A brief history of the science of catalysis II: from polytopes to hyperstructures
The possibility of detecting active sites and molecules in complex environments, and following their evolution, has revolutionized the field of heterogeneous catalysis over recent decades. It has been shown that active centres are not static at solid surfaces and may interact with each other through transport phenomena and structural flexibility. These interactions lead to spatio-temporal self-organization phenomena, in which both structural and kinetic effects play a role. As a consequence, catalytic reactions typically result in non-linear combinations of complexity structures of active sites. In this scenario, hyperstructures (namely Hv-structures) have recently been proposed as a model to describe the complexity of catalysts.
ACTIVE SITES AND COMPLEXITY STRUCTURES
During the last few decades, significant developments have been made in the study of reactions at solid surfaces, showing that a catalytic surface may change under experimental conditions (1-3).
Thanks to the development of in-situ spectroscopies and to advanced scanning probe techniques, it has been possible to investigate the active sites under reaction conditions and to identify the elementary steps underlying a reaction on an atomic scale (4, 5).
Nowadays, the most recent milestone in the development of these methods is represented by four-dimensional ultrafast electron microscopy (4D UEM), in which ultrafast laser techniques are combined with electron microscopy, allowing the motion of single atoms to be studied on a timescale of femtoseconds (6). The transmission electron microscope (TEM) is a powerful tool that enables the visualization of materials with length scales smaller than the Bohr radius, though a high resolution visualization of structural changes occurring on sub-millisecond time scales is not possible. In UEM, images are obtained either stroboscopically with coherent single-electro ...