Johnson Ben A, Ott Sascha
Department of Chemistry, Ångström Laboratory, Uppsala University, Box 523, 751 20 Uppsala, Sweden.
Chem Sci. 2020 Jul 28;11(28):7468-7478. doi: 10.1039/D0SC02601H. Epub 2020 Jun 26.
Metal-organic frameworks (MOFs) are becoming increasingly popular as heterogenous support matrices for molecular catalysts. Given that reactants, or potentially holes/electrons, need to diffuse into the porous framework as the reaction proceeds, the reaction can possibly take place within the bulk of the particle or be confined to a thin layer at the surface due to transport limitations. Herein, a simple steady-state reaction-diffusion kinetic model is developed to diagnose these two mutually exclusive behaviors in MOF-based systems. The oxygen evolution reaction (OER) driven by a chemical oxidant is presented as an example mechanism. Quantitative metrics for assigning either bulk or surface reactivity are delineated over a wide variety of conditions, and numerical simulations are employed to verify these results. For each case, expressions for the turnover frequency (TOF) are outlined, and it is shown that surface reactivity can influence measured TOFs. Importantly, this report shows how to transition from surface to bulk reactivity and thus identifies which experimental parameters to target for optimizing the efficiency of MOF-based molecular catalyst systems.
金属有机框架（MOFs）作为分子催化剂的非均相载体正变得越来越受欢迎。由于随着反应进行反应物或潜在的空穴/电子需要扩散到多孔框架中，反应可能在颗粒内部发生，也可能由于传质限制而局限于表面的薄层。在此，开发了一个简单的稳态反应-扩散动力学模型来诊断基于MOF的体系中的这两种相互排斥的行为。以化学氧化剂驱动的析氧反应（OER）为例进行说明。在各种条件下描绘了用于确定体相或表面反应活性的定量指标，并采用数值模拟来验证这些结果。对于每种情况，都概述了周转频率（TOF）的表达式，并表明表面反应活性会影响测得的TOF。重要的是，本报告展示了如何从表面反应活性转变为体相反应活性，从而确定了哪些实验参数可用于优化基于MOF的分子催化剂体系的效率。
