Operando electron microscopy in heterogeneous catalysis

Abstract

Visualizing nanoparticles at atomic‐resolution by transmission electron microscopy (TEM) is beneficial for the understanding of their physico‐chemical properties. However, nanoparticles respond dynamically to changes in the surrounding gas or liquid environment, and such changes can have profound impact on their properties. It has therefore remained a long‐standing challenge in heterogeneous catalysis and related chemical fields to functionalize TEM for operando studies in which the state and functionality of nanoparticles are simultaneously evaluated at the atomic‐scale under catalytically meaningful reaction conditions [1]. Recent, we have developed a nanoreactor that enables atomic‐resolution TEM of catalysts at ambient pressure conditions and elevated temperatures [2–5]. The nanoreactor is a micro‐electro‐mechanical system (MEMS) device that integrates a micrometer‐sized and unidirectional gas‐flow channel with a micro‐heater and an array of electron‐transparent SiN x windows (Fig. 1). This nanoreactor design also allows concurrent mass‐spectrometry, calorimetry and electron energy loss spectroscopy of changes occuring in the gas phase during catalysis. Hereby, the nanoreactor enables operando TEM studies in heterogeneous catalysis [6]. Here, we showcase operando TEM movies that correlate the atomic‐scale dynamics of Pt nanoparticles with their catalytic activity for the CO oxidation at relevant pressure and temperature conditions (Fig. 2) [6,7]. This reaction has long served as the prototypical example for oscillating chemical reactions. The movies surprisingly show that the catalytically active surface sites undergo periodic and reversible changes that are synchronous with the oscillations in the reaction rate. The atomic‐resolution TEM was performed under low electron dose‐rate conditions to surpress beam‐induced alterations [6,8], and the observations were location‐dependent due to the mass‐ and heat‐distributions across the reaction zone [6,7]. In interplay with density functional theory, microkinetic and mass‐ and heat‐transport calculations, the operando TEM observations unveiled a mechanism for the oscillatory behavior based on the difference in CO adsorption energy and oxidation rate on the prevalent Pt surface terminations. Thus, operando TEM can extend the description of dynamics and functionality in catalysis with atomic‐scale information that is specific to the surface sites, and bridge simultaneously both the so‐called materials and pressure gap in catalysis and surface science.