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三维石墨烯负载Pd催化剂的制备及加氢性能研究
论文作者:童鞋论文网  论文来源:www.txlunwenw.com  发布时间:2019/11/1 9:12:16  

摘要:硝基苯加氢是工业上广泛应用制备苯胺的有效途径之一。Pd催化剂用于硝基苯加氢制备苯胺拥有极高的选择性。但是,一种高比表面积,稳定性好的载体是制备高分散金属Pd,获得高活性的关键。石墨烯由于其高的比表面积以及优异的热力学稳定性成为负载Pd的潜在理想载体。但二维(2D)石墨烯片层之间的重复堆叠会掩盖活性组分,阻碍活性组分与底物接触,影响催化剂性能。而二维石墨烯组装成的具有三维(3D)架构的石墨烯不仅继承了石墨烯原有的优异性能,且特殊的孔道结构有效避免片层堆叠,获得更高的比表面积,使金属Pd均匀分散。

本文使用氧化石墨烯(GO)水溶液、PdCl2、抗坏血酸为原料,通过一步化学还原法在低温常压下制备催化剂Pd/3D-RGO(Pd负载三维还原氧化石墨烯)。通过X射线衍射(XRD)、氮气吸–脱附法、场发射扫描电子显微镜(SEM)、透射电子显微镜(TEM)等表征手段对材料晶相、比表面积和孔体积、粒子形貌、结构形态进行表征。结果显示,三维石墨烯相比于二维石墨烯具有更高的比表面积,负载Pd之后用于硝基苯加氢拥有更高的催化活性。通过调节GO水溶液浓度和抗坏血酸添加量优选出最佳合成条件制备高活性的加氢催化剂。同时,该催化剂在6次重复实验中苯胺收率几乎没有下降,说明其拥有优异的稳定性。此外,通过简单的离心回收,催化剂回收率都保持在95%以上。

将上文的催化剂进行改进,添加环己烷作为油相与水形成乳液(Emulsion),加入乙醇作为助表面活性剂,同样采用化学还原法制备三维石墨烯前驱体,再通过油浴回流还原PdCl2制备Pd/3D-ERGO。在这里,乳液有两重作用:一是辅助形成相互交错的三维石墨烯;二是乳液胶团微粒之间的碰撞导致水核内物质发生交换,引起在水核内的金属还原,并且水核半径限制了金属颗粒的生长,对金属起到分散作用。制备的Pd/3D-ERGO催化剂Pd的粒径更小,分布均匀,在硝基苯加氢反应中,60 °C下,反应0.25 h,苯胺收率就可达到98.5%。

为了得到比表面积更高的催化剂,在晶化釜中采用水热合成法,添加适量苯酚(Phenol)和甲醛(Formaldehyde)合成酚醛树脂(PF)作为交联剂交联GO组装成PF-3D-RGO前驱体,再通过硼氢化钠在常温下还原PdCl2得到Pd/PF-3D-RGO。通过表征得出PF-3D-RGO的比表面积最高达到504 m2/g,孔体积为0.356 cm3/g,远大于前面制备的三维石墨烯基催化剂。并且,Pd纳米粒子的粒径约为2–3 nm,在石墨烯片层上达到高分散。此外,酚醛作为交联剂不仅能与GO片之间形成共价键连接氧化石墨烯来稳固三维石墨烯的结构,还引入大量含氧官能团与部分还原的氧化石墨烯相结合为Pd金属提供良好的固载位点。将催化剂用于硝基苯加氢,60 °C下反应0.25 h,苯胺收率达到100%。6次重复使用中,苯胺收率没有明显下降,说明催化剂具有优异的稳定性。

Nitrobenzene hydrogenation is one of themost effective industrial ways to prepare the important chemical raw material,aniline. Pd catalyst for the nitrobenzene hydrogenation has a very highselectivity to aniline. It is necessary to find a carrier with high specificsurface area and good stability to disperse Pd particles for getting a highactivity. Graphene is a potential supporting for Pd because of its largespecific surface area and good thermal stability. However, restacking betweentwo-dimensional graphene sheets can mask the active component, hinder theactive component from contacting thereactant molecule. Three-dimensionalgraphene not only inherits the excellent performance of graphene, but alsoeffectively avoids lamella stacking and obtains larger specific surface area,which is favorable for the dispersion of Pd particles.

In this paper, the catalyst Pd/3D-RGO wasprepared by one-step chemical reduction method using graphene oxide aqueoussolution, PdCl2, and ascorbic acid as raw materials at low temperature andatmospheric pressure. X-ray diffraction, nitrogen adsorption-desorption, fieldemission scanning electron microscopy, and transmission electron microscopywere used to characterize the crystal phase, specific surface area, porevolume, particle morphology, and morphology of the materials. The results showthat the three-dimensional graphene has a higher specific surface area than thetwo-dimensional graphene, and has higher catalytic activity for thehydrogenation of nitrobenzene after Pd loading. By adjusting the concentrationof GO aqueous solution and the amount of ascorbic acid, an optimal synthesiscondition was used to prepare a highly efficient hydrogenation catalyst. At the same time, the as-synthesizedcatalyst showed an excellent stability during catalytic cycle for 6 times. Inaddition, the catalyst recovery rate reaches above 95% through simplecentrifugal recovery.

Then, the above catalyst is modified.Cyclohexane is added as an oil phase to form an emulsion with water; ethanol isadded as a cosurfactant. A chemical reduction method is used to prepare athree-dimensional graphene precursor, and then the PdCl2 is reduced by an oilbath under reflux to prepare Pd/3D-ERGO. Here, the emulsion has two effects.First, the emulsion can assist to form interdigitated three-dimensionalgraphene. Then, collisions between micelle particles result in the exchange ofsubstances in the water coreto cause the reduction of metals. The radius of thewater core limits the growth of the metal particles and improves the dispersionthe metal. The prepared Pd/3D-ERGO catalyst with very smaller Pd particlesgives a high yield of aniline (98.5% ) at 60 °C for 0.25 h.

In order to futher increase the specificsurface area and pore volume of three-dimensional graphene, an appropriateamount of phenol and formaldehyde are added. The synthesized phenolic as acrosslinking agent to crosslink GO to assemble into PF-3D-RGO precursor. Then,PdCl2 is reduced at room temperature by sodium borohydride to obtainPd/PF-3D-RGO. The specific surface area and the pore volume of PF-3D-RGO reach504 m2/g and 0.356 cm3/g. They are much larger than those of the previouslyprepared three-dimensional graphene-based catalyst. The Pd nanoparticles arehighly dispersed (2–3 nm) on the graphene sheets. In addition, phenolic as across-linking agent not only forms a covalent bond with the GO sheet tostabilize the structure of the three-dimensional graphene, but also introducesa large number of oxygen-containing functional groups to provide good anchoringsites for the Pd particles. In the hydrogenation of nitrobenzene, the catalystgives 100% yield of aniline at 60 °C for 0.25 h. After 6 cycles, the yield ofaniline did not decrease significantly, indicating that the catalyst hadexcellent stability.

关键词:硝基苯加氢;加氢催化剂;Pd;氧化石墨烯;三维石墨烯

Nitrobenzene hydrogenation; Hydrogenationcatalyst; Pd; Graphene oxide; Three-dimensional graphene

上一篇:层状双金属(氢)氧化物/碳基复合材料的制备及其光催化应用     下一篇:图像句子标注方法研究
 
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