Revealing the Role of sp2@sp3 Structure of Nanodiamond in Direct Dehydrogenation: Insight from DFT study

To understand the superior performance of nanodiamond (ND) catalyst in dehydrogenation reactions in comparison with other nanostructured carbon catalysts, first principles calculations are performed to study the direct dehydrogenation of isobutane catalyzed by ND catalyst. The NDs form a unique sp(2)@sp(3) core-shell structure because of the diminishment of the surface dangling bond. The calculations show that, in comparison to carbon nanotubes (CNTs), NDs have a much lower activation-barrier of the first C-H bond activation, which is the rate-limiting step in the reaction. Moreover, the complete reaction pathways revealed from the calculations and the adsorption of isobutene further verify the better activity and selectivity of ND catalyst. The investigation of different morphologies, ND sizes, and the presence of surface hydrogens indicates that the sp(2)@sp(3) core shell structure is crucial for the observed excellent reactivity. The Bader charge analysis shows that the oxygen functional groups on ND have less charge than those on CNT, which favors the homolytic cleavage of the C-H bond of isobutane. Moreover, the carbon atoms on ND could accept more charge than their counterparts on CNT, revealing the active role of surface carbon during the C-H bond activation. The current work establishes the relation between the structures of ND catalyst and the catalytic performance of dehydrogenation reactions, which paves the way for further optimization.