【作者】 张志翔；

【导师】 上官文峰；

【作者基本信息】 上海交通大学 ， 环境工程， 2010， 博士

【摘要】 随着汽车保有量的持续上升,机动车排放尾气成为城市环境空气污染的主要污染源之一,引起越来越多的关注。传统的三效催化剂（TWC）对稀燃发动机、柴油发动机等排放的富氧、低温尾气不再有效;此外,城市公路隧道、地下停车场等一些特殊场所,持续地向周围大气环境排放低浓度、富氧、低温的废气,是一类容易被忽视却又具较大危害的尾气排放污染源。虽然,通过引入具有毒害性的NH3作为还原剂,在低温下利用选择性催化还原反应（SCR）可去除NOx,但该技术不适合用于机动车移动源以及隧道、停车场等固定源;采用贵金属负载氧化物、分子筛等催化剂,以与NOx共存的碳氢化合物（HC）作为还原剂,不仅可以实现污染物的同时去除,而且具有良好的低温催化活性。本文通过实验室配气模拟富氧、低污染物浓度的机动车排放废气,采用湿式浸渍法制备了Pt负载金属氧化物MOx（M= Al、Ce、Ti、W、Zr）及不同贵金属（Pt、Pd、Rh、Ru）负载TiO₂、Pt负载分子筛（MCM-41、ZSM-5）等3个系列催化剂,通过XRD、TEM、BET、in situ DRIFTS等分析表征方法及低温吸附（ADS）、程序升温脱附（TPD）、程序升温氧化反应（TPO）、SCR等催化剂活性评价方法,分别考察并比较了催化剂低温选择性催化C3H6还原NOx的催化活性,揭示了富氧条件下Pt/TiO₂、Pt/ZSM-5催化剂的低温C3H6-SCR反应的可能机理。本文研究的主要内容及结果如下:1. C3H6在Pt/MOx（M=Al、Ce、Ti、W、Zr）及Pt、Pd、Rh、Ru负载TiO₂等氧化物催化剂上的吸附能力均很微弱,Pt/TiO₂与TiO₂对C3H6的吸脱附实验结果表明,C3H6在催化剂样品及其载体上仅发生简单的物理吸附过程,影响催化剂对C3H6吸附的主要因素是载体,而不是对载体的改性（Pt负载）;另一方面,Pt/ZSM-5催化剂样品具有比载体（ZSM-5）更强的C3H6吸附能力,主要是由于Pt负载过程对载体ZSM-5微孔结构的轻微改变而引起的。2.催化剂及其载体的NO-ADS、NO-TPD等实验结果表明,Pt负载显著提高了MOx（M=Al、Ce、Ti、W、Zr）及MCM-41、ZSM-5等载体对NO的吸附能力,也相应地增加了NO-TPD的脱附量。在贵金属负载催化剂的NO-TPD低温段出现的NO脱附峰,主要来自于载体对NO的物理吸附以及NO在贵金属表面的解离吸附,而高温段脱附的NO和NO₂来自于催化剂表面硝酸盐的分解。催化剂对NO吸附能力及其NO-TPD脱附量大小,是由载体的BET比表面积、酸碱度等物理化学特性、以及对载体的改性（如Pt负载）等多方面因素共同决定的。3.反应物C₃H₆和NO的TPO的实验结果表明,Pt/MOx（M=Al、Ce、Ti、W、Zr）及Pt/MCM-41、Pt/ZSM-5等催化剂均显著地提高了载体对C₃H₆-TPO的低温催化氧化活性,同时也提高了载体对NO的低温催化氧化活性,催化剂对C₃H₆具有良好的选择性氧化能力,C₃H₆在TPO、SCR反应中均被完全氧化成CO₂。催化剂的SCR催化活性与其对反应物（C₃H₆、NO）的低温氧化催化性能密切相关,催化剂的SCR反应性能随反应温度的变化呈现火山口状的变化形式。反应物（C₃H₆、NO）和反应中间产物之间对催化剂表面活性氧（如Pt-O、吸附氧）的竞争反应,导致C₃H₆与NO的氧化反应在C₃H₆-SCR中相互抑制。4.通过对Pt/TiO₂催化剂制备条件的对比实验发现,在450°C时制备的Pt/TiO₂催化剂具有最高低温C₃H₆-SCR催化活性。但载体预处理温度过高（>450°C）,会引起载体BET比表面积的下降,以及具有较高催化活性的锐钛矿相TiO₂的减少,从而降低了催化剂的催化活性;增加Pt负载量能够显著提高催化剂的低温C₃H₆-SCR催化活性,0.5 wt%是Pt/TiO₂催化剂的有效Pt负载量。5.在Pt/MOx（M= Al、Ce、Ti、W、Zr）及不同贵金属（Pt、Pd、Rh、Ru）负载TiO₂等氧化物型催化剂中,0.5 wt% Pt/TiO₂具有最高的低温C₃H₆-SCR催化活性,在140°C下,实现了63.4%的NOx转化（N2+N2O）率和83.7%的C₃H₆去除率,C₃H₆的选择性还原效率C3 H6?为75.7%。Pt/ZSM-5催化剂具有比Pt/TiO₂更高的低温催化活性和C3 H6? ,在140°C下,达到了77.1%的NOx转化率（转化为N2+N2O）和79.7%的C₃H₆去除率,C3 H6?则为96.7%。富氧条件下,Pt/TiO₂、Pt/ZSM-5的C₃H₆-SCR反应具有相同的反应特性,增加O₂浓度能提高C₃H₆-SCR低温活性,并增大了C₃H₆选择性还原NOx的能力;增加C₃H₆的浓度,可以提高NOx的最高转化率,促使SCR反应温度窗口向高温方向移动,并导致C3 H6?下降,更多的C₃H₆直接发生了非选择性催化燃烧。6. HC-SCR的反应机理因催化剂载体物理化学性质的不同而有显著的差异,推断富氧条件下Pt/TiO₂、Pt/ZSM-5的低温C₃H₆-SCR的反应机理分别属于NO解离机理和硝酸盐反应机理,载体表面的酸碱对、电荷转移等物化特性影响C₃H₆-SCR反应对机理路径的选择。在Pt/TiO₂低温C₃H₆-SCR的反应过程中,N2和N2O的生成来自于NO*和N*自由基的复合反应。NO*与CO*等碳氧中间体（CxHyOz）之间的相互反应,是Pt/TiO₂选择性催化C₃H₆还原NOx反应中的重要步骤。而Pt/ZSM-5的SCR反应,是NO在催化剂表面形成的具有氧化性的硝酸盐物种,与C₃H₆生成的还原性碳氧中间体（CxHyOz）之间的反应。NO和C₃H₆在催化剂表面的活化是C₃H₆-SCR反应机理中两个关键反应步骤。总之,采用贵金属Pt负载的氧化物TiO₂、分子筛ZSM-5作为低温C₃H₆-SCR催化剂,不仅实现了与NH3-SCR相当的反应活性温度,而且达到了HC和NOx污染物同时去除的效果,在柴油发动机、稀燃汽油发动机等移动污染源,以及城市公路隧道、地下停车场等固定源的污染物控制领域,具有良好的发展前景。更多还原

