Bifunctionality of Re Supported on TiO2 in Driving Methanol Formation in Low-Temperature CO2 Hydrogenation

Low temperature and high pressure are thermodynamicallymore favorableconditions to achieve high conversion and high methanol selectivityin CO2 hydrogenation. However, low-temperature activityis generally very poor due to the sluggish kinetics, and thus, designinghighly selective catalysts active below 200 & DEG;C is a great challengein CO2-to-methanol conversion. Recently, Re/TiO2 has been reported as a promising catalyst. We show that Re/TiO2 is indeed more active in continuous and high-pressure (56and 331 bar) operations at 125-200 & DEG;C compared to an industrialCu/ZnO/Al2O3 catalyst, which suffers from theformation of methyl formate and its decomposition to carbon monoxide.At lower temperatures, precise understanding and control over theactive surface intermediates are crucial to boosting conversion kinetics.This work aims at elucidating the nature of active sites and activespecies by means of in situ/operando X-ray absorptionspectroscopy, Raman spectroscopy, ambient-pressure X-ray photoelectronspectroscopy (AP-XPS), and diffuse reflectance infrared Fourier transformspectroscopy (DRIFTS). Transient operando DRIFTSstudies uncover the activation of CO2 to form active formateintermediates leading to methanol formation and also active rheniumcarbonyl intermediates leading to methane over cationic Re singleatoms characterized by rhenium tricarbonyl complexes. The transienttechniques enable us to differentiate the active species from thespectator one on TiO2 support, such as less reactive formateoriginating from spillover and methoxy from methanol adsorption. TheAP-XPS supports the fact that metallic Re species act as H-2 activators, leading to H-spillover and importantly to hydrogenationof the active formate intermediate present over cationic Re species.The origin of the unique reactivity of Re/TiO2 was suggestedas the coexistence of cationic highly dispersed Re including singleatoms, driving the formation of monodentate formate, and metallicRe clusters in the vicinity, activating the hydrogenation of the formateto methanol.

Automated summary

Low temperature and high pressure are advantageous conditions for achieving high conversion and selectivity in CO2 hydrogenation. Re/TiO2 has been identified as a promising catalyst with higher activity than the industrial Cu/ZnO/Al2O3 catalyst at high pressure and low temperature. The study provides insights into the nature of active sites and active species, demonstrating the active role of cationic Re species in CO2 activation and formate intermediate formation.