Formation, dynamics, and long-term stability of Mn- and Fe-promoted Rh/SiO2 catalysts in CO hydrogenation

Abstract

The conversion of syngas (CO/H₂) to ethanol (StE) is one promising example to generate a high-value fuel and key intermediate for various base chemicals, preferably from non-fossil carbon resources. Rh-Based catalysts demonstrated the highest selectivities towards C₂₊ oxygenates and ethanol, in particular. However, the accomplished yields still must be increased, and the catalyst's stability must be improved for industrial application. One primary strategy to improve C₂₊ oxygenate yields over Rh is the addition of one or several promoters. Specifically, Mn and Fe are among the most frequently used metals to improve rhodium's catalytic performance in binary and ternary systems. To date, experimental studies primarily focused on increasing the C₂₊ oxygenate yields, but long-term catalytic investigations are only rarely reported. Consequently, Mn and Fe's specific role as promoter and their influence on the long-term and thermal stability of supported Rh catalysts are not clarified as yet. A holistic view of atomistic promoter effects and their impact on the stability and dynamics of Rh-based catalysts under reaction conditions is thereby highly desired. Herein, we report a comprehensive study about the stability and dynamics of Mn- and Fe-promoted Rh/SiO₂ catalysts at industrially relevant high-pressure conditions (54 bar). For this purpose, unpromoted Rh/SiO₂, single-promoted RhMn/SiO₂ and RhFe/SiO₂, and complex multi-promoted RhMnFe/SiO₂ catalysts were systematically investigated in four different states: calcined, reduced, after a long-term catalytic study (>22 days on stream), and after a high temperature stability investigation (T = 243–320 °C). The thorough analysis of each catalyst in the different states with integral and local characterization methods led to specific structural models before and after long-term catalytic investigations. These structural models provide a detailed view on compositions, electronic properties, and morphologies of promoted Rh/SiO₂ catalysts and serve as a basis for improved catalyst design strategies and more sophisticated computational modeling efforts.