一种典型钒钛系SCR催化剂SO3生成特性研究

选择性催化还原（SCR）技术由于脱硝效率高、选择性好而被广泛应用于烟气氮氧化物排放控制;然而,目前广泛采用的钒钛系SCR脱硝催化剂会使烟气中SO₂氧化成SO₃,烟气中过高的SO₃对电厂安全运行会造成严重影响,也会对环境造成污染。以典型V₂O₅-WO₃/TiO₂催化剂为研究对象,系统研究了SCR脱硝过程中烟气流量、温度、O₂浓度、SO₂浓度等对催化剂表面SO₃生成特性的影响,并进一步对SO₃生成的反应动力学特性进行了分析。研究表明：催化剂表面SO₃生成反应中SO₂的反应级数为0.59,当O₂浓度大于3%时,O₂的反应级数为0,该反应的表观活化能为70.39 kJ/mol;实验条件下,烟气中SO₂浓度增加会使SO₃生成的反应速率提高;O₂浓度对催化剂表面SO₃生成影响并不显著;烟气温度对催化剂表面SO₃生成具有显著影响,高温会促进SO₃的生成。     

V2O5-MoO3/TiO2 催化剂的NOx选择性催化还原及SO2氧化活性

采用浸渍法以TiO₂为载体制备V₂O₅-MoO₃/TiO₂ 选择性催化还原催化剂,研究V₂O₅和MoO₃负载量对于催化剂选择性催化还原反应及SO₂氧化活性的影响,并考察氧含量、氨氮物质的量比和反应空速对3%V₂O₅-6%MoO₃/TiO₂催化剂选择性催化还原脱硝活性的影响。结果表明,随着催化剂中V₂O₅负载质量分数增加,V₂O₅-MoO₃/TiO₂ 催化剂的选择性催化还原活性和SO₂氧化活性均呈上升趋势。MoO₃的负载对催化剂的SO₂氧化活性有明显抑制作用。MoO₃负载质量分数超过9%,制备的催化剂既保持较高的低温选择性催化还原活性,又使选择性催化还原反应中的SO₂转化率小于1%。     

溶胶-凝胶原位合成宽活性温度V2O5/TiO2脱硝催化剂

利用溶胶-凝胶技术原位合成一系列不同V₂O₅担载量的V₂O₅/TiO₂催化剂,通过BET、XRD、NH₃-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,V₂O₅在TiO₂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V₂O₅/TiO₂催化剂进行活性评价,结果显示随着V₂O₅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）V₂O₅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。     

钒钛系脱硝催化剂抗硫酸氢铵中毒改进措施研究进展

NH₃选择性催化还原(SCR)技术具有较高的脱硝效率、优良的选择性和实用性,是当前燃煤电厂去除NO_x的主流方法。其中V₂O₅/TiO₂催化剂在中温段(300～450℃)具有较高的脱硝活性和抗硫性,被广泛应用。但是,烟气中的SO₃、NH₃和水蒸气会发生反应生成硫酸氢铵(ABS)和硫酸铵(AS),其中硫酸氢铵在低温条件下因毛细冷凝现象沉积在V₂O₅/TiO₂催化剂表面致其中毒,活性降低。为了改善低温条件下催化剂中毒问题,本文通过分析ABS在催化剂表面的生成机理、对催化剂的危害及催化剂抗ABS中毒改性研究进展,发现钒钛系脱硝催化剂抗ABS中毒改进措施主要集中在抑制硫酸氢铵生成、促进硫酸氢铵分解两方面。最后,总结了合理调控催化剂壁厚、孔径和隔离层等物理结构以及添加MoO₃、BaO、Nb₂O₅、Fe₂     

负载型钒钛脱硝催化剂酸化处理与性能

以商业TiO₂为载体,采用浸渍法制备了V₂O₅-WO₃-TiO₂/SO₄^2-催化剂,考察了硫酸酸化载体TiO₂的顺序及硫酸酸化量对氨气选择性催化还原NO活性的影响,采用XRD、BET、FT-IR、TG-DTA、TPD等手段对催化剂进行了表征。BET表征结果表明,随着酸化处理的用酸量增大,催化剂表面积降低;但FT-IR、TG结果表明,增大酸化处理用硫酸量提高了硫酸根与钨之间的电子交互作用。程序升温的活性测试结果表明,硫酸酸化处理对催化剂活性具有促进作用,结合NH₃-TPD表征证实了具有酸化处理程序制备的催化剂可以增强表面酸性位,从而使催化剂具有高活性。     