【Abstract】 With the number of motor vehicles increasing instantly, exhaust gas pollution is being more and more serious. The traditional three-way catalysts （TWC） cannot be effective for lean burn engines and diesel vehicles. Moreover, low-concentration, oxygen-rich and low-temperature exhausts emitted from urban road tunnels and underground parking lots, are easily ignored sources of air pollution and can evoke serious environmental problems. Low-temperature removal of NOx by SCR can be achieved with the application of the toxic reducing agent NH3, but it is not suitable for the mobile engines and stationary constructions. Alternatively, low-temperature and simultaneous removal of pollutants can be accomplished by noble metal supported oxide and zeolite catalysts, using hydrocarbons （HC） as reducing agent.In this work, the oxygen-rich and low-concentration vehicle exhaust was simulated in lab scale. A series of catalysts were prepared by wet-impregnation method, including Pt supported MOx （M= Al, Ce, Ti, W, Zr）, noble metals （Pt, Pd, Rh, Ru） supported TiO₂ and Pt supported zeolites （MCM-41, ZSM-5）. The catalysts were characterized by BET, TEM,XRD and in situ DRIFTS techniques and low-temperature adsorption （ADS）, temperature programmed desorption （TPD）, temperature programmed oxidation （TPO）, selective catalytic reduction （SCR）. The main contents and results of this paper are as follows:1. From the results of the adsorption and desorption of C₃H₆ over Pt/MOx, it can be observed that only simple physical adsorption process happened on the catalyst, as well as the supports. The corresponding desorption process was also simple, suggesting that the support played important role in C₃H₆ adsorption, not the promotion of support （precious metal loading）. Otherwise, Pt/ZSM-5 showed stronger ability of C₃H₆ adsorption than ZSM-5. This is possibly due to the slight change of microspores structure of ZSM-5 during the impregnation of Pt.2. Results of the NO-ADS and NO-TPD over the catalysts and supports indicated that Pt loading enhanced the NOx adsorption ability of MOx and MCM-41, ZSM-5 dramaticly as well as the NO-TPD amounts. The NO desorption peak displayed in low temperature range, attributing to the physical adsorption of NO and the dissociative adsorption of NO on Pt sites; while NO and NO₂ desorbed at higher temperature can be assigned to the decomposition of surface nitrates. The NO-ADS capacity and NO-TPD amounts depends on the physiochemical properties of supports （e.g., BET area, acidic and basic properties）, as well as the modification of supports （Pt loading）.3. It can be observed from the C₃H₆/NO-TPO experiments that the low-temperature oxidation activity of C₃H₆/NO were increased distinctly over Pt/MOx, Pt/MCM-41 and Pt/ZSM-5. The SCR activity of catalyst correlated well with its catalytic performance for TPO of reactants （C₃H₆, NO） and displayed a volcanic variation versus temperature. Owing to high selectivity for oxdization of C₃H₆, no CO was detected during the reactions of TPO and SCR. The competitive reaction of reactants （C₃H₆ and NO） and intermediates with the active oxygen species on catalyst