Quo Vadis Micro-Electro-Mechanical Systems for the Study of Heterogeneous Catalysts Inside the Electron Microscope?

Abstract

During the last decade, modern micro-electro-mechanical systems (MEMS) technology has been used to create cells that can act as catalytic nanoreactors and fit into the sample holders of transmission electron microscopes. These nanoreactors can maintain atmospheric or higher pressures inside the cells as they seal gases or liquids from the vacuum of the TEM column and can reach temperatures exceeding 1000 °C. This has led to a paradigm shift in electron microscopy, which facilitates the local characterization of structural and morphological changes of solid catalysts under working conditions. In this review, we outline the development of state-of-the-art nanoreactor setups that are commercially available and are currently applied to study catalytic reactions in situ or operando in gaseous or liquid environments. We also discuss challenges that are associated with the use of environmental cells. In catalysis studies, one of the major challenge is the interpretation of the results while considering the discrepancies in kinetics between MEMS based gas cells and fixed bed reactors, the interactions of the electron beam with the sample, as well as support effects. Finally, we critically analyze the general role of MEMS based nanoreactors in electron microscopy and catalysis communities and present possible future directions.

Operando Surface Spectroscopy and Microscopy during Catalytic Reactions: From Clusters via Nanoparticles to Meso‐Scale Aggregates

“…Which processes lead to deactivation and how can they be avoided? Significant efforts have thus been devoted to study the catalytic properties of catalyst particles by various microscopic or locally-resolved spectroscopic/diffractive methods, for example, by electron microscopy, [50,91,271,272] nuclear magnetic resonance, [273] (nano-) infrared, [274] X-ray microscopy [85c] and tomography, [51,275,276] fluorescence, [277] and plasmonic nanospectroscopy. [278] Recently, photoemission electron microscopy (PEEM) directly revealed a long-ranging communication effect of the metal/oxide interface with the internal surface sites of metal particles, as discussed below for two oxidation reactions.…”

“…[85c] Operando electron microscopy can be performed in scanning (ESEM) or transmission (OTEM), the latter utilizing dedicated microreactors with electron transparent windows, coupled to sensitive MS analysis. [91][92][93]271,272] NAP-PEEM is still a niché, but will find more applications for sure. [197,198] Altogether, the ongoing technological development of new and more sensitive operando methods, both in timeand spatially-resolved modes, will enable new insights on active centers in catalytic systems.…”

Complexions at the Electrolyte/Electrode Interface in Solid Oxide Cells

“…when the electron beam interacts with the gas phase. [43] This example shows that spatially resolved operando techniques alone may be of limited use and should be complemented by quasi in situ imaging approaches and theory. [44,45]…”

Dynamic interplay between metal nanoparticles and oxide support under redox conditions

“…Depending on the system studied, the pressure-dependent chemical potential of reactive species might thus be too low to trigger specific processes and reactions that are relevant for catalytic function at higher pressure. However, the development and commercial availability of microelectromechanical-based reactors for in situ electron microscopy has extended the accessible pressure range by roughly two orders of magnitude ( 11 , 40 , 41 ) and enabled partially bridging the so-called pressure gap ( 42 ).…”