Comprehensive Understanding of the Effects of Carbon Nanostructures on Redox Catalytic Properties and Stability in Oxidative Dehydrogenation

Abstract

The intrinsic redox catalytic properties of metal-free carbons have been widely investigated due to their fundamental interest as well as potential practical applications. Although a large variety of nanostructured carbons are now available, the effects of carbon nanostructures on redox properties have not been comprehensively understood. In this work, the redox catalytic properties and thermochemical stabilities of 16 different types of carbons, including activated carbon, carbon nanotubes, onion-like carbons, and microporous/mesoporous templated carbons were systematically investigated using n-butane oxidative dehydrogenation as a model reaction. The results demonstrate that the overall catalytic activity increases with increasing content of C═O active sites. However, with increasing C═O content, the activity per site (i.e., turnover frequency) gradually decreases, while the alkene selectivity increases due to the decreased reducibility of each C═O site. Since more C═O sites are present in a thermochemically less stable amorphous framework, the carbons generally exhibit a trade-off relationship between catalytic activity and stability. However, a graphitic carbon with “coin-stacking” carbon layers showed exceptionally high activity and stability simultaneously. This is attributed to its unique carbon structure that simultaneously provides high graphitic order and abundant carbon edge sites where C═O active sites are grafted.