Catalysts are used in essentially all chemical processes to increase their efficiency and lower energy costs. They help by making the desired reaction pathways faster, or more favorable compared to undesired reaction pathways. To do so, the catalyst must associate with the molecules that need to be converted using active sites at the catalyst surface. Some catalysts consist of small metal nanoparticles, others of metal atoms, and others again act as solid acids or bases. Not all possible sites at the surface of a catalyst material are (equally) active, and direct geometric effects, electronic effects, and confinement effects for example in narrow pores (as observed in zeolites and enzymes) play major roles in determining catalyst activity. Irving Langmuir thought we can best describe the active sites as planar surfaces, like a chessboard, that nowadays can be accessed experimentally with a.o., single-crystal surface sensitive techniques. Hugh Stott Taylor, in contrast, believed that the active sites act as oligarchs and can be found mainly at surface defects, with a small number of sites dominating the activity.
In a review article published in Nature Reviews Chemistry, Charlotte Vogt and Bert Weckhuysen from Utrecht University and the Technion Institute for Technology discuss the meaning of the term “active site” in heterogeneous catalysis. In light of recent discoveries, they postulate that many of the concepts often used to describe active sites deserve some further thoughts and even revision. By comparing the different schools of thought (i.e., those of Langmuir and Taylor), the authors conclude that multi-scale effects play an important role. These larger-scale multi-dimensional effects can account for various orders of magnitude in observed catalytic activity. So, we have to expand our chessboard to multiple dimensions, and start playing 3D-chess. This also implies that the catalyst material must be characterized by a wide range of dedicated analytical tools that account for the intrinsic multiscale heterogeneity of the solid catalyst, by combined complex, and convoluted different electronic, geometric and confinement effects.
The concept of active site in heterogeneous catalysis, Charlotte Vogt and Bert Weckhuysen, Nature Reviews – Chemistry, January 6, 2022, DOI: https://doi.org/10.1038/s41570-021-00340-y