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 intensely 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 precursors (SOMC/TMP) approach to rationalize the promotional effect of Mn in CO2 hydrogenation. We found 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 MnII is atomically dispersed in the vicinity of metallic Rh nanoparticles, and enables to induce the oxidation of Rh to form Mn-O-Rh interface under reaction conditions. The formed interface is key to maintain Rh+ site to promote the formation of CO, CH3OH and CH3CH2OH.