Promotional effect of Fe on Rh-based catalyst for CO hydrogenation to ethanol over FeOx/Rh(111)

FeOx/Rh(111) model catalysts were prepared and used to investigate the promotional effects of Fe on Rh catalysts for CO hydrogenation. X-ray photoelectron spectroscopy (XPS), low energy ion scattering (LEIS), low energy electron diffraction (LEED), infrared reflection absorption spectrometer (IRAS) and Auger electron spectroscopy (AES) were used to characterize the model surfaces. The results show that in 1 x 10-7 Torr O2 at 1023 K, the iron oxide can grow on the Rh(111) surface up to 1 monolayer (ML) with a structure of nonstoichiometric FeO denoted as O-Fe-. It can be interchanged to an O-Fe-O- structure by oxidized in oxygen at various temperatures. The catalytic performance shows that the Fe additive significantly increases the formation rates of ethanol and ethylene, while suppresses the formation rate of methane. It reaches a maximum for ethanol at the FeOx coverage of 1ML, but continuously increases for ethylene up to 2 ML. XPS, LEIS and IRAS results reveal the formation of Fe-Rh alloy and iron carbides under CO hydrogenation reaction conditions. These combined results demonstrate that the Rh surface is highly active for methane formation, FeRh alloys promote ethanol formation, while iron carbides may be good for ethylene formation. The present model study allows to more directly evaluate the active sites without involving some effects from the oxide supports.