(499f) Exploring Fundamental Properties of Fe- and Mn-Promoted Rh Catalysts for Syngas Conversion to Higher Alcohols

As of today, large parts of the chemical industry rely on crude oil as a feedstock. However, in recent years dwindling supplies of this carbon source have led to the exploration of alternative feedstocks. A particularly promising candidate is syngas, a mixture of CO and H₂, which can either be produced from biomass or natural gas. Most commonly, syngas is either converted to methanol or to longer alkanes using the Fischer-Tropsch process, but the direct conversion of syngas to C₂₊ oxygenates would be highly desirable, due to their wide range of applications in chemical, fuel and polymer industries. A catalyst with particular promise in this regard is Rh. While monometallic systems show selectivity towards acetaldehyde, promoters, such as Fe and Mn, significantly influence selectivity. However, the atomic scale nature of the active site and its impact on the reaction network and selectivity has not been explored in detail.

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.