环境友好的选择性催化还原氮氧化物催化剂

富氧条件下的氮氧化物（NO。）选择性催化还原（SCR）是目前环境催化领域的研究热点,其核心问题是研发环境友好、高效稳定的SCR催化剂．目前,以NH3为还原剂的NH3-SCR已经大规模应用于固定源烟气脱硝和柴油车尾气净化,以碳氢化合物（Hc）为还原剂的HC-SCR也有望实际应用．针对NH3-SCR,本文以作者研究的铁钛复合氧化物催化剂、铈基氧化物催化剂以及国际上新兴的小孔分子筛催化剂为例,从催化剂结构、SCR反应机理、催化剂低温活性改进以及抗中毒性能等诸多方面对该领域的研究进展做了较为全面的论述．针对HC-SCR催化剂,本文在综述长链HC及柴油选择性还原NQ研究现状的基础上,结合作者在HC-SCR反应机理方面的研究成果,展望了实现柴油-SCR的发展方向．     

基于钛锡载体的SCR低温脱硝催化剂的表面性质研究

采用共沉淀法制备TiO₂-SnO₂固溶体,浸渍法负载CeO₂得到一系列xCeO₂/TiO₂-SnO₂负载型催化剂,在模拟NH₃选择性催化还原NO_x(NH₃-SCR)反应条件下考察催化剂低温脱硝活性。通过X射线衍射(XRD)、比表面积测定(BET)、程序升温还原(H₂-TPR)、程序升温脱附(NH₃-TPD)、高分辨率透射电子显微镜(HRTEM)、原位漫反射傅里叶变换红外光谱(in situ DRIFTS)等表征技术,研究了氧化铈负载后催化剂的微观结构、表面物种的存在状态、表面酸位等表面性质及NH₃吸附特性。结果表明,Ce:Ti物质的量比为0.1时,催化剂催化脱硝反应活性最高,同时具有较宽的温度窗口(250~300℃)和热稳定性;铈的过量负载会导致催化剂比表面积减小、活性窗口变窄,同时其氧化还原能力和NH₃吸附能力也减弱。NH₃-TPD结果显示,CeO₂的负载导致催化剂NH₃在弱酸及中等酸位的吸附显著增强,与催化剂NH₃-SCR最佳反应物温度降低有关。in situ DRIFTS表明,xCeO₂/TiO₂-SnO₂催化剂的Lewis酸位和Brønsted酸位强度均明显增强,同时,在1657~1666cm^-1处出现新的Brønsted酸位,参与SCR反应的主要物质是NH₄⁺分子。     

Preparation of apatite-type-silicate-supported precious metal catalysts for selective catalytic reduction of NOx

Apatite-type silicate supported precious metal catalysts were prepared and investigated for their catalytic activity in selective catalytic NO reduction. Single-phase La_9.33Si₆O₂₆ and La_8.33ASi₆O_25.5 (A=Ca, Sr, Ba) were obtained by a sol-gel method. Pd/La_9.33Si₆O₂₆ catalyst exhibited high activity for oxidation of C₃H₆, comparable to Pd/Al₂O₃ catalyst, although the specific surface area of La_9.33Si₆O₂₆ was lower than that of Al₂O₃. In addition, Pt/La_9.33Si₆O₂₆ catalyst exhibited higher activity for selective catalytic reduction of NO than Pt/Al₂O₃ catalyst. Substitution of Ba²⁺ for La³⁺ of La_9.33Si₆O₂₆ led to increased catalytic activity at low temperature.     

Low-temperature selective catalytic reduction of NOx with NH3 over zeolite catalysts: A review

The selective catalytic reduction (SCR) of NO_x by NH₃ is one of the most mature technologies for NO_x treatment. Catalysts are the main factors affecting denitrification efficiency. Zeolites as low-temperature NH₃-SCR catalysts have been extensively studied in the past few years. In this work, the mechanism of zeolites for NH₃-SCR reaction was reviewed and the denitrification performances of zeolite catalysts prepared by different methods were compared. The effects of sulfur and water poisoning on zeolite catalysts in NH₃-SCR reaction were also analyzed. Several ways to address the problems in low-temperature NH₃-SCR reaction were discussed. Hopefully, this review could provide a fundamental understanding of SCR reaction on zeolite catalysts and pave the way toward similar studies to realize its commercial applications.     

