(499f) Exploring Fundamental Properties of Fe- and Mn-Promoted Rh Catalysts for Syngas Conversion to Higher Alcohols
In this contribution, we present our efforts in carefully controlling the synthesis and preparation of Fe- and Mn-promoted Rh catalysts supported on SiO2. Specific amounts of either one or both promoters are deposited using controlled surface reactions and the impact of systematically varying the amounts of deposited Fe, Mn and Fe/Mn on the product selectivity is explored. This procedure reveals that both, Fe and Mn, increase the selectivity towards ethanol, an effect that is additive and leads to highest selectivity towards ethanol for a ternary system with molar ratios of 1:0.15:0.10 (Rh:Fe:Mn). In a subsequent step, the nature of the active sites is interrogated using a combination of XPS, CO-FTIR spectroscopy and density functional theory calculations. Under reaction conditions, Fe is found to be in a metallic state in a subsurface site. Mn on the other hand is present as an oxide on the 211 step edge. Finally, the implications of the structural changes due to the presence of the promoters on the reaction network and selectivities will be discussed.
The systematic study presented here is a step towards elucidating the effects and interactions of promoters on the selectivity of the studied catalysts. Especially the insights into the nature of the active site are a stepping-stone towards the computational exploration of the impact of various promoters on the activity, which is prerequisite for rational, modeling-based catalyst design.