【作者】 高珊；

【导师】 王新平；

【作者基本信息】 大连理工大学 ， 物理化学， 2013， 博士

【摘要】 柴油发动机汽车尾气中NOx的排放对大气环境产生了严重的污染,而选择催化还原法(SCR)是消除上述NOx的有效途径。大量研究表明,具有较多强B酸中心的分子筛对于烃选择催化还原NOx (HC-SCR)及氢气选择催化还原NOx (H2-SCR)是较优的催化剂载体。因此,具有更强B酸位的新型分子筛被预期可成为更有效的HC-SCR及H2-SCR反应的催化剂载体。目前,分子筛的合成大都在以有机大分子为模板剂的条件下进行。这些有机分子的加入不仅增加了沸石分子筛的合成成本,还对环境造成了严重的污染。因此,人们亟需得到B酸性更强的分子筛及其在成本低廉,环境友好条件下的合成路线。本论文对具有强B酸位的微孔硅铝沸石SUZ-4分子筛的合成进行了深入研究,利用晶种法实现了在不外加有机模板剂的条件下合成该分子筛。通过对该合成系统中各参数的详细研究,不仅缩短了晶化时间,降低了晶化温度,还提出了一条可实现SUZ-4分子筛形貌及硅铝比可控的绿色合成路线。另外,本论文还研究了具有不同形貌的HSUZ-4分子筛负载Pt所得催化剂对H2-SCR反应催化活性的差异,建立了载体的形貌结构与催化剂催化性能之间的构效关系。主要研究结果如下：1.优化了制备SUZ-4晶种胶的晶化时间。研究提出,在系统晶化曲线上,晶体生长阶段与Ostwald成熟阶段的交叉转折点所对应的时间应为制备晶种胶的最佳晶化时间。在该时间点上所得的晶种胶中几乎不含有无定形物质,且所含的SUZ-4微晶数量最多。在初始凝胶组成为7.9KOH:1.0Al2O3:21.2SiO2:498.6H2O,晶化温度为150℃,转速为20rpm的条件下,利用在上述最佳晶化时间点所得到的晶种胶(S16),首次成功合成出了纳米纤维状的SUZ-4分子筛。2.通过XRD、SEM及TEM对晶种法合成SUZ-4分子筛系统的晶化过程进行了监测。提出并论证了,S16晶种在晶化初期阶段(≤3h)的大量溶解是该纳米纤维SUZ-4分子筛形成的关键步骤。HR-TEM的分析确认了,SUZ-4分子筛是沿着骨架低密度的轴向方向,即c-轴方向进行生长的。3.以S16为晶种,在无模板剂的条件下,实现了SUZ-4分子筛的硅铝比及形貌的可控以及该分子筛在较短时间内、较低温度下的合成。研究发现,通过简单地调节凝胶中Al的投料量及K+浓度,可使合成得到SUZ-4分子筛的SiO2/Al2O3比在10.6到20之间可控；增加凝胶中H20量,Al量以及K+浓度,可有利于纳米纤维状SUZ-4分子筛的形成；通过减少系统中水的含量,可将合成100%SUZ-4分子筛所需的时间有效地缩短至12h(文献中报道的至少需要96h)；组成为7.9KOH:1.0Al2O3:21.2SiO2:498.6H2O的初始凝胶可在90℃下生成纯SUZ-4分子筛(文献中报道的至少需要150℃)。4.对Na+可否作为SUZ-4分子筛孔道内的平衡阳离子作了深入探讨。研究提出,在有晶种存在的条件下,无论系统中Na+含量的多寡,它都无法充当SUZ-4分子筛孔道的平衡阳离子；该离子的存在并不影响系统生成SUZ-4分子筛相的选择性以及最终产品的形貌。5.首次将Pt/HSUZ-4催化剂用于H2-SCR反应。在相同Pt担载量的条件下(0.1wt.%),纳米纤维HSUZ-4分子筛载Pt催化剂的活性显著高于相应的棒状分子筛载Pt催化剂。研究发现,对于利用等体积浸渍法制备的Pt/HSUZ-4催化剂,纳米纤维HSUZ-4分子筛较长的十元环孔道及其外表面上较多的B酸位使Pt更易以较高活性的Pt纳米颗粒的形式较均匀地分散在该载体的外表面上；当棒状HSUZ-4为载体时,较多Pt以非活性的Pt离子的形式进入该载体孔道内,而在其外表面上的Pt颗粒因以团聚态的形式存在而活性较差。6.首次提出,通过吸附NOx和红外光谱测定,可用1652cm-1处的吸收峰识别位于HSUZ-4分子筛孔道中的过渡金属阳离子。更多还原