Metal oxide promoted TiO2 catalysts for photo-assisted selective catalytic reduction of NO with NH3

The photo-assisted selective catalytic reduction (SCR) of NO with NH₃ (Photo-SCR) was performed over TiO₂ modified by supporting 1 wt% of various transition metal (V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Ta or W) oxides aiming at the improvement of the photo-SCR activity. The addition of Nb, Mo or W oxide to TiO₂ was found to enhance the photo-SCR activity. We have reported that the amount of acid sites on TiO₂ is one of the key factors to the photo-SCR activity. The increase in the activity depends on the enhancement of acidity of catalyst by the addition of Nb, Mo or W oxide. In contrast, the addition of V, Cr, Mn, Fe, Co, Ni or Cu oxide to TiO₂ lowered the photo-SCR activity, although addition of metal cations also changed the acidity of TiO₂. We guess that the reduction of the activity was caused by two reasons; the first is that the sites newly formed on these transition metal oxides is not photoactive and the second is that TiO₂ supporting V, Cr, Mn, Fe, Co, Ni or Cu oxides had low stabilities under the reaction conditions, i.e., the chemical state of the cations changed during the reaction. Therefore, we concluded that the increase in the acid sites that are active sites for photo-SCR and the stability of the catalysts are important for the photo-SCR.     

CuSO_4/TiO_2催化剂在氨选择性催化还原氮氧化物反应中的性能(英文)

氮氧化物(NO_x)是主要的大气污染物之一.氨气选择性催化还原法(NH_3-SCR)是目前去除固定源排放的氮氧化物的最有效方法,被广泛用于燃煤或者生物质的火电厂中.催化剂是NH_3-SCR法的核心,其中V_2O_5-WO_3/TiO_2催化剂是主要的商业SCR催化剂;但是V_2O_5有毒,对环境的影响很大;另外,该催化剂具有较高的SO_2氧化性能.因而研究者一直在探索新型的SCR催化剂.SO_2是燃煤电厂烟气中的典型气体之一,所以抗硫性能是催化剂的一个重要指标.在SCR反应条件下,SO_2和O_2容易与氧化物催化剂发生反应生成稳定性较高的硫酸盐,覆盖在催化剂表面从而引起催化剂失活.但已有研究发现,硫化会提高K中毒后的V_2O_5-WO_3/TiO_2催化剂的活性.并且,短时间的硫化可以明显提高CuO/Al_2O_3的NH_3-SCR活性.硫酸盐催化剂或许具有较低毒性和较高抗硫性能,应该是一种有前景的SCR催化剂.本文以商业纳米TiO_2为载体,采用湿式浸渍法制备了一系列的CuSO_4/TiO_2催化剂.在自制的活性评价装置上测试了样品的NH_3-SCR活性并且在340℃下连续24 h测试了SO_2、水蒸气及两者共同作用对催化剂活性的影响.使用N_2等温吸附-脱附、X射线衍射(XRD)、X射线光电子能谱(XPS)、H_2程序升温还原(H2-TPR)和NH_3程序升温脱附(NH_3-TPD)对催化剂进行了表征.另外,采用原位红外漫反射光谱研究了CuSO_4/TiO_2催化剂上的NH_3-SCR反应过程.N_2等温吸附-脱附结果表明,负载的CuSO_4没有明显改变载体的孔结构.而XRD结果仅显示锐钛矿TiO_2的衍射峰,说明CuSO_4在载体上有较好的分散度或者CuSO_4的含量低于检测限.XPS结果显示,催化剂中的铜主要以Cu~(2+)形式存在,硫主要以SO_4~(2-)形式存在,而氧主要以晶格氧和吸附氧两种形式存在,并且CuSO_4的存在会增加催化剂中吸附氧的含量.H_2-TPR结果表明,随着CuSO_4含量的增加,催化剂的氧化还原能力逐渐增强.NH_3-TPD结果表明,催化剂表面的酸性位数目随着样品中CuSO_4含量的增加而增加.纯TiO_2的NH_3-SCR活性很差,当温度从300℃增加到450℃时,最高NO_x转化率仅为32.7%.但当CuSO_4负载到TiO_2上以后,催化剂活性明显提高.在反应温度高于340℃时,CuSO_4/TiO_2催化剂的NO_x转化率在94%以上,与商业V_2O_5-WO_3/TiO_2催化剂相当,并且其N_2O生成量低于商业催化剂.不过,当温度低于340℃时,CuSO_4/TiO_2催化剂的NO_x转化率明显低于商业催化剂,说明CuSO_4/TiO_2催化剂的活性仍有待改善.连续24 h测试了SO_2、水蒸汽及两者的共同作用对CuSO_4/TiO_2催化剂活性的影响.结果显示,单独的水蒸气会导致活性轻微下降,但SO_2以及两者共同存在时对催化剂的活性基本没有影响.CuSO_4/TiO_2催化剂的NH_3吸附红外光谱表明,催化剂上存在Lewis和Bronsted两种酸性位,但Bronsted酸性位上的NH_4~+稳定性较差,280℃时即基本消失.在高温时,NH_3主要吸附在Lewis酸性位上且CuSO_4/TiO_2催化剂对NO_x的吸附能力较差,红外光谱未检测到NO_x的吸附峰.380℃下,当NO和O_2通入预吸附NH_3的催化剂样品时,属于Lewis酸性位上NH_3的红外峰明显下降,说明Lewis酸性位上吸附的NH_3参与了反应.CuSO_4/TiO_2显示出高的抗硫抗水性能和比较好的NH_3-SCR活性,应该是一种有应用前景的SCR催化剂.CuSO_4可以增加催化剂的酸性位数目和吸附氧量.根据原位红外漫反射结果,CuSO_4/TiO_2上的SCR反应遵循Eley-Rideal机理.气相的NO与吸附在Lewis酸性位上的NH_3反应生成N_2和H_2O或许是主要的反应途径,并且吸附氧可能会促进这个过程.     

