqueous NH₄Cl had a higher activity than that of the catalyst treated by dilute H₂SO₄. The main reason is that the NH₃ generated from the decomposition of NH₄Cl at high temperatures can be adsorbed onto the catalyst which promotes the reaction. The aggregated V₂O₅ were partially re-dispersed during the regeneration process, and the intrinsic oxidation of ammonia with high concentrations of O₂ is a factor that suppresses the catalytic activity.     

MgO修饰的V2O5-MoO3/TiO2低温SCR催化剂

V系催化剂具有良好的抗中毒能力,广泛应用于燃煤电厂、工业锅炉烟气(固定源)和车辆尾气(移动源)脱硝领域.本文采用水热法制备了V2O5-MoO3/TiO2催化剂,并通过挤出成型得到蜂窝式V2O5-MoO3/TiO2催化剂,通过与共混法相比,发现采用水热法表现出来的较高的低温SCR活性,并研究了MgO的掺杂对V2O5-MoO3/TiO2催化剂性能的影响,发现MgO的掺杂能够降低SO2的转化率.并通过表征发现采用水热反应法得到的V2O5-MoO3/TiO2催化剂能使活性组分高度分散于TiO2载体表面上,具有较高的比表面积,从而使催化剂表现出最佳的低温活性,同时MgO修饰的V2O5-MoO3/TiO2催化剂的抗硫抗水性以及抗碱金属中毒性也做出了研究.     

Reactivity of V2O5-WO3/TiO2 catalysts in the selective catalytic reduction of nitric oxide by ammonia

The physico-chemical characteristics and the reactivity of sub-monolayer V₂O₅-WO₃/TiO₂ deNO_x catalysts is investigated in this work by EPR, FT-IR and reactivity tests under transient conditions. EPR indicates that tetravalent vanadium ions both in magnetically isolated form and in clustered, magnetically interacting form are present over the TiO₂ surface. The presence of tungsten oxide stabilizes the surface V^IV and modifies the redox properties of V₂O₅/TiO₂ samples. Ammonia adsorbs on the catalysts surface in the form of molecularly coordinated species and of ammonium ions. Upon heating, activation of ammonia via an amide species is apparent. V₂O₅-WO₃/TiO₂ catalysts exhibits higher activity than the binary V₂O₅/TiO₂ and WO₃/TiO₂ reference sample. This is related to both higher redox properties and higher surface acidity of the ternary catalysts. Results suggest that the catalyst redox properties control the reactivity of the samples at low temperatures whereas the surface acidity plays an important role in the adsorption and activation of ammonia at high temperatures.     

V2O5负载量对V2O5／TiO2催化剂SCR法脱硝的影响

以TiO2载体,采用浸渍法制备了不同V2O5负载量的用于选择性催化还原NOx的V2O5／TiO2催化剂。利用BET,SEM和Ⅺ①,对不同V2O5负载量的催化剂组成、结构、形貌和性能进行了表征,考察不同V2O5负载量对催化剂制备的影响。结果表明制备的催化剂具有较多的中孔和微孔,催化剂中V2O5含量的增加,会降低催化剂的表面积;V2O5含量为2％的V2O5／TiO2催化剂样品比表面积最大,但是其活性非常低;V2O5含量为4％催化剂比表面积较大,NOx脱转化率高;V2O5的负载量小时,V2O5主要以等轴聚合的钒基型式（V3O7和V6O13）存在,这些钒基是催化剂的活性中心;当负载量超过6％,V2O5主要以结晶相存在,占据大量活性位,降低催化效果。     

Polyfunctionality of DeNOx catalysts in other pollutant abatement

The total oxidation of o-dichlorobenzene over differently loaded V₂O₅/TiO₂-based catalytic materials was studied. A series of vanadium-supported catalysts have been prepared, by incipient wetness impregnation, on different commercial supports (bare TiO₂, TiO₂/WO₃ and TiO₂/WO₃/SiO₂). The prepared materials were characterized by XRD, H₂-TPR, Raman spectroscopy and surface area measurements. All the catalysts exhibited high oxidation activity and the content of vanadium in the system was demonstrated to be important in controlling the catalyst activity and selectivity. Isolated and well-dispersed vanadium sites resulted beneficial for o-DCB conversion. Thus, in spite of the lower ability of SiO₂ to spread metal oxides the higher resistance to sintering of silica-containing materials also at high vanadium content, favors VO_x dispersion and leads to superior catalytic performance. Nevertheless, the presence of tungsten on the support and of high amount of vanadium also lead to the formation of partial oxidation products. In particular, dichloromaleic anhydride was formed and its production seems to be connected to the distribution of acidic sites.     