Superior resistance to alkali metal potassium of vanadium-based NH3-SCR catalyst promoted by the solid superacid SO42--TiO2

The significant decrease of acid sites caused by alkali metal poisoning is the major factor in the deactivation of commercial V₂O₅-WO₃/TiO₂NH₃-SCR catalysts.In this work,the solid superacid SO₄^2--TiO₂ modified by sulfate radicals,was selected as the catalyst support,which showed superior potassium resistance.The physicochemical properties and K-poisoning resistance of the V₂O₅-WO₃     

MnOx/WO3/TiO2低温选择性催化还原NOx机理的原位红外研究

浸渍法制备15% MnO_x/5% WO₃/TiO₂低温脱硝催化剂,利用原位傅里叶变换红外（in situ FT-IR）设计包括多种吸附反应以及不同预处理方式的微观暂态试验与微观稳态试验,研究其NH₃-SCR脱硝反应机理,并推测反应路径。结果表明,催化剂的NH₃-SCR反应主要以Eley-Rideal机理方式进行,仅在一定温度条件下可以看到Langmuir-Hinshclwood反应路径。催化剂表面Lewis酸位的NH₃吸附是还原剂的主要来源,Brønsted酸位吸附的NH₄⁺随温度上升参与反应的比例略有提高。NH₃的吸附活化是整个反应的控制步骤,吸附态NH₃更易与NO₂发生反应,NO与催化剂表面的相互作用明显弱于NO₂。NO会在催化剂表面氧化活性中心形成大量双齿配位型硝酸盐,阻碍NH₃的吸附和活化,O₂存在条件下促进NH₃-SCR反应进行,阻止NO在催化剂表面形成双齿硝酸盐。NO与NH₃在催化剂表面存在吸附竞争,NO的吸附作用强于NH₃,温度达到100℃后吸附的NH₃方可大量活化并与NO_x发生进一步反应。     

选择催化还原(SCR)反应机理研究进展

简述了NH₃和NO在催化剂表面吸附、转化活化和反应历程及H₂O和SO₂对以上反应行为的影响。分析表明,NH₃氧化脱氢进而与NO反应是决定NH₃反应性和最终产物的关键。NO以气态（Eley-Rideal机理）或硝基类物质等吸附态（Langmuir-Hinshelwood机理）形式参与选择催化还原（SCR）反应。提高催化剂酸性和氧化还原循环性能,利于NH₃和NO吸附和转化及相互间反应。高温时,H₂O影响轻微,而SO₂增强催化剂酸性,提高脱硝活性。低温时,H₂O和SO₂抑制NO吸附和转化活化,导致硫铵盐累积和活性位转变为硫酸盐使催化剂失活。因此,提高抗H₂O、抗SO₂性能是低温脱硝催化剂研发的重要方向。而发展在线升温等再生工艺以解决硝酸盐或含硫化合物导致的失活问题,对保障低温脱硝系统长期稳定运行具有重要意义。     

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。     

不锈钢板负载V2O5-WO3-TiO2催化剂制备及其催化还原脱硝性能

以溶胶-凝胶法制备的TiO₂粉末为载体,偏钒酸铵和水合钨酸铵溶液为浸渍液,采用分步浸渍法制备了V₂O₅-WO₃-TiO₂催化剂,以聚乙烯醇-硅溶胶为黏合剂,采用涂覆法将催化剂粘合于经硫酸和钛酸丁酯溶胶处理过的不锈钢板板材表面,获得不锈钢板负载的V₂O₅-WO₃-TiO₂催化剂。采用XRD、FT-IR和SEM等表征手段对催化剂进行表征,结果表明,V₂O₅-WO₃-TiO₂催化剂可均匀负载于不锈钢板表面。采用氨选择性催化还原氮氧化物法研究了催化剂的脱硝性能,结果表明,在空速8 000 L·（kg·h）^-1和反应温度360 ℃的条件下,NOx脱除率超过92%,且制备的催化剂具有良好的稳定性和耐硫性。     