The structure and catalytic activity of anatase and rutile titania supported manganese oxide catalysts for selective catalytic reduction of NO by NH3

The structure and catalytic properties of anatase and rutile supported manganese oxide catalysts prepared by impregnation method have been studied by using X-ray diffraction (XRD), laser Raman spectroscopy (LRS), X-ray photoelectron spectroscopy (XPS), H(2) temperature-programmed reduction (H(2)-TPR) and BET surface area measurements combined with activity testing of selective catalytic reduction (SCR) of NO by NH(3). It has been shown that the manganese oxide loadings on the two TiO(2) supports exert great influences on the SCR activity. For the rutile supported manganese oxide catalysts, increasing manganese oxide loading leads to the increase of reducibility of dispersed manganese oxide species and the rate constant k, which reaches a maximum around 9.6 × 10(-6) mol g(Mn)(-1) s(-1) at 0.5 mmol Mn per 100 m(2) TiO(2). When the manganese oxide loading is beyond this value, the existence of amorphous MnO(x) multiple layers will certainly reduce the ratio of manganese oxide species exposed on the surface and the reducibility of dispersed manganese oxide species, resulting in the rapid decrease of rate constant k. The LRS and XPS results have revealed that for the anatase supported manganese oxide catalysts manganese oxide species exist in Mn(+4) as a major species with Mn(+3) species and partially undecomposed Mn-nitrate as the minor species. Under the SCR reaction conditions, Mn(+3) species on anatase are oxidized to Mn(+4) species, inserting in the surface of anatase and promoting the anatase-to-rutile transformation in the surface layers of the anatase support. Since these Mn(4+) cations are actually dispersed on the support with a rutile shell-anatase core structure and its concentration is very near to that of MnO(x)/TiO(2) (R) catalyst, the relation between the rate constant k and the MnO(x) loading on the anatase support is similar to that on the rutile support, and that the rate constant k values for anatase and rutile supported manganese oxide catalysts are very close at the same MnO(x) loading.     

Fe-Mn/Al2O3催化剂低温 NH3选择性催化还原 NO的性能

本文制备了一系列 Fe-Mn/Al2O3催化剂,并在固定床上考察了其 NH3低温选择性催化还原 NO的性能.首先考察了不同 Fe负载量制备的催化剂的脱硝性能,优选出最佳的 Fe负载量;在此基础上,研究了 Mn负载量对催化剂脱硝效率的影响;最后,对优选催化剂的抗 H2O和抗 SO2性能进行了实验研究;同时,对催化剂由于 SO2所造成的失活机制进行了考察.采用 N2吸附-脱附、X射线衍射、透射电镜、能量弥散 X射线谱、程序升温还原、程序升温脱附、X射线光电子能谱、热重和傅里叶变换红外光谱等方法对催化剂进行了表征.结果表明,最佳的 Fe和 Mn负载量均为8%,所制的8Fe-8Mn/Al2O3催化剂在150°C的脱硝效率可达近99%;同时,在整个低温测试区间(90–210°C)的脱硝效率均超过了92.6%. Fe在催化剂表面主要以 Fe3+形态存在,而 Mn主要包括 Mn4+和 Mn3+; Mn的添加提高了 Fe在催化剂表面的积累,促进了催化剂比表面积增大和活性物种分散,改善了催化剂氧化还原性能和对 NH3的吸附能力.催化剂的高活性主要是由于其具有较大的比表面积、高度分散的活性物种、增加的还原特性和表面酸性、较低的结合能、较高的 Mn4+/Mn3+和增强的表面吸附氧.此外,8Fe-8Mn/Al2O3的催化性能受 H2O和 SO2影响较小,抗 H2O和 SO2能力较强.同时,反应温度对催化剂的抗硫性有重要影响,在较低的反应温度下,催化剂抗硫性更好; SO2造成催化剂活性降低主要是由于催化剂表面硫酸盐物种的生成.一方面,表面硫酸铵盐的生成造成催化剂孔道堵塞和比表面积降低,减少了反应中的气固接触从而导致活性降低;另一方面,催化剂表面的活性物种被硫酸化,造成反应中的有效活性位减少,从而降低了催化剂活性.     

