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

•N-doped mesoporous carbon hollow spheres (N-MCHSs) support for RuCl3 catalysts were examined for CO2 hydrogenation.•At low pyrolysis temperatures (700–800 °C), the dominant N species were shown to be pyrrolic, pyridinic N however, at high pyrolysis temperatures (900–1000 °C) were graphitic N.•The Ru/N-MCHS-900 catalyst, the turn-over number was as high as 7550 after the hydrogenation for 2 h at 120 °C and 8 MPa.•The Ru/N-MCHS-700 catalyst exhibited excellent stability in both batch and continuous reactors. 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 (Npyrrolic and Npyridinic). However, the dominant N species on the samples produced from 900 to 1000 °C was graphitic N (Ngraphitic). The highest catalytic activities on the batch hydrogenation experiments were recorded on the catalysts with the predominant Ru–Ngraphitic 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 Npyridinic and Npyrrolic 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.