碱金属活性组分催化碳烟燃烧性能及机理

发布时间：2018-08-08 18:58

【摘要】：柴油车尾气中排放的碳烟颗粒物对环境和人类健康带来严重的危害。因此,如何有效控制颗粒物的排放受到研究者的广泛的关注。目前,解决这一问题最有效的技术是DPF(diesel particulate filters)技术。该技术的关键是开发一种高效的催化剂以降低碳烟的起燃温度。研究表明,碱金属催化剂具有优异的催化碳烟燃烧活性。然而对于其催化作用机理一直存在争议。大部分人认为碱金属能够增加催化剂与碳烟的接触效率,从而有效地促进碳烟氧化。而我们课题组在前期工作的基础上认为,碱金属物种除了作为助剂改善接触之外,其本身也具有催化活性。为了证明这一猜想,我们对催化剂进行了设计:将碱金属限域在具有孔道材料的催化剂中,使其不与碳烟接触。由于ZSM-5分子筛具有有序的三维微孔结构和阳离子交换能力。因此我们选取ZSM-5为载体,将碱金属(Na、K、Cs)用离子交换法引入ZSM-5微孔中,制备了M-ZSM-5-25(M=Na/K/Cs)催化剂,通过XRD、BET、SEM、ICP、FT-IR、Raman、XAFS、NH_3-TPD等手段对其物理化学性质进行表征,证明碱金属的存在位置位于离子交换位。并通过O_2-TPO和等温反应对其催化碳烟氧化表观活性和内在活性进行了探究。发现该类催化剂展现出了碳烟氧化活性,并且其活性与碱金属本身的性质有关。此外,对其催化碳烟氧化机理进行了研究,对不同K含量的ZSM-5的活性进行比较,发现随着K含量的增加,催化剂催化碳烟燃烧活性增强。并通过同位素示踪实验证实了K-ZSM-5能够使气相氧活化。气相氧的活化来源于活性组分K。将碱金属引入分子筛中,已经证明碱金属物种可以作为活性组分用于催化碳烟氧化。为了证明这一结论在其他体系中的适用性,我们设计了另外一种催化剂,将K引入到具有隧道结构的Ti O_2载体中,作进一步的研究。首先利用固相法制备了棒状的KTi_8O_(16)催化剂,发现K的加入,有助于改善Ti O_2催化碳烟燃烧活性。此外,为了进一步提高该类催化剂的活性,利用不同的方法对K含量进行了调控,制备了一系列形貌为颗粒状的K_xTi_8O_(16)催化剂。利用XRD、FT-IR、XAFS等技术确定了制备样品中K物种的存在状态。结合XPS和O_2-TPO表观活性测试,排除了氧空位对催化活性的影响。并且对催化剂内在活性进行测试,发现其与表观活性结果一致。该体系进一步证明了碱金属物种能够作为活性组分起到催化碳烟燃烧的作用。
[Abstract]:The soot particles emitted from diesel exhaust are harmful to the environment and human health. Therefore, how to effectively control the emission of particulate matter has been widely concerned by researchers. At present, the most effective technology to solve this problem is DPF (diesel particulate filters) technology. The key of this technology is to develop an efficient catalyst to reduce the ignition temperature of soot. The results show that alkali metal catalysts have excellent catalytic activity for soot combustion. However, the mechanism of its catalytic action has been controversial. Most people think that alkali metal can increase the contact efficiency between the catalyst and soot, thus effectively promoting soot oxidation. On the basis of the previous work, our group thinks that alkali metal species have catalytic activity in addition to improving contact as auxiliaries. In order to prove this conjecture, the catalyst is designed: the alkali metal is confined to the catalyst with pore material so that it is not in contact with soot. Because ZSM-5 molecular sieve has ordered three dimensional micropore structure and cation exchange ability. Therefore, M-ZSM-5-25 (M=Na/K/Cs) catalyst was prepared by using ZSM-5 as the carrier and introduced into the ZSM-5 micropore by ion exchange method. The physicochemical properties of M-ZSM-5-25 (M=Na/K/Cs) catalyst were characterized by means of XRDX BETSMEM, FT-IR, Ramanhe, XAFSS-NH3-TPD, etc. It is proved that the location of alkali metal is in the ion exchange site. The apparent and intrinsic activity of soot oxidation was investigated by O_2-TPO and isothermal reaction. It is found that this kind of catalyst exhibits soot oxidation activity, and its activity is related to the properties of alkali metal itself. In addition, the mechanism of catalytic soot oxidation was studied, and the activity of ZSM-5 with different K content was compared. It was found that the catalytic soot combustion activity increased with the increase of K content. The isotopic tracer experiments show that K-ZSM-5 can activate oxygen in gas phase. The activation of gaseous oxygen originates from the active component K. It has been proved that alkali metal species can be used as active component to catalyze soot oxidation by introducing alkali metal into molecular sieve. To prove the applicability of this conclusion in other systems, we have designed another catalyst to introduce K into TiO2 carrier with tunnel structure for further study. Firstly, the rod-like KTi _ 8O _ (16) catalyst was prepared by solid phase method. It was found that the addition of K was helpful to improve the catalytic activity of TIO _ 2 for soot combustion. In addition, in order to further improve the activity of these catalysts, K content was regulated by different methods, and a series of K _ XTi _ 8O _ (16) catalysts with granular morphology were prepared. The state of K species in the prepared samples was determined by using XRDX FT-IR and XAFS techniques. The effect of oxygen vacancy on catalytic activity was excluded by XPS and O_2-TPO. The intrinsic activity of the catalyst was tested and it was found that it was consistent with the apparent activity. It is further proved that alkali metal species can act as active components to catalyze soot combustion.
【学位授予单位】：济南大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：X734.2;O643.36

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