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过渡金属催化剂的球磨法制备及其催化CO氧化性能研究
中文摘要

CO氧化反应可用于制备CO检测器、气体净化、汽车尾气处理等领域。另外因CO氧化反应过程相对简单且具有代表性,所以在理论研究中常用做探针反应。目前,CO氧化使用最多的催化剂为铜系和贵金属催化剂,然而合成这些催化剂大多数在溶液条件下进行。溶液合成法一般存在环境污染、耗时、操作过程复杂等缺点,同时它还要求催化剂的前驱体要具有一定的溶解性,这极大地限制了前驱体的选择。针对该研究领域存在的这些缺点,本论文采用球磨合成技术实现了多孔金属氧化物催化剂以及高分散贵金属催化剂的合成,且利用球磨的特性成功合成了具有高热稳定性的金属氧化物催化剂。本论文开发的方法很好地克服了溶液合成方法的缺点,具有潜在的应用前景。主要研究工作如下: (1)多孔催化剂有利于反应物的扩散,可增加反应物与活性中心的接触。因此在催化剂中创造孔道是一种有效提高过渡金属催化剂催化性能的方法。我们采用球磨合成技术,用商用的多孔SiO₂为模板剂,成功合成了一系列具有丰富孔道的多孔金属氧化物(ZrO₂:293 ㎡ g⁻¹,Fe₂O₃:163 ㎡ g⁻¹,CeO₂:211 ㎡ g⁻¹, CuO〓-CeO〓 catalyst:237 ㎡ g⁻¹and CuO〓-CoO〓-CeO〓 catalyst:202 ㎡ g⁻¹),并将铜系氧化物应用于催化CO氧化。实验结果表明两个催化剂都具有较好的CO氧化活性。本论文中将这种制备多孔金属氧化物的方法称为机械纳米铸造法。同溶液法相比,机械纳米铸造法可在60 min内完成金属前驱体对孔道的填充,这远快于溶液法所需要的数天时间,且该方法合成过程中不需要使用溶剂,可忽略金属前驱体的溶解性,极大拓展了金属前驱体的选择。 (2)贵金属是性能优异的CO氧化催化剂,为了提高贵金属的催化效率,最为有效的策略是将贵金属高度分散在载体上。本论文中选用NaCl作为辅助剂,采用球磨技术,合成了TiO₂(P25)负载Pd催化剂。结构表征表明,合成催化剂中的Pd高度分散在TiO₂上,且NaCl的加入有利于保持载体原有的结构特征。将其用于催化CO氧化反应发现该催化剂展现出了优异的低温CO氧化活性。同样该方法也成功制备了Pt和Ru负载在TiO₂上的催化剂。该方法仅用无毒、价廉的NaCl作为辅助剂,且焙烧前的合成过程只需要30 min。而在溶液中合成高度分散贵金属负载型催化剂时常依赖于溶剂的选择、合成条件的精确控制以及稳定剂的加入。这些使得溶液合成方法过程繁琐,且稳定剂的加入也增加了合成成本。因此,采用NaCl辅助球磨合成贵金属负载型催化剂的方法具有潜在的应用前景。 (3)考虑到球磨法在合成多孔金属氧化物和贵金属催化剂中的优势,本论文尝试用球磨技术一步制备了多孔过渡金属氧化物负载贵金属催化剂。以ZrCl₄为金属氧化物前驱体,加入NaCl在球磨的条件下合成了Pd、Pt、Ru/ZrO₂催化剂。结构表征说明,合成的催化剂具有较大的比表面积(180 ㎡/g),且这些贵金属高度分散在多孔ZrO₂中。将这些催化剂用于催化CO氧化反应,它们均展现出了很好的CO氧化活性。传统合成多孔氧化物负载贵金属催化剂方法通常需要先制备多孔氧化物,然后再通过浸渍、共沉淀或者沉积沉淀等方法对贵金属进行负载。此合成过程十分的繁琐、耗时。而本论文所开发的一步合成方法十分简单且快速,有着很好的应用前景。 (4)处理汽车尾气中的CO所使用的催化剂不仅需要好的低温催化活性,并且也要求好的热稳定性,因此开发高活性、高热稳定性的CO氧化催化剂是当今研究的热点。上面的工作基础表明球磨合成法具有众多的优势,同时我们也调研到球磨法经常被用于合成性能优异的高温陶瓷。因此,我们尝试利用球磨法制备高热稳定的CO氧化催化剂。文献报道共沉淀制备的CuO〓-CoO〓-CeO〓催化剂具有优异的CO氧化活性。这里本文以Cu(OAc)₂、Co(OAc)₂和Ce(OAc)₃为原料,分别采用球磨法和共沉淀法合成了CuO〓-CoO〓-CeO〓催化剂,并将其用于催化CO氧化反应。实验结果表明:球磨和共沉淀合成的催化剂在600 ℃时均表现出了优异的CO氧化活性。然而当焙烧温度升至900和1000 ℃球磨比共沉淀合成的催化剂表现出了更好的CO氧化性能。这说明球磨法合成的催化剂具有更好的热稳定性。结构表征表明球磨合成可以阻碍催化剂中CuO的生长,抑制Co₃O₄的分解,且可以很好地保持晶格的稳定性。这为合成高热稳定性的CO氧化催化剂提供了新的合成思路。 关键词:机械化学;一氧化碳;多孔氧化物;贵金属;热稳定