Cr改性钒基催化剂对NH3低温选择性催化还原NOx的影响

火电厂和机动车辆等的NOx排放量与日俱增, NOx的治理已成为环境保护的重要组成部分.以NH3作为还原剂的选择性催化还原(SCR)技术是目前消除NOx最为高效的方法之一.该反应最为典型的催化剂是V2O5–WO3(MoO3)/TiO2,催化活性窗口为250–400 oC.国外通常将SCR系统置于省煤器之后,此时烟气温度在300 oC以上,催化剂能保持较高的活性,但易受到烟气中高浓度烟尘、SO2和碱金属等的影响,寿命相对较短.此外,高温工艺中副产物硫酸铵的堵塞也是一个不可忽视的问题.因此,将SCR脱硝装置设在脱硫除尘之后成为一种优选技术之一,但烟气温度会降至250 oC以下,而常规的钒基催化剂不能满足低温活性要求.通过添加助剂或改变载体可改善钒基催化剂的低温活性,同时保持其高效的抗硫能力.
　　本文以Cr和V为活性组分, TiO2为载体,采用浸渍法制备了铬钒钛(Cr–VOx/TiO2)系列催化剂,考察了它们的低温脱硝活性和抗水抗硫性,并通过N2吸附-脱附、X射线衍射、NH3程序升温脱附(NH3-TPD)、H2程序升温还原(H2-TPR)和X射线光电子能谱等手段对催化剂进行了表征,分析了Cr–V催化剂的作用机制.
　　结果显示,当n(Cr)：n(V)为0.2：0.8,活性组分负载量为10 wt%时, Cr–VOx/TiO2催化剂表现出最佳的低温催化活性;当反应温度为160°C时, NOx转化率达到90%以上,明显优于其他催化剂,同时活性窗口(160–300°C)得到拓宽. NH3-TPD结果表明, VOx/TiO2催化剂表面呈中弱酸性,随着Cr的添加,钒基催化剂的NH3脱附峰向高温拓宽,说明其表面强酸量有所增加, Cr0.2–V0.8/TiO2在160–300°C范围内均出现了NH3的脱附峰,此时催化剂表面弱酸量最大.当n(Cr)：n(V)大于0.2：0.8时,催化剂表面出现强酸位,这种强酸位不利于NH3脱附,从而不利于SCR反应的进行.因此适量Cr的添加有助于提高钒基催化剂表面弱酸及中性酸量. H2-TPR结果发现,助剂Cr的添加使得催化剂表面氧量增加,这可能是由于Cr的添加形成了较多的氧空穴和未饱和的化学键.催化剂表面化学吸附氧是氧化还原反应最活跃的氧物种,在SCR反应中,表面氧可作为NO的吸附介质参与到催化剂表面反应中,可有效提高SCR反应速率.通过考察活性组分负载量对催化剂活性的影响,发现不同负载量催化剂的催化活性依次为10 wt%>20 wt%>50 wt%>5 wt%. XPS分析发现,当负载量为10 wt%时,催化剂表面非计量的钒离子和化学性质活跃的自由电子最多,因此表现出最佳的SCR活性;当负载量过高时,大量氧化物堆积烧结形成V2O5和Cr2V4O13,覆盖了钒离子活性位点,降低了催化剂脱硝效率.
　　催化剂在220°C表现出良好的抗硫性,在通入100 ppm SO212 h后NOx的转化率仍可达99%以上,并未出现硫中毒现象.同时该催化剂也表现出较好的抗水性,在通入10 vol%水蒸气12 h后, NOx转化率仍能达85%以上.     

