MFI分子筛限域空间内Pd催化剂上甲烷燃烧

甲烷是一种重要的温室气体,其开发利用过程中不完全燃烧所残留的气体排放到大气中会造成严重的环境问题,因此提高甲烷燃烧效率显得尤为重要.与传统燃烧方式相比,催化燃烧在低温区表现出高的燃烧效率,成为甲烷燃烧理想的选择.在实际应用时,甲烷燃烧催化剂应在低温区具备高的催化活性,同时在过量水蒸气存在下具备好的稳定性.负载型Pd基催化剂是当前研究最多的甲烷燃烧催化剂,Pd粒子尺寸、载体类型、酸性位点以及金属与载体的相互作用是影响甲烷燃烧活性与稳定性的关键因素.本文设计了原位水热合成路线将孤立的Pd离子稳定封装于MFI分子筛孔道内(Pd@MFI),以期获得高活性、高稳定性的甲烷燃烧催化剂,并揭示其反应机理与构效关系.通过X射线粉末衍射、高分辨透射电子显微镜以及球差校正扫描透射电子显微镜分析了Pd@MFI催化剂的基本结构,并直接观测了Pd物种在分子筛晶体中的分布;进而利用氨气程序升温脱附、固体核磁共振、氢气程序升温还原、X射线光电子能谱(XPS)和CO吸附红外光谱等表征技术研究了催化剂的酸性以及Pd在分子筛中的存在状态.表征结果证实,通过原位水热合成方法可将Pd物种以pd2+和Pd(OH)+的形式封装在MFI分子筛孔道内,孤立的Pd离子与分子筛骨架之间存在着强相互作用,有效稳定Pd离子并实现贵金属Pd的最大化利用.在甲烷燃烧反应中,Pd@H-ZSM-5在高空速下表现出较好的催化活性与较低的表观活化能(70.7 kJ/mol).热稳定性及耐水性测试结果表明,Pd@H-ZSM-5在400℃下连续反应100 h后甲烷燃烧活性无明显下降,且反应后Pd物种在分子筛孔道内仍保持高度分散,说明该催化剂在甲烷燃烧过程中具备优异的稳定性和抗烧结性能.通过反应动力学、程序升温脱附以及原位红外光谱等技术手段研究了甲烷催化氧化机理,结果表明,Brφnsted酸性位点的存在有利于甲烷吸附并促进其在相邻Pd位点上活化,在MFI分子筛限域空间内形成Pd位点和Brφnsted酸性位点的有效协同.原位近常压XPS分析结果表明,Pd@H-ZSM-5催化的甲烷燃烧过程中存在着pd2+-pdn+-pd2+的可逆氧化还原循环.综合分析上述结果,最终可阐明Pd@H-ZSM-5模型催化剂上甲烷燃烧的反应机理.

Methane combustion over palladium catalyst within the confined space of MFI zeolite

Isolated cationic Pd species encapsulated in MFI zeolite,i.e.,Pd@MFI,have been successfully pre-pared via in situ hydrothermal route followed by oxidative treatment.The as-prepared Pd@MFI samples are investigated as promising catalysts in the reaction of methane combustion.Typically,Pd@H-ZSM-5 shows remarkable activity in methane catalytic combustion with a low apparent activation energy value of 70.7 kJ/mol as well as good catalytic stability even in excess water vapor.Detailed characterization results demonstrate the strong interaction between Pd sites and zeolite framework in Pd@ZSM-5 and the efficient stabilization of isolated Pd sites by zeolite thereof.Spec-troscopy analyses reveal that the presence of Brφnsted acid sites is beneficial to methane adsorption and its subsequent activation on adjacent Pd sites,constructing cooperation between Brφnsted acid sites and Pd sites within the confined space of MFI zeolite toward high-efficiency methane catalytic combustion.The reaction mechanism of methane combustion catalyzed by Pd@H-ZSM-5 model catalyst is finally discussed.