SCR反应过程中NO/NH3在γ-Al2O3表面吸附特性

采用密度泛函理论（DFT）方法研究了NO和NH₃在完整和有缺陷的γ-Al₂O₃（110）表面吸附与SCR（选择催化还原）反应特性。研究表明,NO在完整的（110）表面的吸附作用较弱,而NH₃分子的吸附作用较强,NH₃分子在Al原子顶位可形成稳定吸附。反应路径研究结果表明完整的（110）表面上SCR反应的决速步为-NH₂NO基团的分解,反应的最大能垒为235.75 kJ·mol^-1。对于产生氧空穴的有缺陷（110）表面,NO和NH₃均可稳定吸附,NH₃在吸附过程中可直接裂解成NH₂和H。另外,SCR反应在有缺陷（110）表面的最大能垒明显较低,说明氧空穴的存在促进了SCR脱硝反应的进行。     

Research advance in mechanism of low-temperature SCR of NO on carbon materials

简述了不同反应物组合在碳材料表面的行为特征,单组分NO可以形成吸附态的NO₂、二聚体(NO)₂、—NO₂或吡啶类的化合物;O₂存在时NO被吸附态的氧氧化成NO₂;NO、O₂和NH₃同时存在时,反应发生在吸附态的NH₃和吸附态的NO₂之间。着重详述了活性碳纤维（activated carbon fibers,ACF）催化剂上的选择性催化还原（selective catalytic reduction,SCR）NO的机理为：低温时以NH₃为还原剂的SCR（NH₃-SCR）遵循Langmuir-Hinshelwood机理,较高温度时NH₃-SCR 遵循Eley-Rideal机理;分析指出了催化剂孔结构特征和表面化学官能团是ACF能低温选择性催化还原NO的主要影响因素。     

Mechanistic insights into the formation of N2O and N2 in NO reduction by NH3 over a polycrystalline platinum catalyst

The reaction pathways of N₂ and N₂O formation in the direct decomposition and reduction of NO by NH₃ were investigated over a polycrystalline Pt catalyst between 323 and 973 K by transient experiments using the temporal analysis of products (TAP-2) reactor. The interaction between nitric oxide and ammonia was studied in the sequential pulse mode applying ¹⁵NO. Differently labelled nitrogen and nitrous oxide molecules were detected. In both, direct NO decomposition and NH₃–NO interaction, N₂O formation was most marked between 573 and 673 K, whereas N₂ formation dominated at higher temperatures. An unusual interruption of nitrogen formation in the ¹⁵NO pulse at 473 K was caused by an inhibiting effect of adsorbed NO species. The detailed analysis of the product distribution at this temperature clearly indicates different reaction pathways leading to the product formation. Nitrogen formation occurs via recombination of nitrogen atoms formed by dissociation of nitric oxide or/and complete dehydrogenation of ammonia. N₂O is formed via recombination of adsorbed NO molecules. Additionally, both products are formed via interactions between adsorbed ammonia fragments and nitric oxide.     

Selective catalytic reduction of NO and NO2 at low temperatures

The fast SCR reaction using equimolar amounts of NO and NO₂ is a powerful means to enhance the NO_x conversion over a given SCR catalyst. NO₂ fractions in excess of 50% of total NO_x should be avoided because the reaction with NO₂ 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 NO_x 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/NO_x 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.     

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.     

燃煤烟气脱硝副产物硫酸氢铵/硫酸铵沉积与分解特性研究

搭建模拟空预器多段控温实验台,研究不同SO₃浓度、SO₃/NH₃比、不同温度下硫酸氢铵（ABS）与硫酸铵（AS）的生成、沉积与分解特性。在高温条件下发生的初始沉积部位,沉积物均为液态ABS,其失重特性与纯ABS基本一致。而在低温条件下,则根据SO₃/NH₃比的不同沉积物特性有所变化。SO₃/NH₃比为2∶1时,沉积物为H₂O、H₂SO₄与少量ABS的混合物,以密集液滴形态存在,呈多段分解特性;SO₃/NH₃比为1∶1与1∶2时,沉积物为分散性较强的干AS粉末,分解特性与纯AS基本一致。研究结果可为ABS防控提供指导。     

Competition between NO reduction and NO decomposition over reduced V–W–O catalysts

The SCR of NO and NO decomposition were investigated over a V–W–O/Ti(Sn)O₂ catalyst on a Cr–Al steel monolith. The conversions of NO and NH₃ over the reduced and oxidised catalysts were determined. The higher conversion of NO than of NH₃ 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: V₁₀O₃₁H₁₂ (V–V) and V₉WO₃₁H₁₂ (V–W) modelling (0 0 1) surfaces of vanadia and WO₃–V₂O₅ 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.