【Abstract】 Environmental pollution caused by the nitrogen oxides being contained in diesel vehicle exhaust has been serious, and selective catalytic reduction of NOx (SCR) has been believed to be the effective way to remove the NOx under the lean-burn condition. It has been found that, the zeolite with large population of Bronsted acids in strong acidic is the superior support for the selective catalytic reduction of NOx by hydrocarbons (HC-SCR) or by hydrogen (H2-SCR). Hence, the novel zeolite with much more Bronsted acids in stronger acidic is expected to be a better support for the HC-SCR and H2-SCR reactions. Until now, for the zeolite synthesis, organic molecules have had to be used as template, at the expense of high cost and serious environmental pollution. Thus, strategies to synthesize the zeolite with stronger Bronsted acid sites in an economical and environmental-friendly way are urgently desired.In this thesis, the microporous SUZ-4zeolite with strong Bronsted acids was synthesized with the assistance of seed slurry under the template-free condition. Based on the systematic investigation on the parameters of the synthesis system, we not only shortened the crystallization time, reduced the crystallization temperature, but also provided a route to synthesize the SUZ-4zeolite with controllable morphology and SiO2/Al2O3ratio. Furthermore, HSUZ-4zeolites with different morphology were used as the catalyst supports for loading Pt in H2-SCR reaction, and the catalytic performances of the catalysts were well associated with their support morphology. The main work and findings are as follows:1. The crystallization time for preparing SUZ-4seed slurry was optimized. It was found that, the crystallization time at the turning point of the crystal growth stage and the Ostwald ripening stage on the crystallization curve was optimum. The seed slurry being obtained at the time contained almost no amorphous material, but the most population of SUZ-4crystallites. With the assistance of the seed slurry (S16), the nanofibrous SUZ-4zeolite was firstly synthesized from the initial gel with7.9KOH:1.0Al2O3:21.2SiO2:498.6H2O at150℃, under the rotation of20rpm.2. The crystallization process of the SUZ-4synthesis seeded by S16was monitored by XRD, SEM and TEM. The seed dissolution at the early stage (≤3h) of the crystallization process was proposed to be the key step for the formation of the nanofibrous SUZ-4zeolite. The orientation of the nanofibrous SUZ-4zeolite to be along c-axis was identified by HR-TEM. 3. With the assistance of S16, the synthesis of SUZ-4zeolite with controllable morphology and SiO2/Al2O3ratio and the synthesis of SUZ-4zeolite in a shorter time or at a lower temperature were realized under the template-free condition. It was found that, by simply regulating the Al content and the K+concentration in the initial gel, the SiO2/Al2O3ratio of the obtained SUZ-4zeolite can be manipulated in the range of10.6-20; the initial gel with larger H2O content, larger Al content and higher K+concentration was favorable for the nanofibrous SUZ-4zeolite formation; by decreasing the H2O content in the initial gel, the crystallization time for obtaining the100%SUZ-4zeolite can be effectively shortened to12h (it is at least96h reported in literature); from the initial gel of7.9KOH:1.0Al2O3:21.2SiO2:498.6H2O, the crystallization temperature for obtaining pure SUZ-4zeolite can be lowered to90℃(it is at least150℃reported in literature).4. Whether Na+in the initial gel can act as balanced ions in the channels of SUZ-4zeolite or not was investigated. It was found that, with the assistance of seed, no matter how much the Na+existing in the gel, it can not incorporate into the SUZ-4framework as the balanced ions; the existence of Na+did not influence the SUZ-4zeolite synthesis.5. Pt/HSUZ-4catalyst was firstly used in H2-SCR reaction. With the same Pt loading (0.1wt.%), the catalyst of nanofibrous HSUZ-4zeolite supporting Pt (Pt/NF) exhibited much higher activity than the corresponding catalyst of rod-like HSUZ-4supporting Pt (Pt/ROD). It was found that, for the Pt/HSUZ-4catalysts prepared by impregnation, in the case of Pt/NF, Pt primarily presents as more active Pt nanoparticles well dispersing on the external surface of the support, due to the longer10-ring channels and the much more Bronsted acids on the external surface of the support; whereas in the case of Pt/ROD, more Pt presents as inactive Pt ions locating in the support channels, and less Pt exists as aggregated particles on the external surface of the support, which were less acive for the reaction.6. It is firstly proposed that, after the catalyst adsorbed NOx, the FTIR absorption band at1652cm-1can be used to identify the transition metal ions in SUZ-4zeolite channels.更多还原