Micro-Spectroscopy to Interrogate Solid Catalysts at Work

Heterogeneous catalysis encompasses a broad range of catalytic solids and highly relevant industrial processes for the production of materials, chemicals, and fuels.1,2 Therefore, subjects of academic and industrial research in heterogeneous catalysis span from the atomic scale (i.e., picometers) to the scale of catalytic reactors (i.e., meters), from fast bond making/breaking processes (i.e., femtoseconds) to slow catalyst deactivation timescales (i.e., hours, days, and years).3 The majority of catalytic studies break down to one central theme of surface science that ultimately determines the performance of a catalyst material—that is, catalytically active sites and their chemical nature, number, distribution, and accessibility.4,5 Nevertheless, catalytic solids possess a 3-D structure that is rarely uniform and often imposes difficulties to determine the active sites and their change in relation to the underlying catalytic mechanisms.6,7 The complexity of catalyst particles can range from well-defined supported metal nanoparticles to millimeter-sized, multicomponent catalyst bodies with a multitude of often very distinct functionalities. Importantly, the relationship between surface structure, composition, and catalytic properties needs to be established under operating conditions.8 Reaction conditions in catalytic reactors vary from gas to liquid phase, from ambient to highly elevated pressures and temperatures. As a result, a catalytic process happens under very dynamic conditions, with constant reactivity changes in space and time.3,9 Therefore, to completely understand the highly heterogeneous and dynamic nature of a catalytic solid, informative single-point spectroscopic measurements0055; 0060 ; 0065 should be further complemented by microscopic methods.9,11,13 Ideally, we would like to follow catalytic processes under operating conditions with high spatiotemporal resolution and sensitivity to their structural constituents and reactivity changes, preferably down to the level of atomic constituents and catalytic turnovers.8,14 The past two decades have witnessed an explosive growth of characterization tools that have transformed a somewhat static picture of the catalytic surface into dynamic movies of constantly changing surfaces that are very sensitive to reaction conditions.0080; 0085; 0090 ; 0095 This complex multidisciplinary scope of heterogeneous catalysis requires a plethora of characterization approaches that are capable of studying various aspects of surface science mentioned earlier.