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 (NG) and pyridine nitrogen (NP) are identified as the primary active sites. However, at higher nitrogen contents, the synergistic interaction between NP and NG 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.


A techno-economic analysis was performed for a hydrogen-driven calcium looping (CaL) process capable of capturing 5 × 104–7 × 104 metric tons of CO2 per year from flue gas. The study investigated the use of coke oven gas (COG) and wind-photovoltaic to hydrogen (WPTH) as hydrogen sources. With COG as the hydrogen source, the CaL process yielded an annual production of 1.49 × 108 Nm3 CH4, an energy efficiency of 84.77%, and a payback period of 5.49 years. Conversely, using WPTH as the hydrogen source resulted in a lower annual CH4 output of 3.9 × 107 Nm3, a reduced energy efficiency of 65.04%, and annual losses of 62.10 million USD. In the near to mid-term, the hydrogen-driven CaL process enabled by COG is practically viable for industrial-scale operation. Using WPTH as the hydrogen source provides some improvements in certain aspects but drawbacks in others compared to COG. However, the considerably higher cost of producing green hydrogen remains a substantial hindrance to the process’s economic feasibility.