Atomic-Scale Observations of Catalyst Structures under Reaction Conditions and during Catalysis

Abstract

Heterogeneous catalysis is a chemical process performed at a solid-gas or solid-liquid interface. Direct participation of catalyst atoms in this chemical process determines the significance of the surface structure of a catalyst in a fundamental understanding of such a chemical process at a molecular level. High-pressure scanning tunneling microscopy (HP-STM) and environmental transmission electron microscopy (ETEM) have been used to observe catalyst structure in the last few decades. In this review, instrumentation for the two in situ/operando techniques and scientific findings on catalyst structures under reaction conditions and during catalysis are discussed with the following objectives: (1) to present the fundamental aspects of in situ/operando studies of catalysts; (2) to interpret the observed restructurings of catalyst and evolution of catalyst structures; (3) to explore how HP-STM and ETEM can be synergistically used to reveal structural details under reaction conditions and during catalysis; and (4) to discuss the future challenges and prospects of atomic-scale observation of catalysts in understanding of heterogeneous catalysis. This Review focuses on the development of HP-STM and ETEM, the in situ/operando characterizations of catalyst structures with them, and the integration of the two structural analytical techniques for fundamentally understanding catalysis.

Future Challenges in Heterogeneous Catalysis: Understanding Catalysts under Dynamic Reaction Conditions

“…Especially electron microscopy has evolved to a true in situ method which can nowadays even be used for catalysts under gas atmospheres and at higher temperatures 12, 85, 86, 87. For this purpose, differentially pumped microscopes or systems making use of special window cells which confine the gas atmosphere are used.…”

“…The new boundary conditions contrast most of the former and current chemical processes that are continuously run in a single, more or less optimal operation window. Notwithstanding, even under steady‐state conditions, dynamic changes are omnipresent in heterogeneous catalysis and electrocatalysis 11, 12, 13, 14, 15, 16…”

“…As one has to unravel catalyst structure, concentration and heat gradients in the reactor while the process is running (“operando”)17, 18, 20, 21, 22 we have only recently begun to grasp these complex structure‐function relationships. With the availability of new analytical methods, it is now possible to gain further insights into (electro‐)catalytic systems and to obtain a deep molecular‐level understanding even for variable reaction conditions 12. Furthermore, theoretical approaches have evolved to an extent that they can now predict active sites and give input for kinetic modeling which will substantially contribute to understanding the complex relationships in dynamically operated systems 23, 24, 25.…”

In Situ Industrial Bimetallic Catalyst Characterization using Scanning Transmission Electron Microscopy and X‐ray Absorption Spectroscopy at One Atmosphere and Elevated Temperature

“…This has led to an explosion of interest in in situ TEM, where materials are studied under more realistic environmental conditions 21, 22, 23. Dedicated environmental transmission electron microscope (ETEM) systems allow for atomic resolution imaging at moderate temperature but are limited to pressures of less than ≈50 mbar 24, 25, 26.…”

“…Since different nanoparticle structures may have varying catalytic activity,13 an understanding of the different particle types observed gives additional insight into catalytic performance. This is particularly important in cases where particle morphology is affected by the environment 20, 21, 22. In systems such as these, in situ high spatial resolution EDS is required to observe the true nanostructures and their compositional distribution as shown in Figure 3 f. …”