Atomically dispersed Ru(III) on N-doped mesoporous carbon hollow spheres as catalysts for CO₂ hydrogenation to formate

Atomically dispersed Ru(III) catalysts on hierarchical and porous supports show great potential for application in the catalysis of the hydrogenation of CO₂ to formate due to their high catalytic activity and stability. In this study, heterogeneous Ru(III) catalysts on N-doped mesoporous carbon hollow spheres (N-MCHS) were synthesized through a SiO₂-templated resorcinol–formaldehyde polymer pyrolysis at different temperatures. At low pyrolysis temperatures (700–800 °C), the dominant N species on the catalysts were found to be pyrrolic and pyridinic N (N_pyrrolic and N_pyridinic). However, the dominant N species on the samples produced from 900 to 1000 °C was graphitic N (N_graphitic). The highest catalytic activities on the batch hydrogenation experiments were recorded on the catalysts with the predominant Ru–N_graphitic active sites. Using the Ru/N-MCHS-900 catalyst, the turn-over number (TON) was as high as 7550 after the hydrogenation for 2 h at 120 °C and 8 MPa. On the other hand, the Ru/N-MCHS-700 catalyst exhibited excellent stability in both batch and continuous reactors. No noticeable deactivation was observed even after 72 h of continuous hydrogenation, which can be attributed to the strong binding energy between the Ru(III) species, and the N_pyridinic and N_pyrrolic sites on the N-MCHS supports. This study provided significant insights for the development of novel heterogeneous catalyst systems for the hydrogenation of CO₂ to formate.