surface （e.g., Pt-O, adsorbed oxygen） resulted in mutual inhibitition for the oxidation of NO and C₃H₆ in SCR reaction.4. It was found that Pt/TiO₂ catalyst calcined at 450 oC showed the best low-temperature C₃H₆-SCR catalytic activity,which can be explained by better dispersion of Pt on the catalyst surface, as well as unchanged phase compositions of the support. However, high pretreatment temperature of support （ > 450 oC） resulted in drop of the BET surface area and derease of the active rutile TiO₂, then caused the decrease of catalytic activity. With Pt content increasing, the low-temperature C₃H₆-SCR activity increased obviously. 0.5 wt% Pt was found to be proper for the enhancement of catalytic activity, while more Pt loading did not have profound effect on the catalytic performace any more.5. Among noble metal supported oxide catalysts, 0.5 wt% Pt/TiO₂ displays outstanding catalytic activity for C₃H₆-SCR and selective reduction efficiency of C₃H₆ （C3 H6 ）, in which 63.4% NOx was reduced to N2+N2O and 83.7% C₃H₆ was oxidized with 75.7% C3 H6? at 140 oC. At the same temperature, Pt/ZSM-5 showed higher catalytic activity and C3 H6? than oxide catalysts, with 77.1% NOx reduction, 79.7% C₃H₆ conversiton and 96.7%C3 H6? . Under the oxygen-rich condition, Pt/TiO₂ and Pt/ZSM-5 showed almost the same C₃H₆-SCR catalytic properties. With the increase of O₂ concentration, the low-temperature C₃H₆-SCR activity and C3 H6? was enhanced. With the concentration of C₃H₆ increasing, the maximum conversion of NOx was elevated, but the SCR reaction temperature range shifted to higher temperatures, bringing about the decrease ofC3 H6 .6. The low-termperature SCR of NO with C₃H₆ in the excess of O₂ was investigated by in situ DRIFTS. NO dissociation mechanism and nitrates reaction mechanism had been proposed for the SCR over Pt/TiO₂ and Pt/ZSM-5 respectively; and the C₃H₆-SCR mechanism varies from the physicochemical property （e.g., acid-base pair, charge transfer） of support. During the reaction of C₃H₆-SCR over Pt/TiO₂, the formation of N2 and N2O was from the combinations of NO* and N* radicals. The interaction of NO*, CO* and other CxHyOz species was thought to be the important step during the SCR reaction. On the other hand, oxidizing nitrates induced by the oxidation of NO to NO₂, could react with the reducing CxHyOz species over Pt/ZSM-5 catalyst. The activations of NO and C₃H₆ were crucial steps in the SCR reaction.In conclution, the Pt supported metal oxide （ Pt/TiO₂ ） and zeolite （ ZSM-5 ） catalysts for the low-temperature C₃H₆-SCR catalysts showed high catalytic activity in excess O₂ at temperature as low as that of NH3-SCR. Moreover, HC and NOx were removed simultantly in C₃H₆-SCR. The low-temperature C₃H₆-SCR is a promising technology for NOx emission control in the fields of motor vehicles, even urban road tunnels and underground parking lots.更多还原