The promotional role of Mn in CO 2 hydrogenation over Rh-based catalysts from a surface organometallic chemistry approach.

Rh-based catalysts modified by transition metals have been intensively studied for CO 2 hydrogenation due to their high activity. However, understanding the role of promoters at the molecular level remains challenging due to the ill-defined structure of heterogeneous catalysts. Here, we constructed well-defined RhMn@SiO 2 and Rh@SiO 2 model catalysts via surface organometallic chemistry combined with thermolytic molecular precursor (SOMC/TMP) approach to rationalize the promotional effect of Mn in CO 2 hydrogenation. We show that the addition of Mn shifts the products from almost pure CH 4 to a mixture of methane and oxygenates (CO, CH 3 OH, and CH 3 CH 2 OH) upon going from Rh@SiO 2 to RhMn@SiO 2 . In situ X-ray absorption spectroscopy (XAS) confirms that the Mn II is atomically dispersed in the vicinity of metallic Rh nanoparticles and enables to induce the oxidation of Rh to form the Mn-O-Rh interface under reaction conditions. The formed interface is proposed to be key to maintaining Rh + sites, which is related to suppressing the methanation reaction and stabilizing the formate species as evidenced by in situ DRIFTS to promote the formation of CO and alcohols.