The effect of Fe–Rh alloying on CO hydrogenation to C2+ oxygenates

• Rh/TiO2 catalysts were made with varying Fe-promoter concentrations.
• The active phase in Fe-promoted Rh/TiO2 during CO hydrogenation was identified.
• Fe–Rh surface alloy formation is confirmed by XRD and TEM.
• Fe–Rh alloy promotes ethanol production over methane formation.
• Metallic Fe undergoes carburization and oxidation to Fe3C and FeOx, respectively.

A combination of reactivity and structural studies using X-ray diffraction (XRD), pair distribution function (PDF), and transmission electron microscopy (TEM) was used to identify the active phases of Fe-modified Rh/TiO2 catalysts for the synthesis of ethanol and other C2+ oxygenates from CO hydrogenation. XRD and TEM confirm the existence of Fe–Rh alloys for catalyst with 1–7 wt% Fe and ∼2 wt% Rh. Rietveld refinements show that FeRh alloy content increases with Fe loading up to ∼4 wt%, beyond which segregation to metallic Fe becomes favored over alloy formation. Catalysts that contain Fe metal after reduction exhibit some carburization as evidenced by the formation of small amounts of Fe3C during CO hydrogenation. Analysis of the total Fe content of the catalysts also suggests the presence of FeOx also increased under reaction conditions. Reactivity studies show that enhancement of ethanol selectivity with Fe loading is accompanied by a significant drop in CO conversion. Comparison of the XRD phase analyses with selectivity suggests that higher ethanol selectivity is correlated with the presence of Fe−Rh alloy phases. Overall, the interface between Fe and Rh serves to enhance the selectivity of ethanol, but suppresses the activity of the catalyst which is attributed to the blocking or modifying of Rh active sites.

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