英文摘要

CO oxidation has been applied to prepare CO sensors, purify the gas and clean up the automobile exhaust. The copper- and noble metal-based catalysts are two kinds of common catalysts for the CO oxidation. To obtain the catalysts, almost preparations are operated in the solvents, which always cause damages to the environment, consume a lot of time and need tedious preparation procedures. At the same time, they also require catalyst's precursors to be soluble in solvents, limiting the scope of precursors. Considering this, the current works employed the ball milling methods to synthesize a series of highly porous metal oxide, well dispersed noble metal and highly thermal stable catalysts. These developed methods can overcome the drawbacks of solution preparation and have the potential applications in catalyst preparation. We introduce our works as follows: (1)Embedding porosity in the transitional metal catalysts can advance their catalytic activities because the porous structures facilitate the diffusion of reactants which enhances the contact between the reactants and active sites. Herein, employing the ball milling method, a series of highly porous metal oxides were successfully synthesized using the commercial porous silica as template. Their specific surface areas are comparable to the reported metal oxides with high surface areas, ZrO₂: 293 ㎡ g⁻¹, Fe₂O₃: 163 ㎡ g⁻¹, CeO₂: 211 ㎡g⁻¹, CuO〓-CeO〓 catalyst: 237 ㎡ g⁻¹ and CuO〓-CoO〓CeO〓 catalyst: 202 ㎡ g⁻¹. And CuO〓-CeO〓 and CuO〓-CoO〓-CeO〓 are active for the CO oxidaton. This developed method is named mechanochemical nanocasting. The mechanochemical method can complete the casting procedure within 60 mins, much shorter than that of soulution process (2-3 days). Moreover, no slovents are needed in the mechanochemical nanocasting. Hence this developed technology in principle can ignore the solubility issue of metal precusors, and tolerate metal precursors with poor or even no solubility. (2)The noble metal catalysts show excellent catalytic activities for the CO oxidation, especially for the highly dispersed noble metal. In the current work, employing the ball milling technique, a supported Pd-P25 (P25 is the a commercial TiO₂) heterogeneous catalyst was prepared with the aid of NaCl. The structural analysis reveals that the Pd is highly dispersed in the P25, and the P25 structure can be maintained in the preparation process with the aid of NaCl. The as-synthesized catalyst displays excellent activity for the CO oxidation. This developed method is also successfully used to prepare the highly dispersed Pt- and Ru-P25 catalysts. For the solution procedure, the synthesis of highly dispersed noble metal catalysts heavily depends on the solvents, accurate modulation of synthesis conditions and protection reagents, resulting in tedious and high-cost preparation process. Our developed method is operated in the solvent free condition, and the anchoring process can be complete within 30 mins. To obtain the very active catalysts, the non-toxic and low-cost NaCl is only needed. Compared to the solution process, the developed method has the potential applications in the synthesis of noble metal catalysts. (3)The above works have demonstrated the advantages of ball milling method for the synthesis of porous metal oxide and highly dispersed noble metal catalysts. Thus, we try to prepare noble metal/porous metal oxide catalysts. Herein, ball milling the mixture of noble metal precursors, ZrCl₄ and NaCl, then calcinating the mixture and removing NaCl with amount of water, the Pd, Pt, Ru-ZrO₂ catalysts were prepared. The structure analysis reveals that the noble metals are highly dispersed on porous ZrO₂ and these catalysts exhibit high specific surface areas (180 ㎡/g). And these catalysts show excellent catalytic activities for CO oxidation. To obtain the highly porous noble metal supported catalysts, the traditional methods always need two steps, the preparation of porous supports and anchoring of noble metals. For the solution preparation, the two steps are all tedious and consume a lot of time. While the developed method can finish these in one step and only needs 30 mins. This provide an efficient route for the synthesis of porous supported noble metal catalysts. (4)To clean up the CO gas in the automobile exhaust demands the catalysts not only possessing high catalytic activities but also exhibiting highly thermal stability. The ball milling method presents many advantages to synthesize catalysts for the CO oxidation. And we also notice that it also has been utilized to synthesis of ceramics with outstanding properties. Considering this, we try to synthesize highly thermal stable catalysts for the CO oxidation. The CuO〓-CoO〓-CeO〓 catalyst (CCC) prepared by co-precipiation has been reported to show excellent CO oxidation activity. Herein, empolying the ball milling method, a CuO〓-CoO〓-CeO〓 catalyst (CCC-B) were prepared by using the Cu(OAc)₂, Co(OAc)₂ and Ce(OAc)₃ as the precursors. The CO oxidation experiments reveal that activity of CCC-B is similar with the CCC catalyst after 600 ℃ calcination. When rising the calcination temperatures to 900 and 1000 ℃, the CCC-B displays higher activity than the CCC, indicating that the catalyst prepared by ball milling is more thermal stable than that prepared by co-precipitation. The structure analysis reveals that the ball milling synthesis can impede the growth of CuO, inhibit the decomposition of Co₃O₄ and maintain the surface lattice oxygen. This provides a new route for the synthesis of highly thermal stable catalysts towards the CO oxidation. Key words: mechanochemistry; carbon monoxide; porous metal oxide; noble metal; thermal stability

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