Influence of ceria modification on the properties of TiO2-ZrO2 supported V2O5 catalysts for selective catalytic reduction of NO by NH3

TiO(2)-ZrO(2) (hereafter denoted as Ti-Zr) supported V(2)O(5) catalysts with different loadings of CeO(2) were synthesized, and their physicochemical properties were characterized by Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), in situ Fourier transform infrared spectroscopy (in situ FT-IR) and temperature-programmed reduction (TPR). Their catalytic activities toward the NO(x) reduction reaction with NH(3) were tested. We found that with the addition of CeO(2), more NO was removed in a wide temperature range of 220-500 °C. As the CeO(2) content was increased from 10% to 20% (i.e., the molar ratio of Ce to Ti-Zr), NO conversion increased significantly; after that, increasing CeO(2) content, however, decreased NO conversion. In particular, the addition of CeO(2) to V(2)O(5)/Ti-Zr suppressed the coke deposition and rendered a stable and high catalytic activity. The characterization results indicated that: (1) the deposited vanadium and cerium oxides were highly dispersed over the Ti-Zr support, and in addition to ZrV(2)O(7), a common binary compound observed in V(2)O(5)/Ti-Zr, CeVO(4) and Ce(3)ZrO(8) was formed upon increasing CeO(2) content; (2) the introduction of CeO(2) to V(2)O(5)/Ti-Zr sample promoted the redox ability of the resulting catalysts; and (3) the Ce-containing catalysts possessed the greater amount of surface acidic and active intermediate.     

In situ FT-IR studies on the mechanism of selective catalytic reduction of NOx by propene over SnO2/Al2O3 catalyst

The mechanism of the selective catalytic reduction (SCR) of NOx by propene over SnO2/Al2O3 catalyst in the presence of oxygen has been investigated using in situ Fourier transform infrared (FT-IR) spectroscopy. In situ IR measurements indicate that acetate and formate, which are the derivatives of the partial oxidation of propene, play a crucial role in the formation of NCO by reacting with the reactive monodentate nitrate species. The resulting NCO species subsequently reacts with NOx to form N2. The presence of oxygen substantially contributes to the partial oxidation of propene and thus shows a promoting effect for the NOx reduction.     

Structural investigation of the promotional effect of hydrogen during the selective catalytic reduction of NOx with hydrocarbons over Ag/Al2O3 catalysts

In situ EXAFS has been used to examine the hydrogen effect on the selective catalytic reduction of NOx over silver/alumina catalysts. For all SCR conditions used, with or without co-reductant (H2 or CO), the catalyst structure remained the same. Significant changes in the catalyst were only found under reducing conditions. The enhanced activity found in the presence of hydrogen is thought to be due to a chemical effect and not the result of a change in the structure of the active site.     

Promotional effect of acidic oxide on catalytic activity and N2 selectivity over CeO2 for selective catalytic reduction of NOx by NH3

The CeO₂, CeO₂‐ZrO₂ and CeO₂‐WO₃ catalysts were prepared by hydrothermal method and used to the selective catalytic reduction of NO_x by NH₃. The addition of ZrO₂ or WO₃ into CeO₂ was favorable for pore structure, and then improved the number of active sites. Besides, the introduction of ZrO₂ into CeO₂ could improve the Lewis acid sites while WO₃ could contribute to the generation of Brønsted and Lewis acid sites, which could improve the catalytic performance and N₂ selectivity. The CeO₂‐WO₃ catalyst exhibited optimal catalytic activity with above 90% NO_x conversion performance at 220–425 °C and approximately 100% N₂ selectivity at 150–425 °C.     

Fe-Ag/Al_2O_3催化丙烯还原NO的实验研究

以蜂窝状陶瓷为载体,采用溶胶凝胶法和浸渍法制备了不同Fe/Ag负载量的Fe-Ag/Al_2O_3催化剂。以C_3H_6为还原剂,在模拟烟气条件下和200-700℃范围内,程序控温的陶瓷管流动反应器上进行了催化还原NO的性能评估。结果表明,7.2Fe/1.9Ag/20Al_2O_3/CM在500和550℃时催化C_3H_6还原NO的脱硝效率分别超过90%和达到100%。铁离子能有效地提高Ag/20Al_2O_3/CM催化剂抵抗烟气中的SO_2和H_2O的能力。结果表明,当烟气中含有体积分数为0.02%的SO2和8%的H_2O时,在500℃时7.2Fe/1.9Ag/20Al_2O_3/CM催化C_3H_6还原NO的脱硝效率不受影响,在6 h的连续实验中保持90%的脱硝效率而没有下降。而未经铁离子修饰的2Ag/20Al_2O_3/CM的催化活性则受烟气中的SO2和H_2O影响很大,0.02%的SO2和8%的H_2O分别使2Ag/20Al_2O_3/CM在500℃时催化C_3H_6还原NO的脱硝效率迅速从70%分别下降至46%和25%。XRD和SEM表征结果表明,经铁离子修饰后的7.2Fe/1.9Ag/20Al_2O_3/CM催化剂中,形成了AgFeO_2以及Fe~(3+),催化剂表面变得疏松多孔,形成以Fe_3O_4为主的针状和片状晶体。H_2-TPR结果表明,7.2Fe/1.9Ag/20Al_2O_3/CM比Ag/20Al_2O_3/CM在更宽的温度范围内具有更好的还原特性。吡啶吸附红外光谱(Py-FTIR)实验结果显示,Fe增加了催化剂表面的Lewis酸性位。     

