Transport Phenomena: Challenges and Opportunities for Molecular Catalysis in Metal–Organic Frameworks

Abstract

Metal–organic frameworks (MOFs) are appealing heterogeneous support matrices that can stabilize molecular catalysts for the electrochemical conversion of small molecules. However, moving from a homogeneous environment to a porous film necessitates the transport of both charge and substrate to the catalytic sites in an efficient manner. This presents a significant challenge in the application of such materials at scale, since these two transport phenomena (charge and mass transport) would need to operate faster than the intrinsic catalytic rate in order for the system to function efficiently. Thus, understanding the fundamental kinetics of MOF-based molecular catalysis of electrochemical reactions is of crucial importance. In this Perspective, we quantitatively dissect the interplay between the two transport phenomena and the catalytic reaction rate by applying models from closely related fields to MOF-based catalysis. The identification of the limiting process provides opportunities for optimization that are uniquely suited to MOFs due to their tunable molecular structure. This will help guide the rational design of efficient and high-performing catalytic MOF films with incorporated molecular catalyst for electrochemical energy conversion.

Enhancing photovoltages at p-type semiconductors through a redox-active metal-organic framework surface coating

“…The amount of the film that is electrochemically probed can be calculated using Eq. (2) [40][41][42]…”

“…The amount of the film that is electrochemically probed can be calculated using Eq. ( 2 ) 40 – 42 where δ is the diffusion-layer thickness (cm), D app is the apparent diffusion coefficient (cm 2 s − 1 ), R is the gas constant (J mol − 1 K − 1 ), T is temperature (K), F is Faraday’s constant (C mol − 1 ), and υ is the scan rate (V s − 1 ). With an apparent diffusion coefficient of 10 − 10 cm 2 s −1 and a scan rate of 100 mV s − 1 , about 50 nm of the film is probed during the timescale of the measurement.…”

Open Framework Material Based Thin Films: Electrochemical Catalysis and State‐of‐the‐art Technologies

“…Electron mobility in OFM films is critical in improving kinetic processes and turnover frequencies in electrocatalysis. [ 28 ] In general, OFM thin films tend to exhibit low electrical conductivity due to their intrinsic low density, mix of metal–organic and/or organic bonds coupled with an ordered arrangement of building blocks (e.g., hard metal ions, carboxyl‐based organic ligands, and so on). However, it is important to note that low electrical conductivity does not preclude charge‐transfer chemistry upon applied potentials.…”

A versatile heterogeneous photocatalyst: nanoporous gold powder modified with a zinc(II) phthalocyanine derivative for singlet oxygen [4 + 2] cycloadditions

“…A discussion of the observed conversions and activities, respectively, must involve the lifetime and diffusion length of the excited singlet oxygen as well as the diffusion of the substrates. Mass transport becomes increasingly important when dealing with heterogeneous catalysts [54,55]. The intrinsic lifetime of 1 O 2 in the absence of a chemical reaction is limited by its deactivation to the triple state by energy transfer to solvent vibrations (physical quenching) [36,56].…”