Quantitative study on catalysis of unpaired electrons in carbon edge sites

Abstract

Carbon materials show incredible potential as environmentally friendly catalysts for numerous oxidation reactions. The presence of unpaired electrons (radicals) has also attracted significant research attention as they can act as catalytically active sites. However, the origin of these unpaired electrons in complex sp2-based carbon materials is still under debate, and this has hampered the development of high-performance carbon catalysts. Herein, we reveal the location of these unpaired electrons by advanced temperature-programmed desorption (TPD) coupled with magnetic susceptibility measurements. For this purpose, we use various porous carbon materials with vastly different numbers of edge sites, including activated carbon, carbon black, edge-site-free graphene mesosponge, and edge-site-rich zeolite-templated carbon. Interestingly, the ratio of unpaired electrons to the total edge-site number falls in the 0.52–1.83% range, regardless of the carbon materials investigated. A positive correlation exists between the number of unpaired electrons and the activity of the dehydrogenative oxidation reaction. The unpaired electron's turnover number is estimated to be up to 43, demonstrating that the unpaired electrons are not reactants but catalysts. This work provides the design guidelines for the development of radical-based carbon catalysts.