V2O5-CeO2/TiO2-ZrO2催化剂表征及NH3还原NOx性能

商业选择性催化还原(SCR)催化剂V2O5-WO3(MoO3)/TiO2存在反应温度窗口窄(300–400 oC)和SO3转化率高等缺点,同时占催化剂总质量80%以上的载体TiO2比表面积小,热稳定性差.已有研究发现TiO2-ZrO2固溶体具有较大的比表面积和较强的表面酸性, TiO2与ZrO2的摩尔比为1：1时其比表面积达到最大. CeO2作为SCR催化剂的组成部分,由于其优良的储氧和放氧能力受到广泛关注.研究表明, CeO2-CuO, Ce/Ti-Si-Al和Mo2O3(Co2O3)/Ce-Zr等催化剂具有优良的SCR脱硝活性,同时对V2O5-WO3/TiO2催化剂进行CeO2改性,可提高催化剂的抗SO2中毒能力.实际烟气组分中同时存在SO2和H2O,必定会导致催化剂硫酸盐中毒,而目前对含Ce催化剂的硫酸盐中毒情况研究较少,因此开发新型高效脱硝催化剂十分必要.前期我们研究了xCeO2-3%V2O5/TiO2-ZrO2催化剂,发现CeO2可以显著拓宽脱硝温度窗口,同时增强催化剂酸性位点,但是V2O5含量较高时对环境及人体健康均有较大危害.本文采用共沉淀法制备摩尔比为1：1的TiO2-ZrO2固溶体,用浸渍法负载不同摩尔比的CeO2和1%的V2O5,得到一系列V-xCe/Ti-Zr催化剂,结合X射线衍射(XRD)、比表面积测试(BET)、高分辨透射电镜(HRTEM)、程序升温还原(H2-TPR)、原位漫反射红外光谱(in situ DRIFTS)和程序升温脱附(NH3-TPD)等手段分析催化剂的晶相、活性物质分散程度、氧化还原性质及表面酸性,在200–450 oC范围内考察Ce掺杂催化剂选择性催化还原NOx的脱硝活性,并在250 oC测试催化剂在NH3+NO+O2+SO2+H2O气氛中的脱硝活性,研究催化剂抗硫酸盐中毒能力.研究发现,CeO2掺杂可以拓宽脱硝反应活性窗口, V-0.2Ce/Ti-Zr (摩尔比Ce：Ti =0.2)表现出最优的脱硝性能,在250–350oC范围内脱硝效率均在92%以上,同时与前期研究结果对比发现CeO2含量较高时会导致高温段NOx转化率下降. XRD和HRTEM结果表明,ZrO2的添加可以显著降低载体TiO2的结晶度,复合氧化物TiO2-ZrO2呈无定形态, CeO2高度分散于载体之上,并且催化剂以单晶形式存在. H2-TPR结果表明,CeO2能显著提高催化剂的还原能力,主要的还原反应发生在CeO2的α(200–430oC)和β(430–600 oC)还原峰上,总体而言, V-0.2Ce/Ti-Zr表现出最大的氢气消耗量,即其还原性最强.低V2O5负载有利于较低温度SCR反应, V-0.3Ce/Ti-Zr的钒氧化物还原峰强度最大,其次是V-0.2Ce/Ti-Zr. NH3-TPD测试发现V2O5/TiO2主要存在中强酸及强酸,而V2O5/TiO2-ZrO2主要是弱酸, CeO2负载后随着其含量提高,弱酸强度增加.结合氨气原位漫反射红外光谱发现, CeO2可以增加催化剂Br?nsted和Lewis酸位数量,同时出现反应中间物–NH2, V2O5的负载量较高会抑制1660 cm–1处Br?nsted酸吸收峰的出现. BET结果发现, TiO2-ZrO2和V2O5/Ti-ZrO2比表面积分别可达255.73和143.77 m2/g, V2O5/TiO2仅为66.1 m2/g,表明ZrO2的添加可以显著增大催化剂比表面积,进而有利于SCR反应进行,沉积的氧化物进入载体孔道导致催化剂比表面积降低. V2O5-xCeO2/TiO2-ZrO2表现出较强的抗SO2中毒能力,但是在H2O存在条件下脱硝活性较差,可能是生成的硫酸铵盐及亚硫酸盐阻塞催化剂孔道所致. SO2和H2O停止通入后, V2O5-0.3CeO2/TiO2-ZrO2活性恢复至原有水平, V2O5-0.2CeO2/TiO2-ZrO2恢复至最初的84%.对中毒催化剂进行不同反应温度下的活性测试,发现V2O5-0.2CeO2/TiO2-ZrO2在中温段反应活性显著降低,可能是由于Ce(SO4)2的形成所致,由于V2O5-0.3CeO2/TiO2-ZrO2的Ce含量较高,其在此温度范围内活性依旧较高.两者在高温段NOx转化率均较高,推测是V2O5开始发挥活性组分作用的缘故.     

