Catalytically active sites and their complexity: a micro-review
This paper reports a short historical introduction to the concept of active sites in heterogeneous catalysis, followed by a brief illustration of certain features of active centres. Three different typologies of heterogeneous catalysts, having different complexity levels, are reported in this work to show the important role of self-organizing phenomena in catalysis: i) single-site heterogeneous catalysts (TS-1); ii) metal supported catalysts (small metal clusters and nanoparticles); iii) transition metal oxide catalysts (operating via a Mars-van Krevelen mechanism). Indeed, a catalytic system may regenerate itself through the self-repair and reorganization of the active site and its environment after each catalytic cycle. This provides information on the “complexity” of a catalytic system similar to phenomena that occur in bio-catalysis.
In the last few decades, thanks to the development of advanced in-situ techniques and to modern scanning probe techniques, it has been possible to study the behaviour of active sites under working conditions and to observe the processes on an atomic scale (1). For example, the scanning tunnelling microscope allows us to directly image atomic steps as “active sites” in dissociation, or to analyze the surface diffusion of adsorbates on an atomic scale. Similarly, the more recent 4D electron microscopy, in which ultrafast laser techniques are combined with electron microscopy, allows us to follow the motion of single atoms at femtoseconds timescale (2). This has reinforced the awareness that active sites are dynamic entities that can continuously change their properties according to operative conditions (1, 2).
During a catalytic reaction, many sites or regions on the catalyst surface are able to interact with one another and thus lead to self-organizing phenomena that occur in both space and time (1, 3). Then, the catalyst may regenerate itself through the products desorption, the self-repair and reorganization of the active site ...