摘要
In the present work,a Pd/graphene/cordierite(Pd/Gr/Cor)composite was prepared as a monolithic catalyst for low-temperature combustion of toluene.We mainly focused on understanding the role of graphene coating through investigation of catalytic performance and adsorption behavior of the composite.Compared with the traditional Pd/Cor catalyst without graphene coating,Pd/Gr/Cor catalyst delivered much higher activity and stability for toluene catalytic combustion in both dry and moist conditions.Transmission electron microscopy(TEM)and hydrophobic characterizations indicated that graphene coating can considerably improve the dispersity of Pd nanoparticles and enhance the hydrophobicity of the cordierite support.The adsorption behavior of the above two catalysts,including adsorption isothermal,adsorption kinetics,and adsorption thermodynamics were carefully investigated.The simulation results indicated that a large amount of toluene was adsorbed on graphene surface through relatively weak interaction,whereas only a relatively small amount of toluene was adsorbed on Pd surface with strong affinity.The adsorption thermal calculation indicated that the adsorption of toluene on graphene was a process with reduced entropy,indicating highly-ordered assembly of toluene molecular on graphene.It is the significant concentration and affinity gap between graphene and Pd that ensures a simultaneously and rapid transfer of toluene during the reaction process.
对于传统整体催化剂而言,堇青石等基体比表面积低,往往需先涂覆活性氧化铝等高比表面涂层,此外低温催化燃烧反应生成的水和周围空气中的水分会大量吸附于亲水性氧化物涂层表面,导致贵金属催化活性降低,同时,贵金属的分散度也是影响催化剂活性的主要因素.我们利用石墨烯高疏水性、二维平面结构、对苯环强吸附及对贵金属颗粒的高分散与锚定作用等独特性能,发展基于石墨烯涂层的高活性纳米Pd整体催化剂,以改善上述问题.所制备的Pd/石墨烯/堇青石(Pd/Gr/Cor)复合材料作为整体催化剂用于甲苯低温燃烧反应,通过考察催化性能和吸附行为,重点研究了石墨烯涂层的作用.催化性能结果表明,与无石墨烯涂层的传统Pd/Cor催化剂相比,Pd/Gr/Cor催化剂对甲苯的起燃温度从175℃降至132℃,且在水蒸气存在的情况下表现出更好的稳定性.TEM和吸水速率表征表明,石墨烯涂层可显著提高Pd纳米粒子的分散性,提高堇青石载体的疏水性.动力学研究表明,Pd/Gr/Cor催化剂上甲苯催化燃烧符合一级反应动力学规律,活化能为60.93 k J/mol.此外,研究了其吸附行为,包括吸附等温线,吸附动力学和吸附热力学.模拟结果表明,Pd/Gr/Cor催化剂对甲苯具有优异的吸附性能,对甲苯的吸附符合Freundlich模型,为化学吸附.FLm双点位吸附模型表明,石墨烯表面吸附了大量的甲苯,而Pd粒子表面吸附的甲苯相对较少,但亲和力较强.吸附热力学计算表明,石墨烯对甲苯的吸附是一个自发的放热反应,是一个熵减小的过程,表明甲苯分子可在石墨烯上高度有序组装.石墨烯与Pd之间的显著浓度差和亲和力的差距确保了反应过程中甲苯在石墨烯上的快速转移.吸附动力学研究表明,催化剂对甲苯的吸附为快速过程,催化反应为控速步骤.综上,石墨烯涂层不仅可以提高Pd纳米粒子的分散性,提高催化剂的疏水性,在催化反应过程中,还可利用其强吸附能力提高催化剂表面的甲苯浓度,而显著的浓度差和亲和力的差距可作为驱动力为Pd粒子提供甲苯,从而发挥吸附-催化协同作用优势,进一步提高催化性能.
基金
supported by the National Natural Science Foundation of China(21276284,21676303,21706292)~~