钴铝复合氧化物负载金催化剂的制备及催化分解N2O

分别用离子交换法和共沉淀法制备了钴铝复合氧化物负载金催化剂,用于催化分解N₂O。离子交换法制备的催化剂活性优于共沉淀法制备的催化剂。对于离子交换法制备的催化剂,考察了金负载量、HAuCl₄溶液的预处理方式、焙烧温度对催化剂活性的影响,用BET、XRD、H₂-TPR等技术对催化剂进行了表征,优化出了催化剂的最佳制备参数：金负载量1.1%、预调节HAuCl₄溶液的pH值至9.0、300℃焙烧。在钴铝氧化物中加入适量的助剂Na,提高了Co³⁺的还原性和催化剂的低温活性,在此基础上制备的1.5%Na/1.1%Au/Co-Al催化剂的低温活性优于1.1%Au/Co-Al和1.5%Na/Co-Al。     

Novel non-hydrolytic synthesis of a V2O5-TiO2 xerogel for the selective catalytic reduction of NOx by ammonia

A vanadia-titania mesoporous xerogel (10.5 wt% V(2)O(5)) was prepared from chloride precursors using a one-step non-hydrolytic sol-gel route and subsequent drying at ambient pressure; after calcination at 773 K for 5 h no crystalline V(2)O(5) was detected and the resulting mixed oxide exhibited remarkable activity in the selective reduction of NO with NH(3).     

Cl–改性对Ag/Al2O3催化剂结构及其催化C3H6-SCR和H2/C3H6-SCR反应性能的影响

以烯烃为还原剂的NOx选择性催化还原(HC-SCR)是重要的环境催化反应之一.Ag/Al2O3催化剂(SA)因在HC-SCR反应中表现高活性、高N2选择性及中等H2O和SO2耐受性等优点,而被广泛研究.SA催化剂中存在不同的Ag物种,包括孤立Ag+离子,带部分正电荷Agnδ+团簇和金属态Agn0团簇.文献研究结果表明,SA催化剂中Agnδ+团簇是催化HC-SCR反应的活性Ag物种,而Ag物种类型与Ag的负载量密切相关.因此文献中研究SA催化HC-SCR反应的结构-性能关系主要是通过改变Ag负载量来开展的,最优Ag负载量约为1％～2％.本文以Cl–改性的γ-Al2O3作为载体,采用传统的浸渍法制备了Ag/Al2O3-Cl催化剂(SA-Cl),通过XRD、TEM、H2-TPR、UV-Vis DRS以及XPS对催化剂进行了结构表征,并结合C3H6-SCR和H2/C3H6-SCR活性测试,建立催化剂结构-催化性能关系;同时利用原位傅里叶变换红外光谱(DRIFTS)和在线同步辐射单光子电离质谱(SVUV-PIMS)研究了SA催化HC-SCR的反应机理.结构表征结果表明,在SA催化剂中,Ag负载量的提高主要是增加了Agn0物种,而在SA-Cl催化剂中,Ag负载量的提高主要是增加了Agnδ+物种,因此Cl?改性能促进SA催化剂中Agnδ+物种的形成.活性测试结果表明,在相同Ag负载量下,SA-Cl催化剂表现出比SA催化剂更好的HC-SCR催化性能.Cl?改性对SA催化剂中Ag物种的调控作用和HC-SCR催化性能的促进作用随Ag负载量的增加变得更为明显.原位DRIFTS结果表明,γ-Al2O3载体(Al位点)是NO氧化形成硝酸盐物种的主要活性位点;Agnδ+物种催化丙烯适度氧化主要生成乙酸盐类物种,可以还原表面硝酸盐物种;而Agn0催化丙烯过度氧化主要生成羧酸盐类物种,进而生成CO2,不能还原表面硝酸盐物种.由此可见,Agnδ+是催化HC-SCR反应的活性Ag物种,而Agn0是催化烃类完全氧化反应的活性Ag物种;Cl–改性能有效促进Agnδ+的形成,进而提高HC-SCR催化反应活性.在线SVUV-PIMS结果检测到H2/C3H6-SCR反应中存在气态中间物种丙烯腈(CH2=CHCN).–CN和–NCO物种被认为是HC-SCR反应的关键中间物种,能够直接与气相NO+O2反应生成N2.因此,CH2=CHCN的存在说明HC-SCR反应涉及到气相反应机理.基于SA和SA-Cl催化剂,进一步研究了H2对C3H6-SCR低温活性的促进作用.结果表明,H2的促进作用是通过作用于Agnδ+物种,而不是通过Agn0物种来实现的;H2的引入有利于低温下强吸附硝酸盐物种的脱附或分解以及中间体向–NCO和–CN物种的转化,从而提高HC-SCR低温催化活性.综上,基于Cl–改性的Ag/Al2O3-Cl催化剂,本文成功证实了Agnδ+物种是催化HC-SCR反应的活性Ag物种,并结合原位DRIFTS在线SVUV-PIMS谱分别鉴定了催化反应表面中间物种和气相中间物种.这些结果加深了对SA催化HC-SCR反应构-效关系和反应机理的基础理解.     

