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Abstract:
As-prepared materials tested for a catalytic reaction are usually only precatalysts that become active and/or selective under specific conditions. During this initial formation phase, catalysts can undergo a change in their structure, morphology, chemical state, or even composition. This dynamic behavior has a vital impact on reactivity, and we identified that this initial formation phase is also critical for Rh in the catalytic conversion of synthesis gas to oxygenates and ethanol in particular. The syngas-to-ethanol reaction (StE) is a promising alternative route to ethanol from fossil and nonfossil carbon resources. Despite heavy research efforts, rates and selectivities still need to be improved for industrial operations. For this reason, structure–function relationships at industrially relevant reaction conditions must be clarified. Although some in situ and operando studies have been reported, a pressure gap still exists between experimental and process-relevant high-pressure conditions. To overcome this pressure gap and investigate the dynamic behavior of Rh-based catalysts under reaction conditions, we applied a generic method for formation phase studies at high partial pressures of synthesis gas where standard operando methods are inapplicable. Combining integral and local characterization methods before and after a long-term catalytic test of a RhFeOx/SiO2 catalyst (>140 h on stream) allowed us to ascribe a drastic decrease in ethanol formation to a structural change from an unalloyed RhFeOx to an alloyed RhFe/FeOx nanostructure. Our investigation provides an explanation for the great variation of reported catalytic results of RhFe catalysts and their nanostructures in synthesis gas conversion. The structure–function relationship we identified finally provides the opportunity for improved catalyst design strategies: stabilizing the Rh–FeOx interface by preventing RhFe nanoalloy formation. As one example, we report a RhFeOx catalyst on a high surface area Mn2O3 support which decreases the Fe mobility and reducibility through the formation of a (Fe,Mn)Ox mixed surface oxide. Stabilizing the Rh–FeOx interface finally led to stable ethanol selectivity, and the formation of RhFe nanoalloy structures was not observed.