Regeneration of commercial SCR catalysts by washing and sulphation: effect of sulphate groups on the activity

The use of bio-fuels is becoming more important because of the environmental benefits associated with these fuels. Deactivation of SCR catalysts applied in bio-fuel plants is a major problem due to the high potassium content of bio-fuels and therefore, great potential lies in finding regeneration processes that can be used commercially. Exposing the catalyst surface to sulphate groups generated by SO₂ or H₂SO₄ is a promising way to rejuvenate the initial activity of the catalyst. The chemical stability of the sulphate groups was investigated by the interaction of the SCR reactant gases with the sulphate-promoted catalysts. Sulphate ions deposited on the surface of the TiO₂/V₂O₅/WO₃ were thermally stable at 420°C. The introduced sulphate groups were chemically unstable when the catalyst was treated with the SCR reactants at 400°C, but were chemically stable when the catalyst was exposed for the SCR reactants at 350°C. Sulphation after water treatment provided more chemically stable surface sulphate groups at 400°C. The presence of sulphate groups enhanced the number and the strength of the surface acid sites. The amount of ammonia bound to the Brønsted acid sites decreased with the potassium content of the catalyst while the amount of ammonia adsorbed on the Lewis acid sites was almost unaffected. Since potassium both retarded the redox potential of the surface vanadia species and decreased the amount of ammonia bound to the Brønsted acid sites, it is important to wash the strongly deactivated catalyst before sulphation.     

The influence of sulphate on the catalytic properties of V2O5-TiO2 and WO3-TiO2 in the reduction of nitric oxide with ammonia

The effect of sulphate on the catalytic properties of V₂O₅/TiO₂ and WO₃/TiO₂ in the selective reduction of NO with NH₃ has been investigated. For both catalytic systems, the presence of sulphate results in the enhancement of catalytic activity without reduction of selectivity to nitrogen. The rate of NO reduction depends on the sulphate content, which is affected by the original composition of titania, the method of catalyst preparation and the metal oxide loading.     

Impact of copper on the performance and sulfur tolerance of barium-based NOx storage-reduction catalysts

The presence of sulfur in automotive exhaust is known to be detrimental to lean-NO_x traps as SO₂ is oxidized to SO₃ that competes with NO₂ for sites on the trap and is difficult to remove. In this study the effect of adding Cu to the prototypical Pt–BaO/γ-Al₂O₃ 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 N₂ during regeneration. In the presence of SO₂, 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 SO₂ 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 SO₂ 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 SO₃ reacts with the BaO to form BaSO₄, and there is a partial loss of storage capacity.     

Potentialities and limitations of lean de-NOx catalysts in reducing automotive exhaust emissions

The performance of Pt–γ-Al₂O₃ and CuZSM5 de-NO_x catalysts was evaluated at the exhaust of a lean-burn gasoline engine. It is shown that durability for zeolite catalysts and N₂O formation for Pt catalysts are the main limitations which have to be still overcome to successfully apply the lean de-NO_x technology.     

NO–NH3 coadsorption on vanadia/titania catalysts: determination of the reduction degree of vanadium

Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) has been used to study NH₃ and NO adsorption over a 15% w/w vanadia/titania catalyst. NH₃ is adsorbed as coordinate NH₃ and NH₄⁺ species over the oxidised catalyst, leading to the reduction of the vanadia surface. At 300°C, adsorbed nitrosyls species are detected, suggesting that the oxidation of gaseous or adsorbed ammonia species takes place over the V=O sites. Coadsorption experiments show that NO is able to reoxidise about the 57% of the reduced V=O groups, resulting in N₂, according to a NO+V_□→1/2N₂+V=O reaction. On the other hand, NO is only adsorbed over vanadia reduced surfaces. The measure of the area of the 2ν(V=O) bands results in an estimate of the oxidation state of vanadium. From this estimate it can be concluded that nitrosyls species are adsorbed on the catalyst surface for vanadium atoms having an oxidation state ranging from +4 to +3.1.     

Other catalytic properties

The behavior of fresh and aged EUROCAT V₂O₅–WO₃/TiO₂ 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.     

Characterization and catalytic activity of Pd/V2O5/Al2O3 catalysts on benzene total oxidation

The role of vanadium oxide and palladium on the benzene oxidation reaction over Pd/V₂O₅/Al₂O₃ catalysts was investigated. The Pd/V₂O₅/Al₂O₃ catalysts were more active than V₂O₅/Al₂O₃ and Pd/Al₂O₃ catalysts. The increase of vanadium oxide content decreased the Pd dispersion and increased the benzene conversion. A strong Pd particle size effect on benzene oxidation reaction was observed. Although the catalysts containing high amount of V⁴⁺ species were more active, the Pd particle size effect was responsible for the higher activity.     

The effect of the nature of vanadium species on benzene total oxidation

The nature of the vanadium species present on V₂O₅/Al₂O₃ catalysts was investigated by using solid state ⁵¹V NMR, diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD) and temperature programmed reduction (TPR). ⁵¹V NMR and DRS analyses indicated the presence of V⁵⁺ in tetrahedral symmetry at low vanadium loading. A surface polymeric vanadium species and/or the bulk crystalline V₂O₅ were mainly observed at high vanadium loading as also detected by XRD. The positions of the absorption edges determined through the UV–VIS spectra allowed distinguishing between various tetrahedral symmetries. After TPR, the average oxidation state of vanadium depended on the vanadium content. The nature of vanadium species was related to the catalyst behavior on the benzene oxidation reaction. The catalysts containing high vanadium content were more active suggesting that a high amount of V⁴⁺ is responsible for the higher activity.