Fe-Beta分子筛催化剂在选择催化还原氮氧化物反应中的活性:Fe含量的影响

NOx是主要的大气污染物之一,对环境和人体健康具有极大的危害.其主要来源之一是柴油机尾气排放,V2O5-WO3/TiO2催化剂是现阶段大规模商用的SCR催化剂,但V2O5-WO3/TiO2催化剂相对较窄的温度窗口和V2O5的生物毒性使得迫切需要新型的环境友好的高效SCR催化剂,其中分子筛因其特殊的孔道结构和催化性能受到广泛的关注.用于SCR过程的分子筛主要包括ZSM-5,Beta,MOR,SAPO-34和SSZ-13等,通常采用Cu,Fe,Mn和Co等过渡金属对其进行改性,通过调变分子筛的表面酸性和氧化还原性能,提高催化剂的SCR活性.Beta分子筛具有三维12元环孔道结构,相对其它分子筛具有较好的水热稳定性,而且制备工艺成熟,价格低廉,因此该类分子筛催化剂在SCR过程中具有很好的应用前景.我们采用离子交换法制备了系列Fe-Beta催化剂,发现将相同质量硝酸铁溶解在不同体积去离子水中,配制成不同浓度的硝酸铁溶液后与分子筛进行离子交换反应,制备得到的Fe-Beta催化剂中Fe的含量和NH3-SCR催化活性均存在显著差别.在此基础上,我们固定硝酸铁溶液浓度(0.02mol/L),通过增加溶液的体积,分别制备了Fe含量为(2.6,6.3和9)wt%的Fe-Beta分子筛.结果表明,Fe负载量为6.3 wt%时,Fe-Beta催化剂表现出最好的催化活性,NOx转化率大于80%的温度窗口为202-616℃.虽然三个催化剂在比表面积,孔径和Fe的价态上没有明显的差别,但Fe含量为6.3 wt%的催化剂在保持相对较高的Fe负载量的同时具有更多的孤立Fe3+物种,同时具有较好的NH3和NO吸附性能以及NO氧化能力,这些特性使得该催化剂相对于其它两个催化剂表现出更高的NH3-SCR催化活性.当Fe含量增加到9 wt%时,催化剂中FexOy纳米颗粒的含量大幅增加,使得NH3非选择性氧化能力加强,从而导致高温NH3-SCR反应活性大幅下降.     

Redox couples in the selective catalytic reduction of NOx with hydrocarbons over Co-ZSM-5 and Ni-ZSM-5 catalysts: an FT-IR study

On the basis of IR spectroscopy studies it is concluded that the redox couples Co(2+)/Co(+) and Ni(2+)/Ni(+) operate in the selective catalytic reduction of nitrogen oxides with hydrocarbons over Co-ZSM-5 and Ni-ZSM-5.