The promotional role of Mn in CO2 hydrogenation over Rh-based catalysts from a surface organometallic chemistry approach

Rh-based catalysts modified by transition metals have been intensively studied for CO2 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@SiO2 and Rh@SiO2 model catalysts via surface organometallic chemistry combined with thermolytic molecular precursor (SOMC/TMP) approach to rationalize the promotional effect of Mn in CO2 hydrogenation. We show that the addition of Mn shifts the products from almost pure CH4 to a mixture of methane and oxygenates (CO, CH3OH, and CH3CH2OH) upon going from Rh@SiO2 to RhMn@SiO2. 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.