A redox catalyst can be present in the solution phase or immobilized on the electrode surface. When the catalyst is present in the solution phase the process can proceed via inner- (with bond formation, chemical catalysis) or outer-sphere mechanisms (without bond formation, redox catalysis). For the latter, log k is linearly proportional to the redox potential of the catalysts, E°. In contrast, for inner-sphere catalyst, the values of k are much higher than those predicted by the redox potential of the catalyst. The behaviour of these catalysts when they are confined on the electrode surface is completely different. They all seem to work as inner-sphere catalysts where a crucial step is the formation of a bond between the active site and the target molecule. Plots of (log i)E versus E° give linear or volcano correlations. What is interesting in these volcano correlations is that the falling region corresponding to strong adsorption of intermediates to the active sites is not necessarily attributed to a gradual surface occupation of active sites by intermediates (Langmuir isotherm) but rather to a gradual decrease in the amount of M(II) active sites which are transformed into M(III)OH inactive sites due to the applied potential.
Part of the book: Redox
The general trends in the construction of highly active electrode devices are focused on the science of materials. These are useful for developing 2D nanostructured electrodes, with well-defined active sites, which are excellent approaches for understanding the fundamentals of electrocatalytic reactions. Here we present an overview of the experimental self-assembled molecular catalyst configurations to develop excellent electrode materials containing molecular catalysts for energy conversion device applications. First, by applying well-known reactivity descriptors for electrocatalysis, nanoarchitectonics, and the self-assembled concept, we summarize the main molecular building blocks to achieve a technology system for arranging by a rational design, nanoscale structural units configuration that promotes electrocatalytic reactions such as oxygen reaction reduction (ORR) and water-splitting reactions. We focus the discussion on the MN4 molecular catalyst linked to electrode surfaces with the help of the axial blocks, bio-inspired self-assembled approaches such as biomimetic models of metalloenzymes active sites, and molybdenum sulfide clusters for hydrogen evolution reaction (HER). We briefly discuss the advantages of developing host-guest self-assembled molecular catalyst systems based on cyclodextrins anchored to electrodes to get well-defined active sites with local environment control.
Part of the book: Self-Assembly of Materials and Their Applications