Unveiling the Eley–Rideal mechanism: H₂ activation for nitrobenzene hydrogenation on nitrogen-doped carbon nanotubes

Nitrogen-doped carbon nanotubes (NCNTs) emerge as an efficient metal-free catalyst for the hydrogenation of nitrobenzene to aniline, utilizing molecular hydrogen (H₂) as the reducing agent. However, the mechanism of H₂ activation on NCNTs is under debate so far. Here, we unveil the catalytic mechanism of NCNTs in this reaction through a comprehensive approach combining experimental and theoretical investigations. Our findings indicate that NCNTs are unable to directly dissociate H₂ molecules, suggesting that the hydrogenation reaction does not proceed via the conventional Horiuti–Polanyi mechanism. Instead, we propose an Eley–Rideal mechanism, where H₂ molecules are activated upon adsorption on the surface of NCNTs with adsorbed nitrobenzene molecules. Graphitic nitrogen (N_G) and pyridine nitrogen (N_P) are identified as the primary active sites. However, at higher nitrogen contents, the synergistic interaction between N_P and N_G is detrimental to catalytic activity, emphasizing that increased nitrogen content does not necessarily enhance performance. Further experiments demonstrate that, in addition to direct H₂ activation, the transfer hydrogenation in protic solvents significantly boosts the overall reaction rate. Our work provides deep insights into the mechanism of H₂ activation for the nitrobenzene hydrogenation catalyzed by NCNTs and offers theoretical guidance for the rational design of high-performance metal-free catalysts in catalytic hydrogenation reactions.