Kinetic and Thermodynamic Factors Influencing Palladium Nanoparticle Redispersion into Mononuclear Pd(II) Cations in Zeolite Supports
Kinetic and Thermodynamic Factors Influencing Palladium Nanoparticle Redispersion into Mononuclear Pd(II) Cations in Zeolite Supports
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DOI：
10.1021/acs.jpcc.2c01613
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发表时间：
2022-05-19
影响因子：
3.7
通讯作者：
Gounder, Rajamani
中科院分区：
文献类型：
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作者：
Lardinois, Trevor M.;Mandal, Keka;Gounder, Rajamani
负载在氧化物和沸石上的钯离子和纳米颗粒被用作催化剂和吸附剂，用于许多具有实际意义的化学反应，具有不同和不同的活性中心要求。因此，Pd离子与团聚的纳米粒子相互转化的烧结和再分散过程是催化剂活化和失活现象的基础，然而，Pd纳米粒子的粒度分布和气体条件对影响这种相互转化的热力学和动力学因素的影响还不是很清楚。在这里，我们制备了不同颗粒大小和分布(正态、对数正态)的Pd纳米颗粒负载在高对称性的分子筛上，以获得定义良好的材料，这些材料的Pd位分布可以通过实验进行定量表征，并从理论上进行准确的建模。分子筛的从头算热力学模拟和等温(593-973K)湿空气和干空气处理表明，在低温含水环境中广泛观察到的失活反映了有利于孤立的Pd离子聚集成纳米颗粒的位置相互转化热力学。在高温无水条件下，实验动力学测量和动力学蒙特卡罗模拟表明，动力学因素对Pd纳米粒子的再分散具有显著的影响，Pd纳米粒子的再分散速率受到初始Pd颗粒尺寸分布的强烈影响，并通过底物扩散介导的Ostwald熟化过程，Pd单体在分子筛载体的阴离子交换中心(骨架Al)以原子捕获过程被捕获。这些发现解决了关于Pd再分散过程中H2O和载体相互作用的长期存在的问题，并确定了通过改变氧化载体和气体条件来增强或抑制Pd位置相互转化的策略。
Palladium cations and nanoparticles supported on oxides and zeolites areused as catalysts and adsorbents for a wide range of chemical reactions of practicalimportance, with various and distinct active site requirements. Consequently, sinteringand redispersion processes that interconvert site-isolated Pd cations and agglomeratednanoparticles underpin catalyst activation and deactivation phenomena, yet theinfluence of Pd nanoparticle size distribution and gas conditions on the thermodynamicand kinetic factors influencing such interconversion is imprecisely understood. Here, weprepare Pd nanoparticles of different particle size and distribution (normal, log-normal) supported on high-symmetry crystalline zeolites to access well-definedmaterials whose Pd site distributions can be quantitatively characterized by experiments and modeled accurately by theory. Abinitio thermodynamic modeling and isothermal (593-973 K) wet and dry air treatments of Pd-zeolites reveal that the widelyobserved deactivation in low-temperature hydrous environments reflects site interconversion thermodynamics that favor theagglomeration of isolated Pd cations into nanoparticles. Under high-temperature anhydrous conditions, experimental kineticmeasurements and kinetic Monte Carlo simulations evince the preeminence of kinetic factors on Pd nanoparticle redispersion intocations, which proceeds at rates that are strongly influenced by the initial Pd particle size distribution and via a substrate diffusion-mediated Ostwald ripening process whereby Pd monomers are captured in an atom trapping process at anionic exchange sites(framework Al) in the zeolite support. Thesefindings resolve longstanding questions regarding the roles of H2O and supportinteractions in Pd redispersion processes and identify strategies to enhance or suppress Pd site interconversion by modifying oxidesupports and gas conditions.